Pound–Drever–Hall laser frequency locking technique based on orthogonal demodulation

Abstract

A Pound-Drever-Hall laser frequency-locking scheme is developed based on the principle of orthogonal demodulation. In the orthogonal demodulation Pound-Drever-Hall system, three sine signals are generated simultaneously using a direct digital synthesizer. A 0° phase sine signal is used to drive an electro-optic modulator to produce the phase sidebands, and 180° and 270° phase sine signals are used as reference signals for phase demodulation. The phase-modulated laser beam is coupled with a reference Fabry–Pérot cavity, and the reflected beam is sent into a photo-detector whose output is mixed with two orthogonal reference signals to obtain in-phase and orthogonal components of the error signal. An analog switch is used to eliminate the offset of the analog demodulator and channel mismatch. Using an analog-to-digital converter, the two orthogonal components are processed using orthogonal phase sensitive detection to obtain the error signal and PI controlled to acquire the correction signal on a host computer. To increase the resolution and SNR of analog-to-digital conversions, oversampling and averaging is utilized. The Pound-Drever-Hall laser frequency discrimination and tracking system is established and investigated experimentally using the orthogonal demodulation method. A frequency discrimination curve is obtained, and it is observed that the resonant frequency of the Fabry–Pérot cavity can automatically track laser frequency variation.