Optical fiber gyro.

BACKGROUND OF THE INVENTION

0001This invention relates to an integrated optical fiber gyro.

0002An optical fiber gyro for detecting rotational speed,utilizing the Sagnac effect, has various merits. More specifically, the optical fiber gyro is rigid against acceleration because it has no movable parts. Furthermore, it is high in sensitivity, excellent in linearity, simple in construction and low in power consumption. However, it has not been put in practical use.

0003Figure 7 is a plan view showing the arrangement of a conventional optical fiber gyro..

0004As shown in Fig. 7, a single mode optical fiber 40 is wound several turns to form an optical fiber loop. Reference numeral 41 designates a laser. The laser beam is divided into two light beams by a half-mirror 42. One of the two light beams is applied through a lens 43 to one end of the optical fiber 40, thus passing through the latter clockwise (hereinafter referred to as "the clockwise (or CL) beam"), and the other is applied through a lens 44 to the other end of the optical fiber loop, thus passing through the latter counterclockwise (hereinafter referred to as "the counterclockwise (or CCL) beam"). The two light beams passed through the optical fiber loop are then put together by the half-mirror 42 in a manner such that the optical axes thereof coincide.

0005The optical path of the clockwise light beam is equal in length to that of the counterclockwise beam.

0006When the gyro is turned about a normal to the optical fiber loop at an angular velocity Q, the phase difference Δθ between the two light beams is:<maths id="math0001" num=""><img file="EP0092831A2_D0001.tif" /></maths>where L is the length of the optical fiber, a is the radius of the optical fiber, X is the wavelength of the laser beam, and c is the light velocity.

0007The photo-detector 45 provides an output proportional to the square of the sum of the clockwise light beam and the counterclockwise light beam. If the two light beams are equal in amplitude, then the output of the photo-detector is proportional to (1 + cos Δθ), as explained hereafter.

0008Ideally, the amplitude of the clockwise light beam is equal to that of the counterclociwise light beam. The waves of the clockwise and counterclockwise light beams are represented by the following expressions (2) and (3), respectively:<maths id="math0002" num=""><img file="EP0092831A2_D0002.tif" /></maths><maths id="math0003" num=""><img file="EP0092831A2_D0003.tif" /></maths>where ω is the light angular frequency. The output of the photo-detector, being equal to the square of the sum of both, is:<maths id="math0004" num=""><img file="EP0092831A2_D0004.tif" /></maths>

0009The output includes a term in cos Δθ. When Δθ is small, the variation of the component cos A8 is extremely small. That is, when Δθ is small, the sensitivity is low.

0010Accordingly, it should be included, in the form of sin Δθ, in the output. An optical output in such a functional form can be obtained by employing an method in which two light beams are separated and one of the two light beams is shifted by π/2 in phase.

0011Figure 7 is a plan view showing a conventional optical fiber gyro of this type.

0012Similarly as in Fig. 7, a light beam from a laser 41 is divided into two light beams by a beam splitter 42, and the two light beams are applied through condenser lenses 43 to both ends of a single mode optical fiber loop 40.

0013The aforementioned beam splitter 42 and three additional beam splitters 46, 46 and 47 are arranged at the four corners of a square. The clockwise (CW) light beam passes along two opposed sides of the square, and the counterclockwise (CCW) light beam passes along the remaining two opposed, sides. That is, the optical path for the CW light beam is separated from that for the CCW light beam. A phase shifter is inserted in one of the optical paths to shift the phase of the laser beam by π/2.

0014The shifted wave can be represented by the following expression (5) instead of (3):<maths id="math0005" num=""><img file="EP0092831A2_D0005.tif" /></maths>Expression (2) is added to expression (5), and the result of addition is squared, so that the output I₂(t) of photo-detector 45 is provided:<maths id="math0006" num=""><img file="EP0092831A2_D0006.tif" /></maths>

0015In the optical systems of Figs. 7 and 7a, the laser, the photo-detector and the half-mirrors are made up of ordinary optical components.

0016Since the optical fiber gyro must have excellent vibration resistance, the optical components are fixed on a large, heavy bench by means of magnet.stands. Accordingly, the optical fiber gyro is bulky, heavy and incovenient to handle as a whole, thus not being practical. Furthermore, the optical fiber gyro is disadvantageous in that it is rather difficult to adjust the optical axes, which are liable to shift.

