Essay: Fiber optic communication

INTRODUCTION

Background

Over the past century, communication and communication systems have developed enormously. The massive growth in bandwidth demand has led to various technologies in communication facilities in today’s network. One of the main factors for continuous research to create more efficient communication systems is the need to send more data (information) through a communication medium (channel) [1], communication through fiber optic is one of them.

Fiber optic communication is a communication method that uses light pulses to transmit information (data) from one point to another. Figure 1.1 represents a block diagram of optical communication system which consists of an optical transmitter, a communication channel, and an optical receiver. Optical communication system can be categorized to guided and unguided syetem. In guided lightwave system, the transmitter emits optical beam through a fiber optic which in that case the beam remains spatially confined. Unguided ligtwave system is similar to the spreading of microwave system because the optical beam spreads in space but in forward direction because of its short wavelength. Unguided optical systems are less proper for broadcasting applications than microwave systems because of beam short wavelength they need very accurate pointing between the transmitter and receiver [2]. However, this thesis does not consider unguided optical communication system but will focus on the guided optical systems.

Figure 1.1: a generic optical communication system [2].

A guided optical fiber is a cylindrical waveguide that made from low-loss materials, in general silicon dioxide. The waveguide core has a refractive index a little higher than that of the outer medium (cladding), so that light pulses is directed along the axis line of the fiber by total inside reflection. Generally a guided fiber optic communication systems consist of an optical transmitter to change an electrical signal to an optical signal for transmission through inside the optical fiber, a cable has several bundles of optical fibers, optical amplifiers to amplify the power of the optical signal, and an optical receiver to reconvert the received optical signal back to the original electrical signal [3].

Figure 1.2: guided optical communication system [3].

Optical fiber technology development is considered as a major driver to the revolution of the information technology and tremendous progress on telecommunication that has been seen in recent years. Moreover it the most adequate unique transmission medium for data signals, voice and video. Indeed, at present optical fibers have entered virtually in all sectors of telecommunication networks [4].

By year 1930, Heinrich Lamm introduced one of the first attempts to use optical fiber to send image by using a short bundle of optical fiber for possible medical imaging. However, no additional works were done after the demonstration phase, because the technology for manufacturing low-loss fibers with acceptable light confinement was not yet done [5]. Further work and tests for image transmission via fiber optic continued till 1960 when the attenuation of glass-clad degraded to about1 dB/m. Although this attenuation makes fibers to be used for medical imaging, it is too high for communication until 1970 when it decreased less than 20 dB/km by using a novel fabrication technique. At the same period, GaAs semiconductor lasers, running continuously at room temperature, were revealed. Low loss (attenuation) optical fibers together with optical so-
urces led the efforts for enhancing the fiber-optic communication systems [6]. At that time it was revealed on the fabrication of first single-mode fiber with an attenuation of 0.017dB/m at the 633-nm helium-neon line and that declaration was considered a great breakthrough with many others developments which opened the door to fiber optic communication.

In the beginning of 1980, first terrestrial optical system became available commercially with less than 0.1 Gb/s as a data rate. Within period of less than quarter of a century the development has been rapid as by an increasing in the system capacity by a factor of 100,000 [6].
To increase the benefits of fiber optic many technologies have been developed to get more network capacity, reliability, scalability and less cost and one of the first technological advances was the capability to carry multiple light channels through one fiber optic cable. It achieves buy combining (multiplexing) different optical frequencies (wavelengths) onto a single fiber and that what is known Wavelength Division Multiplexing (WDM) [7].
First WDM created in the late 1980s by using the two widely spaced wavelengths in the 850 nm and 1310 nm (or 1310 nm and 1550 nm) regions. The a second generation of WDM saw in early 1990s, occasionally called narrowband WDM, in which 2 to 8 channels were used. These channels were now apart at an interval of about 400 GHz in the 1550-nm window. With 16 to 40 channels and spacing from 100 to 200 GHz Dense Wavelength Division Multiplexing (DWDM) systems were emerging in mid-1990s [8], since year 2000 when the number of wavelengths reached to over 100 per fiber and that number provides massive network capacity [7].

Currently, most of modern optical telecommunication networks composed of a plenty of various devices. Working such a network is a challenge task. It needs an accurate planning; taking into account all technological needs and offers of the optics, to end up with an operating network which provides the best return on the spent investments.

