UNIT II OPTICAL FIBRE LASERS CHAPTERA Optical Fibre

UNIT II OPTICAL FIBRE & LASERS CHAPTER-A Optical Fibre

What is Fiber Optic Technology? Also called Lightwave Technology Fiber Optic Technology uses light as the primary medium to carry information. The light often is guided through optical fibers. Most applications use invisible (infrared) light; LEDs or LDs

Why Fiber Optic Technology? During past three decades, remarkable and dramatic changes took place in the electronic communication industry. A phenomenal increase in voice, data and video communication demands for larger capacity and more economical communication systems. Lightwave Technology : Technological route for achieving this goal Most cost-effective way to move huge amounts of information (voice, video, data) quickly and reliably.

Why Optical Transmission ? Capacity ! and More Capacity ! A technical revolution in Communication Industry to explore for large capacity, high quality and economical systems for communication at Global level. Radio-waves and Trrestrial Microwave systems have long since reached their capacity Satellite Communication Systems can provide, at best, only a temporary relief to the ever-increasing global demand. extremely high initial cost of launching geometry of suitable orbits, available microwave frequency allocations and if needed repair is nearly impossible Next option: Optical Communication Systems !

Optical Region

Potential of Optical Transmission ? The information carrying capacity of a communications system is directly proportional to its bandwidth; Wider the bandwidth, greater the information carrying capacity. Theoretically; BW is 10% of the carrier frequency Signal Carrier VHF Radio system; 100 MHz. Microwave system; 6 GHz Lightwave system; 106 GHz Bandwidth 10 MHz 0. 6 GHz. 105 GHz. A system with light as carriers has an Excessive bandwidth (more than 100, 000 times than achieved with microwave frequencies) Meet the today’s communication needs or that of the foreseeable future

Fiber Optics Timeline 1951: Light transmission through bundles of fibers- flexible fibrescope used in medical field. 1957 : First fiber-optic endoscope tested on a patient. 1960 : Invention of Laser (development, T Maiman) 1966: Charles Kao et al; proposed cladded fiber cables with lower losses as a communication medium. 1970: (Corning Glass, NY) developed fibers with losses below 20 d. B/km. 1972: First Semiconductor diode laser was developed 1977: GT&E in Los Angeles and AT&T in Chicago send live telephone signals through fiber optics (850 nm, MMF, 6 Mbps, 9 km ) -World’s first FO link 1980 s: 2 nd generation systems; 1300 nm, SM, 0. 5 d. B/km, O-E-O 3 rd generation systems; 1550 nm, SM, 0. 2 d. B/km, EDFA, 5 Gb/s 1990 s : Bell Labs sends 10 Billion bits through 20, 000 km of fibers using a Soliton system & WDM Techniques. 2000 s : NTT, Bell Labs and Fujitsu are able to send Trillion bits per second through single optical fiber Þ All optical networks.

ØIntroduction • Communication is defined as the transfer of information from one point to another. • Main constraints in the communication are transmission fidelity, data rate, distortions, and distance between relay stations. • In order to meet out the demands of telecommunication companies worldwide, optical fibers are used as a dominant transmission system. • This optical communication system consists of hair-thin glass fibers that guide light signals with minimum losses over long distances. • An optical fiber is a cylindrical waveguide system consisting of three regions. The centre is a the core, the middle region is a cladding and the outer region is a protective sheath. • Fibers fabricated with recently developed technology are characterized by extremely low losses (less than 0. 2 d. B/km) as a consequence of which, the distance between two successive repeaters could be as large as 250 km. • Due to the low cost and better response, optical fibers are replacing the traditional copper cables.

ØWhy we use optical fibers? There are many advantages of optical fibers over conducting wires. The main advantages are: (1) Cheaper: • Silicon (Si) is the main component in the manufacturing of optical fibers. • It is one of the most abundant materials on earth. • Due to this the overall cost of optical fiber is lower than that of an equivalent cable used in communication. (2) Not hazardous: • In optical fiber cables there is no chance of sparking and short circuit. • This removes the risk of high damage. (3) Immune to RFI and EMI: • The information is carried by photons in the optical fiber communication. • Due to this signals propagating through fibers suffer less loss and are immune to electromagnetic interference and radio frequency interference.

