Acousto-optic fiber-optic frequency shifter using periodic contact with a surface acoustic wave

Abstract: An acoustic wave is propagated colinearly with the opti­ cal signals in an optical fiber (330) to cause coupling of an optical signal in one propagation mode of the optical fiber (330) to the other propagation mode in the optical fiber (330). The acoustic wave is selected to have an acoustic wavelength which is shorter in the direction of optical propagation than the optical beat length between the two propagation modes of the fiber. In order to cause phase matching between the two optical propagation modes, a periodic structure (340) is used to cause the acoustic wave to contact the optical fiber (330) at periodic locations (342) so that the coupling between the two propagation modes is periodically enabled and dis­ abled. The periodic contact of the acoustic wave with the opti­ cal fiber (330) has the effect of adding a spatial propagation constant. The periodicity of the periodic structure (340) is selected so that the spatial propagation constant of the per­ iodic structure (340) is equal to the mismatch in the propaga­ tion constants of the two optical propagation modes and the acoustic wave. The present invention thereby provides a means of obtaining greater shifts in the optical frequency while using a colinearly propagating acoustic wave.

Description: Field of the Invention The present invention relates to acousto-optic frequency shifters, and particularly to fiber-optic frequency shifters utilizing surface acoustic waves to cause an optical signal to be coupled from one propagation mode to another propagation mode. Background of the Invention An optical fiber typically can support two or more propagation modes for an optical signal. For example, the light energy comprising the optical signal can travel in either the LP₀₁ or the LP₁₁ spatial propagation modes of a two-mode optical fiber. As another example, the light energy of an optical signal can travel in either of two orthogonal polarization modes in a birefringent single-mode optical fiber. In either of the two examples presented, the optical fiber provides two propagation paths for the optical signal, and one of the propagation paths in each example is faster than the other propagation path (i.e., the LP₁₁ propagation mode is faster than the LP₀₁ propagation mode, and light polarized along one axis of a birefringent fiber travels faster than light polarized along the other axis of a birefringent fiber). These known phenomena have been used to advantage to couple light between the two propagation modes by periodically stressing the fiber. For example, see "Active Polarization Coupler for Birefringent Fiber," J.L. Brooks, et al., OPTICS LETTERS , Vol. 9, No. 6, June 1984, pp. 249-251; "Birefringent Fiber Polarization Coupler," R.C. Youngquist, et al., OPTICS LETTERS , Vol. 8, No. 12, December, 1983, pp. 656-658; "Two-Mode Fiber Modal Coupler," R.C. Youngquist, et al., OPTICS LETTERS , Vol. 9, No. 5, May, 1984, pp. 177-179; and European Patent Publication Number 0143583, published June 5, 1985, "Birefringent Fiber Narrow Band Polarization Coupler", and assigned to the assignee of the present application. Each of the foregoing articles and the patent application are incorporated herein by reference. It has also been found that if an optical fiber having two paths for propagation of an optical signal is periodically stressed by a traveling surface wave (e.g., a traveling surface acoustic wave) or a simulated traveling wave, the light energy of the optical signal traveling along one propagation mode can be coupled to another propagation mode and shifted in frequency by the magnitude of the frequency of the traveling wave. The frequency-shifted optical signal can be separated from the unshifted optical signal by using a modal coupler or a polarizer. Such frequency shifting devices have been described in the following articles: "Acousto-Optic Frequency Shifter for Single Mode Fibers," Nosu, et al., published at 47th International Conference on Integrated Optics and Optical Fiber Communications in Tokyo, June 27-30, 1983, and in ELECTRONICS LETTERS , Vol. 19, No. 20 (29 September, 1983); "Single-Sideband Frequency Shifting in Birefringent Optical Fiber", W.P. Risk, et al., SPIE, Vol. 478, FIBER OPTICS AND LASER SENSORS II , May, 1984, pp. 91-97; "Acousto-Optic Frequency Shifting in Birefringent Fiber," W.P. Risk, et al., OPTICS LETTERS , Vol. 9, No. 7, July 1984, pp. 309-311; "Acousto-Optic Birefringent Fiber Frequency Shifters," W.P. Risk, et al., INTEGRATED AND GUIDED WAVE OPTICS CONFERENCE , sponsored by the Quantum Electronics Group of IEEE and by the Optical Society of America, Kissimmee, Florida, (April 24-26, 1984); and "Acoustic Fiber-Optic Modulators," W.P. Risk, et al., PROCEEDINGS OF THE IEEE ULTRASONICS SYMPOSIUM , November 14-16, 1984, pp. 318-327. Acousto-optic frequency shifters are also disclosed in European Patent Publication Number 0144190, published June 12, 1985, "Single Mode Fiber Optic Single Sideband Modulator,"; in European Patent Publication Number 0153124, published August 28, 1985, "Acousto-Optic Frequency Shifter"; and in European Patent Publication Number 0190923, Published August 13, 1986, "Acousto-optic Frequency Shifter Utilizing Multi-Turn Optical Fiber," all of which are assigned to the assignee of the instant application. The foregoing articles and patent applications are incorporated herein by reference. As discussed in European Patent Publication Number 0144190, published June 12, 1985, one approach to frequency shifting is to launch an actual acoustic wave (either a surface wave or a bulk wave) for propagation longitudinally along the length of an optical fiber. This approach has the advantage of providing a continuous, virtually infinite, number of coupling points which travel along the length of the fiber. However, it has been found that in order to achieve maximum coupling between propagation modes in an acousto-optic frequency shifter constructed in accordance with European Patent Publication Number 0144190, published June 12, 1985, the acoustic wavelength should be substantially equal to the beat length of the optical energy traveling in the two propagation modes of the optical fiber. For example, in an exemplary commercially available high birefringence fiber, the minimum beat length between the optical energy traveling in the two polarization modes of the fiber is on the order of 1 mm. An acoustic wavelength of 1 mm corresponds to an acoustic frequency of about 1-5 MHz, depending upon the acoustic propagation velocity in the acoustic propagation medium (e.g., 3.4 MHz at 3411 meters/sec.). In order to obtain greater shifts in the optical frequency, a higher acoustic frequency must be used. However, since a higher acoustic frequency corresponds to a shorter acoustic wavelength, phase matching between the acoustic wave and the beat length of the optical fiber cannot be obtained by directing the acoustic wave colinearly with the propagation direction of the optical signals in the optical fiber. Thus, a number of techniques have developed in the art for propagating the acoustic wave at an angle with respect to the direction of propagation of the optical signal in the optical fiber. For example, the above-referenced European Patent Publication Number 0153124, published August 28, 1985, discloses a fiber optic frequency shifter in which an acoustic transducer is positioned at an angle relative to an optical fiber such that the acoustic wave fronts of the acoustic wave generated by the transducer acoustically contact the fiber at an angle of incidence which is less than 90° and greater than 0°. When the acoustic wave fronts of the acoustic wave acoustically contact the fiber at an angle of incidence of 90°, the acoustic wave is propagating colinearly with the optical signals (i.e., the acoustic wave is propagating in the same direction as the optical signals) and when the acoustic wave fronts acoustically contact the fiber at an angle of incidence of 0°, the acoustic wave is propagating in a direction perpendicular to the direction of propagation of the optical signals. The angle of incidence can be chosen such that the shorter wavelengths of higher frequency acoustic waves can be matched with the longer beat lengths of the optical signal in the fiber. Thus, the maximum possible frequency shift can be substantially increased. A number of techniques are disclosed in European Patent Publication Number 0153124, published August 28, 1985, to increase the concentration of acoustic energy impinging upon the optical fiber. For example, European Patent Publication Number 0190923, published August 13, 1986, discloses an alternative apparatus for increasing the percentage of acoustic energy applied to the optical fiber by wrapping the optical fiber around the acoustic propagation means so that the propagating acoustic waves contact the optical fiber at a plurality of locations. The two techniques described in the two copending applications are further described in "Acousto-Optic Birefringent Fiber Frequency Shifters," W.P. Risk, et al., INTEGRATED AND GUIDED WAVE OPTICS CONFERENCE , sponsored by the Quantum Electronics Group of IEEE and by the Optical Society of America, Kissimmee, Florida, (April 24-26, 1984); "Single-Sideband Frequency Shifting in Birefringent Optical Fiber", W.P. Risk, et al., SPIE, Vol. 478, FIBER OPTICS AND LASER SENSORS II , May, 1984, pp. 91-97; "Acousto-Optic Frequency Shifting in Birefringent Fiber," W.P. Risk, et al., OPTICS LETTERS , Vol. 9, No. 7, July 1984, pp. 309-311; and "Acoustic Fiber-Optic Modulators," W.P. Risk, et al., PROCEEDINGS OF THE IEEE ULTRASONICS SYMPOSIUM , November 14-16, 1984, pp. 318-327. The devices described in the foregoing articles and copending patent applications require alignment of the optical fiber at an angle with the direction of propagation of the acoustic wave in order to achieve proper phase matching. In order to achieve the angular alignment, the acoustic wave must have broad wave fronts in order to affect a substantial length of the optical fiber. For an acoustic wave of a given width, the closer that the direction of propagation of the acoustic wave is to being perpendicular to the direction of propagation of the optical signal, the more power is required to couple optical energy from one propagation mode to the other propagation mode. Thus, a large amount of acoustic energy must be used to obtain substantial acoustic coupling. Alternatively, as described in European Patent Publication Number 0190923, published August 13, 1986, the optical fiber can be precisely wound around the acoustic propagation medium so that the propagating acoustic wave provides a cumulative effect each time it passes across the optical fiber at the selected angle. Summary of the Present Invention The present invention is an apparatus and a method