JPH05332775A - Photo interference angular velocity meter - Google Patents

Abstract

PURPOSE:To maintain the stability of input and output scale factors, and a frequency response characteristic against a temperature fluctuation by alternately or intermittently applying a rectangular wave phase modulation as alternate phase modulation to right-turn and left-turn light propagating through an optical passage. CONSTITUTION:A phase modulator 16 performs phase modulation A=pi/4 (rad) for light-turn and left-turn light during the period I, and phase modulation A (rad) in the period II following the period I, alternately with the first rectangular wave modulation signal. Furthermore, phase modulation Bnot equal to A (rad) is applied to both of the light with the second rectangular wave modulation signal in the modulator 16 during the period III, and then phase modulation - B (rad) is applied during the succeeding period IV. This phase modulation with the second rectangular wave modulation signal is inserted alternately or intermittently, together with the phase modulation with the first rectangular wave modulation signal. This selection of phase modulation is performed by a clock CK8. As a result, even if a loss in an optical system fluctuates due to a change in ambient temperature or the like, the scale factors of input and output and a frequency characteristic can be kept stable.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical interference for measuring the angular velocity applied around the axis of a loop-shaped optical path by detecting the phase difference between the clockwise light and the counterclockwise light propagating in the optical path. The present invention relates to an angular velocimeter, and more particularly to one that imparts rectangular wave-like biasing to the phase difference between the two lights, and particularly to improve the stability of the input / output scale factor or frequency response characteristic with respect to temperature fluctuations.

[0002]

2. Description of the Related Art A conventional optical interference angular velocity meter (hereinafter referred to as FOG) will be described with reference to FIG. Light I from the light source 11
Is sequentially introduced through the optical coupler 12, the polarizer 13, and the optical coupler 14, and is input from both ends of an optical path 15 constituted by, for example, an optical fiber coil wound a plurality of times in a loop. Optical path 1
Both the left-handed and right-handed lights propagating through 5 are phase-modulated by a phase modulator 16 arranged between one end of the optical path 15 and the optical coupler 14. The two lights subjected to the phase modulation are combined by the optical coupler 14 and interfere with each other, and are branched to the light receiver 17 by the optical coupler 12 and photoelectrically converted. The optical integrated circuit 18 shown in the figure is manufactured by vapor-depositing titanium Ti or the like on an optical crystal of lead niobate LiNbO ₃ and thermally diffusing it, and has an optical coupler 14, a phase modulator 16 and the like integrated therein.

In a state where the angular velocity in the circumferential direction is not applied to the optical path 15, the phase difference between both lights in the optical path 15 is ideally zero, but the circumference of the optical path 15 is rounded. When the angular velocity Ω is applied to the light source, a so-called Sagnac effect is generated by the angular velocity Ω, and a phase difference ΔΦs is generated between the two lights. This phase difference ΔΦs is expressed by the following equation.

ΔΦs = 4πRLΩ / (Cλ) (1) where C: speed of light λ: wavelength of light in vacuum R: radius of optical fiber coil 15 L: optical fiber length of optical fiber coil 15 phase modulator 16 is optical Positive and negative π / 4 (rad) rectangular wave phase modulations having a width of light propagation time τ in the path 15 are alternately applied to both lights passing through the phase modulator 16. The propagation time τ is represented by τ = nL / C, where n is the refractive index of the optical path 15. As a result, in the phase difference biasing ΔΦb between the two lights, as shown in FIG. 4, positive and negative π / 2 (rad) rectangular wave phase differences appear. In FIG. 4, a curve 19 shows the characteristic of the intensity I ₀ of the interference light of both lights with respect to the phase difference ΔΦ between the clockwise light and the counterclockwise light. The positive and negative half cycles I ₁ and II ₁ of the phase modulation are in a state where the input angular velocity is not applied to the optical path 15, that is, the Sagnac phase difference Δ.
Phase difference biasing when Φs is zero is shown, and the outputs of the interference light at that time are shown as I ₁ ′ and II ₁ ′, respectively. Each of the positive and negative half-cycles I ₂ and II ₂ of the phase modulation has an optical path 1
5 shows the phase difference biasing in the state where the input angular velocity is applied, that is, the state where the Sagnac phase difference ΔΦs occurs, and the outputs of the interference light at that time are shown by I ₂ ′ and II ₂ ′. These interference lights are photoelectrically converted by the light receiver 17 and then converted into A / D.
It is input to the converter 21.

