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Fiber Optics Communications

Source: David R. Goff. Fiber Optic Video Transmission, 1st ed. Focal Press: Woburn, Massachusetts, 2003

and other private writings.

Typical Fiber Optic Communication Systems

 

Modern fiber optic data transmission systems can be extraordinarily complex as the data rates are increased along with a large number of channels over very long distances. In order to understand how a fiber optic data transmission system operates, let’s first look at the most basic possible example. Let’s first review some basic fiber optic terminology.


Fiber optic data transmission systems send information over fiber by turning electronic signals into light. Light refers to more than the portion of the electromagnetic spectrum that is near to what is visible to the human eye. The electromagnetic spectrum is composed of visible and near-infrared light like that transmitted by fiber, and all other wavelengths used to transmit signals such as AM and FM radio and television. Figure 1 illustrates the electromagnetic spectrum. As one can see, only a very small part of it is perceived by the human eye as light.

 

Electromagnetic Spectrum

Figure 1 - Electromagnetic Spectrum

(Click to Enlarge Image)

The term wavelength refers to the wavelike property of light, a characteristic shared by all forms of electromagnetic radiation. Wavelength is the measurement of the distance a single cycle of an electromagnetic wave covers as it travels through a complete cycle. Wavelengths for fiber optics are measured in nanometers (the prefix nano meaning one-billionth) or microns (the prefix micro meaning one-millionth). Wavelengths of light used in fiber optic applications can be broken into two main categories: near-infrared and visible. Visible light, as defined by the human eye, ranges in wavelengths from 400 to 700 nanometers (nm) or 0.4 μm or 400 nm to 0.7 μm or 700 nm and has very limited uses in fiber optic applications, due to the high optical loss. Near-infrared wavelengths range from 700 to 1,700 nanometers; these wavelengths are almost always used in modern fiber optic systems.


The principles behind fiber optic data transmission systems are relatively simple. Figure 2 shows the most basic fiber optic links contain three basic elements common to all fiber optic transmission systems: the transmitter (Tx) that accepts one or more electrical inputs and generates an optical output signal, the optical fiber that carries the data (in the form of light), and the receiver (Rx) that accepts the optical input signal and generates one or more electrical outputs that corresponds to the electrical signal that was first applied to the input of the transmitter.

 

The system shown in Figure 2 is, of course, a very simple example that is no longer representative of modern fiber optic transmission systems.

Elements of a Fiber Optic Transmission System

Figure 2 - Basic Elements of a Fiber Optic Transmission System

The next level of complexity that was added to fiber optic transmission systems was the addition of multiple wavelengths of light on each fiber. Each wavelength of light is a distinct "color" that can be separated out at the receiver end by using various optical filters. This technique is referred to as WDM (Wavelength Division Multiplexing). Figure 3 shows a basic WDM system with two wavelengths being transmitted in the same direction on the fiber. The first WDM systems that were deployed used wavelengths of 1310 nm and 1550 nm for λ1 and λ2. WDM systems were popular upgrades in the telecommunications industry in the late 1980's and early 1990's. They allowed the information carrying capacity of each fiber to be doubled.

WDM Fiber Optic Transmission System

Figure 3 - Basic WDM Fiber Optic Transmission System

As the load from Internet traffic increased, WDM systems evolved into systems that could carry a multitude of wavelengths (colors). These systems were called DWDM (Dense Wavelength Division Multiplexing) systems. A key development that made DWDM systems practical was the EDFA (Erbium-Doped Fiber Amplifier). The EDFA allowed the multitude of wavelengths on the fiber to boosted simultaneously making the systems very economical. Early systems carried four to eight wavelengths and then sixteen. The increase in capacity per fiber was dramatic, but was still insufficient to handle the exponentially growing Internet traffic load. A typical small DWDM system configuration is shown in Figure 4.

4-Channel DWDM Fiber Optic Transmission System

Figure 4 - Basic 4-Channel DWDM Fiber Optic Transmission System

(Click to Enlarge Image)

Modern DWDM systems incorporate a number of sophisticated elements that allow the channel count and distances to be very large. Some common elements that are incorporated into today's DWDM systems are:
System Element Benefit/Usage
Soliton Transmitter Solitons have a special pulse shape that allows them to travel very long distances compared to other formats.
FEC Coding FEC is Forward Error Corrections. It involves sending additional information along with the primary data load to allow some errors that may occur in transmission to be corrected.
LiNbO3 Modulators Lithium Niobate Modulators are used to generate very clean signals at very high data rates. At these rates it is no longer possible to directly modulate the laser.
Pre-Chirp DCF This is a special length of fiber that imparts the opposite distortion from the long length of transmission fiber. This allows the maximum distance to be dramatically increased.
Raman Laser Pump Also called a Raman Optical Amplifier. This innovation allows the transmission fiber itself to amplify the signal in transit. This is another innovation that allows the maximum distance of the fiber to be increased.
VOA Variable Optical Attenuator.
Dispersion Compensation A special length of fiber or a grating that cancels out distortion that occurs in the transmission fiber.
Gain Equalization This is a special filter that keeps the amplitude of all of the different wavelengths equal.
PMD Compensation Polarization Mode Dispersion compensation. PMD is a factor to be overcome on long lengths of fiber at data rates of OC-192 and higher. Solitons have a special pulse shape that allows them to travel very long distances compared to other formats.
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