Small silicon-photonics-based optical scanner could work well for automotive lidar

A simple, small, and very compact optical beam scanner that is potentially useful for automotive lidar systems has been designed and fabricated by researchers at Toyota Central Research and Development Labs (Nagakute, Japan). The device consists of an imaging lens along with a photonic integrated circuit with numerous optical switches and grating arrays. Light at a 1551 nm wavelength fiber-coupled into the photonic circuit is optically switched from one grating array to another in a sequence—the grating arrays are placed in a line and couple the beam out of the plane of the circuit so that the beam is linearly translated. After passing through the lens, the sequentially scanned beams all converge at a focal point and then angularly diverge into the far field, resulting in a field scanning angle of 6°. The chip size of the circuit is only 7 × 12 mm.

The researchers designed the grating shape so that the gratings could be packed as densely as possible to minimize gaps in the field—they determined that a diamond shape worked best. The pitch, duty cycle, efficiency, and overall size of each grating are 0.62 μm, 0.5, 35%, and 14 μm, respectively. The optical switches, which have a response time of 0.3 μs, are each made up of a ring resonator, a waveguide, and a thin-film heater. When the thermally controlled resonator’s optical path length equals a multiple of the input light wavelength, the light exits the output port—otherwise, the output port is dark, achieving switching. The resonators’ bandwidth is about 0.3 nm. The experimental setup was very “preliminary,” with the lens held in front of the scanning circuit with a pair of pliers. In addition, the singlet lens had some aberration, which could be corrected in the future by use of a doublet. Lenses with 3 or 4 mm focal lengths produced a 6° or 4.6° scanned field, respectively, with a resolution of 0.3° or 0.23°, respectively. The researchers are also working on a 2D version of the scanner. Future versions of this device, which could have thousands of individual emitters, will require refinements to the driver circuit and other parts, as well as the addition of intensity monitors for the resonator switches. Reference: D. Inoue et al., Opt. Express (2019); https://doi.org/10.1364/oe.27.002499.