Device and method for measuring the SBS gain coefficient of an optical fiber system

Abstract

A device and method for measuring the SBS gain coefficient of an optical fiber system are disclosed. The device includes a single-frequency fiber laser source, a phase modulation system, an optical fiber amplifier, a circulator, an optical fiber splitter, an optical fiber adjustable attenuator, a photodetector, an oscilloscope, the optical fiber system under test (ODT), and an isolator. This invention obtains the SBS build-up time constant under different injected laser powers by measuring the SBS transient response excited by the ODT at Port 3 of the optical fiber splitter and by fitting the SBS build-up process using an exponential function. Based on the relationship between the SBS build-up time constant and the laser power, the SBS gain coefficient of the ODT optical fiber system can be obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to an optical fiber system, in particular to a device and method for measuring SBS gain coefficient of an optical fiber system. BACKGROUND

[0002] In the field of narrow linewidth high power fiber lasers and fiber communication, the stimulated Brillouin scattering (hereinafter referred to as SBS) effect seriously limits the laser working power of the fiber system due to the formation of a large pulse in the backward direction. In order to suppress the SBS effect, researchers have proposed techniques such as phase modulation broadening, stress gradient and temperature gradient. These techniques broaden the SBS gain spectrum to reduce the SBS peak gain, thereby increasing the SBS threshold of the fiber system. When designing a narrow linewidth high power fiber laser or a fiber communication system, the SBS threshold of the fiber system needs to be investigated to give feedback for further optimization of the fiber system, and the SBS threshold is affected by the SBS gain coefficient of the fiber system. Although the fiber included in the fiber system usually gives a reference value of the SBS gain coefficient, the SBS gain coefficients of different fibers differ significantly due to different fiber doping, and the SBS gain coefficient of the overall fiber system composed of these fibers cannot be known. Therefore, one of the keys to accurately predicting the SBS threshold of the fiber system is to study the measurement method of the SBS gain coefficient of the fiber system. SUMMARY

[0003] To solve the above problems and fill the technical gap, the present application provides a device and method for measuring the SBS gain coefficient of a fiber system. The present application measures the SBS transient response excited by the fiber system to be measured, and simultaneously uses an exponential function to fit the SBS establishment process to obtain the SBS establishment time constant of the fiber system to be measured under different injected laser powers. According to the relationship between the SBS establishment time constant and the laser power, the measurement of the SBS gain coefficient of the fiber system to be measured can be realized.

[0004] The technical solution of the present application is as follows:

[0005] A device for measuring the SBS gain coefficient of a fiber system, characterized in that it comprises a single-frequency fiber laser light source, a phase modulation system, a fiber amplifier, a circulator, a fiber optical splitter, a fiber adjustable attenuator, a photodetector, an oscilloscope, a fiber system to be measured and an isolator.

[0006] The single-frequency fiber laser light source emits continuous laser light, and the phase modulation system, the fiber amplifier, the circulator, the fiber optical splitter, the fiber system to be measured and the isolator are arranged in the transmission direction of the laser light in sequence, and adjacent systems and devices are connected to each other by optical fibers.

[0007] The optical fiber splitter comprises four ports, namely Port1, Port2, Port3 and Port4, wherein the Port3 port is connected with the input end of the optical fiber adjustable attenuator, the output end of the optical fiber adjustable attenuator is connected with the input end of the photodetector, and the output end of the photodetector is connected with the input end of the oscilloscope through the radio frequency signal transmission line.

[0008] Another input channel of the oscilloscope is connected with the modulation signal output end of the phase modulation system through the radio frequency signal transmission line.

[0009] There is no signal delay between the modulation signal of the phase modulation system and the output signal of the photodetector, and the trigger of the oscilloscope is the rising edge of the output signal of the photodetector.

[0010] The circulator has the function of the mode field adapter, the mode field of the input end of the circulator is matched with the mode field of the optical fiber amplifier, the mode field of the output end of the circulator is matched with the mode field of the to-be-measured optical fiber system, and the mode fields of the isolator and the optical fiber splitter are matched with the to-be-measured optical fiber system.

[0011] The connection between adjacent devices and systems is realized by means of flanges or fusion.

[0012] The laser power injected into the to-be-measured optical fiber system is measured by measuring the output power of the Port2 port of the optical fiber splitter, and then the beam splitting ratio of the optical fiber splitter is obtained.

