CN114674302B - Dual-polarization optical fiber gyroscope with dead-end optical power recycling function - Google Patents
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Abstract
The embodiment of the invention discloses a dual-polarization fiber optic gyroscope for 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 of keeping the input light power of the light source unchanged, and improves the signal-to-noise ratio of the gyroscope by about 40% at 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
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a dual-polarization optical fiber gyroscope capable of recycling dead-end optical power.
Background
The fiber-optic gyroscope is sensitive to the rotation angular velocity of a carrier, is mainly used for inertial navigation and attitude control of a motion carrier, and is increasingly applied to multiple important fields such as flight navigation, automobile automatic driving, robot control and the like. Along with the expansion of the application field, the market puts higher and higher demands on the gyro precision, the most important of which is the improvement of the signal-to-noise ratio and the stability, and the two indexes are respectively evaluated by Angle Random Walk (ARW) and zero Bias Instability (BI) and are core indexes of the fiber-optic gyro.
Generation of fiber optic gyroscope interference signals based on the optical Sagnac effect, with interference phase
Can be represented by the following formula:
whereinLIs the length of the optical fiber and,Dis the diameter of the optical fiber loop,λis the wavelength in the vacuum of the light source,cthe light velocity in vacuum and Ω are the carrier rotation angular velocity. By rotating the interference phaseThe detection of the rotation signal can be realized by replacing the interference light intensity signal I.
WhereinI 0 The input light intensity signal attenuated by the optical path is determined by the input light power of the light source and the loss on the optical path.
For modulating the phase, provided by a phase modulator, for modulation and demodulation.
The stability of the gyroscope is determined for the non-reciprocal phase error.
First, the gyro signal-to-noise ratio is explained, and as can be seen from equation (1), the signal intensity is proportional to the input light intensity.
Generally, the noise of the fiber-optic gyroscope is composed of shot noise of a detector, relative intensity noise of a light source, thermal phase noise and the like, and when the light intensity is low, the ultimate sensitivity of the fiber-optic gyroscope is composed of the shot noise
And (6) determining.
Wherein,Ito reach the detector, the intensity of light is proportional to the intensity of the input lightI 0 ，hIs the constant of the Planck, and is,h=6.63X10 - 34 Js,vis the frequency of the light, and the frequency of the light,
，cin order to achieve the speed of the vacuum light,λis the wavelength in the vacuum of the light source.
Bandwidth is acquired for the signal.
Signal to noise ratio of gyroscopeRCan be expressed as
It can be seen that the higher the optical power reaching the detector, the higher the gyro signal-to-noise ratio. Under the condition of certain optical path loss, the most effective means for improving the signal-to-noise ratio of the gyroscope is to improve the input optical power of the gyroscope. The traditional scheme is to improve the light intensity of the light source, but the light source with high power is often higher in price, and the power consumption of the gyroscope can be improved, so that the difficulty is brought to the development of a navigation system. Therefore, the light intensity of the detector is improved on the premise of not changing the light power of the light source, so that the signal-to-noise ratio of the gyroscope is improved, and the practical significance is greater.
In a conventional fiber optic gyroscope, in order to ensure reciprocity of an optical path, a 3dB coupler needs to be installed behind a light source, as shown in fig. 1, half of light enters a dead end through the coupler, and scattering loss occurs. If the part of light can be recycled and reused and the scattered loss light is changed into signal light, the signal-to-noise ratio of the gyroscope can be improved on the premise of not changing the output light intensity of the light source
And (4) multiplying.
The stability of the gyroscope (or called zero-bias instability) is mainly limited by the nonreciprocal phase error in the gyroscope light path
. Such phase error is caused by modulation-demodulation and angular velocity interference
Decoupling cannot be achieved, the stability of the gyroscope is greatly influenced, and the gyroscope can be restrained only through a good light path design. The nonreciprocal phase error mainly comprises polarization error, Rayleigh scattering, thermo-optic error, elasto-optic error and the like, whereinThe polarization dependent error is the dominant error of the gyroscope.
