MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI - 621213 DEPARTMENT : ECE SUBJECT NAME : OPTICAL COMMUNICATION & NETWORKS SUBJECT CODE : EC 2402 UNIT II: TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS PART -A (2 Marks) 1. What is intra model dispersion?[auc NOV 2007] Intra model dispersion is pulse spreading that occurs with a single mode. The spreading arises from finite spectral emission width of an optical fiber. This phenomenon is also called as group velocity dispersion. 2. What is group delay? [AUC MAY 2009] In an optical fiber there are various modes present. Then the optical input, which is propagated along the fiber, will travel in various modes. Because of these modes the velocity of the signal will vary also there may be a delay in the optical signal of these various modes. This is called as the Group Delay. 3. Define dispersion flattening? The reduction of fiber dispersion by spreading the dispersion minimum out over a widen range.this approach is known asd dispersion flattering.. 4. Write a note on scattering losses. Scattering losses in glass arise from microscopic variation in the material density from compositional fluctuation and from structural in homogeneities or defects occurring during fiber manufacture 5. What is Rayleigh scattering? [AUC MAY 2012] The index variation causes a Rayleigh type of scattering of light. Rayleigh scattering in glass in the same phenomenon that scatters light from sun in the atmosphere, giving rise to blue sky. 6. Define microscopic bending? Fiber losses occur due to small bending arise while the fiber is inserted into a cable.
7. Define direct band gap materials and indirect band gap materials. In direct band gap materials direct transition is possible from valence band to conduction band.e.g.gaas,inp,ingaas In indirect band gap materials direct transition is not possible from valence band to conduction.e.g.silicon,germanium. 8. Define attenuation co-efficient of a fiber [AUC NOV 2011] Signal attenuation with in optical fibers can be defined as for a particular optical wavelength as the ratio of the input optical power P i into a fiber to the output Optical power P o from the fiber. Signal attenuation in number of decibel (db) = 10log 10 P i / P o 9. Define dispersion [AUC NOV 2013] Dispersion of the transmitted optical signal causes distortion for both digital and analog transmission along optical fibers. 10. What is wave guide dispersion? [AUC April 2004,MAY2010] Wave guide dispersion which occurs because of a single mode fiber confines only about 80% of optical power to the core. Dispersion this arises since 20% of light propagates in cladding travels faster than the light confined to the core. Amount of wave guide dispersion depends on fiber design. 11. What is the reason for chromatic dispersion?[auc MAY2012] 1. It arises due to variation of the refractive index of core material as a function of wavelength. 2. This causes a wavelength dependence of the group velocity of any given mode. 12. Define mechanical splice. [AUC MAY 2013] Mechanical spice alignment can be achieved by various methods like tube splices or V grooves. 13. Define 3 db coupler. [AUC MAY 2011] A 3 db coupler can be used to distribute an input signal equally among two output ports if the coupling length. L is adjusted such that half of the power from each input appears at each output. 14. Fiber couplers. [AUC NOV 2012] An optical fiber coupler is a device that distributes light from a main fiber into one or more branch fibers. to combine and split optical signals in an optical network a directional coupler is used.
PART (B) 1. List the requirements that be satisfied by materials used to manufacture optical fiber? [AUC NOV 2010] ANS: Fiber Materials Most of the fibers are made up of glass consisting of either Silica (SiO2) or.silicate. Highloss glass fibers are used for short-transmission distances and low-loss glass fibers are used for long distance applications. Plastic fibers are less used because of their higher attenuation than glass fibers. Glass Fibers The glass fibers are made from oxides. The most common oxide is silica whose refractive index is 1.458_at 850 nm. To get different index fibers, the dopants such as GeO2, P2O5 are added to silica. GeO2 and P2O3 increase the refractive index whereas fluorine or B203 decreases the refractive index. Few fiber compositions are given below as follows, (i) GeO2 SiO2 Core: SiO2 Cladding (ii) P2Q5 SiO2, Core; SiO2 Cladding The principle raw material for silica is sand. The glass composed of pure silica is referred to as silica glass, nitrous silica or fused silica. Some desirable properties of silica are, (i) Resistance to deformation at temperature as high as 1000 C. (ii) High resistance to breakage from thermal shock. (iii) Good chemical durability. (iv) High transparency in both the visible and infrared regions. Basic Requirements and Considerations in Fiber Fabrication (i) Optical fibers should have maximum reproducibility. (ii) Fibers should be fabricated with good stable transmission characteristics i.e., the fiber should have invariable transmission characteristics in long lengths. (iii) Different size, refractive index and refractive index profile, operating wavelengths material. Fiber must be available to meet different system applications. (iv) The fibers must be flexible to convert into practical cables without any degradation of their characteristics. (v) Fibers must be fabricated in such a way that a joining (splicing) of the fiber should not affect its transmission characteristics and the fibers may be terminated or connected together with less practical difficulties.
