On-chip heterodyne measurement method and device for laser frequency chirp

Abstract

The application relates to a kind of on-chip heterodyne measurement method and device of laser frequency chirp, wherein, method includes: with pulsed current drive first terahertz quantum cascade laser, make the first terahertz quantum cascade laser generate laser signal with frequency chirp;With continuous wave current drive second terahertz quantum cascade laser, make the second terahertz quantum cascade laser generate stable local signal;With the nonlinear mixing characteristics of the second terahertz quantum cascade laser itself, the laser signal emitted by the first terahertz quantum cascade laser and the local signal are heterodyned down-converted, and radio frequency beat signal is output;The time-domain waveform of the radio frequency beat signal is collected, the time-domain waveform is seamlessly continuous time slice and Fourier transform, and bidirectional peak tracking is used to extract the chirp curve of frequency change with time.The application can measure laser transient frequency chirp dynamic with sub-microsecond time resolution.

Description

Technical Field

[0001] This invention relates to the field of semiconductor optoelectronic device application technology, and in particular to an on-chip heterodyne measurement method and apparatus for laser frequency chirp. Background Technology

[0002] Frequency chirp refers to the phenomenon of the output light frequency of a laser drifting over time, and is an inherent dynamic characteristic of semiconductor lasers during pulse driving or modulation. This characteristic has a significant impact on the performance of laser systems: on the one hand, random frequency chirp can lead to increased bit error rates in coherent optical communication, decreased spectral peak resolution in molecular spectroscopy detection, and reduced spatial resolution in non-destructive imaging; on the other hand, controllable frequency chirp is of great value in applications such as frequency-modulated continuous wave radar and chirped pulse amplification. Therefore, the dynamic measurement and characterization of the transient frequency chirp of lasers is a key technical aspect of laser performance evaluation and application optimization.

[0003] Terahertz (THz) quantum cascade lasers (QCLs) have become one of the most competitive semiconductor light sources in the terahertz band due to their advantages such as high output power, narrow linewidth, compact size, and flexible design of lasing wavelengths through bandgap engineering. Current research on the frequency characteristics of terahertz QCLs mainly focuses on their frequency tuning characteristics under steady-state operating conditions, such as achieving continuous tuning of the lasing frequency by adjusting the drive current or operating temperature. However, existing research has not yet addressed the transient frequency chirp dynamics of terahertz QCLs during the pulse-driven startup phase.

[0004] The main reason for this research gap is the lack of efficient and practical coherent detection methods in the terahertz band. In the mid-infrared band, mature high-performance detectors (such as mercury cadmium telluride detectors and quantum well infrared detectors) make frequency chirp measurement of QCLs under pulse drive relatively convenient. However, in the terahertz band, existing coherent detection techniques have significant limitations: Schottky barrier diode mixers lack sufficient sensitivity to meet the requirements of weak signal detection; superconducting thermal electron radiometers and quantum well infrared detectors require low-temperature operation, resulting in complex and bulky systems unsuitable for integrated and portable applications. Therefore, there is an urgent need for a method that can achieve high time resolution transient frequency chirp measurement in the terahertz band. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an on-chip heterodyne measurement method and apparatus for laser frequency chirp, which can measure the transient frequency chirp dynamics of laser with sub-microsecond time resolution.

[0006] The technical solution adopted by this invention to solve its technical problem is: to provide an on-chip heterodyne measurement method for laser frequency chirp, comprising the following steps:

[0007] A pulsed current drives a first terahertz quantum cascade laser, causing the first terahertz quantum cascade laser to generate a laser signal with frequency chirp.

[0008] A second terahertz quantum cascade laser is driven by a continuous wave current, so that the second terahertz quantum cascade laser generates a stable local oscillator signal;

[0009] By utilizing the nonlinear mixing characteristics of the second terahertz quantum cascade laser, the laser signal emitted by the first terahertz quantum cascade laser and the local oscillator signal are heterodyne downconverted to output a radio frequency beat frequency signal.

[0010] The time-domain waveform of the radio frequency beat frequency signal is acquired, and the time-domain waveform is subjected to seamless continuous time slicing and Fourier transform. The chirp curve of frequency change over time is extracted by bidirectional peak tracking.

