SOLID-STATE OPTICAL INTERFEROMETER

Abstract: In a solid-state optical interferometer having a light path composed of a loop-shaped optical fiber (4, 4A, 4B) and a solid-state light wave guide (30) optically coupled to said optical fiber, electrodes (51A, 51B) for producing a surface acoustic wave (SAW) are formed as a pair on a surface of a substrate (3) on which said solid-state light wave guide is formed, and said surface acoustic wave is used to apply optical modulation to a light beam passing through said solid-state light wave guide.

Description: The present invention relates to a solid-state interferometer such as it is described in the precharacterizing part of claim 1. Components of the light path except for the optical fiber are integrated in form of a solid body and the interferometer is useful for detecing various physical quantities such as angular velocity. Such a solid-state optical interferometer is known from EP-A2-0020066. Interferometers are used for observing light interference. In a so-called double-beam interferometer, a beam of light emitted from a light source is divided into two beam portions. These two beam portions are combined into one beam after being passed through light paths having different conditions. From the interference between the two beam portions, the physical change in the light path is measured. These double-beam interferometers have been used for a long time for the precise measurement of various physical quantities. In recent years, optical fibers have been used as light paths. Since the size of the entire double-beam interferometer can thus be reduced, they have recently been used in even wider measurement fields. Figs. 1 to 3 illustrate some methods of such known interferometers utilizing optical fibers. Fig. 1 shows a so-called ring interferometer. An optical fiber (hereafter referred to as OF) is wound in a loop form to make an OF ring R. The beam emitted from a laser source L is divided into two beam portions by a beamsplitter BS1 including a half mirror. One portion of the beam is directed into one end of the OF of the ring R and the other portion of the beam is directed into the other end of the OF. After travelling through the OF of the ring R the two portions of the beam are combined together by the beamsplitter BS1. The resultant combined beam is directed by a beamsplitter BS2 to be detected by a detector D. The detector D is a photodiode, for example. When all of the components including the OF ring R are immobile, the two beam portions directed into two ends of the OF of the ring R travel around light paths formed by the same OF and are then combined together to be sent to the detector D. Since there is no phase difference between the two beam portions, no interference occurs. When the OF ring R is rotated at an angular velocity Ω, the path length of the OF of the ring R appears to be lenghtened for one beam portion travelling around the ring R in the direction of angular velocity Ω while the path length appears to be shortened for the other beam portion travelling around the ring R in the counterclockwise direction. Accordingly, there exists a phase difference between the two beam portions which have been combined by the beam splitter BS1, resulting in interference. Therefore, the angular velocity Ω can be determined by detecting the change in light quantity caused by the interference using the detector D. Accordingly, the ring intereferometer can be used as a gyro, for example. Letter P denotes a polarizer. Fig. 2 shows a Mach interferometer (also referred to as Mach-Zehnder interferometer). Two OF rings R1 and R2 comprise OF's having identical lengths. A beam of light emitted from a laser source L is divided into two portions by a beamsplitter BS1. The two beam portions thus split are directed into OF rings R1 and R2, respectively. After travelling around OF rings R1 and R2 the two beam portions are recombined by a beamsplitter BS2 to be directed into a detector D. As long as the physical condition of the OF ring R1 is the same as that of the OF ring R2, the time required for one beam portion to travel around the ring R1 is exactly the same as the time required for the other beam portion to travel around the ring R2. Therefore, there is no phase difference between the two beam portions after they are recombined by the beam splitter BS2. Therefore, no interference arises. When there is a difference between the physical conditions of the OF rings R1 and R2, the time required by light to travel around each of the rings is different. The interference caused by the phase difference is detected by the detector D. Therefore, it is possible to detect a physical quantity from the interference provided that one of the OF rings, say, the ring R2 is maintained under constant physical conditions as a reference ring and the other OF ring R1 is provided with the physical quantity to be measured as a detection ring. If the physical quantity to be measured is a current or a magnetic field, a change in the index of refraction of the OF ring R1 caused by the Faraday effect produces interference. If the physical quantity to be measured is vibration such as underwater sound or temperature, a change in the refractive index caused by the change in stress produces interference. In either case, the interference can be detected. Fig. 3 shows a Michelson interferometer, in which reflected beam portions which are returned from the ends of two OF rings R1 and R2 are recombined by a beamsplitter BS to produce interference. If the OF ring R1 is used as a detection probe and a fluid is adopted as the reflection object at the end of the OF ring R1, the current speed of the fluid can be measured. If a vibrating body is adopted as the reflection object, its amplitude displacement or vibration mode can be detected. If the OF ring R1 is also terminated by a mirror, the interferometer can be used similarly to the Mach interferometer for measuring the temperature, for example. Thus, these interferometers make it possible to measure various physical quantities and can be used as sensors. Known interferometric gyrometers utilizing phase modulation are disclosed in US-P 4,286,878 and US-P 4,265,541. The interferometer known from US-P 4,286,878 , however, forms the light path except the OF by the beam splitter comprising the half mirror as evident from Figs. 1 to 3. Accordingly, this interferometer has such a structure that the concept of an optical bench used for optical experiments is applied thereto. Thus, it is difficult to reduce the size of such a device and its construction and adjustment require a great deal of skill. Even after the construction and adjustment, the optical system is apt to get out of alignment due to slight changes in temperature or vibration. Thus, it is difficult to maintain precision. As a result, it is extremely difficult to provide the interferometer in form of various modular sensors. As disclosed in Japanese Patent Application Laid-Open Nos. 5694687 and 57113297 for example, the above described