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We examine the effect of photodarkening-induced pump light absorption on Yb-doped fiber thermal loading. In these experiments, the fiber is cladding pumped at 915 nm, air cooled by natural convection, and monitored with a midinfrared thermal camera. The fiber temperature is found to correlate with progressive photodarkening. The maximum observed fiber temperature was 120 [degrees]C, recorded at a pump power of 10.5 W. The observed increase in fiber temperature can be explained by a model that takes into account the combined effects of the Yb-silica quantum defect, the temperature dependence of the Yb-absorption cross section, and photodarkening-induced loss at 915 nm. We hypothesize that the latter effect results in the progressive activation of temperature-dependent photochemical processes (e.g., bleaching) and is ah important consideration with regard to the accurate modeling of Yb-doped fiber photodarkening kinetics. OCIS codes: 140.3510, 160.5690, 140.6810, 140.3615.

Yb-doped fibers are widely used in laser applications requiring high average output powers and high-peak-power pulse amplification. Photodarkening (PD) is recognized as one limiting factor in these fibers when pumped with high-intensity radiation. We describe an approach for performing quantitative PD studies of fibers, and we present measurements of the rate of PD in Yb-doped single-mode fibers with varying inversion levels. The method is applicable to large-mode-area fibers. We observed a seventh-order dependence of the PD rate on the excited-state Yb concentration for two different fibers; this result implies that PD of a Yb-doped fiber source fabricated using a particular fiber will be strongly dependent on the configuration of the device. OCIS codes: 140.3380, 140.3510, 140.5680, 060.2270, 060.2290.

A real-time, noninvasive approach for detecting trace amounts of vapor-phase mercuric chloride (Hg[Cl.sub.2]) in combustion flue gas is demonstrated using a near-infrared pulsed fiber amplifier that is frequency converted to the ultraviolet. Excitation of the Hg[Cl.sub.2] ([1.sup.1][[PHI].sub.[micro]] [left arrow] [1.sup.1] [[SIGMA].sup.+.sub.g]]) transition at 213 nm generates 253.7 nm emission from the Hg ([6.sup.3][P.sub.1]) photoproduct that is proportional to the concentration of Hg[Cl.sub.2]. A measured quadratic dependence of the Hg[Cl.sub.2] photofragment emission (PFE) signal on the laser irradiance indicates that the photodissociation process involves two-photon excitation. Additionally, low concentrations of Hg[Cl.sub.2] are detected with the PFE approach in an environment characteristic of coal-fired power-plant flue gas using this compact solid-state laser source. A detection limit of 0.7 ppb is extrapolated from these results. OCIS codes: 280.1120, 300.2530, 120.0280.

A new procedure for pumping double-clad rare-earth-doped fiber sources is described. The procedure uses diode lasers, diode bars, or fiber-coupled pump sources.

A compact, lightweight, low-power-consumption source of tunable, narrow-bandwidth blue and UV radiation is described. In this source, a single-longitudinal-mode diode laser seeds a pulsed, GaAlAs tapered amplifier whose [approximately] 860-nm output is frequency quadrupled by two stages of single-pass frequency doubling. Performance of the laser system is characterized over a wide range of amplifier duty cycles (0.1-1.0), pulse durations (50 ns-1.0 [[micro]second]), peak currents ([less than or equal to]14 A), and average currents ([less than or equal to]2.0 A). The capabilities and limitations of this source are discussed. We recorded high-resolution, Doppler-limited absorption spectra of nitric oxide and sulfur dioxide near 215 nm; the S[O.sub.2] spectrum was found to have significantly more structure and higher peak absorption cross sections than previously reported.

Fiber dispersion plays a significant role in spectral broadening of incoherent continuous-wave light. We develop a self-consistent stochastic model for spectral broadening of incoherent continuous-wave light through nonlinear wave mixing and apply this model to numerical simulations of spectral broadening in a continuous-wave fiber Raman laser. The results of these numerical simulations agree very well with carefully conducted laboratory measurements. Under a wide range of operating conditions, these numerical simulations also exhibit striking features, such as damped oscillatory spectral broadening (during the initial stages of propagation) and eventual convergence to a stationary, steady-state spectral distribution at sufficiently long propagation distances. We analyze the important role of fiber dispersion in such phenomena. We also derive an analytical rate equation expression for spectral broadening, whose numerical evaluation is far less computationally intensive than the fully stochastic simulation, and a mathematical criterion for the applicability of this analytical expression. [ABSTRACT FROM AUTHOR]

We examine the temperature dependence of photodarkening in ytterbium-doped silica fibers. A sequence of consecutive photodarkening experiments are performed over the same fiber sample, which shows good repeatability with no apparent changes in the glass structure. We find that during infrared irradiation, the level of saturation of the losses can be determined by the fiber core temperature, independent of the previous state of photodarkening losses and fiber temperature, and also at low temperatures where the thermal bleaching is not activated. We observe that variations in the fiber core temperature, induced by pump absorption due to photodarkening, affect the inversion level and photodarkening processes. These effects in turn cause a discrepancy in determining the ion dependence. We highlight the importance of performing the experiments under isothermal conditions and we propose a new approach to control the fiber temperature at room temperature and at elevated temperatures. The approach is based on an isothermal Galinstan bath. The appropriateness of this method is shown by comparing it to different cooling methods, and the results are supported by simulations. [ABSTRACT FROM AUTHOR]

We report a multimode, double-clad, Yb-doped fiber amplifier that produces diffraction-limited, 0.8-ns pulses with energies of 255 muJ and peak powers in excess of 300 kW at a repetition rate of ~8 kHz . Single-transverse-mode operation was obtained by bend-loss-induced mode filtering of the gain fiber.