2. What are the basic attenuation mechanisms in the optical fiber communication? Explain in brief on what factors this mechanism depends? [AUC MAY 2011] Ans: Attenuation When a decrease in light power occurs during light propagation along an optical fiber then such a phenomenon is called attenuation. The major causes for attenuation in fiber optic communications are, 1. Bending loss 2. Scattering loss 3. Absorption loss 1. Bending Loss Bending loss is further classified into, (i) Macro bending loss-and (ii) Micro bending loss. (i) Macro bending Loss The light travels in fiber due to occurrence of total internal reflection inside the fiber at the interface of core and cladding. However the light beam forms a critical angle with the fiber's central axis at the fiber face. When the fiber is bend and the light beam travelling through fiber strikes at the boundary o f core at an angle greater than critical angle then the beam fails to achieve total internal reflection. Hence this beam is lost through the cladding. Get scattered and due to this total internal reflection is not achieved hence, the beam is lost through the cladding.
Absorptions Loss Whenever a beam of light photon having energy equal to energy band gap then the light photon is absorbed by the material resulting in absorption loss. Absorption loss occur due to presence of anions OH~ in silica fibers and due to metallic ions like Iron (Fe), Chromium (Cr) and Nickel (Ni). The absorption loss peak is observed in the region of 2700 nm and 4200 nm wavelength with low-loss at 7200 nm, 9500 nm and 13800 nm wavelength windows. 3.Explain in detail about signal distortion and attenuation in optical fiber? [AUC NOV 2009] Ans: Signal Distortion in Optical Fibers One of the important property of optical fiber is signal attenuation. It is also known as fiber loss or signal loss. The signal attenuation of fiber determines the maximum distance between transmitter and receiver. The attenuation also determines the number of repeaters required, maintaining repeater is a costly affair. Another important property of optical fiber is distortion mechanism. As the signal pulse travels along the fiber length it becomes broader. After sufficient length the broad pulses starts overlapping with adjacent pulses. This creates error in the receiver. Hence the distortion limits the information carrying capacity of fiber Attenuation Attenuation is a measure of decay of signal strength or loss of light power that occurs as light pulses propagate through the length of the fiber. In optical fibers the attenuation is mainly caused by two physical factors absorption and scattering losses. Absorption is because of fiber material and scattering due to structural imperfections within the fiber. Nearly 90% of total attenuation is caused by Rayleigh scattering only. Micro bending of optical fiber also contributes to the attenuation of signal. Attenuation Units: As attenuation leads to a loss of power along the fiber, the output power is significantly less than the coupled power. Let the coupled optical power is P (0) i.e. at origin (z = 0) Then the power at distance z is given by
This parameter is known as fiber loss or fiber attenuation. Attenuation is also a function of wavelength. Optical fiber wavelength as a function of wavelength is shown in fig. 4. Explain the following [AUC MAY 2012] (i) Mode field diameter (ii) Modal Birefringence Mode field diameter: Mode field diameter is\a primary parameter of single-mode fibers. It is obtained from the mode field distribution of the fundamental mode. The figure shows, the distribution of light in a single mode fiber.
(ii) Modal Birefringence The propagation of two approximately degenerate modes with orthogonal polarizations is allowed in single mode fibers with nominal circular symmetry about the core axis. Thus, these are referred as bimodal supported and modes. Here, the super scripts x and y denotes the principle axes and are calculated using the symmetry elements of the fiber cross section. The difference in the effective refractive indices and phase velocities for these orthogonally polarized modes makes the fiber to function as a birefringent medium. The independency of fiber cross section with the fiber length in the z-direction yields the expression for modal birefringence BF as, Where, βx = Propagation constant for the mode x βy= Propagation constant for the mode 'y' λ = Optical wavelength. The difference in phase velocities is responsible for linear retardation Φ (z) exhibited by the fiber. The expression for linear retardation is given by, Φ (z) = (βx βy) L Where, L = Length of the fiber. If the coherence time of the source is greater than the delay between the two transit times then only, the phase coherence of the two mode components is achieved. However, the expression for coherence time of the source is given by, tc = 1/ δf Where, δf = Uncorrelated source frequency width Then, the length of fiber over which birefringent coherence is maintained is given by
Where, c = Velocity of light in vacuum δλ = Source line width 5. Commonly available single mode fiber have beat length in the range 10cm<LP<2m.What rate of refractive index difference does this corresponds to for λ =1300nm? Ans: Give that For a single mode fiber, Beat length LP = 10cm to 2cm Operating wavelength λ = 1300nm
6. A 10 km length of fiber is 100 μw and the average output power is 25 (J.W. Calculate, (i) The signal attenuation in db through the fiber. It is assumed that there are no connectors or splices (ii) Signal attenuation per km of the fiber (iii) Overall signal attenuation for the 11 km optical link using the same fiber with 3 splices, each having an attenuation of 0.8 db (iv) Numerical value of the ratio between input and output power. Ans: Given that L = 10Km P input =100μm P output = 25μm
7. A graded index fiber with a parabolic refractive index profile core has a refractive index at the core axis of 1.5 and a relative index difference of 1%.Estimate the maximum possible core diameter which allows single mode operation at a wave length of 1.3μm? Ans: Given that, For a graded index fiber with parabolic refractive index profile, Refractive index of core is n1=1.5. Relative index difference, = 1% = 0.01 Operating wave length, λ =1.3μm Maximum possible core diameter = 2a =?
