A method for measuring the laser linewidth of a multi-channel short-delay optical fiber

Abstract

The application relates to a kind of multi-channel short delay optical fiber laser linewidth measurement methods, the laser to be measured is divided into multiple channels by coupler after passing through optical isolator, wherein the first group of optical signals are frequency-shifted by acousto-optic modulator, the remaining optical signals are coupled with the first group of frequency-shifted optical signals after passing through different lengths of short delay optical fiber, converted into optical signals after input to photodetector, finally output to oscilloscope or spectrum analyzer to extract the power difference of power spectrum first-order envelope peak and second-order envelope peak, and finally the linewidth parameter of the laser to be measured is obtained by correlation processing. The application can effectively reduce the length of delay optical fiber, reduce the sensitivity of the system to environmental vibration interference by 100 times, and the multi-channel information collection of different lengths of delay optical fiber can obtain the average level of the laser linewidth output in a time period and under a working state at one time, which is beneficial to eliminate the accidental error influence in linewidth measurement.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser linewidth measurement, and in particular to a laser linewidth measurement scheme for forming beat frequency by combining a plurality of different lengths of short delay optical fiber channels with an optical signal after frequency shifting by an acousto-optic modulator, and obtaining the power difference of the first and second maximum points of the spectrum of each channel to obtain the laser linewidth of the laser to be measured. BACKGROUND

[0002] Due to the advantages of good coherence and high integration, semiconductor lasers have become a research hotspot in various fields in recent years. With the gradual compression of laser linewidth to kHz, Hz, and sub-Hz levels, the current mainstream method of measuring laser linewidth by delay self-coherence requires several hundred kilometers of long delay optical fiber. On the one hand, the loss of the optical fiber will bring errors to the linewidth measurement, and on the other hand, the longer the delay optical fiber, the more sensitive it is to environmental vibration, which is difficult to control in experiments. Therefore, the current delay self-coherence method has great limitations.

[0003] Invention 1 (201910306123.0) proposes a laser linewidth measurement method and device using power spectrum double characteristic parameters, which can avoid the problem of inaccurate laser linewidth measurement caused by measurement error of the length of the delay optical fiber in the prior art. Journal article 2 (Zhao Z, Bai Z, Jin D, Qi Y, Ding J, Yan B, Wang Y, Lu Z, Mildren RP. Narrow laser-linewidth measurement using short delay self-heterodyne interferometry. Opt Express. 2022 Aug 15;30(17):30600-30610. doi: 10.1364 / OE.455028. PMID: 36242160.) discloses a laser linewidth measurement method that can effectively shorten the length of the delay optical fiber. For a KHz linewidth, only 100m of optical fiber is required.

[0004] However, the above methods are only suitable for lasers that output constant linewidth lasers stably. If the laser output or the experimental environment fluctuates, it will introduce a certain amount of error to the linewidth measurement, thereby affecting the accuracy of the measurement results. SUMMARY

[0005] The application is a kind of multi-channel short-delay optical fiber laser linewidth measurement scheme, which combines the light signals after frequency shift by the acousto-optic modulator through a plurality of short-delay optical fiber channels with different lengths to form beat frequency, and obtains the power difference of the first and second maximum points of the spectrum of each channel to obtain the laser linewidth measurement scheme of the laser to be measured.

[0006] The technical scheme adopted by the application is as follows:

[0007] Step 1: Establish a multi-channel short-delay optical fiber self-coherent measurement system;

[0008] Step 2: Divide the laser to be measured into two beams, frequency shift one of the beams, and then delay the remaining laser through different delay times, combine the frequency-shifted and delayed lasers, input the photoelectric current of the combined probe light, and obtain the power spectrum thereof;

[0009] Step 3: Extract the power difference of the first and second maximum points of the spectrum from the power spectrum;

[0010] Step 4: Data processing to obtain the linewidth of the laser to be measured.

[0011] Further, the multi-channel short-delay optical fiber self-coherent measurement system is as shown in Figure 1 , which comprises a laser to be measured, an optical isolator, a plurality of optical couplers, an acousto-optic modulator, a plurality of groups of short-delay optical fibers with different lengths, a plurality of groups of polarization controllers, a plurality of photodetectors, and a multi-channel spectrum analyzer. Figure 1 As shown in , the output beam of the laser to be measured is divided into n+1 equal-intensity optical signals through the optical isolator, one of which is frequency-shifted by the acousto-optic modulator, and the remaining optical signals are delayed through different lengths of delay optical fibers, and the polarization controllers are connected on each optical path to ensure the polarization consistency of the optical signals. Finally, the optical signals are combined through the couplers to generate beat frequency, and then connected to the multi-channel photodetector to convert into electrical signals, and then input into the spectrum analyzer for frequency domain information extraction.

