Apparatus and method for measuring laser linewidth

Abstract

This application provides a device and method for measuring the linewidth of a laser, which can be applied in the field of laser technology. The measuring device includes: a filter configured to have a tunable transmission center wavelength such that the variation in the transmission center wavelength covers the linewidth of the laser under test, filtering the output laser light from the laser under test, wherein the filter's transmission spectrum has a predetermined lineshape and a predetermined linewidth, and the laser's output spectrum has a predetermined lineshape; a detector configured to detect the filtered laser light and obtain the power of the output laser light passing through the filter at different transmission center wavelengths; and a processor configured to: fit each transmission center wavelength and the corresponding power based on the predetermined lineshape to generate a power spectrum; and determine the linewidth of the output spectrum of the laser under test based on the linewidth of the power spectrum and the predetermined linewidth of the transmission spectrum.

Description

Technical Field

[0001] At least one embodiment of this application relates to the field of laser technology, and more specifically to a device and method for measuring the linewidth of a laser. Background Technology

[0002] Lasers, based on the principle of stimulated emission, are devices that generate highly coherent, monochromatic, and collimated beams, and are widely used in optical communication, spectral analysis, optical measurement, quantum information, lidar, and biomedical imaging, among other fields. With the deepening application of laser technology in these fields, high-performance lasers have become indispensable core components. Scientifically evaluating the comprehensive performance of lasers requires systematic characterization through multiple parameters. Laser linewidth is a core indicator for measuring its spectral purity and coherence characteristics, and accurate measurement of this indicator plays a crucial role in improving laser performance. However, existing methods for measuring laser linewidth still have various drawbacks, such as complex equipment and limited applicability. Summary of the Invention

[0003] According to a first aspect of this application, a laser linewidth measurement apparatus is provided, the apparatus comprising: a filter configured to have a tunable transmission center wavelength such that the variation of the transmission center wavelength covers the linewidth of the laser under test, for filtering the output laser of the laser under test, wherein the transmission spectrum of the filter has a predetermined lineshape and a predetermined linewidth, and the output spectrum of the laser under test has the predetermined lineshape; a detector configured to detect the filtered laser and obtain the power of the output laser passing through the filter at different transmission center wavelengths; and a processor configured to: fit each transmission center wavelength and the power corresponding to each transmission center wavelength based on the predetermined lineshape to generate a power spectrum; and determine the linewidth of the output spectrum of the laser under test based on the linewidth of the power spectrum and the predetermined linewidth of the transmission spectrum.

[0004] According to an embodiment of this application, the predetermined line shape includes a Lorentz line shape; the linewidth is characterized by the full width at half maximum (FWHM); the processor is configured to: based on the Lorentz line shape, fit each of the transmission center wavelengths and the power corresponding to each of the transmission center wavelengths to generate a Lorentz power spectrum; subtract the full WHM of the transmission spectrum from the full WHM of the Lorentz power spectrum to obtain the full WHM of the output spectrum of the laser under test.

[0005] According to an embodiment of this application, the predetermined line shape includes a Gaussian line shape; the linewidth is characterized by the full width at half maximum (FWHM); the processor is configured to: based on the Gaussian line shape, fit each of the transmission center wavelengths and the power corresponding to each of the transmission center wavelengths to generate a Gaussian power spectrum; and take the square root of the difference between the square of the FWHM of the Gaussian power spectrum and the square of the FWHM of the transmission spectrum to obtain the FWHM of the output spectrum of the laser under test.

[0006] According to an embodiment of this application, the parameters of the filter include tuning parameters; the filter is configured to: adjust the value of the transmission center wavelength by adjusting the tuning parameters, so that the change in the transmission center wavelength covers the linewidth of the laser under test, thereby filtering the output laser of the laser under test.

[0007] According to an embodiment of this application, the tuning parameters include temperature.

[0008] According to an embodiment of this application, the predetermined linewidth of the transmission spectrum of the filter is smaller than the linewidth of the output spectrum of the laser under test.

[0009] A second aspect of this application provides a method for measuring the linewidth of a laser. The method includes: tuning the transmission center wavelength of a filter such that the variation in the transmission center wavelength covers the linewidth of the laser under test; filtering the output laser light from the laser under test, wherein the transmission spectrum of the filter has a predetermined line shape and a predetermined linewidth, and the output spectrum of the laser under test has the predetermined line shape; detecting the filtered laser light using a detector to obtain the power of the output laser light passing through the filter at different transmission center wavelengths; fitting each transmission center wavelength and the corresponding power based on the predetermined line shape to generate a power spectrum; and determining the linewidth of the output spectrum of the laser under test based on the linewidth of the power spectrum and the predetermined linewidth of the predetermined transmission spectrum.

