JP2548044B2 - Optical interference gyro - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Industrial field of application" The present invention divides light from a light source into two parts by a first optical branching device and further divides one light by a second optical branching device into two loops. The light is supplied to both ends of the optical transmission line as right-handed light and left-handed light, respectively, and the two lights emitted from the loop-shaped optical transmission line are combined and interfered with each other. The present invention relates to an optical interference angular velocimeter that branches to the detector side and detects the angular velocity applied to the loop optical transmission line around its axis from the output of the photodetector.

“Prior Art” FIG. 3 shows a conventional optical interference gyro. The light emitted from the light source 11 is divided into two in the optical branching device 12, one of which is passed through the polarizer 13 and then guided to the optical branching device 14, and the other is terminated by the terminating element 15. The light that has passed through the optical splitter 14 is split into right-handed light and left-handed light.
After being subjected to phase modulation by the phase modulator 16 immediately after exiting from, it is incident on one end of the optical fiber coil 17 as a loop-shaped optical transmission line, propagates counterclockwise through the optical fiber coil 17, and then splits again. Reach vessel 14. On the other hand, the clockwise light exits the optical splitter 14 and enters the other end of the optical fiber coil 17, propagates counterclockwise in the optical fiber coil 17, and then the phase modulator.
After being phase-modulated by 16, it reaches the optical splitter 14 again. In the optical branching device 14, the clockwise light and the counterclockwise light propagating through the optical fiber coils 17 meet and interfere with each other. At this time, the right-handed light and the left-handed light undergo a periodic phase shift by the phase modulator 16, so that a periodic phase difference occurs between the right-handed light and the left-handed light. For example, phase modulator 16
When the frequency f _{m of the} modulation signal that drives the optical fiber is 1 / (2τ) (τ is the time during which light propagates through the optical fiber coil 17), the optical branching device 14
The phase difference between the right-handed light and the left-handed light combined in step 1 changes in 2τ cycles. For this reason, the interference light obtained by combining these two lights is repeatedly strengthened or weakened with each other in the cycle τ, that is, the light whose intensity changes in the cycle τ. The intensity of the interference light changes in the cycle τ according to the phase difference between the clockwise light and the counterclockwise light.

The interference light from the optical splitter 14 passes through the polarizer 13,
The light reaches 12 and is split into two lights, one of which returns to the light source 11 and the other is converted into an electric signal by the photodetector 21. This electric signal becomes a signal that changes at a frequency twice the phase modulation frequency f _m and 1 / τ.

When an angular velocity about the axis is applied to the optical fiber coil 17, a phase difference corresponding to the input angular velocity is generated between the clockwise light and the counterclockwise light due to the Sagnac effect. Therefore, a component of the phase modulation frequency f _m appears in the output electric signal of the photodetector 21 according to the phase difference. Photo detector
The output of 21 is synchronously detected by the synchronous detection circuit 22 with the reference signal of the phase modulation frequency. When the input angular velocity is zero, as described above, the output of the photodetector 21 is only an even multiple component of the phase modulation frequency, mainly the double component, so the output of the synchronous detection circuit 22 is zero. Is input, a component having the same frequency as the phase modulation frequency is generated at the output of the photodetector 21, and an output of polarity and level according to the direction and magnitude of the input angular velocity is obtained from the synchronous detection circuit 22. It is supplied to the output terminal 23, and the input angular velocity can be detected. The phase modulation signal supplied to the phase modulator 16 and the reference signal supplied to the synchronous detection circuit 22 are generated by the modulation signal generator 24.

[Problems to be Solved by the Invention] In the conventional optical interference angular velocity meter, since the branching ratio of the optical branching device 12 that branches the light emitted from the light source 11 is set to 1: 1, the loss of the optical element is caused. Is zero and the incident light amount from the light source 11 is 100, the light amount of the light branched into the polarizer 13 and the terminating element 15 is 50, and the light amount of 50 is returned from the optical fiber coil 17 to the light source 11. The amount of return light and the amount of signal light to the photodetector 21 are 25, respectively, and the signal-to-noise ratio of the optical system can be large, but at the same time, the amount of return light to the light source 11 is also maximized, and the optical interference angular velocity There is a drawback that the performance of the meter deteriorates.

That is, the light source 11 used in the optical interference angular velocity meter often uses an optical resonator such as a semiconductor laser.
Usually, a semiconductor laser constitutes a resonator of light by using cleavage planes at both ends of a laser chip as reflecting mirrors, and the light resonated by this resonator is taken out as laser light. As described above, in a light source utilizing the resonance of light such as a semiconductor laser, when there is a reflected light or a returning light from a portion other than the cleavage plane of the semiconductor laser chip, the reflected light or the returning light causes the laser chip to return. When incident on, another resonator will be formed in addition to the resonator forming the semiconductor laser. This other resonator is called an external resonator because it is formed outside the semiconductor laser. When this external resonator is formed, the spectrum shape, center wavelength, coherence, etc. of the light source change. It has been reported that such a phenomenon is also caused in a super luminescent diode (SLD) which is often used in an optical interference angular velocity meter.

The spectrum shape of the light source 11 in the optical interference gyro,
The fluctuation of the center wavelength and coherence is fatal, and the fluctuation of the center wavelength is a problem because it directly leads to the fluctuation of the scale factor of the optical interference gyro. Because the scale factor of the optical coherence gyro is a function of the wavelength of the light source 11, the wavelength variation leads to the scale factor variation. Further, the fluctuation of the coherence of the light source 11 also changes the bias error amount caused by the reflected and scattered light in the optical interference gyro, which is also a big problem.

