Weak light signal measuring device and method based on pump-probe structure

Abstract

The present application relates to a kind of weak light signal measurement device and method based on pump-probe structure.The device includes: structured light spatial encoding module, time delay control module, 4-f imaging system and sensor, structured light spatial encoding module generates two beams of light of pump pump light and probe probe light, wherein, pump pump light is incident on sample after spatial modulation, probe probe light is incident on sample after time delay control module, time delay control module is used to adjust the optical path difference of two beams of light;Sample generates transient optical signal after being irradiated by light and stimulated radiation, optical signal is received by sensor after 4-f imaging system.This application realizes signal light extraction under the environment of strong background light by the introduction of spatial frequency, can greatly improve image signal-to-noise ratio.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pump-probe optical path structure and biological sample stimulated radiation, and particularly relates to a weak light signal measurement device and method based on a pump-probe structure. BACKGROUND

[0002] The pump-probe technology was first proposed by Nobel Prize winner Zewail in 1987. The method was initially used to observe the transient process of chemical reactions, and achieved the time resolution of human observation of transient dynamic processes to the femtosecond scale. On the one hand, compared with the time resolution limit of SPAD array or streak camera and other devices in the picosecond level, the pump-probe technology can improve the time resolution to the femtosecond scale. On the other hand, the technology can realize the simultaneous observation of multiple optical dimensions such as time, space, spectrum, polarization and angular momentum, greatly improving the observation bandwidth of the optical system. This technology has extremely significant role and significance in the scene of non-reproducible experiments or observation of damaged samples.

[0003] However, in the process of interaction of biological cells or proteins and other samples with light signals, the reaction signal is often extremely weak, and is limited by the interference of ambient light signals and pump optical paths in conventional experimental conditions. Therefore, it is urgent to develop a weak light signal measurement device with high signal-to-noise ratio and high precision. SUMMARY

[0004] Therefore, the present application provides a weak light signal measurement device based on a pump-probe structure and a measurement method thereof.

[0005] The technical scheme adopted by the present application is as follows:

[0006] The weak light signal measurement device based on the pump-probe structure comprises a structured light spatial encoding module, a time delay control module, a 4-f imaging system and a sensor. The structured light spatial encoding module generates two beams of light, pump light and probe light. The pump light is modulated in space and then incident on the sample, and the probe light is incident on the sample after being controlled by the time delay control module. The time delay control module is used to adjust the optical path difference of the two beams of light. The sample generates transient optical signals after being stimulated by light irradiation, and the optical signals are received by the sensor after passing through the 4-f imaging system.

[0007] Further, the structured light spatial encoding module comprises a femtosecond laser, a light splitting prism and a spatial light modulator, the femtosecond laser generates an ultrashort light pulse, the light splitting prism splits the light into two beams with perpendicular polarization directions, one of which is used as pump light and the other is used as probe light; the spatial light modulator optically encodes the light signal and introduces spatial frequency.

[0008] Further, the spatial light modulator is one or more of a phase modulation type or an amplitude modulation type.

[0009] Further, the time delay control module comprises a right-angle prism mirror group and a one-dimensional translation stage, and the one-dimensional translation stage is used to adjust the position of the right-angle prism mirror group.

[0010] Further, the right-angle prism mirror group adopts four identical bevel-coated isosceles right-angle prisms, wherein the first and second right-angle prisms are assembled back to back, the included angle between the bevel-coated reflecting surfaces is 90°, the third and fourth right-angle prisms are placed in a straight line, the included angle between the bevel-coated surfaces is also 90°, the hypotenuse of the first and third right-angle prisms is parallel, and the hypotenuse of the second and fourth right-angle prisms is also parallel; the third and fourth right-angle prisms are fixed on the one-dimensional translation stage.

[0011] Further, the 4-f imaging system comprises two lenses and an aperture stop, and the aperture stop is placed on the focal plane between the two lenses.

