Fiber Optics and Laser Instrumentation Lecture PDF - Indian Institute of Technology

Fiber Optics and Laser Instrumentation (ECD 409) Dr. Swati Rajput Assistant Professor Department of Electronics Engineering Indian Institute of Technology (Indian School of Mines) Dhanbad Fiber Optics and Laser I...

Fiber Optics and Laser Instrumentation (ECD 409) Dr. Swati Rajput Assistant Professor Department of Electronics Engineering Indian Institute of Technology (Indian School of Mines) Dhanbad Fiber Optics and Laser Instrumentation: Course Goals and Key Takeaways Course Objective To present an introduction to Fiber optics and laser instrumentation. It emphasizes on understanding of the basic knowledge, how fiber optic will be used for communication as well as sensing applications. It will also give an idea how Laser will be used in instrumentation and measurement to meet the demand of industry. Learning Outcomes Upon successful completion of this course, students will: Have a broad understanding of optical fiber as a transmission line as well as a sensor. Have a high-level understanding of different types of fiber optics sensor. Be able to design fiber optic sense network to measure the different type of physical parameter. Be able to know the different application of Laser in the field of instrumentation and measurement which will be helpful to full fill the requirement of industry, medicine, and society. 03-02-2025 2  Fiber Optics and Laser Instrumentation : Key Topics at a Glance Unit No. Topics to be Covered Lecture Hours Learning Outcome 1. Introduction: Introduction to optical fibers; Overview of an optical fiber 07 This will help student to understand the basic of optical communication; Transmission characteristics of optical fiber. fiber and its application in high speed communication system. 2. Optical Fiber Sensors: Intrinsic, extrinsic, and interferometric fiber optic 10 This unit will help student in understanding how optical sensors for the measurement of strain, temperature, pressure, fiber can be used as modern sensor which has advantage displacement, velocity, acceleration, acoustic sensors, sensors for over conventional electronic/electrical sensor. measurement of magnetic field and current, humidity, pH, rotation, gyroscope. 3. Optical Sensors for Remote Detection: Magnetic and electric field 04 This unit will help student to design fiber optics sensor measurements based on –Intensity, Phase, Polarization, Frequency, system to measure and control some physical as well as Wavelength modulation. electrical parameter from remote distance. 4. Optical Devices and Equipment: Optical source and detector, Optical 07 This unit will help student in understanding the Time Domain Reflectometer (OTDR), Optical Spectrum Analyzer, Optical construction working and operation of basic optical Power Meter. devices and equipment used in instrumentation and measurement. 5. Laser Instrumentation: Applications of Laser for the measurement of 07 This will help student how the optical components like distance, velocity, acceleration, current and voltage. Medical applications laser can be used in commercial application. of lasers. 6. Industrial Applications of Laser: Application of Laser in material 04 This will help student how the optical components like processing and design. laser can be used for industrial applications. Textbook: Reference Books: 1. Senior J.M, “Optical Fiber Communications- 1. Kesier G, “Optical Fiber Communication”, Tata McGraw Hill, New Delhi. Principle and Practice”, 2. John F. Read, “Industrial Applications of Laser”, Academic Press. 03-02-2025 3. Monte Ross, “Laser Applications”, Tata McGraw Hill. 3  Fiber Optics and Laser Instrumentation : Weightage Weightage of Different Components 1. Quiz/ Assignment 10 Marks 2. Mid Semester Examination 30 Marks 3. Quiz/ Assignment 10 Marks 4. End Semester Examination 50 Marks Total Marks = 100 03-02-2025 4  Fiber Optics and Laser Instrumentation Lecture 1: 2nd January 2025 03-02-2025 5  Introduction to Optical Communication History of Communication Communication ➔ Transfer of information from one point to another. When the info is conveyed over any distance a communication system is usually required. Within the communication system the information transfer is frequently achieved by superimposing or modulating the information onto an electromagnetic wave (carrier of the info signal). This modulated carrier is then transmitted to the required destination where its is received and the original information signal is obtained by demodulation. Sophisticated techniques have been developed for this process using electromagnetic carrier waves operating at radio frequencies, microwave and millimetre wave frequencies. 03-02-2025 6  Introduction to Optical Communication History of Optical Fiber Communication Fiber optic communication is the most modern of communication for last 40 to 50 years. Search of wideband medium for communication, scientists explored the optical window➔ Fiber Optic Communication History of Communication➔ Graham Bell’s ➔ First revolution in communication ➔ where the audio signals were converted into electrical signals and were transmitted on electrical wires, and were converted back into the audio form. At that time, the primary objective ➔ carry voice from one point to another by means of electrical medium. As the time progresses, the need for the communication increased ➔ more and more people wanted to communicate from one point to another ➔ Requirement of larger and larger bandwidth. Frequency of operation has consistently increased from the audio frequencies to higher and higher. 03-02-2025 7  Introduction to Optical Communication History of Communication Why there is an increase in the Frequency ?? Initially communication started at low frequency bands i.e., few KHz, and then increased to few MHz ➔ few 100s of MHz ➔ few GHz and so on. Fundamental Quantities to Assure Reliable Communication 1. Signal to Noise Ratio: Ratio of the power of the signal to power of the noise Assuming noise to be additive nature that noise is generated by independent source then if we increase signal power, SNR increases. For a given input signal if the medium has a very low loss then at the receiving end the signal amplitude will be large. Transmission medium should have as low loss as possible. 2. Bandwidth: Maximum amount of data that can be transmitted over a communication channel. Larger number of user can send data then ➔ Larger Bandwidth is required. 03-02-2025 8  Introduction to Optical Communication 𝒇𝒐 𝑩𝒂𝒏𝒅𝒘𝒊𝒅𝒕𝒉 = ➔ Bandwidth ∝ 𝒇𝒐 𝑸 If more and more information needs to be send on the channel, then I require more and more bandwidth. Since the Quality Factor (Q) is more or less independent of frequency, the bandwidth is proportional to the operating frequency. If 𝒇𝒐 increases then the bandwidth will scale proportional to the operating frequency. Communication scenario, In second world war time the most of the communication was essentially based on microwave region. In mid 1950s and 1960s, ➔ demand for bandwidth was still increasing and then the further extension of the waveguide technology was getting more and more difficult to accommodate larger and larger bandwidth. Pushing the technology further, we may get an increase in bandwidth by a factor of 10➔ which can take us forward only may be a decade not beyond that. Could we now look for the other Technology..?? People started looking for another window which was used widely by the Physicist for experimentation and that as Optical Window. Optical frequency can increase the bandwidth by the factor of 1000 to 10000, ➔ Attractive Proposition. Higher bandwidth was the major motivation for exploring the optical communication window. Optical Communication started sometimes in 1960s. Optical frequency is a medium of transmission of info from one point to another. 