Silicon Mach-Zehnder modulator using low-loss phase shifter with bottom PN junction formed by restricted-depth doping

A silicon Mach-Zehnder modulator with a low-loss phase shifter formed by restricted-depth doping is fabricated and characterized. The phase shifter has a PN junction at the bottom of the rib waveguide, whereas the top of the rib is un-doped. Device simulations confirm that the phase-shifter loss is reduced by 26-28% with an increased phase-shifter length to keep a pi phase shift with the same bias condition in comparison with a conventional PN junction through the whole depth. An optical loss as low as 1.6 dB has been achieved in the fabricated phase shifter with a 6-mm length. The Mach-Zehnder modulator with the phase shifter has a fiber-to-fiber loss of 9 dB. 10-Gbps non-return-to-zero on-off keying with extinction ratio of 11 dB is demonstrated under push-pull operation with a 6V(pp) driving voltage on each arm.