0017When the relative position of the laser, beam splitter, condenser lens and optical fiber varies, the coupling efficiency of the optical fiber is varied, and the output of the photo-detector drifts accordingly.

0018For the same reason, the overlapping part of the clockwise and counterclockwise light beams is changed in area on the photo-detector, and accordingly the output of the photo-detector drifts, making measurement unreliable.

0019Even when a system of detecting (1 + sin Δθ) is employed, the output drift due to the instability of the optical system cannot be distinguished from the variation of the phase difference Δθ, because the DC component of the photo-detector output is observed. The drift results in . the occurrence of an error in the measurement of A6.

0020It is rather difficult to manufacture and assemble the conventional optical fiber gyro.

0021In the conventional optical fiber gyro, the optical system is made up of discrete optical components. Accordingly, it is difficult to adjust the optical axis during assembly.

0022It is also difficult to adjust the angle of the phase: shifter. With the phase shifter in the conventional optical fiber gyro, the parallel glass plate is inclined to adjust the substantial thickness in the optical path direction, thereby to obtain the desired phase difference π/2. However, since the optical wavelength is extremely short, it is considerably difficult ot obtain the phase difference π/2 with high accuracy. Thus, when the glass plate is inclined, the output of the photo-detector drifts.

SUMMARY OF THE INVENTION

0023Accordingly, one object of this invention is to provide an optical fiber gyro eliminating the above problems, in one embodiment of which a laser diode, a photo-diode, gratings and a modulating element are provided on one substrate,to modulate one light beam, whereby the S/N ratio may be improved.

0024Three modulation methods, namely, amplitude modulation, phase modulation and frequency shift modulation are available, and any one of these modulation methods may be employed in the invention.

0025In an optical fiber gyro according to further embodiments, optical wave-guides and a light branching element or elements are formed on one substrate by a photolithographic technique, and the light emitting element and the light receiving element are integral with the substrate.

0026In a further fiber gyro of the invention,optical wave-guides, light branching elememts, a phase shifter, etc. are formed on one substrate having an electro-optic effect by photolithography, and a light emitting element and a light receiving element are bonded to the end faces of the substrate or formed on the substrate, so that the optical systems are provided in monolithic form.

BRIEF DESCRIPTION OF THE DRAWINGS

0027<ul id="ul0001" list-style="none"><li>Fig. 1 is a perspective view of the substrate of an optical fiber gyro and its relevent components according to a first embodiment of this invention;</li><li>Fig. 2 is a perspective view of the substrate of an optical fiber gyro and its relevent components according to a second embodiment of the invention;</li><li>Fig. 3 is an enlarged plan view of a directional coupler type amplitude modulator of the gyro in Fig. 2;</li><li>Fig. 4 is an enlarged plan view of a Mach-Zehnder interferometer type amplitude modulator of the gyro in Fig. 1;</li><li>Fig. 5 is a plan view outlining an optical fiber gyro using a phase modulator;</li><li>Fig. 6 is a plane view outlining an optical fiber gyro in which a surface acoustic wave transducer is set near an optical wave-guide in order to shift the frequency;</li><li>Fig. 7 is a plan view of a conventional optical fiber gyro fabricated by combining individual optical elements;</li><li>Fig. 7a is an explanatory diagram showing the optical system of another conventional gyro using a phase shifter;</li><li>Fig. 8 is a perspective view of an optical fiber gyro according to another embodiment on this invention, in which the light branching element is a diffraction grating;</li><li>Fig. 9 is a perspective view of an optical fiber gyro according to an embodiment of the invention which employs directional coupler type light branching elements;</li><li>Fig. 10 is a perspective view of an optical fiber gyro according to an embodiment of the invention which employs Y-shaped branching paths;</li><li>Fig. 11 is a perspective view of an optical fiber gyro according to an embodiment of the invention in which the optical systems are all monolithically provided on a GaAs single crystal substrate;</li><li>Fig. 12 is a plan view showing the arrangement of another example of an optical fiber gyro according to this invention;</li><li>Fig. 13 is a perspective view showing a still further example of the optical fiber gyro according to the invention, in which optical elements are formed on one and the same substrate;</li><li>Fig. 14 is a plan view outlining a light branching element provided according to a Y-shaped branching system; and</li><li>Fig. 15 is a plan view outlining a light branching element provided according to an evanescent coupling system.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0028This invention will now be described with reference to preferred embodiments thereof.

