Coupled-wave theory of multiple-stripe semiconductor injection lasers

We apply coupled-wave theory to describe the lateral modes of semiconductor lasers with a periodic gain and refractive-index variation across their widths. The model is relevant to devices whose complex index of refraction is determined by current injection from closely spaced parallel electrodes. Good agreement is observed between the analytical modes and those computed numerically for comparison

Saturable nonlinear dielectric waveguide with applications to broad-area semiconductor lasers

Self-focusing in a passive dielectric waveguide with a saturable nonlinearity is studied. The eigensolutions constitute a good approximation to the lateral modes of broad-area semiconductor lasers under low-duty-cycle pulsed conditions. The laser modes are predicted to consist of adjacent filaments coupled in phase, leading to a single-lobed far field, and to be stable with increased current injection above saturation intensity. The ultimate filament spacing is inversely proportional to the threshold gain, and thus wider filaments are expected in low-threshold broad-area lasers

Self-stabilized Nonlinear Lateral Modes of Broad Area Lasers

The lateral modes of broad area lasers are investigated theoretically. The nonlinear interaction between optical field and effective refractive index leads to a saturable nonlinearity in the governing field equation, so that self-modulated solutions are found to be stable with increased current injection above saturation intensity. We derive approximate analytical solutions for traveling wave fields within the broad area laser. The field amplitude consists of a small ripple superimposed on a large dc value. Matching fields at the boundary determines the modulation depth and imparts an overall phase curvature to the traveling wave mode. There are multiple lateral modes for a given set of operating conditions, and modes with successively more lobes in the ripple have greater overall phase curvature. In contrast to the linear problem, several lateral modes can achieve the same modal gain, for a given injected current density, by saturating the gain to different extent. Thus, these modes would exhibit slightly different optical powers

Broadband tunability of gain-flattened quantum well semiconductor lasers with an external grating

Quantum well lasers are shown to exhibit flattened broadband gain spectra at a particular pumping condition. The gain requirement for a grating-tuned external cavity configuration is examined and applied to a semiconductor quantum well laser with an optimized length of gain region. The predicted very broadband tunability of quantum well lasers is confirmed experimentally by grating-tuning of uncoated lasers over 85 nm, with single longitudinal mode output power exceeding 200 mW

Linear tailored gain broad area semiconductor lasers

Tailored gain semiconductor lasers capable of high-power operation with single-lobed, nearly diffraction limited beamwidths only a few degrees wide have been demonstrated in proton implanted chirped arrays and "halftone" broad area lasers. We analyze lasers with a linear gain gradient, and obtain analytic approximations for their unsaturated optical eigenmodes. Unlike a uniform array, the fundamental mode of a linear tailored gain laser is the lasing mode at threshold. Mode discrimination may be controlled by varying the spatial gain gradient. All modes of asymmetric tailored gain waveguides have single-lobed far-field patterns offset from 0°. Finally, we utilize tailored gain broad area lasers to make a measurement of the anti-guiding parameter, and find b = 2.5 +/- 0.5, in agreement with previous results

Erosión por labranza con arado de disco en suelos volcánicos de ladera en Costa Rica

Este trabajo midió el movimiento de suelo por labranza y erosión al utilizar el arado de disco, el implemento de labranza primaria más común en la región cultivada en las faldas del volcán Irazú. Los resultados revelaron que el potencial de erosión por labranza es muy alto, tanto así que los valores de erosividad de la labranza por el arado de disco fueron el doble de los reportados en implementos de labranza primaria utilizados en Europa y Norte América. Se concluyó que la reducción de erosión del suelo por labranza y agua es necesaria para mantener una producción agrícola viable a largo plazo en esta región de Costa Rica

InternationaL cross-sectIonAl and longItudinal assessment on aSthma cONtrol in European adult patients : the LIAISON study protocol

The study is funded by Chiesi Farmaceutici S.p.A., Parma, ItalyPeer reviewedPublisher PD

Comparison of serious inhaler technique errors made by device-naïve patients using three different dry powder inhalers: a randomised, crossover, open-label study

