A method and device for measuring dynamic frequency of a tunable laser

Abstract

The present invention generates a frequency marker within a large frequency tuning range through a digital bandpass filter and a frequency reference light source, and dynamically measures the output frequency of a tunable laser within the large frequency tuning range through the frequency marker. This scheme of dynamically measuring the output frequency of a laser through a frequency marker device has the characteristics of simple structure, small size, and low cost. At the same time, it can meet the requirements of a large test range and has the excellent performance of high measurement resolution, fast test speed, and strong anti-interference ability. More importantly, the measurement accuracy is very high. Therefore, the technical solution of the present invention can be used for detailed analysis or evaluation of the performance of a laser within a large frequency tuning range, as well as for frequency calibration including pre-measurement and real-time online measurement in the specific application of the laser.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical measurement, and more particularly, relates to a method for dynamically measuring the output frequency of a tunable laser, and a device for implementing the method. Background Art

[0002] Tunable lasers can output narrow-linewidth laser light whose frequency varies continuously over time. This characteristic makes them widely used in technologies such as lidar, optical frequency domain reflectometry, optical coherence tomography, swept-frequency spectrometers, and insertion loss measurement. In these applications, a large frequency tuning range can achieve better results, such as high resolution in lidar, optical frequency domain reflectometry, and optical coherence tomography, and a large test range in swept-frequency spectrometers and insertion loss measurement. Today, commercial tunable lasers can easily achieve tuning ranges exceeding 10 THz. However, with such a large frequency tuning range, the laser's frequency tuning speed is difficult to maintain stable, making it difficult to ensure the accuracy of the laser's instantaneous laser frequency. This makes it crucial to dynamically measure the laser's output frequency over a large frequency range in order to better understand the performance of tunable lasers and evaluate their applicability in different application scenarios.

[0003] Currently, there are several methods for realizing dynamic measurement of laser output frequency over a large range, but the disadvantages of these methods are also very obvious.

[0004] 1. Measurement method based on wavelength meter. The wavelength meter is a mature commercial instrument that can measure the frequency of the input optical signal. However, the measurement speed is very slow, only achieving a measurement speed of 500Hz. Obviously, it cannot accurately measure the frequency of a tunable laser that changes at a high speed.

[0005] 2. A measurement method that uses an optical frequency comb as a frequency reference and uses Hilbert transform to read the beat frequency between the tunable laser and the optical frequency comb, thereby calculating the output frequency of the laser. During the tuning process, the laser will produce a beat frequency with the optical frequency comb. Since the frequency of the laser is continuously changing, the beat frequency will also change continuously. Therefore, reading the frequency change of this beat frequency can restore the frequency change of the laser. This method uses Hilbert transform when reading the beat frequency, and divides the beat frequency signal into two paths, one of which introduces a 90-degree phase delay. These two signals are received by two receivers. However, this method has the following problems. Ideally, only the beat frequency of the laser and the nearest comb tooth is considered, and the beat frequency signal can be written as Where A is the beat signal amplitude and ω(t) is the beat frequency, which changes as the laser is tuned. is the phase of the beat frequency. After introducing a 90-degree phase, the resulting signal is The instantaneous phase can be calculated by dividing these two signals Taking the derivative of the instantaneous phase, we can get Φ'(t) = ω(t) + ω'(t)t. However, the Hilbert transform method ignores the latter term when reading the beat frequency, and assumes that the derivative of the instantaneous phase is the beat frequency, which will introduce errors. More seriously, since the laser will produce a beat frequency with all the comb teeth, the actual beat frequency signal is The corresponding other signal is Then through The instantaneous phase obtained does not simply correspond to the ideal case. Therefore, the instantaneous frequency of the laser determined by the Hilbert transform method is inaccurate. Furthermore, the actual signal sampled contains noise, which further affects the Hilbert transform result. This method is very sensitive to phase, resulting in poor anti-interference capabilities. Furthermore, the Hilbert transform method requires the device to split the signal into two paths and apply a 90-degree phase delay to one of the paths, making the device complex, bulky, and costly.

