Laser frequency drift measurement system and method based on time-delayed self-heterodyne

Abstract

The application provides a laser frequency drift measurement system and method based on time-delay self-heterodyne, wherein the system comprises: a laser for outputting stable single-frequency laser; a delay stabilization structure for detecting and compensating the time delay variation of an optical fiber link according to a radio frequency signal and single-frequency laser output by an auxiliary laser; an optical delay self-heterodyne interferometer for modulating single-frequency laser output by a to-be-measured laser based on the optical fiber link with adjusted length to obtain intermediate frequency signal light; and a laser frequency drift calculation device for calculating the laser frequency drift of the to-be-measured laser according to the intermediate frequency signal light. The system detects and compensates the time delay variation of the optical fiber link through the delay stabilization structure, and converts the measurement of the to-be-measured laser frequency drift into the measurement of the phase difference between the transmission signals of the delay branch and the reference branch, so that the long-term real-time measurement of the laser frequency drift of the to-be-measured laser becomes possible, and the measurement precision of the laser frequency drift is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a laser frequency drift measurement system and method based on time-delay self-heterodyne. Background Technology

[0002] Lasers, with their superior characteristics such as high beam quality, long coherence length, and narrow spectral linewidth, are widely used in many fields, including laser Doppler wind radar, fiber optic sensing, and fiber optic communication. These applications require lasers to have very high frequency stability. For example, to achieve a measurement accuracy of 0.5 m / s in laser Doppler wind radar, the laser frequency drift should not exceed 1 MHz.

[0003] However, in practical applications, the output frequency of a laser changes over time due to environmental factors such as temperature and mechanical vibration. Measuring the laser's output frequency drift characteristics can, on the one hand, facilitate the evaluation of the laser's performance; on the other hand, if the laser's frequency drift is known, the collected data can be processed to counteract the effects of the frequency drift.

[0004] Existing laser frequency drift characteristic measurement methods commonly include direct measurement, beat frequency method, and time-delay self-heterodyne method.

[0005] Direct measurement methods typically involve using a spectrometer to directly measure the output light frequency of the laser under test. However, due to the limitations of spectrometer measurement accuracy, the accuracy of direct measurement methods is generally in the range of MHz to GHz. Furthermore, spectrometers cannot simultaneously meet the requirements of high frequency resolution and fast scanning rate, which limits the real-time performance of the measurement.

[0006] The beat frequency method typically uses the interference of a laser with the same or similar frequency to the laser under test. By analyzing the beat frequency signal, the relative frequency stability between the two lasers can be obtained. However, it is necessary to find a reference laser in advance, and the frequency stability of the reference laser must be more stable than that of the laser under test.

[0007] The time-delay self-heterodyne method splits the laser under test into two paths, subjecting each path to a different delay before interferometric beat frequency analysis. Frequency drift information is then obtained by analyzing the beat frequency signals. Compared to the previous two methods, the time-delay self-heterodyne method offers advantages such as high measurement accuracy, simple structure, and no need for additional references.

[0008] However, during long-term continuous measurements, the delay fiber used in this method is affected by changes in ambient temperature and pressure, which in turn changes the delay and affects the laser frequency drift measurement results.

[0009] Therefore, it is essential to address the problem that existing laser frequency drift measurement methods are susceptible to changes in ambient temperature and pressure, making it impossible to continuously and accurately measure the output frequency drift characteristics of lasers over long periods. Summary of the Invention

[0010] This invention provides a laser frequency drift measurement system and method based on time-delay self-heterodyne, which overcomes the shortcomings of existing laser frequency drift measurement methods that are easily affected by changes in ambient temperature and pressure, resulting in the inability to continuously and accurately measure the output frequency drift characteristics of lasers for a long time. This invention achieves long-term continuous and high-precision measurement of the output frequency drift characteristics of lasers.

[0011] On one hand, the present invention provides a laser frequency drift measurement system based on delay self-heterodyne, comprising: a laser, including a laser under test and an auxiliary laser, for outputting a stable single-frequency laser; a delay stabilization structure for detecting and compensating for delay changes in an optical fiber link based on a radio frequency signal and the single-frequency laser output by the auxiliary laser, and adjusting the length of the optical fiber link; an optical delay self-heterodyne interferometer connected to the delay stabilization structure for modulating the single-frequency laser output by the laser under test based on the adjusted length of the optical fiber link to obtain an intermediate frequency signal light; and a laser frequency drift calculation device connected to the optical delay self-heterodyne interferometer for calculating the laser frequency drift of the laser under test based on the intermediate frequency signal light; wherein, both the delay stabilization structure and the optical delay self-heterodyne interferometer include a multiplexing structure, the multiplexing structure being used to synthesize, transmit, and separate the single-frequency laser output by the laser under test.

[0012] Furthermore, the delay stabilization structure includes: a modulation unit for modulating the radio frequency signal onto a single-frequency laser output by the auxiliary laser to obtain an optical radio frequency signal; and a delay compensation unit connected to the multiplexing structure for detecting and compensating for delay changes in the optical fiber link based on the signal light to be detected output by the multiplexing structure, and adjusting the length of the optical fiber link.

[0013] Furthermore, the modulation unit includes: a radio frequency signal generator for generating a radio frequency signal; and a Mach-Zehnder modulator connected to the radio frequency signal generator for modulating the radio frequency signal onto the single-frequency laser output by the auxiliary laser to obtain an optical radio frequency signal.

