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The chair of semiconductor technology at the Walter Schottky Institute, TUM is fabricating novel long wavelength vertical-cavity surface-emitting laser diodes (VCSEL). These devices cover the communications wavelengths from 1.3 to 1.6 µm and provide record-high modulation bandwidth above 10 GHz. Prof. Chang-Hasnain’s research group at the University of California, Berkeley, recently presented a new class of VCSEL at 850 nm. This device, fabricated on a third party epitaxial wafer, uses a novel sub-wavelength grating instead of a conventional DBR mirror. Both groups started a scientific cooperation on the development, fabrication and characterisation of 1.55 µm high-speed lasers using sub-wavelength gratings. These novel lasers are expected to show higher modulation performance and polarisation mode stability, which will be a significant progress for this class of lasers.
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Final report:
Even though the goal of the project was very ambitious, all targets have been finally achieved.
During the exchange of scientists via BaCaTeC it turned out, that the proposed goals would not be
achievable within the timeframe of the project. Therefore, the collaboration was continued via a
postdoctoral fellowship of the travelling junior scientist Werner Hofmann.
Sub-wavelength gratings with high refractive index contrast to the surrounding media (HCG, highcontrast
grating) can provide high broadband reflectivity and are therefore ideally suited as a
VCSEL mirror and polarization stabilizer. In addition, they can be used to achieve a single
fundamental mode with larger apertures than it is possible with DBRs. Last but not least it can
replace a 10-µm thick Bragg-mirror stack by a grating layer in the nanometer-range.
The HCG long-wavelength VCSEL realized due to this BaCaTeC project, is based on the latest
high-speed long-wavelength VCSEL structure with short cavity and record modulation bandwidth
[1]. The active region was tailored to emit at 1320 nm, an interesting wavelength for the 10G
Ethernet long-range IEEE standard. As proposed, an HCG long-wavelength VCSEL was fabricated
in Berkeley on top of a VCSEL structure manufactured in Bavaria. The HCG was fabricated of
amorphous silicon on top of a SiO2 spacer. For the first time, no additional DBR layers have been
used [3]. These amorphous layers were deposited by e-beam evaporation. The gratings were defined
by e-beam lithography in a university lab for prototyping; however, could be also done by
conventional lithography in a silicon facility for mass fabrication.
The HCG long-wavelength VCSELs showed CW operation up to 18°C and pulsed operation up to
60°C. No visible polarization modes can be observed.
The incorporation of nano-photonic components technology enables novel device-concepts as e.g.
an HCG VCSEL arrays. To continue this very promising technology, the former junior BaCaTeC
scientist Werner Hofmann now continues his research on this topic as Junior Professor for
nanophotonic devices at TU Berlin.
Summary:
In a novel fabrication process, BCB-passivated lasers with low parasitics and optimized design for
high-speed data-transmission, with sub-wavelength grating as top-DBR-mirror were co-developed
and co-fabricated in cooperation at both institutes in Bavaria and California.
Not only all goals were achieved, but also a long-term collaboration was established and young
academic careers were successfully supported.
References:
[1] Müller M, Hofmann W, Böhm G, and Amann M, 2009, Short-Cavity Long-Wavelength VCSELs with
Modulation-Bandwidth in Excess of 15 GHz, IEEE Photon. Technol. Lett. 21 1615-1617.
[2] Hofmann W, 2011, Evolution of high-speed long-wavelength vertical-cavity surface-emitting lasers,
Semicond. Sci. Technol. 26 014011.
[3] Hofmann W, Chase C, Müller M, Rao Y, Grasse C, Böhm G, Amann M, and Chang-Hasnain C, 2010, Long-
Wavelength High-Contrast Grating Vertical-Cavity Surface-Emitting Laser, IEEE Photon. J. 2 415-422.
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