A method and device for measuring laser linewidth based on digital dephasing

Abstract

The application provides a laser linewidth measurement method and device based on digital de-coherence, and is suitable for the technical field of laser linewidth analysis. The method is as follows: a to-be-measured laser source is divided into first delay light and first reference light; digital de-coherence is performed based on at least the first delay light and the first reference light to determine the frequency noise power spectral density of a to-be-measured laser corresponding to the to-be-measured laser source; and the laser linewidth of the to-be-measured laser is determined according to the frequency noise power spectral density. The frequency noise power spectral density used to determine the laser linewidth is determined through digital de-coherence, without requiring the to-be-measured laser source and the auxiliary laser source to have consistent central wavelengths, and without needing complex physical fiber links to avoid introducing a large amount of environmental noise, thereby improving the flexibility, universality and precision of laser linewidth measurement.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser linewidth analysis, and particularly relates to a laser linewidth measurement method and device based on digital de-coherence. BACKGROUND

[0002] The phase noise and linewidth of laser are key parameters for measuring the frequency stability and phase noise characteristics of a laser, and are of great significance to the fields of high-precision laser measurement, optical communication and quantum technology. Traditional measurement methods of the phase noise and linewidth of laser mainly include a beat frequency method and a delay self-heterodyne method.

[0003] However, when the beat frequency method is used to measure a laser, one or more reference lasers are needed, and the reference lasers and the laser to be measured need to have consistent central wavelengths, which results in poor flexibility and universality of the beat frequency method in measuring the laser linewidth.

[0004] When the delay self-heterodyne method is used to measure a laser, the optical fiber link used in the delay process introduces a large amount of environmental noise, thereby affecting the measurement accuracy of the laser linewidth. SUMMARY

[0005] Therefore, the embodiments of the present application provide a laser linewidth measurement method and device based on digital de-coherence to solve the problems of poor flexibility, poor universality and poor measurement accuracy in the traditional measurement methods of the laser linewidth.

[0006] To achieve the above object, the embodiments of the present application provide the following technical solutions.

[0007] The first aspect of the embodiments of the present application discloses a laser linewidth measurement method based on digital de-coherence, and the method comprises the following steps.

[0008] The laser source to be measured is divided into first delay light and first reference light;

[0009] Digital de-coherence is performed based on at least the first delay light and the first reference light to determine the frequency noise power spectral density of the laser to be measured corresponding to the laser source to be measured.

[0010] The laser linewidth of the laser to be measured is determined according to the frequency noise power spectral density.

[0011] Preferably, the digital de-coherence is performed based on at least the first delay light and the first reference light to determine the frequency noise power spectral density of the laser to be measured corresponding to the laser source to be measured, and the method comprises the following steps.

[0012] The auxiliary laser source is divided into second delay light and second reference light;

[0013] demodulating first phase difference data of the to-be-tested laser source and second phase difference data of the auxiliary laser source by using the first delayed light, the first reference light, the second delayed light and the second reference light;

[0014] eliminating noise in the fiber link based on the first phase difference data and the second phase difference data to obtain third phase difference data;

[0015] performing digital de-coherence on the first phase difference data to obtain fourth phase difference data and performing digital de-coherence on the third phase difference data to obtain fifth phase difference data;

[0016] determining the frequency noise power spectral density of the to-be-tested laser corresponding to the to-be-tested laser source by using the fourth phase difference data and the fifth phase difference data.

[0017] Preferably, at least based on the first delayed light and the first reference light, digital de-coherence is performed to determine the frequency noise power spectral density of the to-be-tested laser corresponding to the to-be-tested laser source, comprising:

[0018] demodulating first phase difference data of the to-be-tested laser source by using the first delayed light and the first reference light;

[0019] performing digital de-coherence on the first phase difference data to obtain fourth phase difference data;

[0020] determining the frequency noise power spectral density of the to-be-tested laser corresponding to the to-be-tested laser source based on the fourth phase difference data.

[0021] Preferably, demodulating first phase difference data of the to-be-tested laser source and second phase difference data of the auxiliary laser source by using the first delayed light, the first reference light, the second delayed light and the second reference light, comprising:

[0022] summarizing and transmitting the first delayed light and the second delayed light into a delay fiber link and performing frequency shift to obtain third delayed light;

[0023] summarizing and performing interference measurement on the third delayed light and the first reference light to obtain first interference signal and summarizing and performing interference measurement on the third delayed light and the second reference light to obtain second interference signal;

[0024] demodulating the first interference signal to obtain first phase difference data and demodulating the second interference signal to obtain second phase difference data.

[0025] Preferably, demodulating first phase difference data of the to-be-tested laser source by using the first delayed light and the first reference light, comprising:

[0026] transmitting the first delayed light into a delay fiber link and shifting a frequency of the first delayed light to obtain fourth delayed light;

[0027] summing the fourth delayed light and the first reference light and performing interferometric measurement by a photodetector to obtain a third interference signal;

[0028] demodulating the third interference signal to obtain first phase difference data.

[0029] Preferably, digitally de-cohering the first phase difference data to obtain fourth phase difference data, comprising:

[0030] delaying the first phase difference data by a specified time delay to form new phase difference data;

[0031] comparing the new phase difference data with the first phase difference data to obtain output phase difference data with 2 times the specified time delay;

[0032] delaying the new phase difference data by the specified time delay to update the new phase difference data;

[0033] comparing the updated new phase difference data with output phase difference data with (i-1) times the specified time delay to obtain output phase difference data with i times the specified time delay, where i is initially 3;

[0034] increasing i by 1 and returning to the step of delaying the new phase difference data by the specified time delay to update the updated new phase difference data again until i is greater than m to obtain fourth phase difference data;

[0035] wherein the fourth phase difference data is output phase difference data with m times the specified time delay.

[0036] Preferably, determining frequency noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source by using the fourth phase difference data and the fifth phase difference data, comprising:

[0037] calculating cross power spectrum of the fourth phase difference data and the fifth phase difference data for multiple times;

[0038] averaging the cross power spectrum calculated for multiple times to obtain phase noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source;

[0039] determining frequency noise power spectral density of the to-be-measured laser by using the phase noise power spectral density.

[0040] Preferably, the frequency noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source is determined based on the fourth phase difference data, and the method comprises the following steps of:

[0041] The fourth phase difference data is subjected to Fourier transform to obtain the phase noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source;

[0042] The frequency noise power spectral density of the to-be-measured laser is determined by using the phase noise power spectral density.

[0043] Preferably, the laser linewidth of the to-be-measured laser is determined according to the frequency noise power spectral density, and the method comprises the following steps of:

[0044] The laser linewidth of the to-be-measured laser is determined by using the β line method, the direct integration method or the piecewise integration method to perform weighted integration on the frequency noise power spectral density.

