Abstract

We describe a simple Erbium fiber based master oscillator power amplifier generating record 50 kW peak power in a diffraction and bandwidth limited pulse (schematic in Fig.1(a)). The fiber oscillator uses carbon nanotubes for saturable absorption and modelocking. It is followed by a single mode Er fiber amplifier. The amplifier outputs 1.1 ps pulses with 0.136 nJ pulse energy at a 22 MHz repetition rate. A mode matched splice couples pulses into an 45/125 μιη core/clad diameter Erbium fiber with a record 875 μιη² effective area, and anomalous dispersion D « 22 ps/nm-km. Spectral multi-path interference measurements show that ~ 99% energy is coupled into the fundamental mode. The output beam is diffraction limited with M² <1.1. This fiber is core pumped with a 1480 nm Raman pump laser to amplify the pulses. A maximum pulse energy of ~ 30 nJ at an average power of 660 mW is obtained (Fig. 1(b)). Most interestingly the pulse spectrum increases in width from 2.4 nm at the source to 3.7 nm at the maximum amplifier output (Fig. 1(c),(e)), while the pulse duration decreases from 1.1 ps to 0.61 ps (Fig.1(d)), i.e. the pulse sees nonlinear spectral broadening accompanied by temporal compression during amplification. The pulse peak intensity is therefore ~ 50 kW which is a record high at Λ = 1.5 μηι for pulses at the tip of a fiber. The time-bandwidth product of ~ 0.278 indicates that the output pulses may have an asymmetric Sech shape due to gain shaping [1,2]. Nonlinear Schrodinger equation (NLSE) simulations ignoring gain confirm that interaction between self Fig. 1. (a) Amplifier schematic, (b) Average power and pulse energy versus pump power, (c) Comparison of oscillator (dashed), and amplifier spectra at low (dotted) and high (solid line) amplification, (d) Autocorrelation of oscillator and highest amplified pulses. Inset: NLSE calculated decrease in pulse duration with pulse power, (e) Pulse peak power and spectral width versus pump power. phase modulation and anomalous dispersion at these intensities leads to simultaneous spectral broadening and temporal compression (inset of Fig.1(d)) to close to bandwidth limit. At higher nonlinearities, spectral modulations and pulse breakup occur experimentally and numerically.

© 2007 IEEE