1.2. Optical Transmission Systems

1.2.1. Overview

The first step in the development of fiber optic transmission over meaningful distances was to find light sources that were sufficiently powerful and narrow. The light-emitting diode (LED) and the laser diode proved capable of meeting these requirements. Lasers went through several generations in the 1960s, culminating with the semiconductor lasers that are most widely used in fiber optics today.

The next step was to overcome the loss of signal strength, or attenuation, seen in glass. In 1970, Corning produced the first communication-grade fibers. With attenuation less than 20 decibels per kilometer (dB/km), this purified glass fiber exceeded the threshold for making fiber optics a viable technology.

Innovation at first proceeded slowly, as the telephone companies—the main users of the technology—were rather cautious. AT&T first standardized transmission at DS3 speed (45 Mbps) for multimode fibers. Soon thereafter, single-mode fibers were shown to be capable of transmission rates ten times that of the older type, as well as to support spans of up to 32 km (20 miles). In the early 1980s, MCI, followed by Sprint, adopted single-mode fibers for its long-distance network in the United States.

Further developments in fiber optics were closely tied to the use of the specific regions on the optical spectrum where optical attenuation is low. These regions, called windows, lie between areas of high absorption. The earliest systems were developed to operate around 850 nm, the first window in silica-based optical fiber. A second window (S band), at 1310 nm, soon proved to be superior because of its lower attenuation, followed by a third window (C band) at 1550 nm with an even lower optical loss. Today, a fourth window (L band) near 1625 nm, is under development and early deployment.

Transmission of light in optical fiber presents several challenges that must be dealt with. These fall into the following three broad categories [Agrawal97]:

These are described in more detail below.

1.2.2. Attenuation

Attenuation in optical fiber is caused by intrinsic factors, primarily scattering and absorption, and by extrinsic factors, including stress from the manufacturing process, the environment, and physical bending. The most common form of scattering, Rayleigh scattering, is caused by small variations in the density of glass as it cools. These variations are smaller than the wavelengths used and therefore act as scattering objects. Scattering affects short wavelengths more than long wavelengths and limits the use of wavelengths below 800 nm.

Attenuation due to absorption is caused by a combination of factors, including the intrinsic properties of the material itself, the impurities in the glass, and any atomic defects in the glass. These impurities absorb the optical energy, causing the light to become dimmer. While Rayleigh scattering is important at shorter wavelengths, intrinsic absorption is an issue at longer wavelengths and increases dramatically above 1700 nm. Absorption due to water peaks introduced in the fiber manufacturing process, however, is being eliminated in some new fiber types.

The primary factors affecting attenuation in optical fibers are the length of the fiber and the wavelength of the light. Attenuation in fiber is compensated primarily through the use of optical amplifiers.

1.2.3. Dispersion

Dispersion is the spreading of light pulses as they travel down optical fiber. Dispersion results in distortion of the signal, which limits the bandwidth of the fiber. Two general types of dispersion affect DWDM systems. One of these effects, chromatic dispersion, is linear, while the other, Polarization Mode Dispersion (PMD), is nonlinear.

Chromatic dispersion occurs because different wavelengths propagate at different speeds. In single-mode fiber, chromatic dispersion has two components, material dispersion and waveguide dispersion. Material dispersion occurs when wavelengths travel at different speeds through the material. A light source, no matter how narrow, emits several wavelengths within a range. When these wavelengths travel through a medium, each individual wavelength arrives at the far end at a different time. The second component of chromatic dispersion, waveguide dispersion, occurs because of the different refractive indices of the core and the cladding of fiber (see section 1.2.5). Although chromatic dispersion is generally not an issue at speeds below 2.5 Gbps, it does increase with higher bit rates.

Most single-mode fibers support two perpendicular polarization modes, vertical and horizontal. Because these polarization states are not maintained, there occurs an interaction between the pulses that results is a smearing of the signal. PMD is generally not a problem at transmission rates below 10 Gbps.

