CN116136450A - Laser linewidth measuring method for multichannel short-delay optical fiber - Google Patents
Laser linewidth measuring method for multichannel short-delay optical fiber Download PDFInfo
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- CN116136450A CN116136450A CN202310275871.3A CN202310275871A CN116136450A CN 116136450 A CN116136450 A CN 116136450A CN 202310275871 A CN202310275871 A CN 202310275871A CN 116136450 A CN116136450 A CN 116136450A
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Abstract
The invention relates to a laser linewidth measurement method of a multichannel short-delay optical fiber, wherein a laser to be measured is divided into a plurality of channels through a coupler after passing through an optical isolator, a first group of optical signals are subjected to frequency shift through an acousto-optic modulator, the rest optical signals respectively pass through short-delay optical fibers with different lengths and are coupled with the first group of frequency-shifted optical signals, the signals are input into a photoelectric detector and converted into optical signals, finally, the optical signals are output to an oscilloscope or a spectrum analyzer to extract the power difference value between a first-order envelope peak value and a second-order envelope peak value of a power spectrum, and finally, the linewidth parameter of the laser to be measured is obtained through correlation processing. On one hand, the invention can effectively reduce the length of the delay optical fiber by 100 times, and reduce the sensitivity of the system to the environmental vibration interference; on the other hand, multichannel information acquisition of different length delay optical fibers can obtain the average level of laser linewidth output in one working state in one time period at a time, and is beneficial to eliminating accidental error influence in linewidth measurement.
Description
Technical Field
The invention relates to the field of laser linewidth measurement, in particular to a linewidth measurement scheme of laser linewidth to be measured, which is characterized in that a beat frequency is formed by combining a plurality of short-delay optical fiber channels with different lengths and optical signals subjected to frequency shift by an acousto-optic modulator, and the linewidth laser linewidth measurement scheme of the laser to be measured is obtained by extracting the power difference of the first-stage and second-stage maximum points of each channel frequency spectrum.
Background
The semiconductor laser has the advantages of good coherence, high integration level and the like, so that the semiconductor laser becomes a research hot spot in various fields in recent years, and as the linewidth of the laser is gradually compressed to kHz, hz and sub-Hz, the current mainstream delay self-phase dry method for measuring the linewidth of the laser needs hundreds of thousands of kilometers of delay optical fibers.
The invention 1 (201910306123.0) provides a laser linewidth measurement method and device for extracting dual characteristic parameters of a power spectrum, which can avoid the problem of inaccurate linewidth measurement of a laser caused by delay fiber length measurement errors in the prior art. Journal document 2 (ZhaoZ, baiZ, jinD, qiY, dingJ, yanB, wangY, luZ, mildrenRP.Narrow laser-linewidth measuring using short wavelength-heteodyneinterface measuring, optexpress.2022Aug15;30 (17): 30600-30610.Doi:10.1364/OE.455028.PMID: 36242160.) discloses a laser linewidth measuring method capable of effectively shortening the delay fiber length, which requires only 100m of optical fiber for KHz linewidth.
However, the method is only suitable for lasers which stably output constant linewidth lasers, and if the laser outputs or experimental environments fluctuate, a certain amount of errors can be introduced into linewidth measurement, so that the accuracy of measurement results is affected.
Disclosure of Invention
The invention relates to a laser linewidth measurement scheme of a multichannel short-delay optical fiber, which is characterized in that a beat frequency is formed by combining a plurality of short-delay optical fiber channels with different lengths and optical signals subjected to frequency shift by an acousto-optic modulator, and the linewidth laser linewidth measurement scheme of a laser to be measured is obtained by extracting the power difference of the first-stage and second-stage maximum points of each channel spectrum. According to the invention, through extraction of the extreme points of the frequency domain power spectrum of the multiple channels, on one hand, the length of the delay optical fiber can be effectively reduced, compared with the traditional delay self-phase dry method, the length of the delay optical fiber is reduced by more than 100 times, and the sensitivity of the system to environmental vibration interference is effectively reduced; on the other hand, multichannel information acquisition of different length delay optical fibers can obtain the average level of laser linewidth output in one working state in one time period at a time, and is beneficial to eliminating accidental error influence in linewidth measurement.
