An apparatus and method for detecting the polarization dependence of fiber Bragg gratings based on scattered light

Abstract

The present invention discloses a device and method for detecting the polarization dependence of fiber Bragg gratings based on scattered light. The device includes an incident laser light source, a scattering plane, a fiber rotation fixture, and an information extraction tool. By obtaining the characteristic profile information of the scattered light, calculating the discrete degree of the scattered light intensity, and based on the linear relationship between the discrete degree of the scattered light intensity and the central wavelength offset, the polarization dependence result of the fiber Bragg grating device is obtained, realizing the rapid detection of the polarization dependence of fiber Bragg grating devices processed by femtosecond lasers. The detection method of the present invention is based on the characteristic profile of the scattered light distribution, and can quickly identify the polarization-dependent characteristics caused by the ellipticity of the modulation cross-section in the grating region, providing an efficient and convenient means for the identification of fiber Bragg gratings with low polarization-dependent loss.

Description

Technical Field

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

[0002] Fiber gratings are very important passive devices in the field of fiber optic sensing. As sensors, they sense changes in the external environment by monitoring the shift of the center wavelength of the reflected light, and have the advantages of small size, light weight, high sensitivity, good corrosion resistance, strong anti-electromagnetic interference ability, and support for distributed monitoring. They have 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 gratings. It is a physical phenomenon that describes the change in the intensity of optical signals due to different polarization states of light in optical devices or optical systems, and is mainly used to measure the sensitivity of optical devices to the polarization state of transmitted light. For fiber grating devices, when the polarization state of the input light changes, lights with different polarization states will exhibit different wavelength shifts. When the polarization-dependent loss is too large, it will affect the stability of the reflection spectrum of the fiber grating, resulting in obvious errors in the measurement results, thus leading to a decline in the sensing performance of the fiber grating. Therefore, how to quickly identify low-PDL fiber grating devices is of great significance for the production of high-quality fiber grating devices.

[0004] Due to its unique advantages, femtosecond laser direct writing technology has received increasing attention in the field of fiber grating processing. Compared with traditional fiber grating processing methods (such as ultraviolet exposure method), femtosecond laser direct writing technology has shown significant advantages in terms of process flexibility, writing accuracy, processing efficiency, etc. Since the refractive index modulation range of femtosecond laser processing is relatively small, usually localized at certain specific positions in the fiber core and it is difficult to achieve full-core modulation, it is necessary to consider the geometric characteristics of the processed grating structure. When there is a large ellipticity in the refractive index modulation cross-section, it will cause a large polarization-dependent loss, making the fiber grating extremely sensitive to incident light with different polarization states, directly affecting the sensing accuracy. Especially in the dynamic sensing process, the polarization-dependent loss may mask the true environmental change signal, resulting in a large systematic error. Currently, there are problems such as high cost, complex operation, and long time consumption for the polarization analysis equipment for fiber gratings. Summary of the Invention

[0005] Aiming at the problems of complex operation and long time consumption in detecting the polarization dependence of fiber Bragg gratings by existing polarization analysis devices, the purpose of the present invention is to provide a device and a detection method for detecting the polarization dependence of femtosecond laser processed fiber Bragg gratings based on scattered light. The detection device can be embedded in the preparation process of fiber Bragg gratings, 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:

[0007] A device for detecting the polarization dependence of fiber Bragg gratings based on scattered light provided by the present invention includes: an incident laser light source, a scattering plane, a fiber rotation fixture, and an information extraction tool;

[0008] The incident laser light source is connected to one end of the fiber on which the fiber Bragg grating device is inscribed, so that the incident laser is coupled into the fiber and transmitted in the fiber;

[0009] The scattering plane is used to receive the light beam radiated from the fiber Bragg grating region in the fiber to form a scattering characteristic profile, and the fiber vertically passes through the scattering plane;

[0010] There are two fiber rotation fixtures, which are respectively located on both sides of the scattering plane, and are used to clamp the fiber and synchronously rotate the fiber axially;

[0011] The information extraction tool is used to extract the scattered light intensity information of the measurement points on the scattering plane, and the measurement points are selected in the scattering characteristic profile;

[0012] By rotating the fiber to obtain multiple scattered light intensity information data of the measurement points, calculating the discrete degree of the scattered light intensity, and according to the linear relationship between the discrete degree of the scattered light intensity and the central wavelength offset, the polarization dependence result of the fiber Bragg grating device is obtained.

[0013] Furthermore, an adapter ferrule through which the fiber can pass is provided on the scattering plane.

[0014] Furthermore, angle scales are marked on the fiber rotation fixture.

[0015] Furthermore, the information extraction tool is selected from a combination device of a photodiode power probe and a power meter, a combination device of a photodetector and an oscilloscope, a CMOS image sensor, and a CCD image sensor.

