CN115389441A - High-precision mid-infrared spectrum detection method - Google Patents
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Abstract
The invention discloses a high-precision mid-infrared spectrum detection method, which comprises the following steps: the method comprises the steps that a nonlinear crystal based on a chirp polarization structure is used for realizing broadband frequency up-conversion, and broadband mid-infrared light is converted into a visible light wave band; a Virtual Imaging Phased Array (VIPA) is combined with a grating, and visible light wave band signal spectrums are respectively expanded in the vertical direction and the horizontal direction; the silicon-based camera with high efficiency and low noise is used for detection, and the spectrum detection of the mid-infrared broadband and high sensitivity is realized. The invention uses single-frequency narrow-linewidth continuous light as pumping light, and can realize high-fidelity frequency conversion, thereby maintaining the spectrum information of mid-infrared light; and the combination of a virtual imaging phased array and a grating is adopted, so that the high-precision spectrum detection of the up-conversion signal is realized. The invention can obtain wide spectrum coverage range and high spectral resolution, has the advantages of ultrahigh detection sensitivity and rapid data acquisition, and provides an effective means for realizing broadband, high-precision and high-sensitivity mid-infrared high-speed spectrum detection.
Description
Technical Field
The invention relates to the field of infrared spectrum detection, in particular to a high-precision intermediate infrared spectrum detection method.
Background
Mid-infrared spectroscopy is a well established analytical tool commonly used to characterize gases and liquids, as well as solid compounds and mixtures. The middle infrared region is called as a molecular fingerprint region and covers the basic vibration conversion energy levels of a plurality of molecules, the fundamental frequency absorption band of most organic compounds and inorganic ions appears in the middle infrared region, and the fundamental frequency vibration is the vibration with the strongest absorption in the infrared spectrum, so the region is widely used for substance trace analysis. In addition, mid-infrared spectroscopy has strong detection specificity, can accurately identify analytes, and can qualitatively and quantitatively determine the composition of analytes by analyzing the optical attenuation of different wavelengths through the analytes. Therefore, mid-infrared spectroscopy is widely used in important fields such as trace gas analysis, chemical and biological sensing, environmental monitoring, industrial process control, medical diagnosis, national defense safety, astronomical observation, and the like.
Mid-infrared spectroscopy techniques are generally based on dispersive spectroscopy detection or detection using Fourier Transform Infrared (FTIR). In the former, a grating or a prism and other dispersive elements are used for carrying out spatial separation on an incident spectrum, and then a linear array detector is used for detecting the mid-infrared spectrum. The formed dispersive spectrometer is mature in development and high in commercialization degree, the acquisition speed is greatly improved in recent years, however, due to the limitation of a grating preparation process technology for a long time, the grating groove density and the effective size of the grating are difficult to improve, the resolution of the spectrometer is usually about 0.1nm, and high-precision spectral measurement is difficult to achieve. In contrast, FTIR provides a highly accurate spectroscopic measurement by measuring the autocorrelation interferogram and obtaining an ir spectrum of absorbance or transmittance as a function of frequency or wavelength after fast fourier transform. However, FTIR is limited to mechanical scanning, and the acquisition time is usually long, and although high accuracy can be achieved, it is not suitable for fast acquisition, and it is difficult to achieve real-time high-accuracy spectral measurement. Therefore, the existing mid-infrared spectroscopy measurement technology still faces the mutual restriction between high resolution and fast spectrum formation, and a new spectrum technology is urgently needed to be developed so as to meet the requirements of innovative applications such as transient molecular spectroscopy, high-speed spectral imaging and the like on high-speed and high-resolution spectrum measurement.
In the aspect of detecting mid-infrared light, the existing mid-infrared spectrum technology has a great promotion space. The sensitivity of the mid-to-outer detectors is typically 3-4 orders of magnitude lower than the near-infrared and visible bands. The mid-infrared detector generally adopts a semiconductor material with a narrow band gap, such as indium antimonide (InSb) or mercury cadmium telluride (HgCdTe), because the band gap is narrow, the detector is easily affected by thermal noise and dark current, so that the background noise is very large, the sensitivity is limited, liquid nitrogen or stirling refrigeration is usually needed, and the complexity and the manufacturing cost of the detector are greatly increased. In addition, the mid-infrared array detector required by the rapid spectrum measurement generally adopts a linear array or area array form, and faces the bottlenecks to be solved, such as limited pixel points, high noise, low frame frequency and the like. Therefore, the development of a novel intermediate infrared detection mode has important significance for further enhancing the resolution, improving the spectrum forming rate and improving the sensitivity.
