Strong Bragg Gratings Research Articles

Strong cladding mode excitation in ultrathin fiber inscribed Bragg grating with ultraviolet photosensitivity.

Strong UV-written Bragg gratings written in 50 µm-diameter cladding single mode fibers compatible with conventional fiber couple core guided light to dozens of cladding modes distributed across 140 nm in the 1400-1600 nm region, without the need for complex symmetry breaking mechanisms such as tilted, laterally offset, or localized gratings. The extent of the coupling to high order modes and the smaller cladding diameter both contribute to increasing the sensitivity to surrounding refractive index changes by more than one order of magnitude, and to an increased spacing between mode resonances to facilitate unambiguous measurements of larger index changes between 1.3 and 1.44. These improvements are confirmed by theoretical and experimental studies that also cover the temperature and strain differential sensitivities of the cladding mode resonances for complete multiparameter sensing capability.

Inscription of strong Bragg gratings in pure silica photonic crystal fibers using UV femtosecond laser pulses

We demonstrate our experimental results on inscription of strong first-order Bragg gratings in hydrogen-free and hydrogen-loaded pure silica photonic crystal fibers (PCFs) using high-intensity 160 femtosecond (fs) UV laser pulses at 266nm and a two-beam Talbot interferometer. Bragg reflectivities up to 37dB for the H2-loaded fiber as well as Bragg wavelength tuning over the telecom C-band using a single phase mask were achieved. The in-situ nonlinear filamentation of UV fs laser pulses, scattered by the fiber’s micro-structured cladding, is demonstrated by direct refractive index profile measurement across the cross section of a written fiber. The results of thermal annealing measurements for a H2-loaded PCF are also presented.

Fiber Bragg grating inscription based on optical filamentation of UV femtosecond laser pulses

We present the fabrication and characterization of strong first-order Bragg gratings operating around 1550nm in standard telecom fibers (SMF-28) by use of 160fs UV laser pulses at 266nm and a Talbot interferometer. Bragg reflectivities exceeding 50dB are achieved in both hydrogen-free and hydrogen-loaded fibers. In addition, Bragg wavelength tuning over the telecom C-band using a single phase mask is demonstrated. We also show that the formation of Bragg gratings, relying on a two-photon absorption process, is intimately accompanied by the optical filamentation process of UV fs laser pulses during nonlinear propagation in the fiber.

Strong Bragg Gratings in Highly Photosensitive Photo-Thermo-Refractive-Glass Optical Fiber

A new type of photosensitive fiber is demonstrated. Long lengths <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$({&gt;}{\rm 100}~{\rm m})$</tex></formula> of coreless optical fiber are fabricated from highly photosensitive photo-thermo-refractive glass. A minimum loss of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${&lt;}{\rm 2}~{\rm dB}/{\rm m}$</tex></formula> is measured. A holographic technique using low power near-UV two-beam interference patterns is applied to record strong and robust Bragg gratings inside the fiber. The gratings show no degradation when heated up to 425 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}{\rm C}$</tex></formula> for several hours.

Femtosecond direct-write überstructure waveguide Bragg gratings in ZBLAN

Strong waveguide Bragg gratings (10.5dB transmission dip) were fabricated using the femtosecond (fs) laser direct-write technique in ZBLAN glass. The Bragg gratings are based on depressed cladding waveguides and consist of planes, periodic according to the Bragg condition, which are constructed from a transverse hexagonal lattice of smaller point features. Such gratings are a key step toward the realization of mid-infrared monolithic waveguide lasers using the fs laser direct-write technique.