for shifting the frequency of an optical signal propagating in an optical fiber using an acoustic wave having a vector component propagating in the direction of propagation of the optical signal. Unlike the above-referenced earlier devices, the vector component of the acoustic wave propagating in the direction of propagation of the optical signal is not phase matched with the beat length of the optical signal. Rather, the vector component of the acoustic wave traveling in the direction of the optical signal has a wavelength in the direction of the optical signal which is smaller than the optical beat length. The acoustic wave contacts the optical fiber at plural spaced locations along the fiber, such that the acoustic energy is concentrated at the plural locations, thereby causing spatially periodic stress of the optical fiber by the acoustic wave. The periodic contact of the acoustic wave introduces a space harmonic into the coupling effect of the acoustic wave on the optical signal to compensate for the lack of phase matching between the acoustic wavelength and the fiber beat length. As a result of the periodic contact, an optical signal introduced into the optical fiber in one of two propagation modes is cumulatively coupled at the spaced apart locations to the other of the two propagation modes. The coupled light is shifted in frequency by an amount equal to the acoustic frequency. The apparatus of the present invention is defined in Claim 1, and comprises an optical fiber having first and second optical propagation modes, an acoustic generator which generates acoustic energy, and an acoustic medium in acoustic contact with the fiber. The acoustic energy propagates along the fiber such that at least a portion of the applied acoustic energy is transferred to the fiber to stress the fiber. The acoustic medium and optical fiber are arranged to provide periodic acoustic contact between the fiber and the acoustic medium at spaced plural locations to concentrate the transferred acoustic energy at the spaced plural locations. This configuration causes the acoustic energy to preferentially stress the fiber at each of the plural locations. The propagation of the acoustic energy in the fiber causes light at a first frequency to be coupled at each of the plural locations from a first optical path in the fiber to a second optical path in the fiber and shifted to a second frequency. Preferably, there is little or no acoustic contact between the plural locations so that substantially no coupling of light occurs in the fiber between the plural locations. The acoustic generator produces an acoustic wave which has a wavelength less than the beat length of an optical signal propagating along the two propagation paths of the optical fiber. Preferably, there are a sufficient number of plural locations to cause substantially all of the light in the first optical path to be coupled from the first path to the second path. In a preferred embodiment, the acoustic medium comprises a substrate having a surface for the propagation of acoustic energy. Preferably, the acoustic energy propagates as a surface acoustic wave in the surface. The surface is formed to provide an elongated narrow wave guide which interfaces with the fiber at the spaced locations to provide the acoustic contact therebetween. Preferably, the width of the wave guide is not substantially larger than the width of the optical fiber, thus tightly confining the acoustic energy to the interface region between the wave guide and the, fiber. In a preferred embodiment, the spatially periodic contact of the acoustic wave with the optical fiber is caused by a periodic structure which pushes the optical fiber against an acoustic propagation medium at the spaced apart locations. Preferably, the distance between adjacent spaced apart locations and the length of each spaced apart location are selected so that the spaced apart locations are spatially periodic along the length of the optical fiber. The periodicity of the periodic structure is selected as a function of the optical beat length of the optical fiber at the optical frequency and as a function of the selected frequency shift. In a preferred embodiment, the length between corresponding portions of the periodic locations (i.e., the length of the periodicity), designated as Λ g , is selected according to the following relationship: where Λ a is the acoustic wavelength of the acoustic signal in the direction of propagation of the optical signal in the optical fiber, and where L B is the beat length of the optical signal propagating in the two propagation modes of the optical fiber. In a preferred embodiment, the present invention can be understood in terms of the propagation constants of the optical signal, the propagation constant of the acoustic signal, and a static constant, which will be referred to herein as the spatial propagation constant of a periodic structure employed to cause the periodic contact of the acoustic wave with the optical fiber. The relationship among the foregoing constants can be represented mathematically as: k₂ - k₁ = k a - k g , where k₂ is the propagation constant of the second optical propagation mode, k₁ is the propagation constant of the first optical propagation mode, k a is the propagation constant of the acoustic wave, and k g (the spatial propagation constant of the periodic contact of the acoustic wave) is equal to 2π/Λ g . The acoustic propagation constant depends on the acoustic wave frequency, while the spatial propagation constant depends on the periodicity of the acoustic contact. Thus, the spatial propagation constant of the periodic structure provides a means for matching the propagation constant of an acoustic wave with the difference in the propagation constants of the two optical propagation modes. Accordingly, the use of the periodic structure in conjunction with the traveling acoustic wave provides a means for using an acoustic wave of higher frequency to achieve a larger shift in the frequency of an optical signal that would otherwise be possible. In the preferred embodiment, the direction of propagation of the acoustic wave is colinear with that of the optical signal. This advantageously permits the acoustic wave to be concentrated in a small area proximate to the optical fiber. Thus, for a given amount of acoustic power, a greater percentage of the optical signal can be coupled from an input propagation mode to an output propagation mode at a shifted frequency. In a preferred embodiment, the fiber has a portion of the cladding removed between each of the plural locations. This geometry inhibits acoustic contact of the fiber with the acoustic medium between each of the plural locations. In a preferred embodiment, the surface of the substrate has a periodically varying elevation which inhibits acoustic contact of the fiber with the surface between each of the plural locations. In a preferred embodiment, the acoustic energy propagates as a surface acoustic wave having wave fronts which are perpendicular to the longitudinal axis of the optical fiber. In a preferred embodiment, the optical fiber has only first and second propagation paths. Preferably, these two propagation paths are the two polarization modes of a single mode birefringent fiber. The invention also includes a method of frequency shifting as define in Claim 13, using an optical flier having two propagation modes. Acoustic energy is applied to the fiber so that it propagates along the fiber and at least a portion of the acoustic energy is transferred to the optical fiber to stress the fiber. The transferred acoustic energy is concentrated at spaced plural locations along the fiber so that the acoustic energy stresses the fiber preferentially at each of these locations. The stressing causes coupling of frequency shifted light from one of the modes to the other. In a preferred embodiment of the method, a periodic structure is used to hold the fiber against an acoustic medium at the spaced plural locations. In a preferred embodiment of the method, the fiber has a larger cross sectional diameter at the spaced plural locations than between the spaced plural locations. The step of concentrating the acoustic energy transferred to the fiber is accomplished by holding the larger cross sectional diameter portions of the fiber against the acoustic medium at the spaced plural locations. In a preferred embodiment of the method, the concentrating step is performed by holding the fiber against an acoustic medium having a periodic structure. Preferably the acoustic energy propagates in a surface of an acoustic medium as a surface acoustic wave. The acoustic wave has a wavelength which is less than the beat length of the optical fiber. In a preferred embodiment the optical fiber is a single mode birefringent fiber. Brief Description of the Drawings These and other features of the present invention may best be understood through reference to the drawings in which: Figure 1 is a cross-sectional view illustrating the two orthogonal axes of birefringence of a single-mode high birefringence fiber; Figure 2 is an elevation view in partial cross-section which schematically shows a polarization coupler comprising a ridge structure for creating alternate stressed and unstressed regions in an optical fiber to cause coupling between the polarization modes of the optical fiber; Figure 3 is a graph of acoustic pressure versus the distance along the axis of an optical fiber, illustrating regions of compression and rarefaction caused by the wave front of a traveling acoustic wave which provide corresponding stressed and unstressed regions in the optical fiber; Figure 4 is an ω-k diagram illustrating the frequency shifting effect of a traveling acoustic wave when optical energy is caused to be coupled from the fast propagation mode to the slow propagation mode; Figure 5 is an ω-k diagram illustrating the frequency shifting effect of a traveling acoustic wave when optical energy is caused to be coupled from the slow propagation mode to the fast propagation mode; Figure 6 is an ω-k diagram illustrating the mismatching effect which occurs when the frequency shifted optical energy cannot be supported in the mode to which it has been coupled; Figure 7 is a schematic diagram of an optical fiber and an acoustic transducer, showing the acoustic wave fronts generated by such transducer directed to acoustically contact the fiber at an angle thereto, and illustrating that, for proper phase matching of the acoustic wave to the beat pattern of the fiber, the angle should be selected such that the distance between adjacent wave fronts as measured along the fiber is equal to the beat length of the fiber; Figure 8 is a diagram showing the relationship between the angle of incidence, beat length, and acoustic wave length required for proper phase matching of the acoustic wave to the beat pattern of the fiber; Figure 9 is a