Free-space beam steering using optical phased arrays is a promising method for implementing free-space communication links and Light Detection and Ranging (LIDAR) without the sensitivity to inertial forces and long latencies which characterize moving parts. Implementing this approach on a silicon-based photonic integrated circuit adds the additional advantage of working with highly developed CMOS processing techniques. In this work we discuss our progress in the development of a fully integrated 32 channel PIC with a widely tunable diode laser, a waveguide phased array, an array of fast phase modulators, an array of hybrid III-V/silicon amplifiers, surface gratings, and a graded index lens (GRIN) feeding an array of photodiodes for feedback control. The PIC has been designed to provide beam steering across a 15°x5° field of view with 0.6°x0.6° beam width and background peaks suppressed 15 dB relative to the main lobe within the field of view for arbitrarily chosen beam directions. Fabrication follows the hybrid silicon process developed at UCSB with modifications to incorporate silicon diodes and a GRIN lens.
Free-space beam steering using optical phase arrays are desirable as a means of implementing Light Detection and
Ranging (LIDAR) and free-space communication links without the need for moving parts, thus alleviating vulnerabilities
due to vibrations and inertial forces. Implementing such an approach in silicon photonic integrated circuits is
particularly desirable in order to take advantage of established CMOS processing techniques while reducing both device
size and packaging complexity.
In this work we demonstrate a free-space diode laser together with beam steering implemented on-chip in a silicon
photonic circuit. A waveguide phased array, surface gratings, a hybrid III-V/silicon laser and an array of hybrid III/V
silicon amplifiers were fabricated on-chip in order to achieve a fully integrated steerable free-space optical source with
no external optical inputs, thus eliminating the need for fiber coupling altogether. The chip was fabricated using a
modified version of the hybrid silicon process developed at UCSB, with modifications in order to incorporate diodes
within the waveguide layer as well as within the III-V gain layer. Beam steering across a 12° field of view with ±0.3° accuracy and 1.8°x0.6° beam width was achieved, with background peaks suppressed 7 dB relative to the main lobe within the field of view for arbitrarily chosen beam directions.
In this paper we outline recent results which combine defect mediated Photo-Detectors (PDs) in a Ring Resonator (RR)
structure. By exploiting the multiple-pass of the optical signal through the detector, we are able to significantly decrease
the size of the detector structure while maintaining good responsivity (typically 0.1 A/W). In such a geometry the
detector bandwidth is not capacitance limited, while the leakage current is reduced toward 1 nA. We also show that these
PDs may be used in the drop port of a RR to monitor the propagating signal. These devices have applicability in
multiplexing and potential for integration with high speed modulation functionality.
Recently, low threshold Raman silicon lasers based on ring resonator architecture have been demonstrated. One of the
key elements of the laser cavity is the directional coupler that couples both pump and signal light in and out of the ring
resonator from the bus waveguide. The coupling coefficients are crucial for achieving desired laser performance. In this
paper, we report design, fabrication, and characterization of tunable silicon ring resonators for Raman laser and amplifier
applications. By employing a tunable coupler, the coupling coefficients for both pump and signal wavelength can be
tailored to their optimal values after the fabrication, which significantly increases the processing tolerance and improves
the device performance.
We report simulation results for a directional coupler between silicon waveguides in different layers of a three-dimensional (3D) optical circuit. The coupling length is 1.4 mm. The device is manufacturable using standard CMOS technology provided individual waveguide layers can be vertically stacked. In simulations of coupling efficiency the design exhibits negligible loss with respect to translational and rotational misalignments of up to 0.5 μm. Efficiency degradation is less than 5% for etch depth and waveguide width variations of 0.4 μm, and less than 1 dB over the range of standard lithographic tolerances for variations from layer to layer in feature width, depth, and alignment.
This paper describes work investigating the impact of lattice defects on the attenuation of optical signals at wavelengths
around 1550nm in silicon rib waveguides. Using Fourier transform infrared spectroscopy it is shown that high energy proton irradiation of silicon induces excess optical absorption peaked at a wavelength of 1800nm, but extending below 1600nm. This absorption is related to the introduction of silicon divacancy defects. It is further demonstrated that silicon divacancy concentration is accurately determined for a range of proton doses using positron annihilation spectroscopy and successfully predicted using an analytical expression proposed previously. Low loss rib waveguides
were fabricated in silicon-on-insulator substrates. These waveguides were subsequently implanted with silicon ions at an energy of 2.8MeV through photolithographically defined mask windows of various lengths. The additional optical loss as a result of the defects introduced by the implantation process was accurately determined. For a dose of 2.5x10<sup>14</sup>cm<sup>-2</sup>, the loss is greater than 500dBcm<sup>-1</sup>. Finally, it is shown that excess absorption can be predicted using the same analytical expression for the determination of vacancy concentration, thus providing a straightforward method for the design of integrated, on-chip optical absorbers in silicon photonic circuits.