B.SC.II PAPER-B (OPTICS and LASERS) Submitted by Dr. Sarvpreet Kaur Assistant Professor PGGCG-11, Chandigarh

Unit-IV Lasers and Fiber optics

Atomic Line Widths The widths of atomic lines are of considerable importance in atomic spectroscopy. Narrow lines are highly desirable for both absorption and emission because they reduce the possibility of interference due to overlapping spectra. The line width ½ of an atomic absorption or emission line is defined as its width in wavelength units when measured at one half the maximum signal.

Line broadening Spectral lines are not infinitely narrow, but show a certain width. A large variety of phenomena contribute to the observed linewidths. In principle we distinguish homogeneous and inhomogeneous broadening. In the first case all molecules that contribute to the absorption line suffer from the same broadening. In the second case different molecules absorb at slightly different frequencies due to small variations in their direct environment For the homogeneous contribution to the linewidth we have T1=Excited state lifetime T2=pure dephasing time For the inhomogeneous contribution: the transition frequencies are taken from some distribution: Inhomogeneous distribution function (IDF)

Line broadening (Homogeneous and non-homogeneous) arises from four sources: The uncertainty effect (because of uncertainties in the transition times)(homogeneous) The Doppler effect (because of rapid movement of atoms)(non-homogeneous) Pressure effect due to collision between atoms of the same kind and with foreign atoms.(non-homogeneous) Electric and magnetic field effects.

Line broadening Classical theory leads to the following equation for the shape of a line (transition). It is called the Lorentz profile Since the Lorentz profile is normalized we find by integrating over all frequencies

Line broadening The line width (full width at half maximum) of the Lorentz profile is the damping parameter, . For an isolated molecule the damping parameter can be interpreted as the inverse of the lifetime of the excited quantum state. This is consistent with the Heisenberg Uncertainty Principle If absorption line is dampened solely by the natural lifetime of the state this is natural broadening

Pressure broadening For an isolated molecule the typical natural lifetime is about 10-8 s, 5x10-4 cm-1 line width Collisions between molecules can shorten this lifetime These collisions can be viewed as ‘billiard ball’ reactions, or as the overlapping of the potential fields of the two molecules. The collision process leads to a Lorentz line shape.

Pressure broadening Clearly the line width will depend on the number of collisions per second,i.e. on the number density of the molecules (Pressure) and the relative speed of the molecules (the square root of the temperature)

Doppler broadening Second major source of line broadening Molecules are in motion when they absorb. This causes a change in the frequency of the incoming radiation as seen in the molecules frame of reference (Doppler effect) Let the velocity be v, and the incoming frequency be , then

Doppler broadening In the atmosphere the molecules are moving with velocities determined by the Maxwell Boltzmann distribution

Doppler broadening The cross section at a frequency  is the sum of all line of sight components

Doppler broadening We now define the Doppler width as

Comparison of line shapes

The Uncertainty Principle Classical physics Measurement uncertainty is due to limitations of the measurement apparatus There is no limit in principle to how accurate a measurement can be made Quantum Mechanics There is a fundamental limit to the accuracy of a measurement determined by the Heisenberg uncertainty principle If a measurement of position is made with precision Dx and a simultaneous measurement of linear momentum is made with precision Dp, then the product of the two uncertainties can never be less than h/2p

Two-level Laser System Unimaginable as absorption and stimulated processes neutralize one another. The material becomes transparent.

Three-level Laser System Initially excited to a short-lived high-energy state . Then quickly decay to the intermediate metastable level. Population inversion is created between lower ground state and a higher-energy metastable state.

Three-level Laser System Nd:YAG laser He-Ne laser

Four-level Laser System Laser transition takes place between the third and second excited states. Rapid depopulation of the lower laser level.

Four-level Laser System Ruby laser

Common Components of all Lasers Active Medium The active medium may be solid crystals such as ruby or Nd:YAG, liquid dyes, gases like CO2 or Helium/Neon, or semiconductors such as GaAs. Active mediums contain atoms whose electrons may be excited to a metastable energy level by an energy source. Excitation Mechanism Excitation mechanisms pump energy into the active medium by one or more of three basic methods; optical, electrical or chemical. High Reflectance Mirror A mirror which reflects essentially 100% of the laser light. Partially Transmissive Mirror A mirror which reflects less than 100% of the laser light and transmits the remainder. Mirrors form an optical resonator

Laser Components Gas lasers consist of a gas filled tube placed in the laser cavity. A voltage (the external pump source) is applied to the tube to excite the atoms in the gas to a population inversion. The light emitted from this type of laser is normally continuous wave (CW).

Laser Construction

Pump Source Provides energy to the laser system Examples: electrical discharges, flashlamps, arc lamps and chemical reactions. The type of pump source used depends on the gain medium. →A helium-neon (HeNe) laser uses an electrical discharge in the helium-neon gas mixture. →Excimer lasers use a chemical reaction.

Gain Medium Major determining factor of the wavelength of operation of the laser. Excited by the pump source to produce a population inversion. Where spontaneous and stimulated emission of photons takes place. Example: solid, liquid, gas and semiconductor.

Optical Resonator Two parallel mirrors placed around the gain medium. Light is reflected by the mirrors back into the medium and is amplified . The design and alignment of the mirrors with respect to the medium is crucial. Spinning mirrors, modulators, filters and absorbers may be added to produce a variety of effects on the laser output.