1. High Speed Data Transmission through Optical Fiber using 1550nm Semiconductor Laser A Thesis submitted by 1. Zihad, Fakir Sheik [09-14949-3] 2. Mahfuz, Muhammad [09-14839-3] 3. Bhowmik, Bijoy Kumar [09-14832-3] 4. Amin, Md. Sadeq -Ul- [09-14526-3] Under the Supervision of Mr. Rinku Basak Head, Graduate Program & Assistant Professor Department of EEE Electrical and Electronic Engineering Department Faculty of Engineering American International University - Bangladesh Fall Semester 2012-2013 May 2013

2. High Speed Data Transmission through Optical Fiber using 1550nm Semiconductor Laser A Thesis submitted to the Electrical and Electronic Engineering Department of the Engineering Faculty, American International University - Bangladesh (AIUB) in partial fulfillment of the requirements for the degree of Bachelor of Science in Electrical and Electronic Engineering. 1. Zihad, Fakir Sheik [09-14949-3] 2. Mahfuz, Muhammad [09-14839-3] 3. Bhowmik, Bijoy Kumar [09-14832-3] 4. Amin, Md. Sadeq -Ul- [09-14526-3] Electrical and Electronic Engineering Department Faculty of Engineering American International University - Bangladesh Fall Semester 2012-2013 May 2013

3. Declaration This is to certify that this project and thesis is our original work. No part of this work has been submitted elsewhere partially or fully for the award of any other degree or diploma. Any material reproduced in this thesis has been properly acknowledged. Students names & Signatures 1. ________________________________________ Zihad, Fakir Sheik ID: 09-14949-3 Electrical and Electronic Engineering Department 2. ________________________________________ Mahfuz, Muhammad ID: 09-14839-3 Electrical and Electronic Engineering Department 3. ________________________________________ Bhowmik, Bijoy Kumar ID: 09-14832-3 Electrical and Electronic Engineering Department 4. ________________________________________ Amin, Md. Sadeq Ul- ID: 09-14526-3 Electrical and Electronic Engineering Department i

4. Approval The Thesis titled High Speed Data Transmission through Optical Fiber using 1550nm Semiconductor Laser has been submitted to the following respected members of the Board of Examiners of the Faculty of Engineering in partial fulfillment of the requirements for the degree of Bachelor of Electrical and Electronic Engineering on May 2013 by the following students and has been accepted as satisfactory. (Supervisor) Mr. Rinku Basak Head, Graduate Program Assistant Professor Department of EEE American International University Bangladesh Prof. Dr. ABM Siddique Hossain Dr. Carmen Z. Lamagna Dean Vice Chancellor Faculty of Engineering American International University Bangladesh American International University Bangladesh ii

5. Acknowledgements At first we would like to thank our honorable supervisor Mr. Rinku Basak, Head, Graduate Program and Assistant Professor, Faculty of Engineering department, American International University Bangladesh, who has given us such an innovative topic. The idea of this thesis has been founded by him. By following his advices, we have completed this thesis paper successfully. He provided valuable inputs and parameters to solve the rate equations. Also he has given us some data which help us to create some new graphs with multiple curves and without his help and encouragement we could not complete this thesis paper successfully. Special thanks goes to our honorable Vice Chancellor, Dr. Carmen. Z. Lamagna, American International University Bangladesh for her support which also motivated us to write thesis paper. We would also like to thank our honorable Dean, Professor Dr. ABM Siddique Hossain, American International University Bangladesh for his motivation and support. We would also like to thank our parents for their blessings during the times of this thesis. Ultimately, we would like to express our height gratitude to Almighty God for his divine blessing which provided us the courage to embark on this thesis. The Authors iii

6. Contents Page No. Title Declaration i Approval ii Acknowledgement iii Abstract vi Chapter 1: Introduction 1.1 Introduction 1-4 1.2 Laser Fundamentals 5 1.3 Laser action 6-8 1.4 Applications of Semiconductor Laser 9-18 1.5 Objectives of this work 19 1.6 Introduction to this thesis 20 Chapter 2: Theory on Semiconductor Laser 2.1 Introduction 21 2.2 Edge Emitting Laser 22 2.3 Vertical Cavity Surface Emitting Laser (VCSEL) 22-25 2.4 Chart of Laser wavelengths with applications 25 2.5 Semiconductor Laser Structure 26-30 2.6 Intrinsic and Extrinsic Semiconductor materials 31-33 2.7 N-type and P-type materials 33-35 2.8 Formation of p-n junction 35-37 2.9 Energy Band for materials 37-42 2.10 Absorption, Spontaneous Emission, Stimulated Emission of a Laser 42-43 2.11 Population Inversion of a Laser 44-45 2.12 Threshold Current and its Temperature dependence 45 2.13 Summary 46 iv

7. Chapter 3: Performance Characteristics of a 1550nm Semiconductor Laser 3.1 Introduction 47-48 3.2 Structure of a 1550 nm VCSEL 48-52 3.3 Calculation of structural and performance parameters of the semiconductor laser 52-56 3.4 Characteristics of the 1550 nm VCSEL 56-61 3.5 Summary 62 Chapter 4: High speed Performance of the 1550nm Semiconductor Laser 4.1 Introduction 63 4.2 Calculation of modulation response and performance of parameters of the semiconductor laser 64-66 4.3 Resonance frequency and bandwidth 66-67 4.4 Modulation response analysis 68 4.5 Effects of variation of injunction current on modulation response of laser 69-70 4.6 Summary 70 Chapter 5: Discussions and Conclusions 5.1 Discussions 71 5.2 Conclusions 71-72 References 73-74 v

8. Abstract This work presents the prototype of both the analytical and the simulation based study on high speed communication system particularly high speed data transmission through optical fiber using 1550nm InGaAsP laser. In recent years, vertical cavity surface emitting lasers (VCSELs) are prerequisites for future ultrahigh data-rate systems. This paper presents an overview of our recent work on high speed with an innovative semiconductor laser. The design and characteristics of a 1550 nm Multi Quantum Well (MQW) VCSEL using InGaAsP materials have been obtained through computation and simulation is using MATLAB simulation Tool. The obtained characteristics have been analyzed and represented in this work with the aim of obtaining better performance and better speed of data transmission through optical fiber using VCSEL semiconductor laser. For obtaining better performance, concentrations of InGaAsP QW material have been chosen. At 300K temperature, the threshold current and Injection current have been obtained. With this threshold current VCSELs allow high-speed operation. Maximum output power has been obtained for this designed VCSEL using injection current. Corresponding to this the modulation bandwidth has been obtained which indicates a high speed performance of the designed VCSEL for applications in optical fiber communication. Some multiple graphs such as carrier density Vs photon density Vs output power have been plotted successfully. Further increasing the injection current maximum bandwidth is obtained which is helpful to transmit high speed data through optical fiber. vi

9. Chapter 1 Introduction 1.1 Introduction: A laser is a different kind of light source that emits electromagnetic radiation through a process. Because of laser properties it becomes an extraordinary device. It is one of the most important inventions of 20th century. It is not an exaggeration if we say that next to computer it is the laser that is bringing a revolutionary change in our daily life Also lasers are the generator of light which is biased on the amplification of light by means of stimulated radiation of atoms. The strongest laser is purple. Laser is profitably used in almost every field. High power lasers are used in materials processing, nuclear fusion, medical field, defense etc. In other hand low power lasers are used in CD players, laser printer, optical memory cards, data processing, range finders, optical communications etc application. Lasers are used in fundamental researches. Due to the development of advance semiconductor material, a remarkable change has been made in our communication engineer such like optical fiber sending high data transmission through optical fiber using semiconductor laser. Fig.1.0: Transmitting Laser as a light source through Optical Fiber to Receiver

10. Table 1.1: History of laser Year Discover Types of laser/Principle 1917 Albert Einstein Stimulated the emission process which is a milestone of laser invention. 1952 N.G. Basov, A.M. Prokhorov and Townes Found the principle of Maser 1954 Townes, Gordon, Zeiger Invented the Maser 1958 Townes, Schawlow, Basov, Prokhorov Found the Laser principle 1960 Theodore Maiman Invented Ruby Laser which is the first types of laser. 1961 A. Javan, W. Bennett and D. Harriott Invented Helium-Neon Laser 1961 L.F. Johnson and K, Nassau Developed Neodymium Laser 1962 R. Hall Semiconductor Laser 1963 C.K.N. Patel Developed a new Carbon dioxide Laser 1964 W. Bridges Argon Ion Laser 1966 W. Silfvast, G.R. Fowles and B.D. Hopkins He-Cd Laser 1966 P.P. Sorokin and J.R Lankard Tunable Dye Laser 1975 J.J. Ewing and C. Brau Excuner Laser 1976 J.M.J. Madey and coworkers Free-electron Laser 1979 Walling and coworkers Alexandrite Laser 1985 D. Mathews and coworkers X-ray Laser

