A computational model of gas-particle flows has been extended to predict venturi scrubber performance as measured by particle collection efficiency and pressure drop. The concept of regarding particle and liquid phases as sources of momentum and energy to the gaseous phase was incorporated into the model's computational scheme. Predicted pressure drop results showed good agreement with available experimental data, particularly when uniform liquid distribution across the venturi cross-section was achieved. Our model was also more successful in predicting particle collection efficiency than several other models previously reported in the literature. Differences between model predictions and experimental results were chiefly caused by maldistribution of injected liquid into the test scrubbers.

Ytterbium-doped fiber lasers are making impressive leaps in power production. Yet in spite of fiber’s large surface area to volume ratio which is beneficial for cooling, such power inevitably leads to high core temperatures that in turn affect the laser performance. In this paper, the temperature effects on the emission and fluorescence lifetime of ytterbium-doped optical fibers are investigated. From these the temperature dependent emission and absorption cross-sections are calculated.

We report studies of SBS in optical fibers with the goal of using SBS phase conjugation as a passive beam combiner to build high power (>100 W cw) all fiber laser sources. We propose the development of a near infrared high power fiber laser by phasing two Er doped amplifiers in parallel using stimulated Brillouin scattering in a multimode fiber. We use a 1.5 p.m master oscillator-power amplifier configuration (MOPA) to generate SBS in a multimode fiber. The fiber amplifier consists of two Er doped multimode fiber amplifiers (diode pumped), in parallel, which will combine to generate SBS in the multimode fiber.

We investigate stimulated Brillouin scattering (SBS) threshold in single mode and multimode fibers in an all fiber network. The pump is a single mode fiber pigtail attached to a diode. We find the theory and experiment agree for both single mode and multimode GRIN fibers. We modify the bulk SBS threshold equation for use with fibers by properly accounting for mode sizes and modal dispersion.

Phytochemical studies on the roots of Hemidesmus indicus resulted in the isolation of six new pentacyclic triterpenes including two oleanenes identified as olean-12-en-21 beta -yl acetate, and olean-12-en-3 alpha -yl acetate, three ursenes characterized as 16(17)-seco-urs-12,20(30)-dien-18 alphaH-3 beta -yl actetate, urs-20(30)-en-18 betaH-3 beta -yl acetate and 16(17)-seco-urs-12,20(30) dien-18-alphaH-3 beta -ol and a lupene formulated us lup-1,12-dien-3-on-21-ol including a known compound, beta -amyrin acetate, on the basis of spectroscopic techniques and chemical means.

We model lasing in a high finesse, doubly resonant cw Raman laser. We derive expressions for threshold, power extraction, and the conditions for maximum Stokes output power. Our model confirms recent measurements [Brasseur et al., Optics Lett. 23 (1998) 367] of the Stokes output power as a function of the input pump power.

Self-pulsing in passively Q-switched microchip lasers is investigated in detail. The typical range of values of the parameters motivates a new analysis of the laser rate equations. We determine basic properties of the laser intensity oscillations such as threshold conditions, repetition rate, pulsewidth, peak power, and pulse energy. We show that these oscillations appear through a quasi-vertical Hopf bifurcation located slightly above the lasing threshold. Our bifurcation results are verified numerically by modeling a microchip laser experiment with Nd:YAG as the gain medium and Cr:YAG as saturable absorber. Our results agree with the experiment to within 10%.

We show that a change from contradirectional pumping to back-reflection pumping in upconversion lasers can reduce the threshold by 50%, increase the slope efficiency by 87%, and increase the laser output power by a factor of two to six depending on the pump powers. Our predictions are anchored to an upconversion Pr 3+ ZBLAN fiber laser experiment to within 10%.

We model the steady-state threshold and extracted power of a two-photon incoherently pumped upconversion fibre laser. Our threshold analysis is entirely analytic, and along with this derivation we obtain an analytic threshold cutback formula. This takes a particularly simple form when the ground state pump absorption follows exponential absorption. We also numerically simulate the extracted laser power. The experiment which we simulate is upconversion in Pr3+ doped ZBLAN fibre lasing at 491 nm and pumped with 1017 nm and 835 nm diode lasers. Our formulas and simulations are mutually consistent and agree with the experiment to within 10%.

We simulate Yb:YAG laser tuning curves using analytic equations for a two manifold laser. Our simulations agree with experiment.

We model and compare with experiment the threshold, extraction efficiency and temperature effects in Tm:YAG lasers. In particular, we are concerned with high pump powers where lasing can cease abruptly. We simulate the pump depletion equation with the forward and reverse lasing equations combined with the CW rate equations which include cross-relaxation. The resonator is single-pass, end-pumped Fabry-Perot. Consequently, the equations must be solved numerically with a shooter. Our simulation of these equations gives the z-dependence of the intensities, of the populations and of the heat source. Heating is created by nonradiative decay from the two upper manifolds and by the cross-relaxation energy deficit. This leads to thermal lensing as a function of incident pump power which causes lasing shut-off as the resonator becomes unstable. Our simulations for the slope efficiency, threshold and laser shut-off agree with experiment. Additionally, we show that for pump powers in the range of 15–20 W the core temperature rise is about 40 K and the thermal focal length is in the range 5–6 cm.

