He-Ne Laser: Construction and working with Diagram

Explain He-Ne Laser under following heads: Construction – working with Diagram.

He-Ne Laser

Construction:

(i) Active medium:

It is a gas laser, which consist of a narrow quartz tube filled with the mixture of helium and neon gases in the ratio 10:1 respectively, at a low pressure (~0.1 mm of Hg). Ne atoms act as active centers and responsible for the laser action, while He atoms are used to help in the excitation process. The length of quartz tube is about 50 cm and diameter is about 1 cm.

(ii) Optical resonator:

To construct the optical resonator cavity, two parallel mirrors are placed at the ends of the quartz tube one of them is partly transparent while the other is fully reflecting. The spacing between the mirrors is adjusted such that it should be equal to the integral multiple of half-wavelengths of the laser light.

(iii) Pumping system:

The pumping is done through electrical discharge by using electrodes that are connected to a high frequency alternating current source.

Working of He-Ne

It is a four energy level laser system. The electrons produced from electric discharge collide with He and Ne atom and excite them to the higher energy levels He2 and Ne4 at 20.61 eV and 20.66 eV respectively. These two states are metastable so the atoms may stay there for a longer time. They are very close to each other, thus some of the He atoms at He2 state may transfer their energy to ground-state Ne atoms through collisions and excite them to the higher energy level Ne4. The kinetic energy of atoms provides the additional energy of 0.05 eV. Thus, He atoms help to achieve a population inversion in Ne atoms.


Now some of the Ne atoms will decay spontaneously to the lower state Ne3 at 18.70 eV by emitting photons of wavelength 6328 Å. The photons those are moving parallel to the axis of the tube will reflect back and forth by the end mirrors of the rod and stimulate other excited Ne atoms to radiate another photon with same phase. Thus, due to successive reflections of these photons at the ends of the tube the number of photons multiplies. After a few microsecond a monochromatic, intense and collimated beam of red light of wavelength 6328 Å emerges through the partially silvered mirror.
The atoms at state Ne3 are unstable and decay spontaneously to a lower state Ne2 and finally they come to the ground state by losing their remaining energy through collision with the tube walls. He-Ne laser operates continuously.

Ruby Laser: Construction and working with Diagram

Explain Ruby Laser under following heads: Construction – working with Diagram.

Ruby Laser

Construction:
(i) Active medium:

It is a solid-state laser, in which a rod of synthetic ruby crystal is used as active medium. The ruby crystal is obtained by doping small amount (about 0.05% by weight) of chromium oxide (Cr2O3) in Aluminum oxide (Al2O3), so that some of the aluminium ions (Al3+) are replaced by chromium ions (Cr3+). These chromium ions give the crystal a pink or red color depending upon the doping concentration. Al2O3 only acts as the host while the chromium ions act as active centers in ruby crystal and responsible for the laser action. The length of ruby rod is usually 2 cm to 30 cm and diameter is 0.5 cm to 2 cm.

 

(ii) Optical resonator:

To construct the optical resonator cavity, the ends of the rods are polished such that they become flat and parallel to each other. Now the one of the ends is coated with silver completely while the other one is partially silvered. Thus, the two silver-coated ends of the rod act as optical resonator system.

(iii) Pumping system:

The ruby rod is placed inside a helical shaped xenon flash lamp to excite the Cr3+ ions. Thus, in ruby laser population inversion is achieved by using optical pumping.

Working of Ruby LASER:

Ruby is a three energy level laser system. After absorbing light photons of wavelength 5500 Å from xenon flash lamp, some of the Cr3+ ions at ground energy level E1 get excited to higher energy level E3. At this energy level, they are unstable and by losing a part of their energy to the crystal lattice, they fall to the metastable energy level E2, whose lifetime is much longer (about 10-3 s). Therefore, the number of Cr3+ ions goes on increasing in E2 state while the number of these ions in ground state E2 goes on decreasing due to pumping by flash lamp and soon the population inversion is achieved between states E2 and E1.


Now some of the Cr3+ ions will decay spontaneously to the ground state E1 by emitting photons of wavelength 6943 Å. The photons those are moving parallel to the axis of the rod will reflect back and forth by the silvered ends of the rod and stimulate other excited Cr3+ ions to radiate another photon with same phase. Thus, due to successive reflections of these photons at the ends of the rod the number of photons multiplies. After a few microsecond a monochromatic, intense and collimated beam of red light of wavelength 6943 Å emerges through the partially silvered end of the rod. The Ruby laser is a pulsed laser that emits light in the form of very short pulses.

Characteristics of LASER light | Types of LASER

  • What are the Characteristics of LASER light? What are the properties of LASER Light?

  • How many types of LASER?

