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

Abstract

The invention relates to a weak light signal measuring device and method based on a pump-probe structure. The device comprises a structured light space coding module, a time delay regulation and control module, a 4-f imaging system and a sensor, the structured light space coding module generates pump pump light and probe light, the pump pump light enters a sample after being spatially modulated, the probe light enters the sample after passing through the time delay regulation and control module, and the probe light enters the sample after passing through the time delay regulation and control module. The time delay regulation and control module is used for regulating the optical path difference of the two beams of light; a transient optical signal is generated after a sample is irradiated by light and stimulated, and the optical signal is received by a sensor after passing through a 4-f imaging system. According to the invention, through introduction of the spatial frequency, signal light extraction in a strong background illumination environment is realized, and the signal-to-noise ratio of an image can be greatly improved.

Description

Technical Field

[0001] The invention relates to the technical fields of pump-probe optical path structure and stimulated radiation of biological samples, and in particular to a weak light signal measurement device and method based on the pump-probe structure. Background Art

[0002] The pump-probe technology was first proposed by Zewail, the Nobel Prize winner in Chemistry, in 1987. The method was originally used to observe the transient process of chemical reactions, bringing 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 devices such as SPAD arrays or streak cameras at the picosecond level, the pump-probe technology can improve the time resolution to the femtosecond scale; on the other hand, the technology can achieve 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 effects and significance in scenarios of non-repeatable experiments or observation of damaged samples.

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

[0004] In view of this, the present invention provides a weak light signal measurement device based on a pump-probe structure and a measurement method thereof.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A weak light signal measurement device based on a pump-probe structure comprises: a structured light spatial encoding module, a time delay control module, a 4-f imaging system and a sensor, wherein the structured light spatial encoding module generates two beams of light, namely, a pump light and a probe light, wherein the pump light is incident on a 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.

[0007] Furthermore, the structured light spatial coding module includes a femtosecond laser, a beam splitter prism and a spatial light modulator. The femtosecond laser generates an ultrashort light pulse, which is divided into two beams of light with mutually perpendicular polarization directions by the beam splitter prism, one beam of light being used as a pump light and the other beam of light being used as a probe light; the spatial light modulator optically encodes the optical signal and introduces a spatial frequency.

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

[0009] Furthermore, 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.

[0010] Furthermore, the right-angle prism reflector group uses four identical isosceles right-angle prisms with coated bevels, wherein the first and second right-angle prisms are assembled back to back, the angle between the coated reflecting bevels 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 bevels is also 90°, the hypotenuses of the first and third right-angle prisms are parallel, and the hypotenuses 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.

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

[0012] Furthermore, the pump light after spatial modulation and the probe light after time delay control module are combined by a beam splitter prism and incident on the sample.

[0013] Furthermore, the device also includes an image processing module connected to the sensor and used for performing weak signal analysis on the optical signal.

[0014] The present invention also provides a measurement method for the above-mentioned weak light signal measurement device based on the pump-probe structure. First, a laser generates two beams of light, namely, a pump pump light and a probe probe light. The pump pump light is spatially modulated to load periodic stripes or speckle patterns, so the light beam carries high-frequency information in space. The light intensity is irradiated on the sample according to the loaded pattern structure in the transmission direction, so that the sample regionally absorbs the pump light. At the same time, the probe probe light does not carry spatial high-frequency information and uniformly irradiates the sample, so that the sample area that has absorbed the pump light energy produces stimulated radiation transition. Then, a 4-f optical system is used to isolate the low-frequency signal with a larger light intensity, retain the high-frequency spatial information modulated by space, and obtain the light field information of the signal.

[0015] Furthermore, 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.

[0016] The present invention introduces the spatial frequency information generated by the spatial modulator and the 4-f signal processing system, so that the decomposition of the pump light and the probe light signal based on the stimulated emission scene can be realized, so that the target weak light signal can be separated from the ultra-strong light background, and then the stimulated emission signal can be collected in a single exposure on the spatial scale; at the same time, combined with the time delay system composed of a right-angle reflector group, the transient optical process can be analyzed on the time scale, and the time accuracy can reach the order of ~10fs. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0018] Figure 1 A schematic diagram of a weak light signal measuring device based on a pump-probe structure provided in an embodiment of the present invention.

[0019] Figure 2 It is a top view schematic diagram of the assembly of the reflector assembly and the one-dimensional translation stage according to an embodiment of the present invention.

[0020] Figure 3 A 4-f optical filtering system is provided in an embodiment of the present invention.

[0021] Figure 4 The present invention provides a flowchart of a method for measuring weak light signals based on a pump-probe structure. DETAILED DESCRIPTION

[0022] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0023] Please refer to the following Figure 1 A weak light signal measuring device and method based on a pump-probe structure according to an embodiment of the present invention are described.

