Heterodyne detection of spatially distorted laser signals with a two-dimensional detector array for optical imaging of tissues

Abstract

Optical coherence imaging techniques based on heterodyne detection and interferometry have proven to be useful for imaging in highly scattering media, including biological tissues and materials. Their performances, however, can be affected by several factors. For example, speckle noise caused by interference between scattered photons may yield significant fluctuations in the signal intensity, while loss in the spatial coherence for the laser signals through turbid media also contributes to the reduction in heterodyne detection efficiency. Here, the authors present a two-dimensional (2-D) heterodyne detection technique that is capable of enhanced detection and speckle averaging of spatially distorted signals emerging from turbid media such as biological tissues. A figure illustrates the implementation of the 2-D heterodyne detection technique in a transillumination imaging system. In this Mach-Zehnder interferometer-based system, a continuous-wave, single-frequency Nd:YAG laser at 1.064 /spl mu/m is used as the light source, whose output is collimated to 1 mm in diameter and split into the signal and local oscillator beams. The signal light emerging from the sample, which generally spreads out rapidly due to scattering at the coarse surface, is collected by a lens. Although the collected laser signal is spatially distorted, it can be effectively detected by using a 2-D heterodyne detector array with finite element sizes in conjunction with incoherent and/or coherent summation methods.