As shown in fig. 1, an input light is filtered into a linearly polarized light by a polarizer and enters a polarization-maintaining fiber ring, and the fiber ring mainly transmits a linear polarization state (s-polarization state or p-polarization state). If the polarization state can be maintained over a fiber ring up to several hundred meters long, the polarization error is 0. However, in practical applications, due to the imperfection of the optical fiber and the influence of stress temperature, etc., the linearly polarized light will cross-talk to its vertical polarization state (e.g. from s polarization state to p polarization state), and the cross-talk of the polarization maintaining optical fiber can be indicated by the polarization cross-talkhEvaluation was about 10 -5 And/m. Because the refractive indexes of two polarization states of the polarization maintaining fiber are different (s state and p state are respectively marked as n) o And n is e ）
The crosstalk light and the intrinsic light have different phase differences, thereby generating a polarization error.
The traditional solution for suppressing polarization errors is to increase the extinction ratio of the polarizer to suppress the light intensity in the vertical polarization state, but the polarizer with a high extinction ratio usually requires a complicated process and higher cost. For single polarization fiber-optic gyroscope, its polarization error
Can be estimated by the following equation:
wherein,pis the degree of polarization of the input light,
，T s ，T p the light intensity of s polarization state and p polarization state respectively. For highly polarized or single polarized gyroscopesp≈1。
In order to determine the extinction ratio of the polarizer,
in order to achieve the polarization crosstalk of the optical fiber,Lis the fiber loop length.
If the light path design is adopted, the requirement of the single-polarization gyroscope on a polarizer with a high extinction ratio is reduced, the polarization error is effectively inhibited, and the method has great significance for the development of a fiber-optic gyroscope with high cost performance.
Disclosure of Invention
The technical problem to be solved by the embodiments of the present invention is to provide a dual-polarization fiber optic gyroscope with a recycled dead-end optical power, so as to reduce the requirement of a single-polarization gyroscope on a polarizer with a high extinction ratio and effectively suppress polarization errors.
In order to solve the technical problem, an embodiment of the present invention provides a dual-polarization fiber optic gyroscope with a dead-end optical power recycling function, including a polarization-maintaining fiber optic ring, a coupler, a first polarizer, a light source, a detector and a polarization beam splitter, wherein the first polarizer, the light source and the detector are connected to the coupler; 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.
Furthermore, the light at the two output ports of the coupler is used for gyro interference, and the light at the two ports passes through different ports of the polarization beam splitter, so that the light entering the polarization-maintaining fiber ring is respectively s-state linearly polarized light and p-state linearly polarized light, and the polarization crosstalk errors of the two polarization states are equal in amplitude and opposite in direction.
Further, the polarizers are all 45 ° polarizers.
Further, the coupler is a 2X2 fused taper type coupler or a diaphragm type coupler.
The invention has the beneficial effects that:
1. the invention adopts the polarization beam splitter to combine with the conversion of the polarization state, recycles the light at the dead end of the traditional fiber-optic gyroscope coupler, doubles the interference light intensity on the premise of keeping the input light power of the light source unchanged, and improves the signal-to-noise ratio of the gyroscope by about 40% at low cost.
2. In the polarization maintaining optical fiber ring, the s-polarization state and p-polarization state polarization light transmission channels are used simultaneously, and the characteristics of opposite amplitudes of polarization errors of the two polarization states and the like are utilized to offset the polarization related errors, so that the stability of the gyroscope is greatly improved.
3. The invention has compact structure, high stability and easy assembly, and can be applied to various wavelength systems and multi-precision gyroscopes.
Drawings
Fig. 1 is a light path diagram of a conventional fiber optic gyroscope.
Fig. 2 is an optical path diagram of a dual-polarization fiber optic gyroscope for recycling dead-end optical power according to an embodiment of the present invention.
FIG. 3 is a schematic view of the polarization axis of an optical fiber according to an embodiment of the present invention.
FIG. 4 is a schematic diagram of a polarization interference light combination according to an embodiment of the present invention.
FIG. 5 is a polarization error cancellation test chart of an embodiment of the present invention.
Detailed Description
It should be noted that the embodiments and features of the embodiments in the present application can be combined with each other without conflict, and the present invention is further described in detail with reference to the drawings and specific embodiments.
Referring to fig. 2, the dual-polarization fiber optic gyroscope with dead-end optical power recycling according to the 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.
The dead end of the coupler is connected with a second polarizer, and the first polarizer and the second polarizer are connected with a polarization beam splitter. 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 adopts a brand-new optical path architecture 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 the stability of the gyroscope.
