Unsaturated Optical Gain Research Articles

Homogeneous and inhomogeneous broadening effects on static and dynamic responses of quantum-dot semiconductor optical amplifiers

In this paper, the effects of homogeneous and inhomogeneous broadenings on the response of quantumdot semiconductor optical amplifier (QD-SOAs) are investigated. For the first time, the state space model is used to simulate static and dynamic characteristics of the QD-SOA. It is found that with decreasing the homogeneous and inhomogeneous broadenings, the saturation power of the QD-SOA decreases and the optical gain and the ultrafast gain compression increase. Simulation results show that with decreasing the homogeneous broadening from 20 to 1 meV, the gain compression increases from 40% to 90%, the unsaturated optical gain becomes approximately tripled, and the saturation power becomes two times less. Also, simulations demonstrate that with decreasing the inhomogeneous broadening from 50 to 25 meV, the gain compression increases from less than 50% to more than 90%, the unsaturated optical gain becomes approximately 10-fold, and the saturation power becomes three times less. In addition, it is found that the homogeneous and inhomogeneous linewidths should be small for nonlinear applications. The homogeneous and inhomogeneous broadenings need to be large enough for linear applications.

Vertical coupling in multiple stacks quantum-dot semiconductor optical amplifiers

The characteristics of vertically coupled multiple stacks quantum-dot (QD) semiconductor optical amplifiers (SOAs) are studied taking into account the effect of carrier coupling between adjacent stacks. We find that QD-SOAs that consist of a few numbers of stacks provide higher optical gain at lower applied current. Our analysis shows that the optical gain of the amplifier can be considerably enhanced due to short tunnelling lifetimes between adjacent stacks. Also, our analysis reveals that short coupling lifetimes can significantly reduce the transparency current and improve the uniformity of the active region since the net coupling rate is a linear function of the injection rate. Long coupling lifetime, on the other hand, produces non-linear and non-uniform coupling rates, which reduce the optical gain and increase the transparency current of the amplifier. We also find that the degradation in the unsaturated optical gain due to long coupling lifetimes is more severe in p-type doped QD-SOA compared with un-doped QD-SOA.

Low-Current N-Type Quantum Dash Semiconductor Optical Amplifiers

In this letter, we propose the use of light N-type doping to enhance the optical gain of the quantum dash semiconductor optical amplifier (SOA) at low applied current. We find that doping the dashes with light donor concentration increases the unsaturated optical gain and widens the optical gain spectrum range at low applied current. Also, our calculation reveals that the use of N-type doping is associated with a reduction in the slope efficiency and a decrease in the transparency current, which makes N-type quantum dash SOA attractive for low power dissipation applications.

Effect of Doping on the Optical Characteristics of Quantum-Dot Semiconductor Optical Amplifiers

The influence of p-type and n-type doping on the optical characteristics of a quantum-dot semiconductor optical amplifier (SOA) is studied using a rate equation model that takes into account the effect of the multidiscrete energy levels and the charge neutrality relation. Our calculations show that the amplifier optical gain can be greatly enhanced through p-type doping where the doping concentration should not exceed the certain level. We find that increasing the acceptor concentration increases the unsaturated optical gain but at the same time decreases the saturation density and the effective relaxation lifetime. Also our calculation reveals that the use of p-type doping will be associated with an increase in the transparency current where the increase in the transparency current depends on the incident photon energy. On the other hand, we find that it is possible to increase the saturation density and enhance the linearity of the SOA by using n-type doping.

Optical gain and saturation characteristics of quantum-dot semiconductor optical amplifiers

Detailed theoretical analysis of the gain characteristics of quantum-dot semiconductor optical amplifiers (QD-SOA) is presented. An analytical expression for the optical gain is derived from the quantum dot and wetting layer rate equations. Due to the better confinement of carriers in the quantum dots, our calculation shows that large unsaturated optical gain can be obtained at low operating current. Also, we found that the output saturation intensity of QD-SOA is higher than the output saturation intensity of bulk-SOA. This fact lends itself to the design of efficient low-power SOAs.

An analytical model for quantum dot semiconductor optical amplifiers

An analytical model for the optical gain of quantum dot-semiconductor optical amplifier (QD-SOA) is presented. The model is derived by analytically solving the rate equations of the laser amplifier taking into account inhomogeneous line broadening, dot-to-dot electron tunneling and gain saturation due to spontaneous emission. Because of the unique electronic structure of quantum dots (QDs), we observed that using QD in the active region of a semiconductor optical amplifier increases the unsaturated optical gain and the output saturation intensity of the amplifier.

