InP-based quantum dot (QD) laser devices emitting at 1.3 µm were realized by incorporating a GaAs nucleation layer underneath the InAs QD layers. A good carrier confinement while retaining the waveguiding properties is achieved by embedding the QDs in In0.528Al0.371Ga0.101As. Length dependent P-I characteristics yielded static parameters, which were comparable to static parameters obtained for InP-based lasers emitting at 1.55 µm. Additionally, temperature dependent measurements were conducted and evaluated. The lasers show ground mode lasing up to high operation temperatures with good temperature stability of the threshold current density and external quantum efficiency.
The small and large signal responses of InP-based 1.55 μm high-speed quantum dot (QD) lasers with and without tunnel-injection (TI) quantum well (QW) and/or p-type doping in the active region (incorporating nominally identical QDs) were designed, manufactured and compared. The structures were grown by a molecular beam epitaxy system equipped with group-V valved cracker cells. In all cases, the active region consisted of six QD or TI-QD structures, which were embedded in InAlGaAs barriers lattice matched to InP. The InGaAs TI-QWs were separated by a thin InAlGaAs tunnel barrier from the InAs QDs. The laser structures were processed into ridge waveguide lasers and analyzed. The results show, that the bandwidth and maximum data rates were reduced by incorporation of TI-QWs. P-doping resulted in slightly worse performance of the simple QD laser, but in an improvement of the TI QD laser. Furthermore, the large signal response of the tunneling injection QD laser is one of the first reports of digital modulation of such a laser. An optimization of the doping profile is promising to further improve the laser performance over the undoped counterparts.
Semiconductor DFB or DBR lasers with narrow linewidths are of outmost importance for a variety of applications, the most important ones being communication and LIDAR. Conventional single mode lasers based on quantum wells have linewidths of the order of one to a few MHz; reducing the linewidth requires the addition of an external feedback, and a stabilization scheme by means of some control circuitry which enables to reach linewidths of about 100 kHz.
A much better solution is a diode laser chip that can provide very narrow linewidths without the need for complex external additions. Recent works suggest that such lasers are possible provided that the gain medium comprises quantum dots (QDs). This paper describes the spectral properties of state of the art 1550 nm InAs/InP QD DFB lasers grown by solid source MBE and comprising five highly uniform dot layers. The linewidth of these lasers was tested using delayed self-heterodyne as well as by beating against a highly stabilized optical frequency comb. The lasers exhibit linewidths of the order of 20 kHz at room temperature and below 70 kHz at 80 degrees C.
The integration of optical sources in Si photonic transceivers has relied so far on externally coupled III-V laser dies within the assembly. These hybrid approaches are however complex and expensive, as there are additional cost-increasing factors coming from the redundant testing of the pre- and post-coupled laser photonic chips. Further optimization of Photonic Integrated Circuits (PICs) cost and performance can be obtained only with radical technology advancements, such as the “holy grail” of Silicon Photonics; the monolithic integration of III-V sources on Si substrates. MOICANA project funded by EU Horizon 2020 framework targets to develop the technological background for the epitaxy of InP Quantum Dots directly on Si by Selective Area Growth with the best-in-class, in terms of losses and temperature sensitivity, in a CMOS fab, i.e. the SiN waveguide technology. In addition, MOICANA will develop the necessary interface for the seamless light transition between the III-V active and the SiN passive part of the circuitry featuring advanced multiplexing functionality and a combination of efficient and broadband fiber coupling. Through this unique platform, MOICANA aims to demonstrate low cost, inherent cooler-less and energy efficient transmitters, attributes stemming directly from the low loss SiN waveguide technology and the QD nature of the laser’s active region. MOICANA is targeting to exploit the advantages of the monolithic integrated PICs for the demonstration of large volume single-channel and WDM transmitter modules for data center interconnects, 5G mobile fronthaul and coherent communication applications.
A comparison between QD lasers with and without tunnel-injection QW designs was performed. In both cases, six layers of a QD or TI-QD design were grown by molecular beam epitaxy equipped with group-V valved cracker cells. The InAs QDs are embedded in InAlGaAs barriers lattice matched to InP. The TI-QW consists of InGaAs separated by a thin InAlGaAs tunnel barrier. The lasers were processed into broad area and ridge waveguide lasers. Both laser designs exhibited high modal gain values in the range of 10-15 cm−1 per dot layer. The static and dynamic device properties of the different QD laser designs were measured and compared against each other.
An overview is given about the recent improvement in 1.55 μm QD lasers for direct modulation. Based on improved QD epitaxy, which reduces the inhomogeneous size distribution, record values in small signal modulation bandwidth of more than 15 GHz and in digital modulation of up to 35 GBit/s were obtained. Due to the high modal gain and robust ground state transition, the temperature dependence of the laser performance could be very much improved with characteristic temperatures of T0 = 125 K and T1 near to 400 K. This allow a temperature stable modulation bandwidth between 15-60 °C of (14 +/- 1) GHz sufficient for 25 GBit/s digital modulation.
