Reflection properties of a Gaussian laser beam from multilayer dielectric films

Abstract

A laser microphone is a surveillance device that uses a laser beam to detect sound vibrations in a distant object. The object is typically inside a room where a conversation is taking place, and can be anything that can vibrate (for example, picture or window) in response to the sound waves of the conversation. The object preferably has a smooth surface. The laser beam is directed into the room through a window, reflected off the object and returned to a receiver that converts the beam to audio signal. The beam is mostly bounced off the window itself. Usually these kinds of devices are used by surveillance intelligence in some parts of governments and these kinds of weapons analyze the laser beam which reflects from window. In this thesis a countermeasure to the detection of laser beam is analyzed. In order to make this possible, the reflection from dielectric stratified surfaces of a Gaussian laser beam needs to be described. The reflected beam profile of electromagnetic radiation exposes to various effects different from reflected plane waves. Gaussian beams which reflect from a dielectric slab experience in a shifting maximum point in one direction; lateral shift, focal shift and angular divergence are the shift and distortion of the beam profile. The Gaussian beam propagates in z direction and broadens in transverse plane, in two dimensions and is decomposed into plane wave components. Upon analyzing the reflection coefficient and beam profile, reduction of beam power after reflection from stratified films is described.