Optical System Simulation and experimental results

In this section, we first simulated the optical performance of a LED chip with a microlens in Figure 4-3 by using the commercial computer aid design software,

Figure 4-2 The fabrication process. (a) The bared LED chip. (b) Wire bonding and PDMS encapsulation. (c) PDMS hardening. (d) A layer of SU-8 base layer. (e) UV/ozone treatment through shadow mask. (f) Removing the shadow mask. (g) Dipping out from diluted SU-8 solution. (h) UV curing.

base layer, and SU-8 microlens. The lens diameter was 182 mm and the LED chip was 200 μm by 300 μm. The radius of curvature of the microlens was 1.83 mm. The detector plan (30 mm x 30 mm) was 30 mm away from the LED chip. A close-up drawing of the simulation model is also shown at the right hand side of Figure 4-3. The simulation results of optical illuminance are presented in Figure 4-4. The illumination pattern form a LED without a microlens was shown in Figure 4-4(a) and Figure 4-4(b) is the illumination patter after a microlens was placed on the LED chip. From the simulation result, it reveals that the peak luminance was approximately 15 % improved.

Figure 4-3 The optical system of microlens on LED.

As described above, the contact angle became smaller with longer UV/ozone treatment time. Figure 4-5(a) shows the experimental result of contact angles of diluted SU-8 photoresist with zero, one, two, three, and four minutes UV/ozone treatment time on an SU-8 photoresist base layer before UV curing. The measurement system was based on the Sessile Drop Method, which was set up by a uniform backlight, a digital camera (R10, Ricoh, Japan), and the software programmed by LabVIEW^®, as shown in Figure 4-5(b). The program detected the two endpoints of the footprint and the spherical centroid of the profile of droplet, where the profile was assumed to be spherical because the drop was small so that the gravity can be neglected. Thereafter, the contact angle was derived by the three points. From the result, the contact angles reached its minimum value when UV/ozone treatment time was longer than three minute. The scanning electron microscope (SEM) photograph is shown in Figure 4-6. It shows that the 200-μm diameter MLA with three minutes UV/ozone treatment time was fabricated

Figure 4-4 The simulation result of luminance map (30 mm x 30 mm). (a) Without microlens (b) With microlens.

Figure 4-5 (a) The experimental results of contact angles between a non-UV-exposed diluted SU-8 photoresist droplet and an SU-8 photoresist base layer. (b) The

experiment setup of Sessile Drop method.

Two dimensional profile of MLA which was measured by surface profiler (Alpha Step 500, TENCOR) showed that the lenses had good surface smoothness. The surface roughness was less than 70 nm. This is a typical value for microlenses without going

,where f is the focal length, r is the footprint radius, h is the sag height, and the α is the Figure 4-6 (a) The SEM photograph of 200-μm diameter MLA under three

minutes UV/ozone treatment time. (b) The cross section profile of an SU-8 microlens.

photoresist after UV curing.

We have fabricated microlens direct on the top of LED chip successfully by using this method. The opening of the shadow mask was 200 μm and the UV/ozone treatment time was three minutes. The size of LED chip was approximately 200 μm and 300 μm.

Figure 4-7 (a) and (b) shows the optical microscopy (ECLIPSE 50i with 50X objective, Nikon, Japan) image of the fabrication result of the microlens on a LED chip. It was captured by a CCD camera (CoolPix4500, Nikon, Japan). Because of a high NA (= 0.8) objective lens, the depth of focus (DOF) of a microscopy is very small (∼0.6 μm).

Figure 4-7(a) is a LED chip with aluminium wire bonding. Figure 4-7(b) is a microlens on the LED chip. Figure 4-7(c) shows the stereo microscopy (PS-130a, Potop, Taiwan) image of the microlens on the LED. It shows both clear image on LED chip and microlens. Figure 4-7(d) shows the tilt view of the stereo microscopy image.

In order to measure the angular light field distribution, we setup a measurement system as shown in Figure 4-8. It included a computer, a step-motor controller (DS102, Suruga, Japan), a motorized rotation stage (KS101, Suruga, Japan), a current meter (Kathley 2400, USA), a power supply, and a photo detector. We measured the light field distribution from -90 degree to +90 degree, where zero degree stood for the normal direction, with a step of one degree, as shown in Figure 4-9. The intensity was Figure 4-7 The optical microscope image of fabrication result. (a) The LED chip with aluminum (Al) wire bonding. (b) The 200-μm diameter microlens on LED. (c) The clear stereo microscope image on both LED chip and microlens. (d) The tile view of the stereo microscope image of the microlens on LED.

the maximum intensity). Figure 4-10 shows the illumination map. It reveals that the luminance was improved as well.

