Fiber optic amplifier
An amplifier for use with fiber optic systems comprises a neodymium YAG crystal placed in series with a signal-carrying optical fiber. The ND:YAG crystal is supplied by the optical fiber with both the signal to be amplified, and pumping illumination. The pumping illumination is coupled onto the optical fiber by a multiplexing coupler which is used to combine the signal to be amplified and illumination from a pumping illumination source onto a single optical fiber. The pumping illumination inverts the neodymium ions within the ND:YAG crystal. The signal to be amplified propagates through this crystal to stimulate emission of coherent light from the neodymium ions, resulting in amplification of the signal. Because this arrangement permits the ND:YAG crystal to be end-pumped with pumping illumination, and because the length of the ND:YAG crystal may be substantially greater that the absorption length for the crystal at the wavelength of the pumping illumination, virtually all of the pumping illumination may be absorbed within the ND:YAG crystal and used for amplification of the signal carried by the optical fiber.
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The concept of optical amplifiers, based upon the capability of certain materials, particularly on a macroscopic level, is well known. Thus, for example, it is known to place a pumping light source and a single crystal neodymium-yttrium aluminum garnet (ND:YAG) rod, several millimeters in diameter and several centimeters in length, in a tubular reflective cavity. For example, the light source and ND:YAG rod may be located, respectively, to extend along the two foci of a cavity having an elliptical cross section. In such an arrangement, light emitted by the light source and reflected from the cavity walls will impinge upon the ND:YAG rod. The light source is preferably selected to emit wavelengths corresponding to the absorption spectra of the ND:YAG crystal so that the neodymium ions of the crystals are inverted to an energy level above the upper lasing level. After inversion, an initial relaxation of the neodymium ions through phonon radiation yields an ion population at the upper lasing level. From this level, the ions will relax to a lower lasing level, emitting light of a wavelength which is characteristic of the ND:YAG material. Advantageously, the lower lasing level is above the ground level for the ions so that a rapid, phonon-emitting relaxation will occur between the lower lasing level and the ground level, enabling a high inversion ratio to exist between the upper and lower lasing levels within the pumped ions.
With the population so inverted, as is well known from laser technology, the ND:YAG will provide a very slow fluorescence, that is, random emission of incoherent light. This spontaneous radiation, however, has a minimal effect on the amplifying rod, since the average lifetime of ions in the inverted state is 230 microseconds, in ND:YAG.
If, after some of the neodymium ions of the ND:YAG rod have been inverted, a light signal at the lasing frequency is transmitted through the rod, the light signal will trigger the relaxation of the neodymium ions, causing coherent emission of stimulated radiation, which will effectively add to the transmitted signal, thus amplifying this signal.
The absorption length of the pumping illumination within the ND:YAG crystal (i.e., the length of material through which the illumination must traverse before about 65% of the illumination is absorbed) is typically in the range between 2 and 3 millimeters, and thus the ND:YAG crystals used in transverse pumping structures such as described previously have had diameters at least this large so that the crystal could absorb a substantial portion of the pumping radiation during the initial reflection from the cavity walls and passage through the crystal. If, during this initial traverse through the crystal, the pumping illumination is not absorbed, it is likely to be reflected by the cavity walls back to the light source, where it will be reabsorbed, generating heat in the light source and reducing the overall efficiency of the amplifier.
When such amplifiers are used in fiber optic systems, it has been thought necessary, because of the large difference in diameter between the optical fiber and the ND:YAG crystal, to use optical components, such as lenses, to focus light from the optical fiber into the ND:YAG rod, and the amplified light signal from the ND:YAG rod back into another fiber. Such optical systems require careful alignment and are susceptible to environmental changes, such as vibration, and thermal effects. Additionally, the optical components and the size of the ND:YAG rod make the amplifying system relatively large, and thus impractical for certain applications. Furthermore, the relatively large size of the ND:YAG rod introduces beam wander within the rod due to thermal effects. Thus, the signal from the input fiber optic element will traverse different paths through the rod, a characteristic which is temperature related and varies with time, so that the output light may be lost due to the fact that the fiber will accept only light within a small acceptance angle. Thus, as the beam within the ND:YAG rod wanders, the output signal may vary in an uncontrollable manner. Furthermore, the large size of the ND:YAG rod requires a large amount of input energy in order to maintain a high energy density within the rod. Such large pump power requires high output light sources, generating substantial heat which must be dissipated, typically by liquid cooling of the cavity.
While amplifiers of this type are useful in many applications, such as some communications applications, a use which puts severe limitations upon the amplification system is a recirculating fiber optic gyroscope. With such gyroscopes, an optical fiber, typically a kilometer or more in length, is wound into a loop, and a light signal is recirculated within the loop in both directions. Motion of the loop causes a phase difference between the counter-propagating light signals which may be used to measure gyroscope rotation. It is advantageous, because the phase shift induced in one rotation is relatively small and because periodic outputs relating to rotation are required, to recirculate input light within the loop as many times as possible.
In traversing a kilometer of optical fiber, an optical signal will typically lose 30 to 50 percent of its intensity. An amplifier, if capable of amplifying the bidirectional counter-propagating light signals, would permit a light signal to propagate many more times within the loop, if the amplifier were placed in a series with the loop, and provided a gain equal to the propagation loss.
Unfortunately, the relatively large size, high power requirements caused by relatively inefficient performance, beam wander effects, and cooling requirements of prior art ND:YAG rod amplifiers makes such amplifiers relatively impractical for high accuracy gyroscopes. These factors, of course, also limit the utility of such amplifiers in other applications, such as communication networks.SUMMARY OF THE INVENTION
The disadvantages associated with crystal rod amplifiers are alleviated in the present invention. This invention permits end pumping of the ND:YAG material, and thus, completely avoids the requirement for a large diameter for this crystal which is inherent in side pumping arrangements. The ND:YAG fiber may thus be made extremely small in diameter in comparison with prior art rod amplifiers, since the pumping illumination is absorbed along the length of the fiber, rather than across its width. This results in a higher concentration of pumping illumination within the small diameter of the ND:YAG crystal and thus a higher potential gain for the amplifying structure.
In order to accomplish this end pumping, the ND:YAG material is formed as small diameter fiber and is placed in series with the optical fiber which is transmitting the signal to be amplified.
Adjacent to the ND:YAG fiber, the optical fiber passes through a multiplexing optical coupler. Within this multiplexing coupler, a pair of optical fibers are arranged with a carefully selected interaction length to provide a high fiber-to-fiber coupling efficiency at the wavelength of the pumping source, but a low coupling efficiency at the wavelength of the signal to be amplified. This results in a coupling of the pumping illumination into the optical fiber which is carrying the signal, and thus into the ND:YAG fiber, while substantially eliminating loss to the optical signal which is to be amplified, since this signal is not coupled from the optical fiber of the multiplexing coupler.
Because the present invention permits the pumping illumination to be coupled into the optical signal fiber for guiding to the end of the ND:YAG fiber, the diameter of the ND:YAG fiber need not exceed the absorption length at the pumping wavelength, since the pumping illumination is effectively absorbed in a direction along the axis of the ND:YAG fiber rather than perpendicular to that axis.
Using this arrangement, pumping illumination can be continuously supplied to the amplifying ND:YAG fiber without interfering with its signal carrying characteristics. Thus, since a four-port coupler is used for multiplexing the pumping illumination to the amplifying fiber, the ends of the amplifying fiber are always available for direct signal coupling to the optical fibers within the optical system so that careful time sequencing between the application of pumping illumination and the signal to be amplified is not necessary.
In order to achieve uniform bidirectional amplification within the ND:YAG crystal, pumping illumination may be supplied by multiplexing couplers arranged at both ends of the ND:YAG fiber, providing a symmetrical inversion population along the length of this fiber.BRIEF DESCRIPTION OF THE DRAWINGS
These and other advantages of the present invention are best understood through reference to the drawings, in which:
FIG. 1 is a cross sectional view of the fiber optic coupler used as a multiplexer in the present invention showing a pair of fiber optic strands mounted in respective arcuate grooves with a given radius of curvature of respective bases;
FIGS. 2 and 3 are cross sectional views of the coupler of FIG. 1 taken along lines 2--2 and 3--3, respectively;
FIG. 4 is a perspective view of the lower base of the coupler of FIG. 1 separated from the other base to show its associated fiber mounting and the oval-shaped facing surface of the fiber;
FIG. 5 is a schematic diagram showing the evanescent fields of the pair of fibers overlapping at the interaction region;
FIG. 6 is a chart showing relative coupled power versus signal wavelength for a fiber coupler having a minimum fiber spacing of 4 microns, an offset of 0 micron, and a fiber radius of curvature of 25 centimeters;
FIG. 7 is a chart of relative coupled power versus signal wavelengths similar to FIG. 6 but with a fiber radius of curvature of 200 centimeters;
FIG. 8 is a chart of relative coupled power versus signal wavelength for a fiber coupler having a minimum fiber spacing of 4 microns, a fiber radius of 200 centimeters, and a selectable fiber offset;
FIG. 9 is a schematic diagram showing the directional amplifier of the present invention;
FIG. 10 is a diagram showing the absorption spectrum of ND:YAG at 300.degree. K.;
FIGS. 11a and 11b are a simplified energy level diagrams of a four-level laser using a doped material, such as ND:YAG;
FIG. 12 is a schematic diagram of a symmetrical bidirectional amplifier in accordance with the present invention;
FIG. 13 is a schematic diagram of an amplifier system, including a multiplexing coupler for signal insertion and signal sensing in a closed fiber loop.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In order to gain a detailed understanding of the operation of the fiber optic amplifier of this invention, it is necessary to understand the manner in which a fiber optic coupler can be constructed to selectively couple a first optical frequency while not coupling a second optical frequency. The apparatus, as well as methods for constructing the apparatus, required for such selective coupling, are disclosed in copending patent application entitled "Passive Fiber Optic Multiplexer", filed in the U.S. Patent Office Nov. 9, 1981, bearing Ser. No. 319,301, and listing Herbert J. Shaw and Michel J. F. Digonnet as inventors. That application is assigned to the assignee of the present invention. That application is hereby incorporated herein by reference. Nonetheless, the principal characteristics of that device and its method of manufacture are described below.
This invention utilizes a passive multiplexer which utilizes a fiber optic coupler. This coupler 10 is illustrated in FIGS. 1-4, and includes two strands 12A and 12B of a single mode fiber optic material. The fiber strands 12A and 12B, in the embodiment shown, have a core radius of 2 microns, a core refractive index of 1.46 and a cladding index of 1.456. The strands 12A and 12B are mounted in longitudinal arcuate grooves 13A and 13B, respectively, formed in optically flat confronting surfaces 14A and 14B, respectively, of rectangular bases or blocks 16A and 16B, respectively. The block 16A with the strand 12A mounted in the groove 13A will be referred to as the coupler half 10A and the block 16B with the strand 12B mounted in the groove 13B will be referred to as the coupler half 10B.
