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.
You can read this content in various ways:
- Just read it in your browser.
Make a bookmark to remember this page.
(Disadvantage: you may still forget to come back.)
- Use an RSS reader. We offer an RSS feed for that: spotlight.xml
- Get it via e-mail as a newsletter.
And here are the articles:
Mediation in Disputes on Laser Technology
Posted on 2014-01-17 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2014_01_17.html
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:
- Is the beam quality consistent with the specifications, which may be somewhat vague in that point?
- Does it have to be considered as acceptable that the laser stays within specifications only if its alignment is optimized quite often by the user?
- Is it unusual that certain parts needs to be replaced quite often? Is it a sign of poor workmanship?
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
Posted on 2013-12-13 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2013_12_13.html
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:
- How strict do we have to be when judging such issues? As that is usually not defined in quotations, purchase orders and similar documents, one has to find a reasonable judgment based on various circumstances. For example, is it an industrial laser, a medical or scientific laser? Is it a prototype, where you have to be somewhat more tolerant, or a production machine?
- What shall happen if specifications are not fulfilled? A manufacturer may say: Well, we will just repair the device as often as necessary within the warranty period. An industrial user, however, may have great problems with that if such repairs are required too frequently – that may render the laser useless for the intended application.
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
- when you are a supplier, formulating specifications for your products, and
- when you want a buy a laser and need to formulate what exactly you need.
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:
- Systematically check the specifications (or more generally, the descriptions) of a certain range of products. Identify unclear details and suggest how to remedy them. Give warnings where problems may arise.
- Work out the detailed requirements on a laser for a certain application. Discuss with those who will apply the laser what exactly they want to do. Make them aware of possible issues. (Note that the end users are often not aware what specifications are vital for their application, as they don't know the challenges of laser development).
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.
Beam Quality Limit for Multimode Fibers
Posted on 2013-11-12 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2013_11_12.html
- How good does the beam quality of my light source have to be – for example in terms of the M2 factor – for efficient launching into the fiber?
- What will be the beam quality of the light coming out of the fiber?
Surprisingly, it is not so easy to find helpful and reliable advice on such essential things in the Internet or in textbooks, but this is what I will try to provide in the following.
Clearly, the input light to the fiber should essentially be limited to the region of the fiber core; we don't want to launch light into the cladding. In addition, the angular distribution needs to be limited according to the numerical aperture of the fiber. Can we now calculate the maximum M2 factor of the fiber input or output from these two conditions?
Strictly speaking, we can't – basically because these two values do not determine the details of the beam intensity profile. However, one may assume that the optical power is well spread over all the guided modes of the fiber. For that case, one can show that the M2 factor of the output can be well estimated by the equation
where rcore is the core radius, NA the numerical aperture, and λ the vacuum wavelength. (In case you don't see the equation, please make sure that images are displayed.) I have confirmed this with some numerical tests.
As the high-order modes contribute more to a high M2 value than the low-order ones, reality might be worse than according to the equation, if one preferentially launches into high-order modes. However, that is unlikely in typical practical cases. One will then usually not obtain more than the calculated limit, or even less.
The maximum M2 factor of the input light (for efficient launching) can be estimated with the same equation, but again, it is an estimate, which is often very reasonable, but not a strict limit.
The figure below shows the intensity profile of a monochromatic beam from a numerical simulation, where the power is well spread over all guided modes.
Note that much smoother intensity profiles are possible for polychromatic light, where different wavelength components have similar intensity patterns as shown above, but the minima and maxima of different components largely average out. That averaging effect has not impact on the beam quality, however.
Will the Output Beam Quality be the same as that of the Input?
From a simple picture, one might expect that the M2 value for the output of the fiber should be the same as for the input. That is not always true, however:
- If some of the launched light is “stripped off” by the fiber, i.e., lost in the cladding, the output beam quality might be better. The fiber then acts as a kind of spatial filter.
- If you launch primarily into low-order modes, using a light source with good beam quality, you may also get that good beam quality from the fiber end. However, there might also be some mode mixing in the fiber, e.g. as a result of bending. That may shuffle light into higher-order modes, leading to a higher M2 value.
- If you do not illuminate the whole area of the fiber core at the input, you will nevertheless get light from essentially the whole core area at the output. The increased beam area together with the usually not decreased beam divergence leads to a higher M2 value.
How About Few-mode Fibers and Single-mode Fibers?
