The optical beam steering device is essential for LiDARs and non-mechanical ones have been developed extensively. We have studied the one based on a Si photonic crystal waveguide (PCW) that guides slow light. In LiDARs, the beam hits a distant object. Then, reflected light is scattered hemispherically and a part of it is returned and received by the PCW. In this process, a long PCW aperture is expected to increase the reception intensity. However, since the PCW has a propagation loss of the order of 10 dB/cm, the reception intensity is not increased by simply lengthening the PCW. In this study, in order to suppress the total loss of the PCW, we proposed and fabricated a serial array of PCWs, in which light is received by multiple and short PCWs and then summed by using Si wire waveguide and coupler. We first estimate the transmission and reception characteristics of the PCW array. The effective aperture radiating light is lengthened by dividing the PCW, so the beam divergence becomes small and the reception intensity is improved. Also, we measured the transmission characteristics of the PCW array. We obtained a 0.046° beam divergence by controlling the phase between the PCWs. In the beam steering by the wavelength scanning or heating, we confirmed that the phase matching angular step appears stepwise. If we use the angular step as a resolution point, we can obtain the beam steering without the phase control.
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.
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.
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