Monopulse ladar: super-resolved 3D localization with Si-photonic serpentine optical phased arrays.

We present an optical ranging and super-resolution object localization method, monopulse ladar, used to determine the angle of a point target in two dimensions to a few percent of an optical beam width from differential measurements of four just-resolved waveform-encoded beams while simultaneously providing target range via either coherent or incoherent coded waveform correlation. A common optical carrier is shifted by four GHz-scale tones, each modulated with distinct ranging waveforms, which when transmitted from a Si-photonic 2D wavelength-steered serpentine optical phased array (SOPA) aperture form an encoded rectangular beam cluster that propagates to and scatters from a distant point target. Superposed backscattered target returns from each beam are decoded by correlation with reference waveforms at the receiver. The angular position of the target along the two orthogonal axes is calculated from pairwise ratios of beam amplitudes, while target range is determined from the round-trip time delay of each beam as measured with a wideband correlation peak. The analysis of coherent and incoherent monopulse ladar architectures presented herein indicates that a 50-fold increase in angular resolution-to the tens of arcseconds level-of a point target located within a wide field of regard is achievable while maintaining cm-scale resolution-limited ranging using a single SOPA tile transmitter, with further improvement in angular resolution possible through arrayed tiling of SOPAs. Implementation of monopulse ladar with a SOPA aperture enables non-mechanically steered high-resolution 3D object localization in a compact, low-control complexity form factor.

Afocal catadioptric optical assembly for Fourier-sampling computational microscopy.

This paper describes a fast, wide-angle, afocal, catadioptric optical assembly designed and used for the projection of coherent collimated beams in Fourier-sampling computational microscopy, which demands an unorthodox set of optical requirements unmet by traditional imaging designs. The system accepts a diverging set of collimated beams as an input and produces a converging set of collimated beams that overlap on the surface of a target at 5 m scale distances. We derive equations for the focal surfaces relevant for system alignment and report the results of simulations of the optical performance of the system for axially symmetric and asymmetric beam interferometry. We also describe a method to vary the microscope imaging distance by up to one meter through small positional shifts in the optical elements.

Box-bender: a 3D dispersion and pathlength-matched polarization interferometer.

We present the 3D box-bender interferometer, which folds a Mach-Zehnder into a symmetric box using a polarization-rotating periscope with a 90° geometric phase shift to rotate the polarization in one arm for interfering two broadband orthogonally-polarized beams with equal optical pathlength and dispersion. We demonstrate its utility by interfering the orthogonally-polarized diffracted and undiffracted beams from an acousto-optic tunable filter.

Measuring photoelastic coefficients with Schaefer-Bergmann diffraction.

A novel technique for measuring the relative photoelastic coefficients using Schaefer-Bergmann diffraction is introduced and applied to fused silica and α-BaB2O4. The measurements of fused silica agree with the accepted values to within 0.4%, and the α-BaB2O4 measurements are verified with results presented in this paper from the established Dixon method.

Complete elastic constants of α-BaB2O4: Schaefer-Bergmann acousto-optic diffraction and resonant ultrasound spectroscopy.

Both Schaefer-Bergmann diffraction and resonant ultrasound spectroscopy were used to measure the six independent elastic-stiffness coefficients of the trigonal, non-piezoelectric crystal α-BaB2O4. The two measurement sets resulted in a root-mean-square variance of 1.2%. This paper provides a detailed analysis of the two different measurement techniques and discusses the similarities and differences.

Single-shot afocal three-dimensional microscopy.

Fourier-basis agile structured illumination sensing (F-BASIS) employs acousto-optically synthesized moving interference patterns, sparse RF-encoded aperture synthesis, nonredundant spatiotemporal frequency multiplexing, and single-pixel detection to measure dense clouds of three-dimensional (3D) Fourier samples without scanning, enabling high-speed focus-free volume microscopy. We present 3D fluorescence imaging results using F-BASIS, including an unprecedented wide-field single-shot volumetric measurement in under 10 ms. The unique capabilities provided by F-BASIS could prove instrumental for capturing fleeting dynamic processes such as neuron signaling in 3D.

Polarization instability of Raman solitons ejected during supercontinuum generation.

We numerically investigate polarization instability of soliton fission and the polarization dynamics of Raman solitons ejected during supercontinuum generation in a photonics crystal fiber using the coupled vector generalized nonlinear Schrödinger equations for both linear and circular birefringent fibers. The evolution of the state of polarizations of the ejected Raman soliton as representated on the Poincaré sphere is affected by both nonlinear and linear polarization rotations on the Poincaré sphere. The polarization dynamics reveal the presence of a polarization separatrix and the emergence of stable slow and unstable fast eigen-polarizations for the Raman solitons ejected in the supercontinuum generation process. Circularly birefringent fiber is investigated and found to simplify the nonlinear polarization dynamics.

Doppler encoded excitation pattern tomographic optical microscopy.

