Question

What are the typical output characteristics of Nd-YAG laser? What are the pumping mechanisms employed in the fabrication of a Nd-YAG laser? Compare different pumping mechanism in terms of their advantages and disadvantages.

Answer 1

Nd:YAG (neodymium-doped yttrium aluminum garnet; Nd:Y3Al5O12) is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, Nd(III), typically replaces a small fraction (1%) of the yttrium ions in the host crystal structure of the yttrium aluminum garnet (YAG), since the two ions are of similar size. It is the neodymium ion which provides the lasing activity in the crystal, in the same fashion as red chromium ion in ruby lasers.

Output Characteristics:

Nd: YAG laser generates laser light commonly in the near-infrared region of the spectrum at 1064 nanometers (nm). It also emits laser light at several different wavelengths including 1440 nm, 1320 nm, 1120 nm, and 940 nm.

1.     Type: It is a four level solid state laser.

2.     Active medium: The active medium is Nd: YAG laser.

3.     Pumping method: Optical pumping is employed for pumping action.

4.     Pumping source: Xenon or Krypton flash tube is used as pumping source.

5.     Optical resonator: Two ends of Nd: YAG rod is polished with silver (one end is fully silvered and the other is partially silvered) are used as optical resonator.

6.     Power output: The power output is approximately 70 watt.

7.     Nature of output: The nature of output is pulsed or continuous beam of light.

8.     Wavelength of the output: The wavelength of the output beam is 1.06μm(infra-red)

pumping mechanism

Nd: YAG lasers are optically pumped using a flashtube or laser diodes.

Figure shows the energy level diagram for Nd: YAG laser. These energy levels are those of Neodymium (Nd3+) ions.

1.     When the krypton flash lamp is switched on, by the absorption of light

radiation of wavelength 0.73μm and 0.8μm, the Neodymium(Nd3+) atoms are raised from ground level E0 to upper levels E3and E4 (Pump bands).

2.     The Neodymium ions atoms make a transition from these energy levels E2 by non-radiative transition. E2 is a metastable state.

3.     The Neodymium ions are collected in the level E2 and the population inversion is achieved between E2 and E1.

4.     An ion makes a spontaneous transition from E2 to E1, emitting a photon of energy hγ. This emitted photon will trigger a chain of stimulated photons between E2 and E1.

5.     The photons thus generated travel back and forth between two mirrors and grow in strength. After some time, the photon number multiplies more rapidly.

6.     After enough strength is attained (condition for laser being satisfied),

an intense laser light of wavelength 1.06μm is emitted through the partial reflector. It corresponds to the transition from E2to E1.

Types of Pumping

Optical pumping: light is used to raise the atoms to higher energy states.

Solid state laser gain media often exhibit slightly non-degenerate Stark level manifolds. Due to the energy variations within each manifold, optically pumped laser operation is possible even with only two involved manifolds: ions are pumped from the lower manifold (usually the ground-state manifold) to some higher manifold, and the laser transition directly leads from there back to the lower manifold, with no intermediate manifolds.

This pump scheme, called in-band pumping, can be used with various laser-active ions

example : RUBY LASER

Chemical reaction pumping: chemical reactions are used to raise the atoms.

EXAPLE : CO2 LASER

Electric discharge  pumping: A strong field is applied to the atomic system with the use of high voltage power supply. The high energy electrons collide with the atoms and transfer their kinetic energy to the later. As a result, atoms rise to the higher states

EXAMPLE : He -Ne laser

Direct conversion: In this method the electrical energy directly creates the state of population inversion and laser is produced.

example : SEMI CONDUCTOR LASER