The effects of upconversion and pump excited state absorption (ESA) on a three-manifold continuous wave (CW) laser are investigated by solving the laser and pump differential equations subject to two-point boundary conditions. This technique is applied to erbium germanosilicate fibre lasers in low-finesse cavities. First, the three-manifold laser problem in a low-finesse cavity is solved analytically, without ESA and upconversion. This shows that the finite value of the upper pump manifold decay rate causes the extracted power to saturate, and it also shows that pump threshold is strongly influenced by the cavity reflectivity. Next, upconversion and ESA are included. This leads to a study of the extracted power and pump threshold as functions of fibre length and outcoupling. This simulation is numerical and is successfully anchored to experiment.

We develop the cw extraction equations for diode pumped solid state lasers when the pumping field is also constrained by a resonant cavity. That is, both the pump and the laser fields are described by forward and reverse fields and two point boundary conditions. Our model includes depleted pump with direct-manifold pumping characterized by the pump photon connecting the two manifolds from which lasing occurs. We derive the condition for minimum threshold pumping and show that it occurs when the reflected pump field vanishes. We compare the optimal crystal lengths, thresholds, and laser output rates for resonant pumping with their single pass pump counterparts. We show that with the recirculating pump the laser output can be at least ten times larger than the single pass output for the parameters chosen.

Morphologic characteristics of circulating platelets were studied in 20 patients with primary Budd-Chiari syndrome (BCS) without any known etiology, using transmission electron microscopy (TEM). Significant platelet ultrastructural changes were observed in all the patients (in 10-->90% platelets) as compared to 20 normal healthy controls (in up to 4% of their platelets). The prominent changes in the platelets were paucity or absence of alpha granules, hypertrophy of the open canalicular systems (OCS) and clumping and fusion of the granules and other organelles in the centre of platelets. Some other changes observed in platelets were dilated channels of OCS, pseudopodial protrusion of cytoplasm and presence of prominent masses of glycogen particles. Platelets from 20 normal controls processed along with the patients' platelets showed only a few such abnormalities. Most of these changes observed in patients' platelets were akin to the changes observed in platelets undergoing activation. Assay of plasma beta-thromboglobulin showed significantly higher levels in all the patients (p < 0.001) further confirming on-going in vivo platelet activation with morphologic changes most likely reflecting the thrombotic process present in BCS patients.

In this paper, we study the reflection coefficient of a periodic Bragg reflecting structure in which there exists both an index-of-refraction grating as well as a gain grating. Most past studies have assumed that these two gratings were in phase with each other. In the present work we investigate the effects that arise when a phase shift is purposely introduced between the two gratings. We use a normal coupled-wave analysis to calculate the reflection coefficient as a function of the detuning away from the Bragg frequency when this grating phase difference is varied from 0 to 2/spl pi/. We find particular phase difference values for which essentially a single-line reflection coefficient results, which in turn should produce narrow-band, single-mode operation of any device utilizing this Bragg structure for distributed feedback purposes. >

We study the continuous-wave (cw) characteristics of both two-manifold and three-manifold Tm: YAG laser pumped at λp ≈ 1.8 µm or λp = 0.785 µm and lasing at λ1 = 2.02 µm. The three-manifold rate equations are adiabatically reduced to their two-manifold form. For each pumping scheme, the steady-state rate equations are combined with the cw differential equations for the forward- and reverse-lasing fields and the pump-depletion differential equation. These three coupled cw differential equations are solved analytically. This gives the linear flux-conservation law between the input pump and the laser output, the minimum crystal length, and optimal output couplings. We show that the major difference between these two pumping schemes is due to the different pump effective absorption cross sections and not the two-for-one cross relaxation. Our example shows that the minimum intensity threshold and optimal crystal length are smaller for pumping at λtp = 0.785 µm than pumping at λp ≈ 1.8 µm.

We develop a general plane wave theory for cw diode-pumped solid state lasers which is valid for arbitrary saturation and outcoupling. Our model is based on the steady state rate equations for a two-manifold system coupled to the growth of the laser field and the depletion of the pump. This model is entirely analytic and predicts: a linear photon flux conservation law; the modification to Beer's law pump depletion due to outcoupling, and its influence on the pump threshold; optimal crystal lengths and outcouplings as functions of temperature. We give some examples which show optimal lengths and reflectivities for Tm:YAG.

Modern crystal-growing techniques allow nearly any imaginable heterostructure to be grown. We are interested here in a periodic Bragg reflector in which there exists both an index-of-refraction grating as well as a gain grating. Unlike in past studies, however, we will investigate the effects due to any phase shift that might exist between these two gratings. For example, we can let the index n ( z ) and the gain g ( z ) be harmonic functions of z , as n ( z ) = n + n 1 cos2β 0 z and g ( z ) =g + g 1 cos(2β 0 z + ϕ), with ϕ being the phase difference between the functions. We then proceed with a normal coupled-wave analysis to calculate the reflection coefficient as a function of the detuning between the incident field frequency and the grating frequencies for various phase differences.