interferometers can detect angular velocity and hence can be used as gyros. As increasing attention has been paid to automobile navigation system in recent years, application thereto becomes an important problem. Angular velocity detection by the ring interferometer is carried out by using the Sagnac effect in the OF ring as the basic principle. The output characteristic of the detector of the ring interferometer with respect to angular velocity is extremely undesirable when a wide dynamic range is required. For example, a gyro used in the automobile navigation system requires a wide dynamic range amounting to six figures. In a gyro using a ring interferometer (hereafter referred to as optical fiber gyro or OF gyro), it is necessary to use an optical modulator as the interferometer and control the output of the optical modulator by the zero method in order to expand the dynamic range. Such a zero method is disclosed in Japanese Patent Application Laid-Open No. 5593010. In an interferometer for an OF gyro, therefore, an optical modulator is required for portions of the light path except the OF light path. This is also true of interferometers other than ring interferometers. As a result, the structure of the light path outside the OF becomes even more complicated, and the above described drawbacks become more and more serious. The interferometric laser gyrometer known from the above mentioned US-P 4,265,541 contains a solid light wave guide substrate formed by the same material as described in the following description. An object of the present invention is to provide a solid-state interferometer having an interference optical system which can be easily constructed and adjusted and which maintains its alignment after the construction and adjustment and being suitable for mass production because of modular structure of the entire interferometer. The above object is solved in accordance with the present invention by the features of claim 1. The dependent claims characterize advantageous developments thereof. Once the Bragg diffraction angle (2 ϑ B ) is set during production, it never becomes misaligned thereafter. Thus the interferometer has a feature of stable operation. The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the present invention in connection with the accompanying drawings in which: Fig. 1 shows the fundamental configuration of a conventional ring interferometer using an optical fiber; Fig. 2 shows the fundamental configuration of a conventional Mach interferometer; Fig. 3 shows the fundamental configuration of a conventional Michelson interferometer; Fig. 4 shows the configuration of an embodiment of a ring interferometer according to the present invention; Fig. 5 is a schematic drawing illustrating an embodiment of a light wave guide; Fig. 6 is a schematic drawing illustrating a coupling portion between the optical fiber and the light path; Fig. 7 is a schematic drawing illustrating another embodiment of a light wave guide; Fig. 8 is a schematic drawing illustrating an embodiment of an optical modulator; Fig. 9 is a drawing illustrating the operation of the optical modulator; Fig. 10 is a block diagram of an embodiment of a phase modulation control circuit; Fig. 11 is an oblique view of an embodiment of a solid-state interferometer according to the present invention; Fig. 12 is a drawing illustrating an embodiment of a coupling portion; Fig. 13 shows the configuration of an embodiment of a Mach interferometer according to the present invention; Fig. 14 shows the configuration of an embodiment of a Michelson interferomter according to the present invention; Fig. 15 is a drawing for illustrating a standing wave caused by laser coherency; Fig. 16 is an operation characteristic diagram of a semiconductor laser; Fig. 17 is a circuit diagram of an example of a laser activation circuit which prevents occurrence of a standing wave; Fig. 18 shows drawings illustrating the drift in an optical fiber gyro; Fig. 19 is a circuit diagram illustrating an example of a method used to eliminate the effect of the drift; Fig. 20 is a circuit diagram illustrating another example of a method used to eliminate the effect of the drift; Fig. 21 is a block diagram of an example of a circuit for detecting the average of drift by means of a microcomputer; Fig. 22 is a flow chart for the circuit illustrated in Fig. 21; Fig. 23 is a block diagram illustrating an example of a readout method when the gyro has an analogue output; Fig. 24 is a block diagram illustrating an example of a readout method when the gyro has a digital output; Fig. 25 is a curve diagram representing the angular rotation velocity of an automobile as a function of its turning radius; and Fig. 26 is a drawing illustrating a position at which an optical fiber gyro is mounted. Solid-state interferometers according to the present invention will now be described by referring to the illustrated embodiments. Fig. 4 shows an embodiment of the present invention, which has been obtained by applying the present invention to the ring interferometer illustrated in Fig. 1. The interferometer illustrated in Fig. 4 includes a laser 1, an isolator 2, a solid-state light wave guide substrate 3, a loop-shaped OF ring 4, a surface acoustic wave element 5, a photodetector 6, a buffer amplifier 7, a phase modulation control circuit 8, an assembly substrate 9, and a coupling portion 10. A solid-state light wave guide 30 is formed on the substrate 3. Light beam splitting portions 30a, 30b, and 30c as well as a light combination portion 30d are formed on the solid-state light wave guide 30. The OF ring 4 is coupled to the solid-state light wave guide 30 at the ends 4a and 4b of the OF ring 4. A semiconductor laser is used as the laser 1 to emit monochromatic light having high convergence. As the isolator 2, an isolator using the Faraday effect, for example, is adopted. The isolator 2 prevents the beam returning from the OF ring 4 from entering the laser 1. The transparent solid-state light wave guide substrate 3 is made of a ferroelectric material such as lithium niobate LiNbO₃. The substrate 3 is provided for the solid-state light wave guide 30 and for the surface acoustic wave element 5 to be formed thereon as will be later described in detail. The OF ring 4 functions in the same way as the OF ring R of the prior art illustrated in Fig. 1. The surface acoustic wave element 5 is also referred to as a surface elastic wave element. The element 5 propagates surface acoustic waves on one surface of the solid-state light wave guide substrate 3, on which the solid-state light wave guide 30 has been formed, and forms an optical modulator using Bragg diffraction as will be later described in detail. The photodetector 6 is a photoelectric conversion element such as a photodiode and is fixed in a dent or hole 6a provided on the solid-state light wave guide substrate 3. The photodetector 6 detects the light beam emitted from the solid-state light wave guide 30. The buffer amplifier 7 converts a current signal fed from