Passive laser heterodyne double-balance spectrum detection method and device

Abstract

The invention relates to the technical field of spectrum detection, and particularly provides a passive laser heterodyne double-balance spectrum detection method and device, and the method comprises the steps: carrying out the frequency mixing and differential processing of an input narrow-linewidth laser local oscillator signal and a wide-spectrum incoherent target optical signal through employing an optical mixer and a photodiode, and obtaining a laser heterodyne double-balance spectrum. Two paths of random time beat frequency photocurrent signals with the phase difference of 90 degrees are generated, the random time beat frequency photocurrent signals comprise information such as the spectrum of signal light, and the spectrum information of the signal light can be recovered by processing the output photocurrent signals through methods such as filtering and integration. The narrow linewidth and wavelength tunable characteristics of the narrow linewidth laser local oscillation signal can be utilized to perform high spectral resolution information acquisition on the wide spectrum incoherent optical signal, the utilization efficiency of the local oscillation light is effectively improved, common mode noise and part of system noise are eliminated, and the signal-to-noise ratio of the output signal is enhanced.

Description

Technical Field

[0001] The invention relates to the technical field of spectrum detection, and specifically provides a passive laser heterodyne double-balance spectrum detection method and device. Background Art

[0002] The basic principle of laser heterodyne detection technology is to introduce a beam of high-power, narrow-linewidth, frequency-adjustable local oscillator light to mix and superimpose the incident signal light on the photosensitive surface, and use photoelectric conversion devices and filtering devices to convert the frequency magnitude of The infrared signal is converted to The heterodyne RF signal is processed to obtain the spectrum, frequency and other information of the signal light.

[0003] One of the core advantages of laser heterodyne detection technology is the high-sensitivity amplification of weak signals. Specifically, when the power of the local oscillator light is adjusted to a higher level, the local oscillator light can be used as a carrier to effectively amplify the weak signal. The amplitude of the heterodyne signal is proportional to the product of the local oscillator light power and the signal light power. In practical applications, through carefully designed system parameters, the intensity of the final heterodyne signal can be increased by several orders of magnitude compared to the original signal light, usually up to 6 to 9 orders of magnitude. It not only greatly expands the dynamic range of signal detection, but also significantly reduces the impact of noise on the signal, so that even in extremely weak or complex interference environments, accurate and reliable signal extraction and analysis can be achieved.

[0004] Another core advantage of laser heterodyne detection technology is its efficient frequency selectivity. This technology only responds to and processes difference frequency signals that strictly conform to the preset detectable bandwidth limit, thus achieving accurate screening and identification of signal frequency components. In other words, only when the difference between the frequency of the signal light and the frequency of the local oscillator light is exactly within the preset bandwidth of the system, the difference frequency signal generated by the interaction between the two will be effectively identified and responded to. Conversely, if the frequency difference exceeds the bandwidth limit of the system, the corresponding heterodyne signal will not be captured by the system, thereby achieving automatic rejection of irrelevant frequency components. This mechanism gives laser heterodyne detection technology a natural filtering performance. It does not need to rely on complex narrowband filters to eliminate stray light interference like direct detection technology, which greatly simplifies the complexity of the system and improves the accuracy and reliability of detection.

[0005] With the development of semiconductor technology and laser technology, laser heterodyne technology has gradually been applied to the field of spectroscopy with its advantages of high environmental adaptability, high spatial resolution, and high spectral resolution. For example, Tan Tu and others from the Hefei Institutes of Physical Science, Chinese Academy of Sciences, have developed an all-fiber laser heterodyne solar radiometer that can measure solar spectrum data with high spectral resolution in real time, and has the advantages of small size and easy use. The radiometer connects solar trackers, optical fibers, high-speed heterodyne detectors, RF processors and other components, combined with tunable narrow linewidth lasers, can not only achieve high signal-to-noise ratio heterodyne signal acquisition, but also conveniently provide local oscillator light and its real-time output wavelength information. Andrew Sappi and others invented a new combustion efficiency monitoring device and its operation method. The device uses laser heterodyne technology to capture the light emitted by the combustion zone and mix it with the local oscillator signal to generate a photocurrent, and then filter and separate the difference frequency signal proportional to the concentration of the target substance, thereby monitoring the combustion efficiency in real time.

[0006] Although traditional heterodyne detection technology plays a fundamental role in many optical signal detection scenarios, its inherent limitations are becoming increasingly prominent. Specifically, one of the shortcomings of this technology is that it fails to fully utilize the power of the local oscillator light, and the noise has a greater impact. As a key link in heterodyne detection, the effective utilization of the power of the local oscillator light is directly related to the sensitivity and accuracy of the detection. In traditional heterodyne detection, due to the loss of light waves during transmission and the inadequacy of the power allocation mechanism, the power of the local oscillator light is often not fully utilized. In the process of actual application, the environment is complex and changeable, and the impact of noise increases. These noise sources include but are not limited to thermal noise, shot noise, and various electronic and mechanical noises within the system. These noise components are intertwined with the target signal, and traditional heterodyne detection technology is difficult to distinguish, which will cause a decrease in the signal-to-noise ratio. Therefore, when performing high-precision measurements or weak signal detection, traditional heterodyne detection causes energy waste and reduced detection efficiency, and its sensitivity is severely restricted, thereby affecting the improvement of detection performance.