[0024] [Structured light spatial coding module] The femtosecond pulses emitted by the femtosecond laser (FsLaser) are divided into two beams of light with mutually perpendicular polarization directions by the polarization beam splitter PBS according to the method of orthogonal polarization projection. One beam of light is used as the pump light, which is modulated by the spatial light modulator SLM to load one-dimensional sinusoidal distribution stripes or speckle patterns. Therefore, the beam carries high-frequency information in space. The light intensity is irradiated to the sample according to the loaded pattern structure in the transmission direction, so that the sample absorbs the pump light regionally; at the same time, the other beam of light is used as the probe light, which does not carry spatial high-frequency information and uniformly irradiates the sample, so that the sample area that has absorbed the pump light energy produces stimulated radiation transition. Among them, the pulse time width of the femtosecond light source belongs to one or more of the 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 an ordinary 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 probe light time delay control module includes a rectangular prism reflector group (RectangularMirrors) and a one-dimensional translation stage, wherein the one-dimensional translation stage includes a knob-type translation stage, a piezoelectric ceramic type, etc., which have micrometer to nanometer levels and can achieve one or more of the functions of fine position adjustment. This embodiment uses a spiral micrometer device. The rectangular prism reflector group is composed of four isosceles rectangular prisms (such as Figure 2 ), forming an adjustable optical delay system for adjusting the optical path difference of the two paths of light; specifically, ①② right-angle prisms are assembled back to back, and the angle between the reflective bevels of the coating is 90°; the right-angle sides of ③④ right-angle prisms are placed in a straight line, and the angle between the bevels of the coating is also 90°; at this time, the bevels of ① and ③ right-angle prisms are parallel, and the bevels of ② and ④ right-angle prisms are also parallel. The reason for this arrangement is that after the light is incident from the bevel of ① right-angle prism, it can be reflected to the bevel of ③ right-angle prism, and then the light is reflected to ④ right-angle prism, and then reflected to ② right-angle prism. At this time, the propagation direction of the light reflected from ② right-angle prism is parallel and coaxial with the incident light incident on ① right-angle prism, which is convenient for subsequent optical path debugging.

[0026] Fix the ③④ right-angle prism reflector set on a one-dimensional translation stage (such as Figure 2 As shown in the figure, by moving the translation stage (the translation stage moves back and forth along the right-angle side of the right-angle prism ③), the optical path between the right-angle prism reflector groups changes accordingly. The one-dimensional translation stage can achieve a resolution of 1μm displacement accuracy. Therefore, by changing the optical path difference by displacement, the minimum adjustable light pulse time interval can be 6.66fs (calculated as: Δt = Δx / c0 = 2×1×10 -6 m / 3.0×108 m / s=6.66fs). Considering the influence of device error, the fine-tuning time accuracy of the system is about ~10fs.

[0027] [Control the 4-f system to select the target weak signal and block the background noise] Since the polarization, phase, wavelength and other information of the photons generated by the stimulated radiation process are the same as those of the pump light, the general method cannot separate the stimulated radiation light from the pump light after passing through the sample. Therefore, the goal of the 4-f system is to distinguish the light emitted by stimulated radiation from the transmitted light, so as to realize the collection and processing of weak light signals under strong background illumination conditions. The mathematical language is described as:

[0028] The original unmodulated stimulated emission 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 passing through the SLM spatial light modulator, the stripe I s (e.g. sinusoidal stripes), the process of interaction between the pump light and the stripe signal is equivalent to the process of point multiplication of the corresponding positions, i.e. I x I s , at this time, after the modulated signal passes through the equivalent Fourier transform of lens L1, we get:

[0029]

[0030] Among them, FT() means Fourier transform of the variables in the brackets, "·" means point multiplication operation, and "*" means convolution process. This process is equivalent to introducing high-frequency components into the spatial light field through SLM. It can be seen from the spectrum that the target signal frequency term is moved to the high-frequency position.

[0031] At this time, a bandpass filter I is introduced at the rear focal plane of lens L1 (also the front focal plane of lens L2). mask , so that the frequency term after the signal and the stripe convolution can pass, blocking other signals (including pump light, probe light and ambient light that are not absorbed by the sample); the signal after bandpass filtering is converted into a spatial domain light intensity signal after the equivalent inverse Fourier transform of lens L2, that is:

[0032] FT -1 {[FT(I x I s )]·I mask}

[0033] Among them, FT -1 () represents the inverse Fourier transform operation. Finally, the signal is received by the detection device. Since the image collected at this time only retains the high-frequency part of the optical signal, it is necessary to optimize the image by post-processing and other methods.

[0034] Under ideal conditions, in the Cartesian coordinate system ( Figure 3 The coordinate system on the right side is set to 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 (i.e., the location of the aperture), and the x' coordinate to describe the imaging surface at the CCD / CMOS location). The sinusoidal fringes g with a spatial frequency of u0 and periodically distributed 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 through lens L1, the obtained 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] Among them, λ is the wavelength of the incident light, f is the focal length of the convex lens, and g 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 A structural detail diagram of a 4-f optical filtering system provided in an embodiment of the present invention.

