JPH04110718A - Photointerference angular velocity meter - Google Patents

Abstract

PURPOSE:To improve S/N by inserting a second phase modulator in series with a first phase modulator, which is inserted between a photo branch unit and one end of a loop phototransmission passage, and driving both the modulators with the modulation signal having the same frequency. CONSTITUTION:Phase modulators 15, 26 are inserted between an optical branch unit 14 and both ends of an optical fiber coil 16, and the modulators 15, 26 are connected in series through the coil 16. The modulators 15, 26 have the same characteristic, and the modulator 26 is driven by the modulation signal, which has a periodic function and a frequency as same as the modulation signal against the modulator 15 and of which phase is delayed by 2pitaufm, so that the clockwise light receives a phase displacement from the modulator 26 in the same direction with the phase displacement received from the modulator 15. The clockwise light and the counterclockwise light receive the phase deflection to be added by the modulators 15, 26, and the clockwise light and the counterclockwise light composed by the branch unit 14 have a phase difference, which is changed with periodic function phi(2pifmt). Consequently, the interference light composed of both the light is converted into the electrical signal by a photo detecting unit 21, and is detected synchronously to detect an angular velocity input to the coil 16.

Description

Detailed Description of the Invention "Industrial Application Field" This invention splits light from a light source into two and supplies them as clockwise light and counterclockwise light to both ends of a loop-shaped optical transmission line, respectively. At one end of the optical transmission line, a phase modulator applies a phase shift to the light entering the optical transmission line and the light exiting from the optical transmission line using a modulation signal, and the two lights output from the loop-shaped optical transmission line are combined. interfere with each other, convert the interference light into an electrical signal,
The present invention relates to an optical interference gyrometer that detects the angular velocity applied to a loop-shaped optical transmission line around its axis by synchronously detecting the electrical signal with a modulation signal of a phase modulator.

``Conventional Technology'' Figure 6 shows a conventional optical interference gyrometer. The light emitted from the light source 11 is split into two by the optical splitter 12, one of which passes through a polarizer 13 and then guided to the optical splitter 14, and the other is terminated. The light that has passed through the optical splitter 14 is split into clockwise light and counterclockwise light, and the clockwise light is phase-modulated by the phase modulator 15 immediately after exiting the optical splitter 14, and then is loop-shaped. Optical fiber coil 16 as an optical transmission line
The light enters one end of the light beam, propagates clockwise through the optical fiber coil 16, and then reaches the optical splitter 14 again.

On the other hand, the counterclockwise light exits the optical splitter 14 and passes through the optical fiber coil 1.
6, propagates counterclockwise through the optical fiber coil 16, undergoes phase modulation by the phase modulator 15, and immediately reaches the optical splitter 14 again. In the optical splitter 14,
The clockwise light and counterclockwise light propagating through the optical fiber coil 16 meet and interfere. At this time, the clockwise light and the counterclockwise light undergo a periodic phase shift by the phase modulator 15, so a periodic phase difference occurs between the clockwise light and the counterclockwise light. For example, phase modulation When the frequency f of the modulation signal that drives the modulator 15 is 1/(2τ) (τ is the time for light to propagate through the optical fiber coil 16), the clockwise light receives a phase shift φ6 in the phase modulator 15. If the phase shift of the clockwise light when it passes through the optical fiber coil 16 and returns to the optical splitter 14 is sinusoidal as shown in FIG. The deviation φccw is shown in Fig. 7A.
Since the time τ has elapsed for the modulated signal of the seventh
The result is as shown in Figure B. Therefore, the phase difference φ between the clockwise light and the counterclockwise light that are combined in the optical splitter 14. Eφccw changes at a period of 2τ as shown by curve 17 in FIG. Therefore, the interference light obtained by combining these two lights strengthens and weakens each other repeatedly at a period τ, that is, becomes light whose intensity changes at a period τ. Phase difference φ between both lights. 8
-φccw, the intensity of the interference light changes as shown by a curve 18, and therefore the intensity change is repeated at a period τ as shown by a curve 19.

In FIG. 6, the interference light from the optical splitter 14 is transmitted to the polarizer 13.
The light reaches the optical splitter 12 and is split into two lights, one of which is converted into an electrical signal by the photodetector 21. As shown by curve 19 in FIG. 7, this electrical signal becomes a signal whose frequency is twice the phase modulation frequency f, which in the example of FIG. 7 changes at 1/τ.

