Determination of coherence length in biological tissues

Abstract

Lately in phototherapy the use of diodes instead of lasers was suggested for economical and practical reasons. It has been argued that lasers have no preference over diodes since they lose their coherence once penetrating biological tissues. However, this point has never been experimentally proven. In this work we, for the first time, have experimentally validated the conditions affecting the spatial coherence of a laser illumination going through a biological tissue. In our experiments we measured the spatial coherence of the light passing through phantoms containing intralipid and ink component as well as through uncooked turkey meat. We do this measuring the changes of the contrast of the speckle patterns generated due to laser illumination. Flow tunnels inside the phantoms were generated by needles in two different diameters. The measurements were performed for varied integration time, varied thickness of phantoms, and for varied flow rates. The measurement system included two excitation sources: a green doubled Nd:YAG laser at wavelength of 532 nm and an ultra high power green LED at a wavelength of 520 nm. It was experimentally validated that the thickness of the tissue does not change the coherence while there is no flow. Furthermore, the flow velocity and the flow volumetric rate highly affect the coherence length. Previously developed mathematical expression, in which the contrast depends on the correlation and the exposure time, was found to be compatible with the obtained experimental results. We found that the coherence of the laser is not lost when the light goes through a static tissue but it is partially lost when there is a flow of fluid through the tissue. The volumetric flow rate is directly correlated to the loss of spatial coherence. Higher flow rate produces shorter coherence length.