[0007] In addition, since the core of laser heterodyne spectrum detection technology is to obtain the difference frequency component between the target signal and the local oscillator signal, there are high requirements for the stability of the local oscillator signal. Specifically, its stability is directly related to the accuracy of the detection results. In practical applications, the stability of the local oscillator signal is easily disturbed by many factors. External environmental factors, such as slight fluctuations in ambient temperature, vibrations of mechanical structures, or interference from electromagnetic fields, may act on the laser through physical effects, causing unpredictable changes in the frequency, phase, or intensity of its output light waves. Internal factors of the system, such as noise from electronic devices, aging of the laser, and minor flaws in the optical path design, will also introduce additional disturbances, further affecting the stability of the local oscillator signal. Once these interference factors act on the local oscillator signal, they will destroy the phase locking relationship between it and the target signal, resulting in phase errors and amplitude fluctuations in the process of extracting the difference frequency component. These errors will not only reduce the signal-to-noise ratio of the detection signal, but may also distort the true form of the spectral characteristics, causing the final detection results to deviate from the true value, thereby affecting the accuracy and reliability of data analysis. Summary of the invention

[0008] In order to solve the above problems, the present invention provides a passive laser heterodyne double-balanced spectral detection method and device, which not only improves the signal-to-noise ratio of the detection system, but also enables the system to maintain high detection accuracy and stability when the local oscillator light stability fluctuates, significantly reducing the requirements for the local oscillator light stability.

[0009] In a first aspect, the present invention provides a passive laser heterodyne double-balanced spectrum detection method, comprising: The target optical signal is collected by using an optical system, wherein the target optical signal is a wide-spectrum incoherent light, and is composed of optical signals of different frequencies, wherein the complex amplitudes of the optical signals of different frequencies are random functions with respect to time; The target optical signal is chopper-modulated by using a chopper, and the modulated target optical signal is input into the target signal input end of the optical mixer, and the narrow linewidth laser local oscillator signal is input into the local oscillator input end of the optical mixer. In the optical mixer, the target optical signal and the narrow linewidth laser local oscillator signal are superimposed, and the superimposed target optical signal includes a DC part and an AC part, wherein the part with a frequency magnitude exceeding the recording limit of the photodetector is the DC part, and the component in the target optical signal with a frequency close to that of the narrow linewidth laser local oscillator signal is coherently superimposed, wherein the part detected by the photodetector is called an AC signal, and the optical mixer outputs four photocurrent signals, each of which includes a DC part and an AC part, wherein the DC part is proportional to the sum of the light intensities of the narrow linewidth laser local oscillator signal and the target optical signal, and there is a relative phase difference between the AC parts, which are 0 , 90 , 180 and 270 ; Select a suitable filtering range to perform bandpass filtering on the four outputs, input the two signals of 0° and 180° into a pair of photodiodes, perform differential processing on the outputs of the photodiodes, and the output electrical signals are called in-phase signals, and input the two signals of 90° and 270° into another pair of photodiodes, and the outputs of 90° and 270° are orthogonal signals, wherein the in-phase signal and the orthogonal signal are twice the AC part of the input signal and do not contain the DC part, and there is a 90° phase delay between the in-phase signal and the orthogonal signal; The in-phase signal and the orthogonal signal are subjected to harmonic detection by a phase-locked amplifier, wherein the reference signal of the phase-locked amplifier comes from the frequency of the chopper. The two signals are input into an oscilloscope for sampling, and the square sum is calculated based on the amplitudes of the two signals collected by the oscilloscope. The result obtained is proportional to the target light signal intensity at the current moment, the target light signal intensity within a preset time is obtained, the calculated average value of the target light signal intensity within the preset time is obtained, and the light intensity information with high spectral resolution is obtained.

[0010] As a preferred solution, the method of collecting the target light signal by using an optical system includes: The target light signal is converged at the rear focus of the first convex lens through the first convex lens, and a first concave lens and a second convex lens with appropriate focal lengths are placed to convert the target light signal into parallel light. The parallel light enters the third convex lens, and a pinhole of a preset size is placed at the rear focus of the third convex lens. The pinhole is used to filter the target light signal and modulate the spot size. A fourth convex lens is placed behind the pinhole, and the distance between the pinhole and the fourth convex lens is twice the focal length of the fourth convex lens. The image of the pinhole formed by the fourth convex lens is at twice the focal length behind the fourth convex lens. A mechanical chopper blade is placed at twice the focal length behind the fourth convex lens to convert the target light signal into an AC signal. A fifth convex lens is placed behind the mechanical chopper blade, and the distance between the mechanical chopper blade and the fifth convex lens is the focal length of the fifth convex lens. Finally, the target light signal is converted into parallel light after passing through the fourth convex lens and is coupled into the single-mode optical fiber through a collimator.

[0011] As a preferred solution, it also includes: The narrow linewidth laser local oscillator signal output by a DFB laser with tunable central wavelength is used.