[0016] The method for detecting the polarization dependence of fiber Bragg gratings by using the above device provided by the present invention includes the following steps:

[0017] 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;

[0018] S2. Use the information extraction tool to read the scattered light intensity information of the measurement points in the scattered feature profile on the scattering plane. Rotate the optical fiber at a fixed angle, read the scattered light intensity information of the measurement points after each rotation, obtain multiple data, and calculate the discrete degree of the scattered light intensity;

[0019] S3. Repeat steps S1 and S2, read and calculate the discrete degree of the scattered light intensity of multiple calibrated fiber Bragg grating devices, and measure the central wavelength offset of the calibrated fiber Bragg grating devices. Perform a linear fit on the central wavelength offset and the discrete degree of the scattered light intensity to obtain the fitting curve equation;

[0020] S4. Repeat steps S1 and S2, read and calculate the discrete degree of the scattered light intensity of the fiber Bragg grating device to be measured, substitute it into the fitting curve equation obtained in step S3, and calculate the central wavelength offset. Determine whether the central wavelength offset exceeds the threshold. If it does not exceed the threshold, it is determined as a low polarization-dependent fiber Bragg grating device.

[0021] Further, in step S2, obtaining multiple data means obtaining more than 6 data.

[0022] Further, in step S2, the index of the discrete degree is the standard deviation or variance.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] The present invention provides a simple and intuitive device and method for detecting the polarization dependence of fiber Bragg grating devices. By obtaining the characteristic profile information of the scattered light, calculating the discrete degree of the scattered light intensity, and based on the linear relationship between the discrete degree of the scattered light intensity and the central wavelength offset, the polarization dependence result of the fiber Bragg grating device is obtained, realizing the rapid detection of the polarization dependence of fiber Bragg grating devices processed by femtosecond lasers. The detection method of the present invention is based on the characteristic profile of the scattered light distribution, and can quickly identify the polarization-dependent characteristics caused by the ellipticity of the modulation cross-section in the grating region, providing an efficient and convenient means for the identification of low polarization-related loss fiber Bragg gratings. Compared with traditional methods such as using a polarization-dependent loss measuring instrument or a high-precision polarization analyzer, the method of the present invention does not require complex experimental equipment or operation procedures, and can accurately judge the polarization characteristics of fiber Bragg gratings only through a simple detection device 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 fiber Bragg gratings, providing a guarantee for the production of low polarization-dependent fiber Bragg grating devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of a fiber Bragg grating polarization dependence detection device;

[0026] Figure 2 Schematic diagram of the scattering plane structure;

[0027] Figure 3 Schematic diagram for extracting the scattered light intensity information on the scattering feature profile;

[0028] Figure 4 Schematic diagram for testing the central wavelength offset of the fiber grating device;

[0029] Figure 5 Fitting curve equation of Example 1;

[0030] Figure 6 Scattering feature profiles of device A and device B;

[0031] Figure 7 Test results of the polarization-dependent loss of device A and device B. Detailed implementation manners

[0032] 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 following further details the specific implementation manners, structures, features and their effects of the present invention in conjunction with the accompanying drawings and preferred embodiments. The examples given 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 all conventional means well-known to those skilled in the art.

[0033] A device for detecting the polarization dependence of fiber gratings based on scattered light provided by an embodiment of the present invention, as Figure 1 shown, includes: an incident laser light source 1, a scattering plane 2, a fiber rotation fixture 3, and an information extraction tool 4.

[0034] One end of the incident laser light source 1 is connected to 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.

[0035] A fiber grating device 6 processed by femtosecond laser is inscribed in the optical fiber 5. The inscription position of the fiber grating device 6 can be the core of the optical fiber 5 or the cladding of the optical fiber 5; it can be a fiber grating processed at the center of the core or a fiber grating processed offset from the center of the core.

[0036] Due to the strong radiation characteristics of the fiber grating processed by femtosecond laser, when the laser is incident on the fiber grating region, 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.

[0037] 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 Figure 2As shown Figure 2 In (a) is the front view and (b) is the side view. There is a round hole 201 for the optical fiber to pass through in the middle of the scattering plane 2. An optical ferrule 202 is arranged in the round 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 angular deviation. There is also a fixture 203 for fixing the scattering plane 2 at the lower end of the scattering plane 2 to ensure that the scattering plane 2 is perpendicular to the horizontal plane. A adjustable fixing bracket 204 is also connected below the fixture 203 to adjust the height and horizontal placement position of the scattering plane 2 to ensure that the optical fiber can pass through the optical ferrule horizontally.

[0038] There are two optical fiber rotation fixtures 3, which are respectively located on both sides of the scattering plane 2, used to clamp the optical fiber 5 and can synchronously rotate the optical fiber 5 axially to ensure that the optical fiber 5 is in a straight state and there is no stress influence in the grating area. Angle scales are marked on the optical fiber rotation fixtures 3, which can ensure that the two optical fiber rotation fixtures 3 rotate the same angle synchronously each time.