03-02-2025 9  Introduction to Optical Communication If light is used as the carrier, then do we have a medium to transmit the light with low loss form one point to another ? Do we have sources of light which could carry information from one point to another? Medium around us air the light propagates through that and we feel light propagates very efficiently. Medium light air seems to be very good medium for transporting light. Example: A 100 V bulb on a terrace of a house, you can see that bulb may be from a distance ok km, may be 10 km but not beyond that. Medium which we think that it is reasonably transparent is not really transparent➔ if you want to send the light over a distance of 100s of km. Explore the possibility of finding the medium, which can carry light over the very long distances (100s to 1000s of km). Look for a medium which has as low loss as possible After Air, the next alternative that was explored was glass (appears to be a very transparent medium) Physicists have been using glass for guiding lights and focusing lights in the form of prisms and lenses. Glass seems to be one of the very promising medium for transporting light. However, when the glass was used in the form of the prisms and lenses, the distances over which the light was carrying ➔ short distance (few metres) Glass losses is reasonable low when shorter distance is considered. If glass is taken as the medium and the light propagates through the glass for 1000 Km, would it still be treated as the low loss medium ?? Loss characterization in glass gave the attenuation/ loss of 1000 dB/Km. 10 dB is a reduction in power by a factor of 10 20 dB is a reduction of power by a factor of 100. 30 dB is a reduction of power by a factor of 1000. 03-02-2025 Glass is not a medium for Communication. 10  Introduction to Optical Communication However, scientists realized very soon, that this loss in Glass is not due to the intrinsic nature of the glass, however due to the impurities present in the glass. The impurities present in the glass were not removed for the glass used in the prisms and lenses, because their presence was not harmful as far as the loss concerns in the laboratory experiments. People tried to purify glass to the best possible level. In the first purification process, they could reduce the loss from the glass from 1000 dB/km to 20 dB/km (still a very large loss in today’s scenario) Comparing this loss, with the other alternative available at that time (coaxial cable, waveguides), this loss was comparable. If I take a glass rod, which is purified to a good level, then the light will attenuate by a factor of 100 over a distance of 1 km, and similar attenuation will happen in waveguides. It was very attractive to at least explore the possibility that if this medium glass does not give the superior performance compared to coaxial cables and waveguides as far as attenuation is concerned. At least this glass medium will give the bandwidth which is thousands time more. Optical communication people started exploring, the glass is a medium for sending information over long distances. And later this glass was moulded into the form ➔ Optical Fibers which now can carry the information over a very very long distances. 03-02-2025 11  Introduction to Optical Communication Do we have sources of light which could carry information from one point to another? Can we use light bulb to carry information on that..?? Carrier carries the information when it has some variation in its characteristics like amplitude, frequency, or in time. If the light source whose amplitude or frequency cannot change with respect to the time, ➔ then the light source cannot carry information. So if we need to change the amplitude or frequency of the light, how simple is it to change the light properties. If we have light bulb, and try to switch on and off, we cannot to it a very faster rate. So if we want to have a higher bandwidth or more data rate, then we need to have the sources which can be switched on and off at the very fast rate. Optical sources switching on and off rate depends on the spectral width. White light has a very large spectral width➔ cannot be switched on and off at very fast rate. Switching on and off at a very fats rate ➔ optical sources with a narrow spectral width Sources like Lasers intrinsically have a very narrow spectral width➔ can send information at a very fast rate. Incidentally when the optical fibers were explored at that time, lasers were not invented. However, it happened that the lasers and optical fibers were invented more or less at the same time. By 1960s, we had a low loss medium known as Optical Fibers for carrying information over larger distances and The optical Sources such as LASERS which could carry information at a faster rate. 03-02-2025 Optical Fiber Communication 12  Introduction to Optical Communication ❖ Information Source: Sends an electrical signal to the transmitter. ❖ Transmitter: Has an electrical part that drives an optical source to modulate the lightwave signal. ❖ Optical Source: Changes the electrical signal into an optical (light) signal. This could be a semiconductor laser or a light-emitting diode (LED). ❖ Transmission Medium: The signal travels through an optical fiber cable. ❖ Receiver: Has an optical detector that turns the optical signal back into an electrical signal. This helps to demodulate (decode) the optical signal. ❖ Detection and Conversion: Devices like photodiodes (p-n, p-i-n, or avalanche types) are used to detect the light signal and convert it into an electrical signal. In some cases, phototransistors or photoconductors may also be used. ❖ Electrical Interfacing: Both ends of the optical link need electrical connections. 03-02-2025 13 Signal processing is usually done electrically in current systems.  Fiber Optics and Laser Instrumentation Lecture 2: 3rd January 2025 03-02-2025 14  Comparison between Optical Fiber Comm and Other Technologies Transmission Medium Twisted Pair (Point-to-Pint) ❖ Two conductor system which could carry signal from one point to another. ❖ Medium used for point to point communication (wired communication). ❖ Two wires are twisted ➔ seen in telephone lines. ❖ Supports a very low data rate. ❖ Telephone lines use typically use data rate of 10 Kbps. ❖ Since the structure is completely open structure ➔relatively high electromagnetic interference Electromagnetic Interference (EMI) refers to the disturbance caused by electromagnetic fields that disrupt the normal operation of electrical equipment or communication systems. It occurs when external electromagnetic signals interfere with the functioning of electronic devices, causing degradation in performance or even malfunction. EMI can be caused by various sources, such as: Electrical equipment: Motors, transformers, or power lines. Radiofrequency signals: Broadcasting stations, mobile phones, and Wi-Fi networks. Static discharge: Sparks from electronic devices or lightning. ❖ High Loss as the frequency increases ❖ Twisted pairs are the good medium for carrying the signals which are at low frequency. 03-02-2025 15  Comparison between Optical Fiber Comm and Other Technologies Transmission Medium Co-axial Cable (Point-to-Pint) ❖ For high frequency, co-axial cable is more useful. ❖ Medium used for point to point communication (wired communication). ❖ Signal is connected between the centre conductor and outer shell. ❖ Energy is confined between the region of the outer shell and the inner rod. ❖ Does not have the exposure to the electromagnetic fields ➔ low electromagnetic interference. ❖ Co-axial cables is used for connections in Local Area Networks. ❖ Data rate is relatively higher of the order Mbps. ❖ Moderately low loss. ❖ Further going to higher frequency side, we have microwave link ❖ Point to point communication but this is a wireless communication. Microwave Link (Point-to-Pint) ❖ Signal is transmitted by using a highly directional microwave antenna. ❖ Received on the other side by another antenna. ❖ Without laying the actual cables, the signals can be transmitted over the long distances. ❖ Situation where laying of the cable is difficult, the microwave link is a very attractive proposition. ❖ Frequency is in the microwave range then the bandwidth is larger (few 100s of MHz) compared to co-axial cable. ❖ Line of sight communication, ➔ high loss due to the free space propagation ❖ Larger and larger distance the signal attenuates very rapidly. ❖ Because of the curvature of the earth, the transmitting and receiving antennas have to be mounted on very high towers. 