Wavelength control system and fiber optic gyroscope capable of achieving ultra-high scale factor stability

Abstract

The present invention relates to a wavelength control system and a fiber optic gyroscope (FOG) capable of achieving ultra-high scale factor stability. The invention relates to the following: a single-mode fiber collimator and a wavelength tracking module are added to a conventional optical path. The input end of the single-mode fiber collimator is connected to one of the output ports of a 2×2 coupler, so that the other beam of light, which is split 50:50, is projected into the wavelength tracking module via the single-mode fiber collimator. This achieves real-time sensing of changes in the average wavelength of a broadband light source, which is used as a feedback signal to adjust the driving current of the broadband light source. The average wavelength of the broadband light source is related to the magnitude of the driving current, thereby achieving closed-loop control of the average wavelength of the broadband light source in the FOG system. This can improve the stability of the average wavelength of the broadband light source by orders of magnitude, thereby significantly improving the stability of the scale factor of the FOG.

Description

Technical Field

[0001] The present invention belongs to the technical field of fiber optic gyroscopes, and in particular relates to a wavelength control system and a fiber optic gyroscope capable of achieving ultra-high scale factor stability. Background Art

[0002] The fiber optic gyroscope (FOG) is an all-solid-state optical gyroscope based on the Sagnac effect. It detects the phase difference between two light beams transmitted in opposite directions in a circular manner through interference to characterize the angular rate of rotation. It uses a broadband light source with wide spectral characteristics (low coherence) to suppress the amplitude noise and drift caused by the Kerr effect, backscattering, and polarization coupling in the circular light path, so that the gyroscope's accuracy level meets the requirements of engineering applications.

[0003] In actual engineering applications, ambient temperature changes significantly impact the stability of the fiber optic gyroscope's scale factor. Therefore, full-temperature scale factor modeling and compensation must be performed before the fiber optic gyroscope is delivered for use. Changes in the stability of the fiber optic gyroscope's full-temperature scale factor primarily stem from the thermal expansion and contraction of the angular rate-sensitive annular optical path and the change in the broadband light source's full-temperature average wavelength. The thermal expansion and contraction effect is linearly related to temperature, while the change in the broadband light source's full-temperature average wavelength exhibits both a linear and nonlinear relationship with temperature. Therefore, the currently widely used linear model for fiber optic gyroscope full-temperature scale factor compensation is unable to completely eliminate the impact of the broadband light source's full-temperature average wavelength change on the fiber optic gyroscope's full-temperature scale factor stability.

[0004] There are two main types of broadband light sources used in fiber optic gyroscopes: superluminescent diodes (SLDs) and erbium-doped fiber sources (ASEs). SLDs are semiconductor light sources in which spontaneous photon emission is amplified in a single pass. They offer high output power, a narrow beam divergence, and a wide output spectrum. However, their average wavelength stability does not meet high-precision requirements, making them suitable for low- to medium-precision fiber optic gyroscopes. ASEs are broadband spontaneous emission sources amplified by erbium-doped gain fiber. They feature high output power, good average wavelength stability, and a compact size, making them primarily used in high-precision fiber optic gyroscopes.

[0005] Although ASE sources offer the advantage of better average wavelength stability compared to SLD sources, the inherent spontaneous emission mechanism of broadband sources prevents further improvement in average wavelength stability. The average wavelength stability of a source directly reflects the stability of the fiber optic gyroscope's scale factor. Therefore, the scale factor stability of a fiber optic gyroscope is difficult to match that of a laser gyroscope using a narrow-linewidth source, limiting the expansion of fiber optic gyros into higher-precision applications. Summary of the Invention

[0006] In view of the deficiencies of the prior art, the present invention proposes a wavelength control system and a fiber optic gyroscope that can achieve ultra-high scale factor stability.

