Rayleigh scattering-based weak light detector nonlinear measurement and correction method and system

Abstract

The invention discloses a weak light detector nonlinear measurement and correction method and system based on Rayleigh scattering, and belongs to the technical field of optical measurement. A weak light detector is an optical signal device used for detecting extremely low light intensity, and when the weak light detector works under the low light intensity condition, nonlinear response is often shown, and the accuracy and the stability of a measurement result can be remarkably influenced. Therefore, non-linear correction is crucial to guarantee of reliability and repeatability of measured data. According to the invention, a Rayleigh scattering-based weak light detector nonlinear measurement system is established, the incident laser power is measured by using a high-precision thermoelectric detector, and then the scattered light power is measured by using a weak light detector-photomultiplier. Different laser incident powers are set to obtain Rayleigh scattering signals and incident light powers under different working conditions, and the scattered light power is corrected by using an incident laser power measurement value.

Description

(I) Technical field

[0001] The present invention proposes a nonlinear measurement and correction method and system for a weak light detector based on Rayleigh scattering, which belongs to the field of optical measurement technology. The method uses a high-precision pyroelectric detector to measure the incident laser power, uses a weak light detector-photomultiplier tube to measure the scattered light power, and uses the incident laser power to correct the scattered light power. The problem of nonlinear response curve of the photomultiplier tube in the weak light detection equipment is solved, the system error is reduced, the measurement accuracy is improved, and the measurement equipment is better used in a wide range of input signals. (II) Background technology

[0002] Weak light detectors are specifically used to detect extremely low-intensity light signals, such as Rayleigh scattering. Rayleigh scattering is an elastic light scattering phenomenon that occurs when light interacts with particles smaller than its wavelength (such as gas molecules or microparticles). The intensity of the scattered light is related to the wavelength of the incident light, the concentration of the particles and the temperature. It can capture single photons or even photon flux intensities lower than those visible to the human eye. For example, a typical example is a photomultiplier tube that can convert incident photons into electrons through the photoelectric effect. The incident photons hit the photocathode, releasing initial electrons, which are then amplified by a secondary electron multiplier to output a stronger electrical signal. Nonlinear measurement refers to the process in which the relationship between the output signal and the input signal of the system is not linear during the measurement process, but rather presents a certain nonlinear relationship. This means that the output value does not change proportionally with the input value, but rather describes the corresponding relationship between the two in a more complex manner (such as a polynomial, exponential or logarithmic relationship). The present invention adopts a method of correcting the scattered light power with the incident light power to solve the nonlinear problem of the photomultiplier tube when detecting Rayleigh scattering signals. Before the experiment started, the ambient temperature was tested with a platinum resistor to ensure environmental stability and improve correction accuracy. (III) Summary of the invention

[0003] Aiming at the nonlinear problem of traditional weak light signal detection, the present invention proposes a weak light detector nonlinear measurement and correction method and system based on Rayleigh scattering. First, a platinum resistor is used to measure the air temperature in front of the signal receiver to evaluate the stability of the ambient temperature and ensure the signal measurement accuracy. Then, the detection signal is received by a power meter and a photomultiplier tube, and finally the signal is corrected to solve the nonlinear problem of weak light detection.

[0004] The system consists of a laser, a chopper, an aperture, a sleeve, a half-wave plate, a high-power laser polarization beam splitter, a darkroom, a power meter and a probe, a convex lens, a filter, a photomultiplier tube, an acquisition system, a host computer, a digital multimeter, and a four-wire platinum resistor. Before the experiment, a platinum resistor is used in conjunction with a digital multimeter to measure the temperature stability of the Rayleigh scattering detection area. Then the high-power continuous laser emits a 532nm light signal, which is modulated by the chopper and becomes an intermittent pulse signal with a fixed frequency. The aperture and sleeve control the size of the laser transmission. When the laser reaches the half-wave plate and the high-power laser polarization beam splitter, it is divided into P polarization and S polarization. The P polarization is reflected, and the S polarization is transmitted and reaches the Rayleigh scattering detection area along the coaxial optical path. The scattered light is used to detect and collect the Rayleigh scattering signal, and the remaining light enters the power meter along the main optical axis to collect the power size. In the Rayleigh scattering detection part, there is a convex lens in front to converge the scattered light onto the receiving surface of the photomultiplier tube. The distance between the two is the focal length of the convex lens, so that the scattered signal can be detected to the maximum extent; there is a 532nm filter behind the convex lens to filter out stray light in other bands. The convex lens, filter and photomultiplier tube are connected in the coaxial system through a sleeve, which ensures coaxiality while blocking light that can enter the photomultiplier tube from other directions. The detection light path forms a darkroom. After the signal is received, it is transmitted to the host computer for signal acquisition and demodulation; in the power meter measurement part, the coaxial light path after the Rayleigh scattering detection area, and then enters the probe of the power meter after the aperture and sleeve beam, and finally the power meter value is collected by the host computer. The detection of the power meter and the photomultiplier tube is carried out simultaneously. After the acquisition, a scatter plot is drawn, and the measurement correction is performed by fitting. The specific measurement steps are as follows:

[0005] Step 1: A nonlinear measurement and correction method and system for weak light detectors based on Rayleigh scattering, characterized in that a platinum resistor is used to measure the room temperature in front of the Rayleigh scattering detection area, the reading of a digital multimeter is recorded every one minute, the resistance value is converted into a temperature value by looking up a platinum resistor graduation table, and the temperature change is recorded.

[0006] Step 2: A method and system for nonlinear measurement and correction of weak light detectors based on Rayleigh scattering, characterized in that a power meter is used to collect the incident light power. After the coaxial light beam passes through the Rayleigh scattering detection area, it is combined by the aperture and the sleeve, and then the signal is received by the thermal probe, and then the computer collects data. Multiple groups of measurements are performed under different laser powers to obtain I1, I2, ..., I i ,…,I n And take a set of data as an example to examine the signal stability

[0007] Step 3: A method and system for nonlinear measurement and correction of weak light detectors based on Rayleigh scattering, characterized in that a coaxial optical path is constructed using a half-wave plate, a beam splitter, an aperture and a sleeve, etc., to facilitate reception by a power meter probe for nonlinear correction. First, the laser is started for preheating. After the preheating is completed, the laser is turned on to emit a continuous laser, and a coaxial optical path for combined beams is formed through the aperture and the sleeve. A half-wave plate and a high-power laser polarization beam splitter are added. The half-wave plate introduces a phase delay into the polarized light to change the polarization direction of the light. The polarization beam splitter divides the light beam into S-polarized light and P-polarized light. The P-polarized light is reflected and the S-polarized light is transmitted. The combination of the two optical elements can adjust the direction of the polarized light, which can be achieved by rotating the half-wave plate, so that the S light is finally in the coaxial direction, and the maximum value can be obtained in the direction of receiving the Rayleigh scattering signal. When the optical path reaches the measurement area of ​​Rayleigh scattering, the scattered light is converged to the signal receiving section of the photomultiplier tube through a convex lens and a filter, eliminating the influence of stray light in other bands. The accuracy of the signal measurement is ensured according to the formula The signals R1, R2, ..., R received by the photomultiplier tube are measured at different laser powers. i , …, R n The stability of the signal is examined by taking one of the powers as an example, and the Rayleigh scattering measurement signal is corrected by the formula R=AI. (IV) Description of the drawings

[0008] Figure 1 The invention is a schematic diagram of the overall structure of a nonlinear measurement and correction method and system of a weak light detector based on Rayleigh scattering, which is composed of the following parts: a laser light source (1), an optical chopper (2), a sleeve (3), a half-wave plate (4), a high-power laser polarization beam splitter (5), a four-wire platinum resistor (6), an aperture (7), a thermoelectric power sensor (8), a full-band optical power meter (9), a host computer (10), a photomultiplier tube (11), an acquisition module (12), a digital multimeter (13), a convex lens (14), and a filter (15).

[0009] Figure 2 is the incident laser monitoring power curve.

[0010] Figure 3 is the Rayleigh scattering measurement curve.

[0011] Figure 4 It is a nonlinear measurement and correction curve. (V) Specific implementation methods

[0012] The present invention will be further described below in conjunction with examples.