Background: Serious inhaler technique errors can impair drug delivery to the lungs. This randomised, crossover, open-label study evaluated the proportion of patients making predefined serious errors with Pulmojet compared with Diskus and Turbohaler dry powder inhalers. Methods: Patients ≥18 years old with asthma and/or COPD who were current users of an inhaler but naïve to the study devices were assigned to inhaler technique assessment on Pulmojet and either Diskus or Turbohaler in a randomised order. Patients inhaled through empty devices after reading the patient information leaflet. If serious errors potentially affecting dose delivery were recorded, they repeated the inhalations after watching a training video. Inhaler technique was assessed by a trained nurse observer and an electronic inhalation profile recorder. Results: Baseline patient characteristics were similar between randomisation arms for the Pulmojet-Diskus (n = 277) and Pulmojet-Turbohaler (n = 144) comparisons. Non-inferiority in the proportions of patients recording no nurse-observed serious errors was demonstrated for both Pulmojet versus Diskus, and Pulmojet versus Turbohaler; therefore, superiority was tested. Patients were significantly less likely to make ≥1 nurse-observed serious errors using Pulmojet compared with Diskus (odds ratio, 0.31; 95 % CI, 0.19–0.51) or Pulmojet compared with Turbohaler (0.23; 0.12–0.44) after reading the patient information leaflet with additional video instruction, if required. Conclusions These results suggest Pulmojet is easier to learn to use correctly than the Turbohaler or Diskus for current inhaler users switching to a new dry powder inhaler

Linear, nonlinear, and tunable guided wave modes for high-power (GaAl)As semiconductor lasers

High-power, coherent radiation from semiconductor lasers is attractive for such diverse applications as free-space communication, optical data storage, and microsurgery. However, several factors conspire to prevent near-ideal performance from broad area devices and laser arrays. Waveguides wider than a few microns support many lateral modes with poor gain discrimination. Consequently, such modes are easily "mixed" by perturbations in gain and refractive index caused by gain saturation, thermal gradients, and inhomogeneities that are due to imperfect crystal growth. This causes spatially localized modes, multimode operation, and reduced spatial coherence, all of which lead to farfields broader than the "diffraction limit." In this thesis, we have investigated the influence of gain saturation on the lateral modes of broad area structures and laser arrays. Analytical and numerical techniques have been developed to solve self-consistently for mode shapes and propagation constants as a function of injected current density above threshold. For example, our analysis indicates that the gain-saturated modes of broad area lasers consist of an integer number of phase-locked "filaments." In gain-guided quantum well lasers, these nonlinear modes are observed to oscillate into narrow, single-lobed farfields, which broaden only slightly with increased power output up to the 500mW level. Conversely, laser arrays have been widely touted as structures that suppress unwanted filamentation in favor of spatial mode control. Indeed, in this work we have demonstrated supermode control at the 100 mW power level by varying the diffraction region length in diffraction-coupled arrays. Both theoretically and experimentally, however, we have found the lateral modes of laser arrays to be unstable with increased current injection. Waveguides that are phase-matched below threshold become detuned under the influence of gain saturation, so that interguide power transfer is reduced. This decreases the injection-locking bandwidth, and ultimately, the spatial coherence. While undesirable for a laser oscillator, this property may be attractive for all-optical switching in nonlinear directional couplers. Finally, we have considered marrying the high-power, coherent output of broad area lasers and laser arrays with the broadband tunability possible in semiconductor lasers. In particular, the steplike density of states unique to quantum well structures results in gain spectra that are broader and flatter than comparable spectra of double heterostructure lasers. Experimentally, we have tuned uncoated, single quantum well stripe lasers in a grating-coupled external cavity over a range >125 nm centered about 800 nm. Similarly tuned broad area lasers output in excess of 200 mW (pulsed) into a single longitudinal mode over 80 nm, and buried heterostructure lasers were operated continuously over 90 nm. We expect that in the future, such devices could provide a compact, rugged, more efficient alternative to dye lasers