Studying interference patterns of laser diode light under different pump current and feedback conditions for speckle reduction in double pass imaging

2019 edition

Donatus Halpaap

The double pass (DP) imaging technique is a diagnostic method used to obtain an overall estimation of the optical quality of an eye, containing information on scattering as well as higher order aberrations. [1]

In DP imaging, the point spread function of the eye is determined by recording the image of a point source on the retina. A collimated light beam is directed into the eye, where it passes the ocular media, and is reflected at the retina, then passes through the ocular media again in reverse direction and is recorded after exiting the pupil.

DP images usually suffer from speckle noise, if we do not take further action to suppress it.

In the past, we compared several methods of speckle reduction in DP imaging, some involving undesired mechanical vibrations, as well as different semiconductor light sources, e.g. a semiconductor laser diode, or a superluminescent light emitting diode, which is based on amplified spontaneous emission.

Now, we study the use of semiconductor optical chaos for cost-effective speckle reduction in this application. To this end, we first investigate the use of optical chaos for reducing speckle in a full-field imaging setup. We obtain optical chaos from a semiconductor laser diode by subjecting it to different amounts of optical feedback as well as modulating its pump current, with the aim of reducing the coherence length of the source and thus reducing speckle formation. [2]

For example, the speckle contrast, C, at the end of a multimode fiber is reduced from C = 0.41 to C = 0.30 simply by subjecting the laser diode to optical feedback, [3] where C is the standard deviation of an intensity pattern divided by its mean. [4]

Once we have discovered the feedback parameters for best speckle reduction, we will apply the chaotic source in our DP imaging experiment.

 

[1] J. L. Güell, J. Pujol, M. Arjona, F. Diaz-Douton, and P. Artal, Optical quality analysis system: Instrument for objective clinical evaluation of ocular optical quality, J. Cataract Refract. Surg. 30, 15981599 (2004).

[2] J. Ohtsubo, Semiconductor lasers: stability, instability and chaos (Springer, 2012).

[3] B. Dingel and S. Kawata: “Speckle-free image in a laser-diode microscope by using the optical feedback effect”, Optics Letters, Vol. 18, No. 7 (1993).

[4] J. W. Goodman, Speckle Phenomena in Optics: Theory and Applications (Roberts & Company, 2007).