Simulation Software: Use Commercial Products of Home-Made Software?

Posted on 2012-02-03 as a part of the Photonics Spotlight.

Permanent link: http://www.rp-photonics.com/spotlight_2012_02_03.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

The article helps to find a rational decision on this issue, considering a number of important aspects, such as the time to get started, the issue of software validation, flexibility and user friendliness.

When you need to do some simulation and modeling, a basic question often arises: should you develop your own software to do this, or better take a commercial product?

I am admittedly not neutral on this issue, as I am obviously happy if people decide to buy my own modeling software, such as

• RP Fiber Power for fiber laser and amplifier design
• RP Resonator for resonator design (mainly for bulk lasers)
• RP ProPulse for pulse propagation simulations, e.g. in mode-locked lasers, regenerative amplifiers and fibers

Anyway, I can contribute some thoughts on that question which may help you to arrive a rational conclusion. It turns out that a number of aspects should be considered in this context.

Time to Get Started

In most cases, the time to get familiar with some commercial software is quite small compared with the time to develop some software oneself. So in this respect, commercial software often has a big plus. However, it can also turn into a big waste of time if you start with some software which works for the simple cases but later out turns out not to be sufficiently flexible (see below) so that you still have to turn to something else later on.

Is the Code Correct?

That can be a big issue. Anyone can easily make errors when developing simulation and design software, and obviously such errors can severely hurt you. For example, you may be mislead by them and subsequently try out things in the lab which cannot work.

Definitely, any such software should be carefully validated before it is used. This is a process which takes substantial time and experience. (I have actually devoted another article to the problem of software validation). If you develop such software yourself, you may be under pressure to get results quickly and do only some superficial testing. On the other hand, you may find it hard to trust some anonymous developer at some software company. So preferably you will use software developed by somebody who is known to be most competent, experienced and careful.

User Friendliness and Flexibility

Obviously, it will help you if a software is user friendly, as this makes your work easier, more efficient and also safer. (The risk of introducing errors in the software handling is much greater if you don't have a well engineered user interface!) On the other hand, you want something really flexible, working also in more sophisticated cases, as it is no point to do only some simple things with a software and have no solutions for the most interesting cases.

Unfortunately, it is not easy to achieve user friendliness and flexibility, let alone to combine them in one product. Some products come with a really beautiful graphical interface, where you simply put together some model with a few mouse clicks. However, this approach tends to severely limit your flexibility, as some general structures are hard-coded and cannot be changed. For example, there are optimizations where you can only adjust parameters but not the general type of the used figure of merit. In other cases, you may easily set up some chain of optical components, but cannot deviate from a linear chain.

As a more concrete example, imagine that you want to simulate the operation of a regenerative amplifier, where you inject a pulse, let it circulate say 50 times, then eject it and replenish the energy by pumping the crystal for some time. Now you may want to vary some parameter (e.g., the initial pulse energy) and check out what you get after multiple amplification/pumping cycles. If you have to arrange all that manually for each parameter value, it may drive you crazy before your results are complete.

Scripting vs. Forms and Graphical User Interfaces

Initially, my software always had to be controlled by writing scripts, containing certain commands and mathematical expressions. That gave the user the full flexibility, but required some initial investment to get acquainted to the script language. Well, starting out with some demo scripts one gets into it quite quickly, but some potential users were concerned to spend too much time. Other companies offered software with a purely graphical interface, which was easier to get started with, but totally lacked flexibility. I then considered to offer forms in addition, but I still didn't want to confront the user with a hard choice between two totally different approaches, each one still having its characteristic limitations. I finally managed to find the right combination, best implemented in my RP Fiber Power software:

• You can work with simple forms, but you can put mathematical expressions instead of plain numbers into most form fields. For example, this allows you to specify any time-dependent powers, and not only choose from some given selection of simple temporal shapes.
• When a simulation is executed, the software writes a script based on your form inputs and then executes that. If you do really complicated stuff, just go as far as possible with the forms and then refine further the automatically generated script - which is much easier than starting from scratch with script programming.
• The forms also offer various opportunities to inject additional script code for special purposes such as adding a special graph or certain calculated labels to some diagram, or to import some input data from files.