[0006] 3. A method that uses an optical frequency comb as a frequency reference and a hardware bandpass filter to generate a frequency mark. This method combines the optical signals of a tunable laser and an optical frequency comb, and then uses a photodetector to read the beat frequency. The electrical signal generated by the beat frequency is divided into two paths, which are respectively passed through two hardware bandpass filters and then read by an oscilloscope. Since the laser is continuously sweeping, the beat frequency generated by it and the optical frequency comb is constantly changing. Only at specific moments is the beat frequency equal to the passband frequency of the filter. Therefore, these moments can be found through the bandpass filter, and the output frequency of the laser can be obtained. However, the use of hardware bandpass filters will also increase the complexity of the device. This method uses two hardware bandpass filters, which can only achieve a frequency resolution of 60MHz, and there is a problem of low resolution. The frequency measurement resolution can be improved by increasing the bandpass, but it is necessary to increase the channels collected by the oscilloscope, which increases the cost, volume and system complexity.

[0007] 4. Use the Hilbert transform of the fiber optic unequal-arm interferometer for measurement. This method divides the output of the tunable laser into two arms, and then combines the two arms after passing through the two arms of different lengths. If the arm length difference between the two arms is L, there will be a time difference when the laser passes through the combined beam. Where n is the refractive index and c is the speed of light in a vacuum. Since the laser output frequency is tuned, there will be a frequency difference between the two beams. Where α(t) is the frequency tuning speed of the laser. Then the combined light will form a beat frequency signal Where A is the beat frequency amplitude, is the phase of the beat frequency. Next, ω(t) is read using the Hilbert transform. Due to the Hilbert transform's sensitivity to noise, this method also suffers from poor anti-interference capabilities. Furthermore, implementing the Hilbert transform requires a complex, bulky, and costly device. Furthermore, due to the dispersion of optical fibers—that is, the refractive index n varies at different frequencies—the time difference τ also changes when the frequency is swept over a wide range. This results in inaccurate measurements of the laser's output frequency over a wide range using this method.

[0008] 5. Use the transmission spectrum of the fiber optic unequal arm interferometer to measure the frequency. This method measures the frequency by measuring the transmission spectrum of the fiber optic unequal arm interferometer when the laser frequency is tuned. As the laser frequency changes with time, the transmission spectrum of the fiber optic unequal arm interferometer changes sinusoidally, such as Figure 1 The period of the sinusoidal signal is the free spectral range (FSR) of the unequal-arm interferometer. Where c is the speed of light in a vacuum, n is the refractive index, and L is the arm length difference. This method measures the laser frequency by calculating the period of the transmission spectrum. However, over a large frequency tuning range, optical fiber is affected by dispersion, and the FSR of the unequal-arm interferometer varies, resulting in inaccurate frequency measurements. Furthermore, the frequency resolution of this method is proportional to the FSR. To increase the resolution to 1 MHz, an arm length difference of approximately 200 m is required. Such a long optical fiber is severely affected by temperature, vibration, and other interferences, resulting in poor stability. Therefore, this method can only achieve a trade-off between resolution and stability.

[0009] 6. Use the dispersion-calibrated fiber ring cavity transmission spectrum to measure frequency. This method measures the frequency by measuring the transmission spectrum of the fiber ring cavity when the laser frequency is tuned. As the laser frequency changes with time, the transmission spectrum of the fiber ring cavity will be as follows: Figure 1 As shown in b. Its resonance peaks also appear at approximately equal intervals, and its free spectral range (FSR) is Where c is the speed of light in a vacuum, n is the refractive index, and L is the cavity length. This method measures frequency by counting the resonant peaks in the transmission spectrum. Unlike the unequal-arm interferometer, the dispersion of the fiber ring cavity can be calibrated using a modulator, which can eliminate the influence of dispersion during measurement. However, the same thing is that the frequency resolution of this method is the size of the FSR. If you want to increase the resolution to 1MHz, the fiber ring cavity length needs to reach about 200m. Such a long fiber is severely affected by temperature, vibration, and other interferences, and has poor stability. In addition, because the fiber cavity loss is very small and the cavity life is very long, when the laser is tuned at high speed, the light newly entering the cavity will interfere with the light that entered the cavity before, and oscillations can be observed in the transmission spectrum, which makes the measurement accuracy poor.