[0012] In a second aspect, the present invention provides a passive laser heterodyne double-balanced spectrum detection device, comprising an optical structure and a radio frequency signal processing structure, wherein: The optical structure includes a target light signal, a first convex lens, a first concave lens, a second convex lens, a third convex lens, a pinhole, a fourth convex lens, a mechanical chopper blade, a fifth convex lens, and a collimator. The target light signal is converged at the rear focus of the first convex lens through the first convex lens. The first concave lens and the second convex lens with appropriate focal lengths are placed to convert the target light signal into parallel light. The parallel light enters the third convex lens. A pinhole of a preset size is placed at the rear focus of the third convex lens. The pinhole is used to filter the target light signal and modulate the spot size. A A fourth convex lens, the distance between the pinhole and the fourth convex lens is twice the focal length of the fourth convex lens, the image of the pinhole formed by the fourth convex lens is at twice the focal length behind the fourth convex lens, a mechanical chopper blade is placed at twice the focal length behind the fourth convex lens to convert the target optical signal into an AC signal, a fifth convex lens is placed behind the mechanical chopper blade, the distance between the mechanical chopper blade and the fifth convex lens is the focal length of the fifth convex lens, and finally the target optical signal is converted into parallel light after passing through the fourth convex lens, and is coupled into the single-mode optical fiber through a collimator; The radio frequency signal processing structure comprises: The target optical signal is collected by using an optical system, wherein the target optical signal is a wide-spectrum incoherent light, and is composed of optical signals of different frequencies, wherein the complex amplitudes of the optical signals of different frequencies are random functions with respect to time; The target optical signal is chopper-modulated by using a chopper, and the modulated target optical signal is input into the target signal input end of the optical mixer, and the narrow linewidth laser local oscillator signal is input into the local oscillator input end of the optical mixer. In the optical mixer, the target optical signal and the narrow linewidth laser local oscillator signal are superimposed, and the superimposed target optical signal includes a DC part and an AC part, wherein the part with a frequency magnitude exceeding the recording limit of the photodetector is the DC part, and the component in the target optical signal with a frequency close to that of the narrow linewidth laser local oscillator signal is coherently superimposed, wherein the part detected by the photodetector is called an AC signal, and the optical mixer outputs four photocurrent signals, each of which includes a DC part and an AC part, wherein the DC part is proportional to the sum of the light intensities of the narrow linewidth laser local oscillator signal and the target optical signal, and there is a relative phase difference between the AC parts, which are 0 , 90 , 180 and 270 ; Select a suitable filtering range to perform bandpass filtering on the four outputs, input the two signals of 0° and 180° into a pair of photodiodes, perform differential processing on the outputs of the photodiodes, and the output electrical signals are called in-phase signals, and input the two signals of 90° and 270° into another pair of photodiodes, and the outputs of 90° and 270° are orthogonal signals, wherein the in-phase signal and the orthogonal signal are twice the AC part of the input signal and do not contain the DC part, and there is a 90° phase delay between the in-phase signal and the orthogonal signal; The in-phase signal and the orthogonal signal are subjected to harmonic detection by a phase-locked amplifier, wherein the reference signal of the phase-locked amplifier comes from the frequency of the chopper. The two signals are input into an oscilloscope for sampling, and the square sum is calculated based on the amplitudes of the two signals collected by the oscilloscope. The result obtained is proportional to the target light signal intensity at the current moment, the target light signal intensity within a preset time is obtained, the calculated average value of the target light signal intensity within the preset time is obtained, and the light intensity information with high spectral resolution is obtained.

[0013] Compared with the prior art, the present invention can achieve the following beneficial effects: A passive laser heterodyne double-balanced spectral detection method and device provided in an embodiment of the present invention uses an optical mixer and a photodiode to mix and differentially process the input narrow linewidth laser local oscillator signal and the wide spectrum incoherent target optical signal, and generate two random time beat frequency photocurrent signals with a phase difference of 90°. The random time beat frequency photocurrent signal contains information such as the spectrum of the signal light. The output photocurrent signal is filtered, integrated, and processed to restore the spectrum information of the signal light. By using this method, the narrow linewidth and wavelength tunable characteristics of the narrow linewidth laser local oscillator signal can be used to obtain high spectral resolution information of the wide spectrum incoherent optical signal, and the utilization efficiency of the local oscillator light can be effectively improved, common mode noise and part of the system noise can be eliminated, and the signal-to-noise ratio of the output signal can be enhanced. The system can also maintain high detection accuracy and stability when the stability of the local oscillator light fluctuates, significantly reducing the requirements for the stability of the local oscillator light. In addition, balanced coherent detection is also combined with subsequent signal processing steps such as filtering and amplification to further improve the signal quality and dynamic response range. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic flow chart of a passive laser heterodyne double-balanced spectrum detection method provided according to an embodiment of the present invention; Figure 2 is a schematic diagram of the overall optical field spectrum density of the photocurrent in the passive laser heterodyne double-balanced spectral detection method provided according to an embodiment of the present invention; Figure 3is a schematic diagram of an optical structure in a passive laser heterodyne double-balanced spectral detection method provided according to an embodiment of the present invention; Figure 4 It is a schematic diagram of a radio frequency signal processing structure in a passive laser heterodyne double-balanced spectrum detection method provided according to an embodiment of the present invention.

