UNIT 4
PHYSICS OF LASER AND ITS APPLICATIONS
Introduction
LASER stands for “Light Amplification by Stimulated Emission of Radiation” .
L = Light
A = Amplification (by)
S = Stimulated
E = Emission (of)
R = Radiation
Theodore H. Maiman of the Hughes Research Laboratory, California, was the first scientist who experimentally demonstrated laser by flashing light through a ruby crystal in 1960. But the basic idea behind the development of laser was given by the great scientist “Albert Einstein” in his 1917.
SPONTANEOUS AND STIMULATED EMISSION
Let us discuss Einstein’s theory of interaction of electromagnetic radiation with matter. He proposed that electromagnetic radiation interacts with matter in following three steps.
- Stimulated Absorption
- Spontaneous Emission
- Stimulated Emission
Stimulated Absorption:
Let E1 and E2 be the energies of ground and excited states of an atom. Suppose, if a photon of energy hν= E1− E2 interacts with an atom present in the ground state, the atom gets excitation form ground state E1 to excited state E2. This process is called stimulated absorption. Stimulated absorption rate depends upon the number of atoms available in the lowest energy state as well as the energy density photons.
Stimulated absorption rate ∝ Number of atoms in the ground state
∝ Density of photons Spontaneous emission
Spontaneous Emission:
Let E1 and E2 be the energies of ground and excited states of an atom. Suppose, if a photon of energy hν= E1− E2 interacts with an atom present in the ground state, the atom gets excitation form ground stateE1 to excited state E2. The excited atom does not stay in a long time in the excited state. The excited atom gets de-excitation after its life time by emitting a photon of energy hν= E1− E2. This process is called spontaneous emission. Also Spontaneous means by its own. Here excited atom comes to ground state by its own so it is named as spontaneous emission.
The spontaneous emission rate depends up on the number of atoms present in the excited state.
Spontaneous emission ∝ rate number of atoms in the excited state
Stimulated Emission:
This phenomena is responsible for producing laser light. Let E1and E2 be the energies of ground and excited states of an atom. Suppose, if a photon of energy hν= E1− E2 interacts with an atom present in the ground state, the atom gets excitation form ground stateE1 to excited state E2. Let, a photon of energy hν= E1− E2 interacts with the excited atom with in their life time; the atom gets de-excitation to ground state by emitting of another photon. These photons have same phase and it follows coherence. This phenomenon is called stimulated emission.
Stimulated emission rate depends upon the number of atoms available in the excited state as well as the energy density of photons.
Stimulated emission rate ∝ number of atoms in the excited state
∝ Density of photons
PROPERTIES OF LASER
The laser light exhibits some peculiar properties than compare with the convectional light. Those are
1. Highly monochromatic
2. Highly coherence
3. Highly directionality
4. Highly intense
5. Laser Speckles
1. Highly monochromatic
Monochromatic light means a light containing a single colour or wavelength. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colours. Ordinary light is a mixture of waves having different frequencies or wavelengths. The light waves of laser have single wavelength or colour.
Therefore, laser light covers a very narrow range of frequencies or wavelengths.
Hence The laser light is more monochromatic than that of a convectional light source. This may be due to the stimulated characteristic of laser light. The band width of convectional monochromatic light source is 1000 Å. But the band width of ordinary light source is 10 Å. For high sensitive laser source is 10-8 Å.
2. Highly coherence
Definition:- A predictable correlation of the amplitude and phase at any one point with other point is called coherence.
Two waves are said to be coherent, the waves must have
In case of convectional light, the property of coherence exhibits between a source and its virtual source where as in case of laser the property coherence exists between any two of more light waves.
There are two types of coherence
i) Temporal coherence
Ii) Spatial coherence
Temporal coherence (or longitudinal coherence):-
The predictable correlation of amplitude and phase at one point on the wave train w .r. t another point on the same wave train, then the wave is said to be temporal coherence
To understand this, let us consider two points P1 and P2 on the same wave train, which is continuous as in shown in figure.
Suppose the phase and amplitude at any one point is known, then we can easily calculate the amplitude and phase for any other point on the same wave train by using the wave equation
y= a sin ( (ct-x))
Where ‘a’ is the amplitude of the wave and ‘x’ is the displacement of the wave at any instant of time‘t’.
Spatial coherence (or transverse coherence) The predictable correlation of amplitude and phase at one point on the wave train w. r .t another point on a second wave, then the waves are said to be spatial coherence (or transverse coherence)
3. Highly directionality
The light ray coming ordinary light source travels in all directions, but laser light travels in single direction. For example the light emitted from torch light spreads 1km distance it spreads 1 km distance. But the laser light spreads a few centimetres distance even it travels lacks of kilometre distance.