[0013] The method for measuring the SBS gain coefficient of the optical fiber system by using the above-mentioned device for measuring the SBS gain coefficient of the optical fiber system comprises the following steps:

[0014] 1) Turn on the single-frequency optical fiber laser source, adjust the output power of the optical fiber amplifier under the condition that the phase modulation system is not turned on, ensure that the power injected into the to-be-measured optical fiber system exceeds the SBS threshold value while the backward Stokes light power is lower than the damage threshold value of the system device, and record the optical power at this time at the Port2 port of the optical fiber splitter;

[0015] 2) Adjust the optical fiber adjustable attenuator to ensure that the power input into the photodetector is within its working range;

[0016] 3) Turn on the phase modulation system, and then turn off the phase modulation system after the SBS process is completely suppressed;

[0017] 4) Record the output waveform of the photodetector in the oscilloscope and the output signal of the phase modulation system;

[0018] 5) Take the time when the phase modulation system output signal is off as the zero time of the time axis of the two signals in step 4, intercept the part of the photodetector output signal from the zero time to the rising edge of the waveform and fit it with an exponential function, and the time constant of the exponential function obtained by fitting is the SBS establishment time constant;

[0019] 6) Adjust the output power of the fiber amplifier, repeat steps 1-5, and obtain multiple data points of the injected laser power and the SBS establishment time constant;

[0020] 7) Use an inverse proportional function to fit the data points of the injected laser power and the SBS establishment time constant to obtain an inverse proportional coefficient k;

[0021] 8) The inverse proportional coefficient and the SBS gain coefficient satisfy the following relationship:

[0022]

[0023] Where g is the SBS gain coefficient, n is the refractive index of the fiber system to be measured, A is the effective mode field area of the fiber system to be measured, and c is the speed of light in vacuum. The SBS gain coefficient of the fiber system to be measured is calculated by substituting the above parameters. eff

[0024] Compared with the prior art, the technical effects of the present application are as follows:

[0025] 1) The system is all-fiber, easy to integrate;

[0026] 2) The measurement of the overall SBS gain coefficient of the fiber system is realized. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 It is a structural schematic diagram of the fiber system SBS gain coefficient measurement device of the present application.

[0028] 101 is a single-frequency fiber laser light source; 102 is a phase modulation system; 103 is a fiber amplifier; 104 is a circulator; 105 is an isolator; 106 is a fiber adjustable attenuator; 107 is a fiber optical splitter; 108 is a fiber system to be measured; 109 is an isolator; 110 is an oscilloscope; 111 is a fiber; and 112 is a radio frequency signal transmission line. DETAILED DESCRIPTION

[0029] The present application will be further described in detail below in combination with the drawings and examples, and the examples are only used to explain the present application and should not limit the protection scope of the present application.

[0030] First, refer to Figure 1 , Figure 1 ​A structural schematic diagram of the SBS gain coefficient measuring device of the optical fiber system, as shown in the figure, the SBS gain coefficient measuring device of the optical fiber system comprises a single-frequency fiber laser light source 101, a phase modulation system 102, a fiber amplifier 103, a circulator 104, a fiber optical splitter 105, a fiber adjustable attenuator 106, a photoelectric detector 107, an oscilloscope 110, a fiber system to be measured 108 and an isolator 109.

[0031] The single-frequency fiber laser light source 101 emits continuous laser, and the phase modulation system 102, the fiber amplifier 103, the circulator 104, the fiber optical splitter 105, the fiber system to be measured 108 and the isolator 109 are sequentially arranged along the laser transmission direction, and adjacent systems and devices are connected to each other through optical fibers 111.

[0032] The fiber optical splitter 105 comprises four ports, namely Port1, Port2, Port3 and Port4, wherein the Port3 port is connected with the input end of the fiber adjustable attenuator 106, the output end of the fiber adjustable attenuator 106 is connected with the input end of the photoelectric detector 107, and the output end of the photoelectric detector 107 is connected with the input end of the oscilloscope 110 through the radio frequency signal transmission line 112.

[0033] The other input end of the oscilloscope 110 is connected with the modulation signal output end of the phase modulation system 102 through the radio frequency signal transmission line 112.

[0034] There is no signal delay between the modulation signal of the phase modulation system 102 and the output signal of the photoelectric detector 107, and the trigger of the oscilloscope 110 is the rising edge of the output signal of the photoelectric detector 107.

[0035] Embodiment

[0036] In the embodiment, the line width of the single-frequency fiber laser light source 101 is less than 20 kHz, and the output power is 10 mW. The phase modulation system 102 comprises an electro-optical modulator, a radio frequency amplifier and a pseudo-random binary sequence (PRBS) signal generator. In order to suppress ASE, the fiber amplifier 103 adopts a three-stage amplification structure, and the maximum output power is about 8 W.

