Four level laser

In the four level laser system, the atoms from ground state E1 are raised to excited state E4 with the help of pumping. From the E4, the atoms decay to energy state E3 by spontaneous emission. The transition rate of atoms from E4 to E3 is much faster as compared to transition rate of atoms from E3 to E2. This is due to the reason that E4 is an excited state with life time of the atoms of the order of 10-8 seconds.

E3 is the metastable state. Thus the number of atoms in E3 exceeds the number of atoms in E2. The population inversion is achieved between E3 and E2. The laser action takes place between E3 and E2 by stimulated emission. The atoms from energy state E2 get deexcited to E1. The atoms from E1 are again pumped to E4.

Example: Helium-Neon laser is four level laser system.

Advantage of four level laser:

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Three level lasers

In the three level laser system, the atoms are pumped from the ground state E1 to higher state E3 with the help of pumping source. The E3 is called pumping state. The life time of atoms is least in the energy level E3. It means E3 is unstable stable state and here atoms stay for 10-9 (ten power -9) to 10-8 seconds.

Atoms make transition from level E3 to E2. Energy level E2 is the metastable state (having life time 10-5 to 10-3 seconds). Thus, population of atoms become more in the energy state E2 as compared to E1.

The stimulated emission occurs between E2 and E1 producing laser. E2 is known as upper laser level (ULL) and E1 is known as lower laser level (E1).

The example of three level laser is ruby laser.

Drawback of three level laser system: Continue reading “Three level lasers”

temporal and spatial coherence

Coherence can be classified into two ways:

a) temporal coherence: consider a light wave traveling along +X axis. Consider two different points A and B along the same wave train that is along +X axis.

If Φ(A) is phase of point A at any time and Φ(B) is phase of point B at any time, then

phase difference between these point is given by

Φ = Φ(A) – Φ(B)

If Φ is independent of time then points A and B are said to exhibit temporal coherence or longitudinal coherence

In other words, a beam of laser is said to exhibit temporal coherence if the phase difference of the waves crossing the two points lying on a plane parallel to the direction of the propagation of beam is independent of time.

Note: try the make the diagram yourself.

a) spatial coherence: consider a light wave traveling along +X axis. Draw a line perpendicular to the direction of the beam. Consider two different points C and D on this line.

If Φ(C) is phase of point C at any time and Φ(D) is phase of point D at any time, then

phase difference between these point is given by

Φ = Φ(C) – Φ(D)

If Φ is independent of time then points C and D are said to exhibit spatial coherence or transverse coherence or lateral coherence.

In other words, a beam of laser is said to exhibit spatial coherence if the phase difference of the waves crossing the two points lying on a plane perpendicular to the direction of the propagation of beam is independent of time.

Note: try the make the diagram yourself.

Reference: This article is referred from my authored book “optics and lasers” having ISBN 81-272-2948-2. In case of any doubt, post in the comment section.

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