Ultra-wideband mid-infrared spectrum measurement method and system based on quantum correlation

Abstract

The invention relates to the technical field of mid-infrared spectroscopy, and particularly discloses an ultra-wideband mid-infrared spectroscopy measurement method and system based on quantum correlation, and the method comprises the steps: enabling pump light to enter a sulfur-gallium-silver crystal to achieve spontaneous parametric down-conversion, changing the phase matching angle of the sulfur-gallium-silver crystal, and generating ultra-wideband mid-infrared photons and correlated near-infrared forecast photons; the ultra-wide-band near-infrared forecast photons are directly detected by a detector, the ultra-wide-band mid-infrared photons are converted into a near-infrared band by a second sulfur-gallium-silver crystal through a frequency up-conversion method, and the other detector detects the ultra-wide-band near-infrared forecast photons; and the phase matching angle of the sulfur-gallium-silver crystal is changed to cover the intermediate infrared range of 3-10 [mu] m, and intermediate infrared spectrum acquisition is realized. The device has the advantages of simple light splitting mode, simple structure and the like, breaks through the measurement bandwidth of a traditional intermediate infrared nonlinear spectrum based on a lithium niobate crystal or a potassium titanyl phosphate crystal, is easy to operate, and can realize ultra-sensitive, low-noise and high-resolution ultra-wideband intermediate infrared spectrum measurement.

Description

Technical Field

[0001] The invention belongs to the technical field of infrared spectroscopy, and in particular relates to an ultra-wideband mid-infrared spectroscopy measurement method and system based on quantum correlation. Background Art

[0002] The mid-infrared band with a wavelength between 2.5 and 25 μm plays a vital role in many applications. This band covers the vibration and rotation resonance phenomena of many molecules, reflects the fundamental frequency absorption characteristics of various organic and inorganic substances, and can directly obtain the spectral information of substances, further revealing their chemical structure and physical properties. Mid-infrared spectroscopy has a wide range of applications in important fields such as environmental monitoring, medical diagnosis, and food industry. In environmental monitoring, mid-infrared spectroscopy can be used to monitor pollutants in air and water, such as volatile organic compounds, and can provide real-time data to help environmental protection agencies monitor environmental quality and take corresponding measures; in medical diagnosis, especially in non-invasive disease detection and biomarker analysis, early signs of diseases such as cancer can be identified by analyzing the mid-infrared spectra of blood or tissue samples, achieving non-contact and non-staining measurements; in the food industry, mid-infrared spectroscopy can be used to detect adulterated ingredients in food, such as detecting other vegetable oils in olive oil.

[0003] However, in mid-infrared spectral measurement, spectral resolution structures and detectors still face severe challenges. In the traditional measurement process, spectral resolution needs to be achieved by relying on gratings or interferometers, but these spectroscopic methods increase the instability and complexity of the system. In recent years, time stretching technology has developed rapidly. Time stretching technology achieves spectral resolution by utilizing group velocity dispersion in dispersive media, and no longer requires complex operating methods such as gratings or interferometers, which greatly improves the stability of the system. However, in the process of spectral resolution, time stretching usually requires the use of long-distance optical fibers. Due to chromatic aberration, the coupling efficiency of light of different frequencies in the same optical fiber is different, and the transmission loss is relatively large, resulting in the loss of light flux used for spectral resolution. At the same time, the dispersion coefficient of the optical fiber is easily affected by changes in ambient temperature, thereby affecting the accuracy of spectral resolution. In addition, in terms of detectors, compared with the more mature visible light and near-infrared detectors, mid-infrared detectors still have great deficiencies in detection sensitivity, dark noise, detection response speed, etc. In order to break through the limitations of the insufficient performance of mid-infrared detectors on the sensitivity of mid-infrared spectral measurement, researchers have developed a mid-infrared quantum correlation spectroscopy measurement scheme combined with frequency up-conversion technology. This solution improves the sensitivity of spectral measurement by utilizing the time-frequency correlation characteristics of quantum spectroscopy, and further combines frequency up-conversion technology to convert the mid-infrared band into the near-infrared or visible light band, thereby circumventing the bottleneck of insufficient performance of current mid-infrared detectors. However, this type of solution usually relies on lithium niobate crystals or potassium titanyl phosphate crystals. These crystal materials have strong absorption in the mid-infrared band above 5μm, so this type of solution cannot measure mid-infrared spectra above 5μm, and cannot cover the fingerprint spectral region of molecules, limiting the application scope of mid-infrared nonlinear spectroscopy. In addition, this type of solution still relies on the above-mentioned gratings, interferometers or long-distance optical fibers in terms of spectral resolution, which limits the stability of the system to a certain extent and increases system losses. Therefore, how to achieve ultra-wideband, high-sensitivity, high-resolution, and high-light flux spectral detection in the mid-infrared band is still quite challenging. Summary of the invention

[0004] The object of the present invention is to provide an ultra-wideband mid-infrared spectrum measurement method and system based on quantum correlation to solve the problems raised in the above background technology.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for measuring ultra-wideband mid-infrared spectra based on quantum correlation comprises the following steps:

[0007] Step 1: inject pump light into the silver gallium sulfide crystal to achieve spontaneous parametric down-conversion, generating correlated photon pairs of ultra-wide-band mid-infrared photons and near-infrared forecast photons, where the forecast photons are detected by superconducting nanowire single-photon detectors;

[0008] Step 2: Introduce a pump light beam that is time-domain synchronized with the pump light described in step 1 and has the same repetition frequency, and combine it with the ultra-wide-band mid-red photons that carry spectral information after passing through the sample and inject it into the silver gallium sulfide crystal to achieve frequency up-conversion, thereby generating sum-frequency photons in the near-infrared band, and using another superconducting nanowire single-photon detector for detection;

[0009] Step 3: Mid-infrared spectral resolution is achieved by rotating two silver gallium sulfide crystals, and a time-correlated single photon counting module is used to achieve correlated spectral measurement of sum-frequency photons and predicted photons using coincidence measurement, thereby achieving 3-10μm ultra-wideband mid-infrared spectral measurement.

[0010] Preferably, the two silver gallium sulfide crystals can respectively realize the generation of a 3-10 μm mid-infrared light source and a 3-10 μm mid-infrared frequency up-conversion process by changing their respective phase matching angles.

[0011] Preferably, the spontaneous parametric down-conversion and frequency up-conversion processes both satisfy the laws of conservation of energy and conservation of momentum. For the spontaneous parametric down-conversion process: where ω p ,ω MIR and ω h are the angular frequencies of the pump light, mid-infrared photons, and predictor photons, respectively; and are the wave vectors of the corresponding light fields; for the frequency up-conversion process: ω p′ +ω MIR =ω SFG , where ω p′ ,ω MIR and ω SFG are the angular frequencies of the new pump light field, mid-infrared photons and sum-frequency photons, and They are angular frequencies ω p′ ,ω MIR and ω SFG The wave vector of the light field.

