Mizuki Shinkawa

10 Gb/s operation of photonic crystal silicon optical modulators

We report the first experimental demonstration of 10 Gb/s modulation in a photonic crystal silicon optical modulator. The device consists of a 200 μm-long SiO2-clad photonic crystal waveguide, with an embedded p-n junction, incorporated into an asymmetric Mach-Zehnder interferometer. The device is integrated on a SOI chip and fabricated by CMOS-compatible processes. With the bias voltage set at 0 V, we measure a V(π)L < 0.056 V∙cm. Optical modulation is demonstrated by electrically driving the device with a 2(31) - 1 bit non-return-to-zero pseudo-random bit sequence signal. An open eye pattern is observed at bitrates of 10 Gb/s and 2 Gb/s, with and without pre-emphasis of the drive signal, respectively.

Compact and fast photonic crystal silicon optical modulators

We demonstrate the first sub-100 μm silicon Mach-Zehnder modulators (MZMs) that operate at >10 Gb/s, by exploiting low-dispersion slow-light in lattice-shifted photonic crystal waveguides (LSPCWs). We use two LSPCW-MZM structures, one with LSPCWs in both arms of the MZM, and the other with an LSPCW in only one of the arms. Using the first structure we demonstrate 10 Gb/s operation with a operating bandwidth of 12.5 nm, in a device with a phase-shifter length of only 50 μm. Using the second structure, owing to a larger group index as well as lower spectral noise, we demonstrate 40 Gb/s operation with a phase-shifter length of only 90 μm, which is more than an order-of-magnitude shorter than most 40 Gb/s MZMs.

Nonlinear enhancement in photonic crystal slow light waveguides fabricated using CMOS-compatible process.

We have studied low-dispersion slow light and its nonlinear enhancement in photonic crystal waveguides. In this work, we fabricated the waveguides using Si CMOS-compatible process. It enables us to integrate spotsize converters, which greatly simplifies the optical coupling from fibers as well as demonstration of the nonlinear enhancement. Two-photon absorption, self-phase modulation and four-wave mixing were observed clearly for picosecond pulses in a 200-μm-long device. In comparison with Si wire waveguides, a 60-120 fold higher nonlinearity was evaluated for a group index of 51. Unique intensity response also occurred due to the specific transmission spectrum and enhanced nonlinearities. Such slow light may add various functionalities in Si photonics, while loss reduction is desired for ensuring the advantage of slow light.

Photonic Crystal Silicon Optical Modulators: Carrier-Injection and Depletion at 10 Gb/s

We demonstrate 10 Gb/s modulation in a 200 μm photonic crystal silicon optical modulator, in both carrier-injection and depletion modes. In particular, this is the first demonstration of 10 Gb/s modulation in depletion mode and without pre-emphasis, in a Mach-Zehnder type modulator of this length, although moderate pre-emphasis can improve the signal quality. This is made possible by utilizing the slow-light of the photonic crystal waveguide, where the group index ng is ~ 30 and gives ~ 7 times enhancement in the modulation efficiency compared to rib-waveguide devices. We observe 10 Gb/s modulation at drive voltages as low as 1.6 V and 3.6 V peak-to-peak, in injection- and depletion-modes, respectively.

Photonic crystal tunable slow light device integrated with multi-heaters

We fabricated photonic crystal slow light waveguides integrated with multi-heaters, using CMOS-compatible process. By optimizing heating powers and adjusting the index distribution, a clear delay peak was observed, which suggests that the fabrication errors were compensated for completely. When a linear index chirp was added to this condition, the delay was tuned by 54 ps. When a quadratic chirp was added, arbitrary group delay dispersion was generated at wavelengths around 1550 nm within a 3 nm bandwidth. The continuously tunable range was from −32 to 54 ps/nm/mm. Using this as a dispersion compensator, we compressed pre-chirped pico-second pulses.

Photonic crystal slow light devices fabricated by CMOS-compatible process

We have studied wideband dispersion-free slow light in photonic crystal waveguides and other photonic nanostructures, which allows tunable delays in short optical pulses and the enhancement of light-matter interaction. This paper demonstrates the state-of-the-art devices fabricated by CMOS-compatible process, where compact devices of 200 – 400 μm lengths are integrated with Si photonics components and controlled by DC and AC electronics. They are applied for dispersion controllers, optical correlators, DQPSK receivers, retiming of pulse train, two-photon absorption photo-diodes and Mach-Zehnder modulators.

Sub-100 µm, 40 Gb/s photonic crystal silicon optical modulators

We demonstrate a 90 μm, 40 Gb/s silicon MZI modulator, exploiting large group index (>;30) slow-light photonic crystal phase-shifters at 5.3 Vpp drive voltage. We also demonstrate a 50 μm device with 12.5 nm bandwidth.

Nonlinear enhancement in photonic crystal waveguides with spot-size converter

We evaluated the nonlinear enhancement in slow-light photonic crystal waveguides. The devices butt-joined to Si wire waveguides were fabricated on SOI substrate using CMOS-compatible process. Integrating spot size converter to the device allows high input power and clear enhancement. The coupling loss between lensed fiber and the wire was 3 - 4 dB. We observed two-photon absorption, self-phase modulation and four-wave mixing in the low power regime of less than 1 W, which is assisted by low-dispersion slow light. Such slow light can be exploited for compact functional components such as optical limiters and wavelength converters.

Nonlinearity induced ultrafast slow-light tuning in photonic crystal waveguide

We demonstrated ultrafast delay tuning of slow-light pulse with a response time < 10 ps. This is achieved using two types of slow light: dispersion-compensated slow light for the signal pulse; and low-dispersion slow light to enhance the nonlinear effect of the control pulse. These two slow light are generated simultaneously in lattice-shifted photonic crystal waveguides, arising from flat and straight sections of the photonic band, respectively. The control pulse blue-shifts the signal pulse spectrum, through dynamic tuning and cross-phase modulation caused by the plasma effect of two-photonabsorption carriers. This changes the delay by up to 10 ps only when the two pulses overlap within the waveguide, and enables an ultrafast tuning that is not limited by the carrier lifetime. Using this, we succeeded in the delay tuning of one target pulse within a pulse train with 12 ps intervals.

Electrically-controlled photonic crystal slow light device and its application to optical correlator

Lattice-shifted photonic crystal waveguides (LSPCWs) are effective for generating wide-band on-chip slow light at room temperature. In this study, we integrated the LSPCW with multi-heaters using CMOS-compatible process, and demonstrated electrically-tunable slow light. Seven heaters were placed on each side of the LSPCW with air slots for thermal isolation. On-demand temperature distributions were formed by controlling each heating power independently. When the heating powers were optimized, a clear delay peak of slow light corresponding to the flat photonic band of the LSPCW was observed, which suggests that fabrication errors in the air-hole diameters were less than 5 nm and related band fluctuation was well compensated by the heating. When a linear temperature distribution was added to this condition, the delay was reduced up to 54 ps. When a quadratic distribution was added, the group delay dispersion was generated in the range from -10.2 to 17.5 ps/nm. We applied the tunable delay to the delay scanning in optical correlator. Here, output pulse were compressed to 0.6 ps through self-phase modulation and dispersion compensation in external fibers, and its delay was tuned in range of 17 ps. At a scanning frequency of 100 Hz, which is <10 times faster than that of conventional mechanical delay scanners, pulse lengths of 0.3 - 6 ps were measured with a <95% accuracy. We are also fabricating all on-chip optical correlator consisting of this slow light delay scanner and two-photon-absorption photo-diodes.