In the latter case, the angular distribution for the guided mode normally remains well below the limit calculated from the numerical aperture. In other words, one can construct light beams which would reasonably well fulfill the condition of a maximum angular spread, and nevertheless cannot be efficiently launched into the fiber.
A nice example is that of a TEM10 beam, which is well aligned to the fiber and has a reasonable beam radius. Although only some small fraction of its angular distribution is outside the limit set by the fiber's NA, the launch efficiency is zero. That already follows from symmetry reasons: the overlap with the single guided fiber mode is zero, since that mode is radially symmetric, whereas the TEM10 beam is anti-symmetric. Curiously, some of the light can be launched if the center of the TEM10 beam is somewhat offset against the fiber axis – although that misalignment surely does not modify the angular distribution.
You may also gradually increase the core diameter and/or the NA, and suddenly you will reach a point where a substantial part of the power of the TEM10 beam can be launched into the fiber core.
So the wave optics details become more subtle for single-mode and few-mode fibers, even though simple estimates as provided above generally work well for fibers with many modes, where we nearly have a continuum of modes.
Fiber Optics Tutorial
You find such things and more in our tutorial on fiber optics, in particular in its section 4 on multimode fibers. I have worked hard to provide more substance, particularly concerning the physics, than the dozens of other fiber optics tutorials which you can find in the Internet.
I hope you will agree that our newsletter and our tutorials provide useful high-quality information, and that you can recommend them to your colleagues – possible also using your website and social media.
- 2013-09-24: Simulation of a Q-switched Nd:YAG Laser:
Numerical Beam Propagation Reveals What Happens, Analytical Reasoning Explains It
- 2013-08-26: Frequency Doubling and the Reverse Process
- 2013-07-08: Amplified Spontaneous Emission in Fiber Amplifiers
- 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
- 2012-02-03: Simulation Software: Use Commercial Products or Home-Made Software?
- 2011-12-23: Kerr-lens Mode-locked Thin-disk Laser
- 2011-06-10: Are Compact Resonators More Stable?
- 2011-05-28: Explanation for the Mode Instability in High-power Fiber Amplifiers with Few-mode Fibers
- 2011-03-13: What if Solid-State Laser Transitions Would Be Much Stronger?
- 2011-02-10: Fiber Lasers: More Difficult to Design than Bulk Lasers
- 2011-01-05: Femtosecond Fiber Amplifiers: Unlimited Peak Power?
- 2010-09-02: Why LEDs are Energy-efficient, and Why They Could Well Increase Energy Consumption
- 2010-07-27: Special SESAMs for Mode-locked High-power Lasers?
- 2010-07-12: Laser Development: Get an Expert Early on!
- 2010-06-09: Poor Man's Isolator
- 2010-05-14: Plagiarism, Exploiting the Encyclopedia of Laser Physics and Technology
- 2010-04-26: Resolution and Accuracy of Measurements
- 2010-04-16: Why Large Mode Area Waveguides Do Not Work for Laser Diodes
- 2010-04-08: Creating a Top-hat Laser Beam Focus
- 2010-03-22: All-in-one Concepts versus Modular Concepts
- 2010-03-15: Spatial Walk-off and Beam Quality in Nonlinear Frequency Conversion
- 2010-03-09: Nonlinearities in Fiber Amplifier Modeling
- 2010-03-03: Thresholds for Nonlinear Effects in Fiber Amplifiers
- 2010-02-26: New Scientific Paper: Timing Jitter and Phase Noise of Mode-locked Fiber Lasers
- 2010-02-06: Scientific Conferences and Publications: Emphasize Device Performance or Insight?
- 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
- 2009-12-08: Increased Output Power of a Laser with Forced Tuning
- 2009-11-22: The Beam Focus – Not Just a Demagnified Version of Your Beam
- 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?
- 2009-10-19: Risk Factors for Science Fraud and the Scientist's Responsibility
- 2009-10-12: Cold Light from the Hottest Bodies and from Cool Devices
- 2009-10-08: Nobel Prize for Charles K. Kao for Pioneering Work on Optical Fibers
- 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?
- 2009-09-07: Anniversary: 5 Years of the Encyclopedia of Laser Physics and Technology
- 2009-09-01: Fresnel Reflections from Double Interfaces
- 2009-08-22: Jitter and Phase Noise of Mode-locked Fiber Lasers
- 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
- 2009-07-23: Submit Photographs for the Encyclopedia of Laser Physics and Technology
- 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
- 2009-05-22: Interference Effects with Imbalanced Intensity Levels
- 2009-05-13: The Minimum Time–Bandwidth Product
- 2009-04-28: SPIE Field Guides
- 2009-04-19: Last Chance to Get the Encyclopedia of Laser Physics and Technology Cheaper
- 2009-04-17: Miniature Laser Projectors – The Next Big Laser Thing?
