S. Nolte

High average power femtosecond fiber CPA system

We report on a diode-pumped ytterbium-doped low-NA large mode area fiber based chirped pulse amplification system delivering 125 W average power at a repetition rate of 75 MHz and diffraction limited beam quality. Femtosecond laser pulses at 1060 nm center wavelength are stretched in a single mode fiber and recompressed after amplification to 350 fs pulse duration.

Comparison of different processes for separation of glass and crystals using ultrashort pulsed lasers

We investigate cutting of transparent materials using ultra short laser pulses with pulse durations in the sub to a few ps regime. All compared methods base on nonlinear absorption including ablation cutting and cleaving or selective etching supported by laser induced modification inside the bulk material. For most of the experiments samples of hardened glass (Corning Gorilla®) with thickness up to 700 μm were used, ablation cutting of sapphire is presented additionally. Absorption and modification inside the volume is analyzed in detail, aiming for tailored modifications. Besides optical microscopy a pump probe setup was used. We show results of time resolved absorption measurements of 6 ps pulses focused into the volume. We observe shielding due to the interaction region and accumulation effects influencing the modifications. First results on inscribing and cutting by using beam shaping indicate the importance of tailoring the shape and arrangement of the pulses temporally and spatially. The results presented for the different cutting methods supports an assessment of the individual potential and a selection of the applicable method based on the requirements.

Ultrashort pulse laser drilling of metals using a high-repetition rate high average power fiber CPA system

We present an experimental study of the drilling of metal targets with ultrashort laser pulses with pulse durations from 800 fs to 19 ps at repetition rates up to 1 MHz, average powers up to 70 Watts, using an Ytterbium-doped fiber CPA system. Particle shielding and heat accumulation have been found to influence the drilling efficiency at high repetition rates. Particle shielding causes an increase in the number of pulses for breakthrough. It occurs at a few hundred kHz, depending on the pulse energy and duration. The heat accumulation effect is noticed at higher repetition rates. Although it overbalances the particle shielding thus making the drilling process faster, heat accumulation is responsible for the formation of a large amount of molten material that limits the hole quality. The variations of the pulse duration reveal that heat accumulation starts at higher repetition rates for shorter pulse lengths. This is in agreement with the observed higher ablation efficiency with shorter pulse duration. Thus, the shorter pulses might be advantageous if highest precision and processing speed is required.

Second generation OH suppression filters using multicore fibers

Ground based near-infrared observations have long been plagued by poor sensitivity when compared to visible observations as a result of the bright narrow line emission from atmospheric OH molecules. The GNOSIS instrument recently commissioned at the Australian Astronomical Observatory uses Photonic Lanterns in combination with individually printed single mode fibre Bragg gratings to filter out the brightest OH-emission lines between 1.47 and 1.70μm. GNOSIS, reported in a separate paper in this conference, demonstrates excellent OH-suppression, providing very “clean” filtering of the lines. It represents a major step forward in the goal to improve the sensitivity of ground based near-infrared observation to that possible at visible wavelengths, however, the filter units are relatively bulky and costly to produce. The 2nd generation fibre OH-Suppression filters based on multicore fibres are currently under development. The development aims to produce high quality, cost effective, compact and robust OH-Suppression units in a single optical fibre with numerous isolated single mode cores that replicate the function and performance of the current generation of “conventional” photonic lantern based devices. In this paper we present the early results from the multicore fibre development and multicore fibre Bragg grating imprinting process.

Industrial perspectives of ultrafast fiber lasers

The current status and perspectives of ultrafast fiber laser systems to be applied in production technology are reviewed. An ytterbium-doped ultrafast fiber chirped pulse amplification system generating 500 fs pulses with a pulse energy of 100 muJ at repetition rates up to almost 1 MHz is reported. Possibilities for a further power scaling using novel fiber designs based on microstructuring of the fiber are discussed. With such systems a significant increase in the processing speed in ultrashort pulse high precision micro-machining will become possible. Hence, fiber lasers offer the potential to revolutionise the high precision production technology by enabling an economic use of ultrashort pulse technology.

Coupling management of fs laser written waveguides

The evanescent coupling of femtosecond laser written waveguides with elliptical and circular shape is investigated in detail. Elliptical waveguides are used to investigate directional tuning of the coupling properties in a square array by tilting the elliptical waveguides. This allows to specifically pronounce diagonal coupling. In contrast, directional insensitive coupling is demonstrated in a circular waveguide array based on circular waveguides.

Rod-Type Fiber Laser

We report on a novel ytterbium-doped fiber design which combines the advantages of rod and fiber gain media. The fiber design has the outer dimensions of a rod laser, meaning a diameter in the range of a few millimeters and a length of just a few ten centimeters, but including two important waveguide structures, one for the pump radiation and one for the laser radiation. 120 W output power in single-mode beam quality are obtained from a 48 cm long fiber cane, corresponding to an extracted power of 250 W per meter. The fiber has a significantly reduced nonlinearity and allowing therefore scalability of the performance of high peak power fiber laser and amplifier systems.

High power photonic crystal fiber laser systems

We review the advantages of rare-earth-doped air-clad photonic crystal fibers for power scaling of fiber laser systems with excellent beam quality. Most recently, the output power of single-mode fiber lasers has been significantly increased (>500 W). Further power scaling is usually limited by damage to fiber end faces, thermo-optical problems or nonlinear effects. Microstructuring the fiber adds several desirable features to the fiber to overcome these restrictions.

Photonic Crystal Structures in Ultrafast Optics

During the past several years the study of light propagation in photonic crystals has attracted a steadily growing interest. Here we evaluate the potential of low index contrast photonic crystals realized in film waveguides and fibers. Based on the characterization of these structures (attenuation, mode profiles and dispersion) we discuss various applications in ultrafast optics.

Precise laser ablation with femtosecond pulses

The advantages of ultrashort laser pulses in material processing has already been demonstrated for a variety of materials.1-3