Www.uptunotes.com OPTICAL COMMUNICATION (EEC-701) UNIT-1 Introduction: Block diagram of optical fiber communication system, Advantages of optical fiber communication Optical fiber waveguides: structure of optical wave guide, light propagation in optical fiber using ray theory, acceptance angle, numerical aperture, skew rays, wave theory for optical propagation, modes in a planar and cylindrical guide, mode volume, single ode fibers, cutoff wavelength, mode field diameter, effective refractive index and group and mode delay factor for single mode fiber. Communication may be defined as the transfer of information from one point to another. Within the communication system the information transfer is frequently achieved by superimposing or modulating the information transfer on to an electromagnetic wave Which acts as a cartier for the information signal. This modulated signal is then transmitted to the required destination where itis seceived and the original information signal is obtained by demodulation. The use of visible light to carry the information is called optical communication and the light travels through a optical fiber eable. Optical Fiber Communication: Wavelength — 1,7um to 0.5m Frequeney —+ 10" to 10° Hz Attenuation — 0.2 dB/km 1.1 Block Diagram of Optical Communication System: The optical fiber communication system is similar in basic concepts, the block 1 diagram is chown: ‘ Optical Optical fiber The block diagram represents the information seurce [7] cable souree provides an electrical signal to a transmitter comprising an electrical signal to a transmitter comprising an electrical signal whieh erives an optical source to give modulation of light wave camrier. The optieal sources (LED LASER) which provide the optical conversion used to convey the light travels through fiber cable in a particular manner. At the receiver end a optical detector exists, it can be a PIN or APD. photodiode, it converts light energy 10 elaetcical signal. Electrical receiver receives the signal and converts it into a mescage format. This is the working of a optical fiber communication system, Gectrial Destination] Optical fiber communication system i ' ' ' It I t 1 I ' ' 1 1 Leswar L.l.d Advantages: Optical fiber commmmnication offers a qumber of advantages over other communication systems because it has very low attenuation: a) Enormous potential bandwidth: The optical fiber communication offers frequency in the range of 10" ro 10" Fiz which yields a far greater potential transmission bandwidth, At present, the bandwidth available to fiber system is not fully utilized but modulation over three hnadred kilometer without repeaters is possible bb) Small size and Weight: Fiber cables have very small diameter, just li protection still they are very light and small diameter. €) Electrical Isolation: Optical fibers are fabricated from glass or sometimes a plastic polymer, they are electrical insulators and they da not exhibit earth loop. 4) Immunity to interface and cross talk Optical fiber form a dielectric waveguide and are therefore free from electromagnetic interference (EMD), radio frequency interference (RFD or switching transients electromagnetic pulses. €) Signal Security: The light from optical fiber does not radiate significantly and therefore they provide a high degree of signal Low transmission loss: The attenuation in optical fiber cable is very low (around 0.2 4B/km) as compare to other communication channels. 2) Ruggedness and flexibility: Optical fibers are manufactured with very b radii or twisted without damage hair, rather than when it cover with jackets for tensile strength. The fiber may bent to quite smallWww.uptunotes.com bh) System reliability and ease of maintenance: It reduces the requirement of intermediate repeaters or live amplifiers to boost the transmitted signal strength. The reliability is high due to predicted life time of 20 years to 30 years. 1.1.2 Disadvantages: 4)” Optical cables can be handle with a skilled hand, the perfect joining of eable is mast important otherwise signal lost in betw the cable. 'b) The detection of faulty area is very hard because these cable situated under Mud. c) The whole establishment of optical eables is very 4) Bending loss occurs, so cable must be properly aligned. 1.2 Optical Fiber Waveguide: The light travels in cable, which have a wansparent core with a refiactive index 1, surrounded by wansparent cladding of slightly lower reizactive index np. The cladding supports the waveguide structure and reducing the radiation loss into the surrounding air ‘Refractive Index = ratio of speed of lisht in a vacuum to that in matter (speed v) sally the valve of light in a vacuum of refractive indexes of nel, for ai 1171.33, for water for glass 42, for diamond 1.24 Ray theory transmission: 1.2.1.1 Reflection and refraction: When a light ray encounters a boundary separation of two different media, either the ray reflected back into the first medium: i's called Reflection, or it will beat owards second medium called Refraction Refraction affects the refractive index of the medium, The relationship atthe interface is known as Snell's Law. ny sini= of m1: 605 i=; 605 r w 1.2.1.2 Total Intermal Reflection: As mis areater than m;. the angle of refiaction is always greater than the augle of incidence Thus when angle of refraction is 90° and seftacted emerges parallel to axis, the angle is called critical angle. The critical angle is given by sin .