A terahertz spectrum measurement system and method based on incoherent light mixing

Abstract

The application provides a terahertz spectrum measurement system and method based on incoherent light mixing, and belongs to the technical field of spectrum measurement. The system comprises an incoherent light source, first and second laser sources, first, second and third optical beam splitters, a balanced photodetector, an optical mixer, an amplifier, a first band-pass filter, a second band-pass filter, an acquisition unit and a cross-correlation processing module. The broadband incoherent light is split into two paths, one path is combined with a single-frequency continuous wave laser, passes through the third optical beam splitter, the balanced photodetector and the first band-pass filter to generate a reference signal. The other path is combined with an adjustable continuous wave laser, passes through the second optical beam splitter to serve as a local oscillator signal of a to-be-measured light path, is input into the optical mixer to be mixed with a to-be-measured terahertz signal, passes through the second band-pass filter and the amplifier to obtain an intermediate frequency signal. The cross-correlation processing module calculates the absolute frequency and power of the terahertz signal to obtain a broadband terahertz spectrum. The application improves the measurement speed of the measurement system and the accuracy of the measurement result.

Description

Technical Field

[0001] This invention belongs to the technical field of spectrum measurement, and more specifically relates to a terahertz spectrum measurement system and method based on incoherent optical mixing. Background Technology

[0002] Most existing terahertz spectrum measurement systems are based on electric mixers and employ subharmonic mixing technology. Their local oscillator drive links consist of frequency multipliers, amplifiers, and filters. Each electric mixer corresponds to a specific frequency band, and multiple electric mixers are required to cover the entire terahertz band (0.1–10 THz). For example, in the range of 100 GHz to 1.5 THz (the highest commercially available frequency range), at least seven expansion modules are needed, resulting in high investment costs across the entire frequency band. Furthermore, the complexity of the local oscillator drive link increases significantly with increasing frequency.

[0003] Besides electronic spectrum analysis techniques, current research on terahertz spectrum measurement techniques both domestically and internationally includes interferometry and photonics techniques. Interferometry primarily splits the input terahertz signal into two beams of equal amplitude using a beam splitter: one beam passes through a fixed mirror, and the other through a movable mechanical delay line. The two beams then rejoin at the beam splitter and are input to a power detector. When the optical path difference between the two terahertz waves is an integer multiple of the wavelength (i.e., an even multiple of half the wavelength), constructive interference occurs, resulting in the strongest output signal power. When the optical path difference is an odd multiple of half the wavelength, destructive interference occurs, resulting in the weakest output power. By continuously moving the delay line to change the optical path difference, the intensity fluctuation curve of the detector's output power as a function of the delay time (or optical path difference) can be recorded, i.e., an interferogram. After removing the DC background, this interferogram mathematically corresponds precisely to the time autocorrelation function of the terahertz electric field signal. According to the Wiener-Khinchin Theorem, the power spectrum of the terahertz signal under test can be directly obtained by performing a Fourier transform on the autocorrelation function. While interferometry can measure signal intensity or power, it cannot directly obtain phase information. Because it requires moving a mirror to achieve scanning, the measurement system demands high mechanical precision and has limited resolution. Furthermore, based on the square-law detection principle, liquid nitrogen cooling is typically required, resulting in a low signal-to-noise ratio.

[0004] Photonics technologies mainly include electro-optic sampling, frequency combing, and superheterodyne techniques based on optical mixing. In electro-optic sampling or frequency combing, electro-optic sampling uses repetitive pulses to sample continuous wave signals, acquiring only one point at a time. In the spectrum, this is equivalent to shifting the continuous signal frequency by n times the pulse frequency to the intermediate frequency (IF). Although the time sequence of the continuous signal may be submerged in noise, since the signal is not pure noise, its autocorrelation function yields a time-intensity map. A Fourier transform is then performed to obtain the spectrum of the signal shifted to the IF. Superimposing an integer multiple of the pulse frequency recursively reconstructs the absolute frequency of the original terahertz signal. Frequency combing, on the other hand, uses a femtosecond laser to illuminate a photoconductive antenna to generate an optical frequency comb signal. This signal serves as the local oscillator for detecting terahertz signals. After mixing, the signal is also shifted to the IF. By superimposing an integer multiple of the pulse repetition frequency, the absolute frequency of the original terahertz signal can be calculated. However, this technology requires the use of expensive femtosecond lasers and strict hardware phase-locking to ensure the accuracy and stability of the local oscillator signal; otherwise, it is easy to misjudge the sequence order n, making it difficult to accurately measure the absolute frequency of the terahertz signal.

[0005] The heterodyne technique of optical mixing uses two coupled light beams as local oscillator signals, which are then mixed with the detected terahertz signal in an optical mixer. The signal is down-converted to an intermediate frequency (IF) for processing to obtain the terahertz signal information. Existing continuous-wave terahertz heterodyne analysis techniques have achieved sub-kilohertz resolution and ultra-wideband coverage. These techniques employ a femtosecond laser optical frequency comb with both repetition and bias frequencies locked to an atomic clock as a precise reference frequency. Two continuous-wave lasers are locked to two teeth of the optical frequency comb via two sets of optical phase-locked loops (PLLs), and these two signals are used as local oscillator inputs to the optical mixer for detection of the terahertz signal. This extreme performance heavily relies on a complex optical PLL and frequency stabilization system to suppress the phase noise of the single-frequency laser, and the scanning method is still a single-frequency step-scan, making the overall testing system extremely complex and costly.

[0006] In heterodyne terahertz detection systems based on optical mixing, resolution is directly limited by the relative linewidth of the laser. Furthermore, optical mixing techniques using free-running lasers are highly susceptible to light source phase noise, leading to intermediate frequency (IF) signal broadening and a decrease in signal-to-noise ratio, thus limiting the application of high-resolution spectral measurements. To obtain a broadband spectrum, the system must slowly and continuously change the laser wavelength through mechanical delay or temperature tuning, making snapshot-style detection impossible. Simultaneously, photoconductive antennas inevitably contain dark current, slowly temperature-drifting DC bias, and 1 / f noise, with low-frequency noise directly aliasing with the weak terahertz signal. Moreover, during local oscillator scanning, the frequency of the terahertz signal to be measured is typically a fixed characteristic frequency. Due to the absolute value characteristic of heterodyne mixing, local oscillator signals with the same frequency offset above and below the measured signal will output the same IF frequency. Without additional hardware (such as an IQ mixer or phase shifter), false peaks at mirror frequencies will inevitably occur, making it difficult to distinguish the true terahertz characteristic spectral lines.

[0007] In summary, existing terahertz spectrum measurement systems are susceptible to phase noise and low-frequency interference, and suffer from slow scanning speed and blurred image frequencies. This results in decreased measurement resolution and signal-to-noise ratio, difficulty in identifying the true frequency, and an inability to achieve fast and accurate terahertz spectrum measurement. Summary of the Invention

[0008] To address the issues of slow scanning speed and inaccurate measurement results in existing terahertz spectrum measurement systems, this invention proposes a terahertz spectrum measurement system and method based on incoherent optical mixing, which improves the measurement speed and accuracy of terahertz spectrum measurement results.