The invention adopts the technical scheme that: a laser linewidth measurement method of a multichannel short-delay optical fiber comprises the following steps:
step 1, establishing a multichannel short-delay optical fiber self-coherent measurement system;
step 3, extracting power differences of the first-order maximum value points and the second-order maximum value points of each frequency spectrum from the power spectrum;
and 4, data processing to obtain the line width of the laser to be measured.
Further, the multichannel short-delay optical fiber self-coherent measurement system is shown in fig. 1, and 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 photoelectric detectors and a multichannel spectrum analyzer, wherein, as shown in fig. 1, an output beam of the laser to be measured is divided into n+1 paths of equal-intensity optical signals after passing through the optical isolator, one path of signals is subjected to frequency shift through the acousto-optic modulator, each path of optical signals is left to be delayed through delay optical fibers with different lengths, the polarization controllers are connected on each path of optical signals to ensure polarization consistency of the optical signals, and finally each path of optical signals and a first path of frequency shift signal are combined through the couplers to generate beat frequencies, then are connected onto the multichannel photoelectric detectors to be converted into electric signals, and frequency domain information extraction is carried out by the spectrum analyzer.
Further, the input end of the optical isolator is connected with the output end of the laser, and the output end of the optical isolator is connected with the input end of the first coupler and used for protecting the laser.
Further, the optical couplers respectively realize beam splitting and beam combining functions, and meanwhile, the light intensity of each channel is required to be the same during beam splitting.
Further, the acousto-optic modulator should include a frequency shift module including a frequency shifter and a dc 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 of the frequency shifter is connected with the 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; the frequency shifter is used for shifting the frequency of the laser transmitted by the first coupler;
further, the delay fiber length of each channel should satisfy:
wherein L represents the length of the delay fiber; c represents the light velocity; n represents the refractive index of the optical fiber; deltav represents the linewidth value of the laser to be measured; Δsmin represents the minimum measurable value of the power difference between the peak and valley 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 an optical power value of the combined beam; Δfr represents the power value of the measured system noise floor; alpha represents the magnitude of the detector responsivity; g represents the gain of the amplifier in the detector; r represents the size of the output resistor of the detector; sn represents the resolution magnitude of the measured power spectrum.
Further, the number of system channels includes 2 and more, and is described herein primarily as three channels.
Further, the input ends of the multiple groups of polarization controllers are connected with the output ends of the delay optical fibers.
Further, the detection wavelength of the optical detectors of the plurality of photodetectors is to cover the wavelength of the laser to be detected, and the detection bandwidth is larger than the frequency shift of the acousto-optic modulator.
Further, the data processing part obtains the linewidth of the laser to be measured through the following formula after obtaining the power difference of the maximum value points of the first-order power spectrum and the second-order power spectrum of each channel:
wherein c represents the light velocity; n represents the refractive index of the optical fiber; representing the linewidth value of the laser to be measured; representing the power difference between the first-order envelope peak value and the second-order envelope peak value of each channel; the length of each channel delay fiber is represented, and m represents the number of channels.
Compared with the prior art, the technical scheme of the invention has the following beneficial effects:
1. compared with the current mainstream line width measurement method which is a delay self-phase method, the method can effectively reduce the length of the delay optical fiber, the length of the delay optical fiber is reduced by more than 100 times, the traditional delay self-phase method is faced with KHz, and the Hz line width needs more than hundreds of delay optical fibers to meet the requirement that the delay is longer than the coherence time, so that the longer optical fiber brings great loss on one hand, and on the other hand, the longer optical fiber has greater amplification on the influence of environmental vibration and the like, and the influence on experimental results is greater; the invention can utilize the optical fiber with the length of about 100m to realize the measurement of the line width, and effectively reduces the interference of environmental fluctuation and the extra line width broadening caused by transmission loss.
