CN104062004B - Extremely-weak light signal detection device and method - Google Patents

Abstract

The invention belongs to the field of signal detection, in particular to a high-precision extremely weak light signal detection device and method based on a digital signal processor (DSP). The device includes a mechanical shutter, a photomultiplier tube, a low-noise preamplifier, a high-pass filter circuit, a high-resolution A/D conversion circuit, a DSP processing circuit, a shutter drive circuit and a computer; the extremely weak optical signal is modulated, The processing of eliminating background noise and digital sampling integration realizes the precise detection of extremely weak incident light. At the same time, the device of the invention has low production cost, simple structure and convenient use.

Description

An extremely weak optical signal detection device and method

technical field

The invention belongs to the field of optical signal detection, and relates to an extremely weak optical signal detection device and method, in particular to a high-precision extremely weak optical signal detection device and method based on a digital signal processor (DSP).

Background technique

In the measurement of extremely weak optical signals, it often occurs that the background noise or interference is very large, but the measured signal is very weak or even obliterated by the noise. In this case, the traditional method of converting the weak light signal into a weak electrical signal through a photodetector and then amplifying the weak electrical signal through a low-noise preamplifier cannot extract and restore the measured signal from the noise. At present, the commonly used method is to realize the detection and extraction of extremely weak optical signals through a lock-in amplifier or a sampling integrator.

The lock-in amplifier utilizes the frequency and phase uncorrelated characteristics of noise and signal, and realizes a large suppression of noise by compressing the bandwidth of the detection channel, thereby improving the signal-to-noise ratio of the signal. The sampling integrator uses a reference signal consistent with the signal repetition frequency to sample the noise-containing signal. After repeated sampling and extraction, the statistical average of the noise tends to zero, and the signal is extracted.

These two methods have the following disadvantages in actual use:

1) When using a lock-in amplifier to detect weak light signals, it is necessary to install a chopper at the front end of the detector. The function of the chopper is to modulate the incident optical signal into a sinusoidal AC signal and output a reference signal with the same frequency and phase as the modulated signal to the lock-in amplifier. This puts forward high requirements for the chopper, which can not only modulate the optical signal into a sinusoidal signal in the time domain, but also output the reference signal with the same frequency and phase as the modulated signal, and also require the reference signal to have high frequency stability to avoid drift out of the narrowband. Usually, a chopper that can meet the above requirements is very expensive, increasing the development task cost.

2) Since the lock-in amplifier and the chopper are two independent devices, when the reference signal is transmitted to the reference terminal of the lock-in amplifier through the cable, the phase shift of the signal may be caused by the influence of the distribution parameters of the cable. In addition, if the reference signal is subjected to electromagnetic interference during transmission, its frequency and phase will be modulated by the interference signal. This will undoubtedly cause inaccuracy in the final test, especially when the long-distance transmission or the electromagnetic environment is poor, these effects are very obvious.

3) The sampling integrator is an instrument for measuring weak periodic repetitive signals in noise, so when measuring extremely weak optical signals, it is also necessary to periodically modulate the measured light, which has similar problems to the lock-in amplifier.

4) The core of the sampling integrator is the sampling analog switch and the RC integration circuit, wherein the sampling analog switch samples the weak analog signal at intervals under the control of the gate pulse, and the RC integration circuit samples the weak analog signal within the time when the analog switch is turned on. The analog signal value is integrated, and then output after being amplified by the final stage. The analog switch is a semiconductor device, and there is a leakage current, and it cannot be turned off in a true sense, and additional noise will be introduced. The RC integrating circuit determines the integral time constant τ. Usually, the capacitance of the capacitor C is determined. By adjusting the resistance of the resistor R, the adjustment of τ is realized. However, since the resistance of R cannot be accurately measured, the The integral time constant τ cannot be adjusted precisely.

5) Both the lock-in amplifier and the sampling integrator need to amplify the output signal of the detector through a low-noise pre-amplifier circuit and then extract the extremely weak useful signal. It is usually required that the detector output be as close as possible to the input end of the preamplifier, and the transmission distance between the output end of the preamplifier and the input end of the lock-in amplifier or sampling integrator be shortened as much as possible. On the one hand, it can reduce signal attenuation, and on the other hand, it can reduce the interference of long-distance transmission from the external electromagnetic environment. However, this will limit the application in some special occasions, especially when the detector is placed far away from the preamplifier or the preamplifier is far away from the test instrument, the signal attenuation and the interference of the external electromagnetic environment will seriously affect the test results.

