Patent Description:
This invention was made with Government support under contract <NUM> awarded by the National Science Foundation and under contract N00014-<NUM>-<NUM>-<NUM> awarded by the Office of Naval Research. The Government has certain rights in the invention.

The present disclosure relates to optical beam steering in general, and, more particularly, to optical-phased-array-based beam steering.

Two-dimensional optical beam steering is an important technology that has found widespread use in a variety of applications, including telecommunications, LiDAR, three-dimensional imaging, hyperspectral imaging, and optical sensing applications, among others.

Historically, most beam-steering systems employed slow and expensive macro-mechanical-beam-deflection systems such as two-dimensional gimbal-based movable mirrors, pairs of one-dimensional movable mirrors, position-controllable bulk optics, and the like. Unfortunately, such systems are typically complex to implement, are quite costly, have limited response speeds, and are fraught with reliability issues.

In contrast, on-chip optical phased arrays (OPAs) offer a substantially solid-state approach to optical beam steering that promise faster, more robust, and less expensive beam-steering systems. Prior-art OPAs have been demonstrated using widely disparate technologies, such as liquid-crystal phase shifters, MEMS-based piston or grating-based mirrors, and integrated-optics-based surface waveguide arrays having integrated grating couplers and thermo-optic phase shifters. Unfortunately, while such prior-art systems offer lower cost and higher reliability than macro-mechanical beam-deflection systems, they still have significant drawbacks.

Liquid-crystal-based phase shifters, for example, are known to have relatively slow response times and are highly sensitive to changes in temperature and wavelength.

Micromechanical (i.e., MEMS-based) OPAs employing piston-actuated mirrors can achieve fast response times; however, it can be difficult to realize a MEMS-based OPA having small inter-element pitch, which limits the size of the field-of-view (FOV) that can be achieved.

OPAs based on integrated-optics-based surface waveguide gratings and tunable lasers also show promise. In such systems, rapidly tuned lasers are used to change the direction of the light by exploiting the high dispersion of the surface waveguide gratings. Unfortunately, this requires implementing rapidly tunable lasers in silicon photonics, gives rise to cross-talk between transmit and receive on the same antenna, and limits the RF bandwidth that can be transmitted on the light field due to angular dispersion.

Furthermore, the geometric perturbations that define the surface waveguide gratings suitable for OPA use are preferably defined using standard lithography techniques to define of the gratings. To realize an output light beam with low beam divergence, however, long waveguides (several millimeters or more) and high-resolution patterning (on the order of one nanometer) can be required. Unfortunately, patterning nanometer-scale features over large areas can be difficult, if not impossible, using standard patterning techniques. As a result, high-cost patterning methods, such as e-beam lithography, x-ray lithography, and the like, are required. Alternatively, the surface waveguides and the gratings therein must be enlarged to increase the required lithography dimensions, thereby reducing waveguide packing density.

Fixed-frequency lasers have been employed to mitigate these disadvantages; however, this leads to other factors arise that can make on-chip two-dimensional (2D) beam steering extremely challenging. For example, the number of phase shifters and couplers required scales unfavorably with the area of the aperture. In addition, the most common phase shifter is a thermo-optic phase shifter, which has poor power efficiency. As a result, milliwatts of static power consumption is typically required per element. For a millimeter-scale aperture, therefore, 2D beam steering consumes kilowatts of power just for powering the phase shifters themselves.

Still further, unlike phased arrays at microwave frequencies where microstrip patch antennas can radiate efficiently and have footprints smaller than the free space wavelength of the electromagnetic field, in the optical domain, it is difficult to fully scatter light from an optical guided wave to a radiating mode with a small footprint (sub-free space-wavelength). As a result, the radiating elements or antennae in a photonic system are typically large and spaced in two dimensions with a pitch greater than the wavelength of the light. The large spacing leads to significant sidelobes and a reduction in phased array performance.

<CIT> discloses a full-parallax acousto-optic/electro-optic holographic video display, which includes a control layer and a piezo-electric layer electrically and optically connected to the control layer. The piezo-electric layer includes a substrate and an array of anisotropic waveguide elements residing on the substrate. Each waveguide element is configured to guide light into a single polarization and includes a horizontal grating comprising surface acoustic waves that diffracts the light in the waveguide element in the horizontal direction and a vertical grating including an electro-optic phased array that diffracts the light in the waveguide element in the vertical direction.

<CIT> discloses a light beam deflector or modulator including an optical waveguide and an interdigital transducer for generating surface eleastic waves in the optical waveguide.