The present invention provides an optical coherent angular velocity meter that reduces the scale factor fluctuation and the amount of bias error by suppressing the returning light to the light source that causes the scale factor fluctuation, the wavelength fluctuation of the light source causing the bias error, and the coherence fluctuation. The purpose is to

[Means for Solving the Problem] According to the present invention, the optical branching device for branching the returned interference light from the loop-shaped optical transmission line to the photodetector side and the light source side is The branching ratio is selected so that the amount of light branched to the photodetector side is large. That is, the branching ratio of this optical branching device is shifted from 1: 1 and the light amount on the light source side: the light amount on the photodetector side is, for example, 5:95 to 40:60, preferably about 20:80 to 30:70. Selected.

Since such a branching ratio is selected, the light from the light source is branched to the terminating element side more than the loop optical transmission line side, the light amount on the loop optical transmission line side is reduced, and the signal-to-noise ratio of the optical system is reduced. Therefore, the light amount of the light source is made larger than the conventional one so that the signal-to-noise ratio becomes a required value or more. On the other hand, to reduce the amount of signal light (interference light) propagating through the loop optical transmission line, or Rayleigh scattered light from each point in the optical fiber, reflected light from the fiber fusion point, etc. Therefore, it is possible to reduce the fluctuation of the spectrum shape of the light source, the fluctuation of the central wavelength, and the fluctuation of the coherence caused by the fluctuation. As a result, it is possible to reduce the scale factor error and the bias error of the optical interference gyro which are caused by the variation of these light source characteristics.

[Embodiment] FIG. 1 shows an embodiment of the present invention, in which parts corresponding to those in FIG. In the present invention, the light from the light source 11 has a branching ratio of 1: 4 in this example, and the smaller the amount of light in the light split by the light splitter 25 is toward the polarizer 13 side. It is incident, and the larger one is incident on the terminating element 15. The interference light returned from the optical fiber coil 17 is an optical branching device.
It is incident on 25 and is branched to the light source 11 side and the photodetector 21 side.

With this configuration, assuming that the loss of the optical element is zero and the amount of light from the light source 11 is 100, the polarizer 13
The amount of light to the side is 20, the amount of light to the side of the terminator 15 is 80, and the amount of interference light that has returned is 20, so the amount of return light to the light source 11 is 4 and light detection The quantity of signal light to the device 21 is 16. In this way, the return light to the light source 11 becomes considerably smaller than before, and the spectrum shape of the light source 11, the center wavelength, and the coherence are less likely to change,
Variations in the scale factor and bias error of the optical fiber gyro are reduced.

Although the present invention is applied to the open-loop optical interference angular velocity meter in the above description, it can also be applied to the closed-loop optical interference angular velocity meter. An example thereof is shown in FIG. 2 with the same reference numerals attached to the parts corresponding to those in FIG. That is, the output of the synchronous detection circuit 22 is supplied to the sawtooth wave generation circuit 27, and the sawtooth wave generation circuit 27 has a sawtooth wave (in a tilt direction according to the polarity of the input and at a tilt angle according to the magnitude of the input). Signal (which may be a stepped sawtooth wave), and the sawtooth wave signal phase modulates a phase modulator 28 inserted in series between one end of the optical fiber coil 17 and the optical splitter 14. Negative feedback control is performed so that the output of the synchronous detection circuit 22 becomes zero. As a result, a sawtooth wave signal having a polarity according to the polarity of the input angular velocity and a frequency corresponding to the magnitude of the input angular velocity is generated by the sawtooth wave generation circuit 27.
A pulse having the same frequency as that of the sawtooth signal is obtained from the output terminals 29a and 29b according to the polarity of the sawtooth signal.
Is output to any of

As the optical branching device 25, a so-called bulk type using a prism is used. An optical fiber coupler in which a clough of two optical fibers is polished and bonded to each other to be bonded to each other, and two optical fibers are fused and extended. An optical fiber coupler, an optical directional coupler including an optical waveguide, a double Y-shaped branching device including an optical waveguide, or the like can be used.

[Advantages of the Invention] As described above, according to the present invention, the light from the light source is branched to the side of the loop-shaped optical transmission line, and the interference light of the left-handed and right-handed light returning from the loop-shaped optical transmission line is detected. As an optical branching device that splits the interference light into the light source side and the light source side, the branching ratio is selected so that the amount of interference light that branches from the light source side to the photodetector side increases, so the amount of return light to the light source As a result, the fluctuation amount of the light source characteristics such as the spectrum shape of the light source, the central wavelength, and the coherence becomes smaller than the conventional one, and as a result, the scale factor fluctuation and the bias error of the optical interference angular velocity meter are reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a block diagram showing another embodiment of the present invention, and FIG. 3 is a block diagram showing a conventional optical interference angular velocity meter.

Claims (1)

(57) [Claims] 1. A light from a light source is branched by a first optical branching device, one of the branched output lights is branched by a second optical branching device into two lights, and one of the light beams is output from a loop optical transmission line. The light is incident on one end as clockwise light, the other light is incident on the other end of the loop-shaped optical transmission line as counterclockwise light, and the clockwise light and the counterclockwise light that have passed through the loop-shaped optical transmission line are The second optical branching device synthesizes and causes interference, and the interference light is branched and supplied to the light source and the photodetector by the first optical branching device, and the loop-shaped optical transmission line is output from the output of the photodetector. In the optical interference gyroscope for detecting the angular velocity applied around its axis, the first optical branching device is configured so that the branching light amount to the photodetector side is larger than the branching light amount to the light source side in the first optical branching device. An optical interference angular velocity meter characterized in that a branching ratio of one optical branching device is selected.