In recent years, a nonlinear frequency up-conversion technology provides a promising mid-infrared spectrum detection technology, the mid-infrared spectrum is up-converted into a near-infrared or visible light region through a nonlinear sum frequency process, and then a high-performance silicon-based detector is used for detection, so that the advantages of high sensitivity, fast frame frequency, multiple pixel points and the like of the silicon-based detector can be fully utilized, and the defects of the existing mid-infrared detector are overcome. However, the upconversion infrared spectrum technology is limited by phase matching, the conversion bandwidth is narrow, generally about 10-100nm, and the requirement of broadband mid-infrared spectrum detection is difficult to meet. In addition, the existing up-conversion spectrum detection usually uses a grating as a dispersion means, and the spectral resolution is only 1-10cm -1 And the magnitude is difficult to meet the requirement of medium infrared high-precision spectral resolution. In summary, the mid-infrared nonlinear up-conversion spectrum technology still faces the difficult problems of narrow conversion bandwidth and low resolution precision, and the mid-infrared spectrum detection with wide band, high resolution, high sensitivity and high speed is still the mid-infrared precise lightA major challenge in the field of spectroscopy.
Disclosure of Invention
The invention aims to provide a high-precision mid-infrared spectrum detection method for solving the defects of the prior art, and the method uses a single-frequency narrow-linewidth continuous optical pump to realize high-fidelity frequency conversion so as to keep mid-infrared spectrum information; by combining an optical external cavity enhancement technology, the remarkable improvement of the continuous pumping light power is obtained, the nonlinear quantum conversion efficiency is improved, and the detection signal-to-noise ratio is improved; the nonlinear crystal based on the chirp polarization structure is adopted to realize broadband frequency up-conversion, so that broadband mid-infrared signals are effectively up-converted to visible light wave bands; the silicon-based detector is used for detecting the up-conversion signal, so that the mid-infrared spectrum detection with high frame frequency, high efficiency and low noise is realized; the spectrum of the up-conversion light field is accurately mapped to a two-dimensional space by adopting a spectrum dispersion technology combining a Virtual Imaging Phased Array (VIPA) and a grating, a camera detection array is fully utilized, and finally ultra-high precision mid-infrared spectrum detection is realized.
In order to achieve the purpose, the invention adopts the following technical scheme:
a high-precision mid-infrared spectrum detection method is used for realizing the fast, high-sensitivity, high-resolution and high-precision mid-infrared spectrum detection of a mid-infrared broadband, and comprises the following steps:
step 2: the spectrum of the up-converted visible light wave band signal is dispersed in the vertical direction by a virtual imaging phased array, namely VIPA, and then is expanded in the horizontal direction by a diffraction grating to obtain a visible light two-dimensional space intensity distribution light field;
and step 3: the obtained visible light two-dimensional space intensity distribution light field is captured by a silicon-based camera with excellent performance, a one-dimensional spectrum is obtained by reading the intensity of each pixel, the spectral intensity of a sample is divided by the spectral intensity of a sample to finally obtain a transmittance curve of mid-infrared light passing through the sample, and mid-infrared high-speed spectrum detection with broad band, high resolution and high sensitivity is realized.
In the step 1, frequency up-conversion is adopted, so that mid-infrared light can be up-converted to a visible light wave band, and the defects of large dark noise, low-temperature refrigeration and the like of the existing mid-infrared detector are avoided.
In the step 1, the chirped polarized crystal is used as a nonlinear medium, so that efficient conversion of broadband mid-infrared light is obtained, and meanwhile, the sensitivity to parameters such as a light beam incident angle and a crystal working temperature can be reduced, so that the system robustness is improved.
In the step 1, single-frequency narrow-linewidth continuous light is used as pumping light, and because the linewidth is narrow, the frequency corresponding relation between the input mid-infrared signal light and the up-conversion visible light can be kept in the frequency conversion process, so that the influence of the pumping bandwidth on the mid-infrared spectral resolution is avoided.
