FR2587810A1 - OPTICAL REFLECTOR AND METHOD FOR REFLECTING A PART OF THE ENERGY OF A LIGHT WAVE PROPAGATING IN AN OPTICAL FIBER - Google Patents

Abstract

THE INVENTION CONCERNS A METHOD AND AN APPARATUS USING THE ACOUSTO-OPTICAL INTERACTION PHENOMENON TO REFLECT A PART OF A LIGHT WAVES PROPAGATING ALONG AN OPTICAL FIBER. THE REFLECTOR INCLUDES TRANSDUCERS 10, 11 INTENDED TO PRODUCE TWO ACOUSTIC WAVES, AND A SUBSTRATE 12 THROUGH WHICH THE ACOUSTIC WAVES CAN PROPAGATE AND ENTER A FIBER 16 TO WHICH SUBSTRATE 12 IS BOUND. THE FIRST ACOUSTIC WAVE SPREADS IN AN INTERACTION TRUNCON OF THE FIBER FORMING AN ANGLE OF 45 WITH THE AXIS OF THE FIBER AND THE SECOND ACOUSTIC WAVE SPREADS IN THIS SAME TRUNCON ALSO FORMING A 45 ANGLE WITH THE AXIS FIBER, BUT IN A DIRECTION PERPENDICULAR TO THAT OF THE FIRST ACOUSTIC WAVE. FIELD OF APPLICATION: TRANSMISSION OF INFORMATION BY OPTICAL FIBERS.

Description

The invention relates to a method and an apparatus

  to reflect a part of a light wave propagates

  along an optical fiber. More particularly, the invention relates to a method and apparatus in which two acoustic waves are caused to propagate in the fiber at selected angles so that a portion of a light wave propagating in a first direction along the fiber interact with the fiber. the two acoustic waves and that, under the effect of this interaction, it is reflected by reflection in the opposite direction to the first sense. One or more reflectors according to the invention may be bonded on an optical fiber to produce reflected light signals following an order by the activation of selected ones of the reflectors so that the acoustic waves produced by the

  activated reflectors propagate in the fiber.

  In various data transmission applications and fiber sensors in which an optical fiber is used, it is desirable that a device can be placed along the optical fiber to produce a reflection when activated by an order, and that this device has virtually no loss when not activated. In a large number of devices with these properties and placed along

  of fiber, one can use techniques of reflection

  time domain tometry to time division multiplexed data from a large

number of sensors.

  Conventional fiber couplers have been used to produce a reflected light signal in a fiber by placing a mirror at the unused output of the coupler. However, the excessive loss occurring

  in such a coupler and the resulting additional losses

  both of the attachment of the coupler to the fiber are too important to allow the realization of a large number of such points of reflection on a single fiber. In addition, these losses are permanent because the couplers can not be disabled to eliminate losses at

desired moments.

  Another method for producing reflection in an optical fiber is described in U.S. Patent Application No. 596,889, filed Apr. 1984. This method utilizes evanescent coupling between two fiber segments separated by a cross-sectional area. fiber forming a loop so that a portion of the light propagating along the fiber is transmitted from a

  segment to the other segment so as to come back into

  paging along the fiber in the opposite direction. In this process, the fiber may not be broken. even though

  excess losses in this arrangement are

  Although they are smaller than those due to the presence of a coupler, they are still too large to allow the presence of hundreds of these points of reflection on a single fiber. In addition, the reflectors of the above-mentioned application N 596 889 are permanent in the sense that the excess loss results in each time a crossing of the reflectors by the light and that these losses affect the operation of all the reflectors formed downstream. of any particular reflector on a

only fiber.

  Other methods known to produce a

  reflection in an optical fiber include: the introduc-

  tion of a discontinuity in the fiber, for example by breaking the fiber and reconnecting the broken ends with a low quality connector; the mechanical introduction of a microscopic conicity into the fiber; and exposing a portion of the fiber to spatially periodic disturbances of the optical refractive index of the cladding surrounding the core of the fiber. The latter method is described in British Patent Application No. 2,145,237, published March 20, 1985, page 5, line, page 6, line 35. These known methods for forming a reflector on a fiber all have disadvantage of leading to a permanent reflector. Optical loss occurs to each permanent reflector as light passes therethrough, and these losses affect all the sensors associated with all the reflectors downstream of a particular reflector and on a fiber. As a result of the losses, we can not set up a large

  many of these permanent reflectors on a single fiber.

