JPH1096634A - Phase modulation type optical fiber gyro - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain low cost and high performance by intermittently supplying drive current for coherence light source by the pulse signals with different base frequencies and detecting input angular velocity and outputting based on low frequency signals. SOLUTION: Photoelectric current generated by signal light changed from the current of coherence light source 1 into pulse type with a switch SW2 flows intermittently with a delay for a transmission time τ. this is converted to voltage with a resistor R1 , averaged with a low-pass filter of a resistor R3 and a capacitor C2 and input 12. Here, pulse signal Ef2 is added to a switch SW1 to close contacts A and C for only the period of flow of the photoelectric current and contacts B and C are closed for the other period, the current by the signal light is sent to an operation amplifier OA1 , which is converted to voltage in the resistor R2 to be an output Eoe-1 . This output waveform becomes a pulse series with a long period and is smoothed with the low-pass filter to be an output Eoe-2 with time axis of N times. This frequency becomes a low frequency the same as 1/N times of the base frequency of drive voltage of a phase modulator 3 and a signal processor 10 pays attention to this to detect and output the input angular velocity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber gyro for detecting an angular velocity using an optical fiber, and particularly to a small-sized light suitable for being mounted on a moving body such as an automobile and detecting an azimuth. It relates to a fiber gyro.

[0002]

2. Description of the Related Art As a sensor for detecting an angular velocity, a gyroscope (gyroscope) has been conventionally known. In recent years, an optical fiber gyroscope has been put into practical use as a solid-state gyroscope. Japanese Patent Application Laid-Open No. Sho 61-29715, "Apparatus for measuring irreversible phase shift generated in an open loop interferometer" and Japanese Patent Application Laid-Open No. 63-314410, "Phase modulation type optical fiber gyro" can be cited.

[0003] The one shown here is a closed-loop system of a phase modulation type optical fiber gyro (Japanese Patent Laid-Open No. 61-29).
No. 715) and an open loop system (Japanese Patent Laid-Open No. 63-3 / 1988).
In this case, the optimum value of the fundamental frequency of the drive voltage of the phase modulator is determined by the total length of the optical fiber of the optical fiber loop, and the optimum value of the fundamental frequency of the drive voltage is determined by the total length of the optical fiber. Inversely proportional. When the total length of the optical fiber is several tens of meters, the optimum value of the fundamental frequency of the driving voltage is several MHz, and when the waveform of the driving voltage is a square wave, the photoelectric conversion unit and the signal processing unit handle a high frequency of several tens MHz. Had to use something special that could.

[0004]

As the above-mentioned conventional photoelectric conversion unit and signal processing unit use special ones that can handle high frequencies, the apparatus becomes expensive, and
There has been a problem such as generation of an output bias due to a sneak of the drive voltage of the phase modulator into the photoelectric conversion unit.

An object of the present invention is to provide an inexpensive and high-performance phase modulation type optical fiber gyro.

[0006]

In order to achieve the above object, the drive current of the coherent light source is intermittently switched between two frequencies by a pulse signal having a fundamental frequency different from the fundamental frequency of the drive voltage applied to the phase modulator. Multiplication is performed to generate a beat frequency of the sum and difference of the two frequencies, and a detection output of the input angular velocity is obtained based on the beat frequency of the difference between the two beat frequencies, that is, the signal with the lower frequency. To do.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of a phase modulation type optical fiber gyro according to the present invention will be described in detail.

First, the configuration will be described. The embodiment shown in FIG. 1 is roughly divided into an optical system including a coherent light source 1, an optical splitter 2, a phase modulator 3, and an optical fiber loop 4; Other than the signal processing unit. As an example, the optical system uses an optical integrated circuit in which the optical splitter 2 and the phase modulator 3 are integrated as surrounded by a broken line, and the photodiode PD is on the opposite side of the light output end of the coherent light source 1. , And receives the signal light passing through the coherent light source 1 in the simplest configuration.

In FIG. 1, a coherent light source 1 of an optical system is, for example, a general laser diode or a super-light emitting diode, and has a built-in monitoring photodiode PD. The optical splitter 2 is formed by forming a Y-shaped optical waveguide on a lithium niobate substrate, and is a beam splitter utilizing the effect of a mode filter. Further, the phase modulator 3 is provided with electrodes on both sides so as to sandwich the optical waveguide, and changes the refractive index of the optical waveguide (equivalently, changes the optical path length) by applying a drive voltage Em to the electrode, thereby changing the optical waveguide section. To change the propagation time when passing through
It functions as AC light phase bias means. The optical fiber loop 4 is formed by winding, for example, a single-mode polarization-maintaining single-mode optical fiber having a total length of several tens of meters in a coil shape, and is a sensing unit for input angular velocity Ωin. The coherent light source 1 and the optical splitter 2 are connected by an optical fiber.