graph of acoustic frequency as a function of the angle of incidence, illustrating that small angles of incidence permit the use of acoustic frequencies which are quite high, and yield correspondingly high frequency shifts; Figure 10 is an ω-k diagram showing the effect of angling the acoustic wave propagation direction with respect to the direction of propagation of the optical signals so that the light coupled from one propagation mode to the other propagation mode is shifted in frequency and has a propagation constant which matches the propagation constant of the slow propagation mode for the shifted frequency; Figure 11 is a perspective drawing illustrating the present invention in which a periodic structure is used to cause an optical fiber to periodically contact an acoustic wave propagating colinearly with the optical signals in the optical fiber; Figure 12 is an enlarged view of one end of the embodiment of Figure 11 further illustrating the construction of the transducer that generates the acoustic signals in the acoustic propagation medium; Figure 13 is a partial elevational view of the embodiment of Figures 11 and 12 illustrating the alternating acoustically stressed and acoustically unstressed regions caused by the contact of the fiber with the acoustic propagation medium; Figure 14 is a graphical representation of the coupling of optical energy from a first propagation mode to a second propagation mode and back to the first propagation mode, illustrating the difference between a phase-matched acoustic wave and an acoustic wave which is not phase-matched; Figure 15 is a graphical representation of the cumulative effect of selectively enabling coupling from the first propagation mode to the second propagation mode and disabling coupling from the second propagation mode back to the first propagation mode; Figure 16 is a graph of the optical energy coupled from the first propagation mode to the second propagation mode, having an expanded distance scale on the horizontal axis to show the relationship between the coupling in the present invention and the coupling caused by a phase-matched acoustic wave; Figure 17 is a graph of the coupling between the optical propagation modes, as in Figure 16, with a further expanded horizontal scale to show the cumulative effect of the coupling caused by the periodic structure; Figure 18 is an ω-k diagram illustrating the effect of the spatial propagation constant of the periodic structure to cause phase-matched coupling from the fast propagation mode to the slow propagation mode when the acoustic wave has a frequency too high for colinear phase-matching; Figure 19 is an alternative embodiment of the present invention in which the optical fiber has portions of its cladding periodically removed to create spaced apart locations of the optical fiber which can be positioned in acoustic contact with a traveling acoustic wave; Figure 20 is an alternative embodiment of the present invention in which the acoustic propagation means is notched to create periodic locations for contact with the optical fiber so that the acoustic wave propagating in the acoustic propagation means causes periodic stresses to the optical fiber; and Figure 21 is a pictorial representation of the present invention in which the periodic structure and the acoustic waves are positioned at angles with respect to the optical propagation axis of an optical fiber to provide a means for fine tuning the coupling from one propagation mode to the other propagation mode, Detailed Description of the Preferred Embodiment The theory of operation of the present invention will be presented initially, followed by a description of the physical structure of the embodiment of the present invention. Additional information regarding the theory of frequency shifting in birefringent optical fibers can be found in "Acousto-optic Frequency Shifting in Birefringent Fiber," W.P. Risk, et al., OPTICS LETTERS , Vol. 9, No. 7, July 1984, pps. 309-311; "Single-Sideband Frequency Shifting in Birefringent Optical Fiber," W.P. Risk, et al., SPIE Vol. 478, FIBER OPTICS AND LASER SENSORS II , May 1984, pgs. 91-97; "Acousto-Optic Birefringent Fiber Frequency Shifters," W.P. Risk, et al., INTEGRATED AND GUIDED WAVE OPTICS CONFERENCE , sponsored by the Quantum Electronics Group of IEEE and by the Optical Society of America, Kissimmee, Florida, (April 24-26, 1984); "Acoustic Fiber-Optic Modulators," W.P. Risk, et al., PROCEEDINGS OF THE IEEE ULTRASONICS SYMPOSIUM , November 14-16, 1984, pp. 318-327. In one embodiment of the frequency shifter of the present invention, a birefringent single mode optical fiber is used. A cross-section of an exemplary birefringent single mode optical fiber 10 is illustrated in Figure 1. The optical fiber 10 has an elliptical central core 12 having a relatively high index of refraction and has a surrounding cladding 14 which has a relatively low index of refraction. As is well known, a birefringent optical fiber has two orthogonal principal axes of birefringence, each of which corresponds to a polarization mode or optical propagation path through the fiber 10. These axes are labeled X and Y in Figure 1. Each axis has a different effective refractive index. The difference in effective refractive indices (i.e., Δn eff ) is referred to as the birefringence of the optical fiber 10. In the absence of perturbations of these axes, light which is linearly polarized along either of these two axes will remain linearly polarized as it propagates down the fiber 10. In general, light of any polarization can be regarded as a superposition of these two linearly polarized modes. The two polarization modes of birefringent single mode optical fiber propagate light at slightly different velocities because of the differences in the refractive indices. Therefore, the phase of light propagating in the X-axis polarization mode will change relative to that in the Y-axis polarization mode as the light propagates down the fiber. The distance, measured longitudinally along the fiber, required for light in one mode to separate in phase by 360 degrees (2π radians) relative to the light in the other mode is commonly referred to as the "beat length" of the fiber. Mathematically, the beat length may be defined as follows: where L B is the beat length, λ is the free-space wavelength of the light, and Δn is the difference in the indices of refraction for the two polarization modes. From Equation 1, it may be seen that the beat length is inversely proportional to the difference in the indices of refraction of the two optical propagation paths corresponding to the two polarization modes. Thus, the beat length is inversely proportional to the birefringence of the fiber 10. Consequently, a fiber having a relatively high birefringence has a shorter beat length than a fiber having a relatively low birefringence. One common technique for fabricating a high birefringence fiber is to draw the fiber such that the core has an elliptical shape, as illustrated by the core 12 in Figure 1. In an exemplary embodiment of the present invention, an elliptical core fiber manufactured by Andrew Corporation has a Y-axis measuring approximately one micrometer and has an X-axis measuring approximately two micrometers. The birefringence of the two axes of the core is approximately 3.72 x 10⁻⁴ which provides a beat length of 1.7 millimeters at an optical wavelength of 633 nanometers. The outer diameter of the cladding of the fiber is approximately 50 micrometers. High birefringence fibers are advantageous in that the polarization modes are well-defined, and thus, the polarization of the applied light will be maintained over relatively long lengths of fiber, without significant coupling of light between the polarization modes. Thus, the polarization modes of a high birefringence fiber may be viewed as independent optical paths through the fiber, which are normally uncoupled such that light is not transferred between them. As discussed in European Patent Publication Number 0143583, published June 5, 1985, entitled "Birefringent Fiber Narrow Band Polarization Coupler"; European Patent Publication Number 0144190, published June 12, 1985, entitled "Single Mode Fiber Optic Single Sideband Modulator"; and an article entitled "Birefringent Fiber Polarization Coupler, OPTICS LETTERS Vol. 8, No. 12 (December, 1983) pp. 656-658, selective coupling of light between the polarization modes of a birefringent single mode fiber may be achieved by applying a force to the birefringent fiber at an angle of about 45° relative to the principal axes of birefringence. Such force may be applied by squeezing the fiber between two plates on opposing sides of the fiber. Application of such force perturbs the axes of birefringence at the point of force, and causes the axes of birefringence to be rotated through a small angle. Consequently, when light launched in one linearly polarized mode reaches the localized perturbation, the light will decompose into a superposition of modes linearly polarized along the perturbed axes of birefringence, effectively coupling light from one polarization mode to the other. The coupled light is not frequency shifted because the stresses in the fiber are static and do not travel down the fiber. The foregoing may be more fully understood through reference to Figure 2 which schematically depicts a ridge structure 20 comprising plural ridges 22, 24, 26 at spaced intervals. The fiber 10 is disposed between the ridges 22, 24, 26 and a base block 28, so that the fiber 10 may be squeezed therebetween. Application of force to the ridge structure 20 in a direction normal to the longitudinal axis of the fiber 10 perturbs the axes of birefringence at each of the ridges 22, 24, 26 and provides alternate stressed and unstressed regions along the fiber 10 which cause coupling between the two polarization modes of the fiber 10. For maximum coupling between the modes, it is preferable that the ridges 22, 24, 26 be spaced at beat length intervals and that the length of each ridge be one half beat length. Satisfying these conditions causes the coupling at each ridge 22, 24, 26 to be cumulative with the coupling at the other ridges 22, 24, 26. By providing a sufficient number of ridges 22, 24, 26, one hundred percent of light input to one of the polarization modes can be coupled to the other of the polarization modes. A more complete description of this coupling phenomenon may be found in the above referenced articles and patent applications, which are hereby incorporated by reference herein. The alternate stressed and unstressed regions provided by the ridge structure 20 of Figure 2 may be alternatively provided by an actual traveling acoustic wave, such as the acoustic wave 30 of Figure 3, which is launched to propagate longitudinally along the central axis of the fiber 10. The periodicity of the traveling acoustic wave 30 provides alternating regions