The invention can adopt SLEDs with multiple wavelengths of 830nm, 850nm, 1310nm, 1550nm and the like or ASE light sources with spontaneous radiation.
The coupler of the present invention is preferably a 2X2 fused taper type coupler or a diaphragm type coupler. Different from the traditional dead-end processing of the coupling arm of the fiber-optic gyroscope, the direct connection arm and the coupling arm of the coupler are both used for the gyroscope to work, so that the recovery and the reutilization of dead-end optical power are realized, and the signal optical power for the gyroscope to be demodulated is doubled.
The straight arm and the coupling arm of the coupler are respectively welded with the optical fiber polarizer. The polarizer of the present invention may be of a birefringent type, a tilted grating type, an integrated device type, or the like. The tail fiber of the polarizer is a polarization maintaining fiber.
The polarized light of the polarizer of the invention is transmitted on the fast axis or the slow axis of the polarization-maintaining tail fiber, in order to ensure low welding loss, the tail fiber of the polarization beam splitter can adopt the same polarization-maintaining tail fiber as the polarizer of the invention, the corresponding polarization-maintaining tail fiber of the polarizer and the respective fast axis of the polarization-maintaining tail fiber of the polarization beam splitter form an angle of 45 degrees or a welding angle of 135 degrees, and the length of the polarization-maintaining tail fiber away from the polarization beam splitter after the welding point is more than 10 times of the decoherence length of the light source. The polarization-maintaining tail fiber can be directly twisted by 45 degrees at the root of the corresponding polarizer, so that the polarizing axis of the polarizer forms an included angle of 45 degrees with the fast axis or the slow axis of the polarization-maintaining tail fiber, the other end of the polarization-maintaining tail fiber of the polarizer used in the mode can be directly used as the tail fiber of the polarization beam splitter and used for manufacturing the polarization beam splitter, and under the condition, the length of the polarization-maintaining tail fiber between the polarizer and the polarization beam splitter is required to be more than 10 times of the decoherence length of the light source; in addition, the polarization-maintaining tail fiber of the polarizer used in the mode can be welded with the polarization-maintaining tail fiber of the polarization beam splitter, in order to ensure low welding loss, the polarization-maintaining tail fiber of the polarization beam splitter can adopt the same polarization-maintaining tail fiber as the polarizer, before welding, the fast axes of the polarization-maintaining tail fiber of the polarizer and the polarization-maintaining tail fiber of the polarization beam splitter are aligned or the fast axis of one of the polarization-maintaining tail fiber and the slow axis of the other polarization-maintaining tail fiber are aligned, and after welding, the length of the distance between the welding point and the polarization-maintaining tail fiber of the polarization beam splitter is ensured to be larger than 10 times of the decoherence length of a light source.
And respectively connecting the depolarized straight-through tail fiber and the coupling tail fiber with two input ports of the polarization beam splitter. And two output ports of the polarization beam splitter are connected with the optical fiber ring, and the polarization main axis of one of the output ports is twisted by 90 degrees to form the polarization mode converter.
The optical fiber ring is provided with a modulator which can be an electro-optical modulator or a piezoelectric modulator and the like. The modulation mode can be square wave modulation, triangular wave modulation or sine wave modulation.
The invention leads the light at the dead end of the traditional fiber-optic gyroscope coupler to the polarization beam splitter, realizes the reciprocal interference of optical signals through the control of the polarization state, the interference light intensity is doubled on the premise of not changing the input light power of the light source, thereby improving the signal-to-noise ratio of the gyroscope by more than 40 percent, two mutually vertical polarization states are introduced into the light path, and statistically, the coupling probability from the s state to the p state is equal to the coupling probability from the p state to the s state, therefore, the two polarization crosstalk paths have similar amplitude values, the phase shift of the crosstalk light is increased when the s state is coupled to the p state, the phase shift of the crosstalk light is reduced when the p state is coupled to the s state, the two polarization crosstalk paths have opposite directions, the intensities of the two polarization crosstalk paths are superposed, the polarization compensation method can realize the compensation of the polarization error, can simultaneously promote the light intensity and improve the signal to noise ratio through the design of the scheme, realizes the dual-polarization work, and realizes the compensation of the polarization error to improve the stability of the gyroscope.