Each of the strands 12A and 12B comprise an optical fiber which is doped to have a central core and an outer cladding. The strands 12A and 12B may comprise a commercially available fiber of quartz glass which is doped to have a central core and an outer cladding. The index of refraction of the fibers 12A and 12B should be as nearly as possible identical, and both of the strands 12A and 12B should include a central core which is sufficiently small to provide single mode transmission at the optical frequencies to be used. Thus, these strands 12A and 12B typically have a core diameter on the order of 10 microns or less and a cladding diameter on the order of 125 microns. In the embodiment disclosed, the diameter of the strands 12 and their respective cores are exaggerated. As will be understood in more detail form the description which follows, the fiber 12B is used to transmit the signal to be amplified while the fiber 12A is used to couple pumping illumination to the fiber 12B. For this reason, the fiber 12B will be referred to as the signal fiber while the fiber 12A will be referred to as the pumping fiber.
The arcuate grooves 13A and 13B have a radius of curvature which is very large compared to the diameter of the fibers 12, and have a width slightly larger than the fiber diameter to permit the fibers 12, when mounted therein, to conform to a path defined by the bottom walls of the grooves 13. The depth of the grooves 13A and 13B varies from a minimum at the center of the blocks 16A and 16B, respectively, to a maximum at the edges of the blocks 16A and 16B, respectively. This advantageously permits the fiber optic strands 12A and 12B, when mounted in the grooves 13A and 13B, respectively, to gradually converge toward the center and diverge toward the edges of the blocks 16A, 16B, thereby eliminating any sharp bends or abrupt changes in direction of the fibers 12 which may cause power loss through mode perturbation. In the embodiment shown, the grooves 13 are illustrated as being rectangular in cross section, however, it will be understood that other suitable cross sectional contours which will accommodate the fibers 12 may be used alternatively, such as a U-shaped cross section or a V-shaped cross section.
At the centers of the blocks 16, in the embodiment shown, the depth of the grooves 13, which mount the strands 12, is less than the diameter of the strands 12, while at the edges of the blocks 16, the depth of the grooves 13 is preferably at least as great as the diameter of the strands 12. Fiber optic material was removed from each of the strands 12A and 12B e.g. by polishing, to form the respective oval-shaped planar surfaces 18A, 18B, which are coplanar with the confronting surfaces 14A, 14B, respectively. These surfaces 18A, 18B, will be referred to herein as the fiber "facing surfaces". Thus, the amount of fiber optic material removed increases gradually from zero towards the edges of the block 16 to a maximum towards the center of the block 16. This tapered removal of the fiber optic material enables the fiber cores to converge and diverge gradually, which is advantageous for avoiding backward reflection and excess loss of light energy.
In the embodiment shown, the coupler halves 10A and 10B are identical, and are assembled by placing the confronting surfaces 14A and 14B of the blocks 16A and 16B together, so that the facing surfaces 18A and 18B of the strands 12A and 12B are in facing relationship.
An index matching substance (not shown), such as index matching oil, is provided between the confronting surfaces 14. This substance has a refractive index approximately equal to the refractive index of the cladding, and also functions to prevent the optically flat surfaces 14 from becoming permanently locked together. The oil is introduced between the blocks 16 by capillary action.
An interaction region 32 is formed at the junction of the strands 12, in which light is transferred between the strands by evanescent field coupling. It has been found that, to insure proper evanescent field coupling, the amount of material removed from the fibers 12 must be carefully controlled so that the spacing between the core portions of the strands 12 is within a predetermined "critical zone". The evanescent fields extend into the cladding and decrease rapidly with distance outside their respective cores. Thus, sufficient material should be removed to permit each core to be positioned substantially within the evanescent field of the other. If too little material is removed, the cores will not be sufficiently close to permit the evanescent fields to cause the desired interaction of the guided modes, and thus, insufficient coupling will result. Conversely, if too much material is removed, the propagation characteristics of the fibers will be altered, resulting in loss of light energy due to mode perturbation. However, when the spacing between the cores of the strands 12 is within the critical zone, each strand receives a significant portion of the evanescent field energy from the other strand, and good coupling is achieved without significant energy loss. The critical zone is illustrated schematically in FIG. 5 as including that area, designated by the reference numeral 33, in which the evanescent fields, designated by reference numerals 34A and 34B, of the fibers 12A and 12B, respectively, overlap with sufficient strength to provide coupling, i.e., each core is within the evanescent field of the other.
The blocks or bases 12 may be fabricated of any suitable rigid material. In one presently preferred embodiment, the bases 12 comprise generally rectangular blocks of fused quartz glass approximately one inch long, one inch wide, and 0.4 inch thick. In this embodiment, the fiber optic strands 12 are secured in the slots 13 by suitable cement 38, such as epoxy glue. One advantage of the fused quartz blocks 16 is that they have coefficient of thermal expansion similar to that of glass fibers, and this advantage is particularly important if the blocks 16 and fibers 12 are subjected to any heat treatment during the manufacturing process. Another advantage of the fused quartz blocks is that, as they are made of the same material as the optical fiber, they polish at the same rate as the optical fiber, and thus, provide a continuous support to the fiber during polishing. Other suitable materials for the block 16 include silicon, which also has excellent thermal properties for this application.OPERATION OF THE COUPLER 10
The coupler 10 includes four ports, labeled A, B, C, and D in FIG. 1. When viewed from the perspective of FIG. 1, ports A and C, which correspond to strands 12A and 12B, respectively, are on the left-hand side of the coupler 10, while the ports B and D, which correspond to the strands 12A and 12B, respectively, are on the right-hand side of the coupler 10. For the purposes of discussion, it will be assumed that input light is applied to port A. This light passes through the coupler and is output at port B and/or port D, depending upon the amount of power that is coupled between the strands 12. In this regard, the term "normalized coupled power" is defined as the ratio of the coupled power to the total output power. In the above example, the normalized coupled power would be equal to the ratio of the power at port D to the sum of the power output at ports B and D. This ratio is also referred to as the "coupling efficiency", and when so used is typically expressed as a percent. Thus, when the term "normalized coupled power" is used herein, it should be understood that the corresponding coupling efficiency is equal to the normalized coupled power times 100. In this regard, tests have shown that the coupler 10 has a coupling efficiency of up to 100%. However, it will also be seen that the coupler 10 may be "tuned" to adjust the coupling efficiency to any desired value between zero and the maximum. Such turning may be accomplished e.g. by relatively laterally sliding the fibers in a direction perpendicular to their length. The relative positions of the fibers may be defined in terms of their offset, i.e. the distance between the central axes of the fiber cores, measured in the direction perpendicular to their length, along the surfaces 14. Thus, when the oval surfaces 18 are superimposed, the offset is zero, and the offset increases as the fibers 12 are laterally separated by relatively sliding the blocks 16.
The coupler 10 is highly directional, with substantially all of the power applied at one side of the coupler being delivered to the other side of the coupler. The coupler directivity is defined as the ratio of the power at port D to the power at port C, with the input applied to port A. Tests have shown that the directionally coupled power (at port D) is greater than 60 db above the contra-directionally coupled power (at port C). Further, the coupler directivity is symmetrical. That is, the coupler operates with the same characteristics regardless of which side of the coupler is the input side and which side is the output side. Moreover, the coupler 10 achieves these results with very low throughput losses. The throughput loss is defined as the ratio of the total output power (ports B and D) to the input power (port A) subtracted from one (i.e., 1-(P.sub.B +P.sub.D)/P.sub.A). Experimental results show that throughput losses of 0.2 db have been obtained, although losses of 0.5 db are more common. Moreover, these tests indicate that the coupler 10 operates substantially independently of the polarization of the input light applied.
The coupler 10 operates on evanescent field coupling principles in which guided modes of the strands 12 interact, through their evanescent fields, to cause light to be transferred between the strands 12. As previously indicated, this transfer of light occurs at the interaction region 32. The amount of light transferred is dependent upon the proximity and orientation of the cores, as well as the effective length of the interaction region 32. As will be described in detail below, the amount of light transferred is also dependent of the wavelength of the light. The length of the interaction region 32 is, in turn, dependent upon the radius of curvature of the fibers 12, and, to a limited extent, the core spacing, although it has been found that the effective length of the interaction region 32 is substantially independent of core spacing. However, the "coupling length", i.e., the length within the interaction region 32 which is required for a single, complete transfer of a light signal from one fiber 12 to the other, is a function of core spacing, as well as wavelength. In one exemplary embodiment, employing an edge-to-edge core spacing of about 1.4 microns, and a radius of curvature on the order of 25 centimeters, the effective interaction region is approximately one millimeter long at a light signal wavelength of 633 nm. Because the coupling length at 633 nm is also one millimeter in such a coupler, the light makes only one transfer between the strands 12 as it travels through the interaction region 32. However, if the length of the interaction region 32 is increased, or core spacing decreased, a phenomenon referred to herein as "overcoupling" will occur, since the coupling length is shorter than the effective interaction length. Under these circumstances, the light will transfer back to the strand from which it originated. As the interaction length is further increased, and/or the core spacing further decreased, the effective interaction length increases relative to the coupling length, and at least some of the light transfers back to the other strand. Thus, the light may make multiple transfers back and forth between the two strands 12 as it travels through the region 32, the number of such transfers being dependent on the length of the interaction region 32, the light wavelength (as described below), and the core spacing.
Since the coupling length in a single mode fiber coupler, as described above, shows a strong dependence on signal wavelength, as described in detail in the copending application incorporated above, it is possible with properly chosen geometrical parameters for the coupler 10, to totally couple one signal wavelength while a second signal wavelength remains essentially uncoupled. This phenomenon permits the combination of two signals fed into the ports on one side of the coupler 10. Thus, as shown in FIG. 1, if a pumping signal having a wavelength .lambda..sub.1 is fed into port A of coupler 10, and a signal to be amplified, having a wavelength .lambda..sub.2 is coupled to port C, and the geometry is properly selected, both signals can be combined at port D, with virtually no light output at port B.
To illustrate this wavelength dependence, FIG. 6 provides a plot of coupled power versus signal wavelength in the visible and near infrared spectrum for a particular coupler geometry. Because for this coupler configuration the effective interaction length of the coupler is an odd multiple of the coupling length for the wavelength 720 nm, but an even multiple of the coupling length for the wavelength 550 nm, the wavelength 720 nm will be 100% coupled, while the wavelength 550 nm will be effectively uncoupled. With different efficiencies, different wavelengths may be combined or separated. For instance, 590 nm and 650 nm may be separated or combined at an 80% efficiency.
