JPH0731059B2 - Ultra-sensitive optical fiber gyroscope - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical fiber propagating in an optical fiber loop of a gyroscope that is important in the configuration of a rotational angular velocity measuring device, a flight navigation device, an earthquake prediction detection system, and the like. An optical fiber gyroscope that detects the rotational angular velocity by the change of the wavelength loop due to the rotation, especially the ultra-sensitive optical fiber
With regard to gyroscopes, in addition to downsizing the equipment mainly consisting of laser light sources and optical fiber loops, it was affected by AM and FM noise of laser light sources, sensitivity difference of photodetectors, temperature drift of optical fiber optical path length, etc. Instead, it is possible to achieve ultra-high sensitivity up to the theoretical limit determined by the shot noise of light detection.

(Prior Art) Generally, as a gyroscope which is a rotation angular velocity detecting means, a non-reciprocal phase difference caused by the rotation of the optical fiber loop, that is, between the light beams propagating in opposite directions in the optical fiber loop, that is, The optical gyroscope utilizing the so-called Sagnac effect has many advantages such as long life, low energy consumption, small size and light weight as compared with the conventional mechanical gyroscope. In particular, an optical gyroscope is extremely promising, in which an optical fiber loop, which is a main part, is formed by using a low-loss quartz optical fiber for downsizing and weight reduction.

However, most of these conventional optical fiber gyroscopes use a gas laser as a light source for injecting a light beam into the optical fiber loop, or use a low-coherence semiconductor laser. As described above, as the format of the optical fiber interferometer for detecting the non-reciprocal phase difference between the lights propagating in the opposite directions, there are many technically required techniques for improving the phase difference detection sensitivity, as described in detail below. Most of them use a so-called Matsuha-Zender interferometer, which has the above-mentioned drawback, and there is a drawback that sufficient performance cannot be obtained as a gyroscope.

Here, in order to make the effects of the present invention relatively clear, a detailed description will be given of the Matsu-Zender interferometer, which is an essential part of the conventional optical fiber gyroscope, and this interferometer is as shown in FIG. Is configured. Ie current source
The light beam of the laser LS driven by CS is an isolator I
And the beam splitters BS1 and BS2 via the checker Ch to inject the optical beams propagating in opposite directions into the optical fiber loop FL. One of the two light beams is guided to the photodetectors D1 and D2 through the non-reciprocal phase shifter PS and photoelectrically converted, and the converted output voltage is obtained by the differential amplifier DA and the divider.
The output voltage corresponding to the non-reciprocal phase difference between the two light beams in the optical fiber loop FL is taken out through the lock-in amplifier LIA by sequentially guiding to DIV.

The non-reciprocal phase difference due to the so-called Sagnac effect between the two light beams in the optical fiber loop FL is given by the following equation (1).

= (8πnNA / λc) Ω (1) where n, N, A, λ, c and Ω are the optical fiber refractive index, the optical fiber winding number, the optical fiber loop area, the laser light wavelength, the optical speed, and Optical fiber loop rotation angular velocity.

Then, the detection sensitivity of the rotational angular velocity Ω by the measurement of the non-reciprocal phase difference, when using a semiconductor laser of low coherence as in the past, in order to miniaturize the light source of the light beam to be injected into the optical fiber loop FL, A limit is caused by the following factors.

(A) FM noise of laser LS (B) AM noise of laser LS (C) Sensitivity difference between two photodetectors D1 and D2 and temperature drift (D) Optical fiber caused by optical axis adjustment error of semiconductor laser Optical path length difference between two light beams outside the loop FL Among the limiting factors of the rotational angular velocity detection sensitivity, the influence of (A) laser FM noise is relatively small, but the influence of other limiting factors (B) to (D) It is difficult to reduce, and especially (D) optical path length difference Δl, when Δl> λ, Δl
The sensitivity limit increases in proportion to> λ, but the optical path length difference Δl =
It is actually extremely difficult to set it to 0.

When all of these limiting factors (A) to (D) are removed, the ultimate sensitivity limit Ω _S is determined by the following equation (2) due to the shot noise of the photodetectors D1 and D2.

