Digital decoherence-based laser linewidth measurement method and device

Abstract

Provided in the present disclosure are a digital decoherence-based laser linewidth measurement method and device, which are applicable to the technical field of laser linewidth analysis. The method comprises: splitting into first delayed light and first reference light a laser source to be measured; at least on the basis of the first delayed light and the first reference light, performing digital decoherence to determine frequency noise power spectral density of a laser to be measured corresponding to said laser source; and, on the basis of the frequency noise power spectral density, determining a laser linewidth of said laser. By means of digital decoherence, obtaining frequency noise power spectral density for determining a laser linewidth does not require a laser source to be measured and an auxiliary laser source to have a consistent central wavelength, and does not require a complex physical optical fiber link, so as to avoid introducing a large amount of environmental noise, thereby improving the flexibility, universality and precision of laser linewidth measurement.

Description

A method and apparatus for measuring laser linewidth based on digital decoherence

[0001] This invention claims priority to Chinese Patent Application No. 202510478148.4, filed on April 16, 2025, entitled "A Laser Linewidth Measurement Method and Apparatus Based on Digital Decoherence", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This invention relates to the field of laser linewidth analysis technology, specifically to a laser linewidth measurement method and apparatus based on digital decoherence. Background Technology

[0003] Phase noise and linewidth are key parameters for evaluating the frequency stability and phase noise characteristics of lasers, and are of great significance for high-precision laser measurement, optical communication, and quantum technology. Traditional methods for measuring laser phase noise and linewidth mainly include the beat frequency method and the time-delay self-heterodyne method.

[0004] However, when using the beat frequency method to measure a laser, one or more reference lasers are required, and the reference laser and the laser under test must have the same center wavelength. This results in poor flexibility and universality of the beat frequency method for measuring laser linewidth.

[0005] When measuring a laser using the time-delay self-heterodyne method, the fiber optic link used during the delay process introduces a large amount of environmental noise, which affects the measurement accuracy of the laser linewidth. Summary of the Invention

[0006] In view of this, embodiments of the present invention provide a laser linewidth measurement method and apparatus based on digital decoherence to solve the problems of poor flexibility, poor universality and poor measurement accuracy of existing traditional laser linewidth measurement methods.

[0007] To achieve the above objectives, the embodiments of the present invention provide the following technical solutions:

[0008] The first aspect of this invention discloses a laser linewidth measurement method based on digital decoherence, the method comprising:

[0009] The laser source under test is divided into a first delay light and a first reference light;

[0010] Digital decoherence is performed at least based on the first delayed light and the first reference light to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test.

[0011] The laser linewidth of the laser under test is determined based on the frequency noise power spectral density.

[0012] Preferably, digital decoherence is performed at least based on the first delayed light and the first reference light to determine the frequency noise power spectral density of the laser source under test, including:

[0013] The auxiliary laser source is divided into a second delay light and a second reference light;

[0014] Using the first delayed light, the first reference light, the second delayed light, and the second reference light, the first phase difference data of the laser source under test and the second phase difference data of the auxiliary laser source are demodulated.

[0015] Based on the first phase difference data and the second phase difference data, noise in the optical fiber link is eliminated to obtain the third phase difference data;

[0016] The first phase difference data is digitally decohered to obtain the fourth phase difference data, and the third phase difference data is digitally decohered to obtain the fifth phase difference data.

[0017] Using the fourth phase difference data and the fifth phase difference data, the frequency noise power spectral density of the laser under test corresponding to the laser source under test is determined.

[0018] Preferably, digital decoherence is performed at least based on the first delayed light and the first reference light to determine the frequency noise power spectral density of the laser source under test, including:

[0019] The first phase difference data of the laser source under test is obtained by demodulation using the first delayed light and the first reference light;

[0020] Digital decoherence is performed on the first phase difference data to obtain the fourth phase difference data;

[0021] Based on the fourth phase difference data, the frequency noise power spectral density of the laser under test corresponding to the laser source under test is determined.

[0022] Preferably, the first phase difference data of the laser source under test and the second phase difference data of the auxiliary laser source are demodulated using the first delayed light, the first reference light, the second delayed light, and the second reference light, including:

[0023] The first and second delay times are combined and transmitted to the delayed fiber link and frequency-shifted to obtain the third delay time;

[0024] The third delayed light and the first reference light are combined and interferometric measurements are performed to obtain a first interference signal, and the third delayed light and the second reference light are combined and interferometric measurements are performed to obtain a second interference signal;

[0025] The first interference signal is demodulated to obtain the first phase difference data, and the second interference signal is demodulated to obtain the second phase difference data.

[0026] Preferably, the first phase difference data of the laser source under test is obtained by demodulating the first delayed light and the first reference light, including:

[0027] The first delayed time is transmitted to the delayed fiber link and frequency shifted to obtain the fourth delayed time;

[0028] The fourth delayed light and the first reference light are combined and interferometrically measured using a photodetector to obtain a third interference signal;

[0029] The third interference signal is demodulated to obtain the first phase difference data.

[0030] Preferably, digital decoherence is performed on the first phase difference data to obtain the fourth phase difference data, including:

[0031] The first phase difference data is delayed by a specified time to form new phase difference data;

[0032] The new phase difference data is compared with the first phase difference data to obtain output phase difference data with twice the specified time delay;

[0033] The new phase difference data is delayed by the specified time delay in order to update the new phase difference data;

[0034] The updated new phase difference data is compared with the output phase difference data having i-1 times the specified time delay to obtain the output phase difference data having i times the specified time delay, where the initial value of i is 3;

[0035] Increment i by 1, return to the step of delaying the new phase difference data by the specified time to update the updated new phase difference data again, until i is greater than m to obtain the fourth phase difference data;

[0036] The fourth phase difference data is: output phase difference data with m times the specified time delay.

[0037] Preferably, the frequency noise power spectral density of the laser under test corresponding to the laser source under test is determined using the fourth phase difference data and the fifth phase difference data, including:

[0038] The cross-power spectrum of the fourth phase difference data and the fifth phase difference data is calculated multiple times;

[0039] The cross power spectrum obtained from multiple calculations is averaged to obtain the phase noise power spectral density of the laser under test corresponding to the laser source under test;

[0040] The frequency noise power spectral density of the laser under test is determined using the phase noise power spectral density.

[0041] Preferably, determining the frequency noise power spectral density of the laser under test corresponding to the laser source under test based on the fourth phase difference data includes:

[0042] The fourth phase difference data is subjected to Fourier transform to obtain the phase noise power spectral density of the laser under test corresponding to the laser source under test.

[0043] The frequency noise power spectral density of the laser under test is determined using the phase noise power spectral density.

[0044] Preferably, determining the laser linewidth of the laser under test based on the frequency noise power spectral density includes:

[0045] The laser linewidth of the laser under test is determined by weighted integration of the frequency noise power spectral density using the β-line method, direct integration method, or piecewise integration method.

[0046] A second aspect of this invention discloses a laser linewidth measurement device based on digital decoherence, the device comprising:

[0047] The processing module is used to divide the laser source under test into a first delayed light and a first reference light;

[0048] A decoherence module is used to perform digital decoherence based at least on the first delayed light and the first reference light to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test.

