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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.

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And here are the articles:

Shortages of Rare Earth Materials – a Problem for Photonics?

Ref.: Marcius Extavour, “Rare Earth Elements: High Demand, Uncertain Supply”, Optics and Photonics News 22 (7), 40 (2011); editorial “The new oil?” of Nature Photonics 5, 1 (2011)

As an introductory remark, this article presents an example for issues where a first glance suggests certain conclusions which then turn out to be wrong when taking a closer look. Therefore, I recommend to read this to the end, if you are interested!

In recent years, there has been a growing awareness of the fact that various technology sectors heavily depend on certain materials for which a reliable long-term supply at reasonable prices is not guaranteed. Specifically, rare earth elements (REE) are more and more widely used in diverse technological sectors, while various sites worldwide have closed their production; about 95% of the total worldwide production is now in China, which uses more and more of these materials for its own production. That situation creates a concern in other countries about a dependencies which might be exploited, particularly in case of political tensions. In fact, that has already happened in 2011 in a dispute with Japan, which lead to rocketing REE prices on global markets.

The Use of Rare Earth Elements in Photonics

In laser technology and other fields of photonics, we also heavily depend on rare-earth materials. In particular, various rare earth ions like Nd3+, Yb3+, Er3+ and Tm3+ are essential laser-active dopants in laser crystals and glasses as used e.g. in rare-earth-doped fibers. Furthermore, even the host materials often are rare earth materials – in particular, yttrium as a constituent of YAG (yttrium aluminum garnet) and vanadates like YVO4. Besides, rare earths like europium and cerium are contained in phosphors for lighting applications and displays, and cerium is used for optics polishing agents. Various modern optical clocks utilize rare earth ions as optical frequency standards.

The situation is reminiscent of that in electronics, where some other rare elements like gallium, indium, tantalum and niobium fulfill essential functions.

Are We Facing Big Trouble?

Based on these facts, serious concerns have been raised, fearing troubles for the whole photonics industry if we don't either secure more reliable REE sources or learn to replace rare earth elements with others, where the supply situation is less critical. For example, an OPN article of Marcius Extavour (see the reference above) in 2011 created that impression. He named recycling efforts as a possible path, and even raised the idea that rare-earth-free solid-state lasers might be needed in the future, and stated: “With myriad laser technologies available today, rare-earth-free solid-state lasers are certainly a practical alternative.” My opinion is that in most cases they would certainly not be a practical alternative, as that would imply e.g. that we essentially give up all well-working fiber lasers – but what could replace ytterbium-doped fiber lasers and erbium-doped fiber amplifiers, for example?

Given this doubt, you may now think that I consider the situation even more threatening than Extavour and other authors described it. That is not the case, however. In particular, I believe that Extavour and other authors did not sufficiently take into account that although photonics and in particular laser technology are indeed critically dependent on REE, only tiny amounts of them are needed in photonics – in particular, for most essential functions such as laser-active ions:

  • As an example, consider an Yb-doped double-clad fiber as used in high-power fiber lasers and amplifiers; this contains only of the order of 5 μg ytterbium per meter of fiber for a typical doping concentration and core size. So you if you need 10 m for a laser, it is just 0.05 mg – close to nothing.
  • We need substantially more when using a laser crystal with a rare-earth host medium. Let's consider a Nd:YAG crystal with dimensions of 10 mm × 2 mm × 2 mm and a concentration of 1 % wt (by Nd2O3). That contains around 1.6 mg of neodymium.
  • About the worst case is a 150 mm long and 10 mm thick Nd:YAG rod for a lamp-pumped laser, which may contain several hundred milligrams of neodymium, and more importantly tens of grams of yttrium.
  • A color TV screen may contain around 1 g of europium.
  • Compare all that with neodymium content of an electric generator or motor in an electric car or hybrid engine car – this may easily be of the order of 1 kg. It's in the neodymium magnets, which could in principle be replaced with other materials, but at the cost of larger size and/or lower efficiency.
  • Some wind energy generators contain even much more neodymium; only, there are not so many of them.

These examples demonstrate that in photonics and particularly in laser technology we use only tiny amounts of rare earth elements, which make up only a tiny fraction of the overall demand. And that changes the conclusions in essential ways. Even if China would suddenly stop exporting any rare earth elements, a small portion of the smaller REE production of other countries would suffice to meet the whole demand of photonics. As we a talking about high-value applications requiring tiny amounts, the price of a gram of REE essentially does not matter; we will always have enough for such purposes. The situation is entirely different for applications like magnets (e.g. NdFeB in neodymium magnets), where you (a) need much more and (b) cannot pay excessive prices.

In the longer term, we will also probably see new mines in various countries; only this takes some time, and efforts have started only recently, after a long time where the demand was simply not sufficiently high to amortize big investments.

How about recycling? This is certainly a wise thing to do, where large amounts of REE remain packed into compact devices. For example, it would be more than silly not to recycle the neodymium content of the generators and motors of cars, where you can hold with two hands a machine containing 1 kg or more. On the other hand, it is hard to see how recycling of rare earth elements from laser crystals or even active fibers could have a significant impact, except perhaps for the yttrium in the bigger laser crystals.

My conclusion is that there may well be trouble ahead with the supply of rare earth elements, but not for laser technology. The sectors which might be hit are those requiring large amounts – for example, catalysts and magnets. Some sectors of photonics may also be affected to a smaller extent – perhaps the display and lighting sector requiring phosphors. For polishing of optics, we may have to use alternatives to cerium, which however should not be a major problem.

Environmental Concerns about Rare Earth Mining

As a side remark, there are substantial environmental concerns about the mining of rare earth materials. Again, however, it makes a big difference whether you need some milligrams for a device or kilogram amounts. Therefore, good ways to counteract serious pollution issues are not developing rare-earth-free lasers, but building better mines and possibly more efficiently using (and recycling) REE at those places where a lot is used.

10-Year Anniversary of RP Photonics

Ref.: anniversary article

An an exception, the Photonics Spotlight today covers a non-technical topic which is related to our company history: RP Photonics is celebrating its 10-year anniversary. End of May 2004, the company was founded and officially started in Zürich, Switzerland. Hardly anybody in the laser industry knew about some Dr. Paschotta, who had a reputation only as a scientific researcher. But that changed rapidly – nowadays, RP Photonics is a worldwide very well known company. Our anniversary article tells you how this worked based on a somewhat unusual business model, and what is expected to happen next. Therefore, I can keep this newsletter article very short, hoping you will visit our anniversary article with a single click and learn more there!

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.

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