  On the other hand, the apparatus according to the invention produces a reflection in an optical fiber when it is activated by an order, with an excess loss practically zero when it is in a deactivated state. The operating principle of the invention is based on the acousto-optical interaction of light and sound waves. Given

  the sheathing material does not have to be removed close

  In the core of the fiber, excess losses can be rendered extremely low. As in the reflection device of the invention, the device well known as the acoustooptic Bragg cell name uses acousto-optical interactions. Figure 1 of the accompanying drawings described below is a schematic diagram showing the characteristics of a cell

  from Bragg. In FIG. 1, a light beam 1 is

  paging in an optical medium 3 such as glass or quartz, meets a train of acoustic waves forming

  an angle O with the plane wave fronts of 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: o (1) Sin =% 2A o> is the wavelength of the light in the media 3 and 4 is the wavelength of the acoustic wave train, the light wave 4 (which comprises a portion of the energy of the light beam 1) is then diffracted by an angle Q with respect to the light ray 1. Acoustic waves

  in a Bragg cell are generated by a trans-

  The driver (such as the transducer 5 of FIG. 1) is acoustically coupled to the optical medium 3. Laser modulators and ray deflectors have been made using Bragg cells. However, in a conventional use, the light is removed by being deviated by a very small angle from its

  O10 direction of initial propagation.

  Retrograde reflection or desired feedback

  in a fiber corresponds to O - 90 'in equation (1).

  If A-> / 2, we obtain a retrograde reflection (for O = 90). The amplitude of the optical wave reflected back is maximum when the periodic variations of the optical refraction index induced by the acoustic energy are spaced half an optical wavelength. In general, maxima exist when A = M H / 2, where M is any non-negative odd integer (i.e. H - 1, 3, 5 ...). The principal maximum appears for A = X / 2 or M - 1. Since V - Af, where V is the speed of the acoustic wave and f is the acoustic frequency, and since X = o / n, o 0 is the wavelength of light in free space, and n is the refractive index of the medium in which the acousto-optical interaction takes place, then we have: (2) F 2nV Xo. 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 raised or lowered by a value equal to f depending on whether the acoustic wave is moving towards or away from the light wave. In most optical fibers, the Ume is in fused silica, for which n = 1.46 and V = 5.96x 103 meters / second. For light having a wavelength, in the free space,> o = 1.3 x 10- 6 meters, the acoustic frequency required to induce the lowest order diffraction in the direction of return is f = 13.38 GHz. The difficulty of designing a device that produces retrograde or return reflection on the basis of an acousto-optic interaction results from the high frequency required in the direct collinear Bragg regime described above. It is very difficult to manufacture and bond acoustic transducers that operate at these frequencies and most materials exhibit extreme losses at this high frequency. The first problem to be faced in such a device, in which light propagates in an optical fiber, is to cause the sound wave to propagate from the transducer through a substrate to the fiber core area with a loss

as low as possible.

  A disagreement in the acoustic impedance, pV, where P = the density and V = the acoustic velocity, between the substrate and the fiber is also at the origin of a loss of acoustic reflection at the interface. One way to prevent loss of acoustic reflection is to

  forming both the fiber and the fused silica substrate.

  However, the loss of ultrasound in fused silica is described as 12 dB / cm-GHz2 in an article

  by D. A. Pinnow, "Guidelines for the Selection of Acousto.-

  Optical Materials, "IEEE Journal of Quantum Electronics, Volume QE-6, No. 4, April 1970. This loss of 2028d / cm at a frequency of 13 GHz This very large loss makes it very difficult to design a configuration that can inject

  an ultrasonic wave colinear to the core of the fiber. Linen-

  jection at very small angles or focusing devices all require long journeys in

  substrates. Crystals causing

  Although these crystals have a significantly lower acoustic impedance than fused silica optical fibers, they exhibit the required acoustic propagation properties only along certain axes. As before, the injection of the acoustic waves into the fiber core at small angles causes very large losses due to the impedance mismatch, and the

  focusing is not possible due to anisotropy.

  The apparatus according to the invention is a device that can be positioned in the immediate vicinity of an optical fiber and bonded to this fiber to reflect a portion of a light wave propagating along the longitudinal axis of the fiber. The device includes means for generating two acoustic waves that propagate in the fiber at selected angles, such that each propagates in an interaction portion of the fiber in a 45-axis direction relative to the axis.

  and perpendicular to the direction of the other.

  In one embodiment, the apparatus according to the invention comprises two acoustic transducers connected to a substrate through which acoustic signals produced by the transducers can propagate. The transducers are oriented so that when the substrate is placed in close proximity to and bound to an optical fiber, a first acoustic wave emanating from one of the transducers penetrates the fiber and propagates therein in a first direction at 45 with the longitudinal axis of the fiber, and that a second acoustic wave, emanating from the other transducer, enters the fiber and propagates in a second direction to 45

  with the axis and perpendicular to the first direction.

