JPH0513447B2 - - Google Patents

Description

Detailed Description of the Invention "Industrial Application Field" This invention splits phase-aligned light into two parts using an optical directional coupler, and propagates the light in opposite directions through an optical waveguide that goes around at least once. This invention relates to an optical interference gyrometer that measures the angular velocity of an optical waveguide by extracting it from a waveguide and detecting the phase difference that occurs between the two beams.

"Prior Art" FIG. 4 is a block diagram showing an example of a conventional optical interference gyrometer.

Coherent light with uniform phase is emitted from the light source 11.
L ₁ is emitted. This coffee rent light L ₁ is Φ
It is expressed as (t)=2πνt (where ν is the frequency of the light), and is coherent light. The emitted light L ₁ is the first
The second optical directional coupler 1 passes through the optical directional coupler 12
The light enters the coupling end 13C of No.3. This incident light
L ₁ is divided into two lights L ₂ and L ₃ , and the branch end 13A,
The light is emitted from the light source 13B and enters the optical waveguide 14 made of, for example, an optical fiber, which makes at least one circuit through separate input/output paths 14A and 14B. These two beams L ₂ and L ₃ propagate in the optical waveguide 14 in opposite directions as clockwise light and counterclockwise light, respectively, and enter an input/output path 14B different from the respective input/output paths 14A, 14B to the optical waveguide 14. , 14A to the branch ends 13B, 13A of the second optical directional coupler 13,
It is again combined into one light L ₄ , and its combined end 13C
It is emitted from. This coupled light L ₄ enters from the coupling end 12C of the first optical directional coupler 12, and the distributed light L ₅ is emitted from the distributing end 12B, and the emitted distributed light L ₅ enters the photodetector 15. , converted into an electrical signal. This electric signal passes through a gate circuit 16 and is supplied to a synchronous detection circuit 17.

If there is no angular velocity Ω around the axis perpendicular to the plane formed by the optical waveguide 14, the optical waveguide 14 will produce clockwise light.
They are reciprocal with respect to L ₂ and the counterclockwise light L ₃ , and when the two beams are combined by the second directional optical coupler 13 as a beam splitter, the phases match each other. However, when the angular velocity Ω is present, the optical waveguide 14 transmits clockwise light L ₂ propagating in opposite directions.
It becomes non-reciprocal with respect to the counterclockwise light _L3 , and therefore a phase difference occurs due to the sanyac effect.

When the input angular velocity of the optical waveguide 14 is Ω, the area surrounded by the annular optical waveguide 14 is A, the refractive index of the optical waveguide is n, the wavelength of light in vacuum is λ, and the total length of the optical waveguide is L.
Then, the phase difference φ _r of the sannyac effect occurring between the right-handed light L ₂ and the left-handed light L ₃ is expressed as φ _r =8πAΩ/λc (1).

A phase modulator 18 is provided near one input/output path 14B of this optical waveguide 14. In this example, a piezoelectric cylinder 18 is used as the phase modulator 18, and an optical fiber 14B that forms input and output paths 14A and 14B to the optical waveguide 14 is wound around the cylinder 18. It is configured to modulate the light of. That is, a composite signal of the alternating signal output from the first oscillator 19 and the sawtooth wave signal output from the second oscillator 21 is amplified by the amplifier 22, and the composite amplified drive signal is supplied to the phase modulator 18. and modulates the clockwise light L ₂ and the counterclockwise light L ₃ that propagate in the optical waveguide 14 in opposite directions.

In this optical interference gyrometer, the alternating signal from the first oscillator 19 is, for example, a sine wave signal with a frequency f _n .

