Optical fiber gyroscope

Abstract: In an optical fiber gyroscope, the sensitivity thereof is optimalized with a phase bias afforded by utilizing the phase characteristics of an optical branching/coupling unit (20). Further, the fluctuation of the intensity of a light source (10) and the error and secular change of the phase characteristics of the optical branching/coupling unit (20) are compensated by an electronic circuit. Moreover, an optical fiber loop (30) is received in a heat insulation casing (3, 4, 5) so as not to be affected by an ambient temperature change, thereby to prevent the drift of the optical fiber gyroscope attributed to the temperature change. Thus, the optical fiber gyroscope of high measurement accuracy is provided.

Description: BACKGROUND OF THE INVENTION: The present invention relates to improvements in a small-sized optical fiber gyroscope wherein an azimuth etc. are detected by utilizing an optical interference. Heretofore, an optical fiber gyroscope which utilizes an optical interference for detecting an azimuth from the change of a rotating angular velocity has had its measurement based on the interference of two light waves which propagate in directions reverse to each other through an optical fiber loop. In other words, the rotating angular velocity is found by measuring the phase difference of interference light which is developed by rotations. In measuring the phase difference by means of the optical fiber gyroscope in this manner, a phase bias needs to be given beforehand. As an expedient for affording the phase bias, there has been known a technique wherein, as stated in the official gazette of Japanese Patent Application Laid-open No. 135912/ 1983 or "Applied Optics", 20, (1981) pp. 4313-4318, the two light waves to form the interference light are once guided to separate optical paths and passed through phase plates in the optical fiber gyroscope, thereby to be endowed with the predetermined phase bias. Besides, for enhancing the detection accuracy of the optical fiber gyroscope, it is necessary to consider the influence of feedback light, the compensation of a drift attendant upon any change in the environment of the gyroscope, etc. In these regards, there have been known techniques disclosed in the official gazettes of Japanese Patent Applications Laid-open No. 94680/1981, No. 228113/1984, No. 134513/1990, etc. SUMMARY OF THE INVENTION: An object of the present invention is to provide, an optical fiber gyroscope, having a construction which can afford a stable phase bias without splitting an optical path. Another object of the present invention is to provide an optical fiber gyroscope in which the influence of light fed back from the optical system of an interferometer to a light source can be eliminated without reducing the average light quantity of the light source by half. These objects are accomplished with an optical fiber gyroscope as claimed in claim 1. Dependent claims are directed on features of preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS: The present invention will be more apparent from the following detailed description, when taken in conjunction with the accompanying drawings, in which: Fig. 1 shows the first embodiment of an optical fiber gyroscope according to the present invention; Fig. 2 shows the second embodiment of the optical fiber gyroscope of the present invention; Fig. 3 shows the third embodiment of the optical fiber gyroscope of the present invention; Fig. 4 is a diagram for explaining the operation of the third embodiment; Fig. 5 shows the fourth embodiment of the optical fiber gyroscope of the present invention; Fig. 6 shows an example of a method of fabricating an optical branching/coupling unit according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS: Now, an embodiment of an optical fiber gyroscope according to the present invention will be described with reference to Fig. 1. Referring to the figure, numeral 10 designates a coherent light source, numeral 11 a light source driving circuit, numeral 20 an optical branching/ coupling unit, numeral 30 an optical fiber loop, numeral 40 a photodetector, and numeral 50 a signal processing circuit. In the embodiment of Fig. 1, the coherent light source 10 is driven by the light source driving circuit 11. Light from the coherent light source 10 is guided to the first terminal 21 of the optical branching/ coupling unit 20. The light having entered the optical branching/coupling unit 20 via the terminal 21 is bifurcated in this unit, and the resulting light beams emerge from the second terminal 22 and the third terminal 23, respectively. The first light beam which is the emergent light beam from the second terminal 22 is light transmitted through the optical branching/coupling unit 20, while the second light beam which is the emergent light beam from the third terminal 23 is light coupled in the optical branching/coupling unit 20. The phase difference between the first and second light beams is determined by the phase characteristics of the optical branching/coupling unit 20. In