[0010] According to an embodiment of this application, the predetermined line shape includes a Lorentz line shape; the linewidth is characterized by the full width at half maximum (FWHM); the process of fitting each transmission center wavelength and the power corresponding to each transmission center wavelength to generate a power spectrum based on the predetermined line shape includes: fitting each transmission center wavelength and the power corresponding to each transmission center wavelength to generate a Lorentz power spectrum based on the Lorentz line shape; the process of determining the linewidth of the output spectrum of the laser under test based on the linewidth of the power spectrum and the predetermined linewidth of the transmission spectrum includes: subtracting the full WHM of the transmission spectrum from the full WHM of the Lorentz power spectrum to obtain the full WHM of the output spectrum of the laser under test.

[0011] According to an embodiment of this application, the predetermined line shape includes a Gaussian line shape; the linewidth is characterized by the full width at half maximum (FWHM); based on the predetermined line shape, fitting each transmission center wavelength and the power corresponding to each transmission center wavelength to generate a power spectrum includes: fitting each transmission center wavelength and the power corresponding to each transmission center wavelength to generate a Gaussian power spectrum based on the Gaussian line shape; determining the linewidth of the output spectrum of the laser under test based on the linewidth of the power spectrum and the predetermined linewidth of the transmission spectrum includes: taking the square root of the difference between the square of the FWHM of the Gaussian power spectrum and the square of the FWHM of the transmission spectrum to obtain the FWHM of the output spectrum of the laser under test.

[0012] According to an embodiment of this application, the parameters of the filter include tuning parameters; the tuning of the tunable transmission center wavelength of the filter so that the change in the transmission center wavelength covers the linewidth of the laser under test, and the filtering of the output laser of the laser under test, includes: adjusting the value of the transmission center wavelength by adjusting the tuning parameters so that the change in the transmission center wavelength covers the linewidth of the laser under test, and the filtering of the output laser of the laser under test. Attached Figure Description

[0013] The above-mentioned contents, other objects, features and advantages of this application will become clearer from the following description of embodiments of this application with reference to the accompanying drawings.

[0014] Figure 1 A schematic diagram of a laser linewidth measuring device according to an embodiment of this application is shown.

[0015] Figure 2 A schematic diagram of the first and second Lorentz functions and their convolution result is shown.

[0016] Figure 3 A schematic diagram of the first Gaussian function, the second Gaussian function, and their convolution result is shown.

[0017] Figure 4 A flowchart illustrating the operation of a laser linewidth measurement method according to an embodiment of this application is shown.

[0018] Figure 5 A flowchart illustrating the operation of a measurement experiment according to an embodiment of this application is shown.

[0019] Figure 6 A schematic diagram showing experimental data of temperature and power based on a Lorentz linear filter according to an embodiment of this application is illustrated.

[0020] Figure 7 A schematic diagram of experimental data on temperature and power based on a Gaussian linear filter according to an embodiment of this application is shown. Detailed Implementation

[0021] The embodiments of this application will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of this application. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of this application for ease of explanation. However, it will be apparent that one or more embodiments may be implemented without these specific details. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concepts of this application.

[0022] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0023] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0024] When using expressions such as "at least one of A, B and C", they should generally be interpreted in accordance with the meaning that is commonly understood by those skilled in the art (e.g., "a system having at least one of A, B and C" should include, but is not limited to, a system having A alone, a system having B alone, a system having C alone, a system having A and B, a system having A and C, a system having B and C, and / or a system having A, B and C, etc.).

[0025] Laser linewidth is defined as the width of the laser spectrum, reflecting both the random fluctuations in laser frequency (i.e., phase noise) and determining the temporal coherence of the optical field. Spectrometry is a typical measurement method. Its core principle is to use dispersive elements such as gratings to spatially spread the incident light according to wavelength, and then record the intensity distribution at different wavelengths using a detector. This directly obtains the spectral profile of the laser, and the full width at half maximum (FWHM) of this spectrum is considered the laser linewidth. Spectrometry offers advantages such as ease of operation and rapid response, enabling real-time spectral display and measurement without complex scanning. However, the resolution of grating-type spectrometers is typically limited to the tens of picometers, and they require high input light intensity, generally reaching the nanowatt level. Therefore, the applicability of this method is significantly limited.

[0026] For measuring narrower linewidths, the beat frequency method is a commonly used high-sensitivity measurement technique. The basic principle is as follows: the laser to be measured is combined with a reference laser or its own beam after time delay, and then incident on a high-speed photodetector. A beat frequency signal is generated through optical mixing, the frequency of which is equal to the difference in frequencies between the two beams. Subsequently, the spectral broadening of this beat frequency signal is analyzed using a radio frequency spectrum analyzer, thereby calculating the actual linewidth of the laser. The beat frequency method offers extremely high measurement accuracy, with resolution reaching the kilohertz (kHz) to hertz (Hz) range, making it one of the most sensitive techniques for measuring extremely narrow linewidths. However, this method has several inherent limitations: firstly, the measurable linewidth range is limited by the electronic bandwidth of the detector and spectrum analyzer, typically only applicable to laser sources with relatively narrow linewidths; secondly, the system relies on a reference light source with extremely high frequency stability, often requiring another narrow-linewidth reference laser or a long fiber delay line, resulting in a complex system structure and high construction costs.

