Compact Single and Double Pass Yb Amplifier Using a High-Brightness Multi-Watt Tapered Laser Diode

Many applications of femtosecond lasers typically require pulse energies that cannot be reliably obtained directly from oscillators. For this reason, the research of efficient and cost-competitive ultrafast amplifiers is still a relevant topic for both scientific and industrial research. In this work, we present a novel amplification module relying on a high-brilliance, tapered pump laser diode. A monolithic distributed Bragg grating permits to stabilize the diode wavelength at 978.5 nm with a full-width at half-maximum bandwidth < 0.2 nm. The main feature of the tapered diode pump module is its very good spatial beam quality (M2 < 1.5 at 5 W power with linearly polarized beam). This property well fits the goal of building a compact amplifier designed to maximize the gain/Watt of available pump power, exploiting tight pump focusing in a short, highly-doped crystal. To this purpose we employed as active medium a 3-mm-long, 9.5%-doped N
g
-cut Yb:KYW, exploiting the Nm-axis for maximum gain [1]. The unusually high length×doping concentration product was meant to compensate for the strong saturation of the pump absorption
$(\mathrm{w}_{\mathrm{p}}\approx 25\mu \mathrm{m}$, Ip $\sim 250\text{kW}/\text{cm}^{2} > > \mathrm{I}_{\text{sat},\mathrm{p}})$
. The amplifier was seeded by a either a cw or a femtosecond Yb:KYW oscillator providing a maximum average power of ~ 0.5 W with output wavelength tunable between 1023–1035 nm and < 270 fs pulses in mode-locking regime. The design of the amplifier was eased by the use of a rate-equation-based numerical model we successfully developed and employed in previous investigations on quasi-three-level multi-pass amplification experiments [2].