The directionality of laser beam is expressed in terms of divergence
∆θ =
Where r1 and r2 are the radii of laser bam spots at distances of D1 and D2 respectively from laser source.
4. Highly Intense or Brightness
We know that the intensity of a wave is the energy per unit time flowing through a unit normal area. Laser light is highly intense than the convectional light. An one mill watt He-Ne laser is highly intense than the sun intensity. This is because of coherence and directionality of laser. Suppose when two photons each of amplitude a are in phase with other, then young’s principle of superposition, the resultant amplitude of two photons is 2a and the intensity is 4a2. Since in laser many number of photons are in phase with each other, the amplitude of the resulting wave becomes na and hence the intensity of laser is proportional to n2a2. So 1mW He-Ne laser is highly intense than the sun.
In an ordinary light source, the light spreads out uniformly in all directions. If you look at a 100 Watt lamp filament from a distance of 30 cm, the power entering your eye is less than 1/1000 of a watt. If you look laser beam (caution: don’t do it at home, direct laser light can damage your eyes) then all the power in the laser would enter your eye. Thus, even a 1 Watt laser would appear many thousand times more intense than 100 Watt ordinary lamp.
5. Laser Speckles
The term speckle refers to a random granular pattern which can be observed when a highly coherent light beam is diffusely reflected at a surface with a complicated structure. This phenomenon results from the interference of different reflected portions of the incident beam with random relative optical phases.
Even minor changes of the conditions, such as of the illuminated spot or the direction of the incident laser beam, can change the detailed shape of a speckle pattern.
When laser light which has been scattered off a rough surface falls on another surface, it forms an "objective speckle pattern". If a photographic plate or another 2-D optical sensor is located within the scattered light field without a lens, a speckle pattern is obtained whose characteristics depend on the geometry of the system and the wavelength of the laser.
The distribution of atoms in the two energy levels will change by absorption or emission of radiation. Einstein introduced three empirical coefficients to quantify the change of population of the two levels. Let N1 be the number of atoms per unit volume with energy E1 and N2 be the number of atoms per unit volume with energy E2. Let ‘n’ be the number of photons per unit volume at frequency ‘υ’ such that hυ= E1− E2.
Then, the energy density of photons ρ(υ) = nhυ
When these photons interact with atoms, both upward (absorption) and downward (emission) transition occurs.
At the equilibrium the upward transitions must be equal downward transitions.
Upward Transition
Stimulated absorption rate depends upon the number of atoms available in the lowest energy state as well as the energy density photons.
We have seen above that
Stimulated absorption rate ∝ N1 i.e. Number of atoms in the ground state
∝ ρ(υ) i.e. Density of photons spontaneous emission
Stimulated absorption rate = B12N1ρ(υ) ………(1)
Where B12 is the Einstein coefficient of stimulated absorption.
Downward transition
The spontaneous emission rate depends up on the number of atoms present in the excited state.
Spontaneous emission rate ∝ N2 i.e. number of atoms in the excited state
Spontaneous emission rate = A21N2 ………(2)
Where A21 is the Einstein coefficient of spontaneous emission.
Stimulated emission rate depends upon the number of atoms available in the excited state as well as the energy density of photons.
Stimulated emission rate ∝ N2 i.e. number of atoms in the excited state
∝ ρ(υ) i.e. Density of photons
Stimulated emission rate = B21N2ρ(υ) ………(3)
If the system is in equilibrium the upward transitions must be equal downward transitions.
Upward transitions = downward transitions
B12N1ρ(υ) = A21N2 + B21N2ρ(υ) ………(4)
B12N1ρ(υ) - B21N2ρ(υ) = A21N2
(B12N1- B21N2) ρ(υ) = A21N2
ρ(υ) = ………(5)
Divide with B21N2 in numerator and denominator in right side of the above equation,
ρ(υ) = = ………(6)
ρ(υ) = = = ………(7)
We know from Maxwell Boltzmann distribution law
= ………(8)
And also from Planck’s law, the radiation density
ρ(υ) = ………(9)
Comparing the two equations (7) and (9)
= and =1 ………(10)
The above relations referred to as Einstein relations.
From the above equation for non degenerate energy levels the stimulated emission rate is equal to the stimulated absorption rate at the equilibrium condition.