[0011] The first terahertz quantum cascade laser and the second terahertz quantum cascade laser are integrated on a single monolithic integrated chip.

[0012] Before acquiring the time-domain waveform of the radio frequency beat frequency signal, the method further includes:

[0013] The radio frequency beat signal is amplified by an amplifier and then input to a spectrum analyzer. The spectrum analyzer is used to observe the spectrum of the radio frequency beat signal, and the drive current is adjusted so that the center frequency of the radio frequency beat signal falls within the range that the sampling rate of the time-domain waveform acquisition device can cover.

[0014] The process of performing seamless continuous time slicing and Fourier transform on the time-domain waveform, and extracting the chirp curve with frequency variation over time using bidirectional peak tracking, specifically includes:

[0015] The time-domain waveform is divided into several consecutive short-time slices in chronological order. A window function is applied to each short-time slice, and a fast Fourier transform is performed to obtain the spectrum corresponding to each short-time slice.

[0016] Peak detection is performed on the spectrum of each short-time slice, and frequency points whose peak values ​​exceed a preset threshold are extracted as the candidate frequency set of the short-time slice;

[0017] A short-time slice with stable frequency change and a unique candidate frequency is selected as a reference point. Starting from the reference point, the slices are traversed one by one in both the time-increasing and time-decreasing directions. In the candidate frequency set of the short-time slices adjacent to the current short-time slice, the frequency point that is closest to the frequency value determined by the current short-time slice and changes continuously is searched as the frequency value of the adjacent short-time slice, forming the trajectory of frequency over time, and the frequency chirp curve of the first terahertz quantum cascade laser is obtained.

[0018] When searching for the frequency point in the candidate frequency set of the short-time slices adjacent to the current short-time slice that is closest to the frequency value determined by the current short-time slice and changes continuously, if a frequency point that meets the conditions cannot be found in the candidate frequency set of a certain short-time slice, then the frequency values ​​of the short-time slices before and after the current short-time slice are used to supplement it by linear interpolation or extrapolation.

[0019] When the first terahertz quantum cascade laser operates in multimode, the radio frequency beat signal contains multiple frequency components. For each frequency component, bidirectional peak tracking is used to extract the chirp curve of the frequency changing over time, resulting in multiple chirp curves. For any chirp curve, the frequency values ​​of adjacent short-time slices are compared. If the difference between the two is greater than the frequency resolution, it is determined that a frequency jump has occurred at that position. The frequency resolution is the reciprocal of the duration of the short-time slice.

[0020] The technical solution adopted by this invention to solve its technical problem is: to provide an on-chip heterodyne measurement device for laser frequency chirp, comprising:

[0021] The first terahertz quantum cascade laser is used to generate laser signals with frequency chirps by being driven by pulsed current.

[0022] The second terahertz quantum cascade laser is used to drive the laser with a continuous wave current to generate a stable local oscillator signal, and to perform heterodyne downconversion with the laser signal with frequency chirp to generate a radio frequency beat frequency signal.

[0023] A time-domain waveform acquisition device is used to acquire the time-domain waveform of the radio frequency beat frequency signal;

[0024] A computer is used to perform seamless continuous time slicing and Fourier transform on the time-domain waveform, and to extract the chirp curve of frequency variation over time using bidirectional peak tracking.

[0025] The first terahertz quantum cascade laser and the second terahertz quantum cascade laser are integrated on a single monolithic integrated chip.

[0026] The on-chip heterodyne measurement device for laser frequency chirp also includes:

[0027] Amplifier, used to amplify the radio frequency beat signal;

[0028] A spectrum analyzer is used to observe the spectrum of the radio frequency beat signal in order to adjust the drive current so that the center frequency of the radio frequency beat signal falls within the range that the sampling rate of the time-domain waveform acquisition device can cover.

[0029] The computer includes:

[0030] The partitioning and transformation module is used to divide the time-domain waveform into several consecutive short-time slices in chronological order, apply a window function to each short-time slice and then perform a fast Fourier transform to obtain the spectrum corresponding to each short-time slice.

[0031] The peak detection module is used to perform peak detection on the spectrum of each short-time slice and extract frequency points whose peak values ​​exceed a preset threshold as the candidate frequency set of the short-time slice.

