JPH0323845B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical fiber iron, and more specifically, to an optical fiber iron having a stable scale factor and a wide dynamic range.

BACKGROUND OF THE INVENTION Currently, gyros are used as angular velocity sensors for navigation and attitude control of aircraft, flying objects, automobiles, robots, and the like. By using this gyro, you can obtain not only angular velocity but also data such as direction by integrating it.

Among these types of gyroscopes, optical fiber gyroscopes can be used in any environment without shielding problems because the light and the optical fiber through which the light propagates are not easily affected by magnetic fields or electric fields, and they have no moving parts. Furthermore, it is superior to conventional gyroscopes in terms of minimum detectable angular velocity (sensitivity), drift, measurable range (dynamic range), and scale factor stability. Therefore, it has been attracting attention and being developed in recent years.

Examples of such fiber optic tires include, for example, gear range gear. G. , Bukalo J. A. Others = Optical Fiber Sensor Technology = IEEJ Annual of Quantum Electronics (Giallorenzi TG, Bucaro J.
A. et al “Optical Fiber Sensor Technology”,
IEEE J. of Quantum Electronics) QE−18,
No. 4, pp626-662 (1982) and Kurashiyo and I. P. Culshawand IPGiles “Fiber Optic” Journal of Physics Electronics Science Instruments
Gyroscopes”J.Phys.E:Sci Instrument) 16 pp5
−15, (1983), Tsubokawa, Otsuka, “Optical Fiber Gyroscope” Laser Research, 11 , No. 12, pp889−
902 (1983) and others.

(a) Principle of optical fiber coil The principle of optical fiber coil will now be explained with reference to FIG. 6.

The light from the light emitting element 10 is transmitted to the beam splitter 1
The optical fiber loop is made by winding the single mode optical fiber 18 many times in a coil shape, that is, the input is input to both ends of the sensor coil 20. Propagate light. At this time, if the sensor coil 20 is rotating at an angular velocity Ω,
A phase difference △θ occurs between clockwise light and counterclockwise light, and △θ
The angular velocity Ω is detected by measuring the .

The electric field strength E _cw of the light propagated clockwise and the light propagated counterclockwise in the sensor coil 20,
_Eccw is expressed as follows.

E _cw = E _r sin (ωt + △θ/2) E _ccw = E _t sin (ωt − △θ/2) However, E _r , E _t : Amplitude of counterclockwise light and clockwise light ω : Angular frequency of light t: time △θ: phase difference due to sannyac effect The counterclockwise light and clockwise light with such a phase difference Δθ are combined by the beam splitter 12,
The light is incident on the light receiving element 26. The phase difference Δθ can be determined from the detected intensity of the light receiving element 26. The phase difference Δθ can be expressed as follows.

△θ＝4πLa／cλΩ ……(1) However, L: fiber length of sensor coil a: Radius of sensor coil c: speed of light in vacuum λ: wavelength of light Ω: rotational angular velocity This is called the sanyatsuk effect.

There are various methods of detecting the phase difference Δθ, and various methods have been proposed.

Most simply, when the sum of the counterclockwise light and the clockwise light is square-law detected using a light receiving element, the output I becomes I∝{1+cos(△θ)}...(2).

This is because △θ is in cos, so △θ is 0
The disadvantage is that the sensitivity is poor when close to .

Therefore, an optical mechanism has been proposed in which the phase of either the counterclockwise or clockwise light is shifted by 90 degrees and square-law detection is performed. In this case, the output I takes the form I∝{1+sin(△θ)} (3), so the sensitivity is good when △θ is close to 0.

However, in order to separate one of the lights, three new beam splitters are required to separate the optical paths. Furthermore, the lengths of the separated optical paths must always be made equal.

Improving the sensitivity when Δθ is close to 0 by using a static optical detection mechanism as described above has the above-mentioned difficulties.

(b) Phase modulation type optical fiber irons Therefore, many optical fiber irons that attempt to detect △θ using a dynamic mechanism have been proposed. For example, a phase modulation method, a frequency modulation method, etc. are used. Among them, the phase modulation optical fiber iron is the most superior in terms of minimum detectable angular velocity.

The phase modulation type optical fiber iron is a type in which a phase modulation element is provided at one end of an optical fiber sensor coil, and the phase difference Δθ is determined by measuring the magnitude of the modulation signal.

The phase modulation type optical fiber iron will be explained with reference to FIG.

