Excited-state Absorption Losses Research Articles

Lasing in the space charge limited current regime

We introduce an analytical model for ideal organic laser diodes based on the argument that their intrinsic active layers necessitate operation in the bipolar space charge-limited current regime. Expressions for the threshold current and voltage agree well with drift-diffusion modeling of complete p-i-n devices and an analytical bound is established for laser operation in the presence of annihilation and excited-state absorption losses. These results establish a foundation for the development of organic laser diode technology.

Effect of the excited state absorption on the efficiency of erbium-doped DFB fiber lasers

In this paper, a model of erbium-doped distributed feedback fiber laser is proposed and discussed. The model is based on a fundamental matrix approach and accounts for all types of losses including the active fiber background loss and the loss originated from excited state absorption immanent to erbium ions. It is shown that the excited state absorption loss is a main limiting factor to the laser efficiency. The model permits to optimize the laser parameters including fiber Bragg grating strength and position of π-phase defect. It is shown that applying our optimization procedure the pump-to-signal conversion ratio can be increased by 60% for the same device length and pumping conditions.

Passive Q-switching of an erbium-doped glass laser by using a Co2+ : ZnSe crystal

The operation of a passively Q-switched laser emitting the TEMmn mode is described. The excited-state absorption losses and nonlinear absorption in the shaded ends of the active element are taken into account. We measured the laser pulse energy for the TEM00 mode, the ratios of the output energy in transverse TEMmn modes with m = 0–4, n = 0 and m = 0–3, n = 1 to the TEM00 mode energy in an erbium-doped glass laser and determined its temporal parameters in the passive Q-switching regime achieved with the help of a Co2+ : ZnSe crystal. It is shown that on passing from one type of the transverse mode to another for invariable parameters of the resonator, the laser pulse energy changes discretely. A change in the radius of the transverse distribution of the TEM00 mode at the output mirror of the passively Q-switched laser during the development of lasing is estimated. It is found that the relative change in the laser beam cross section on the output mirror under our conditions is less than 2%. The experimental data obtained in the study are in good agreement with our calculations.

Emission Regimes of a Green Er:${\hbox {YLiF}}_{4}$ Laser

We present an analysis of the emission regimes of an Er:YLiF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> laser on the transition <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3/2</sub> rarr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15/2</sub> , under direct pumping and various upconversion pumping mechanisms. The analysis is based on the study of the steady-state solutions of a rate equation system. The thresholds of the various emission regimes are calculated. The influence of the pump mechanism and the excited-state absorption losses on the emission regime is discussed.

Thermal lens and Auger upconversion losses' effect on the efficiency of Nd^3+-doped lead lanthanum zirconate titanate transparent ceramics

A thorough investigation of optical losses for the 1064nm emission in Nd3+-doped lead lanthanum zirconate titanate (PLZT) transparent ceramics is presented. Thermal lens experiments were carried out to evaluate thermo-optical properties and the fluorescence quantum efficiency of the emitting level 4F3/2. Excited-state absorption losses were measured in the emitting wavelength region, and the Auger upconversion energy transfer parameter γ was calculated. By using γ, the pump-intensity dependence of the optical gain at 1064nm, the fluorescence quantum efficiency, and the generation of heat in the ceramic were simulated for a high 803nm pump-power regime. Since the radiative and nonradiative losses in Nd:PLZT were verified to be considerably lower than in various commercial laser crystals and glasses, it is suggested that this material might become an interesting alternative for high-power laser emission.

Structure and spectroscopy of Cr3+ defects in KMgF3, KZnF3, and CsCaF3 crystals. An ab initio model potential embedded cluster study

The ab initio model potential (AIMP) method has been proven to produce effective one-electron operators that accurately represent the embedding effects of the cations and anions that consititute crystalline lattices like the KMgF3, KZnF3, and CsCaF3 cubic fluoroperovskites. The combination of these quantum mechanical embedding potentials with highly sophisticated molecular quantum chemical calculations of the CrF\documentclass{article}\pagestyle{empty}\begin{document}$^{3-}_{6}$\end{document} defect cluster enables theoretical study of the structure and spectroscopy of promising laser materials like the Cr3+-doped KMgF3, KZnF3, and CsCaF3 crystals at the level of quality attainable in molecular quantum chemistry. The combination of the theoretical results with available experimental data [electron paramagnetic resonance (EPR), electron spin resonance (ESR), absorption, and emission spectra at ambient and high pressures] can contribute to clarifying the electronic structure of these systems, where different interpretations of the very sophisticated spectroscopic data exist, mainly due to the difficulty associated with the existence of cubic, trigonal, and tetragonal Cr3+ defects contributing to the spectra. The results of AIMP embedded-cluster studies of the cubic defects are presented here as one more useful and independent source of information that serves to clarify the divergent assignments and to provide new spectroscopic information, which shows that the laser emission of Cr3+ defects in all three crystals should be free from excited state absorption losses if the pumping process is done through selective excitation to the 4T2g laser level, below the 2Eg higher excited state. Since the Cr3+ substitutional impurities create an excess positive charge, the effects of lattice site relaxation and dipole polarization on the local structure have been investigated and are presented here. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 961–972, 2000

