Fiber Optics PDF

# MODERN PHYSICS ## CHAPTER - 5 ## FIBER OPTICS ### 5.1 Introduction Fiber optics is a branch of optics that deals with the study of propagation of light through dielectric waveguides eg. optical fibers. The transmission medium in fiber-optic communications system is an optical fiber. An optical f...

# MODERN PHYSICS ## CHAPTER - 5 ## FIBER OPTICS ### 5.1 Introduction Fiber optics is a branch of optics that deals with the study of propagation of light through dielectric waveguides eg. optical fibers. The transmission medium in fiber-optic communications system is an optical fiber. An optical fiber is a transparent flexible filament that guides light from a transmitter to a receiver. A fiber-optic communications system is the most preferred telecommunications system for several reasons that will be discussed in what follows. An optical fiber forms the heart of the fiber communications system that possesses a lot of advantages compared to copper or co-axial or even satellite communications. ### 5.2 Advantages of Fiber-Optic Communications - The bandwidth, which is the information carrying capacity, of optical communication system is very large. Theoretically a bandwidth of the order of 50 Tb/s is possible. - Owing to the fact that photons carry the information in optical fibers, the speed at which the data are transmitted is quite high. - The attenuation is very less (of the order of 0.15 dB/km). - Fibers are light in weight and small in size. - Digital communication is possible. ### 5.3 A Fiber - Optic Communication System The block diagram represents the generalized configuration of a fiber - optic communication system. | Source | Input coupler | Repeater | Output coupler | Detector | Optical fibers | Transmitter | Information input | Information output | Receiver | |---|---|---|---|---|---|---|---|---|---| | | | | | | | | | Voice, Video, Data | | | | | | | | | | Voice, Video, Data | | | #### Figure 5.1: Generalized configuration of a fiber-optic communication system The information input may be either voice or video or data. Therefore an input transducer is required for converting the nonelectrical input into an electrical input. The transmitter (or the modulator, as it is often called) converts the electric signal into the proper form and impresses this signal onto the electromagnetic wave generated by an optoelectronic source. The light from the source is coupled to the fiber by a coupler. The light is transmitted through the fiber by the phenomenon of total internal reflection. As the optical signals propagate through the fiber, they get attenuated due to scattering, absorption and bending of the fiber. Hence the regeneration of signal is necessary. A repeater just does this. The conventional electronic repeaters are being replaced, nowadays, by optical amplifiers. There is another coupler at the output which directs the light to the detector. The detector converts light into electricity back. The receiver section filters out undesired frequencies and amplifies the photocurrent. Finally, the information must be presented in a form that can be interpreted by a human observer. Suitable output transducers are required for achieving this transformation. ### .4 Structure of an Optical Fiber Optical fibers are thin, transparent, flexible strands that consists of a core surrounded by cladding. The core and cladding of an optical fiber are made from the same material (a type of glass called silica) and they differ only in their refractive indices. The refractive index of the core (n₁) will be slightly greater than that of the cladding (n₂). Normally an elastic plastic buffer encapsulates the fiber. #### Figure 5.2: Schematic of a single-fiber structure ### 5.5 Light Propagation through Fibers The basic principle behind the transmission of light through the optical fibers is 'total internal reflection' which is described below: #### Figure 5.3: Representation of critical incident angle and total internal reflection ### 5.6 Conditions for Light Propagation in Optical Fibers As has been discussed already, an optical fiber has an inner core of higher refractive index, n₁, surrounded by an outer core of relatively lower refractive index n₂. Hence the refractive index of the core is uniform throughout the core and decreases abruptly at core-cladding boundary. This fiber is called a step-index fiber as the refractive index profile shows that the refractive index decreases step wise. This is clearly shown in the Figure 5.4. #### Figure 5.4: Basic structure of a step-index fiber ### 5.7 Dispersion Dispersion is the spreading of a light pulse as it travels down the length of an optical fiber. Dispersion ultimately limits the information-carrying capacity of a fiber. In fiber-optic communications, there are two major types of dispersions, namely, - Intermodal dispersion and - Intramodal dispersion #### 5.7.1 Intermodal Dispersion When an optical pulse is launched into a fiber, the optical power in the pulse is distributed over all of the modes of the fiber. The propagation of light