Stefan Nolte

InN as THz emitter excited at 1060 nm and 800 nm

InN, a novel semiconductor material, is used as THz surface emitter. The material is irradiated with fs-laser pulses at 1060 nm and 800 nm and the emitted ultrashort THz pulses are measured by phase sensitive detection. Pulsforms, amplitudes and spectra are compared to the THz emission of p-doped InAs, the standard material for THz surface emission.

Simultaneously spatially and temporally focusing light for tailored ultrafast micro-machining

Simultaneous spatially and temporally focussing (SSTF) of ultrashort pulses allows for an unprecedented control of the intensity distribution of light. It has therefore a great potential for widespread applications ranging from nonlinear microscopy, ophthalmology to micro-machining. SSTF also allows to overcome many bottlenecks of ultrashort pulse micro-machining, especially non-linear effects like filamentation and self-focussing. Here, we describe and demonstrate in detail how SSTF offers an additional degree of freedom for shaping the focal volume. In order to obtain a SSTF beam, the output of an ultrafast laser is usually split by a grating into an array of copies of the original beam, which we refer to as beamlets. The ratio of the beamlet array width to the width of the invidual beamlet is the beam aspect ratio. The focal volume of the SSTF beam can now be tailored transversally by shaping the cross-section of the beamlets and axially by choosing the right beam aspect ratio. We will discuss the requirements of the setup for a successful implementation of this approach: Firstly, the group velocity dispersion and the third order dispersion have to be compensated in order to obtain a high axial confinement. Secondly, the beamlet size and their orientation should not vary too much spectrally. Thirdly, beamlet and SSTF focus should match. We will hence demonstrate how SSTF allows to inscribe tailored three-dimensional structures with fine control over their aspect ratio. We also show how the SSTF focus can be adapted for various glasses and crystals.

Mitigation of stimulated Raman scattering in high power fiber lasers using transmission gratings

The average output power of fiber lasers have been scaled deep into the kW regime within the recent years. However a further scaling is limited due to nonlinear effects like stimulated Raman scattering (SRS). Using the special characteristics of femtosecond laser pulse written transmission fiber gratings, it is possible to realize a notch filter that mitigates efficiently this negative effect by coupling the Raman wavelength from the core into the cladding of the fiber. To the best of our knowledge, we realized for the first time highly efficient gratings in large mode area (LMA) fibers with cladding diameters up to 400 μm. The resonances show strong attenuation at design wavelength and simultaneously low out of band losses. A high power fiber amplifier with an implemented passive fiber grating is shown and its performance is carefully investigated.

High Repetition Rate Fiber Laser Systems for High Field Physics

We demonstrate the generation of XUV radiation with wavelengths down to 23.9 nm (52 eV) at 50 kHz via high harmonic generation of a fiber chirped pulse amplification system. At repetition rates of 100 kHz and 200 kHz we were able to obtain 27.8 nm (45 eV) and 35.5 nm (35 eV), respectively. Furthermore, we show latest experimental results on post compression of the same fiber chirped pulse amplification system via self-phase modulation in hollow core fibers pathing the way towards more efficient high harmonic generation with even shorter wavelengths. Pulses as short as 70 fs with 200 μJ pulse energy at 50 kHz and with 77 fs have been obtained owing a peak power of 2 GW.

Fast THz Imaging of Styrofoam

Imaging of styrofoam with the help of ultrashort Terahertz pulses is investigated. With a combination of pulse amplitude and time delay imaging it is possible to speed up the measurement about two orders of magnitudes.

Femtosecond written buried waveguides in silicon

The laser inscription of waveguides into the volume of crystalline silicon is presented. By using sub-ps laser pulses at a wavelength of 1552 nm highly localized light guiding structures with an average diameter ranging from 1 – 3 μm are achieved. The generated waveguides are characterized in terms of mode field distribution, damping losses and permanent refractive index modification. First investigations indicate an induced increase of the refractive index in the order of 10-3 to 10-2. Depending on the applied laser pulse energy single-mode to multimode like propagation behavior can be observed. At optimized processing parameters, the damping losses can be estimated below 3 dB/mm.

