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Figure 2. Development Milestones
Early Technologies
As fiber optics came into use, network providers soon found that some improvements in technology could greatly increase capacity and reduce cost in existing networks. These early technologies eventually led to the optical network as it is today.
Broadband WDM
The first incarnation of WDM was broadband WDM. In 1994, by using fused biconic tapered couplers, two signals could be combined on the same fiber. Because of limitations in the technology, the signal frequencies had to be widely separated, and systems typically used 1,310-nm and 1,550-nm signals, providing 5 Gbps on one fiber. Although the performance did not compare to today's technologies, the couplers provided twice the bandwidth out of the same fiber, which was a large cost savings compared to installing new fiber.
Optical Amplifiers
The second basic technology, and perhaps the most fundamental to today's optical networks, was the erbium-doped optical amplifier. By doping a small strand of fiber with a rare earth metal, such as erbium, optical signals could be amplified without converting the signal back to an electrical state. The amplifier provided enormous cost savings over electrical regenerators, especially in long-haul networks.
Current Technologies
Systems deployed today use devices that perform similar functions to earlier devices but are much more efficient and precise. In particular, flat-gain optical amplifiers have been the true enabler for optical networks by allowing the combination of many wavelengths across a single fiber. Dense Wavelength Division Multiplexing (DWDM)
As optical filters and laser technology improved, the ability to combine more than two signal wavelengths on a fiber became a reality. Dense wavelength division multiplexing (DWDM) combines multiple signals on the same fiber, ranging up to 40 or 80 channels. By implementing DWDM systems and optical amplifiers, networks can provide a variety of bit rates (i.e., OC–48 or OC–192), and a multitude of channels over a single fiber (see Figure 3). The wavelengths used are all in the range that optical amplifiers perform optimally, typically from about 1,530 nm to 1,565 nm (see Figure 4).
Figure 3. DWDM Systems and Optical Amplifiers
Figure 4. ITU Channel Spacing
Two basic types of DWDM are implemented today: unidirectional and bidirectional DWDM (see Figure 5). In a unidirectional system, all the wavelengths travel in the same direction on the fiber, while in a bidirectional system the signals are split into separate bands, with both bands traveling in different directions.
Figure 5. Unidirectional and Bidirectional DWDM
Optical Amplifiers
The performance of optical amplifiers has improved significantly—with current amplifiers providing significantly lower noise and flatter gain—which is essential to DWDM systems. The total power of amplifiers also has steadily increased, with amplifiers approaching +20–dBm outputs, which is many orders of magnitude more powerful than the first amplifiers.
Narrowband Lasers
Without a narrow, stable, and coherent light source, none of the optical components would be of any value in the optical network. Advanced lasers with narrow bandwidths provide the narrow wavelength source that is the individual channel in optical networks. Typically, long-haul applications use externally modulated lasers, while shorter applications can use integrated laser technologies.
These laser sources emit a highly coherent signal that has an extremely narrow bandwidth. Depending on the system used, the laser may be part of the DWDM system or embedded in the SONET network element. When the precision laser is embedded in the SONET network element, the system is called an embedded system. When the precision laser is part of the WDM equipment in a module called a transponder, it is considered an open system because any low-cost laser transmitter on the SONET network element can be used as input (see Figure 6).
Figure 6. Embedded vs. Open DWDM Systems
Fiber Bragg Gratings
Commercially available fiber Bragg gratings have been important components for enabling WDM and optical networks. A fiber Bragg grating is a small section of fiber that has been modified to create periodic changes in the index of refraction. Depending on the space between the changes, a certain frequency of light—the Bragg resonance wavelength—is reflected back, while all other wavelengths pass through (see Figure 7). The wavelength-specific properties of the grating make fiber Bragg gratings useful in implementing optical add/drop multiplexers. Bragg gratings also are being developed to aid in dispersion compensation and signal filtering as well.
Figure 7. In-Fiber Bragg Grating Technology: Optical A/D Multiplexer
Thin Film Substrates
Another essential technology for optical networks is the thin film substrate. By coating a thin glass or polymer substrate with a thin interference film of dielectric material, the substrate can be made to pass through only a specific wavelength and reflect all others. By integrating several of these components, many optical network devices are created, including multiplexers, demultiplexers, and add/drop devices.
