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 Dr. Rüdiger Paschotta 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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Attenuating Laser Beams – not That Easy
Posted on 2015-02-05 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2015_02_05.html
Ref.: encyclopedia article on optical attenuators
In principle, attenuating a laser beam, i.e., reducing its optical power, is an easy thing: simply send it through a partially absorbing medium, or exploit a partial reflection. In practice, however, various nasty problems can arise, some of which are discussed in the following.
Thermally Induced Distortions
Because laser beams often carry substantial optical powers, absorption of a significant part of that power can lead to substantial thermal effects. Therefore, e.g. absorbing neutral density filters are often not suitable for such purposes: the increased temperature in the glass would lead to strong thermal lensing effects, which can focus the beam and distort its spatial profile.
For higher powers, the glass could even be fractured; a few watts would usually be sufficient for that effect.
For attenuating the output of a single-frequency laser with a moderate optical power (a few hundred milliwatts), I once (being a beginner in the field) used several neutral density filters in series. I was then very astonished and frightened to see the transmitted power dropping strongly within a few seconds after turning on the laser; initially, I thought the laser had been damaged. It turned out that due to the significant reflectivity of these filters, I had actually realized a Fabry–Perot interferometer. When this got into resonance, a relatively high optical power was circulating between the surfaces of two filters, and that heated these filters such that their surfaces were somewhat bulged. That in turn tuned the resonance frequency and thus influenced the circulating power. Due to the resonance effect, the transmitted power was also far higher than expected.
It turned out that I had to somewhat tilt the filters against the beam such that no light could circulate between them. That was an easy measure, but I had already spoiled some spots on the filters by overheating. Only the resonance effect produced enough heat for damage of the parts.
It may sound clever if you utilize not the actual output beam, but rather a parasitic beam getting through a highly reflecting mirror of the laser resonator due to the non-perfect reflectivity. Then you do not have to place additional things into the output beam and do not lose any useful output power.
However, it can be problematic that the residual transmission of a highly reflecting mirror can strongly depend on the exact position on the mirror. Therefore, if you align the laser resonator for maximum power on your photodiode, you may actually spoil the laser alignment because you actually optimize concerning the spot on the mirror having the highest transmission! (If the responsivity of your photodiode is not uniform, e.g. due to damaged spots, you can have the same effect even with perfect beam attenuation.)
Similar problems can occur when you want to measure the laser beam quality. The beam transmitted through a highly reflected mirror may have a better suited power for such a measurement, but can be severely distorted, thus exhibiting a substantially lower beam quality than the actual output beam.
For such reasons, it is better to use several mirrors in series, where each mirror does not attenuate the beam that strongly.
Another seemingly clever idea would be to reflect a linearly polarized laser beam with p polarization at a glass surface, choosing an incidence angle close to Brewster's angle. There, the reflectivity is very small, so that the reflected beam is strongly attenuated.
The caveat is that in this configuration you get a far higher reflectivity for s polarization, and even a nominally p-polarized beam will in practice have some fraction of its power in the other polarization direction. Therefore, the reflected beam may be stronger than you expect, and it may also exhibit a curious beam profile. This is because light in the nominally absent polarization direction often gets there by thermally induced depolarization effects e.g. in a laser crystal, which are not radially symmetric.
Therefore, it is again better to use reflections on subsequent surfaces, where the attenuation per reflection is not too strong.
A common method of obtaining an adjustable degree of attenuation is to use a half waveplate in combination with a polarizer. This works quite well for linearly polarized input beams, but again there are limitations due to non-perfect properties of the waveplate and the polarizer. You may not be able to reliably achieve a very high degree of attenuation, and of course you depend on a stable polarization state of the input.
In some cases, you need to attenuate a laser beam without changing its direction. Many methods of attenuation, however, deflect the beam or at least cause a parallel beam offset, the magnitude of which may vary if you change the degree of attenuation. Some kinds of variable optical attenuators have been constructed where such effects are avoided by compensation. For example, you may have a certain beam offset upon transmission through an angled plate, which is compensated by transmission through another plate oriented at the same angle. Of course, one requires high-quality fine mechanics for preserving the beam direction and position precisely.
