I. Project Overview
1.1 Introduction
In optical measurement applications, it is sometimes necessary to modulate the optical signal for frequency selective amplification and coherent detection after photoelectric conversion. We can use an optical chopper to modulate the optical signal. The optical chopper modulates the continuous light from the light source by controlling the rotation of the blade to modulate the light signal into an isochronous optical signal.
In optical research, sometimes the optical signal is measured, and the optical signal to be measured may be weak, and it is interfered by many signals, and even the interference signal strength is much higher than that of the optical signal to be measured. At this point, if you know the frequency value of the wanted signal, you can use a lock-in amplifier to measure these signals that are submerged in the noise. The optical chopper can provide a reference signal for the lock-in amplifier. The optical chopper modulates the optical signal to modulate the optical signal, and outputs a reference voltage square wave synchronized with the modulation frequency, thereby providing an accurate reference signal for the lock-in amplifier, so that the lock-in amplifier can recognize A useful weak signal. The process of a single optical path experiment using an optical chopper is shown in Figure 1.
Figure 1 Single light path experiment process
Since the optical chopper can modulate the measured light and simultaneously output a reference voltage square wave synchronized with the modulation frequency as a reference signal for the lock-in amplifier, the optical chopper is particularly suitable for a laser using a lock-in amplifier. Optical or microwave measurement system.
1.2 Project Background
At present, mature optical chopper products on the market mainly include: SR540 of Stanford Research Systems, USA; Signal Recovery of the United States (formerly EG&G) includes Model 650 series (651-1, 652-1, 652-2), Model 197 , Model 198A; British Scitec Model-300C series (300CD, 310CD, 320CD, 340CD, 350CD, 360C), Model-C995; American TTI C-995 optical chopper; Japan HORIBA ACH-C. The accuracy and stability of these products are very high, and their main technical indicators are shown in Table 1.
Table 1 Mainstream optical chopper technical parameters
At present, the domestic market only sees the HIT-3 optical chopper studied by the Institute of Automatic Detection and Process Control Systems of Harbin Institute of Technology, which can adjust the measured light in the frequency range of 20 Hz to 6 kHz. Frequency stability: "0.1% . Due to the high stability and reliability of optical choppers, they can be recognized by the market. Nowadays, domestic optical choppers have not been widely promoted.
The optical choppers used in the experiments are now imported, which are more expensive and inconvenient to purchase. Now take this opportunity to study optical choppers, make optical choppers and apply them to experiments, replacing the current laboratory optical chopper.
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