Scientific Conferences and Publications: Emphasize Device Performance or Insight?

Posted on 2010-02-06. Permanent link: http://www.rp-photonics.com/spotlight_2010_02_06.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

When attending the conferences Photonics West in San Francisco and Advanced Solid-State Photonics (ASSP) in San Diego, I learned a lot, but also thought about some deficits which are typical for such conferences as well as for scientific journals. The selection process for the submitted papers often favors strongly such papers which report advances in laser performance, even though many of these papers do not generate any significant new insight. In many cases, good performance results from more or less systematically applying the already available knowledge. I then often think that such presentations help me mostly to see that certain persons, research groups of companies can do certain things, but not really to learn much about the scientific technical subject – which would be more useful, of course.

Different Types of Papers

There are other types of papers which transfer really interesting and valuable information but nevertheless are more difficult to get accepted for presentation or publication. For example, there are papers presenting a theoretical analysis of certain technical aspects which are crucial for further progress in the field. Instead of appreciating such contributions, some committee members tend to derate them on the ground that an improved laser devices has not been demonstrated yet.

Admittedly, if someone wanted to build a better laser, didn't succeed for some reason and wants to present only the idea as such, this can rightly be considered less valuable than the completed experiment. Sooner or later, ideas need to be tested in our discipline. However, there can be very valuable information extracted from a theoretical analysis only. Even when the core result is a negative one, saying for example that a certain strategy to overcome some common problem does not work in certain situations, or does not work as expected, this can be very useful to know. As an example, I would definitely appreciate a paper which explains clearly why the common understanding of stimulated Brillouin scattering in optical fibers is not accurate, and what that means for strategies (based on new fiber designs, for example) for raising the Brillouin threshold. (Some people have strong views on such issues, but I found interesting information on those only in discussions with colleagues, but nothing new in the conference programs.) On the other hand, I may well live without being told that some research group tweaked a little more power out of a laser using a well-known technique with slightly improved components. So why are we then getting so much stuff of that kind?

Criteria for Scoring Papers

I think it is important to think carefully about the right criteria for scoring (and finally accepting or rejecting) papers. The final criterion should always be to which extent the community will presumably profit from a certain paper being presented or published. Various typically considered aspects are not always very relevant for this:

• Is the achieved device performance really convincing? This is rightly asked when a paper emphasizes performance itself, but even then one should also ask other questions: Did they just use better components, or do we learn about any new ideas, concepts, problems, etc.? Will the paper help others to get better, or is it more about raising the prestige of a certain research group or company?
• Have they got experimental results, or only theory? This question can be annoying when it means that we are interested only in devices and not in thoughts, analysis, concepts, etc. If you like, science is all about constructing mental models of reality, not primarily about building things. Therefore, to be contemptuous of theory (a system of mental models) is hardly compatible with being a scientist. The point is whether any experiment or any theoretical treatment is valuable, i.e., helpful, convincing, original, etc. We should pick the neat experiments as well as the helpful theory, while filtering out the poor stuff of any kind.
• Is the presented material well described? This can apply to experimental circumstances, to the characterization of devices, or to the assumptions underlying a theoretical model. We like to get informed clearly, and do not appreciate to get only preliminary results. However, we shouldn't insist on completeness where it is not relevant for the actual question of interest.
• Is it all correct? Obviously, I would have difficulties supporting a paper which I am sure is based on wrong reasoning. However, there is a risk that we let all non-controversial (but often boring) stuff pass while losing some of the important materials which may be somewhat more controversial. Particularly at conferences, it may not be a problem to have something presented which turns out to be questionable: even more people may have the opportunity to get some views corrected if such views are presented but then criticized.
• Are the authors already known to be good in their area? If yes, this may increase the confidence that something useful will be presented. However, for obvious reasons it is highly problematic to score more highly the well-known guys.

Inviting Useful Contributions

Having realized what kind of papers we need to advance our science and technology, we may not only score papers more diligently, but also encourage the submission of useful papers. Calls for papers usually define the subject areas which are considered suitable for some conference or some special issue of a journal. This is obviously needed, but it may help also to name specifically what types of presentations are welcome: not just reports of advances in performance, but also anything which improves our understanding, corrects problematic views and points out new perspectives.