  Light traveling along the axis of the fiber interacts with the acoustic field resulting from the superposition of the first and second acoustic waves so as to be partially reflected back along the axis of the fiber. The area of the fiber in which the acousto-optic interaction occurs

  hereinafter called "interaction zone" (or "section -

  The substrate is preferably bonded to the optical fiber and has an acoustic impedance selected to match as closely as possible to that of the fiber to reduce acoustic reflection loss occurring as the acoustic waves propagate. In a preferred embodiment, the substrate has two zones each separating the different transducers of the fiber, the two zones being advantageously separated.

  from each other by an acoustically 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 propagate in the fiber in directions at 45 relative to the longitudinal axis of the fiber and at 900 relative to one another. The first acoustic wave is a part of the wave acoustic energy generated by the single transducer, which propagates through the substrate directly into the fiber. The second acoustic wave is another part of the wave acoustic energy produced by the single tranducer, which is

  reflected on a surface of the substrate and then enters.

  by refraction in the fiber. Since the second embodiment only requires a transducer, it is simpler to manufacture than the first embodiment. In addition, the second embodiment

  allows a more favorable geometry of the area of inter-

  action, which also reduces the acoustic loss in the fiber sheath for an interaction zone of sufficient length along the longitudinal axis of the fiber, and therefore results in a higher diffraction efficiency. A disadvantage is that the two acoustic waves (i.e., the reflected and unreflected portions of the wave acoustic energy produced by the single transducer) have the same frequency, so that the second embodiment of the the invention can only be used as a reflector,

  and not as an optical frequency shift device.

  According to the method of the invention, the acoustic transducers (or the acoustic transducer) associated with an interaction section of a fiber are activated to introduce the first and second acoustic waves described in the interaction section, at desired times. . The transducer (s) can be turned off at selected times, so that light can

  propagate freely through the interaction section.

  When desired, the first and second acoustic waves are both produced to have substantially the same frequency, f, of = (2nV) / MA Ccos 45) where X0 is the wavelength, in space, of the light that it is desired to reflect during its propagation in the fiber, V is the speed of the first acoustic wave in the fiber, n is the optical refractive index of the fiber, and M is an interpositive integer. In another embodiment, the first acoustic wave is generated to have a frequency different from that of the second acoustic wave. As described in detail below, the frequency of the reflected light signal is shifted up or down by a frequency proportional to the difference between the

  frequencies of the first and second acoustic waves.

  One or more interaction sections (or points of reflection), 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 light reflections, each reflector is kept in a disabled state, except during periods of

  chosen times during which it is activated.

  The invention will be described in more detail with reference to the appended drawings as non-limiting examples and in which: FIG. 1 is a schematic diagram showing the operating mode of a conventional Bragg cell in which a light wave of input meets, at an angle of incidence 9, an acoustic wave,

  part of the light wave being reflected by diffraction

  in a direction at an angle to the input light wave; FIG. 2 is a cross-section of an embodiment of the reflector according to the invention and of an associated optical fiber, in a plane containing the longitudinal axis of the fiber; FIG. 3 is an enlarged view of the acoustooptic interaction zone of the reflector of FIG. 2, showing the acoustic wave fronts encountered by an incident light wave propagating through the interaction zone along the soul of the fiber; FIG. 4A is a cross section of a

  optical fiber bonded to a plate, in a plane perpendicular to

  eyepiece to the longitudinal axis of the optical fiber; FIG. 4B is a cross-section of the assembly shown in FIG. 4A, after the upper surface thereof has been ground and polished, the ground and polished upper surface of the assembly of FIG. 4 being ready to be bonded to the reflector substrate of the invention; FIG. 5 is a cross-section of another embodiment of the reflector of the invention and an associated optical fiber, in a plane which contains the longitudinal axis of the fiber; and FIG. 6 is an enlarged view of the acousto-optical interaction zone of the reflector of FIG. 5, showing the acoustic wave fronts encountered by an incident light wave propagating through the interaction zone, along the axis of the fiber. Figure 2 shows in cross section a first preferred embodiment of the reflector according to the invention. The reflector includes a first

  acoustic transducer 10, a second acoustic transducer

  It is a substrate 12. The latter is bonded to a sheath 17 of an optical fiber 16. The core 18 of the fiber 16 extends

  along the central longitudinal axis of this fiber 16.

  The substrate 12 includes a first portion 13 through which a first acoustic wave 19 produced by the transducer 10 can propagate and penetrate the fiber 16, a second portion 14 through which a second acoustic wave 20 produced by the transducer II can occur. propagate and penetrate the fiber 16, and an acoustically absorbing member disposed between the first

Part 13 and the second part 14.

  Substrate 12 is simply the medium through which acoustic waves are transported to the

  fiber and is advantageously characterized by a

  low acoustic noise. Low attenuation materials suitable for the first and second parts of the substrate

  12 include Li NbO 3, YIG, Al 2 O 3, Ti O 2 YAG, or MgAl 2 O 4.

  Materials suitable for use in the substrate of the invention include the anisotropic materials that were used in conventional Bragg cells. Because of their anisotropy, these materials have a preferred orientation with respect to the transducer and the fiber to minimize acoustic losses. Studies of materials for conventional Bragg cells have resulted in convergence on materials having suitable optical properties. However, in the reflector of the invention, it is not necessary that the substrate has any particular optical properties and it can be optically opaque. Therefore, other materials suitable for the substrate, including isotropic materials, can be used. For the purposes of

  the description of Figure 2, it is assumed that the substrate

  is in lithium niobate (Li NbO 3). The absorbent element may consist of the material used to join the two parts of the substrate, for example an epoxy resin. Transducers 10 and may be conventional ultrasonic transducers of the type used for Bragg cells. Such transducers can

  be made of Li Nb 03 or other piezoelectric material.