[0010] In summary, although some existing methods for measuring laser output frequency exist, they suffer from numerous drawbacks. For example, wavelength meters are very slow; Hilbert transforms have poor anti-interference capabilities and are accompanied by complex, bulky, and costly devices; transmission spectrum measurements also suffer from poor stability and even require sacrificing resolution; and some measurement methods themselves, such as approximate Hilbert transform processing, dispersion-affected transmission spectrum processing, and interferometry within fiber ring cavities, can, in principle, lead to inaccurate measurements. Therefore, a new approach to dynamic frequency measurement of tunable lasers that overcomes these drawbacks and is applicable over a wide frequency range is needed. Summary of the Invention

[0011] In response to the defects of related technologies, the present invention proposes a technical solution that generates frequency marks within a large frequency tuning range through a digital bandpass filter and a frequency reference light source, and dynamically measures the output frequency of a tunable laser within a large frequency tuning range through the frequency marks. The invention aims to solve the problems of poor measurement accuracy, low measurement resolution, slow test speed, weak anti-interference ability, and complex structure, large size and high cost of the existing technology.

[0012] To achieve the above object, the present invention provides a method for measuring the dynamic frequency of a tunable laser, wherein the method uses an output optical signal of the tunable laser and a frequency reference light source to form a beat signal, and processes the beat signal to measure the frequency of the output optical signal. The method is characterized in that the method comprises the following steps:

[0013] S1. Combining the output optical signal with the frequency reference light source to form the beat frequency signal;

[0014] S2. Detecting the beat signal using a photodetector to obtain a digital signal corresponding to the beat signal;

[0015] S3, passing the digital signal through a digital bandpass filter to obtain a series of pulse signals as generated frequency markers;

[0016] S4. Calculate the frequency of the output optical signal using the frequency marker and the known frequency of the frequency reference light source.

[0017] Due to the use of digital bandpass filters, some hardware limitations are largely eliminated, and the filters can be set more precisely, thereby greatly improving the accuracy, measurement speed, resolution and other performance of the tunable laser frequency measurement. The frequency measurement device is also simpler, lower in cost and smaller in size.

[0018] Furthermore, in the step S4, it also includes obtaining the pulse apex position of the pulse signal corresponding to the frequency mark, and obtaining beat frequency information according to the pulse apex position; and further calculating the frequency of the output optical signal according to the beat frequency information and the known frequency of the frequency reference light source.

[0019] Furthermore, before step S3, the method further includes setting the passband center frequency of the digital bandpass filter. When the frequency reference light source is an optical frequency comb, the passband center frequency is selected from the beat frequency generated by the optical comb with the nearest comb tooth, the beat frequency generated by the optical comb with the next nearest comb tooth, or the beat frequency generated by the optical comb with the comb tooth farther away. This allows for a highly flexible design of the digital bandpass filter, and selecting the beat frequency with the nearest comb tooth can reduce the requirement for the device's sampling bandwidth.

[0020] Preferably, before step S3, the method further includes setting different widths of the digital bandpass filter according to the frequency tuning speed of the tunable laser. This will make the frequency mark narrower, thereby facilitating more accurate locating of the pulse peak position and improving the accuracy of frequency measurement.

[0021] Furthermore, step S3 specifically involves passing the digital signal through multiple digital bandpass filters with different passband center frequencies, where the interval between the multiple passband center frequencies is greater than the instantaneous linewidth of the tunable laser. For example, the passband center frequencies of the multiple digital bandpass filters range from 1 MHz to 99 MHz and are consecutively spaced at 1 MHz intervals. In this way, high-resolution frequency measurement with a resolution of 1 MHz can be achieved without increasing the complexity, volume, or cost of the device.