Committee members regularly have to think about possible invited speakers. I would warmly recommend to think not only about which research groups or companies are leading in terms of impressive performance advances. We should also think about who should be able to give us new insight and perspectives by explaining and discussing clearly certain relevant aspects. It can also be a good starting point to ask what open questions we would like to be addressed. For example, these could be questions about some physical mechanisms or about the suitability of certain measures, techniques and technologies.

I am quite sure that our conferences and journals will become even more useful if we think more carefully about such things. We could have more of the stimulating ideas – including some controversial ones –, more precise reasoning and judgment, and less of the boring routine stuff which does not really bring forward our discipline.

Far From Maturity: The Photonics Industry

Posted on 2010-01-29. Permanent link: http://www.rp-photonics.com/spotlight_2010_01_29.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on photonics, lasers, optical fibers

The world saw the first laser about 50 years ago, and since then a laser industry with multi-billion yearly turnover has been developed, which is considered a major innovation driver. One would such an industry expect to have developed a fair degree of maturity, but surprisingly, in some sense it has not, or at least not in many sectors. This came to my mind when walking through the many aisles of the exhibition at Photonics West in San Francisco, which ended today.

Complete and Correct Specifications – a Dispensable Luxury?

Can you imagine the following: someone wants to procure a kind of electronic integrated circuit, for example, and asks the manufacturer about various vital technical details, such as some switching speeds and the stability of some output voltages. The manufacturer replies that the switching is rather fast, and the output is stable, but he cannot give any numbers. He would be delighted, however, if the customer buys the chips, does the measurements and tells him the results. I think one would be rather surprised about such a reply.

This, however, is exactly what is quite typical in the photonics industry. People are selling rare-earth doped fibers, for example, while not completely knowing themselves their parameters. They also do not know whether the performance of their fibers matches theoretical expectations, as they are not able to produce such theoretical predictions. For example, they do not have suitable modeling software for doing such calculations, and maybe not even the expertise for that. So they are selling products for which they are unable to predict how they would perform in the fiber lasers and amplifiers for which they are made. I guess that any electronics expert would be rather surprised to learn how people in the photonics industry operate.

Making Life Easier

Admittedly, maturity of our industry should not be the only goal, and overly pedantic expectations might slow down innovation. Still, one should expect the industry as a whole to work much better if manufacturers generally did their homework more carefully before throwing things on the market. It would be less tedious then to procure some number of components and put them together to obtain a device (some laser source, for example) with predictable performance – just as developers of electronic devices can do it.

At least in some sectors, this has been achieved to quite some extent. In particular, the telecom business has gone through some substantial standardization of components, as system builders require it, and by the way, this hasn't put an end to innovation in this sector.

Where to Improve Things

It may be instructive to consider some reasons for a lack of maturity in certain sectors of photonics, or say some related phenomena. Education is certainly an important aspect. You can study electrical engineering at many places worldwide, where you can expect to get to some standard level of expertise. In contrast, few places offer the same in areas like laser technology or optical fibers. Actually, there is not even a consensus on what exactly should belong to such courses; professors often just focus on their particular area of interest.

Another thing is setting standards. Sure, we do have some important technical standards, such as the ITU standards for telecom fibers, or the ISO Standard 11146 for the beam quality M² factor. Take the latter: some very competent people have worked out very reasonable guidelines for measuring beam quality. Only, I wonder how many people in our business care to learn at least the basic rules to be observed in order to produce meaningful M² values.

A sometimes incredible level of sloppiness is found even in scientific research. Some researchers should be reminded that the essence of science is to create reliable objective knowledge by investigating and communicating things with great care, excluding errors as much as possible. If you look at the scientific literature, however, you regularly find papers where lots of uncertainties are totally unnecessarily introduced, just because of a blatant lack of care. For example, people regularly specify values for a “beam size” (radius or diameter???), report measurement results from largely unspecified experiments, or abuse scientific terms in ways that destroy all clarity. Some even consider issues like semantics largely irrelevant – as if it wouldn't matter in science what exactly words or technical terms mean. Such habits are then taken over by those making data sheets of laser products which say more about the author's competence than about the product.