[0045] The second aspect of the embodiment of the application discloses a laser linewidth measurement device based on digital de-coherence, and the device comprises:

[0046] A processing module is configured to divide a to-be-measured laser source into first delayed light and first reference light;

[0047] A de-coherence module is configured to perform digital de-coherence based on at least the first delayed light and the first reference light to determine the frequency noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source;

[0048] A determination module is configured to determine the laser linewidth of the to-be-measured laser according to the frequency noise power spectral density.

[0049] The laser linewidth measurement method and device based on digital de-coherence provided by the embodiment of the application are used to divide a to-be-measured laser source into first delayed light and first reference light, perform digital de-coherence based on at least the first delayed light and the first reference light to determine the frequency noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source, and determine the laser linewidth of the to-be-measured laser according to the frequency noise power spectral density. The frequency noise power spectral density used to determine the laser linewidth is determined by digital de-coherence, and the to-be-measured laser source and the auxiliary laser source do not need to have a consistent central wavelength, and a complex physical optical fiber link is not needed to avoid introducing a large amount of environmental noise, thereby improving the flexibility, universality and precision of the laser linewidth measurement. BRIEF DESCRIPTION OF DRAWINGS

[0050] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description only are a part of the embodiments of the present application, and for those skilled in the art, other drawings can be obtained without creative labor based on the provided drawings.

[0051] Figure 1 The principle architecture diagram based on digital de-coherence for acquiring power spectral density is provided for the embodiments of the present application.

[0052] Figure 2 The flow chart of a laser linewidth measurement method based on digital de-coherence is provided for the embodiments of the present application.

[0053] Figure 3 The flow chart of a first way to obtain frequency noise power spectral density is provided for the embodiments of the present application.

[0054] Figure 4 The flow chart of a second way to obtain frequency noise power spectral density is provided for the embodiments of the present application.

[0055] Figure 5 The flow chart of digital de-coherence for the first phase difference data to obtain the fourth phase difference data is provided for the embodiments of the present application.

[0056] Figure 6 The structure example diagram of a hybrid triangular cap system is provided for the embodiments of the present application.

[0057] Figure 7 The structure block diagram of a laser linewidth measurement device based on digital de-coherence is provided for the embodiments of the present application. DETAILED DESCRIPTION

[0058] The technical solutions in the embodiments of the present application will be described clearly and completely below with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the present application.

[0059] In this application, the terms "comprises," "comprising," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not preclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0060] Laser phase noise and linewidth are key parameters for measuring a laser's frequency stability and phase noise characteristics, and are of great significance for fields such as high-precision laser measurement, optical communications, and quantum technology. Phase noise refers to fluctuations in the laser's output phase and is typically characterized in the frequency domain by power spectral density (including phase noise power spectral density and frequency noise power spectral density). Phase noise reflects short-term random variations in the laser's phase and frequency.

[0061] Linewidth is a parameter that describes the width of a laser's output spectrum. It is closely related to the laser's phase noise. Lasers with narrow linewidths offer higher frequency stability. High-precision phase noise and linewidth measurements are crucial for evaluating the quality of laser sources, particularly in optical atomic clocks, lidar, and precision instruments.

[0062] Traditional methods for measuring laser phase noise and linewidth mainly include direct spectrum analysis, beat frequency analysis, and time-delay self-heterodyne analysis. The following is a brief description of these traditional measurement methods.

[0063] Direct spectrum analysis measures the linewidth of a laser's output signal by directly examining its spectrum. Specifically, the laser signal is fed into a spectrum analyzer, which displays the laser signal's frequency distribution. Linewidth refers to the width of the power distribution within the spectrum, typically expressed as the half-power width (3dB bandwidth) of the laser frequency. Direct spectrum analysis can capture the laser's spectral profile and is commonly used to evaluate laser sources with relatively wide linewidths.

[0064] The evaluation of narrow linewidth laser sources is generally achieved through the beat frequency method. Specifically, the laser to be measured is interferometrically with a reference laser with a similar central wavelength, the same noise level, or a more stable one, so as to obtain phase difference data or frequency offset. The principle is that after the interference of two laser signals with similar central wavelengths, a low-frequency beat frequency signal is obtained, and the frequency of the low-frequency beat frequency signal is equal to the difference between the two frequencies. By detecting and analyzing the phase fluctuations of the beat frequency signal, the phase noise of the laser to be measured can be obtained, and the linewidth of the laser to be measured can be further obtained. When the reference laser is consistent with the laser to be measured, the phase noise spectrum of the beat frequency signal is twice the phase noise spectrum of the laser to be measured; when the reference laser is better than the laser to be measured, the phase noise spectrum of the beat frequency signal is the phase noise spectrum of the laser to be measured. The beat frequency method has a relatively simple structure and is suitable for measuring lasers with extremely narrow linewidths.

[0065] The time-delayed 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-delayed self-heterodyne method, the laser signal is divided into two beams through an optical coupler. One of the beams is delayed beyond the coherence length through the optical fiber link and can be regarded as another reference light source that is incoherent with the original laser source. The delayed light interferes with the undelayed light to obtain a beat frequency signal. By detecting and analyzing the phase fluctuations of the beat frequency signal, the phase noise of the laser to be measured can be obtained, and the line width of the laser to be measured can be further obtained. At this time, the reference laser is consistent with the laser to be measured, and the phase noise spectrum of the beat frequency signal is twice the phase noise spectrum of the laser to be measured. The time-delayed self-heterodyne method does not require an auxiliary laser source, which reduces the complexity of the system and is widely used in the evaluation of laser sources with a line width of kHz and above.

[0066] The inventors have discovered that direct spectrum analysis has some limitations when measuring narrow-linewidth lasers. Due to the limited frequency resolution of spectrum analyzers, this limited resolution may not be sufficient to accurately measure the laser linewidth, especially for lasers with very small frequency fluctuations. Therefore, for narrow-linewidth lasers, beat frequency analysis or time-delay self-heterodyne methods are generally used.

[0067] When using the beat frequency method to measure lasers, one or more reference lasers are often required. To achieve interferometric measurement between the reference laser and the laser under test, the reference laser and the laser under test must have the same central wavelength and a linewidth comparable to or superior to that of the laser under test. Furthermore, due to the inherent frequency drift of the laser, the beat frequency method often struggles with long-term interferometric measurements. These factors limit its application in evaluating narrow-linewidth lasers.

[0068] The delay self-heterodyne method uses the delay light of the laser itself as a reference light source, and does not have the limitations of the beat frequency method. However, when measuring a laser with a line width of Hz level, 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, during the delay process, the used fiber link will introduce a large amount of environmental noise, which limits the measurement accuracy of the delay self-heterodyne method.

[0069] In general, in the traditional measurement methods of the phase noise and line width of the laser, the problems can be mainly divided into two categories. One problem is that the reference source has too high a requirement (the beat frequency method), so that the flexibility and universality of the measurement method are poor. The other problem is that the physical system is complex, the measurement accuracy is not enough, and the measurement method is easily disturbed by noise (the delay self-heterodyne method), so that the measurement method is not suitable for the laser with a narrow line width of Hz level.