= ………(11)
He-Ne LASER
The first He-Ne gas laser was fabricated in 1961 by Ali Javan, Bennett and Herriott at Bell Telephone Laboratories. others. Helium-Neon laser is a type of gas laser in which a mixture of helium and neon gas is used as a gain medium. Helium-Neon laser is also known as He-Ne laser. The helium-neon laser was the first continuous wave laser ever constructed. The helium-neon laser operates at a wavelength of 632.8 nanometres (nm), in the red portion of the visible spectrum.
Ruby laser is a pulse laser, even it have high intense output. For continuous laser beam gas lasers are used. Using gas lasers, we can achieve highly coherence, high directionality and high monochromacity beam. The output power of the gas laser is generally in few milliwatts.
CONSTRUCTION
The helium-neon laser consists of three essential components:
- Pump source (high voltage power supply)
- Gain medium (laser glass tube or discharge glass tube)
- Resonating cavity
Pump source
The gain medium of a helium-neon laser is made up of the mixture of helium and neon gas contained in a glass tube at low pressure. In He-Ne gas laser, the He and Ne gases are taken in the ratio 10:1 in the discharge tube.
Gain medium
In He-Ne laser 80cm length and 1cm diameter discharge is generally used. The out power of these lasers depends on the length of the discharge tube and pressure of the gas mixture. Therefore, in order to achieve population inversion, we need to pump electrons from lower energy state of the helium. In He-Ne laser, neon atoms are the active centres and have energy levels suitable for laser transitions while helium atoms help in exciting neon atoms.
Resonating cavity
Two reflecting mirrors are fixed on either ends of the discharge tube, in that, one is partially reflecting and the other is fully reflecting. The fully silvered mirror will completely reflect the light whereas the partially silvered mirror will reflect most part of the light but allows some part of the light to produce the laser beam.
WORKING
When the electric discharge is passing through the gas mixture, the electrons accelerated towards the positive electrode. During their passage, they collide with He atoms and excite them into higher levels. 23s1 and 21s0 form ground state of He atom. In higher levels 23s1 and 21s0, the life time of He atoms is more. So there is a maximum possibility of energy transfer between He and Ne atoms through atomic collisions. When He atoms present in the levels 23s1 and 21s0 collide with Ne atoms present ground state, the Ne atoms gets excitation into higher levels 4s and 5s.
Due to the continuous excitation of Ne atoms, we can achieve the population inversion between the higher levels 4s and 5s and lower levels 3p and 4p. The various transitions 5s to 4p, 4s to 3p and 5s to 3p leads to the emission of wavelengths 3.93μm, 1.51μm and 6328 Å or 632.8μm.
The first two corresponding to the infrared region while the last wavelength is corresponding to the visible region. The Ne atoms present in the 4s level are de-excited into 3s level, by spontaneously emitting a photon of around wavelength 6000 Å. When a narrow discharge tube is used, the Ne atoms present in the level 3s collide with the walls of the tube and get de-excited to ground state energy level.
ADVANTAGES OF HELIUM-NEON LASER
- Helium-neon laser emits laser light in the visible portion of the spectrum.
- High stability
- Low cost
- Operates without damage at higher temperatures
DISADVANTAGES OF HELIUM-NEON LASER
- Low efficiency
- Low gain
- Helium-neon lasers are limited to low power tasks
APPLICATIONS OF HELIUM-NEON LASERS
- Helium-neon lasers are used in industries.
- Helium-neon lasers are used in scientific instruments.
Helium-neon lasers are used in the college laboratories
SEMICONDUCTOR LASER
It is a solid state semiconductor laser. It is specifically fabricated p-n junction diode. This diode emits laser light when it is forward biased. A semiconductor laser is a device that causes laser oscillation by flowing an electric current to semiconductor. The mechanism of light emission is the same as a light-emitting diode (LED). Light is generated by flowing the forward current to a p-n junction. In forward bias operation, the p-type layer is connected with the positive terminal and the n-type layer is connected with the negative terminal, electrons enter from the n-type layer and holes from the p-type layer. When the two meet at the junction, an electron drops into a hole and light is emitted at the time.
Semiconductor lasers represent one of the most important class of lasers in use today, not only because of the large variety of direct applications in which they are involved but also because they have found a widespread use as pumps for solid state lasers.
Semiconductor lasers require, for the active medium, a direct gap material and accordingly, the normal elemental semiconductors like Si or Ge cannot be used. The majority of semiconductor-laser materials are based on a combination of elements belonging to the third group of the periodic table such as Al, Ga, In with elements of the fifth group such as N, P,As, Sb. Examples include the best known GaAs as well as some ternary AlGaAs, InGaAs and quaternary InGaAsP alloys.