1.2.4. Nonlinear Effects

In addition to PMD, there are other nonlinear effects. Because nonlinear effects tend to manifest themselves when optical power is very high, they become important in DWDM (see section 1.2.9). Linear effects such as attenuation and dispersion can be compensated, but nonlinear effects accumulate. They are the fundamental limiting mechanisms to the amount of data that can be transmitted in optical fiber. The most important types of nonlinear effects are stimulated Brillouin scattering, stimulated Raman scattering, self-phase modulation, and four-wave mixing [Agrawal97]. In DWDM, four-wave mixing is the most critical of these types. Four-wave mixing is caused by the nonlinear nature of the refractive index (see the next section) of the optical fiber. Nonlinear interactions among different DWDM channels create sidebands that can cause interchannel interference. Three frequencies interact to produce a fourth frequency, resulting in cross talk and signal-to-noise level degradation. Four-wave mixing cannot be filtered out, either optically or electrically, and increases with the length of the fiber. It also limits the channel capacity of a DWDM system.

1.2.5. Optical Fiber

The main requirement on optical fibers is to guide light waves with a minimum of attenuation (loss of signal). Optical fibers are composed of fine threads of glass in layers, called the core and cladding, in which light can be transmitted at about two-thirds its speed in vacuum. Although admittedly an oversimplification, the transmission of light in optical fiber is commonly explained using the principle of total internal reflection. With this phenomenon, 100 percent of light that strikes a surface is reflected. By contrast, a mirror reflects about 90 percent of the light that strikes it.

Light is either reflected (it bounces back) or refracted (its angle is altered while passing through a different medium) depending on the angle of incidence (the angle at which light strikes the interface between an optically denser and optically thinner material). Total internal reflection happens when the following conditions are met:

An optical fiber consists of two different types of very pure and solid glass (silica): the core and the cladding. These are mixed with specific elements, called dopants, to adjust their refractive indices. The difference between the refractive indices of the two materials causes most of the transmitted light to bounce off the cladding and stay within the core. The critical angle requirement is met by controlling the angle at which the light is injected into the fiber (see Figure 1-1). Two or more layers of protective coating around the cladding ensure that the glass can be handled without damage.

There are two general categories of optical fiber in use today, multimode and single-mode. Multimode, the first type of fiber to be commercialized, has a larger core than single-mode fiber. It gets its name from the fact that numerous modes, or light rays, can be simultaneously carried by it. The second general type of fiber, single-mode, has a much smaller core that allows only one mode of light at a time through the core. As a result, the fidelity of the signal is better retained over longer distances. This characteristic results in higher bandwidth capacity than achievable using multimode fibers. Due to its large information-carrying capacity and low intrinsic loss, single-mode fibers are preferred for longer distances and higher bandwidth applications, including DWDM.

Designs of single-mode fiber have evolved over several decades. The three principle types are:

As discussed earlier, there are four windows within the optical spectrum that have been exploited for fiber transmission. The first window, near 850 nm, was used almost exclusively for short-range, multimode applications. Non-dispersion-shifted fibers, commonly called standard single-mode (SM) fibers, were designed for use in the second window, near 1310 nm. To optimize the fiber's performance in this window, the fiber was designed so that chromatic dispersion would be close to zero near the 1310-nm wavelength.

The third window, or C band, has much lower attenuation. However, its dispersion characteristics are severely limiting. To alleviate this problem, manufacturers came up with the dispersion-shifted fiber design, which moved the zero-dispersion point to the 1550-nm region. Although this solution meant that the lowest optical attenuation and the zero-dispersion points coincided in the 1550-nm window, it turned out that there were destructive nonlinearities in optical fiber near the zero-dispersion point for which there was no effective compensation. Because of this limitation, these fibers are not suitable for DWDM applications.

