Absolute metrology techniques have been proposed for and used in a variety of optical systems as a means of precisely measuring distances. Typically the metrology concepts utilize frequency stabilized multiple wavelength lasers. We describe a multi-wavelength approach that utilizes the strengths of Photonic Integrated Circuits (PIC) to implement an absolute metrology sensor. The metrology sensor is a self-contained system that forms a multi-wavelength distance measuring interferometer. The PIC includes the waveguide circuitry, laser source, detectors and a National Institute of Standards and Technology (NIST) traceable reference in a compact form. The PIC is a hybrid combination of Indium Phosphide circuitry with active elements (detectors and laser) and a passive Silicon Nitride circuit of waveguides. The PIC implementation is presented along with measurement results showing an unambiguous range of 30 mm with nanometer precision.
We have developed a technique for extracting atmospheric turbulence induced wavefront error by means of digital
holography. The technique enables wavefront error determination as a function of field angle. Closed form expressions
for the anisoplanatic wavefront error caused by atmospheric turbulence have been developed for comparison. We show
very good agreement between experimental data and the closed form solution. The comparison is made over Cn2 values
from approximately 10-12 to 10-15 m-2/3.
Multi-aperture coherent LADAR techniques can be applied to generate high resolution images. When setting up a system
with multiple sub-apertures, misalignment of the sub-apertures causes the beams entering the sub-apertures to have
mismatched optical path lengths, which will degrade the image resolution. Post-processing using image sharpening
techniques to correct for piston phase, as well as other aberration corrections, require computing power and time. We
study whether the processing time can be shortened by providing measured piston phase information to the image
sharpening algorithms. Simulations are used to demonstrate the usefulness of piston phase measurements. Simulations
are presented showing the benefits of piston phase measurements for two or more sub-apertures. The speed of
convergence for the sharpening algorithm both with and without the piston phase measurements are compared for
multiple sub-aperture imaging.
We report the results of experiments demonstrating large-format coherent imaging at a range of 1.5 km. For this work,
we use digital holographic detection to record coherent data and are thus able to measure the complex-valued optical
field from a flood-illuminated scene over an extended aperture. Images are formed using digital Fourier processing. The
return light is interfered with a coherent reference beam, and the intensity of the interference pattern is recorded using a
conventional large-format detector array. Experimental results obtained using a coherent, pulsed laser source operating at
1.6 microns are presented. We also demonstrate the utility of this technique for advanced imaging functions such as 3D
Current optical phased arrays produce images by adaptively phasing the output of several telescopes on a common focal
plane. Image based phasing techniques such as Phase Diversity, are used to maintain the phasing in real time. This
requires both a computationally intensive algorithm for estimating the phasing errors as well as a means for rapidly
adjusting the optical path length through each telescope. In this paper we will compare the adaptive technique of phasing
multiple telescopes with the analytic technique of digital holography. Digital holography provides a means of digitally
estimating and correcting the phasing errors between the multiple telescopes. The process can occur long after the data
has been acquired which relaxes the requirements on the stability of the phased array as well as the mechanical
complexity. Experimental results will be shown for adaptive and analytical image formation in remote sensing
An important issue in synthetic aperture ladar is phase noise mitigation, since phase noise corrupts image quality. There
are many phase noise contributors including, residual platform motion, local oscillator phase/frequency instability,
atmospheric turbulence, and additive receiver noise. The Phase Gradient Autofocus (PGA) algorithm is a common phase
noise correction algorithm utilized in synthetic aperture radar. The Cramer-Rao Lower Bound for the phase-difference
estimate variance of PGA can be found in the radar literature. This lower bound describes the precision of the phasedifference
estimate between any two pulses as a function of the carrier-to-noise ratio (CNR). However, this lower bound
does not account for speckle saturation limitations, present in both synthetic aperture ladar and radar.
This paper extends the PGA performance theory to include a high CNR saturation term which accounts for speckle
decorrelation. This term is shown to be proportional to the ratio of the image spot size to the laser pulse repetition
frequency (PRF). This paper also describes impact of PGA estimate variance on image cross-range resolution. We
show, given a fixed PRF and fixed PGA phase-difference estimate variance, that resolution initially improves with
increasing dwell times but eventually saturates to a level proportional to the product of the PGA estimate variance and
the laser PRF.
Digital Holography is a technique which provides a measurement of the complex field reflecting from a coherently
illuminated object. When the measurement is performed with two carefully chosen wavelengths a phase difference map
can be created providing a three dimensional map of the object. We present results from a laboratory experiment where
the surface contours of coral are measured in seawater. Contour maps with step sizes on the order of 0.1 mm can easily
be obtained. We propose that this technique be used to remotely monitor the growth of coral in an effort to quantify the
health of coral beds. The technique is effective from space, aircraft, ships, buoys or rigid platforms such as a pier. In the
last few years we have been successfully using this technique to measure objects through very turbulent atmosphere at
ranges of up to 700 meters and we are now applying the concept to shoreline applications.
