Basic facts about LASER

Basic facts about LASER

Laser is acronym for Light Amplification by Stimulated Emission of Radiation.

Introduction and History:

    Laser is like magic light machine that can cut, drill, weld metals, perform bloodless and painless surgeries, destroy satellites, help in computer use and provide sophistication to almost all phases of life. The discovery of laser changed drastically the scientific and technology world during second half century of 20^thcentury.

    As it is seen that the conventional light obtained from sun, starts, electric lamp, fire, etc. is un-polarized, incoherent, divergent and weak. Hence, researcher focused on getting polarized, coherent, directional and bright light.

     In 1917, Albert Einstein predicted theoretically the process of stimulated emission. This process is basic principle of laser. But due to practical problems involved in development practical laser took 43 years. The first working laser was built by an American Physicist Theodore Maiman in 1960.

    But before that practical laser discovery some developments took place which led to this discovery. In 1939, USSR Physicist Fabrikant suggested about amplification of electromagnetic radiation by stimulated emission. Then MASER (Microwave Amplification by Stimulated Emission of Radiation) theory was developed by American Physicist Charles Townes and Arthur Schawlow and independently by USSR Physicist Basov and Prokhorov in 1954. Townes, Basov Prokhorov and were awarded the Nobel Prize in 1964. Townes and Schawlow built first maser in 1954. These scientists suggested about the possibility of an optical laser in 1959.

   ALL these efforts produced the result of successful demonstration of working ruby laser by T. H. Maiman in 1960. In the same year Ali Javan discovered He-Ne gas laser. In 1962, Robert Hall and Nathan developed semiconductor laser. In 1964, Kumar Patel invented CO₂ laser.

     The lasers find applications in many areas. They brought amazing changes in the field of communication in particular. The laser is leading the way in new field of science called “Photonics".

Some basic concepts to study Laser principle

1. Nature principle

    According to nature principle,

   “Every system in nature is in stable condition when it possesses minimum energy”.

    If there is shortage or excess of energy then the system is in unstable or excited state. The extent of excitation is depending upon extent of shortage or excess of energy with the system. But in nature every system is always trying to acquire stable state by giving excess energy or giving shortage of energy from suitable system. The life time of excitation is 10^-8 second in normal condition.

    Within 10^-8 second system goes to stable state if it is disturbed from stability.

2. Fermions and Bosons:

 “Fermions are sub-atomic particle with half odd spin (1/2), governed by the Fermi-Dirac statistics.”

An important characteristic of fermions is that there is only one particle to occupy the same quantum state. The particles like electron, proton, quarks and leptons are fermions.

      “Boson is sub-atomic particle with integral spin (0, 1), governed by the Bose-Einstein statistics.”

  An important characteristic of bosons is that there is no restriction on the number of them that occupy the same quantum state. The particles like photons, pions and nuclei of even mass number are bosons.

3. Atomic energy states:

 a. Ground state:  The electrons revolving in different orbits round the nucleus constitutes stable atom. Let us consider the sodium atom ₁₁Na²³.

Fig A

The electron distribution is 1S2 , 2S2 , 2P6 , 3S1 , 3P0 . In this case the entire atom has minimum energy, and the atom is said to be in ground state, which is stable state.  This ground state energy can be represented by E1.  In the ground state: 1. The atom is stable. 2. The electron distribution is normal. 3. The atom has minimum energy.

b. Excited state: 

Fig B

Consider one electron in 2S state, which is excited to 3P state by supplying energy. Along with the electron the atom also has excess energy and the atom is in excited state. This excited state of the atom is represented by E2. In the excited state: 1. The atom is unstable. 2. The electron distribution is not normal. 3. The atom has excess energy.

In further discussion, the atom as whole is taken in to consideration, with two energy states: Ground state E₁ and Excited state E_2..  

4. Boltzmann's Distribution Law:

   Every matter is made up of large number of atoms or molecules. Normally majority atoms are in ground state, but some atoms, in ordinary conditions also, stay in excited state. The number atoms in any atomic state are governed by Boltzmann's distribution law.

   Let N₁ be the number of atoms (or population) in ground state of energy E_1, N₂ be the number of atoms (or population) in excited state of energy E_2, k be Boltzmann's constant and T be the Kelvin temperature of the system.

Boltzmann's distribution law is stated in mathematical form:

N₂ = N₁ e^{-(E2 – E1) / k T}

Fig C

This law indicates that the population of ground state is very large as compared to the population of excited state (N1 ˃˃ N2).

