Gordon Craggs

Standardized speckle measurement method matched to human speckle perception in laser projection systems

We present a standardized procedure to measure the amount of speckle in laser based projection systems. The parameters of the measurement procedure are chosen such that the measured speckle contrast values are in correspondence with the subjective speckle perception of a human observer, independent of the particularities of the laser projectors illumination configuration. The resulting measurement configuration consists of a single digital image sensor in combination with a camera lens of which the settings are related to the human eye. In addition, a standardized measurement procedure and speckle pattern analysis method are suggested. Finally, the speckle measurement set-up is applied to a laser projection system and corresponding subjective speckle perception results of a large test public are discussed.

Spatially Resolved Characterization of the Coherence Area in the Incoherent Emission Regime of a Broad-Area Vertical-Cavity Surface-Emitting Laser

We present direct measurements of the spatial coherence area of a pulsed broad-area vertical-cavity surface-emitting laser using a reversing wavefront interferometer. With this technique, we can assess the size and uniformity of the coherence area across the laser aperture, being of importance for projection applications. We show that the output beam can be considered quasi-homogeneous and that the measured coherence area corresponds well with the coherence area deduced from the far-field emission profile. We demonstrate that the coherence area is limited in size by the radial temperature gradient in the device and discuss the origin of coherence variations.

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.

Thermally Controlled Onset of Spatially Incoherent Emission in a Broad-Area Vertical-Cavity Surface-Emitting Laser

We present an experimental study of the physical process that leads to spatially incoherent, nonmodal emission in broad-area vertical-cavity surface-emitting lasers. We show that this special emission regime that occurs in pulsed operation of these lasers is due to a combination of a spatially distributed thermal or refractive index gradient (thermal lens) and thermal expansion of the cavity during the pulse (thermal chirp). Our measurements are based on preinstalling a thermal lens through a current bias, and subsequently, modulating a pulse onto the bias. This approach allows us to independently investigate the role of both thermal effects in the onset of nonmodal emission.

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.

Evaluation of an extensive speckle measurement method

The introduction of lasers for projection applications is hampered by the emergence of speckle. In order to evaluate the speckle distorted image quality, it is important to devise an objective way to measure the amount of speckle. Mathematically, speckle can be described by its speckle contrast value C, which is given by the ratio between the standard deviation of the intensity fluctuations and the mean intensity. Because the measured speckle contrast strongly depends on the parameters of the measurement setup, in this paper we propose a standardized procedure to measure the amount of speckle in laser based projection systems. To obtain such a procedure, the influence of relevant measurement set-up parameters is investigated. The resulting measurement procedure consists of a single digital image sensor in combination with a camera lens. The parameters of the camera lens are chosen such that the measured speckle contrast values correspond with the subjective speckle perception of a human observer, independent of the projectors speckle reduction mechanism(s). Finally, the speckle measurement procedure was performed with different cameras and the results were compared.

Low speckle line generation using a semiconductor laser source

In this contribution we show that we can design a low speckle line generator based on the reduced spatial coherence of a broad-area vertical-cavity surface-emitting laser (BA-VCSEL). This type of semiconductor laser can be driven into a special operation regime, a regime of low spatial coherence, which has already been shown to have a speckle reducing eect for image projection applications. However, the eectiveness of reducing speckle strongly depends on the line generating optics. Therefore we compare dierent line generating optical systems on their potential to use spatially incoherent laser emission for speckle reduction. These systems are: a single cylindrical lens, a tandem cylindrical lens array or a diuser based line generator. We also compare these results with the speckle occurring if we replace the partial spatially coherent VCSEL with a single mode laser or with a multi-mode VCSEL. We nally draw conclusions on the design of the most optimal optical system. The results of our study are also valid for other sources of partial spatially coherent emission.

Characterization of a low-speckle laser line generator.

The goal of our investigation is to design a low-speckle laser line generator based on partial spatially coherent laser light. Low speckle is achieved by exploiting a regime of strongly reduced spatial coherence of a broad-area vertical-cavity surface-emitting laser, which is used as the line generators light source. A comparative experimental study of different optical configurations is conducted, leading to the design of an optimal optical system. The results of our study are also valid for other sources of partial spatially coherent emission.

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.