SYSTEMS, METHODS AND STRUCTURES FOR OPTICAL PHASE-CONTROLLED ARRAY CALIBRATION VIA INTERFERENCE

Abstract

Optical structure comprising: a substrate on which is formed: an array of optical phase shifters; an array of optical antennas in optical communication with the array of optical phase shifters; an optical output distribution network in optical communication with the array of optical antennas; and a detector element in optical communication with the optical output distribution network;wherein the optical structure is configured such that light passing through the optical phase shifters is phase-shifted, the phase-shifted light is directed to the optical antennas where at least a portion is transmitted through them, the transmitted light from all the phase shifters is directed to the optical output distribution network and subsequently to the detector element, which detects the transmitted light from each phase shifter as an interference signal, wherein the interference signal contains light from all the phase shifters in the array of optical phase shifters, wherein individual optical paths between the optical antennas and the detector element all have the same length.

Description

CROSS REFERENCE TO RELATED APPLICATIONSThis application claims priority to U.S. Provisional Patent Application No. 62 / 631,576, filed February 16, 2018.TECHNICAL FIELDThis disclosure relates generally to optical phased arrays. More particularly, it relates to systems, methods, and structures for optical phased array calibration via interference.PRIOR ARTAs is known, chip-size optical phased arrays (OPAs) are of enormous current interest because they have been found to be useful for a number of applications, including light detection and ranging (LiDAR), free space communication, holographic displays, and biomedical imaging. Known advantages of OPAs for such applications include reduced size and weight, relatively low cost, and high reliability. Given the social importance of such applications, it follows that calibration of OPAs is of utmost importance. Accordingly, systems, methods, and structures that provide for the improved calibration of OPAs would represent a willary augmentation to the art.JP 2017-187649 A discloses: an optical phased array comprising: a distributing unit for inputting output light from a laser source via an input terminal as an input light pin and distributing it into a plurality to output it from a plurality of output terminals; a phase modulating unit having a plurality of optical waveguides connected to each of the plurality of output terminals and extending in a prescribed direction, and a plurality of heating elements supplying heat to each of the plurality of optical waveguides to modulate each phase of the transmitted input light Pinalong the plurality of optical waveguides; and a diffraction grating array comprising a plurality of diffraction gratings respectively arranged in each of the plurality of optical waveguides and radiating the input light Pinof which phase is modulated as radiation light Pout.US 2014 / 0376001 A1 discloses optical integration technologies, designs, systems and methods directed to optical coherence tomography (OCT) and other interferometric optical sensor, range finding and imaging systems, such systems, methods and structures using tunable optical sources. coherent detection and other structures on monolithic single or multichip integration. Unlike modern prior art OCT systems and structures that use simple miniature optical bench technology using small optical components positioned on a substrate, systems and methods according to the present disclosure use one or more photonic integrated circuits (PICs) and use swept source techniques and use one or more widely tunable optical sources. In another embodiment, the system uses an optical photonic phased array. The phased array may be a static phased array to eliminate or enlarge the lens that couples light to and from a sample of interest, or it may be static and use a spectrally dispersive antenna and a tunable source to perform angle scanning. The phased array may be active one- or two-dimensionally in order to scan the light beam at an angle. The phased array may also adjust focus. The phased array may implement an optical waveform that extends the focus of focus of focus for imaging. The phased array may also be a separate discrete element fed by one or more optical fibers. The phased array can be used to scan a biomedical sample in conjunction with a swept source OCT system, can be used in free space coherent optical communication system for beam alignment or tracking, is used in LIDAR applications, or many other beam steering or other applications.US 2018 / 0039153 A1 discloses an optical apparatus comprising an array of optical unit elements. Each of the unit optical elements includes a plurality of amplitude adjustment (AA) elements, each of the plurality of AA elements configured to process one of different phases of an optical signal; an optical antenna; and a combination mechanism for optically coupling the plurality of AA elements to the optical antenna. The optical device also includes a controller coupled to the plurality of AA elements, the controller configured to generate control signals for the plurality of AA elements