[0007] One of the above-mentioned purposes of the present invention is achieved by the following technical solutions:

[0008] A wavelength control system capable of achieving ultra-high scale factor stability, comprising a broadband light source, a 2×2 coupler, a single-mode fiber collimator, a wavelength tracking module, and a modulation, demodulation, and wavelength monitoring control circuit board;

[0009] The 2×2 coupler is used to achieve 50:50 splitting of the light emitted by the light source, wherein one beam of split light is output from one output port of the 2×2 coupler to the single-mode fiber collimator;

[0010] The single-mode fiber collimator is used to convert the corresponding beam of light output by the 2×2 coupler into parallel light, so that the light is coupled into the wavelength tracking module with maximum efficiency;

[0011] The wavelength tracking module adopts a 50:50 special-shaped beam splitter structure to form two anti-phase interference signals that can characterize the average wavelength change, and completes the photoelectric conversion and sends it to the modulation and demodulation and wavelength monitoring control circuit board;

[0012] The modulation, demodulation and wavelength monitoring control circuit board is used to perform differential subtraction processing on the two interference electrical signals from the wavelength tracking module to obtain a real-time voltage value representing the wavelength change, and use the difference between the obtained real-time voltage value and the preset voltage value as a feedback signal for adjusting the broadband light source driving current to send it to the broadband light source through a cable connection, so that the voltage difference is always kept at zero, thereby realizing closed-loop control of the average wavelength of the light source.

[0013] Moreover, the wavelength tracking module adopts a 50:50 special-shaped beam splitter. During the light splitting process, there is a phase difference of π between the two horizontal interference light waves, and the phases of the vertical and horizontal interference light waves remain in anti-phase.

[0014] The second object of the present invention is achieved by the following technical solutions:

[0015] A fiber optic gyroscope capable of achieving ultra-high scale factor stability, characterized by comprising a broadband light source, a 2×2 coupler, a Y-waveguide, a fiber optic ring, a photodetector, a single-mode fiber collimator, a wavelength tracking module, and a modulation, demodulation, and wavelength monitoring control circuit board;

[0016] The 2×2 coupler is used to achieve 50:50 splitting of the light emitted by the light source, wherein one beam of split light is transmitted to the Y waveguide through one output port of the 2×2 coupler; the other beam of split light is output to the single-mode fiber collimator through the other output port of the 2×2 coupler;

[0017] The two pigtails of the Y-waveguide are optically connected to the two pigtails of the optical fiber ring by fusion splicing, forming a closed Sagnac interference optical path for polarizing and splitting light and outputting it through the detection port of the 2×2 coupler after interference.

[0018] The photoelectric detector is used to perform photoelectric conversion on the optical signal output from the detection port of the 2×2 coupler and transmit it to the modulation, demodulation and wavelength monitoring control circuit board;

[0019] The single-mode fiber collimator is used to convert the corresponding beam of light output by the 2×2 coupler into parallel light, so that the light is coupled into the wavelength tracking module with maximum efficiency;

[0020] The wavelength tracking module adopts a 50:50 special-shaped beam splitter structure to form two anti-phase interference signals that can characterize the average wavelength change, and completes the photoelectric conversion and sends it to the modulation and demodulation and wavelength monitoring control circuit board;

[0021] The modulation, demodulation and wavelength monitoring control circuit board is used to realize the modulation and closed-loop control of the fiber optic gyroscope and the closed-loop control of the average wavelength of the light source.

[0022] Moreover, the modulation, demodulation and wavelength monitoring control circuit board includes an amplifier, an analog-to-digital converter, a field programmable gate array (FPGA), a digital-to-analog converter and a serial port chip. The modulation, demodulation and wavelength monitoring control circuit board sequentially amplifies, isolates, performs analog-to-digital conversion and differential demodulation on the electrical signal from the photodetector to obtain a phase error digital signal after closed-loop compensation. After digital integration, the digital signal serves as the gyroscope output signal sent by the serial port chip and as a closed-loop feedback signal to form a feedback step wave. After the feedback step wave is superimposed on the bias modulation square wave, it is converted into a voltage signal through a digital-to-analog conversion and applied to the Y waveguide via a cable connection to achieve modulation and closed-loop control of the fiber optic gyroscope.

