JP2739191B2 - Optical interference angular velocity meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention detects a phase difference between right-handed light and left-handed light propagating in at least one round of an optical path, thereby detecting the phase difference applied to the optical path. The present invention relates to an optical interference gyro for detecting angular velocity.

[0002]

2. Description of the Related Art A conventional optical interference gyro (hereinafter referred to as FOG) will be described with reference to FIG. Light I from light source 11
Are sequentially passed through the optical coupler 12, the polarizer 13, and the optical coupler 14, and are incident on both ends of an optical path 15 constituted by, for example, an optical fiber coil wound in a loop a plurality of times. Optical path 15
Is phase-modulated by a phase modulator 16 disposed between the optical path 15 and the optical coupler 14. The two lights that have undergone the phase modulation are combined by the optical coupler 13, interfere with each other, are branched by the optical coupler 12 to the light receiver 17, and are photoelectrically converted. When the angular velocity in the circumferential direction is not applied to the optical path 15, the phase difference between the two lights in the optical path 16 is ideally zero, but the optical path 1
When an angular velocity Ω is applied to the circumference of the light 5, 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 Δ
Φs is represented by the following equation.

ΔΦs = 4πRLΩ / (Cλ) (1) C: speed of light λ: wavelength of light in a vacuum R: radius of an optical fiber coil L: length of an optical fiber On the other hand, the output Vp of the photodetector 17 at this time is Phase modulator 16
If P (t) = A sin ωmt, the phase modulation signal is expressed by the following equation.

[0004] Vp = (Po / 2) · Kpd {1+ cosΔΦs (Σε n · (-1) n · J 2n (X) · cos2nωmt ') - sinΔΦs (2Σ (-1) n · J 2n + 1 (X ) · Cos (2n + 1) ωmt ′)｝ (2) where Σ is from n = 0 to infinity t ′ = t− (τ / 2) ε _n = 1; n = 0, 2; n > 1 Kpd: A constant determined by a photoelectric conversion coefficient, an amplifier gain, and the like Po: Maximum light amount reaching the light receiver 17 Jn: Bessel function of the first kind X: 2A sin πfm τ ΔΦ: Phase difference between left and right round lights in the optical fiber coil 15 ωm: Phase Modulation angular frequency (ωm = 2πfm) τ: light propagation time in optical fiber coil 15 As is apparent from equation (2), the photoelectric conversion signal from photodetector 17 has a term proportional to sin ΔΦs and a term cos And a term proportional to ΔΦs. Therefore, the angular velocity Ω can be detected by measuring the intensity of the interference light. As the output of the conventional FOG, the primary sin ΔΦs component of the output of the light receiver 17 is synchronously detected and used. The primary sin ΔΦs output from the synchronous detection circuit 18 below
The components are shown.

V ₁ = Po · Kpd · J ₁ (X) · sinΔΦs · cosωmt ′ (3) The component represented by the equation (3) is calculated by the synchronous detection circuit 18.
Synchronous detection is performed by a rectangular wave having cos ωmt ′ as a fundamental wave. That is, the output of the light receiver 17 is input to the synchronous detection circuit 18, where the same component as the phase modulation frequency, that is, the primary component (n = 1) in the equation (1) receives the reference signal Vf ₁ from the clock circuit 21. Taken out. Synchronous detection circuit 18
The output of the ripple component in the low-pass filter 19 is filtered, it is taken out to an output terminal 20 as the FOG output V _1.

The output V ₁ of the FOG is expressed by the following equation. Vo = Po · Kpd · J ₁ (X) · K _A1 · sin ΔΦs (4) where K _{A1 is} the gain of the synchronous detection circuit 18 The phase modulator 16 controls the clockwise and counterclockwise light passing therethrough. Although it has a function of performing phase modulation, it has an optical transmission loss at the same time as phase modulation as a practical problem, resulting in undesired light intensity modulation. In other words, in equation (4), the amount of light Po reaching the light receiver is accompanied by an intensity modulation component. Here, assuming that the intensity modulation is Im (t) = Bsin ωmt, the amount Po (t) reaching the light receiver is expressed by the following equation.

Po (t) = Po + Im (t) + Im (t−τ) (5) The intensity modulation component is synchronously detected by the synchronous detection circuit 18,
It is converted to a direct current and becomes a bias error. Therefore, conventionally, the phase modulation frequency fm is reduced to 1 /
(2τ). By doing so, the second item and the third item on the right side of the equation (5) cancel each other out to be zero, and are not affected by the intensity modulation at all.

[0008]

In the conventional FOG, the phase modulation frequency fm is reduced to 1 / (2
τ). Therefore, if the optical fiber of the optical path 15 is shortened for low cost and miniaturization, τ becomes small, the phase modulation frequency fm becomes high, and it becomes difficult to achieve the required performance of the synchronous detection circuit 18, and the speed of the element is increased. Is required, the current consumption increases, which hinders a reduction in current consumption.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical interference gyro which can suppress a bias drift and can use low power consumption components.