this embodiment, the optical branching/coupling unit 20 is designed and fabricated so as to afford 45° as the phase difference. The first and second light beams enter both the ends of the optical fiber loop 30, respectively, and they propagate in directions reverse to each other through the optical fiber loop 30. Thereafter, the first light beam reaches the terminal 23, while the second light beam reaches the terminal 22. Accordingly, both the light beams enter the optical branching/coupling unit 20 again via the opposite terminals, respectively. The first light beam having entered the optical branching/coupling unit 20 via the third terminal 23 is bifurcated in this unit into the fourth light beam which is transmitted light to emerge from the fourth terminal 24, and the fifth light beam which is coupled light to emerge from the first terminal 21. On the other hand, the second light beam having entered the optical branching/coupling unit 20 via the second terminal 22 is similarly bifurcated in this unit into the sixth light beam which is transmitted light to emerge from the first terminal 21, and the seventh light beam which is coupled light to emerge from the fourth terminal 24. The fourth and seventh light beams emergent from the terminal 24 form interference light, which is detected by the photodetector 40. On the other hand, the fifth and sixth light beams emergent from the terminal 21 are fed back to the coherent light source 10. Here, the optical branching/coupling unit 20 used in this embodiment is a symmetric reciprocal transducer wherein, whichever terminal the light beam enters this unit via, the phase difference between the transmitted and coupled light beams is equal and is 45° in this case. In the propagation process of the light stated above, the phase difference between the fourth and seventh light beams constituting the interference light will be investigated. The fourth light beam has been transmitted through the optical branching/ coupling unit 20 twice, while the seventh light beam has been coupled in the optical branching/coupling unit 20 twice. Accordingly, the total phase difference of the fourth and seventh light beams based on the optical branching/coupling unit 20 is 45° × 2 = 90° in the case of this embodiment. When the whole optical system is rotating, a phase difference  s owing to the Sagnac effect is added to the aforementioned phase difference 90°, and the phase difference of the interference light becomes: ( s + 90°) in all. Meanwhile, letting  s denote a phase difference to-be-measured and letting I and ν denote the intensity and visibility of interference light, respectively, the output signal P of an optical interference sensor is generally expressed by Eq. (1): P = K I {1 + ν cos  s } where K denotes a constant. Thus, the output signal P responds to the variation of the phase difference to-be-measured  s in terms of a cosine function. In the vicinity of  s = 0, accordingly, the sensitivity of the optical interference sensor lowers drastically, and the detection of a low rotational angular velocity becomes difficult in the case of the optical fiber gyroscope. It is therefore favorable for the detection of the very small phase difference  s to bias the phase difference of the optical interference sensor by any means so that the output signal P of the optical interference sensor may vary as a sine function in response to the phase difference to-be-measured  s as indicated by Eq. (2): P = K I {1 + ν cos ( s + 90°)} = K I {1 + ν sin  s } Herein, according to the optical fiber gyroscope of this embodiment, the phase bias of 90° can be afforded without especially installing a phase modulator, as described before, and an interference signal P which is delivered as the output of the photodetector 40 varies versus the phase difference  s in terms of the sine function as indicated by Eq. (2). That is, the optimum sensitivity is achieved. Further, here will be explained the phase difference between the fifth and sixth light beams which form the feedback light to the coherent light source 10. Each of these light beams has been transmitted through the optical branching/coupling unit 20 once and coupled therein once. It is accordingly understood that both the light beams have no phase difference caused by the propagation through the optical branching/coupling unit 20. That is, when the feedback light is handled as interference light, it is always in a cosine function with respect to the phase difference  s . This fact is utilized in the fourth embodiment of the present invention to be described later. In Fig. 1, the signal processing circuit 50 receives as its input the interference signal of the photodetector 40 varying as the sine function versus the phase difference  s , and it delivers as its output a rectilinear signal voltage concerning the phase difference  s through the calculation of an inverse sine function. By the way, in a case where polarized light fluctuates in the optical system, a drift is sometimes involved in the measured value of the phase difference to-be-measured  