Thermally controlled Si photonic crystal slow light waveguide beam steering device.

The doubly periodic Si photonic crystal waveguide radiates the guided slow light into free space as an optical beam. The waveguide also functions as a beam steering device, in which the steering angle is changed substantially by a slight variation in the wavelength generated due to the large angular dispersion of the slow light. A similar function is obtained when the wavelength is fixed and the refractive index of the waveguide is changed. In this study, we tested two kinds of integrated heater structures and observed the beam steering using the thermo-optic effect. For a p-i-p doped waveguide, the heating current was made to flow directly across the waveguide and a beam steering range of 21° was obtained with a relatively low heating power and high-speed response of the order of 100 kHz, maintaining a narrow beam divergence of 0.1-0.3° and a 120 resolution points. We also performed a preliminary life test of the device but did not observe any severe degradation in the temperature variation of 80-430 K for the duration up to 20‒40 h. For a TiN heater device, we obtained the comparable beam steering characteristics, but the required heating power increased, and the response speed decreased drastically.

Electro-optic phase matching in a Si photonic crystal slow light modulator using meander-line electrodes.

We demonstrate a Si photonic crystal waveguide Mach-Zehnder modulator that incorporates meander-line electrodes to compensate for the phase mismatch between slow light and RF signals. We first employed commonized ground electrodes in the modulator to suppress undesired fluctuations in the electro-optic (EO) response due to coupled slot-line modes of RF signals. Then, we theoretically and experimentally investigated the effect of the phase mismatch on the EO response. We confirmed that meander-line electrodes improve the EO response, particularly in the absence of internal reflection of the RF signals. The cut-off frequency of this device can reach 27 GHz, which allows high-speed modulation up to 50 Gbps.

Full C-band Si photonic crystal waveguide modulator.

The slow-light effect in silicon lattice-shifted photonic crystal waveguide (LSPCW) Mach-Zehnder modulators allows compact phase shifters, while limiting the working spectrum Δλ. We optimized the structural parameters of the LSPCW and extended Δλ to cover the full C-band in exchange for moderately decreasing the group index ng. We obtained Δλ=42 nm with ng=8-9 in a fabricated device and observed 25 Gbps eye opening in the 200 µm modulator in the full C-band.

Optimized optical coupling to silica-clad photonic crystal waveguides.

Silica-clad silicon photonic crystal waveguides (PCWs) are promising components for various applications because of their simple fabrication and generation of slow light. However, an optical loss higher than 4 dB occurs when they are simply coupled to input/output silicon wire waveguides. To reduce the optical loss, we proposed a junction structure in which light in the waveguide is first coupled to a high-group-velocity radiation mode at an expanded core and subsequently converted to the slow-light mode in a tapered core of the PCW. The coupling loss at a junction is calculated to be 0.28 dB at its minimum and less than 0.5 dB for the wavelength range of 12 nm. We measured a coupling loss of 0.46 dB for the device fabricated by the silicon photonics process. This low-loss junction well supports the practical use of PCWs.

Slow-light effect in a silicon photonic crystal waveguide as a sub-bandgap photodiode.

We demonstrate a Si sub-bandgap photodiode in a photonic crystal slow-light waveguide that operates at telecom wavelengths and can be fabricated using a Ge-free, standard Si-photonics CMOS process. In photodiodes based on absorption via mid-bandgap states, the slow-light enhancement enables performance that is well balanced among high responsivity, low dark current, high speed, wide working spectrum, and CMOS-process compatibility, all of which are otherwise difficult to achieve simultaneously. Owing to the slow-light effect and supplemental gain at a high reverse bias, the photodiode shows a responsivity of 0.15 A/W with a low dark current of 40 nA, which is attributed to no particular processes such as ion implantation and excess exposure of the Si surface. The maximum responsivity was 0.36 A/W. The modest gain allows for sufficient frequency bandwidth to observe an eye opening at up to 30 Gb/s.

Hitless tunable WDM transmitter using Si photonic crystal optical modulators.

We have developed compact Si Mach-Zehnder modulators that are assisted by wideband low-dispersion slow light in lattice-shifted photonic crystal waveguides. We have also developed Si triangular-shaped coupled-microring multiplexers that allow a box-like spectrum, a wide free spectral range, and an efficient thermal tuning. In this study, we integrated three sets of these devices in a small footprint of 2.0 × 0.7 mm(2) and achieved their 25 Gbps/ch operation as a wavelength division multiplexing transmitter. Moreover, we demonstrated hitless wavelength tuning using thermo-optic switches loaded in the bus waveguide.

Slow-light Mach-Zehnder modulators based on Si photonic crystals.

Mach-Zehnder optical modulators are the key devices for high-speed electrical-to-optical conversion in Si photonics. Si rib waveguides with a p-n diode structure operated in the carrier depletion mode have mainly been developed as their phase shifters. Their length is usually longer than millimeters due to the limited change in the refractive index due to the carrier depletion in a Si p-n diode. This length is shorter than commercial LiNbO3 modulators, but still much shorter devices are desired for large-scale integration and for simplifying the high-speed RF modulation. A promising solution is to use slow light in photonic crystal waveguides, which enhances the modulation efficiency in proportion to the group-velocity refractive index ng. In particular, dispersion-engineered slow light allows more than five-fold enhancement, maintaining a wide working spectrum as well as large temperature tolerance. The devices with a phase shifter length of around 100 µm are fabricated by a standard process compatible with complementary metal-oxide semiconductors. The operation at 10 Gbps and higher speeds are obtained in the wavelength range of 16.9 nm and temperature range of 105 K.