[0009] To achieve the above-mentioned technical effects, the technical solution of the present invention is as follows: In a first aspect, the present invention provides a terahertz spectrum system based on incoherent optical mixing, comprising: an incoherent light source, a first laser source, a second laser source, a first optical beam splitter, a second optical beam splitter, a third optical beam splitter, a balanced photodetector, an optical mixer, an amplifier, a first bandpass filter, a second bandpass filter, a data acquisition unit, and a cross-correlation processing module. The output end of the incoherent light source is connected to the input end of the first optical beam splitter, the first output end of the first optical beam splitter is connected to the first input end of the third optical beam splitter, and the second output end of the first optical beam splitter is connected to the first input end of the second optical beam splitter. The output of the first laser source is connected to the second input of the third optical beam splitter, the output of the third optical beam splitter is connected to the input of the balanced photodetector, the output of the balanced photodetector is connected to the input of the first bandpass filter, and the output of the first bandpass filter is connected to the first input of the acquisition unit. The output of the second laser source is connected to the second input of the second optical beam splitter, and the output of the second optical beam splitter is connected to the first input of the optical mixer. The second input of the optical mixer is used to receive the terahertz signal to be measured. The output of the optical mixer is connected to the input of the second bandpass filter. The output of the second bandpass filter is connected to the input of the amplifier. The output of the amplifier is connected to the second input of the acquisition unit. The output of the acquisition unit is connected to the input of the cross-correlation processing module.

[0010] Furthermore, the first laser source is a fixed-wavelength continuous-wave laser source, used to generate a stable continuous-wave laser with a fixed wavelength difference from the center wavelength of the incoherent light source.

[0011] Furthermore, the second laser source is an adjustable continuous wave laser source, used to generate a stable continuous wave laser with an adjustable wavelength difference from the center wavelength of the incoherent light source.

[0012] Furthermore, the incoherent light source includes: a distributed feedback laser source, a polarization state controller, an optical beam splitter, an erbium-doped fiber amplifier, an optical fiber circulator, and a variable optical attenuator; The output of the distributed feedback laser source is connected to the input of the polarization state controller; the output of the polarization state controller is connected to the input of the optical beam splitter; the first output of the optical beam splitter is connected to the input of the erbium-doped fiber amplifier; the second output of the optical beam splitter is connected to the first input of the fiber circulator; the output of the fiber circulator is connected to the input of the variable optical attenuator; and the output of the variable optical attenuator is connected to the second input of the fiber circulator.

[0013] Furthermore, the third optical beam splitter is a 2×2 fiber coupler; the third optical beam splitter also includes: a first output end and a second output end; the first output end and the second output end of the third optical beam splitter are connected to the input end of the balanced photodetector.

[0014] Secondly, the present invention also provides a terahertz spectrum measurement method based on incoherent optical mixing, comprising the following steps: S1: Control the incoherent light source to generate broadband incoherent light, and the first laser source generates single-frequency continuous wave laser; use the first optical beam splitter to split the broadband incoherent light into a first broadband incoherent light and a second broadband incoherent light. S2: Input the first broadband incoherent light and the single-frequency continuous wave laser into the third optical beam splitter to output two signals; use a balanced photodetector to detect the two signals to obtain the detected current signal; use a first bandpass filter to filter the detected current signal to obtain a reference signal; S3: Control the second laser source to generate a tunable continuous wave laser. Use the second optical beam splitter to couple the tunable continuous wave laser with the second broadband incoherent optical path as the local oscillator signal of the optical path under test. Input the local oscillator signal of the optical path under test into the optical mixer and perform heterodyne mixing with the terahertz signal under test to output a current signal. Use the second bandpass filter to filter the current signal to obtain the filtered current signal. Use the amplifier to amplify the filtered current signal to obtain the intermediate frequency mixing signal. S4: Using the acquisition unit, acquire the reference signal and the intermediate frequency mixing signal; S5: Using the cross-correlation processing module, perform Hilbert transform on the reference signal to construct a complex analytic signal; perform down-conversion processing on the complex analytic signal to extract the incoherent optical envelope; perform complex cross-correlation operation on the incoherent optical envelope and the intermediate frequency mixing signal to obtain the cross-correlation power spectrum; S6: Based on the cross-correlation power spectrum, read the frequency offset corresponding to the peak value, and calculate the absolute frequency and power of the terahertz. S7: Repeat steps S2 to S6, control the difference frequency of the second laser source to jump to the frequency point corresponding to the next scanning step, until the full spectrum scan within the range of the terahertz spectrum to be measured is completed, and stitch together the frequencies and powers measured under all scanning steps to obtain the broadband terahertz spectrum.

[0015] Furthermore, the expression for the broadband incoherent light described in step S1 is:

[0016] In the formula, This represents the random envelope fluctuations of incoherent light. The angular frequency representing the center frequency of incoherent light. This represents broadband incoherent light; The expression for the single-frequency continuous wave laser is:

[0017] In the formula, Indicates a single-frequency continuous wave laser. This represents the amplitude of the electric field of a laser at a fixed wavelength. The angular frequency representing the center frequency of incoherent light. The angular frequency representing the difference frequency from the center frequency of incoherent light; The expression for the tunable continuous wave laser is:

[0018] In the formula, This indicates a tunable continuous wave laser. This indicates the adjustable laser electric field amplitude. This indicates the local oscillator angular frequency offset.

[0019] Furthermore, the expression for the reference signal is:

[0020] In the formula, This indicates the first broadband incoherent light path. This represents the conjugate expression for a single-frequency continuous-wave laser. This represents the random envelope fluctuations of incoherent light. This indicates the operation of taking the real part. This represents the electric field amplitude of a continuous-wave laser with a fixed wavelength. This represents the current signal after detection. This represents the optical signal from the first output port of the third optical beam splitter 122. This indicates the optical signal from the second output port of the third optical beam splitter. It represents the angular frequency that differs from the center frequency of incoherent light.

[0021] Furthermore, the intermediate frequency mixing signal is expressed as follows:

[0022] In the formula, Indicates an intermediate frequency (IF) mixing signal. This represents the electric field amplitude of the second laser source. This represents the equivalent complex amplitude of the terahertz signal under test. The complex conjugate representing the random envelope fluctuations of incoherent light. This indicates the intermediate frequency.

[0023] Furthermore, the expression for the cross-correlation power spectrum is:

[0024] In the formula, Represents the cross-correlation power spectrum. This represents the electric field amplitude of the second laser source. This represents the equivalent complex amplitude of the terahertz signal under test. The complex conjugate representing the random envelope fluctuations of incoherent light. Indicates intermediate frequency. The complex conjugate representing the equivalent complex amplitude of the terahertz signal under test. This represents the random envelope fluctuations of incoherent light.