New Tutorial: Modeling of Fiber Amplifiers and Lasers
Posted on 2015-01-09 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2015_01_09.html
Ref.: tutorial “Modeling of Fiber Amplifiers and Lasers”
Today, I have published on our website a new tutorial with the title “Modeling of Fiber Amplifiers and Lasers”. Last year, I already published tutorials on passive fiber optics and on fiber amplifiers. In contrast to these, the new tutorial mainly focuses on various aspects of developing and using laser and amplifier models. It is intended to help particularly those who would like to get into this area – either by developing the required software themselves or with commercial software.
I believe that computer modeling will become more and more important in the field of photonics. We still have many development teams and research groups not using such powerful tools, instead relying a lot on trial and error in the laboratory. This is often highly inefficient, however, and this in many ways. One spends time and money for purchasing parts which may not work in the end (e.g. due to wrong design of the setup), and perhaps more importantly spend a lot of time in the lab without making efficient progress. Spent salaries during the wasted time are often only the smaller part of the damage, as there are also infrastructure costs, and lost opportunities in terms of exploiting market potentials or publishing findings in time can even be more damaging than that.
Even if you clearly see in the lab that some setup doesn't work, it is often far from obvious why it doesn't or what you should do to make progress. You then always have to somehow interpret the situation based on some kind of theoretical model – be it a set of vague ideas in your mind or quantitative computer model with which you can really check certain ideas, explanations or possible improvements.
I regularly see people considering whether they can afford to spend a certain amount of money and time for getting into modeling of their devices. Actually, I believe that this is the wrong question. One should determine what is the most efficient way of reaching the central goals, and you cannot afford not to do it in the most efficient way!
Anyway, I hope that this tutorial will help many industry people and researchers to develop clear thoughts on many issues, including both technical and non-technical ones, and thus to make good progress in their jobs. Enjoy the reading!
How PhD Students Should Get Supported by Supervisors
Posted on 2014-12-17 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2014_12_17.html
We are dealing with a hot topic, even with relations to ethical guidelines and science fraud.
Today I would like to cover a topic which is not specific to photonics, but of general interest for the scientific community. I would like to emphasize at this point already that my article does not only touch the interests of students, but also hot topics like authorship and science fraud. (For some, the topic may even be too hot to share the article with their supervisor!) Hopefully, these thoughts will find some resonance, as I believe that this could result in great benefits not only for the quality of life of many researchers, but also for the quality and effectiveness of scientific research.
In my career as a researcher, I obtained insight into a number of research groups. There, I recognized substantial differences concerning the conditions under which PhD students are working. Some of them are related to the structure of the research group, e.g. whether there are staff members in intermediate positions (e.g. senior research assistants) in addition to a professor as the head of the group. In addition, there appear to be quite different ideas concerning what to expect from PhD students and from their supervisors. Maybe not all of these ideas have resulted from thorough reflections.
What Students Can Do, and What is Difficult
Certainly, PhD students play a vital role in most research groups. They often do the largest part of the laboratory work, like working out detailed plans for experiments, ordering needed equipment or even setting up whole labs, performing much of the experiments and interpreting experimental results. Many of them work quite hard and acquire a rather solid level of scientific and technical competence during these years.
Formulating a concrete plan for a suitable research project is a difficult task – normally requiring more experience than a beginning PhD student can have.
What is really difficult for beginning PhD student is to identify a suitable research project. After all, this requires a substantial knowledge on pre-existing work, a vision of what could and should be accomplished next, and to a realistic appreciation of the possible difficulties to be encountered in a certain project. Even though one rightly expects a PhD student to accomplish substantially more than only thoroughly carrying out a predefined plan, I believe it would normally be far too much to ask from a PhD student to work out the whole plan for his studies himself. Particularly in the early stages, vital inputs from an experienced supervisor are deadly needed.
A research project can get into a crisis, the management of which often requires more than can be expected from a student alone.