8. Explain about fiber optic connectors and types of connectors in detail? [AUC NOV 2011] Ans: Fiber Optic Connectors: Connectors are mechanisms or techniques used to join an optical fiber to another fiber or to a fiber optic component. Different connectors with different characteristics, advantages and disadvantages and performance parameters are available. Suitable connector is chosen as per the requirement and cost. Various fiber optic connectors from different manufacturers are available SMA 906, ST, Biconic, FC, D4, HMS-10, SC, FDDI, ESCON, EC/RACE, Principles of good connector design 1. Low coupling loss. 2. Inter-changeability.
3. Ease of assembly. 4. Low environmental sensitivity. 5. Low cost. 6. Reliable operation. 7. Ease of connection. Connector Types Connectors use variety of techniques for coupling such as screw on, bayonetmount, push-pull configurations, butt joint and expanded beam fiber connectors. Butt Joint Connectors Fiber is epoxies into precision hole and ferrules arc used for each fiber. The fibers are secured in a precision alignment sleeve. Butt joints are used for single mode as well as for multimode fiber systems. Two commonly used butt-joint alignment designs are: 1. Straight-Sleeve. 2. Tapered-Sleeve/Bi conical. In straight sleeve mechanism, the length of the sleeve and guided ferrules determines the end separation of two fibers. Fig. 3.1 shows straight sleeve alignment mechanism of fiber optic connectors. In tapered sleeve or bi conical connector mechanism, a tapered sleeve is used to accommodate tapered ferrules. The fiber end separations are determined by sleeve length and guide rings. Fig. 3.2 shows tapered sleeve fiber connectors
9. Write a short notes on dispersion shifted fiber and dispersion compensating fiber? [AUC NOV 2009] Ans: Dispersion Shifted Fiber Single mode fibers which are designed to offer simultaneously zero dispersion and minimum attenuation at λ = 1.55μm is called dispersion shifted fibers. The dispersion classifications of various fibers are shown in figure 8.1, which depicts the shifting of zero dispersion wavelength from λ = 1.33 um to λ= 155 mm. This can be achieved by changing the fiber parameters, namely, the refractive index dispersion shifted fiber.
For example, by reducing the fiber core diameter from 8-10μm to 4.5μm and increasing the refractive index difference between core and cladding from 0.003 to greater than 0.01 yields zero dispersion wavelength shifted from 1.33μm to 1.55μm. This may lead to substantial excess loss. Triangular core profile also yields dispersion shifted fibers and moreover it solves the above excess loss problem. So, for better results we have to modify the triangular profile. These Figure 8.2 shows that the convex index profile also gives the dispersion shifted fiber. Dispersion shifted fibers have the advantage of increased guiding strength, increase in the cut-off wavelength of second order mode and better resistance to bending losses. Such dispersion shifted fibers have been produced by BTRL and others and are now commercially available from any glass company. Table (1) compares the characteristics of triangular refractive index profile dispersion shifted fiber with that of simple step index fiber.
Dispersion Compensating Fiber The process of dispersion compensation and the fiber loop is referred as dispersion compensating fiber. A large base of dispersion shifted fiber has been installed throughout the world for use in the single wavelength transmission systems. For these kinds of links the complexity arises from Four Wave Mixing (FWM), when one attempt to upgrade them with high speed dense WDM technology in which the channel spacing is less than 100 GHz and the bit rates are in excess of 2.5 Gb/s. By using the passive dispersion compensation technique we can reduce the effect of FWM (four wave mixing). This consists of inserting into the link a loop of fiber having a dispersion characteristic that negates the accumulated dispersion of the transmission fiber. This process is known as dispersion compensation. If the transmission fiber has a low positive dispersion, the dispersion compensating fiber will have a large negative dispersion. By using this technique, the total accumulated dispersion will become zero after some distance, but the absolute dispersion per length is non-zero at all points along the fiber. Figure 8.3 depicts the Dispersion Compensating Fiber (DCF) which can be inserted at either the starting (or) the end of an installed fiber span between two optical amplifiers. A third option is to have DCF (Dispersion Compensating Fiber) at both ends. In In pre-compensation schemes, the DCF is located just after the optical amplifier and just before the transmission fiber. Where as in post compensation schemes, the DCF is located just after the transmission fiber and just before the optical amplifier. Figure 8.3 also depicts the plots of accumulated dispersion and optical power level as functions of distance along the fiber.