Recent progress in gas detection with hollow-core microstructured optical fibers (HC-MOFs) and direct absorption/photothermal interferometry spectroscopy are reported. For direct-absorption sensors, the issue of mode interference noise is addressed and techniques to minimize such a noise are experimentally demonstrated. Large-scale drilling of hundreds of low-loss micro-channels along a single HC-MOF is demonstrated, which reduces the diffusion-limited response time from hours to ~40 seconds for sensing HC-MOFs of 2.3 meters. For photothermal inteferometry sensors, novel detection configurations based on respectively a Sagnac interferometer and an in-fiber modal interferometer are proposed and experimentally demonstrated. The Sagnac configuration avoids the need for complex servo-control for interferometer stabilization while the in-fiber configuration simplifies the detection, reducing the size and cost of the sensor system. Sub-ppm gas detection can be achieved easily with photothermal interferometry spectroscopic HC-MOF sensors but is difficult to achieve for direct-absorption sensors with the current commercial HC-MOFs.
Highly birefringent (Hi-Bi) microfiber-based fiber loop mirrors (FLMs) were studied for tunable comb filters and refractive index (RI) sensors. The use of two cascaded Hi-Bi microfibers instead of a single microfiber allows more flexibility in controlling the transmission/reflection characteristics of the FLM. The length of Hi-Bi microfibers is of the order of centimeters, one or even more than two orders of magnitude shorter than the conventional Hi-Bi fiber-based FLM devices. The transmission/reflection spectra are sensitive to the RI surrounding the microfibers, and RI sensitivity of 20 Optical comb filters as multichannel filters have been studied intensively for application in wavelength division multiplexing (WDM) communication systems [7], and filters with flattop are particularly attractive for DWDM systems due to signal fidelity and tolerance of signal wavelength drift. Especially, all-fiber comb filters based on a Sagnac fiber loop mirror (FLM) are popularly used for all-optical signal processing and multiwavelength fiber lasers [8][9][10] due to lower insertion loss and better performance.In this Letter, we investigate the use of highly birefringent (Hi-Bi) microfiber-based FLMs for tunable comb filters and high sensitivity refractive index (RI) sensors. Two different configurations are studied. One incorporates a single piece of twisted Hi-Bi microfiber in a Sagnac loop, while the other includes two pieces of Hi-Bi microfibers cascaded along a fiber with a rotation of their birefringence axes. This work on microfibers may result in novel applications, such as wavelengthscale light transmission and interconnection, which are useful for photonic integration, computing, and nanoscale sensing.The Hi-Bi microfiber is made from a commercial SMF-28 (outer diameter D ∼ 125 μm, core diameter d ∼ 8.2 μm, Δn ∼ 0.36%) [11]. The SMF-28 fiber is first "processed" by use of a femtosecond IR laser system (Fig. 1) to cutting away parts of the silica cladding on opposite sides of the SMF, resulting in an approximately square-like cross section [inset (a) in Fig. 1]. Figure 2 shows photos of a "processed" SMF with a cutting away depth of ∼26 μm, and a length of ∼3 mm. The "processed" SMF is then taper-drawn to micrometer size by use of a flame-brushing technique. The cross-sectional shape of the "processed" region is well preserved during the initial taper-drawn process and eventually turns to an approximately elliptical shape when the fiber diameter is reduced down to wavelength scale, which generates a high birefringence of the order of 10 −2 [11], much bigger than conventional polarization maintaining fibers. The insertion loss of the tapered microfibers with diameter above 2 μm is in general considerably smaller than 0.2 dB. The ellipticity depends on the depth of the cut and the parameters used in the tapering process.To fabricate two cascaded Hi-Bi microfibers along the same SMF, the SMF is first cut at one location and the Fig. 1. Schematic of the femtosecond IR laser system. Insets: artistic view of the idealiz...
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