Compact all-solid-state femtosecond Yb amplifier using a high-brightness multi-watt tapered laser diode

In this work, we present a novel amplification module based on a high-brilliance tapered pump laser diode. A monolithic distributed Bragg grating permits to stabilize the diode wavelength, that was kept at 978.5 nm with a full-width at half-maximum bandwidth <0.2 nm during our experiments. Such a compact and reliable single laser source achieves a maximum output power of about 6.5 W, in a linearly polarized beam with high spatial quality (M2 < 1.5). This allows to optimize the performance of compact amplification modules designed to maximize the gain-per-Watt of absorbed pump power and the optical-to-optical extraction efficiency. To assess the potential of such pump unit, we designed a single-pass and double-pass amplification module based on a 3-mm-long 9.5%-doped Yb:KYW crystal, seeded by a either a cw or a femtosecond oscillator providing a maximum average power of ∼500 mW with output wavelength tunable between 1025 nm and 1035 nm. By tightly focusing the pump beam and carefully optimizing the mode-matching between the almost TEM00 pump and seed beam we achieved a single pass small signal gain G0 > 7 at about 3.5 W absorbed pump power and a maximum output power of 2.2 W, corresponding to an optical-to-optical extraction efficiency close to 50%. The excellent beam quality of the seeder was very well preserved during amplification, resulting in a measured Mx2× My2 <1.1 both in single and double pass.