Have you ever wondered, how do we write a CD in a CD writer for copying the information or data stored in one onto the other or reading of it? Or for that matter, did you ever realize what a showroom manger does to tell you the price of a product by simply reading into the price code bars printed on a product through an illuminating device & so on….? Infact, there is examples galore wherein, a specially created source of light comes into play making us simply to gesticulate with amazement to know that here comes the science of physics in live action…!

Without a surprise, in either of the two cases as being exemplified above and in many other similar instances familiar to us, it is practically the use of some specially created and modified beams of light what we call as LASERS.

What is a LASER

What is a LASER? LASER is basically an acronym for (Light Amplification by Stimulated Emission of Radiation) and is essentially a device that makes the said amplification of light in the sense that it helps in producing an intensified, monochromatic electromagnetic radiation. As we already know that the light in its particulate nature comes to us in the form of small packets of energy called as photons. As a substance absorbs these photons of light, its atoms gets excited and electrons jumps to a high energy level. When these electrons at a high energy level are again stimulated by bombarding photons onto them, they jump back to a lower energy level and in the process, releases a photon or photons that has the same frequency as the one being used for stimulating them and even travel in the same direction. With a sufficient number of electrons at high energy levels (in excited atoms) once stimulated by bombarding photons onto them such that they will cause further emission of photons of the same energy and frequency while returning back to lower energy levels. This process of stimulating and stimulated release of photons will cause a stimulated emission of radiations that come as a narrow beam of monochromatic radiations whose waves are parallel and in phase such that the said beam of light or radiation shall be noted for having extremely high energy with almost negligible spread or diffraction. A beam of light produced this way through a device is what we call a LASER beam.

Going by the many virtues of laser light or beams, most importantly of its being very powerful in nature, it has found its use in diverse applications right from medical diagnostic instruments to playing music in your CD player or reading price code bars to cutting & welding metal pieces and right well up to its use in the information transmission. Not too far, LASER does find its application in repairing damaged eyes (LASIK surgery) to even guiding a missile onto its target.

A typical laser device is made up of the following four essential primary components:                                            

• Active medium: The active medium may be in the form of solid crystals such as ruby or as liquid dyes or even gases like CO2, Helium or Neon. Sometimes even semiconductors such as GaAs are also being used as active medium. As such, the active mediums contain atoms or molecules whose electrons are thus excited to an elevated energy source by means of any external source of energy, the first pre-requisite to produce a Laser beam.
• Excitation mechanism or an energy source: As the name indicates, the excitation mechanism is meant for pumping in the energy into the active medium so as to make its atoms to have their electrons excited to a higher energy level. In fact, this so called excitation mechanism is achieved by one or a combination of methods such as optical, electrical or chemical.
• High Reflectance Mirror: A component called as mirror that is placed at the end of the optical cavity through which energy is pumped into. This mirror is essentially meant for reflecting back almost 100% of the laser light.
• Mirror Allowing Partial Transmission: As the name being self explanatory, it reflects somewhere, less than 100% of the laser light and transmits the remainder of it.
• Noted that on the basis of the kind of active medium being used for producing a laser beam, the lasers are broadly classified into continuous and pulsed laser types. While the former use gases as an active medium to produce a continuous laser beam and thus, called as continuous laser. On the other hand, the later makes use of the solids as an active medium that may be solid crystals, glass or may even be a semiconductor as being the light amplifying material. This thus, produces a pulsating (time varying) laser beam and hence, referred to as the pulsed lasers.
• Using all these four components, lasers ultimately perform the task of light amplification that manifests as being a thin, but quite intense beam of light and noted for being highly directional (coherent) in nature. The sheer advantage of a laser beam is that of its being inherently a pure source of light in the sense that it is almost a monochromatic beam of light comprising of a single wavelength. Being so, the laser waves besides, traveling in a narrow path always move in a single direction unlike the light waves of an ordinary light beam which tend to spread in different directions and does not remain focused on a single spot. This is how that the ordinary light is generally described as an incoherent light while, LASER as being the coherent one. This unique property of coherence in a laser beam makes it to travel too far without even loosing its intensity. Today, we have lasers that work under different wavelengths right from the ultraviolet through the visible and up to the infrared light spectra and hence, their respective applications. The lasers that work with UV or IR light produce the unified beams of light that are invisible to the human eye and also have distinct intensities of their own based on their emitted power. However, from the view point of intensity that a laser has today, we have the following four classes of the lasers mentioned in accordance with their power:
• Class 1 lasers: They represent the weakest lasers and considered safe for humans and human use. This class in fact includes all those lasers or laser systems that are known for not emitting optical radiations beyond a level at which their exposure may be detrimental to the human eyes i.e. they do not emit optical radiations at a level that is above the exposure limits for the eye howsoever the inherent design of the laser product may be.
• Class 2 lasers: A class 2 laser or laser system is accustomed to emit a visible laser beam, but because of its extreme brightness, it is too bright and dazzling to be stared at for any extended period of time. However, class 2 lasers are also considered safe for human use because, their upper radiant limit is well below the so called MPE (Maximum Permissible Exposure). Yet it permits only for a momentary exposure of 0.25 seconds or even less than this and does not hold good for a long standing exposure.
• Class 3 lasers: This laser or laser system can actually emit any wavelength of radiations, but it does not produce a reflection hazard as such unless, it is focused or viewed at a close range for an extended period of time.
• Class 4 lasers: A laser or laser system that exceeds the output limits of class 3 laser type or the one produced by a class 3 laser device is a class 4 laser. Given thus, it can potentially be a hazardous either in terms of causing some skin burns or fire. Therefore, very stringent and controlled measures are called for the application of such a class of lasers or laser devices for all practical purposes.