[0012] Further, the input end of the optical isolator is connected to the output end of the laser, and the output end is connected to the input end of the first coupler, which is used for protecting the laser.

[0013] Further, the plurality of optical couplers respectively implement beam splitting and beam combining functions of the light beams, while requiring the same light intensity of each channel when splitting.

[0014] Further, the acousto-optic modulator should include a frequency shift module including a frequency shifter and a direct current power supply; the output end of the direct current power supply is connected with one input end of the frequency shifter; the other input end of the frequency shifter is connected with the first output end of the first coupler, and the output end thereof is connected with the first input end of the power spectrum acquisition module; the direct current power supply is used to output a signal to drive the frequency shifter; the frequency shifter is used to shift the frequency of the laser transmitted by the first coupler;

[0015] Further, the length of the delay optical fiber of each channel should satisfy:

[0016]

[0017] wherein L represents the length of the delay optical fiber; c represents the size of the light speed; n represents the size of the refractive index of the optical fiber; Δv represents the line width value of the laser to be measured; ΔSmin represents the minimum measurable value of the power difference between the peak value and the valley value of the first-order envelope; Δfmin represents the minimum measurable value of the frequency difference between the zero-order minimum point and the center frequency; P0 represents the optical power value of the combined light; Δfr represents the power value of the measured system noise base; α represents the size of the responsivity of the detector; g represents the gain size of the amplifier in the detector; R represents the size of the output resistance of the detector; Sn represents the resolution size of the measured power spectrum.

[0018] Further, the number of system channels includes 2 or more, and this paper mainly takes three channels as an example for elaboration.

[0019] Further, the input ends of the plurality of groups of polarization controllers are connected with the output ends of the delay optical fibers.

[0020] Further, the detection wavelength of the plurality of photodetectors should cover the wavelength of the laser to be measured, and the detection bandwidth should be greater than the frequency shift amount of the acousto-optic modulator.

[0021] Further, the data processing part, after obtaining the power difference between the first-order and second-order power spectrum maximum points of each channel, obtains the line width of the laser to be measured through the following formula:

[0022]

[0023] wherein c represents the size of the light speed; n represents the size of the refractive index of the optical fiber; Δv represents the line width value of the laser to be measured; ΔS represents the power difference between the first-order envelope peak value and the second-order envelope peak value of each channel; Lm represents the length of the delay optical fiber of each channel, and m represents the number of channels.

[0024] Compared with the prior art, the above technical scheme of the present application can achieve the following beneficial effects:

[0025] 1. Compared with the current mainstream line width measurement method of delay self-coherence method, the present application can effectively reduce the length of the delay optical fiber, and the length of the delay optical fiber is reduced by more than 100 times. The traditional delay self-coherence method needs hundreds or thousands of delay optical fibers to meet the delay greater than the coherence time when facing KHz, Hz line width. Such a long optical fiber brings great loss on the one hand, and the longer the optical fiber, the greater the amplification of the influence of environmental vibration, etc., and the greater the influence on the experimental results. The present application can realize line width measurement by using an optical fiber of about 100 m, effectively reducing the interference of environmental fluctuations and the additional line width broadening introduced by transmission loss.

[0026] 2. Compared with the measurement method mentioned in the present application 1, the present application can obtain the average line width state of the laser under a time period and working state at one time by sampling in a time period through the design of multiple channels and different delay optical fibers, thereby improving the reliability of line width measurement. On the other hand, through the sampling of multiple channels, the influence of accidental error and environmental fluctuations is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 A measurement system schematic diagram used by the present application of a multi-channel short delay optical fiber laser line width measurement method. DETAILED DESCRIPTION

[0028] The present application will be further described below in combination with the drawings and specific embodiments.

[0029] The specific embodiment of the present application of a multi-channel short delay optical fiber laser line width measurement method is shown in Figure 1 , which includes:

[0030] 1. Establishing a multi-channel short delay optical fiber self-coherence measurement system;

[0031] 2. Dividing the laser to be measured into two beams, shifting the frequency of one of the beams, and passing the remaining laser through different delays. Then, the laser after frequency shifting and delay is combined respectively, and the photocurrent of the probe combined light is inputted and its power spectrum is obtained;

[0032] 3. Extracting the power difference of the first-order and second-order maximum points of each frequency spectrum from the power spectrum;

[0033] 4. Data processing to obtain the line width of the laser to be measured.

