A large core polarization maintaining optical fiber extinction ratio testing device

Abstract

The application discloses a polarization extinction ratio testing device for large-core polarization maintaining optical fiber, and is characterized by comprising the following parts: a non-coherent polarized light source, a polarization maintaining mode field adapter, a focusing coupling system, a cladding light filter, a collimating mirror, a polarimeter and a power meter; the non-coherent polarized light source outputs high-quality non-coherent polarization maintaining linear polarized light, the light enters the large-core polarization maintaining optical fiber to be tested through the polarization maintaining mode field adapter and the focusing coupling system, and is emitted under the action of the cladding light filter, the energy of the output parallel light of the polarization maintaining fiber core is concentrated through the collimator, and the light enters the power meter through the polarimeter. The application uses a non-coherent light source as a testing light source, thereby reducing the coupling between modes, simultaneously uses a mode field adapter to control the mode of the completely polarized signal light to be a base mode state input, simultaneously matches the mode field, reduces the component of high-order modes, thereby cooperatively reduces the coherent superposition between different modes, thereby stabilizes the power test results under different polarization states, makes the extinction ratio test results stable and accurate, and has high consistency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical fiber testing, in particular to a polarization extinction ratio testing device for large-core polarization maintaining optical fiber. BACKGROUND

[0002] Fiber lasers have the advantages of high conversion efficiency, good beam quality, small size, stability and reliability, and have been developing steadily since the 1980s. As an important branch of lasers, ultra-short pulse fiber lasers have developed rapidly in the field of fine processing due to their high average power and single pulse energy. Compared with non-polarization maintaining optical fiber, all-fiber amplification system based on all-polarization maintaining optical fiber devices has better polarization stability and higher single pulse energy, and is less affected by external interference. Combined with the small size and high beam quality of fiber lasers, polarization maintaining optical fiber for industrial laser also has a wide application prospect.

[0003] Polarization extinction ratio describes the ability of polarization maintaining optical fiber to maintain the polarization state of signal light transmitted in the core, and is one of the key indicators for measuring the performance of polarization maintaining optical fiber. Most of the current polarization maintaining optical fiber testing systems are for polarization maintaining optical fiber used in fiber gyroscopes. This type of polarization maintaining optical fiber has a small size and a simple cladding structure. However, optical fibers used in the industrial laser field are usually double-clad or multi-clad fibers with rich core-cladding sizes and complex transmission modes. There are currently few polarization extinction ratio testing devices for large-core multi-clad polarization maintaining optical fiber.

[0004] Chinese patent document CN111830632A uses an extinction ratio detection system to ensure the axial fusion of large-core polarization maintaining optical fiber, and determines the extinction ratio by measuring the power of linearly polarized light after passing through the polarization maintaining optical fiber. The extinction ratio testing principle is to use optical power to calculate the optical power of the fast and slow axes. The device is relatively simple. However, in practice, it is found that the optical power reading is unstable during testing, and the data fluctuates back and forth, which cannot stably display the positions of the fast and slow axes. SUMMARY

[0005] In view of the above defects or improvement needs of the prior art, the present application provides a polarization extinction ratio testing device for large-core polarization maintaining optical fiber, which aims to reduce the coherence and high-order mode power components of signal light by using a non-coherent light source and a mode field adapter, and to reduce the superposition of mode components, thereby stabilizing the power under different polarization states, reducing and eliminating the fluctuation of extinction ratio test results, thereby solving the technical problems of large-core polarization maintaining optical fiber extinction ratio test devices such as large reading fluctuation, instability and poor accuracy.

[0006] To achieve the above-mentioned purpose, according to one aspect of the present application, a polarization extinction ratio testing device for large-core polarization maintaining optical fiber is provided, which comprises: a non-coherent polarized light source, a polarization maintaining mode field adapter, a focusing coupling system, a cladding light filter, a collimating mirror, a polariscope, and a power meter.

