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Plesiochronous Digital Hierarchy And of Synchronous Digital Hierarchy (PDH and SDH)

Overview of Plesiochronous Digital Hierarchy And of Synchronous Digital Hierarchy (PDH and SDH)

With the introduction of PCM technology in the 1960s, communications networks were gradually converted to digital technology over the next few years. To cope with the demand for ever higher bit rates, a multiplex hierarchy called the plesiochronous digital hierarchy (PDH) evolved. The bit rates start with the basic multiplex rate of 2 Mbit/s with further stages of 8, 34 and 140 Mbit/s. In North America and Japan, the primary rate is 1.5 Mbit/s. Hierarchy stages of 6 and 44 Mbit/s developed from this. Because of these very different developments, gateways between one network and another were very difficult and expensive to realize. PCM allows multiple use of a single line by means of digital time-domain multiplexing. The analog telephone signal is sampled at a bandwidth of 3.1 kHz, quantized and encoded and then transmitted at a bit rate of 64 kbit/s.  PDH and SDH PDH and SDH PDH and SDH PDH and SDH

A transmission rate of 2048 kbit/s results when 30 such coded channels are collected together into a frame along with the necessary signaling information. This so-called primary rate is used throughout the world. Only the USA, Canada and Japan use a primary rate of 1544 kbit/s, formed by combining 24 channels instead of 30.

The growing demand for more bandwidth meant that more stages of multiplexing were needed throughout the world. A practically synchronous (or, to give it its proper name: plesiochronous) digital hierarchy is the result. Slight differences in timing signals mean that justification or stuffing is necessary when forming the multiplexed signals. Inserting or dropping an individual 64 kbit/s channel to or from a higher digital hierarchy requires a considerable amount of complex multiplexer equipment.

Traditionally, digital transmission systems and hierarchies have been based on multiplexing signals which are plesiochronous (running at almost the same speed). Also, various parts of the world use different hierarchies which lead to problems of international interworking; for example, between those countries using 1.544 Mbit/s systems (U.S.A. and Japan) and those using the 2.048 Mbit/s system.

To recover a 64 kbit/s channel from a 140 Mbit/s PDH signal, it’s necessary to demultiplex the signal all the way down to the 2 Mbit/s level before the location of the 64 kbit/s channel can be identified. PDH requires “steps” (140-34, 34-8, 8-2 demultiplex; 2-8, 8-34, 34-140 multiplex) to drop out or add an individual speech or data channel (see Figure 1).

Plesiochronous Digital Hierarchies (PDH)

The main problems of PDH systems are:

  1. Homogeneity of equipment
  2. Problem of Channel segregation
  3. The problem cross connection of channels
  4. Inability to identify individual channels in a higher-order bit stream.
  5. Insufficient capacity for network management;
  6. Most PDH network management is proprietary.
  7. There’s no standardised definition of PDH bit rates greater than 140 Mbit/s.
  8. There are different hierarchies in use around the world. Specialized interface equipment is required to interwork the two hierarchies.

1988 SDH standard introduced with three major goals:

– Avoid the problems of PDH

– Achieve higher bit rates (Gbit/s)

– Better means for Operation, Administration, and Maintenance (OA&M)

SDH is an ITU-T standard for a high capacity telecom network.  SDH is a synchronous digital transport system, aim to provide a simple, economical and flexible telecom infrastructure. The basis of Synchronous Digital Hierarchy (SDH) is synchronous multiplexing – data from multiple tributary sources is byte interleaved.

SDH brings the following advantages to network providers:

  1. High transmission rates

Transmission rates of up to 40 Gbit/s can be achieved in modern SDH systems. SDH is therefore the most suitable technology for backbones, which can be considered as being the super highways in today’s telecommunications networks.

  1. Simplified add & drop function

Compared with the older PDH system, it is much easier to extract and insert low-bit rate channels from or into the high-speed bit streams in SDH. It is no longer necessary to demultiplex and then remultiplex the plesiochronous structure.

  1. High availability and capacity matching

With SDH, network providers can react quickly and easily to the requirements of their customers. For example, leased lines can be switched in a matter of minutes. The network provider can use standardized network elements that can be controlled and monitored from a central location by means of a telecommunications network management (TMN) system.

  1. Reliability

Modern SDH networks include various automatic back-up and repair mechanisms to cope with system faults. Failure of a link or a network element does not lead to failure of the entire network which could be a financial disaster for the network provider. These back-up circuits are also monitored by a management system.

  1. Future-proof platform for new services

Right now, SDH is the ideal platform for services ranging from POTS, ISDN and mobile radio through to data communications (LAN, WAN, etc.), and it is able to handle the very latest services, such as video on demand and digital video broadcasting via ATM that are gradually becoming established.

  1. Interconnection

SDH makes it much easier to set up gateways between different network providers and to SONET systems. The SDH interfaces are globally standardized, making it possible to combine network elements from different manufacturers into a network. The result is a reduction in equipment costs as compared with PDH.