0029Fig. 1 is a perspective view of the substrate of an optical fiber gyro according to a first embodiment of the invention.

0030The substrate 1 has electro-optic effects, and is formed of a crystal of GaAs, InP, LiNb0₃, Bi₁₂SiO₂₀ or Bi₁₂GeO₂₀, etc.

0031Optical wave-guides 2, 3, 4 and 5 are formed on the substrate 1 in a manner such that they cross one another as shown in Fig. 1.

0032A grating 6 serving as a half-mirror is provided at the intersection of the two optical wave-guides 2 and 5 such that it extends in the direction of the bisector of the angle formed by the two optical wave-guides 2 and 5. Similarly, gratings 7, 8 and 9 serving as half-mirrors are provided at the intersections of the optical wave-guides 2 and 3, 3 and 4, and 4 and 5, respectively. Each grating, having a number of parallel lines arranged at equal intervals, can diffract a light beam of a predetermined wavelength. In this embodiment, each grating operates to bend a light beam from one optical wave-guide to another.

0033A laser diode 10 in the form of a chip is bonded to the "starting" end of the optical wave-guide 5. The light beam from the laser diode 10 is divided into a clockwise light beam r and a counterclockwise light beam 1. The optical wave-guide 5, in which the clockwise light beam r propagates, is partially deformed, or branches partially, to form a modulator 11 for light amplitude modulation.

0034Fig. 4 is an enlarged plan view of the modulator. The optical wave-guide 5 branches into optical wave-guides 5a and 5b on which electrodes 12a and 12b are provided by vacuum-evaporation, printing or other methods. A modulation signal generating IC (integrated circuit) 13 (Fig. 1) is fixedly secured to the substrate. The IC 13 operates to apply a voltage to the electrodes 12a and 12b, to thereby change the refractive indexes of the optical wave-guides 5a and 5b by the electro-optic effect.

0035In each of the optical wave-guides 5a and 5b, the light velocity is inversely proportional to the refractive index. Accordingly, when different electric fields are applied to the optical wave-guides 5a and 5b, the light velocity in the optical wave-guide 5a is different from that in the optical wave-guide 5b. The two optical wave-guides 5a and 5b are thereafter joined into one optical wave-guide; however, the light beam passed through the former is different in phase from that passed through the latter. Accordingly, the amplitude of the combined light beams is not the simple sum of the amplitudes of the light beams from the two optical wave-guides 5a and 5b. In the ideal case, the amplitude of the combined light beams is a function including the cosine of the phase difference ¢.

0036Accordingly, if the voltages applied to the two electrodes 12a and 12b are changed periodically so as to periodically change the phase difference φ, then the amplitude of the light beams passed through the optical wave-guide branches 5a and 5b are periodically changed.

0037The optical wave-guide branches 5a and 5b form a Mach-Zehner interferometer.

0038The clockwise light beam is subjected to amplitude-modulation by the modulator 11 as described. An optical element for applying an alternating electric field to optical wave-guide branches to perform such amplitude-modulation is well known in the art.

0039The counterclockwise light beam is not amplitude-modulated.

0040A photo-diode 14 is bonded to one end of the optical wave-guide 4, which is close to the.intersection of the optical wave-guides 4 and 3. The output of the photo-diode 14 is amplified and suitably processed by a detection signal processing IC 15. The clockwise light beam r, after being modulated while passing through the optical wave-guide 5, enters an optical fiber 16. The optical fiber 16 is wound several turns to form an optical fiber loop (in Fig. 1, the loop is not shown).