[0023] Moreover, the modulation, demodulation and wavelength monitoring control circuit board performs differential subtraction processing on the two interference electrical signals from the wavelength tracking module to obtain a real-time voltage value representing the wavelength change, and uses the difference between the obtained real-time voltage value and the preset voltage value as a feedback signal for adjusting the broadband light source driving current and sends it to the broadband light source through a cable connection, so that the voltage difference is always kept at zero, thereby realizing closed-loop control of the average wavelength of the light source.

[0024] The advantages and positive effects of the present invention are:

[0025] 1. The present invention adds a single-mode fiber collimator and a wavelength tracking module to the traditional optical path of the fiber optic gyroscope. The input end of the single-mode fiber collimator is connected to one of the output ports of the 2×2 coupler, so that the other beam of light with a 50:50 split is projected into the wavelength tracking module through the single-mode fiber collimator. This achieves real-time perception of changes in the average wavelength of the broadband light source, which is used as a feedback signal to adjust the driving current of the broadband light source. The average wavelength of the broadband light source is related to the magnitude of the driving current, thereby achieving closed-loop control of the average wavelength of the broadband light source in the fiber optic gyroscope system. This can improve the stability of the average wavelength of the broadband light source by an order of magnitude, thereby significantly improving the stability of the scale factor of the fiber optic gyroscope.

[0026] 2. The present invention utilizes the idle port of the original 2×2 coupler in the traditional optical path of the fiber optic gyroscope as the average wavelength detection port of the broadband light source, which does not destroy the classical optical path structure of the fiber optic gyroscope. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a diagram showing the structure of the fiber optic gyroscope of the present invention;

[0028] Figure 2 is a structural diagram of the wavelength tracking module used in the present invention;

[0029] Figure 3 This is a comparison of the full-temperature scale factor changes of the fiber optic gyroscope before and after wavelength closed-loop control. DETAILED DESCRIPTION

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and examples.

[0031] Figure 1 This is a schematic diagram of the fiber optic gyroscope, which consists of a broadband light source 1, a 2×2 coupler 2, a Y waveguide 3, a fiber ring 4, a photodetector 5, a single-mode fiber collimator 6, a wavelength tracking module 7, and a modulation, demodulation, and wavelength monitoring control circuit board 8.

[0032] A broadband light source is used to suppress polarization, backreflection, and other related errors in the gyroscope optical path system. Light emitted by the light source enters the input port of a 2×2 coupler for 50:50 splitting. One beam of light is transmitted from the output port of the 2×2 coupler to a Y-waveguide for polarization and beam splitting. The two Y-waveguide pigtails are fusion-spliced ​​with the two fiber pigtails of the fiber loop to form a closed optical path, forming a Sagnac interference optical path. The two light waves split by the Y-waveguide propagate in opposite directions in this Sagnac interference optical path. After a round of propagation, they finally reach the Y-waveguide again to converge and interfere. The phase difference between the two light beams is proportional to the angular velocity of rotation. After passing through the Y-waveguide in the opposite direction, the interference light signal is transmitted to the detection port of the 2×2 coupler, where it is converted to photoelectricity by a photodetector. The electrical signal is decoded by the modulation, demodulation, and wavelength monitoring control circuit board to form the gyroscope output signal representing the angular velocity of rotation.

