Wafer-level calibration of large-scale integrated optical phased arrays.

We present the wafer-level characterization of a 256-channel optical phased array operating at 1550 nm, allowing the sequential testing of different OPA circuits without any packaging steps. Using this, we establish that due to random fabrication variations, nominally identical circuits must be individually calibrated. With this constraint in mind, we present methods that significantly reduce the time needed to calibrate each OPA circuit. In particular, we show that for an OPA of this scale, a genetic optimization algorithm is already >3x faster than a simple hill climbing algorithm. Furthermore, we describe how the phase modulators within the OPA may be individually characterized 'in-situ' and how this information can be used to configure the OPA to emit at any arbitrary angle following a single, initial calibration step.

Characterization of SiN Integrated Optical Phased Arrays on a Wafer-Scale Test Station.

Optical phased arrays (OPAs) can produce low-divergence laser beams and can be used to control the emission angle electronically without the need for moving mechanical parts. This technology is particularly useful for beam steering applications. Here, we focus on OPAs integrated into SiN photonic circuits for a wavelength in the near infrared. A characterization method of such circuits is presented, which allows the output beam of integrated OPAs to be shaped and steered. Furthermore, using a wafer-scale characterization setup, several devices can easily be tested across multiple dies on a wafer. In this way, fabrication variations can be studied, and high-performance devices identified. Typical images of OPA beams are shown, including beams emitted from OPAs with and without a uniform waveguide length, and with varying numbers of channels. In addition, the evolution of output beams during the phase optimization process and beam steering in two dimensions is presented. Finally, a study of the variation in the beam divergence of identical devices is performed with respect to their position on the wafer.

Sub-decibel silicon grating couplers based on L-shaped waveguides and engineered subwavelength metamaterials.

The availability of low-loss optical interfaces to couple light between standard optical fibers and high-index-contrast silicon waveguides is essential for the development of chip-integrated nanophotonics. Input and output couplers based on diffraction gratings are attractive coupling solutions. Advanced grating coupler designs, with Bragg or metal mirror underneath, low- and high-index overlays, and multi-level or multi-layer layouts, have proven less useful due to customized or complex fabrication, however. In this work, we propose a rather simpler in design of efficient off-chip fiber couplers that provide a simulated efficiency up to 95% (-0.25 dB) at a wavelength of 1.55 µm. These grating couplers are formed with an L-shaped waveguide profile and synthesized subwavelength grating metamaterials. This concept jointly provides sufficient degrees of freedom to simultaneously control the grating directionality and out-radiated field profile of the grating mode. The proposed chip-to-fiber couplers promote robust sub-decibel coupling of light, yet contain device dimensions (> 120 nm) compatible with standard lithographic technologies presently available in silicon nanophotonic foundries. Fabrication imperfections are also investigated. Dimensional offsets of ± 15 nm in shallow-etch depth and ± 10 nm in linewidth's and mask misalignments are tolerated for a 1-dB loss penalty. The proposed concept is meant to be universal, which is an essential prerequisite for developing reliable and low-cost optical couplers. We foresee that the work on L-shaped grating couplers with sub-decibel coupling efficiencies could also be a valuable direction for silicon chip interfacing in integrated nanophotonics.

SiN integrated optical phased arrays for two-dimensional beam steering at a single near-infrared wavelength.

In this work, we present two-dimensional beam steering in the near-infrared using a SiN integrated circuit, containing optical phased arrays. Beam steering was achieved over a range of 17.6° × 3°, at a fixed wavelength of 905 nm. The first dimension was steered via phase differences between the optical phased array channels. The second dimension was accessed by actively switching between various optical phased array sub-devices containing output diffraction gratings with different periods. The characterisation was performed on a wafer-level test station.

Low loss poly-silicon for high performance capacitive silicon modulators.

Optical properties of poly-silicon material are investigated to be integrated in new silicon photonics devices, such as capacitive modulators. Test structure fabrication is done on 300 mm wafer using LPCVD deposition: 300 nm thick amorphous silicon layers are deposited on thermal oxide, followed by solid phase crystallization anneal. Rib waveguides are fabricated and optical propagation losses measured at 1.31 µm. Physical analysis (TEM ASTAR, AFM and SIMS) are used to assess the origin of losses. Optimal deposition and annealing conditions have been defined, resulting in 400 nm-wide rib waveguides with only 9.2-10 dB/cm losses.

Partially localized hybrid surface plasmon mode for thin-film semiconductor infrared photodetection.

We use numerical simulations to show that a suitably dimensioned periodic arrangement of vertical metallic metal-dielectric-metal nanocavities supports a hybrid plasmonic mode whose spatial electric field distribution is suitable for use in infrared photodetectors based on an unpatterned semiconductor thin-film absorbing layer. The partially localized nature of the hybrid mode offers reduced sensitivity to the angle of incoming light and smaller pixel sizes compared with surface plasmonic modes coupled by diffraction.