Virtually any pair of wavelengths (.lambda..sub.1, .lambda..sub.2) may be efficiently combined or separated so long as the effective interaction length is an even multiple of the coupling length for one wavelength and an odd multiple for coupling length for the other wavelength. As the number of coupling lengths within the effective interaction length increases, the resolution of the multiplexer is enchanced. As is described in detail in the incorporated reference, the multiplexer resolution may be enhanced by incresing the radius of curvature of the fibers 12A, 12B. Provided that the interaction length of the coupler is large enough, virtually any two signals may be exactly mixed or separated, regardless of how closely spaced their wavelengths are.
The interaction length is a function of wavelength, and the resolution is approximately proportional to (R)-1/2. As R increases, the effective interaction length increases, and becomes a higher multiple of the coupling length, improving resolution. This result is illustrated in FIG. 7, which is comparable to the graph of FIG. 6, except that the radius of curvature has been increased to 200 centmeters. As anticipated, this increase in radius improves the coupler resolution near g=600 nm from approximately 170 nm in the 25 centemeter radius example of FIG. 6 to approximately 60 nm in the 200 centimeter case.
The resolution of a multiplexing coupler depends on two independent parameters, H (fiber spacing) and R (radius of curvature of the fibers). For a given pair of signa wavelengths, efficient mixing may be achieve by first properly selecting a fiber spacing H for the coupler which yields a large wavelength dependence for the wavelengths of interest (choice of H), and then by selecting a radius of curvature which yields a resolution equal to the difference between the wavelengths (choice of R).
After the resolution of the coupler has been set in accordance with the wavelengths to be separated, the coupler may be tuned to precisely adjust the coupling lengths for the wavelengths of interest so that the effective interaction length is an even multiple of the coupling length of one wavelength and an odd multiple of the coupling length of the other wavelength. This is accomplished by offsetting the fibers by sliding the blocks 16A, 16B (FIG. 1) relative one another in a direction normal to the axis of the fibers 12A, 12B. Such an offset has the effect of increasing the minumum fiber spacing H and increasing the effective radius of curvature of the fibers. If the required offset is small enough, it will not upset the multiplexer resolution. This stems from the fact that the separation H of a large radius coupler changes rapidly with fiber offset in comparison to changes in the effective radius of curvature with fiber offset.
To illustrate this tunability of multiplexing couplers, FIG. 8 provides a plot of relative coupled power versus wavelength for three increasing values of fiber offset (0 microns, 0.5 microns, and 1.0 microns). The curve is seen to shift toward increasing wavelengths as the offset increases, while the period of oscillation (or resolution) remains virtually unchanged. In this particular example (R=200 cm, H=4 microns), a 1-micron offset shifts the curve by approximately 45 nm.OVERALL OPERATION OF THE AMPLIFIER
Referring now to FIG. 9, the manner in which the amplifier of the present invention utilizes the wavelength multiplexing properties of the coupler 10 to provide pumping illumination to energize ND:YAG fiber will be described.
A source of pumping illumination 42 is coupled to the fiber 12A of the coupler 10 to provide pumping illumination at port A of the multiplexing coupler 10, and a signal to be amplified is coupled to one end of the fiber 12B at port C of the coupler 10. The pumping illumination from the source 42 and the signal to be amplified are combined at port D at the coupler 10 through the multiplexing action of the coupler, as explained above. Thus, the coupler 10 is adjusted to have a 100% coupling efficiency at the wavelength of the source 42, and a 0% coupling efficiency at the wavelength of the signal input at port C. This pair of signal wavelengths is supplied to an ND:YAG crystal 44 which is coupled to the fiber 12B in the manner described below. The signal input at the port C will be amplified in the ND:YAG crystal 44 and the amplified signal will be coupled from this crystal 44 to an optical fiber 46 for transmission within the optical fiber system. The diameter of the ND:YAG crystal 44 may be extremely small in comparison with the diameter of ND:YAG rods used in prior art amplifiers. For example, an amplifier has been constructed in which the crystal 44 has a diameter of 100 microns. Even smaller diameters are feasible, approaching the diameter of the single mode fiber 12B. Coupling between the fiber 12B and the crystal 44 is enhanced as the diameter of the crystal 44 is reduced and signal gain is increased since the density of pumping illumination from the source 42 within the crystal 44 increases as the crystal 44 diameter is reduced.ND:YAG AMPLIFICATION
Referring now to FIG. 10, which is a diagram of the absorption spectrum of the ND:YAG crystal 44 at 300.degree. K., it can be seen that ND:YAG material has a relatively high optical density, and thus a short absorption length, at selected wavelengths. For this reason, it is advisable to select the wavelength of the pumping illumination source 42 in order to permit the absorption length to be as short as possible. This will permit substantially complete absorption of the pumping illumination within a very short length of the ND:YAG crystal 44. As can be seen from FIG. 10, the wavelength 0.58 microns is best suited for pumping illumination, although the wavelengths 0.75 and 0.81 microns are relatively well suited.
Referring now to FIG. 11A, which is an energy level diagram for the ND:YAG crystal 44, it will be understood that, when pump light at the absorption wavelength, described above, is absorbed by the ND:YAG crystal 44, the neodymium ions are excited from the ground state to the pump band. From the pump band, the ions quickly relax, through phonon interactions, to the upper lasing level. From this upper lasing level, the neodymium ions will undergo a relatively slow fluorescence to the lower lasing level. From this latter level, a final, rapid phonon relaxation occurs to the ground state. This latter rapid relaxation in a four-level laser system of the type shown in FIG. 11A is advantageous, since the rapid phonon relaxation between the lower lasing level and the ground state provides a practically empty lower lasing level. This feature is shown in FIG. 11B, in which the population densities at the pump band upper lasing level, lower lasing level, and ground state are shown for the ND:YAG crystal 44 during continuous pumping. Because the rate of fluorescence between the upper lasing level and lower lasing level is relatively slow in comparison with the phonon relaxation between the pump band and the upper lasing level, as well as between the lower lasing level and the ground state, the population density at the upper lasing level is substantially higher than that at the lower lasing level, yielding a high inversion ratio. The average lifetime of neodymium ions at the upper lasing level, prior to spontaneous fluorescence, is about 230 microseconds at 300.degree. K. in ND:YAG.
The signal to be amplified is selected to have a wavelength at the laser transition wavelength (1.064 microns), i.e., the wavelength of light emitted by the ND:YAG ions during relaxation between the upper and lower lasing levels. When this signal is supplied to the crystal 44 by the coupler 10 (FIG. 9), it will trigger the emission of stimulated photons at the same frequency, coherent with the signal, and the signal is thereby amplified. Thus, the passage of light at this frequency will cause a photon emitting relaxation between the upper lasing level and lower lasing level of FIG. 11A, in phase with the light signal to be amplified, yielding an effective gain for the input light signal.
The gain which can be achieved in the amplifier of this invention is dependent upon the density of the inverted neodymium ion population within the ND:YAG crystal 44. Initially, the ultimate inversion population is limited by the lattice structure of the YAG material 44 itself since in ND:YAG material, some yttrium atoms are replaced with neodymium atoms in the crystal lattice. Only approximately 1 yttrium atom in each 100 yttrium atoms may be replaced by a neodymium ion without distorting the lattice structure on the ND:YAG material.
Theoretical calculations of the small gain signal (g.sub.0 of the amplifier of this invention can be made, using the relation g.sub.0 =.sigma..DELTA.N, where .sigma. is the stimulated emission cross section, for ND:YAG, 8.8.times.10.sup.-19 cm.sup.2, and .DELTA.N is the population inversion density given by: ##EQU1## where P.sub.P is the absorbed pump power, V is the crystal volume and thus, P.sub.P /V is the absorbed pump power per unit of fiber volume, t.sub.sp is the spontaneous radiative lifetime, that is, the 230-microsecond fluorescence relaxation time of the neodymium ions, .eta..sub.1 is the effective spectral overlap of pump output with an ND:YAG absorption line, as shown in FIG. 10, .eta..sub.2 is equal to the quantum efficiency of 1.06-micron fluorescence, namely 0.63, and h.sup..nu. is equal to the energy of one pump photon.
Combining the above relationship provides: ##EQU2## for the dependence of gain on pump power. It should be recognized that the value P.sub.P is the absorbed pump power and that an increase in the length of the crystal 44 does not necessarily increase the gain. Thus, if the pumping radiation from the source 42 is coupled completely to the ND:YAG crystal 44, and travels in the crystal 44 a distance which is sufficient to permit this crystal 44 to nearly completely absorb the pumping radiation, then the value P.sub.P in this equation may be replaced by the input power level. To obtain the net gain, however, one must subtract from g.sub.0 the propagation losses within the ND:YAG crystal 44 at 1.06 microns. A loss of 100 db per kilometer would reduce the gain by only 0.001 db per centimeter. Thus, if the overall length of the crystal 44 can be maintained relatively short, while still absorbing substantially all of the input pump power, the propagation losses within the amplifier can be maintained at a low level.DETAILED OPERATION OF THE AMPLIFIER
Referring again to FIG. 9, the pumping source 42 coupled to the fiber 12A at the port A of the coupler 10, through the multiplexing action of the coupler 10, provides pumping illumination for the ND:YAG crystal 44. The pumping source 42 may be, for example, a long-life LED, such as those currently available which operate at a current density of approximately 1,000 amps per cm.sup.2 and have a radiance of approximately 5 watts per sr.cm.sup.2. In fact, some LEDs have been reported with a radiance of approximately 50 watts/sr./cm.sup.2. Because of the size differential between the single mode fiber 12A and these LEDs, a lens may be useful in focusing the output of the LED source into the fiber 12A.
Alternatively, the pump source 42 may be a laser diode which permits even higher concentrations of pump power in the fiber 12A and thus in the ND:YAG crystal 44.
Regardless of the type of pumping source 42 utilized, the efficiency of the system will be enhanced if the wavelength of the radiation from this source 42 corresponds with a peak in the absorption spectrum of the ND:YAG crystal 44, shown in FIG. 10. Electroluminescent diodes are commercially available with appropriate dopings to emit spectra in the 0.8 micron range which match quite well the absorption spectrum of room temperature ND:YAG material. For example, commercially available GaAlAs LEDs provide radiation spectra which are strong at the 0.8 micron region. Similarly, laser diode structures are commercially available which emit energy over the 0.8 to 0.85 micron range. In addition, the pump wavelength should be as close to the signal wavelength as allowed by the spectroscopy of the ND:YAG, to maximize the overall pumping efficiency.
It will be recalled that the lasing frequency of the ND:YAG material of the crystal 44 is 1.06 micron. The multiplexing coupler 10 is thus fabricated for use in this invention to provide virtually complete coupling at the wavelength of the pumping source 42, 0.8 microns in the above example, while providing substantially no coupling at the lasing frequency of the ND:YAG crystal 44, 1.06 microns in this same example.
