NO863776L - REFLECTOR FOR OPTICAL FIBERS. - Google Patents

Description

The invention relates to a method and device for

reflection of a portion of a light wave as the light wave propagates along an optical fiber. More specifically, the invention relates to a method and device where two acoustic waves are caused to propagate in the fiber at selected angles so that part of a light wave propagating in a first direction along the fiber will interact with the two acoustic waves, and as a result of such cooperation will reflect back in the direction opposite to the first direction. One or more of the inventive reflectors can be bonded onto an optical fiber to produce reflected light signals on command by activating selected of the reflectors to cause the acoustic waves produced by the activated reflectors to propagate in the fiber.

For a range of fiber sensing and data transmission applications

where an optical fiber is used, it would be desirable to have a device which can be placed along the optical fiber to produce a reflection when activated on command, and which has virtually no loss when activated. With a large number of devices having these properties located along the fiber,

time domain reflectometry techniques will be able to be used for time division multiplexing of data from a large number of sensors.

Conventional fiber couplers have been used to produce

a reflected light signal in the opposite direction in a fiber,

by placing a mirror at the unused coupler output port. However, the additional loss in such a coupler and those

additional losses due to the attachment of the coupler to the fiber, too great to allow a significant number of such reflection points to be formed on a single fiber. Moreover, these losses are permanent in that the couplers cannot be made inactive to eliminate the losses at will

times.

Another method of producing a reflection in an optical fiber is described in US Patent Application No. 596,889, filed April 5, 1984 by the applicant. This method uses transient coupling between two fiber segments separated by a looped fiber section to cause a portion of light propagating down the fiber to be coupled from one segment to the other segment to propagate back along the fiber in the opposite direction. The fiber can be unbroken by this procedure. Although the additional losses with this solution are significantly lower than with a coupler, they are still too large to prevent hundreds of such reflection points on a single fiber. Moreover, the reflector of US patent application 596,889 is permanent in that the additional loss results each time light travels over the reflectors and such losses affect the operation of all reflectors formed downstream from any particular reflector on a single fiber.

Other known methods of producing a reflection in an optical fiber include: introducing a discontinuity into the fiber, such as by breaking the fiber and reconnecting the broken ends using a low-quality connector, mechanically introducing a microscopic taper to the fiber , and to subject part of the fiber to spatial perturbations of the optical refractive index in the cladding surrounding the fiber core. The latter method is described in British patent application GB 2,145,237Å by Chevron Research Company, published March 20, 1985, from page 5, line 65 to page 6, line 35. These known methods for forming a reflector on a fibers all have the disadvantage that they result in a permanent reflector. There is an optical loss at each permanent reflector as light passes through it, and such losses affect all sensors associated with all reflectors downstream relative to any particular reflector on a fiber. As a result of the losses, a large number of such permanent reflectors could not be accommodated on a single fiber.

In contrast, it prompts the inventive

device a reflection in an opticalVrior activated on command, with virtually zero additional loss when in the deactivated state. The operating principle according to the invention is based on the acousto-optic interaction between light and sound waves. Because the cladding material does not need to be removed close to the optical fiber core, the additional losses can be made extremely low. Like the inventive reflection device, the well-known device known as the acousto-optic Bragg cell uses acousto-optic interactions. Figure 1 is a schematic diagram showing the features of a Bragg cell. In Figure 1, the light beam 1 propagates into an optical medium, such as glass or quartz, encounters. acoustic wave trains 2 at an angle 9 relative to them. plane wave fronts in the wave train. The wave train 2 produces a modulation of the refractive index of the optical medium 3. When the conditions are such that:

where X is the wavelength of the light in medium 3 and A is the wavelength of the acoustic wave train, the light wave 4 (which includes part of the energy in the light beam 1) is then deflected at an angle 20 relative to the light beam 1. The acoustic waves in a Bragg -cell is generated by a transducer (such as the transducer 5 in Figure 1) acoustically coupled to the optical medium 3. Laser modulators and beam deflectors have been constructed using Bragg cells. In conventional use, however, the light is deflected at a very small angle away from its original direction of propagation.

The desired back-reflection in a fiber corresponds to 0 = 90" in equation (1). If A = X/2, then back-reflection (where 0 = 90°) will be achieved. The maximum amplitude of the back-reflected optical wave is a maximum when the periodic optical refractive index variations induced by the acoustic energy are separated by half an optical wavelength from each other. There is generally a maximum when A = M X/2, where M is any non-negative odd integer (ie M = 1, 3 , 5 ...).The main maximum occurs for A = X/2, or M = 1. Since V = Af, where V is the velocity of the acoustic wave and f the acoustic frequency, and since X = Xg/n, where Xq is the wavelength of light in free space, and n is the refractive index of the medium in which the acousto-optic interaction occurs, we then get

This represents the frequency of the acoustic wave required to produce a reflected signal of significant amplitude in the case where the light beam propagates collinearly with the acoustic wave. The optical frequency of the scattered or reflected light is either shifted up or shifted down by an amount equal to f depending on whether the acoustic wave moves towards the light wave or away from the light wave. In most optical fibers the core is fused silicon for which n = 1.46 and V = 5.96 x IO<3>m/sec. For the light with free-space wavelength equal to Xq = 1.3 x 10~^ m, the required acoustic frequency to produce the lowest order deflection in the backward direction is equal to f= 13.38 GHz.

