Scalable two-dimensional optical phased arrays with reduced side lobes

Abstract

An emission to free space of a beam with suppressed sidelobes includes providing light to optical antennas separated by a set of spacings substantially larger than a wavelength of light and forming a two-dimensional optical phased array. Phases of light emitted by the optical antennas are distributed uniformly with equalized weights within a full range in substantially all directions of a far-field distribution except a direction of a single beam characterized by an intensity distribution that comprises a central lobe with substantially suppressed sidelobes. The suppression of the sidelobes is at least 10 decibels compared to the central lobe.

Description

Scalable Two-dimensional Optical Phased Arrays with Reduced Side LobesTECHNICAL FIELD

[0001] The present disclosure relates to optical phased arrays.BACKGROUND

[0002] Optical phased arrays (OPAs) have enabled applications ranging from light detection, Light Detection and Ranging (LiDAR), and free-space optical communications. The ability of integrated OPAs to generate arbitrary far-field radiation patterns and perform two-dimensional (2D) beam steering has long been pursued for optical satellite communications because OPAs are able to provide miniatured footprint, high mechanical stability, and steering accuracy.

[0003] Short optical wavelengths impose enormous challenges on the design of two-dimensional OPAs when it comes to generation of a desired far-field radiation profile while integrating electronic phase shifting units for waveform control. Meeting stringent requirements on beam collimation and optical aperture for satellite communications is particularly challenging. To date concrete solutions that address these challenges are lacking.SUMMARY

[0004] According to an aspect of the present disclosure, a method for performing an emission to free space of a beam with suppressed sidelobes includes providing light to a plurality of optical antennas separated by a set of spacings substantially larger than a wavelength of light and forming a two-dimensional optical phased array. Phases of light emitted by the optical antennas are distributed uniformly with equalized weights within a full range in substantially all directions of a far-field distribution except a direction of a single beam characterized by an intensity distribution that comprises a central lobe with substantially suppressed sidelobes. The suppression of the sidelobes is at least 10 decibels compared to the central lobe.

[0005] The array of optical antennas may be arranged with a variable spacing in a first dimension and a uniform spacing in a second dimension.

[0006] The array of optical antennas may be arranged with a uniform spacing in a first dimension and a uniform spacing in a second dimension.

[0007] An optical phased array may include a two-dimensional array of optical antennas disposed according to the method.

[0008] According to another aspect of the present disclosure, an optical phased array includes a two-dimensional array of optical antennas separated by a set of spacings substantially larger than a wavelength of light. The optical antennas are configured to emit phases of light distributed uniformly with equalized weights within a full range in substantially all directions of a far-field distribution except a direction of a single beam characterized by an intensity distribution that comprises a central lobe with substantially suppressed sidelobes The suppression of the sidelobes is at least 10 decibels compared to the central lobe.

[0009] The optical phased array may further include a plurality of in-array phase shifters. Each in-array phase shifter may be positioned between adjacent optical antennas.

[0010] The optical phased array may further include a plurality of bus waveguides. Each bus waveguide may extend in a first dimension of the two-dimensional array. The bus waveguides may be spaced apart in a second dimension of the two-dimensional array. A respective subset of the optical antennas and a respective subset of the in-array phase shifters may be coupled to each bus waveguide.

[0011] The optical antennas of each respective subset may have a uniform pitch in the first dimension. The optical antennas of different respective subsets coupled to different bus waveguides may be offset in the first dimension.

[0012] The bus waveguides may have a uniform pitch in the second dimension.

[0013] The optical antennas of each respective subset may have a variable pitch in the first dimension.

[0014] Each bus waveguide may be coupled to an external phase shifter.

[0015] The optical antennas may have a pitch within a range of about 5 micrometers to about 1 millimeter.

[0016] According to another aspect of the present disclosure, an optical phased array includes a two-dimensional array of optical antennas. Each optical antenna is coupled to a bus waveguide. The array further includes a plurality of in-array phase shifters. Each in-array phase shifter is positioned between adjacent optical antennas. The positions of the optical antennas in the two-dimensional array are selected according to a constraint that position-induced relative phases of the optical antennas are evenly distributed over the full range of phase.

[0017] The optical phased array may further include a plurality of bus waveguides. Each bus waveguide may extend in a first dimension of the two-dimensional array. The bus waveguides may be spaced apart in a second dimension of the two-dimensional array. A respective subset of the optical antennas and a respective subset of the in-array phase shifters may be coupled to each bus waveguide.

[0018] The optical antennas of each respective subset may have a uniform pitch in the first dimension. The optical antennas of different respective subsets coupled to different bus waveguides may be offset in the first dimension to satisfy the constraint.