The coherent light from the light emitting element 10 is split into two by a beam splitter 12 and coupled to both ends of an optical fiber 18 . The optical fiber 18
is divided into a portion wound to constitute a sensor coil 20 and a portion 24 wound around a phase modulation element 22 such as a piezo element driven at an angular frequency ω _n . The light coupled from both ends of the optical fiber is
It propagates clockwise and counterclockwise within the optical fiber sensor coil 20, exits from the opposite end, is combined by the beam splitter 12, and is sent to the light receiving element 2.
6.

When the phase modulation element is installed at an asymmetric position with respect to the sensor coil, the light emitted from the light emitting element at the same time passes through the sensor coil and the phase modulation element winding part in clockwise and counterclockwise directions, but the light is not modulated. Since the times are different, when the output is square-law detected by the light receiving element, a modulated signal appears in the output. Since Δθ is included in the amplitude of the modulation signal, Δθ can be determined by knowing the magnitude of the modulation signal.

For example, assume that a phase modulator is provided near the input end of counterclockwise light. When the length of the optical fiber sensor coil is L, the refractive index of the fiber core is n, and the speed of light is c, the time τ required for light to pass through the sensor coil is τ=nL/c (4).

As mentioned above, the modulation signal has an angular frequency ω _n
Suppose that it is a sine wave. The phase difference φ of the modulation signal when the light emitted from the light emitting element at the same time is divided into clockwise light and counterclockwise light, each undergoing phase modulation.
φ=ω _n τ =nLω _n /c =2πf _n nL/c (5) However, ω _n =2πf _n .

Due to the sannyac effect, the clockwise light and the counterclockwise light have a phase difference of ±Δθ/2, but the phase is further modulated by the phase modulation element. If the amplitude of the phase modulation element is b, the electric field strengths E _cw and E _ccw of the clockwise and counterclockwise lights are E _cw = E _r sin {ωt + △θ/2 + bsin (ω _n t + φ)} ...( 6) E _ccw = E _L sin {ωt−△θ/2+bsin(ω _n t)} ...(7).

The clockwise light and counterclockwise light having the electric field strengths as described above are combined by the beam splitter 12 and square-law detected by the light receiving element 26, so the output S(Δθ, t) of the light receiving element is E It is proportional to the square of the sum of _cw and _Eccw .

S (△θ, t) − {E _cw + E _ccw } ² ...(8) Calculating this, S (△θ, t) = E _r E _L cos {△θ + 2bsin (φ / 2) cos
(ω _n t+φ/2)}+D.C.+{2ω or more}...(9) However, DC means a direct current component.

{2ω or more} means a term with a frequency twice the angular frequency of light. Note that this is 0 because it is not applied to the detector.

becomes. Thus, because of the phase difference φ provided by the phase modulation element, Δθ can be obtained in relation to the amplitude of the modulation signal.

Therefore, DC is omitted and S(△θ, t) is expanded into a series using the Betzel function. First, (9)
The formula is expressed as follows.

S(△θ,t)=E _r E _L [cos△θcos{2bsinφ/2cos(
ω _n t+φ/2)}−sin△θsin{2bsiφ/2cos(ω _n
t+φ/2)}...(10) On the other hand, from the generating function expansion of the Betzel function, It is. By setting t=e ⁱ 〓, it can be expressed as e ^ixsin 〓= _∞ 〓 ^n=-∞ J _o (x) e ⁿⁱ 〓 …(12). From the expansion of the real and imaginary parts of equation (12), we can obtain the series expansion numbers of the cos and sin parts of equation (10). Divide S (△θ, t) into these parts and write S (△θ, t) = (S _c cos △ θ + S _s sin △ θ) E _r E _L … (13), then θ → θ + π /2 conversion, J _-o (x)=(-) ⁿ J _o (x) ...(14) However, using the property that n is a positive integer, ξ=2bsinφ/2 ...(15) Then, writing S _c and S _s above, S _c = J _p (ξ) + 2 _∞ 〓 ⁿ⁼¹ (−) ⁿ J _2o (ξ) cos2nω _n t
…(16) S _o =2 _∞ 〓 ⁿ⁼⁰ (−) ⁿ J2 _o+1 (ξ)cos(2n+1)ω _n t
…(17) becomes. Therefore, S(Δθ, t) is expressed as follows again.