Where h, ν, α, P _C , η and τ are Planck's constant, laser frequency, optical transmittance determined by fiber transmission loss, laser incident power of optical fiber, photodetector quantum efficiency and laser This is the integration time for measuring the variance value that represents the magnitude of FM and AM noise.

FIG. 2 shows the results of the rotational angular velocity detection sensitivity of the Matsu-Zender interferometer having the configuration shown in FIG. 1 obtained under the following conditions.

λ = 0.8μm, ν = 375THz, P C = 3mW, α = -1dB, n = 1.5, N
= 335, A = 283 cm ² , Δl = 1 mm, η = 1 In addition, an AlGaAs laser is used as the laser LS, and as the optical fiber loop FL, an optical fiber having a total length L = 200 m is formed into a loop having a diameter of 19 cm and 335 times. It wound.

As can be seen from the figure, by setting the optical path length difference Δl = 0 and stabilizing the laser power to the quantum noise level, it is possible to approach the ultimate limit sensitivity Ω _S determined by the shot noise of the photodetector D1. However, the use of two photodetectors D1 and D2 having slightly different sensitivities has a serious drawback that the sensitivity limit is rather increased.

As is clear from the above, in order to improve the rotational angular velocity detection sensitivity of the conventional Matsu-Zender interferometer type optical fiber gyroscope, suppression of the above-mentioned limit factors, that is,
Elimination of optical path length difference Δl outside the optical fiber loop FL,
It is necessary to stabilize the laser power and suppress the sensitivity difference of the photodetector. However, it is extremely difficult to satisfy suppression of each of these limiting factors at the same time, and therefore the conventional optical fiber gyroscope reaches the ultimate rotational angular velocity detection sensitivity limit determined by the photodetector shot noise. However, in practice, it is technically extremely difficult, and there is a fatal drawback that the detection sensitivity is 100 times or more the ultimate detection sensitivity limit.

(Main points of the invention) An object of the present invention is to eliminate the above-mentioned conventional drawbacks, downsize the device by using a high-coherence semiconductor laser as a light source of a light beam to be injected into an optical fiber loop, and improve its performance. In addition, it is theoretically determined by the shot noise of the photodetector without being affected by the noise of the light source, the difference in detection sensitivity of the photodetector, the temperature drift of the optical fiber loop optical path length, the difference in the optical path length outside the optical fiber loop, etc. Ultra-high-sensitivity optical fiber gyroscope capable of detecting rotational angular velocity with limit sensitivity, earthquake prediction detection device, compact and lightweight ultra-high sensitivity with sufficient rotational angular velocity detection sensitivity that can be practically used for inertial navigation devices It is to provide an optical fiber gyroscope.

That is, the ultra-sensitive fiber gyroscope of the present invention uses a single longitudinal mode oscillation semiconductor laser device having high coherence and an output light beam of the semiconductor laser device.
An optical beam splitter for splitting and an optical splitter for inputting the two output optical beams of the optical beam splitter to two and splitting each of the two input optical beams into two split output optical beams are provided at the input end and the output end, respectively. A ring-Fabry-Perot interferometer consisting of a pair of quartz fibers that are connected and wound in opposite directions to form the same ring, and the two-branch output optical beams of the output end optical brancher in the interferometer are photoelectrically converted. In a fiber optic gyroscope that stabilizes the resonance frequency of the interferometer by negatively feedback controlling the input optical frequency of the interferometer with at least one of the converted outputs, the drive current converted by adding and mixing two input voltages. And a semiconductor laser driving circuit for driving the semiconductor laser device, and one of the two input voltages of the semiconductor laser driving circuit. An oscillator that modulates the oscillation frequency of the semiconductor laser device by an output voltage, and a pair of a pair that performs synchronous detection and amplification by the oscillation output voltage of the oscillator after photoelectrically converting the two-branch output light beam of the output end optical branching device, respectively. The oscillation frequency of the semiconductor laser is controlled by negatively feeding back the output voltage of one of the synchronous detection amplification means and one of the pair of synchronous detection amplification means as the other of the two input voltages of the semiconductor laser drive circuit to control the oscillation frequency of the semiconductor laser. A frequency control stabilizing means for fixing the output vibration frequency of the synchronous detection amplification means is provided, and the rotational angular velocity of the ring-Fabry-Perot interferometer is obtained from the output voltage of the other synchronous detection amplification means. It is characterized by.