  In an embodiment wherein the first and second transducers are identical ultrasonic transducers, applying a radio frequency pulse, of frequency f and duration t, to the first transducer causes transmission in the substrate of a wave

  ultrasound 19 of frequency f and duration t, and the application

  cation of a pulse similar to the second transducer

  causes the emission, in the substrate, of an ultrasonic wave

  sound 20 of frequency f and duration t. Ultrasonic waves

  Sounds 19 and 20 propagate through the substrate and penetrate the fiber as shown in Figure 2. The ultrasonic waves intersect at right angles in the fiber, and the two wavefronts in the fiber each form an angle of 45. with the axis of the fiber. The section of the fiber in which the acoustic waves intersect is referred to herein as the "interaction section" (or "interaction one") of the fiber, for the waves 19 and 20 to traverse a fiber of the fiber. at an angle of 45 to the axis of the fiber, the angle c "between the axis of the fiber and the surface of the substrate on which the ultrasonic transducer is mounted must be: (3) X-Sin -I (V Sin 451 / V) L so VL and V are the velocities of the lithium niobiate and the silica, respectively. This relationship stems from Snell's law. Since VL = 6.57 x 103 m / s and V = 5.96 x 103 m / s, t should be 51.2 in s

  this embodiment of the invention.

  Fig. 3 is an enlarged view of the interaction zone of the embodiment shown in Fig. 2. Fig. 3 shows the geometry of the acousto-optical interaction resulting in the desired retrograde or return reflection. The acoustic wave fronts 19 and 20, which propagate through the interaction zone in the fiber, must be substantially planar so that light propagating through the interaction zone encounters substantially flat acoustic wave fronts. , such as the wavefront X of the wave train 19 and the wavefront Y of the wave train 20. First, the light ray 21, of wavelength in the fiber, is considered. propagates along the axis of the fiber to the right in Figure 3 and which meets the points of intersection _____R, S, etc., of the two acoustic wave trains. The gradient of the refractive index in the fiber at these points is in the direction in which the light wave is

  displaces and so a reflection occurs at 180 (that is,

  to say retrograde or return thinking). The amplitude of the reflection reaches a maximum when L, the distance between R and S, is equal to 2/2. Higher order maxima exist at intervals corresponding to L = 3% / 2, 5X / 2, ... MI / 2 (where M is any positive odd integer). Moreover, since L - A / Cos 0, where o \ is the acoustic wavelength in the fiber, 0 = 45, and A = V / f, is the acoustic velocity in the fiber, it follows that = 2V / M 9Cos0, where (4) f 2nv is the average optical refractive index of the fiber (that is, the optical refractive index of the fiber in the absence of any acoustic wave propagating in the fiber) and. is the wavelength in the space of the light wave 21. If% o: 1.3 x 10 M, n = 1.46

  and V = 5.96 x 103 m / s, then f = (1 / M) 18.93 GHz.

  The ultrasonic deformation produces the effect of a diffraction grating on the light wave 21. The possible operating frequencies of the device of the invention are f = 18.93 GHz, 6.31 GHz, 3.78 GHz and so on, in the example described. Due to the difficulties of working at higher frequencies (including transducer manufacturing problems and the fact that the attenuation increases with the square of the frequency),

  and since the intensity of the reflected light wave

  In higher diffraction orders, the drop falls very rapidly, the choice of the best working frequency is the subject of a compromise. A working frequency of 6.31 GHz, in the example described, is a possibility. This corresponds to a difference in path length of 3 X / 2. Another possibility is to start from a transducer having a lower fundamental frequency and make it work at a higher even harmonic frequency. It is well known that Bragg cells can function in this way, but that the bandwidths on which they work are correspondingly reduced to higher harmonics. A wide bandwidth is not essential to the present invention. Thus, in the example described, it is possible to work at 18.93 GHz using transducers having a fundamental frequency of 2.7 GHz, but excited at the seventh harmonic, ie

18.93 GHz.

  Referring again to FIG. 3, it can be seen that the difference in path length for a light beam 22, after two reflections, is the same as for a light beam 21 and that, therefore, the conditions for maximum reflection are the same for all the light rays propagating along

  the axis of the fiber and penetrating the interaction zone.

  The acoustic field, resulting from the superposition of the acoustic waves injected in the interaction zone, acts in a very similar way to that of a Porro prism that possesses the retro-refl-ion property.

of light in a plane.

  In a conventional Bragg cell, the frequency of the diffracted optical beam is shifted

  Doppler with a value equal to the acoustic frequency.