according to an optical aiming beam pattern.SUMMARYAdvancement is made in the art in accordance with aspects of the present disclosure directed to systems, methods, and structures for optical phased array calibration via interference.In sharp contrast to the prior art - and in accordance with aspects of the present invention - calibration of optical phased arrays can be advantageously performed as a single measurement of phase offset with respect to only a single interference signal. In further contrast to the prior art, systems, methods, and structures in accordance with aspects of the present disclosure advantageously generate phase offsets and phase functions of each element without time consuming, repeating procedures or multiple detector signals as required in the prior art.The invention relates to an optical structure according to claim 1, an optical structure according to claim 10, and a calibration method according to claim 21. Advantageously, optical path length equality allows valid calibration for a wide range of wavelengths.BRIEF DESCRIPTION OF THE DRAWINGSA more complete understanding of the present disclosure will be obtained by reference to the accompanying drawings, in which: FIG. 1 is a schematic diagram illustrating an illustrative optical phased array (OPA) structure with an on-chip detector, in accordance with aspects of the present disclosure; FIG. 2 is a schematic diagram illustrating an alternative illustrative OPA structure with an on-chip detector and tap(s), in accordance with aspects of the present disclosure; FIG. 3 is a schematic diagram illustrating an alternative illustrative OPA structure with output distribution network and multiple photodetectors, in accordance with aspects of the present disclosure; FIG. 4 is a schematic diagram illustrating another alternative illustrative OPA structure with output distribution network and multiple photodetectors, in accordance with aspects of the present disclosure; FIG. 5 is a schematic diagram showing an illustrative OPA structure with an on-chip detector and reflector(s) disposed between phase shifters and antennas, in accordance with aspects of the present disclosure; FIG. 6 is a schematic diagram illustrating an alternative illustrative OPA structure with an on-chip detector and reflector(s) arranged after antennas, in accordance with aspects of the present disclosure; FIG. 7 is a schematic diagram illustrating an illustrative OPA structure with an on-chip detector and distributed reflector(s) in accordance with aspects of the present disclosure; FIG. 8 is a schematic diagram illustrating an illustrative OPA structure with a coherent / symmetric on-chip detector and reflector(s), in accordance with aspects of the present disclosure; FIG. 9 is a schematic diagram illustrating an illustrative OPA structure with a coherent / symmetric on-chip detector and reflector(s) arranged after antennas, in accordance with aspects of the present disclosure; FIG. 10 is a schematic diagram illustrating an illustrative OPA structure with a coherent / symmetric on-chip detector and distributed reflector(s), in accordance with aspects of the present disclosure; FIG. 11 is a diagram illustrating generation of a complex field value interference signal having 64 elements, in accordance with aspects of the present disclosure; FIG. 12(A) is a graph showing initial angular transmit far field power versus far field degrees of a one-dimensional optical phased array with 64 elements with a randomized phase front, in accordance with aspects of the present disclosure; FIG. 12(B) is a zoom-in graph showing initial far field power versus far field degrees near 0° and indicating where an interference measurement will take place, in accordance with aspects of the present disclosure; FIG. 13(A) is a graph showing the effect of the phase shifters at 0° far field power versus applied phase in scanning optical phase shifters for 3 illustrative phase shifters, in accordance with aspects of the present disclosure; FIG. 13(B) is a graph showing the effect of the phase shifters at 0° far field power versus applied phase in scanning optical phase shifters for 64 illustrative phase shifters, in accordance with aspects of the present disclosure; FIG. 14(A) is a graph showing far field degrees versus far field power of a calibrated illustrative optical phased array, in accordance with aspects of the present disclosure; FIG. 14(B) is a zoom-in diagram showing calibrated far field degrees versus far field power of a calibrated illustrative optical phased array, in accordance with aspects of the present disclosure; FIG. 15 is a flow diagram illustrating an illustrative calibration method for optical phased arrays, in accordance with aspects of the present disclosure.The illustrative embodiments are more fully described by the figures and the detailed description. However, embodiments according to this disclosure may be embodied in various forms and are not limited to specific or illustrative embodiments described in the drawings and the detailed description.DESCRIPTIONThe following merely illustrates the principles of the disclosure. It is therefore to be understood that various arrangements may be devised by those skilled in the art, which, although not expressly described or shown herein, embody the