[0032] The traversal module is used to select a short-time slice with stable frequency change and unique candidate frequency as a reference point. Starting from the reference point, it traverses slice by slice in both the time-increasing and time-decreasing directions. In the candidate frequency set of the short-time slices adjacent to the current short-time slice, it searches for the frequency point that is closest to the frequency value determined by the current short-time slice and changes continuously as the frequency value of the adjacent short-time slice, forming a trajectory of frequency over time, and obtaining the frequency chirp curve of the first terahertz quantum cascade laser.

[0033] When the traversal module searches for the frequency point in the candidate frequency set of the short-time slices adjacent to the current short-time slice that is closest to the frequency value determined by the current short-time slice and changes continuously, if a frequency point that meets the conditions cannot be found in the candidate frequency set of a certain short-time slice, the frequency values ​​of the short-time slices before and after that short-time slice are used to supplement it by linear interpolation or extrapolation.

[0034] When the first terahertz quantum cascade laser operates in multimode, the radio frequency beat signal contains multiple frequency components. The computer is used to extract the chirp curve of the frequency change over time for each frequency component using bidirectional peak tracking, resulting in multiple chirp curves. For any chirp curve, the frequency values ​​of adjacent short-time slices are compared. If the difference between the two is greater than the frequency resolution, it is determined that a frequency jump has occurred at that position. The frequency resolution is the reciprocal of the duration of the short-time slice.

[0035] Beneficial effects

[0036] Due to the adoption of the above-mentioned technical solution, this invention has the following advantages and positive effects compared with the prior art: This invention utilizes the nonlinear mixing characteristics of the second terahertz quantum cascade laser to achieve self-detecting heterodyne downconversion, converting the terahertz frequency chirped signal to the radio frequency band for acquisition, eliminating the need for an external terahertz coherent detector, overcoming the technical bottleneck of the lack of efficient room temperature detectors in the terahertz band, and resulting in a compact system structure that is easy to integrate; at the same time, this invention can obtain the frequency chirped dynamic evolution curve with sub-microsecond time resolution by performing short-time slicing, Fourier transform, and peak tracking on the time-domain waveform of the radio frequency beat frequency signal, meeting the measurement requirements of rapid frequency changes during the start-up phase of the pulsed laser, thereby accurately extracting the chirped curve with continuously changing frequency. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of an on-chip heterodyne measurement device for laser frequency chirp according to an embodiment of the present invention;

[0038] Figure 2 This is a flowchart of an on-chip heterodyne measurement method for laser frequency chirp according to an embodiment of the present invention;

[0039] Figure 3 This is a schematic diagram of the frequency chirp curves of the laser under different pulse driving currents measured in an embodiment of the present invention;

[0040] Figure 4 This is a graph showing the results of the multimode signal mode transitions measured in an embodiment of the present invention. Detailed Implementation

[0041] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0042] Embodiments of the present invention relate to an on-chip heterodyne measurement device for laser frequency chirp, such as... Figure 1 As shown, the system includes: a first terahertz quantum cascade laser (QCL1), a second terahertz quantum cascade laser (QCL2), a pulsed power supply, a continuous-wave power supply, a T-type bias resistor, an amplifier, a DC blocker, a spectrum analyzer, a time-domain waveform acquisition device, and a computer. The first and second terahertz quantum cascade lasers are integrated on a single monolithic chip. The first terahertz quantum cascade laser is connected to the pulsed power supply; the mixing port of the T-type bias resistor is connected to the second terahertz quantum cascade laser, the DC port is connected to the continuous-wave power supply, and the AC port is connected to the amplifier; the amplifier's output can be switched to the spectrum analyzer and, via the DC blocker, to the time-domain waveform acquisition device; the time-domain waveform acquisition device acquires the time-domain signal, which is then processed by the computer.

[0043] A monolithic integrated chip containing two terahertz quantum cascade lasers is placed in a vacuum, cryogenic environment. A continuous-wave power supply powers the second terahertz quantum cascade laser, enabling it to generate a frequency-stable local oscillator signal; a pulsed power supply powers the first terahertz quantum cascade laser, causing it to emit light and inducing it to generate a laser signal with frequency chirping.