Connecting concepts: Application of LASERS in communication technology:

As the use of optical fibers in communication technology is well understood given the fact of their information carrying capacity over large distances. The signal carrying capacity of these optical fibers has further been increased and that too with considerably a far less signal wastage by making the use of laser beams inside optical fibers as information carrying signals, the use of which has indeed, offered many advantages. Fibre optic laser systems today are being used in various communication networks. Many long-haul fibre communication networks making use of Lasers, for both transcontinental connections and, through undersea cables, international connections are in operation. One advantage of optical fibre systems is the long distances that can be maintained before signal repeaters are needed to regenerate signals. These are currently separated by about 100 km (about 62 miles), compared to about 1.5 km (about 1 miles) for electrical systems. Newly developed optical fibre amplifiers can extend this distance even farther. Local areas networks (LAN) are another growing application for fibre optics. Unlike long-haul communications, these systems connect many local subscribers to expensive centralized equipment such as computers and printers. This system expands the utilization of equipment which can easily accommodate new users on a network. Development of new electro-optic and integrated-optic components will further expand the capability of optical fibre systems. 

Lasers in Everyday Physics: As noted above, the very nature of laser beams make their indispensable applications in as diverse fields as in equipments used in homes, factories, offices, hospitals and libraries right up to the high end technologies associated with information transmission via fibre optics, storage and copying of the information on compact discs and missile guidance in defence technology. Yet, some of the noteworthy and most glaring applications of lasers today deserve a special mention herein below:

Storage & transmission of Information: The most common uses of lasers include the recording of music, motion pictures, computer data and other material on compact discs. Bursts of laser light record such material on the discs in patters of tiny pits. A laser beam’s tight focus allows much more information to be stored on a CD or DVD than on a phonograph record thereby, making them good agents for holding data as well as music and movies.

Reading stored information: Lasers can also be employed for reading and playing back information recorded on discs. In a CD/DVD player, a laser beam reflects off the pattern of pits as the compact disc spins. Other devices in the player change the reflections into electrical signals and decode them as music. More amounts of lasers are used in CD/DVD players than in any other electronic product or gadget.            

Holography: Laser beams can produce three-dimensional images in a photographic process called as holography. Holography is a method for storing and displaying a three-dimensional image usually on a photographic plate or any other light-sensitive material. Since the laser exposed plate is called a hologram and hence, the name “holography” is given to the technique. Some typical examples where holography is being used is the credit cards that carry so called holograms to prevent counterfeiting.

Fibre-optics: One of the laser’s greatest use is in the filed of fibre-optics communication. This technology changes electrical signals of telephone calls and television pictures into pulses (bursts) of laser light that is then transmitted by the long strands made of glass what we call as optical fibres.