The A / D converter 21 receives the interference light I ₁ ′, I _{1 in} response to the clock signal CK ₁ from the clock circuit 22.
The photoelectric conversion signals of I ₁ ′, I ₂ ′, II ₂ ′, ... Are converted as digital quantities. The output of the A / D converter 21 is output by K.
Calculate (In'-IIn '). As a result, when the input angular velocity is zero, K · (I ₁ ′ −II ₁ ′) = 0, and when the input angular velocity is applied, K · (I ₂ ′ −II ₂ ′) ≠
It becomes 0. This relational expression is shown below.

K · In ′ = (Po / 2) · K · (1-sinΔΦ) (2) K · IIn ′ = (Po / 2) · K · (1 + sinΔΦ) (3) ∴K · (In′− IIn ') =-Po * K * sin [Delta] [Phi] (4) Here, K represents the gain of the photodetector 17, the A / D converter 21, and the third logic circuit 23.

Since the phase difference ΔΦ between the two lights can be regarded as the Sagnac phase difference ΔΦs, K · (In′-II)
By calculating n '), the input angular velocity can be detected with high sensitivity. The output of the logic circuit 23 is directly output to the output terminal 24 as the output of the FOG, and the D / A
An analog quantity is converted by the converter 25 and output terminal 26
You can also output to. The phase modulator 16 is driven by the phase modulation drive circuit 27, and the light source 11 is driven by the light source drive circuit 2
Driven by 8. The logic circuit 23, the D / A converter 25, and the phase modulation drive circuit 27 are supplied from the clock circuit 22 to the clocks CK ₂ , CK ₃ , and CK for their operations.
₄ are being supplied.

In FIG. 5, the step difference ramp waveform shown in FIG. 6A is generated by the output from the logic circuit 23, and the phase difference ΔΦs generated in the optical path 15 is generated by feeding back the step difference ramp waveform to the phase modulator 29. 3 shows a zero loop closed loop type FOG that cancels. In this case, the input angular velocity is generally measured by counting the repetition frequency of the stepped ramp waveform.

First, the integrating circuit 31 integrates the output of K (In'-IIn ') from the logic circuit 23. The output from the integrating circuit 31 is determined by the reset circuit 32 to have a step height ΔΦ _f shown in FIG. 6A, and the maximum phase Φ _R of the step ramp waveform is 2π (ra.
It is reset at d). The digital amount of this stepped ramp waveform in the reset circuit 32 is D /
The A converter 33 converts the analog quantity as shown in FIG. 6A and inputs the analog quantity to the phase modulator 29. The width of one step of the stepped ramp waveform is the same as the propagation time τ of light, and as a result, as shown in FIG. 6A, the clockwise light 34 and the counterclockwise light 39, which are deviated by τ time, interfere with each other in the optical coupler 14. , The feedback phase difference between the two lights is ΔΦ _f and (ΔΦ _f
Two states of _f- 2π) occur. For this system,
(4) a phase difference .DELTA..PHI between both light in the expression, ΔΦ = ΔΦs -ΔΦ _f, is represented by _{ΔΦ = ΔΦs -ΔΦ f + 2π (} 5).

If the step ramp waveform is controlled so that ΔΦ _f = ΔΦ s is obtained from the equation (5), the equation (4) is always zero and the closed loop of the null method can be achieved. Here, if the number of steps of the stepped ramp waveform is N and the cycle is T,
The repetition frequency f is expressed by the following equation. f = 1 / T = 1 / (Nτ) = CΔΦ _f / (2πnL) (6) When the null method is achieved, as described above, Δ
Φ _f = ΔΦ s holds, and the following formula is obtained by substituting the formula (1) into the formula (6).

F = 2RΩ / (nλ) (7) As shown above, the repetition frequency f of the stepped ramp waveform
By measuring, the input angular velocity can be detected.
The output of the reset circuit 32 is the output terminal 3 as the FOG output.
6 is output. Integration circuit 31, reset circuit 32, D
To the A / A converter 33 and clock C for the respective operations.
K ₅ , CK ₆ , and CK ₇ are supplied from the clock circuit 22.

On the other hand, the closed loop transfer function G (S)
Is expressed by the following equation. G (S) = (2R / nλ) · (1 / (1 + STs)) (8) where _{Ts = To / (K T)} K T: loop transfer function gain (K _T αPo) To: integrator 31 The gain K _T of the open loop transfer function of the time constant (8) is proportional to the amount of light Po reaching the light receiver 17. That is, the amount of light reaching the light receiver 17 has a 1: 1 relationship with the frequency response of the closed loop.

