CN100557399C - Polarization maintaining fiber beat length measuring apparatus - Google Patents

Abstract

The invention discloses a polarization-maintaining optical fiber beat length measuring instrument, which comprises an optical path system, a force applying device, and a data acquisition and processing system. The optical path system includes a light source, a crystal polarizer, a focusing lens, and a crystal analyzer, and its function is to provide linearly polarized light for measurement; The tightening mechanism and the force-applying mechanism with a micrometer card are used to fix the measured polarization-maintaining fiber, and apply a certain radial pressure to the measured polarization-maintaining fiber in a moving manner along the length direction of the measured polarization-maintaining fiber, so that the measured polarization-maintaining fiber The output optical power after polarizing the optical fiber changes periodically; the data acquisition and processing system collects the optical power output by the optical fiber under test through the optical power meter, and the computer performs corresponding processing and calculation on the output of the optical power meter to obtain the measured The average beat length of polarization-maintaining fiber and the change curve of optical power with acquisition time. The main advantages of the present invention are that the structure is simple, the operation is convenient and the measurement precision is high.

Description

Polarization Maintaining Optical Fiber Beat Length Meter

technical field

The invention belongs to the technical field of inertial device measurement, and mainly relates to a polarization-maintaining optical fiber beat-length measuring instrument, in particular to a polarization-maintaining optical fiber beat-length measuring device with an extinction ratio test component as the core and the principle of a modulation pressure moving power method.

technical background

Polarization-maintaining fiber is a kind of single-mode fiber, which refers to the fiber whose core cross-section is deliberately elliptical, or which uses non-axisymmetric birefringence to transmit two orthogonal polarizations independently in an uncoupled manner. . Polarization-maintaining fiber is widely used in the field of fiber optic sensing, and its main applications include fiber optic gyroscopes, etc. Beat length is an important technical index of polarization maintaining fiber, which characterizes the birefringence performance of polarization maintaining fiber.

The core of polarization-maintaining optical fiber is not an ideal circle, but has a certain degree of ellipticity, and the refractive index of its section is not absolutely isotropic but has certain differences. In fact, what is transmitted in the polarization maintaining fiber is not a true single mode, but two polarization modes, namely HE ₁₁ ^x mode and HE ₁₁ ^y mode. As the length of the fiber varies, the phase difference between the two orthogonal polarization modes propagating in a degenerate state also changes. Therefore, when the incident light is linearly polarized, on the output end section of the single-mode fiber, the outgoing light synthesized by these two orthogonal polarization modes is polarized light whose polarization direction changes with the length of the fiber, and its polarization state is linearly polarized , elliptical polarization and circular polarization evolve periodically. The polarization evolution period is the cross-sectional spacing L _B with a phase difference of 2π, that is, the beat length L _B . According to the characteristic of polarization maintaining fiber, if a periodical modulation pressure is applied to a section of polarization maintaining fiber, then as the applied force moves along the length of the polarization maintaining fiber, the polarization state of the output light of the polarization maintaining fiber (with a certain The optical power of a polarization direction) will change periodically, and the waveform and wavenumber of the polarization state change can be collected by the optical power meter (see Figure 1). These characteristics collected by the optical power meter After sending it into a computer programmed with a test program, the length of the optical fiber that changes the polarization state for one cycle can be calculated according to the distance that the modulation pressure mechanism moves along the length of the optical fiber, that is, the beat length of the polarization-maintaining optical fiber.

