CN115308154A - Medium-infrared hyperspectral imaging method based on single photon time-frequency correlation - Google Patents
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Abstract
The invention provides a middle infrared hyperspectral imaging method based on single photon time-frequency correlation, which is characterized in that a method of nondegenerate entangled photon time-frequency correlation imaging and broadband frequency conversion is adopted to realize ultrasensitive middle infrared spectral imaging under the condition of extremely weak illumination, and the method specifically comprises the following steps: the method comprises the steps of generating a nondegenerate entangled photon pair, converting the nondegenerate entangled photon pair into a visible/near infrared band, enabling the time of an up-conversion signal photon reaching a camera to be in a camera acquisition gate width, setting different idler frequency light filtering wavelengths, enabling an ultrafast gate width camera to acquire a corresponding spectrum image, carrying out scale correction through an image scaling factor, and comparing the measurement results of the existence of an imaging object to obtain the absorption rate of the imaging object to each infrared wavelength. Compared with the prior art, the method has the advantages of single photon sensitivity, high spectral resolution and the like, and through spontaneous parametric down-conversion and broadband frequency up-conversion, the generation of wide wavelength range nondegenerate entangled photon pairs, the up-conversion of broadband mid-infrared photons and sensitive imaging are realized.
Description
Technical Field
The invention relates to the technical field of hyperspectral imaging, in particular to a mid-infrared hyperspectral imaging method based on single-photon time-frequency correlation.
Background
The hyperspectral imaging is a multidimensional information acquisition means, combines an imaging technology and a spectrum technology, has the important characteristic of map integration, and an acquired map image data cube simultaneously contains contour information and spectrum information of a target object. In particular, the spectrum domain of the mid-infrared band is wide, the spectrum contains vibration-energy conversion level transition characteristic spectral lines of a plurality of important molecules, the spectrum imaging of the band can realize the fingerprint identification of a target object, and a plurality of application scenes have urgent needs for realizing sensitive hyperspectral imaging based on mid-infrared weak signals. For example, in cancer cell pathology analysis imaging, in order to reduce phototoxicity of the detection signal to the sample, it is generally necessary to image the cells under very low light conditions; when airborne laser surveys the ground, signal beams are seriously attenuated through long-distance transmission, and effective information is generally required to be obtained under the condition of extremely low signal-to-noise ratio; in some ceramic material compositional analyses, the illumination source power requirements are extremely low in order to avoid damage to the surface photocatalytic/photodegradable material. The development of an ultra-sensitive mid-infrared hyperspectral imaging technology is always an international research hotspot, and the advantages of non-destructiveness, low phototoxicity and the like provide powerful support for medical diagnosis, environmental exploration, biological tissue detection and other applications.
The mid-infrared hyperspectral imager in the prior art is limited by an infrared detector and a spectral dispersion light splitting technology, and the acquisition sensitivity and the spectral resolution of a mid-infrared spectrum are difficult to be considered at the same time. In the aspect of infrared detectors, detectors for infrared hyperspectral imaging usually adopt semiconductor materials with narrow band gaps (such as mercury cadmium telluride and indium antimonide), have large intrinsic dark noise, need complex and expensive low-temperature refrigeration devices, and are far less than silicon-based devices in visible light bands in terms of core performance indexes such as pixel number, pixel size and working frame frequency, so that the aspects of infrared imaging sensitivity, spatial resolution and the like are greatly limited. In the aspect of spectral imaging technology, the scanning spectral imaging technology generally uses a grating as a light splitting means to complete two-dimensional spatial imaging in cooperation with a scanning device, and overlapping of grating multi-level diffraction can interfere spectral line acquisition and restrict spectral resolution; the snapshot type spectral imaging technology can acquire two-dimensional image information at one time through an area array detector, but the spectral information is acquired by a narrow-band filter band by band, the number of pixels of a mid-infrared area array detector is small, and the detection sensitivity is low; the chip coating method realizes hyperspectral imaging by respectively coating filtering films with different wave bands on the detector pixels, and the spectral resolution is low and is usually more than ten nanometers; fourier-infrared spectroscopy imaging techniques can achieve higher spectral resolution by scanning a long-range interferometric arm, but often at the expense of scan rate and system size. Therefore, in order to realize the ultra-sensitive and high-resolution spectral imaging result and expand and push the result to wider application, a new mid-infrared hyperspectral imaging technology is certainly required to be developed.