Fiber optic has vast advantages over other transmission media because:
Unlike the copper that depletes the natural earth resources, it is made of cheapest resources in the earth (sand).
The information travels with the speed of light.
Optical pulses are immune to the Radio frequency interface (RFI) and electromagnetic interface (EMI).
It is not affected by atmospheric conditions.
Very low Bit Error Rate (BER) 10^-11.

The only limitation that limits the bandwidth of an optical fiber is dependent on the equipment that creates the light in the fiber [9].
Optical Fiber Structure

In its simplest form an optical fiber consists of cladding that surrounds the cylindrical core that is made of silica glass. The core which is a central part of the fiber, has a refractive index (refractive index term refers to the change in light speed in vacuum to its speed in other media) of N1, is higher than refracting index N2 of cladding. So when the light beam enters the fiber, the cladding forces it to remain in the core, and the beam travels down the fiber because of internal reflection between of cladding and the core [10].

Figure 1.3: Optical Fiber Structure [10].

Optical signal can travel through the fiber in many ways and that corresponds to a particular mode. Each mode is determined by the angle of incident of the optical beam. Internal reflection can take place at any angle greater than the critical angle but optical light may not propagate through the fiber on all angles and that because some of incident angles result in destructive interference and that leads to prohibit light propagation, in contrast with other angles that cause constructive interferences which allows the light to propagate.

The mode of the fiber is the number of paths for the light rays inside the cable and according to that numbers and structure, fiber cables can be classified as a single mode and multimode fiber. Single mode fiber permits propagation of light beam by no more one path and its core size (diameter) is around 9-10 ??m. Multimode term refers to the matter of fact that many light beams (mode) are carried at same time through the waveguide (over more than 100 modes at a time) with size of its core diameter around 50??m or 62.5??m. In general, the diameter of the single mode is smaller than multimode diameter and because of that the number of reflections in single mode for a light passes inside the core is lesser than in multimode. Such reasons leads to fact that optical pulses experience very low attenuation and makes the signal to travel further (long distance) and faster and higher bandwidth compared to multimode fiber that is used in short distance because of high dispersion and attenuation [2].
Figure 1.4: Single mode and multimode fiber optic.
The main advantage of multimode fibers over the single mode is that a light could be inserted simply on bigger core and allows lesser coupling loss for example, through an LED. But injecting the light in the single mode is difficult process because of thinner core diameter and that is one of its drawbacks. The one of problems in multimode fiber is intermodal dispersion. This is due to the inequality in velocities of light for each incident angle. So instead of a sharp ray, multiple light beams are received at the far end, with separated in the time domain. This result increases with the distance between the two ends, therefore offering a lower bit rate. However dispersion problem can be lessen by decreasing the number of modes which can be reduced by diminishing the core radius, reducing numerical aperture, and increasing the light wavelength and that can be seen in the following equation:
To find the number mode m in multimode fiber [11]:

m’1/2 V^2 ”””””””’ 1.1
Where V is the normalized frequency

V= Ko a'(2&n??core-n^2 clad) ””””””’1.2

Ko =2 ??/?? , and a is the core radius.
?? represents the wavelength of light in a vacuum.

Fiber loss

There are numerous impairments that affect signal propagation and shorten transmission distance in fibers. The major impairment that affects signal propagation is attenuation. It is a property of the fiber and it is a consequence of the several material, structural, and modular impairments in a fiber. It is given as a specification for a particular fiber type. Attenuation means loos of light energy (It is called optical loss) when the light pulses travel through fiber from one point to the other point and it is directly proportional to the length of the cable [12].

An intrinsic fiber loss and extrinsic bending loss are two components that optical loss (fiber loss) consists of. Intrinsic loss can be characterized to [1] [13]:

Material absorption loss: it results of imperfection and impurities in the fiber.
Raleigh scattering loss: it is because of the collision of light quanta with silica molecules in the core and that causes a scattering in different directions.

Extrinsic bending loss includes [1] [13]:

Microbending loss: due to imperfection in the cylindrical geometry of fiber through the manufacturing cycles.
Macrobending loss: bending of fiber (core and cladding) in small radius leads to certain modes not to be reflected and causes loss.