(4) Small size, light weight, flexible, and strong: • The size of optical fiber is very small. It is of the order of few hundred microns. • Its weight is very less. • Optical fibers are flexible. They can be molded at any place with the help of suitable connectors and splices. • An optical fiber has an outer jacket, which protects it from any outer damage and hence, makes it strong. (5) No crosstalk: • There is no chance of crosstalk in the optical fiber communication because the information propagating through the optical fiber is trapped within the fiber and cannot leak out. .

(6) High information-carrying capacity: • A light source, acting as a carrier wave is capable of carrying far more information than radio waves and microwaves. • It has been observed that the light signals used instead of electric signals in the process of communication can transmit 45 million pulses per second (7) Low loss: • Optical fibers are characterized by extremely low losses (less than 0. 2 d. B/km) as a consequence of which, the distance between two successive repeaters can be as large as 250 km. (8) Higher data-rate transmission: • Optical fiber communication permits the transmission of data over longer distances and at higher data rates than other forms of wired and wireless communications.

FUNDAMENTALS OF OPTICAL FIBERS: What is an optical fiber? ØOptical fibers are dielectric waveguides which are fabricated from glass or plastic and are operated on optical frequencies. Structure of an optical fiber: Ø Optical fibers are normally of cylindrical form. It has three principal sections: (i) Core (ii) Cladding (iii) Jacket Fig 1: An optical fiber waveguide showing core, cladding, and protective jacket

(i) Core • It is the innermost region of the fiber which has specific property of conducting an optical beam. • Core is usually made of glass or plastic. • It is covered with another layer of glass or plastic having slightly different chemical composition known as cladding. (ii) Cladding • It is the region just above the core region of the optical fiber. • It has lower refractive index than the core region. • The optical fiber may have an abrupt boundary between the core and the cladding or there may be a gradual change in the material between the two. (iii) Jacket • The outermost section of the optical fiber is known as jacket. • It is made up of plastic or special kind of polymer and other materials usually opaque in nature. • It protects the core from abrasion, interaction with environment, moisture, absorption, crushing, and other adversities of the terrestrial atmosphere and thus, enhances its tensile strength.

ØPROPAGATION OF LIGHT THROUGH OPTICAL FIBER: • In the optical fiber, the arrangement of core and cladding regions is done in such a way that the core acts like a continuous layer of two parallel mirrors. • The message which has to be sent through fiber is first encoded into a light wave and then fed into the fiber where it is propagated as a result of multiple internal reflections. Fig 2: Propagation of light in an optical fiber • The end at which the light enters the fiber is known as the launching end.

Let n 1 = the refractive index of the core and n 2 = the refractive index of cladding (n 2 < n 1). n 0 = refractive index of outside medium from where the light is launched And θi be the angle made by light with the axis of fiber at launching end and θr be the angle made by the refracted ray with the axis Φ be the angle at which refracted ray strikes the core–cladding interface If Φ > critical angle (θc), then the ray undergoes total internal reflection at the interface. As long as the angle Φ > θc the light remains within the fiber. Applying Snell’s law at the launching face of the fiber, we get (1) Now, the largest value of θi will be at Φ = θc

From the right-angled triangle ABC, we have sin θr = sin (90°– Φ) = cos Φ From Eq. (1), we know that By putting the value of sin θr , we get When Φ = θc , θi = θmax Now, (2) Using Snell’s law at point B or cladding boundary, or, (because for total internal reflection, reflection angle will be 90°)

So, (3) Using the value of cos θc from Eq. (3) in Eq. (2), we get (4) For the conditions when for all values of angle of incidence, total internal reflection will occur. For special condition, when n 0 = 1, the maximum value of angle of incidence (θi) for the ray to be guided is given by (5) In the above expression, θm is known as the acceptance angle of the fiber. Ø Acceptance angle is defined as the maximum angle which incident light makes with the axis of fiber at which the ray is propagated (guided) through the fiber. ØThe light rays contained within the cone having a full angle are accepted and transmitted along the fiber. This cone is known as acceptance cone.

