Moe Takeuchi

Two-dimensional beam steering device using double periodic Si photonic crystal waveguide

We demonstrate a non-mechanical on-chip optical beam steering device using the photonic crystal waveguide with a double periodic structure that repeats the increase and decrease of hole diameter. Guided slow light in this waveguide is radiated to be a light beam. Slow light shows strong dispersion, which allows a deflection angle of approximately 10 times that of a normal diffraction grating. We fabricated the device using complementary metal oxide semiconductor process and observed a beam deflection angle of 24° in the longitudinal direction with maintaining the divergence angle of 0.3° when the wavelength was changed by 27 nm. Four such waveguides were integrated, and one of them was selected by a Mach-Zehnder optical switch. Then, the lateral beam steering was obtained when a cylindrical lens was placed above these waveguides. By combining the longitudinal and lateral beam steering, the collimated beam was scanned two-dimensionally with 80 × 4 resolution points.

Thermally controlled Si photonic crystal waveguide slow light beam steering device

The double-periodic Si photonic crystal waveguide radiates guided slow light into free space as an optical beam. It also functions as a beam steering device, in which the steering angle is changed widely by the slight wavelength variant thanks to the large dispersion of slow light. A similar function is obtainable when the wavelength is fixed and the refractive index of the waveguide is changed. In this study, we integrated two kinds of heater structures in the waveguide and demonstrated the beam steering by the thermo-optic effect. For a p-i-p doped heater structure, we implanted a p-type dopant except around the waveguide core, and observed a beam steering angle Δθ = 26°, which is close to a theoretical value, with a relatively low heating power P = 1.6 W and high-speed response of 100 kHz order. However, the beam divergence increased up to δθ = 5°, which seemed to reflect the temperature nonuniformity in the Si slab. On the other hand, for the TiN heaters placed away from the waveguide core, we obtained a comparable steering angle with a narrower beam divergence of δθ < 0.3°. However, the required heating power was as large as P = 4.8 W, and the response speed was slow, reflecting its low heating efficiency and large heat capacity. We expect these problems to be solved by homogenizing the current and temperature distributions for the former and by optimizing the positioning of the heaters for the latter.

Improvement of modulation speed in Si slow light modulator by optimizing doping profile

We theoretically investigated the modulation speed of Si photonic crystal slow light modulators. Optimizing the doping profile of p/n junction reduces the RC time constant and enhances the speed to 40 Gbps.