Quantum dot (QD) diode lasers attract currently much attention due to their ability to emit light in the advanced near-
infrared region at extraordinarily low threshold current densities. A vertical-cavity surface emitting laser (VCSEL),
having a superior beam quality, improved temperature stability, low threshold current, and cost-effective planar
fabrication, is also an attractive device variant. Here we discuss the state of the art of these lasers intended for the use in
1.3-μm fiber-optic communications. The discussion is centered on an InAs/GaAs semiconductor QD system. Basic
issues of the QD synthesis in the system are addressed. The achievement of the control over the 1.3-μm QD emission is
demonstrated. Both, wide-stripe and single-mode edge-emitting lasers are described. The lasers designed have a very low
threshold current density, high differential efficiency, and a high output power. Narrow-stripe 1.3-μm QD lasers generate
in a single mode, have a record-low threshold current, and produce the continuous-wave (CW) power output in excess of
100 mW. Also, we report on QD VCSELs emitting at 1.3 μm. The design of their cavity and active region are described.
The room-temperature CW output power of these lasers is as high as 2 mW. Both, the edge- and surface-emitting lasers
satisfy the demands of the fiber optical communication technology.
Quantum dot (QD) is one of the most perspective candidates to be used as an active region of temperature-insensitive 1.3-micron GaAs based lasers for optical networks. However, the limited optical gain achievable in QD ground state hindered their practical use. In this work we have demonstrated that using of high number of QDs stacks grown under proper conditions by MBE is an effective way to considerably increase the optical gain of QD lasers. Ridge waveguide laser diodes with width of 2.7 μm and 4.5 μm based on various numbers of QD layers (N=2, 5, 10) were fabricated and studied in this work. Ultra-low threshold current of 1.43 mA was achieved for 2-stack QD. Regime of simultaneous lasing at ground- and excited-states was discovered. This effect was accounted for the finite time of carriers capture to the ground-state in QD. Multi-stack QD structures enabled to maintain continuous work ground-state lasing up to the current density of 10 kA = 100xJth. Enhanced optical gain allowed us to unite very high differential efficiency (>75%) with low threshold current (<100 A/cm2) and characteristic temperature (T0>100K). For example, laser diode of 1-mm cavity length has shown single mode output power of 100mW at operating current of 195 mA and at high operation power demonstrated insensibility to the changes of temperature. The combination of parameters achieved is quite competitive to all technologies currently used for 1.3-micron lasers including traditional InP-based lasers and makes QD gain medium very promising for VCSEL and telecom laser applications.
Recent results on molecular-beam epitaxy growth of the quantum dot InGaAs/GaAs heterostructures for long-wavelength lasers on GaAs substrates are presented. As a result of optimization of the growth procedure for active region and emitter layers low-threshold current density (45 - 80 A/cm2) long-wavelength (1.27 - 1.3 μm) laser diodes may be fabricated with high reproducibility.
The development of 1.3 micron VCSELs is currently considered to give a strong impulse for a wide use of ultra-fast local area networks. In the present work we discuss MBE growth and characteristics of InAs/GaAs quantum dot (QD) lasers, we also give characteristics of 1.3 micron QD VCSELs grown on GaAs and compare them with those of 1.3 micron InGaAsN/GaAs QW VCSELs. Overgrowing the InAs quantum dot array with thin InGaAs layer allows us to achieve 1.3 micron emission. Long stripe lasers showed low threshold current density (<100 A/cm2), high differential efficiency (>50%), and low internal loss (1-2 cm-1). Maximum continuous wave (CW) output power for wide stripe lasers was as high as 2.7 W and 110 mW for single mode devices. Uncoated broad area lasers showed no visible degradation of characteristics during 450 hours (60C, ambient environment). 1.3 micron InGaAsN/GaAs QW VCSELs are characterized by higher optical loss and lower differential efficiency than QD VCSELs. Due to high gain in the active region QW VCSELS demonstrate high output power (1 mW). QW VCSELs show extremely low internal round-trip optical loss (<0.05%), low threshold currents (<2 mA), high differential efficiency (40%) and output power (600 microW).
Inter-sublevel transitions in InGaAs/AlGaAs quantum dots (QDs) in the mid-infrared (MIR) wavelength range are investigated by means of absorption and optically and electrically pumped emission spectroscopy. Charging dependent energy shifts of inter-sublevel transitions observed in calorimetric absorption spectra are attributed to few-particle effects in the QDs. MIR emission from near-infrared QD lasers is observed in the MIR lasing mode below threshold, which is confirmed by a theoretical modelling of such a bipolar lasing device. In contrast, spontaneous MIR emission is recorded for optically pumped Qds.
Continuous wave room-temperature output power of approximately 3 W for edge-emitters and of about 1 mW for vertical-cavity surface-emitting lasers is realized for GaAs-based devices using InAs quantum dots (QDs) operating at 1.3 micrometers . Long operation lifetimes are manifested. The breakthrough became possible due to development of self- organized growth and defect-reduction techniques in QD technology. We show that the basic parameters of QD lasers outperform the parameters of the devices fabricated using competing GaAs-based `quantum well' technologies.
The physical mechanism for creation of intraband population inversion between levels of quantum dots under injection of electron-hole pairs of suggested. The method is based on employment of generation of interband radiation providing fast depopulation of quantum dot ground level. Spontaneous far-IR radiation from diode laser structures with InGaAs/AlGaAs quantum dots connected with intraband hole and/or electron transitions between levels of size quantization in quantum dots was found and investigated for the first time. Spontaneous far-IR radiation is observed only under simultaneous generation of stimulated near-IR radiation connected with interband carrier transitions. Far- IR emission is observed also from laser structures with InGaAs/GaAs quantum wells. Intensity of this radiation is about of order less then intensity of radiation from structures with quantum dots. Qualitative explanations of phenomena observed are proposed.