[0049] The determination module is used to determine the laser linewidth of the laser under test based on the frequency noise power spectral density.

[0050] Based on the above embodiments of the present invention, a laser linewidth measurement method and apparatus based on digital decoherence are provided. The method comprises: dividing the laser source under test into a first delay light and a first reference light; performing digital decoherence based at least on the first delay light and the first reference light to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test; and determining the laser linewidth of the laser under test based on the frequency noise power spectral density. Determining the frequency noise power spectral density used to determine the laser linewidth through digital decoherence does not require the laser source under test and the auxiliary laser source to have the same center wavelength, nor does it require complex physical fiber optic links to avoid introducing a large amount of environmental noise, thus improving the flexibility, universality, and accuracy of laser linewidth measurement. Attached Figure Description

[0051] To more clearly illustrate the technical solutions in the embodiments of the present 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 only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0052] Figure 1 is a schematic diagram of the principle architecture for obtaining power spectral density based on digital decoherence according to an embodiment of the present invention;

[0053] Figure 2 is a flowchart of a laser linewidth measurement method based on digital decoherence provided in an embodiment of the present invention;

[0054] Figure 3 is a flowchart of the first method for obtaining frequency noise power spectral density provided by an embodiment of the present invention;

[0055] Figure 4 is a flowchart of the second method for obtaining frequency noise power spectral density provided in an embodiment of the present invention;

[0056] Figure 5 is a flowchart of digital decoherence of the first phase difference data to obtain the fourth phase difference data according to an embodiment of the present invention.

[0057] Figure 6 is a structural example diagram of the hybrid triangular cap system provided in an embodiment of the present invention;

[0058] Figure 7 is a structural block diagram of a laser linewidth measurement device based on digital decoherence provided in an embodiment of the present invention. Detailed Implementation

[0059] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0060] In this application, the terms "comprising," "including," or any other variations thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0061] Phase noise and linewidth are key parameters for evaluating the frequency stability and phase noise characteristics of lasers, and are of great significance for high-precision laser measurement, optical communication, and quantum technology. Phase noise refers to the fluctuation of the laser output phase, and is usually characterized in the frequency domain by power spectral density (including phase noise power spectral density and frequency noise power spectral density). Phase noise reflects the short-time random changes in the phase and frequency of the laser.

[0062] Linewidth is a parameter describing the width of the laser output spectrum. It is closely related to the phase noise of the laser, and lasers with narrow linewidths can provide higher frequency stability. High-precision phase noise and linewidth measurements are crucial for the quality assessment of laser sources, especially in optical atomic clocks, lidar, and precision instruments.

[0063] Traditional methods for measuring the phase noise and linewidth of lasers mainly include direct spectral analysis, beat frequency method, and time-delay self-heterodyne method. The following is a brief explanation of these traditional measurement methods.

[0064] Direct spectral analysis measures the linewidth of a laser by directly detecting the spectrum of its output signal. Specifically, the laser signal is fed into a spectrum analyzer, which displays the frequency distribution of the laser signal. The linewidth refers to the width of the power distribution in the spectrum, typically expressed as the half-power point width (3dB bandwidth) of the laser frequency. Direct spectral analysis can obtain the spectral morphology of a laser and is commonly used to evaluate laser sources with relatively wide linewidths.

[0065] The evaluation of narrow-linewidth laser sources is generally achieved through the beat frequency method. Specifically, the laser under test is subjected to interferometry with a reference laser that has a similar center wavelength and a more stable noise level, thereby obtaining phase difference data or frequency shift. The principle is that the interference of two laser signals with similar center wavelengths produces a low-frequency beat frequency signal, the frequency of which is equal to the frequency difference between the two signals. By detecting and analyzing the phase fluctuations of the beat frequency signal, the phase noise of the laser under test can be obtained, and further, the linewidth of the laser under test can be determined. When the reference laser and the laser under test are at the same level, the phase noise spectrum of the beat frequency signal is twice that of the laser under test; when the reference laser is superior to the laser under test, the phase noise spectrum of the beat frequency signal is the same as the phase noise spectrum of the laser under test. The beat frequency method has a relatively simple structure and is suitable for measuring lasers with extremely narrow linewidths.

[0066] The time-delay self-heterodyne method is another commonly used method for measuring laser phase noise spectrum. Its principle is similar to the beat frequency method, but it does not require an additional auxiliary laser source. In the time-delay self-heterodyne method, the laser signal is split into two beams by an optical coupler. One beam is delayed beyond the coherence length via an optical fiber link and can be considered as another reference light source incoherent with the original laser source. The interference between the delayed and undelayed beams yields a beat frequency signal. By detecting and analyzing the phase fluctuations of the beat frequency signal, the phase noise of the laser under test can be obtained, and the linewidth of the laser under test can be further acquired. At this point, the reference laser and the laser under test are horizontally aligned, and the phase noise spectrum of the beat frequency signal is twice that of the phase noise spectrum of the laser under test. The time-delay self-heterodyne method does not require an auxiliary laser source, reducing system complexity, and is widely used in the evaluation of laser sources with linewidths of kHz and above.

[0067] The inventors discovered through research that direct spectral analysis has some limitations when measuring narrow-linewidth lasers. Because the frequency resolution of a spectrum analyzer is limited, especially for lasers with very small frequency fluctuations, the limited resolution may be insufficient to accurately measure the laser linewidth. Therefore, for narrow-linewidth lasers, beat frequency analysis or time-delay self-heterodyne analysis is generally used.

[0068] When measuring lasers using the beat frequency method, one or more reference lasers are often required. To achieve interferometric measurements between the reference laser and the laser under test (DUT), both lasers typically need to have the same center wavelength, and the linewidth of the reference laser should be comparable to or better than that of the DUT. Furthermore, due to the inherent frequency drift of the laser, the beat frequency method is often difficult to perform long-term interferometric measurements. These factors limit the application of the beat frequency method in evaluating narrow-linewidth lasers.

[0069] The time-delay self-heterodyne method uses the laser's own delayed light as a reference source, and it is not limited by the beat frequency method. However, when measuring lasers with Hz-level linewidths, the theoretical delay fiber length needs to reach 200,000 kilometers, which is a huge challenge for the implementation of the physical system. In addition, the fiber optic link used during the delay process introduces a large amount of environmental noise, which limits the measurement accuracy of the time-delay self-heterodyne method.

[0070] In general, the problems in traditional methods for measuring phase noise and linewidth of lasers can be divided into two main categories. One problem is the excessively high requirements for the reference source (beat frequency method), which makes the measurement method less flexible and universal. The other problem is the complexity of the physical system, insufficient measurement accuracy, and susceptibility to noise interference (delay self-heterodyne method), which makes the measurement method unsuitable for Hz-level narrow linewidth lasers.