[0033] At the same time, the other beam, split 50:50, is projected from the other output port of the 2×2 coupler through a single-mode fiber collimator into the wavelength tracking module to detect changes in the light source's average wavelength. The wavelength tracking module generates two anti-phase interference signals to represent the change in average wavelength. After photoelectric conversion, these signals are sent to the modulation and demodulation and wavelength monitoring control circuit board for signal differential subtraction. The difference between the real-time voltage value and the preset voltage value after differential subtraction serves as the feedback signal for adjusting the broadband light source's drive current, thereby achieving closed-loop control of the light source's average wavelength. During closed-loop control of the broadband light source's average wavelength, adjusting the light source's drive current will cause low-frequency fluctuations in the output optical power. The operating frequency of the fiber optic gyroscope's fully digital closed-loop modulation and demodulation is much higher than the frequency of the light source's output optical power fluctuations. Therefore, the fiber optic gyroscope's output stability is not affected by low-frequency fluctuations in the broadband light source's output optical power.

[0034] The modulation, demodulation, and wavelength monitoring control circuit board used includes an amplifier, an analog-to-digital converter, a field-programmable gate array (FPGA), a digital-to-analog converter, and a serial port chip. The modulation, demodulation, and wavelength monitoring control circuit board sequentially amplifies, blocks, performs analog-to-digital conversion, and performs differential demodulation on the electrical signal from the photodetector, generating a closed-loop compensated digital signal for the phase error. This digital signal undergoes digital integration and serves as both the gyro output signal transmitted by the serial port chip and the closed-loop feedback signal, forming a feedback step wave. This feedback step wave is superimposed with a bias-modulated square wave, converted into a voltage signal, and applied to the Y-waveguide via a cable connection, achieving modulation and closed-loop control of the fiber optic gyroscope. Furthermore, the modulation, demodulation, and wavelength monitoring control circuit board performs differential subtraction on the two interferometric electrical signals from the wavelength tracking module. The difference between the resulting real-time voltage and a preset voltage value is transmitted to the broadband light source via a cable connection as a feedback signal for adjusting the broadband light source's drive current, ensuring that the voltage difference remains zero, thus achieving closed-loop control of the light source's average wavelength.

[0035] Figure 2 The figure shows a schematic diagram of a wavelength tracking module. The module utilizes a 50:50 shaped beam splitter. The input lightwave is split into two beams on the reflection / transmission surface and transmitted vertically and horizontally, respectively. The vertical and horizontal ends of the shaped beam splitter form triangular reflective structures. Both vertical and horizontal beams are reflected and returned, splitting into four beams at the same location on the reflection / transmission surface. The two vertical lightwaves interfere and are received by a photodetector in the vertical direction, converting them into voltage signals. The two horizontal lightwaves interfere and are received by a photodetector in the horizontal direction, converting them into voltage signals. The two vertical interference lightwaves are both reflected and transmitted once on the reflection / transmission surface. The beam splitting process does not produce a phase difference between the two vertical interference lightwaves. One of the two horizontal interference lightwaves is reflected twice on the reflection / transmission surface, while the other is transmitted twice. Reflection introduces a π / 2 phase delay. Therefore, the beam splitting process creates a π phase difference between the two horizontal interference lightwaves. This ensures that the phases of the vertical and horizontal interference signals remain in opposite phases. Differential processing of the two interference signals improves wavelength tracking accuracy.

[0036] The 50:50 shaped beam splitter is slightly longer horizontally than vertically to ensure a certain optical path difference Δl between the horizontally propagating light waves and the vertically propagating light waves. For example, when the average wavelength is 1550nm, this optical path difference forms a phase difference of π / 2. Therefore, the above-mentioned two interference signals will operate at the point with the maximum slope, which can improve the sensitivity of wavelength change perception.

[0037] The interference signal of one channel can be expressed as formula (1). When the average wavelength of the light source is When the phase of the interference signal changes By monitoring the amplitude change of the interference signal, the change in the average wavelength of the light source can be determined. The difference between the voltage signal representing the amplitude of the interference signal and the preset voltage value is used as the feedback signal for adjusting the driving current of the broadband light source, thus achieving closed-loop control of the light source's average wavelength.

