Christof Pruss

Interferometer for precise and flexible asphere testing

A novel non-null interferometer for the precise measurement of aspheric surfaces is presented. In contrast to a classical interferometer, where only one test wavefront is used, the proposed system makes use of multiple test beams propagating under different angles through the interferometer. This allows the measurement of aspheric surfaces in a very short time, even for strong aspheres with deviations from the best-fit sphere of up to 900 microm. The non-null test configuration implies that any additional aberrations introduced by the interferometer have to be well characterized to precisely measure the asphere. Experimental measurements of a calibrated non-null interferometer on an aspheric element with 900 microm SAG deviation are presented.

Absolute interferometric test of aspheres by use of twin computer-generated holograms.

A complete absolute interferometric test of axially symmetric aspheres is presented. The method is based on a specially designed computer-generated hologram (CGH) that reconstructs an aspherical wave as well as a spherical auxiliary wave. Since both phase functions have the same symmetry and their pattern is simultaneously encoded, we call this type of multiplex hologram a Twin-CGH. The spherical wave is used for calibration. The aberrations of the spherical auxiliary wave are measured absolutely with either a spherical mirror or an absolute test for Fresnel zone plates. Thus the two types of aberration inherent in the CGH can be identified and separated from each other. The errors of the spherical wave can be transferred to those of the aspherical wave. Two different methods thatuse Twin-CGHs for absolute testing of aspheric surfaces are described. Test procedures are explained, equations are derived, and experimental results are presented. A mutual comparison of the two results and a comparison with the established N-position rotation test are given.

High-power radially polarized Yb:YAG thin-disk laser with high efficiency

Radially polarized beams with an output power of 275 W, M2=2.3 and an efficiency of about 52.5% were generated from an Yb:YAG thin-disk laser. An intra-cavity circular resonant waveguide grating was used as a polarization selective mirror inside the laser cavity. We report on the design and the fabrication using a scanning beam interference lithography system and discuss the calculated and measured performances of the presented polarizing grating mirrors.

Computer-generated holograms in interferometric testing

With surface-relief structures, optical functions that are required for radiation power management such as antireflection, light trapping, or light distribution and redirection can be obtained for new applications in solar energy systems and in displays. There, structures with submicrometer features must be distributed over large areas homogeneously. We address the design and the whole experimental process chain from the microstructure origination on large areas to the replication and the system integration in the specific application. Topics are antireflective surfaces for solar systems and displays, light trapping in polymer solar cells, sun protection systems for facades, and diffusers for projection displays and in glazing. For the microstructure origination we investigate the suitability of holographic recording in photoresist using a large-scale interferometer. We use an argon ion laser as a coherent light source at a wavelength of 364 nm. Periodic and stochastic interference patterns are recorded in positive photoresist with the interferometer setup. In the case of periodic structures, grating periods between 200 nm and 20 µm are realized. By carefully modeling the resulting resist profiles it is possible to originate even prismatic surface-relief profiles. Structures with good homogeneity are originated on areas of up to 4800 cm2 by optimizing the interferometer setup and the photoresist processing.

Calibration of a non-null test interferometer for the measurement of aspheres and free-form surfaces

The measurement of aspheric and free-form surfaces in a non-null test configuration has the advantage that no compensation optics is required. However, if a surface is measured in a non-null test configuration, retrace errors are introduced to the measurement. We describe a method to calibrate the test space of an interferometer, enabling to compensate retrace errors. The method is effective even for strong deviations from null test configuration up to several 100 waves, enabling the fast and flexible measurement of aspheres and free-form surfaces. In this paper we present the application of the method to the calibration of the Tilted Wave Interferometer. Furthermore, the method can be generalized to the calibration of other setups.

