Performance Of Quantum Dot Lasers Research Articles

A self-locking structure for quantum dot DFB and FP lasers based on optical feedback

This paper presents a self-coherent optical feedback system that achieves resonance between the external cavity mode and the relaxation oscillation frequency of distributed feedback (DFB) laser and the repetition frequency of Fabry–Perot (FP) laser, all in an effort to achieve optical feedback self-locking. Results using this system show that for FP lasers, feedback is able to reduce the 3 dB radio frequency linewidth from 2 MHz to 600 kHz. In the case of DFB lasers, an electric frequency comb with an external cavity repetition rate of 17.25 MHz is achieved. This research presents a significant step toward the improvement of the performance of quantum dot lasers in optical communication systems.

Red-emitting InP quantum dot micro-disk lasers epitaxially grown on (001) silicon.

Direct epitaxy of InP quantum dot (QD) lasers on silicon (Si) provides an on-chip red laser source for integrated Si photonics with different applications. Here, we demonstrate the first, to the best of our knowledge, InP QD lasers directly grown on (001) Si. Combining highly emissive InP QDs and a GaAs/Si template with low defect density, continuous-wave (CW) lasing of micro-disk lasers (MDLs) on Si is achieved at room temperature. The lowest threshold of MDLs on Si is ∼500nW, without considering the micro-disk surface absorption efficiency of the pump power. The MDLs grown on the native GaAs substrate with the same growth and fabrication process are compared using statistical data analysis. Similar material characterization results and device performances on these two substrates further confirm the performance of QD lasers on Si. This demonstration paves the way for future realization of integrated photonic circuits with red and near-infrared (NIR) lasers on Si.

The Importance of p-Doping for Quantum Dot Laser on Silicon Performance

$p$ -type modulation doping of the quantum dot active region is known to improve high temperature and dynamic performance of quantum dot lasers. These improvements are critical to realizing commercially relevant quantum dot devices on silicon and are shown to enable continuous wave operation over 100°C, nearly complete insensitivity to optical feedback, and orders of magnitude improvement in device reliability relative to unintentionally doped active regions. Also described is a spectrally resolved analysis of the effect of p-modulation doping on the optical gain revealing anomalous behavior that explains the high characteristic temperatures commonly observed in literature for similar devices on native substrate.

Optimization of 1.3 µm InAs/GaAs quantum dot lasers epitaxially grown on silicon: taking the optical loss of metamorphic epilayers into account

Taking the optical loss caused by the metamorphic layers into account, we have proposed an optimization strategy for 1.3 µm InAs/GaAs quantum dot (QD) laser structures directly grown on silicon. We have investigated the effects of the QD layer number, the thickness and the composition of AlGaAs cladding layers on QD laser performance. The results demonstrate, with respect to the net modal gain and the differential quantum efficiency, that the optimized QD layer number is 7 for lasers grown on silicon, which is different from the optimized number of 5 for the counterparts grown on native substrates. The optimized thickness is obtained for the cladding layers of Al0.4Ga0.6As, Al0.6Ga0.4As and Al0.8Ga0.2As. Further, the optimized QD layer number for the net modal gain is nearly independent of the material gain of active regions. More importantly, the optimization strategy provides a comprehensive method to understand the differences in design between QD laser structures on silicon and those on native substrates.

Quantum dot laser optimization: selectively doped layers

Edge emitting quantum dot (QD) lasers are discussed. It has been recently proposed to use modulation p-doping of the layers that are adjacent to QD layers in order to control QD's charge state. Experimentally it has been proven useful to enhance ground state lasing and suppress the onset of excited state lasing at high injection. These results have been also confirmed with numerical calculations involving solution of drift-diffusion equations. However, deep understanding of physical reasons for such behavior and laser optimization requires analytical approaches to the problem. In this paper, under a set of assumptions we provide an analytical model that explains major effects of selective p-doping. Capture rates of elections and holes can be calculated by solving Poisson equations for electrons and holes around the charged QD layer. The charge itself is ruled by capture rates and selective doping concentration. We analyzed this self-consistent set of equations and showed that it can be used to optimize QD laser performance and to explain underlying physics.

