Pierre Laperle

Yb-doped LMA triple-clad fiber for power amplifiers

High-energy pulsed narrow-linewidth diffraction-limited ytterbium-doped power amplifiers in the 1030 to 1100 nm wavelength range and in the nanosecond regime require large mode area (LMA) fibers to mitigate stimulated Brillouin scattering (SBS). However, typical LMA fibers with mode-field diameters larger than 20 &mgr;m are inherently multimode. To achieve a diffraction-limited output, several techniques are available such as low core numerical aperture, fiber coiling and selective doping. The triple-clad fiber design takes advantage of the three techniques. The first clad located next to the core allows a reduction and a better control of the effective numerical aperture for high ytterbium doping that is difficult to achieve with the standard double-clad fiber design. Also, the thickness of the first clad gives an extra degree of freedom that allows either a nearly bending-insensitive output or mode filtering through bending losses that can be enhanced by a depressed-clad design. Incorporating to the triple-clad design an optimized selective rare-earth doping of the core favors the fundamental mode over higher-order modes by the gain differential. Using the right dopants, it can also favor SBS suppression by reducing the overlap between optical and acoustic field distributions. Ytterbium-doped LMA triple-clad fibers with a large depressed first clad and selective ytterbium doping are tested in a power amplifier configuration. Also, ytterbium-doped polarization-maintaining LMA triple-clad fibers with a thin first clad are tested for SBS.

Yb-doped LMA triple-clad fiber laser

The ytterbium-doped large mode area triple-clad fiber design allows for a high concentration of ytterbium in the fiber core which is difficult to achieve with a standard double-clad design. The novelty of the triple-clad fiber design consists in adding to the double-clad fiber design, a first clad next to its core. This first clad offers a better control of the core effective area. With this design a low numerical aperture is achievable (~0.06) for highly rare earth doped large mode area fiber. A 33-μm core ytterbium doped fiber has been fabricated using MCVD and solution doping processes. Selective doping and optimized first clad thickness have been used in the triple-clad design to obtain a nearly bending insensitive and nearly diffraction-limited fiber output. The fiber has been tested in a free-running laser configuration and its slope efficiency is 84% with a laser threshold of 1.4 W. A maximum output power of 26 W at 1070 nm has been achieved for a launched pump power of 34 W at 976 nm. The mode-field diameter has been measured at 18 μm and the output beam M2 quality factor is below 1.1. Both output power and beam quality were not significantly affected by fiber bending with loops diameter as small as 2.5 cm. The optical performance of the triple-clad fiber design is robust to mechanical stress and well suited for building very compact high power fiber lasers and amplifier sources.

Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers

Photodarkening is presently a major concern for the long term reliability and efficiency of high power Yb-doped fiber lasers and amplifiers. This phenomenon has been associated with the formation of color centers in the fiber core of single-clad and large mode area Yb-doped fibers. However, its origin is still not well understood and to date no comprehensive model that could predict the lifetime of Yb-doped fiber-based devices has been put forward. A semi-empirical approach seems at the moment the best way to gain a better understanding of the growth behavior of photo-induced losses in Yb-doped fibers in the presence of both photodarkening and photobleaching processes. A rate equation describing the activation and deactivation of color centers involving stretched exponential functions has been developed. For this approach to be effective and reliable, a minimum of parameters is used, four to describe photodarkening and three for photobleaching. A large mode area Yb-doped fiber fabricated at INO using the MCVD process has been characterized. By properly choosing the initial pumping conditions, each parameter of the stretched exponential functions has been measured separately from the others. The model has then been used to simulate the power decay from a 1 kW, 10 ns-pulse, 100 kHz Yd-doped LMA fiber power amplifier. We show that the photodarkening behavior predicted by the model is in good agreement with the experimental results over more than 6000 hours. Such a model is general in its application but the stretched exponential parameters are unique to the type of fiber tested. The model will be a useful characterization tool for developing photodarkening-resistant fibers and for evaluating the lifetime of Yb-doped fiber-based devices affected by photodegradation.

Modeling of Tm-doped ZBLAN blue upconversion fiber lasers operating at 455 nm

We present a model for 455-nm thulium-doped fluorozirconate fiber lasers co-pumped at 645 nm and 1064 nm. Twelve radiative transitions are accounted for in our model, along with cross- relaxation and cooperative upconversion processes. Blue laser output power is computed using a rate equation analysis. Relevant spectroscopic data used in our model are given, including cross-section measurements that we have performed. The results of our simulation show a good agreement with previously published experimental data. The importance of cross-relaxation processes is discussed. The dependence of output laser power on fiber length, output mirror reflectivity, and pump powers is also addressed.