[0022] Corresponding to the tunable laser dynamic frequency measurement method, the present invention also proposes a tunable laser dynamic frequency measurement device, which uses the output optical signal of the tunable laser and a frequency reference light source to form a beat signal, and measures the frequency of the output optical signal by processing the beat signal. The device includes a frequency marking device and a response analysis device; the frequency marking device includes a frequency reference light source and a photodetector; the response analysis device includes a digital bandpass filter and a control unit thereof, and the parameters of the digital bandpass filter are set by the control unit; the response analysis device is also provided with a calculation unit, and the calculation unit calculates the frequency of the output optical signal based on the known frequency of the frequency marker and the frequency reference light source.

[0023] Preferably, the frequency reference light source is an optical frequency comb; and / or the digital bandpass filter is a finite impulse response digital bandpass filter based on a Kaiser window design; the digital bandpass filter is implemented via software or replaced by a field programmable gate array; and / or the photodetector is a balanced photodetector. This improves the performance of the entire measurement device.

[0024] Based on the excellent performance of the measurement method and measurement device of the present invention, a laser tuning dynamic measurement device is further proposed, which includes the above-mentioned tunable laser dynamic frequency measurement device and a user interaction device. The user interaction device includes a display, a processor, and a storage device, which is used to display the frequency measurement results of the tunable laser dynamic frequency measurement device according to user settings, store the measurement results, and further compare and analyze the measurement results, thereby evaluating the performance of the tunable laser. In addition, a tunable laser frequency correction unit is further proposed, which includes the above-mentioned tunable laser dynamic frequency measurement device and corrects the output frequency of the tunable laser according to the measurement results of the tunable laser dynamic frequency measurement device. This further improves the performance of the product in different application scenarios.

[0025] The above technical solution conceived by the present invention has achieved the following beneficial effects compared with the prior art. Such a solution of dynamically measuring the output frequency of a laser through a frequency marker device has the characteristics of simple structure, small size, and low cost. At the same time, it can meet the needs of a large test range and has the excellent performance of high measurement resolution, fast test speed, and strong anti-interference ability. More importantly, the measurement accuracy is very high. Therefore, the technical solution of the present invention can be used for detailed analysis or evaluation of the performance of a laser within a large frequency tuning range, as well as for frequency calibration including pre-measurement and real-time online measurement in the specific application of the laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 a is a schematic diagram of frequency markers generated by an unequal-arm interferometer in the prior art;

[0027] Figure 1 b is a schematic diagram of frequency markers generated by a ring resonator in the prior art;

[0028] Figure 2 It is the basic process of dynamic frequency measurement and application of tunable laser;

[0029] Figure 3 It is a schematic diagram of the specific test optical path;

[0030] Figure 4 This is a schematic diagram of the principle of frequency marking generated by a tunable laser and an optical frequency comb;

[0031] Figure 5 This is a schematic diagram of frequency markers generated by digital bandpass filters with different passband widths for a frequency tuning speed of 0.625 THz / s;

[0032] Figure 6This is a schematic diagram of the relationship between multiple bandpass filters with different passband center frequencies and frequency resolution;

[0033] Figure 7 The results are obtained by measuring the frequency of a commercial tunable laser at different tuning speeds within the 10THz frequency tuning range. DETAILED DESCRIPTION

[0034] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0035] Example 1

[0036] See also Figure 2 This is the basic process for dynamic frequency measurement and application of tunable lasers. First, the output of the laser to be tested is input into a frequency marking device, which includes but is not limited to frequency marking devices for frequency reference light sources such as optical frequency combs. The response of the frequency marking device is then processed by a subsequent response analysis device to obtain the precise output frequency f(t) of the laser at different times. After obtaining the precise output frequency, f(t) can be further analyzed to evaluate the tuning accuracy of the tunable laser. The measurement results can also be used to perform frequency correction on the output of tunable lasers in application scenarios including but not limited to lidar, optical frequency domain reflectometry, optical coherence tomography, swept spectrum spectrometers, and insertion loss measurement.

[0037] The following specific embodiments describe the measurement scheme in detail using an optical frequency comb as an example of a frequency marker device. Those skilled in the art can expect that other frequency marker devices based on a frequency reference light source are also applicable.