[0027] While each of the aforementioned measurement methods has its advantages, they remain difficult to adapt to certain specialized applications. For instance, in fields such as quantum communication, it is often necessary to prepare specific quantum states, resulting in quantum light output power typically as low as the picowatt (pW) level or even lower. Simultaneously, to suppress noise interference, spectral filtering is required, which reduces the typical spectral linewidth of quantum light to the order of approximately 10 pM. Under these demanding conditions of extremely low output power and narrow linewidth, achieving precise measurement of the laser linewidth is particularly challenging.

[0028] This application proposes a device and method for measuring the linewidth of a laser, aiming to solve at least one of the above-mentioned technical problems.

[0029] Figure 1 A schematic diagram of a laser linewidth measuring device according to an embodiment of this application is shown.

[0030] like Figure 1 As shown, the laser linewidth measurement device includes a filter 1, a detector 2, and a processor 3.

[0031] Filter 1 is configured to have a tunable transmission center wavelength such that the variation in the transmission center wavelength covers the linewidth of the laser under test 4, thereby filtering the output laser light from the laser under test 4. The transmission spectrum of filter 1 has a predetermined line shape and a predetermined linewidth, and the output spectrum of the laser under test 4 has the predetermined line shape.

[0032] Detector 2 is configured to detect the filtered laser and obtain the power of the output laser passing through filter 1 at different transmission center wavelengths.

[0033] The processor 3 is configured to: fit each transmission center wavelength and the power corresponding to each transmission center wavelength based on the predetermined line shape to generate a power spectrum; and determine the linewidth of the output spectrum of the laser under test 4 based on the linewidth of the power spectrum and the predetermined linewidth of the transmission spectrum.

[0034] The line shape and linewidth of the filter's transmission spectrum can be pre-calibrated through calibration experiments. The predetermined line shape represents the curve shape of the spectral power distribution of the laser output light. The predetermined line shape can be a Lorentz line shape, a Gaussian line shape, a Vogt line shape, etc. A filter with the same transmission spectrum line shape can be selected based on the line shape of the laser's output spectrum under test.

[0035] The type of detector can be selected based on the output power of the laser under test. For example, if the output power of the laser under test is in the range of +30dBm to -70dBm, a power meter can be used as the detector. Conversely, if the output power of the laser under test is less than -70dBm, a single-photon detector can be used. Single-photon detectors have high sensitivity to light intensity and can measure the linewidth of lasers at the single-photon level, solving the problem of directly measuring the linewidth of ultra-low-power light sources such as quantum communication sources.

[0036] The power spectrum is the power spectrum of the detector after the output laser passes through the filter. Since the output spectrum of the laser under test and the transmission spectrum of the filter have the same line shape, the line shape of the generated power spectrum can be the same as that of the output spectrum of the laser under test. The output spectrum of the laser under test, the transmission spectrum of the filter, and the power spectrum of the detector exhibit a convolution relationship. The power spectrum is the convolution of the output spectrum of the laser under test and the transmission spectrum of the filter.

[0037] According to embodiments of this application, a filter with the same line shape as the output spectrum of the actual laser under test can be selected; a detector matching the output power of the actual laser under test can be selected; the output laser of the laser under test is scanned and filtered using the filter, and the filtered laser is detected using the detector to obtain the power of the output laser passing through the filter at different transmission center wavelengths; since the power spectrum, transmission spectrum and output spectrum of the laser under test have the same line shape, the linewidth of the output spectrum of the laser under test can be determined based on the linewidth of the power spectrum and the predetermined linewidth of the transmission spectrum, making the measurement of the laser linewidth more convenient.

[0038] The laser linewidth measurement device of this application embodiment is easy to operate, requiring no complex optical path construction and precision adjustment. It can measure the laser linewidth by using tunable filters and detectors and combining data processing.

[0039] The laser linewidth measurement device in this application is flexibly designed and can be adaptively adjusted according to the line shape and output power of the actual laser under test, thereby reducing the time cost and technical threshold of measurement.

[0040] The laser linewidth measurement device of this application embodiment can cover the measurement range from nanometers (nm) to 10 fm by changing filters with different linewidths. That is, it can measure both wide linewidth light sources and narrow linewidth lasers.

[0041] Linewidth can be characterized using the full width at half maximum (FWHM). The full width at half maximum (FWHM) refers to the difference between two wavelengths when the power reaches 50% of the peak value in the spectral power distribution curve of a laser.

[0042] When the output spectrum of the laser under test and the transmission spectrum of the filter are Lorentz-shaped, the first convolution relationship and the first linewidth relationship can be determined. The first convolution relationship indicates that if the output spectrum of the laser under test and the transmission spectrum of the filter are Lorentz-shaped, then the power spectrum generated by the detector is also Lorentz-shaped. The first linewidth relationship indicates that the linewidth of the power spectrum is the sum of the linewidth of the transmission spectrum and the linewidth of the output spectrum of the laser under test.

[0043] The theoretical derivation of the first convolution relation and the first linewidth relation is shown below.

