 # The divergence of the laser

Mar 09, 2021

Laser divergence is used to measure the speed of beam divergence from the waist. The beam with a very small divergence angle, such as the beam radius is close to constant in a long transmission distance, is called a collimated beam.

Some divergence in the beam is inevitable (assuming that the beam propagates in an isotropic medium). The divergence angle of the tightly focused beam is larger. If the divergence angle of a beam is much larger than the physically determined divergence angle, the beam will have poor beam quality. The divergence angle is the derivative of the beam radius to the far-field axial position, that is, the distance from the beam waist is much larger than the Rayleigh length. This definition extends the concept of semi divergence angle (in radians), which depends on the definition of beam radius. For Gaussian beams, the beam radius is usually defined as the radius at the peak intensity.

The radius corresponding to. For non-Gaussian beams, the integral formula can be used, which is discussed in the entry of beam radius. Sometimes full angle is used, which is twice the half divergence angle. Except in Gaussian beams.

Besides the angle of the corresponding point at the peak intensity as the divergence angle, the FWHM divergence angle can also be used. It is usually used in the data table of the laser diode and light-emitting diode. In the Gaussian beam, the divergence angle defined by this definition is 1.18 times the half divergence angle determined by the radius of the Gaussian beam.

In order to measure the beam divergence angle, usually, measure the beam defocusing degree, that is, use the beam analyzer to measure the beam radius at different positions.

The beam divergence angle can also be obtained from the complex amplitude distribution of a certain plane. These data can be obtained by shack Hartmann wavefront sensor.

Beam divergence angle is an index that can be measured in the far-field, and it can be used to determine how much the beam is expanded compared with the original radius. In laser physics, the beam divergence half-angle can be obtained by measuring the ratio of the expanded beam radius to the axial distance in the far-field.

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