Mitsuru Ishida

Recent progress in self-assembled quantum-dot optical devices for optical telecommunication: temperature-insensitive 10?Gb?s?1 directly modulated lasers and 40?Gb?s?1 signal-regenerative amplifiers

This paper reviews the recent progress of self-assembled quantum-dot optical devices, highlighting temperature-insensitive 10 Gb/s directly modulated lasers at 1.3 mum and 40 Gb/s signal-regenerative amplifiers in the 1.5 mu band

Temperature-Insensitive Eye-Opening under 10-Gb/s Modulation of 1.3-µm P-Doped Quantum-Dot Lasers without Current Adjustments

We demonstrate temperature-insensitive eye-opening under 10-Gb/s direct modulation of 1.3-µm p-doped quantum-dot lasers without using any current adjustments. The lasers show a 6.5-dB extinction ratio between 20°C and 70°C. An active region consisting of ten quantum-dot layers with p-type doping enabled this highly temperature-stable dynamic performance, which was much superior to conventional 1.3-µm quantum-well lasers. These results make it possible to use uncooled 1.3-µm quantum-dot lasers without any current adjustments.

Modeling room-temperature lasing spectra of 1.3-μm self-assembled InAs∕GaAs quantum-dot lasers: Homogeneous broadening of optical gain under current injection

We studied the injection current dependence of room-temperature lasing spectra of a 1.3-μm self-assembled InAs∕GaAs quantum-dot laser both experimentally and theoretically. Starting from the ground-state lasing with a few longitudinal modes, the spectra showed splitting, broadening, excited-state lasing, and quenching of the ground-state lasing as the current increased. We could explain this unique current dependence by numerical simulation based on our quantum-dot laser theory, taking into account the inhomogeneous and homogeneous broadening of the optical gain as well as the carrier relaxation processes in the spatially isolated quantum dots. Through the simulation, we found that the homogeneous broadening of the ground state is kept between 5 and 10 meV under the ground-state lasing, while it increases up to 20 meV under the excited-state lasing.

Photon lifetime dependence of modulation efficiency and K factor in 1.3μm self-assembled InAs∕GaAs quantum-dot lasers: Impact of capture time and maximum modal gain on modulation bandwidth

We studied small-signal modulation characteristics of 1.3μm InAs∕GaAs self-assembled quantum-dot lasers in terms of the modulation efficiency and the K factor as a function of the photon lifetime. We could explain the measured photon-lifetime dependence based on the rate equation model considering explicitly the carrier-capture process and Pauli blocking in quantum dots. Our model shows how the modulation bandwidth of quantum-dot lasers is limited by the carrier-capture time and by the maximum modal gain via the K factor. We present prerequisite designs of quantum-dot active regions for over 10GHz modulation.

Systematic Study of the Effects of Modulation p-Doping on 1.3-

The effects of modulation p-doping on 1.3-mum InGaAs-InAs quantum-dot (QD) lasers are systematically investigated using a series of wafers with doping levels from 0 to 18 acceptors per QD. Various characterization techniques for both laser diodes and surface-emitting light-emitting diode structures are employed. We report: 1) how the level of modulation p-doping alters the length dependant laser characteristics (in turn providing insight on various key parameters); 2) the effect of modulation p-doping on the temperature dependence of a number of factors and its role in obtaining an infinite T0; 3) how increasing concentrations of modulation p-doping affects the saturated gain, differential gain, and gain profile of the lasers; and finally, 4) the effect modulation p-doping has on the small signal modulation properties of 1.3-mum QD lasers. In each of these areas, the role of modulation p-doping is established and critically discussed.