5. Pumping:

    The population of lower state is always greater than the population of higher state in normal condition. But by supplying proper energy some of the atoms may be taken from lower state to higher state, by exciting the electrons of those atoms. This process is called pumping.(It is well known practice that water at lower tank is pumped to upper tank by pump.)

“The energy required to take atoms from lower energy state to higher energy state is pumping energy”.

Fig D

Fig D represents the process of pumping by which atoms in lower energy level E1 is taken upper energy level E2 by proper pumping energy. Optical, electrical, chemical, thermal etc. energies are used for pumping. The pumping process is one of important process for laser action.

6. Meta-stable State:

   Normally the excited electrons, in turn the atoms of that electron, remain in excited state for the duration of 10^-8 second. This electron and atom attain stable state within 10^-8 second. Due to this there is no accumulation of atoms in excited state for longer duration. For laser action accumulation in excited state little longer time is required. This purpose is served by some of energy levels of some of the atoms ( e.g. Cromium). Such levels are called meta-stable levels or states.

   “ The energy level having longer duration of excitation life time (of the order 10^-3 second) is called meta-stable level or state”.

    The excitation life time of meta-stable level is nearly 10⁵ times than that of life time of excitation of usual energy level. This helps in bring accumulation of atoms in excited state little longer time.

7. Population Inversion:

   Normally population of higher state is always lower than population of lower state ( N₁ ˃ N₂ ).

“If the population of atoms in higher energy state is greater than the population of atoms in lower state then this situation is called Population Inversion. ( N₂ ˃ N₁ )”

 

Fig E

Fig E shows that when number of atoms (N2) of upper state E2 is greater than that of the number of atoms (N1) in lower state E1. The population inversion can be achieved by using pumping energy. By pumping large number of atoms to meta-stable state population inversion is brought. Laser operation requires this non-equilibrium condition of population inversion.

8. Radiation:

  “The energy which is propagating in the form the electromagnetic wave is called radiation.”

   The electromagnetic wave is travelling with the speed 3 x 10⁸ m/s.  Based on range of frequency or wave length the radiation can be classified as: radio waves, micro waves, infrared, visible light, ultraviolet, X-rays, γ-rays, and cosmic rays.

9. Light:

       “The electromagnetic waves of the wavelength range between 4000 ˚A to 7000 ˚A are called visible light.”

      These wavelengths are sensitive to human eye. The visible light originates from the source by jumping of electrons from higher orbit to the lower orbit in an atom. This gives one packet of energy called photon. But this photon while travelling from one point to another point forms the electromagnetic wave. Thus light behaves as particle and as well as wave .i.e. light has dual nature-wave and particle. A ray of light may be treated as a collection of plane, monochromatic, polarized waves of many frequencies, direction of propagation and polarization planes.

     The photon state is defined by three projections of momentum (P_x, P_y, P_z) and polarization σ. Thus photon state is represented by (P_x, P_y, P_z, σ) . To change the photon state at least one of the four qualities must be changed.

THE EFFECT OF INCIDENT PHOTON ON THE COLLECTION OF ATOMS:

     To know the effect of incident photon the collection of atoms, let us consider one common experience. There is a mango tree with full of mangoes. To detach one mango from the tree, when the stone is thrown toward mango, the kinetic energy with the stone, if it over comes the binding energy of mango, and then mango will fall down. Different things will happen when stone is going towards mango.

a. The stone will not hit the mango.

b. The stone will hit the mango but mango will not detach due to insufficient supplied energy.

c. The stone will hit the mango and detaching it, when stone kinetic energy matches with the binding energy of mango.

        In view of this example let consider the effect of incident photon on the collection of atoms.

a. Photon Passes Away:

Fig F

Let E1 be energy of ground state of atoms with population N1 and E2 be energy of excited state with population N2. ( N1 ˃ N2) If the photon of energy hν not equal to E2 –E1, is incident on collection of atoms then the photon simply passes through the collection (Fig F).

 b. Stimulated Absorption:

   “The process in which the atom absorbs photon energy and get excited is called stimulated absorption or simply absorption”

Fig G

The photon of energy hν = E2 - E1 incident on the collection of atoms (N1˃ N2). There is maximum probability of the photon to collide with the atoms in ground state, because population of ground state is higher. This photon energy is matching with energy level difference of atoms. Hence this photon is completely absorbed by one of the atom is ground state, when that photon collides with that atom. Due to this the atom gets excited and shift to level E2(Fig G).

 

The conditions required for stimulated absorption:

a. The population of lower state (N₁) must be greater than the population of excited state (N₂) i.e. N₁ ˃ N₂.

b. The incident photon energy must match with energy level difference of the atom i.e. hν=E₂-E₁.