[0014] Furthermore, the delay compensation unit includes: a first photodetector, used to perform photoelectric conversion on the signal light to be detected output from the multiplexing structure, and demodulate it to obtain a signal to be detected, the signal to be detected including the delayed radio frequency signal; a mixer, used to mix the signal to be detected with the untransmitted radio frequency signal output from the radio frequency signal generator to obtain an error signal; and a phase-locked loop, used to integrate and amplify the error signal, and negatively feedback control the adjustable optical delay line in the multiplexing structure to compensate for the delay variation of the optical fiber link, so as to adjust the length of the optical fiber link.

[0015] Furthermore, the optical delayed self-heterodyne interferometer includes: a first fiber coupler for separating the single-frequency laser output from the laser under test by a preset ratio to obtain a signal light and a reference light; a delay branch for delaying the transmission of the signal light to obtain an interference signal light; a reference branch for transmitting the reference light; and a second fiber coupler for combining the reference light and the interference signal light to obtain an intermediate frequency signal light.

[0016] Furthermore, the delay branch includes: an acousto-optic frequency shifter for applying a fixed frequency shift to the signal light; and a polarization tracker for performing polarization state tracking control on the delayed signal light output by the multiplexing structure to maintain the polarization state of the delayed signal light.

[0017] Further, the multiplexing structure includes: a first wavelength division multiplexer, used to synthesize the optical radio frequency signal output by the modulation unit in the delay-stabilized structure and the frequency-shifted signal light to obtain a synthesized signal light; a delay fiber, connected to the first wavelength division multiplexer, used to transmit the synthesized signal light and delay it; a tunable optical delay line, connected to the delay fiber, used to compensate for the jitter caused by the environmental influence of the fiber link during the transmission of the delayed synthesized signal light to obtain a compensated signal light; and a second wavelength division multiplexer, connected to the tunable optical delay line, used to separate the compensated signal light of different wavelengths to obtain a signal light to be detected and a delayed signal light.

[0018] Furthermore, the laser frequency drift calculation device includes: a second photodetector connected to the optical delayed self-heterodyne interferometer, used to perform photoelectric conversion on the intermediate frequency signal light to obtain an intermediate frequency electrical signal; and a data acquisition and processing unit connected to the second photodetector, used to calculate the laser frequency drift of the laser under test based on the intermediate frequency electrical signal.

[0019] Secondly, the present invention provides a laser frequency drift measurement method based on time-delay self-heterodyne, applied to the laser frequency drift system based on time-delay self-heterodyne described in any of the above claims, comprising: acquiring the laser to be tested output by the laser under test and the auxiliary laser output by the auxiliary laser; detecting and compensating for the delay change of the optical fiber link using the radio frequency signal and the auxiliary laser, and adjusting the length of the optical fiber link; modulating the laser to be tested based on the optical fiber link after the length adjustment to obtain an intermediate frequency signal light; and calculating the laser frequency drift of the laser under test based on the intermediate frequency signal light.

[0020] Further, the radio frequency signal includes a first radio frequency signal transmitted via the optical fiber link and a second radio frequency signal not transmitted via the optical fiber link; the step of using the radio frequency signal and the auxiliary laser to detect and compensate for the delay change of the optical fiber link includes: modulating the first radio frequency signal onto the auxiliary laser to obtain an optically carried radio frequency signal; transmitting the optically carried radio frequency signal sequentially through a delay fiber and a tunable optical delay line, then separating it and performing photoelectric conversion by a photodetector to obtain a signal light to be detected, the signal light to be detected including the delayed first radio frequency signal; mixing the second radio frequency signal with the signal light to be detected to obtain an error signal; and compensating for the delay change of the optical fiber link based on the error signal.

[0021] This invention provides a laser frequency drift measurement system based on time-delay self-heterodyne, comprising a laser under test (DUT) and an auxiliary laser for outputting a stable single-frequency laser; a delay stabilization structure for detecting and compensating for delay variations in the fiber optic link based on the radio frequency signal and the single-frequency laser output from the auxiliary laser, and adjusting the length of the fiber optic link; an optical time-delay self-heterodyne interferometer connected to the delay stabilization structure for modulating the single-frequency laser output from the DUT based on the adjusted fiber optic link to obtain an intermediate frequency (IF) signal; and a laser frequency drift calculation device connected to the optical time-delay self-heterodyne interferometer for calculating the laser frequency drift of the DUT based on the IF signal. This system detects and compensates for delay variations in the fiber optic link through the delay stabilization structure, and further transforms the measurement of the laser frequency drift into the measurement of the phase difference between the transmitted signals of the delay branch and the reference branch through the time-delay self-coherence structure. This enables long-term real-time measurement of the laser frequency drift of the DUT and effectively improves the measurement accuracy. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] Figure 1 A schematic diagram of the laser frequency drift measurement system based on time-delay self-heterodyne provided by the present invention;

[0024] Figure 2 A schematic diagram of the overall structure of the laser frequency drift measurement system based on time-delay self-heterodyne provided by the present invention;

[0025] Figure 3 A schematic diagram of the overlap Allen deviation of the time-delayed optical fiber jitter over time provided by the present invention;

[0026] Figure 4 A schematic diagram illustrating the measurement accuracy of laser frequency drift provided by the present invention;

[0027] Figure 5 This invention provides schematic diagrams illustrating the laser frequency drift characteristics of different types of lasers.

