T.C. Newell

Optical characteristics of 1.24-μm InAs quantum-dot laser diodes

The optical characteristics of the first laser diodes fabricated from a single-InAs quantum-dot layer placed inside a strained InGaAs QW are described. The saturated modal gain for this novel laser active region is found to be 9-10 cm/sup -1/ in the ground state. Room temperature threshold current densities as low as 83 A/cm/sup 2/ for uncoated 1.24-/spl mu/m devices are measured, and operating wavelengths over a 190-nm span are demonstrated.

Room-temperature operation of InAs quantum-dash lasers on InP [001]

The first self-assembled InAs quantum dash lasers grown by molecular beam epitaxy on InP (001) substrates are reported. Pulsed room-temperature operation demonstrates wavelengths from 1.60 to 1.66 μm for one-, three-, and five-stack designs, a threshold current density as low as 410 A/cm2 for singlestack uncoated lasers, and a distinctly quantum-wire-like dependence of the threshold current on the laser cavity orientation. The maximal modal gains for lasing in the ground-state with the cavity perpendicular to the dash direction are determined to be 15 cm–1 for single-stack and 22 cm–1 for five-stack lasers.

The influence of quantum-well composition on the performance of quantum dot lasers using InAs-InGaAs dots-in-a-well (DWELL) structures

The optical performance of quantum dot lasers with different dots-in-a-well (DWELL) structures is studied as a function of the well number and the indium composition in the InGaAs quantum well (QW) surrounding the dots. While keeping the InAs quantum dot density nearly constant, the internal quantum efficiency /spl eta//sub i/, modal gain, and characteristic temperature of 1-DWELL and 3-DWELL lasers with QW indium compositions from 10 to 20% are analyzed. Comparisons between the DWELL lasers and a conventional In/sub 0.15/Ga/sub 0.85/As strained QW laser are also made. A threshold current density as low as 16 A/cm/sup 2/ is achieved in a 1-DWELL laser, whereas the QW device has a threshold 7.5 times larger. It is found that /spl eta//sub i/ and the modal gain of the DWELL structure are significantly influenced by the quantum-well depth and the number of DWELL layers. The characteristic temperature T/sub 0/ and the maximum modal gain of the ground-state of the DWELL structure are found to improve with increasing indium in the QW It is inferred from the results that the QW around the dots is necessary to improve the DWELL lasers /spl eta//sub i/ for the dot densities studied.

Transition dipole moment of InAs/InGaAs quantum dots from experiments on ultralow-threshold laser diodes

Semiconductor ultralow-threshold InAs quantum-dot lasers are investigated operating at 1230–1250 nm at room temperature (laser threshold range is of 16–83 A/cm2 for ground-state emission). The dependence of gain on current is derived from measurements of the threshold current as a function of the cavity length. The ground-state gain appears at very low current: the inversion threshold of ∼13 A/cm2 is a record low value. Analysis of these data for diodes of different molecular beam epitaxial-grown wafers leads to a squared dipole moment of the transition of ∼9.2×10−57 C2 m2 that corresponds to the length of elementary dipole of ∼0.6 nm.

Ground-state emission and gain in ultralow-threshold InAs-InGaAs quantum-dot lasers

Emission spectra and modal optical gain are investigated in ultralow-threshold MBE-grown InAs-InGaAs quantum dot (QD) structures. The record lowest room-temperature inversion current is found to be /spl sim/13 A cm/sup -2/. The rate-equation model is proposed describing the optical gain related to the ground-state (GS) transitions in QDs. The ground-state gain goes to the maximum value that corresponds to the total inversion of available levels. The gain cross section for the GS emission is estimated as /spl sim/7/spl times/10/sup -15/ cm/sup 2/.

High external feedback resistance of laterally loss-coupled distributed feedback quantum dot semiconductor lasers

External optical feedback effects on quantum dot (QD) laterally loss-coupled (LLC) distributed feedback (DFB) lasers are reported for the first time in this letter. The critical external feedback ratio that causes coherence collapse of the QD DFB is measured to be -14 dB. No spectral broadening at this feedback level is observed within the 0.06-nm resolution of the optical spectrum analyzer (OSA). Self-homodyne measurements also confirm that the rebroadened linewidth of the QD DFB under -14-dB feedback is still much smaller than the feedback-free linewidth. Under 2.5-Gb/s modulation, eye-diagram measurements show that the signal-to-noise ratio starts to degrade at a feedback ratio of -30 dB in the QD LLC-DFB, about 20 dB higher than a typical quantum-well DFB at the same output power and extinction ratio.

150-nm tuning range in a grating-coupled external cavity quantum-dot laser

An antireflection (AR) coated single-stack quantum-dot (QD) laser in a grating-coupled external cavity is shown to operate across a tuning range from 1.095 /spl mu/m to 1.245 /spl mu/m. This 150-nm range extends from the energy levels of the ground state to excited states. At any wavelength, the threshold current density is no greater than 1.1 kA/cm/sup 2/. This large tunable range is the product of the rapid carrier filling of the higher energy states under a low pumping current and homogeneous broadening in the QD ensemble. The possibility of a larger tuning range is discussed with the further improvement of the AR-coating.

1.58-μm lattice-matched and strained digital alloy AlGaInAs-InP multiple-quantum-well lasers

A versatile, digital-alloy molecular beam epitaxy (MBE) technique is employed to grow lattice-matched and strained AlGaInAs multiple-quantum well (MQW) 1.58-/spl mu/m laser diodes on InP. A threshold current density as low as 510 A/cm/sup 2/ has been demonstrated for broad area lasers with 1% strained AlGaInAs MQWs, which is the best MBE result in this material system. A single facet pulsed power as high as 0.56 W is obtained. It is also found that the efficiency and internal loss of digital alloy AlGaInAs QW devices are comparable to lasers grown by conventional MBE.

The effect of increased valence band offset on the operation of 2 μm GaInAsSb-AlGaAsSb lasers

Two and four quantum-well (QW) GaInAsSb-AlGaAsSb lasers emitting at 2 /spl mu/m are reported. In comparison to previously published data, it is found that higher Al content in the QW barrier improves the internal efficiency, saturated modal gain, and characteristic temperature of the lasers. These results are attributed to an increased valence band offset that provides superior hole confinement in the GaInAsSb QW. A differential efficiency of 74% is observed at 25/spl deg/C under pulsed conditions for a 900-/spl mu/m cavity length, 2-QW device, and a record characteristic temperature of 140 K is measured for a 4-QW laser.

Linewidth study of InAs-InGaAs quantum dot distributed feedback lasers

The linewidth of laterally loss-coupled distributed feedback (DFB) lasers based on InAs quantum dots (QDs) embedded in an InGaAs quantum well (QW) is investigated. Narrow linewidth operation of QD devices is demonstrated. A linewidth-power product less than 1.2 MHz /spl middot/ mW is achieved in a device of 300-/spl mu/m cavity length for an output power up to 2 mW. Depending on the gain offset of the DFB modes from the QD ground state gain peak, linewidth rebroadening or a floor is observed at a cavity photon density of about 1.2-2.4/spl times/10/sup 15/ cm/sup -3/, which is much lower than in QW lasers. This phenomenon is attributed to the enhanced gain compression observed in QDs.