the photodetector 6 to a voltage signal. The phase modulation control circuit 8 controls the phase of the drive signal which has been supplied to the surface acoustic wave element 5 in accordance with the signal fed from the photodetector 6 and produces the sensor output by the zero method, as will be described in detail later. The assembly substrate 9 is made of ceramic material such as alumina porcelain and functions to assemble the optical system apart from the OF ring 4 into a unit as will be described later in detail along with the coupling portion 10. By using the zero method, a measurement output can be obtained with high precision. Fig. 5 shows one embodiment of the solid-state light wave guide substrate 3 along with the solid-state light wave guide 30 and the light division portion 30a formed thereon in detail. The embodiment illustrated in Fig. 5 has a substrate for the light wave guide of Ti diffused LiNbO₃. As the substrate 3, a LiNbO₃ crystal quarried in the Z plane is used. On the surface of the substrate 3, titanium Ti is diffused to form the light wave guide 30. At first, a titanium film having the surface shape of the light wave guide to be made is formed on the surface of the substrate 3 by using a method such as sputtering. The titanium film is subjected to a thermal diffusion process to diffuse titanium into the substrate. Thus, a part of the substrate ranging from the titanium film to a predetermined depth will have a refractive index which is slightly different from that of the substrate itself, the light wave guide 30 thus being formed. At this time, the light beam splitting portion 30a can also be produced only by forming the titanium film in accordance with the shape of the portion 30a. The light combination portions 30b to 30d can also be made in the same way as the light division portion 30a. The solid-state light wave guide thus formed is 5 µm in width W and several hundred Å in depth D, for example. The relationship between the solid-state light wave guide 30 and the OF core 40 is illustrated in Fig. 6. Fig. 6 shows the coupling between the OF ring 4 and the solid-state light wave guide 30 at ends 4a and 4b of the OF ring 4. The OF is assumed to be 125 µm in cladding diameter and 5 µm in core diameter. Fig. 6(a) is a side view, Fig. 6(b) is a top view, and Fig. 6(c) is a front view. A light beam can be continuously propagated by coupling an end face of the OF core 40 with an end face of the solid-state light wave guide 30 in close contact and confronting each other. Fig. 7 shows another embodiment of a solid-state light wave guide 30, which is referred to as a bridge type or wedge type. Those parts of the surface of the substrate 3 which are to be formed as the light wave guide are left unchanged and the environs thereof is removed by a method such as ion beam machining. Onto the part thus removed are placed filling layers 31 comprising material having a suitable refractive index (such as polyimide resin) to form the light wave guide 30. When the solid-state light wave guide illustrated in Fig. 7 is used, it is not possible to obtain light propagation by means of reflection in the thickness direction as illustrated in Fig. 6(a). The OF ring 4 is obtained by winding the above described OF, which has a cladding diameter of 125 µm, a core diameter of 5 µm, and a length of approximately 500 m, around a ring which is approximately 30 cm in diameter. The OF ring 4 provides light beams propagating in both directions (Sagnac effect). Fig. 8 shows an embodiment of a surface acoustic wave element (hereafter referred to as SAW element) in detail, and depicts a part of the substrate 3 illustrated in Fig. 4. In Fig. 8, reference numeral 50 denotes a diffraction portion and reference numerals 51A and 51B denote comb electrodes. Similarly to the solid-state light wave guide 30, the diffraction portion 50 is formed on the surface of the substrate 3 with diffused titanium or by a ridge. The comb electrodes 51A and 51B are interdigitated comb-shaped electrodes formed on the surface of the substrate 3 by means of evaporation or sputtering. Upon receiving a high frequency drive signal fed from a reference oscillator 80 which is included in the phase modulation control circuit 8, the comb electrodes 51A and 51B produce a surface acoustic wave (referred to as SAW) and propagate the wave into a region including the diffraction portion 50. Operation of the SAW element 5 will now be described in more detail by referring to Fig. 9. When the high frequency signal from the reference oscillator 80 is supplied to the electrodes 51A and 51B, a high frequency electric field is produced between the combs of the electrodes. Due to the piezoelectric effect of the high frequency electric field, the surface of the substrate 3 contracts and expands locally. The SAW thus produced is propagated through the diffraction portion 50. If a light beam LB1 is then applied to the diffraction portion 50 as illustrated in Fig. 9, a part of the light beam LB1 is subjected to Bragg diffraction caused by stress which is generated near the surface of the substrate 3 by the SAW. The Bragg diffraction light LB1" thus produced proceeds in a direction which forms a predetermined angle 2ϑ B with respect to a light beam LB1' which proceeds straight without being subjected to diffraction. The angle 2ϑ B is defined as: Sinϑ B = 1 / 2· K / k = 1 / 2· λ / Λ (1) where: K = the number of SAW waves k = wavelength of SAW Λ = the number of the light λ = wavelength of the light Assuming that the diffraction light LB1" has a frequency ω₂, ω₂ is displaced from the frequency ω₁ of the original light beam LB1 by the frequency ΩE of the SAW as: ω₂ = ω₁ ± ΩE (2) The frequency ΩE of the SAW is the same as the frequency of the drive signal fed from the reference oscillator 80 to electrodes 51A and 51B. In the SAW element, therefore, the light beam LB1 is subjected to frequency modulation by the signal fed from the reference osillcator 80, the function of the optical modulator thus being realized. The sign ± in the expression (2) depends upon the propagation direction of the SAW and the incident direction of LB1. In the above described example, the sign is +. At the same time, another light beam LB2 is applied to the diffraction portion 50 at a predetermined angle with respect to the light beam LB1. If the incident direction of the light beam LB2 with respect to the light beam LB1 is set to 2ϑ B , it is possible to combine the diffracted light LB1" with the light beam LB2 and take the combined light out of the diffraction portion 50. Thus, the function of the light combination portion 30d is fulfilled. In this embodiment, the light beam LB1' and a light beam which is produced by diffraction of the light beam LB2 and is not illustrated in Fig. 9 are not used. Fig. 10 shows one embodiment of the phase modulation control circuit 8. As already described, reference numeral 80 denotes a high frequency reference oscillator for supplying the drive signal F to the electrodes 51A and 51B of the SAW