[0028] Figure 6 The present invention provides a laser frequency drift measurement method based on time-delay self-heterodyne.

[0029] Figure label:

[0030] 110: Laser; 111: Laser under test; 112: Auxiliary laser; 120: Delay stabilization structure; 130: Optical delay self-heterodyne interferometer; 140: Laser frequency drift calculation device; 150: Multiplexing structure. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0032] It should be noted that common existing laser frequency drift characteristic measurement methods include direct measurement, beat frequency method, and time-delay self-heterodyne method.

[0033] Compared to the previous two methods, the time-delayed self-heterodyne method has advantages such as high measurement accuracy, simple structure, and no need to introduce additional references.

[0034] However, during long-term continuous measurements, the delay fiber used in this method is affected by changes in ambient temperature and pressure, which in turn changes the delay and affects the laser frequency drift measurement results.

[0035] In view of this, the present invention provides a laser frequency drift measurement system based on time-delay self-heterodyne, specifically, Figure 1 A schematic diagram of the laser frequency drift measurement system based on time-delay self-heterodyne provided by the present invention is shown.

[0036] like Figure 1 As shown, the system includes:

[0037] Laser 110 includes laser under test 111 and auxiliary laser 112, used to output stable single-frequency laser;

[0038] The delay stabilization structure 120 is used to detect and compensate for delay changes in the fiber optic link based on the radio frequency signal and the single-frequency laser output from the auxiliary laser 112, and to adjust the length of the fiber optic link.

[0039] An optical delay self-heterodyne interferometer 130, connected to a delay stabilization structure 120, is used to modulate the single-frequency laser output from the laser under test 111 based on the fiber optic link after adjustment of length, to obtain intermediate frequency signal light.

[0040] The laser frequency drift calculation device 140 is connected to the optical delay self-heterodyne interferometer 130 and is used to calculate the laser frequency drift of the laser under test 111 based on the intermediate frequency signal light.

[0041] Both the delay stabilization structure 120 and the optical delay self-heterodyne interferometer 130 include a multiplexing structure 150, which is used to synthesize, transmit and separate the single-frequency laser output from the laser under test 111.

[0042] It is understood that the laser 110 includes a laser under test 111 and an auxiliary laser 112. The laser under test 111 is the object of laser frequency drift measurement, while the auxiliary laser 112 is used to assist in detecting delay changes in the fiber optic link.

[0043] The delayed stabilization structure 120 is a new structure added to the existing interferometer in this invention, and it is the core focus of this invention.

[0044] Specifically, the processing objects of the delay stabilization structure 120 include two parts: one part is the radio frequency signal, and the other part is the single-frequency laser (auxiliary laser) output by the auxiliary laser 112.

[0045] Similarly, the delay stabilization structure 120 also has two functions: one is to detect the delay change of the optical fiber link based on the radio frequency signal and the single-frequency laser, and the other is to compensate for the delay change of the optical fiber link based on the detected delay change, thereby adjusting the length of the optical fiber link, making it possible to perform long-term real-time measurement of the laser frequency drift of the laser under test 111.

[0046] In this embodiment, since the delay change of the optical fiber link cannot be directly detected by radio frequency signal, the radio frequency signal is modulated onto the single-frequency laser output by the auxiliary laser 112, and then the modulated optical signal is used to detect and compensate for the delay change of the optical fiber link.

[0047] An optical delay self-heterodyne interferometer is connected to a delay stabilization structure 120. After the length of the optical fiber link is adjusted in the delay stabilization structure 120, the laser frequency drift of the laser under test 111 is measured based on the adjusted optical fiber link.

[0048] Specifically, the optical delay self-heterodyne interferometer can be divided into two parts: a delay branch and a reference branch. Through the delay self-coherent structure, the measurement of the laser frequency drift of the laser under test 111 is transformed into the measurement of the phase difference between the transmitted signals of the delay branch and the reference branch, thereby obtaining the intermediate frequency signal light.

[0049] The laser frequency drift calculation device 140 is connected to the optical delay self-heterodyne interferometer 130. Based on the intermediate frequency signal light obtained by the optical delay self-heterodyne interferometer 130, the laser frequency drift of the laser under test 111 is calculated.

[0050] The output of the laser under test can be expressed as:

[0051]

[0052] Where, ω c It is the fixed angular frequency of the output laser. This is the phase noise caused by frequency variation. The beat frequency intermediate frequency signal light can be represented as:

[0053]

[0054] Where, ω fs For the fixed frequency shift introduced by the acousto-optic frequency shifter, when the delay τ introduced by the delay fiber remains constant, the frequency drift of the laser under test can be calculated by measuring the phase change of the intermediate frequency signal light using the following formula:

[0055]

[0056] in, It is the phase change of the intermediate frequency signal.

[0057] It should be noted that there is a part of the structure that overlaps and is used between the delay stabilization structure 120 and the optical delay self-heterodyne interferometer 130, which is referred to as the multiplexing structure 150.

[0058] The multiplexing structure 150 is used to synthesize the single-frequency laser output from the laser under test 111 and the single-frequency laser output from the auxiliary laser 112 (the single-frequency laser is modulated with a radio frequency signal), and to transmit the synthesized signal light, as well as to separate the transmitted optical signal to detect and compensate for the delay changes of the optical fiber link, and to measure the laser frequency drift of the laser under test 111.

