Slow Light Tuning in Photonic Crystals

One attractive application of slow light is tuning the delay of optical pulses. It is achieved by controlling the chirping of photonic crystal waveguides whose photonic band exhibits a flat band sandwiched by the opposite dispersion characteristics. We call the so-obtained tunable slow light dispersion-compensated (DC) slow light. In these years, we have fabricated the devices using CMOS-compatible process, demonstrated the delay tuning of sub-ps pulses with a tuning resolution over 100, and applied it to varying the demodulation rate of DQPSK receiver. The other attractive application of slow light is enhancing the nonlinearity. It is achieved by the particular design of photonic crystal waveguides, which exhibits a straight photonic band with a small slope, producing low-dispersion (LD) slow light. We have demonstrated strong two-photon absorption (TPA), self-phase modulation and four-wave mixing in Si-based devices at fiber communication wavelengths. In this presentation, we demonstrate two advanced devices that utilize both DC and LD slow light. One is the on-chip optical correlator and the other is the all-optical ultrafast delay tuning. In the former, the delay scanner based on DC slow light and nonlinear-enhanced TPA photodiode based on LD slow light were integrated. The auto-correlator action was confirmed for ps pulses. In the latter, the delay of the DC signal pulse was tuned through the intensity and timing of the LD control pulse with a maximum tuning range and response time of 10 ps. It potentially achieves the retiming of disordered pulses by using LD pulses as a clock.