[0070] In order to solve the above problems, the embodiment of the present application proposes a laser line width measurement method and device based on digital de-coherence. The frequency noise power spectral density used to determine the laser line width is determined by digital de-coherence, without requiring the to-be-measured laser source and the auxiliary laser source to have consistent central wavelengths, and without needing a complex physical fiber link to avoid introducing a large amount of environmental noise, thereby improving the flexibility, universality and accuracy of the laser line width measurement.

[0071] That is, the present scheme does not require a reference laser with consistent line width level and central wavelength as the to-be-measured laser source, and does not need a complex physical system composed of an ultra-long fiber link, so that the laser line width can be measured for a long time, thereby improving the flexibility, universality and accuracy of the laser line width measurement.

[0072] It should be noted that the basic idea of the digital de-coherence used in the present scheme is that the physical time delay of the fiber link is replaced by digital delay, and on the basis of the short fiber link delay self-heterodyne detection, the multiplication of the physical time delay is realized by using a signal processing means, and finally the acquisition of the comparison information of the two incoherent light sources is realized.

[0073] In the traditional delay self-heterodyne method measurement, a fiber link longer than the coherence length of the laser source is needed to make the delay light and the reference light incoherent. Therefore, the evaluation of the laser source with Hz level theoretically needs a fiber link longer than 200,000 kilometers, which not only makes the physical system impossible to realize, but also introduces a large amount of environmental noise. The digital de-coherence used in the present scheme can realize the multiplication of the physical delay on the basis of the short delay fiber.

[0074] In order to better understand the main content of the present scheme, the basic principle of the digital de-coherence used in the present scheme will be briefly introduced (taking the to-be-measured laser source as an example) first.

[0075] It should be noted that the to-be-measured laser source is divided into delay light and reference light.

[0076] Step 1, time-delayed self-heterodyne interferometry is performed on the laser source to be measured, and the time delay of the optical fiber link is set to (that is, the specified time delay mentioned later, which is less than the coherence time of the laser source to be measured). Specifically, by using a short time-delayed self-heterodyne interferometry system, the interference signal of the time-delayed light and the reference light can be measured at a photodetector, and the interference measurement result (i.e., the measured interference signal) is subjected to IQ demodulation after passing through low-noise radio frequency power amplifiers (abbreviated as low-noise amplifiers), filters and other radio frequency devices, and the initial phase difference data between the time-delayed light and the reference light can be output , where IQ represents in-phase and quadrature, and the specific content of the initial phase difference data is shown in formula (1).

[0077] (1).

[0078] In formula (1), is the fluctuation of the laser source phase with time, i.e., the fluctuation of the reference light phase with time; t is an independent variable representing time; is the fluctuation of the time-delayed light phase with time, and compared with the reference light, the time-delayed light has a time delay of .

[0079] Step 2, the obtained initial phase difference data is time-delayed by the specified time delay (i.e., the time delay ), to form new phase difference data ; the new phase difference data is compared with the initial phase difference data (the comparison process can be referred to as comparison), and the output phase difference data corresponding to the time delay of is output, i.e., the output phase difference data has a double specified time delay.

[0080] It should be noted that from the content of the initial phase difference data and the new phase difference data formed in Step 2, and have the same part of , so comparing and can eliminate the phase noise , so as to output the output phase difference data corresponding to the time delay of , and the specific content of the output phase difference data is shown in formula (2) below.

[0081] (2);

[0082] Step 3, repeat the above Step 2 from each step after Step 3 until Step m.

[0083] For example: i represents the step number, at the i-th step, the phase difference data (in the third step, i.e. i = 3, this term is ) is delayed by a specified time delay (i.e. the time delay ), to form new phase difference data (in the third step, i.e. i = 3, this term is );

[0084] It should be noted that the output phase difference data obtained at the i-1-th step (i.e. the previous step) is (in i = 3, this term is ).

[0085] The new phase difference data is compared with the output phase difference data obtained at the i-1-th step , it can be seen that and have the same part , so the phase noise can be eliminated, thereby outputting the output phase difference data corresponding to the time delay , the specific content of the output phase difference data corresponding to the time delay is shown in the following formula (3).

[0086] (3);

[0087] The above steps can accurately multiply the physical delay caused by the short fiber link through the digital system, and by executing the above steps until the m-th step (until the i = m-th step is executed), the delay of the output phase difference data output is expanded from to .

[0088] It should be noted that the coherence time of the laser is determined by the nature of the laser source itself, if is greater than the coherence time of the laser source to be measured, then digital de-coherence can be achieved, i.e. the measurement result of the laser source itself (t ) and another beam of light obtained by delaying the laser source by more than the coherence time (t ).

[0089] For example: when the laser source to be measured is a Hz-level laser, then >1s can achieve digital decoherence; when the laser source to be measured is a sub-Hz laser, Digital decoherence can be achieved in >10s.

[0090] Furthermore, the obtained output phase difference data can be subjected to Fourier transform (FFT) to obtain the power spectrum density of the phase difference data (including the phase noise power spectrum density and the frequency noise power spectrum density). The specific content of the power spectrum density of the phase difference data is shown in formula (4).

[0091] (4);

[0092] In formula (4), is the power spectrum density of the phase difference data, the independent variable is the frequency f, is the phase noise power spectral density of the reference light, For delay The phase noise power spectral density of the delayed light is is the cross power spectrum of the reference light and the delayed light.

[0093] Since the reference light and the delay light come from the same laser source, ,in, is the phase noise power spectrum density of the laser source to be measured. Since the delay time exceeds the coherence time of the laser source to be measured, the reference light and the delayed light are incoherent at this time, that is, the cross power spectrum is a random fluctuation, which can be ignored in the long-term measurement process. Therefore .

[0094] The power spectrum density of the phase noise of the laser source to be measured can be solved according to the power spectrum density of the phase difference data. The frequency noise power spectrum density of the laser source to be measured can be obtained , and the laser line width is calculated according to the β-line method.

[0095] In practical applications, since the amount of data of the power spectrum density of the full frequency domain is large, the Fourier transform can generally be performed in segments by filtering and downsampling, so as to obtain the power spectrum density of the high frequency band, the medium frequency band and the low frequency band respectively.

[0096] In general, the principle of obtaining power spectral density based on digital decoherence is as follows Figure 1 As shown, Figure 1 The "delayed self-heterodyne interference signal" is the interference signal of 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 Latency The new phase difference data obtained , the "short delay phase difference data 2-3" is time delay The new phase difference data obtained , and so on.

[0097] Figure 1 The "long delay phase difference data 0-2" in the long delay phase difference data 0-2 is the output phase difference data , the long delay phase difference data 0-3 is the output phase difference data , and so on; the Fourier transform is performed in a segmented manner by filtering and downsampling, so as to obtain the power spectrum density of the high frequency band, the medium frequency band and the low frequency band respectively.

[0098] The above is an explanation of the principle of obtaining the power spectrum density based on digital de-coherence.

[0099] It is worth noting that the fluctuation of the laser output frequency, i.e. the frequency noise of the laser source, has a frequency domain of the frequency noise power spectrum density. The phase noise and the frequency noise of the laser source can be obtained according to different measurement methods, and can be converted between each other.