[0007] The incoherent polarized light source outputs high-quality incoherent polarization maintaining linear polarized light, which enters the large-core polarization maintaining optical fiber to be measured through a polarization maintaining mode field adapter and a focusing coupling system, and exits under the action of a cladding light filter, and the energy of the output light is concentrated in the fiber core through a collimator, and then enters a power meter through a polarizer.

[0008] Preferably, in the polarization extinction ratio testing device for the large-core polarization maintaining optical fiber, the output light signal of the incoherent polarized light source is preferably completely polarized ASE light with constant power.

[0009] Preferably, in the polarization extinction ratio testing device for the large-core polarization maintaining optical fiber, the wavelength of the incoherent polarized light source matches the operating wavelength of the large-core polarization maintaining optical fiber to be measured.

[0010] Preferably, in the polarization extinction ratio testing device for the large-core polarization maintaining optical fiber, when the optical fiber to be measured is a ytterbium-doped and related passive matching optical fiber, an ASE light source with a center wavelength of 1060 nm is selected.

[0011] When the optical fiber to be measured is an erbium-doped and related passive matching optical fiber, an ASE light source with a center wavelength of 1550 nm is selected.

[0012] When the optical fiber to be measured is a thulium-doped and related passive matching optical fiber, an ASE light source with a center wavelength of 1920 nm is selected.

[0013] Preferably, in the polarization extinction ratio testing device for the large-core polarization maintaining optical fiber, the mode field adapter is a polarization maintaining mode field adapter, which connects the light source and the large-core polarization maintaining optical fiber to be measured with a mismatched mode field and fiber size, and the output end tail fiber is a single-clad polarization maintaining optical fiber with a core / clad size consistent with that of the large-core polarization maintaining optical fiber to be measured.

[0014] Preferably, in the polarization extinction ratio testing device for the large-core polarization maintaining optical fiber, the tail fiber of the mode field adapter is in a coiled state.

[0015] Preferably, in the polarization extinction ratio testing device for the large-core polarization maintaining optical fiber, the number of turns and the diameter of the coiled tail fiber of the mode field adapter are adjustable.

[0016] Preferably, in the polarization extinction ratio testing device for the large-core polarization maintaining optical fiber, a light splitting and collimating system and a CCD camera are included; after passing through the polarization maintaining mode field adapter, the light is split into two beams of polarization maintaining parallel light through the light splitting and collimating system, one beam of light enters the CCD camera, and the other beam of light enters the large-core polarization maintaining optical fiber to be measured through a focusing coupling system.

[0017] The mode state of the output light source is controlled by adjusting the coiled state of the single-clad polarization-maintaining tail fiber behind the mode field adapter, and the CCD camera observes the current polarization-maintaining linear light mode state, so as to ensure that the light source state entering the large-core polarization-maintaining optical fiber is the base mode transmission.

[0018] Preferably, the polarization extinction ratio testing device of the large-core polarization-maintaining optical fiber, the cladding light filter is a liquid mold with a higher refractive index than the cladding of the optical fiber, and the bare fiber after stripping the coating of the optical fiber is immersed in the high-refractive-index liquid mold after being roughened.

[0019] Preferably, the polarization extinction ratio testing device of the large-core polarization-maintaining optical fiber, the light collimation system is a composite optical system composed of a double-cemented lens and a dichroic mirror, the double-cemented lens can convert the divergent light at the focal point into collimated parallel light, and the dichroic mirror is a semi-transparent mirror, which divides the parallel light into two paths of transmission and reflection, and the optical performance of the two paths of light is consistent with that of the collimated parallel light after the double-cemented lens, and the power is equally divided into the two paths of transmission and reflection.

[0020] The focusing coupling system is used for clamping the large-core polarization-maintaining optical fiber to be tested, and includes a focusing lens and a five-dimensional adjustment clamp, the focusing lens has a numerical aperture less than 0.3 and a focal length less than 75 mm.

