Axicons are interesting optical elements for a variety of applications. Optical metrology, micro manipulation or laser beam shaping are examples for their wide field of use. Typically these optical elements are realized with refractive conical surfaces. An alternative are circular concentric diffractive structures that have several advantages in terms of fabrication and their application. Compared to refractive axicons the in-plane property of the diffractive variant allows for higher accuracy with regard to the deflection angle. The attainable deflection is directly dependent on the minimum structure size that can be realized. Direct laser lithography is a very convenient and cost effective method for the production of diffractive optics since it is very flexible in terms of substrate geometries that can be used. However, there is a limitation with regard to the smallest critical dimensions that can be manufactured. The limit for conventional laser direct writing system working with visible light sources is typical in the range of 0.5 µm. Thus for a wavelength of 633 nm the maximum first order diffraction angle is approx. 40 degrees. If one wants to increase the angle the structure period has to be reduced. There are several approaches for the realization of sub-micrometer periodical structures that make use of ultraviolet light sources or use advanced highly nonlinear photo resists that allow to shrink the area that interacts with the focused laser beam.
At the ITO a completely different approach that is optimized for the fabrication of rotational symmetric periodic structures has been realized. Instead of reducing the spot size, the substrate is exposed with a tiny interference pattern (figure 1) whose period matches the one of the final structure.
Fig. 1: Simulation of the expected exposure pattern performed with ZEMAX (a). Picture of the real interference pattern taken with the camera of the writing system (b)
The pattern is scanned ring wise over the substrate. Through careful subsequent stitching of a large number of ring patterns, the substrate is completely exposed. The advantage of this technique is the high writing speed that can be maintained through the comparably large interference pattern and does not drop for smaller periods.
Fig. 2: Subsequent stitching of rings
Furthermore, by means of an active fringe locking system one can achieve a high uniformity of the final structure.
The following two examples show applications that benefit from this novel technique:
In material processing, especially in laser cutting and drilling, it is beneficial to use radially polarized laser radiation. It has been shown that the process speed can be increased roughly by a factor of two. In a joint research project together with the “Institut für Strahlwerkzeuge” (IFSW) we implemented this technique with the goal to generate radially polarized laser light with state of the art thin disc lasers.
The working principle of the realized laser is based on intra cavity polarization selection by means of a sub wavelength axicon grating structure. Compared to the refractive solution the diffractive approach benefits from the low losses in the grating structures which is essential for the use in thin disc lasers.
Fig. 3: Schematic of the radially polarized thin disc laser. The polarizing grating mirror consists of sub wavelength axicon structures.
A schematic of the laser built at the IFSW is shown in figure 3. At the current state it delivers up to 275W of radially polarized light .
As a second example we present the application of diffractive axicon structures in optical metrology. Figure 4 shows the setup for the measurement of a precision conical mirror.
Fig. 4: Setup for measurement of a precision conical mirror (a). Photograph of the diffractive axicon structure integrated into the setup (b).
Measuring an optical surface with an interferometric null test requires a reference structure with the inverse optical function of the specimen to be tested, typically a computer generated hologram (CGH). Testing steep surfaces such as right angle cones requires a deflection angle of 45°, which leads to extremely fine hologram structures, making the CGH difficult to manufacture in a conventional writing process. Our novel writing technique allows to easily fabricate such structures with high uniformity and high writing speed Thus one can now focus on the optimization of the metrological aspects of this challenging task .
- Abdou Ahmed, M., Haefner, M., Vogel, M., Pruss, C., Voss, A., Osten, W., and Graf, T., "High-power radially polarized Yb: YAG thin- disk laser with high efficiency," Optics Express 19, 47-49 (2011)
- J. Ma, C. Pruss , M. Häfner , B. Heitkamp , R. Zhu , Z. Gao , C. Yuan , W. Osten, "A systematic analysis of the measurement of cone angles using high line density Computer-generated Holograms," Opt. Eng 50( 5), (2011)