Valerie C. Coffey
The future of large-capacity wavelength-division multiplexed (WDM) networks depends on investigation into the best pulse modulation formats, such as return-to-zero (RZ) and the conventional nonreturn-to-zero (NRZ) format. Routers, ATM switches, and SONET equipment use the simpler NRZ, whereas soliton-based systems use RZ (see related story, page 47). Network devices for long-haul, large-capacity networks need either to operate within the same format or convert efficiently between formats.
Researchers with NTT Network Innovation Laboratories and NTT Telecommunications Energy Laboratories (both Kanagawa, Japan) have demonstrated a novel configuration for RZ-transmission systems based on an optical-disk-shaped bandpass filter configured with a modelocked laser diode. This system enables higher repetition rates necessary for increased bit rate/channel, as well as increased wavelength tunability. The disk filter is a variable optical-density tunable interference filter with a dielectric-multilayer-film layer deposited on one surface of a disk-shaped silicon dioxide (SiO2) substrate.
Wavelength tunability has been demonstrated previously in modelocked laser diodes by using external-cavity configurations with various alternative wavelength selectors, such as diffraction gratings and Fabry-Perot (FP) filters. But gratings or FP filters need physical space in which to tilt about an axis parallel to the rulings. However, a disk-shaped interference filter tuned by rotation allows the external cavity to be shorter. The external-cavity configuration consists of an antireflection-coated Fabry-Perot laser diode coupled via a ball lens to a gold-coated external mirror with a reflectivity of >90%. The disk filter, which acts as the wavelength selector, is inserted perpendicularly to the rulings in this external cavity. The oscillation wavelength of the modelocked laser diode can be chosen to correspond to the transmission wavelength of the disk filter, and can be tuned widely by rotating the disk filter.
This configuration allows wider tunability, lower loss, and better polarization independence than either the grating or FP configuration. This scheme retains the advantage of tuning by rotation, which means the positioning can be made precise and reliable by using optical storage technology for the disk operation. Preliminary experimental demonstrations performed at a repetition rate of 8 GHz generated modelocked pulses ranging from 1535-1587 nm. For more details, contact Koji Akimoto at [email protected].The proposed disk-shaped wavelength-tunable external-cavity mode-locked laser diode allows the external cavity length to be reduced, enabling higher-repetition-rate optical pulses for telecommunications.