The phase modulator 18 uses this signal to generate two beams of light L ₂ ,
When phase modulation is applied to each of L ₃ , two lines of light,
In other words, the clockwise light L ₂ and the counterclockwise light L ₃ are φ(t) = φ _n sin (2πf _n・t) ... (2) where φ _n : maximum phase deviation f _n : phase modulation of the modulation frequency receive. The clockwise light L ₂ that has undergone this phase modulation is transmitted to the branch end 13 of the second optical directional coupler 13.
The counterclockwise light _L3 enters the branch end 13A with a time delay of τ after going around the optical waveguide 14. Phase-modulated clockwise light L ₂ and counterclockwise light L ₃
are coherent with each other, and the second optical directional coupler 1
3, and this interference light L ₄ is the first
The distributed light L ₅ enters the photodetector 15 from the coupling end 12A of the optical directional coupler 12.

Interference light L ₄ changes in intensity due to interference,
The intensity I(t) is expressed as I(t)˜P ₀ [1+cos {Δφ +φ(t)−φ(t−τ)}] (3). However, 2P ₀ is the maximum value of interference light L ₄ ,
Δφ is the phase difference between the two lights due to the angular velocity, and τ is the difference in time until the clockwise light L ₂ and the counterclockwise light L ₃ reach the phase modulator 18 .

ψ(t) in equation (3) is a modulation signal of frequency f _n as shown in equation (2), and equation (3) is a spectral sequence of harmonics of frequency f _n .

In the optical interference gyrometer, the first harmonic component included in this spectral train signal 2J ₁ (2φ _n sin (πf _n τ)) sinΔφ ・cos {2πf _n (t−τ/2)} ...(4) and 2 The harmonic component 2J ₂ (2φ _n sin (πf _n τ)) cosΔφ ・cos {4πf _n (t−τ/2)} ...(5) is extracted and used as a measurement signal of the angular velocity Ω. (Here, J ₁ and J ₂ are first-order and second-order Betzell functions.) When the optical waveguide 14 has no angular velocity, the optical waveguide 14 has a high speed for the clockwise light L ₂ and the counterclockwise light L ₃ . is reciprocal, and Δφ=0. Therefore, the first harmonic component in equation (4) is zero, and only the second harmonic component in equation (5) is obtained. This first harmonic signal is synchronously detected by the synchronous detection circuit 17 at the oscillation frequency f _n of the first oscillator 19, but as shown in FIG. 5, the detection output becomes zero. In other words, it can be seen that the interference light _L4 does not contain a phase difference component due to the Sannyac effect, and the angular velocity Ω of the optical waveguide 14 is zero (see Japanese Patent Laid-Open No. 47512/1983).

When an angular velocity Ω is applied to the optical waveguide 14, the optical waveguide 14 becomes non-reciprocal with respect to the clockwise light _L2 and the counterclockwise light _L3 , and a phase difference occurs between the two lights.
As mentioned above, the phase difference φ _r caused by this sannyac effect is expressed as φ _r =8πAΩ/λc (=Δφ≠0). Therefore, when the first and second harmonic components shown in equations (4) and (5) are synchronously detected at the frequency f _n , the position of the f _n component according to the phase difference φ _r is obtained as shown in Figure 6. A phase difference detection signal is obtained.

On the other hand, the phase modulator 18 is also driven by the sawtooth wave signal output from the second oscillator 21 (Reference, August 1984/Vol.9.No.8/OPTICS)
LETTERS 375-377). By the phase modulator 18 driven by this sawtooth wave signal, the clockwise light L ₂ and the counterclockwise light L ₃ undergo a phase shift according to the slope of the sawtooth wave. The phase shift ψ(t) is ψ(t)=ψ _s t, (0<t<t ₁ ) (6) where ψ _s is expressed by a constant corresponding to the slope of the sawtooth wave. FIG. 7A is a diagram showing this situation, and this phase shift is serrodyne modulation that is repeated every period _t1 .