[0059] In this embodiment, the laser frequency drift measurement system based on delay self-heterodyne includes a laser under test 111 and an auxiliary laser 112 for outputting a stable single-frequency laser; a delay stabilization structure 120 for detecting and compensating for delay changes in the fiber optic link and adjusting the length of the fiber optic link based on the radio frequency signal and the single-frequency laser output by the auxiliary laser 112; an optical delay self-heterodyne interferometer 130 connected to the delay stabilization structure 120 for modulating the single-frequency laser output by the laser under test 111 based on the adjusted fiber optic link to obtain an intermediate frequency signal light; and a laser frequency drift calculation device 140 connected to the optical delay self-heterodyne interferometer 130 for calculating the laser frequency drift of the laser under test 111 based on the intermediate frequency signal light. The system detects and compensates for delay changes in the optical fiber link through a delay stabilization structure 120. Based on this, it transforms the measurement of the laser frequency drift under test into the measurement of the phase difference between the transmitted signals of the delay branch and the reference branch through a delay self-coherent structure. This makes long-term real-time measurement of the laser frequency drift of the laser under test 111 possible and effectively improves the measurement accuracy of laser frequency drift.

[0060] Based on the above embodiments, this embodiment further describes the delay stabilization structure 120 in detail. Specifically, the delay stabilization structure 120 includes:

[0061] The modulation unit is used to modulate the radio frequency signal onto the single-frequency laser output by the auxiliary laser 112 to obtain an optical radio frequency signal;

[0062] The delay compensation unit, connected to the multiplexing structure 150, is used to compensate for the delay variation of the optical fiber link based on the signal light to be detected output by the multiplexing structure 150, and to adjust the length of the optical fiber link.

[0063] It should be noted that during long-term measurement of laser frequency drift, the fiber transmission delay of the fiber optic link changes with temperature and stress fluctuations, resulting in an additional phase change on top of the actual laser frequency change. Therefore, it is necessary to eliminate its influence after detecting the delay change of the fiber optic link.

[0064] It is understandable that the delay stabilization structure 120 consists of three main parts: the modulation unit, the multiplexing structure 150, and the delay compensation unit.

[0065] In one specific embodiment, the modulation unit further includes a radio frequency signal generator and a Mach-Zehnder modulator.

[0066] The function of the modulation unit is to modulate the radio frequency signal onto the single-frequency laser (optical carrier) output by the auxiliary laser 112, thereby obtaining the optical radio frequency signal.

[0067] The radio frequency signal generator is used to generate radio frequency signals. The Mach-Zehnder modulator is connected to the radio frequency signal generator and is used to modulate the radio frequency signal onto the single-frequency laser output by the auxiliary laser 112 to obtain an optical radio frequency signal.

[0068] according to Figure 2 It can be seen that the radio frequency signal generated by the radio frequency signal generator is divided into two paths. One path is modulated by the Mach-Zehnder modulator onto the single-frequency laser output by the auxiliary laser 112, while the other path is not transmitted through the fiber optic link.

[0069] In some embodiments, the radio frequency signal generator is a microwave source synchronized with a cesium clock, used to generate a 26 GHz radio frequency probe signal.

[0070] In another specific embodiment, the multiplexing structure 150 includes a first wavelength division multiplexer, a delay fiber, a tunable optical delay line, and a second wavelength division multiplexer.

[0071] The first wavelength division multiplexer is used to synthesize the optical radio frequency signal output from the modulation unit in the delay-stabilized structure 120 and the frequency-shifted signal light (the intermediate output signal of the optical delay self-heterodyne interferometer 130) to obtain the synthesized signal light.

[0072] The delay fiber, connected to the first wavelength division multiplexer, is used to transmit the synthesized signal light and delay its transmission duration.

[0073] A tunable optical delay line, connected to a delay fiber, is used to compensate for the jitter caused by environmental factors affecting the fiber optic link during the transmission of the synthesized signal light after delay, thus obtaining the compensated signal light (the specific compensation process is implemented through a delay compensation unit).

[0074] In some embodiments, the tunable optical delay line consists of a piezoelectric fiber stretcher and an electrically driven delay line.

[0075] The second wavelength division multiplexer, connected to the tunable optical delay line, is used to separate the compensated signal light of different wavelengths to obtain the signal light to be detected and the delayed signal light.

[0076] In yet another specific embodiment, the delay compensation unit includes a first photodetector, a mixer, and a phase-locked loop.

[0077] The first photodetector is used to perform photoelectric conversion on the signal light to be detected output from the multiplexing structure 150 and demodulate it to obtain the electrical signal to be detected, which includes a delayed radio frequency signal.

[0078] Specifically, the first photodetector is connected to the second wavelength division multiplexer. After the delayed radio frequency signal is separated from the second wavelength division multiplexer, the first photodetector is used to perform photoelectric conversion on the delayed radio frequency signal and demodulate it to obtain the electrical signal to be detected.

[0079] A mixer is used to mix the electrical signal to be detected (corresponding to one of the radio frequency signals generated by the radio frequency signal generator mentioned above) with the radio frequency signal output by the radio frequency signal generator that has not been transmitted (i.e., another radio frequency signal generated by the radio frequency signal generator mentioned above) to obtain an error signal.