[0071] To address the aforementioned issues, this invention proposes a laser linewidth measurement method and apparatus based on digital decoherence. By using digital decoherence to determine the frequency noise power spectral density used to determine the laser linewidth, it eliminates the requirement for the laser source under test and the auxiliary laser source to have the same center wavelength, and also eliminates the need for complex physical fiber optic links to avoid introducing a large amount of environmental noise, thereby improving the flexibility, universality, and accuracy of laser linewidth measurement.

[0072] In other words, this solution does not require a reference laser with the same linewidth and center wavelength as the laser source under test, nor does it require a complex physical system consisting of ultra-long fiber optic links. It can measure laser linewidth for a long time, thus improving the flexibility, universality and accuracy of laser linewidth measurement.

[0073] It should be noted that the basic idea of ​​digital decoherence adopted in this scheme is to use digital delay to replace the physical delay of the optical fiber link. Based on the self-heterodyne detection of short optical fiber link delay, signal processing is used to multiply the physical delay, and finally the comparative measurement information of two incoherent light sources is obtained.

[0074] In traditional time-delay self-heterodyne measurements, an optical fiber link exceeding the coherence length of the laser source is required to decoherently separate the delayed light from the reference light. Therefore, theoretically, evaluating a Hz-level laser source requires an optical fiber link exceeding 200,000 kilometers, which not only makes the physical system impractical but also introduces significant environmental noise. The digital decoherence method used in this solution, however, can multiply the physical delay through digital delay on a short-delay optical fiber.

[0075] To better understand the main content of this scheme, a brief introduction to the basic principle of digital decoherence used in this scheme is given below (taking the laser source under test as an example):

[0076] It should be noted that the laser source under test consists of a delayed light beam and a reference light beam.

[0077] Step 1: Perform time-delay self-heterodyne interferometry on the laser source under test. The time delay of the fiber optic link is set to τ0 (i.e., the specified time delay mentioned later, where τ0 is less than the coherence time of the laser source under test). Specifically, using a short-delay self-heterodyne interferometry system, the interference signal between the delayed light and the reference light can be measured at the photodetector. The interference measurement result (i.e., the measured interference signal) is then demodulated using IQ after passing through a low-noise RF power amplifier (LNA), filters, and other RF devices to output the initial phase difference data between the delayed light and the reference light. Where IQ represents in-phase and quadrature, the initial phase difference data. The specific details are shown in formula (1).

[0078] In formula (1), The phase fluctuation of the laser source over time is the same as the phase fluctuation of the reference light over time; t is the independent variable characterizing time. To account for the phase fluctuations of the delayed light over time, there is a time delay of τ0 compared to the reference light.

[0079] Step 2: Obtain the initial phase difference data The delay is specified by a time delay (i.e., delay τ0), which forms new phase difference data. New phase difference data Compared with the initial phase difference data The comparison is performed (the comparison process can be simply referred to as comparison measurement), and the corresponding output phase difference data with a time delay of 2τ0 is output. That is, the output phase difference data It has a delay of twice the specified time.

[0080] It should be noted that, as can be seen from the initial phase difference data and the new phase difference data formed in step 2, and have This identical part, therefore and Comparison can eliminate phase noise This results in the output phase difference data with a corresponding time delay of 2τ0. Output phase difference data The specific details are shown in formula (2) below.

[0081] Step 3: Repeat the operation of step 2 above for each subsequent step until step m.

[0082] For example: i represents the step number; in the i-th step, the phase difference data is... (In step 3, i.e., when i = 3, this term is) The delay is specified by a time delay (i.e., delay τ0), which forms new phase difference data. (In step 3, i.e., when i = 3, this term is)

[0083] It should be noted that the output phase difference data obtained in step i-1 (i.e., the previous step) is (When i = 3, this term is)

[0084] This new phase difference data The output phase difference data obtained in step i-1 By comparison, we can see and Having the same parts Therefore, phase noise can be eliminated. This outputs the corresponding phase difference data with a time delay of iτ0. The corresponding output phase difference data with a time delay of iτ0 The specific details are shown in formula (3) below.

[0085] The above steps can accurately multiply the physical delay caused by short fiber optic links through a digital system. By executing the above steps up to the m-th step (until the i=m-th step is completed), the delay of the output phase difference data is extended from τ0 to mτ0.

[0086] It should be noted that the coherence time of a laser is determined by the properties of the laser source itself. If mτ0 is greater than the coherence time of the laser source under test, digital decoherence can be achieved, that is, the coherence time of the laser source under test can be obtained. Another beam of light obtained by delaying the time beyond the coherence time The comparison results.

[0087] For example, when the laser source under test is a Hz-level laser, digital decoherence can be achieved if mτ0 > 1s; when the laser source under test is a sub-Hz-level laser, digital decoherence can be achieved if mτ0 > 10s.

[0088] Furthermore, by performing a Fourier transform (FFT) on the obtained output phase difference data, the power spectral density of the phase difference data (including the phase noise power spectral density and the frequency noise power spectral density) can be obtained. The specific contents of the power spectral density of the phase difference data are shown in formula (4).

[0089] In formula (4), The power spectral density of the phase difference data is given by the independent variable f. For the phase noise power spectral density of the reference light, Let mτ0 be the phase noise power spectral density of the time delay. The cross-power spectrum of the reference light and the delayed light.

[0090] Since the reference light and the delayed light both originate from the same laser source, they satisfy... in, Let be the phase noise power spectral density of the laser source under test. Since the delay time exceeds the coherence time of the laser source under test, the reference light and the delayed light are incoherent at this point, meaning the cross-power spectrum exhibits random fluctuations, which can be neglected during long-term measurements.

[0091] The phase noise power spectral density of the laser source under test can be solved from the power spectral density of the obtained phase difference data. Furthermore, based on... The frequency noise power spectral density S of the laser source under test can be obtained. ν (f) and calculate the laser linewidth according to the β-line method.

[0092] In practical applications, since the power spectral density data in the full frequency domain is large, it is generally possible to perform Fourier transform in segments by filtering and downsampling to obtain the power spectral density of the high-frequency, mid-frequency and low-frequency bands respectively.

[0093] In summary, the principle of obtaining power spectral density based on digital decoherence is shown in Figure 1. In Figure 1, the "delayed self-heterodyne interference signal" is the interference signal between the delayed light and the reference light, and the "short delay phase difference data 0-1" is the initial phase difference data. "Short delay phase difference data 1-2" is... New phase difference data obtained by time delay τ0 "Short delay phase difference data 2-3" is... New phase difference data obtained by time delay τ0 And so on.

[0094] The "long-delay phase difference data 0-2" in Figure 1 is the output phase difference data. The long delay phase difference data 0-3” is the output phase difference data. Similarly, by using filtering and downsampling, Fourier transforms are performed in segments to obtain the power spectral density of the high-frequency, mid-frequency, and low-frequency bands, respectively.

[0095] The above is an explanation of the principle of obtaining power spectral density based on digital decoherence.

[0096] It is worth noting that the fluctuation of the laser output frequency is the frequency noise of the laser source, and its frequency domain is the frequency noise power spectral density. The phase noise and frequency noise of the laser source can be obtained according to different measurement methods, and the two can be converted to each other.