[0100] The present scheme measures the laser linewidth based on the "principle of obtaining the power spectrum density based on digital de-coherence" given above, see Figure 2 , a flowchart of a laser linewidth measurement method based on digital de-coherence is shown, Figure 2 comprising the following steps:

[0101] Step S201: divide the to-be-measured laser source into first delay light and first reference light.

[0102] In the process of implementing step S201, the to-be-measured laser source is divided into two paths, one path is delay light, and the other path is reference light, wherein the delay light and the reference light divided from the to-be-measured laser source are called first delay light and first reference light.

[0103] Step S202: at least based on the first delay light and the first reference light, digital de-coherence is performed to determine the frequency noise power spectrum density of the to-be-measured laser corresponding to the to-be-measured laser source.

[0104] In the process of implementing step S202, at least based on the first delay light and the first reference light, digital de-coherence is performed to determine the frequency noise power spectrum density of the to-be-measured laser corresponding to the to-be-measured laser source.

[0105] It is worth noting that the present scheme provides two ways to obtain the frequency noise power spectrum density, which are respectively called "first way to obtain the frequency noise power spectrum density" and "second way to obtain the frequency noise power spectrum density".

[0106] The "first method of obtaining the frequency noise power spectral density" requires starting an auxiliary laser source (also called a reference laser source), while the "second method of obtaining the frequency noise power spectral density" does not require starting the auxiliary laser source.

[0107] In practical applications, the “first method of obtaining the frequency noise power spectral density” or the “second method of obtaining the frequency noise power spectral density” can be selected according to actual conditions.

[0108] For example, when measuring a laser to be tested whose noise level is better than that of the test environment, the auxiliary laser source is started and the "first method of obtaining the frequency noise power spectrum density" is used to determine the frequency noise power spectrum density of the laser to be tested.

[0109] For another example, when measuring a laser to be tested whose noise level is worse than that of the test environment, the auxiliary laser source is not started, and the "second method of obtaining the frequency noise power spectrum density" is used to determine the frequency noise power spectrum density of the laser to be tested.

[0110] Regarding the implementation process of the above-mentioned "first method of obtaining the frequency noise power spectrum density", in the subsequent Figure 3 The embodiments shown are described in detail.

[0111] Regarding the implementation process of the above-mentioned "second method of obtaining the frequency noise power spectrum density", Figure 4 The embodiments shown are described in detail.

[0112] Step S203: determining the laser linewidth of the laser to be tested according to the frequency noise power spectrum density.

[0113] In the specific implementation of step S203 , after obtaining the frequency noise power spectrum density of the laser to be tested, the frequency noise power spectrum density is weightedly integrated using the β-line method, direct integration method or piecewise integration method to determine the laser linewidth of the laser to be tested.

[0114] Specifically, the frequency noise power spectrum density of the laser to be measured is weightedly integrated to determine the laser line width of the laser to be measured. The weighted integration method can adopt the β line method, direct integration method or segmented integration method.

[0115] In an embodiment of the present invention, the frequency noise power spectral density used to determine the laser linewidth is determined by digital decoherence. This does not require the laser source to be measured and the auxiliary laser source to have the same central wavelength, nor does it require a complex physical optical fiber link to avoid introducing a large amount of environmental noise. This improves the flexibility, universality, and accuracy of laser linewidth measurement.

[0116] For the above embodiments of the present invention Figure 2The first way of obtaining the frequency noise power spectral density involved in step S202 is described below with reference to Figure 3 FIG. 3 shows a flowchart of the first way of obtaining the frequency noise power spectral density provided by the embodiment of the present application, Figure 3 comprising the following steps:

[0117] Step S301: Dividing the auxiliary laser source into second delayed light and second reference light.

[0118] In the process of implementing step S301, when measuring the noise level of the to-be-tested laser source is better than the test environment, the auxiliary laser source is started, and the auxiliary laser source is divided into two paths, one path is the delayed light, and the other path is the reference light, wherein the delayed light and the reference light divided from the auxiliary laser source are called the second delayed light and the second reference light.

[0119] Step S302: 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 to-be-tested laser source and the second phase difference data of the auxiliary laser source.

[0120] In the process of implementing step S302, the first delayed light and the second delayed light are summarized and transmitted into the delay fiber link and frequency-shifted to obtain the third delayed light.

[0121] The third delayed light is summarized with the first reference light and interference measurement is performed to obtain the first interference signal, and the third delayed light is summarized with the second reference light and interference measurement is performed to obtain the second interference signal.

[0122] 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.

[0123] Specifically, the first delayed light of the to-be-tested laser source and the second delayed light of the auxiliary laser source are summarized together through the fiber coupler and transmitted into the delay fiber link, and then frequency-shifted through the acousto-optic modulator to obtain the third delayed light.

[0124] The third delayed light is divided into two beams, and the two beams of the third delayed light are summarized with the first reference light and the second reference light, respectively, and interference measurement is performed at the photodetector after passing through the optical filter. The power of the interference signal obtained by the interference measurement is amplified by using a low-noise amplifier, and the excess noise is filtered out by using a filter, and finally the first interference signal and the second interference signal are acquired by using a high-speed data acquisition card.

[0125] The first phase difference data and the second phase difference data are obtained by performing digital IQ demodulation on the acquired first interference signal and second interference signal, respectively.

[0126] It should be noted that the frequency of the delay light and the reference light (such as the third delay light and the first reference light) involved in the interference has a deviation, and the function of the acousto-optic modulator is to shift the frequency of the light, which will cause the frequency deviation of the delay light and the reference light. The process of interference on the photodetector is called heterodyne interference.

[0127] In some embodiments, the above-mentioned obtained first phase difference data and second phase difference data have a specified time delay (i.e., have In the case of using the "first way of obtaining the frequency noise power spectral density", the specific content of the first phase difference data (denoted as ) is shown in formula (5), and the specific content of the second phase difference data (denoted as ) is shown in formula (6).

[0128] (5);

[0129] (6);

[0130] In formula (5) and formula (6), is the first phase difference data (i.e., the delay self-heterodyne interference result of the to-be-tested laser source), is the second phase difference data (i.e., the delay self-heterodyne interference result of the auxiliary laser source); and is the link noise introduced by the fluctuation of the environment in which the optical fiber link is located, is the wavelength of the auxiliary laser source, is the wavelength of the to-be-tested laser source, and n is the refractive index of the light transmitted in the optical fiber, is the fluctuation of the length of the optical fiber link with time.

[0131] It should be noted that the third delay light is composed of two parts, one part is the result obtained after the first delay light is delayed, and the other part is the result obtained after the second delay light is delayed; when the third delay light and the first reference light interfere, the aforementioned two parts of the third delay light will interfere with the first reference light.

[0132] Through one RF filter, other interference results are filtered out, and only the interference result of the "first reference light" and the "light obtained after the first delay light is delayed" in the third delay light is retained.

