Jan Aubrecht

Self-swept holmium fiber laser near 2100 nm

Self-sweeping of laser wavelength corresponding to holmium emission near 2100 nm is reported. The sweeping occurred in ~4 nm interval with rate ~0.7 nm/s from longer towards shorter wavelengths. Origins of the selection of the sweeping direction are discussed. The laser wavelength drift with time was registered by Fourier transform infrared spectrometer. To our knowledge it is the first observation of self-swept fiber laser beyond 2000 nm.

Enhanced pump absorption efficiency in coiled and twisted double-clad thulium-doped fibers.

Results of the first experimental demonstration of the recently proposed technique for improvement of the pump absorption in double-clad fibers by their simultaneous coiling and twisting are reported. The peak absorption (14 dB) of 3-m long hexagonal thulium-doped fiber was increased by 8 dB by its simultaneous coiling and twisting. Explanation of the effect is given by numerical modelling of the pump absorption in hexagonal and panda-type double-clad fibers. Improvement of fiber laser performance was also proved. The slope efficiency increased from 19.6% of the straight fiber to 23.9% of the coiled only fiber and 29.4% of the simultaneously coiled and twisted fiber.

Preparation and characterization of Bragg fibers with air cores for transfer of laser radiation

This paper presents results of preparation and characterization of Bragg fibers with silica and air cores designed for delivery of laser radiation at 1060 nm. The fibers consist of cores with a refractive index equal to that of air or silica which is surrounded by three pairs of Bragg layers. Each pair is composed of one layer with a high and one layer with a low refractive index and is characterized by a refractive-index contrast up to 0.03. Three structures of Bragg fibers are presented in the paper, namely the structure with a silica core of 26 μm in diameter, structure with an air core of 5 μm in diameter and that with an air core of a diameter of 72 μm. Preforms of the Bragg fibers in the form of a rod or tube have been prepared by the MCVD method using germanium dioxide, phosphorous pentoxide and fluorine as silica dopants. The fibers have been drawn from the preforms under controlled temperatures in order to obtain fibers with air or solid cores. Results of characterization of prepared fibers with optical microscopy and by measuring their refractive-index profiles, losses and angular distributions of the output optical power are presented. The characterization of fibers for delivery radiation of a Nd:YAG laser with nanosecond pulses at 1060 nm, namely the transmission, attenuation coefficient, spatial profiles of transmitted beams, and bending losses are also presented. Fiber damage thresholds in a range 25-30 GW/cm2 have been determined.

Development of Ammonia Gas Sensor Using Optimized Organometallic Reagent

Reliable, continuous, and spatially distributed monitoring of dangerous or irritating chemical substances belongs to standard functions of contemporary industrial and public security systems. Fiber-optic-based detection provides feasible platform to fulfill such aims. This paper deals with characterization of ammonia sensing elements based on multimode polysiloxane-clad silica-core optical fibers sensitized with 5-(4′-dioctylamino phenylimino) quinoline-8-1 cobalt bromide complex reagent immobilized into the cross-linked polymer matrix from a proper mixture of organic solvents and a radical scavenger contributing to the desired long-term stability of optical properties. The applied sensing mechanism combines optical detection principle with chemical reaction of the reagent and ammonia resulting in changes in the visible near-infrared optical absorption spectrum of the cladding layer, influencing via evanescent optical field interactions the spectral distribution of the guided light intensity. Reaction kinetics of short fiber sections exposed to ammonia/nitrogen mixture of various ammonia concentrations is tested and evaluated. The obtained sensitivity, limit of detection, and forward response time of the prepared sensors amount to ppm−1, 31 ppm, and 25 s, respectively. The obtained results are promising for fabrication of distributed fiber-optic sensors applicable to detection and location of ammonia gas leaks in industrial as well as general public premises.

Development of absorption fiber optic sensor for distributed measurement of ammonia gas

Polymer-clad silica optical fibers are employed for development of different absorption optic fiber sensors of gaseous analytes. In our case, the physical principles of the detection are combined with a chemical reaction between analyte and suitable opto-chemical absorption reagents. Selected organometallic complex reagents with different lengths of lateral aliphatic chains are studied with respect to the type of central ions and their coordinative conditions to surrounding ligands. The effect of solvent type on solubility and the long-term stability of the prepared reagents in solid matrix are presented and discussed. Various methods are also tested in order to achieve an effective reagent immobilization into the polymer matrix, which creates optical fiber cladding. The chemical reaction of the reagents with ammonia based on ligand exchange process is accompanied by changes of visible-near-infrared optical absorption influencing via evanescent field on the guided light intensity. Experimental results suggest that the selected reagents provide optical properties suitable for practical sensing applications and that the sensitized PCS optical fibers could be used for detection of ammonia gas.

