Rayleigh Range (Z_R) is the distance from the beam waist where the beam radius has increased 1.414 times its initial value. This is the point at which the area of cross-section is doubled.

If we focus a laser beam, at the focal point, theoretically the intensity goes to infinity. But this is practically not possible because a laser beam cannot have infinite intensity.

It is found that practically the laser beam cannot focus on a point, instead, there is a small beam thickness at the focal point. This smallest distance up to which a laser beam can converge is known as the beam waist (w₀). It is basically the focal point of a laser beam.

So the minimum value of the beam radius is the beam waist (w₀) and the beam radius goes on increasing on either side of the beam waist. The value of the beam waist determines the quality of a laser beam. It quantifies how sharply a laser beam can be focused. A low beam waist signifies a good laser quality.

Confocal Parameter (b) is the distance up to which the collimated waist region of the laser beam extends. It is twice the Rayleigh range.