The third type, non-zero dispersion-shifted fiber, is designed specifically to meet the needs of DWDM applications. The aim of this design is to make the dispersion low in the 1550-nm region, but not zero. This strategy effectively introduces a controlled amount of dispersion, which counters nonlinear effects such as four-wave mixing that can hinder the performance of DWDM systems [Dutton99].

1.2.6. Optical Transmitter and Receivers

Light emitters and light detectors are active devices at opposite ends of an optical transmission system. Light emitters, are transmit-side devices that convert electrical signals to light pulses. This conversion is accomplished by externally modulating a continuous wave of light based on the input signal, or by using a device that can generate modulated light directly. Light detectors perform the opposite function of light emitters. They are receive-side opto-electronic devices that convert light pulses into electrical signals.

The light source used in the design of a system is an important consideration because it can be one of the most costly elements. Its characteristics are often a strong limiting factor in the final performance of the optical link. Light-emitting devices used in optical transmission must be compact, monochromatic, stable, and long lasting. Two general types of light-emitting devices are used in optical transmission: light-emitting diodes (LEDs) and laser diodes or semiconductor lasers. LEDs are relatively slow devices, suitable for use at speeds of less than 1 Gbps; they exhibit a relatively wide spectrum width, and they transmit light in a relatively wide cone. These inexpensive devices are often used in multimode fiber communications. Semiconductor lasers, on the other hand, have performance characteristics better suited to single-mode fiber applications.

Requirements for lasers include precise wavelength, narrow spectrum width, sufficient power, and control of chirp (the change in frequency of a signal over time). Semiconductor lasers satisfy nicely the first three requirements. Chirp, however, can be affected by the means used to modulate the signal. In directly modulated lasers, the modulation of the light to represent the digital data is done internally. With external modulation, the modulation is done by an external device. When semiconductor lasers are directly modulated, chirp can become a limiting factor at high bit rates (above 10 Gbps). External modulation, on the other hand, helps to limit chirp.

Two types of semiconductor lasers are widely used: monolithic Fabry-Perot lasers and Distributed Feedback (DFB) lasers. The latter type is particularly well suited for DWDM applications for several reasons: it emits a nearly monochromatic light, it is capable of high speeds, it has a favorable signal-to-noise ratio, and it has superior linearity property. DFB lasers also have center frequencies in the region around 1310 nm, and from 1520 to 1565 nm. There are many other types and subtypes of lasers. Narrow spectrum tunable lasers are available, but their tuning range is limited to approximately 100–200 GHz. Wider spectrum tunable lasers, which will be important in dynamically switched optical networks, are under development.

On the receive end, it is necessary to recover the signals transmitted on different wavelengths over the fiber. This is done using a device called the photodetector. Two types of photodetectors are widely deployed, the Positive-Intrinsic-Negative (PIN) photodiode and the Avalanche Photodiode (APD). PIN photodiodes work on principles similar to, but in the reverse of, LEDs. That is, light is absorbed rather than emitted, and photons are converted to electrons in a 1:1 relationship. APDs are similar devices to PIN photodiodes, but provide gain through an amplification process: One photon acting on the device releases many electrons. PIN photodiodes have many advantages, including low cost and reliability, but APDs have higher reception sensitivity and accuracy. APDs, however, are more expensive than PIN photodiodes. They may also have very high current requirements and they are temperature sensitive.

1.2.7. Regenerators, Repeaters, and Optical Amplifiers

Optical signals undergo degradation when traversing optical links due to dispersion, loss, cross talk, and nonlinearity associated with fiber and optical components. Regenerators are devices consisting of both electronic and optical components to provide “3R” regeneration—Reamplification, Reshaping and Retiming. Retiming and reshaping detect the digital signal that is distorted and noisy, and re-create it as a clean signal (see Figure 1-2). In practice, signals can travel for up to 120 km (74 miles) between amplifiers. At longer distances of 600 to 1000 km (372 to 620 miles), the signal must be regenerated. This is because an optical amplifier merely amplifies the signals and does not perform the other 3R functions (reshape and retime). Recent advances in transmission technology have increased the distance that can be traversed without amplification and 3R regeneration. It should be noted that amplifiers are purely optical devices whereas regenerators require optical-to-electrical (O/E) conversion and electrical-to-optical (E/O) conversion.