[0019] The bus waveguides may have a uniform pitch in the second dimension.

[0020] The optical antennas of each respective subset may have a variable pitch in the first dimension.

[0021] Each bus waveguide may be coupled to an external phase shifter.

[0022] The optical antennas have a pitch within a range of about 5 micrometers to about 1 millimeter.

[0023] According to another aspect of the present disclosure, a method of making an optical phased array includes fabricating a two-dimensional array of optical antennas, including coupling each optical antenna to a bus waveguide, and positioning the optical antennas according to a constraint that position-induced relative phases of the optical antennas are evenly distributed over a full range of phase. The method further includes fabricating a plurality of in-array phase shifters, including positioning each in-array phase shifter between adjacent optical antennas.BRIEF DESCRIPTION OF THE FIGURES

[0024] FIG. 1 is a schematic view of an example two-dimensional optical phased array.

[0025] FIG. 2A is a plot of an ideal far-field profile for the optical phased array of FIG. 1 according to the techniques discussed herein.

[0026] FIG. 2B is a plot of an example relative phase distribution according to the techniques discussed herein.

[0027] FIG. 2C is a plot of example relative phases of antennas as a function of antenna index according to the techniques discussed herein.

[0028] FIG. 3 is a table showing a comparison between a conventional Dirac delta function and a sum-field function of an optical phased array.

[0029] FIG. 4 is a plan view of an example two-dimensional optical phased array that applies the techniques discussed herein and that may be used as a framework for various example arrays.

[0030] FIG. 5 A is a plot of dimensions for an example two-dimensional optical phased array that applies the framework of FIG. 4 with randomized positioning.

[0031] FIG. 5B is a plot of a two-dimensional far-field profile (linear scale) of the example of FIG. 5A, with an inset showing a one-dimensional far-field profile (logarithmic scale) showing side-lobe suppression.

[0032] FIG. 5C is a plot of relative phases produced by antennas of the example of FIG. 5 A.

[0033] FIG. 5D is a plot of phase distribution of the example of FIG. 5 A.

[0034] FIG. 6A is a plot of dimensions for another example two-dimensional optical phased array that applies the framework of FIG. 4 with randomized positioning.

[0035] FIG. 6B is a one-dimensional far-field profile (logarithmic scale) of the example of FIG. 6A showing side-lobe suppression.

[0036] FIG. 6C is a plot of a two-dimensional far-field profile (linear scale) of the example of FIG. 6A.

[0037] FIG. 7A is a plot of dimensions for an example two-dimensional optical phased array that applies the framework of FIG. 4 with reversely compensated elliptical positioning.

[0038] FIG. 7B is a plot of a two-dimensional far-field profile of the example of FIG. 7 A.

[0039] FIG. 7C is a one-dimensional far-field profile (logarithmic scale) of the example of FIG. 7A showing side-lobe suppression.

[0040] FIG. 7D is a plot of relative phase of the example of FIG. 7 A.

[0041] FIG. 7E is a plot of phase distribution of the example of FIG. 7A.

[0042] FIG. 8A is a plot of dimensions for another example two-dimensional optical phased array that applies the framework of FIG. 4 with reversely compensated elliptical positioning.

[0043] FIG. 8B is a one-dimensional far-field profile (logarithmic scale) of the example of FIG. 8A showing side-lobe suppression.

[0044] FIG. 8C is a plot of a two-dimensional far-field profile of the example of FIG. 8A.

[0045] FIG. 8D is a plot of phase distribution of the example of FIG. 8A.

[0046] FIG. 9A is a plot of dimensions for another example two-dimensional optical phased array that applies the framework of FIG. 4 with reversely compensated elliptical positioning.

[0047] FIG. 9B is a one-dimensional far-field profile (logarithmic scale) of the example of FIG. 9A showing side-lobe suppression.

[0048] FIG. 9C is a plot of a two-dimensional far-field profile of the example of FIG. 9A.

[0049] FIG. 9D is a plot of phase distribution of the example of FIG. 9A.

[0050] FIG. 9E is a plot of a two-dimensional far-field profile of the example of FIG. 9A when the beam is steered a certain angle.

[0051] FIG. 10 is a plan view of another example two-dimensional optical phased array that applies the techniques discussed herein and that may be used as a framework for various example arrays.

[0052] FIG. 11 A is a plot of relative phase at a first grating lobe angle for the example of FIG. 10 with constant column pitch.

[0053] FIG. 1 IB is a plot of a one-dimensional far-field profile for a periodic array for the example of FIG. 10 with constant column pitch.

