RU2734999C1 - Fiber-optic gyroscope photodetector current stabilization device - Google Patents

Abstract

FIELD: fiber optics.SUBSTANCE: invention relates to fiber optics, in particular to photodetector current stabilization devices and can be used in designing fiber-optic gyroscopes and other physical quantity sensors based on single-mode fiber waveguides. An electronic feedback processing unit of the fiber-optic gyroscope is completed with an additional feedback loop, with a pass band of at least 4 kHz, which includes differential amplifier, current-to-voltage converter, output of which is connected to first input of differential amplifier, a source of stabilized reference voltage, the output of which is connected to the second input of the differential amplifier, and a regulator, the input of which is connected to the output of the differential amplifier, and the output is connected to the power supply unit of the optical radiation source.EFFECT: technical result consists in improvement of accuracy of fiber-optic gyroscope under action of external destabilizing factors due to stabilization of constant component of optical signal on photodetector.1 cl, 4 dwg

Description

The invention relates to the field of fiber optics and can be used in the design of fiber-optic gyroscopes and other sensors of physical quantities based on single-mode fibers.

Fiber-optic gyroscope (hereinafter referred to as FOG) contains an optical unit and an electronic information processing unit. The optical unit contains an optical radiation source, a photodetector, and a fiber ring interferometer (hereinafter referred to as VKI). The FRI includes an optical circulator, an integrated-optical circuit (hereinafter referred to as IOS), and a multi-turn sensitive coil. The IOS contains a Y-power divider of optical radiation based on polarizing channel waveguides and a phase modulator located on the output arms of the Y-divider. The channel waveguides of the Y-divider are formed in the lithium niobate substrate using the proton-exchange technology, which allows the waveguides to acquire polarizing properties. A phase modulator of optical beams passing through the channel waveguides is located on the output channel waveguides. The phase modulator consists of channel waveguides formed in a lithium niobate substrate and metal electrodes deposited on both sides of the channel waveguides. When an electric voltage is applied to the electrodes, the refractive index in the material of the channel waveguides changes due to the electro-optical effect, which leads to the effect of phase modulation of the optical rays propagating along the channel waveguides. The ends of the fibers of the sensitive gyroscope coil are docked to the output waveguides of the Y-divider.

An interference pattern is observed on the FRI photodetector, formed by two optical beams that have passed the sensitive gyroscope coil in two mutually opposite directions. When the ring interferometer rotates between these two beams, due to the Sagnac effect, a phase difference arises, which is expressed as follows:

where R is the radius of the sensitive gyroscope coil;

L is the length of the coil light guide;

λ is the weighted average radiation wavelength of the source;

c is the speed of light in vacuum;

Ω is the angular velocity of the gyroscope rotation.

Thus, at the photodetector, the optical radiation power can be represented as:

where Р ₀ is the power of beams interfering on the photodetector.

The power P _{0 of the} interfering beams depends on the output power of the optical radiation source. The FOG uses sources of optical radiation with a short coherence length. These can be either superluminescent laser diodes (SLDs) or erbium fiber superluminescent sources (EWSS). In high-precision FOGs, EVSI are predominantly used. The EVSI includes an erbium fiber, a fiber multiplexer and a laser diode for pumping an erbium fiber. The output power of the EVSI is determined by the output power of the pump laser diode, which, in turn, is determined by the current of the pump laser diode. The pump laser diode current is provided by the EVSI power supply unit. Auxiliary phase modulation is used to increase the FOG sensitivity near zero angular velocities. To achieve the effect of phase modulation of beams in a ring interferometer using the IOS phase modulator, the time lag of the interfering beams on the photodetector is used when passing the IOS phase modulator. This time delay is equal to the travel time of the FRI light beams through the fiber of the sensitive coil and amounts to:

where n ₀ is the refractive index of the light guide material of the sensitive coil.

When applying to the phase modulator voltage pulses of auxiliary phase modulation (APM) following with a frequency of 1/6 τ [1], the photodetector current can be represented as:

where η _f - current sensitivity of the photodetector;

Δ is the HFM parameter that determines its amplitude;

- амплитуда сигнала вращения ВОГ.

is the amplitude of the FOG rotation signal.

Further, the signal from the photodetector goes to the input of the current amplifier of the photodetector, at the output of which there is a voltage proportional to the value:

where R _n - load resistance of the current amplifier of the photodetector;

- сигнал вращения ВОГ.

- FOG rotation signal.