  In a variant of a conventional Bragg cell in which a diffraction of 180 is produced, with the same values as those used above for o, n and V, the resulting optical frequency offset is

18.93 GHz.

  In the device of the invention, it does not

  produces no optical frequency shift if the frequencies

  of the optical waves emitted by the two trans-

  ductors are the same. As can be understood by referring to FIG. 3. In FIG. 3, a first acoustic wave train 19 propagates in the sheath 17 of the fiber and in the core 18 of the fiber, in the direction of the boom 30, and a second acoustic wave train 20 propagates in the sheath 17 and in the core 18, in the direction of the arrow 31. The wave trains 19 and 20 have identical frequencies. The "mirror" at a point R is formed by the intersection of the X front of the wave 19 and the Y edge of the wave 20 at a particular instant. Some time later, the point R is moved downward in the direction of arrow 35 (i.e., perpendicular to the longitudinal axis of the fiber). The "mirror" therefore has no velocity component in the parallel direction

at the light radius 21.

  The device according to the invention, however, 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 V in the direction s away from the approaching light and thus a downward shift of the reflected light beam will occur. The light beam will reflect shifted upward if the frequency of the second acoustic wave train 20 is higher. The amplitude of the optical frequency shift would be: (5) A = 2n V (fl-f2) Sine Ao fof is the frequency in the fiber of the first acoustic wave train 19, f2 is the frequency in the fiber of the second acoustic train acoustic waves 20, and all other symbols are as defined above. The frequency of the reflected light produced in the reflector of the invention can be adjusted by varying the frequency difference, f1-f2, for example by exciting the first and second transducers at unequal and selected frequencies. This possibility gives rise to applications, cations, for the embodiment of FIG.

  reflector of the invention, in the field of telecommunications

  munications. The first and second acoustic transducers of the embodiment of FIG. 2 of the device of the invention must be oriented and the substrate must be configured in such a way that the acoustic waves emitted by the first and second acoustic transducers each enter the fiber under the angle needed to produce an acoustic field in the interaction zone of the fiber

  of the type described above with reference to FIG.

  The substrate advantageously comprises an acoustic element

  Absorbent material, such as an epoxy resin, bonded between two parts formed by the substrate material and having low sound attenuation properties. Such an absorbent element reduces undesired reflections indoors

  of the substrate and at the substrate-fiber interface.

  Figure 5 is a cross-section of a second preferred embodiment of the reflector of the invention. An acoustic transducer 101 emits wave acoustic energy into a substrate 100 so that the acoustic wave propagates initially in the ray direction 108, 109 and 110. The portion of the acoustic wave propagating initially through the substrate in the zone between the spokes 109 and 110 is refracted directly in the sheath 105 of an optical fiber 104 in order to propagate in the sheath 105 and in the core 106 of the fiber 104, in the direction of the spokes 112. This Unreflected wave energy propagating in the fiber will be called the first acoustic wave. The portion of the acoustic wave propagating initially through the substrate 100, in the area between the spokes 108 and 109, is reflected on the surface 102 of the substrate 100 and then penetrates by refraction into the fiber 104 so as to propagate in the direction of radii 111. This reflected part propagating in the fiber will be called hereinafter the second acoustic wave. The management of

  111 must form an angle of 45 with the longitudinal axis

  tudinal of the fiber and the direction of the rays 112 should be about 45 relative to the longitudinal axis of the fiber and should be perpendicular to the direction of the

111 rays.

  The transducer 101 and the substrate 100 may be of the same type, respectively, as the transducer 10 and the substrate portion 13 of the embodiment of FIG. 2, and they may be connected to each other and to the same as that shown in FIG. Z. Care must be taken to properly orient the substrate 100 and to properly position the transducer 101 and the surface 102 relative to the fiber 104

  so that the thoughtful and unthinking parts of the energy

  acoustic waves arrive at the substrate-

  fiber in the appropriate angle and to minimize acoustic losses in the substrate (which, in general, depends on the orientation of the substrate with respect to the direction

  propagation of an acoustic wave in this substrate).

  FIG. 6 is an enlarged view of the interaction zone of the embodiment of FIG. 5. The wave acoustic energy propagates through the substrate 100 in the direction of the rays 108, 109 and 110. acoustic wave energy propagating in the area between the spokes 108

  and 109 is reflected by the surface 102 of the substrate 100.

  The surface 102 is part of the interface between the substrate 100 and the surrounding medium 116. The latter is generally air. The reflected acoustic radiation refracts into sheath 117 of the fiber so as to propagate in the form of a second wave 120, in the direction of the rays 111 and 112. Part of the wave acoustic energy propagating in the substrate 100, in the zone between the spokes 109 and 110, refractly penetrates into the sheath 117 of the fiber so as to propagate therein as the first wave 121 in the direction of the radii 122 and 123. A wave light 130 propagating along the fiber meets the interaction part in which are propagated to the

  times the first wave 121 and the second wave 120.

  An advantage of the embodiment of Figures 5 and 6 is that only one transducer is needed

  so that the device is simpler to manufacture.

