JP2993678B2 - Optical fiber gyroscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fiber optic gyroscope.

It is known that optical fibers are configured to include a light source, a beam splitter, a coil, a phase modulator, and an electronic unit. Generally, the electronic unit has a demodulator that demodulates the signal generated by the detected light such that a signal that depends on the irreversibility of the interferometer is transmitted.

There are many types of fiber optic gyroscopes with open and closed loop mechanisms with various modulation and demodulation techniques. In a closed loop configuration, the irreversible phase "well-known" is forced to zero by the demodulator since the signal from the demodulator is used for cancellation.
In the open-loop structure, the irreversible phase caused by the rotation of the coil cannot be canceled at all. Therefore, the measurement is simply performed by observing the output of the demodulator. The limiting factor for all these devices is apparently the rotation rate (wandering) due to demodulator cancellation. This wander can be reduced by increasing the gain before the demodulator, but there are substantial limitations, such as amplifier and demodulator saturation. The difficulty is that demodulator rates are typically 100KHz
Since it is in the range of ~ 10MHz, it is further increased. Because these frequencies are high, it is necessarily difficult to design a demodulator with a sufficiently low offset voltage.

Accordingly, one of the objects of the present invention is to provide a system that reduces the gyro sensitivity to demodulator offset.

According to a first aspect of the present invention, a coil of an optical fiber, a light source, a light beam from a phase modulation means and a light source are passed through the coil from two opposite directions, and a light beam splitting / combining device combines the light beams. A photodetector coupled to the coil via the means, and demodulator means connected to the output of the photodetector for generating an electrical signal indicative of an amount of irreversible phase received by light passing through the coil; A phase that modulates the phase of light according to a modulation waveform that has a frequency f that is half the frequency that light passes through the coil 2 of the optical fiber and that periodically reverses the phase at a frequency substantially equal to or less than f. A fiber optic gyroscope having a signal generator connected to the modulating means.

According to a second aspect of the present invention, light passing through the fiber is modulated by a first waveform demodulated from second and third periodic waveforms, wherein the third waveform is the second waveform.
The frequency f of the second waveform has approximately half the period for light to pass through the gyroscope, and the light is provided by the first detection. An optical fiber gyroscope is provided wherein the light is substantially demodulated and detected by both the first and second waveforms to minimize various deviations.

Hereinafter, an embodiment of the present invention will be described with reference to FIGS.

In FIG. 1, light passes from an edge emitting light emitting diode (ELED) light source 1 through an optical fiber 3, an optical fiber coupler 4, a polarization splitter 5, an integrated optical fiber (Y-coupled) 7, phase modulators 8 and 9, and so on. To reach the optical fiber coil 2. The light is split into two beams at coupler 7 which pass through coil 2 from modulator 2 and modulator 9 in two opposite directions, and this beam returns to coupler 7 where the beams recombine and interfere. Happens. The recombined beam then passes through the polarizer 5 and the optical fiber coupler 4 to the detector.
Return to 10. As a variant of the device shown in FIG. 1, the optical device assembly 6 has only modulators 8 and 9, and the coupler 7 is a separate component between the optical device assembly 6 and the polarizer 5. Provided. Alternatively, in addition to the modulators 8 and 9 and the coupler 7, one or both of the polarization demultiplexer 5 and the optical device assembly 6 are provided. When an assembly of optical devices made of III-V semiconductors is formed, this assembly can also incorporate a light source and a detector.

The fiber coupler 4 is a fused two-cone tapered coupler made of ordinary fiber. The polarizer 5 can be designed on the basis of glass or metal, with the indium-gallium positioned in the D-shaped hole inside the base of the fiber and a single-mode fiber core near it. Is provided. The optical assembly is made of lithium niobate and can be provided with two phase modulation portions. As the high birefringence fiber, a coil having a length of 200 m is used. Polarization is stably performed by this optical device assembly.

A similar gyroscope could be constituted by a depolarizer or a coil made from a common optical fiber.

The phase difference between the counterpropagating waves in the gyroscope is measured by modulating the phase of the light in the coil with the signal 15 in the manner described below. Fig. 2
A diagram of the interference fringe 22, the modulation waveform 23 and the zero, lag phase and lead phase differences 24, 25, 26, respectively, without the chopper signal 14A (the effect of this signal will be described later) is shown. The signal obtained by the detector 10 has a component of the modulation frequency, and for small errors is proportional to the phase difference around the coil.

