CN114674302A - Dual-polarization fiber optic gyroscope for dead-end optical power recovery and reuse - Google Patents

Abstract

The embodiment of the invention discloses a dual-polarization fiber-optic gyroscope capable of recycling dead-end optical power, which comprises a polarization-maintaining fiber-optic ring, a coupler, a first polarizer, a light source, a detector and a polarization beam splitter, wherein the dead end of the coupler is connected with a second polarizer; two output ports of the polarization beam splitter are connected with the polarization-maintaining optical fiber ring, and one output port and the polarization main shaft of the polarization-maintaining optical fiber are twisted by 90 degrees to form the interchange of the s state and the p state of a polarization mode. The invention recycles the light at the dead end of the traditional fiber-optic gyroscope coupler, doubles the interference light intensity on the premise that the input light power of a light source is unchanged, and improves the signal-to-noise ratio of the gyroscope by about 40% with low cost; the invention uses two optical transmission channels of s polarization state and p polarization state in the optical fiber ring at the same time, and utilizes the characteristic of reverse amplitude of polarization errors of the two polarization states, and the like to cancel the polarization related errors, thereby greatly improving the stability of the gyroscope.

Description

Dual-polarization fiber optic gyroscope for dead-end optical power recovery and reuse

technical field

The invention relates to the technical field of optical fiber sensing, in particular to a dual-polarization fiber optic gyroscope for recycling and reusing dead-end optical power.

Background technique

Fiber optic gyroscopes can be sensitive to the rotational angular velocity of carriers and are mainly used for inertial navigation and attitude control of moving carriers. With the expansion of application fields, the market has put forward higher and higher requirements for gyro accuracy. The most important of which is the improvement of signal-to-noise ratio and stability. These two indicators are respectively determined by angle random walk (ARW) and zero Bias Instability (BI) evaluation is the core indicator of fiber optic gyroscopes.

可由下式表示： The generation of FOG interference signals is based on the optical Sagnac effect, whose interference phase

It can be represented by the following formula:

------------------------（1）

------------------------(1)

Where L is the length of the fiber, D is the diameter of the fiber ring, λ is the wavelength of the light source in vacuum, c is the speed of light in the vacuum, and Ω is the rotational angular velocity of the carrier. The detection of the rotation signal can be realized by converting the interference phase into the light intensity signal I after the interference.

-------------------（2）

-------------------(2)

为调制相位，由相位调制器提供，用于调制与解调。

为非互易相位误差，决定了陀 螺的稳定性。 Among them, I ₀ is the input light intensity signal attenuated by the optical path, which is determined by the input optical power of the light source and the loss on the optical path.

To modulate the phase, it is provided by the phase modulator for modulation and demodulation.

For the non-reciprocal phase error, it determines the stability of the gyro.

Firstly, the signal-to-noise ratio of the gyro is explained. According to formula (1), the signal intensity is proportional to the input light intensity.

决定。 Generally speaking, the noise of fiber optic gyroscope is composed of detector shot noise, light source relative intensity noise, thermal phase noise, etc. When the light intensity is low, the limit sensitivity of fiber optic gyroscope is determined by shot noise.

Decide.

---------------------（3）

--------------------- (3)

，c为真空光速，λ为光源真空中的波长。

为信号采集带宽。 Among them, I is the light intensity reaching the detector, which is proportional to the input light intensity I ₀ , h is Planck's constant, h =6.63X10 ^{- 34} Js, v is the light frequency,

, c is the speed of light in vacuum, λ is the wavelength of the light source in vacuum.

Acquisition bandwidth for the signal.

Then the signal-to-noise ratio R of the gyro can be expressed as

----------------------（4）

---------------------- (4)

It can be seen that the higher the optical power reaching the detector, the higher the gyro SNR. In the case of a certain optical path loss, the most effective way to improve the signal-to-noise ratio of the gyroscope is to increase the input optical power of the gyroscope. The traditional solution is to increase the light intensity of the light source, but the high-power light source is often expensive and will increase the power consumption of the gyroscope, which brings difficulties to the development of the navigation system. Therefore, under the premise of not changing the optical power of the light source, the research of the present invention improves the light intensity of the detector, thereby improving the signal-to-noise ratio of the gyro, which has great practical significance.