[0097] This scheme measures laser linewidth based on the aforementioned principle of "obtaining power spectral density based on digital decoherence". Referring to Figure 2, a flowchart of a laser linewidth measurement method based on digital decoherence provided by an embodiment of the present invention is shown. Figure 2 includes the following steps:

[0098] Step S201: Divide the laser source to be tested into a first delay light and a first reference light.

[0099] In the specific implementation step S201, the laser source under test is divided into two paths: one is the delay light and the other is the reference light. The delay light and the reference light separated from the laser source under test are called the first delay light and the first reference light, respectively.

[0100] Step S202: Perform digital decoherence at least based on the first delayed light and the first reference light to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test.

[0101] In the specific implementation of step S202, digital decoherence is performed at least based on the first delayed light and the first reference light to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test.

[0102] It is worth noting that this scheme provides two methods for obtaining the frequency noise power spectral density, which are referred to as "the first method for obtaining the frequency noise power spectral density" and "the second method for obtaining the frequency noise power spectral density," respectively.

[0103] The first method of obtaining frequency noise power spectral density requires the activation of an auxiliary laser source (also known as a reference laser source), while the second method does not require the activation of an auxiliary laser source.

[0104] In practical applications, you can choose to use either the "first method to obtain the frequency noise power spectral density" or the "second method to obtain the frequency noise power spectral density" depending on the actual situation.

[0105] For example, when measuring a laser under test with a noise level better than the test environment, an auxiliary laser source is activated and the "first method for obtaining the frequency noise power spectral density" is used to determine the frequency noise power spectral density of the laser under test.

[0106] For example, when measuring a laser under test with a noise level worse than that of the test environment, the auxiliary laser source is not activated, and the frequency noise power spectral density of the laser under test is determined by the "second method of obtaining frequency noise power spectral density".

[0107] The implementation process of the above-mentioned "first method for obtaining frequency noise power spectral density" will be described in detail in the embodiment shown in Figure 3 below.

[0108] The implementation process of the above-mentioned "second method for obtaining frequency noise power spectral density" will be described in detail in the embodiment shown in Figure 4 below.

[0109] Step S203: Determine the laser linewidth of the laser under test based on the frequency noise power spectral density.

[0110] In the specific implementation step S203, after obtaining the frequency noise power spectral density of the laser under test, the frequency noise power spectral density is weighted and integrated using the β-line method, direct integration method, or piecewise integration method to determine the laser linewidth of the laser under test.

[0111] Specifically, the laser linewidth of the laser under test is determined by weighted integration of the frequency noise power spectral density. The weighted integration can be performed using the β-line method, direct integration method, or piecewise integration method.

[0112] In this embodiment of the invention, the frequency noise power spectral density used to determine the laser linewidth is obtained by digital decoherence. This does not require the laser source under test and the auxiliary laser source to have the same center wavelength, nor does it require a complex physical fiber optic link to avoid introducing a large amount of environmental noise, thereby improving the flexibility, universality and accuracy of laser linewidth measurement.

[0113] Regarding the "first method for obtaining frequency noise power spectral density" involved in step S202 of Figure 2 of the above embodiment of the present invention, referring to Figure 3, a flowchart of the first method for obtaining frequency noise power spectral density provided by the embodiment of the present invention is shown. Figure 3 includes the following steps:

[0114] Step S301: Divide the auxiliary laser source into a second delay light and a second reference light.

[0115] In the specific implementation of step S301, when measuring a laser under test with a noise level better than the test environment, an auxiliary laser source is activated and the auxiliary laser source is divided into two paths: one is a delay light and the other is a reference light. The delay light and the reference light separated from the auxiliary laser source are called the second delay light and the second reference light.

[0116] Step S302: Using the first delayed light, the first reference light, the second delayed light, and the second reference light, demodulate to obtain the first phase difference data of the laser source under test and the second phase difference data of the auxiliary laser source.

[0117] In the specific implementation step S302, the first delay time and the second delay time are combined and transmitted to the delayed optical fiber link and frequency shifted to obtain the third delay time.

[0118] The third delayed light and the first reference light are combined and interferometric measurements are performed to obtain the first interference signal, and the third delayed light and the second reference light are combined and interferometric measurements are performed to obtain the second interference signal.

[0119] The first interference signal is demodulated to obtain the first phase difference data, and the second interference signal is demodulated to obtain the second phase difference data.

[0120] Specifically, the first delay time of the laser source under test and the second delay time of the auxiliary laser source are combined by an optical fiber coupler and transmitted to the delay optical fiber link, and then frequency-shifted by an acousto-optic modulator to obtain the third delay time.

[0121] The third delayed light beam is split into two beams, which are then combined with the first and second reference beams, respectively. After passing through an optical filter, interferometry is performed at a photodetector. The power of the interference signal obtained from the interferometry is amplified using a low-noise amplifier, and redundant noise is filtered out using a filter. Finally, a high-speed data acquisition card is used to acquire the first and second interference signals.

[0122] By performing digital IQ demodulation on the first and second interference signals respectively, the first phase difference data and the second phase difference data can be obtained.

[0123] It should be noted that the frequencies of the delayed light and the reference light (such as the third delayed light and the first reference light) involved in the interference are deviated. The function of the acousto-optic modulator is to shift the frequency of the light, which will cause the frequencies of the delayed light and the reference light to deviate. The subsequent interference process on the photodetector is called heterodyne interference.

[0124] In some embodiments, the first phase difference data and the second phase difference data obtained above have a specified time delay (i.e., have τ0). In the case of using the "first method of obtaining frequency noise power spectral density", the first phase difference data (denoted as x in this case) T The specific content of (t) is shown in formula (5), and the second phase difference data (referred to as x at this time) is... R The specific content of (t) is shown in formula (6).

[0125] In formulas (5) and (6), x T (t) represents the first phase difference data (i.e., the delayed self-heterodyne interference result of the laser source under test), x R (t) represents the second phase difference data (i.e., the delayed self-heterodyne interference result of the auxiliary laser source); and Link noise is introduced by fluctuations in the environment in which the fiber optic link operates. λ R To assist the wavelength of the laser source, λ T λ is the wavelength of the laser source to be tested, n is the refractive index of the light transmitted in the optical fiber, and l(t) is the fluctuation of the length of the optical fiber link over time.

[0126] It should be noted that the third time delay consists of two parts: one part is the result obtained after delaying the first time delay, and the other part is the result obtained after delaying the second time delay. When the third time delay interferes with the first reference light, both of the aforementioned parts of the third time delay will interfere with the first reference light.

[0127] By filtering out other interference results through a radio frequency filter, only the interference results of the "first reference light" and the "light obtained after delaying the first delay light" in the third delay light are retained.

[0128] Similarly, other interference results are filtered out by another RF filter, and only the interference results of the "second reference light" and the "light obtained after delaying the second delay light" in the third delay light are retained.

[0129] In formulas (5) and (6), The phase fluctuation of the laser source under test over time is the same as the phase fluctuation of the first reference light over time. The phase of the portion of the third delayed time that is summed with the first reference light, corresponding to the first delayed time, fluctuates with time. To assist in the fluctuation of the laser source phase over time, i.e. the fluctuation of the second reference light phase over time; The phase of the portion of the third delay time that corresponds to the second delay time and is summed with the second reference light fluctuates with time.

