Applications of Optical Fibers : Fiber Optic Intelligent Traffic Systems

Traffic control has been an issue since humans put the first wheels on the first cart. The modern world demands mobility. Cars represent the main method of mobility, but today's congested highways and city streets don't move fast, and sometimes they don't move at all. Intelligent traffic systems (ITS), sometimes called intelligent transportation systems, apply communications and information technology to provide solutions to this congestion as well as other traffic control issues. Intelligent transportation systems offer many types of information. They may offer real-time information about traffic conditions, such as variable message signs (Figure 1) to warn of Amber Alerts, accidents, or other delays. ITS controls the flow of traffic via traffic signals, or by opening and closing special gated lanes that allow commuters to access additional traffic lanes in one direction or the other, depending on the time of day, and the direction of the heaviest commuter traffic flow. Some applications provide fog sensors that activate road lights in areas where heavy fog can occur and cause extremely hazardous driving conditions. These fog sensors may also be used to send a message to a variable message sign located before the foggy section to warn motorists of the upcoming hazard.

 

                                                 Figure 1 — Variable Message Sign 

  

Other forms of ITS include special radio channels for traffic updates, web sites that map driving routes or provide information on road construction. ITS features include "pay as you go" toll collections system that scan an electronic tag on the vehicle's bumper and futuristic advanced vehicle control systems that act automatically to avoid collisions, improve vision in poor weather conditions, or wake up drowsy drivers who have fallen asleep behind the wheel.

                             Figure 2 — Traffic Sensor (left of center traffic signal)

                                and Traffic Camera (right of center traffic signal). 

Regardless of the exact function of the ITS, fiber optic links offer a valuable component in the overall traffic network. Modern ITS networks require ever-increasing data rates and payload carrying capabilities to facilitate real-time communications between a wide variety of field devices and traffic control centers (TCCs). Single-mode optical fiber-based ITS infrastructures are displacing twisted pair copper and coax for both data and video transmission requirements in urban and rural jurisdictions worldwide. Video transmission for surveillance of intersections, ramps and tunnels, incident detection or verification, and replacement of traffic signal loop sensors is an increasingly popular ITS tool. Lately, communities have been installing cameras on traffic signals to record the license plates of cars whose drivers run a red light. All of these applications require distance between the site where the information is collected and the location where the information gets stored. Video transmission that incorporates 2-way data has grown as an ITS application. This system transmits video to a control center as well as data. The control center sends data to the remote camera that allows a PTZ device to be custom positioned as needed by the person at the control center. Fiber optic links for point-to-point FM baseband transmission over single-mode fiber from fixed or PTZ-equipped roadside cameras are widely available for distances up to 90 km. Intelligent transportation systems, as with many fiber optic applications, require a network of nodes, controls and signal paths. "Fiber Optic Network Topologies" discusses the various forms this and other types of networks can take.

Fiber, Wireless, and Free Space Optics: The Ties That Bind

The new industry buzz word these days is wireless. Defined by IEEE standard 8.02.11, wireless refers to the telecommunication technology, in which radio waves and microwaves carry signals to connect communications devices. These devices include pagers, cell phones, portable PCs, computer networks, GPS, satellite systems and handheld personal digital assistants (PDAs). This term evokes the idea of a network detached from wire with a transmission scheme that consists of voice and data quickly whizzing through the air from point A to point B. This idea is partly true. Voice and data can be sent "whizzing through the air," a technology dubbed Free Space Optics (FSO), via lasers and high powered LEDs that transmit through the air rather than through an optical cable. The desire to develop wireless networks lies in the ease of installation, and the rapidity of its technological advances. Breaking away from optical fiber allows a network to go virtually anywhere by eliminating the need to dig ditches and break up streets to install the fiber cable. In addition, over-air wireless transmission is free because wireless optics use the 300 GHz spectrum and above, which includes infrared frequencies, a range that remains unlicensed. Currently, the only regulation on these transmission frequencies is that the radiated power cannot exceed the limits established by the International Electrotechnical Commission or the United States' FDA. Soon, the United States is expected to adopt the IEC standard, creating a global wireless transmission standard.

