Todd S. Rose

Diode-pumped 1-W continuous-wave Er:YAG 3-?m laser

We have demonstrated 1.15 W of Gaussianlike (M(2) = 2) cw output at 2.94 mum from a diode laser end-pumped monolithic laser crystal composed of Er-doped yttrium aluminum garnet (YAG) bonded to undoped YAG. The laser was pumped with two polarization-coupled 2.5-W flared laser diodes that provided a 45-mum pump waist with a low N.A. (<0.04) . Output at 2.94 mum was generated with a 34% slope efficiency and a greater-than-unity quantum efficiency.

Gamma and proton radiation effects in erbium-doped fiber amplifiers: active and passive measurements

Commercially available Er-doped fibers were irradiated with 5.6 and 28 MeV protons and /sup 60/Co gamma rays, up to levels of 50 krad. White-light transmission spectra under passive conditions (no pump or signal) were measured at several radiation levels for the six types of fibers that were tested. The spectra were used to evaluate the relative radiation sensitivity of the fibers and compare gamma versus proton-induced damage for two fiber types. The amount of radiation damage for the fibers was observed to scale inversely with the Ge concentration. Samples from three of the fiber types were configured as optical amplifiers using 980-nm and 1550-nm pump and input signals. In situ measurements of the gain, noise figure, and amplified spontaneous emission (ASE) were made as a function of pump power at several levels of radiation. A computer code, based on a conventional Er-doped fiber amplifier (EDFA) model, was written to simulate performance, using input data provided by the fiber vendor and anchored to measurements made prior to radiation. A comparison between the simulations and experimental data shows that, in certain fibers where the damage is significant, the radiation-induced loss determined from amplifier measurements can be substantially less than that determined from passive transmission spectra.

Multiwatt continuous-wave and Q-switched Er:YAG lasers at 1645 nm: performance issues

The cw and Q-switched performance of Er:YAG lasers operating at 1645 nm were measured. Guided by previous work in the literature, we sought to improve efficiency at low pulse repetition frequencies by decreasing the doping level from 0.5 to 0.25 at.% to reduce upconversion losses. Only a small improvement was obtained with this first-time-tested lower-doped material. Measurements of the fluorescence due to upconversion directly indicated that loss due to this process could not account for the observed power loss at low pulse repetition frequencies. Enhanced green emission during Q-switched operation, resulting from two-photon absorption of 1645 nm intracavity laser light, is reported for what we believe to be the first time. Measurements indicated that the output loss from this process is negligible.

Narrowband Er:YAG nonplanar ring oscillator at 1645 nm

We report 1645 nm narrowband operation of a monolithic Er:YAG nonplanar ring oscillator resonantly pumped at 1532 nm. Unidirectional cw power up to 0.5 W was obtained with a measured linewidth of 21 kHz.

Secondary energy transfer and nonparticipatory Yb 3+ ions in Er 3+ -Yb 3+ high-power amplifier fibers

The energy exchange processes within Er3+–Yb3+-codoped silica fibers manufactured for high-power optical amplifiers are investigated through luminescent decay measurements and rate-equation modeling. The Er3+ and Yb3+ luminescence, as functions of pump power, exhibit an unquenchable Yb3+ to Er3+ cooperative energy transfer (CET) that does not subside with saturation of the Er3+ inversion. Consistent with the experimental results is the occurrence of secondary energy transfer to excited Er3+ ions in their metastable 4I13/2 state. The transfer coefficient for this secondary 4I13/2 CET is determined to be of the same order of magnitude as that for the initial Yb3+ to ground-state Er3+ CET. Additionally, the decay measurements and modeling indicate that a fraction of Yb3+ ions does not participate in energy exchange with the Er3+. These nonparticipatory Yb3+ ions amounted to ~15% of the total Yb3+ concentration and could constitute Yb3+ clusters. Both secondary CET and nonparticipatory Yb3+ will lower Er3+–Yb3+ fiber amplifier efficiencies.

Low noise 10-W cw OPO generation near 3 /spl mu/m MgO doped PPLN

Over 10 W of near diffraction-limited output at 3-/spl mu/m was achieved from a singly resonant OPO pumped by a 50-W ytterbium fiber laser. Excellent frequency and power stabilities were observed using 5% MgO doped PPLN.

Semimonolithic Nd:YAG ring resonator for generating cw single-frequency output at 1.06 μm

We report 2.1 W of unidirectional single-frequency laser output from a novel Nd:YAG ring laser longitudinally pumped by a laser diode bar. The resonator requires only a single discrete optic and magnet, in addition to the laser medium, for stable multiwatt single-frequency operation. Undoped YAG is diffusion bonded to the Nd:YAG laser crystal at the pump face to eliminate residual spatial hole burning and improve thermal conductivity. This new laser design can be scaled to higher powers through more facile diode access and is useful for cw intracavity doubling.

LEO to ground optical communications from a small satellite platform

A pair of 2.2 kg CubeSats using COTS hardware is being developed for a proof-of-principle optical communications demo from a 450-600 km LEO orbit to ground. The 10x10x15 cm platform incorporates a 25% wall-plug efficient 10-W Yb fiber transmitter emitting at 1.06 μm. Since there are no gimbals on board, the entire spacecraft is body-steered toward the ground station. The pointing accuracy of the LEO craft, which governs the data rate capability, is expected to be ~ 0.1-0.2 deg. Two optical ground stations, located at the Mt. Wilson observatory, have receiver apertures of 30 and 80 cm. Launch of the CubeSat pair is anticipated to be mid to late 2015.

High Performance 1645-nm Er:YAG Laser

The performance of the resonantly fiber-laser pumped Er:YAG lasers at 1645 nm using 0.25% and 0.5% doped crystals were compared in cw and Q-switched operation. Superior performance of the 0.25% crystals was observed.

High-efficiency longitudinal diode bar pumping of solid state lasers

A longitudinal diode bar pumping scheme for a solid state laser has been conceived which can concentrate tens of watts of pump power into a 300 micrometers spot with 90 to 95% efficiency.