CN103743392B - A kind of unicoupler double polarizing light fiber gyroscope - Google Patents

Abstract

The invention provides a single-coupler dual-polarization fiber optic gyroscope, which includes a wide-spectrum light source, a polarization-maintaining fiber, a coupler, a polarization-maintaining fiber ring, and a photodetector, wherein the output end of the wide-spectrum light source is connected to a single-mode fiber through a single-mode fiber. The polarization-maintaining fiber is connected, the polarization-maintaining fiber is divided into two sections and fused at a 45° angle, the other end of the polarization-maintaining fiber is coupled with the first port of the coupler, and the third port and the fourth port of the coupler are respectively connected to the polarization-maintaining fiber through a single-mode fiber. The two ports of the polarized fiber ring are coupled, and the second port of the coupler is coupled with the input end of the photodetector through a single-mode fiber. The gyroscope structure saves the polarizer and one of the couplers in the minimum reciprocal structure of the gyroscope, and generates dual polarized light through the polarization-maintaining fiber fused at a 45° rotation angle to compensate and eliminate the non-reciprocal error, thereby greatly reducing the Cost, while having lower noise and better zero bias stability.

Description

A single-coupler dual-polarization fiber optic gyroscope

technical field

The invention belongs to the technical field of gyroscopes, in particular to a single-coupler dual-polarization fiber optic gyroscope.

Background technique

A gyroscope is a rotational sensor used to measure the rotational angular velocity of the carrier on which it is mounted. Gyroscopes are widely used in the guidance of aircraft and weapons, precision measurement in industry and military, and other fields. The early gyroscopes were mechanical gyroscopes. Mechanical gyroscopes are directional devices manufactured using the physical principle that the rotating shaft of a high-speed rotating body has a tendency to maintain its direction. Because mechanical gyroscopes contain moving parts (such as high-speed rotors), their structures are complex, their process requirements are high, and their accuracy is restricted in many ways.

In the 1960s, with the advent of lasers, the research on using lasers to manufacture optical gyroscopes developed rapidly. Optical gyroscopes are directional devices based on the Sagnac effect. Specifically, in the rotating closed optical path, two beams of light with the same characteristics emitted by the same light source interfere when they are transmitted in the clockwise (CW) direction and counterclockwise (CCW) direction respectively. By detecting the phase of the two beams of light The rotational angular velocity of the closed optical path can be measured by the difference or the change of the interference fringes. The above phase difference is called the Sagnac phase shift φ _S , which is proportional to the rotational angular velocity Ω of the closed optical path:

${\mathrm{\φ}}_S=\frac{4\mathrm{\ωA}}{c^2}\mathrm{\Ω}$ Formula (1) where, ω is the frequency of light, c is the speed of light in vacuum, and A is the area enclosed by the closed light path.

Optical gyroscopes have no moving parts and are compact, sensitive, reliable, and long-lived. In 1963, the first generation of optical gyroscope - laser gyroscope came out. The basic element of a laser gyroscope is a ring laser. For example, a laser gyroscope may include a triangular closed optical path made of quartz within which a HeNe laser tube, two mirrors, and a semi-transparent mirror are located. The two oppositely transmitted laser beams emitted from the helium-neon laser tube are respectively reflected by two mirrors, and then led out of the circuit by a semi-transparent mirror. The rotational angular velocity of the closed optical path can be obtained by measuring the phase difference between the two beams of light.

In 1976, the second generation of optical gyroscope - fiber optic gyroscope appeared. Fiber optic gyroscopes have higher sensitivity and stability, lower cost and power consumption, and are smaller in size. Fiber optic gyroscopes are roughly divided into interferometric fiber optic gyroscopes and resonant fiber optic gyroscopes. At present, interferometric fiber optic gyroscopes are the most widely used.

In interferometric fiber optic gyroscopes, longer fibers are often wound into multi-turn coils to form a closed optical path. Using multi-turn coils can enhance the Sagnac effect. In this case, the expression for the Sagnac phase shift _φS is:

${\mathrm{\φ}}_S=2\mathrm{\π}\frac{\mathrm{LD}}{\mathrm{\λc}}\mathrm{\Ω}$ Formula (2) where L is the length of the fiber, D is the diameter of the fiber coil, and λ is the wavelength of the light wave.