InGaN semiconductor lasers the best candidates for semiconductor laser emission in the very important blue-green spectral region. Semiconductor laser materials are not limited to III–V compounds, however. For the blue-green end of the spectrum we note that there are wide-gap semiconductors using a combination between elements of the second group (such as Cd and Zn) and of the sixth group (S, Se).
PRINCIPLE
When a p-n junction diode is forward biased, the electrons from n – region and the holes from the p- region cross the junction and recombine with each other.
During the recombination process, the light radiation (photons) is released from a certain specified direct band gap semiconductors like Ga-As. This light radiation is known as recombination radiation.
The photon emitted during recombination stimulates other electrons and holes to recombine. As a result, stimulated emission takes place which produces laser.
CONSTRUCTION
The basic structure of a semiconductor laser is shown in Figure. The active medium is a p-n junction diode made from the single crystal of gallium arsenide. The active layer called as light emission layer sandwiched between the p- and n-type clad layers i.e. double hetero structure is formed on an n-type substrate and voltage is applied across the p-n junction from the electrodes. This crystal is cut in the form of a platter having thickness of 0.5μmm. Both edges of the active layer has mirror-like surface. When forward voltage is applied, electrons combine with holes at the p-n junction, and emit the light.
This light is not a laser yet; it is confined within the active layer because the refractive index of the clad layers is lower than that of the active layer. In addition both ends of the active layer act as a reflecting mirror where the light reciprocates in the active layer. Then, the light is amplified by the stimulated emission process and laser oscillation is generated.
The photon emission is stimulated in a very thin layer of PN junction (in order of few microns). The electrical voltage is applied to the crystal through the electrode fixed on the upper surface. The end faces of the junction diode are well polished and parallel to each other. They act as an optical resonator through which the emitted light comes out.
WORKING
When the PN junction is forward biased with large applied voltage, the electrons and holes are injected into junction region in considerable concentration
The region around the junction contains a large amount of electrons in the conduction band and a large amount of holes in the valence band.
If the population density is high, a condition of population inversion is achieved. The electrons and holes recombine with each other and this recombination’s produce radiation in the form of light.
When the forward – biased voltage is increased, more and more light photons are emitted and the light production instantly becomes stronger. These photons will trigger a chain of stimulated recombination resulting in the release of photons in phase.
The photons moving at the plane of the junction travels back and forth by reflection between two sides placed parallel and opposite to each other and grow in strength.
After gaining enough strength, it gives out the laser beam of wavelength 8400Å. The nature of output is continuous wave or pulsed output. The power output from this laser is 1mW. The wavelength of laser light is given by
Where Eg is the band gap energy in joule.
ADVANTAGES
- It is very small in dimension.
- The arrangement is simple and compact.
- It exhibits high efficiency.
- It is operated with lesser power than ruby and CO2 laser.
- The laser output can be easily increased by controlling the junction current
- It requires very little auxiliary equipment
- It can have a continuous wave output or pulsed output.
DISADVANTAGES
- It is difficult to control the mode pattern and mode structure of laser.
- The output is usually from 5 degree to 15 degree i.e., laser beam has large divergence.
- The purity and monochromacity are power than other types of laser
- Threshold current density is very large (400A/mm2).
- It has poor coherence and poor stability.
APPLICATION
- It is widely used in fibre optic communication
- It is used to heal the wounds by infrared radiation
- It is also used as a pain killer
- It is used in laser printers and CD writing and reading.
APPLICATIONS OF LASERS
Applications of lasers because of unique property of laser beam such as coherence, monochromacity, directionality, and high intensity, they are widely used in various fields like
1. Communication
2. Computers
3. Chemistry
4. Photography
5. Industry
6. Medicine
7. Military
8. Scientific Research
1. Communication
In case of optical communication semiconductors laser diodes are used as optical sources and its band width is (1014Hz) is very high compared to the radio and microwave communications. More channels can be sent simultaneously Signal cannot be tapped as the band width is large, more data can be sent. A laser is highly directional and less divergence, hence it has greater potential use in space crafts and submarine. It is used in optical fiber communications to send information over large distances with low loss. Laser light is used in underwater communication networks. Lasers are used in space communication, radars and satellites.
2. Computers
In LAN (local area network), data can be transferred from memory storage of one computer to other computer using laser for short time. Lasers are used in CD-ROMS during recording and reading the data. Lasers are used in computer printers.
3. Chemistry
Lasers are used in molecular structure identification Lasers are also used to accelerate some chemical reactions. Using lasers, new chemical compounds can be created by breaking bonds between atoms are molecules.