The LASER has following characteristics:
i) Monochromatic:

We know that the wavelength (or the frequency) of light decides its color. The ordinary light contains several electromagnetic waves of different colors (or different wavelengths). Generally, in a laser the light is produced by a specific atomic transition. Hence, the laser light is nearly monochromatic, means it has only one specific color or wavelength rather than so many wavelengths.

ii) Coherent:

In a laser, the process of stimulated emission produces photons of electromagnetic radiation. Thus, it is highly coherent, i.e. all the waves of a laser beam are in phase with one another.

iii) Collimated:

Due to the use of optical resonator, all the electromagnetic waves emitted by a laser travel in the same direction, exactly parallel to one another. This means that laser beams are very narrow and does not spread out much.

iv) Highly intense:

Since the laser produces a collimated beam of coherent electromagnetic waves, travelling in the same direction. Thus, they interfere constructively with each other and hence the intensity (power per unit area) of a laser beam is much greater than the intensity of any other source of electromagnetic radiation.

Types of Lasers

Generally, the lasers are categorized based on the type of active medium used. The active medium may be a solid, gas, liquid or semiconductor:

Solid-state lasers:

Generally, a rod of suitable material is used as active medium in a solid- state laser (e.g. ruby laser and Nd:YAG laser).

Gas lasers:

The active medium of a gas laser is in the gaseous form (e.g. He-Ne laser and CO2 laser).

Dye lasers:

They use liquid solution or suspension of an organic dye (e.g. rhodamine or fluorescein) as active media. They are tunable and capable to produce a wider range of wavelengths.

Semiconductor lasers:

In these lasers, the laser action is performed in a p-n junction diode. They are very compact in size and required very low power.

Lasers may also be categorised based on light produced by them as pulsed laser and continuous wave laser. As the names suggested, the pulsed laser emits light in the form of very short pulses (typically of the order of a few hundred microseconds to a few milliseconds) while the continuous wave laser emits light in the continuous wave form.

Components of LASER: active medium, Pumping system and Optical resonator

What are the main components of LASER?

Components of LASER

There are three basic components of a LASER:

(i) active medium, (ii) pumping system and (iii) optical resonator

(i) Active medium:

The active medium in LASERS may be a solid, liquid, or gas. Different active media emit different energies or wavelengths of light. The basic requirement for the active medium of a LASER is that it should have suitable energy levels to achieve the condition of population inversion or it must have a metastable energy states to support stimulated emission. Those atoms of active medium, which are responsible for LASER action are called active centers and rest of the medium is called as host.

(ii) Pumping System

The pumping system consists of an external source that supplies energy to active medium and helps in obtaining the population inversion. The excitation of atoms may occur directly or through atom-atom collision. It can be optical, electrical or thermal in nature.

(iii) Optical Resonator:

It consists of a pair of parallel mirrors enclosing the active medium in between them. The reflectivity of one of the mirrors near to 100% and the other is partially transparent. It is basically a feedback device that reflects undesirable (off-axis) photons out of the system and directs the desirable (on-axis) photons back and forth through the active medium and in the process the number of photon is multiplied due to stimulated emission causing thereby amplification.

Working of Optical Resonator:-

Atoms (active centers) of the lasing material normally reside in ground energy state.
These atoms can be excited to a higher energy state by supplying external energy. The atoms are unstable at this state so they drop spontaneously to a metastable state in which they can stay longer in compare to the ordinary excited state and hence the population inversion can be achieved at this state. Some of the atoms can be de-excited spontaneously from the metastable state to their ground state emitting photons in random directions. Each spontaneous photon can stimulate other excited atoms to fall to their ground state by emitting a photon that travel in phase and in the same direction as the incident photon. If the direction of emitted photons is parallel to the optical axis, the emitted photons travel back and forth in the optical cavity through the lasing material between the totally reflecting mirror and the partially reflecting mirror. The light energy is amplified in this manner until sufficient energy is built up for a burst of laser light to be transmitted through the partially reflecting mirror.

LASER: Population inversion & Meta stable state

Define Population inversion & Meta stable state in LASER.

Population inversion:

Under normal thermal equilibrium conditions, the number of atoms in the lower energy state (N1).of a material is always larger than the number of atoms in the excited energy state (N2). In this situation, the probability of stimulated emission is much less than the probability of stimulated absorption and spontaneous emission and hence the LASER action is not possible. The population inversion refers to the state of the system in which the number of atoms in the higher energy levels (N2) is more than that in the lower energy levels (N1), i.e. N2 > N1. At this state, the probability of stimulated emission is more as compare to absorption or spontaneous emission.