In order to accurately measure the Sagnac effect (that is, the Sagnac phase shift φ _S ), it is necessary to ensure that the closed optical path has reciprocity, that is, to ensure that the light transmitted along the clockwise direction of the closed optical path (hereinafter referred to as CW light) and the light transmitted along the counterclockwise direction of the closed optical path (hereinafter referred to as CCW light) have the same mode, polarization and phase delay, so that the phase difference between CW light and CCW light is only related to the rotational angular velocity of the closed optical path, It has nothing to do with the transmission, thereby improving the accuracy of the measurement.

Figure 1 shows the minimum reciprocity structure of an interferometric fiber optic gyroscope. As shown in Figure 1, the minimum reciprocity structure includes a light source, a light source end coupler, a polarizer, a ring end coupler, a fiber ring, and a photodetector. The above-mentioned coupler can realize beam splitting and recombination of beams. After passing through the polarizer, the light beam of the light source is divided into CW light and CCW light by the ring end coupler and transmitted in the fiber ring. After being transmitted in the fiber ring, the CW light and CCW light are recombined by the ring end coupler and form an interference wave. , the interference wave finally enters the photodetector through the light source end coupler. The ring-end coupler has reciprocity, and the phase delay caused by it to CW light and CCW light is the same. In addition, the polarizer is used to perform polarization filtering on the light wave to ensure that the CW light and the CCW light have the same polarization, thereby achieving polarization reciprocity. In fiber optic gyroscopes, polarization maintaining fibers can be used to ensure polarization reciprocity.

Since the phase and amplitude of CW light and CCW light are exactly the same when the fiber ring is stationary, the power P ₀ of the interference light is the maximum. When the fiber ring rotates, the interference optical power P is a function of the phase difference φ _S between the CW light and the CCW light caused by the rotation P(φ _S )=P ₀ (1+cosφ _S ). In order to obtain high sensitivity, a bias Δφ should be applied to φ _S to make the system work near the point where the optical power slope is not zero: P(φ _S )=P ₀ [1+cos(φ _S +Δφ)]. For this reason, it is necessary to add a phase modulator (for example, a PZT phase modulator) at one end of the fiber ring to modulate the phase of the CW light and CCW light transmitted in the fiber ring, so that a phase difference occurs when the fiber ring is stationary Δφ.

Interferometric fiber optic gyroscopes are divided into different precision levels according to their application needs. Table 1 shows the technical requirements of each precision level.

Table 1. Technical requirements for each precision level

Among them, the bias stability is the most important technical index to measure the accuracy of the interferometric fiber optic gyroscope. Zero-bias correlated noise includes quantization noise, angle random walk, rate random walk, rate ramp, etc.

As mentioned above, in the fiber optic gyroscope, the structural principle of two couplers and one polarizer can eliminate the noise component caused by polarization non-reciprocity and ensure good bias stability, but it also increases the cost.

Contents of the invention

The object of the present invention is to provide a single-coupler dual-polarization fiber optic gyroscope, which has very low complexity, low cost, high precision and zero bias stability.

In order to achieve the above object, the present invention provides a single-coupler dual-polarization fiber optic gyroscope, which includes a wide-spectrum light source, a polarization-maintaining fiber, a coupler, a polarization-maintaining fiber ring, and a photodetector, wherein the output end of the wide-spectrum light source The polarization-maintaining fiber is connected to the polarization-maintaining fiber through a single-mode fiber. The polarization-maintaining fiber is divided into two sections and fused at a 45° angle. The other end of the polarization-maintaining fiber is coupled to the first port of the coupler. The optical fiber is respectively coupled with two ports of the polarization maintaining optical fiber ring, and the second port of the coupler is coupled with the input end of the photodetector through a single-mode optical fiber.

Preferably, the coupler is a 3dB polarization maintaining fiber coupler.

Preferably, a phase modulator is inserted into the polarization maintaining fiber ring. Further preferably, the phase modulator is a PZT phase modulator.

