Ali Saliminia

Bragg gratings photoinduced in ZBLAN fibers by femtosecond pulses at 800 nm.

Fiber Bragg gratings were written in thulium-doped and undoped single-mode ZBLAN fibers by focusing femtosecond laser pulses on the fiber core through a phase mask. Maximum index modulation of the order of 1 x 10(-3) was induced in both types of fibers. Measurements of the transverse refractive index changes across the core and cladding regions indicate that the grating formation originates from a negative index change.

Optical breakdown versus filamentation in fused silica by use of femtosecond infrared laser pulses.

The competition between optical breakdown (OB) and laser-pulse filamentation (FL) in bulk fused silica is investigated by using a 1-kHz femtosecond infrared laser. We measure input powers corresponding to the threshold of OB and FL in terms of external focusing conditions. The results demonstrate that OB precedes FL for tight focusing, whereas for sufficiently long focal lengths FL takes places at a lower power than OB does.

First- and second-order Bragg gratings in single-mode planar waveguides of chalcogenide glasses

First- and second order Bragg reflectors at telecommunication wavelength (1.5 /spl mu/m) were fabricated in single-mode monolayer (As/sub 2/S/sub 3/) and multilayer (AsSSe-AsS) chalcogenide glass (ChG) planar waveguides with near bandgap illumination using an interferometric technique. Reflectivities as high as 90% near 1555 nm, and index modulations up to 3/spl times/10/sup -4/ were achieved. The volume photodarkening effect is the principal mechanism involved in the formation of the Bragg gratings.

Writing optical waveguides in fused silica using 1 kHz femtosecond infrared pulses

We have investigated the writing of waveguides in bulk pure fused silica glass with femtosecond Ti:Sapphire laser at 1 kHz repetition rate. The photoinduced tracks were characterized in terms of writing geometry (parallel and perpendicular), pulse duration (45 fs, 140 fs, and 200 fs), pulse energy (1–10 μJ), and translation speed (5–150 μm/s) of the sample. Under specific writing conditions, uniform buried waveguides with circular cross section, core diameter of 3–4 μm, and refractive-index change as large as 5×10−3 between core and cladding were achieved.

Ultra-broad and coherent white light generation in silica glass by focused femtosecond pulses at 1.5 μm

We report on the observation of efficient and ultra-broadband white light supercontinuum generated by focusing femtosecond pulses from an optical parametric amplifier at 1.5 microm in silica glass. The characteristic white light spectrum is extending from 400 nm up to at least 1750 nm. At sufficiently high input powers stable white light patterns associated with the interference of spatially coherent filamentary sources were observed and analyzed. Unlike focusing with 800 nm pulses from a Ti-sapphire laser, the stable fringes formed for each spectral component were pronounced owing to significantly reduced destructive impact of optical breakdown on filamentation of femtosecond pulses at 1.5 microm. By taking advantage of this property, the formation of optical waveguides in silica glass with considerably broader range of writing parameters as compared to those fabricated with 800 nm pulses, was demonstrated.

Ytterbium fiber laser based on first-order fiber Bragg gratings written with 400nm femtosecond pulses and a phase-mask

A Fiber Bragg grating of 369 nm pitch was inscribed in a germanium-free double-clad ytterbium doped silica fiber using a femto-second pulse train at 400 nm wavelength and a phase mask. The photo-induced refractive index modulation of higher than 4 x 10(-3) was obtained and the accompanying photo-induced losses were subsequently removed by thermal annealing, resulting in a low loss (<0.1 dB), stable and high reflectivity (>40 dB) FBG. Based on this FBG, a monolithic Ytterbium fiber laser operating at 1073 nm with slope efficiency of 71% and output power of 13 W was demonstrated.

Densification of silica glass induced by 0.8 and 1.5μm intense femtosecond laser pulses

We investigate the physical mechanisms responsible for waveguide formation in silica glass induced by 1kHz intense femtosecond laser pulses from a Ti-sapphire laser at 0.8μm as well as from a femtosecond optical parametric amplifier at 1.5μm. It is demonstrated that the densification taking place at the irradiated region is the principal cause for refractive index change in the waveguides written with both 0.8 and 1.5μm pulses. The birefringence induced by the stress arising from such densification and its behavior against thermal annealing are also studied.

Microchannel fabrication in silica glass by femtosecond laser pulses with different central wavelengths

A newly developed tunable visible femtosecond laser source was used to fabricate microchannels in silica glass. For comparison, femtosecond laser pulses from a Ti-sapphire laser at 800 nm as well as from a femtosecond optical parametric amplifier at 1.3 µm were also employed to fabricate microchannels. We found that it is much more efficient to drill micro-channels using the visible pulses because of their lower damage threshold. The quality of the cross section of the microchannel is also better with this visible femtosecond laser source because of the high beam quality of the visible pulses.

Temperature dependence of Bragg reflectors in chalcogenide As 2 S 3 glass slab waveguides

The temperature dependence of the spectral response of Bragg grating filters at 1550 nm, written in single-mode planar waveguides of As2S3 chalcogenide glasses, is presented. It was found that Bragg reflectance increases with temperature (corresponding to an increase as high as 4 dB of the transmission depth, from 20 °C to 50 °C), whereas the resonant wavelength shifts by ∼1 nm. The difference between temperature dependence of the structural properties for exposed and unexposed areas is thought to be responsible for the increase of refractive-index modulation depth. Two experimental techniques are used to further study the observed phenomena.

Photoinduced Bragg reflectors in As-S-Se/As-S based chalcogenide glass multilayer channel waveguides

We have fabricated Bragg reflectors at 1550 nm in multilayer channel waveguides of As-S-Se/As-S chalcogenide glasses using near-bandgap light (∼514 nm). A reflectivitity up to 86% (∼8.5 dB), and a linewidth of ∼0.3 nm were achieved for a 3.0 mm long first order Bragg grating. The polarization dependence of these Bragg filters was also studied.In recent years, amorphous chalcogenide glasses (ChG) have attracted much attention owing to their potential application in infrared communication systems and integrated photonics [1,2]. Because of important properties of ChG, such as high transparency in the 1–10 m m wavelength region, large nonlinear index of refraction (almost two orders) of magnitude higher than silica glasses, and low phonon energy, numerous applications for these materials have emerged, such as low-loss fibers and waveguides, fast nonlinear all optical switches, and optical amplifiers [3–5]. Chalcogenide glasses have also shown high photosensitivity upon exposition at a wavelength corresponding to bandgap energy [6,7]. In particular, photodarkening and photoexpansion have been a focus of research due to the high potential in realizing a variety of holographic elements, such as microlenses, gratings, and channel waveguides in bulk and thin films of ChG [8–10].