within the fiber can be described in terms of a set of guided electromagnetic waves called the 'modes'. Roughly, we can say that a mode is the path taken by the light rays while entering into the fiber. Hence different modes will travel with different propagation angles, the range being zero (corresponding to the fundamental mode) to the critical value, a (corresponding to the critical mode). These modes take different routes but travel with the same velocity and hence, at the end of the fiber, they arrive at different timings. This ultimately causes the pulse widening. This is called intermodal dispersion or simply modal dispersion since it is the dispersion that arises within the different modes of a single pulse. #### Figure 5.6: Modes in an optical fiber #### Measuring Intermodal Dispersion Consider a beam propagating inside a fiber, taking into account the mode concept, as in Figure 5.6. For digital transmission, a light pulse represents logic 1, and no light pulse represents logic 0. Such light pulses, radiated by light source, enter a fiber, where each pulse breaks down into a set of small pulses carried by an individual mode. At the fiber output, individual pulses recombine and, since they are overlapping, the receiver sees one long light pulse whose rising edge is from the fundamental mode and whose falling edge is from the critical mode. This explanation is depicted in Figure 5.6, where four modes are shown as an example. #### 5.7.2 Intramodal Dispersion As the name indicates, it is the dispersion that arises within a mode of the pulse. It is otherwise called chromatic dispersion. This dispersion is further classified as follows: - Material dispersion - Waveguide dispersion #### i) Material Dispersion As no optical source is perfectly monochromatic, each pulse of light has several spectral components. These spectral components experience different indices inside the fiber and travel with different speeds. Hence they arrive at the end of the fiber with different timings. This dispersion is called material dispersion since it results from the refractive index variation of the material of the fiber with the wavelength of light propagating through the fiber. Material dispersion plays a major role in limiting the bandwidth of a single mode fiber. #### ii) Waveguide Dispersion It is caused by the fact that light is guided by a structure - here, an optical fiber. This dispersion, though exists in multimode fibers, is seen prominently only with single mode fibers. After entering a single-mode fiber, an information-carrying light pulse is distributed between the core and the cladding. Its major portion travels within the core and the rest within the cladding. Both the portions propagate at different velocities since the core and cladding have different indices. The pulse will spread simply because light is confined within a structure having different refractive indices - the core - cladding combination of the fiber. This dispersion is called waveguide dispersion. However, waveguide dispersion in a single-mode fiber is relatively small compared with material dispersion. It should be noted that multimode fibers also suffer from chromatic dispersion. However modal dispersion is the major factor limiting multimode fiber bandwidth. ### 5.8 Meridional and Skew beams All beams (modes) propagating within an optical fiber are divided into two categories: meridional beams and skew beams. Meridional beams are those that intersect the centerline of the fiber; skew beams propagate without intersecting the fiber's central axis. Skew rays are not confined to a single plane, but instead tend to follow a helical-type path along the fiber. These rays are more difficult to track as they travel along the fiber, since they do not lie in a single plane. Although skew rays constitute a major portion of the total number of guided rays, their analysis is not necessary to obtain a general picture of rays propagating in a fiber. Hence, it is sufficient to consider the meridional rays for all practical purposes. In all the discussions of this chapter, only meridional rays have been considered ### 5.9 Types of Fibers Optical fibers are classified based on the - material with which they are made - refractive index profile and - number of modes employed Based on the material used, we have following types of fibers. - Low loss fiber - Medium loss fiber - Higher loss fiber In low and medium loss fibers the core material is generally glass and is surrounded by either a glass or a plastic cladding. In the case of higher loss fiber, b core and cladding are made of plastic. ### 5.10 Applications of Fiber Optics in Communication Owing to the several advantages of fiber-optic communications that were discussed in the beginning of this chapter, telecommunication industries consider optical communication as their primary technology. Several novel fibers designed especially to tackle the problem of dispersion have made a lot of improvements in communications technology. We are living in an information society, where the efficient