The Photonics Spotlight > 2015-05-14

Alignment Sensitivity of Laser Resonators – an Important Design Criterion

Ref.: encyclopedia articles on optical resonators, alignment sensitivity of optical resonators, resonator design

Everyone in the field knows that an essential design criterion for laser resonators is to have appropriate mode radii, particularly within a laser crystal. For example, the mode radius in the laser crystal should approximately match the mode radius of the pump beam if one wants to achieve transverse single-mode operation, which results in a high beam quality.

What is much less known is the importance of the alignment sensitivity of laser resonators, and that it can be greatly influenced by the resonator design. The easy part is to understand why a low alignment sensitivity is very desirable; obviously, you do not want a resonator which needs to be aligned with extreme care, and it needs to be realigned when ever some optomechanical parts (e.g. mirror holders) are slightly affected e.g. by thermal expansion, or when the thermal lens is modified by changes of the pump beam profile. Particularly for an industrial product, an excessive alignment sensitivity cannot be tolerated.

In extreme cases, one would not even achieve any reasonable performance, even when working hard on a fine alignment, if thermal effects in the laser crystal are strong. Interestingly, the alignment affects the power and position of the circulating laser beam, which can also lead to modifications of thermal lensing, and that again affects the beam position and power; if such mutual influences are strong enough, you can get a very strange behavior of the laser which prevents any reasonable performance.

Calculating Alignment Sensitivities and Applying that Knowledge

Experts in the field of laser resonator design know very well that the alignment sensitivity of a laser resonator, e.g. concerning angular positions of laser mirrors, can be calculated with suitable software, which does not use a simple ABCD matrix algorithm but rather an extended algorithm using 3×3 matrices. The alignment sensitivity can then be used as a criterion for the quality of the laser design; it can (and often should) even be included in a figure of merit within an automatic optimization procedure. Ignoring this important aspect in laser design can easily lead to designs with unnecessarily high alignment sensitivity which do not work well in practice.

Mode Areas are Important – What Else?

It is also known that lasers with large mode areas tend to have a higher alignment sensitivity. (This is the essential reason why high-power lasers are often much more delicate to align; the wide-spread believe that this results simply from the large size of the resonator itself is wrong, as discussed in an earlier posting.) However, there is no fixed relation between mode size and alignment sensitivity; one can have two different laser resonator designs with the same mode area in the laser crystal which differ very much (e.g. by a factor larger than 5) concerning alignment sensitivity. In the case of linear resonators (standing-wave resonators), one has two different stability zones in terms of the focusing power of the thermal lens, and these can have very different alignment sensitivities. Unfortunately, one cannot always use the less sensitive zone because that involves limitations in other aspects.

The issue of alignment sensitivities even at the heart of an often encountered trade-off: high-power laser can often be designed either for a high power conversion efficiency and robustness or for highest beam quality, but not both at the same time. Obviously, one can hardly find optimized designs without understanding these issues very well.

How to Find a Suitable Resonator Design?

Even in seemingly simple cases, it is very desirable that the person developing a laser resonator design understands the matter well. The required knowledge goes far beyond a basic understanding of resonator modes; it should definitely include a precise knowledge on alignment sensitivity issues and substantial experience concerning various typical trade-offs. Simply having a heavy textbook in the office, or even having read it, will often not be sufficient.

A suitable laser resonator design software (such as our product RP Resonator) must definitely be able to calculate alignment sensitivities and to take them into account in optimizations. However, no software in this area can replace a decent technical understanding of the person using it; I think it is not possible to make it such that it takes into account all important issues without bothering the user with it. For example, a software can hardly “know” the importance of various resonator properties for the particular application, i.e., it could not put appropriate weights on certain factors in the trade-offs which are necessary. At least, however, software from a good source comes with very helpful technical support, giving you crucial pieces of advice.

If you need a proper laser resonator design, you basically have two different options:

• You can try to acquire all the required expertise (which is certainly not easy and will require substantial time) and also get a good resonator design software.
• You can try to find an experienced expert who can do that job for you. That person would first closely analyze the concrete requirements in a dialog with you, then translate that into appropriate resonator properties such as mode sizes and maximum alignment sensitivities, and finally work out a suitable design using proper design software.

If you quite often need resonator designs, you will probably try to get into the position of doing it yourself. If that is not the case, however, it will often be much more economical (and also lead to better results) to have an external expert doing it.

In any case, I warmly recommend taking the question of resonator design very serious because this very much contributes to an efficient product development, avoiding a lot of possible problems causing delays and cost overruns.

By the way, the fact that a very simple resonator has been used so far for certain lasers does in no way prove that more elaborate laser design considerations would be wasting resources. After all, how can you know that the simple type of resonator is doing its job well and could not be improved? Also, even seemingly simple resonators are not so easy to understand. Finally, a few hours of good work by a competent expert cost far less than what you might well pay for wrong decisions on such matters.

This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page and cite it, because its location is permanent. See also the RP Photonics Encyclopedia.

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