s because of a change in the interference state of the interferometer. For the purpose of eliminating the evil effect of the polarization fluctuation, it is desirable to insert a polarizer/ polarizers 75 between the light source 10 and the first terminal 21 of the optical branching/coupling unit 20 or/and between the fourth terminal 24 of the optical branching/coupling unit 20 and the photodetector 40. Meanwhile, in the case of the first embodiment described above, when the quantity of light from the coherent light source 10 changes, the quantity of light received by the photodetector 40 (the intensity I in Eq. (2)) fluctuates immediately. On the other hand, in the interference sensor employing an optical fiber, the quantity of light sometimes fluctuates due to the temperature change, the secular change or the like of the coupling efficiency between the light source and the optical fiber. Accordingly, the first embodiment can pose the problem that the proportionality coefficient between the output voltage and input phase difference  s of the signal processing circuit 50, in other words, the voltage scale thereof changes depending upon the the quantity of light entering the photodetector 40. The second and third embodiments of the present invention to be described below compensate the fluctuation of the voltage scale in the first embodiment. Now, the second embodiment of the present invention will be described. Fig. 2 shows the construction of an optical fiber gyroscope according to the second embodiment. In the interference sensor employing the optical fiber, the most important factor of the fluctuation of the light quantity is usually the temperature or secular change of the coupling efficiency between the light source and the optical fiber. In the second embodiment, therefore, a second optical branching/coupling unit 60 and a second photodetector 41 are disposed in addition to the first emhodiment as illustrated in Fig. 2. Referring to Fig. 2, light from the coherent light source 10 enters the optical branching/coupling unit 60 via the first terminal 61 thereof, and it is bifurcated to emerge from the second terminal 62 and third terminal 63. The second photodetector 41 receives light from the terminal 63, and it adjusts and controls the light source driving circuit 11 so as to keep the quantity of this light constant. As a result, output light from the second terminal 62 of the second optical branching/coupling unit 60 is also held at a constant light quantity. The output light from the terminal 62 is guided to the first optical branching/coupling unit 20. The subsequent operation is quite the same as in the first embodiment. As stated above, in the second embodiment, the light quantity is stabilized, so that the voltage scale does not fluctuate. By the way, as in the first embodiment, it is desirable in the second embodiment to insert the polarizer/polarizers 75 between the second terminal 62 of the optical branching/coupling unit 60 and the first terminal 21 of the optical branching/coupling unit 20 or/and between the fourth terminal 24 of the optical branching/coupling unit 20 and the photodetector 40. Now, the third embodiment of the present invention will be described. Fig. 3 shows the construction of an optical fiber gyroscope according to the third embodiment. The third embodiment is such that the fluctuation of the voltage scale is compensated by an expedient different from the expedient of the second embodiment. In the third embodiment, the first embodiment is additionally furnished with an optical phase modulator 70 which is fabricated by winding an optical fiber about 1 meter long round a PZT (piezoelectric transducer or piezo-ceramic) cylinder and which is incorporated at one end of the optical fiber loop 30. With the optical fiber gyroscope in Fig. 3, when the temporal change of a light phase based on the optical phase modulator 70 is denoted by ψ(t), an interference signal P(t) which is delivered as the output of the photodetector 40 is expressed by Eq. (3): P(t) = K {I + ν sin ( s + ψ(t) + ψ(t - τ))} Here, τ denotes the propagation delay time of the light wave attributed to the fiber loop, and it is given by Eq. (4) by the use of the length ℓ of the fiber loop, the refractive index n of the optical fiber and the velocity c of light: τ = n ℓ/c Here, it is the same as in the first embodiment that, since the interferometer undergoes the phase bias of 90° owing to the phase characteristics of the optical branching/coupling unit 20, the interference signal P(t) is in a sine function with respect to the phase difference  s . It is now assumed that the optical phase modulator 70 is driven with a sinusoidal wave signal expressed by Eq. (5): ψ(t) = ψ µ sin ω µ t Here, ψ µ denotes the amplitude of optical phase modulation, and ω µ the angular frequency of the optical phase modulation. On this occasion, the interference signal P(t) is expressed by Eqs. (6) and (7): P(t) = K I {1 + ν sin ( s + η cos ω µ (t - τ/2))} η = 2 ψ µ sin (ω µ τ/2) Accordingly, when the optical phase modulation amplitude ψ µ is set so as to render the effective phase modulation index η sufficiently large, a certain time t 0 can exist to satisfy Eq. (8) in correspondence with any desired phase difference  s :  s + η cos ω µ (t 0 - τ/2) = π/2 On this occasion, Eq. (9) holds: P PEAK = P(t 0 ) = K I {1 + ν} Thus, the interference signal P(t) becomes its peak value at the time t 0 . As understood from Eq. (9), the peak value P PEAK of the interference signal does not depend upon the phase difference  s . With note taken of this fact, the third embodiment compensates the voltage scale fluctuation in the first embodiment. In Fig. 3, an optical phase modulator driving circuit 80 and a peak detecting circuit 90 are operated in accordance with a gate signal which is generated by a compensation timing circuit 100. More specifically, as illustrated in Fig. 4, the optical phase modulator driving circuit 80 drives the optical phase modulator 70 only when the gate signal is in its ON state, and the peak detecting circuit 90 detects the peak of the interference signal at this time. When the gate signal is OFF, the optical phase modulator driving circuit 80 stops operating. Besides, while the gate signal is OFF, the peak detecting circuit 90 holds the peak value detected before and continues to deliver it as a peak value signal. Meantime, a signal processing circuit 110 is supplied as inputs with the interference signal and the peak value signal, the ratio of which is calculated and delivered as an output. That is, while the gate signal is OFF, the interference signal is: P(t) = K I {1 + ν sin  s } similarly to that of the first embodiment. This interference signal is divided by the peak value signal obtained when the gate signal is ON: P PEAK = K I {1 + ν} Thus, the interference light intensity I is removed, so that the output of the signal processing circuit 110 becomes regardless of the change of the quantity of the coherent light. The ON time of the gate signal is set at about 10 ms. in this embodiment in consideration of the response characteristics of the optical phase modulator of PZT. The OFF time may be sufficiently long for the compensation of a slow fluctuation, and the gate signal is turned ON repeatedly at a period of 1 second in this embodiment. Also in the third embodiment, it is desirable to employ the polarizer(s) likewise to the first embodiment. Now, the fourth embodiment of the present invention will be described. In the first embodiment of the present invention, the phase characteristics of the optical branching/ coupling unit are set at 45°. In the fabrication of the optical branching/coupling unit, however, it is sometimes difficult to set the phase characteristics exactly at the predetermined value. Also, fluctuation in the phase characteristics of the optical branching/ coupling unit can occur in a long term. Therefore, the fourth embodiment solves this problem. Fig. 5 shows the construction of an optical fiber gyroscope according to the fourth embodiment. As illustrated in the figure, likewise to the second embodiment, the fourth embodiment is so constructed that the second optical branching/coupling unit 60 is inserted between the coherent light source 10 and the first optical branching/coupling unit 20, in addition to the first embodiment. Unlike the second emhodiment, however, the second photodetector 41 is connected to the fourth terminal 64 of the second optical branching/coupling unit 60. Besides, a signal processing circuit 200 comprises a variable-gain amplifier 201 of gain A, adders/ subtracters 202, 203 and a linearization circuit 204. Further, the phase characteristics of the first optical branching/coupling unit 20 have any desired value α and need not be exactly 45° as in the first, second or third embodiment. As understood from the detailed description of the first embodiment, the phase difference of interference light which emerges from the terminal 24 of the optical branching/coupling unit 20 is 2 α in the fourth embodiment. The first interference signal P 1 which is delivered from the first photodetector 40 is expressed by Eq. (12): P 1 = K I {1 + ν cos ( s + 2 α)} = K I {1 + ν cos 2 α · cos  s - ν sin 2 α · sin  s } On the other hand, emergent light from the terminal 64 of the second optical branching/coupling unit 60 is part of the feedback light to the coherent light source 10 as has emerged from the terminal 21 of the first optical branching/coupling unit 20. As referred to in the description of the first embodiment, this emergent light is the interference light which is always in the cosine function irrespective of the phase characteristics of the first optical branching/coupling unit 20. Accordingly, the second interference signal P 2 which is delivered from the second photodetector 41 becomes as indicated by Eq. (13): P 2 = L I {1 + ν cos  s } Here, L denotes a constant which is determined by the branching characteristics of the second optical branching/coupling unit 60 and the gain of the second photodetector 41. With the fourth embodiment, the first and second interference signals P 1 , P 2 