of compression and rarefaction so as to provide corresponding alternating stressed and unstressed regions in the fiber, and thus, cause coupling between the polarization modes of the fiber. For maximum coupling, it is preferable that the wavelength of the acoustic wave 30 be selected such that it is equal to the beat length of the fiber 10. Since the acoustic wave is sinusoidal, each of the alternating regions of compression and rarefaction will then be one-half beat length in length, and thus, each of the alternating stressed and unstressed regions will also be one-half beat length in length. From the foregoing, it will be understood that the acoustic wave 30 of Figure 3, by providing alternating half beat length stressed and unstressed regions along the fiber 10, cumulatively couples light between the polarization modes of the fiber inch the same manner as the half beat length ridges 22, 24, 26 of Figure 2. In addition, it should be understood that, while an acoustic wavelength which is equal to the fiber beat length is preferred for maximum coupling, cumulative coupling will also occur if the wavelength is an odd multiple of beat lengths. In contrast to the ridged structure 20 of Figure 2 in which the fiber stresses produced by the ridges 22, 24, 26 are static, the fiber stress pattern produced by the traveling acoustic wave 30 of Figure 3 travels down the fiber 10. Such travel of the stress pattern causes the light coupled from one polarization mode to the other to be shifted in frequency much as light from a moving source is Doppler shifted. In effect, the optical carrier wave and the acoustic wave are heterodyned such, that the acoustic frequency and the optical carrier frequency combine either additively or subtractively to provide a side band at either the sum or the difference frequency. If the acoustic wave propagates in the same direction as the light, light coupled from the faster polarization mode to the slower polarization mode is upshifted in frequency, while light propagating from the slower polarization mode to the faster polarization mode is downshifted in frequency. If the acoustic wave propagates in a direction opposite to that of the optical carrier, these relationships are reversed, so that light coupled from the faster mode to the slower mode is downshifted, while light coupled from the slower mode to the faster mode is upshifted. Frequency shifted coupling requires that the acoustic wave be properly phase matched to the beat pattern of the optical modes in the fiber. In Figure 3, such phase matching occurs when the acoustic wavelength, as measured along the axis of the fiber 10 is equal to the beat length of the fiber. Stated differently, the propagation constant k₁ of the faster mode (lower effective refractive index), the propagation constant k₂ of the slower mode (higher effective refractive index), and the propagation constant k a of the acoustic wave should satisfy the following relationship: k₁ + k a = k₂ (2) The acoustic propagation constant k a is the component of the acoustic propagation constant that is aligned with the optical propagation axis of the fiber. The frequency shifted coupling of light between the polarization modes may be examined mathematically by representing the light in the fast optical mode as cos(ω₁t - k₁z ) where ω₁ is the angular frequency of the light, t is time, and z is the distance along the fiber axis. The acoustic wave may be represented as cos(ω a t - k a z) , where ω a is the angular frequency of the acoustic wave. The interaction of these two waves leads to a product term proportional to: ½ {cos[(ω₁+ω a )t-(k₁+k a )z] + cos[(ω₁-ω a )t-(k₁-k a )z]} (3) The second term of Expression 3 does not satisfy the phase matching condition of Equation 2, and thus, it is expected that the signal represented by this term will die away. The first term, however, is phase matched to the slow mode, in accordance with expression (2), and explicitly indicates that the slow mode is upshifted. A similar analysis shows that if the slow mode interacts with the acoustic wave, the expression for the resulting interaction is: ½ {cos[ω₁-ω a )t-(k₂-k a )z] + cos[(ω₁+ω a )t-(k₂+k a )z]} (4) The second term of Expression 4, like the second term of Expression 3, is not phase matched, however, the first term is phase matched to the fast mode in accordance with equation (2) and explicitly describes a downshifted wave. The foregoing analyses can be represented graphically as illustrated in Figure 4. The horizontal axis of the graph represents the magnitude of the propagation constant of the optical signal in the optical fiber 10. The vertical axis represents the magnitude of the frequency of the optical signal in the optical fiber 10. The relationship between the propagation constant and the frequency of an optical signal is dependent upon the propagation velocity in accordance with the following equations: where v₁ and v₂ are the propagation velocities of the optical signals in the fast propagation mode and the slow propagation mode, respectively; ω₁ and ω₂ are the frequencies of the optical signals in the fast propagation mode and slow propagation mode,respectively; and k₁ and k₂ are the propagation constants of the optical signals in the fast propagation mode and slow propagation mode, respectively. A phantom line 100, representing the velocity v₁ of the optical signal in the fast propagation mode, has a greater slope (i.e., Δω/ωk) than the slope of a phantom line 102, representing the velocity v₂ of the slow propagation mode. A vector 110, superimposed on the velocity line 100, has a length determined by the frequency ω₁ and propagation constant k₁ of the optical signal input into the fiber 10 in the fast propagation mode. A vector 112, represents the acoustic wave 30 and has a slope, representing the velocity v a of the acoustic wave 30, in accordance with the following equation: The length of the vector 112 is determined by the frequency ω a and is thus determined by the propagation constant k a . The vector 112 is added to the vector 110 to obtain a resulting vector 114 having a length on the k-axis equal to the sum of the optical propagation constant k₁ plus the acoustic propagation constant k a (i.e., k₂ = k₁ + k a ); and having a length along the ω-axis equal to the sum of the input optical frequency ω₁ plus the acoustic frequency ω a (i.e., ω₂ = ω₁ + ω a ). In the example presented, the resulting vector has a slope equal to the velocity v₂ of the slow propagation mode 60 that the vector satisfies the phase matching condition for propagation along the optical path provided by the slow propagation mode. In like manner, Figure 5 illustrates the phasematching condition of Expression 4. A vector 120 represents the input optical signal in the slow propagation mode having a velocity v₂, a frequency ω₂ and a propagation constant k₂. In accordance with Expression 4, a vector 122, representing the acoustic wave 30, is subtracted from the vector 120 to obtain a vector 124 having a length along the ω-axis of ω₁=ω₂-ω a ; and having a length along the k-axis of k₁=k₂-k a . As illustrated, the resulting vector has a slope of ω₁/k₁ which corresponds to the slope of the graph 100 of the velocity v₁ for the fast propagation mode. Thus,the vector 124 represents an optical signal satisfying the phase-matching conditions for propagation in the fast optical propagation mode. One skilled in the art will appreciate that the graphical relationships depicted in Figures 4 and 5 have been exaggerated for illustrative purposes. The actual optical frequencies and velocities are so much greater than the acoustic frequencies and velocities that it is not practical to graph the relationship with dimensions proportional to the respective frequencies and velocities. One skilled in the art will also understand that the graph 100 and the graph 102 representing the two polarization modes are not necessarily straight lines as illustrated. Thus, it should be understood that the phase-matching conditions are met when the tip of the resulting vector 114 in Figure 4 is on the graph 102. Similarly, phasematching conditions are met when the resulting vector 124 in Figure 5 is on the graph 100. Figure 6 graphically illustrates the result of using a higher frequency acoustic signal (e.g., an acoustic signal having a frequency ω a ') such that the phase matching conditions are not met. A vector 130 represents the input optical signal in the fast propagation having a frequency ω₁ and a propagation constant k₁, corresponding to the velocity v₁. A vector 132 represents the acoustic signal having a frequency ω a ' and a propagation constant k a '. Assuming that the propagation velocity V a of the acoustic wave is the same as before, the acoustic vector 132 is necessarily longer than the corresponding acoustic vector 112 in Figure 4. Thus, a vector 134 representing the sum of the vector 130 and the vector 132, has a magnitude along the ω-axis of ω₂' = ω₁ + ω a ' and a magnitude along the k-axis of k₂, = k₁ + k a ' . However, unlike the vector 114 in Figure 4, the vector 134 does not have a slope corresponding to the slope of the velocity graph 102 for the slow propagation mode. Thus, the frequency-shifted optical signal is not properly phase-matched for propagation in the slow propagation mode and will not be supported in that propagation mode. The foregoing phase matching requirements indicate that, for an acoustic wave propagating longitudinally down the fiber with the wave fronts normal to the fiber axis (i.e., an acoustic wave that propagates colinearly with the direction of propagation of the optical signals in the fiber), the acoustic wave frequency should be such that its wavelength is equal to the fiber beat length for the optical signal. Since fiber beat lengths are typically on the order 1 mm or more, (e.g., 1.7 mm) the maximum available frequency shift from such longitudinally propagating acoustic waves is on the order of only a few megahertz. Figure 7 schematically illustrates a frequency shifter which circumvents this limitation by positioning an acoustic transducer 140 to produce an acoustic wave 142 that propagates in a direction, e.g., as indicated by the arrow 143, which is at an angle to the central axis 148 of a high birefringence single-mode optical fiber 146. Such positioning causes the wave fronts 144 of the wave 142 to be directed to acoustically contact the highly birefringent single-mode optical fiber 146 at an angle ϑ, referred to herein as "the angle of incidence." As used herein, the term "angle of incidence is defined as the acute angle between a wave front of an acoustic wave impinging on the fiber and the longitudina] central axis 148 of that fiber. The wavelength λ a (which is measured in the direction of propagation 143) of the acoustic wave 144 and the angle of incidence, ϑ, are preferably selected such that adjacent wave fronts 144 are spaced by one