[0012] Preferably, the spontaneous parametric down-conversion is realized by irradiating the first silver gallium sulfide crystal with an 850nm pulsed laser to generate an ultra-wideband mid-infrared quantum correlation light source, and mid-infrared photons with a wavelength covering 3-10μm and forecast photons with a wavelength covering the 929-1186nm band are realized by changing the phase matching angle of the crystal. In the spontaneous parametric down-conversion process, the angular frequency is ωp The first pump light is incident on the first silver gallium sulfide crystal to cause spontaneous parametric down-conversion, generating an angular frequency of ω MIR The mid-infrared photon with an angular frequency of ω h The predicted photon follows the conservation of energy and momentum in this process: ω p =ω MIR +ω h , in and They are angular frequencies ω p ,ω MIR and ω h The wave vector of the light field satisfies the phase matching condition. When , the phase matching angle θ of the angle phase matching in the spontaneous parametric down-conversion process can be obtained as:

[0013]

[0014] where λ p , MIR , h They are angular frequencies ω p ,ω MIR ,ω h The wavelength corresponding to the light field; n MIR and n h They are angular frequencies ω MIR and ω h The refractive index corresponding to the light field, n po and n pe They are angular frequencies ω p The refractive index of the ordinary light o and extraordinary light e corresponding to the light field can be further obtained by the phase matching angle θ under the condition of generating mid-infrared photons of different wavelengths. incident The curve of the change of; and the emission angles θ of mid-infrared photons and predicted photons of different wavelengths exit The change curve of the first silver gallium sulfide crystal is shown in Figure 1. By rotating the angle of the first silver gallium sulfide crystal, that is, changing the phase matching angle θ, the spontaneous parametric down-conversion process of different frequencies can be achieved.

[0015] The method uses a broadband frequency up-conversion technology to combine broadband mid-infrared photons in a quantum correlation light source with a 1550nm pulse laser and then up-convert the frequency to a near-infrared band. In the broadband frequency up-conversion process, the second pump light source and the first pump light source are firstly triggered by a clock synchronization mode to keep their repetition frequency the same and time domain synchronized, and then the angular frequency is ω p′ The second pump light has an angular frequency of ω MIR The combined mid-infrared photons are incident on the second silver gallium sulfide crystal, and finally, the frequency up-conversion process generates an angular frequency of ωSFG The sum frequency photons still obey the conservation of energy and momentum during the frequency conversion process: ω p′ +ω MIR =ω SFG , in and They are angular frequencies ω p′ ,ω MIR and ω SFG The wave vector of the light field satisfies the phase matching condition. When , the phase matching angle θ′ inside the silver gallium sulfide crystal during the frequency up-conversion process is obtained as:

[0016]

[0017] where λ p′ , MIR and λ SFG They are angular frequencies ω p′ ,ω MIR and ω SFG The wavelength corresponding to the light field; n p′ and n MIR They are angular frequencies ω p′ and ω MIR The refractive index corresponding to the light field, n SFGo and n SFGe They are angular frequencies ω SFG The refractive index of the ordinary light o and extraordinary light e corresponding to the light field; The incident angle θ of the pump light and the mid-infrared photon in the frequency up-conversion process of mid-infrared photons of different wavelengths can be obtained by using the phase matching angle θ′. i ' ncident Change curve and frequency photon emission angle θ e ' xit By rotating the angle of the second silver gallium sulfide crystal, the phase matching angle θ′ of different frequencies is achieved, thereby realizing frequency up-conversion of different frequencies in the wavelength range of 3-10μm, and then realizing frequency resolution;

[0018] Finally, the first superconducting nanowire single-photon detector is used to detect the predicted photons, and an electrical signal is output to the time single-photon counter as a trigger signal. The mid-infrared photons of different wavelengths carrying the sample spectral information are converted to the near-infrared band by the second silver gallium sulfide crystal and detected by the second superconducting nanowire single-photon detector, which also outputs an electrical signal to the time single-photon counter for coincidence measurement. By continuously changing the first silver gallium sulfide crystal and the second silver gallium sulfide crystal, 3-10μm ultra-wideband mid-infrared spectrum measurement can be achieved.

[0019] A quantum-correlated ultra-wideband mid-infrared spectrum measurement system, comprising: a first pump light source, a half-wave plate, a plano-convex lens, a first silver gallium sulfide crystal, an off-axis parabolic reflector, a dichroic mirror, a sample to be measured, a reflective silver mirror, a superconducting nanowire single-photon detector, a second pump light source, a half-wave plate, a dielectric film reflector, a dichroic mirror, an off-axis parabolic reflector, a second silver gallium sulfide crystal, an off-axis parabolic reflector, a filter group, a superconducting nanowire single-photon detector, and a time-correlated single-photon counter;

[0020] The first pump light source is an 850nm pulse laser with a repetition frequency of 40MHz and an output power of 1W. This light source is used as a spontaneous parametric down-conversion pump light source to generate correlated photon pairs of mid-infrared photons in the wavelength range of 3-10μm and forecast photons in the wavelength range of 929-1186nm. The repetition frequency is the same as that of the second pump light source and is synchronized in the time domain through clock synchronization triggering.

[0021] The half-wave plate is an 850nm half-wave plate, which is used to change the polarization direction of the first pump light source so that it meets the first type I phase matching condition of the first silver gallium sulfide crystal;

[0022] The plano-convex lens is an N-BK7 plano-convex lens, the purpose of which is to focus the first pump light source into the first silver gallium sulfide crystal to achieve efficient spontaneous parametric down-conversion; the plano-convex lens has a focal length of 50 mm and a lens diameter of 25.4 mm.

[0023] The first silver gallium sulfide crystal is a spontaneous parametric down-conversion nonlinear medium for generating 3-10 μm mid-infrared photons and 929-1186 nm prediction photons. Its coating band covers 2.6-11 μm, and the cutting angle is θ=50° and The crystal size is 6mm (length) × 6mm (width) × 2mm (thickness); and it is placed on a rotating translation stage with an accuracy of 1mrad. The angle of the translation stage is changed to achieve different phase matching angles in the first silver gallium sulfide crystal, thereby realizing spontaneous parametric down-conversion of different frequencies.

[0024] The off-axis parabolic reflector is a 90° off-axis parabolic reflector coated with a silver film, and its purpose is to collimate and output mid-infrared photons and forecast photons generated by spontaneous parametric down-conversion without chromatic aberration; the off-axis parabolic reflector has a reflection focal length of 50.8 mm and an off-axis diameter of 25.4 mm;

[0025] The dichroic mirror spatially separates the mid-infrared photons generated by spontaneous parametric down-conversion from the forecast photons. The dichroic mirror is a 2 μm short-wave pass dichroic mirror that reflects 3-10 μm mid-infrared photons and transmits 929-1186 nm forecast photons.