(Examples) Examples of the present invention will be described in detail below with reference to the drawings.

First, FIG. 3 shows a structural example of the optical fiber gyroscope of the present invention. In the optical fiber gyroscope having the configuration shown in the figure, a small single longitudinal mode oscillation semiconductor laser device 1 having high coherence and less generation of FM noise and AM noise is used as a light source of a light beam. The high coherent light beam from the laser element 1 is collimated into a parallel beam by the lens 2 and then guided to the optical isolator 3, and the reflected light from the subsequent fiber input terminal or the like goes back to the laser element 1 to disturb the oscillation state. To prevent noise. The parallel light beam that has passed through the optical isolator 3 is guided to a light beam splitter 4 composed of, for example, a semi-transparent mirror and divided into two parallel light beams, and these two parallel light beams are guided to lenses 5 and 6, respectively, and focused. Then, it is incident on the optical fibers 7 and 8. In addition,
These optical fibers 7 and 8 function as mere optical transmission lines,
It is irrelevant to the action of the subsequent optical fiber loop, which is the main part of the so-called optical fiber gyroscope, and its performance, optical path length, mutual optical path length difference, etc. are arbitrary. The two light beams guided by the optical fibers 7 and 8 are connected to the optical splitter 9
To each of the two input ports. As shown in FIG. 3a, the optical branching device 9 has two optical branching devices, as in a hybrid circuit in which two strip lines are closely arranged in parallel with each other over an appropriate section length in electronic engineering. A part of the clad of the optical fiber is removed over an appropriate section length and the cores are arranged in parallel and close to each other. In the optical branching device having such a configuration, for example, one input port 7 The incident light is guided to the other input port 8 in the form of reflected light with the boundary surface 9 of both claddings being a mirror surface, and is also guided to one output port 11 in the form of transmitted light leaking between the cores. The branching ratio between
Generally, it is determined by the distance between the cores and the material of the clad.

However, when an optical resonance loop is connected to any of the input / output ports of the optical branching device having such a configuration as described later, the input and output of the optical resonance loop at that port are in phase. Will be emphasized,
When the branching ratio to the port to which the optical resonance loop is connected is extremely increased and the optical resonance loop is connected to the output ports 10 and 11 of the optical branching device 9 as shown in FIG. ,
For example, most of the incident light from the input port 7 is transmitted light from the output port 11, and most of the incident light from the port 10 as a loop input port is reflected light from the output port 11. Similarly, incident light from the input port 8 and a port as a loop input port
Incident light from 11 becomes transmitted light and reflected light from the output port 10, respectively. The two-output light beam of the optical branching device 9 is, for example, a low-loss quartz optical fiber having a length of several meters, which is divided into two and wound in a ring shape in opposite directions to each other.
0, 11 and returns to the optical branching device 9 through the optical fiber 10, the optical branching device 12, and the optical fiber 11 in this order from the optical branching device 9 indicated by a chained arrow in FIG. It propagates in the optical fiber loop formed by the optical fibers 10 and 11 in opposite directions such as a counterclockwise counterclockwise direction indicated by a dashed arrow. Therefore, if this optical fiber loop rotates at high speed in either direction, between the light beams propagating in the optical fibers 10 and 11, respectively, as described above, the non-reciprocal position due to the Sagnac effect is generated. The phase difference and therefore the effective optical path length difference occurs. The two light beams that have passed through the optical fibers 10 and 11 are guided to an optical branching device 12, which is exactly the same as the optical branching device 9 at the input end of the optical fiber loop. However, the optical splitters 9 and 12 at both ends of the input and output are
Since it is controlled so as to resonate in the loop, if the branching ratio is properly selected and a low insertion loss is used, as described above, a high reflection for the input light in the optical fiber loop is achieved. Each acts as a mirror having an index. Therefore, the light beams are repeatedly reflected in the optical fiber loops 10 and 11 equipped with the optical branching devices 9 and 12 at the input and output ends, and the high reflection that causes interference fringes of fine pitches due to the repeated reflection. It will act as a finesse ring-fabri-perot interferometer.

