Widely bandwidth-tunable silicon filter with an unlimited free-spectral range

Jonathan St-Yves, Hadi Bahrami, Philippe Jean, Sophie LaRochelle, Wei Shi
2015 Optics Letters  
Next-generation high-capacity optical networks require flexible allocation of spectrum resources, for which low-cost optical filters with an ultra-wide bandwidth tunability beyond 100 GHz are desired. We demonstrate an integrated band-pass filter with the bandwidth continuously tuned across 670 GHz (117-788 GHz) which, to the best of our knowledge, is the widest tuning span ever demonstrated on a silicon chip. The filter also features simultaneous wavelength tuning and an unlimited free
more » ... range. We measured an out-of-band contrast of up to 55 dB, low in-band ripples of less than 0.3 dB, and in-band group delay variation of less than 8 ps. This result was achieved using cascaded Bragg-grating-assisted contra-directional couplers and micro-heaters on the 220 nm silicon-on-insulator platform with a very compact footprint of less than 7000 μm 2 . Another design with the bandwidth continuously tunable from 50 GHz to 1 THz is also presented. Next-generation optical transmission systems applying flexible networking and the super-channel technique will require highly dynamic channel allocation to drastically increase the spectral efficiency and transmission capacity [1, 2] . Tunable optical filters, reconfigurable in both center wavelength and bandwidth with scalability toward terahertz [2], are essential for these applications. Such large bandwidth tunability is currently only available in bulky bench-top systems using diffractive grating spectrometers or liquid crystals. Integrated solutions are desired for lower cost and power consumption. In particular, silicon photonics based on the sub-micron silicon-on-insulator platform allows for CMOS compatible mass fabrication, enabling low-cost, high-yield, and high-density chip-scale integration. Existing solutions for tunable filters on silicon include devices based on microring resonators [3, 4] and Mach-Zehnder interferometers (MZIs). These devices have relatively small tunable bandwidths (less than 200 GHz) and small free spectral ranges (FSR), typically less than 10 nm, unable to cover the entire C-band, which are not suitable for high-capacity transmission applications. Contra-directional couplers (contra-DCs) are grating assisted add-drop filters [5] . Analogous to waveguide Bragg gratings, the wavelength selectivity in contra-DCs is based on periodic dielectric perturbations. However, instead of back reflections in the same waveguide, the selected wavelength in a contra-DC is dropped to another waveguide through contra-directional coupling. This allows add-drop operation without the need of a circulator. Contra-DCs have merits of compactness, flat-top response, flexible filter design (e.g., through apodization), and nearinfinite FSR (in the case of first-order gratings). In particular, they allow for very high bandwidths (greater than 10 nm) and, thus, can support very high baud rate super-channel signals [1] . In this Letter, we demonstrate a broadband filter with a large tunability in both wavelength and channel bandwidth, using thermally controlled cascaded contra-DCs on a silicon chip. The filter has flattop responses, low insertion loss, low in-band ripples, and high contrast between the pass-band and the stop-band. The schematic of the proposed device is shown in Fig. 1 . It consists of a pair of cascaded contra-DCs, each operating as a drop filter. The drop port of the first contra-DC is connected to the input port of the second contra-DC. Therefore, the finally dropped signal ("drop port of the system") is determined Fig. 1. Schematic of the device. The dropped wavelength of the first contra-DC is re-filtered in an identical component. Both contra-DCs are temperature controlled with metal heaters. Plots show examples of spectra at each port on a logarithmic scale.
doi:10.1364/ol.40.005471 pmid:26625028 fatcat:7xxtrc7a7bbd5a4gtejpv3unse