Optically amplified systems
Laurel Clark AT&T Bell Laboratories
The experts of the ITU-T Working Party 15/4 Question 25 group met in Geneva last October to discuss three draft recommendations regarding optical interfaces for synchronous digital hierarchy equipment in systems slated for completion by the end of 1996 that use optical amplifiers. The group`s charter encompasses optical systems for inter-office and long-distance networks.
The recommendations are known as G.mcs, "Optical interfaces for multichannel systems with optical amplifiers"; G.scs, "Optical interfaces for single-channel systems with optical amplifiers"; and G.lon, "Functional characteristics of inter-office and long-haul line systems using optical amplifiers, including wavelength-division multiplexing."
The first two recommendations, which focus on optical interface parameter values, are divided into two parts--systems without line amplifiers and systems with line amplifiers.
In Recommendation G.mcs, discussions focused on systems using line amplifiers that have amplifier spacings of 80 and 120 kilometers for total distances between regeneration of 360 and 600 km, respectively.
In addition, a frequency grid approach was accepted as the first step toward establishing channel frequencies. The grid will comprise a mathematical construct with even-grid frequency spacings of 25 gigahert¥and no established end points. Channel frequencies will be based on this grid, where selected grid frequencies will be the nominal central frequencies of the channels.
The discussion surrounding the assignment of eight signal channels within the optical fiber amplifier passband was general in coverage. Optical fiber amplifier gain flatness, concatenation effects and laser wavelength distribution were cited as drivers toward limiting the amplifier spectrum over which the signal channels can be distributed.
Fiber nonlinearities, multiplexing component complexity and laser wavelength stability requirements were enumerated as drivers that would push for increased channel separation. Provisional values were assigned to channel central frequency separation and allowable frequency deviation, which meant a range of channel spacings--125 to 375 GHz--would be considered in future discussions.
Several tentative agreements were reached regarding Recommendation G.scs for single-channel amplified systems. The target distances for long-distance, very long-distance and ultra long-distance systems in the 1550-nanometer window were 80, 120 and 160 km, respectively. To reduce the number of application codes, the group also endorsed the approach to specify only the interfaces to the main optical path and to define only one implementation (using a booster amplifier) for very long-distance type systems.
In addition, a polarization mode dispersion requirement was introduced into the optical path specification, which would provisionally require mean values of PMD of less than 0.1 of a bit period.
Methods for dispersion accommodation (techniques to counteract the deleterious effects of chromatic dispersion) represent challenges to system-transverse compatibility. Based on practicality and maturity, broadband passive compensation devices and specialized modulation techniques seem to offer likely platforms for standardization. It was agreed, however, that any permitted dispersion accommodation approach should ensure maximum transparency, allow use of line amplifiers and permit equipment compatibility with dispersion-shifted fiber.
Progress was also made on Recommendation G.lon.
Laurel Clark is a member of the technical staff at AT&T Bell Laboratories in Holmdel, NJ. She holds a B.S. in Electrical Engineering from the University of Colorado at Boulder and an M.ENG from Cornell University, Ithaca, NY.