Device and method for detecting polarization dependence of fiber bragg grating based on scattered light

Abstract

The invention discloses a device and a method for detecting polarization dependence of a fiber bragg grating based on scattered light. The device comprises an incident laser light source, a scattering plane, an optical fiber rotating clamp and an information extraction tool, the method comprises the following steps: acquiring characteristic contour information of scattered light, calculating scattering light intensity dispersion degree, and obtaining a polarization dependence result of the fiber grating device according to a linear relation between the scattering light intensity dispersion degree and central wavelength offset, thereby realizing rapid detection of the polarization dependence of the fiber grating device for femtosecond laser processing. According to the detection method, based on the characteristic contour of scattered light distribution, the polarization dependence characteristic caused by the grid region modulation section ovality can be rapidly identified, and an efficient and convenient means is provided for identification of the low-polarization-dependent-loss fiber bragg grating.

Description

Technical Field

[0001] The invention relates to the technical field of fiber grating device detection, and in particular to a device and method for detecting polarization dependence of fiber grating based on scattered light. Background Art

[0002] Fiber Bragg grating is a very important passive device in the field of fiber optic sensing. As a sensor, it senses changes in the external environment by monitoring the deviation of the central wavelength of reflected light. It has the advantages of small size, light weight, high sensitivity, good corrosion resistance, strong anti-electromagnetic interference ability, and support for distributed monitoring. It has broad application prospects in the biomedical industry, power battery safety monitoring and traditional large-scale projects.

[0003] Polarization Dependent Loss (PDL) is one of the important factors affecting the sensing performance of fiber Bragg gratings. It is a physical phenomenon that describes the change in light signal intensity due to different polarization states of light in optical devices or optical systems. It is mainly used to measure the sensitivity of optical devices to the polarization state of transmitted light. For fiber Bragg grating devices, when the polarization state of the input light changes, light in different polarization states will show different wavelength shifts. When the polarization-dependent loss is too large, it will affect the stability of the fiber Bragg grating reflection spectrum, causing obvious errors in the measurement results, thereby reducing the sensing performance of the fiber Bragg grating. Therefore, how to quickly identify low PDL fiber Bragg grating devices is of great significance for the production of high-quality fiber Bragg grating devices.

[0004] Femtosecond laser direct writing technology has received more and more attention in the field of fiber Bragg grating processing due to its unique advantages. Compared with traditional fiber Bragg grating processing methods (such as ultraviolet exposure method), femtosecond laser direct writing technology has shown significant advantages in process flexibility, writing accuracy, processing efficiency, etc. Since the refractive index modulation range of femtosecond laser processing is small and usually localized in certain specific positions of the fiber core, it is difficult to achieve full core modulation, so the geometric characteristics of the processed grating structure need to be considered. When the refractive index modulation cross-section has a large ellipticity, it will cause a large polarization-related loss, making the fiber Bragg grating extremely sensitive to incident light of different polarization states, which directly affects the sensing accuracy. Especially in the dynamic sensing process, polarization-related loss may mask the real environmental change signal, resulting in a large systematic error. At present, polarization analysis equipment for fiber Bragg gratings has problems such as high cost, complex operation, and long time consumption. Summary of the invention

[0005] In view of the problems that the polarization analysis equipment in the prior art is complex and time-consuming to detect the polarization dependence of fiber grating, the purpose of the present invention is to provide a device and a detection method for detecting the polarization dependence of fiber grating processed by femtosecond laser based on scattered light. The detection device can be embedded in the preparation process of fiber grating, and the detection method is simple to operate and has high detection efficiency.

