Patent Description:
The present description relates in general to coherent free space optical communications (FSOC) and remote sensing coherent LIDAR, and more particularly to, for example, without limitation, a monolithically or nearly monolithically formed multi-aperture optical system ("optical head") for high speed FSOC and coherent LIDAR.

A primary use of the subject technology is free space optical communications (FSOC) and the descriptions will primarily relate to this application. However, the technology can also be applied to coherent LIDAR as well as to other optical system types, such as optical illuminators or designators.

FSOC systems can enable high-speed wireless communications over a sizable range (e.g., many kilometers). In terrestrial applications, such systems can achieve very high (e.g., more than <NUM> gigabits per second - Gbps) data rates. Multiplexing several (N) optical frequencies in a single system enables the data rate of the system to be multiplied by N.

Unlike communications over fiber-optic transmission lines, FSOC must deal with atmospheric turbulence. This can significantly degrade performance by creating optical phase variations across the optical aperture used to transmit and receive light. Conventional FSOC systems have a single optical aperture ("monostatic" configuration) or may have separate transmit and receive apertures ("bistatic" configuration) through which light is transmitted and received. When turbulence effects are substantial enough that the transverse scale of the phase fluctuations (typically measured by the so-called Fried parameter r<NUM>) become comparable to or smaller than the aperture diameter D then the system performance begins to degrade, resulting in signal fluctuations (fades) and/or data drop-outs. Conventional FSOC systems also typically need mechanical beam steering assemblies for coarse beam pointing as well as to mitigate pointing errors due to, for example, jitter of the platform to which it is attached. These mechanical assemblies add considerable weight, are frequently bulky, and often consume high electrical power.

<CIT> refers to an optical photonic device comprising a planar semiconductor substrate having a light-guiding layer thereon, a primary laser light source in said light-guiding layer and a vertical coupler optically coupled to the primary laser light source by waveguide portions of the light-guiding layer. The vertical coupler is configured to receive a light beam from the primary laser light source and redirect the light beam in a direction that is substantially perpendicular to a surface of the planar substrate. <CIT> relates to mode coupling and mode transformation in wave propagation. <CIT> relates generally to technology, designs, and methods applicable to optical imaging, ranging, sensor and communication technology including swept-source optical coherence tomography systems including optional photonic phased arrays. <CIT> discloses a phased array that comprises a predetermined number of emitter/receiver elements; said emitter/receiver elements being arranged on an array formed of stacked rows, wherein the emitter/receiver elements in each row of the array are distributed according to a pseudo-random pattern; and the heights of the rows vary according to a pseudo-random pattern.

A multi-aperture optical system is provided according to claim <NUM>.

A method of manufacturing a multi-aperture optical system is provided according to claim <NUM>.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the subject technology as claimed. It is also to be understood that other aspects may be utilized, and changes may be made without departing from the scope of the invention as defined by the claims.

The following figures are included to illustrate certain aspects of the present disclosure and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, without departing from the scope of the invention as defined by the claims.

The detailed description set forth below is intended as a description of various implementations and is not intended to represent the only implementations thereof.

As those skilled in the art would realize, the described implementations may be modified in various different ways, all without departing from the scope of the invention as defined by the claims.

Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive.

In an effort to address the deficiencies associated with the conventional FSOC systems described above, alternative FSOC systems have been proposed, such as that described in <CIT>. The alternative FSOC systems described in <CIT> replace the single aperture of the conventional FSOC systems with multiple smaller apertures ("sub-apertures"). By making the sub-apertures smaller than the anticipated worst-case Fried parameter each sub-aperture sees a linear phase across it. By incorporating optical phase shifters in each sub-aperture "channel" and a means to measure phase variations it is possible to counter the phase variability across the set of sub-apertures and reduce or eliminate turbulence impact.

The alternative FSOC systems described in <CIT> includes an array of lenslets for transmitting or receiving light. Each lenslet is optically coupled to a single-mode optical fiber. A drawback associated with optical fibers is that they are susceptible to environmental effect, including pathlength changes due to mechanical and thermal disturbances. Unless the thermal and mechanical environment is controlled carefully these pathlength changes may add to the problem of controlling phases across the channels. Furthermore, coupling light from free space into single-mode fibers necessitates high alignment precision, which can make large arrays costly to fabricate.

Various aspects of the present disclosure are directed to addressing the deficiencies of the alternative FSOC systems described in <CIT> and the conventional single aperture architectures by constructing a substantially monolithic optical system (head) that does not require a multitude of discrete optical components and complex construction techniques. The various embodiments of the present disclosure described herein enable construction of systems far smaller and lightweight than is possible prior architectures. The system can furthermore incorporate non-mechanical beam steering to enable continuous beam steering, or pointing, over large angular ranges, such as +/-<NUM> degrees or more.