element 5. A phase comparator 81 compares the phase of a detected signal S, which is supplied from the photodetector 6 to the buffer amplifier 7, with that of the output F of the reference oscillator 80 to produce a signal P representing the phase difference. A voltage controlled oscillator (hereafter referred to as VCO) 82 produces an output signal Q having a frequency corresponding to the phase comparison signal P. A shift register 83 produces a shift output signal F' in response to a shift input signal F and a shift clock signal Q. The shift register 83 outputs the signal F' which lags behind the signal F by a predetermined delay time, i.e., a predetermined lag phase. The phase lag can be arbitrarily controlled by the frequency of the signal Q. As a result, the shift register 83 operates as a variable phase shifter. Operation for detecting an angular velocity in the embodiment illustrated in Fig. 4 will now be described. The light beam (hereafter abbreviated as LB) fed from the laser 1 passes through the isolator 2 and enters the solid-state light wave guide (hereafter abbreviated as SLG) 30 formed on the solid-state light wave guide substrate 3. The light beam is then divided into two beam portions LB1 and LB2 by the light beam splitting portion 30a formed on the SLG 30. LB1 passes through the beam splitting division portion 30b as it is and is directed into the OF of the ring 4 through the end 4a of the OF in the OF ring 4 located in the coupling portion 10. After travelling around the OF ring 4 in the clockwise direction (Fig. 4), LB1 reenters the SLG substrate 3 through the other end 4b of the OF ring 4. LB1 is then directed into the diffraction portion 50 (Figs. 8 and 9) of the SAW element 5 through the light beam splitting portion 30c. The resultant Bragg diffraction light LB1" is supplied from the SAW to the photodetector 6. On the other hand, LB2 passes through the light beam splitting portion 30c and is directed into the OF ring 4 through the end 4b. After travelling around the OF ring 4 in the counterclockwise direction, LB2 reenters the SLG 30 through the end 4a. After being divided by the light beam splitting portion 30b, LB2 is directed into the diffraction portion 50 of the SAW element 5 to become LB2' as it is. LB2' is combined with LB1" and enters the photodetector 6. As described before, a part of LB1 or LB2 which is divided at the light beam splitting portion 30b or 30c to be directed into the light beam spliting portion 30a is prevented from returning to the laser 1 by the isolator 2. From the light beams LB1" and LB2' thus directed into the photodetector 6, LB1" is subjected to frequency modulation in the SAW element 5 as described before. The frequency ω₂ of LB1" deviates from the frequency ω₁, of the original LB1 and LB2 by ΩE. Because of the frequency difference ΩE between LB1" and LB2', a beat signal having a frequency ΩE is produced between them. Thus, the output signal S (Fig. 10) of the photodetector 6 has a frequency ΩE. The phase of the signal S is compared with that of the output signal F of the reference oscillator 80. If the OF ring 4 is not rotated (its angular rotation speed Ω is zero), no phase difference due to the Sagnac effect is produced between the two counterpropagating beams LB1 and LB2. Accordingly, the phase difference between LB1 and LB2 when directed into the SAW element 5 only depends upon constants of this system and can be practically considered to be zero. If the OF ring 4 is rotated at an angular velocity Ω₁ [rad/s], the Sagnac effect produces a phase difference Δϑ between beams LB1 and LB2 counterpropagating through the OF ring 4 as: where: A = area enclosed by the OF ring [m²] N = the number of turns on the OF ring 4 L = length of OF in the OF ring 4 [m] R = radius of the OF ring 4 [m] λ = wavelength of laser light [m] C = light velocity (= 3 x 10⁸[m/s]) When L = 10³ m, R = 0.3 m, and λ = 0.83 x 10⁻⁶ m, for example, it follows that: Δϑ ≃ 1.2 Ω₁ (4) Such phase relationship between LB1 and LB2 is completely preserved in the LB1" and LB2' produced after passing through the SAW element 5. Further, the relationship is also preserved in the beat signal S having a frequency ΩE as diferential frequency between LB1" and LB2'. If the phase difference between LB1 and LB2 directed into the SAW element 5 is zero, the phase of the beat signal S detected by the photodetector 6 is coincident with the phase of the frequency change determined by the signal F' having a frequency ΩE (Fig. 10) which is supplied to the SAW element 5. The method is such, where the phase of the signal S deviates from the phase of the signal F' supplied to the comb electrodes 51A and 51B, a feedback of the signal S to compare the phase between the beat signal S and the output signal F of the reference oscillator 80 is carried out. If the OF ring 4 is provided with an angular rotation speed Ω₁, a phase difference Δϑ is generated between LB1 and LB2. The phase difference between the beat signal S fed from the photodetector 6 and the output signal F of the reference oscillator 80 also changes by Δϑ. As a result, the comparison signal P fed from the phase comparator circuit 81 varies corresponding to Δϑ. Thus, the frequency of the output signal Q of the VCO 82 is varied to control the shift time of the shift register 83. Accordingly, the phase difference between the signals F and F' is varied to change the phase of the SAW produced by the SAW element 5. Thus, control is so effected as to cancel the phase change Δϑ of the beat signal S. As a result, the phase of the signal F' is varied according to the phase difference generated between the LB1 and LB2. Control is so effected as to converge the phase difference between signals S and F of the phase comparator circuit 81 to zero. Thus, the phase detection operation is carried out by the so-called zero method. From the frequency of the output singal Q of the VCO 82, therefore, the phase difference between the LB1 and LB2, i.e., the angular rotation speed Ω₁ can be measured with a predetermined precision under a sufficiently wide dynamic range. Since almost all elements of the optical system apart from the OF ring 4 are included in the solid-state light wave guide substrate 3 in this embodiment, almost all assembly steps of the optical system required for constructing the ring interferometer are completed by carrying out the manufacturing process of the substrate 3. Thus, construction and adjustment of the interferometer are extremely simple. In addition, the optical axis hardly gets out of alignment in use. The assembly substrate 9 (Fig. 4) and the coupling portion 10 will now be described. Fig. 11 is an oblique view of one embodiment of the present invention. Reference numerals 11, 12 and 13 denote an OF holding member, a stop member, and an integrated circuit, respectively, The members 11 and 12 form the coupling portion 10. As already described, the assembly substrate 9 is made of alumina ceramics, for example. In the integrated circuit 13 formed on one surface of the assembly substate 