I. Collimate A Laser Diode Beam
A smaller collimated laser beam means accepting greater divergence; conversely, long-distance collimation requires a larger beam size.
The laser beam is focused through the focal lens. The focal lens acts like a magnifying glass and sunlight. A 55mm EFL lens focuses the laser beam, which converges to its smallest spot 55mm from the lens edge. The laser beam is concentrated to the smallest size at this “spot”.
II. For different types of lasers (diode and DPSS), the beam specifications are completely different.
Quite often, CW lasers have a short cavity. The resonator of microchip DPSS lasers may vary from less than a millimeter to a 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.
For microscopy and spectroscopy, divergence requirements typically stay below 2 mrad (full angle)—or even 1.5 mrad. To meet the requirements of modern analytical instruments, laser beams must be collimated. This process involves placing a single lens or lens assembly in front of the laser cavity—whether a semiconductor laser cavity or a short DPSS resonator.However, beam specifications vary significantly between diode and DPSS lasers.
III. Astigmatism characteristics of diode laser beams, core objectives of collimation and focusing, and commonly used techniques.
A diode laser beam has low wavefront quality and high astigmatism—fast-axis divergence is far higher than that of the slow axis. Engineers employ various techniques to collimate such an astigmatic beam, and in this process, several key objectives are paramount.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 simplest and most popular way is to collimate a laser diode beam by using a single aspheric lens. (see Fig. 1). The larger the focal length of this lens, the larger the beam diameter after collimation. Furthermore, when specific beam adjustments are needed—for example, expanding the beam radius of a collimated beam—engineers often adopt a two-lens system, commonly called a 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 focuses it. Alternatively, engineers can use a single aspheric lens for direct beam focusing, but in most cases, this lens introduces severe aberrations, a larger beam spot, and significant diffraction. By definition, beam quality refers to a measure of how effectively a laser beam can be focused.
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