Preferably, the segment length of the two polarization-maintaining fibers is (L ₀ , 2L ₀ ), where L ₀ =L _d /Δn, Δn is the distance between the x-axis and the y-axis of the birefringent crystal of the polarization-maintaining fiber or The difference in refractive index between the x' axis and the y' axis, is the decoherence length of the broadband light source, _λ0 is the central wavelength of the broadband light source, and Δλ is the spectral width of the broadband light source.

As mentioned above, the present invention breaks through the limitation of the minimum reciprocity structure of the fiber optic gyroscope based on the principle of polarization error compensation. The single-coupler dual-polarization fiber optic gyroscope of the present invention does not need a polarizer, but only one coupler, which greatly reduces the The cost of the structure is reduced, and the cost of the structure is lower than that of the mainstream fiber optic gyro structure currently on the market. The dual-polarized light is generated by two-stage 45° fusion-spliced polarization-maintaining optical fiber, which can achieve a good non-reciprocal error compensation effect and achieve high gyro accuracy and stability.

Description of drawings

Fig. 1 is the schematic diagram of the minimum reciprocity structure of interferometric fiber optic gyroscope;

Fig. 2 is the schematic diagram of the structure of single coupler dual polarization fiber optic gyroscope described in an embodiment of the present invention;

Figure 3 is a schematic diagram of the principle of 45° fusion splicing of two polarization-maintaining fibers;

Fig. 4 is the time-domain data diagram of the gyroscope output angular velocity value in Fig. 2;

Fig. 5 is an error analysis diagram of the gyroscope output angular velocity data in Fig. 2; and

FIG. 6 is a linearity analysis diagram of the output angular velocity data of the gyroscope in FIG. 2 .

detailed description

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident, however, that these embodiments may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more embodiments.

Various embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Fig. 2 is a schematic diagram of the structure of a single-coupler dual-polarization fiber optic gyroscope according to an embodiment of the present invention. As shown in FIG. 2 , the single-coupler dual-polarization fiber optic gyroscope of the present invention includes: a wide-spectrum light source, a polarization-maintaining fiber, a coupler, a phase modulator, a polarization-maintaining fiber ring, and a photodetector. The wide-spectrum light source can be a wide-spectrum light source commonly used in interferometric fiber optic gyroscopes, for example, an ASE wide-spectrum light source can be used, with a central wavelength of 1550 nm and a spectral width of 40 nm. The output end of the wide-spectrum light source is connected to the two-section 45° fusion-spliced polarization-maintaining optical fiber through a single-mode optical fiber. The other end of the polarization-maintaining fiber is connected to the first port of the coupler. Preferably, a polarization-maintaining fiber coupler with a splitting ratio of 50:50 is used, that is, a 3dB polarization-maintaining fiber coupler.

Fig. 3 is a schematic diagram of the principle of 45° fusion splicing of the two sections of polarization-maintaining optical fibers. The segment length of the two polarization-maintaining fibers is (L ₀ , 2L ₀ ), where L ₀ =L _d /Δn, and Δn is the distance between the x-axis and the y-axis of the birefringent crystal of the polarization-maintaining fiber or the x' axis and the refractive index difference between the y' axis, is the decoherence length of the broadband light source, _λ0 is the central wavelength of the broadband light source, and Δλ is the spectral width of the broadband light source. During fusion splicing, the included angle between the main axes of the two optical fibers is 45°. The light wave of the light source will become dual polarized light after passing through this part of the polarization maintaining fiber.

The third port and the fourth port of the coupler in Fig. 2 are coupled to the two ports of the polarization maintaining optical fiber ring respectively. The second port of the coupler is coupled with the input end of the photodetector through a single-mode optical fiber. Preferably, the photodetector can be a semiconductor PIN photodiode, for example.

As mentioned above, the present invention utilizes dual polarized light to avoid using the polarizer in the existing fiber optic gyroscope, and realizes the compensation effect of the phase error (details will be described later). In addition, the fiber optic gyroscope of the present invention only needs to use one coupler instead of two couplers used in the existing fiber optic gyroscope, so the cost of the fiber optic gyroscope of the present invention is greatly reduced.