transfer of information is highly relevant to our well-being. Fiber-optic systems form the very means of such information transfer and have a very important role, directly or indirectly, in the development of almost every sphere of life. The following are the areas in which fiber-optic technology has made a considerable impact: - Voice communication - Inter-office - Video communication - TV Broadcast - Remote monitoring - Videophones - Data transfer - Inter-office data link - Computers - Internet - Intercity - Intercontinental links - Cable television (CATV) - Wired city - Local area networks - Satellite ground stations - E-mail - Video conferencing ### 5.11 Source #### 5.11.1 Light emitting diode (LED) LEDs have been around for more than 35 years. They have found application in nearly every consumer-electronic device: TV sets, VCRs, telephones, car electronics, and many others. They are used in fiber-optic communications, mostly because of their small size and long life. #### Figure 5.9: Energy Vs momentum for (a) direct and (b) indirect bandgap semiconductors #### Figure 5.11: The energy band diagram of a degenerately doped p-n junction with no bias #### Figure 5.12: Band diagram with a sufficiently large forward bias #### Figure 5.14: Structure of a homojunction diode laser #### Figure 5.15: Output optical power Vs. diode current (I) characteristics and the corresponding output spectrum of a laser diode #### 5.12 Detector #### 5.12.1 p-n photodiode The detectors used in fiber-optic communications are semiconductor photodiodes or photodetectors, which get their name from their ability to detect light. The simplest semiconductor detectors are solar cells, where incident light energy raises valence band electrons to the conduction band, generating an electric voltage. Unfortunately, such photovoltaic detectors are slow and insensitive. #### Figure 5.16: Ap-n photodiode #### 5.12.2 p-i-n photodiode The major feature of this positive-intrinsic-negative photodiode is that it consists of a thick, lightly doped intrinsic layer sandwiched between thin p and n regions. #### Figure 5.17: A simple p-i-n Photodiode ### 5.13 Endoscope The term 'endo' means 'inside' and 'scope' means 'device to look or see'. Hence an endoscope is a device to see or look inside. #### Figure 5.18: Endoscope ### Solved problems - In an optical fiber, the core refractive index is 1.4513 and the cladding refractive index is 1.4468. What is: - Critical incident angle? - Critical propagation angle? - Acceptance angle ? - The numerical aperture? - Calculate the numerical aperture of an optical fiber whose core index is 1.48 and relative index is 0.02. - Calculate the numerical aperture of a step-index fiber having n₁=1.48 and n₂ = 1.46. What is the maximum entrance angle for this fiber if the outer medium is air with n = 1.00 - A Si p-i-n photodiode has a quantum efficiency of 0.7 at a wavelength of 0.85 µm. Calculate its responsivity. - A particular photodectector has a responsivity of 0.6A/W for a light of wavelength 1.3µm. Calculate its quantum efficiency. - If a step-index fiber has a core of refractive index 1.5 and a cladding of refractive index 1.48, calculate, assuming that the fiber is kept in air, the: - NA of the fiber, - angles θ, α and θ. - A p-n photodiode has a quantum efficiency of 70% for photons of energy 1.52 × 10-19J. Calculate, (a) the wavelength at which the diode is operating and (b) the optical power required to achieve a photocurrent of 3 µA when the wavelength of the incident photons is that calculated in part (a). - A pin photodiode, on an average, generates one electron-hole pair per two incident photons at a wavelength of 0.85 µm. Assuming all the photo-generated electrons are collected, calculate (a) the quantum efficiency of the diode, (b) the maximum possible bandgap energy (in eV) of the semiconductor, assuming the incident wavelength to be a long-wave length cut-off; and (c) the mean output photocurrent when the incident optical power is 10 μW. - A step index fiber has higher core index and lower cladding index of 1.5 and 1.45, respectively. Calculate (a) critical incident angle of the fiber, (b) the corresponding acceptance angle of the fiber in air and (c) NA of the fiber. - When 2.5 × 1012 photons generated by a laser source of wavelength 0.85 µm are incident on a photodiode, 1.5 × 1012 electrons on an average are collected at the output terminal. Calculate the quantum efficiency and the responsivity of the photodiode at the above wavelength. - A silica fiber with a core diameter large enough to be considered by ray theory analysis has a core refractive index 1.50 and a cladding refractive index 1.47. Determine: - the critical angle at the core-cladding interface. - the NA of the fiber - the acceptance angle in air for the fiber. - A step-index fiber has an acceptance angle of 20° in air and a relative refractive index difference of 3%. Estimate the NA and the critical angle at the core- cladding interface. - Photons of wavelength 0.90 µm are incident on a p-n photodiode at a rate of 