This approach nicely combines the simplicity of filling out forms with the great flexibility of scripting. It even avoids the hard break when moving from forms to scripting.

Obviously, such features are really difficult to implement. You surely cannot invest that much time when developing some own code for a project, and many software companies cannot offer it either.

Can the Next Colleague Take Over?

A typical situation at universities (but also in some companies) is that some clever guy puts together some ingenious programs which allow him to efficiently to do his job. Only, when he later leaves the place, nobody can take this over, as nobody will understand his source code within a reasonable time. So the next guy will start from scratch. Obviously, this is very inefficient in the long run, and leads to periods of time where the productivity of the team is severely reduced. When working with some well engineered software, this won't happen: the work is not done by tweaking the source code, but on a level which is much easier to access. Also, technical support is available to sort out any remaining problems. So the continuity is much more easily maintained.

Some Conclusions

There can be very good reasons to stick to home-made code, particularly when available commercial software is too limited in its flexibility. If it is not really usable to do the job also in more complicated cases, don't bother to start with it.

On the other hand, if you know a trustworthy commercial software combining a convenient user interface with high flexibility, it is hard to defend the do-it-yourself approach. That will take much more time, will presumably lead to something which is more prone to errors (both in the code and during the handling), and makes it difficult to transfer the job to somebody else. Well, if you want to force your boss to keep you employed forever, this may be the way to go! Your boss however, should want to avoid that, even if it costs some money upfront, as it would be silly to ignore the difference in productivity. One cannot spend that much time on developing software before making this effectively more expensive than buying a commercial software. After all, it is obviously more efficient if one guy (or team) makes an excellent software to be used by many, compared with everyone implementing a quick-and-dirty solution himself.

Sometimes, people say that it would great to have some good commercial software, but they cannot afford it. The question is then whether they can afford to work in less efficient ways, such as paying salaries over long times for the same amount of work and not fully utilizing some expensive infrastructure. Obviously, if you are under pressure to be economically efficient, you should choose the most efficient solution rather than the one which requires the smallest possible initial investment. If you can't, you may get out of business some time later.

Kerr-lens Mode-locked Thin-disk Laser

Posted on 2011-12-23 (revised on 2011-12-28) as a part of the Photonics Spotlight.

Permanent link: http://www.rp-photonics.com/spotlight_2011_12_23.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: O. Pronin et al., “High-power 200 fs Kerr-lens mode-locked thin-disk oscillator”, Opt. Lett. 36 (24), 4746 (2011)

The history of mode-locked thin-disk lasers has a curious twist. Originally, there was a patent saying that semiconductor saturable absorber mirrors (SESAMs) would be unsuitable for mode locking of such lasers; only Kerr lens mode locking (KLM) would work. However, a research team at ETH Zürich (which I supervised at that time) then demonstrated the first mode-locked thin-disk laser exactly with a SESAM (J. Aus der Au et al., Opt. Lett. 25 (11), 859 (2000)), while the attempts of another group with Kerr lens mode locking apparently failed. 11 years later, there is now the paper by Oleg Pronin et al. at the Max-Planck Institute for Quantum Optics in Garching, Germany (see the reference above), reporting a Kerr-lens mode-locked thin-disk laser. It is based on an Yb:YAG thin-disk laser head, as are most other mode-locked thin-disk lasers so far. There is a version with KLM alone, and one which uses a (weak) SESAM in addition for self-starting mode locking and better stability.

What makes that result particularly interesting is the very short pulse duration – down to 200 fs, or 270 fs with a higher output power of up to 45 W. Previously, mode-locked thin-disk Yb:YAG lasers could only be operated with pulse durations around 700 to 800 fs, because only there we obtain a helpful effect from spatial hole burning in the disk (see R. Paschotta et al., Appl. Phys. B 72 (3), 267 (2001)). Forcing such a laser to a pulse duration like 200 fs requires a saturable absorber with larger modulation depth (because spectral filtering by the limited gain bandwidth). With a SESAM, this can lead into trouble with Q-switching instabilities and with excessive heating. KLM does not involve additional heating and may have saturation characteristics which make it easier to avoid Q-switching instabilities.