[0026] The sample to be tested is a biological tissue or a chemical material, and the sample to be tested has different absorbances to light intensities of different mid-infrared wavelengths. The system measures the changes in coincidence events when there is no sample and when there is a sample at different mid-infrared wavelengths by simultaneously changing the phase matching angles of the first silver gallium sulfide crystal and the second silver gallium sulfide crystal, so as to obtain different transmittances of the sample to be tested at different wavelengths, thereby realizing spectral detection;

[0027] The plane reflector is a reflective silver mirror, which is used to change the direction of the light path, and the reflectivity of the reflective silver mirror to the forecast photons is more than 90%;

[0028] The superconducting nanowire single-photon detector can achieve ultra-sensitive detection of the forecast photons generated by spontaneous parametric down-conversion, with a detection wavelength range of 929-1190nm, a detection efficiency of 90%, and a dark count of less than 20cps.

[0029] The second pump light source is a 1550nm pulsed laser, which can accept external electrical triggering of the first pump light source, has a repetition frequency of 40MHz, and an output power of 1W; the light source is used as a pump light source for frequency up-conversion to achieve frequency up-conversion of 3-10μm mid-infrared photons and generate sum frequency photons;

[0030] The half-wave plate is a 1550nm half-wave plate, which is used to change the polarization direction of the second pump light source so that it meets the first type phase matching condition of the second silver gallium sulfide crystal;

[0031] The plane reflector is a dielectric film reflector, which is used to change the direction of the light path. The reflectivity of the dielectric film reflector to the forecast photons is more than 99%;

[0032] The dichroic mirror spatially combines the 3-10 μm mid-infrared photons generated by spontaneous parametric down-conversion with the 1550 nm second pump light source. The dichroic mirror is a 2 μm long-wave pass dichroic mirror that transmits 3-10 μm mid-infrared photons and reflects 1550 nm pump light;

[0033] The off-axis parabolic reflector is a 90° off-axis parabolic reflector coated with a silver film, and its purpose is to focus the 3-10μm mid-infrared photons and the second pump light source in the second gallium sulfide silver crystal without chromatic aberration to achieve efficient frequency conversion. The off-axis parabolic reflector has a reflection focal length of 50.8mm and an off-axis diameter of 25.4mm;

[0034] The second silver gallium sulfide crystal is a frequency up-conversion nonlinear medium for converting 3-10 μm mid-infrared photons to near-infrared bands. Its coating band covers 2.6-11 μm, and the cutting angle is θ=50° and The crystal size is 6mm (length) × 6mm (width) × 2mm (thickness). It is placed on a rotating translation stage with an accuracy of 1mrad, and the phase matching angle in the second silver gallium sulfide crystal is changed by changing the angle of the translation stage, thereby achieving frequency up-conversion of different mid-infrared wavelengths;

[0035] The off-axis parabolic reflector is a 90° off-axis parabolic reflector coated with a silver film, the purpose of which is to collimate the output of the sum frequency photons generated by the frequency up-conversion without chromatic aberration, so as to facilitate subsequent detection. The reflection focal length of the off-axis parabolic reflector is 50.8mm, and the off-axis diameter is 25.4mm;

[0036] The filter set is a combination of a 1000nm long-pass filter and a 1350nm short-pass filter, and the filter set is used for frequency up-conversion photon filtering to filter out 1550nm pump light, pump light up-conversion fluorescence, and ambient stray light;

[0037] The superconducting nanowire single-photon detector can achieve ultra-sensitive detection of sum-frequency photons generated by frequency up-conversion, with a detection wavelength range of 1020-1350nm, a detection efficiency of 70%, and a dark count of less than 100cps;

[0038] The time-correlated single photon counter is used to record the correlation detection of sum-frequency photons and forecast photons, collect the changes of coincidence events caused by the angle change of two silver gallium sulfide crystals, with a maximum resolution of 1ps and a measurement time window of 65ns.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] This scheme realizes the generation and spectral detection of mid-infrared quantum light sources with a spectral range of 3-10μm based on silver gallium sulfide crystals, breaking through the limitations of the light transmission range of traditional spectral measurement systems based on lithium niobate crystals or potassium titanyl phosphate crystals. The mid-infrared photons in the quantum light source have quantum characteristics of time correlation and frequency correlation with the predicted photons, which can effectively reduce the impact of noise disturbances on the measurement in spectral measurement. By calibrating the relationship between the wavelength and the rotation angle of the silver gallium sulfide crystal in advance, spectral resolution can be performed by simply rotating the silver gallium sulfide crystal, which no longer relies on infrared gratings, interferometers or long-distance optical fibers, and has the characteristics of high light flux and high resolution. Through the frequency up-conversion process based on silver gallium sulfide crystals, the need for mid-infrared detection devices is avoided, the noise in the measurement process is reduced, and the detection sensitivity is further improved. The device has a simple structure and is easy to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a schematic diagram of the ultra-wideband mid-infrared spectrum measurement method based on quantum correlation of the present invention;

[0042] Figure 2Definitions of the incident angle, the exit angle and the phase matching angle in the spontaneous parametric down-conversion process (a) and the frequency up-conversion process (b) of the present invention;

[0043] Figure 3 The graph is a relationship between the incident angle (a) and the exit angle (b) and the mid-infrared wavelength in the spontaneous parametric down-conversion process of the present invention;

[0044] Figure 4 The graph is a relationship between the phase matching angle of the silver gallium sulfide crystal and the mid-infrared wavelength during the spontaneous parametric down-conversion (a) and frequency up-conversion (b) processes of the present invention.

[0045] Figure 5 The figure is a relationship diagram of the incident angle (a) and the exit angle (b) as a function of the mid-infrared wavelength during the frequency up-conversion process of the present invention.

[0046] In the figure: 1. Pump light source; 2. Half-wave plate; 3. Plano-convex lens; 4. Gallium sulfur silver crystal; 5. Off-axis parabolic reflector; 6. Dichroic mirror; 7. Sample to be tested; 8. Reflective silver mirror; 9. Superconducting nanowire single-photon detector; 10. Pump light source; 11. Half-wave plate; 12. Dielectric film reflector; 13. Dichroic mirror; 14. Off-axis parabolic reflector; 15. Gallium sulfur silver crystal; 16. Off-axis parabolic reflector; 17. Filter set; 18. Superconducting nanowire single-photon detector; 19. Time-correlated single-photon counter. DETAILED DESCRIPTION

[0047] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0048] The present invention provides an ultra-wideband mid-infrared spectroscopy measurement scheme based on quantum correlation. An ultra-wideband quantum correlation light source is generated through a spontaneous parametric down-conversion process based on silver gallium sulfide (AgGaS2) crystal. The time-frequency correlation characteristics of the quantum correlation light source help to significantly reduce the impact of environmental noise disturbances on spectral measurements. Combined with a broadband frequency up-conversion process based on silver gallium sulfide crystals, broadband mid-infrared photons are converted to a near-infrared band, thereby avoiding the use of mid-infrared detectors. Through the angle phase matching characteristics of the silver gallium sulfide crystal itself, it is only necessary to rotate the crystal to achieve spectrometry, and spectroscopic elements such as gratings, interferometers or long-distance optical fibers are no longer required. In addition, the ultra-wide light bandwidth of the silver gallium sulfide crystal breaks through the bandwidth limitations of traditional mid-infrared nonlinear spectroscopy based on lithium niobate crystals and potassium titanyl phosphate crystals. The scheme has the advantages of high sensitivity, ultra-wideband and high resolution, and the method is simple and the structure is stable.