Two sections of polarization-maintaining fiber spliced at 45° can produce two polarization states with equal light intensity and independent of each other. It has non-reciprocity, but the influence of non-reciprocity can be eliminated by error compensation. The following theoretical analysis to illustrate this point.

Define the degree of polarization of light at point C, that is, the point between the polarization-maintaining fiber segment and the coupler, as d, so that the normalized light field input into the polarization-maintaining fiber ring is:

${\mathrm{E.}}_C=\left[\begin{array}{c}\sqrt{1+d/2}{e}^{-\mathrm{j\Δ\β}{L}_0}\\ \sqrt{(1-d)/2}\end{array}\right]e^{{\mathrm{j\ω}}_{0}t}$ Formula (3)

where Δβ is the difference in the propagation constant between the two axes of the polarization-maintaining fiber, and the length L ₀ eliminates the coherence between the two polarization states. The degree of polarization of light waves is d, and the range of values is -1~1. d=-1 means the linear polarization in the y direction, d=0 means the amplitudes of the x direction and the y direction are equal, and d=1 means the linear polarization in the x direction.

The symbol M _CW is used to represent the total transmission matrix of the light wave transmitted clockwise through the coupler and the fiber ring, and the symbol M _CCW is used to represent the total transmission matrix of the light wave transmitted counterclockwise through the coupler and the fiber ring, then:

$m_{\mathrm{CW}}=\left[\begin{array}{cc}{C}_1& C_2\\ C_3& C_4\end{array}\right]$ Formula (4)

$m_{\mathrm{CCW}}=\left[\begin{array}{cc}{C}_1& C_3\\ C_2& C_4\end{array}\right]$ Formula (5)

From this, the interference light wave formed by the return wave can be calculated:

${\mathrm{E.}}_{\mathrm{CW}}+{\mathrm{E.}}_{\mathrm{CCW}}=m_{\mathrm{CW}}{\mathrm{E.}}_C{e}^{\mathrm{i\φ}}+m_{\mathrm{CCW}}{\mathrm{E.}}_C$ Formula (6)

Among them, the non-reciprocal phase φ= _φS +Δφ(t) includes the Sagnac phase difference _φS and the phase offset Δφ(t) introduced by the modulator. The specific form of each item in the formula is as follows

${\mathrm{E.}}_{\mathrm{CW}}=m_{\mathrm{CW}}{\mathrm{E.}}_C{e}^{\mathrm{j\φ}}=\left[\begin{array}{c}{C}_1\sqrt{(1+d)/2}{e}^{-\mathrm{j\Δ\βL}}+C_2\sqrt{(1-d)/2}\\ C_3\sqrt{(1+d)/2}{e}^{-\mathrm{j\Δ\βL}}+C_4\sqrt{(1-d)/2}\end{array}\right]e^{{\mathrm{j\ω}}_{0}t}{e}^{\mathrm{j\φ}}$ Formula (7)

${\mathrm{E.}}_{\mathrm{CCW}}=m_{\mathrm{CCW}}{\mathrm{E.}}_C=\left[\begin{array}{c}{C}_1\sqrt{(1+d)/2}{e}^{{-\mathrm{j\Δ\βL}}_0}+C_3\sqrt{(1-d)/2}\\ C_2\sqrt{(1+d)/2}{e}^{{-\mathrm{j\Δ\βL}}_0}+C_4\sqrt{(1-d)/2}\end{array}\right]e^{{\mathrm{j\ω}}_{0}t}$ Formula (8)

In order to clearly explain the principle of error compensation, we calculate the x-polarization component and y-polarization component of the obtained interference light wave respectively. where the interference light intensity of the x-polarized component is

