Research And Design Of Nearinfrared Spectrum Signal Detecting System Based On AOTF

The near infrared spectrometer which based on AOTF has the advantages of stable structure, fast measurement, high precision, high accuracy. In this paper, a near-infrared spectrometer detecting system based on AOTF(Acousto-optic Tunable Filter) is designed, using the correlation detection method design lock-in amplifier circuit to realize weak signal detection, and using DDS(direct digital frequency synthesis technology) to realize the automatic scanning wavelength of procedural control.This paper complete background intensity measurement of AOTF near infrared spectrometer detection system, and get the background intensity figure of different wavelength.In this paper, the main work includes:1) Introduce the based on AOTF near infrared spectrometer detection system structure: Analysis the AOTF principle and its tuning relationship between diffraction light wavelength and driving frequency; Using monochromatic light modulation technology, determine the design of AOTF near-infrared spectrometer system based on monochromatic light modulation; Using correlation detection method, determine the part of signal detection scheme based on lock-in amplifier design.2) By using the correlation detection method design lock-in amplifier circuit for weak signal extraction: DDS is used to generate the frequency-adjustable reference signal; Monochromatic light modulation technology is used to convert the aperiodic useful signal to a periodic signal under the background noise; Phase-shifting circuit is used to realize the useful signal and reference signal phase synchronization; Phase-sensitive detection circuit is used to extract the weak signal from the noise background. On the basis of the lock-in amplifier circuit, to improve the accuracy of measurement, double lock-in amplifier circuit is designed which using phase-locked loop to realize dual reference signal has a fixed 90° phase difference.3) By using the DDS realize AOTF driving signal generation and control: The wavelength of AOTF spectrometer and its driving signal frequency has a one-to-one relationship, using this characteristics and DDS to produce specific frequency of AOTF drive signal; Compile the microcontroller procedures to realize wavelength scanning from 900 nm to 1700nm; It not only can choose a certain wavelength to scan within the scanning scope, but also can realize the auto-scanning in the whole spectrum, scanning step in 0.1nm to 10 nm wavelength is adjustable.4) Complete the hardware and software part of the control system design, including microcontroller, DDS, AD converter analog to digital, communication UART and voltage regulator, etc.Finally, this paper verify the lock-in amplifier circuit and AOTF near infrared spectrometer detecting system function, and analyze the testing result. Through the experiment and debugging, this paper complete the implementation of AOTF near-infrared spectrometer detection system, the system based on lock-in amplifier curcuit signal to noise ratio is 2605:1, basic to achieve the expected design goal.