0041After passing through the optical fiber loop 16, the clockwise light beam r is returned into the optical wave-guide 2 and is then bent by the grating 7, to pass into the optical wave-guide 3. The light beam r is further bent by the grating 8, to reach the photo-diode 14. The counterclockwise light beam 1 which is provided by the grating 6 is applied through the optical wave-guide 2 to the optical fiber loop 16. The counterclockwise light beam 1, after passing through the optical fiber loop 16 counterclockwise, reaches the end of the optical wave-guide 5, and is then bent from the optical wave-guide 5.into the optical wave-guide 4 by the grating 9, to reach the photo-diode 14.

0042The photo-diode receives the clockwise beam r and the counterclockwise beam 1 simultaneously. One of the two light beams r and 1 has been subjected to amplitude-modulation.

0043When the fiber loop turns at an angular velocity Q, a phase difference Δθ is provided between the light beams r and 1.

0044The output of the photo-diode includes an electrical signal having an angular frequency equal to the modulation angular frequency wm. The amplitude of the signal includes the phase difference Δθ. Accordingly, the phase difference Δθ can be obtained by detecting the amplitude.

0045This function will be described in more detail.

0046When the clockwise beam and the counterclockwise beam come out of the optical wave-guides 5 and 2, respectively, they can be represented by the following expressions: <ul id="ul0002" list-style="none"><li>(clockwise) (1 + a sin _wm t) e^iωt</li><li>(counterclockwise) _eⁱw^t</li></ul> where w is the angular frequency of the laser beam, and wm is the modulation angular frequency of the modulator 11.

0047It is assumed that, when the clockwise light beam and the counterclockwise light beam pass through the optical fiber loop respectively clockwise and counterclockwise, the former light beam leads by Δθ/2 in phase and the latter light beam lags by Δθ/2 in phase. As is clear from expression (1), the phase difference Δθ is proportional to the angular velocity Ω of the fiber loop.

0048Therefore, then the light beams r and 1 enter the photo-diode, they can be represented by the following functional expressions: (clockwise) E_r(1 + a _si_n ωm t) e^{i(ωt + Δθ/2)} (counterclockwise) _Er e^{i(ωt - Δθ/2)}

0049The photo-diode performs square-law detection. Therefore, the square of the sum of both input signals is the value of the electrical signal output by the photo-diode. Thus, the output electrical signal I is:<maths id="math0007" num=""><img file="EP0092831A2_D0007.tif" /></maths>That is, the output electrical signal includes DC components, ωm components and 2ωm components.

0050The ωm components in the output electrical signal can be obtained by generating an electrical signal having an angular frequency ωm as a reference signal, and applying the same to the output electrical signal I (expression (7)) to electrically perform synchronous detection. The amplitude of the ωm components is: 2 E_r² a + 2 E E1 a cos Δθ Thus, the phase difference Δθ can be detected.

0051Either the clockwise or counterclockwise light beam can be amplitude-modulated as in a second embodiment of the invention shown in Fig. 2. In this embodiment, a directional coupler type amplitude modulator 17 is employed.

0052In the second embodiment, the optical wave-guide 5 is divided into optical wave-guide parts 5c and 5d (_Fig. 3), which are adjacent to each other at end portions where electrodes 18c and 18d are provided.

0053When two optical wave-guides are juxtaposed adjacent to each other on the order of the wavelength, they are coupled to each other through evanescent waves in the intermediate region of low refractive index therebetween. This is known as "evanescent wave coupling". When the two optical wave-guides are equal in optical phase constant, the coupling is maximum, and, ideally, all the energy in one optical wave-guide is transferred to the other optical wave-guide.

0054The optical phase constant is proportional to the refractive index. The refractive index can be varied by controlling the electric field when the optical wave-guide has an electro-optic effect.

0055A modulation signal generating IC 19 applies a modulation voltage to the electrode 18c and 18d. The amplitude of the light beam transmitted to the optical wave-guide 5d from the optical wave-guide 5c changes with the modulation angular frequency oom. That is, in this embodiment, the clockwise beam is subjected to amplitude-modulation.

0056In the embodiment shown in Fig. 2, no grating is employed. The optical wave-guides 2 and 3 are not perpendicular to each other at the intersection, but they are smoothly connected in such a manner that the directions of the center lines are made to coincide with each other. The optical wave-guides 3 and 4, 4 and 5, and 5 and 2 are connected in the same manner.