9, the buffer amplifier 7 and the phase modulation control circuit 8 have been integrated. On that surface of the assembly substrate 9, the laser 1, the isolator 2, and the solid-state light wave guide substrate 3 are mounted to form a unit. The solid-state light wave guide substrate 3 has the optical system required for the ring interferometer, which includes the light wave guide 30 formed in a predetermined shape, the SAW element 5, and the photodetector 6. Before (or after) the solid-state light wave guide 3 is attached to the substrate 9, the ends 4a and 4b of the OF ring 4 are coupled to the solid-state light wave guide 3 via the coupling portion 10. Details of coupling are illustrated in Fig. 12. The OF holding member 11 is made of a silicon plate. In a part of the OF holding member 11, V-shaped grooves 11A and 11B are formed to meet the incident ends of the light wave guide 30 formed on the solid-state light wave guide 3. The ends 4a and 4b of the OF with the jacket layer (used to guard the external surface of the clad layer) removed are inserted into the grooves 11A and 11B, and bound and held in the grooves by solder glass, for example. The grooves 11A and 11B are formed with high dimensional precision. The distance between the core centers of the two pieces of the OF is made to be coincident with the distance between the light wave guides 30 formed on the solid-state light wave guide substrate 3 with predetermined precision. In addition, the core center of the OF is aligned with the surface of the member 11 which is attached to the solid-state light wave guide plate 3 with predetermined precision. As the processing method of the member 11, anisotropic silicon etching, for example, is used to produce the V-shaped grooves 11A and 11B with high precision. The stop member 12 is made of suitable glass and formed in a dent shape. When both ends of the stop member 12 are bound to the holding member 11, the stop member presses the ends 4a and 4b of the two pieces of the OF, which are inserted in the V-shaped grooves 11A and 11B of the holding member 11, against the V-shaped grooves 11A and 11B, respectively. Thereby, the OF is put between the member 12 and each of the V-shaped grooves 11A and 11B of the member 11 to be held in a stable and reliable manner. By using the electrostatic bonding, i.e., anode bonding, for example, members 11 and 12 can be bound together with stability, high precision, and low thermal stress. In this embodiment, therefore, the ends 4a and 4b of the OF are fixed to the holding member 11 and the stop member 12 is bound to the holding member 11. Then the holding member 11 is placed on a surface of the substrate 3, on which the SLG 30 is formed, so that an end face of the SLG substrate 3 and an end face of the stop member 12 may confront each other. An end face of the SLG substrate and an end face of the stop member 12 are adhered closely together. (This adhered portion is represented by "P" in Figs. 11 and 12(b).) Only by sliding the holding member 11 with respect to the SLG substrate 3 while keeping the lower face of the holding member 11 adhered to the upper face of the SLG substrate 3 (as represented by Q), the optical axes of the two pieces of SLG 30 formed on the SLG substrate can be aligned with those of the ends 4a and 4b of the OF attached to the holding member 11, respectively. Thus, the optical axis alignment can be easily conducted with high precision by simple procedure. After the optical axis alignment has been conducted by using the holding member 11 and the stop member 12 as guides, binding at portions P and Q is conducted by a suitable technique to fix the members 11 and 12 to the SLG substrate 3. In this way, construction and adjustment work is completed. In the above described embodiments, the present invention has been applied to a ring interferometer which is suitable for OF gyros, for example. It is a matter of course that the present invention can be applied to other kinds of interferometers. Fig. 13 shows an embodiment obtained by applying the present invention to the Mach interferometer. And Fig. 14 shows an embodiment obtained by applying the present invention to the Michelson interferometer. In the embodiments of Figs. 13 and 14, 4A and 4B are OF rings which correspond to the OF rings R1 and R2 illustrated in Figs. 2 and 3. Remaining components in Figs. 13 and 14 are the same as those in Fig. 4. As the interferometer, the embodiment illustrated in Figs. 13 and 14 operate in the same way as the embodiments illustrated in Figs. 2 and 3. For measurement by the zero method, the embodiments illustrated in Figs. 13 and 14 operate in the same way as the embodiment illustrated in Fig. 4. Accordingly, further description is omitted. In the above described embodiments, the light beam splitting portion 30a and the light combination portion 30c formed on the SLG substrate 3 are simple branches. In the embodiments of the present invention, however, the light beam splitting portion and the light combination portion may be optic directional couplers formed by two pieces of SLG which run in parallel by a predetermined length with a predetermined short spacing. As evident from the above explanation, the OF gyro using the ring interferometer demands a light beam having sufficient monochromatism and convergence. Therefore, a laser is mainly used as the light source. On the other hand, the laser light has a high degree of coherency. If some reflection should exist within the optical system originating from the laser, a standing wave due to interference between the reflected light and the incident light appears between the part where reflection occurs and the laser. Thus, the oscillation mode of the laser is affected and the operation of the laser becomes unstable, resulting in an output error of the gyro. In the embodiment of the present invention illustrated in Fig. 4 and the OF gyro using the ring interferometer illustrated in Fig. 1, therefore, some countermeasures must be taken against the problems caused by the occurrence of the standing wave. An example of a method for preventing occurrence of a gyro detection error caused by a standing wave will now be described. Fig. 15 illustrates how the standing wave is generated. A laser light Pt having a wavelength λ which is sent out from a laser diode LD is directed into an optical system of an interferometer including lenses L₁ and L₂ as well as a beam splitter BS and is used for measurement. This optical system is composed of the SLG and OF in the embodiment of Fig. 4. If some part of the optical system has a discontinuity face in refractive index, a reflected light Pr is generated to produce the above described standing wave. On the other hand, the wavelength λ of the laser light Pt can be varied by changing the drive voltage V F of the laser diode LD as illustrated in Fig. 16. In this method, the wavelength λ of the laser light Pt emitted from the laser diode LD is continuously varied by a small amount by utilizing the characteristics of the laser diode LD