Network Elements of SDH

Figure 2 is a schematic diagram of a SDH ring structure with various tributaries. The mixture of different applications is typical of the data transported by SDH. Synchronous networks must be able to transmit plesiochronous signals and at the same time be capable of handling future services such as ATM.

Schematic diagram of hybrid communications networks

Current SDH networks are basically made up from four different types of network element. The topology (i.e. ring or mesh structure) is governed by the requirements of the network provider.

  1. Regenerators as the name implies, have the job of regenerating the clock and amplitude relationships of the incoming data signals that have been attenuated and distorted by dispersion. They derive their clock signals from the incoming data stream. Messages are received by extracting various 64 kbit/s channels (e.g. service channels E1, F1) in the RSOH (regenerator section overhead). Messages can also be output using these channels.
  2. Terminal multiplexers Terminal multiplexers are used to combine plesiochronous and synchronous input signals into higher bit rate STM-N signals.
  3. Add/drop multiplexers (ADM) Plesiochronous and lower bit rate synchronous signals can be extracted from or inserted into high speed SDH bit streams by means of ADMs. This feature makes it possible to set up ring structures, which have the advantage that automatic back-up path switching is possible using elements in the ring in the event of a fault.
  4. Digital cross-connects (DXC) This network element has the widest range of functions. It allows mapping of PDH tributary signals into virtual containers as well as switching of various containers up to and including VC-4.
  5. Network element management The telecommunications management network (TMN) is considered as a further element in the synchronous network. All the SDH network elements mentioned so far are software-controlled. This means that they can be monitored and remotely controlled, one of the most important features of SDH. Network management is described in more detail in the section “TMN in the SDH network”

SDH Rates

SDH is a transport hierarchy based on multiples of 155.52 Mbit/s. The basic unit of SDH is STM-1. Different SDH rates are given below:

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Environment Effect on Optical Fiber | OFC Splicing

The Environment Effect on Optical Fiber | OFC Splicing

THE ENVIRONMENT EFFECT

  1. There are however always small defects at the surface of the fiber, called microcracks. These cracks grew when water vapour is present and the fiber simultaneously is under strain, hence shortening the life of the fiber.
  2. Another effect ingress of water, which may increase of concentration of water vapour around the fiber.
  3. Temperature variation may cause Expansion/ Contraction of fibers and affect the performance to some extent. By proper choice of materials and by adjusting the excess length of fiber in the loose tube, the temperature variation effect can be neglected.

CABLE DRUM LENGTH :

Cables come reeled in various length, typically 1 to 2 km, although lengths of 5 or 6 kms are available for single mode fibers. Long lengths are desirables for long distance applications, since cable must be spliced end to end over the run. Each splice introduce additional loss into the system. Long cable lengths mean fewer splices and less loss.

METALLIC OR NON-METALLIC CABLES :

Fiber optic cables sometimes also contain copper conductors, such as twisted pair. One use of these conductors is to allow installers to communicate with each other during installation of the fiber especially with long distance telephone installation. The other use is to power remote equipment such as repeaters. Sub-marine cables, cables for overhead mounting, highly, armoured cables of railways etc are also coming in category of metallic cables. In such cables strength member will typically be of steel wire and the cable will also contain one or two copper service pairs. It is also common to include an aluminium water barrier.

It is possible to construct completely metal free cables, used in areas suffering from high frequency of lightening. Strength member is made of fiber glass rod. Induction effect due to lightening or power line parallelism is not at all on such non-metallic cables.

 

OFC Splicing

Splices

Splices are permanent connection between two fibers. The splicing involves cutting of the edges of the two fibers to be spliced.

Splicing Methods

Single–Fiber Mechanical Splicing

  • Single Fiber Capillary
  • Aligns two fiber ends to a common centerline, thereby aligning cores.
  • Clean, cleaved fibers are butted together and index matched.
  • Permanently secured with epoxy or adhesive.

 

Examples : Siecor, See Splice GTE Elastomeric Splice.

See Splice Mechanical Splice.png

Splicing Methods

The following three types are widely used :

  1. Adhesive bonding or Glue splicing.
  2. Mechanical splicing.
  3. Fusion splicing.
  4. Adhesive Bonding or Glue Splicing

1. Adhesive bonding or Glue splicing.

This is the oldest splicing technique used in fiber splicing. After fiber end preparation, it is axially aligned in a precision V–groove. Cylindrical rods or another kind of reference surfaces are used for alignment. During the alignment of fiber end, a small amount of adhesive or glue of same refractive index as the core material is set between and around the fiber ends. A two component epoxy or an UV curable adhesive is used as the bonding agent. The splice loss of this type of joint is same or less than fusion splices. But fusion splicing technique is more reliable, so at present this technique is very rarely used.

  1. Mechanical Splicing

This technique is mainly used for temporary splicing in case of emergency repairing. This method is also convenient to connect measuring instruments to bare fibers for taking various measurements.

The mechanical splices consist of 4 basic components :

(i)         An alignment surface for mating fiber ends.

(ii)        A retainer

(iii)       An index matching material.