0057In the embodiments shown in Figs. 1 and 2, the S/N ratio is increased by employing amplitude modulation. However, phase modulation may be employed. More specifically, a phase modulator 20 may be provided for one of the clockwise and counterclockwise light beams as shown in Fig. 5.

0058As the optical wave-guide has an electro-optic effect, the refractive index changes with the electric field. When a voltage is applied across electrodes 21 and 22, the refractive index of that part of the wave-guide which is between the two electrodes is changed, to change the light velocity, whereby phase modulation can be achieved.

0059The magnitude of phase modulation is: b sin ωm t The counterclockwise beam and the clockwise beam can be expressed as follows: <ul id="ul0003" list-style="none"><li>(counterclockwise beam) e^{i(ωt + Δθ/2)}</li><li>(clockwise beam) e^{i(ωt - Δθ/2} + b sin wm t)</li></ul>

0060When square-law detection is effected with a photo-detector, then the output signal includes the following term: J₁(b) sin Δθ sin ωm t where J₁(b) is the Bessel function, and wm is the modulation frequency.

0061Therefore, sin Δθ can be obtained by conducting electrical synchronous detection with a reference signal having the frequency ωm.

0062In the above-described embodiments, one of the clockwise beam or the counterclockwise beam is amplitude-modulated or phase-modulated.

0063An optical hererodyne method utilizing optical frequency shift may also be employed in the invention.

0064Fig. 6 is an explanatory diagram outlining a surface acoustic wave transducer - utilizing an optical frequency shift method.

0065An interdigital electrode 23 is provided beside an optical wave-guide 5e. A drive circuit applies an AC voltage having a frequency ωm to the electrode 23. The substrate 1 should exhibit an acousto-optic effect.

0066When the AC voltage is applied to the interdigital electrode 23, a surface acoustic wave advancing in the direction of the broken line is generated. As the substrate 1 has an acousto-optic effect, it has a region whose refractive index is changed by the surface acoustic wave. Thus, the surface acoustic wave and the light beam interact with each other. The light beam is modulated so that the conservation law of the light beam and surface acoustic wave with respect to wave number and frequency is established.

0067The light beam in the wave-guide 5e advances to the wave-guide 5f, which is inclined with respect to the wave-guide 5e, and simultaneously the angular frequency is shifted to (ω + ωm).

0068When the frequency is shifted as described above, the light beam in the optical wave-guide 5f can be expressed as follows: e^i(ω + wm)t

0069After passing through the optical fiber loop, the two light beams are: <ul id="ul0004" list-style="none"><li>(counterclockwise) E₁ e^{i(ωt + Δθ/2)}</li><li>(clockwise) E_r e^i(ω+ ωm) t - Δθ /2. (clockwise) E_r e^{i(ω + ωm) t - Δθ /2}.</li></ul>

0070As the photo-diode subjects the sum to square-law detection, the output signal of the photo-diode includes the folowing term: ^E₁ E_r cos (wm t - Δθ)

0071This output is electrically compared with the reference signal (cos ωm t), to obtain the phase difference Δθ.

0072This method is disadvantageous in that the signal processing system is rather intricate; however, it is advantageous in that the operation is scarcely affected by level drift because the output of the photo-detector is an AC signal (having the frequency wm). Furthermore, the method has merit in that its phase difference detection range is wide.

0073Thus, according to the invention, an optical fiber gyro is fabricated monolithically by providing the optical wave-guides, the laser diode, the photo-diode and the modulator on a substrate having an electro-optic effect. Therefore, the optical fiber gyro according to the invention has the following merits or effects: <ul id="ul0005" list-style="none"><li>(1) The device is small in size, small in weight and durable against vibration. In the device, adjustment of the optical axis is unnecessary, because the active elements, namely, the laser diode, the photo-diode, the optical wave-guides and the modulator are intergrated on one substrate.</li><li>(2) The optical fiber gyro is high in S/N ratio, because a modulator, such as an optical amplitude modulator, an optical phase modulator or a frequency shifter is inserted in the optical path of one of the clockwise and counterclockwise light beams.</li><li>(3) The modulation signal generating circuit and the detection signal electrical processing circuit can be monolithically arranged by employing a semiconductor such as GaAs or InP to form a substrate having the electro-optic effect.</li></ul>

0074Fig. 8 shows an optical fiber gyro according to another embodiment of the invention.

