Previous papers have demonstrated that anomalies observed in synthetic aperture radar (SAR) images can be attributed
to perturbations in the clear-air refractive index. The atmospheric effect is small, and is more frequent, or apparent, in
long range imaging. A geometrical optics (ray-trace) analysis is applied to potential atmospheric perturbations. The
results correlate with observations, and demonstrate that the effect is strongly dependent on the azimuth orientation of
the atmospheric perturbation. The analysis also demonstrates that the magnitudes of the spatial size of the anomalies
and index perturbations can be much smaller than expected.
Firing systems typically incorporate isolation-based architectures that are established by the safety themes of particular weapon systems. Robust electrical diversion barriers are implemented to isolate energy from detonation-critical components until the event of intended use of the system. An optical trigger assembly is being developed to enhance the safety of new firing systems. It couples a fast trigger signal through an exclusion region barrier without compromising the integrity of the barrier in abnormal environment situations. A laser diode generates an optical pulse that is coupled through a sapphire stub to a photoconductive semiconductor switch (PCSS). The PCSS drives a vacuum switch tube to complete the triggering chain in the firing system. A general discussion and comparison of triggering technology options, and the design characteristics and performance parameters of the specific optical trigger point design are presented in this paper.
Industrial, military, medical, and research and development applications of lasers frequently require a beam with a specified irradiance distribution in some plane. A common requirement is a laser profile that is uniform over some cross-section. Such applications include laser/material processing, laser material interaction studies, fiber injection systems, optical data/image processing, lithography, medical applications, and military applications. Laser beam shaping techniques can be divided in to three areas: apertured beams, field mappers, and multi-aperture beam integrators. An uncertainty relation exists for laser beam shaping that puts constraints on system design. In this paper we review the basics of laser beam shaping and present applications and limitations of various techniques.
Injection of high power, multi-mode laser profiles into a fiber optic delivery system requires controlling a number of injection parameters to maximize throughput and minimize concerns for optical damage both at the entrance and exit faces of the fiber optic. A simple method for simultaneously achieving a compact fiber injection geometry and control of these injection parameters, independent of the input source characteristics, is provided by a refractive lenslet array and simple injection lens configuration. Design criteria together with analytical and experimental results for the refractive lenslet array and short focal length injection lens are presented. This arrangement provides a uniform spatial intensity distribution at the fiber injection plane to a large degree independent of the source mode structure, spatial profile, divergence, size, and/or alignment to the injection system. This technique has application to a number of laser systems where uniform illumination of a target or remote delivery of high peak power is desired.
Micromachine accelerometers offer a way to enable critical functions only when a system encounters a particular acceleration environment. This paper describes the optical readout of a surface micromachine accelerometer containing a unique 24-bit code. The readout uses waveguide-based optics, which are implemented as a photonic integrated circuit (PIC). The PIC is flip-chip bonded over the micromachine, for a compact package. The shuttle moves 500 micrometer during readout, and each code element is 17 micrometer wide. The particular readout scheme makes use of backscattered radiation from etched features in the accelerometer shuttle. The features are etched to create corner reflectors that return radiation back toward the source for a 'one' bit. For a 'zero' bit, the shuttle is not etched, and the radiation scatters forward, away from the detector. This arrangement provides a large signal difference between a 'one' and 'zero' signal, since the 'zero' signal returns virtually no signal to the detector. It is thus superior to schemes that interrogate the code vertically, which have a limited contrast between a 'one' and a 'zero.' Experimental results are presented for mock shuttle features etched into a silicon substrate. To simulate the shuttle moving under a fixed PIC, a commercially available waveguide source was scanned over the mock code.
Sandia National Laboratories is developing a MEMS-based trajectory safety subsystem, which allows enablement of critical functions only after a particular acceleration environment has been achieved. The device, known as an Environmental Sensing Device (ESD), consists of a suspended moving shuttle that translates a given distance when exposed to an appropriate acceleration environment. The shuttle contains an embedded code, consisting of grating structures, hat is illuminated and optically read using a semiconductor laser and detector integrated together in a GaAs-based Photonic Integrated Circuit (PIC) flip-chip bonded to the assembly. This paper will describe the optical design and performance analysis of the embedded code features in the shuttle.
A novel method for the measurement of the change in index of refraction vs. temperature (dn/dT) of fused silica and calcium fluoride at the 193 nm wavelength has been developed in support of thermal modeling efforts for the development of 193 nm-based photolithographic exposure tools. The method, based upon grating lateral shear interferometry, uses a transmissive linear grating to divide a 193 nm laser beam into several beam paths by diffraction which propagate through separate identical material samples. One diffracted order passing through one sample overlaps the undiffracted beam from a second sample and forms interference fringes dependent upon the optical path difference between the two samples. Optical phase delay due to an index change from heating one of the samples causes the interference fringes to change sinusoidally with phase. The interferometer also makes use of AC phase measurement techniques through lateral translation of the grating. Results for several samples of fused silica and calcium fluoride are demonstrated.
Optical lithography (OL) is the manufacturing base for patterning of semiconductor integrated circuits (IC). Fundamental limits on optical imaging at features below 200- nm potentially limit its continued applicability. Interferometric lithography (IL) techniques provide a demonstrated, low-cost, large area nanoscale patterning capability with dense feature resolution to approximately 50 nm. An important next step is demonstrating the capability of applying IL to ann existing pattern defined by OL. We report on a mix-and-match scheme for writing a critical layer in an electrical linewidth test structure. Two important issues are the accuracy and the overlay of the grids for the two exposure techniques. Moire techniques are used to evaluate the grid absolute accuracy for Il with expanding spherical wavefronts. For a 1-m distance to the wafer plane, the period variation is less than 5 X 10-5 over a 1 cm2 field leading to a placement error of +/- 60 nm over the field. This error scales as the inverse square of the distance to the wafer plane and can be reduced to arbitrarily levels. Collimation will eliminate this systematic error to the extent that lens aberrations are compensated. Preliminary experimental results in qualitative agreement with the analysis are presented.
The most common tool used by aircraft inspectors is the personal flashlight. While it is compact and very portable, it is generally typified by poor beam quality which can interfere with the ability for an inspector to detect small defects and anomalies, such as cracks and corrosion sites, which may be indicators of major structural problems. A Light Shaping Diffuser TM (LSD) installed in a stock flashlight as a replacement to the lens can improve the uniformity of an average flashlight and improve the quality of the inspection. Field trials at aircraft maintenance facilities have demonstrated general acceptance of the LSD by aircraft inspection and maintenance personnel.
We present two new devices that contain a lateral shear-plate interferometer, held in a mount that can be rotated about the centerline of the incident laser beam. This configuration ensures constant shear while allowing the shear orientation to be varied. One of these new systems relays the sheared image to a fixed video, 35 mm film, or other camera. With the proper camera, it can record the wavefront quality of optical systems of almost any wavelength. The other system holds a large, 150-mm-diameter shear plate in a light-weight structure that can be set on any of several surfaces. This instrument will shear a 100 mm dia. beam horizontally, vertically, and at +/- 45 degree(s). These tools allow quick and easy measurement of the defocus, and third order spherical aberration, coma, and astigmatism in a system without computerized data reduction. The accuracy is about a quarter of a wave.