Typical Wireless Applications

Many applications use wireless technology. The most prevalent applications include those in a local area network (LAN) where right-of-ways are unable to accommodate fiber or copper. The following are the main wireless applications implemented today: 
Last-Mile Access: High-speed links that connect end-users with Internet Service Providers or Satellite services. This applications remains the most popular implementation for wireless technology, eliminating, where applicable, the need for fiber to the curb or fiber to the home. 
Metropolitan Area Network extensions: Used to connect new networks, their core infrastructure, to complete. 
Enterprise Connectivity: Used to connect Local Area Network segments housed within buildings that do not have easily accessible right-of-ways for fiber. 
Fiber Backup: Act as a backup for a fiber based system. 
Backhaul: Wireless system used to carry cellular phone traffic from antenna tower back to facilities wired into the public switch telephone. 
Service Acceleration: Used to provide instant service to fiber optic customers while the fiber infrastructure is being laid.

Fiber Optic Transport in Wireless Networks

Fiber Optic transceivers may be used to connect the Uplink/Downlink equipment to the transmission towers. This allows a greater increase in the distance between the base station and the wireless transmission towers. By incorporating fiber optic links, the system's EMI sensitivity is greatly reduced while reliability and signal quality go up. Figure 1 illustrates a typical use of fiber optic transceivers in a wireless network.

Figure  - Typical use of fiber optic links in a wireless network.

Analog Fiber Optic CATV System Design

Analog AM fiber optic systems have begun to replace coax cable for local distribution within a CATV network, while digital systems are being used for headend or hub site elimination and for transmitting various data services. In the past, these analog and digital transmissions systems are operated separately from each other over separate optical fibers. However, as these CATV systems grow and expand, the current trend in CATV system design incorporates wavelength-division multiplexing to combine both the analog and digital signals for transmission using the same fiber. This allows system expansion by increasing the number of signals transmitted on fiber currently installed. As these systems grow, the forward path transmission ceases to be the only required path. Today's CATV system may also require a return path network to handle data from the Internet via cable modems. This article will focus on both two fiber and single fiber two-signal WDM CATV system design. For additional information on WDM systems using more than two signals, see articles on CWDM and DWDM. Unidirectional CATV Transmission (Forward Path) Before 1980, most CATV systems were coax based, but by the early 1980's the CATV industry began using direct modulated 1310 nm VSB/AM links for distribution super trunks. Figure 1 illustrates a typical system architecture including a super trunk. By transporting a high quality replica of the headend signals, this system reduced the number of cascaded amplifiers required.

                                Figure 1 - Typical Super Trunk CATV Architecture 


By the early 1990's, CATV providers began using multichannel digital systems to transport large numbers of uncompressed, broadcast-quality, digitized video channels between the headends. Still operating in the 1310 nm wavelength window, in this configuration, a previous separate headend is replaced by very high quality signals that are transported by a multichannel digital system from a "master" headend. Figure 2 illustrates this configuration. The advent of high performance externally modulated 1550 nm VSB/AM transmitters and erbium-doped fiber amplifiers (EDFAs) changed the architecture of CATV system design once again. These 1550 nm links are used to carry signals between headend sites over long distances, using the EDFA as an in-line amplifier.

                          Figure 2 - Hybrid Analog/Digital CATV Architecture


The high performance 1550 nm systems vary slightly in that a few additional optical components are required. Illustrated in Figure 3, this system also incorporates optical splitters in addition to the EDFA. In this configuration, the transmitter is assumed to have dual outputs, a common feature for these new transmitters. The first optical output of the 1550 nm transmitter feeds a secondary headend 1310 nm transmitter. The second optical output goes into a 1 x 2 optical splitter. The first output feeds directly into a 1550 nm receiver for distribution from the main headend to a 1310 nm transmitter. The second output of the optical splitter feeds an EDFA. The signal is amplified optically and forwarded to the optical receiver which supplies a third headend located many miles away in the system.