[0013]

Generally, it is considered that the optical system of the FOG fluctuates by nearly 30% with a temperature change of -20 ° C to + 70 ° C. That is, the light amount P reaching the light receiver 17
o changes by the same amount. Therefore, in the case of the open loop type FOG shown in FIG.
Since this constitutes the input / output scale factor, the scale factor of FOG fluctuates by 30%, which is the same amount as the fluctuation amount of Po. Further, in the case of the closed loop type FOG shown in FIG. 5, a change of 30% in Po does not affect the output of the FOG as shown in the equation (7), but the frequency characteristic as shown in the equation (8). Will be affected by 1: 1.

Therefore, an object of the present invention is to provide an optical interference angular velocity meter in which the temperature stability of the input / output scale factor of the open loop type FOG is improved and the temperature stability of the frequency characteristic of the closed loop type FOG is improved. It is in.

[0015]

According to the present invention, the phase modulation means causes A = 2nπ ± π / 4 (ra
d) phase modulation is applied, and −A (r
In addition to the first rectangular wave phase modulation of the alternating phase modulation which gives the phase modulation of (ad), the phase modulation of B ≠ A (rad) is given in the third period, and the phase modulation of −B (rad) is given in the following fourth period. The second rectangular wave-like phase modulation of alternating phase modulation that gives the output of the photodetector at the time of the first rectangular wave-like phase modulation and the second rectangular wave-like wave modulation is given alternately or intermittently. The fluctuation of the output is detected from the output of the light receiver at the time of phase modulation, and the fluctuation is corrected.

Therefore, as the correction means, according to the invention of claim 2, the difference ΔV ₁ between the output from the photoreceiver in the I period and the output from the photoreceiver in the III period, or the IIth period.
Difference between the output from the photoreceiver during the period IV and the output from the photoreceiver during the fourth period ΔV ₂ or the amount of light reaching the photoreceiver such that the average value of ΔV ₁ and ΔV ₂ becomes constant, or A control means for automatically adjusting the gain of the electric circuit is used.

Alternatively, the correction means is the same as the invention according to claim 3.
According to the above,₁Or the above ΔV ₂, Or the above Δ
V₁And ΔV₂The average value of the transfer function gain of the optical interference gyro
As information, calculate the ratio between this and the reference signal, and calculate the ratio.
Hand to multiply input / output scale factor of optical interference gyro
Steps are used.

[0018]

FIG. 1 shows an embodiment of an optical interference angular velocity meter according to the present invention, in which parts corresponding to those in FIGS. 3 and 5 are designated by the same reference numerals. As in the case of FIG. 3, the phase modulator 16 uses the first rectangular wave modulation signal to rotate clockwise (CW) light and counterclockwise (CCW).
For light, A = π / 4 (rad) phase modulation is alternately performed in the I-th period, and −A (rad) phase modulation is alternately performed in the subsequent II-th period. In the present invention, further, the phase modulator 16 applies the phase modulation of B ≠ A (rad) to both lights by the second rectangular wave modulation signal in the third period III, and the subsequent phase modulation is performed.
In the IV period, −B (rad) phase modulation is applied. The phase modulation by the second rectangular wave modulation signal is inserted alternately or intermittently with the phase modulation by the first rectangular wave modulation signal. This switching is performed by the clock CK ₈ .

In FIG. 2, In, IIn (n = 1, 2, ...
Each half cycle of ..) indicates the first rectangular wave phase modulation, and the phase difference biasing between the two lights at that time appears as a rectangular wave phase difference of positive and negative π / 2 (rad). II
Each cycle of Im, IVm (m = 1, 2, ...)
It shows rectangular wave phase modulation, and the phase difference bias ΔΦ _b2 between both lights at that time is positive and negative (π / 2 + Φ a) (rad).
Appears as a rectangular wave phase difference of. The respective outputs of the interference light in these half cycles In, IIn, IIIm, IVm are indicated by In ', IIn', IIIm ', IVm', respectively. These interference lights are photoelectrically converted by the light receiver 17 and then input to the A / D converter 21.

