A method for testing the stimulated Brillouin scattering gain coefficient of a passive transmission optical fiber

Abstract

The present invention relates to the field of optical fiber testing. The technical problem to be solved is to provide a method for testing the stimulated Brillouin scattering gain coefficient of a special passive transmission optical fiber with a large core diameter. In the present invention, a light source stably outputs laser to the first port of a circulator with three ports. The second port of the circulator is fusion-connected to the optical fiber to be tested. The optical fibers at the other ports of the circulator all use passive optical fibers of the same type as the optical fiber to be tested. An optical power meter is used to measure the return optical power and the output optical power at the third port of the circulator and the output end of the optical fiber to be tested respectively. When the return optical power is equal to 1% of the input optical power of the injected light at the front end of the optical fiber to be tested, the input optical power of the injected light at the front end of the optical fiber to be tested at this time is the SBS threshold of the optical fiber to be tested. Then, according to the formula, the Brillouin gain coefficient of the optical fiber to be tested can be deduced. The testing method of the present invention is applicable to special passive transmission optical fibers with large core diameters, filling the blank.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber testing, and particularly to a method for testing the stimulated Brillouin scattering gain coefficient of special passive transmission optical fibers suitable for large core diameters. Background Art

[0002] Passive transmission optical fibers can be used as the output pigtails of lasers, the pigtails of high-power pump combiners, etc., and have the function of high-power laser transmission, and are widely used in the fields of weapon equipment and industrial processing. Although passive transmission optical fibers can transmit high power, serious nonlinear effects will occur at a certain usage length, mainly including stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and mode instability (TMI). These nonlinear effects greatly hinder the improvement of laser beam combining and the final high-power laser beam combining power. In high-energy laser transmission systems, SBS is the most obvious.

[0003] The Brillouin gain coefficient is a key test index of passive transmission optical fibers, which can intuitively reflect the SBS effect of optical fibers made by different processes. The smaller the gain coefficient, the weaker the SBS; the larger the gain coefficient, the stronger the SBS. In the prior art, the method of measuring the stimulated Brillouin scattering threshold is mostly used to indirectly calculate the Brillouin gain coefficient. The higher the threshold, the smaller the gain coefficient; the lower the threshold, the higher the gain coefficient.

[0004] The existing method for testing the stimulated Brillouin scattering threshold generally uses an ordinary high-power laser plus a coupler to form a test optical path. This test optical path is suitable for communication optical fibers with small core diameters. The core diameter of communication optical fibers with small core diameters is generally below 10 micrometers, and the transmission power is generally in the milliwatt range. Since the existing test optical path is difficult to achieve the high-power injected light required for testing the stimulated Brillouin scattering threshold of large-core passive optical fibers, and the use of a coupler will cause optical power loss due to optical path splitting, which will bring problems of inaccurate measurement, so it cannot be directly used for testing the Brillouin gain coefficient of large-core (50 micrometers), high-power (tens of watts) passive optical fibers.

[0005] Therefore, there is an urgent need to research and develop a method for testing the Brillouin gain coefficient of special passive transmission optical fibers suitable for large core diameters to fill the blank of the testing method for the Brillouin gain coefficient of large-core passive optical fibers in China. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the problems existing in the prior art and provide a method for testing the stimulated Brillouin scattering gain coefficient of passive transmission optical fibers, which can be used for testing the Brillouin gain coefficient of special passive transmission optical fibers with large core diameters.

[0007] To solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A method for testing the stimulated Brillouin scattering gain coefficient of a passive transmission optical fiber, characterized in that:

[0009] Use a light source 1 to stably output laser light to the first port of a circulator 3 with three ports. The second port of the circulator 3 is fusion-spliced to the optical fiber 4 to be measured. The optical fibers at other ports of the circulator 3 all use passive optical fibers of the same type as the optical fiber 4 to be measured. The output end of the optical fiber 4 to be measured outputs at an angle of 4° to 12°, which can ensure that the intensity of the end-face reflected light is the lowest when the optical fiber power is output; use an optical power meter to measure the return light power and the output light power at the third port of the circulator 3 and the output end of the optical fiber 4 to be measured respectively; when the return light power is equal to 1% of the input optical fiber power of the injected light at the front end of the optical fiber 4 to be measured, the input optical fiber power of the injected light at the front end of the optical fiber 4 to be measured at this time is the SBS threshold of the optical fiber 4 to be measured.

[0010] Furthermore, the Brillouin gain coefficient of the optical fiber 4 to be measured can be derived according to the following formula:

[0011]

[0012] In the formula, A eff is the effective area of the optical fiber to be measured, g B is the Brillouin gain coefficient that needs to be derived, L eff is the effective length of the optical fiber to be measured, P th is the measured SBS threshold of the optical fiber.

[0013] The above-mentioned test method obtains the input optical fiber power of the injected light at the front end of the optical fiber to be measured by the following steps:

[0014] When the optical fiber 4 to be measured is not connected, splice a small section of passive optical fiber of the same type as the optical fiber 4 to be measured at the second port of the circulator 3, and test the output power at the second port of the circulator 3 under different light source powers. This power is the input optical fiber power after connecting the optical fiber 4 to be measured, so as to obtain the corresponding data table of the light source power and the input optical fiber power.