Most far-field optical imaging systems rely on lenses and spatially resolved detection to probe distinct locations on the object. We describe and demonstrate a high-speed wide-field approach to imaging that instead measures the complex spatial Fourier transform of the object by detecting its spatially integrated response to dynamic acousto-optically synthesized structured illumination. Tomographic filtered backprojection is applied to reconstruct the object in two or three dimensions. This technique decouples depth of field and working distance from resolution, in contrast to conventional imaging, and can be used to image biological and synthetic structures in fluoresced or scattered light employing coherent or broadband illumination. We discuss the electronically programmable transfer function of the optical system and its implications for imaging dynamic processes. We also explore wide-field fluorescence imaging in scattering media by coherence gating. Finally, we present two-dimensional high-resolution tomographic image reconstructions in both scattered and fluoresced light demonstrating a thousandfold improvement in the depth of field compared to conventional lens-based microscopy.

Wavelength-compensated photonic multibeam-forming system for two-dimensional wideband radio-frequency phased-array antennas.

Traditional lens-based photonic Fourier beam-forming systems can be used to steer multiple beams for narrowband radio-frequency (RF) phased-array antennas. For wideband RF phased-array antennas, such Fourier beam-forming systems suffer from frequency-dependent beam steering, known as beam squint. We present a novel squint-free Fourier-based photonic multibeam-forming system for wideband two-dimensional RF phased-array antennas using a lens and frequency-mapped modulation. In this new beam-forming system, we modulate the receiving wideband RF signals onto a broadband light source in a frequency-mapped manner by a traveling-wave tunable filter at each antenna element. These modulated signals are launched in a miniaturized topology of the RF antenna array, and the wavelength-scaling factor in the lens Fourier transform exactly compensates the frequency dependence of beam steering. Heterodyne detection at the Fourier plane between the focused modulated multicolor spots and the broadband laser reference beams from the same light source recovers the received RF signals. An analysis with numerical simulations and then demonstrated with preliminary experimental results of this beam-forming system is presented.

Pulsed ultrasound-modulated optical tomography using spectral-hole burning as a narrowband spectral filter.

We applied a submegahertz nonlinear optical filter afforded by a cryogenically cooled spectral-hole burning crystal to ultrasound-modulated optical tomography. Our experimental results show that this technique, having the largest etendue among all available ultrasound-modulated optical tomography techniques and being immune to speckle decorrelation, offers potential for imaging in vivo and forming high resolution optical tomograms in real time. It opens an opportunity for the development of a clinically applicable high resolution optical imaging modality.

Doppler-free, multiwavelength acousto-optic deflector for two-photon addressing arrays of Rb atoms in a quantum information processor.

We demonstrate a dual wavelength acousto-optic deflector (AOD) designed to deflect two wavelengths to the same angles by driving with two RF frequencies. The AOD is designed as a beam scanner to address two-photon transitions in a two-dimensional array of trapped neutral Rb87 atoms in a quantum computer. Momentum space is used to design AODs that have the same diffraction angles for two wavelengths (780 and 480 nm) and have nonoverlapping Bragg-matched frequency response at these wavelengths, so that there will be no cross talk when proportional frequencies are applied to diffract the two wavelengths. The appropriate crystal orientation, crystal shape, transducer size, and transducer height are determined for an AOD made with a tellurium dioxide crystal (TeO(2)). The designed and fabricated AOD has more than 100 resolvable spots, widely separated band shapes for the two wavelengths within an overall octave bandwidth, spatially overlapping diffraction angles for both wavelengths (780 and 480 nm), and a 4 micros or less access time. Cascaded AODs in which the first device upshifts and the second downshifts allow Doppler-free scanning as required for addressing the narrow atomic resonance without detuning. We experimentally show the diffraction-limited Doppler-free scanning performance and spatial resolution of the designed AOD.

Tunable vertical-cavity surface-emitting laser with feedback to implement a pulsed neural model. 2. High-frequency effects and optical coupling.

The operation of an optoelectronic dynamic neural model implementation is extended to higher frequencies. A simplified model of thermal effects in vertical-cavity surface-emitting lasers correctly predicts the qualitative changes in the nonlinear mapping implementation with frequency. Experiments and simulations show the expected resonance properties of this model neuron, along with the possibility of other dynamic effects in addition to the ones observed in the original FitzHugh-Nagumo equations. Results of optical coupling between two similar pulsing artificial neurons are also presented.

Tunable vertical-cavity surface-emitting laser with feedback to implement a pulsed neural model. 1. Principles and experimental demonstration.

An optoelectronic implementation of a modified FitzHugh-Nagumo neuron model is proposed, analyzed, and experimentally demonstrated. The setup uses linear optics and linear electronics for implementing an optical wavelength-domain nonlinearity. The system attains instability through a bifurcation mechanism present in a class of neuron models, a fact that is shown analytically. The implementation exhibits basic features of neural dynamics including threshold, production of short pulses (or spikes), and refractoriness.