principles of the disclosure and are included within its scope and spirit.Moreover, all examples and conditional language recited herein are intended for purpose of teaching only to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor(s) to further developing the art, and are not to be interpreted as limiting such specific recited examples and conditions.Moreover, all statements herein that set forth principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Moreover, such equivalents are intended to include both currently known equivalents and equivalents to be developed in the future, i.e., any elements developed that perform the same function, regardless of structure.Thus, for example, those skilled in the art will understand that any block diagrams herein represent conceptual views of illustrative circuits embodying the principles of the disclosure.Unless expressly specified otherwise herein, the figures that make up the drawing are not to scale.As an additional background, it is noted that optical phased arrays have found wide-spread applicability for transmitting and receiving light with diffraction limited output beams. Thousands of elements are required to build a large opening. To produce a desired phase distribution across the elements for beamforming, a fast, efficient and robust calibration procedure is required. Ideally, such a procedure(s) is performed on-chip so that calibration can be performed in the field.As will be readily apparent to those skilled in the art upon examination of the present disclosure, systems, methods, and structures according to the present disclosure advantageously and directly measure unknown individual phase offsets and phase / control signals from multiple optical elements forming at least a portion of an optical phased array. It is of particular advantage that only a single detector element is required for performing the measurement(s), although more than one detector may be used.As will be further understood by those skilled in the art, these unknown phase offsets of the optical elements may be caused by variations in wafer characteristics, manufacturing-induced phase noise, thermal gradients, or other processes.Briefly, aspects of the present disclosure include measuring a varying intensity of interference of the elements during individual sampling of the phase of each element. It is noted that such interference, as used herein, is a sum of optical signals in the optical domain with a single photodetector or a sum of optical signals in the optical and electrical domain with multiple photodetectors. A background field caused by the elements is used as a reference for determining the phase of each element. The background field may be randomly assigned or selected by modifying phase shifter control signals. When an individual element is scanned, the background remains nearly constant due to the large number of elements throughout the optical phased array.After this detailed discussion, we now turn to some specific configurations and operations of systems, methods, and structures in accordance with aspects of the present disclosure.FIG. 1 is a schematic diagram illustrating an illustrative optical phased array (OPA) structure with an on-chip detector, in accordance with aspects of the present disclosure.Referring to this figure, there is shown an optical structure generally including a substrate having an optical waveguide pattern formed thereon, the pattern including a light receiving end configured to receive and distribute light to an array of phase shifting elements. Further, an array of antennas configured to receive phase shifted light and emit at least a portion of the received light is shown in the figure.Depending on the antenna configuration(s) and construction, those skilled in the art will appreciate that not all of the phase shifted light received by the antennas is necessarily emitted. Instead, at least a portion of the light is transmitted through the antennas, whereafter it is directed by the action of an output optical distribution network and to a detector element.Operationally, and in accordance with an illustrative aspect of the present disclosure, light received by the input optical distribution network is phase shifted by the phase shifters and directed toward the antenna elements. Portions of the phase shifted light transmitted through the antennas are then directed to the output optical distribution network and then directed to an on-chip detector as an interference signal containing light from each of the phase shifters. As stated previously, such an interference signal is the sum of the light signals output by the output optical distribution network.At this point, it is noted that the waveguide pattern formed on the substrate generally interconnects and includes the elements described above, and may advantageously be implemented on a substrate(s) that / utilize known, i.e., CMOS, manufacturing techniques and is / are constructed from Si (or Sin) waveguides.Further, it is to be noted that while the distribution networks are shown as including a binary tree distribution network, this is for illustrative purposes only. As those skilled in the art readily appreciate, such distribution networks according to the present disclosure may be constructed from any one or