$\begin{array}{c}{I}_x=<{|{\mathrm{E.}}_{\mathrm{wxya}}+{\mathrm{E.}}_{\mathrm{wxya}}|}^2>\\ =I_{x0}+{\left|C_1\right|}^2(1+d)\cos \mathrm{\&phi;}+(1-d)\left|C_2{C}_3\right|\mathrm{\&Gamma;}\left(z_{\mathrm{twenty\; three}}\right)\cos \mathrm{\&phi;}\cos {\mathrm{\&phi;}}_{\mathrm{twenty\; three}}\\ -(1-d)\left|C_2{C}_3\right|\mathrm{\&Gamma;}\left(z_{\mathrm{twenty\; three}}\right)\sin \mathrm{\&phi;}\sin {\mathrm{\&phi;}}_{\mathrm{twenty\; three}}\end{array}$ Formula (9)

In this step, the phase-independent DC component and the weak coherence term are written into I _x0 , and high-order small quantities are discarded. Where φ ₂₃ and z ₂₃ are the phase and birefringence delay of C ₂ C ₃ ^* , respectively, and Γ(z) is the coherence function of the light source.

In order to analyze the error of the obtained signal, we can write the result of formula (9) in the following equivalent form

$I_x=I_{x0}+q_x\cos \mathrm{\&phi;}+p_x\sin \mathrm{\&phi;}=I_{x0}+\sqrt{p_x^2+q_x^2}\cos (\mathrm{\&phi;}-{\mathrm{\&phi;}}_{\mathrm{err}}^x)$ Formula (10)

p _x =-(1-d)|C ₂ C ₃ |Γ(z ₂₃ )sinφ ₂₃ formula (11)

q _x =(1+d)|C ₁ | ² +(1-d)|C ₂ C ₃ |Γ(z ₂₃ )cosφ ₂₃ formula (12)

in Represents the magnitude of the phase error introduced by polarization non-reciprocity in the x direction.

Similarly, we can get the interference light intensity I _y in the y direction and the phase error in the signal

${\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; err</span>}}^{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arctan</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; .</span>$

Formula (10)

p _y =(1+d)|C ₂ C ₃ |Γ(z ₂₃ )sinφ ₂₃ formula (11)

q _y =(1-d)|C ₄ | ² +(1+d)|C ₂ C ₃ |Γ(z ₂₃ )cosφ ₂₃ formula (12)

Finally, the form of the total light intensity of the interference light obtained after the superposition of the two components I _x and I _y can be expressed as a direct current component (DC) plus an interference correlation component:

$\begin{array}{c}{I}_x+I_{\mathrm{the\; y}}=I_{x0}+I_{\mathrm{the\; y}0}+(q_x+q_{\mathrm{the\; y}})\cos {\mathrm{\&phi;}}_{\mathrm{the\; s}}+(p_x+p_{\mathrm{the\; y}})\sin {\mathrm{\&phi;}}_{\mathrm{the\; s}}\\ =I_0+\cos (\mathrm{\&phi;}-{\mathrm{\&phi;}}_{\mathrm{err}})\end{array}$ Formula (13)

The error introduced by polarization non-reciprocity is:

$\begin{array}{c}{\mathrm{\&phi;}}_{\mathrm{err}}=\arctan \left(\frac{p_x+p_{\mathrm{the\; y}}}{q_x+q_{\mathrm{the\; y}}}\right)\\ =\arctan \frac{2d\left|C_2{C}_3\right|\mathrm{\&Gamma;}\left(z_{\mathrm{twenty\; three}}\right)\sin {\mathrm{\&phi;}}_{\mathrm{twenty\; three}}}{{\left|C_1\right|}^2(1+d)+{C_4|}^2(1-d)+2\left|C_2{C}_3\right|\mathrm{\&Gamma;}\left(z_{\mathrm{twenty\; three}}\right)\cos {\mathrm{\&phi;}}_{\mathrm{twenty\; three}}}\end{array}$ Formula (14)

It can be seen that when the light is split uniformly (d=0), the phase errors in I _x and I _y are just equal to the opposite sign, and the final phase error is zero at this time. That is, when d=0, φ _err =0. This shows that the fiber optic gyro structure in the present invention can effectively reduce the error introduced by polarization non-reciprocity, so that there is no need for two couplers to strictly ensure reciprocity, and only one coupler can also obtain stable angular velocity detection results .