That is, the two light beams that have entered the optical fiber loops 10 and 11 from the optical fibers 7 and 8 through the optical branching device 9 have directions opposite to each other, for example, in the ring Fabry-Perot interferometer as described above. As shown in FIG. 3b, while repeatedly reciprocating in the clockwise direction and the counterclockwise direction, two light beams of an amount proportional to the transmittance according to the branching ratio at the time of optical loop resonance of the optical branching device 12 are emitted. It will be injected into the fiber 13,14. The two output light beams are sequentially guided to the pair of lenses 15 and 16 and the photodetectors 17 and 18, respectively, and, for example, the resonance frequencies ν _r and _r Each has ν _l . The output light beam is photoelectrically converted into a two-voltage signal.

Therefore, when the ring Fabry-Perot interferometer described above is stationary, the resonance frequencies ν _r and ν _l of the Fabry-Perot interferometer for two light beams, for example, clockwise and counterclockwise, are equal. Become. However, this Ring-Fabry-Perot interferometer has, for example, a third
When rotating at an angular velocity Ω as indicated by a solid arrow in the figure, a difference proportional to the rotational angular velocity Ω occurs between the resonance frequencies ν _r and ν _{l of} both due to the above-mentioned Sagnac effect. . Therefore, the value of the rotational angular velocity Ω can be obtained by measuring the frequency difference.

The oscillation frequency ν of the semiconductor laser device 1 as the light beam light source in the configuration shown in FIG. 3 is modulated by the oscillation output of the oscillator 21, and the converted output voltage signals of the photodetectors 17 and 18 are applied to the oscillation output of the oscillator 21. And a pair of synchronous detectors that are driven respectively, that is, a so-called lock-in
When guided to the amplifiers 19 and 20 and amplified for detection, the frequency-dependent characteristics of the light quantities of the two counterclockwise and clockwise light beams after passing through the ring Fabry-Perot interferometer are expressed by the form of the differential signal. , As shown in FIGS. 4 (a) and 4 (b), respectively. Therefore, the resonance frequency ν for two clockwise and counterclockwise light beams
_In order to measure the difference between _r and ν _l , the oscillation frequency ν of the semiconductor laser device 1 is set to one of the two resonance frequencies ν _r and ν _l , for example, the resonance frequency of the counterclockwise light beam. In order to fix it at ν _l , the output voltage V ₁ of the synchronous detection amplifier 19 is set to the frequency control circuit 22.
Is supplied to the semiconductor laser driving circuit 23 shown in the form of a mixer and the deviation of the output voltage V ₁ when the reference zero is made to correspond to the resonance frequency ν _l is output as a frequency control voltage. Drive current, which has been mixed and added to the oscillation output voltage of the oscillator 21, is negatively fed back to the semiconductor laser element 1 and its oscillation frequency ν is controlled, as shown in FIGS. 4 (a) and 4 (b). The frequency stabilization control is performed so that the laser frequency ν follows the center frequency ν _l of the resonance frequency characteristic curve of the counterclockwise light beam of the ring Fabry-Perot interferometer. Based on the frequency stabilization by such load return control, the output voltage V ₂ of the other synchronous detection amplifier 20
To a recorder 24 to detect a signal proportional to the difference between the center frequencies ν _r and ν _l of the clockwise and counterclockwise resonance frequency characteristic curves of the ring Fabry-Perot interferometer. That is, as shown in FIG. 4 (b), the signal waveform representing the frequency dependence of the output voltage V ₂ of the synchronous detection amplifier 20 has an S shape, and is linear in the vicinity of the central resonance frequency ν _r . Since it is a characteristic curve, sharp frequency discrimination can be performed.

That is, the output voltage V ₂ of the synchronous detection amplifier 20 is expressed by the following equation (3).

V ₂ = K (ν _r −ν ₁ ) (3) Here, K is a constant proportional to the detection sensitivity of the photodetector 18 and the amplification factor of the synchronous detection amplifier 20. Further, the resonance frequency difference (ν _r −ν _l ) is the rotational angular velocity Ω due to the Sagnac effect.
And have a relationship according to the following equation (4).

Therefore, the output voltage V ₂ according to equation (3) is
It will be represented by a formula.

That is, the output voltage V ₂ consists of only a term proportional to the rotational angular velocity Ω. Therefore, the Ring-Fabry-Perot interferometer does not include the offset amount independent of the rotational angular velocity Ω as in the Matsuha-Tender interferometer that constitutes the conventional optical fiber gyroscope.
It is completely unaffected by the various limiting factors, particularly the limiting factors (C) and (D), which have been problems of the prior art, with respect to the rotational angular velocity detection sensitivity described above for the conventionally used Matsu-Zehnder interferometer. As a result, it is no longer necessary to make the detection sensitivities of the pair of photodetectors 17, 18 uniform, and it becomes irrelevant to the temperature drift, and even if there is an optical path length difference outside the optical fiber loop, the effect is completely absent. You will not receive it.

Further, as can be seen from FIG. 4, since the rotational angular frequency Ω is discriminated by the differential waveform of the resonance frequency characteristic curve of the interferometer, a high discrimination sensitivity can be obtained even when Ω = 0. It is not necessary to perform π / 2 non-reciprocal phase shift by the non-reciprocal phase shifter PS as used in the configuration shown in the first drawing for the used Matsu-Zehnder interferometer. Further, the phase difference between the two light beams that perform relative counterclockwise rotations such as clockwise rotation and counterclockwise rotation is determined only by the inside of the fiber loop, so that the light used in conventional Matsuha-Tender interferometers is not affected. It is also irrelevant to the optical path length difference Δl outside the fiber loop, and the rotational angular velocity detection sensitivity is an ideal type in which the sensitivity is limited only by the shot noise of the photodetector.

However, according to the result of studying the limit of the rotational angular velocity detection sensitivity due to only the sailboat noise of such a photodetector, the low loss quartz optical fiber is remarkably compared with several hundred meters in the conventional Matsuha-Zender interferometer. It is clear that the limit of rotational angular velocity detection sensitivity, which is determined by the photodetector's shot noise, is reduced by using the ring Fabry-Perot interferometer consisting of an optical fiber loop wound for only a few meters. It was

As described above, a characteristic example in which the change of the rotational angular velocity Ω of the optical fiber gyroscope of the present invention using the ring Fabry-Perot interferometer having excellent performance with respect to the integration time τ for measuring the noise variance value is obtained. FIG. 5 shows various values of the single-transmissivity α _t determined by the total loss of the optical fiber loop including the insertion loss of the optical branching device.
As is apparent from the figure, in the optical fiber gyroscope of the present invention, the limit sensitivity Ω _S itself determined by the shot noise of the photodetector is significantly reduced compared to the same type limit sensitivity in the conventional device. Note that the output voltage V ₂ of the synchronous detection amplifier 20 in the gyroscope of the present invention is only proportional to the rotational angular velocity Ω, and since no other offset amount is added, the influence of noise from the laser element is also present. Extremely small. Also, 4 I / O port optical splitter
Insertion loss of 9,12 is extremely small.

(Effect) As is apparent from the above description, in the ultra-high-sensitivity optical fiber gyroscope of the present invention, various problems that the conventional optical fiber gyroscope of this kind has are as follows. All resolved for a reason.

(1) Since a large-sized gas laser is not used as a light source for injecting a light beam, but a semiconductor laser is used, the size of the device can be reduced.

(2) The conventional low-coherence semiconductor laser is not used, the high-coherence single-longitudinal-mode oscillation semiconductor laser is used, and the conventional Matsu-Zender interferometer is not used. Since a Ring-Fabry-Perot interferometer is used, ultra high sensitivity is obtained.

(3) Since the laser oscillation frequency is fixed to one of the resonance frequencies in the ring Fabry-Perot interferometer,
The effects of AM noise, FM noise of the laser element or temperature drift of the optical fiber optical path length can be eliminated.

(4) Since it is not necessary to measure the output voltage difference between the pair of photodetectors 17 and 18, the detection sensitivity difference between the two is irrelevant.