This selective coupling is accomplished, in accordance with the techniques described above, by properly selecting the fiber spacing H to yield a large wavelength dependence for wavelengths between 0.8 microns and 1.06 microns, and then by selecting a radius of curvature for the fibers 12A, 12B which yields a resolution equal to the difference between 1.06 and 0.8 microns, or 0.26 microns. After the resolution of the coupler has been set in this manner, the coupler may be tuned, as previously described, to adjust the coupling length for the wavelengths 0.8 microns and 1.06 microns so that the effective interaction length is an even multiple of the coupling length for one of this pair of wavelengths and an odd multiple of the coupling length for the remaining wavelength. In the example shown in FIG. 9, since it is desired to couple the output of the pump source 42 into the fiber 12B, the effective interaction length for the coupler should be adjusted to be an odd multiple of the coupling length at the wavelength of the pump source 42, i.e., 0.8 microns, and to be an even multiple of the signal frequency 1.06 microns. This will result in a complete coupling of the illumination from pump source 42, from the fiber 12A into the fiber 12B, with essentially no coupling of the signal to be amplified from the fiber 12B to the fiber 12A. It will be understood, of course, that no coupling in this instance means an even number of complete couplings so that, for example, if the effective interaction length at the region 32 is twice the coupling length at 1.06 microns, the signal to be amplified will be coupled two complete times, once from the fiber 12B to the fiber 12A, and then from the fiber 12A to the fiber 12B. If this signal enters the coupler at port C, as shown on the left of FIG. 9, it will exit uncoupled at port D. However, at port D, the signal to be amplified will coexist with light from the pumping source 42, which will be completely coupled from the fiber 12A to the fiber 12B.
Since light from the pumping source 42 will be transmitted along the fiber 12B, after coupling, this pumping illumination will invert the neodymium ions in the ND:YAG crystal 44. Thus, a signal, which is injected at port C and exits, uncoupled, from port D, will be amplified in the manner previously described as it passes through the crystal 44, since this signal will excite spontaneous lasing relaxation of the ND:YAG material of the crystal 44, which lasing relaxation will provide light coherent with the signal to be amplified.
The amplifier of the present invention therefore provides a convenient means to transfer pumping illumination from the pump source 42 by wavelength dependent coupling to the ND:YAG crystal 44, while prohibiting coupling of the signal to be amplified from the fiber 12B to the fiber 12A. It should be recognized that the results achieved in the present invention may also be realized using a coupler in which the coupling efficiency at the pumping illumination wavelength is 0%, while the coupling efficiency at the light signal wavelength is 100%. In this case, the pumping source would be coupled to port C of the coupler 10, while the input light signal to be amplified would be coupled to port A.BIDIRECTIONAL SYMMETRY
In order to make the amplifier symmetrically bidirectional, a pair of pump sources 42,48 should be utilizied, as shown in FIG. 12, along with a pair of multiplexing couplers 50,52. It will be understood that, if such bidirectional symetry is not necessary, either of the pump sources 42,48 will invert ions at one end of the ND:YAG crystal 44 and will thus yield gain for signals transmitted in either direction in the crystal 44.
If only one of the pump sources 42,48 is utilized, it should be recognized that the ND:YAG crystal 44 will not be uniformly illuminated. Thus, the inverted population of neodymium ions will not be uniformly distributed along the length of the crystal 44. Because this non-uniform or non-symmetrical state within the amplifier may yield different gain for signals input at the fiber 54, than for signals input at the fiber 56 (particularly when these signals occur simultaneously), it is advantageous to utilize the pair of sources 42,48.
The phenomenon of dissimilar gain for signals traversing the fiber crystal 44 in different directions with a non-symmetrical inversion population of neodymium ions occurs as follows. It will be recognized that, as a signal to be amplified propagates from left to right in the crystal 44 of FIG. 12, it will trigger the emission of stimulated photons within the ND:YAG crystal 44. Such triggering emission, of course, lowers the inversion population within the crystal 44. If, for example, in a gyroscope, a pair of waves propagate simultaneously through the crystal 44 in opposite directions, the signal input at the left end will deplete the inversion population adjacent this end before the signal input at the right end arrives at the left end of the crystal 44, as viewed in FIG. 12. If the inversion population is higher at the left end of the crystal 44, than at the right end, as would be the case if only the pump source 42 were utilized, the signal input at the left will undergo a greater amplification, since it will deplete the inversion population before the signal which is input at the right end arrives at the high density left end.
It should also be recognized that the pumping illumination supplied by the pump sources 42,48 should be sufficient, on a continuing basis, to replace the depleted population within the crystal 44 which occurs when the signals are amplified. Thus, for example, in a gyroscope where a pulse signal circulates through a kilometer of fiber, a counter-propagating signal will traverse the amplifier, shown in FIG. 12, approximately once each 5 microseconds. If continuous pump sources 42,48 are used, they should provide sufficient output so that, during each 5-microsecond period, they are capable of reinverting the neodymium ion population which is relaxed during each successive traverse of the signals to reinvert a population equal to that which has relaxed, such that the amplification factor or gain of the amplifier will remain relatively constant.
As will be recognized from the above-description, a proper selection of fiber spacing and radius of curvature will yield a coupler which permits pumping sources 42,48 to illuminate the crystal 44 to invert the neodymium population therein. With a proper selection of the coupler parameters, the signal to be amplified is not coupled from the fibers 54,56, and thus traverses the crystal 44 to be amplified by stimulating relaxation of neodymium ions in the crystal 44 which produces light coherent with the signal to be amplified.SIGNAL INSERTION
Referring now to FIG. 13, a system arrangement is shown in which the multiplexing properties of a pair of couplers 58,60 is used to permit a pumping source 42 to illuminate the ND:YAG crystal 44 while also permitting an injection of signals into a recirculating fiber optic system. In this instance, for example, it may be assumed that the fiber ends 62,64 are a part of a recirculating fiber loop and that the system, in addition to supplying pumping illumination for the crystal 44, to amplify signals circulating in this loop, must supply signals for injection into the loop.
In the system shown in FIG. 13, the coupler 58 is arranged to couple light wavelengths from the illumination source 42 with a coupling efficiency of 100%, while leaving the input signal at an input fiber 66 effectively uncoupled. This will yield a combination signal on the fiber segment 68 which includes both the pumping illumination from the source 42 and the input signal from the fiber 66. If the characteristics of the coupler 60 are properly selected in the manner described above, and this coupler 60 is properly tuned, the coupler 60 may be arranged to couple 100% of light wavelengths from the pumping source 42 but only 1% of wavelengths at the signal input from the fiber segment 68. Thus, 1% of the input signal from the fiber segment 66 will be injected into the fiber recirculating loop and will initially be transmitted through the crystal 44 to the fiber segment 64 for recirculation. At the same time, 100% of the pumping illumination from the source 42 will be injected into the end of the crystal 44 to provide amplification for the recirculating signal.
When this recirculating signal appears at the fiber segment 62, only 1% of the signal will be lost at the coupler 60 and this 1% may supply an output for a sensor 70 to monitor the recirculating light. The remaining 99% of the recirculating light signal will be injected into the crystal 44 from the coupler 60, for reamplification and recirculation within the fiber loop. This system, therefore, with the use of two couplers 58,60, provides continuous sensing of the light circulating in the fiber loop, signal injection into the loop, and continuous pumping for the amplifying crystal 44. The coupling efficiencies provided above are exemplary only, but it should be recognized that the coupling efficiency of the coupler 60 should be maintained relatively low at the wavelength of the signal propagating in the loop so that only a small portion, smaller than the amplification added to the signal at the crystal 44, is subtracted from the recirculating signal by the coupler 60.SUMMARY
The combination of the multiplexing coupler with an ND:YAG amplifying crystal permits end pumping of the amplifying crystal with simultaneous signal injection at the end of this fiber so that the pumping illumination need not be carefully timed with respect to the signal to be amplified. Symmetrical bidirectional amplification is possible and signal injection using the same multiplexing coupler may be achieved.
1. A fiber optic amplifier, comprising:
- an optical fiber doped with material which will possess a laser transition at a single wavelength when said material is pumped with illumination; and
- means for superimposing pumping illumination and a signal to be amplified, and for coupling the resulting superimposed signal on one end of said optical fiber.
2. A fiber optic amplifier, as defined in claim 1, wherein said superimposing means comprises an optical coupler having a different coupling efficiency for said signal to be amplified than for said pumping illumination.
3. A fiber optic amplifier, as defined in claim 2, wherein said optical coupler provides said different coupling efficiency in accordance with the difference in wavelength of said pumping illumination and said signal to be amplified.
4. A fiber optic amplifier, as defined by claim 1, wherein said optical fiber is formed of a crystal material.
5. A fiber optic amplifier, comprising:
- an optical fiber doped with material which will possess a laser transition at a signal wavelength when said material is pumped with pumping illumination, wherein said optical fiber has a diameter which is less than the absorption length of said crystal material at the wavelength of said pumping illumination; and
- means for superimposing said pumping illumination and a signal to be amplified, and for coupling the resulting superimposed signal on one end of said optical fiber.
6. A fiber optic amplifier, as defined by claim 5, wherein said optical fiber is formed of a crystal material.
7. A fiber optic amplifier, comprising:
- an optical fiber formed of material having a laser transition at a signal wavelength when said material is pumped with illumination at a pump wavelength;
- a signal source at said signal wavelength;
- a pump source at said pump wavelength; and
- means providing optical paths for simultaneously optically connecting an end of the laser optical fiber to (a) said signal source and (b) said pump source.
|3230474||January 1966||Keck et al.|
|3731225||May 1973||Wild et al.|
|3914709||October 1975||Pike et al.|
|3957341||May 18, 1976||Taylor|
|3975692||August 17, 1976||Mego, Jr. et al.|
|4136929||January 30, 1979||Suzaki|
|4243297||January 6, 1981||Elion|
|4258336||March 24, 1981||Fletcher et al.|
|4300811||November 17, 1981||Ettenberg et al.|
|4307933||December 29, 1981||Palmer et al.|
|4315666||February 16, 1982||Hicks, Jr.|
|4335933||June 22, 1982||Palmer|
|4342499||August 3, 1982||Hicks, Jr.|
|4343532||August 10, 1982||Palmer|
|4362359||December 7, 1982||Dammann et al.|
|4383318||May 10, 1983||Barry et al.|
|4515431||May 7, 1985||Shaw et al.|
|4546476||October 8, 1985||Shaw et al.|
|4553238||November 12, 1985||Shaw et al.|
|4554510||November 19, 1985||Shaw et al.|
|4603940||August 5, 1986||Shaw et al.|
|4674830||June 23, 1987||Shaw et al.|
- Kapany et al, "Coherent Interactions Between Optical Waveguides . . . ", J.O.S.A., vol. 58, No. 9, Sep. 1968. Burrus et al, "Single-Crystal Fiber Optical Devices: A Nd:YAG Fiber Laser", Appl. Phys. Lett., vol. 26, No. 6, Mar. 15, 1975, pp. 318-320. Bergh et al, "Single Mode Fiber Optic Directional Coupler", Electronics Letters, vol. 16, No. 7, Mar. 27, 1980, pp. 260-261. Periasamy et al, "Laser Amplification in an Optical Fiber by Evanescent Field Coupling", Appl. Physics, vol. 24, No. 3, Mar. 1981, pp. 201-203. Digonnet et al, "Analysis of a Tunable Single Mode Optical Fiber Coupler", IEEE J.Q.E., vol. QE-18, No. 4, Apr. 1982. Injeyan et al, "Light Amplification by Evanescent Wave Coupling in a Multimode Fiber", Appl. Optics, vol. 21, No. 11, Jun. 1982, pp. 1928-1932. Alferov et al., Multichannel Duplex Fiber-Optic Communication Line . . . ", Soviet J. of Quantum Electronics, vol. 12, No. 8, Aug. 1982, pp. 1088-1090.
Filed: Nov 12, 1986
Date of Patent: Feb 9, 1988
Assignee: The Board of Trustees of the Leland Stanford Junior University (Stanford, CA)
Inventors: Herbert J. Shaw (Stanford, CA), Michel J. F. Digonnet (Palo Alto, CA)
Primary Examiner: John Lee
Law Firm: Knobbe, Martens, Olson & Bear
Application Number: 6/930,136