 

Meta stable state:

Atoms of a material can be excited to a higher energy state from their ground state when they absorb electrical, optical, or thermal energy. At the excited level, atoms are unstable and spontaneously return to their ground state by emitting a photon in a very short time of the order of 10-9 s. A meta stable is an excited state in which an atom can stay for a long time interval of 10-6 – 10-3 s in compare to the ordinary excited state. The population inversion can be achieved only through meta stable state.

Relation between Einstein`s Coefficients A and B

Derive Relation between Einstein`s Coefficients A & B in LASER.

Relation between Einstein`s Coefficients A & B

Let us consider an atomic system in thermal equilibrium at absolute temperature T. Let N1 and N2 be the number of atoms per unit volume in the ground energy state E1 and excited energy state E2 respectively. Let Ju is the energy density of the incident radiation corresponding to frequency u.

Energy density Ju is defined as the incident energy on an atom as per unit volume in a state.

According to Einstein,

1) The rate of absorption of light (R1) is proportional to the number of atoms N1 per unit volume in the ground energy state E1 and energy density Ju, of the incident radiation corresponding to frequency u.

That is                R1  N1 Ju

or                         R1 = B12N1Ju                                                                                                            (1)

Where B12 is known as the Einstein’s coefficient of stimulated absorption and it represents the probability of absorption of radiation.

2) The rate of spontaneous emission (R2) is independent of energy density Ju of the incident radiation and is proportional to number of atoms N2 in the excited state E2 thus

R2  N2

or                         R2 = A21N2                                                                                                               (2)

Where A21 is known as Einstein’s coefficient for spontaneous emission and it represents the probability of spontaneous emission.

3) The rate of stimulated emission (R3) is proportional to the energy density Ju, of the incident radiation corresponding to frequency u and number of atoms N2 in the excited energy state E2, thus

R3  N2 Ju

or

R1 = B21N2Ju                                                                                                            (3)

Where B21 is known as the Einstein coefficient for stimulated emission and it represents the probability of stimulated emission.

In steady state (at thermal equilibrium), the two emission rates (spontaneous and stimulated) must balance the rate of absorption. Thus

R1 = R2 + R3

Using equations (1, 2, and 3), we get

                            B12N1Ju = A21N2 + B21N2Ju

or                         B12N1JuB21N2Ju = A21N2

or                         (B12N1B21N2) Ju = A21N2

Ju = A21N/ (B12N1B21N2)                                                            (4)

Einstein proved thermodynamically, that the probability of stimulated absorption is equal to the probability of stimulated emission, thus

B12 = B21

Then from equation (4),

Ju = A21N/ B12(N1N2)

Ju = A21 / {B12(N1/N2)-1}

According to Boltzman’s distribution law, at absolute temperature T the probability that an atom is occur in an energy state E is proportional to eE/KT , where K is the Boltzman`s constant. Thus the ratio of populations of two energy levels (E1 and E2) at temperature T can be expressed as

This is the relation between Einstein’s coefficients in laser.

Significance of relation between Einstein`s coefficient:

It is clear from the above relation that the ratio of Einstein’s coefficient of spontaneous emission to the Einstein’s coefficient of stimulated absorption is proportional to cube of frequency u. It means that at thermal equilibrium, the probability of spontaneous emission increases rapidly with the energy difference between two states.

LASER | Principle of LASER | LASER Process | Transition Probabilities

Define LASER. What are the transition probabilities of LASER?

LASER

Lasers are the source of a special form of light. The word LASER is an acronym for “Light Amplification by Stimulated Emission of Radiation”. Laser is a device in which we use process of stimulated emission to amplify the infrared, visible or ultraviolet electromagnetic radiation.

Principle of LASER:

To understand the principle of LASER it is essential to understand the three quantum processes through which an electromagnetic radiation interacts with the atoms of matter, viz. (i) Stimulated absorption, (ii) Spontaneous emission and (iii) Stimulated emission.

  • Stimulated absorption:

Generally, the atoms of any material reside in ground energy state (E1) at low temperatures. These atoms can be excited to a higher energy state (E2) when they absorb a photon of energy ΔE = E2E1 = hu, where h is the Planck’s constant and u is the frequency of photon. This process is known as stimulated absorption of radiation (Fig.1).

  • Spontaneous emission:

At the E2 level, atoms are unstable and spontaneously return to their ground state E1, with the emission of a photon of energy ΔE in a very short time of the order of 10-9 s. This process is referred to as spontaneous emission of radiation (Fig.2).

  • Stimulated emission:

If in a system where there exist atoms in excited state, we allow a photon of energy E2E1, then the incident light photon may force excited atom to go back to its ground state emitting thereby an another light photon of same frequency, phase and same state of polarization. These two photons are perfectly coherent. This process is known as
stimulated emission of radiation (Fig. 3).