S (△θ, t) = 1/2 (E _r ² + E _L ² ) + (components over 2ωt) + E _r E _L J ₀ (ξ) cos △ θ + E _r E _L 2 _∞ 〓 ⁿ⁼¹ (-1 ) ⁿ J _2o
(ξ)・cos2nω _n t・cos△θ +E _r E _L 2 _∞ 〓 〓 ⁿ⁼⁰ (−1) ⁿ J2 _o+1 (ξ)cos(2n+1)ω _n t・sin△θ
……(10)′ = DC component + 2E _r E _L J ₁ (ξ)cosω _n t・sin△θ−2E _r E _L J ₂ (
ξ) cos2ω _n t·cosΔθ+higher-order component...(10)'' This is the sum of the series of the fundamental wave of the modulation signal ω _n and the high-frequency signal.

By using an appropriate filter, it is possible to extract the fundamental wave ω _n or a high frequency signal of any order.
No matter which signal is adopted, the magnitude of cos Δθ or sin Δθ can be known.

In that case, the amplitude b of modulation by the phase modulation element, the modulation angular frequency ω _n , and the sensor coil transit time τ should be set so that the value of the Betzel function J _o (ξ) of that order becomes large.

The highest sensitivity can be expected from the first-order term (n=0) in equation (17), that is, the second term on the right side of equation (10).This is the fundamental wave component.This fundamental wave component If P (△θ, t), then P (△θ, t) = 2E _r E _L J ₁ (ξ) cosω _n t・sin△θ
...(18). In this way, an output proportional to sin △θ is obtained, and △θ can be found by finding the amplitude of the fundamental wave component.

Furthermore, since sensitivity improves when J ₁ (ξ) is maximized, ξ is set to 1.8. At this time, the DC component J _o
(ξ) is approximately 0.

The above is the basic configuration of a phase modulation type optical fiber pilot.

Problems to be Solved by the Invention As can be seen from equation (18) above, the amplitudes E _r and E _L of the clockwise light and counterclockwise light are within the amplitude of the fundamental wave component P of the light receiving element output. included. Also, J ₁
There is also a coefficient called (ξ).

In order to accurately determine Δθ from such an output, the values of E _r , E _L , and J ₁ (ξ) must be stable.

However, the values of E _r , E _L , and J ₁ (ξ) vary. In particular, the light amplitudes E _r and E _L tend to fluctuate.
E _r and E _L are fluctuations in the output of the light source, fluctuations in the amount of light passing through the polarizer inserted to remove the optical path difference,
It can easily change due to misalignment of the optical system, etc.

Furthermore, even if the direction of polarization when entering the fiber is determined, the state of polarization will fluctuate as it propagates through the fiber due to the effects of temperature, pressure, strain, etc.
The polarization of the emitted light is not constant.

As a result, the amplitudes of light E _r and E _L for the light receiving element are
will change.

In addition to such fluctuations in scale factors, △
Since θ is detected in the form of sin△θ, the detection range is limited to the range in which △θ can be approximated to sin△θ with extremely small errors, and there are problems such as it is difficult to obtain a wide dynamic range. .

Therefore, the present invention provides an optical fiber coil whose scale factor does not depend on the above-mentioned E _r and E _L and which has a wide dynamic range without being limited to the range in which △θ can be approximated to sin △θ. That is.

Means for Solving the Problems According to the present invention, as shown in FIG. An optical fiber 1 whose light is split by a beam splitter 28, coupled to both ends, and which propagates through the sensor coil 20 in both directions and outputs the light from both ends.
8 and a light-receiving element 26 that receives the bidirectional light propagated through the optical fiber 18, and measures the rotational angular velocity from the phase difference between the bidirectional light generated when the sensor coil rotates. A phase modulator 3 is installed near one end of the sensor coil.
0, and when the time difference from when the two-way light is split until it reaches the phase modulator 30 is τ,
While the phase modulator 30 is driven by a triangular wave pulse 32 with a period of 2τ and a duty of 1/2,
Comparing the magnitude of the phase difference caused by rotation from the output of the light receiving element that receives the combined light of the light emitted from the optical fiber and the phase difference caused between the two directions of light due to modulation by the phase modulator, An optical fiber gyroscope is provided which is characterized in that a rotational angular velocity is detected from the ratio of a large time to a small time.

Effect In the above configuration, if the length of the optical fiber from the beam splitter 28 to the phase modulator 30 on the side that does not pass through the sensor coil 20 is ignored, the length of the optical fiber 18 is L, and the refractive index is When n is the speed of light and c is the speed of light, τ is given by nL/c from the above equation (4).