倍。In the traditional fiber optic gyroscope, in order to ensure the reciprocity of the optical path, a 3dB coupler needs to be installed after the light source, as shown in Figure 1, half of the light enters the dead end through the coupler, resulting in scattering loss. If this part of the light can be recycled and reused, and the scattered loss light can be converted into signal light, the signal-to-noise ratio of the gyro can be improved without changing the output light intensity of the light source.

times.

。此种相位误差，通过调制解调与角速度干涉相位

无法解耦，极大的影响陀螺 稳定性，仅能通过良好的光路设计予以抑制。非互易相位误差主要由偏振误差，瑞利散射， 热光误差、弹光误差等组成，其中偏振相关误差是陀螺的主要误差。 For the stability of the gyroscope (or known as the bias instability), it is mainly limited by the non-reciprocal phase error in the optical path of the gyroscope.

. This phase error, through modulation and demodulation and angular velocity interference phase

It cannot be decoupled, which greatly affects the stability of the gyro, and can only be suppressed by a good optical path design. The non-reciprocal phase error is mainly composed of polarization error, Rayleigh scattering, thermo-optic error, elastic-optic error, etc. Among them, the polarization-related error is the main error of the gyroscope.

The traditional single-polarization fiber optic gyroscope is shown in Figure 1. The input light is filtered into linearly polarized light by a polarizer and enters the polarization-maintaining fiber ring. The fiber ring mainly transmits one linear polarization state (s-polarization state or p-polarization state). If the polarization state can be maintained over hundreds of meters of fiber loops, the polarization error is zero. However, in practical applications, due to the imperfection of the fiber and the influence of stress temperature, the linearly polarized light will crosstalk to its vertical polarization state (such as from s-polarization to p-polarization). The crosstalk of polarization-maintaining fibers can be determined by polarization crosstalk The sound index h is estimated to be about 10 ^-5 /m. Since the refractive indices of the two polarization states of the polarization-maintaining fiber are different (the s state and the p state are denoted as no _o and _ne respectively)

Therefore, the crosstalk light and the intrinsic light will have different phase differences, resulting in polarization errors.

可由下式估算： A traditional solution for suppressing polarization errors is to increase the extinction ratio of the polarizer to suppress the light intensity of the vertically polarized state, but a polarizer with a high extinction ratio often requires complicated processes and higher costs. For a single polarization fiber optic gyroscope, its polarization error

It can be estimated by the following formula:

-------（5）

-------(5)

，T _s，T _p 分别为s偏振态、p偏振态的光 强。对于高偏振或单偏振陀螺p≈1。

为偏振器消光比，

为光纤偏振串音，L为光纤环长 度。 where p is the degree of polarization of the input light,

, T _s , and T _p are the light intensities of the s-polarized state and the p-polarized state, respectively. p ≈ 1 for highly polarized or single-polarized gyroscopes.

is the polarizer extinction ratio,

is the fiber polarization crosstalk, and L is the length of the fiber loop.

It is of great significance to develop cost-effective fiber optic gyroscopes to reduce the requirements of single-polarization gyroscopes for high extinction ratio polarizers and effectively suppress polarization errors through optical path design.

SUMMARY OF THE INVENTION

The technical problem to be solved by the embodiments of the present invention is to provide a dual-polarization fiber optic gyroscope that can recover and reuse dead-end optical power, so as to reduce the requirement of a single-polarization gyroscope for a polarizer with a high extinction ratio, and effectively suppress polarization errors.

In order to solve the above technical problems, an embodiment of the present invention proposes a dual-polarization fiber optic gyroscope for recycling and reusing dead-end optical power, including a polarization-maintaining fiber ring, a coupler, and a first polarizer, a light source, and a detector connected to the coupler. , also includes a polarization beam splitter, the dead end of the coupler is connected with a second polarizer, the first polarizer and the second polarizer are connected to the polarization beam splitter; the two output ports of the polarization beam splitter are connected to the polarization maintaining fiber ring , one of the output ports and the polarization main axis of the polarization-maintaining fiber are twisted by 90°, forming the exchange of the polarization mode s-state and p-state.

