Dense Wavelength Division Multiplexing

Conventional fiber-optic systems use a single wavelength or color injected by an optical transmitter that is a light-emitting diode (LED) in the case of multimode fiber (MMF) or a laser diode in the case of single-mode fiber (SMF). Laser diodes or LEDs perform an electrical-to-optical (EO) conversion of the electrical signal. The light is injected at a precise angle into the core of the fiber-optic cable using a lens, which has a higher refractive index than the cladding. Light pulses are transmitted along the fiber-optic line due to the principle of total internal reflection, which states that when the angle of incidence exceeds a critical value, light cannot leave the core. Instead, the light reflects inward toward the core and bounces back in as a result. Figure 10-2 shows conventional fiber-optic cable types.

Figure 10-2. Conventional Fiber-Optic Cable Types

Figure 10-2. Conventional Fiber-Optic Cable Types

Multi-mode liber

The fiber-optic core is the photonic carrier element at the center of the optical fiber. It is commonly made from a combination of silica and germania. Single-mode fiber has a much smaller core than multimode fiber, typically 5 to 10 microns. This uses a graded index scheme in which the refractive index diminishes gradually from the center axis out toward the cladding.

Multimode fiber core diameters typically are 50, 62.5, or 100 micrometers. MMF uses a step index scheme in which there is a distinct refractive index difference between the core and the cladding. Surrounding the core is the optic cladding, made of pure silica. This combination makes the principle of total internal reflection possible. This creates a waveguide effect, which guides the light signal to its destination, where it is detected by a photodetector. The photodetector performs an optical-to-electric (OE) conversion of the signal.

MMF cable segments have a typical distance limitation of 2 km, and intermediate-reach (IR) SMF segments have a distance limitation of 20 km. Long-reach (LR) SMF segments have a typical distance limitation of 40 km. Signal attenuation and degradation are caused primarily by the dispersion and scattering of light within the cable itself. This necessitates the use of fiber-optic amplifiers between rated segments.

DWDM is the process of multiplexing signals of different wavelengths onto a single fiber. This operation creates many virtual fibers, each capable of carrying a different signal. A schematic of a DWDM system is shown in Figure 10-3.

Figure 10-3. DWDM Schematic

DWDM employs wavelengths to transmit data parallel-by-bit or serial-by-character, which increases the fiber's capacity by assigning incoming optical signals to specific frequencies (wavelength, lambda) within a designated frequency band and then multiplexing the resulting signals out onto one fiber. Each signal can be carried at a different rate (such as OC-3/12/48) and in a different format (SONET, ATM, data, and so on). This can increase the capacity of existing networks without the need for expensive recabling and can tremendously reduce the cost of network infrastructure upgrades. DWDM supports point-to-point, ring, and mesh topologies. Existing fiber in a SONET fiber plant can be easily migrated to DWDM.

Most DWDM systems support standard SONET/SDH short-reach optical interfaces to which any SONET/SDH-compliant client device can attach. There are two kinds of DWDM systems—metro DWDM and long-haul DWDM. In today's long-haul DWDM systems, the interfaces are most often OC-48c/STM-16c interfaces operating at the 1310 nm wavelength. In addition, other interfaces important in metropolitan areas and access networks are commonly supported: Ethernet, Fast Ethernet, Gigabit Ethernet, ESCON, Sysplex Timer, Sysplex Coupling Facility Links, and Fibre Channel. The new 10 Gigabit Ethernet standard is supported using a very-short-reach (VSR) OC-192 interface over MMF between 10 Gigabit Ethernet and DWDM equipment.

On the client side, there can be SONET/SDH terminals or ADMs, ATM switches, or routers. By converting incoming optical signals into the precise ITU-standard wavelengths to be multiplexed, transponders are currently a key determinant of the openness of DWDM systems. A schematic of the DWDM system with transmission elements is illustrated in Figure 10-4.

Figure 10-4. DWDM System

Figure 10-4. DWDM System

The DWDM system performs the following main functions:

• Generating the signal— The source, a solid-state laser, must provide stable light within a specific, narrow bandwidth that carries the digital data, modulated as an analog signal. Stability of a light source is a measure of how constant its intensity and wavelength are. The distributed feedback (DFB) laser is well-suited for DWDM applications, because it emits a nearly monochromatic light, is capable of high speeds, and has a favorable signal-to-noise ratio. It has center frequencies in the region around 1310 nm, and from 1520 to 1565 nm. The latter wavelength range is compatible with Erbium Doped Fiber Amplifier (EDFA) fiber-optic amplifiers.

• Combining the signals— DWDM systems employ multiplexers to combine the signals. Some inherent loss is associated with multiplexing and demultiplexing. This loss depends on the number of channels, but it can be mitigated with optical amplifiers, which boost all the wavelengths at once without electrical conversion. Arrayed Waveguide Grating (AWG) devices perform multiplexing and demultiplexing operations simultaneously and are also suited for large channel counts.

• Transmitting the signals— The effects of crosstalk and optical signal degradation or loss must be reckoned with in fiber-optic transmission. Controlling variables such as channel spacings, wavelength tolerance, and laser power levels can minimize these effects. Over a transmission link, the signal might need to be optically amplified.

• Separating the received signals— At the receiving end, the multiplexed signals must be separated or demultiplexed. Demultiplexing must be performed before the light is detected, because photodetectors are inherently broadband devices that cannot selectively detect a single wavelength. An AWG is used for demultiplexing.

• Receiving the signals— The demultiplexed signal is received by a photodetector. Two types of photodetectors are widely deployed—the positive-intrinsic-negative (PIN) photodiode and the avalanche photodiode (APD).

In addition to these functions, a DWDM system must also be equipped with client-side interfaces to receive the input signal. This function is performed by transponders. On the DWDM side are interfaces to the optical fiber that links DWDM systems.

Within the DWDM system, a transponder converts the client optical signal back to an electrical signal and performs the 3R (reshape, retime, retransmit) functions. This electrical signal is then used to drive the WDM laser. Each transponder within the system converts its client's signal to a slightly different wavelength. The wavelengths from all the transponders in the system are then optically multiplexed. In the receive direction of the DWDM system, the reverse process takes place.

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