03-02-2025 16  Comparison between Optical Fiber Comm and Other Technologies Satellite Communication (Point-to-Multi-Point) ❖ Large bandwidth and wireless mode ➔ Point to multipoint communication. ❖ Broadcasting applications ➔ without laying cables. ❖ Signals are transmitted from the earth station to the satellite ➔ satellite reflects those signals back in different frequency➔ signals are received on the earth by different receiving antennas. ❖ Frequencies used here are typically microwave frequencies, ➔ large bandwidth (GHz) ❖ Signal scattered by the satellite is received by the transmitting station itself ➔ data monitoring capability ❖ Large delay ➔ geostationary satellite (located at a very high altitude) ➔ because of the travel ❖ Mobile environment (cables laid or microwave towers installed) ➔ no mobility ❖ Moderate lifetime Comparing the transmission medium, the two technologies can compete with each other: 1. Satellite Communication Technology (Bandwidth in GHz) 2. Fiber Optic Technology (Bandwidth in THz) 03-02-2025 17  Satellite Vs. Fiber Optics Satellite Fiber Optics Point to Multi Point Point to Point (cabled medium) Bandwidth ~ GHz Bandwidth ~ THz Maintenance Free Needs Maintenance (link, status of fiber, peripheral equipment Short Life : 7 to 8 Years Long Life No upgradeability Upgradeable Mobile, Air, Sea On Ground Only Both the technologies could be of competing nature but are also complementary to each other. Combination of both is the best combination for transmission of a very high quality 03-02-2025 information over very, very long distance. 18  Optical Fiber Technology : History of Attenuation for Glass ❖ Initially when glass was tested in early 1970s ➔ Attenuation (dB/km) with respect to wavelength ➔ low attenuation region around 800 nm. ❖ During this testing, the Lasers based on GaAs were also invented simultaneously which were capable of emitting light at 800 nm. ❖ Combination of GaAs based Laser and Low Loss Region of Glass ➔ Made Optical Communication possible in 1970s. ➔ Referred to as the First Window of Optical Communication. ❖ Loss at 800 nm is 3 dB/km. ❖ As the time progressed, the glass was purified further and further ➔ Loss profile modified. ❖ No window that has the minimum around 800 nm. ❖ Now the window of low loss has been shifted to 1300 nm or 1550 nm. ❖ Between 1300 nm an d1550 nm, there is a region where the attenuation is very large. ❖ In 1980’s, there was 2 windows available for communication i.e., around 1300 nm and 1550 nm. ❖ By the time the material technology improved significantly, and thus we can identify which semiconductor material can emit the light in this low loss windows. ❖ 1300 nm has very high data rates i.e., large bandwidth. ❖ 1550 nm has a loss of 0.2 dB/km whereas the 300 nm window has a loss of 0.4 dB/km. 03-02-2025 19  Optical Fiber Technology : History of Attenuation for Glass 03-02-2025 20  Advantages of Optical Fiber Communications Enormous Potential Bandwidth ❖ The optical carrier frequency in the range 1013 to1016 Hz (generally in the near infrared around 1014 Hz or 105 GHz) yields a far greater potential transmission bandwidth than metallic cable systems (i.e. coaxial cable bandwidth ypically around 20 MHz over distances up to a maximum of 10 km) or even millimeter wave radio systems (i.e. systems currently operating with modulation bandwidths of 700 MHz over a few hundreds of meters). Small Size and Weight ❖ Optical fibers have very small diameters which are often no greater than the diameter of a human hair. Hence, even when such fibers are covered with protective coatings they are far smaller and much lighter than corresponding copper cables. Electrical Isolation ❖ Optical fibers which are fabricated from glass, or plastic polymer, are electrical insulators and therefore, unlike their metallic counterparts, they do not exhibit earth loop and interface problems. Furthermore, this property makes optical fiber transmission ideally suited for communication in electrically hazardous environments as the fibers create no arcing or spark hazard at abrasions or short circuits. Immunity to interference and crosstalk ❖ Optical fibers form a dielectric waveguide and are therefore free from electromagnetic interference (EMI), radio-frequency interference (RFI), or switching transients giving electromagnetic pulses (EMPs). ❖ Operation of an optical fiber communication system is unaffected by transmission through an electrically noisy environment and the fiber cable requires no shielding from EMI. ❖ The fiber cable is also not susceptible to lightning strikes if used overhead rather than underground. Moreover, it is fairly easy to ensure that there is no optical interference between fibers and hence, unlike communication using electrical conductors, crosstalk is negli-gible, even when many fibers 03-02-2025 21 are cabled together.  Advantages of Optical Fiber Communications Low Transmission Loss ❖ The development of optical fibers over the last 20 years has resulted in the production of optical fiber cables which exhibit very low attenuation or transmission loss in comparison with the best copper conductors. Fibers have been fabricated with losses as low as 0.15 dB/km and this feature has become a major advantage of optical fiber communications. Ruggedness and flexibility ❖ Although protective coatings are essential, optical fibers may be manufactured with very high tensile strengths. Perhaps surprisingly for a glassy substance, the fibers may also be bent to quite small radii or twisted without damage. ❖ Furthermore, cable structures have been developed which have proved flexible, compact and extremely rugged. Taking the size and weight advantage into account, these optical fiber cables are generally superior in terms of storage, transportation, handling and installation to corresponding copper cables, while exhibiting at least comparable strength and durability. System reliability and ease of maintenance ❖ Low-loss property of optical fiber cables which reduces the requirement for intermediate repeaters or line amplifiers to boost the transmitted signal strength. ❖ Fewer optical repeaters or amplifiers, system reliability is generally enhanced in comparison with conventional electrical conductor systems. ❖ The reliability of the optical components is no longer a problem with predicted lifetimes of 20 to 30 years being quite common. ❖ Both these factors also tend to reduce maintenance time and costs. 03-02-2025 22  Fiber Optics and Laser Instrumentation Lecture 3: 8th January 2025 03-02-2025 23  Optical Fibers Structure of the Optical Fiber & Propagation of the light inside the Optical Fiber ❖ Optical fiber is a solid glass rod which consist of an inner structure ➔ Core; outer shell➔ Cladding. ❖ Essentially for the propagation of the light➔ core and cladding are the important regions. ❖ To support the structure mechanically, some other layers are present ➔ buffering layers ❖ The cladding supports the waveguide structure while also, when sufficiently thick, substantially reducing the radiation loss into the surrounding air. A transparent core with a refractive index n1 surrounded by a transparent cladding of slightly lower refractive index n2. Ray Theory Transmission: Total Internal Reflection ❖ Refractive Index: Ratio of the velocity of light in a vacuum to the velocity of light in the medium. ❖ Ray of light travels more slowly in an optically dense medium than in one that is less dense. ❖ Ray approaching the interface is propagating in a dielectric of refractive index n1 and is at an angle 𝜙1 to the normal at the surface of the interface. ❖ If the dielectric on the other side of the interface has a refractive index n2 < n1, refraction is such that the ray path in this lower index medium is at an angle 𝜙2 to the normal, where 𝜙2 > 𝜙1. ❖ The angles of incidence 𝜙1 and refraction 𝜙2 are related to each other and to the refractive indices of the dielectrics by Snell’s law of refraction. ❖ Small amount of light is reflected back into the originating dielectric medium (partial internal 03-02-2025 24 reflection).  