A method for non-contact 3D form testing of aspheric surfaces including determination of decenter and wedge errors and lens thickness is presented. The principle is based on the absolute measurement capability of multi-wavelength interferometry (MWLI). The approach produces high density 3D shape information and geometric parameters at high accuracy in short measurement times. The system allows inspection of aspheres without restrictions in terms of spherical departures, of segmented and discontinuous optics. The optics can be polished or ground and made of opaque or transparent materials.
A non-contact optical scanning metrology solution measuring aspheric surfaces is presented, which is based on multi wavelength interferometry (MWLI). The technology yields high density 3D data in short measurement times (including set up time) and provides high, reproducible form measurement accuracy. It measures any asphere without restrictions in terms of spherical departures. In addition, measurement of a large variety of special optics is enabled, such as annular lenses, segmented optics, optics with diffractive steps, ground optics, optics made of opaque and transparent materials, and small and thin optics (e.g. smart phone lenses). The measurement instrument can be used under production conditions.
Quality control in the fabrication of high precision optics these days needs nanometer accuracy. However, the fast
growing number of optics with complex aspheric shapes demands an adapted measurement method as existing
metrology systems more and more reach their limits.
In this contribution the authors present a unique and highly flexible approach for measuring spheric and aspheric optics
with diameters from 2mm up to 420mm and with almost unlimited spheric departures. Based on a scanning point
interferometer the system combines the high precision and the speed of an optical interferometer with the high form
flexibility of a classical tactile scanning system. This enables the measurement of objects with steep or strongly changing
slopes such as “pancake” or “gull wing” objects. The high accuracy of ±50nm over the whole surface is achieved by
using a full reference concept ensuring the position control even over long scanning paths.
The core of the technology is a multiwavelength interferometer (MWLI); by use of several wavelengths this sensor
system allows for the measurement of objects with polished as well as with ground surfaces. Furthermore, a large
absolute measurement range facilitates measuring surfaces with steps or discontinuities like diffractive structures or even
segmented objects. As all the measurements can be done using one and the same system, a direct comparison is possible
during production and after finishing an object.
The contribution gives an insight into the functionality of the MWLI-sensor as well as into the concept of the reference
system of the scanning metrology system. Furthermore, samples of application are discussed.
An acrylamide-based photopolymer formulated in the Centre for Industrial and Engineering Optics has been investigated with a view to further optimisation for holographic optical storage. Series of 15 to 30 gratings were angularly multiplexed in a volume of the photopolymer layers with different thickness at a spatial frequency of 1500 lines/mm. Since the photopolymer is a saturable material, an exposure scheduling method was used to exploit the entire dynamic range of the material and allow equal strength gratings to be recorded. From this investigation the photopolymer layer's M/# was determined with regard to the recording geometry used. The temporal stability of photopolymer layers was studied in terms of diffraction efficiency and change of the reconstructed angle due to material shrinkage. In addition, the potential of the photopolymer as a holographic data storage medium was demonstrated by recording bit data-pages.
We review the crucial properties of a phase-encoded volume holographic storage system in terms of data quality and security, which are the key issues of any bulk memory system. Two major problems which need to be tackled in holographic storage systems in terms of data quality are the hologram erasure during readout and the data encoding schemes for error-free reconstruction.
We present a novel storage material, (bismuth tellurite crystals - Bi2TeO5), which has the potential to overcome the volatility problem and avoiding the need of any further fixing. Regarding data encoding schemes, we present a general approach of gray scale modulation coding in order to improve the data capacity in comparison to normal modulation coding, while the bit error rate maintains low.
Data security in a phase encoded system can be realized by exploiting its special multiplexing characteristics. We present different encryption techniques and investigate their decryption probability.
We present several encryption techniques for holographic data storage, using orthogonal phase-code multiplexing and random phase encoding. Our system is capable of storing page-oriented data based on the selectivity of orthogonal phase-codes in a photorefractive LiNbO3 storage crystal. In order to encrypt data, random phase multiplexing is added to the system. We proof that the combination of deterministic and random phase code multiplexing is possible with low cross-talk and take advantage of this technique for an extremely secure data encryption. Moreover, the potential of phase-code multiplexing to realize arithmetic operations is exploited for data encryption purposes.