D. Nodop

Efficient high-power generation of visible and mid-infrared light by degenerate four-wave-mixing in a large-mode-area photonic-crystal fiber

An efficient and simple approach for converting pulsed near-IR laser radiation into visible and mid-IR light by exploiting degenerate four-wave-mixing in an endlessly single-mode, large-mode-area photonic-crystal fiber is presented. Coupling a 1 MHz, 200 ps, 8 W average power pulsed source emitting at 1064 nm into this fiber results in average powers of 3 W at 673 nm signal wavelength and of 450 mW at 2539 nm idler wavelength, respectively. The excellent pulse energy conversion efficiencies of 35% for the signal and 6% for the idler wavelength are due to the unique combination of characteristics of this type of fiber.

Compensation of pulse-distortion in saturated laser amplifiers.

We derive an expression describing pre-compensation of pulse-distortion due to saturation effects in short pulse laser-amplifiers. The analytical solution determines the optimum input pulse-shape required to obtain any arbitrary target pulse-shape at the output of the saturated laser-amplifier. The relation is experimentally verified using an all-fiber amplifier chain that is seeded by a directly modulated laser-diode. The method will prove useful in applications of high power, high energy laser-amplifier systems that need particular pulse-shapes to be efficient, e.g. micromachining and scientific laser-matter-interactions.

High-pulse-energy passively Q-switched quasi-monolithic microchip lasers operating in the sub-100-ps pulse regime

We present passively Q-switched microchip lasers with items bonded by spin-on-glass glue. Passive Q-switching is obtained by a semiconductor saturable absorber mirror. The laser medium is a Nd:YVO(4) crystal. These lasers generate pulse peak powers up to 20 kW at a pulse duration as short as 50 ps and pulse repetition rates of 166 kHz. At 1064 nm, a linear polarized transversal and longitudinal single-mode beam is emitted. To the best of our knowledge, these are the shortest pulses in the 1 microJ energy range ever obtained with passively Q-switched microchip lasers. The quasi-monolithic setup ensures stable and reliable performance.

Reduction of timing jitter in passively Q -switched microchip lasers using self-injection seeding

We present an efficient, simple, and passive technique for the reduction of timing jitter in passively Q-switched microchip lasers via self-injection seeding using a fiber delay line. The presented approach mitigates one inherent issue of passively Q-switched lasers without the need for active stabilization. At a repetition rate of a few hundred kilohertz and pulse duration of approximately 200 ps delivered by a microchip laser, the rms jitter is reduced from several nanoseconds down to 20 ps, hence, significantly below the pulse duration of the laser source.

Suppression of stimulated Raman scattering employing long period gratings in double-clad fiber amplifiers

We report on the suppression of stimulated Raman scattering (SRS) in a double-clad fiber amplifier using long-period gratings (LPGs). The LPGs, fabricated with a CO(2) laser, achieve SRS suppression by coupling the Stokes wavelength from the active core into the cladding. With only three LPGs inserted into a fiber pulse amplifier, the extractable Raman-free output power was nearly doubled. A numerical simulation of the setup shows good agreement with the experimental results.

Nonlinear compression of Q-switched laser pulses to the realm of ultrashort durations.

Mode-locked lasers have an undisputed position in the ultrafast domain, though they are fairly expensive for miscellaneous applications. Thus, laser consumers revert to more cost-effective systems like Q-switched lasers. Here we report on the nonlinear compression of passively Q-switched laser pulses that allows accessing the time domain of sub-10-picoseconds, which has been so far the realm of mode-locked lasers. Laser pulses with an initial duration of 100 ps from a passively Q-switched microchip laser are amplified in a photonic crystal fiber and spectrally broadened from 20 pm to 0.68 nm by self-phase modulation. These pulses are compressed in a grating compressor to a duration of 6 ps with a pulse energy of 13 µJ.

Modeling the inhibition of stimulated Raman scattering in passive and active fibers by lumped spectral filters in high power fiber laser systems.

We report on the simulation of stimulated Raman scattering inhibition by lumped spectral filters both in passive optical transport fibers and in fiber amplifiers. The paper includes a detailed theoretical study that reveals the parameters that have the strongest influence on the suppression of the Raman scattering, such as the filter distribution and the insertion losses at the signal wavelength. This study provides guidelines for the use of spectral filtering elements, such as long period gratings, for Raman scattering inhibition in real-world high power fiber amplifiers.

Compensation of pulse-shaping due to saturation in fiber-amplifiers

We derive an expression describing pre-compensation of pulse-distortion due to saturation effects in short pulse laseramplifiers. The analytical solution determines the optimum input pulse required to obtain any arbitrary target pulse at the output of the saturated laser-amplifier. The relation is experimentally verified using an all-fiber amplifier chain that is seeded by a directly modulated laser-diode.

Improved parametric generation of light in optical fibers

In this work we report on a method to substantially improve the conversion of near-infrared pulsed light into visible or mid-infrared radiation exploiting degenerate Four-Wave Mixing (FWM) in an endlessly single-mode photonic crystal fiber (PCF). In our previous work [1] we showed that it is possible to exploit FWM to simultaneously obtain Mid-infrared (MIR) and Visible (Vis) radiation. However, the pulses coming out of the system are not clean, exhibiting multiple peaks due to successive energy back-transfer processes. This multi-peak nature of the output pulses reduces, on the one hand, their applicability and, additionally, the overall converted pulse energy. The multi-peak structure originates from the periodic changes in direction of the energy flow in the FWM process (first the energy flows from the pump towards the signal and idler waves until substantial pump depletion is reached, and then, from that moment on, the energy flows from the signal and idler in direction to the pump) [2]. These periodic changes of direction of the energy flow can be recognized as an oscillatory behavior of the energy conversion efficiency along the fiber. Crucially, these periodic changes also limit the output peak power of the converted pulses. Thus, for the idler wave (at 670nm for an LMA-10 endlessly singlemode fiber when pumped at 1064nm) the maximum peak power lies slightly below half of the pump peak power, which is a serious limitation of this technique.

Modeling the suppression of stimulated Raman scattering in active and passive fibers by lumped spectral filtering elements

Stimulated Raman scattering in optical fibers can be suppressed by different techniques, which include the use of special fiber designs (that work as distributed spectral filters) [1] or the use of lumped filtering elements. Fiber designs like the w-type profile are limited in maximum fiber core size and provide Raman attenuations of a few dB/m. These characteristics imply that these designs are not suitable for high power fiber amplifiers with short fiber lengths. In these applications lumped filters could provide a better and more flexible solution. Thus, in this work lumped filters will be evaluated as Raman suppression elements in passive fibers as well as in active Yb-doped fibers for amplifier and laser applications. Both the influence of the number of equidistant discrete filters in various setups and the impact of their insertion losses will be theoretically studied.