Fiber optic gyroscope

Abstract: A fiber optic gyroscope includes a light source, an optical fiber coil, a photodetector and an optical integrated circuit substrate (30) having formed thereon an optical waveguide, whereby light from said light source is split into two for supply to the two ends of the optical fiber coil. A reinforcing plate (50) of a material having a thermal expansion coefficient nearly equal to that of said optical integrated circuit substrate (30) is fixed to the optical integrated circuit substrate. The reinforcing plate is held by flexible holding means (12, 52) for displaceably holding said reinforcing plate on a support structure (10). Thus, even if a thermal stress develops in the reinforcing plate, the reinforcing plate is displaced relative to the support structure to thereby absorb a thermal stress.

Description: BACKGROUND OF THE INVENTION The present invention relates to a fiber optic gyroscope which employs an optical waveguide formed as an optical integrated circuit for splitting and coupling light. There has been proposed a fiber optic gyroscope which utilizes, in place of an optical fiber coupler, an optical waveguide formed as an optical integrated circuit, for splitting light from a light source into two for propagation to an optical fiber coil forming a ring interferometer and for coupling two beams of light having propagated through the optical fiber coil for guiding it as interference light to a photodetector (see e.g. GB-A-2 121 532). Fig. 1 shows another example of such a fiber optic gyroscope, in which an optical fiber coil 61 is wound in layers around the periphery of a disc-shaped support structure 20 near one side thereof and a light source 62, a photodetector 63 and an optical integrated circuit substrate 70 are mounted on the support structure 60 inside the optical fiber coil 61. The optical integrated circuit substrate 70 is a rectangular electro-optic crystalline plate as of lithium niobate and has on its one surface an optical waveguide 74 formed by diffusing thereinto, for example, titanium in a desired strip pattern. The optical waveguide 74 comprises a trunk 71, a pair of branches 72a and 72b which diverge from the trunk 71 at one end thereof, and another pair of branches 73a and 73b which similarly diverges from the trunk 71 at the other end. The pair of branches 72a and 72b and the trunk 71 constitute a 3dB coupler, whereas the pair of branches 73a and 73b and the trunk 71 also constitute a 3dB coupler. Optical connectors 76 and 77 are provided at opposite ends of the optical integrated circuit substrate 70. The branches 72a and 72b of the optical waveguide 74 are connected to one and the other ends 61a and 61b of the optical fiber coil 61, and the branches 73a and 73b are connected via optical fibers 78 and 79 to the light source 62 and the photodetector 63, respectively. An electric circuit unit 80 is mounted on the other side of the support structure 60. Though not shown, an electrode pair for phase modulation is provided for either one or both of the branches 72a and 72b of the optical waveguide 74 and the electric circuit unit 80 includes a phase modulation signal generator for supplying a phase modulation signal to the above-mentioned electrode pair and a synchronous detector for synchronous/detection of an output signal from the photodetector 63 by the phase modulation signal to obtain the output of the fiber optic gyroscope. The trunk 71 of the optical waveguide 74 is so formed as to produce a polarization effect. Light from the light source 62 passes through the optical fiber 78 and is applied from the branch 73a of the optical waveguide 74 via the trunk 71 to the branches 72a and 72b, that is, the light is split into two beams. At this time, either one or both of the beams of light are phase modulated by the application of the above mentioned phase modulation signal to the above-mentioned electrode pairs. As a result, the one light is supplied as left-handed light to the optical fiber coil 61 from its one end 61a and the other light is supplied as right-handed light to the optical fiber coil 61 from the other end 61b. The left-handed light and the right-handed light having propagated through the optical fiber coil 61 are supplied to the trunk 71 of the optical waveguide 74 via its branches 72b and 72a from the other and the one end 61b and 61a of the optical fiber coil 61, respectively, whereby they are coupled together. The resulting interference light is applied from the branch 73b of the optical waveguide 74 via the optical fiber 79 to the photodetector 63, wherein it is converted into an electric signal, which is subjected to synchronous modulation by the above-mentioned phase modulation signal, providing the output of the fiber optic gyroscope. Further, a base 91 is mounted on one side of the support structure 60 and the optical integrated circuit substrate 70 is fixedly mounted directly on the base 91 with an adhesive 92 as shown in Figs. 2 and 3. Since the prior art fibre optic gyroscope