So it seems to me that have to fight the mentioned problems at many fronts. This begins with senior researchers, who should give young people a good example in being reasonably precise and careful. Then we definitely need to establish more systematically high-quality degree courses for subjects like laser technology or optical fibers. In companies, the leading figures could put some more emphasis on continuing education, for example in the form of training courses (see also the Spotlight article of 2006-12-09). And generally, it wouldn't hurt to occasionally compare the habits in our industry with those in others.

Pumping Fiber Lasers with Fiber Lasers

Posted on 2010-01-22. Permanent link: http://www.rp-photonics.com/spotlight_2010_01_22.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on fiber lasers, high-power fiber lasers and amplifiers, fiber lasers versus bulk lasers

In recent years, the possible output power for fiber lasers has been increased enormously. At multi-kW power levels, however, things get difficult. Thermal effects could be kept under control by using longer fibers, but fiber nonlinearities force one to go for shorter fibers, and the limited brightness of the pump diodes introduces further restrictions.

In this situation, IPG has chosen a route which may be surprising: using several ytterbium-doped fiber lasers, emitting at 1018 nm, for pumping a very high-power ytterbium-doped fiber laser with emission around 1070 nm. At a first glance, one may think that pumping at 1018 nm, where the pump absorption is much weaker than at 975 nm or 940 nm, for example, is no good idea. However, the outputs of several 1018-nm fiber lasers can be combined into a single fiber core with only 100 μm diameter – rather small, comparing with the usual pump cores as needed in conjunction with high-power laser diodes. Due to the small cladding-to-core area ratio of the double-clad fiber which one can then use, the pump absorption is in the end quite good, and the small quantum defect for 1018-nm pumping mitigates the thermal problems. Therefore, a relatively short fiber for the final laser can be used. A 10-kW laser with nearly diffraction-limited beam quality has been demonstrated by IPG this year.

Unfortunately, this technical approach does not only lead to a more complex setup, but also reduces significantly the wall-plug efficiency. Comparing with other types of high-power lasers, however, the efficiency is still rather good.

Clearly, the times are over where great further power increases are possible with fiber lasers just by optimizing design and components. But there is plenty of stuff, of course, which can be done with the power level reached already. So we can expect a lot of progress on the side of laser applications.

Beams of Laser Pointers: Visible in Air?

Posted on 2010-01-11. Permanent link: http://www.rp-photonics.com/spotlight_2010_01_11.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on laser pointers, laser beams; Wikipedia article on Luminosity function

I have two laser pointers: a small red one, and a larger green one (see the figure).

Figure 1: A green-emitting laser pointer, containing a tiny diode-pumped frequency-doubled solid-state laser.

When I take the green laser and shoot into the night sky, I very clearly see the beam in the air over a large distance. When I do that with the red laser, however, hardly anything is seen.

There are three reasons for that remarkable difference:

• The green laser has a few milliwatts, the red one only about 1 mW.
• The beam is visible in air only due to scattering. In clean air, that is Rayleigh scattering, the strength of which scales with the inverse fourth power of the wavelength. This means that the difference in scattering efficiency between 530 nm and 670 nm is a factor of 2.6.
• The human eye is much more sensitive to the green light (around 530 nm) than for the red light (670 nm). Interestingly, the difference in sensitivity depends strongly on the overall brightness of the scene. Under normal daylight conditions, one has to use the photopic sensitivity curve, which tells us that the sensitivity at 530 nm is roughly 20 times higher than at 670 nm. With the eye adapted to the dark night, however, that difference becomes much stronger. The reason for this is that the eye uses different sensors. Under normal daylight conditions, it uses three types of cones for color vision, whereas in the dark it uses the rods. The latter are more sensitive for green light, but have a very poor response at 670 nm.

In combination, one should expect that the visible brightness of the scattered light from the green laser is much larger than for red light. And this perfectly fits to the experimental experience!