In the step 1, cavity enhancement is used, the power of the single-frequency narrow-linewidth continuous light is improved by several orders of magnitude, and a high-quality Gaussian beam can be formed in the cavity, so that the conversion efficiency of nonlinear frequency up-conversion is improved.
The spectral resolution method combining the Virtual Imaging Phased Array (VIPA) and the grating used in the step 2 has a plurality of potential advantages compared with the common diffraction grating, including large angular dispersion, low polarization sensitivity, simple structure, low cost and compactness, and the combination of the VIPA and the grating can realize higher-precision spectral detection.
The broadband mid-infrared light source is generated in a mode that: optical parametric oscillators, mid-infrared difference frequencies, supercontinuum generation, and thermal light sources.
The sample to be detected comprises gas, liquid, medical slices and biological samples.
The invention has the beneficial effects that:
(1) The invention adopts a nonlinear frequency up-conversion technology, overcomes the problems of low sensitivity, low refrigeration temperature, limited number of pixels and the like of the existing mid-infrared detection array, utilizes a multi-pixel silicon-based camera with excellent performance, has detection sensitivity reaching a single photon level and imaging frame frequency reaching million frames per second, and provides an effective way for realizing high-speed and high-sensitivity mid-infrared spectrum detection.
(2) The nonlinear crystal with the chirp polarization structure is adopted, so that infrared signals can be matched to different inversion periods, the phase matching bandwidth can be remarkably expanded, and the tolerance on the incident light angle and the working temperature of the crystal is improved; the single-frequency laser pumping technology combined with the optical external cavity enhancement can realize high-fidelity spectrum information wavelength conversion and improve the spectrum detection efficiency.
(3) The invention adopts a two-dimensional dispersion technology combining a Virtual Imaging Phased Array (VIPA) and a grating, the angular dispersion of the VIPA is 30 to 40 times of that of the grating, extremely high spectral resolution is provided, meanwhile, the problem of VIPA spectrum order overlapping is solved by using the dispersion effect of the grating in the orthogonal direction, two-dimensional mapping of the spectrum can be realized, and the high dispersion of the VIPA is combined with the wide spectrum detection range characteristic of the diffraction grating, so that ultra-high precision mid-infrared spectrum detection is realized.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a two-dimensional spatial intensity distribution plot of a visible light field on a silicon-based camera plane;
FIG. 3 is a schematic diagram of an embodiment of a method of the present invention.
Detailed Description
The features of the present invention and other related features are described in further detail below by way of example in connection with the accompanying drawings for the purpose of facilitating understanding by persons skilled in the art:
FIG. 1 is a flow chart of the present invention, which is based on a chirp quasi-phase matching broadband nonlinear optical parametric conversion technique, and realizes broadband frequency up-conversion by a nonlinear crystal of a chirp polarization structure, so as to up-convert broadband mid-infrared light to visible light bands; the single-frequency narrow-linewidth continuous light is used as a pumping source, so that the frequency one-to-one correspondence relationship between the mid-infrared signal and the up-conversion light can be realized, and the high-fidelity mid-infrared spectrum information mapping is obtained; by combining the optical external cavity enhancement technology, the average power of continuous pump light can be remarkably improved, and the nonlinear conversion efficiency is further improved; and finally, a two-dimensional dispersion module consisting of a VIPA and a grating accurately disperses the up-conversion spectrum in the vertical and horizontal directions respectively, the obtained two-dimensional spatial intensity distribution is captured by a silicon-based camera with excellent performance, and finally, the broadband, high-resolution and high-sensitivity mid-infrared high-speed spectrum detection is realized.
In the frequency up-conversion process, the frequency omega of the signal light is according to the conservation of energy S Pump light frequency omega p Up-conversion optical frequency omega u The following relation is satisfied:
ω p +ω S ＝ω u
because the single-frequency narrow linewidth continuous light is used as the pumping light, the linewidth is narrow, thereby satisfying the following relation:
Δω S ＝Δω u
in the process of frequency conversion, the one-to-one correspondence relationship between the frequencies of the input mid-infrared signal light and the up-conversion visible light can be kept, the influence of the bandwidth of the pump light on the mid-infrared spectrum resolution is avoided, and therefore high-fidelity mid-infrared spectrum information mapping is obtained.