ØFRACTIONAL REFRACTIVE INDEX CHANGE Fractional refractive index change is defined as the ratio of the difference between the refractive indices of the core and the cladding to the refractive index of the core. It is denoted by Δ and is given as (6) The value of Δ is always positive and less than one because n 1> n 2 (always), otherwise the phenomena of total internal reflection will not be fulfilled. Ø WHAT IS NUMERICAL APERTURE? Numerical aperture (NA) is a number, which defines the light acceptance or light propagating capacity of a fiber. It is also known as figure of merit. It is expressed as

Usually n 0 is the refractive index of the air and is given as n 0 = 1. Hence, the value of NA can be given as (7) where n 1 and n 2 are the refractive indices of core and cladding, respectively. Numerical aperture can also be expressed in terms of fractional refractive index change (Δ)as follows: Since the difference in n 1 and is n 2 small, so we can write Hence Δ or, (8)

• The assembly is placed in a furnace capable of heating the crucible contents to a temperature between 80°C and 1200°C. • Clad fiber is drawn directly from the melt through the nozzles in the bases of crucibles. Index grading is achieved by the diffusion of mobile ions across the core–cladding interface within the molten glass. • Graded index fibers produced by this technique are subsequently less dispersive than step index fibers.

ØTYPES OF FIBERS: On the basis of refractive index profile and modes propagated through core, optical fibers are classified mainly into two categories. These are as follows: (i) Step index (SI) optical fiber (ii) Graded index (GI) optical fiber 1. Step Index Optical Fiber: • In step index optical fiber, there is a step discontinuity of the refractive index profile at the core–cladding interface. • The refractive index of these fibers is defined as n(r) = n 1 when r < a (where n 1 > n 2) n 2 when r > a where n 1 and n 2 are the refractive indices of core and cladding respectively, and a is the core radius. Ø SI optical fibers can be further classified into two categories: (i) Single-mode step index (SMSI) optical fiber (ii) Multimode step index (MMSI) optical fiber

(i) Single-Mode Step Index Optical Fiber: • In the SMSI optical fiber the refractive index difference between core and cladding is very small. • Due to this, only a single mode is propagated through the core of fiber (Fig. 16. 4). These fibers are also known as mono mode fibers. • In general, the mono mode optical fiber has a core diameter of 8 mm to 10 mm and is designed for use in the infrared region. • Single-mode optical eliminates the modal dispersion because only single mode can travel through the core. • Hence, the information transmission capacity of a single-mode fiber is much larger than that of a multimode fiber.

• Such fibers are used for communication longer than 200 m, and these are frequently used under sea water. Fig. 4 (a) Structure of a single-mode optical fiber, (b) refractive index profile, (c) input pulse, (d) pulse propagation, and (e) output pulse

(ii) Multimode Step Index Optical Fiber • In general, an MMSI optical fiber has larger core diameter than an SMSI optical fiber. • The diameter of core is about 20 mm– 100 mm and the diameter of cladding is about 100 mm– 200 mm. The standard overall diameter of the MMSI optical fiber is about 125 mm. • In order to achieve the minimum angle for total internal reflection, the difference between the refractive index of core and cladding material is kept relatively large. • The interface between the core and the cladding acts as a cylindrical mirror at which the reflection of the transmitted light takes place. • Due to this structure, there are many paths available for light signals to travel through the fiber.

• Since the refractive index of the core is constant, so all the rays making an angle equal to or greater than critical angle travel with the same velocity in the core. • They take different times to reach the output end of the fiber as their path lengths are different (shown in Fig. 5). • Due to the time difference between the rays arriving at the end of the optical fiber, modal dispersion takes place. • It reduces the information carrying capacity of the fiber. • Hence, Multimode optical fibers are used for short distances (less than 200 m), where high power transmission is needed. Fig. 5 (a) Structural view of MMSI, (b) refractive index profile, (c) input pulse, (d) pulse propagation, and (e) output pulse

2. Graded Index Optical Fiber In graded index optical fiber, refractive index of the core region decreases with the radial distance from the maximum value of n 1 at the starting boundary (inner side) to a constant value n 2 beyond the core radius (a). The index variation may be given as Where,