[0054] FIG. 11C is a plot of position as a function of antenna index for one row of antennas of the example of FIG. 10 with variable column pitch.

[0055] FIG. 1 ID is a plot of relative phase at a first grating lobe angle for the example of FIG. 10 with variable column pitch.

[0056] FIG. 11E is a plot of a one-dimensional far-field profile for a periodic array for the example of FIG. 10 with variable column pitch.

[0057] FIG. 12 is a flowchart of manufacturing an optical phased array according to the techniques discussed herein.DETAILED DESCRIPTION

[0058] Optical phased arrays capable of functional beam forming, wavefront sensing, and beam steering are an important technology in state-of-the-art applications of fight detection and ranging, such as LiDAR. Integrated OPAs using electro-optic phase shifters are immune to the mechanical vibrations and radiation pressure in space, are thus appealing to optical satellite communication where bulk optics and mechanical parts are preferably avoided due to the limited power budgets, carrying capacity, and the stringent requirements on the control of steeringaccuracy and stability. A high-performance 2D OPA, capable of operating at a constant wavelength, is highly desirable for optical Satcom.

[0059] However, there remain fundamental challenges in designing a practical 2D OPA that meets the practical requirements for Satcom. In 2D OPAs, in order to form and collimate a beam in the target direction, the antennas are theoretically required to be packed closely, down to a subwavelength pitch, to reduce or eliminate grating lobes. This requirement poses a major challenge in optical waveguide routing and the placement of phase shifters. The implementation of waveguide routing and phase control creates unwanted grating sidelobes, generating an undesired optical far-field profile with multiple beams. Moreover, the large transmission distance in Satcom, which may be up to hundreds of kilometers, requires an optical aperture as large as several square centimeters. Such a large aperture may require thousands of antennas or more, if packed with a density sufficient to avoid sidelobe formation. Hence, a key issue for any 2D OPA design for Satcom is the scalability in aperture size. In principle, out-of-chip collimation can be done using lenses, but in this case the steering range will be compromised. In brief, there are at least three fundamental design challenges for 2D OPAs in Satcom: 1) Single beam far-field generation with high side-lobe suppression; 2) Feasibility of optical waveguide routing and phase control; and 3) Scalability. The state-of-the-art does not overcome all of these challenges.

[0060] Artificial intelligence (Al), specifically a genetic algorithm, has been shown by Fatemi et al. to be useful in designing an OPA with up to 128 antennas to optimize the far-field and reduce the space required for waveguide routing and external phase control. This design appears theoretically scalable for up to 512 antennas. However, further scalability is highly questionable due to the increasing difficulty in optical routing as the dimension of the array increases, e.g., to 1000s of elements. Optimization of the far-field and automatic routing based on Al design becomes extremely time-consuming for larger array sizes. Al-based designs are foreseeably infeasible for the aperture sizes typically required by Satcom applications.

[0061] The present disclosure relates to deterministic design that simultaneously integrates a good optical far-field, sufficient room for phase shifters, and scalability in size. The techniques discussed herein may provide practical solutions for Satcom, LiDAR, and other potential applications, as well as a ground-breaking advance in the field of OPAs and free-space optical communications.

[0062] This present disclosure provides advances in the understanding of optical far-field formation and describes new techniques for OPA design, which involves phase equalization, and new OPAs that embody these techniques. Examples discussed herein provide 1) single-beam far- field formation with high side-lobe suppression; 2) a large antenna pitch and provide sufficient room for phase shifters, without complicated waveguide routing; and 3) scalability in the array size, i.e., the size can be scaled up by straightforwardly by increasing the quantity of rows and columns without fundamental issues or the need for Al-based optimization. For example, the techniques have demonstrated that high background suppression of 25 decibels (dB) can be obtained with a periodic pitch up to 750 micrometers (pm) in a 2x1024 OPA, which is further scalable. This represents a fundamental advance in the field of integrated OPAs, opening new possibilities for the use of 2D OPAs in important applications such as LiDAR, metrology, 5G, and, particularly, satellite communications (Satcom).

[0063] FIG. 1 shows an example optical phased array or OPA 100. The array 100 is formed as a two-dimensional array of optical antennas 102. The optical antennas 102 may be nanophotonic antennas or similar. The optical antennas 102 may be aligned with a plane (e.g., x-y plane) in a rectangular (shown) or non-rectangular arrangement. The optical antennas 102 may be formed on a substrate, such as a semiconductor wafer, circuit board, or similar.

[0064] The optical antennas 102 are separated by a set of spacings substantially larger than the wavelength of light operable with the array 100.