The difficulty in constructing a device that produces a back reflection based on acousto-optic interaction is due to the high frequency required in the forward collinear Bragg regime described above. It is very difficult to manufacture and bond acoustic transducers that operate at these frequencies and most materials are extremely lossy at this high frequency. The first problem to be addressed in such a device, in which the light propagates through an optical fiber, is to cause the sound wave to propagate from the transducer through a substrate to the fiber core region with as little loss as possible. A mismatch in acoustic impedance, pV, where p = density and W = acoustic velocity, for the substrate and fiber, would also cause an acoustic reflection loss at the boundary layer. A solution to avoid the acoustic reflection loss would be to form both the fiber and the substrate from fused silicon. However, the ultrasonic loss in fused silica is described to be 12 db/cm-GHz<2>in D.A. Pinnow, "Guidelines for the Selection of Acousto-Optic Materials", IEEE Journal of Quantum Electronics, vol. Qe-6, No. 4, April 1970. This loss is 2028 db/cm at a frequency equal to 13 GHz. This very large loss makes it very difficult to design a configuration that can introduce an ultrasound wave collinear with the fiber core. Introduction at very small angles and focusing solutions require all long paths in the substrate. Crystals have been identified as causing far less attenuation than fused silicon, but these crystals have acoustic impedance significantly different from that of fused silicon optical fibers and have the necessary acoustic propagation properties only along certain axes. Again, introducing the acoustic waves into the fiber core at shallow angles is very lossy due to the impedance mismatch, and focusing is not practical due to the anisotropy.

The device according to the invention is a device which is capable of being placed adjacent to and tied to an optical fiber to reflect part of a light wave as the light wave propagates along the longitudinal axis of the fiber. The device comprises means for generating two acoustic waves which will propagate in the fiber at selected angles so that each propagates in a cooperating part of the fiber in a direction 45° relative to the axis and perpendicular to the direction of the other.

In one embodiment, the device according to the invention comprises two acoustic transducers bound to a substrate through which acoustic signals produced by the transducers can propagate. The transducers are oriented so that when the substrate is placed adjacent to and bonded to an optical fiber, a first acoustic wave that originates at one of the transducers will propagate in the fiber in a first direction 45° relative to the longitudinal axis of the fiber, and a second acoustic wave that originating at the second of the transducers will propagate into the fiber and propagate into the fiber in a second direction 45° relative to the axis and perpendicular to the first direction. Light moving along the fiber axis will interact with the acoustic field resulting from the superimposition of said first and second acoustic waves to thereby be partially reflected 180° back along the fiber axis. The fiber region in which the acousto-optic interaction occurs will be referred to herein as the "interaction region" (or "interaction part").

Preferably, the substrate will be bonded to the optical fiber and will have acoustic impedance chosen to thereby match that of the fiber as closely as possible to reduce acoustic reflection loss that occurs when the acoustic waves propagate from the substrate into the fiber. In a preferred embodiment, the substrate will include two regions, each of which separates a different one of the transducers from the fiber. The two regions are preferably separated from each other by means of an acoustic absorbing element.

In a second embodiment, a single acoustic transducer is used to produce the two acoustic waves. As in the first embodiment, the two acoustic waves will propagate in the fiber in directions 45° relative to the longitudinal axis of the fiber, and 90° relative to each other. The first acoustic wave is part of the acoustic wave energy generated by the single transducer that propagates through the substrate directly into the fiber. The second acoustic wave is another part of the acoustic wave energy generated by the single transducer, which reflects from a surface of the substrate and is then refracted into the fiber. Because the second embodiment only requires a single transducer, it is simpler to manufacture than the first embodiment. Furthermore, the second embodiment allows a more favorable interaction region geometry which allows reduced acoustic loss in the fiber cladding for an interaction region of sufficient length along the longitudinal axis of the fiber, and which therefore results in greater diffraction efficiency. A disadvantage is that the two acoustic waves (i.e. the reflected and non-reflected parts of the acoustic wave energy generated by the one transducer) will have identical frequency, so that the second embodiment of the device according to the invention can only be used as a reflector, and not an optical frequency-f or pusher

According to the method according to the invention, the acoustic transducers (or the transducer) linked to an interaction part of a fiber are activated to introduce the described first and second acoustic waves into the interaction part at desired times. The transducer (or transducers) can be made inactive at selected times, so that light can propagate unimpeded through the interaction part. When it is desired, both the aforementioned first and second acoustic waves will be generated to thereby have essentially the same frequency, f, where f. = (2nV)/M\g ( cos 45° ) , where Xq is the free-space wavelength of light desired to be reflected as it propagates through the fiber, V is the velocity of the first acoustic wave in the fiber, n is the optical index of refraction for the fiber, and M is a positive odd number. In an alternative embodiment, the first acoustic wave will be generated so that it receives a frequency that is different from that of the second acoustic wave. As will be explained in detail below, the frequency of the reflected light signal will be shifted up or down by a frequency proportional to the difference between the frequencies of said first and second acoustic waves.

