Energy scaling of ultrafast fiber systems using chirped and divided pulse amplification

The technique known as chirped pulse amplification (CPA) is the most popularly used technique to scale the energy of ultrashort amplification systems. This technique consists in stretching and compressing the pulse upstream and downstream the amplifier to mitigate various deleterious effects and still recover ultrashort pulse duration. However, its implementation is increasingly difficult and costly when the desired stretched pulse duration is greater than 1 ns, due to the increasing size of the gratings and overall optical setup. Such long stretched pulse duration is therefore not used for compact tabletop or industrial systems. The divided-pulse amplification (DPA) idea was proposed more recently [1], and is similar to CPA in the sense that it also consists in redistributing the pulse energy over a time interval larger than the initial pulsewidth to reduce the peak power. In this scheme, a train of several orthogonally polarized delayed pulse replicas is generated and amplified, before final recombination. In this contribution, we propose and demonstrate for the first time to our knowledge a femtosecond fiber amplifier that uses both concepts simultaneously to scale the output energy of a compact (1.2 m × 0.4 m²) femtosecond fiber system. The experimental arrangement is depicted in Fig. 1. The DPA setup is included inside a moderately nonlinear fiber CPA in which the impact of nonlinearities can be partially compensated by the dispersion mismatch of the stretcher and compressor units [2]. Temporal pulse divisions are implemented using two free-space Mach-Zehnder interferometers in which the splitting and recombining elements are thin-film polarizers. Depending on the orientation of the input linear polarization, the power fraction sent in each arm can be adjusted. In particular, one arm can be completely bypassed, reducing the number of replicas generated. The arm length differences in both interferometers are 1.32 m and 0.66 m respectively. On- the way to the amplifier, this arrangement therefore generates a train of up to 4 orthogonally polarized chirped pulse replicas separated by a delay of 2.2 ns and be equivalent to a stretching ration > 2 ns.