[0047] The stimulated emission light signal carries spatial frequency information, and its energy is concentrated at high frequencies; the background light does not carry high-frequency information, so its energy is concentrated at the central low-frequency position; at this time, we are in this Fourier plane ( Figure 3 Place an aperture stop at the Aperture position in the middle of the image, so that the center of the aperture is in the high-frequency energy area, and the aperture size of the aperture 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 combination respectively; the polarizer is used for modulation of the linear polarization state, and has multiple functions of polarization and analysis, because PBS beam splitting cannot guarantee a completely linear polarization state, and the polarization degree of the light path can be further improved in combination with the polarizer; 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, and has one or more functions of high reflectivity for the used light band.

[0049] [Collecting signal light field information] Optical sensor devices (including one or more of CCD, CMOS, SCMOS, EMCCD) are used to receive stimulated radiation light signals after 4-f system optical filtering, and are used to transmit the collected signals to a computer for display and combine with subsequent image enhancement and low-light signal detection and other image processing methods 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, the image processing module connected to the sensor can continue to perform frequency shifting to improve the imaging effect. The processing method used by the image processing module is 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 4In the description. By recording the optical signal of the target scene, optically encoding the optical signal, controlling the right-angle reflector group to control the relative time delay of the two beams of light, and then realizing the time resolution scanning imaging at the femtosecond level, controlling the 4-f system to block the low-frequency information of the stimulated radiation signal generated in the biological tissue sample, selecting the high-frequency light information with a certain spatial frequency, receiving the light field information, and performing image processing on the light field information to obtain the stimulated radiation information of the target scene. Generally speaking, after the laser beam irradiates the sample, three effects of absorption, scattering and transmission are generated. Among them, the energy of the transmitted light is three or four orders of magnitude greater than the radiation light generated after the sample absorbs. Therefore, in this scene, the light signal radiated by the sample is called a weak light signal. The signal light is submerged in the extremely strong background light. If it is not processed by 4-f optical filtering, the optical sensor cannot distinguish the signal light. Through the measuring device of the present invention, the background light and part of the signal light can be physically blocked. At this time, the signal collected in the optical sensor is all signal light, and the measurement of the weak light signal is realized. The present invention realizes the extraction of signal light in a strong background illumination environment by introducing spatial frequency, which can greatly improve the image signal-to-noise ratio.

[0052] In this specification, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0053] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A weak light signal measurement device based on a pump-probe structure, characterized in that: The device comprises: a structured light spatial coding module, a time delay control module, a 4-f imaging system and a sensor. The structured light spatial coding module generates two beams of light, namely, a pump light and a probe light. The pump light is incident on a sample after being spatially modulated, and the probe light is incident on the sample after passing through the time delay control module. 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.

2. The weak light signal measuring 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 an ultrashort light pulse, which is divided into two beams of light with mutually perpendicular polarization directions by the beam splitter prism, one beam of light being used as a pump light and the other beam of light being used as a probe light; the spatial light modulator optically encodes the optical signal and introduces a spatial frequency.

3. The weak light signal measuring 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 measuring device based on the pump-probe structure according to claim 1, characterized in that: The time delay control module comprises 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 measuring device based on the pump-probe structure according to claim 4, characterized in that: The right-angle prism reflector group adopts four isosceles right-angle prisms with identical bevel coatings, wherein the first and second right-angle prisms are assembled back to back, the angle between the coated reflective bevels 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 bevels is also 90°, the hypotenuses of the first and third right-angle prisms are parallel, and the hypotenuses 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 measuring device based on the pump-probe structure according to claim 1, characterized in that: The 4-f imaging system includes two lenses and an aperture stop, which is placed on the focal plane between the two lenses.

7. The weak light signal measuring device based on the pump-probe structure according to claim 1, characterized in that: The pump light after spatial modulation and the probe light after time delay control module are combined by a beam splitter and incident on the sample.

8. The weak light signal measuring device based on the pump-probe structure according to claim 1, characterized in that: The device also includes an image processing module connected to the sensor and used for performing weak signal analysis on the optical signal.

9. A measurement method using the weak light signal measurement device based on a pump-probe structure as claimed in claim 1, characterized in that: First, the laser generates two beams of light, the pump light and the probe light. The pump light is spatially modulated to load periodic stripes or speckle patterns, so the light beam carries high-frequency information in space. The light intensity is irradiated to the sample according to the loaded pattern structure in the transmission direction, so that the sample absorbs the pump light regionally. At the same time, the probe light does not carry spatial high-frequency information and irradiates the sample uniformly, so that the sample area that has absorbed the pump light energy produces stimulated radiation transition. Then, the low-frequency signal with larger light intensity is isolated through the 4-f optical system, and the high-frequency spatial information modulated by space is retained to obtain the light field information of the signal.

10. The measuring method according to claim 9, characterized in that: After obtaining the light field information of the signal, the stimulated radiation signal generated in the sample is calculated using a weak signal processing algorithm.

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