[0065] The optical antennas 102 are configured to emit phases of light distributed uniformly with equalized weights within a full range of phase, i.e., within 0 to 2n (or multiple thereof), in substantially all directions of a far-field distribution except a direction of a single beam. The optical antennas 102 are configured to provide this single beam with an intensity distribution that includes a central lobe with substantially suppressed sidelobes. The suppression of the sidelobes is at least 10 decibels compared to the central lobe.

[0066] As shown in FIG. 1, an emitted light beam has an azimuthal angle (p in the plane of the arrangement (x-y plane) and an elevation angle 0. The Dirac delta function S (or simply delta function), shown in FIG. 2A, is considered an ideal shape for far-field distribution and is used as the basis for the phase-equalization techniques discussed herein. Accordingly, FIG. 2B shows an example relative phase distribution and FIG. 2C shows example relative phases of antennas 102 as a function of antenna index.

[0067] The techniques discussed herein aim to generate such a delta function in the far field without compromising the phase control and scalability. The convolution of the delta function 8(t - r) to a continuously differentiable function M(t) will yield the singular output of M(T) at T in the time domain. With angular frequency co in the frequency domain, the Fourier transform f of the delta function d(t- r) is thus:

[0069] from which the composition of d(t - T) function in time domain may be deduced:

[0071] which is the integration of field eia>(t~T)over an infinitely wide frequency co spectrum.Meanwhile, the sum field E (cp, 0) and far-field intensity profile FF(cp, 0) of an N-antenna optical phased array may be calculated using the following equation:

[0073] where Xi and yi are the positions of the antennas and a (p, 0) the field pattern of radiation of a single antenna. The is the external phase introduced by phase shifters for beam steering.Considering the simplest case where a (<p, 0) = 1 and evl= 0 and looking at azimuthal angle <p =0, for the main electric field component of linearly polarized light the equation is reduced to a scalar form:

[0075] The similarity of Equation (2) and (4) is useful to configure and optimize the optical phased array. A comparison between the two scenarios (equations) is shown in FIG. 3, specifically, a comparison between a conventional Dirac delta function in co - 1 space and a sumfield function of an optical phased array. The limited window width in the time t domain and sin(6) domain results from the discrete sampling (finite resolution) in the frequency domain, i.e., co and x,.

[0076] Position-induced phase koXi corresponds to the frequency component, conjugated to the sin(6) domain. The discretization of position-induced phase koXi restricts the formation of the delta function within range in a periodic optical phased array, where A is the period.Equation 2 makes it clear that generating a central peak At to a delta function requires thesummation of uniform ca-components over an infinite range. In this case, for any t not equal to r, the product cot will yield phases equally distributed within the range [0, 2TT] .

[0077] Based on this understanding of the Dirac delta function (Equation 2) and the calculation of the far-field distribution of the optical phased array (Equation 4), the following phase equalization techniques for an optical phased array are provided:1. To have single nonzero output at 9o = 0 without background noise, the positions of the antennas should be arranged such that the position-induced relative phase to the first antenna, i.e., the product ko Xi sin( 9), is distributed uniformly with equal weights within the range [0, 2TT], as shown in FIG. 2B.2. For a specific angle 0, the required phase set can be created by ko Xi sin( 6) or ko [xt sin( 6)+ nn A] where is the wavelength of the optical wave and mi is an integer. Both positions set Xi will yield the same phase set at angle 9. Therefore, by well arranging thelayout, a sparse set of positions are also able to suppress the signal at the desired angle 0 as long as the phase equalization condition is satisfied, regardless of how phases are given from a position set, as shown in FIG. 2C.3. By increasing the quantity of the phase components ko Xi sin( 6) without having aggregation, the level of phase equalization is improved statistically, according to Equation 4.

[0078] The above teachings consider the calculation of the sum-field at a specific azimuthal angle <p using Equation 4 and may readily be applied to a complete two-dimensional form using Equation 3. In addition, “phase,” as used herein, refers to the position-induced phase of the antenna i relative to the first antenna, unless otherwise mentioned.

[0079] FIG. 4 shows an example two-dimensional optical phased array 400 that applies the techniques discussed above and that may be used as a framework for various specific example arrays.

[0080] The optical phased array 400 includes a two-dimensional array of optical antennas 402. Each optical antenna 402 is coupled to a bus waveguide 404.

