A goal of many modern long-haul optical transport systems is to provide for the efficient transmission of large volumes of voice traffic and data traffic over trans-continental distances at low costs. Various methods of achieving these goals include time-division multiplexing (TDM) and wavelength-division multiplexing (WDM). In time division multiplexed systems, data streams comprised of short pulses of light are interleaved in the time domain to achieve high spectral efficiency, high data rate transport. In wavelength division multiplexed systems, data streams comprised of short pulses of light of different carrier frequencies, or equivalently wavelength, co-propagate in the same fiber to achieve high spectral efficiency, high data rate transport.
The transmission medium of these systems is typically optical fiber. In addition there is a transmitter and a receiver. The transmitter typically includes a semiconductor diode laser, and supporting electronics. The laser is often a DFB laser stabilized to a specified frequency on the ITU frequency grid. The laser may be directly modulated with a data train with an advantage of low cost, and a disadvantage of low reach and capacity performance. In many long-haul systems, the laser is externally modulated using a modulator. A single stage modulator is sufficient for a non-return-zero (NRZ) modulation format. A two-stage modulator is typically used with the higher performance return-to-zero (RZ) modulation format. An example of a modulator technology is the Mach-Zehnder lithium niobate modulator. Alternatively, an electro-absorptive modulator may be used. After binary modulation, a high bit may be transmitted as an optical signal level with more power than the optical signal level in a low bit. Often, the optical signal level in a low bit is engineered to be equal to, or approximately equal to zero. In addition to binary modulation, the data can be transmitted with multiple levels, although in current optical transport systems, a two-level binary modulation scheme is predominantly employed. The receiver is located at the opposite end of the optical fiber, from the transmitter. The receiver is typically comprised of a semiconductor photodetector and accompanying electronics.
Typical long-haul optical transport dense wavelength division multiplexed (DWDM) systems transmit 40 to 80 channels at 10 Gbps (gigabit per second) across distances of 3000 to 6000 km in a single 35-nm spectral band. In a duplex system, traffic is both transmitted and received between parties at opposite end of the link. In a DWDM system, different channels operating at distinct carrier frequencies are multiplexed using a multiplexer. Such multiplexers may be implemented using arrayed waveguide grating (AWG) technology or thin-film technology, or a variety of other technologies. After multiplexing, the optical signals are coupled into the transport fiber for transmission to the receiving end of the link. The total link distance may, in today's optical transport systems, be two different cities separated by continental distances, from 1000 km to 6000 km, for example. To successfully bridge these distances with sufficient optical signal power relative to noise, the signal is periodically amplified using an in-line optical amplifier. Typical span distances between optical amplifiers are 50-100 km. Thus, for example, 30 100-km spans would be used to transmit optical signals between points 3000 km apart. Examples of in-line optical amplifiers include erbium doped fiber amplifiers (EDFAs) and semiconductor optical amplifiers (SOAs).
At the receiving end of the link, the optical channels are demultiplexed using a demultiplexer. Such demultiplexers may be implemented using AWG technology or thin-film technology, or a variety of other technologies. Each channel is then optically coupled to separate optical receivers.
Other common variations include the presence of post-amplifiers and pre-amplifiers just before and after the multiplexer and de-multiplexer. Often, there is also included dispersion compensation with the in-line amplifiers. These dispersion compensators adjust the phase information of the optical pulses in order to compensate for the chromatic dispersion in the optical fiber while appreciating the role of optical nonlinearities in the optical fiber. Another variation that may be employed is the optical dropping and adding of channels at cities located in between the two end cities. The invention disclosed applies in any of these variations, as well as others.
Traditionally, optical transport systems are either long haul systems, for traffic between distant cities, or metropolitan (“metro”) systems for traffic in and around a city. Typically the terminals of a long-haul optical transport system are located in one location such as a central office, and all the channels in a DWDM system are terminated. The traffic is then sorted by electronic identification of data and routed to different parts of the metropolitan area using metropolitan optical transport systems. In many practical circumstances, there is a space, power and cost inefficiency in terminating the long haul signal and retransmitting over a second metro-system. For this reason, the concept of a distributed terminal architecture was invented, and is disclosed in co-pending U.S. patent application Ser. No. 10/402,840, hereafter referred to as Jaggi.
As taught by Jaggi, there was no provision for duplex traffic between distributed terminals in the same metropolitan area. It would be highly desirable for a terminal in one section of a city to exchange traffic with a second terminal in a second section of the city while also providing scalable communication with cities a great distance away.