Physics Vidyapith ✍️

Brief Description: (Liquid Lasers) Due to their homogeneous properties and a very high optical cavity of liquids, these are also used as active materials in lasers. Liquid lasers are four-level lasers that use liquids as active material or lasing medium. In these lasers, laser tubes are filled with liquid instead of laser rods as in solid-state lasers or gas in gas lasers. Liquid laser medium has some advantages like very high gain, no cracking for high output power, feasibility of cooling the liquid by circulation, narrow frequency spectrum, etc. In liquid lasers, optical pumping is required for laser action. Optical pumping includes flash tubes, nitrogen lasers, excimer lasers, etc. A rare earth ion dissolved in a solution makes it possible to obtain optically pumped laser action in liquids. The first successful liquid laser was reported by using europium ions ($Eu^{+3}$) in which a sharp and strong laser transition was observed at $6131 A^{\circ}$ wavelength. In this laser, a eur

Some of the differences between spontaneous and stimulated emission of radiation are given as follows: 1. In spontaneous emission, an atom in excited state falls to the ground state on its own without any incident photon while in stimulated emission transition takes place by stimulating photons or by an external agency. 2. In stimulated emission for each incident photon there are two outgoing photons in the same direction while in spontaneous emission the emitted photons move randomly in any direction. 3. The photons emitted in spontaneous emiss ion have a random phase and hence are incoherent while in stimulated emission the emitted photons are in phase and hence are coherent. 4. The rate of spontaneous emission is proportional to only the number of atoms in the excited state while the rate of stimulated emission is proportional to the number of atoms left in the excited state as well as on the energy density of the incident radiation. 5. In stimulated emission of

Characteristics of Ruby Laser → Some of the characteristics of ruby laser are given as follows: Ruby laser is the first working laser that was developed in 1960. Ruby lasers are three-level solid-state pulsed lasers with pulse lengths of the order of a millisecond. This laser uses a synthetic Ruby crystal that is Aluminium oxide as its gain medium. A triply ionized chromium $$Cr^{+3} is used as a dopant for active ion, concentration bring of the order of $0.055%$. Ruby crystals are hard and durable, chemically stable and it has good thermal conductivity. Ruby lasers are optically pumped using a flash lamp. In a ruby laser, water or liquid nitrogen is used as a coolant. These lasers produce pulses of visible light at wavelength $6928A^{\circ}$ and $6943A^{\circ}$, with $6943A^{\circ}$ as dominant wavelength which is a deep red color. Ruby laser is highly temperature-dependent.

Principle of Ruby Laser → Ruby laser is the first working laser that was invented by T.H.Maima in 1960. It is a three-level solid-state pulsed laser that uses a synthetic ruby crystal or sapphire$(Al_{2}O_{3})$ as its gain medium and triply ionized chromium$(Cr^{+3})$ is used as a dopant. Construction of Ruby Laser → There are the following main components of ruby laser: Active Medium Resonant Cavity Pumping and Cooling Device Ruby laser diagram 1. Active Medium → The active medium or gained medium in ruby laser is a synthetic ruby crystal or Aluminium oxide $(Al_{2}O_{3})$ in the form of a cylindrical rod having size $2-30cm$ in length and $0.5-2.0cm$ in diameter. The size of the rod main varies depending upon the use. This gain medium falls in the category of 'narrow line width' laser material. A triply ionised chromium $(Cr^{+3})$ is used as doping material or dopant which works as

Description: In four-level pumping, atoms of ground energy state go to upper energy state$(E_{4})$ by pumping transition to achieve the population inversion. Due to the short time of the upper energy state atoms go to metastable state by nonradiative transitions or spontaneous emission. Atoms of metastable state come to lower lasing level by laser transition process. The atoms come from lower lasing level to ground state by nonradiative transition or spontaneous emission. This process is repeated continuously. Four-level pumping in Laser In contrast to level pumping, the lower lasing transition level in the four-level scheme is not the ground state and is virtually vacant. As soon as some atoms are pumped to the upper lasing level, population inversion is achieved. So it is required less pumping energy than a three-level laser system. this is the major disadvantage of this scheme. Further, the lifetime of the lower lasing level is shorter as it is not a metastable state. H

Description: Three-level pumping in laser is suitable for attending population inversion. When atoms of ground energy state observe the photon from incident energy. It goes from lower energy or ground energy state two to a higher energy state but the lifetime of a high energy state is very short that is $10^{-8}$ $sec$ i.e. So an atom cannot stay for a long time in high energy state i.e.$E_{3}$ and then the atom goes for non-radiative transition and reach to the metastable state. In a metastable state, Atoms cannot go to a lower energy state or ground energy state directly. Therefore, These atoms come from a metastable state to a lower energy state or ground energy state by lasing transition. Three-level pumping in Laser This is the process of three-level pumping in a laser. For better pumping efficiency, The level $E_{3}$ should be the band of energy levels instead of being a single arrow line. It allows the use of pumping radiation of wider bandwidth to excite more

