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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And here are the articles:
Self-phase Modulation Causes Spectral Broadening – Does it Really?
Posted on 2015-07-01 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2015_07_01.html
For many, it seems to be common wisdom that the effect of self-phase modulation (SPM), which results from the Kerr nonlinearity, always increases the optical bandwidth of an ultrashort pulse. After all, it creates a so-called chirp, i.e., a temporal variation of the instantaneous frequency, which then runs through a wider range of frequencies. This effect is utilized, for example, in a method of temporal pulse compression, where one first broadens the bandwidth using SPM and then temporally compresses the pulse by applying an appropriate amount of chromatic dispersion which removes the created chirp.
However, it should then be irritating that various example cases quite clearly contradict the mentioned belief:
- When fundamental soliton pulses propagate through an optical fiber, these continuously experience self-phase modulation. Nevertheless, their optical bandwidth does not change at all! Well, here we also have chromatic dispersion acting on the pulse, but that linear effect is known not to affect the optical bandwidth. Therefore, how could the chromatic dispersion remove the bandwidth-broadening effect of the fiber nonlinearity?
- If one injects a higher-order soliton pulse into such a fiber, its optical bandwidth changes periodically, i.e., it regularly expands and contracts again.
- There are indeed cases where e.g. a broadband down-chirped pulse enters a fiber and leaves it with much reduced optical bandwidth and reduced chirp.
All this can be resolved by considering more carefully the generation of additional frequency components by the Kerr nonlinearity. The essential point is to realize that the Kerr nonlinearity adds certain complex amplitudes to other frequency components. (The typically used differential equations for light propagation in fibers clearly show that.) How the intensities of these frequency components change, depends on the relative signs of existing and added complex amplitudes:
- If the pulse is unchirped (i.e., it has a constant instantaneous frequency), the complex amplitudes added in the frequency domain are 90° out of phase with the already existing frequency components. As a result, the magnitude of these frequency components does not change to first order. This explains, for example, how a fundamental soliton pulse can have a constant optical bandwidth: the chirp introduced by the nonlinearity is constantly removed by the (anomalous) chromatic dispersion, and unchirped pulses are effectively not broadened by SPM.
- Without chromatic dispersion, the situation is different: the pulse can acquire a growing chirp, and here the magnitude of the generated frequency components can indeed grow, so that the bandwidth increases.
- If, however, a pulse initially has a down-chirp, the outer frequency components are indeed reduced by SPM, because the added amplitudes are out of phase with the existing ones. In that situation, SPM leads to spectral compression rather than broadening.
We see that the sign of the chirp of the pulse is essential for the nonlinear effects on the pulse spectrum, as is also illustrated in the following two diagrams:
It is also instructive to consider a soliton mode-locked laser where the pulse bandwidth is constantly reduced by the finite gain bandwidth (→ gain narrowing). In the steady state of the laser, there must be an effect to compensate for this. Apart from a modulator or a saturable absorber, SPM can take over that function. The pulse then must develop a positive chirp, as spectral broadening is possible only with that. Indeed one can observe in computer simulations that pulses in soliton mode-locked lasers exhibit a slight up-chirp, depending on the magnitude of bandwidth-reducing effects.
The presented thoughts demonstrate that one can learn a lot by thinking about basic effects e.g. in ultrafast optics or laser physics a little more closely. Many people are too quickly satisfied with inaccurate descriptions of effects, which are in contradiction even with quite common observations.
The probably most effective way for detecting and subsequently revising inaccurate beliefs is to deal with numerical models. Here, existing beliefs are continuously put to test. Seeing quite easily what exactly goes on in various systems (e.g., in a transparent laser realized in the form of a computer model), one quite quickly realizes that certain thoughts cannot be correct. At the same time, this is one of the greatest opportunities to obtain new ideas.
Alignment Sensitivity of Laser Resonators – an Important Design Criterion
Posted on 2015-05-14 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2015_05_14.html
Everyone in the field knows that an essential design criterion for laser resonators is to have appropriate mode radii, particularly within a laser crystal. For example, the mode radius in the laser crystal should approximately match the mode radius of the pump beam if one wants to achieve transverse single-mode operation, which results in a high beam quality.
What is much less known is the importance of the alignment sensitivity of laser resonators, and that it can be greatly influenced by the resonator design. The easy part is to understand why a low alignment sensitivity is very desirable; obviously, you do not want a resonator which needs to be aligned with extreme care, and it needs to be realigned when ever some optomechanical parts (e.g. mirror holders) are slightly affected e.g. by thermal expansion, or when the thermal lens is modified by changes of the pump beam profile. Particularly for an industrial product, an excessive alignment sensitivity cannot be tolerated.
