Aiming at the shortcomings of the traditional optical power meter on the market, such as small dynamic range, low test accuracy, obvious nonlinear error and slow gear switching speed, a high precision optical power meter based on STM32 is designed, which adopts advanced large dynamic wavelength response. The range of INGAAS-PIN photodetectors, together with the AD795 photoelectric preamplifier AD795, TI's 24-bit analog-to-digital converter ADS1232 and Maxim's high-speed multi-channel analog switch MAX4051 for system design, using STM32 to control the amplification range The automatic gain switching technology eliminates the measurement error caused by the nonlinear response of the photodetector to light at different light intensities at the same wavelength, which can greatly improve the accuracy and reliability of the optical power meter test.
2, the system worksThe principle block diagram of the system is shown in Figure 1. The INGAAS-PIN photodetector converts the detected optical signal into a current signal, performs I/V (current-voltage) conversion, and outputs a voltage signal. After amplification and filtering, the voltage signal is sent. Into the A / D for analog-to-digital conversion, according to the size of the converted data, using the microprocessor to determine the automatic switching of the range of the control amplifier circuit to obtain the appropriate amount of digital calculation, and finally by the STM32 controller to carry out the data Processing and analysis, and then sent to the LCD LCD for power display, and achieve system button control, data storage and serial communication operations.
Figure 1 system principle
3, PM hardware 3.1, INGAAS-PIN photodetectorAt present, the popular photodetectors include PIN photodiodes and APD avalanche photodiodes. APD has a high detection speed and plays a major role in the high-speed photodetection application industry, but its dark current is large, requiring a high bias voltage and noise. Larger, not suitable for use in accurate measurements. PIN photodiodes with low dark current, high sensitivity, and can operate in a biased state are widely used in optical power design. The PIN tube currently used is mainly made of materials such as SI, Ge, INGAAS, and covers a wavelength range from 400 to 1800 NM. The relationship between the responsiveness R and the wavelength λ of the PIN photodetectors of different materials is shown in Fig. 2.
Figure 2 Relationship between responsivity R and wavelength λ of PIN photodetector
Absolute responsivity is an important indicator to measure the performance of photodetectors, which determines the relationship between photocurrent and optical power. The responsiveness formula is defined as the ratio of the primary photocurrent IP to the incident optical power P.
R=IP/P(1)
Where: R is the responsiveness of the photodiode.
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