6) Usually when using a lock-in amplifier or a sampling integrator, it is often impossible to determine the noise of the signal preamplifier itself, because this part of the noise is not modulated and is not related to the measured signal, and belongs to the background noise of the circuit.

Contents of the invention

In order to solve the deficiencies and limitations of common methods in the technical background, the present invention proposes a very weak light signal detection device and method with low cost, simple operation, and accurate detection of very weak light signals.

Concrete technical scheme of the present invention is:

An extremely weak optical signal detection device is characterized in that it includes a mechanical shutter, a photomultiplier tube, a low-noise preamplifier, a high-pass filter circuit, a high-resolution A/D conversion circuit, a DSP processing circuit, a shutter drive circuit and a computer;

A mechanical shutter is set between the photomultiplier tube and the incident extremely weak light. The output end of the photomultiplier tube is connected to the input end of the low-noise preamplifier, and the output end of the low-noise preamplifier is connected to the input end of the high-pass filter circuit. The output end of the filter circuit is connected with the input end of the high-resolution A/D conversion circuit, the output end of the high-resolution A/D conversion circuit is connected with the input end of the DSP processing circuit, and the DSP processing circuit is connected with the computer; the mechanical A shutter driving circuit is arranged between the shutter and the DSP processing circuit.

The above-mentioned low-noise preamplifier is a low-current-to-voltage preamplifier; the bias current of the low-noise preamplifier is less than 2pA, and the noise is less than or equal to 2mV _pp when the conversion gain is 10 ⁶ I/V; the high-pass filter circuit The filtering can meet the requirements of filtering out low frequency components less than 100 Hz; the resolution level of the high-resolution A/D conversion circuit is 16 bits.

The computer and the DSP processing circuit are connected to each other through a shielded network cable.

According to above-mentioned detection device, propose a detection method of this device now, it is characterized in that, comprises the following steps:

1) Set parameters such as the modulation period, sampling time, sampling interval and sampling times of the extremely weak optical signal on the computer;

2) The computer transmits the parameters set in step 1) to the DSP processing circuit;

3) The DSP processing circuit controls the shutter drive circuit according to the set modulation period to make the mechanical shutter modulate the extremely weak light signal;

4) The photomultiplier tube detects the modulated extremely weak light signal and outputs a weak current signal;

5) Low-noise pre-amplification circuit (bias current less than 2pA, noise ≤2mV _p - _p when converting 10 ⁶ I/V;) receiving the weak current signal output by the photomultiplier tube, converting it into a voltage signal and amplifying it, and then amplifying it The voltage signal output;

6) The high-pass filter circuit receives the amplified voltage signal and performs high-pass filtering on the signal to filter out low-frequency components below 100 Hz, and then outputs the filtered voltage signal;

7) The high-resolution A/D conversion circuit receives the filtered voltage signal and converts it into a 16-bit digital signal and transmits the signal to the DSP processing circuit;

8) The DSP processing circuit collects the digital signal, removes the background noise, digital sampling, band-pass filtering and digital integration to obtain the final extremely weak light signal data, and then transmits the data to the computer through a shielded network cable;

Wherein, the background noise removal is to remove the noise of the low-noise preamplifier circuit.

Above-mentioned step 8) in the step of removing background noise as follows:

8.1) The mechanical shutter is closed and there is no optical signal input;

8.2) The low-noise preamplifier circuit amplifies the weak voltage signal of the body and outputs it to the high-pass filter circuit; the amplified weak voltage signal is filtered by the high-pass filter circuit to filter out low-frequency components, and the filtered voltage signal is transmitted to the high-resolution A/D Conversion circuit; the high-resolution A/D conversion circuit converts the filtered voltage signal into a digital signal and transmits the digital signal to the DSP processing circuit;

8.3) The DSP processing circuit records the data when the mechanical shutter is closed as the background noise of the preamplifier circuit;

8.4) The DSP processing circuit subtracts the background noise of the preamplifier circuit from the digital signal when the optical signal is incident.