Junction-Temperature Measurement in InAs/InP(100) Quantum-Dot Lasers

We report on the measurement of junction temperature of the InAs/InP(100) quantum dot lasers working in the 1.55 μm wavelength region. The measurement is based on analyzing the temperature induced mode shift of the Fabry—Perot cavity. Under pulsed operation mode, more than 20°C junction temperature rise is measured for the quantum-dot (QD) laser when the duty cycle is increased from 1% to 95%. For a reference quantum well laser, the junction temperature rise is obtained as only around 3°C. The large junction temperature rise might be a crucial factor to improve the performance of QD lasers.

Optimizing InAs/InP (113) B quantum dot lasers with considering mutual effects of coverage factor and cavity length on two-state lasing

Optimizing InAs/InP (113) B quantum dot lasers with considering mutual effects of coverage factor and cavity length on two-state lasing

Fabrication of narrow-striped InAs/GaAs quantum dot laser with wet etching technique

An InAs/GaAs quantum dot laser, fabricated with a narrow-striped width of 6 μm by a wet etching technique, is reported. The etching solutions are composed of three components, i.e. phosphoric acid, hydrogen peroxide, and deionized water. We observed that the unavoidable undercutting was changed with the ratio of etching solution in the GaAs materials. By taking a suitable ratio of etching solution, good performance of quantum dot laser with a size of 6 μm × 700 μm was achieved for fabrication at room temperature. Under continuous wave mode, the lasing wavelength exhibited a single mode, which is located in the region of 1051 nm. In contrast, multimode lasing with a series of non-lasing gaps appeared and the spectra were gradually broadened to the high energy side by increasing the injection current. The laser has one facet power more than 22 mW, with a slope efficiency of 140 mW/A, just a little above threshold current.

Effect of inter-level relaxation and cavity length on double-state lasing performance of quantum dot lasers

Double-state lasing phenomena are easily observed in self-assembled quantum dot (QD) lasers. The effect of inter-level relaxation rate and cavity length on the double-state lasing performance of QD lasers is investigated on the basis of a rate equation model. Calculated results show that, for a certain cavity length, the ground state (GS) lasing threshold current increases almost linearly with the inter-level relaxation lifetime. However, as the relaxation rate becomes slower, the ratio of excited state (ES) lasing threshold current over the GS one decreases, showing an evident exponential behavior. A relatively feasible method to estimate the inter-level relaxation lifetime, which is difficult to measure directly, is provided. In addition, fast inter-level relaxation is favorable for the GS single-mode lasing, and leads to lower wetting layer (WL) carrier occupation probability and higher QD GS capture efficiency and external differential quantum efficiency. Besides, the double-state lasing effect strongly depends on the cavity length.

High-performance 1.3 µm InAs/GaAs quantum-dot lasers with low threshold current and negative characteristic temperature

A high-growth-temperature GaAs spacer layer (HGTSL) is shown to significantly improve the performance of 1.3 µm multilayer InAs/GaAs quantum-dot (QD) lasers. The HGTSL inhibits threading dislocation formation, resulting in enhanced electrical and optical characteristics and hence improved performance of QD lasers. To further reduce the threshold current density and improve the room-temperature characteristic temperature (T0), the high-reflection (HR) coating and p-type modulation doping have been incorporated with the HGTSL technique. A very low continuous-wave room-temperature threshold current of 1.5 mA and a threshold current density of 18.8 A cm−2 are achieved for a three-layer device with a 1 mm HR/HR cavity, while a very low threshold current density of 48 A cm−2 and a negative T0 are achieved in the p-doped lasers.