[0080] Specifically, the mixer mixes radio frequency signals transmitted via fiber optic links and radio frequency signals not transmitted via fiber optic links.

[0081] By observing the difference between the two radio frequency signals, we can understand the current delay variation of the fiber optic link, which can be reflected by the error signal.

[0082] A phase-locked loop is used to integrate and amplify the error signal. The tunable optical delay line in the negative feedback control multiplexing structure 150 compensates for the delay variation of the optical fiber link in order to adjust the length of the optical fiber link.

[0083] Understandably, the error signal obtained by the mixer enters the phase-locked loop (PLL), which drives the adjustable optical delay line to compensate for the change in fiber optic link delay. Once the loop is locked, the delay branch of the optical delay autoheterodyne interferometer 130 will maintain a fixed transmission delay.

[0084] As can be seen from the above, the modulation unit, the multiplexing structure 150, and the delay compensation unit form a loop.

[0085] In this embodiment, the delay stabilization structure 120 includes a modulation unit, a multiplexing structure 150, and a delay compensation unit. The modulation unit modulates the radio frequency signal onto the single-frequency laser output by the auxiliary laser 112 to obtain an optical radio frequency signal. The delay compensation unit detects and compensates for the delay variation of the optical fiber link based on the signal light to be detected output by the multiplexing structure 150, and adjusts the length of the optical fiber link. With the assistance of high-frequency probe signal and error signal amplification, the phase-amplitude conversion of this system significantly improves the phase detection accuracy, thereby ensuring the measurement accuracy of laser frequency drift. Furthermore, with the help of a self-designed high-performance phase-locked loop, a highly stable adjustable optical delay line is obtained, achieving high-precision length measurement of laser frequency drift.

[0086] Based on the above embodiments, this embodiment further describes the optical delayed self-heterodyne interferometer 130 in more detail. Specifically, the optical delayed self-heterodyne interferometer 130 includes:

[0087] The first fiber coupler is used to separate the single-frequency laser output from the laser under test 111 by a preset ratio to obtain the signal light and the reference light.

[0088] The delay branch is used to delay the transmission of the signal light to obtain an interference light signal;

[0089] Reference branch, used for transmitting reference light;

[0090] The second fiber coupler is used to combine the reference light and the interference signal light to obtain the intermediate frequency signal light.

[0091] Understandably, the optical delayed self-heterodyne interferometer 130 comprises four parts: a first fiber coupler, a delay branch, a reference branch, and a second fiber coupler. The stable single-frequency laser (the laser to be tested) generated by the laser under test 111 is split into two parts by the first fiber coupler. One part is injected into the delay branch for delayed transmission to obtain the interference light signal, while the other part is injected into the reference branch for transmission only, without any further processing.

[0092] In some embodiments, the delay branch uses long optical fibers, which can help improve the measurement accuracy of the system.

[0093] Then, the optical signals from the two branches are combined through a second fiber coupler to obtain intermediate frequency signal light.

[0094] The first fiber coupler separates the laser light to be detected according to a first preset ratio to obtain signal light and reference light, and the second fiber coupler combines the optical signals of the two branches according to a second preset ratio to obtain intermediate frequency signal light.

[0095] The first and second preset ratios can be set according to actual conditions, and no specific limitations are made here.

[0096] For example, in some embodiments, the first fiber coupler is a 95:5 polarization-maintaining coupler that divides the laser to be detected into signal light and reference light in a 95:5 ratio; the second fiber coupler is a 50:50 coupler that combines the optical signals of the two branches in a 50:50 ratio to obtain intermediate frequency signal light.

[0097] More specifically, the delay branch includes an acousto-optic frequency shifter, a multiplexing structure 150, and a polarization tracker.

[0098] An acousto-optic frequency shifter is used to apply a fixed frequency shift to signal light;

[0099] A polarization tracker is used to track and control the polarization state of the delayed signal light output from the multiplexing structure 150, and to maintain the polarization state of the delayed signal light.

[0100] Understandably, in order to shorten the reference branch and eliminate its instability, this embodiment places the acousto-optic frequency shifter on the delay branch and integrates the first fiber coupler and the second fiber coupler into a steel tube for encapsulation.

[0101] In the delay branch, after the laser to be detected is separated by the first fiber coupler, the signal light passes sequentially through an acousto-optic frequency shifter, multiplexing structure 150 (first wavelength division multiplexer, delay fiber, tunable optical delay line, second wavelength division multiplexer) and a polarization tracker. The polarization tracker maintains the polarization state of the delayed signal light.

[0102] In this embodiment, the optical delayed self-heterodyne interferometer 130 includes a first fiber coupler, a delay branch, a reference branch, and a second fiber coupler. The first fiber coupler is used to separate the single-frequency laser output from the laser under test 111 by a preset ratio to obtain a signal light and a reference light. The delay branch is used to delay the transmission of the signal light to obtain an interference light signal. The reference branch is used to transmit the reference light. The second fiber coupler is used to combine the reference light and the interference signal light to obtain an intermediate frequency signal light. By introducing a delayed self-coherent structure, this system can avoid introducing a laser 110 or a standard with higher frequency stability as a reference, thus reducing system complexity.

[0103] Based on the above embodiments, this embodiment further describes the laser frequency drift calculation device 140 in more detail. Specifically, the laser frequency drift calculation device 140 includes:

[0104] The second photodetector is connected to the optical delay self-heterodyne interferometer 130 and is used to perform photoelectric conversion on the intermediate frequency signal light to obtain the intermediate frequency electrical signal.