Scanning: Scanning involves the movement of a laser beam across a surface. Scanning beams are often used to read information. Laser scanners used at supermarket checkout counters are a familiar sight. What that looks like merely a line of light is actually a rapidly moving laser beam scanning a bar code. A bar code in fact, consists of a pattern of lines and spaces on packages that identifies the product. The scanner reads the pattern and sends the information to a computer in the store. The computer identifies the item’s price and sends the information to the register. In addition, such scanners keep track of books in libraries, sort mail in post offices as well as read account numbers on cheques in banks. Laser printers use a scanning laser beam to produce copies of documents. Other scanners make printing plates for newspapers.

Entertainment: In entertainment, laser light shows are aerated with scanning laser beams. These beams can “draw” spectacular patterns of red, yellow, green and blue light on buildings or other outdoor surfaces. The laser beams here move so rapidly that they produce what otherwise looks like a stationary picture.

Heating: A laser beam’s highly focused energy can produce a great amount of heat. Industrial lasers, for example, produce beams of thousands of watts of power. They thus are used for cutting and welding metals or for drilling holes.

Medicine: In medicine, the heating power of lasers is often used in eye surgery; a high end surgical procedure called as LASIK (Laser in-situ keratomalacia). In this case, highly focused laser beams are used to close off broken blood vessels in the retina, a tissue in the back of the eyeball or can re-attach a loose retina. Laser beams pass through the cornea (front surface of the eye) but cause no pain or damage to it because, the cornea is transparent and does not absorb light. Doctors also use lasers to treat skin disorders, remove birthmarks or to shatter gallstones or kidney stones in a procedure called as Lithotripsy.

Nuclear technology: In nuclear energy research, scientists use lasers to produce controlled, miniature hydrogen bomb explosions. They focus many powerful laser beams onto a pellet of frozen forms of hydrogen. The intense beams compress the pellet and heat it by millions of degrees. These actions cause the nuclei to fuse and release energy, a prototype experiment to display fusion bomb technology.

Biochemistry: Some laser systems, through the process of mode locking, can produce extremely brief pulses of light – as short as picoseconds or femtoseconds. Such pulses can be used to initiate and analyze chemical reactions. This method is particularly useful in biochemistry where it is used to analyze details of protein folding and their function.            

Measurements: Lasers are also used to measure distance. An object’s distance can be determined by measuring the time, a pulse of laser light takes to reach and reflect back from the object. Laser beams directed over long distances also can detect small movements of the ground. Such measurements help geologists involved in earthquake warning systems. Laser devices used to measure shorter distances are called range finders. Surveyors use these devices to get information needed to make maps. Military personnel however, use them to calculate the distance to an enemy target.  

Guidance: A laser’s strong, straight beam makes it as a valuable tool for guidance. For example, construction workers use laser beams as “weight less strings” to align the walls and ceilings of a building and to lay straight sewer and water pipes. Moreover, the special instruments called laser gyroscopes make use of laser beams to detect changes in direction and thus, find their extensive use in ships, airplanes and guided missiles to help them stay on course. Another military use of lasers is in a guidance device called a target designator. A millitary personnel using this device aims a laser beam at an enemy target. Missiles, artillery, shells and bombs equipped with laser beam detectors seek the reflected beam and adjust their flight to hit the spot where the beam is aimed.

Laser cooling: A technique that has had met with recent success is laser cooling. This involves ion or atom trapping wherein a number of ions or atoms are confined in a specially shaped arrangement of electric and magnetic fields. Shining particular wavelengths of laser light slows them down, thus cooling them off. If the process is continued eventually, they all are slowed down and have the same energy level thereby, forming an unusual arrangement of matter known as a Bose-Einstein condensate (first conceptualized by S.N. Bose).

The Science of Photonics

What is Photonics? “Photonics” broadly, refers to the study of light, but where “light” includes much more than just the visible wavelengths of the light spectrum just because, in photonics the energy and information is being carried or transmitted by the so called particles of light what we call as photons (particles of light) and not by the electrons as it is in conventional electronics. Photonics thus, uses the wave/particle nature of light to create new and high end technology laced optical materials and devices.

