Introduction. Hollow-core Bragg fiber (HC-BF) is a kind of photonic band-gap fiber with one-dimensional photonic crystal. (1DPC) cladding on the inner wall.
OSA/ CLEO 2011
CThM4.pdf
Quaternary One-Dimensional Photonic Crystal Cladding Hollow-Core Bragg Fiber Lichao Shi1, Wei Zhang1, Jie Jin1, Yidong Huang1, Jiangde Peng1 1
Tsinghua National Laboratory for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing 100084, P.R. China slc07@mails.tsinghua.edu.cn, zwei@mail.tsinghua.edu.cn
Abstract: We proposed and fabricated hollow-core Bragg fiber with quaternary one-dimensional photonic crystal cladding. The new cladding increases the design flexibility. The transmission measurements revealed that it guides light mainly by the higher order photonic bandgaps. ©2010 Optical Society of America OCIS codes: (060.2280) Fiber design and fabrication; (060.5295) Photonic crystal fibers
1. Introduction Hollow-core Bragg fiber (HC-BF) is a kind of photonic band-gap fiber with one-dimensional photonic crystal (1DPC) cladding on the inner wall. The concept of the Bragg fiber was proposed as early as in 1978 [1] while the fabrication of it was started in the end of 1990s. In 2002, HC-BF based on semiconductor glass and polymer was fabricated by Y. Fink et al [2]. Its ability of mid-infrared and high power transmission has attracted much attention. The typical cladding of the HC-BF is a bilayers structure 1DPC formed by two kinds of materials with different refractive indices and we call it as the “binary 1DPC”. It provides a fundamental photonic bandgap (PBG) and several higher order PBGs which are equally spaced in frequency above the light line by which the light is confined in the hollow-core of the fiber. This kind of 1DPC structure is relatively simple and easy to fabricate. While the simplicity of the binary 1DPC restricts its flexibility in fiber design to extend the transmission characteristics. To increase its design flexibility, we proposed and fabricated a novel HC-BF, which has 1DPC cladding with four layers in one period, so-called quaternary 1DPC HC-BF (Q-1DPC HC-BF). Theoretical and experimental results indicated that the fiber is also PBG guiding. The cladding provides much more designable structure parameters, which provide the possibility to extend the high order transmission bands and reduce its loss, showing great potential to expand the application areas of HC-BF. 2. Structure of the Q-1DPC HC-BF The cross-section structure of the Q-1DPC HC-BF is shown in Fig. 1. The Q-1DPC surrounding the hollow-core of the fiber is a one-dimensional periodic structure, containing four layers in one period. The hollow-core diameter is indicated as tc. The thicknesses of these four layers are indicated as th1, tl1, th2, and tl2, respectively. Correspondingly, the period of the 1DPC is th1+tl1+th2+ tl2, indicated as . Currently, these four layers in one period was considered as forming by two kinds of materials, of which the refractive indices are indicated as nh for higher one and nl for lower one. This structure can also be composed by four materials with different refractive indices, if their glass transition temperatures are compatible with each other and can be drawn together.
r tl2 tl1
. . .
. . .
th2 th1
tc
nc
nl
nh
n(r)
Fig. 1: The cross-section of the Q-1DPC HC-BF
3. Fabrication of the Q-1DPC HC-BF Thin arsenic triselenide (As2Se3) layer (~ 2 m) was deposited on one side of a piece of polyetherimide (PEI) thin film (10 m) by thermal evaporation. Then another As2Se3/PEI deposited bilayer thin film was fabricated with the
OSA/ CLEO 2011
CThM4.pdf thickness of As2Se3 and PEI as ~7 m and 25 m, respectively. The thinner As2Se3/PEI bilayer film was put upon the thicker one to form a four layers film. These four layers film was rolled on a glass tube, forming the Q-1DPC correspondingly. Then, PEI films without As2Se3 were rolled up to form the fiber preform. The fiber preform was consolidated in a furnace in vacuum and then the glass tube was removed by hydrofluoric acid (HF) etching. Finally, the preform was drawn into fiber with a diameter of several hundred micrometers. Nitrogen gas was injected into the hollow fiber preform, providing a small pressure in the hollow-core to avoid hollow-core collapse during the drawing process. Fig 2 is the SEM picture of the Q-1DPC HF-BF sample we fabricated. The white and dark gray layers on the insert are As2Se3 and PEI layers, respectively, showing its clear Q-1DPC structure (The hollow-core of the fiber was filled with epoxy to prevent the collapse of the core when we were preparing samples for SEM).
As2Se3/PEI layers
Epoxy PEI
Fig. 2: cross-section SEM picture of the C-1DPC HC-BF sample
4. Transmission Characteristics of the Q-1DPC HC-BF sample The transmission characteristics of the Q-1DPC HC-BF were measured by a FTIR spectrometer. The transmission spectrum is shown in Fig 3 a). The fiber has two clear transmission peaks around 2800 cm-1 and 2100 cm-1, respectively. The PBGs of the cladding was theoretically calculated using the structure parameters estimated from the SEM picture and plotted in Fig. 3 b). The theoretical results indicate that these two transmission peaks are formed by the 2nd and 3rd PBGs of the Cladding. The transmission of the 1st PBGs is extremely suppressed. The transmission losses of these two peaks were measured by cut-back method, which are ~7 dB/m for the 2nd PBG and ~4.6 dB/m for the 3rd PBG. 5. Conclusion The hollow-core Bragg fiber with quaternary one-dimensional photonic crystal cladding was proposed and fabricated. Unlike the common binary 1DPC HC-BF which mainly guides light by the 1st PBG, the Q-1DPC HC-BF we fabricated guides light mainly by the 2nd and 3rd PBGs. The transmission of the 1st PBG is extremely suppressed. The transmission loss of the 3rd PBG of the Q-1DPC HC-BF is at the same level or even better comparing with the one of the common binary 1DPC HC-BF’s 1st PBG we ever fabricated. This new HC-BF shows great potential to enrich the transmission characteristics by the increased design flexibility. This work is supported in part by National Natural Science Foundation of China under Grant No. 60777032, 973 Programs of China under Contract No. 2010CB327600, Science Foundation of Beijing under Grant No. 4102028, and Basic Research Foundation of Tsinghua National Laboratory for Information Science and Technology (TNList).
Fig 3: a) Transmission spectrum of the Q-1DPC HC-BF sample and b) PGBs of the cladding
6. References [1] P. Yeh, A. Yariv, E. Marom, “Theory of Bragg fiber,” J. Opt. Soc. Am., 68, 1196-1201 (1978) [2] B. Temelkuran, S. D. Hart, G. Benoit, J. D. Joannopoulos, Y. Fink, “Wavelength-scalable hollow optical fibres with large photonic bandgaps for CO2 laser transmission,” Nature, 420, 650-653 (2002)