Figure 1.5: Extrinsic bending losses type [13].
Attenuation factor ?? represents the loss in dB per fiber length in kilometer

Power attenuation can calculate by using [1]:

dp/( dz)=- ?? p ””’. 1.3

dp/dz Represents the power change with respect to the length.

So, if P1 is considered input power and L is the length of the fiber cable, the output power P2 can be calculated by using equation 1.4

P2=P1e^(-??L)””’1.4

?? in dB/Km can be calculated by using equation 1.5

?? = – 10/L log P2/P1””’.1.5

Typically ”’??0.25 dB/km for a single mode fiber in the 1550 nm band.
and
?? = 0.5 dB/km for a single mode fiber in the 1310 nm band.

Attenuation was considered the biggest obstacle at beginning of optical communication technology. It was very high (was more than 1000 dB/km) in the glass optical fibers but new process was invented by using silica as the main material, reduced the attenuation to less than 20 dB/km at carrier wavelength about 1??m. Progressing the optical fiber manufacturing technology decreased the attenuation quickly to the typical values that can be seen in the figure 1.6. The curve in the figure refers to the region around 1300nm and 1550nm where the attenuation at these region has minimum values whereas at region between 800nm and 900nm the attenuation at maximum value and that refers to the historical values before the development when the light source operated at the wavelengths from that region [14].

Figure 1.6: attenuation curve of silica based optical fiber [14].
Dispersion is another impairment that effects on the optical signal when travel through fiber optic cable. Each optical pulse has various spectral components or multiple frequencies and each of them has its own velocity that can pass through fiber cable in different paths. Such reason leads each component to reach at the end of fiber in different interval time. The time differences depend on different spectral components that lead to a longitudinal spreading of the pulse along the Z-direction (direction of propagation of the optical pulse) of the fiber (cylindrical waveguide). The optical spread amount depends on the length of fiber and the bite rate [1]

Over adequately long fiber the dispersion may cause intersymbol interference (ISI) whereas in high bit rate, two consecutive pulses are adjacent to each other causing difficulty in detecting the pulses and that means increasing in bit error rate (BER).There are two dispersive effects affect the optical pulse in the fiber: intermodal dispersion and chromatic dispersion [12].
Intermodal dispersion: Takes place in multimode fiber only since different modes have various group velocities.
Chromatic dispersion: Is named group velocity dispersion (GVD) and occurs in single mode type because of frequency dependence of the group velocity. The sources of chromatic dispersion are [13]:
1- Material dispersion: Occurs because of variation on account of refractive index of core as a function of wavelength and takes place even when different wavelengths pass at same path.
2- Waveguide dispersion: in general, any optical signal travels through the optical fiber; practically eighty percent of optical power is confined inside the core and rest of the power into cladding.

Absorption is other type of loss that may affect the light propagation. This type comes when some molecule of the hydroxide anion OH??, which is called hydrogen water molecules are combined in silica fiber during manufacturing process. Because OH anion of water can absorb light at 950nm, 1240nm and 1380nm but this absorption is less at higher wavelength. Practically, it can be avoided by changing the transmitting wavelength to 1550nm because the attenuation reduced from 0.5 dB/km at1300nm to 0.001db/km at 1550nm [15].

Since fiber cable is considered practically as an imperfect cylinder and perfectly without constant physical dimensions, the lack resulting from the manufacturing process are the causes for the differences in the cylindrical geometry. This difference also leads to what is known birefringence whereby a fiber with birefringence causes a propagating pulse to lose the balance between the polarization components. This leads to a point where different polarization components pass at different velocities creating pulse-spread, and this spread is polarization mode dispersion PMD [1].

Optical technology

Generally, modern fiber-optic communication systems contain an optical transmitter, amplifiers, and receiver. Transmitter is used to convert an electrical signal into an optical signal to send into the optical fiber, and use multiple kinds of amplifiers to recover and boost the weak optical signals that experience from different loses that mentioned before. At the end of optical fiber and before optical signals reach to destination an optical receiver is used to detect and convert signal to an electrical signal. The information transmitted is typically digital information generated by computers, telephone systems, and cable television companies.
Optical Transmitters

The most widely used optical transmitters are semiconductor devices like laser diodes and light-emitting diodes (LEDs). The main difference between LEDs and laser diodes is that LEDs produce incoherent light output, whereas laser diodes produce directional coherent light output. When it in optical communication, it is necessary be efficient, supply high optical output power, compact, and reliable, when operating in an optimal wavelength range, directly modulated at high frequencies [16].