First, the basic operation of the optical system will be described. First, light emitted from the coherent light source 1 enters an optical splitter 2 via an optical fiber and splits in two directions.
Each light loops through the optical fiber loop 4 to return to the original branch point, and the two lights are combined into one light. This combined light indicated by the dashed arrow in the figure will be referred to as signal light. The phase relationship immediately before the two lights are combined is determined by the magnitude of the drive voltage Em applied to the electrodes when the respective lights pass through the phase modulator 3, and is, for example, clockwise. The driving voltage Em of the electrode when the light in the direction (CW) passes through the phase modulator 3 and the driving voltage Em of the electrode when the light in the counterclockwise direction (CCW) starting from the branch point passes through the phase modulator 3 simultaneously. If the driving voltages Em are different, a phase difference always occurs between the two lights, and a phase bias is applied. The phase bias can be applied most efficiently by making the half cycle of the change in the drive voltage Em applied to the electrode equal to the time for light to propagate through the optical fiber loop 4, and the reciprocal of one cycle of the change at this time is Phase modulator 3
Is the optimum driving frequency of the driving voltage Em.

Here, assuming that the refractive index of the optical fiber is 1.5, the total length of the optical fiber loop 4 is 50 m, and the propagation speed of the light in a vacuum is 300,000 km / s, the light propagates through the optical fiber loop 4. The time (propagation time τ), that is, a half cycle for changing the driving voltage Em is 0.2 which is equal to the propagation time τ.
5 μs, so one cycle of the optimal driving frequency is 0.5 μ
s, the frequency becomes 2 MHz. Usually, the magnitude of the phase bias is set to about 1 / 2π to maximize the sensitivity of the phase change of the light to the input angular velocity Ωin. The frequency component equal to the fundamental frequency of the drive voltage Em included in the signal light and the odd harmonic component increase and decrease in proportion to the Sin of the input angular velocity Ωin, and drive the phase modulator 3 included in the signal light.
The even harmonic component of m increases or decreases in proportion to the Cos of the input angular velocity Ωin.

Next, the photoelectric conversion unit and the signal processing unit will be described. First, the signal light that has exited the optical splitter 2 causes the coherent light source 1 to have an amplification factor of approximately one-fold (strictly speaking, an optical fiber and an optical fiber). (There is an amplifying action that compensates for the coupling loss of the coherent light source 1), enters the photodiode PD, and the photoelectric current flows through the resistors R1 and R3. (However, the contact points A and C are always connected to the switch SW1.)
3, capacitor C2, optical output controller 12, switch SW
The portion consisting of 2 (in this case, it is always ON) keeps the light output of the coherent light source 1 constant at a value commensurate with the command value Ep. The circuit including the resistor R3 and the capacitor C2 is a negative feedback circuit having a low-pass filter function, and controls the average value of the photoelectric current to be constant.
The operation of the switch SW2 will be described later.

The portion including the operational amplifier OA1, the resistors R1 and R2, the capacitor C1, and the switch SW1 is the most general current-voltage conversion circuit, and forms a photoelectric conversion circuit together with the photodiode PD. Photoelectric conversion unit output E
oe is amplified to a required voltage level by the amplifier 5,
The input voltage Ein is applied to the A / D converter 6, where the analog amount is converted into a digital amount and converted into a digital amount.
Is being entered. The signal processor 10 is connected to a transmitter 11, a waveform shaper 7 and a D / A converter 8, and the signal processor 10 executes a simple numerical operation in synchronization with a clock signal Ef0 output from the transmitter 11. And the frequency of the clock signal Ef0 is divided to generate pulse signals Ef1 to Ef3 having a predetermined fundamental frequency.
, And exchanges signals with the A / D converter 6 and the D / A converter 8 to output a numerical output Ωout to the outside as a final output. The waveform shaper 7 attenuates or amplifies the pulse signal Ef1 output from the signal processing unit 10, converts the pulse signal Ef1 into a square wave pulse signal Ef1m having a predetermined size, and adds the pulse signal Ef1m to the mixer 9. Here, assuming that the sawtooth voltage Eramp is zero, the output of the mixer 9, that is, the magnitude of the driving voltage Em for driving the phase modulator 3 is the same as that of the clockwise light and the counterclockwise light. In general, the phase difference is set to a value that becomes approximately π / 2. The D / A converter 8 generates a sawtooth voltage Eramp in accordance with the numerical output of the signal processing unit 10 and adds the sawtooth voltage Eramp to the mixer 9.