Chinese patent application CN1912564A discloses a pressure perturbation method polarization maintaining optical fiber beat length measuring instrument invented by Nankai University in my country. The beat length measuring instrument includes precision displacement control device, signal processing analysis control system, LCD touch screen, host and analysis processing system, optical fiber transmission polarization state direct coupling adjustment device, force application mechanism, optical path with collimating lens and polarization analyzer In the system, the power source of the force-applying mechanism that clamps the optical fiber is controlled by a magnet, and the magnitude of the pressure is changed by the magnitude of the current, and the pressure block directly contacts the bare optical fiber. At the same time, the polarization analyzer uses a fiber-optic polarizer, and an optical signal pre-processing module is installed at the end of the optical system. However, the beat length tester still has the following problems: the force-applying mechanism is controlled by a magnet, and due to the existence of a close-range magnetic field, the influence of the magnetic field on the polarization state of the light in the polarization-maintaining fiber cannot be ignored; the force-applying mechanism is in direct contact with the optical fiber, The optical fiber is easy to be damaged; the polarization analyzer uses a fiber optic polarization device, and the polarization performance is not good enough; the optical system uses a collimator lens and an optical signal pre-processing module, and its structure is too complicated. The above problems all affect the measurement accuracy of the beat length measuring instrument, and bring inconvenience to the operation.

Contents of the invention

The technical problem to be solved by the present invention is to provide a polarization-maintaining optical fiber beat-length measuring instrument using an all-crystal polarizing device and an all-mechanical force application device in view of the deficiencies in the prior art. Features; and high measurement accuracy, easy to operate.

In order to solve the above technical problems, the polarization maintaining optical fiber measuring instrument provided by the present invention includes an optical path system, a force applying device, a data acquisition and processing system, and the optical path system includes a light source, a polarizer, a focusing lens, a polarizer, and an optical fiber adapter , the fiber adapter is connected to the measured polarization-maintaining fiber, the light beam emitted by the light source passes through the polarizer to form the first linearly polarized light, and then enters the measured polarization-maintaining fiber after being focused on the interface end face of the fiber adapter by the focusing lens. After the first linearly polarized light is transmitted through the measured polarization-maintaining optical fiber that is elastically deformed due to the applied force, its polarization characteristics change, and then after passing through the analyzer, it becomes the second linearly polarized light that has the same polarization characteristics as the first linearly polarized light light; the described force applying device contains a clamping mechanism, a displacement mechanism, a pressure regulating mechanism, and a bottom plate: the described clamping mechanism contains a strip body fixed on the bottom plate and two locking mechanisms, and the two locking mechanisms have a certain The spacing between them is placed on the strip body, and the polarization-maintaining optical fiber to be tested is placed on the upper end surface of the strip body and kept flat and stationary under the cooperation of the strip body and the locking mechanism; the displacement mechanism contains two-way stepping The motor, the rack drive assembly, the guide rail assembly, the fixed part of the guide rail assembly is fixedly connected to the bottom plate, the sliding part is fixedly connected to the rack of the rack drive assembly, and the bidirectional stepping motor provides power for the rack drive assembly; The pressure regulating mechanism includes a micrometer card, a joint and a connecting plate. The micrometer card stands upright above the joint and its telescoping head is fixedly connected to the upper end of the joint. The part is fixed, and the groove at the lower end of the connector is facing the bar-shaped body; the rotational motion of the bidirectional stepping motor is converted into a translational motion through the rack drive assembly and driven by the guide rail assembly to be stuck on the measured polarization maintaining optical fiber Mobile; the data acquisition and processing system includes an optical power meter, a computer with a built-in measurement software package, the input end of the optical power meter is connected to the described polarizer through an optical fiber jumper, and the output end is connected to the computer through a cable; The measurement software package includes a control module, a data acquisition module, a parameter module written with bidirectional stepping motor step size, optical power extreme value criterion, and optical power extreme value acquisition accumulation number, and a calculation module with polarization-maintaining optical fiber beat length algorithm , the main function of this software is to control the operation and stop of the bidirectional stepping motor, collect the extreme value of the optical power output by the optical power meter and record the corresponding time, and calculate the average beat length of the measured polarization-maintaining fiber according to the formula.

According to the present invention, the polarizer is a crystal polarizer, and the polarizer is a crystal analyzer.

According to the present invention, the two end surfaces of the strip-shaped body in contact with the locking mechanism and the two end surfaces of the strip-shaped body in contact with the joint are provided with elastic gaskets.

