The use of ultrashort laser pulses has found widespread attention in the microstructuring of transparent materials. Specifically,
the origin of refractive index changes in glasses and crystalline materials was extensively investigated. In LiNbO<sub>3</sub>,
which is an important material for nonlinear optical applications, the possibility of waveguide fabrication with fs laser
pulses was also shown. Recently, two distinct types of waveguides were discovered in LiNbO<sub>3</sub> which show different thermal
stability and optical properties. In one type, frequency doubling of 1064-nm radiation was demonstrated. Here, we
discuss the different origins of the two waveguide types and present results of thermal annealing experiments. Furthermore,
the influence of the processing parameters and the focussing on the properties of the waveguides was investigated. The
electrooptic coefficient of the waveguide was measured and gives evidence that the nonlinear properties of the crystal are
depleted by the laser structuring.
We developed a fabrication process for microoptical elements with continuous profiles. In contrast to gray tone lithography with the cost intensive HEBS-glass or direct writing by laser or electron beam and the closely connected expensive equipment, the presented technique allows a low budget fabrication of continuous profiles with smooth surfaces. We use conventional binary photolithography with standard DNQ-Novolak based photoresist, simple smoothing techniques and proportional transfer by dry etching. All variations of the procedure are based on the local depth control with the local filling factor of a periodic pattern in a binary photomask. The filling factor of the mask defines the resist volume, which corresponds to an effective layer thickness. With the aid of smoothing techniques after development, the effective resist layer thickness is transformed to the real local profile thickness. Thus, continuous change of the filling factor in the periodic mask pattern results in a smooth height profile. Furthermore, it is possible to fabricate continuous height profiles with just one lithographic step if the mask pattern can not be resolved by the exposure-system. This can be achieved by the use of smaller periods or by increasing the gap between mask and substrate. The need of further surface smoothing depends on the smoothness demands. With the help of continuous resist profiles, fabricated by smoothing of binary resist patterns and also by using non-resolvable masks, combined with further binary structuring after the proportional transfer, three dimensional waveguide taper for low loss fiber-waveguide coupling via mode matching were successfully manufactured.
We present a technique for the fabrication of small period structures using a near field holography setup. Using a two-dimensionally structured phase mask, the creation of two-dimensional hole or dot arrays was possible with one single exposure step. In order to get a high contrast interference pattern, the mask parameters were optimized by rigorous calculation to achieve equal transmission efficiency in the respective diffraction orders. The mask generation was done by electron beam lithography and ion beam etching. We have made exposures with two different setups. The first setup is an exposure with normal incidence, where the interference of the four first diffraction orders is used. The second setup uses the zeroth and first diffraction order interference of a conical incident beam.
We report on a novel technique for hardening micro-optical resist elements, like beam-shaping elements or micro lenses, with large profile depths before proportional transfer into the fused silica substrate. This technique allows to harden the resist with only small distorsions of the height profile. For demonstration a refractive beam shaper was designed and fabricated in photo-resist using gray-tone lithography. This element was transfered into fused silica with high etch rates using an optimized set of parameters in an ICP-etcher.
In many laser diode applications, it is necessary to make a beam shaping or beam transformation. One example is the collimation, but often we wish to achieve additional properties like special shapes of the beam. Such beams can be designed with high efficiency and signal quality by means of refractive beam shaping elements. Frequently, we have to vary the beam propagation parameters significantly to fulfil the beam shaping task. If we want to use refractive beam shaping elements, the design results in an element with a large profile depth. A well suited fabrication method for refractive beam shaping elements is the gray tone lithography, however, it is limited by the achievable depth of profile. This means that design and fabrication methods should be taken into account to achieve the advantages of refractive elements. On the one hand, we have to improve fabrication technique for enlarging the producible profile depth. On the other hand, we have to use all of the design freedoms to reduce the profile depth. We will present results of the design and fabrication of a refractive beam shaping element with a profile depth up to 60micrometers to transform a laser diode beam into a line intensity distribution.