mentioned above has the optical integrated circuit substrate 70 joined directly to the base 91 with the adhesive 92, accidental heating from the outside, such as an ambient temperature rise, will develop a high thermal stress in the optical integrated circuit substrate 70 owing to the difference in thermal expansion coefficient between the electro-optic crystal forming the integrated circuit substrate 70 and the base 91 to thermally distort the electro-optic crystal and hence change its refractive index, introducing an error in the output of the fiber optic gyroscope. The document US-A-4,750,800 discloses a mounting structure for an integrated optic chip, in which the LiNbO₃ is bonded to a substrate, the substrate in turn being attached to a support package structure. An ultraviolet cured adhesive is used for bonding the chip to the substrate while a compliant adhesive, such as silicone rubber is used for mounting the substrate to the support package structure. The compliant adhesive serves to limit (by absorption) stress transmission from the support package structure to the substrate. The substrate material is chosen from materials having anisotropic thermal expansion characteristics similar or exactly matching those of the IO chip. SUMMARY OF THE INVENTION An object of the present invention is to provide a fiber optic gyroscope of a structure which reduces thermal distortion of an electro-optic crystal forming an optical integrated circuit used for splitting and coupling light. This object is achieved with a fiber optic gyroscope as claimed. According to the present invention, there is provided a fiber optic gyroscope which includes a light source, an optical fiber coil constituting a ring interferometer, a photodetector and an optical integrated circuit substrate whereby light from the light source is split into two beams for supply to the optical fiber coil and the two beams of light having propagated through the optical fiber coil are coupled together for supply to the photodetector. In this fiber optic gyroscope the optical integrated circuit substrate is fixed to a reinforcing plate made of the same electro-optic crystal as that used for the optical integrated circuit substrate, or a material of about the same thermal expansion coefficient as that of the electro-optic crystal, and this reinforcing plate is held by flexible means on a support structure. In such a fiber optic gyroscope in which the reinforcing plate is displaceably held by the flexible means on the support structure, even if a thermal stress develops in the reinforcing plate owing to the difference in thermal expansion between the reinforcing plate and the support structure when the velocity meter is heated externally, the reinforcing plate is displaced relative to the support structure to thereby absorb the thermal stress. At the same time, since the optical integrated circuit substrate and the reinforcing plate are formed of the same electro-optic crystal, or since the reinforcing plate is formed of a material whose thermal expansion coefficient is about the same as that of the optical integrated circuit substrate, no high thermal stress will be produced directly in the optical integrated circuit substrate due the difference between their thermal expansion coefficients. Accordingly, substantially no thermal distortion is developed in the electro-optic crystal forming the optical integrated circuit substrate, practically eliminating the possibility of inducing errors in the output of the fiber optic gyroscope by a change in the refractive index of the electro-optic crystal due to its thermal distortion. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view showing an example of a conventional fiber optic gyroscope; Fig. 2 is a perspective view showing a support structure for an optical integrated circuit substrate in the conventional fiber optic gyroscope; Fig. 3 is a side view of the support structure depicted in Fig. 2; Fig. 4 is a plan view illustrating an example of the optical integrated circuit substrate support structure in the fiber optic gyroscope of the present invention; Fig. 5 is a side view of the support structure depicted in Fig. 4; Fig. 6 is a sectional view showing deformation of a part of the support structure of Fig. 4 when a thermal stress is induced therein; and Fig. 7 is a sectional view illustrating another example of the optical integrated circuit substrate support structure in the fiber optic gyroscope of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 4 and 5 illustrate an example of the optical integrated circuit substrate support structure in the fiber optic gyroscope of the present invention. The optical integrated circuit substrate 30 is fixed to the reinforcing plate 50 with an adhesive or the like. On the top of the support structure 10 there are protrusively provided, by cutting, a bed 11 shorter and narrower than the optical integrated circuit substrate 30 and two pairs of thin plate-like support pieces 12 taller than the bed 11 and