as illustrated in Fig. 16 in order to prevent the occurrence of the standing wave. One embodiment of the method is illustrated in Fig. 17. A transistor Tr₁ varies voltage V F fed to the laser diode LD in accordance with the base voltage thereof. A capacitor C₁ is provided for noise suppression. An operational amplifier OP constitutes an integration circuit in conjunction with a capacitor C₂ and a resistor R₇. The output of the integration circuit controls the transistor Tr₁. Inverters IV₁ and IV₂ constitute an astable multivibrator in conjunction with a capacitor C₅ and a resistor R₈. The multivibrator supplies a square wave to the operational amplifier OP via a capacitor C₄. When the circuit of Fig. 17 is activated, the square wave generated by the inverters IV₁ and IV₂ is integrated by the operational amplifier OP to produce a triangular wave. Since the transistor Tr₁ is controlled by the triangular wave, the voltage V F fed to the laser diode LD varies in the triangular form. Since the wave-length λ of the laser light Pt is continuously varied over a predetermined range, no standing wave is generated even if the reflected wave Pr emerges. Thus, it becomes possible to prevent occurrence of a gyro detection error Further, in the embodiment illustrated in Fig. 17, the output of a monitor diode MD associated with the laser diode LD is smoothed by resistors R₂ and R₃ as well as a capacitor C₃ and supplied to the + input of the operational amplifier OP. Thus, the drive voltage V F is so controlled in accordance with the intensity of the laser light Pt emitted from the laser diode LD as to hold constant the intensity of the output laser light. As a result, the laser light output is stabilized. The monitor diode MD is a photodiode and resistor R₁ is the load resistor of the monitor diode MD. The voltage E₂ is provided for setting the DC bias for the laser diode LD. Resistor R₄, R₅ and R₆ are provided for level setting. In an OF gyro using such a ring interferometer, the output is not completely zero but always drifts slightly near zero level, even if the angular rotation velocity to be detected is zero. If such an OF gyro is used in an automobile navigation system, therefore, the output ΩGY does not assume zero even in a period ST during which the car stops and the angular rotation speed Ωfed to the gyro is zero, as illustrated in Fig. 18(a). As expanded in Fig. 18(b), the average value is not zero. The deviation of the average value from zero might be increased as the stoppage-period ST is increased. When a car with this navigation system restarts, the departure direction shown by the interferometer also depends upon the output of the gyro. If the average value is deviated from zero at stoppage time as described above, the departure direction gets out of alignment, resulting in a large position error. Fig. 19 shows an example of a circuit which avoids a large position error, when the solid-state interferometer according to the present invention is applied. The circuit illustrated in Fig. 19 includes a car velocity sensor 15, a comparator 16, a retriggerable monostable multivibrator (hereafter referred to as RMM) 17, a digital AND gate 18, a hand brake lamp 19, a hand brake switch 20, an OF gyro 21, a gyro reference signal generator (hereafter referred to as GYR ) 22, AND gates 23 and 24, an inverter 25, and an OR gate 26. The car velocity sensor 15 functions to measure the car velocity and detect the car stoppage. For example, a well known sensor composed of a plurality of magnets attached to the thrust shaft of the car and magnetic flux detection coils may be used. A comparator 16 functions to convert the output of the car velocity sensor 15 into pulses. While the car is running, therefore, pulses appear at the output of the comparator 16 with a repetition period corresponding to the car velocity. The RMM 17 operates with a time constant predetermined by a capacitor C and a resistor R. As long as the comparator 16 supplies pulses to the RMM 17 within a predetermined repetition period defined by the time constant, the output Q of the RMM 17 continues to be "1". One end of the hand brake lamp 19 is connected to a battery BAT. The other end of the lamp 19 is connected to the switch 20 which is closed when the hand brake is pulled. The GYR 22 produces a reference signal which indicates that the angular velocity Ω is zero. Operation of the circuit illustrated in Fig. 19 will now be described. While the car is running, the car velocity sensor 15 produces its output. Since the RMM 17 is thus continuously triggered by the output pulse of the comparator 16, the output Q continues to be "1". While the car is running, the hand brake is also released, the hand brake switch 20 being in the open state. Accordingly, the upper terminal of the switch 20 is held to the power supply voltage to indicate the state "1". Accordingly, the output of the AND gate 18 assumes state "1" to enable the AND gate 23. On the other hand, the AND gate 24 remains closed because of the inverter 25. While the car is running, therefore, the angular rotation velocity signal fed from the OF gyro 21 is sent out as the gyro output GYO as it is via the AND gate 23 and the OR gate 26, thereby activating the navigation system. When the car is stopped, the output of the car velocity sensor 15 disappears. Since the pulses fed from the comparator 16 are also removed, the Q output of the RMM 17 turns "0" after a predetermined time defined by the CR time constant has elapsed. Thereby, the output of the AND gate 18 also turns "0". When the hand brake is applied, the hand brake switch 20 is closed. The hand brake lamp 19 is lit. One input of the AND gate 18 is grounded through the switch 20 to change from "1" to "0". The output of the AND gate 18 is held to "0" by the input as well. At this time, therefore, the AND gate 23 is closed. On the other hand, the AND gate 24 is enabled by the output of the inverter 25. Thus, the reference signal representing zero angular velocity fed from the GYR 22 is sent out as the gyro output GYO, the navigation system being activated. In summary, the detection signal of the OF gyro 21 is sent as it is to the navigation system to be used for position detection, for example, while the car is running. When the car stops and the angular rotation speed Ω becomes zero, the reference signal indicating zero speed is sent to the navigation system. As a result, it is possible to prevent direction setting at the time of car departure from getting out of order because of the output drift of the OF gyro 21. And the position can be correctly measured at all times. Fig. 20 shows another example of a circuit attaining the same object as Fig. 19. In the circuit of Fig. 19, the reference value is sent out instead of the output of the gyro during the stoppage time. Meanwhile, every time the car stops, the output of the OF gyro in the circuit of Fig. 20 during stoppage becomes the zero level of a new angular rotation speed when the car restarts. Thereby, occurrence of error in the