(iv)       A protective housing

A very good mechanical splice for M.M. fibers can have an optical performance as good as fusion spliced fiber or glue spliced. But in case of single mode fiber, this type of splice cannot have stability of loss.

  1. Fusion Splicing

The fusion splicing technique is the most popular technique used for achieving very low splice losses. The fusion can be achieved either through electrical arc or through gas flame.

The process involves cutting of the fibers and fixing them in micro–positioners on the fusion splicing machine. The fibers are then aligned either manually or automatically core aligning (in case of S.M. fiber) process. Afterwards the operation that takes place involve withdrawal of the fibers to a specified distance, preheating of the fiber ends through electric arc and bringing together of the fiber ends in a position and splicing through high temperature fusion.

If proper care taken and splicing is done strictly as per schedule, then the splicing loss can be minimized as low as 0.01 dB/joint. After fusion splicing, the splicing joint should be provided with a proper protector to have following protections:

  • Mechanical protection
  • Protection from moisture.

Sometimes the two types of protection are combined. Coating with Epoxy resins protects against moisture and also provides mechanical strength at the joint.

Now–a–days, the heat shrinkable tubes are most widely used, which are fixed on the joints by the fusion tools.

The fusion splicing technique is the most popular technique used for achieving very low splice losses. The introduction of single mode optical fiber for use in long haul network brought with it fiber construction and cable design different  from those of multimode fibers.

The splicing machines imported by BSNL begins to the core profile alignment system, the main functions of which are :

  • Auto active alignment of the core.
  • Auto arc fusion.
  • Video display of the entire process.
  • Indication of the estimated splice loss.

The two fibers ends to be spliced are cleaved and then clamped in accurately machined vee–grooves. When the optimum alignment is achieved, the fibers are fused under the microprocessor contorl, the machine then measures the radial and angular off–sets of the fibers and uses these figures to calculate a splice loss. The operation of the machine observes the alignment and fusion processes on a video screens showing horizontal and vertical projection of the fibers and then decides the quality of the splice.

The splice loss indicated by the splicing machine should not be taken as a final value as it is only an estimated loss and so after every splicing is over, the splice loss measurement is to be taken by an OTDR (Optical Time Domain Reflectometer). The manual part of the splicing is cleaning and cleaving the fibers. For cleaning the fibers, Dichlorine Methyl or Acetone or Alcohol is used to remove primary coating.

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Losses in Optical Fiber: Attenuation | Dispersion | Tube Buffering

What is the Attenuation? How many types of Dispersion? How many types of Fiber Buffering Loose? What is the Tube Buffering?

ATTENUATION : loss of optical power

Attenuation is defined as the loss of optical power over a set distance, a fiber with lower attenuation will allow more power to reach a receiver than fiber with higher attenuation.

Attenuation may be categorized as intrinsic or extrinsic.

INTRINSIC ATTENUATION

It is loss due to inherent or within the fiber. Intrinsic attenuation may occur as

(i) Absorption – Natural Impurities in the glass absorb light energy.

(II) Scattering – Light rays travelling in the core reflect from small imperfections into a new pathway that may be lost through the cladding.

  1. Absorption – Natural Impurities in the Glass Absorb Light Energy.

 

Absorption Scattering

Or

(2) Scattering – Light Rays Travelling in the Core Reflect from small Imperfections into a New Pathway that may be Lost through the cladding.

EXTRINSIC ATTENUATION

It is loss due to external sources. Extrinsic attenuation may occur as –

  • Macrobending – The fiber is sharply bent so that the light travelling down the fiber cannot make the turn & is lost in the cladding.

Extrinsic attenuation

(II) Microbending – Microbending or small bends in the fiber caused by crushing contraction etc. These bends may not be visible with the naked eye.

Attenuation is measured in decibels (dB). A dB represents the comparison between the transmitted and received power in a system.

DISPERSION

It is defined as the spreading of light pulse as it travels down the fiber. ecause of the spreading effect, pulses tend to overlap, making them unreadable by the receiver.

 

BANDWIDTH

It is defined as the amount of information that a system can carry such that each pulse of light is distinguishable by the receiver.

System bandwidth is measured in MHz or GHz. In general, when we say that a system has bandwidth of 20 MHz, means that 20 million pulses of light per second will travel down the fiber and each will be distinguishable by the receiver.

NUMBERICAL APERTURE

Numerical aperture (NA) is the “light – gathering ability” of a fiber. Light injected into the fiber at angles greater than the critical angle will be propagated. The material NA relates to the refractive indices of the core and cladding.

NA  =  n12 – n22

where n1 and n2 are refractive indices of core and cladding respectively.

NA is unitless dimension. We can also define as the angles at which rays will be propagated by the fiber. These angles form a cone called the acceptance cone, which gives the maximum angle of light acceptance. The acceptance cone is related to the NA

Æ  = arc sing (NA) or

NA = sin Æ

where Æ  is the half angle of acceptance.

The NA of a fiber is important because it gives an indication of how the fiber accepts and propagates light. A fiber with a large NA accepts light well, a fiber with a low NA requires highly directional light.