Another interesting point will be whether a Kerr lens mode-locked thin-disk laser can be sufficiently stable for making a commercial product. One may say that titanium–sapphire lasers with KLM have been sold for many years, but that alone would not be fully convincing, since (a) this is mostly for applications in research, where the demands on stability are lower than for industrial applications, and (b) the issues may be more serious for the large effective mode areas required in a high-power laser.

Are Compact Resonators More Stable?

Posted on 2011-06-10 as a part of the Photonics Spotlight.

Permanent link: http://www.rp-photonics.com/spotlight_2011_06_10.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

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

One of the widespread myths in photonics is that the more compact an optical resonator (e.g., a laser resonator) is, the more robust and stable it will be. As with nearly every myth, there is some truth in it. Surely, it is easier to make a compact mechanical setup stable and thus to prevent vibrations, for example, from having strong effects on the alignment. However, the sensitivity of the resonator to small misalignments (called alignment sensitivity) is in fact often increased when the resonator length is reduced. As a result, the overall robustness may be decreased, despite the tentatively more robust mechanical properties.

A Simple Example

A simple example demonstrates that point convincingly, without using complicated calculations or software. Imagine that we need a simple linear resonator, consisting of a plane mirror and a concave (focusing) mirror. The latter must have a suitable radius of curvature such that the fundamental mode has a certain beam radius in the resonator. (For a laser resonator, the right mode size is often very important for obtaining good beam quality, a high slope efficiency, etc.) The shorter the resonator, the weaker will be the required curvature of the focusing mirror.

Now imagine a small angular misalignment of the curved mirror, while the plane mirror remains fixed. This misalignment simply makes the resonator mode shift transversely to a new position, where it is again perpendicular to the surface of the curved mirror. (Obviously, the mode always has to be perpendicular to both mirror surfaces.) It becomes clear now that a weak curvature – as needed for a short resonator – implies a larger transverse shift of the mode for a given misalignment angle. And this is bad. Consider, for example, such a shift in the resonator of an end-pumped laser: it will decrease the overlap between the laser beam and the pumped region.

The Problem is Common

One may believe that the discussed example is only an unusual pathological case, but it isn't: any type of resonator will exhibit this problem if you try to make it shorter and shorter while maintaining the mode size. Fundamentally, you are then pushing the resonator into a regime where the effect of diffraction becomes weak (simply because there is little propagation distance), mirror curvatures have to become correspondingly weaker, and the alignment becomes very sensitive. It is not good to have a resonator length far below the Rayleigh length of the intracavity beam.

A common case is that of a Q-switched laser, which we like to be compact partly because this gives us the shortest pulse duration. The mode size, however, cannot be reduced arbitrarily, because we often need to extract energy from the whole pumped volume in the laser crystal without employing higher-order modes (which would spoil the beam quality), and possibly because a too small mode leads to optical damage of resonator components. We then run exactly into that problem: the resonator will be compact and mechanically stable, but its alignment sensitivity may nevertheless be high.

Well, you might think, this simply means that we need a very good mechanical setup, and initially align that very carefully. However, misalignment is not only caused by mechanical tilts, but also by thermal effects such as thermal lensing in the laser crystal. Even exceptional mechanical stability can therefore not fully solve this problem.

A Deep Understanding Helps

Even though the problem is fundamental and cannot be eliminated with a simple design trick, it is essential to precisely understand such issues when designing laser resonators. First of all, one needs to be aware of the problem, and know that compactness is not necessarily the key to stability. Furthermore, one can at least select the best possible laser resonator design. What “best possible” means, depends on the concrete requirements. Therefore, finding the best solution is only possible after carefully analyzing these requirements, including the relative importance of various design goals. Having done this, one can decide for a type of design and finally employ advanced laser resonator design software to optimize the design.