First, the relationship between the input angular velocity Ωin, the input voltage Ein of the A / D converter 6 and the sawtooth voltage Eramp of the output of the D / A converter 8 will be described. , Input voltage E
Since the fundamental frequency component of the drive voltage Em included in and the same component as the odd harmonic component thereof are zero, the optical reversing mechanism for canceling the input angular velocity Ωin does not operate, and the sawtooth voltage Eramp has a slope. It is zero.

On the other hand, when the input angular velocity Ωin is not zero, a fundamental frequency component and an odd harmonic component of the drive voltage Em are temporarily generated in the input voltage Ein. The slope of the voltage of the sawtooth voltage Eramp changes so as to cancel the voltage of the frequency component, and finally, the frequency component becomes zero. Sawtooth voltage Er
Since amp cannot be increased indefinitely, it needs to be reset on the way. As an example of the reset, there is a method in which the PP of the sawtooth voltage Eramp is converted to the phase difference of light to always be 2π. In this case, the number of resets is proportional to the input angle, and the number of resets per unit time (number / s) is proportional to the input angular velocity Ωin. Therefore, the input angular velocity Ωin can be easily detected and the output Ωout can be output. . That is, the configuration up to this point in which the switches SW1 and SW2 do not operate is the basic configuration of the closed-loop phase modulation type optical fiber gyro, and performs a well-known operation. See the description of JP-A-61-29715). Therefore, according to this configuration, when the total length of the optical fiber loop is short as described above, the fundamental frequency of the drive voltage Em of the phase modulator 3 increases, so that the frequency characteristics of the parts other than the optical output control unit are changed to the high frequency range. It is necessary to have good characteristics, and there is a problem that the apparatus becomes expensive because special parts are used.

Next, the basic operation of the present invention will be described with reference to FIG. First, as described above, the switch SW1,
When SW2 is not operating and the input angular velocity Ωi
If n is zero and the drive voltage Em of the phase modulator 3 is a square wave having a predetermined magnitude of 50% duty and the phase bias of the light becomes substantially π / 2, the phase bias of the light and the waveform of the signal light become As shown, the waveform of the signal light is Ao
The output levels of both the region and the Bo region are each １ ／ of the maximum output, and the waveform excluding the spike o portion is flat. The spike o occurs when the propagation time τ and the fundamental frequency of the drive voltage Em do not match or the duty is not 50%, and the phase bias becomes zero, and the peak value becomes the maximum value.

On the other hand, when the input angular velocity Ωin is not zero, the output levels of the Ao region and the Bo region of the signal light waveform are different, and a step is formed. When the above-described reverse rotation mechanism operates, the slope of the sawtooth voltage Eramp is controlled so that the step becomes zero, and the waveform of the signal light becomes exactly the same as when the input angular velocity Ωin is zero. Although the waveform of the photoelectric current when the switches SW1 and SW2 are not operating is not shown,
A waveform is obtained by adding the DC output of the non-output end of the coherent light source 1 to the waveform of the signal light.

Here, considering the case where only the switch SW2 is intermittent in response to the pulse signal Ef3, first,
Since the light output of the coherent light source 1 changes substantially similar to the current of the coherent light source 1, the light output changes in a pulse shape by changing the current of the coherent light source 1 in a pulse shape by the switch SW2. As a result, the photoelectric current due to the signal light flows intermittently with a time delay equal to the propagation time τ as shown in the illustrated Ai, Bi and spike i portions. Further, a current due to the light output on the non-light output side of the coherent light source 1 that hardly delays is added to the photoelectric current, and the waveform of the photoelectric current is as shown in the figure. The photoelectric current due to this light output is several tens times larger than that due to the signal light. The photoelectric current is the resistance R1
Is converted to a voltage, averaged by the low-pass filter including the resistor R3 and the capacitor C2 as described above, and applied to the optical output controller 12 as a negative feedback signal.
The light output is kept constant at a value corresponding to the light output command value Ep. This operation does not change even when the switch SW1 operates.