[0038] The optical frequency comb has a stable frequency component with narrow linewidth in the frequency domain. For the optical frequency comb, its frequency component can be expressed as f n =f ceo +n*f rep , where the system frequency shift signal f ceo and the optical frequency comb repetition frequency signal f rep These frequency components are equally spaced and distributed in a wide range up to 10 THz. They can be used as a reference light source to meet the needs of frequency measurement in a wide range. And in the full range, the repetition frequency f rep Remain stable and unaffected by dispersion.

[0039] See also Figure 3The figure is a schematic diagram of the specific test optical path. The output of the tunable laser 1 to be tested is input into the frequency marking device 2 and then combined with the optical frequency comb 21. The balanced photodetector 22 receives the signal, and the response analysis device 3 then processes the detector signal. It is easy for those skilled in the art to know that an ordinary photodetector can also be used to receive the optical signal. When the laser is frequency tuned, it will generate a beat frequency b(t) = |f n -f l (t)|, where f l (t) is the instantaneous frequency of the tunable laser, f n is the reference frequency. Consider f l (t) is the beat frequency between the nearest reference frequency, then when f l When (t) is continuously tuned, the frequency difference between it and the reference frequency will go from far to near and then become farther, and then beat with the next reference frequency. In this process, the beat frequency b(t) is changed from f rep / 2 decreases to 0 and then increases to f rep / 2 and repeat continuously, such as Figure 4 a and Figure 4 As shown in b.

[0040] The corresponding response analysis device 3 is composed of a signal acquisition device, such as an oscilloscope, an acquisition card, etc., and a processing unit for processing the algorithm. A digital bandpass filter is used to process the beat frequency of the tunable laser and the optical frequency comb. Specifically, a finite impulse response (FIR) digital bandpass filter based on the Kaiser window design is used here. The Kaiser window can be used for the most flexible bandpass design, which is convenient for optimizing the design parameters. For example, the steepness of the filter can be easily controlled. It is easy for those skilled in the art to know that similar effects can be achieved by replacing other digital bandpass filter algorithms. In addition to using software to implement the filter here, it can also be replaced by a field programmable gate array (FPGA). For a passband center frequency of f F The bandpass filter only works when the beat frequency signal is within f F Therefore, if Figure 4 As shown in c, when the laser is continuously tuned, the beat signal generates a series of pulses after passing through the bandpass filter, and the peaks of these pulses mark the beat frequency b(t) = f F These pulses are the generated frequency markers. F and the frequency f of the optical frequency comb n It is known that, and the laser is continuously tuned, the vertices of these frequency marks correspond to the output frequency f of the laser. n ±f F .f F The selection of is very flexible, and we can choose the beat frequency generated by the optical comb and the nearest comb teeth, that is, You can also choose the beat frequency between the optical comb and the next adjacent comb teeth, that is, Or the beat frequency with a more distant comb tooth. However, the beat frequency with the nearest comb tooth is usually selected to reduce the demand on the device sampling bandwidth.

[0041] The width of the frequency mark is determined by the frequency tuning speed and the passband width of the bandpass filter. Therefore, it is necessary to design different bandpass filter widths B according to different frequency tuning speeds. By simulating a frequency sweep signal y(t)=cos(πkt 2 ), where k is the frequency tuning speed and t is time. For a specific k, by changing the passband width B of different bandpass filters, frequency marks of different widths can be obtained. By scanning the value of the passband width B, the passband width B corresponding to the narrowest frequency mark can be obtained. After finding the optimal passband width B under different k, the empirical formula for selecting the passband width can be fitted from the obtained data. In this embodiment, for the bandpass filter based on the Kaiser window, the empirical formula that can be fitted is B=1.087*k^0.5031, where the unit of k is THz / s. The influence of passband width on frequency marking is as follows Figure 5 As shown in the figure, for a frequency tuning speed of 0.625 THz / s, a 0.86 MHz bandpass filter passband width yields the narrowest frequency marker. This width is 27.5% smaller than a 1.5 MHz bandpass and 35.8% smaller than a 0.5 MHz bandpass. This significantly improves measurement accuracy. The narrower the frequency marker, the more accurately the vertex is found, and the more accurate the frequency calibration. Furthermore, the steepness of the digital filter is set to 0.9 during design to achieve the narrowest frequency marker.

