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Optoelectronics & Communications

A Bragg grating in an optical fiber with a nanostructure core

A reflection grating written on a novel, all-solid, silica-based fiber with a 2D core offers advantages over previous devices.
1 December 2008, SPIE Newsroom. DOI: 10.1117/2.1200809.1249

The first photonic crystal fiber (PCF) opened a new era for fiber optics.1 The potential device applications of this novel, microstructured fiber in optical-communication systems and sensing technologies have attracted research attention and intensive study worldwide. Most previous studies have concentrated on PCFs with a photonic crystal (PC) cladding surrounding a homogeneous core. If the wavelength of the light falls in the forbidden band gap of the PC, the light is confined to the core, which can even be hollow.

In contrast, index-guiding PCFs confine light in the same way as traditional fibers do, by exploiting total internal reflection at the effective-index mismatch between core and cladding. Index-guiding fibers that employ a PC cladding were initially appealing because the fundamental space-filling mode index of the cladding lattice varies strongly with wavelength. However, as shown in our earlier work,2 a fiber with a PC core can also guide light. A birefringent fiber with a finite, 1D-PC core has even been fabricated.3 By the same token, only a few numerical modeling efforts have been extended to a 2D-PC core,4 and we know of no report on fabrication and characterization of silica-based fibers with a 2D-PC core. Moreover, grating devices based on such fiber are rare, both theoretically and experimentally. Here, we report the first fiber Bragg grating (FBG), to our knowledge, written on silica-based, all-solid, nanostructure-core fiber (NCF). Instead of the PC cladding, the fiber core is formed by a periodic 2D array of high-index rods.

To make the fiber, we used a doped-silica preform to build the periodic structure of the core, which we then overclad with a pure silica jacket. The maximum refractive-index difference between the germanium-doped region and pure silica is approximately 3%. We then drew the preform into a cane and stacked the cane in a hexagonal pattern. The stack was jacketed in a silica tube and drawn to form the nanostructure core. The resulting fiber was etched in 50% hydrofluoric acid for 2min, to an outer diameter of 114μm, as shown in Figure 1(a). The scanning electron micrograph (SEM) of the fiber cross-section close to the core and the mode profile are shown in Figure 1(b) and (c), respectively. The lattice spacing is ∼1μm. The diameter of the high-index rods in the core is ∼800nm. The attenuation spectrum of a 500m-long NCF, measured by the cutback method, is shown in Figure 1(d). At 1550nm, the attenuation is 3.5dB/km. The high attenuation peak at around 1385nm, and the peaks at 944 and 1247nm, are water peaks due to OH absorption. It is believed that the loss is mainly caused by water contamination accumulated during the stacking process.

Figure 1. (a) Scanning electron microscope (SEM) picture of the fabricated nanostructure-core fiber. (b) The details of the core region. The black regions are germanium-doped. (c) Contour plot of the measured mode profile of the guided mode for the fiber shown in (a). (d) Attenuation spectrum, measured using the cutback method.

For the inscription of gratings along the length of the fiber, the NCF was loaded with hydrogen for 3 weeks. High-power UV light was generated by a frequency-doubled argon laser at a wavelength of 244nm, with an average power level 66mW. This beam was directed via an acousto-optic modulator and a series of UV mirrors to the device under fabrication. There it was focused on a phase mask to generate fringe patterns that created a grating structure in the fiber. The phase mask, with a period of 1070.2nm, was moved along the fiber length by affixing it to a motion-controlled stage. The total grating length was 28.5mm, with 100 sampling points. During fabrication, the transmission spectrum of the grating was monitored by a conventional-band and long-wavelength-band (C+L) broadband amplified-spontaneous-emission source and an optical spectrum analyzer to provide real-time information about the FBG under fabrication. The final reflection spectrum of the grating is shown in Figure 2.

Figure 2. Measured reflection spectrum of the fabricated fiber Bragg grating.

Tunable optical components based on FBG devices are of great interest for application in optical-fiber telecommunication and sensor networks. To select and manipulate different WDM channels, tuning of the Bragg wavelength is necessary. One way to achieve tuning is to heat a section of the fiber that contains a Bragg grating, using the thermo-optic effect. We measured the temperature sensitivity of the FBG using a sensor head in a temperature-controlled container. Measurements with both increasing and decreasing temperature showed a sensitivity ∼10pm/°C. A second tuning method uses the photoelastic effect of the silica fiber. We fixed one end of the grating and stretched the other end by using a precision translation stage. A linear fitting to the experimental data gives a wavelength-strain sensitivity of 0.96pm/με (ppm strain).

In conclusion, we have fabricated an FBG in a novel all-solid silica fiber with a nanostructure formed with a 2D periodic lattice of high-index rods. The FBG was written by a phase mask in a UV-laser system. Compared with FBGs based on single-mode fibers, our device has the potential advantage of a large mode area. Moreover, the core mode has stronger leakage, which can lead to sensitive devices made using side polishing.

The authors are grateful to the R&D department, YOFC Wuhan China, for helping to fabricate the NCF. We also acknowledge Cheng Xueping and Zhou Jinlong for discussions and help with making the grating.

Xia Yu, Ping Shum 
Nanyang Technological University
Singapore, Singapore

Xia Yu received her BEng and PhD in the school of electrical and electronic engineering (EEE) from Nanyang Technological University (NTU), Singapore, in 2003 and 2006, respectively. Since June 2006, she has been working as a research fellow at EEE, NTU. Her research interests include photonic crystal-based devices, microstructured fibers, fiber sensors, and surface plasmons.