Mohsen Khoshnevisan

Miniaturized micro-optical scanners

Optical beam scanners are critical components for airborne and space-based laser radar, on-machine-inspection systems, factory automation systems, and optical communication systems. We describe here a laser beam steering system based on dithering two complementary (positive and negative) microlens arrays. When the two microlens arrays are translated relative to one another in the plane parallel to their surfaces, the transmitted light beam is scanned in two directions. We have demonstrated scanning speeds up to 300 Hz with a pair of 6-mm-aperture microlens arrays designed for input from a HeNe laser. The output beam covers a discrete 16 x 16 spot scan pattern with about 3.6 mrad separation and only 400 μrad of beam divergence, in close agreement with design predictions. This demo system is relatively compact; less than 2 in. on a side. We also describe several near-term applications, some critical design trade-offs, and important fabrication and design issues.

Fabrication of refractive microlens arrays

Fabrication issues of microlens arrays, made by first forming photoresist microlenses, by patterning and reflowing photoresist islands under temperature, and then transferring this into the substrate by a dry etch process, were studied. Photoresist microlenses were reliably fabricated within a range of aspect ratios. The desired sag of the microlenses in the substrate was controllably achieved by adjusting the etch selectivity. Etching behavior of fused silica in mixtures of fluoroform with oxygen or sulfur hexafluoride was studied in detail. High quality microlens arrays were fabricated in fused silica, silicon and germanium, and selected lenses were characterized.

Ferroelectric tungsten bronze crystals and their photorefractive applications

Abstract The photorefractive properties of tungsten bronze ferroelectric crystals are reviewed with respect to host character and dopant. A classification into four groups is proposed for tungsten bronze materials based on their structural, ferroelectric and optical properties. The use of dopants is found to control the speed of response and coupling strength as well as spectral range. In particular, the availability of different crystallographic sites is shown to provide opportunities to separately adjust these properties. The best performance has been established for Ce3+ and Cr3+ doped SBN:60 crystals in the visible and IR regions, respectively.

Thin-film birefringent devices based on form birefringence

The twisted nematic liquid crystal display (TN-LCD) is the leading technology for high performance flat panel displays. However, the region of high contrast for TN-LCDs is limited. Birefringent elements, of compensators, may be used to provide improved contrast at high viewing angles. The phenomenon of form birefringence has been used to design a compensator that can be fabricated by physical vapor deposition of silica and titania, two common coating materials. Improved viewing angle characteristics, particularly in the horizontal direction, have been demonstrated using the compensator. The 20:1 isocontrast region has been extended to +/- 50 degree(s) in displays incorporating the compensator, an improvement of 10 degree(s) or more relative to uncompensated displays. The compensator design includes integrated antireflection coatings to reduce glare. In this way, display legibility is maximized in the presence of both high and low ambient illumination.

Binary optics thin-film microlens array

Binary optics can produce microlenses and lens arrays with theoretical diffraction efficiency as high as 95% for eight-phase level devices. Due to shadowing, mask misalignment, and etching errors that accumulate during fabrication, the actual diffraction efficiency can be reduced to less than 70%. Advances in mask design and e-beam writing have reduced mask misalignment errors to less than 0.2 micrometers but the major issue is the accuracy of the RIE process that is used to transfer a lithographic pattern into the substrate. RIE has two limitations for binary optic applications. First, it cannot be readily employed for the wide range of possible optical substrates of interest (Al2O3 for example), and second, since the pattern is etched directly into the substrate, there is no simple means to calibrate the etch depth during the process. Thin film deposition of the binary structure addresses both of these limitations. It is applicable to a wide range of materials, and accurate in process monitoring of the deposit permits precise control of the feature height. In this paper, we report on eight-phase level binary optic microlenses processed by deposition of SiO2 on fused silica and Al2O3 on sapphire using a projection lithography system. Photoresist processing was achieved by image reversal and lift-off technique. The microlens arrays (in a square format) were designed for (lambda) equals 0.632 micrometers with two microlens sizes of 120 micrometers X 120 micrometers and 240 micrometers X 240 micrometers having speeds of F/12 and F/6 (at the corners), respectively. Optical characterization has demonstrated that the microlens arrays are near diffraction limited and diffraction efficiency is in excess of 80%.

Micro-optic laser beam scanner

Laser beam scanners are used to modulate the direction of laser light, and are critical components of airborne and space-based LIDAR and optical communications systems. We report here a laser beam steering device design based on dithering two complementary (positive and negative) binary optic microlens arrays. When the two microlenses are translated relative to one another in the plane parallel to their surfaces, a light beam can be scanned and controlled in two directions. The first demonstration of this device concept was reported by Lincoln Laboratory. We have demonstrated a miniaturized version of this concept consisting of a pair of 6-mm-aperture binary optic microlens arrays designed for HeNe laser wavelength.

Nonlinear optical techniques for obtaining high brightness from diode lasers

How nonlinear optics can be used to obtain high brightness from diode lasers is described. The use of two-wave mixing to combined and clean up the output of any frequency-locked high-power device is addressed. The application of phase conjugation for coupling to lock lasers is considered.

Multiple quantum wells for nonlinear optical applications in infrared

In this paper, we describe the development (which includes theoretical modeling, structure design, material growth, and sample characterizations) of SiGe superlattices (SL) for nonlinear optical applications in infrared. Our theoretical model predicts that the two-photon absorption coefficient in SiGe (110) SL structures can be on the order of 1 cm/KW, with very fast response (approximately 1 ps). In addition, we also show that quantum well materials with enhanced nonlinear refraction and moderate absorption offer potential advantages for applications in a lowest order Fabry-Perot etalon.