[0044] The first Lorentz function Second Lorentz function Perform convolution.

[0045] First Lorentz function It can be represented as:

[0046] (1);

[0047] Second Lorentz function It can be represented as:

[0048] (2);

[0049] in, Describing the first Lorentz function variables, Describing the first Lorentz function The center, Describing the first Lorentz function Half the height; Represents the second Lorentz function variables, Represents the second Lorentz function The center, Represents the second Lorentz function Half the height.

[0050] Figure 2 A schematic diagram of the first and second Lorentz functions and their convolution result is shown.

[0051] like Figure 2 As shown in one example, the center of the first Lorentz function is zero, the half-height and half-width of the first Lorentz function are 1, and the full width and half-height of the first Lorentz function are... The value is 2. The center of the second Lorentz function is zero, the half-height and half-width of the second Lorentz function are 1.5, and the full width and half-height of the second Lorentz function are... The value is 3.

[0052] The definition of convolution can be expressed as: for the first function to be convolved Second convolution function Perform convolution, and convolve the first function to be convolved. Second convolution function The independent variable is replaced with an integral dummy variable. And for the second convolution function Perform a flip and translation. Convolution result. It can be represented as:

[0053] (3);

[0054] in, Represents the convolution result The independent variable.

[0055] The first Lorentz function Second Lorentz function Substituting into formula (3), we obtain the first Lorentz function. Second Lorentz function Convolution results , can be represented as:

[0056] (4);

[0057] Simplifying formula (4), we get:

[0058] (5);

[0059] make and set By transforming formula (5), we can obtain:

[0060] (6);

[0061] Formula (6) is a standard integral, which can be solved using the residue theorem in complex analysis or by partial fraction decomposition. Calculating the integral part of formula (6), we can obtain:

[0062] (7);

[0063] Substituting formula (7) into formula (6), we get:

[0064] (8);

[0065] Known We can obtain:

[0066] (9).

[0067] According to formula (9), the first Lorentz function can be determined. Second Lorentz function Convolution results It was thought Center, with The half-height Lorentz function is the sum of the half-heights of the two Lorentz functions.

[0068] The convolution result of the first and second Lorentz functions is called the third Lorentz function, such as... Figure 2 As shown, the center of the third Lorentz function is zero, the half-height and half-width of the third Lorentz function are 2.5, and the full width at half-height of the third Lorentz function is... The value is 5. The half-height of the third Lorentz function is the sum of the half-heights of the first and second Lorentz functions, and the full width of the half-height of the third Lorentz function is 5. It is the full width at half maximum (FWHM) of the first Lorentz function. Full width at half maximum (FWHM) of the second Lorentz function sum.

[0069] Given that the output spectrum of the laser under test is of Lorentz shape, if a filter with a transmission spectrum of Lorentz shape is selected, and the power spectrum is the convolution of the output spectrum of the laser under test and the transmission spectrum of the filter, then the power spectrum is also of Lorentz shape. The full width at half maximum (FWHM) of the Lorentz shape multiplied by 2 is the full width at half maximum (FWHM) of the Lorentz shape, i.e., the linewidth of the Lorentz shape. The linewidth of the power spectrum is the sum of the predetermined linewidth of the transmission spectrum and the linewidth of the output spectrum of the laser under test.

[0070] Given that the output spectrum of the laser under test and the transmission spectrum of the filter are Gaussian, the second convolution relationship and the second linewidth relationship can be determined. The second convolution relationship indicates that if the output spectrum of the laser under test and the transmission spectrum of the filter are Gaussian, then the power spectrum generated by the detector is also Gaussian. The second linewidth relationship indicates that the square of the linewidth of the power spectrum is the sum of the square of the linewidth of the transmission spectrum and the square of the linewidth of the output spectrum of the laser under test.

[0071] The theoretical derivation of the second convolution relation and the second linewidth relation is shown below.

[0072] For the first Gaussian function Second Gaussian function Perform convolution.

[0073] First Gaussian function It can be represented as:

[0074] (10);

[0075] Second Gaussian function It can be represented as:

[0076] (11);

[0077] in, Indicates the independent variable. Represents the first Gaussian function standard deviation Represents the second Gaussian function standard deviation Represents the natural constant.

[0078] Figure 3 A schematic diagram of the first Gaussian function, the second Gaussian function, and their convolution result is shown.

[0079] like Figure 3 As shown, the standard deviation of the first Gaussian function is 0.8, and the full width at half maximum (FWHM) of the first Gaussian function is... The standard deviation of the second Gaussian function is 0.7, and its full width at half maximum (FWHM) is 1.88. It is 1.65.

[0080] The definition of convolution can be expressed as: for the first Gaussian function Second Gaussian function Perform convolution, using the first Gaussian function Second Gaussian function The independent variable is replaced with an integral dummy variable. And for the second Gaussian function Perform a flip and translation. First Gaussian function. Second Gaussian function Convolution results It can be represented as:

[0081] (13).

