1308 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 12, JUNE 15, 2006 Self-Written Waveguide on a VCSEL-Emitting Window Using a Photomask Transfer Method Yusuke Obata, Masahiro Kanda, and Osamu Mikami Abstract—This study is focused on a self-written waveguide (SWW) fabricated using a new photomask transfer method. With the proposed photomask transfer method, an SWW array was successfully fabricated on vertical-cavity surface-emitting laser (VCSEL) diode-emitting windows. It was found that the output power of a VCSEL having the SWW increased as compared to that of VCSEL without the SWW. We have confirmed that the optical output is independent of the placement of the emitting window inside the SWW. Optical coupling between an SWW and a graded-index multimode fiber was also demonstrated. Index Terms—Photomask transfer method, self-written waveguide (SWW), self-written waveguide (SWW) array, vertical-cavity surface-emitting laser (VCSEL). R ECENTLY, the data traffic through networks has in- creased dramatically. As a result, the signal transmission through metallic wiring has become bottlenecked, even inside of a box [1]. The issues involved in electrical transmission include bandwidth, impedance matching, and crosstalk. As a solution, the introduction of optical wiring into the box and printed-wiring board (PWB) has attracted much attention. But this approach is not a simple one [2]. The reason is, optical signals have characteristics much different from electrical ones. These characteristics require precise alignment between optical devices and optical wirings. This may induce a considerable increase in the cost and time for packaging and mounting optical devices. The key behind optical interconnection is the development of a simple packaging and coupling technology with high efficiency. One method to make the technology more efficient is to apply optical-surface mount technology (O-SMT) and a self-written waveguide (SWW). The O-SMT is an optical device-mount technology at the board level that corresponds with the surface mount technology (SMT) employed in elec- tronic circuits [3]. It has been shown that SWWs are very useful for optical coupling between optical components [4]–[7]. The mechanism of SWW is based on the fundamental property of photopolymerization. Irradiating a UV light through an optical fiber into photopolymerized resin, the irradiated part begins to solidify from higher power parts along the optical axis. The so- lidified part has a higher refractive index than the resin so that it has a function as the waveguide core. The process of how a UV light from a fiber writes an optical waveguide is best described as a self-writing formation. In this study, we have proposed a new SWW array formation using a photomask transfer method Manuscript received September 26, 2005; revised March 7, 2006. The authors are with the Tokai University, Information Technology and Elec- tronics, Hiratsuka-shi 259-1292, Japan (e-mail: mikami@keyaki.cc.u-tokai. ac.jp). Digital Object Identifier 10.1109/LPT.2006.876772 Fig. 1. SWW array fabrication using photomask. and applied this method to the formation of SWWs on ver- tical-cavity surface-emitting laser (VCSEL)-emitting windows. Typical SWW formation is performed by irradiating a UV light from a single fiber. When SWWs are applied to an optical connection between multichannel wirings and devices, this fiber-irradiation method has several problems, one being the amount of time required for fabrication. We have proposed an innovative SWW array fabrication method using a photomask transfer method. An outline of this fabrication method is schematically shown in Fig. 1. UV curable resin is filled into a space underneath a photomask. Then, a UV light having a large beam flux is irradiated onto a photomask. After a few seconds of exposure, the SWW array having cross sections close to the mask apertures can be formed. This photomask transfer method would be very suitable for SWW array connections where channels layers optical wirings come into optoelec- tronic (OE) PWBs. We have already shown some applications of the photomask transfer method involving O-SMT. SWW array formations having square and round cross sections with 50–100 m and 1.0-mm length were confirmed. In addition, we demonstrated that the formation of SWW having an up/down taper or a straight shape, depends on the irradiation conditions [8]. Recently, fabrication of polymer pillars was demonstrated by Bakir and Meindl [6]. They obtained polymer pillars having dimensions 180 m tall with a 55- m diameter, by employing a spin coating and baking process before UV irradiation. Con- versely, our proposed process enables fabrication of SWWs as long as 1000 m produced by a single UV irradiating process at room temperature. No baking process is required. It is hypothesized that highly efficient optical coupling between a VCSEL and a waveguide can be achieved through applying the proposed method. An SWW formed on an OE-device’s window seems likely as a very effective coupling tool. By forming SWW on an OE-device’s window, it is possible to increase the optical coupling efficiency while reducing the problems related to alignment and mounting. 1041-1135/$20.00 © 2006 IEEE