Alexander Steinmetz

High average power large-pitch fiber amplifier with robust single-mode operation.

Ytterbium-doped large-pitch fibers with very large mode areas are investigated in a high-power fiber amplifier configuration. An average output power of 294 W is demonstrated, while maintaining robust single-mode operation with a mode field diameter of 62 μm. Compared to previous active large-mode area designs, the threshold of mode instabilities is increased by a factor of about 3.

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.

Sub-5-ps, multimegawatt peak-power pulses from a fiber-amplified and optically compressed passively Q-switched microchip laser

We report on high-energy picosecond pulse generation from a passively Q-switched and fiber-amplified microchip laser system. Initially, the utilized microchip lasers produce pulses with durations of around 100 ps at 1064 nm central wavelength. These pulses are amplified to energies exceeding 100 μJ, simultaneously chirped and spectrally broadened by self-phase modulation using a double stage amplifier based on single-mode LMA photonic crystal fibers at repetition rates of up to 1 MHz. Subsequently, the pulse duration of chirped pulses is reduced by means of nonlinear pulse compression to durations of 2.7 ps employing a conventional grating compressor and 4.7 ps using a compact compressor based on a chirped volume Bragg grating.

High-power efficient generation of visible and mid-infrared radiation exploiting four-wave-mixing in optical fibers

We report on the generation of 17.6W of visible radiation at 650 nm using four-wave-mixing in an endlessly single-mode silica fiber. The conversion efficiency was as high as ~30%. This high efficiency could be obtained by exploiting the natural absorption of silica for the mid-infrared radiation >2.5 µm. In a separate experiment 1.6 W of mid-IR radiation at 2570 nm were generated simultaneously with 14.4 W at 672 nm. These power levels of picosecond red radiation are among the highest reported so far for a diffraction limited beam quality in this wavelength region.

Dispersion-free pulse duration reduction of passively Q-switched microchip lasers.

We present a dispersion-free method for the pulse duration reduction of passively Q-switched microchip laser (MCL) seed sources. This technique comprises two stages: one that carries out the self-phase modulation induced spectral broadening in a waveguide structure and a subsequent spectral filtering stage in order to shorten the pulses in time domain. The setup of a proof-of-principle experiment consists of a fiber-amplified passively Q-switched MCL, a passive single-mode fiber used as nonlinear element in which the spectrum is broadened, and a reflective volume-Bragg-grating acting as bandpass filter. A reduction of the pulse duration from 118 to 32 ps with high temporal quality has been achieved with this setup.

All-fiber pulse shortening of passively Q-switched microchip laser pulses down to sub-200 fs

We present an all-fiber concept that generates ultrashort pulses using a passively Q-switched microchip seed laser. A proof-of-principle configuration combines nonlinear pulse compression applying a chirped fiber-Bragg-grating, dispersion-free pulse shortening by means of a fiber-integrated spectral filtering, and a final hollow-core-fiber compression to reach the sub-200-fs pulse-duration region. In a compact all-fiber pulse-shortening unit, initial 100 ps long microchip pulses at 1064 nm wavelength have been shortened to 174 fs and shifted to 1034 nm while preserving a high temporal quality.

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.

Wavelength-tunable, sub-picosecond pulses from a passively Q-switched microchip laser system

We present a novel concept to generate sub-picosecond pulses from a passively Q-switched Nd:YVO4 microchip laser system with an adjustable wavelength shift up to a few tens of nanometers around the original emission wavelength of 1064 nm. This concept comprises two stages: one that carries out a nonlinear compression of fiber-amplified microchip pulses and a subsequent stage in which the compressed pulses are coupled into a further waveguide structure followed by a bandpass filter. In a proof-of-principle experiment, pedestal-free 0.62 ps long pulses have been demonstrated with a wavelength shift to 1045 nm.

Smoothed spectra for enhanced dispersion-free pulse duration reduction of passively Q-switched microchip lasers

We present an enhanced technique for dispersion-free pulse shortening, which exploits the interplay of different third-order nonlinear effects in a waveguide structure. When exceeding a certain value of the pulse energy coupled into the waveguide, the typical oscillations of self-phase modulation (SPM)-broadened spectra vanish during pulse propagation. Such smoothed spectra ensure a high pulse quality of the spectrally filtered and, therefore, temporally shortened pulses independently of the filtering position. A reduction of the pulse duration from 138 to 24 ps has been achieved while preserving a high temporal quality. To the best of our knowledge, the nonlinear smoothing of SPM-broadened spectra is used in the context of dispersion-free pulse duration reduction for the first time.

650 fs pulses at 1045 nm from a passively Q-switched Nd:YVO 4 microchip laser system

Summary form only given. In this contribution we present a novel concept to produce ultrashort pulses from a passively Q-switched Nd:YVO4 microchip [1] laser system reaching the sub-ps range with a tunable emission wavelength from 1030nm to 1050nm.This method comprises two stages: one that carries out the nonlinear compression of the spectrally broadened microchip pulses with a grating compressor [2] and a second stage, in which these pulses are coupled into a further waveguide for additional spectral broadening through self-phase modulation (SPM) followed by a narrow band-pass filter [3]. The high peak power and the short pulse duration of the compressed pulses result in wide SPM spectra with a nearly un-chirped region at the spectral edges, which is used for this concept. Therefore, the spectrally broadened pulses can be filtered afterwards far away from the original central wavelength, which leads in addition to temporal pulse shortening and cleaning to an adjustable wavelength shift. To the best of our knowledge the combination of nonlinear compression and subsequent spectral filtering is used the first time for generating wavelength-tunable sub-ps pulses from a Q-switched laser source.The setup of a proof-of-principle experiment (Fig. 1a) consists of a fiber-amplified Nd:YVO4 microchip laser source, which emits SPM-broadened 72 ps pulses followed by a grating compressor leading to pulse durations of 5.9 ps with a central wavelength of 1064 nm. Afterwards, these compressed pulses are coupled into a passive single-mode fiber (0.3 m long, 10 μm core diameter), in which SPM broadens the spectrum to a bandwidth of approximately 40 nm (-20 dB) (Fig. 1c). The used band-pass has a spectral transmission window of 6 nm with a tunable central filter wavelength. Subsequent filtering at the lower edge of the pulse spectrum results in pedestalfree pulses with a 650 fs full width at half maximum (FWHM) duration corresponding to an autocorrelation trace of 0.9 ps (FWHM) (Fig. 1b). At the same time the filtered pulses have been shifted from 1064 nm to 1045 nm (Fig. 1c), which is very advantageous for subsequent power amplification in Yb-doped fibers. In conclusion, the presented method appears very suitable for passively Q-switched microchip laser systems to reach the sub-ps range. Moreover, this concept seems very suitable for the integration into an all-fiber system, which would result in very compact and stable lasers delivering ultrashort pulses. Such a laser can find many applications, especially in the field of micromachining and might be a very promising alternative to conventional mode-locked laser systems.