GAIN MEDIA & Q- SWITCHING

PRESENTED BY-ANAMIKA BANERJEEMSc –CHEMISTRY115217CHM- 801

GAIN MEDIA

  A gain medium is required in a laser to compensate for the resonator losses, and is also called an active laser medium. It can also be used for application in an optical amplifier. The term Gain refers to the amount of amplification.

TYPES OF LASER GAIN MEDIA

There are 3 medium – Gas, Liquid, Solid. a laser transition in the desired wavelength

region, results in maximum gain. GAS LASERS- e.g. in CO2 and Excimer

LasersThey have narrow wavelength region where

there is appreciable optical gain because the spectroscopic transitions are sharp.

LIQUID LASERS- e.g in Dye LasersHave broad region for optical gain

corresponding to their flurescence.

SOLID STATE LASERS- e.g Nd: YAGHave either narrow or broad range

region depending upon the nature of fluorescence.

PUMPING

As the gain medium adds energy to the amplified light, it must itself receive some energy through a process called pumping.

There are 2 basic pumping mechanism

1) OPTICAL PUMPING

2) ELECTRICAL PUMPING

OPTICAL PUMPING

Optically pumping some medium essentially means to inject light in order to electronically excite the medium or some of its constituents into other (usually higher-lying) energy levels. In the context of lasers or laser amplifiers, the goal is to achieve a population inversion in the gain medium and thus to obtain optical amplification via stimulated emission for some range of optical frequencies.

One can use strong and incoherent Black body source or Coherent Laser for exciting laser medium to excited states.

ARC lamps & Tungsten Lamps are used in many continous lasers while flash lamps are used in Pulse lasers.

A ruby laser head. The photo on the left shows the head unassembled, revealing the pumping cavity, the rod and the flash lamps. The photo on the right shows the head assembled.

ELECTRICAL PUMPING

Discharge is created in Gain media which excites the population inversion.

Violent discharge like Spark discharge & Gentle Discharge like Glow Discharge are used.

These gas discharge lamps  show the spectral line outputs of the various noble gases.

LASER RESONATOR

A laser requires a laser resonator (or laser cavity), in which the laser radiation can circulate and pass a gain medium which compensates the optical losses.

It provides a means by which an operator can control a Laser. Control is achieved by adjusting the losses experienced by Cavity . High losses are chosen for most which are not desired and low losses for selected band of frequencies.

Q- SWITCHING

Q-switching, sometimes known as giant pulse formation, is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high (giga watt) peak power, much higher than would be produced by the same laser if it were operating in a continuous wave (constant output) mode. Compared to mode locking, another technique for pulse generation with lasers, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations.

It’s a technique that provide very high output power from pulsed laser. Crossed polariser are placed within cavity to prevent any lasing as every time the light travel to & fro in cavity so its phase is changed. Such photon are mutually out of phase & interact destructively never allowing laser to amplify.

POCKEL CELL

A pockel cell is placed within the cavity between analyzer so that when appropriate wavelength is impinged It will rotate the plane of polarized light to pass

through it with no loss.In actual operation the flash lamp which is

usually used to cause population inversion in turn on & population inversion is build.

When population inversion is at maximum the pockel cell is burst so that lasing occurs. By this time the excited population has become so great that a strong burst of laser emission is emitted causing rapid depletion of population inversion. This process is called Q- Switching.

Q- Switching is a method of storing

energy to increase the output power. In this method energy is stored in the form of excited state. It is applicable for laser material having relative longer lived excited state such as Ruby Laser, Nd: Yag laser.

THANK YOU