Vernier optical phased array lidar transceivers.

Optical phased arrays (OPAs) which beam-steer in two dimensions (2D) are currently limited to grating row spacings well above a half wavelength. This gives rise to grating lobes along one axis which limit the field of view (FOV), introduce return signal ambiguity, and reduce the optical efficiency in lidar applications. We demonstrate a Vernier transceiver scheme which uses paired transmit and receive phased arrays with different row periodicities, leading to mismatched grating lobe angular spacings and only a single aligned pair of transmit and receive lobes. This permits a return signal from a target in the desired lobe to be efficiently coupled back into the receive OPA while back-scatter from the other grating lobes is rejected, removing the ambiguity. Our proposal goes beyond previously considered Vernier schemes in other domains like RF and sound, to enable a dynamic Vernier where all beam directions are simultaneously Vernier aligned, and allow ultra-fast scanning, or multi-beam, operation with Vernier lobe suppression. We analyze two variants of grating lobe suppressing beam-steering configurations, one of which eliminates the FOV limitation, and find the conditions for optimal lobe suppression. We present the first, to the best of our knowledge, experimental demonstration of an OPA Vernier transceiver, including grating lobe suppression of 6.4 dB and beam steering across 5.5°. The demonstration is based on a pair of 2D-wavelength-steered serpentine OPAs. These results address the pervasive issue of grating lobes in integrated photonic lidar schemes, opening the way to larger FOVs and reduced complexity 2D beam-steering designs.

Fourier-basis structured illumination imaging with an array of integrated optical phased arrays.

Active imaging and structured illumination originated in "bulk" optical systems: free-space beams controlled with lenses, spatial light modulators, gratings, and mirrors to structure the optical diffraction and direct the beams onto the target. Recently, optical phased arrays have been developed with the goal of replacing traditional bulk active imaging systems with integrated optical systems. In this paper, we demonstrate the first array of optical phased arrays forming a composite aperture. This composite aperture is used to implement a Fourier-based structured-illumination imaging system, where moving fringe patterns are projected on a target and a single integrating detector is used to reconstruct the spatial structure of the target from the time variation of the back-scattered light. We experimentally demonstrate proof-of-concept Fourier-basis imaging in 1D using a six-element array of optical phased arrays, which interfere pairwise to sample up to 11 different spatial Fourier components, and reconstruct a 1D delta-function target. This concept addresses a key complexity constraint in scaling up integrated photonic apertures by requiring only N elements in a sparse array to produce an image with N2 resolvable spots.

Optical isolation using microring modulators.

Optical isolators, while commonplace in bulk and fiber optical systems, remain a key missing component in integrated photonics. Isolation using magneto-optic materials has been difficult to integrate into complementary metal-oxide-semiconductor (CMOS) fabrication platforms, motivating the use of other paths to effective non-reciprocity such as temporal modulation. We demonstrate a non-reciprocal element comprising a pair of microring modulators and a microring phase shifter in an active silicon photonic process, which, in combination with standard bandpass filters, yields an isolator on-chip. Isolation up to 13 dB is measured with a 3 dB bandwidth of 2 GHz and insertion loss of 18 dB. We also show transmission of a 4 Gbps optical data signal through the isolator while retaining a wide-open eye diagram. This compact design, in combination with increased modulation efficiency, could enable modulator-based isolators to become a standard 'black-box' component in integrated photonics CMOS foundry platform component libraries.

Integrated optical isolators using electrically driven acoustic waves.

We propose and investigate the performance of integrated photonic isolators based on non-reciprocal mode conversion facilitated by unidirectional, traveling acoustic waves. A triply-guided waveguide system on-chip, comprising two optical modes and an electrically-driven acoustic mode, facilitates the non-reciprocal mode conversion and is combined with spatial mode filters to create the isolator. The co-guided and co-traveling arrangement enables isolation with no additional optical loss, without magnetic-optic materials, and with low power consumption. The approach is theoretically evaluated with simulations predicting over 20 dB of isolation and 2.6 dB of insertion loss with a 370 GHz optical bandwidth and 1 cm device length. The isolator uses only 1 mW of electrical drive power, an improvement of 1-3 orders of magnitude over the state of the art. The electronic drive and lack of magneto-optic materials suggest the potential for straightforward integration with drive circuits, including in monolithic CMOS electronic-photonic platforms, enabling a fully contained 'black box' optical isolator with two optical ports and DC electrical power.

Mode multiplexer for guided optical and acoustic waves.

Integrated acousto-optic (AO) devices utilize the strong overlap of acoustic and optical fields in a waveguide to facilitate efficient photon-phonon (Brillouin) interactions. For example, acoustic waves offer a lossless modulation mechanism for light. "Brillouin active" photonic platforms are currently being developed that may see optical, acoustic, and AO waveguide circuits on the same chip, where guided light and sound come together in active interaction regions. A key missing component for such a platform is a device that can multiplex modes across these two physical domains. We propose and describe a new class of optical and acoustic components, the "acoustic-optical mode multiplexer" (AOMM), a device that takes respective optical and acoustic waveguides as input ports and couples their excited guided modes into a single, joint output waveguide. We show an example suspended silicon-silicon dioxide design that combines two optical modes and a spatially separate acoustic mode into a single, co-guided output port with low insertion loss down to 0.3 dB for both optical and acoustic modes, and reflection below -20dB and -11dB, respectively. The AOMM may enable new, efficient integrated AO devices, such as isolators and circulators, where the acoustic wave generation and opto-acoustic interaction are separated.

Acoustic Waveguide Eigenmode Solver Based on a Staggered-Grid Finite-Difference Method.

A numerical method of solving for the elastic wave eigenmodes in acoustic waveguides of arbitrary cross-section is presented. Operating under the assumptions of linear, isotropic materials, it utilizes a finite-difference method on a staggered grid to solve for the acoustic eigenmodes (field and frequency) of the vector-field elastic wave equation with a given propagation constant. Free, fixed, symmetry, and anti-symmetry boundary conditions are implemented, enabling efficient simulation of acoustic structures with geometrical symmetries and terminations. Perfectly matched layers are also implemented, allowing for the simulation of radiative (leaky) modes. The method is analogous to that in eigenmode solvers ubiquitously employed in electromagnetics to find waveguide modes, and enables design of acoustic waveguides as well as seamless integration with electromagnetic solvers for optomechanical device design. The accuracy of the solver is demonstrated by calculating eigenfrequencies and mode shapes for common acoustic modes across four orders of magnitude in frequency in several simple geometries and comparing the results to analytical solutions where available or to numerical solvers based on more computationally expensive methods. The solver is utilized to demonstrate a novel type of leaky-guided acoustic wave that couples simultaneously to two independent radiation channels (directions) with different polarizations - a 'bi-leaky' mode.