4. Photography
Lasers can be used to get 3-D lens less photography. Lasers are also used in the construction of holograms.
5. Industry
Lasers can be used to blast holes in diamonds and hard steel. Lasers are also used as a source of intense heat Carbon dioxide laser is used for cutting drilling of metals and non-metals, such as ceramics plastics glass etc. High power lasers are used to weld or melt any material. Lasers are also used to cut teeth in saws and test the quality of fabric. It is used to cut glass and quartz, used in electronic industries for trimming the components of Integrated Circuits (ICs).Lasers are used for heat treatment in the automotive industry. Laser light is used to collect the information about the prefixed prices of various products in shops and business establishments from the bar code printed on the product. Ultraviolet lasers are used in the semiconductor industries for photolithography. Photolithography is the method used for manufacturing printed circuit board (PCB) and microprocessor by using ultraviolet light. It is also used to drill aerosol nozzles and control orifices within the required precision.
6. Medicine
Pulsed neodymium laser is employed in the treatment of liver cancer. Argon and carbon dioxide lasers are used in the treat men of liver and lungs. Lasers used in the treatment of Glaucoma.
Lasers used in endoscopy to scan the inner parts of the stomach. Lasers used in the elimination of moles and tumors which are developing in the skin tissue and hair removal. It is also used for bloodless surgery.
Lasers are used to destroy kidney stones, in cancer diagnosis and therapy also used for eye lens curvature corrections. Lasers are used to study the internal structure of microorganisms and cells. It is used to create plasma. Lasers are used to remove the caries or decayed portion of the teeth.
7. Military
Lasers can be used as a war weapon. High energy lasers are used to destroy the enemy air-crofts and missiles. Lasers can be used in the detection and ranging likes RADAR. Laser range finders are used to determine the distance to an object. The ring laser gyroscope is used for sensing and measuring very small angle of rotation of the moving objects.
Lasers can be used as a secretive illuminators for reconnaissance during night with high precision.
8. Scientific research
Lasers are used in the field of 3D-photography Lasers used in Recording and reconstruction of hologram. Lasers are employed to create plasma. Lasers are used in Raman spectroscopy to identify the structure of the molecule and to count the number of atoms in a substance. Lasers are used in the Michelson- Morley experiment. A laser beam is used to confirm Doppler shifts in frequency for moving objects. A laser helps in studying the Brownian motion of particles. With the help of a helium-neon laser, it was proved that the velocity of light is same in all directions. Lasers are used to measure the pollutant gases and other contaminants of the atmosphere. Lasers help in determining the rate of rotation of the earth accurately. Lasers are used for detecting earthquakes and underwater nuclear blasts. A gallium arsenide diode laser can be used to setup an invisible fence to protect an area.
HOLOGRAPHY
An important application of laser beam is the production of true three-dimensional pictures in space without the use of lens. The record of' this three-dimensional image of the object on a film is called a hologram.
The phenomena called interference produced due to interaction of two beam monochromatic light waves under certain conditions. The interference pattern is produced by mixing two beams of the laser light. Laser light is split into two beams. One beam called the reference beam is directed toward photographic plate. The other beam is direct towards the object to be recorded, so that suitably reflected from the object. The reference beam and the reflected beam are made to with each other to form an interference patter the photographic plate.
To get a high quality hologram, radiation of in high coherence is required. Generally, the helium-neon gas laser which gives highly coherent beam and Its continuous output beam is used for this purpose. The holograms are recorded on special photographic plates with good resolution. The plate is developed and fixed in the usual way.
Holography is a two-stage process of photography using coherent light from a laser to illuminate the scene. In the first stage a hologram is formed by combining the light scattered from the object and the direct laser beam on a photographic plate. In the second stage a three-dimensional image is reconstructed without the use of lenses, by directing the laser beam through the hologram.
CONSTRUCTION AND RECONSTRUCTION OF HOLOGRAM
Holography technique to obtain 3D image of an object:
- Holography is the science and practice of making holograms. Holography is actually a recording of interference pattern formed between two beams of coherent light coming from the same source.
- In this process, both the amplitude and phase components of a light wave are recorded on a light sensitive medium such as a photographic plate. The recording is known as a hologram.
- Holography requires an intense coherent light source. Lt became a practical proposition only after the invention of LASERS.
- Holography is a two-step process. In the first step, recording of hologram is done where the object is transformed into a photographic record and the second step is the reconstruction in which the hologram is transformed into image.
During the recording process we superimpose on the scattered wave emanating from the object, another coherent wave called as reference beam of the same wavelength. These two waves interfere in the plane of recording medium and produce interference fringes. This is the recording process of hologram.