In general, fibers with a high bandwidth have a lower NA. They thus allow fewer modes means less dispersion and hence greater bandwidth. A large NA promotes more modal dispersion, since more paths for the rays are provided NA, although it can be defined for a single mode fiber, is essentially meaningless as a practical

Single mode multimode

characteristic. NA in a multimode fiber is important to system performance and to calculate anticipated performance.

Total Internal Reflection

(Summary)

* Light Ray A : Did not Enter Acceptance Cone – Lost

* Light Ray B : Entered Acceptance Cone – Transmitted through the Core by Total Internal Reflection.

NA = 0.275 (For 62.5 mm Core Fiber)

DISPERSION : Types of dispersion in a fiber

Dispersion is the spreading of light pulse as its travels down the length of an optical fiber. Dispersion limits the bandwidth or information carrying capacity of a fiber. The bit-rates must be low enough to ensure that pulses are farther apart and therefore the greater dispersion can be tolerated.

There are three main types of dispersion in a fiber –

(I) Modal Dispersion

(II) Material dispersion

(III) Waveguide dispersion

MODAL DISPERSION

Modal dispersion occurs only in Multimode fibers. It arises because rays follow different paths through the fiber and consequently arrive at the other end of the fiber at different times. Mode is a mathematical and physical concept describing the propagation of electromagnetic waves through media. In case of fiber, a mode is simply a path that a light ray can follow in travelling down a fiber. The number of modes supported by a fiber ranges from 1 to over 100,000. Thus a fiber provides a path of travels for one or thousands of light rays depending on its size and properties. Since light reflects at different angles for different paths (or modes), the path lengths of different modes are different. Thus different rays take a shorter or longer time to travel the length of the fiber. The ray that goes straight down the center of the core without reflecting, arrives at the other end first, other rays arrive later. Thus light entering the fiber at the same time exist the other end at different times. The light has spread out in time.

The spreading of light is called modal dispersion. Modal dispersion is that type of dispersion that results from the varying modal path lengths in the fiber. Typical modal dispersion figures for the step index fiber are 15 to 30 ns/ km. This means that for light entering a fiber at the same time, the ray following the longest path will arrive at the other end of a 1 km long fiber 15 to 30 ns after the ray, following the shortest path. Fifteen to 30 billionths of a second may not seem like much, but dispersion is the main limiting factor on a fiber’s bandwidth. Pulse spreading results in a pulse overlapping adjacent pulses as shown in figure. Eventually, the pulses will merge so that one pulse cannot be distinguished from another. The information contained in the pulse is lost Reducing dispersion increases fiber bandwidth.

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Types of Fiber Optics

How many types of Fiber Optics?

Types of Fiber Optics:

The refractive Index profile describes the relation between the indices of the core and cladding. Two main relationship exists :

(I) Step Index

(II) Graded Index

The step index fiber has a core with uniform index throughout. The profile shows a sharp step at the junction of the core and cladding. In contrast, the graded index has a non-uniform core. The Index is highest at the center and gradually decreases until it matches with that of the cladding. There is no sharp break in indices between the core and the cladding.

By this classification there are three types of fibers :

(I) Multimode Step Index fiber (Step Index fiber)

(II) Multimode graded Index fiber (Graded Index fiber)

(III) Single- Mode Step Index fiber (Single Mode Fiber)

(1) STEP INDEX MULTIMODE FIBER

This fiber is called “Step Index” because the refractive index changes abruptly from cladding to core. The cladding has a refractive index somewhat lower than the refractive index of the core glass. As a result, all rays within a certain angle will be totally reflected at the core-cladding boundary.  Rays striking the boundary at angles grater than the critical angle will be partially reflected and partially transmitted out through the boundary. After many such bounces the energy in these rays will be lost from the fiber.

The paths along which the rays (modes) of this step index fiber travel differ, depending on their angles relative to the axis. As a result, the different modes in a pulse will arrive at the far end of the fiber at different times, resulting in pulse spreading which limits the bit-rate of a digital signal which can be transmitted.

The maximum number of modes (N) depends on the core diameter (d), wavelength and numerical aperture (NA)

STEP INDEX MULTIMODE FIBER

This types of fiber results in considerable model dispersion, which results the fiber’s band width.

(2) GRADED INDEX MULTI-MODE FIBER

This fiber is called graded index because there are many changes in the refractive index with larger values towards the center. As light travels faster in a lower index of refraction. So, the farther the light is from the center axis, the grater is its speed. Each layer of the core refracts the light. Instead of being sharply reflected as it is in a step index fiber, the light is now bent or continuously refracted in an almost sinusoidal pattern. Those rays that follow the longest path by travelling near the outside of the core, have a faster average velocity. The light travelling near the center of the core, has the slowest average velocity.

As a result all rays tend to reach the end of the fiber at the same time. That causes the end travel time of different rays to be nearly equal, even though they travel different paths.

The graded index reduces model dispersing to 1ns/km or less.

