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We report the fabrication of long-period gratings (LPGs) in wavelength-scale microfibers with diameters from 1.5 to 3 m. The LPGs were fabricated by use of a femtosecond IR laser to periodically modify the surface of the fibers. These LPGs have grating periods of a few tens of micrometers, much smaller than those in conventional optical fibers. A compact 10-period LPG with a device length of only ϳ150 m demonstrated a strong resonant dip of Ͼ20 dB around 1330 nm. These microfiber LPGs would be useful in-fiber components for microfiber-based devices, circuits, and sensors.

We investigated experimentally and theoretically an invertible fiber-type transformation from a photonic bandgap fiber into a nonideal waveguide and then into an index-guiding photonic crystal fiber via the thermo-optic effect of the fluid filled in the air holes. Such a transformation could be used to develop an in-fiber optical switch/attenuator with a high-extinction ratio of more than 35 dB over an extremely broad wavelength range from 600 to 1700 nm via a small temperature adjustment. -6]. The fiber type transformation from a PCF into a PBF should be an invertible process resulting from increasing/decreasing the refractive index of the filled materials. Such an invertible transformation in the fiber types (PCF/PBF) may lead to many potential applications. Research [1][2][3][4][5][6] was, however, focused on the transformation from an index-guiding fiber to a bandgap-guiding fiber instead of an opposite process. In this Letter, we filled a fluid into the air holes of a solid-core PCF and investigated experimentally and theoretically an invertible fiber-type transformation from a PBF into an unideal waveguide and then into an index-guiding PCF via the thermo-optic effect of the filled fluid. Such a fluid-filled PCF could be used to develop an in-fiber optical switch/attenuator with a high extinction ratio of more than 35 dB over an extremely broad wavelength range from 600 to 1700 nm.We employed a large-mode area PCF (LMA-10 from Crystal Fibre) with a core diameter of about 5.9 m in which the air holes with a diameter of 3.2 m are arranged in a hexagonal pattern with a pitch of 7.5 m, as shown in Fig. 1(a). One end of the PCF with a length of 500 mm was spliced to a standard single-mode fiber (SMF) with a splice loss of about 1.5 dB using the arc fusion-splicing technique [7]. Another end of the PCF was cleaved and then immersed into a refractive-index-matching liquid with a thermo-optic coefficient of −4.15ϫ 10 −4 /°C from Cargille Labs (n = 1.550 at room temperature, http:// www.cargille.com). The fluid was filled into air holes in the PCF with the well-known capillary action, as shown in Figs. 1(b)-1(d). The fully filled PCF has a total length of about 200 mm.Then the opening end of the fluid-filled PCF was butt-coupled to another conventional SMF in order to investigate its transmission spectrum with a supercontinuum white-light source (KOHERAS SuperK Compact) and an optical spectrum analyzer (ANDO AQ6317B). The fluid-filled PCF was placed in a column oven (LCO 102) to investigate its transmission spectra at different temperatures. Figure 2(a) illustrates the measured transmission spectra of the fluid-filled PCF at temperatures of 20°C, 60°C, and 100°C, where the total insertion loss of about 5 dB is attributed to the coupling losses between the PCF and the two SMFs. As shown in the transmission spectrum at 20°C, three clear attenuation gaps were observed within the wavelength range from 750 to 820 nm, from 950 to 1050 nm, and from 1300 to 1550

We present a new method for fabricating structural long-period gratings (LPGs) in photonic-crystal fibers (PCFs). The method is based on periodically drilling holes into the PCF cladding along the length of the fiber by use of a focused femtosecond infrared laser. A very short LPG with only 9 periods and a grating length of < 4 mm exhibited resonance strength of over 20 dB and a polarization dependent loss of 25 dB. The high resonance strength is attributed to the strong modulated mode-field profile caused by the significant perturbation of the fiber geometry. The mechanism of LPG formation is discussed based on coupled local-mode theory.

A half-filling technique was demonstrated to improve the bending properties of a fluid-filled photonic crystal fiber. Such a technique can realize to fill selectively a fluid into half of air holes in a PCF. The bending properties of the half-filled PCF are quite different from those of the fully-filled PCF. Distinct bending properties were observed when the half-filled PCF was bent toward different fiber orientations. Especially, the transmission spectrum of the half-filled PCF was hardly affected while the fiber was bent toward the filled-hole orientation.