fiber beat length, L, which is measured in a direction along the fiber axis 148. This beat length spacing of the wave fronts 144 satisfies the phase matching conditions set forth above, and thus, the component of acoustic propagation along the fiber axis 148 will satisfy Equation 2, above. This arrangement permits the use of much shorter acoustic wavelengths, and thus, much higher acoustic frequencies than with a colinearly propagating acoustic wave, while still satisfying the phase matching condition. As illustrated in Figure 8, simple trigonometry shows that the phase matching condition will be satisfied when: λ a = L sin ϑ (8) Accordingly, by varying the angle of incidence, ϑ, the acoustic wavelength, and thus, the acoustic frequency may be readily changed. Equation 8 may be rewritten in terms of the acoustic frequency ω a , as follows: where v a is the propagation velocity of the acoustic wave in the direction 143. Figure 9 graphically illustrates the relationship between the acoustic frequency ω a and the angle of incidence ϑ, and shows that for very small angles of incidence (e.g., a few degrees or a fraction of a degree), the acoustic frequency utilized may be quite high. Note that the acoustic frequency ω a is at a minimum when ϑ equals 90° (i.e., when the acoustic wave fronts are normal to the fiber). By way of example, an angle of incidence of 3° would permit use of an acoustic frequency of 95 MHz for a fiber having a 1 mm beat length. The device of Figure 7 may also be analyzed in terms of the "propagation phase velocity" of the acoustic wave. The term "propagation phase velocity," as used herein, is defined as the velocity, measured along a particular line of measurement (e.g., the direction 143 or axis 148), with which the intersection of a wave front (e.g., one of the wave fronts 144) and that axis of measurement move. Thus, for a particulur wave front 144, that wave front will have a "propagation phase velocity" which is peculiar to the axis of measurement which is selected. The propagation phase velocity along the line 143 is equal to the rate at which the wave front moves along that line. Similarly, the propagation phase velocity along the fiber axis 148 is the rate at which the wave front moves along that axis. The propagation phase velocity of the acoustic wave in the direction of propagation 143 is independent of the angle of incidence. However, the propagation phase velocity of the acoustic wave fronts in the direction of the fiber axis 148 increases as the angle of incidence decreases and decreases as the angle of incidence increases. Thus, by directing the wave 142 at an angle relative to the fiber 146, the propagation phase velocity of the wave fronts 144 will be higher in the direction along the fiber axis 148 than along the direction of propagation 143. In general, for a given acoustic propagation velocity and frequency, the closer the angle of incidence is to 0°, the higher the propagation phase velocity along the fiber axis. Further, the higher the propagation phase velocity, the higher the acoustic frequency can be, while still maintaining the proper phase matching conditions. Thus, directing the acoustic wave at an angle to the fiber increases the propagation phase velocity in the direction of the fiber axis and thereby permits a high frequency acoustic signal to be matched with a fiber having a relatively long beat length. The foregoing can be demonstrated graphically as illustrated in Figure 10. In Figure 10, the vector 130 represents the optical signal propagating in the fast optical propagation mode as in Figure 6. A vector 132' (in phantom) corresponds to the vector 132 in Figure 6 and a vector 134' (in phantom) corresponds to the vector 134 in Figure 6. In Figure 10, a vector 152 represents the acoustic signal 143 of Figure 7 directed at an angle with respect to the fiber 146 so that the effective propagation phase velocity of the acoustic wave is increased. Thus, as illustrated, the slope of the acoustic wave vector 152 is greater than the slope of the acoustic wave vector 132'. Since the frequency of the acoustic wave represented by the vector 152 has not changed with respect to the frequency of the acoustic wave represented by the vector 132 in Figure 6, the magnitude of the acoustic frequency is again shown as ω a '. However, increasing the velocity of the acoustic wave has the effect of reducing the propagation constant from k a ' (shown in phantom) to k a ''. Thus, a vector 154, representing the frequency shifted optical signal has a length along the ω-axis of ω₂'' = ω₁ + ω a ' and has a length along the k-axis of k₂'' = k₁ + k a '' . As illustrated in Figure 10, the acoustic propagation velocity along the length of the optical fiber is selected so that the vector 154 has a slope (i.e., velocity) corresponding to the velocity v₂ of an optical signal propagating in the slow propagation mode. Thus, the vector 154 represents an optical signal that has been phase-matched with the slow propagation mode by angling the acoustic wave 143 with respect to the optical propagation axis 148 of the optical fiber 146. While the above-described angled approach has produced nearly 100% coupling from one polarization to the other, the electrical input power required to achieve this efficiency is quite large (e.g., approximately 25 watts). Furthermore, since the acoustic wave has to be directed at an angle to the optical fiber, the width of the acoustic propagation mode must be relatively large so that the acoustic wave has a width sufficient to impinge upon a significant length of optical fiber. Thus, it is desirable to find a method to achieve phase matching to a high frequency acoustic wave propagating in the direction of a fiber so that the acoustic wave can be confined to a waveguide. This would increase the acoustic power density in the region of the fiber and thereby reduce the input power needed to obtain a given degree of coupling. The present invention, in its preferred embodiment, utilizes an acoustic wave having a propagation direction which is colinear with that of the optical signal. The preferred embodiment achieves colinear phase matching between a propagating acoustic wave at frequencies above the usual colinear phase matching frequency. For example, for the above-described elliptical core birefringent fiber, a beat length of approximately 1.7 millimeters and an acoustic propagation velocity of approximately 3411 meters per second (the value for a Rayleigh wave on fused quartz) would limit the acoustic frequency to 2.0 MHz to obtain proper phase matching between the optical beat lengths and the acoustic wave when the acoustic wave is directed to propagate colinearly with the axis of the fiber. Thus, the optical frequency could only be upshifted or downshifted by a maximum of approximately 2 MHz. In order to obtain increased frequency shifting without directing the acoustic wave at an angle with respect to the direction of propagation of the optical fiber, the present invention is constructed so as to cause the colinearly propagating acoustic wave to affect the optical fiber only at periodically spaced locations on the fiber. Although the preferred embodiment utilizes colinear propagation of the acoustic wave and optical signal, it will be understood that the invention way also be used with an angled wave such as was described in connection with Figure 7. Figure 11 illustrates one embodiment of the present invention. The embodiment of Figure 11 includes a substrate 300 having a top surface 302 which provides an acoustic propagation path for an acoustic wave 304, shown pictorially in Figure 11. An acoustic transducer 306, shown in more detail in Figure 12, is positioned at one end 308 of the substrate 300. The acoustic transducer 306 comprises a slab of piezoelectric material, such as PZT (lead-zirconate-titanate) mounted in a conventional manner at the end 308 of the substrate 300. A pair of electrodes 310, 312 are formed on opposing sides of the transducer 306 to provide a driving means for the transducer 306. In the illustrated embodiment, the end 308 of the substrate 300 has a thin layer of Cr-Au deposited thereon to form the first electrode 310. The PZT transducer 306 is mounted thereon by conventional means (e.g., by bonding one side of the transducer 306 to the Cr-Au electrode 310). The second electrode 312 is formed by depositing a layer of Cr-Au on the opposite side of the PZT transducer 306 from the electrode 310. When an alternating electric voltage (not shown) is applied to the first electrode 310 and to the second electrode 312, the PZT transducer 306 oscillates at the frequency of the applied electrical voltage. The PZT material of the transducer 306 is selected to oscillate in the shear mode represented by the double-headed arrow 314. Since the PZT material 306 is bonded to the end of the substrate 300, a surface acoustic wave is generated on the top surface 302 of the substrate 300 and propagates away from the transducer 306, as illustrated in Figure 11. Preferably, the second electrode 312 is formed on only a portion of the PZT material 306 so that the shearing action, represented by the double-headed arrow 314, is concentrated near the top surface 320 of the substrate 300. Thus, the generated acoustic energy is concentrated near the top surface 302 of the substrate 300. Preferably, the top surface 302 is configured as a narrow acoustic waveguide to confine the acoustic wave energy to propagate in a narrow channel. Thus, the acoustic energy generated by the transducer 306 is wellguided as it propagates away from the transducer. An optical fiber 330 is placed on the top surface 302 of the substrate 300 in contact with the traveling acoustic wave 304 generated by the transducer 306. In order to provide periodic acoustic contact between the top surface 302 and the optical fiber 330, a periodic structure, such as a ridged grating 340, is provided to apply a small amount of force to the optical fiber 330 to force it against the top surface 302 of the substrate 300 at periodic locations. Except for the periodicity of its ridges, the ridged grating 340 is similar to the ridged block 20, described above in connection with Figure 2. However, the force applied to the ridged grating 340 is sufficient only to provide good acoustic contact between the optical fiber 330 and the top surface 302 substantially exclusively at the periodic locations defined by the ridges. Preferably, such force is not sufficient to stress the fiber in the manner described in connection with Figure 2. In the embodiment illustrated in Figures 11 and 12, the ridged grating 340 comprises a series of ridges 342 separated by a series of groves 344. The optical fiber 330 makes good acoustic contact with the top surface 302 of the substrate 300 where the ridges 342 are pressing against the optical fiber 330. The optical fiber 330 makes poor or no acoustic contact