Wavelength-selectable lasers share a resemblance
John Wallace
Two independent groups of Japanese researchers seem to have converged on similar solutions for wavelength-selectable lasers intended as WDM light sources. Both of their designs start with eight distributed-feedback laser diodes monolithically fabricated out of indium gallium arsenide phosphide, and both combine all eight outputs using an 8 × 1 multimode interference (MMI) optical combiner. In addition, both designs pass the resulting combined output through a semiconductor optical amplifier (SOA). However, there are differences in the details.
One of the lasers, developed at Fujitsu Laboratories (Atsugi, Japan) contains eight individual diode-laser waveguides spaced laterally on the substrate across a 240-µm distance; their outputs are brought together into the much-narrower (40-µm-wide) MMI combiner through S-curved waveguide sections having a 300-µm radius of curvature (see figure). Active and passive waveguide structures were integrated via butt-coupling and regrowth. The laser outputs have facet outputs up to 30 mW and a wavelength spacing of 3.18 µm over a 1535-1558-nm range; fiber-coupled output reaches 10 mW with a maximum side-mode suppression ratio (SMSR) of 50 dB.
The other device, made at NEC`s Optoelectronics and High Frequency Device Research Laboratory (Tsukuba, Japan), contains an additional element-an electro-absorption modulator that achieves high-quality 2.5-Gbit/s modulation with clear eye diagrams. The eight diode lasers take up a lateral space on the substrate of only 80 µm, allowing their outputs to be sent into a MMI modulator without waveguide bends. The laser wavelengths span a range of 1552.5-1567.8 nm and are temperature-tunable within that range. Output power is 7 mW, SMSR is 40 dB, and the extinction ratio is 17 dB.
The differences in these two devices do not overshadow their similarities, which extend to their common size of 0.5-0.6 mm wide and 2-3 mm long. It remains to be seen whether this convergence in design is a reflection of a larger trend. For details, contact M. Bouda at bouda@flab. fujitsu.co.jp or Koji Kudo at [email protected].