Passive large-scale nanophotonic phased arrays are described by <NPL>.

A stress-optic modulator in a silicon nitride-based waveguide platform in the telecommunication C-band is described by <NPL>; [<NPL>], <NPL>.

<CIT> discloses a bulk acoustic wave integrated optical deflector and monolithic A/D converter using such deflector. The deflector comprises an optical wave guide disposed on a substrate materiel, a plurality of parallel spaced bulk acoustic wave transducer elements disposed on the optical waveguide and a plurality of controllable phase shifters.

The need for a device technology that enables practical optical beam steering in two dimensions remains, as yet, unmet in the prior art.

The present disclosure enables optical beam steering by employing dynamic grating structures in each of an array of surface waveguides. The dynamic gratings are formed by imparting mechanical waves having a desired frequency in the surface waveguides. As a result, in each surface waveguide, the optical wave propagating through it is subject to an acousto-optical interaction that causes the optical wave to scatter out of the surface waveguide as free-space optical radiation. The light scattered from the array of surface waveguides collectively defines one or more optical beams that can be independently steered in a first dimension (the longitudinal dimension) by controlling the wavelength of the mechanical waves. Steering in a second dimension (the transverse dimension) is performed by controlling the relative phases of the mechanical and optical waves in each surface waveguide. Furthermore, by employing materials compatible with standard silicon photonics processes, embodiments in accordance with the present disclosure are compatible with previously developed technologies in silicon photonics (e.g., lasers, phase-shifters, detectors, etc.) and enable monolithic integration of electronic circuitry, thereby affording system-scale integration. Embodiments in accordance with the present disclosure are particularly well suited for use in applications such as light detection and ranging (LIDAR), autonomous vehicles, robotics, light-field imaging, plenoptic cameras, free-space optical communications and switching systems, wireless power transfer, medical diagnostics, and the like.

In contrast to the prior art, embodiments in accordance with the present disclosure employ one or more high-confinement waveguides, which affords significant advantages over prior-art acousto-optic systems including: <NUM>) more efficient light scattering; and <NUM>) higher waveguide packing density, which enables better suppression of undesirable sidelobes in the output signals of the beam steering systems.

An illustrative embodiment is an optical beam-steering system comprising an array of high-confinement waveguides, each of which is operatively coupled with a mechanical transducer that imparts a mechanical wave in the material of the surface waveguide. The mechanical transducer is coupled with each surface waveguide through a mechanical phase shifter that enables control over the phase of the mechanical wave in its respective surface waveguide. Two-dimensional beam steering is enabled by controlling the wavelength of the mechanical waves in the surface waveguides, as well as the relative phase of the mechanical waves across the surface waveguide array.

In some embodiments, a high-index-contrast slab waveguide is used instead of an array of individually defined waveguides. Optical and/or mechanical phase shifters control the direction of light and/or mechanical waves in the slab thereby directing outgoing radiation.

In some embodiments, the surface waveguides are held above the substrate by narrow support structures such that most of the structural material of the surface waveguides is movable relative to the substrate.

In some embodiments, each surface waveguide includes a phase shifter that controls the phase of its respective light signal. In such embodiments, the inclusion of mechanical phase shifters is optional.

In some embodiments, at least one of the surface waveguides is optically coupled with a photonic lightwave circuit that includes one or more photonic elements such as lasers, phase shifters, detectors, gain elements, modulators, diffraction elements, Bragg mirrors, splitters, combiners, and the like. In some embodiments, the surface waveguide array is part of a photonic integrated circuit that also includes electronic circuitry, logic elements, and the like.

An embodiment in accordance with the present disclosure is an optical beam steering system comprising: a substrate; a plurality of surface waveguides disposed on the substrate, each surface waveguide conveying a light signal, and each surface waveguide being a high-confinement waveguide; an acoustic transducer that is operatively coupled with the plurality of surface waveguides, the acoustic transducer being configured to induce a plurality of mechanical waves such that each mechanical wave of the plurality thereof is coupled with a different surface waveguide of the plurality thereof; and a phase controller for controlling the relative phase of each mechanical wave of the plurality thereof and the light signal conveyed by its respective surface waveguide.

Another embodiment in accordance with the present disclosure is a method for steering an optical beam, the method comprising: conveying a plurality of light signals having wavelength λ in a plurality of surface waveguides disposed on a substrate, each surface waveguide conveying a different light signal of the plurality thereof, and each surface waveguide being a high-confinement waveguide; coupling a plurality of mechanical waves into the plurality of surface waveguides such that interaction between the plurality of mechanical waves and the plurality of light signals gives rise to emission of an optical beam from the plurality of surface waveguides, wherein the mechanical waves are characterized by first frequency, ω; and controlling the first frequency, ω.