[0082] The first Gaussian function Second Gaussian function Substituting into formula (13), we can obtain:

[0083] (14);

[0084] Expanding the exponent part of formula (14) and completing the square, we get:

[0085] (15);

[0086] Substituting formula (15) into formula (14) and integrating, we get:

[0087] (16);

[0088] Simplifying the coefficients of the radical part of formula (16), we get:

[0089] (17);

[0090] Substituting formula (17) into formula (16), we get:

[0091] (18);

[0092] Further simplification of formula (18) yields:

[0093] (19);

[0094] According to formula (19), the first Gaussian function can be determined. Second Gaussian function Convolution results Therefore A Gaussian function with standard deviation is a Gaussian function; that is, the convolution of two Gaussian functions will still result in a Gaussian function.

[0095] Therefore, if the output spectrum of the laser under test and the transmission spectrum of the filter are Gaussian, then the power spectrum generated by the detector is also Gaussian.

[0096] Gaussian line half-height full width Standard deviation of Gaussian line The relationship can be represented as:

[0097] (20).

[0098] Therefore, the first Gaussian function half height full width With the first Gaussian function Standard deviation The relationship can be represented as:

[0099] (twenty one);

[0100] Second Gaussian function half height full width With the second Gaussian function Standard deviation The relationship can be represented as:

[0101] (twenty two);

[0102] First Gaussian function Second Gaussian function Convolution results half height full width With the first Gaussian function Second Gaussian function Convolution results Standard deviation The relationship can be represented as:

[0103] (twenty three);

[0104] Squaring formula (23) yields:

[0105] (twenty four);

[0106] Squaring formula (21) yields:

[0107] (25);

[0108] Squaring formula (22) yields:

[0109] (26);

[0110] According to formulas (24), (25), and (26), we can obtain:

[0111] (27).

[0112] According to formula (27), the first Gaussian function can be determined. Second Gaussian function Convolution results half height full width The square of is the first Gaussian function half height full width With the second Gaussian function half height full width The sum of the squares, that is, the square of the full width at half maximum (FWHM) of the convolution result of two Gaussian functions is equal to the sum of the squares of the full width at half maximum (FWHM) of the two Gaussian functions.

[0113] The convolution result of the first and second Gaussian functions is called the third Gaussian function, such as... Figure 3 As shown, the full width at half maximum (FWHM) of the third Gaussian function It is 2.5. The full width at half maximum (FWHM) of the third Gaussian function. The square of is the full width at half maximum (FWHM) of the first Gaussian function. The square of the second Gaussian function and its full width at half maximum (FWHM). The sum of the squares of .

[0114] Therefore, the square of the power spectrum linewidth is the sum of the square of the filter transmission spectrum linewidth and the square of the output spectrum linewidth of the laser under test.

[0115] According to an embodiment of this application, the predetermined linewidth of the transmission spectrum of the filter is smaller than the linewidth of the output spectrum of the laser under test.

[0116] According to embodiments of this application, the filter parameters include tuning parameters. The filter is configured to adjust the value of the transmitted center wavelength by adjusting the tuning parameters, such that the change in the transmitted center wavelength covers the linewidth of the laser under test, thereby filtering the output laser light from the laser under test.

[0117] The relationship between the filter's tuning parameters and the transmission center wavelength can be obtained through calibration experiments.

[0118] According to an embodiment of this application, the tuning parameter can be temperature. By adjusting the temperature, the value of the transmitted center wavelength is adjusted so that the change in the transmitted center wavelength covers the linewidth of the laser under test, thereby filtering the output laser light of the laser under test. The relationship between the filter temperature and the transmitted center wavelength can be obtained through calibration experiments.

[0119] According to an embodiment of this application, the predetermined line shape can be a Lorentz line shape. The linewidth is characterized by the full width at half maximum (FWHM). The processor is configured to fit each transmission center wavelength and the power corresponding to each transmission center wavelength based on the Lorentz line shape to generate a Lorentz power spectrum; the full WHM of the transmission spectrum is obtained by subtracting the full WHM of the transmission spectrum from the full WHM of the Lorentz power spectrum.

[0120] According to an embodiment of this application, the predetermined line shape can be a Gaussian line shape. The linewidth is characterized by the full width at half maximum (FWHM). The processor is configured to fit a Gaussian power spectrum to each transmission center wavelength and the power corresponding to each transmission center wavelength based on the Gaussian line shape; and to obtain the full WHM of the output spectrum of the laser under test by taking the square root of the difference between the square of the full WHM of the Gaussian power spectrum and the square of the full WHM of the transmission spectrum.

[0121] Embodiments of this application also provide a method for measuring the linewidth of a laser.

[0122] Figure 4 A flowchart illustrating the operation of a laser linewidth measurement method according to an embodiment of this application is shown.

[0123] like Figure 4 As shown, the laser linewidth measurement method includes operations S410 to S440.

[0124] In operation S410, the transmission center wavelength of the filter is tuned so that the change in the transmission center wavelength covers the linewidth of the laser under test, and the output laser of the laser under test is filtered. The transmission spectrum of the filter has a predetermined line shape and a predetermined linewidth, and the output spectrum of the laser under test has a predetermined line shape.

