CN114624981B - Ultra-fast holographic microscopic imaging method and system based on time broadening - Google Patents
Ultra-fast holographic microscopic imaging method and system based on time broadening Download PDFInfo
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- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/0005—Adaptation of holography to specific applications
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- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/04—Processes or apparatus for producing holograms
- G03H1/0443—Digital holography, i.e. recording holograms with digital recording means
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- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/04—Processes or apparatus for producing holograms
- G03H1/0465—Particular recording light; Beam shape or geometry
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- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/04—Processes or apparatus for producing holograms
- G03H1/08—Synthesising holograms, i.e. holograms synthesized from objects or objects from holograms
- G03H1/0866—Digital holographic imaging, i.e. synthesizing holobjects from holograms
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- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/0005—Adaptation of holography to specific applications
- G03H2001/0033—Adaptation of holography to specific applications in hologrammetry for measuring or analysing
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- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/0005—Adaptation of holography to specific applications
- G03H2001/005—Adaptation of holography to specific applications in microscopy, e.g. digital holographic microscope [DHM]
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- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/04—Processes or apparatus for producing holograms
- G03H1/0443—Digital holography, i.e. recording holograms with digital recording means
- G03H2001/0445—Off-axis recording arrangement
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- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/04—Processes or apparatus for producing holograms
- G03H1/0465—Particular recording light; Beam shape or geometry
- G03H2001/0467—Gated recording using pulsed or low coherence light source, e.g. light in flight, first arriving light
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- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/04—Processes or apparatus for producing holograms
- G03H1/08—Synthesising holograms, i.e. holograms synthesized from objects or objects from holograms
- G03H1/0866—Digital holographic imaging, i.e. synthesizing holobjects from holograms
- G03H2001/0883—Reconstruction aspect, e.g. numerical focusing
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Abstract
The invention discloses an ultrafast holographic microscopic imaging method and system based on time broadening, wherein the system comprises the following steps: the ultra-short pulse sequence generating module is used for generating a multi-wavelength chirped pulse sequence for illumination; the holographic imaging spectrometer is used for generating a spatially discrete multi-wavelength off-axis hologram sequence according to the multi-wavelength chirped pulse sequence; and the hologram reconstruction module is used for reconstructing the multi-wavelength off-axis hologram sequence to obtain a quantitative optical thickness sequence of the sample. The method is used for reconstructing the multi-wavelength hologram sequence generated by the holographic imaging spectrometer to obtain the phase sequence, and then taking each sub-pulse wavelength of the multi-wavelength chirped pulse sequence as a standard to obtain a quantitative sample optical thickness sequence, so that ultra-short time resolution and ultra-fast imaging speed three-dimensional space imaging can be realized under the condition of single exposure, the problem that only two-dimensional space information of a detection target can be obtained by the existing method is solved, and the imaging effect is greatly improved.
Description
Technical Field
The invention belongs to the field of ultrafast optical imaging, and particularly relates to an ultrafast holographic microscopic imaging method and system based on time broadening.
Background
Optical imaging that achieves ultra-short time resolution and ultra-fast imaging speed has been an important direction of research in the field of optical imaging. Existing detectors, such as Charge Coupled Devices (CCDs) and Complementary Metal Oxide Semiconductor (CMOS) cameras, can only achieve time resolution imaging speeds on the order of sub-microseconds, making it difficult to detect transient, non-repetitive phenomena using conventional optical architectures and light sources.
The frequency-time-space mapping ultrafast imaging based on space broadening is an all-optical ultrafast imaging technology with resolution reaching picosecond/subpicosecond magnitude, however, the existing frequency-time-space mapping ultrafast imaging device is complex, high in construction cost, and can only acquire two-dimensional space information of a detection target, and cannot detect phase/depth information of the target. It is very urgent to realize the ultra-fast imaging technology of higher dimension.
Disclosure of Invention
The invention aims to: in order to overcome the defects in the prior art, the ultra-fast holographic microscopic imaging method and the system based on time broadening are provided, and the ultra-short time resolution and the ultra-fast imaging speed three-dimensional space imaging can be realized under the condition of single exposure.