Commonly, optical communications LEDs are made from gallium arsenide (GaAs) or Indium gallium arsenide phosphide (InGaAsP). Since InGaAsP LEDs work at a longer wavelength than GaAs LEDs (1.3 micrometers vs. 0.81-0.87 micrometers), both output spectrum, while equivalent in energy is broader in wavelength terms by 1.7. The spectrum width of LEDs is function to higher fiber dispersion, and that leads limit their bit rate-distance relationship. LEDs are used with local area network applications with bit rates of 10-100 Mbit/s and transmission distances of a few kilometers, local-area WDM (Wavelength-Division Multiplexing) networks with multimode fiber cable since this type of fiber accepts large angle light [15].

Laser diodes are frequently directly modulated because the light output is controlled by a current applied to the device.

Optical Receiver

A photodetector, typically is a semiconductor-based photodiode, is the main component in the optical receiver which uses photoelectric effect to convert light source into electrical signal. There are different types of photodetector, the first type was made from Indium gallium arsenide, then several type follows the first generation like p-n photodiodes, p-i-n photodiodes and Metal-semiconductor-Metal (MSM) photodetectors which are used for circuit integration in generators and wavelength-division multiplex as a result of their stability [17].

A converters (Optical to electrical) are usually connected with a limiting amplifier and a transimpedance amplifier to produce a digital signal ( electrical domain) from the incoming optical signal, which may be attenuated and distorted during travelling through the fiber optic cable. Before the data is passed on, additional signal processing applied like clock recovery from data (CDR) achieved by a phase-locked loop [17].
Optical Amplifier

Because of the limitation of nonamplified fiber span to approximately 200 km for bit rate in the Gbs, using optical amplifiers in the optical networks are very important to work perfectly. Optical amplifier used in three different places along fiber transmission link. Amplifiers operating as power amplifier, their function are to amplify the power signal before it is launched on the line to extend the transmission distance before further amplification is required. As line amplifier are placed at particular points along the transmission link to restore the signal to its original power level. Finally, as a preamplifier, it increases the signal level at the input of an optical receiver to improve the detection performance for the signal [12]. Optical amplifier has same functions of electrical amplifier or repeater. Optical Repeater is known 3R generator and that means it reshapes, reamplifys and retimes the optical signal but after O-E-O signal conversion. In contrast the optical amplifiers amplify data stream totally in the optical domain and that make them entirely transparent to signals and protocols [1]. There are three types of amplifier:

Doped fiber amplifiers: they amplify light at specific wavelength and make use the cores of fibers which are doped with elements. They amplify an optical signal by a doped optical fiber as a gain medium. Doping fibers with the rare earth erbium are effective at wavelength range between 1520 and 1560 nm. Erbium doped fiber amplifiers or EDFAs ( Figure 1.6) is common example for doped fiber amplifier and has gain up to 30 db and that means it outs 1000 photon for each 1 photon in [16]. In past before EDFA deployment, individual wavelength through fiber had regenerated one time each 40 km long by using expensive electronic device and increased to 1500-2500 km long when use more recent EDFA system [7].

Figure 1.7: Erbium doped fiber amplifiers [1].
Semiconductor Optical Amplifiers: These types make use of an adapted semi-conductor laser. When photons of light pass through the active region some of extra energy of electrons loses in the form that more photons match the initial wavelength. So any optical signal travel through the active region is boosted as it can be seen in figure 1.7 [18].

Figure 1.8: Semiconductor Optical Amplifiers [18]
Raman Amplifier: the amplification process is taken place by nonlinear interaction
between a pump laser and the signal within an optical fiber. Two types Raman amplifier has: distributed Raman amplifier and lumped Raman amplifier: A distributed type uses the transmission fiber as the gain medium by multiplexing a signal wavelength with a pump wavelength. Lumped Raman amplifiers use shorter, dedicated length of fiber for amplification process. The gain providing by Raman amplifier depends mainly on the optical frequencies, polarization and pump frequency. It is noticed from the figure 1.8 that maximum gain when pump-signal frequency difference 13.2 THz [12].

Figure 1.9: Raman Amplifier gain [12].

