New Horizontal Frustum Optical Waveguide Fabrication Using UV Proximity Printing

NEW HORIZONTAL FRUSTUM OPTICAL WAVEGUIDE FABRICATION USING UV PROXIMITY PRINTING

Tsung-Hung Lin1, Hsiharng Yang 2, Ruey Fang Shyu3, and Ching-Kong Chao1

1 Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan 105 2Institute of Precision Engineering, National Chung Hsing University, Taichung, Taiwan 402 3Department of Mechanical Manufacturing Engineering, National Formosa University, Taiwan 632

ABSTRACT

This paper presents a novel method to fabricate the horizontal frustum structure as a planar optical waveguide by using the proximity printing technique. A horizontal frustum optical waveguide with a both lateral and vertical taper structure was produced. The orthogonal and inclined masks with the diffraction effect were employed in lithography process. This method can precisely control each horizontal frustum optical waveguide geometric profile in the fabrication process. The horizontal frustum optical waveguide and its array with the same inclined angle were generated. The beam propagation simulation software (BPM_CAD) was used to modeling the optical performance. The simulation results reveal that the mode profile matched into horizontal frustum optical waveguide and fiber from the laser diode. The optical loss of horizontal hemi-frustum structure of optical waveguides was less than 0.2dB. The horizontal hemi- frustum waveguide will be used for fiber coupling on boards for further optical communication systems.

1. INTRODUCTION

Integrated Optical Circuits (IOCs) have been under development in many laboratories and companies for over three decades. New access technologies such as Gigabit Ethernet, 10 Gigabit Ethernet, and passive optical access systems are investigated [1]. Optical Broadband Access Technologies (OBAT) with the current dominant broadband access technology of Hybrid-Fiber-Coax (HFC) systems is very interested. In recent years, an optical device integrated with a spot-size converter has been paid much attention for its direct coupling to an optical fiber without a micro-lens [2], tapered fiber [3] or lens fibers [4]. The laser diode generally has a small field radius in order to minimize the pumping current, and also has an elliptically shaped mode profile. An optical fiber has lager dimensions and is circularly symmetric with a mode radius of about 4.5 µm. The main problems associated coupling light from a semiconductor laser diode to an optical fiber lie in the mismatch between the mode profiles of the laser and the fiber, as well as establishing the alignment between them. A laser diode integrated with a spot-size converter is much more attractive for low-cost packaging due to its large spot-size, which is well matched to that of a single- mode fiber [5]. Three main classes of spot-size converter have been developed to expand the optical mode. The first is the vertical spot-size converter in which the waveguide thickness is decreased along the output direction. The second is laterally spot-size converter in which the waveguide width is decreased. The third is the lateral and the vertical dimensions of the guiding layer are changed. Such a combination allows us to easily control the beam divergence at the output facet [6]. These kinds of tapered waveguides have been proposed and demonstrated to improve the optical coupling among optoelectronic devices [7, 8, 9].
The taper waveguide exist much fabrication methods. The laterally taper can be achieved by standard photolithography followed by wet chemical etching, reactive ion etching (RIE) or reactive ion beam etching (RIBE) [10]. To fabricate vertical taper methods are including shadow mask techniques [11, 12], selective growth [13, 14, 15], sulfuric acid dip-etching [16], and diffusion-limited etching with selective area epitaxy [17]. The other type uses both lateral and vertical structuring, which is generally harder to fabricate since most current processes are planar. It is very difficult to achieve tapers in the vertical direction, but it has been done using such techniques as utilizes the selective area growth to add reactive ion etching or Stepped etching to add Cl2 chemical dry etching and regrowth [18, 19]. However, their processes are limited to semiconductor materials. Using a planar technology, such as spin coating of polymers, it is not easy to make structures which have a physical vertical taper shape. It can be done using laser ablation or RIE, but with an increased fabrication cost. The two guiding layers are then patterned using different masks. This is very hard to do using semiconductors, since it involves two mask, etch and regrowth steps and various resists and etches have to be applied to and cleaned from the intermediate surfaces [20]. In this study, ©EDA Publishing/DTIP 2007 ISBN: 978-2-35500-000-3

Tsung-Hung Li

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