[0130] It should be noted that for a laser beam, the perceived fiber optic link noise is inversely proportional to the laser wavelength. Since both the auxiliary laser source and the laser source under test enter the same fiber optic link, the perceived fiber optic link noise is directly proportional to the "fluctuation of the fiber optic link length over time l(t)". ​​Therefore, the waveform shape is consistent and the magnitude is inversely proportional to the laser wavelength.

[0131] Step S303: Based on the first phase difference data and the second phase difference data, eliminate noise in the optical fiber link to obtain the third phase difference data.

[0132] In the specific implementation step S303, noise in the optical fiber link is eliminated using the first phase difference data and the second phase difference data to obtain the third phase difference data x. T-R (t).

[0133] Specifically, in the process of obtaining the first and second phase difference data, the phase noise of the laser source under test and the auxiliary laser source are coupled together with the noise introduced by the fiber optic link. By effectively eliminating the noise in the fiber optic link, a measurement result containing the phase noise of the laser source under test and the auxiliary laser source can be obtained (i.e., the third phase difference data). The third phase difference data x T-R The specific content of (t) is shown in formula (7).

[0134] As can be seen from the content of the above formula (7), the third phase difference data has a specified time delay (i.e., it has τ0).

[0135] Step S304: Perform digital decoherence on the first phase difference data to obtain the fourth phase difference data, and perform digital decoherence on the third phase difference data to obtain the fifth phase difference data.

[0136] In the specific implementation step S304, the "principle of obtaining power spectral density based on digital decoherence" given above is used to process the first phase difference data x. T (t) Perform digital decoherence to obtain the fourth phase difference data (denoted as X). T (t)), for the third phase difference data x T-R (t) Perform digital decoherence to obtain the fifth phase difference data X T-R (t).

[0137] It should be noted that, when using the "first method to obtain the frequency noise power spectral density", the first phase difference data is equivalent to the "initial phase difference data" in the above "principle of obtaining power spectral density based on digital decoherence", and the fourth phase difference data is equivalent to the output phase difference data obtained in the m-th step (i=m) in the above "principle of obtaining power spectral density based on digital decoherence".

[0138] Similarly, the third phase difference data is equivalent to the "initial phase difference data" in the above "principle of obtaining power spectral density based on digital decoherence", and the fifth phase difference data is equivalent to the output phase difference data obtained in the m-th step (i=m) in the above "principle of obtaining power spectral density based on digital decoherence".

[0139] When using the "first method for obtaining frequency noise power spectral density", the fourth phase difference data X T The specific content of (t) is shown in formula (8), the fifth phase difference data X T-R The specific content of (t) is shown in formula (9).

[0140] As can be seen from Equations (8) and (9), after digital decoherence, the time delay of the phase noise of the laser source expands from τ0 (a specified time delay) to mτ0 (m times the specified time delay). The noise of the fiber optic link is expressed as N. fiber (t), N fiber (t) refers to the "link noise introduced by fluctuations in the environment of the optical fiber link" mentioned in formula (5) above. "The result obtained after digital decoherence."

[0141] In some embodiments, the fourth phase difference data and the fifth phase difference data have a specified time delay of m times, which is greater than the coherence time of the laser under test.

[0142] Step S305: Using the fourth phase difference data and the fifth phase difference data, determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test.

[0143] In the specific implementation step S305, the cross-power spectrum of the fourth and fifth phase difference data is calculated multiple times. The cross-power spectra obtained from the multiple calculations are averaged to obtain the phase noise power spectral density of the laser under test corresponding to the laser source under test. Using the phase noise power spectral density, the frequency noise power spectral density of the laser under test is determined.

[0144] Specifically, due to the fourth phase difference data X T (t) and the fifth phase difference data X T-R (t) all contain the phase noise of the laser under test, and X is obtained through multiple measurements. T (t) and X T-R (t) and calculate the cross-power spectrum of the two; average the cross-power spectra obtained from multiple calculations to extract X. T (t) and X T-R The common-mode component in (t) is the phase noise power spectral density of the laser under test.

[0145] The specific calculation process of "averaging the cross power spectrum obtained from multiple calculations" is shown in formula (10).

[0146] In formula (10), <·> K S refers to the average of K times. T(T-R) For X T (t) and X T-R The cross-power spectrum of (t), S TT for and The cross power spectrum (i.e., the power spectral density of the phase difference data of the laser under test), S TR for and Cross power spectrum;

[0147] S Rfiber The noise N of the fiber optic link fiber (t) and The cross power spectrum, S Tfiber The noise N of the fiber optic link fiber (t) and The cross power spectrum. It refers to A specific multiple of , decreases as K increases.

[0148] It should be noted that when calculating the cross power spectrum, the result will be a positive real number when the two terms involved in the calculation are the same; otherwise, the result will be a random number. As can be seen from the above explanation of formula (10), S TT It is a positive real number with stable amplitude; multiple averaging will not change S. TT Size.

[0149] And besides S TT All other items are random numbers, and their values ​​gradually decrease as the average number of occurrences increases, decreasing by a factor of 1 / 3 of the original value. Therefore, the measurement result after averaging the cross power spectrum multiple times is [number] times. in, The power spectral density S of the phase difference data of the laser under test decreases as K increases until it becomes negligible; therefore, the result obtained after multiple averaging is the power spectral density S of the phase difference data of the laser under test. TT In other words, the phase noise power spectral density of the laser under test is S. TT .

[0150] It is worth noting that in the actual calculation of cross-power spectrum, a higher sampling rate yields a higher highest frequency in the cross-power spectrum. However, excessively high sampling rates place higher demands on system resources. Therefore, it is often necessary to first process two sets of high-sampling-rate data X. T (t) and X T-R (t) Perform a Fast Fourier Transform (FFT), and then multiply the two Fourier Transform results by conjugate to obtain the cross power spectrum of the high frequency band; then downsample the original data and perform the same operation to obtain the cross power spectrum of the mid frequency band, and so on to obtain the cross power spectrum of the lower frequency band.

[0151] The phase noise power spectral density S of the laser under test is obtained. TT Then, through "S" ν (f)=f 2 S TT (f) / 2” Calculate the frequency noise power spectral density S of the laser under test. ν (f), where f is the frequency (i.e., the independent variable of the power spectral density).

[0152] The content in Figure 3 of the above embodiment of the present invention is a description of the "first method of obtaining frequency noise power spectral density". After determining the frequency noise power spectral density of the laser under test using the "first method of obtaining frequency noise power spectral density", the frequency noise power spectral density can be weighted and integrated using the β-line method, the direct integration method or the piecewise integration method to determine the laser linewidth of the laser under test.

[0153] For example: based on the β-line method and the frequency noise power spectral density S ν (f) Determine the laser linewidth of the laser to be tested.

[0154] It should be noted that the β-line method is a way to calculate the laser linewidth based on the frequency noise power spectral density, by drawing a β-line. reserve S at the frequency point ν (f), and by integrating the retained part, the laser linewidth can be calculated from the frequency noise power spectral density.

