Laser Beam Transformation: Propagation, Amplification, Frequency Conversion, Pulse Compression and Pulse Expansion

Abstract

Before it is put to use, a laser beam is generally transformed in some way. The most common type of transformation is that which occurs when the beam is simply made to propagate in free space or through a suitable optical system. Since this produces a change in the spatial distribution of the beam (e.g., the beam may be focused or expanded), we shall refer to this as a spatial transformation of the laser beam. A second type of transformation, also rather frequently encountered, is that which occurs when the beam is passed through an amplifier or chain of amplifiers. Since the main effect here is to alter the beam amplitude, we shall refer to this as amplitude transformation. A third, rather different, case occurs when the wavelength of the beam is changed as a result of propagating through a suitable nonlinear optical material (wavelength transformation or frequency conversion). Finally the temporal behavior of the laser beam can be modified by a suitable optical element. For example, the amplitude of a cw laser beam may be temporally modulated by an electro-optic or acousto-optic modulator or the time duration of a laser pulse may be increased (pulse expansion) or decreased (pulse compression) using suitably dispersive optical systems or nonlinear optical elements. This fourth and last case will be referred to as time transformation. It should be noted that these four types of beam transformation are often interrelated. For instance, amplitude transformation and frequency conversion often result in spatial and time transformations occurring as well.