The technical scheme is as follows: to achieve the above object, the present invention provides an ultrafast holographic microscopic imaging system based on time broadening, comprising:
the ultra-short pulse sequence generating module is used for generating a multi-wavelength chirped pulse sequence for illumination;
the holographic imaging spectrometer is used for generating a spatially discrete multi-wavelength off-axis hologram sequence according to the multi-wavelength chirped pulse sequence;
and the hologram reconstruction module is used for reconstructing the multi-wavelength off-axis hologram sequence to obtain a quantitative optical thickness sequence of the sample.
Further, the ultra-short pulse sequence generating module comprises an ultra-short pulse laser, a first diffraction grating, a second diffraction grating, a first convex lens, a second convex lens, a digital micro-mirror device and a glass rod;
the first diffraction grating is used for dispersing pulses generated by the ultra-short pulse laser;
the first convex lens is used for collimating the dispersed pulse into a one-dimensional spectral line on the digital micro-mirror device;
the digital micro-mirror device is used for carrying out selective filtering on the one-dimensional spectral line and reflecting the one-dimensional spectral line to the second convex lens;
the second convex lens and the second diffraction grating are used for combining the filtered pulse into a parallel light pulse;
the glass rod is used for carrying out time domain broadening on parallel optical pulses.
Further, the holographic imaging spectrometer comprises a third convex lens, an imaging lens, a spectroscope, a first micro objective lens, a second micro objective lens, a first reflecting mirror, a second reflecting mirror, a third diffraction grating and a CCD camera;
the third convex lens is used for converging the pulse sequences generated by the ultra-short pulse sequence generating module;
the spectroscope is used for dividing the converged pulse sequence into object light and reference light;
the first microscope objective is used for projecting object light on the sample, and collecting scattered light of the sample and returning the scattered light to the spectroscope;
the second microscope objective is used for projecting the reference light to the first reflecting mirror and collecting the reference light reflected by the first reflecting mirror to the spectroscope;
the first reflecting mirror is used for reflecting the reference light to the second micro objective lens;
the second reflector and the third reflector are used for reflecting the object light and the reference light which are transmitted in parallel and then making the object light and the reference light incident to the third diffraction grating;
the imaging lens is used for projecting the sub-pulse objects and the reference light emitted by the third diffraction grating to different positions on the CCD camera;
the CCD camera is used to capture discrete multi-wavelength off-axis hologram sequences.
Further, the first reflecting mirror is obliquely arranged, and the oblique direction is orthogonal to the dispersion direction of the third diffraction grating, so that the reference light on the imaging surface can be obliquely incident and interfere with object light to form an off-axis hologram, and the influence of dispersion on the visibility of interference fringes of the hologram is reduced.
An ultrafast holographic microscopic imaging method based on time broadening comprises the following steps:
s1: generating a multi-wavelength chirped pulse sequence through an ultrashort pulse sequence generating module;
s2: the holographic imaging spectrometer uses the multi-wavelength chirped pulse sequence to illuminate, and generates a spatially discrete multi-wavelength off-axis hologram sequence;
s3: and reconstructing the multi-wavelength off-axis hologram sequence through a hologram reconstruction module to obtain a quantitative optical thickness sequence, thereby realizing three-dimensional holographic microscopic imaging.
Further, the generating process of the multi-wavelength chirped pulse sequence in the step S1 is as follows:
the ultra-short pulse laser emits parallel light pulses, the pulses are dispersed through a first diffraction grating, collimated into one-dimensional spectral lines on a digital micro-mirror device of a back focal plane through a first convex lens, the digital micro-mirror device is loaded with a binary image, the one-dimensional spectral lines are subjected to selective filtering, the filtered pulses are combined through a second convex lens and a second diffraction grating and then become parallel light pulses again, and a glass rod is used for carrying out time domain broadening on the parallel light pulses to form a multi-wavelength chirped pulse sequence.
