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The Photonics Spotlight

The Photonics Spotlight – associated with the Encyclopedia of Laser Physics and Technology – is a “blog” (web log) with the purpose of highlighting interesting news and useful information in the area of photonics, particularly laser technology and applications. The content can be related to particularly interesting scientific papers or to other forms of publications, reporting for example cute new techniques, special achievements, or useful hints.

Note that the Spotlight articles (as well as those of the Encyclopedia) are citable. Permanent links are given for each article.

This blog is operated by of RP Photonics Consulting. Comments and suggestions are welcome. The news items are definitely not available for advertising, but advertisers can order banners on the right column of this page.

You can read this content in various ways:

And here are the articles:


Lower Emission Cross-section Leads to Higher Pulse Energy?!?

Ref.: encyclopedia articles on saturation energy and Q-switched lasers

You may have heard it: if you take a passively Q-switched solid-state laser, the emitted pulse energy per unit area in the laser crystal is approximately inversely proportional to its emission cross-section. So you get more energy out if you choose a material with lower emission cross-sections. Doesn't this look weird? After all, lower emission cross-sections mean a weaker interaction of the excited laser-active ions with the optical field, so one should expect a weaker rather than a stronger output.

Of course, one can understand this. As a first step, let us consider the saturation fluence: this is the pulse fluence which decreases the stored energy in the gain medium to 1 / e of its initial value. That saturation fluence is inversely proportional to the emission cross-section, which cannot be surprising. (That rule is not strictly true for (quasi-)three-level gain media, but let's assume for simplicity that we have a four-level system as in Nd:YAG or Nd:YVO4.)

Next, we can see that the gain efficiency, which can be defined as the gain (e.g. in dB) per millijoule of energy stored in the gain medium, is obviously proportional to the emission cross-section, and thus inversely proportional to the saturation fluence. A low emission cross-section means that you need to pump more energy into the crystal to get a certain gain.

Further, a passively Q-switched laser always starts to emit a pulse once the round-trip gain exceeds the losses. Therefore, a crystal with low emission cross-sections will have to be pumped longer before a pulse is generated. But once this happens, the pulse will be more intense. How much energy can be extracted by it, does normally not depend that much on the emission cross-section, but mainly on the amount of saturable absorption and the output coupler transmission, apart from the beam area in the crystal.

Ultimately it gets clear: the crystal with low emission cross-sections gives us more intense pulses, but fewer of them, i.e., it has a lower pulse repetition rate.

How about Active Q-switching?

In an actively Q-switched laser, things are different. There, the pulse is emitted when we trigger it. As a result, the pulse energy will not depend much on the emission cross-sections. With low emission cross-sections, you may get a longer pulse due to a lower laser gain – unless the gain efficiency is preserved or even increased by a longer upper-state lifetime.

Laser Designs Need Understanding

We are not just talking about some curious phenomena. Understanding such things is essential for efficiently developing good laser designs. You can then quickly find out what beam area in the laser crystal will be required for a certain pulse energy. (To some extend, you can also adjust the parameters of the saturable absorber, but often only in a limited range.) Also, one can then calculate the resonator length required for a certain pulse duration. If you don't know these things, you are prone to start out with a too high or too low beam area, or even with the wrong crystal material, and you will have to change a lot of things after first experiments. That results in an inefficient and thus often too costly laser design and/or in non-ideal designs. Many people in the laser industry are wasting a lot of resources by working out such product designs with insufficient understanding.

Are Numerical Simulations Required?

In simple cases, one can do it without numerical simulations. A set of not too complicated equations often allows me to develop a design rather systematically and quickly. But these equations are based on a number of assumptions, which are not always well fulfilled. For example, you might have gain guiding effects in a high-gain laser, and it hard to tell without simulations how strong these are. Particularly if high performance including high pulse energy and high beam quality is essential, one wants to check more reliably how the design should behave – in order to change it before the first experiments if it isn't working well.

