MIXSEL: Vertical integration of ultrafast semiconductor lasers for wafer-scale mass production

Eli Kapon of EPFL/SB/IPEQ/LPN, expert in VCSELs

Pierre Thomann of Uni Neuch/IPH/LTF, expert in Frequency comb characterization

Bernd Witzigmann of University of Kassel, expert in VCSELs modelling for narrow line-width and broad tuning with current

The recent development in novel ultrafast semiconductor lasers using the MIXSEL concept has the potential to further reduce the complexity and size of ultrafast lasers. Our research work on such new developments will support and strengthen a field that is significant in value creation.

Lasers generating short pulses – referred to as ultrafast lasers – enable many applications in science and technology. Numerous laboratory experiments have confirmed that ultrafast lasers can significantly increase telecommunication data rates, improve computer interconnects, and optically clock microprocessors. New applications in metrology, supercontinuum generation, and life sciences with two-photon microscopy only work with ultrashort pulses but have relied on bulky and complex ultrafast solid-state lasers. Semiconductor lasers are ideally suited for mass production and widespread applications, because they are based on a wafer-scale technology with a high level of integration. In 2007 a novel type of ultrafast semiconductor laser has been demonstrated - referred to as the modelocked integrated external-cavity surface emitting laser (MIXSEL) - using vertical integration of gain and saturable absorber layers (Appl. Phys. B 88, 493, 2007). Vertical integration supports a diffraction-limited circular output beam, transform-limited pulses, lower timing jitter, simple synchronization to an external clock and adustable pulse repetition rate in the 1 to 100 GHz regime.

Figure shows vertical integration scheme, progressing from conventional VECSEL-SESAM modelocking with (a) large mode area ratios and thus large cavities (Phys. Rep. 429, 67, 2006), (b) to absorber-gain integration in a MIXSEL (Appl. Phys. B 88, 493, 2007). The MIXSEL semiconductor wafer structure contains two high reflectors (HR), quantum dot (QD) saturable absorber, quantum well (QW) gain and an anti-reflective (AR) coating. The QW gain can also be replaced with QD gain layers.

In this project we want to confirm power scaling and pulse repetition rate scaling of optically pumped MIXSELs (OP-MIXSELs). The pulse duration should be further reduced to first the 1 - 10 ps regime and second the sub-picosecond regime. The pulses should be transform-limited. Such lasers will be used for continuum and frequency comb generation and fully characterised in terms of amplitude and phase noise. This work then will be extended to electrically pumped MIXSELs (EP-MIXSELs) and their power scaling capabilities should be explored.

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