Stable trains of ultrashort light pulses with large instantaneous intensities from mode-locked lasers are key elements for many important applications such as nonlinear frequency conversion [1-3], time-resolved measurements [4, 5], coherent control [6, 7], and frequency combs [8]. To date, the most common approach to generate short pulses in the mid-infrared (3.5-20 µm) molecular "fingerprint" region relies on the down-conversion of short-wavelength mode-locked lasers through nonlinear processes, such as optical parametric generation [9-11] and four-wave mixing [12]. These systems are usually bulky, expensive and typically require a complicated optical arrangement. Here we report the unequivocal demonstration of mid-infrared mode-locked pulses from a semiconductor laser.

The impact of upper state lifetime and spatial hole burning on pulse shape and stability in actively mode locked QCLs is investigated by numerical simulations. It is shown that an extended upper state lifetime is necessary to achieve stable isolated pulse formation per roundtrip. Spatial hole burning helps to reduce the pulse duration by supporting broadband multimode lasing, but introduces pulse instabilities which eventually lead to strongly structured pulse shapes that further degrade with increased pumping. At high pumping levels gain saturation and recovery between pulses leads to suppression of mode locking. In the absence of spatial hole burning the laser approaches single-mode lasing, while in the presence of spatial hole burning the mode locking becomes unstable and the laser dynamics does not reach a steady state anymore.

We show that nonlinear phase shifts and third-order dispersion can compensate each other in short-pulse fiber amplifiers. This compen-sation can be exploited in any implementation of chirped-pulse amplification, with stretching and compression accomplished with diffraction gratings, single-mode fiber, microstructure fiber, fiber Bragg gratings, etc. In particular, we consider chirped-pulse fiber amplifiers at wavelengths for which the fiber dispersion is normal. The nonlinear phase shift accumulated in the amplifier can be compensated by the third-order dispersion of the combination of a fiber stretcher and grating compressor. A numerical model is used to predict the compensation, and experimental results that exhibit the main features of the calculations are presented. In the presence of third-order dispersion, an optimal nonlinear phase shift reduces the pulse duration, and enhances the peak power and pulse contrast compared to the pulse produced in linear propagation. Contrary to common belief, fiber stretchers can perform as well or better than grating stretchers in fiber amplifiers, while offering the major practical advantages of a waveguide medium.

We demonstrate that the compensation of self-phase modulation by third-order dispersion can be exploited in the design of fiber amplifiers with tens of microjoules pulse energy. At the highest energies, the amplified pulse accumulates nonlinear phase shift as large as 17 pi. Gain-narrowing occurs in the final amplifier stage, but shorter pulses are still generated with larger nonlinear phase shifts. A large-mode-area Yb-doped photonic crystal fiber amplifier generates diffraction-limited 30 microJ pulses, which are compressed to 240 fs duration. These are converted to the second harmonic with 48% conversion efficiency, as expected theoretically, which confirms the pulse quality.

By optimizing the cavity dispersion map, 1.5-nJ pulses as short as 36 fs are obtained from a Yb-doped fiber laser. Residual higher-order dispersion currently limits the pulse duration, and it should be possible to generate pulses as short as 25-30 fs with Yb-doped fiber.