Generating and measuring device and generating and measuring method for narrow-linewidth intermediate infrared frequency modulation continuous waves

Abstract

The invention relates to the technical field of optics, in particular to a narrow-linewidth intermediate infrared frequency modulation continuous wave generating and measuring device and a generating and measuring method.The generating device comprises an arbitrary function generator, an ultra-low noise driving source and an inter-band cascade laser, the arbitrary function generator is connected with the ultra-low noise driving source, and the ultra-low noise driving source is connected with the inter-band cascade laser; the ultra-low noise driving source is connected with the inter-band cascade laser; two simple and economical intermediate infrared frequency modulation continuous wave signal generation schemes are provided, and comprise a current modulation scheme and a temperature modulation scheme; the invention provides a testing device capable of analyzing intermediate infrared frequency modulation continuous wave signals. The device can be used for analyzing time-frequency signals, linearity and spectrum information of intermediate infrared frequency modulation continuous waves. The defects in the prior art are overcome.

Description

Technical Field

[0001] The present invention relates to the field of optical technology, and in particular to a device and method for generating and measuring a narrow-linewidth mid-infrared frequency-modulated continuous wave. Background Art

[0002] The mid-infrared band (2.5μm-25μm) contains many absorption lines of gas molecules, and has low loss, weak scattering, and strong anti-interference ability in the atmosphere, so it has attracted the attention of many researchers.

[0003] The interband cascade laser is a high-quality mid-infrared laser source that produces light in the 3-6μm band. It has the advantages of narrow linewidth, high modulation efficiency, high quantum efficiency, low power consumption, and stable operation at room temperature. Currently, this laser has been widely used in military and civilian fields such as gas detection, free-space optical communications, and infrared laser weapons.

[0004] The mid-infrared frequency modulated continuous wave source combines the unique advantages of the mid-infrared band. It can ensure that the radar source can resist the interference of stray light while avoiding the interference of scattered coherent light, thereby accurately identifying the necessary targets. Secondly, due to the unique advantages of the inter-band cascade laser, namely narrow linewidth and high modulation efficiency, the frequency modulated continuous wave signal generated by this laser can also have a longer coherence distance and a larger sweep bandwidth compared to other laser sources. In addition, the current lack of optical fibers, measuring instruments, etc. in the mid-infrared band (3-6μm) makes it difficult to analyze and optimize the generated signals. Therefore, proposing or inventing a device that can measure mid-infrared frequency modulated continuous signals in real time is also the focus of current research.

[0005] Researchers have already conducted research on mid-infrared FM continuous wave (FMCW) signal generators. Patent CN214124313U proposes a mid- and far-infrared parallel FMCW signal generator. This device primarily uses a chalcogenide photonic chip to generate an optical comb, which modulates the photonic chip to produce multiple parallel FMCW signals.

[0006] However, this solution is not yet mature and is mainly affected by source power, external interference, environmental vibration, etc. Based on this, the present invention proposes a narrow-linewidth mid-infrared frequency-modulated continuous wave generation and measurement device. Summary of the Invention

[0007] The purpose of the present invention is to provide a narrow-linewidth mid-infrared frequency-modulated continuous wave generation and measurement device and method, which solves the problem that the existing mid- and far-infrared parallel frequency-modulated continuous wave signal generation device is affected by source power, external interference, environmental vibration, etc.

[0008] To achieve the above objectives, the present invention provides a narrow-linewidth mid-infrared frequency-modulated continuous wave generator, comprising an arbitrary function generator, an ultra-low noise driving source, and an inter-band cascade laser, wherein the arbitrary function generator is connected to the ultra-low noise driving source, and the ultra-low noise driving source is connected to the inter-band cascade laser;

[0009] The arbitrary function generator is used to generate a triangular wave or sawtooth wave voltage signal with adjustable frequency and amplitude, and the signal is used to modulate the current input or temperature input of the ultra-low noise driving source;

[0010] The ultra-low noise driving source is used to control the current and temperature parameters of the inter-band cascade laser, and also modulate the current and temperature parameters in conjunction with an external arbitrary function signal;

[0011] The inter-band cascade laser is used to output a narrow-linewidth, mid-infrared frequency-modulated continuous-wave optical signal.

[0012] Wherein, the ultra-low noise driving source includes a current module, and the current module is connected to the arbitrary function generator and the inter-band cascade laser respectively.

[0013] Wherein, the ultra-low noise driving source includes a temperature module, and the temperature module is connected to the arbitrary function generator and the inter-band cascade laser respectively.

