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Definition: the description of the optical components from which an optical resonator is made, and their exact arrangement
The term resonator design (or cavity design) is most often used in the context of laser resonators for solid-state bulk lasers. The design of such an optical resonator basically means to define the arrangement of optical elements (e.g. laser mirrors) including their exact distances, so that the resonator modes have the desired properties. The mechanical design can also be an important aspect.
Important Resonator Properties
The design goals for resonators, particularly for laser resonators, can include a large number of aspects which can affect various performance details:
- A frequently encountered requirement is that of suitable mode sizes in or at certain optical components such as the gain medium (in a laser resonator), a saturable absorber (in a mode-locked laser), or a mirror.
- In many cases, it is important to avoid an excessive sensitivity of mode sizes to mechanical tolerances (length deviations or misalignment), and also to thermal lensing effects (including aberrations). For a laser, this can help to achieve a good beam quality over a wide range of pump powers.
- The stability zones should be as wide as possible, but this property is connected to the minimum mode size e.g. in the laser gain medium.
- A minimum sensitivity to misalignment and to asymmetries of thermal lensing is also desirable.
- The resonator should have an appropriate length, e.g. a short length for generating short pulses with Q switching or for easy single-frequency operation, or a certain length for a suitable pulse repetition rate of a mode-locked laser.
- In some cases, a resonator needs to have the potential to be operated with a variable resonator length, e.g. for obtaining a tunable pulse repetition rate.
- In many cases, the round-trip losses should be minimized, e.g. for a low threshold pump power and low laser noise.
- A minimum number of optical components is often desirable.
- The overall size may be optimized, also the mechanical stability, and the ease of manufacturing.
The importance of these criteria should not be underestimated. There are cases where very significant improvements of performance of a laser as well as greatly reduced alignment sensitivity can be achieved with an optimized resonator. However, the optimization can involve various tradeoffs:
Typical Tradeoffs in Resonator Design
- A particularly compact resonator design may not be ideal in terms of beam quality or alignment sensitivity.
- Operation in stability zone I can strongly reduce the alignment sensitivity, but this may require a longer resonator and/or smaller spot sizes on the resonator mirrors, which can cause problems e.g. in the case of Q-switched lasers.
- A high-power laser may be designed for diffraction-limited beam quality, but then have a lower tolerance for misalignment.
Therefore, successful optimization is possible only on the basis of a solid understanding of the concrete requirements, the relevant physical effects, and the involved tradeoffs.
The Design Problem
While it is normally not very difficult to evaluate the properties of a given resonator, it can be much more challenging to find a resonator design which satisfies multiple criteria such as those listed above. In many cases, finding a good solution requires
- a deep understanding of resonator physics
- a flexible resonator design software with the option for numerical optimization
- significant experience with different kinds of resonator designs and their performance (not just with the handling of some software)
A powerful numerical method for resonator design is to define a "figure of merit", calculated as the sum of "penalties" for all non-ideal properties, and to minimize that with numerical techniques by varying resonator arm lengths and possibly mirror curvatures. Because the figure of merit often has a huge number of local extrema, a Monte-Carlo method is sometimes used for finding the global optimum or at least a good solution. Note that the procedure starts with some basic resonator structure, which may have to be varied as long as no good solution is found.
Particularly challenging resonator design tasks arise in the context of mode-locked lasers, with combinations of various mode sizes, resonator length constraints, and other complications.
Bibliography
| [1] | D. Metcalf et al., "Laser resonators containing self-focusing elements", Appl. Opt. 26 (21), 4508 (1987) |
| [2] | A. E. Siegman, "New developments in laser resonators", Proc. SPIE 1224, 2 (1990) |
| [3] | V. Magni et al., "Recent developments in laser resonator design", Opt. Quantum Electron.. 23, 1105 (1991) |
| [4] | R. Paschotta, "Beam quality deterioration of lasers caused by intracavity beam distortions", Opt. Express 14 (13), 6069 (2006) |
| [5] | For German readers: R. Paschotta, "Resonatordesign – unterschätztes Potenzial für bessere Laser", http://www3.interscience.wiley.com/cgi-bin/fulltext/116313857/PDFSTART, Laser Technik Journal 4 (4), p. 50 (2007) |
| [6] | A. E. Siegman, "Lasers", University Science Books, Mill Valley, CA (1986), ISBN 0-935702-11-3 |
| [7] | N. Hodgson and H. Weber, "Laser resonators and beam propagation", 2nd edition, Springer 2005, ISBN 0-387-40078-8 |
See also: optical resonators, lasers, laser resonators, enhancement cavities, stability zones, alignment sensitivity, laser design
Categories: methods, resonators
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!
