Hans Lauth

In-process surface measurement of replication material during UV curing

UV curing plays an important role in the optical shop for cementing optics or for the fabrication of replicated optics. Material shrinkage is a common but unwanted effect during UV curing, because it causes stress and form modifications. In order to minimize these effects it is useful to measure shrinkage and surface changes that occur during the UV curing process. For replicated optics, shrinkage and form modifications are usually measured by comparing the replica to the mould geometry after the curing process has finished. In this paper, a measuring method is presented, which enables the observation of shrinkage and changes of the surface shape in situ. The surface of UV hardening material, in this case a commercially available optical adhesive, is monitored interferometrically during UV curing. This enables observing the material effects that cause stress and form modifications in real time.

Novel method for computing reference wave error in optical surface metrology

Despite advances in various metrology tools, interferometry remains the method of choice for measurements of optical surfaces. Fizeau interferometers can achieve precisions of X/100 PV (and better) with proper environmental control. The quality of the reference surface, however, usually limits the uncalibrated accuracy to merely X/10 PV or so. Various methods have been developed for absolute (unbiased) surface testing, including the N-position, 3- flat, 2-sphere, and random average tests. The basic principle of these tests is that the reference wave error remains invariant when the part is moved. These tests as a rule require multiple parts and/or measurements at different positions. Sub-aperture stitching requires measurements at multiple positions, and thus in principle can measure reference wave error. QEDs stitching algorithm exploits this possibility to produce a measurement of the reference surface along with the stitched full-aperture phase. The precision mechanics of QEDs stitching workstation make it an excellent platform for performing conventional reference wave calibrations as well. Results obtained from the QED stitching algorithm are compared with other calibration methods performed on the same workstation. The mean results and uncertainties of the various methods are evaluated, and limitations discussed.

Compensation technology of glass molding accuracy

To realize high-precision glass molding, some themes are yet to be settled, including performance of molding machine, molding conditions, material and accuracy of molds. To get a desired product profile with predetermined accuracy, molds must be manufactured according to specified accuracy. The present molding machine has a high transferability of fine profile, but causes minor difference in profiles by thermal expansion and contraction of mold and glass material as it involves a heating process. We clarify the results of experiment and verification of the two methods , in consideration of a heat expansion difference, molds is manufactured beforehand and feedback of geometrical error of prototype mold product to mold profile.

Cleaning of parts for precision-optic and glass substrates before coating

The state of the art cleanliness of substrate surfaces prior to a coating process is decisive for its success. Any contamination of the surface affects the adhesion of the coating and leads to defects. The degree of cleanliness of the surface can not be expressed in numerical terms and can only be demonstrated by the use of suitable aids. The objective sought is freedom from residues and freedom from particles, as perfectly as possible. Uniform, reproducible quality is indispensable, even when the products supplied from the preceding stages of manufacture vary within relatively wide tolerances in terms of shape, size, nature and degree of contamination. The solution to this type of problem requires well-tried process technologies in user-friendly equipment which operates safely and economically. One well-tried cleaning procedure prior to coating is based on cleaning with aqueous solutions plus ultrasound, followed by drying. In the course of todays increasing awareness of environmental matters, processes which make use of solvents prior to coating have meanwhile disappeared completely from factories. Cleaning in aqueous solutions is carried out in accordance with precise cleaning mechanisms. Cleaning is always a multistage process, in which cleaning and rinsing stages alternate repeatedly. Modern multi-chamber cleaning plants are to be found in the optical and electronics industries and fine mechanics as well as in the high-vacuum coating area. (coated lenses and hard coating)

Effect of process parameters on surface morphology in MRF

Spot surface morphology can be considered as a footprint of the removal process in magnetorheological finishing (MRF). When properly processed, it can account for the performance of the MR fluid in polishing. Experiments were done using different conditions to vary removal rate and evaluate the resulting effect on microroughness and the overall spot surface morphology. Such experiments have been performed on two optical glasses, with several different MR fluids and a wide range of machine parameter settings. Atomic force microscope (AFM) measurements show that the surface morphology has a strong dependence on the abrasive type in the MR fluid. Interferometry measurements show that the roughness inside the spot increases with the rate of the material removal.

Characterizing optical polishing pitch

Characterization data for five experimental optical polishing pitch products were compared to those for corresponding standard commercial optical polishing pitches. The experimental pitches were tested for three physical properties: hardness, viscosity at 90°C, and softening point. A Shore A Durometerl test was used to measure hardness. Viscosity data were collected using a Stony Brook Scientific falling needle viscometer. Softening point was determined using the ASTM D3104-97 method. Results demonstrate that the softest and the hardest batches of the experimental grades of optical pitch are comparable to the industry-accepted standards, while the other grades of pitch are not. The experimental methodology followed in this research may allow opticians to rapidly compare different brands of pitch to help identify batch- to- batch differences and control pitch quality before use.

Development and demonstration of precision optical contouring by wet etch figuring

Wet etch figuring (WEF) utilizes surface tension gradients to confine and stabilize the footprint of an etchant solution contacting an optic, and uses real-time optical interferometry for closed-loop control the local dwell time of etchant on the optic. This enables precision figuring of arbitrary continuous contour optical shapes with gradients of up to 0.5 waves/ mm. This stress-free figuring method enables the figuring of ultrathin optics, and allows inexpensive thin sheet glass to become raw material for precision optics such as phase plates, aberration correctors and beam shapers. We present data on surfaces generated and discuss surface smoothness of optics manufactured by WEF.

Polishing rate of fused silica, compared to glasses BK7 and SF6

We discuss micromechanics contributions to the loose abrasive polishing removal rate in fused silica, and the multicomponent glasses (crown) BK7 and (flint) SF6. Nanoindentation reveals the contributions of hardness at depths in the range 2-6 nm.

Mechanical behaviour of silica fibers in various moisture conditions

The strength of silica optical fibers depends on the water activity at the glass surface. Polymer coatings are currently applied to optical fibers to protect them from mechanical damage during handling. They also act as a barrier diffusion of the surrounding humidity reaching the glass surface. Water is known to be one major factor of the propagation of cracks at fiber glass surface because it makes much easier the breaking of the Si-O bonds which build the vitreous network. The strength of the fiber is related to the water concentration at the glass surface. It is well known that flaws in glass subject to stress in humid condition grow subcritically. The crack velocity is related to the applied stress and also to the humidity rate [1]. The kinetic of the reaction between silica and water changed at very low water concentration [2].

Progress of modern interference layer design methods and coating technologies demonstrated by the manufacturing of uncemented, wide-angle thin film polarizers

The progress of modern interference layer design methods and coating technologies is demonstrated by the manufacturing of uncemented, wide-angle thin film polarizers. The requirements are to produce uncemented polarizers for 655nm and an angle of incidence of i=45° +/-6°. The dimensions are 9,5 x 8 [mm] and the surface flatness should be less than 2J15. The items should be manufactured in huge numbers to reasonable costs. The multilayer system was developed by modem design methods. The realisation of such complex design needs very stable properties of the coatings and a sophisticated monitoring system. For this purpose an ion-assisted coating technology was applied. The resulting coatings are shiftfree and mechanically resistant, which is important for the following cutting process. The cutting of large substrates into the required dimension allows the manufacturing of demanding elements to comparatively low costs. Special methods for mechanical stress compensation of the coating system were used to fulfil the demands on the surface flatness.