2. Compared with the measuring method in the invention 1, the invention can obtain the average linewidth state of the laser in one time period and working state at one time by sampling in one time period through the design of the multichannel different delay optical fibers, thereby improving the reliability of linewidth measurement; on the other hand, by multi-channel sampling, the influence caused by accidental errors and environmental fluctuation is reduced.
Drawings
Fig. 1 is a schematic diagram of a measurement system utilized in a multi-channel short-delay fiber laser linewidth measurement method according to the present invention.
Detailed Description
The invention is further described below with reference to the drawings and detailed description.
The specific implementation mode of the multi-channel short-delay fiber laser linewidth measuring method of the invention is shown in figure 1, comprising the following steps:
1. establishing a multichannel short-delay optical fiber self-coherent measurement system;
2. dividing laser to be measured into two beams, wherein one beam of laser frequency-shifting and the rest laser are subjected to different time delays, respectively combining the frequency-shifting and time-delayed lasers, inputting photocurrent of detected combined beam light, and obtaining a power spectrum of the photocurrent;
3. extracting power differences of first-order and second-order maximum points of each frequency spectrum from the power spectrum;
4. and (5) processing data to obtain the line width of the laser to be measured.
The multichannel short-delay optical fiber self-coherent measurement system is shown in fig. 1, and comprises a laser 1 to be measured, 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 (6 a, 6b and 6 c), a plurality of photodetectors (7 a, 7b and 7 c) and a multichannel spectrum analyzer 8. The plurality of optocouplers are a first optocoupler 3a, a second optocoupler 3b, a third optocoupler 3c, a fourth optocoupler 3d and a fifth optocoupler 3e, the plurality of groups of short-delay optical fibers with different lengths 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, and the plurality of photodetectors are a first photodetector 7a, a second photodetector 7b and a third photodetector 7c. As shown in fig. 1, an output light beam of a laser 1 to be measured is divided into n+1 paths of equal-intensity light signals by a coupler 3a after passing through an optical isolator 2, one path of the light signals is subjected to frequency shift by an acousto-optic modulator 4, the rest paths of the light signals are respectively delayed by a first delay optical fiber 5a, a second short delay optical fiber 5b and a third short delay optical fiber 5c with different lengths, a first polarization controller 6a, a second polarization controller 6b and a third polarization controller 6c are connected on each light path to ensure the polarization consistency of the light signals, and finally, the signals of each path of light signals and the first path of frequency shift signals are subjected to beam splitting by the second optical coupler 3b, then are subjected to beam combining by the third optical coupler 3c, the fourth optical coupler 3d and the fifth optical coupler 3e to generate beat frequencies, and then are connected to a multichannel photoelectric detector to be converted into electric signals, and frequency domain information is further input into a spectrum analyzer to be extracted.
The input end of the optical isolator 2 is connected with the output end of the laser, and the output end of the optical isolator is connected with the input end of the first coupler and is used for protecting the laser.
The optical couplers respectively realize beam splitting and beam combining functions, and meanwhile, the light intensity of each channel is required to be the same during beam splitting. As shown in fig. 1, the left end of the first optical coupler 3a is connected with an optical isolator, and the right end is divided into multiple paths of outputs which are respectively connected with the acousto-optic modulator 4 and multiple groups of short delay optical fibers (a first short delay optical fiber 5a, a second short delay optical fiber 5b and a third short delay optical fiber 5 c) with different lengths; the left end of the second optical coupler 3b is connected with the polarization controller of the same path of the acousto-optic modulator, and the right end is divided into multiple paths of equal-intensity optical signals and then connected with the third optical coupler 3c, the fourth optical coupler 3d and the fifth optical coupler 3 e; the third optical coupler 3c, the fourth optical coupler 3d and the fifth optical coupler 3e are connected with the second optical coupler 3b at the left side, one end is connected with the polarization controller on each delay optical fiber optical path, and the right side light beams are respectively connected with the first photoelectric detector 7a, the second photoelectric detector 7b and the third photoelectric detector 7c after being combined.
