Ultrafast Diode-Pumped Ti:Sapphire Laser Generating 200-mW Average Power in 68-fs Pulses

Abstract

We present the highest average power from a diode-pumped Ti:Sapphire laser. Using self-starting SESAM-modelocking we obtain 200 mW in 68-fs pulses at 378 MHz. The laser is pumped by two air-cooled 520 nm AlInGaN laser diodes. OCIS codes: (140.7090) Ultrafast Lasers; (140.4050) Mode-locked lasers; (140.3590) Lasers, titaniumSimple, compact and cost-efficient ultrafast diode-pumped solid-state lasers (DPSSLs) have revolutionized numerous areas in science and technology. Most femtosecond DPSSLs are currently based on Yb-doped gain materials, reliably mode-locked with a Semiconductor Saturable Absorber Mirror (SESAM), and pumped by diodes in the 940-980 nm regime. Although large research efforts target to improve the gain bandwidth of Yb-materials, the shortest achievable pulse duration directly from an oscillator is still obtained from Ti:Sapphire lasers. This gain material combines excellent mechanical strength, good thermal conductivity and an extremely broad gain bandwidth. However, Ti:Sapphire requires a pump wavelength in the blue-green spectral region, which until recently required the use of complex and expensive pump lasers. Frequency-doubled green DPSSLs are typically used, and an additional modulator is often required for fast control of the pump power in applications like CarrierEnvelope-Offset (CEO) Phase stabilization.With the advent of gallium nitride (GaN) laser diodes, a major breakthrough was achieved in 2009, when Roth et al. demonstrated the first directly diode-pumped Ti:Sapphire laser [1]. Initially, a pump wavelength of 452 nm was used, which led to slow deterioration of the output power due to a pump-induced loss mechanism. Furthermore, the relatively low pump absorption cross-section in the region around 440-nm makes efficient laser operation challenging. The highest output power of an ultrafast Ti:Sapphire DPSSL demonstrated so far was 105 mW in 50 fs pulses, which required two 445 nm pump diodes with a total power of 4 W [2]. The recent availability of green pump diodes enabled the first demonstration of a directly green pumped Ti:Sapphire DPSSL. It achieved 23.5 mW average power and 62-fs pulses using a 1-W laser diode at 520 nm [3], corresponding to an optical-to-optical efficiency of <2.5%.Here, we report on a green-pumped ultrafast Ti:Sapphire DPSSL generating 200 mW average power, which is twice as high as any previously published diode-pumped Ti:Sapphire laser, and it has 5-times higher efficiency in comparison to [3]. We use two 520-nm pump diodes with a total power of 2 W incident onto the laser crystal. We achieve self-starting SESAM-modelocked operation with 68 fs pulses. The laser operates at a high repetition rate of 378 MHz, resulting in a very compact cavity layout. No complex cooling system is required: neither the SESAM nor the Ti:Sapphire crystal are actively cooled, only the diodes are cooled by a small fan. Fig. 1. Laser Diagram