The main advantages of Raman amplifier are possibility to amplify variable wavelength and can be used to extend the Erbium-Doped Fiber Amplifiers EDFAs and it is used with single mode fiber. But high pump power needs is the main disadvantage of Raman amplifier also the need of complex gain control and noise are two points add to the disadvantages [19].
Wavelength Division Multiplexing (WDM)

Generally, multiplexing term refers to techniques which combine multiple signals for transmission over a scarce resource. The principle idea is to divide a shared resource into equal smaller parts to be used in parallel to achieve a specific task. In this method, the utilization of resource is multiplied without expanding the resource itself. Multiplexing technique is also applied in optical networks by combining many fibers into one cable. TIME Division Multiplexing (TDM) is prominent example of that method [1].

Wavelength-division multiplexing (WDM) is a method of multiplying the available optical fibers capacity by using parallel channels, each channel uses a dedicated wavelength of light as it cleared in figure 1.9. WDM and Frequency Division multiplexing ( FDM) both share same principles, where WDM relates to light wavelength in fiber cable FDM corresponds to analogue transmission but WDM is more reliable than FDM since it is totally passive. The main advantage of WDM is increasing a transmission speed without increasing the capacity of the connections, and that done by dividing a high bandwidth bitstream into several time slots which each one carries traffic on a lower bitrate. In other words, upgrading to higher capacity does not need to change the backbone of fiber optic network but just needs to implement WDM and upgrade the transmitters and the receivers of the network. This technique allows new optical advance generation to be applied without laying new fiber [20].

Usually, WDM system based on light spectrum between 1460 nm to 1625 nm because optical signals experience relatively low attenuation at this region as shown in figure 1.1x. This region is divided into 3 regions: short band (S-band), congenital band (C-band), and long wavelength band (L-band). Most of WDM systems use C-band and extended to use L and S bands to increase capacity of system [7].

Figure 1.1x WDM regin

Figure 1.10: Wavelength division multiplexing (WDM)
Before proceeding to examine different type of WDM, it will be necessary to summarize the usage of WDM technology advantages:

Upgrade the capacity of current fiber networks without adding new fibers.
Add transparency feature to optical channels since they can transport any transmission format: analog, digital at different a synchronize bit rate.
Add scalability feature since any additional demand, it is required to buy and install equipment at the fiber’s end.
Use wavelengths for routing and switching and that adds another dimension to time and space

Depends on wavelength pattern, WDM is divided into two types:

Dense WDM (DWDM): It is chosen for transporting very large amounts of data traffic over long distances in the telecommunication system. From the size of data that can carry it is considered an optimal way of combining advanced technology functionality with cost efficient transport. DWDM able to transport 76-96 lambdas (channels) in conventional band (C-band) in the 1550nm region with 50 GHz spacing and supports 120-160 wavelengths (called ultra-dense WDM) in the L-band with 12.5 GHz spacing. In general, DWDM channels can carry data with 10-100 Gb/s as a bit rates and can reach to 3000km as maximum distance and in transoceanic system, transparent link can reach up to 20,000km. Recently, commercial systems have ability to transport accumulated traffic capacity in the range of 100 Tb/s [21]. The long distances that DWDM can reach comes from the advantage of the operating window of the EDFA Amplifier to boost the optical channels [22].

Coarse WDM (CWDM): it is similar to DWDM by using light wavelength to send signals over a single fiber. It uses 20nm wavelength spacing and it is wider than DWDM and that is why CDWM is less costly than DWDM. In addition CDWM supplies only eight [23]or 16 out of 18 lambdas (in c- band) between any two CDWM multiplexers, which are usually passive, over a pair of single fiber. Wavelengths of CDWDM can carry payload 1.2, 25, 4, 5, or 10 Gb/s which [21].

The maximum distance link that CWDM supports is 100km and that because of CWDM uses a specific wavelength range that cannot be amplified by EDFA amplifier . CWDM is suitable for low-latency and high bandwidth but it is low scalable, complex installation, cost, and operation than DWDM [23].
WDM enabling equipment

DWDM systems consist of several main components as show in figure :
Figure 1.11 DWDM system structure.

