We have demonstrated a high-power, high-energy femtosecond fiber source based on direct amplification of parabolic pulses from an environmentally stable passively mode-locked fiber oscillator in an Yb-doped single-polarization photonic crystal fiber. The special pulse shape allows for the generation of high-quality femtosecond pulses beyond nonlinearity limits. The system delivers a pulse energy of 1.2 µJ (21 W average power) at a repetition rate of 17 MHz and a pulse duration of 240 fs in a linearly polarized beam with diffraction-limited quality. To our knowledge this is the first experimental confirmation that parabolic pulses can lead to a significant improvement of pulse quality in fiber amplifier systems.
High-peak-power and high-average-power femtosecond laser systems are required in several industrial and scientific applications: prominent examples are nonlinear frequency conversion, waveguide writing in transparent material, and high-precision material processing. In past years the average output power of such ultrafast laser systems has been continually increased. This was mainly due to the development of new concepts of diode-pumped solid-state-lasers, which displace upward the power-limiting thermo-optical problems. Rare-earth-doped fiber amplifier systems also offer a promising alternative to conventional solid-state lasers. Their main performance advantages are due to the excellent heat dissipation characteristics of the long and thin gain medium, along with the high efficiencies (often more than 80%). Power scaling of ultrafast single-mode fiber amplifiers is restricted due to nonlinear pulse distortions, which are enforced by the large product of intensity and interaction length inside the fiber core. However, the nonlinearity during the propagation of optical pulses in a high-power fiber amplifier can even be used to overcome this limitation. The combined interaction of normal dispersion, gain, and nonlinearity (self-phase modulation) can create linearly chirped parabolic pulses, which resist optical wave breaking. The opposite approach is sufficient pulse stretching in the time domain and enlargement of the mode field diameter of the fiber to reduce the peak power and therefore nonlinear effects such as stimulated Raman scattering and self-phase modulation. This approach leads to a rare-earth-doped-fiber–based chirped-pulse-amplification system employing low-NA large-mode-area (LMA) fibers.
Compact CPA System
We have developed a femtosecond fiber laser for medical applications. The laser generates > 5 μJ-pulses at an ultra -short pulse duration of < 400 fs and a pulse repetition rate of 200 kHz. The average power of the laser is ~ 1 W and the pulse repetition rate can be scaled to > 1 MHz. The laser system is compact and is dimensioned for air-cooled operation. For high stability the laser is monolithically integrated from the oscillator to the booster amplifier stage relying mostly on qualified and economic Telecom-grade polarization maintaining components. For simplicity and flexibility reasons we chose a modular setup of the femtosecond fiber laser source. The principal laser set-up consists of the six modules oscillator, pulse stretcher, pre-amplifier, acousto-optic pulse picker, booster amplifier, and pulse compressor, and is shown in Figure.
High-Power CPA System
To our knowledge we have demonstrated the first solid-state femtosecond laser system producing average output powers in excess of 100 W. The fiber CPA described here delivers 131 W of average power of 220 fs pulses at a repetition rate of 73 MHz, corresponding to a peak power of 8.2 MW. No limitations owing to nonlinearity were observed up to this power level.
High-Energy CPA System
We report on a large-mode-area ytterbium-doped photonic crystal fiber (PCF) based CPA system generating up to 90 W of average power of 500 fs pulses at 0.9 MHz repetition rate corresponding to 100 µJ of pulse energy. The employed diffraction grating compressor consisting of multilayer dielectric reflection gratings is able to handle this power level without any degradation or imposed beam distortion. To our knowledge this is the highest average power ever reported for a high energy ultra-short-pulse solid-state laser system