[0025] Compared with existing technologies, the beneficial effects of this method are: This invention provides a terahertz spectrum measurement system and method based on incoherent optical mixing. The terahertz spectrum measurement system includes: an incoherent light source, a first laser source, a second laser source, a first optical beamsplitter, a second optical beamsplitter, a third optical beamsplitter, a balanced photodetector, an optical mixer, an amplifier, a first bandpass filter, a second bandpass filter, a data acquisition unit, and a cross-correlation processing module. The broadband incoherent light is split into two paths. One path, along with a single-frequency continuous-wave laser, passes through the third optical beamsplitter, the balanced detector, and the first bandpass filter to generate a reference signal. The other path, along with a tunable continuous-wave laser, is coupled through the second optical beamsplitter and serves as the local oscillator signal for the optical path under test. This signal is then input to the optical mixer and mixed with the terahertz signal under test. After passing through the second bandpass filter and the amplifier, an intermediate frequency signal is obtained. The cross-correlation processing module is used to calculate the absolute terahertz frequency and power to obtain the broadband terahertz spectrum. This invention eliminates image frequency ambiguity, suppresses DC bias and low-frequency noise, and achieves fast, high-resolution terahertz spectrum measurement. Attached Figure Description

[0026] Figure 1 This diagram illustrates the structure of the terahertz spectrum measurement system based on incoherent optical mixing proposed in this embodiment of the invention. Figure 2 This diagram illustrates the structure of the incoherent light source proposed in this embodiment of the invention. Figure 3 A flowchart illustrating the terahertz spectrum measurement method based on incoherent optical mixing proposed in this embodiment of the invention; Figure 4 A schematic diagram illustrating the terahertz signal proposed in the embodiments of the present invention; Figure 5 A schematic diagram illustrating a local oscillator signal with a local oscillator frequency of 122 GHz as proposed in an embodiment of the present invention; Figure 6 A schematic diagram illustrating the reference path signal proposed in an embodiment of the present invention; Figure 7 A schematic diagram illustrating the signal to be tested in an embodiment of the present invention; Figure 8 This refers to the broadband incoherent envelope proposed in the embodiments of the present invention; Figure 9 This represents the final cross-correlation power spectrum converted to a terahertz spectrum as proposed in the embodiments of the present invention. Detailed Implementation

[0027] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this patent. To better illustrate this embodiment, some parts of the accompanying drawings may be omitted, enlarged, or reduced, and do not represent the actual dimensions; It is understandable to those skilled in the art that some well-known details may be omitted from the accompanying drawings.

[0028] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] The positional relationships depicted in the accompanying drawings are for illustrative purposes only and should not be construed as limiting this patent. Example 1 This embodiment provides a terahertz spectrum measurement system based on incoherent optical mixing, such as Figure 1 As shown, it includes: an incoherent light source 100, a first laser source 110, a second laser source 111, a first optical beam splitter 120, a second optical beam splitter 121, a third optical beam splitter 122, a balanced photodetector 140, an optical mixer 150, an amplifier 160, a first bandpass filter 170, a second bandpass filter 171, an acquisition unit 180, and a cross-correlation processing module 190; The output terminal of the incoherent light source 100 is connected to the input terminal of the first optical beam splitter 120, the first output terminal of the first optical beam splitter 120 is connected to the first input terminal of the third optical beam splitter 122, and the second output terminal of the first optical beam splitter 120 is connected to the first input terminal of the second optical beam splitter 121. The output terminal of the first laser source 110 is connected to the second input terminal of the third optical beam splitter 122, the output terminal of the third optical beam splitter 122 is connected to the input terminal of the balanced photodetector 140, the output terminal of the balanced photodetector 140 is connected to the input terminal of the first bandpass filter 170, and the output terminal of the first bandpass filter 170 is connected to the first input terminal of the acquisition unit 180. The output of the second laser source 111 is connected to the second input of the second optical beam splitter 121, and the output of the second optical beam splitter 121 is connected to the first input of the optical mixer 150. The second input of the optical mixer 150 is used to receive the terahertz signal 50 to be measured. The output of the optical mixer 150 is connected to the input of the second bandpass filter 171. The output of the second bandpass filter 171 is connected to the input of the amplifier 160. The output of the amplifier 160 is connected to the second input of the acquisition unit 180. The output of the acquisition unit 180 is connected to the input of the cross-correlation processing module 190.

[0030] In this embodiment, the incoherent light source 100 is used to generate broadband incoherent light with a center frequency.

[0031] In this embodiment, the incoherent light source 100 is used to generate incoherent light with a determined center frequency and a preset bandwidth. For example... Figure 2The diagram of the incoherent light source 100 shown includes: a distributed feedback laser source 101, a polarization state controller 102, an optical beam splitter 103, an erbium-doped fiber amplifier 104, an optical fiber circulator 105, and a variable optical attenuator 106. The output of the distributed feedback laser source 101 is connected to the input of the polarization state controller 102; the output of the polarization state controller 102 is connected to the input of the optical beam splitter 103; the first output of the optical beam splitter 103 is connected to the input of the erbium-doped fiber amplifier 104; the second output of the optical beam splitter 103 is connected to the first input of the fiber circulator 105; the output of the fiber circulator 105 is connected to the input of the variable optical attenuator 106; and the output of the variable optical attenuator 106 is connected to the second input of the fiber circulator 105.

[0032] The incoherent light generation principle is based on an external cavity delayed feedback structure in an optical fiber. It achieves this by precisely controlling the polarization state and intensity of the feedback light to excite the nonlinear dynamic evolution within the semiconductor laser. First, the distributed feedback (DFB) semiconductor laser, driven by a DC bias current, outputs an initial narrow-linewidth continuous optical signal. This signal is coupled into a polarization state controller 102 (PC), which is configured to precisely adjust the polarization state of the transmitted optical field. This ensures that the polarization direction of the feedback light subsequently injected back into the laser resonant cavity achieves optimal vector matching with the polarization direction of the dominant lasing mode in the laser's active region, thereby maximizing the interference coupling efficiency between the external feedback light and the intracavity optical field.

[0033] The polarization-modulated optical signal then enters the optical beam splitter 103, where it is split into a feedback branch and an output branch according to a preset splitting ratio. In the feedback branch, the optical signal sequentially enters an external cavity feedback loop composed of a fiber circulator and a fiber variable optical attenuator (VOA). The fiber circulator is configured to control the unidirectional flow path of the optical signal, effectively preventing interference from unwanted backscattered light. After propagating through this feedback loop, the optical signal carries a fixed external cavity time delay and is then re-injected back into the output end of the DFB laser.

