A new method to create refractive index gradients in inorganic glass.

Barz, Andrea ; Bliedtner, Jens ; Heineck, Volker 等

1. INTRODUCTION

Conventional optical lenses mostly consist of homogeneous optical media; their refractive index is constant. Their functional qualities are generated by complicated surface geometries (spheric, aspheric). The market, however, more and more demands optics whose qualities can be systematically influenced. Complex lens shaping is no longer necessary through the application of gradient index (GRIN) lenses, as the lensing effect is generated by a constantly changing refractive index gradient in the material and thus only plane surfaces are required. In optical engineering such components are used for e.g. improving imaging performance, a better aberration correction as well as for collimating or focussing of laser beams.

Currently there are different methods to generate a refractive index gradient in glass. Besides the CVD procedure (chemical vapor deposition) other procedures are as well used such as neutron radiation, ion exchange (Hornschuh & Russel, 2004; Knittel et al., 2001; Messerschmidt, 2003; Messerschmidt, 2005) and electrophoresis of sol-gel mixtures (Schmidt, 2005). However, the dimensions of achievable gradients are limited. Moreover, the technologies are complicated and expensive.

2. EXPERIMENTS

The development of a new procedure had become necessary in order to manufacture GRIN-optics, which have a refractive index gradient over the whole material surface. The method is base on the assumption that ions diffuse in the glass through the impact of a centrifugal force. The diffusion is influenced by the viscosity of the glass as well as the molecular weight and the binding strength of ions in glass network.

This way refractive index gradients with linear or rotation-symmetric distribution can be generated depending on how the sample is positioned towards the rotation axis. Figure 1 illustrates the achievable gradients.

Glasses with low transformation temperatures and ions with high molecular weights (Pb, Ba) were selected for the experiments i.e. mainly heavy flint glasses, light flint glasses and heavy crown glasses. Besides their chemical composition especially the rheological behaviour of these glasses was interesting for the setting of the process parameters. The viscosity was determined through different procedures in the range from [10.sup.2]-[10.sup.14,5] dPaxs. Figure 2 shows the viscosity curves.

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A special device had been developed and mounted for processing the glasses under the influence of temperature and centrifugal force. It basically consists of two units with different functionalities. The first unit guarantees the rotation of the glass samples and contains a motor driving, by means of a v-belt, two metal shafts which are positioned one upon the other and guided through ball bearings. The upper shaft goes into a chamber furnace through a hole in the bottom and is linked to a metal flange which holds the mould for the glass samples to be processed. The mould which is centrically positioned on the shaft can be closed by a lid. The second unit controls the temperature and consists of a chamber furnace with integrated processor for realising defined heating and cooling rates. Figure 3 illustrates the described device.

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3. RESULTS

The measuring device SAG 80 of the company Zeiss, using the Toepler method, enabled a visualisation of the refractive index gradient within a certain range.

With this method an image is created through an illuminated slit. If a sample shows different refractive indexes, a further shifted slit image is created due to the light deflexion. Depending on the refractive index the deflexion with different angles occurs. A gradual fading out of the deflected light by means of a slit diaphragm enables an observation of the gradient through the shadow contours. Figures 4 a-d show the radial and continuous course of a refractive index gradient on a glass sample SF15.

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Refractive index profiles can be determined by the RNF (Refracted-Near-Field)-method. A focussed laser beam is coupled perpendicularly to the refractive index gradient into the glass substrate to be measured. A part of the light is refracted by the reference glasses lying under the glass substrate. Subsequently, the light passing the pin diaphragm is focussed onto a photo diode through a split lens system. The measured light intensity enables a calculation of the refractive index. As the refractive indexes of the immersion layer and the reference glasses are known, they serve for the calibration of the refractive index of the glass substrate to be measured. The refractive index profile for axially spun samples is shown in fig.5. Apparently the course is rotationally symmetric and spreads over the whole sample volume.

Within a narrow area adjacent to the centre of the spin axis a reduction of the refractive index could be detected, for which a reason cannot be given at present. It is assumed that only from the point of reduction the centrifugal force is high enough to induce diffusion and to push ions outwards. This would lead to the determined continuous rise of the refractive index up to the sample's edge.

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4. CONCLUSION

The presented investigations show the successful creation of a method to generate refractive index gradients over the whole sample volume. Depending on the positioning of the glass the resulting refractive index profile shows a rotationally symmetric or linear distribution. This method allows a simultaneous processing of several glasses with similar or different geometries which leads to a significant reduction of manufacturing costs.

5. REFERENCES

Hornschuh, S & Russel, C. (2004). Glasses fort he preparation of gradient index lenses in the [Na.sub.2]O-[Al.sub.2][O.sub.3]-[B.sub.2][O.sub.3]-Si[O.sub.2] system --hydrolytic durability, thermal and optical properties, Glass Sci. Technol., Vol77, No. 6, 283-288

Knittel, J.; Schnieder, L; Buess, G.; Messerschmidt, B. & Possner, T. (2001). Endoscope-compatible confocal microscope using a gradient index-lens system, Opt. Commun., V. 188, 267-273

Messerschmidt, B. (2003). Gradientenoptik--eine innovative Mikrooptik fur die Optoelektronik und die medizinische Bil derfassung, Photonik, No. 6, 54-57

Messerschmidt, B. (2005). Gradientenoptik--Innovative Mik rooptiken fur Laserdiodenstrahlformung und Sensorik, Laser Technik Journal, No. 3, 47-50

Schmidt, T. (2005). Herstellung von optischen GRIN-Komponenten durch Elektrophorese, doctor thesis, Universi tat Saarbrucken