[0014] A narrow-linewidth mid-infrared frequency-modulated continuous wave measurement device comprises an optical isolator, a Michelson interferometer, a beam splitter, a Fourier transform infrared spectrometer, and a photodetector, wherein the optical isolator is connected to the interband cascade laser and is connected in series with the Michelson interferometer; the Michelson interferometer is directly connected to the beam splitter; the Fourier transform infrared spectrometer and the photodetector are respectively connected to both ends of the beam splitter;

[0015] The optical isolator is used to isolate the feedback light from the testing device to the generating device;

[0016] The Michelson interferometer is used to convert the frequency modulated continuous wave optical signal into a time optical beat signal;

[0017] The optical beam splitter is used to split the frequency modulated continuous wave optical signal input by the interband cascade laser and combine the optical signals reflected from the two reflectors. The optical signal is further split by the optical beam splitter, and a part of the optical signal is transmitted to the Fourier infrared spectrometer, and the other part of the optical signal is transmitted to the photodetector;

[0018] The Fourier infrared spectrometer is used to analyze the scanning spectrum of the frequency modulated continuous wave optical signal and read the sweep width;

[0019] The photoelectric detector is used to receive the temporal optical beat signal from the Michelson interferometer and generate a beat frequency electrical signal, which is subsequently transmitted to a digital storage oscilloscope and a computer terminal.

[0020] The narrow-linewidth mid-infrared frequency-modulated continuous wave measuring device further comprises a digital storage oscilloscope and a computer terminal, wherein the digital storage oscilloscope is connected to the photodetector, and the computer terminal is connected to the digital storage oscilloscope;

[0021] The digital storage oscilloscope is used to record and observe real-time beat frequency electrical signals;

[0022] The computer terminal is used for off-line processing of beat frequency electrical signals.

[0023] A method for generating a narrow-linewidth mid-infrared frequency-modulated continuous wave comprises the following steps:

[0024] Use an arbitrary function generator to generate a triangle wave or sawtooth wave voltage signal with adjustable frequency and amplitude;

[0025] The generated signal is used to modulate the current input or temperature input of an ultra-low noise driving source to control the current and temperature parameters of the interband cascade laser;

[0026] Through modulation, the inter-band cascade laser outputs a narrow-linewidth, mid-infrared frequency-modulated continuous-wave optical signal.

[0027] A method for measuring narrow-linewidth mid-infrared frequency-modulated continuous wave (FMCW) comprises the following steps:

[0028] The frequency modulated continuous wave optical signal is input into the Michelson interferometer through an optical isolator to generate an optical beat signal;

[0029] Part of the optical signal is transmitted to the Fourier infrared spectrometer to analyze the scanning spectrum of the frequency modulated continuous wave optical signal and read the sweep width. The other part of the optical signal is transmitted to the photodetector to receive the time optical beat signal and generate a beat frequency electrical signal, which is then transmitted to the digital storage oscilloscope and the computer.

[0030] The collected beat frequency signal is processed on the computer side, including beat frequency signal de-skewing, cutting, Hilbert transform, instantaneous frequency determination and output.

[0031] The present invention provides a narrow-linewidth mid-infrared frequency-modulated continuous wave (FMCW) generation and measurement device and method, comprising a generation device and a measurement device. The generation device includes the arbitrary function generator, the ultra-low-noise current drive source, and the mid-infrared interband cascade laser. The measurement device includes the optical isolator, the Michelson interferometer, the photodetector, the oscilloscope, the Fourier transform infrared spectrometer, the computer, and other components. In the generation scheme, the arbitrary function generator generates a triangular wave signal to modulate the output of the drive source (including current modulation and temperature modulation). This signal acts on the interband cascade laser to generate a FMCW signal with a power that varies little over time and a frequency that varies in a triangular wave pattern. In the measurement scheme, the FMCW signal passes through the optical isolator and generates an optical beat signal in the Michelson interferometer. This signal is observed by the Fourier transform infrared spectrometer or converted by the photodetector into a beat frequency signal and then sent to the computer for processing to obtain a time-frequency analysis result. Two simple and economical mid-infrared frequency-modulated continuous wave signal generation schemes are provided, including current modulation and temperature modulation schemes; a test device for analyzing mid-infrared frequency-modulated continuous wave signals is provided, which can analyze the time-frequency signal, linearity, and spectral information of the mid-infrared frequency-modulated continuous wave, thereby solving the shortcomings of the existing technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art.

[0033] Figure 1 It is a structural schematic diagram of a narrow-linewidth mid-infrared frequency-modulated continuous wave generating device and a narrow-linewidth mid-infrared frequency-modulated continuous wave measuring device of the present invention.