combination of known structures, including cascaded 1 x N splitters, binary trees, star couplers, a single 1 x M splitter, directional couplers, free space and other known waveguide structures such as slab waveguides.According to aspects of the present disclosure, for transmissive structures, optical path lengths between individual antenna elements and the detector element all have the same length.It should also be noted that while an equal number of phase shift and antenna elements are shown, this is not required. The exact number of phase shifters and antennas will depend on the particular waveguide interconnection employed. Moreover, while the figures, such as FIG. 1, show illustrative structures with eight (8) phase shifters and antennas, this disclosure is not so limited, and as many as any number of elements are possible as manufacturing techniques and application dictate. Accordingly, any feature size is contemplated and may be advantageously fabricated on a single semiconductor integrated chip or photonic integrated circuit. Systems built in accordance with the present disclosure may include, interconnect, and / or integrate both electronic integrated circuits and photonic integrated circuits, as appropriate.Those skilled in the art will readily understand and appreciate that any phase shifters employed in the phase shifting elements may include thermo-optical, electro-optical, mechanical, fluidic, liquid crystal, non-linear, acousto-optical, and stress related phase shifters, while antennas may include grid-based design(s), plasmonic emitters, metal antennas, mirror facet, and axial beam facet as appropriate.Finally, the detector element may include one or more individual photodetectors of any type known in the art that generally receives an optical signal as an input and outputs an electrical signal in response thereto.FIG. 2 is a schematic diagram illustrating an alternative illustrative OPA structure with on-chip detector and tap(s), in accordance with aspects of the present disclosure. As can be seen from this figure, the structure shown includes an array of individual taps that tap at least a portion of phase shifted light from each of the phase shifters. The tapped light is directed to an on-chip detector element where it is detected as an interference signal. As before, such an interference signal contains light from all of the phase shifters. Advantageously, optical power taps such as those shown may be formed with a directional coupler, multimode interference coupler, adiabatic coupler or other device as appropriate.FIG. 3 is a schematic diagram illustrating an alternative illustrative OPA structure with output distribution network with multiple individual photodetectors, in accordance with aspects of the present disclosure. In this illustrative configuration, the multiple photodetectors are electrically connected in parallel such that a single electrical signal is output from the multiple photodetectors and a single detector element is formed. It should be noted that the distribution network optically connecting the multiple photodetectors to the array of antennas includes a number of 1xM and 2x1 structures, and those skilled in the art will recognize that the disclosure is not so limited.It should also be noted that, as before, individual optical path length(s) between the antennas and the individual one of the pluralities of photodetectors are all the same. Accordingly, any output network structure / configuration that provides this same path length characteristic and optically connects the antennas to the photodetectors is contemplated. It is further noted that each of the individual photodetectors is associated with a particular collection of the antennas, and that in the configuration shown, each of the individual photodetectors receives as input light transmitted through an equal number of individual antennas.FIG. 4 is a schematic diagram illustrating another alternative illustrative OPA structure with output distribution network and multiple photodetectors, in accordance with aspects of the present disclosure. In this illustrative configuration, the individual photodetectors are grouped into sets and the sets are summed in parallel. It is noted that while two sets are shown in this illustrated illustrative configuration, this is for illustrative purposes only and a larger number of sets are considered in other configurations. However, two sets are useful for quadrature detection of the coherent measurement.Up to this point, structures and operations have been disclosed and discussed that generally involve the transmission of light output by phase shifting elements and transmitted to one or more photodetectors where it is detected as interference signal(s). As described below, alternative structures according to the present disclosure may advantageously employ reflective rather than transmissive elements.FIG. 5 is a schematic diagram illustrating an illustrative optical phased array (OPA) structure with an on-chip detector and reflector(s) disposed between phase shifters and antennas, in accordance with aspects of the present disclosure. In this configuration shown, at least a portion of the light output from each of the individual phase shifters is reflected back through the phase shifters and then directed