[0049] The following is combined with Figure 1 The basic principles and advantages of this solution are explained in detail:

[0050] First, the scheme uses 850nm pulsed laser to irradiate the first silver gallium sulfide crystal 4 to achieve spontaneous parametric down-conversion, generating an ultra-wideband mid-infrared quantum correlation light source, and by changing the phase matching angle of the crystal, mid-infrared photons with wavelengths covering 3-10μm and forecast photons with wavelengths covering the 929-1186nm band are achieved. In the process of spontaneous parametric down-conversion, the angular frequency is ω p The pump light 1 is incident on the first silver gallium sulfide crystal 4 to cause spontaneous parametric down-conversion, generating an angular frequency of ω MIR The mid-infrared photon with an angular frequency of ω h The predicted photon. In this process, energy conservation and momentum conservation are followed: ω p =ω MIR +ω h , in and They are angular frequencies ω p ,ω MIR and ω h The wave vector of the light field. When the phase matching condition is met When , the phase matching angle θ of the angle phase matching in the spontaneous parametric down-conversion process can be obtained as:

[0051]

[0052] where λ p , MIR , h They are angular frequencies ω p ,ω MIR ,ω h The wavelength corresponding to the light field; n MIR and n h They are angular frequencies ω MIR and ω h The refractive index corresponding to the light field, n po and n pe They are angular frequencies ω p The refractive index of the ordinary light o and extraordinary light e corresponding to the light field. The incident angle θ of the pump light under the condition of generating mid-infrared photons of different wavelengths can be further obtained by the phase matching angle θ. incident The change curve of Figure 3 (a) As shown. And the emission angles θ of mid-infrared photons and forecast photons of different wavelengths exit The change curve of Figure 3(b) As shown. By rotating the angle of the first silver gallium sulfide crystal 4, that is, changing the phase matching angle θ, the spontaneous parametric down-conversion process of different frequencies can be achieved. Figure 4 (a) is shown. The definitions of the incident angle, the exit angle and the phase matching angle in spontaneous parametric down-conversion are as follows: Figure 2 (a) shown.

[0053] The method uses broadband frequency up-conversion technology to combine broadband mid-infrared photons in the quantum correlation light source with a 1550nm pulsed laser and then up-convert the frequency to the near-infrared band. In the broadband frequency up-conversion process, the second pump light source 10 and the first pump light source 1 are firstly triggered by clock synchronization to keep their repetition frequency the same and time domain synchronized, and then the angular frequency is ω p′ The pump light is 10 and the angular frequency is ω MIR The combined mid-infrared photon beam is incident on the second silver gallium sulfide crystal 15, and finally, a frequency up-conversion process is performed to generate an angular frequency of ω SFG The sum frequency photons still obey the conservation of energy and momentum during the frequency conversion process: ω p′ +ω MIR =ω SFG , in and They are angular frequencies ω p′ ,ω MIR and ω SFG The wave vector of the light field satisfies the phase matching condition. When , the phase matching angle θ′ inside the silver gallium sulfide crystal during the frequency up-conversion process is obtained as:

[0054]

[0055] where λ p′ , MIR and λ SFG They are angular frequencies ω p′ ,ω MIR and ω SFG The wavelength corresponding to the light field; n p′ and n MIR They are angular frequencies ω p′ and ω MIR The refractive index corresponding to the light field, n SFGo and n SFGe They are angular frequencies ω SFG The refractive index of the ordinary light o and extraordinary light e states corresponding to the light field. The incident angle θ of the pump light and the mid-infrared photon in the frequency up-conversion process of mid-infrared photons of different wavelengths can be obtained by using the phase matching angle θ′. i ' ncident Change curve and frequency photon emission angle θe ' xit The change curve of Figure 5 By rotating the angle of the second silver gallium sulfide crystal 15, the phase matching angle θ′ of different frequencies is achieved, thereby achieving frequency up-conversion of different frequencies in the wavelength range of 3-10 μm, and then achieving frequency resolution, as shown in FIG. Figure 4 (b) As shown in the figure. During the frequency up-conversion process, the incident angle, the exit angle and the phase matching angle are defined as follows: Figure 2 (b) as shown.

[0056] Finally, the forecast photons are detected by using the superconducting nanowire single-photon detector 9, and an electrical signal is output to the time single-photon counter 19 as a trigger signal. The mid-infrared photons of different wavelengths carrying the sample spectral information are converted to the near-infrared band by the second silver gallium sulfide crystal 15 and detected by the superconducting nanowire single-photon detector 18, and the electrical signal is also output to the time single-photon counter 19 for coincidence measurement. By continuously changing the first silver gallium sulfide crystal 4 and the second silver gallium sulfide crystal 15, 3-10μm ultra-wideband mid-infrared spectrum measurement can be achieved.

[0057] Spectral resolution is achieved by rotating the crystal, which no longer requires the use of complex and expensive devices such as gratings, interferometers or long-distance dispersive optical fibers. Spectral resolution can be achieved by calibrating the relationship between the mid-infrared wavelength and the rotation angle of the two crystals in advance.

[0058] The correlation measurement of up-converted mid-infrared photons and near-infrared prediction photons can effectively improve the sensitivity of spectral measurement and reduce the impact of noise on spectral measurement.

[0059] The structure of the present invention is not limited to silver gallium sulfide crystals, and can use crystals such as gallium selenide (GaSe), silver gallium selenide (AgGaSe2) and the like according to different needs to achieve ultra-wideband mid-infrared spectroscopy measurements in different bands.

