Sun Do Lim

Tunable acoustic gratings in solid-core photonic bandgap fiber

We investigate acousto-optic long period grating resonances in a fluid-filled solid-core photonic bandgap fiber (PBGF). The acoustic grating design enables electrically tunable notches in each of the PBGF transmission bands, where both the center frequency and depth of the resonances can be varied. The measured intermodal beat length and resonance bandwidth are in good agreement with numerical simulations based on multipole method. We show that the highly dispersive nature of PBGF modes results in very narrow-band rejection for a given grating pitch.

Combined effects of optical and acoustic birefringence on acousto-optic mode coupling in photonic crystal fiber

We investigate in detail the origin of multiple resonance peaks experimentally observed in an all-fiber acousto-optic tunable filter built with a photonic crystal fiber having slightly deformed air-hole structure and non-circular outer cladding. A model for the acousto-optic mode coupling in the PCF is formulated taking into account both the acoustic and the optical birefringence. All experimental results are in good agreement with numerical calculations.

Twist effect on spectral properties of two-mode fiber acousto-optic filters

The splitting and the shift of resonance peaks in the output spectrum of two-mode fiber acousto-optic devices caused by twist perturbation are described. An elliptical-core two-mode fiber and a two-mode photonic-crystal fiber are used for the analysis. The splitting of the resonance peaks are found to be almost linearly proportional to the twist angle of the fiber with the slopes of about 0.58 nm/(rad/m) for the elliptical-core fiber and about 0.29 nm/(rad/m) for the photonic crystal fiber. The experimental results compare well with the theoretical predictions.

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.

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.

A Novel Birefringent Photonic Crystal Fiber and Its Application to Curvature Measurement

We demonstrate a curvature sensor with a newly designed photonic crystal fiber of high birefringence (Hi-Bi PCF). The proposed PCF has two large air holes in the outer cladding region that give rise to core ellipticity during the fabrication process. The curvature sensor is based on the PCF Sagnac interferometer, the output spectrum shift of which provides information on the PCF curvature. It turns out that the curvature sensitivity depends on the bending directions with respect to the large-air-hole alignment. For the two orthogonal bending directions, the curvature sensitivities near 1480 nm are -1.87 nm/m-1 (parallel to the axis of the two large air holes) and 1.24 nm/m-1 (perpendicular to the axis of the two large air holes), respectively, and are also insensitive to environmental temperature variation. The proposed curvature sensor may be useful in thin structural deflection measurement.

An optical microfiber Sagnac interferometer with adjustable transmission

A Sagnac interferometer based on an optical microfiber is demonstrated. It shows good adjustability of transmission, which enables it to be used as linear edge filter, optical variable attenuator, or gain flattening filter.

Noise Reduction in Multiwavelength SOA-Based Ring Laser by Coupled Dual Cavities for WDM Applications

A novel scheme for the suppression of intra- wavelength-channel mode competition in a multiwavelength SOA-based ring laser is proposed and experimentally demonstrated. The scheme is based on coupled ring cavities, which act as mode filters to suppress the side mode frequencies. The proposed scheme performance is evaluated in terms the output power, long term stability, and side-mode suppression ratio of the proposed SOA-based ring laser. A side mode suppression ratio lager than 10 dB is readily achieved by incorporating a subcavity inside a main ring cavity, due to Vernier effect induced by the coupled dual cavities. Also is investigated the possibility of the use of the noise-reduced multiwavelength laser as a transmitter light source within wavelength division-multiplexed passive optical network.

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.

In situ gas sensing using a remotely detectable probe with replaceable insert

We demonstrate a spectroscopic gas sensor using an optical fiber probe with a replaceable insert. The probe consists of a hollow-core photonic bandgap fiber (HC-PBGF) with a core diameter of 10.9 μm and a glass tube where a 2-μm hollow core fiber (HCF) with a gold coated end facet can be inserted. The HCF is designed to function as both a gate where gases can enter the HC-PBGF and a mirror reflecting the guided light back to the HC-PBGF. The opposite distal end of the probe is also designed to be able to regulate the gas pressure within the HC-PBGF for a high gas flow rate, while still transmitting the reflected light to the analysis instrument. The remote sensing probe, we believe, has much potential for detecting gases in hazardous environments.