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**Beam Divergence** of a laser beam is an angular measure of the increase in the beam diameter or the radius with the distance from the **beam waist**, from where the beam diverges.

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 (w _{0})**. It is basically the focal point of a laser beam.

Beam diameter is calculated in terms of beam diameters at two different points far from the rayleigh range and the distance between these points.

Let D_{f} and D_{i} be the beam diameters at two different points away from the rayleigh range and let l be the distance between them. Then **beam divergence (θ)** is given by,

If a collimated beam is focused using a lens with a focal length f, then the diameter D_{m} of the beam at a point far from the rayleigh range is related to the beam divergence θ as,

**Rayleigh range (Z _{R})** is the distance from the beam waist where the beam radius has increased √2 times its initial value. This is the point at which the area of cross-section is doubled.

where 𝛌 is the wavelength of the laser beam.

**Parallel (Horizontal) beam divergence** is the angular measure of the increase in the beam diameter along the optic axis, i.e. along the Z-axis as in the figure.

**Perpendicular (Vertical) beam divergence** is the angular measure of the increase in the beam diameter along the axis perpendicular to the optic axis, i.e. along the Y-axis as in the figure.