[0037] In the embodiment, the fiber system to be measured 108 is a 1060XP optical fiber with a length of 1 km, and the core diameter and the cladding size of the optical fiber are 10 / 125 μm. The optical fiber sizes of the corresponding circulator 104, fiber optical splitter 105 and isolator 109 are also 10 / 125 μm.

[0038] In the embodiment, the maximum working power of the circulator 104, the fiber optical splitter 105 and the isolator 109 is 20W, the maximum working power of the fiber optical attenuator 106 is 500mW, and the maximum working power of the photoelectric detector 107 is 1mW.

[0039] In the embodiment, the response bandwidth of the photoelectric detector 107 is 45GHz, and the rise time is 14ps, the response bandwidth of the oscilloscope 110 is 2.5GHz, the sampling rate is 20GSa / s, the delay between the two input channels of the oscilloscope 110 is calibrated, and the lengths of the radio frequency signal transmission lines 112 connected with the photoelectric detector 107 and the phase modulation system 102 are the same.

[0040] In the embodiment, the output end of the single-frequency fiber laser source 101, the input end and the output end of the phase modulation system 102, the input end of the fiber amplifier 103, the input end and the output end of the fiber optical attenuator 106, and the Port3 port of the fiber optical splitter 105 are all fiber jumper heads, and the fibers 111 are connected through flanges.

[0041] In the embodiment, the fiber 111 between the output end of the fiber amplifier 103 and the input end of the circulator 104 is connected through fusion splicing, the fiber 111 between the output end of the circulator 104 and the Port4 port of the fiber optical splitter 105 is connected through fusion splicing, the fiber 111 between the Port1 port of the fiber optical splitter 105 and the input end of the fiber system under test 108 is connected through fusion splicing, and the fiber 111 between the output end of the fiber system under test 108 and the input end of the isolator 109 is connected through fusion splicing.

[0042] In the embodiment, the excess port of the circulator 104, the Port2 port of the fiber optical splitter 105, and the output end of the isolator 109 are all cut at an oblique angle.

[0043] In the embodiment, the splitting ratio of the fiber optical splitter 105 is 1:9, and the proportion of the Port1 port is 90%, so the laser power injected into the fiber system under test 108 is 9 times the output power of the Port2 port.

[0044] In the embodiment, the measurement steps are as follows:

[0045] 1) Turn on the single-frequency fiber laser source 101, adjust the output power of the fiber amplifier 103 in the case that the phase modulation system 102 is not turned on, monitor the laser power injected into the fiber system under test 108 at the Port2 port, and monitor the backward Stokes light power at the Port3 port. If the backward power fluctuates significantly, it is considered that the SBS threshold is reached. The maximum injected laser power of this measurement is determined, and the power at this time is recorded, with the backward power being lower than the damage threshold of the system device as the standard;

[0046] 2) Connect the fiber adjustable attenuator 106 with the fiber optical splitter 105, and adjust the fiber adjustable attenuator 106 to ensure that the power input to the photodetector 107 is lower than 1 mW;

[0047] 3) Set the working mode of the PRBS signal generator in the phase modulation system 102 to PRBS9, the modulation frequency is 10 GHz, turn on the phase modulation system 102, and then turn off the phase modulation system 102 after the SBS process is completely suppressed;

[0048] 4) Record the output waveform of the photodetector 107 in the oscilloscope 110 and the output signal of the phase modulation system 102;

[0049] 5) Repeat steps 3) and 4) 20 times, take the moment when the output signal of the phase modulation system 102 is turned off as the zero moment of the time axis of the two signals in step 4), average the 20 sampling results to eliminate the time domain burr, obtain the final time domain waveform, and cut off the part from the zero moment to the rising edge of the waveform and fit it with an exponential function. The time constant of the exponential function obtained by fitting is the SBS establishment time constant;

[0050] 6) Adjust the output power of the fiber amplifier 103, repeat steps 1)-5), and obtain 15 data points of the injected laser power and the SBS establishment time constant;

[0051] 7) Fit the data points of the injected laser power and the SBS establishment time constant with an inverse proportional function to obtain the inverse proportional coefficient k;

[0052] 8) The inverse proportional coefficient k and the SBS gain coefficient satisfy the following relationship:

[0053]

[0054] Wherein, g is the SBS gain coefficient, n=1.45 is the refractive index of the fiber system under test 108, A eff = 5.6 x 10 -11 m 2 is the effective mode field area of the fiber system under test 108, and c is = 3 x 10 8The SBS gain coefficient of the optical fiber system 108 to be measured is calculated by substituting the above parameters into the speed of light in a vacuum.

