DeepLightSeekdefinition:
A laser beam with a very small divergence angle.
A collimated beam refers to a beam of light (usually a laser beam) with a very small beam divergence angle, so that the beam radius does not significantly change after a certain propagation distance. The simplest and most common scenario is a Gaussian beam, which means that the Rayleigh length is longer than the assumed propagation distance.
For example, a 1064 nm beam with a beam radius of 1 mm at the waist and a Rayleigh length of approximately 3 m in air can be collimated in ordinary laboratory devices. Due to the fact that the Rayleigh length is proportional to the square of the waist radius, a larger waist radius is more conducive to the long-distance propagation of the beam.
For beams with non idealized beam quality, the Rayleigh length will decrease, and the degree of reduction is determined by the M2 factor. Therefore, compared to the ideal situation, in order to achieve beam collimation, the waist radius needs to be larger.
Divergent beams can be easily converted into collimated beams through lenses or curved mirrors, simply by selecting the appropriate focal length or curvature radius to flatten the originally curved wavefront.
The beam radius at the lens or curved mirror should be large enough to obtain a relatively small divergence angle. By adjusting the position of the lens or curved mirror in the direction of the beam, residual divergence can be eliminated.
The use of Shack Hartmann sensors or specific interferometers can measure the radius change of a beam propagating over a distance in free space, thereby verifying whether the beam is collimated.
In fiber optics, fiber collimators are usually used. Suitable for bare optical fibers and optical fibers that need to be connected, that is, optical fibers that need to be connected to fiber optic connectors.
Figure 1: A lens can collimate the light emitted from an optical fiber or direct the collimated beam into the fiber.
Collimated beams are very useful in typical experiments because their beam radius remains relatively constant during propagation, making it easy to vary the distance between various optical devices in experiments without the need for any additional optical components, and without increasing the beam radius.
The output beam of most solid-state lasers is collimated, and a planar output coupler can obtain an output with a wavefront that is planar. Additionally, the beam waist radius is large enough to avoid excessive divergence angles.
However, for laser diodes, the divergence of their output beam is significant, so a collimating device (at least a fast axis collimator, which can significantly reduce the divergence in the fast axis direction) is usually required.
For the light output from the optical fiber, a simple optical lens can be used to obtain a collimated beam. Of course, using a non spherical lens can also improve the beam quality, especially for single-mode fibers with large numerical apertures.