[0155] Regarding the "second method for obtaining frequency noise power spectral density" involved in step S202 of Figure 2 of the above embodiment of the present invention, referring to Figure 4, a flowchart of the second method for obtaining frequency noise power spectral density provided by the embodiment of the present invention is shown. Figure 4 includes the following steps:

[0156] Step S401: Demodulate the first phase difference data of the laser source under test using the first delayed light and the first reference light.

[0157] In the specific implementation step S401, when measuring a laser under test with a noise level worse than the test environment, the auxiliary laser source is not activated. The first delayed light is transmitted to the delayed fiber link and frequency-shifted to obtain the fourth delayed light. The fourth delayed light is combined with the first reference light and subjected to interferometry through a photodetector to obtain the third interference signal. The third interference signal is demodulated to obtain the first phase difference data.

[0158] It should be noted that, when using the "second method for obtaining frequency noise power spectral density," the first phase difference data is equivalent to the "initial phase difference data" in the aforementioned "principle of obtaining power spectral density based on digital decoherence." ".

[0159] Step S402: Perform digital decoherence on the first phase difference data to obtain the fourth phase difference data.

[0160] In the specific implementation step S402, the first phase difference data is digitally decohered to obtain the fourth phase difference data. In the case of using the "second method to obtain the frequency noise power spectral density", the fourth phase difference data is equivalent to the output phase difference data obtained in the m-th step (i=m) of the above "principle of obtaining power spectral density based on digital decoherence".

[0161] Step S403: Determine the frequency noise power spectral density of the laser source under test based on the fourth phase difference data.

[0162] In the specific implementation step S403, after obtaining the fourth phase difference data, a Fourier transform is performed on the fourth phase difference data to obtain the phase noise power spectral density of the laser under test corresponding to the laser source under test. Using this phase noise power spectral density, the frequency noise power spectral density of the laser under test is determined.

[0163] The content in Figure 4 of the above embodiment of the present invention is a description of the "second method of obtaining frequency noise power spectral density". After determining the frequency noise power spectral density of the laser under test using the "second method of obtaining frequency noise power spectral density", the frequency noise power spectral density can be weighted and integrated using the β-line method, the direct integration method or the piecewise integration method to determine the laser linewidth of the laser under test.

[0164] Regarding the step S304 in Figure 3 of the above embodiment of the present invention, which involves "digitally decohering the first phase difference data to obtain the fourth phase difference data", and the step S402 in Figure 4 of the above embodiment of the present invention, which involves "digitally decohering the first phase difference data to obtain the fourth phase difference data", taking the digital decohering of the first phase difference data as an example, Figure 5 will be used to explain how to perform digital decohering on the phase difference data.

[0165] Referring to Figure 5, a flowchart is shown to obtain fourth phase difference data by digital decoherence of the first phase difference data according to an embodiment of the present invention. Figure 5 includes the following steps:

[0166] Step S501: Delay the first phase difference data by a specified time to form new phase difference data.

[0167] In the specific implementation step S501, the "principle of obtaining power spectral density based on digital decoherence" given above is used to delay the first phase difference data as "initial phase difference data" by a specified time delay (τ0) to form new phase difference data.

[0168] Step S502: Compare the new phase difference data with the first phase difference data to obtain output phase difference data with a specified time delay of twice.

[0169] In the specific implementation step S502, the new phase difference data is compared with the first phase difference data to obtain the output phase difference data with a specified time delay of twice.

[0170] Step S503: Delay the new phase difference data by a specified time to update the new phase difference data.

[0171] Step S504: Compare the updated new phase difference data with the output phase difference data having an i-1 times specified time delay to obtain the output phase difference data having an i times specified time delay, where the initial value of i is 3.

[0172] In the specific implementation step S504, i starts from 3, and the updated new phase difference data is compared with the output phase difference data with a specified time delay of i-1 times to obtain the output phase difference data with a specified time delay of i times.

[0173] Step S505: Increment i by 1, return to step S503 to update the new phase difference data again, until i is greater than m to obtain the fourth phase difference data.

[0174] In the specific implementation of step S505, i = i + 1, and return to step S503 to update the new phase difference data again until i is greater than m to obtain the fourth phase difference data. The fourth phase difference data is: the output phase difference data with m times the specified time delay.

[0175] It should be noted that "updating the new phase difference data again" specifically means delaying the "new phase difference data" used before executing step S505 by the specified time delay.

[0176] In other words, when returning to step S503, the "updated new phase difference data" used in step S504 of the previous round will be delayed by the specified time again, and then step S504 of the current round will be executed, and so on until i is greater than m to obtain the fourth phase difference data.

[0177] In other words, when returning to execute step S503, the "new phase difference data" mentioned in step S503, which refers to the "updated new phase difference data" used in the previous round of step S504, refers to the "new phase difference data".

[0178] It should be noted that the execution principle of steps S501 to S505 can be found in the "Principle of Obtaining Power Spectral Density Based on Digital Decoherence" given above, and will not be repeated here.

[0179] It should be further explained that the specific implementation method of digitally decohering the third phase difference data to obtain the fifth phase difference data can be found in Figure 5 above and the "Principle of Obtaining Power Spectral Density Based on Digital Decoherence" given above, which will not be repeated here; the obtained fifth phase difference data is: the output phase difference data with a specified time delay of m times.

[0180] The above is a description of the laser linewidth measurement method based on digital decoherence presented in this solution. In practical applications, when using the "first method to obtain the frequency noise power spectral density," the "laser linewidth measurement method based on digital decoherence" can be implemented by building a hybrid triangular cap system.

[0181] For example, Figure 6 shows a structural example of one of the hybrid triangular cap systems provided in an embodiment of the present invention. The hybrid triangular cap system shown in Figure 6 consists of an optical unit, a radio frequency unit, and a high-speed data processing unit.

[0182] The optical unit of the hybrid triangular cap system mainly consists of an auxiliary laser source, a delay fiber, an acousto-optic modulator, a photodetector, a fiber filter (referred to as the optical filter in Figure 6), and a fiber coupler. The optical unit can realize the delayed self-heterodyne interferometry measurement of the laser source under test and the delayed self-heterodyne interferometry measurement of the auxiliary laser source, respectively. The auxiliary laser source is not required to have the same center wavelength as the laser source under test, nor is it required that the laser linewidth of the auxiliary laser source and the laser source under test be equivalent.

[0183] The RF unit of the hybrid triangular cap system consists of two parallel low-noise RF power amplifiers (referred to as low-noise amplifiers in Figure 6), an RF filter (referred to as filter in Figure 6), and a 4-channel high-speed data acquisition card. The RF unit can amplify, filter, and acquire the two sets of interference signals at high speed.

[0184] It should be noted that when the signal-to-noise ratio requirement is extremely high or the quality of the laser source under test is extremely high, and it is necessary to suppress the noise of radio frequency devices, it is advisable to consider that each low-noise radio frequency power amplifier, radio frequency filter, and 4-channel high-speed data acquisition card consists of two sets of devices, that is, a total of four sets of instruments. In this case, the noise of radio frequency devices can be suppressed by specifying the method.