Optical Transmitters and Transponders

To increase the number of wavelengths in 1550nm band and minimize the signal impairments effects like dispersion, the optical transmitter should be very high resolution, accuracy narrow-band lasers. Such lasers allow close channel spacing. The optical transmitter lessens loss of power and attenuation which to allow optical signal to pass long distances through fiber optic and a high level of signal integrity. Optical amplifiers can be used by these lasers to amplify the signal strength to extend the transmission distance [24] [14]. The design of the laser systems should be:

Follow the ITU-T wavelength frequencies for interoperability and component selection issues [24].
Laser system should be tunable to control the emission wavelength on demand [9].

Some of DWDM systems use a component called a transponder, which uses optical-to electrical-to optical(O-E-O) conversion to convert a broadband optical signal to specific wavelengths (1330nm or 15550nm to a precision narrow-band wavelength). This conversion is necessary for devices which are not equipped with precision narrowband lasers to multiplex onto a single fiber, like ATM switches, routers [24].

Optical amplifiers

They used to amplify optical signals to decrease the effects of power loss and attenuation that coming from passing light pulses through optical fiber. Use the optical amplifier or not in the DWDM depends on the optical transmitter technology and fiber optic long because not all DWDM network types need to boost the optical signals before sending since the signal reaches to fiber end’s terminal at level which photodetector at receiving side can detect and recover. Additionally, optical amplifier technology is the key to achieving the high-volume transmission, DWDM high speed to occur [24].

Optical Multiplexers and Demultiplexers

Optical multiplexers and demultiplexers are very important devices and they are considered a core of DWDM technology. Optical multiplexers are used to combine transmit optical signals that have different wavelengths into a composite signal which have boosts by optical amplifier if need to amplify before sending onto single optical amplifier. Optical demultiplexrs have opposite function to multiplexer by splitting the composite optical signals into their component wavelengths at the receiving terminal [25]. [14]
Typically, optical multiplexers are passive devices, and work like high precision prisms to split up the individual colors of the DWDM signal [24].

Optical Receivers

Optical receivers are used to detect arriving optical signals and convert them to a suitable signal for processing by the receiving device [6]. Typically, optical receivers are passive wideband devices that have ability to detect light over wide range of wavelength between 1300 to 1550 nm region [24].
Remote Optical add/remove Dense multiplexer (ROADM)

Remote Optical Add/Drop Multiplexers (ROADMs) are essential devices in DWDM for adding or dropping specific wavelengths (channels) from a fiber that is dense wave division multiplexed and bypass other remaining wavelengths. Usually, they are installed in a multi wavelength fiber span [7]. (

Figure 1.12: Remote Optical add/remove Dense multiplexer (ROADM) [7].

Hierarchy of optical network
Geographically, optical networks are segmented into four tiers, each one depends on the number of users served, demand capacity, geographic area which the tier covers. Figure 1.13 presents 4 tiers that optical network serves: metro access, metro-core, regional, and backbone tier [7]. Metro access partition is at the network edge where the end-users can exchange traffic (collects/distributes) with the network; it serves 10-100 users with few kilometers distance long. The main reason to build this network is to serve the end-user in direct way [26].
Moving up the hierarchy, metro-core tier aggregates the traffic from thousands of customers that relate to access networks and interconnects number of central offices of telecommunication or cable distribution head-end office by passing 10s to100s of kilometers. Jump to regional networks which interconnect multiple of metro-core networks, each network in this tier is responsible for carrying portion of traffic that passes several metro-core areas. The entire regional networks serve about100s to 1000s of users who cover hundred to a thousand kilometers [7].
Last tier in the four-layer is Backbone (long-haul) networks which span 1000s of kilometers and share millions of users because of carrying the inter-regional networks traffic [27].
Figure 1.13: Hierarchy of optical network [7].

There is other network architecture based on three-layer as in figure 1.14. Application layer is the top of this model which contains voice, video, and data services. The next layer is called intermediate layer which based on electronic technology such as Asynchronous Transfer Mode (ATM) switches, SONET/SDH switches, and IP routers. These electronic devices pass their payloads to Optical layer (configurable WDM layer) to be carried into wavelengths and routed dynamically through optical nodes by using WDM technology and optical switches [7].

Figure 1.14: three layer model [7].