Optical Fibers Ray Theory Transmission: Total Internal Reflection Snell’s Law of Refraction: 𝒏𝟏 𝐬𝐢𝐧 𝝓𝟏 = 𝒏𝟐 𝐬𝐢𝐧 𝝓𝟐 Or 𝑺𝒊𝒏 𝝓𝟏 𝒏𝟐 = 𝑺𝒊𝒏 𝝓𝟐 𝒏𝟏 ❖ As n1 is greater than n2, the angle of refraction is always greater than the angle of incidence. ❖ When the angle of refraction (𝝓𝟐 ) is 90° and the refracted ray emerges parallel to the interface between the dielectrics, the angle of incidence (𝝓𝟏 ) must be less than 90°. ❖ This is the limiting case of refraction and the angle of incidence is now known as the critical angle 𝝓𝒄 Critical Angle: 𝒏𝟐 𝐬𝐢𝐧 𝝓𝒄 = 𝒏𝟏 ❖ At angles of incidence (𝝓𝟏 ) greater than the critical angle (𝝓𝒄 )the light is reflected back into the originating dielectric medium (total internal reflection) with high efficiency (around 99.9%). ❖ Total internal reflection occurs at the interface between two dielectrics of differing refractive indices when light is incident on the dielectric of lower index from the dielectric of higher index, and the angle of incidence of the ray exceeds the critical value. 03-02-2025 25  Optical Fibers Transmission of a Light Ray in a Perfect Optical Fiber ❖ Total Internal Reflection: Mechanism by which light at a sufficiently shallow angle (less than 90° − 𝜙𝑐 ) may be considered to propagate down an optical fiber with low loss. ❖ Above Figure illustrates the transmission of a light ray in an optical fiber via a series of total internal reflections at the interface of the silica core and the slightly lower refractive index silica cladding. ❖ The ray has an angle of incidence 𝜙 at the interface which is greater than the critical angle and is reflected at the same angle to the normal. ❖ The light ray shown in Figure is known as a meridional ray as it passes through the axis of the fiber core. This type of ray is the simplest to describe and is generally used when illustrating the fundamental transmission properties of optical fibers. ❖ It must also be noted that the light transmission illustrated assumes a perfect fiber, and that any discontinuities or imperfections at the core–cladding interface would probably result in refraction rather than total internal reflection, with the subsequent loss of the light ray into the cladding. 03-02-2025 26  Optical Fibers Acceptance Angle What is the angular zone from which the light is accepted by the optical fiber …?? ❖ Launch a ray from the tip of the optical fiber such that it lies in a plane containing the axis of the optical fiber. ❖ Since only rays with a sufficiently shallow grazing angle (i.e. with an angle to the normal greater than 𝝓𝒄 ) at the core–cladding interface are transmitted by total internal reflection; it is clear that not all rays entering the fiber core will continue to be propagated down its length. ❖ The geometry concerned with launching a light ray into an optical fiber is shown in Figure, which illustrates a meridional ray A at the critical angle 𝝓𝒄 within the fiber at the core–cladding interface. ❖ It may be observed that this ray enters the fiber core at an angle θa to the fiber axis and is refracted at the air–core interface before transmission to the core– cladding interface at the critical angle. ❖ Hence, any rays which are incident into the fiber core at an angle greater than θa will be transmitted to the core–cladding interface at an angle less than 𝝓𝒄 , and will not be totally internally reflected. The incident ray B at an angle greater than θa is refracted into the cladding and eventually lost by radiation. ❖ Thus, for rays to be transmitted by total internal reflection within the fiber core they must be incident on the fiber core within an acceptance cone defined by the conical half angle θa. Hence θa is the maximum angle to the axis at which light may enter the fiber in order to be propagated, and is often referred to as the acceptance angle* for the fiber. 03-02-2025 27  Optical Fibers Numerical Aperture ❖ The numerical aperture (NA) of an optical fiber is a measure of the fiber's ability to collect light and how efficiently it can transmit light through the fiber core. ❖ To continue the ray theory analysis to obtain a relationship between the acceptance angle and the refractive indices of the three media involved, namely the core, cladding and air. This leads to the definition of a more generally used term, the numerical aperture of the fiber. ❖ A light ray incident on the fiber core at an angle θ1 to the fiber axis which is less than the acceptance angle for the fiber θa. The ray enters the fiber from a medium (air) of refractive index n0, and the fiber core has a refractive index n1, which is slightly greater than the cladding refractive index n2. ❖ Assuming the entrance face at the fiber core to be normal to the axis, then considering the refraction at the air–core interface and using Snell’s law given by: 𝒏𝟎 𝐬𝐢𝐧 𝜽𝟏 = 𝒏𝟏 𝐬𝐢𝐧 𝜽𝟐 ……………….(1) ❖ Considering the right-angled triangle ABC: 𝝅 𝝓 = − 𝜽𝟐 ……………………………(2) 𝟐 ❖ where 𝜙 is greater than the critical angle at the core–cladding interface. Hence Equation 1 becomes: 𝝅 𝒏𝟎 𝐬𝐢𝐧 𝜽𝟏 = 𝒏𝟏 𝐬𝐢𝐧 𝜽𝟐 ➔ 𝒏𝒐 𝐬𝐢𝐧 𝜽𝟏 = 𝒏𝟏 𝐬𝐢𝐧 𝟐 − 𝝓 ⇒ 𝒏𝒐 𝐬𝐢𝐧 𝜽𝟏 = 𝒏𝟏 𝐜𝐨𝐬 𝝓 ……………………………………………………………………………..(3) ❖ Using the trigonometrical relationship 𝑆𝑖𝑛2 𝜙 + 𝐶𝑜𝑠 2 𝜙 = 1; Equation 3can be written as: 03-02-2025 𝒏𝒐 𝐬𝐢𝐧 𝜽𝟏 = 𝒏𝟏 (𝟏 − 𝑺𝒊𝒏𝟐 𝝓) …………………………………(4) 28  Optical Fibers Numerical Aperture ❖ When the limiting case for total internal reflection is considered,𝜙 becomes equal to the critical angle for the core–cladding interface. ❖ Also, in this limiting case θ1 becomes the acceptance angle for the fiber θa. Combining these limiting cases into Eq. (4) gives: NA= 𝒏𝒐 𝐬𝐢𝐧 𝜽𝒂 = 𝒏𝟐𝟏 − 𝒏𝟐𝟐 Light collection efficiency ➔ Numerical Aperture of the optical fiber.➔ The Numerical Aperture (NA) of an optical fiber is a measure of the fiber's ability to collect light and how effectively it can transmit that light through its core. It determines the range of angles at which the fiber can accept and transmit light. A higher NA means the fiber can accept light from a wider range of angles, which typically improves the fiber's light-gathering capacity. ❖ For larger Numerical Aperture / Light Collection Efficiency; 𝒏𝟐𝟏 − 𝒏𝟐𝟐 should be as large as possible. ❖ For the transmission of the light the material is glass (core) with a refractive index of n1 ❖ So, to have the higher NA; we cannot change n1 (core refractive index) ➔ so we have to reduce the refractive index of the cladding n2 as lower value as possible. ❖ But n2 to be always greater than or equal to 1. 03-02-2025 29 ❖ If n2 = 1; then we get the maximum collection efficiency.  Fiber Optics and Laser Instrumentation Lecture 4: 9th January 2025 03-02-2025 30  Optical Fibers Numerical Aperture ❖ If n2 = 1; it means removal of the cladding. ❖ As far as the numerical aperture is concerned, cladding is undesirable feature. ❖ In a first look; it appears that although the optical fiber structure is consisting of core and cladding ➔ the presence of the cladding reduces the Numerical aperture. Light launching is one of the aspects of the Optical Fiber➔ Was our prime goal was to put the light inside the optical fiber …?? If we have a light source and you can put that light efficiently inside the optical fiber ➔ but it doesn’t carry any information then that propagation is not of any use to communication. ❖ The numerical aperture may also be given in terms of the relative refractive index difference ∆ = 𝑛1 − 𝑛2 and assumes that this difference is very less. 𝑛 −𝑛 ❖ The fractional index difference can be given as : ∆= 1𝑛 2 1 ❖ 𝑁𝐴 = 𝑛1 (2∆)1Τ2 03-02-2025 31  Optical Fibers Numerical Aperture Problem 1: A silica optical fiber with a core diameter large enough to be considered by ray theory analysis has a core refractive index of 1.50 and a cladding refractive index of 1.47. Determine: (a) the critical angle at the core–cladding interface; (b) the NA for the fiber; (c) the acceptance angle in air for the fiber. Answer: (a) 78.5 Degree; (b) 0.30; (c) 17.4 Degree Skew Rays ❖ Another category of ray which is transmitted without passing through the fiber axis. ❖ These rays, which greatly outnumber the meridional rays, follow a helical path through the fiber, and are called Skew Rays. ❖ The helical path traced through the fiber gives a change in direction of 2γ at each reflection, where γ is the angle between the projection of the ray in two dimensions and the radius of the fiber core at the point of reflection. ❖ Hence, unlike meridional rays, the point of emergence of skew rays from the fiber in air will depend upon the number of reflections 03-02-2025 32  Optical Fibers Numerical Aperture Problem 2: An optical fiber in air has an NA of 0.4. Compare the acceptance angle for meridional rays with that for skew rays which change direction by 100° at each reflection. Answer: 𝜽𝒂 = 𝟐𝟑. 