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Baden-Württemberg Stiftung





twip optical solutions GmbH



Institut f. Technische Optik
Universität Stuttgart
Pfaffenwaldring 9
70569 Stuttgart

T: +49 (0)711/685-66074
F: +49 (0)711/685-66586



The race to the realization of efficient, simple, cost-effective and novel diffractive optical elements (DOE) for processing or forming cylindrical waves has just started. One particular example of such DOE is a polarizer that filters out azimuthal polarization distribution from an unpolarized or circularly polarized incident beam while allowing the radially polarized distribution to pass through. This has gained a lot of interest because of its several foreseen applications for example, in laser material processing. Such DOE already exist; however, the fabrication process to manufacture them still requires improvement in the context of simplicity and cost while not compromising the DOE’s performance.

Within the SubWell project, the fusion of novel DOE conceptual designs and straightforward, cost-effective corresponding fabrication processes is studied. Three design variants were provided by the Institut für Strahlwerkzeuge (IFSW). Depending on the design, the DOE components can be used in terms of either the internal or external resonances in laser operation. The first design is the grating waveguide output coupler (GWOC) which is used inside the resonator and aims to select radially polarized distribution. The second design is a broadband intra-cavity grating mirror that converts the polarization and is based on sub-λ lattice structures. The third design is a transmissive polarization converter with a lattice periodicity that is much smaller than the wavelength of light.

These designs are implemented in Institut für Technische Optik (ITO) in which laser lithography and plasma etching techniques are utilized. In the first design, the circular grating lines were written using a direct laser writing technique on
a resist-coated wedged fused silica (FS) substrate and then patterned by inductively-coupled plasma (ICP) etching. Alternating layers of Ta2O5 and SiO2 were then deposited on the etched FS substrate with an antireflection (AR) coating at the backside. Table 1 shows the clean intensity distribution (LG01/ doughnut mode) of a beam was extracted out of the 1st generation of GWOCs [1].

Grating period (nm)
Etch depth (nm)
Intensity profile of the extracted beam
Grating waveguide
output coupler
900 30 subwell2

Table 1: Grating parameters for the 1st generation of GWOCs.
A clean and perfectly uniform LG01 (doughnut mode) intensity
distribution was extracted out of the 1st generation of GWOCs [1].

Scanning beam interference lithography (SBIL) will be used to realize the structures with relatively small periodicities of the second design. To implement the third design which is the transmissive extra-cavity polarization converter, a new interference-lithographic method, the socalled Scanning Mask Interference Lithography Exposure (SMILE) has been developed and tested successfully in ITO. The motivation behind this development is to reach the requirements set by the
design which demands a grating period of 300 nm and an etch depth of 1 μm. Figure 1 shows the schematic of the lattice design, the actual setup and an element after photoresist (PR) patterning. With this setup, the required complex lattice
structure was written in a few exposure steps at a wavelength of 405 nm.

subwellFig. 1: (a) Schematic of the designed extra-cavity polarization
convertes utilizing the form- birifringence with Ta2O5
grating. (b) SMILE setup used to fabricate the (c) sector
structure with linear gratings.

Supported by: AiF
Project: SUBWELL (IGF 18728 N) In cooperation with: IFSW, University Stuttgart, within the programme for sponsorship by Industrial Joint Research (IGF) of the German Federal Ministry of Economic Affairs and Energy based on an enactment
of the German Parliament.


[1] Eckerle, M., Dietrich, T., Schaal, F., Pruß, C., Osten, W., Ahmed, M.A.; Graf, T.
“Novel thin-disk oscillator concept for the generation of radially polarized femtosecond laser pulses”,
Opt. Lett. 41, 1680 (2016).