In this way, the phase changes experienced by the clockwise and counterclockwise lights propagating through the sensor coil of the fiber optic gyroscope as described above are illustrated as shown in Figure 2 a and b, and the difference between the clockwise and counterclockwise lights is as follows. There is a time difference of τ.

The clockwise and counterclockwise lights of such an optical fiber gyroscope can be expressed as E _cw = E _r sin (ω + φ _cw −△θ/2) E _ccw = E _L sin (ωt + φ _ccw + △θ/2) . Therefore, its clockwise light E _cw ,
When the counterclockwise light E _ccw is square-law detected, S(△θ) is: S(△θ) = 1/2 (E _r ² + E _L ² ) + E _r E _L cos {(
It can be expressed as φ _cw −φ _ccw )−△θ} (19). Here φ _cw −φ _ccw is a function of time. If we set this as φ ₀ (t), then φ ₀ (t)
can be expressed as shown in Figure 2c.

Figures 3a, c and e show the relationship between Δθ and φ ₀ (L) for various values of Δθ.

Therefore, if we generate a pulse train that turns ON when the cos of the last term on the left side of equation (19) is positive and turns OFF when it is negative, then for each △θ, Figure 3 b and d Pulses as shown in and f are obtained.

Here, if the ON and OFF times in one cycle are T ₂ and T _1, respectively, then T ₁ + T ₂ = 2τ, and if the maximum width of phase modulation is φ _nax (Figure 2 a) , φ ₀ (t) with respect to time change is ±2φ _nax /τ.

From this, it is expressed as T ₁ =τ−τ/φ _nax Δθ T ₂ =τ+τ/φ _nax Δθ.

For example, if we set D(△θ)=T ₂ −T ₁ /T ₁ +T ₂ ...(20), then D(△θ)=(τ/φ _nax )・△θ/τ=1/φ _nax
△θ ……(20a) becomes. In other words, the phase difference △θ caused by rotation
and the phase difference (φ _cw - φ _cw ), that is, φ ₀ (t), generated between the two directions of light due to the modulation by the phase modulator, and determine the time T ₂ (or T ₁ ) that is large. A small ratio of time T ₁ (or T ₂ ) results in an output proportional to Δθ, ie the sannyac phase difference. In this case, the scale factor depends only on φ _nax and is not affected by changes in the amount of light, etc.

Embodiments Hereinafter, embodiments of the optical fiber sensor according to the present invention will be described with reference to the accompanying drawings.

FIG. 4 is a diagram showing the configuration of one embodiment of a phase modulation type optical fiber coil embodying the present invention. This phase modulation type optical fiber coil has a minimum configuration that meets the basic requirements of an optical fiber coil. Regarding the minimum configuration, please refer to Izekiel S. and ADT H. J.A. 〓Optical fiber rotation sensor〓 Springer-Verlag Berlin (EzeKil S. and Arditty
HJ “Fiber Optic Rotation Sensors”
Springer-Verlag Berlin.) 1982 has a detailed explanation.

In the illustrated phase modulation type optical fiber iron, a light source such as a triangular wave pulse 32 is provided and is driven by a power source (not shown) to generate a light beam. Note that as a light source, a He--Ne laser, a semiconductor laser, a superluminescent diode, etc. can be used. The light beam generated by the light emitting element 32 is sent to beam splitters 34 and 36, which are like half mirrors arranged in series.
The beam splitter 34 is for splitting the light to the light receiving element 38, and the beam splitter 34 is for splitting the light from the light source 32 into two.
It is coupled to both ends of the optical fiber 40.

Optical fiber 42 is wound into a coil a number of times to form an optical fiber sensor, and includes a sensor coil 42 and a portion coupled to phase modulators 44 and 46, respectively.

One of the phase modulators 46 is composed of, for example, a piezoelectric vibrating element, is connected to an AC excitation power source 48 for phase modulation, and is configured to be driven by a sinusoidal AC wave having an angular frequency ω _n . In this case, the optical fiber 40 is wound around the piezoelectric vibrating element 46, for example.

The other phase modulator 44 is for performing phase modulation with a triangular wave. However, phase modulation using a triangular wave is difficult to achieve using a method that utilizes the resonance of a piezoelectric vibrator. For this reason, a method using an electro-optic crystal such as LiNbO ₃ is considered to be effective. From the viewpoint of modulation efficiency, it is desirable to use a waveguide type element. For example, as shown in FIG. 5, an optical waveguide 52 is provided on the surface of a LiNbO ₃ substrate 50, and electrodes 54 and 56 are embedded so as to sandwich the optical waveguide 52. In this case, an optical waveguide 52 is inserted into one end of the optical fiber 40 and electrodes 54 and 56
During this period, a triangular wave modulation voltage with a period of 2τ is applied from the triangular wave generator 48A.

