Falko Riechert

Laser speckle reduction via colloidal-dispersion-filled projection screens

We use projection screens filled with colloidal dispersions to reduce laser speckle in laser projection systems. Laser light is multiply scattered at the globules of the colloidal dispersions internal phase, which do Brownian movement. The integration time of the human eye causes a perception of a reduced laser speckle contrast because of temporal averaging. As a counteracting effect, blurring of projected images occurs in the colloidal dispersion, which degrades image quality. We measure and compare speckle reduction and blurring of three different colloidal dispersions filled into transmission screens of different thicknesses. We realized a high speckle contrast reduction at simultaneously low blurring with a thin screen filled with a highly scattering colloidal dispersion with forward-peaked scattering. We realize speckle contrast values below 3% at acceptable blurring.

Low-speckle laser projection with a broad-area vertical-cavity surface-emitting laser in the nonmodal emission regime

We demonstrate low-speckle laser projection using a broad-area vertical-cavity surface-emitting laser (VCSEL) emitting at 840 nm wavelength as the illumination source. By driving the source in a nonmodal emission regime, we were able to achieve speckle contrast values as low as 3.5% in a realistic projection setup. This was done by driving the VCSEL with specific current pulses without using any additional or mechanically moving components to destroy the coherence of the laser beam. We quantitatively model the speckle contrast reduction based on polarization scrambling and the reduced temporal and spatial coherence of the VCSEL.

Ray-based simulation of the propagation of light with different degrees of coherence through complex optical systems

We show the functional extension of a standard ray tracer to be capable of tracing light fields of different degrees of coherence through complex optical systems. The light fields are represented by spherical waves. An approximate reconstruction of the optical field is possible at arbitrary positions in an optical system under investigation. Therefore, we can calculate the intensity distribution as well as the complex degree of coherence between two points at arbitrary positions. Simulations of the coherence properties of basic optical systems, which can be described analytically, show excellent agreement with theory. Furthermore, we show simulations of the coherence properties of a two-tandem-array microlens beam homogenizer under illumination with fully and partially coherent light.

Method to determine the speckle characteristics of front projection screens.

We present a method to determine the speckle properties of front projection screens. Seven different screens are investigated in a backscattering geometry for 808 nm light. The speckle contrast reduction that results from polarization scrambling and reduced temporal coherence is modeled for the case of volume scattering in the screens. For this purpose, the screens volume scattering path length distributions and depolarization characteristics are determined. This is done via a streak camera setup to measure the temporal broadening of ultrashort 50 fs light pulses scattered in the screens. We show that it is essential to properly select a projection screen with large volume roughness in order to achieve low speckle contrast values for moderate illumination bandwidths.

Far-Field Nonmodal Laser Emission for Low-Speckle Laser Projection

In this letter, we discuss how the nonmodal emission regimes far field of a broad-area vertical-cavity surface-emitting laser (BA-VCSEL) can be used for low-speckle laser projection. A microlens beam homogenizer is used to exploit the low spatial coherence of the VCSELs far field in the nonmodal emission regime. Using far-field instead of near-field illumination of the homogenizer has some important advantages for a practical projection system: the field emitted by the VCSEL can be directly projected onto the homogenizer without the need for additional lenses or accurate alignment. Speckle contrast values as low as 2.5% are measured and in good agreement with modeled contrast values.

Low-speckle laser projection using farfield nonmodal emission of a broad-area vertical-cavity surface-emitting laser

We discuss how the nonmodal emission regimes farfield of a broad-area vertical-cavity surface-emitting laser (BAVCSEL) can be used for low-speckle laser projection. More specifically we investigate how the farfield of a BA-VCSEL in its nonmodal emission regime can be used for low-speckle laser projection. A microlens beam homogenizer is used to exploit the low spatial coherence of the VCSEL. Speckle contrast values as low as 2.5% are measured without using any additional or mechanically moving components to destroy the coherence of the laser beam. We explore and explain the effect on the speckle contrast of the beams size on the homogenizer. We successfully modeled the speckle contrast reduction, taking into account all contributing speckle reducing factors.

Numerical simulation tool for synthetic speckle pattern images and their intensity-based integration under variable conditions for metrology applications

In many metrological applications speckle can be used to determine surface properties where several complex effects and parameters are of concern. However, an analytical modelling of the decorrelation of speckle patterns created e.g. by two wavelength illumination of a surface is possible only for Gaussian-distributed surface height values and in illumination regimes where the separation of the two illumination wavelengths is much smaller than the mean value of the two wavelengths. Depending on the fabrication process of the illuminated surface the statistical distribution of the surface height values can differ significantly from a Gaussian distribution. We present a numerical simulation tool for the synthetic generation of laser-speckle patterns which a non-imaging observer (e.g. a bare CCD-chip) detects in the farfield of an illuminated surface. We investigate for which illumination wavelengths separation the analytical model is valid with a centre wavelength of 650 nm and compare simulated speckle images originating from surfaces with equal- and Gaussian-distributed height values under two-wavelength illumination. We show which differences from the analytical model for the correlation of the two emerging speckle images occur and consider the implications for an experimental situation.

Speckle characteristics of a laser projector using nonmodal laser emission of a semiconductor laser

In this contribution we show how spatially incoherent emission from a broad-area vertical-cavity surface-emitting laser (BAVCSEL) can be used for low-speckle laser projection. In our projection setup, we use a microlens beam homogenizer in order to homogenize the intensity distribution and to exploit the low spatial coherence of the VCSEL. In order to investigate the speckle in a projection setup using a BA-VCSEL as light source, we compare speckle values in case of modal and nonmodal emission of the BA-VCSEL. Furthermore, the microlens beam homogenizer can either be illuminated with the lasers near field or far field, leading to comparable results. Speckle contrast values as low as 3.5% in case of nearfield illumination, and 2.5% in case of farfield illumination, are measured without using any additional or mechanically moving components to destroy the coherence of the laser beam. The microlens array in the setup is essential in order to obtain speckle reduction, since it generates an overlap of mutually independent speckle patterns, thus reducing the overall speckle in the projected image. We successfully model the speckle contrast reduction, taking into account all contributing speckle reducing factors.

P-250L: Late News Poster: Low-Speckle Laser Projection with a Broad-Area VCSEL in the Incoherent Emission Regime

We demonstrate low speckle laser projection using a 850nm-VCSEL driven in an incoherent emission regime as illumination source. We achieve speckle contrast values as low as 4.4 % without using any additional or mechanically moving components to destroy the coherence of the laser beam.

Speckle phenomena in pulsed broad-area vertical-cavity surface-emitting laser emission under different driving and illumination conditions

We have investigated the speckle contrast using different configurations: we place the screen either in the imaged near-field or in the far-field of the VCSELs emission whereas the CCD is placed in the near-or the far-field of the screen. The measured speckle contrast is strongly dependent on both the configuration and the driving conditions of the VCSEL.