- The reproduction of the image from the hologram is known as reconstruction of the hologram.
- In this process, a wave identical to reference beam is used.
- When the hologram is illuminated by the reconstruction wave, two waves are produced.
- One wave appears to diverge from the object and provides the virtual image of the object.
- The second wave converges to form the real image of the object.
APPLICATIONS OF HOLOGRAPHY
Applications of holography include information storage, recording of images in depth, the use of holograms as optical elements and as a means of performing precise interferometric measurements on three-dimensional objects of any shape and surface finish.
Holography is being used for non-destructive testing, holographic information storage, display devices and pattern matching procedures for tasks as credit card and identity card verification.
Holographic methods can also be used for is secret communication of information by recording the holograms of secret documents, maps and objects, and restructuring the images only at the receiver end. Interference holography can be used to measure accurately how structures deform under the effect of mechanical stress or thermal gradient.
Standard holograms may be used in industrial production processes to check high precision components with regard to their shape dimensional accuracy.
OPTICAL FIBRE
A cable which is used to transmit the data through fibres (threads) or plastic (glass) is known as optical fibre cable. This cable includes a pack of glass threads which transmits modulated messages over light waves.
Principle: Optical Fibre works on the principle of Total Internal Reflection.
Total internal reflection:-
When the light ray travels from denser medium to rarer medium the refracted ray bends away from the normal. When the angle of incidence is greater than the critical angle, the refracted ray again reflects into the same medium. This phenomenon is called total internal reflection. The refracted ray bends towards the normal as the ray travels from rarer medium to denser medium. The refracted ray bends away from the normal as it travels from denser medium to rarer medium.
Figure – Total Internal Reflection
CHARACTERISTICS OF OPTICAL FIBRE
- It has a large bandwidth.
- The optical frequency of 2 x 1014 Hz can be used and hence the system has higher bandwidth.
- Thus optical fibres have greater information-carrying capacity due to greater bandwidth.
- In optical fibre system transmission losses are as low as 0.1 db/km.
- Optical fibre is of small size and light weight as compared to electrical fibre.
- Optical fibre communication is free from electromagnetic interference.
- Optical fibre does not carry high voltage and current hence they are safer than electrical cable.
- Optical Fibres are flexible and have high tensile strength. Thus can be bent or twisted easily.
Figure: Optical Fibre
CONSTRUCTION OF OPTICAL FIBRE
It consists of a very thin fibre of silica or glass or plastic of a high refractive index called the core. The core has a diameter of 10 um to 100 um. The core is enclosed by a cover of glass or plastic called cladding. The refractive index of the cladding is less than that of the core (which is a must condition for the working of the optical fibre). The difference between the two indicates is very small of order 10-3. The core and the cladding are enclosed in an outer protective jacket made of plastic to provide strength to the optical fibre. The refractive index can change from core to cladding abruptly (as in step-index fibre) or gradually (as in graded-index fibre).
Figure Representation of Optical Fibre
WORKING OF OPTICAL FIBRE
When a ray of light is incident on the core of the optical fibre at a small angle, it suffers refraction and strikes the core-cladding interface, As the diameter of the fibre is very small hence the angle of incidence is greater than the critical angle. Therefore, the ray suffers total internal reflection at the core-cladding interface and strikes the opposite interface. At this interface also, the angle of incidence is greater than the critical angle, so it again suffers total internal reflection. Thus, the ray of light reaches the other end of the fibre after suffering repeated total internal reflections along the length of the fibre. At the other end, the ray suffers refraction and emerges out the optical fibre.
We can see that the light travels in the core in a guided manner. Hence the communication through the optical fibre is sometimes referred as an optical waveguide.
ACCEPTANCE ANGLE
Definition:- Acceptance angle is defined as the maximum angle of incidence at the interface of air medium and core medium for which the light ray enters into the core and travels along the interface of core and cladding.
Let n0 be the refractive indices of air
n1 be the refractive indices of core
n2 be the refractive indices of cladding
Let a light ray OA is incident on the interface of air medium and core medium with an angle of incidence θ0
The light ray refracts into the core medium with an angle of refraction θ1 and the refracted ray AB is again incidenting on the interface of core and cladding with an angle of incident (90- θ1)
If (90- θ1) is equal to the critical angle of core and cladding media then the ray travels along the interface of core and cladding along the path BC. If the angle of incident at the interface of air and core θ1< θ0 then (90- θ1) will be greater than the critical angle. Therefore,
The total internal reflection takes place.