The light path diagram of the present invention is shown in fig. 2.
The working principle of the invention is as follows:
the light emitted by the light source is divided into 2 parts by the coupler, and the light is directly transmitted and coupled.
Taking the panda-type polarization maintaining fiber shown in FIG. 3 as an example, the X-axis is the fast axis of the fiber, and the refractive index n thereof o Low, the electric field component of s wave is parallel to X axis, Y axis is the slow axis of optical fiber, its refractive index n e The higher, electric field component of the p-wave is parallel to the Y-axis.
Firstly, analyzing the light of the straight-through arm of the coupler, converting the straight-through light into linearly polarized light through the first polarizer, twisting the polarization main shaft of the polarization-maintaining fiber behind the first polarizer by 45 degrees (as shown in fig. 3), gradually transitioning from the linearly polarized light of 45 degrees to elliptically polarized light and circularly polarized light along the length direction of the polarization-maintaining fiber due to the birefringence of the polarization-maintaining fiber, and when the propagation distance exceeds the decoherence length L d Then the light is converted into s-state linearly polarized light (s wave) and p-state linearly polarized light (p wave) which are not coherent with equal amplitude,respectively transmitted along the optical fiber birefringence main shaft and enters the polarization beam splitter. Coherence length L d Is determined by the following formula,
the polarization beam splitter can reflect s-waves and transmit p-waves, and unlike the conventional beam splitter, the beam splitter projects end-pigtails twisted by 90 degrees for interconversion of s-waves and p-waves.
The s-wave is reflected by the polarization beam splitter, enters the optical fiber ring clockwise, is transmitted in the optical fiber ring in the form of s-wave, is converted into p-wave before returning to the beam splitter, is transmitted from the polarization beam splitter, and returns to the first polarizer in the polarization state of the p-wave.
After transmitting through the polarization beam splitter, the p wave projected behind the first polarizer enters the fiber ring counterclockwise, is converted into s wave, is transmitted in the fiber ring in the form of s wave, is reflected to the polarization beam splitter, and returns to the first polarizer in the polarization state of the s wave.
Note that the incident s-wave propagates in the ring of reflected fiber as an s-wave, and the transmission returns to the first polarizer as a p-wave. The incident p-wave is transmitted into the fiber ring, propagates in the ring as an s-wave, and is reflected back to the first polarizer as an s-wave. The phase evolution is as follows:
wherein,
the phase of the clockwise light for the coupler through arm,
in order to be the phase of the light counterclockwise,L 1 is the length of fiber, L, passing through the first polarizer of the arm to the polarizing beam splitter 3 The length of the fiber loop.
The added phase shift at the reflective end of the polarizing beam splitter,
the additional phase shift for the transmission end of the polarizing beam splitter,
is the modulation phase. As can be seen from the above equation, clockwise and counter-clockwise light experience equal optical paths with phases other than the modulation phase
Besides, only the sagnac phase shift to be demodulated is differed
And phase error
And the mutual difference of the optical path structures is realized. The two beams re-establish coherence before returning to the first polarizer, but due to the phase difference they are reconstructed into elliptically polarized lightThe ellipse degree of which is composed of
It was determined that demodulation of the phase can be achieved by polarization detection, as shown in fig. 4.
When a sinusoidal modulation is used in the case of,
；
I 1 in order to obtain the intensity of the light after detection,I 0 in order to input the light intensity,
is a 1 st order bessel function of the modulation depth. When the input light intensity is kept constant, the phase can be calculated by measuring the light intensity after detection.
The light of the coupling arm is analyzed, similar to the straight-through light, the polarization main axis of the polarization-maintaining optical fiber behind the second polarizer is twisted by 45 degrees, and after the light is transmitted to the decoherence length, the light is converted into s-state linearly polarized light (s wave) and p-state linearly polarized light (p wave) which are incoherent with equal amplitude, and the s-state linearly polarized light and the p-state linearly polarized light (p wave) are respectively transmitted along the optical fiber birefringence main axis and enter the polarization beam splitter.
The s-wave is reflected by the polarization beam splitter, enters the fiber loop counterclockwise, is converted into a p-wave, is transmitted in the fiber loop in the form of a p-wave, and returns to the second polarizer in the form of a p-wave from the transmission port of the polarization beam splitter.