[0034] Figure 2 This is a theoretical verification diagram of the generating device of the implementation case of the present invention. When there is no modulation, (a1) the change of the number of photons over time, (a2) the change of the number of carriers over time, and (a3) ​​the change of the instantaneous frequency over time; when modulation is applied, (b1) the change of the number of photons over time, (b2) the change of the number of carriers over time, and (b3) the change of the instantaneous frequency over time.

[0035] Figure 3 It is a theoretical verification diagram of the test device of the implementation case of the present invention, (a1) the change of the instantaneous frequency of the short arm output of the Michelson interferometer over time, (a2) the change of the instantaneous frequency of the long arm output of the Michelson interferometer over time, and (a3) ​​the change of the beat frequency over time.

[0036] Figure 4 : is a spectrum diagram of the interband cascade laser of the embodiment of the present invention when it is not modulated, and a broken line statistics of the lasing frequency changing with time, wherein, Figure 4 (a) shows the change of light intensity at the same wavelength and different currents; Figure 4 (b) Shows the change of current with frequency.

[0037] Figure 5 is a spectrum diagram of the interband cascade laser at different modulation frequencies according to an embodiment of the present invention, wherein: Figure 5 (a) shows the change of light intensity at different temperatures with the same wavelength; Figure 5 (b) Shows the change of temperature with frequency.

[0038] Figure 6 This is a comparison diagram of the frequency modulated continuous wave spectrum output under different modulation amplitudes of the present invention and the spectrum without modulation amplitude, wherein: Figure 6 (a) shows the changes of different wavelengths with different adjustment amplitudes; Figure 6 (b) Different adjustment amplitudes correspond to different bandwidths.

[0039] Figure 7 This is a comparison diagram of the frequency modulated continuous wave spectrum output at different modulation frequencies of the present invention and the spectrum without modulation.

[0040] Figure 8 The present invention is a flowchart of the steps of the method for generating narrow-linewidth mid-infrared frequency-modulated continuous waves.

[0041] Figure 9 The present invention is a flowchart of the steps of the method for measuring narrow-linewidth mid-infrared frequency-modulated continuous wave.

[0042] In the figure: 1-arbitrary function generator, 2-ultra-low noise driving source, 3-interband cascade laser, 4-optical isolator, 5-Michelson interferometer, 6-beam splitter, 7-photodetector, 8-Fourier infrared spectrometer, 9-digital storage oscilloscope, 10-computer terminal. DETAILED DESCRIPTION

[0043] The embodiments of the present invention are described in detail below. Examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, but should not be understood as limiting the present invention.

[0044] The first embodiment of this application is:

[0045] See also Figure 1 ,in, Figure 1 It is a structural schematic diagram of a narrow-linewidth mid-infrared frequency-modulated continuous wave generating device and a narrow-linewidth mid-infrared frequency-modulated continuous wave measuring device of the present invention.

[0046] The present invention provides a narrow-linewidth mid-infrared frequency-modulated continuous wave generating device, comprising an arbitrary function generator 1, an ultra-low noise driving source 2 and an inter-band cascade laser 3, wherein the ultra-low noise driving source 2 comprises a current module or a temperature module.

[0047] For this specific embodiment, the arbitrary function generator 1 is connected to the ultra-low noise driving source 2, and the ultra-low noise driving source 2 is connected to the interband cascade laser 3;

[0048] The arbitrary function generator 1 is used to generate a triangular wave or sawtooth wave voltage signal with adjustable frequency and amplitude. The signal is loaded into the AC modulation port of the current source to modulate the current output or temperature output of the ultra-low noise driving source 2. At this time, the current controlling the laser can be expressed as: ; The temperature of the laser can be expressed as: Where Ib and T0 represent the initial control current and initial control temperature, A represents the modulation amplitude, and F is the triangle wave modulation function.

[0049] The ultra-low noise driving source 2 controls the current and temperature parameters of the inter-band cascade laser 3 through the electric control line and the temperature control line, and also modulates the current and temperature parameters in conjunction with an external arbitrary function signal, and the modulation method is consistent with the above;

[0050] The inter-band cascade laser 3 is used to output a narrow-linewidth, mid-infrared frequency-modulated continuous wave optical signal.

[0051] The ultra-low noise driving source 2 includes a current module, and the current module is connected to the arbitrary function generator 1 and the interband cascade laser 3 respectively.

[0052] Secondly, the ultra-low noise driving source 2 includes a temperature module, which is connected to the arbitrary function generator 1 and the interband cascade laser 3 respectively.