<FIG> depicts a functional block diagram of a beam-steering system in accordance with an illustrative embodiment in accordance with the present disclosure. System <NUM> comprises acousto-optic antenna array <NUM>, light source <NUM>, acoustic source <NUM>, phase controller <NUM>, and processing circuit <NUM>. System <NUM> is operative for forming and steering output beam <NUM> in two dimensions.

Acousto-optic antenna array <NUM> is a planar lightwave circuit (PLC) comprising a plurality of high-index-contrast, high-confinement surface waveguides formed on a substrate, where each surface waveguide functions as a different antenna element of the antenna array, and where the PLC is configured to efficiently couple mechanical energy provided by acoustic source <NUM> into the surface waveguides.

As will be understood by one skilled in the art, an integrated-optics-based optical waveguide (referred to herein as a "surface waveguide" or "waveguide") is a light-guiding structure formed on the surface of a substrate. The light-guiding structure comprises a light-guiding core that is surrounded by cladding layers that serve to confine optical energy within the surface waveguide. The materials of the core and cladding are selected such that the refractive index of the core material is at least slightly higher than the refractive index of the cladding material(s). The difference in these refractive indices dictates how tightly the optical energy of a light signal propagating through the surface waveguide is confined to the core region.

Typically, prior-art acousto-optic beam-steering systems are based on surface waveguides whose core and cladding materials have only a slight difference in refractive index resulting in low-confinement of the light. Typically, prior-art acousto-optic beam-steering systems are based on surface waveguides that are not released from their substrate resulting in low-confinement of the mechanical waves. These surface waveguides are referred to, herein, as "low-confinement surface waveguides. " In contrast, embodiments in accordance with the present disclosure employ high-confinement waveguides, which affords them significant advantages over the prior art, as discussed below. For the purposes of this Specification, including the appended claims, a "high-confinement waveguide" is defined as an integrated-optics-based optical waveguide whose light-guiding core comprises a material having a refractive index that is at least <NUM>% higher than the refractive index of the cladding material that surrounds the core and is released from its substrate rendering it "mechanically active. " As a result, the optical or mechanical energy of a light or acoustic signal propagating through a high-confinement waveguide has a mode field that extends only slightly (if at all) into the cladding regions of the surface waveguide. In the depicted example, the high-confinement waveguides employed in system <NUM> are air-cladded, silicon-core surface waveguides has a core comprising silicon, which is surrounded by air, where the air functions as the cladding material of the surface waveguide. Silicon has a refractive index of approximately <NUM>, while air has a refractive index of <NUM>. Other examples of high-confinement waveguides suitable for use in embodiments in accordance with the present disclosure include, without limitation, surface waveguides having cores of silicon nitride and claddings of air and silicon dioxide, silicon-core surface waveguides having at least one silicon dioxide-based cladding layer, certain silicon oxynitride-core waveguides, and the like.

Light source <NUM> is a laser that provides light signals <NUM>, each of which has a wavelength, λ<NUM>, of approximately <NUM> microns and is optically coupled into the surface waveguides of acousto-optic antenna array <NUM>. In some embodiments, light source <NUM> is a different source of coherent light. In some embodiments, light signals have a wavelength other than <NUM> microns.

Acoustic source <NUM> is a generator that is operative for generating acoustic energy <NUM> that couples into each antenna (i.e., high-confinement waveguide) of acousto-optic antenna array <NUM> in the form of a mechanical wave having frequency, ω.

Phase controller <NUM> is a controller operative for controlling the phases of each of the mechanical waves in the surface waveguides of acousto-optic antenna array <NUM>. In some embodiments, phase controller <NUM> includes a plurality of thermal tuning elements operative for controlling the elasticity and/or density of material between acoustic source <NUM> and each surface waveguide. In some embodiments, phase controller includes a plurality of elements that controls the phase of the mechanical waves in each surface waveguide via another physical mechanism. In some embodiments, phase controller <NUM> includes an array of optical phase controllers that control the optical phase of each of the light signals propagating through the surface waveguides of acousto-optic antenna array <NUM>.

Processing circuit <NUM> is an electronics circuit that includes a conventional processor operative for, among other things, controlling the output of light source <NUM>, acoustic source <NUM>, and phase controller <NUM> to steer output beam <NUM> in two dimensions. In the depicted example, processing circuit <NUM> is monolithically integrated on the same substrate as acousto-optic antenna array <NUM>. In the depicted example, the processing circuit is depicted as a single, discrete component. In various other embodiments, the processing circuit can be distributed, at least in part, among multiple components of system <NUM>, implemented, in part or in full, in a remote or cloud-based computing system, or otherwise implemented in a suitable arrangement for carrying out the functions described herein.