The light beams propagated clockwise and counterclockwise through the optical fiber 42 are output from both ends of the optical fiber 42, are combined by the beam splitter 36, and are incident on the light receiving element 38 via the beam splitter 32.

The electrical output of the light receiving element is connected to the input of a synchronous detector 58. This synchronous detector 58 is
Similar to the piezoelectric vibrating element 46 described above, it is connected to an AC excitation power source 48 for phase modulation, and is adapted to receive alternating current having an angular frequency ω _n . The output of the synchronous detector 58 is then input to the signal processor 60. The signal processor 60 is supplied with a signal representing the maximum amplitude φ _nax of the phase modulation from the triangular wave generator 48A.

The reason for adopting the phase modulation method as described above is
In the basic configuration shown in Figure 1, if the output of the light-receiving element is in the form shown in equation (19), if detection is performed when the cos of equation (19) is near 0, the cos may be positive or negative. This is because the value of cos is positive, and it is not possible to distinguish between times T ₁ and T ₂ . As shown in FIG. 4, when the modulation angular frequency of the sine wave phase modulation of the second phase modulator 46B is ω _n , the ω _n component in the output of the light receiving element is obtained as follows.

S(△θ)=2E _r E _L J ₁ (2bsinφ/2) cosω _n t・
sin {(φ _cw −φ _ccw )−△θ}……(21) However, b: modulation degree, φ=nL/c・ω _n As a result, if the inside of sin changes near 0,
It can be easily confirmed that the sign of the sin can be determined by the sign of the sin value.

The phase modulation type optical fiber coil constructed as described above operates as follows.

A light beam from a light emitting element 32 driven by a power source passes through a beam splitter 34 and is split into two by a beam splitter 36 and then connected to an optical fiber 40.
is connected to both ends of the

The light beam input to the optical fiber 40 has a phase difference at the part of the sensor coil 42 undergoing rotation, and a piezoelectric vibrating element driven by a sinusoidal alternating current with an angular frequency ω _n from an alternating current excitation power source 48. 46
and the portions coupled to the electro-optic effect phase modulator 44 are phase modulated.

The clockwise light beam and the counterclockwise light beam, which have a phase difference and are phase-modulated in the optical fiber 40, are output from both ends of the optical fiber 40, are combined by the beam splitter 36, and are further combined by the beam splitter 36. The light receiving element 38 via 32
incident on .

The output of the light receiving element 38 is synchronously detected by a synchronous detector 58 at ω _n , and the output expressed by the above equation (21) is output to the signal processor 60. The signal processor 60 calculates the time T ₁ when the input signal is negative and the time T ₂ when it is positive, calculates the above equation (20), and further divides by φ _nax according to equation (20a). Outputs a signal representing θ.

Here, when the required modulation voltage of the electro-optic effect phase modulator 44 is calculated, it is as follows. The phase change due to the electro-optic effect in the electro-optic effect phase modulator 44 is expressed as follows.

φ=k△nl =1/2kn ³ γEl ...(22) However, k=2π/λ λ: wavelength n: refractive index γ: electro-optic constant E: electric field (V _nax /t) V _nax : electrode 54 and Maximum voltage applied between the electrodes 56 t: Distance between the electrodes 54 and 56 l: Length of the electrodes 54 and 56 In addition, by appropriately selecting the polarization of the light and the direction of applying the electric field, r of LiNbO ₃ can be maximized. Naru R ₃₃ is available. At this time, the maximum value φ _nax in equation (22) is as follows: λ=0.83×10 ^-6 (m), n=2.29, γ=30.8×10 ^-12 (m/v), t=10×10 ^-6 ( m), l=5×10 ^-3 (m), φ _nax =1/2kn ³ γV _nax /tl=1/2×2π/0
.83×10 ^-6 × (2.29) ³ × (30.8×10 ^-12 ) ×V _nax /10×10 ^-6 ×5×10 ^-3 =0.7V _nax (rad
)...(23) becomes. On the other hand, from Fig. 3, the detection range of △θ is φ _nax
It can be seen that up to Therefore, for example, if we consider a fiber length of 200 m and a coil radius of 25×10 ^-3 (m), then according to equation (1), △θ=4πLa/cπΩ=4π×200×25×10 ^-3 /3×10
⁸ ×0.83×10 ^-6 Ω=0.25Ω (rad)……(24) Therefore, from equations (23) and (24), V _nax
The relationship is V _nax = 0.25/0.7Ω _nax = 0.36Ω _nax (V) with respect to the measured maximum angular velocity Ω _nax . Therefore, Ω _nax =300°/sec=
Assuming 5.2 rad/sec, the above measurement can be achieved with a sufficiently low voltage of V _nax =1.8 (V).

