Carson G. Valdez

While partial coherence often degrades interference and information transfer, its correlations can be engineered to enhance imaging and optical information processing. However, approaches based on tomography and numerical reconstruction do not physically separate unknown fields into their coherence modes—that is, their orthogonal and mutually incoherent components. Here we demonstrate the separation of partially coherent light into coherence modes. We use variational processing in layered self-configuring interferometers on a silicon photonic circuit. This process finds the eigenvectors and eigenvalues of the coherency matrix, characterizing the state while leaving it intact and separated after optimization. We show that spatially orthogonal, mutually incoherent beams can be routed to separate outputs even when their intensity profiles overlap. Our experiment separates the two strongest coherence modes from a nine-input sampling of overlapping fields from two independent lasers; operation beyond two modes is verified in a rank-3 experiment. The number of physical components scales linearly with the rank r of the coherency matrix, through r in situ gradient-based optimizations enabled by electronic drive-frequency multiplexing of the phase shifters. We benchmark performance against an on-chip mixture-based tomographic method. These results establish a scalable framework for programmable coherence analysis and control.

Polarization-resolved control and measurement of optical fields are essential for a wide range of photonic systems, including coherent communication, polarimetric sensing, and quantum information processing. We present a photonic integrated circuit architecture that enables the generation and analysis of arbitrary polarization states. The device provides reconfigurable access to the full polarization degree of freedom of coherent light within a single integrated platform. We experimentally demonstrate arbitrary polarization state generation spanning the Poincaré sphere, as well as Stokes vector measurement on a chip. Unlike conventional Stokes measurements that rely on direct detection, the polarization state in this architecture is inferred from the phase settings required to interferometrically combine polarization-demultiplexed optical fields. As a result, the optical signal itself does not need to be detected or absorbed and remains available for subsequent optical-domain processing. The devices are fabricated in a commercial foundry using CMOS-compatible processes, enabling scalable and reproducible integration. By combining polarization generation and analysis in a compact and stable photonic circuit, this work eliminates the need for external polarization optics and provides a foundation for robust, polarization-enabled photonic integrated systems.

High-efficiency optical coupling between fibers or free-space beams and silicon photonic circuits is critical for reducing system-level losses and enabling scalable packaging. Conventional grating couplers typically require illumination off-normal incidence, limiting their compatibility with vertically oriented sources and fibers. We report the design, fabrication in a commercial foundry, and experimental results of high-efficiency, normal incidence grating couplers for silicon photonics. We observe a maximum coupling efficiency of 85.4% (−0.69dB) with a 1 dB bandwidth of 20 nm at a central wavelength of 1546 nm. These experimental results verify earlier theoretical and simulation results and pave the way for the use of perfectly vertical grating couplers, as an alternative to edge coupling, in silicon photonics applications that are sensitive to input coupling loss. Further, these results enable the use of grating couplers for vertically oriented elements, such as multicore fibers and VCSELs, and address challenges associated with coupling to free space beams.

Spectral filters are widely used in sensing and communicating with light, such as for separating wavelength channels in communications or sensing the specific spectra of some object or material of interest. The filter function is, however, often fixed, and precise filtering can require precise manufacturing. We propose an approach to integrated optical spectral filtering that allows arbitrary programmability, can compensate automatically for imperfections in filter fabrication, allows multiple simultaneous and separately programmable filter functions on the same input, and can configure itself automatically to the problem of interest, for example, to filter or reject multiple arbitrarily chosen frequencies. The approach exploits splitting the input light into an array of multiple waveguides of different lengths that then feed a programmable interferometer array that can also self-configure. It can give a spectral response similar to arrayed waveguide gratings but offers many other filtering functions, as well as supporting other structures based on non-redundant arrays for precise spectral filtering. Simultaneous filtering also allows an automatic measurement of the temporal coherency matrix and physical separation into the Karhunen–Loève expansion of temporally partially coherent light fields. With this approach, a wide range of spectral operations can be controllably, automatically, and precisely performed by an integrated photonic device with simple programmability.