𝟔 Degree; 𝜽𝒂𝒔 = 𝟑𝟖. 𝟓 Degree Group Delay If we want to send the information on the optical fiber; then the light cannot be of continuous nature, you have to change the parameters of the light. Light which is pulsed ➔ light pulses is sent inside the core. Any ray which is launched within this cone can be totally internally reflected and will get guided. A pulsed signal travels by multiple paths within the NA cone. Ray launched at different angles and that’s why they travelled different distances. Pulse energy is divided into different rays. 𝐿 𝑛1 (𝑛1 − 𝑛2 ) ∆𝑇 = × 𝑐 𝑛2 Group delay governs the time taken by different spectral components of a pulse to propagate 03-02-2025 33  Optical Fibers Problem : A light signal propagates through an optical fiber with the following properties: Path length (L) = 10 cm; Core refractive index (n1​) = 1.48 Calculate the Group Delay (ΔT) and Bandwidth (B) for the following cases: Cladding refractive index n2=1. Cladding refractive index n2=1.2. Cladding refractive index n2=1.47 Compare the bandwidth values for these cases. 03-02-2025 34  Fiber Optics and Laser Instrumentation Lecture 5: 10th January 2025 03-02-2025 35  Optical Fibers Contradictory requirements as far as numerical aperture is considered ➔ difference between n1 and n2 should be as large as possible As far as bandwidth is concerned the difference between n1 and n2 should be as small as possible We should be guided by bandwidth constraint or by the numerical aperture constraint..??? ✓ Depends upon the application for which you are using the optical fiber ✓ If you are using the optical fiber for the sensor kind of application where we want to measure very weak light ➔ then we cannot to lose any light or in other words we must have very high launching efficiency inside the optical fiber ➔ Use the optical fiber which have a very high numerical aperture or the fiber which do not have any cladding. ✓ If you go for the communication ➔ bandwidth is a rather important parameter; launching efficiency of course, is but, bandwidth is of much higher importance➔ for sending the information onto the optical fiber. 03-02-2025 36  Optical Fibers ❖ 𝑛1 − 𝑛2 ~10−3 − 10−4 ; ➔ core and cladding although they are two different regions ➔ the difference in the refractive indices of these two region is extremely small. ❖ The same glass which have the core and cladding; so only inner portion of this glass rod is doped with some material; so, the refractive index of the material increases little bit. Total Internal Reflection ❖ In ray model, it appears there is a solid cone of rays enters the optical fiber and essentials to multiple internal reflection. ❖ Let us now put the phase fronts behind the rays. ❖ A phase front is a surface where all points on the wave have the same phase of oscillation at a given instant. ❖ Rays are the fictitious lines actually the phase fronts moves. 03-02-2025 37  Optical Fibers Total Internal Reflection ❖ Phase fronts are supporting the rays. ❖ The green and red lines tells the phase of the phase fronts. ❖ Let us say green shows the zero and red shows the π ❖ The phase fronts corresponding to the transmitted and reflected ray they overlap (intersect). 0 3π ❖ Whenever a red and green phase fronts intersects ➔ two optical fields which are out of π 2π 2π π phase. 3π ❖ Red line has a phase of odd multiple of π 0 ❖ Green lines has a phase of even multiple of π ❖ Whenever a red and green phase fronts intersects ➔ the light field cancels each other➔ we have a zero intensity. ❖ Whenever a green and green line intersect, we have a constrictive interference (maximum intensity). Similarly for the intersection of the red line. ❖ Intersection of red and green line➔ Destructive interference ➔ zero intensity. ❖ In the medium where the total internal reflection is taking place, light essentially varies from maximum to zero, maximum to zero and so on. ❖ Standing wave kind of behavior of the light intensity pattern. ❖ Inside the core, the intensity distribution of the light will have maxima, minima, and so on. At Total Internal Reflection: 1. Standing wave type of fields in the core. 2. Decaying fields in the cladding. 03-02-2025 3. Sudden phase change of the ray at the core cladding interference. 38  Optical Fibers Phase and Group Velocity ❖ Within all electromagnetic waves, whether plane or otherwise, there are points of constant phase. ❖ As a monochromatic Lightwave propagates along a waveguide in the z direction these points of constant phase travel at a phase velocity υp given by: where ω is the angular frequency of the wave. ❖ However, it is impossible in practice to produce perfectly monochromatic light waves, and light energy is generally composed of a sum of plane wave components of different frequencies. ❖ Often the situation exists were a group of waves with closely similar frequencies propagate so that their resultant forms a packet of waves. ❖ The formation of such a wave packet resulting from the combination of two waves of slightly different frequency propagating together. ❖ This wave packet does not travel at the phase velocity of the individual waves but is observed to move at a group velocity υg given by: ❖ The group velocity is of greatest importance in the study of the transmission characteristics of optical fibers as it relates to the propagation characteristics of observable wave groups or packets of light. 03-02-2025 39  Optical Fibers Phase and Group Velocity ❖ If propagation in an infinite medium of refractive index n1 is considered, then the propagation constant may be written as: 03-02-2025 40  Fiber Optics and Laser Instrumentation Lecture 6: 11th January 2025 03-02-2025 41  Optical Fibers ❖ 2 of the phase fronts is common to Ray 2 ❖ It means that these two-phase fronts should satisfy the phase condition ➔ for sustained constructive interference ❖ So, the distance between the two-phase fronts i.e., s2 should be multiples of 2π ❖ d is the thickness of the core 2𝜋𝑛1 𝑑 sin 𝜃 ❖ + 𝛿 = 𝜋𝑚 ; this condition needs to be satisfied for the 𝜆 light ray to propagate inside the structure. ❖ Phase difference between the two rays plus the phase difference happening at the total internal reflection should be equal to multiples of 2π ❖ 2𝛿 is the phase difference happening at the total internal reflection ❖ From numerical aperture point of view; any ray launched at an angle less than the maximum acceptance angle ➔ would have sustained propagation of light through TIR. ❖ Now we have seen that is not enough; even within the cone of acceptance angle; if the light is launched at an angle which does not satisfy the condition of the phase difference ➔ then the ray cannot propagate inside the fiber. ❖ Now this departure from the continuous theta to the discrete theta ❖ m is an integer which gives you the discrete angles. domain➔ essentially leads to the modes inside the optical fiber. ❖ So, within the acceptance angel cone; the ray launched at very ❖ Discrete patterns of the intensity of the light inside the optical fiber. discrete angles then only there will be the sustained propagation of light. 03-02-2025 42  Optical Fibers Optical Modes A mode refers to a specific light path or electromagnetic field distribution that can propagate through the fiber core. Modes represent the solutions to Maxwell’s equations for light traveling through a waveguide, such as an optical fiber. 