Today's optical systems support interfaces which run at up to 100 Gbit/s Ethernet (GE), per wavelength channel. Therefore a 40 wavelength system can support up to 40 channels, has a combined capacity 4,000 Gbits per second on a single fiber pair. A 160 wavelength channel system could theoretically support 16 Terabits per second on each fiber pair. Indeed, 6.4 Terabit commercial systems were demonstrated as early as 2001.
The IEEE standards committees are currently defining Optical Ethernet systems capable of support 400 Gigabits per second, per channel; and 1 Terabit per second per channel; 400 GE is considered to be an interim step on the path to reaching 1 Terabit Ethernet.
Multiwavelength Terabit Ethernet supports could therefore support hundreds of Terabits of bidirectional data transmission on a single optical fiber pair.
Current optical communication systems typically use an electrical signal to directly modulate the intensity of an optical signal (carrier). The optical signal is directly directed by a photodiode which converts the optical signal directly back to an electrical signal. These schemes are called intensity modulation with direct detection (IM/DD). They can be likened to AM radio. IM/DD systems are relatively simple to engineer.
Microwave and radio frequency (RF) systems have long used other techniques including frequency and phase modulation, in order to get better spectral efficiency and to recover weak signals which might be buried in background noise. They detect the signal using homodyne or heterodyne detection techniques. Since these techniques rely on the coherence of the signals, they are called coherent communication technques. Examples include phase shift keying (PSK), frequency shift keying (FSK) etc.
As we push the boundaries of data rate and spectral efficiency, coherent lightwave systems are being developed; they borrow heavily from the techniques developed for microwave and RF systems over the last 40 years.
Coherent optical communications may be one of the enabling technoologies to reach 1 Terabit Ethernet. Coherent lightwave systems allow wavelength channels to be pack closer together (when compared with IM/DD systems); they use the frequency spectrum more efficiently. Coherent systems also have improved receiver sensitivit; they can recover signals which would otherwise be buried in background noise. However, they are more complex than IM/DD systems.
An optical add drop multiplexer is a optical system component which allows one or more signals or channels to be introduced or removed from a optical fiber system that contains many channels or signals. Recononfigurable Optical Multiplexers include tunable lasers, so that wavelength channels can be rapidly added or reconfigured. ROADMS may be key components in optical switching systems.