High Speed Evanescent Quantum-Dot Lasers on Si

Distributed feedback (DFB) lasers represent a central focus for wavelength-division-multiplexing-based transceivers in metropolitan networks. Here, the first 1.3 μm quantum dot (QD) DFB lasers grown on a complementary metal-oxide-semiconductor (CMOS)-compatible (001) Si substrate are reported. Temperature-stable, single-longitudinal-mode operation is achieved with a side-mode suppression ratio of more than 50 dB and a threshold current density of 440 A cm−2. A single-lane rate of 128 Gbit s−1 with a net spectral efficiency of 1.67 bits−1 Hz−1 is demonstrated, with an aggregate total transmission capacity of 640 Gbit s−1 using five channels in the O-band. Apart from the QD active region growth, the overall fabrication is essentially identical to the commercial process for quantum well (QW) DFB lasers. This provides a process-compatible path for QD technology into commercial applications previously filled by QW devices. In addition, the capability to grow laser epi across entire CMOS-compatible (001) Si wafers adds extra benefits of reduced cost, improved heat dissipation, and manufacturing scalability. Through direct epitaxial integration of III–Vs and Si, one can envision a revolution of the photonics industry in the same way that CMOS design and processing revolutionize the microelectronics industry. This is discussed from a system perspective for on-chip optical interconnects.

1.3 µm Quantum Dot-Distributed Feedback Lasers Directly Grown on (001) Si

This paper describes a theory for mode locking and frequency comb generation by four-wave mixing in a semiconductor quantum-dot active medium. The derivation uses a multimode semiclassical laser theory that accounts for fast carrier collisions within an inhomogeneous distribution of quantum dots. Numerical simulations are presented to illustrate the role of active medium nonlinearities in mode competition, gain saturation, carrier-induced refractive index and creation of combination tones that lead to locking of beat frequencies among lasing modes in the presence of cavity material dispersion.

Multimode description of self-mode locking in a single-section quantum-dot laser.

This work reports on an investigation of the dynamics of $1.3\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\text{m}$ epitaxial quantum dot (QD) lasers on silicon subject to delayed optical feedback. Operating the device under different feedback conditions, we experimentally identify various dynamical states of periodic oscillations. In the long-cavity feedback regime, the device remains chaos-free up to $\ensuremath{\approx}70% (\ensuremath{-}1.55\phantom{\rule{0.16em}{0ex}}\mathrm{dB})$ feedback strength. This remarkable result is in agreement with prior studies and is attributed to the particular design and properties of the QD-based active region. Shortening the external cavity length to the short-cavity regime being on the scale of photonics integrated circuits (PICs), the onset of periodic oscillations only takes place under extremely high feedback strength, which is much higher than those in PICs. The devices studied exhibit strong resistance to chip-scale back reflections in absence of any unstable oscillation. Our results also demonstrate that p doping is an efficient technique to further improve the feedback tolerance. These results point out the potential of QD lasers as an on-chip light source not requiring an optical isolator and gives insights for developing ultrastable silicon transmitters for PIC applications.

Dynamic and nonlinear properties of epitaxial quantum-dot lasers on silicon operating under long- and short-cavity feedback conditions for photonic integrated circuits

Semiconductor nanostructures with low dimensionality like quantum dots and quantum dashes are one of the best attractive and heuristic solutions for achieving high performance photonic devices. When one or more spatial dimensions of the nanocrystal approach the de Broglie wavelength, nanoscale size effects create a spatial quantization of carriers leading to a complete discretization of energy levels along with additional quantum phenomena like entangled-photon generation or squeezed states of light among others. This article reviews our recent findings and prospects on nanostructure based light emitters where active region is made with quantum-dot and quantum-dash nanostructures. Many applications ranging from silicon-based integrated technologies to quantum information systems rely on the utilization of such laser sources. Here, we link the material and fundamental properties with the device physics. For this purpose, spectral linewidth, polarization anisotropy, optical nonlinearities as well as microwave, dynamic and nonlinear properties are closely examined. The paper focuses on photonic devices grown on native substrates (InP and GaAs) as well as those heterogeneously and epitaxially grown on silicon substrate. This research pipelines the most exciting recent innovation developed around light emitters using nanostructures as gain media and highlights the importance of nanotechnologies on industry and society especially for shaping the future information and communication society. Quantum dot are one of the best practical examples of nanotechnologies. Owing to the discrete energy levels, quantum dot lasers output unique features like thermal stability, feedback insensitivity and spectral purity.

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Uncovering recent progress in nanostructured light-emitters for information and communication technologies.

With a new generation of quantum dot (QD) optical gain material comprising atom-like features, the fundamental spectral characteristics of laser emission have been improved significantly. We describe the spectral and power characteristics of continuous wave (CW) single-mode InAs/AlGaInAs/InP QD distributed feedback lasers operating at 1.5 μm. Linewidths as narrow as 60 kHz (30  kHz±10  kHz intrinsic linewidth) at 20°C, which broadens to only 280 kHz (80  kHz±10  kHz intrinsic linewidth) at 80°C, have been achieved. The laser exhibits high output powers of 58 mW at 20°C and 26 mW at 80°C with side mode suppression ratios exceeding 50 dB. These record values stem from high uniformity of the QDs and a large dot density. The linewidth was measured by two techniques that confirm each other: delayed self-heterodyne interferometry and optical frequency comb interferometry. A model fits the experimental results well and enables extraction of the bias and temperature dependent α parameter. At 20°C, α is less than 0.5 at threshold and increases to only 0.9 at 150 mA above threshold. The corresponding values at 80°C are 2 and 2.5. These results imply a great potential of QD lasers for the most demanding applications in terms of spectral purity, such as coherent optical communication systems and optical metrology.

Large linewidth reduction in semiconductor lasers based on atom-like gain material

Comparison between InP-based quantum dot lasers with and without tunnel injection quantum well and the impact of rapid thermal annealing

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.

Comparison of quantum dot lasers with and without tunnel-injection quantum well

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.

On the differences in dynamical properties of quantum-dot lasers with and without p-doping in the active region and tunneling injection quantum wells

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. 

Anna Sengül

Papers

Large linewidth reduction in semiconductor lasers based on atom-like gain material

Comparison between InP-based quantum dot lasers with and without tunnel injection quantum well and the impact of rapid thermal annealing

Comparison of quantum dot lasers with and without tunnel-injection quantum well

On the differences in dynamical properties of quantum-dot lasers with and without p-doping in the active region and tunneling injection quantum wells

Emission characteristics and temperature stability of InP-based quantum-dot lasers emitting at 1.3 µm