navigation system is prevented. A car velocity sensor 15, a comparator 16, an RMM 17, a hand brake lamp 19, a hand brake switch 20, and an OF gyro are the same as those included in the circuit of Fig. 19. Therefore, the output of the NAND gate 31 turns "0" only while the car is running. When the car stops to remove the output of the car velocity sensor 15 and/or the hand brake is applied to close the switch 20, the output of the NAND gate turns "1", which can be used as the angular velocity detection signal. In the circuit of Fig. 20, operational amplifiers 32 and 33, an analog switch 34, an integrating resistor 35, an integrating capacitor 36, and a data hold capacitor 37 are also included. The operational amplifier 32 operates as a buffer amplifier. When the analog switch 34 turns on, the amplifier 32 transfers the voltage appearing at the capacitor 36 to the capacitor 37. The operational amplifier 33 amplifies the output of the OF gyro 21 with respect to the voltage of the capacitor 37 fed to its minus input. The amplifier 33 sets the zero level of the output of the OF gyro 21 to the voltage of the capacitor 37. The analog switch 34 turns on when the output of the NAND gate 31 is "1" and turns off when the output of the NAND gate 31 is "0". The associated navigation system acquires the output of the NAND gate 31 as the zero angular velocity signal. When the output of the NAND gate 31 turns on, the gyro output GYO is recognized as the signal when the angular rotation speed Ω is zero. Operation of the circuit illustrated in Fig. 20 will now be described. An integration circuit composed of the resistor 35 and the capacitor 36, which may be considered to be a low-pass filter, smoothes the output of the OF gyro 21. The smoothed output is supplied to the operational amplifier 32 functioning as a buffer amplifier. When the car is stopped and the angular rotation velocity Ω is zero, therefore, the average value of change caused by the drift of the OF gyro 21 as illustrated in Fig. 18(b) always appear at the output of the operational amplifier 32. As a result, every time the car stops and the output of the NAND gate 31 turns "1" to turn on the analog switch 34, the voltage across capacitor 37 is updated by an average value of new drift of the OF gyro 21. The updated voltage becomes the zero level of the gyro output GYO by the function of the operational amplifier 33. Even if the car stops and the average level of the OF gyro output changes due to drift, therefore, the zero level of the gyro output is always compensated automatically by the changed average level in this circuit. Thus, error in detection operation is prevented from occurring in the navigation system. In the circuit of Fig. 20, the output of the OF gyro is an analogue signal. When the circuit of Fig. 20 is applied to the embodiment of Fig. 4, the frequency output must be converted into an analogue signal by using a frequency discrimination circuit, for example. In a navigation system using such an OF gyro, a microcomputer is generally used for signal processing. The angular velocity is represented by the frequency of the gyro output in the embodiment illustrated in Fig. 4. In such a case, an example of a method for calculating the average value of drift as illustrated in Fig. 18(b) wil now be described. Fig. 21 shows the schematic block diagram of the signal processing part of a microcomputer. A frequency counter 41, an input/output device (I/O) 42, an MPU 43 of the microprocessor, and a memory 44 are included in Fig. 21. An OF gyro 21 may be, for example, one embodiment of the present invention illustrated in Fig. 4, in which the angular rotation velocity detection signal is a frequency signal. NAND gate 31 is the same as that in the circuit of Fig. 20. The frequency counter 41 always counts the output Q of the OF gyro 21 to send out its frequency data. On the other hand, the MPU 43 monitors the output of the NAND gate 31 via the I/O 42. Each time the output of the gate 31 changes from "0" to "1", i.e., each time the car stoppage is detected, the MPU 43 starts execution of a series of processing steps as illustrated in a flow chart of Fig. 22. After the processing of Fig. 22 is started, predetermined memory area A and a counter T serving as a timing means which both are beforehand prepared in the RAM of the memory 44 are cleared at step 1 ○. The counter (T) is also a soft counter using a predetermined memory area of the RAM. The output data of the frequency counter 41 is acquired at step2 ○ and added to the above described memory area A at step 3 ○. At step 4 ○, the timer counter (T) is incremented. At step 5 ○, it is determined whether the car starts running or not. If the result is "NO", processing returns to the step2 ○ and steps2 ○ to4 ○ are repeated. If the result at the step5 ○ is "YES", processing proceeds to the step 6 ○, where the data in the memory is divided by the count value T of the timer counter to yield the average value. At the step 7 ○, the average value thus computed is stored in the RAM as the zero reference data. Thereafter, the processing illustrated in Fig. 22 is terminated. At the step 5 ○, the car is considered to have started running if the output of the NAND gate 31 is "0". It the steps2 ○ to5 ○ are repeated at a predetermined time interval, the drift value averaged over the car stoppage period, i.e., the zero reference data can be obtained. A general method for aquiring the angular rotation velocity data from the OF gyro in the navigation system using such a microcomputer will now be described. If the OF gyro is one which outputs the angular rotation speed signal in the form of analogue data, the data GYO may be acquired into the MPU 43 simply via an analogue-digital converter (A/D) 45 as illustrated in Fig. 23. If the OF gyro is one which outputs the angular rotation speed signal in the form of frequency data Q as the embodiment of the present invention illustrated in Fig. 4, the frequency data Q must be converted into digital data DD by using a frequency counter 46 before acquiring data into the MPU 43 as illustrated in Fig. 24. In an alternative method, the frequency data Q is fed to the MPU 43 and processed by soft count means. In this case, however, a large part of processing time of MPU 43 is occupied by the soft count processing. Accordingly, it is more desirable to provide an external frequency counter 46 as illustrated in Fig. 24. In Figs. 23 and 24, the A/D 45 and the frequency counter 46 have a capacity of 12 bits, and the length of digital data DD is 12 bits. The reason will now be explained. While the car is running, the maximum speed at which the car can turn safely without skidding is defined by: where m = mass of the car V = velocity of the car r = turning radius of the car g = acceleration of gravity C f = coefficient of friction between car tire and road surface Fig. 25 shows the maximum angular rotation speed Ωmax of the car, which can be obtained at a speed satisfying the expression (4), as a function of the turning radius r. The friction