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The buckling load of a latticed structure is determined by considering the equilibrium of the joints in the deformed state, taking into account the effect of axial loads and deformation. The displacements U, V, W and the rotations ox, oy, oz are considered as unknowns at each joint. A stiffness matrix is developed based on ‘second order theory’. The loads causing buckling are those for which the determinant of the coefficients of the equilibrium equations at the joints vanish. As this results in a nonlinear eigen value problem, it is solved by iteration. A general computer program is developed to find the load causing instability of latticed structure of any geometry and for any support conditions.
Mr. Williams in introducing his paper quoted from an unpublished illustrated report on the first submerged tube tunnel under the River Thames at Rotherhithe which it was proposed to build in 1811 but was later abandoned because of differences among its promoters.
Mr. R. E. Landau (F): The researches referred to in this valuable paper are relevant to the design of retaining walls and bridge abutments, which have 'flexural corners' at the junction of stem and base. Three cases may be considered.


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Beyoncé Shoots New Music Video In Paris
Beyoncé reportedly spent time in Paris recently to shoot a new music video, which is set to be included on a video anthology for her upcoming album.
News of Beyonce's time in France spread quickly across social media, with Bey's lead dancer Ashley Everett posting a photo of the Eiffel Tower on Instagram along with the caption "Oh just my view today.#Paris."
That wasn't the only posting of Beyoncé sightings, as another dancer, Claudia Dolez, posted "When your agency sends you a casting for the clip of Beyoncé, you reread the email 3 times to make sure they were not wrong."
"My brother just made a cast for the next clip of Beyoncé!" another post read, while two more read, "Beyoncé will be in Paris this weekend for the filming of her new video!" and "Some French sources are reporting that Beyoncé is going on some video-shooting in Paris this weekend. The castings were today made things a little more obvious."
Copyright(c) 2013 RTTNews.com. All Rights Reserved