with the top surface 302 of the substrate 300 in the areas of the groves 344. This is illustrated more clearly in Figure 13 which is a partial cross-sectional view of the invention illustrated in Figure 11. The acoustic wave 304 is represented pictorially as a solid undulating line in Figure 13. One skilled in the art will understand that the acoustic wave 304 physically represents traveling undulations in the top surface 302 of the substrate 300 caused by the traveling acoustic wave 304. As Illustrated In Figure 13, the ridges 342 of the preferred embodiment have a length in the direction of propagation of the optical signal in the fiber 330 shown as Λ g /2. Similarly, the groves 344 have a distance in the direction of propagation of the optical signal in the optical fiber 330 of Λ g /2. Thus, the distance between corresponding locations on the spaced-apart ridges 342 is shown as Λ g . In other words, the distance from the beginning of one ridge 342 to the beginning of the next ridge 342 is Λ g . As further illustrated In Figure 13, the optical fiber 330 Is acoustically stressed in the areas where the ridges 342 push the optical fiber 330 against the top surface 302 of the substrate 300. In the areas under the grooves 344, the optical fiber 330 is not pushed against the top surface 302. Thus, the optical fiber 330 is not acoustically stressed in the areas under the grooves 344. Thus, an optical signal traveling through the fiber 330 passes through alternate acoustically stressed locations and acoustically unstressed locations, corresponding to the ridges 342 and the grooves 344, respectively. As will be set forth in detail below, the use of a periodic structure to cause the acoustic wave to contact the optical fiber 330 at spatially periodic locations provides a means for causing cumulative coupling between the two optical propagation modes. In order to more fully understand the present invention, It is helpful to first review the coupling provided by above-described phase-matched device of Figures 2 and 3. Figure 14 illustrates a graph of coupled optical power versus distance (z) along the fiber 10 (Figure 2) in an interaction region defined by the length of the ridged pusher 20 (Figure 2) or the distance along which the acoustic wave 30 (Figure 3) stresses the fiber. As set forth above, the alternate stressed and unstressed regions caused by the ridged pusher of Figure 2 or the acoustic wave of Figure 3 cause cumulative coupling of light between the two polarization modes of the fiber 10 as the optical signal passes through the interaction region. First, assume that all of the optical energy is propagating in one of the two propagation modes prior to entering the interaction region of the fiber, and that input optical energy can be represented as P₁(0), indicating the power in the first polarization mode at zero-distance into the interaction region. As the optical signal travels through the portion of the optical fiber 10 which is stressed by the ridged pusher or the acoustic wave, optical power is gradually coupled to the second propagation mode. This power can be represented as P₂(z) wherein the subscript "2" indicates the power in the second propagation mode and z represents the distance from the beginning of the acoustic interaction region. As the optical signal passes through the interaction region, the coupled power increases gradually and continuously until, for example, 100% of the power is coupled to the second propagation mode. It has been shown that for a properly phased matched acoustic wave, the coupled power is a sine-squared (sin²) function of the coupling coefficient κ₀ and the distance traveled in the stressed region of the fiber. This is represented graphically in Figure 14, wherein the horizontal axis of the graph represents the distance from the beginning of the interaction region and is shown as z, and the vertical axis is the percent of power coupled to the second propagation mode normalized with respect to the power initially in the first polarization mode at the beginning of the stressed regions. The vertical axis is shown as P₂(z)/P₁(0). Thus, a graph 400 in Figure 14 is the graph of the equation: In Equation (10), the coupling coefficient κ₀ is determined by the magnitude of the applied acoustic energy. As illustrated in Figure 14, as the distance along the length of the fiber increases, the energy coupled to the second propagation mode gradually increases to 100% (i.e., P₂(z)/P₁(0) = 1.0 ), and then begins to decrease along the same sine-squared curve. The location where 100% of the optical energy has been coupled to the second propagation mode is shown in Figure 14 as L max . For the ridged pusher structure of Figure 2, the applied force and the number of ridges can be selected so that the length of the interaction region is equal to L max , thereby causing all the optical energy input to one mode to be coupled to the other mode. Similarly, for the acoustic wave structures of Figures 3 and 7, the amount of acoustic energy applied to the optical fiber (e.g., the optical fiber 146 in Figure 7), can be adjusted so that L max is equal to the length of the interaction region so that no further coupling occurs once 100% of the input signal has been coupled to the second propagation mode. Since sin²(κ₀z) is at a maximum when κ₀z=π/2 , then the applied forces are adjusted so that κ₀=π/(2NL B ) . Thus, when the optical signals exit the interaction region, substantially 100% of the optical energy will have been coupled from the first propagation mode to the second propagation mode. Continuing to refer to Figure 14, a graph 410 represents a graph of the coupling for the devices of Figures 2, 3, and 7 when the acoustic wavelength is not properly matched with the beat length of the fiber, for example, when the acoustic frequency is increased to a frequency higher than the frequency corresponding to proper phase matching between the acoustic wavelength and the beat length of the fiber. When this occurs, only a small fraction of the power initially in one propagation mode is coupled to the other propagation mode, then it is coupled back to the other propagation mode in a relatively short distance. The periodic transfer of power between the propagation modes continues as the optical signal travels down the optical fiber. Thus, as illustrated by the graph 410 in Figure 14, the maximum power transferred to the second propagation mode is substantially less than 100% of the input power in the first propagation mode. The periodicity of the graph 410 indicates that the amount of power coupled to the second propagation mode cannot be increased by increasing the length of the acoustic interaction region. Thus, it has heretofore been believed that the only way to obtain efficient coupling between the propagation modes using an acoustic wave or a simulated traveling wave to cause the coupling is to match the wavelength of the traveling acoustic wave or simulated traveling wave with the beat length of the fiber, as described above, either by using a relatively low frequency acoustic wave or by angling the propagation path of the acoustic wave with respect to the direction of propagation of the optical signals in the optical fiber. However, Applicants' novel structure, illustrated in Figures 11-13, unexpectedly provides an alternative to the phase matching requirements of the above-described structures. Figure 15 graphically illustrates an explanation for the operation of the present invention. Figure 15 is a graph on an enlarged scale of the periodic coupling graph 410 caused by the mismatched acoustic wave. The periodic coupling graph 410 comprises first coupling portions 412a, 412b, 412c, 412d, 412e, etc., which represent coupling from the first propagation mode to the second propagation mode and second coupling portions 414a, 414b, 414c, 414d, etc., which represent coupling from the second propagation mode back to the first propagation mode. The device of the present invention eliminates the second coupling portions 414a-d, etc., so that the coupling portions 412a-e are cumulative, thus causing a cumulative transfer of optical energy from the first propagation mode to the second propagation mode. This is graphically illustrated by a coupling graph 420 which comprises a series of cumulative coupling portions 422a, 422b, 422c, 422d, 422e, etc., representing the coupling of optical energy from the first propagation mode to the second propagation mode, alternating with a series of non-coupling portions 424a, 424b, 424c, 424d, etc., which represent no coupling in either direction. Since no energy is coupled back to the first propagation mode, energy coupled during each coupling portion 422a, 422b, etc. is cumulative. It should be noted that the percentage of the optical signal in the first propagation mode coupled to the second propagation mode by each coupling portion 422a, 422b, 422c, etc., is substantially smaller for a given distance than for the coupling which occurs when the acoustic wave length is matched with the beat length of the optical fiber. Furthermore, it should be noted that the cumulative coupling increases nonlinearly as a sine-squared function of distance. This is represented by the increasing coupling amounts for each of the coupling steps 422a, 422b, 422c, etc. As further illustrated in Figure 16, in which the graph 400 and the graph 420 are shown with an expanded distance scale on the horizontal axis, the graph 420 increases in a series of increasing steps corresponding to the incremental steps 422a, 422b, 422c, etc., in Figure 15. Further expansion of the horizontal axis, as illustrated in Figure 17, shows that the graph 420 is a sine-squared function similar to the sine-squared function of the graph 400, except that for the same acoustic power applied to the optical fiber, the coupling represented by the graph 420 requires a length approximately π times the length of the interaction region required to accomplish 100% coupling with a continuous coupler having the acoustic wave length matched with the optical beat length of the signal propagating in the optical fiber (i.e., πL max ). Returning again to Figure 13, the distance Λ g /2 is selected to correspond to the length of the first coupling portions 412a, 412b, 412c, 412d, 412e, etc., of Figure 15, so that the series of acoustically stressed regions under the ridges 342 of the periodic grating structure 340 cause the cumulative coupling effect represented by the graph 420 in Figure 15. In other words, when an optical signal in the optical fiber 330 in the first propagation mode enters the the portion of the fiber under the first ridge 342a in Figure 13, a portion of the power of the optical signal in the first propagation mode is coupled to the second propagation mode, as illustrated by the portion 422a of the graph 420 in Figure 15. When the optical signal passes from the acoustically stressed