Further, the generating process of the multi-wavelength off-axis hologram sequence in the step S2 is as follows:
the pulse sequence is converged by the third convex lens and then is separated into object light and reference light by the spectroscope, the object light is projected on a sample on the front focal plane of the object light after passing through the first microscope objective, and scattered light of the sample is collected again by the first microscope objective and returns to the spectroscope;
the reference light is projected on the inclined first reflecting mirror on the rear focal surface of the second microscope objective lens, and the reference light is reflected by the first reflecting mirror, collected by the second microscope objective lens and returned to the spectroscope;
due to the effect of the spectroscope, the returned object light and the reference light are transmitted in parallel, reflected by the second reflector and the third reflector and then are injected into the third diffraction grating, the sub-pulse objects with different wavelength-time mapping and the reference light are separated in the x direction and are projected and converged at different positions on the CCD camera of the rear focal plane through the imaging lens, the sub-pulse objects with the same wavelength-time mapping and the reference light are projected at the same position on the CCD camera with a certain included angle to interfere to form off-axis holograms, and the CCD camera captures a series of discrete off-axis holograms in one exposure time to obtain a multi-wavelength off-axis hologram sequence.
Further, in the step S3, the off-axis hologram is reconstructed using a hologram reconstruction algorithm:
the corresponding sub-pulse wavelength is lambda i The hologram is expressed as g (x, y) =a (x, y) +c (x, y) exp (j 2 pi f) y x)+c * (x,y)exp(-j2πf y x), wherein a is a direct current component,b is modulation degree, & gt>In order to be a phase of the light, * representing complex conjugate, f y Spatial frequencies for hologram fringes;
fourier transforming G to obtain G (u, v) =a (u, v) +c (u-f) x ,v)+C * (u+f x V) wherein A, C * A, c respectively * Is a fourier transform of (C) using a low pass filter x V) filtering and moving to the origin of frequency domain to obtain C, performing inverse Fourier transform to obtain C, and calculating the phase asimag represents the imaginary part, real represents the real part, and the optical thickness of the sample is calculated after two-dimensional unwrapping the phase: />
The beneficial effects are that: compared with the prior art, the method has the advantages that the phase sequence is obtained by reconstructing the multi-wavelength hologram sequence generated by the holographic imaging spectrometer, and then the quantitative sample optical thickness sequence is obtained by taking each sub-pulse wavelength of the multi-wavelength chirp pulse sequence as a standard, so that the ultra-short time resolution and the ultra-fast imaging speed of three-dimensional space imaging can be realized under the condition of single exposure, the problem that the existing method can only obtain the two-dimensional space information of the detection target is solved, and the imaging effect is greatly improved.
Drawings
FIG. 1 is a schematic diagram of an ultrafast holographic microscopy imaging system based on time broadening provided by the invention;
FIG. 2 is a frequency domain feature diagram of a virtual array of multi-wavelength chirped pulses generated by the ultrashort pulse sequence generating module in the present embodiment;
FIG. 3 is a schematic diagram of the timing characteristics of the multi-wavelength chirped pulse generated by the ultra-short pulse sequence generating module according to the present embodiment;
FIG. 4 is a diagram of 10 spatially discrete off-axis holograms acquired by a camera under one exposure in this embodiment;
fig. 5 is a sequence of three-dimensional images reconstructed using the hologram reconstruction algorithm in the present embodiment.
In fig. 1, 1 is an ultrashort pulse sequence generating module, 2 is a holographic imaging spectrometer, 101 is an ultrashort pulse laser, 102 is a first diffraction grating, 106 is a second diffraction grating, 209 is a third diffraction grating, 103 is a first convex lens, 105 is a second convex lens, 107 is a glass rod, 104 is a digital micro-mirror device, 201 is a third convex lens, 210 is an imaging lens, 202 is a spectroscope, 203 is a first micro-objective, 205 is a second micro-objective, 204 is a sample, 206 is a first mirror, 207 is a second mirror, 208 is a third mirror, 211 is a CCD camera, and 212 is a computer.
Detailed Description
The present invention is further illustrated in the accompanying drawings and detailed description which are to be understood as being merely illustrative of the invention and not limiting of its scope, and various modifications of the invention, which are equivalent to those skilled in the art upon reading the invention, will fall within the scope of the invention as defined in the appended claims.