[0034] When feedback light with a specific time delay and matched polarization state is re-injected into the resonant cavity of a DFB laser, this external optical perturbation disrupts the original steady-state balance between carrier and photon densities within the laser. By adjusting the variable optical attenuator to change the intensity of the feedback light injected into the cavity, the nonlinear dynamic state of the laser undergoes a regular bifurcation. As the feedback intensity increases, the laser successively experiences a continuous steady state, a single-period oscillation, and a quasi-periodic oscillation, eventually entering a fully developed chaotic state. In this fully developed chaotic state, the laser's intrinsic relaxation oscillation frequency is strongly excited and subjected to dense modulation by the external cavity mode, resulting in highly unpredictable noise-like fluctuations in the output optical signal in the time domain. Consequently, the corresponding radio frequency and optical spectra are significantly broadened, thus generating an optically chaotic signal with continuous broadband characteristics at the physical source.

[0035] Finally, due to the fluctuations in the output optical power of the DFB laser in the chaotic state, and the attenuation after splitting by the external cavity passive devices and beam splitter, the original chaotic optical power extracted from the output branch is low. Therefore, this original chaotic light is fed into the erbium-doped fiber amplifier 104 (EDFA) for processing. The EDFA provides high-gain power amplification across the entire frequency band while strictly maintaining the broadband spectral profile and temporal random fluctuation characteristics of the input chaotic signal, ultimately providing a stable high-power, high-quality broadband chaotic optical signal to the downstream detection system.

[0036] In this embodiment, a distributed feedback laser source 101 (DFB) is used to generate a laser beam at 1550 nm. A polarization state controller 102 is used to control the polarization state of the laser output light. An optical beam splitter 103 is used to split the light into two beams. An optical fiber circulator 105 is used to construct a unidirectional optical feedback loop or form a ring laser cavity. A variable optical attenuator 106 (VOA) is used to control the feedback intensity. An erbium-doped fiber amplifier 104 (EDFA) is used to amplify the optical power.

[0037] In this embodiment, the first laser source 110 (Continuous-wave Laser Source With Fixed Wavelength) is a fixed-wavelength continuous-wave laser source used to generate a stable continuous-wave laser with a fixed wavelength difference from the center wavelength of the incoherent light source 100.

[0038] In this embodiment, the second laser source 111 (Tunable Continuous-wave Laser Source) is a tunable continuous-wave laser source used to generate a stable continuous-wave laser with an adjustable center wavelength difference from the incoherent light source 100.

[0039] In this embodiment, the third optical beam splitter 122 is a 2×2 fiber coupler; the third optical beam splitter 122 further includes: a first output terminal and a second output terminal; the first output terminal and the second output terminal of the third optical beam splitter 122 are connected to the input terminal of the balanced photodetector 140.

[0040] In this embodiment, the input terminal of the balanced photodetector 140 is divided into a first input terminal and a second input terminal, and the first output terminal and the second output terminal of the third optical beam splitter 122 are respectively connected to the first input terminal and the second input terminal of the balanced photodetector 140.

[0041] In this embodiment, the first optical beam splitter 120 and the second optical beam splitter 121 are also 2×2 fiber couplers. The second optical beam splitter 121 is used to couple the two input signals and output them to the optical mixer in a 1:1 ratio. The first optical beam splitter 120 can split only one light into two beams in a 1:1 ratio.

[0042] In this embodiment, the balanced photodetector 140 is used to perform square-law detection on the two input optical signals and convert them into electrical signals, and then output current by the phase difference between the two signals.

[0043] In this embodiment, the optical mixer 150 (Photomixer) receives the coupled optical signal as the optical local oscillator (LO) and performs heterodyne mixing with the terahertz wave to be measured irradiated from space, wherein the difference frequency of the coupled optical signal is used as the local oscillator frequency.

[0044] In this embodiment, amplifier 160 (Low-Noise Amplifier) ​​is used to amplify the weak input signal through impedance matching and then output it.

[0045] In this embodiment, the first band-pass filter 170 and the second band-pass filter 171 are used to perform frequency cutoff processing on the input electrical signal, retaining the signal within the selected frequency band for output and filtering out the signal outside the selected frequency band.

[0046] In this embodiment, the acquisition unit 180 is a multi-channel high-speed acquisition unit (Digitizer / ADC) used to acquire input analog signals through multiple channels and convert the analog signals into digital signals.

[0047] In this embodiment, the cross-correlation processing module 190 is used to perform Hilbert transform on the reference real signal acquired by the reference optical path, combine it with the acquired real signal to generate a complex analytic signal, extract the accurate carrier frequency, perform digital down-conversion to strip the carrier frequency, extract the incoherent optical envelope, and finally perform complex cross-correlation operation with the intermediate frequency signal of the optical path under test to obtain the cross-correlation spectrum and obtain the accurate image-free terahertz frequency.

[0048] Even if the terahertz measurement system uses incoherent light generated by a common semiconductor laser, the large bandwidth of the incoherent light will not affect the final output, and it can still achieve extremely high-resolution terahertz spectrum reconstruction.

[0049] The incoherent light source of the terahertz spectrum measurement system has broadband characteristics. Combined with a bandpass filter, a wide-bandwidth spectral "snapshot" can be obtained with a single local oscillator dwell. The local oscillator does not require dense continuous frequency tuning; it only needs to perform a skip scan with a large step size matching the width of the viewing window. The feature envelope can be restored and extracted, and the ultra-wideband terahertz full spectrum can be seamlessly stitched together, which greatly shortens the time for full spectrum scanning and data acquisition.

[0050] Example 2 This embodiment provides a terahertz spectrum measurement method based on incoherent optical mixing, using the terahertz spectrum measurement system based on incoherent optical mixing described in Embodiment 1, such as... Figure 3 The flowchart of the method shown includes the following steps: S1: Control the incoherent light source 100 to generate broadband incoherent light, and the first laser source 110 to generate single-frequency continuous wave laser; use the first optical beam splitter 120 to split the broadband incoherent light into a first broadband incoherent light and a second broadband incoherent light. S2: Input the first broadband incoherent light and the single-frequency continuous wave laser into the third optical beam splitter 122 and output two signals; use the balanced photodetector 140 to detect the two signals and obtain the detected current signal; use the first bandpass filter 170 to filter the detected current signal and obtain the reference signal. S3: Control the second laser source 111 to generate a tunable continuous wave laser. Use the second optical beam splitter 121 to couple the tunable continuous wave laser with the second broadband incoherent light as the local oscillator signal of the optical path under test. Input the local oscillator signal of the optical path under test into the optical mixer 150 and perform heterodyne mixing with the terahertz signal 50 under test to output a current signal. Use the second bandpass filter 171 to filter the current signal to obtain the filtered current signal. Use the amplifier 160 to amplify the filtered current signal to obtain the intermediate frequency mixing signal. S4: Use the acquisition unit 180 to acquire the reference signal and the intermediate frequency mixing signal; S5: Using the cross-correlation processing module 190, perform Hilbert transform on the reference signal to construct a complex analytic signal; perform down-conversion processing on the complex analytic signal to extract the incoherent optical envelope; perform complex cross-correlation operation on the incoherent optical envelope and the intermediate frequency mixing signal to obtain the cross-correlation power spectrum. S6: Based on the cross-correlation power spectrum, read the frequency offset corresponding to the peak value, and calculate the absolute frequency and power of the terahertz. S7: Repeat steps S2 to S6, control the difference frequency of the second laser source 111 to jump to the frequency point corresponding to the next scanning step, until the full spectrum scan of the terahertz spectrum to be measured is completed, and stitch together the frequencies and powers measured under all scanning steps to obtain the broadband terahertz spectrum.