                                    Hybrid 1310 nm & 1550 nm VSB/AM CATV Architecture 


The first three architectures use no WDM components and represent completely analog architectures. As CATV systems grow, the need to expand each fiber's transmission capacity grows with it. Wavelength-division multiplexing allows both analog and digital signals to co-exist on a single fiber. Figure 4 illustrates a unidirectional WDM AM CATV/Digital transport system.

Figure 4 - Unidirectional Analog/Digital CATV Transport using WDM 


In the configuration shown in Figure 4, the signal from the 1310 nm CATV AM transmitter and the 1550 digital transmitter are wavelength-division multiplexed onto one fiber. At the receive, the signals are demultiplexed and output to the correct receivers. In order to maintain system quality, the WDM must be a high isolation type that prevents interference between the 1310 nm analog signal and the 1550 nm digital signal. A bidirectional configuration of this analog/digital CATV transport system is illustrated in Figure 5.

                Figure 5 - Bidirectional Analog/Digital CATV Transport Using WDM

Fiber Optic Security Control and Data Acquisition (SCADA) Networks

SCADA (Supervisory Control And Data Acquisition) networks, originally developed in the 1960s, are used for industrial measurement, monitoring, and control systems, especially by electricity and natural gas utilities, water and sewage utilities, railroads, telecommunications, and other critical infrastructure organizations. They enable remote monitoring and control of an amazing variety of industrial devices, such as water and gas pumps, track switches, and traffic signals. A SCADA system gathers information, such as where a leak on a pipeline has occurred, transfers the information back to a central site, alerting the home station that the leak has occurred, carrying out necessary analysis and control, such as determining if the leak is critical, and displaying the information in a logical and organized fashion. SCADA systems range from relatively simple networks that monitor environmental conditions of a given location to incredibly complex systems that monitor all the activity in a power plant or a municipal water system. for example. 

SCADA Network Components

A SCADA network consists of one of more Master Terminal Units (MTUs) which the operators utilize to monitor and control a large number of Remote Terminal Units (RTUs). The MTU is often a general purpose computing platform, like a PC, running SCADA management software. The RTUs are generally small dedicated devices which are hardened for outdoor use and industrial environments. Fiber optic data transceivers are ideal in SCADA networks because they offer EMI immunity. When transceivers are used for the master and remote terminal units, a fault tolerant self-healing ring network is easy to configure. Figure 1 illustrates a self-healing ring network topology.

Reliable operations of SCADA systems depends on proper configuration, cyber security measures, and other factors.

DWDM - Dense Wavelength-division Multiplexing

Dense Wavelength-division Multiplexing

Dense wavelength-division multiplexing (DWDM) revolutionized data transmission technology by increasing the capacity signal of embedded fiber. This increase means that the incoming optical signals are assigned to specific wavelengths within a designated frequency band, then multiplexed onto one fiber. This process allows for multiple video, audio, and data channels to be transmitted over one fiber while maintaining system performance and enhancing transport systems. This technology responds to the growing need for efficient and capable data transmission by working with different formats, such as SONET/SDH, while increasing bandwidth. The fiber optic amplifier component of the DWDM system provides a cost efficient method of taking in and amplifying optical signals without converting them into electrical signals. In addition, DWDM amplifies a broad range of wavelengths in the 1550 nm region. For example, with a DWDM system multiplexing 16 wavelengths on a single optical fiber, carriers can decrease the number of amplifiers by a factor of 16 at each regenerator site. Using fewer regenerators in long-distance networks results in fewer interruptions and enhanced efficiency. 

DWDM System Considerations

Important components for a DWDM systems are transmitters, receivers, fiber amplifiers, DWDM multiplexers, and DWDM demultiplexer. These components, along with conforming to ITU channel standards, allow a DWDM system to interface with other equipment and to implement optical solutions throughout the network.