area under the ridge 342a into the acoustically unstressed area under the grove 344a, the optical power coupled to the second propagation mode remains in the second propagation mode, as represented by the constant magnitude portion 424a of the graph 420 In Figure 15. Similarly, when the optical signal next passes into the region of the optical fiber 330 acoustically stressed by the acoustic contact caused by the second ridge 342b, coupling occurs from the first propagation mode to the second propagation mode, as illustrated by the increasing portion 422b of the graph 420 in Figure 15, and so on. Thus, the optical power coupled to the second propagation mode will gradually increase as was illustrated in Figures 16 and 17. The distance Λ g referred to hereinafter as the periodicity of the periodic grating structure 340, is determined by the propagation constants of the two optical propagation modes and the propagation constant of the acoustic wave at the selected frequency shift. This can be more fully understood by referring to Figure 18, which graphically illustrates the present invention in which the acoustic wave 304 is periodically coupled to the optical fiber 330 to provide the on/off optical coupling, described above. The graph 100 represents the fast optical propagation mode, and the graph 102 represents the slow optical propagation mode, as in Figures 4, 5, 6 and 10. A vector 450 represents the optical signal propagating in the fast propagation mode at a velocity v₁, a frequency ω₁, and a propagation constant k₁. A vector 452 represents the acoustic wave 304 having a frequency ω a and a propagation constant k a as indicated by the length ω a along the ω-axis of the graph and the length k a along the k-axis of the graph. A vector 454 (shown in phantom) represents the sum of the vector 450 and the vector 452. As set forth above, when the acoustic frequency is greater than the phase-matched acoustic frequency, the vector 454 does not have a slope that corresponds to the slope of the graph 102. Thus, the optical energy coupled to the slow propagation mode does not propagate in the slow propagation mode and is coupled back to the fast propagation mode, as was illustrated by the graph 410 in Figure 14. In Figure 18, a vector 456 is subtracted from the sum of the vector 450 and the vector 452. The vector 456 represents the spatial propagation constant of the periodic grating 340 In Figures 11-13. Since the periodic grating 340 is stationary, it has no time-varying component and is therefore shown as having no component along the ω-axis (i.e., it is shown as having zero frequency and is thus shown as having a slope of zero representing no velocity). The vector 456 has a length along the k-axis corresponding to its effective propagation constant k g . Since the periodic grating 340 is stationary, the propagation constant k g in radians/meter is determined according to the following equation: where Λ g is the distance between corresponding portions of the ridges 342 of the periodic grating structure 340, as illustrated in Figure 13. In other words, Λ g is the periodicity of the ridges 342 in meters/cycle. The periodicity Λ g of the grating structure is selected so that k₁ + k a - k g = k₂ (12) as shown In Figure 18, and wherein a vector 458, representing the vector 450 plus the vector 452 minus the vector 456, has a slope corresponding to the slope of the graph 102 of the slow optical propagation mode. In other words, the velocity of an optical signal represented by the vector 458 is substantially equal to the velocity v₂ of the slow optical propagation mode. Thus, the difference between the acoustic propagation constant and the grating constant (i.e., k a -k g ) is equal to the difference between the two optical propagation constants (i.e., k₂-k₁). Equation 12 above can be rewritten as follows: k g = k a + k₁ - k₂ (13) It is known that where Λ a is the acoustic wavelength of the acoustic wave 304. It is also known that. where L B is the beat length of the two optical signals at the frequencies ω₁ and ω₂, respectfully. Thus Equation (13) can be rewritten as: Equation (16) can be rewritten so that the periodicity Λ g of the periodic grating structure 340 can be expressed as follows: As a specific example of an embodiment of the present Invention, an optical fiber 330 having a beat length of approximately 1.7 mm at an optical wavelength of 633 nm was used. As set forth above, an acoustic signal propagating at a velocity of 3411 meters/second at a frequency of approximately 2.0 MHz has a wavelength of approximately 1.7 mm. Thus, if the acoustic wave propagates colinearly with the optical signal, a frequency greater than 2.0 MHz would not phase-match the acoustic wavelength to the fiber beat length. If it is desired to increase the wavelength to, for example, 5.6 MHz, the acoustic wavelength would become 0.61 mm which is substantially different from the 1.7 mm required for phase-matching. However, solving Equation (17) above, one finds that a periodic grating structure 340 having a periodicity Λ g of 0.94 mm causes cumulative coupling of the optical signal from the first propagation mode to the second propagation mode in accordance with Figures 14-18. The present invention is particularly advantageous in that it enables colinear propagation of a high frequency acoustic wave with the optical signal, and thus, the acoustic wave can be guided to propagate along a narrow channel so that the acoustic power is confined to an elongate strip which forms a waveguide to maintain the acoustic power in close proximity to the optical fiber. It can be shown that the fraction of power coupled from a first propagation mode to a second propagation mode can be expressed mathematically by the following expression: where L is the interaction length between the acoustic wave and the optical fiber, κ₀ is a coupling coefficient per unit length characteristic of the acoustic wave if it were in continuous contact with the optical fiber, and Δk is defined by: Δk = k₁ + k a - k₂ - k g (19) In the case where the acoustic frequency and the periodicity of the grating are properly selected so that Δk equals 0, equation (18) above reduces to: Note that the Equation (20) above is similar to the Equation (10) for the continuous coupling devices described above except that for the same amount of coupling, a length of π times as long an interaction region is required. In an experimental embodiment of the present invention, using the above-described dimensions and using a ridged grating structure having 28 ridges, an optical coupling efficiency of 26% was obtained using an acoustic power of 32 watts. In the experimental embodiment, the fused quartz substrate had cross-sectional dimensions of one inch by one inch. In preferred embodiments of this invention, the cross-sectional dimensions of the quartz substrate will be substantially smaller so that the input acoustic energy is more tightly confined to propagate along the interface between the substrate and the optical fiber. Thus, since the acoustic wave propagates colinearly with the optical fiber, the width of the substrate acting as the acoustic propagation means does not have to be significantly wider than the optical fiber. It is therefore expected that the acoustic power required in the experimental embodiment can be significantly reduced by utilizing a substrate having an acoustic propagation surface which is no wider than is necessary for propagation of the acoustic wave. Although described above in connection with a ridged pusher, the present invention can be constructed by using other means of providing periodic acoustic contact. For example, as illustrated in Figure 19, portions of the cladding of an optical fiber 500 can be periodically removed to form a series of grooves 502 so that the cladding of the fiber only contacts the upper surface 510 of an acoustic substrate 512 at the larger diameter unremoved portions 504 of the fiber. A flat plate (not shown) can be used to hold the larger diameter optical fiber portions 504 against a flat upper surface 510 of the substrate 512 to provide sufficient acoustic contact. Alternatively, the optical fiber 500 may be bonded to the upper surface 510 of the substrate 512 so that the portions 504 of the optical fiber 500 are in good acoustic contact with the substrate 512, thereby eliminating the need for the flat plate. An acoustic wave is caused to propagate along the upper surface 510 of the substrate 512 by a transducer 514 mounted at one end 516 of the substrate as described above in connection with Figures 11-13. In a further alternative embodiment illustrated in Figure 20, a substrate 520 has a notched upper surface 522 to provide the periodic grating structure. The upper surface comprises a series of notches 524 which form a series of raised ridges 526 therebetween. Preferably, the notches 524 are sufficiently deep so as to avoid contact with the fiber 500, yet not so deep so as to significantly perturb the propagation of an acoustic wave generated by the transducer 514. A flat pusher plate (not shown) can be used to force the optical fiber (500) into sufficient acoustic contact with the upper surface 522 of the substrate 520, or the optical fiber can be bonded to ridges 524 formed by the notches. Although described above in terms of the acoustic wave propagating precisely colinearly with the optical signals in the optical fiber, the acoustic wave can be propagated at an angle with respect to the optical fiber to provide additional tuning. Furthermore, the ridges and grooves of the periodic grating structure (e.g., the grating structure 340 in Figures 11-13) can also be positioned at an angle with respect to the fiber. This is illustrated schematically In Figure 21, wherein a series of acoustic wavefronts 600 are shown striking an optical fiber 602 at an angle ϑ with respect to the fiber axis, and a periodic grating structure 604 is shown positioned at an angle  with respect to the fiber axis. Equation (13) above can be rewritten in terms of the angles ϑ and  as follows: k₂ - k₁ - k a cos ϑ + k g cos  = 0 (21) When ϑ and  are both equal to zero, we have: k₂ - k₁ - k a + k g = 0 as above. Since positioning the periodic structure at an angle  with respect to the optical fiber 602 effectively increases the periodicity Λ g , the operational frequency would thus be lowered. Thus, angling the periodic structure 604 with respect to the optical fiber 602 can be used as a means of fine tuning for a particular acoustic frequency. Angling the acoustic waves 600 with respect to the optical fiber 602 has the effect of increasing the operational frequency as described above in connection with Figure 7. Thus, with a fixed periodicity for the grating structure 604, the direction of propagation of the acoustic waves 600 can be angled to increase the operational frequency above the frequency required for colinear propagation.