[0081] For explanatory purposes a bus waveguide 404 generally extends along an x (first) dimension (axis) and plural bus waveguides 404 may be arranged along a perpendicular y (second) dimension (axis). For a given bus waveguide 404, optical antennas 402 may be distributed in a row that extends along the x dimension. Optical antennas 402 of different waveguides 404 may be considered to form a column that extends along the y dimension. Such acolumn is not linear arrangement of optical antennas 402, as will be discussed, due to the offsets of the antennas 402 in the x dimension. However, it may be useful to consider the first optical antenna 402 of each waveguide 404 to form a first column, the second optical antenna 402 of each waveguide 404 to form a second column, and so on. In other examples, other two- dimensional coordinate systems may be used.

[0082] The bus waveguides 404 may extend in the x dimension and may be spaced apart in the y dimension of the two-dimensional array 400. Each bus waveguide 404 may serve multiple optical antennas 402. A subset of the optical antennas 402 of the array 400 is coupled to each of the bus waveguides 404. Each bus waveguide 404 may be coupled to an external phase shifter 406.

[0083] The optical phased array 400 further includes a plurality of in-array phase shifters 408. A subset of the in-array phase shifters 408 of the array 400 is coupled to each of the bus wave guides 404. Each in-array phase shifter 408 may be coupled to a respective bus waveguide 404 at a position between adjacent optical antennas 402.

[0084] The bus waveguides 404 have a uniform pitch (spacing) Ayin the y dimension. Such a pitch may be selected to be within about 1 to 5 pm, for example, 1.5 pm. The optical antennas 402 have a pitch Axin the x dimension. The optical-antenna pitch Axmay be uniform for all bus waveguides 404. The optical-antenna pitch Axmay be selected to be within a range of about 5 pm to about 1 millimeter (mm).

[0085] Each row of optical antennas 402, as defined by the respective coupled bus waveguide 404, may be offset in the x dimension by an offset distance Ax. The offset distance Ax of each row of optical antennas 402 may be individually selected. The offset distance Ax may be selected to evenly distribute optical antennas 402 in the x dimension. The offset distance Ax may be random within a predefined range. The two-dimensional array 400 may thus be configurable within these geometric limits.

[0086] The positions of the optical antennas 402 in the two-dimensional array 400 are selected according to a constraint. The constraint is that position-induced relative phases of the optical antennas 402 are evenly distributed over the full range of phase. The waveguide pitch Ay, the optical-antenna pitch Ax, and the offset distances Ax may be selected to satisfy the constraint.

[0087] The two-dimensional array 400 provides a readily configurable and scalable framework evenly distributing position-induced relative phases of the optical antennas 402 over the fullrange of phase. Numerous specific designs may be realized by selecting the number of bus waveguides 404, the number of optical antennas 402 coupled to each bus waveguide 404, the waveguide pitch Ay, the optical-antenna pitch Ax, and the offset distances Ax.

[0088] FIGs. 5 A - 5D show an example configuration of the framework provided by two- dimensional array 400 to realize a specific optical phased array design.

[0089] In this example, the OPA includes 256 rows, i.e., 256 bus waveguides 404, and two columns, i.e., two optical antennas 402 per waveguide 404. The waveguide pitch Ayis selected to be 1.5 pm and the optical-antenna pitch Axis selected to be 150 pm. To equalize the relative phases for the grating lobe angle, the offset distances Ax are selected to be within the range of 0 to some ratio of the optical-antenna pitch, i.e., gAx. The factor g controls the coverage level of phase equalization. Offsets Ax may be selected as randomly uniform within the range of 0 to gAx.

[0090] In this example the factor g is selected to be 1. FIG. 5 A shows selected values for an example layout geometry. FIG. 5B shows a corresponding far-field, on linear scale, to show the process of phase equalization. The phase equalization condition is satisfied up to the first grating lobe for q> = 0. Any angles above the first grating lobe (0.595°) will have relative phases covering the full range. There are no visible grating sidelobes other than the central peak.

[0091] FIGs. 6A - 6C show another example configuration of the framework provided by two- dimensional array 400 to realize a specific optical phased array design.

[0092] This example configuration is similar to that of FIGs. 5A - 5D with only differences discussed in detail. In this example, the OPA includes 1024 rows, i.e., 1024 bus waveguides 404, and two columns, i.e., two optical antennas 402 per waveguide 404. The waveguide pitch Ayis selected to be 1.5 pm and the optical-antenna pitch Axis selected to be 750 pm.

[0093] Background noise is reduced to 66 dB at the angle where phase equalization is perfectly satisfied (FIG. 6B). At angles other than (p = 0, equalization degrades slightly due to the involvement of y-position induced relative phases. Noise is suppressed to nearly 25 dB over the whole far-field window, as shown in FIG. 6C.

[0094] FIGs. 7A - 7E show another example configuration of the framework provided by two- dimensional array 400 to realize a specific optical phased array design.