[0060] The present invention is realized by the following technical solutions:

[0061] The main idea of ​​the present invention is to inject 850nm pulse pump light into the sulfur gallium silver crystal to realize spontaneous parametric down conversion, and by changing the crystal phase matching angle, generate mid-infrared photons covering a wide band of 3-10μm and associated prediction photons. The 850nm laser triggers the 1550nm pump light source through a synchronous clock signal, and the two lasers maintain the same repetition frequency. The generated mid-infrared photon pulse is synchronized with the light pulse output by the 1550nm pump light source in the time domain, and enters another sulfur gallium silver crystal for frequency up conversion to generate sum frequency photons. The generated sum frequency photons and prediction photons are subjected to coincidence measurement, and their corresponding wavelengths are determined according to the angle changes of the two sulfur gallium silver crystals. In the presence or absence of samples, by collecting the changes in coincidence events caused by the angle changes of the sulfur gallium silver crystals, the coincidence count rate under the condition of sample with different wavelengths is compared with the coincidence count rate when there is no sample, and the transmittance of the sample to be measured can be obtained to achieve spectral measurement. For example, if the angular phase matching of the first silver gallium sulfide crystal is 60.62°, the corresponding mid-infrared wavelength is 4μm. At this time, the phase matching angle of the second silver gallium sulfide crystal is 44.89°. The 4μm mid-infrared photons are converted into sum-frequency photons through the frequency up-conversion process, and their wavelength is 1117nm. At this time, the coincidence peak when the predicted photon wavelength is 1.08μm and the sum-frequency wavelength is 1117nm can be obtained. By repeating the above operation, the spectrum line of the sample to be tested can be obtained.

[0062] The quantum-correlated ultra-wideband mid-infrared spectrum measurement system of the present invention comprises: a first pump light source 1, a half-wave plate 2, a plano-convex lens 3, a first gallium sulfide silver crystal 4, an off-axis parabolic reflector 5, a dichroic mirror 6, a sample to be measured 7, a reflective silver mirror 8, a superconducting nanowire single-photon detector 9, a second pump light source 10, a half-wave plate 11, a dielectric film reflector 12, a dichroic mirror 13, an off-axis parabolic reflector 14, a second gallium sulfide silver crystal 15, an off-axis parabolic reflector 16, a filter group 17, a superconducting nanowire single-photon detector 18, and a time-correlated single-photon counter 19.

[0063] The first pump light source 1 is an 850nm pulse laser with a repetition frequency of 40MHz and an output power of 1W. Using this light source as a spontaneous parametric down-conversion pump light source can generate mid-infrared photons with a wavelength range of 3-10μm and associated photon pairs of forecast photons with a wavelength range of 929-1186nm. And through clock synchronization triggering, the repetition frequency is the same as that of the second pump light source 10 and is synchronized in the time domain.

[0064] The half-wave plate 2 is an 850nm half-wave plate, which is used to change the polarization direction of the first pump light source 1 so that it meets the type I phase matching condition of the first silver gallium sulfide crystal 4.

[0065] The plano-convex lens 3 is an N-BK7 plano-convex lens, and its purpose is to focus the first pump light source 1 into the first silver gallium sulfide crystal 4 to achieve efficient spontaneous parametric down-conversion. The plano-convex lens has a focal length of 50 mm and a lens diameter of 25.4 mm.

[0066] The first silver gallium sulfide crystal 4 is a spontaneous parametric down-conversion nonlinear medium for generating 3-10 μm mid-infrared photons and 929-1186 nm prediction photons. Its coating band covers 2.6-11 μm, and the cutting angle is θ=50° and The crystal size is 6mm (length) × 6mm (width) × 2mm (thickness). It is placed on a rotating displacement platform with an accuracy of 1mrad, and the angle of the displacement platform is changed to achieve different phase matching angles in the first silver gallium sulfide crystal 4, thereby achieving spontaneous parametric down-conversion of different frequencies.

[0067] The off-axis parabolic reflector 5 is a 90° off-axis parabolic reflector coated with a silver film, and is used to collimate and output mid-infrared photons and forecast photons generated by spontaneous parametric down-conversion without chromatic aberration. The off-axis parabolic reflector has a reflection focal length of 50.8 mm and an off-axis diameter of 25.4 mm.

[0068] The dichroic mirror 6 spatially separates the mid-infrared photons generated by spontaneous parametric down-conversion from the forecast photons. The dichroic mirror is a 2 μm short-wave pass dichroic mirror that reflects 3-10 μm mid-infrared photons and transmits 929-1186 nm forecast photons.

[0069] The sample to be tested 7 is a biological tissue or a chemical material. The sample to be tested has different absorption degrees to light intensities of different mid-infrared wavelengths. The system measures the changes in coincidence events with and without samples at different mid-infrared wavelengths by simultaneously changing the phase matching angles of the first silver gallium sulfide crystal 4 and the second silver gallium sulfide crystal 15, thereby obtaining different transmittances of the sample to be tested 7 at different wavelengths, thereby realizing spectral detection.

[0070] The plane reflector 8 is a reflective silver mirror, which is used to change the direction of the light path. The reflectivity of the reflective silver mirror to the forecast photons is above 90%.

[0071] The superconducting nanowire single-photon detector 9 achieves ultra-sensitive detection of the forecast photons generated by spontaneous parametric down-conversion, with a detection wavelength range of 929-1190nm, a detection efficiency of 90%, and a dark count of less than 20cps.

[0072] The second pump light source 10 is a 1550nm pulse laser, which can accept external electrical triggering of the first pump light source 1, has a repetition frequency of 40MHz, and an output power of 1W. This light source is used as a frequency up-conversion pump light source to achieve frequency up-conversion of 3-10μm mid-infrared photons and generate sum frequency photons.

[0073] The half-wave plate 11 is a 1550nm half-wave plate, which is used to change the polarization direction of the second pump light source 10 so that it meets the type I phase matching condition of the second silver gallium sulfide crystal 15.

[0074] The plane reflector 12 is a dielectric film reflector, which is used to change the direction of the light path. The reflectivity of the dielectric film reflector to the forecast photons is above 99%.

[0075] The dichroic mirror 13 spatially combines the 3-10 μm mid-infrared photons generated by spontaneous parametric down-conversion with the 1550 nm second pump light source 10. The dichroic mirror is a 2 μm long-wave pass dichroic mirror that transmits 3-10 μm mid-infrared photons and reflects 1550 nm pump light.

[0076] The off-axis parabolic reflector 14 is a 90° off-axis parabolic reflector coated with a silver film, and its purpose is to focus the 3-10 μm mid-infrared photons and the second pump light source 10 in the second gallium silver sulfide crystal 15 without chromatic aberration to achieve efficient frequency conversion. The off-axis parabolic reflector has a reflection focal length of 50.8 mm and an off-axis diameter of 25.4 mm.

[0077] The second silver gallium sulfide crystal 15 is a frequency up-conversion nonlinear medium for converting 3-10 μm mid-infrared photons to near-infrared bands. Its coating band covers 2.6-11 μm, and the cutting angle is θ=50° and The crystal size is 6mm (length) × 6mm (width) × 2mm (thickness). It is placed on a rotating displacement platform with an accuracy of 1mrad, and the phase matching angle in the second silver gallium sulfide crystal 15 is changed by changing the angle of the displacement platform, thereby realizing frequency up-conversion of different mid-infrared wavelengths.