[0133] Similarly, through another RF filter, other interference results are filtered out, and only the interference result of the "second reference light" and the "light obtained after the second delay light is delayed" in the third delay light is retained.

[0134] In formula (5) and formula (6), the phase fluctuation of the first reference light over time; the phase fluctuation of the part of the third delayed light corresponding to the first delayed light over time; the phase fluctuation of the second reference light over time; the phase fluctuation of the part of the third delayed light corresponding to the second delayed light over time.

[0135] It should be noted that for a laser, the size of the fiber link noise perceived is inversely proportional to the wavelength of the laser. Since the auxiliary laser source and the laser under test both enter the same fiber link, the fiber link noise they perceive is proportional to the "fluctuation of the length of the fiber link over time ", so the waveform shape is consistent, and the size is inversely proportional to the wavelength of the laser.

[0136] Step S303: Eliminate the noise in the fiber link based on the first phase difference data and the second phase difference data to obtain third phase difference data.

[0137] In the process of implementing step S303, the noise in the fiber link is eliminated based on the first phase difference data and the second phase difference data to obtain third phase difference data .

[0138] Specifically, in the process of obtaining the first phase difference data and the second phase difference data, the phase noise of the laser under test and the auxiliary laser source is coupled with the noise introduced by the fiber link, and by effectively eliminating the noise in the fiber link, the measurement result (i.e. third phase difference data) containing the phase noise of the laser under test and the auxiliary laser source can be obtained, and the specific content of the third phase difference data is shown in equation (7).

[0139] (7);

[0140] As can be seen from the content of the above equation (7), the third phase difference data has a specified time delay (i.e. has ).

[0141] Step S304: Perform digital de-coherence on the first phase difference data to obtain fourth phase difference data, and perform digital de-coherence on the third phase difference data to obtain fifth phase difference data.

[0142] In the process of implementing step S304, the "principle of obtaining power spectral density based on digital de-coherence" given in the foregoing content is used to perform digital de-coherence on the first phase difference data to obtain the fourth phase difference data (which is denoted as ), the third phase difference data is digitally de-coherent to obtain the fifth phase difference data .

[0143] It should be noted that in the case of adopting the "first way of obtaining the frequency noise power spectral density", the first phase difference data is equivalent to the "initial phase difference data" in the above-mentioned "principle of obtaining the power spectral density based on digital de-coherence", and the fourth phase difference data is equivalent to the output phase difference data obtained at the mth step (i = m) in the above-mentioned "principle of obtaining the power spectral density based on digital de-coherence".

[0144] Similarly, the third phase difference data is equivalent to the "initial phase difference data" in the above-mentioned "principle of obtaining the power spectral density based on digital de-coherence", and the fifth phase difference data is equivalent to the output phase difference data obtained at the mth step (i = m) in the above-mentioned "principle of obtaining the power spectral density based on digital de-coherence".

[0145] In the case of adopting the "first way of obtaining the frequency noise power spectral density", the specific content of the fourth phase difference data is shown in formula (8), and the specific content of the fifth phase difference data is shown in formula (9).

[0146] (8);

[0147] (9);

[0148] From the content of formula (8) and formula (9), it can be seen that after digital de-coherence, the time delay of the phase noise of the laser source is expanded from (the specified time delay) to (m times the specified time delay), and the noise of the optical fiber link is represented as , which is the result obtained after digital de-coherence of the "link noise introduced due to the fluctuation of the environment in which the optical fiber link is located" mentioned in the above formula (5).

[0149] In some embodiments, the fourth phase difference data and the fifth phase difference data have m times the specified time delay, and m times the specified time delay is greater than the coherence time of the to-be-tested laser.

[0150] Step S305: using the fourth phase difference data and the fifth phase difference data, determining the frequency noise power spectral density of the to-be-tested laser corresponding to the to-be-tested laser source.

[0151] During the specific implementation of step S305, the cross power spectra of the fourth phase difference data and the fifth phase difference data are calculated multiple times. The cross power spectra obtained by the multiple calculations are averaged to obtain the phase noise power spectrum density of the laser under test corresponding to the laser source under test. The phase noise power spectrum density is used to determine the frequency noise power spectrum density of the laser under test.

[0152] Specifically, due to the fourth phase difference data and the fifth phase difference data Both include the phase noise of the laser to be measured, obtained through multiple measurements and And calculate the cross power spectrum of the two; average the cross power spectrum obtained by multiple calculations to extract and The common mode component in the phase noise is extracted, that is, the phase noise power spectrum density of the laser to be measured is extracted.

[0153] The specific operation process of "averaging the cross power spectra obtained by multiple calculations" is shown in formula (10).

[0154] (10);

[0155] In formula (10), Refers to K-times average, for and The cross power spectrum of for" "and" "The cross power spectrum (that is, the power spectrum density of the phase difference data of the laser to be measured), for" "and" "The cross power spectrum of

[0156] is the noise of the optical fiber link and" The cross power spectrum of is the noise of the optical fiber link and" The cross power spectrum of . refers to It is a specific multiple of , which decreases as K increases.

[0157] It should be noted that when calculating the cross power spectrum, when the two items involved in the calculation are the same, the result will be a positive real number, otherwise the result will be a random number. From the above explanation of formula (10), it can be seen that Is a positive real number with a stable amplitude, which does not change after multiple averaging size.

[0158] In addition to All other items except are random numbers, which will gradually decrease with the increase of the average number of times, and the reduction ratio is 1 / 2 of the original size. times, so the measurement result after multiple averaging of the cross power spectrum is ,in, As K increases, it decreases until it is negligible; therefore, the result obtained after multiple averaging is the power spectrum density of the phase difference data of the laser to be measured. , that is, the phase noise power spectrum density of the laser to be measured is .

[0159] It is worth noting that in the actual calculation of the cross power spectrum, the higher the sampling rate, the higher the highest frequency of the cross power spectrum obtained. However, when the sampling rate is too high, the system resources will be more demanding. Therefore, two sets of high sampling rate data are often first and Perform a fast Fourier transform (FFT), and then conjugate multiply the two Fourier transform results 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.

[0160] Get the phase noise power spectral density of the laser to be measured Afterwards, through Calculate the frequency noise power spectral density of the laser to be tested , f is the frequency (i.e., the independent variable of the power spectral density).

[0161] The above embodiments of the present invention Figure 3 The content in the figure is related to the "first method for obtaining the frequency noise power spectral density". After using the "first method for obtaining the frequency noise power spectral density" to determine the frequency noise power spectral density of the laser to be tested, the β-line method, direct integration method, or piecewise integration method can be used to perform weighted integration on the frequency noise power spectral density to determine the laser linewidth of the laser to be tested.

[0162] For example: According to the beta line method and the frequency noise power spectral density , determine the laser linewidth of the laser to be tested.

[0163] It should be noted that the β line method is a way to calculate the laser line width based on the frequency noise power spectrum density. ",reserve" " , and by integrating the retained part, the laser linewidth can be calculated through the frequency noise power spectral density.