[0105] The data acquisition and processing unit is connected to the second photodetector and is used to calculate the laser frequency drift of the laser under test 111 based on the intermediate frequency electrical signal.

[0106] Understandably, the second photodetector in the laser frequency drift calculation device 140 is connected to the second fiber coupler in the optical delay self-heterodyne interferometer 130. The output of the second fiber coupler (intermediate frequency signal light) is used as the input to perform photoelectric conversion on it in order to detect the beat frequency signal and obtain the intermediate frequency electrical signal.

[0107] Then, the second photodetector is connected to the data acquisition and processing unit, which calculates the laser frequency drift of the laser under test 111 based on the intermediate frequency electrical signal output by the photodetector.

[0108] It should be noted that the data acquisition and processing unit can calculate the laser frequency drift of the laser under test 111 in real time, or it can calculate the laser frequency drift of the laser under test 111 according to a set time interval, without making specific limitations here.

[0109] In this embodiment, the laser frequency drift calculation device 140 includes a second photodetector and a data acquisition and processing unit. The second photodetector is used to perform photoelectric conversion on the intermediate frequency signal light to obtain an intermediate frequency electrical signal. The data acquisition and processing unit is used to calculate the laser frequency drift of the laser under test 111 based on the intermediate frequency electrical signal. This system detects and compensates for the delay changes of the optical fiber link through the delay stabilization structure 120, and on this basis, through the delay self-coherent structure, transforms the measurement of the laser frequency drift under test into the measurement of the phase difference between the transmitted signals of the delay branch and the reference branch. This makes long-term real-time measurement of the laser frequency drift of the laser under test 111 possible and effectively improves the measurement accuracy of the laser frequency drift.

[0110] In some embodiments, Figure 2 A schematic diagram of the overall structure of the laser frequency drift measurement system based on time-delay self-heterodyne provided by the present invention is shown.

[0111] like Figure 2 As shown, firstly, on the left are two lasers 110, namely the laser under test 111 and the auxiliary laser 112.

[0112] In the circuit formed by the delay-stabilized structure 120, the radio frequency signal generated by the radio frequency signal generator is modulated onto the optical carrier output by the auxiliary laser 112 by the Mach-Zehnder modulator on the left, thus obtaining the optical carrier radio frequency signal.

[0113] In the loop of the optical delay autoheterodyne interferometer 130, the stable single-frequency laser output from the laser under test 111 is separated by the first fiber coupler to obtain the signal light transmitted on the delay branch and the reference light transmitted on the reference branch. The signal light is then passed through an acousto-optic frequency shifter to obtain a fixed-frequency shifted signal light.

[0114] Both the delay stabilization structure 120 and the optical delay self-heterodyne interferometer 130 have a multiplexing structure 150. Specifically, the optical radio frequency signal modulated by the Mach-Zehnder modulator and the signal light obtained by fixed frequency shifting by the acousto-optic frequency shifter are combined by the first wavelength division multiplexer in the multiplexing structure 150 to obtain the synthesized signal light.

[0115] The synthesized signal light passes sequentially through the delay fiber and the tunable optical delay line in the multiplexing structure 150 to obtain the compensated signal light (here the compensated signal light is obtained after delay compensation by the delay stabilization structure 120), and the second wavelength division multiplexer separates the compensated signal light of different wavelengths to obtain the corresponding detection signal light and delayed signal light.

[0116] After being separated by the second wavelength division multiplexer, the separated signal light to be detected is used as the input of the first photodetector for photoelectric conversion and demodulation to obtain the electrical signal to be detected, which contains a delayed radio frequency signal.

[0117] Then, the electrical signal to be detected and another radio frequency signal generated by the radio frequency signal generator that has not been transmitted through the optical fiber link are mixed by a mixer. The delay change of the optical fiber link is detected to obtain an error signal. Then, the phase-locked loop uses the error signal output by the mixer to negatively feedback control the adjustable optical delay line to compensate for the delay change of the optical fiber link and adjust the length of the optical fiber link.

[0118] The entire loop of the delay stabilization structure 120 repeats as described above, achieving an extremely stable tunable optical delay line.

[0119] The delayed signal light obtained by the second wavelength division multiplexer is polarized by a polarization tracker to obtain the interference signal light, which is the output of the delayed branch.

[0120] Next, the second fiber coupler combines the output of the delay branch with the output of the reference branch to obtain the intermediate frequency signal light.

[0121] Finally, the intermediate frequency signal light is used as the input of the second photodetector for photoelectric conversion to obtain an intermediate frequency electrical signal. Then, the data acquisition and processing unit calculates the laser frequency drift of the laser under test 111 based on the intermediate frequency electrical signal output by the second photodetector.

[0122] In some other embodiments, Figure 3A schematic diagram of the overlap Allen deviation of the time-delay optical fiber provided by the present invention is shown.

[0123] Understandably, the stability of the delay arm (delay branch) is crucial for laser frequency drift measurement. Therefore, this embodiment evaluates the stability of the delay branch. Specifically, a network analyzer can be used to generate a 24 GHz radio frequency signal, which is then transmitted through the delay branch, and its phase change is measured. The overlap Allen error is calculated using the measured phase data.

