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Applied Physics Letters

LIGHT AMPLIFICATION IN SATURABLE ABSORBERS

M. Mack

Published May 15, 1968• 16 citations
Publisher

Abstract

It has been found that an absorber driven into saturation by a strong light pulse can amplify a weak light pulse simultaneously incident upon the medium. Gains as high as 20 cm−1 have been observed. This effect is believed to be related to the ability of a saturable absorber to bring about mode locking in a laser cavity.
DOI
10.1063/1.1651839

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The interaction between waves in an idealized saturable absorber is discussed. It is shown that the presence of a strong wave will cause the line to appear to be ``hole-burned'' when probed by a second wave, even though the broadening mechanisms are conventionally homogeneous. This effect would be expected to work against the tendency of dye-switched ruby lasers to operate in a single mode. Related effects should play a role in optical pulse generators and in other devices using saturable absorbers.
In a laser with a saturable absorber (LSA), two spatially separated cells are placed in the laser cavity as shown in Figure 8.1. The roles of the two cells are quite different: one of them is pumped so that the atoms have a positive population inversion (active or amplifying medium); the other one is left with a negative population inversion (passive or absorbing medium). As these two media are in general different, they saturate at different power levels. The most interesting case corresponds to the situation where the absorber saturates more easily than the active medium, introducing nonlinear losses inside the cavity. This new nonlinearity is responsible for two phenomena. An LSA may exhibit optical bistability, i.e. two distinct stable steady states may coexist for a range of values of a parameter. It may also produce pulsating intensity oscillations which have been called “passive Q-switching” (PQS) in contrast to “active Q-switching” experiments such as the “gain switching” experiments discussed in Section 1.3.2. The interest in LSAs varied very much over time, with peaks in the late 1960s for their large intensity pulses, in the mid 1980s for their chaotic outputs, and in the late 1990s for the design of compact microlasers. Historically, physicists trying to explain the irregular intensity pulses delivered by the ruby laser suspected the possible destabilizing role of a saturable absorber. Shimoda proposed that the nonlinear losses generated by a saturable absorber could explain this phenomenon.
Experiments are reported that measure the pulse velocity of a mode-locked 6328-Å He-Ne laser pulse in a resonant absorber as a function of the pulse intensity. At low intensities, pulse velocities greater than the velocity of light in the passive medium are observed. These results are compared with a simple theory based on the saturation of the dispersion associated with the absorption. At higher intensities where the pulse approaches a 2π pulse, pulse delays (that is pulse velocities less than the velocity of light in the passive medium) are observed.
The optical absorption of several substances has been found to depend in a reversible way on the incident light flux [1]-[5] of a high intensity laser. Moreover, it has been observed [6]-[7] that the absorption decreases not only at the laser wavelength but simultaneously within a large band around this wavelength. These effects may be explained by a time-dependant population distribution among the different excited states of absorbing molecules during the strong laser excitation [8]. The following experimental results enable us to determine the different time constants of importance in this phenomenon.
A comparative analysis of two approaches to the description of the light modulation pulse in a saturable absorber is presented. According to the simplest model, the delay of the optical pulse is a result of the distortion of its shape due to the self-modulation of absorption in the nonlinear medium. The second model of the effect, arisen at the beginning of our century, relates the pulse delay to the so-called “slow light,” attributing this delay to the group velocity reduction under the condition of coherent oscillations of the population. It is shown that the data of all known experiments on the light pulse delay in saturable absorbers can be comprehensively described in terms of the simplest model of saturable absorber and do not require invoking the effect of population coherent oscillations with spectral hole-burning and anomalous modifications of the light group velocity. It is concluded that the effect of the group velocity reduction under the condition of population coherent oscillations has not received so far any experimental confirmation, and the assertions about real observation of the slow light based on this mechanism are unfounded.