5x1010/s and, on an average, the electrons are collected at the terminals of the diode at the rate of 2x1010/s. Calculate: - the quantum efficiency and - the responsivity of the diode at this wavelength. - Calculate the responsivity of an ideal p-n photodiode at 0.85µm. - Explain, with the block diagram, the operation of a typical fiber-optic communication system. List the advantages of fiber-optic communications. - Describe the conditions for the light propagation through optical fibers and hence explain the following terms: - Critical incident angle - Critical propagation angle - Acceptance angle - Numerical aperture - Draw the structure of an optical fiber and explain its parts. Explain, in detail, the different types of optical fibers with suitable diagrams. - What is dispersion? Calculate the pulse spreading due to intermodal dispersion in the case of a step-index fiber. - Describe the two dispersions that appear under the title of intramodal dispersion. Explain why single mode fibers are the most preferred one for long-distance communications. - Explain the construction and the operation of a light emitting diode. Describe about both the direct and indirect bandgap semiconductors. - Describe the construction and working of a semiconductor laser. Mention its merit and demerits. - Explain about a p-i-n photodiode with a suitable diagram. What are its advantages compared with a simple p-n photodiode? Arrive at the relationship connecting responsivity and quantum efficiency of a photodiode. - Explain the operation of an endoscope with a neat sketch. ### Points to Remember - Light is guided through optical fibers by total internal reflection if it enters the fiber within an acceptance angle, measured directly or as the numerical aperture. - The core of an optical fiber must have a higher refractive index than the cladding surrounding it. - An optical fiber that supports only the fundamental mode is called a single-mode fiber. - Intermodal dispersion is maximum in the case of step index fibers, minimum in the case of graded index fibers and totally absent in single-mode fibers. - Single mode fibers suffer from chromatic dispersion only. - Both inter and intramodal dispersions arise in multimode fibers. - Meridional beams intersect the centerline of the fiber. - Skew beams don't intersect the centerline of the fiber. - LEDs are realized using direct bandgap semiconductors. - Detectors always need to be reverse-biased. - Responsivity is measured in A/W. - Quantum efficiency of a photodiode is the ratio of the number of produced electrons to the total number of falling photons. ### MODERN PHYSICS LAB #### CONTENTS | Sl.No. | List of Experiments | Page No. | |---|---|---| | | | | | 1) | Traveling microscope - Length of a glass plate | 1 | | | Spectrometer - Direct ray measurement | 4 | | 1. | Spectrometer - Angle of prism | 10 | | 2. | Planck's constant - LED method | 14 | | 3. | Refractive index of liquid | 19 | | 4. | Laser - grating - Determination of wavelength | 23 | | 5. | Spectrometer - Refractive index of a glass prism | 27 | | 6. | Sonometer - Frequency of AC main | 35 | | 7. | Optical fiber - Numerical aperture and acceptance angle | 39 | | 8. | Ultrasonic interferometer - Velocity of ultrasonic waves in liquid | 42 | | 9. | Air wedge - Thickness of a thin wire | 47 | | 10. | Laser-Particle size determination | 52 | # CHAPTER - 3 # LASER PHYSICS ## Introduction Laser, perhaps one of the most exciting discoveries of the twentieth century, is an acronym for light amplification by stimulated emission of radiation. The first successful operation of a laser was done by Maiman in 1960 using a ruby crystal. Laser light, like light from any other ordinary source, is emitted when atoms make a transition from quantum state of higher energy to a state of lower energy. However, it has unique properties not found in the light from ordinary sources. Let us now discuss its basic principle, working of some laser systems, and its important applications. ## 3.1 Characteristics of Laser ### i) Monochromaticity If light coming from a source has only one frequency of oscillation, the light is said to be monochromatic and the source, a monochromatic source. In practice it is not possible to produce light with only one frequency. ## 3.1.1 Coherence Conventional light sources produce incoherent light. This means that the light that emerges from a conventional light source is mixture of waves at various frequencies that reinforce or cancel each other in a random fashion. Figure 3.1(b) depicts this situation. Obviously, the wavefront thus produced varies from point to point and changes from time to time. The wave from a laser is called almost coherent because it is an orderly wave of one frequency where the whole beam is spatially in phase. There are, thus, two independent facets of this coherence: namely, temporal coherence and spatial coherence. ### 3.1.2 Temporal Coherence This type of coherence refers to the correlation between the radiation field at a point and the radiation field at the same point at a later time, i.e. the relation between E(x, y, z, t₁) and E(x, y, z, t), where these represent the radiation field at the point (x, y, z) at times t₁ and t₂ respectively. ### 3.1.3 Spatial Coherence Two fields at two different points on a wavefront of a given electromagnetic wave are said to be space coherent if they preserve a constant phase difference over any time t, i.e., space coherency requires that the waves not only are of the same frequency, but that they are in phase in space. Figure 3.1 shows these conditions. In figure 3.1 (c) the waves are monochromatic (time coherent) but are not in sequence in space. This can occur when the source of the wave is physically broad rather than a point source. Figure 3.1(d) shows a spatially and time-coherent wave. The whole wavefront is in step and each cycle take the same length of time. The very nature of laser mechanism produces this coherent signal. ### 3.2 Einstein's Prediction When we see light from any source, we actually see electrons jumping from excited states to lower states. This type of emission of light which occurs on its own is known as spontaneous emission and is responsible for the light coming from candles, electric bulbs, fire, sun etc. Einstein predicted in 1917, there must be a second emission process to establish thermal equilibrium. For example, if we illuminate a material with light of suitable frequency, the atoms in it absorb light and go to higher energy state. The excited atoms tend to return randomly to the lower energy state. As the ground state population is very large, more and more atoms are excited under the action of incident light and it is likely that a stage may be reached where all atoms are excited. This violates thermal equilibrium condition. Therefore Einstein suggested that there could be an additional emission mechanism, by which the excited atoms can make downward transitions. He predicted that the photons in the light field induce the excited atoms to fall to lower energy state and give up their excess energy in the form of photons. He called this type of second emission as stimulated emission. ### 3.3 The Three Fundamental Processes Let us consider a medium consisting of identical atoms capable of being excited from the energy level 1 to the energy level 2 by absorption of photons. Let the levels be denoted by E, and E, and their populations be N, and N, respectively. Let the atoms be in thermal equilibrium. In the equilibrium condition the number of atomic-upward transitions must be equal to the number of downward transitions. Thus no net photons are generated or lost. However, when the atoms are subjected to an external light of frequency, 'v', the following three processes occur in the medium. ### 3.4 Einstein's A and B Co-efficients #### Figure 3.5: A blackbody at a temperature 'T' emits radiation that interacts with the atoms in the blackbody #### Figure 3.6: Radiative processes that affect the number of atoms at energy E, and E₂. ### 3.5 Population Inversion When an atomic system is in equilibrium, absorption and spontaneous emission take place side by side. But, because N₂ <N₁, absorption dominates. However laser operation requires obtaining stimulated emission exclusively. To achieve a high percentage of stimulated emission, a majority of the atoms should be at the higher energy level than at the lower level. The non equilibrium state in which the population N, of the upper energy level exceeds the population N, of the lower energy level is known as population inversion. #### Figure 3.7 ### 3.6 Pumping In order to realize and maintain the state of population inversion, it is necessary that atoms must be continuously promoted from the lower level to the higher level. So, energy is supplied by some means to the laser medium to raise atoms from the lower level to the excited level, thus maintaining population at the excited level at a value greater than that of the lower level. ### 3.6.1 Active Medium An active medium is a medium, which when excited, reaches the state of population inversion, and eventually causes light amplification. The active medium may be solid, a liquid or a gas. ### 3.6.2 Two Level Pumping Scheme It seems that the simple and straight forward method to achieve population inversion is to pump and maintain excess of atoms into the excited state by applying intense radiation. But basically a two level pumping scheme is not suitable for attaining population inversion. This is due to two basic reasons. - It is difficult to keep a collection of atoms in their excited states until they are stimulated to emit photon. - The atoms that happen to be in their ground state will undergo absorption and will thus remove photons from the beam as it builds up. ### 3.6.3 Three Level Pumping Scheme #### Figure 3.8:(a) A three-level laser ### 3.6.4 Four Level Pumping Scheme #### Figure 3.8:(b) Four-level laser #### Figure 3.9: Buildup of intense beam in a laser. Each emitted photon interacts with an excited atom and produce two photons. Multiplication of stimulated photons. ### 3.7 Gain