Since the counterclockwise light L ₃ goes around the optical waveguide 14 after receiving this shift, it is accompanied by a time delay of τ with respect to the clockwise light L ₂ at the coupling end 13C of the second optical directional coupler 13. Therefore, the second optical directional coupler 1
When combined by 3, clockwise light L ₂ and counterclockwise light L ₃
The phase difference ψ _s τ of the phase shift due to the sawtooth wave signal is expressed as ψ(t)−ψ(t−τ)=ψ _s τ (7). FIG. 7B is a diagram showing this situation. The two lights are repeatedly and continuously given a constant phase difference for a period of time t ₁ −τ within period t ₁ .

In this manner, a certain phase difference occurs between the two lights due to the phase shift in the slope portion of the sawtooth wave. In this optical interference gyrometer, the phase difference φ _r generated by the sannyac effect when the angular velocity Ω is given, and the phase difference φ _s 〓 generated by driving the phase modulator 18 with a sawtooth wave and adding a phase shift to the light. The phase difference Δφ obtained by adding these is extracted by bias-driving the phase modulator 18 at a frequency f _n . In other words, Δφ=φ _r +ψ _s τ ...(8) is detected.

Therefore, the sannyac effect φ _r can be canceled out by the phase difference ψ _s τ caused by the phase shift due to the sawtooth wave. The phase deviation ψ _s and the input angular velocity Ω when this cancellation occurs
What is (1/2π)ψ _s = (4A/λnL)Ω ...(9) However, A: Area surrounded by the optical waveguide λ: Wavelength of light in vacuum n: Refractive index of the optical waveguide L: Area of the optical waveguide There is a full-length relationship.

In order to find this ψ _s , a conventional optical interference gyrometer detects the phase difference between the two beams of light, and adjusts ψ _s so that the detected output becomes zero. In other words, ψ _s τ=
−φ _r . This ψ _s has the following relationship with the drive signal V of the phase modulator 18: ψ _s =(4π ² /λ)·dN(V) (10). However, d is a piezoelectric constant, N is the number of turns of the fiber, V is the driving voltage, a is the electrode spacing, and λ is the wavelength of light in vacuum.

In other words, ψ _s is proportional to the drive voltage V applied to the phase modulator 18, and ψ _s can be found by measuring this drive voltage V when the detection output of the synchronous detection circuit 17 becomes zero. It is said that it is possible to know the angular velocity Ω from that value.

"Problems to be Solved by the Invention" As described above, in the method of determining the drive voltage V of the phase modulator to determine the amount of phase shift due to Serrodyne modulation, the temperature characteristics, frequency characteristics, or dielectric constant of the phase modulator cannot be determined. Drive voltage V and phase deviation ψ _s due to changes, etc.
There is a problem in that the relationship between the two changes and it is not possible to obtain an accurate amount of phase shift.

"Means for solving the problem" Two lines of light are propagated in opposite directions from different input/output ends into a loop-shaped optical waveguide, and the two lines of light have frequencies near one input/output end. Provides shift modulation, for example serrodyne modulation. The interference signals of the two beams extracted from the input and output ends of the loop-shaped optical waveguide are synchronously detected at the phase modulation frequency to obtain a signal corresponding to the phase difference between the two beams.

In the present invention, a signal corresponding to the obtained phase difference is fed back as a frequency shift feedback signal, and the frequency shift by the frequency shifter is controlled.

Further, in the present invention, an optical directional splitter is provided in the optical waveguide, and one light that has undergone frequency shift and the above-mentioned alternating modulation from inside the optical waveguide and the other light that has not undergone frequency shift and alternating modulation. Take out each part. These two extracted lights are caused to interfere, creating a beat signal with the difference in frequency between the two lights.

An alternating signal frequency component or one of its harmonic components is extracted from this beat signal output, and the extracted alternating signal frequency component or its harmonic component is a signal from an alternating signal source or a signal corresponding to its harmonic signal. Extract the difference frequency.
The extracted difference frequency corresponds to the phase shift component to be measured.

Embodiment FIG. 1 is a block diagram showing an embodiment of the present invention. Portions corresponding to those in FIG. 4 described above are designated by the same reference numerals.

