Kent Erik Mattsson

Photo darkening of rare earth doped silica

The photo darkening (PD) absorption spectra from unseeded amplifier operation (by 915 nm pumping) of ytterbium / aluminum and co-doped silica fibers is after prolonged operation observed to develop a characteristic line at 2.6 eV (477 nm). This line is proposed to be due to inter center excitation transfer from type II oxygen deficiency centers ODC(II) to Tm3+ trace impurities. The ODC(II) is proposed to be the result of a displacive transition of a 4-fold silica ring hosting two 3-fold silicon units that presents two non-bridging oxygen to Yb3+ (as part of its 6-fold coordination by oxygen). The displacive transition is initiated by a charge disproportionation process which leads to NBO transfer in forming dioxasilirane (2-fold coordinated silicon with two NBO attached) next to silylene (2-fold coordinated silicon with a lone electron pair). In collaboration with a valence electron of Yb3+ a new ½ / 1½ chemical bond is formed on dioxasilirane which comprises the PD color center for the visible and near-infrared. Difference in solid acidity of the silica material co-doped with Yb/Al and Yb/P may explain the observed difference in spectral shapes by change of bond order to the formed chemical bond.

Plasma‐enhanced growth, composition, and refractive index of silicon oxy‐nitride films

Secondary ion mass spectrometry and refractive index measurements have been carried out on silicon oxy‐nitride produced by plasma‐enhanced chemical vapor deposition (PECVD). Nitrous oxide and ammonia were added to a constant flow of 2% silane in nitrogen, to produce oxy‐nitride films with atomic nitrogen concentrations between 2 and 10 at. %. A simple atomic valence model is found to describe both the measured atomic concentrations and published material compositions for silicon oxy‐nitride produced by PECVD. A relation between the Si–N bond concentration and the refractive index is found. This relation suggest that the refractive index of oxy‐nitride with a low nitrogen concentration is determined by the material density. It is suggested that the relative oxygen concentration in the gas flow is the major deposition characterization parameter, and that water vapor is the predominant reaction by‐product. A model, that combine the chemical net reaction and the stoichiometric rules, is found to agree with me...

Gain-switched CW fiber laser for improved supercontinuum generation in a PCF

We demonstrate supercontinuum generation in a PCF pumped by a gain-switched high-power continuous wave (CW) fiber laser. The pulses generated by gain-switching have a peak power of more than 700 W, a duration around 200 ns, and a repetition rate of 200 kHz giving a high average power of almost 30 W. By coupling such a pulse train into a commercial nonlinear photonic crystal fiber, a supercontinuum is generated with a spectrum spanning from 500 to 2250 nm, a total output power of 12 W, and an infrared flatness of 6 dB over a bandwidth of more than 1000 nm with a power density above 5 dBm/nm (3 mW/nm). This is considerably broader than when operating the same system under CW conditions. The presented approach is attractive due to the high power, power scalability, and reduced system complexity compared to picosecond-pumped supercontinuum sources.

Airclad fiber laser technology

High-power fiber lasers and amplifiers have gained tremendous momentum in the last five years, and many of the traditional manufactures of gas and solid-state lasers are pursuing the attractive fiber-based systems, which are now displacing the old technology in many areas. High-power fiber laser systems require specially designed fibers with large cores 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 70 μm single-mode polarization-maintaining rod-type fiber capable of amplifying to MW 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. Finally, we briefly touch upon the subject of photo darkening and its origin.

Low photo darkening single mode RMO fiber.

A new reduced mode overlap (RMO) single mode fiber design is proposed and demonstrated. For the first time saturated photo darkening operation is observed in a nominal 350 W Yb / Al co-doped silica fiber laser. After 1500 hours operation less than 7% slope efficiency degradation is found and further 500 hours operation show no degradation.

Photo darkening in ytterbium co-doped silica material

A model description of photo darkening based on 30 characterised fibres in an un-seeded amplifier setup is presented. Photo darkening of ytterbium / aluminium and/or phosphorous co-doped silica fibres is found to saturate following prolonged exposure to pump radiation. The photo darkening is associated with non-binding oxygen at surfaces of ytterbium / aluminium clusters. The dominant colour centre at near infrared wavelengths in MCVD material is a combination of 1.9 eV (FWHM of 0.62 eV) and 2.4 eV (FWHM of 0.85 eV) absorption dependent on average phonon energy of the glass material.

The all-fiber cladding-pumped Yb-doped gain-switched laser

Gain-switching is an alternative pulsing technique of fiber lasers, which is power scalable and has a low complexity. From a linear stability analysis of rate equations the relaxation oscillation period is derived and from it, the pulse duration is defined. Good agreement between the measured pulse duration and the theoretical prediction is found over a wide range of parameters. In particular we investigate the influence of an often present length of passive fiber in the cavity and show that it introduces a finite minimum in the achievable pulse duration. This minimum pulse duration is shown to occur at longer active fibers length with increased passive length of fiber in the cavity. The peak power is observed to depend linearly on the absorbed pump power and be independent of the passive fiber length. Given these conclusions, the pulse energy, duration, and peak power can be estimated with good precision.

Surface micromachined scanning mirrors

Both aluminum cantilever and torsional scanning mirrors have been fabricated and their static and dynamic properties are studied experimentally and theoretically. The experiments showed resonance frequencies in the range of 163 k-Hz - 632 kHz for cantilever beams with Q values between 5 and 11. Torsional mirrors showed resonance frequencies in the range of 410 kHz - 667 kHz with Q values of 10 - 17. All measurements performed at atmospheric pressure. Both types of mechanical structures were deflected electrostatically at large angles (± 5°) more than 1011 times without breaking and without any noticeable deterioration due to fatigue. A number of different light modulator arrays made up of adjacent devices was investigated and the measured performance parameters were in good agreement with calculations.

Photo darkening of ytterbium cw fiber lasers

A model description of photo darkening of ytterbium cw fiber lasers based on long term tested fiber lasers is presented. Photo darkening of Yb/Al co-doped silica fibres is found to saturate following prolonged exposure to pump and signal radiation. The observed slope efficiency of Al co-doping is compared with modeled slope efficiency of P co-doping.

The three-electron bond =Si<O2:∙Yb absorption center of pre-darkened ytterbium-doped silica.

The formation and bleaching of color centers during annealing of pre-darkened ytterbium-doped silica fibers is modeled by three-electron bond (TEB) = Si<O(2):∙Yb absorption centers. The nature of a center and underlying mechanism for its annealing in formation, shift and dissociation of chemical bonds is described in terms of a Markov statistical model with state change set by Bose-Einstein phonon statistics. The center hold one terminal and four active states with activation energies for transitions among these found to match bond energies of molecular oxygen in ionic character bonds of 1 and 1½ bond order. Experimentally observed in- and decrease in absorption during ramp and isothermal annealing of pre-darkened ytterbium co-doped silica fibers are hereby matched by a set of = Si<O(2):∙Yb centers.