**Acknowledgments**

This work was supported in part by EC Framework 7 (FP7) project DAPHNE (www.fp7daphne.eu) − Developing aircraft photonic networks (grant ACP8-GA-2009-233709). We acknowledge funding from FCT and program POCTI/FEDER under the National Plan for Scientific Hardware Renewal with grant REEQ/1272/EEI/2005. D. Coelho also acknowledges support from FCT through a PhD grant.