Coefficient - Threshold Gain Coefficient The gain of a laser depends on several factors. Foremost among them is the separation of energy levels that produce laser transition. If the two levels are too apart, the gain is higher because the laser transition energy is a larger fraction of the pump transition energy. If the two levels are closer together, the gain is lower. #### Figure 3.10: Laser medium ### 3.9 Ruby Laser Ruby laser is a three level solid state laser and was constructed by Maiman. It is a Pulsed Laser having very high power of hundred of mega wall in a single with 10 nanosecond duration. It is used for various industrial applications like surface hardening, cladding of various industrial products, etc. Recently erbium (Er³+) doped ruby lasers are available and have higher merits than ordinary chromium (Cr³+) doped ruby laser. #### Figure 3.11: Ruby laser #### Figure 3.12: Three levels in Ruby Laser "Transition" ### 3.10 Neodymium Laser This is a rare earth laser system. There are two types of Neodymium lasers - Nd: YAG laser - Nd: glass laser These are four level solid state lasers with high power pulses having shorter pulse width and high repetition rate. These can give continuous power output also. Among these, Nd: YAG lasers are widely used in the industries for cutting, welding, drilling and surface hardening of the industrial products. In medicine it is used to treat gastrointestinal bleeding and to do intracular eye surgeries. Further Nd glass lasers are used to produce nuclear fusion process. #### Figure 3.13: Energy levels of Nd³ YAG laser #### Figure 3.14: Schematic diagram of He-Ne laser #### Figure 3.15: Energy level diagram of He-Ne laser ### 3.11 He-Ne Laser The first successful demonstration of a gas laser was done by Ali Javan in 1961. Helium - Neon is an atomic laser which employs four - level pumping scheme. The active medium is a mixture of 10 parts of Helium and one part of Neon. Neon atoms are active centres and have the energy levels suitable for laser transitions while helium atoms help efficient excitation of neon atoms. ### 3.12 CO, Laser The CO, laser is the most important laser of its class and in terms of technological applications it unquestionably ranks first. It exhibits both high efficiency (upto 40%) and high power output. CO, laser is an example of molecular gas laser and it was designed by CKN patel. #### Figure 3.16: Vibrational modes of CO, molecule #### Figure 3.17:(a) Schematic diagram of CO, laser ### 3.13 Excimer Laser They also belong to a family of gas lasers that produce nanosecond long powerful pulses at UV region (308, 248 or 193 nm) of electromagnetic spectrum. These lasers use mixture of gases. The interest in excimer lasers principally of heavy noble gases (Xe, Kr, Ar) and the halogens (F, Cl, Br, I) is due to the relatively efficient production of their excited state by electron beam collisions and the fast that their emission wavelengths lie in the ultraviolet (0.2 <2 <0.4 µm) region of the spectrum, a region not covered well by other types of lasers. The important excimer laser gases are krypton fluoride, xenon fluoride, argon fluoride and xenon chloride. It can generate billion watt of power pulses. ### 3.14 Dye Laser Dye lasers, as the name suggests, use liquid organic dyes. Liquid dye laser beam covers a wide range of wavelengths and thus have the great advantage of being tunable. The user can select a fine tuned wavelength as required. Liquid dye lasers can emit laser beams from 250 nm (UV) through the whole visible spectrum to 1800 nm (IR). In the liquid dye laser, the dye is the active medium. ### 3.15 Free Electron Laser The conventional microwave sources produce waves at wavelength 2 ≥ 1 cm and the different types of lasers operate at infrared and optical wavelengths. Hence, there remained a big void of sources in the range 1 cm ≥ 2 ≥ 30 mm. ### 3.16 Chemical Laser Some lasers are stimulated by chemical reactions instead of an outside source of energy. Chemical lasers most often utilize gases as active medium and the end products of the reaction are excited energy states that are capable of emitting photons. Some chemical lasers can produce pulses of energy as enormous as 200 gigawatts. (~ of 102 i.e., Billion watt). #### Figure 3.18: Laser welding ### 3.18 Laser Drilling Hole perforating by the laser relies on the intense evaporation of the material heated by powerful light pulses of 104 to 10 sec duration. Use of series of short pulses minimises the energy diffused laterally into the workpiece and assists in controlling the size and shape of the hole. ### 3.19 Laser Cutting In cutting, the aim is to vaporize the material as quickly as possible and to produce 1 narrow heat affected zone as possible with minimum distortion of workpiece. Compared o other cutting techniques, lasers offer the following advantages. - Minimal amount of mechanical distortion and thermal damage introduced in the material being cut. - The process does not introduce any contamination. - Possibility of cutting in two and even three dimensions