Graded index fibers have core diameter of 50, 62.5 or 85 mm and a cladding diameter of 125 mm. The fiber is used in applications requiring a wide bandwidth a low model dispersion. The number of modes in the fiber is about half that of step index fiber having the same diameter  & NA.

GRADED INDEX MULTI-MODE FIBER

(3) SINGLE MODE FIBER

Another way to reduce model dispersion is to reduce the core’s diameter, until the fiber only propagates one mode efficiently. The single mode fiber has an exceedingly small core diameter of only 5 to 10 m m. Standard cladding diameter is 125 mm. Since this fiber carries only one mode, model dispersion does not exists. Single mode fibers easily have a potential bandwidth of 50to 100GHz-km.

The core diameter is so small that the splicing technique and measuring technique are more difficult. High sources must have very narrow spectral width and they must be very small and bright in order to permit efficient coupling into the very small core dia of these fibers.

One advantage of single mode fiber is that once they are installed, the system’s capacity can be increased as newer, higher capacity transmission system becomes available. This capability saves the high cost of installing a new transmission medium to obtain increased performance and allows cost effective increases from low capacity system to higher capacity system.

As the wavelength is increased the fiber carries fewer and fewer modes until only one remains. Single mode operation begins when the wavelength approaches the core diameter. At 1300 nm, the fiber permits only one mode, it becomes a single mode fiber.

As optical energy in a single mode fiber travels in the cladding as well as in the core, therefore the cladding must be a more efficient carrier of energy. In a multimode fiber cladding modes are not desirable, a cladding with in efficient transmission characteristic can be tolerated. The diameter of the light appearing at the end of the single mode fiber is larger than the core diameter, because some of the optical energy of the mode travels in the cladding. Mode field diameter is the term used to define this diameter of optical energy.

OPTICAL FIBER PARAMETERS

Optical fiber systems have the following parameters.

(I) Wavelength.

(II) Frequency.

(III) Window.

(IV) Attenuation.

(V) Dispersion.

(VI) Bandwidth.

WAVELENGTH

It is a characterstic of light that is emitted from the light source and is measures in nanometers (nm). In the visible spectrum, wavelength can be described as the colour of the light.

For example, Red Light has longer wavelength than Blue Light, Typical wavelength for fiber use are 850nm, 1300nm and 1550nm all of which are invisible.

FREQUENCY

It is number of pulse per second emitted from a light source. Frequency is measured in units of hertz (Hz). In terms of optical pulse 1Hz = 1 pulse/ sec.

Wavelength frequency window

WINDOW

A narrow window is defined as the range of wavelengths at which a fiber best operates. Typical windows are given below :

Window Operational Wavelength
800nm – 900nm 850nm
1250nm – 1350nm 1300nm
1500nm – 1600nm 1550nm

 

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Fiber Optics in Brief

What is the Fiber Optics Communication System?

Fiber Optics:

Optical Fiber is new medium, in which information (voice, Data or Video) is transmitted through a glass or plastic fiber, in the form of light, following the transmission sequence give below :

(1) Information is encoded into electrical signals.

(2) Electrical signals are converted into light signals.

(3) Light travels down the fiber.

(4) A detector changes the light signals into electrical signals.

(5) Electrical signals are decoded into information.

ADVANTAGES OF FIBER OPTICS :

Fiber Optics has the following advantages :

(I) Optical Fibers are non conductive  (Dielectrics)

–  Grounding and surge suppression not required.

– Cables can be all dielectric.

(II) Electromagnetic Immunity :

– Immune to electromagnetic interference (EMI)

– No radiated energy.

– Unauthorised tapping difficult.

(III) Large Bandwidth (> 5.0 GHz for 1 km length)

– Future upgradability.

– Maximum utilization of cable right of way.

– One time cable installation costs.

(IV) Low Loss (5 dB/km to < 0.25 dB/km typical)

–  Loss is low and same at all operating speeds within the fiber’s specified bandwidth long, unrepeated links (>70km is operation).

(v) Small, Light weight cables.

–  Easy installation and Handling.

– Efficient use of space.

(vi) Available in Long lengths (> 12 kms)

–   Less splice points.

(vii) Security

– Extremely difficult to tap a fiber as it does not radiate energy that can be received by a nearby antenna.

– Highly secure transmission medium.

(viii) Security – Being a dielectric

–  It cannot cause fire.

–  Does not carry electricity.

–  Can be run through hazardous areas.

(ix) Universal medium

–  Serve all communication needs.

–  Non-obsolescence.

APPLICATION OF FIBER OPTICS IN COMMUNICATIONS :

–  Common carrier nationwide networks.

–  Telephone Inter-office Trunk lines.

–  Customer premise communication networks.

–  Undersea cables.

–  High EMI areas (Power lines, Rails, Roads).

–   Factory communication/ Automation.

–   Control systems.

–   Expensive environments.

– High lightening areas.

–   Military applications.

–   Classified (secure) communications.

 Transmission Sequence :

(1) Information is Encoded into Electrical Signals.

(2) Electrical Signals are Coverted into light Signals.