[0053] Using a narrow-linewidth mid-infrared frequency-modulated continuous wave generator of this embodiment,

[0054] The invention comprises two devices for generating a mid-infrared frequency-modulated continuous wave signal. The first and second devices for generating a mid-infrared frequency-modulated continuous wave signal both comprise the arbitrary function generator 1 (AFG), the ultra-low noise driving source 2 (ULN-LDC, Ultra-Low Nosie Laser Diode Controller) with a current and temperature module, and the interband cascade laser 3 (ICL, Interband Cascade Laser). Among them, the arbitrary function generator 1 is used to generate a frequency-adjustable and amplitude-adjustable triangular wave or sawtooth wave voltage signal, which is subsequently used to modulate the current input or temperature input of the ultra-low noise driving source 2; the ultra-low noise driving source 2 with current and temperature modules is mainly used to control the current and temperature parameters of the inter-band cascade laser 3 (the current control range of the inter-band cascade laser is 0-200mA; the temperature control range is 18-25°C). At the same time, the ultra-low noise driving source 2 can also modulate the current and temperature parameters in conjunction with an external arbitrary function signal; the inter-band cascade laser 3 is used to output a narrow-linewidth, mid-infrared frequency-modulated continuous wave optical signal.

[0055] In the first generating device, the arbitrary function generator 1 is directly connected to the current module of the ultra-low noise driving source 2, and the output of the driving source is directly connected to the interband cascade laser 3;

[0056] In the second generating device, the arbitrary function generator 1 is directly connected to the temperature module of the ultra-low noise driving source 2 , and the output of the driving source is directly connected to the interband cascade laser 3 .

[0057] The first generating device is a current-modulated interband cascade laser 3, which mainly uses carrier effect and thermal effect to change the laser emission frequency; the second is a temperature-modulated interband cascade laser 3, which mainly uses thermal effect to change the laser emission frequency.

[0058] In some implementation examples, the arbitrary function generator 1 generates a triangular waveform with a modulation period of 1-100 kHz and a modulation amplitude of 4 Vpp. This provides two simple and economical mid-infrared frequency-modulated continuous wave signal generation solutions: current modulation and temperature modulation. These solutions address shortcomings in existing technologies.

[0059] The second embodiment of this application is:

[0060] Based on the first embodiment, please refer to Figure 1 ,in, Figure 1 It is a structural schematic diagram of a narrow-linewidth mid-infrared frequency-modulated continuous wave generating device and a narrow-linewidth mid-infrared frequency-modulated continuous wave measuring device of the present invention.

[0061] The narrow-linewidth mid-infrared frequency-modulated continuous wave measuring device of this embodiment includes an optical isolator 4, a Michelson interferometer 5, an optical beam splitter 6, a Fourier transform infrared spectrometer 8, a photodetector 7, a digital storage oscilloscope 9 and a computer terminal 10.

[0062] In this specific embodiment, the optical isolator 4 is connected to the interband cascade laser 3 and is connected in series with the Michelson interferometer; the Michelson interferometer is directly connected to the optical beam splitter 6; the Fourier transform infrared spectrometer 8 and the photodetector 7 are respectively connected to the two ends of the optical beam splitter 6;

[0063] The optical isolator 4 is used to isolate the feedback light from the test device to the generating device;

[0064] The Michelson interferometer 5 is used to obtain a self-heterodyne differential signal. This signal is a low-frequency interference signal generated by the different lengths of the two arms of the Michelson interferometer;

[0065] The optical beam splitter 6 is used to split the frequency modulated continuous wave optical signal input by the interband cascade laser 3 and combine the optical signals reflected from the two reflectors. The optical signal is further split by the optical beam splitter 6. Part of the optical signal is transmitted to the Fourier transform infrared spectrometer 8, and the other part of the optical signal is transmitted to the photodetector 7.

[0066] The Fourier infrared spectrometer 8 is used to collect the scanning spectrum of the frequency modulated continuous wave optical signal and read the sweep width;

[0067] The photodetector 7 is used to receive the temporal optical beat signal from the Michelson interferometer and generate a beat frequency electrical signal which is subsequently transmitted to the digital storage oscilloscope 9 and the computer terminal 10 .

[0068] Wherein, the digital storage oscilloscope 9 is connected to the photoelectric detector 7, and the computer terminal 10 is connected to the digital storage oscilloscope 9;

[0069] The digital storage oscilloscope 9 is used to record and observe real-time beat frequency electrical signals;

[0070] The computer terminal 10 is used for off-line processing of the beat frequency electrical signal.

