Dagmar Esser

Time dynamics of burst-train filamentation assisted femtosecond laser machining in glasses.

Bursts of femtosecond laser pulses with a repetition rate of f = 38.5MHz were created using a purpose-built optical resonator. Single Ti:Sapphire laser pulses, trapped inside a resonator and released into controllable burst profiles by computer generated trigger delays to a fast Pockels cell switch, drove filamentation-assisted laser machining of high aspect ratio holes deep into transparent glasses. The time dynamics of the hole formation and ablation plume physics on 2-ns to 400-ms time scales were examined in time-resolved side-view images recorded with an intensified-CCD camera during the laser machining process. Transient effects of photoluminescence and ablation plume emissions confirm the build-up of heat accumulation effects during the burst train, the formation of laser-generated filaments and plume-shielding effects inside the deeply etched vias. The small time interval between the pulses in the present burst train enabled a more gentle modification in the laser interaction volume that mitigated shock-induced microcracks compared with single pulses.

In-volume waveguides by fs-laser direct writing in rare-earth-doped fluoride glass and phosphate glass

Refractive index modifications are fabricated in the volume of rare-earth-doped glass materials namely Er- and Pr-doped ZBLAN (a fluoride glass consisting of ZrF4, BaF2, LaF3, AlF3, NaF), an Er-doped nano-crystalline glass-ceramic and Yb- and Er-doped phosphate glass IOG. Femtosecond laser radiation (τ=500fs, λ=1045nm, f=0.1-5MHz) from an Ybfiber laser is focused with a microscope objective in the volume of the glass materials and scanned below the surface with different scan velocities and pulse energies. Non-linear absorption processes like multiphoton- and avalanche absorption lead to localized density changes and the formation of color centers. The refractive index change is localized to the focal volume of the laser radiation and therefore, a precise control of the modified volume is possible. The width of the written structures is analyzed by transmission light microscopy and additionally with the quantitative phase microscopy (QPm) software to determine the refractive index distribution perpendicular to a waveguide. Structures larger than 50μm in width are generated at high repetition rates due to heat accumulation effects. In addition, the fabricated waveguides are investigated by far-field measurements of the guided light to determine their numerical apertures. Using interference microscopy the refractive index distribution of waveguide cross-sections in phosphate glass IOG is determined. Several regions with an alternating refractive index change are observed whose size depend on the applied pulse energies and scan velocities.

In-volume waveguides by femtosecond laser double pulses in glass materials

Direct writing with focused laser radiation in transparent materials is a remarkable tool for a wide range of products in optical technologies. Optical properties like the refractive index of transparent materials are changed locally with the laser direct writing technique in a very precise way. By increasing the refractive index in the volume of glass materials beam guiding components like waveguides are realizable [1]. Waveguide lasers are producible by writing waveguides in laser active materials. Such passive and active components may be applied in different fields of optical technologies like diagnostic and optical systems in medicine, illumination, therapy or cosmetics.

3D nano and micro structures in transparent materials by in-volume femtosecond laser processing

Nano as well as micro structuring in the volume of transparent materials is enabled by ultrafast laser radiation. By laser radiation with pulse durations in the fs and ps regime multi photon processes are efficiently induced resulting in a high resolution of less than 1 µm3, a very low heat input and a high writing flexibility in all three dimensions. High transparent materials such as sapphire and glasses are locally modified in the volume to change the refractive index for optical applications or to increase the corrodibility selectively for the manufacturing of micro channels and micro structured parts for the use in micro systems and medical technology.The miniaturization of products for micro optics, the medical technology and micro systems engineering requires transparent components with structure sizes in the micrometer range and accuracies of approx. 100 nm. In-volume Selective Laser-induced Etching (ISLE) is an appropriate manufacturing process for micro machining of transparent materials such as sapphire and glasses, e.g. fused silica. By focusing the laser radiation (wavelength 1040 nm, pulse duration 500 fs, repetition rate 0.1-5 MHz, pulse energy <1 µJ) in the volume the material is locally modified. By scanning the laser focus with pulse overlap inside the material, connected volumes of modified material are created. The modified volumes are subsequently removed by chemical etching in a second processing step using aqueous solution of e.g. HF or KOH.Periodical nano structures (ripples, nanoplanes) are fabricated inside transparent materials exploiting fs-laser induced nano optics with potential applications such as nano sieves for filtering, optical gratings or functionalized internal surfaces.Direct laser writing of waveguides in transparent materials by fs laser radiation is well known. Further applications include crack-free 3D micro markings in transparent materials consisting e.g. of micro-gratings, which resu lt in a colorful experience in the observers eye due to diffraction and interference.To exploit the high manufacturing velocities possible with ultrafast lasers with repetition rates of several MHz a high speed scanning system with large numerical aperture and pre-compensation of spherical aberrations has been developed for in-volume nano and micro structuring. With this system high writing velocities (50-400 mm/s), small focus size (0.6-2.2 mm) and high precision (100-400 nm) are combined for the first time on rat her large scanning fields (0.6-1.5 mm) and material depth of up to 2 mm.Nano as well as micro structuring in the volume of transparent materials is enabled by ultrafast laser radiation. By laser radiation with pulse durations in the fs and ps regime multi photon processes are efficiently induced resulting in a high resolution of less than 1 µm3, a very low heat input and a high writing flexibility in all three dimensions. High transparent materials such as sapphire and glasses are locally modified in the volume to change the refractive index for optical applications or to increase the corrodibility selectively for the manufacturing of micro channels and micro structured parts for the use in micro systems and medical technology.The miniaturization of products for micro optics, the medical technology and micro systems engineering requires transparent components with structure sizes in the micrometer range and accuracies of approx. 100 nm. In-volume Selective Laser-induced Etching (ISLE) is an appropriate manufacturing process for micro machining of transparent materials such as...

Fabrication of micro-channels and waveguides by fs-laser direct writing in glasses

Compact lasers with emission e.g. in the visible spectral range are eligible for display technology, medicine and cosmetics. Erbium- and Praseodym-doped fluoride glass materials are eligible candidates for visible emission. Non-doped waveguides may find their use for beam guiding and shaping of diode laser radiation. For applications in micro-fluidics, micro mechanical systems and medical diagnostics micro-channels and hollow microstructures in fused silica are necessary. Micro-channels in fused silica and waveguides in laser active glass materials are fabricated with fs-laser pulses. The use of a high repetition rate laser and a fast scanning system potentially allow the fabrication of laser sources and micro-mechanical components with high speed.

Erbium-doped fluoride glass waveguides

Since green laser diodes are still not commercially available, alternative technologies like frequency doubling have to be used for compact green laser sources. The goal of this work is to develop diode pumped green erbium-doped glass up-conversion waveguide lasers, instead. Planar fluoride glass waveguides are fabricated using a spin-coating technology. Until now, planar fluoride glass film waveguides with thicknesses are achieved down to 30 mum. The waveguides were doped with up to 3 mol% erbium. Scattering losses of 0.2 dB/cm at 975 nm and strong green emission at 520 - 550 nm is observed in erbium-doped waveguides 50 mum in thickness.