In [2], a method for linearizing the output characteristics of the FOG is proposed. A stepped sawtooth voltage is applied to the phase modulator simultaneously with the auxiliary phase modulation (APM) voltage to compensate for the Sagnac phase difference. With the help of a sawtooth voltage applied to the phase modulator, a controlled phase difference between the FRI beams is introduced, with the help of which the Sagnac phase difference is compensated. For this purpose, a closed feedback loop (FOG with a closed feedback loop OC-1) is organized to zero the signal at the output of the gyroscope rotation signal demodulator. The signal at the output of the rotation signal demodulator is automatically zeroed due to the selection of the voltage step value of the sawtooth step voltage (SPV). Due to this, the output characteristic of the FOG becomes linear. In this case, the signal at the output of the current amplifier of the photodetector can be represented as:

The efficiency of the IOS phase modulator is strongly dependent on changes in the ambient temperature. Changes in the efficiency of the IOS phase modulator leads to instability of the FOG scale factor. To increase the stability of the FOG scale factor, a second feedback loop [1] (OC-2 loop) is used, based on the second demodulator, which selects the amplitude of the error signal that is present on the photodetector when the efficiency of the phase modulator changes. With the help of the OS-2 circuit, the parameter Δ VFM is stabilized.

On the photodetector, in addition to the rotation signal and the mismatch signal, there is a constant component of these signals, the amplitudes of which are rigidly related to its value. To increase the sensitivity of the FOG, the current amplifier of the photodetector must have a large gain, and therefore the constant component of the total signal at the photodetector brings the amplifier to a saturation state. To ensure the operability of the amplifier, it is necessary to compensate for the constant component of the signal at the input of the amplifier and, as a consequence, at its output. To compensate for the DC component of the signal at the amplifier output, a third feedback loop (OC-3 circuit) is organized on the basis of the third demodulator, which selects the DC component at the output of the photodetector current amplifier [3]. With the help of the OS-3 circuit, a voltage is formed, which is fed to the second input of the differential current amplifier of the photodetector (a common signal from the photodetector is fed to the first input, which contains a rotation signal, an error signal and their constant component) in order to zero the constant component of the total signal at the amplifier output FOG.

It is believed that in a FOG with a closed loop OS-1, the output signal does not depend on changes in the constant component of the optical radiation power at the photodetector if they occur within small limits and change slowly compared to the speed of the OS-1 circuit. However, experimental data indicate that such a dependence still takes place under the influence of external destabilizing factors, for example, when the ambient temperature changes, vibrations, radiation, etc. Let us assume that in the FOG there is a certain parasitic displacement of the angular velocity, which can be introduced into the right side of the equation for the OS-1 circuit [4]:

where Ф _s (t) is the Sagnac phase difference;

Ф _k (t) is the phase difference compensating for the Sagnac phase difference introduced by the phase modulator when a stepped sawtooth voltage is applied to it;

G (t) - bandwidth of the OS-1 contour;

t is time;

θ (t) is a parasitic quantity, which is determined by the parasitic phase difference of the beams in the ring interferometer of the FOG, the bias at the output of the photodetector current amplifier, as well as distortions of the rotation signal, which are determined by imperfections in the characteristics of the phase modulator. It is also taken into account that, due to changes in the constant component of optical radiation at the photodetector, the FOG bandwidth (G) also depends on time. If these changes are small and slow in time, then for the steady state (t >> G) and constant angular velocity of rotation, the solution to the equation for the OS-1 contour looks like this:

Here, all derivatives of the quantity θ (t) are considered very small due to the fact that it slowly depends on time. Also for Q (t) the following relation is valid:

where P ₀ (t) is the optical power at the photodetector, depending on time;

χ (t) is the time-dependent current sensitivity of the photodetector.

The angular velocity measurement error depends on the change in θ (t), A, P ₀ (t). The parasitic displacement θ (t) is minimized by eliminating parasitic effects in the optical components of the FRI. The VFM parameter Δ is stabilized due to the operation of the OS-2 circuit. Thus, the main disadvantage of the known design of a high-precision FOG is the instability of the zero signal (a decrease in its accuracy) due to the instability of the amplitude of the FOG rotation signal, which is determined by the value Q (t) = P ₀ (t) χ {t). The stability of the amplitude of the FOG rotation signal depends on the output power of the radiation source, changes in the optical radiation loss in the optical components of the FRI and upon a change in the current sensitivity of the photodetector with a change in the ambient temperature, as well as on the effect of vibrations and radiation on the radiation source and optical components of the FRI. The main disadvantage of the known information processing schemes [4] is that when the current sensitivity of the photodetector changes, the optical power loss in the components of the FRI optical circuit changes under the influence of external destabilizing factors, the amplitude of the rotation signal changes and, as a consequence, this leads to the instability of the zero signal of the FOG, then there is a deterioration in its accuracy.