One or more interacting parts (or reflection points), each associated with an acousto-optical reflector in any of the embodiments described herein, may be established along a single optical fiber. To eliminate unwanted reflections, each reflector is maintained in a deactivated state except during selected time periods when it is activated. Figure 1 is a schematic drawing showing the mode of operation of a conventional Bragg cell, where an incoming light wave meets, at an angle of incidence, 0, an acoustic wave. Part of the light wave is deflected in a direction with an angle 20 relative to the incoming light wave. Figure 2 is a cross-sectional view of an embodiment of the reflector according to the invention, and an associated optical fiber, taken in a plane that includes the longitudinal fiber axis. Figure 3 is an enlarged view of the acousto-optic interaction region for the reflector of Figure 2, showing the acoustic wavefronts encountered by an incident light wave propagating through the interaction region along the core of the fiber. Figure 4A is a cross-sectional view of an optical fiber bonded to a surface, taken in a plane perpendicular to the longitudinal optical fiber axis. Figure 4B is a cross-sectional view of the assembly shown in Figure 4A, after the top surface of the assembly has been ground and polished. The ground and polished top surface of the device in Figure 4B is ready to be bonded to the substrate of the reflector of the invention. Figure 5 is a cross-sectional view of another embodiment of the reflector according to the invention, and an associated optical fiber, taken in a plane that includes the longitudinal fiber axis. Figure 6 is an enlarged view of the acousto-optic interaction region of the reflector in Figure 5, which

shows the acoustic wavefronts encountered by an incident light wave propagating through the interaction region along the axis of the fiber.

A cross-sectional view of a preferred embodiment of the reflector, according to the invention, is shown in figure 2.

The reflector includes first acoustic transducer 10, second acoustic transducer 11 and substrate 12. The substrate 12 is bonded to the cladding 17 of an optical fiber 16. The core 18 of the fiber 16 extends along the central longitudinal axis of the fiber 16. The substrate 12 comprises a first region 13 through which a first acoustic wave 19 produced by trans-

the diffuser 10 can propagate into the fiber 16, a second region 14 through which a second acoustic wave 20

produced by the transducer 11 can propagate into the fiber 16, and acoustic absorbing element 15 placed between said first region 13 and said second region 14.

The substrate 12 is simply the medium through which the acoustic waves are transported to the fiber, and preferably has low acoustic attenuation characteristics. Suitable low attenuation materials for said first and second regions of the substrate 12 include LiNbO 3 , YIG, Al 2 O 3 , TiO 2 , YAG or MgAl 2 O 4 .

Suitable materials for use in the substrate according to the invention include the anisotropic materials that have been used in conventional Bragg cells. Due to their anisotropy,

these materials will have a preferred orientation relative to the transducer and the fiber to minimize acoustic attenuation. Studies of materials for conventional Bragg cells have been concentrated on those with suitable optical properties^ In the reflector "according to the invention, however, the substrate need not have any special optical properties, and may be optically opaque. Therefore, it can

be other more suitable substrate materials, including isotropic materials. For the purpose of the description in connection with figure 2, the substrate shall be assumed to be lithium niobate (LiNbC^). The absorbent element 15 can be the material used to join the two substrate components, such as epoxy.

The transducers 10 and 11 can be conventional ultrasound transducers of the type used for Bragg cells. Such transducers can be made of LiNb03 or other piezoelectric material. In an embodiment where said first and second transducers are identical ultrasound transducers, the supply of an RF pulse with frequency f and duration t to the first transducer will emit in the substrate the ultrasound wave 19 with frequency f and duration t, and supply of a similar pulse to the secondly, the transducer will emit the ultrasound wave 20 with frequency f and duration t into the substrate. The ultrasound waves 19 and 20 will propagate through the substrate and into the fiber as shown in figure 2. The ultrasound waves intersect at right angles in the fiber, and the wave fronts in the fiber are both in a angle of 45° relative to the axis of the fiber. The portion of the fiber in which the acoustic waves intersect will be referred to throughout this application as the "interaction portion" (or "interaction region") of the fiber. In order for the waves 19 and 20 to move in a silicon fiber at an angle of 45° relative to the fiber's axis, the angle a between the fiber axis and the substrate surface to which the ultrasound transducer is mounted must be:

where Vlog Vser the sound speeds in lithium niobate and silicon respectively. This relationship follows from Snell's law. Because VL= 6.57 x IO<3>m/s and Vs= 5.96 x IO<3>m/s, a should be 51.2° in this embodiment of the invention. Figure 3 shows an enlarged view of the interaction region according to the embodiment shown in Figure 2. Figure 3 shows the geometry of the acousto-optic interaction which results J. the desired back reflection. The wave fronts for the acoustic waves 19 and 20 that propagate through the interaction region in the fiber should be essentially flat, so that light propagating through the interaction region will meet in all. essentially flat acoustic wave fronts, such as the wave front X of the wave train 19 and the wave front Y of the wave train 20. First, the light beam 21 with wavelength X r.i-- the fiber, which propagates along the fiber axis to the right in Figure 3, and meets the intersection the points R, S, etc. for the two acoustic wave trains. The refractive index gradient in the fiber at these points is in the direction that the light beam is moving and therefore a reflection at 180° (ie a back reflection) will occur. The reflection amplitude is at a maximum when L, the distance between R and S, is X/2. Higher order maxima exist at the distances corresponding to L = 3X/2, 5X/2, ..., MX/2 (where M is any positive odd number). Also, since L = A/Cos $, where A is the acoustic wavelength in the fiber, = 45°, and A = V/f, where V is the acoustic velocity in the fiber, it follows that f=2V/M X Cos 0, or

where n is the average optical refractive index of the fiber (ie the optical refractive index of the fiber in the absence of any acoustic wave propagating through the fiber), and Lg is the free-space wavelength of the light wave 21. If X0equal to 1.3 x IO-<6>M , n = 1.46 and V = 5.96 x10<3>m/s, f 0 (l/M) becomes 18.93 GHz. The ultrasonic deformation has the effect of a refraction grating on the light wave 21. The possible operating frequencies for the device according to the invention are f = 18.93 GHz, 6.31 GHz, 3.78 GHz, etc. in the described "example. Because of the difficulties" which is "nature Week" when operating at higher GHz frequencies (including transducer fabrication problems and the fact that attenuation

increases with the square of frequency), and because the intensity of the reflected light wave in higher refractive orders decreases very rapidly, a trade-off is involved in choosing the best operating frequency. An operating frequency equal to 6.31 GHz in the described example is a choice. This corresponds to a path length difference equal to 3X/2. There is another possible solution, and that is to start with a transducer at a low fundamental frequency and to operate it at a higher even-number harmonic. It is well known that Bragg cells can be operated in this way, but that the bandwidths over which they operate are correspondingly reduced at the higher harmonics. In the invention according to this application, wide bandwidth is not required. Thus, in the described example, one can operate at 18.93 GHz using transducers with a fundamental frequency equal to 2.7 GHz, but which are operated at the seventh harmonic or 18.93 GHz.

Referring again to figure 3, it will be seen that the path length difference for the light beam 22 after two reflections is the same as the path length difference for the light beam 21, and therefore the conditions for reflection maxima are the same for all light beams that propagate along the fiber axis into the interaction region. The acoustic field, which is due to the superposition in the interaction region of the acoustic waves introduced there, acts to a large extent as a Porro prism which has the property of retro-reflecting light in a plane.

In a conventional Bragg cell, it becomes deflected optically

the beam Doppler-shifted in frequency by an amount equal to the acoustic frequency. In a variant of a conventional Bragg cell where a 180° deflection is produced, with the same X0,

n and V used above, the resulting optical frequency shift would be 18.93 GHz.

With the device according to the invention, there is no optical frequency shift if the frequencies of the acoustic waves emitted from the two transducers are the same. This can be understood with reference to Figure 3. In Figure 3, the first acoustic wave train 19 propagates in the fiber cladding 17 and the fiber core 18 in a direction of the arrow 30 and the second acoustic wave train 20 propagates in the cladding 17 and the core 18 in the direction of the arrow 31. The wave trains 19 and 20 have identical frequencies. The "mirror" at the point R is formed by the intersection of the wave front X of the wave 19 and the wavefront Y of the wave 20 at that particular instant. At some time later, the point R will have moved downward; in the direction parallel to the light beam 21.

However, the device according to the invention produces an optical frequency shift in the reflected light beam if the frequencies of the two acoustic beams are not identical. In this embodiment of the invention, if the frequency of the first acoustic wave train 19 is higher, the "mirror" formed by the intersection of the two wave fronts will have a velocity component, Vp, in the direction away from the approaching light and therefore a downward shift in the reflected the light beam. The reflected light beam should be pushed up if the frequency of the second acoustic wave train 20 was higher. The.amount.of.the.optical.frequency.shift.would.be

where fi is the frequency in the fiber for the first acoustic wave train 19, fg is the frequency in the fiber for the second acoustic wave train 20, and all other symbols are as previously defined. The reflected light frequency produced in the reflector, according to the invention, can be controlled by varying the frequency difference, fi-fg» such as by

drive said first "and second transducers" at selected different frequencies. This ability gives rise to applications for

the embodiment in figure 2 for the reflector according to the invention, within the area of telecommunications.

Said first and second acoustic transducers in the embodiment in Figure 2 of the device, according to the invention, must be oriented and the substrate must be shaped so that acoustic waves emitted from said first and second acoustic transducers will each enter the fiber at the required angle to thereby produce in the fiber's interaction region an acoustic field of the type described above with reference to figure 3. Preferably, the substrate will comprise an acoustic absorbing element, such as epoxy, bonded between two regions of the substrate material which have low acoustic attenuation properties. Such an absorbing element would reduce unwanted reflections within the substrate and on the substrate-fiber boundary layer.