In fact, owing to the considerable research currently being undertaken in this newly emerging field of experimental Physics, photonics is likely to replace conventional electronics as well as electronic components with that of the optical components manufactured under the aegis of photonics thereby, offering a wide range of revolutionary possibilities in the domain of information storage or transmission including a significant increase in data processing speed in computational systems. Given thus, a new offshoot of electronics is very much in offing what we will soon acquaint ourselves with called as “optoelectronics.”

Everyday Physics in the Application of Photonics:

The field of photonics is huge and applications can be found in virtually all technological industries. The long list of photonic applications has grown practically by the day.

Some examples of the major application areas of photonics are mentioned below:

Electronics:

• Conventional cable products and optical patch cords, and multi fibre cables
• Physical Layer ICs
• Specialized fibre cables, custom terminations, high-technology assemblies, custom cables and cable harnesses.

Health:

• Bio-medical sensors and instrumentation
• Capillary electrophoresis, columns and accessories

Industrial Process Control:

• Accelerated component life-testing equipment
• Lab-on-a-chip
• Optical networking components for machine vision and industrial inspection industries
• Specialized illumination systems for machine vision and industrial inspection industries.

Instrumentation:

• Digital multimetre, power supplies,, function generators, counters
• RF and Microwave instrument and Systems.
• Sensors and instrumentation for foundry services.
• Detection of cracks and corrosion under paint. This work has led to prototype detection devices for use on airplane wings that are in the process of being brought to market.

Telecom:

• Carrier grade data networks
• Flat Super Raman with extra 2.5 dB of margin increase.
• High performance, intelligent, valued-added Internet services
• Low and High power C-band, L-band and dual-band ASE sources
• Raman fibre lasers, C-band and L-band EDFAs (gain blocks, rack mount or bench top units)
• Telecom networking products
• Telecom-ready Raman amplifier subsystems  

Health Applications of Photonics

• Disease Detection and Diagnosis Using Minimally/Non-Invasive Optical Techniques: Optical sensors that use photons as sensing elements are becoming increasingly important and relevant in the field of non-invasive diagnostics. Techniques include spectroscopic techniques for minimally invasive early detection of cervical, prostate, oral, gastrointestinal and skin cancers and breast cancer imaging technology that uses safe ultra fast lasers to see deep into the breast.
• Photoplethysmography (PPG): a noninvasive method of detecting the blood volume pulse. The hardware for picking up the blood volume pulse from human finger is developed by using the principle of transmission mode of photoplethysmography. Analysis of the signal is done after interfacing the pulse to a computer.
• Bacteria and Virus Detection Defense: This technology recognizes spectroscopic signatures of bacteria and viruses.
• Compact Photonic Explorer: Scientists are developing a sophisticated “Photonic pill” that can perform remote diagnostics at such places inside the body such as the digestive tract and send information back to doctors.  

Communication satellites

Telecommunication is in fact, a wider term and refers to a system or a set of devices that can send or transmit the information over long distances after converting the same into electronic or electrical signals. The telecommunication devices thus, convert different types of information such as sound (audio), video (motion pictures), or words etc into electronic signals. The said information what we also call as messages once being converted into electronic signals are then sent to the respective media devices such as sound over the telephone, video over the TV & words or pictures over the PC  and hence, the recipient receives the same.

In short, Telecommunications begin with messages that are converted into electronic signals. The signals are then sent over a medium to a receiver, where they are decoded back into a form that the person who message is being sent to, could understand. There are a variety of ways to create and decode signals and even many different ways to transmit signals.

Significantly, all telecommunication services whether telephone, television or radio or for that matter, IT (including internet etc) have their functional basis in the communication satellites because; all these telecommunication services make the use of radio frequency waves better, called as radio-frequency signals that are essentially transmitted or relayed down to the earth by a communication satellite up there orbiting the earth in a fixed orbit.

A communication satellite in fact, includes any earth-orbiting spacecraft that provides communication over long distance by reflecting or relaying radio-frequency signals. Satellite relay systems today, have in a way revolutionized the communication by making worldwide telephone links and live broadcasts, a common occurrence.

 A communication satellite actually works by receiving a microwave signal (a signal of different frequency) from a ground station on the earth (the uplink), amplifies the same and then retransmits the signal back to a receiving station or stations on earth but at a different frequency (generally at radio frequency range) (the downlink).