(3) Light Travels Down the Fiber.

(4) A Detector Changes the Light Signals into Electrical Signals.

(5) Electrical Signals are Decoded into Information.

–  Inexpensive light sources available.

–  Repeater spacing increases along with operating speeds because low loss fibers are used at high data rates.

Fiber Optics

Principle of Operation – Theory

  • Total Internal Reflection – The Reflection that Occurs when a Ligh Ray Travelling in One Material Hits a Different Material and Reflects Back into the Original Material without any Loss of Light.

Fiber Optics Principle of Operation

THEORY AND PRINCIPLE OF FIBER OPTICS

Speed of light is actually the velocity of electromagnetic energy in vacuum such as space. Light travels at slower velocities in other materials such as glass. Light travelling from one material to another changes speed, which results in light changing its direction of travel. This deflection of light is called Refraction.

The amount that a ray of light passing from a lower refractive index to a higher one is bent towards the normal. But light going from a higher index to a lower one refracting away from the normal, as shown in the figures.

As the angle of incidence increases, the angle of refraction approaches 90o  to the normal. The angle of incidence that yields an angle of refraction of 90o is the critical angle. If the angle of incidence increases amore than the critical angle, the light is totally reflected back into the first material so that it does not enter the second material. The angle of incidence and reflection are equal and it is called Total Internal Reflection.

PRINCIPLE OF FIBER OPTICS

PROPAGATION OF LIGHT THROUGH FIBER

The optical fiber has two concentric layers called the core and the cladding. The inner core is the light carrying part. The surrounding cladding provides the difference refractive index that allows total internal reflection of light through the core. The index of the cladding is less than 1%, lower than that of the core. Typical values for example are a core refractive index of 1.47 and a cladding index of 1.46. Fiber manufacturers control this difference to obtain desired optical fiber characteristics.

Most fibers have an additional coating around the cladding. This buffer coating is a shock absorber and has no optical properties affecting the propagation of light within the fiber.

Figure shows the idea of light travelling through a fiber. Light injected into the fiber and striking core to cladding interface at grater than the critical angle, reflects back into core, since the angle of incidence and reflection are equal, the reflected light will again be reflected. The light will continue zigzagging down the length of the fiber.

Light striking the interface at less than the critical angle passes into the cladding, where it is lost over distance. The cladding is usually inefficient as a light carrier, and light in the cladding becomes attenuated fairly. Propagation of light through fiber is governed by the indices of the core and cladding by Snell’s law.

Such total internal reflection forms the basis of light propagation through a optical fiber. This analysis consider only meridional rays- those that pass through the fiber axis each time, they are reflected. Other rays called Skew rays travel down the fiber without passing through the axis. The path of a skew ray is typically helical wrapping around and around the central axis. Fortunately skew rays are ignored in most fiber optics analysis.

The specific characteristics of light propagation through a fiber depends on many factors, including

– The size of the fiber.

– The composition of the fiber.

– The light injected into the fiber.

Total internal reflection in an optical fiber

FIBER GEOMETRY

An Optical fiber consists of a core of optically transparent material usually silica or borosilicate glass surrounded by a cladding of the same material but a slightly lower refractive index.

Fiber themselves have exceedingly small diameters. Figure shows cross section of the core and cladding diameters of commonly used fibers. The diameters of the core and cladding are as follows.

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Analog and Digital Communications

What is the analog and digital communications? Draw the basic block diagram of transmitter and
receivers.

Answer: Analog and digital communication

Analog communication :

Analog communication is that types of communication in which the transmitted message or information signal is analog in nature. This means that in analog communication the modulating signal i.e. base‐band signal is an analog signal. This analog message signal may be obtained from sources such as speech, video shooting etc.

Digital communication :

In digital communication, the message signal to be transmitted is digital in nature. This means that digital communication involves the transmission of information in digital form. The immunity to channel noise and external interference is better than analog communication.

Block diagram of transmitter and receiver :

block diagram of transmitter and receiver

  1. Radio transmitter : A radio transmitter consists of several elements that work together to generate radio waves that contain useful information such as audio, video, or digital data.

(i)  Power supply : It provides the necessary electrical power to operate the transmitter.

(ii) Oscillator : It creates alternating current at the frequency on which the transmitter will transmit. The oscillator usually generates a sine wave, which is referred to as a carrier wave.

(iii) Modulator : It adds useful information to the carrier wave. There are two main ways to add this information. The first, called amplitude modulation or AM, makes slight increases or decreases to the amplitude of the carrier wave. The second, called frequency modulation or FM, makes slight increases or decreases the frequency of the carrier wave.

(iv) Amplifier : It amplifies the modulated carrier wave to increase its power. The more powerful the amplifier, the more powerful the broadcast.

(v) Antenna : It converts the amplified signal to radio waves.