[0071] A narrow-linewidth mid-infrared frequency-modulated continuous wave (FMCW) measurement device using this embodiment includes an optical isolator 4 (ISO), a Michelson interferometer 5 (MCI), a beam splitter 6 (BS), a gold mirror (GM), a Fourier transform infrared spectrometer 8 (Fourier transform infrared spectroscopy), a photodetector 7 (PD), a digital storage oscilloscope 9 (DSO), and a personal computer 10 (PC). The optical isolator 4 is used to isolate feedback light from the test module to the generation module; the Michelson interferometer 5 is used to down-convert the FMCW optical signal (high-frequency signal) into a temporal optical beat signal (low-frequency signal); and the beam splitter 6 is a key component of the Michelson interferometer, which is used to split the FMCW optical signal input from the interband cascade laser 3 and combine the optical signals reflected from the two mirrors. The optical signal is then further split by the beam splitter, with one portion transmitted to the Fourier transform infrared spectrometer 8 and the other portion to the photodetector 7. The Fourier transform infrared spectrometer 8 is used to analyze the scanned spectrum of the frequency-modulated continuous wave optical signal and read the sweep width. The photodetector 7 is used to receive the temporal optical beat signal (low-frequency signal) from the Michelson interferometer and generate a beat frequency electrical signal, which is subsequently transmitted to the digital storage oscilloscope 9 and the computer 10. The digital storage oscilloscope 9 is used to record and observe the real-time beat frequency electrical signal, while the computer 10 is used to process the beat frequency electrical signal offline. The optical isolator 4 is connected in series with the Michelson interferometer, which is directly connected to the optical beam splitter 6. The Fourier transform infrared spectrometer 8 and the photodetector 7 are respectively connected to the ends of the optical beam splitter 6. In the measurement device of some embodiments, a Michelson interferometer 5 is used to obtain the beat frequency signal. Its structure includes a beam splitter and two gold total reflection mirrors. The beam splitter has a splitting ratio of 5:5 or 3:7; the reflectivity of the gold total reflection mirror is greater than 99%. In the measurement device of some implementation cases, the arm length difference of the Michelson interferometer 5 is 60 cm (2L1-2L2). L1 is the long arm, and L2 is the short arm. In the measurement device of some implementation cases, the frequency modulated signal is measured by a Fourier transform infrared spectrometer 8; the beat frequency signal is measured and recorded by a photodetector 7 and a digital storage oscilloscope 9. When measuring the frequency modulated signal, one arm of the Michelson interferometer 5 needs to be removed.The resolution of the Fourier infrared spectrometer 8 is 0.5 cm-1; the bandwidth of the digital oscilloscope is 2.5 GHz, and the sampling rate is 20 Gsa / s; the bandwidth of the photodetector 7 is 500 MHz. In the measurement device of some implementation cases, the two optical signals for measuring the spectrum and time series are shared through a beam splitter, and the beam splitter has a splitting ratio of 5:5. In the measurement device of some implementation cases, the optical isolator 4 is used to isolate the light returned to the laser by the Michelson interferometer 5 to prevent the laser from mode hopping during the frequency modulation process. In the measurement device of some implementation cases, the computer terminal 10 is used to process the collected beat frequency signal, and the processing content includes beat frequency signal de-skewing, cutting, Hilbert transform, instantaneous frequency determination, and output. Deskewing refers to removing the slowly varying envelope signal from the beat signal due to modulation, primarily using polynomial fitting. Slicing involves slicing the upper and lower chirps of the beat signal to extract the beat frequency corresponding to each chirp segment. The Hilbert transform uses this algorithm to extract the orthogonal signal from the beat signal, using it to determine the laser phase change over time. Determining the instantaneous frequency uses the differential relationship between phase and frequency, and differentiating the instantaneous phase to obtain the instantaneous frequency signal. A test device for analyzing mid-infrared frequency-modulated continuous wave (FMCW) signals is provided. This device can analyze the time-frequency signal, linearity, and spectrum of the FMCW signal, addressing shortcomings in existing technologies.

[0072] The third embodiment of this application is:

[0073] Based on the first embodiment, please refer to Figure 8 ,in, Figure 8 The flowchart of the method for generating narrow-linewidth mid-infrared frequency-modulated continuous wave of the present invention is as follows. The method for generating narrow-linewidth mid-infrared frequency-modulated continuous wave of this embodiment comprises the following steps:

[0074] S101: using an arbitrary function generator 1 to generate a triangular wave or sawtooth wave voltage signal with adjustable frequency and amplitude;

[0075] S102: Using the generated signal to modulate the current input or temperature input of the ultra-low noise driving source 2 to control the current and temperature parameters of the interband cascade laser 3;

[0076] S103: Through modulation, the inter-band cascade laser 3 outputs a narrow-linewidth, mid-infrared frequency-modulated continuous-wave optical signal.

