Monolithic passively mode-locked lasers are investigated based on chirped multilayer InAs/InGaAs QDs. Three chirped wavelengths, with stacking numbers of 2, 3 and 5 layers, are designed with capped InGaAs thickness of 4, 3 and 1 nm, respectively. The ridge-waveguide devices of 5-μm width and 3-mm length are fabricated to have absorber-to-gain length ratio of 1:9. A curve tracer is used to analyze the hysteresis on the light-current curve. Two kinks in the L-I curve are observed at threshold current near 50 mA and at higher current of about 150 mA. The lasing wavelength just above threshold is centered at 1268 nm and the RF spectrum of mode-locking is peaked at 13.32 GHz. At well above threshold of 200 mA, another RF peak at 13.21 GHz occurs that corresponds to shorter lasing wavelength around 1233 nm. The two lasing wavelengths are originated from ground-state transitions of two groups of InAs/InGaAs QDs. Simultaneous dual-wavelength mode-locking is therefore achieved at rather low forward current and low reverse bias by incorporating this novel design of QD structure.
The chirped multilayer quantum-dot (QD) gain media are arranged in Fourier-transform external-cavity laser (FT-ECL) configuration. Novel slit designs select 2, 3, and 4 different wavelengths that are diffracted from the grating for optical feedback. Therefore, the dual-, triple- and quadruple-wavelength ECLs are demonstrated in this study. The resulted multi-wavelength lasing emissions are achieved under injected current of 100 mA (or 1.33 kA/cm2) with signal to amplified spontaneous emission (ASE) ratio over 20 dB. Around peak-gain wavelength of 12xx-nm range, the adjacent wavelength separation is over 50 nm for dual-wavelength lasing, up to 13 nm for triple-wavelength lasing, and about 4-5 nm for quadruple-wavelength lasing emissions. To further extend the wavelength separation for dual-wavelength lasing emissions, another modified scheme with two separate external mirrors are adopted and the achieved maximum value is about 126 nm in wavelength separation or over 25 THz in frequency difference. The terahertz (THz) generation by photomixing of dual-wavelength ECLs is also discussed in this study.
Low threshold and widely tunable InAs/InGaAs/GaAs quantum-dot external-cavity lasers are implemented with gratingcoupled Littrow configuration. Throughout the tuning range of 130 nm, from 1160 to 1290 nm, the threshold current density is less than 0.9 kA/cm2 and without noticeable threshold jump. For a shorter-cavity device, the injection current is kept at a record low value of 90 mA but the tuning range is further extended from 1143 to 1293 nm. We discuss the effect of cavity length on the tuning characteristics and propose the strategy for design and optimization of multilayer
Chirped multilayer (N=10) QD lasers with 2-, 3- and 5-layer of longer-, medium-, and shorter-wavelength QD stacks,
respectively, were grown in this work. Low threshold current density and high saturated modal gain were achieved in
this specially designed QD structure. Empirical gain-current analysis was performed on this chirped multilayer QD
structure for the first time. It was consistent with our spectral observations and provided valuable information on carrier
recombination in chirped multilayer QD structure. Two novel spectral characteristics were discovered also for the first
time. First, simultaneous two-wavelength lasing around threshold was observed under particular gain-loss condition at
this specific multilayer structure of QD stacking numbers. Second, at cryogenic temperature, simultaneous two-wavelength
lasing emissions switched from longer-wavelength lasing first to shorter-wavelength lasing first with
increasing current injection. Non-uniform carrier distribution among chirped multilayer QD structure is evident at low
temperature below 200 K from our analysis.
MBE growth of high quality diluted Nitride materials have been investigated. Photoluminescence intensity of high nitrogen content InGaAsN/GaAs SQW can be improved significantly by decreasing the growth temperature due to suppressd phase separation of InGaAsN alloy. The longest room temperature PL peak wavelength obtained in this study is 1.59 μm by increasing the nitrogen composition up to 5.3%. High performance ridge-waveguide InGaAsN/GaAs single quantum well lasers at wavelength 1.3 μm have been demonstrated. Threshold current density of 0.57 KA/cm2 was achieved for the lasers with a 3-μm ridge width and a 2-mm cavity length. Slope efficiencies of 0.67 W/A was obtained with 1 mm cavity length. The cw kink-free output power of wavelength 1.3 μm single lateral mode laser is more than 200 mW, and the maximum total wallplug efficiency of 29% was obtained. Furthermore, monolithic MBE-grown vertical cavity surface emitting lasers (VCSELs) on GaAs substrate with an active region based on InGaAsN/GaAs double quantum wells emitting at 1304 nm with record threshold current density below 2 KA/cm2 also have been demonstrated. The CW output power exceeds 1 mW with an initial slope efficiency of 0.15 W/A. Such low threshold current density indicates the high quality of InGaAsN/GaAs QW active region.
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.
Development of submonolayer deposition technique can offer significant flexibility in creation of strained heterostructures of different types and material systems. It was found that under certain growth conditions the deposition of InAs insertions of less than 1 monolayer (ML) thickness in GaAs matrix forms so-called sub-monolayer quantum dots (SML QDs). The energy spectrum of these QDs can be varied over a wide range by tuning the InAs coverage and the thickness of GaAs spacers. Stranski-Krastanow (In,Ga)As QDs (SK QDs), which have been investigated in more details, have proved theoretically predicted lower threshold current density of 26 A/cm2 in compare with QW lasers. However, strong size variation of SK QDs in combination with the relatively low sheet density leads to low peak gain achievable in the ground state. This problem is the reason of typically low efficiency of SK QD-based lasers. Due to higher gain, SML QDs have proved their potential for high power laser application. In this presentation we report on further progress in the technology of SML QD lasers demonstrating high output power (6W) from 100-μm-wide laser diode emitting at 0.94 μm. High power QW-based lasers of the state-of-the-art performance are also presented for comparison.