The operative principle of embodiments in accordance with the present disclosure arises from the fact that an acoustic wave coupled into a surface waveguide can efficiently scatter light propagating as a guided optical wave in the waveguide into a beam that propagates in free space. This is because phase-matching between two optical waves propagating in the longitudinal direction can be achieved using the acoustic wave momentum, while out-of-plane momentum does not need to be conserved for optical waves at a surface that breaks translational symmetry in the vertical direction. As discussed below, the longitudinal angle at which the output beam propagates depends on the relative wavelengths of the acoustic wave and guided optical wave.

<FIG> demonstrates the generation of free-space radiation via interaction between a light signal propagating in a single surface waveguide and a mechanical wave. Plot <NUM> shows guided optical wave <NUM> as injected into surface waveguide <NUM> such that it is characterized by frequency, ω, within the surface waveguide.

Mechanical wave <NUM> is induced in surface waveguide <NUM> such that the mechanical wave and guided optical wave <NUM> interact over an interaction length, which gives rise to scattering of the guided optical energy into free space. The mechanical wave induces changes in the optical properties of the surface waveguide materials by, for example, increasing the path length, inducing an index change due to a volume elasto-optic effect, and/or realizing a boundary perturbation effect.

Because energy and momentum conservation requirements must be obeyed, interaction between the mechanical wave and guided optical wave give rise to free-space radiation <NUM>, which is Doppler shifted such that it is characterized by frequency ωrad, where ωrad = w+Ω and Ω is the frequency of mechanical wave <NUM>.

In the ideal case, waveguide momentum conservation yields the phase matching condition: <MAT> where β(Ω) is the wavevector of guided optical wave <NUM>, K(Ω) is the wavevector of mechanical wave <NUM>, θ is the coupling angle (i.e., angle in the x-z plane relative to the surface of the waveguide) at which the output radiation propagates, and k<NUM>=ωrad/C (where c is the speed of light).

Therefore, by controlling the phase relationship between mechanical wave <NUM> and guided optical wave <NUM>, the magnitude of coupling angle θ can be controlled.

<FIG> depicts the relationship between phase-matching between the guided optical and mechanical waves and the coupling angle, θ. Plot <NUM> depicts coupling angle, θ, for three different phase-matching conditions in the system shown in plot <NUM>.

It should be noted that the induced index changes due to the volume elasto-optic effect, as well as the boundary perturbation effect are significantly enhanced in embodiments in accordance with the present disclosure due to the fact that the surface waveguides included in acousto-optic antenna array <NUM> are high-confinement waveguides rather than low-confinement waveguides as used in prior-art systems. As a result, embodiments in accordance with the present disclosure require lower amplitude mechanical motion to effect a desired amount of light scatter out of the surface waveguides.

Returning now to the illustrative embodiment, <FIG> depicts a schematic diagram of a detailed perspective view of system <NUM>. As shown in <FIG>, system <NUM> is an example of a beam-steering system in which acousto-optic antenna array <NUM>, acoustic source <NUM>, and phase controller <NUM> are monolithically integrated on substrate <NUM>. In the depicted example, system <NUM> also includes monolithically integrated processing circuit <NUM>. In some embodiments, at least one of acousto-optic antenna array <NUM>, acoustic source <NUM>, phase controller <NUM>, and processing circuit <NUM> is formed on a separate substrate and integrated in a hybrid manner (or otherwise operatively coupled) with the remaining elements of system <NUM>. In some embodiments, light source <NUM> is monolithically integrated on substrate <NUM>.

<FIG> depicts operations of a method for forming a beam-steering system in accordance with the illustrative embodiment. Method <NUM> begins with operation <NUM>, wherein the structure of the surface waveguide elements of acousto-optic antenna array <NUM> is defined on substrate <NUM>. Method <NUM> is described with continuing reference to <FIG> and <FIG>.

Substrate <NUM> is a conventional substrate suitable for use in a planar processing fabrication method. In the depicted example, substrate <NUM> is a conventional silicon-on-insulator (SOI) substrate comprising conventional silicon handle wafer <NUM>, buried oxide layer (BOX) <NUM>, and single-crystal-silicon active layer <NUM>, where the thickness of active layer <NUM> is approximately <NUM> nanometers (nm).