When an angular velocity centered on the optical fiber coil 16 is applied to its axis, a phase difference is generated between the clockwise light and the counterclockwise light according to the input angular velocity due to the Sagnac effect. Therefore, in FIG. 7, the phase difference is superimposed on the input angular velocity with respect to the curve 17. When the phase difference is superimposed in a DC manner in this way, a component of the phase modulation frequency f appears in the output electrical signal of the photodetector 21 in accordance with the DC phase difference. The output of the photodetector 21 is synchronously detected in a synchronous detection circuit 22 using a reference signal having a phase modulation frequency. When the input angular velocity is zero, as mentioned above, the output of the photodetector 21 is only the even multiple components of the phase modulation frequency, mainly only the double component, so the output of the synchronous detection circuit 22 is zero, but the angular velocity is input, a component with the same frequency as the phase modulation frequency is generated in the output of the photodetector 21, and an output with a polarity and level corresponding 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. A phase modulation signal supplied to the phase modulator 15 and a reference signal supplied to the synchronous detection circuit 22 are generated by a modulation signal generator 24.

"Problem to be Solved by the Invention" When light passes through the phase modulator 15, the light not only undergoes a phase shift due to a modulation signal, but also undergoes intensity modulation. This is because phase modulation is performed by changing the refractive index of the medium through which light propagates, but when the refractive index of the medium changes, the confinement state of light in that medium changes. The optical confinement state changes, and in synchronization with this, the intensity of the passing light is modulated.

Therefore, both the clockwise light and the counterclockwise light that have passed through the phase modulator 15 are subjected to intensity modulation at a frequency of 11, and the clockwise light and counterclockwise light that have undergone intensity modulation are recombined at the optical splitter 14. The combined interference light also has intensity modulation of the component of the modulation frequency f. Therefore, even when the input angular velocity is zero, a component of frequency f is detected by the synchronous detection circuit 22, and this causes an offset error in the bias value of the optical interference angular velocity needle. If this offset error is large, the zero point will also change at a constant rate if some factor changes due to disturbance, resulting in poor zero point stability.

As mentioned above, when the input angular velocity is zero, the interference light is subjected to intensity modulation with a frequency twice the modulation signal frequency f2, and this interference light is split by the optical splitter 12, and one side is sent to the photodetector 21.
while the other one returns to the light source 11. This returned light adds intensity modulation to the light emitted from the light source 11 at a frequency twice the modulation signal frequency f. Alternatively, the returned interference light is detected by a photodiode for controlling the amount of light emitted from the light fi11, and an automatic light amount stabilization circuit including this detection output operates to keep the amount of light 'all constant. Therefore, intensity modulation is applied to the light emitted from the light source 11 at a frequency twice the frequency f.

When the light from the light source 11 is intensity-modulated at 2f in this way, when this light passes through the phase modulator 15 and is also intensity-modulated by the phase modulation signal, the phase modulator 15
Due to the frequency mixing effect of the modulated wave in the phase modulator 15, the light that has passed becomes 2 f, +f, = 3 f, and 2 f, -f, = f.

intensity modulation of each frequency occurs. ) Therefore, the interference light that passes through this phase modulator 15 and is combined by the optical splitter 14 also has intensity modulation of the f component, and as mentioned above, even if the input angular velocity is zero, the output from the synchronous detection circuit 22 is This results in an offset error in the bias value.

An object of the present invention is to provide an optical system that can improve the zero point stability of the bias without causing bias value errors even if there is intensity modulation by a phase modulator or even if the light from the light source is intensity modulated. An object of the present invention is to provide an interferometric angular velocity meter.

"Means for Solving the Problem" According to the present invention, the first phase modulator is inserted in series between the optical branching means and one end of the loop-shaped optical transmission line as in the conventional case, and the first phase modulator is A second phase modulator is inserted in series with the modulator. The characteristics of the second phase modulator are preferably the same as those of the first phase modulator, and the modulation signal of the same frequency and the same periodic function as the modulation signal for the first phase modulator is transmitted to the second phase modulator.
The phase shift that the light in the same direction receives in the first phase modulator is combined with the phase shift in the same direction in the second phase modulator.
The phase of the modulation signal supplied to the second phase modulator is selected to be received by the phase modulator. In other words, the modulation signal supplied to the first phase modulator is φ(2πf).

t), the modulation signal supplied by the second phase modulator is φ
(2πf, t −2πτf,).

τ is the optical propagation time of the loop-shaped optical transmission line. periodic function φ
A sine wave, a rectangular wave, or the like is used as (2πf, t).