The acousto-optic modulator 4 should include a frequency shift module including a frequency shifter and a dc 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 of the frequency shifter is connected with the 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; the frequency shifter is used for shifting the frequency of the laser transmitted by the first coupler;
the delay fiber length of each channel (first short delay fiber 5a, second short delay fiber 5b, third short delay fiber 5 c) should satisfy:
wherein L represents the length of the delay fiber; c represents the light velocity; n represents the refractive index of the optical fiber; deltav represents the linewidth value of the laser to be measured; Δsmin represents the minimum measurable value of the power difference between the peak and valley 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 an optical power value of the combined beam; Δfr represents the power value of the measured system noise floor; alpha represents the magnitude of the detector responsivity; g represents the gain of the amplifier in the detector; r represents the size of the output resistor of the detector; sn represents the resolution magnitude of the measured power spectrum.
The number of system channels includes 2 or more, and the invention is mainly described by taking three channels as an example.
The input ends of the multiple groups of polarization controllers are connected with the output ends of the delay optical fibers. Specifically, as shown in fig. 1, the input ends of the first polarization controller 6a, the second polarization controller 6b and the third polarization controller 6c are respectively connected with the output ends of the first short-delay optical fiber 5a, the second short-delay optical fiber 5b and the third short-delay optical fiber 5 c;
the detection wavelength of the plurality of photodetectors is to cover the wavelength of the laser to be detected, and the detection bandwidth is greater than the frequency shift of the acousto-optic modulator 4.
The data processing part acquires the linewidth of the laser to be measured through the following formula after acquiring the power difference of the maximum value points of the first-order power spectrum and the second-order power spectrum of each channel:
wherein c represents the light velocity; n represents the refractive index of the optical fiber; representing the linewidth value of the laser to be measured; representing the power difference between the first-order envelope peak value and the second-order envelope peak value of each channel; the length of each channel delay fiber is represented, and m represents the number of channels.
Claims (8)
1. A laser linewidth measuring method of a multichannel short-delay optical fiber is characterized in that: the method comprises the following steps:
step 1, establishing a multichannel short-delay optical fiber self-coherent measurement system;
step 2, dividing the laser to be detected into two beams, wherein one beam of laser frequency-shifting and the rest of laser are subjected to different time delays, respectively combining the frequency-shifting and time-delayed lasers, and then inputting the photocurrent of the detected combined light to obtain a power spectrum of the photocurrent;
step 3, extracting power differences of the first-order maximum value points and the second-order maximum value points of each frequency spectrum from the power spectrum;
and 4, data processing to obtain the line width of the laser to be measured.
2. The method for measuring the laser linewidth of the multichannel short-delay optical fiber according to claim 1, wherein the method comprises the following steps: in the step 1, the measuring system comprises a laser (1) to be measured, an optical isolator (2), a plurality of optical couplers (3 a, 3b, 3c, 3d, 3 e), an acousto-optic modulator (4), a plurality of groups of short delay optical fibers (5 a, 5b, 5 c) with different lengths, a plurality of groups of polarization controllers (6 a, 6b, 6 c), a plurality of photodetectors (7 a, 7b, 7 c) and a multichannel spectrum analyzer (8); the processing of the measurement data comprises the steps of calculating the linewidth value of the laser to be measured according to the extracted characteristic parameters after the power difference value of the first-order envelope peak value and the second-order envelope peak value of the power spectrum of each channel, wherein the output light beam of the laser to be measured is divided into n+1 paths of equal-intensity light signals after passing through an optical isolator, one path of the light signals is subjected to frequency shift through an acousto-optic modulator, the rest paths of the light signals are respectively delayed through delay optical fibers with different lengths, the polarization controllers are connected on each light path to ensure the polarization consistency of the light signals, finally, each path of light signals and the first path of frequency shift signals are subjected to beam combination through a coupler to generate beat frequency, and then are connected to a multi-channel photoelectric detector to be converted into electric signals, and then the electric signals are input into a spectrum analyzer to extract frequency domain information.