11. Table 1.2: History of optical fiber Year Description 1880 Alexander Graham Bell experimented with an apparatus he called a photophone.The photophone was a device constructed from mirrors and selenium detectors that transmitted sound waves over a beam of light- 1930 J.L. Baird, an English scientist, and C.W.Hansell, a scientist from the United States, were granted patents for scanning and transmitting television images through uncoated fiber cables. After few years of 1930 German scientist named H. Lamm successfully transmitted images through a single glass fiber. 1950 Before that year people considered fiber optics more of a toy or a laboratory stunt but after that year people that any substantial breakthrough was made in the field of fiber optics. 1951 A.C.S. van Heel of Holland and H. H. Hopkins and N. S. Kapany of England experimented with light transmission through bundles of fibers. 1958 Charles H. Townes, an American, and Arthur L. Schawlow, a Canadian, wrote a paper describing how it was possible to use stimulated emission for amplifying light waves (laser) as well as microwaves (maser). 1960 Theodore H. Maiman, a scientist with Hughes Aircraft Company, built the first optical maser. 1967 K. C. Kao and G. A. Bockham of the Standard Telecommunications Laboratory in England proposed a newcommunications medium using cladded fiber cables

12. Year Description 1970 Kapron, keck, and Maurer of Corning Glass Works in Corning, New York, developed an optical fiber with losses less than 2dB/km. That was the big breakthrough needed to permit practical fiber optics communications systems. Early 1980s The refinement of optical cables and the development of high-quality, affordable light sources and detectors opened the door to the development of high-quality, high capacity, efficient, and affordable optical fiber communications systems. 1988 NEC Corporation set a new long-haul transmission record by transmitting 10 gigabytes per second over 80.1 kilometers of optical fiber. Also in that year, the American National Standards Institutes (ANSI) published the Synchronous Optical Network (SONET). Mid-1990s Optical voice and data networks were commonplace throughout the United States and much of the world.

13. 1.2 Laser Fundamentals: The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation. This is a device to produce a beam of monochromatic light in which all the waves are in phase or are coherent. Lasers contain four primary components regardless of style, size or application (Fig.1). 1. Active Medium The active medium may be solid crystals such as ruby or Nd: YAG, liquid dyes, gases like CO2 or Helium/Neon, or semiconductors such as GaAs. Active mediums contain atoms whose electrons may be excited to a metastable energy level by an energy source. 2. Excitation Mechanism Excitation mechanisms pump energy into the active medium by one or more of three basic methods; optical, electrical or chemical. 3. High Reflectance Mirror A mirror which reflects essentially 100% of the laser light. 4. Partially Transmissive Mirror A mirror which reflects less than 100% of the laser light. Fig.1.1: Laser components

14. 1.3 Laser action The laser medium transfers its external energy into laser beam. The molecules or atoms of a crystal are excited in the laser cavity; as a result more of the molecules or atoms are at a higher level of energy than at the lower energy level. If the frequency of the photon is related to the difference in energy between the ground states and excited states strikes an excited atom, the atom is stimulated as it returns to the lower state of energy to release a second photon with the same frequency, in the phase with the bombarding photon as well as the same direction. This quantum mechanical process is referred to as stimulated emission. When electricity and light such as flash lamp or another laser is added to the atoms of laser medium, the majority of electrons are excited to a higher level of energy state. When that number exceeds the number of electrons in the lower level of energy state, this will lead to a phenomenon known as population inversion. This energy state is unstable for the electrons, the electrons will remain in the high energy state for a short period of time and then they decay back to the low energy level. The decay or emission can happen in two ways: spontaneous or stimulated. The light is amplified when the amount of stimulated emission or decay due to light that passes through is greater than the amount of absorption. This amplification will continue until there is a built up of sufficient energy for a burst of laser to be transmitted through the optical cavity. Strictly speaking, these are the necessary and crucial component of a laser. The laser output can occur in several forms. It may be continuous constant-amplitude output, more commonly referred to as continuous wave or CW. It may also be pulsed output. A laser is different in three main ways: (i). The light emitted from a laser is monochromatic, that is, it only has one wavelength (color). Ordinary light on the other hand, is a combination of many different wavelengths (colors) and led light is also monochromatic.

15. (ii) It is coherent, that is, the waves of light of the laser are in phase. Just like two sea waves when they are both at their maximum height, all waves of the light in a laser are exactly at the same amplitude, all time but light of the led are not in phase(in-coherent). (iii) It is highly directional. Laser light is emitted as a narrow beam in a specific direction. Ordinary light coming from the sun, a light bulb or a candle is emitted in various directions from the source. Since laser rays are so concentrated they also have much more brilliance than rays from other sources. Fig.1.2: LASER, One color (monochromatic) and waves in phase (coherent). Fig.1.3: Ordinary light waves (many different colors) and LED, one color (monochromatic) and waves not in phase (non-coherent).

16. Table 1.3: Differences between LED and LASER Parameter LED LASER Wavelength Content: A monochromatic source consists of radiation of a single wavelength or a very small range of wavelength. Large Small (desirable; more monochromatic) Principle of Emission Spontaneous; random photon emission (hence, not directional) Stimulated Coherence : A constant phase difference between two waves Incoherent; since emission is spontaneous Coherent; since emission is stimulated; same phase as the stimulating photon Output Power Low High Directionality Low Highly directional Speed of Operation Slow Faster Numerical Aperture : determines the output pattern Higher Lower Ease of Use Easier to use; less complex circuitry Complex circuitry; needs thermal & optical stabilization circuits due to light amplification Lifetime Long Shorter Cost Cheaper Expensive

17. 1.4 Applications of Semiconductor Laser: LASERS IN MECHANICAL INDUSTRY Materials working are most important and fundamental in mechanical industry. It consists of cutting, drilling, welding etc. Jobs to be done on both metals and nonmetals. These processes require transfer to energy from the laser beam to the work piece. It can happen only if the material has high absorption at the wavelength corresponding to the laser beam. A material working requires large amounts of energy to be localized at specific areas in order to cause heating there. Therefore, the lasers should deliver large amount of power. The intensity of the laser beam can be enhanced with a suitable optical system that can focus the beam into a spot of about 10 to 100 m diameter. The lasers that are widely used in material working are the co laser and ND: YAG laser. Co lasers operate at 10.6 m and metals have high reflectivity at this wavelength. Drilling Drilling holes by a laser beam is based on the intrinsic evaporation of material heated by a series of powerful light pulses of short duration. The energy supplied for drilling should be such that raped evaporation of material takes place radial distribution of heat into the work piece occurs. Laser drilling is a non contact process and does not require a physical drill bit. The problems of wear and broken drills do not arise. As there is no physical contact between drill and work piece, the process becomes faster and is also highly reproducible. The drilling operation can be done with extremely high precision and in any desired direction. Lasers are routinely used for drilling in ceramic materials. Ceramic materials become brittle after they are burnt and therefore conventional drilling has to be performed prior to firing. The size and position of the hole may change after firing. Laser drilling is carried out after firing and therefore size variation does not take place.

18. Cutting A wide range of materials can be cut by Co lasers. The materials include paper, wood, cloth, glass, quartz, ceramics, composites, steel etc. The cutting process essentially consists of removing material. Laser cutting is done with the assistance of air, oxygen or dry nitrogen gas jet as shown in Fig.1.1. The role of the jet is to cool the surface of the material and blow away the debris from the cutting zone. The advantage of laser cutting is that it is fine and precise. It introduces a minimum mechanical distortion and thermal shock in the material being cut. The process does not introduce any contamination. It is easily automates and high production rates can be achieved. Brittle materials such as ceramics and glass are cut using pulsed lasers in order to minimized micro cracking. In metal cutting, the laser beam heats the metal to a high temperature where it burns as oxygen passes over it. It is therefore accentually the oxygen that does the job of cutting. It is widely used in fabrication of aircraft engine parts. Co lasers are used for selective removal of information from electrical wires and cables. While the insulation is stripped off, the electrical wire in side is not affected because of its high reflectivity. Fig.1.4: An arrangement for a gas-laser cutter.

