For approximately ten years a number of researchers have been investigating the use of quantum well (QW) or multiple quantum well structures (MQWSs) as saturable absorbers for mode-locking low gain lasers. The interest in this approach lies in the potential for engineering the characteristics of such an absorber and optimizing it for a particular laser wavelength. Both intracavity elements and nonlinear reflectors have been studied.
More recently a saturable Bragg reflector incorporating a single quantum well to provide a small amount (e.g. 1-2%) of saturable loss has been proposed. Tsuda et al. Opt. Lett. 20, 1406 (1995) describe such a device, based on AlAs/AlGaAs, which they have used successfully to mode-lock both Ti:Sapphire and Cr:LiSAF lasers operating near 850 nm. The Bragg reflector illustrated in their paper, was grown by molecular beam epitaxy (MBE) and consisted of 30 high/low index pairs with a single GaAs quantum well buried in one of the layers. The entire structure, which was grown epitaxially on a GaAs substrate, acted as a high reflectivity mirror having an intensity dependent reflectivity governed by the saturable absorption behaviour of the quantum well. An important feature of this scheme is the fact that the effective saturation intensity can be controlled by varying the location of the quantum well within the multi-layer reflector structure. The Quantum Well was located at an electric field maximum in the first layer of the Bragg reflector.
A drawback of this saturable reflector, however, is the fact that the high reflectance stack has to be deposited epitaxially in order to deposit the epitaxial Quantum Well structure. Hence great care has to be taken in the fabrication of such a mirror to ensure that non saturable losses, particularly scattering, are extremely small, especially as a large number of layers, typically around 60, have to be grown to achieve the desired reflectance (typically around 99%). The fabrication of low loss reflectors by molecular beam epitaxy (MBE) is thus very difficult. Furthermore, the MBE deposition rate is slow compared to other processes.
A hybrid reflector, incorporating both epitaxially deposited semiconductor layers and evaporated dielectric layers, has been developed by U. Keller, et al. Opt. Lett. 17, 505 (1992). It is not capable of operating as a saturable Bragg reflector.
An object of the invention is to alleviate this problem.