Difference-frequency pulse generation in quantum well heterolasers

It is shown that mid-to far-infrared (IR) and terahertz (THz) pulse generation via difference-frequency mixing in quantum well (QW) dual-wavelength heterolasers can be rather efficient under the modelocking regime for one or both lasing fields even at room temperature. In such a device, the long-wavelength field is produced in the process of intracavity difference-frequency mixing of two optical fields: continuous wave (CW) and pulsed (or both pulsed), due to the resonant intersubband quantum coherence in QWs, as well as due to the nonresonant second-order semiconductor bulk nonlinearity. The mode-locking regime of the optical generation allows one to significantly enhance the pulsed driving fields in comparison with those under CW operation and, therefore, substantially increase the output difference-frequency power. Within a simple model, an explicit formula for the intensity and shape of the generated IR or THz pulse is derived. It is shown that this method is capable of producing picosecond pulses at a ∼ 1-GHz repetition rate with a peak power of the order of 1 W and ≲0.2 mW at 10 and 50 μm wavelengths, respectively. Original Text ? Astro, Ltd., 2007.     

A time-compensated spectral filter for transform-limited tunable picosecond pulse generation from spectro-temporal-selection dye lasers

Diffraction and transform-limited picosecond tunable pulses are generated from Spectro-temporal-Selection (STS) dye lasers by using a new extra-cavity filter. This filter is based on a grazing-incident grating and arranged in the configuration of a folded dispersive delay line. Thus, it provides both high spectral selectivity and controllable temporal compensation for elimination of pulse broadening. Direct production of diffraction- and transform-limited picosecond dye laser (10 µJ, 50 ps) pulses spectrally adjustable between 398 and 702 nm is demonstrated in a compact device, with 8 ns pump pulses from a nanosecond nitrogen laser.     

Double heterojunction lasers and quantum well lasers

The improved technology of compound semiconductor heterojunction preparation has resulted in very reliable CW, room temperature diode lasers for optical information read-out grown on p-type substrates on the one hand and very abrupt double heterojunction diode lasers based on quantum effects on the other hand. The influence of quantization effects on the emission wavelength, the threshold current and its temperature dependence are discussed. A distinction has been made between quantization due to strong magnetic fields giving rise to a one-dimensional electron gas (quantum wire) and quantization resulting from electrostatic and/or compositional changes (quantum well). The double heterojunction as a test structure to study carrier scattering into quantum wells, the phonon participation in the hot carrier relaxation process and optical flux guiding in graded heterojunctions have been emphasized.     

490 fs pulse generation from a passive C-band AlGaInAs/InP quantum well mode-locked laser

We report femtosecond pulses from a passive C-band two-section AlGaInAs/InP mode-locked laser with a monolithically integrated passive waveguide made by quantum well intermixing. Without any external pulse compression, Lorentzian pulses are generated at a repetition frequency of ~38 GHz with 490 fs pulse duration, which is, to the best of our knowledge, the shortest pulse from any directly electrically pumped quantum well semiconductor mode-locked laser. The mode-locking range is relatively large and the ultranarrow pulse width is very stable over a broad range of driving conditions.     

Stable 10 ns, kilowatt peak-power pulse generation from a gain-switched Tm-doped fiber laser

We report what we believe to be the first demonstration of stable short pulse (10 ns) generation from a gain-switched Tm-doped fiber laser in the 2 microm spectral region. A modulated 1.55 microm pump laser was used for fast gain switching to regulate the conventionally chaotic spiking in the gain-switched fiber laser. From a 6.3 microm core all-fiber oscillator, over a kilowatt of peak power was obtained at the slope efficiency of 50% and freely varying repetition rate up to 500 kHz.     

Inhomogenous broadening in quantum well lasers

The effect of structural inhomogeneities in multi-quantum-well lasers caused by composition and well width fluctuations on the line-shape and gain of the lasers is discussed based on gaussian and uniform distribution models. Quantitative results of wide applicability are deduced. The results show that the structural inhomogeneities will reduce the gain and broaden the lineshape.     

Lead salt quantum well diode lasers

Lead-salt diode lasers are useful for spectroscopic applications in the 2.5–30 μm wavelength range. These devices have previously required cryogenic cooling <100 K) for CW operation. The use of quantum well, large optical cavity structures has improved the operating temperatures to 174 K CW (at 4.39 μm) and to 270 K pulsed (at 3.88 μm). These diodes have a single PbTe quantum well with lattice-matched Pb_1?xEu_xSeyTe_1?y confinement layers grown by molecular beam epitaxy. The emission energy shifts have been calculated using a finite square well with nonparabolicity effects included. Initial work has also been done on multiple quantum well lasers. The maximum operating temperatures were comparable to those of single quantum well lasers, with leakage current and possibly Auger recombination limiting device performance.     

Ultrashort pulse generation in diode lasers

The basic ideas and current state of the art of ultrashort pulse generation by injection lasers are reviewed. All developed techniques, including gain switching,Q-switching, and mode-locking are described and compared. A simple theoretical treatment of a diode laser which emits picosecond light pulses is discussed. Some fundamental limits of the pulse parameters are discussed. Finally, compression of chirped optical pulses by optical fibres and the soliton effect is considered.     

Tunable picosecond pulse generation from the passively mode-locked coumarin 6 dye laser

Using the saturable absorber 2-(p-Dimethylaminostyryl)-benzothiazolylethyl iodide, coumarin 6 has been passively mode-locked for the first time to give fully modulated trains of pulses of 4 ps duration and with peak powers of 3 MW tunable over the spectral range 526–547 nm.     