19. Welding Welding is the joining of two or more pieces into a single unit. Let us consider the welding of metal plates. The metal plates are held in contact at their edges and a laser beam is made to move along the line of contact of the plates. The laser beam heats the edges of the two pates of their melting points and causes them to fuse together where they are in contact. The main advantage of laser welding is that it is a contact less process and hence there is no possibility of introduction of impurities into the joint. The heat affected zone is relatively small because of rapid cooling. Laser welding can be done even at difficult at reach places as illustrated in Fig.1.2. The process is easily automated. The most common laser used in welding is the Co laser. Both CW and pulsed lasers are used. Fig.1.5: Inert gas laser welding for otherwise locations. Heat Treating Heat treating is a process which consists of heating metals and certain other materials for some time to harden them. Heat treating converts the surface layer to a crystalline state that is harder and more resistant to wear. As metals are more reflecting at 10.6mm, an absorptive coating such as graphite or zinc phosphate is applied on the surface of the work piece to help it absorb the laser energy more efficiently. Laser heat treating requires a low amount of energy input to the work piece. Laser processing is advantageous as it can provide selective treatment of the desirable areas. Heat treatment is used to strengthen cylinder blocks, gears, camshafts etc in the automobile industry.

20. LASERS IN ELECTRONICS INDUSTRY Lasers are widely used for material processing in electronics industry also. They are mainly employed I wafer scribing, in repairing photo masks, in making electrical connections for thick film hybrid circuits etc areas. Investigations are being carried out on deposition of conducting metal films by decomposing metal-containing gases just above the surfaces of semiconductors and insulators. Scribing Scribing of silicon and ceramic substrates is very important in the electronics industry. Lasers are extensively used for breaking silicon chips having hundreds of ICs on them. Scribing involves drawing fine lines in brittle ceramic and semiconductor wafers. Computer controlled scribing with a laser is a non contact process and does not cause any damage to the surface. Low power signal mode Co lasers are generally used in these operations. Because of high absorption of nonmetallic materials at 10.6 m, Co laser is highly suitable. In order to obtain a sufficiently small diameter of the beam after focusing, single mode operation is preferred. Soldering Lasers are used in welding of leads of materials such as platinum, silver, palladium which are usually difficult to solder. Nd: YAG laser is used for this process of flux less soldering. The main advantages of laser soldering are as follows: (1) It is a non contact method and areas visible but difficult to reach can also be soldered. (2) Leads as thin as 0.001 inch. Diameter can be soldered without causing damage to them. (3) The process is highly cost effective.

21. Trimming Existing methods of manufacturing the thick and thin film resistors are not capable of producing required values of resistance. The resistance depends on the surface area of the film and as such the desired resistance value can be obtained by removing some surface material. Initially, a greater amount of film is deposited on the surface and the initial resistance is of low value. Then, the resistor is trimmed by removing part of the film material using a laser beam while the value of the resistor is continuously monitored. Use of lasers gives about 50% cost saving in comparison to other techniques. Photolithography Photolithography is a process used to form patterns on the surface of semiconductor wafers. In this process, the surface of a wafer is coated with a photo resist, which is a light sensitive material. Selective exposure of certain areas of the resist to light through a circuit pattern leaves the pattern in the resist material. Unexposed resist material is developed and washed. With growing miniaturization, smaller and smaller lines are to be made on the semiconductor wafer. The minimum width of the lines depends on the wavelength of light that exposes the photo resist. The shorter the wavelength, the finer are the features. Pulsed exciter lasers are used now a day in making circuits with fine features. LASERS IN NUCLEAR ENERGY Isotope Separation The natural uranium are used to fuel nuclear reactors contains mainly two principle isotopes, U-238 and U-235. U-238 is the more abundant isotope but it cannot sustain the fission chain reaction needed to drive the nuclear reactor. It is the U-235 isotope that sustains a fission reaction. Natural uranium contains only 0.7% of U-235 and to be useful for nuclear power generation or for production of atomic bombs, it is essential that about 3% U-235 is present. The isotope enrichment of uranium is a very important problem and in general this enrichment is performed using gas con diffusion.

22. Fig.1.6: Schematic of a Uranium isotope separation facility. Nuclear Fusion Fusion refers to releasing enormous amounts of energy through the fusion of nucleus of lighter elements to form nuclei of heavier elements. It is alternative source of energy and its chief attractions over nuclear fission area as follows: (1) It offers a low cost and pollution free energy. (2) It produces less radioactive nuclear waste materials. (3) The light nuclei required for a fusion reaction are available in abundance on earth than the heavy elements needed for fission. The main disadvantage is that the right conditions for caring out controlled fusion reactions are very hard to produce. Extremely high temperatures and pressures are required in order to make light nuclei to overcome their mutual repulsion and combine to release energy. LASERS IN MEDICINE Lasers are playing nowadays a very important role both in diagnostics and surgery. Lasers have become in association with optical fibers a powerful tool in diagnostics. Interiors of body which would otherwise be difficult or impossible to see can now be seen very easily with the help of optical fibers. Fiber-optic sensors are very useful both before during surgery. The focused laser beam proved to be a new and unique scalpel in the hands of surgeons. Laser surgery is a highly

23. sterile process since contact does not occur between surgical tool and the tissues being cut. The first big success of lasers in medicine was in the treatment of eye. Argon laser has been in use for several years to treat the detachment of the retina. The retina is the light sensitive layer at the back of the eye. Sometimes it becomes torn and gets detached from the back of the eye ball. The damage can spread and lead to total blindness. Conventional open-eye surgery was very risky. Laser treatment, on the other hand, is very simple and fast. Argon laser beam is focused on the desired point of the retina. The green beam of the laser is strongly absorbed by the red block cells of the retina and causes thermal effects which weld the retina back to the eye ball. Laser photocoagulation of retinopathies is a common treatment. Laser photocoagulation is used to destroy areas of new blood vessels and areas of hypoxic retina which is believed to further increase blood vessel proliferation. Fig.1.7: Lasers are used to rupture the cloudy membrane behind the eye lens. LASERS IN DEFENCE Application of lasers in military involves mainly ranging, guiding weapons to the intended target and laser beam itself acting as a weapon. Lasers are used in measurement of distances in the same way as micro wave do in radars. The optical radars, using laser beams, are employed in detecting distant objects and collecting information about them. This is known as range finding. In the optical range finding system, a laser transmits a signal which is typically a pulse

24. or a sequence of pulses. Nowadays, low flying aircrafts are used in ground attacks. They are fitted with laser instrumentation for measuring the range of the target and guiding the bomb onto it thereby reducing the human element to the minimum. The principle of laser guidance is as follows: The target is selected by a viewing telescope. The bomb is released from the aircraft at a certain height from where it falls freely under gravity. It carries a direction sensing seeker head and a servo loop. It is required from the laser weapons that their beam may either disable the enemy weapons or destroy them. It is easier to achieve the goal of damaging the infrared sensors on guided missiles or the sensitive electronic eyes of spy satellites. Laser powers of laser beam suffice for such type of operation. One of the strategic defense systems chalked out by USA is Star War where a possible nuclear attack by Intercontinental Ballistic Missiles from USSR is proposed to be thwarted. When a missile is launched, it can hit the target within a few minutes. It is required that the missile should be detected and destroyed before it can reach the target. It is only laser weapons that can do the job of intercepting the missile and make it inoperative. MEASUREMENT OF DISTANCE Interferometric Methods Modern engineering requires large-size items manufactured to very high accuracy. It involves measurement of distances from a meter to several meters accurate to within a few microns. Optical methods are used for such measurements, where Michelson interferometer or a variation of this instrument is employed. The retro reflector is essentially a truncated triangular prism where an incident ray is internally reflected twice and is redirected along a direction parallel to its incidence path. An accuracy of 0.1 m can be achieved and this technique can be used for a distance up to 100m. This technique is widely used for calibration, testing and measurements of machine tools. It controls tool motions, automatically compensating for the errors due to wear, facilitates marking- off of points of engagement for a cutting tool on the surface to be machined, precisely aligns fixtures in aircraft engine manufacturing and facilitates a number of similar jobs.

