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The silicon-on-insulator (SOI) platform allows for a miniaturization of optical elements at the micron size. It is now a mature technology, with high quality material and well-known fabrication processes. Another advantage stems in its compatibility with the CMOS facilities. The SOI platform is used already for numerus applications such as datacom, sensing or manipulation of quantum objects. Bragg filters are often used for on-chip the rejection of pump lasers. They can also be used for sensing purposes. By periodically modulating a standard waveguide width, it is possible to realize a 1-D photonic crystal with a forbidden wavelength band. In principle, the bandwidth and central wavelength of this bandgap can be tailored just by a proper design of the introduced corrugation. However, the very large index contrast of Si-wires makes the realization of narrowband rejection filters a technological challenge, requiring multiple etching steps or corrugation widths of a few tens of nanometers. Sub-wavelength nanostructuration of Si waveguides has shown to allow narrowband operation with a single-etch process, but reported rejection levels remained limited.
Here we present an innovative differential corrugation approach that allows the realization of narrowband rejection optical filters with relaxed fabrication constraints. By sub-wavelength engineering of the waveguide geometry we experimentally demonstrated simultaneous high rejection of 50dB and narrowband operations less than 3nm.
We propose new subwavelength designs based on subwavelength structures having two subperiods in each Bragg period. We also investigate the equivalent asymmetric structure to reduce the index contrast in the periods and further reduce the bandwidth without adding any new fabrication constraints. We have fabricated the sub-wavelength engineered filters in standard SOI wafer with a 220 nm thick Si guiding layer and bottom oxide layer of 2 µm. We have used electron beam lithography with 5 nm step-size and have patterned the structurse by dry etching with an inductively coupled plasma etcher. Finally, we have covered the devices with PMMA to provide symmetric cladding.
We report results showing that subwavelength Bragg filter geometries allow a drastic reduction of the operating optical bandwidth to the 0.6nm-2.5nm range if compared with regular Bragg filters (20 nm) while retaining still a strong rejection level of around 40dB. Similarly, each asymmetric version was observed to be bandwidth narrower.
To sum up, this paper is an investigation of advanced SOI waveguide Bragg mirrors. We report that the use of subwavelength corrugations and a judicious of waveguide Bragg asymmetry allow to push the extinction ratio/operating bandwidth beyond its traditional limit.
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