The phase modulator 8 is driven by the square wave signal 12 from the generator 11, which signal has a half period of light passing through the coil of the optical fiber and has a phase of ± π / 4 of the light passing through the modulator. The signal 12 has an amplitude that causes a shift, and the signal 12
Passed through. The signal 12 is inverted in phase in synchronization with the chopper 14, and the signal 14A of the chopper 14 has a lower frequency than the frequency of the signal 12. The obtained signal 15 and signals 12, 14A are the third
It is shown in the figure. This signal is transmitted to a modulator 8 in the optics assembly 6 of the system.

The detector output signal 16 is demodulated by a first rate demodulator 17 in a known manner.

The signal 18 from this rate demodulator will have any wave signal formed from a large number of irreversible amplitudes typically occurring in a gyroscope, at least in the rate demodulator, And a relatively high direct current (DC). FIG. 3 shows the zero, negative and positive rotation rate signals of the gyroscope. Signal 18 is demodulated by a second demodulator 19 at a low chopper frequency (such as generated by chopper 14). Since the demodulator 19 is operated at a low frequency, it can be designed to have a relatively lower DC offset than the frequency of the demodulator 17 and not increase production costs. Therefore, the output 20 from the second demodulator 19 has a small offset. FIG. 3 shows the zero, negative and positive rotation rates of the gyroscope. This signal 20 is passed through a suitable filter device 21 to a cellodyne voltage controlled oscillator (VCO) 31
Will be introduced. This oscillator generates a sawtooth signal 29,
The slope of this signal changes by adjusting the phase difference imbalance until the phase difference around the coil is zero. The amplitude of serrodyne is controlled to 2mπ when m is an integer.

The rate and amplitude control loop controls the slope of the serrodyne signal 29 and its height. Thus, the frequency of the signal is proportional to the rate, and the gyroscope output can be obtained by summing the number of positive and negative resets in the VCO. These are calculated, converted and output to the user as increments of angles or rates in the usage portion.

The output of the gyroscope is typically a serodyne VCO frequency that is linearly proportional to the rate of rotation (negative frequencies have negative sawtooth slopes, positive frequencies have positive sawtooth slopes) Defined as).

The detector waveform 16 has a periodic signal 32, shown in FIG. 3, which occurs when the chopper 14 reverses the phase signal 12. The amplitude of the signal is an indication of the output at the detector and can be used in a feedback loop to adjust the gain or power source of amplifier 28 to maintain a constant output. Demodulation is blocked in a known manner during period 40.

In the second embodiment of the present invention shown in FIG. 4, the phase of the signal 12 changes in synchronization with the signal 14A output from the chopper 14, and becomes the reference signal 33 for the rate demodulator 17. Yes, this structure is a simplification of the previous embodiment.

The second embodiment does not work as well as the previous embodiment, and in that respect is not a preferred embodiment.

There are many variations of the first embodiment. For example, when the output of the first demodulator 17 is amplified, an AC connection (usually a connection from which a DC component is removed) is made. This includes a reduction in the bandwidth of the signal at the output of the first demodulator 17 so that the amplification before the second demodulator 19 does not suffer from the negative side of the saturation phenomenon and the offset generated from the first demodulator 17 There is the advantage of reducing the effect. Furthermore, the AC connection allows the second demodulator 19 to cut off unwanted DC components from the first demodulator 17 by ensuring that the average value of the output to the second demodulator 19 is just zero. .

The double demodulation process can be performed digitally if an analog-to-digital converter is placed before the first demodulator 17. Alternatively, the analog-to-digital converter is located before the second demodulator 19. In the latter case, the first demodulator 17
Performed by analog electronics, while a second demodulator
19 is performed by digital technology.

The double demodulation process can be applied to both closed and open loop systems where the gyro is demodulated. The additional demodulated signal 14A
Can be a fixed frequency, a variable frequency, a pseudo-random frequency or a random frequency. The initial demodulation waveform applied to the gyro (signal 15) can be rectangular, sinusoidal, or some other shape.

Although the first embodiment is a preferred embodiment, this is not the only embodiment.