25. Laser Rangers The distances that can be measured using interferometric technique are limited to a few hundred meters. For measuring laser distances, amplitude modulated CW laser beams are employed. The beam from a He-Ne or Ga As laser is modulated by a sinusoidal wave. As a result, the amplitude of the beam varies along the direction of beam propagation. The modulated beam is directed to the object whose distance is to be measured. Lidar Large distances can be measured using pulse echo techniques. The technique is often known as optical radar or lidar and is used as a range finder in military applications. It consist a Q-switched ruby laser yielding giant pulses, a telescope, a photo detector and an accurate timer. The sharply collimated laser pulse is reflected by the target and by measuring the round trip transit time, the distance can be determined. Surveying A laser beam is used as a reference to measure the angle of deflection from required alignment. A low power signal mode He-Ne laser is used as the source. Using lasers, small deflection of large structures under varying load and long term small movements of dam structures can be measured. Other surveying applications include pipe laying, level grading, tunnel boring, fledging, alignment in hazardous environment like coal mines etc. VELOCITY MEASUREMENT It is well known that when a light beam gets scattered by a moving object, the frequency of the scattered wave is different from that of the incident wave; the shift is the frequency depends on the velocity of the object. Laser Doppler velocimeters are used for measuring fluid flow rates. The beam from a CW laser is split by a beam splitter; one of the components is reflected back from a fixed mirror and the other component undergoes scattering from the moving object. The

26. two beams are then combined and of interfere. The change in frequency leads to beating. The beam frequency is a direct measure of the velocity of motion of the object. HOLOGRAPHY In conventional photography a negative is made first and using it a positive print is produced later. The positive print is only a two dimensional record of light intensity received from a three dimensional object. It contains information about the square of the amplitude of the light wave that produced the image. A hologram does not contain a distinct image of the object. It is only a record of the interference pattern formed by the superposition of two content light beams. The interference pattern on a hologram consists of a complete pattern of alternate regions of dark and bright fringes. Holography is a two state process. In the first state a hologram is recorded in the form of interference pattern. In the second state the hologram acts as a diffraction grating for the reconstruction beam and the image of the object is reconstructed from the hologram. Fig.1.8: Generation of a hologram.

27. 1.5 Objectives of this work: The objectives of this work are to study the structures and materials of 1550nm semiconductor laser. to solve the rate equations of semiconductor laser for analyzing its characteristics. to analyze the modulation performance characteristics of InGaAsP laser. to introduce the optical fiber communication system. to study high speed data transmission through optical fiber using semiconductor laser.

28. 1.6 Introduction to this thesis: The chapter of this book has been divided into five parts. In these chapters all the topics have been discussed briefly and some topics have been explained by using graphs, figures and tables. Chapter 2, starting with the structures of semiconductor laser then described the materials and energy band theory with also the temperature effect of laser. After that the rate equations and the output power of laser have been demonstrated. Chapter 3, described briefly the performance characteristics of 1550 nm VCSEL laser by exposing some graphs such like PHOTON vs. TIME, CARRIER vs. TIME, OUTPUT POWER vs. TIME etc. Chapter 4, represents modulation response for high speed data transmission. By completing the modulation part successfully this laser will transmit data through optical fiber. Chapter 5, discussed the whole work and conclusion of this thesis.

29. Chapter 2 Theory on Semiconductor Laser 2.1 Introduction Semiconductor device such as laser now plays an important role in our daily life because of research and development of optoelectronic devices and optointegrated circuits have received a tremendous boost with development of low loss of optical fiber for long distance communication. The semiconductor lasers with optoelectronics market are projected to grow at an annual rate of 9-10%. Lasers are in fact generators of light. They are based on the amplification of light by means of stimulated radiation of atoms or molecules. In this chapter, the basic understanding of VCSEL laser with the properties of intrinsic and extrinsic semiconductor has derived. In an intrinsic semiconductor, the number of thermally generated electrons is equal to the number of holes. In an extrinsic semiconductor, impurities are added to the semiconductor that can contribute either excess electrons or excess holes and the operations of semiconductor materials are based on the creation or annihilation of electron- hole pairs. Starting with the principle of edge emitting and surface emitting laser then, sequentially the long wavelength and short wavelength dependent lasers applications, 4homojunction and heterojunction structure, p-n junction, p-type and n-type materials, energy band diagram with conduction and valence band, introduced doping, emission and absorption, population inversion, temperature dependence of a laser have derived. Finally, the rate equations of VCSEL semiconductor laser have discussed.

30. 2.2 Edge Emitting Laser Laser diodes consist of a p-n diode with an active region where electrons and holes recombine resulting in light emission. In addition, a laser diode contains an optical cavity where stimulated emission takes place. The laser cavity consists of a waveguide terminated on each end by a mirror. As an example, the structure of an edge-emitting laser diode is shown in Fig.2.1. Photons, which are emitted into the waveguide, can travel back and forth in this waveguide provided they are reflected at the mirrors. The distance between the two mirrors is the cavity length, labeled L. Fig.2.1: Structure of an edge-emitting laser diode. 2.3 Vertical Cavity Surface Emitting Laser (VCSEL) A VCSEL has the optical cavity axis along the direction of current flow rather than perpendicular to the current flow as in conventional laser diodes. The active region length is very short compared with the lateral dimensions so that the radiation emerges from the surface of the cavity rather than from its edge. The reflectors at the ends of the cavity are dielectric mirrors made from alternating high and low refractive index quarter wave thick multi layers.

31. Such dielectric mirrors provide a high degree of wavelength selective reflectance at the required free-space wavelength if the thickness of alternating layers d1 and d2 with refractive indices n1 and n2 are such that + = (2.1) which then leads to the constructive interference of all partially reflected waves at the interfaces. Fig.2.2: (a) A simplified schematic illustration of a Vertical Cavity Surface Emitting Laser (VCSEL). (b) The field distribution of the laser mode inside a vertical cavity laser with L = n with three quantum wells. Since the wave is reflected because of a periodic variation in the refractive index as in a grating, the dielectric mirror is essentially a Distributed Bragg Reflector (DBR). The wavelength in Eq. (2.1) is chosen to coincide with the optical gain of the active layer. High reflectance end mirrors are needed because the short cavity length L reduces the optical gain of the active layer in as much as the optical gain is proportional to exp (gL) where g is the optical gain coefficient.

32. There may be 20-30 or so layers in the dielectric mirrors to obtain the required reflectance (~99%). The whole optical cavity in Fig.2.2 looks vertical if we keep the current flow the same as in a conventional laser diode cavity. The active layer is generally very thin ( < 0.1 m) and is likely to be a Multiple Quantum Well (MQW) for improved threshold current. The required semiconductor layers are grown by epitaxial growth on a suitable substrate which is transparent in the emission wavelength. For example, a 980 nm emitting VCSEL device has InGaAsP as the active layer to provide the 980 nm emission, and a GaAs crystal is used as substrate which is transparent at 980 nm. The dielectric mirrors are then alternating layers of AlGaAs with different compositions and hence different band gaps and refractive indices. The top dielectric mirror is etched after all the layers have been epitaxially grown on GaAs substrate to arrive at the structure shown in Fig.2.2 In practice, the current flowing through the dielectric mirrors give rise to an undesirable voltage drop and methods are used to feed the current into the active region more directly, for example, by depositing peripheral contacts close to the active region. There are various sophisticated VCSEL structures and Fig2.2 is only one simplified example. The vertical cavity is generally circular in its cross section so that the emitted beam has a circular cross-section, which is an advantage. The height of the vertical cavity may be as small as several microns. Therefore the longitudinal mode separation is sufficiently large to allow only one longitudinal mode to operate. However, there may be one or more lateral (transverse) modes depending on the lateral size of the cavity. In practice there is only one single lateral mode (and hence one mode) in the output spectrum for cavity diameters less than~8m. Various VCSELs in the market have several lateral modes but the spectral width is still only~0.5 nm, substantially less than a conventional longitudinal multimode laser diode. With cavity dimensions in the microns range, such a laser is referred to as a micro laser. One of the most significant advantages of micro lasers is that they can be arrayed to construct a matrix emitter that is a broad area surface emitting laser source. Such laser arrays have important potential applications in optical interconnect and optical computing technologies. Further, such laser arrays can provide a higher optical power than that available from a single

33. conventional laser diode. Powers reaching a few watts have been demonstrated using such matrix lasers. 2.4 Chart of Laser wavelengths with applications

34. Table 2.1: Wavelength ranges of different materials. Material systems active layer/confining layers Useful wavelength range(m) Substrate GaAs/AlGaAs 0.8-0.9 GaAs GaAs/InGaP 0.9 GaAs AlGaAs/AlGaAs 0.65-0.9 GaAs InGaAs/InGaP 0.85-1.1 GaAs GaAsSb/GaAlAsSb 0.9-1.1 GaAs GaAlAsSb/GaSb 1.0-1.7 GaSb InGaAsP/Inp 0.92-1.7 Inp 2.5 Semiconductor Laser Structure Homo-junction Structure The first semiconductor lasers consisted of two layers made from the same compound, generally gallium arsenide, one doped with a material that added extra electrons to the conduction band to make an n-type semiconductor, the other with a material that produced holes in the valence band to make a p-type material. A junction zone separated them. These devices were called homostructure or homojunction lasers because their two layers were made from the same compound semiconductor. An addition of p-n junction forward biased material is injected with the requirement of stimulated emission and optical feedback. For simplicity degeneration the p region and n region which are essential for creating a large density of electrons and holes. After that using band flat conditions a large density of electrons and holes are injected to the opposite side. For a semiconductor laser action is made to occur on p-side by electron injection. Over the active region electron and holes recombine radiatively to produce photon and the radioactive recombination rate can be high depending on the injection level. The photon can be emitted stimulate upward or downward transmission. The former is the absorption process, whereby an electron is raised from the valance band to the conduction band.