    This is similar to absorbing kinetic energy of stone by stem in our mango tree example.

                         Atom + hν = excited atom

 The rate of absorption transition R_ab is the number of atoms per unit volume per second which are shifted from lower level to higher level. R_ab is proportional to the population N₁ of lower state and the energy density of photons ρ(ν). 

R_ab = B₁₂  ρ(ν) N₁

Where, B₁₂ is known as the Einstein coefficient for stimulated absorption. B₁₂ represents the probability of stimulated transition from state 1 to state 2.

 c. Spontaneous Emission:

   “The emission of photon by an excited atom spontaneously is called spontaneous emission

Fig H

After photon absorption the atom which is in excited state has life time of excitation of the order 10-8 second. So, the time duration the atom will remain in excited state E2 is 10-8 second. This atom will drop back to ground state E1 spontaneously. While doing so the electron of that atom jumps from level E2 to level E1 by emitting the photon of energy hν = E2 –E1 spontaneously (Fig. H) .

  The spontaneous emission process is random in nature and completely disorder and uncontrolled process. The light given out by usual sources like sun, star, electric bulb, fire etc. is due to spontaneous emission. Hence light from these sources is incoherent and polarized. The spontaneous emission is like public moving in the bazaar.

                          Excited atom= atom + hν

The rate of spontaneous transition R_sp is the number of atoms per unit volume per second which are emitted from higher level to lower level. R_sp is proportional to the population N₂ of excited state only.

R_sp = A₂₁  N₂

Where, A₂₁ is known as the Einstein coefficient for spontaneous emission. A₂₁ represents the probability of spontaneous transition from state 2 to state 1.

d. Stimulated Emission:

     “The emission of photon by an excited atom by the stimulus ( or induction ) given by incident photon of matching energy is called stimulated emission.”

    Now consider the situation in the collection of atoms where in the population inversion (N₂ ˃ N₂) is brought between meta-stable level (E₂) and ground level (E₁).

Fig I

The photon of energy hν = E2 – E1 is incident on the collection of atoms. Then there is maximum probability that the photon will collide with atoms in meta-stable level. There is resonance between the photon energy and excess energy possess by atoms in E2.  This led to triggering action by the incident to the atom to emit photon an excited atom. Due to this stimulus given by incident photon the atom jumps from level E2 to level E1 by emitting the photon of energy hν = E2 - E1.  (Fig I)

     The boson nature of photon leads that the incident photon and induced photon occupy same state. In other words, the induced photon finds itself in the same state as that of induced photon. This stimulated photon has same frequency, direction, phase and polarization as that of incident photon.

      In this process incident photon is one but output photons are two. Thus stimulated emission brings amplification. These two photons have same phase, they emerge together as coherent. The stimulated process is thus ordered, and controlled process. This process was discovered theoretically by Einstein in 1917, while re-deriving Planck’s law of radiation using the concepts of probability coefficients (called Einstein coefficient) for absorption, spontaneous and stimulated emission. This process is like military march. The stimulated emission is basic principle of laser action. Albert Einstein laid foundation for the invention of the laser.

The conditions required for stimulated emissions are:

1.     There should be population inversion (N₂ ˃ N₁)

2.     To bring the population inversion, the upper level E_2, must be meta-stable state.

3.     The incident photon must have energy hν = E₂-E₁ which brings resonance.  

                                Excited atom + hν = Atom + 2hν

 The rate of stimulated emission R_st is the number of atoms per unit volume per second which are shifted from excited level to lower level. R_st is proportional to the population N₂ of excited state and the energy density of photons ρ(ν). 

R_st = B₂₁  ρ(ν) N₂

Where, B₂₁ is known as the Einstein coefficient for stimulated emission. B₂₁ represents the probability of stimulated emission from state 2 to state 1.

Pumping schemes:

          By the process of stimulation emissions the laser action takes place. The energy levels involved for laser actions are:

a. Ground level

b. Pumping level

c. Upper lasing level (usually meta-stable state)

d. Lower lasing level (usually ground state)

But, however, same level can be used to serve two purposes. Accordingly, there are three pumping schemes:

a. Two levels system:

      The ground level also acts as lower lasing level. Whereas pumping level and upper lasing level both together forms upper level.

b. Three levels system:

     The atoms are pumped to an excited level, there by these excited atoms dropped to meta-stable state. Then laser action takes place between this level and ground level.

c. Four level system:

   For each state, as mentioned above, there are four separate levels.