Various aspects of the present disclosure described herein are directed to an optical phased array assembly (OPAA) and a beam steering assembly (BSA). In some embodiments, the OPAA is a multi-aperture optical system (head) that may include a photonic integrated circuit (PIC), a spacer substrate, and an array of optical cells. The PIC incorporates waveguides for transporting light, light beam folding elements, optical phase shifters, a beam combiner, and may incorporate a beam splitter. The PIC may also incorporate a photodetector coupled to the beam combiner. However, the various embodiments described herein are not limited to the aforementioned configuration. Alternatively, in other embodiments the photodetector may be positioned external to the PIC, in which case light may be coupled to it using, for example, an optical fiber. Similarly, the PIC may contain a laser for transmitting light through the structure and into free space or the laser may also be positioned external to the PIC and an optical fiber used to couple light into the PIC. The PIC may also incorporate an optical amplifier, for example, a semiconductor optical amplifier (SOA). A controller may also be connected electrically to the photodetector and used to control the optical phase shifters based on the detected photodetector signal. As shall be described in further detail below, the substrate, the PIC, and the lensing elements may be constructed as a single monolithic assembly.

In accordance with some embodiments, the OPAA as described above may be optically coupled to one or more beam steering devices to enable beam steering over much greater ranges than is possible with just the OPAA. The beam steering device may be a mechanical mirror assembly, or it could alternatively be a non-mechanical beam steerer. As an example, a polarization grating liquid crystal (PGLC) beam steerer could be used to steer in discrete steps. For example, such a device could steer over +/-<NUM> degrees or more with a step size of <NUM> degree. In cases where the PGLC step angle is greater than the OPAA steering angle a third steering mechanism could be inserted between the OPAA and the PGLC. This enables continuous steering from small angles to large angles. Such a third steering mechanism could be, for example, a mechanical mirror or it could be a liquid crystal OPA or a liquid crystal spatial light modulator (SLM).

The monolithically formed multi-aperture optical system is designed to enable minimization of the adverse effects of atmospheric turbulence which can significantly degrade performance of the system, as described above. In addition, the optical phased array nature of the system allows for fine angle beam steering. In particular, the planar FSOC optical head of the various embodiments described herein combines a monolithically formed OPAA with solid state, wide-angle beam steering which reduces the complexity of the mechanical structure as compared with conventional FSOC optical heads.

The multi-aperture optical system of the various embodiments described herein allows for many improvements across the FSOC portfolio. For example, the alternative FSOC systems described in <CIT> are generally configured with a plurality of sub-apertures which use discrete lenses to focus light onto corresponding single-mode fibers, whereby each of the optical fibers needs to be separately and precisely aligned with and coupled to corresponding phase shifters. In these embodiments, the optical fibers are used as waveguides to couple each of the sub-apertures to the phase shifters, and to guide light from the sub-apertures to the phase shifters. This configuration is disadvantageous in that optical fibers are traditionally extremely sensitive to thermal and mechanical disturbances. This type of optical head is generally complex to construct and requires greater control to minimize the impact of disturbances.

In contrast, a multi-aperture optical system whose components are fabricated in a substantially monolithic form, as described herein, eliminates the need for incorporating optical fibers to couple the focusing optical elements to phase shifters in order to perform the phase correction. Further, the monolithic nature of the described systems allows for more complexity in a single substrate and results in a solid-state system that is more jitter resistant and geometrically conformal than conventional FSOC systems.

Additionally, since the multi-aperture optical system of the various embodiments described herein is entirely solid state, unlike conventional FSOC systems, mechanical actuators are not necessary to perform the phase correction. Therefore, the disclosed multi-aperture optical systems can run at a significantly higher bandwidth and may be substantially more robust as compared with conventional FSOC systems. Further, the multi-aperture optical system of the various embodiments described herein is substantially smaller and lighter and offers substantial size, weight, and power (SWaP), as well as cost saving features over conventional configurations. For example, for a fixed light collection area, the disclosed multi-aperture optical system has a fraction of a depth of the traditional FSOC systems, thereby resulting in a substantial volume and weight savings, e.g., by an order of magnitude or more. Moreover, the entire disclosed beam combiner can be integrated into the PIC.

Furthermore, contrary to the alternative FSOC systems described in <CIT> in which high precision alignment of the individual components of the system (e.g., alignment of the phased array) needs to be carried out, the multi-aperture optical system of the various embodiments described herein may be fabricated using lithographically defined chips which may eliminate the need for performing high precision alignment of individual components. Thus, the assembly/manufacturing process of the multi-aperture optical system of the various embodiments described herein is significantly easier and can be done reliably and repeatedly without spending extensive expert labor hours. Additionally, since potential complexity occurring during assembly are kept at the lithographically manufactured stage, labor costs, issues with reliability, and other similar problems associated with conventional or alternative FSOC assembly/manufacturing processes are substantially reduced. Further advantageously, cost may be reduced as high volume multi-aperture optical systems can be fabricated at low cost using existing chip foundries.

