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Frequency dynamics of transverse mode instabilities (TMIs) are investigated by testing three 285/100 rod fibers in a single-pass amplifier setup reaching up to ~200W of extracted output power without beam instabilities. The pump power is increased well above the TMI threshold to uncover output dynamics, and allowing a simple method for determining TMI threshold based on standard deviation. The TMI frequency component is seen to appear on top of system noise that may trigger the onset. A decay of TMI threshold with test number is identified, but the threshold is fully recovered between testing to the level of the pristine fiber by thermal annealing the fiber output end to 300°C for 2 h.

Photonic crystal bandgap fibers employing distributed mode filtering design provide near diffraction-limited light outputs, a critical property of fiber-based high-power lasers. Microstructure of the fibers is tailored to achieve single-mode operation at specific wavelength by resonant mode coupling of higher-order modes. We analyze the modal regimes of the fibers having a mode field diameter of 60 µm by the cross-correlated (C2) imaging method in different wavelength ranges and evaluate the sensitivity of the modal content to various input-coupling conditions. As a result, we experimentally identify regimes of resonant coupling between higher-order core modes and cladding band. We demonstrate a passive fiber design in which the higher-order modal content inside the single-mode guiding regime is suppressed by at least 20 dB even for significantly misaligned input-coupling configurations.

We present a semi-analytic numerical model to estimate the transverse modal instability (TMI) threshold for photonic crystal rod amplifiers. The model includes thermally induced waveguide perturbations in the fiber cross section modeled with finite element simulations, and the relative intensity noise (RIN) of the seed laser, which seeds mode coupling between the fundamental and higher order mode. The TMI threshold is predicted to ~370 W – 440 W depending on RIN for the distributed modal filtering rod fiber.

We demonstrate a high power fiber (85μm core) amplifier delivering up to 292Watts of average output power using a mode-locked 30ps source at 1032nm. Utilizing a single mode distributed mode filter bandgap rod fiber, we demonstrate 44% power improvement before the threshold-like onset of mode instabilities by operating the rod fiber in a leaky waveguide regime. We investigate the guiding dynamics of the rod fiber and report a distinct bandgap blue-shifting as function of increased signal power level. Furthermore, we theoretically analyze the guiding dynamics of the DMF rod fiber and explain the bandgap blue-shifting with thermally induced refractive index change of the refractive index profile.

High-power fiber amplifiers for pulsed applications require large mode area (LMA) fibers having high pump absorption and near diffraction limited output. Photonic crystal fibers allow realization of short LMA fiber amplifiers having high pump absorption through a pump cladding that is decoupled from the outer fiber diameter. However, achieving ultra low NA for single mode (SM) guidance is challenging, thus different design strategies must be applied. The distributed modal filtering (DMF) design enables SM guidance in ultra low NA fibers with very large cores, where large preform tolerances can be compensated during the fiber draw. Design optimization of the SM bandwidth of the DMF rod fiber is presented. Analysis of band gap properties results in a fourfold increase of the SM bandwidth compared to previous results, achieved by utilizing the first band of cladding modes, which can cover a large fraction of the Yb emission band including wavelengths of 1030 nm and 1064 nm. Design parameters tolerating refractive index fabrication uncertainties of ± 10-4 are targeted to yield stable SM bandwidths.

A large-mode-area Ytterbium-doped photonic crystal fiber amplifier with build-in gain shaping is presented. The fiber cladding consists of a hexagonal lattice of air holes, where three rows are replaced with circular high-index inclusions. Seven missing air holes define the large-mode-area core. Light confinement is achieved by combined index and bandgap guiding, which allows for single-mode operation and gain shaping through distributed spectral filtering of amplified spontaneous emission. The fiber properties are ideal for amplification in the long wavelength regime of the Ytterbium gain spectrum above 1100 nm, and red shifting of the maximum gain to 1130 nm is demonstrated.

We report 216W of average output power from a photonic crystal rod fiber amplifier. We demonstrate 44% power improvement before onset of the mode instability by operating the rod fiber in a leaky guiding regime.

We present recent development of photonic crystal fiber amplifiers containing photonic bandgap structures for enhanced spectral and modal filtering functionality.

Photonic crystal fibres (PCFs) emerged as a research topic in the mid 1990'ies [1]. Today, 15 years later, these fibres are increasing deployed in various commercial markets. Here, we will address three of these markets; medical imaging, materials processing and sensors. We will describe how the PCFs provide radical improvements and illustrate the strong diversity in the evolution of PCFs to serve these different markets.

We demonstrate modal characterization using spatial and spectral resolved (S2) imaging, on an Ytterbium-doped large-mode-area photonic crystal fiber (PCF) amplifier and compare results with conventional cut-off methods. We apply numerical simulations and step-index fiber experiments to calibrate our mathematical and experimental routines of our S2 imaging system. We systematically analyze higher order mode (HOM) content of a polarizing 40μm core double-clad PCF amplifier with various launching and coiling configurations. We demonstrate a HOM suppression of more than -24dB with variance of 2.3dB.

Enabling Single-Mode (SM) operation in Large-Mode-Area (LMA) fiber amplifiers and lasers is critical, since a SM output ensures high beam quality and excellent pointing stability. In this paper, we demonstrate and test a new design approach for achieving ultra-low NA SM rod fibers by using a spatially Distributed Mode Filter (DMF). This approach achieves SM performance in a short and straight rod fiber and allows preform tolerances to be compensated during draw. A low-NA SM rod fiber amplifier having a mode field diameter of ~60μm at 1064nm and a pump absorption of 27dB/m at 976nm is demonstrated.