0075In this optical fiber gyro, the light branching element is a diffraction grating, and the substrate 61 is made of a material such as Li_Nbo₃ which allows the passage of a laser beam and can form optical wave-guides 62.

0076The optical wave-guides 62 are provided in the form of a cross (+) on the substrate. In this embodiment, _Ti is diffused in a substrate of LiNbo₃, to form two straight parts in the substrate which are higher in refractive index than the substrate, thus providing the optical wave-guides.

0077A diffraction grating 63 is provided at the intersection of the two optical wave-guides, in a manner such that it forms a 45° angle with each of the optical wave-guides 62.

0078The function of the diffraction grating 63 corresponds to that of the beam splitter in the above-described conventional optical fiber gyro.

0079The diffraction grating 3 is formed by the following method: Photoresist, which is lower in refractive index than the substrate, is coated on the substrate, and is then subjected to two-beam interference exposure, to form interference fringes arranged at certain intervals. The portions of the photo resist which have been subjected to the exposure, or those portions not subjected to the exposure are removed by developing, to form the diffraction grating,

0080The two-beam interference technique mentioned above is as follows: A laser beam is divided into two beams, which are changed into parallel beams of larger diameter with the aid of collimator lens. The two light beams thus provided are applied to the photo-sensitve material in a manner such as to form an incident angle + 6 with respect to the normal, so that interference fringes are formed on the photo-sensitive material.

0081A light emitting element 64 and a light receiving element 65 are connected to ends of the crossed optical wave-guides 62, 62, respectively. The light emitting element 65 may be a PIN photo-diode or an avalanche photo-diode. These elements are connected, in the form of a chip, to the ends of the optical wave-guides 62.

0082Similarly to the conventional optical fiber gyro, an optical fiber loop is fabricated by winding a single mode otical fiber several turns, the two ends of the optical fiber loop being connected to the remaining ends of the optical wave-guides 62.

0083Instead of the above-described diffraction grating, a directional coupler type light branching element may be employed.

0084Fig. 9 is a perspective view of an optical fiber gyro according to a further embodiment of the invention, which utilizes such a light branching element.

0085As shown in Fig. 9, four optical wave-guides 77, 78, 79 and 70 are provided on a substrate 1. Each wave- guide is bent through 90° near the center of the substrate 1. None of the optical wave-guides are crossed. However, pairs of optical wave-guides are adjacent to each other, and a light beam is shifted from one of the optical wave-guides to the other via evanescent waves.

0086In this embodiment, the optical wave-guide 77 is laid between the light emitting element 74 and the fiber end A. The clockwise (C_W) light beam goes through a the optical wave-guide 77 to the optical fiber loop 76.

0087The optical wave-guide 78 consists of a first part adjacent to the optical wave-guide 77 and a second part extended to the fiber end B. Therefore, the counterclockwise (CCW) beam is transferred from the optical wave-guide 77 to the optical wave-guide 78 through evanescent waves, thus entering the optical fiber loop 76 through the fiber end B.

0088The CW light beam, after passing through the optical fiber loop, enters the optical wave-guide 78 through the fiber end B. The optical wave-guide 78 is parallel with the optical wave-guide 70 with a small gap therebetween. Therefore, the light beam is shifted from the optical wave-guide 78 to the optical wave-guide 70 through evanescent waves.

0089The CCW light beam, after passing through the optical fiber loop, is shifted to the optical wave-guides 77, 79 and 70, in the stated order, through evanescent waves.

0090As is apparent from the above description, directional couplers are formed between the optical wave-guides 77 and 78, 78 and 70, 79 and 70, and 79 and 77, respectively. These four directional couplers serve as light bianching elements, thus corresponding to the beam splitter in the conventional optical fiber gyro.

0091The light branching element may also be a Y-shaped branching path.

0092Fig. 10 is a perspective view of an optical fiber gyro according to a further embodiment of the invention, which employs such light branching elements.

