Speckle free laser projection With Optotune s Laser Speckle Reducer October 2013 Dr. Selina Casutt, Application Engineer Bernstrasse 388 CH-8953 Dietikon Switzerland Phone +41 58 856 3011 www.optotune.com info@optotune.com
Speckle-free laser projection with an -axicon-integrator rod combination A great speckle reduction can be achieved with a configuration described in this document with - an axicon as a focusing lens - Optotune s - directly followed by a beam homogenizer Such a setup is compact, cost-saving and easy to align Red Laser Collimation lenses Optotune Optotune s Collimation lenses Red Laser homogenizer homogenizer Optotune DLP/LCOS homogenizer DLP/LCOS DLP/LCOS Projection Optics Projection optics Optics Green Laser Green Laser Fiber Blue Laser Collimation optics Axicon combiner Blue Laser Collecting lens Homogen, collimated, Homogenous, speckle free combiner speckle-free beam beam Homogen, Collecting lens collimated, speckle free beam 1
Benefits of an axicon for speckle-reduction The aim is to generate a homogenous, speckle-free beam incident on the DLP/LCOS after the homogenizer For optimal speckle reduction, a uniform angle distribution (= flat intensity distribution across the beam profile) of the angles incident on the homogenizer is beneficial However, the beam profile of a common laser source is Gaussiandistributed With an axicon, the intensity of a beam can be redistributed, resulting in a profile with a flat intensity distribution with a uniform angle Optotune distribution: Fiber Red Laser Green Laser Blue Laser Collimation optics Collimation lenses Axicon combiner Optotune s Collimation lenses Red Laser Green Laser Blue Laser Collecting lens homogenizer homogenizer Optotune Homogen, collimated, Homogenous, speckle free combiner speckle-free beam beam Homogen, Collecting lens collimated, speckle free beam Projection Optics Projection DLP/LCOS optics Optics homogenizer DLP/LCOS DLP/LCOS 2
Working principle of an axicon The output beam from an axicon in the far-field is a ring At the focal point, an initially Gaussian distributed beam becomes a beam with a flat intensity profile: 3
Design considerations for an axicon setup D d d b d Q Spot size: minimally achievable spot diameter d is given by d = D/2; D: diameter of the incident beam Angle of incidence: the angle of incidence b is given by 2b = 90 -Q/2; Q: apex angle of the axicon 4
Exemplary setup (1/2) 5
Exemplary setup (2/2) Fiber Collimated laser source Axicon Homogenizer Imaging optics DLP/ LCOS Projection optics 6
Design guidelines Collimation optics Axicon Homogenizer DLP/ LCOS Projection optics Fiber The should be placed as close as possible to the homogenizer rod The spot on the should be slightly smaller than the aperture of the homogenizer rod The collimation optics should be chosen such that the collimated laser beam diameter is ~2times the desired focus spot diameter The apex angle Q* of the axicon should be chosen such that the angle incident on the α in fulfills: α in 2 + α 2 < α accept. ; α : diffusion angle, α in : acceptance angle homogenizer *Q/2=90-2α in 7
Exemplary performance of a despeckled laser projection system (1/2) off on Camera settings: F-stop: f/8 Exposure: 1/20 sec Focal length: 24 mm 8
Exemplary performance of a despeckled laser projection system (2/2) off on Camera settings: F-stop: f/4.2 Exposure: 1/20 sec Focal length: 34 mm 9
Alternative setup for large laser source fibers In case it is not possible to collimate the laser source such that the collimated laser beam diameter is ~2times the desired focus spot diameter, an alternative setup with lens arrays is recommended: Optical setup: L1 L2 Fly s eye lens pair L3 L4 L5 Projection optics Optical fiber Homogenizer rod DLP/LCOS The combination Fly s eye lenses/focusing lens provides a uniform angle distribution at the position of the out of an originally Gaussiandistributed beam profile 10
Despeckling screen reducing the subjective speckles The configuration described above reduces the objective speckles For reduction of the subjective speckles, Optotune provides a passive subjective speckle reducing screen with a specially designed surface structure, enabling the removal of subjective speckles Working principle: The microstructure of the screen introduces a spatially varying phase shift for different spatial parts of the illuminating beam. When the distance between the screen and the observer is large enough, a sufficient number of independent beam parts with differing phases are superimposed in the observers imaging system (e.g. eye) and the speckle pattern is smeared out, resulting in a reduced speckle contrast. Please contact us for further information and for screen samples to test in your setup 11
Difference between subjective and objective speckles Objective speckles are formed when coherent light from the laser source is scattered by a projection optics and interferes on the projection screen. Thus, the interference pattern on the projection screen is made up of contributions from all scatterings from the projection optics and the structure of the screen. The resulting speckle pattern on the projection screen stays the same, regardless of how or from which direction the projection screen is viewed. In contrast, in the case of subjective speckles, the scattered light from the projection screen is imaged by an imaging system (e.g., the human eye) onto an imaging surface (e.g., the retina). Here, the detailed structure of the speckle pattern on the imaging surface depends on imaging parameters such as, e.g., imaging direction, aperture, or resolving power of the imaging system. 12
Optotune 1 slide Optotune AG Bernstrasse 388 CH-8953 Dietikon Switzerland Phone: +41 58 856 3000 Fax: +41 58 856 3001 www.optotune.com info@optotune.com