Two-level pumping occurs between two energy levels. All the process of laser (absorption, spontaneous emission, or stimulated emission) occurs between two energy level. The absorption of light or emission of light energy is the difference between two energy levels. If two energy levels are $E_{1}$ and $E_{2}$ so absorption or emission of a photon → $E_{2}-E_{1}=h\nu$ Where$h$ → Planck's Constant$\nu$ → Frequency of photon Two-level pumping in laser is not suitable for attaining the population inversion. The transition of atoms between two energy levels by stimulated emission is called a lasing transition. The lower level is known as the lower lasing level and the upper level is known as the upper lasing level. The upper lasing level must be a metastable level. The uppermost level to which atoms are in the excited state is known as the pumping level. The transition between the ground level and pumping level is called the pumping transition. Two-leve

Absorption → When a photon (or light) incident on atoms then atoms absorb the energy from the photon and jump from a lower energy state to a higher energy state. This transition is known as induced absorption or stimulated absorption or simply as absorption. The process is represented as $A+h\nu=A^{*}$ Where$A$ → Lower energy state atom$A^{*}$ → Excited or Higher energy state atom Absorption Transition If $N_{1}$ and $N_{2}$ is the population of energy $E_{1}$ and $E_{2}$, the number of atoms per unit volume that makes upward transitions from the lower levels to the upper level per second is called the rate of absorption transitions. It is represented by $R_{abs}=-\frac{dN_{1}}{dt} \qquad(1)$ Where $-\frac{dN_{1}}{dt}$ is rate of decrease of population at the lower energy level $E_{1}$ The rate of absorption transition can also be represented by the rate of the increase of population at the upper energy level $E_{2}$. i.e $R_{abs}=\frac{dN_{2}}{dt} \qquad(2)$

Derivation of Einstein Coefficient Relation→ Let us consider the $N_{1}$ and $N_{2}$ is the mean population of lower energy state and upper energy state respectively. If the energy density of incident light is $\rho(\nu)$ then The rate of transition of number of atoms due to absorption process: $R_{abs}=B_{12} \: \rho(v) \: N_{1} \qquad(1)$ The above equation shows the number of atoms absorbing the photon per second per unit volume Where $B_{12}$= Einstein Absorption Coefficent The rate of transition of number of atoms due to sponteneous emission process: $R_{sp}=A_{21} \: N_{2} \qquad(2)$ The above equation shows the number of atoms emitting the photon per second per unit volume due to spontaneous emission Where $A_{21}$= Einstein Spontaneous Emission Coefficient The rate of transition of the number of atoms due to stimulated emission process: $R_{st}=B_{21} \: \rho(v) \: N_{2} \qquad(3)$ The above equation shows the number of atoms emitting the photon per

Laser→ LASER is an acronym for  Light Amplification by Stimulated Emission of Radiation . It is a device that produces a highly intense monochromatic, collimated, and highly coherent light beam. Laser action mainly depends on the phenomenon of population inversion and stimulated emission. The first successful Laser is a solid-state laser which was built by TH Maiman in 1960 using Ruby as an active medium. Note→ The laser has often been referred to as an optical MASER because it operates in the visible spectrum portion of the spectrum. In general, when the variation occurs below the infrared portion of the electromagnetic spectrum, the term MASER will be employed, and when stimulated emission occurs in the infrared, visible, or ultraviolet portion of the spectrum the term laser or optical MASER will be used. Properties of a Laser Beam→ The laser beam has the following main characteristics properties: A laser beam has high directionality and can be em

Difference between Stable and Unstable Resonators: The oscillating beam is converged in stable resonator while in unstable resonator is spreads out of the the resonator. In stable resonator laser output is from the centre of optical axis while in unstable resonator laser output comes from the edge of the output mirror. The field is confined to the axis in stable resonator while it is not so in unstable resonator. Stable resonators are used for low power lasers while unstable resonators are used for high power lasers. In stable resonator these remains risk of breakage of the m  irrors while it is reduced to unstable resonators. The mode volume is is small in stable resonators while it is large in unstable resonators. The geometrical losses are large in unstable resonator in comparison to stable resonators. In unstable resonators better beam quality may be achieved in comparison to stable resonators.

Description of Application of Lasers: There are widespread applications of lasers in various disciplines such as medicine, industries, astronomy, communication, chemistry, etc. Some of the laser applications are given below in short: 1.) Lasers in Medicine:  Some of the applications of lasers in medical care such as in: Controlling haemorrhage. Treatment of the liver and lungs and for the elimination of moles and tumours developing on the skin tissues. Therapy and stomatology. Microsurgery for virtually painless treatment. Ophthalmology to reattach a detached retina. Penetration of blood vessels in the eye for treating glaucoma. Treatment of cancer. Dentistry etc. 2.) Lasers in Industries: Some of the industrial applications of lasers are as follows: Testing the quality of optical components such as lenses, prism, gratings etc. More accurate measurement of the sizes of physical quantities, precision length measurement. Gel