Incidentally, the photoelectric current flowing due to the light output on the non-light output side of the coherent light source 1 has an input angular velocity Ωi
There is no information on n, which hinders subsequent signal amplification and signal processing. Therefore, the problem that the amplifier 5 is saturated by eliminating this harmful component and selectively amplifying only the component flowing by the signal light is solved. The selection of the signal is realized by the action of the switch SW1 as a toggle switch, a pulse signal Ef2 is applied to the switch SW1, and the contacts A and C are closed only during the period when the photoelectric current by the signal light flows as shown in FIG. By closing the contact points B and C during the period of, it is possible to selectively flow only the current due to the signal light to the operational amplifier OA1, and convert the current into a voltage by the resistor R2 (assuming that there is no capacitor C1). Of the photoelectric conversion unit. Here, the pulse signal E
f2 and the pulse signal Ef3 have the same pulse period and different phases and pulse widths. Also, the pulse period is α of period = (2τ ± α) in order to facilitate understanding of the operation.
Is considerably large, the repetition period of the waveform of the photoelectric conversion unit output Eoe-1 is short,
Although the fundamental frequency is shown so as not to be substantially different from the fundamental frequency of the signal light, the actual value of α is 2τ of 1 to 1.
2%, so that the repetition period of the waveform of the photoelectric conversion unit output Eoe-1 is long, so that several tens of Ae parts continue, several spike e parts continue, and several tens Be parts continue. Pulse train, and the pulse train is connected to the capacitor C1.
When the signal is smoothed by a primary low-pass filter composed of a resistor and a resistor R2, the output becomes a photoelectric conversion unit output Eoe-2, and the time axis is N
The waveform becomes similar to the waveform of the signal light. Ao portion of the signal light waveform and Ae portion of the photoelectric conversion portion output Eoe-2 waveform,
Similarly, the Bo portion and the Be portion correspond to the spike o portion and the spike e.
The components correspond to each other, and the frequency of the signal light waveform becomes 1 / N, which is exactly the same as when the fundamental frequency of the drive voltage Em of the phase modulator 3 is 1 / N.

Here, N is represented by N = 2τ / α. For example, if α is 1% of 2τ, then N = 100.
As described above, when the phase modulator 3 is driven with the optimum driving voltage Em of 2 Mhz whose fundamental frequency is determined by the entire length of the optical fiber loop 4, the frequency of the waveform of the photoelectric conversion unit output Eoe-2 is the basic frequency of the driving voltage Em. This is exactly the same as when the frequency is set to 20 kHz. Therefore, the operational amplifier OA1
, The amplifier 5, the A / D converter 6
It is sufficient to handle a low frequency of 00 kHz or less, and the signal processing unit 10 may perform signal processing by focusing on this low frequency signal, so that inexpensive general-purpose components can be used. Also,
Since the electrostatic coupling is reduced, the phenomenon that the drive voltage Em wraps around the photoelectric conversion unit and the amplification unit is eliminated, and no bias is generated in the output Ωout, thereby achieving the object of the present invention. it can. When N is increased, the response speed is equivalently reduced, but does not affect the accuracy (especially, resolution, etc.) of the output Ωout.

As shown in FIG. 3, the light output control unit omits the switch SW1, the resistor R1, the resistor R3 and the capacitor C2 of the components shown in FIG.
And the operational amplifier OA1 are directly connected to each other, and the operational amplifier OA1 converts the photoelectric current obtained by adding the output of the non-light output terminal of the coherent light source 1 to the signal light into a voltage, and at the same time, smoothes (first-order low-pass filter) the signal light. The output Eoe of the photoelectric conversion unit is applied to the optical output controller 12 as a negative feedback signal, so that the optical output of the coherent light source 1 is maintained at a value corresponding to the command value Ep. Is inserted in series between the operational amplifier OA1 and the amplifier 5 to cut a DC component (a component of an output of the non-light output terminal of the coherent light source 1), and the amplifier 5 amplifies only an AC component. The same effect as the effect of the configuration shown in FIG. 1 can be obtained.

In the above description, the object of the present invention is a closed-loop type phase-modulation optical fiber gyro using an optical integrated circuit, and the signal light extraction section is set as the non-light output end of the coherent light source 1. However, the subject of the present invention is not limited to this. For example, the present invention can be applied to a general open loop type phase modulation type optical fiber gyro having two optical splitters. In that case, the effect of the present invention does not change.

As described above, the first, fourth and fifth terms
According to the invention described in the paragraph, since low frequency can be handled, inexpensive parts can be used, and no bias is generated due to the sneak of the driving voltage Em, and a low-cost and high-performance phase-modulation optical fiber gyro is used. Can be provided.

According to the inventions described in the second and sixth aspects, the number of optical splitters can be reduced to one, so that an inexpensive phase modulation type optical fiber gyro can be provided.

Further, according to the inventions described in the third and seventh aspects, it is possible to provide an inexpensive phase modulation type optical fiber gyro without losing the high resolution characteristic of the closed loop type phase modulation type optical fiber gyro. it can.