According to this configuration, 1. In the second phase modulator, the clockwise light and the counterclockwise light each receive an added phase shift, so that the same effect as conventional phase modulation can be obtained, and in addition, the first.
Each intensity modulation in the second phase modulator acts as a frequency mixer, with one intensity modulation serving as a command for the other intensity modulation. does not include the component of the phase modulation frequency f.

Embodiment FIG. 1 shows an embodiment of the present invention, and parts corresponding to those in FIG. 6 are given the same reference numerals. In this invention, in series between the optical splitter 14 and one end of the optical fiber coil 16,
While the phase modulator 15 is inserted, a phase modulator 26 is also inserted in series between the optical splitter 14 and the other end of the optical fiber coil 16. In this way, the phase modulator 26 is connected in series to the phase modulator 15 via the optical fiber coil 16. The phase modulator 26 has the same characteristics as the phase modulator 15, so that the clockwise light receives a phase shift in the same direction as the phase shift received by the phase modulator 1-5. , the same periodic function and the same frequency as the modulation signal for the phase modulator 15, and the phase is 2πτ
The phase modulator 26 is driven by the modulation number 11 delayed by f1. This modulation signal is also generated by the modulation signal generator 24. In other words, the modulation signal for the phase modulator 15 is φ(2πf
, t), and as shown in FIG. 7, f, == 1/
(2τ), the modulation signal to the phase modulator 26 becomes φ(2πf, −π).

According to this configuration, the clockwise light is subjected to an additive phase shift in the phase modulator 15.26, and the counterclockwise light is also subjected to an additive phase shift in the phase modulator 15.2.
The clockwise light and the counterclockwise light that are subjected to an additive phase shift in step 6 and synthesized by the optical splitter 147 are periodic functions φ(2πf, t)
There is a phase difference that changes with

Therefore, by converting the interference light obtained by combining the 20 lights into an electrical signal by the photodetector 21 and synchronously detecting the electrical signal, the angular velocity input to the optical fiber coil 16 can be detected as in the conventional case.

When the clockwise light passes through the phase modulator 15, it also receives intensity modulation at the frequency f1 as described above, but when the clockwise light that has undergone this intensity modulation passes through the phase modulator 26, it also receives the intensity modulation at the frequency f, is also subjected to intensity modulation, i.e. phase modulator 15.
26, both intensity modulations are multiplied, and the frequency components of the intensity modulation are f, +f.

=2f, and f, -f, =0. Therefore, the intensity modulation of the clockwise light returned to the optical splitter 14 does not include the f1 configuration. Similarly, the intensity modulation of the counterclockwise light returned to the optical splitter 14 does not include the f2 configuration.

Therefore, when the input angular velocity is zero, the change component of the light intensity of the interference light between the clockwise light and the counterclockwise light does not include the f component.
At this time, the output of the synchronous detection circuit 22 is zero, and no offset error occurs in the bias value. Therefore, there is no offset error in the optical interference gyrometer, and the stability of the bias zero point is improved.

When the light from the light source 11 is intensity-modulated by 2f as described above, the clockwise light is transmitted to the phase modulator 15.
When passing through, it receives intensity modulation at f, and the intensity modulation of the clockwise light passing through it is 2f, %f, = 3 f, l, and 2f.
, ~f,=f.

When this clockwise light passes through the phase modulator 26, f
, and the intensity modulation of the clockwise light that has passed through the phase modulator 26 is 3 f, 10 f, = 4 f.

, 3 f, -f, = 2 f, and, r11+r, = 2
f.

Then, flI−f,−0, and the component of f disappears. Similarly, the fy component no longer exists in the intensity modulation of the counterclockwise light that has passed through the phase modulator 15.

Therefore, when the input angular velocity is zero, there is no f component in the intensity modulation of the interference light obtained by combining these two returned lights by the optical splitter 14, and the output of the synchronous detection circuit 22 becomes zero.
The stability of the bias zero point of the optical interference gyrometer is improved.

In the above description, the present invention was applied to an oven-loop optical interference angular velocity meter, but it can also be applied to a closed-loop optical interference angular velocity needle. An example of this is shown with the same reference numerals attached to parts corresponding to those in FIGS. In the direction of inclination depending on the polarity,
A sawtooth wave (sometimes a stepped sawtooth wave) signal is generated with an inclination angle depending on the magnitude of the input, and the sawtooth wave signal is used to connect one end of the optical file coil 16 and the optical splitter 14. The phase modulator 28 inserted in series with the synchronous detection circuit 22 is phase-modulated, and 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 corresponding to the polarity of the input angular velocity and a frequency corresponding to the magnitude of the input angular velocity is obtained from the sawtooth wave generation circuit 27, and is outputted to the output terminal 29.