[0124] like Figure 3 As shown, when the transmission delay of the fiber optic link changes with the environment, its stability is within 10. -14 Approximately 6.39 × 10⁻⁶ after compensating for delay jitter using a delay-stabilized structure. The overlap Allen bias decreases linearly with increasing averaging time. Within a 1000s averaging period, the overlap Allen bias reaches 6.39 × 10⁻⁶. -18 It exhibits excellent long-term frequency stability.

[0125] In yet other embodiments, Figure 4 A schematic diagram illustrating the measurement accuracy characterization of laser frequency drift provided by the present invention is shown.

[0126] It is understood that, in order to characterize the measurement accuracy of the laser frequency drift measurement system based on time-delay self-heterodyne provided by this invention, this embodiment applies a known periodic sawtooth wave modulation to the output laser of the laser under test, wherein the sweep frequency magnitude is 5kHz and the sweep period is 0.5s. The frequency domain diagram of the measurement result is shown below. Figure 4 As shown.

[0127] Due to the stability of the delay fiber and the high sampling rate of TIA, this small and fast frequency change can be clearly observed in the frequency domain, and even the third harmonic can be distinguished, indicating that the measurement system provided by this invention can accurately measure kHz frequency changes.

[0128] In the last few embodiments, Figure 5 A schematic diagram illustrating the laser frequency drift characteristics of different types of lasers provided by this invention is shown.

[0129] It is understood that this embodiment illustrates the frequency drift characteristics of three different lasers after being powered on, such as... Figure 5 As shown, although all lasers slow down their frequency drift and tend to stabilize within a few hundred seconds after being powered on, they behave differently.

[0130] Among them, the fiber laser E15 has a large frequency spike after startup; the RIO semiconductor laser's output frequency always drifts in the same direction; the FITEL distributed feedback laser module has the largest frequency drift and large phase noise.

[0131] The above results can help study long-term real-time laser behavior after startup and provide more information for laser selection, which also demonstrates the long-term measurement capability of the laser frequency drift measurement system proposed in this invention.

[0132] Based on the laser frequency drift measurement system based on time-delay self-heterodyne described above, this invention also provides a corresponding measurement method, namely, a laser frequency drift measurement method based on time-delay self-heterodyne.

[0133] Specifically, Figure 6 This invention illustrates a laser frequency drift measurement method based on time-delay self-heterodyne, as provided by the present invention. Figure 6 As shown, the method includes:

[0134] S610, acquire the laser to be tested output from the laser under test, and the auxiliary laser output from the auxiliary laser;

[0135] The S620 uses radio frequency signals and auxiliary lasers to detect and compensate for delay variations in fiber optic links and adjust the length of the fiber optic links.

[0136] S630 modulates the laser to be detected based on the fiber optic link with adjusted length to obtain intermediate frequency signal light;

[0137] S640 calculates the laser frequency drift of the laser under test based on the intermediate frequency signal light.

[0138] It should be noted that this method uses a laser frequency drift measurement system based on time-delay self-heterodyne as the execution subject.

[0139] In some embodiments, the radio frequency signal includes a first radio frequency signal transmitted via an optical fiber link and a second radio frequency signal not transmitted via an optical fiber link;

[0140] Accordingly, the delay variation of the fiber optic link is detected and compensated using radio frequency (RF) signals and an auxiliary laser, including: modulating a first RF signal onto an auxiliary laser to obtain an optical RF signal; transmitting the optical RF signal sequentially through a delay fiber and a tunable optical delay line, then separating it and performing photoelectric conversion by a photodetector to obtain a signal light to be detected, the signal light to be detected including the delayed first RF signal; mixing a second RF signal with the signal light to be detected to obtain an error signal; and compensating for the delay variation of the fiber optic link based on the error signal.

[0141] It should also be noted that the laser frequency drift measurement method based on time-delay self-heterodyne provided in this embodiment can be referred to in correspondence with the laser frequency drift measurement system based on time-delay self-heterodyne described above, and will not be elaborated on here.

[0142] In this embodiment, the method acquires the laser output from the laser under test (DUT) and the auxiliary laser output from the auxiliary laser. Using radio frequency signals and the auxiliary laser, it detects and compensates for delay variations in the fiber optic link, adjusts the length of the fiber optic link, and then modulates the DUT based on the adjusted link to obtain an intermediate frequency (IF) signal. The laser frequency drift of the DUT is then calculated from this IF signal. This method detects and compensates for delay variations in the fiber optic link using a delay-stabilized structure. Furthermore, by employing a delay-self-coherent structure, it transforms the measurement of the DUT's frequency drift into the measurement of the phase difference between the transmitted signals of the delay branch and the reference branch. This enables long-term real-time measurement of the DUT's laser frequency drift and effectively improves the measurement accuracy.

[0143] The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Those skilled in the art can understand and implement this without any creative effort.

[0144] Through the above description of the embodiments, those skilled in the art can clearly understand that each embodiment can be implemented by means of software plus necessary general-purpose hardware platforms, and of course, it can also be implemented by hardware. Based on this understanding, the above technical solutions, in essence or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods described in the various embodiments or some parts of the embodiments.

