Generation of sub-150-fs, 100 nJ pulses from a low-cost cavity-dumped Cr:LiSAF laser Citation

We report a low-cost, cavity dumped Cr:LiSAF laser, generating 135-fs pulses at 825 nm, with 105 nJ pulse energies and ~0.78 MW of peak power at 10 kHz, using only 600 mW of pump power. 2010 Optical Society of America OCIS codes: (140.3460) Lasers; (140.3480) Lasers, diode pumped; (140.4050) Mode-locked lasers Several application areas of ultrafast laser technology such as white-light generation, micromachining, and deep multiphoton microscopy imaging, require high peak power laser sources. Compared to multipass cavity lasers or complex amplifying schemes, cavity dumping is a relatively simple technique which can be used to scale up the available peak powers from mode-locked laser oscillators. Cavity dumping has been successfully applied to increase the pulse energies of Ti:Sapphire lasers [1-3], ytterbium-doped lasers [4] and neodymium-doped lasers [4]. However, Ti:Sapphire lasers can not be directly diode-pumped, which increases the cost and complexity of the system. Also, for the diode-pumped ytterbium-doped and neodymium-doped lasers, the obtainable pulsewidths are limited to hundreds of femtoseconds to picoseconds [4]. As an alternative, Cr-doped colquiriite gain media can be directly diode pumped around 650 nm, and have broad emission bandwidths around 800-850 nm, enabling 10-fs long pulse generation [5]. Moreover, direct-diode pumping of Cr:Colquiriite lasers also allows high electrical-to-optical conversion efficiencies (∼10%), compactness, and ease of use [5]. Pumping Cr:Colquiriites with four single-mode diodes, ∼50-100 fs pulses with ∼1-2.5 nJ pulse energies and ∼20 kW peak powers have been generated from standard ∼100 MHz cavities [5], where the obtained peak powers were limited by the total available pump power (∼600 mW). Slightly higher peak powers can be obtained with multimode diode pumping (∼40 kW) [6], at the expense of increased complexity. In this work, we report the first cavity-dumping experiments with a simple, low-cost, single-mode diode-pumped Cr:Colquiriite laser. As the gain medium, we have chosen Cr:LiSAF among the Cr:Colquiriite family, since it has a higher emission cross-section. This increases the gain and reduces Q-switching instabilities. The crystal was pumped by four ∼150-mW single-mode laser diodes at 660 nm, each costing only $150. A semiconductor saturable absorber mirror (SESAM) [7] (also referred as saturable Bragg reflectors (SBR) [8]) was used for initiating and sustaining modelocking, making stable turn-key mode-locked operation possible. By cavity dumping at 10 kHz repetition rate, the laser generated ∼135 fs pulses at ∼825 nm, with 105 nJ of pulse energy and ~0.78 MW of peak power. At higher dumping rates approaching 1 MHz, the pulse energy was reduced to 62 nJ, due to the limitations imposed by Q-switching instabilities. This study demonstrates that low-cost Cr:Colquiriite lasers have the potential to generate ∼MW level peak powers with very modest pump requirements. (a) (b) (c) Fig. 1. (a) Schematic of the cavity dumped, single-mode diode-pumped Cr:LiSAF laser. PBS: polarizing beam splitting cube. (b) Measured dynamics of intracavity pulse train at a dumping rate of 10 kHz. (c) Contrast ratio between the dumped pulse and neighboring pulses (>20:1). Fig. 1 shows the schematic of the Cr:LiSAF laser. The 5-mm-long, 1.5% Cr:LiSAF crystal was pumped by four linearly-polarized, AlGaInP single-mode diodes, and up to 600 mW of pump power was incident on the crystal. An astigmatically-compensated, x-folded laser cavity, with curved dichroic mirrors (ROC=75 mm, ROC=radius of curvature) (M1-M2 in Fig. 1(a)) was used in the laser experiments. A second Z-fold focus was created by use of 100 mm ROC mirrors, where we placed the ∼3-mm thick, fused silica acousto-optic cavity dumper. The cavity dumper (64380-SYN-9.5-2, Neos Technologies, Inc.) had a single-pass diffraction efficiency of ~30%, and was used in doublepass configuration to obtain 50-60% dumping efficiency. The dumped beam was picked up with a small metallic high reflector after its second pass through the dumper. A 250 mm ROC curved mirror was used to focus onto the Dumping rate = 10 kHz