[0034] The multi-channel short delay optical fiber self-coherence measurement system is shown in Figure 1As shown, it comprises a to-be-tested laser 1, an optical isolator 2, a plurality of optical couplers, an acousto-optic modulator 4, a plurality of groups of short delay optical fibers with different lengths, a plurality of groups of polarization controllers (6a, 6b, 6c), a plurality of photodetectors (7a, 7b, 7c), and a multi-channel spectrum analyzer 8. The plurality of optical couplers are a first optical coupler 3a, a second optical coupler 3b, a third optical coupler 3c, a fourth optical coupler 3d, and a fifth optical coupler 3e. The plurality of groups of short delay optical fibers are a first short delay optical fiber 5a, a second short delay optical fiber 5b, and a third short delay optical fiber 5c. The plurality of groups of polarization controllers are a first polarization controller 6a, a second polarization controller 6b, and a third polarization controller 6c. The plurality of photodetectors are a first photodetector 7a, a second photodetector 7b, and a third photodetector 7c. As shown in the figure, Figure 1 As shown, the output beam of the to-be-tested laser 1 passes through the optical isolator 2 and is split into n+1 equal-intensity optical signals by the coupler 3a. One of the signals passes through the acousto-optic modulator 4 to be frequency-shifted, and the remaining signals are delayed by the first delay optical fiber 5a, the second short delay optical fiber 5b, and the third short delay optical fiber 5c with different lengths, respectively. The polarization controllers 6a, 6b, and 6c are connected in each optical path to ensure the polarization consistency of the optical signals. Finally, the signals in each path and the frequency-shifted signal in the first path are split by the second optical coupler 3b, and then combined by the third optical coupler 3c, the fourth optical coupler 3d, and the fifth optical coupler 3e to generate beat frequencies. The signals are then converted into electrical signals by the multi-channel photodetector, and the frequency domain information is extracted by the spectrum analyzer.

[0035] The input end of the optical isolator 2 is connected to the output end of the laser, and the output end is connected to the input end of the first coupler, for protecting the laser.

[0036] The plurality of optical couplers realize the functions of beam splitting and beam combining, and require equal optical intensity in each channel during splitting. Figure 1 As shown, the left end of the first optical coupler 3a is connected to the optical isolator, and the right end is split into multiple paths and connected to the acousto-optic modulator 4 and the plurality of groups of short delay optical fibers (the first short delay optical fiber 5a, the second short delay optical fiber 5b, and the third short delay optical fiber 5c). The left end of the second optical coupler 3b is connected to the polarization controller in the same path as the acousto-optic modulator, and the right end is split into multiple equal-intensity optical signals and connected to the third optical coupler 3c, the fourth optical coupler 3d, and the fifth optical coupler 3e. The left end of the third optical coupler 3c, the fourth optical coupler 3d, and the fifth optical coupler 3e is connected to the second optical coupler 3b, and the right end is connected to the polarization controllers in the optical paths of the delay optical fibers. The combined beams on the right side are connected to the first photodetector 7a, the second photodetector 7b, and the third photodetector 7c, respectively.

[0037] The acousto-optic modulator 4 should include a frequency shift module including a frequency shifter and a direct current power supply; an output end of the direct current power supply is connected with an input end of the frequency shifter; another input end of the frequency shifter is connected with a first output end of the first coupler, and an output end thereof is connected with a first input end of the power spectrum acquisition module; the direct current power supply is used for outputting a signal to drive the frequency shifter; and the frequency shifter is used for frequency-shifting the laser transmitted by the first coupler.

[0038] The delay fiber length of each channel (the first short delay fiber 5a, the second short delay fiber 5b and the third short delay fiber 5c) should satisfy:

[0039]

[0040] Wherein, L represents the length of the delay fiber; c represents the size of the light speed; n represents the size of the fiber refractive index; Δv represents the line width value of the laser to be measured; ΔSmin represents the minimum measurable value of the power difference between the peak value and the valley value of the first-order envelope; Δfmin represents the minimum measurable value of the frequency difference between the zero-order minimum point and the center frequency; P0 represents the optical power value of the combined light; Δfr represents the measured power value of the system noise base; α represents the size of the detector responsivity; g represents the gain size of the amplifier in the detector; R represents the size of the detector output resistance; and Sn represents the resolution size of the measured power spectrum.

[0041] The number of system channels includes 2 and more, and the present application mainly takes three channels as an example for illustration.

[0042] The input ends of the multiple sets of polarization controllers are connected with the output ends of the delay fibers. Specifically, as shown in Figure 1 the input ends of the first polarization controller 6a, the second polarization controller 6b and the third polarization controller 6c are connected with the output ends of the first short delay fiber 5a, the second short delay fiber 5b and the third short delay fiber 5c respectively;

[0043] The detection wavelength of the multiple photodetectors should cover the wavelength of the laser to be measured, and the detection bandwidth should be greater than the frequency shift amount of the acousto-optic modulator 4.

[0044] After the power difference between the first-order and second-order power spectrum maximum points of each channel is acquired, the data processing part acquires the line width of the laser to be measured through the following formula:

[0045]

[0046] Wherein, c represents the size of the light speed; n represents the size of the fiber refractive index; Δv represents the line width value of the laser to be measured; ΔS represents the power difference between the first-order envelope peak value and the second-order envelope peak value of each channel; Lm represents the length of the delay fiber of each channel, and m represents the number of channels.