In recent years, a novel method is provided for mid-infrared hyperspectral imaging by a broadband frequency upconversion technology. Based on the chirped polarization nonlinear crystal, the intermediate infrared image/spectrum information is converted into a visible/near infrared band by means of a sum frequency process, and then a silicon-based device with excellent performance is fully utilized to realize high-sensitivity detection. When the signal light radiated in unit area in the imaging scene is extremely weak, the number of signal photons reaching the detection surface from an object is as low as single photon magnitude, and the signal photons are highly coupled with noise photons in the environment, how to accurately discriminate the signal-noise photons under extremely low signal-to-noise ratio and realize image reconstruction and spectrum detection becomes a key technical problem in the ultra-sensitive mid-infrared up-conversion hyperspectral imaging scheme.
Disclosure of Invention
The invention aims to provide a single photon time-frequency correlation-based intermediate infrared hyperspectral imaging method, which aims at overcoming the defects of the prior art, adopts a non-degenerate entangled photon to time-frequency correlation imaging method, utilizes a broadband frequency conversion technology to realize broadband, high-precision and ultrasensitive intermediate infrared hyperspectral imaging, combines ICCD (integrated computer system compact disc) ultrafast door width to accurately discriminate signal noise photons under an extremely low signal-to-noise ratio, effectively filters background noise irrelevant to an illumination light source in a time domain, greatly improves detection sensitivity, is favorable for realizing ultrasensitive intermediate infrared imaging under an extremely weak illumination condition, and breaks through the limitations on imaging waveband, sensitivity, signal-to-noise ratio and spectral resolution of the traditional imaging scheme. The method is based on a nonlinear crystal of a chirp polarization structure, realizes conversion under spontaneous parameters, generates a non-degenerate entangled photon pair in a wide waveband range, widens the detection wavelength to a middle infrared waveband, adopts an adjustable filter to select the wavelength of an idler photon, utilizes a mature light splitting device of a near infrared waveband to improve the spectral resolution, combines a correlated photon trigger imaging technology, effectively inhibits background noise by utilizing the time-frequency correlation characteristic of the entangled photon, extracts an infrared signal, and has the advantages of simplicity and convenience, super sensitivity, high resolution and the like, and has wide application prospect.
The specific technical scheme for realizing the aim of the invention is as follows: a middle infrared hyperspectral imaging method based on single photon time-frequency correlation is characterized in that a non-degenerate entangled photon is adopted to image the time-frequency correlation, a broadband frequency conversion technology is utilized, and ultra-sensitive middle infrared spectral imaging under the condition of extremely weak illumination is realized, and the method specifically comprises the following steps:
step 1: based on the nonlinear crystal with the chirp polarization structure, spontaneous parametric down-conversion is realized under narrow-band pumping, a nondegenerate entangled photon pair of mid-infrared signal photons and idler photons is generated, and the detection wavelength is widened to a mid-infrared band.
And 2, step: the intermediate infrared signal photons acquire spectral information through an object to be imaged, are converted into visible/near infrared bands through broadband frequency, and ultra-sensitive associated spectral imaging is realized by utilizing a silicon-based ultra-fast gate width ICCD.
And step 3: the tunable filter is adopted to select the wavelength of the idler photons, the spectral resolution is improved by utilizing the light splitting device with mature near-infrared waveband, the idler photons are detected by the indium gallium arsenic detector after being selected by the wavelength of the tunable filter, the electric signal output by the indium gallium arsenic detector is used as a camera shutter trigger signal, and the delay module is adjusted, so that the time for the up-conversion signal photons to reach the camera is within the camera acquisition gate width.
And 4, step 4: and setting different idle frequency filtering wavelengths to enable the ultrafast door wide camera to acquire a corresponding spectral image, performing proportional correction through an image scaling factor, and comparing the measurement results of the imaging object to obtain the absorption rate of the imaging object to each infrared wavelength.