According to [12], multiwave optical network can be divided into two layers: logical layer and physical layer and each layer is expanded to sub- layers as introduced in figure 1.15. Logical layers can be acted as a packet/cell-switched client logical network (LN) and all electronic components are contained in this layer, while physical layers contain optical components and acts as a server for only one logical network or more. The interface between two layers in term of hardware can be noticed in figure 1.16 which is located at external port of Network Access Station (NAS). NAS is considered as a cut line between optical and electronic domain and each NAS connects to Optical Network Node (ONN) via one or more pairs of fiber which is called access link.

Physical layer is spitted to: Optical layer and Fiber layer. Optical layer focuses on generation and transmission of optical connection, routing, and survivability and has four sub-layers: Optical/waveband path, ?? channel , Optical connection, and Transmission channel.

Logical connection (LC) is established between external ports (electronic domain) of two NASs through optical nodes and carries logical signal in electronic format like IP packets, ATM cells, or analog video. Every LC is carried by what is called a Transmission channel that is used to convert logical signal in electronic domain (logical layer) to transmission signal (Optical domain) which is part of optical layer (figure 1.15). The conversion process usually happens in the transmission processor (TP) that exists in NAS with taking into account the requirements of the optical device that transports it for example the bandwidth of the optical transmitter (OT), optical receiver (OR), and spectrum channel.

Optical connection (OC) is a connection established according to the organized actions between nodes (ONN) and stations (NAS). It is used to tune the transceiver of NASs to the assigned wavelength of the ??-channel.

Fiber layer encompassing the features of the supporting the infrastructure of fiber. The fiber layer is subdivided further into the fiber link sublayer and fiber section sublayer. Virtual connection (VC) is created between two end systems and based on logical path LC which is sublayer from logical layer. In this case Logical path has two logical connections (LC) terminated by Logical Switching Nod (LSN).

Figure 1.15: Optical network layer [12].
Figure 1.16: Layered view of optical network connections [12].
Figure 1.17:Typical connection with multiple hope [12].

Optical network concepts

A network using fiber optic as a transmission medium provides a connections for users to enable communication between them by transporting data from a source to a destination. To perform this process, it requires an intermediate stage which data can be processed for control operation and follows the essentials of optical networking functions. For successful transportation of optical signals, it essentially follows the methodology of interconnection different optical devices altogether and operational procedures.

The structure of simple optical network can be seen in the figure 1.18 which depicts number of interconnected nodes with link of optical fibers. Each optical node may act as a transceiver unit with multifunction. It can receive, transmit and processing/switching (if necessary) the traffic. Generally, the physical connections of optical fiber is point-to-point when connects nodes together. Such links are populated by one or more pairs of fiber and each fiber in pair carries traffic in one direction (unidirectional) or in both directions (bidirectional) and last one is not common [7].

Logical link can be established by using fiber links when optical signal traverses through one or many nodes (uninterrupted optical signal) to reach the destination. Ideally, a single wavelength ?? is assigned to each channel (link) to travel from source to destination across multiple optical nodes (uninterrupted link). Lightpath is a term used to refer to the signal carried on a specific wavelength, data can be transmitted over these lightpaths in time of the connections set up and apply controlling mechanism to validate the whole data transmission between each transmitting and receiving node [28].

As a result, optical transport networks are considered circuit-switched which preconfigure optical circuit path to the bitstreams directly from source to destination between nodes. Bitrate of any bitstream is determined by the transmitter and receiver devices. Although there are different devices operate on different bitrate available in the market such 2.5Gb/s or 10Gb/s, different bitrate circuits can be used with each other on the same WDM network because of its transparently for different bitrates. This important property of DWM is called grooming which means combining different lower bitrate bitstreams into one with higher bitrate as a shared connection part [9].

Figure 1.18: Optical network structure [28].

Optical network node and switching elements

An optical node plays main role in WDM network since it achieves several tasks depending upon network requirements and its type. Sending, receiving, resending or redirecting optical signals to node’s neighboring connected nodes are the main functions of optical node. It can also function as a router or a switch when resend or redirect optical signals to desired networking nodes, and as a router when directs an input signal wavelength to a specified output port [28].

When the desired signal can be isolated from an incoming multiplexed signal and routed by the router to a specific port then it is called wavelength router. But if the wavelength of the isolated signal is changed before routing to the output port, then router is known wavelength converting router and in other case is wavelength routing switch( optical switch) in case of switching the wavelength to the complaint signal wavelength [29].
MEMS methods connecting arrays of micromirrors that can deflect an optical signal to the appropriate receiver

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