Figure 5 is a cross-sectional view of a second preferred embodiment of the reflector according to the invention. An acoustic transducer 101 emits acoustic wave energy into the substrate 10, so that the acoustic wave first propagates in the direction of the beams 108, 109 and 110. The part of the acoustic wave that first propagates through the substrate 100 in the region between the beams 109 and 110 is deflected directly into the cladding 105 of an optical fiber 104 to thereby propagate in the cladding 105 and core 106 of the fiber 104 in the direction of the rays 112. This unreflected wave energy will be referred to as the first acoustic wave as it propagates in the fiber. The part of the acoustic wave that first propagates through the substrate 100 in the region between the rays 108 and 109 is reflected from the surface 102 of the substrate 100 and is then deflected into the fiber 104 to thereby propagate in the fiber 104 in the direction of the rays 111. The reflected portion will be referred to as the second acoustic wave as it propagates in the fiber."The direction of the rays 111 should be 45° relative to the longitudinal fiber axis, and the direction of the rays 112 should be substantially 45° relative to the longitudinal fiber axis and perpendicular to the direction of the rays 111.

The transducer 101 and the substrate 100 can be of the same type, respectively, as the transducer 10 and the substrate region 13 in

the embodiment in figure 2, and can be bound to each other and to the fiber in the same way as in figure 2. Care must be taken that the substrate 100 orients itself correctly, and that the transducer 101 and the surface 102 are correctly positioned, relative to the fiber 104, so that the reflected and non-reflected parts of the acoustic wave energy arrive at the substrate-fiber boundary layer at the correct angle and thereby minimize acoustic losses in the substrate (which will generally depend on the orientation of the substrate relative to the direction of propagation of an acoustic wave therein) .

Figure 6 is an enlarged view of the interaction region for the embodiment of Figure 5. Acoustic wave energy propagates through substrate 100 in the direction of beams 108, 109 and 110. The portion of the acoustic wave energy that propagates in the region between beams 108 and 109 will be reflected back. from the surface 102 of the substrate 100. The surface 102 is part of the boundary layer between the substrate 102 and the surrounding medium 116. The surrounding medium will typically be

air. The reflected acoustic radiation will be deflected into the fiber cladding 117 to propagate therein as a second wave 102 in the direction of the rays 111 and 112. Part of the acoustic wave energy that propagates in the substrate

100 in the region between the rays 109 and 110 will be deflected into the fiber cladding 117 to thereby propagate therein as a first wave 121 in the direction of the rays 122 and 123. The light wave 130 propagating along the fiber will encounter the interaction part in which both the first wave 121 and the second wave 120 propagates.

An advantage of the embodiment in Figures 4 and 5 is that only a single transducer is needed so that the device is easier to manufacture. There are also other important advantages. The refractive efficiency of the reflector (ie the percentage of light deflected per unit of acoustically applied power) increases with increasing interaction region length. Moreover, the acoustic power loss in the fiber cladding 117 increases with increasing acoustic path length through the cladding 117. It can be seen in Figure 6 that as the fiber core moves upwards relative to the substrate (ie as the cladding thickness decreases), the length of the interaction region increases. In the embodiment of Figure 2, the opposite is true, since, in the embodiment of Figure 2, as the core is moved upward by increasing the cladding thickness, the length of the interaction region decreases. Therefore, the embodiment according to Figure 6 would allow greater diffraction efficiency as the cladding thickness is reduced.

Figures 4A and 4B show a method of providing a flat surface on the fiber to which the substrate (in any of its embodiments) can be bonded. As shown in Figure 4A, the optical fiber 40 (which includes cladding 44 and core 45) is bonded into a plate 41. The plate 41 may be made of fused silicon. The top surface of the assembly in Figure 4A is then ground and polished to produce the assembly shown in Figure 4B. A typical fiber has an outer diameter of approx. 125 pm, and a single-mode fiber operating at an optical wavelength of 1.3 pm will typically have a core diameter of approx. 10 p.m. Preferably, after grinding and polishing, a thin cladding layer remains between the core 45 and the top surface 46. For a fiber of typical dimensions, this thin cladding layer should have a thickness of approx. 30 pm in the embodiment in figure 2. In the embodiment in figure 6, for a fiber of typical dimensions, the optimal cladding layer thickness will be less than 30 pm, and will desirably be in the range of approx. 5-10 p.m. When the substrate is placed against the top surface 46 in the assembly of Figure 4B, if a cladding layer remains between the substrate and the core, there is no additional optical loss associated with the device. If the acoustic waves pass through a distance that is no more than approx. 30 pm in silicon cladding to reach the core, the associated attenuation loss in acoustic energy is less than 3 db, if the operating frequency is 6.3 GHz.

The substrate of the reflector according to the invention can be bonded to the ground and polished top surface 46 of the unit in Figure 4B using the same techniques used to bond the GHz transducers to the Bragg cells. The substrate, the bonding material, and the cladding 44 and the core 45 will preferably have closely matched acoustic impedance in order to thereby reduce acoustic reflection losses that occur when acoustic waves propagate from the substrate into the fiber 40.