[0044] In this embodiment, both terahertz quantum cascade lasers have a cavity length of 1 mm. The operating current of the second terahertz quantum cascade laser is set to 230 mA DC, ensuring stable operation in a single-mode continuous wave state and providing a stable single-frequency local oscillator signal. The first terahertz quantum cascade laser is driven by a pulsed power supply with a pulse width of 1 ms, a duty cycle of 10%, and an adjustable pulse current amplitude ranging from 160 mA to 174 mA (in 2 mA steps) to generate chirped laser signals with different starting frequencies. The chirped laser signal generated by the first terahertz quantum cascade laser and the local oscillator signal of the second terahertz quantum cascade laser beat in the second terahertz quantum cascade laser and down-converted to the radio frequency band to generate an radio frequency beat signal.

[0045] The amplifier in this embodiment is a radio frequency low-noise amplifier (RF low-noise amplifier) ​​whose operating bandwidth covers the frequency of the RF beat signal, providing a 30dB signal gain to meet the observation and measurement requirements of the time-domain waveform acquisition device and the spectrum analyzer. A DC blocker is used to prevent excessively strong leaked DC signals from directly entering the time-domain waveform acquisition device, providing protection for it. The time-domain waveform acquisition device can be an oscilloscope, used to measure the time-domain waveform of the RF beat signal, which is then processed by a computer. The oscilloscope's sampling rate is set to be greater than twice the highest frequency of the RF beat signal, here 20GS / s. The spectrum analyzer is used to monitor the state of the RF beat signal, facilitating the determination of its approximate frequency position.

[0046] In this embodiment, the spectrum analyzer and oscilloscope work together by switching between them: During the measurement preparation phase, the amplifier output is connected to the spectrum analyzer to observe the frequency and power of the beat signal in real time, so as to adjust the pulse current and / or continuous wave current to a suitable bias point; after confirming that the signal state meets the requirements (i.e., the center frequency of the RF beat signal falls within the range that the oscilloscope's sampling rate can cover), the amplifier output is reconnected to the DC blocker and the oscilloscope to acquire the time-domain waveform for subsequent chirp extraction. The two connection methods can be switched using the same high-frequency cable. During the measurement preparation phase, the spectrum analyzer can be used to monitor the beat signal state in real time to quickly lock the measurement conditions. After confirmation, the oscilloscope is switched to acquire the time-domain waveform, eliminating the need for an additional power divider, which simplifies the circuit and avoids signal attenuation introduced by the power divider.

[0047] The computer in this embodiment includes:

[0048] The partitioning and transformation module is used to divide the time-domain waveform into several consecutive short-time slices in chronological order. After applying a window function to each short-time slice, a fast Fourier transform is performed to obtain the spectrum corresponding to each short-time slice. The duration Δτ of each short-time slice is determined according to the required frequency resolution. Adjacent short-time slices are seamlessly connected. In this embodiment, the frequency resolution is the reciprocal of the duration of the short-time slice.

[0049] The peak detection module is used to perform peak detection on the spectrum of each short-time slice and extract frequency points whose peak values ​​exceed a preset threshold as the candidate frequency set of the short-time slice.

[0050] The traversal module is used to select a short-time slice with stable frequency changes and a unique candidate frequency as a reference point. Starting from the reference point, it traverses slice by slice in both the increasing and decreasing time directions. In the candidate frequency set of the short-time slice adjacent to the current short-time slice, it searches for the frequency point that is closest to the frequency value determined by the current short-time slice and changes continuously. This is used as the frequency value of the adjacent short-time slice, forming a trajectory of frequency over time, and obtaining the frequency chirp curve of the first terahertz quantum cascade laser. If a frequency point that meets the conditions cannot be found in the candidate frequency set of a certain short-time slice, it is supplemented by linear interpolation or extrapolation using the frequency values ​​of the adjacent short-time slices before and after the current short-time slice.