  This device also has other important advantages. The diffraction efficiency of the reflector (i.e., the percentage of diffracted light per unit of acoustic power applied) increases with

  increasing the length of the interaction zone.

  In addition, the loss of acoustic power in the sheath 117 of the fiber increases with the increase in the length of the acoustic path through the sheath 117. It can be seen in Figure 6 that, when the core of the fiber

  is displaced upwards with respect to the substrate (that is,

  that is, when the thickness of the sheath is reduced), the length of the interaction zone increases. In the embodiment of FIG. 2, the opposite is true because, in this embodiment of FIG. 2, when the core is displaced upwards by a decrease of

  the thickness of the sheath, the length of the area of inter-

  action decreases. Therefore, the embodiment of FIG. 6 allows a greater diffraction efficiency

  because the thickness of the sheath is reduced.

  Figs. 4A and 4B illustrate a method of making a flat surface on the fiber, to which surface the substrate (in any of its embodiments) can be bonded. As shown in FIG. 4A, an optical fiber 40 (which comprises a sheath 44 and a -me45) is fixed in a plate 41 in which a groove 42 is machined. The plate 41 may be made of fused silica. The upper surface of the assembly of Figure 4A is then ground and polished to obtain the assembly shown in Figure 4B. A typical fiber has an outer diameter of about 125 μm, and a monomode fiber, working at an optical wavelength of 1.3 μm, generally has a core with a diameter of about 10 μm. After grinding and polishing, a thin layer of

  sheathing between the core 45 and the upper surface 46.

  For a fiber of typical dimensions, this thin cladding layer should have a thickness of about 1 m in the embodiment of FIG. 2. In the embodiment of FIG. 6, for a fiber of typical dimensions, The optimum thickness of the cladding layer should be less than 30 μm and is advantageously between about 5 and 10 μm. When the substrate is placed against the upper surface 46 of the assembly shown in FIG. 4B, there is a cladding layer between the substrate and the core, and no excessive optical loss occurs in the device. If the acoustic waves travel a distance not exceeding about 30 Nm in the silica sheath to reach the core, the associated loss, by weakening, of acoustic energy is

  less than 3 dB if the working frequency is 6.3 GHz.

  The reflector substrate according to the invention can be bonded to the rectified and polished top surface 46 of the assembly of FIG. 4B by means of the same techniques as those used to bind transducers working at frequencies of the order of 1 GHz, to Bragg cells. The substrate, the bonding material, the sheath 44 and the core 45 advantageously have closely matching acoustic impedances to reduce the acoustic reflection losses that occur when acoustic waves propagate from the substrate.

in the fiber 40.

  We can set up along a single

  optical fiber several reflectors according to the invention.

  In this configuration, it is desirable to prevent the optical loss of a reflector from affecting the reflected light signal produced by other reflectors associated with the single fiber. To obtain this desired result, one can choose the transducers used in the reflectors among those available commercially, which can be

Z587810

  switched between an enabled state and a disabled state, on command. In operation, a light wave (such as a laser pulse) is emitted into the fiber and a selected reflector is activated by "turning on" the associated transducer (or a pair of associated transducers). All other reflectors placed along the fiber between the light source and the chosen reflector (ie the reflectors placed "upstream"

  should be switched off to minimize the

  of the light wave as it passes through these upstream reflectors. Subsequently, after a subsequent light wave has been emitted into the fiber, if desired, any desired combination of active reflectors can be obtained by appropriate activation or deactivation.

individual reflectors.

  It should be noted that by way of illustration, an optical frequency corresponding to a wavelength of 1300 nm has been used here, but that current optical fibers also have a low optical loss at wavelengths of 850 and 1550 nm. bare. Some efforts

  important efforts are made to produce representative fibers.

  feeling much lower losses at longer wavelengths. If these fibers are made, the acousto-optical reflectors described will be much easier to manufacture, because of the lower frequencies

Operating.

  It goes without saying that many modifications can be made to the optical reflector described and

  represented, without departing from the scope of the invention.

Claims (31)