[0125] When operating the S420, the detector detects the filtered laser and obtains the power of the output laser passing through the filter at different transmission center wavelengths.

[0126] In operation S430, based on a predetermined line shape, the power spectrum is generated by fitting each transmission center wavelength and the power corresponding to each transmission center wavelength.

[0127] In operation S440, the linewidth of the output spectrum of the laser under test is determined based on the linewidth of the power spectrum and the predetermined linewidth of the transmission spectrum of the filter.

[0128] According to embodiments of this application, the predetermined line shape includes a Lorentz line shape. The linewidth is characterized by the full width at half maximum (FWHM). Based on the Lorentz line shape, a Lorentz power spectrum is generated by fitting each transmission center wavelength and the corresponding power. The FWHM of the transmission spectrum is subtracted from the FWHM of the Lorentz power spectrum to obtain the FWHM of the output spectrum of the laser under test.

[0129] According to an embodiment of this application, the predetermined line shape includes a Gaussian line shape. The linewidth is characterized by the full width at half maximum (FWHM). Based on the Gaussian line shape, a Gaussian power spectrum is generated by fitting each transmission center wavelength and the power corresponding to each transmission center wavelength. The square root of the difference between the square of the FWHM of the Gaussian power spectrum and the square of the FWHM of the transmission spectrum is taken to obtain the FWHM of the output spectrum of the laser under test.

[0130] According to an embodiment of this application, the filter parameters include tuning parameters. By adjusting the tuning parameters, the value of the transmitted center wavelength is adjusted so that the change in the transmitted center wavelength covers the linewidth of the laser under test, thereby filtering the output laser light from the laser under test.

[0131] Figure 5 A flowchart illustrating the operation of a measurement experiment according to an embodiment of this application is shown.

[0132] like Figure 5 As shown, equipment preparation is performed first. The linewidth of the laser under test is denoted as f0. The linewidth of the filter is denoted as f1. The linewidth of the filter and the mapping relationship λ=f(X) between the tuning parameter X and the transmission center wavelength λ are determined in advance through calibration experiments. When the output power of the laser under test is within the range of the power meter, the power meter is used as the detector. When the output power of the laser under test is outside the range of the power meter, a single-photon detector is used as the detector.

[0133] The laser, filter, and detector under test are arranged according to... Figure 1 Connect the components as shown. Set the operating state of the laser under test to stabilize its output power and wavelength. Record the filter tuning parameter X and the detected power P at this point. Change the filter tuning parameter X sequentially, scanning the output spectrum of the laser under test. Repeat this process several times until the scanning range covers the entire spectrum of the laser under test. Record the filter tuning parameter Xn and the corresponding detected power Pn for each iteration.

[0134] Then, data processing is performed. Based on the mapping relationship λ=f(X) between the tuning parameter X and the transmission center wavelength λ, and the recorded tuning parameters Xn and the corresponding detected power Pn for each filter, the data pair λn-Pn of the transmission center wavelength λn and the corresponding detected power Pn for each test can be obtained. According to the shape of the laser under test, curve fitting is performed on the data pair λn-Pn to obtain the λn-Pn curve. Based on the λn-Pn curve, the linewidth f2 of the filtered power spectrum is calculated. According to the shape of the laser under test, the corresponding linewidth relationship f0=f(f1,f2) is selected, and the linewidth f0 of the laser under test is calculated.

[0135] In one embodiment, a filter with a Lorentz-shaped transmission spectrum is selected as the filter, based on the fact that the output spectrum of the laser under test is Lorentz-shaped. For example, a Lorentz-shaped filter could be a Fabry-Perot filter. When the output power of the laser under test is within the range of the power meter, the power meter is used as the detector. When the output power of the laser under test is outside the range of the power meter, a single-photon detector is used as the detector.

[0136] The tuning parameter X is temperature T. The linewidth of the Lorentz linear filter and the mapping relationship between temperature T and the transmission center wavelength λ are pre-calibrated through calibration experiments. The linewidth of the Lorentz linear filter, i.e., its full width at half maximum (FWHM) LF1, can be determined based on the filter's transmission spectrum. The mapping relationship between temperature T and the transmission center wavelength λ can be determined by the change in the filter's transmission center wavelength corresponding to each degree Celsius change in temperature.

[0137] A Lorentz line filter with a suitable linewidth can be selected. The linewidth of the Lorentz line filter is on the same order of magnitude as the linewidth of the laser under test, or much smaller than the linewidth of the laser under test.

[0138] The laser under test, Lorentz linear filter, power meter or single-photon detector, and processor are arranged according to... Figure 1 The connections shown are made sequentially. The processor controls the laser under test to emit an output laser beam. This output laser beam passes through a Lorentz linear filter and reaches a power meter or single-photon detector. The power meter or single-photon detector can detect the power of the output laser beam passing through the Lorentz linear filter. Record the temperature and power of the Lorentz linear filter at this point. The temperature of the Lorentz linear filter is changed sequentially, and this process is repeated several times until the filtered power can cover the entire output spectrum of the laser under test. Record the temperature and power of the Lorentz linear filter each time.