35. The latter occur when the photon transmit to downward. The emitting photon is the same phase and frequency as the original photon. The losses in the active regions of the lasers can be occurred due to free carrier absorption, scattering at defects and in homogeneities and other non radiative transmissions. Fig.2.3: A schematic illustration of a GaAs homojunction laser diode. The cleaved surfaces act as reflecting mirrors. In the active region of the junction laser, the gain will eventually exceed the losses with an enhancement in the rate of stimulated emission. Outside the active region the losses will dominate. Heterojunction Structure (i) Single Heterojunction Structure A contact between two semiconductors that differ in their chemical composition. At the interface there usually is a change in, for example, the width of the energy gap, the mobility of the charge carriers, and the effective masses of the carriers. In abrupt single heterojunctions the change in properties occurs over a distance that is comparable to or less than the width of the space-charge region. Depending on the doping of the two sides of the heterojunction, it is possible to obtain a p-n, a p-p, or an n-n heterojunction. Combinations of various single semiconductor heterojunctions and p-n junctions form single heterostructures.

36. An ideal match of crystal lattices is possible in a heterojunction only if the crystal lattices of the materials being joined coincide in their type, orientation, and spacing. Also, in an ideal heterojunction the interface must be free from structural and other defects, such as dislocations and charged centers, and from mechanical stresses. The most widely used semiconductor heterojunctions are single-crystal heterojunctions between semiconductor compounds of the type solid solutions thereof based on arsenides, phosphides, and antimonides of Ga and Al. Since there is only a small difference between the covalent radii of Ga and Al, the change in chemical composition occurs without any change in lattice spacing. The manufacture of single- crystal semiconductor heterojunctions became possible as a result of the development of methods for the epitaxial growth of semiconductor crystals. Semiconductor single heterojunctions are used in various semiconductor devices, such as semiconductor lasers, light- emitting diodes, photocells, and optrons. A thin electron accumulation layer is generated along a single heterojunction which has a different electron affinity. This electron accumulation layer suffers less ionized-impurity scattering, because the thickness does not exceed the spread of an electron wave. A channel constituted with this electron accumulation enjoys excellent electron mobility, particularly at cryogenic temperatures. Fig.2.4: Single Heterojunction Structure.

37. (ii) Double Heterojunction Structure In Double Heterostructer laser, AlGaAs has Eg of 2 eV and GaAs has Eg of 1.4 eV. p-GaAs is a thin layer (0.1 0.2 um) and is the Active Layer where lasing recombination occurs. Both p regions are heavily doped and are degenereate with EF in the VB. With an adequate forward bias Ecof n-AlGaAs moves above Ec of p-GaAs which develops a large injection of electrons from the CB of n-AlGaAs to the CB of p-GaAs. These electrons are confined to the CB of the p- GaAs due to the difference in barrier potential of the two materials. Due to the thin p-GaAs layer a minimal amount of current only is required to increase the concentration of injected carriers at a fast rate. This is how threshold current is reduced for the purpose of poulation inversion and optical gain. A semiconductor with a wider band gap (AlGaAs) will also have a lower refractive index than GaAs. This difference in refractive index is what establishes an optical dielectric waveguide that ultimately confines photons to the active region. Fig.2.5: A double heterostructure diode has two junctions which are between two different badgap semiconductors (GaAs and AlGaAs). Fig.2.6: Simplified energy band diagram under a large forward bias Lasing recombination takes place in the p-GaAs layer, the active layer.

38. Fig.2.7: Higher band gap materials have lower refractive index and AlGaAs layers provide lateral optical confinement. In Fig.2.8 substrate is n-GaAs, confining layers are n-AlGaAs and p-AlGaAs, active layer is p- GaAs (870-900nm) and additional contacting layer is p-GaAs allows better electrode contact and avoids Schottky junctions which limit current. The p and n-AlGaAs layers provide carrier and optical confinement by forming heterojunctions with the p-GaAs. Current density J is not uniform laterally from the stripe contact and current is maximum along the central path and diminishes on either side with confinement between path 2 and 3. population inversion and therefore optical gain occurs where current density exceeds threshold current values. Fig.2.8: Schematic illustration of the structure of a double heterojunction laser diode.

39. 2.6 Intrinsic and Extrinsic Semiconductor materials Intrinsic material A perfect semiconductor crystal with no impurities or lattice defects is called an intrinsic semiconductor. In such material there are no charge carriers at 0K, since the valence band is filled with electrons and the conduction band is empty. Fig.2.9: Energy band diagram for intrinsic material. At higher temperatures electron-hole pairs are generated as valence band electrons are excited thermally across the band gap to the conduction band. These EHPs are the only charge carriers in intrinsic material. The generation of EHPs can be visualized in a qualitative way by considering the breaking of covalent bonds in the crystal lattice (Fig.2.10). If one of the Si valence electrons is broken away from its position in the bonding structure such that it becomes free to move about in the lattice, a conduction electron is created and a broken bond (hole) is left behind. The energy required to break the bond is the band gap energy Eg. This model helps in visualizing the physical mechanism of EHP creation, but the energy band model is more productive for purposes of quantitative calculation. One important difficulty in the broken bond model is that the free electron and the hole seem deceptively localized in the lattice. Actually, the positions of the free electron and the hole are spread out over several lattice spacings and should be considered quantum mechanically by probability distributions.

40. Since the electrons and holes are created in pairs, the conduction band electron concentration n (electron per cm3 ) is equal to the concentration of holes in the valence band p (holes per cm3 ). Each of these intrinsic concentrations is commonly referred to as . Thus for intrinsic material = = (2.2) Fig.2.10: Electron-hole pairs in the covalent bonding model of the Si crystal. At a given temperature there is a certain concentration of electron-hole pairs ni. Obviously, if steady state carrier concentration of electron-hole pairs ni. obviously, if a steady state carrier concentration is maintained, there must be recombination of EHPs at the same rate at which they are generated. Recombination occurs when an electron in the conduction band makes a transition (direct or indirect) to an empty state (hole) in the valence band, thus annihilating the pair. If we denote the generation rate of EHPs as gi (EHP/cm3 s ) and the recombination rate as ri, equilibrium requires that = (2.3)

41. Each of these rates is temperature dependent. For example, gi (T) increases when the temperature is raised, and a new carrier concentration ni is established such that the higher recombination rate ri (T) just balances generation. At any temperature, we can predict that the rate of recombination of electrons and holes ri is proportional to the equilibrium concentration of electrons n0 and the concentration of holes p0: = = = (2.4) The factor is a constant of proportionality which depends on the particular mechanism by which recombination takes place. Extrinsic Material In addition to the intrinsic carriers generated thermally, it is possible to create carriers in semiconductors by purposely introducing impurities into the crystal. This process, called doping, is the most common technique for varying the conductivity of semiconductors. By doping, a crystal can be altered so that it has a predominance of either electrons or holes. Thus there are two types of doped semiconductors, n-type (mostly electrons) and p-type (mostly holes). When a crystal is doped such that the equilibrium carrier concentration n0 and p0 are different from the intrinsic carrier concentration ni, the material is said to be extrinsic. 2.7 N-type and P-type materials N-type material When impurities or lattice defects are introduced into an otherwise perfect crystal, additional levels are created in the energy band structure, usually within the band gap. For example, an impurity from column V of the periodic table (P, As and Sb) introduces an energy level very near the conduction band in Ge or Si. This level is filled with electrons at 0K, and very