to a detector as an interference signal. A tap coupler may be used to direct a portion of the reflected light into the detector. As will be readily appreciated by those skilled in the art, such reflected light passes through its respective phase shifter twice and is then distributed through the input distribution network to the detector. Advantageously, such a configuration requires only a single distribution network that distributes both light to the array of phase shifters and the reflected light back to the detector element. It should be appreciated that reflections may be generated with any / any of a number of known reflectors / structures, including discontinuities in waveguides, resonators, Bragg gratings, rigid reflectors, tunable reflectors, liquid crystals, or other structures as appropriate.Those skilled in the art will understand and recognize that yet another way of generating reflection with commonly used grating-based antennas is at the wavelength that satisfies the Bragg condition. Advantageously, in such a configuration, the antennas act as a reflector element.FIG. 6 is a schematic diagram illustrating an alternative illustrative optical phased array (OPA) structure with an on-chip detector and reflector(s) arranged after antennas, in accordance with aspects of the present disclosure. Those skilled in the art will appreciate that a variety of reflector positions may be employed in structures in accordance with aspects of the present disclosure. As illustratively shown, reflector elements in this figure are arranged in an optical path after the antennas. As a result, light transmitted through the antennas is reflected back through the antennas, reflected by the phase shifters, and then directed to the detector element as an interference signal.FIG. 7 is a schematic diagram illustrating an illustrative optical phased array (OPA) structure with an on-chip detector and distributed reflector(s), in accordance with aspects of the present disclosure. In this illustrative configuration shown, the antennas are configured to serve as a distributed reflector and may advantageously be wavelength dependent.FIG. 8 is a schematic diagram illustrating an illustrative optical phased array (OPA) structure with a coherent / symmetric on-chip detector and reflector(s), in accordance with aspects of the present disclosure. Similar to the structures disclosed previously, this figure shows an alternative structure with a coherent detector, which advantageously contains a local oscillator.FIG. 9 is a schematic diagram illustrating an illustrative optical phased array (OPA) structure with a coherent / balanced on-chip detector and reflector(s) arranged after antennas, in accordance with aspects of the present disclosure.FIG. 10 is a schematic diagram illustrating an illustrative optical phased array (OPA) structure with a coherent / symmetric on-chip detector and distributed reflector (distributed reflectors), in accordance with aspects of the present disclosure.At this point, those skilled in the art will understand and recognize that interference as employed in the present disclosure may be advantageously measured at any point after traversing phase shifting elements. And, unlike the prior art, systems, methods, and structures according to the present disclosure do not require or use time-consuming, repetitive optimization procedures and instead measure phase offset of each element directly in a single predetermined operation along with phase shifting as a function of a control signal (also referred to as a "phase function"). Finally, and as a particular distinction, only a single interference measurement of all antennas is required, i.e. a single pixel detector, and therefore multiple detector elements measuring the interference from adjacent antennas or groups of antennas are unnecessary.After the phase offsets and phases of each element are measured, the optical phased array can be easily adjusted to any element phase distribution to perform beam steering and beam formation as specified (dictated) by a particular application(s). Finally, and in addition to the phase versus control signal ratio(s), calibration according to the present disclosure enables extraction of intensity variations versus control signal by comparing the interference with an expected sinusoidal shape of the interference.To more thoroughly understand our disclosed method, an optical power measurement, S, interference of N optical elements with amplitude a n( real valued) and an unknown but desired phase offset, b n. A portion of b n is device inherent due to imperfections, but may also contribute due to interference measurement techniques, such as far field measurement not to 0°.Generally, the tunable phase of the elements may be set to any initial known voltage (or control signal), v n, but the resulting phase, θ n( v n), may be unknown. The initial measurement S is as follows: where i is the imaginary unit and C≅is the initial complex field value of the measurement. If v m, the voltage of element m, is changed, then the resulting power measurement, S m( v m), can be written as follows:Thus, S m( v m) can be considered an interference measurement of element m against a reference signal C m. For large N, C is nearly equal to C m, i.e., the extraction of