<FIG> is a block diagram illustrating a multi-aperture optical system for free-space optical communication (FSOC), according to some embodiments of the present disclosure. As depicted, the multi-aperture optical system <NUM> includes a PIC <NUM>, a spacer substrate <NUM> (illustrated in <FIG>), and a plurality of optical cells <NUM> monolithically formed with the PIC <NUM>. In accordance with some embodiments, a plurality of multi-aperture optical systems <NUM> may optically communicate with one another through open space. To this effect, each multi-aperture optical system <NUM> may be coupled to an FSO modem (not shown), which in turn is in communication with a network (not shown) via, for example, a network switch (not shown). Examples of the network include the Internet, a local area network (LAN), an Ethernet network, or other networks). In some embodiments, each multi-aperture optical system <NUM> receives optical signals from the FSO modem and transmits electrical signals to the FSO modem. Communications between the FSO modem and the switch and between the switch and the network is through electrical signals. In this manner, each multi-aperture optical system <NUM> is able to correct the phase of the received signal to compensate for atmospheric disturbance.

In some embodiments, each cell includes a focusing optical element <NUM> formed on the spacer substrate <NUM> (illustrated in <FIG> and <FIG>), a phase shifter <NUM>, and a waveguide <NUM>, all monolithically integrated onto the PIC <NUM>. The focusing optical element <NUM> may be configured to receive light and focus the light through the PIC <NUM> and onto the folding element <NUM>. The phase shifter <NUM> may be embedded in the PIC, and the waveguide <NUM> may be coupled to the phase shifter <NUM> to transport the light through the phase shifter <NUM>. As depicted, the multi-aperture optical system may further include a beam combiner <NUM> coupled to the phase shifters <NUM>, and configured to combine light output from the phase shifters <NUM>. A photodetector <NUM> may be coupled to the beam combiner <NUM> to receive the combined light output from the beam combiner <NUM> and output a corresponding signal. In some embodiments, the multi-aperture optical system <NUM> may optionally include a beam splitter for splitting the signal output from the beam combiner <NUM> into first and second portions. As further depicted, the multi-aperture optical system <NUM> may further include a controller <NUM> coupled or otherwise connected to the photodetector <NUM> and each of the phase shifters <NUM> to control phase shifting of each phase shifter <NUM> based on the signal output from the photodetector <NUM>. In some embodiments, the controller may be a general-purpose microprocessor, a microcontroller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a programmable logic device (PLD), a state machine, gated logic, discrete hardware components, or any other suitable device that can perform calculations or other manipulations of information.

As shall be described in further detail below, the PIC <NUM>, the spacer substrate <NUM>, the plurality of optical cells <NUM>, the beam combiner <NUM>, and the photodetector <NUM> may be integrated as a single monolithically formed optical head. In particular, in some embodiments, the folding elements <NUM>, the phase shifters <NUM>, the waveguides <NUM>, the optional amplifier <NUM>, the beam combiner <NUM>, and the photodetector <NUM> may be lithographically formed on the PIC as a single monolithic unit. A laser input port <NUM> may further be provided for coupling an optical source such as a laser (not shown herein for simplicity), or an on-chip laser may be used.

In accordance with various embodiments of the present disclosure, the multi-aperture optical system <NUM> may be operated in either receiving or transmitting modes. In receiving mode light is captured by each optical cell <NUM>, and focused by the focusing optical element <NUM> onto the folding element <NUM>. The folding element <NUM> may redirect the light at substantially <NUM> degrees to fold it into the PIC <NUM>. Light may then be transported by the waveguides <NUM> through the phase shifter <NUM> and, if present, through the amplifier <NUM>, and then to the beam combiner <NUM>. In some embodiments, the relative positions of the phase shifter <NUM> and the amplifier <NUM> may be reversed. In transmission mode light may propagate in the opposite direction from the beam combiner <NUM> to the focusing optical element <NUM> and into free space. The beam combiner <NUM> may coherently combine the light from all optical cells <NUM> and couple the combined light to the photodetector <NUM> which may then output a corresponding signal. In operation, the controller <NUM> may be coupled or connected to the photodetector <NUM> and to the phase shifters <NUM> to control the phase of each phase shifter <NUM> based on the signal output from the photodetector <NUM>. In particular, in some embodiments, the controller may execute various instructions in the form of algorithms to maximize the signal output by the photodetector, or may be used to impose specific phase shifts in each optical cell <NUM>. For example, in some embodiments, the controller may be used to impose linear phase shifts across the array of optical cells <NUM> to effect optical phased array (OPA) beam steering over an angular range A, whose magnitude may be proportional to the transverse dimension of the optical beam at each sub-aperture. In some embodiments, smaller beams provide larger OPA angular range and vice versa.