In the technology for realizing broadband frequency up-conversion by the nonlinear crystal based on the chirped polarization structure, in order to relieve uneven spectral response and angle-dependent phase matching, compared with the use of a periodically polarized nonlinear crystal, the chirped polarization structure in the nonlinear crystal can lead to wider and more uniform spectral response, the flatness of spectral conversion is maintained, the chirped polarization structure nonlinear crystal also eliminates the need for inclined mid-infrared beams, and the design of a spectrometer after up-conversion is simplified, because all up-converted light is collinear. In a nonlinear crystal of a chirped polarization structure, the polarization period is linearly reduced by the length of the crystal, so that different mid-infrared wavelengths achieve phase matching at different positions.
The invention uses the combination technology of VIPA and grating to form a high-precision spectral resolution detector, light enters the VIPA etalon to be reflected back and forth, transmitted light looks like coming from a phased array light source, and interference of the light source array causes large-angle dispersion of light transmitted from the VIPA etalon. A diffraction grating is placed behind the etalon and disperses the transmitted beam into a 2D pattern, with each spatial element corresponding to a unique frequency. The 2D spatial pattern is then imaged onto a CCD camera to record the intensity distribution of the frequency resolved image. Light dispersed along the tilt direction of the VIPA etalon provides high spectral resolution, while light dispersed along the orthogonal direction by the grating is used to resolve different mode orders transmitted through the VIPA etalon.
This equation describes the two-dimensional spatial intensity distribution of the visible light field in the plane of the silicon-based camera. Since the grating dispersion and VIPA dispersion are orthogonal, the above equation is a simple product of the grating intensity distribution (x-direction) and the VIPA intensity distribution (y-direction). x and y represent the abscissa and ordinate of the two-dimensional spatial intensity distribution of the light field, ω is the frequency of the incident light, I in Is the intensity of the incident light, f c And f is the focal length of the cylindrical lens and the imaging lens, τ is the incident beam radius, τ 0 Is the diffraction limited spot size, R, of incident light in the plane of a silicon-based camera 1 And R 2 High reflectance of the incident surface and partial reflectance of the exit surface, respectively, k =2 pi/λ is the wave number of the light beam,
ω 0 is the frequency of illumination to the center of the silicon-based camera;
where λ is the wavelength of the incident light, c is the speed of light in vacuum, d is the line spacing of the grating, θ dg The diffraction angle of light incident on the grating, and most of the physical phenomena determining the dispersion of VIPA are included in the parameter deltaThe method comprises the following steps:
where t is the thickness of the VIPA, n r Is the refractive index, theta iv Is the angle of incidence, θ, of the focused beam on the VIPA ν Is the propagation angle of the light beam within the VIPA.
High-precision spectrometers formed by combining VIPA and gratings generally use a characteristic wavelength calibration method to achieve spectral calibration. For example, 800nm continuous light with a bandwidth of about 1kHz is used as a characteristic spectral component, the characteristic spectral component is mixed with up-converted signal light, the signal light is subjected to VIPA and grating light splitting and then captured by a silicon-based CCD, as shown in fig. 2, the wavelengths represented by two light spots with high brightness circled in the figure are both 800nm, the vertical distance represents an FSR, the accurate position of the frequency on the image is determined by extracting the peak pixel point of the light spot, the spectral resolution of the system can be measured by the pixel point corresponding to the continuous light and the full width at half maximum of an intensity curve, each light spot on the plane of the silicon-based camera corresponds to 5 pixel points, and the one-to-one correspondence between the positions and the wavelengths on the two-dimensional area-array CCD can be obtained by calculating the difference between the values of the remaining spectral components and the characteristic components. The VIPA in the present invention has a thickness of 3mm, a refractive index n =1.5, and then according to FSR = c/2nt, the free spectral range is 33.3GHz, and the grating constant d =1/1200mm, the spectral resolution of the system can be expressed as:
R 1 、R 2 high reflectance of the incident surface and partial reflectance of the exit surface, R 1 ＝98.5％，R 2 =96%, the resolution of the system can reach 0.6GHz (0.02 cm) -1 )。
The invention uses the multi-pixel silicon-based camera with excellent performance to detect the up-conversion spectrum after light splitting, the selected camera pixel size is smaller than the diffraction limit spot size of the focused light beam on the camera as much as possible, the influence of the pixel size on the resolution ratio of the system is reduced, and the high precision of the system is fully shown. By selecting the silicon-based EMCCD camera, the sensitivity of the system detection spectrum can realize single photon response, and if the silicon-based CMOS camera is selected, the imaging frame frequency can reach million frames per second, so that the realization of the detection of the mid-infrared spectrum with single photon sensitivity and MHz frame frequency is facilitated.