[0026]

According to the present invention, an inexpensive and high-performance phase modulation type optical fiber gyro is provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a phase modulation type optical fiber gyro according to an embodiment of the present invention.

FIG. 2 is a waveform chart showing an operation example of the embodiment of FIG.

FIG. 3 is a block diagram of a phase modulation type optical fiber gyro showing another embodiment based on the present invention.

[Explanation of symbols]

1: coherent light source, 2: optical splitter, 3: phase modulator,
4: optical fiber loop, 10: signal processing unit, 11: oscillator, 12: optical output controller, SW1, SW2: switch.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tatsuya Kumagai 880 Sunasawa-cho, Hitachi City, Ibaraki Prefecture Inside the Takasago Plant, Hitachi Cable Co., Ltd. (72) Inventor Katsuaki Oto 880, Sunasawa-machi, Hitachi City, Ibaraki Prefecture Hitachi Cable Co., Ltd. Inside the Takasago Plant

Claims (7)

[Claims] A coherent light source; light output control means for adjusting the light output of the coherent light source; and a light branching unit for splitting light from the coherent light source in two directions and recombining the light. ,
An optical fiber loop for propagating the branched light in opposite directions to each other, and a first fundamental frequency divided from a predetermined oscillation source for obtaining an AC phase bias to the branched light to obtain a signal light Phase modulation optical fiber including: a phase modulation control unit driven by a pulse signal; a photoelectric conversion unit that converts the signal light into an electric signal; and a signal processing unit that outputs a detected value of an input angular velocity from the electric signal. In the gyro, the light output control means is provided with signal intermittent means driven by a pulse signal of a second fundamental frequency divided from the same oscillation source as the oscillation source, and the second fundamental frequency and the first A fundamental frequency is equivalently multiplied to generate a beat frequency of the sum and difference of the fundamental frequencies, and the input angular velocity is calculated by the signal processing unit based on a signal of a lower frequency of the beat frequencies. Phase modulation type optical fiber gyro and outputs the detected value. 2. The phase modulation type optical fiber gyro according to claim 1, wherein the signal light is taken out at a position opposite to a light output end of the coherent light source. 3. An optical integrated circuit comprising only the phase modulation control means or the optical branching unit and the phase modulation control means, and the signal processing unit applies the phase bias as a signal to be applied to the phase modulation control means. A sawtooth signal for superimposing the input angular velocity is superimposed on the original signal to be given in a pseudo manner, and the number of repetitions of the sawtooth wave signal is used as an element for determining the detection value of the input angular velocity. 2. A phase modulation type optical fiber gyro according to claim 1, wherein: 4. A coherent light source, light output control means for adjusting the light output of the coherent light source, and an optical branching unit for splitting light from the coherent light source in two directions and recombining the light. ,
An optical fiber loop for propagating the branched light in opposite rotations, and an AC phase bias to the branched light to obtain a signal light, of a first fundamental frequency divided from a predetermined oscillation source. Phase modulation type optical fiber gyro including phase modulation control means driven by a pulse signal, a photoelectric conversion unit for converting the signal light into an electric signal, and a signal processing unit for outputting a detected value of an input angular velocity from the electric signal In each of the optical output control means and the photoelectric conversion unit, a signal intermittent means driven by a pulse signal of a second fundamental frequency divided from the same oscillation source as the oscillation source is provided, and the second fundamental frequency And the first fundamental frequency are equivalently multiplied to generate a beat frequency of the sum and difference of the fundamental frequencies, and the signal is generated based on the signal of the lower frequency of the beat frequencies. Phase modulation type optical fiber gyro and outputs the detected value of the input angular velocity in physical unit. 5. A signal for driving the signal interrupting means of the light output control means and a signal for driving the signal interrupting means of the photoelectric converter have a predetermined amount of phase difference. 4. A phase modulation type optical fiber gyro according to 4. 6. The phase-modulated optical fiber gyro according to claim 4, wherein the signal light is extracted at a position opposite to a light output end of the coherent light source. 7. An optical integrated circuit comprising only said phase modulation control means or said optical branching section and phase modulation control means, and said signal processing section applies said phase bias as a signal to be applied to said phase modulation control means. A sawtooth signal for superimposing the input angular velocity is superimposed on the original signal to be given in a pseudo manner, and the number of repetitions of the sawtooth wave signal is used as an element for determining the detection value of the input angular velocity. 5. A phase modulation type optical fiber gyro according to claim 4, wherein: