Narrow linewidth lasers have many applications, such as higher order coherent communications, optical sensing, and metrology. While semiconductor lasers are typically unsuitable for such applications due to relatively low coherence, recent advances in heterogeneous integration of III-V with silicon have shown that this is no longer true. In this tutorial, we discuss in-depth techniques that are used to drastically reduce the linewidth of a laser. The heterogeneous silicon-III/V platform can fully utilize these techniques, and fully integrated lasers with Lorentzian linewidth on the order of 100 Hz and tuning range of 120 nm are shown.

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Tutorial on narrow linewidth tunable semiconductor lasers using Si/III-V heterogeneous integration

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

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

Narrow linewidth InAs/InP QD DFB lasers with linewidths of less than 50kHz at 20°C which broadens to less than 80kHz at 80°C were demonstrated using delayed self-heterodyne as well as by optical frequency comb interferometry.

Narrow Linewidth InAs/InP Quantum Dot DFB Laser

We report spectral characteristics of InAs/InP QD DFB lasers. Owing to low linewidth enhancement factor and high modal gain, lasers with 5 QD layers exhibit record linewidth of 55 kHz measured by the heterodyne beat note between the laser and an ultra-stable optical frequency comb.

Sutapa Gosh

Papers

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

Narrow Linewidth InAs/InP Quantum Dot DFB Laser

Spectral Characteristics of Narrow Linewidth InAs/InP Quantum Dot Distributed Feedback Lasers