[0015] Reference numerals include: 1 is the target optical signal, 2 is the first convex lens, 3 is the first concave lens, 4 is the second convex lens, 5 is the third convex lens, 6 is the pinhole, 7 is the fourth convex lens, 8 is the chopper blade, 9 is the fifth convex lens, 10 is the collimator, 11 is the DFB laser, 12 is the optical modulator, 13 is the target signal input terminal, 14 is the local oscillator input terminal, 15 is the optical mixer 90 Mixer, 16 for optical mixer 90 The 0° output terminal of the mixer, 17 is the optical mixer 90 The 90° output terminal of the mixer, 18 is the 90° output terminal of the optical mixer The 180° output terminal of the mixer, 19 is the optical mixer 90 The 270° output terminal of the mixer, 110 is a photodiode, 111 is an in-phase signal output terminal, 112 is an orthogonal signal output terminal, 113 is a bandpass filter, 114 is a lock-in amplifier, 115 is the frequency of the chopper, and 116 is an oscilloscope. DETAILED DESCRIPTION

[0016] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same modules are represented by the same reference numerals. In the case of the same reference numerals, their names and functions are also the same. Therefore, the detailed description thereof will not be repeated.

[0017] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation of the present invention.

[0018] Combination Figure 1 As shown, an embodiment of the present invention provides a passive laser heterodyne double-balanced spectrum detection method, comprising: S101. Collect a target optical signal using an optical system. The target optical signal is a wide-spectrum incoherent light composed of optical signals of different frequencies, wherein the complex amplitudes of the optical signals of different frequencies are random functions with respect to time.

[0019] S102, chopping and modulating the target optical signal by using a chopper, inputting the modulated target optical signal into the target signal input end of the optical mixer, inputting the narrow linewidth laser local oscillator signal into the local oscillator input end of the optical mixer, in the optical mixer, the target optical signal and the narrow linewidth laser local oscillator signal will be superimposed, the superimposed target optical signal includes a DC part and an AC part, wherein the part with a frequency magnitude exceeding the recording limit of the photodetector is the DC part, according to which the component in the target optical signal with a frequency close to that of the narrow linewidth laser local oscillator signal will be coherently superimposed, wherein the part detected by the photodetector is called an AC signal, the optical mixer outputs four photocurrent signals, each signal includes a DC part and an AC part, wherein the DC part is proportional to the sum of the light intensities of the narrow linewidth laser local oscillator signal and the target optical signal, and there is a relative phase difference between the AC parts, which are 0 , 90 , 180 and 270 .

[0020] S103. Select a suitable filtering range to perform bandpass filtering on the four outputs, input the two signals of 0° and 180° into a pair of photodiodes, perform differential processing on the outputs of the photodiodes, and the output electrical signals are called in-phase signals. Input the two signals of 90° and 270° into another pair of photodiodes, and the outputs of 90° and 270° are orthogonal signals, wherein the in-phase signal and the orthogonal signal are twice the AC part of the input signal and do not contain the DC part, and there is a 90° phase delay between the in-phase signal and the orthogonal signal.

[0021] S104, performing harmonic detection on the in-phase signal and the quadrature signal through a lock-in amplifier, wherein a reference signal of the lock-in amplifier comes from the frequency of the chopper, inputting the two signals into an oscilloscope for sampling, performing square summing based on the amplitudes of the two signals collected by the oscilloscope, and obtaining a result proportional to the target light signal intensity at the current moment, obtaining the target light signal intensity within a preset time, obtaining a calculated average value of the target light signal intensity within the preset time, and obtaining light intensity information with high spectral resolution.

[0022] In some embodiments, the target optical signal is collected by using an optical system in S101. The target optical signal is a wide-spectrum incoherent light composed of light of different frequencies, wherein the complex amplitude of the optical signal of different frequencies is a random function of time. The collection process specifically includes: The target light signal is converged at the rear focus of the first convex lens through the first convex lens, and a first concave lens and a second convex lens with appropriate focal lengths are placed to convert the target light signal into parallel light. The parallel light enters the third convex lens, and a pinhole of a preset size is placed at the rear focus of the third convex lens. The pinhole is used to filter the target light signal and modulate the spot size. A fourth convex lens is placed behind the pinhole, and the distance between the pinhole and the fourth convex lens is twice the focal length of the fourth convex lens. The image of the pinhole formed by the fourth convex lens is at twice the focal length behind the fourth convex lens. A mechanical chopper blade is placed at twice the focal length behind the fourth convex lens to convert the target light signal into an AC signal. A fifth convex lens is placed behind the mechanical chopper blade, and the distance between the mechanical chopper blade and the fifth convex lens is the focal length of the fifth convex lens. Finally, the target light signal is converted into parallel light after passing through the fourth convex lens and is coupled into the single-mode optical fiber through a collimator.

[0023] In some embodiments, a DFB laser with tunable central wavelength is used to output a narrow linewidth laser local oscillator signal.

[0024] In S102, the target optical signal is chopped and modulated by a chopper, and the modulated target optical signal is input into the target signal input end of the optical mixer, and the narrow linewidth laser local oscillator optical signal is input into the local oscillator input end of the optical mixer. In the optical mixer, the target optical signal and the narrow linewidth laser local oscillator optical signal are superimposed, and the superimposed optical signal includes two parts, wherein the part whose frequency magnitude exceeds the recording limit of the photodetector is the DC part, and the DC part can be considered as a constant and does not change with time. According to the components in the target optical signal with a frequency close to that of the narrow linewidth laser local oscillator optical signal, the frequency magnitude is reduced to , which can be detected by the photodetector. The part that can be detected is called the AC part, which can be considered as random beat frequency information. The optical mixer outputs four photocurrent signals, each of which includes a DC and an AC part, wherein the DC part is proportional to the sum of the light intensities of the local oscillator light signal and the target light signal, and there is a relative phase difference between the AC parts, which are 0 , 90 , 180 and 270 .

