Michael J. DePrenger

Two-dimensional beam steering using a thermo-optic silicon photonic optical phased array

Abstract. Many components for free-space optical (FSO) communication systems have shrunken in size over the last decade. However, the steering systems have remained large and power hungry. Nonmechanical beam steering offers a path to reducing the size of these systems. Optical phased arrays can allow integrated beam steering elements. One of the most important aspects of an optical phased array technology is its scalability to a large number of elements. Silicon photonics can potentially offer this scalability using CMOS foundry techniques. A phased array that can steer in two dimensions using the thermo-optic effect is demonstrated. No wavelength tuning of the input laser is needed and the design allows a simple control system with only two inputs. A benchtop FSO link with the phased array in both transmit and receive mode is demonstrated.

Imaging with multi-spectral mosaic-array cameras.

The emerging class of multi-spectral mosaic-array cameras combines opportunities of spectral data processing and full-motion video color display. We explore capabilities of such sensors and propose the novel demosaicking algorithm capable of enhancing resolution of equally sampled multi-spectral mosaic imagery. We present experimental results of the proposed processing using the imagery acquired with a nine-band short-wave infrared mosaic-array camera.

Interferometric microscopy of silicon photonic devices

Silicon photonics provides the ability to construct complex photonic circuits that act on the amplitude and phase of multiple optical channels. Many applications of silicon photonics depend on maintenance of optical coherence among the various waveguides and structures on the chip. Other applications can depend on the modal structures of the waveguides. All these application require the ability to characterize the amplitude and phase of individual optical channels. Fourier imaging with high numerical aperture microscope objectives has been used to image the intensity of individual channels of photonic structures in both real and Fourier space. In other work, holographic imaging of multimode fibers has allowed modal decomposition. In this work we use interferometric microscopy to image the amplitude and phase of a variety of silicon photonic structures. These include a multimode interference splitter and a multimode waveguide under various excitation conditions.

Super-Resolution Based Demosaicking For Full Motion Video SWIR Multi-Spectral Sensor

We present novel demosaicking algorithm for the first full motion video SWIR 9-band sensor based on pixel size filter array. Spatial resolution enhancement of each spectral band is achieved using modified super-resolution technique.

Multiple-order staircase etalon spectroscopy

Traditional Fabry-Perot (FP) spectroscopy is bandwidth limited to avoid mixing signals from different transmission orders of the interferometer. Unlike Fourier transformation, the extraction of spectra from multiple-order interferograms resulting from multiplexed optical signals is in general an ill-posed problem. Using a Fourier transform approach, we derive a generalized Nyquist limit appropriate to signal recovery from FP interferograms. This result is used to derive a set of design rules giving the usable wavelength range and spectral resolution of FP interferometers or etalon arrays given a set of accessible physical parameters. Numerical simulations verify the utility of these design rules for moderate resolution spectroscopy with bandwidths limited by the detector spectral response. Stable and accurate spectral recovery over more than one octave is accomplished by simple matrix multiplication of the interferogram. In analogy to recently developed single-order micro-etalon arrays (Proc. of SPIE v.8266, no. 82660Q), we introduce Multiple-Order Staircase Etalon Spectroscopy (MOSES), in which micro-arrays of multiple order etalons can be bonded to or co-fabricated with a sensor array. MOSES enables broader bandwidth multispectral and hyperspectral instruments than single-order etalon arrays while keeping a physical footprint insignificantly different from that of the detection array.

Demosaicking for full motion video 9-band SWIR sensor

Short wave infrared (SWIR) spectral imaging systems are vital for Intelligence, Surveillance, and Reconnaissance (ISR) applications because of their abilities to autonomously detect targets and classify materials. Typically the spectral imagers are incapable of providing Full Motion Video (FMV) because of their reliance on line scanning. We enable FMV capability for a SWIR multi-spectral camera by creating a repeating pattern of 3x3 spectral filters on a staring focal plane array (FPA). In this paper we present the imagery from an FMV SWIR camera with nine discrete bands and discuss image processing algorithms necessary for its operation. The main task of image processing in this case is demosaicking of the spectral bands i.e. reconstructing full spectral images with original FPA resolution from spatially subsampled and incomplete spectral data acquired with the choice of filter array pattern. To the best of authors knowledge, the demosaicking algorithms for nine or more equally sampled bands have not been reported before. Moreover all existing algorithms developed for demosaicking visible color filter arrays with less than nine colors assume either certain relationship between the visible colors, which are not valid for SWIR imaging, or presence of one color band with higher sampling rate compared to the rest of the bands, which does not conform to our spectral filter pattern. We will discuss and present results for two novel approaches to demosaicking: interpolation using multi-band edge information and application of multi-frame super-resolution to a single frame resolution enhancement of multi-spectral spatially multiplexed images.

An analytic method for spectrum recovery from wedge or staircase spectrometers

Wedge or staircase micro-optics have become important components for building miniature optical spectrometers. These devices create spectral discrimination through interference between beams resulting from reflections at the surfaces of the optic. The literature has examples of low reflectance wedge spectrometer system where the Fourier transform is used to recover the spectrum (with no inherent bandwidth limit), and high-reflectance, band-limited simplex spectrometers where no data processing is required. Instruments in the first category tend to be for the thermal infrared range, and instruments in the second category are more often encountered in the visible band. This second category includes linear variable filters and discrete etalon staircases. Though in practice, the signal treatment for these two types of spectrometers is radically different, the underlying interference mechanism is identical. It follows, that a single signal processing algorithm must exist which correctly treats the two types of signals. We present a mathematical description of the signal model for such spectrometers. We show that in the case of spectrally uniform reflectance, the signal has a specific relationship to the spectrum’s Fourier transform. We cast the spectral recovery problem as a matrix inversion, and derive formulas for calculating the solution matrix. The solution matrix is shown to yield the exact spectrum when applied to modeled wedge spectrometer signals in both low and high reflectance cases.

The effect of lens aperture for remote sensing of trace gases using Fabry-Perot interferometer-based cameras

We report an approximate simplified calculation of transmission spectra for a telecentric cone of light impinging on a Fabry-Perot interferometer. We model sulfur dioxide sensing and show that the F-number affects the optimum parameters. OCIS codes: (120.2230) Fabry-Perot; (280.1120) Air pollution monitoring

Increasing Resolution of Multi-Spectral Mosaic-Array Cameras

We present approaches to spatial resolution recovery of multi-spectral mosaic-array sensors based on pixel size filter arrays. Resolution enhancement is demonstrated using imagery collected with 9-band SWIR full motion video camera.

Free space optical communication link using a silicon photonic optical phased array

Many components for free space optical communication systems have shrunken in size over the last decade. However, the steering systems have remained large and power hungry. Non-mechanical beam steering offers a path to reducing the size of these systems. Optical phased arrays can allow integrated beam steering elements. One of the most important aspects of an optical phased array technology is its scalability to a large number of elements. Silicon photonics can potentially offer this scalability using CMOS foundry techniques. In this paper a small-scale silicon photonic optical phased array is demonstrated for both the transmitter and receiver functions in a free space optical link. The device using an array of thermo-optically controlled waveguide phase shifters and demonstrates one-dimensional steering with a single control electrode. Transmission of a digitized video data stream over the link is shown.