Claims

1. A method of laser linewidth measurement of a multi-channel short-haul fiber, characterized by: It comprises the following steps: Step 1, a multi-channel short delay fiber self-coherent measurement system is established; the measurement system comprises a laser to be measured (1), an optical isolator (2), a plurality of optical couplers (3a, 3b, 3c, 3d, 3e), an acousto-optic modulator (4), a plurality of groups of short delay optical fibers with different lengths (5a, 5b, 5c), a plurality of groups of polarization controllers (6a, 6b, 6c), a plurality of photodetectors (7a, 7b, 7c), and a multi-channel spectrum analyzer (8); the processing of the measurement data comprises the power difference of the first-order envelope peak and the second-order envelope peak of the power spectrum of each channel, and then the linewidth value of the laser to be measured is calculated according to the extracted characteristic parameters, wherein the output beam of the laser to be measured is divided into n+1 equal intensity optical signals after passing through the optical isolator, one of which is shifted in frequency by the acousto-optic modulator, and the remaining optical signals are delayed by different lengths of delay optical fibers, and polarization controllers are connected in each optical path to ensure the polarization consistency of the optical signals, finally each optical signal and the first frequency-shifted signal are combined by the coupler to generate beat frequency, and then connected to the multi-channel photodetector to convert into electrical signal, and then input into the spectrum analyzer for frequency domain information extraction; Step 2, the laser to be measured is divided into n+1 beams, one of which is shifted in frequency, and the remaining lasers are delayed by different lengths, then the frequency-shifted and delayed lasers are combined respectively, and the photoelectric current of the combined light is inputted and the power spectrum is obtained; Step 3, the power difference of the first-order and second-order maximum points of the power spectrum is extracted; Step 4, data processing, the linewidth of the laser to be measured is obtained.

2. The method of claim 1, wherein: The number of channels of the multi-channel measurement system comprises 2 or more.

3. The method of claim 2, wherein: the plurality of channels are provided by a plurality of lasers; and the plurality of lasers are configured to emit light having a same wavelength. In each channel, the length of each delay optical fiber is taken as any multiple of different lengths within the length interval, as the length of the delay optical fiber of each channel, and the length interval of the delay optical fiber is: ； Wherein, L represents the length of the delay fiber; c represents the size of the light speed; n represents the size of the fiber refractive index; represents the line width value of the laser to be measured; represents the minimum measurable value of the power difference between the peak and valley of the first order envelope; represents the minimum measurable value of the frequency difference between the zero order minimum point and the center frequency; represents the optical power value of the combined light; represents the measured power value of the system noise floor; α represents the size of the detector responsivity; g represents the gain size of the amplifier in the detector; R represents the size of the detector output resistance; represents the resolution size of the measured power spectrum.

4. The method of claim 3, wherein: the plurality of channels are provided by a plurality of lasers; and the plurality of lasers are configured to emit light having a same wavelength. The output of each channel is passed through the spectrum analyzer to obtain the power spectrum, and by obtaining the power difference between any order maximum points or minimum points, the relationship between the laser linewidth and the power difference can be obtained: ； Wherein, c represents the size of the speed of light; n represents the size of the refractive index of the optical fiber; represents the line width value of the to-be-tested laser; represents the power difference value between the first-order envelope peak value and the second-order envelope peak value of each channel; represents the length of the delay optical fiber of each channel, k and m represent the parameters of the nth-order extreme point, when the nth-order extreme point is a maximum point, m=2n-1, k=2n-1, when the nth-order extreme point is a minimum point, m=2n-2, k=2n-2.

5. The method of claim 4, wherein: the plurality of channels are provided by a plurality of lasers; and the plurality of lasers are configured to emit light having a same wavelength. In the power spectrum, the power difference between the first-order and second-order maximum points is taken, so that the relationship between the power spectrum difference and the laser linewidth is: 。 6. The method of claim 5, wherein: the plurality of channels are provided by a plurality of lasers; and the plurality of lasers are configured to emit light having a same wavelength. The output of each channel is passed through the spectrum analyzer to obtain the power spectrum, and by obtaining the power difference between any order maximum points or minimum points, the relationship between the laser linewidth and the power difference can be obtained: ； Wherein, c represents the size of the speed of light; n represents the size of the refractive index of the optical fiber; represents the line width value of the to-be-tested laser; represents the power difference value between the first-order envelope peak value and the second-order envelope peak value of each channel; represents the length of the delay optical fiber of each channel, and m represents the number of channels.

7. The laser linewidth measurement method for multi-channel short-delay optical fibers as described in claim 6, characterized in that, The characteristic parameters of the power spectrum include the power difference between the first-order envelope peak and the second-order envelope peak of each channel signal.

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