[0095] In this example, the OPA includes 256 rows, i.e., 256 bus waveguides 404, and two columns, i.e., two optical antennas 402 per waveguide 404. The two columns of optical antennas 402 extending in the y dimension are positioned according to two semi-elliptical curves ofopposite orientation. In this example, two ellipses with an axis ratio of two (a / b = 2) are used. Unlike a complete ellipse, in which the density of antennas would be high at the edges and sparse at the center, reversed elliptical curves mutually compensate for density and therefore collectively flatten the distribution of relative phases. A further feature for equalizing phase isX2 shown at right in FIG. 7A. While the lower branch of the elliptical curve uses the trajectory — 4- V2A— — 1, the upper branch (antennas Ul, U2, etc.) contains a half y-pitch offset - according to x2(y+^)2the trajectory — 4 - — = 1, so as to introduce deterministic x-position shifts compared to the lower branch (antennas LI, L2, etc.). As shown, while antennas Ul and LI have the same vertical distance to antenna 0, antenna Ul is disposed between antennas LI and L2 in the x dimension. Similarly, antenna U2 is positioned between L2 and L3 in the x dimension, and the pattern thus repeats.

[0096] Optical-antenna pitch Axmay be controlled by the number of the bus waveguides 404 and the axis ratio of the ellipse. In this example, the geometric proportions and ratios selected provide for a nearly square optical aperture (~ 400 pm x 400 pm). The calculated 2D and far- field profiles are shown in FIG. 7B and 7C, respectively. As shown, noise is suppressed nearly -20 dB over the whole window. The relative phases and the distribution at one of the high-order grating lobes (5°) are shown in FIGs. 7D and 7E. Despite the elliptical curve not providing a perfectly uniform phase distribution, by combing the reverse curves and differentiating overlap between the upper and lower branches of a curve, the relative phases are distributed within the range 0 to 2n without notable aggregation. As such, this example simultaneously provides good far-field performance, space to accommodate phase shifters 408, and scalability of array size.

[0097] FIGs. 8A - 8D show another example configuration of the framework provided by two- dimensional array 400 to realize a specific optical phased array design.

[0098] This example configuration is similar to that of FIGs. 7A - 7E with only differences discussed in detail. In this example, the OPA includes 1024 rows, i.e., 1024 bus waveguides 404, and two columns, i.e., two optical antennas 402 per waveguide 404. The optical-antenna pitch Axis selected to be 700 pm. The optical aperture is 1500 pmxl500 pm.

[0099] In this example, the space provided among the waveguides 404 and optical antennas 402 is sufficient to accommodate state-of-the-art electro-optic and or thermo-optic phase shifters. Inaddition, there is a clean main beam with background suppression, as shown in FIGs. 8B and 8C down to -25 dB.

[0100] FIGs. 9 A - 9E show another example configuration of the framework provided by two- dimensional array 400 to realize a specific optical phased array design.

[0101] This example configuration is similar to that of FIGs. 8 A - 8D with only differences discussed in detail. In this example, the OPA includes 2048 rows, i.e., 2048 bus waveguides 404, and one column, i.e., one optical antennas 402 per waveguide 404. The optical antennas 402 are positioned according to a similar elliptical pattern as with the example above. However, in this example, the axis ratio is four (a / b = 4). This example demonstrates the scalability of the framework with respect to the example of FIGs. 8A - 8D.

[0102] Due to the relatively large number of antennas and the equalized phase distribution (FIG. 9D), a high signal-to-noise ratio of 27 dB is obtained in the far-field, as shown in FIGs. 9B and 9C. A set of dynamical phase offsets may be provided using external phase shifter 406 to steer the beam to improve the far-field profile. As shown in FIG. 9E, when the beam is steered to <p = 45° and 6 = 45°, beam shape remains useful and background noise remains reduced.

[0103] This example, as with others discussed herein, may be used as a fundamental building block of 2D OPAs, i.e., a new type of “artificial antenna.” Two-dimensional OPAs based on a building block, such as this example, may be readily integrated with phase shifters and scaled up by duplicating the block in one or both or the x and y dimensions. A 2D array of such blocks may be provided.

[0104] In the OPA 400 and the above examples that conform to the framework provided by the OPA 400, phase equalization is enabled by optical antennas 402 within the same column. Small transverse distances between antennas 402 provide for dense usage of the phase space, thereby providing greater efficiency. The techniques discussed herein are not limited to this framework. Phase distribution may be alternatively or additional provided by antennas 402 in the same row.