Room Temperature Performance of (311) GaAs Quantum-Wire Structures

Quantum wire structures are directly formed during the growth of GaAs/AlAs multilayer structures by molecular beam epitaxy due to the natural evolution of nanometer-scale corrugations on (311)-oriented GaAs and AlAs surfaces. The pronounced optical anisotropy clearly observable at 300 K reaches a value as high as 30% for the heavy-hole exciton resonance in 43 Å GaAs quantum-wire structures. The luminescence intensity of (311) GaAs quantum-wire structures with average GaAs layer thicknesses below 66 Å is generally higher than that of conventional (100) GaAs quantum-well structures. In 56 Å GaAs quantum-wire structures no reduction of the integrated luminescence intensity is observed up to 400 K. In the regime of unsaturated optical gain, the amplified luminescence intensity of (311) GaAs quantum-wire structures exceeds that of (100) GaAs quantum-well structures by fourfold. These findings highlight the potential of these novel low-dimensional structures for advanced semiconductor device concepts.

Room-temperature stimulated emission in optically pumped narrow ZnSe/ZnSxSe1−x multiple-quantum-well structures

Room-temperature stimulated emission in ZnSe/ZnS0.18Se0.82 multiple-quantum-well structure is reported. An optical excitation threshold around 300 kW/cm2 at 300 K and 8.4 kW/cm2 at 10 K was measured. An unsaturated optical gain coefficient of the order of 5×102 cm−1 has been measured at low temperature.

Optical gain and loss processes in GaInAs/InP MQW laser structures

The experimental determination of unsaturated optical gain in GaInAs/InP multiple-quantum-well heterostructures is reported. A pronounced step-like line shape of the optical gain spectra is observed, directly reflecting the two-dimensional (2-D) nature of the carrier system. An excellent description of all experimental data is obtained by applying a microscopic theory for the radiative recombination processes. From detailed line shape analyses, two-dimensional carrier densities up to 11*10/sup 2/ cm/sup -2/ and internal absorption coefficients up to 1000 cm/sup -1/ at room temperature are derived. A steep decrease of the differential quantum efficiency above 240 K reveals the importance of intervalence band absorption in quantum-well (QW) lasers. Auger processes are quantitatively investigated in 2-D lasers. The threshold behavior of QW lasers is discussed using the experimentally determined absorption and recombination coefficients, and an optimization of the threshold current density with respect to the number of wells is given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical gain in strain-free and strained layer Ga XIn 1−XAs/InP superlattices

We report on the experimental determination of unsaturated optical gain spectra in Ga XIn 1−XAs/InP multiple quantum well heterostructures (MQWHs). For the first time a pronounced step-like lineshape of the optical gain spectra is observed, directly reflecting the two dimensional (2D) nature of the carrier system. The optical gain (lineshape and peak gain values) in QWHs is completely different for TE and TM polarization in contrast to bulk material where it is isotropic. An excellent description of all experimental data is obtained by applying a microscopic theory for the radiative recombination processes. Two-dimensional carrier densities are derived from detailed lineshape fits and range up to 9.6·10 12 cm −2 at room temperature. In In-rich QWHs (x<0.47) the gain is suppressed for TM polarization as compared to GaInAs layers lattice-matched to InP. The enlarged anisotropy of the optical gain is explained by higher separations between light and heavy hole subbands which result from the biaxial compression of the GaInAs wells.

Optical gain spectra of high density electron-hole plasma in GaSe and InSe

We have performed stimulated emission and unsaturated optical gain measurements at low temperature in the indirect semiconductors GaSe and InSe at excitation intensities above 0.5 MW/cm2, i.e., when the critical Mott density is overcome and an electron-hole plasma (EHP) is generated.In both GaSe and InSe the gain spectra show only the band due to zero phonon recombination of indirect plasma (IEHP). No optical gain due to direct plasma (DEHP) is observed. The comparison of experimental results with theoretical models is very satisfactory. In InSe it becomes also a tool for evaluating the consistence of proposed band structures with observed data.

Optical gain by self-trapped excitons inCdI2

The stimulated emission in ${\mathrm{CdI}}_{2}$ at temperatures between 10 K and room temperature has been studied under ${\mathrm{N}}_{2}$-laser and dye-laser optical pumping. The observed spectra consist of only one broad band due to a strong saturation effect, but the unsaturated optical gain spectrum shows that the stimulated emission is due to the overlapping of four main amplified transitions. These transitions are highly consistent with the model of self-trapped excitons in ionic crystals.

Stimulated emission and E-H plasma in gallium selenide

The stimulated emission from extremely high quality GaSe crystals is investigated at very high values of excitation intensity by means of a nitrogen laser. The rising of the laser action due to electron-hole plasma recombination is reported. The unsaturated optical gain spectrum confirms the stimulated effect either from excitonic interaction processes or from electron-hole plasma. The role of the surface quality of the samples in the competition among these two amplification mechanisms is particularly discussed.

Investigations on unsaturated optical gain spectra of GaAs-GaAlAs-DHS lasers at above-threshold conditions

Directly measured unsaturated gain spectra of GaAs-GaAlAs double heterostructure wafers at above-threshold conditions are reported for different temperatures. Maximum gain values are in the order of 100 cm−1 for room temperature and up to 1000 cm−1 for liquid helium bath temperature. Best fit of the gain spectra is obtained using lineshapes corresponding to band-to-band transitions without validity ofk-selection rule and including tail states. The temperature dependence of the maximum gain agrees with these assumptions.