[0038]

[0039] Where I1 and I2 are the light intensity of the light wave propagating in the horizontal direction and the light intensity of the light wave propagating in the vertical direction respectively, n is the refractive index, Δl is the optical path difference between the two light waves, is the average wavelength of the light source.

[0040] The main reason for the change in the average wavelength of the broadband light source for the fiber optic gyroscope is the change in ambient temperature. For example, there are three temperature-related sources that cause the average wavelength to change in the ASE light source, as shown in formula (2).

[0041]

[0042] The first term is the inherent average wavelength caused by the temperature characteristics of the erbium-doped fiber. The second and third terms are the pump wavelength λ p and pump power P p The change in average wavelength caused by this; T is the ambient temperature.

[0043] It can be seen that adjusting the light source drive current, that is, changing the pump power, can change the light source's average wavelength. It can also be seen that the full-temperature average wavelength variation of the ASE light source is determined by three temperature-related factors, not a single linear relationship. Consequently, the average wavelength stability of the ASE light source exhibits a certain nonlinear correlation with temperature variation, and this nonlinear relationship with temperature variation cannot be eliminated in the subsequent linear model for temperature compensation of the fiber optic gyroscope scale factor. Implementing a wavelength tracking module to achieve closed-loop control of the broadband light source's average wavelength can effectively eliminate the linear and nonlinear variations in the average wavelength caused by temperature changes, significantly improving the stability of the fiber optic gyroscope scale factor.

[0044] Figure 3 This figure compares the full-temperature scale factor variation of a fiber optic gyroscope (FOG) before and after wavelength closed-loop control. The FOG was placed on a single-axis rate turntable with a rotational speed of 30° / s. This speed is significantly greater than the gyro's thermally induced nonreciprocal noise, minimizing gyro drift caused by temperature changes and thus not affecting the observed full-temperature scale factor variation. Temperature cycling was performed with the turntable rotating at high speed. The gyro output fluctuations were entirely due to the gyro scale factor variation. The temperature cycling conditions were -40°C to 60°C, with a ramp rate of 1°C / min. For the same FOG, the FOG output varied significantly with temperature before wavelength closed-loop control. However, the temperature-induced fluctuations were significantly reduced after wavelength closed-loop control. The gyro scale factor variation caused by the full-temperature variation of the broadband light source's average wavelength was eliminated, leaving only the linear variation of the gyro scale factor caused by thermal expansion and contraction of the angular rate-sensitive annular optical path. This linear relationship with temperature is fully suppressed in the subsequent FOG scale factor temperature compensation linear model. It can be seen that the fiber optic gyroscope technology proposed in this patent has an ultra-high scale factor stability characteristic.

[0045] In summary, the present invention proposes a fiber optic gyroscope with closed-loop control of the average wavelength of a broadband light source, which can solve the bottleneck problem that the stability of the fiber optic gyroscope's scale factor is limited by the stability of the average wavelength of the broadband fiber optic light source. A wavelength tracking module is provided in the gyroscope to sense the change of the average wavelength in real time as a feedback signal for the light source driving current control to achieve closed-loop control of the average wavelength, which can greatly improve the stability of the fiber optic gyroscope's scale factor.

Claims

1. A wavelength control system capable of achieving ultra-high scale factor stability, characterized in that: Includes broadband light source, 2×2 coupler, single-mode fiber collimator, wavelength tracking module, modulation and demodulation and wavelength monitoring control circuit board; The 2×2 coupler is used to achieve 50:50 splitting of the light emitted by the light source, wherein one beam of split light is output from one output port of the 2×2 coupler to the single-mode fiber collimator; The single-mode fiber collimator is used to convert the corresponding beam of light output by the 2×2 coupler into parallel light, so that the light is coupled into the wavelength tracking module with maximum efficiency; The wavelength tracking module adopts a 50:50 special-shaped beam splitter structure to form two anti-phase interference signals that can characterize the average wavelength change, and completes the photoelectric conversion and sends it to the modulation and demodulation and wavelength monitoring control circuit board; The modulation, demodulation and wavelength monitoring control circuit board is used to perform differential subtraction processing on the two interference electrical signals from the wavelength tracking module to obtain a real-time voltage value representing the wavelength change, and use the difference between the obtained real-time voltage value and the preset voltage value as a feedback signal for adjusting the broadband light source driving current to send it to the broadband light source through a cable connection, so that the voltage difference is always kept at zero, thereby realizing closed-loop control of the average wavelength of the light source.