Figure 4-8 The measurement setup of the angular light field distribution.

Figure 4-9 The normalized angular distribution of light intensity (from -90 to 90 degree). It improved the extraction efficiency of 15 % and 28% in maximum and total accordingly, and increased the viewing angle of 17 degree (θ1/2).

4.6 Conclusions

A transparent and self-assembled microlens on the top of LED fabricated by using hydrophilic effect under UV/ozone treatment was presented. This method provides a fast, low cost, no etch-transfer, no photo lithography fabrication processes. MLA by the use of a shadow mask of 200 μm diameter for three minutes UV/ozone treatment time has been fabricated successfully. The LED chips were encapsulated by a layer of thick PDMS (n = 1.44) and the MLA was made of negative photoresist SU-8 (n=1.63). The high refractive index material of PDMS and SU-8 photoresist and the microlens on the top of LED improved the extraction efficiency of 15 % and 28% in maximum and total accordingly, and increased the viewing angle of 17 degree.

Figure 4-10 The experimental illumination map. (a) Without microlens and (b) With microlens.

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Chapter 5 An Optical Wavefront Sensor Based on a Double Layer Microlens Array

In order to determine light aberrations, Shack-Hartmann optical wavefront sensors make use of microlens arrays (MLA) to divide the incident light into small parts and focus them onto image planes. In this work, we present the design and fabrication of long focal length MLA with various shapes and arrangements based on a double layer structure for optical wavefront sensing applications. A longer focal length MLA could provide high sensitivity in determining the average slope across each microlens under a given wavefront, and spatial resolution of a wavefront sensor is increased by numbers of microlenses across a detector. In order to extend focal length, we used polydimethysiloxane (PDMS) above MLA on a glass substrate. Because of small refractive index difference between PDMS and MLA interface (UV-resin), the incident light is less refracted and focused in further distance. Other specific focal lengths could also be realized by modifying the refractive index difference without changing the MLA size. Thus, the wavefront sensor could be improved with better sensitivity and higher spatial resolution.

In this chapter, Vinna Lin was the one who has the primary contribution including the fabrication and discussion of double layer microlens array and the comparison between commercial SHWS and ours. I was the coauthor with the contribution to the part of wavefront sensor computing algorithm programing and the wavefront reconstruction. Most contents were adopted from the published paper in Sensors with

5.1 Introduction

An optical wavefront sensor is an important device in determining wavefront aberration if fields ranging from the astronomy to any optical testing application. There are several types of wavefront sensors including the Michelson interferometer, Shearing interferometer, Fizeau interferometer, and even the Foucault knife-edge test. Among these interferometric methods, the Shack-Hartmann wavefront sensor (SHWS) is the most popular sensor because of its simplicity and elegance for measuring the shape of a wavefront. It has been applied to adaptive optics for high-energy lasers and astronomy for many years [2]. The main goal is to improve image quality taken by ground-based telescopes which might be distorted by atmospheric turbulence. However, as the technology has been developed, the technique has been implemented in many other fields. Over time, applications in quality laser beam measurement, optics testing, and optical system calibration and alignment have been discussed. Furthermore, this technology quickly led to the evolution of more sophisticated sensors focused around ophthalmic applications [3], CCD cameras, and micro-optics. The applications of SHWS have become widespread throughout the World, with hundreds of millions of astronomical images benefiting from the process to millions of corrective surgeries that will be performed in upcoming years to enhance vision. It is amazing for a technology to have such a dramatic impact and evolution from a single field to multiple fields as the SHWS has.

The most critical element of an SHWS is its microlens array (MLA). The incident light is divided into a number of small samples by the lenslet arrays, which then are focused onto a detector array. These focal spots of light are the key principle in the

displacement of the measured focal spots from their reference spot positions. In this work, we first propose a design and method for extending the focal length of MLA effectively based on double layer structure; the evaluation of the MLA is presented accordingly. Furthermore, we integrated the MLA with an image sensor to build a SHWS. Not only the development of the SHWS is discussed but also the experimental results of the SHWS performance is explained by comparing the measured wavefront with the commercial one. The experimental results of the system are discussed and compared between the long focal length, the shorter focal length and the commercial SHWS.