[0012] Further, the pump light is modulated by the spatial modulation, and the light after the time delay control of the probe light is combined by the light splitting prism and then incident on the sample.

[0013] Further, the device further comprises an image processing module connected with the sensor, which is used for weak signal analysis of the optical signal.

[0014] The application also provides a measurement method of the above-mentioned weak light signal measurement device based on a pump-probe structure, first, laser generates pump light and probe light, the pump light is modulated by the spatial modulation to load periodic stripes or speckle patterns, so that the light beam carries high-frequency information in space, and the light intensity is irradiated on the sample according to the transmission direction according to the loaded pattern structure, so that the sample region absorbs the pump light; at the same time, the probe light does not carry spatial high-frequency information, and uniformly irradiates the sample, so that the sample region which has absorbed the pump light energy produces stimulated emission transition; then the 4-f optical system isolates the low-frequency signal with large light intensity, retains the high-frequency spatial information modulated by the spatial modulation, and obtains the light field information of the signal.

[0015] Further, after obtaining the light field information of the signal, the stimulated radiation signal generated in the sample is calculated by combining a weak signal processing algorithm.

[0016] The present application can realize the decomposition of the pump light and probe light signals based on the stimulated radiation field scene due to the introduction of the spatial frequency information generated by the spatial modulator and the 4-f signal processing system, so that the target weak light signal can be separated in the super strong light intensity background, and then the stimulated radiation signal can be collected in a single exposure in the spatial scale; and the time delay system composed of the right-angle mirror group is combined, so that the transient optical process can be analyzed in the time scale, and the time accuracy can reach the order of 10 fs. BRIEF DESCRIPTION OF DRAWINGS

[0017] The above and other aspects and advantages of the present application will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, in which:

[0018] Figure 1 A schematic diagram of the weak light signal measurement device based on the pump-probe structure provided by the embodiments of the present application.

[0019] Figure 2 A top view schematic diagram of the mirror group and the one-dimensional displacement table after assembly in the embodiments of the present application.

[0020] Figure 3 A 4-f optical filtering system provided by the embodiments of the present application.

[0021] Figure 4 A flowchart of the weak light signal measurement method based on the pump-probe structure provided by the embodiments of the present application. DETAILED DESCRIPTION

[0022] The embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, in which the same or similar notations represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the accompanying drawings are exemplary and are intended to explain the present application, and cannot be understood as a limitation of the present application.

[0023] The embodiments of the present application are described below by referring to the accompanying drawings. Figure 1 The weak light signal measurement device and method based on the pump-probe structure in the embodiments of the present application are described.

[0024] The femtosecond laser (FsLaser) emits a femtosecond pulse, which is divided into two beams of orthogonal polarization by a polarization beam splitter prism PBS according to the method of orthogonal polarization projection. One of the beams is used as pump light, which is modulated by a spatial light modulator SLM to load a one-dimensional sinusoidal distribution stripe or speckle pattern. Therefore, the beam carries high-frequency information in space, and the light intensity is irradiated on the sample according to the transmission direction according to the loaded pattern structure, so that the sample region absorbs the pump light. At the same time, the other beam is used as probe light, which does not carry spatial high-frequency information, and uniformly irradiates the sample, so that the sample region that has absorbed the pump light energy produces stimulated radiation transition. The femtosecond light source pulse time width belongs to one or more of the order of hundreds of femtoseconds; the spatial light modulator SLM is one or more of the phase modulation type or the amplitude modulation type; the beam splitter prism can also be a common non-polarized beam splitter prism; the sample (Sample) is used to absorb the pump light energy, and produces stimulated radiation effect after being irradiated by the probe light.

