Ondřej Podrazký

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.

In vivo optical detection of pH in microscopic tissue samples of Arabidopsis thaliana.

Minimally invasive in vivo measurement of pH in microscopic biological samples of μm or μl size, e.g. plant cells, tissues and saps, may help to explain complex biological processes. Consequently, techniques to achieve such measurements are a focus of interest for botanists. This paper describes a technique for the in vivo measurement of pH in the range pH5.0 to pH7.8 in microscopic plant tissue samples of Arabidopsis thaliana based on a ratiometric fluorescence method using low-loss robust tapered fiber probes. For this purpose tapered fiber probes were prepared and coated with a detection layer containing ion-paired fluorescent pH-transducer 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (c-HPTS). A fluorescence ratiometric approach was employed based on excitation at 415 nm and 450 nm and on the comparison of the fluorescence response at 515 nm. The suitability of tapered fiber probes for local detection of pH between 5.0 and 7.8 was demonstrated. A pH sensitivity of 0.15 pH units was achieved within the pH ranges 5.0-5.9 and 7.1-7.8, and this was improved to 0.04 pH units within the pH range 5.9-7.1. Spatial resolution of the probes was better than 20 μm and a time response within 15-20s was achieved. Despite the minute dimensions of the tapered fiber probes the setup developed was relatively robust and compact in construction and performed reliably. It has been successfully employed for the in vivo local determination of pH of mechanically resistant plant tissues of A. thaliana of microscopic scale. The detection of momentary pH gradients across the intact plant seems to be a good tool for the determination of changes in pH in response to experimental treatments affecting for example enzyme activities, availability of mineral nutrients, hormonal control of plant development and plant responses to environmental cues.

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.

Double-clad rare-earth-doped fiber with cross-section tailored for splicing to the pump and signal fibers: analysis of pump propagation

In this contribution we present numerical study of propagation of the multimode pump radiation in the inner cladding of the developed double-clad fiber. Field evolution was simulated using full vector finite element beam propagation method. Due to reeling of the fiber on the spool, the curvature of the fiber is involved in the model. The longitudinal dependence of power distribution was then analyzed. The simulations showed that after the length of propagation of about 40 mm the field becomes homogeneously distributed in the structure. Over 90 % of pump energy is absorbed in 3 m of the rare-earth-doped fiber with core-absorption of 2000 dB/m and cladding-core diameter ratio of 18.5. This confirms suitability of the tailored cross section for effective pump absorption along the rare-earth-doped fiber.

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.

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.

Comparison of characteristics of Bragg fibers with silica and air cores

This paper presents the comparison of experimental characteristics of two types of Bragg fibers, one with a silica core and second with an air core. The claddings of the fibers consist of three pairs of Bragg layers. Each pair is composed of one layer with a high and one layer with a low refractive index with a refractive-index contrast of about 0.03. The diameter of the silica core is of about 30 μm and that of the air core is of about 55 μm. Preforms of the Bragg fibers in the form of a tube have been prepared by the MCVD method by using germanium dioxide, phosphorous pentoxide and fluorine as silica dopants. In the case of the fiber with the air core only Bragg layers were applied inside a substrate tube while in the case of the fiber with the silica core one additional silica layer was deposited over the Bragg layers. The fibers were drawn from the preforms under controlled temperatures in order to obtain fibers with air or solid cores. Results of characterization of prepared fibers by optical microscopy are presented. Several laser sources including a pulse and continuous-wave Nd:YAG lasers at 1064 nm and a laser diode at 1550 nm were used for testing the transmission of laser radiation through the fibers. Transmittances, attenuation coefficients, bending losses and spatial profiles of output beams from the fibers were determined from such measurements.

Evaluation of energy transfer coefficients in Tm-doped fibers for fiber lasers

In this work we report on the determination of the cross-relaxation energy-transfer coefficients from the measurements of the fluorescence lifetimes of the 3F4 and 3H4 energy levels of Tm3+ ions in the experimentally prepared optical fibers. Optical fiber preforms were prepared by solution-doping of Tm3+ ions with either Al3+ ions or dispersed alumina nanoparticles. Optical fibers were characterized by means of Tm, Al and Ge concentrations, refractive index profiles, optical spectral attenuations, luminescence spectra and fluorescence lifetimes. Highly aluminium-codoped optical fibers exhibited fluorescence lifetimes of up to 756 μs.