- 2009-04-06: Laser Pointers in Soccer Games: Not Necessarily Harmless
- 2009-04-05: Stability of Resonators – an Ambiguous Term
- 2009-03-19: Scientific Progress: not as Straight a Process as it Seems
- 2009-03-07: Complicated Pulse Shapes from Q-switched Fiber Lasers
- 2009-03-02: User Interfaces for Simulation Software
- 2009-02-13: Laser Beam Deflections: The Angle–Diameter Product
- 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
- 2008-10-20: Rate Equations – An Example for Stiff Sets of Differential Equations
- 2008-10-03: Wavelength-Tunable Lasers: Does the Tuner Degrade the Power Efficiency?
- 2008-09-24: Decoupling Pulse Duration and Pulse Energy
- 2008-09-10: Unpolarized Single-Frequency Output
- 2008-08-28: Photographs for the Encyclopedia of Laser Physics and Technology
- 2008-08-15: Print Version of the Encyclopedia of Laser Physics and Technology
- 2008-07-26: Beat Signals with Zero Linewidth
- 2008-07-13: The Simplified History of the Michelson–Morley Experiment
- 2008-07-02: Stronger Focusing Avoids SESAM Damage
- 2008-06-20: All-in-One Ultrafast Laser Systems
- 2008-06-13: Heisenberg's Uncertainty Principle and the Transform Limit
- 2008-06-06: Fiber Lasers Which Are No Fiber Lasers
- 2008-05-25: Einstein and the Laser
- 2008-05-13: Easier Self-Starting Passive Mode Locking for Short Lasers
- 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
- 2008-04-02: Solitons: Lower Dispersion, Stronger Dispersive Effects!
- 2008-03-26: Mode-Locked Lasers: Lower Average Powers in Shorter Pulses
- 2008-03-17: Ultrafast Fiber Lasers: Re-Inventing Mode Locking
- 2008-03-10: Automatic Phase Matching
- 2008-03-04: What is a “High” Laser Beam Quality?
- 2008-02-22: Launching Light from a Bulb into a Single-Mode Fiber
- 2008-02-14: How Laser Development Can Go Wrong
- 2008-02-12: Factor 2 in the Equation for Cross-Phase Modulation
- 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
- 2008-01-06: Saturation Intensity or Saturation Fluence of a Saturable Absorber or a Laser Gain Medium: What Matters?
- 2007-12-18: The Role of Laser Safety Goggles
- 2007-12-11: The Idler Wave - Essential for Parametric Amplification and Oscillation
- 2007-12-03: New Paper on Power Scaling of Lasers
- 2007-11-26: Solving Laser Problems Step by Step
- 2007-11-19: Walk-Off and Phase-Matching Bandwidth in Nonlinear Crystals
- 2007-11-10: Retirement of Prof. David C. Hanna
- 2007-11-02: Ultrafast Laser Kills Viruses
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- 2007-07-06: Promoting Dangerous Practices in Laser Labs
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- 2007-04-28: Easier Launching into Fibers with Large Mode Area?
- 2007-04-16: Questions and Answers on Shot Noise
- 2007-04-01: The Ideal Pump Intensity Distribution in an End-Pumped Solid-State Laser
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- 2007-03-16: Time To Market and the Economics of Laser Development - or How to Cause Great Financial Damage without Spending Money
- 2007-03-11: Divided-Pulse Amplification
- 2007-03-09: The Trouble with Crystal and Coating Damage
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- 2007-02-16: The Science of Biophotons
- 2007-02-09: Papers Reporting Yet Another Laser Crystal
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- 2007-01-15: Origins of Heating in Laser Crystals
- 2007-01-09: The Myth of Fiber-Optic Polar Bears
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- 2006-07-23: Single-Atom Lasers
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- 2006-07-14: Lasers Like it Cool
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- 2006-07-05: Laser Design: Side Product or the Basis of Laser Development?
- 2006-07-01: Lifting the Confusion Concerning Doping Concentrations
- 2006-07-01: Characterizing a Cavity with a Frequency Comb
- 2006-07-01: With Wavelength Combs to Picometer Resolution