[0025] In S103, the four outputs are bandpass filtered, and a suitable filtering range is selected to reduce the influence of the noise of the local oscillator light itself, environmental noise, etc. on the signal. The two signals of 0° and 180° are input into a pair of photodiodes, and the output of the photodiodes is differentially processed. The output electrical signal is called an in-phase signal, and the two signals of 90° and 270° are input into another pair of photodiodes. The outputs of 90° and 270° are orthogonal signals, wherein the in-phase signal and the orthogonal signal are twice the AC part of the input signal and do not contain the DC part, and there is a 90° phase delay between the in-phase signal and the orthogonal signal.

[0026] In S104, the in-phase signal and the quadrature signal are subjected to harmonic detection through a lock-in amplifier, wherein the reference signal of the lock-in amplifier comes from the frequency of the chopper, and the two signals are input into an oscilloscope for sampling, and the square sum is calculated based on the amplitude of the two signals collected by the oscilloscope, and the result obtained is proportional to the signal intensity of the target light at the current moment. The signal intensity within a period of time is obtained, and the calculated average value within the period of time is obtained, which can be regarded as the target light intensity within the bandpass filter range. By changing the filter range and the local oscillator light frequency, high spectral resolution light intensity information can be obtained.

[0027] A passive laser heterodyne double-balanced spectral detection method provided in an embodiment of the present invention realizes coherent superposition of partial target light with a frequency close to that of the local oscillator light and the local oscillator light by selecting appropriate local oscillator light and coupling it with target light reflected or emitted by the target, thereby realizing passive spectral subdivision, and the output difference frequency signal is completely consistent with the spectral distribution of the target signal.

[0028] Since the target signal is a wide-spectrum incoherent light, the signal light is considered to be a random wave, that is, the polarization unit vector and complex amplitude are random functions of time and position. The target light is collected through the optical system, and the local oscillator light is generated inside the device. On the surface of the photodetector, the two beams of light are superimposed, and the intensity of the synthesized light field is proportional to the modulus size after the two beams of light are superimposed, forming a photocurrent on the detector surface. The photocurrent includes two parts: the DC component and the AC component. The DC component is proportional to the light intensity of the two beams of light and has nothing to do with time. It can be considered as background noise. The AC component is related to the light field after the signal light and the local oscillator light are superimposed, and changes with the angular frequency difference of the two beams of light. The AC signal contains the intensity, frequency difference and phase difference information of the target light and the local oscillator light. When the local oscillator light information is known, the intensity, frequency and phase of the target light can be derived from the AC component. The DC signal can be regarded as the sum of the light intensities of the target light and the local oscillator light. The phase, frequency and other information of the modulation signal will not change the amplitude of the generated photocurrent. Therefore, when performing heterodyne detection, the DC signal must be suppressed or filtered.

[0029] In general, the polarization state and complex amplitude of natural light are statistically independent functions. Therefore, the average of their product can be decomposed into the average of independent random functions of polarization and amplitude. And the phase of the signal field is , so if the time average of the photocurrent is directly calculated, its AC part is 0. However, this does not mean that the amplitude of the AC part of the heterodyne detection output is 0. In order to understand this, we first make the following assumptions: 1. All random variables of the received light are considered to be stationary in time. 2. The polarization of the reference wave can be considered to be independent of the spatial coordinates and time. 3. The intensity of the reference wave is always much greater than the intensity of the signal wave. 4. It is assumed that the signal wave and the reference wave are spatially coherent in the sensitive area of ​​the photodetector. Since the AC part is related to the frequency difference, and the Fourier transform of the spectral density is a correlation function, we first calculate the photocurrent at two different times. , The correlation function of the photocurrent includes the following four parts: the correlation function of the background noise, the correlation function of the local oscillator light intensity, the correlation function of the coherent superposition amplitude coefficient, and the correlation function of the target field complex amplitude. The coherent superposition light intensity coefficient refers to the ratio of the projection of the target light on the local oscillator light polarization direction at a certain moment to the amplitude of the target light itself, which can be understood as the cosine of the angle between the polarization direction of the target signal and the polarization direction of the local oscillator light. Then perform Fourier transform on the correlation function of the photocurrent to obtain the spectral density of the photocurrent, including the noise part, the spectral density of the local oscillator light intensity, , the spectral density of the output difference frequency signal .in is the spectral density of the coherent sum amplitude coefficients, , are the spectral densities of the target light and the local oscillator light amplitudes, respectively. and The convolution physically means the spectral density of the projection of the signal complex amplitude vector in the direction of the local oscillator light vector. If the local oscillator light is a monochromatic wave, the spectral density of the local oscillator light can be considered as a delta function, and the spectrum of the output difference frequency signal can be considered to be completely consistent with the spectral distribution of the target. The overall optical field spectral density of the photocurrent is as follows: Figure 2 shown.