Examples
Fig. 3 is a schematic diagram of an embodiment for implementing a high-precision mid-infrared spectrum detection method, and the system includes a broadband mid-infrared light source 1, a plano-convex lens 2, a sample 3 to be detected, a plano-convex lens 4, a dichroic mirror 5, a pumping light source 6, a concave mirror 7, a chirped and polarized lithium niobate crystal 8, a concave mirror 9, a piezoelectric ceramic actuator 10, a feedback control system 11, a dichroic mirror 12, a lock detector 13, a plano-convex lens 14, a filter 15, a cylindrical lens 16, a VIPA17, a grating 18, a plano-convex lens 19, a silicon-based camera 20, and an image processing computer 21.
The broadband mid-infrared light source 1, which may be an active light source, is generated by a supercontinuum, such as: waveguide, soft glass fiber, etc., or passive light source such as thermal light source, etc., with common wavelength range of 2.5-5 μm.
The plano-convex lens 2 is a calcium fluoride lens, aims to collimate intermediate infrared signal light generated by a silicon nitride waveguide, has relatively high transmittance for an intermediate infrared light source, and has a focal length of 100mm and a lens diameter of 50.8mm.
The sample 3 to be detected, i.e. the target to be detected, includes but is not limited to: gases, liquids, biological tissue cells, and the like. The detection target has different light absorptivities to different wavelengths, and the experimental system can obtain the absorptivities of each pixel of the detected target to each wavelength by measuring the imaging results of the samples without or with.
The plano-convex lens 4 is a calcium fluoride lens and aims to focus a mid-infrared wide-spectrum light source passing through a target to be detected into a nonlinear frequency up-conversion medium so as to realize efficient frequency conversion. The focal length of the plano-convex lens is 50mm, and the lens diameter is 50.8mm.
The dichroic mirror 5 is used for spatially combining the infrared wide-spectrum light source passing through the target to be detected and the pumping light source, so that a subsequent conversion signal on a broadband frequency can be generated conveniently. The dichroic mirror is a 2.4-micron long-wave-pass dichroic mirror, and is used for filtering components below 2.4 microns in signal light while spatially combining beams so as to prevent the wavelength components from being mixed with up-converted signal components.
The pump light source 6 is high-power 1-micron continuous laser, the output power of the pump light source can reach 10W, and the pump light source is used as pump light of a broadband frequency up-conversion part to realize effective conversion of a wide-spectrum infrared light source.
The concave mirror 7 is a calcium fluoride lens, and aims to enhance the oscillation of a 1-micron pump light source in a cavity, thereby improving the conversion efficiency on broadband frequency. It has high transmittance to 2.5-5 μm and 0.7-0.9 μm, and has a reflectance to 1 μm of 97%.
The chirped and polarized lithium niobate crystal 8 is used as a nonlinear frequency up-conversion medium to complete the effective conversion of a wide-spectrum infrared light source. The polarization period of the chip covers 16-24 μm, and the chirp polarization stepping length is 0.01mm. The crystal size was 5mm (length) × 3mm (width) × 1mm (thickness). The chirped crystal polarization period used in this embodiment can achieve 2.5-5 μm infrared signal conversion under 1 μm high power laser pumping.
The concave mirror 9 is a calcium fluoride lens, and aims to enhance the oscillation of a 1-micron pump light source in a cavity, so that the conversion efficiency on broadband frequency is improved. It has high transmittance to 2.5-5 μm and 0.7-0.9 μm, and has a reflectance to 1 μm of 97%.
The piezoelectric ceramic actuator 10 can perform mechanical displacement according to an electrical signal provided by a feedback system to compensate the cavity length and realize long-time stable operation of the resonant cavity, and has the characteristics of small volume, high displacement resolution, high response speed, low-voltage driving, large output force and the like.