coefficient C f is assumed to be 0.8 in Fig. 25. Assuming the minimum possible turning radius r min of the car is 5 m, the maximum value of the angular rotation velocity Ω to be detected by the OF gyro in the navigation system is approximately 70 deg/s as evident from Fig. 25. On the other hand, the minimum value of the angular rotation velocity to be detected by the OF gyro is not defined when viewed from the standpoint of the navigation system side. Since the minimum detection precision of the OF gyro is approximately 0.05 deg/s however, this value is taken as the minimum value. Thus, the dynamic range of the angular rotation speed data required for the navigation system becomes 1,400. For representing the dynamic range by binary data, 11 bits are required. In addition, one bit is required for determining the rotation direction. As a result, 12 bits are required for the data of the OF gyro. That is the reason why the 12-bit digital data DD is used in Figs. 23 and 24. How to mount the OF gyro will now be described. The solid-state interferometer according to the present invention is suitable for the OF gyro and hence is expected to be applied to automobile navigation system in many cases. In these cases, the OF gyro must be mounted on the car. Since the OF gyro is sensitive to temperature change and stress change, it is inadvisable to mount the OF gyro in the engine compartment of the car. For practical use, the OF gyro can be mounted on positions illustrated in Fig. 26. In Fig. 26, reference numeral 60 denotes the entire car and reference numeral 61 denotes one of the seats. The first mounting position A is located on the roof of the car 60. An OF gyro mounted on this position is represented by GY₁. Since a large-sized OF ring can be used in this position, the sensitivity of the OF gyro can be raised. However, the temperature rise of the roof in summer might cause problems. The second mounting position B is located beneath a seat 61. An OF gyro mounted on this position is represented by GY₂. The third mounting position C is located in the trunk of the car 60. An OF gyro mounted on this position is represented by GY₃. It is difficult to say which of positions B or C is better. This problem requires further examination. In most of the description, it has been assumed that a solid-state interferometer according to the present invention is used as the OF gyro and the OF gyro is applied to an automobile navigation system. This OF gyro is effective in detecting the position and orientation of a moving object. Accordingly, this OF gyro is useful not only to the automobile navigation system but also to control units such as an industrial robot or various manipulators. Owing to the present invention as described before, the optical system including the optical modulator can be integrated on a single substrate with the exception of the OF ring. Thus, it is possible to provide a solid-state interferometer which has none of the drawbacks of the prior art, which can be produced in a small-sized modular form, which requires only extremely simple construction and adjustment, which exhibits little change in charactristics in use, which can be used as an OF gyro operating stably with high precision, and which is low in cost.

Claims: A solid-state optical interferometer comprising: a loop-shaped optical fiber (4) having end portions (4a, 4b); a solid-state light wave guide (30) optically coupled to said optical fiber (4), said solid-state light wave guide (30) forming a light path in conjunction with said optical fiber (4) for enabling mixing of light received from the end portions (4a, 4b) said optical fiber (4); electrodes (51A, 51B) for producing a surface acoustic wave (SAW), said surface acoustic wave being used to apply acousto-optical modulation to a light beam passing through said solid-state light wave guide (30), said electrodes (51A, 51B) being formed as an interdigitated pair on a surface of a substrate (3) on which said solid-state light wave guide (30) is formed, and a detector circuit for detecting rotation of a body to which the interferometer is fixedly mounted, characterized in that said solid-state light wave guide (30) includes a light diffraction portion (50) provided adjacent to said electrodes (51A, 51B), two input sections of said light wave guide (30) being arranged on said substrate (3) so as to converge under a predetermined angle (2 ϑ B ) and enter said light diffraction portion (50), at least one output section of said light wave guide (30) being coupled to said light diffraction portion (50), wherein light beams (LB₁, LB₂) fed from said two input sections are recombined into an interference beam (LB₁") by said surface acoustic wave (SAW) within said light diffraction portion (50), said interference beam (LB₁") being sent out through said at least one output section of said light wave guide (30). A solid-state optical interferometer according to claim 1, wherein said electrodes (51A, 51B) formed on said substrate (3) are connected to an optical modulation oscillator unit (80), and means are provided for controlling the oscillation output of said oscillator unit (80) by said interference beam (LB₁") as to always cancel a phase difference produced by said recombination of said two interfering light beams (LB₁, LB₂). A solid-state optical interferometer according to claim 1, further comprising a laser beam generating circuit for feeding a laser beam to said light path, said laser beam generating circuit comprising a circuit for continuously varying the wavelength of said laser beam so as to prevent a standing wave from appearing in said light path. A solid-state optical interferometer according to claim 1, wherein the end portions of said loop-shaped optical fiber (4) include first and second end portions respectively coupled to first and second light wave guide portions at confronting end surfaces thereof, said solid-state light wave guide (30) including a holding member (11) having V-shaped grooves (11A, 11B) formed therein for receiving the end portions of said loop-shaped optical fiber (4) therein, the end portions of said loop-shaped optical fiber (4) being fixedly secured within said grooves (11A, 11B). A solid-state optical interferometer according to claim 4, wherein said V-shaped grooves (11A, 11B) are anisotropic silicon etched grooves, and said solid-state light wave guide (30) includes portions coupled together by electrostatic bonding. A vehicle including a rotary motion sensor comprising a solid-state optical interferometer according to any of the preceding claims. A vehicle including a rotary motion sensor according to claim 6, wherein the sensor comprises a circuit for compensating a signal generated in the output of said detector circuit, while no rotational movement is applied to the interferometer.

Inventors: ENDO, AKIRA, KANKE, ATSUSHI, OHO, SHIGERU, SASAYAMA, TAKAO, SHIBATA, TAKANORI
Applicants: HITACHI, LTD.
Application year: 1984
Main classification: G01C19
IPC codes: G02F1, G02B6, G01P9, G01C19, G01B9, G01J3