Lenses made from fused silica avoid the heat and focusing problems associated with optical glass. Read on to find out more.
Aspheric lenses made from fused silica, a form of synthetic quartz, are used to focus the laser beams involved in a variety of manufacturing processes. Often a high-energy beam has to be focussed on a very small and precise spot without any aberrations.
Standard optical lenses made from glass have purely spherical surface shapes. This is a straightforward and very economic manufacturing process using just one rotating tool. However, problems begin when such a lens is used to focus a beam of collimated light on a precise point. Collimated light is the principle of a laser beam. Light rays are made to travel in parallel along a narrow beam and, unlike natural light, do not disperse away from it.
Collimated light cannot be focussed correctly with a classical glass lens, as it results in a blur around the focal point. This blur worsens as the ratio of beam’s diameter to the lens’s focal length increases. But in order to achieve a small laser spot on a working surface, this ratio has to be as large as possible. In the past, this has been corrected by using two or three lenses that provide corrections in series.
A further problem is that high-energy laser beams heat up the glass and cause it to expand. This changes the radius of the lens surface and alters its focal length. As a result, the intended laser spot on the working surface tends to wander away from its true position.
These problems can be avoided by using fused silica instead of standard optical glass. Fused silica conducts heat at ten times the rate of optical glass and so avoids the expansion and aberration of images.
In addition, the shape of the lens surface has to be redesigned to ensure that all of the rays in the laser beam pass through it to arrive at the same point. This means that the surface curvature has to flatten out with distance from the centre of the lens. The advantage here is that such a surface requires a single lens element rather than two or three in the case of optical glass.
The surfaces are manufactured with precision using CNC (computer numerical control) techniques. These CNC techniques were originally designed to make complex metal shapes but have been adapted for use on fused silica material.
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