Multiplexers and Demultiplexers

The recent explosion of DWDM technology forced the fiber optic manufacturers to develop DWDM multiplexers and demultiplexers that can handle closely spaced optical wavelengths. These designs require narrow passbands, usually 0.4 nm wide, steep roll-off to reject adjacent channels, and stable operation over increased temperature. Recently, multiplexers have gained versatility, moving beyond the "wideband" wavelengths and into densely packed wavelengths that can be integrated into a multiple high frequency, 192 to 200 THz, transmission system. This type of system can maintain up to 16 channels, acting as a 16 fiber channel cable with each frequency channel operating to serve a STM-16/OC-48 carrier. Demultiplexers need to eliminate crosstalk and channel interference. Couplers and dichroic filter, both passive devices, are the most favorable demultiplexers today. The first DWDM coupler design is based on fiber Bragg grating (FBG) filters illustrated in Figure 2. Bragg gratings are comprised of a length of optical fiber with the index of the core permanently modified periodically usually when exposed to an ultraviolet interference pattern. As a result, the fiber grating behaves as a wavelength dependent reflector and lends itself to precise wavelength separation.

The second design is based on cascaded dichroic filters much like those used in the WDM system shown below in Figure 3. In a DWDM coupler, a second dichroic filter would be placed where the fiber 2 is located, and additional dichronic filters would be cascaded until all wavelengths have been combined or separated. At moderate cost, the dichronic filter method assures stability and excellent isolation between channels.

Fiber Amplifiers for DWDM

Because DWDM systems handle information optically rather than electrically, it is imperative that long-haul applications do not suffer the effects of dispersion and attenuation. Erbium-doped fiber amplifiers (EDFAs) counteract these problems. EDFAs are silica based optical fibers that are doped with erbium. This rare earth element has the appropriate energy levels in its atomic structure for amplifying light at 1550 nm. A 980 nm "pump" laser is used to inject energy into the doped fiber. When a weak signal at 1310 nm or 1550 nm enters the fiber, the light stimulates the rare earth atoms to release their stored energy as additional 1310 nm or 1550 nm light. This process continues as the signal passes down the fiber, continually growing stronger. Figure 4 illustrates an erbium-doped fiber.

The photons amplify the incoming signal optically, boosting the wavelength, and avoiding almost all of the active components. The output power of the EDFA is large, and thus, fewer amplifiers may be needed in any given system design. The amplification process is independent of the data rate. Because of this benefit, upgrading a system means only changing the launch/receive terminals. As demands for wider bandwidth grow there is a call for more efficient and reliable optical amplifiers. The usable bandwidth of an EDFA is only about 30 nm (1530 nm-1560 nm), but the minimum attenuation is in the range of 1500 nm to 1600 nm. The dual-band fiber amplifier (DBFA) solves the usable bandwidth problem. It is broken down into two sub-band amplifiers. The DBFA is similar to the EDFA, but its bandwidth ranges from about 1528 nm to 1610 nm. The first range is similar to that of the EDFA and the second is known as extended band fiber amplifier (EBFA). Some features of the EBFA include flat gain, slow saturation, and low noise. The EBFA can achieve a flat gain over a range of 35 nm which is comparable to the EDFAs. EBFAs have the advantage of reaching a slower saturation keeping the output constant even though the input increases.

Channel Spacing

DWDM channel spacing governs system performance; 50 GHz and 100 GHz outline the standards of ITU channel spacing. Currently, 100 GHz is the most commonly used and reliable channel spacing. This spacing allows for several channel schemes without imposing limitations on available fiber amplifiers. However, channel spacing depends on the system's components. Channel spacing is the minimum frequency separation between two multiplexed signals. An inverse proportion of frequency versus wavelength of operation calls for different wavelengths to be introduced at each signal. The optical amplifiers bandwidth and receivers ability to identify two close wavelength, sets the channel spacing. Figure 5 illustrates the typical DWDM specifications.