= Ataugles of incidence greater thaa the critical angle the liht is reflect back ino the originating dielectric medivin tat is called total intemal Reflection. This isthe mechanisn by which light at a suficieat shallow angle (Less than 90° - 4.) may be cousidered to propagite down an optical fiber with low lossWww.uptunotes.com AV. A. Cladding:n2 Core:n 1.2.1.3 Numerical Aperture and Acceptance Angle: As per the diagram # meridional ray A enters at the critical angle e within the fiber at the core cladding interface. The ray enters the fiber core at an angle @, to the fiber axis and refracted at the alr — core interface at critical angle, Hence any ray which are incident into the fiber core at an angle greater than ®, will be transmitted to the core cladding interface at an angle less than $, and will not be totally reflected, This 0, is called ‘Acceptance Angle’. Quy west CABIN) ‘Numerical Aperture’ (VA) is relationship been acceptance angle and refractive indexes. Fig shows a light ray incident on the fiber core at an angle @a to the fiber axis which is less than the acceptance angle forthe fiber. The ray enters to the fiber from medium (air) of refractive index aumber. AS snell’s Law rosin 8a ~n: sin ® wo ‘Skew ray Consider the tight angle tangle, thea -@ e) 33 where § is greater than the eritieal angle at core clad interface. nisin a =n, 60s @ ? nysin 6 = (Ls? g)!? ose ‘When the Limiting case for TIR is considered, $ becomes equal to critical angle, so sin Ge= mo! Ta so the limiting case wll be . rosin 8a = (nV? «) This equation serves as a basic forthe definition of Numerical Apertrure, so NA no sin Ga ~ On? = m2? The NA may also be given in the terms of relative reffactive index difference between core and claddiWww.uptunotes.com Henee, NA=m 20)? 1.2.1.4 Skew Rays: Skew rays are the rays which does not follow the fiber axis. These says are not easy to visualize, only the direction ean be predicted in helical path of direction change of 2y at each reflection. y isthe angle between the projection of the ay in the two dimension and the radii ofthe fiber core atthe point of reflection When the light input to the fiber is non uniform, rays will therefore tend to have a smoothing effect on the distribution of light as i is transmitted, giving more information output. NA in case of skew rays, NA= ny sim 0as cos 7 1.2.2 Optical Fiber Modes: ‘Modes: Ina planar guide, TE(E, = 0) and TM(H,=0) modes are obtained within the dielectric oylindér. Thus to integers, | and mare necessary in order to specify the modes, the single integer (m) required for the planar guide, for cylindrical waveguide we refer TE y-and TM modes, ‘Modes in Fiber: There are two fiber modes exists. Fist is a). Single mode Fiber b) Multi mode Fiber The optical fiber is a dielectric waveguide that operates at optical frequencies. The fiber waveguide i normally cylindrical in form. Single mode fiber sustains only one mode of propagation, whereas multimode fibers eontain many hundreds of modes. The diameter of core of SMF is comparatively very small from MMF. A disadvantage of MMfis that they suffer from intermodal dispersion but it ean be reduced, SINGLE MODE FIBER ‘MULTIMODE FIBER 1.23 Mode Theory for Cireular Waveguide: Ln Optical fibers, the core cladding boundary conditions lead to a coupling berween the electric and nmgoeti eld componea, This gives diet hybrid modes, which means optical wavegvide analysis is tore complex than metallic waveguide analysis. Fibers afeeonstrcted so tint the difference in the core and cladding indexes of reffaction is very sul ie"ny-9y 2" The field components ave called lneslypolasized (LP) modes sad labeled as LP = where j & mate integers designing made solitons Figure shows a clectie id distibution for several ofthe lower order guided modes in asymmetrical slab waveguide \ 1 | xponentiat LesaWww.uptunotes.com The core of this waveguide is a dielectric slab of index 1 that is sandwiched between two dielectric layers which have refractive indexes n: < m. Fig shows the field patterns of several of the lower order transverse electric (TE) modes. The order of a mode is equal to the ray congruence or same corresponding to this mode makes with the plane of the waveguide. The plot shows that the electzc fields of the guided modes are not completely confined to the central dielectic slab. The field varies harmonically in the guiding region of the refractive index 1; and decay exponentially outside of the region. For low order modes the fields are tightly concentrated near the center of the slab, will little penetration into the cladding region. On the other hand, for higher order modes the fields ate distributed more towards the edges of the guide and penetrate faster into the cladding ‘Mode Couplings As the core and cladding modes propagates along the fiber order core modes. This coupling occurs because the electri fields of the gu but expend partially into the claddi node coupling occurs between the cladding and higher ied core modes are not completely confined to the core of modes in a fiber with cut off conditions: INA; ideally its V2 405 total optical power Pass avg optical power residing in eladding 1.24 Step Index Fiber and Graded Index Fiber: 1.2.4.1 Step Index Fiber: The optical fiber with a core of constant reffaetive index v, and a cladding ofa slightly lower refractive index mp is known as step index fiber. This is because the refractive index profile for this type of fiber makes a step change at the core cladding interface, The refractive index profile, mi, Pea (Core) 1 =F FB lading) @ ) The figure shows 2 multimode step index fiber (a) and a single mode index fiber (b). The core diameter of SMF is around 2 to 10 uu. The modes in step index fiber is, 1.2.4.2 Graded Index Fiber: Graded index fiber do not have a constant refractive index in the core but a decreasing core index ‘(2 with radial distance from a maximum value of 7; at the axis toa constant value n, beyond the core radius a in the cladding. The seffactiva index profile, my (A= 2a(r/a"4?, —cladding 2) PsOs - SiO: — core, SiO; ~ cladding 3) SiO; ~ core, B.0;- SiO; ~ cladding ‘Active Glass Fiber: Some glass material (atomic no $7.71) resulting new optical and magnetic properties. These new properties allow the material to perform amplification, attenuation and phase retardation on light passing through it. Doping can be carried out for silica, tellucte and halide glasses. 