42. littlethermal energy is required to excite these electrons to the conduction band (Fig.2.11). Thus at about 50-100K virtually all of the electrons in the impurity level are donated to the conduction band. Such an impurity level is called a donor level, and the column V impurities in Ge or Si are called donor impurities. For (Fig.2.11) we note that the material doped with donor impurities can have a considerable concentration of electrons in the conduction band, even when the temperature is too low for the intrinsic EHP concentration to be appreciable. Thus semiconductors doped with a significant number of donor atoms will have n0 (ni, p0) at room temperature. This is n-type material. Fig.2.11: Donation of electrons from a donor level to the conduction band. P-type material Atoms from column III (B, Al, Ga and In) introduce impurity levels in Ge or Si near the valence band. These levels are empty of electrons at 0K (Fig.2.12). At low temperatures, enough thermal energy is available to excite electrons from the valence band into the impurity level accepts electrons from the valence band. Since this type of impurity level accepts electrons from the

43. valence band, it is called an acceptor level, and the column III impurities are acceptor impurities in Ge and Si. As Fig.2.12 indicates, doping with acceptor impurities can create a semiconductor with a hole concentration p0 much greater than the conduction band electron concentration n0. This is p-type material. Fig.2.12: Acceptance of valence band electrons by an acceptor level, and the resulting creation of holes. 2.8 Formation of p-n junction The p-n junction diode is formed by creating adjoining p and n type semiconductor layers in a single crystal, as shown in Fig.2.13 (a) A thin depletion region or layer is formed at the junction through carrier recombination which effectively leaves it free of mobile charge carriers (both electrons and holes). This establishes a potential barrier between the p and n type regions which restricts the interdiffusion of majority carriers form their respective regions, as illustrated in Fig.2.13 (b). In the absence of an externally applied voltage no current flows as the potential barrier prevents the net flow of carriers from one region to another. When the junction is in this

44. equilibrium state the Fermi level for the p and n type semiconductor is the same as shown in Fig.2.13 (b). Fig.2.13: (a) The impurities and charge carriers at a p-n junction. (b) The energy band diagram corresponding to (a). The width of the depletion region and thus the magnitude of the potential barrier are dependent upon the carrier concentrations (doping) in the p and n type regions, and any external applied voltage. When an external positive voltage is applied to the p type region with respect to the n type, both the depletion region width and the resulting potential barrier are reduced and the diode is said to be forward biased. Electrons from the n type region and holes from the p type

45. region can flow more readily across the junction into the opposite type region. These minority carriers are effectively injected across the junction by the application of the external voltage and form a current flow through the device as they continuously diffuse away from the interface. However, this situation in suitable semiconductor materials allows carrier recombination with the emission of light. 2.9 Energy Band for materials In an isolated atom the valence electrons can exist only in one of the allowed orbitals each of a sharply defined energy called energy levels. But when two atoms are brought nearer to each other, there are alterations in energy levels and they spread in the form of bands. Now, some basic structures are, Band A range of some physical variable, as of radiation wavelength or frequency. A range of very closely spaced electron energy levels in solids, the distribution and nature of which determine the electrical properties of a material. Dopant A small quantity of a substance, such as phosphorus, added to another substance, such as a semiconductor, to alter the latter's properties. Energy The work that a physical system is capable of doing in changing from its actual state to a specified reference state, the total including, in general, contributions of potential energy, kinetic energy, and rest energy. Gap A suspension of continuity; hiatus. A conspicuous difference; disparity.

46. Model A tentative description of a system or theory that accounts for all of its known properties. Energy bands are of following types The Conduction Band The conduction band is the upper band of allowed states. When it is drawn it is represented by a line labeled by Ec which represents the lowest possible energy state in the conduction band. This band is usually empty, it contains few or no electrons since energy is required for them to get there from the valence band. Electrons in the conduction band are free to move about the crystal, thus the name conduction band. If an electron does manage to get to the conduction band, it resides there for mere fractions of a second (an average lifetime). When it loses its energy it drops back down to the valence band emitting its energy as heat, light or by transferring it to another electron. The higher energy level band is called the conduction band. (i) It is also called empty band of minimum energy. (ii) This band is partially filled by the electrons. (iii) In this band the electrons can gain energy from external electric field. (iv) The electrons in the conduction band are called the free electrons. They are able to move anywhere within the volume of the solid. (v) Current flows due to such electrons. iv) The number of atoms of impurity element is about 1 in atoms of the semiconductor. (v) ne is not equal to nh.

47. (vi) In these fermi level shifts towards valence or conduction energy bands. (vii) Their conductivity is high and they are practically used. The Valence Band The valence band is the lower band of allowed states. In the drawings it is depicted by a line labeled by Ev which represents the highest energy state in the valence band. Since electrons have a tendency to fill the lowest available energy states, the valence band is always nearly completely filled with electrons, especially as the temperature falls toward 0K. As the temperature rises or light is introduced, electrons can absorb the energy and leave the valence band to rise up to the conduction band. When an electron gains enough energy, greater than the band gap energy, and gets to the upper band, it is free to move, becoming a carrier and therefore increasing the conductivity of the semiconductor. When electrons leave the valence band they leave behind a hole which can move about the crystal, also adding to the conductivity. The energy band formed by a series of energy levels containing valence electrons is known as valence band. At 0 K, the electrons fill the energy levels in valence band starting from lowest one. (i) This band is always filled by electrons. (ii) This is the band of maximum energy. (iii) Electrons are not capable of gaining energy from external electric field. (iv) No flow of current due to such electrons. (v) The highest energy level which can be occupied by an electron in valence band at 0 K is called Fermi level. The Band Gap Energy The band gap energy is the energy needed to break a bond in the crystal. When a bond is broken, the electron has absorbed enough energy to leave the valence band and "jump" to the

48. conduction band. The width of the band gap determines the type of material (conductor, semiconductor, insulator) you are working with. This is shown pictorially using a band diagram. Crystalline materials can be classified according to their band gap. An insulator is a poor conductor since it requires a lot of energy, 5-8 eV, to excite the electrons enough to get to the conduction band. We can say that the width of the band gap is very large, since it requires that much energy to traverse the band gap, and draw the band diagram respectively. A metal is an excellent conductor because, at room temperature, it has electrons in its conduction band constantly, with little or no energy being applied to it. This may be because of its narrow or nonexistent band gap, the conduction band may be overlapping the valence band so they share the electrons. The band diagram would be drawn with Ec and Ev very close together, if not overlapping. The reason semiconductors are so popular is because they are a medium between a metal and an insulator. The band gap is wide enough to where current is not going through it at all times, but narrow enough to where it does not take a lot of energy to have electrons in the conduction band creating a current. Fig.2.14: The band structures of insulator, conductor and semiconductor.

49. Introducing Dopants When a semiconductor is doped, energy states are introduced in the band gap. If it is doped with donors, the energy states are called donor states. Because it takes very little energy, much less than the band gap energy, to free the electron that inhabits the donor state, the states are shown close to the conduction band. Adding donors, therefore, adds more electrons to the conduction band (without adding holes to the valence band) making the semiconductor more conductive. Acceptor states are introduced into the forbidden gap if the semiconductor is doped with acceptors. These initially empty states readily accept an electron to complete its bonds with the four nearest neighbors in the crystal. When an electron from the valence band transitions to an acceptor state, it leaves behind a hole. The energy required for an electron to move to an acceptor state is much less than the band gap energy so it is shown close to the valence band. Holes are created without creating electrons. This note finds the number of electrons in the conduction band of a semiconductor, and the number of holes in the valence band. By definition, the density of states is the number of single-particle states per unit energy range and unit volume. The fraction of electrons in those states is given by . Therefore the number of electrons in the conduction band per unit volume is given by = (2.5) where, is the energy at the bottom of the conduction band and that at the top of the band.