Under the pump light with fixed wavelength, the chirp polarization structure has larger phase matching bandwidth compared with a periodic polarization structure, is favorable for generating a non-degenerate entangled photon pair with a wide wavelength range, and realizes spontaneous parametric down-conversion.
The narrow-band pump is a continuous laser or a pulse laser, and preferably, a narrow-linewidth continuous laser is used as a pump light source in consideration of the influence of the bandwidth of the pump light source on the high spectral imaging spectral resolution.
The tunable filter selects the wavelength of the idler frequency near-infrared photons, and makes full use of a light splitting device with mature near-infrared band, so that the subsequent correlation spectrum imaging is realized in the mid-infrared wavelength range, the resolution of the existing near-infrared tunable filter can reach 0.1nm, and the corresponding mid-infrared photon spectrum resolution is 0.4cm -1 。
The invention adopts an ultra-fast door width enhanced camera as a detector, and takes an electric signal output by an indium gallium arsenic detector as external trigger to control a shutter switch. And a time delay module is adopted for adjusting the time of the trigger signal reaching the camera so as to realize the time consistency of the associated spectrum detection.
The broadband frequency up-conversion realizes the wavelength conversion of the intermediate infrared signal photons, and further realizes detection imaging by utilizing a sensitive silicon-based single photon camera. The absorption rate of each position of the object to be imaged to the non-transmission wavelength is different, and the spectral intensity R when the object is passed or not passed c(x,y) Dividing to obtain a filter frequency omega f And adjusting the filtering frequency of the filter to realize mid-infrared hyperspectral imaging on the interested area. In the process, the conversion rate and the detectivity of the conversion detection system to each wavelength are divided by data without influencing the hyperspectral imaging absorptivity calibration result.
The pair of nondegenerate entangled photons is subjected to time-frequency correlation imaging, idler photons and intermediate infrared signal photons are generated in pairs, and the transmission time and the frequency of the idler photons have correlation consistency. In single acquisition, the filter wavelength of the tunable filter is fixed, and after the idler photons are detected, the signal photons are collected within a preset camera acquisition gate width, and the image measured by the camera is a monochrome image. The filtering wavelength of the tunable filter is adjusted band by band to acquire data, and spectral absorption information of each position of an imaging object is obtained by comparing spectral imaging results of the imaging object. The correlation imaging method effectively inhibits the statistical noise irrelevant to the illumination light source in the time domain, greatly improves the signal-to-noise ratio of spectral imaging, and is beneficial to realizing the ultra-sensitive mid-infrared hyperspectral imaging under the condition of extremely weak illumination.
Compared with the prior art, the invention has the following remarkable technical effects and progresses:
1) The non-degenerate entangled photon to time frequency correlation imaging method is utilized, and the ICCD ultrafast gate width is combined to accurately discriminate the signal noise photons under the extremely low signal to noise ratio, so that the background noise irrelevant to the illumination light source is effectively filtered in the time domain, the detection sensitivity is greatly improved, and the ultra-sensitive mid-infrared spectrum imaging under the extremely weak illumination condition is favorably realized.
2) By utilizing the quantum correlation imaging method, mature light splitting devices in near-infrared wave bands can be fully utilized, the wavelength of idler frequency near-infrared photons is selected by adopting a high-resolution (about 0.1 nm) adjustable filter, the intermediate infrared spectrum resolution is about 0.4nm, and the realization of high-resolution intermediate infrared hyperspectral imaging is facilitated.
3) Compared with the nonlinear crystal with periodic polarization in the traditional scheme, the nonlinear crystal with the chirp polarization structure has the advantage that the phase matching bandwidth is greatly expanded. On one hand, the method is beneficial to the generation and conversion of mid-infrared photons in a wide wavelength range; on the other hand, the wide-angle intermediate infrared photon conversion imaging is facilitated.
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FIG. 1 is a schematic diagram of an imaging system constructed in accordance with the present invention;
fig. 2 is a schematic structural diagram of the spectral imaging system of embodiment 1.