Before the arrival of optical amplifiers (OAs), every signal transmitted had to be individually regenerated or amplified using repeaters. The OA has made it possible to amplify all the wavelengths at once and without optical-electrical-optical (OEO) conversion. Besides being used on optical links, optical amplifiers can also be used to boost signal power after multiplexing or before demultiplexing, both of which can introduce loss in the system. The Erbium-Doped Fiber Amplifier (EDFA) is the most commonly deployed OA.

Erbium is a rare-earth element that, when excited, emits light around 1.54 micrometers—the low-loss wavelength for optical fibers used in DWDM. A weak signal enters the erbium-doped fiber, into which light at 980 nm or 1480 nm is injected using a pump laser. This injected light stimulates the erbium atoms to release their stored energy as additional 1550-nm light. As this process continues down the fiber, the signal grows stronger. The spontaneous emissions in the EDFA also add noise to the signal; this determines the noise figure of an EDFA.

The key performance parameters of optical amplifiers are gain, gain flatness, noise level, and output power. The target parameters when selecting an EDFA, however, are low noise and flat gain. Gain should be flat because all signals must be amplified uniformly. Although the signal gain provided by the EDFA technology is inherently wavelength-dependent, it can be corrected with gain flattening filters. Such filters are often built into modern EDFAs. Low noise is a requirement because noise, along with the signal, is amplified. Because this effect is cumulative and cannot be filtered out, the signal-to-noise ratio is an ultimate limiting factor in the number of amplifiers that can be concatenated. This limits the length of a single fiber link.

1.2.8. Characterizing Optical Signals and Performance

The basic measure of digital signal transmission performance is a probabilistic quantity known as the Bit Error Rate (BER). Given a large sample of received bits, the BER gives the percentage of those received in error. The following basic phenomena affect the bit error rate of a signal:

Analysis of 1–4 for general communication systems is highly dependent on the modulation method used and the type of detection employed. In optical systems, the modulation method most frequently used is On-Off-Keying (OOK). As its name suggests, it is just like turning on and off the light (albeit very rapidly) according to the bits being sent (that is, “on” if a bit is 1 and “off” if the bit is 0). BER of the OOK modulated signal increases with the increasing bit rate of the signal.

Intersymbol interference takes place when a signal interferes with itself. This can happen in a couple of ways. First, if the channel the signal passes through is bandwidth-limited, then the nice square edges on the signal can get “rounded” so that the various individual bits actually interfere with each other. This band limiting may take place at the transmitter, receiver, or within the channel. Within the fiber, dispersion occurs when the different wavelengths that compose the signal travel at different velocities down the fiber.

Interchannel interference occurs when signals based on different wavelengths interfere with each other, that is, their individual spectrums overlap. Hence, the channel spacing in a Wavelength Division Multiplexing (WDM) system must be wide enough to prevent the signal spectrums from overlapping. For a system with 100 GHz spacing carrying OC-48 signals (2.5Gbps bit rate), this is not a problem. As we shrink the spacing down to 12.5 GHz for OC-192 signals, it is more challenging to prevent interchannel interference.

Thus, both the wavelength of an optical signal and its bandwidth affect interference. Moreover, the BER is dependent on the signal modulation method and the bit rate. Thus, impairments such as line noise, loss, dispersion, and nonlinear effects must be taken into consideration when selecting a route to achieve the required performance criteria.

