Detection of a weak Near-Infrared signal using a digital orthogonal-vector lock-in amplifier

A Near-Infrared (NIR) measurement method based on a digital orthogonal-vector lock-in amplifier (LIA) is presented in this paper. NIR sky background radiation is very weak; to detect the signals obscured by noise, our approach is to use a chopper to modulate the detected signal and demodulate it using an LIA . The effect of the 1/f noise of the detector, dark current and other noise sources can be reduced to improve Signal-to-Noise Ratio (SNR) and the detected signal will be obtained. The orthogonal vector LIA avoids phase shift and achieves high precision measurements using two orthogonal components. In order to simplify the system, data are digitized by an Analog-to-Digital Converter (ADC) and a digital algorithm, running in a Microcontroller Unit (MCU) with ARM cortex-M4, is adopted to implement the LIA . In our scheme, the signal of the detector is amplified and filtered. Then, Phase Sensitive Detection (PSD), Low-Pass Filtering (LPF) and amplitude phase calculation are performed using the digital LIA method. The digital method leads to a greatly simplified circuit design, and adjustment of the time constant of the LPF allows achieving different Equivalent Noise Bandwidths (ENBs) conveniently. The method has the advantage of high precision, flexible usage, simple implementation and low computational resource requirements. Using this method, weak infrared signal submerged in the noise can be picked up easily, which extremely improves the detection capability of the system.