[0015] Preferably, the light source 1 is connected to the first port of the circulator 3 through a mode matcher 2.

[0016] Preferably, the light source 1 used in the above-mentioned test method is a single-frequency laser.

[0017] Preferably, the output end of the optical fiber 4 to be measured outputs at an angle of 8°.

[0018] The present invention has the following beneficial effects compared with the prior art:

[0019] 1. The test method of the present invention is applicable to the test of the Brillouin gain coefficient of special passive transmission optical fibers with large core diameters, filling a blank.

[0020] 2. The light source adopted by the test method of the present invention is a single-frequency laser, which can significantly reduce the requirement for the output optical power of the light source, stimulate the Brillouin nonlinear effect of the optical fiber to be tested at a lower power, reduce the test difficulty, and improve the safety.

[0021] 3. The test method of the present invention adopts a Brillouin gain coefficient test optical path design based on the circulator coupling method. Compared with the traditional optical path design using a coupler, the circulator optical path design can avoid the optical power loss caused by optical path splitting, receive the weak backward scattered light to the greatest extent, and improve the test accuracy. Description of the Drawings

[0022] Figure 1 . The test optical path diagram adopted by a preferred embodiment of the present invention

[0023] Description of the reference numerals:

[0024] 1: Light source 2: Mode matcher 3: Circulator

[0025] 4: Optical fiber to be tested 5: Small-range optical power meter 6: Large-range optical power meter Detailed Embodiment

[0026] The present invention will be further described in detail below with reference to the drawings and embodiments.

[0027] The design idea of the present invention is to first measure the SBS threshold of the optical fiber by building an SBS threshold test system, and then calculate the Brillouin gain coefficient of the optical fiber according to the formula.

[0028] Referring to Figure 1 , the light source 1 is connected to the first port of the circulator 3 through the mode matcher 2, so as to reduce the mode instability phenomenon caused by the difference in size between the output pigtail of the light source 1 and the optical fiber 4 to be tested; the second port of the circulator 3 is fusion-connected to the input end of the optical fiber 4 to be tested; the small-range optical power meter 5 is used to measure the return optical power at the third port of the circulator 3, and the large-range optical power meter 6 is used to measure the output optical power at the output end of the optical fiber 4 to be tested; when the return optical power is equal to 1% of the input optical power of the injected light at the front end of the optical fiber 4 to be tested, the input optical power of the injected light at the front end of the optical fiber 4 to be tested at this time is the SBS threshold of the optical fiber 4 to be tested.

[0029] Because it is impossible to separately test the power of the injection arm in the optical path, it is impossible to directly measure the input fiber power of the injected light. Considering that: the optical fibers used in the optical path are all passive optical fibers with very small background losses; at the same time, the insertion losses of the mode matcher 2 and the circulator 3 are both within 0.8 dB. Although the loss of the entire optical path is not large, for this test system, the maximum power of the light source 1 is only 50 W. If the output power of the light source 1 is regarded as the input fiber power of the injected light to be measured, the measured threshold is larger than the actual value, and the calculated Brillouin gain coefficient is smaller. These losses have a huge impact on the error of the test results, which affects the correct test of the experimental indicators.

[0030] To eliminate the influence of these losses, when the fiber 4 to be measured is not connected, it is necessary to calibrate the output power of the light source and the input fiber power of the injected light to be measured to obtain a data correspondence table of the light source power and the input fiber power. When calibrating the input fiber power, a small section of passive fiber of the same type as the fiber 4 to be measured needs to be fused at the second port of the circulator 3, which can avoid the influence of the melting point loss, so as to obtain the input fiber power of the fiber 4 to be measured under different light source powers. This step does not need to be repeated in subsequent tests.

[0031] It can be seen from the Brillouin gain coefficient calculation formula that the size of the SBS threshold is related to the line width of the laser. The narrower the line width, the lower the SBS threshold, the easier it is to occur the SBS effect, and the shorter the test fiber required.

[0032] The calculation formula of the SBS threshold in the optical fiber is as follows:

[0033]

[0034] In the formula, A eff is the effective area of the optical fiber, g B is the Brillouin gain coefficient, L eff is the effective length, Δv p is the pump laser spectral width, Av B is the Brillouin gain spectral width. By measuring the SBS threshold, the Brillouin gain coefficient of the optical fiber can be obtained.

[0035] L eff can be expressed by the formula:

[0036]

[0037] Among them, α is the loss of the optical fiber, and L is the length of the optical fiber.

[0038] When the pump source line width is narrow enough and the injection power is high, the threshold calculation formula can be simplified to:

[0039]

[0040] Wherein, A eff is the effective area of the optical fiber to be measured, g B is the Brillouin gain coefficient to be derived, L eff is the effective length of the optical fiber to be measured, P th is the measured SBS threshold of the optical fiber.