It should be noted that, while substrate <NUM> is an SOI substrate in the depicted example, myriad alternative substrates can be used for substrate <NUM> without departing from the scope of the present disclosure. Substrates suitable for use in embodiments in accordance with the present disclosure include, without limitation, substrates comprising silicon, lithium niobate, compound semiconductors (e.g., gallium arsenide, aluminum gallium arsenide, indium phosphide, cadmium telluride, etc.), semiconductor compounds (e.g., silicon carbide, silicon germanium, etc.), dielectrics, dielectric stacks, glasses, composite materials, and the like.

Acousto-optic antenna array <NUM> includes a linear array of surface waveguides <NUM>. Each of surface waveguides <NUM> includes at least a portion that is a straight waveguide, and these straight-waveguide portions are arranged in parallel along the y-direction. For exemplary purposes, acousto-optic antenna array <NUM> is depicted in <FIG> as having only five antenna elements (i.e., surface waveguides <NUM>). It should be noted that, typically, the number of antennae included in acousto-optic antenna array <NUM> is within the range of <NUM> to <NUM>,<NUM>; however, acousto-optic antenna arrays in accordance with the present disclosure can include any practical number of antennae.

<FIG> depicts a schematic drawing of a cross-sectional view of a portion of acousto-optic antenna array <NUM>. The view depicted in <FIG> is taken through line a-a as shown in <FIG>. Each of surface waveguides <NUM> is an air-cladded, silicon-core ridge-type surface waveguide that is held above handle wafer <NUM> by distance d1 within mechanically active region <NUM>.

The structure of surface waveguides <NUM> is formed in mechanically active region <NUM> by defining the lateral extent of the ridge portion of each waveguide structure in a photoresist mask disposed on active layer <NUM> and partially etching the exposed regions of the active layer in a conventional reactive-ion etch (RIE).

In the depicted example, each of surface waveguides <NUM> is a ridge-type waveguide comprising ridge portion <NUM> and slab region <NUM>. Ridge portion <NUM> has width w1 and thickness t1 and slab region <NUM> has width w2 and thickness t2. The ridge portions project from a substantially continuous slab region with uniform pitch, p1 along the y-direction. In the depicted example, w1 and w2 are <NUM>, t1 is <NUM>, t2 is approximately <NUM>, and waveguide pitch, p1 is <NUM>. Typically, surface waveguides <NUM> are configured to guide the optical waves of light signals <NUM> as well as the mechanical waves of acoustic energy <NUM>.

It should be noted that the dimensions provided above are merely exemplary and that a wide range of waveguide dimensions can be used without departing from the scope of the present disclosure. Furthermore, surface waveguides <NUM> can have any practical waveguide structure, such as a channel waveguide structure, etc., and/or include any practical core and cladding materials (e.g., silicon nitride, silicon-rich silicon nitride, silicon dioxide, lithium niobate, compound semiconductors, etc.) Furthermore, as discussed below, in some embodiments, the surface-waveguide array of acousto-optic antenna array <NUM> is replaced by a slab waveguide.

Preferably, surface waveguides <NUM> are formed in a "mechanically active" region of substrate <NUM>. As a result, at optional operation <NUM>, mechanically active region <NUM> is formed. In some embodiments, surface waveguides <NUM> are not disposed on a mechanically active layer.

Mechanically active region <NUM> is formed by removing BOX layer <NUM> from underneath it, thereby "releasing" that portion of the active layer from handle wafer <NUM>. To form mechanically active region <NUM>, release holes <NUM> are etched through slab regions <NUM> to underlying BOX layer <NUM>. A release etchant (e.g., hydrofluoric acid) is used to attack the BOX layer through release holes <NUM>, thereby undercutting the active layer <NUM> and defining mechanically active region <NUM>. This release etch is timed such that a portion of BOX layer <NUM> remains in place to define supports <NUM>, which inhibit lateral propagation of acoustic energy between adjacent surface waveguides <NUM>.

In some embodiments, mechanically active region <NUM> is supported above handle wafer <NUM> via a plurality of anchors, which are formed of structural material (e.g., polysilicon, silicon nitride, etc.) conformally deposited into vias formed through structural layer <NUM> and BOX layer <NUM> before the release etch is performed.