[0042] For a passband center frequency of f F1 A bandpass filter can be used to obtain a set of frequency markers whose frequencies correspond to f n ±f F1 ,like Figure 6 As shown in a. When the passband center frequency is f F2 By bandpass filter processing, another set of frequency markers can be obtained, such as Figure 6 By using different center frequency bandpass filters, different frequency markers can be obtained, and they can be combined to obtain dense frequency markers, such as Figure 6c. The frequency interval of the frequency marker is the resolution of the frequency calibration. Therefore, increasing the number of bandpass filters can increase the resolution of the laser frequency measurement. If the center frequency interval of the selected multiple bandpass filters is 1MHz, then the frequency interval of the frequency marker is 1MHz, and the corresponding frequency measurement resolution reaches 1MHz. For digital bandpass filters, the center frequency of the bandpass can be set by software in the response analysis device or configured in the FPGA, which does not increase the complexity of the device. Therefore, the overall device structure is very simple and will not cause problems of large size or high cost. In the example, f is realized F The continuous setting of 1MHz intervals from 1MHz to 99MHz enables high-resolution frequency measurement with a resolution of 1MHz. For higher-resolution frequency measurement requirements, smaller frequency intervals can be selected as long as the frequency interval is larger than the instantaneous linewidth of the laser.

[0043] This resolution is unaffected by the sweep speed. Therefore, when the laser is tuned at high speeds, such as 25 THz / s, the frequency measurement speed can reach 25 MHz, enabling high-speed measurement. Furthermore, although the laser beats with all the comb teeth of the optical frequency comb during tuning, the use of a bandpass filter effectively filters out the signal generated by the laser beating against the distant comb teeth, ensuring accuracy. Similarly, the use of a bandpass filter eliminates most noise signals, resulting in excellent anti-interference capabilities.

[0044] Example 2

[0045] The device for implementing the method for measuring the dynamic frequency of a tunable laser as described in the first embodiment is as follows: Figure 2 and 3As shown, the main body includes a frequency marking device 2 and a response analysis device 3. The frequency marking device includes a frequency reference light source, represented by an optical frequency comb 21, and a photodetector for detecting the beat signal formed by the tunable laser's output light signal and the frequency reference light source. Preferably, a balanced photodetector 22 is used. This balanced detection technology helps eliminate common-mode noise, thereby improving signal quality. The photodetector receives and detects the light signal and converts it into a corresponding electrical signal. If the converted electrical signal is in analog form, it is converted to a digital signal by an analog-to-digital converter for subsequent processing. The analog-to-digital converter can be integrated into the frequency marking device or provided in the response analysis device. The digital signal is then further processed in the response analysis device. The response analysis device includes a digital bandpass filter and its control unit. Its parameters, such as the passband center frequency, passband width, and cutoff frequency, can be set using dedicated hardware in the control unit, allowing the user to set a series of digital bandpass filters with different passband center frequencies according to the solution of Example 1. The response analysis device also includes a calculation unit that performs further signal analysis and calculations on the output signal of the digital filter, as described in Example 1, to obtain the output frequency of the tunable laser.

[0046] Example 3

[0047] The tunable laser output frequency measurement device of Example 2 is used to implement the dynamic frequency measurement method of Example 1, and the frequency of a commercial tunable laser at different tuning speeds (0.125THz / s to 25THz / s) within the 10THz frequency tuning range is measured. Figure 7 Figure 1 shows the analysis results of a laser at a 2.5 THz / s frequency tuning speed using an optical frequency comb to generate frequency markers. Different bandpass filter passbands were designed for different frequency tuning speeds, as shown in Table 1. The steepness of the bandpass filters was designed to be 0.9.

[0048] Table 1 Optimal passband width at different tuning speeds

[0049]

[0050] This method uses a frequency comb f rep =200MHz, digital bandpass filter f F The frequency range is from 1 MHz to 99 MHz in 1 MHz intervals. The frequency resolution achieved is 1 MHz, which is not affected by the frequency tuning speed. At a frequency tuning speed of 25 THz / s, the measurement speed reaches 25 MHz.