[0006] To achieve the above object, the present invention adopts the following technical solutions: The present invention provides a device for detecting polarization dependence of fiber grating based on scattered light, comprising: an incident laser light source, a scattering plane, a fiber rotating fixture, and an information extraction tool; The incident laser light source is connected to one end of an optical fiber on which a fiber grating device is inscribed, so that the incident laser is coupled into the optical fiber and transmitted in the optical fiber; The scattering plane is used to receive the light beam radiated from the fiber grating region in the optical fiber to form a scattering characteristic profile, and the optical fiber vertically passes through the scattering plane; There are two optical fiber rotation clamps, which are respectively located on both sides of the scattering plane and are used to clamp the optical fiber and synchronously rotate the optical fiber axially; The information extraction tool is used to extract scattered light intensity information of a measurement point on the scattering plane, and the measurement point is selected to be located in the scattering feature profile; The multiple scattered light intensity information data of the measuring points are obtained by rotating the optical fiber, the scattered light intensity dispersion is calculated, and the polarization dependence result of the fiber grating device is obtained according to the linear relationship between the scattered light intensity dispersion and the central wavelength offset.

[0007] Furthermore, a core for the optical fiber to pass through is provided on the scattering plane.

[0008] Furthermore, the optical fiber rotation fixture is marked with an angle scale.

[0009] Furthermore, the information extraction tool is selected from a combination of a photodiode power sensor and a power meter, a combination of a photodetector and an oscilloscope, a CMOS image sensor, and a CCD image sensor.

[0010] The method for detecting the polarization dependence of a fiber grating using the above device provided by the present invention comprises the following steps: S1. Start the incident laser light source to input the incident laser into the fiber Bragg grating device, so that the light beam radiated from the fiber Bragg grating region forms a scattering characteristic profile on the scattering plane; S2. Using the information extraction tool to read the scattered light intensity information of the measurement point in the scattering feature profile on the scattering plane, rotating the optical fiber at a fixed angle, reading the scattered light intensity information of the measurement point after each rotation, obtaining multiple data, and calculating the degree of dispersion of the scattered light intensity; S3 repeats steps S1 and S2, reads and calculates the scattered light intensity discreteness of multiple calibration fiber Bragg grating devices, and measures the center wavelength offset of the calibration fiber Bragg grating device, and linearly fits the center wavelength offset and the scattered light intensity discreteness to obtain a fitting curve equation; S4. Repeat steps S1 and S2, read and calculate the discrete degree of scattered light intensity of the fiber Bragg grating device to be tested, substitute it into the fitting curve equation obtained in step S3, and calculate the center wavelength offset; determine whether the center wavelength offset exceeds the threshold value, if not, determine it to be a low polarization-dependent fiber Bragg grating device.

[0011] Furthermore, in step S2, the obtaining of multiple data is obtaining more than 6 data.

[0012] Furthermore, in step S2, the indicator of the degree of dispersion is a standard deviation or a variance.

[0013] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a simple and intuitive device and method for detecting the polarization dependence of a fiber Bragg grating device. By obtaining the characteristic profile information of the scattered light, the discrete degree of the scattered light intensity is calculated, and the polarization dependence result of the fiber Bragg grating device is obtained according to the linear relationship between the discrete degree of the scattered light intensity and the central wavelength offset, so as to realize the rapid detection of the polarization dependence of the fiber Bragg grating device processed by femtosecond laser. The detection method of the present invention is based on the characteristic profile of the scattered light distribution, and can quickly identify the polarization dependence characteristics caused by the ellipticity of the grating area modulation cross section, and provides an efficient and convenient means for the identification of low polarization-dependent loss fiber Bragg gratings. Compared with traditional methods such as polarization-dependent loss measuring instruments or high-precision polarization analyzers, the method of the present invention does not need to rely on complex experimental equipment or operating procedures, and can achieve accurate judgment of the polarization characteristics of the fiber Bragg grating only through simple detection devices and optical distribution information. The detection method of the present invention not only reduces the experimental complexity and significantly improves the detection efficiency, but also can be embedded in the preparation process of the fiber Bragg grating, providing a guarantee for the production of low polarization-dependent fiber Bragg grating devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of a fiber Bragg grating polarization-dependent detection device; Figure 2 is a schematic diagram of the scattering plane structure; Figure 3A schematic diagram for extracting scattered light intensity information on a scattering feature profile; Figure 4 This is a schematic diagram of the test of the center wavelength offset of the fiber Bragg grating device; Figure 5 is the fitting curve equation of Example 1; Figure 6 is the scattering characteristic profile of device A and device B; Figure 7 These are the test results of polarization-dependent loss of device A and device B. DETAILED DESCRIPTION