3. The method for measuring the laser linewidth of the multichannel short-delay optical fiber according to claim 1, wherein the method comprises the following steps: the number of channels of the multi-channel measurement system includes 2 or more.
4. The laser linewidth measurement method of the multichannel short-delay optical fiber according to claim 3, wherein: in each channel, the length of each delay fiber takes any multiple lengths with distinguishability in the length interval to be used as the length of the delay fiber of each channel, and the length interval of the delay fiber is as follows:
wherein L represents the length of the delay fiber; c represents the light velocity; n represents the refractive index of the optical fiber; deltav represents the linewidth value of the laser to be measured; ΔS min A minimum measurable value representing a power difference between a peak and a valley of the first-order envelope; Δf min A minimum measurable value representing a frequency difference between the zero-order minimum point and the center frequency; p (P) 0 An optical power value representing the combined beam light; s is S n Representing the measured power value of the system noise floor; alpha represents the magnitude of the detector responsivity; g represents the gain of the amplifier in the detector; r represents the size of the output resistor of the detector; Δf r Representing the resolution magnitude of the measured power spectrum.
5. The laser linewidth measurement method of the multichannel short-delay optical fiber according to claim 4, wherein the method comprises the following steps: the output of each channel is passed through a spectrum analyzer, a power spectrum is obtained from the output, and the relation between the laser linewidth and the difference value can be obtained by obtaining the power difference between the maximum value point or the minimum value point of any order:
wherein c represents the light velocity; n represents the refractive index of the optical fiber; Δf represents the linewidth value of the laser to be measured; Δs represents the power difference between the first-order envelope peak and the second-order envelope peak of each channel; l represents the length of each channel delay fiber, k and m represent the parameters of the n-th order extreme point, and when the maximum value point is the point, m=2n-1, k=2n-1, and when the minimum value point is the point, m=2n-2, and k=2n-2.
6. The laser linewidth measurement method of the multichannel short-delay optical fiber according to claim 5, wherein the method comprises the following steps: and taking the difference value between the first-order maximum value point and the second-order maximum value point in the power spectrum, so that the relation between the power spectrum difference value and the line width of the laser is as follows:
7. the laser linewidth measurement method of the multichannel short-delay optical fiber according to claim 6, wherein: the output of each channel passes through a spectrum analyzer, the power difference value of a first-order envelope peak value and a second-order envelope peak value of a power spectrum is obtained from the output, the difference value of m channels is counted, and then the line width of the laser to be measured can be obtained according to a formula:
wherein c represents the light velocity; n represents the refractive index of the optical fiber; Δf represents the linewidth value of the laser to be measured; ΔS i Representing the power difference between the first-order envelope peak value and the second-order envelope peak value of each channel; l (L) i The length of each channel delay fiber is represented, and m represents the number of channels.
8. The method for measuring the laser linewidth of the multichannel short delay fiber according to claim 7, wherein the characteristic parameters of the power spectrum comprise: the power difference between the first-order and second-order envelope peaks of each channel signal.
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| CN117073990A (en) * | 2023-10-16 | 2023-11-17 | 常州灵动芯光科技有限公司 | Linewidth testing system and method for narrow linewidth laser |
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| CN117073990A (en) * | 2023-10-16 | 2023-11-17 | 常州灵动芯光科技有限公司 | Linewidth testing system and method for narrow linewidth laser |
| CN117073990B (en) * | 2023-10-16 | 2024-01-26 | 常州灵动芯光科技有限公司 | Linewidth testing system and method for narrow linewidth laser |
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