The inclusion of mechanically active region <NUM> mitigates coupling of acoustic energy <NUM> into the handle wafer, thereby enabling a highly efficient scatter process (as much as three orders of magnitude more efficient than prior-art systems). As a result, the amount of power required for device operation to effect light emission from the surface waveguides is reduced. It should be noted that a more efficient scatter process also reduces the required interaction length (i.e., the length over which mechanical waves <NUM> and light signals <NUM> interact). However, it is typically preferable for the interaction length to remain long to effect good far-field resolution and reduce power consumption. One skilled in the art will recognize, after reading this Specification, that forming mechanically active region <NUM> gives rise to a mechanical effect that is somewhat analogous to the total internal reflection of a light signal within a surface waveguide and reduces the mechanical damping of the mechanical energy by the substrate, each of which can increase the distance over which acoustic energy <NUM> can propagate on chip.

In the depicted example, the slab regions of the surface waveguides are mechanically connected along the entire length of the surface waveguides; however, in some embodiments, the slab regions are patterned to define discrete tethers that are distributed along the length of each surface waveguide such that that they extend between adjacent waveguides. As a result, the tethers support the surface waveguides above handle wafer <NUM> and also mitigate mechanical coupling between adjacent surface waveguides. In addition, mechanical and/or optical cross-talk between the surface waveguides can be inhibited by patterning the slab in more complex ways (e.g. to make subwavelength surface waveguides), as well as by varying waveguide core dimensions to inhibit coupling of optical and/or mechanical waves between distinct surface waveguides. Furthermore, surface waveguides <NUM> can be formed in any practical material system, including, without limitation, silicon on oxide (suspended or unsuspended), silicon nitride, diamond, silicon carbide, gallium nitride, gallium arsenide and its compounds, indium phosphide and its compounds, aluminum nitride, lithium niobate, lithium tantalite, and the like.

It is well understood that phased antenna arrays exhibit sidelobes whose size depends strongly on the spacing between their antennae elements. If the antennae are spaced by more than the wavelength of the light signals being operated on, the sidelobes become comparable to the main beam. Furthermore, not only do the sidelobes become large, the angular density of these lobes increases.

Because a low-confinement surface waveguide has a propagating optical mode that extends well into its cladding, adjacent low-confinement surface waveguides must be spaced apart by large distances to avoid cross-coupling of their optical modes. An acousto-optic antenna array based on low-confinement waveguides, therefore, would exhibit large sidelobes, reducing the optical power in the main output beam and the field of view of the system.

As noted above, however, embodiments in accordance with the present disclosure employ acousto-optic antenna arrays based on high-confinement waveguides. Since their optical modes do not extend significantly into their claddings, high-confinement waveguides can be spaced apart by much smaller distances - on the scale of the wavelength, λ, of light signals <NUM>. In fact, typically, surface waveguides <NUM> are arrayed with a pitch that is within the range of approximately λ/<NUM> to approximately 2λ. In embodiments, p1 is within the range of approximately <NUM> to approximately <NUM> microns. As a result, the scattering process is more efficient (by <NUM> orders of magnitude or more), thereby reducing the RF power required to effect beam steering, as well as the length of the surface waveguides required.

Furthermore, in contrast to acousto-optic deflection systems based on low-confinement waveguides, the length of surface waveguides <NUM> can be shorter - typically within the range of approximately <NUM> microns to approximately <NUM>. It should be noted that shorter length can be critical for integrated-optics-based optical waveguides due to the fact that it is difficult to maintain optical coherence over more than a few millimeters due to imperfections in the surface waveguides, which normally arise during fabrication. It also enables faster beam steering. In some embodiments, the effect of waveguide non-uniformity is mitigated by fabrication through ultraviolet (UV) lithography on large wafers.

In some embodiments, at least one of surface waveguides <NUM> is optically coupled with a photonic lightwave circuit that includes one or more photonic elements such as lasers, phase shifters, detectors, gain elements, modulators, diffraction elements, Bragg mirrors, splitters, combiners, and the like.

At operation <NUM>, acoustic transducer <NUM> is formed on substrate <NUM> such that it is operatively coupled with acousto-optic antenna array <NUM>.

In the depicted example, acoustic transducer <NUM> is an interdigitated transducer (IDT <NUM>) that comprises piezoelectric slab <NUM> and interdigitated electrodes <NUM>. Piezoelectric slab <NUM> is a layer of piezoelectric material (e.g., aluminum nitride, lithium niobate, lead zirconium titanate (PZT), indium phosphide, gallium arsenide and its compounds, etc.), which is formed on the top surface of active layer <NUM>. In some embodiments, acoustic transducer <NUM> comprises a different mechanical transducer, such as a chirped IDT, electro-static actuation, electro-thermal actuation, thermal actuation, magneto-strictive actuation, optical excitation, etc. In some embodiments, acoustic transducer <NUM> includes a piezo-optic material, such as lithium niobate (LiNbOs), and the like.