[0010]

According to the present invention, at least one round optical path, branching means for passing clockwise light and counterclockwise light to the optical path, and clockwise light propagating through the optical path. And an interfering means for interfering with the counterclockwise light, and a cascade disposed between the branching means and one end of the optical path so as to modulate the clockwise light and the counterclockwise light with the frequency fm.
A phase modulation means which gives a phase change in the waves, a photodetector for detecting the light intensity of the interference light as an electrical signal, is the detection
The detected electrical signal is synchronously detected at the frequency fm, and the input angular velocity
Synchronous detection means for detecting a degree component, at least a light interference gyro of the prior distichum wavenumber fm, when the elapsed time of light propagating through the optical path was tau, fm = 1 /
(4τ) is set. As a result, the influence of the intensity modulation accompanying the phase modulation is reduced, and a low-frequency operation at half the conventional phase modulation frequency is made possible.

[0011]

In the configuration of FIG. 2, according to the present invention, the frequency fm of the voltage Vpm applied to the phase modulator 16 is reduced to 1 / (4
τ). In this case, the expression (5) becomes the following expression. Po (t) = Po + B · sin ωmt + B · sin ωm (t−τ) = Po · (1+ (2B / Po) · cos (π / 4) · sin (ωmt− (π / 4)) (6) On the other hand The primary sin ΔΦs component input to the synchronous detection circuit 18 is expressed by the following equation from the equation (3).

V ₁ = Po · Kpd · J ₁ (X) · sin ΔΦs · cos (ωmt− (π / 4)) (7) By the way, in the synchronous detection circuit 18, the same phase as the primary sin ΔΦs component V ₁ is referred. Since the signal is synchronously detected by the signal Vpm, that is, cosωmt, the first-order sin ΔΦs expressed by the equation (7) is used.
Component V ₁ was, as in the conventional FOG (4) signal shown in expression is output. On the other hand, the intensity modulation component expressed by the equation (6) is
Since the phase of the reference signal Vpm (that is, cosωmt) is 90 ° out of phase, no DC component appears and is filtered by the low-pass filter 19 at the next stage.

FIG. 1 shows the relationship between the input and output signals of the synchronous detection circuit 18. A indicates a first-order sin ΔΦs component represented by the equation (7), B indicates an intensity modulation component, and C indicates a reference signal Vpm.
And a waveform having the same phase as Po (t) in (6). In the synchronous detection circuit 18, for example, a signal input when the reference signal Vpm is in the region I is output as it is, and when the reference signal Vpm is in the region II, the input signal is inverted and output. Then the primary sin Δ
The Φs component is output as indicated by D, and the AC component is filtered by the low-pass filter 19 in the next stage, and the DC component is
Is output to On the other hand, the intensity modulation component is output as shown in FIG. E, is filtered by the low-pass filter 19 in the next stage, and does not appear at the terminal 20.

In the above description, the optical coupler 13 may be configured as an optical fiber coupler, and the phase modulator 16 using an optical fiber may be used. The present invention can be applied not only to an open-loop optical interference gyro but also to a closed-loop optical gyro.

[0015]

As described above, according to the present invention, the modulation frequency fm for driving the phase modulation means is set to fm = 1 / (4.tau.), Where τ is the elapsed time of light propagating through the optical path. By doing so, it is possible to reduce the influence of intensity modulation due to phase modulation, and to reduce the influence of the conventional FOG by one.
Since the phase modulation frequency fm is / 2, the synchronous detection circuit can be operated at a lower frequency, and the bias drift can be reduced. Also, low power consumption components can be used. Furthermore, if the conventional phase modulation frequency is allowed, the optical fiber as the optical path 15 can be further shortened by twice and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a time chart showing an example of an input / output relationship of a synchronous detection circuit according to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating an example of an optical interference gyro.

Claims (1)

(57) [Claims] 1. An optical path that makes at least one round, a branching unit that passes clockwise light and a counterclockwise light with respect to the optical path, and an interference unit that interferes the clockwise light and the counterclockwise light propagating through the optical path. Phase modulating means disposed between the branching means and one end of the optical path in cascade with them to change the phase of the clockwise light and the counterclockwise light with a modulating wave having a frequency fm; A light receiver for detecting the light intensity of the light as an electric signal, and the detected electric signal at the frequency fm.
Synchronous detection means for detecting the input angular velocity component by performing phase detection
When, in optical interference gyro having, before distichum wavenumber fm is, when the elapsed time of light propagating through the optical path was tau, characterized in that it is set to fm = 1 / (4τ) Optical interference gyro.