[0185] The high-speed data processing unit of the hybrid triangular hat system mainly includes an FPGA (Field Programmable Gate Array) high-speed processing board and a high-speed data memory. It can perform digital IQ demodulation on the sampled signal to obtain the initial phase difference data, and then perform digital decoherence on the initial phase difference data to obtain the output phase difference data (that is, the long-delay phase difference data in Figure 1). Further, it performs Fast Fourier Transform (FFT), filtering, downsampling and other operations on the output phase difference data to obtain the phase noise power spectral density / frequency noise power spectral density of the laser, and finally calculates the laser linewidth. At the same time, the high-speed data memory stores the phase difference data, phase noise power spectral density / frequency noise power spectral density and other effective data to meet other analysis needs.

[0186] When using the hybrid triangular cap system shown in Figure 6 to measure the laser linewidth, the first delay time of the laser source under test and the second delay time of the auxiliary laser source are combined together by an optical fiber coupler and transmitted to the delay fiber link, and then frequency-shifted by an acousto-optic modulator to obtain the third delay time.

[0187] The third delayed light beam is split into two beams, which are then combined with the first and second reference beams via fiber optic couplers. After passing through an optical filter, interferometry is performed at a photodetector. The power of the interference signal obtained from the interferometry is amplified using a low-noise amplifier, and redundant noise is filtered out using a filter. Finally, the first and second interference signals are acquired using a high-speed data acquisition card.

[0188] The first and second interference signals acquired are demodulated by digital IQ to obtain the first phase difference data and the second phase difference data. The subsequent process of calculating the laser linewidth of the laser under test based on the first and second phase difference data can be found in the relevant content in Figure 3 of the above embodiment of the present invention, and will not be repeated here.

[0189] It should be noted that the hybrid triangular cap system provided in this embodiment of the invention can obtain a combined result containing only the noise of any two of the three parts: the laser source under test, the auxiliary laser source, and the fiber optic link.

[0190] The dual-laser structure shown in Figure 6 is just one example of the hybrid triangular cap system given in the embodiments of the present invention. The key feature of the hybrid triangular cap system is that it consists of three independent units formed by the laser source and the fiber optic link. Therefore, similar functions can also be achieved by using a single laser with two different fiber optic links or three lasers. Here, various hybrid triangular cap systems will not be described in detail.

[0191] Based on the above embodiments, this solution can achieve the following beneficial effects:

[0192] 1. This scheme utilizes digital delay to multiply the physical delay of the optical fiber link. Compared with the traditional delay self-heterodyne method, this scheme does not require a complex physical optical fiber link. By using digital delay, the delay introduced by the short physical optical fiber link can be multiplied, ultimately achieving decoherence of the laser source and thus obtaining an accurate power spectral density.

[0193] 2. This scheme does not require one or more auxiliary laser sources with the same center wavelength as the laser source under test and a linewidth close to or better than that of the laser source under test. The laser source under test does not perform direct interferometry measurement with the auxiliary laser sources, but rather performs time-delayed self-heterodyne interferometry on its own. This allows for greater freedom in the selection of auxiliary laser sources, with almost no limitations on center wavelength and linewidth. At the same time, it enables long-term continuous measurement without being affected by the frequency drift of the laser itself.

[0194] 3. The hybrid triangular cap system proposed in this solution can be implemented not only with one reference laser and one delay link, but also with two delay links or two reference lasers. The implementation of the hybrid triangular cap system is quite flexible. The hybrid triangular cap system can achieve accurate measurement of the noise spectrum and linewidth of narrow linewidth lasers.

[0195] Corresponding to the laser linewidth measurement method based on digital decoherence provided in the above embodiment of the present invention, referring to Figure 7, the present invention also provides a structural block diagram of a laser linewidth measurement device based on digital decoherence. The laser linewidth measurement device includes: a processing module 100, a decoherence module 200, and a determination module 300.

[0196] The processing module 100 is used to divide the laser source under test into a first delay light and a first reference light.

[0197] The decoherence module 200 is used to perform digital decoherence based at least on a first delay light and a first reference light to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test.

[0198] The determination module 300 is used to determine the laser linewidth of the laser under test based on the frequency noise power spectral density.

[0199] In some embodiments, the determining module 300 is specifically used to: perform weighted integration of the frequency noise power spectral density using the β-line method, direct integration method, or piecewise integration method to determine the laser linewidth of the laser under test.

[0200] In this embodiment of the invention, the frequency noise power spectral density used to determine the laser linewidth is obtained by digital decoherence. This does not require the laser source under test and the auxiliary laser source to have the same center wavelength, nor does it require a complex physical fiber optic link to avoid introducing a large amount of environmental noise, thereby improving the flexibility, universality and accuracy of laser linewidth measurement.

[0201] Preferably, as shown in Figure 7, the decoherence module 200 includes a partitioning submodule, a first demodulation submodule, an elimination submodule, a first decoherence submodule, and a first determination submodule. The execution principle of each submodule is as follows:

[0202] The auxiliary laser source is divided into a second delay light and a second reference light.

[0203] The first demodulation submodule is used to demodulate the first phase difference data of the laser source under test and the second phase difference data of the auxiliary laser source using the first delayed light, the first reference light, the second delayed light and the second reference light.

[0204] In some embodiments, the first demodulation submodule is specifically used to: combine the first delayed light and the second delayed light and transmit them to the delayed fiber link and perform frequency shifting to obtain the third delayed light; combine the third delayed light and the first reference light and perform interferometry to obtain the first interference signal, and combine the third delayed light and the second reference light and perform interferometry to obtain the second interference signal; demodulate the first interference signal to obtain the first phase difference data, and demodulate the second interference signal to obtain the second phase difference data.

[0205] The noise elimination submodule is used to eliminate noise in the optical fiber link based on the first phase difference data and the second phase difference data to obtain the third phase difference data.

[0206] The first decoherence submodule is used to perform digital decoherence on the first phase difference data to obtain the fourth phase difference data, and to perform digital decoherence on the third phase difference data to obtain the fifth phase difference data.

[0207] The first determination submodule is used to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test by using the fourth phase difference data and the fifth phase difference data.

[0208] In some embodiments, the first determining submodule is specifically used to: calculate the cross power spectrum of the fourth phase difference data and the fifth phase difference data multiple times; average the cross power spectrum obtained from the multiple calculations to obtain the phase noise power spectral density of the laser under test corresponding to the laser source under test; and use the phase noise power spectral density to determine the frequency noise power spectral density of the laser under test.

[0209] Preferably, as shown in Figure 7, the decoherence module 200 includes a second demodulation submodule, a second decoherence submodule, and a second determination submodule. The execution principle of each submodule is as follows:

[0210] The second demodulation submodule is used to demodulate the first phase difference data of the laser source under test using the first delayed light and the first reference light.