In extreme cases, one would not even achieve any reasonable performance, even when working hard on a fine alignment, if thermal effects in the laser crystal are strong. Interestingly, the alignment affects the power and position of the circulating laser beam, which can also lead to modifications of thermal lensing, and that again affects the beam position and power; if such mutual influences are strong enough, you can get a very strange behavior of the laser which prevents any reasonable performance.
Calculating Alignment Sensitivities and Applying that Knowledge
Experts in the field of laser resonator design know very well that the alignment sensitivity of a laser resonator, e.g. concerning angular positions of laser mirrors, can be calculated with suitable software, which does not use a simple ABCD matrix algorithm but rather an extended algorithm using 3×3 matrices. The alignment sensitivity can then be used as a criterion for the quality of the laser design; it can (and often should) even be included in a figure of merit within an automatic optimization procedure. Ignoring this important aspect in laser design can easily lead to designs with unnecessarily high alignment sensitivity which do not work well in practice.
Mode Areas are Important – What Else?
It is also known that lasers with large mode areas tend to have a higher alignment sensitivity. (This is the essential reason why high-power lasers are often much more delicate to align; the wide-spread believe that this results simply from the large size of the resonator itself is wrong, as discussed in an earlier posting.) However, there is no fixed relation between mode size and alignment sensitivity; one can have two different laser resonator designs with the same mode area in the laser crystal which differ very much (e.g. by a factor larger than 5) concerning alignment sensitivity. In case of linear resonators (standing-wave resonators), one has two different stability zones in terms of the focusing power of the thermal lens, and these can have very different alignment sensitivities. Unfortunately, one cannot always use the less sensitive zone, because that involves limitations in other aspects.
The issue of alignment sensitivities even at the heart of an often encountered trade-off: high-power laser can often be designed either for a high power conversion efficiency and robustness or for highest beam quality, but not both at the same time. Obviously, one can hardly find optimized designs without understanding these issues very well.
How to Find a Suitable Resonator Design?
Even in seemingly simple cases, it is very desirable that the person developing a laser resonator design understands the matter well. The required knowledge goes far beyond a basic understanding of resonator modes; it should definitely include a precise knowledge on alignment sensitivity issues and substantial experience concerning various typical trade-offs. Simply having a heavy textbook in the office, or even having read it, will often not be sufficient.
A suitable laser resonator design software (such as our product RP Resonator) must definitely be able to calculate alignment sensitivities and to take them into account in optimizations. However, no software in this area can replace a decent technical understanding of the person using it; I think it is not possible to make it such that it takes into account all important issues without bothering the user with it. For example, a software can hardly “know” the importance of various resonator properties for the particular application, i.e., it could not put appropriate weights on certain factors in the trade-offs which are necessary. At least, however, software from a good source comes with very helpful technical support, giving you crucial pieces of advice.
If you need a proper laser resonator design, you basically have two different options:
- You can try to acquire all the required expertise (which is certainly not easy and will require substantial time) and also get a good resonator design software.
- You can try to find an experienced expert who can do that job for you. That person would first closely analyze the concrete requirements in a dialogue with you, then translate that into appropriate resonator properties such as mode sizes and maximum alignment sensitivities, and finally work out a suitable design using proper design software.
If you quite often need resonator designs, it will probably try to get into the position of doing it yourself. If that is not the case, however, it will often be much more economical (and also lead to better results) to have an external expert doing it.
In any case, I warmly recommend to take the question of resonator design very serious, because this very much contributes to an efficient product development, avoiding a lot of possible problems causing delays and cost overruns.
By the way, the fact that a very simple resonator has been used so far for certain lasers does in no way prove that more elaborate laser design considerations would be wasting resources. After all, how can you know that the simple type of resonator is doing its job well and could not be improved? Also, even seemingly simple resonators are not so easy to understand. Finally, a few hours of good work by a competent expert cost far less than what you might well pay for wrong decisions on such matters.
The Enormous Popularity of the RP Photonics Website
Posted on 2015-04-10 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2015_04_10.html
As a reader of this newsletter, you are probably aware that the website of RP Photonics is one of the most popular ones in the area of photonics. I thought that some may be interested to learn a bit more in detail what kind of traffic numbers we have reached after a bit over 10 years. Of course, such information is most relevant for those selling photonics products, but others may just enjoy a look at some mind-boggling numbers.