The beneficial effects of the present invention are embodied in the following aspects.

(1) The present invention adopts a purely mechanical force applying device and adds an elastic gasket for protecting the optical fiber at the contact position between the force applying device and the measured polarization-maintaining optical fiber. Compared with the prior art, there is no additional The influence of the magnetic field on the birefringence measurement results of the polarization-maintaining optical fiber ensures that the measured polarization-maintaining optical fiber will not break during the measurement process, and also simplifies the composition and structure of the present invention.

(2) The present invention adopts the concept of modular design, that is, the whole measuring instrument is composed of three independent parts, and most of the functional parts in the extinction ratio testing device are mature technologies, which not only reduces the design cost of the present invention, but also expands the It expands the application range of the extinction ratio test device and is convenient to carry.

(3) The light source drive circuit with integrated circuit control is selected in the extinction ratio test device, so that the optical power output accuracy of the light source can reach 0.5%, and a high-precision bidirectional stepping motor and a crystal polarizer with a high extinction ratio are used at the same time. A crystal analyzer ensures the accuracy of the measurement results of the present invention.

Description of drawings

Figure 1 is a graph of optical power versus acquisition time.

Fig. 2 is a schematic composition diagram of the polarization-maintaining optical fiber beat-length measuring instrument of the present invention.

Fig. 3 is a diagram of a driving circuit of a light source in a preferred embodiment of the present invention.

Fig. 4 is a schematic composition diagram of the force applying device shown in Fig. 1 .

5a and 5b are schematic diagrams of the tightening mechanism shown in FIG. 3 .

Fig. 6a and Fig. 6b are schematic diagrams of the composition of the displacement mechanism shown in Fig. 3 .

Fig. 7 is a motor drive circuit diagram in a preferred embodiment of the invention.

8a and 8b are schematic diagrams of the composition of the pressure regulating mechanism shown in FIG. 3 .

Fig. 9 is a working flow chart of the measurement software package of the present invention.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and preferred embodiments.

As shown in Figure 2, the preferred embodiment of the present invention includes an optical path system, a force applying device 4, and a data acquisition and processing system. The optical path system includes a light source, a crystal polarizer, a focusing lens, and a crystal analyzer. The light source, crystal polarizer, and focusing lens are individually integrated into an extinction ratio test device. The light source is composed of a semiconductor laser and a light source driving circuit. Through experimental analysis, it is found that the fluctuation of optical power is mainly determined by the ambient temperature and the constant current source. When the ambient temperature is within the refrigeration control range, the constant current source plays a major role. When the ambient temperature changes greatly, the latter plays a major role. . The light source driving circuit of this preferred embodiment starts from the constant current source and controls the power environment temperature of the semiconductor laser, and designs a driving circuit with an integrated chip as the core, thereby providing a constant DC power supply and a stable operating temperature for the semiconductor laser to drive the light source A constant power is issued, and its detailed circuit is shown in Figure 3. The stability of the output power of the light source ensures that the optical power threshold in the test process will not drift beyond the error range and affect the test results. Both crystal polarizer and crystal analyzer are made of high extinction ratio crystal material. The focusing lens is installed inside the extinction ratio testing device through a three-dimensional adjustable bracket. The light source switch and three adjustment knobs connected to the three-dimensional adjustable bracket are installed outside the housing of the extinction ratio testing device. The extinction ratio testing device is connected with the optical fiber adapter 2 One end of the measured polarization-maintaining fiber 3 is connected, and the other end of the measured polarization-maintaining fiber 3 is connected to the crystal polarization analyzer 5 through another fiber adapter 2 (the length of the measured polarization-maintaining fiber is generally 2m). Turn on the light source switch, and the light-emitting diode emits a laser beam of sufficient power under the drive of the light source drive circuit; the beam passes through the crystal polarizer to form the first linearly polarized light and focuses through the focusing lens; by turning the three adjustment knobs, the focusing lens can The focal point of the optical fiber is accurately located on the input end face of the fiber adapter 2, and the first linearly polarized light enters the crystal analyzer 5 after passing through the measured polarization-maintaining fiber 3. When the first linearly polarized light beam is transmitted through the measured polarization-maintaining optical fiber 3, the measured polarization-maintaining optical fiber 3 is elastically deformed because the force applying device applies pressure to the radial direction of the measured polarization-maintaining optical fiber 3 in a moving manner along the length direction. , thus resulting in a change in the polarization characteristics of the first linearly polarized light transmitted therein, and therefore, the output beam of the polarization-maintaining fiber 3 under test is no longer the first linearly polarized beam. The function of setting the crystal analyzer 5 is to make the light beam output by the crystal analyzer 5 become linearly polarized light having the same vibration direction as the first linearly polarized light again, so as to ensure that the optical power meter receives light with the same vibration direction. The power correctly reflects the change period (beat length) of the measured polarization-maintaining optical fiber 3 from linearly polarized light to elliptically polarized light to linearly polarized light, ensuring test accuracy.