In addition, in the description of the above embodiment, a description of the specific configuration of the signal processor 60 that obtains T ₁ and T ₂ and performs calculations using equations (20) and (20a) is omitted. However, it will be clear to those skilled in the art that both digital and analog implementations are readily possible. For example, in the case of a digital type, the high-speed clock may be counted during each period when the input signal is positive and when the input signal is negative, and the necessary arithmetic processing may be performed using the counted values as T ₁ and T ₂ .

Further, the phase modulator using a triangular wave is not limited to one that utilizes an electro-optic effect, and any element may be used as long as it is capable of phase modulation using a triangular wave.

Effects of the Invention As is clear from the above description, in the optical fiber coil according to the present invention, the scale factor does not depend on the amplitudes E _r and E _L of the light propagating through the optical fiber.
Moreover, it has a wide dynamic range without being limited to the range in which Δθ can be approximated to sin Δθ. Therefore, the optical fiber according to the present invention can be used in a wide range of applications.

[Brief explanation of drawings]

FIG. 1 is an optical system diagram illustrating the principle of the optical fiber coil according to the present invention, and FIG. 2 shows the phase modulation by a triangular wave of the light in both directions propagating through the optical fiber of the fiber optic coil in FIG. 3 is a waveform diagram showing the relationship between the phase difference due to the triangular wave and the phase difference due to rotation. FIG. 4 is a waveform diagram showing the relationship between the phase difference due to the triangular wave and the phase difference due to rotation. FIG. FIG. 5 is a schematic perspective view illustrating an example of an electro-optic phase modulator used in the phase modulation type optical fiber coil shown in FIG. 4, and FIG. FIG. 7 is a basic configuration diagram illustrating the principle of a fiber optic iron. FIG. [Main reference numbers] 10...Light emitting element, 12...
... Beam splitter, 14, 16 ... Coupling lens, 18 ... Optical fiber, 20 ... Sensor coil, 22 ... Phase modulation element, 26 ... Light receiving element,
30... Phase modulator, 32... Light source, 34, 36
... Beam splitter, 38 ... Light receiving element, 40
...Optical fiber, 42...Sensor coil, 44...
... Electro-optic effect phase modulator, 46 ... Piezoelectric vibration element, 48 ... Sine wave AC excitation power supply, 48A ... Triangular wave generator, 50 ... LiNbO ₃ substrate, 52 ... Optical waveguide, 54, 56 ... Electrode , 58... synchronous detector, 60... signal processor.

Claims (1)

[Scope of Claims] 1. A device comprising a light emitting element and a sensor coil portion wound in a coil shape many times, and in which light from the light emitting element is branched and coupled to both ends, and light propagated in both directions through the sensor coil is split. An optical fiber coil is equipped with an optical fiber that outputs from both ends and a light receiving element that receives light in both directions propagated through the optical fiber, and measures the rotational angular velocity from the phase difference between the lights in both directions generated when the sensor coil rotates. In this case, a phase modulator is provided near one end of the sensor coil, and when the time difference between the two directions of light branching and reaching the phase modulator is τ, the period is 2τ and the duty is 1/2. While the phase modulator is driven by a triangular wave pulse, from the output of the light-receiving element that receives the composite light of the light emitted from the optical fiber, a bidirectional light is generated due to the phase difference caused by rotation and the modulation by the phase modulator. An optical fiber gyro is characterized in that the rotational angular velocity is detected from the ratio of the time when the phase difference is large and the time when it is small by comparing the magnitude of the phase difference generated between the two. 2. The optical fiber gyroscope according to claim 1, wherein the phase modulator is a phase modulator that utilizes an electro-optic effect. 3. The optical fiber is further provided with a second phase modulator that performs phase modulation using a sine wave, and the output of the light receiving element is synchronously detected at the modulation frequency of the second phase modulator. An optical fiber gyroscope as claimed in claim 1 or 2.