Carrier Distribution, Gain, and Lasing in 1.3->tex<$mu hboxm$>/tex<InAs–InGaAs Quantum-Dot Lasers

In this paper, we present the results of a theoretical model built to describe the temperature-dependent lasing characteristics of InAs-InGaAs quantum-dot (QD) lasers operating at 1.3 /spl mu/m. From the model, we find that traditional carrier distribution theories are inadequate to describe the performance of these lasers. We therefore introduce an improved model that allows for both free carriers and excitons in the dots. The new model provides threshold current and characteristic temperature T/sub 0/ values that are in good agreement with experimental data. The results of our modeling reveal that, while it is the excitons that mainly contribute to the gain, the ratio of excitons to free carriers significantly affect the T/sub 0/ of QD lasers. Our model results also indicate that the wetting layer current plays little role in QD laser performance. In addition, the model correctly predicts other experimental observations such as; increased T/sub 0/ for increased number of QD layers and p-doped structures, and the oscillatory behavior of T/sub 0/, lending further credibility to the model.

Recent advances in semiconductor quantum-dot lasers

Within the last few years a breakthrough in the device performance of quantum dot lasers occurred and new application areas were opened. Recent advances in the understanding and realisation of quantum dot lasers are reviewed. To cite this article: J.P. Reithmaier, A. Forchel, C. R. Physique 4 (2003).

Lateral-cavity spectral hole burning in quantum-dot lasers

Spectral hole burning effects are observed as strong spectral intensity modulations in the emission spectra of broad and narrow stripe quantum-dot lasers with ridge waveguide. The modulation is attributed to lateral-cavity resonances burning holes in the inhomogeneously broadened spectral gain profile of the quantum dots. Lateral cavity engineering is expected to be crucial for optimizing quantum-dot laser performance and for potential realizing of wavelength-stabilized devices.

Performance of lasers containing three, five and seven layers of quantum dots

The authors have analysed the performance of quantum dot lasers containing three, five and seven layers of InGaAs quantum dots. At room temperature the threshold current density of devices of length between 320 μm and 2000 μm, with uncoated facets, decreases with device length and is lower for higher layer numbers. They explain this behaviour by analysis of measured modal gain spectra, internal optical mode loss and the temperature dependence of the threshold current density. They find that increasing the number of dot layers can improve device performance because the optical mode loss does not increase while the modal gain at a particular wavelength does increase with more dot layers. However, at low temperatures <200 K they find that the three-layer structure exhibits the lowest threshold current density for devices of length 550 μm and longer. They conclude that the improved threshold current behaviour of the higher layer number devices at room temperature is not primarily due to intrinsic gain saturation but because the required modal gain can be achieved in high layer number samples with a lower quasi-Fermi level separation and hence a reduced population of higher lying energy states.

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 laser's /spl eta//sub i/ for the dot densities studied.

MBE growth and characterisation of InGaAs quantum dot lasers

High quality self-assembled InGaAs quantum dots have been formed on GaAs by molecular beam epitaxy via Stranski–Krastonov growth mode, and have been employed to produce quantum dot lasers with reasonably good properties. The effects of growth conditions, substrate misorientation, and doping in quantum dots on the characteristics of quantum dots and quantum dot lasers are presented. It has been shown that higher density of quantum dots is obtained under higher As flux because the diffusion length of Ga adatoms is reduced. Higher degree of substrate misorientation also leads to higher density of quantum dots since the kinks on the surface have similar effect on the diffusion of cations. It is also found that doping in the quantum dots plays an important role in the performance of quantum dot lasers. Room temperature continuous wave operation has been achieved on Be-doped quantum dot lasers. Under pulse operation, characteristic temperature as high as 121 K between 20 and 70°C has been obtained.

Effect of phonon bottleneck on quantum-dot laser performance

The effect of phonon bottleneck on quantum-dot laser performance is examined by solving the carrier-photon rate equations including the carrier relaxation process into the quantum-dot ground state. We show that the retarded carrier relaxation due to phonon bottleneck degrades the threshold current and the external quantum efficiency. We also show that quantum-dot lasers are quite sensitive to the crystal quality outside as well as inside quantum dots. Our results clarified that the relaxation lifetime should be less than about 10 ps to fully utilize the laser potential originating from the quantum-dot discrete energy states.