[Brief description of the drawings]

FIG. 1 is a block diagram of an optical fiber gyroscope system using double demodulation processing according to a first embodiment of the present invention. FIG. 2 is a diagram showing zero, positive and negative based imbalanced fringe modulation waveforms and detector signals of the system of FIG. FIG. 3 is a diagram showing some typical signals in the system of FIG. FIG. 4 is a block diagram of a gyroscope using double demodulation processing according to a second embodiment of the present invention. DESCRIPTION OF SYMBOLS 1 ... Light source 2 ... Coil 3 ... Fiber 4 ... Coupler 5 ... Polarizer / demultiplexer 6 ... Optical device assembly 7 ... Coupler 8, 9 ... Phase modulator 10 ... Detector 11 ... Generator 12 Signal 13 Phase inverter 14 Chopper 14A Signal 17, 19 Demodulator 21 Filter 28 Amplifier 29 Sawtooth signal 31 Oscillator 33 Reference signal

Claims (11)

(57) [Claims] An optical fiber coil (2) connected to the light source (1) for receiving light from the light source (1); and separating the light from the light source (1) into an optical fiber. Coil (2)
Means for beam splitting and combining (7) in opposite directions around said light and said optically combining; a second chopper signal having a first signal of a first frequency lower than the first frequency; Phase modulation means (8, 9) for causing a phase shift in light passing through the coil (2) of the optical fiber in accordance with a coupling signal generated by coupling with the optical fiber; and coupling light passing through the coil (2) of the optical fiber. And a photodetector (10) for generating an output signal; demodulating the output from the photodetector with a first signal, and partially producing a gyro and a dc signal due to an offset in the demodulator. A first rate demodulator for generating an output signal having an amplitude representing the magnitude of the irreversibility; and an output signal from the first rate demodulator using a second chopper signal having a frequency lower than the operating frequency of the first rate demodulator. Demodulate and add offset A second demodulator that produces a signal of an amplitude substantially unaffected by any of the dc signals of the gyro and substantially representing the magnitude of the irreversibility that occurs in the gyro. scope. 2. The method according to claim 1, wherein the first signal is a half cycle of the light passing through the coil of the optical fiber, and a ± 1 cycle of the light passing through the phase modulator.
2. The fiber optic gyroscope according to claim 1, wherein the fiber optic gyroscope has an amplitude that produces a phase of π / 4. 3. The first chopper signal is synchronized with a second chopper signal.
2. The optical fiber gyroscope according to claim 1, further comprising an inverting means for inverting the phase of the signal. 4. A cellodyne (phase modulator) voltage controlled oscillator (VCO) for receiving an output signal from a second demodulator and generating a null signal that cancels irreversibility in the gyro caused by rotation. The optical fiber gyroscope according to claim 1, wherein: 5. A cellodyne (phase modulator) voltage controlled oscillator (VCO) for receiving an output signal from a second demodulator and generating a null signal for canceling irreversibility in a gyro caused by rotation. The fiber optic gyroscope according to claim 1, wherein the null signal has an amplitude and a sawtooth wave of approximately 2mπ (m: an integer). 6. A cellodyne (phase modulator) voltage controlled oscillator (VCO) for receiving an output signal from a second demodulator and generating a null signal to cancel irreversibility in a gyro caused by rotation. The null signal has an amplitude of about 2mπ (m: an integer) and a sawtooth wave, and has means for controlling the amplitude of a serrodyne waveform obtained from an error pulse generated in synchronization with the reset of serrodyne. The fiber optic gyroscope according to claim 1. 7. An apparatus for generating an error signal for correcting the amplitude of a cellodyne waveform by synchronous demodulation of a signal of a photodetector using a reference signal derived from both a dither and a chopper waveform. Item 7. An optical fiber gyroscope according to Item 6. 8. The fiber optic gyroscope according to claim 1, wherein the first and second demodulators are ac-coupled. 9. A gain between the first and second demodulators.
The optical fiber gyroscope according to claim 1, comprising: 10. The fiber optic gyroscope according to claim 1, wherein the first and / or second demodulator performs the demodulation digitally. 11. The fiber optic gyroscope according to claim 1, wherein a sample-and-hold device and a narrow band filter are arranged before the second demodulator.