## 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.

Note from winnerscience: If you want the e-notes of all the laser  articles that will include the basics of lasers like stimulated absorption, difference between spontaneous and stimulated emission, Einstein  Coefficients, properties and applications of lasers, complete construction and working of lasers like Ruby laser, He-Ne laser, Carbon dioxide laser, Nd:YAG laser, dye laser, semiconductor laser, holography and additional articles of Q-switching and mode locking, then please contact winnerscience@gmail.com. You can post your queries also there.