[0051] It is easy to see that this invention utilizes the nonlinear mixing characteristics of the second terahertz quantum cascade laser to achieve self-detecting heterodyne downconversion, converting the terahertz frequency chirped signal to the radio frequency band for acquisition. This eliminates the need for an external terahertz coherent detector, overcoming the technical bottleneck of the lack of efficient room-temperature detectors in the terahertz band. The system structure is compact and easy to integrate. At the same time, by performing short-time slicing, Fourier transform, and peak tracking on the time-domain waveform of the radio frequency beat signal, this invention can obtain the frequency chirped dynamic evolution curve with sub-microsecond time resolution, meeting the measurement requirements of rapid frequency changes during the start-up phase of pulsed lasers. This allows for the accurate extraction of chirped curves with continuously changing frequencies.

[0052] It is worth mentioning that the first terahertz quantum cascade laser in this embodiment can also operate in a multimode state. In this state, the obtained radio frequency beat frequency signal contains multiple frequency components. The computer can use bidirectional peak tracking to extract the chirp curve of the frequency changing with time for each frequency component, thereby obtaining multiple chirp curves. For any chirp curve, by comparing the frequency values ​​of adjacent short-time slices, if the difference between the two is greater than the frequency resolution, it is determined that a frequency jump has occurred at that position. This can provide a technical means for studying the nonlinear dynamic process inside the laser.

[0053] This embodiment also provides an on-chip heterodyne measurement method for laser frequency chirp, such as... Figure 2 As shown, it includes the following steps:

[0054] Step S1: Provide a monolithic integrated dual terahertz quantum cascade laser chip. Place the chip, which integrates the first and second terahertz quantum cascade lasers, in a cryogenic vacuum environment (a liquid helium-cooled vacuum chamber), and maintain the heat sink temperature stably by a temperature control system. Connect the first terahertz quantum cascade laser to a pulsed power supply, and connect the second terahertz quantum cascade laser to a continuous wave power supply via a T-type bias. Connect the amplifier, DC blocker, oscilloscope, spectrum analyzer, and computer as described above.

[0055] Step S2: Simultaneously drive two terahertz quantum cascade lasers: Turn on the continuous wave power supply and apply a stable DC current to the second terahertz quantum cascade laser through a T-type biaser, enabling it to operate in single-mode continuous wave mode and generate a frequency-stable local oscillator laser signal. Turn on the pulsed power supply and apply a pulsed current to the first terahertz quantum cascade laser, causing it to generate a laser signal with frequency chirp.

[0056] Step S3: Pre-observe the beat frequency signal and optimize parameters using a spectrum analyzer: Connect the amplifier output to the spectrum analyzer. The chirped light emitted by the first terahertz quantum cascade laser and the local oscillator light emitted by the second terahertz quantum cascade laser undergo nonlinear mixing within the second terahertz quantum cascade laser, outputting a radio frequency beat frequency signal. Observe the spectrum of the beat frequency signal using the spectrum analyzer, adjust the pulse current amplitude and / or the continuous wave current magnitude to ensure that the center frequency of the beat frequency signal falls within the range covered by the oscilloscope's sampling rate, and ensure sufficient signal power and a good signal-to-noise ratio. Record the optimal bias parameters.

[0057] Step S4, switch to time-domain waveform acquisition: Change the amplifier output from the spectrum analyzer to the DC blocker and high-speed oscilloscope. Set the oscilloscope's sampling rate (greater than twice the highest frequency of the RF beat frequency signal) and recording duration (covering the entire pulse width). Trigger the oscilloscope to acquire the complete time-domain waveform of the RF beat frequency signal during the pulse duration and transmit the data to the computer.

[0058] Step S5, Time-Domain Waveform Slicing and Fourier Transform: The computer divides the received time-domain waveform into several consecutive short-time slices in chronological order. The duration Δτ of each short-time slice is determined according to the required frequency resolution, and adjacent short-time slices are seamlessly connected. After applying a window function to each short-time slice, a Fast Fourier Transform (FFT) is performed to obtain the spectrum corresponding to each short-time slice.

[0059] Step S6, Peak Detection and Candidate Frequency Set Generation: Perform peak detection on the spectrum of each short-time slice, and extract frequency points whose amplitude exceeds a preset threshold as the candidate frequency set of the short-time slice.

