Optical Filter Design and Analysis: A Signal Processing Approach

CHAPTER EIGHT

Future Directions

Optical filters are evolving at a rapid pace. This transformation is arising from both a pull by the development needs of optical communication systems and the technology push coming from research in materials and fabrication processes. These two areas are discussed with respect to their impact on future directions for optical filters.

8.1 COMMUNICATION SYSTEM APPLICATIONS

8.1.1 Ultra-Dense WDM Systems and Optical Networks

Dense WDM systems are widely deployed and provide a wealth of applications for optical filters. The filter requirements for the bandpass, gain equalization, and dispersion compensating filters become more demanding as the number of channels, the bandwidth utilization, and the system wavelength range increase.

Different transmission formats are used in WDM systems, for example, non-retum-to-zero (NRZ) and solitons. The transmission format impacts channel spacing, dispersion compensation and gain equalization; thus, impacting filter requirements. The best spectral efficiency of 0.6 bits/s/Hz was demonstrated using duo-binary coding [1]. A 2.6 Tb/s WDM system was achieved using 132 channels at 20 Gb/s each over 120 km of standard fiber. The channels were separated by 33.3 GHz and demultiplexing was accomplished by using three WGRs with FWHM = 0.32 nm, temperature control for center wavelength alignment, and manually turned double cavity thin-film filters with 0.3 nm bandwidths.

To date, point-to-point and fixed add/drop commercial ...