An overview is given about the recent improvement in 1.5 μm QD lasers for direct modulation. Based on improved QD epitaxy with a reduced inhomogeneous size distribution, record values in small signal modulation bandwidth of more than 15 GHz and in digital modulation of up to 36 GBit/s were obtained. Due to the high modal gain and robust ground state transition, the temperature dependence of the laser performance could be very much improved with characteristic temperatures of T0 = 125 K and T1 near to 400 K. Also the impact of the temperature on the digital modulation speed will be discussed.
Self-organized InAs quantum dot (QD) lasers based on InP substrate were grown by means of solid source molecular beam epitaxy (SSMBE). Six InAs QD layers with high dot density and highly uniform dot sizes were used as active medium. Broad area (BA) and ridge waveguide (RWG) lasers with different cavity lengths were processed and characterized. Also the influence of a post-growth rapid thermal annealing (RTA) process on the laser characteristics was investigated. The lasers showed a high modal gain of 12 - 14.5 cm-1 per dot layer and a threshold current density for infinite cavity length of 120 A/cm2 per dot layer. In pulsed operation, as-cleaved BA lasers with a cavity length of 292 μm can be operated up to 120 °C. High characteristic temperature values were obtained with T0 = 125 K (20 °C to 45 °C) and T0 = 100 K up to 120 °C. The slope efficiency of about 0.28 W/A can be kept constant over a wide operating temperature range of up to 100 °C. Mounted RWG lasers with 388 μm cavity length and operated in pulsed mode showed a maximum output power of 120 mW a slope efficiency of 0.42 W/A at 15 °C. The lasers can be operated at 150 °C with 25 mW output power. These results demonstrate very well the temperature insensitive lasing performance expected in nearly ideal QD lasers due to the high density of states localized at the transition energy, which allow a very robust ground state lasing.
Due to the discrete density of states distribution and spatial localization of carriers in quantum dot (QD) material, the
dynamics should be strongly enhanced in comparison to quantum well material. Based on improved 1.5 μm
InAs/InGaAlAs/InP QD gain material short cavity ridge waveguide lasers were fabricated. Devices with cavity, lengths
of 230 to 338 μm with high reflection coatings on the backside exhibit record value for any QD laser in small and large
signal modulation performance with up to 15 GHz and 36 GBit/s, respectively, obtained at 14 °C. Due to the high
temperature stability of threshold current and external differential efficiency, the lasers exhibit also nearly constant
modulation bandwidth between 14-60 °C.
The ever-growing need for higher data rates is a driving force for the implementation of higher order coherent communication formats. A key element in coherent detection is the local oscillator (LO) of the decoding unit. This device has to provide coherent light with a narrow linewidth in order to distinguish between different phase and amplitude states of the incoming signal. As predicted by theory, a drastic linewidth reduction is expected from quantum dot (QD) laser materials by the quasi zero-dimensional nature of the gain function. The impact of different gain materials consisting of different numbers of QD layers on the linewidth of distributed feedback (DFB) lasers was investigated and shows an unambiguous dependence on the layer design. Intrinsic linewidths as low as 110 kHz could be determined.
Based on a novel quantum dot (QD) growth technique, high density dot-like QDs were grown on (100)InAlGaAs/InP surfaces, which resulted in a strongly improved modal gain in 1.55 μm QD lasers. The influence of the number of QD layers on static properties, e.g., modal gain, threshold current density and spectral properties, are presented and discussed. For a large number of QD layers, e.g., 6 QD layers, a high modal gain of > 70cm-1 could be obtained. By reducing the number of QD layers, i.e., lowering the modal gain, the wavelength shift with temperature can be reduced to < 0.2 nm/K. Systematic dependence of laser properties on structural parameters is observed.
In this work, a theoretical and model study of the temperature effects on threshold current, as tuning technique, and the
comparison with experimental results of quantum dot (QD) diode lasers is presented. It is well known the dependence of
output wavelength with temperature in semiconductor lasers. This property can be highly useful in order to obtain stable
and easy tuning lasers getting two different specific wavelengths to achieve signals in the millimetre (mmW) and
terahertz (THz) ranges by photomixing. Our model and study over QD lasers allow us to understand the behaviour of
temperature inside the device and thus, we can estimate the best characteristics to obtain the desired results.
In this work, we study and investigate the thermally effects on the compact continuous wave (CW) distributed feedback
(DFB) laser as a tuning method using an external platinum μ-heater film in a vertical and lateral configurations. A low
injection current into platinum heater produces the variation temperature inside the active and grating regions to shift the
lasing wavelength. The frequency is continuously tuned up to 3 THz at operation wavelength of 937 nm, by controlling
the temperature of the laser to achieve sub-millimetre (sub-mmW) and terahertz (THz) signals generation by
photomixing.
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