Abstract

Optical imaging through scattering media with sub-mm spatial resolution using short light pulses and suitable correlation techniques has been proven successfully by several groups (see for example Refs. 1–4), with interesting application potentials in biology and medicine. The short light pulse (or broadband coherent light) illumination serves to separate the imaging signal from the scattered background. To achieve µm spatial resolution these techniques need to be combined with microscopic imaging. Here we describe a linear correlation technique that not only provides the gating but also spatial resolution similar to ordinary confocal imaging. The schematic diagram of the apparatus is shown in Fig. 1. Pulses from a fs Ti:sapphire laser (800 nm, 100 fs) are coupled into a Michelson interferometer that contains a (home-made) laser scanning microscope. The reference arm is terminated by a loudspeaker that is driven by a frequency of f = 2kHz, producing an oscillation amplitude of about 5µm. The fringe modulation (if both arms have the same length) is detected by a photodiode D1. The amplified and rectified signal is then sent to a lock-in amplifier operating at 2/. An improvement of the signal to noise ratio can be achieved by using a second detector D2 at the other output of the Michelson and processing the difference signal D1-D2.

© 1994 IEEE