Innovative laser beam splitter: Holographic lens

A new type of beam splitter enables optical applications in laser technology and in continuous beam analysis of high-power lasers that up to now could not be implemented. The optical characteristics of this innovative beam splitter can be integrated into the lens subassemblies of laser processing centers.

The new beam splitter uses a buried holographic lens. With the function patterns tested to date, the division ratio reached was 1:1,000 and 1:10,000. The division ratio is virtually independent of the polarization. This innovative beam splitter [1] consists of two wedge prisms that are cemented with a gap; a holographic lens which acts as a thin phase lattice is positioned in the front prism (Fig. 1). In the first diffraction order, with a phase modulation of λ/40, an output coupling of about 0.1% from the beam is obtained. The output-coupled monitor beam is simultaneously deflected and focused. At the gap between the prisms the monitor beam is reflected and coupled out under 90° to the laser beam. The beam spot from the holographic lens corresponds exactly to the spot created by the laser lens of the processing laser.

Low beam output coupling

The division ratio and the focal length of the holographic lens can be adapted to specific customer requirements. One of the main characteristics of this product is the very low output coupling, achieved by calculating a computer-generated hologram in the first step and then simulating the holographic lens numerically. The holographic lens is calculated as an off-axis asphere. The hologram is plotted in the form of a photomask and transferred to the surface of the front glass prism using established lithographic processes. Thermic ion exchange [2] is used to create a thin phase lattice below the surface of the glass prism. The well-known and highly transparent BK-7 glass is suitable for this purpose [3]. The holographic lens is formed within the glass. Thus the beam splitting hologram is protected from external factors (buried) and the division ratio is non-sensitive to surface films (absorbates). The optical prism surfaces are non-reflecting in accordance with the state of the art.

Relief hologram

When fused silica prisms are used, plasma etching is a good thin-layer technique for manufacturing the holographic lens. Plasma etching creates very uniform etching profiles, even in hard materials.

The result is a relief hologram in the glass surface. To achieve a refraction efficiency of 0.1% for Nd:YAG lasers, an etching depth of about 45 Nm is required. Thus the relief hologram has a very low aspect ratio. An anti-reflective coating is applied to the structured surface at the same time as to the other glass surfaces. The output limits are expanded by the use of fused silica. This innovative beam splitter can be used in the UV spectral range and the NIR/SWIR range.

Successful testing

Functional prototypes made of BK7 glass were developed for the Nd:YAG laser within the scope of this research project. These were successfully tested in two laser systems in conjunction with a beam analysis system and a permanent output measurement system. The test setup includes a further laser beam analysis system, integrated directly before the laser lens in an Nd:YAG manufacturing laser. For this purpose, the available surface of the beam splitter is enlarged accordingly and the holographic lens is adjusted to a diameter of 14 mm.

The innovative beam splitter can also be used in a universal gauge (OEM) for optical output measurement and laser beam analysis.

These studies were supported by the European Regional Development Fund (ERDF).

References

[1] Patent DE-102 17 657, (2004) Patent DE-10 2006 059 217, (2008)

[2] C. Thoma, H. Schütte, T. Vahrenkamp, H. Kreitlow: "Diffractive optical elements in glass," Proceedings of the DGG Symposium, Leipzig, Glass Sci. Tech. 76 C2, pp. 71-76 (2003)

[3] C. Thoma, H. Schütte: "Strahlteiler mit diffraktiv optischem Element aus BK7-Glas," (Beam splitter with diffractive optical element made of BK-7 glass), Forschung und Innovation, 12/2006, pp. 10 -11 (2006)

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Authors

Professor Christoph Thoma worked in the optics industry for 9 years after completing his doctorate. In 1995, he was appointed a professorship at the Fachhochschule University of Applied Sciences at Oldenburg/Ostfriesland/Wilhelmshaven (FHOOW). He is currently head of the Labor für Technische Optik (Technological Optics Laboratory) in Wilhelmshaven, where his work involves diffractive optical elements (DOEs) in glass, within the scope of several EFRE projects.

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Graduate Engineer Helmut Schütte has been at the FHOOW since 1996, and is responsible for microtechnology and nanotechnology, as well as for microlithography, as an assistant in the Labor für Microfertigung (Microproduction Laboratory).

Graduate Engineer Ulrich Dickel was one of the inventors who worked on the development and qualification of the new beam splitter. Today he is on the scientific staff at the Institut für Technologietransfer Institute of  Technology Transfer in Wilhelmshaven.

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