[0051] Specifically, the incoherent light source 100 generates a center frequency of... (angular frequency is) ), bandwidth is Broadband incoherent light, the first laser source 110 generates a single-frequency continuous wave laser beam, whose frequency is consistent with... difference And requires Considering the filter margin and to prevent baseband spectrum folding, the reference is... These two beams of light are input into the third beam splitter 122 in the reference optical path.

[0052] In this embodiment, the expression for the broadband incoherent light in step S1 is:

[0053] In the formula, This represents the random envelope fluctuations of incoherent light. The angular frequency representing the center frequency of incoherent light. This represents broadband incoherent light; The expression for the single-frequency continuous wave laser is:

[0054] In the formula, Indicates a single-frequency continuous wave laser. This represents the complex amplitude of the electric field of a laser at a fixed wavelength. The angular frequency representing the center frequency of incoherent light. It represents the angular frequency that differs from the center frequency of incoherent light.

[0055] In this embodiment, the second laser source 111 is controlled to generate tunable continuous wave laser, generating one beam of single-frequency continuous wave laser at a time, and the wavelength of the light is changed in a jump manner. The step size of the frequency scan corresponding to each wavelength is set to... ,Require ,in It is the high-frequency cutoff frequency of the second bandpass filter 171, and the wavelength scanning range is set to the difference frequency between the center frequencies of continuous wave laser and broadband incoherent light. Traversing the terahertz spectrum range to be measured, this beam of light and incoherent light are coupled through the second optical beam splitter 121 and then input into the optical mixer 150 of the optical path to be measured.

[0056] In this embodiment, the expression for tunable continuous wave laser is:

[0057] In the formula, This indicates a tunable continuous wave laser. This indicates the adjustable laser electric field amplitude. This indicates the local oscillator angular frequency offset.

[0058] In this embodiment, the two beams of light input to the reference optical path and Two signals are output through the third optical beam splitter 122 and then input into the balanced photodetector 140. After square-law detection, the two signals are phased to obtain a current signal, which is then set to a frequency greater than or equal to... After the first bandpass filter 170, the signal is input to one of the channels of the acquisition unit 180.

[0059] Specifically, the expressions for the two signals are:

[0060]

[0061] In the formula, This represents the optical signal at the first output terminal of the third optical beam splitter 122. This represents the optical signal at the second output terminal of the third optical beam splitter 122. This represents the complex amplitude of the optical signal at the first input terminal of the third optical beam splitter 122. This represents the complex amplitude of the optical signal at the second input terminal of the third optical beam splitter 122.

[0062] In this embodiment, the expression for the reference signal is:

[0063] In the formula, This indicates the first broadband incoherent light path. This represents the conjugate expression for a single-frequency continuous-wave laser. This represents the random envelope fluctuations of incoherent light. This indicates the operation of taking the real part. This represents the electric field amplitude of a continuous-wave laser with a fixed wavelength. This represents the current signal after detection. This represents the optical signal from the first output port of the third optical beam splitter 122. This indicates the optical signal from the second output port of the third optical beam splitter 122. It represents the angular frequency that differs from the center frequency of incoherent light.

[0064] Specifically, the coupled continuous-wave laser and incoherent light input to the optical path under test are used as local oscillator signals and input into the optical mixer 150. When the terahertz signal under test illuminates the optical mixer, heterodyne mixing occurs, and an output current signal is generated. This current signal then passes through a low-frequency cutoff frequency. High-frequency cutoff frequency After passing through the second bandpass filter 171, the signal passes through a response interval with a cutoff frequency less than or equal to... Greater than or equal to The low-noise amplifier 160 amplifies the signal and inputs it into another channel of the multi-channel fast acquisition unit 180.

[0065] Specifically, the intermediate frequency mixer signal is expressed as follows:

[0066] In the formula, Indicates an intermediate frequency (IF) mixing signal. This indicates that the terahertz signal to be measured is 50. This represents the local oscillator signal.

[0067] The expression for the local oscillator signal is:

[0068] In the formula, This indicates that the terahertz signal to be measured is 50. This represents the equivalent complex amplitude of the terahertz signal 50 to be measured. This indicates the frequency of the terahertz signal to be measured is 50.

[0069] The expression for the local oscillator signal is:

[0070] In the formula, This represents the local oscillator signal frequency, which is the frequency difference between the continuous wave laser frequency emitted by the second laser source 111 and the center frequency of the incoherent light.

[0071] Based on the expressions for the local oscillator signal and the intermediate frequency mixer signal, the expression for the intermediate frequency mixer signal is derived as follows:

[0072] In the formula, Indicates an intermediate frequency (IF) mixing signal. This represents the electric field amplitude of the second laser source 111. This represents the equivalent complex amplitude of the terahertz signal 50 to be measured. The complex conjugate representing the random envelope fluctuations of incoherent light. This indicates the intermediate frequency.

[0073] The expression for the intermediate frequency is:

[0074] In the formula, This indicates the terahertz signal to be measured at a frequency of 50 Hz. This indicates the frequency of the local oscillator signal.

[0075] In this embodiment, the sampling rate of the acquisition unit 180 satisfies the Nyquist sampling theorem and is greater than or equal to 1. Analog bandwidth is greater than or equal to To ensure that the two signals travel through essentially the same line length, the reference optical path signal and the optical path signal under test are acquired simultaneously.

[0076] The cross-correlation processing module 190 transforms the optical path signal under test into a complex signal in the digital domain using Hilbert transform, and extracts the carrier frequency using fast Fourier transform or digital phase-locked loop. The envelope signal is obtained by down-converting a complex signal by multiplying it by a digitally inverted carrier wave. The complex conjugate of the intermediate frequency mixing signal and the envelope signal acquired by the optical path under test. Cross-correlation calculations are performed to obtain the cross-correlation power spectrum. The orthogonality of zero-mean incoherent signals is used to filter out DC bias and low-frequency noise in the intermediate frequency mixer signal. The expression for the signal in the optical path under test is:

[0077] in, This represents the Hilbert transform.

[0078] The formula for the actual intermediate frequency signal is expressed as follows:

[0079] In the formula, This represents DC bias, dark current, and low-frequency 1 / f noise that drifts slowly with temperature. This indicates thermal noise in the environment, etc.