[0105] FIG. 10 shows an example two-dimensional optical phased array 1000 that applies the techniques discussed above and that may be used as a framework for various specific example arrays. The OPA 1000 provides phase distribution with optical antennas 402 in the same row. The OPA 1000 may be used as a framework for various example arrays instead of or in conjunction with the principles discussed with regard to the OPA 400. The OPA 400 and related description may be referenced for details not repeated here.

[0106] The optical phased array 1000 includes a plurality of bus waveguides 404 arranged along a y dimension. Coupled to each bus waveguide 404 are a plurality of optical antennas 402 that extend along an x dimension. In-array phase shifters 408 may be positioned between adjacent antennas 402. A bus waveguide 404 may be connected to an external phase shifter 406.

[0107] The optical antennas 402 are aligned along the y dimension. This forms a number of linear columns of antennas 402 in the y dimension. The pitch of the antennas 402 along the x dimension is varied. In other words, the columns of antennas 402 are not uniformly spaced. This variable column pitch is denoted as Axi, AX2, AX3, etc.

[0108] Variable column pitch may be selected to meet a constraint that position-induced relative phases of the optical antennas 402 are evenly distributed over the full range of phase.

[0109] FIGs. 11 A - 1 IE show an example configuration of the framework provided by two- dimensional array 1000 to realize a specific optical phased array design.

[0110] In this example, 2048 rows of optical antennas 402 are arranged in five columns. That is, 2048 bus waveguides 404 are provided with five antennas 402 are coupled to each bus waveguide 404. A minimum pitch along the x dimension of the optical antennas 402 is selected to be 50 pm.

[0111] If x-pitch is constant, all antennas 402 would have 0 relative phase (FIG. 11A) at the grating lobe angles (FIG. 11B). To equalize the phase at these angles, without strongly influencing phase at other angles, a small step, such as 5 nm, may be applied to the x-pitch. That is, if n denotes column number, then this small step provides a difference in pitch between two adjacent columns, such that AX(n)- AX(n-i) = 5 nm or other suitable value. The positions of the antennas 402 and the phase distribution are shown in FIGs 11C and 11D. As can be seen, the phase gradually drifts as antenna index increases and fills out the whole space multiple times. Such a small step size can yield considerably uniform phase distribution incorporating a useful number of inline antennas, e.g., 2048. The one-dimension field profile is shown in FIG. HE, which indicates that background noise is suppressed by -27 dB.

[0112] The OPA 1000 of FIG. 10 and related example of FIGs. 11 A - 1 IE demonstrate that sidelobes and background can be suppressed by selecting a suitable set of emitter positions to satisfy the phase equalization condition, regardless of how sparsely the antenna positions are arranged in the layout. This is consistent with at least point 2 in the above list of phase equalization techniques.

[0113] In operation, a method for performing an emission to free space of a beam with suppressed sidelobes using an optical phased array, such as any of the OPAs discussed above, includes providing light to the plurality of optical antennas, which are separated by a set of spacings substantially larger than a wavelength of light. Phases of light emitted by the optical antennas are distributed uniformly with equalized weights within a full range in substantially all directions of a far-field distribution except a direction of a single beam. An intensity distribution includes a central lobe with substantially suppressed sidelobes. Suppression of the sidelobes is at least 10 decibels compared to the central lobe.

[0114] FIG. 12 shows a method 1200 of making an optical phased array, such as any of the OPAs discussed above. Semiconductor fabrication techniques may be used to perform the method 1200.

[0115] At block 1202, a two-dimensional array of optical antennas is fabricated. This includes coupling each optical antenna to a bus waveguide and positioning the optical antennas according to a constraint that position-induced relative phases of the optical antennas are evenly distributed over a full range of phase.

[0116] At block 1204, a plurality of in-array phase shifters is fabricated. This includes positioning each in-array phase shifter between adjacent optical antennas.

[0117] Blocks 1202, 1204 may be performed in a different order than discuss above or may be performed simultaneously.

[0118] The techniques discussed above utilize phase equalization to improve far-field generation in two-dimensional optical phased arrays. Deterministic frameworks and designs are provided with geometry that is large-pitch, intrinsically scalable, and compatible with electronic phase shifters, without complex optical waveguide routing. Sidelobes are suppressed and a single-beam far-field radiation profile is generated. Background suppression of over 25 dB may be obtained with, for example, a periodic pitch up to 750 pm in a 2x1024 scalable OPA. The fully phase- equalized direction has suppression of up to 66 dB. The techniques discussed herein provide a useful advance in the field of integrated OPAs and enhance various functions such as transceiver, beam steering, and waveform sensing and holograms, impacting applications in LiDARs, metrology, Satcom, 5G, to name a few.