1. A mode is a stable propagation pattern of light in the fiber core. 2. Each mode corresponds to a particular field distribution (electric and magnetic fields) across the fiber cross-section. Types of Modes in Optical Fibers: 1. Single Mode: 1. The fiber supports only one mode of light (the fundamental mode). 2. Occurs when the core diameter is very small (typically 8–10 µm for single-mode fibers). 3. Used in long-distance communication. 2. Multimode: 1. The fiber supports multiple modes of light (higher-order modes). 2. Occurs when the core diameter is larger (e.g., 50 µm or 62.5 µm for multimode fibers). 3. Suitable for short-distance communication. 03-02-2025 43  Optical Fibers Step Index and Graded Index Fibers n2 n1 3 (a) Multimode step 2 index fiber. Ray paths 1 n are different so that O rays arrive at different times. n2 (b) Graded index fiber. 3 Ray paths are different 2 but so are the velocities O 1 n along the paths so that O' O'' 2 n1 all the rays arrive at the 3 same time. n2 © 1999 S.O. Kasap,Optoelectronics (Prentice Hall) 03-02-2025 44  Optical Fibers r Buffer tube: d = 1mm Protective polymerinc coating Cladding: d = 125 - 150 m n n1 Core: d = 8 - 10 m n2 The cross section of a typical single-mode fiber with a tight buffer tube. (d = diameter) © 1999 S.O. Kasap,Optoelectronics (Prentice Hall) Only one propagation mode is allowed in a given wavelength. This is achieved by very small core diameter (8-10 µm) SMF offers highest bit rate, most widely used in telecom 03-02-2025 45  Fiber Optics and Laser Instrumentation Lecture 7: 14th January 2025 03-02-2025 46  Optical Fibers Step Index Fibers ❖ The optical fiber considered with a core of constant refractive index n1 and a cladding of a slightly lower refractive index n2 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 may be defined as: 03-02-2025 47  Optical Fibers Normalized Frequency (V-Number) ❖ The normalized frequency may be expressed in terms of the numerical aperture NA and the relative refractive index difference Δ. ❖ The normalized frequency is a dimensionless parameter and hence is also sometimes simply called the V number or value of the fiber. It combines in a very useful manner the information about three important design variables for the fiber: namely, the core radius a, the relative refractive index difference Δ and the operating wavelength λ. ❖ The V number determines the fraction of the optical power in a certain mode which is confined to the fiber core. Significance of the V-number: 1.Single-Mode and Multi-Mode Operation: 1. When V2.405, the fiber operates in multi-mode, meaning multiple modes can propagate. 2.Mode Cutoff: 1. The V-number determines the cutoff condition for higher-order modes. 2. As V increases, more modes are allowed to propagate. 3.Fiber Design: 1. The value of V helps design fibers to achieve single-mode or multi-mode operation depending on the application. ❖ The total number of guided modes or mode volume Ms for a step index fiber is related to the V value for the fiber by the approximate expression: 03-02-2025 48  Optical Fibers Problem 1: A multimode step index fiber with a core diameter of 80 μm and a relative index difference of 1.5% is operating at a wavelength of 0.85 μm. If the core refractive index is 1.48, estimate: (a) the normalized frequency for the fiber; (b) the number of guided modes. Answer: 2873 Graded Index Fibers ❖ Graded index fibers do not have a constant refractive index in the core* but a decreasing core index n(r) with radial distance from a maximum value of n1 at the axis to a constant value n2 beyond the core radius a in the cladding. This index variation may be represented as: where Δ is the relative refractive index difference and α is the profile parameter which gives the characteristic refractive index profile of the fiber core. step index profile when α = ∞, a parabolic profile when α = 2 and a triangular profile when α = 1. 03-02-2025 49  Optical Fibers ❖ The total number of guided modes or mode volume Mg supported by the graded index fiber: Problem 2: A graded index fiber has a core with a parabolic refractive index profile which has a diameter of 50 μm. The fiber has a numerical aperture of 0.2. Estimate the total number of guided modes propagating in the fiber when it is operating at a wavelength of 1 μm. Answer: 247 Problem 3: Estimate the maximum core diameter for an optical fiber with the same relative refractive index difference (1.5%) and core refractive index (1.48) as the fiber given in Problem 1 in order that it may be suitable for single-mode operation. It may be assumed that the fiber is operating at the same wavelength (0.85 μm). Further, estimate the new maximum core diameter for single-mode operation when the relative refractive index difference is reduced by a factor of 10. Answer: 8 µm 03-02-2025 50  Optical Fibers ❖ Graded index fibers may also be designed for single-mode operation and some specialist fiber designs do adopt such non step index profiles. However, it may be shown that the cutoff value of normalized frequency Vc to support a single mode in a graded index fiber is given by: Problem 4: 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 wavelength of 1.3 μm. Answer: 6.6 µm Cutoff Wavelength ❖ Single-mode operation only occurs above a theoretical cutoff wavelength λc given by: Thus, for step index fiber where Vc = 2.405, the cutoff wavelength is given by: 03-02-2025 51  Optical Fibers Problem 5: Determine the cutoff wavelength for a step index fiber to exhibit single-mode operation when the core refractive index and radius are 1.46 and 4.5 μm, respectively, with the relative index difference being 0.25%. Answer: 1214 nm 03-02-2025 52  Fiber Optics and Laser Instrumentation Lecture 8: 16th January 2025 03-02-2025 53  Optical Fiber Fabrication ❖ As the refractive index of the Core and Cladding is slightly different, so the material is distributed along such small dimensions. ❖ If we see Graded index fibers, even in the core itself the refractive index changes with radial position. How to obtain or fabricate such a structure on a such a small micrometer scale. 03-02-2025 54  Optical Fiber Fabrication ❖ Materials suitable for optical fibers. ❖ Fiber Preform: Blown up version of optical fiber. Optical fiber cladding diameter is 125 microns and length is in kilometers. Fiber preform is blown up to few centimeters in transverse dimensions and length can be around in meters. ❖ Preform fabrication techniques ❖ Fiber drawing from the preform ❖ Fiber Cables Materials for Optical Fiber Fabrication Requirements ❖ It should be possible to draw flexible, thin, kilometers long fibers from the material. ❖ Since the core and cladding are of the different materials; two physically compatible materials should be available to make them➔ otherwise there would be deformation in the structure. ❖ The material should be having as low loss as possible in the wavelength range of the interest primarily for the telecommunication purpose. Materials Glass ❖ Fused Silica Glass/Silicate Glass ❖ Most widely used material for telecom fiber ❖ Low loss, chemical inertness, high stability, and it is resistant to deformation at high ambient temperatures. 03-02-2025 55  Materials for Optical Fiber Fabrication Materials Halide Glass ❖ Fluoride Glasses ❖ Extremely low transmission losses at mid-IR wavelength. ❖ Major components in this glass is ZrF4 and other constituents are BaF2, LaF3, AlF3, and NaF. ❖ Potential candidate for futuristic telecom fiber, which can work around 2 micrometer wavelength. Active Glass ❖ Dope certain rare earth materials into Silica glass and out of which you make active glass. ❖ Doping can be of Er: Erbium; Nd: Neodymium; Tm: ThuliumFluoride in fused Silica. ❖ Used for making optical fiber amplifiers Soft Glasses : Chalcogenide Glass ❖ Core can be made of As40S58Se2/As2S3 (Cladding) ❖ High Optical Nonlinearity over long interaction length ❖ Very high losses ~ 1 dB/m ❖ Not good for the transmission of the optical signal ❖ But can be used to make light sources in mid-IR range, fiber amplifiers and optical switches. 