[0164] For the above embodiment of the present application Figure 2 The second way of obtaining the frequency noise power spectral density involves the following steps: Figure 4 The second way of obtaining the frequency noise power spectral density involves the following steps: Figure 4 The second way of obtaining the frequency noise power spectral density involves the following steps:

[0165] Step S401: demodulation is performed using the first delayed light and the first reference light to obtain first phase difference data of the to-be-measured laser source.

[0166] In the process of implementing step S401, when the to-be-measured laser is in a measurement noise level lower than a test environment, the auxiliary laser source is not started, the first delayed light is transmitted to the delay fiber link and frequency-shifted to obtain fourth delayed light. The fourth delayed light and the first reference light are combined and subjected to interference measurement by the photodetector to obtain a third interference signal. The third interference signal is demodulated to obtain the first phase difference data.

[0167] It should be noted that, in the case of using the second way of obtaining the frequency noise power spectral density, the first phase difference data is equivalent to the initial phase difference data in the above-mentioned principle of obtaining the power spectral density based on digital de-coherence .

[0168] Step S402: digital de-coherence is performed on the first phase difference data to obtain fourth phase difference data.

[0169] In the process of implementing step S402, digital de-coherence is performed on the first phase difference data to obtain the fourth phase difference data. In the case of using the second way of obtaining the frequency noise power spectral density, the fourth phase difference data is equivalent to the output phase difference data obtained in the mth step (i=m) in the above-mentioned principle of obtaining the power spectral density based on digital de-coherence.

[0170] Step S403: the frequency noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source is determined based on the fourth phase difference data.

[0171] In the process of implementing step S403, after obtaining the fourth phase difference data, Fourier transform is performed on the fourth phase difference data to obtain the phase noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source. The frequency noise power spectral density of the to-be-measured laser is determined using the phase noise power spectral density.

[0172] The above embodiment of the present application Figure 4The content in the above embodiment of the present application is related to the description of the second way of obtaining the frequency noise power spectral density, and after the frequency noise power spectral density of the to-be-tested laser is determined by using the second way of obtaining the frequency noise power spectral density, the frequency noise power spectral density can be weighted integrated by using the beta line method, the direct integration method or the piecewise integration method, so as to determine the laser linewidth of the to-be-tested laser.

[0173] For the above embodiment of the present application Figure 3 The step S304 related to the digital de-coherence of the first phase difference data to obtain the fourth phase difference data, and for the step S402 related to the digital de-coherence of the first phase difference data to obtain the fourth phase difference data in the above embodiment of the present application Figure 4 The digital de-coherence of the phase difference data is explained by taking the digital de-coherence of the first phase difference data as an example. Figure 5

[0174] Referring to Figure 5 , a flowchart of the digital de-coherence of the first phase difference data to obtain the fourth phase difference data provided by the embodiment of the present application is shown, Figure 5 including the following steps:

[0175] Step S501: time delay the first phase difference data by a specified time delay to form new phase difference data.

[0176] In the process of specifically implementing the step S501, the first phase difference data is taken as the initial phase difference data to be time delayed by a specified time delay (as shown in the above embodiment of the present application) to form new phase difference data by using the principle of obtaining the power spectral density based on the digital de-coherence given in the foregoing content.

[0177] Step S502: compare the new phase difference data with the first phase difference data to obtain output phase difference data with a 2-fold specified time delay.

[0178] In the process of specifically implementing the step S502, the new phase difference data is compared with the first phase difference data to obtain output phase difference data with a 2-fold specified time delay.

[0179] Step S503: time delay the new phase difference data by a specified time delay to update the new phase difference data.

[0180] Step S504: compare the updated new phase difference data with the output phase difference data with an i-1-fold specified time delay to obtain output phase difference data with an i-fold specified time delay, and the initial value of i is 3.

[0181] ​​In the implementation of step S504, i starts from 3, the updated new phase difference data is compared with the output phase difference data with i-1 times of the specified time delay to obtain the output phase difference data with i times of the specified time delay.

[0182] Step S505: i increases by 1, and step S503 is executed again to update the updated new phase difference data until i is greater than m to obtain the fourth phase difference data.

[0183] In the implementation of step S505, i=i+1, and step S503 is executed again to update the updated new phase difference data until i is greater than m to obtain the fourth phase difference data, which is the output phase difference data with m times of the specified time delay.

[0184] It should be noted that the "updating the updated new phase difference data again" specifically refers to that the "updated new phase difference data" used before step S505 is time delayed again by the specified time delay.

[0185] That is, when step S503 is executed again, the "updated new phase difference data" used in the last execution of step S504 is time delayed again by the specified time delay, and then step S504 in this round is executed, and so on until i is greater than m to obtain the fourth phase difference data.

[0186] In other words, when step S503 is executed again, the "new phase difference data" in the "time delay of the new phase difference data by the specified time delay" mentioned in step S503 refers to the "updated new phase difference data" used in the last execution of step S504.

[0187] It should be noted that the execution principle of steps S501 to S505 can be referred to the "principle of obtaining power spectral density based on digital de-coherence" given in the foregoing content, which will not be repeated here.

[0188] It should be further noted that the specific implementation of the digital de-coherence of the third phase difference data to obtain the fifth phase difference data can be referred to the content of the foregoing Figure 5 and the "principle of obtaining power spectral density based on digital de-coherence" given in the foregoing content, which will not be repeated here; and the obtained fifth phase difference data is the output phase difference data with m times of the specified time delay.

[0189] The above describes the laser linewidth measurement method based on digital decoherence proposed in this solution. In practical applications, using the first method for obtaining the frequency noise power spectral density, the "laser linewidth measurement method based on digital decoherence" can be implemented by building a hybrid triangular hat system.

[0190] For example Figure 6 FIG. 1 shows a structural example of a hybrid triangular hat system provided by an embodiment of the present invention. Figure 6 The hybrid triangular hat system shown consists of an optical unit, a radio frequency unit and a high-speed data processing unit.

[0191] The optical unit of the hybrid triangular hat system mainly consists of an auxiliary laser source, a delay fiber, an acousto-optic modulator, a photodetector, and a fiber filter ( Figure 6 The optical unit is composed of an optical filter (abbreviated as optical filter in the text), an optical fiber coupler, and the optical unit can respectively realize the delayed self-heterodyne interferometry measurement of the laser source to be measured and the delayed self-heterodyne interferometry measurement of the auxiliary laser source. The auxiliary laser source is not required to be consistent with the central wavelength of the laser source to be measured, nor is it required that the laser linewidth level of the auxiliary laser source and the laser source to be measured be equivalent.

[0192] The RF unit of the hybrid triangular hat system consists of two parallel low-noise RF power amplifiers ( Figure 6 LNA), RF filter ( Figure 6 The RF unit is capable of amplifying, filtering, and high-speed acquisition of two sets of interference signals.