  1. Radio receiver : A radio receiver is the opposite of a radio transmitter. It uses an antenna to capture radio waves, processes those waves to extract only those waves that are vibrating at the desired frequency, extracts the audio signals that were added to those waves, amplifies the signals.
  •  Antenna : It captures the radio waves. Typically, the antenna is simply a length of wire. When this wire is exposed to radio waves, the waves induce a very small alternating current in the antenna.
  •  RF amplifier : A sensitive amplifier that amplifies the very weak radio frequency (RF) signal from the antenna so that the signal can be processed by the tuner.
  •  Tuner : A circuit that can extract signals of a particular frequency from a mix of signals of different frequencies. On its own, the antenna captures radio waves of all frequencies and sends them to the RF amplifier, which dutifully amplifies them all.
  •  Detector : It is responsible for separating the audio information from the carrier wave. For AM signals, this can be done with a diode that just rectifies the alternating current signal.
  •  Audio amplifier : This componentʹs task is to amplify the weak signal that comes from the detector so that it can be heard. This can be done using a simple transistor amplifier circuit.
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Nuclear Fission and Nuclear Fusion

Explain Nuclear Fission and Nuclear Fusion.

Answer:

Nuclear Fission and Nuclear Fusion:

Isotopes: are atoms that have same atomic number (number of protons) but different mass numbers (numbers of neutrons) or both elements having the same atomic number.

isotopesare isotopes (17 Protons and 18, 20 Neutrons)

Isobars: are atoms that have same mass numbers (numbers of neutrons) but different atomic number (number of protons) or both elements having the same atomic mass.

isobars are isobars (40 Neutrons and 19, 20 Protons)

Nuclear fission: –

In nuclear fission process, the atom splits generating large amounts of energy; For example uranium (U-235) absorbs a neutron, fission occurs as it splits into two particles of uneven mass (smaller nuclei), huge amount of energy (~200Mev) with several neutrons. A chain reaction is produced as fission continues and the neutrons emitted bombard more uranium (U-235) nuclei. Experimentally, we find that spontaneous fission reactions occur for only the very heaviest nuclides those with mass numbers of 230 or more. Even when they do occur, these reactions are often very slow. The half-life for the spontaneous fission of U-238 is 1016 years. In these process isotopes of elements having atomic numbers greater than 80 are capable of undergoing fission and large amount of energy is released that an atomic explosion occurs. The Nuclear power plants operate under this principle.

A diagram of uranium-235 undergoing nuclear fission is shown below.

Nuclear fission

Nuclear Fusion:

The nuclear fusion is reverse process of nuclear fission; i.e the nuclear fusion is the joining of two lighter nuclei and forms a heavier one. Mass is lost and a huge amount of energy is released (even energy amount is grater than the energy released in nuclear fission). Solar energy is a form of fusion energy. When two isotopes of hydrogen combine at very high temperatures, fusion occurs. The fusion of four protons to form a helium nucleus, two positrons (and two neutrinos) generates 24.7 MeV of energy. Nuclear Fission and Nuclear Fusion

Nuclear Fusion

Most of the energy radiated from the surface of the sun is produced by the fusion of protons to form helium atoms within its core.

Ex: hydrogen bomb

Properties of Nuclear Fission and Nuclear Fusion

Properties of Nuclear Fission

  1. In the nuclear fission the splitting of a heavy nucleus into lighter nuclei.
  2. It is a chain reaction process.
  3. There is High temperature is not essential (Even Takes place at room temperature).
  4. Fission process is a controlled process and energy released can be used for peaceful purposes.
  5. It produced the large number of radioisotopes and large amount of nuclear waste.

Properties of Nuclear Fusion

  1. In nuclear fusion two or more lighter nuclei combine to form a heavy nucleus.
  2. The nuclear fusion not a chain reaction process.
  3. It required high temperature (106 oC).
  4. Fusion process is very difficult to be carried out in a controlled manner.
  5. In this process there is no nuclear waste is left.
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 Semi Empirical Mass Formula

 What is the Semi Empirical Mass Formula?

Answer:

Semi Empirical Mass Formula

V. Weizsäcker proposed an equation in terms of atomic mass of the nuclide and series of binding energy term in 1935. This equation is known as Semi Empirical Mass Formula or V. Weizsäcker formula. This equation contains five factors that contribute to the binding energy of nuclei i.e. Surface energy term, Volume energy term, Coulomb Energy term, Asymmetry energy term and Pairing energy term. The volume term, being directly proportional to the number of nucleons, illustrates the idea that each nucleon only interacts with its nearest neighbors and binds to the nucleus at a specific binding energy.

So the mass of atom is defined by: Semi Empirical Mass Formula

ZMA = (Z*Mp + N*Mn) – (Binding Energy in terms of mass unit)

Z = Number of protons; Mp = Mass of proton; N = Number of neutron; Mn = Mass of neutron; ZMA =Atomic mass of nuclide

Semi Empirical Mass Formula

Total Binding Energy = Surface energy term Esa + Volume energy term EVol + Coulomb Energy term Ecoul + Asymmetry energy term Easy + Pairing energy term EPa.