according to complicated profile. - Easy automation of the process and high production rates. ### 3.20 Optical Disk Systems In recent years optical disks have been used increasingly for educational programs, entertainment and general audio-visual communications. In the field of data storage direct optical recording systems are becoming popular as computer peripherals, where the combination of very high information capacity and rapid random access makes optical disks an attractive alternative to other forms of computer memory store. #### Figure 3.19: Schematic diagram of laser beam recorder #### Figure 3.20: Readout from an optical disk ### 3.21 Holography Using lasers, we can get three dimensional lensless photography called holography. Using interference techniques we can take hologram which is analogous to negative of the photographic film. In one hologram we can store so much information and if a hologram is broken, then each piece will act an individual hologram. #### Figure 3.21: Holographic recording and reconstruction ### Solved Problems - The He-Ne system is capable of lasing at several different IR wavelengths, the prominent one being 3.3913 µm. Determine the energy difference (in eV) between upper and lower levels for this wavelength. - Find the ratio of population of the two states in He-Ne laser that produces light of wavelength 6328 Å at 27° С. - The CO, laser is one of the most powerful lasers. The energy difference between the two levels is 0.117 eV. Determine the frequency and wavelength of radiation. - A He-Ne laesr produces an output power of 5 mW. If it emits light of wavelength 632.8 nm, calculate the number of photons emitted by the laser in one second. - A transition between the energy level E, and E, produces a light of wavelength 632.8 nm, calculate the energy of the emitted photons. - A system has three energy levels E,, E, and E,. The energy levels E, and E₂ are at 0 eV and 1.4 eV respectively. If the lasing action takes place from the energy level E, to E₂, and emits a light of wavelength 1.15 µm, find the value of E - A laser transition takes place from an energy level at 3.2 eV to another level at 1.6 eV. Calculate the wavelength of the laser beam emitted. - The band gap of GaAs is 1.42 eV. What is the wavelength of the laser beam emitted by a GaAs diode laser? - Calculate the relative population of the energy levels N, and N, at 300 K, λ = 500 nm. - Examine the possibility of of stimulated emission at 300 K, and 2 = 600 nm. - Calculate the efficiency of a He-Ne laser, if it produces an output power of 5 mW and if it is operated with a current of 10 mA at 3 kV. - A laser beam emits an output power of 1 mW. If it is focused as a spot having a diameter of 1µm, calculate the intensity of the laser beam. - A laser beam of wavelength 632.8 nm is made to fall on a wall that lies at a distance of 5 m and if it produces a spot having a diameter of 1 mm, calculate the angular spread and the divergence of the laser beam. - The coherence length for sodium D, line is 2.5 cm. Determine (a) the spectral width of the line, Δλ, (b) the purity factor, Q, (c) the coherence time t. Take wavelength of light 6000Å. ### Points to Remember - Laser stands for Light Amplification by Stimulated Emission of Radiation. - Situation when N₂>N, is population inversion. - The process of achieving N₂ > N₁ is pumping. - Ruby laser produces pulsed output. - He-Ne laser produces continuous output. - Helium acts as coolant in CO2 laser. - Information is recorded using high power laser and data is read out using low power laser. - The role of He in a He-Ne laser is to produce population inversion with Ne atoms. - The role of N₂ in a CO, laser is to produce population inversion with CO₂ molecules. - Hologram is constructed by means of interference of light between the reference beam and the light reflected from the object. ### Formulae - E = - hc - λ = - 12400 (eV) - (A) - Number of photons in each pulse = - Energy - hv - Number of photons emitted per minute = - Total energy emitted per minute - energy of 1 photon - Ratio of population of two states, - N₂ - N₁ = - e-(E,-E)/kT - e-(hv)/kT - Intensity = - Power - Area - Angular spread, dθ = - Wavelength - Aperture diameter - Area of spread = - π (distance × d0)2 - Efficiency, η = - optical output - electrical input × 100% ### Review Questions #### Short Answer Questions - What is absorption of light? - What is meant by emission of light? - What is stimulated emission? What is spontaneous emission? - How laser is different from conventional light sources? - Write the mathematical expression for the stimulated and spontaneous emissions and explain the terms. - Distinguish between spontaneous and stimulated emission. - What is meant by the population inversion? - What is optical pumping? - What is electron excitation? - What is inelastic atom-atom collision? - What is a resonator? - Mention about the different types of resonators used for laser production. - Mention the distinct properties of laser. - What is monochromaticity? - What is intensity of laser