Even a well planned research project pursued by a good student can get into trouble e.g. if certain unexpected technical difficulties arise or if utterly unexpected findings are made which make the original plan obsolete. A student who is left alone in such a situation without substantial help – or even worse, with additional pressure coming from the boss, can easily get into despair. If this results in the failure of the project, the whole career can be at stake. It can be an invaluable benefit in such a situation to receive competent help and moral support from an experienced supervisor, for whom it is easier to judge a complex situation, to analyze the magnitude of the encountered difficulties and to identify the best way out of the crisis.
Benefits Provided by Supportive Supervisors
I would like to emphasize that a supervisor who sees his task in supporting students will not only avoid serious trouble and a very uncomfortable time for the student, but also contribute greatly to the success of the research group as a whole. He or she will bring important contributions to the productivity of the students' work and will also contribute to the team spirit which makes the research more pleasant for everybody and more productive due to fruitful cooperation, which can arise only on the basis of mutual trust. In the end, everybody will profit from that.
There are group leaders, however, who are too strongly focused on their personal career and seem to feel that they have no resources left for supporting others. Some of these leave students alone in trouble, while others even increase the trouble by mounting additional pressure – sometimes with the doubtful idea that this may lead to harder work by the student which will eventually solve the problem.
The Relation to Authorship Practices
An important question is who really deserves authorship in scientific publications. There is normally no question whether the student, having done most of the work, obtains a prominent position in the list of authors, but whether supervisors are to be listed there as well is a more delicate question. Generally, nobody should be an author who has not brought concrete important contributions to the published work. Amongst those who seriously dealt with this question, there seems to be a widespread consensus that it is absolutely not sufficient for authorship
- to be the leader of a group,
- to have produced the original idea for the project without having worked out any details, or
- to have read the final publication and made a few minor corrections to it.
Breaching theoretically valid authorship guidelines is still quite common, unfortunately. Some are lacking the power to fight, while others have different priorities.
Unfortunately, many supervisors (senior research assistants and particularly professors) still believe that they are automatically entitled for authorship on anything being done in their group. They consider it sufficient to have produced some vague ideas or to have paid (of course not with their own money!) for the used infrastructure, for example. Of course, a PhD student is normally not in the position to argue about such points, given the enormous discrepancy of power between him or her and the professor. Unfortunately, at many places there appears to be nobody who would be able and willing to enforce the authorship practices which are theoretically demanded from everyone. I have even experienced a case where most obvious abuse by a professor (demanding the authorship on a whole book chapter without having written a single sentence of it) has been explicitly defended rather than prosecuted by the highest ranks in a highly esteemed research institute. Obviously, these people were being less interested in upholding ethical standards than in avoiding trouble with that professor.
If serious engagement were the condition of authorship of supervisors, supervisors would be more likely to do their job.
You may ask what that problem has to do with supporting students, the topic of my article. Imagine that justifiable authorship practices would be enforced throughout in universities. As a result, professors (particularly those having large groups) could no more claim authorship in many cases where they did before. This would create an incentive not to set up huge groups without having any time to support the students, but rather to form smaller groups and to actively participate in the research. I believe that this would in the long term lead to a stronger cooperation between the different ranks in a group and that way to more productive and efficient research.
I know that not every senior researcher is able to really bring such valuable contributions as outlined above; in some cases, the students may actually be more happy to be left alone. However, if that were a serious counterargument, one might as well stop employing students on the basis that some of them turn out not to be sufficiently competent.
Supporting Students and Preventing Science Fraud
There is also a relation between the support for students and the prevention of science fraud – but not what many might think! I sometimes read that students would be prevented from forging results if they were under closer control, but I believe that this view is distorted:
A substantial risk for science fraud arises only in a serious crisis – and is in that case strongly dependent on how the supervisor performs!
The risk for fraud is surely far higher than under normal conditions if a student's project gets into a crisis (as described above) and there is no support available. Even worse, the supervisor may mount additional pressure and in that way make the student feel that the only way out very serious trouble is to produce results somehow – be it with fraud or at least doubtful practices if no other way can be seen. On the other hand, imagine a helpful supervisor, where a student has a very attractive option: just to go there, openly discuss the problems and obtain valuable help for solving the problems. Which student would be crazy enough to forge results rather than to accept valuable help?
