CIC-Assisted FIR Digital Lock-In Demodulation for Multi-Spot Position Detection with a Quadrant Detector

Abstract

Multi-spot position detection with a quadrant detector (QD) is useful for compact multi-beam optical tracking in maritime optical terminals, distributedaperture sensing, and beam combining systems for future space-air-sea integrated networks. In such receivers, several optical beams may be monitored by one shared detector, and their photocurrents are superimposed on the same four electrodes. Frequency-division modulation can separate these components, but existing schemes commonly rely on fast Fourier transform (FFT)-based spectral extraction. When the modulation frequencies are assigned in advance, direct carrier-selective demodulation is also possible. This paper proposes a multi-spot QD position-detection method based on digital quadrature lock-in demodulation with cascadedintegrator-comb (CIC) decimation and finite impulse response (FIR) low-pass filtering. The proposed demodulator extracts the assigned carrier labels, reduces the baseband data rate after quadrature mixing, controls the detection bandwidth through FIR filtering, and combines I/Q components to obtain phase-insensitive quadrant amplitudes. Monte Carlo simulations show that three simultaneous spots modulated at 0.8, 1.0, and 1.2 kHz can be separated and localized with sub-micrometer accuracy.