REVIEW ARTICLE Guided properties and applications of photonic bandgap fibers Zhi WANG (*), Yange LIU, Guiyun KAI, Bo LIU, Chunshu ZHANG, Long JIN, Qiang FANG, Shuzhong YUAN, Xiaoyi DONG Institute of Modern Optics, Nankai University, Tianjin 300071, China E Higher Education Press and Springer-Verlag 2008 Abstract The authors have reviewed some of their recent studies on photonic bandgap fibers (PBGFs). PBGFs that confine light in the core by the photonic bandgap effect of cladding have potential applications in various photonic devices. In this paper, the guided properties and tuned mechanics of anti-resonant PBGFs are theoretically illus- trated. The special coupling properties in multi-core PBGFs, such as decoupling and resonant coupling effect, are then introduced. Finally, fiber Bragg grating inscribed in all-solid PBGFs is theoretically and experimentally studied, and special resonant characteristics are also observed. Keywords photonic crystal fibers (PCFs), photonic bandgap, directional coupler, fiber Bragg grating 1 Introduction In recent years, photonic crystal fibers (PCFs) have become more attractive due to unique properties such as endless single-mode guiding [1], high nonlinearity [2], and tailorable group velocity dispersion (GVD) [3]. PCFs can be commonly classified as index-guided PCFs, which guide light through total internal reflection (TIR), and photonic bandgap fibers (PBGFs), which guide light in a low-index core through the photonic bandgap (PBG) effect [4]. One of the most interesting PBGFs is the hol- low-core PBGF, in which light can be confined in an air core by a periodic array of air holes in silica [5,6]. Hollow- core PBGFs have been widely studied in various applica- tions such as gas nonlinearity [7], high power soliton deliv- ery [8], and pulse compression [9]. Another kind of PBGF, the so called anti-resonant PBGF, is commonly composed of a solid core and a two-dimensional array of high-index rods in a silica background [10]. Anti-resonant PBGFs can be achieved by infiltrating the air-holes of TIR- PCFs with high-index liquid material. If the refractive index of the infiltrating material, such as liquid crystal, is sensitive to temperature or electric fields, the guided properties of the anti-resonant PBGFs will be tunable [11]. The all-solid PBGF, another kind of anti-resonant PBGF, is usually composed of a two-dimensional array of high-index germanium-doped silica rods in a pure silica background [12]. All-solid PBGFs are easier to fabricate and couple with conventional fibers than hollow-core PBGFs, and have potential applications in integrated fiber communication devices. A study on PBGFs has been carried on by our research group in recent years [13–21]. This paper gives an over- view of the progress of our work. In Sect. 2, the guided properties and design of anti-resonant PBGFs are demon- strated. Coupling properties of multi-core PBGFs are investigated in the next section. The fiber Bragg grating (FBG) inscribed in PBGFs is then shown in Sect. 4. Finally, conclusions are given in Sect. 5. 2 Guided properties of anti-resonant PBGFs When high index materials such as liquid crystals are filled in the air holes of general TIR-PCFs, as shown in Fig. 1(a), the fiber cannot guide light by TIR since the fiber now has a low-index silica core surrounded by high-index rods. However, this infiltrated fiber can support a number of guided-wavelength bands due to the PBG effect formed by the anti-resonance in high index rods [13]. In Fig. 2, the bandgaps of the pattern are depicted by using the plane-wave method. In this case, the holes have diameter d and pitch length L of 1.8 and 3.38 mm, respec- tively. The refractive index of silica and infiltration mate- rial is assumed to be 1.444 and 1.653, respectively. The diagram reveals the existence of two PBGs and a silica line crossing the fundamental and secondary gap regions. For a specific propagation constant value, no fundamental modes are allowed to propagate in the anti-resonant PBGFs if their frequencies are not located in the PBGs and below the silica line. Received October 22, 2007; accepted November 17, 2007 E-mail: zhiwang@nankai.edu.cn Front. Optoelectron. China 2008, 1(1–2): 25–32 DOI 10.1007/s12200-008-0040-2