The coherent light from light source 11 is two rays of light.
Divided into L ₂ and L ₃ , different input and output ends 14A, 1
The light enters the loop-shaped optical waveguide 14 from 4B. Two rays of light propagated through the optical waveguide 14 in opposite directions
L ₂ and L ₃ are input/output paths 14B different from those at the time of input,
It is emitted to 14A.

The output lights L ₂ and L ₃ are transmitted to the second optical directional coupler 13
The two interfere with each other to become one light L, which enters the photodetector 15 from the distribution end 12B of the first optical directional coupler 12. This incident light L ₅ is converted into an electrical signal,
The signal is supplied to a synchronous detection circuit 17 via a gate circuit 16.

The gate circuit 16 is supplied with a signal from the second oscillator 21 as a gate control signal, and as in the conventional case, the period is calculated by subtracting the delay time τ in the optical waveguide 14 from the rise of the sawtooth wave signal transmitted by the second oscillator 21. (See Figure 7).
The output of this gate circuit 16 is supplied to a synchronous detection circuit 17 and synchronously detected at the oscillation frequency f _n of the first oscillator 19 .

In this invention, in this way, the synchronous detection circuit 1
The phase difference output between the two beams L ₂ and L ₃ obtained from the oscillator 7 is supplied to an integrator 23, and the integrated output from the integrator 23 is supplied to the second oscillator 21 as a feedback signal.

Further, in this example, a comparator 24 is provided. This comparator 24 operates during the negative period of the signal shown in FIG.
It is checked whether the sawtooth wave generated by the oscillator 21 is properly controlled, and the check signal is given to the second oscillator 21. In other words, the sawtooth wave is controlled so that its period is maintained at 2π·k, where k is an integer. By controlling in this way, it is possible to reduce measurement errors in angular velocity.

In this invention, for example, the loop-shaped optical waveguide 14
By providing an optical directional coupler 29 in the loop-shaped optical waveguide 14, parts L ₂ A and L ₃ A of the two beams L ₂ and L ₃ propagating in opposite directions to each other are taken out.

In this example, an optical fiber is used as the optical directional coupler, and another optical fiber core 29 is brought out close to the optical fiber core of the loop-shaped optical waveguide 14. A portion of _the light L ₂ A is extracted from one end 29A of the optical fiber 29, and a portion of _the light L ₃ A is extracted from the other end 29B. The extracted distributed lights L ₂ A and L ₃ A have coherency, interfere with each other, become one light L ₆ , and enter the light receiver 26 .

One of the lights L ₂ A is emitted from the light source 11, propagates through the optical waveguide 14, and is not modulated at all before being taken out. However, the other light L ₃ A is converted into a sawtooth wave signal by the phase modulator 18. The light intensity of the interfering light is as understood from equations (4) and (5).
Contains a sine wave signal component of frequency f _n and its harmonic components.

This photoelectrically converted signal is applied to a bandpass filter 32 via an amplifier 31, and a component necessary for measuring the angular velocity Ω, for example, a first harmonic component, is extracted from the electrical signal. This first harmonic component is supplied to mixer 33.

On the other hand, the sine wave oscillation signal of frequency f _n from the first oscillator 19 is supplied to the mixer 33 through the phase shifter 35 as required, and the mixer 33 mixes it with the supplied first harmonic component and outputs the mixture. . In this example, the sine wave signal from the first oscillator 19 is transmitted to the phase shifter 3.
In this example, the water is supplied to the mixer 33 via 5. The output mixed signal is passed through a low pass filter 36
The low frequency components are extracted. This low frequency component is a frequency signal corresponding to a phase shift due to serrodyne modulation, and this frequency signal is counted by a frequency counter 37.

“Effect” The two beams of light L ₂ and L ₃ are each ψ(t)=ψ _s t due to serodyne modulation. (11) However, ψ _s is subjected to constant modulation, and due to sine wave modulation, φ(t)=φ _n sin (2πf _n t) ...(12) However, φ _n : Maximum phase shift _n : Receives modulation at the modulation frequency.