<FIG> is a perspective view illustrating the multi-aperture optical system of <FIG>, according to some embodiments of the present disclosure. As depicted in <FIG> and <FIG>, the multi-aperture optical system <NUM> may further include a coarse beam steering element <NUM> positioned in front of, or depending on orientation, directly above the plurality of optical cells <NUM>. The coarse beam steering element <NUM> may be a non-mechanically steered beam steering device, or a mechanically steered beam steering device. In some embodiments, the non-mechanically steered beam steering device may be a liquid crystal beam steering device including a plurality of liquid crystal polarization gratings (LCPGs). The LCPGs may, for example be thin birefringent films that steer light to one of two deflection angles, depending on the polarization of the input light. Advantageously, the plurality of LCPGs may be stacked against each other to create a wide-angle non-mechanical beam control system with significant improvements over mechanically steered systems in size, weight, and power (SWaP), beam agility, and pointing stability. In operation, the LCPGs use polarization modulation instead of phase or amplitude modulation (as done with traditional diffraction gratings), resulting in increased first-order efficiencies, for example, exceeding <NUM>%. Beams are diffracted into a positive or a negative order with a pass-through zero (undeflected) order possible. Because each LCPG of the plurality of LCPGs can be switched, deflection angles can be added or subtracted as light propagates through the plurality of LCPGs. A relatively small number of LCPGs can provide a large set of deflection angles, enabling a wide range of angles in two dimensions to be achieved with a small number of LCPGs. The high efficiency and compact size advantageously yields a multi-aperture optical system <NUM> having size and weight savings. In some embodiments, the mechanically steered beam steering device may be a mechanically steered mirror, e.g., a mechanical mirror-based gimbal or a mirror steered by a galvanometer mechanism, or any other form of Microelectromechanical systems (MEMS).

<FIG> is a perspective view illustrating a configuration of three optical cells <NUM> of the multi-aperture optical system <NUM> of <FIG>, according to some embodiments of the present disclosure. As depicted in <FIG>, with continued reference to <FIG>, the plurality of optical cells <NUM>, the beam combiner <NUM>, and the photodetector <NUM> may be integrated as a single monolithically formed optical head. To this effect, the folding elements <NUM>, phase shifters <NUM>, and waveguides <NUM> of each of the optical cells <NUM> may be lithographically fabricated or grown onto the PIC <NUM>, and coupled to the beam combiner <NUM>, and the photodetector <NUM> - each of which may be fabricated directly on the PIC <NUM>. Each folding element <NUM> may be integrated into the PIC <NUM>, and optically coupled with the respective waveguide <NUM>. Each optical waveguide <NUM> may be fabricated on the PIC <NUM> and coupled to the corresponding phase shifter <NUM> which may be embedded in the PIC <NUM>. Each focusing optical element <NUM> may be fabricated or grown onto the spacer substrate <NUM>, which is coupled to the PIC above the corresponding folding element <NUM>. For example, the focusing optical elements <NUM> may be lithographically formed on the spacer substrate <NUM> (illustrated in <FIG>) as a layer above the folding elements <NUM>.

In some embodiments, the monolithically formed multi-aperture optical system <NUM> is a compact assembly, for example, with a depth, D, of less than <NUM>, as compared to the aperture of conventional FSOC systems that may have a depth of about <NUM>.

In operation, each phase shifter <NUM> imposes a phase shift on an optical signal received by the corresponding focusing optical element <NUM>. In accordance with some embodiments, each phase shifter <NUM> may be an electro-optical (EO) phase shifter such as a lithium niobate crystal shifter. In other embodiments, each phase shifter <NUM> may be another type of phase shifter, such as a thermal phase shifter or a phase shifter fabricated using silicon (Si) or other materials, including indium phosphide (InP). Each phase shifter <NUM> may receive a control signal (e.g., a phase command signal) from the controller <NUM>, and shift a phase of a respective input optical signal received from a respective focusing element <NUM> based on the control signal.

In some embodiments, the phase-shifted optical signals from each of the phase shifters <NUM> are coherently combined by the beam combiner <NUM> and output to the photodetector <NUM>. The processing of the phase of a respective optical signal input to each phase shifter <NUM> results in correcting the phase of the respective input optical signal to remove adverse effects of atmospheric turbulence on the optical signal. The atmospheric turbulence disturbs, for example, the phase of the optical signal while traveling in open space. The processed phase of a respective optical signal input to each phase shifter <NUM> may also be used to steer the beam over small angles.