  An optical reflector which can be placed in the immediate vicinity of an optical fiber (16) and connected to this optical fiber which has a longitudinal axis so as to reflect part of a light wave.   propagating along the axis through a section of inter-   action of the fiber adjacent to the reflector, characterized in that it comprises a substrate (12) in which acoustic signals can propagate, and means (10, 11) for producing a first acoustic wave (19) which propagates the substrate in the interaction section for propagating in the fiber in a first direction at an angle of about 45 with the axis; and a second acoustic wave (20) propagating from the substrate in the d-section. interaction to propagate in the fiber in a second direction at an angle of about 45 to the axis and approximately perpendicular to the first direction. 2. Reflector according to claim 1, characterized in that the means producing acoustic waves comprise a first transducer (10) and a second transducer (11) each connected to the substrate and each of which can be connected selectively to an active state in which the transducer produces wave acoustic energy, and an inactive state in which the transducer   does not produce wave acoustic energy.   3. Reflector according to claim 2, characterized in that the substrate comprises a first portion (13) to which the first transducer is connected and through which the first acoustic wave propagates,   a second part (14) to which the second trans-   duct and through which the second acoustic wave propagates, and an acoustically absorbing element (15)   placed between the first part and the second part.   4. Reflector according to claim 1, characterized in that the acoustic wave production means comprise a transducer (101) connected to the substrate (100) and selectively switchable between an active state in which the transducer produces energy wave acoustic and an inactive state in which the transducer does not produce wave acoustic energy, the first acoustic wave   being a first part of the wave acoustic energy   a transducer produced by the transducer and propagating in the interaction section, the substrate having a surface from which a second portion of the wave acoustic energy produced by the transducer is reflected and then propagates in the interaction section; second acoustic wave being the second part of wave acoustic energy.   5. An optical reflector which can be placed in the immediate vicinity of an optical fiber (16) and which can be connected to this fiber which has a longitudinal axis, so as to reflect part of a light wave.   propagating along the axis through a section of inter-   action of the fiber, adjacent to the reflector, characterized in that it comprises a substrate (12) through which acoustic signals can propagate, a first transducer (10) bound to the substrate and capable of producing a first acoustic wave (19) which propagates through the substrate and enters the interaction section so that this first acoustic wave propagates in the fiber in a first direction forming an angle of about 45 with the axis, and a second transducer (11) bound to the substrate and capable of producing a second acoustic wave (20) which propagates through the substiat and enters the interaction section so that this second acoustic wave propagates in the fiber in a second direction forming   an angle of about 45 with the axis and approximately in the first direction.   Reflector according to one of the claims   2 and 5, characterized in that the portion of the first acoustic wave propagating through the interaction section of the fiber is a plane acoustic wave having a frequency f 1, and that part of the second   acoustic wave propagating through the section of inter-   action of the fiber is a plane acoustic wave having a   frequency f2, o f2 is different from fl.   7. Reflector according to claim 6, characterized in that the first transducer has a fundamental frequency f3, where fl is a harmonic of f3 '   and the second transducer has a fundamental frequency   tale f4 o f2 is a harmonic of f4 ' 8. Reflector according to claim 5, characterized in that the substrate comprises a first portion (13) through which is propagated the first acoustic wave, a second portion (14) through which is   propagates the second acoustic wave, and an acoustic element   absorbing material (15) placed between the first part and the second part.   9. Reflector according to one of the claims   I and 5, characterized in that the substrate has an acoustic impedance selected to match that of the fiber to reduce acoustic reflection loss occurring when the first acoustic wave and the second acoustic wave propagate from the substrate in the fiber.   10. Optical reflector, characterized in that it comprises an optical fiber (16) having an optical refractive index n and a longitudinal axis, a substrate (12) bonded to the fiber, a first acoustic transducer (10) bonded to the substrate and capable of producing a first planar acoustic wave (19) propagating through the substrate to the interior of the fiber to propagate in the fiber in a first direction forming an angle of about 25878 10     with the axis, the first acoustic wave having a frequency f1 in the fiber, and a second acoustic transducer (II) bound to the substrate and capable of producing a second acoustic wave (20) which propagates through the substrate to the interior of the the fiber in order to propagate in the fiber in a second direction forming an angle   about 45 with the axis and approximately perpendicular   in the first direction, the second acoustic wave   having a frequency f2 in the fiber.   Reflector according to one of the claims   and 10, characterized in that the first acoustic transducer and the second acoustic transducer can each be selectively switched between an active state in which the transducer produces an acoustic wave, and an inactive state in which the transducer does not produce. acoustic wave.   12. Reflector according to claim 10,   characterized in that the frequencies f1 and f2 are different rents from each other.   13. Reflector according to claim 10,   characterized in that the frequencies f1 and f2 are sensi-   equal to and fl (2nV) / M) 0 (cos 45), where o> o is the wavelength in the space of the light that it is desired to be reflected by the reflector, when it propagates in the fiber, V is the speed of the first wave   acoustic in the fiber and M is a positive odd integer.   A reflector according to claim 10, characterized in that the substrate comprises a first portion (13) through which the first acoustic wave propagates, and a second portion (14) through which   propagates the second acoustic wave.   15. Reflector according to claim 14, characterized in that the substrate also comprises an acoustically absorbing element (15) placed between the first part and the second part.   16. Reflector according to claim 10, characterized in that the substrate has an acoustic impedance chosen so as to reduce the loss of acoustic reflection occurring when the first and second acoustic waves propagate through the interface between the substrate and the fiber.   