0093As shown in Fig. 10, optical wave-guides 811, 812, 813, 814 extend from the light emitting element 84, fiber end A, light receiving element 85 and fiber end B perpendicularly to the end faces of the substrate, respectively. Optical wave-guides 815, 816,817 and 818 are formed on the substrate in a manner such that they connect the optical wave-guides 811, 812, 813 and 814 as shown in Fig. 10.

0094These optical wave-guides provide the aforementioned Y-shaped branching paths 819, 820, 821 and 822.

0095The Y-shaped branching path 819 is provided at the joint of the optical wave-guides 811, 815 and 818.

0096The laser beam from the light emitting element 4 is divided into two light beams at the Y-shaped branching path 819. One of the two light beams goes through the optical wave-guides 815 and 812 to the fiber end A, while the other goes through the wave-guides 818 and 814 to the fiber end B. The light beams, after passing through the optical fiber loop 86, return through the fiber ends B and A to the optical wave-guides 814 and 812, respectively. Parts of the light beams, which have been allowed to pass through the optical wave-guides 817 and 816 at the Y-shaped branching paths 22 and 20, respectively, enter the light receiving element 85.

0097In the above-described embodiment, the light emitting element and the light receiving element are not integrated in the substrate; that is, the chips are bonded to the substrate. However, the light emitting element, the receiving element and the substrate may all.be made from one compound semiconductor crystal. That is, they can be provided in monolithic form.

0098Fig. 11 is a perspective view of an optical fiber gyro according to a fourth embodiment of the invention, which uses a GaAs substrate.

0099The substrate 91 comprises an electrode 923, an n-GaAs layer 924, an n-Ga_0.2Al_0.8As layer 925 and an n-GaAs layer 926, an optical waveguide, which are arranged in the stated order from bottom to top,

0100A light emitting element 94 has the same layer structure as a light receiving element 95.

0101The light emitting element 94 is a double heterostructure junction laser which comprises an active layer, namely, an n-GaAs layer 928, a p-Ga_0.2Al_0.8As layer 929, a p-GaAs layer 930 and an electrode 931 which are arranged in the stated order from bottom to top. The laser beam is oscillated from the active (n-GaAs) layer 928, and is transmitted to the optical wave-guide by evanescent coupling.

0102In the light receiving element 95, the n-GaAs layer 928 is an absorption layer, and coupling with the optical wave-guide is effected through evanescent waves.

0103As the optical systems are optically integrated, the optical fiber gyro according to the invention is small in size, small in weight, high in reliability and high in stability.

0104In the optical systems, the number of elements requiring optical axis adjustment is decreased, or it becomes completely unnecessary to adjust the optical axes thereof. Accordingly, the optical fiber gyro can be readily manufactured or assembled.

0105Fig. 12 is a plan view showing another example of an optical fiber gyro according to this invention.

0106Two parallel optical wave-guides 102 are formed horizontally on a substrate 101 having an electro-optic effect, and two parallel optical wave-guides 102 are formed vertically on the substrate, so that the wave-guides cross at four points.

0107Four light branching elements 103 are disposed at the intersections of the optical wave-guides 102; that is, at the four corners of the square. The two groups of optical wave-guides 102 are separated from each other. A phase shifter 106 is provided on one of the two groups of optical wave-guides 102.

0108The material of the substrate is, for instance, crystalline LiNbo₃.

0109The optical wave-guides may be formed by a Ti diffusion method, for instance. More specifically, each optical wave-guide is provided by forming a linear part on the substrate, which is higher in refractive index than the remaining portions.

0110In this example, diffraction gratings are employed as light branching elements 103. The diffraction gratings may be provided by forming parallel grating lines by mechanical means. However, the diffraction gratings can be more readily formed by photolithography. A photoresist lower in refractive index than the optical wave-guide is applied to the intersections of the optical wave-guides, interference fringes of desired period are formed by two-beam interference exposure, and the photoresist which is not hardened is then washed away. Thus, the diffraction Gratings formed of photoresist may be employed as the light branching elements.

0111The light emitting element 104 is a laser diode or a superluminescent diode in the form of a chip bonded to one end face of the substrate. It goes without saying that a container including the chip may be bonded to the end face of the substrate. In this connection, it is essential that the light emitting element be firmly bonded to the substrate.