Further, the light of the two output ports of the coupler is used for gyroscopic interference, and the light of the two ports passes through different ports of the polarization beam splitter, so that the light entering the polarization-maintaining fiber ring is s-state linearly polarized light and p-state respectively. For linearly polarized light, the polarization crosstalk errors of the two polarization states have equal amplitudes and opposite directions.

Further, the polarizers are all 45° polarizers.

Further, the coupler is a 2X2 fused taper type coupler or a diaphragm type coupler.

The beneficial effects of the present invention are:

1. The present invention uses a polarization beam splitter combined with the conversion of polarization states to recycle and reuse the light at the dead end of the traditional fiber optic gyroscope coupler. On the premise that the input optical power of the light source remains unchanged, the interference light intensity is doubled, and the gyro is low-cost. The signal-to-noise ratio is improved by about 40%.

2. In the polarization-maintaining fiber loop, the s-polarized state and the p-polarized state polarized light transmission channel are used at the same time, and the polarization-related errors of the two polarization states are canceled by using the polarization error of the two polarization states and other characteristics of reverse amplitude, thereby greatly improving the stability of the gyro.

3. The present invention has compact structure, high stability, easy assembly, and can be applied to various wavelength systems and multi-precision gyroscopes.

Description of drawings

Figure 1 is an optical path diagram of a conventional fiber optic gyroscope.

FIG. 2 is an optical path diagram of a dual-polarization fiber optic gyroscope for recycling and reusing dead-end optical power according to an embodiment of the present invention.

FIG. 3 is a schematic diagram of the polarization axis of an optical fiber according to an embodiment of the present invention.

FIG. 4 is a schematic diagram of polarization interference combining according to an embodiment of the present invention.

FIG. 5 is a test chart of polarization error cancellation according to an embodiment of the present invention.

Detailed ways

It should be noted that the embodiments in the present application and the features of the embodiments may be combined with each other without conflict, and the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

Referring to FIG. 2 , the dual-polarization fiber optic gyroscope for recycling and reusing dead-end optical power according to an embodiment of the present invention includes a polarization-maintaining fiber ring, a coupler, a first polarizer, a light source, a detector, a polarization beam splitter, and a second polarizer .

A second polarizer is connected to the dead end of the coupler, and the first polarizer and the second polarizer are connected to a polarization beam splitter. The two output ports of the polarization beam splitter are connected to the polarization-maintaining fiber ring, and one of the output ports and the polarization-maintaining fiber are twisted by 90° to form the exchange of the s-state and the p-state of the polarization mode. The invention adopts a brand-new optical path structure to simultaneously realize the reuse of dead-end optical power and the simultaneous utilization of two polarized light transmission channels, so as to improve the signal-to-noise ratio and stability of the gyro.

In the present invention, multi-wavelength SLEDs of 830 nm, 850 nm, 1310 nm, 1550 nm, etc., or ASE light sources of spontaneous radiation can be used.

The coupler of the present invention is preferably a 2X2 fused taper type coupler or a diaphragm type coupler. Different from the traditional fiber optic gyro coupling arm for dead-end processing, the straight-through arm and coupling arm of the coupler are both used for gyro operation, thus realizing the recovery and reuse of dead-end optical power, which is a signal that can be used for gyro demodulation. Double the optical power.

The straight-through arm and the coupling arm of the coupler of the present invention are respectively welded to the optical fiber polarizer. The polarizer of the present invention may be a birefringent type, a tilted grating type, an integrated device type, or the like. The polarizer pigtail of the present invention is a polarization-maintaining fiber.