The invention provides an ultrafast holographic microscopic imaging system based on time broadening, as shown in fig. 1, comprising: an ultrashort pulse sequence generation module 1, a holographic imaging spectrometer 2 and a hologram reconstruction module.
The ultra-short pulse train generating module 1 includes an ultra-short pulse laser 101, a first diffraction grating 102, a second diffraction grating 106, a first convex lens 103, a second convex lens 105, a digital micro-mirror device 104, and a glass rod 107.
The holographic imaging spectrometer 2 comprises a third convex lens 201, an imaging lens 210, a spectroscope 202, a first microscope objective 203, a second microscope objective 205, a first reflecting mirror 206, a second reflecting mirror 207, a third reflecting mirror 208, a third diffraction grating 209 and a CCD camera 211. Wherein the first mirror 206 is obliquely disposed about the x-axis, and the oblique direction is orthogonal to the dispersion direction of the third diffraction grating 209.
The hologram reconstruction module is disposed in the computer 212, and is specifically a hologram reconstruction system.
In this embodiment, the holographic microscopic imaging system performs three-dimensional holographic microscopic imaging on the sample 204, and provides an ultrafast holographic microscopic imaging method based on time broadening, where the sample 204 is a reflector plated with an NJNU-shaped relief, and the specific imaging method is as follows in comparison with fig. 1:
s1: the multi-wavelength chirped pulse sequence is generated by an ultrashort pulse sequence generating module 1:
the ultra-short pulse laser 101 emits parallel light pulses with a center wavelength of 800nm, a 3dB bandwidth of 100nm and a pulse width of 5.5ps, the pulses are dispersed through the first diffraction grating 102, collimated into one-dimensional spectral lines on the digital micro-mirror device 104 of the back focal plane through the first convex lens 103, the digital micro-mirror device 104 is loaded with binary images to selectively filter the one-dimensional spectral lines, the filtered pulses are combined through the second convex lens 105 and the second diffraction grating 106 and changed into parallel light pulses again, and the glass rod 107 expands the time domain of the parallel light pulses to form a multi-wavelength chirped pulse sequence.
In this embodiment, the first diffraction grating 102 and the second diffraction grating 106 are 600 lines/mm, the focal length of the first convex lens 103 and the second convex lens 105 is 100mm, and the glass rod 107 is made of 3m long N-SF10 material.
As shown in FIG. 2, the one-dimensional spectral lines after filtering have 10 spectral lines, namely 750nm,760nm, 460 nm,780nm, 800nm,810nm, 630 nm,830nm and 840nm, and the width of each spectral line is 0.25 nm. As shown in fig. 3, the multi-wavelength chirped pulse sequence formed after the time domain broadening has 10 sub-pulses, each of which has a pulse width of 5.3ps, and the adjacent pulses have a spacing of 10ps.
S2: the holographic imaging spectrometer 2 is illuminated with a sequence of multi-wavelength chirped pulses, producing a sequence of spatially discrete multi-wavelength off-axis holograms:
the multi-wavelength chirped pulse sequence is separated into object light and reference light by the spectroscope 202 after being converged by the third convex lens 201, the object light is projected on a sample 204 of the front focal plane of the object light after passing through the first micro objective lens 203, and scattered light of the sample 204 is collected again by the first micro objective lens 203 and returns to the spectroscope 202;
the reference light is projected on the inclined first reflecting mirror 206 on the rear focal plane of the second microscope objective 205, and the reference light is reflected by the first reflecting mirror 206, collected by the second microscope objective 205 and returned to the spectroscope 202;
due to the effect of the spectroscope 202, the returned object light and the reference light propagate in parallel, are reflected by the second reflecting mirror 207 and the third reflecting mirror 208 in sequence, and then are injected into the third diffraction grating 209, the sub-pulse objects with different wavelength-time mappings and the reference light are separated in the x direction and are projected and converged at different positions on the CCD camera 211 of the rear focal plane through the imaging lens 210, the sub-pulse objects with the same wavelength-time mappings and the reference light are projected at the same position on the CCD camera 211 with a certain included angle to interfere to form off-axis holograms, and the CCD camera 211 captures a series of discrete off-axis holograms in one exposure time to obtain a multi-wavelength off-axis hologram sequence.