[0030] Combination Figure 3As shown, the passive laser heterodyne double-balanced spectrum detection method provided by the present invention includes an optical structure and a radio frequency signal processing structure. The optical structure mainly includes: the target light signal 1 is converged at the rear focus of the first convex lens 2 through the first convex lens 2, and then the first concave lens 3 and the second convex lens 4 with appropriate focal lengths are placed to convert the signal light into parallel light, and the parallel light enters the third convex lens 5, and a pinhole 6 of appropriate size is placed at the rear focus of the third convex lens 5, and the pinhole 6 is used to filter the target light signal 1 and modulate the spot size, and a fourth convex lens 7 is placed behind the pinhole 6, and the distance between the pinhole 6 and the fourth convex lens 7 is twice the focal length of the fourth convex lens 7, and the image of the pinhole 6 formed by the fourth convex lens 7 is at twice the focal length behind the fourth convex lens 7, and a mechanical chopper blade 8 is placed at the position of twice the focal length behind the fourth convex lens 7 to convert the target light signal 1 into an AC signal, and a fifth convex lens 9 is placed behind the chopper blade 8, and the distance between the chopper blade 8 and the fifth convex lens 9 is the focal length of the fifth convex lens 9. Finally, the target optical signal 1 is transformed into parallel light after passing through the fourth convex lens 9, and is coupled into the single-mode optical fiber through the collimator 10, and the single-mode optical fiber transmits the target optical signal to the radio frequency signal processing structure.

[0031] The target optical signal 1 is characterized by a wide spectrum of incoherent light, which can be sunlight, halogen lamp or spontaneous radiation of an object, etc. The first concave lens 3 and the second convex lens 4 are used to adjust the signal light to parallel light, which is convenient for subsequent optical path adjustment. The size of the pinhole 6 should match the blade width of the mechanical chopper blade 8, so that the modulation frequency and extinction ratio of the signal light meet the requirements.

[0032] Combination Figure 4 As shown, the RF signal processing structure mainly includes: firstly connecting the target optical signal collected by the optical structure to the optical modulator 12. The optical modulator 12 can adopt a semiconductor amplifier. The optical modulator 12 amplifies the target optical signal, and then inputs the amplified target optical signal to the target signal input terminal 13 of the optical mixer 15. In addition, the narrow linewidth laser local oscillator signal output by the DFB laser 1 with adjustable central wavelength is input to the local oscillator input terminal 14 of the optical mixer 15. The optical mixer 15 can adopt a 90° optical mixer. The function of the optical mixer 15 is to coherently superimpose the local oscillator light and the signal light, and output four signals. Each signal includes a DC part and an AC part, wherein the DC part is proportional to the sum of the light intensities of the local oscillator light and the signal light, and there is a relative phase difference between the AC parts. 16 is the optical mixer 90 The 0° output terminal of the mixer, 17 is the optical mixer 90 The 90° output terminal of the mixer, 18 is the 90° output terminal of the optical mixer The 180° output terminal of the mixer, 19 is the optical mixer 90 The 270° output of the mixer, where the optical mixer is 90 The 0° output terminal 16 of the mixer and the optical mixer 90 The 180° output of the mixer is 180 18 The output port branch is called the in-phase branch I-arm, the optical mixer 90 The 90° output terminal 17 of the mixer and the optical mixer 90 The 270° output port 19 branch of the mixer is called the quadrature branch Q-arm.

[0033] The signals of the in-phase branch and the orthogonal branch are respectively input into the photodiode 110. The two photodiodes 110 form a detection unit. The outputs of the two detection units are respectively called the in-phase signal 111 and the orthogonal signal 112. The DC part of the input signal is eliminated from the two output signals, and the AC part becomes twice the input signal.

[0034] The two signals are then input into a bandpass filter 113 with the same parameters to perform bandpass filtering on the signals. The two signals are subjected to harmonic detection in a lock-in amplifier 114, wherein the reference signal of the lock-in amplifier 114 comes from the frequency 115 of the chopper. Finally, the two signals are input into an oscilloscope 116 for sampling. The square sum of the amplitudes of the two signals collected by the oscilloscope 116 is calculated, and the result obtained is proportional to the intensity of the target light.

[0035] As an embodiment, a passive laser heterodyne double balanced spectral detection method provided in an embodiment of the present invention specifically uses a 90° mixer and four photodiodes to mix and differentially process the input narrow linewidth laser local oscillator signal and the wide spectrum incoherent target light signal, and generate two random time beat frequency photocurrent signals with a phase difference of 90°. The random time beat frequency photocurrent signal contains information such as the spectrum of the signal light. The output photocurrent signal is filtered, integrated, and processed to restore the spectrum information of the signal light. Using this method, the narrow linewidth and wavelength tunable characteristics of the narrow linewidth laser local oscillator signal can be used to obtain high spectral resolution information of the wide spectrum incoherent light signal, and effectively improve the utilization efficiency of the local oscillator light, eliminate common mode noise and part of the system noise, enhance the signal-to-noise ratio of the output signal, and enable the system to maintain high detection accuracy and stability when the stability of the local oscillator light fluctuates, significantly reducing the requirements for the stability of the local oscillator light. In addition, balanced coherent detection is also combined with subsequent signal processing steps such as filtering and amplification to further improve the signal quality and dynamic response range.

