1128 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 9, MAY 1, 2009 Low-Loss Y-Branch Waveguides Designed by Wavefront Matching Method Yohei Sakamaki, Member, IEEE, Takashi Saida, Member, IEEE, Toshikazu Hashimoto, and Hiroshi Takahashi, Member, IEEE Abstract—In this paper, we describe the procedure for designing low-loss Y-branch waveguides by the wavefront matching (WFM) method, and report experimental results as proof of concept. The designed Y-branches were fabricated using silica-based planar lightwave circuit (PLC) technology. A Y-branch fabricated with a 0.45%- waveguide exhibited a low excess loss of less than 0.2 dB over a wide wavelength range of 1250 to 1650 nm. In addition, we demonstrate that the WFM-designed Y-branches enable us to provide compact 1 32 splitters with an average insertion loss of 16.0 dB at a wavelength of 1550 nm. In addition, we present some experimental results obtained using samples with different values, and show that our design method is more efficient for a higher waveguide suitable for functional PLC devices. Index Terms—Design methodology, optical planar waveguides, waveguide junctions. I. INTRODUCTION Y -BRANCH waveguides play important roles as optical signal dividers/combiners in optical waveguide devices. Optical splitters consisting of cascaded Y-branches are essential components for optical signal distribution in a broad- band subscriber network [1]. The performance requirements for the splitters include a low insertion loss, a wide operational wavelength range, a uniform splitting ratio, a low polariza- tion-dependent loss and a compact size. Silica-waveguide splitters made with planar lightwave circuit (PLC) technology have been installed in access infrastructure thanks to their mass-producibility, long-term reliability, and good optical characteristics [2]. Now, with the increase in the scale of access networks, we must further reduce both splitter loss and size to enable us to construct more cost-effective optical communication systems. To realize splitters with lower loss, the excess loss of a Y-branch must be greatly reduced, because the excess losses of single Y-branches accumulate in a cascade configuration. And, since the chip size of the splitter is mainly determined by the radius of the bent waveguides used for cascading the Y-branches and ex- panding the output waveguides, for miniaturization it is effective to fabricate them with a higher refractive index difference waveguide, namely a strongly guiding waveguide. However, for the following reason, it is difficult to obtain low-loss Y-branches Manuscript received March 11, 2008; revised June 18, 2008. Current version published April 24, 2009. The authors are with the NTT Photonics Laboratories, NTT Cor- poration, Kanagawa 243-0198 Japan (e-mail: sakamaki@aecl.ntt.co.jp; saida@photo.nel.co.jp; t_hashi@aecl.ntt.co.jp; hiroshi@aecl.ntt.co.jp). Digital Object Identifier 10.1109/JLT.2008.928183 Fig. 1. Y-branch patterns of (a) conventional tapered waveguide and (b) WFM- designed waveguide. with a high waveguide. The excess loss of the conventional Y-branch shown in Fig. 1(a) results from the mode mismatch at the interface between a tapered waveguide and two output waveguides. The excess loss decreases as the gap between the output waveguides becomes narrower, because the fundamental local normal mode of two output waveguides becomes closer to the fundamental mode at the end of the tapered waveguide. However, the gap cannot be zero because the minimum gap width is limited by the resolution of the photolithographic and etching processes. As increases, the gap becomes relatively wider, because the single-mode waveguide width becomes nar- rower while the minimum gap width remains approximately constant. Thus, the excess loss resulting from the mode mis- match increases for higher waveguides. Several approaches have been proposed for resolving this mode mismatch [3]–[11]. For instance, Gamet et al. introduced a segment design in the branching region [3] and Wang et al. designed a Y-branch struc- ture with a multimode waveguide transition section [4] to reduce the Y-branch loss. However, the segmented structures require precise shaping with a gap of a few micrometers, and multimode interference waveguides have considerable wavelength depen- dence. In this paper, we propose a Y-branch waveguide design based on the wavefront matching (WFM) method and describe low- loss Y-branch waveguides with a new shape designed by this method. First, we explain the principle of the WFM method and the Y-branch design procedure. Then, we present some ex- perimental results and demonstrate low-loss and compact 1 32 splitters composed of our designed Y-branches. In addition, we show that the loss reduction realized by using the WFM method is more substantial for higher waveguides suitable for functional PLC devices. As an example of the application of the WFM-designed Y-branches to functional PLC devices, we 0733-8724/$25.00 © 2009 IEEE