Tunable diode-pumped and frequency-doubled Cr:LiSAF lasers

The performance of a diode-pumped Cr:LiSAF laser and its intracavity frequency-doubled operation is reported. Close to 1 W of quasi-cw power at 870 nm was obtained in multimode operation. Power in excess of 20-mW cw of TEM(00) second-harmonic power at 435 nm has also been obtained by using a lithium triborate crystal with an infrared-to-visible conversion efficiency of 33%. This result is believed to be the highest yet reported for a true cw blue output from a diode-pumped Cr:LiSAF system. However, the maximum attainable output power was limited by thermal fluorescence quenching. The effect of the pump size on the temperature rise and on the small-signal gain is investigated.

Stability aspects in the operation of a 2500-ppm thulium-doped ZBLAN fiber laser at 481 nm

We report on the stable operation of a 2500-ppm thulium-doped ZBLAN upconversion fiber laser at 481 nm and the related problem of photodegradation associated with the formation of color centers. The output coupling is observed to affect the level of induced absorption in the fiber, hence the output power level. The start-up laser threshold is shown to increase significantly over time after lasing as a result of a thermally driven relaxation of photobleached color centers. Three techniques are investigated for restoring the transparency of the darkened fiber prior to lasing operation: visible photobleaching at 514-nm, near-infrared photobleaching from the 780-nm laser transition of thulium, and annealing at temperatures above 100°C.

Relations between phosphorus/aluminum concentration ratio and photodarkening rate and loss in Yb-doped silica fibers

The relations between dopant concentrations (phosphorus and aluminum) and photodarkening rate, excess loss, and activation energies in ytterbium-doped silica fibers are experimentally investigated. It is shown that increasing the concentration of phosphorus from 0.2 to 2.5 mol% in phosphorus/aluminum codoped fiber cores decreases the photodarkening excess loss by a factor of 8 and the photodarkening rate by a factor of 10. Moreover, the effective number of ytterbium ions involved in the photodarkening process increases from 4 to more than 6 for tested phosphorus/aluminum concentration ratios varying from 0.1 to 1 respectively. In contrast, increasing the aluminum concentration from 2 to 5 mol% for a fixed phosphorus concentration of 0.2 mol% has negligible effect on the initial photodarkening rate or the effective number of ytterbium ions involved in the process, but still decreases the photodarkening excess loss by a factor of 5. Those results suggest photodarkening activation energies of 5.2 eV for ytterbium/aluminum-codoped silica fibers and more than 7.8 eV for ytterbium/phosphorus/aluminum-codoped silica fibers. The net improvement in photodegradation of fiber amplifiers based on such phosphorus and aluminum codoping is measured experimentally and numerically simulated. The output power loss of 1064-nm ytterbium-doped LMA fiber amplifiers with phosphorus/aluminum ratios of 0.1 and 0.6 is reduced after 10 000 hours from 17% to less than 2%, respectively. Better understanding of the effects of phosphorus and aluminum on photodarkening will help to design reliable and efficient ytterbium-doped fiber amplifiers.

Thulium-doped ZBLAN fiber laser

The thulium-doped ZBLAN fiber laser is emitting visible radiation through an upconversion process which allows for the generation of hundreds of milliwatts of output power at 480 nm. The main features of this laser are presented and analyzed on the basis of an equation rate analysis. Cross- relaxation processes are shown to play an important role in the dynamical evolution of the population levels. The problem of the onset of a photo-induced absorption in the fluoride-glass matrix is also addressed in connection with its detrimental effects on laser operation. Practical solutions to this problem are explored.

Ultraviolet-induced color centers and annealing in thulium-doped fluorozirconate optical fibers

We have investigated the dynamics of color center formation (darkening) in a 1000 ppm thulium-doped ZBLAN (ZrF 4 -BaF 2 .-LaF 3 -AlF 3 -NaF) optical fiber by ultraviolet (285, 300, 351, 364 nm) and infrared (1112 nm) exposures. We observe that color centers are simultaneously created and photobleached by the UV light and that upon exposure to 1112 nm the color centers are thermally activated. We make the hypothesis that the mechanism responsible for the formation of color centers is a charge transfer to trap sites following the ionization of the glass matrix by an excitation from the 5d level of the thulium ion by ∼55 000 cm -1 . Also, thermal annealing of the color centers is investigated with the use of a standard stretched-exponential model. The annealing results are consistent with the assumption of a distribution of energies of color center states.

Improvement in the beam quality of large-mode-area fiber amplifier output through fiber tapering

It is shown that the introduction of an adiabatic taper at the output end of a large-mode-area fiber amplifier can greatly improve the beam quality factor of the single-mode output beam when the core refractive index profile departs from an ideal step-index distribution. The deleterious impact of higher-order modes is also analyzed.