In accordance with some embodiments, the controller <NUM> receives the output signal from the photodetector <NUM>, and generates control signals that are used by the phase shifters <NUM> to shift the phase of each respective input optical signal received from the respective focusing optical elements <NUM>, based on the control signal. In some embodiments the control signals to the phase shifters may be dithered in order to maximize the photo-detector signal, indicating the contributions from all subapertures are mutually coherent. Additional phase shifts may be applied to the individual channels, for example to impose linear phase gradients across the full aperture.

<FIG> is a perspective view illustrating light incident on one of the optical cells of the multi-aperture optical system of <FIG>, according to some embodiments of the present disclosure. As briefly described above, each optical cell <NUM> may include a focusing optical element <NUM>, a folding element <NUM>, a phase shifter <NUM>, and a waveguide <NUM>, all monolithically integrated onto the PIC <NUM>. As depicted, each focusing optical element <NUM> may be configured to receive an incident beam of light <NUM> and to focus the light <NUM> through the PIC <NUM>, and onto the folding element <NUM>. As such, each of the focusing optical elements <NUM> may be a micro-lens, a lithographically defined lens, a gradient-index lens, a holographically formed lens, a refractive lens, or diffractive optics. In some embodiments, however, the focusing optical elements <NUM> may be meta materials lenses, thereby providing the advantage of reduced reliance on traditional lenslets. Since the meta materials lenses are printed directly onto the substrate surface they advantageously provide a thinner and more compact configuration as compared with traditional lenslets. Further advantageously, the meta materials lenses may be printed onto the substrate using a lithographic process, thereby eliminating the need for labor-intensive alignment commonly used with traditional lenslets.

Each focusing optical element <NUM> is aligned to a corresponding waveguide <NUM> (e.g., via the spacer substrate <NUM> (illustrated in <FIG>)) to maintain a fixed relative position thereto. To this effect, a method of manufacture may include fusing each focusing optical element <NUM> to the spacer substrate <NUM>, and optically coupling each focusing optical element to the corresponding waveguide <NUM> on the PIC <NUM> to form a monolithic structure. As previously discussed, each focusing optical element <NUM> may be fabricated or grown onto the spacer substrate <NUM> above the corresponding folding element <NUM>. For example, the focusing optical elements <NUM> may be lithographically formed on the spacer substrate <NUM> as a layer above the folding elements <NUM> and on top of the PIC <NUM>. In some embodiments, the focusing optical elements <NUM> can be positioned within a common plane. Each focusing optical element <NUM> may be formed with a common focal length and a distance from each of the focusing optical elements <NUM> to the substrate may be equal.

The focusing optical element <NUM> of the various embodiments described herein may advantageously be more compact in size as compared to focusing elements or apertures of conventional FSOC systems. In particular, the focusing element <NUM> may have a diameter raging from about <NUM> to about <NUM>, as compared to focusing elements of prior art systems which typically have diameters of <NUM> or greater. Advantageously, the compact size of the focusing elements described herein allows for a greater number of focusing elements <NUM> to be monolithically formed on the spacer substrate. A greater number of focusing elements yields a corresponding increase in the number of channels through which light may propagate through the multi-aperture optical system. For example, the multi-aperture optical system <NUM> of the various embodiments described herein may incorporate <NUM> or more focusing elements <NUM> based on the compact size of the focusing elements, as compared to prior art systems having larger focusing elements/apertures. Additionally, due to the increased number of focusing elements, the multi-aperture optical system <NUM> of the various embodiments described herein may be capable of compensating for more severe turbulence and may be more fade resistant with respect to the optical signal as compared to conventional FSOC systems.

In some embodiments, each folding element <NUM> is a boundary surface (e.g., a mirror or grating coupler) defined in the PIC <NUM>, and configured to receive and reflect the incident beam of light <NUM> within the PIC <NUM>. For example, each folding element <NUM> can reflect light transmitted from a corresponding focusing optical element <NUM> to a corresponding waveguide <NUM>. By further example, where the waveguides <NUM> is oriented in a direction that is orthogonal to an orientation of the focusing optical element <NUM>, the folding element <NUM> can be formed at an angle of <NUM>° within the PIC <NUM> in order to reflect the light at a right angle. Thus, the reflected beam may exit the folding element <NUM> at a <NUM>° angle with respect to the incident light beam. It will be recognized that other angles can be used to reflect light transmitted from each of the focusing optical elements <NUM> to the corresponding waveguides <NUM>. Accordingly, the folding elements <NUM> can act as prisms to direct light from the focusing optical elements <NUM> to the corresponding waveguides <NUM> which may be oriented in a transverse (e.g., orthogonal) direction with respect to the direction of incident light. Advantageously, this enables construction of a flat, thin monolithic FSOC system, in the form of a PIC device, with the focusing optical elements <NUM> overlaying the rest of the elements/components of the multi-aperture optical system <NUM>.