[0013] The structure of the present invention is as follows Figure 1As shown, before the experiment, a platinum resistor (6) is used to measure the indoor temperature, and a digital multimeter (13) reads the voltage value. The laser (1) emits a continuous laser, which is modulated by a chopper (2) into an intermittent pulse signal, and a coaxial light beam is formed by a combination of a sleeve (3) and an aperture (7). The half-wave plate (4) cooperates with a polarization beam splitter (5) to split the coaxial light into P polarized light and S polarized light. The S polarized light continues to propagate along the axis direction, and the P polarized light is reflected. After passing through the Rayleigh scattering signal measurement area, the S polarized light is combined again by the aperture (7) and the sleeve (3), and finally irradiated onto the rake surface of the power meter probe (8), transmitted to the power meter (9) to display the power size, and the data is recorded by the host computer (10). The Rayleigh scattered signal is combined through the aperture (7) and the sleeve (3) and enters the receiving module. The convex lens (14) is used to converge the light signal to the detection surface of the photomultiplier tube (11). The detection surface is at the focus of the convex lens (14) to ensure that the signal enters the detector to the greatest extent. The filter (15) ensures that the stray light of the remaining bands will not enter the photomultiplier tube (11). The detected signal is collected by the signal acquisition module (12) in cooperation with the host computer (10). The specific nonlinear measurement and correction process of the weak light detector based on Rayleigh scattering includes the following steps:

[0014] Step 1: In this example, before the experiment, a platinum resistance is used to measure the indoor temperature. The digital multimeter readings are recorded every 1 minute. The specific readings are: 110.586Ω, 110.607Ω, 110.628Ω, 110.647Ω, 110.668Ω, 110.683Ω, 110.698Ω, 110.712Ω, 110.736Ω, 110.751Ω. The corresponding temperature values ​​are obtained by looking up the platinum resistance graduation table.

[0015] Step 2: In this example, 12 different working conditions are selected, including 100mW, 200mW, 300mW, 400mW, 500mW, 1W, 1.5W, 2W, 2.5W, 3W, 3.5W, and 4W. In each working condition, the incident light power is measured using a power meter (9). Figure 2 It is the incident laser monitoring power measurement curve. Taking the laser incident power of 100mW as an example, after measuring for ten minutes, the incident laser monitoring power fluctuation is within 2%, which meets the nominal indicators of the laser.

[0016] Step 3: In this example, the Rayleigh scattering signal is measured using a photomultiplier tube (11), and recorded for ten minutes at 12 different powers. Taking 100 mW as an example, the Rayleigh scattering measurement signal result is as follows: Figure 3 As shown in Figure 1, the demodulation value of the Rayleigh scattering measurement results is stable within 0.5% over time. The Rayleigh scattering method proposed in this paper is used to measure and correct the nonlinearity of the weak light detector. The incident light power and the demodulation value of the photomultiplier tube (11) measured at 12 different powers are shown in Figure 1. Figure 4 As shown, the dotted straight line segment is the measured value obtained through the experiment, and the non-dotted straight line segment is the corrected nonlinear coefficient curve obtained by calculating the experimental data.

[0017] The above description of the present invention and its embodiments is not limited thereto, and the accompanying drawings are only one embodiment of the present invention. Without departing from the purpose of the present invention, any structure or embodiment similar to the technical solution designed without creativity shall fall within the protection scope of the present invention.

Claims

1. A method and system for nonlinear measurement and correction of weak light detectors based on Rayleigh scattering. The system consists of a laser light source, an optical chopper, an aperture, a half-wave plate, a high-power laser polarization splitter prism, a darkroom, a power meter and a probe, a convex lens, a filter, a photomultiplier tube, an acquisition system, a host computer, a four-wire platinum resistor and a digital multimeter. A continuous laser is used as a light source with a wavelength of 532nm. The light emitted by the laser is modulated into an intermittent pulse signal by a chopper. The modulated laser is beamed through a sleeve, an aperture and other objects, which reduces The reflection of the laser also suppresses the stray light in the optical system. The modulated laser is split by a half-wave plate and a high-power polarization beam splitter. The S-polarized light is transmitted and the P-polarized light is reflected. The transmitted S-polarized light is re-beamed by the aperture, sleeve and other equipment and then irradiated onto the receiving surface of the power meter probe. At this time, the power meter displays the received laser power in real time. The receiving position of the Rayleigh scattering signal is perpendicular to the direction of the incident light. The Rayleigh scattering signal receiving device is composed of a sleeve, an aperture, a lens, a filter and a photomultiplier tube. The sleeve and aperture in front of the photomultiplier tube shield the stray light in the environment. There is a convex lens in front of the photomultiplier tube rake face to converge the scattered light received in the direction perpendicular to the light path, and there is a filter behind the lens to filter the scattered light. The acquisition system uses an acquisition card with a sampling frequency of 204.8Hz. The Labview program is used to collect the Rayleigh scattering signal. The signal is collected in dual-channel mode, which mainly analyzes the signal value received by the photomultiplier tube, fits the signal value of the photomultiplier tube and the power size curve of the power meter, and corrects the measurement.