\mu{\hbox {m}}

We report the device characteristics of stacked InAs∕GaAs quantum-dot lasers cladded by Al0.4Ga0.6As layer grown at a low temperature by metalorganic chemical vapor deposition. A blueshift in emission energy by the effect of postgrowth annealing can be suppressed when the annealing temperature is below 570°C. We achieved the 1.28μm continuous-wave lasing at room temperature of five layer stacked InAs∕GaAs quantum dots embedded in In0.13Ga0.87As strain-reducing layer whose p-cladding layer is grown at 560°C. From the experiments and calculations of the gain spectra of fabricated quantum-dot lasers, the observed lasing originates from the first excited state of stacked InAs quantum dots.

Quantum-Dot Lasers

The authors report the fabrication of GaAs-based quantum dot (QD) lasers grown by metal organic chemical vapor deposition above 1.30μm. They fabricated a laser diode with five stacked InAs∕Sb:GaAs(100) QD layers, grown by antimony-surfactant-mediated growth. Ground state lasing was obtained at 1.34μm, with internal quantum efficiency of 62%, internal loss of 4.5cm−1 and ground state modal gain above 12cm−1. Lasing above 1.30μm could be achieved because of the beneficial effects of antimony on both the coherent InAs∕Sb:GaAs QD density and the suppression of the emission blueshift, usually observed for InAs∕GaAs QDs during postgrowth annealing at 600°C.

1.28μm lasing from stacked InAs∕GaAs quantum dots with low-temperature-grown AlGaAs cladding layer by metalorganic chemical vapor deposition

We present a method that improves the emission efficiency of InAs quantum dots (QDs) fabricated by antimony surfactant-mediated metal organic chemical vapor deposition. This process consists of removing the excess segregated antimony from the surface of InAs/Sb:GaAs QDs by applying a high arsenic pressure before capping. In such a way, one benefits from the advantages of InAs/Sb:GaAs QDs (high density, low coalescence) without the formation of antimony-induced nonradiative defects. Finally, we show that this better QD interface quality results in a strong decrease of the threshold current densities of InAs/Sb:GaAs QD lasers in the 1.3 μm band.

Ground state lasing at 1.34μm from InAs∕GaAs quantum dots grown by antimony-mediated metal organic chemical vapor deposition

This paper presents a theoretical study of the high-speed modulation response of Fabry-Perot (FP) and distributed-feedback (DFB) quantum-dot lasers based on the rate equation models, making reference to available experimental data. We show that the K-factor-limited maximum modulation bandwidth increases with the maximum optical gain and that there is an optimum cavity loss to maximize the bandwidth at a given maximum gain, enabling us to design the bandwidth of FP lasers as well as DFB lasers with and without a phase shift. We present modulation wave forms of FP quantum-dot lasers to indicate that the maximum modal gain of 30–40cm−1 is sufficient for 10Gbit∕s eye opening, which explains the recent success of 10Gbit∕s modulation of the quantum-dot laser with ten dot layers in the active region having the maximum modal gain of 35cm−1. We show a design for low-driving-current 10Gbit∕s operation by shortening the cavity length with the optimum cavity loss maintained by the high-reflectivity coating.

Interface properties of InAs quantum dots produced by antimony surfactant-mediated growth: Etching of segregated antimony and its impact on the photoluminescence and lasing characteristics

We report the device characteristics of stacked InAs-GaAs quantum dot (QD) lasers cladded by an Al/sub 0.4/Ga/sub 0.6/As layer grown at low temperature by metal-organic chemical vapor deposition. In the growth of quantum dot lasers, an emission wavelength shifts toward a shorter value due to the effect of postgrowth annealing on quantum dots. This blueshift can be suppressed when the annealing temperature is below 570/spl deg/C. We achieved 1.28-/spl mu/m continuous-wave lasing at room temperature of five layers stacked InAs-GaAs quantum dots embedded in an In/sub 0.13/Ga/sub 0.87/As strain-reducing layer whose p-cladding layer was grown at 560/spl deg/C. From the experiments and calculations of the gain spectra of fabricated quantum dot lasers, the observed lasing originates from the first excited state of stacked InAs quantum dots. We also discuss the device characteristics of fabricated quantum dot lasers at various growth temperatures of the p-cladding layer.