Exploring different speckle reduction techniques for double pass ocular imaging. Donatus Halpaap Advisers: Meritxell Vilaseca, Cristina Masoller

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1 Exploring different speckle reduction techniques for double pass ocular imaging Donatus Halpaap Advisers: Meritxell Vilaseca, Cristina Masoller Biophysics by the Sea, Alcúdia, Mallorca, October 12, 2018

2 Outline Introduction to double pass imaging Goal of project Quantifying speckle Effect of different speckle reducing techniques Discussion and outlook

3 Introduction Double pass imaging technique: evaluate optical quality of patient s eye by looking at point source reflection from retina eye (by M. Vilaseca) camera Imaged on CCD: eye s point spread function DP image is an overall measure for optical quality of the eye, including scattering and aberrations Speckle pattern due to coherence of light and unevenness of retina.

4 Goal of project Using optical chaos from semiconductor laser with feedback to reduce speckle in double pass images Idea: feedback excites different cavity modes à spectral broadening reduces speckle Previous results: Compare: Source λ FWHM Δλ Laser 780 nm ~1 nm SLED 800 nm ~34 nm à LD + feedback not sufficient for speckle reduction

5 Quantifying speckle Difficulty of quantifying speckle in double pass images: superposition of Gaussian intensity distrib. and speckle à study speckle in a different setup: measure speckle contrast C = % & ' of camera image Ground glass diffuser polished with 1500 grit (CAMI standard: 3 μm particle size of abrasive material)

6 LD pump current change: spectral maps With feedback No feedback Speckle contrast Speckle contrast hardly dependson feedback Very small decrease in SC in the case of no feedback, coinciding with the transition between different longitudinal modes (I ) 63 ma and I ) 78 ma) SC depends on image brightness (not according to formula, but because of finite number of discrete brightness steps (8 bit))

7 Polarizing display A display that polarizes incoming light wave in a definable direction at every pixel We achieve maximum speckle reduction for a 25*25 pixel random pattern, changing every frame

8 Vibrating mirror An often used method for speckle reduction in DP imaging, but undesired because of mechanical moving parts and some influence on the resulting DP image, is a vibrating mirror (f = 100 Hz in this case).

9 2 laser diodes of different wavelengths We couple the light of two laser diodes of different wavelengths into the fiber in order to reduce speckle λ 8 = 656 nm, λ ; = 685 nm Similar output intensity of both wavelengths at fiber end Similar intensity

10 2 laser diodes of different wavelengths The speckle pattern of two LDs is simply the linear superposition of the speckle patterns of each of them LD 1 LD 1 & 2 LD 2

11 2 LDs and current modulation With the same setup as before, one or both of the two LDs are current modulated Idea: broaden their spectra by turning LDs on and off We do not observe any reduction of the speckle contrast compared to using 2 unmodulated LDs We also don t observe a dependence of the SC on the modulation frequency (2 MHz corresponds to the exposure time of the camera, 0.5 ms)

12 Speckle contrast comparison With fixed camera settings 1 and similar source intensities, we measure the speckle contrast, C = % &, for different ' methods of specklereduction LD (no speckle reduction) C 0.89 Superluminescent diode C = 0.53 Vibrating mirror C = 0.30 (5 ms exposure time, laser intensity reduced accordingly) Polarizing display C = 0.87 Two LDs at different λ C = 0.63 Two LDs + current modulation C = settings: rolling shutter, exposure time: 0.5 ms, no gain, black level offset: 234

13 Speckle reducer Optotune speckle reducer, device in which light passes scatterers moving in an electrical field (300 Hz) Setup adjusted because of large beam divergence after speckle reducer LSR off C = 0.61 LSR on C = 0.12 Image from

14 Discussion Why not LED? à impossible to collimate without huge intensity loss Optotune laser speckle reducer is also not viable for DP imaging because of large beam divergence LD + optical feedback: Δλ < 3 nm; far from the SLED (34 nm), or two LDs of different λ Restrictions to be considered: Directionality & sufficient intensity of light source, chromatic aberrations of eye, price of solution

15 Outlook Possible next steps: Check specklereduction by: Combining more lasers of different wavelengths Optical injection of laser diodes of similar wavelength Apply results from these studies on speckle to DP imaging

16 Thank you for your attention Universitat Politècnica de Catalunya Biophysics by the Sea, Alcúdia, Mallorca, October 12, 2018