2. A Rayleigh scattering-based weak light detector nonlinear measurement and correction method and system according to claim 1, characterized in that: Stability of ambient temperature, nonlinear measurement Before the experiment begins, use a standard platinum resistor to measure the room temperature, record the reading of the digital multimeter at this time and compare it with the graduation table of the standard platinum resistor to obtain the room temperature value at this time. During the experiment, place the front end of the platinum resistor probe in the air in front of the signal receiver, measure the temperature change during the experiment, and record the reading of the digital multimeter every one minute.

3. A Rayleigh scattering-based weak light detector nonlinear measurement and correction method and system according to claim 1, characterized in that The incident light power is monitored by connecting a power meter to a host computer to monitor and collect power. In order to obtain a stable incident light and collect a larger Rayleigh scattering signal value, a combination of a half-wave plate and a beam splitter prism is used to allow the S light to be transmitted and the P light to be reflected. With the restriction of the aperture and the sleeve, the light is propagated along the main axis and finally received by the thermal sensor. The incident laser power is set to n groups respectively to obtain I1, I2, ..., I i ,…,I n .

4. A Rayleigh scattering-based weak light detector nonlinear measurement and correction method and system according to claim 1, characterized in that: Measure and correct the scattered light power value measured by the photomultiplier tube; Rayleigh scattering is the elastic scattering of incident light by molecules, and the internal energy of the molecules does not change. The central wavelength of the Rayleigh scattering spectrum is the same as the incident wavelength and can be expressed as: Where I0 is the incident light intensity, S k (ν,T) and g k (ν,T,M k ) are the Rayleigh scattering intensity and linear function, S k (ν,T) can be expressed as: Where N is the number density of the measured area, X k is the mole fraction of the kth gas component in the air, is the differential scattering cross section of the kth gas component, which can be expressed as: Among them, R L is the molar refractive index, ρ0 is the depolarization rate, and the molar refractive index and depolarization rate of air can be obtained from the literature, so as to accurately calculate the differential scattering cross section value of each component in the air, and obtain the Rayleigh scattering intensity value combined with the concentration of each component in the air. g k (ν,T,M k ) is the Rayleigh scattering line function, expressed as: Where ν0 is the incident laser wavelength, Δν T,k is the half-width of the scattered spectrum, expressed as: In the actual measurement process, the detection value is the integration of the spectrum amplitude over the wavelength. Since the linear function satisfies: Then the detected values ​​of scattered light power and incident light power satisfy the relationship: in: Rayleigh scattered light is one hundred millionth of the incident laser power. When the incident light power is 10W, the Rayleigh scattered light is about tens of nW. When the air temperature and the concentration of each molecular component are stable, the incident laser power is proportional to the scattered light power. Therefore, a high-power laser with stable and adjustable frequency and power is selected to construct the Rayleigh scattering measurement optical path, and the incident laser power is set to n groups respectively. The incident light power is directly monitored by a high-precision pyroelectric detector to obtain I1, I2, ..., I i ,…,I n . While measuring the incident light power, measure the 90° Rayleigh scattered light power and record them as: R1, R2, …, R i , …, R n . Then the measurement data can be written in matrix form: R=AI (9) Among them, A represents the coefficient matrix determined by temperature and concentration, which is expressed as: The n groups of measured R and I data are plotted into a dot-line graph, and the relationship curve between the incident light power and the scattered light power is fitted according to R=AI to correct the original data graph.

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