Cover Picture: Phys. Status Solidi C 2/2012

In their article on pp. 282 Naokatsu Yamamoto et al. describe the characteristics of a wavelength tunable quantum dot optical frequency comb laser (QD‐CML) as a newly proposed attractive photonic device. The generation of multiple‐wavelength fine peaks in the 1.3‐μm waveband with wide wavelength tunability and power controllability is successfully demonstrated for the QD‐CML integrated with an absorptive optical attenuator by employing a broadband optical injec‐ tion seeding technique. Ultra‐high density (8.2 × 10 10 /cm 2 ) InAs/InGaAs QDs in the optical gain media can be effectively obtained on a GaAs substrate using a sandwiched sub‐nano separator (SSNS) growth technique. A wide wavelength tuning range between 1.274 and 1.302 μm was achieved at a low current operation of 50 mA. In addition, a high extinction ratio of –20 dB at a low attenuator voltage (–2.5 V) was achieved in the fabricated QD‐CML. It is expected that the developed QD‐CML will become a suitable candidate for constructing a compact and low‐power consumption wavelength‐division multiplexing light source for optical communications and bio/medical‐imaging.