ZMA = (Z*Mp + N*Mn) – (Esa  + EVol  + Ecoul + Easy + EPa )

  • Surface energy: A nucleon at the surface of a nucleus interacts with fewer other nucleons than one in the interior of the nucleus and hence its binding energy is less.This surface energy term takes that into account and is therefore negative and is proportional to the surface area.

Esa = -As* A2/3

As = Propostional constant A = total number of nucleons

  •  Volume energy: When an assembly of nucleons of the same size is packed together into the smallest volume, each interior nucleon has a certain number of other nucleons in contact with it. So, this nuclear energy is proportional to the volume.

EVol = Av*A   ; Av = constant.

  • CoulombEnergy: The electric repulsion between each pair of protons in a nucleus contributes toward decreasing its binding energy.

Ecoul = -AcZ (Z-1) / A1/3   ; Ac = Constant, Z = Number of protons.

  • Asymmetry energy:Energy associated with the Pauli Exclusion Principle. Were it not for the Coulomb energy, the most stable form of nuclear matter would have the same number of neutrons as protons, since unequal numbers of neutrons and protons imply filling higher energy levels for one type of particle, while leaving lower energy levels vacant for the other type.

Easy = -Aas (A-2Z)2 / A ;       Aas = Constant.

  • Pairing energy: An energy which is a correction term that arises from the tendency of proton pairs and neutron pairs to occur. An even number of particles is more stable than an odd number.

So the Semi Empirical mass formula is

ZMA = (Z*Mp + N*Mn) – (-As* A2/3 + Av*A -AcZ (Z-1) / A1/3 – Aas (A-2Z)2 / A + Epa)

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Nuclear Liquid Drop Model

What is the liquid drop model? Write Comparison and difference between Nucleus and Liquid Drop.

Answer:

Liquid Drop Model

The liquid drop model is one of the models which explain the different properties of the nucleus. It was first proposed by George Gamow and then developed by Niels Bohr and J. A. Wheeler. According to Bohr et al the nucleus as a drop of incompressible nuclear fluid and fluid is made of protons and neutrons (nucleons), which are held together by the strong nuclear force. This is a crude model that does not explain all the properties of the nucleus, but does explain the spherical shape of most nuclei. It also helps to predict the nuclear binding energy and to assess how much is available for consumption.

Liquid Drop Model

The liquid drop model, developed from the observation of similar properties between a nucleus and a drop of incompressible fluid, helps explain nuclear phenomena such as the energetic of nuclear fission and the binding energy of nuclear ground levels which cannot be illustrated by the shell model. The idea of considering the nucleus as a liquid drop originally carne from considerations about its saturation properties and from the fact that the nucleus has a very low compressibility and a well defined surface. One reason for this big difference as compared with an ordinary Liquid is that the nucleons obey Fermi statistics and a nucleus is thus a quantum fluid.

Comparison between Nucleus and Liquid Drop:

  • The potential barrier acts on the surface of the nucleus similarly surface tension acts on the surface of a liquid drop.
  • Nuclear and Liquid drop both are in spherical shape.
  • The short range force acts on both cases.
  • The near constant binding energy per nucleon may be compared to the constant heat of evaporation of a liquid.
  • Two small liquid drops combine to make a bag drop and a big drop breaks into small drop similar behavior shows in case of nuclear fusion and fission of nuclei.

Difference between Nucleus and Liquid Drop:

  • There is positive charge (proton) present in the nucleus but not found in liquid drop.
  • Nucleus contains two components like proton and neutron but not in liquid drop.
  • The number of nucleons in the heaviest stable nucleus is much smaller than the molecules in an average liquid drop.
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Nuclear Force

What is the Nuclear Force?

Answer:

Nuclear Force:

An absorption or emission of nuclear energy occurs in nuclear reactions or radioactive decay; those that absorb energy are called endothermic reactions and those that release energy are exothermic reactions. The best-known classes of exothermic nuclear change process are fission and fusion.

The strong forces of attraction (Nuclear binding Energy) which firmly hold the nucleons in the nucleus are known as nuclear forces. Nuclear interactions or strong forces or Nuclear forces are the forces that act between two or more nucleons. They bind nucleons (protons and neutrons) into atomic nuclei. The nuclear force is powerfully attractive force between nucleons at distances of about 1 fm (fm = femtometer = 1.0 × 10−15 metres) between their centers, but rapidly decreases to insignificance at distances beyond about 2.5 fm. At distances less than 0.7 fm, the nuclear force becomes repulsive force. This repulsive component is responsible for the physical size of nuclei, since the nucleons can come no closer than the force allows. The nuclear force is about 10 millions times stronger than the chemical binding that holds atoms together in molecules. This is the reason why nuclear reactors produce about a million times more energy per kilogram fuel as compared to chemical fuel like oil or coal.

Nuclear forces

Properties of Nuclear forces:

Nuclear forces are extremely strong.

Nuclear forces are attractive in character.

Nuclear forces are short range forces.

Nuclear forces do not depend upon the charge on the nucleons.

Nuclear forces are non-central forces.

Nuclear forces do not increase with the increase in the number of nucleons.

Nuclear forces are dependent on the spin of the nuclei.