Temperature resistant anti-reflective coating on Si-wafer for long-wave infra-red imaging.

A micromachined Silicon lid, sealed by CuSn solid liquid interdiffusion bonding is a promising approach for hermetic sealing of microbolometers for use in low-cost thermal cameras. However, since ∼30% of long-wave infrared light is reflected at an uncoated single Si-air interface, anti-reflective treatments are required. Traditional anti-reflective coatings are inapplicable since CuSn solid liquid interdiffusion bonding requires heating to about 270 °C and these multi-layer coatings fail due to differing coefficients of thermal expansion for the different layers and the substrate. For this purpose, an anti-reflective coating that maintains its anti-reflective properties after being heat-cycled to 300 °C has been developed. This coating was developed using a simple 2-layer structure composed of ZnS and YF3 and deposited at 100 °C. The development process that led to the successful coating has also been described in this paper. The final sample shows a 30% average increase in transmission in the 8-12 µm wavelength range as compared to an uncoated wafer.

Speckle reduction using a motionless diffractive optical element.

Speckle reduction by moving diffuser has been previously studied in display systems with coherent light sources, such as lasers. In this Letter, we propose a motionless diffractive optical element (DOE) for speckle reduction. The DOE was designed based on finite-element method simulations, fabricated using micromachining technology, and characterized for despeckle efficiency. Experiments using a DOE with two gratings have indicated that the speckle was suppressed to 50%, which shows fair agreement with theoretical analysis. With some modification of this DOE, the speckle noise can be reduced to 10% according to the theory.

Laser speckle reduction due to spatial and angular diversity introduced by fast scanning micromirror.

We utilize spatial and angular diversity to achieve speckle reduction in laser illumination. Both free-space and imaging geometry configurations are considered. A fast two-dimensional scanning micromirror is employed to steer the laser beam. A simple experimental setup is built to demonstrate the application of our technique in a two-dimensional laser picture projection. Experimental results show that the speckle contrast factor can be reduced down to 5% within the integration time of the detector.

Simulation and control of narcissus phenomenon using nonsequential ray tracing. II. Line-scan camera in 7-11 microm waveband.

A nonsequential ray tracing technique is used to calculate the narcissus signature in infrared (IR) imaging cameras having cooled detectors operating in the 7-11 microm waveband. Imaging cameras based on a one-dimensional linear detector array with a scan mirror are simulated. Circularly symmetric diffractive phase surfaces commonly used in modern IR cameras are modeled including multiple diffraction orders in the narcissus retroreflection path to correctly estimate the stray light return signal. An optical design example based on a step-zoom dual field of view optical system is discussed along with the performance curves to elaborate the modeling technique. Optical methods to minimize the narcissus return signal are explained, and modeling results presented. The nonsequential ray tracing technique is found to be an effective method to accurately calculate the narcissus return signal in complex IR cameras having diffractive surfaces.

Simulation and control of narcissus phenomenon using nonsequential ray tracing. I. Staring camera in 3-5 microm waveband.

A nonsequential ray tracing technique is used to simulate the narcissus phenomenon in infrared (IR) imaging cameras having cooled detectors. Imaging cameras based on two-dimensional focal plane array detectors are simulated. In a companion article, line-scan imaging cameras based on one-dimensional linear detector arrays are simulated. Diffractive phase surfaces commonly used in modern IR cameras are modeled including multiple diffraction orders in the narcissus retroreflection path to correctly simulate the stray light return signal. Practical optical design examples along with their performance curves are given to elucidate the modeling technique. Optical methods to minimize the narcissus return signal are thoroughly explained, and modeling results are presented. It is shown that the nonsequential ray tracing technique is an effective method to accurately calculate the narcissus return signal in complex IR cameras having diffractive surfaces.

Speckle reduction in line-scan laser projectors using binary phase codes.

A Barker binary phase code of maximum length 13 has previously been used for speckle reduction in line-scan laser projectors, and a speckle contrast factor decrease down to 13% has been achieved. In this Letter, Barker-like binary phase codes of lengths longer than 13 are used at an intermediate image plane. It is shown by theoretical calculation that a much better speckle reduction with a speckle contrast factor up to 6% can be achieved by using longer binary phase codes other than the Barker code.