[0025] The pump light and the probe light time delay control module includes a rectangular mirror group and a one-dimensional translation stage. The one-dimensional translation stage includes one or more of a knob type displacement stage, a piezoelectric ceramic type, etc., which has a micron to nanometer level and can realize fine adjustment of the position function. The embodiment adopts a screw micrometer device. The rectangular mirror group is composed of four same size bevel-coated isosceles right-angle prisms (such as Figure 2 ), which form an adjustable optical delay system for adjusting the optical path difference of the two light beams. Specifically, ① and ② are assembled back to back, and the angle between the coated reflecting bevels is 90°; ③ and ④ are placed in a straight line, and the angle between the coated bevels is also 90°; at this time, the bevels of ① and ③ are parallel, and the bevels of ② and ④ are also parallel. The reason for such placement is to realize that the light is reflected from the bevel of ① to the bevel of ③, and then reflected to ④, and then reflected to ②. At this time, the propagation direction of the light reflected by ② is parallel and coaxial with the incident light incident to ①, which is convenient for subsequent optical path debugging.

[0026] The ③ and ④ rectangular mirror groups are fixed on the one-dimensional translation stage (as shown in Figure 2 ), and the optical path between the rectangular mirror groups changes with the movement of the displacement stage (the displacement stage moves back and forth along the right angle edge of ③). The one-dimensional translation stage can realize the accuracy of 1 μm displacement resolution. Therefore, by changing the optical path difference through displacement, the minimum adjustable optical pulse time interval can be realized as 6.66 fs (the calculation method is: Δt = Δx / c0 = 2 × 1 × 10 -6 m / 3.0 × 108 m / s = 6.66 fs). Considering the influence of device error, the system fine-tuning time accuracy is about ~10 fs.

[0027]

Control 4-f system to select target weak signal and block background noise

[0028] The original unmodulated stimulated radiation signal is denoted as I x At this time, since the light signal is uniformly irradiated, the energy of the signal spectrum is concentrated at the base frequency position; after the SLM spatial light modulator, the stripe I s (such as sinusoidal stripe) is loaded, at this time the process of pump light interacting with stripe signal is equivalent to the process of point multiplication at corresponding position, that is, I x ·I s At this time, after the equivalent Fourier transform of the modulated signal through the lens L1, we get:

[0029]

[0030] Where, FT() represents the Fourier transform of the variable in the parentheses, and "·" represents the point multiplication operation, and "*" represents the convolution process. This process is equivalent to introducing high-frequency components in the spatial light field through SLM, and in the spectrum diagram, it can be obtained that the target signal frequency term is moved to the high-frequency position.

[0031] At this time, a band-pass filter I mask is introduced at the back focal plane of the lens L1 (also the front focal plane of the lens L2), so that the frequency term after the signal and stripe convolution can pass through, and other signals (including pump pump light, probe light and environmental light which are not absorbed by the sample, etc.) are blocked; after the band-pass filtered signal passes through the equivalent inverse Fourier transform of the lens L2, it is converted into a spatial domain light intensity signal, that is:

[0032] FT -1 {[FT(I x ·I s )]·I mask}

[0033] Where, FT -1 () represents the inverse Fourier transform operation. Finally, the signal is received by the detector. Since the image collected at this time only retains the high-frequency part of the light signal, it is necessary to optimize the image by post-processing method.

[0034] Under ideal conditions, in the Cartesian coordinate system ( Figure 3 The coordinate system on the right side is set as the x direction along the up and down direction of the paper; for a clearer description, we use the x coordinate to describe the sample surface, the x' coordinate to describe the Fourier spectrum surface at a distance of 2f from the sample (that is, the location of the aperture), and the x' coordinate to describe the imaging surface at the CCD / CMOS position). The sinusoidal fringes g with a spatial frequency of u0 and periodic distribution in the x direction are in (x) is used as an example for analysis:

[0035] The sinusoidal light field can be described as:

[0036]

[0037] After Fourier transform of lens L1, the frequency spectrum G in (u) has only three peaks (in Fourier space, the up and down direction along the paper is set as the u direction):

[0038]

[0039] Where δ(u) represents the pulse impulse function; the light field distribution g before bandpass filter filtering f- (x”):

[0040]

