Coherent Backscattering and Laser Feedback Microscopy

Abstract

Laser Feedback Interferometry — It is well known in the laser field that laser light re-entering a laser cavity can produce interference, in some cases sufficient to produce chaotic effects even to the extent of extinguishing laser action [1]. What is less well known is that it is possible to use controlled re-entry light as the basis for laser interferometry where the laser acts simultaneously as light source, detector, amplifier, and interferometer reference and measuring arm [2-5]. If a target surface, which may be a mirror or a diffuse reflector, intercepts the laser beam and coherent laser light is retro-reflected (back-scattered) back into the laser cavity resonant mode (i.e., TEMoo), interference occurs with the circulating energy [6-8] affecting the intensity of the laser. Milliwatt He-Ne lasers, with an emission wavelength of 632.8nm, typically incorporate end mirrors of >98% reflectivity. With a specular reflector as target, intensity modulations of ≤30% are observed and interferometric information (phase and amplitude) is readily measured with diffuse targets and back-scattered light intensities as low as 10-6% of the incident beam intensity [9,10]. One useful model of this effect, called laser-feedback interferometry, is to regard the emergent-end mirror and the target surface as two “mirrors” forming a Fabry-Perot etalon; therefore, displacement of the target (mirror) produces changes in the effective reflectivity of the laser-end mirror pair resulting in modulation of the laser cavity-resonator properties.