Excited-state absorption in the range of pumping and laser efficiency of Cr^4+:forsterite

The absorption saturation and the laser efficiency of Cr(4+): forsterite were measured with 1064-nm (E || b), 980-nm (E || c), and 780-nm (E || b) pulsed laser pumping. A slope efficiency of 32% was obtained for the 980-nm (E || c) pump wavelength, where the excited-state absorption losses were shown to be negligible.

Cross sections of excited-state absorption at 800 nm in erbium-doped ZBLAN fiber

Excited-state absorption (ESA) fromthe metastable levels 4I13/2 and 4I11/2 of erbium is measured in a fluorozirconate fiber in the wavelength range 780–840 nm. Using a pump- and probe-beam technique and choosing the pump wavelength such that the perturbation by pump ESA is minimized in the measurement, it is possible to determine the effective ESA cross sections, despite the fact that the excitation is distributed among two metastable levels. The derived ESA cross sections at 793 nm of 1.4x10−21 cm2 from the 4I13/2 level and less than 0.1x10−21 cm2 from the 4I11/2 level are in reasonable agreement with former results obtained from a rate-equation simulation of the erbium 3-um laser. The corresponding ESA spectrum under 3-um lasing conditions is derived. At the strongest ground-state absorption around 799 nm, decreasing ESA from the 4I13/2 level is compensated by increasing ESA from the 4I11/2 level, i.e., ESA losses cannot be avoided when pumping around 800 nm. This result is of relevance for possible high-power diode pumping of an erbium 3-um double-clad fiber laser.

Influence of upconversion and pump excited state absorption losses on the performance of co- and contradirectionally pumped erbium-doped fiber amplifiers

The combined effects of upconversion and pump excited state absorption losses on the gain from co- and contradirectionally pumped erbium-doped fiber amplifiers have been studied experimentally and simulated numerically. Our results indicate that for all fiber lengths, contradirectional gain is higher than codirectional gain at low pump powers, and codirectional gain becomes higher than contradirectional gain at higher pump powers. © 1996 John Wiley & Sons, Inc.

Highly efficient emerald laser

The laser-relevant spectroscopic parameters in emerald are characterized. Polarized spectra of the absorption cross sections and excitation and emission cross sections were measured. Highly efficient laser operation has been achieved. In a quasi-cw laser-pumped emerald laser, a 64% output-slope efficiency was measured at 768 nm, corresponding to a laser quantum yield of 76%. The maximum output power reached 1.6 W, at 3.6 W of pump power at 647.1 nm, and was pump-power limited. The emerald laser was tuned from 720 to 842 nm. Emerald has good pump and fluorescence efficiencies. The round-trip laser loss, excluding the excited-state absorption (ESA), is 0.4%/cm. In high-optical-quality emeralds, the ESA loss becomes the main factor that limits laser efficiency.

Saturable absorber properties of a laser dye; stimulated propagation of the pump flux

Inefficiencies in a laser dye resulting from excited state absorption are examined analytically with respect to its pump saturation behavior under conditions of small (extracted) signal gain. The spectrum of the lowest excited singlet state for such a dye shows a crossover point from excited state absorption sigma* to gain sigma(e), and this point can occur blue-shifted from the onset of ground-state absorption sigma(o). At wavelengths where sigma(o). sigma*, and sigma(e) operate simultaneously and at flux densities used for laser pumped dye oscillators and amplifiers, stimulated propagation of the pump flux compensates, in part, for excited state absorption losses. Under certain conditions, higher transmissions, flux densities, and excited state population densities are possible than for even a pure saturable absorber. A laser dye pumped between the crossover point and the onset of ground-state absorption sacrifices high excited state densities at the pump surface in order to give a uniform distribution deep into the medium. The percentage photon loss from excited state absorption is a function of sigma* and the initial pump flux only and is independent of sigma(e) and concentration.