[0145] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A laser frequency drift measurement system based on time-delay self-heterodyne, characterized in that, include: A laser, including the laser under test and an auxiliary laser, is used to output a stable single-frequency laser. A delay stabilization structure is used to detect and compensate for delay changes in the fiber optic link based on the radio frequency signal and the single-frequency laser output from the auxiliary laser, and to adjust the length of the fiber optic link. An optical delay self-heterodyne interferometer, connected to the delay stabilization structure, is used to modulate the single-frequency laser output by the laser under test based on the fiber optic link with adjusted length, to obtain intermediate frequency signal light; A laser frequency drift calculation device, connected to the optical delay self-heterodyne interferometer, is used to calculate the laser frequency drift of the laser under test based on the intermediate frequency signal light. Both the delay stabilization structure and the optical delay self-heterodyne interferometer include a multiplexing structure, which is used to synthesize, transmit, and separate the single-frequency laser output from the laser under test. The delay-stabilizing structure includes: A modulation unit is used to modulate the radio frequency signal onto the single-frequency laser output by the auxiliary laser to obtain an optical radio frequency signal; A delay compensation unit, connected to the multiplexing structure, is used to detect and compensate for the delay changes of the optical fiber link based on the signal light to be detected output by the multiplexing structure, and to adjust the length of the optical fiber link. The modulation unit includes: Radio frequency (RF) signal generator, used to generate RF signals; A Mach-Zehnder modulator, connected to the radio frequency signal generator, is used to modulate the radio frequency signal onto the single-frequency laser output by the auxiliary laser to obtain an optical radio frequency signal. The delay compensation unit includes: The first photodetector is used to perform photoelectric conversion on the signal light to be detected output by the multiplexing structure and demodulate it to obtain the electrical signal to be detected, wherein the electrical signal to be detected includes the radio frequency signal after a delay. A mixer is used to mix the electrical signal to be detected with the radio frequency signal output by the radio frequency signal generator that has not been transmitted, in order to obtain an error signal; A phase-locked loop is used to integrate and amplify the error signal, and to use negative feedback to control the adjustable optical delay line in the multiplexing structure to compensate for the delay variation of the optical fiber link, so as to adjust the length of the optical fiber link. The reuse structure includes: The first wavelength division multiplexer is used to synthesize the optical radio frequency signal output by the modulation unit in the delay-stabilized structure with the frequency-shifted signal light to obtain the synthesized signal light. A delay fiber, connected to the first wavelength division multiplexer, is used to transmit the synthesized signal light and delay it; A tunable optical delay line is connected to the delay fiber and is used to compensate for the delay jitter caused by the environmental influence of the fiber link during the transmission of the synthesized signal light after delay, so as to obtain the compensated signal light. The second wavelength division multiplexer, connected to the tunable optical delay line, is used to separate the compensated signal light of different wavelengths to obtain the signal light to be detected and the delayed signal light.

2. The laser frequency drift measurement system based on time-delay self-heterodyne according to claim 1, characterized in that, The optical delayed self-heterodyne interferometer includes: The first fiber coupler is used to separate the single-frequency laser output from the laser under test by a preset ratio to obtain signal light and reference light; A delay branch is used to delay the transmission of the signal light to obtain interference signal light; A reference branch is used to transmit the reference light; The second fiber coupler is used to combine the reference light and the interference signal light to obtain the intermediate frequency signal light.

3. The laser frequency drift measurement system based on time-delay self-heterodyne according to claim 2, characterized in that, The delay branch includes: An acousto-optic frequency shifter is used to apply a fixed frequency shift to the signal light; A polarization tracker is used to perform polarization state tracking control on the delayed signal light output by the multiplexing structure, and to maintain the polarization state of the delayed signal light.

4. The laser frequency drift measurement system based on time-delay self-heterodyne according to any one of claims 1-3, characterized in that, The laser frequency drift calculation device includes: The second photodetector is connected to the optical delayed self-heterodyne interferometer and is used to perform photoelectric conversion on the intermediate frequency signal light to obtain an intermediate frequency electrical signal. The data acquisition and processing unit, connected to the second photodetector, is used to calculate the laser frequency drift of the laser under test based on the intermediate frequency electrical signal.

5. A laser frequency drift measurement method based on time-delay self-heterodyne, applied to the laser frequency drift measurement system based on time-delay self-heterodyne as described in any one of claims 1-4, characterized in that, include: Acquire the laser output from the laser under test and the auxiliary laser output from the auxiliary laser; The delay variation of the fiber optic link is detected and compensated using radio frequency signals and the auxiliary laser, and the length of the fiber optic link is adjusted. Based on the fiber optic link with adjusted length, the laser to be detected is modulated to obtain intermediate frequency signal light; The laser frequency drift of the laser under test is calculated based on the intermediate frequency signal light.

6. The laser frequency drift measurement method based on time-delay self-heterodyne according to claim 5, characterized in that, The radio frequency signal includes a first radio frequency signal transmitted via the optical fiber link and a second radio frequency signal not transmitted via the optical fiber link; The method of detecting and compensating for delay variations in the fiber optic link using radio frequency signals and the auxiliary laser includes: The first radio frequency signal is modulated onto the auxiliary laser to obtain an optical radio frequency signal; The optical radio frequency signal is transmitted sequentially through a delay fiber and a tunable optical delay line, then separated, and photoelectrically converted by a photodetector to obtain the signal light to be detected, which includes the first radio frequency signal after delay. The second radio frequency signal is optically mixed with the signal to be detected to obtain an error signal; The delay variation of the optical fiber link is compensated based on the error signal.

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