03-02-2025 56  Materials for Optical Fiber Fabrication Materials Polymer/Plastic ❖ PMMA (Polymethyl Methacrylate) and Perfluorinated polymer. ❖ High Losses ~ 0.2 dB/m ❖ Low cost, easy handling, and light weight. ❖ Fiber dimensions are much larger than glass fiber. ❖ Less expensive components ❖ Useful for short distance communication (100 m) or for sensing purposes. Plastic Clad Silica (PCS) Fiber Materials is very attractive for making optical fiber sensors. Silica Glass Fiber Fabrication Two-Step Process ❖ First is fabrication of preform ❖ Second is drawing fiber from the preform 03-02-2025 57  Silica Glass Fiber Fabrication Preform ❖ Optical fiber cladding is 125 microns; and 10 micron core ❖ In this micron scale; we need a refractive index variation. ❖ Not possible to start with something that is in the micron scale ❖ First make a large structure which contains this geometry and refractive index distribution ❖ Dimension of preform is 10 cm to 20 cm and length can be 1 m. ❖ And when we draw fiber out of this, this structure is retained over kilometers of the length. ❖ Quality of the fiber now depends upon the quality of the preform. (RI profile, geometry of the core and cladding, and propagation characteristics decided by the preform. ❖ Preform fabrication is a very important stage. Fabrication of Preform ❖ Plot shows the refractive index of Silica glass as a function of dopant concentration. ❖ Refractive index of silica glass changes with the dopants ❖ Core we need high refractive index; cladding we need low refractive index. First Possibility 03-02-2025 58  Silica Glass Fiber Fabrication ❖ Ge is most widely used to dope Silica to attain high refractive index in the core region ❖ Most commonly used telecom fiber ❖ Fluorine or Boron can be used to make Cladding Second Possibility 03-02-2025 59  Silica Glass Fiber Fabrication Fabrication of Preform ❖ In 1960s the major bottleneck in the development of the optical fiber communication was losses in the glass. ❖ Primary reason for the losses was the impurities in the glass. ❖ Make a fiber which has very small loss ➔ make glass itself in the laboratory ❖ Start with chemicals and make glass from chemicals itself ➔ don’t have any impurities embedded in it ➔ need to do this in extremely clean environment. ❖ The chemical reactions exhibits the process for glass and doped glass. 03-02-2025 60  Silica Glass Fiber Fabrication Fabrication of Preform 1. Flame Hydrolysis: Vapor Axial Deposition (VAD); Outside Vapor Phase Oxidation (OPVO)/ Outside Vapor Deposition (OVD) SiCl4​+2H2​O Flame​ SiO2​+4HCl 2. Chemical Vapor Deposition (CVD): MCVD (Modified Chemical Vapor Deposition); Plasma Modified Chemical Vapor Deposition (PMCVD); Plasma Activated Chemical Vapor Deposition (PCVD) 03-02-2025 61  Silica Glass Fiber Fabrication Modified Chemical Vapor Deposition ❖ Take a tube of Silica Glass ❖ Flow reactants into this tube➔ make Silica glass cladding ❖ Flow in SiCl4 in oxygen environment and heat up. ❖ For heating: Burner is kept around 1600 Degree centigrade ❖ Burner translates in horizontal direction and simultaneously this tube is rotated. ❖ Burner translates and tube is rotated, so that it can cover the whole surface area. ❖ When the reactants are flown into this tube; and they are heated; as a result of chemical reaction the glass particles are formed. ❖ Since it is inside vapor deposition; first you deposit the cladding layers inside this tube ❖ After cladding, start depositing core layers; for core layers you need ❖ Glass particles have the typical size of 0.1 microns to dope germanium ❖ These particles are called soot ❖ Along with SiCl4 you start flowing GeCl4 ❖ These particles are deposited on the internal wall of the tube ❖ Now we deposit Ge doped Silica layers on top pf cladding layers ❖ And when the burner translates this tube at 1600-degree ❖ Now when the deposition of core layers gets completed; now increase centigrade, then these particles gets vitrified and form a thin the temperature of the burner to about 2000 Degree centigrade. glass layer. ❖ Collapse the tube to solid preform➔ from hollow tube you get a solid ❖ Keep on depositing cladding layer by layer. rod ❖ And when done with the deposition of enough layer of cladding; ❖ The solid rod in the center has the core layers and outside the ➔ required for certain thickness of cladding cladding layers 03-02-2025 62  Silica Glass Fiber Fabrication Glass Working Lathe 03-02-2025 63  Silica Glass Fiber Fabrication Outside Vapor Deposition Technique ❖ Exclusively used by Corning glass work ➔ first company to produce optical fiber with a very low loss. ❖ Also called Soot Process. ❖ Central mandrel made up of Aluminum Oxide or Graphite. ❖ Flow in the reactants and heat it up ❖ Burner translates and mandrel is rotated ❖ As a result of the chemical reaction again the soot is formed. ❖ Soot is deposited in the mandrel layer by layer ❖ A preform is formed ❖ In last step; called sintering; a hollow preform is dehydrated and collapsed in controlled atmosphere to form preform. 03-02-2025 64  Silica Glass Fiber Fabrication Fiber Drawing ❖ Fiber draw tower is as high as 3-storey building ❖ On top of that we have a furnace ❖ Lower the preform using precise feed mechanism into the furnace ❖ Initially when it starts melting; a gob of glass is formed ❖ Gob of glass falls under gravity ➔ it pulls the fiber down ❖ Now this fiber goes through various stages ❖ Diameter monitor ❖ Coat the fiber. ❖ Speed of the drum (take-up drum) and speed of lowering feed mechanism of the preform 03-02-2025 65  Fiber Cables 03-02-2025 66  Fiber Optics and Laser Instrumentation Lecture 9: 17th January 2025 03-02-2025 67  Transmission Characteristics of an Optical Fiber ❖ What happens to the optical pulses when they propagate through an optical fiber? ❖ Attenuation of the signal ❖ Loss mechanisms in the fused silica glass fiber ❖ Lowest loss wavelength ❖ Various communication bands ❖ Futuristic system we are looking at Data Transmission ❖ Data encoded in a pulse train ❖ When pulse train is sent though an optical fiber, then at the output end the pulse broadens ❖ Time delay refers to how long it takes for a light pulse to travel from the transmitter to the receiver through the optical fiber. ❖ Different parts of the pulse might experience different delays due to the nature of the fiber, leading to pulse broadening. ❖ Amplitude of the pulses goes down and at the same time duration of the pulses increases. ❖ Need to pay attention that the pulses do not overlap ❖ Also, the power level of the pulses should remain at such as a level that the optical detector should be able to detect them otherwise the information 03-02-2025 will be lost. 68  Transmission Characteristics of an Optical Fiber ❖ So, before the power levels go down beyond a certain limit, we need to reshape the pulses ❖ Bring them to their original form and this is done using a device known as Repeater. ❖ In a communication link, we send pulses through an optical fiber and then after a certain distance you will have to reshape the pulses and then bring them to their original form➔ and then you again send it to the next segment and so on. L is the repeater-less length of the link; then this L the repeater –less length of the link depends upon tow things: ❖ Ho much the pulses have attenuated ❖ What is the spread of this pulses➔ broadening in these pulses 03-02-2025 69  Transmission Characteristics of an Optical Fiber Attenuation ❖ The power at the input end we sent through an optical fiber (Pin) ; the power we receive at the output end is (Pout) So, before the power levels go down beyond a certain limit, we need to reshape the pulses ❖ Then the loss of the fiber through the length L is : 03-02-2025 70  Transmission Characteristics of an Optical Fiber Attenuation Problem: When the mean optical power launched into an 8 km length of fiber is 120 μW, the mean optical power at the fiber output is 3 μW. Determine: (a) the overall signal attenuation or loss in decibels through the fiber assuming there are no connectors or splices; (b) the signal attenuation per kilometer for the fiber. (c) the overall signal attenuation for a 10 km optical link using the same fiber with splices at 1 km intervals, each giving an attenuation of 1 dB; (d) the numerical input/output power ratio in (c).The power at the input end we sent through an optical fiber (Pin) ; the power we receive at the output end is (Pout) So, before the power levels go down beyond a certain limit, we need to reshape the pulses Then the loss of the fiber through the length L is : 03-02-2025 71  Transmission Characteristics of an Optical Fiber Attenuation Problem: When the mean optical power launched into an 8 km length of fiber is 120 μW, the mean optical power at the fiber output is 3 μW. Determine: (a) the overall signal attenuation or loss in decibels through the fiber assuming there are no connectors or splices; (b) the signal attenuation per kilometer for the fiber. (c) the overall signal attenuation for a 10 km optical link using the same fiber with splices at 1 km intervals, each giving an attenuation of 1 dB; (d) the numerical input/output power ratio in (c). 