[0039] The information extraction tool 4 is used to extract the scattered light intensity information of the measurement points on the scattering plane 2, and the measurement points are selected on the scattering characteristic profile. The information extraction tool 4 can be selected from a combined device of a photodiode power probe and a power meter or a combined device of a photodetector and an oscilloscope, or can also be selected from a CMOS image sensor or a CCD image sensor.

[0040] Figure 3 It is a schematic diagram of using a combined device of a photodiode power probe 401 and a power meter 402 to extract the scattered light intensity information on the scattering characteristic profile. The photodiode power probe 401 is installed on an adjustable bracket 403. After determining the measurement points on the scattering plane 2, the position is fixed. The photodiode power probe 401 does not need to be absolutely perpendicular to the scattering plane 2 and there can be a certain angle tolerance range.

[0041] A method for detecting the polarization dependence of an optical fiber grating by using the detection device of the embodiment of the present invention includes the following steps:

[0042] Step 1: Start the incident laser light source 1 to input incident laser light into the optical fiber grating device 6, so that the light beam radiated from the optical fiber grating area forms a scattering characteristic profile on the scattering plane 2.

[0043] Step 2: Use the information extraction tool 4 to read the scattered light intensity information of the measurement points in the scattering characteristic profile on the scattering plane 2, denoted as P 1; then rotate the optical fiber at an angle θ and read the scattered light intensity information of the measurement points after rotation, denoted as P 2; repeat the above operation, rotate the optical fiber at a fixed angle θ to obtain n data and calculate the discrete degree of the scattered light intensity.

[0044] The index of the degree of dispersion is the standard deviation or variance. The more measurement point data is extracted, the more accurate the calculated result is. Therefore, n should be greater than 6. θ = 2π / n.

[0045] Step 3: Take multiple (at least 5) calibrated fiber Bragg grating devices, repeat Step 1 and Step 2, read and calculate the degree of dispersion of the scattered light intensity of the calibrated fiber Bragg grating devices; and measure the central wavelength shift of the calibrated fiber Bragg grating devices; perform a linear fit on the central wavelength shift and the degree of dispersion of the scattered light intensity to establish a linear relationship between the central wavelength shift and the degree of dispersion of the scattered light, and obtain the fitting curve equation.

[0046] The central wavelength shift is mainly used to characterize the polarization dependence of the fiber Bragg grating device. Its testing method is as Figure 4 shown. The testing device includes a linearly polarized light source 7, a circulator 8, and a spectrometer 9. First, the linearly polarized light source 7 outputs light with a specific linear polarization, and then it reaches the fiber Bragg grating device 6 through the circulator 8. The reflected light of the device to be measured passes through the circulator 8 again and finally is transmitted to the spectrometer 9. Two states of the fiber Bragg grating device are mainly tested, namely the stable state and the non-stable state.

[0047] Step 4: Repeat Step 1 and Step 2, read and calculate the degree of dispersion of the scattered light intensity of the fiber Bragg grating device to be measured, substitute it into the fitting curve equation obtained in Step 3, and calculate the central wavelength shift; determine whether the central wavelength shift exceeds the threshold. If it does not exceed the threshold, it is determined as a fiber Bragg grating device with low polarization dependence.

[0048] Generally, when the central wavelength shift of the fiber Bragg grating device caused by the polarization state of the incident light is within 3 pm, it is considered to have good sensing performance. Therefore, the degree of dispersion of the scattered light corresponding to the 3 pm central wavelength shift can be determined as the effective threshold to judge whether the polarization dependence of the processed device exceeds the allowable range.

[0049] The following is a specific example of detecting the polarization dependence of the fiber Bragg grating using the detection device of the present invention.

[0050] Example 1

[0051] Take 5 calibrated fiber Bragg grating devices with the central wavelength shift already measured, 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 scattered light characteristic profile on the scattering plane 2, select the measurement points in the scattered light characteristic profile, fix the photodiode power probe, and read the scattered light intensity of the measurement points P 1; then control the fiber rotation fixture 3 to drive the fiber to rotate axially by 45°, and read the scattered light intensity of the measurement points again P2; Repeat the above operation, rotate at a rotation angle of 45° for 7 times, and read the scattered light intensity at the measurement points 8 times; calculate the standard deviation of the 8 measured scattered light intensity data σ ; Perform a linear fit on the central wavelength offset and the standard deviation of the scattered light intensity, establish a linear relationship between the central wavelength offset and the standard deviation of the scattered light intensity, and obtain the fitting curve equation, as Figure 5 shown, y = 0.8037x - 2.4963, R 2 = 0.9873.

[0052] Figure 6 are the scattering characteristic profiles of device A and device B. By reading and calculating, 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. Substitute them into the fitting curve equation, and 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 are the results of the polarization-dependent loss test of device A and device B using the Figure 4 device. In the non-steady state, the central wavelength offset caused by polarization of device A is 16 pm, and the central wavelength offset caused by polarization of device B is 3 pm. It can be seen that the detection method of the present invention has high accuracy.

[0053] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments by using the disclosed technical content within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence 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 on 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 on 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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