Compact QPSK and PAM Modulators With Si Photonic Crystal Slow-Light Phase Shifters

We demonstrate quadrature phase-shift keying (QPSK) modulation and four-level pulse-amplitude modulation (4-PAM) in Si Mach-Zehnder modulators whose phase shifter length is reduced to 300 μm by the slow-light effect in photonic crystal lattice-shifted waveguides and increased modal overlap with interleaved p-n junctions. The QPSK constellation patterns were observed up to 15 Gbaud (30 Gb/s) with an error-vector magnitude of <;20% and 28 Gbaud (56 Gb/s) using an RF equalizer filter. In the 4-PAM, four separate eyes were observed up to 15 Gbaud (30 Gb/s). Improving the modulation efficiency and frequency bandwidth and suppressing the insertion loss may deliver practical modulator performance.