According to Snell’s law at point A
n0 Sin θ0 = n1 Sin θ1
Sin θ0= (n1 / n0) Sin θ1 ………(1)
According to Snell’s law at point B
n1 Sin(90- θ1) = n2 Sin90 ………(2)
n1 Cosθ1 = n2 as (Sin90=1)
Cosθ1 = n2 /n1
Sinθ1 = (1-Cos2 θ1)1/2
Sinθ1= (1- (n2 /n1)2)1/2
Sinθ1= ( n12- n22 )1/2/ n1 ………(3)
We know Sin θ0= (n1 / n0) Sin θ1 from equation (1)
Substitute the value of Sinθ1 from equation (3)
Sinθ0= (n1 / n0) *( n12- n22 )1/2/ n1
On simplification
Sinθ0= ( n12- n22 )1/2/ n0
θ0=Sin-1 ( n12- n22 )1/2/ n0
Acceptance Angle is θ0=Sin-1 ( n12- n22 )1/2/ n0 ………(4)
NUMERICAL APERTURE
Definition: -Numerical aperture is defined as the light gathering capacity of an optical fibre and it is directly proportional to the acceptance angle. Numerically it is equal to the sin of the acceptance angle.
NA = Sin(acceptance angle)
NA = Sin {Sin-1 (( n12- n22 )1/2/ n0)} from equation (4)
NA = (( n12- n22 )1/2/ n0) ………(5)
If the refractive index of the air medium is unity i.e. n0=1 put in (5)
NA = ( n12- n22 )1/2 ………(6)
Fractional change in refractive index
∆= (n1- n2)/ n1
n1∆ = (n1- n2) ………(7)
From equation (6), we have
NA = {( n1- n2 )( n1+n2 )}1/2
NA = { n1∆ (n1+n2 )}1/2 as n1∆ = (n1- n2) by Eq(7)
NA = { n1∆ 2n1}1/2 n1 ≈ n2, so n1+n2 =2n1
NA = n1{2∆}1/2
This gives the relation between Numerical aperture and Fractional change in refractive index.
TYPES OF OPTICAL FIBRES
The types of optical fibres depend on the refractive index, materials used, and mode of propagation of light.
The classification based on the materials used is as follows:
- Plastic Optical Fibres: The polymethylmethacrylate is used as a core material for the transmission of the light.
Example:
Core: polymethyl methacrylate : Cladding: Co- Polymer
Core: Polystyrene : Cladding: Methyl methacrylate
- Glass Fibres: It consists of extremely fine glass fibres.
Example:
Core: SiO2 Cladding: SiO2
Core: GeO2- SiO2 Cladding: SiO2
The classification based on the mode of propagation of light is as follows:
Mode of propagation:
Light propagates as electromagnetic waves through an optical fibre. All waves, having ray directions above the critical angle will be trapped within the fibre due to total internal reflection. However, all such waves do not propagate through the fibre. Only certain ray directions are allowed to propagate. The allowed directions correspond to the modes of the fibre. In simple terms, modes can be visualized as the possible number of paths of light in an optical fibre.
- Single-Mode Fibres:
These fibres are used for long-distance transmission of signals. In general, the single mode fibres are step – index fibres. These types of fibres are made from doped silica. It has a very small core diameter so that it can allow only one mode of propagation and hence called single mode fibres.
The cladding diameter must be very large compared to the core diameter. Thus in the case of single mode fibre, the optical loss is very much reduced. The structure of a single mode fibre is given below.
Structure:
Core diameter : 5-10μm
Cladding diameter : Generally around 125μm
Protective layer : 250 to 1000μm
Numerical aperture : 0.08 to 0.10
Band width : More than 50MHz km.
Application:
Because of high bandwidth, they are used in long haul communication systems.
- Multimode Fibres:
These fibres are used for short-distance transmission of signals. The multi-mode fibres are useful in manufacturing both for step – index and graded index fibres. The multi-mode fibres are made by multi-component glass compounds such as Glass – Clad Glass, Silica – Clad – Silica, doped silica etc. Here the core diameter is very large compared to single mode fibres, so that it can allow many modes to propagate through it and hence called as Multi mode fibres. The cladding diameter is also larger than the diameter of the single mode fibres. The structure of the multimode fibre is as shown in the figure above.
Structure:
Core diameter : 50-350μm
Cladding diameter : 125μm - 500μm
Protective layer : 250 to 1100μm
Numerical aperture : 0.12 to 0.5
Band width : Less than 50MHz km.
The total number of modes possible for such an electromagnetic wave guide is
Application:
Because of its less band width it is very useful in short haul communication systems.