are noticed and are electrically composed, thereby to produce an optical fiber gyroscope output which varies in a sine function with respect to the phase difference  s in the same manner as in the bestowal of the 90° phase bias. More specifically, in the signal processing circuit 200, one input signal B of the adder/subtracter 203 serves to adjust a D.C. bias level, and the output signal P o thereof is expressed by Eq. (14): P o = A P 2 - P 1 - B Accordingly, when the gain A of the variable-gain amplifier 201 is set as indicated by Eq. (15): A = K cos 2 α/L the output signal P o is expressed by Eq. (16): P o = K I ν sin 2 α · sin  s + (AL - K) I - B Thus, the output signal with which the sensitivity of the optical fiber gyroscope is optimalized with respect to the phase difference  s is produced. Besides, when the D.C. bias B is set as indicated by Eq. (17) here: B = (AL - K) I the output signal becomes P o = 0 for the phase difference  s = 0. If the phase characteristics α of the optical branching/coupling unit 20 are sufficiently stable, the gain constant A may be set by adjusting it only at the stage of the fabrication of the optical fiber gyroscope. However, in a case where the phase characteristics α fluctuate to the extent of posing a problem in practical use, the constant A needs to be set again. In the fourth embodiment, the gain constant A of the variable-gain amplifier 201 is occasionally adjusted. A monitor circuit 205 is disposed for this purpose, and in response to a reset signal input externally applied, it adjusts the gain constant A so that the output signal P o of the adder/subtracter 203 may agree with a predetermined value. The fourth embodiment shall be supplied with the reset signal only at the point of time at which a moving object or the like carrying the optical fiber gyroscope thereon is not rotating, so the Sagnac phase difference  s 0 may be deemed. At this time, the monitor circuit 205 resets the gain constant A and further the D.C. bias B so as to establish: P o ( s = 0) = 0 thereby to compensate the fluctuation of the phase characteristics α of the optical branching/coupling unit 20. By the way, also in the fourth embodiment, it is desirable to employ the polarizer(s) likewise to the second embodiment. Fig. 6 illustrates a fabrication method for the optical branching/coupling unit according to the invention. The optical branching/coupling unit is fabricated by the fusion stretching process. Referring to the figure, light from a light source 10 is caused to enter the first optical fiber 400, and emergent light from the first optical fiber 400 is detected by the first photodetector 410. The first optical fiber 400 is supported by a stationary clamp 420 and a movable clamp 430. On the other hand, the second optical fiber 401 is structurally identical to the first optical fiber 400, and it is similarly supported by a stationary clamp 421 and a movable clamp 431. The light from the light source 10 is coupled to the second optical fiber 401 in a fusion stretching portion 440, and the coupled light is detected by the second photodetector 411. A burner 450 is supported on a movable stage 460, and the melted state of the fusion stretching portion 440 is controlled by moving the movable stage 460 vertically and horizontally. In the fusion stretching process, photocoupling is achieved in such a way that the claddings of the two optical fibers are brought into contact and are unitarily melted by heating with the burner until the cores of the two optical fibers come into close proximity. For the realization of the proximate state of the cores, it is common practice that, under the melted state of the fusion stretching portion 440, the cores of the two optical fibers 400, 401 are pressed toward each other, whereupon the movable clamps 430, 431 are moved axially of the optical fibers, thereby to stretch these optical fibers. On this occasion, the output intensities of the photodetectors 410, 411 are compared so as to attain a desired branching ratio. The present invention realizes the asynchronous optical branching/coupling unit in such a way that, during the fusion stretching stated above, the first optical fiber 400 and the second optical fiber 401 are stretched under different stretching conditions. As shown in Fig. 6, the movable clamps 430, 431 for stretching the first and second optical fibers 400, 401 are individually disposed and are separately moved, thereby to perform asymmetric stretching. In the fusion stretching portion 440, the optical fibers are melted and stretched and therefore have their propagation constants changed. Herein, since the stretch amounts of the first and second optical fibers are different, a difference in the propagation constants arises in the fusion stretching portion 440. Incidentally, when means for measuring the phase characteristics and branching ratio of an optical branching/coupling unit is conjointly used in the fabricating process for the fiber coupler, a fiber coupler having phase characteristics and a branching ratio as desired can be fabricated.