Testing of aspheric surfaces

Aspheric surfaces are becoming interesting for the reduction of elements in optical systems as well as for improving the quality of the image forming system. The fabrication process of aspheric surfaces has been improved. For optical testing of aspheric surfaces computer generated holograms (CGHs) are interesting and already used. To perform aspheric testing in the same accuracy as spherical surface testing, further improvements of the CGH-null test method are required. A new concept for testing aspheric surfaces with CGH-nulls, including a calibration of the system, will be described. To specify and verify CGH quality, systematic errors due to fabrication inaccuracies of the CGHs will be analysed. On the other hand, alternative methods that provide more flexibility but possibly less accuracy than the CGH-null technique are required. Potential alternative testing methods of aspherics will be discussed.

Single-layer resonant-waveguide grating for polarization and wavelength selection in Yb:YAG thin-disk lasers.

A single-layer resonant-waveguide grating consisting of a sub-wavelength grating coupler etched into a waveguide is proposed in order to achieve high polarization and high spectral selectivity inside an Yb:YAG thin-disk laser resonator. The designed structure was fabricated with the help of a Lloyds-mirror interference lithography setup followed by reactive ion beam etching down to the desired grating groove depth. The wavelength and polarization dependent reflectivity is measured and compared to the design results. The behaviour of the device at higher temperatures is also investigated in the present work. The device is introduced as the end mirror of an Yb:YAG thin-disk laser cavity. Output powers of up to 123 W with a spectral bandwidth of about 0.5 nm (FWHM) is demonstrated in a multimode configuration (M2~6). In fundamental-mode operation (TEM00 with M2~1.1) 70 W of power with a spectral bandwidth of about 20 pm have been obtained. Moreover, the degree of linear polarization was measured to be higher than 99% for both multimode and fundamental mode operation.

New design techniques and calibration methods for CGH-null testing of aspheric surfaces

The entire process of designing the test setup with a computer-generated hologram (CGH) and performing the CGH-null test of an asphere will be described in detail. Critical aspects in testing aspherics with CGH-nulls e.g. caustic of the asphere, influence of alignment errors, lateral distortion and lateral resolution of the measurement are discussed. Detection, specification and calibration methods of fabrication errors of the CGH-null are analyzed with respect to the fabrication technology of the CGH. For a high accuracy CGH-null test of an asphere, the errors of the CGH must be negligible or well calibrated. Techniques to calibrate the systematic error of the test setup are presented with examples. Experimental results of interferometric CGH-null tests of an asphere are presented.

Circular grating waveguide structures for intracavity generation of azimuthal polarization in a thin-disk laser

We report on the generation of beams with azimuthal polarization using resonant grating waveguide structures (GWSs) inside an Yb:YAG thin-disk laser (TDL) oscillator. Two different GWS concepts were used to select the polarization of the emitted beam. The first uses the resonant reflection principle, and the second is based on the leaky-mode approach already reported in our previous work. Up to 93 W and 103 W of output power were extracted from a TDL with an optical efficiency, η(oo), of 36.2% and 40.1% using the first and the second approaches, respectively. In both cases, a pure azimuthal polarization and a beam quality factor, M2, of about 2.2 were measured. The design, fabrication, and different experimental results, as well as the laser performances for both GWSs, are discussed in the present Letter.

Optical metrology: from the laboratory to the real world

Optical metrology has shown to be a versatile tool for the solution of many inspection problems. The main advantages of optical methods are the noncontact nature, the non-destructive and fieldwise working principle, the fast response, high sensitivity, resolution and accuracy. Consequently, optical principles are increasingly being considered in all steps of the evolution of modern products. However, the step out of the laboratory into the harsh environment of the factory floor was and is a big challenge for optical metrology. The advantages mentioned above must be paid often with strict requirements concerning the measurement conditions and the object under test. For instance, the request for interferometric precision in general needs an environment where high stability is guaranteed. If this cannot be satisfied to a great extent special measures have to be taken or compromises have to be accepted. But the rapid technological development of the components that are used for creating modern optical measurement systems, the unrestrained growth of the computing power and the implementation of new measurement and inspection strategies give cause for optimism and show that the high potential of optical metrology is far from being fully utilized. In this article current challenges to optical metrology are discussed and new technical improvements that help to overcome existing restrictions are treated. On example of selected applications the progress in bringing optical metrology to the real world is shown.