[0036] Accordingly, an embodiment of the present invention provides a passive laser heterodyne double-balanced spectrum detection device, including an optical structure and a radio frequency signal processing structure, wherein: The optical structure includes a target light signal, a first convex lens, a first concave lens, a second convex lens, a third convex lens, a pinhole, a fourth convex lens, a mechanical chopper blade, a fifth convex lens, and a collimator. The target light signal is converged at the rear focus of the first convex lens through the first convex lens. The first concave lens and the second convex lens with appropriate focal lengths are placed to convert the target light signal into parallel light. The parallel light enters the third convex lens. A pinhole of a preset size is placed at the rear focus of the third convex lens. The pinhole is used to filter the target light signal and modulate the spot size. A A fourth convex lens, the distance between the pinhole and the fourth convex lens is twice the focal length of the fourth convex lens, the image of the pinhole formed by the fourth convex lens is at twice the focal length behind the fourth convex lens, a mechanical chopper blade is placed at twice the focal length behind the fourth convex lens to convert the target optical signal into an AC signal, a fifth convex lens is placed behind the mechanical chopper blade, the distance between the mechanical chopper blade and the fifth convex lens is the focal length of the fifth convex lens, and finally the target optical signal is converted into parallel light after passing through the fourth convex lens, and is coupled into the single-mode optical fiber through a collimator; The radio frequency signal processing structure comprises: The target optical signal is chopper-modulated by using a chopper, and the modulated target optical signal is input into the target signal input end of the optical mixer, and the narrow linewidth laser local oscillator signal is input into the local oscillator input end of the optical mixer. In the optical mixer, the target optical signal and the narrow linewidth laser local oscillator signal are superimposed, and the superimposed target optical signal includes a DC part and an AC part, wherein the part with a frequency magnitude exceeding the recording limit of the photodetector is the DC part, and the component in the target optical signal with a frequency close to that of the narrow linewidth laser local oscillator signal is coherently superimposed, wherein the part detected by the photodetector is called an AC signal, and the optical mixer outputs four photocurrent signals, each of which includes a DC part and an AC part, wherein the DC part is proportional to the sum of the light intensities of the narrow linewidth laser local oscillator signal and the target optical signal, and there is a relative phase difference between the AC parts, which are 0 , 90 , 180 and 270 ; Select a suitable filtering range to perform bandpass filtering on the four outputs, input the two signals of 0° and 180° into a pair of photodiodes, perform differential processing on the outputs of the photodiodes, and the output electrical signals are called in-phase signals, and input the two signals of 90° and 270° into another pair of photodiodes, and the outputs of 90° and 270° are orthogonal signals, wherein the in-phase signal and the orthogonal signal are twice the AC part of the input signal and do not contain the DC part, and there is a 90° phase delay between the in-phase signal and the orthogonal signal; The in-phase signal and the orthogonal signal are subjected to harmonic detection by a phase-locked amplifier, wherein the reference signal of the phase-locked amplifier comes from the frequency of the chopper. The two signals are input into an oscilloscope for sampling, and the square sum is calculated based on the amplitudes of the two signals collected by the oscilloscope. The result obtained is proportional to the target light signal intensity at the current moment, the target light signal intensity within a preset time is obtained, the calculated average value of the target light signal intensity within the preset time is obtained, and the light intensity information with high spectral resolution is obtained.

[0037] A passive laser heterodyne double-balanced spectrum detection device provided in an embodiment of the present invention uses an optical mixer and a photodiode to mix and differentially process the input narrow linewidth laser local oscillator signal and the wide spectrum incoherent target optical signal, and generate two random time beat frequency photocurrent signals with a phase difference of 90°. The random time beat frequency photocurrent signal contains information such as the spectrum of the signal light. The output photocurrent signal is filtered, integrated, and processed to restore the spectrum information of the signal light. By using this method, the narrow linewidth and wavelength tunable characteristics of the narrow linewidth laser local oscillator signal can be used to obtain high spectral resolution information of the wide spectrum incoherent optical signal, and the utilization efficiency of the local oscillator light can be effectively improved, common mode noise and part of the system noise can be eliminated, and the signal-to-noise ratio of the output signal can be enhanced. The system can also maintain a high detection accuracy and stability when the stability of the local oscillator light fluctuates, significantly reducing the requirements for the stability of the local oscillator light. In addition, balanced coherent detection is also combined with subsequent signal processing steps such as filtering and amplification to further improve the signal quality and dynamic response range.

[0038] Although the embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and cannot be understood as limiting the present invention. Those skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

[0039] The above specific implementations of the present invention do not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A passive laser heterodyne double-balanced spectrum detection method, characterized in that: include: The target optical signal is collected by using an optical system, wherein the target optical signal is a wide-spectrum incoherent light, and is composed of optical signals of different frequencies, wherein the complex amplitudes of the optical signals of different frequencies are random functions with respect to time; The target optical signal is chopper-modulated by using a chopper, and the modulated target optical signal is input into the target signal input end of the optical mixer, and the narrow linewidth laser local oscillator signal is input into the local oscillator input end of the optical mixer. In the optical mixer, the target optical signal and the narrow linewidth laser local oscillator signal are superimposed, and the superimposed target optical signal includes a DC part and an AC part, wherein the part with a frequency magnitude exceeding the recording limit of the photodetector is the DC part, and the component in the target optical signal with a frequency close to that of the narrow linewidth laser local oscillator signal is coherently superimposed, wherein the part detected by the photodetector is called an AC signal, and the optical mixer outputs four photocurrent signals, each of which includes a DC part and an AC part, wherein the DC part is proportional to the sum of the light intensities of the narrow linewidth laser local oscillator signal and the target optical signal, and there is a relative phase difference between the AC parts, which are 0 , 90 , 180 and 270 ; Select a suitable filtering range to perform bandpass filtering on the four outputs, input the two signals of 0° and 180° into a pair of photodiodes, perform differential processing on the outputs of the photodiodes, and the output electrical signals are called in-phase signals, and input the two signals of 90° and 270° into another pair of photodiodes, and the outputs of 90° and 270° are orthogonal signals, wherein the in-phase signal and the orthogonal signal are twice the AC part of the input signal and do not contain the DC part, and there is a 90° phase delay between the in-phase signal and the orthogonal signal; The in-phase signal and the orthogonal signal are subjected to harmonic detection by a phase-locked amplifier, wherein the reference signal of the phase-locked amplifier comes from the frequency of the chopper. The two signals are input into an oscilloscope for sampling, and the square sum is calculated based on the amplitudes of the two signals collected by the oscilloscope. The result obtained is proportional to the target light signal intensity at the current moment, the target light signal intensity within a preset time is obtained, the calculated average value of the target light signal intensity within the preset time is obtained, and the light intensity information with high spectral resolution is obtained.

