If you want a smaller collimating laser beam, you must accept a larger divergence; On the contrary, if we want to keep the collimation of light over a long distance, it must get a larger beam size.
The laser beam is focused through the focal lens. The focal lens acts like a magnifying glass and sunlight. For a 55mm lens, the laser beam passes through the lens and converges to the smallest point at about 55mm from the edge of the lens. The laser beam is concentrated to the smallest size at this "spot".
Fig. 1. Aspheric lens is being used to collimate a laser diode beam.
Quite often CW lasers have a short cavity. The resonator of microchip DPSS lasers may vary from less than a millimeter to few millimeters. Cavities of single-mode laser diodes are in the range of hundreds of microns. Generally speaking, such short cavities produce highly divergent beams, which are not very usable in optical systems.
The divergence requirement in microscopy and spectroscopy is often less than 2 mrad (full angle) or even less than 1.5 mrad. In order to meet this requirement of modern analytical instruments, laser beams have to be collimated. This can be understood as putting a lens or a set of lenses in front of the laser cavity – does not matter be it a semiconductor laser cavity or a short DPSS resonator. However, for different types of lasers (diode and DPSS) the beam specifications are completely different.
A diode laser beam features low wavefront quality and high astigmatism - the divergence in the so-called fast axis is much higher than divergence in the slow axis. Various techniques are used for collimating such an astigmatic beam and in this consideration several objectives are important. The primary goal of collimation is to reduce divergence of a beam, the secondary goal is to eliminate astigmatism as much as possible, third – to improve wavefront quality, fourth – to make the beam less elliptical, fifth – to maintain good focusability.
The most simple and popular way is to collimate a laser diode beam by using a single aspheric lens. (see Fig. 1). The larger is the focal length of this lens, the larger will the beam diameter be after collimation. Furthermore, if a certain beam adjustment has to be made, for example in order to expand the beam radius of a collimated beam, two lens system is often used - the so-called telescope. One lens with a negative focal length and the other with a positive one creates a setup to collimate and expand or shrink the beam.
Quite frequently the most popular way to focus a laser diode beam is to use a two lens system where one lens collimates the highly divergent beam and the second lens focusses it. Alternatively, a single aspheric lens can be used to focus the beam for direct focusing, but in most cases, it causes severe aberrations, larger beam and lots of diffractions. By definition, beam quality implies a measure for how well a laser beam can be focused.
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