Ø Multimode Graded index(MMGI) Optical Fiber • In the multimode graded index optical fiber, the index of refraction in the core decreases continuously in a parabolic manner from a maximum value at the centre of the core to a minimum constant value of the core–cladding interface (Fig. 16. 6). • The rays travelling close to the fiber axis have shorter paths in comparison to the rays travelling into the outer regions of the core. • Since, the velocity of light ray is inversely proportional to the refractive index and the axial rays are transmitted through a region of higher refractive index, so they travel with a lower velocity than the extreme rays. • This compensates for the shorter path lengths and reduces the dispersion in the fiber. MMGI optical fibers have the advantage of large core diameters (greater than 30 mm) coupled with the bandwidths suitable for long-distance communication.

Fig. 6 (a) Structural view of MMGI, (b) refractive index profile, (c) input pulse (d) pulse propagation, and (e) output pulse

ØCOMPARISON OF SINGLE-MODE AND MULTIMODE INDEX FIBRES Table 1 Comparison of single-mode and multimode index fibers

ØDIFFERENCES BETWEEN STEP INDEX AND GRADED INDEX FIBERS Table 2 Differences between step index and graded index fibers

Ø FIBER ATTENUATION (LOSSES) • Attenuation is defined as the reduction in the signal strength or power when it is transmitted (or guided) through an optical fiber. • Signal attenuation within an optical fiber is usually expressed in terms of logarithmic unit of the decibel (d. B). • Decibel is defined for a particular optical wavelength as the ratio of the input optical power Pi into a fiber to the output optical power Po from the fiber. Mathematically, it is expressed as (20) • In optical fiber communications, attenuation is usually expressed in decibels per unit length (i. e. , d. B/km) as (21) • Where αd. B is the signal attenuation per unit length in decibel and L is the fiber length.

On the basis of several mechanisms which are responsible for signal attenuation within optical fibers, we can categorize the losses in terms of the following points: (i) Absorption losses (ii) Scattering losses (iii) Bending losses (iv) Dispersion losses 1) Absorption Losses The absorption of light by core and cladding materials of a fiber during wave propagation is the main source of attenuation. Absorption of light is caused by the following three different mechanisms: (i) Atomic imperfection in the glass composition (ii) Intrinsic absorption (iii) Extrinsic absorption

ØAtomic Imperfections v Atomic defects or imperfections in the atomic structure of fiber materials is due to the missing of molecules, high density clusters of atom groups, or oxygen defects in the glass structure. These losses increase up to a significant value if the fiber is exposed to ionizing radiations. v Radiations damage a material by changing its internal structure. Up to what extent a material is damaged depends on the energy of ionizing particles, rays, and radiation flux. v The basic response of a fiber to ionizing radiation is an increase in the attenuation, owing to the creation of atomic defects that absorb optical energy. v The higher the radiation level, the larger the attenuation. Ø Intrinsic Absorption v. It is caused by interaction of the propagating light wave with one or more major components of the glass which is used in the composition of the fiber. v. A pure silicate glass has small intrinsic absorption due to its basic material structure in the near infrared region. However, it does have two major intrinsic absorption mechanisms at optical wavelengths which leave a low intrinsic absorption window over 0. 8 mm– 1. 7 mm wavelength range as shown in Fig. 10.

Fig. 10 The attenuation spectra for the intrinsic loss mechanism in pure Ge. O 2—Si. O 2 glass The absorption due to the strong electronic and molecular transition band is characterized by peak loss in the ultraviolet and diminishing loss as the visible region is approached.

Ø Extrinsic Absorption v It is caused by the presence of minute quantity of metallic ions and the hydroxyl ion from the water dissolved in glass. v In the fabrication of various types of fibers, Ge. O 2, P 2 O 5, B 2 O 3, etc. , are used as dopants in silica to modify its refractive index. v B 2 O 5 produces strong absorption at 3. 2 mm and P 2 O 5 at 3. 8 mm wavelength. However, in both the cases, absorption tails extend below 1. 3 mm v Loss increases considerably when the operating wavelength is beyond 1. 55 mm. v The Hydroxyl ion absorption in optical fibers is due to the presence of trapped hydroxyl ions remaining in water as a contaminant. Hydroxyl ion absorption produces a significant attenuation of discrete wavelength, e. g. , centered at 1. 383 mm. v Impurities such as Fe+3, Cr+2, and copper, present in the glass may create unacceptable losses within the usable portion of the spectrum. However, these impurities can be reduced considerably by using a good refining technique for purifying the raw materials for silica.