[0078] The off-axis parabolic reflector 16 is a 90° off-axis parabolic reflector coated with a silver film, and its purpose is to collimate and output the sum frequency photons generated by frequency up-conversion without chromatic aberration, so as to facilitate subsequent detection. The off-axis parabolic reflector has a reflection focal length of 50.8 mm and an off-axis diameter of 25.4 mm.

[0079] The filter set 17 is a combination of a 1000nm long-pass filter and a 1350nm short-pass filter. The filter set is used for frequency up-conversion photon filtering to filter out 1550nm pump light, pump light up-conversion fluorescence, ambient stray light, and the like.

[0080] The superconducting nanowire single-photon detector 18 achieves ultra-sensitive detection of sum-frequency photons generated by frequency up-conversion, with a detection wavelength range of 1020-1350nm, a detection efficiency of 70%, and a dark count of less than 100cps.

[0081] The time-correlated single photon counter 19 is used to record the correlation detection of sum frequency photons and prediction photons, and collect the changes of coincidence events caused by the angle change (wavelength change) of the two silver gallium sulfide crystals. The maximum resolution is 1ps and the measurement time window is 65ns.

[0082] Implementation 1:

[0083] First, spontaneous parametric down-conversion is realized based on nonlinear crystals to generate correlated photon pairs in a wide band. That is, the first pump light source 1 outputs a pulsed laser of 850 nm, and the plano-convex lens 3 is used to focus the pulsed laser of 850 nm to the center of the first silver gallium sulfide crystal 4. The change curve of the incident angle of the pump light before entering the first silver gallium sulfide crystal 4 with the generated mid-infrared photons of different wavelengths is shown as follows: Figure 3 By changing the polarization direction of the half-wave plate 2 to satisfy the type I phase matching condition of the first silver gallium sulfide crystal 4, high-efficiency spontaneous parametric down-conversion is achieved.

[0084] By rotating the angle of the first silver gallium sulfide crystal 4 translation stage, different phase matching angles in the first silver gallium sulfide crystal 4 are achieved. The corresponding relationship between the phase matching angle and the mid-infrared wavelength is as follows: Figure 4 As shown in (a), it can be seen from the figure that by changing the phase matching angle, mid-infrared photons and prediction photons of different frequencies can be generated. The 3-10μm mid-infrared photons and 929-1186nm prediction photons generated by spontaneous parametric down-conversion are collimated and output without chromatic aberration through the off-axis parabolic reflector 5, and the output angle is as follows Figure 3 (b) is shown. The dichroic mirror 6 is then used to separate the mid-infrared photons and the forecast photons in space, so that the mid-infrared photons are transmitted and the forecast photons are reflected. The forecast photons change the optical path through the reflective silver mirror 8 and are incident on the superconducting nanowire single photon detector 9 for detection.

[0085] Then, the generated 3-10μm mid-infrared photons pass through the sample to be tested 7 and are transferred to the near-infrared band through broadband frequency up-conversion. That is: the 1550nm pulse laser light path generated by the second pump light source 10 is changed by the dielectric film reflector 12, and the mid-infrared photons carrying the spectral information of the sample 7 are spatially combined using the dichroic mirror 13 (3-10μm mid-infrared photons are transmitted, 1550nm pump light is reflected). Here, the first pump light source 1 triggers the 1550nm pulse light generated by the second pump light source 10 through clock synchronization, and keeps synchronization with the output pulse of the first pump light source 1 in the time domain. Afterwards, the combined mid-infrared photons and the 1550nm pulse laser are focused together through the off-axis parabolic reflector 14 and incident into the second gallium silver sulfide crystal 15. The incident angles of the two beams are as follows: Figure 5 (a) shows that the frequency up-conversion process generates 1022-1342nm sum frequency photons, whose emission angle is as follows: Figure 5(b) In this process, the polarization direction of the half-wave plate 11 is changed to meet the type I phase matching condition of the second silver gallium sulfide crystal 15, thereby achieving high-efficiency frequency up-conversion. And by rotating the angle of the second silver gallium sulfide crystal 15 translation stage, different phase matching angles in the second silver gallium sulfide crystal 15 are achieved, such as Figure 4 (b) is shown, thereby realizing frequency up-conversion of different mid-infrared wavelengths. Afterwards, the sum frequency photons generated by the frequency up-conversion are collimated and output using an off-axis parabolic reflector 16 for subsequent detection. However, in the process of frequency up-conversion, not only sum frequency photons are generated, but also 1550nm pump light, pump light up-conversion fluorescence, ambient stray light, etc. are introduced, so they are filtered out by a 1000nm long-pass filter and a 1350nm short-pass filter group 17. The sum frequency photons after filtering out the noise photons are incident on a superconducting nanowire single-photon detector 18 for detection.

[0086] Finally, according to the different absorbances of the sample 7 to light intensities of different mid-infrared wavelengths, the time-correlated single photon counter 19 records the correlation detection of the sum frequency photons and the prediction photons, and collects the changes in the coincidence events caused by the angle changes of the two silver gallium sulfide crystals to achieve spectral measurement. When the sum frequency photons and the prediction photons reach the two detectors at the same time, a coincidence peak will be obtained at the zero delay position, forming a coincidence event, and the detected photon pairs will meet the time correlation and frequency correlation characteristics. Specifically, the entire experimental device measures the changes in coincidence events with and without samples at different mid-infrared wavelengths by simultaneously changing the phase matching angles of the first silver gallium sulfide crystal 4 and the second silver gallium sulfide crystal 15. Each time the phase matching angles of the two crystals are changed, the transmittance of the sample 7 to be tested at a certain wavelength can be obtained. By repeatedly changing the phase matching angles of the two crystals, the relationship between the coincidence light intensity of the sum frequency photon and the prediction photon and the phase matching angle of the two silver gallium sulfide crystals can be measured, which can be further converted into the relationship between the coincidence measurement light intensity of the sum frequency photon and the prediction photon and the wavelength, and the transmittance of the sample 7 to be tested at different wavelengths can be obtained, thereby realizing spectral detection. The relationship between the wavelength and the rotation angle is calibrated in advance, and the rotation angle is related to the phase matching angle. Only the rotation angle and the coincidence measurement results need to be recorded later.

