Ivan Kasik

Theoretical modeling of fiber laser at 810 nm based on thulium-doped silica fibers with enhanced 3H4 level lifetime.

A compact upconversion fiber laser operating around 810 nm is proposed using thulium-doped silica-based fiber with locally modified thulium environment by high alumina codoping. Using a comprehensive numerical model of thulium doped fiber we investigate performance of the proposed laser. Comparison with two other thulium hosts, fluoride glass and standard silica, is presented. Efficient lasing can be expected even for silica based fiber for specific ranges of the fiber and laser cavity parameters, especially when 3H4 lifetime is enhanced. With moderate pump power of 5 W at wavelength of 1064 nm, the predicted output power of the upconversion laser is about 2 W at 810 nm.

Experimental demonstration of novel end-pumping method for double-clad fiber devices

We present experimental demonstration of an end-pumping scheme based on splicing the multimode pump and single-mode signal fibers directly to a double-clad fiber with a tailored cross section. The method is used to pump a double-clad, erbium- and ytterbium-doped, fiber ring laser. The efficiency of the end-pumping method is tested by determining the slope efficiencies of the fiber ring laser and the fiber laser in a Fabry-Perot configuration. Comparable slope efficiencies are found when both laser configurations have similar output coupler ratios. The developed pumping scheme and double-clad fiber can find applications in cost-effective power fiber amplifiers and lasers.

In situ study of the epoxy cure process using a fibre-optic sensor

Real-time, in situ monitoring for quality control of the polymer cure process is of great interest. In fact, high-performance composites, made with polymer matrixes, are used extensively in high-tech areas, such as the aircraft, space and automobile industries. In particular, epoxy resin reinforced with fibre is a system with good mechanical properties and low density. In this paper, a fibre-optic sensor to monitor the cure of an epoxy resin is studied. Optical fibres are, in fact, compatible with the reinforcing fibre in laminate composites. This sensor is based on the measurement of the angular distribution of light transmitted through an optical fibre inside the cured polymer. The light guiding properties of this sensor are linked to the difference between the refractive index of the core and that of the cladding. So, by partially removing the cladding and placing a sample of curing epoxy around the stripped region, it is possible to monitor the extent of the cure. In fact, the refractive index of the DGEBA/amine system varies from 1.54 to 1.58, mainly due to the increase in density during the cure. For manipulation, an experimental fibre made of special glass was prepared so as to have a high refractive index core (n = 1.62). The sensitivity to the refractive index was tested with aniline (n = 1.58) mixed with various concentrations of toluene (n = 1.50). The response of the sensor to this liquid test showed that it would be possible to measure the refractive index in the index range of epoxy during cure. Tests with an epoxy system were also performed. In order to understand the angular distribution of the transmitted light, a model based on geometrical optics was developed using the reflection coefficient of the optical ray on a boundary between two dielectrics. This allowed us to obtain real and imaginary parts of the cladding refractive index, thus the cross-link density during the epoxy cure can be monitored with a single fibre-optic sensor.

Optical fiber with novel geometry for evanescent-wave sensing

Abstract First results in the preparation and analysis of an optical fiber with a novel geometry which facilitates the access of chemical species to the evanescent field for sensing purposes are presented. This ‘s-fiber’ is of approximately sectorial cross section with the core located in the carefully rounded vertex of the sector. Using a perturbation method, the dependence of the attenuation coefficient of the fundamental mode in a weakly-guiding, step-index s-fiber on the fiber normalized frequency, vertex angle and cladding thickness are determined. Attenuation coefficients several times higher than in D-fibers are theoretically attainable. Preforms for drawing s-fibers are prepared from standard MCVD preforms by accurate grinding and polishing the preforms to a desired sectorial shape. Multimode s-fibers with core dimension of about 30 μm and cladding size of about 170 μm and exhibiting satisfactory strength have been drawn. Resulting shapes of the fiber and core depend on the shape, structure and composition of the preform, drawing temperature and drawing velocity. Results have proved the feasibility of the chosen approach to the laboratory preparation of s-fibers. In preliminary experiments the sensing ability of the drawn fibers has been examined.

