In-line, microstructured optical-fiber devices for sensing applications

Air-silica microstructured optical fibers (MOFs) possess unique properties, such as endlessly single- and two-mode operation, novel dispersion characteristics, light guidance in an air core, and strong light-sample (gas or liquid) interactions inside air columns. These properties are not found in conventional optical fibers and may be exploited for development of novel photonic devices and sensors.¹

Active research has focused on developing and integrating in-line photonic devices with MOFs by employing carbon dioxide, electrical arc-discharge, acousto-optic-interaction, and femtosecond lasers.^2–4 We have used a femtosecond IR laser (see Figure 1) to modify the fiber geometries. Our laser setup is based on a titanium:sapphire regenerative amplifier system, which produces linearly polarized light with a maximum pulse energy of 1mJ at a wavelength of 800nm (with a repetition rate of 1kHz and a pulse width of 120fs). The laser beam is focused onto a MOF by a microscope objective. The MOF is mounted on a computer-controlled three-axis translation stage.

Figure 1. Femtosecond-laser setup for in-line device fabrication. MOF: Microstructured optical fiber. SMF: Single-mode optical fiber. OSA: Optical-spectrum analyzer.

With our femtosecond-laser setup, we fabricated a number of devices in MOFs. Figure 2(a) shows a cross section of a cone-shape microhole drilled from the surface to the core of a MOF. By drilling multiple holes periodically along the MOF, a long-period grating (LPG) is formed that resonantly couples the fundamental to a higher-order mode. Figure 2(b) shows the evolution of the LPG's transmitted spectra with increasing numbers of drilled holes. Because of the very strong perturbation of the mode fields around the drilled regions, LPG formation is highly efficient and we obtained a very strong resonance with a very short (~4mm) grating length. The near-field measurement—see Figure 2(c)—indicates that the LPG couples light to an LP₁₁-like (higher-order) cladding mode.⁵

Figure 2. (a) Scanning-electron micrograph (SEM) of a side hole drilled with the femtosecond laser. (b) Transmitted spectra of a long-period grating (LPG) with increasing numbers of holes. The separation between the holes (LPG period) is 420μm. (c) Near-field intensity profile at the resonant wavelength.

Instead of drilling holes, the femtosecond-laser beam can be focused onto a selected region of the MOF's inner cladding to modify only this region, while keeping the solid outer cladding and fiber core almost unaffected. Figure 3(a) shows cross sections of the modified MOF for a focal point ~5μm above the core center, scanned transversely across the MOF. The cladding holes in the focal region can be closed using a micro-explosion and redeposition process. Figure 3(b) shows the transmitted spectrum of an LPG made based on such a method.⁶ The thermal response of the LPG shows good stability and repeatability over a temperature range of up to 800°C: see Figure 3(c).

Figure 3. (a) SEM of the cladding-modified MOF cross section. (b) Transmitted spectra of the cladding-modified LPG with increasing numbers of grating periods. The period of the LPG is 360μm. (c) Measured resonant wavelength as a function of temperature.

We also drilled multiple micrometer-sized side holes along an air-core photonic-bandgap fiber and demonstrated a methane-gas sensor with a fast (~3s) response time.⁷ In addition, we used the femtosecond laser to cut grooves on MOFs for selective filling of air columns with various fluids. These air-fluid-silica fibers possess novel properties such as sensitivity to directional bend.⁸

Combining the holey structure of MOFs, the fabrication power of femtosecond lasers, and techniques for selected filling of sample materials into the hole columns enables creation of a range of novel photonic devices. Our research currently focuses on developing liquid-filled hybrid-MOF devices and MOF-based gas detectors with increased sensitivity and distributed-sensing capability.

The authors acknowledge support from the government of the Hong Kong Special Administrative Region through General Research Fund grant PolyU5182/07E and from the Hong Kong Polytechnic University through grant J-BB9K.