The aim of the invention is to improve the accuracy of the FOG when exposed to external destabilizing factors.

This goal is achieved by the fact that a fourth feedback loop is introduced into the electronic information processing unit, with a bandwidth of at least 4 kHz, which includes a differential amplifier, a current-voltage converter, the output of which is connected to the first input of the differential amplifier, a source of stabilized reference voltage, the output of which is connected to the second input of the differential amplifier, and the regulator, the input of which is connected to the output of the differential amplifier, and the output is connected to the power supply unit of the optical radiation source.

An increase in the accuracy of the FOG is achieved by reducing the instability of the zero signal under the influence of external factors by stabilizing the amplitude of the rotation signal by stabilizing the constant component of the optical radiation power at the photodetector.

The essence of the invention is illustrated by drawings. FIG. 1 shows a block diagram of a fiber optic gyroscope. FIG. 2 shows a block diagram of a fiber optic gyroscope with three closed feedback loops. FIG. 3 shows a block diagram of a fiber-optic gyroscope with a circuit for stabilizing the constant component of the photodetector current. FIG. 4 shows a circuit for regulating the current of an SLD or a laser pumping diode EVSI.

FIG. 1 shows a block diagram of a fiber optic gyroscope. The FOG contains an optical radiation source 1, VKI 2, which includes an optical circulator 3, IOS 4, a sensitive fiber coil 5, a photodetector 6 and an electronic information processing unit 7.

FIG. 2 shows a block diagram of a fiber optic gyroscope with three feedback loops. The EVSI is used as a source of optical radiation. The EVSI includes a pumping laser diode 8, a power supply for a pumping laser diode 9, a multiplexer 10, a piece of erbium fiber 11 and an optical radiation reflector 12. The electronic unit for processing information from the photodetector contains a differential current amplifier of the photodetector 13 (hereinafter referred to as DU1), an analog-to-digital converter (hereinafter referred to as ADC) 14 and a programmable logic integrated circuit 15 (hereinafter referred to as FPGA). In the FPGA, a rotational signal demodulator D ₁ 16, a step height regulator 17 of a step sawtooth voltage (hereinafter referred to as SPN), a code generator SPN18, a voltage code generator for auxiliary phase modulation (hereinafter referred to as VFM) 19, an adder of codes SPN and VFM 20 are formed. at the output of the adder goes to the input of the digital-to-analog converter 21 (hereinafter referred to as DAC1). The FPGA also has a second demodulator D2 22 for demodulating the error signal, a voltage amplitude regulator VFM. The VFM voltage amplitude is adjusted by changing the reference current of DAC1. The voltage of the SPN and VFM is amplified by the operational amplifier 24 (hereinafter referred to as OU1) and then fed to the electrodes of the IOS phase modulator. On the basis of the D ₁ demodulator, the operation of the closed loop OS- _{1 is} organized, designed to linearize the output characteristics of the FOG. By adjusting the step of the SPN code at the output of the demodulator D _{1, the} code is kept equal to zero. On the basis of the D ₂ demodulator, the operation of a closed loop OS- _{2 is} organized, designed to stabilize the scale factor of the FOG when the efficiency of the phase modulator changes. The code at the output of the demodulator D _{2 is} maintained equal to zero by changing the amplitude of the VFM voltage by adjusting the reference current of DAC1

To compensate for the DC component at the output of the differential current amplifier of the photodetector, a third OS-3 circuit is organized on the basis of the third demodulator D ₃ 25, which selects the DC component at the output of the differential current amplifier of the photodetector. The OC-3 circuit includes a regulator 26 of the adjustable code cell 27, the output code of which is fed to the input of the DAC 28 (hereinafter referred to as TsPA2). The voltage from the output of DAC2 is fed to the input of the operational amplifier 29 (hereinafter referred to as OU2), and from its output to the second input of DU1. The voltage at the second input of the amplifier DU1 is fixed when there is a zero code at the output D ₃ .

FIG. 3 shows a block diagram of a fiber-optic gyroscope with a circuit for stabilizing the constant component of the photodetector current. The magnitude of the optical amplitude of the FOG rotation signal is rigidly related to the magnitude of the constant component of the optical power at the photodetector. Thus, to stabilize the amplitude of the rotation signal, it is sufficient to achieve stability of the constant component of the signal at the photodetector. In this circuit, the photodetector is connected to a current-to-voltage converter (hereinafter referred to as STP) 30. The voltage from the STP is supplied to the first input of the second differential amplifier 31 (hereinafter referred to as DU2). The second input of DU2 is connected to a stabilized voltage source (hereinafter referred to as ISN). The voltage from the DU2 output is fed to the input of the current regulator 33 of the laser pumping diode EVSI. On the basis of DU2, a fourth feedback loop is organized (loop OS-4). The regulator changes the current of the laser pumping diode EVSI (with the help of an electric power source EVSI) until zero voltage is established at the output of DU2. Thus, the stability of the constant component of the optical power mainly depends only on the transmission coefficient of the STP and the stability of the voltage provided by the ISN. The stabilization of the constant component of the optical signal at the photodetector when using a superluminescent laser diode (SLD) as a source of optical radiation is also performed using the same regulator by changing the consumption current of the SLD using its power source.