17. Reflector according to claim 10, characterized in that the fiber comprises a central core (18) surrounded by a sheath (17) and in that the part of the sheath into which penetrates, in propagation, the   first and second acoustic waves has a thick approximately 30 μm.   An optical reflector capable of being placed in the immediate vicinity of an optical fiber (104) and bonded to that fiber having a longitudinal axis, for reflecting a portion of a light wave propagating along the axis through an optical fiber interaction section of the fiber adjacent to the reflector, characterized in that it comprises a substrate (100) through which acoustic signals can propagate and which has a first surface and a transducer (101) bonded to the substrate and capable of producing an acoustic wave energy of which a first portion propagates through the substrate to enter the interaction section in the form of a first acoustic wave propagating in the interaction section of the fiber, in a first direction forming an angle about 45 with the axis, a second part of the wave acoustic energy being reflected by the first surface and then propagating to   traves the substrate to enter the section of inter-   action in the form of a second acoustic wave propagating in the interaction section of the fiber, in a second direction forming an angle of about 45 with   the axis and approximately perpendicular to the first direction.   Reflector according to one of the claims   1, 2, 4 and 18, characterized in that the portion of the first acoustic wave propagating through the   fiber interaction section is an acoustic wave   plane of frequency f1, and in that the part of the second acoustic wave propagating through the   fiber interaction section is an acoustic wave   plane tick also of frequency f1, of] = (2nV) / MX (cos 45), one is the index of optical refraction of the i = bre, V is the speed of the first acoustic wave in the fiber,> o is the wavelength in the space of the light that is to be reflected by the reflector as the light propagates through the interaction section   of the fiber, and M is a positive odd integer.   20. Reflector according to claim 19, characterized in that f! is a harmonic of the frequency fundamental of the transducer.   21. Reflector according to claim 18, characterized in that the substrate has an acoustic impedance chosen to match that of the fiber in order to reduce the loss of acoustic reflection occurring when undulatory acoustic energy from the transducer passes, spreading, from substrate in the fiber.   22. Optical reflector, characterized in that it comprises an optical fiber (104) having an optical refractive index n and a longitudinal axis, a substrate (100) bonded to the fiber and having a first surface, and an acoustic transducer ( 101) bound to the substrate and capable of producing planar wave acoustic energy, a first portion of which propagates through the substrate to penetrate the fiber to propagate in the fiber in the form of a first acoustic wave, in a first direction forming an angle of about 45 with the axis, a second portion of the wave acoustic energy being reflected by the first surface, then propagating through the substrate to penetrate the fiber where it propagates in the form of a second acoustic wave, in a second direction forming   an angle of about 45 with the axis and about   in the first direction.   23. Reflector according to one of the claims   18 and 22, characterized in that the acoustic transducer can be selectively switched between an active state in which the transducer produces wave acoustic energy, and an inactive state in which the transducer   does not produce wave acoustic energy.   24. Reflector according to claim 22, characterized in that the frequency of the acoustic wave is substantially equal to (2nV) M Xo (cos 45) - oo is the wavelength in the space of the light that the it is desired to be reflected by the reflector when it propagates in the fiber, V is the speed of the first acoustic wave in the fiber and M is a positive odd integer. 25. Reflector according to any one of   Claims 13, 19 and 24, characterized in that M is equal to 3.   26. Reflector according to claim 19,   with claims 1, 2 and 5, characterized in that   what fl is a harmonic of the fundamental frequency of the first transducer and f1 is also a harmonic   of the fundamental frequency of the second transducer.   27. A method for reflecting a portion of the energy of a light wave propagating in an optical fiber having a longitudinal axis, characterized in that it comprises the. steps of positioning, in the immediate vicinity of the fiber, a substrate through which acoustic waves can propagate, causing a first acoustic wave to propagate through the substrate to penetrate the fiber so that the first acoustic wave is propagating in the fiber in a first direction at an angle of about 45 with the axis, causing a second acoustic wave to propagate through the substrate to penetrate the fiber so that the second acoustic wave propagates through the fiber in the fiber. a second direction forming an angle of about 45 with the axis and approximately perpendicular to the first direction, and emitting the light wave into the fiber so that it propagates along the axis and interacts with the first and second acoustic waves for a portion of the light wave to be reflected along the axis in an opposite direction of 180 to the initial direction of propagation of the light wave.   28. The method of claim 27, characterized in that the substrate has an acoustic impedance chosen to match that of the fiber to reduce the loss of acoustic reflection occurring when the first acoustic wave and the second acoustic wave pass, spreading, from substrate in the fiber.   29. The method of claim 28, wherein the fiber comprises a central core surrounded by a sheath, characterized in that it also consists of removing by grinding a portion of the sheath of the fiber to produce a flat surface separated from the liner. 'soul by a thin zone of cladding material, and to bind the substrate on the flat surface.   30. Method according to claim 27, characterized in that the first acoustic wave and the second acoustic wave have a substantially equal frequency f, in the fiber, o f = (2nV) / MX (cos 45), where   n is the optical refractive index of the fiber, V is -   the speed of the first acoustic wave in the fiber,   is the wavelength in the space of the light wave   neuse and M is a positive odd integer, 31. The method of claim 27, characterized in that the frequency of the first acoustic wave in the fiber differs from the frequency of the   second acoustic wave in the fiber.