[0041] Where λ is the wavelength of the incident light, f is the focal length of the convex lens, and g is the wavelength of the incident light. f- (x”) represents the light field function on the front surface of the aperture stop, g f+ (x”) represents the light field function after passing through the aperture. The aperture device blocks the low-frequency signal energy and retains the high-frequency signal introduced by the structured light. At this time, the light field function g f+ (x”) is:

[0042]

[0043] After the Fourier transform of the second lens, the light field signal G on the imaging surface is out (u), at this time the signal only contains the signal light we want to collect:

[0044]

[0045] It can be seen from the above formula that the present invention can achieve spatial frequency modulation of the light field by adding spatial modulation, and further can filter out the desired weak signal.

[0046] Figure 3 This is a structural detail diagram of the 4-f optical filtering system provided by an embodiment of the present invention.

[0047] The stimulated emission light signal carries spatial frequency information, and its energy is concentrated in the high-frequency position; the background light does not carry high-frequency information, so its energy is concentrated in the central low-frequency position; at this time, we are in this Fourier plane ( Figure 3 Place an aperture stop at the middle Aperture position, so that the center of the aperture is in the high-frequency energy area, and the aperture size should block the central low-frequency area. The position of the hole and the light that can pass through are as follows: Figure 3 The middle aperture is described in Aperture.

[0048] Figure 1 Other optical elements and mechanical parts: the front and rear aperture diaphragms are used for circular modulation of the output light and optical filtering in the 4-f system respectively; the front and rear polarization beam splitters (PBS) are used for beam decomposition and beam combining respectively; the polarizer is used for modulating the linear polarization state, and has multiple functions of polarization and analysis. This is because PBS splitting cannot guarantee a completely linear polarization state. Combining with a polarizer can further improve the polarization degree of the light path; the half-wave plate is used to adjust the polarization angle of the light path, and has one or more functions of rotating the linear polarization direction of the light path, mainly because the spatial light modulator (SLM) is more efficient in modulating polarized light in a specific polarization direction; the reflector is used to change the propagation direction of the light beam, and is a silver-plated reflector, a gold-plated reflector or a dielectric film reflector with one or more high reflectivity for the light band used.

[0049] [Collecting signal light field information] Optical sensor devices (including one or more of CCD, CMOS, SCMOS, and EMCCD) are used to receive stimulated emission light signals after 4-f system optical filtering. The collected signals are then transmitted to a computer for display and combined with subsequent image processing methods such as image enhancement and low-light signal detection to improve the image signal-to-noise ratio and complete the measurement process.

[0050] [Image Processing of Light Field Information] Since the collected light field information only contains high-frequency information after optical filtering, it can be further frequency-shifted by an image processing module connected to the sensor to improve imaging. This image processing module uses one or more weak signal processing methods, such as grayscale histogram equalization, contrast stretching, image enhancement, and ultra-low illumination image processing.

[0051] The above overall process can be summarized as Figure 4The present application discloses a kind of measurement devices and methods for weak light signal, which can realize the measurement of weak light signal in the strong background light environment.The measurement device comprises a light source, a target scene, a light sensor, a 4-f optical filter and an image processing unit.The light source is used to irradiate the target scene, and the light sensor is used to receive the light field information of the target scene.The 4-f optical filter is used to control the low-frequency information of stimulated radiation signal generated in biological tissue sample, and to select the high-frequency light information with certain spatial frequency.The image processing unit is used to process the light field information received by the light sensor, and to obtain the stimulated radiation information of the target scene.In general, after laser beam irradiates sample, three effects of absorption, scattering and transmission are generated, and the energy of transmitted light is three or four orders of magnitude larger than that of radiation light generated after sample absorption, so in this scene, the light signal of sample radiation is called weak light signal.The signal light is submerged in extremely strong background light.If not processed by 4-f optical filter, the optical sensor cannot distinguish the signal light.By the measurement device of the present application, the background light and part of the signal light can be physically blocked, and at this time, the signal collected in the optical sensor is all signal light, realizing the measurement of weak light signal.The present application realizes the extraction of signal light in the strong background light environment by introducing spatial frequency, which can greatly improve the image signal-to-noise ratio.