The polarized light of the polarizer of the present invention propagates on the fast axis or the slow axis of the polarization-maintaining pigtail. In order to ensure low splice loss, the pigtail of the polarizing beam splitter can use the same type of protection as the polarizer of the present invention. Polarization pigtail, splicing the corresponding polarizer polarization maintaining pigtail with the respective fast axes of the polarization maintaining pigtail of the polarization beam splitter at an angle of 45° or 135°, and the length of the polarization maintaining pigtail from the polarization beam splitter after the fusion point Should be greater than 10 times the decoherence length of the light source. The polarization-maintaining pigtail can also be directly twisted by 45° at the root of the corresponding polarizer, so that the polarizing axis of the polarizer and the fast or slow axis of the polarization-maintaining pigtail form an included angle of 45°. The other end of the polarization pigtail can be directly used as the pigtail of the polarization beam splitter to make the polarization beam splitter. In this case, the length of the polarization maintaining pigtail between the polarizer and the polarization beam splitter should be greater than 10 times the decoherence length of the light source; in addition, the polarizer polarization-maintaining pigtail used in this way can be spliced with the polarization-maintaining polarization-maintaining fiber of the polarization beam splitter. For polarization-maintaining pigtails of the same type as the polarizer, before splicing, align the fast axes of the polarization-maintaining pigtails of the polarizer and the polarization-maintaining pigtail of the polarization beam splitter, or the fast axis of one and the slow axis of the other. After splicing, ensure that the length of the polarization-maintaining pigtail from the polarization beam splitter behind the splicing point should be greater than 10 times the decoherence length of the light source.

Connect the depolarized straight-through pigtail and the coupled pigtail to the two input ports of the polarization beam splitter respectively. The two output ports of the polarization beam splitter are connected to the fiber ring, and the polarization principal axis of one of the output ports is twisted by 90° to form a polarization mode converter.

A modulator is installed on the optical fiber ring of the present invention, and the modulator can be an electro-optic modulator or a piezoelectric modulator or the like. The modulation mode can be square wave modulation, triangular wave modulation or sine wave modulation.

The invention leads the light from the dead end of the traditional fiber optic gyro coupler to the polarization beam splitter, realizes the reciprocal interference of the optical signal through the control of the polarization state, and doubles the interference light intensity on the premise of not changing the input optical power of the light source. Thereby, the signal-to-noise ratio of the gyro is improved by more than 40%, and two mutually perpendicular polarization states are introduced into the optical path. From a statistical point of view, the coupling probability of the s state to the p state is equal to the coupling probability of the p state to the s state, so the two The polarization crosstalk of the two channels has a similar amplitude, and the s state is coupled to the p state, the phase shift of the crosstalk light increases, and the p state is coupled to the s state, and the phase shift of the crosstalk light decreases. The superposition of beam light intensity can realize the cancellation of polarization error. Through the design of this scheme, the light intensity can be improved to improve the signal-to-noise ratio at the same time, and dual-polarization work can be realized, and the polarization error can be canceled to improve the stability of the gyro.

The optical path diagram of the present invention is shown in FIG. 2 .

The working principle of the present invention is as follows:

The light emitted by the light source is divided into two parts by the coupler, the straight light and the coupled light.

Taking the panda-type polarization-maintaining fiber shown in Figure 3 as an example, the X axis is the fast axis of the fiber, and its refractive index _n is low, the electric field component of the s-wave is parallel to the X axis, and the Y axis is the slow axis of the fiber, and its refractive index n _e is higher and the electric field component of the p-wave is parallel to the Y axis.

First analyze the light from the straight-through arm of the coupler, the straight-through light is converted into linearly polarized light by the first polarizer, and the polarization axis of the polarization-maintaining fiber after the first polarizer is twisted by 45° (as shown in Figure 3). Refraction, along the length of the polarization-maintaining fiber from 45° linearly polarized light, and gradually transitions to elliptically polarized light and circularly polarized light, when the propagation distance exceeds the decoherence length L _d , it is converted to equal-amplitude incoherent s-state linearly polarized light ( s-wave) and p-state linearly polarized light (p-wave), respectively, are transmitted along the optical fiber birefringence principal axis and enter the polarization beam splitter. The coherence length L _d is determined by the following equation,

--------------------（6）

-------------------- (6)

为光源真空中的中心波长，

为光源光谱3dB带宽。

为保偏光纤的双折 射差，

。

is the central wavelength of the light source in vacuum,

is the 3dB bandwidth of the light source spectrum.

is the birefringence difference of the polarization-maintaining fiber,

.

The polarization beam splitter can reflect the s-wave and transmit the p-wave. Different from the traditional beam splitter, the pigtail at the projection end of the beam splitter is twisted by 90° for the mutual conversion of the s-wave and the p-wave.