From the loop-shaped optical waveguide 14 to the optical directional coupler 2
Of the two lights L ₂ A and L ₃ A taken out by the optical directional coupler 29 , one of the lights L ₂ A is emitted from the light source, propagates through the optical waveguide 14 , and is taken out by the optical directional coupler 29 . Since the light is not modulated, if the frequency when emitted from the light source 11 is ν, the phase change of the light L ₂ A at the light receiving surface of the light receiver 26 is Φ(t) = 2πνt ...(13) can be expressed.

The other light L ₃ A is subjected to serrodyne modulation ψ _s t and phase modulation φ _n sin (2π _n t) by the phase modulator 18, so the phase when it enters the light receiver 26 is Φ'(t) =2πν(t+t ₀ )+ψ _s t +φ _n sin(2πf _n t)+φ ₀ (14). However, t ₀ is the time difference when the two rays of light are incident on the light receiving surface, and φ ₀ is the phase difference between the two rays of light received from the optical directional coupler, optical fiber, etc.

Therefore, the phase difference Ψ between the two lights at the light receiving surface is expressed by the formula
From (13) and (14), Ψ=2πνt ₀ +ψ _s t +φ _n sin (2πf _n t) + φ ₀ ...(15).

When two lights having a phase difference expressed by equation (15) enter the light receiving surface of the light receiver 26, they interfere with each other,
The light intensity I(t) of the interfered light is I(t)=P ₀ [1+cos{ψ _s t +φ _n sin (2πf _n t) +2πνt ₀ +φ ₀ }] ...(16) However, 2P ₀ is the light receiving surface Maximum luminous intensity of the light The intensity I(t) of this light is expressed in frequency components as I(t)=P ₀ [1+cos (ψ _s t+φ′ ₀ ) ・{J ₀ (φ _n )+2J ₂ (φ _n )cos4π _n t+...} −sin (ψ _s t+φ' ₀ ) ・{2J ₁ (φ _n )・sin2πf _n t +2J ₃ (φ _n ) sin6πf _n t+...}] =P ₀ [1+J ₀ (φ _n )cos (ψ _s t+φ′ ₀ ) +J ₁ (φ _n )cos (2πf _n t+ψ _s t+φ′ ₀ ) −J ₁ (φ _n )cos (2πf _n t−ψ _s t−φ′ ₀ ) +J ₂ (φ _n ) cos (4πf _n t+ψ _s t+φ′ ₀ ) −J ₂ (φ _n )cos (4πf _n t−ψ _s t−φ′ ₀ ) +……] …(17). FIG. 3 is a diagram showing this frequency spectrum. However, J ₀ , J ₁ , J ₂ ... are respectively 0th order,
First-order, second-order, ... Betzel functions, and φ′ ₀ =
We set it as 2πνt ₀ +φ ₀ .

This interfered incident light is converted into an electrical signal according to the intensity of the incident light by the photoreceiver 26, and the converted electrical signal is passed through the amplifier 31 and passed through the bandpass filter 32.
from the bandpass filter 32 to the equation f
Only the necessary harmonic components are extracted from among the many harmonic components of the frequency f _n shown in .

When the passband of this bandpass filter 32 is set, for example, to f _n ±(φ _SMAX /2π), the output from the bandpass filter is the first harmonic component of I(t) I(t) ₁ = P ₀ J ₁ (φ _n ) ・{cos (2π _n t+ψ _s t+φ′ ₀ ) −cos (2π _n t−ψ _s t−φ′ ₀ )} (18) is output.

