M. Nadeem Akram

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.

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.

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.

Sinusoidal rotating grating for speckle reduction in laser projectors: feasibility study

This paper describes a novel idea for reduction of speckle contrast in laser display projectors using the rotation of a diffraction pattern whose zeroth order has been canceled out without loosing power. The feasibility of the proposed method was investigated by illuminating gratings with a sinusoidal phase on two spatial light modulators (SLMs) in series for minimal intensity modulation, where the phase grating pattern was rotated with respect to the previous one on both SLMs. Two series of measurements were done with different periods of the sinusoidal grating. For each series, an image of the speckle pattern was recorded at discrete rotation angles of the phase grating, and then an average image was calculated. Experimental results were compared with a new theoretical model for speckle contrast of N partially correlated speckle patterns. The experimental measurement results compare well with the theoretical predictions resulting in a minimum speckle contrast of 0.36, with further reduction possible. Parameters necessary to achieve target contrast (0.08 or less) are discussed.

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

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

Effect of projection lens numerical aperture and aberrations on speckle reduction in line-scan laser projectors

Barker and Barker-like binary phase codes have previously been used for speckle reduction in line-scan laser projectors, and a speckle contrast factor decreased down to 6% has been theoretically achieved assuming ideal imaging conditions. In this article, it is shown by theoretical simulations that the speckle reduction performance of the the binary phase codes is adversely affected by the finite numerical aperture and the aberrations of the projections lens. A minimum numerical aperture of the projection lens is needed to faithfully reproduce the binary phase code placed at an intermediate image plane onto the final display screen. The effects of the cell dimension of the binary phase code are also considerd in simulations.

Spatial coherence measurement of high power broad area edge emitting laser

We use double reversing wavefront Michelson interferometer to measure the spatial coherence properties of high power broad area Fabry Perot edge emitting blue laser. The measurements are done at different bias current, from lasing threshold up to 1A. The laser is driven in both continuous-wave, CW and pulsed-wave modes. It is found that the magnitude of complex degree of coherence decrease when the laser is driven in pulsed mode. Among driving currents for each mode just for 700 mA bias current more uniform and rather visible interference fringes attained.

Reducing unwanted intensity fluctuations in laser-projected images

Compared to typical display projectors based on incandescent lamps, laser sources have several advantages. Among them are directionality, miniaturization potential, multiplexing possibilities, improved color gamut, higher contrast, deeper focus, and higher brightness. However, laser light is coherent— it has a well-defined wavefront—which creates constructive and destructive interference in the observer’s eye. This manifests as a grainy, noiselike pattern over the image,1 called speckle. The main challenge with laser projectors is to reduce the presence of speckle to an acceptable level. However, existing methods are often expensive, cause loss of power, or degrade the image. One common approach to speckle reduction is to place a diffuser in the light path, between the light source and imaging system. Making a small alteration to the surface of the diffuser changes the wavefront of the beam and, consequently, the speckle pattern. Summing the resulting speckle patterns in the eye during its exposure time diminishes the unwanted intensity fluctuations in the image. A challenge is to find a suitable way of changing the speckle pattern. Here, we present a novel method2 based on discrete rotation of a diffraction pattern on a diffuser, where the stationary zeroth-order diffraction maximum is extinguished without losing overall power. Imagine that a beam of coherent light is passed through a sinusoidal phase grating and diffracted into a line of spots on a diffuser (see Figure 1). Rotating the grating causes the spots to cover different areas of the diffuser and produce different speckle patterns. However, the zeroth-order maximum will remain stationary. A sinusoidal phase grating with a peak-topeak phase delay of 4.8 radians cancels out the zeroth order Figure 1. Diffraction from a sinusoidal phase grating with a peak-topeak phase delay of 4.8 radians produces a pattern with a vanishing zeroth order. Rotation of the diffraction pattern onto new areas on the diffuser creates independent speckle patterns.

Speckle reduction in line-scan laser projectors using binary phase codes: theory and experiments

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 decreased down to 13% has been achieved. In this article, Barker-like binary phase codes of length longer than 13 are used at an intermediate image plane. It is shown by theoretical calculation that much better speckle reduction with speckle contrast factor up to 6% can be achieved by using longer binary phase codes other than the Barker code. Preliminary experimental results are also presented indictaing good speckle reduction.