Tilt Tuning of Etalons

Posted on 2009-12-31. Permanent link: http://www.rp-photonics.com/spotlight_2009_12_31.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on etalons

Let us consider a solid etalon, consisting of a transparent plate of a material like fused silica, for example, with flat and well polished surfaces. These surface can have reflecting dielectric coatings for increasing the finesse. Maximum transmission (near unity) is only possible for wavelengths where the etalon is resonant, i.e., the round-trip phase shift is an integer multiple of 2π. Such resonances occur at approximately equally spaced frequencies with a spacing which is called the free spectral range. The transmission bandwidth is the free spectral range divided by the finesse, with the latter being determined by the surface reflectivities (at least in the simplest cases).

In Which Direction Do the Transmission Peaks Move?

It is a common method to tune such an etalon to a certain transmission wavelength by tilting it. A first interesting question is whether an increasing tilt angle increases or decreases the wavelengths of the transmission peaks. A seemingly plausible but wrong argument is that tilting makes the beam path longer, and that this must be compensated by a longer wavelength. In reality, however, the round-trip phase change is decreased rather than increased! A somewhat formal (but correct) argument for this is that the projection of the k vector to the direction perpendicular to the surfaces is reduced. But what is wrong about the previous argument? This is explained in the Spotlight article of 2006-11-02. The correct conclusion is then that the transmission peaks move to shorter wavelengths when the tilt angle is increased. See Figure 2 for a calculated example case.

Figure 3: Transmission curves of a 50 μm thick fused silica etalon with 60% surface reflectivity. The curves correspond to tilt angles from 0 to 5° in steps of 1°. Increasing angles shift the transmission to shorter wavelengths, with an increasing rate.

The Overlap Issue

Another issue is that tilting causes a transverse offset of the circulating beam after each round trip. Obviously, this can affect the interference conditions and thus the etalon's performance. In an extreme case, where the etalon is quite thick, the tilt angle is large and the beam radius is small, there is no overlap at all, so we have no interference, and the etalon will not function as a wavelength filter. On the other hand, the transverse offset may be negligible when the etalon is thin, the angle is small, and the incident beam is large. But where does tilting begin to cause a problem? The answer to that question is often essential when applying etalons, e.g., for wavelength tuning of lasers.

A customer of mine recently directed my attention to claims of an etalon manufacturer, where they use a simple rule for calculating the resulting loss. They take the finesse as the effective number of round trips and calculate the total transverse beam offset within these round trips. Then they divide this total offset by the beam radius, and claim that the result is the fraction of the power which is lost due to the tilt effect: the maximum transmission is reduced by that amount.

Well, I didn't believe that this is true, and thus made my own calculation. The right way of doing this: take the electric field distribution of the incident beam and calculate the field distributions after multiple round trips, where the total power is reduced depending on the surface reflectivities, and the transverse offset increases with each round trip, depending on the thickness and the tilt angle. By adding up all reflected field components, one obtains the total electric field reflected by the device, for example for a wavelength where the device is resonant. Without any tilt, the first reflection is exactly canceled by the sum of all other reflected components, resulting in zero reflection and thus total transmission. (I assume the ideal case where both surfaces have the same reflectivity, the mirror coatings have no absorption and scattering losses, the surface quality and parallelism is perfect, etc.) With some finite tilt, this is no longer true due to the transverse beam offset.

Figure 4: Reduction of the maximum transmission of a 50 μm thick fused silica etalon with 60% surface reflectivity. The incident Gaussian beam has a beam radius of 200 μm.

Figure 3 shows results for an example case. For small tilt angles (few degrees), the transmission losses are much smaller than those calculated with the crude technique explained above. For example, we obtain ∼ 0.33% loss for a tilt of 5°, whereas the crude method predicts nearly 18%! For increasing tilt angles, however, the losses increase more than in proportion to the tilt angle, so that eventually they can get significant.

It may be surprising that the crude estimate is so far off. However, consider the example case with numbers as used for Figure 3 and a tilt angle of 5°. The finesse is ∼6, and within 6 round trips there is a total transverse offset of 36 μm, which is 18% of the 200-μm beam radius. However, within 6 round trips the optical field amplitude is reduced to only 4.7% of the initial amplitude. Therefore, it doesn't matter so much if there is some transverse offset by then.