1.2.9. DWDM Systems

WDM [Green92, Agrawal97, Saleh+91] is an analog multiplexing technique where the original signals are “frequency shifted” to occupy different portions of the frequency spectrum of the transmission media. For example, commercial broadcast radio stations take a base audio signal (typically with a frequency between 20 and 20,000 Hz), use this to modulate a higher frequency carrier signal, and then broadcast this new signal into free space. This signal does not interfere with other signals if there is sufficient “spacing” between the carriers of the different radio stations. The required spacing is a function of both the spectral characteristics, spectrum, of the original signal and the modulation method.

In optical networking, the transmission medium is an optical fiber rather than free space. The signals of interest, rather than being analog audio content, are typically digital signals of various types. The carrier, rather than being an electromagnetic signal in the KHz or MHz range, is an electromagnetic signal with a frequency around 193 THz, that is, 1.93 × 10¹⁴ Hz! And like the radio case, there are a variety of different modulation methods that can be used to apply the digital signal to this carrier for transmission. As we go from radio to optical signals, the technologies change completely, for example, from electronic oscillators to lasers for generating the carrier signal.

The commercial U.S. AM radio broadcasts have carrier frequencies in the range of 560–1600 KHz with a spacing of 10 KHz, that is, signals may exist at 560 KHz, 570 KHz, 580 KHz, and so on. The spectral content of the audio signal is restricted to be between approximately 100–5000 Hz. Early WDM began in the late 1980s using the two widely spaced wavelengths in the 1310 nm and 1550 nm (or 850 nm and 1310 nm) regions, sometimes called wideband WDM. The early 1990s saw a second generation of WDM, sometimes called narrowband WDM, in which two to eight channels were used. These channels were spaced at an interval of about 400 GHz in the 1550 nm window. By the mid-1990s, dense WDM (DWDM) systems were emerging with sixteen to forty channels and spacing from 100 to 200 GHz. By the late 1990s, DWDM systems had evolved to the point where they were capable of sixty-four to 160 parallel channels, densely packed at 50 or even 25 GHz intervals.

In the WDM case, there has been some initial standardization of frequencies and spacing. In particular, ITU-T has specified the frequencies in terms of offsets from the reference frequency of 193.1 THz [ITU-T98c]. The standard offsets are 200 GHz, 100 GHz, and 50 GHz. It should be noted that there are deployed WDM systems operating with grid spacing as narrow as 25 GHz and even narrower spacing is in the works. The ITU-T specification [ITU-T98c] does not preclude these systems.

Unfortunately, even with a standard set of frequencies, WDM systems from different vendors currently do not interoperate. To understand why this is so, consider a WDM system as shown in Figure 1-3. The current trend in the long haul market place has three aspects: (1) longer distances between regenerators, (2) denser spacing between channels, and (3) higher data rates carried per channel. These trends push the capabilities of the fiber and the systems so that linear and nonlinear effects rather than just signal attenuation must be compensated for or otherwise taken into consideration. Vendors use proprietary techniques to address these issues leading to lack of interoperability. Please refer to [Ramaswamy+02] for more information on transmission impairments in optical networks.

At its core, DWDM involves a small number of physical-layer functions. A typical 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.

• Combining the signals: Modern DWDM systems employ multiplexers to combine the signals. There is some inherent loss associated with multiplexing and demultiplexing. This loss is dependent on the number of channels but can be mitigated with optical amplifiers, which boost all the wavelengths at once without electrical conversion.

• Transmitting the signals: The effects of cross talk and optical signal degradation or loss must be dealt with in fiber optic transmission. These effects can be minimized by controlling channel spacing, wavelength tolerance, and laser power levels. The signal may need to be optically amplified over a transmission link.

• Separating the received signals: At the receiving end, the multiplexed signals must be separated out. Although this task would appear to be simply the opposite of combining the signals, it is actually more difficult.

• Receiving the signals: The de-multiplexed signal is received by a photodetector.

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. Interfaces to the optical fiber that links DWDM systems are on the other side.