[0060] Step S7, bidirectional traversal tracing to extract the chirp curve: Select a short-time slice with stable frequency change and a unique candidate frequency as a reference point. Starting from the reference point, traverse slice by slice in both the increasing and decreasing time directions. Search for the frequency point in the candidate frequency set of the adjacent short-time slices that is closest to the frequency value determined by the current short-time slice and changes continuously. This is used as the frequency value of the adjacent short-time slice, forming a frequency trajectory over time. If a frequency point that meets the conditions cannot be found in a certain short-time slice, it is supplemented by linear interpolation or extrapolation using the frequency values ​​of the adjacent short-time slices before and after it. The final trajectory is the frequency chirp curve of the first terahertz quantum cascade laser, as shown below. Figure 3 As shown.

[0061] In this embodiment, the first terahertz quantum cascade laser can operate in a multimode state. The spectrum of the radio frequency beat signal contains multiple significant peaks, each corresponding to a different lasing mode. Steps S6 to S7 are executed independently for each peak, resulting in multiple parallel chirped curves. For any given curve, the frequency values ​​of adjacent short-time slices are compared. If the difference exceeds the frequency resolution, a mode hopping is determined to have occurred at that location. Figure 4 The diagram shows the results of mode switching during the measured chirping process of the multimode signal.

[0062] Through the above steps, this embodiment achieves high time resolution measurement of the transient frequency chirp process during the pulse start-up phase of a terahertz quantum cascade laser. It can accurately extract the frequency chirp curves under different operating conditions and identify mode switching behavior in multimode states, providing a reliable technical means for evaluating the dynamic characteristics of lasers and for frequency domain applications.

Claims

1. A method for on-chip heterodyne measurement of laser frequency chirp, characterized in that, Includes the following steps: A pulsed current drives a first terahertz quantum cascade laser, causing the first terahertz quantum cascade laser to generate a laser signal with frequency chirp. A second terahertz quantum cascade laser is driven by a continuous wave current, so that the second terahertz quantum cascade laser generates a stable local oscillator signal; By utilizing the nonlinear mixing characteristics of the second terahertz quantum cascade laser, the laser signal emitted by the first terahertz quantum cascade laser and the local oscillator signal are heterodyne downconverted to output a radio frequency beat frequency signal. The time-domain waveform of the radio frequency beat frequency signal is acquired, and the time-domain waveform is subjected to seamless continuous time slicing and Fourier transform. The chirp curve of frequency change over time is extracted by bidirectional peak tracking. The first terahertz quantum cascade laser and the second terahertz quantum cascade laser are integrated on a single monolithic integrated chip.

2. The on-chip heterodyne measurement method for laser frequency chirp according to claim 1, characterized in that, Before acquiring the time-domain waveform of the radio frequency beat frequency signal, the method further includes: The radio frequency beat signal is amplified by an amplifier and then input to a spectrum analyzer. The spectrum analyzer is used to observe the spectrum of the radio frequency beat signal, and the drive current is adjusted so that the center frequency of the radio frequency beat signal falls within the range that the sampling rate of the time-domain waveform acquisition device can cover.

3. The on-chip heterodyne measurement method for laser frequency chirp according to claim 1, characterized in that, The process of performing seamless continuous time slicing and Fourier transform on the time-domain waveform, and extracting the chirp curve with frequency variation over time using bidirectional peak tracking, specifically includes: The time-domain waveform is divided into several consecutive short-time slices in chronological order. A window function is applied to each short-time slice, and a fast Fourier transform is performed to obtain the spectrum corresponding to each short-time slice. Peak detection is performed on the spectrum of each short-time slice, and frequency points whose peak values ​​exceed a preset threshold are extracted as the candidate frequency set of the short-time slice; A short-time slice with stable frequency change and a unique candidate frequency is selected as a reference point. Starting from the reference point, the slices are traversed one by one in both the time-increasing and time-decreasing directions. In the candidate frequency set of the short-time slices adjacent to the current short-time slice, the frequency point that is closest to the frequency value determined by the current short-time slice and changes continuously is searched as the frequency value of the adjacent short-time slice, forming the trajectory of frequency over time, and the frequency chirp curve of the first terahertz quantum cascade laser is obtained.

