Research Review: Nanolithography Promises Cheaper Telecom Lasers
Engineers produce 20 mW lasers with a side-mode suppression ratio of 60 dB using nano-imprint lithography
A PARTNERSHIP between Tampere University of Technology and the laser manufacturer Modulight promises to cut the cost of manufacturing single-mode, 1.5 μm InP-based lasers.
This class of laser, which is used in telecom networks, normally employs a buried Bragg grating for selective optical feedback. One major downside of this approach is that it requires epitaxial re-growth, which complicates fabrication and can degrade device performance. The established way for avoiding re-growth is to turn to electronbeam lithography. Thanks to the efforts of the Finnish team, nano-imprint lithography can now provide a cheaper alternative. “While EBL lithography requires an investment in the range of € 2 million, modern UV-mask aligners require an investment of less than € 0.1 million for the UV tooling," explains lead-author Jarkko Telkkälä from Tampere University of Technology.
According to Telkkälä, nano-imprint lithography should also cut production costs. Electron beam lithography is a direct write technology, and exposure time for the wafer can be several hours. In contrast, nano-imprint lithography can imprint a whole wafer in a few minutes. “In general, nanoimprint lithography is suitable for roll-to-roll process implementation, and hence is more suitable for volume production and costeffective processing of distributed feedback laser than any other method," says Telkkälä.
He and his co-workers produced their InP lasers using a fabrication approach that is similar to the one they have previously used to make GaAs lasers emitting at 894 nm and 980 nm, and GaSb-based variants operating at 1945 nm.
One major step forward with the latest batch is the improved coupling of the optical field to the surface gratings, which results from an adjustment to the laterally corrugated ridge-waveguide layout.
By switching to a grating with a large lateral extension, aspect-ratio-dependent etching was avoided, preventing formation of non-etched pockets at the bottom of the grating trenches that dramatically reduce coupling efficiency.
Single-mode 1.5 μm lasers feature gratings that were formed using a EVG 620 mask aligner with tooling for nano-imprint lithography
The engineers fabricated 1.5 μm lasers by taking a commercial InP-based epi-wafer from Modulight and applying a grating pattern with a 726 nm period. The soft stamp patterned and UV-curved nanoimprint lithography resist – a mr-UVCCur06 liquid polymer from Micro Resist Technology, formed an etch mask for the PECVD grown 100 nm-thick SiN layer. This nitride film was used again as the etch mask for InP.
Dry etching to a depth of 1400 nm left 350 nm of p-side cladding untouched. The SiN layer was then removed, and the wafer passivated with SiO2, which also filled the grating trenches. Conventional ridge waveguide fabrication steps followed to yield a 300 μm-long laser that was attached, p-side up, to a copper mount.
These devices produce 20 mW with a sidemode suppression ratio of 60 dB and an emission linewidth less than 200 kHz. Preliminary lifetime tests revealed no degradation in the first 1000 hours of operation.
“These results are comparable of better to most 1550 nm DFB lasers available on the market," claims Telkkälä.
J. Telkkälä et al. Electron Lett. 47 400 (2011)