50. Fig 2.15: Dopants cycle. Fig.2.16: Band diagram and the electron-hole distribution between conduction band and valence band. 2.10 Absorption, Spontaneous Emission, Stimulated Emission of a Laser An electron in an atom can be excited from an energy level E1 to a higher energy level E2 by the absorption of a photon of energy h = E2 E1 as shown in Fig.2.17 (a). When an electron at a

51. higher energy level transits down in energy to an unoccupied energy level, it emits a photon. There are essentially two possibilities for the emission process. The electron can undergo the downward transition by itself quite spontaneously, or it can be induced to do so by another photon. Fig.2.17: Absorption, Spontaneous emission and stimulated emission. In spontaneous emission, the electron falls down in energy from level E2 to E1 and emits a photon of energy hv = E2 E1 in a random direction as indicated in Fig.2.17 (b). Thus, a random photon is emitted. The transition is spontaneous provided that the state with energy E1 is not already occupied by another electron. In stimulated emission, an incoming photon of energy hv = E2 E1 stimulates the whole emission process by inducing the electron at E2 to transit down to E1. The emitted photon is in phase with the incoming photon, it is in the same direction, it has the same polarization and same energy since hv = E2 E1 as shown in Fig.2.17 (c). During stimulated emission, the electric field of the incoming photon coupling to the electron and there by driving it with the same frequency as the photon. The forced oscillation of the electron at a frequency v = (E2 E1)/h causes it to emit electromagnetic radiation whose electric field is in total phase with that of the stimulating photon. When the incoming photon leaves the site, the electron can return to E1 because it has emitted a photon of energy hv = E2 E1. Stimulated emission is the basis for obtaining photon amplification since one incoming photon results in two outgoing photons which are in phase.

52. 2.11 Population Inversion of a Laser To obtain stimulated emission, the incoming photon should not be absorbed by another atom at E1. For a collection of atoms, the majority must be at a higher energy level E2 to avoid absorption at E1. If there are more atoms at E2 than at E1 then it is called population inversion. But this is not possible with only two energy levels, because at steady state, the incoming photon will cause as many upward excitations as downward stimulated emissions. Let us consider the three energy level system shown in Fig.2.18. Suppose that an external excitation causes the atoms in this system to become excited to the energy level E3. This is called the pump energy level and the process of exciting the atoms to E3 is called pumping. Fig.2.18: (a) Atoms in the ground state are pumped up to the energy level E3 by incoming photons of energy h13 = E3 E1. (b) Atoms at E3 rapidly decay to the long-lived state at energy level E2by emitting photons or emitting lattice vibrations h32 = E3 E2. (c) As the states at E2 are long-lived, they quickly become populated and there is a population inversion between E2 and E1. (d) A random photon (from a spontaneous decay) of energy h21 = E2 E1can initiate stimulated emission. Photons from this stimulated emission can themselves further stimulate emissions leading to an avalanche of stimulated emissions and coherent photons being emitted. From E3 the atoms decay rapidly to an energy level E2 ( a long-lived state )Since the atoms cannot decay rapidly from E2 to E1 they accumulate at this energy level causing a population inversion between E2 and E1 as pumping takes more and more atoms to E3 and hence E2. When one atom at E2 decays spontaneously, it emits a photon (a random photon) which can go on to a

53. neighboring atom and cause that to execute stimulated emission. The photons from the latter can go on to the next atom at E2 and cause that to emit by stimulated emission and so on. The result is an avalanche effect of stimulated emission processes with all the photons in phase so that the light output is a large collection of coherent photons. 2.12 Threshold Current and its Temperature dependence Inspite of very efficient heat sinking, the temperature of a laser diode may increase with time because of the large injection current density. It is desirable to have the lasing characteristics & in particular the threshold current, independent of device or ambient temperature. Unfortunately, all the junction lasers exhibit a temperature dependent threshold current. For an example, In this thesis much stronger temperature dependence has been observed in InGaAsP laser used for optical fiber communication. As band gap is reduced, the efficiency of Auger recombination, which is a non radiative process, increases. It is believed that this is mainly responsible for low values of T (40-80) K in device made with this material. Carrier leakage from the active region is also cited as a cause for low T.

54. 2.13 Summary In this chapter, different types of laser and characteristics of different wavelengths of different frequency are observed for different applications. Semiconductor laser structure (homojunction structure, single and double heterojunction structure) and semiconductor materials which are Intrinsic and Extrinsic have discussed. In an intrinsic semiconductor, no impurities are added. In an extrinsic semiconductor, impurities are added to the semiconductor that can contribute either excess electrons or excess holes and the operations of semiconductor materials are based on the creation or annihilation of electron-hole pairs. n-type and p-type materials are also discussed. When semiconductor dopes with a significant number of donor atoms will have n0 (ni, p0) at room temperature, then it is called n-type material and when semiconductor dopes with acceptor impurities which create a hole concentration much greater than electron concentration, then it is called p-type material and presented the formation of p-n junction. In this chapter, the characteristics of absorption, spontaneous emission, stimulated emission and population inversion of a laser are presented and also briefly discussed about the temperature dependent threshold current.

55. Chapter 3 Performance Characteristics of a 1550nm Semiconductor Laser 3.1 Introduction Vertical-Cavity Surface-Emitting Lasers (VCSELs) are a relatively recent type of semiconductor lasers. Very soon, VCSELs gained a reputation as a superior technology for short reach applications such as fiber channel, Ethernet intra-systems links and local area networks. In this work, the design and characteristics of a 1550 nm Multi Quantum Well (MQW) VCSEL using InGaAsP materials have been obtained by using MATLAB simulation tool. The obtained characteristics have been analyzed for obtaining better performance. The concentrations of InGaAsP QW material have chosen for achieving a superior performance. The structure of a top emitting VCSEL, presented in Fig.3.1 which consists three Quantum Wells with active region, two cladding layers and upper and bottom DBR stacks. Current is injected through the upper p-type contact of the device and the lower n-type contact is connected with InP substrate. The active layer is sandwiched by two Separate Confinement Hetero-structure (SCH) layers of InP where the photons are confined which is shown in Fig.3.2. The SCH layers are sandwiched by two cladding layers of GaInP. The top cladding layer is p-doped while the bottom layer is n- doped. At 300K the band gap energies of InGaAsP and InP and transparency carrier densities of InGaAsP and InP materials are presented in this chapter. List of parameters are presented which are used for computing and simulating the calculation. For achieving a superior performance, the concentrations of InGaAsP QW material have been chosen using the results of another research works. The rate equations of the VCSEL, carrier density, photon density, transparency carrier density, output power, threshold current, confinement factor, group velocity, material gain at threshold condition are solved by using these parameters. The obtained results have been analyzed for obtaining better performance.

56. Using these results rate of change of carrier density, rate of change of photon density, output power and injection current of a VCSEL are plotted in this chapter and injection current has also varied for observing their characteristics. 3.2 Structure of a 1550 nm VCSEL The laser resonator consists of two Distributed Brag Reflector (DBR) mirrors parallel to the wafer surface. The planar DBR-mirrors consist of layers with alternating high and low refractive indices. Each layer has a thickness of a quarter of the laser wavelength in the material, yielding intensity reflectivity above 99%. High reflectivity mirrors are required in VCSELs to balance the short axial length of the gain region. The structure of a top emitting VCSEL, presented in Fig.3.1, consists of an active region with three quantum wells which is separated by two cladding layers. The internal structure of the VCSEL cavity consist InGaAsP QWs with band gap energy of 0.8014 eV, refractive index (n1) of 3.5048. Each of the QWs has a thickness of 19.5nm and surrounding these there are two barriers of InP with band gap of 1.351 eV, a refractive index (n2) of 3.146. The VCSEL consists of 8 layer of Si/SiO2 material in the top DBR stacks (p-type) and 77 layers of InGaAsP/InP materials in the bottom DBR stacks (n-type) which is placed on an InP substrate. Current is injected through the upper p-type contact of the device and the lower n-type contact is connected with InP substrate as shown in this figure.

57. Fig.3.1: Schematic structure of 1550nm top emitting InGaAsP MQW VCSEL. Theoretically, for designing a VCSEL an optimum choice of material gain and transparency carrier density of InP and GaInP materials has to be made. The VCSEL is suitable for transmitting data at high speed through optical fiber. The structure of the VCSEL cavity is presented in Fig.3.2. Fig.3.2: The structure of the InGaAsP MQW VCSEL cavity consisting 3 QWs of 195 each.

