Wen-Fung Liu

Q-switched all-fiber laser with an acoustically modulated fiber attenuator

An actively Q-switched all-fiber laser based on an acoustically modulated fiber attenuator was implemented for producing 1550.5 nm pulses of 3 /spl mu/J in pulse energy, 150 ns in pulse width, and 5 kHz in pulse repetition frequency. The fiber attenuator was combined with a fiber Bragg grating, which was used as the end mirror and the wavelength selector of the laser system. The fiber attenuator was implemented with the excitation of transverse vibration by a modulated acoustic wave. The transverse vibration or micro bending led to the coupling of the core mode and cladding modes. Through this mechanism, the feedback level from the fiber grating or the Q factor of the laser can be well controlled. The Q-switched fiber laser based on this scheme can be quite simple.

Reflectivity-tunable fiber Bragg grating reflectors

We report a novel device that modulates the Bragg reflectivity of a fiber grating by exciting the transverse vibration of the fiber through an acoustic wave. The excitation of the transverse vibration, leading to fiber microbending, induces the coupling of the fiber core mode into cladding modes. This leads to the reduction of core-mode power and hence that of Bragg reflection. This mechanism provides us a means to control the reflectivity after a fiber Bragg grating is fabricated. The numerical results based on a simple micro-bending model agree well in trend with the experimental data.

Temperature compensation of optical fiber Bragg grating pressure sensor

A temperature-insensitive sensing scheme for pressure measurement is presented. In the sensing scheme, a single optical fiber Bragg grating (FBG) is axially strained by its surrounding pressure, and is thermally compensated by a bimaterial effect. The FBG is kept tight (i.e., in a positive strain) initially in the sensing structure. When the surrounding temperature increases, a negative strain (to loosen the FBG) is incurred on the FBG and produces a blue shift of the Bragg wavelength. Temperature compensation is, thus, made by balancing such blue shift and the inherent thermally induced red shift of the FBG. Experimental results show a pressure sensitivity of the fractional change in the Bragg wavelength of 1.8/spl times/10/sup -2/ (MPa)/sup -1/, and demonstrate quite a low response to the temperature varying from 10/spl deg/C to 60/spl deg/C.

High-sensitivity temperature-independent differential pressure sensor using fiber Bragg gratings.

By means of novel packaged-structure design, a temperature independent differential pressure sensor based on fiber Bragg gratings with high sensitivity is experimentally demonstrated. The differential pressure sensitivity of the sensor can reach to 821.87nm/MPa. This device can also be used for simultaneous measurement of temperature and differential pressure, which is suitable for applications involving measurement of liquid level, liquid density or specific gravity detection.

Excimer-laser-induced activation of Mg-doped GaN layers

In this study, we investigated the 248 nm excimer-laser-induced activation of the Mg-doped GaN layers. According to the observed photoluminescence results and the x-ray photoelectron spectroscopy measurements, we found that the dissociation of the Mg–H complexes and the formation of hydrogenated Ga vacancies (i.e., VGaH2) and/or the Ga vacancies occupied by interstitial Mg during the laser irradiation process, led to an increase in the hole concentration.

Multipoint temperature-independent fiber-Bragg-grating strain-sensing system employing an optical-power-detection scheme

A temperature-independent fiber-Bragg-grating strains-sensing system, based on a novel optical-power-detection scheme, is developed and analyzed. In this system a pair of fiber Bragg gratings with reflection spectra either partially or substantially overlapping is placed side by side to form a temperature-independent strain-sensor unit. Conventional wavelength-interrogation techniques are not used here, and instead an optical-power-detection scheme is proposed to directly calibrate the measurand, i.e., the strain. Unlike the conventional approach in a multiplexed sensing system, the presented power-detection-based interrogation method does not need the fiber-Bragg-grating sensors to be spectrally separate. The only requirement is that the spectra of the two fiber Bragg gratings of each sensor unit in a multiplexed system be identical or slightly separate (slightly overlapping spectra would also work in the sensing scheme) and the sources optical power be sufficient for sensitive measurement. Based on a three-sensor-unit system, we demonstrate simple strain measurements of high linearity (+/- 0.4%), good sensitivity [2 microstrains (microS)], high thermal stability (+/- 0.8%), and zero cross talk. The effects of light source spectral flatness and fiber bending loss on measurement accuracy are also discussed.

High-sensitivity simultaneous pressure and temperature sensor using a superstructure fiber grating

In this paper, we show that both pressure and temperature can be measured simultaneously by using a high-sensitivity fiber sensor. This sensor has a superstructure fiber grating (SFG) encapsulated in a polymer-half-filled metal cylinder, which has two openings on opposite sides of the wall of the polymer, to sense the pressure. The sensed pressure is transferred into axial extended-strain. The variation of pressures and temperatures will cause the variation of the center-wavelength and reflection of the SFG simultaneously due to the optical response of the SFG composed by the fiber Bragg grating (FBG) as well as long-period grating (LPG). Thus, the sensor can be used for measuring pressure and temperature simultaneously. It has a pressure sensitivity of 3 times 10-2 MPa-1, better than that using only a bare FBG. Temperature sensitivities in both 0.02 nm per degC and 0.16 dBm per degC have experimentally been obtained. This fiber sensor can be applied for boiler as well as for the underwater depth measurement

Hybrid AG-FFPI/RLPFG for Simultaneously Sensing Refractive Index and Temperature

This work presents a novel, simple, and sensitive device for simultaneously sensing refractive index (RI) and temperature (T) of its surrounding environment. The sensing elements are based on a hybrid air-gap fiber Fabry-Pérot interferometer (AG-FFPI) with a reflective long-period fiber grating (RLPFG). An air gap of around 10 μm is formed in an Sn-overlaying process on the fiber endface to make a fiber Fabry-Pérot interferometer whose interferometric cavity is formed between the fiber endface and the surface of the Sn metal. The proposed devices are characterized by the simple fabrication and high sensitivity to both T and RI, which can be simultaneously measured. Additionally, the proposed device can readily recognized the response from ambient variations in T or the RI.

Analysis of phase-matching conditions in flexural-wave modulated fiber Bragg grating

Effective refractive indexes of the core mode and various cladding modes of a tapered single-mode step-index fiber were calculated for identifying the cladding modes of phase matching for coupling with the core mode, caused by a prewritten Bragg grating together with an applied flexural wave (microbending) in the tapered region. With these coupling mechanisms, the previously reported reflection wavelength switching could be well interpreted. Between the core mode and a cladding mode, the Bragg grating caused contradirectional coupling and the flexural wave resulted in codirectional coupling of either first- or second-order diffraction. Meanwhile, the period of the acoustic-induced flexural wave was calibrated to be a few hundreds of micrometers. Based on the phase-matching calculations, the relationship between cladding radius and flexural wave period with a chosen wavelength for reflection switching was provided. Such design considerations should be helpful in implementing wavelength switches, based on flexural wave modulation of fiber Bragg grating (FBG), for the applications of wavelength division multiplexing (WDM) fiber communications.

An Ultra-Sensitive Liquid-Level Indicator Based on an Etched Chirped-Fiber Bragg Grating

A novel ultra-sensitive liquid-level indicator using an etched chirped fiber Bragg grating is experimentally demonstrated with a sensitivity of 1.214 nm/mm for detecting tens of micro-meter liquid-level variation. The operation mechanism is based on the superposition wavelength-peak shift created by the overlap between the spectrum of the etched CFBG section immersed in the liquid and the spectrum of the rest of the grating in air. This sensor may be used to precisely measure the liquid-level variation in bio-hazardous systems or industrial containers that demand accurate interrogation of the amounts of the contents.