Narrow-directed fast-ion flow generation from targets irradiated by a picosecond laser pulse

The proton and deuteron yields from thin targets irradiated by a picosecond laser pulse with an average radiation intensity of ≤4×10¹⁸ W/cm² at the target were measured in the megaelectron-volt energy range. A ring structure was observed for the outgoing ions, and the angular ion-beam divergence was found to be extremely small (0.5°). The fast-ion generation mechanism allowing for the appearance of ring structure is discussed, and the characteristic energies and spatioangular ion-beam distribution are estimated.     

Simulation of carrier capture in quantum well lasers due to strong inelastic scattering

The effects of strong inelastic scattering on carrier transport over and capture into the quantum wells of quantum well lasers are simulated. In contrast to most semiconductor devices, strong scattering is beneficial to the operation of quantum well lasers. However, such strong inelastic scattering in nanostructures can be expected to produce intermediate degrees of phase coherence, limiting the applicability of both classical models, such as Bethe thermionic emission theory, and commonly used quantum mechanical treatments, such as Fermi's Golden Rule. Two computational approaches are demonstrated for simulating such transport with intermediate degrees of phase coherence. First, absorbing potentials are used within Schrödinger's equation to represent inelastic scattering. This simple approach both exhibits much of the essential physics of such transport and is computationally efficient. Then a more rigorous approach, Schrödinger equation (based) Monte Carlo (SEMC), is demonstrated. While SEMC is rigorously quantum mechanical, the numerical algorithm has more in common with semiclassical Monte Carlo methods than path integral-based quantum Monte Carlo methods. Both of these methods demonstrate nonlinear variations in carrier capture with variations in scattering, and the destruction of quantum resonances for transmission over the quantum well.     

Theoretical analysis of picosecond pulse development of passively mode-locked Nd-glass lasers

A realistic numerical theory of the temporal and spectral development of picosecond light pulses in a passively mode-locked Nd-glass laser is presented. The effects of the inhomogeneous gain profile of the active medium are considered. The calculations include the pulse development in the prelaser, the linear laser and the mode-locking region. In the mode-locking region twophoton absorption and self-phase modulation in the active medium as well as the bleaching dynamics of the mode-locking dye are included. The influence of self-phase modulation on the temporal pulse shape in active media of finite spectral gain width is analysed. The theoretical results are compared with the experimental findings.This paper is dedicated to Professor W.Kaiser for his 60th birthday.     

Carrier transport effects in quantum well lasers

EditorialSpecial Issue

Carrier transport effects in quantum well lasers     

Resonant injection quantum well diodes and lasers

Simple modifications of the standard GaAsGaAlAs double barrier tunneling diode and quantum well laser structures are considered. Calculations show that by replacing the outer GaAs layers of the diode by small aluminum concentration GaAlAs, the peak-to-valley ratio of the negative resistance can be increased. A similar structure is formed if thin high aluminum content barriers are added on one or both sides of the quantum well in a quantum well laser. The barriers then create a resonance in the well region. The alloy concentration outside of the barriers can be chosen to line up the incoming electron energy with the resonance, creating a greatly enhanced charge density in the well. Electrons are thereby captured by the well more efficiently and threshold currents may be lowered.     

Short pulse physics of quantum well structures

This paper reports some recent results of time-resolved studies of the carrier dynamics in GaAs/GaAlAs quantum well structures with picosecond and subpicosecond time resolution. These experiments have provided insight into carrier trapping, energy relaxation, and carrier recombination processes. Carrier trapping into the quantum well layers is very efficient and determines the decay of the GaAlAs luminescence even for 1 μm thick cladding layers. Carrier recombination is enhanced particularly at low temperatures. This effect has been attributed to the increased overlap of electron and hole (exciton) wavefunctions in the quasi-two-dimensional carrier system.     

Multiple quantum well mixing and index-guided quantum well heterostructure lasers by MeV ion implantation

In recent years considerable interest has been shown in impurity-induced compositional disordering of III–V compound semiconductor devices, especially in efforts directed towards fabricating index-guided buried heterostructure lasers and other unique photonic and electronic devices. In this work we describe the study of compositional disordering induced by MeV implantation of oxygen and krypton into AlAs-GaAs superlattices and the fabrication of index-guided quantum well heterostructure lasers by MeV oxygen ion implantation.     

Stable gain-switched 845 nm pulse generation by a weak 1550 nm seed laser

We report what we believe to be the first demonstration of stable gain-switched 845 nm pulse generation (Er(3+): (4)S(3/2)->(4)I(13/2) transition) by a weak modulated 1550 nm seed laser, from an Er(3+)-doped fluoride fiber pumped by a cw 974 nm laser diode. When the pump power of the 974 nm laser diode is set to a certain value, the injection of a weak (<1 mW) modulated 1550 nm seed laser into the 845 nm lasing cavity can depopulate the lower state (4)I(13/2) of 845 nm lasing, switch the gain to regulate the conventionally chaotic spiking, and generate stable synchronous 845 nm pulse trains with the repetition rate up to 100 kHz.     

Analysis of tunable picosecond pulse generation from a distributed feedback Ti：sapphire laser

A distributed feedback Ti:sapphire laser (DFTL) pumped by a 532nm Q-switched pulse is proposed for the generation of tunable picosecond pulses. With coupled rate equation model, the temporal characteristics of DFTL are obtained. The numerical solutions show that the DFTL pulse with a 50-ps pulse duration and as much as 3.SmJ pulse energy can be obtained under 40-m J, 5-ns pulse pumping. The dependence of output pulse width on the laser crystal‘s length, pumping pulse duration, and pumping rate is also discussed in detail.