58. QW material InGaAsP Bandgap energy, Egw = 0.8014 eV, Refractive index, n = 3.5048 Lattice constant, a = 5.869 , Transparency carrier density, Ntr = 1.341 1018 cm3 Barrier material InP Egb = 1.351 eV, n = 3.146, a = 5.869 , Ntr = 2.5301 1018 cm3 ( = . , = . ) Ecladd = 1.395eV, n = 3.139, a = 5.848 , Ntr = 2.6143 1018 cm3 Average refractive index in the cavity region, navg = ((33.5048)+(43.146)+(23.139)) 9 = 3.264 Length of the cavity, Lcavity = navg = (1550107) 3.264 = 474.877 107 cm Length of the cladding layers, Lcladding = 2 ( 4navg ) = 2 ( (1550107) (43.264) ) = 237.438 107 cm = 2 1187.19 Length of the active layers, barriers and SCH regions,

59. L = ( 2navg ) = (1550 107 ) (2 3.264) = 237.438 107 cm Length of the active region, Lactive = 3 195 = 58.5 107 cm Length of the barrier region, Lbarrier = 2 170 = 34 107 cm Length of the SCH region, LSCH = 2 724.69 = 144.938 107 cm The energy of the confined state in the conduction band (CB) well of thickness lw is calculated as E1 = h2 8mclw 2 = ((6.626 1034 )2 /(8 (4.163 1032) (19.5 109 )2))/(1.6 1019 ) = 0.0217eV where, h is the Plancks constant and mc is the effective mass of electron in the conduction band. The energy of the confined state in the valence band (VB) well of thickness lw is calculated as E2 = h2 8mvlw 2 = ((6.626 1034 )2 /(8 (4.0227 1031) (19.5 109 )2))/(1.6 1019 ) = 0.00224eV where, mv is the effective mass of hole in the valence band. The emission wavelength of the laser is related to the band gap energy of the material in the well as = hc Egw + E1 + E2 = { (6.626 1034 3 108) (0.8014 + 0.0217 + 0.00224) } 1.6 1019 = 1505nm where, Egw is the band gap energy of the material in the well and c is the velocity of the light.

60. For the above QW thickness the corresponding emission wavelength of the InGaAsP MQW VCSEL is obtained as 1505nm. 3.3 Calculation of structural and performance parameters of the semiconductor laser For finite order differential solution of the rate equation we use the following table of parameters. Some of the values have been taken from reference. Table 3.1: List of parameters to be used to find out the solution to rate equations. Types of Parameter Values Plancks constant(h) 6.626 1034 m2 kgs1 Boltzmann constant(k) 1.38 1023 m2 kgs2 Free space permittivity(0) 8.854 1012 F/m Speed of light(c) 3 1010 cm Energy band gap of the material,(Egw) 0.8014eV Active region radius, (r) 5.65 104 cm Refractive index of the QW material (InGaAsP) 3.5048 Refractive index of the Barrier material(InP) 3.146 Refractive index of the Cladding material(GaInP) 3.139 Temperature in Kelvin 300K Electron charge(q) 1.6 1019 c Injection current in the laser system(I) 6.1mA Current injection efficiency(i ) 0.8 Peak material gain coefficient(g0) 1620.2cm1

61. Types of Parameter Values Carrier lifetime(c) 2.63 109 sec Differential gain(a) 5 1016 cm2 Intrinsic absorption loss(i) 20cm1 Stationary electronic mass(m0) 9.1 1031 kg Reflectivity (R)of both the mirrors 0.999 Gain compression factor() 1.5 1017 cm3 Spontaneous emission factor(sp ) 1.69 104 Thickness of each QW, (lw) 19.5 109 m Transparency carrier density(Ntr) 1.34 1018 cm3 Carrier density at threshold point(Nth) 1.741 1018 cm3 The threshold current(Ith) .78mA Effective mass of electron at the conduction band(mc) 4.163 1032 kg Effective mass of hole at the valence band(mv) 4.0227 1031 kg All of these parameters are gathered for solving the rate equations. - Lasing wavelength, = 1550nm = 1550 107 cm - Refractive index (Si), nh1 = 3.6 - Refractive index (SiO2), nl1 = 1.45 - Refractive index (InGaAsP) nh2 = 3.5048 - Refractive index (InP), nl2 = 3.146 -Refractive index (GaInP) = 3.139 - Refractive index difference in the top DBR stack, n1 = 3.6 1.45 = 2.15 - Refractive index difference in the bottom DBR stack, n2 = 3.5048 3.146 = 0.3588

62. - Effective length of top Bragg reflector, LeffTop = 4n1 = 1550107 42.15 = 1.8023 105 cm (3.1) - Effective length of bottom Bragg reflector, LeffBottom = 4n2 = 1550107 40.3588 = 1.080 104 cm (3.2) - The average refractive index of the cavity, navg = 3.264 - Cavity length, Lcavity = navg = 1550107 3.264 = 4.749 105 cm (3.3) - Effective cavity length, L ff = Lcavity + LeffTop + LeffBottom (3.4) = (4.749 105) + (1.8023 105) + (1.080 104) = 1.73513 104 cm - NQW = 3, DQW = 19.5 107 - Active region length, Lactive = NQW DQW = 3 19.5 107 = 5.85 106 cm (3.5) - Active region radius, r = 5.65 104 cm - Area of the active region, A = r2 = 3.1416 (5.65 104 )2 = 1.0029 106 cm2 (3.6) - Active region volume, Va = Lactive A = 5.85 106 1.0029 106 = 5.867 1012 cm3 (3.7)

63. - Confine factor, = 2 Lactive Leff 0.9 = 2 5.85106 1.73513104 0.9 = 0.0607 (3.8) - The group velocity, vg = C navg = 31010 3.264 = 9.1912 109 cm/s (3.9) - ()= would be h 2 = 6.6261034 2 = 1.055 1034 Js1 (3.10) - The transparency carrier density, Ntr = 2 ( KT 22) 3 2 (mc mv) 3 4 (3.11) = 2 ( 1.38 1023 300 2 3.1416 (1.055 1034)2 ) 3 2 (4.163 1032 4.0227 1031 ) 3 4 = 1.341 1018 cm3 - Mirror loss, m = 1 Leff ln ( 1 R ) = 1 (1.73513104) ln( 1 0.999 ) = 5.766cm1 (3.12) - Photon life time, p = 1 {vg(i+m)} = 1 {(9.1912109)(20+5.766)} = 4.223 1012 sec (3.13) - The carrier density at threshold point (Nth) is written as, Nth = Ntr e +m g0 (3.14)

64. = 1.341 1018 e 20+5.766 0.06071620.2 = 1.743 1018 cm3 -The threshold current, Ith = qVaNth ic = 1.610195.86710121.7431018 0.82.63109 (3.15) =0.78mA - At threshold condition, the material gain of a Laser is expressed as, g = gth = (+m) = (20+5.766) 0.0607 = 424.48cm1 (3.16) -At steady state the photon density of a Laser can be written as, S = i (IIth) qvggthVa = 1.12 1015 cm3 (3.17) -For evaluating the performance of the designed 1550nm VCSEL the differential quantum efficiency is, d = i m +m = 0.8 5.766 20+5.766 = 0.1790 = 17.90% (3.18) 3.4 Characteristics of the 1550 nm VCSEL Characteristics of a 1550nm VCSEL is presented in this work. The rate equations of the VCSEL The rate of change of carrier density can be written as, = ( ) ( +) (3.19)

65. Using equation 3.19 the rate of change of carrier density of a VCSEL is plotted which is shown in Fig.3.3 Fig.3.3: Plot of carrier density vs. time of a 1550 nm InGaAsP 195 QW VCSEL at 300K, where, the injection current is 6.1mA. From the above figure it is found that the steady value of carrier density of the VCSEL is 2.204 1018 cm3 after a threshold value of carrier density where, Nth=1.743 1018 cm3 , where, the threshold current is 0.78mA and transparency carrier density is 1.341 1018 cm3 . The rate of change of photon density can be written as, = ( ) (+) + (3.20)

66. Using equation 3.20 the rate of change of photon density of a VCSEL is plotted which is shown in Fig.3.4 Fig.3.4: Plot of photon density vs. time of a 1550 nm InGaAsP 195 QW VCSEL where, the injection current is 6.1mA. From the above figure, it is found that the steady value of photon density of the VCSEL is 1.12 1015 cm3 for a value of injection current of 6.1mA at 300K. The output power (Pout) of a VCSEL can be expressed in terms of mirror loss coefficient m, group velocity vg, Plancks constant, the optical frequency , photon density, reflectivity and volume of the cavity. Photons escape out of the Laser cavity at a rate of vgm the output power from the mirrors is expressed as = (3.21) = (9.1912 109 5.766 6.626 1034 1.94 1014 1.12 1015 5.8671012 0.0607 ) = .7358 mW

67. Using equation (3.21) the output power of a VCSEL is plotted which is shown in Fig.3.5 Fig.3.5: Plot of output power vs. time of a 1550 nm InGaAsP 195 QW VCSEL, where the time is 2.63 109 sec and the injection current is 6.1mA. From the above figure it is found that the output power of the VCSEL is 0.7358 mW for a value of injection current of 6.1mA at 300K.

68. Using equation 3.19, 3.20, 3.21 and by changing the injection