High-power Yb- and Tm-doped fiber amplifiers seeded by a femtosecond Er:Fiber system

Abstract

Summary form only given. The generation of ultrafast optical pulses that are used for many applications is increasingly based on Er:fiber lasers. The inherent advantages of this technology are compactness, stability, and turn-key operation. Tm- and Yb-doped fiber systems are promising candidates for reaching microjoule pulse energies [1,2,3].We present a setup exploiting the well-established Er:fiber technology that provides a femtosecond pulse train suitable for coherent seeding of both Yb: and Tm:amplifiers. A seed source generates pulses with a wavelength centered at 1550 nm. Four parallel amplification branches each provide 8 nJ pulse energy at a repetition rate of 40 MHz. This source implements also a passive phase-locking scheme [4]. The pulses are compressed in a silicon prism pair and then coupled into a highly nonlinear fiber (HNF). This scheme allows us to generate tailor-cut spectra with components spanning from 800 to 2300 nm that can be finely tuned by material insertion in the prism sequence [5]. The solitonic part of the spectrum is centered at 1970 nm and optimized to cover the entire gain bandwidth of Tm:silica. The dispersive part of a second HNF is designed to fit the gain maximum of Yb-doped fibers at a wavelength of 1030 nm. The benefit of Er:fiber seeding has been proven experimentally by confirming the full coherence of the spectral components generated in the HNFs [6].MHz and the transform limit for the pulse duration is 110 fs. The Er:fiber system acts as seed source for the parallel Yb: and Tm:fiber amplifiers. We reduce the repetition rate to 10 MHz via electro-optic modulators. To amplify the spectrum delivered by the HNF at a center wavelength of 1030 nm, we stretch the pulses with a grating pair to a temporal duration of 340 ps. The amplification occurs in a first Yb:fiber preamplifier pumped at a wavelength of 976 nm. The energy is then boosted in a 1.5 m long Yb-doped photonic crystal fiber (PCF) amplifier stage. The PCF double cladding structure enables pumping with high-power multimode laser diodes. At a pump power of 40 W, we measured pulse energies of 2.2 μJ. The spectrum after amplification has a bandwidth of 12 nm (FWHM) centered at 1032 nm (see Fig. 1.a). Recompression of these pulses leads to a pulse duration of 185 fs (see SHG FROG characterization in Fig. 1.b). The seed for the Tm:amplifier is stretched in 30 m of single-mode fiber directly spliced to the HNF. In a monolithic and truly single-mode Tm:fiber amplifier pumped at 796 nm, we demonstrate pulse energies of 250 nJ at a repetition rate of 10 MHz [6]. Fig. 1.c shows the pulse spectrum after amplification. It is centered at a wavelength of 1950 nm with a bandwidth of 50 nm. Both amplifiers are operating in a linear regime and there are no signs of a significant influence of nonlinear effects. The combination of high-energy, sub 200-fs pump pulses together with passive carrier-envelope phase stability of ultrabroadband seed pulses for parametric amplification paves the way towards extremely nonlinear optics and attosecond technology at unprecedented repetition rates and stability.