element m from the interference does not greatly change the magnitude or phase of the initial complex field value, and when m is changed, each coherent antenna measurement is against a similar reference signal.FIG. 11 is a diagram illustrating the generation of a complex field value having 64 elements, in accordance with aspects of the present disclosure. In particular, Fig. 11 shows the value Cξ after being generated from the sum of 64 elements. For a larger number of elements, C≅has the possibility of being larger and the individual involvement of each element becomes smaller.The use of the approximation C m ≈C≅ makes it possible to simplify S m( v m) in the following: where ∠C≅ is the angle of the complex number C≅.Here, it can be seen that S m( v m) generates a sine curve with a phase offset that is the difference between the initial desired phase offset of the optical element, b m, and a constant phase value across all elements, C≅. In addition, the phase response of the element as a function of voltage, θ m( v m), is also measured directly. If θ m( v m) is complex (has gain / attenuation), this can also be measured with this method. The relative amplitudes of the elements, a m, are also compared.Accordingly, experimental insertion of this sine curve generates the relative phase offset, the relative amplitude, and the phase function of each element. Insertion can be performed with heuristic knowledge of the system for generating an exact result. In addition, the phase shifters may also allow measurement of the conducted light power (such as leakage current in a pn junction). Utilizing this knowledge in conjunction with this procedure enables accurate insertion of the complex value of θ m( v m).By performing a single step and measuring S m( v m) for each element 1 to N, one experiences the initial phase offset b m( with a constant of C≅ ) of each optical element and the tunable phase function θ m( v m) as a function of voltage. If this voltage is set equal to the opposite of (b m- ∠C) then the interference measurement, S, is maximized and calibration is completed.As will be appreciated, this interference measurement according to the present disclosure is general and can be achieved in numerous phased array configurations, including those shown and described herein. A significant aspect of the interference measurement is that it relates to a desired phase distribution of the optical elements. That is, generating the maximum of this interference measurement generates the desired phase distribution or results in the desired phase distribution via a known phase transformation. Since the phase shift pattern, θ m( v m), is measured for each optical element, beam steering or beam formation from the initial desired phase distribution to another phase distribution after calibration can be easily achieved. It is also important to note that the mathematical discussions herein are for phase control of each individual element, but this procedure can also be performed with phase shifters that control groups of elements.An illustrative example of calibration of a one-dimensional optical phased array having 2μm pitch and 64 elements at a wavelength of λ=1.55μm will now be described. The phase offset of the elements is set to a uniformly random phase distribution within 0-2π. The resulting far field is shown in Fig. 12(A).For simplicity, the interference measurement is the center of the far field at 0° (FIG. 12(B) ). The power at this point when the first three phase shifters are sampled is shown in Fig. 13(A). Sinusoids are generated with phase offsets that are approximately equal to b m- C≅. Fig. 13(B) shows all the sine curves generated when the 64 elements are sampled on a single chart.The sinusoids shown in FIG. 13(B) have theoretical phase offsets of b m- C≅. When the phase shifters are set to the opposite of the inserted offset value, the far field is changed as shown in Figure 14(A). the optical phased array is now calibrated to maximize the interference measurement, which is the power at 0°.Fig. 14(B) shows zoom In at 0° and represents the theoretical maximum far field at this point. The calibrated optical phased array has less than 0.05 dB loss compared to the theoretical maximum.The generalized calibration procedure will be further understood with reference to a flow chart shown in Fig. 15. As shown in this figure, for an optical structure containing an optical phased array, for each individual phase shifter, the phase shift is varied over a predetermined pattern while an interference signal is detected for that structure. From such detections, an overall phase offset with respect to the reference signal is determined. It should be noted that this method / procedure according to the present disclosure may be advantageously modified such that groups of phase shifters are varied instead of individually. With such an operation, the phase shifters are varied at different frequencies, and a phase offset of each frequency component in the interference signal can be advantageously determined.At this point, while we have presented this disclosure using some specific examples, those skilled in the art will recognize that our teachings are not so limited. Accordingly, this disclosure is limited only by the scope of the claims appended hereto.