The s-wave is reflected by the polarization beam splitter, enters the fiber ring clockwise, and is transmitted in the fiber ring in the form of s-wave. the polarization state back to the first polarizer.

The p-wave projected after the first polarizer is transmitted through the polarization beam splitter, enters the fiber ring counterclockwise, is converted into an s-wave, and is transmitted in the fiber ring in the form of an s-wave, and is reflected to the polarization beam splitter as an s-wave. the polarization state back to the first polarizer.

Note that the incident s-wave propagates as an s-wave in the reflective fiber loop, and the transmission returns to the first polarizer as a p-wave. Incident p-waves are transmitted into the fiber ring, propagate as s-waves in the ring, and reflect back to the first polarizer as s-waves. Its phase evolution is as follows:

---------（7）

---------(7)

-----------（8）

-----------(8)

为耦合器直通臂顺时针光的相位，

为逆时针光的相位，L ₁为直 通臂第一偏振器到偏振分束器的光纤长度，L₃为光纤环的长度。

为偏振分束器反射端所 附加的相移，

为偏振分束器透射端所附加的相移，

为调制相位。由上式可知，顺时针 光与逆时针光经历了对等的光程，其相位除了调制相位

之外，仅差了待解调的sagnac 相移

以及相位误差

，实现了光路结构的互异性。两束光回到第一偏振器前重新建立 相干性，但是由于存在上述的相位差，它们会重构为椭圆偏振光，其椭度由

决定，通过偏振检波，可以实现相位的解调，如图4所示。 in,

is the phase of the clockwise light in the thru-arm of the coupler,

is the phase of the counterclockwise light, L ₁ is the length of the fiber from the first polarizer of the straight-through arm to the polarization beam splitter, and L ₃ is the length of the fiber ring.

is the phase shift added at the reflection end of the polarizing beam splitter,

is the phase shift added at the transmission end of the polarizing beam splitter,

is the modulation phase. It can be seen from the above formula that the clockwise light and the counterclockwise light have experienced the same optical path, and their phases are in addition to the modulation phase.

, the only difference is the sagnac phase shift to be demodulated

and phase error

, realizing the mutual dissimilarity of the optical path structure. The coherence of the two beams is re-established before returning to the first polarizer, but due to the above-mentioned phase difference, they are reconstructed as elliptically polarized light whose ellipticity is given by

It was decided that the demodulation of the phase can be achieved through polarization detection, as shown in Figure 4.

； When using sinusoidal modulation,

;

为调制深度的1阶贝塞尔函数。当输入光 强保持恒定时，可以通过检波后光强的测量，实现相位的计算。 I ₁ is the light intensity after detection, I ₀ is the input light intensity,

is the first-order Bessel function of the modulation depth. When the input light intensity remains constant, the phase calculation can be achieved by measuring the light intensity after detection.

The light from the coupling arm is analyzed below. Similar to the straight-through light, the polarization axis of the polarization-maintaining fiber after the second polarizer is twisted by 45°. ) and p-state linearly polarized light (p-wave), respectively, are transmitted along the optical fiber birefringence principal axis and enter the polarization beam splitter.

The s-wave is reflected by the polarization beam splitter, enters the fiber ring counterclockwise, is converted into a p wave, and is transmitted in the fiber ring as a p wave, from the transmission port of the polarization beam splitter, and returns to the second polarizer as a p wave .

The p wave transmits the polarization beam splitter, enters the fiber ring clockwise, and transmits in the fiber ring in the form of p wave. After the transmission is completed, it is converted into s wave, reflected by the polarization beam splitter, and returns to the second polarizer in the form of s wave .

Its phase evolution is as follows:

---------（9）

---------(9)

-----------（10）

------------- (10)

为耦合器耦合臂顺时针光的相位，

为逆时针光的相位，L ₂为 耦合臂第二偏振器到偏振分束器的光纤长度。由上式可知，顺时针光与逆时针光经历了对 等的光程，其相位除了调制相位

之外，仅差了待解调的sagnac相移

以及相位误差

，实现了光路结构的互异性。类似的，通过45°的偏振检偏器，完成相位的解调。

，同样通过检波后光强的测量，实现相位的计算。 akin,

is the phase of the clockwise light of the coupler arm,

is the phase of the counterclockwise light, and L ₂ is the fiber length from the second polarizer of the coupling arm to the polarizing beam splitter. It can be seen from the above formula that the clockwise light and the counterclockwise light have experienced the same optical path, and their phases are in addition to the modulation phase.

