# Laser Collimation

The beam of a laser source can be characterized by the divergence θ × θII measured perpendicular and parallel to the active surface area at the 1/e²-level ( = 13.5%).

Beam characteristics can also be described at the 50% intensity level and are then defined by the divergence θ⊥ FWHM × θII FWHM (FWHM: full width at half maximum).

For laser diodes, the parameters θ⊥ FWHM × θII FWHM are usually specified, and for a collimated beam, a description at the 1/e²- level is more suitable.

## Collimation Optics

Collimation optics transform a divergent beam with the divergence θ × θII into a collimated beam, retaining both its Gaussian intensity distribution and elliptical beam profile with diameters Ø × ØII. The beam diameter Ø⊥ II at the collimator is also given at the 1/e²-level and is defined by the focal length f of the collimating lens and the divergence θ⊥ II FWHM of the laser diode.

The differing definitions account for the factor 1.7 in the equations below.

Even a collimated beam has a non-vanishing divergence, the beam diameter varies (for large distances) with the distance A from the laser diode collimator. The resulting beam divergences of the collimated beam θ and θII depend on the beam diameter at the collimator Øor ØII respectively and on the wavelength λ of the emitted radiation. For an ideal Gaussian beam (M² = 1):

## Collimating Lenses

Collimating lenses by Schäfter+Kirchhoff are manufactured from high quality glass. Beam collimation and beam shape are up to 30 times more stable compared to plastic lenses showing variation in refractive index and shape caused by temperature changes.

Bi-asphere lenses used for collimating monochromatic radiation show the same correction and imaging quality compared to microscope lenses with three and four elements. Caused by their specific manufacturing process, these lenses have micro structures on their surfaces, which also appear in the collimated beam, but not in a focussed spot. Triplet lenses are three lens systems with spherical elements and high surface quality. The lenses are characterized by a very good spherical correction and a high numerical aperture.

For wavelengths in the range 370–2300 nm the lenses are provided with anti-reflex coatings of a few hundred nm bandwidth, respectively.

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