The feedback control system 11 is a high-speed intelligent acquisition and control system, and performs digital bandwidth feedback and programming control on the piezoelectric ceramic actuator 10 by receiving an error signal of the locking detector 13, so as to realize high-precision locking of the resonant cavity length and long-time stable operation of the resonant cavity.
The dichroic mirror 12 is designed to separate the 1 μm pump light output from the cavity from the upconverted signal light and to transmit the 1 μm pump light to the lock detector 13 to obtain the intra-cavity error signal. The dichroic mirror is a 0.95 μm long-wave pass dichroic mirror.
The lock detector 13 is used for recording the error signal in the cavity through the pump light with the diameter of 1 μm output in the cavity and transmitting the error signal to the feedback control system.
The plano-convex lens 14 is a calcium fluoride lens for spatially collimating the up-converted signal light, and has a focal length of 50mm and a lens diameter of 50.8mm.
The filter 15 is a band-pass filter, and the transmission wavelength of the filter is 700-900nm. The filter is used for up-conversion signal filtering and filtering high-power 1 mu m pump light, up-conversion fluorescence of the pump light, environmental stray light and the like.
The cylindrical lens 16, which is an achromatic cylindrical lens, can focus light in the same dimension, and has a focusing wavelength range of 700-900nm, and is used for focusing light into the VIPA through an anti-reflection coating groove in an R =100% coating of the VIPA.
The virtual imaging phased array 17, vipa itself is a tilted etalon disperser, made of Si. With a reflectivity of 96% and an angle of incidence of 3 deg., a thickness of 3mm and an fsr of 33.3GHz.
The grating 18 has a grating constant d =1/1200mm, and the size of the reflecting surface is 25 × 25mm 2 Light scattered by the grating in the orthogonal direction is used to resolve different mode orders transmitted through the VIPA etalon.
The plano-convex lens 19 is an achromatic lens coated with an antireflection film and used for collimating light split by the grating and projecting the collimated light onto a silicon-based camera, and has a focal length of 50mm and a lens diameter of 50.8mm.
The silicon-based camera 20 is a silicon-based CMOS camera and is designed to collect a two-dimensional image obtained by splitting a VIPA and a grating and convert an optical signal into a digital signal, the pixel size is 8.3 μm, the pixel array size is 2048 × 2048, and the maximum quantum conversion efficiency is greater than 95%.
The image processing computer 21 is intended to process a two-dimensional image acquired by a camera and convert the two-dimensional image into one-dimensional spectral information.
The following is a specific procedure of the example:
firstly, a broadband mid-infrared light source of a sample to be detected and pump light after cavity enhancement generate sum frequency in a nonlinear crystal of a chirp polarization structure to generate visible light. Specifically, 1550 μm light is collimated by a broadband mid-infrared light source 1 generated by waveguide through a plano-convex lens 2, passes through a sample 3 to be measured, is focused by a plano-convex lens 4 after acquiring spectral information of the sample, passes through a dichroic mirror 5, and is spatially combined with a high-power narrow-linewidth 1 μm pump light source 6 to enter a chirped and polarized lithium niobate crystal 8. In order to enable the conversion efficiency of the broadband frequency up-conversion process to be higher, piezoelectric ceramics (PZT) is adhered beside the concave mirror 9, 1 micron pump light is oscillated back and forth through the concave mirror 7 and the concave mirror 9 to form a resonant cavity, the output pump light is separated from the up-conversion light through the dichroic mirror 12, and the pump light is transmitted to the locking detector 13. The locking detector 13 records an intra-cavity error signal through the 1 μm pump light output from the cavity and transmits the intra-cavity error signal to the feedback control system 11, so as to perform digital bandwidth feedback and programming control on the piezoelectric ceramic actuator 10, thereby realizing high-precision locking of the resonant cavity length and long-time stable operation of the resonant cavity. The sum frequency of the 2.5-5 μm signal light and the cavity-enhanced 1 μm pump light occurs in the chirped and polarized lithium niobate crystal, and the spectrum range after upconversion is 714-877nm.