[0021] The collimator is an aspheric lens, and the polarization-maintaining signal light output from the cladding light filter is output as collimated parallel light after passing through the aspheric lens at the focal point of the lens, and the focal length of the lens is not greater than 75 mm.

[0022] The polarization analyzer is composed of a high-precision rotary motor and a polarizing plate, and the polarizing plate is placed on the rotary motor and can be precisely rotated by 0.5° per step.

[0023] Overall, compared with the prior art, the above technical solutions conceived by the present application can achieve the following beneficial effects:

[0024] The present application uses incoherent light as the test light source, thereby reducing the coupling between modes, and uses a mode field adapter to control the mode of the completely polarized signal light to be the base mode state input, and simultaneously matches the mode field to reduce the components of high-order modes, thereby cooperatively reducing the coherent superposition between different modes, thereby stabilizing the power test results under different polarization states, making the extinction ratio test results stable and accurate, and having high consistency.

[0025] The preferred scheme is that a CCD camera is used to observe the state of the test light source, and the light is split into two beams of polarization maintaining parallel light by a collimating light splitting system after passing through a polarization maintaining mode field adapter, one of the beams enters the camera, and the other beam of polarization maintaining parallel light whose energy is concentrated in the core after passing through a collimator enters a power meter after passing through a polarizer. The mode state of the output light source is controlled by adjusting the coiled state of the single-clad polarization maintaining fiber after the mode field adapter, the CCD camera is used to observe the current polarization maintaining linear polarization mode state, the state of the light entering the large core polarization maintaining fiber to be tested is ensured, and thus the test result is accurate and reliable.

[0026] The polarization extinction ratio testing device of the application is suitable for polarization extinction ratio testing of polarization maintaining fibers with different cladding structures, different stress region designs and different sizes, and meets the technical conditions required for polarization extinction ratio testing of large core polarization maintaining fibers. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is a schematic diagram of the optical path structure of the polarization extinction ratio testing device of the large core polarization maintaining fiber. In all the drawings, the same reference signs are used to represent the same elements or structures, wherein:

[0028] 1 is a polarized light source, 2 is a polarization maintaining mode field adapter, 3 is a light splitting collimating system, 4 is a focusing coupling system, 5 is a large core polarization maintaining fiber to be tested, 6 is a cladding light filter, 7 is a collimator, 8 is a polarizer, 9 is a power meter, and 10 is a CCD camera. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical scheme and advantages of the application more clear, the application is further described in detail below with examples. It should be understood that the specific examples described herein are only used to explain the application and not to limit the application. In addition, the technical features involved in each embodiment of the application described below can be combined with each other as long as they do not conflict with each other.

[0030] The large core polarization maintaining fiber to be tested generally has a core size of 5-50 μm and a cladding size of 80-600 μm. Some of the cladding structures are multi-cladding structures, the optical fiber generates birefringence by increasing a plurality of stress regions, and maintains the polarization characteristics of the transmitted signal light. Repeated experiments show that, due to the large core diameter of the large core polarization maintaining fiber to be tested, the optical transmission mode is generally non-single mode operation, that is, there is high-order mode transmission, at this time, the components between different modes will be coherently superimposed, and then the power under different polarization states will change obviously, which is the main factor causing the up and down fluctuation of the extinction ratio test result. At the same time, the cladding light also interferes with the extinction ratio test result.

[0031] The application provides a large-core polarization maintaining optical fiber extinction ratio testing device, comprising: a non-coherent polarized light source, a polarization maintaining mode field adapter, a light splitting collimation system, a focusing coupling system, a cladding light filter, a collimator, a polarimeter, a power meter and a CCD camera.

[0032] The non-coherent polarized light source outputs high-quality non-coherent polarization maintaining linearly polarized light, which is split into two beams of polarization maintaining parallel light by the polarization maintaining mode field adapter and the light splitting collimation system, one of which enters the CCD camera; the other enters the large-core polarization maintaining optical fiber to be tested through the focusing coupling system, and exits under the action of the cladding light filter, and the energy output by the collimator is concentrated in the polarization maintaining parallel light in the core, which enters the power meter after the polarimeter.

