Large Diameter Optics
For some applications, particularly large optical elements such as mirrors, beam splitters, optical flats and windows, lenses or diffraction gratings are required. In many cases, they need to be made as custom optics, because they are used in small numbers and are often subject to additional very special specifications. On the other hand, there are also some very frequently required elements such as large household mirrors and illumination lenses where the quality requirements are quite low and the fabrication is correspondingly simple and cheap. This article focuses mainly on large objects with high quality requirements.
For mounting large optical elements, particularly sophisticated opto-mechanics are often required in addition, so that one achieves precise positioning while avoiding detrimental effects of vibrations and gravity-induced bending.
Large Optical Elements for Imaging Purposes
- The input aperture of an imaging system limits the possible angular resolution due to diffraction.
- A large input aperture also allow one to collect more light, which implies that images of very faint objects can be made with shorter exposure times and lower influence of various noise sources.
For such applications, an extraordinarily high precision of the surface shape is also usually required: the surface elevation should deviate from its ideal value by far less than one wavelength, and this is particularly difficult to achieve for such large elements. One of the challenges is to minimize the effect of gravity, e.g. from bending of mirror substrates due to their own weight. For that purpose, one needs to develop sophisticated ways of suspending such mirrors.
The largest currently used astronomical telescopes have primary mirrors with diameters larger than 10 m; even substantially larger instruments with diameters even above 30 m are currently designed and developed. Such huge mirrors can usually not be fabricated as monolithic optical elements. Instead, one increasingly uses segmented mirrors, typically with segments of hexagonal shape which are arranged in a honeycomb pattern. The fabrication of segments with limited size is much easier, but additional challenges arise from the need to very precisely combine them. Examples for very large (≥10 m) telescopes with segmented primary mirrors are the Keck Telescope, the Southern African Large Telescope, and the Gran Telescopio Canarias. The support structure of such a primary mirror requires sophisticated technology, including a multitude of actuators for controlling the mirror shape.
Laser Beam Expanders
In some cases, laser beams need to be formed with very large beam radius – for example, in order to evenly illuminate large areas, to send a beam over large distances with low beam divergence, or for avoiding problems with laser-induced damage or heating.
Typically, lasers produce beams with relatively small diameter, and one applies a suitable kind of beam expander, often in the form of a kind of telescope. The output lens or mirror of such a device then has to be correspondingly large. The size and quality requirements depend very much on the particular application.
Large Optics for Ultrashort Pulse Compressors
A completely different application area of large optics is the dispersive compression of ultrashort pulses with extremely high peak power. Here, very large beam cross sections are required in order to limit the peak intensity such that laser-induced damage is safely avoided. Laser-induced breakdown in air can also be an issue.
Due to the obvious disadvantages of a large setup – high cost of elements, space requirements, etc. – one typically tries to limit the size of the required optics by maximizing the threshold for laser-induced damage in terms of peak intensity or fluence. This implies that the optical surfaces including their metallic or dielectric coatings need to have a very high quality on a microscopic scale, since microscopic defects can often trigger light-induced damage at substantially lower fluence levels. Therefore, optimized fabrication methods need to be employed, for example refined polishing techniques and sophisticated magnetron or ion beam sputtering techniques for coating fabrication. Careful fabrication must be followed by standardized measurements of the laser-induced damage threshold (LIDT).
Large Interferometer Mirrors
There are large interferometers, used e.g. for detecting gravitational waves, we also relatively large mirrors are required. Here, the peak intensities are actually not that high, since one is dealing with continuous radiation, not with low duty cycle pulses. However, the average optical power is very large, and another reason is excessive beam divergence for smaller diameter beams.
In addition to a high optical quality, thermal vibrations of the mirror surface need to be minimized. While operation at low temperatures already substantially mitigates the problem, an interesting additional measure is to use crystalline mirrors, i.e., mirrors with reflective coatings made from crystalline semiconductor materials. Due to the reduced mechanical loss of those materials, the thermal fluctuations are more concentrated to certain narrow regions of noise frequency, and very low noise is achieved at other frequencies.
There is a variety of applications where relatively large lenses are required, but with only quite moderate quality requirements. Examples are overhead projectors and (on a much larger scale) lighthouses. Here, one often uses Fresnel lenses, which can easily be produced in large sizes – if necessary, by combining multiple segments.
High precision optical metrology is often particularly important for large optical elements with strict specifications. On the other hand, it is also particularly challenging to precisely characterize large optical elements and systems. In addition to the fabrication challenges, manufacturers need to find suitable solutions for optics characterization, since without that they could not prove their capability to meet specifications.
A frequently used method is laser interferometry – not only for precisely measuring surface shapes, but also for detecting vibrations. With dynamic interferometry, it is possible not only to detect vibrations, but also to determine the average position of the surface, thus effectively eliminating the impact of vibrations on the measurement results. Greatly simplify the optics characterization, allowing to perform that before realizing the final low-noise mounting. Besides, the performance of the used opto-mechanics for mounting can also be assessed with that technology.
It is often not sufficient to characterize such elements only once after fabrication. In addition, it also needs to be done at least occasionally at the location of the application, for example in a telescope, or even permanently during operation. This implies that metrology equipment must be built into the final device.
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