, the only difference is the sagnac phase shift to be demodulated

and phase error

, realizing the mutual dissimilarity of the optical path structure. Similarly, the phase demodulation is accomplished through a 45° polarization analyzer.

, and also through the measurement of the light intensity after detection, the calculation of the phase is realized.

倍，约为40%。 The only difference between the optical path of the straight-through arm and the optical path of the coupling arm is that the light of the straight-through arm is transmitted in the fiber ring in the form of s-wave, and the light of the coupling arm is transmitted in the form of p-wave in the fiber ring. Because the ring length of the fiber ring is generally several hundred meters, which is much longer than the decoherence length of the light source, it is firstly ensured that the straight-through arm and the coupling arm are incoherent, and the combined light at the coupler is only the superposition of the intensity. Through the simultaneous use of this dual-polarized light channel technology, the light intensity reaching the detector is doubled compared to the traditional fiber optic gyroscope, and the greater the signal-to-noise ratio

times, about 40%.

The mutual crosstalk between s-wave and p-wave on the fiber ring is mainly caused by fiber defects, temperature, stress and other factors. Since the fiber ring is hundreds of meters long, from a statistical point of view, the coupling probability of s-wave to p-wave is the same as that of p-wave to s-wave. The coupling probability of the waves is equal, that is, the coupling amplitudes are similar. And because the p-wave refractive index is high, the propagation constant is large, the s-wave refractive index is low, and the propagation constant is small, when the s-wave is coupled to the p-wave, the phase of the coupled wave is greater than the phase of the main wave, that is, the phase difference is positive, when the p-wave is coupled to the p-wave When the s-wave is coupled, the phase of the coupled wave is smaller than the phase of the main wave, that is, the phase difference is negative.

The error caused by such polarization crosstalk accounts for the main component of the optical path error, so it can be approximated as

；

;

When the intensities of the two beams of light are superimposed,

------------------（11）

------------------(11)

The cancellation of the error is realized, thereby greatly improving the gyro stability, as shown in Figure 5.

Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, and substitutions can be made in these embodiments without departing from the principle and spirit of the invention and modifications, the scope of the invention is defined by the appended claims and their equivalents.

Claims (4)

1. a dual-polarization fiber optic gyroscope of dead-end optical power recycling and reuse, comprising polarization maintaining fiber ring, coupler and the first polarizer, light source, detector connected with coupler, it is characterized in that, also comprises polarization beam splitting A second polarizer is connected to the dead end of the coupler, and the first polarizer and the second polarizer are connected to a polarization beam splitter; the two output ports of the polarization beam splitter are connected to the polarization maintaining fiber ring, and one of the output ports and The polarization main axis of the polarization-maintaining fiber is twisted by 90° to form the exchange of the s-state and p-state of the polarization mode, and the two optical transmission channels of the s-polarization state and the p-polarization state in the fiber ring are used in parallel at the same time. 2. The dual-polarization fiber optic gyroscope of dead-end optical power recycling and reuse as claimed in claim 1, wherein the light of the two output ports of the coupler is all used for gyro interference, and the light of the two ports is separated by polarization. Different ports of the beamer make the light entering the polarization-maintaining fiber ring be s-state linearly polarized light and p-state linearly polarized light, respectively, and the polarization crosstalk errors of the two polarization states have equal amplitudes and opposite directions. 3 . The dual-polarization fiber optic gyroscope for recycling and reusing dead-end optical power as claimed in claim 1 , wherein the polarizers are all 45° polarizers. 4 . 4 . The dual-polarization fiber optic gyroscope for recycling and reusing dead-end optical power according to claim 1 , wherein the coupler is a 2×2 fused taper-type coupler or a diaphragm-type coupler. 5 .

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