2. The passive laser heterodyne double-balanced spectrum detection method according to claim 1, characterized in that: The method of collecting the target light signal by using an optical system comprises: The target light signal is converged at the rear focus of the first convex lens through the first convex lens, and a first concave lens and a second convex lens with appropriate focal lengths are placed to convert the target light signal into parallel light. The parallel light enters the third convex lens, and a pinhole of a preset size is placed at the rear focus of the third convex lens. The pinhole is used to filter the target light signal and modulate the spot size. A fourth convex lens is placed behind the pinhole, and the distance between the pinhole and the fourth convex lens is twice the focal length of the fourth convex lens. The image of the pinhole formed by the fourth convex lens is at twice the focal length behind the fourth convex lens. A mechanical chopper blade is placed at twice the focal length behind the fourth convex lens to convert the target light signal into an AC signal. A fifth convex lens is placed behind the mechanical chopper blade, and the distance between the mechanical chopper blade and the fifth convex lens is the focal length of the fifth convex lens. Finally, the target light signal is converted into parallel light after passing through the fourth convex lens and is coupled into the single-mode optical fiber through a collimator.

3. The passive laser heterodyne double-balanced spectrum detection method according to claim 1, characterized in that: Also includes: The narrow linewidth laser local oscillator signal output by a DFB laser with tunable central wavelength is used.

4. A passive laser heterodyne double-balanced spectrum detection device, characterized in that: It includes an optical structure and a radio frequency signal processing structure, wherein: The optical structure includes a target light signal, a first convex lens, a first concave lens, a second convex lens, a third convex lens, a pinhole, a fourth convex lens, a mechanical chopper blade, a fifth convex lens, and a collimator. The target light signal is converged at the rear focus of the first convex lens through the first convex lens. The first concave lens and the second convex lens with appropriate focal lengths are placed to convert the target light signal into parallel light. The parallel light enters the third convex lens. A pinhole of a preset size is placed at the rear focus of the third convex lens. The pinhole is used to filter the target light signal and modulate the spot size. A A fourth convex lens, the distance between the pinhole and the fourth convex lens is twice the focal length of the fourth convex lens, the image of the pinhole formed by the fourth convex lens is at twice the focal length behind the fourth convex lens, a mechanical chopper blade is placed at twice the focal length behind the fourth convex lens to convert the target optical signal into an AC signal, a fifth convex lens is placed behind the mechanical chopper blade, the distance between the mechanical chopper blade and the fifth convex lens is the focal length of the fifth convex lens, and finally the target optical signal is converted into parallel light after passing through the fourth convex lens, and is coupled into the single-mode optical fiber through a collimator; The radio frequency signal processing structure comprises: The target optical signal is chopper-modulated by using a chopper, and the modulated target optical signal is input into the target signal input end of the optical mixer, and the narrow linewidth laser local oscillator signal is input into the local oscillator input end of the optical mixer. In the optical mixer, the target optical signal and the narrow linewidth laser local oscillator signal are superimposed, and the superimposed target optical signal includes a DC part and an AC part, wherein the part with a frequency magnitude exceeding the recording limit of the photodetector is the DC part, and the component in the target optical signal with a frequency close to that of the narrow linewidth laser local oscillator signal is coherently superimposed, wherein the part detected by the photodetector is called an AC signal, and the optical mixer outputs four photocurrent signals, each of which includes a DC part and an AC part, wherein the DC part is proportional to the sum of the light intensities of the narrow linewidth laser local oscillator signal and the target optical signal, and there is a relative phase difference between the AC parts, which are 0 , 90 , 180 and 270 ; Select a suitable filtering range to perform bandpass filtering on the four outputs, input the two signals of 0° and 180° into a pair of photodiodes, perform differential processing on the outputs of the photodiodes, and the output electrical signals are called in-phase signals, and input the two signals of 90° and 270° into another pair of photodiodes, and the outputs of 90° and 270° are orthogonal signals, wherein the in-phase signal and the orthogonal signal are twice the AC part of the input signal and do not contain the DC part, and there is a 90° phase delay between the in-phase signal and the orthogonal signal; The in-phase signal and the orthogonal signal are subjected to harmonic detection by a phase-locked amplifier, wherein the reference signal of the phase-locked amplifier comes from the frequency of the chopper. The two signals are input into an oscilloscope for sampling, and the square sum is calculated based on the amplitudes of the two signals collected by the oscilloscope. The result obtained is proportional to the target light signal intensity at the current moment, the target light signal intensity within a preset time is obtained, the calculated average value of the target light signal intensity within the preset time is obtained, and the light intensity information with high spectral resolution is obtained.

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