2. The wavelength control system capable of achieving ultra-high scale factor stability according to claim 1, characterized in that: The wavelength tracking module adopts a 50:50 special-shaped beam splitter. During the light splitting process, there is a phase difference of π between the two horizontal interference light waves, and the phases of the vertical and horizontal interference light waves remain in anti-phase.

3. A fiber optic gyroscope capable of achieving ultra-high scale factor stability, characterized by: Includes broadband light source, 2×2 coupler, Y waveguide, fiber ring, photodetector, single-mode fiber collimator, wavelength tracking module, modulation and demodulation and wavelength monitoring control circuit board; The 2×2 coupler is used to achieve 50:50 splitting of the light emitted by the light source, wherein one beam of split light is transmitted to the Y waveguide through one output port of the 2×2 coupler; the other beam of split light is output to the single-mode fiber collimator through the other output port of the 2×2 coupler; The two pigtails of the Y-waveguide are optically connected to the two pigtails of the optical fiber ring by fusion splicing, forming a closed Sagnac interference optical path for polarizing and splitting light and outputting it through the detection port of the 2×2 coupler after interference. The photoelectric detector is used to perform photoelectric conversion on the optical signal output from the detection port of the 2×2 coupler and transmit it to the modulation, demodulation and wavelength monitoring control circuit board; The single-mode fiber collimator is used to convert the corresponding beam of light output by the 2×2 coupler into parallel light, so that the light is coupled into the wavelength tracking module with maximum efficiency; The wavelength tracking module adopts a 50:50 special-shaped beam splitter structure to form two anti-phase interference signals that can characterize the average wavelength change, and completes the photoelectric conversion and sends it to the modulation and demodulation and wavelength monitoring control circuit board; The modulation, demodulation and wavelength monitoring control circuit board is used to realize the modulation and closed-loop control of the fiber optic gyroscope and the closed-loop control of the average wavelength of the light source.

4. The fiber optic gyroscope capable of achieving ultra-high scale factor stability according to claim 3, wherein: The modulation, demodulation, and wavelength monitoring control circuit board includes an amplifier, an analog-to-digital converter, a programmable gate array, a digital-to-analog converter, and a serial port chip. The modulation, demodulation, and wavelength monitoring control circuit board sequentially amplifies, DC isolates, performs analog-to-digital conversion, and performs differential demodulation on the electrical signal from the photodetector to obtain a closed-loop compensated phase error digital signal. After digital integration, the digital signal serves as a gyro output signal sent by the serial port chip and as a closed-loop feedback signal to form a feedback step wave. After the feedback step wave is superimposed on the bias modulation square wave, it is converted into a voltage signal through digital-to-analog conversion and applied to the Y waveguide through a cable connection to achieve modulation and closed-loop control of the fiber optic gyroscope.

5. The fiber optic gyroscope capable of achieving ultra-high scale factor stability according to claim 4, characterized in that: The modulation, demodulation and wavelength monitoring control circuit board performs differential subtraction processing on the two interference electrical signals from the wavelength tracking module to obtain a real-time voltage value representing the wavelength change, and uses the difference between the obtained real-time voltage value and the preset voltage value as a feedback signal for adjusting the broadband light source driving current and sends it to the broadband light source through a cable connection, so that the voltage difference is always kept at zero, thereby realizing closed-loop control of the average wavelength of the light source.

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