This first harmonic component I(t) ₁ is supplied to the mixer 33. The mixer 33 is supplied with a sine wave signal of frequency f _n output from the first oscillator 19, and the first harmonic component I(t) ₁ is mixed with the sine wave signal. In this example, the sine wave signal to the mixer 33 is supplied via a phase shifter 35, e.g.
The phase difference between the two signals supplied to the mixer 33 is α
In this case, the output I _r (t) from the phase shifter 35 is
If expressed as I _r (t) = Asin (2πf _n t + α), mixer 3
3, the signal output I _n (t) mixed with frequency _n is I _n (t) = I (t) ₁ × I _r (t) = P ₀ J ₁ (φ _n ) ・{cos (2πf _n t + ψ _s t+φ' ₀ ) −cos (2π _n t−ψ _s t−φ′ ₀ )} ・Asin (2πf _n t+α) = (1/2) AP ₀ J ₁ (φ _n ) ・{sin (4πf _n t+α+ψ _s t+φ ' ₀ ) −sin (4πf _n t+α−ψ _s t−φ′ ₀ ) −sin (α−ψ _s t−φ′ ₀ ) +sin (α+ψ _s t+φ′ ₀ )} (19). This phase-detected output I _n (t) is supplied to the low-pass filter 36, and the output of the low-pass filter 36 is I _L (t)=(1/2)AP ₀ J ₁ (φ _n ) ・{sin(α+ψ _s t+φ′ ₀ ) −sin(α−ψ _s t−φ′ ₀ )} =AP ₀ J ₁ (φ _n )cosα·sin(ψ _s t+φ′ ₀ ) ……(20). That is, this signal is a sine wave with a frequency F=ψ _s /2π. (Note that the phase shift difference α by the phase shifter 35 is selected so that this term is maximized.) Therefore, this obtained sine wave signal is transmitted to the frequency counter 3.
7 can be counted. Counting is performed over a predetermined period of time, and the magnitude of the count value during that period corresponds precisely to the magnitude of the phase shift due to serrodyne modulation.

Up to now, the first-order harmonic component has been used as the measurement signal, but if a higher-order harmonic component is used as the measurement signal, a multiplier is provided between the first oscillator 19 and the phase shifter 35. . For example, the bandpass filter 32
When extracting harmonic components, the multiplier 34
The magnification is set to 3 when the second and third harmonic components are extracted.

FIG. 2 is a diagram showing another embodiment of the present invention, in which a light source 11, a photodetector 15, and a loop-shaped optical waveguide 1 are shown.
This is an example in which the coupling with 4 is performed using a pair of Y-branch optical waveguides formed on the substrate 38 instead of an optical fiber. Further, in this example, a frequency shifter such as an electro-optical element may be used instead of the phase modulator 18 to impart a frequency shift to the light, thereby canceling out the sannyac effect.

Although not shown in the figure, in this invention,
Means for detecting the direction of the angular velocity Ω is provided as necessary. For example, since the two lights L ₂ A and L ₃ A enter the light receiver 26 at a predetermined angle of incidence, an interference pattern of bright and dark light is formed on the light receiving surface of the light receiver 26 . The interference pattern moves on the interference plane on the intersection line where the plane and the plane formed by the two incident lights L ₂ and L ₃ intersect, and the angular velocity Ω input to the optical waveguide 14 is clockwise or counterclockwise. Change the direction of movement depending on the surroundings. Therefore, by arranging two light receivers at a distance of 1/4 of the pitch of the bright and dark interference patterns and detecting which of the signals from the light receivers is in the leading phase, the angular velocity can be calculated. You can know the direction of

In addition, the direction signal of this detected angular velocity is
It can also be a signal that is supplied to an oscillator to control whether to increase or decrease the oscillation amplitude of the sawtooth wave oscillation signal.

In the above description, one optical directional coupler 29 is used to transfer two lights L ₂ A,
Although we have explained the case where L ₃ A is taken out, using separate optical directional couplers, the light that has undergone frequency shift or serrodyne modulation and the received light are It may also be configured to extract light that is not present. It is also possible to use two modulators to apply frequency shift or serrodyne modulation and phase modulation to light.