Detailed Description
Referring to fig. 1, the invention adopts a method of non-degenerate entangled photon to time-frequency correlation imaging, and utilizes a broadband frequency conversion technology to realize ultra-sensitive mid-infrared spectrum imaging under the condition of extremely weak illumination, and specifically comprises the following steps: 1) Based on the nonlinear crystal with the chirp polarization structure, spontaneous parametric down-conversion is realized under narrow-band pumping, and a non-degenerate entangled photon pair is generated in a wide-band range; 2) The intermediate infrared signal photons pass through an object to be imaged to obtain spectral information, are converted into visible/near infrared bands through a broadband frequency up-conversion method, and are detected and imaged by an ultra-fast door width camera; 3) The method comprises the steps that idler frequency photons are subjected to wavelength selection through an adjustable filter and then detected by an indium gallium arsenic detector, an electric signal output by the indium gallium arsenic detector is used as a camera shutter trigger signal, and a time delay module is adjusted, so that the time of the up-conversion signal photons reaching a camera is within the camera acquisition gate width; 4) Different idler frequency light filtering wavelengths are set, so that the corresponding spectral image is acquired by the ultra-fast door width camera and is subjected to scale correction through an image scaling factor. And the absorption rate of the imaging object to each infrared wavelength can be obtained by comparing the measurement results of the existence of the imaging object. The invention realizes the generation of non-degenerate entangled photon pairs in a wide wavelength range, the up-conversion of infrared photons in a broadband and sensitive imaging by spontaneous parametric down-conversion and broadband frequency up-conversion. And a light splitting device with mature near-infrared wave band is adopted, and the imaging spectral resolution is greatly improved by the high-precision filtering of idler frequency light. Meanwhile, by combining the non-degenerate entangled photon time-frequency correlation characteristic, the ultra-fast door wide camera is adopted to effectively filter the randomly distributed background noise on the time domain, and the ultra-sensitive hyperspectral imaging is realized.
Under the pump light with fixed wavelength, the chirp polarization structure has a larger phase matching bandwidth compared with a periodic polarization structure, so that a non-degenerate entangled photon pair with a wide wavelength range can be generated, spontaneous parametric down-conversion is realized, and the entangled photon pair generated in the process can be represented by a wave function of the following formula (a):
a (upsilon) is spectrum amplitude converted under spontaneous parameters and is determined by phase matching of wavevector in the nonlinear crystal;
respectively representing the central frequencies of signal light and idler frequency light; v + satisfies the following parameter (b):
although the energy and momentum of each photon generated by spontaneous parametric down-conversion are uncertain, the energy conservation and momentum conservation condition of the following formula (c) is satisfied between the photon pairs generated by the down-conversion process:
wherein, ω is p Is the pump light frequency; k is a radical of formula p Is the pumping light wave vector; k is a radical of s Is the signal light wave vector; k is a radical of formula i Is the idler wavevector.
Therefore, the energy/momentum information of a certain photon can be obtained through measurement, and the energy/momentum information of the corresponding entangled photon can be determined.
The narrow-band pumping light source can be a continuous laser or a pulse laser, and preferably, a narrow-linewidth continuous laser is used as the pumping light source in consideration of the influence of the pumping light source bandwidth on the high spectral imaging spectral resolution.