The focal length of the third convex lens 201 in this embodiment is 200mm, and the focal length of the imaging lens 210 is 150mm; first microscope objective 203, first mirror 206 is of a specification of focal length 40mm, na=0.26; the third diffraction grating 209 is 600 reticle/mm; the CCD camera 211 has a pixel size of 3um and a pixel number of 5000×1500; each hologram has an imaging field of view of 200um x 200um. The first mirror 206 is placed at the front focal plane of the second microscope objective 205 and the CCD camera 211 is placed at the back focal plane of the imaging lens 210.
In this example, a sequence of 10 off-axis holograms was captured, as shown in particular in FIG. 4.
S3: reconstructing the multi-wavelength off-axis hologram sequence through a hologram reconstruction module to obtain a quantitative optical thickness sequence, and realizing three-dimensional holographic microscopic imaging:
reconstructing 10 off-axis holograms using a hologram reconstruction algorithm:
taking one of the 10 holograms as an example, the corresponding sub-pulse wavelength is lambda i The hologram is expressed as g (x, y) =a (x, y) +c (x, y) exp (j 2 pi f) y x)+c * (x,y)exp(-j2πf y x), wherein a is a direct current component,b is the degree of modulation of the light beam,/>in order to be a phase of the light, * representing complex conjugate, f y Spatial frequencies for hologram fringes;
fourier transforming G to obtain G (u, v) =a (u, v) +c (u-f) x ,v)+C * (u+f x V) wherein A, C * A, c respectively * Is a fourier transform of (C) using a low pass filter x V) filtering and moving to the origin of frequency domain to obtain C, performing inverse Fourier transform to obtain C, and calculating the phase asimag represents the imaginary part, real represents the real part, and the optical thickness of the sample is calculated after two-dimensional unwrapping the phase: />
Finally, a three-dimensional image sequence diagram shown in fig. 5 is obtained.
In this embodiment, the imaging system provided by the present invention adopts a scheme that the final imaging speed is 6.8x1010 frames/second, the time resolution is 5.4ps, each field of view is 200 micrometers x 200 micrometers, and the spatial resolution is 4.38 micrometers. All of which are superior to existing imaging systems.
Claims (3)
1. An ultrafast holographic microscopy imaging system based on temporal broadening, comprising:
the ultra-short pulse sequence generating module is used for generating a multi-wavelength chirped pulse sequence for illumination;
the holographic imaging spectrometer is used for generating a spatially discrete multi-wavelength off-axis hologram sequence according to the multi-wavelength chirped pulse sequence;
a hologram reconstruction module for reconstructing the multi-wavelength off-axis hologram sequence to obtain a quantitative optical thickness sequence of the sample;
the ultra-short pulse sequence generating module comprises an ultra-short pulse laser, a first diffraction grating, a second diffraction grating, a first convex lens, a second convex lens, a digital micro-mirror device and a glass rod;
the first diffraction grating is used for dispersing pulses generated by the ultra-short pulse laser;
the first convex lens is used for collimating the dispersed pulse into a one-dimensional spectral line on the digital micro-mirror device;
the digital micro-mirror device is used for carrying out selective filtering on the one-dimensional spectral line and reflecting the one-dimensional spectral line to the second convex lens;
the second convex lens and the second diffraction grating are used for combining the filtered pulse into a parallel light pulse;
the glass rod is used for carrying out time domain broadening on the parallel light pulses;
the holographic imaging spectrometer comprises a third convex lens, an imaging lens, a spectroscope, a first micro objective, a second micro objective, a first reflecting mirror, a second reflecting mirror, a third diffraction grating and a CCD camera;
the third convex lens is used for converging the pulse sequences generated by the ultra-short pulse sequence generating module;
the spectroscope is used for dividing the converged pulse sequence into object light and reference light;
the first microscope objective is used for projecting object light on the sample, and collecting scattered light of the sample and returning the scattered light to the spectroscope;
the second microscope objective is used for projecting the reference light to the first reflecting mirror and collecting the reference light reflected by the first reflecting mirror to the spectroscope;
the first reflecting mirror is used for reflecting the reference light to the second micro objective lens;
the second reflector and the third reflector are used for reflecting the object light and the reference light which are transmitted in parallel and then making the object light and the reference light incident to the third diffraction grating;
the imaging lens is used for projecting the sub-pulse objects and the reference light emitted by the third diffraction grating to different positions on the CCD camera;
the CCD camera is used to capture discrete multi-wavelength off-axis hologram sequences.