[0015] In order to enable those skilled in the art to more clearly understand the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the specific implementation methods, structures, features and effects of the present invention are further described in detail below in conjunction with the accompanying drawings and preferred embodiments. The examples cited are only used to explain the present invention and are not used to limit the scope of the present invention. In the embodiments of the present invention, unless otherwise specified, all raw material components are commercially available products well known to those skilled in the art; if not specifically specified, the technical means used are conventional means well known to those skilled in the art.

[0016] An embodiment of the present invention provides a device for detecting polarization dependence of fiber Bragg grating based on scattered light, such as Figure 1 As shown, it includes: an incident laser light source 1, a scattering plane 2, an optical fiber rotation clamp 3, and an information extraction tool 4.

[0017] The incident laser light source 1 is connected to one end of the optical fiber 5 , so that the incident laser can be coupled into the optical fiber 5 and can be transmitted in the optical fiber 5 .

[0018] The optical fiber 5 is inscribed with a fiber grating device 6 processed by a femtosecond laser. The fiber grating device 6 can be inscribed at the core of the optical fiber 5 or at the cladding of the optical fiber 5; it can be a fiber grating processed at the center of the core or at a position offset from the center of the core.

[0019] Since the fiber grating processed by femtosecond laser has strong radiation characteristics, when the laser is incident on the fiber grating area, part of the light can radiate out of the optical fiber 5, be transmitted to the scattering plane 2 and form a characteristic profile with polarization-dependent information.

[0020] The scattering plane 2 is used to receive the light beam radiated from the fiber grating region in the optical fiber, and the structure is as follows: Figure 2 As shown, Figure 2(a) is a front view, (b) is a side view, there is a circular hole 201 in the middle of the scattering plane 2 for the optical fiber to pass through, and a core 202 is arranged in the circular hole 201 to accurately position the optical fiber and ensure that the optical fiber is perpendicular to the scattering plane 2, avoiding the non-uniformity of the scattering profile caused by the angle offset. There is also a clamp 203 at the lower end of the scattering plane 2 for fixing the scattering plane 2 to ensure that the scattering plane 2 is in a vertical state with the horizontal plane. An adjustable fixing frame 204 is also connected below the clamp 203 to adjust the height and horizontal placement of the scattering plane 2 to ensure that the optical fiber can pass through the core horizontally.

[0021] There are two optical fiber rotation clamps 3, located on both sides of the scattering plane 2, for clamping the optical fiber 5 and synchronously rotating the optical fiber 5 axially, ensuring that the optical fiber 5 is in a straight state and the grid area is free of stress. Angle scales are marked on the optical fiber rotation clamps 3 to ensure that the two optical fiber rotation clamps 3 rotate synchronously at the same angle each time.

[0022] The information extraction tool 4 is used to extract scattered light intensity information of a measurement point on the scattering plane 2, and the measurement point is selected on the scattering feature profile. The information extraction tool 4 can be selected from a combination of a photodiode power sensor and a power meter or a combination of a photodetector and an oscilloscope, or can be selected from a CMOS image sensor or a CCD image sensor.

[0023] Figure 3 The schematic diagram is of using a combination of a photodiode power sensor 401 and a power meter 402 to extract scattered light intensity information on a scattering feature profile. The photodiode power sensor 401 is mounted on an adjustable bracket 403. Once the measurement point on the scattering plane 2 is determined, the position is fixed. The photodiode power sensor 401 does not need to be absolutely perpendicular to the scattering plane 2, and a certain angle tolerance range may exist.