FIG. 4 shows a circuit of a current-voltage converter for regulating the current of an SLD or a laser pumping diode EVSI. A measuring resistor 35 is installed at the output of the reference voltage source 34, which is used to power the FOG photodetector. The output of the measuring resistor is connected to a low-pass filter. The voltage drop across the measuring resistor and low-pass filter is amplified by the amplifier 37. Thus, the voltage at the amplifier output is proportional to the average current of the photodetector, that is, the constant component of the photodetector current. Due to the regulation of the output optical power of the EVSI, when the current of the pump laser diode or the SLD current is changed, the average current of the photodetector is stabilized. The stability of the constant component of the photodetector current due to a change in the output power of the radiation source is achieved due to a small number of electronic components that make up the OC-4 circuit. The stability of the parameters of the measuring resistor, the low-pass filter circuit and the voltage drop amplifier across the measuring resistor ensure the stability of the DC component of the photodetector current at a level not exceeding 100 ppm / 1 ° C. The stability of the constant component of the signal at the photodetector depends on the bandwidth of the OC-4 circuit. The proposed electrical circuit of the OS-4 circuit should have a wide bandwidth to eliminate rapid changes in the amplitude of the rotation signal, which make a significant contribution to the instability of the zero signal of the FOG. When the FOG is exposed to vibrations, a parasitic displacement of the zero signal of the FOG occurs, which can be expressed as follows:

where Ф ₀ is the amplitude of the parasitic phase difference of the ring interferometer when exposed to the sensitive vibration coil;

MK is the scale factor of the ring interferometer;

ΔР - changes in the amplitude of the optical rotation signal;

Р ₀ - the amplitude of the optical signal of rotation.

To eliminate the parasitic displacement of the zero rotation signal when exposed to vibrations, it is necessary to zero either Ф ₀ or the value of ΔР. Zeroing of the ΔР value is carried out using the OS-4 circuit.

The feedback loop OS-4 should have a passband of at least 4 kHz, which will effectively stabilize the DC component of the FOG photodetector when the rotation signal amplitude changes with a frequency of up to 2 kHz. Rapid changes in the amplitude of the rotation signal can occur when the FOG is subjected to vibration loads. In this case, when using the OS-4 circuit, the stability of the zero signal is not worse than 0.001 deg / h. when exposed to vibrations with a frequency of up to 2 kHz.

Literature:

[1] A.M. Kurbatov, R.A. Kurbatov, A.M. Goryachkin “Improving the accuracy of a fiber-optic gyroscope by suppressing parasitic effects in integrated-optical phase modulators” Gyroscopy and navigation. Volume 27, No. 2 (105). 2019.

[2] G.A. Pavlath "Closed-loop fiber optic gyros" SPIE v. 2837, 1996, pp 46-60.

[3] A.M. Kurbatov, P.A. Kurbatov "A method for increasing the accuracy of a fiber-optic gyroscope when exposed to vibrations." Application No. 2016134633 priority dated August 25, 2016. RF patent No. 2627020 dated August 2, 2017

[4] Kurbatov A.M., Kurbatov R.A. Vibration error of angular velocity of a fiber-optic gyroscope and methods of its suppression // Radiotekhnika i Elektronika. - 2013. - No. 8. - from. 842.

Claims (1)

A device for stabilizing the current of the photodetector of a fiber-optic gyroscope, containing an optical radiation source, a fiber ring interferometer, a photodetector and an electronic information processing unit containing a first feedback loop for linearizing the output characteristics of the gyroscope, a second feedback loop for stabilizing its scale factor and a third feedback loop to compensate for the constant voltage component at the output of the photodetector current amplifier, characterized in that a fourth feedback loop is introduced into the electronic information processing unit, with a bandwidth of at least 4 kHz, which includes a differential amplifier, a current-voltage converter, the output of which is connected with the first input of the differential amplifier, a stabilized reference voltage source, the output of which is connected to the second input of the differential amplifier, and a regulator whose input is connected to the output of the differential amplifier, and the output connected to the power supply unit of the optical radiation source.

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