Fig. 1.

The integrated Pound−Drever−Hall laser frequency stabilization system. a The block diagram of the reported Pound−Drever−Hall (PDH) chip implemented in 180 nm GF7RFSOI CMOS silicon-on-insulator process. The laser output is phase modulated using a p-doped-intrinsic-n-doped (PIN) phase modulator, passed through the electronically reconfigurable Mach−Zehnder interferometer (MZI) with 80 ps delay imbalance, and photo-detected. The photo-current is amplified and converted to a voltage using a trans-impedance amplifier (TIA) and mixed with the same electrical signal used to phase modulate the laser. The mixer output is filtered, amplified, converted to a current and injected to the laser gain section to lock the laser to the notch in the MZI frequency response. The output of the voltage-to-current converter, the read-out signal, is asymmetric with respect to the frequency of the notch in the MZI frequency response, $f_{\mathrm{notch}}$. Therefore, the read-out signal indicates both the difference between the laser frequency and $f_{\mathrm{notch}}$ and whether the laser frequency is greater or less than $f_{\mathrm{notch}}$ (Supplementary Note 1). Note that the modulation sidebands are essential to detect the sign of the read-out (error) signal required to tune the laser toward $f_{\mathrm{notch}}$. b The structure of the reported PDH system. c Micro-photograph of the CMOS chip with a photodiode mounted on top. The details of the integrated CMOS chip are included in Supplementary Note 5 and Supplementary Fig. 6. The hybrid integration of the photodiode chip with the CMOS chip is discussed in Supplementary Note 8. Scale bar, 0.5 mm