The p-wave is transmitted through the polarization beam splitter, enters the fiber loop clockwise, is transmitted in the fiber loop in the form of the p-wave, is converted into the s-wave after transmission is completed, is reflected by the polarization beam splitter, and returns to the second polarizer in the form of the s-wave.
The phase evolution is as follows:
in a similar manner, the first and second substrates are,
for the coupler to couple the phase of the clockwise light of the arm,
in order to be the phase of the light counterclockwise,L 2 the length of fiber coupling the second polarizer to the polarizing beam splitter. As can be seen from the above equation, clockwise and counter-clockwise light experience equal optical paths with phases other than the modulation phase
Besides, only the phase shift of sagnac to be demodulated is differed
And phase error
And the mutual difference of the optical path structures is realized. Similarly, demodulation of the phase is accomplished by a 45 ° polarization analyzer.
And the phase calculation is realized by measuring the light intensity after detection.
The only difference between the optical path of the through arm and the optical path of the coupling arm is that the through arm light is transmitted in the form of s-wave in the optical fiber ring, and the coupling arm light is transmitted in the form of p-wave in the optical fiber ring. Because the length of the optical fiber ring is generally hundreds of meters and is far longer than the decoherence length of the light source, the incoherence of the straight arm and the coupling arm is firstly ensured, and the light combination in the coupler is only the superposition of intensity. By the technology of using the double polarized light channels simultaneously, the light intensity reaching the detector is doubled compared with the traditional fiber-optic gyroscope, and the signal-to-noise ratio is increased
And about 40% of the total weight.
The mutual crosstalk between the s wave and the p wave on the optical fiber ring is mainly caused by factors such as optical fiber defects, temperature, stress and the like, and the coupling probability of the s wave to the p wave is equal to that of the p wave to the s wave in statistics, namely the coupling amplitude is close to that of the p wave to the s wave, because the optical fiber ring is hundreds of meters long. And because p-wave refractive index is high, propagation constant is large, s-wave refractive index is low, propagation constant is small, when s-wave couples to p-wave, the phase of coupled wave is greater than the phase of main wave, i.e. phase difference is positive, when p-wave couples to s-wave, the phase of coupled wave is less than the phase of main wave, i.e. phase difference is negative.
The error caused by such polarization crosstalk is the main component of the optical path error, and can be approximated as
When the two light beams are superimposed in intensity,
cancellation of errors is achieved, and therefore gyro stability is greatly improved, as shown in fig. 5.
Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.
Claims (4)
1. A dual-polarization fiber-optic gyroscope with a recycled dead-end optical power comprises a polarization-maintaining fiber ring, a coupler, a first polarizer, a light source and a detector, wherein the first polarizer, the light source and the detector are connected with the coupler; two output ports of the polarization beam splitter are connected with the polarization-maintaining optical fiber ring, one output port and a polarization main shaft of the polarization-maintaining optical fiber are twisted by 90 degrees to form the interchange of s-state and p-state of a polarization mode, and two optical transmission channels of the s-state and the p-state in the optical fiber ring are simultaneously utilized in parallel.
2. The dual-polarization fiber-optic gyroscope for recycling dead-end optical power of claim 1, wherein the light from the two output ports of the coupler is used for gyroscope interference, and the light from the two output ports passes through different ports of the polarization beam splitter, so that the light entering the polarization-maintaining fiber-optic ring is respectively s-state linearly polarized light and p-state linearly polarized light, and the polarization crosstalk error between the two polarization states has equal amplitude and is in opposite direction.
3. The dual-polarization fiber optic gyroscope for dead-end optical power recovery and reuse of claim 1, wherein the polarizers are all 45 ° polarizers.
4. The dual-polarization fiber optic gyroscope for dead-end optical power recovery and reuse of claim 1, wherein the coupler is a 2X2 fused taper coupler or a diaphragm coupler.
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| CN104965317A (en) * | 2015-06-25 | 2015-10-07 | 佛山科学技术学院 | Electronic-control polarized-light-splitting-ratio-adjustable polarization beam splitter and working method thereof |
| KR101978444B1 (en) * | 2017-03-13 | 2019-05-15 | 서강대학교산학협력단 | Optical fiber Sagnac interferometer using a polarizing beam splitter |
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