The tunable filter selects the wavelength of the idler frequency near-infrared photons, and makes full use of a light splitting device with mature near-infrared band, so that the subsequent correlation spectrum imaging is realized in the mid-infrared wavelength range, the resolution of the existing near-infrared tunable filter can reach 0.1nm, and the corresponding mid-infrared photon spectrum resolution is 0.4cm -1 。
The invention adopts an ultra-fast door width enhanced camera as a detector, and takes an electric signal output by an indium gallium arsenic detector as external trigger to control a shutter switch. And the time delay module is used for adjusting the time of the trigger signal reaching the camera so as to realize the time consistency of the associated spectrum detection. The existing ultra-fast door width enhanced camera has a door width of 50ps, can effectively eliminate background noise in a sparse photon time-frequency correlation detection imaging process, and realizes ultra-sensitive single photon imaging. For a certain pixel (x, y) on the camera, i.e. a certain point (x ', y') on the corresponding object, a measurement count according to the following equation (d) is counted:
wherein, T is the integral time of the light,
is a second order correlation function and is expressed by the following formula (e):
wherein, the first and the second end of the pipe are connected with each other,
filtering a near infrared path for an electric vector at a photoelectric detector (namely an ICCD (integrated circuit compact disc) and an indium gallium arsenic detector) by combining a converted wave function under a spontaneous parameter, and assuming that the filtering width is far smaller than the phase matching width, obtaining a second-order correlation function G (2) Can be further represented by formula (f):
wherein, the first and the second end of the pipe are connected with each other,
as a function of the signal measurement;
as a filter function, the near infrared idler photon filter center frequency is omega f Two-way delay difference t = | t 1 -t 2 L. Therefore, the coincidence count rate can be further simplified as represented by the following formula (g):
according to the formula (g), the center frequency of the filter is ω f The coincidence measurement count of pixel (x, y) will be proportional to the spontaneous parametric down-conversion generation intensity and the signal detection efficiency introduced in the measurement path. The coincidence counting rate R can be strictly calculated by calibrating the generation efficiency of the converted entangled photon pair under the spontaneous parameters, the transmittance of an optical element in a measuring path, the frequency up-conversion efficiency in a signal measuring path and the detection efficiency of an indium gallium arsenic detector/ICCD (integrated circuit charge coupled device) c(x,y) (ω f ). In the specific implementation process, in order to conveniently calibrate the absorption rate of each point of the sample to each wavelength, the coincidence counting intensity R of the sample without spectral imaging is generally measured c(x,y) And divided to obtain a certain filtering frequency omega f The absorption rate at this point.
The broadband frequency up-conversion realizes the wavelength conversion of the intermediate infrared signal photons, and further realizes detection imaging by utilizing a sensitive silicon-based single photon camera. The absorption rate of each position of the object to be imaged to the non-passing wavelength is different, and the spectral intensity R when the object is passed or not passed c(x,y) Dividing to obtain a filter frequency omega f And the absorption rate of the pixel is reduced, and the spectral imaging of the interested wavelength region can be realized by adjusting the filtering frequency of the filter. In the process, the conversion rate and the detection rate of the up-conversion detection system to each wavelength are divided by data without influencing the calibration result of the hyperspectral imaging absorptivity.
The time-frequency correlation imaging of the pair of non-degenerate entangled photons is realized, idler photons and intermediate infrared signal photons are generated in pairs, and the transmission time and the frequency of the idler photons have correlation consistency. In single acquisition, the filtering wavelength of the tunable filter is fixed, and after the idler photons are detected, the signal photons are collected in a preset camera acquisition gate width, and an image measured by the camera is a monochrome image. The filtering wavelength of the adjustable filter is adjusted band by band to acquire data, and spectral absorption information of each position of an imaging object is obtained by comparing spectral imaging results of the imaging object. The correlation imaging method effectively inhibits the statistical noise irrelevant to the illumination light source in the time domain, greatly improves the signal-to-noise ratio of spectral imaging, and is beneficial to realizing the ultra-sensitive mid-infrared hyperspectral imaging under the condition of extremely weak illumination.
The present invention will be described in further detail with reference to specific examples.
Example 1
Referring to fig. 2, the ultra-sensitive mid-infrared hyperspectral imaging system adopting the framework of the invention comprises: the device comprises a pumping light source 1, a beam splitter 2, a first silver reflecting mirror 3, a second silver reflecting mirror 4, a first chirped polarized lithium niobate crystal 5, an a dichroic mirror 6, a first filter 7, an imaging object 8, a first plano-convex lens 9, a b dichroic mirror 10, a first concave lens 11, a second chirped polarized lithium niobate crystal 12, a second concave lens 13, a second filter 14, a second plano-convex lens 15, an ultrafast door wide camera 16, a third filter 17, a third silver reflecting mirror 18, an adjustable filter 19, an indium gallium arsenic detector 20 and a time delay module 21.
The pump light source 1 is a high-power 1064nm continuous light source, the output power of the pump light source can reach 10W, and the pump light source is used as a spontaneous parametric down-conversion and frequency up-conversion pump light source, so that the generation and conversion of mid-infrared photons in the wavelength range of 3-5 μm can be realized.