In some embodiments, each waveguide <NUM> is coupled to a phase shifter <NUM>, and configured to transport the light reflected by the folding element <NUM> through the corresponding phase shifter <NUM>. The alternative FSOC systems described in <CIT> employ waveguides in the form of optical fibers, however as discussed above, the multi-aperture optical system of the various embodiments described herein obviates the need to use optical fibers to couple the focusing element to the phase shifters. In some embodiments, the waveguide <NUM> is lithographically formed on the PIC <NUM>. Alternatively, the waveguide <NUM> may be fabricated on the PIC <NUM> using ultrafast laser inscription (ULI). Thus, the waveguide <NUM> may be fabricated directly on the PIC <NUM> in the desired position, as opposed to the alternative FSOC systems described in <CIT>, in which the waveguide is in the form of optical fibers which need to be individually aligned precisely. Accordingly, the aforementioned configuration yields a monolithic, pre-aligned (based on location of fiducial indicators) multi-aperture optical system which eliminates the tedious process of manual alignment of separate focusing optical elements and optical fiber waveguides. Further, the aforementioned configuration provides a product with improved thermal stability and jitter resistance, as compared to conventional FSOC systems. The alternative FSOC systems described in <CIT> employ a mechanical array of lenslets coupled into single-mode optical fiber, thereby necessitating meticulous single micron alignment of separate focusing optical elements and optical fiber waveguides. In contrast, the multi-aperture optical system of the various embodiments described herein may be fabricated using lithographically defined chips, thereby eliminating the need for high precision alignment of the individual components and drastically reducing manufacturing costs.

In some embodiments, each phase shifter <NUM> processes a phase of an optical signal received by the corresponding focusing optical element <NUM>. Each phase shifter <NUM> may receive a control signal (e.g., a phase command signal) from the controller <NUM>, and process (e.g., shift) the phase of a respective input optical signal received from a respective focusing element <NUM> based on the control signal. In accordance with some embodiments, each phase shifter <NUM> may be a thermal phase shifter, a semiconductor phase shifter, or an electro-optic phase shifter.

Each optical cell <NUM> may further include an optical amplifier <NUM> disposed in series with the phase shifter <NUM>. Due to insertion losses in the optical components fabricated on the PIC <NUM>, particularly at points of their coupling, the optical amplifier <NUM> may be included in each optical path to boost output channel signals from the respective phase shifter <NUM>. The optical amplifier <NUM> may be a semiconductor optical amplifier (SOA) or an optically pumped doped crystalline or ceramic or glass amplifier.

<FIG> is an exemplary partial cross-sectional view of the optical cell of <FIG>, according to some embodiments of the present disclosure. <FIG> is an exemplary partial cross-sectional view of the optical cell of <FIG>, according to some embodiments of the present disclosure. <FIG> is an exemplary partial cross-sectional view of the optical cell of <FIG>, according to some embodiments of the present disclosure. As briefly described above, each of the focusing optical elements <NUM> may be a micro-lens, a lithographically defined lens, a gradient-index lens, a holographically formed lens, a refractive lens, diffractive optics, or a grating coupler. <FIG> depicts a configuration in which the focusing optical element <NUM> is a micro-lens. In these embodiments, the micro-lens may be fabricated on the substrate <NUM>, for example, using ink-jet printing or laser direct writing in order to produce a spherical micro-lens. <FIG> depicts a configuration in which the focusing optical element <NUM> is a lithographically defined lens. In these embodiments, each focusing optical element <NUM> may be fabricated on the substrate <NUM> by etching multiple layers on top of each other to produce a roughly spherical lens. Lithographically forming the focusing optical elements <NUM> on the substrate <NUM> is advantageous in that the focusing optical elements <NUM> can be created in extremely small patterns (for example sizes in the magnitude of <NUM>). Further, since lithographic formation of the focusing optical elements <NUM> on the substrate <NUM> affords exact control over the shape and size of the focusing optical elements, the focusing optical elements may be fabricated on the entire substrate <NUM> cost-effectively. <FIG> depicts a configuration in which the focusing optical element <NUM> is a gradient-index (GRIN) lens.

<FIG> is a cross-sectional view of a focusing element incorporating mode conversion using printed GRIN technology. Using closely spaced conventional lens elements may, in transmission, produce an intensity profile across the full aperture that is not uniform. This results from the transverse mode profile exiting waveguides being non-uniform, frequently having an approximately Gaussian shape. As this shape propagates to the lens element the Gaussian shape is retained. If the construction of the subaperture array is such that the mode is smaller than the subaperture then there will be non-uniformities in intensity across the array. If the mode is made much larger than the subaperture to minimize intensity variations then there will be losses associated with clipping of the mode. One method to avoid this problem is to fabricate a mode converter that converts the Gaussian mode near the waveguide exit (input transverse mode) to a top-hat or super-Gaussian shape at the focusing element plane (output transverse mode). This produces a much more uniform intensity profile across the array while not incurring high losses. This approach can be implemented as illustrated in <FIG>. Here the focusing element is fabricated using printed-GRIN technology, such as available from Voxel, Inc. , which enables fabrication of largely arbitrary refractive index profiles in three dimensions. This in turn enables fabrication of mode converters that transforms a small Gaussian input beam to a nearly flat-topped beam at the output that approximately fills the subaperture and optimizes efficiency.

