Effect of metal coating in all-fiber acousto-optic tunable filter using torsional wave.

Torsional mode acousto-optic tunable filter (AOTF) is demonstrated using a metal-coated birefringent optical fiber for an improved robustness. The changes in acoustic and optical properties of a metal-coated birefringent optical fiber induced by the thin metal coating were analyzed experimentally and theoretically. The filter wavelength shift is successfully explained as a result of combined effect of acoustic wavelength change and optical birefringence change. We also demonstrated a small form-factor configuration by coiling the fiber with 6 cm diameter without performance degradation. The center wavelength of the filter can be tuned >35 nm by changing the applied frequency, and the coupling efficiency is higher than 92% with <5 nm 3-dB bandwidth.

Design of waveguide grating with ultrafast tunable index contrast.

A long-period waveguide grating (LPWG) with a tunable index contrast is proposed. The design features a simple configuration that consists of a two-mode waveguide formed on periodically poled lithium niobate with an angle with respect to its domain wall and a traveling-wave electrode. In the design, the electrical traveling wave introduces a periodic change in the refractive index of waveguide, which functions as a long-period waveguide grating that couples between symmetric and anti-symmetric core modes. The index contrast of grating can be controlled by the traveling-wave intensity. For application to ultrafast device, structural parameters satisfying velocity and impedance matching conditions are numerically calculated.

Experimental excitation and characterization of cladding modes in photonic crystal fiber.

We extend the previous theoretical study on the effect of outer silica cladding to the analysis on real field profiles in a practical PCF. Clear field profiles for the first higher-order cladding modes are presented and discussed. The observed mode fields are reproduced by numerical calculations, and it turns out that they correspond to LP(16) mode groups. Optical properties of the observed modes such as lobe direction and polarization are also investigated. The results of this study will be useful in the design of the PCF-based optical devices utilizing cladding mode coupling.

Fabrication of a highly efficient core-mode blocker using a femtosecond laser ablation technique.

We present a method for the fabrication of a highly efficient core-mode blocker based on a laser micromachining technique. The process for the fabrication is as follows. A micron-sized crater is made by irradiation of an ultra-short pulse laser on the end face of a single-mode fiber, and then a defect that acts as a core-mode blocker is formed by splicing the cratered fiber with a normal fiber. The attenuation of the core mode was adjustable up to 25 dB according to the initial crater size and the splicing condition. An all-fiber acousto-optic tunable bandpass filter built with the core-mode blocker is also demonstrated, which exhibits a low insertion loss of 1.8 dB with non-resonance light suppression greater than 23 dB.