More than one of the reflectors, according to the invention, can be placed along a single optical fiber. With such a configuration, it is desirable that optical loss at one reflector is prevented from affecting the reflected light signal produced at every other reflector linked to said single fiber. To achieve this desired result, the transducers used in the reflectors can be selected from those commercially available, which can be switched between an activated state and a deactivated state on command. In operation, a light wave (such as a laser pulse) will be emitted into the fiber and a selected reflector will be activated by switching "on" the associated transducer (or pair of transducers). All other reflectors located along the fiber between the light source and the selected reflector (ie, the "upstream" reflectors) should be switched "off" to minimize attenuation of the light wave as it passes these upstream reflectors. At later times, possibly after a subsequent light wave has been emitted into the fiber, any desired combination of active reflectors can be achieved by appropriately activating or deactivating the individual reflectors.

It should be understood for the purposes of the illustration that an optical frequency equal to 1300 nm has been used here, but that today's optical fibers also have low optical losses at 850 and 1550 nm. Extensive effort has been made to produce fibers that have far less loss at longer wavelengths. If these fibers are realised, the described acousto-optical reflector devices will be easier to manufacture due to the lower operating frequencies.

It should be understood that the various embodiments described here are only illustrative of the inventive concept and that these embodiments should not be considered limitations of the invention. Various changes to the methods and devices described here may lie within the scope of the appended patent claims without deviating from the idea of the invention.

Claims (1)