The beam splitter 2 splits the pumping light source, the high-power pumping light source is used for realizing the up-conversion of mid-infrared photons, and the low-power pumping light source is used for realizing the down-conversion of spontaneous parameters.
The silver reflectors 3, 4 and 18 have large reflectivity to infrared wave bands and small loss, and are used for changing the trend of a light path.
The chirp polarization lithium niobate crystals 5 and 12 are used as a spontaneous parameter down-conversion nonlinear medium for generating 3-5 mu m mid-infrared photons. The polarization period covers 25-32 μm.
The dichroic mirror 6 a is used for spatially separating the signal photons generated by the spontaneous parameter down-conversion from the idler frequency photons, is a 2-micron long-wave-pass dichroic mirror, transmits 3-5-micron signal light and reflects 1.35-1.65-micron idler frequency light.
The filters 7, 14 are 2.4 μm long pass filters. The filter is used for filtering mid-infrared photons and filtering 1064nm pump light, up-conversion fluorescence of the pump light, environmental stray light and the like.
The imaging subject 8, i.e. the object to be imaged, includes, but is not limited to: biological tissue, tumor cells, chemical materials. The absorption rates of the imaging object to light with different wavelengths are different, and the absorption rates of the detected target to the wavelengths can be obtained by comparing the pixel counts when no object exists and the pixel counts when the object exists through an experimental system.
The plano-convex lenses 9 and 15 are CaF 2 The lens aims to focus mid-infrared photons passing through a sample 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 diameter of the lens is 50.8mm.
The dichroic mirror 10 is designed to spatially combine signal photons generated by spontaneous parametric down-conversion with high-power 1064nm pump laser light. The dichroic mirror is a 2-micron long-wave-pass dichroic mirror, is transparent to 3-5-micron signal light and has high reflectivity to 1064nm pump light.
The concave mirrors 11 and 13 are CaF 2 The lens aims to enhance the oscillation of the 1064nm pump light source in the cavity, thereby improving the conversion efficiency on broadband frequency. It has high transmittance to 3-5 μm and 0.7-0.9 μm, and reflectivity to 1064nm of 97%.
The chirped polarized lithium niobate crystals 5 and 12 are used as frequency up-conversion nonlinear media for converting 3-5 mu m mid-infrared photons into visible/near-infrared bands. The polarization period of the chip covers 16-24 μm, and the chirp polarization step length is 0.01mm. The crystal size is 25mm (length) × 3mm (width) × 1mm (thickness).
The filters 7 and 14 are 700-900nm band-pass filters, and are used for up-conversion photon filtering and filtering 1064nm pump light, up-conversion fluorescence of the pump light, environmental stray light and the like.
The plano-convex lenses 9 and 15 are CaF 2 The lens aims at collimating and outputting visible/near-infrared photons generated by up-conversion so as to facilitate subsequent detection, the focal length of the plano-convex lens is 50mm, and the diameter of the lens is 50.8mm.
The ultra-fast gate wide phase 16 realizes ultra-sensitive single photon imaging on visible/near-infrared photons generated by up-conversion, and the detection wavelength range of the ultra-sensitive single photon imaging covers 400-1100nm. The existing ultra-fast door width enhanced camera has a door width of 50ps, and background noise randomly distributed in a time domain cannot enter the camera in the image acquisition process if idle frequency photons are not detected, namely, no external trigger is used for controlling the opening of a shutter. When the idler photons are detected, the shutter switching time is controlled to be 50ps by external triggering, and only a few background noise photons can be detected while the up-conversion signal photons are ensured to enter.
The filter 17 is a near-infrared band-pass filter, and the transmission wavelength range is 1.3-1.7 μm. The filter is used for filtering idler frequency photons and filtering 1064nm pump light, up-conversion fluorescence of the pump light, environmental stray light and the like.
The tunable filter 19 is designed to select the wavelength of the idler photons to achieve subsequent correlation spectral imaging. The existing near-infrared light splitting device is developed, for example, the filtering resolution of a near-infrared monochromator can reach 0.1nm, and the tuning wavelength range covers 1.35-1.65 mu m.