2) Scattering Losses v Scattering is another parameter for optical attenuation. Such losses in glass arise due to microscopic variation in material density, random variation in refractive index, and structural in homogeneities or defects occurring during fiber manufacturing. v Depending upon the various factors responsible for scattering losses, we can classify them in the following two types: (i) linear scattering losses and (ii) non-linear scattering losses. (i) Linear scattering losses v Linear scattering mechanisms cause the transfer of some or all of the optical power contained within one propagating mode to be transferred linearly (proportionally to mode power) into a different mode. This process tends to result in attenuation of the transmitted light. v This transfer may be to a leaky or a radiation mode, which does not continue to propagate within the fiber core, but is radiated from the fiber. There is no change in the frequency on scattering. v Linear scattering may be categorized into two major types: (a) Rayleigh and (b) Mie scattering.

(ii) Nonlinear Scattering Losses v It is observed that in optical waveguide the output optical power does not always increase in the proportion of the input power. v Several nonlinear effects occur, which in the case of scattering cause disproportionate attenuation, usually at high optical power levels. This nonlinear scattering depends critically upon the optical power density within the fiber and hence, becomes significant only above threshold power levels. v There are two types of nonlinear scattering: (a) Stimulated Brillouin scattering (SBS): v Stimulated Brillouin scattering may be regarded as the modulation of light through thermal molecular vibrations within the fiber. v In this scattering, the incident photon produces a phonon of acoustic frequency as well as a scattered photon, which produces an optical frequency shift according to the variation in the scattering angle. v If the polarization state of the transmitted light is not maintained, then the threshold power density can be given as; (22) where d is the core diameter, λ is the operating wavelength (both are measured in μm), αd. B is the fiber attenuation in db/km, and ν is the source bandwidth in GHz.

(b) Stimulated Raman scattering (SRS): v Stimulated Raman scattering is similar to stimulated Brillouin scattering except that a high frequency optical phonon rather than an acoustic phonon is generated in the scattering process. v Similar to Brillouin scattering threshold, the threshold optical power for SRS in a long single-mode fiber is given as (23) where PSRS is the threshold optical power for SRS, d is the core diameter, λ is the operating wavelength, and αd. B is the fiber attenuation in decibels.

3) Bending Losses Bending losses occur due to imperfections and deformations present in the fiber structure. Generally, there are two types of bending losses: micro bending losses and macro bending losses. (i) Micro bending losses: v These losses occur when the core surface has small variations in shape. These variations change the angle at which light strikes the core–cladding interface and can cause the light to refract into the cladding rather than reflect into the core. (ii) Macro bending losses: v. Macro bending losses depend on the core radius and the bend radius. v. As the radius of curvature decreases, the loss increases exponentially until at a certain critical radius. v. If the bend radius is made a little bit smaller, then a threshold point may be reached at which losses suddenly become extremely large. But practically, we never reach this critical value. The critical value of radius of curvature Rc is expressed as (16. 24) where all the symbols have their usual meanings.

4)Dispersion Losses v. Dispersion loss is defined as the spreading of light pulse as it travels down along the length of the fiber causing the pulses to overlap and thus making the pulses undetectable at the receiving end. Mainly, there are two types of dispersions: (i) Intramodal dispersion and (ii) Intermodal dispersion. (i)Intramodal Dispersion v. Intramodal or chromatic dispersion occurs in all types of fibers. v. It is further divided in two categories: (a) material dispersion and (b) waveguide dispersion. (a) Material dispersion: v Material dispersion is based on the wavelength of the optical signal and its interaction with the glass of which the fiber is made. v Every laser source has a range of optical wavelengths. The refractive index of silica is different for different wavelengths of wave. v Hence, different spectral components of an optical pulse have different speeds which lead the pulse to spread out in time after travelling some distance in the fiber.

