Fig. 1. Schematic diagram of the FPGA-based frequency stabilization system. Mode-matching lenses in front of the PBS and a quarter-wave plate between the PBS and cavity are not shown in the figure. VOA, variable optical attenuator; EOM, electro-optic modulator; OFC, optical fiber collimator; PD, photodetector; PBS, polarization beam splitter; FPGA, field-programmable gate array; PZT, piezo-electric transducer; DAC, digital-to-analog converter; ADC, analog-to-digital converter; DDS, direct digital synthesizer; LPF, low-pass filter; LD, laser diode.

Fig. 2. (a) PZT actuation, (b) cavity transmission, and (c) error signal during the establishment of laser frequency locking. The coarse and fine PZT voltage scans cover frequency ranges of 20 and 2.5 MHz, respectively. Around 5.8 s, a perturbation is introduced and the locking is recovered after 100 ms.

Fig. 3. Laser frequency noise and individual noise contributions. (a) In-loop noise of PDH frequency locking and the noise floor of electronic origin. (b) Laser frequency noise and contributions from RIN and RAM. The noise floor is measured by blocking the beam impinging on PD1 and then measuring the demodulated signal. The frequency noise spectrum is obtained from 12-h data of heterodyne beat between two locked Nd:YAG lasers, and individual noise contributions from laser intensity fluctuation and RAM are from separate out-of-loop measurements. Digital low-pass filters with a corner frequency of 1 kHz are used for the RAM measurements.

Fig. 4. Optical heterodyne beat of two Nd:YAG lasers independently locked to two 20-cm cavities. (a) Beat frequency; (b) frequency instability. A linear drift of 0.016 Hz/s is removed from the beat frequency.