The indium gallium arsenic detector 20 realizes ultra-sensitive detection on the filtered idler frequency photons, the detection wavelength range of the detector covers 900-1700nm, and output signals of the detector are used as external trigger of the ultra-fast-door wide camera to control a shutter switch.
The delay module 21 adjusts the time when the trigger signal reaches the camera, so as to realize the time-frequency consistency of the associated spectrum detection.
The invention adopts a method of nondegenerate entangled photon to time-frequency correlation imaging and broadband frequency conversion to realize ultrasensitive mid-infrared spectrum imaging under the condition of extremely weak illumination, and the specific implementation process is as follows:
1) Nonlinear crystal based on chirp polarization structure to realize spontaneous parametric down-conversion
Non-degenerate entangled photon pairs are generated over a wide wavelength band. Specifically, the pump light source 1 is split by spatial power, the low-power part participates in spontaneous parametric down-conversion, 3-5 μm mid-infrared signal photons and 1.35-1.65 μm near-infrared idler photons are generated by the chirped polarized lithium niobate crystal 5, and beam spatial separation is realized by the dichroic mirror 6. The intermediate infrared signal photon and the idler photon are entangled photon pair with wavelength frequency relation of omega p ＝ω s +ω i Wherein, the spectral width of the narrow-linewidth pump light is much smaller than that of the signal light/idler light, which can be regarded as the pump light frequency ω p Is a constant. Therefore, signal photon wavelength information can be derived according to the idler photon filtering wavelength.
2) Obtaining spectral information by passing mid-infrared signal photons through an object to be imaged
Converted to visible/near infrared bands by a broadband frequency up-conversion method, and detected and imaged by an ultra-fast-door wide camera. Specifically, the middle infrared signal photons are filtered by the filter 7 to remove 1064nm pump light, pump light up-conversion fluorescence and environmental stray light, and sample spectrum information is obtained through the imaging object 8. The intermediate infrared sparse photons carrying the object spectral information are focused by the plano-convex lens 9 and spatially combined with the high-power 1064nm pump light source by the dichroic mirror 10 to enter the chirped polarized lithium niobate crystal 12. In order to make the conversion efficiency of the broadband frequency up-conversion process higher, the pump light is oscillated back and forth through the concave cavity mirrors 11 and 13 to form a resonant cavity. Wherein the temperature of the chirped and polarized lithium niobate crystal 12 is set at 30 deg.C, and the plano-convex lenses 9 and 15 are both CaF 2 A lens having in the mid-infrared band>A transmittance of 95%. Visible/near-infrared signal light generated in the nonlinear frequency up-conversion process passes through the filter 14 to remove stray light, and is spatially collimated by the plano-convex lens 15 to enter the ultrafast door wide camera 16. Meanwhile, the idler photons are wavelength-selected by the tunable filter 19 and then detected by the InGaAs detector 20. The electrical signal output by the indium gallium arsenic detector 20 is used as an external trigger signal to control the shutter switch of the ultrafast door width camera 16. The near infrared idler photons are filtered by the optical filter 17 to remove stray light, and the stray light is spatially input into the tunable filter 19. After the narrow-band wavelength selection of the tunable filter 19, the idler photons are output to the InGaAs detector 20 and sensitively detected. The ultrafast door wide camera 16 is in the collecting state (the shutter is closed), in order to obtain the entangled signal photons corresponding to the idler-frequency photons through detection, the electric signal output by the InGaAs detector 20 is used as external trigger to control the shutter of the ultrafast door wide camera 16 to be opened, the time delay module 21 is adjusted, and the photons of the up-conversion signal reach the cameraIs within the camera capture gate width.
Different scaling factors can be introduced into different wavelength components of signal photons in broadband upconversion imaging, and the image proportion after upconversion needs to be corrected. If the frequency upconversion is imaged by adopting a 4f system, an image scaling factor M exists in the upconversion image compared with the original image, and the following expression (1) is expressed as follows:
wherein f is 2 Is the focal length of the focusing lens behind the crystal; f. of 1 Is the focal length of the crystal front focusing lens; lambda [ alpha ] u Is an up-converted optical wavelength; lambda [ alpha ] s The wavelength of the intermediate infrared signal to be detected.