The classification based on the refractive index is as follows:
- Step Index Single mode Fibres
It consists of a core surrounded by the cladding, which has a single uniform index of refraction. Step index-single mode fibres: A single mode step index fibre consists of a very thin core of uniform refractive index surrounded by a cladding of refractive index lower than that of core. The refractive index abruptly changes at the core cladding boundary. Light travels along a side path, i.e., along the axis only. So zero order modes is supported by Single Mode Fibre.
- Step index-Multimode fibres
A multimode step index fibre consists of a core of uniform refractive index surrounded by cladding of refractive index lower than that of the core. The refractive index abruptly changes at the core cladding boundary. The core is of large diameter. Light follows zigzag paths inside the fibre. Many such zigzag paths of propagation are permitted in Multi-Mode Fibre. The Numerical Aperture of a Multi-mode fibre is larger as the core diameter of the fibre is larger
- Graded Index Fibres:
The refractive index of the optical fibre decreases as the radial distance from the fibre axis increases. GRIN fibre is one in which refractive index varies radially, decreasing continuously in a parabolic manner from the maximum value of n1, at the center of the core to a constant value of n2 at the core cladding interface.
In graded index fibre, light rays travel at different speeds in different parts of the fibre because the refractive index varies throughout the fibre. Near the outer edge, the refractive index is lower. As a result, rays near the outer edge travel faster than the rays at the center of the core. Because of this, rays arrive at the end of the fibre at approximately the same time. In effect light rays arrive at the end of the fibre are continuously refocused as they travel down the fibre. All rays take the same amount of time in traversing the fibre. This leads to small pulse dispersion.
For a parabolic index fibre, the pulse dispersion is reduced by a factor of about 200 in comparison to step index fibre. It is because of this reason that first and second generation optical communication systems used near parabolic index fibres.
ADVANTAGES OF FIBRE OPTIC COMMUNICATION
The optical fibre communication has more advantages than convectional communication.
1. Enormous Bandwidth
2. Low Transmission Loss
3. Electric Isolation
4. Signal Security
5. Small Size and Less Weight
6. Immunity Cross Talk
1. Enormous bandwidth:- The information carrying capacity of a transmission system is directly proportional to the frequency of the transmitted signals. In the coaxial cable transmission the bandwidth range is up to around 500MHz only. Where as in optical fibre communication, the bandwidth range is large as 105 GHZ.
2. Low transmission loss:- The transmission loss is very low in optical fibres (i.e.KmdB/2.0) than compare with the conventional communication system. Hence for long distance communication fibres are preferred.
3. Electric isolation:- Since fibre optic materials are insulators, they do not exhibit earth and interface problems. Hence communicate through fibre even in electrically danger environment.
4. Signal security:- The transmitted signal through the fibre does not radiate, unlike the copper cables, a transmitted signal cannot be drawn from fibre without tampering it. Thus the optical fibre communication provides 100% signal security.
5. Small size and less weight:- The size of the fibre ranges from 10μm to 50μm, which is very small. The space occupied by the fibre cable is negligibly small compared to conventional electrical cables. Optical fibres are light in weight.
6. Immunity cross talk:- Since the optical fibres are dielectric wave guides, they are free from any electromagnetic interference and radio frequency interference. Since optical interference among different fibres is not possible, cross talk is negligible even many fibres are cabled together.
DISADVANTAGES OF OPTICAL FIBRE
The disadvantages of optical fibre include the following
- The main disadvantages of these cables are installation is expensive and difficult to fix together.
- The optical fibre cables are very difficult to merge & there will be a loss of the beam within the cable while scattering.
- Fibre optic cables are compact and highly vulnerable while fitting
- These cables are more delicate than copper wires.
- Special devices are needed to check the transmission of fibre cable.
APPLICATION OF OPTICAL FIBRE
- Optical fibres are extensively used in communication system.
- Optical fibres are in exchange of information between different computers
- Optical fibres are used for exchange of information in cable televisions, space vehicles, submarines etc.
- Optical fibres are used in industry in security alarm systems, process control and industrial auto machine.
- Optical fibres are used in pressure sensors in biomedical and engine control.
- Optical fibres are used in medicine, in the fabrication in endoscopy for the visualization of internal parts of the human body.
- Sensing applications of optical fibres are Displacement sensor, Fluid level detector, Liquid level sensor, Temperature/pressure sensor and Chemical sensors
- Medical applications of optical fibres are Gastroscopy, Orthoscopic, Couldoscope, Peritonescope and Fibrescope.