Folded diffractive laser resonators with user-defined fundamental mode

The adjustment effort required for the alignment of laser resonators with arbitrarily shaped end mirrors can be significantly reduced by folding the resonator path half-way between the two end mirrors and by slightly tilting the resonator axis. Thus the resulting resonator consists only of one diffractive mirror and a plane mirror, and the separate alignment of both diffractive end mirrors with respect to each other can be avoided. This principle is demonstrated by a Nd:YAG laser with super-Gaussian output beam. Furthermore, it is shown theoretically that the influence of phase quantization effects on the beam quality can be strongly reduced as a consequence of the tilted resonator axis.     

Short-periodic measuring of great retardations with a modified Senarmont-method

Up to now measurements of retardations and of birefringence on flexible, anisotropic media (fibres, films) are done contactlessly, automatically and on-line. For new manufacturing processes of such media first of all fast (short-periodic) and free of false colours measurements are required for very large retardations. A simultaneous combination of the multicolour-Senarmont-method with the discrete Fourier-Analysis (DFA) is presented in this paper.     

Improving the brightness of a multi-kilowatt single thin-disk laser by an aspherical phase front correction

We report on results obtained with an aspherical mirror to compensate for the phase front aberrations of a cw thin-disk laser with a single disk in the resonator. A record output power of 5?kW with a beam quality suitable for laser cutting (beam propagation factor M2=9.2) has been achieved.     

Hybrid optics for focusing ultrashort laser pulses

We present a specially designed hybrid optics comprising refraction and diffraction effects for tight spatial and temporal focusing of ultrashort laser pulses. Both aims can be put into practice by having a high numerical aperture (NA=0.45) and low internal dispersion at the same time. We are presenting what we believe is the first experimental realization of such a hybrid short plus focusing optics. The focusing properties are compared with a commonly used microscope objective (20 X, NA = 0.45) in theory and experimentally.     

High performance diffraction gratings made by e-beam lithography

Gratings are essential components in different high performance optical set-ups such as spectrometers in space missions or ultrashort-pulse laser compression arrangements. Often such kinds of applications require gratings operating close to the technological accessible limits of today??s fabrication technology. Typical critical parameters are the diffraction efficiency and its polarization dependency, the wave-front error introduced by the grating, and the stray-light performance. Additionally, space applications have specific environmental requirements and laser application typically demand a high damage threshold. All these properties need to be controlled precisely on rather large grating areas. Grating sizes of 200?mm or even above are not unusual anymore. The paper provides a review on how such high performance gratings can be realized by electron-beam lithography and accompanying technologies. The approaches are demonstrated by different examples. The first example is the design and fabrication of the grating for the Radial-Velocity-Spectrometer of the GAIA-mission of the ESA. The second grating is a reflective pulse compression element with no wavelength resonances due to an optimized design. The last example shows a three level blazed grating in resonance domain with a diffraction efficiency of approximately 86?%.     

Extending standard mask lithography exposure technique to spherical surfaces

Similar to planar lithography, the use of a mask to produce multiple copies of a binary master sample is also possible in the case of spherical surfaces. Evidently, the spherical mask needs to have the opposite radius of curvature of the desired substrate, and additional problems arising from the curved geometry have to be taken into consideration. Inhomogeneities of the illumination impinging on the resist-coated surface negatively influence the exposure result. Ways of overcoming these difficulties to obtain satisfactory results for the implementation of the exposure in a conventional mask aligner are shown. Despite a lowered contrast due to back reflections and a varying distance between mask and substrate, exposure results of sufficient quality are achieved with the help of an adapted aperture and the use of water as an immersion fluid.     

Inscription of fiber Bragg gratings with femtosecond pulses using a phase mask scanning technique

We report on the fabrication of highly efficient fiber Bragg gratings (FBG) in non-photosensitive fibers based on nonlinear absorption of fs laser light. Up to 40 mm long gratings with a transmission of T=-25 dB at the Bragg reflection wavelength were obtained and their coupling constant determined by spectral analysis. Therefore, a phase mask scanning technique with appropriate control of the focus was established. PACS  42.60.-v; 42.62.-b; 42.65.Re; 42.79.Dj; 42.81.-i     

Unambigous measuring of great retardations with a modified Senarmont method

Tiltable compensators are often used for measuring of great retardations of media with birefringence. In case false colours do appear a destruction-free measuring would not be possible with this method. That is why we present a multi-colour or multi-Senarmont-method basing on a simultaneous or sucsessive measuring of the Senarmont-angle with two or more wavelengths and a subsequent minimization calculation. Two principal advantages will result:1. Retardations Γ λ will be measured unambigously. 2. Samples with false colours will be also measured without any destruction and unambigously. Important condition: The difference of the measuring-wavelengths can amount to at most 10 nm. Further advantages of the multicolour-Senarmont-method (opportunities for automatic contactless and short-period measuring [minimal integration time]) will be published later.