As shown in FIG. 4 , the force applying device 4 in this preferred embodiment is an independent device, mainly composed of a clamping mechanism, a displacement mechanism, a pressure regulating mechanism, and a bottom plate 10 . The bottom plate 10 is square and made of aluminum. According to Fig. 5a and Fig. 5b, the clamping mechanism includes a strip body 13, two locking mechanisms and a first felt sheet 12 with a thickness of 0.5 mm. The bottom of the bar-shaped body 13 is located in a longitudinal groove of the base plate 10 and is fixedly connected with the base plate 10 by screws, and its upper end surface is bonded with the first felt sheet 12 with rubber liquid. Locking mechanism contains elbow pressing plate 14, fixed block 16, direct pin 15. The upward elbow at one end of the elbow pressing plate 14 has a U-shaped end sleeve and a pin hole, and the other end has a downward protrusion, and the bottom of the middle position has a groove corresponding to the width of the strip body 13, and the bottom surface of the groove is Take a second felt sheet 0.5 mm thick. Two elbow pressure plates 14 are separated on the strip body 13 at a distance of 10 cm, and the downward protrusion of each elbow pressure plate 14 is fixed on the bottom plate 10 by screws 11 and its groove is stuck on both sides of the strip body 13 , the U-shaped end cover of the other end is then enclosed within the both sides of the fixed block 16 that is connected with the base plate 10, and the U-shaped end cover of the elbow pressure plate 14 is connected in series with the fixed block 16 with the straight pin 15. One section of the measured polarization-maintaining optical fiber 3 is placed on the first felt sheet 12, and the second felt sheet on the bottom surface of the groove of the elbow pressure plate 14 is pressed against the first felt sheet 12 by adjusting the screw 11, and the two locks are ensured. The measured polarization-maintaining optical fiber 3 between the tightening mechanisms remains flat and motionless.

According to Fig. 6a and Fig. 6b, the displacement mechanism includes a bidirectional stepper motor 27, a motor drive circuit, a rack drive assembly including a spur gear 25 and a rack 24, and a guide rail assembly including a dovetail groove 22 and a dovetail 23. The dovetail 23 is fixedly connected to the bottom plate 10 by screws, and its length direction is parallel to the length direction of the strip body 13; the dovetail groove 22 is slidingly matched with the dovetail 23. The two-way stepper motor 27 is fixedly connected to the bottom plate 10 through a press buckle 28, and its transmission shaft is fixedly connected to the spur gear 25 through a pin 26. The spur gear 25 meshes with the rack 24, and the rack 24 is fixedly connected to the upper end surface of the dovetail groove 22. superior. Fig. 7 shows the motor drive circuit diagram of this preferred embodiment. The main function of the circuit is to receive test instructions given by the data acquisition and processing system, drive the bidirectional stepper motor 27 to rotate forward and reverse, control the speed of the bidirectional stepper motor 27 and determine the step size of the bidirectional stepper motor 27 . The step length of the stepping motor is an important parameter that directly affects the measurement accuracy. According to multiple tests, the step length of the bidirectional motor 27 controlled by the motor drive circuit of this preferred embodiment is 0.164 mm. Usually, the beat length of the polarization-maintaining optical fiber is generally on the order of 2 to 3 mm, and the beat length algorithm adopted in the present invention is actually to obtain the average beat length. The long value can fully meet the high-precision test requirements.