The advantages of the present invention are:

1. The invention modulates the incident weak light through the mechanical shutter, and all the work is controlled by the timing of the DSP processing circuit, so that the timing relationship between the modulation circuit and the acquisition circuit is clear and relevant, and the modulation method is simple and low in cost;

2. The present invention can determine the background noise of the low-noise preamplifier in the state of the mechanical shutter CLOSE, and can subtract the influence of the background noise through the DSP processing circuit to make the result more real and credible;

3. The present invention adopts a high-resolution A/D conversion circuit to make the sampling result closer to the real value;

4. The present invention uses a DSP processing circuit to determine the sampling time and sampling interval for data, avoiding the noise introduced by the sampling analog switch itself and the inaccuracy brought by the RC integration circuit;

5. The present invention uses a high-pass filter circuit to digitally filter the data, which reduces the noise introduced by the circuit hardware filter;

6. The present invention adopts DSP processing circuit to digitally integrate the sampled data, and the processing speed is fast;

7. The DSP processing circuit in the present invention is connected to the computer by a shielded network cable, the output data is digital, and the anti-interference ability is strong, and the longest data transmission distance of 90m can be realized by the shielded network cable;

8. The present invention has the advantages of low cost, good repeatability, easy operation and convenient system integration.

Description of drawings

Fig. 1 is a schematic diagram of the structure of the present invention.

1-Mechanical shutter, 2-High voltage power supply, 3-Photomultiplier tube, 4-Low noise preamplifier, 5-High-pass filter circuit, 6-High resolution A/D conversion circuit, 7-DSP processing circuit, 8-Shutter Drive circuit, 9-computer.

detailed description

The invention provides a detection device capable of accurately detecting extremely weak light signals and a detection method of the device.

The structure of this device is described below in conjunction with Fig. 1:

The detection device includes a mechanical shutter 1, a photomultiplier tube 3, a low-noise preamplifier 4, a high-pass filter circuit 5, a high-resolution A/D conversion circuit 6, a DSP processing circuit 7, a shutter drive circuit 8 and a computer 9;

A mechanical shutter 1 is arranged between the photomultiplier tube 3 and the incident extremely weak light, the output end of the photomultiplier tube 3 is connected to the input end of the low-noise preamplifier 4, and the output end of the low-noise preamplifier 4 is connected to the high-pass filter circuit 5 The input end of the high-pass filter circuit 5 is connected with the input end of the high-resolution A/D conversion circuit 6, and the output end of the high-resolution A/D conversion circuit 6 is connected with the input end of the DSP processing circuit 7, and the DSP The processing circuit 7 is connected to the computer 9; a shutter driving circuit 8 is arranged between the mechanical shutter 1 and the DSP processing circuit 7 .

Wherein, those skilled in the art know that when using a photomultiplier tube, a high-voltage power supply 2 must be connected to the photomultiplier tube 3 to satisfy its normal operation.

The low-noise preamplifier 4 is a low-current-to-voltage preamplifier; the bias current of the low-noise preamplifier is less than 2pA, and the noise is less than or equal to 2mV _p - _p when the conversion gain is 10 ⁶ I/V; the high-pass filter circuit 5 filters out It can satisfactorily filter out low-frequency components less than 100 Hz; the resolution level of the high-resolution A/D conversion circuit 6 is 16 bits.

In addition, when the device is working, in order to avoid external interference, the computer 9 and the DSP processing circuit 7 are connected by a shielded network cable, and the shielded network cable can realize a data transmission distance of up to 90m, realizing interference-free, long-distance data transmission. transmission.

Through the above description of the device structure, the specific working process of the device includes the following steps:

Step 1) Setting parameters such as the modulation period, sampling time, sampling interval and sampling times of the extremely weak optical signal on the computer;

Step 2) The computer transmits the parameters set in step 1) to the DSP processing circuit;

Step 3) The DSP processing circuit controls the shutter drive circuit according to the set modulation period to make the mechanical shutter modulate the extremely weak light signal;

Step 4) The photomultiplier tube detects the modulated extremely weak light signal and outputs a weak current signal;

Step 5) Low-noise preamplifier circuit (bias current less than 2pA, noise ≤2mV _pp during 10 ⁶ I/V conversion) receives the photomultiplier tube output weak current signal and converts it into a voltage signal and amplifies it, and then the amplified voltage signal output;

Step 6) The high-pass filter circuit receives the amplified voltage signal and performs high-pass filter on the signal to filter out low-frequency components, and then outputs the filtered voltage signal; Components are filtered out to ensure more accurate sampling results)

Step 7) The high-resolution A/D conversion circuit receives the filtered voltage signal to convert the digital signal and transmits the signal to the DSP processing circuit;

Step 8) The DSP processing circuit collects the digital signal, removes background noise, digital sampling, band-pass filtering and digital integration to obtain the final extremely weak light signal data, and then transmits the data to the computer through a shielded network cable;

Wherein, the background noise removal is to remove the noise of the low-noise preamplifier circuit.

In step 8), the steps of removing background noise are as follows:

Step 8.1) The mechanical shutter is in a closed state, and there is no optical signal input;

Step 8.2) The low-noise pre-amplification circuit amplifies the weak voltage signal of the body and outputs it to the high-pass filter circuit; the amplified weak voltage signal is filtered by the high-pass filter circuit to remove low-frequency components, and the filtered voltage signal is transmitted to the high-resolution A/ D conversion circuit; the high-resolution A/D conversion circuit converts the filtered voltage signal into a digital signal and transmits the digital signal to the DSP processing circuit;

(Note: Usually, the high-pass filter circuit we use filters out the low-frequency components below 100Hz, so as to ensure more accurate sampling results)

Step 8.3) The DSP processing circuit records the data when the mechanical shutter is closed as the background noise of the preamplifier circuit;

Step 8.4) The DSP processing circuit subtracts the digital signal of the body noise of the low-noise preamplifier circuit received by the DSP processing circuit from the received digital signal of the extremely weak incident light.

Claims (3)

1. A very weak optical signal detection device is characterized in that: comprise mechanical shutter, photomultiplier tube, low-noise preamplifier, high-pass filter circuit, high-resolution A/D conversion circuit, DSP processing circuit, shutter drive circuit and computer; A mechanical shutter is set between the photomultiplier tube and the incident extremely weak light. The output end of the photomultiplier tube is connected to the input end of the low-noise preamplifier, and the output end of the low-noise preamplifier is connected to the input end of the high-pass filter circuit. The output end of the filter circuit is connected with the input end of the high-resolution A/D conversion circuit, the output end of the high-resolution A/D conversion circuit is connected with the input end of the DSP processing circuit, and the DSP processing circuit is connected with the computer; A shutter drive circuit is arranged between the mechanical shutter and the DSP processing circuit; the low-noise preamplifier is a low-current-to-voltage preamplifier; the bias current of the low-noise preamplifier is less than 2pA, at 10 ⁶ Noise ≤ 2mV _pp during I/V conversion gain; the high-pass filter circuit can filter out low-frequency components less than 100Hz; the resolution level of the high-resolution A/D conversion circuit is 16bit; The computer and the DSP processing circuit are connected to each other through a shielded network cable. 2. A detection method of the detection device according to claim 1, characterized in that, comprising the following steps: 1) Set the modulation period, sampling time, sampling interval and sampling times of the extremely weak optical signal on the computer; 2) The computer transmits the parameters set in step 1) to the DSP processing circuit; 3) The DSP processing circuit controls the shutter drive circuit according to the set modulation period to make the mechanical shutter modulate the extremely weak light signal; 4) The photomultiplier tube detects the modulated extremely weak light signal and outputs a weak current signal; 5) The low-noise preamplifier circuit receives the weak current signal output by the photomultiplier tube and converts it into a voltage signal and amplifies it, and then outputs the amplified voltage signal; the bias current of the low-noise preamplifier circuit is less than 2pA, at 10 ⁶ Noise ≤2mV _pp at I/V conversion gain; 6) The high-pass filter circuit receives the amplified voltage signal and performs high-pass filtering on the signal to filter out low-frequency components below 100 Hz, and then outputs the filtered voltage signal; 7) The high-resolution A/D conversion circuit receives the filtered voltage signal and converts it into a 16bit digital signal and transmits the signal to the DSP processing circuit; 8) The DSP processing circuit collects the digital signal, removes background noise, digital sampling, band-pass filtering and digital integration to obtain the signal data of the final extremely weak light, and then transmits the data to the computer through a shielded network cable; The noise floor is to remove the noise of the low-noise preamplifier circuit. 3. the detection method of detection device according to claim 2, is characterized in that: the step of removing background noise in described step 8) is as follows: 8.1) The mechanical shutter is closed and there is no optical signal input; 8.2) The low-noise preamplifier circuit amplifies the weak voltage signal of the body and outputs it to the high-pass filter circuit; the amplified weak voltage signal is filtered by the high-pass filter circuit to filter out low-frequency components, and the filtered voltage signal is transmitted to the high-resolution A/D Conversion circuit; the high-resolution A/D conversion circuit converts the filtered voltage signal into a digital signal and transmits the digital signal to the DSP processing circuit; 8.3) The DSP processing circuit records the data when the mechanical shutter is closed as the background noise of the preamplifier circuit; 8.4) The DSP processing circuit subtracts the background noise of the preamplifier circuit from the digital signal when the optical signal is incident.