16 fs, 350 nJ pulses at 5 MHz repetition rate delivered by chirped pulse compression in fibers

We demonstrate a simple approach for broadening and compression of intense pulses at megahertz repetition rates by self-phase modulation in nonlinear photonic crystal fibers. In order to avoid damage by self-focusing, we positively chirp the input pulses, which allows coupling of significantly more energy into the fiber, while maintaining the same spectral bandwidth and compression as compared to the Fourier-limited case at lower energy. Using a commercial long-cavity Ti: sapphire oscillator with 55 fs, 400 nJ pulses at 5 MHz, we generate 16 fs, 350 nJ pulses, which is a factor of 4 more energy than possible with unchirped input pulses. Self-phase-modulated spectra supporting 11 fs duration are also shown with 350 nJ pulse energy. Excellent stability is recorded over at least 1 h. (C) 2011 Optical Society of America