 A communication satellite used for all telecommunication services such as telephone, television etc. is always placed in a geosynchronous orbit, which means that the same is orbiting the earth at the same speed as the earth is revolving on its own axis. The satellite thus, stays in the same position relative to the surface of the earth, so that the broadcasting station on the ground will never lose contact with the receiver.

 Some of the first communications satellites were designed to operate in passive mode. That means, instead of their actively transmitting the radio signals down to the earth, they served merely to reflect signals that were beamed up to them by transmitting stations on the ground of course, without subjecting them to any change in their frequency on their part. Signals thus, were reflected by them in all directions, so they could be picked up by any or all receiving stations around the world.

How does a Satellite Work in Geosynchronous Orbit? (Question asked in Pre-GS-2011).

A satellite in a geosynchronous orbit follows a circular orbit over the equator at an altitude of 35,800 kms, an equivalent of (22,300 miles) or to be rounded off to approx.36000 kms thereby, completing one orbit around the earth, every 24 hours, the time that it takes the earth to rotate once on its own axis. Moving in the same direction as the earth’s rotation, the satellite thus, remains in a fixed position over a point on the equator, thereby, providing uninterrupted contact between ground stations in its line of sight. Most communications satellites that followed over the years were also placed in geosynchronous orbit.

AM and FM Radio

 As we already know that a radio anatomically, is made, but of the same heart of the electronic devices as what constitutes the anatomy of almost all or any other electronic device for that matter. To this heart of an electronic device, we call a transistor. However, functionally as being a telecommunication device, it makes the use of same type of signals carrying sound called as radio signals for the purposes of transmitting information electronically. The conventional radio although, used the same radio waves by making a modification in the amplitude of radio waves only. Whereas, the new breed of radio transmission called FM works by modifying the radio waves in respect of their frequency and hence, the name. Of these two, of course, the latter and latest, FM, certainly has the sheer advantage of its own as compared to the former, AM as are being enumerated below for a clear understanding…

What is AM (Amplitude Modulation)?

AMPLITUDE MODULATION OR AM:

Amplitude modulation or AM is a form of modulation used for radio transmissions for broadcasting and two-way radio communication applications. Although, it is one of the earliest used forms of modulation yet, it is still in widespread use even today.  

How it works? In order that a radio signal can carry audio or other information for broadcasting or for two way radio communication, it must be modulated or changed in some way. Although, there are a number of ways in which a radio signal may be modulated, but generally we just change its amplitude in line with variations of the sound. In fact, the basic concept surrounding the AM is quite straight-forward. The amplitude of the signal is changed in line with the instantaneous intensity of the sound. In this way, the radio frequency signal has a representation of the sound wave superimposed on it. In view of the way in which the basic signal “carries” the sound or modulation, the radio frequency signal is often termed as the “carrier”.

AM broadcasting: It is the process of radio broadcasting using amplitude modulation. AM receiver detects amplitude variations in the radio waves at a particular frequency. It then amplifies changes in the signal voltage to drive a loudspeaker or earphones.

Advantages of AM: There are several advantages of amplitude modulation, and some of these advantages reasons out why it is still in widespread use today:

I) It is simple to implement;

II) It can be demodulated using a circuit consisting of very few components;

III) AM receivers are very cheap as no specialized components are needed.

Disadvantages of AM: AM is a very basic form of modulation and although, its simplicity is one of its major advantages, other more sophisticated systems provide a number of advantages. Accordingly, it is worth looking at some of the disadvantages: (i) it is not efficient terms of its power usage; (ii) It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that of the highest audio frequency; and (iii) it is prone to high levels of noise because most noise is amplitude based and obviously AM detectors are sensitive to it.

What is FM (Frequency Modulation)?

FREQUENCY MODULATION OR FM:

While changing the amplitude of a radio signal is the most obvious method to modulate it, it is by no means the only way. It is also possible to change the frequency of a radio signal to give frequency modulation or FM. Frequency modulation is widely used on frequencies of about 30 MHz, and it is particularly well known for its use for VHF FM broadcasting. Although it may not be quite as straightforward as AM, nevertheless FM, offers some distinct advantages vis a vis AM. It is able to provide near interference free reception, and it was for this reason that it was adopted for the VHF sounds broadcasts. These transmissions could offer high fidelity audio, and for this reason, FM is far more popular than the older transmissions on the long, medium and short wave bands. In addition to its widespread use for high quality audio broadcasts, FM is also used for a variety of two way audio communication systems. Whether for fixed or mobile radio communication systems or for use in portable applications, FM is widely used at VHF and above.

