Pulse duration limitations in a diode-pumped femtosecond Kerr-lens mode-locked Cr:LiSAF laser

Abstract

We describe the results of experimental investigations into the minimisation of the pulse duration from a diodepumped Kerr-lens mode-locked Cr:LiSAF laser. Specifically, the effects on performance of changing the intracavity dispersion characteristics and the laser cavity mirrors are investigated and described. Pulse durations of less than 20 fs have been recorded. PACS: 42.55R; 42.60F In recent years there have been exciting developments in femtosecond laser technology, exploiting the wide bandwidth of solid-state laser materials such as Ti:sapphire and Cr:LiSAF. There have been significant improvements in both the convenience and performance of femtosecond pulse lasers, and this has encouraged a rapid expansion in the application of such sources, e.g. in time-resolved spectroscopic studies, biological imaging, and nonlinear optics. One aspect of femtosecond laser technology that has attracted special attention is the development of all-solid-state diode-pumped femtosecond solid-state lasers (DPFSSLs). These systems bring well-known advantages such as efficiency, compactness, reliability, and stability, all of which will help bring about the fullest exploitation of the potential of ultrashort pulse lasers. A number of alternative approaches to DPFSSLs have been demonstrated. The most attractive option, at least in terms of convenience and simplicity, is the direct-diode pumping of a suitable broadband gain material. Systems to date have typically used the material Cr3+:LiSrAlF6 (Cr:LiSAF) [or its sister material Cr3+:SrGaF6 (Cr:LiSGAF)] pumped with visible 670-nm laser diodes. Recently, the technique of Kerr-lens modelocking (KLM) (also known as self-mode locking) has been optimised for use in these diode-pumped lasers [1, 2] and proven to be both reliable and flexible. Semi-conductor saturable absorbers have been used as an alternative mode∗Current address: Microlase Optical Systems Ltd., 141 St. James Road, Glasgow, G4 ONG, UK locking technique, and several designs have been used successfully, see e.g [3–5]. Although a wide range of DPFSSLs have been reported, most of the progress in reducing the minimum pulse duration obtained from femtosecond lasers has occurred in systems which are not all-solid-state. Work in this field has been dominated by KLM Ti:sapphire laser systems, pumped with an argon ion laser (or frequency doubled flashlamp pumped Nd:YAG laser). Recently, there has been considerable progress in this field, for example [6, 7], and pulses as short as 7.5 fs have been produced [7]. This dramatic progress has been important in enabling new applications and studies to be made in previously inaccessible regimes. Ti:sapphire is used because it has a very wide emission bandwidth, and because its excellent thermal and spectroscopic properties allow high output powers. However, the absorption regions of Ti:sapphire are not compatible with direct diode pumping and this may act as a barrier to the full exploitation of the very short pulses which have been produced in these systems. In this paper we report on investigations into minimising the pulse duration from a diode-pumped Cr:LiSAF laser. Cr:LiSAF clearly has the required bandwidth, at 1900 cm−1 to support much shorter pulse durations than the 30 fs or longer typically seen in systems reported to date [2, 8–11], and there are many applications which would benefit from obtaining pulse durations closer to the sub-10-fs regime, in a convenient all-solid-state laser source. 1 Experimental background 1.1 The diode-pumped KLM Cr:LiSAF laser The Cr:LiSAF laser used for the experiments described in this paper is similar in design to the first reported KLM diodepumped Cr:LiSAF laser [1]. In a higher-power version of this laser we have previously reported routine operation with output powers of up to 42 mW at pulse durations down to 34 fs [2]. Also, in a similar laser we have reported on the generation of pulses as short as 18 fs in duration [12], al-