Mediation in Disputes on Laser Technology

As in other areas of business, disputes can arise e.g. when some company purchases a laser from some supplier. Frequently, some technical issues are at the core of the dispute. For example, the following types of questions may occur, where the two parties develop conflicting views:

As lasers cost a lot of money, the involved parties may not be perfectly relaxed when discussing such things. In some cases, they end up going to court. However, the resulting litigation is usually not at all satisfying for any party; it costs enormous money and time, does not at all reliably bring the expected results, distracts from the actual focus of the company, and destroys business relations.

It is a good idea in such a situation to seriously try mediation in order to greatly reduce the cost, risk and distraction. A caveat is only: how to find an expert who could act as an mediator? While it is normally quite easy to find out which law court has to be chosen, finding a suitable expert can be difficult. You would like that person to have an indisputable technical competence, and also a reputation and standing such that both parties can trust in his or her honest attempt to be impartial and just.

Mediation belongs to the services I offer. It is not that I am often asked about such things, but it is an area where I can be quite useful (provided, of course, that I don't have too close relations to one of the parties). Although I do not have a formal training in mediation or arbitration, I am familiar with the underlying principles and confident that I can properly handle such cases to the benefit of both parties. If you want a fully professional mediator, I am also happy to work together with that person, complementing the expertise in the technical area.

You may prefer in such a case to first ask an legal adviser. It cannot hurt to know more precisely what your legal position would be. However, note that lawyers earn substantially more money when going through a court case with you. So they may be tempted to make you more optimistic than you should actually be concerning the chances at court. That's different with a mediator, who's explicit task is to find a settlement in a cheaper way. Anyway, your lawyer will also hardly understand the technical issues, so you need a technical expert in any case.

A company's boss may actually believe that the internal technical know-how of the company is sufficient for such purposes. However, an internal expert can obviously not be impartial and thus not convincing for the other party. Even for internal advice, internal experts may get into difficult situations. For example, they may feel uneasy about certain mistakes they did themselves, and thus be somewhat biased witnesses. That may exactly lead to overoptimistic assessments of chances, which can lead into financial disasters. So I would warmly recommend to be careful with that.

Avoiding Trouble with Laser Specifications

Ref.: encyclopedia article on laser specifications

Can you imagine how much trouble can arise from incomplete and unclear specifications of lasers? Apparently, most people cannot – otherwise, it would not be so common that such specifications are quite poor. And indeed a lot of bad things happen because people are too careless with specifications.

I recently had to form an expert opinion in a case where a multi-million dollar battle had developed. Different opinions on the exact meaning of certain laser specifications were at the heart of that conflict. Due to a substantial lack of clarity of these specifications, it required rather complicated work to clarify that in hindsight.

Note that the exact meaning of specifications is just one of the possible issues. Further questions can come into play:

What also makes it difficult is that there is often not a clear line between purely technical and legal issues. Obviously, legal expertise alone is not sufficient, but if you have a technical expert who finds it hard to understand certain legal principles, it may result in a difficult communication between him and the lawyers.

Of course, the very best thing is to avoid such trouble from the beginning by being very careful with laser specifications

As always, those dealing with the matter may or may not be sufficiently competent for that. If not, it is very advisable to get an experienced external expert involved. That can avoid a lot of trouble, and costs a very tiny fraction of what is at stake. An employee might not be keen to tell his boss that additional external competence is required to be secure; however, he may be even less keen to report huge trouble as a consequence of a lack of diligence!

What you can ask an expert may be, for example:

I am happy to do such jobs, which are particularly useful. Curiously, however, I am not often asked to do such things. I suppose this is because most people are not aware how important these matters are. Many buyers would not be willing to pay a little money for clarifying their requirements, and many suppliers don't seem to care that their published specifications make no good impression and invite trouble.

As a free but valuable piece of information, I offer my recently written encyclopedia article on laser specifications.

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… and keep in mind that the competent technical consulting services of RP Photonics could be very useful for your business!

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