[0080] In this embodiment, the expression for the cross-correlation power spectrum is:

[0081] In the formula, Represents the cross-correlation power spectrum. This represents the electric field amplitude of the second laser source 111. This represents the equivalent complex amplitude of the terahertz signal 50 to be measured. The complex conjugate representing the random envelope fluctuations of incoherent light. Indicates intermediate frequency. The complex conjugate of the equivalent complex amplitude of the terahertz signal 50 under test is represented. This represents the random envelope fluctuations of incoherent light.

[0082] It is an energy peak, which will appear in the mid-frequency range of the relevant spectrum. A clearly visible energy peak is generated at that location. It still fluctuates randomly; the mathematical expectation of its integral over time on the relevant spectrum is 0, allowing for accurate differentiation. frequency.

[0083] It is a constant (or a straight line that changes very slowly). It is a high-speed oscillating random noise with zero mean. Multiplying the two together still results in a high-speed random noise with zero mean. It can turn the interference peaks generated by DC bias, dark current, and low-frequency 1 / f noise that slowly drifts with temperature into extremely low broadband noise floor.

[0084] and Statistically, they are completely independent and unrelated, so the expected value of their cross-correlation function is 0. This part of the energy will not accumulate in the spectrum and will instead become background noise.

[0085] In this embodiment, the cross-correlation power spectrum is observed, and the effective observation region frequency is set at... If a relevant peak exists in the power spectrum within the observation area, then read the positive and negative frequency offsets corresponding to that peak. The frequency of this frequency offset is the difference frequency between the current center frequencies of the continuous wave laser and the incoherent light. Add them together to obtain the accurate absolute terahertz frequency. The calculation is repeated until the full spectrum splicing scan within the terahertz frequency range to be measured is completed.

[0086] The terahertz spectrum measurement method based on incoherent optical mixing proposed in this invention utilizes incoherent light with its own wide bandwidth characteristics as the local oscillator light, and adopts a terahertz detection architecture that uses the envelope of co-originating incoherent light for digital cross-correlation demodulation. This completely eliminates image frequency ambiguity in the spectrum detection frequency sweep process, eliminates the limitation of resolution by the phase noise of the light source, solves the interference of dark current and DC drift, and realizes rapid broadband spectrum splicing in a leapfrog manner.

[0087] In this embodiment, the signals from broadband incoherent light and continuous-wave laser light after entering the third optical beam splitter and passing through the balanced photodetector are used as references, and The hardware architecture blocks baseband zero-frequency folding and signal aliasing. Another incoherent light coupled with the continuous wave laser is used as a local oscillator (LO). In the probe optical path, it is heterodyne mixed with the terahertz wave by an optical mixer, then amplified by a bandpass filter and a low-noise amplifier to filter out most of the DC bias and 1 / f noise.

[0088] The acquired single-channel real-valued incoherent reference signal is reconstructed into a complex analytic signal using digital Hilbert transform and other methods. The incoherent envelope is extracted, and the complex conjugate of the envelope is cross-correlated with the mixed intermediate frequency real signal. This algorithm achieves separation of positive and negative frequencies (upper and lower sidebands) and image frequency suppression to determine a unique frequency, while also spreading residual detector DC bias and noise to an extremely low noise floor.

[0089] This embodiment also proposes a "large step size skip-type" terahertz frequency domain scanning method, namely, the scanning step size of the difference frequency between the center frequencies of continuous wave laser and incoherent light. , set as ,in It is the high-frequency cutoff frequency of the second bandpass filter 171. With an effective bandwidth of incoherent light, each local oscillator dwell time can acquire an extremely wide double-sideband detection window, thereby achieving seamless stitching snapshot detection in the terahertz broadband frequency domain.

[0090] In this embodiment, a complex (phase-information-preserving) incoherent envelope is used for cross-correlation demodulation. Utilizing the difference in the conjugate states of the envelopes corresponding to positive and negative frequency offsets in heterodyne mixing, the terahertz waves to be measured above and below the local oscillator frequency are clearly separated in the complex cross-correlation spectrum. The frequency of the signal to be measured is uniquely determined, and no mirror false peaks or spectral folding ambiguity occur during full-spectrum scanning.

[0091] To address the significant DC drift and low-frequency 1 / f noise commonly found in terahertz systems, this invention employs a dual suppression mechanism. Firstly, a bandpass filter isolates the DC bias at the physical layer. Secondly, during cross-correlation demodulation, the residual background noise and dark current are multiplied by a broadband complex reference envelope with zero mean. Based on the orthogonality principle, this multiplication is completely spread out into an extremely low broadband noise floor, preventing interference peaks from forming in the cross-correlation spectrum, resulting in a high system signal-to-noise ratio.

[0092] Example 3 In this embodiment, the terahertz spectrum measurement system based on incoherent optical mixing proposed in Embodiment 1 and the terahertz spectrum measurement method based on incoherent optical mixing proposed in Embodiment 2 are illustrated by way of example.

[0093] like Figure 4 The diagram shows a terahertz signal, assuming a 125 GHz terahertz signal as the terahertz signal to be measured, 50. In the terahertz spectrum measurement system, a DFB laser generates a continuous wave laser at 1550 nm, which is output to the input port of a polarization controller 102. The output of the polarization controller 102 is connected to the first input port of an optical beam splitter 103. The first output port of the optical beam splitter 103 is connected to the forward input port of an optical fiber circulator 105. The output port of the optical fiber circulator 105 is connected to the input port of a variable optical attenuator 106 (VOA). The output port of the variable optical attenuator 106 (VOA) is connected back to the reverse input port of the circulator. The second output port of the optical beam splitter 103 is connected to an erbium-doped fiber amplifier 104 (EDFA), which amplifies the optical power and outputs a broadband incoherent light beam located at 1550 nm with a bandwidth of 2 GHz.

[0094] A first laser source 110 with a fixed wavelength (continuous wave laser) is used to generate a stable, narrow-linewidth single-frequency continuous wave laser with a frequency 3 GHz different from the center frequency of the broadband incoherent light.

[0095] A second, wavelength-tunable laser source 111 (continuous wave laser) is used to generate a stable, narrow-linewidth, single-frequency continuous wave laser with an initial frequency that differs from the center frequency of the broadband incoherent light by 98 GHz.

[0096] Broadband incoherent light passes through a 1:1 first optical beamsplitter 120, then enters a third optical beamsplitter 122 and another 1:1 second optical beamsplitter 121. In the 1:1 second optical beamsplitter 121, it is coupled with a stable, narrow-linewidth, single-frequency continuous-wave laser generated by an tunable continuous-wave laser source, and input into the optical mixer 150 of the probe optical path as the local oscillator signal. Assuming the laser frequency generated by the tunable continuous-wave laser source is continuously scanned until the difference frequency with the center frequency of the broadband incoherent light, i.e., the local oscillator signal frequency fLO, is 122 GHz, then... Figure 5 The diagram shown is a schematic of a local oscillator signal with a local oscillator frequency of 122 GHz.

