Analysis of optical properties of fundamental-mode in waveguide tapered fibers Cheng-Ling Lee * , Kuo-Hsiang Lin, Nan-Kuang Chen Department of Electro-Optical Engineering and Optoelectronics Research Center, National United University, No.1 Lien-Da, Kung-Ching Li, Miaoli 360, Taiwan, ROC article info Article history: Received 30 November 2009 Received in revised form 7 January 2010 Available online 20 February 2010 abstract An analysis of optical properties of fundamental-mode in waveguide tapered fibers is theoretically inves- tigated and realized in this paper. The waveguide device is tapering an SMF-28 fiber to few tens of micrometers of diameter. For discussing the cladding size of waveguide structure affects the fundamen- tal-mode cutoff (FMC) and the optical characteristics of the devices, to etch outer cladding to reduce the pure-silica cladding diameter proceeded and to compare optical properties of FMC with non-etched clad- ding tapered fibers. Numerical results show the cutoff wavelength of FMC is mainly dominated by the size of squeezed core and slope of FMC could be influenced by the size of pure-silica cladding. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction There are many research works and applications have been studied towards the optical characteristics of the tapered fibers be- cause they have great applications in different technologies such as supercontinuum generation, pulse compression applications, use- ful optical fiber components and sensitive sensors [1–6]. A tapered waveguide must be regarded as a three-layer structure (core–clad- ding and external medium). The transmission spectra of optical characteristics are influenced by the refractive index of the exter- nal medium, refractive index profile of fiber and the tapered shape of the device. The dispersion characteristics of the tapered fiber can be easily modified by applying new optical materials such as opti- cal liquids or optical polymers surrounding the tapered fiber. When the mode fields of the guiding lights penetrate the pure-sil- ica cladding deep enough to overlap the outer new materials, the effective indices of guiding lights can be modulated and this is called the material dispersion engineering [5]. An optimal tapered fiber structure for a short-wavelength-pass filter with sharp and high cutoff efficiency has been achieved and demonstrated [7]. However, characteristics of SMF-28 tapered fiber of fundamental- mode cutoff (FMC) influenced by the waveguide structures (clad- ding diameters) have not discussed in the published papers so far. Therefore, in this paper, for the first time, an analysis of optical properties of fundamental-mode in waveguide tapered fibers is theoretically investigated and realized. The presented device is based on SMF-28 tapered fibers with few tens of micrometers of pure-silica cladding diameter which plays as a new core after tapering. To discuss the cladding size affects the fundamental- mode cutoff and the optical characteristics (transmission spectra) of the devices; the pure-silica cladding of the tapered fibers are etched to reduce different sizes and immersed in the same optical liquid for comparing the waveguide properties. 2. Principle and tapered fiber structure To investigate the influences of cladding size of the waveguide structure to the fundamental-mode in tapered fibers. Mode fields should be extended and covered over the whole waveguide struc- tures. The simplest way to expose the mode fields of guiding lights is tapering technique and then modifies the structure of the wave- guide tapered devices; therefore, propagation constant: b of the fundamental-mode will be changed. That means effective index (n eff = b/k, k is wave number: k =2p/k), delay time: t (by differenti- ating b with k) and chromatic dispersion (by differentiating t with k) of the device are varied, as well as fundamental-mode cutoff (FMC) and the optical characteristics of the tapered filters. The studied tapered fiber filters are made by tapering standard sin- gle-mode fibers (SMF-28) with the original ratio of diameters of core and cladding, D co 8.2 lm and D cl 125 lm, respectively. For simplification and practical considerations, three samples of ta- pered fibers with original cladding diameter D cl of 50, 40, and 30 lm which means that corresponding remained-core size D co is about 3.28, 2.624, and 1.968 lm, respectively. In our simulation study, we assume that germanium in the fiber core does not dif- fuses out into pure-silica cladding through the tapering processes. The tapered fibers are all chemically etched using hydrofluoric acid to remove part of silica cladding for reducing cladding diameter D cl and to compare the curves of FMC wavelengths, FMC slopes and transmission losses with different waveguide structures of tapers. Fig. 1 shows diagram of the structure for the proposed devices: (a) before and (b) after etching with the original cladding diameter D cl of 50 lm and etched cladding diameter D cl of 40 lm with the iden- tical core size. 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.01.032 * Corresponding author. Tel.: +886 37 381732; fax: +886 37 351575. E-mail address: cherry@nuu.edu.tw (C.-L. Lee). Microelectronics Reliability 50 (2010) 726–729 Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel