Tsung-Ein Tsai

Fiber Bragg reflectors prepared by a single excimer pulse

Narrow-line, permanent Bragg reflection gratings have been created in Ge-doped silica-core optical fibers by interfering beams of a single 20-ns pulse of KrF excimer laser light. Of the fibers studied, the highest reflectance value of ~2% was observed with a linewidth (FWHM) of 0.1 nm, which corresponds to a 2-mm grating length with an index modulation of ~3 x 10(-5).

Index structure of fiber Bragg gratings in Ge-SiO(2) fibers.

The behavior of the concentration of photoinduced color centers in Ge-SiO(2) optical fibers was compared with that of the index modulation associated with fiber Bragg gratings (FBGs) written in the same fibers. We find that the fluence dependence of the photoinduced Ge E? center, its thermal annealing behavior, and its reaction with H(2) are similar to that of the index modulation generated in both H(2)-loaded and unloaded Ge-SiO(2) fibers. The much higher photosensitivity of H(2)-loaded Ge-SiO(2) fibers is attributed to the much higher formation efficiency of Ge E? centers, with an additional contribution from GeH. A diamagnetic structure, possibly densification, is also found to contribute to the index modulation of FBGs.

Photoinduced grating and intensity dependence of defect generation in Ge‐doped silica optical fiber

Ge E’ centers photoinduced in Ge‐doped silica by 5 eV photons of various intensities and fluences were found by electron spin resonance to be induced and bleached by one‐ and two‐photon absorption processes, respectively. The observation that Ge E’‐type centers are the only paramagnetic centers induced by very low intensity 5 eV photons in Ge‐doped silica supports the proposal that Ge E’‐type centers are responsible for the photoinduced gratings observed in both Bragg grating and second‐harmonic generation fibers.

GRATING SENSOR ARRAY DEMODULATION BY USE OF A PASSIVELY MODE-LOCKED FIBER LASER

A fiber Bragg grating sensor array is interrogated by use of a passively mode-locked fiber laser source. A novel demodulation scheme that uses highly dispersive fiber to convert the grating wavelength shift to a temporal shift in the arrival time of the reflected pulses is demonstrated. The source bandwidth of >85 nm permits interrogation of many-grating arrays, and the demodulation technique permits fast sensing of large strains.

Structural origin of the 5.16 eV optical absorption band in silica and Ge‐doped silica

The origin of the 5.16 eV absorption band observed in silica and Ge‐doped silica was studied using optical and electron spin resonance (ESR) measurements. The band was observed only in samples containing Ge, suggesting that it is related to the Ge impurity in silica, while a lack of correlation between the ESR intensity of the induced hydrogen‐associated doublet and the absorption coefficient of the 5.16 eV band indicates that it is not related to two‐coordinated Si or Ge. The observation of the absorption coefficient increased as the square root of the Ge concentration demonstrates that the 5.16 eV band is not related to two‐coordinated Ge defects but that it is an oxygen deficiency center of the divacancy type associated with Ge.

Defect Centers and Photoinduced Self-Organization in Ge-Doped Silica Core Fiber

Photoinduced defect centers by harmonics of 1.06 micrometers photons in Ge-doped silica are discussed and proposed defect models reviewed in relation to the self-organization phenomena, such as Hill gratings and second harmonic generation (SHG) in Ge-doped silica core fibers. In particular, we will show that the reported preparation kinetics of SHG in both seeded and unseeded processes can be explained in terms of defect centers induced by the fourth harmonic of 1.06 micrometers photons. The SHG erasure kinetics can also be understood by the reaction of defect centers responsible for SHG with free excitons.

Radiation effects on a low‐thermal‐expansion glass ceramic

Electron‐spin‐resonance and density gradient column techniques have been used to measure and characterize the effect of radiation on the electronic and mechanical properties of a low‐thermal‐expansion glass ceramic. Defect centers attributed to Zn+, oxygen hole centers (OHCs), Fe3+, Ti3+, and/or Zr3+, and two types of centers associated with arsenic have been observed; the radiation‐induced compaction has been tentatively correlated with the OHC concentration.

Si E′ centers and UV-induced compaction in high purity silica

Abstract Paramagnetic defect centers induced by 6.4 eV excimer-laser photons in high purity silicas at room temperature were studied by electron spin resonance and their line shapes computer simulated. Broad spectra with g-values about the same as those of Si E′ proceeds with an centers were observed at high laser pulse energies in cylindrical samples which strongly focused the laterally impinging UV light to a line in the interior. Data are presented which indicate that this broad line shape is the spectrum of an E

Sensor grating array demodulation using a passively mode-locked fiber laser

center variant located in the focused region of the silica, which has been densified by UV-irradiation.

Radiation‐induced defect centers in glass ceramics

Proven compatibility with civil engineering applications and composite structures has spurred increasing interest in fiber Bragg gratings (FBGs) for distributed optical fibre strain sensing. We demonstrate a fundamentally different demodulation scheme based on the high-energy, ultrafast, broadband pulses generated by a passively mode-locked erbium fiber laser as an alternative approach for high-speed interrogation of large grating arrays.