[0119] In the above description, like reference numerals and / or like terminology denote like components. The description for one example may be referenced to understand another example. Repetition of description is avoided where practical for sake of clarity.

[0120] It should be recognized that features and aspects of the various examples provided above can be combined into further examples that also fall within the scope of the present disclosure. In addition, the figures are not to scale and may have size and shape exaggerated for illustrative purposes.

Claims

CLAIMS1. A method for performing an emission to free space of a beam with suppressed sidelobes, the method comprising: providing light to a plurality of optical antennas separated by a set of spacings substantially larger than a wavelength of light and forming a two-dimensional optical phased array; wherein phases of light emitted by the optical antennas are distributed uniformly with equalized weights within a full range in substantially all directions of a far-field distribution except a direction of a single beam characterized by an intensity distribution that comprises a central lobe with substantially suppressed sidelobes; wherein the suppression of the sidelobes is at least 10 decibels compared to the central lobe.

2. The method of claim 1, wherein the array of optical antennas is arranged with a variable spacing in a first dimension and a uniform spacing in a second dimension.

3. The method of claim 1, wherein the array of optical antennas is arranged with a uniform spacing in a first dimension and a uniform spacing in a second dimension.

4. An optical phased array comprising a two-dimensional array of optical antennas disposed according to the method of claim 1.

5. An optical phased array comprising: a two-dimensional array of optical antennas separated by a set of spacings substantially larger than a wavelength of light; wherein the optical antennas are configured to emit phases of light distributed uniformly with equalized weights within a full range in substantially all directions of a far-field distributionexcept a direction of a single beam characterized by an intensity distribution that comprises a central lobe with substantially suppressed sidelobes; wherein the suppression of the sidelobes is at least 10 decibels compared to the central lobe.

6. The optical phased array of claim 5, further comprising a plurality of in-array phase shifters, wherein each in-array phase shifter is positioned between adjacent optical antennas.

7. The optical phased array of claim 6, further comprising a plurality of bus waveguides; wherein: each bus waveguide extends in a first dimension of the two-dimensional array; the bus waveguides are spaced apart in a second dimension of the two- dimensional array; and a respective subset of the optical antennas and a respective subset of the in-array phase shifters are coupled to each bus waveguide.

8. The optical phased array of claim 7, wherein: the optical antennas of each respective subset have a uniform pitch in the first dimension; and the optical antennas of different respective subsets coupled to different bus waveguides are offset in the first dimension.

9. The optical phased array of claim 8, wherein the bus waveguides have a uniform pitch in the second dimension.

10. The optical phased array of claim 7, wherein the optical antennas of each respective subset have a variable pitch in the first dimension.

11. The optical phased array of claim 7, wherein each bus waveguide is coupled to an external phase shifter.

12. The optical phased array of claim 5, wherein the optical antennas have a pitch within a range of about 5 micrometers to about 1 millimeter.

13. An optical phased array comprising: a two-dimensional array of optical antennas, wherein each optical antenna is coupled to a bus waveguide; and a plurality of in-array phase shifters, wherein each in-array phase shifter is positioned between adjacent optical antennas; wherein positions of the optical antennas in the two-dimensional array are selected according to a constraint that position-induced relative phases of the optical antennas are evenly distributed over the full range of phase.

14. The optical phased array of claim 13, further comprising a plurality of bus waveguides; wherein: each bus waveguide extends in a first dimension of the two-dimensional array; the bus waveguides are spaced apart in a second dimension of the two- dimensional array; and a respective subset of the optical antennas and a respective subset of the in-array phase shifters are coupled to each bus waveguide.

15. The optical phased array of claim 14, wherein: the optical antennas of each respective subset have a uniform pitch in the first dimension; and the optical antennas of different respective subsets coupled to different bus waveguides are offset in the first dimension to satisfy the constraint.

16. The optical phased array of claim 15, wherein the bus waveguides have a uniform pitch in the second dimension.

17. The optical phased array of claim 14, wherein the optical antennas of each respective subset have a variable pitch in the first dimension.

18. The optical phased array of claim 14, wherein each bus waveguide is coupled to an external phase shifter.

19. The optical phased array of claim 13, wherein the optical antennas have a pitch within a range of about 5 micrometers to about 1 millimeter.

20. A method of making an optical phased array, the method comprising: fabricating a two-dimensional array of optical antennas, including coupling each optical antenna to a bus waveguide, and positioning the optical antennas according to a constraint that position-induced relative phases of the optical antennas are evenly distributed over a full range of phase; and fabricating a plurality of in-array phase shifters, including positioning each in-array phase shifter between adjacent optical antennas.

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