Shawn O'Connor

Ytterbium laser with reduced thermal loading

We report a novel design for a high-power ytterbium disk laser. This laser utilizes radial diode pumping of a back surface cooled active-mirror geometry. Wing absorption of the pump light at 0.99 /spl mu/m allows efficient laser operation at 1.05 /spl mu/m with a low quantum defect. Laser performance and thermal loading were characterized for a wide range of conditions. Optimized operation of the laser yielded 490 W in a quasi-continuous-wave mode. Electrical efficiency of the laser was found to be 9.4%, while heating of the laser disk was only 3.2% of the absorbed optical power. Fluorescence re-absorption is identified as the principal heat generation mechanism in this laser. A simplified extension to the conventional rate model is proposed for lasing in radiation-trapped systems. This model allows power flow calculations in a radiation-trapped laser system using a single parameter determined from fluorescence decay waveforms. The revised model agrees with heat load measurements.

Minimizing Heat Generation in Solid-State Lasers

Novel high-power ytterbium YAG lasers are described. These lasers incorporate the principle of anti-Stokes fluorescence cooling to reduce or eliminate detrimental heating. Lasers with net heating and net cooling are demonstrated. By balancing the spontaneous and stimulated emission, we have reduced the net thermal loading to below 0.01% of the lasers average output power. Design, testing, and analysis are reported for lasers up to 500 W average power and pulsed operation up to 30 s. Issues and limitations of this approach are discussed.

Ho-doped fiber for high energy laser applications

Ho-doped fiber lasers are of interest for high energy laser applications because they operate in the eye safer wavelength range and in a window of high atmospheric transmission. Because they can be resonantly pumped for low quantum defect operation, thermal management issues are anticipated to be tractable. A key issue that must be addressed in order to achieve high efficiency and minimize thermal issues is parasitic absorption in the fiber itself. Hydroxyl contamination arising from the process for making the Ho-doped fiber core is the principal offender due to a combination band of Si-O and O-H vibrations that absorbs at 2.2 μm in the Ho3+ emission wavelength region. We report significant progress in lowering the OH content to 0.16 ppm, which we believe is a record level. Fiber experiments using a 1.94 μm thulium fiber laser to resonantly clad pump a triple clad Ho-doped core fiber have shown a slope efficiency of 62%, which we also believe is a record for a cladding-pumped laser. Although pump-power limited, the results of these studies demonstrate the feasibility of power scaling Ho-doped fiber lasers well above the currently-reported 400-W level.1

Demonstration of a radiatively cooled laser

Summary form only given. We have developed a new optical pumping technique that minimizes the thermal loading in a solid-state laser system. This technique incorporates optical cooling directly into a laser medium. Lasing conditions were studied in which the waste heat required by stimulated emission is offset by cooling from anti-Stokes spontaneous emission. We call this condition a radiation balanced laser. Under ideal conditions, the heat load within the lasing medium can be completely eliminated. Perturbation analysis of the RBL condition reveals that thermal loads in the range of 5 W/cm/sup 3/ should be obtainable in scalable solid-state laser designs such as for Yb:KGd(WO/sub 4/)/sub 2/ lasers.

Non-radiative decay of holmium-doped laser materials

Anti-Stokes fluorescence cooling has been demonstrated in a number rare earth doped materials. Ytterbium doped oxides and fluorides, such as ZBLAN, YLF, and YAG, were the first materials to exhibit cooling.1,2,3 These materials were originally developed as laser gain media and fluorescence cooling was eventually incorporated into the 1μm lasers to reduce detrimental thermal loading.4 Anti-Stokes cooling can offset quantum defect heating allowing laser power to be scaled to very high average powers. Since the early work in ytterbium, fluorescence cooling has been demonstrated in both erbium and thulium doped materials.5,6 These materials were also initially developed as lasing media and their fluorescence cooling could be used to increase laser powers at 1.5μm and 2.0μm. In this study we examine the radiative efficiency of holmium and ask the question, “Can anti-Stokes fluorescence cooling be extended beyond 2μm?”

Growth and characterization of direct mid-IR laser materials (Invited Paper)

Rare earth doped ternary lead salts are being studied for use as mid-IR laser materials. We summarize progress at the Naval Research Labs on the production and evaluation of this important class of solid-state laser.

Demonstration and analysis of a high power radiation balanced laser

Detrimental heating in a high power Yb:YAG laser was eliminated by incorporating anti-Stokes optical cooling.

Doped sesquioxide ceramic for eye-safe solid state laser materials

In this paper, we present our recent results in the development of Ho3+ doped sesquioxides for eye-safe solid state lasers. We have synthesized optical quality Lu2O3 nanopowders doped with concentrations of 0.1, 1.0, 2.0, and 5% Ho3+. The powders were synthesized by a co-precipitation method beginning with nitrates of holmium and lutetium. The nanopowders were hot pressed into optical quality ceramic discs. The optical transmission of the ceramic discs is excellent, nearly approaching the theoretical limit. The optical, spectral and morphological properties as well as the lasing performance from highly transparent ceramics are presented.

Growth and mid-infrared laser performance of Er3+: KPb2Cl5

High-quality single crystals of Er³⁺:KPb₃Cl ₅ have been grown by a novel modification of the Bridgman method. These crystals show excellent laser performance at 4.6 mum

High power ytterbium disk laser

A novel diode-pumped disk laser yielded 490 W in a quasi-cw mode. Electrical efficiency of the laser was found to be 9.4%. Heating of the laser disk was only 2.7% of the absorbed optical power.