[0051] The measurement resolution achieved in the example is 1MHz. However, in the prior art, the resolution of the method using a hardware bandpass filter to generate a frequency marker is only 60MHz, and the resolution of the method using a transmission spectrum to measure frequency in the prior art is only 50MHz when ensuring relatively stable parameter settings. This method can achieve a frequency measurement speed of 25MHz at a frequency tuning speed of 25THz / s, while the existing measurement method using a wavelength meter is only 500Hz, the method using a hardware bandpass filter is 417kHz, and the method using a transmission spectrum to measure frequency is only 500kHz when ensuring relatively stable parameter settings. In addition, in principle, when using the Hilbert transform to read the frequency, it will be affected by the beat frequency of the laser and the teeth of multiple optical frequency combs, and the results are inaccurate. This method eliminates the beat frequency of the laser and the teeth of other optical frequency combs through a digital bandpass filter, and the results are accurate. In addition, the use of a digital bandpass filter can simultaneously filter out noise outside the passband frequency, so it has better anti-interference ability.

[0052] Figure 7 a is the instantaneous frequency of the laser at a frequency tuning speed of 2.5 THz / s. For ease of display, only the first 0.01 seconds of data are plotted. By taking the derivative of the frequency, the tuning speed of the laser can be obtained, such as Figure 7 As shown in Figure b, it can be seen that the laser is not tuned at a uniform speed, and the tuning speed has an obvious jitter. By performing Fourier analysis on the laser tuning speed, as shown in Figure 2, Figure 7 As shown in Figure c, the laser tuning speed jitter frequency is 8 kHz, corresponding to a mechanical mode within the laser's internal resonant cavity. This information is obtained thanks to the high resolution, high speed, high accuracy, and strong anti-interference capabilities of the frequency measurement technology of the present invention.

[0053] Example 4

[0054] By using the tunable laser output frequency measurement device of Example 2 to implement the dynamic frequency measurement method of Example 1, an accurate tunable laser output frequency can be obtained. For a frequency tunable laser, one of its measurement criteria is the accuracy of frequency tuning. Therefore, by comparing the set tuning frequency and the instantaneous frequency measured using the technical solution of the present invention, information such as the deviation of the laser output frequency from the set value, the jitter and amplitude of the tuning rate can be obtained. Thus, if Figure 2The frequency marking device 2, response analysis device 3 and user interaction device shown will constitute a laser tuning dynamic measurement device, wherein the user interaction device includes, for example, but is not limited to, a computing processing unit composed of a display, a processor, and a storage device, which is used to display the laser frequency measurement results according to user settings, store the measurement results, and further compare and analyze the laser measurement results. Obtaining the above information through this device is helpful to evaluate the performance of the laser and optimize the design of the laser.

[0055] Tunable lasers are widely used in specific scenarios such as lidar, optical frequency domain reflectometry, optical coherence tomography, swept frequency spectrometers, and insertion loss measurement. In these practical applications, the aforementioned frequency marking device 2 and response analysis device 3 can be used simultaneously to obtain the output frequency of the laser online and use it to correct the frequency of the tunable laser online to form a frequency correction unit of the light source module.

[0056] In use, the frequency correction unit of the light source module can also be used to measure the output frequency of the laser in advance, and the test result obtained in advance can be used to make a rough correction when applied.

[0057] In summary, the present invention generates frequency markers using a digital bandpass filter and a frequency reference light source over a large frequency tuning range. The frequency markers are used to dynamically calculate and measure the output frequency of a tunable laser within this large frequency tuning range. Compared to methods using hardware bandpass filters and optical frequency combs, the present invention's use of digital bandpass filters eliminates hardware limitations, allowing for free design and configuration. While not limited by the number of filters, precise configuration is also possible. As demonstrated in the examples, this significantly improves the accuracy, speed, and resolution of tunable laser frequency measurement, while also enabling simpler, more cost-effective, and smaller frequency measurement devices. Overall, this approach to dynamically measuring the laser output frequency using a frequency marker device offers the advantages of a simple structure, compact size, and low cost. While meeting the requirements of a large test range, it also offers excellent performance features such as high measurement resolution, fast test speed, and strong anti-interference capabilities. More importantly, its measurement accuracy is exceptionally high. Therefore, the technical solution of the present invention can be used for detailed analysis or evaluation of laser performance over a large frequency tuning range, as well as for frequency calibration, including both pre-measurement and real-time online measurement, in specific laser applications.