[0033] The non-coherent polarized light source preferably outputs constant-power fully polarized ASE light. Generally, the signal light used for testing is coherent light, and the ASE light source is used as the testing light source. Since the ASE light source is non-coherent light, the testing fluctuation caused by interference superposition can be improved. In a preferred embodiment, the wavelength of the fully polarized ASE light matches the operating wavelength of the large-core polarization maintaining optical fiber to be tested: when the optical fiber to be tested is a ytterbium-doped and related passive matching optical fiber, an ASE light source with a center wavelength of 1060 nm can be selected; when the optical fiber to be tested is an erbium-doped and related passive matching optical fiber, an ASE light source with a center wavelength of 1550 nm can be selected; and when the optical fiber to be tested is a thulium-doped and related passive matching optical fiber, an ASE light source with a center wavelength of 1920 nm can be selected. The output power is constant, which further ensures the data accuracy of the testing process; and the extinction ratio at the operating wavelength can be more accurately measured to obtain the performance of the large-core polarization maintaining optical fiber in the working state.

[0034] The mode field adapter is a polarization maintaining mode field adapter, which connects the light source and the large-core polarization maintaining optical fiber to be tested with mismatched mode fields and fiber sizes, and the output end tail fiber is a single-clad polarization maintaining optical fiber with a core / cladding size consistent with that of the large-core polarization maintaining optical fiber to be tested, so as to ensure that the polarization maintaining signal light after the mode field adapter is transmitted in the core; the use of the mode field adapter reduces the excitation of high-order modes, better maintains the components of the fundamental mode, thereby reducing the coherent superposition of components between modes, and finally helps to stabilize the power measurement under the polarization state. In a preferred embodiment, the tail fiber of the mode field adapter is in a coiled state, and the number of turns and the diameter of the coil can be adjusted. The mode state of the output light source is controlled by adjusting the coiled state of the single-clad polarization maintaining tail fiber after the mode field adapter, the CCD camera observes the current polarization maintaining linearly polarized mode state, and the light source state entering the large-core polarization maintaining optical fiber to be tested is ensured to be a fundamental mode transmission.

[0035] The light collimation system is a composite optical system composed of a doublet lens and a dichroic mirror. The doublet lens can convert the divergent light at the focal point into collimated parallel light. The dichroic mirror is a semi-transparent and semi-reflective mirror, which divides the parallel light into two paths of transmission and reflection. The two paths of light have the same optical performance as the collimated parallel light after passing through the doublet lens, and the power is evenly distributed to the two paths of transmission and reflection. The focal length of the doublet lens is not more than 75 mm, and the specific focal length range can be freely changed according to the fiber core diameter and numerical aperture, and it is appropriate to fall within the center of the detection device.

[0036] The focusing coupling system is used to clamp the large-core polarization maintaining optical fiber to be measured, and includes a focusing lens and a five-dimensional adjustment clamp. The numerical aperture of the focusing lens is less than 0.3, and the focal length is less than 75 mm. The five-dimensional adjustment clamp precisely controls the position of the optical fiber through five-dimensional angles to realize complete coupling of the signal light. The fixed optical fiber can rotate 360° along the axial direction on the adjustment frame. The focusing lens and the five-dimensional adjustment clamp precisely adjust the spatial coupling precision of the large-core polarization maintaining optical fiber to be measured and the tail fiber of the mode field adapter, so as to ensure that the signal light transmission state is consistent with the mode field adapter as much as possible and is in the fundamental mode state.

[0037] The cladding light filter is a liquid mold with a higher refractive index than the cladding of the optical fiber. After the bare fiber stripped of the optical fiber coating is subjected to roughening treatment and immersed in the high-refractive-index liquid mold, the cladding light outside the fiber core can be completely stripped.