Claims: An optical fiber gyroscope having an optical branching/coupling unit (20) in which light from a coherent light source (10) is bifurcated or conjoined in two directions, an optical system which includes an optical fiber loop (30) for propagating bifurcated light beams in terms of reverse rotations, as well as light phase difference bias means for affording a bias to the bifurcated light beams, the optical output of the gyroscope that consists of interference light having passed the optical branching/ coupling unit twice, a signal processing system (40, 50, 110, 200) which converts the optical output of the optical system (10, 20, 30, 70) into an electric signal and processes the electric signal, and detection means for detecting the angular velocity on the basis of a processed result of the signal processing system, wherein said optical branching/coupling unit (20) develops the light phase difference which affords π/4 as the phase bias between transmitted light and branched light, the optical output of the unit (20) consists of interference light having passed the optical branching/coupling unit twice whereby to function also as said means for affording the light phase bias of π/2 and said optical branching/coupling unit (20) includes a plurality of optical fibers (400, 401) which have been melted and stretched under different stretching conditions. An optical fiber gyroscope as defined in claim 1, comprising means for measuring the intensity of the branched light of said optical branching/coupling unit, and means for controlling an intensity of a light source of said light system in accordance with the measured intensity of the branched light so as to keep the quantity of light entering a path to said optical branching/coupling unit constant. An optical fiber gyroscope as defined in claim 1, comprising optical phase modulation means (70) interposed between said optical fiber loop (30) and said optical branching/coupling unit (20), and means (80, 90) for driving said optical phase modulation means (70) and for compensating the intensity fluctuation of interference light by the use of a peak value of the interference light obtained. An optical fiber gyroscope as defined in claim 1, comprising means for subjecting interference light which is formed of the beams of light-to-interfere endowed with the light phase bias and which is derived from a terminal of said optical branching/coupling unit (20) and feedback light of said optical fiber loop (30) which is derived from another terminal of said optical branching/coupling unit, to predetermined processing based on phase characteristics of said optical branching/coupling unit (20), and for generating a composite signal which varies as a sine function in correspondence with the phase difference between the beams of light-to-interfere in said optical fiber loop (30). An optical fiber gyroscope as defined in claim 1, wherein said optical branching/coupling unit (20) have a path of the transmitted light (22) and that of the branched light (23) made structurally asymmetric. An optical fiber gyroscope as defined in claim 1, wherein said optical branching/coupling unit (20) includes a plurality of polarization plane conserving fibers (500, 501) which give rise to different polarization modes of optical branching/coupling.

Inventors: GUNJI YASUHIRO, KAJIOKA HIROSHI, KUMAGAI TATSUYA, MAKINO JUNICHI, OHO SHIGERU, SANOBE HISAO, GUNJI, YASUHIRO, KAJIOKA, HIROSHI, KUMAGAI, TATSUYA, MAKINO, JUNICHI, OHO, SHIGERU, SANOBE, HISAO
Applicants: HITACHI CABLE, HITACHI LTD, HITACHI CABLE, LTD., HITACHI, LTD.
Application year: 1991
Main classification: G01C19
IPC codes: G01C19