Waveguide Structure

The structure of the waveguide has three essential categories; the geometry of the waveguide in its entirety, including its area and thickness, the geometry of concentrator microstructures, and geometry of the diffuser microstructures. There are numerous configurations for each of these parameters which result in high efficiency controlled guidance of light, and thus this design can be adapted for many applications. The important parameters are discussed in this section, followed by the relevant equations.

4.2.1 Waveguide Layering and Material Selection

Essential to the waveguide’s ability to confine light to the transmission layer of the waveguide, is a difference between the refractive indices of the upper layer (n1) and the

lower layer (n2). In a layered waveguide, it is therefore necessary that the material used

for the waveguide’s transmission layer has a higher refractive index than the upper layer. The difference between refractive indices n1 and n2, will determine the maximum

allowable angle of propagation to ensure confinement of the light to the transmission layer, thus a greater difference will permit a larger range of angles and greater flexibility of the waveguide.

Also important to the material selection is the interface between the layers. It is necessary that the layers are able to bond without the use of an adhesive, which would inhibit the optical performance of the interface. Because PDMS is an optically transparent polymer with an adjustable refractive index, it is an optimal material to use for this flexible, layered design.

4.2.2 Waveguide Area and Thickness

The waveguide’s surface area and thickness are also essential to the optimization of its performance. One of the unique aspects of this particular waveguide design is its ability to cover large areas, it can be used to cover areas up to 1m2 _{and beyond. Not only can the}

micro-features be patterned over large areas, but the design is also fully scalable so it may be scaled up to cover large areas without an accompanying decrease in transmission efficiency, or it can be scaled down to reduce the thickness of the waveguide.

Additionally, the waveguide’s area and thickness are important as the ratio of the concentrator region’s surface area to the thickness of the lower layer (t2) represents the

geometric concentration factor of the collector. The geometric concentration factor is simply the input area divided by the output area, therefore for a higher concentration ratio, the surface area may be increased or the thickness may be decreased. While both these modifications will increase the likelihood that a propagating ray strikes a subsequent coupling pyramid, thereby decreasing the waveguides transmission efficiency, if a high factor of concentration is necessary, it can easily be obtained with minimal changes to the waveguide geometry.

The thickness of the diffusing waveguide must be equivalent to that of the lower layer, t2, to ensure 100% transmission between the concentrating and diffusing regions. A

thinner diffuser waveguide will result in both a higher efficiency waveguide, and a shorter diffusion length, therefore a thinner t2, is desirable. With regard to the diffuser

area, it is presumed that in order to maintain a level of illumination intensity achieved by the collector, the diffuser length, and therefore area, should be minimized. This ensures that the concentration factor of the hybrid waveguide is maintained, while upholding a high diffuser efficiency.

It is important to define the desired factor of concentration and surface area for a given application, as these parameters are used to define the geometry of the optical microstructures and the related waveguide geometry.

4.2.3 Microstructure Functionality

Both the collecting and illuminating regions of the waveguide are patterned with microfeatures which control the ray path in order to guide the ray path as desired. The waveguide’s efficiency and uniformity are largely dependent on the microstructures’ effectiveness, thus their functionality and design must be analyzed thoroughly.

4.2.3.1Concentrator Microfeatures

The role of the concentrator (Figure 4.2) is to collect light over a large region and transmit it efficiently to the edges of the two layer structure where it enters the diffuser that creates an illuminating output on the waveguide sheet. An effective and efficient design for the concentrator depends on the geometry and position of the microlenses imprinted on layer 1 and the optical reflective structures (i.e. pyramids) embedded in layer 2 [38]. Both the geometry of the microfeatures and the geometry of the entire concentrator must be considered for the optimization of the waveguide. The waveguide thickness, surface area and concentration ratio must all be considered when designing the concentrator waveguide. The geometric concentration ratio of the waveguide is given by the input area, or surface area, of the concentrator, divided by the output area or the sum of the area of the concentrator faces. Since only the bottom layer of the waveguide consists of the active concentrator faces, the concentration ratio is dependent on the thickness of the bottom layer only. To achieve a high concentration ratio, a large collection area for the concentrator is desirable, with a thin active layer of the waveguide. While these targets will increase the concentration ability of the waveguide, they will also decrease its efficiency by increasing decoupling losses [4].

Figure 4.2 Illustration of the concentrator’s functionality; the concentrating lenses

focus the light onto the coupling prisms, which reflect the concentrated rays into the waveguide at such an angle that they are confined to the bottom layer of the waveguide.

4.2.3.2Diffuser Microfeatures

The diffuser design (Figure 4.3) incorporates a series of wedge features which reflect the light off their surface, causing the light to refract out of the waveguide. The propagating ray strikes the sloped face of the diffuser feature and is totally internally reflected off this face at such an angle that it refracts out of the waveguide upon striking the illuminating surface. In order to optimize the performance of the diffuser, the efficiency of the waveguide and the uniformity of illumination must be considered. These characteristics are primarily affected by the shape, size and spatial distribution of the wedge features. The shape of the wedges is governed by the condition that the face of the wedge features must be tilted at an angle, θd, small enough that the incident light is reflected off the

features, but large enough that the ray is diffused out of the illuminating face upon reflection [39].

Figure 4.3 The size and distribution of the wedge features must be optimized to enhance the uniformity of the diffuser’s illumination, and the length of the diffuser region must be optimized for both its efficiency and uniformity.