opposed there-across on all sides. The reinforcing plate 50 with the optical integrated circuit substrate 30 fixedly mounted thereon is placed on the bed 11 and joined with an adhesive 52 to the thin plate-like support pieces 12 of the support structure 10. The support pieces 12 form flexible holding means. Incidentally, there are defined around the bed 11 spaces for preventing a flow of the adhesive 52. In this embodiment, even if the reinforcing plate 50 and the support structure 10 have different thermal expansion coefficients, the thin plate-shaped support pieces supporting the reinforcing plate 50 are readily deformed by external heat as shown on an enlarged scale in Fig. 6, affording reduction of the stress which is applied to the reinforcing plate 50. Moreover, since the reinforcing plate 50 is formed of the same electro-optic crystal as that used for the optical integrated circuit substrate 30, or a material having a thermal expansion coefficient substantially equal to that of the electro-optic crystal, no great stress will be induced directly in the optical integrated circuit substrate 30 owing to the difference in their thermal expansion, either. Thus, a possible change in the refractive index of the electro-optic crystal by the stress distortion therein will be so small that the fiber optic gyroscope will produce its output with practically no error. Fig. 7 illustrates another example of the optical integrated circuit substrate support structure in the fiber optic gyroscope according to the present invention. The optical integrated circuit substrate 30 is bonded to the reinforcing plate 50 with an adhesive or the like. The support structure 10 has a bed 13 and a combination of a stepped portion 14 and a thin plate-like support piece 15 which support the reinforcing plate 50 at its opposite ends in its lengthwise direction. The reinforcing plate 50 carrying the optical integrated circuit substrate 30 is mounted on the bed 13 and joined, with an adhesive 53, to the stepped portion 14 and the thin plate-like support piece 15 of the support structure 10. This example also produces the same effect as described previously with respect to Figs. 4, 5, and 6. As described above, according to the present invention, the optical integrated circuit substrate is fixedly mounted on a reinforcing plate made of the same electro-optic crystal as that for the optical integrated circuit substrate, or a material of a thermal expansion coefficient nearly equal to that of the electro-optic crystal, and the reinforcing plate is held on a support structure by flexible holding means. Hence, substantially no thermal distortion occurs in the electro-optic crystal forming the optical integrated circuit substrate, and accordingly, there is practically no possibility of inducing an error in the output of the fiber optic gyroscope by a change in the refractive index of the electro-optic crystal which results from its thermal distortion. It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

Claims: A fiber optic gyroscope which includes: a light source (22), an optical fiber coil (21); a photodetector (23); and an optical integrated circuit substrate (30) having formed thereon an optical waveguide (31,32) whereby light from said light source is split into two for supply to one and the other ends of said optical fiber coil and two rays of light having propagated through said optical fiber coil and emitted from said one end and said other end thereof are coupled together for supply to said photodetector; characterised by: a support structure (10); a reinforcing plate (50) of a material having a thermal expansion coefficient nearly equal to that of said optical integrated circuit substrate, said reinforcing plate having fixed thereto said optical integrated circuit substrate (30); and flexible holding means (12,52;15,53) for displaceably holding said reinforcing plate on said support structure, wherein said flexible holding means includes at least two thin plate-like support pieces (12) protrusively provided on the top of said support structure (10) in opposed relation to at least two opposite sides of said reinforcing plate (50) and said reinforcing plate is disposed between said two thin plate-like support pieces and is bonded thereto with an adhesive (52). The fiber optic gyroscope of claim 1 wherein a bed (11) shorter than said two thin plate-like support pieces (12) is protrusively provided on the top of said support structure (10) between said two thin plate-like support pieces, and said reinforcing plate (50) is mounted on said bed. The fiber optic gyroscope of claim 1 wherein said reinforcing plate (50) is formed of the same material as that for said optical integrated circuit substrate (30).

Inventors: ASAMI EIICHI, KAKU RYOJI, OHNO ARITAKA, SAKUMA KAZUHIRO, ASAMI, EIICHI, KAKU, RYOJI, OHNO, ARITAKA, SAKUMA, KAZUHIRO
Applicants: JAPAN AVIATION ELECTRON, JAPAN AVIATION ELECTRONICS INDUSTRY LIMITED
Application year: 1992
Main classification: G01C19
IPC codes: G02B6, G01C19