The A / D converter 21 receives the interference light I ₁ ′, II according to the clock signal (CK ₁ ) from the clock circuit 22.
_The photoelectric conversion signals of ₁ ', III ₁ ', IV ₁ ', I ₂ ', II ₂ '... A / D converter 2
The output of 1 is K (In'-IIn ') and K (IIIm'-IV) for each cycle of phase modulation by the logic circuit 23.
m ') and either. These calculation results are used as the output of the FOG as shown in the equation (4). On the other hand, the logic circuit 37 outputs one of the following from the output data of each A / D converter 21 during the first rectangular wave phase modulation and the second rectangular wave phase modulation, which are adjacent to each other by the clock signal CK ₉ from the clock circuit 22. Do the calculation.

K · {(I ₁ ′ + II ₁ ′) − (III ₁ ′ + IV ₁ ′)} = Po · K. · SinΦa (9) K · ( I 1 '-III 1') or _{K · (II 1 '+ IV} 1') = (Po / 2) · K · sinΦa (10) (9) formula or, (10) Is converted into an analog amount Vs by the D / A converter 38 and input to the differential amplifier 39, where it is compared with the reference voltage V _R from the reference signal generator 40.

[0022] _{Ve = V R -Vs = V R} -Po · Ko · sinΦa (11) where _{Ko = K · K DA K DA} : Output Ve gain differential amplifier 39 of the D / A converter 38, It is applied to the electric filter 41. The electric filter 41 is, for example, an integrator, and its output is applied to the light source drive circuit 28 that controls the light amount of the light source 11, and the light amount I of the light source 11 is controlled. Here, in the initial stage, V _R = Po · Ko · sin
If it is set to Φa (here, the phase Φa is a known value and a constant value as described above), the light quantity Po,
If the electric circuit gain Ko and the phase Φa are constant, Ve
Is zero. Here, it is assumed that the ambient temperature changes and, for example, the loss of the FOG optical system increases and the light amount I ₀ reaching the light receiver 17 decreases. As a result, Ve produces a positive voltage according to the equation (11). This positive voltage is applied to the next electric filter 41.
To generate a positive integrated voltage. The light source drive circuit 28 is controlled so that the input of the electric filter 41, that is, the output Ve of the differential amplifier 38 is always zero, if the light amount of the light source 11 is adjusted by the positive integrated voltage. To be done.

As a result, the FOG changes due to changes in the ambient temperature.
Even if the loss of the optical system fluctuates, in the open-loop type FOG shown in FIG. 3, the amplitude in the equation (4), that is, the input / output scale factor can always be kept stable, and the closed loop shown in FIG. In loop type FOG,
It is possible to keep the frequency characteristics of the closed loop in the equation (8) stable. The integrating circuit 31, the reset circuit 32, and the D / A converter 33, which are indicated by broken lines in FIG.
The function is the same as that of the closed loop type FOG having the same number. Further, instead of feeding back the output of the electric filter 41 to the light source drive circuit, the output is fed back to the automatic gain control circuit 42 inserted on the output side of the light receiver 17 as shown by the broken line in FIG.
Gain, that is, the circuit gain K in the equations (4) and (8) may be controlled.

Φa is determined by the measurement accuracy of the optical interference angular velocity meter. That is, a value as small as possible, which is about twice the measurement resolution or more, is preferable. If it is too small, the fluctuation cannot be detected correctly, and even if it is too large, the output suddenly changes at the boundary between the first rectangular wave phase modulation and the second rectangular wave phase modulation, but if the circuit does not follow this, the fluctuation is correctly detected. Because you can't.

The reference signal generator 40 and the electric filter 4 are also provided.
It is also possible to easily provide the logic circuit 37 with one calculation function. In that case, the output Vc of the electric filter 41 included in the circuit 37 is D / A converted and fed back to the light source drive circuit 28 as an analog amount. Further, the variation rate of the digital output D ₃ of the logic circuit 37 or the D / A converted output Vs from the reference amount is obtained, and the FOG output represented by the equation (4) is calculated according to the variation rate, for example, by a computer or a DSP ( Numerical correction may be performed using a digital signal processor). That is, ΔV ₁ or ΔV ₂ , or Δ
The average value of V ₁ and ΔV ₂ may be used as the transfer function gain information of the optical interference angular velocity meter, the ratio between this information and the reference signal may be obtained, and the ratio may be multiplied by the input / output scale factor of the optical interference angular velocity meter. The optical coupler 14 and the phase modulators 16 and 29 may be configured using optical fibers.