"Effects" As described above, according to the present invention, a closed loop control system that controls the phase modulator by supplying the output of the sanyac effect caused by the angular velocity to the phase modulator is used to cancel the sanyac effect. did. In addition, means for extracting the light modulated by the phase modulator from within the optical waveguide is provided, and the configuration is such that the degree of modulation of the extracted light is directly measured. That is, compared to the conventional indirect measurement of the drive signal voltage V of the phase modulator, the optical interference angular velocity system of the present invention allows the temperature characteristics, frequency characteristics, or dielectric characteristics of the phase modulator to be measured. This makes it possible to accurately measure angular velocity without being affected by changes in rate, etc. Also, since the beat signals caused by the interference of two rays of light are counted, a digital output can be obtained. Therefore, it is easy to process the extracted angular velocity data using an electronic computer or the like.

Further, according to the invention, the phase modulator and the first
If light is extracted from a loop-shaped optical waveguide as in the embodiment shown in FIGS. I can do it. Since the two extracted beams of light are interfered with each other, converted into electrical signals, and then processed, signal processing can be performed relatively easily using an electrical circuit.

In this way, it has many advantages in terms of manufacturing, price, accurate detection of angular velocity, and ease of data processing of detection signals.
The effects of this invention are significant.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the optical interference gyrometer of this invention, FIG. 2 is a block diagram showing another embodiment of this invention, and FIG. A diagram showing the frequency spectrum of a signal obtained by photoelectrically converting the extracted interference light between clockwise light and counterclockwise light, Figure 4 is a block diagram showing an example of a conventional optical interference gyrometer, and Figure 5 is A diagram showing the waveform of the synchronous detection output from the synchronous detection circuit when the angular velocity is zero, Figure 6 is a waveform diagram of the detection output of the synchronous detection circuit when there is an input angular velocity, and Figure 7A is a diagram showing the waveform of the synchronous detection output from the synchronous detection circuit when the angular velocity is zero. Figure 7B is an explanatory diagram of serodyne modulation of two beams of light by the second phase modulator.
is a diagram showing the difference in phase between two lines of light. 11: light source, 12: first optical directional coupler, 1
3: second optical directional coupler, 14: optical waveguide, 1
5: Photodetector, 16: Gate circuit, 17: Synchronous detection circuit, 18, 18A: Phase modulator, 18B: Second phase modulator (frequency shifter), 19: First oscillator, 21: Second oscillator , 22: amplifier, 23: integrator, 24: comparator, 25: third optical directional coupler, 26: light receiver, 27: fourth optical directional coupler,
28: Counter, 29: Optical directional coupler, 31: Amplifier, 32: Band pass filter, 33: Mixer, 34: Multiplier, 35: Phase shifter, 36: Low pass filter, 37: Frequency counter, 38: Y branch waveguide.

Claims (1)

[Claims] 1. A light source, a beam splitter that splits the light from the light source into two, and a loop-shaped optical waveguide into which the two lights from the beam splitter enter as clockwise light and counterclockwise light. an interference means for interfering clockwise light and counterclockwise light emitted from the optical waveguide; a photodetector to which the interfered light is supplied and outputs an electrical signal according to its intensity; and the optical waveguide. A modulation means inserted between one end and the beam splitter and electrically controlled to apply a frequency shift to the light; a first oscillator supplying an alternating signal as a modulation signal to the modulation means; and an output signal. a second oscillator that supplies the alternating signal as a modulating signal to the modulating means; a frequency component of the alternating signal is detected from the output of the photodetector, and the second oscillator is feedback-controlled so that the frequency component becomes zero by the detected output. a synchronous detection circuit that detects the light modulated by the modulation means and the light that has not been modulated and causes them to interfere, and detects the beat signal; means for extracting one of the harmonic components; phase-detecting the extracted signal using the signal obtained from the first oscillator; alternating current at a frequency corresponding to the phase difference between the clockwise light and the counterclockwise light; and means for obtaining a signal.