[0139] Figure 6 A schematic diagram showing experimental data of temperature and power based on a Lorentz linear filter according to an embodiment of this application is illustrated.

[0140] like Figure 6 As shown, based on the mapping relationship between temperature T and the transmission center wavelength λ, and the recorded temperature and power of each Lorentz line filter, the mapping data of the transmission center wavelength λ and power can be obtained. Based on the Lorentz line shape, the power corresponding to each transmission center wavelength is fitted to generate the Lorentz power spectrum. The full width at half maximum (FWHM) LF2 of the Lorentz power spectrum is calculated.

[0141] Based on the first linewidth relationship, the full width at half maximum (FWHM) of the Lorentz power spectrum (LF2) is subtracted from the full width at half maximum (FWHM) of the transmission spectrum (LF1) of the Lorentz line filter to obtain the full width at half maximum (FWHM) of the output spectrum of the Lorentz line laser under test (LF0).

[0142] When the linewidth of the Lorentz linear filter is much smaller than the linewidth of the laser under test, the linewidth of the Lorentz power spectrum can be used as the linewidth of the laser under test, that is, the full width at half maximum (FWHM) of the Lorentz power spectrum LF2 can be used as the linewidth of the laser under test.

[0143] In another embodiment, a filter with a Gaussian transmission spectrum is selected as the filter, based on the fact that the output spectrum of the laser under test is Gaussian. For example, the Gaussian filter could be a Bragg grating filter using an apodized technique. When the output power of the laser under test is within the range of the power meter, the power meter is used as the detector. When the output power of the laser under test is outside the range of the power meter, a single-photon detector is used as the detector.

[0144] The tuning parameter X is temperature T. The linewidth of the Gaussian linear filter and the mapping relationship between temperature T and the transmission center wavelength λ are pre-calibrated through calibration experiments. The linewidth of the Gaussian linear filter, i.e., its full width at half maximum (FWHM) GF1, can be determined based on the filter's transmission spectrum. The mapping relationship between temperature T and the transmission center wavelength λ can be determined by the change in the transmission center wavelength corresponding to each degree Celsius change in temperature.

[0145] A Gaussian line filter with a suitable linewidth can be selected. The linewidth of the Gaussian line filter is on the same order of magnitude as the linewidth of the laser under test, or much smaller than the linewidth of the laser under test.

[0146] The laser under test, Gaussian linear filter, power meter or single-photon detector, and processor are arranged according to... Figure 1 The connections shown are made sequentially. The processor controls the laser under test to emit an output laser beam. This output laser beam passes through a Gaussian line filter and reaches a power meter or single-photon detector. The power meter or single-photon detector can detect the power of the output laser beam passing through the Gaussian line filter. Record the temperature and power of the Gaussian line filter at this point. The temperature of the Gaussian line filter is changed sequentially, and this process is repeated several times until the filtered power can cover the entire output spectrum of the laser under test. Record the temperature and power of the Gaussian line filter each time.

[0147] Figure 7 A schematic diagram of experimental data on temperature and power based on a Gaussian linear filter according to an embodiment of this application is shown.

[0148] like Figure 7 As shown, based on the mapping relationship between temperature T and the transmission center wavelength λ, and the recorded temperature and power of each Gaussian line filter, the mapping data of the transmission center wavelength λ and power can be obtained. Based on the Gaussian line shape, a Gaussian power spectrum is generated by fitting each transmission center wavelength and its corresponding power. The full width at half maximum (FWHM) GF2 of the Gaussian power spectrum is then calculated.

[0149] Based on the second linewidth relationship, the square root of the difference between the square of the full width at half maximum (FWHM) GF2 of the Gaussian power spectrum and the square of the full width at half maximum (FWHM) GF1 of the transmission spectrum of the Gaussian linear filter is taken to obtain the full width at half maximum (FWHM) GF0 of the output spectrum of the Gaussian linear laser under test.

[0150] When the linewidth of the Gaussian filter is much smaller than the linewidth of the Gaussian laser under test, the linewidth of the Gaussian power spectrum can be used as the linewidth of the Gaussian laser under test, that is, the full width at half maximum (FWHM) of the Gaussian power spectrum (GF2) can be used as the linewidth of the Gaussian laser under test.

[0151] Those skilled in the art will understand that the features described in the various embodiments of this application can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this application. In particular, the features described in the various embodiments of this application can be combined and / or combined in various ways without departing from the spirit and teachings of this application. All such combinations and / or combinations fall within the scope of this application.

[0152] The embodiments of this application have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of this application. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Without departing from the scope of this application, those skilled in the art can make various substitutions and modifications, all of which should fall within the scope of this application.

