GRIN-Wrap

Project GRIN-Wrap

“GRIN-Wrap” is an innovative research project with the aim of developing an advanced manufacturing technique for large aperture GRIN-lenses which possess multiple advantages compared to already-existing GRIN-lenses and conventional lenses.

What is it about?

Materials research, optics simulation, and manufacturing processes have given rise to a global business whose primary products are eyeglasses and telescopes. Optical data storage, optical communication, digital cameras, sensor technologies, and LED lighting have all seen a move toward plastic lenses in recent decades, resulting in substantial cost reductions and the opening of new mass markets. However, very little has changed in the fundamental properties of lenses; lenses are still usually made of a material with a specific refractive index and surface curvature.

But what does this have to do with GRIN-lenses?

The way a GRIN (Gradient Index) lens works can be best explained by taking a closer look at how conventional lenses operate.

With conventional lenses light is refracted when it meets the curved surface of the transparent lens material (typically glass) and then refracted again when emerging through the exit surface of the material. These refractions that are caused by the unique refractive index of the lens material then cause the light rays to be focused on a single spot, resulting in the creation of an image. However, conventional lenses normally either have the characteristic of near- or far-vision or a non-seamless combination of both in the lens (this is typical with reading glasses where one must switch gaze from top to bottom of the glass to read things that are either nearer or further away).

Furthermore, the production of conventional lenses is extremely high. Amongst other things, that is due to the precision required in creating the surface of the lens.

GRIN lenses are fundamentally different. They are typically lenses which are made from polymers and possess a radially varying refractive index– typically radially decreasing from the middle towards the edge –which means the light rays are continuously being bent within the lens material till they all focus on a single spot. All this without needing to have a curved surface such as conventional lenses do. Added curvature, however, can be incorporated as an additional characteristic when creating a GRIN lens, depending on the requests of the costumer and the area of application.

The benefits of this less conventional form of lenses are that there is greater freedom in the materials and curve of the lenses while maintaining the same focal length. Furthermore, they have a seamless near- and far-vision within one lens– similar, but not limited, to how a healthy human eye works. Reduced aberration errors, low-cost manufacturing procedures, and lightweight materials are all advantages.

What is special about our GRIN-lenses?

GRIN lenses are not a new phenomenon. As of today, there already exist some manufacturing processes through which GRIN lenses can be produced. Processes such as using ions that diffuse through the surface of a glass rod such that its refractive index is modified or creating a GRIN lens with the help of partial polymerisation. But so far, these methods have either proven to be exceptionally costly, are not able to produce lenses of sufficient width or cannot develop lenses with a stable, radially varying refractive index.

Project “GRIN-Wrap” aims to create a unique, low-cost manufacturing process that utilizes film extrusion with successive wrapping to create a successful prototype of GRIN lenses with a large aperture.

What is the main idea?

The main idea is to mix two polymers to get a constant linearly change in the materials refractive index. Extrusion is a process in which a material is forced through a series of dies to create the wanted shape. The mixture of the Polycarbonate (PC) and Polymethylmethacrylate (PMMA) is put together with gravimetric dosing. This means the dosing is based on weighing the raw material. In the beginning a so-called Hopper will be filled with the mix of polymer pallets. Then a system of twin-screws forces them through a barrel where the melting process of the pallets takes place. At the end of the screws the molten mixture is pressed through a predefined cross-section area. In this case a thin foil is the shape of desire. The outcoming foil is then redirected a few times over rolling cylinders before it is wrapped around a cylindrical paper roll.

After the extrusion the material is still at a certain heat. The goal is to use this heat for the next step of sintering.

Heat is nothing else than energy and as we know, energy is never lost. It is simply transformed from one form to another.

Every material has a desirable sintering point, which is the temperature and pressure at which the porosity gaps between the material particles are reduced and the loose material is compressed into a solid lump. Sintering is a heat treatment method in which (loose) material is compressed into a solid piece by exposing it to high temperatures and pressure.

This process aids in the formation of a compact mass without melting the material to liquefaction point.

Compressing snow to produce a compact snowball is analogous to this. The heat and pressure required for this sintering process, as we all know, is below the melting point of ice.

During this innovative sintering process, the material will be wrapped with possibly no gaps and then sintered to one compact material.

The next step is to check if the desired change of the refractive index has been achieved. For this a small slice of the compact roll, of the now sintered polymers, is cut off and analysed with Raman spectroscopy. This method is a non-destructive chemical analysis technique. Raman spectroscopy provides detailed information about the molecular interactions within a compound. It is based on an interaction of light with the chemical bonds of the chosen sample. Category wise Raman spectroscopy belongs to the vibrational spectroscopy. The light beam excites molecular vibration which then later can be analysed and interpreted with special software. This light beam has a particular wavelength and almost all this light will remain at this particular wavelength. But a part of it will “scatter” and its frequency will differ. The Raman spectrum results from a range of those scanned wavelengths.

Necessary experiments for the further development of the product and production process

Differential Scanning Calorimetry

DSC is a valuable technique for studying the physical properties and thermal transitions of polymer materials, such as melting and mesomorphic transitions. DSC can be used to calculate their entropies and enthalpies.

Most of the DSC equipment consists of a thermal scanning chamber and a computer. In the thermal scanning furnace chamber, two different trays are used for heating and cooling, respectively. The sample is kept in the first chamber to be processed, while the second chamber which is left with the reference material Iridium.

Tg (glass transition temperatures) and variations in Cp (heat capacity) can be measured using DSC, as well as other impacts in polymer materials. This analytical apparatus uses calorimetry to quantify thermal transitions, Cp, and enthalpy.

Brewster Angle

It indicates the optimal level of polarization that can occur within the refracting material. When light enters a medium, this is the angle at which the least quantity of scattered light occurs due to refraction.

References

https://microbenotes.com/types-of-spectroscopy/#19-raman-spectroscopy

https://www.edinst.com/blog/what-is-raman-spectroscopy/

https://www.bruker.com/en/products-and-solutions/infrared-and-raman/ram…

https://www.sciencedirect.com/topics/materials-science/differential-sca…

https://www.linseis.com/en/methods/differential-scanning-calorimetry-ds…