Consequences of TiO2 doping on the optical properties of porous silica layers coated on silica optical fibers

Abstract The paper deals with the investigation of optical properties of porous silica and TiOTi chains – doped porous silica layers coated on silica optical fibers. The porous layers composed of dried gels have been prepared by the sol–gel method from alkoxides. The refractive indices and optical attenuation of the layers have been determined from the measured angular distributions of the output optical power. For this purpose, a model based on geometrical optics has been developed. The interaction of the porous layers with benzene, toluene, xylene and hexane has been investigated by measuring the changes of the output optical power induced by the effect of vapors of the hydrocarbons in air. On the basis of these measurements, the porosity of the layer, partition coefficients and equilibrium adsorption constants of the hydrocarbons have been estimated. The measured decrease of the optical power for the aromatic hydrocarbons and the increase of the output power for the aliphatic hydrocarbon have been explained on the basis of the developed model.

Twin-core fiber design and preparation for easy splicing

Several methods of preparing twin-core fibers (TCFs) that can be easily spliced to standard single-core single-mode fiber are proposed. Unlike the conventional TCF preparation methods that are used to fabricate a TCF with both cores placed symmetrically with respect to the fiber axis, these methods result in twin-core fibers that have one core in the fiber center. Experimental results obtained with the TCF fabricated by using one of the designs are presented.

All-fiber Ho-doped mode-locked oscillator based on a graphene saturable absorber

In this Letter, we demonstrate a graphene mode-locked, all-fiber Ho-doped fiber laser generating 1.3 nJ energy pulses directly from the oscillator. The graphene used as a saturable absorber was obtained via chemical vapor deposition on copper substrate and immersed in a poly(methyl methacrylate) support. The laser generated ultrashort soliton pulses at 2080 nm with bandwidth up to 6.1 nm. The influence of the output coupling ratio and the SA modulation depth on the mode-locking performance was also investigated.

Generation of high-repetition-rate pulse trains in a fiber laser through a twin-core fiber

We demonstrate the application of a twin-core fiber comb filter to the generation of high repetition rate pulse trains in fiber lasers. We have found experimentally that passive mode locking of the fiber laser can be established due to concurrent effects of a nearly periodic transmission function of the twin-core fiber filter and of the modulational instability. The period of the generated pulse train is determined by the intermodal dispersion of the twin-core fiber inserted into the fiber laser cavity. A repetition rate as high as 206 GHz was achieved. The width of the generated pulses was 2.7 ps.

Detection of aromatic hydrocarbons in air and water by using xerogel layers coated on PCS fibers excited by an inclined collimated beam

This paper deals with experimental results on the sensitivity of silica optical fibers coated with detection xerogel membranes to benzene and toluene both in gaseous mixtures and in aqueous solutions. For this purpose thin polysiloxane and methyl-group-doped polysiloxane xerogel membranes are applied on straight segments of PCS fibers. The interaction of these membranes with hydrocarbons is investigated on the basis of changes of the output optical power from the fibers excited by an inclined collimated beam. The results obtained show that the hydrophobic methyl-group-doped xerogel layers exhibit higher sensitivity to hydrocarbons both in the gaseous phase and in solutions. The detection limits are about 230 ppm vol. for toluene in air and about 9 mg/l for toluene in water. Both the membranes show lower sensitivity to benzene than to toluene.

INVERTED-GRADED INDEX FIBER STRUCTURES FOR EVANESCENT-WAVE CHEMICAL SENSING

Abstract The paper deals with a novel multimode fiber-optic structure, the inverted-graded index profile (IGI) fiber, and its feasibility for evanescent-wave chemical sensing. Results of the theoretical analysis of the sensitivity of the IGI fiber to changes of the refractive index and light absorption coefficient of its cladding are shown. Fabrication of IGI fibers doped in the core with GeO 2 or B 2 O 3 is described. The evanescent-wave sensitivity of the prepared IGI fibers to changes of the cladding refractive index and light absorption coefficient is given showing its increase in comparison with reference fibers with cores of pure silica and with the same polymer claddings as the IGI fibers.