As shown in Fig. 8a and Fig. 8b, the pressure regulating mechanism includes a joint 19, a micrometer card 20, a broken line connecting plate 21 and a third felt sheet with a thickness of 0.5mm. The upper end surface of the joint 19 has a blind hole, the lower end surface has a groove corresponding to the width of the bar body 13, and the side has a pin hole communicating with the blind hole. The third felt sheet is bonded to the bottom surface of the groove of the joint 19 . The centricard 20 stands upright above the joint 19, and its expansion head is inserted into the blind hole of the joint 19 and tightened by the set screw; On the upper end face of the dovetail groove 22, the vertical plate at the lower end is fixedly connected with the side of the dovetail groove 22 by fastening screws, and the groove at the lower end of the joint 19 is facing the strip body 13. By rotating the upper part of the micrometer card 20, the lower part of the expansion head drives the joint 19 to move up and down, so that pressure can be applied to the measured polarization-maintaining optical fiber sandwiched between the first and third felt sheets, and pressure can be applied according to the test requirements. Make adjustments. The motor drive circuit receives the control command output by the data acquisition and processing system, drives the stepper motor 27 to rotate and drives the dovetail groove 22 to slide on the dovetail 23 through the rack 24, thereby also driving the micrometer card 20 between the two tightening mechanisms Moving back and forth along the measured polarization maintaining optical fiber 3, so as to realize the final function of the force applying device. Since the part of the force applying device that is in contact with the optical fiber under test is equipped with a felt sheet, the measured polarization-maintaining optical fiber will not be twisted or broken by the connector 19 during the test, which can ensure the accuracy of the test results or Integrity of the testing process.

Referring to Fig. 2 again, the data acquisition and processing system includes an optical power meter 7 and a computer 8 including a measurement software package. The model of the optical power meter 7 in this preferred embodiment is PMS-12F, its measurement accuracy is ±5%, the resolution reaches 0.001dB, and the measurement range is 10pw～1000uw, which meets the test requirements. The input end of the optical power meter 7 is connected to the crystal analyzer 5 through an optical fiber jumper 6 and an optical fiber adapter, and its output end is connected to a computer 8 through a cable. The optical fiber jumper 6 is a section of multimode optical fiber bundle with a standard length (usually 1m), which mainly plays the role of transmission medium and connection. In order to reduce the loss of optical power in the optical path, an optical fiber jumper 6 that can transmit light waves without loss is selected for this requirement. At the same time, the flexibility of the optical fiber jumper 6 is conducive to the flexible design of the optical path. The main function of the computer 8 is to control the start and stop of the bidirectional stepper motor 27, collect the output signal of the optical power meter 7, and process the collected signal accordingly, and finally calculate the beat length of the measured polarization-maintaining optical fiber 3 . If the test software built in the computer 8 is divided by function, it includes a control module, a data acquisition module, a parameter module, and a calculation module. The data acquisition module collects the voltage signal output by the optical power meter 7 and converts it into a digital signal through A/D. The step length value of the bidirectional stepping motor 27, the criterion of the extreme value of optical power, and the number of collections of the extreme value of optical power set are written in the parameter module. According to the energy loss calculation of the optical path and the test accuracy of the optical power meter 7, the optical power threshold of this preferred embodiment is determined to be 165 ± 5% uw, and the number of the extreme values of the collected optical power is based on the step length of 0.164mm of the bidirectional stepping motor 27 Calculated with the length of the clamping mechanism, the length of the clamping mechanism in this embodiment is 10 cm, and the number of collections should be between 50 and 60. There is a beat length calculation formula in the calculation module, namely

${\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; \&Delta;</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; \&times;</span>\mathrm{<span\; class="notranslate">\; S</span>}$

In the formula, L _Δ is the average beat length of the measured polarization-maintaining fiber; T _N is the acquisition stop time; T ₁ is the acquisition start time; N is the number of acquisitions of the extreme value of optical power; S is the step size of the stepping motor.