<FIG> is a perspective view illustrating light incident on one of the optical cells of the multi-aperture optical system of <FIG>, according to some embodiments of the present disclosure. As illustrated in <FIG>, the folding element may be a grating coupler <NUM>. In these embodiments, the grating coupler <NUM> may be formed directly in the PIC. Grating couplers are commonly used to efficiently couple light between free-space or optical fibers and optical waveguides. Light propagated in a waveguide <NUM> transmits into the grating coupler <NUM> and is diffracted out at nearly normal incidence to the waveguide plane. Very high efficiency devices have been demonstrated, such as ><NUM>% coupling efficiency from waveguides to single-mode fibers. Advantages of fiber couplers over fold mirrors include simple fabrication as well as enabling tailoring of the emitted beam diameter to meet specific needs. In the case of mirrors the beam diameter is determined by the native waveguide mode, which may be only a few hundred nm in diameter. This means that the beam diverges rapidly and makes the spacing to the lens very sensitive to manufacturing and positioning errors. Grating couplers allow creation of larger modes, such as several micrometers in diameter. This reduces the focusing tolerances by a large factor, such as a factor of ten or more.

Methods and systems of the present disclosure can be utilized to provide an array of optical cells <NUM> that are monolithically fabricated on a PIC <NUM> to inject light into the PIC <NUM> without the use of optical fibers as a waveguide. In accordance with some embodiments, the optical cells <NUM> each include a focusing optical element <NUM> which is optically coupled to a folding element <NUM> embedded in the PIC <NUM> to reflect the injected light at substantially right angles. In other embodiments however, as discussed above where grating couplers are used to couple light between free-space the optical waveguides, the injected light may be reflected at angles ranging between <NUM> to <NUM> degrees. In accordance with some embodiments, a method of manufacturing a multi-aperture optical system <NUM> may include providing a PIC <NUM> and monolithically fabricating a plurality of optical cells <NUM> on the PIC <NUM>, where each optical cell <NUM> includes a folding element <NUM>, a focusing optical element <NUM>, a phase shifter <NUM>, and a waveguide <NUM> coupled to the phase shifter <NUM>. Monolithically fabricating the plurality of optical cells <NUM> on the PIC <NUM> may include, for each optical cell <NUM>, integrating a folding element <NUM> into the PIC <NUM>, embedding the phase shifter <NUM> in the PIC <NUM>, fabricating the waveguide <NUM> on the PIC <NUM>, coupling the optical fiber-free waveguide to the phase shifter <NUM>, and lithographically forming the focusing optical element <NUM> on a spacer substrate <NUM> coupled to the PIC <NUM> above the folding element <NUM>.

The method may further include fabricating a beam combiner <NUM> and a photodetector <NUM> on the PIC <NUM>. The photodetector <NUM> may be coupled to the beam combiner <NUM>, and the beam combiner <NUM> may be coupled to each phase shifter <NUM>. The method may further include coupling the controller <NUM> to the photodetector <NUM> and each phase shifter <NUM> to control phase shifting of each phase shifter <NUM> based on a signal output from the photodetector <NUM>. The PIC <NUM>, the spacer substrate <NUM>, the plurality of optical cells <NUM>, the beam combiner <NUM>, and the photodetector <NUM> are integrated as a single monolithically formed optical head. In particular, the focusing optical elements <NUM> may be lithographically formed on a first side of the substrate <NUM>, the folding elements, the phase shifters, the waveguides, the beam combiner, and the photodetector may be lithographically formed on the PIC <NUM>, and the PIC <NUM> may be coupled to a second side of the substrate <NUM> to form a single monolithic structure.

In accordance with some embodiments, the method may further include positioning the plurality of optical cells <NUM> in a one dimensional or two dimensional array in a single plane. The plurality of optical cells <NUM> may be placed in a single plane, and the substrate and each of the focusing elements may form a first layer, and each of the folding elements, waveguides, and phase shifters, and the beam combiner may form a second layer.