[0193] It should be noted that when the signal-to-noise ratio requirement is extremely high or the quality of the laser source to be measured is extremely high, and the noise of the RF device needs to be suppressed, it can be considered that each set of low-noise RF power amplifiers, RF filters, and 4-channel high-speed data acquisition cards consists of two sets of devices, that is, a total of four sets of instruments. In this case, the noise of the RF device can be suppressed in a specified way.

[0194] 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, Figure 1The long-delay phase difference data in the image is obtained by further performing fast Fourier transform (FFT), filtering, downsampling and other operations on the output phase difference data to obtain the phase noise power spectrum density / frequency noise power spectrum density of the laser, and finally calculate the laser linewidth. At the same time, the high-speed data memory stores the phase difference data, phase noise power spectrum density / frequency noise power spectrum density and other valid data to meet other analysis needs.

[0195] In use Figure 6 When the hybrid triangular hat system shown is used to measure laser linewidth, the first delayed light of the laser source to be measured and the second delayed light of the auxiliary laser source are combined together through a fiber coupler and transmitted into a delay fiber link. Then, they are frequency-shifted by an acousto-optic modulator to obtain the third delayed light.

[0196] The third delayed light is split into two beams. These two beams are combined with the first and second reference beams through fiber couplers, then pass through optical filters and undergo interference measurement at a photodetector. A low-noise amplifier amplifies the power of the interference signal obtained by the interferometry, and a filter removes excess noise. Finally, a high-speed data acquisition card is used to acquire the first and second interference signals.

[0197] The collected first interference signal and the second interference signal are respectively subjected to digital IQ demodulation to obtain the first phase difference data and the second phase difference data. The subsequent process of calculating the laser line width of the laser to be measured based on the first phase difference data and the second phase difference data can be referred to the above embodiment of the present invention. Figure 3 The relevant content in will not be repeated here.

[0198] It should be noted that, through the hybrid triangular hat system provided by the embodiment of the present invention, a combination result containing only noise of any two parts among the laser source to be measured, the auxiliary laser source, and the optical fiber link can be obtained.

[0199] Figure 6 The dual-laser structure shown is only one example of a hybrid triangular hat system provided in an embodiment of the present invention. The key feature of the hybrid triangular hat system is that the laser source and the optical fiber link form three independent units. Therefore, similar functions can also be achieved using a single laser in combination with two different optical fiber links or three lasers. Examples of various types of hybrid triangular hat systems will not be provided here one by one.

[0200] In summary, the present invention can achieve the following beneficial effects:

[0201] 1. The scheme utilizes digital time delay to realize the multiplication of the physical delay of the optical fiber link. Compared with the traditional delay self-heterodyne method, the scheme does not need a complex physical optical fiber link, and the delay introduced by the short optical fiber physical link can be multiplied by the digital time delay, and finally the de-coherence of the laser source is realized, so that the accurate power spectral density is obtained.

[0202] 2. The scheme does not need another auxiliary laser source with the same center wavelength and line width close to or better than the to-be-measured laser source. The to-be-measured laser source does not directly interfere with the auxiliary laser source, but each performs delay self-heterodyne interference, which makes the selection of the auxiliary laser source more free, and there is almost no restriction on the center wavelength and line width, and long-time continuous measurement can also be performed without being affected by the frequency drift of the laser itself.

[0203] 3. The hybrid triangular cap system proposed in the scheme can be realized by one reference laser and one delay link, or by two delay links or two reference lasers, and the realization mode of the hybrid triangular cap system is relatively flexible. The hybrid triangular cap system can realize accurate measurement of the noise spectrum and line width of the narrow line width laser.

[0204] Corresponding to the laser line width measurement method based on digital de-coherence provided in the above embodiment of the present application, referring to Figure 7 The embodiment of the present application also provides a structural block diagram of a laser line width measurement device based on digital de-coherence. The laser line width measurement device comprises a processing module 100, a de-coherence module 200 and a determination module 300.

[0205] The processing module 100 is used for dividing the to-be-measured laser source into first delay light and first reference light.

[0206] The de-coherence module 200 is used for performing digital de-coherence based on at least the first delay light and the first reference light, so as to determine the frequency noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source.

[0207] The determination module 300 is used for determining the laser line width of the to-be-measured laser according to the frequency noise power spectral density.

[0208] In some embodiments, the determination module 300 is specifically used for performing weighted integration on the frequency noise power spectral density by adopting a beta line method, a direct integration method or a segmented integration method, so as to determine the laser line width of the to-be-measured laser.

[0209] In the embodiment of the present application, the frequency noise power spectral density used for determining the laser line width is determined by digital de-coherence, which does not require that the to-be-measured laser source and the auxiliary laser source have the same center wavelength, and does not need a complex physical optical fiber link to avoid introducing a large amount of environmental noise, thereby improving the flexibility, universality and precision of the laser line width measurement.

[0210] Preferably, in combination Figure 7 As shown, the decorrelation module 200 includes a dividing sub-module, a first demodulation sub-module, a cancellation sub-module, a first decorrelation sub-module and a first determination sub-module, and the execution principles of each sub-module are as follows:

[0211] The dividing sub-module is configured to divide the auxiliary laser source into the second delayed light and the second reference light.

[0212] The first demodulation sub-module is configured to demodulate the first phase difference data of the to-be-measured laser source and the second phase difference data of the auxiliary laser source by using the first delayed light, the first reference light, the second delayed light and the second reference light.

[0213] In some embodiments, the first demodulation sub-module is specifically configured to: transmit the first delayed light and the second delayed light into the delay fiber link and perform frequency shifting to obtain third delayed light; perform interference measurement on the third delayed light and the first reference light to obtain a first interference signal, and perform interference measurement on the third delayed light and the second reference light to obtain a 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.

[0214] The cancellation sub-module is configured to cancel the noise in the fiber link based on the first phase difference data and the second phase difference data to obtain third phase difference data.

[0215] The first decorrelation sub-module is configured to perform digital decorrelation on the first phase difference data to obtain fourth phase difference data, and perform digital decorrelation on the third phase difference data to obtain fifth phase difference data.

[0216] The first determination sub-module is configured to determine the frequency noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source by using the fourth phase difference data and the fifth phase difference data.

[0217] In some embodiments, the first determination sub-module is specifically configured 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 calculated multiple times to obtain the phase noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source; and determine the frequency noise power spectral density of the to-be-measured laser by using the phase noise power spectral density.

[0218] Preferably, in combination Figure 7 As shown, the decorrelation module 200 includes a second demodulation sub-module, a second decorrelation sub-module and a second determination sub-module, and the execution principles of each sub-module are as follows:

[0219] a second demodulation submodule configured to demodulate the first phase difference data of the laser source under test by using the first delayed light and the first reference light.

[0220] In some embodiments, the second demodulation submodule is specifically configured to: transmit the first delayed light into the delay fiber link and shift the frequency of the first delayed light to obtain fourth delayed light; combine the fourth delayed light with the first reference light and perform interference measurement by using a photodetector to obtain a third interference signal; and demodulate the third interference signal to obtain the first phase difference data.