[0038] The collimator is an aspheric mirror. The polarization maintaining signal light output from the cladding light filter passes through the aspheric mirror at the focal point of the lens to output collimated parallel light. The focal length of the lens is not greater than 75 mm, and the specific focal length is determined according to the fiber core size and numerical aperture, so as to ensure that the spot size of the output collimated parallel light can be completely received by the detection surface of the rear-end power meter.

[0039] The polarization analyzer is composed of a high-precision rotary motor and a polarizing plate. The polarizing plate is placed on the rotary motor and can be precisely rotated by 0.5° for 360°.

[0040] An incoherent light source is used as the test light source to reduce the coupling between modes. At the same time, the mode field adapter is used to control the mode of the completely polarized signal light to be the fundamental mode state, and the mode field is matched to reduce the component of high-order modes, thereby cooperatively reducing the coherent superposition between different modes, stabilizing the power test results under different polarization states, and making the extinction ratio test results stable, accurate and highly consistent.

[0041] The following is an embodiment:

[0042] Figure 1The diagram shows the optical path structure of a large-core polarization-maintaining fiber extinction ratio testing device according to the present invention. 1 is a polarization source, 2 is a polarization-maintaining mode field adapter, 3 is a beam splitting and collimating system, 4 is a focusing and coupling system, 5 is the large-core polarization-maintaining fiber under test, 6 is a cladding light filter, 7 is a collimator, 8 is a polarization analyzer, 9 is a power meter, and 10 is a CCD camera.

[0043] Polarization source 1 generates a constant-power, fully polarized, incoherent ASE light. The output fully polarized polarization-maintaining signal light, after passing through polarization-maintaining mode field adapter 2, has its signal power and polarization state well coupled into a single-clad polarization-maintaining fiber with core / cladding dimensions consistent with the large-core diameter polarization-maintaining fiber under test. The fully polarized core signal light output after passing through polarization-maintaining mode field adapter 2 is then output as two equally polarized parallel lights with unchanged polarization states by beam splitting and collimation system 3. One of these signals enters CCD camera 10 to observe the mode state of the current output polarized signal light. The mode of the fully polarized signal light is controlled by adjusting the bending state of the single-clad polarization-maintaining fiber output by polarization-maintaining mode field adapter 2. When the spot shape in CCD camera 10 is stable, the energy is concentrated in the fiber core, and exhibits a Gaussian or near-Gaussian distribution, the current polarized signal light is considered to be a single-mode or quasi-single-mode output, with a stable mode state and no obvious inter-mode interference. Another signal light enters the focusing coupling system 4. Through precise coupling by the five-dimensional adjustment frame, the large-core polarization-maintaining fiber 5 under test is held stress-free, allowing the parallel output polarized signal light to be fully coupled into the large-core polarization-maintaining fiber 5. By rotating the fiber axially by 360°, the fully polarized polarization-maintaining signal light is made to be coaxially and symmetrically distributed with the stress region in the large-core polarization-maintaining fiber 5, ensuring the uniformity of the inherent stress direction in the entire optical path and better maintaining the transmission polarization state of the signal light. The output end of the large-core polarization-maintaining fiber 5 is placed in the cladding optical filter 6 to strip away all energy outside the fiber core. The signal light, which is transmitted entirely in the fiber core, passes through the collimator 7 and then parallel to the analyzer 8. A high-precision rotary motor rotates the polarizer 360°, obtaining four states where the polarizer's optical axis coincides with the two principal axes of the polarized signal light. The polarized signal light after passing through the analyzer 8 reaches the photosensitive detection surface of the power meter 9, obtaining the output optical power at different rotation angles. The maximum value P of the output power is then taken. max and P min The extinction ratio of the large-core polarization-maintaining fiber under test is obtained by using the formula PER = 10log(Pmax / Pmin).