Electrodes <NUM> are interdigitated electrically conductive traces formed on the top surface of piezoelectric slab <NUM>. When a periodic drive signal, such as a sinusoidally varying voltage, is applied to the interdigitated electrodes, IDT <NUM> generates acoustic energy <NUM> which propagates into surface waveguides <NUM> via mechanical phase shifters <NUM> as mechanical waves <NUM>. Each of mechanical phase shifters <NUM> is located in the intervening portion of active layer <NUM> between acoustic transducer <NUM> and a different surface waveguides <NUM>.

It should be noted that, in the depicted example, mechanical waves <NUM> and light signals <NUM> are counter propagating in the surface waveguides. In some embodiments, the mechanical waves and light signals propagate in the same direction in the surface waveguides.

At operation <NUM>, phase controller <NUM> is formed on substrate <NUM>.

Phase controller <NUM> comprises a plurality of mechanical phase shifters <NUM>, each of which is formed on the top surface of active layer <NUM>. Each mechanical phase shifters <NUM> is an element that is operative for changing the speed at which acoustic energy propagates between acoustic transducer <NUM> and a different one of surface waveguides <NUM> of acousto-optic antenna array <NUM>. As a result, each mechanical phase shifter <NUM> is operative for controlling the phase of the acoustic wave received by the surface waveguide <NUM> with which it is operatively coupled.

In the depicted example, each of mechanical phase shifters <NUM> is a thermal element that tunes the elasticity of the region of active layer <NUM> between acoustic transducer <NUM> and its respective surface waveguide <NUM>. In some embodiments, at least one of mechanical phase shifters <NUM> controls the density of the material of active layer <NUM> in at least a portion of the region between acoustic transducer <NUM> and each of surface waveguides <NUM>. In some embodiments, beam steering in the y-z plane is effected by controlling the phase of light signals <NUM> using optical phase shifters.

Mechanical phase shifters <NUM> are collectively operative for controlling the phase relationship of the light scattered out of the surface waveguides, which dictates the transverse angle, φ, of output beam <NUM> in the y-z plane.

<FIG> depicts a schematic of a cross-section view of acousto-optic antenna array <NUM>. The view depicted in <FIG> is taken through line b-b as shown in <FIG>. The phases, ψ<NUM> through ψ<NUM>, of the mechanical waves <NUM>-<NUM> through <NUM>-<NUM> in surface waveguides <NUM>-<NUM> through <NUM>-<NUM>, respectively, determine the angle, φ, in the y-z plane at which output beam <NUM> propagates.

Although the illustrative embodiment includes an acoustic transducer, a phase controller, and an acousto-optic antenna array that are monolithically integrated on the same substrate (i.e., formed on the same substrate), it will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use alternative embodiments in accordance with the present disclosure in which at least one of acoustic transducer <NUM> and phase controller <NUM> is mechanically affixed to the substrate via hybrid-integration techniques such it is operatively coupled with acousto-optic antenna array <NUM>. In some embodiments, at least one of acousto-optic antenna array <NUM>, acoustic transducer <NUM>, and phase controller <NUM> resides on a different substrate and is operatively coupled with the rest of system <NUM> via an intermediate medium.

At operation <NUM>, light signals <NUM> are injected into surface waveguides <NUM> by optically coupling light source <NUM> and acousto-optic antenna array <NUM>.

<FIG> depicts operations of a method for steering an optical beam in two dimensions in accordance with the illustrative embodiment. Method <NUM> begins with operation <NUM>, wherein light signals <NUM> are launched into surface waveguides <NUM> in response to a drive signal from processing circuit <NUM>.

While not required, light source <NUM> is typically optically coupled with acousto-optic antenna array <NUM> such that light signals <NUM> are collectively in phase as they propagate through the surface waveguides of the antenna array when system <NUM> is in its quiescent state (i.e., when no mechanical energy is generated by acoustic transducer <NUM> and no phase change is proactively imparted on any of the light signals).

At operation <NUM>, processing circuit <NUM> energizes acoustic transducer <NUM> to generate acoustic energy <NUM> that couples into each of surface waveguides <NUM> as a different one of mechanical waves <NUM>, giving rise to free-space scatter energy that defines output beam <NUM>.

At operation <NUM>, processing circuit <NUM> directs phase controller <NUM> to steer output beam <NUM> in the x-z plane by controlling the frequencies of mechanical waves <NUM>, which determines longitudinal angle θ.