[0097] The third optical beam splitter 122 receives broadband incoherent light and a stable, narrow-linewidth single-frequency continuous-wave laser generated by a fixed-wavelength continuous-wave laser source, and outputs the laser into a balanced photodetector 140.

[0098] A balanced photodetector 140 outputs a signal that is connected to a first bandpass filter 170. An optical mixer 150 receives a terahertz signal 50 as its surface and outputs a signal that is connected to a second bandpass filter 171.

[0099] The low-noise amplifier 160 receives the input signal by first passing through a broadband Bias-Tee, and then through a broadband low-noise amplifier cascade gain amplification module to achieve a gain of about 60dB. The output is then connected to channel 2 of the dual-channel high-speed acquisition unit 180.

[0100] The first bandpass filter 170 has a frequency range of 1.5GHz-4.5GHz and its output is connected to channel 1 of the dual-channel high-speed acquisition unit 180. The second bandpass filter 171 has a frequency range of 1GHz-5GHz and its output is connected to the low-noise amplifier 160.

[0101] The dual-channel high-speed acquisition unit 180 selects a sampling rate of 12.5GS / s and an analog bandwidth of 6GHz oscilloscope to simultaneously acquire the reference and probe signals, and outputs them into the cross-correlation processing module 190.

[0102] like Figure 6 The diagram shown illustrates the reference path signal acquired by channel one. Figure 7 The diagram shows the signal acquired by channel two of the path under test. The cross-correlation processing module 190 receives two data streams from the computer or FPGA processor, performs a Hilbert transform on the reference optical path data to obtain a complex reference signal, extracts the carrier frequency through a fast Fourier transform or digital phase-locked loop, multiplies it by a reverse-rotated digital carrier down-converter to remove the carrier frequency, and obtains a broadband incoherent envelope. The envelope is then subjected to digital complex cross-correlation processing with the intermediate frequency signal acquired by the optical path under test to obtain a cross-optical power spectrum (PSD).

[0103] like Figure 8 The diagram shown represents the data processed by the assumed reference optical path, i.e., the broadband incoherent envelope. Figure 9 The final cross-correlation power spectrum is shown as a terahertz spectrum.

[0104] In this embodiment, the specific process of a terahertz spectrum measurement method based on incoherent optical mixing is as follows: First, calibration is performed. The terahertz signal to be measured is input to the 50, which has a known power of 10mW and a frequency of 125GHz. Controlling a DFB laser and its broadband incoherent light generation system to generate an incoherent light source 100 with a center wavelength of 1550nm and a bandwidth of 2GHz.

[0105] A fixed-wavelength continuous-wave laser source (first laser source 110) is controlled to generate a stable, narrow-linewidth single-frequency continuous-wave laser with a frequency 3 GHz different from the center frequency of the broadband incoherent light.

[0106] Controlling the tunable continuous-wave laser source (second laser source 111) to make the difference frequency between its output continuous-wave laser and the center frequency of the broadband incoherent light... It is 122GHz.

[0107] During the center frequency dwell period, the dual-channel high-speed acquisition unit 180 synchronously acquires the real reference signal from channel one and the real intermediate frequency (IF) mixed signal from channel two. The IF signal contains residual DC bias from the photoconductive antenna, low-frequency dark current noise, etc.

[0108] The cross-correlation processing module 190 first performs a Hilbert transform on the reference signal data to obtain an orthogonal imaginary sequence, then combines the two to obtain a complex reference signal. After extracting the carrier frequency through a Fast Fourier Transform or a digital phase-locked loop, it multiplies by a digitally down-converted signal with an inversely rotated phase to remove the carrier frequency, resulting in a broadband incoherent envelope. The complex conjugate of the broadband incoherent envelope is used to perform cross-correlation on the intermediate frequency mixer signal at the current step size, and the cross-correlation power spectrum (PSD) is calculated. The DC bias (DC) introduced by the photoconductive antenna is completely suppressed to an extremely low noise floor because it is uncorrelated with the zero-mean reference signal. Furthermore, due to the introduction of complex phase rotation, the originally overlapping upper and lower sidebands are decoupled and separated in the mid-frequency spectrum.

[0109] Observe the power peak in the cross-correlation spectrum, observe the offset of the signal under test at +3GHz and the power difference with 10dBm, record the data and calibrate the system.

[0110] After calibration, the formal measurement is carried out, with the unknown terahertz signal being input into the terahertz signal 50.

[0111] The first laser source 110, with its output continuous wave laser frequency differing from the center frequency of the broadband incoherent light, is... Perform a skip scan within the terahertz frequency range to be measured, setting a fixed scan step size of 2 GHz (observation view set between 2 GHz and 4 GHz), and execute steps S5, S6, and S7.

[0112] Observe whether there is a power peak in the cross-correlation spectrum. If a power peak exists, read the corresponding frequency offset of the peak within the range of -4GHz to +2GHz. The frequency of the signal under test is higher than the local oscillator frequency. When a positive frequency offset occurs, that is... The frequency of the signal under test is lower than the local oscillator frequency. When a negative frequency offset occurs, that is... Record the spectrum when there is no power peak.

[0113] After calculating the calibration value, the absolute frequency of the terahertz wave to be measured is determined according to the formula. This allows for direct restoration of the true frequency, completely eliminating the image frequency false peak phenomenon during the scanning process and obtaining the absolute terahertz frequency. Simultaneously, the power of the calibrated terahertz signal is read.

[0114] Controlling the adjustable continuous-wave laser source (second laser source 111) to make the difference frequency between the center frequency of its output continuous-wave laser and the incoherent light... Jump to the next 2GHz center frequency point and repeat the above steps to complete the terahertz spectrum measurement of the required broadband range and obtain broadband high-resolution spectrum reconstruction.

[0115] The embodiments described are merely examples to clearly illustrate the present invention and are not intended to limit the implementation of the invention. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively describe all possible implementations. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A terahertz spectrum system based on incoherent optical mixing, characterized in that, include: The system includes an incoherent light source (100), a first laser source (110), a second laser source (111), a first optical beam splitter (120), a second optical beam splitter (121), a third optical beam splitter (122), a balanced photodetector (140), an optical mixer (150), an amplifier (160), a first bandpass filter (170), a second bandpass filter (171), an acquisition unit (180), and a cross-correlation processing module (190). The output end of the incoherent light source (100) is connected to the input end of the first optical beam splitter (120), the first output end of the first optical beam splitter (120) is connected to the first input end of the third optical beam splitter (122), and the second output end of the first optical beam splitter (120) is connected to the first input end of the second optical beam splitter (121). The output of the first laser source (110) is connected to the second input of the third optical beam splitter (122), the output of the third optical beam splitter (122) is connected to the input of the balanced photodetector (140), the output of the balanced photodetector (140) is connected to the input of the first bandpass filter (170), and the output of the first bandpass filter (170) is connected to the first input of the acquisition unit (180). The output of the second laser source (111) is connected to the second input of the second optical beam splitter (121), the output of the second optical beam splitter (121) is connected to the first input of the optical mixer (150), the second input of the optical mixer (150) is used to receive the terahertz signal (50) to be measured; the output of the optical mixer (150) is connected to the input of the second bandpass filter (171); the output of the second bandpass filter (171) is connected to the input of the amplifier (160); the output of the amplifier (160) is connected to the second input of the acquisition unit (180); The output of the acquisition unit (180) is connected to the input of the cross-correlation processing module (190).