An optical reflector capable of being placed adjacent and coupled to an optical fiber having a longitudinal axis to reflect a portion of a light wave as the light wave propagates along the axis through an interacting portion of the fiber adjacent the reflector, characterized by (a) a substrate through which acoustic signals can propagate, and (b) means for generating a first acoustic wave which will propagate from the substrate into the cooperating member to thereby propagate in the fiber in the first direction with a .angle approximately 45° relative to the axis, and a second acoustic wave that will propagate from the substrate into the interaction part to thereby propagate in the fiber in a second direction with an angle approximately equal to 45° relative to the axis and approximately perpendicular to the first direction. 2. Reflector as stated in claim 1, characterized in that the substrate has acoustic impedance chosen to match that of the fiber to reduce acoustic reflection losses that occur when the first acoustic wave and the second acoustic wave propagate from the substrate into the fiber. 3. Reflector as stated in claim 1, characterized in that the acoustic wave generating means comprises a first transducer and a second transducer, each of which is bonded to the substrate and is capable of being selectively switched between an active state where the transducer generates acoustic wave energy, and an inactive state in which the transducer generates no acoustic wave energy. 4. Reflector as specified in claim 3, characterized by part of the first acoustic wave propagation through the interaction part for the fiber is a plane acoustic wave which has frequency fj_, and the part of the second acoustic wave that propagates through the interaction part of the fiber is a plane acoustic wave having frequency f2 , where f2 is different from f^ . 5. Reflector as specified in claim 4, characterized in that the first transducer has fundamental frequency f3 , where f^ is one harmonic of f3 , and the second transducer has fundamental frequency f4 , where f2 is one harmonic of f4. 6. Reflector as specified in claim 3, characterized in that the substrate comprises a first region to which the first transducer is bound and through which the first acoustic wave propagates, a second region to which the second transducer is bound and through which the second acoustic wave propagates, and an acoustic absorbing element placed between the first region and the other region. 7. Reflector as specified in claim 1, characterized in that the acoustic wave generating means comprises a transducer bonded to the substrate and capable of being selectively switched between an active state where the transducer generates acoustic wave energy and an inactive state where the transducer generates no acoustic wave energy, -::wherein the first acoustic wave is a first portion of the acoustic wave energy generated by the transducer that propagates into the interaction portion, and the substrate has a surface from which a second portion of the acoustic wave energy generated by the transducer is reflected and then propagates into in the interaction part, where the second acoustic wave is the second part of the acoustic wave energy. 8. Reflector as stated in claim 7, characterized by the part of the first acoustic wave that propagates gjénnonTthe interaction part in the fiber is a planar acoustic wave having frequency f^, and the part of the second acoustic wave that propagates through the interaction part in the fiber is a plane acoustic wave that also has frequency f^ , where f^ = (2nV)/M Xq (cosine 45' ), where n is the optical refractive index of the fiber, V is the speed of the first acoustic wave in the fiber, Xq is the free-space wavelength for light that is desired to be reflected by. the reflector as the light propagates through the interacting portion of the fiber, and M is an odd positive integer. 9. Reflector as specified in claim 8, characterized in that M = 3. 10. Reflector as specified in claim 8, characterized in that fi is a harmonic of the fundamental frequency for the transducer. 11. Reflector as stated in claim 1 or 3, grade i- certvedat the part of the first acoustic wave that propagates through the interaction part in the fiber is a plane acoustic wave which has frequency fj_, and the part of the second acoustic wave that propagates through the interaction part in the fiber is a plane acoustic wave which also has the frequency f ^ , and where f^ = (2nV)/M \0 (cosine 45°), where n is the optical refractive index of the fiber, V is the speed of the first acoustic the wave in the fiber, Xq is the free-space wavelength of light desired to be reflected by the reflector as the light propagates through the interacting portion of the fiber, and M is an odd positive integer. 12. Reflector as specified in claim 11, characterized in that M = 3. 13. Reflector as specified in claim 11, characterized in that fi is a harmonic of the fundamental frequency for the first transducer, and f^ is also a harmonic of the fundamental frequency for the second transducer. 14. An optical reflector capable of being placed adjacent and bonded to an optical fiber having a longitudinal axis to reflect a portion of a light beam as the light beam propagates along the axis through a cooperating portion of the fiber adjacent the reflector, characterized by (a) a substrate through which acoustic signals can propagate, (b) a first transducer that is bonded to the substrate and is capable of generating a first acoustic wave that will propagate through the substrate into the interface, such that the first acoustic wave propagates in the fiber in a first direction at an angle of approximately equal to 45° relative to the axis, and (c) a second transducer bonded to the substrate and capable of to generate a second acoustic wave which will propagate through the substrate into the interaction part, so that it the second acoustic wave propagates in the fiber in a second direction at an angle approximately equal to 45° relative to the axis and approximately perpendicular to the first direction. • •. • ::.:;" v '. •- ••' 15. Reflector as stated in claim 14, characterized in that the first transducer and the second transducer can each be selectively switched between an active state in which the transducer generates an acoustic wave, and an inactive state in which the transducer does not generate any acoustic wave. 16. Reflector as specified in claim 14, characterized in that the part of the first acoustic wave that propagates; —through the interaction part at: the fiber is a planar acoustic wave having frequency f]_, and the part of the second acoustic wave that propagates through the interacting part of the fiber is a plane acoustic wave having frequency f2 , where f2 is different from f i . 17. Reflector as stated in claim 16, characterized in that the first transducer has fundamental frequency f3, where f-L is a harmonic of f3 , and the second transducer has a fundamental frequency f4 , where f2 is a harmonic of f4 . 18. Reflector as specified in claim 14, characterized in that the part of the first acoustic wave that propagates through the interaction part of the fiber is a plane acoustic wave having frequency f^ , and the part of the second acoustic wave that propagates through the interaction part in the fiber is a plane acoustic wave which also has the frequency f^, and where f^ = (2nV)/M Xq (cosine 45°), where n is the optical refractive index of the fiber, V is the speed of the first acoustic wave in the fiber, Xq is the free-space wavelength of light desired to be reflected by the reflector as the light propagates through the interacting portion of the fiber, and M is an odd positive integer. 19. Reflector as stated in claim 18, characterized in that M = 3. 20. Reflector as stated in claim 18, characterized in that fi is a harmonic of the fundamental frequency for the first transducer, and f^ is also a harmonic of the fundamental frequency for the second transducer. 21. Reflector as stated in claim 14, characterized in that the substrate comprises: a first region through which the first acoustic wave propagate, a second region through which the second acoustic wave propagates, and an acoustic absorbing element placed between the first region and the other region. 22. Reflector as stated in claim 14, characterized in that the substrate has an acoustic impedance chosen to thereby match that of the fiber in order to reduce acoustic reflection losses that occur when the first acoustic wave and the second acoustic wave propagate from the substrate into the fiber. 