At operation <NUM> processing circuit <NUM> directs phase controller <NUM> to control the relative phases, ψ<NUM> through ψ<NUM>, of the mechanical waves <NUM>-<NUM> through <NUM>-<NUM> coupled with surface waveguides <NUM>-<NUM> through <NUM>-<NUM> to control transverse angle φ.

<FIG> depicts a beam-steering system in accordance with an alternative embodiment in accordance with the present disclosure. System <NUM> is analogous to system <NUM> described above; however, system <NUM> includes a high-index-contrast slab waveguide instead of an array of individually defined channel-type surface waveguides.

Slab waveguide <NUM> is a high-confinement slab waveguide that strongly confines the light signal provided by light source <NUM> in the z-direction, thereby resulting in guided wave <NUM>. In the depicted example, slab waveguide <NUM> is a layer of single-crystal silicon having a thickness of approximately <NUM>; however, other dimensions and/or materials can be used for slab waveguide <NUM> without departing from the scope of the present disclosure.

In operation, slab waveguide <NUM> receives light signal <NUM> from light source <NUM>. Mechanical phase shifters <NUM> function as a "phased-array" that provides composite mechanical wave <NUM>, which is steered within slab waveguide <NUM> by controlling the phase shifts imparted at the mechanical phase shifters.

When composite mechanical wave <NUM> and guided wave <NUM> interact, light from the guided wave is scattered into free space as output beam <NUM>, whose transverse angle, φ, is based on the angle of incidence between the composite mechanical wave and the guided wave.

In some embodiments, system <NUM> includes a plurality of optical phase shifters that collectively control the direction of propagation of light signal <NUM> within slab waveguide <NUM>. In such embodiments, the direction of propagation of output beam <NUM>. In some of these embodiments, mechanical phase shifters <NUM> are not included and the direction of propagation of output beam <NUM> is controlled by controlling the direction of propagation of light signal <NUM> and the relative frequencies of mechanical wave <NUM> and light signal <NUM>.

<FIG> depicts a beam-steering system in accordance with another alternative embodiment in accordance with the present disclosure. System <NUM> is analogous to system <NUM> described above; however, system <NUM> includes a phase controller that includes a plurality of optical phase shifters configured to control the phases of a plurality of light portions of light signal <NUM> as the light portions propagate through slab waveguide <NUM>.

Each of optical phase shifters <NUM> is a conventional integrated-optics-based phase shifter suitable for controlling the phase of a portion of light signal <NUM> as it propagates through the region of slab waveguide <NUM> with which it is operatively coupled. In the depicted example, each of phase shifters <NUM> is a thermal phase shifter. Other optical phase shifters suitable for use in embodiments in accordance with the present disclosure include stress-based phase shifters, surface acoustic wave-based (SAW-based) phase shifters, and the like.

The light portion that propagates into slab waveguide <NUM> from each of optical phase shifters <NUM> has a phase that is controlled by that phase shifter. Collectively, the plurality of light portions gives rise to composite optical signal <NUM> whose propagation direction within the slab waveguide is based on their relative phases.

In analogous fashion to the operation of system <NUM>, the transverse angle of output beam <NUM> is based on the angle of incidence between mechanical wave <NUM> and composite guided wave <NUM>, which is controlled by optical phase shifters <NUM>. In some embodiments, both optical phase shifters <NUM> and mechanical phase shifters <NUM> are included.

Claim 1:
An optical-beam steering system (<NUM>) comprising:
a substrate (<NUM>);
a planar lightwave circuit (PLC) (<NUM>) comprising at least one surface waveguide (<NUM>) disposed on the substrate, each of the at least one surface waveguide conveying at least a portion (<NUM>) of a light signal, wherein the at least one surface waveguide includes a core of a first material having a first refractive index and a cladding of a second material having a second refractive index, the first refractive index being at least <NUM>% higher than the second refractive index;
an acoustic transducer (<NUM>) that is configured to induce at least one mechanical wave (<NUM>) in the at least one surface waveguide;
a phase controller (<NUM>) comprising a plurality of phase shifters (<NUM> or <NUM>) that includes a first phase shifter configured to control at least one of (<NUM>) the phase of a first mechanical wave of the at least one mechanical wave and (<NUM>) the phase of a first light portion of the light signal in a first surface waveguide of the at least one surface waveguide; and
the PLC being formed on the substrate such that the PLC and substrate are monolithically integrated; further characterized by:
the at least one surface waveguide being formed in a mechanically active region (<NUM>) that is movable relative to the substrate, the mechanically active region being supported above the substrate by a plurality of supports (<NUM>).