2. The terahertz spectrum system based on incoherent optical mixing according to claim 1, characterized in that, The first laser source (110) is a fixed wavelength continuous wave laser source, used to generate a stable continuous wave laser with a fixed wavelength difference from the center wavelength of the incoherent light source (100).

3. A terahertz spectrum system based on incoherent optical mixing according to claim 1, characterized in that, The second laser source (111) is an adjustable continuous wave laser source used to generate a stable continuous wave laser with an adjustable center wavelength difference from the incoherent light source (100).

4. A terahertz spectrum system based on incoherent optical mixing according to claim 1, characterized in that, The incoherent light source (100) includes: a distributed feedback laser source (101), a polarization state controller (102), an optical beam splitter (103), an erbium-doped fiber amplifier (104), an optical fiber circulator (105), and a variable optical attenuator (106). The output of the distributed feedback laser source (101) is connected to the input of the polarization state controller (102); the output of the polarization state controller (102) is connected to the input of the optical beam splitter (103); the first output of the optical beam splitter (103) is connected to the input of the erbium-doped fiber amplifier (104); the second output of the optical beam splitter (103) is connected to the first input of the fiber circulator (105); the output of the fiber circulator (105) is connected to the input of the variable optical attenuator (106); and the output of the variable optical attenuator (106) is connected to the second input of the fiber circulator (105).

5. A terahertz spectrum system based on incoherent optical mixing according to claim 1, characterized in that, The third optical beam splitter (122) is a 2×2 fiber coupler; the third optical beam splitter (122) also includes: a first output end and a second output end; the first output end and the second output end of the third optical beam splitter (122) are connected to the input end of the balanced photodetector (140).

6. A terahertz spectrum measurement method based on incoherent optical mixing, characterized in that, Includes the following steps: S1: Control the incoherent light source (100) to generate broadband incoherent light, and the first laser source (110) to generate single-frequency continuous wave laser; use the first optical beam splitter (120) to split the broadband incoherent light into a first broadband incoherent light and a second broadband incoherent light. S2: Input the first broadband incoherent light and the single-frequency continuous wave laser into the third optical beam splitter (122) to output two signals; use a balanced photodetector (140) to detect the two signals and obtain the detected current signal; use a first bandpass filter (170) to filter the detected current signal to obtain a reference signal; S3: Control the second laser source (111) to generate a tunable continuous wave laser. Use the second optical beam splitter (121) to couple the tunable continuous wave laser with the second broadband incoherent light as the local oscillator signal of the optical path under test. Input the local oscillator signal of the optical path under test into the optical mixer (150) and perform heterodyne mixing with the terahertz signal under test (50) to output a current signal. Use the second bandpass filter (171) to filter the current signal to obtain the filtered current signal. Use the amplifier (160) to amplify the filtered current signal to obtain the intermediate frequency mixing signal. S4: Use the acquisition unit (180) to acquire the reference signal and the intermediate frequency mixing signal; S5: Using the cross-correlation processing module (190), perform Hilbert transform on the reference signal to construct a complex analytic signal; perform down-conversion processing on the complex analytic signal to extract the incoherent optical envelope; perform complex cross-correlation operation on the incoherent optical envelope and the intermediate frequency mixing signal to obtain the cross-correlation power spectrum; S6: Based on the cross-correlation power spectrum, read the frequency offset corresponding to the peak value, and calculate the absolute frequency and power of the terahertz. S7: Repeat steps S2 to S6, control the difference frequency of the second laser source (111) to jump to the frequency point corresponding to the next scanning step, until the full spectrum scan of the terahertz spectrum to be measured is completed, and splice together the frequencies and powers measured under all scanning steps to obtain the broadband terahertz spectrum.

7. The terahertz spectrum measurement method based on incoherent optical mixing according to claim 6, characterized in that, The expression for the broadband incoherent light in step S1 is: In the formula, This represents the random envelope fluctuations of incoherent light. The angular frequency representing the center frequency of incoherent light. This represents broadband incoherent light; The expression for the single-frequency continuous wave laser is: In the formula, Indicates a single-frequency continuous wave laser. This represents the amplitude of the electric field of a laser at a fixed wavelength. The angular frequency representing the center frequency of incoherent light. The angular frequency representing the difference frequency from the center frequency of incoherent light; The expression for the tunable continuous wave laser is: In the formula, This indicates a tunable continuous wave laser. This indicates the adjustable laser electric field amplitude. This indicates the local oscillator angular frequency offset.

8. The terahertz spectrum measurement method based on incoherent optical mixing according to claim 6, characterized in that, The expression for the reference signal is: In the formula, This indicates the first broadband incoherent light path. This represents the conjugate expression for a single-frequency continuous-wave laser. This represents the random envelope fluctuations of incoherent light. This indicates the operation of taking the real part. This represents the electric field amplitude of a continuous-wave laser with a fixed wavelength. This represents the current signal after detection. This represents the optical signal output from the first output port of the third optical beam splitter (122). This represents the optical signal from the second output port of the third optical beam splitter (122). It represents the angular frequency that differs from the center frequency of incoherent light.

9. A terahertz spectrum measurement method based on incoherent optical mixing according to claim 6, characterized in that, The intermediate frequency mixing signal is expressed as follows: In the formula, Indicates an intermediate frequency (IF) mixer signal. This represents the electric field amplitude of the second laser source (111). This represents the equivalent complex amplitude of the terahertz signal (50) to be measured. The complex conjugate representing the random envelope fluctuations of incoherent light. This indicates the intermediate frequency.

10. A terahertz spectrum measurement method based on incoherent optical mixing according to claim 6, characterized in that, The expression for the cross-correlation power spectrum is: In the formula, Represents the cross-correlation power spectrum. This represents the electric field amplitude of the second laser source (111). This represents the equivalent complex amplitude of the terahertz signal (50) to be measured. The complex conjugate representing the random envelope fluctuations of incoherent light. Indicates intermediate frequency. The complex conjugate of the equivalent complex amplitude of the terahertz signal (50) to be measured is represented. This represents the random envelope fluctuations of incoherent light.

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