Chirped pulse amplification in an erbium-doped fiber oscillator/ erbium-doped fiber amplifier system OPTICS COMMUNICATIONS

As in bulk amplifiers, the onset of nonlinear effects when amplifying ultra-short pulses in fiber amplifiers has been pointed out and investigated by several groups, specifically in erbium-doped fiber amplifiers ( EDFAs) [ l-3 1. Researchers have in fact used the Kerr nonlinearity in conjunction with group velocity dispersion in order to create even shorter pulses via higher-order soliton formation [ 4,5 1. However, as Hodel et al. [ 3 ] point out, efficient distortionless amplification of soliton pulses in EDFAs is actually prevented by the presence of Raman selfscattering and, beyond the limit of adiabatic amplification, by the break-up of pulses due to frequency dependent gain, self-phase modulation and dispersion. Hence it has been suggested to avoid nonlinearities in the amplification process to fully exploit the energy storage capacity of rare-earth amplifiers. Here, we demonstrate the application of chirpedpulse amplification (CPA) [ 6 ] to the amplification of ultra-short pulses generated by an optical fiber oscillator. This erbium-doped fiber based, compact system should in principle be able to produce femtosecond pulses with energies up to a few microjoules, the saturation energy of EDFAs [ 21. This compares quite favorably to diode amplifiers, which are only capable of saturation energies of approximately 100 pJ [ 7 1. The technique of CPA involves temporally stretching a short pulse through a dispersive delay prior to amplification. The pulse peakpower is thereby reduced resulting in a linear amplification process. The amplified pulse is then recompressed by passing it through a dispersive delay line opposite in effect to the stretcher. In the absence of gain narrowing the original pulse-width can theoretically be obtained. The experimental set-up is shown in fig. 1. An environmentally stable Kerr-type mode locked erbium fiber laser [ 81 is the source of seed pulses to the amplifier. This oscillator generates near band-width limited 250 fs pulses at a repetition rate of 33 MHz and a center wavelength of 1.567 urn with an energy content of up to 100 pJ. The autocorrelation and spectrum of a typical oscillator pulse is shown in fig. 2. The seed pulses are launched with about 30% efficiency into the stretcher stage. This is comprised of 450 m of standard telecommunications type Corning SMF-28 fiber, having a core diameter of 8 urn, a numerical aperture of 0.13, and a cutoff wavelength of 1.2 Frn. The second order dispersion /.I2 at 1.56 nm