Introduction

In this chapter we shall examine some of the characteristics of laser radiation that distinguish it from ordinary light. Our discussion will include the monochromaticity and directionality of laser beams, and a preliminary discussion of their coherence properties. Coherence is a measure of the temporal and spatial phase relationships which exist for the fields associated with laser radiation.

The special nature of laser radiation is graphically illustrated by the ease with which the important optical phenomena of interference and diffraction are demonstrated using it. This chapter includes a brief discussion of these two phenomena with some examples of how they can be observed with lasers. Interference effects demonstrate the coherence properties of laser radiation, while diffraction effects are intimately connected with the beam-like properties that make this radiation special.

Diffraction

Diffraction of light results whenever a plane wave has its lateral extent restricted by an obstacle. By definition, a plane wave traveling in the z direction has no field variations in planes orthogonal to the z axis, so the derivatives ∂/∂x or ∂/∂y operating on any field component give zero. Clearly this condition cannot be satisfied if the wave strikes an obstacle: at the edge of the obstacle the wave is obstructed, and there must be variations in field amplitude in the lateral direction. In other words, the derivative operations ∂/∂x and ∂/∂y do not give zero, and the wave after passing the obstacle is no longer a plane wave.