Module 8 : Laser- II
Lecture   : Rate equations for Lasers
 
  • We may arbitrarily define the spatial coherence length $ x$to be the lateral distance at which the waves are out of phase by $ \pi$. Thus
  • $\displaystyle x\frac{d}{z} = \frac{\lambda}{2}$
  • The spatial coherence length is given by
  • $\displaystyle x = \frac{\lambda z}{2d}$
  • Using the above argument, if we have a source of circular shape of diameter $ D$, the lateral coherence length is
  • $\displaystyle l_c = \frac{\lambda z}{2D}$
  • As an illustration, we note that a source of 100 $ \mu$m diameter emitting at 100 nm has a spatial coherence length of $ 2.5\times 10^{-4}$m.
  • Directionality: Laser beam is highly collimated and can travel long distances without significant spread in the beam cross section. As the collimated beam propagates, the beam spreads out. The full angle beam divergence $ \Delta\phi$is defined as the amount by which the beam diameter (D) increases over a distance $ z$after leaving the source
    $\displaystyle \Delta\phi = \frac{D}{z}$
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