Norihiro Ishikura

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.

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.

Wide Range Tuning of Slow Light Pulse in SOI Photonic Crystal Coupled Waveguide via Folded Chirping

In this paper, we demonstrate state-of-the-art slow light in silicon-on-insulator photonic crystal coupled waveguide, which allows slow light pulse transmission and its tunable delay by means of structural chirping. The key idea of this study is the application of a folded chirping profile to the structure, instead of the conventional monotonous chirping. It suppresses unwanted spectral oscillation caused by structural disordering and expands the tuning range. By postprocessing an airhole-diameter-chirped device, we show that 0.9-ps-wide slow light pulses are delayed for 72 ps, corresponding to a buffering capacity of 80 bits. In a separate, unchirped device, we demonstrate a tunable delay by applying thermally induced index chirping. Here, a maximum tuning range of 103 ps and a tunable capacity of 22 bits are obtained.

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.

Continuously tunable slow-light device consisting of heater-controlled silicon microring array

We experimentally demonstrate a tunable slow-light device consisting of all-pass Si microrings. A compact device of 0.014 mm2 footprint is fabricated by using CMOS-compatible process, and its center wavelength, bandwidth and delay are continuously tuned by integrated heaters. The tuning range is 300 ps at fixed wavelengths with a 1 nm bandwidth. Eye opening of 40 Gbps non-return-to-zero signals is observed at up to a 150 ps delay and a 4 bit buffering capacity is confirmed, which corresponds to a spatial buffering density of 0.29 kbit/mm2.

Two-photon-absorption photodiodes in Si photonic-crystal slow-light waveguides

We demonstrate two-photon-absorption photodiodes in Si photonic-crystal waveguides, which shows wideband low-dispersion slow light. The device was fabricated on SOI substrate by CMOS-compatible process. The responsivity was improved by higher group indexes of slow light up to 0.052 A/W for pulses at wavelengths around 1550 nm with a 2.7 ps width and sub-watt peak powers. We applied this device to an optical correlator and dispersion detector. In the former, the correlation waveforms of 0.7−10 ps pulses were observed with small errors. In the latter, photocurrents inversely proportional to the pulse width were detected.

Large tunable fractional delay of slow light pulse and its application to fast optical correlator.

The slow light device based on photonic crystal coupled waveguide was fabricated, and a tunable delay of 72 ps was obtained for 2-ps-wide slow light pulses by local heating, which corresponds to a tunable fractional delay of 36. This value was further enhanced to 110 by compressing the output pulses through self-phase modulation and dispersion compensation in external fibers. We applied this device to optical correlator as a delay scanner, where the fractional delay determines the resolution of the delay scanning. Using this correlator, we successfully observed sub-picosecond pulses at a scan frequency up to 2 kHz, which is 100 times faster than that of mechanical scanners in conventional correlators.

Dispersion-controlled slow light in photonic crystal waveguides.

Slow light with a markedly low group velocity is a promising solution for optical buffering and advanced time-domain optical signal processing. It is also anticipated to enhance linear and nonlinear effects and so miniaturize functional photonic devices because slow light compresses optical energy in space. Photonic crystal waveguide devices generate on-chip slow light at room temperature with a wide bandwidth and low dispersion suitable for short pulse transmission. This paper first explains the delay-bandwidth product, fractional delay, and tunability as crucial criteria for buffering capacity of slow light devices. Then the paper describes experimental observations of slow light pulse, exhibiting their record high values. It also demonstrates the nonlinear enhancement based on slow light pulse transmission.

Silica-Clad Silicon Photonic Crystal Waveguides for Wideband Dispersion-Free Slow Light

We comprehensively calculated the photonic bands of the waveguide modes in practical lattice-shifted photonic crystal waveguides, which are completely cladded by silica. We assumed various lattice shifts and found that the shift of the second rows and the mixed shift of the first and third rows along the waveguide generate low-dispersion slow light with group indices of 34-36, which is higher than those with a conventional shift of the third rows, maintaining a wide bandwidth over 10 nm at telecom wavelengths. We fabricated the waveguides using a CMOS-compatible process and confirmed correspondence with the calculation results. We also compared 25-Gb/s photonic crystal slow light Mach-Zehnder modulators and confirmed the improvement of the modulation efficiency by second-row shifts.