[0052] In the present specification, the terms "first", "second" are only used for descriptive purpose, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features.Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features.In the description of the present application, the meaning of "a plurality of" is at least two, such as two, three, etc., unless otherwise specifically limited.

[0053] Although the embodiments of the present application have been shown and described above, it should be understood that the above-described embodiments are exemplary, and cannot be understood as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above-described embodiments within the scope of the present application.

Claims

1. A weak light signal measurement device based on a pump-probe structure, characterized in that: The device includes: a structured light spatial coding module, a time delay control module, a 4-f imaging system and a sensor, wherein the structured light spatial coding module generates two beams of light, namely, a pump light and a probe light, wherein the pump light is incident on the sample after being spatially modulated, and the probe light is incident on the sample after passing through the time delay control module, and the time delay control module is used to adjust the optical path difference between the two beams of light; the sample generates a transient optical signal after stimulated radiation by light irradiation, and the optical signal is received by the sensor after passing through the 4-f imaging system; the 4-f imaging system includes two lenses and an aperture diaphragm, and the aperture diaphragm is placed on the focal plane between the two lenses; the pump light after being spatially modulated and the probe light after passing through the time delay control module are combined by a beam splitter prism and incident on the sample.

2. The weak light signal measurement device based on the pump-probe structure according to claim 1, characterized in that: The structured light spatial coding module includes a femtosecond laser, a beam splitter prism and a spatial light modulator. The femtosecond laser generates ultrashort light pulses, which are split into two beams of light with mutually perpendicular polarization directions by the beam splitter prism. One beam of light serves as pump light and the other as probe light. The spatial light modulator optically encodes the optical signal and introduces spatial frequency.

3. The weak light signal measurement device based on the pump-probe structure according to claim 2, characterized in that: The spatial light modulator is one or more of a phase modulation type and an amplitude modulation type.

4. The weak light signal measurement device based on the pump-probe structure according to claim 1, characterized in that: The time delay control module includes a right-angle prism reflector group and a one-dimensional translation stage, and the one-dimensional translation stage is used to adjust the position of the right-angle prism reflector group.

5. The weak light signal measurement device based on the pump-probe structure according to claim 4, characterized in that: The right-angle prism reflector assembly uses four identical isosceles right-angle prisms with coated oblique surfaces, wherein the first and second right-angle prisms are assembled back to back, the angle between the coated reflective oblique surfaces is 90°, the right-angle sides of the third and fourth right-angle prisms are arranged in a straight line, the angle between the coated oblique surfaces is also 90°, the oblique sides of the first and third right-angle prisms are parallel, and the oblique sides of the second and fourth right-angle prisms are also parallel; the third and fourth right-angle prisms are fixed on the one-dimensional translation stage.

6. The weak light signal measurement device based on the pump-probe structure according to claim 1, characterized in that: The device further comprises an image processing module connected to the sensor and configured to perform weak signal analysis on the optical signal.

7. A measurement method using the weak light signal measurement device based on the pump-probe structure as claimed in claim 1, characterized in that: First, the laser generates two beams of light: pump light and probe light. The pump light is spatially modulated to load periodic stripes or speckle patterns. Therefore, the beam carries high-frequency information in space. The light intensity is irradiated on the sample according to the loaded pattern structure along the transmission direction, causing the sample to regionally absorb the pump light. At the same time, the probe light does not carry spatial high-frequency information and uniformly irradiates the sample, causing the sample area that has absorbed the pump light energy to produce stimulated radiation transitions. The low-frequency signal with higher light intensity is then isolated through a 4-f optical system, retaining the high-frequency spatial information generated by spatial modulation, and obtaining the signal's light field information.

8. The measuring method according to claim 7, characterized in that: After obtaining the light field information of the signal, the stimulated emission signal generated in the sample is calculated by combining the weak signal processing algorithm.

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