Accordingly, the methods of manufacturing yield a monolithic, pre-aligned and optical fiber-free multi-aperture optical system which eliminates the tedious process of manual alignment of separate focusing optical elements and optical fiber waveguides. Because optical fibers are traditionally extremely sensitive to vibration, FSOC systems which employ optical fibers for example in the form of waveguides, are generally unstable and difficult to employ on moving platforms. Further, FSOC systems which employ optical fibers may generally be susceptible to failure upon exposure to a certain degree of temperature change. In contrast, the methods and systems of the various embodiments described herein provide an FSOC system with improved thermal stability and jitter resistance, as compared to conventional FSOC systems. Additionally, alternative FSOC systems such as those described in <CIT> employ a mechanical array of lenslets coupled into single-mode optical fiber, thereby necessitating performance of meticulous single micron alignment of separate focusing optical elements and optical fiber waveguides. In contrast, the monolithically formed multi-aperture optical system of the various embodiments described herein may be fabricated using lithographically defined chips, thereby eliminating the need for high precision alignment of the individual components, and drastically reducing manufacturing costs.

The preceding description has discussed use of a multi-aperture optical system <NUM> for FSOC. In accordance with some embodiments, the multi-aperture optical system <NUM> described herein may be applied to or used in conjunction with coherent LIDAR systems. As can be appreciated, coherent LIDAR systems typically incorporate similar functional elements as those used wjth FSOC. Consequently, the same technology may be used to fabricate coherent LIDAR systems. Various aspects of the present disclosure enable transmitting of a beam of light in a controlled angular direction. Such capability is desired for additional applications, for example including but not limited to optical illuminators, where light is directed to a remote area, and optical designators, where light is directed to a remote area and the intensity of the light is varied according to a pre-determined temporal code.

Headings and subheadings, if any, are used for convenience only and do not limit the disclosure. To the extent that the term include(s), have, or the like is used, such term is intended to be inclusive in a manner similar to the term comprise as comprise is interpreted when employed as a transitional word in a claim.

A phrase "at least one of" preceding a series of items, with the terms "and" or "or" to separate any of the items, modifies the list as a whole, rather than each member of the list. By way of example, each of the phrases "at least one of A, B, and C" or "at least one of A, B, or C" refers to only A, only B, or only C; any combination of A, B, and C; and/or at least one of each of A, B, and C.

It is understood that the specific order or hierarchy of steps, operations, or processes disclosed is an illustration of exemplary approaches. Unless explicitly stated otherwise, it is understood that the specific order or hierarchy of steps, operations, or processes may be performed in different order. Some of the steps, operations, or processes may be performed simultaneously. The accompanying method claims, if any, present elements of the various steps, operations or processes in a sample order, and are not meant to be limited to the specific order or hierarchy presented. These may be performed in serial, linearly, in parallel or in different order. It should be understood that the described instructions, operations, and systems can generally be integrated together in a single software/hardware product or packaged into multiple software/hardware products.

In one aspect, a term coupled or the like may refer to being directly coupled. In another aspect, a term coupled or the like may refer to being indirectly coupled.

Terms such as top, bottom, front, rear, side, horizontal, vertical, and the like refer to an arbitrary frame of reference, rather than to the ordinary gravitational frame of reference. Thus, such a term may extend upwardly, downwardly, diagonally, or horizontally in a gravitational frame of reference.

The disclosure is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the principles described herein may be applied to other aspects.

All structural and functional equivalents to the elements of the various aspects described throughout the disclosure that are known or later come to be known to those of ordinary skill in the art are intended to be encompassed by the claims.

The title, background, brief description of the drawings, abstract, and drawings are hereby incorporated into the disclosure and are provided as illustrative examples of the invention which is defined by the attached claims.

Claim 1:
A multi-aperture optical system (<NUM>) suitable for free space optical communications, FSOC, and remote sensing coherent LIDAR, comprising:
a photonic integrated circuit (<NUM>);
a spacer substrate (<NUM>) coupled to the photonic integrated circuit;
a folding element (<NUM>) integrated into the photonic integrated circuit;
a plurality of optical cells (<NUM>), each optical cell including:
a phase shifter (<NUM>);
a waveguide (<NUM>);
a focusing optical element (<NUM>) formed on the spacer substrate and configured to receive the light incident on the optical cell, and focus the light through the photonic integrated circuit and onto the folding element to couple light incident on the optical cell into the waveguide, wherein:
the waveguide is integrated into the photonic integrated circuit, coupled to the phase shifter, and configured to transport the light reflected by the folding element through the phase shifter; and
the phase shifter is coupled to the focusing optical element, and configured to shift a phase of an optical signal received by the focusing optical element, suitable for removing adverse effects of atmospheric turbulence on the optical signal;
a beam combiner (<NUM>) coupled to the phase shifter, and configured to combine light output from the phase shifter; and
a photodetector (<NUM>) coupled to the beam combiner to receive the combined light output from the beam combiner and output a corresponding signal,
wherein the photonic integrated circuit, the spacer substrate, the plurality of optical cells, the beam combiner, and the photodetector are integrated as a single monolithically formed optical head.