NEAR-FIELD VS. FAR-FIELD DIVERGENCE

NEAR-FIELD VS. FAR-FIELD DIVERGENCE

Unlike conventional light beams, Gaussian beams do not diverge linearly, as can be seen in figure 36.6. Near the laser, the divergence angle is extremely small; far from the laser, the divergence angle

approaches the asymptotic limit described in equation 36.11 above.

The Raleigh range(z ), defined as the distance over which the  beam radius spreads by a factor of √2, is given by

 

The Raleigh range is the dividing line between near-field diver- gence and mid-range divergence. Far-field divergence (the number quoted in laser specifications) must be measured at a point >zR (usually 10zR will suffice). This is a very important distinction because calculations for spot size and other parameters in an opti- cal train will be inaccurate if near- or mid-field divergence values are used. For a tightly focused beam, the distance from the waist (the focal point) to the far field can be a few millimeters or less. For beams coming directly from the laser, the far-field distance can be measured in meters.