The joining of optical glasses is a challenge even with the latest technology. Welding with ultrashort pulsed lasers is a way to join similar or dissimilar glasses without additives or macroscopic thermal tensions. The laser beam is focused through the work piece on the interface to be welded. Due to the high intensity in the laser focus, the beam is absorbed via nonlinear effects in a volume with about 20 µm in diameter. Several laser pulses heat up the material to the working point, while the thermal conductivity limits the heat affected zone well below 1 mm. This process is usually conducted with microscope objectives. Their short working distance limits the thickness of the work piece, the usable laser power and the feed rate of the process. To increase the possible dimensions of the welding partners and the process speed to industrial levels, we present USP-welding with a galvoscanner and a common F-theta-lens. Despite self-focusing effects, our experiments show that the process is stable and controllable. Furthermore, filamentation of the laser beam occurs and long cylindrical weld zones of some micrometers diameter and several hundreds of micrometers height are generated. The enormous length of this molten zone significantly lowers the demands on the work piece adjustment. Tensile tests were conducted on the welded samples. The tests show that the weld can reach a breaking strength in the order of magnitude of the base material.
We report on nonlinear optical compression of passively Q-switched pulses accessing sub-10 ps domain, which is so far
dominated by mode-locked systems. The concept implements the SPM-induced spectral-broadening of passively Q-switched
microchip pulses in optical waveguides and a supplementary compression with bulk optics e.g. a pair of
diffraction gratings or a chirped-Bragg-grating. Used seed-source is a fiber-amplified, passively Q-switched microchip
laser operating on a single longitudinal mode and consists of a monolithically bonded combination of Nd:YVO4-crystal
and semiconductor saturable absorber mirror. The microchip laser provides pulses with durations of 100-150 ps, pulse
energies of ~200 nJ at various repetition rates from hundreds of kilohertz to more than a megahertz, and line width of
~20 pm at wavelength of 1064nm. During the amplification process in the photonics crystal fiber, the pulses are
spectrally broadened to up to ~0.7nm at energy of 17μJ. Using a diffraction grating compressor with 1740 l/mm, the
pulses are compressed to duration as short as 6ps assuming a numerically calculated de-convolution factor of 0.735. To
the best of our knowledge, this is the first reported realization of nonlinear compression of the Q-switched pulses and the
shortest pulses from a passively Q-switched laser system.
We report on a diode-pumped, monolithic and passively Q-switched microchip laser generating 200 ps pulses at a
wavelength of 1064 nm with a repetition rate of up to 2 MHz. By varying the pump intensity we can change the
repetition rate in the range from 100 kHz to 2 MHz and achieve pulse energies from 400 nJ to 130 nJ respectively, while
still maintaining singe transversal and longitudinal mode operation. The microchip laser is based on Nd:YVO4 as the
gain medium and a SESAM as the passive Q-switch. It is monolithically bonded with spin-on-glass as the bonding
agent. The timing jitter was measured to be shorter than 40 ns for low and 2.5 ns for high repetition rates resulting in a
relative timing jitter smaller than 1%. The output of this type of laser can be amplified easily to the range of few tens of
watts using only one amplification stage based on a photonic crystal fiber. The combination of picoseconds pulses, high
average power and high repetition rates makes this system very interesting for many applications like e.g.
micromachining with high processing speed and nonlinear frequency conversion with high average power.
In this contribution we introduce a simple scheme to spectrally combine four single beams using three low-cost
dielectric interference filters as combining elements. 25 ns pulses from four independent and actively Q-switched fiber
seed-sources are amplified in a single stage fiber-amplifier. Temporally and spatially combined 208 W of average
power and 6.3 mJ of pulse energy are obtained out of a fiber-based laser system. A detailed observation of beam quality
as well as the thermal behavior of the combining elements have been carried out and reveal mutual dependency. The
deterioration of beam quality can be led back to thermal induced wave-front distortions on the part of the interference
filters. This effect as well as other influences on M2 will be discussed and compared to the competing combining
approach with dielectric gratings.
We present a systematic study on the inhibition of stimulated Raman scattering by lumped spectral filters both in passive
optical transport fibers and in fiber amplifiers. This study reveals the parameters that have the strongest influence on the
suppression of the Raman scattering (such as the attenuation at the Raman wavelength and the insertion losses at the
signal wavelength). These parameters have to be optimized in order to achieve the desired Raman inhibition and/or to
minimize the loss in amplifier efficiency. The study is concluded with realistic predictions on the use of spectral filtering
elements for Raman scattering inhibition in real-world high power fiber amplifiers.
Thus, using for example 10 lumped spectral filters with 20 dB effective Raman attenuation and less than 0.25 dB
insertion losses, a maximum Raman threshold increase by a factor of 3 is expected. In this context, long period gratings
are proposed as promising filtering elements for Raman inhibition in high power fiber amplifiers.
In order to experimentally verify the theoretical predictions and the suitability of long period gratings, a fiber amplifier
consisting of 2 m active Ytterbium doped fiber was built. Three long period gratings were consecutively inserted at
different positions along the fiber, and the Raman threshold was determined for each situation. It is shown that, with
three long period gratings, the Raman threshold (defined as the 20 dB ratio of Raman to signal output power) was
increased by about 60%, which offers a good agreement with the theoretical predictions.
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.
We present a Q-switched microchip laser emitting 1064nm pulses as short as 100ps synchronized to a cavity dumped
femtosecond laser emitting 800nm pulses as short as 80fs. The synchronization is achieved by presaturating the
saturable absorber of the microchip laser with femtosecond pulses even though both lasers emit at different
wavelengths. The mean timing jitter is 40ps and thus considerably shorter than the pulse duration of the microchip laser.
We report to the best of our knowledge for the first time on the fabrication and characterization of CO2-laser written
long-period gratings in a large-mode area photonic crystal fiber possessing a core diameter of 25 μm. The gratings have
low insertion losses (<1 dB) and high attenuation (>10 dB) at the resonant wavelengths, making them particularly
interesting for high power applications.
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