4. The on-chip heterodyne measurement method for laser frequency chirp according to claim 3, characterized in that, When searching for the frequency point in the candidate frequency set of the short-time slices adjacent to the current short-time slice that is closest to the frequency value determined by the current short-time slice and changes continuously, if a frequency point that meets the conditions cannot be found in the candidate frequency set of a certain short-time slice, then the frequency values ​​of the short-time slices before and after the current short-time slice are used to supplement it by linear interpolation or extrapolation.

5. The on-chip heterodyne measurement method for laser frequency chirp according to claim 3, characterized in that, When the first terahertz quantum cascade laser operates in multimode, the radio frequency beat signal contains multiple frequency components. For each frequency component, bidirectional peak tracking is used to extract the chirp curve of the frequency changing over time, resulting in multiple chirp curves. For any chirp curve, the frequency values ​​of adjacent short-time slices are compared. If the difference between the two is greater than the frequency resolution, it is determined that a frequency jump has occurred at that position. The frequency resolution is the reciprocal of the duration of the short-time slice.

6. An on-chip heterodyne measurement device for laser frequency chirp, characterized in that, include: The first terahertz quantum cascade laser is used to generate laser signals with frequency chirps by being driven by pulsed current. The second terahertz quantum cascade laser is used to drive the laser with a continuous wave current to generate a stable local oscillator signal, and to perform heterodyne downconversion with the laser signal with frequency chirp to generate a radio frequency beat frequency signal. A time-domain waveform acquisition device is used to acquire the time-domain waveform of the radio frequency beat frequency signal; A computer is used to perform seamless continuous time slicing and Fourier transform on the time-domain waveform, and to extract the chirp curve of frequency variation over time using bidirectional peak tracking. The first terahertz quantum cascade laser and the second terahertz quantum cascade laser are integrated on a single monolithic integrated chip.

7. The on-chip heterodyne measurement device for laser frequency chirp according to claim 6, characterized in that, Also includes: Amplifier, used to amplify the radio frequency beat signal; A spectrum analyzer is used to observe the spectrum of the radio frequency beat signal in order to adjust the drive current so that the center frequency of the radio frequency beat signal falls within the range that the sampling rate of the time-domain waveform acquisition device can cover.

8. The on-chip heterodyne measurement device for laser frequency chirp according to claim 6, characterized in that, The computer includes: The partitioning and transformation module is used to divide the time-domain waveform into several consecutive short-time slices in chronological order, apply a window function to each short-time slice and then perform a fast Fourier transform to obtain the spectrum corresponding to each short-time slice. The peak detection module is used to perform peak detection on the spectrum of each short-time slice and extract frequency points whose peak values ​​exceed a preset threshold as the candidate frequency set of the short-time slice. The traversal module is used to select a short-time slice with stable frequency change and unique candidate frequency as a reference point. Starting from the reference point, it traverses slice by slice in both the time-increasing and time-decreasing directions. In the candidate frequency set of the short-time slices adjacent to the current short-time slice, it searches for the frequency point that is closest to the frequency value determined by the current short-time slice and changes continuously as the frequency value of the adjacent short-time slice, forming a trajectory of frequency over time, and obtaining the frequency chirp curve of the first terahertz quantum cascade laser.

9. The on-chip heterodyne measurement device for laser frequency chirp according to claim 6, characterized in that, When the traversal module searches for the frequency point in the candidate frequency set of the short-time slices adjacent to the current short-time slice that is closest to the frequency value determined by the current short-time slice and changes continuously, if a frequency point that meets the conditions cannot be found in the candidate frequency set of a certain short-time slice, the frequency values ​​of the short-time slices before and after that short-time slice are used to supplement it by linear interpolation or extrapolation.

10. The on-chip heterodyne measurement device for laser frequency chirp according to claim 6, characterized in that, When the first terahertz quantum cascade laser operates in multimode, the radio frequency beat signal contains multiple frequency components. The computer is used to extract the chirp curve of the frequency change over time for each frequency component using bidirectional peak tracking, resulting in multiple chirp curves. For any chirp curve, the frequency values ​​of adjacent short-time slices are compared. If the difference between the two is greater than the frequency resolution, it is determined that a frequency jump has occurred at that position. The frequency resolution is the reciprocal of the duration of the short-time slice.

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