High-efficiency ultraviolet inscription of Bragg gratings in microfibers

In this paper, we demonstrate high-efficiency inscription of Bragg gratings in microfiber. Strong Bragg gratings are easily inscribed using a 193-nm excimer laser and phase mask in microfiber drawn from the 62.5/125- μm standard multimode fiber (MMF) with no additional photosensitization treatment. The enlarged photosensitive core offered by the MMF provides a sufficient overlap between the fundamental mode and the induced refractive index modulation region, which significantly enhances the grating inscription efficiency. The grating inscription in the microfiber is even more efficient than that in the original MMF because the multibeam interference-induced fringe fuzziness in the normal size fiber does not occur as the fiber diameter down to the scale that is comparable with the Talbot length of the ultraviolet (UV)-writing light. The proposed method is beneficial for sensing and device applications of microfiber Bragg grating (mFBGs) because of the ease of fabrication, high coupling strength, and large wavelength separation between individual resonant dips.

Strong photoinduced Bragg gratings in arsenic selenide optical fibre using transverse holographic method

Fabrication of strong fibre Bragg gratings in singlemode arsenic selenide (As2Se3) fibre for use at telecom wavelengths by the transverse holographic method is reported. Photoinduced decrease in the material refractive index was achieved using low-intensity CW illumination at a wavelength of 785 nm. A peak grating strength of 23.7 dB is seen, with inferred change in the average refractive index as large as 4.7×10−3.

Ultrafast IR Laser Writing of Strong Bragg Gratings Through the Coating of High Ge-Doped Optical Fibers

Strong fiber Bragg gratings were written through the standard polymer coatings of off-the-shelf high numerical aperture single-mode optical fibers after a few seconds exposure with femtosecond pulse durations of infrared radiation through a phase mask. While writing through the acrylate coating, we obtained index modulations of up to 1.4 x 10-3 and 7 x 10-4 with and without H2 -loading, respectively.

Stability and collisions of traveling solitons in Bragg-grating superstructures

We investigate stability of moving solitons in a nonlinear optical fiber with short segments of a relatively strong Bragg grating (BG) periodically inserted into it. The model is related to a class of composite and artificial optical media that have recently attracted much interest. The analysis is focused on moving solitons, as this is relevant to the experiment. By means of systematic simulations, we find that, in accordance with a qualitative consideration, the stability region for moving solitons first increases with velocity c but then decreases (in contrast to the uniform fiber BG, where the stability of the solitons depends very weakly on c). As well as in the uniform system, solitons do not exist for c exceeding the group velocity of light in the fiber, nor may they be stable with negative intrinsic frequencies. Collisions between solitons moving in opposite directions are also studied in a systematic way. A difference from the situation in the uniform fiber BG is strong shrinkage and eventual disappearance of a region where the collision results in a merger of the two solitons into a single one, which is explained too. Fiber waveguides with BG superstructures being currently available, moving solitons can be created in them by means of experimental techniques similar to those that have helped to generate solitons in uniform fiber gratings.

Strong Bragg gratings in phosphate glass single mode fiber

Bragg gratings with reflectivities greater than 99% have been fabricated in phosphate glass single mode fibers by irradiating the fibers with 193nm wavelength, high intensity pulses from an ArF excimer laser through a phase mask. Thermal treatment for 1000h at temperatures up to 170°C did not degrade the gratings and resulted in an increase in grating strength. The refractive index modulation amplitudes obtained exceed 10−4.

Fabrication method of Bragg gratings in silica channel optical waveguides

Strong Bragg gratings are realized in silica channel optical channel waveguides by plasma-enhanced chemical vapor deposition and reactive ion etching. In the gratings, the insertion of 0.2-μm-thick SiO 2 layers with a relatively low refractive index between the cores and claddings successfully yields high reflectance, high extinction ratio, and low polarization dependence.

Vacuum-ultraviolet laser-induced refractive-index change and birefringence in standard optical fibers

Strong photosensitivity responses with the use of above-bandgap 157-nm F/sub 2/-laser radiation are reported for standard germanosilicate fiber. A large 1.3/spl times/10/sup -3/ effective index change at 1.55 /spl mu/m was inferred by trimming strong and weak Bragg gratings in hydrogen-free fiber. The F/sub 2/-laser fluence-processing window of 0.01 across a small 1-/spl mu/m penetration depth at the fiber core. The index asymmetry appears to underlie the large >5/spl times/10/sup -5/ value of laser-induced birefringence.

