Development of micro-ring resonator-based optical bandpass filter using SU-8 polymer and optical lithography Swagata Samanta a, * , Pradip K. Dey a , Pallab Banerji b , Pranabendu Ganguly a a Advanced Technology Development Centre, Indian Institute of Technology, Kharagpur, West Bengal, 721302, India b Materials Science Centre, Indian Institute of Technology, Kharagpur, West Bengal, 721302, India article info Article history: Received 5 December 2017 Received in revised form 4 January 2018 Accepted 16 January 2018 Keywords: SU-8 waveguide Micro-ring resonator Chrome mask Filter Lithography abstract Laterally-coupled SU-8 ridge waveguide-based micro-ring resonator was designed, fabricated and characterized for optical filtering applications. Fabrication was done by optical lithography using a patterned chrome mask of SU-8 ridge waveguide and micro-ring resonator structures, which was replicated onto a plasma enhanced chemical vapour deposited (PECVD) silicon-dioxide layer on a silicon wafer. Optical characterization showed that the fabricated micro-ring resonator had a free spectral range (FSR) of ~16nm for a ring radius of 15 mm in TE polarization. A simple semiconductor laser diode and a monochromator were used for the characterization of the device. The measured through port and drop port light output for different wavelengths indicate that the device can be used as an optical filter around 1565 nm centre wavelength, with a 3 dB bandwidth of 5.3 nm and an extinction ratio of ~10.5 dB. © 2018 Elsevier B.V. All rights reserved. 1. Introduction Optical waveguide device like micro-ring resonator (MRR) is basically a ring waveguide acting as the resonant cavity with one or two bus waveguides acting as input and output ports. The coupling mechanism involved in this device is the evanescent coupling be- tween ring and adjacent bus waveguide [1]. This may be vertically coupled or laterally coupled; both configurations have certain pros and cons. In case of laterally coupled resonator, both the ring and bus waveguides lay on the same horizontal plane, thus requires very accurate lithography and etching processes in order to obtain submicron gap between bus and ring - thereby limiting the flexi- bility in the device design. On the other hand, ring and bus wave- guides in vertical configuration don't lie in the same plane. Ring is placed on top or bottom of the bus waveguides, as a result, the ring and bus may be of different material, and the thickness need not be same and can be controlled accurately during deposition e all these enhances the design freedom. However, this vertical configuration is expensive due to the additional processing step of the ring in contrast to lateral configuration which requires only a single layer. Moreover, fabrication of vertically coupled ring resonators is com- plex as wafer bonding and regrowth is required to manufacture these devices; also alignment is an issue as there are two pro- cessing steps [2,3]. In this work, SU-8 ridge waveguide-based air-cladded micro ring resonator (lateral configuration) is considered which was fabricated by photolithography using chrome mask. The choice of SU-8 polymer was due to its optically transparency both in visible and telecommunication region of 1300e1600 nm wavelength, thus can be used for waveguiding purpose; also it is a low cost material which is easily available [4,5]. In most of the previously reported research articles [6e8], SU-8 waveguide based MRR were fabri- cated by costly electron beam lithography, and characterized by very narrow bandwidth tunable laser source (TLS) and photodiode (PD) or optical spectrum analyzer (OSA). In Refs. [6,7], researchers fabricated a free-standing flexible MRR to realize a notch filter. They peeled-off the SU-8 structure from silicon surface to make a high- contrast micro-ring. In addition, some researchers [8] used commercially available UV-15 and OG-125 polymers as lower and upper cladding layers to obtain single-mode waveguide structures. P. Girault et al. [9] fabricated MRR by photolithography using chromium mask, where class 100 clean room and dry etching process were used. Their ring radius was ~120 mm, which resulted * Corresponding author. E-mail address: swagata@atdc.iitkgp.ernet.in (S. Samanta). Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2018.01.024 0925-3467/© 2018 Elsevier B.V. All rights reserved. Optical Materials 77 (2018) 122e126