[0058] It will be easily understood by those skilled in the art that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for measuring the dynamic frequency of a tunable laser, comprising: utilizing an output optical signal of a tunable laser and a frequency reference light source to form a beat signal; and processing the beat signal to measure the frequency of the output optical signal. The method is characterized in that: The steps include: S1. Combining the output optical signal with the frequency reference light source to form the beat frequency signal; S2. Detecting the beat signal using a photodetector to obtain a digital signal corresponding to the beat signal; S3, passing the digital signal through a digital bandpass filter to obtain a series of pulse signals as generated frequency markers; S4. Calculating the frequency of the output optical signal using the frequency marker and the known frequency of the frequency reference light source; In the step S4, the method further includes obtaining a pulse vertex position of the pulse signal corresponding to the frequency marker, obtaining beat frequency information according to the pulse vertex position; and further calculating the frequency of the output optical signal according to the beat frequency information and the known frequency of the frequency reference light source. Before step S3, the method further includes setting the passband center frequency of the digital bandpass filter; when the frequency reference light source is an optical frequency comb, the passband center frequency is selected from the beat frequency generated by the optical comb and the most adjacent comb tooth, or the beat frequency generated by the optical comb and the second adjacent comb tooth.

2. The method for measuring the dynamic frequency of a tunable laser according to claim 1, wherein: Before step S3, the method further includes setting different widths of the digital bandpass filter according to the frequency tuning speed of the tunable laser.

3. The method for measuring the dynamic frequency of a tunable laser according to claim 1, wherein: The step S3 specifically involves passing the digital signal through a plurality of digital bandpass filters with different passband center frequencies.

4. The method for measuring the dynamic frequency of a tunable laser according to claim 2, wherein: The interval between the passband center frequencies of the plurality of digital bandpass filters is greater than the instantaneous linewidth of the tunable laser.

5. A tunable laser dynamic frequency measurement device implementing the tunable laser dynamic frequency measurement method according to any one of claims 1 to 4, wherein the device utilizes an output optical signal of the tunable laser and a frequency reference light source to form a beat signal, and processes the beat signal to measure the frequency of the output optical signal, characterized in that: The apparatus includes a frequency marking device and a response analysis device; The frequency marking device includes a frequency reference light source and a photodetector; The response analysis device includes a digital bandpass filter and a control unit thereof, wherein the parameters of the digital bandpass filter are set by the control unit; the response analysis device is further provided with a calculation unit, wherein the frequency of the output optical signal is calculated based on the frequency marker and the known frequency of the frequency reference light source.

6. The tunable laser dynamic frequency measurement device according to claim 5, characterized in that: The frequency reference light source is an optical frequency comb; and / or, The digital bandpass filter is a finite impulse response digital bandpass filter based on a Kaiser window design; the digital bandpass filter is implemented by software or replaced by a field programmable gate array; and / or, The photodetector is a balanced photodetector.

7. A laser tuning dynamic measurement device, characterized in that: The invention comprises the tunable laser dynamic frequency measurement device according to claim 5 or 6, and a user interaction device, wherein the user interaction device comprises a display, a processor, and a storage device, and is used to display the frequency measurement results of the tunable laser dynamic frequency measurement device according to user settings, store the measurement results, and further compare and analyze the measurement results, so as to evaluate the performance of the tunable laser.

8. A tunable laser frequency correction unit, characterized in that: The device comprises the tunable laser dynamic frequency measuring device according to claim 5 or 6, and the output frequency of the tunable laser is corrected according to the measurement result of the tunable laser dynamic frequency measuring device.

Images