W. Andrew Clarkson

A cladding-pumped, tunable holmium doped fiber laser

We present a tunable, high power cladding-pumped holmium doped fiber laser. The laser generated >15 W CW average power across a wavelength range of 2.043 - 2.171 μm, with a maximum output power of 29.7 W at 2.120 μm. The laser also produced 18.2 W when operating at 2.171 µm. To the best of our knowledge this is the highest power operation of a holmium doped laser at a wavelength >2.15 µm. We discuss the significance of background losses and fiber design for achieving efficient operation in holmium doped fibers.

Efficient operation of a diode-bar-pumped Nd:YAG laser on the low-gain 1123-nm line

Efficient operation of a Nd:YAG laser on the low-gain 1123-nm transition by end pumping with the reshaped output from a 7-W diode bar is reported. Using a simple standing-wave laser configuration and pumping with 5.6 W of incident power yielded a laser output power of 1.7 W in a near-diffraction-limited TEM00 mode with a beam quality factor of M2 < or approximately equal to 1.1. A unidirectional single-frequency ring laser was also constructed, yielding 180 mW of single-frequency output. The prospects for further increase in power by optimization of the resonator design are discussed.

Accurate efficiency evaluation of energy-transfer processes in phosphosilicate Er3+-Yb3+-codoped fibers

A new approach for the prediction of energy-transfer efficiencies in codoped Er-Yb double-clad fiber (EYDCF) is presented. Ab initio calculations have been performed on the basis of migration-assisted energy-transfer models. The two main Yb energy-transfer parameter that is needed in all modelings based on rate equations can be calculated by use of a simple relation. Our approach also permits the prediction of the laser efficiency of EYDCFs, and a comparison of measured and calculated maximum laser slope efficiency shows a good agreement.

Fiber design for high-power fiber lasers

This paper reviews the progress in active fibers suitable for power scaling, highlighting the advances in fiber design that will enable the control of nonlinearities such as SRS and SBS in high power fiber lasers, as well as making feasible a practical high power three-level system.

Stable, high-power, single-frequency generation at 532 nm from a diode-bar-pumped Nd:YAG ring laser with an intracavity LBO frequency doubler.

We obtained 2.5 W of single-frequency TEM(00) output at 532 nm using a Brewster-angled LBO crystal for intracavity second-harmonic generation in a diode-bar-pumped Nd:YAG laser. By inserting a thin uncoated étalon, the 1061.4-nm laser transition can be selected, generating 1.6 W of output at 530.7 nm.

Thulium-ytterbium co-doped fiber laser with 75 W of output power at 2 μm

We report power-scaling of an ytterbium-sensitized thulium-doped silica fiber laser generating up to 75 W of output power in the 2 μm wavelength range when cladding-pumped by a 975 nm diode stack. The slope efficiency is 32% with respect to launched pump power and the beam quality factor (M2) is 1.3. We also investigate the characteristics of this fiber in a tunable laser configuration, operating at ~10 W of output power with the tuning range extended from 2000 to 2080 nm at a launched pump power of 40 W.

Metal clad active fibres for power scaling and thermal management at kW power levels

We present a new approach to high power fibre laser design, consisting of a polymer-free all-glass optical fibre waveguide directly overclad with a high thermal conductivity metal coating. This metal clad active fibre allows a significant reduction in thermal resistance between the active fibre and the laser heat-sink as well as a significant increase in the operating temperature range. In this paper we show the results of a detailed thermal analysis of both polymer and metal coated active fibres under thermal loads typical of kW fibre laser systems. Through several different experiments we present the first demonstration of a cladding pumped aluminium-coated fibre laser and the first demonstration of efficient operation of a cladding-pumped fibre laser at temperatures of greater than 400 °C. Finally, we highlight the versatility of this approach through operation of a passively (radiatively) cooled ytterbium fibre laser head at an output power of 405 W in a compact and ultralight package weighing less than 100 g.

High-power Er:YAG laser at 1646 nm pumped by an Er,Yb fiber laser

In this paper we describe an Er:YAG laser pumped by a tunable, cladding-pumped Er,Yb fiber laser and discuss factors affecting the laser performance. Crystals with different Er3+-concentrations in the range 0.5% to 4 at% and with crystal lengths selected for ~95% absorption of the pump light at 1532nm were used, and the laser performance was investigated for a range of output coupler transmissions (2-30%) at 1646nm. In preliminary experiments we have achieved a maximum output power of 4W at 1646nm for 11W of absorbed pump power corresponding to an efficiency of 36%, using a crystal with 0.5at% Er3+-concentration and an output coupler transmission of 10%. Our experiments have revealed that the cw efficiency decreases quite markedly for higher Er3+-concentrations. The origin this behavior is currently the subject of a detailed experimental investigation and our preliminary findings will be presented. The prospects for further increase in output power and efficiency will also be discussed.

CO 2 laser-fabricated cladding light strippers for high-power fiber lasers and amplifiers

We present and characterize a simple CO2 laser processing technique for the fabrication of compact all-glass optical fiber cladding light strippers. We investigate the cladding light loss as a function of radiation angle of incidence and demonstrate devices in a 400 μm diameter fiber with cladding losses of greater than 20 dB for a 7 cm device length. The core losses are also measured giving a loss of <0.008±0.006  dB/cm. Finally we demonstrate the successful cladding light stripping of a 300 W laser diode with minimal heating of the fiber coating and packaging adhesives.

Efficient holmium-doped solid state lasers pumped by a Tm-doped silica fiber laser

We report on efficient operation of Ho:YAG and Ho:YLF lasers in-band pumped by a tunable Tm-doped silica fiber laser. The Tm-doped fiber laser could be tuned over 150 nm from ~ 1860 to 2010 nm with a relatively narrow linewidth (<0.5 nm) at output power levels in excess of 9 W with a high power stability (RMS < 1.5%). Using a simple standing-wave cavity configuration, >6.4 W of TEMoo output was obtained from a Ho:YAG laser at 2.1 μm at the maximum incident pump power of 9.6 W at 1907nm from the Tm fiber laser, corresponding to an optical-to-optical efficiency of 67%, and the slope efficiency with respect to incident pump power was 80%. For a similar resonator design, 4.8 W of output at 2.07 μm was generated from a Ho:YLF laser at an incident pump power of 9.4 W, corresponding to an optical conversion efficiency of 51%. The different levels of performance of the Ho:YAG and Ho:YLF are compared and their relative merits discussed. Using a simple ring resonator geometry and an acousto-optic modulator to enforce unidirectional operation, we have obtained 3.7 W of single-longitudinal-mode output from a Ho:YAG laser. The prospects for further improvement in performance and higher output power are discussed.