When tunable filter 19 sets the center wavelength of the filter to λ i Then, the corresponding mid-infrared signal photon wavelength λ to be measured s Represented by the following formula (2):
in the process of conversion on the broadband, the following formula (3) relationship exists according to the energy conservation condition:
the wideband up-conversion image scaling factor M is further described by the following equation (4):
3) The filtering wavelength of the tunable filter is adjusted band by band, image acquisition is carried out to obtain hyperspectral map data, each narrow-band image is divided by the scaling factor to correct imaging size distortion, the real size of the image is obtained, a final hyperspectral data cube is obtained, and the absorption rate of the imaging object to each infrared wavelength can be obtained by comparing the measurement results of the imaging object.
The embodiment combines a nonlinear crystal with a chirp polarization structure to realize spontaneous parametric down-conversion, realizes the generation of non-degenerate entangled photon pairs in a wide wavelength range, and the detection range of the mid-infrared spectrum covers 3-5 μm. The adopted broadband frequency up-conversion technology converts the mid-infrared photons to visible/near-infrared bands, makes full use of the existing sensitive silicon-based camera, and can realize the spectral imaging result of single photon sensitivity. The tunable filter with mature near-infrared band has the filtering bandwidth of the idler photons reaching 0.1nm and the filtering bandwidth delta lambda of the tunable filter f Limiting, spectral resolution Δ ν of said correlated spectral imaging s Represented by the following formula (5):
by using a non-degenerate entangled photon pair correlation measurement scheme and combining an ICCD ultra-fast gate width (about 50 ps), background noise is effectively filtered, and ultra-sensitive hyperspectral imaging can be realized. In addition, compared with the traditional up-conversion imaging scheme, the generation of the nondegenerate entangled photon pair avoids a complex broadband mid-infrared light source preparation device, so that the system structure is more compact. The intermediate infrared single photon hyperspectral imaging system realized based on the method has the advantages of wide wavelength, high resolution, high sensitivity and the like.
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, but 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 (4)
1. A middle infrared hyperspectral imaging method based on single photon time-frequency correlation is characterized in that a non-degenerate entangled photon is adopted to image the time-frequency correlation, and ultra-sensitive mid infrared spectral imaging under the condition of extremely weak illumination is realized by utilizing the conversion of broadband frequency, and the method specifically comprises the following steps:
step 1: spontaneous parametric down-conversion is realized under narrow-band pumping by utilizing a nonlinear crystal of a chirped polarization structure, and a nondegenerate entangled photon pair of a mid-infrared signal photon and an idler frequency photon is generated;
step 2: the intermediate infrared signal photons acquire spectral information through an object to be imaged, are converted into visible/near infrared bands through broadband frequency, and realize associated spectral imaging by utilizing a silicon-based ultra-fast gate width ICCD;
and step 3: the idler frequency photons are detected by an indium gallium arsenic detector after being selected by the wavelength of the tunable filter, an electric signal output by the indium gallium arsenic detector is used as a camera shutter trigger signal, and the time delay module is adjusted, so that the time of the up-conversion signal photons reaching the camera is within the camera acquisition gate width;
and 4, step 4: and setting different idle frequency filtering wavelengths to enable the ultrafast door wide camera to acquire a corresponding spectral image, performing proportional correction through an image scaling factor, and comparing the measurement results of the imaging object to obtain the absorption rate of the imaging object to each infrared wavelength.
2. The mid-infrared hyperspectral imaging method based on single photon time-frequency correlation according to claim 1 is characterized in that the narrowband pump is a continuous laser or a pulse laser.
3. The mid-infrared hyperspectral imaging method based on single photon time-frequency correlation according to claim 1, characterized in that the spectral resolution of the correlated spectral imaging is Δ ν s Represented by the following formula (5):
wherein, Δ ω s Is the signal light spectrum resolution; Δ ω f Filtering the spectral width for the tunable filter; delta lambda f Filtering the spectral width for the tunable filter; omega f Filtering the center frequency for the tunable filter; and c is the speed of light.
4. The mid-infrared hyperspectral imaging method based on single photon time-frequency correlation according to claim 1 is characterized in that the polarization period of the nonlinear crystal of the chirped polarization structure is 16-24 μm.
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