(b) Waveguide dispersion: v Waveguide dispersion can occur for waves propagating through any inhomogeneous structure, whether or not waves are confined to some region. v Waveguide dispersion depends on the refractive index difference between core and cladding. The effective refractive index is very close to the refractive index of the core. v Waveguide dispersion is always positive and is not strongly wavelength-dependent. It depends strongly on the core diameter (increases with decrease in core diameter) and on the fiber distance (increases with distance). (ii) Intermodal Dispersion v. Intermodal dispersion occurs due to the propagation delay differences between the modes propagating in a multimode fiber. v. The higher-order modes travel a longer distance and arrive at the receiver end later than the lower-order modes. Hence, different modes have different group velocities. v. The effect of intermodal dispersion can be reduced by taking the parabolic refractive index profile, usually as it is in the case of a GI optical fiber.

ØATTENUATION CONSTANT v. Attenuation losses in optical fibers are generally measured in terms of decibel. v. To obtain the expression for attenuation constant, let us consider that Pout is the output power at the end of 1 km of optical fiber, which is equal to the input optical power (Pin), reduced by a fraction k (say), i. e. , Similarly, after 2 km of optical fiber, the output power is Hence, after L kilometer of optical fiber, the above expression can be given as Or, Taking log on both sides and then multiplying by 10 gives power loss in decibel as where α = (10 log 10 k) is the attenuation coefficient of the fiber in decibel/kilometer.

Therefore, (31) Sometimes, the number of decibel loss is expressed with negative sign and hence, the above equation can also be given as (32)

ØAPPLICATIONS OF OPTICAL FIBERS Optical fibers have wide range of applications in the field of optical communication, medical science, illumination technology, optical sensor, etc. Some important applications of optical fiber are as follows: (i) Optical fibers are widely used in broadcast television, cable TV, remote monitoring, and surveillance. (ii) Fibers are most commonly used for transmission of digital data. (iii)It is frequently used in military operations such as for secret communications, command control links on ship and aircrafts, data links for satellites, etc. (iv) It is widely used in cable TV network and closed circuit TV (CCTV) systems. (v) Fibers are frequently used in illumination technology. (vi) Fibers have a variety of applications in medical services. (vii) Fibers are frequently used for decorative applications. (viii) A coherent bundle fiber is used, sometimes along with lens, for a long, thin imaging device called endoscope. (ix) Fibers are used to transfer infrared energy from the source to the point of application of heat. (x) Fibers are used to form sensors to measure physical and chemical parameters.

ØAPPLICATIONS OF OPTICAL FIBERS

ØAPPLICATIONS OF OPTICAL FIBERS Fibers are Everywhere

Ø OPTICAL FIBER TRANSMISSION LINK The block diagram of an optical communication system is shown in Fig. 9. The system has: (i) Information source: It is the source of input signals which are to be transmitted through the optical fiber up to the destination. (ii) Electrical transmitter: It is the next part of the communication system where the information signals are produced in the form of electrical signals. (iii) Optical source: Optical source is capable of generating an optical signal at desired frequencies. Basically LASER or LEDs are used as the source of light. Fig. 9 Optical fibre communication system

(iv) Optical fiber cable: The optical signal is launched into the optical fiber which is contained inside the cable. The cable provides mechanical and environmental protection to the hair-thin optical fiber. (v) Optical detector: At the receiving end, photo detector is the main component. It is capable of converting the received modulated wave back to the original signal, which has the same wave shape as the optical wave envelope. (vi) Electrical receiver: It is the part of the communication system where original signal is recovered in its suitable form. Usually, electronic amplifiers and signal restorers consisting of signal processor circuits are used in this section of communication system.

OFC- Systems Firstly installed Systems: operating at 1310 nm Low loss; minimum pulse broadening Transmission rate 2 -10 Gb/s Regeneration of Signal after every 30 -60 km Conversion of O-E-O signal Future OFC Systems: 1550 nm Wavelength band Silica has lowest loss, increased dispersion Design of Dispersion Shifted Fibers Lowest loss and Negligible dispersion Erbium Doped Fiber Amplifier (EDFA) Direct amplification of optical signal Flat gain around 1550 nm low loss window BW » 12, 500 GHz ; Enormous potential