Effects of Excited State Absorption on Optical Pulse Propagation

Abstract

In this paper I review the effects of excited state absorption on intense pulse propagation in gas, liquid or solid phase molecular media. I consider the propagation of short (compared with the lifetime of the excited state, τ) intense pulses in media with large excited state absorption, and also discuss briefly propagation of long pulses too. In particular, I consider materials known as reverse saturable absorbers (RSA’s). A RSA is a material whose excited state absorption cross section at wavelength λ, σex(λ), is larger than the ground state absorption cross section, σgr(λ), and which possesses four other necessary properties detailed below. Recently, the optical properties of RSA’s and their applications to a number of areas in optical pulse processing and laser physics have been described.1-6 RSA’s can be used as optical pulse shorteners and optical energy limiters for laser pulses whose temporal widths are small compared with τ.1 A RSA in conjunction with a saturable absorber can be used to mode-lock lasers whose gain media have high saturation energies (the saturable absorber cuts the leading edge of the pulse and the RSA cuts the trailing edge of the pulse)2. Moreover, RSA’s stabilize high gain mode-locked or CW lasers against the onset of relaxation oscillations.2 A combination of RSA and saturable absorber can also be used as an extracavity optical pulse compressor to eliminate the undesirable wings of short optical pulses which may have been produced by the process of superfluorescence.7 Such a pulse compressor can be inserted between the amplifier stages of a laser amplifier chain or after a single amplifier. RSA’s can also be used as power limiters and pulse smoothers for pulses whose rate of change of intensity is slow compared with τ, the decay time of the first excited state of the RSA.1