After certain scandals became public, I asked myself whether I could rule out the possibility that a student could once have forged results without me noticing it. The answer: I am sure there would have been ways to accomplish that, but I also firmly believe that it never happened – simply because there was never an incentive for a student to do so. While I never treated my students as schoolboys who need to be observed in every moment, I was always prepared to work hard in order to bring really helpful contributions when a project encountered difficulties.
When thinking seriously about these things, I believe one can only come to the conclusion that a large part of the responsibility for providing science fraud is on the side of supervisors. As long as these do their job well, I believe there is a minimal risk of fraud. On the other hand such a risk can become substantial if people are sliding into a situation where fraud appears to be the only way to avoid dire consequences. Just telling students that fraud is evil will certainly not solve the problem – I am sure they all know that.
You may also like to read an article which I wrote five years ago: "Risk factors for scientific fraud and the scientist's responsibility".
2014-10-03: Fiber Optics Tutorials
2014-07-28: How to Define the Mode Radius of a Fiber?
2014-05-16: 10-Year Anniversary of RP Photonics
2014-01-17: Mediation in Disputes on Laser Technology
2013-12-13: Avoiding Trouble with Laser Specifications
2013-11-12: Beam Quality Limit for Multimode Fibers
2013-08-26: Frequency Doubling and the Reverse Process
2013-06-13: Two New Photonics Newsletters
2012-08-06: The New RP Photonics Buyer's Guide
2012-03-12: New Raman Lasers
2012-03-03: Conflicting Definitions of s and p Polarization
2011-12-23: Kerr-lens Mode-locked Thin-disk Laser
2011-06-10: Are Compact Resonators More Stable?
2010-07-12: Laser Development: Get an Expert Early on!
2010-06-09: Poor Man's Isolator
2010-04-26: Resolution and Accuracy of Measurements
2010-04-08: Creating a Top-hat Laser Beam Focus
2010-03-22: All-in-one Concepts versus Modular Concepts
2010-03-09: Nonlinearities in Fiber Amplifier Modeling
2010-01-29: Far From Maturity: The Photonics Industry
2010-01-22: Pumping Fiber Lasers with Fiber Lasers
2010-01-11: Beams of Laser Pointers: Visible in Air?
2009-12-31: Tilt Tuning of Etalons
2009-12-13: Johnson–Nyquist Noise in Photodiode Circuits
2009-11-18: Articles and a Quiz on Photonics Issues
2009-11-13: Photodetection: Optical and Electrical Powers
2009-11-03: Coherent Light from a Bulb?
2009-10-03: Peak Intensity of Gaussian Beam
2009-09-27: Lasers with Short Upper-state Lifetime
2009-09-19: Are Laser Resonators Power Scalable?
2009-09-01: Fresnel Reflections from Double Interfaces
2009-08-14: Progress on Green Laser Diodes
2009-08-12: What is an Optical Transistor?
2009-07-29: No Beat Note for Orthogonal Modes
2009-07-21: Signal-to-Noise Ratio and Measurement Bandwidth
2009-07-09: Gain-guiding Index-antiguiding Fibers
2009-06-29: Doing Things Properly: It's the Economy, Stupid!
2009-06-23: Coherence – a Black-or-White Issue?
2009-06-08: Prizes of the European Physical Society
2009-06-02: 5 Years of RP Photonics Consulting
2009-05-13: The Minimum Time–Bandwidth Product
2009-04-28: SPIE Field Guides
2009-04-05: Stability of Resonators – an Ambiguous Term
2009-03-02: User Interfaces for Simulation Software
2009-01-12: Chaotic Lasing Generates Random Numbers
2009-01-05: Extremely Long Mode-locked Fiber Laser
2008-12-16: Why Fiber Amplifiers, not Fiber Lasers?
2008-11-25: The Gouy Phase Shift Speeds up Light
2008-11-08: Validating Numerical Simulation Software
2008-09-24: Decoupling Pulse Duration and Pulse Energy
2008-09-10: Unpolarized Single-Frequency Output
2008-07-26: Beat Signals with Zero Linewidth
2008-07-02: Stronger Focusing Avoids SESAM Damage
2008-06-20: All-in-One Ultrafast Laser Systems
2008-06-06: Fiber Lasers Which Are No Fiber Lasers
2008-05-25: Einstein and the Laser
2008-05-05: Length of a Photon
2008-04-28: Different Kinds of Polarization
2008-04-22: Abused Photonics Terms: Coherence
2008-04-15: Abused Photonics Terms: Modes
2008-03-10: Automatic Phase Matching
2008-03-04: What is a “High” Laser Beam Quality?