With a distributed aperture imaging system, one creates a large imaging aperture by combining the light from a series of
distributed telescopes. In doing this, one can construct a fine-resolution imaging system with reduced volume. In this
paper we present work on distributed aperture, active imaging systems that use coherent detection and digital image
formation. In such a system, the image formation process incorporates digital correction of optical and atmospheric
phase errors. Here we discuss the principles underlying this method and present results from laboratory experiments and
field experiments performed over a 0.5 km outdoor test range.
Three dimensional imaging of planetary and lunar surfaces has traditionally been the purview of Synthetic Aperture Radar payloads. We propose an active imaging technique that utilizes laser frequency diversity coupled with spatial heterodyne imaging. Spatial heterodyne imaging makes use of a local oscillator which encodes pupil plane object information on a carrier frequency. The object information is extracted via Fourier analysis. Snapshots of the encoded pupil plane information are acquired as the frequency of the illumination laser is varied in small steps (GHz). The resulting three-dimensional data cube is processed to provide angle-angle-range information. The range resolution can be adjusted from microns to meters simply by adjusting the range over which the illuminator laser frequency is varied. The proposed technique can provide fine resolution contour maps of planetary surfaces having widely varying characteristics of importance to science exploration, such as the search for astrobiological habitat niches near the surface of heavily irradiated Europa. This information can be used to better understand the geological processes that form the surface features, and help characterize candidate potential habitat sites on the surface of Europa and other planetary bodies of interest. In this paper we present simulations and experimental data that demonstrate the concept.
3D imaging provides important profile information not available with conventional 2D image products. Profile information can be extremely valuable for industrial- inspection and remote target-characterization applications. In this paper, we discuss a novel imaging modality, called PROCLAIM, that utilizes the powerful constraint that opaque objects can be described by a 2D surface embedded in 3D space. Far-field Fourier intensity measurements are collected by flood-illuminating an object with a frequency- tunable laser and direct detecting the backscattered signal with a lensless sensor. This technique allows for precise, non-contract surface measurements, without the stringent coherence and mechanical stability requirements of related interferometric techniques. We present reconstruction results form simulated data and from laboratory measurements.
This paper concerns analysis of imaging sensor called the passive synthetic aperture imaging (PSAI) sensor that is useful for deep-space imaging. The PSAI sensor concept is similar to some other interferometric imaging systems, except that a grating interferometer, which provides improved achromaticity, is used to interfere the light. In this paper we discuss features of the PSAI sensor concept including the optical system and image formation process.
A design approach for developing a staring-dispersive infrared imaging spectroradiometer utilizing a fiber-optic image formatter to simultaneously capture, at rates greater then 100 Hz, 256 high-resolution spectral images of a 2D scene is described. Detailed performance analyses confirm that the spectral radiance within the image scene can be measured to high absolute accuracy. The proposed low-risk prototype, based on commercial off-the-shelf components to minimize the development risk, can be cost-effectively upgraded to a final field instrument through replaceable optical, calibration, and detector modules. Producibility of the design, as described by mechanical layouts and manufacturing drawings of the optical, fiber-optic, and dispersive elements, has been confirmed by independent component manufacturers. The rugged design is sufficiently compact for use in high-performance military aircraft, and can be applied to numerous civilian applications such as environmental cleanup, mineral surveys, vegetation monitoring, and combustion analysis.
Three-dimensional imaging provides profile information not available with conventional 2D imaging. Many 3D objects of interest are opaque to the illuminating radiation, meaning that the object exhibits surface, as opposed to volume, scattering. We investigate the use of an opacity constraint to perform 3D phase retrieval. The use of an opacity constraint in conjunction with frequency-diverse pupil-plane speckle measurements to reconstruct a 3D object constitutes a novel unconventional-imaging concept. This imaging modality avoids the difficulties associated with making phase measurements at a cost of increased computations.
We discuss a 3D imaging modality called pulsed heterodyne-array imaging (PHI). The relationship between PHI and stepped-frequency methods for 3D coherent image formation is derived. For both cases, we consider flood-illumination of the object and detection with a 2D array of coherent receivers located in the pupil plane. It is shown that PHI can recover the same coherent 3D image as a stepped-frequency method such as holographic laser radar.
Experimental results obtained using holographic laser radar are presented. Holographic laser radar is method for obtaining fine-resolution, 3-D images from laser illuminated objects. A series of complex valued holograms is recorded for a series of laser frequencies. These holograms are assembled into a 3-D data array and Fourier transformation yields a 3-D image. Experimental results obtained using a dye laser are presented.
A laser radar using an array of heterodyne detectors offers the possibility of fine resolution angle-angle imaging. The heterodyne measurements, however, are subject to phase errors due to atmospheric turbulence and mechanical misalignment. A method is described that employs digital shearing of the heterodyne measurements as a means to remove phase errors. By this method large phase errors can be corrected without requiring a beacon or a glint. This digital shearing laser interferometry method was investigated theoretically and demonstrated via computer simulations which included photon noise and various types of phase errors. The method was also successfully applied to data collected in a simple laboratory experiment.
Several arguments are made for using higher-order, rather than first-order, kinoforms
for producing low f-number diffractive lenses. It is shown that improved
efficiency can be achieved with higer-order kinoforms when the f-number of the lens
is very low. A procedure for the fabrication of higher-order kinoforms is also presented.