2. The time-broadening-based ultrafast holographic microscopic imaging system of claim 1, wherein the first mirror is tilted and the tilt direction is orthogonal to the third diffraction grating dispersion direction.
3. An ultrafast holographic microscopic imaging method based on time broadening is characterized by comprising the following steps:
s1: generating a multi-wavelength chirped pulse sequence through an ultrashort pulse sequence generating module;
s2: the holographic imaging spectrometer uses the multi-wavelength chirped pulse sequence to illuminate, and generates a spatially discrete multi-wavelength off-axis hologram sequence;
s3: reconstructing the multi-wavelength off-axis hologram sequence through a hologram reconstruction module to obtain a quantitative optical thickness sequence, and realizing three-dimensional holographic microscopic imaging;
the generating process of the multi-wavelength chirped pulse sequence in the step S1 is as follows:
the ultra-short pulse laser emits parallel light pulses, the pulses are dispersed through a first diffraction grating, collimated into one-dimensional spectral lines on a digital micro-mirror device of a back focal plane through a first convex lens, the digital micro-mirror device is loaded with a binary image to selectively filter the one-dimensional spectral lines, the filtered pulses are combined through a second convex lens and a second diffraction grating and then become parallel light pulses again, and a glass rod expands the time domain of the parallel light pulses to form a multi-wavelength chirped pulse sequence;
the generating process of the multi-wavelength off-axis hologram sequence in the step S2 is as follows:
the pulse sequence is converged by the third convex lens and then is separated into object light and reference light by the spectroscope, the object light is projected on a sample on the front focal plane of the object light after passing through the first microscope objective, and scattered light of the sample is collected again by the first microscope objective and returns to the spectroscope;
the reference light is projected on the inclined first reflecting mirror on the rear focal surface of the second microscope objective lens, and the reference light is reflected by the first reflecting mirror, collected by the second microscope objective lens and returned to the spectroscope;
due to the effect of the spectroscope, returned object light and reference light are transmitted in parallel, reflected by the second reflector and the third reflector and then are injected into the third diffraction grating, sub-pulse objects with different wavelength-time mapping and reference light are separated in the x direction and projected on different positions of a CCD camera on the rear focal plane of the CCD camera through the imaging lens, the sub-pulse objects with the same wavelength-time mapping and the reference light are projected on the same position of the CCD camera at a certain included angle to interfere to form off-axis holograms, and the CCD camera captures a series of discrete off-axis holograms in one exposure time to obtain a multi-wavelength off-axis hologram sequence;
reconstructing an off-axis hologram using a hologram reconstruction algorithm in the step S3:
the corresponding sub-pulse wavelength is lambda i The hologram is expressed as g (x, y) =a (x, y) +c (x, y) exp (j 2 pi f) y x)+c * (x,y)exp(-j2πf y x), wherein a is a direct current component,b is modulation degree, & gt>In order to be a phase of the light, * representing complex conjugate, f y Spatial frequencies for hologram fringes;
fourier transforming G to obtain G (u, v) =a (u, v) +c (u-f) x ,v)+C * (u+f x V) wherein A, C * A, c respectively * Is a fourier transform of (C) using a low pass filter x V) filtering and moving to the origin of frequency domain to obtain C, performing inverse Fourier transform to obtain C, and calculating the phase asimag represents the imaginary part, real represents the real part, and the optical thickness of the sample is calculated after two-dimensional unwrapping the phase: />
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