Figure 9 shows the workflow of the measurement software package. As shown in the figure, during the measurement, the measurement software package performs the following steps: the first step is to initialize the optical power meter, and set the test parameters, that is, the optical power threshold M and the accumulated number X of optical power extreme value collection; the second step , send the motor drive command to the motor drive circuit through the control module, and start timing; the third step, the acquisition module collects the output signal of the optical power meter 7 and converts it into a digital signal; the fourth step, calls the optical power threshold judgment in the parameter module According to judging the current optical power data, if the optical power data is greater than or equal to the threshold criterion, then put the optical power data and its corresponding time T _i into the buffer area of the computer and add 1 to the number of extreme value collection, i= 1, 2, 3...N, repeat the above operations until the number N of optical power extreme value collection is equal to the set extreme value collection accumulation number X; the fifth step is to send a stop driving command to the motor drive circuit; the sixth step is to calculate The module calls the relevant data in the parameter module and the buffer, and calculates the average beat length of the measured polarization-maintaining optical fiber 3; in the seventh step, the display of the computer displays the calculation result and the graph of the optical power changing with the acquisition time (see Figure 1) .

It is known through multiple tests that this preferred embodiment can achieve a beat length test accuracy of ±1%.

The usage method of the present invention will be briefly described below in conjunction with the present preferred embodiment.

(1) Calibration

Connect the optical fiber adapter 2 of the output port of the extinction ratio test device 1 with the optical power meter 7 with the optical fiber jumper 6, turn on the switch of the light source and the optical power meter 7, and after ten minutes of warm-up and stabilization as required, if the output of the optical power meter 7 is above 300uw And stabilized, considered normal, and maintained in that state.

(2) Debugging

Replace the optical fiber jumper 6 connected between the optical fiber adapter 2 and the optical power meter 7 with the measured polarization-maintaining optical fiber 3, and adjust the position of the inner focusing lens through the adjustment knob on the housing of the extinction ratio test device 1 until the optical power meter 7 The output value of the polarization maintaining fiber 3 is not less than 150uw; the connection between the output end of the measured polarization maintaining fiber 3 and the optical power meter is disconnected and connected to one end of the crystal analyzer 5, and the other end of the crystal analyzer 5 is connected through an optical fiber jumper 6. Connect with the optical power meter 7, adjust the analysis angle of the crystal analyzer 5, make the polarization direction of the linearly polarized light passing through the crystal polarizer and the crystal analyzer 5 consistent, turn on the power meter 7 and ensure the transparency The passing optical power is always at the maximum value.

(3) Install the force device

First, lift the U-shaped end sleeves of the two elbow clamps 14, place one section of the polarization-maintaining optical fiber 3 under test on the first felt sheet 12 bonded to the upper end of the strip body 13, and then use the straight pin 15 to bend the The U-shaped end sleeve of the head pressure plate 14 is connected in series with the fixed block 16, and the bottom surface of the groove of the elbow pressure plate 14 is pressed against the first felt sheet 12 through the adjustment screw 11, so that the measured polarization-maintaining optical fiber 3 cannot slide or wiggle. Secondly, adjust the screw rod of the micrometer card 20 to move the joint fixedly connected with its expansion head downward until the second felt sheet bonded to the groove at the bottom of the joint contacts with the measured polarization-maintaining optical fiber 3 and applies the corresponding force according to the test requirements. pressure.