The influence of nanostructured optical fiber core matrix on the optical properties of EDFA

In this contribution we demonstrate the effect of the nanostructured optical fiber core matrix, doped with erbium and Al2O3, on the resulting optical properties. Several optical fibers with nanostructured cores were drawn from preforms prepared by different techniques, i.e., by conventional doping from solution of erbium and aluminium chlorides, by deposition of the dispersed alumina nanoparticles with either Er3+ ions or Er2O3. Reference bulk samples were prepared by the solid-state approach and thermally treated by similar way as optical fibers. Prepared optical fibers and bulk samples were investigated by the absorption spectroscopy. Reference samples were analyzed by the X-ray diffraction for the determination of the crystalline properties of formed nanostructures. It was found that nanocrystals inside the optical fiber core matrix improves the homogeneity and decreases the basic optical attenuation. Nanostructured alumina inside the fiber core matrix enhances the absorption properties of Er3+ ions.

Spontaneous laser-line sweeping in Ho-doped fiber laser

Spontaneous laser-line sweeping refers to fiber laser instabilities with regular laser wavelength drift within a broad range that may exceed 10 nm; other characteristics of the laser output are sustained relaxation self-pulsing and narrow spectral linewidth. The laser wavelength drift is caused by standing-wave in the cavity; it can be regarded as a special case of mode instability of longitudinal modes of the laser resonator. Self-sweeping was observed so far in Yb, Er, Tm and Bidoped fiber lasers. We report on Ho-doped fiber laser self-sweeping in interval of 3-5 nm near 2100 nm. The sweeping rate was typically 0.7-0.9 nm/s. The thulium-doped fiber lasers at around 2030 nm and 1950 nm were tested as pump sources. The self-sweeping was registered by FTIR spectrometer and the data processing is discussed.

Characterization of holmium fibers with various concentrations for fiber laser applications around 2.1 μm

In this work, we present experimental results of characterization of the developed holmium-doped silica-based optical fibers with holmium ions concentrations in the range from 1000 to 10000 ppm. The fibers were fabricated by the modified chemical vapor deposition and solution doping method. They were characterized in terms of their spectral attenuation, refractive index profile, and especially performance in fiber laser. Simultaneously, two different fiber laser setups were tested. In the first one, holmium-doped fiber in Fabry-Perot configuration was pumping by in house developed thulium-doped fiber laser in ring arrangement. In the second one, bulk-optic pump-coupling configuration, consisted of a commercially available thulium fiber laser emitting at 1940 nm and system of lenses and mirrors was used. We have focused on comparison of laser output powers, slope efficiencies, and laser thresholds for individual holmiumdoped fiber in these different laser arrangements. Finally, the application of the developed fiber in subpicosecond fiber laser with graphene-based saturable absorber for mode-locking operation was investigated.

Thulium-doped fibre broadband source for spectral region near 2 micrometers

Abstract We demonstrated two methods of increasing the bandwidth of a broadband light source based on amplified spontaneous emission in thulium-doped fibres. Firstly, we have shown by means of a comprehensive numerical model that the full-width at half maximum of the thulium-doped fibre based broadband source can be more than doubled by using specially tailored spectral filter placed in front of the mirror in a double-pass configuration of the amplified spontaneous emission source. The broadening can be achieved with only a small expense of the output power. Secondly, we report results of the experimental thulium-doped fibre broadband source, including fibre characteristics and performance of the thulium-doped fibre in a ring laser setup. The spectrum broadening was achieved by balancing the backward amplified spontaneous emission with back-reflected forward emission.

Special optical fibers doped with nanocrystalline holmium-yttrium titanates (HoxY1-x)2Ti2O7 for fiber-lasers

The paper deals with the preparation and characterization of the silica optical fibers doped with nanocrystalline holmium-yttrium titanates (HoxY1-x)2Ti2O7 with optimized luminescence properties. The sol-gel approach was employed to prepare colloidal solution of (HoxY1-x)2Ti2O7 precursors. The concentration of Ho3+ ions in the compounds was varied up to x=0.4. Prepared sols were calcined at 1000 °C forming xerogels which were characterized by X-ray diffraction to confirm their structure. The xerogels were analyzed by the mean of steady-state luminescence technique to optimize the concentration of Ho3+ ions in the compound. The most intensive emission at 2050 nm was observed for the compound (Ho5Y95)2Ti2O7. Sol of the corresponding composition was soaked into the porous silica frit deposed inside the silica substrate tube which was collapsed into preform and drawn into optical fiber. Single mode optical fiber with the core diameter 12 μm and outer diameter 125 μm was prepared. Numerical aperture of prepared fiber was 0.16. The concentration of Ho3+ ions in the fiber core was 0.03 at %. Background attenuation of prepared fiber at 850 nm was smaller than 0.5 dB⋅m-1.