What is FM? To generate a frequency modulated signal, the frequency of the radio carrier is changed in line with the amplitude of the incoming audio signal. When the audio signal is modulated on to the radio frequency carrier, the new radio frequency signal moves up and down in frequency. The amount by which the signal moves up and down is important. It is known as the deviation and is normally quoted as the number of kilohertz deviation.

What is WBFM? Broadcasts stations in the VHF portion of the frequency spectrum between 88.5 and 108 MHz use large values of deviation, typically ± 75 kHz. This is known as wide-band FM (WBFM). These signals are capable of supporting high quality transmissions, but occupy a large amount of bandwidth. Usually 200 kHz is allowed for each WBFM transmission.

What is NBFM? For communications purposes less bandwidth is used. Narrow band FM (NBFM) often uses deviation figures of around ± 3 kHz. It is NBFM that is typically used for two-way radio communications, but this is not needed for applications such as mobile audio communication.

Advantages of FM: FM is used for a number of reasons and there are several advantages of frequency modulation. In view of this, it is widely used in a number of areas to which it is ideally suited. Some of the advantages of frequency modulation are noted below:

• Resilience to noise: One particular advantage of frequency modulation is its resilience to signal level variations. The modulation is carried only as variations in frequency. This means that any signal level variations will not affect the audio output, provided that the signal does not fall to a level where the receiver cannot cope. As a result this makes FM ideal for mobile radio communication applications including more general two-way radio communication or portable applications where signal levels are likely to vary considerably. The other advantage of FM is its resilience to noise and interference. It is for this reason that FM is used for high quality broadcast transmissions.
•  Easy to apply modulation at a low power stage of the transmitter: Another advantage of frequency modulation is associated with the transmitters. It is possible to apply the modulation to a low power stage of the transmitter, and it is not necessary to use a linear form of amplification to increase the power level of the signal to its final value.
•  It is possible to use efficient RF amplifiers with frequency modulated signals: It is possible to use non-linear RF amplifiers to amplify FM signals in a transmitter and these are more efficient than the linear ones required for signals with any amplitude variations (e.g. AM and SSB). This meant that for a given power output, less batter power is required and this makes the use of FM more viable for portable two-way audio applications.       

Thus, we now clearly understand that frequency modulation or (FM) system of radio transmission is the one in which the carrier wave is modulated so that its frequency varies with the audio signal being transmitted by it.

 The first workable system for radio communication was described by the American inventor Edwin H. Armstrong in 1936, who otherwise has had immense contributions to the science of Electronics particularly, during its nascent stage.

Frequency modulation has several advantages over the system of Amplitude Modulation (AM) used in the alternate form of radio broadcasting. The most important of these advantages is that the FM system has a greater freedom from interference and static. Various electrical disturbances, such as those caused by thunderstorms and automobile ignition systems; create amplitude modulated radio signals (as used in AM transmission) that are received as noise by AM receiver. A well-designed, FM receiver is not sensitive to such disturbances when it is tuned to an FM signal of sufficient strength. Also, the signal-to-noise ratio in an FM system is much higher than that of an AM system. Finally, FM broadcasting stations can be operated in the very high frequency bands at which AM interference is frequently severe; commercial FM radio stations are assigned frequencies between 88 and 108 MHz. The range of transmission on these bands is limited so that stations operating on the same frequency can be located with a few hundred miles of one another without interference.

These features, coupled with the comparatively low cost of equipment for an FM broadcasting station, resulted in its rapid growth in the years following World War II (1939-1945). Because of crowding in the AM broadcast band and the inability of standard AM receivers to eliminate noise, the tonal fidelity of standard stations is purposely limited. FM does not have these drawbacks and therefore, can be used to transmit musical programs that reproduce the original performance with a degree of fidelity that cannot be reached on AM bands.