[0077] Specifically, an arbitrary function generator 1 is used to generate a frequency- and amplitude-adjustable triangular or sawtooth voltage signal. This signal is used to modulate the current or temperature input of an ultra-low-noise driver 2 to control the current and temperature parameters of an interband cascade laser 3. Through modulation, the interband cascade laser 3 outputs a narrow-linewidth, mid-infrared frequency-modulated continuous-wave optical signal. In the first generation device, the arbitrary function generator 1 is directly connected to the current module of the ultra-low-noise driver 2, and the driver output is directly connected to the interband cascade laser 3 to achieve current modulation. In the second generation device, the arbitrary function generator 1 is directly connected to the temperature module of the ultra-low-noise driver 2, and the driver output is directly connected to the interband cascade laser 3 to achieve laser temperature control. The modulation waveform is set to a triangular wave, the modulation period is 1-100kHz, and the modulation amplitude is 4Vpp. Two simple and economical mid-infrared frequency-modulated continuous-wave signal generation schemes are provided: current modulation and temperature modulation. These schemes address the shortcomings of the existing technology.

[0078] The fourth embodiment of the present application is:

[0079] Based on the second embodiment, please refer to Figure 9 ,in, Figure 9 The method for measuring narrow-linewidth mid-infrared frequency-modulated continuous wave of the present invention is a flow chart of the steps. The method for measuring narrow-linewidth mid-infrared frequency-modulated continuous wave of this embodiment comprises the following steps:

[0080] S201: Input the frequency modulated continuous wave optical signal into the Michelson interferometer 5 through the optical isolator 4 to generate an optical beat signal;

[0081] S202: A portion of the optical signal is transmitted to the Fourier infrared spectrometer 8 for analyzing the scan spectrum of the frequency modulated continuous wave optical signal and reading the sweep width. Another portion of the optical signal is transmitted to the photodetector 7 for receiving the temporal optical beat signal and generating a beat frequency electrical signal, which is then transmitted to the digital storage oscilloscope 9 and the computer terminal 10.

[0082] S203: Processing the collected beat frequency signal on the computer terminal 10, including de-skewing, cutting, Hilbert transform, instantaneous frequency determination, and output.

[0083] Specifically, the frequency modulated continuous wave optical signal is input into the Michelson interferometer 5 through the optical isolator 4 to generate an optical beat signal. A portion of the optical signal is transmitted to the Fourier infrared spectrometer 8 for analyzing the scanning spectrum of the frequency modulated continuous wave optical signal and reading the sweep width. The other portion of the optical signal is transmitted to the photodetector 7 for receiving the time optical beat signal and generating a beat frequency electrical signal, which is then transmitted to the digital storage oscilloscope 9 and the computer terminal 10. The collected beat frequency signal is processed on the computer terminal 10, including de-skewing, cutting, Hilbert transform, instantaneous frequency determination, and output of the beat frequency signal. De-skewing refers to removing the slowly changing envelope signal generated by modulation in the beat frequency signal, and the main method used is polynomial fitting. Cutting refers to cutting the upper and lower chirps of the beat frequency signal to extract the beat frequency corresponding to each chirp segment. Hilbert transform refers to using the algorithm to extract the orthogonal signal of the beat frequency signal, and using the orthogonal signal to obtain the change of the laser phase over time. Determining the instantaneous frequency involves utilizing the differential relationship between phase and frequency and differentiating the instantaneous phase to obtain the instantaneous frequency signal. A test device for analyzing mid-infrared frequency-modulated continuous wave (FMCW) signals is provided. This device can analyze the time-frequency signal, linearity, and spectrum of FMCW signals, addressing shortcomings in existing technologies.

[0084] Implementation Case 1: Theoretical Simulation of Infrared Frequency Modulated Continuous Wave Generation:

[0085] The theoretical generation scheme of mid-infrared frequency-modulated continuous wave signals relies on the LK mean field model, which includes the number of photons, the number of carriers, and the instantaneous phase rate equations. It can describe the complex dynamic behavior of the laser, including relaxation oscillations, carrier clamping, phase fluctuations, photon oscillations, etc. The differential equations are as follows:

[0086]

[0087] Where N is the number of carriers, η is the injection current efficiency, I is the bias current, q is the electron charge constant, and Γ p is the mode field confinement factor, v g is the light field group velocity, g is the single-stage differential gain coefficient, S is the number of photons, τ sp is the carrier lifetime, m is the number of cascades, τ p is the photon lifetime, β is the spontaneous emission coefficient, α h Is the linewidth enhancement factor. The analysis of the instantaneous frequency of the FMCW can observe the generated FMCW sweep width and sweep rate. The instantaneous frequency analysis is based on the instantaneous phase of formula (3), which can be expressed as:

[0088]

