Weakly tilted fibre Bragg grating inscription in all-solid photonic bandgap fibres Y. Miao, B. Liu, K. Zhang, Y. Liu, H. Sun and Q. Zhao Weakly tilted fibre Bragg gratings (W-TFBGs) with different tilted angles are fabricated in the cladding rods of all-solid photonic bandgap fibre by UV illumination. The spectral characteristics of the W-TFBGs are investigated. Introduction: Since the advent of photonic crystal fibre (PCF), it has been applied across a number of optical fibre industries. Recently, all- solid photonic bandgap fibre (AS-PBGF), as a new type of PBGF, has attracted much attention owing to its bandgap mechanism and all-solid structure. AS-PBGF is composed of an array of high-index rods embedded in a low-index background, with the core formed by omitting one or more rods [1, 2]. Compared with the hollow-core PBGF (HC- PBGF), AS-PBGF is easy to fabricate and convenient to splice to con- ventional SMFs as no air hole collapse occurs. Moreover, the all-solid fibre structures supply much convenience for fibre grating inscription. Associated devices based on the AS-PBGF have been fabricated in different ways [3, 4]. Tilted fibre Bragg grating (TFBG) is a kind of short period fibre grating, which is useful in order to optimise the interaction between the fundamental guided mode and backward-propagating high-order modes, and has attracted considerable interest in the research area [5]. Most of the applications have focused on TFBG in singlemode fibre and six-hollow microstructure optical fibre (MOF) [6]. However, it is expected that the use of high-order modes can not only offer a greater degree of freedom in the design of the PCF-based optical devices as in the conventional optical fibre, but also lead existing fiber-optic com- ponents to have novel applications. In the work described in this Letter, W-TFBGs induced in AS-PBGF are successfully fabricated in the cladding rods by UV illumination. The spectra of W-TFBGs with different tilted angle and the spectral sens- itivity of W-TFBGs to refractive index liquid are investigated. Fabrication of grating devices: band structure of AS-PBGF: The cross- section of the AS-PBGF used in our experiments, which is fabricated by the Yangtze Optical Fiber and Cable Corporation, is shown in inset (i) of Fig. 1. In the fibre a high index rod surrounded by a low index ring lattice of five layers is embedded in pure silica background; the core is formed by omitting the high index rod and a low index ring. The dia- meter of the fibre is about 125 mm, and the pitch between adjacent rods L is 10 mm. The outer radii of the high index rod and the low index ring are 0.3786L and 0.7572L, respectively. Compared with the pure silica background, the average refractive index differences of the high index rods and the low index rings are approximately 0.028 and 20.008. By means of the plane wave expansion method [7] and a commercial finite-element method, the bandgaps and fundamental core mode are calculated, which are shown in inset (ii) of Fig. 1. The fundamental mode is well limited in the fibre core; there is no energy in high refrac- tive index rods. In addition, owing to the low refractive index ring round- ing the rods, the coupling between the modes in the rods and core mode is very weak. 500 1.440 1.445 1.450 core line core mode LP 11 LP 01 supermode band core mode LP 01 1.455 1.460 1.465 1.470 first bandgap second bandgap 1000 (i) (ii) 1500 2000 l, nm effective index Fig. 1 Dispersion curve of fundamental core mode and LP 11 and distri- bution of fibre bandgap Inset (i): microscopic photograph of cross-section of AS-PBGF, where bright spots represent Ge-doped rods Inset (ii): energy distribution of fundamental core mode in AS-PBGF Fabrication of grating devices: inscription of W-TFBGs: The PBGF is loaded in hydrogen atmosphere at 100atm, 1008C for 48 hours before W-TFBG inscription to enhance its photosensitivity. The inscription process is similar to that mentioned in [8]. The W-TFBGs are inscribed by a 248 nm KrF laser in H 2 -load all-solid photonics fibre by rotating the phase mask at a certain angle in the fibre plane. The tilted angles of the phase mask are 08, 1.58,38 and 48, respectively. The grating growth is monitored by use of an optical spectrum analyser (OSA) and a super- continuum light source. Spectral characteristics and refractive index responses of W-TFBGs: Index modulation can take place in the different layers of high-index rods. Here, only the outermost layer of high-index rods is investigated. Fig. 2 shows the transmission spectra of W-TFBGs with different tilted angles 08, 1.58,38 and 48. We notice the change in W-TFBGs spectral response when the tilt angle increases. –50 –45 –40 –35 –30 transmission, dB 1530 1540 1550 1560 l, nm –60 –55 –50 –45 –40 transmission, dB 1530 1540 1550 1560 l, nm Fig. 2 Transmission spectrum of light propagating in fibre core and rods after UV illumination For tilted angle 08, index modulation in the high refractive index rods has no effect on the transmission of the core mode when the light propagates in the fibre core. However, the supermode LP 01 in high- refractive index rods will be excited when light propagates in the rods. They can induce coupling between the forward-propagating and back- ward-propagating supermode LP 01 . Therefore, there is a resonance peak in the transmission spectrum, corresponding to the so-called Bragg resonance. The resonance wavelength in Fig. 2 is about 1547.1 nm and the transmission loss is 15 dB. Using finite-element analysis, the effective refractive index of LP 01 at the wavelength is approximately 1.4591, and the resonance wavelength is 1553 nm. If the error of the fibre structure parameter is neglected, the experimental result is consistent with the simulated one. For W-TFBG, two definitive resonance peaks can be observed from each transmission spectrum, named peak A and B. There also exist some unobvious peaks in series at the short wavelength side of peak B. Compared with 08-TFBG based on AS-PBGF, W-TFBG based on AS-PBGF has more resonance peaks in its transmission spectra, which indicates that new couplings are generated owing to the angle involved by the TFBG. Besides the coupling from forward-propagating LP 01 mode to backward-propagating LP 01 mode, couplings to the high- order supermode also can be induced. As shown in Fig. 3, with the tilt angle increasing, the coupling coefficient between the forward-propagat- ing supermode and the backward-propagating supermode decreases. So peak A becomes weak too. And the more the tilted angle increases, the more the coupling to the high-order supermode dominates. 1530 –60 –55 –50 –45 –40 4 deg 3 deg 1.5 deg transmission, dB –60 –55 –50 –45 –40 transmission, dB 1540 1550 B B A B A A 1560 l, nm 1530 1540 1550 1560 l, nm –60 –55 –50 –45 –40 transmission, dB 1530 1540 1550 1560 l, nm Fig. 3 Transmission spectra of W-TFBGs with different tilted angles 1.58,38 and 48 We calculated the effective refractive index for several low-order supermodes at this wavelength as 1.4591, 1.4397, 1.40 for LP 01 , ELECTRONICS LETTERS 17th February 2011 Vol. 47 No. 4