Weak laser detection system based on single chip microcomputer and using tuning fork chopping technology

We use photodetectors as the photoelectric conversion element of the system, and use tuning forks for mechanical chopping to directly convert the incident constant (or slowly changing) light signal into a modulated AC signal, which is first amplified by AC coupling to overcome the influence of temperature drift of the photodetector, and then phase-locked amplification is performed. The system's analog output signal is collected by a single-chip microcomputer, and nonlinear compensation is performed to overcome the shortcomings of general low-light detection systems. The system has the characteristics of simple structure and easy use.

System Design

The low-light detection system is mainly composed of an internally modulated photodetector, a signal processing system and a single-chip compensation system. Its overall structure is shown in Figure 1.

Figure 1 Overall principle block diagram of weak laser detection system

Temperature control circuit system

Since temperature changes have an impact on photodetectors, we used a bridge circuit to collect temperature signals through platinum resistance and compare them with the set value, so as to use a semiconductor refrigerator to actively control the temperature of the photodetector, and the temperature is controlled at +10_C with a control accuracy of 0.5_C. This greatly reduces the negative impact of the photodetector brought about by temperature changes.

Signal acquisition and signal conditioning system

First, a photodetector is selected as the photoelectric conversion device. The resonance of the tuning fork is driven by the inductive three-point oscillation, thereby realizing the chopping modulation of the incident weak laser, and the incident constant (or slowly changing) light signal can be converted into a modulated AC signal. Then, through AC coupling, DC and low-frequency temperature drift noise are filtered out, thereby overcoming the impact of temperature drift on the system. In the actual design, the signal processing part of the system includes the preamplifier and the phase-locked amplifier of the AD630 chip. The preamplifier pre-amplifies the weak AC signal output by the detector. The AD630 chip of ADI can easily form a phase-locked amplifier according to the device manual provided by the company. The circuit can extract the signal from the +100dB noise. This design uses this circuit to extract the weak signal. The reference signal is drawn from the center tap of the resonant coil, and then used as the reference signal of the phase-locked amplifier after phase shifting. In this way, the AC signal of a given frequency is amplified and the noise signal of other frequencies is greatly suppressed, thereby obtaining a voltage signal proportional to the light intensity, and this signal is handed over to the single-chip microcomputer system for nonlinear compensation.

Single chip microcomputer system design

In the actual application of weak laser detection system, it is found that when the laser is strong or weak, the nonlinearity of the output signal of the signal processing circuit increases to a certain extent, which makes the output linearity worse, affecting the dynamic range and measurement accuracy of the system. For this reason, we use a single-chip microcomputer system to collect data on its output signal and make linear compensation. The block diagram of the single-chip microcomputer system is shown in Figure 2.

Figure 2 MCU compensation and display system hardware block diagram [page]

The table lookup data is stored in the EEPROM, and then the measured value is read from the A/D converter using a single-chip microcomputer, followed by table lookup, correction, and finally the measured value is displayed on the LED display system through the 8255 parallel port.

Since the nonlinear characteristics of the output signal of the weak laser detection system are related to the characteristics of the photosensitive element itself, there is no good mathematical model, so we use the table lookup method to perform linear compensation on its measured value. The program is written in MCS51 assembly language, mainly including A/D conversion module, linear compensation module and display module. The program flow chart is shown in Figure 3.

Figure 3 Program flow chart

Experimental Results and Discussion

In the experiment, we found that when the incident laser energy is weak, the response of the system deviates from linearity, so it is necessary to compensate for the nonlinearity of its measurement results. Comparison of system correction results When using a single-chip microcomputer to compensate the system linearly, it is necessary to obtain the table data for correction and store it in an external EEPROM. We use a standard laser source as the signal light source, corresponding to the standard laser source with different light intensities, and record its output results. The experimental results show that the tuning fork chopping technology used in the system can effectively overcome the drift of the detector with temperature changes. The linearity of the output signal after correction by the single-chip microcomputer system is also greatly improved, indicating that the system has achieved a relatively ideal detection effect.

Conclusion

The modulated weak laser detection system uses tuning fork chopping technology to overcome the shortcomings of general light detection systems. The single-chip system is used to measure the results and make linearity, which improves the measurement accuracy of the system. The system has strong versatility. Different photosensitive detectors can be used for low-light detection in different bands, and different filters are placed in front of the detector. The system can be used as a single-wavelength energy meter for different occasions and has a wide range of uses in environmental protection, industry, scientific research and other fields.