Enabling Single-Mode (SM) operation in Large-Mode-Area (LMA) fiber amplifiers and lasers is critical, since a SM output ensures high beam quality and excellent pointing stability. In this paper, we demonstrate and test a new design approach for achieving SM LMA rod fibers by using a photonic bandgap structure. The structure allows resonant coupling of higher-order modes from the core and acts as a spatially Distributed Mode Filter (DMF). With this approach, we demonstrate passive SM performance in an only ~50cm long and straight ytterbium-doped rod fiber. The amplifier has a mode field diameter of ∼59Lim at 1064nm and exhibits a pump absorption of 27dB/m at 976nm. © 2011 Optical Society of America.

We demonstrate a Single-Mode (SM) Large-Mode-Area (LMA) ytterbium-doped PCF rod fiber laser with stable and close to diffraction limited beam quality with 110W output power. Distributed-Mode-Filtering (DMF) elements integrated in the cladding of the rod fiber provide a robust spatial mode with a Mode-Field-Diameter (MFD) of 59 mu m. We further demonstrate high pulse energy Second-Harmonic-Generation (SHG) and Third Harmonic Generation (THG) using a simple Q-switched single-stage rod fiber laser cavity architecture reaching pulse energies up to 1mJ at 515nm and 0.5mJ at 343nm. (C) 2011 Optical Society of America

We perform modal characterization on an ytterbium-doped large mode area photonic-crystal-fiber (PCF) amplifier using spatial and spectral (S 2) resolved imaging and compare results to conventional cutoff methods. We apply numerical simulations and step-index fiber experiments to calibrate our mathematical and experimental routines of our S2 imaging system. We systematically analyze higher-order-mode (HOM) content of a polarizing 40-m core double-clad PCF amplifier with various launching and coiling configurations. We demonstrate a HOM suppression of more than -24 dB with variance of 2.3 dB. © 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

High-power fiber lasers and amplifiers have gained tremendous momentum in the last 5 years. Many of the traditional manufacturers of gas and solid-state lasers are now pursuing the fiber-based systems, which are displacing the conventional technology in many areas. High-power fiber laser systems require reliable fibers with large cores, stable mode quality, and good power handling capabilities-requirements that are all met by the airclad fiber technology. In the present paper we go through many of the building blocks needed to build high-power systems and we show an example of a complete airclad laser system. We present the latest advancements within airclad fiber technology including a new 100 m single-mode polarization-maintaining rod-type fiber capable of amplifying to megawatt power levels. Furthermore, we describe the novel airclad-based pump combiners and their use in a completely monolithic 350 W cw fiber laser system with an M2 of less than 1.1. © 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

Tänä päivänä fluoresenssispektroskopia on yksi suurimmista tutkimusmenetelmistä monissa teollisuuden aloilla. Tavallisia fotometrisiä menetelmiä voidaan ainoastaan käyttää mitattaessa ei-fluoresoivaa materiaaleja. Valaistaessa fluoresoivaa materiaalia tietyn suuruisella valolla osa valosta heijastuu materiaalista, mutta osa absorboituu ja emittoituu uudelleen suuremmalla aallonpituudella. Tarve mitata nämä kaksi eri aallonpituuden omaavaa valoa tarpeeksi tarkasti on vaatinut monien kaupallisten fluorometrien kehittämistä. Lukuisat yritykset käyttävät monentyyppisiä fluorometrejä analysoidakseen tuotteitaan, kuitenkaan ei ole olemassa standardimenetelmiä, jolla voitaisiin kalibroida kyseisiä instrumenttejä tarpeeksi tarkasti. Erityisesti referenssimateriaalien puute, stabiilisuus sekä fluoresenssin kvanttihyötysuhteen mittauksen vaikeudet ovat ongelmallisia. Tämä diplomityö on osa tutkimusprojektia, jonka tavoitteena on rakentaa mittauslaitteisto, jolla voidaan karakterisoida standardimateriaaleja teollisuuden tarpeisiin. Mittauslaitteisto voidaan jakaa kahteen pääosaan eksitaatio- sekä emissioyksikköön. Emissioyksikkö koostuu emissiospektrometristä ja goniometrisestä valonkeräysoptiikasta, joka mahdollistaa valonkeräys- sekä valaisukulmien valinnan vapaasti. Tässä työssä rakennettiin ja karakterisoitiin emissiospektrometri sekä goniometrinen valonkeräysoptiikka. Emissiospektrometri rakennettiin yhdistämällä kaksi kaupallista laitetta, monokromaattori ja CCD-detektori. Emissiospektrometrin aallonpituuden tarkkuus, toistettavuus, kaistanleveys sekä spektrinen tehoherkkyys määritettiin karakterisoinnin yhteydessä. Karakterisoinnin jälkeen laitteistolla suoritettiin mittauksia käyttäen tunnettuja fluoresenssinäytteitä, joista määritettiin emissio- ja eksitaatiospektrit. Mittaukset suoritettiin eksitaatio- sekä emissioaallonpituuksilla 300 - 800 nm keräämällä emittoitunut säteily 10° ja valaisemalla näytettä 0° kulmassa näytteen kohtisuoraan nähden. Fluoresoituneen sätei