The phase modulator 28 is omitted, and the feedback sawtooth wave signal from the sawtooth wave generation circuit 27 is transmitted to the phase modulator 2 as shown in FIG.
It may be added to 6. Alternatively, this feedback sawtooth signal may be applied to the phase modulator 15. As shown in FIG. 4 with the same reference numerals assigned to parts corresponding to those in FIG. 2, a phase modulator 31 is further inserted between the optical splitter 14 and one end of the optical fiber coil 16, The phase modulator 31 may be driven by a sawtooth wave signal φ having a polarity opposite to that of the sawtooth wave signal φ that drives the phase modulator 31 . In the configurations shown in FIGS. 2 and 3, the peak value of the feedback sawtooth signal is the voltage value necessary to cause a phase shift of 2π radians in the light, but in the configuration shown in FIG. The peak value of the wave signal may be the voltage necessary to cause a phase shift of π radians in the light, and a low voltage is sufficient, which facilitates the design of the sawtooth wave generation circuit 27.

In FIG. 4, four phase modulators 15, 2628.31
can be integrated on one substrate. That is,
As shown in FIG. 5, an optical branching device 14 is constructed by forming an optical waveguide 33 in a Y-shape on an electro-optic crystal substrate 32 such as lithium niobate. .35 is formed to constitute the phase modulator 15, electrodes 36 and 37 are formed across the branched optical waveguide to constitute the phase modulator 31, and similarly, the phase modulator 31 is constituted by forming the electrodes 36 and 37 across the other branched optical waveguide. A pair of electrodes is formed to constitute a phase modulator 2628. In the configuration of FIG. 4, the phase modulators 28 and 31 may be omitted and the feedback sawtooth wave signals φ8-φ may be supplied to the phase modulators 15 and 15, respectively.

In the above, the phase modulator 26 is connected in series with the phase modulator 15 via the optical fiber coil 16, but the phase modulator 26
may be directly connected in series to the phase modulator 15. Regarding the case where the example is applied to an open loop type, the same reference numerals are given to the parts corresponding to those in FIG.

"Effects of the Invention" As described above, according to the present invention, a phase modulator is further inserted in series with the phase modulator inserted between the optical splitter and one end of the loop-shaped optical transmission line, and the same By driving with a frequency modulation signal, the offset error of the bias value can be reduced to zero due to the light intensity modulation generated by the phase modulator, and the light from the light source is also intensity modulated. In this case, it is also possible to prevent offset errors in the bias value from occurring, thereby increasing the amount of light generated by the light source and increasing the S/N ratio. As a result, a highly stable optical interference gyrometer with a small bias value offset can be obtained.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment in which the present invention is applied to an open loop type, Fig. 2 is a block diagram showing an embodiment in which the invention is applied to a closed loop type, and Figs. 3 and 4 respectively show this embodiment. A block diagram showing another embodiment in which the invention is applied to a closed loop type; FIG. 5 is a perspective view showing an example in which phase modulators 15, 26, 28, and 31 are integrated;
Figure 6 is a block diagram showing a conventional optical interference gyrometer, Figure 7
The figure shows the phase shift received by the clockwise light and the counterclockwise light, the phase difference between these two lights, and the light intensity change of the interference light of both lights. FIG. 3 is a block diagram showing another embodiment.

Claims (1)

[Claims] (1) The light from the light source is split into two lights by the optical branching means, one of the lights is input into one end of the loop-shaped optical transmission line as a clockwise light, and the other light is sent to the loop-shaped optical transmission line. A first phase modulator is inserted in series between one end of the loop-shaped optical transmission path and the optical branching means, and the first phase modulator is used to transmit a periodic function modulated signal. drive to phase modulate the light passing through the first phase modulator, combine the clockwise light and the counterclockwise light that have passed through the loop-shaped optical transmission path with the light branching means, and cause them to interfere. The interference light is converted into an electrical signal by a photodetector, and the electrical signal is synchronously detected by the modulation signal in a synchronous detection circuit, and the angular velocity applied to the loop-shaped optical transmission path around its axis is detected. In the interferometric gyrometer, the light is inserted in series with the first phase modulator, and has the same frequency and the same periodic function as the modulation signal, and passes through the optical transmission path. An optical interference gyrometer comprising: a second phase modulator driven by a modulation signal that modulates with a phase having a phase shift in the same direction as the phase shift received by the second phase modulator.