23. - Optical reflector, characterized by (a) an optical fiber having optical refractive index, n, and having a longitudinal axis, (b) . a .substrate bound. to the fiber, 'r - : (c) a first acoustic transducer bonded to the substrate and capable of generating a first planar acoustic wave that propagates through the substrate into the fiber to thereby propagate in the fiber in a first direction approximately 45° relative to the axis, wherein said first acoustic wave has frequency f^ in the fiber, and (d) a second acoustic transducer bonded to the substrate and capable of generating a second acoustic wave that propagates through the substrate into the fiber to thereby propagate in the fiber in a second direction approximately 45" relative to the axis and approximately perpendicular to r .: in the first direction, where said second acoustic wave has frequency f2 in the fiber. 24. Reflector as stated in claim 23, characterized in that the first acoustic transducer and the second acoustic transducer can each be selectively switched between an active state where the transducer generates an acoustic wave, and an inactive state where the transducer generates no acoustic wave. 25 . Reflector as specified in claim 23, characterized in that the frequencies f^ and f2 are different from each other. 26. Reflector as stated in claim 23, characterized in that the frequencies f^ and f2 are essentially the same, and fj_ = (2nV)M Xq (cos 45°), where Xq is the free-space wavelength for light that is desired to be reflected by the reflector as it propagates through the fiber, V is the velocity of the first acoustic wave in the fiber, and M is an odd positive integer. 27. Reflector as stated in claim 26, characterized by M=3.. 28. Reflector as stated in claim 23, characterized in that the substrate comprises: a first region through which the first acoustic wave propagates, and a second region through which the second acoustic wave propagates. 29. Reflector as stated in claim 28, characterized in that the substrate also includes an acoustic absorbing element placed between the first region and the second region. 30. Reflector as specified in claim 23, characterized in that the substrate has an acoustic impedance chosen to thereby reduce acoustic reflection loss that occurs when said first and second acoustic wave propagates through the boundary layer between the substrate and the fiber. 31. Reflector as stated in claim 23, characterized in that the fiber comprises a central core surrounded by cladding, and the part of the cladding into which said first and second acoustic wave propagates, has a thickness of approx. 30 p.m. 32. An optical reflector capable of being placed adjacent to and bonded to an optical fiber having a longitudinal axis to reflect a portion of a light beam as the light beam propagates along the axis through a cooperating portion of the fiber adjacent the reflector, characterized by : (a) a substrate through which acoustic signals can propagate, said substrate having a first surface, and (b) a transducer bonded to the substrate and capable of generating acoustic wave energy, a first portion of which will propagate through the substrate into the interaction portion as a first acoustic wave propagating in the interaction portion of the fiber in a first direction with an angle of approximately 45° relative to the axis, and a second portion of which acoustic wave energy will be reflected from the first surface and then propagate through the substrate into the interaction portion as a second acoustic wave propagating in the interaction portion of the fiber in a second direction at an angle of approximately 45° relative to the axis and approximately perpendicular to the first direction. 33. Reflector as stated in claim 32, characterized in that the transducer can be selectively switched between an active state where the transducer generates acoustic wave energy, and an inactive state where the transducer does not generate any acoustic wave energy. 34. Reflector as stated in claim 32, characterized in that the part of the first acoustic wave that propagates through the interaction part of the fiber is a plane acoustic wave having frequency f^ , and the part of the second acoustic wave that propagates through the interaction part of the fiber is a plane acoustic wave which also has frequency f-j_, where fi = (2nV)/M Xq (cosine 45°), where n is the optical refractive index of the fiber, V is the speed of the first acoustic wave in the fiber, Xq is the free-space wavelength of light desired to be reflected by the reflector as the light propagates through the interacting portion of the fiber, and M is an odd positive integer. 35. Reflector as specified in claim 34, characterized in that M = 3. 36. Reflector as specified in claim 34, characterized in that f i is a harmonic of the transducer's fundamental frequency. 37. Reflector as stated in claim 32, characterized in that the substrate has an acoustic impedance selected so that it matches that of the fiber in order to reduce acoustic reflection losses that occur when acoustic wave energy from the transducer propagates from the substrate into the fiber. 38. Optical reflector, characterized by (a) an optical fiber having an optical refractive index, n, and having a longitudinal axis, (b) a substrate bonded to the fiber and having a first surface, and. (c) an acoustic transducer bonded to the substrate and capable of generating plane acoustic wave energy, a first portion of which will propagate through the substrate into the fiber to thereby propagate in the fiber as a first acoustic wave in a first direction approximately 45° relative to the axis, and a second part of which acoustic wave energy will/will be reflected from the first surface and then propagate through the substrate into the fiber to thereby propagate in the fiber as a second acoustic wave in a second direction approximately 45° relative to the axis and approximately perpendicular to the first direction. 39. Reflector as stated in claim 38, characterized in that the acoustic transducer can be selectively switched between an active state where the transducer generates acoustic wave energy, and an inactive state where the transducer generates no acoustic wave energy. 40. Reflector .as set forth in claim 38, characterized in that the acoustic wave frequency is approximately equal to (2nV)/M Xg (cos 45°), where Xq is the free-space wavelength of light desired to be reflected by the reflector as it propagates through the fiber, V is the velocity of the first acoustic wave in the fiber, and M is an odd positive integer. 41. Reflector as specified in claim 40, characterized in that M = 3. Method of reflecting part of the energy in a light wave as the light wave propagates through an optical fiber having a longitudinal axis, characterized by the steps: to place adjacent the fiber a substrate through which acoustic waves can propagate, to cause a first acoustic wave to propagate through the substrate into the fiber so that the first the acoustic wave propagates in the fiber in a first direction approximately 45° relative to the axis, to cause a second acoustic wave to propagate see through the substrate into the fiber, such that the second acoustic wave propagates in the fiber in a second direction approximately 45° relative to the axis and approximately perpendicular to the first direction, and to emit the light wave into the fiber so that the light wave propagates itself along the axis and interacts with said first and second acoustic waves, so that part of the light wave is reflected back along the axis in a direction 180° opposite to the first light wave propagation direction. 43. Method as stated in claim 42, characterized in that the substrate has an acoustic impedance selected so that it matches that of the fiber in order to reduce acoustic reflection loss that occurs when the first acoustic wave and the second acoustic wave propagate from the substrate into the fiber. 44. Method as stated in claim 43, characterized in that the fiber includes a central core surrounded by cladding, and also includes the steps: to grind away part of the fiber cladding to produce one flat surface separated from the core by a thin region of cladding material, and to bond the substrate to the flat surface. 45.. Procedure as stated in claim 42, grade i-sertvedat the first acoustic wave and the second acoustic wave have essentially the same frequency, f, in the fiber, where f =- (2nV)/M Xq (cosine 45"), where n is the optical refractive index of the fiber, V is the speed of the first acoustic wave in the fiber, Xq is the free-space wavelength of the light wave, and M is a positive integer, odd number. 46. Method as stated in claim 42, character i^ sees, for example, that the frequency in the fiber for the first acoustic wave deviates from the frequency in the fiber for the second acoustic wave.