1.2.9.2 Plastic Optical Fibers: For high speed services and high bandwidth, eraded index polymer (plasties) optical fiber [OF] designed. The core of these fibers is either polymethacrylate or a perfiuronsted polymer. These polymers are referted to as PMMA. POF and PEPOF. They offer greater optical signal actenuations than a glass fiber. They are tough and durable, COMPARISON BETWEEN PMMA & PF POLYMER OPTICAL FIBER: ‘CHARACTERISTICS | PMMA POF ‘Core Diameter 04 mm ‘Cladding Diameter 1.0 mm’ ‘Numerical Aperture 0.25 ‘Attenuation (0 dBikan at 650 am "<40,dBkm at 650 am ‘Bandwidth 2.5 Ghis over 200 m 25Gb over 530 am 1.2.9.3 Photonic Crystal Fibers: Photonic crystal fibers are basically hollow from center, so it is also called holey fiber initially. The difference between PCF structure aud that of a conventional fiber is that the cladding and air hole in core. The ait bole runs along the entire length of the fiber. The size and spacing of the holes in the microstructure and the refractive index of its constituent material determine the light guiding characteristics of PCF. The basic PCFs are index guiding PCF ane the photonic band gap fiber fa) Index Guiding PCF: This fiber has a solid core that is surrounded by a cladding region which contains air holes running along the length of the fiber. The hole has a diameter d and pitches A. The core and cladding material are same but the air gap hhas lower refractive index of each cladding. E.2. S;Q2 is cladding having 1.45 refractive index but air has refractive index 1 But practically, core cau be made of pute silica. It gives more advantages like low losses, ability to wausmit high optical power levels and many more by Photonic Band gap fiber: The structure of index guiding PCF and photonic band gap fiber are same. The fiber has a hollow core that is surrounded by a cladding region which contains air holes running along the fiber length ©) But the functional principle is analogous to the role of @ periodic crystalline lattice in a semiconductor, which blocks electrons from occupying a band gap region. The hollow core acts as a defect in the photonic band gap structure, which creates a region in which the light can propagate 1.2.10 Fiber Fabrication: The basic techniques for fabrication of al elass optical waveguide are 1.2.10.1 Outside Vapor Phase Oxidation: Ia this method, a layer of SiO; particles called “soot” is deposited from a burner into a rotating graphite or ceramie mandrel. The glass soot adheres 1o this bait rod and layer by layer porous glass preform is built up. ByWww.uptunotes.com property controlling the constituents of the metal halide vapor stream during the deposition process, the glass compositions and dimensions desired for the core and cladding can be incorporated into the perform, When the deposition process is completed, the nandrel is removed and the porous tube is then vitrified in a dry atmosphere at a high temperature (above 1400") to a clear perform and it is mounted in a fiber — drawing tower and made into a fiber une Bait rod (Mandeel) Soot preform | } Glass particles wd 1.2.10.2 Vapor Phase Axial Deposition (VAD): It is nearly like OVPO method. In this method, SiO: particles are formed in the same way. As these particles emerge fiom the torches, they are deposited onto the end of surface of a silica glass rod which acts as a seed. A porous perform is grown in the axial direction by moving the rod upward. When it moves upward, itis transformed into @ solid, transparent rod perform by zone melting with the carbon ring heater. ‘Any fiber, step index or graded index. can be made by this VAD method. Advantages: 1) The preform has no ceatral hole. 2) The preform can be fabricated in continuous lengths whieh can effeet process costs and produet yields, 3) The deposition chamber and zone melting ring heater are tightly connected to each other in the same enclosure allows the clean environment. 1.2.10.3 Modified Chemical Vapor Deposition (MCVD): The MCVD was widely adopted to produce very low loss graded index fibers. The glass vapor particles arising from the reaction of the constimient metal halide gases and oxygen flow through the inside of a revolving silica ube. As SiO; particles are deposited, they are sintered to a clear glass layer by a oxy hydrogen torch which travels back and forth along the tube, When the desired thickness of glass has been deposited, the vapor flow is shut off and the tube is heated strongly to cause it to collapse into a solid rod perform. 1.2.10.4 Plasma Activated Chemical Vapor Deposition: In PCVD, a non isothermal microwave plasma operating at low pressure initiates the chemical reaction. With the silica tube held at temperatures in the range of 1000 ~ 1200°C to reduce mechanical, stresses in the growing glass films, a moving microwave resonator operating at 2.45 GHz generates plasma inside the tube to activate the chemical reaction. This process deposits clear glass material directly oa the tube wall, there is no soot formation. alice tive deposited glass layer | | | | &_ Plasma — ae radio-frequency coil translation