[0211] In some embodiments, the second demodulation submodule is specifically used to: transmit the first delayed light to the delayed fiber link and perform frequency shifting to obtain the fourth delayed light; combine the fourth delayed light with the first reference light and perform interferometry through a photodetector to obtain the third interference signal; and demodulate the third interference signal to obtain the first phase difference data.

[0212] The second decoherence submodule is used to perform digital decoherence on the first phase difference data to obtain the fourth phase difference data.

[0213] The second determining submodule is used to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test based on the fourth phase difference data.

[0214] In some embodiments, the second determining submodule is specifically used to: perform Fourier transform on the fourth phase difference data to obtain the phase noise power spectral density of the laser under test corresponding to the laser source under test; and use the phase noise power spectral density to determine the frequency noise power spectral density of the laser under test.

[0215] Preferably, in some embodiments, the process by which the first decoherence submodule and the second decoherence submodule perform digital decoherence on the first phase difference data to obtain the fourth phase difference data includes the following steps:

[0216] The first phase difference data is delayed by a specified time to form new phase difference data;

[0217] The new phase difference data is compared with the first phase difference data to obtain the output phase difference data with twice the specified time delay;

[0218] Delay the new phase difference data by a specified time interval to update the new phase difference data;

[0219] The updated phase difference data is compared with the output phase difference data with a specified time delay of i-1 times to obtain the output phase difference data with a specified time delay of i times, where the initial value of i is 3;

[0220] Increment i by 1, return to the step of "delaying the new phase difference data by a specified time" to update the new phase difference data again, until i is greater than m to obtain the fourth phase difference data;

[0221] The fourth phase difference data is: the output phase difference data with a specified time delay of m times.

[0222] In summary, the embodiments of the present invention provide a laser linewidth measurement method and apparatus based on digital decoherence. By using digital decoherence to determine the frequency noise power spectral density used to determine the laser linewidth, it does not require the laser source under test and the auxiliary laser source to have the same center wavelength, nor does it require complex physical fiber optic links to avoid introducing a large amount of environmental noise, thereby improving the flexibility, universality and accuracy of laser linewidth measurement.

[0223] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, for system or system embodiments, since they are basically similar to method embodiments, the description is relatively simple, and relevant parts can be referred to the descriptions in the method embodiments. The systems and system 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 creative effort.

[0224] Those skilled in the art will further recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of both. To clearly illustrate the interchangeability of hardware and software, the components and steps of the various examples have been generally described in terms of functionality in the foregoing description. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of this invention.

[0225] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for measuring a laser linewidth based on digital dephasing, characterized in that, The method includes: The laser source under test is divided into a first delay light and a first reference light; Digital decoherence is performed at least based on the first delayed light and the first reference light to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test. The laser linewidth of the laser under test is determined based on the frequency noise power spectral density.

2. The method of claim 1, wherein, Digital decoherence is performed at least based on the first delayed light and the first reference light to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test, including: The auxiliary laser source is divided into a second delay light and a second reference light; Using the first delayed light, the first reference light, the second delayed light, and the second reference light, the first phase difference data of the laser source under test and the second phase difference data of the auxiliary laser source are demodulated. Based on the first phase difference data and the second phase difference data, noise in the optical fiber link is eliminated to obtain the third phase difference data; The first phase difference data is digitally decohered to obtain the fourth phase difference data, and the third phase difference data is digitally decohered to obtain the fifth phase difference data. Using the fourth phase difference data and the fifth phase difference data, the frequency noise power spectral density of the laser under test corresponding to the laser source under test is determined.

3. The method of claim 1, wherein, Digital decoherence is performed at least based on the first delayed light and the first reference light to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test, including: The first phase difference data of the laser source under test is obtained by demodulation using the first delayed light and the first reference light; Digital decoherence is performed on the first phase difference data to obtain the fourth phase difference data; Based on the fourth phase difference data, the frequency noise power spectral density of the laser under test corresponding to the laser source under test is determined.

4. The method of claim 2, wherein, Using the first delayed light, the first reference light, the second delayed light, and the second reference light, demodulation is performed to obtain the first phase difference data of the laser source under test and the second phase difference data of the auxiliary laser source, including: The first and second delay times are combined and transmitted to the delayed fiber link and frequency-shifted to obtain the third delay time; The third delayed light and the first reference light are combined and interferometric measurements are performed to obtain a first interference signal, and the third delayed light and the second reference light are combined and interferometric measurements are performed to obtain a second interference signal; The first interference signal is demodulated to obtain the first phase difference data, and the second interference signal is demodulated to obtain the second phase difference data.

5. The method of claim 3, wherein, Using the first delayed light and the first reference light, the first phase difference data of the laser source under test is demodulated, including: The first delayed time is transmitted to the delayed fiber link and frequency shifted to obtain the fourth delayed time; The fourth delayed light and the first reference light are combined and interferometrically measured using a photodetector to obtain a third interference signal; The third interference signal is demodulated to obtain the first phase difference data.

6. The method according to any one of claims 2-5, characterized in that, Digital decoherence is performed on the first phase difference data to obtain the fourth phase difference data, including: The first phase difference data is delayed by a specified time to form new phase difference data; The new phase difference data is compared with the first phase difference data to obtain output phase difference data with twice the specified time delay; The new phase difference data is delayed by the specified time delay in order to update the new phase difference data; The updated new phase difference data is compared with the output phase difference data having i-1 times the specified time delay to obtain the output phase difference data having i times the specified time delay, where the initial value of i is 3; Increment i by 1, return to the step of delaying the new phase difference data by the specified time to update the updated new phase difference data again, until i is greater than m to obtain the fourth phase difference data; The fourth phase difference data is: output phase difference data with m times the specified time delay.

7. The method of claim 2, wherein, Using the fourth phase difference data and the fifth phase difference data, the frequency noise power spectral density of the laser under test corresponding to the laser source under test is determined, including: The cross-power spectrum of the fourth phase difference data and the fifth phase difference data is calculated multiple times; The cross power spectrum obtained from multiple calculations is averaged to obtain the phase noise power spectral density of the laser under test corresponding to the laser source under test; The frequency noise power spectral density of the laser under test is determined using the phase noise power spectral density.

8. The method of claim 3, wherein, Based on the fourth phase difference data, the frequency noise power spectral density of the laser under test corresponding to the laser source under test is determined, including: The fourth phase difference data is subjected to Fourier transform to obtain the phase noise power spectral density of the laser under test corresponding to the laser source under test. The frequency noise power spectral density of the laser under test is determined using the phase noise power spectral density.

9. The method of claim 1, wherein, Determining the laser linewidth of the laser under test based on the frequency noise power spectral density includes: The laser linewidth of the laser under test is determined by weighted integration of the frequency noise power spectral density using the β-line method, direct integration method, or piecewise integration method.

10. A digital dephasing based laser linewidth measurement apparatus, characterized by, The device includes: The processing module is used to divide the laser source under test into a first delayed light and a first reference light; A decoherence module is used to perform digital decoherence based at least on the first delayed light and the first reference light to determine the frequency noise power spectral density of the laser under test corresponding to the laser source under test. The determination module is used to determine the laser linewidth of the laser under test based on the frequency noise power spectral density.

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