(5) The substantial optical path length difference due to the light propagation direction in the optical fiber loop is determined only inside the ring Fabry-Perot interferometer, and there is essentially no optical path length difference outside the interferometer. Even if it exists, it is not affected at all.

As described above, in the optical fiber gyroscope of the present invention, in order to increase its super sensitivity, a ring-fabri-Perot interferometer having excellent discriminability is used as an optical system,
A highly sensitive synchronous detection amplifier is used as the electronic circuit system, and negative feedback control for frequency stabilization is added to the semiconductor laser, and all the problems in the conventional device are solved. Therefore, the rotational angular velocity detection sensitivity has reached the theoretical limit value determined by the shot noise of the photodetector, and the use of a low-loss quartz optical fiber and a 4-input / output port optical brancher makes it possible to detect the photodetector. The theoretical limit value due to the shot noise of 100 is reduced to 100 times to 10,000 times as high as that of the conventional equipment, and the high-speed angular velocity measurement of about 10 ^-4 degrees / hour is sufficiently possible. Excellent performance is obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing the structure of a conventional optical fiber gyroscope, FIG. 2 is a characteristic curve diagram showing an example of its rotational angular velocity detection sensitivity characteristic, and FIG. 3 is the super-sensitive optical fiber of the present invention.・ A block diagram showing a configuration example of the gyroscope, Fig. 3a is a sectional view schematically showing the configuration of the optical branching device, and Fig. 3b is a block line schematically showing the operation of the optical fiber gyroscope of the present invention. FIGS. 4 (a) and 4 (b) are operation waveform diagrams respectively showing the operation characteristics thereof, and FIG. 5 is a characteristic curve diagram showing an example of the rotational angular velocity detection sensitivity characteristics thereof. CS: current source, LS: laser I: isolator, Ch: chip BS1, BS2: beam splitter FL: optical fiber loop PS: non-reciprocal phase shifter D1, D2 ... optical demultiplexer Device DA …… Differential amplifier DIV …… Divider, LIA …… Lock-in amplifier 1 …… Laser element, 2,5,6,15,16 …… Lens 3 …… Isolator, 4 …… Beam splitter 7 , 8,13,14 …… Optical fiber 9,12 …… Optical splitter 10,11 …… Optical fiber (optical fiber loop) 17,18 …… Photodetector 19,20 …… Synchronous detection amplifier (lock-in Amplifier) 21 …… Oscillator, 22 …… Frequency control circuit 23 …… Semiconductor laser drive circuit 24 …… Recorder

Claims (1)

[Claims] 1. A single longitudinal mode oscillation semiconductor laser device having high coherence, a light beam splitter for splitting an output light beam of the semiconductor laser device into two, and two output light beams of the light beam splitter as two inputs, A ring composed of a pair of quartz fibers in which optical splitters for splitting two input light beams into two split output light beams are connected to the input end and the output end, respectively, and wound in the same ring in opposite directions.・ Has Fabry-Perot interferometer
Negative feedback control of the input optical frequency of the interferometer is performed by at least one of the converted outputs obtained by photoelectrically converting the two-branch output optical beams of the output end optical brancher in the interferometer to stabilize the resonance frequency of the interferometer. In an optical fiber gyroscope, a semiconductor laser drive circuit for driving the semiconductor laser element by a drive current obtained by adding and mixing two input voltages, and an oscillation output voltage as one of the two input voltages of the semiconductor laser drive circuit An oscillator for modulating the oscillation frequency of the semiconductor laser device by means of a pair of synchronous detectors for performing synchronous detection and amplification by the oscillation output voltage of the oscillator after photoelectrically converting the two-branch output light beams of the output end optical branching device, respectively. The output voltage of one of the amplifying means and one of the pair of synchronous detection amplifying means is used as the output voltage of the semiconductor laser driving circuit. And a frequency control stabilizing means for controlling the oscillation frequency of the semiconductor laser by negatively feeding back the output voltage of the other synchronous voltage amplifying means to the output resonant frequency of the one synchronous detecting and amplifying means, and the other synchronous detecting and amplifying means. From the output voltage of the ring
An ultra-sensitive optical fiber gyroscope characterized by being configured to determine the rotational angular velocity of a Perot interferometer.