domain: celebrity


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Hire Irish band in Malpas
Recent review: Booked the Shenanigans for a 60th Wedding Anniversary celebration. A fantastic duo who gave a great performance and had everyone up dancing. They arrived in plenty of time, and although they were kept waiting as our meal went on longer than we anticipated, they were very understanding. Everyone you
About: Tippin it up are an acoustic fplk duo performing mostly Irish traditional music and song with a little Scottish, Welsh and Americana. Instruments include...Guitar, Bouzouki,Mandolin,Mandola and Bodhran...as well as Vocals!
About: As a 5 x piece band The Wee Bag Band's raison d'être is to bring mad, bad, trad, popular and contemporary diddly, diddly Irish / Celtic with a hint of Country and Blue Grass music and song to the masses at pubs, clubs, doos 'n festivals in Merseyeside, UK, infinity & beyond...... We must be hit
Recent review: "Always quick to respond to enquiries and nothing was too much trouble for him. Even when a last minute panic set in over our own playlist, Ben was only too happy to assist and save the day. I can’t recommend these guys enough!"
About: ** NOT AVAILABLE 17th MARCH** Whiskey in the Jar have been entertaining audiences throughout England and Ireland since 1989, but still perform with energy and style. Whatever the occasion, this band have the repertoire and professionalism to make it into a great occasion. Featuring fiddle, banjo,
Recent review: We absolutely loved having Black Velvet Band play our wedding ceilidh at Clearwell Castle. My husband is from Scotland so we wanted a traditional Scottish ceilidh feel with some of the classics, and we were not disappointed! We had a good feeling about Black Velvet band from the moment we booked the
Recent review: Great communication with David since making the booking back in October and a friendly confirmation call the week of the gig too. Set up on time, in a tight space and a great atmosphere created for my friends 40th birthday. Playlist requirements provided by the birthday girl were accommodated for, e
Recent review: Hi, Hope you're all well! Just a quick email to let you know my appreciation for both your music and your passion shown at the weekend. You played at my partners grandparents party on Saturday evening and I must say both I and everyone attending was absolutely blown away by you guys! I will definite
Recent review: “Ben and his band provided a hell of a surprise party for my mum’s 60th! Their ability to integrate with our extended friends and family, both in terms of the ‘surprise’ moment, the food and the speeches was superb, and really meant we were able to make it personal to mum. She hasn’t stopped beaming
Recent review: You made our party (for our Co. Mayo mother's 90th) very memorable and played exceptionally well, all our favourite Irish melodies. You were also very flexible to our set up of the room and integrated well into our family and friends. I would thoroughly recommend you
Recent review: We hired the Stomping Boondocks for our wedding reception and they were absolutely fantastic! Most of the guests had never done a ceilidh before but all of them were raving about it after the event. Very responsive with communication before the event also.
Recent review: "Absolute band were ""ABSOLUTELY"" amazing and far exceeded our expectations and truly made the day extra special. Right from the music for the ceremony through to the first dances and the whole set...they were fantastic. They really looked like they were enjoying it too and their enthusiasm didn't
About: We're a great fun band playing Irish and Scottish music with amusing little stories to compliment the tunes. A night with us will have you smiling and dancing and singing along. Think of The Dubliners and you'll be close to our sound and music. We play weddings, parties, wakes, festivals and ceilidh
Recent review: Absolutely amazing! They did a great job! Best band I've heard and when Roddy played the pipes everyone in the room loved it! Well done and thank you so much for making my babies naming ceremony so memorable! Absolutely fantastic!

domain: artistic


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Breath Work › Increase Focus › Kiranjot
Kriya to Energise
You can do this seated or lying down. Join Kiranjot for energising class through Kundalini yoga body movements and breathwork. This uplifting class works on the ten bodies: the 10 bodies are the kundalini equlivalenet of the koshas, The Soul, Negative Mind, Positive Mind, Neutral Mind, Physical Body, Arc Line aka Halo, Aura, Pranic body, Subtle Body and Radiant Body. With a savasana, resting pose, at the end of the physical movements, then with chanting and mandra. This class is a fantastic all round tonic. No yoga mat needed, suitable for those with wrist issues, and suitable for most levels of mobility and strength (with modifications).
Breath Work For When You Don't Know What To Do

domain: hobby


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Beauty Tips For Teeth Whitening In Urdu
Possibly you desire for having a smile so bright that it can be seen from area, or perhaps you are searching for something a little less blinding. Whether you want an incredibly bright smile or simply to eliminate the coffee discolorations, we have noted the greatest teeth whitening items for you. Beauty Tips For Teeth Whitening In Urdu
The reality is, we just get one collection of teeth to last us throughout our lives. Looking after them as well as keeping them healthy and balanced is actually vital. Being proud of the stamina of your teeth can be shown in just how acne cost-free you keep them. Joy all that see you by trying one of these teeth lightening systems.
Finest Teeth Lightening Products Reviewed
Change your regular tooth paste with this Colgate Optic White Express tooth paste and also you will never ever recall. It assures results (whiter teeth) in as little as three days, and it is as basic as replacing your current tube. This Optic White tooth paste has 2 times of the teeth whitening power of regular tooth paste, yet do not allow its power turn you off. This toothpaste is risk-free to use on a daily basis.
This tooth paste contains the most well-recommended representative for whitening teeth by dental experts, hydrogen peroxide. It also has fluoride, providing the Optic White toothpaste an anti tooth cavity double whammy that will most definitely be beneficial to your oral health.
Crest 3D White Professional Consequences Whitestrips 20 Therapies + Crest 3D White 1 Hr Express Whitestrips 2 Treatments – Teeth Lightening Set
With these whitestrips you can get rid of approximately fourteen years’ well worth of discolorations in less than one month. As opposed to the headache as well as expenditure of going to get your teeth bleached by a expert in a busy workplace, you do not even have to leave your comfy house to utilize this and also get the same results. Beauty Tips For Teeth Whitening In Urdu
Does the idea of using whitestrips and being unable to use your mouth generally frighten you off? These whitestrips utilize an Advanced Seal Innovation so you can speak with whomever you require to and even consume alcohol water! Various other beverages and also definitely food are not suggested as this is, after all, suggested to whiten your teeth. They are also not advised for those with really delicate teeth and also gum tissues, as the bleaching light can aggravate them.
This kit is a little bit various from a lot of, because it consists of a special brush. This brush is enhanced with the incredible blue-light modern technology that is taking dentistry by tornado. Today, it is the only toothbrush available on the market that uses this blue light innovation. The factor this blue light is so exceptional when it involves oral health is that it instantly kills all microorganisms it stumbles upon.
Using the toothbrush along with the tooth lightening gel indicates that you can see outcomes to your teeth in a actually brief amount of time. This is a sonic tooth brush, so it can offer you up to 33,000 strokes per minute, significantly decreasing your chances of coming down with gingivitis or other conditions in your mouth. Making use of the two together makes for a far better, a lot more radiant smile than just one thing alone. It will work actually quickly, providing results in less than two days for lots of people, however it can cause irritability to sensitive teeth. Beauty Tips For Teeth Whitening In Urdu
This might resemble a really futuristic device, however it’s actually easy to use and also even comfortable. It has a hands free professional quality LED light that you can use even if you do not have any aid. Are you on the go a whole lot? This whitening light will work with a mobile phone app so you can even do it
when you are not at home. This comes in a really good, light as well as amazing minty taste so you are constantly sure of fresh breath when you use it. If you seldom go to get it done properly because of the pain triggered by it annoying your delicate teeth, this coincides things, however at a much lower dosage. You can obtain similar results without it bothering you.
A tooth bleaching pen is designed for on the go usage. Possibly it is to brighten your smile after stain-causing agents like coffee or red wine. Due to the fact that it is indicated to be scrambled, there is a much smaller quantity of the energetic tooth lightening component, hydrogen peroxide. Nonetheless, a smaller amount of the active component additionally implies that it will certainly not perturb delicate teeth.
It has a soft brush idea to allow you to get to more persistent discolorations in between teeth, however this style of pen is not created to offer extensive whitening. It is a lot more a upkeep tool, keeping the brilliance of your smile when you swipe it on.
Inquiries regarding the most effective Means to Get Brighter Teeth
Are charcoal bleaching items good for your teeth?
While there is no proof that charcoal has all-natural tooth whitening powers, it is rough sufficient to get up some surface stains. Charcoal is safe to consume as well as make use of occasionally, yet it will certainly not have any type of bleaching benefits for your teeth.
What’s the safest alternative for delicate teeth?
If you have truly sensitive teeth, utilizing a lightening tooth paste instead of strips or lights could be your safest alternative. Find one that is particularly formulated just for delicate teeth.
Will teeth lighten products harm or transform the shade of a fake tooth?
You could be fretted about lightening your teeth, assuming the active agents in tooth lightening systems might have an adverse result on fake teeth, yet they will certainly have little to no effect. Just as they will not hurt a phony tooth though, they will certainly deficient any type of more vibrant. Primarily, once a phony tooth remains in your mouth, what you have is what you obtain. It will certainly not alter in shade, lighter or darker. Beauty Tips For Teeth Whitening In Urdu
Are there any type of wellness advantages to bleach your teeth?
Whether you get your teeth expertly bleached or you do it in your home, there are numerous advantages. One you may not think of is its mental result on your health habits. Having whiter teeth will provide you a lot more satisfaction, so you will certainly endeavor to care for them much better. It will additionally enhance your teeth, which will, subsequently, be beneficial to the health and wellness of your gums.
Why are my teeth yellow?
The discoloration of teeth can be triggered by a fantastic numerous points. When you drink things like soft drink or merlot, the substances can seep into the enamel that is covering your tooth. An additional typical culprit is using tobacco products. Heavy enhancement of fluoride has actually been revealed to trigger yellow teeth in youngsters, however keep in mind, a little is still a good thing.
Exactly how do stars obtain their teeth so white?
Stars are known for having a extremely white, straight set of teeth. The fact is, they frequently have a group of specialists that are paid to work on them. For everyday upkeep, they treat on crunchy veggies to clean stains off the enamel securing their teeth.
What is the most effective Method to Get Your Teeth White
Bleaching your teeth is the very first step to a better, much more certain you. Personally, I like to do points while I get on the go, so the whitening pen would be my suggestion. As soon as you have actually completed a extra complete lightening system, a pen can be used for basic maintenance, and it is so handy. If you intend to take control of your dental hygiene without cutting into your hectic routine, take tooth treatment on the go with you!

