Effects Of Two-Photon Absorption In Saturable Bragg Reflectors In Femtosecond Solid State Lasers

Abstract

The process of two-photon absorption (TPA) in semiconductors plays an important role at frequencies below the bandgap where it can dominate other absorption processes. TPA has been observed and studied in semiconductor materials of different composition and dimensionality1-3, and, while TPA may have useful applications in such devices as optical memories and switches, more often than not it can severely limit the performance of electro-optic devices, especially at high intensities. For example, due to TPA, the insertion loss of saturable absorbers and other optical switches can become prohibitively high for intracavity applications requiring minimal losses. In this paper we will discuss TPA in a saturable distributed Bragg reflectors (DBR) recently used for mode-locking in a femtosecond, a diode-pumped Cr:LiSAF laser4. The focused intensity on the DBR is about 5 GW/cm2, well into the regime for strong TPA. Yet the DBR exhibits a reflectivity in excess of 99%. Clearly, the DBR structure itself must be minimizing the effects of tpa.The saturable DBR used in ref.(4) is simply a DBR with a single quantum well in the first quarter-wavelength layer. However, we will study TPA in a simple DBR structure since the quantum well is very thin. The DBR is made of GaAs/AlAs, a material with a TPA coefficient of β = 0.05cm/MW1. Thus, one expects to see absorption in excess of α = 104 cm-1 (i.e 45% per round trip through reflector) which would certainly degrade the reflectivity of the saturable DBR.