Claims: An apparatus for shifting the frequency of an optical signal from a first optical frequency to a second optical frequency, said apparatus comprising an optical fiber (330,500) having first and second optical propagation modes, said optical fiber having a beat length for said modes at the first frequency of the optical signal, an acoustic generator (306,514) which generates acoustic energy, and an acoustic medium (300,512,520) in acoustic contact with the fiber (330,500) to apply the acoustic energy to propagate along the fiber such that at least a portion of the applied acoustic energy is transferred to the fiber (330,500) to stress the fiber (330,500), the acoustic medium (300,512,520) and optical fiber (330,500) being arranged to provide acoustic contact between the fiber (330,500) and the acoustic medium (300,512,520), characterised in that: the acoustic generator (306,514) is arranged to produce an acoustic wave in the acoustic medium (300,512,520) having a component travelling in the direction of the longitudinal axis of the optical fiber which has a wavelength in the direction of the longitudinal axis of the optical fiber which is less than the beat length of the optical fiber (330,500), and in that the acoustic contact between the fiber and the acoustic medium is at periodically spaced plural locations to concentrate the transferred acoustic energy at the spaced plural locations, and thereby to cause the acoustic energy to stress the fiber (330,500) at each of the plural locations, the propagation of the acoustic energy causing light at the first frequency to be coupled at each of the plural locations from the first optical mode to the second optical mode and shifted to the second frequency. The apparatus as defined in Claim 1, characterised in that there is little or no acoustic contact between the plural locations such that substantially no coupling of light occurs in the fiber between the plural locations. The apparatus as defined in Claim 1, characterised in that the first optical propagation mode has a first optical propagation contant k₁, the second optical propagation mode has a second optical propagation constant k₂, the acoustic wave has an acoustic propagation constant k a in the acoustic medium, and said plural locations are spaced apart from each other by a distance Λ g selected so that said plural locations have an effective static propagation constant k g defined by: k g = 2π/Λ g and wherein the difference between said first and second propagation constants, k₁ and k₂, is substantially equal to the difference between said acoustic propagation constant k a and said effective static propagation constant k g . The apparatus as defined in any one of Claims 1 or 3, characterized in that the distance between corresponding portions of said plural locations is defined as a spatial period Λ g , wherein said acoustic wave has a wavelength of Λ a along said fiber (330, 500), and wherein said optical signal traveling in said fiber (330, 500) has a beat length L B , said distance selected so that said spatial period Λ g is defined by the following relationship: The apparatus as defined in any one of Claims 1-4, characterized in that the plural locations are sufficient in number to cause substantially all of the light in the first mode to be coupled from the first mode to the second mode. The apparatus as defined in any one of Claims 1-5, characterized in that the acoustic medium (300, 512, 520) comprises a substrate having a surface (302, 510, 522) for propagating the acoustic energy, the surface (302, 510, 522) formed to provide an elongated narrow waveguide which interfaces with the fiber (330, 500) at the spaced locations to provide the acoustic contact therebetween, the width of the waveguide being not substantially larger than that of the optical fiber (330, 500) to tightly confine the acoustic energy to the interface between the waveguide and the fiber (330,500). The apparatus as defined in any one of Claims 1-6, characterised in that the acoustic medium (300) comprises a substrate having a surface (302) for propagating the acoustic energy as a surface acoustic wave (304), and the apparatus includes a periodic structure (340) which holds the fiber (330) in acoustic contact with the surface (302) at the spaced plural locations (342) to concentrate the acoustic energy at the spaced plural locations (342). The apparatus as defined in any one of Claims 1-6, characterised in that the fiber (500) has a portion of the cladding removed between each of the plural locations (504) to inhibit acoustic contact of the fiber (500) with the acoustic medium (512) between each of the plural locations (504). The apparatus as defined by Claim 6, characterised in that the surface (522) of the substrate (520) has a periodically varying elevation to inhibit acoustic contact of the fiber (500) with the surface (522) between each of the plural locations. The apparatus as defined in any one of Claims 1-9, characterised in that the acoustic energy propagates as a surface acoustic wave having wave fronts which are perpendicular to the optical fiber (330,500). The apparatus as defined in an one of Claims 1-10, characterised in that the optical fiber (330,500) has only said first and second propagation modes. The apparatus as defined in any one of Claims 1-11, characterised in that the propagation modes comprise the polarisation modes of a single mode birefringent fiber. A method of frequency shifting using an optical fiber (330,500) having two propagation modes by stressing the optical fiber to cause coupling of a light signal from one of the modes to the other, comprising the steps of: applying acoustic wave energy to propagate along the fiber (330,500) so that at least a portion of the acoustic energy is transferred to the optical fiber (330,500) to stress the fiber, said acoustic wave being selected such that it has a component travelling in the direction of the longitudinal axis of the optical fiber having a wavelength in the direction of the longitudinal axis of the optical fiber less than the beat length of the optical fiber (330,500); and concentrating the transferred acoustic energy at periodically spaced plural locations (342,504,522) along the fiber (330,500) so that the acoustic energy stresses the fiber preferentially at each of the locations. The method defined by Claim 13, characterised in that the concentrating step comprises utilising a periodic structure (340) to hold the fiber (330) against an acoustic medium (300) at the spaced plural locations (342). The method defined by Claim 13, characterised in that the fiber (500) has a larger cross-sectional diameter at the spaced plural locations (504) than between the spaced plural locations (502), said concentrating step comprising holding the larger cross-sectional diameter portions (504) of the fiber (500) against an acoustic medium (512) at the spaced plural locations. The method defined by Claim 13, characterised in that the concentrating step comprises holding the fiber (500) against an acoustic medium (520) having a periodic structure. The method defined by any one of Claims 13-16, characterised in that the acoustic energy propagates in a surface of an acoustic medium (300,512,520) as a surface acoustic wave. The method defined by any one of Claims 13-17, characterised in that the optical fiber (330,500) comprises a single mode birefringent fiber.

Inventors: KINO GORDON S, RISK WILLIAM P, KINO, GORDON S., RISK, WILLIAM P.
Applicants: UNIV LELAND STANFORD JUNIOR, THE BOARD OF TRUSTEES OF THE LELAND STANFORD JUNIOR UNIVERSITY
Application year: 1987
Main classification: G02F1
IPC codes: G02B6, H04B10, G02F1, G02F2