[0055] The above merely describes preferred embodiments of the present application, but the protection scope of the present application is not limited thereto. For those skilled in the art to which the present application pertains, several modifications and decorations can be made without departing from the spirit and scope of the present application, and these all belong to the protection scope of the technical scheme of the present application. Therefore, the protection scope of the present application should be defined by the scope of the claims.

Claims

1. A device for measuring the SBS gain coefficient of an optical fiber system, characterized in that, The single-frequency fiber laser light source (101), the phase modulation system (102), the fiber amplifier (103), the circulator (104), the fiber optical splitter (105), the fiber adjustable attenuator (106), the photodetector (107), the oscilloscope (110), the fiber system to be measured (108) and the isolator (109) are sequentially connected by the optical fiber (111) in the order of the single-frequency fiber laser light source (101), the phase modulation system (102), the fiber amplifier (103), the circulator (104), the fiber optical splitter (105), the fiber system to be measured (108) and the isolator (109); The single-frequency fiber laser light source (101) emits continuous laser, and the phase modulation system (102), the fiber amplifier (103), the circulator (104), the fiber optical splitter (105), the fiber system to be measured (108) and the isolator (109) are sequentially connected by the optical fiber (111) in the order of the single-frequency fiber laser light source (101), the phase modulation system (102), the fiber amplifier (103), the circulator (104), the fiber optical splitter (105), the fiber system to be measured (108) and the isolator (109); The fiber optical splitter (105) comprises four ports, namely Port1, Port2, Port3 and Port4, wherein the Port3 port is connected with the input end of the fiber adjustable attenuator (106), the output end of the fiber adjustable attenuator (106) is connected with the input end of the photodetector (107), and the output end of the photodetector (107) is connected with the input end of the oscilloscope (110) through the radio frequency signal transmission line (112); Another input channel of the oscilloscope (110) is connected with the modulation signal output end of the phase modulation system (102) through the radio frequency signal transmission line (112).

2. The apparatus for measuring the SBS gain coefficient of an optical fiber system according to claim 1, characterized in that, There is no signal delay between the modulation signal of the phase modulation system (102) and the output signal of the photodetector (107).

3. The apparatus for measuring the SBS gain coefficient of an optical fiber system according to claim 1, wherein, The trigger of the oscilloscope (110) is the rising edge of the output signal of the photodetector (107).

4. A method for measuring the SBS gain coefficient of an optical fiber system using the measuring device for the SBS gain coefficient of an optical fiber system according to claim 1, characterized in that The method comprises the following steps: 1) turning on the single-frequency fiber laser light source (101), adjusting the output power of the fiber amplifier (103) under the condition that the phase modulation system (102) is not turned on, ensuring that the power injected into the fiber system to be measured (108) exceeds the SBS threshold value, the backward Stokes light power is lower than the damage threshold value of the system device, and the optical power at this time is recorded at the Port2 port of the fiber optical splitter (105); 2) adjusting the fiber adjustable attenuator (106) to ensure that the power input into the photodetector (107) is within its working range; 3) turning on the phase modulation system (102), and then turning off the phase modulation system (102) after the SBS process is completely suppressed; 4) the oscilloscope (110) records the output waveform of the photodetector (107) and the output signal of the phase modulation system (102); 5) taking the closing time of the output signal of the phase modulation system (102) as the zero time of the time axis of the two signals in step 4), intercepting the part of the output signal of the photodetector (107) from the zero time to the rising edge of the waveform and fitting it with an exponential function, and the time constant of the fitted exponential function is the time constant of the SBS establishment. 6) adjusting the output power of the fiber amplifier (103), repeating steps 1) - 5), and obtaining a plurality of data points of the injected laser power and the SBS build-up time constant; 7) fitting the data points of the injected laser power and the SBS build-up time constant by using an inverse proportional function to obtain an inverse proportional coefficient k; 8) the inverse proportional coefficient k and the SBS gain coefficient satisfy the following relation: where g is the SBS gain coefficient, n is the refractive index of the optical fiber system (108) under test, A eff is the effective mode field area of the optical fiber system (108) under test, and c is the speed of light in vacuum. The SBS gain coefficient of the optical fiber system (108) under test is calculated by substituting the above parameters.

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