[0089] Figure 2This is a theoretical verification diagram of the generating device of the present invention, including the change of the number of photons over time (a1), the change of the number of carriers over time (a2), and the change of the instantaneous frequency over time (a3) ​​when no current modulation is applied; and the change of the number of photons over time (b1), the change of the number of carriers over time (b2), and the change of the instantaneous frequency over time (b3) when current modulation is applied. It can be seen from the figure that when there is no modulation, the photons and carriers remain stable after a short relaxation oscillation, and the instantaneous frequency does not change with time. At this time, the laser outputs continuous light and the center frequency does not change with time. When a certain amount of modulation is applied, the carriers and photons will show the same fluctuations as the modulation signal after the relaxation oscillation. At the same time, the instantaneous frequency also changes with time, proving that a frequency-modulated continuous wave signal is generated at this time. In addition, it can be seen that the fluctuation of the number of photons over time is large, that is, the output power fluctuation is large, and due to the carrier clamping effect, the fluctuation of the number of carriers is very small.

[0090] After the above generation process, the output light field of the mid-infrared FMCW signal source can be expressed as:

[0091]

[0092] Where E is the output light field of the FMCW source, is the amplitude of the output light field of the frequency modulated continuous wave source, f is the center frequency of the laser at time t=0, μ is the sweep speed, is the frequency-sweep nonlinear term, and φ is the frequency-modulated continuous wave source phase noise term. In theoretical modeling, the frequency-sweep nonlinearity is set to 0, that is, The phase noise is set to a Gaussian white noise with a very small amplitude. The output light field of the two arms of the Michelson interferometer and the beat signal photocurrent passing through the photodetector 7 can be expressed as (6) and (7):

[0093]

[0094] Where τn represents the length of the Michelson interferometer arm for different delay line lengths (n=1, 2), I represents the beat signal photocurrent, η is the imbalance between the two arms of the Michelson interferometer 5, R is the sensitivity of the photodetector 7, and μ(τ2-τ1) is the beat frequency, which can be adjusted by adjusting the delay length of the two arms. After measuring the beat signal, the instantaneous frequency can be obtained from this signal:

[0095]

[0096] Where arg represents the complex signal phase extraction operation, and H represents the Hilbert transform.

[0097] Figure 3This is a theoretical verification diagram of the test device of the present invention. It includes the temporal variation of the instantaneous frequency of the first arm of Michelson interferometer 5 (a1); the temporal variation of the instantaneous frequency of the second arm of Michelson interferometer 5 (a2); and the temporal variation of the beat frequency (a3). As can be seen from the figure, the different arm lengths of the two Michelson arms in the measurement device result in different delay times in the output frequency-modulated continuous-wave light field, and this delay time is positively correlated with the difference in arm lengths.

[0098] Implementation Case 2: Experimental verification of mid-infrared FM continuous wave generation:

[0099] We built Figure 1 In the experimental setup shown, a narrow-linewidth mid-infrared frequency-modulated continuous wave signal is generated by current-modulating an interband cascade laser 3.

[0100] Figure 4 The following are the output spectra of the interband cascade laser 3 under different current biases and the line graph of the frequency versus current when unmodulated. It can be seen from the figure that the laser output has a high suppression ratio and a narrow spectral linewidth.

[0101] Figure 5 1 is an output spectrum diagram of the interband cascade laser 3 at different temperatures and a line graph showing the change of frequency with current when no modulation is applied.

[0102] Figure 6 The following figure compares the spectrum of the FM continuous wave (FMCW) output at different modulation amplitudes with the spectrum without modulation. The figure shows that varying modulation amplitudes broadens the spectrum, producing an FMCW signal with a continuously sweeping frequency. The sweep half-width increases with increasing modulation amplitude. At a modulation amplitude of 4.5V, the sweep half-width is approximately 49GHz.

[0103] Figure 7 The following figure compares the spectrum of the FMCW output at different modulation frequencies with the spectrum without modulation. The figure shows that varying the modulation frequency broadens the spectrum, creating a frequency-sweeping FMCW signal. The full width at half maximum (FWHM) of the sweep is approximately 57 GHz at a modulation frequency range of 1-200 kHz.

[0104] The above disclosure is merely one or more preferred embodiments of the present application and is not intended to limit the scope of the present application. A person skilled in the art will understand that all or part of the processes of the above embodiments and equivalent changes made in accordance with the claims of the present application are still within the scope of the present application.