Direct characterization of potentially habitable terrestrial exoplanets around Sun-like stars requires extremely precise instrumentation capable of measuring a planet-star contrast ratio of 10-10. This instrumentation challenge pushes the limits of optical technology, with state-of-the-art coronagraphs requiring extremely precise wavefront control and large, complex, and expensive instrument designs. These bulk optics coronagraph instruments typically have limited measurable optical bandwidths and high sensitivity to the overall telescope pupil design (i.e. on-axis vs off-axis telescope configurations) and high sensitivity to optical aberrations. The AstroPIC project is applying programmable integrated photonics to develop a new kind of coronagraph instrument that is capable of reaching the theoretical limits of optimal coronagraph performance. This will result in a coronagraph instrument that is orders of magnitude smaller, less massive, and more flexible compared to traditional bulk optics designs, with the goal of increasing the Exo-Earth science yield of NASA’s Habitable Worlds Observatory flagship mission. The AstroPIC project is leveraging commercial silicon photonics fabrication to develop a coronagraph instrument that couples light from a telescope pupil plane into a mesh of integrated Mach-Zehnder interferometers on a photonic integrated circuit (PIC). The coupling is achieved by using a lenslet array to sample and focus the telescope pupil into an array of grating coupler waveguide inputs on the PIC, as shown in Figure 1a. Once the light from the telescope pupil is coupled onto the PIC, the interferometer mesh can be controlled by tuning thermal phase shifters in each interferometer to implement a mode-sorting algorithm in order to separate the starlight and planet light modes at high contrast levels. An integrated photonic testbed at NASA Ames Research Center is being developed to demonstrate coupling from a realistic telescope pupil into the PIC and will be used to characterize the contrast, throughput, and optical bandwidth of the coronagraph instrument. Figure 1b shows the design of the testbed, which will be used to experiment with different control algorithms for setting the tunable phase shifters in the photonic mesh and assess the factors limiting contrast on the chip to inform future iterations of the PIC design. Figure 1: (a) Diagram of AstroPIC instrument concept coupling the Habitable Worlds Observatory (HWO) pupil into an array of grating couplers using a microlens array. A mesh of Mach Zehnder Interferometers (MZIs) are tuned to separate starlight spatial modes from planet spatial modes. (b) Design of integrated photonics testbed under development at NASA Ames Research Center. The AstroPIC project has also developed simulations of the PIC coronagraph instrument concept in order to predict the coupling from the telescope pupil into the chip and simulate the photonic mesh behavior. Specifically, we are studying the application of the AstroPIC instrument concept to the NASA Habitable Worlds Observatory (HWO) flagship mission, which is currently in the pre-design phase. These simulations have shown that the AstroPIC concept is very robust to telescope pupil shape and aberrations for both on- and off-axis telescope configurations, and that the potential science yield of the AstroPIC coronagraph is very high even for relatively small numbers of input couplers. We find that the AstroPIC concept is particularly advantageous for accessing planets located at small inner working angles relative to the host star, which is extremely challenging for traditional coronagraph designs. Figure 2 shows these simulation results for an example HWO-like pupil, summarizing the predicted exo-Earth science yield as a function of (a) number of PIC input grating coupler channels, (b) starlight suppression order, and (c) end to end transmission efficiency. Figure 2: Predicted number of exo-Earths characterized over the HWO mission lifetime as a function of (a) number of input grating coupler PIC channels, (b) PIC starlight suppression order (number of output channels that are designated as starlight vs signal light), and (c) end-to-end transmission efficiency of the PIC instrument. These simulations are conducted using the FRIDAY yield calculation package.In this presentation, we will describe the AstroPIC instrument concept and how programmable photonics can be used to implement the optimal coronagraph operator. We will then present the development of an integrated photonics testbed at NASA Ames Research Center, which is being built to demonstrate the AstroPIC concept with a realistic telescope pupil directly coupled into the integrated photonic instrument. We will also review recent simulation results predicting the performance of next-generation AstroPIC instrument designs. Finally, we will discuss the next steps needed to mature this technology for future space telescope missions and ground telescope applications, including incorporating novel photonic component designs into the mesh architecture to improve the throughput and optical bandwidth of the instrument.

We demonstrate a silicon photonic architecture comprising double Mach–Zehnder interferometers (DMZIs) designed for high-contrast photonic applications. This configuration significantly enhances the achievable extinction ratio of photonic integrated circuits (PICs), reaching levels exceeding 80 dB. By leveraging the tunable properties of DMZIs and implementing a systematic configuration algorithm, the proposed mesh effectively compensates for fabrication imperfections and mitigates non-idealities such as backreflections. Experimental validation on a silicon-on-insulator platform demonstrates the potential of this approach for applications requiring high-contrast nulling such as astronomical sensing.

We designed a grating coupler optimized for normal incidence and numerically demonstrate near-unity coupling in a standard 220-nm-thick silicon-on-insulator (SOI) technology. Our design breaks the vertical symmetry within the grating region by implementing three scattering sites per local period. This technique removes the need for bottom reflectors or additional material layers and can be realized using only two lithography masks. Using adjoint method-based optimization, we engineer the coupling spectrum of the grating, balancing the trade-off between peak efficiency and bandwidth. Using this technique, we simulate three devices with peak coupling efficiencies ranging between 93.4 (−0.3 dB) and 98.6% (−0.06 dB) with corresponding 1 dB bandwidths between 48 and 8 nm all centered around 1.55 µm.

Optical phase measurement is critical for many applications, and traditional approaches often suffer from mechanical instability, temporal latency, and computational complexity. In this paper, we describe compact phase sensor arrays based on integrated photonics, which enable accurate and scalable reference-free phase sensing in a few measurement steps. This is achieved by connecting multiple two-port phase sensors into a graph to measure relative phases between neighboring and distant spatial locations. We propose an efficient post-processing algorithm, as well as circuit design rules to reduce random and biased error accumulations. We demonstrate the effectiveness of our system in both simulations and experiments with photonics integrated circuits. The proposed system measures the optical phase directly without the need for external references or spatial light modulators, thus providing significant benefits for applications including microscope imaging and optical phased arrays.

Programmable feedforward photonic meshes of Mach–Zehnder interferometers are computational optical circuits that have many classical and quantum computing applications including machine learning, sensing, and telecommunications. Such devices can form the basis of energy‐efficient photonic neural networks, which solve complex tasks using photonics‐accelerated matrix multiplication on a chip, and which may require calibration and training mechanisms. Such training can benefit from internal optical power monitoring and physical gradient measurement for optimizing controllable phase shifts to maximize some task merit function. Here, we design and experimentally verify a new architecture capable of power monitoring any waveguide segment in a feedforward photonic circuit. Our scheme is experimentally realized by modulating phase shifters in a 6 × 6 triangular mesh silicon photonic chip, which can non‐invasively (i.e., without any internal “power taps”) resolve optical powers in a 3 × 3 triangular mesh based on response measurements in only two output detectors. We measure roughly 3% average error over 1000 trials in the presence of systematic manufacturing and environmental drift errors and verify scalability of our procedure to more modes via simulation.