[0221] a second de-coherence submodule configured to perform digital de-coherence on the first phase difference data to obtain fourth phase difference data.

[0222] a second determination submodule configured 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.

[0223] In some embodiments, the second determination submodule is specifically configured 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 determine the frequency noise power spectral density of the laser under test by using the phase noise power spectral density.

[0224] Preferably, in some embodiments, the process of performing digital de-coherence on the first phase difference data by the first de-coherence submodule and the second de-coherence submodule to obtain the fourth phase difference data includes the following steps:

[0225] delaying the first phase difference data by a specified time delay to form new phase difference data;

[0226] comparing the new phase difference data with the first phase difference data to obtain output phase difference data with a 2-fold specified time delay;

[0227] delaying the new phase difference data by the specified time delay to update the new phase difference data;

[0228] comparing the updated new phase difference data with output phase difference data with an (i-1)-fold specified time delay to obtain output phase difference data with an i-fold specified time delay, where i is initially 3;

[0229] increasing i by 1 and returning to the step of delaying the new phase difference data by the specified time delay to update the updated new phase difference data again until i is greater than m to obtain the fourth phase difference data;

[0230] where the fourth phase difference data is output phase difference data with an m-fold specified time delay.

[0231] In summary, the embodiments of the present invention provide a laser linewidth measurement method and apparatus based on digital decoherence. The method uses digital decoherence to determine the frequency noise power spectral density used to determine the laser linewidth. This method does not require the laser source to be measured and the auxiliary laser source to have the same central wavelength, nor does it require a complex physical fiber link to avoid introducing a large amount of environmental noise. This method improves the flexibility, universality, and accuracy of laser linewidth measurement.

[0232] Each embodiment in this specification is described in a progressive manner. The same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on 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. For relevant parts, refer to the partial description of the method embodiment. The system and system embodiments described above are merely schematic, wherein the units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they may be located in one place, or they may be distributed on multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of this embodiment. A person of ordinary skill in the art can understand and implement it without expending creative work.

[0233] Professionals may further appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of the two. In order to clearly illustrate the interchangeability of hardware and software, the above description has generally described the components and steps of each example according to their functions. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professionals and technicians may use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of the present invention.

[0234] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein, but is intended to conform to 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 comprises: dividing the to-be-tested laser source into first delayed light and first reference light; performing digital de-coherence based on at least the first delayed light and the first reference light to determine the frequency noise power spectral density of the to-be-tested laser corresponding to the to-be-tested laser source; determining the laser linewidth of the to-be-tested laser according to the frequency noise power spectral density.

2. The method of claim 1, wherein, The performing of the digital de-coherence based on at least the first delayed light and the first reference light to determine the frequency noise power spectral density of the to-be-tested laser corresponding to the to-be-tested laser source comprises: dividing an auxiliary laser source into second delayed light and second reference light; demodulating, by using the first delayed light, the first reference light, the second delayed light and the second reference light, to obtain first phase difference data of the to-be-tested laser source and second phase difference data of the auxiliary laser source; eliminating noise in the fiber link based on the first phase difference data and the second phase difference data to obtain third phase difference data; performing digital de-coherence on the first phase difference data to obtain fourth phase difference data and performing digital de-coherence on the third phase difference data to obtain fifth phase difference data; determining the frequency noise power spectral density of the to-be-tested laser corresponding to the to-be-tested laser source by using the fourth phase difference data and the fifth phase difference data.

3. The method of claim 1, wherein, The performing of the digital de-coherence based on at least the first delayed light and the first reference light to determine the frequency noise power spectral density of the to-be-tested laser corresponding to the to-be-tested laser source comprises: demodulating, by using the first delayed light and the first reference light, to obtain first phase difference data of the to-be-tested laser source; performing digital de-coherence on the first phase difference data to obtain fourth phase difference data; determining the frequency noise power spectral density of the to-be-tested laser corresponding to the to-be-tested laser source based on the fourth phase difference data.

4. The method of claim 2, wherein, The demodulating, by using the first delayed light, the first reference light, the second delayed light and the second reference light, to obtain first phase difference data of the to-be-tested laser source and second phase difference data of the auxiliary laser source comprises: transmitting the first delayed light and the second delayed light into a delay fiber link and performing frequency shifting to obtain third delayed light; performing interference measurement on the third delayed light and the first reference light to obtain a first interference signal and performing interference measurement on the third delayed light and the second reference light to obtain a second interference signal; demodulating the first interference signal to obtain first phase difference data and demodulating the second interference signal to obtain second phase difference data.

5. The method of claim 3, wherein, The demodulating, by using the first delayed light and the first reference light, to obtain first phase difference data of the to-be-tested laser source comprises: transmitting the first delayed light into a delay fiber link and performing frequency shifting to obtain fourth delayed light; performing interference measurement on the fourth delayed light and the first reference light by an optoelectronic detector to obtain a third interference signal; demodulating the third interference signal to obtain first phase difference data.

6. The method according to any one of claims 2-5, characterized in that, The performing of the digital de-coherence on the first phase difference data to obtain fourth phase difference data comprises: delay the first phase difference data by a specified time delay to form new phase difference data; compare the new phase difference data with the first phase difference data to obtain output phase difference data with 2 times the specified time delay; delay the new phase difference data by the specified time delay to update the new phase difference data; compare the updated new phase difference data with output phase difference data with (i-1) times the specified time delay to obtain output phase difference data with i times the specified time delay, where i is initially 3; increase i by 1 and return to the step of delaying the new phase difference data by the specified time delay to update the updated new phase difference data again until i is greater than m to obtain fourth phase difference data; where the fourth phase difference data is output phase difference data with m times the specified time delay.

7. The method of claim 2, wherein, determining the frequency noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source by using the fourth phase difference data and the fifth phase difference data, including: calculating the cross power spectrum of the fourth phase difference data and the fifth phase difference data multiple times; averaging the cross power spectrum calculated multiple times to obtain the phase noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source; determining the frequency noise power spectral density of the to-be-measured laser by using the phase noise power spectral density.

8. The method of claim 3, wherein, determining the frequency noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source based on the fourth phase difference data, including: performing Fourier transform on the fourth phase difference data to obtain the phase noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source; determining the frequency noise power spectral density of the to-be-measured laser by using the phase noise power spectral density.

9. The method of claim 1, wherein, determining the laser linewidth of the to-be-measured laser according to the frequency noise power spectral density, including: determining the laser linewidth of the to-be-measured laser by weighting integration of the frequency noise power spectral density by using a beta line method, a direct integration method or a segmented integration method.

10. A digital dephasing based laser linewidth measurement apparatus, characterized by, The apparatus includes: a processing module configured to divide a to-be-measured laser source into first delayed light and first reference light; a de-coherence module configured to perform digital de-coherence based on at least the first delayed light and the first reference light to determine the frequency noise power spectral density of the to-be-measured laser corresponding to the to-be-measured laser source; a determination module configured to determine the laser linewidth of the to-be-measured laser according to the frequency noise power spectral density.

Images