An important measure is the number of page views within one month. This tells you how often some user has got one page of the website displayed in his browser software. In March 2015, our statistics software counted as many as 217'428 page views, caused by about 100'000 different visitors. In 2014, we had around 170'000 views per month on average.
The Biggest Photonics Websites
You probably know optics.org, operated by the large institution SPIE over decades. According to their media information of 2014, they got about 108'000 page views per month – certainly respectable, but far behind RP Photonics.
In fact, I'm aware of only a single photonics website reporting more traffic than us: photonics.com with 237'000 page views per month in 2014.
In contrast to others, we publish interesting details concerning how the traffic was spread over different parts of the website:
- The largest traffic driver is the famous Encyclopedia of Laser Physics and Technology, having obtained 181'308 views in March.
- The RP Photonics Buyer's Guide, existing since 2012, got 14'913 views. It happened 8'673 times that a visitor was referred to a supplier's website from there.
By the way, you can always get up-to-date statistical traffic information on our website. That page also tells you how carefully we filter our traffic data, making sure that they are not significantly affected by certain robots, for example.
Comparing with Photonics West
Each year, SPIE organizes Photonics West in San Francisco, the biggest laser show worldwide with a couple of increasingly important conferences around. In 2015, SPIE registered over 21'000 visitors there. It is definitely impressive to see many thousands of visitors walking through the exhibition floors – but keep in mind that we had 100'000 on our website in March alone … So there is one week per year where tens of thousands attend Photonics West, but in each of 52 weeks per year we have on average roughly that amount of traffic on our website.
Advertisers Need to Know Traffic Data – and Think About Them
Statistical data as reported above are most relevant for advertisers. Indeed, RP Photonics does offer online advertising in different forms:
- There are large banners, appearing mostly in the right column of most encyclopedia and buyer's guide pages. These are mostly used by ourselves, but can also be rented by others.
- In the RP Photonics Buyer's Guide, one can have so-called enhanced entries with increased visibility.
With the traffic data above, one can estimate what the value of these offers is. Curiously, it is not published e.g. for photonics.com, as far as I know, which percentage of their traffic relates to their buyer's guide. Concerning transparency, we appear to be leading.
One should also acquire an understanding of what traffic volume one can realistically expect based on what the website offers to their users. In our case, it is the by far most popular encyclopedia in the whole field, creating an invaluable support for many industry people as well as researchers all over the world, and a very handy photonics buyer's guide, providing high-quality information in a nicely presented form. Considering that, one can hardly be surprised about the enormous traffic.
Unfortunately, the large majority of advertisers has not yet realized how attractive our offers are. (One of the problems is that our visitors are mostly the technical people, while the marketing people appear to stroll around somewhere else, not realizing what their target group is using every day.) We regularly see companies spending tens of thousands of dollars per year on print ads in certain journals, where with just a few thousand dollars they could have a great exposure on our website for the whole year.
How the Reputation of Online Marketing is Spoiled
A serious problem is that the reputation of online marketing has been severely hurt by many quite bogus offers. There are certain companies which operate the online marketing for institutions having large websites, trying to monetize the traffic there (and earning their share, of course) – often with rather questionable methods.
A couple of years ago, I myself got convinced that I should spend several thousand dollars for a banner appearing in the Green Photonics Guide which belongs to the OSA website. Monitoring the traffic coming from there, I noticed that it was at least an order of magnitude lower than I could expect based on the claims made when they convinced me on the phone. (I wrote down everything relevant.) When insisting on a clarification, I was finally told that unfortunately there was a bug in their statistic software, leading to unrealistically high traffic numbers. I found that hard to believe for company which operates such buyer's guides on many different websites. Whether or not this is true, it is a disaster. Of course, I informed OSA on that matter; they formally regretted what had happened, but I could until now not convince them that the high reputation of OSA should be protected by ending such practices on their website.
Check the Traffic and Draw Your Consequences!
It is actually amazing that the majority of advertisers seem not to closely check the quality of the offers on which they spend thousands every year. If they did, close to useless banners as mentioned above could never be sold. On the other hand, many companies probably do not know that the RP Photonics website is one of the top ten referrers to their own website – even in many cases where they do not spend a single dollar on enhanced entries or other things on our site. If they knew, probably more of them would be keen to multiply that effect by paying a little. Well, some have got it, and I suppose their number will continue to grow.
2015-02-05: Attenuating Laser Beams – not That Easy
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