Strong Bragg gratings photoinduced by 633-nm illumination in evaporated As2Se3 thin films.

Bragg gratings are used in several photonic devices to reflect, and thus to isolate, specific wavelengths of light. Gratings can be photoinduced in chalcogenide glasses by illumination of bandgap light in an interference pattern. We used holographic interferometry to create Bragg gratings in amorphous As2Se3 thin films with a period of 0.56 microm by illumination with 633-nm light. The quality of the gratings was tested in real time, and refractive-index modulations as high as 0.037 were measured. These gratings were found to be stable over a period of several months if they were kept in the dark.

Geometric variations in high index-contrast waveguides, coupled mode theory in curvilinear coordinates

Perturbation theory formulation of Maxwell's equations gives a theoretically elegant and computationally efficient way of describing small imperfections and weak interactions in electro-magnetic systems. It is generally appreciated that due to the discontinuous field boundary conditions in the systems employing high dielectric contrast profiles standard perturbation formulations fail when applied to the problem of shifted material boundaries. In this paper we developed a novel coupled mode and perturbation theory formulations for treating generic non-uniform (varying along the direction of propagation) perturbations of a waveguide cross-section based on Hamiltonian formulation of Maxwell equations in curvilinear coordinates. We show that our formulation is accurate and rapidly converges to an exact result when used in a coupled mode theory framework even for the high index-contrast discontinuous dielectric profiles. Among others, our formulation allows for an efficient numerical evaluation of induced PMD due to a generic distortion of a waveguide profile, analysis of mode filters, mode converters and other optical elements such as strong Bragg gratings, tapers, bends etc., and arbitrary combinations of thereof. To our knowledge, this is the first time perturbation and coupled mode theories are developed to deal with arbitrary non-uniform profile variations in high index-contrast waveguides.

Polarization control method for ultraviolet writing of advanced Bragg gratings

We present a flexible and simple method for UV writing of Bragg gratings with advanced apodization profiles including discrete phase shifts. The method is based on a p phase shift between the refractive-index modulation profiles induced by s and p polarization of UV light. By changing the ratio of UV intensity in the two polarizations we are able to control the modulation strength and to induce phase shifts while keeping a constant effective refractive index throughout the Bragg grating. We demonstrate strong UV-written Bragg gratings with Gaussian or sinc apodizations with spectral shapes, in good agreement with theoretical predictions.

Suppression of cladding-mode coupling loss in fiber Bragg gratings by independent control of refractive index and photosensitive profiles in a single-mode optical fiber

A new method to suppress the cladding-mode coupling loss in fiber Bragg gratings, by independent control of the core refractive index profile and the photosensitive profile, is proposed and experimentally demonstrated. Across the core and the inner cladding, a uniform step photosensitive profile was introduced by co-doping GeO/sub 2/ and B/sub 2/O/sub 3/. The core refractive index was selectively raised by further doping Al/sub 2/O/sub 3/ in the core that has negligible photosensitivity at 244 nm. For strong Bragg gratings inscribed on the fiber, the cladding-mode coupling loss was suppressed below 0.3 dB.

Gratings written with tripled output of Q-switchedNd:YAG laser

It is demonstrated that strong Bragg and long period gratings can be written using the frequency tripled output from a high repetition rate Q-switch Nd:YAG laser. No loss due to laser induced damage to the core was observed.

Strong Bragg gratings for WDM devices in non-sensitised low-loss Ge-doped waveguides

Bragg gratings have been written with 193 nm UV-light through a phase mask in low-loss germanium-doped buried waveguides. Without any kind of sensitisation, reflectivities of up to 99.98% and background rejection better than 22 dB have been obtained for 20 mm long Bragg gratings.