[0087] The device of this scheme is simple, easy to understand, and easy to implement. Spectral splitting can be achieved only by rotating the crystal, eliminating splitting elements such as gratings, interferometers or long optical fibers. There is no need to introduce large devices such as spectrometers or monochromators, and there is no need to use mid-infrared detectors, which reduces the complexity and instability of the measurement process. Based on the method of spontaneous parametric down-conversion and frequency up-conversion of sulfur gallium silver crystals, the generation and conversion of ultra-wide-band mid-infrared photons are realized, breaking through the limitations of the bandwidth of traditional mid-infrared nonlinear spectral detection. Since the photon pairs generated by spontaneous down-conversion have quantum correlation characteristics, the interference of noise is effectively reduced. Ultra-wideband mid-infrared spectral measurement with high sensitivity, good stability and high resolution is achieved.

[0088] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for measuring ultra-wideband mid-infrared spectra based on quantum correlation, characterized in that: The following steps are involved: Step 1: inject pump light into the silver gallium sulfide crystal to achieve spontaneous parametric down-conversion, generating correlated photon pairs of ultra-wide-band mid-infrared photons and near-infrared forecast photons, where the forecast photons are detected by superconducting nanowire single-photon detectors; Step 2: Introduce a pump light beam that is time-domain synchronized with the pump light described in step 1 and has the same repetition frequency, and combine it with the ultra-wide-band mid-red photons that carry spectral information after passing through the sample and inject it into the silver gallium sulfide crystal to achieve frequency up-conversion, thereby generating sum-frequency photons in the near-infrared band, and using another superconducting nanowire single-photon detector for detection; Step 3: Mid-infrared spectral resolution is achieved by rotating two silver gallium sulfide crystals, and a time-correlated single photon counting module is used to achieve correlated spectral measurement of sum-frequency photons and predicted photons using coincidence measurement, thereby achieving 3-10μm ultra-wideband mid-infrared spectral measurement.

2. The method for measuring ultra-wideband mid-infrared spectra based on quantum correlation according to claim 1, characterized in that: The two silver gallium sulfide crystals can respectively realize the generation of a 3-10 μm mid-infrared light source and a 3-10 μm mid-infrared frequency up-conversion process by changing their respective phase matching angles.

3. The method for measuring ultra-wideband mid-infrared spectra based on quantum correlation according to claim 1, characterized in that: The spontaneous parametric down-conversion and frequency up-conversion processes both satisfy the laws of conservation of energy and conservation of momentum. For the spontaneous parametric down-conversion process: ω p =ω MIR +ω h , where ω p ,ω MIR and ω h are the angular frequencies of the pump light, mid-infrared photons, and predictor photons, respectively; and are the wave vectors of the corresponding light fields; for the frequency up-conversion process: ω p′ +ω MIR =ω SFG , where ω p′ ,ω MIR and ω SFG are the angular frequencies of the new pump light field, mid-infrared photons and sum-frequency photons, and They are angular frequencies ω p′ ,ω MIR and ω SFG The wave vector of the light field.

4. The method for measuring ultra-wideband mid-infrared spectra based on quantum correlation according to claim 1, characterized in that: The method uses 850nm pulsed laser to irradiate the first silver gallium sulfide crystal to realize spontaneous parametric down-conversion, generates an ultra-wideband mid-infrared quantum correlation light source, and realizes mid-infrared photons with a wavelength covering 3-10μm and forecast photons with a wavelength covering the 929-1186nm band by changing the phase matching angle of the crystal. During the spontaneous parametric down-conversion process, the angular frequency is ω p The first pump light is incident on the first silver gallium sulfide crystal to cause spontaneous parametric down-conversion, generating an angular frequency of ω MIR The mid-infrared photon with an angular frequency of ω h The predicted photon follows the conservation of energy and momentum in this process: ω p =ω MIR +ω h , in and They are angular frequencies ω p ,ω MIR and ω h The wave vector of the light field satisfies the phase matching condition. When , the phase matching angle θ of the angle phase matching in the spontaneous parametric down-conversion process can be obtained as: where λ p , MIR , h They are angular frequencies ω p ,ω MIR ,ω h The wavelength corresponding to the light field; n MIR and n h They are angular frequencies ω MIR and ω h The refractive index corresponding to the light field, n po and n pe They are angular frequencies ω p The refractive index of the ordinary light o and extraordinary light e corresponding to the light field can be further obtained by the phase matching angle θ under the condition of generating mid-infrared photons of different wavelengths. incident The curve of the change of; and the emission angles θ of mid-infrared photons and predicted photons of different wavelengths exit The change curve of the first silver gallium sulfide crystal is shown in Figure 1. By rotating the angle of the first silver gallium sulfide crystal, that is, changing the phase matching angle θ, the spontaneous parametric down-conversion process of different frequencies can be achieved. The method uses a broadband frequency up-conversion technology to combine broadband mid-infrared photons in a quantum correlation light source with a 1550nm pulse laser and then up-convert the frequency to a near-infrared band. In the broadband frequency up-conversion process, the second pump light source and the first pump light source are firstly triggered by a clock synchronization mode to keep their repetition frequency the same and time domain synchronized, and then the angular frequency is ω p′ The second pump light and the angular frequency are ω MIR The combined mid-infrared photons are incident on the second silver gallium sulfide crystal, and finally, the frequency up-conversion process generates an angular frequency of ω SFG The sum frequency photons still obey the conservation of energy and momentum during the frequency conversion process: ω p′ +ω MIR =ω SFG , in and They are angular frequencies ω p′ ,ω MIR and ω SFG The wave vector of the light field satisfies the phase matching condition. When , the phase matching angle θ′ inside the silver gallium sulfide crystal during the frequency up-conversion process is obtained as: where λ p′ , MIR and λ SFG They are angular frequencies ω p′ ,ω MIR and ω SFG The wavelength corresponding to the light field; n p′ and n MIR They are angular frequencies ω p′ and ω MIR The refractive index corresponding to the light field, n SFGo and n SFGe They are angular frequencies ω SFG The refractive index of the ordinary light o and extraordinary light e corresponding to the light field; The incident angle θ of the pump light and the mid-infrared photon in the frequency up-conversion process of mid-infrared photons of different wavelengths can be obtained by using the phase matching angle θ′. i ' ncident Change curve and frequency photon emission angle θ e ' xit By rotating the angle of the second silver gallium sulfide crystal, the phase matching angle θ′ of different frequencies is achieved, thereby realizing frequency up-conversion of different frequencies in the wavelength range of 3-10μm, and then realizing frequency resolution; By using the first superconducting nanowire single-photon detector to detect the predicted photons, an electrical signal is output to the time single-photon counter as a trigger signal. Mid-infrared photons of different wavelengths carrying the sample spectral information are converted to the near-infrared band by the second silver gallium sulfide crystal and then detected by the second superconducting nanowire single-photon detector, which also outputs an electrical signal to the time single-photon counter for coincidence measurement. By continuously changing the first silver gallium sulfide crystal and the second silver gallium sulfide crystal, 3-10μm ultra-wideband mid-infrared spectrum measurement can be achieved.