domain: health


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This program is a treadmill workout to lose weight. For this treadmill workout, your going to have to use a treadmill with an incline.
This particular treadmill workout will not only help you lose weight and burn off the fat but it will build your leg strength and prepare you for hard runs outside.
On average, if you were to use the treadmill, you'd burn about 17 calories per minute. Let's say you worked out on the treadmill for 35 minutes, you'd burn about 595 calories. Of course, this all depends on the amount of work you put into the exercise, but all things being equal, this is what you can expect to burn.
Out of all the exercise machines used to burn fat and improve cardiovascular fitness, the treadmill burns calories at the highest rate.
For you runners out there, if you want to mimic the feeling of a good run out on the road, try raising the incline of the treadmill to 1% before starting your run. This is the degree of elevation that most closely approximates out door running.
First, warm up by performing the exercise at low intensity, about 30% to 40% of your full out effort for about 5 minutes.
Now, pick a speed that's about 2 minutes per mile slower than your average outdoor pace. Run at that speed for 2 minutes at an incline of 1%. Then raise the incline to 4% for another 2 minutes. Continue to rase the incline of the treadmill by 2% every 2 minutes until you reach a 10% incline.
Then, lower the incline by 1% at a time, in 2 minute intervals until you complete your 20 minutes worth of exercise. If you are a beginner, try walking first. Start slowly by working at 50% of your full effort for 10 minutes, 3 days a week. Increase your intensity by 10% every other week.
Before you know it, you'll be running for the full 20 minutes.
All the best,
As the owner of Building Muscle 101, I am committed to providing you the best practical weight training advice. I've been training for over 24 years (and still train to this day!) and the advice and guidance I provide comes directly from my experience and knowledge.
Home > Burning Fat


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La Trobe University research has found delivering cancer patients' rehabilitation clinics by telehealth is safe, effective and removes barriers to participation, such as fatigue, travel and parking costs.
Published in Journal of Medical Internet Research - Cancer, the study found 88 per cent of surveyed cancer patients were satisfied with completing the rehabilitation exercise program online, and 82 per cent reported an improvement in their health and wellbeing.
"Availability of exercise programs for cancer patients is very low in Australia, despite global recognition of the enormous benefits," she said.
"Add to that the burden of travel - especially for people in regional areas who might have to make a two over three-hour round trip - and it becomes an impossibility for many.
"We've shown that telerehabilitation isn't a second-best option - for some cancer patients it might be the only way they can participate in a regular exercise program."
IN OTHER NEWS:
"Although telerehabilitation has exploded world-wide out of necessity, this study highlights that it shouldn't necessarily take a back seat if the threat of COVID-19 diminishes," she said.
"Though some patients did mention the challenges of telerehabilitation - such as the lack of social interaction - the benefits were great, and widely acknowledged among study participants.
We might see many more cancer patients with improved recovery and wellbeing.Dr Amy Dennett
"If healthcare providers could incorporate it into their future planning, we might see many more cancer patients with improved recovery and wellbeing."
"They were proud to have risen to the challenge of the rapid switch to telehealth, but some preferred delivering in-person care as they felt better able to assess, monitor and correct patients exercising," she said.

domain: medical


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As you can see, the name has changed and so will the content.
Michael and I have discovered a Secret Garden right in our backyard. Although it is nestled between a major thoroughfare and a busy freeway; by God's providence, our little apartment is at the very back of a large apartment complex and our balcony overlooks a county owned parcel of untouched wilderness. There are squirrels, birds of all sorts, wild turkeys, butterflies, bright red dragon flies, and, unfortunately even an occasional skunk. There is also a small creek that runs through it which is lined with old oak trees; huge eucalyptus trees; and, all different types of wild fruit trees and berry bushes.
We have had the most wonderful two months; more wonderful than I can ever remember having. I will be sharing some of our explorations and adventures in this blog, as well as, writing personal notes around shared experiences.
This is a huge change for me; but, one that I look forward to. My little brain is bursting already with several absolutely delightful stories to share.
Check back on occasion. The first post will be about the mysterious 30 foot tree with miniature blackberries growing right along side of our apartment.


Imagifinity started off as four friends and an idea.
Have you imagined having a 3D model of yourself, that you could share with your friends? Or to shop online for clothes and try them on your 3D model to see how they would look on you before you purchase them?
The expertise of our team is focused into developing an innovative hardware and software solution to create your realistic 3D digital model and to design various services making your life better and more fun - helping you easily and more accurately find the right size for your clothes, keep track of the progress of your health program, or allowing you to play online games with your real 3D Me.
We believe that each one of us has the right to have fun, easy and realistic life in the virtual/online world, where more and more of our existence happens nowadays.
One of the key prerequisites for this enhanced digital life is to be represented in the virtual world by a true 3D image of yourself.
Few of the enhancements such a 3D image brings are the opportunities to try clothes and accessories online and see how well they fit before you buy them, to keep track of the progress you are making with your health program, to fit the main character in an online game with your own face and body, to share with friends 3D profile photos, and more.
The expertise of our team is focused on developing an innovative hardware and software solution for creating your realistic 3D digital self and on designing various services using it to make your virtual life better and more fun.


From trees glimmering with gold lights to embellishments in bright red, the Christmas trees of these influential people bring out the festive feelings this season
For her Christmas tree this year, Grace Baja opted to decorate it with a combination of sparkling orange balls and bronze ornaments. It proves to be a refreshing sight, straying from the reds and greens of this season’s hues.
While Rocio Olbes may have only teased us with a part of her tree, we can still feel the Christmas cheer from the focus on the ornament wrapped in checked fabric. This post certainly had us amplifying our excitement for the 25th.
Though the snow is heavy outside, there is warmth inside the house of Keri and Francis Zamora. Glimmering lights cover their Christmas tree amidst the contrast of red and green embellishments. A further glance also suggests the presence of children as seen with the endearing Santa Claus-themed decor.
The lovely spouses Pinky Tobiano and Juancho Robles adorned their tree with a mix of reds and golds. Anyone seeing the tree would marvel at the complex and detailed decoration. Yet it only seemingly reflects the fullness of love and enthusiasm for the holidays.
While Sofia Borromeo‘s daughter Manunibelle is capturing hearts in this lovely photo, the family’s Christmas tree is something to gaze at as well. Apart from Christmas ornaments, they adorned the tree with their favorite family photos. This turns the tree into something personal and special for them.
Stenie Coyiuto proudly posts a photo of her family’s Christmas tree filled with her favorite flowers like peonies. She knew it had to be special, as, with everyone else, Christmas is her favorite time of the year.
Dressed from head to toe with notice-me-red, the Andanars with Keli Teo pose in front of their majestic Christmas tree. The richness of the green color certainly stands out when contrasted with the decor in other colors. It only proves to be the perfect backdrop for this charming photo.
It is difficult not to notice the splendor of Rachelle Sayed and husband Alex’s Christmas tree. With huge bows, drooping gold ornaments, and enormous balls, the tree certainly reflects the joy of the season.


text:
CHOICE SITE FOR 3 CONDOMINIUMS WITH BREATHTAKING CLOSE UP VIEWS OF OUTER LOS ANGELES HARBOR AND THE SOUTHERN CALIFORNIA COASTLINE. ALLEY ACCESS FROM REAR OF PROPERTY.
Total population of , which has changed by % since 1990.
The median age of the population is , while the median
age of dwellings is .
Out of households, are families.