FIG. 4 shows a time-domain data diagram of output angular velocity values of the gyroscope in FIG. 2 . Among them, the experimental measurement object is the angular velocity of the earth's rotation. On the horizontal plane of the laboratory dimension (40.0 degrees north latitude), the theoretical value to be measured is 9.67 degrees/hour, the data output interval is about 0.12 seconds, and the test time is 2 hours. It can be seen that the data output is stable, and the walk and drift are very small.

FIG. 5 shows an error analysis diagram of the output angular velocity data of the gyroscope in FIG. 2 . According to this figure, the error parameters of the obtained gyroscope are: quantization noise coefficient Q=8.6×10 ^-8 rad, angle random walk Bias stability B=2.3×10 ^-2 О/h, rate random walk K=5.3×10 ^-2 О/h ^3/2 , rate slope R=1.5×10 ^-1 О/h ² .

FIG. 6 is a linearity analysis diagram of the output angular velocity data of the gyroscope in FIG. 2 . This figure shows the results of inclinometers based on the rotation of the earth, showing that the input and output have good linearity. The input and output have a fixed deviation of about 2024О/h. This deviation is introduced by the non-reciprocity of the coupler. It is a fixed deviation that does not change with time. It can be eliminated by simply making a difference without affecting the accuracy and stability of the gyroscope.

In the traditional polarization maintaining gyro and depolarization gyro, if the polarizer is removed, the instability of the zero bias will reach tens to hundreds of О/h. For the single-coupler dual-polarization fiber optic gyroscope of the present invention, the noise and drift caused by the non-reciprocity problem can be almost perfectly compensated, and the result is equivalent to the gyroscope structure with a polarizer. In addition, the single-coupler dual-polarization fiber optic gyroscope of the present invention only needs one coupler, and has lower structural complexity and lower cost than traditional polarization-maintaining gyroscopes and depolarization gyroscopes.

While the foregoing disclosure shows exemplary embodiments of the invention, it should be noted that various changes and modifications can be made without departing from the scope of the invention as defined in the claims. Depending on the structure of the inventive embodiments described herein, constituent elements of the claims may be replaced with any functionally equivalent elements. Therefore, the protection scope of the present invention should be determined by the contents of the appended claims.

Claims (7)

1. a single coupler double polarization fiber optic gyroscope, is characterized in that, comprises wide spectrum light source, polarization maintaining fiber, coupler, polarization maintaining fiber ring and photodetector, wherein, the output end of wide spectrum light source passes single-mode The fiber is connected to the polarization maintaining fiber, the polarization maintaining fiber is divided into two sections and fused at a 45° angle, the other end of the polarization maintaining fiber is coupled with the first port of the coupler, and the third port and the fourth port of the coupler are respectively It is coupled with the two ports of the polarization maintaining fiber ring, and the second port of the coupler is coupled with the input end of the photodetector through the single-mode fiber; the dual-polarized light generated by the two sections of polarization maintaining fiber satisfies the condition of non-reciprocal error compensation. 2. The single-coupler dual-polarization fiber optic gyroscope according to claim 1, wherein the coupler is a 3dB polarization-maintaining fiber optic coupler. 3. The single-coupler dual-polarization fiber optic gyroscope according to claim 1, wherein a phase modulator is inserted into the polarization-maintaining fiber ring. 4. The single-coupler dual-polarization fiber optic gyroscope according to claim 3, wherein the phase modulator is a PZT phase modulator. 5. single coupler dual polarization fiber optic gyroscope according to claim 1, is characterized in that, the section length of described two sections of polarization-maintaining optical fiber is (L ₀ , 2L ₀ ), wherein, L ₀ =L _d /Δn , Δn is the refractive index difference between the x-axis and the y-axis or between the x' axis and the y' axis of the birefringent crystal of the polarization maintaining fiber, is the decoherence length of the broadband light source, _λ0 is the central wavelength of the broadband light source, and Δλ is the spectral width of the broadband light source. 6. The single-coupler dual-polarization fiber optic gyroscope according to claim 1, wherein the wide-spectrum light source adopts an ASE wide-spectrum light source. 7. The single-coupler dual-polarization fiber optic gyroscope according to claim 1, wherein the photodetector adopts a semiconductor PIN photodiode.