Active medium and Active centers:

    The material used for laser is called active medium and atoms involved for actual laser action are called active centers. The active medium may be solid, liquid, and gas. In case ruby laser ruby is active medium and ions Cr⁺⁺⁺ are active centers.

 Resonator:

Fig J

The resonator required for laser action, consists of active medium in the form of long cylinder. One end of this cylinder is perfectly polished to serve as perfect reflector and other end is partially polished to acts as partial reflector.(Fig J)

   Partial reflector in this case has different meaning. This reflector reflects light of intensity below certain threshold limit of intensity. If the intensity of light is above the limit then entire light is transmitted.

Characteristics of laser:

  The laser has all the properties of light such as reflection, refraction, diffraction, interference etc. But it has some special properties:

 Laser is (a) highly intense, (b) perfectly monochromatic, (c) perfectly coherent, (d) highly directional and (e) polarized light.

 a. Highly Intense Light:

    The laser is highly intense or bright light. This is because, the energy of laser source, is focused in only in one direction. The large numbers of photons are travelling in particular direction, whereas, in ordinary light source the photons are distributed in all possible directions.

b. Perfectly Monochromatic Light:

     The laser is highly monochromatic light. This is because; the resonant cavity makes the selectively of only one wave-length of light in laser action.          

                              E₂ – E₁ = hν = hc / λ

   In ordinary source the photons are resulted due to different transitions consisting of different wavelengths.

c. Perfectly Coherent Light:

    Due to stimulated emission process the laser is perfectly coherent. i.e.  All the waves are traveling in same phase due selectively of stimulated emission. Laser has spatial (i.e. coherence with respect to space) as well as temporal (i.e. coherence with respect to time) coherence. All the photons emitted are in same phase.

d. Highly Directional Light:

    Laser beam is in the form of an almost parallel beam emitted only in direction and emerging through very small cross section. The divergence of laser light is very small. The laser can travel very long distance without spreading.

e. Plane Polarized Light:

   Laser light is plane polarized light. All the waves of laser wave only one plane of electric vibrations. The stimulated emission and resonant cavity are responsible for polarization of laser.

 Types of Laser:

       Lasers are commonly designated by the type of active medium used. There are four types of lasers:

1. Solid state laser                    2.  Gas laser

3. Liquid dye laser                   4.  Semiconductor diode laser. 

Some lasers work in continuous wave mode and some lasers work in pulsed mode.

1. Solid state laser:

     In solid state laser the active medium is solid material having active centers. The first historical working laser was ruby laser, built by Maiman in 1960. It is pulsed laser, having three level pumping scheme, operated by optical pumping. Ruby laser is pulsed beam of red color, with wavelength 6943⁰A.

    Most popular solid state laser is Nd: YAG laser built in 1964. It has four level pumping scheme, with optical pumping. This laser emits light in infrared region.

2.  Gas laser:

     In 1961, Ali Javan, Bennett and Herriot built first Helium-Neon(He-Ne) gas laser. It is operated with four level pumping scheme and using electrical discharge. It is continuous wave laser of wavelength 6328⁰A.

   Carbon dioxide (CO₂) laser is one of the high intense lasers used in industries. It has four level pumping scheme. This laser produces a light in infra-red region.

3. Liquid dye laser:

  Dye lasers are liquid lasers that use organic dye solutions as active medium and operated on optical pumping.  

1.    Semiconductor diode laser:

  A semiconductor laser is forward biased p-n junction diode. The first semiconductor laser was built by Hall and Nathen in 1962 using GaAs as active medium. This is small in size and having high efficiency.

Applications of Laser:

     Laser finds applications in wide variety of field fundamental studies of basis sciences, electronics, civil, mechanical engineering, medicine and industries. The fields, in which lasers are used, are multiplying.

1.  Holography is a technique of taking three dimensional (i.e. complete)   picture of a given object or scene. Holography is developed by laser.

2. The medical field uses laser for Bloodless and painless surgery, welding of retina, destruction of malignant tumors, dental field, ophthalmology.

3. The intense beam of laser is very useful for different material working, as laser energy density can be controlled according to required temperature of focusing material. Laser is used in industrial purpose for welding, cutting, soldering, a hole drilling and heat treatment for variety of materials.

4. Laser has widened the range of information capacity of optical communication channel with the help of fiber optics. Optical communication has brought revolution in communication system.

5.There are other applications of laser are Laser printers for computers output ,Optical computing and signal processing, Playing video disks, Reading printed bar codes, Measuring the range of distant objects,  Laser scanning,  Missile guidance, Laser weapons etc.