[0026]

As described above, according to the present invention, when the propagation time of light in the optical path is τ, one period is 2τ for both left-handed and right-handed light in the optical path. 1 rectangular wave phase modulation, the first half cycle (τ time) I
The phase modulation of A = π / 4 (rad) is applied to the period, the phase modulation of −A (rad) is applied to the subsequent period II, and the first half cycle is the second rectangular wave phase modulation in which one cycle is 2τ. A phase modulation of B ≠ A (rad) is applied in the third period (τ time), and a phase modulation of −B (rad) is applied in the subsequent IV period, alternately or intermittently with the first rectangular wave phase modulation. The phase difference biasing, the output from the photodetector in the I-th period of the first rectangular wave phase modulation and the second
The voltage difference ΔV ₁ from the output from the light receiver in the third period of the rectangular wave phase modulation or the first rectangular wave phase modulation
The voltage difference Δ between the output from the photoreceiver in the II period and the output from the photoreceiver in the IV period of the second rectangular wave phase modulation Δ
Since a control circuit for automatically adjusting the amount of light reaching the light receiver or the gain of the light receiver or the electric circuit thereafter is provided so that the average value of V ₂ or ΔV ₁ and ΔV ₂ becomes constant, an open loop type FOG is provided. The temperature stability of the input / output scale factor is improved, and the temperature stability of the frequency characteristic of the closed loop type FOG is improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between phase difference biasing and its interference light intensity in the embodiment of FIG.

FIG. 3 is a block diagram showing a conventional open loop optical interference gyro.

4 is a diagram showing phase difference biasing and interference light intensity in the optical interference angular velocity meter of FIG.

FIG. 5 is a block diagram showing a conventional closed loop optical interference angular velocity meter.

6 is a diagram showing a feedback phase modulation signal and a feedback phase difference between both lights in the optical interference gyro of FIG.

Claims (4)

[Claims] 1. An optical path that makes at least one round, branching means for passing clockwise light and counterclockwise light to the optical path, and interference means for interfering the clockwise light and counterclockwise light propagating through the optical path. A phase modulating means for providing a phase change to the clockwise light and the counterclockwise light, which is arranged in cascade between the branching means and one end of the optical path, and the light intensity of the interference light as an electric signal. In an optical interference gyro having a photodetector for detecting, when the propagation time of light in the optical path is τ, one cycle is 2τ, and A (rad is included in the first period I of the first half cycle (τ time). ) Phase modulation, followed by −A (rad) phase modulation in the second period II, which is an alternating phase modulation, which is the first rectangular wave phase modulation, one cycle is 2τ, and the first half cycle (τ Time)
The second rectangular wave-like phase modulation, which is an alternating phase modulation that gives B (rad) phase modulation in the III period and −B (rad) phase modulation in the subsequent IV period, is inserted alternately or intermittently. Phase difference biasing means, correction means for detecting an output fluctuation by using the photodetector output during the first rectangular wave phase modulation and the photodetector output during the second rectangular wave phase modulation, and correcting the fluctuation. And an optical interference angular velocity meter. 2. The difference between the output from the photoreceiver in the I period of the first rectangular wave phase modulation and the output from the photoreceiver in the III period of the second rectangular wave phase modulation. ΔV ₁ or the output from the photodetector in the second period II of the first rectangular wave phase modulation,
The amount of light reaching the photoreceiver or the photoreceiver or thereafter such that the difference ΔV ₂ of the outputs from the photoreceiver or the average value of the ΔV ₁ and ΔV ₂ in the IV period of the rectangular wave phase modulation becomes constant. The optical interference angular velocity meter according to claim 1, which is control means for automatically adjusting the gain of the electric circuit. 3. The difference between the output from the photoreceiver in the I period of the first rectangular wave phase modulation and the output from the photoreceiver in the III period of the second rectangular wave phase modulation. ΔV ₁ or the difference ΔV ₂ or ΔV ₁ between the output from the light receiver in the second period II of the first rectangular wave phase modulation and the output from the light receiver in the fourth period IV of the second rectangular wave phase modulation The means for outputting the average value of ΔV ₂ as transfer function gain information of the optical interference gyro, obtaining the ratio between the transfer function gain information and the reference signal, and multiplying the ratio by the input / output scale factor of the optical interference gyro. The optical interference angular velocity meter according to claim 1, wherein 4. The A (rad) is 2nπ ± (about π /
4) (n = 0, ± 1, ± 2, ± 3 ...
(Rad) is different from said A (rad), The optical interference angular velocity meter in any one of Claim 1 thru | or 3 characterized by the above-mentioned.