domain: sociology


Bitcoin Could Plummet Below $15K If Expected U.S. Jobs Report Indicates ‘Elevated Employment Figures’
Hedge Fund Predicts a Dip Below $15k for BTC In the Wake of a High U.S. Jobs Report
Bitcoin Could Plummet Below $15K If Expected U.S. Jobs Report Indicates ‘Elevated Employment Figures’, according to AlphaTrAI CIO.
Bitcoin (BTC) remains at the mercy of the bears, as the extensive Crypto Winter continues in its dominance. While a few analysts believe the bottom is already priced in, the CIO of AlphaTraI thinks the community hasn’t seen the worst of the markets yet.
Max Gokhman, Chief Investment Officer at asset management firm AlphaTraI recently noted this in a Bloomberg interview. According to Gokhman, should the U.S. jobs report show heightened values, it could set up a domino effect in the markets which would ultimately send Bitcoin to a 2-year low. The NonFarm Payrolls (NFP) report is due Friday at 12:30 PM (UTC).
AlphaTrAI succumbed to the bear market as August reached a conclusion. The San Diego-based asset manager dumped its crypto holdings late-August amidst the Winter.
Expatiating on his assertion, Gokhman highlighted the expected response from the Federal Reserve to elevated employment figures. The Fed is likely to lean towards further interest rate hikes in response to higher employment rates. Additionally, this would not have the most favourable effects to both the traditional stock market and the crypto markets.
There was a time when macroeconomic conditions had little effect on Bitcoin’s price actions, but that time is no more. Recently, the correlation between BTC and traditional finance has grown, and this has subjected the asset to the damaging effects of bad macro. Gokhman highlighted this reality in his forecast.
Furthermore, he noted that the $20k support level for BTC is a significant point for the asset’s price movement. Gokhman predicts that the dread from a likely interest rate hike announcement from the Fed could send BTC below $20k.
Should BTC break below the $20k support, a price free fall could ensue, pushing the asset further below. Gokhman believes the community could be seeing $15k at such a point.
“The true test for Bitcoin is if it can stay close to the $20,000 level after the NFP release … A hot labor market report and Fed rate hike bets might surge, and that could trigger downward pressure that eyes the summer lows” said Edward Moya, Market Analyst at OANDA, in a note to Bloomberg. Despite not predicting a dip to $15k, Moya’s sentiments appear to align with Gohkhman’s.
Bitcoin’s Price Actions
It is important to note that the last time BTC touched $15k was in November of 2020. The crypto community has already seen BTC’s response to reports of interest rate hikes, and it’s not pretty. Jerome Powell, Fed Chair, on August 26, noted that further interest rate hikes to combat rising inflation was a necessity.
This comment knocked BTC off its $21,700 high on that day. The bearishness spilled into the weekend, as BTC went below $20k for the first time in August. The asset has since then bounced back, but consolidation at this zone highlights a very fragile support at $20k. BTC currently trades at $20,137 at the time of reporting, having gained by 0.96% in the past 24 hours.
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text:
I’ve worked in the Sunnyvale Library since I graduated from high school in 1985, and over the years I’ve noticed that many of my coworkers are aspiring writers, musicians, and artists. One of those coworkers has been working especially hard at writing novels and organizing his coworkers into an effective union. Two years ago, Bob Balmanno published September Snow, a wonderful science fiction book that combines the qualities that I admire in Bob as a person: a strong sense of storytelling and a passionate social conscience.
September Snow takes place in the year 2051 A.D. The world has been devastated by a long war that wrecked havoc on the Earth’s ecosystem, and a new religion called Gaia is being used by a malovent government to manipulate its citizens. Our main character, Tom Novak, remembers a time before the society had transformed itself, a time when people could go outside without suffering solar radiation and poisoning and citizens could think for themselves and read what they want. Tom is one of the last people to have these memories, and the independence of thought that it gives him makes him a threat to the powers that be. He eventually discovers an insurgency of people intent on overthrowing the Gaia system, and he falls in love with their leader, September Snow.
September Snow works on 2 levels. On one level, it’s an exciting science fiction adventure, filled with grand heroics as Tom, September, and a small band of insurgents battle against overwhelming odds to defeat an overarching government. On another level, September Snow is a strong indictment of the path our environmental degradation is taking our society and our planet. It talks about the depletion of the ozone layer and the destruction of various ecosystems and it’s a very timely story. I enjoyed the book on both levels.
This book reflects the heavy strain of social conscience that can be found in all aspects of Bob Balmanno’s life. In reading the bio of the book, I found out that Bob was in the Peace Corps as a young man, herding cattle in Africa. Bob was instrumental in organizing part time workers in the city of Sunnyvale to form a union and be a part of SEIU. I was secretary for 3 years for our chapter in the late 1990s, and I grew to respect the passion of Bob,and the other leaders of the group to help workers with their grievances and work out equitable and fair solutions. I didn’t do much as secretary but take notes of the meetings, but I was able to observe Bob and the other 4 leaders of the union make sure that the members were informed of the issues so that they were knowleadgable enough to make up their minds before voting. When our group negotiated with the city for a contract, I noted the amicable and businesslike way in which the union and the city negotiated. It took away a stereotype that many people have told me about unions, that they were always antagonistic towards managemen and didn’t take into mind the bigger picture. I found the exact opposite was true.
Bob typifies the intersection of literature and activism that I respect. He’s in a line with such writers as Mark Twain, Kurt Vonnegut, Grace Paley, and Doris Lessing, writers who engage in the problems of the society around them through their writings. It’s a tradition in literature that goes back to Steinbeck and Tolstoy and Voltaire.

domain: library


text:
The anniversary of the first battle of the UPA — the battle with the Nazis — free access posted a collection of documents. 61 the document reflects information and armed confrontation between the German occupiers and the Ukrainian insurgent army and the underground organization of Ukrainian nationalists. The collection was prepared by the Centre for research liberation movement (TSIOD) in collaboration with the Archive of the security service and published on the server of the E-archive liberation movement. Its publication is part of the national information campaign «the UPA — response unconquered people.»
As noted on the official website of the Center, the collection of documents includes about 20 original combat reports about armed clashes with the Nazis: ambush convoys, attacks on the garrisons and critical infrastructure, fighting with the punitive expeditions and the like. For example, there is a report that details the attack on the district center in the city of Kamen-Kashirskiy 19 Aug 1943 — one of the most successful operations of the UPA. Then the rebels defeated the garrison, killing up to 100 German soldiers and police, captured a great booty.
As one of the compilers of the collection, historian of the Center of researches of liberation movement , Igor Bigun, the documents can be read about the battles with the German occupiers not only in Western Ukraine and in the Zhytomyr, Kyiv, Khmelnytskyi and Vinnytsia regions. Reports about the raids of hundreds of Smoking «Gordienko», «Lion», «Fast» and «Saluka» in the summer of 1943 and at the turn of 1943-1944 abound in violent clashes with the invaders in the Central Ukrainian lands.
«The activities of the UPA has reached such proportions that the occupants could not respond to it. The collection contains a translation of the order of the commander of SS and police General district «of Volyn and Podillia» Wilhelm günther dated June 7, 1943. In this occupation the official orders
to destroy the «national disorder» and gives a specific order of the German punitive units on how to act against the UPA,» — said the historian.
Along with the military forces of the German invaders used the propaganda apparatus. The collection includes 10 revealing German leaflets against Ukrainian nationalists. They, the Nazis called Ukrainian rebels «bandits», «murderers» and «robbers,» and was also accused of working for the Soviet Union and world Jewry.
At the same time, here you can find information and anti-Nazi materials Ian: «Our path is neither to Berlin, nor in Moscow!», «We threw the Germans,» «To skin the Nazi invaders!» and others.
Also interesting is the materials of archival criminal cases of the Department of the security Service in Rivne oblast. For example, in a criminal case Samchuk Sergei M. — page of the villages Small and Large zholuds’k of Rivne region — he talks about the antics of the first UPA. «The documents States that the first antics of the UPA, lasted for about two weeks. There they studied the geography, the relations of other States with Ukraine. In addition were drill and shooting training with hand weapons,» — said one of the compilers of the collection, historian of the Center of researches of liberation movement Volodymyr Birchak.
In total, the Collection includes documents of the Branch state archive of security Service of Ukraine, the Archive of the SBU in the Rivne region, and the Archive of the Centre for research liberation movement.
Recall, from 7 to 8 February 1943 in the town of vladimirets Rivne region held the first battle of the UPA. Then hundred of Gregory of Perehinka (pseudo «Box») attacked the German garrison.
It is considered the most studied political and military activities of the OUN-UPA in Western and Northern Ukraine.
8 February 2017 at the Ukrainian Institute of national remembrance announced the beginning of the information campaign of the UPA — reply rebellious nation» dedicated to 75th anniversary of UPA «. During the year it is planned to cover information on the main fronts of the struggle of the UPA — anti-Nazi and anti-Communist, what will tell exhibition and international scientific conference. The Institute also participates in the drafting of a new bill on the rehabilitation of victims of political repression.
The state Agency of Ukraine for the movie is planning to hold a special film festival.
Available documents on the battles of the UPA with Nazis in Central Ukraine 08.02.2017


text: The reprogramming of differentiated cells to pluripotent cells (induced pluripotent stem (iPS) cells) is known to be an inefficient process. We recently reported that cells with short telomeres cannot be reprogrammed to iPS cells despite their normal proliferation rates, probably reflecting the existence of 'reprogramming barriers' that abort the reprogramming of cells with uncapped telomeres. Here we show that p53 (also known as Trp53 in mice and TP53 in humans) is critically involved in preventing the reprogramming of cells carrying various types of DNA damage, including short telomeres, DNA repair deficiencies, or exogenously inflicted DNA damage. Reprogramming in the presence of pre-existing, but tolerated, DNA damage is aborted by the activation of a DNA damage response and p53-dependent apoptosis. Abrogation of p53 allows efficient reprogramming in the face of DNA damage and the generation of iPS cells carrying persistent DNA damage and chromosomal aberrations. These observations indicate that during reprogramming cells increase their intolerance to different types of DNA damage and that p53 is critical in preventing the generation of human and mouse pluripotent cells from suboptimal parental cells.