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 optical 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 and also greatly reduced alignment sensitivity can be achieved with an optimized resonator.
Typical Trade-offs in Resonator Design
The optimization of a resonator can involve various trade-offs:
- 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 specific requirements, the relevant physical effects, and the trade-offs involved.
The Design Problem
Whereas 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.
The RP Photonics Buyer's Guide contains 9 suppliers for resonator design software. Among them:
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|||A. E. Siegman, “New developments in laser resonators”, Proc. SPIE 1224, 2 (1990)|
|||V. Magni et al., “Recent developments in laser resonator design”, Opt. Quantum Electron. 23, 1105 (1991)|
|||G. Cerullo et al., “Resonators for Kerr-lens mode-locked femtosecond Ti:sapphire lasers”, Opt. Lett. 19 (11), 807 (1994)|
|||R. Paschotta, “Beam quality deterioration of lasers caused by intracavity beam distortions”, Opt. Express 14 (13), 6069 (2006)|
|||S. Yefet and A. Pe'er, “A review of cavity design for Kerr lens mode-locked solid-state lasers”, Appl. Sci. 3 (4), 694 (2013)|
|||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)|
|||A. E. Siegman, Lasers, University Science Books, Mill Valley, CA (1986)|
|||N. Hodgson and H. Weber, Laser Resonators and Beam Propagation, 2nd edn., Springer, Berlin (2005)|
See also: optical resonators, lasers, laser resonators, enhancement cavities, stability zones, alignment sensitivity, laser design, Spotlight article 2011-06-10
and other articles in the categories optical resonators, methods
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) and software could become very valuable for your business!
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