5. An ultra-wideband mid-infrared spectroscopy measurement system based on quantum correlation, characterized in that: The quantum-correlated ultra-wideband mid-infrared spectrum measurement system comprises: a first pump light source (1), a half-wave plate (2), a plano-convex lens (3), a first sulfur gallium silver crystal (4), an off-axis parabolic reflector (5), a dichroic mirror (6), a sample to be measured (7), a reflective silver mirror (8), a superconducting nanowire single-photon detector (9), a second pump light source (10), a half-wave plate (11), a dielectric film reflector (12), a dichroic mirror (13), an off-axis parabolic reflector (14), a second sulfur gallium silver crystal (15), an off-axis parabolic reflector (16), a filter group (17), a superconducting nanowire single-photon detector (18), and a time-correlated single-photon counter (19); The first pump light source (1) is an 850nm pulse laser with a repetition frequency of 40MHz and an output power of 1W. The light source is used as a spontaneous parametric down-conversion pump light source to generate correlated photon pairs of mid-infrared photons in the wavelength range of 3-10μm and forecast photons in the wavelength range of 929-1186nm. The repetition frequency is the same as that of the second pump light source (10) and is synchronized in the time domain through clock synchronization triggering. The half-wave plate (2) is an 850 nm half-wave plate, and the half-wave plate is used to change the polarization direction of the first pump light source (1) so that it satisfies the first type I phase matching condition of the first silver gallium sulfide crystal (4); The plano-convex lens (3) is an N-BK7 plano-convex lens, and its purpose is to focus the first pump light source (1) into the first silver gallium sulfide crystal (4) to achieve efficient spontaneous parametric down-conversion; the plano-convex lens has a focal length of 50 mm and a lens diameter of 25.4 mm; The first sulfur gallium silver crystal (4) is a spontaneous parametric down-conversion nonlinear medium, used to generate 3-10 μm mid-infrared photons and 929-1186 nm prediction photons, and its coating wavelength band covers 2.6-11 μm, and the cutting angle is θ=50° and The crystal has a size of 6 mm (length) × 6 mm (width) × 2 mm (thickness); and is placed on a rotating displacement platform with an accuracy of 1 mrad, and the angle of the displacement platform is changed to achieve different phase matching angles in the first gallium sulfide silver crystal (4), thereby achieving spontaneous parametric down-conversion of different frequencies; The off-axis parabolic reflector (5) is a 90° off-axis parabolic reflector coated with a silver film, and is used to collimate and output mid-infrared photons and forecast photons generated by spontaneous parametric down-conversion without chromatic aberration; the off-axis parabolic reflector has a reflection focal length of 50.8 mm and an off-axis diameter of 25.4 mm; The dichroic mirror (6) spatially separates the mid-infrared photons generated by spontaneous parametric down-conversion from the forecast photons. The dichroic mirror is a 2 μm short-wave pass dichroic mirror that reflects 3-10 μm mid-infrared photons and transmits 929-1186 nm forecast photons. The sample to be tested (7) is a biological tissue or a chemical material. The sample to be tested has different absorbances to light intensities of different mid-infrared wavelengths. The system measures the changes in coincidence events when there is no sample and when there is a sample at different mid-infrared wavelengths by simultaneously changing the phase matching angles of the first silver gallium sulfide crystal (4) and the second silver gallium sulfide crystal (15). The different transmittances of the sample to be tested (7) at different wavelengths can be obtained, thereby achieving spectral detection. The plane reflector (8) is a reflective silver mirror, which is used to change the direction of the light path, and the reflectivity of the reflective silver mirror to the forecast photons is more than 90%; The superconducting nanowire single-photon detector (9) realizes ultra-sensitive detection of the forecast photons generated by spontaneous parametric down-conversion, and its detection wavelength range covers 929-1190nm, the detection efficiency is 90%, and the dark count is less than 20cps; The second pump light source (10) is a 1550nm pulse laser, which can accept external electrical triggering of the first pump light source (1), has a repetition frequency of 40MHz, and an output power of up to 1W; the light source is used as a pump light source for frequency up-conversion to achieve frequency up-conversion of 3-10μm mid-infrared photons and generate sum frequency photons; The half-wave plate (11) is a 1550 nm half-wave plate, and is used to change the polarization direction of the second pump light source (10) so as to satisfy the first type phase matching condition of the second silver gallium sulfide crystal (15); The plane reflector (12) is a dielectric film reflector, which is used to change the direction of the light path, and the reflectivity of the dielectric film reflector to the forecast photons is above 99%; The dichroic mirror (13) spatially combines the 3-10 μm mid-infrared photons generated by spontaneous parametric down-conversion with the 1550 nm second pump light source (10); the dichroic mirror is a 2 μm long-wave pass dichroic mirror, which transmits the 3-10 μm mid-infrared photons and reflects the 1550 nm pump light; The off-axis parabolic reflector (14) is a 90° off-axis parabolic reflector coated with a silver film, and is intended to focus 3-10 μm mid-infrared photons and the second pump light source (10) in the second gallium silver sulfide crystal (15) without chromatic aberration, so as to achieve efficient frequency conversion. The off-axis parabolic reflector has a reflection focal length of 50.8 mm and an off-axis diameter of 25.4 mm. The second silver gallium sulfide crystal (15) is a frequency up-conversion nonlinear medium, used for converting 3-10 μm mid-infrared photons to near-infrared bands, and its coating band covers 2.6-11 μm, and the cutting angle is θ=50° and The crystal has a size of 6 mm (length) × 6 mm (width) × 2 mm (thickness), and is placed on a rotating displacement platform with an accuracy of 1 mrad. The phase matching angle in the second gallium sulfide silver crystal (15) is changed by changing the angle of the displacement platform, thereby realizing frequency up-conversion of different mid-infrared wavelengths. The off-axis parabolic reflector (16) is a 90° off-axis parabolic reflector coated with a silver film, and is used to collimate and output the sum frequency photons generated by frequency up-conversion without chromatic aberration, so as to facilitate subsequent detection. The off-axis parabolic reflector has a reflection focal length of 50.8 mm and an off-axis diameter of 25.4 mm. The filter set (17) is a combination of a 1000nm long-pass filter and a 1350nm short-pass filter, and the filter set is used for frequency up-conversion photon filtering to filter out 1550nm pump light, pump light up-conversion fluorescence, and ambient stray light; The superconducting nanowire single-photon detector (18) realizes ultra-sensitive detection of sum-frequency photons generated by frequency up-conversion, and its detection wavelength range covers 1020-1350nm, the detection efficiency is 70%, and the dark count is less than 100cps; The time-correlated single photon counter (19) is used to record the correlation detection of sum-frequency photons and forecast photons, collect the changes of coincidence events caused by the angle change (wavelength change) of two silver gallium sulfide crystals, with a maximum resolution of 1ps and a measurement time window of 65ns.

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