Hwa-Yaw Tam

Pressure sensor realized with polarization-maintaining photonic crystal fiber-based Sagnac interferometer

A novel intrinsic fiber optic pressure sensor realized with a polarization-maintaining photonic crystal fiber (PM-PCF) based Sagnac interferometer is proposed and demonstrated experimentally. A large wavelength-pressure coefficient of 3.42 nm/MPa was measured using a 58.4 cm long PM-PCF as the sensing element. Owing to the inherently low bending loss and thermal dependence of the PM-PCF, the proposed pressure sensor is very compact and exhibits low temperature sensitivity.

Stable and uniform multiwavelength erbium-doped fiber laser using nonlinear polarization rotation

A novel and simple technique, based on nonlinear polarization rotation (NPR) effect, to generate stable multiwavelength oscillations in an erbium-doped fiber laser is proposed and successfully demonstrated. The NPR effect effectively induced intensity- and wavelength-dependent loss to alleviate mode competition caused by homogeneous gain broadening in erbium-doped fibers. Up to 28-wavelength lasing operation with wavelength spacing of 0.8 nm has been achieved. The outputs had uniform power distributions, and the power fluctuation in each wavelength is smaller than 0.2 dB within a period of an hour.

Stable and broad bandwidth multiwavelength fiber ring laser incorporating a highly nonlinear photonic crystal fiber

A stable and broad bandwidth multiwavelength fiber ring laser suitable for use in dense wavelength-division-multiplexing systems is demonstrated. A highly nonlinear photonic crystal fiber is inserted in the ring cavity to stabilize the power fluctuation and broaden the output spectrum. A thin-film Fabry-Pe/spl acute/rot etalon filter is used to get stable wavelength lasing. A Sagnac filter is employed to achieve uniform power distribution at each wavelength at the laser output. A laser with a power fluctuation about 0.1 dB in each wavelength within 20 min and with wavelength shift of less than 0.016 nm within 10 min is achieved. A stable 24-wavelength lasing operation with wavelength spacing of 0.8 nm within the 3-dB spectral range of 1573-1594 nm and signal-to-noise ratio better than 48 dB is observed experimentally.

Fundamentals and applications of optical fiber Bragg grating sensors to textile structural composites

In this paper, a review is given on the principles of optical fiber Bragg grating (FBG) sensors and their applications in textile structural composites (TSCs). As a class of novel all-fiber based components, FBG consists of a spatially periodic modulation in the refractive index along a short length of an optical fiber. FBG sensors are immune to electromagnetic interference, small in size, and easily embedded in a variety of composite materials without compromising the host structures. They can be mass-produced in low cost and are unique in their sensing strategies for absolute wavelength coding and multiplexing. The intrinsic wavelength division multiplexing and localized sensing abilities of FBGs are their most important advantages as they provide an effective means for monitoring physical parameters along a single fiber path. This ability to determine quantitatively internal distributions of physical parameters within a TSC integrated with such sensors has been demonstrated by a number of workers and will lead to a better understanding of the relationship between the structure and physical properties.

Trans-4-stilbenemethanol-doped photosensitive polymer fibers and gratings.

We report the development of photosensitive polymeric optical fibers for gratings. The fiber is made of a copolymer of methyl methacrylate and butyl methacrylate, and its core is made of a copolymer of methyl methacrylate, ethyl methacrylate, and benzyl methacrylate (90/4/6 molar ratio) doped with trans-4-stilbenemethanol. Ultraviolet absorption and photoinduced refractive-index change of the doped core system are presented, and its photosensitivity is demonstrated by fabrication of Bragg gratings in the core of the fiber by use of a pulse laser at 325 nm.

Experimental and theoretical analysis of fiber Bragg gratings under lateral compression

Abstract In this paper, an experimental investigation is presented on reflection spectra of fiber Bragg gratings (FBG) under lateral compression together with the theoretical analysis. The coupled mode theory has been employed, by considering the LP x and LP y modes, to obtain accurate results for optical responses of FBG with compression-induced birefringence. The effects of polarization states of the incident light and compression conditions have also been investigated. Good agreements between experimental results and numerical simulations have been obtained.

Fabrication of a compact reflective long-period grating sensor with a cladding-mode-selective fiber end-face mirror

A long-period grating (LPG) based compact optical fiber sensor working in reflection mode is demonstrated. A technique to make a mirror on the cladding region of a fiber end-face to reflect only the cladding modes was realized by growing a polymeric microtip on the core region of the fiber end-face, by photopolymerization, followed by coating the fiber end-face with an aluminum film. Using the cladding-mode-selective fiber end-face mirror, the transmission spectrum of the LPG was inverted and reflected. Preliminary results of using the sensor to measure the refractive index of glycerol/water solutions were successfully demonstrated.

Dual polarization fiber grating laser hydrophone

A novel fiber optic hydrophone based on the integration of a dual polarization fiber grating laser and an elastic diaphragm is proposed and experimentally demonstrated. The diaphragm transforms the acoustic pressure into transversal force acting on the laser cavity which changes the fiber birefringence and therefore the beat frequency between the two polarization lines. The proposed hydrophone has advantages of ease of interrogation, absolute frequency encoding, and capability to multiplex a number of sensors on a single fiber by use of frequency division multiplexing technique.

Linear cavity erbium-doped fiber laser with over 100 nm tuning range.

We report a widely tunable single-frequency linear-cavity erbium-doped fiber laser covering both the conventional wavelength band (C-band) and the long wavelength band (L-band). The laser has low threshold, high slope efficiency and high signal-to-noise radio. A large tuning range of over 100 nm is realized by optimization of the active fiber length.

Cladding-mode-assisted recouplings in concatenated long-period and fiber Bragg gratings.

We investigate new types of mode recouplings in a concatenated grating structure comprising a long-period grating and a fiber Bragg grating. It is demonstrated that the light coupled out to the cladding mode by one of these gratings can be recoupled back to the guided mode by the other grating. Theoretical analysis based on the coupled-mode theory is presented, together with experimental results.