2008-02-14: How Laser Development Can Go Wrong
2008-02-03: Quantifying the Chirp of Ultrashort Pulses
2008-01-27: Beam Quality in Second-Harmonic Generation
2008-01-14: Frequency Doubling: Long Pulses Cause Trouble
2007-12-18: The Role of Laser Safety Goggles
2007-12-03: New Paper on Power Scaling of Lasers
2007-11-26: Solving Laser Problems Step by Step
2007-11-10: Retirement of Prof. David C. Hanna
2007-11-02: Ultrafast Laser Kills Viruses
2007-10-31: Thermal Equilibrium in Laser Crystals
2007-10-25: The Gain Bandwidth of Laser Crystals and Glasses
2007-10-17: Why the Second-Harmonic Beam is Smaller
2007-10-11: Understanding Fourier Spectra
2007-09-21: Optimum Crystal Length for Frequency Doubling
2007-09-07: Power Scaling in Downward Direction
2007-08-27: Distant Healing of Lasers
2007-08-23: An OPO Without Resonator Mirrors
2007-08-15: Light = Electromagnetic Waves?
2007-07-06: Promoting Dangerous Practices in Laser Labs
2007-07-01: Nonsensical Regulations Undermine Laser Safety
2007-06-24: The Plague of a Narrow Emission Linewidth
2007-06-11: Beam Quality Measurements Can Easily Go Wrong
2007-06-01: Characterize Your Pump Beam!
2007-05-19: Why Strong Birefringence in Fibers Helps
2007-04-16: Questions and Answers on Shot Noise
2007-03-23: Explaining the Nature of Photons to Lay Persons
2007-03-11: Divided-Pulse Amplification
2007-03-09: The Trouble with Crystal and Coating Damage
2007-02-26: No Laser, no Result?
2007-02-22: Lossy Laser Cavities
2007-02-16: The Science of Biophotons
2007-02-09: Papers Reporting Yet Another Laser Crystal
2007-02-04: Continuing Struggle for Larger Fiber Mode Areas
2007-01-27: Noise Figure of Amplifiers
2007-01-21: Operation Far Above Threshold
2007-01-15: Origins of Heating in Laser Crystals
2007-01-09: The Myth of Fiber-Optic Polar Bears
2006-12-31: Peak Position of an Optical Spectrum
2006-12-16: Dangerous Green Laser Pointers
2006-12-09: The Laser Industry - High Tech or Low Tech?
2006-12-03: Diffraction in Optical Fibers
2006-11-28: The Role of Diffraction in Optical Resonators
2006-11-21: The Resonator Mystery
2006-11-16: Laser Models - not Always Useful
2006-11-02: Reflection Spectrum of Tilted Dielectric Mirrors
2006-10-22: Lasers Attract Dust to Cavity Mirrors
2006-10-01: Stability Zones of Laser Resonators
2006-09-22: Coherence Length of Ultrashort Pulses
2006-09-16: Q-switched Lasers: YAG versus Vanadate
2006-09-01: Test Yourself with the Photonics Quiz
2006-08-20: Lower Noise from Longer Lasers
2006-08-12: Understanding Quasi-Three-Level Lasers
2006-08-10: Single-Mode Fibers with Large Mode Areas
2006-08-01: Lasers Disturbed by Vacuum?
2006-07-24: Beam Distortions in Laser Cavities
2006-07-23: Single-Atom Lasers
2006-07-22: No Magnetic Field on the Axis of a Coil?
2006-07-16: Spontaneous Emission and Amplifier Noise
2006-07-14: Lasers Like it Cool
2006-07-10: Strength of Thermal Lensing Effects
2006-07-01: Characterizing a Cavity with a Frequency Comb
2006-07-01: With Wavelength Combs to Picometer Resolution