[0041] Through the above simplified formula, the method of measuring the SBS threshold of the optical fiber and then calculating the SBS gain coefficient of the optical fiber is simple and effective, and is applicable to determining the SBS threshold of the transmission optical fiber in the experimental design. The effective length and effective area of the optical fiber can be calculated, and substituting their values into the SBS threshold calculation formula can inversely calculate the Brillouin gain coefficient of the optical fiber.

[0042] The specific embodiment is as follows:

[0043] The light source 1 is a 50W single-frequency laser at 1064nm. The 20 / 130 optical fiber is selected as the output pigtail of the laser and the input optical fiber of the mode matcher 2. The laser output by the laser passes through the mode matcher 2 of 20 / 130-50 / 400 and is fusion-connected to the first port of the circulator 3. The optical fibers of the three ports of the circulator 3 are all 50 / 400 passive optical fibers. The second port of the circulator 3 is fusion-connected to the optical fiber 4 to be measured, and the output end of the optical fiber to be measured outputs at an inclined 8° angle. The return light power and the output light power can be measured respectively by using an optical power meter at the third port of the circulator 3 and the output end of the optical fiber 4 to be measured.

[0044] The central wavelength of the laser is 1064nm, the working mode is continuous output mode, the maximum output power is 50W, the laser power stability is less than 2%, the spectral line width is less than 20kHz, and the specification of the output optical fiber is 20 / 130. The central wavelength of the mode matcher is 1064nm, the specification of the input optical fiber is 20 / 130, the specification of the output optical fiber is 50 / 400, the insertion loss is less than 0.8dB, and the power bearing capacity is greater than 100W. The central wavelength of the circulator is 1064nm, the specifications of the optical fibers of the three ports of the circulator are all 50 / 400, the isolation degree is greater than 30dB, the insertion loss is less than 0.8dB, and the power bearing capacity is greater than 100W.

[0045] The specific implementation steps mainly include:

[0046] (1) Turn on the light source and preheat for more than 10 minutes;

[0047] (2) Fusion-connect any end of the optical fiber 4 to be measured to the second port of the circulator 3;

[0048] (3) Gradually increase the power of the light source 1 and record the input optical power P 0,i (i = 1, 2, 3…);

[0049] (4) Record the reading P of the low-range power meter 5 1,i (i = 1, 2, 3…), which is the return optical power;

[0050] (5) When P 1,i ≥ 1% P 0,i (i = 1, 2, 3…), stop increasing the power of the light source 1, and record the input fiber power P 0,i at this time as the SBS threshold P th of the fiber to be measured.

[0051] (6) Calculate the Brillouin gain coefficient g of the fiber to be measured according to the threshold calculation formula B .

[0052] The implementation manners of the above-described embodiments are only used to illustrate the present invention, rather than limiting the present invention. Any person skilled in the art of this technology field can make various modifications, changes or substitutions without departing from the technical scope disclosed by the present invention. Therefore, all equivalent and similar technical methods should be covered within the patent protection scope of the present invention.

Claims

1. A method for testing the stimulated Brillouin scattering gain coefficient of a passive transmission optical fiber, characterized in that: A light source (1) is used to stably output laser light to the first port of a circulator (3) with three ports. The second port of the circulator (3) is fusion-connected to the optical fiber to be tested (4). The optical fibers at other ports of the circulator (3) all use passive optical fibers of the same type as the optical fiber to be tested (4). The output end of the optical fiber to be tested (4) outputs at an angle of 4° to 12°. A small-range optical power meter (5) is used to measure the return optical power at the third port of the circulator (3), and a large-range optical power meter (6) is used to measure the output optical power at the output end of the optical fiber to be tested (4). When the return optical power is equal to 1% of the input optical power of the injected light at the front end of the optical fiber to be tested, the input optical power of the injected light at the front end of the optical fiber to be tested (4) at this time is the SBS threshold of the optical fiber to be tested (4); Furthermore, the Brillouin gain coefficient of the optical fiber to be tested (4) can be deduced according to the following formula: Wherein, A eff is the effective area of the optical fiber to be measured, g B is the Brillouin gain coefficient to be derived, L eff is the effective length of the optical fiber to be measured, P th is the SBS threshold of the measured optical fiber; The following steps are adopted in the testing method to obtain the input optical power of the injected light at the front end of the optical fiber to be tested: When the optical fiber to be tested (4) is not connected, a small section of passive optical fiber of the same type as the optical fiber to be tested (4) is fusion-connected to the second port of the circulator (3). The output power of the second port of the circulator (3) under different light source powers is measured. This power is the input optical power after connecting the optical fiber to be tested (4), so that a corresponding data table of the light source power and the input optical power can be obtained.

2. The method for testing the stimulated Brillouin scattering gain coefficient of a passive transmission optical fiber according to claim 1, characterized in that: The light source (1) is connected to the first port of the circulator (3) through a mode matcher (2).

3. A method for testing the stimulated Brillouin scattering gain coefficient of a passive transmission optical fiber according to claim 2, characterized in that: The light source (1) adopted in the testing method is a single-frequency laser.

4. The method for testing the stimulated Brillouin scattering gain coefficient of a passive transmission optical fiber according to claim 3, characterized in that: The output end of the optical fiber to be tested (4) outputs at an angle of 8°.

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