[0044] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A polarization extinction ratio testing apparatus for a large core polarization maintaining optical fiber, characterized by, It comprises: Incoherent polarized light source, polarization mode field adapter, focusing coupling system, cladding light filter, collimator, polarimeter, power meter, light splitting collimation system and CCD camera; The incoherent polarized light source outputs high-quality incoherent polarization linearly polarized light, which enters the large-core polarization optical fiber to be measured through the polarization mode field adapter and the focusing coupling system, and is emitted under the action of the cladding light filter, and the output energy is concentrated in the core of the polarization parallel light through the collimator, and enters the power meter after the polarimeter; The incoherent polarized light source outputs constant-power completely polarized ASE light; The mode field adapter is a polarization mode field adapter, which connects the light source with mismatched mode field and fiber size and the large-core polarization optical fiber to be measured, and the output end tail fiber is a single-cladding polarization optical fiber with the same core / cladding size as the large-core polarization optical fiber to be measured; After passing through the polarization mode field adapter, the light splitting collimation system splits the light into two beams of polarization parallel light, one of which enters the CCD camera, and the other of which enters the large-core polarization optical fiber to be measured through the focusing coupling system; The mode state of the output light source is controlled by adjusting the coiled state of the single-cladding polarization tail fiber after the polarization mode field adapter, and the CCD camera observes the current polarization linearly polarized light mode state to ensure that the light source state entering the large-core polarization optical fiber to be measured is the fundamental mode transmission; The cladding light filter is a liquid mold with a higher refractive index than the cladding of the optical fiber, and the bare fiber stripped of the fiber coating is immersed in the high-refractive-index liquid mold after being roughened.

2. The polarization extinction ratio testing apparatus for large core polarization maintaining optical fiber according to claim 1, wherein, The wavelength of the incoherent polarized light source matches the operating wavelength of the large-core polarization optical fiber to be measured.

3. The polarization extinction ratio testing apparatus for large core polarization maintaining optical fiber as claimed in claim 1, wherein When the optical fiber to be measured is a ytterbium-doped and related passive matching optical fiber, an ASE light source with a center wavelength of 1060 nm is selected; When the optical fiber to be measured is an erbium-doped and related passive matching optical fiber, an ASE light source with a center wavelength of 1550 nm is selected; When the optical fiber to be measured is a thulium-doped and related passive matching optical fiber, an ASE light source with a center wavelength of 1920 nm is selected.

4. The polarization extinction ratio testing apparatus for large core polarization maintaining optical fiber as claimed in claim 1, wherein The tail fiber of the mode field adapter is in a coiled state.

5. The polarization extinction ratio testing apparatus for large core polarization maintaining optical fiber as claimed in claim 1, wherein The number of turns and the diameter of the tail fiber of the mode field adapter can be adjusted.

6. The polarization extinction ratio testing apparatus for large core polarization maintaining optical fiber as claimed in claim 1, wherein The light splitting collimation system is a composite optical system composed of a double-cemented lens and a dichroic mirror, the double-cemented lens converts the divergent light at the focal point into collimated parallel light, and the dichroic mirror is a semi-transparent semi-reflective mirror that divides the parallel light into transmitted and reflected light, the two paths of light have the same optical performance as the collimated parallel light after the double-cemented lens, and the power is equally divided into the transmitted and reflected light; The focusing coupling system is used to hold the large-core polarization optical fiber to be measured, and comprises a focusing lens and a five-dimensional adjustment clamp, the focusing lens has a numerical aperture less than 0.3 and a focal length less than 75 mm; The collimator is an aspheric mirror, the polarization signal light output from the cladding light filter passes through the aspheric mirror at the focal point of the lens to output collimated parallel light, and the focal length of the lens is not greater than 75 mm; The polarimeter is composed of a high-precision rotary motor and a polarizing plate, and the polarizing plate is placed on the rotary motor to rotate the polarizing plate by 0.5° in steps by 360°.

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