Claims

An optical structure comprising: a substrate on which is formed: an array of optical phase shifters; an array of optical antennas in optical communication with the array of optical phase shifters; an output optical distribution network in optical communication with the array of optical antennas; and a detector element in optical communication with the output optical distribution network; wherein the optical structure is configured such that light traversing the optical phase shifters is phase shifted, the phase shifted light is directed to the optical antennas where at least a portion is transmitted therethrough, the transmitted light from all of the phase shifters is directed to the output optical distribution network and then to the detector element that detects the transmitted light from each phase shifter as an interference signal, wherein the interference signal includes light from all of the phase shifters in the array of optical phase shifters, wherein individual optical paths between the optical antennas and the detector element are all of the same length.The optical structure of claim 1, wherein the detector element comprises a plurality of individual detectors.The optical structure of claim 2, wherein the plurality of individual detectors are electrically connected in parallel.The optical structure of claim 2, wherein the plurality of individual detectors are arranged in sets of multiple detectors, each set including the same number of individual detectors and each set being electrically connected in parallel.The optical structure of claim 4, wherein there are two sets of multiple detectors electrically connected in parallel.The optical structure of claim 2, wherein each of the individual detectors receives light transmitted through predetermined ones of the array of optical antennas.The optical structure of claim 6, wherein the number of predetermined ones of the array of optical antennas transmitting light to an individual one of the individual detectors is the same for all of the individual detectors.The optical structure of claim 1, further comprising: an input distribution network formed on the substrate and in optical communication with the array of optical phase shifters, the input distribution network configured to receive light from a source and distribute the received light to the array of optical phase shifters.The optical structure of claim 1, wherein certain phase offsets are valid for the structure over a wide band of wavelengths.An optical structure comprising: a substrate on which is formed: an input optical distribution network; an array of optical phase shifters in optical communication with the input optical distribution network; an array of optical antennas in optical communication with the array of optical phase shifters; an array of reflector elements in optical communication with the array of optical phase shifters; a detector element in optical communication with and disposed in an optical path in front of the array of optical phase shifters; wherein the optical structure is configured such that light traversing the optical phase shifters is phase shifted, the phase shifted light is directed to the array of reflector elements where at least a portion of the phase shifted light is reflected back from the reflector elements by the optical phase shifters and then directed as an interference signal to the detector element that detects the interference signal, the interference signal including light phase shifted by all of the optical phase shifters in the array of optical phase shifters.The optical structure of claim 10, wherein the portions of light combined into the interference signal traverse a respective one of the optical phase shifters more than once.The optical structure of claim 10, wherein the detector element comprises a plurality of individual detectors.The optical structure of claim 12, wherein the plurality of individual detectors are electrically connected in parallel.The optical structure of claim 12 or 13, wherein each of the plurality of detectors receives light from predetermined ones of the array of optical reflectors.The optical structure of claim 10, wherein the detector element is a coherent detector having an input local oscillator.The optical structure of claim 15, wherein the coherent detector is a symmetric detector or an IQ detector.The optical structure of claim 10, wherein the array of optical reflectors is disposed in an optical path between the array of optical phase shifters and the array of optical antennas.The optical structure of claim 10, wherein the array of optical reflectors is disposed in an optical path after the array of optical antennas.The optical structure according to claim 10, wherein the array of optical reflectors in the array of optical antennas is formed as a wavelength-independent distributed reflector in each.The optical structure of claim 10, wherein the array of optical reflectors comprises the array of optical antennas, each individual one of the optical antennas having a wavelength dependent Bragg condition.A calibration method for an optical structure according to any preceding claim, the method comprising: directing individual light signals through the phase shift elements such that the individual light signals are phase shifted; directing the phase shifted light signals to the array of transmit antennas; for each individual one of the array of phase shift elements: varying an amount of phase shift reported by the phase shift elements over a predetermined pattern while detecting one or more interference signals generated by the phase shifted light signals from all of the phase shift elements; and determining a phase offset for each phase shift element with respect to at least one of the detected interference signals.The method of claim 21, further comprising: determining a phase versus voltage relationship for each of the individual phase shift elements in the array.The method of claim 21, further comprising: determining an attenuation versus voltage relationship for each of the individual phase shift elements in the array.The method of claim 21, wherein, while varying an amount of phase shift for an individual phase shift element, all other phase shift elements are not varied.The method of claim 21, further comprising: varying an amount of phase shift for a plurality of the phase shift elements at different frequencies; and determining a phase offset of each frequency component in the interference signal.

Images