Nguyen Hong Ky

Efficient miniature fiber-optic tunable filter based on intracore Bragg grating and electrically resistive coating

An electrically tunable reflection filter based on a platinum-coated single-mode optical fiber that contains an intracore Bragg grating has been demonstrated. The device shows a dc tuning range of 2.15 nm with a corresponding electrical power of 0.54 W. Wavelength modulation (WM) has been observed at frequencies lower than 100 Hz. The wavelength shift depends linearly on the electrical input power. A maximum efficiency of 4.1 nm/W is obtained for dc tuning.

A model for the Zn diffusion in GaAs by a photoluminescence study

To study the mechanism of zinc diffusion in GaAs, we diffused zinc from a ZnAs2 source into Si‐doped GaAs samples (n ≊ 1.3 × 1018 cm−3) at different temperatures (from 575 °C up to 700 °C) in sealed evacuated quartz tubes. The samples are characterized by the depth profile of the photoluminescence at different temperatures. The photoluminescence spectra show characteristic emission associated to deep levels of gallium and arsenic vacancies. A detailed analysis of the spectra demonstrates the role played by vacancies in the Zn diffusion process. The spatial correlation between the luminescence spectra and the Zn concentration obtained from secondary ion mass spectroscopy measurements has been demonstrated.

UV-irradiation-induced structural transformation of germanoscilicate glass fiber

Raman spectra of germanosilicate core fibers before and after UV irradiation were investigated. Significant changes of the Raman spectra after irradiation indicate transformation of the glass structure. A possible interpretation of the observed changes is proposed.

Role of Point-Defects in the Silicon Diffusion in Gaas and Al0.3ga0.7as and in the Related Superlattice Disordering

The mechanism of silicon diffusion in GaAs, Al0.3Ga0.7As, and the silicon diffusion‐induced layer disordering of multiquantum wells have been studied by photoluminescence, secondary‐ion‐mass spectroscopy, and transmission electron microscopy across a corner of a wedge‐shaped sample. The diffusion source was a grown in highly Si‐doped layer. The main photoluminescence properties of point defects in GaAs and Al0.3Ga0.7As are reviewed to interpret the experimental data. The depth profile of the photoluminescence allows the spatial correlation between the luminescence spectra and the Si concentration profile obtained from secondary‐ion‐mass‐spectroscopy measurements. On the basis of the photoluminescence results, the physical processes occurring during the Si diffusion are discussed. Frenkel defects (pairs of element‐III vacancies and interstitials) are generated in the highly Si‐doped region. The element‐III interstitials rapidly diffuse towards the surface where they react with the element‐III vacancies gen...

Effects of drawing tension on the photosensitivity of Sn-Ge- and B-Ge-codoped core fibers

The influence of drawing tension on the formation of Bragg gratings in B-Ge- and Sn-Ge-codoped core fibers is investigated by transmission and stress measurements. With increasing drawing tension, the axial stress is reduced in the B-Ge-codoped core but increased in the Sn-Ge-codoped core. A higher drawing tension leads to a photosensitivity enhancement and an increase of the core stress during the grating formation in B-Ge-codoped fibers. For Sn-Ge-codoped fibers, increased drawing tension results in an UV-induced stress decrease accompanied by a reduction of the photosensitivity. The results are discussed on the basis of an UV-induced compaction of the core network.

Efficient broadband intracore grating LP 01 –LP 02 mode converters for chromatic-dispersion compensation

LP(01)-LP(02) mode converters based on UV-written intracore gratings have been fabricated for chromatic-dispersion compensation. The mode converters operate in transmission at wavelengths near 150 nm with spectral bandwidths of 14-25 nm and coupling efficiencies of as much as 90%. A large negative dispersion of -335 ps/(nm km) is obtained for a compensator consisting of the mode converter and a 1490-m-long few-mode fiber. The compensator shows negative dispersion over a wavelength region of 5 nm. Its figure of merit is estimated to be 370 ps/(nm dB).

Amphoteric native defect reactions in Si-doped GaAs

Strong evidence for amphoteric native defect reactions is obtained by photoluminescence analysis of Si-doped GaAs samples (n≈1.5×1018 cm−3) annealed under different conditions. Annealing in excess As4 vapor creates a large concentration of Ga vacancies, making possible the transformation of this defect into an As vacancy and an As antisite defect. Similarly, As vacancies generated at high concentration during annealing under Ga-rich conditions are transformed into Ga vacancies and Ga antisite defects. Photoluminescence intensities associated with the corresponding defects are in qualitative agreement with the predictions of the mass action law applied to the amphoteric native defect reactions.

400 MHz-bandwidth all-fiber phase modulators with ZnO coating on standard telecommunication fiber

Miniature all-fiber phase modulators with a radially symmetrical piezoelectric ZnO jacket have been characterized over a wide frequency range up to 1 GHz. Multiple radial resonances with phase shift values as high as 0.82 rad were observed in the frequency region between 20-400 MHz. A linear dependence of the phase shift on the square root of the driving power is observed up to 125 mW with a phase shift efficiency of 0.21 and 0.175 rad//spl radic/mW per centimeter fiber length at 196.5 and 399.5 MHz, respectively. The dependence of the phase shift efficiency on the modulation length is demonstrated.

HYDROGEN-INDUCED REDUCTION OF AXIAL STRESS IN OPTICAL FIBER CORES

Radial distributions of axial stress across B/Ge- and Sn/Ge-codoped core fibers and a Corning standard single-mode fiber were measured before and after hydrogen loading under different conditions. A significant reduction of axial stress in the core of all investigated fibers is observed after hydrogen loading. The stress reduction in the core of hydrogen-loaded fibers is irreversible and depends strongly on the core dopants and the fiber drawing tension. The hydrogen-induced core stress reduction is believed to be related to the reactions between hydrogen and drawing-induced defects in the fiber core.

Optical performance of miniature all-fiber phase modulators with ZnO coating

Miniature all-fiber phase modulators with a radially symmetrical piezoelectric ZnO jacket have been characterized using a Mach-Zehnder interferometer over a wide frequency range from 50 Hz to 50 MHz. The frequency response of the modulators consists of two flat broad-band regions and various resonances. The maximum phase shifts measured in the broadband and resonance regions are 0.1 and 7.5 rad/V/meter of fiber, respectively. A linear dependence of the phase shift on the applied voltage up to 60 V (peak-to-peak) is observed in the broad-band region.