[0024] The method for detecting polarization dependence of a fiber grating using the detection device of the embodiment of the present invention comprises the following steps: Step 1: Start the incident laser light source 1 to input the incident laser into the fiber grating device 6, so that the light beam radiated from the fiber grating area forms a scattering characteristic profile on the scattering plane 2.

[0025] Step 2: Use the information extraction tool 4 to read the scattered light intensity information of the measurement point in the scattering feature profile on the scattering plane 2, recorded as P 1; then at an angle θ Rotate the optical fiber and read the scattered light intensity information at the measurement point after rotation, recorded as P 2. Repeat the above steps at a fixed angle θ Rotate the optical fiber, obtain n data, and calculate the discrete degree of scattered light intensity.

[0026] The indicator of the degree of dispersion is the standard deviation or variance. The more measurement point data extracted, the more accurate the calculated result will be. Therefore, n should be greater than 6. θ =2π / n.

[0027] Step 3: Take multiple (at least 5) calibration fiber Bragg grating devices, repeat steps 1 and 2, read and calculate the scattered light intensity discreteness of the calibration fiber Bragg grating devices; and measure the center wavelength offset of the calibration fiber Bragg grating devices; perform linear fitting on the center wavelength offset and the scattered light intensity discreteness, establish a linear relationship between the center wavelength offset and the scattered light discreteness, and obtain the fitting curve equation.

[0028] The central wavelength offset is mainly used to characterize the polarization dependence of fiber Bragg grating devices. The test method is as follows: Figure 4 As shown, the test device includes a linear polarization light source 7, a circulator 8 and a spectrometer 9. First, the linear polarization light source 7 outputs a specific linear polarization light, which then passes through the circulator 8 to reach the fiber grating device 6. The reflected light of the device to be tested passes through the circulator 8 and is finally transmitted to the spectrometer 9. The two states of the fiber grating device are mainly tested, namely the stable state and the unstable state.

[0029] Step 4, repeat steps 1 and 2, read and calculate the discrete degree of scattered light intensity of the fiber Bragg grating device to be tested, substitute it into the fitting curve equation obtained in step 3, and calculate the center wavelength offset; determine whether the center wavelength offset exceeds the threshold value, if not, it is determined to be a low polarization-dependent fiber Bragg grating device.

[0030] Generally speaking, fiber Bragg grating devices with a central wavelength offset caused by the polarization state of the incident light within 3 pm are considered to have good sensing performance. Therefore, the degree of scattered light dispersion corresponding to the 3 pm central wavelength offset can be determined as an effective threshold to determine whether the polarization dependence of the processed device exceeds the allowable range.

[0031] The following is a specific example of using the detection device of the present invention to detect the polarization dependence of a fiber grating.

[0032] Example 1

[0033] Take 5 calibrated fiber Bragg grating devices with measured central wavelength offset, connect them to the detection device respectively, start the incident laser light source 1 to input laser with a wavelength of 650 nm in the visible light band, form a scattering feature profile on the scattering plane 2, select the measurement point in the scattering feature profile, fix the photodiode power sensor, and read the scattered light intensity at the measurement point P 1; then control the optical fiber rotating fixture 3 to drive the optical fiber to rotate 45° axially, and read the scattered light intensity of the measuring point again P2. Repeat the above operation, rotate at a 45° angle 7 times, and read the scattered light intensity of 8 measurement points in total; calculate the standard deviation of the 8 scattered light intensity measurement data σ ; Perform linear fitting on the center wavelength offset and the standard deviation of scattered light intensity, establish a linear relationship between the center wavelength offset and the standard deviation of scattered light intensity, and obtain the fitting curve equation, such as Figure 5 As shown, y=0.8037x-2.4963, R 2 =0.9873.