Then, the up-conversion light field is detected by a silicon-based camera after being split by the VIPA and the grating. Specifically, the upconversion light field is collimated by the convex lens 14, and then passes through the band-pass filter 15 to filter out high-power 1 μm pump light, pump light upconversion fluorescence, environmental stray light, and the like. The upconversion light passes through a cylindrical lens 16, so that the light is focused on the same dimension and enters a VIPA17, visible light is spread in the vertical direction due to the dispersion of the VIPA17, then a periodically aliased spectrum is spread in the horizontal direction through a diffraction grating 18, the spectrum dispersion of two spatial dimensions is formed, the visible light is collimated through a plano-convex lens 19 and detected by a silicon-based CMOS camera 20, each pixel contains unique spectrum information, the intensity of each pixel is read through an image processing computer 21 to obtain a traditional one-dimensional spectrum, and the spectrum intensity of a placed sample is divided with the spectrum intensity of a non-placed sample to finally obtain a transmittance curve of medium infrared light passing through the sample.
Because the pump light is continuous light with single frequency and narrow line width, the influence of the pump light on the spectral resolution can not be considered, the spectral coverage range of the combination of the VIPA and the grating is 700-900nm, and the actual resolution of the system can reach 0.6GHz (0.02 cm) -1 ) Compared with the current mature commercial grating spectrometer, the resolution is improved to pm magnitude from about 0.1 nm; as the silicon-based CMOS camera is used, the frame frequency of shooting can reach MHz, however, generally, the time of about one minute is needed for the commercial FTIR to reach the accuracy of pm; in order to improve the sensitivity of the system, the used camera can be replaced by a silicon-based EMCCD camera, so that the sensitivity of the system for detecting the spectrum can realize single photon response, and compared with the situation that the mercury cadmium telluride detector can only detect nW-level optical power, the sensitivity of the system is obviously improved. Therefore, the invention is expected to realize the mid-infrared spectrum detection of single photon sensitivity, MHz frame frequency and pm-level mid-infrared spectrum resolution.
The high-precision mid-infrared spectrum detection method based on the method has the advantages of wide spectrum, high spectral resolution, high frame frequency, high sensitivity and the like, and the system has superior performance and relatively low cost because the detection technology of visible light is mature and related devices are low in manufacturing cost.
Details not described in this specification are within the skill of the art that are well known to those skilled in the art. Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, and not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (5)
1. A high-precision mid-infrared spectrum detection method is characterized by comprising the following steps:
step 1; the broadband mid-infrared light source irradiates a sample to be detected to obtain infrared absorption spectrum information of the sample, single-frequency narrow-linewidth continuous pump light is repeatedly superposed in the optical cavity, and the average power of the pump light is effectively improved through cavity enhancement; nonlinear sum frequency occurs between the broadband mid-infrared light and the cavity-enhanced pump light in the nonlinear crystal based on the chirped polarization structure, so that a mid-infrared signal is converted into a visible light wave band;
step 2: the spectrum of the converted visible light wave band signal is dispersed in the vertical direction by a virtual imaging phased array, namely VIPA, and then is expanded in the horizontal direction by a diffraction grating to obtain a visible light two-dimensional spatial intensity distribution light field;
and step 3: the obtained visible light two-dimensional space intensity distribution light field is captured by a silicon-based camera, a one-dimensional spectrum is obtained by reading the intensity of each pixel, the spectrum intensity of a sample is divided by the spectrum intensity of a sample, a transmissivity curve of mid-infrared light passing through the sample is obtained, and mid-infrared high-speed spectrum detection with wide band, high resolution and high sensitivity is realized.
2. The method according to claim 1, wherein the step 1 of up-conversion employs a chirped and polarized crystal as a nonlinear medium to obtain a high efficiency conversion of broadband mid-infrared light, so that the mid-infrared light is up-converted to a visible light band; the single-frequency narrow-linewidth continuous light is used as the pump light, the linewidth is narrow, and the frequency correspondence of the input mid-infrared signal light and the up-conversion visible light can be kept in the frequency conversion process.
3. The method according to claim 1, wherein the cavity in step 1 is enhanced, the power of the single-frequency narrow-linewidth continuous light is increased by several orders of magnitude, a high-quality gaussian beam can be formed in the cavity, and the conversion efficiency of nonlinear frequency up-conversion is improved.
4. The method according to claim 1, wherein the broadband mid-ir light source is generated by: optical parametric oscillators, mid-infrared difference frequencies, supercontinuum generation, and thermal light sources.
5. The method according to claim 1, wherein the sample to be detected comprises a gas, a liquid, a medical slice, and a biological sample.
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