Tunable lasers are used in optical metrology, but their intrinsic tuning accuracy is sometimes inadequate and an external
wavemeter is then required to measure the wavelength more accurately. In this paper, we present the design of a discrete
step wavemeter to measure the wavelength of the light from a tunable laser during the operation of a multi-wavelength
interferometric shape measuring system. This relatively low-cost wavemeter is embedded in the metrology system and
consists of a discrete set of small retroreflectors mounted at different ranges on a super-invar base, which eases
alignment and allows it to be insensitive to temperature changes. During operation, interference patterns from the
retroreflectors are captured by a camera for each phase shift and commanded wavelength and analyzed to determine the
actual wavelength. The phase measurement uses a least square fitting algorithm. A Fourier Transform peak finding
measurement technique is used for phase unwrapping. Both numerical simulation and experiments indicate improved
system performance using this internal wavemeter technique.
A digital holographic metrology technique is described for measuring the three-dimensional shape of manufactured parts. The technique uses optical fibers to set up a near equal path interferometer, steps through multiple frequencies with a tunable laser, steps through multiple phases using a fiber based phase shifter, uses an off-axis parabolic mirror to collimate the light, and generates a digital hologram that leads to surface flatness measurements accuracies better than 1 micron over large surfaces. An example result for an automobile engine part is given using a Coherix Inc., ShapixTM 2000 instrument.
A signal analyzer is being developed that is based on a periodically tapped optical fiber. The spatial distribution of taps produces a waveform that is weighted by a spatial light modulator (SLM), optically Fourier transformed, and then detected with a video camera to continually display the power spectrum of the signal propagating in the fiber. The analyzer resolution and bandwidth is dependent on the distribution and number of taps. Taps have been formed in the core of fibers using UV light. Both single step beamsplitter and localized phase grating taps have been generated. Bragg grating taps tilted at 45 degrees with respect to the fiber axis were generated in order to diffract light directly out of the side of the fiber.
Using a 3D formulation of inverse synthetic aperture (ISAR), we show how the arbitrary formatting capability of a time- integrating opto-electronic processor can be used to format the data in such a way as to focus the image. The focusing parameters are found from prominent points in the radar data itself assuming a rigid body object. The opto-electronic processor is particularly suited for generated image data such as found with ISAR since it can arbitrarily place data points as needed depending on the object motion without need of interpolations. The processor is a time-integrating architecture that uses direct laser diode intensity modulation for signal insertion, acousto-optic scanners for arbitrary formatting, and a modified Kosters interferometer for stable 2D Fourier transformation. The processor also automatically generates multi-resolution imagery as a by- product of its generation of the final fine resolution image. Thus, there is a sequence of values available for each image pixel that are available for enhanced multi- resolution image processing. This research is funded by the Office of Naval Research.
KEYWORDS: Image processing, Signal processing, Radar, Optoelectronics, 3D image processing, Fourier transforms, 3D acquisition, Scattering, Optical signal processing, Data processing
A 3D formulation of inverse synthetic aperture (ISAR) is presented showing how the various geometry and motion parameters determine the image generation properties. We then show how the arbitrary formatting capability of an opto-electronic processor can be used to format the data in such a way as to focus the image. The focusing parameters are found from tracking prominent points in the radar data itself and using rigid body constraints imposed on the data. The opto-electronic processor is particularly suited for generated image data such as found with ISAR. The processor is a time-integrating architecture that uses acousto-optic scanners for arbitrary formatting and a modified Kosters interferometer for stable Fourier transformation. This research is being funded by the Office of Naval Research.
We describe some performance trades for a hybrid optical processor for real-time synthetic aperture radar (SAR) image formation. A 2D Fourier transforming time-integrating interferometrically based optical processor is a key element of the system. The optical processor uses a modulated laser diode for radar signal insertion, crossed 1D acousto-optic scanners for 2D scanning, a modified Koster interferometer for fringe generation, and fast detector arrays (cameras) for light detection and integration. The image dynamic range is enhanced by processing many camera frames. Digital pre- and post-processing play essential roles in the system enhancement. We present the characteristics of this type of processor and consider some of its performance trades. The optical processor design approach lends itself to the important attributes of high (real-time) data rates, multiple SAR mode processing capabilities, compact and rugged packaging, and power efficiency.
We describe the current status of a hybrid optical processor being developed for real-time synthetic aperture radar (SAR) image formation. The processor is being developed for insertion into the ERIM spotlight mode SAR airborne data collection system under the ARPA TOPS program monitored by the Army Research Lab. A 2D Fourier transforming time-integrating interferometrically based optical processor is a key element of the system. The optical processor uses a modulated laser diode for radar signal insertion, crossed 1D acousto-optic scanners for 2D scanning, a modified Koster interferometer for fringe generation, and fast detector arrays for light detection and integration. The image space-bandwidth-product and dynamic range are enhanced by processing time-multiplexed interlaced image subpatches at real-time rates. Digital pre- and post-processing play essential roles in the system enhancement. The final image is a mosaic of the subpatch images. The optical processor design approach lends itself to the important attributes of high (real-time) data rates, multiple SAR mode processing capabilities, compact and rugged packaging, and power efficiency.
The development and airborne demonstration of a compact realtime optical processor for synthetic aperture radar (SAR) image formation under the DARPA TOPS program is described. The ERIM spotlight mode SAR system and its processing requirements are presented. It is shown that a 2-D Fourier transforming time-integrating interferometrically based optical processor is an attractive solution to the processing requirements. The optical processor uses a modulated laser diode for radar signal insertion, crossed acousto-opto scanners for 2-D scanning, a modified Köster interferometer for fringe generation, fast detector arrays for light detection and integration, and accumulating frame grabbers to build up the dynamic range of the image. Analysis, simulation, and laboratory experimental results are presented.
A two-element holographic optical system is described for converting an elliptically shaped Gaussian-profile laser beam into a rectangularly shaped beam that is uniform in amplitude and phase in the nearfield. Theoretical analysis, design considerations, and experimental results are presented for a compact converter system of less than 6 cm in length.
The spatially-scanning, 2D, time-integrating hybrid interferometric processors presented employ directly-modulated CW laser diodes as input sources and are applicable to complex SAR data processing in real time. Crossed acoustooptic cells scan the input signal over a virtual 2D input space, and an optical interference pattern is detected with a solid-state detector array camera. Continuous transfer of the frames of integrated interference from the camera to a digital image processor, overall dynamic range is increased over that of the camera alone. Envelope detection in the display circuitry generates a continuous real-time representation of image magnitude.
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