03-02-2025 72  Transmission Characteristics of an Optical Fiber Loss in Glass 03-02-2025 73  Transmission Characteristics of an Optical Fiber Spectral Loss Variation in Fused Silica Glass Fiber Water Peaks (OH- Absorption) Lowest Loss of 0.2 dB/km at 1550 nm wavelength 03-02-2025 74  Transmission Characteristics of an Optical Fiber 03-02-2025 75  Transmission Characteristics of an Optical Fiber 03-02-2025 76  Transmission Characteristics of an Optical Fiber Intrinsic Loss ❖ Absorption by the primary constituent materials of fiber ❖ IR Absorption in SiO2 ❖ Towards, longer wavelength side; the IR absorption in Silica glass increases it has a peak beyond 2 microns. ❖ Due to atomic vibration band in the near IR region ❖ Result of the interaction of the vibrating chemical bond and electromagnetic field of the light. 03-02-2025 77  Transmission Characteristics of an Optical Fiber Intrinsic Loss ❖ Absorption by the primary constituent materials of fiber ❖ UV Absorption in SiO2 ❖ Due to the electronic atomic band in the UV region ❖ Increases with the Ge concentration in the core ❖ Is smaller in the IR region 03-02-2025 78  Transmission Characteristics of an Optical Fiber Extrinsic Loss ❖ Caused by various impurities in the fiber 03-02-2025 79  Transmission Characteristics of an Optical Fiber Extrinsic Loss Low Water Peak Fibers 03-02-2025 80  Transmission Characteristics of an Optical Fiber 03-02-2025 81  Transmission Characteristics of an Optical Fiber Radiative Loss Mechanism Rayleigh Scattering ❖ Caused by wavelength scale inhomogeneity which are frozen in the fiber during the fabrication process ❖ Scattering centers developed during the fabrication process ❖ When light hits any of this scattering centers ➔ scatters light in all the directions ❖ When it scatters the light in all directions, then only the rays which make larger angle from the normal of core cladding interface, they are totally internally reflected and guided. ❖ All other rays are refracted into the cladding and all lost. ❖ This causes attenuation in output power from the fiber. 03-02-2025 82 Major contribution to loss at 1550 nm comes from Rayleigh Scattering  Transmission Characteristics of an Optical Fiber Rayleigh Scattering where 𝛾𝑅 is the Rayleigh scattering coefficient, 𝜆is the optical wavelength, n is the refractive index of the medium, p is the average photoelastic coefficient, 𝛽𝑐 is the isothermal compressibility at a fictive temperature TF, and K is Boltzmann’s constant. The fictive temperature is defined as the temperature at which the glass can reach a state of thermal equilibrium and is closely related to the anneal temperature. Furthermore, the Rayleigh scattering coefficient is related to the transmission loss factor (transmissivity) of the fiber Type equation here. following the relation 03-02-2025 83  Transmission Characteristics of an Optical Fiber Rayleigh Scattering 03-02-2025 84  Transmission Characteristics of an Optical Fiber Fresnel Reflection ❖ When the light hits the output end of the fiber, then there is an index contrast 03-02-2025 85  Fiber Optics and Laser Instrumentation Lecture 10: 21st January 2025 03-02-2025 86  Transmission Characteristics of an Optical Fiber How the Pulses are broadened in a Multimode Optical Fiber ❖ Green line represents the sensitivity of the detector ❖ Anything below the green line is recorded as 0, and anything above this is 1. ❖ High pulse and low pulse regions are well distinguished ➔ pulses are resolvable➔ information is intact ❖ If the broadening is much larger ➔ pulses will overlap; everywhere we will have 1 ➔ will lose the information which we want to retrieve at the output end. 03-02-2025 87  Pulse Broadening: Intermodal Dispersion Pulse is centered around t0 ➔ position of the pulse 2𝝉𝟎 is the full width of the pulse where power has dropped to 1/e2 of its peak value. 𝝉𝟎 is the width of the pulse. After travelling the distance z, the pulse becomes: amplitude changes, width changes and position also changes ❖ This type of broadening is known as intermodal dispersion; because different ray paths can be called different modes of propagation of the system 03-02-2025 88  Pulse Broadening: Intermodal Dispersion For the ray making an angle 𝜽 from the fiber axis Time taken in traversing the distance AB➔ Time taken in traversing L length of the fiber (because that L length can be comprising of these several ABs) As the ray path would repeat itself, the time taken in traversing L length of the fiber would be given as above. ❖ Because of the time difference we will have the broadening 03-02-2025 89  Pulse Broadening: Intermodal Dispersion ❖ If all the modes (rays) are excited simultaneously at the input end; the time interval occupied by the rays at the output end would be: Example: Let us send a very narrow pulse or impulse in a typical multimode fiber n1=1.5 (𝒏𝟏 − 𝒏𝟐 )Τ𝒏𝟏 = 𝟎. 𝟎𝟏; for L =1 km; Calculate 𝚫𝝉 03-02-2025 90  Pulse Broadening: Intermodal Dispersion 03-02-2025 91  Pulse Broadening: Intermodal Dispersion Example: Let us now examine the effect of broadening on two consecutive pulses Completely overlapped ➔ At 1 km length cannot distinguish between the two pulses 03-02-2025 92  Pulse Broadening: Intermodal Dispersion If you want to keep the length of the fiber as 1 km and still you want to resolve the pulses at the output end ➔ increase the separation between the pulses Large separation between the pulses are required➔ it means a smaller number of pulses are sent per second ➔ data rate is now smaller. 03-02-2025 93  Pulse Broadening: Intermodal Dispersion Bit Rate Length Product We can relate ∆𝝉 to the information carrying capacity of the fiber measured through Bit Rate B: So, for a given optical fiber where n1 and n2 are fixed then Bit rate Length Product is a constant Case 1: n1 = 1.5; n2 = 0 (Un-cladded Fiber) Case 2: n1 = 1.5; ∆ = 1% 03-02-2025 Smaller the difference between core and cladding ➔ higher would be the data rate 94  Pulse Broadening: Intermodal Dispersion Minimization of the Intermodal Dispersion Using Graded Index Fiber n2 = 1.45; ∆ = 1% 03-02-2025 95  Chromatic Dispersion If the optical source has certain wavelength content ➔ What would be the implication of this on the broadening of pulses Various colors appearing at various angles, and this is purely due to the wavelength dependence of the refractive index of the material of the prism and geometry of the prism➔ Chromatic Dispersion ❖ All the wavelength components which fall in this line width of the light sources; would now experience different refractive index of the material and they will travel at different velocities and that would give rise to dispersion. ❖ Each wavelength component within the line width of the source experiences different refractive index of the medium ➔ Material Dispersion 03-02-2025 96  Chromatic Dispersion Constant Phase Propagating in z-direction Using Graded Index Fiber 03-02-2025 97  Chromatic Dispersion If we superimpose two wave, we make groups And this group of waves travel with some velocity known as Group Velocity➔ velocity of the envelope 03-02-2025 98  Fiber Optics and Laser Instrumentation Lecture 11: 23rd January 2025 03-02-2025 99  Chromatic Dispersion 03-02-2025 100  Chromatic Dispersion All the wavelength components that fall into these line width will now have different transit times. 03-02-2025 101 Chromatic Dispersion Chromat