Claims

1. A device for generating and measuring narrow-linewidth mid-infrared frequency-modulated continuous waves, characterized in that: including a generating device and a measuring device; The generating device includes an arbitrary function generator, an ultra-low noise driving source and an inter-band cascade laser, wherein the arbitrary function generator is connected to the ultra-low noise driving source, and the ultra-low noise driving source is connected to the inter-band cascade laser; The arbitrary function generator is used to generate a triangular wave or sawtooth wave voltage signal with adjustable frequency and amplitude, and the signal is loaded into the AC modulation port of the current source to modulate the current output or temperature output of the ultra-low noise driving source; The ultra-low noise driving source is used to control the current and temperature parameters of the inter-band cascade laser, and also modulate the current and temperature parameters in conjunction with an external arbitrary function signal; The inter-band cascade laser is used to output a narrow-linewidth, mid-infrared frequency-modulated continuous-wave optical signal; The measuring device includes an optical isolator, a Michelson interferometer, a beam splitter, a Fourier infrared spectrometer and a photodetector, wherein the optical isolator is connected to the interband cascade laser and is connected in series with the optical path of the Michelson interferometer; the Michelson interferometer is connected to the optical path of the beam splitter; the Fourier infrared spectrometer and the photodetector are respectively connected to the two ends of the beam splitter; The optical isolator is used to isolate the feedback light from the testing device to the generating device; The Michelson interferometer is used to obtain a self-heterodyne differential signal, which is a low-frequency interference signal generated by the different lengths of the two arms of the Michelson interferometer; The optical beam splitter is used to split the frequency modulated continuous wave optical signal input by the interband cascade laser, and the optical signal is further split by the optical beam splitter, and a part of the optical signal is transmitted to the Fourier infrared spectrometer, and the other part of the optical signal is transmitted to the photodetector; The Fourier infrared spectrometer is used to collect the scanning spectrum of the continuous wave optical signal and read the frequency sweep width; The photoelectric detector is used to receive the temporal optical beat signal from the Michelson interferometer and generate a beat frequency electrical signal, which is subsequently transmitted to a digital storage oscilloscope and a computer terminal.

2. The narrow-linewidth mid-infrared frequency-modulated continuous wave generation and measurement device according to claim 1, characterized in that: The ultra-low noise driving source includes a current module, and the current module is connected to the arbitrary function generator and the inter-band cascade laser respectively.

3. The narrow-linewidth mid-infrared frequency-modulated continuous wave generation and measurement device according to claim 1, characterized in that: The ultra-low noise driving source includes a temperature module, and the temperature module is connected to the arbitrary function generator and the inter-band cascade laser respectively.

4. The narrow-linewidth mid-infrared frequency-modulated continuous wave generation and measurement device according to claim 1, characterized in that: The narrow-linewidth mid-infrared frequency-modulated continuous wave measuring device further includes a digital storage oscilloscope and a computer terminal, wherein the digital storage oscilloscope is connected to the photodetector, and the computer terminal is connected to the digital storage oscilloscope; The digital storage oscilloscope is used to record and observe real-time beat frequency electrical signals; The computer terminal is used for off-line processing of beat frequency electrical signals.

5. The narrow-linewidth mid-infrared frequency-modulated continuous wave generation and measurement device according to claim 4, characterized in that: The beat frequency electrical signal includes: beat frequency signal de-skewing, cutting, Hilbert transformation, instantaneous frequency acquisition, and output.

6. The narrow-linewidth mid-infrared frequency-modulated continuous wave generation and measurement device according to claim 1, characterized in that: Set the modulation waveform to triangle wave, the modulation period to 1-100kHz, and the modulation amplitude to 4Vpp.

7. A method for generating narrow-linewidth mid-infrared frequency-modulated continuous waves, applicable to the generating device according to claim 1, characterized in that: The following steps are involved: Use an arbitrary function generator to generate a triangle wave or sawtooth wave voltage signal with adjustable frequency and amplitude; The generated signal is used to modulate the current input or temperature input of an ultra-low noise driving source to control the current and temperature parameters of the interband cascade laser; Through modulation, the inter-band cascade laser outputs a narrow-linewidth, mid-infrared frequency-modulated continuous-wave optical signal.

8. A method for measuring narrow-linewidth mid-infrared frequency-modulated continuous wave, applicable to the measuring device according to claim 1, characterized in that: The following steps are involved: The frequency modulated continuous wave optical signal is input into the Michelson interferometer through an optical isolator to generate an optical beat signal; Part of the optical signal is transmitted to the Fourier infrared spectrometer to analyze the scanning spectrum of the frequency modulated continuous wave optical signal and read the sweep width. The other part of the optical signal is transmitted to the photodetector to receive the time optical beat signal and generate a beat frequency electrical signal, which is then transmitted to the digital storage oscilloscope and the computer. The collected beat frequency electrical signal is processed on the computer side, including beat frequency signal de-skewing, cutting, Hilbert transform, instantaneous frequency determination and output.

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