Signal Direction

DWDM involved sending a large number of closely spaced optical signals over a single fiber. Standards developed by the ITU (International Telecommunications Union) define the exact optical wavelength used for DWDM applications. The center of the DWDM band lies at 193.1 THz with standard channel spacing of 200 GHz and 100 GHz. The closest "standard" spacing (100 GHz) allows transmission of 45 channels on one fiber. A 45 channel system spaced at 100 GHz would cover a optical span of 35 nm and require a costly wide bandwidth, gain-flattened EDFA. As system designers looked to pack more than the 45 channels at 100 GHz spacing, they started to use closer spaced optical channels. The channel spacing, in GHz, relates to the optical wavelength as follows: A spacing of 200 GHz corresponds to about 1.6 nm, 100 GHz corresponds to about 0.8 nm, and 50 GHz corresponds to about 0.4 nm channels spacing. Most commonly 50 GHz follows 100 GHz, although attempts at 75 GHz and 37.5 GHz show up in literature. While there is nothing magical about any of these numbers, it seems likely that 50 GHz will be the next logical step below 100 GHz. Using a channel spacing of 50 GHz (0.4 nm) allows 45 channels to occupy only 17.5 nm of optical bandwidth. This greatly simplifies the requirement for optical amplifiers in the system. Fiber increases in channels per fiber would likely lead to the use of 25 GHz spacing. Designing the optical demultiplexer to separate the signals at the receive end defines the greatest challenge in closely spaced optical channels. Because of subtle color differences in each of the optical channels, high performance DWDM optical demultiplexers must have three characteristics. First, it must be very stable over time and temperature. Second, it needs to have a relatively flat passband or region of frequencies. Third, it must reject adjacent optical channels so that they do not interfere. Several basic types of designs can be used in optical demultiplexers to separate the optical channels. Many of these designs have an increasingly difficult time separating the optical channels as the spacing becomes very close. Some, however, such as fiber Bragg gratings actually appear better suited for closer channel spacing. The need for close optic channel spacing is a trade-off between the performance required of the optical amplifiers used in the system and the number of channels to be transmitted per fiber. Figure 6 illustrates the transmission spectra of 0.4 nm spacing DWDM FBGs.

Red and Blue Bands

The ITU approved DWDM band extends from 1528.77 nm to 1563.86 nm, and divides into the red band and the blue band. The red band encompasses the longer wavelengths of 1546.12 nm and higher. The blue band wavelengths fall below 1546.12 nm. This division has a practical value because useful gain region of the lowest cast EDFAs corresponds to the red band wavelengths. Thus, if a system only requires a limited number of DWDM wavelengths using the red band wavelength yields the lowest overall system cost.

Optical communications repeater

An optical communications repeater is used in a fiber-optic communications system to regenerate an optical signal by converting it to an electrical signal, processing that electrical signal and then retransmitting an optical signal. Such repeaters are used to extend the reach of optical communications links by overcoming loss due to attenuation of the optical fibre and distortion of the optical signal. Such repeaters are known as 'optical-electrical-optical' (OEO) due to the conversion of the signal.

Repeaters have largely been replaced in long-haul systems by Optical amplifiers since one amplifier can be used for many wavelengths in a Wavelength Division Multiplexing (WDM) system, thereby saving money. Note that this class of device is sometimes called "Optical Amplifier Repeater". 

Due to the high data rates that can be achieved with optical systems, OEO repeaters are expensive to implement as electronics to handle those high data rates are expensive and difficult to construct. Also, since one repeater is required for each wavelength, and many tens of wavelengths may be transmitted down a single fibre, a lot of equipment is required for each fibre. In contrast, an optical amplifier can amplify all of the wavelengths in a single device. An amplifier does not provide the regeneration ability of a repeater, but loss, rather than distortion is generally the limiting factor in the design of communications system.

Figure: Sample MPLS Backbone