Selective laser melting of glass using ultrashort laser pulses

Within the field of laser assisted additive manufacturing, the application of ultrashort pulse lasers for selective laser melting came into focus recently. In contrast to conventional lasers, these systems provide extremely high peak power at ultrashort interaction times and offer both the opportunity of nonlinear absorption and the potential to control the thermal impact at the vicinity of the processed region by tailoring the pulse repetition rate. Consequently, transparent materials like borosilicate glass or opaque materials with extremely high melting points like copper, tungsten or even special composites like AlSi40 can be processed. In this publication, we present the selective laser melting of glass by using 500 fs laser pulses at MHz repetition rates emitted at a center wavelength of 515 nm. In order to identify an appropriate processing window, a detailed parameter study was performed. We demonstrate the fabrication of porous bulk glass parts as well as the realization of structures featuring thicknesses below 30 μm, which is below typical achieved structural sizes using pulsed or CO2 laser [1]. In contrast to alternative approaches [2], due to the nonlinear absorption and therefore complete melting of the material, there was no need for binding materials. This work demonstrates the potential for 3D printing of glass using the powder bed approach.

Selective laser melting of copper using ultrashort laser pulses at different wavelengths

Additive manufacturing gained increasing interest during the last decade due to the potential of creating 3D devices featuring nearly any desired geometry. One of the most widely used methods is the so-called powder bed method. In general, conventional cw and pulsed laser sources operating around 1030 nm and CO2 lasers at 10.6 μm are usually applied. Among other materials like polymers, these systems are feasible for several metals, alloys and even ceramics, but easily reach their limitation at a wide range of other materials, regarding required absorption and intensity. In order to overcome these limits, ultrashort pulse laser systems are one approach. Due to the increased peak power and ultrashort interaction times within the femtosecond and picosecond time range, materials with extraordinary high melting points, increased heat conductivity or new composites with tailored specifications are coming into reach. Moreover, based on the nonlinear absorption effect, also transparent materials can be processed. Here, we present the selective laser melting of pure copper using ultrashort laser pulses. This work involves a comparative study using 500 fs pulses at processing wavelengths of 515 nm and 1030 nm. The repetition rate of the applied laser system was varied within the MHz range in order to exploit heat accumulation. By using the ultrashort interaction times and tailoring the repetition rate, the induced melt pool can be significantly optimized yielding robust copper parts revealing thin-wall structures in the range below 100 μm.

Fs-written fiber Bragg gratings in multicore fibers for astrophotonic applications (Conference Presentation)

For astronomy, the search of exo-planets and spectral analysis of galaxies and stars with ground based telescopes is an ongoing topic especially with the future planned telescopes with even larger mirror diameters. But for the ground based observation the atmosphere is a limiting factor. Besides of air fluctuations also spectral noise is influencing the observation. A forest of emission lines by OH relaxations that are orders of magnitudes stronger than the stars and galaxies appear in the night sky. These lines vary in intensity but are fixed in wavelength. Therefore, fiber Bragg gratings (FBG) are a perfect tool for the suppression of these emission lines. FBGs provide a high filter quality by filtering out magnitudes of intensity in a narrow wavelength window of below 0.5nm bandwidth. Because fibers are already widely used in telescopes to deliver light into spectrographs, the FBGs could be inscribed directly into the fibers. But for astronomy mostly multimode fibers are used where FBGs do not work as needed because of the different propagation constants of higher modes. The solution is the transition to single mode fibers. In terms of compactness and robustness a multicore fiber would be the optimal solution. But the homogeneous modification of a multicore fiber is a challenging task. We report on the ultrashort pulse laser inscription of FBGs into a multicore fiber consisting of 7 cores. Furthermore, investigations on the homogeneity of the inscribed modifications as well as the spectral properties are presented. I would like to participate in the Student Competition.

Implementation of quantum discrete fractional Fourier transform

In this work we experimentally demonstrate the realization of the discrete fractional Fourier transforms (DFrFT) in both the classical and quantum realm. Our approach is fully integrated and free of bulk optical components.