[0034] Figure 6 The scattering characteristic profiles of device A and device B are read and calculated. The standard deviation of the scattered light intensity of device A is 23.2, and the standard deviation of the scattered light intensity of device B is 6.8. Substituting them into the fitting curve equation, the central wavelength offset of device A is 16.15 pm, and the central wavelength offset of device B is 2.97 pm. Figure 7 For device A and device B, Figure 4 The results of the polarization-dependent loss test of the device show that in an unstable state, the center wavelength offset caused by polarization of device A is 16 pm, and the center wavelength offset caused by polarization of device B is 3 pm. It can be seen that the detection method of the present invention has a high accuracy.

[0035] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any person skilled in the art can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A device for detecting polarization dependence of fiber Bragg grating based on scattered light, characterized in that: include: Incident laser light source, scattering plane, optical fiber rotation fixture, information extraction tool; The incident laser light source is connected to one end of an optical fiber on which a fiber grating device is inscribed, so that the incident laser is coupled into the optical fiber and transmitted in the optical fiber; The scattering plane is used to receive the light beam radiated from the fiber grating region in the optical fiber to form a scattering characteristic profile, and the optical fiber vertically passes through the scattering plane; There are two optical fiber rotation clamps, which are respectively located on both sides of the scattering plane and are used to clamp the optical fiber and synchronously rotate the optical fiber axially; The information extraction tool is used to extract scattered light intensity information of a measurement point on the scattering plane, and the measurement point is selected to be located in the scattering feature profile; The multiple scattered light intensity information data of the measuring points are obtained by rotating the optical fiber, the scattered light intensity dispersion is calculated, and the polarization dependence result of the fiber grating device is obtained according to the linear relationship between the scattered light intensity dispersion and the central wavelength offset.

2. The device for detecting polarization dependence of fiber Bragg grating based on scattered light according to claim 1, characterized in that: The scattering plane is provided with an insert core for the optical fiber to pass through.

3. The device for detecting polarization dependence of fiber Bragg grating based on scattered light according to claim 1, characterized in that: The optical fiber rotation fixture is marked with an angle scale.

4. The device for detecting polarization dependence of fiber Bragg grating based on scattered light according to claim 1, characterized in that: The information extraction tool is selected from a combination of a photodiode power sensor and a power meter, a combination of a photodetector and an oscilloscope, a CMOS image sensor, and a CCD image sensor.

5. A method for detecting polarization dependence of a fiber Bragg grating using the device according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1. Start the incident laser light source to input the incident laser into the fiber grating device, so that the light beam radiated from the fiber grating region forms a scattering characteristic profile on the scattering plane; S2. Using the information extraction tool to read the scattered light intensity information of the measurement point in the scattering feature profile on the scattering plane, rotating the optical fiber at a fixed angle, reading the scattered light intensity information of the measurement point after each rotation, obtaining multiple data, and calculating the degree of dispersion of the scattered light intensity; S3 repeats steps S1 and S2, reads and calculates the scattered light intensity discreteness of multiple calibration fiber Bragg grating devices, and measures the center wavelength offset of the calibration fiber Bragg grating device, and linearly fits the center wavelength offset and the scattered light intensity discreteness to obtain a fitting curve equation; S4. Repeat steps S1 and S2, read and calculate the discrete degree of scattered light intensity of the fiber Bragg grating device to be tested, substitute it into the fitting curve equation obtained in step S3, and calculate the center wavelength offset; determine whether the center wavelength offset exceeds the threshold value, if not, determine it to be a low polarization-dependent fiber Bragg grating device.

6. The method for detecting polarization dependence of a fiber Bragg grating according to claim 5, characterized in that: In step S2, the acquiring of multiple data is acquiring more than 6 data.

7. The method for detecting polarization dependence of a fiber Bragg grating according to claim 5, characterized in that: In step S2, the indicator of the degree of dispersion is the standard deviation or the variance.

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