Sluggish light for radio-frequency true-time-delay applications with a large time-bandwidth product.

A new radio-frequency (RF) photonic technique for achieving large RF time delays has been experimentally demonstrated using femtosecond pulses modulated by an acousto-optic tunable filter in a frequency-mapped and Doppler-shifted modulation scheme. A short optical delay line with length of 240 mum produces nearly 3 micros RF time delay after optical heterodyne detection, resulting in an effective slow-light velocity of 86 m/s. A delay-to-pulse-width ratio of 20 based on this technique has been observed, with a larger fractional delay foreseeable.

Ultrawideband coherent noise lidar range-Doppler imaging and signal processing by use of spatial-spectral holography in inhomogeneously broadened absorbers.

We introduce a new approach to coherent lidar range-Doppler sensing by utilizing random-noise illuminating waveforms and a quantum-optical, parallel sensor based on spatial-spectral holography (SSH) in a cryogenically cooled inhomogeneously broadened absorber (IBA) crystal. Interference between a reference signal and the lidar return in the spectrally selective absorption band of the IBA is used to sense the lidar returns and perform the front-end range-correlation signal processing. Modulating the reference by an array of Doppler compensating frequency shifts enables multichannel Doppler filtering. This SSH sensor performs much of the postdetection signal processing, increases the lidar system sensitivity through range-correlation gain before detection, and is capable of not only Doppler processing but also parallel multibeam reception using the high-spatial resolution of the IBA crystals. This approach permits the use of ultrawideband, high-power, random-noise, cw lasers as ranging waveforms in lidar systems instead of highly stabilized, injection-seeded, and amplified pulsed or modulated laser sources as required by most conventional coherent lidar systems. The capabilities of the IBA media for many tens of gigahertz bandwidth and resolution in the 30-300 kHz regime, while using either a pseudo-noise-coded waveform or just a high-power, noisy laser with a broad linewidth (e.g., a truly random noise lidar) may enable a new generation of improved lidar sensors and processors. Preliminary experimental demonstrations of lidar ranging and simulation on range-Doppler processing are presented.

Broadband radio-frequency spectrum analysis in spectral-hole-burning media.

We demonstrate a 20 GHz spectrum analyzer with 1 MHz resolution and >40 dB dynamic range using spectral-hole-burning (SHB) crystals, which are cryogenically cooled crystal hosts lightly doped with rare-earth ions. We modulate a rf signal onto an optical carrier using an electro-optic intensity modulator to produce a signal beam modulated with upper and lower rf sidebands. Illuminating SHB crystals with modulated beams excites only those ions resonant with corresponding modulation frequencies, leaving holes in the crystal's absorption profile that mimic the modulation power spectrum and persist for up to 10 ms. We determine the spectral hole locations by probing the crystal with a chirped laser and detecting the transmitted intensity. The transmitted intensity is a blurred-out copy of the power spectrum of the original illumination as mapped into a time-varying signal. Scaling the time series associated with the transmitted intensity by the instantaneous chirp rate yields the modulated beam's rf power spectrum. The homogeneous linewidth of the rare-earth ions, which can be <100 kHz at cryogenic temperatures, limits the fundamental spectral resolution, while the medium's inhomogeneous linewidth, which can be >20 GHz, determines the spectral bandwidth.

Optoelectronic implementation of a 256-channel sonar adaptive-array processor.

We present an optoelectronic implementation of an adaptive-array processor that is capable of performing beam forming and jammer nulling in signals of wide fractional bandwidth that are detected by an array of arbitrary topology. The optical system makes use of a two-dimensional scrolling spatial light modulator to represent an array of input signals in 256 tapped delay lines, two acousto-optic modulators for modulating the feedback error signal, and a photorefractive crystal for representing the adaptive weights as holographic gratings. Gradient-descent learning is used to dynamically adapt the holographic weights to optimally form multiple beams and to null out multiple interference sources, either in the near field or in the far field. Space-integration followed by differential heterodyne detection is used for generating the system's output. The processor is analyzed to show the effects of exponential weight decay on the optimum solution and on the convergence conditions. Several experimental results are presented that validate the system's capacity for broadband beam forming and jammer nulling for linear and circular arrays.

Optical finite impulse response neural networks.

The finite impulse response neural network is described in detail. Different algorithms capable of temporal back-propagation are considered, including a novel modification to the conventional algorithm, called the delayed-feedback back-propagation algorithm. We present and analyze different optoelectronic processors making use of adaptive volume holograms and three-dimensional optical processing. Two single-layer architectures are presented: the input delay plane architecture and the output delay plane architecture. By combining them it is possible to implement both forward and backward propagation in two multi-layer architectures: the first making use of the conventional temporal back-propagation and the second making use of delayed-feedback back-propagation.