(4) Test

Turn on the power of the computer, input the test command through the keyboard, the computer sends a drive command to the motor drive circuit, the stepper motor 27 starts to run, and drives the centroid card 20 to move along the measured polarization maintaining optical fiber 3 through the rack drive assembly and the guide rail assembly, At the same time, the optical power meter 7 collects the optical power signal transmitted by the crystal polarizer, the measured polarization-maintaining optical fiber 3, and the crystal analyzer 5 and sends it to the computer 8. The power signal is processed and calculated accordingly, and finally the average beat length and optical power of the measured polarization-maintaining optical fiber 3 are obtained and displayed on the display.

Claims (3)

1. A polarization-maintaining optical fiber beat length measuring instrument, including an optical path system, a force applying device [4], a data acquisition and processing system: the optical path system includes a light source, a polarizer, a focusing lens, and a polarizer [5] And the fiber optic adapter [2], the fiber optic adapter [2] is connected with the measured polarization-maintaining fiber [3], the light beam emitted by the light source passes through the polarizer to form the first linearly polarized light, and then focuses on the fiber optic adapter [ 2] into the measured polarization-maintaining fiber [3], the first linearly polarized light passes through the measured polarization-maintaining fiber [3] that is elastically deformed due to force, and its polarization characteristics change after transmission, and then passes through the measured polarization-maintaining fiber. After the analyzer [5], it becomes the second linearly polarized light with the same polarization characteristics as the first linearly polarized light; it is characterized in that: the described forcing device [4] contains a clamping mechanism, a displacement mechanism, and a pressure regulating mechanism And the bottom plate [10]: the clamping mechanism contains a strip body [13] fixed on the bottom plate [10] and two locking mechanisms, and the two locking mechanisms are separated on the strip body [13] with a certain distance Above, the polarization-maintaining optical fiber [3] to be tested is placed on the upper end surface of the strip body [13] and kept flat and stationary under the cooperation of the strip body [13] and the locking mechanism; the displacement mechanism contains two-way The stepper motor [27], the rack drive assembly and the guide rail assembly, the fixed part [23] in the guide rail assembly is fixedly connected with the base plate [10], and the sliding part [22] is connected with the rack [ 24] Fixed connection, bi-directional stepping motor [27] provides power for the rack drive assembly; the pressure regulating mechanism contains centricard [20], joint [19] and connecting plate [21], centricard [20] ] stands upright above the joint [19] and its telescopic head is fixedly connected to the upper end of the joint [19], and the two ends of the connecting plate [21] are respectively fixedly connected to the sliding part [22] of the centricard [20] and the guide rail assembly , and make the groove at the lower end of the joint [19] face the bar [13]; the rotary motion of the bidirectional stepping motor [27] is converted into a translational motion through the rack drive assembly and drives the split card [ 20] Move on the measured polarization maintaining optical fiber [3]; the data acquisition and processing system includes an optical power meter [7], a computer [8] with a built-in measurement software package, and the input end of the optical power meter [7] passes through the optical fiber The jumper [6] is connected with the described polarizer [5], and the output end is connected with the computer [8] through a cable; the measurement software package contains a control module, a data acquisition module, and a bidirectional stepper motor step length , the parameter module of optical power extreme value criterion and optical power extreme value acquisition accumulation number, and the calculation module of polarization-maintaining optical fiber beat length algorithm. The main function of this software is to control the operation and stop of bidirectional stepping motor [27], Collect the extreme value of the optical power output by the optical power meter [7] and record the corresponding moment, and calculate the average beat length of the measured polarization-maintaining fiber [3] according to the formula. 2. The polarization maintaining optical fiber beat length measuring instrument according to claim 1, characterized in that: said polarizer is a crystal polarizer, and said polarizer [5] is a crystal polarizer. 3. The polarization-maintaining optical fiber beat length measuring instrument according to claim 1 or 2, characterized in that: the two end faces of the strip-shaped body [13] in contact with the locking mechanism and the Elastic gaskets are installed on both end faces of the bar [13] and the joint [19] that are in contact.

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