Claims

1. A device for measuring the linewidth of a laser, characterized in that, The measuring device includes: A filter is configured to have a tunable transmission center wavelength such that the variation of the transmission center wavelength covers the linewidth of the laser under test, and to filter the output laser of the laser under test, wherein the transmission spectrum of the filter has a predetermined line shape and a predetermined linewidth, and the output spectrum of the laser under test has the predetermined line shape. The detector is configured to detect the filtered laser and obtain the power of the output laser passing through the filter at different transmission center wavelengths; The processor is configured as follows: Based on the predetermined line shape, a power spectrum is generated by fitting each transmission center wavelength and the power corresponding to each transmission center wavelength. The linewidth of the output spectrum of the laser under test is determined based on the linewidth of the power spectrum and the predetermined linewidth of the transmission spectrum.

2. The measuring device according to claim 1, characterized in that, The predetermined linetype includes the Lorentz linetype; the linewidth is characterized by the full width at half maximum (FWHM); the processor is configured to: Based on the Lorentz line shape, the power corresponding to each transmission center wavelength is fitted to generate the Lorentz power spectrum. The full width at half maximum (FWHM) of the output spectrum of the laser under test is obtained by subtracting the FWHM of the transmission spectrum from the FWHM of the Lorentz power spectrum.

3. The measuring device according to claim 1, characterized in that, The predetermined linetype includes a Gaussian linetype; the linewidth is characterized by half-height and full width; the processor is configured to: Based on the Gaussian line shape, a Gaussian power spectrum is generated by fitting each transmission center wavelength and the power corresponding to each transmission center wavelength. The square root of the difference between the square of the full width at half maximum (FWHM) of the Gaussian power spectrum and the square of the full width at half maximum (FWHM) of the transmission spectrum is taken to obtain the full WHM of the output spectrum of the laser under test.

4. The measuring device according to any one of claims 1 to 3, characterized in that, The filter parameters include tuning parameters; the filter is configured as follows: By adjusting the tuning parameters, the value of the transmitted center wavelength is adjusted so that the change in the transmitted center wavelength covers the linewidth of the laser under test, thereby filtering the output laser of the laser under test.

5. The measuring device according to claim 4, characterized in that, The tuning parameters include temperature.

6. The measuring device according to claim 1, characterized in that, The predetermined linewidth of the transmission spectrum of the filter is smaller than the linewidth of the output spectrum of the laser under test.

7. A method for measuring the linewidth of a laser, characterized in that, The measurement method includes: The transmission center wavelength of the filter is tuned so that the change in the transmission center wavelength covers the linewidth of the laser under test, and the output laser of the laser under test is filtered. The transmission spectrum of the filter has a predetermined line shape and a predetermined linewidth, and the output spectrum of the laser under test has the predetermined line shape. The filtered laser is detected by a detector to obtain the power of the output laser passing through the filter at different transmission center wavelengths; Based on the predetermined line shape, a power spectrum is generated by fitting each transmission center wavelength and the power corresponding to each transmission center wavelength. The linewidth of the output spectrum of the laser under test is determined based on the linewidth of the power spectrum and the predetermined linewidth of the transmission spectrum.

8. The measurement method according to claim 7, characterized in that, The predetermined line type includes the Lorentz line type; the line width is characterized by the full width at half maximum (FWHM). The step of fitting each transmission center wavelength and the power corresponding to each transmission center wavelength based on the predetermined line shape to generate a power spectrum includes: Based on the Lorentz line shape, the power corresponding to each transmission center wavelength is fitted to generate the Lorentz power spectrum. Determining the linewidth of the output spectrum of the laser under test based on the linewidth of the power spectrum and the predetermined linewidth of the transmission spectrum includes: The full width at half maximum (FWHM) of the output spectrum of the laser under test is obtained by subtracting the FWHM of the transmission spectrum from the FWHM of the Lorentz power spectrum.

9. The measurement method according to claim 7, characterized in that, The predetermined line type includes a Gaussian line type; the line width is characterized by half-height and full width. Based on the predetermined line shape, a power spectrum is generated by fitting each transmission center wavelength and the power corresponding to each transmission center wavelength, including: Based on the Gaussian line shape, a Gaussian power spectrum is generated by fitting each transmission center wavelength and the power corresponding to each transmission center wavelength. Determining the linewidth of the output spectrum of the laser under test based on the linewidth of the power spectrum and the predetermined linewidth of the transmission spectrum includes: The square root of the difference between the square of the full width at half maximum (FWHM) of the Gaussian power spectrum and the square of the full width at half maximum (FWHM) of the transmission spectrum is taken to obtain the full WHM of the output spectrum of the laser under test.

10. The measurement method according to claim 7, characterized in that, The parameters of the filter include tuning parameters; The tuning of the tunable transmission center wavelength of the filter, such that the change in the transmission center wavelength covers the linewidth of the laser under test, and the filtering of the output laser of the laser under test, includes: By adjusting the tuning parameters, the value of the transmitted center wavelength is adjusted so that the change in the transmitted center wavelength covers the linewidth of the laser under test, thereby filtering the output laser of the laser under test.

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