0112The optical fiber gyro further comprises a light receiving element 105 which is, for instance, an ordinary silicon photo-diode. The element 105, which is in the form of a chip, or is put in a suitable container, is bonded to the end of the predetermined optical wave- guide 102. The light receiving element may be a photo- diode of a compound semiconductor such as GaAlAs.

0113The optical fiber gyro further comprises a phase shifter 106. The phase shifter includes a pair of electrodes 107 arranged on both sides of the separated optical wave-guide 102 in a manner such that they are parallel with the optical wave-guide, and a power source 108 for applying a DC voltage across the electrodes. The power source is provided outside the substrate 101 and is connected through suitable lead wires to the electoodes 107.

0114The electrodes may, for example, be aluminum plates coated with insulating material.

0115Upon the application of a DC voltage across the electrodes, a strong electric field is generated in the space between the electrodes. As the substrate has an electro-optic effect, the refractive index changes in proportion to the intensity of the electric field. Each optical wave-guide 102 is formedby a Ti diffusion method, so that it is larger in refractive index than other regions of the substrate, and confine light therein, as described before. As the refractive index is changed by the electric field, the velocity of the light passing through the optical wave-guide is changed.

0116Thus, the phase can be shifted as desired by controlling the applied voltage.

0117The optical fiber loop 109 is fabricated by winding a single mode optical fiber several turns. The two ends of the optical fiber loop are connected to the end faces of the predetermined optical wave-guides 102 on the substrate. Rod lenses may be employed so that the light beams in the optical wave-guides are concentrated and then applied to the optical fiber loop 109.

0118The substrate may be of any material which provides an electro-optic effect. For instance, the material of the substrate may be a dielectric such as LiNbO₃ or Bi₁₂Si0₂₀, or a compound semiconductor such as GaAs or InP.

0119In the case where the substrate is made of a compound semiconductor, all of the optical elements can be formed on one and the same substrate as described above; that is, the optical elements can be provided in monolithic form.

0120Fig. 13 shows another example of the optical fiber gyro according to the invention. In the example, the substrate is of n-type GaAs.

0121Similarly to the above-described example, the light emitting element 104 is a GaAs laser diode; however, it should be noted that the element 104 is provided on an optical wave-guide on the substrate.

0122The substrate comprises an electrode 1010, a thick n-GaAs layer 1011, an n-Ga_0.2Al_0.8As layer 1012 and an n-GaAs optical wave-guide layer 1013 which are laid one on another in the stated order.

0123The light receiving element 105 is a GaAs photo- diode, instead of a silicon photo-diode.

0124The Clad layer 102(1014) is an n-Ga0.2 Al_0.8As layer. The light receiving element 105 on the clad layer 102 comprises an n-GaAs layer (absorbing layer) 1015, a p-Ga_0.2Al_0.8As layer 1016, a p-GaAs layer 1016 and an electrode.

0125As the light receiving element corresponds in structure to the light emitting element, all of the optical system can be provided in monolithic form by photolithography.

0126In the example shown in Figs. 12 and 13, the light branching elements 103 are diffraction gratings; however, a Y-shaped branching system 1020 as shown in Fig. 14 may be employed.

0127Furthermore, an evanescent coupling system 1021 as shown in Fig. 15 may be employed. When two optical wave-guides 1022 and 1023 are adjacent to each other, an evanescent wave may be transmitted from one wave-guide to the other, whereby the energy is transmitted. Thus, the branching element may be formed by employing an evanescent coupling system.

0128As the optical systemsare integrated on one substrate, the optical fiber gyro is small in size, small in weight, high in reliability and excellent in stability.

0129It is completely or substantially unnecessary to adjust the optical axes of the optical systems. Accordingly, the optical fiber gyro of the invention can be manufactured and assembled with ease.

0130The amount of phase shift can be continuously changed by controlling the applied voltage; that is, it can be readily adjusted, Thus, the optical fiber gyro of the invention is high in sensitivity in a low angular speed range.

0131The optical fiber gyro of the invention may be installed on automobiles, ships and airplanes, to measure rotational angular speeds or rotational displacements.

0132Furthermore, the optical fiber gyro may be installed on a moving object such as a robot. In this case, it is used as a position sensor for detecting the position of the moving object.