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:
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Make a bookmark to remember this page.
(Disadvantage: you may still forget to come back.)
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And here are the articles:
How to Define the Mode Radius of a Fiber?
Posted on 2014-07-28 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2014_07_28.html
Ref.: encyclopedia article on mode radius; M. Artiglia et al., “Mode field diameter measurements in single-mode optical fibers”, J. Lightwave Technol. 7 (8), 1139 (1989)
Most people who have worked with laser beams know that it is not entirely trivial to find a reasonable definition for the beam radius. Still, you might believe that exactly the same issues apply to the modes of optical fibers. After all, they have similar intensity profiles like laser beams, so why should any new issues arise in this context?
In the early days of fiber optics, one indeed used that approach and simply took the usual 1/e2 intensity criterion for fiber modes. As long as fibers were used which had approximately Gaussian mode profiles, this worked reasonably well. However, some of the fibers – in particular, some dispersion-shifted fibers – exhibited non-Gaussian intensity profiles, and this led to problems. Part of that was related to a common measurement method, where one records the far field intensity profile (which is somewhat simpler to get than the near field profile) and calculates the near field beam radius essentially from the beam divergence as calculated from the far field profile. For that, one uses a simple formula which is valid only for modes with Gaussian profiles. The mentioned formula then leads to a somewhat smaller result than a direct near field measurement.
From this you may actually draw the lesson that in case of doubt you should always do a direct near field measurement, i.e., determine a near field beam radius, ideally using the common D4σ criterion (see my encyclopedia article on beam radius). This, however, is also not true. It turned out, for example, that the beam radius as calculated from the far field divergence (which you may call the far field beam radius, or maybe more clearly the far field derived beam radius) is a better measure for the sensitivity of fiber splices to a small offset of the fiber cores. On the other hand, the beam radius directly measured in the near field profile allows better predictions concerning the sensitivity to angular deviations.
You can see that rather sophisticated issues are encountered in this context (about which the above cited paper of Artiglia gives a good overview). For sure, it is very desirable to know with which method a certain value for the beam radius has been determined. Unfortunately, it is a widespread habit in the industry to quote such numbers without revealing how they have been obtained.
Of course, coupling losses at fiber joints can anyway not be fully determined only from the two involved mode radii, as they depend on the full mode profiles. Only if their shapes are known, the beam radii are sufficient for calculating the coupling losses.
By the way, the near field and far field beam radii can of course also be numerically calculated for a given fiber design. Recently, I have enabled our RP Fiber Power software to calculate far field profiles, and from this you can of course also obtain the beam radius as explained above. You can then check yourself how much the beam radius values from the two definitions deviate from each other, and compare measured values with expected ones.
Shortages of Rare Earth Materials – a Problem for Photonics?
Posted on 2014-06-27 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2014_06_27.html
As an introductory remark, this article presents an example for issues where a first glance suggests certain conclusions which then turn out to be wrong when taking a closer look. Therefore, I recommend to read this to the end, if you are interested!
In recent years, there has been a growing awareness of the fact that various technology sectors heavily depend on certain materials for which a reliable long-term supply at reasonable prices is not guaranteed. Specifically, rare earth elements (REE) are more and more widely used in diverse technological sectors, while various sites worldwide have closed their production; about 95% of the total worldwide production is now in China, which uses more and more of these materials for its own production. That situation creates a concern in other countries about a dependencies which might be exploited, particularly in case of political tensions. In fact, that has already happened in 2011 in a dispute with Japan, which lead to rocketing REE prices on global markets.
The Use of Rare Earth Elements in Photonics
In laser technology and other fields of photonics, we also heavily depend on rare-earth materials. In particular, various rare earth ions like Nd3+, Yb3+, Er3+ and Tm3+ are essential laser-active dopants in laser crystals and glasses as used e.g. in rare-earth-doped fibers. Furthermore, even the host materials often are rare earth materials – in particular, yttrium as a constituent of YAG (yttrium aluminum garnet) and vanadates like YVO4. Besides, rare earths like europium and cerium are contained in phosphors for lighting applications and displays, and cerium is used for optics polishing agents. Various modern optical clocks utilize rare earth ions as optical frequency standards.
The situation is reminiscent of that in electronics, where some other rare elements like gallium, indium, tantalum and niobium fulfill essential functions.
Are We Facing Big Trouble?
Based on these facts, serious concerns have been raised, fearing troubles for the whole photonics industry if we don't either secure more reliable REE sources or learn to replace rare earth elements with others, where the supply situation is less critical. For example, an OPN article of Marcius Extavour (see the reference above) in 2011 created that impression. He named recycling efforts as a possible path, and even raised the idea that rare-earth-free solid-state lasers might be needed in the future, and stated: “With myriad laser technologies available today, rare-earth-free solid-state lasers are certainly a practical alternative.” My opinion is that in most cases they would certainly not be a practical alternative, as that would imply e.g. that we essentially give up all well-working fiber lasers – but what could replace ytterbium-doped fiber lasers and erbium-doped fiber amplifiers, for example?
Given this doubt, you may now think that I consider the situation even more threatening than Extavour and other authors described it. That is not the case, however. In particular, I believe that Extavour and other authors did not sufficiently take into account that although photonics and in particular laser technology are indeed critically dependent on REE, only tiny amounts of them are needed in photonics – in particular, for most essential functions such as laser-active ions:
- As an example, consider an Yb-doped double-clad fiber as used in high-power fiber lasers and amplifiers; this contains only of the order of 5 μg ytterbium per meter of fiber for a typical doping concentration and core size. So you if you need 10 m for a laser, it is just 0.05 mg – close to nothing.
- We need substantially more when using a laser crystal with a rare-earth host medium. Let's consider a Nd:YAG crystal with dimensions of 10 mm × 2 mm × 2 mm and a concentration of 1 % wt (by Nd2O3). That contains around 1.6 mg of neodymium.
- About the worst case is a 150 mm long and 10 mm thick Nd:YAG rod for a lamp-pumped laser, which may contain several hundred milligrams of neodymium, and more importantly tens of grams of yttrium.
- A color TV screen may contain around 1 g of europium.
- Compare all that with neodymium content of an electric generator or motor in an electric car or hybrid engine car – this may easily be of the order of 1 kg. It's in the neodymium magnets, which could in principle be replaced with other materials, but at the cost of larger size and/or lower efficiency.
- Some wind energy generators contain even much more neodymium; only, there are not so many of them.
These examples demonstrate that in photonics and particularly in laser technology we use only tiny amounts of rare earth elements, which make up only a tiny fraction of the overall demand. And that changes the conclusions in essential ways. Even if China would suddenly stop exporting any rare earth elements, a small portion of the smaller REE production of other countries would suffice to meet the whole demand of photonics. As we a talking about high-value applications requiring tiny amounts, the price of a gram of REE essentially does not matter; we will always have enough for such purposes. The situation is entirely different for applications like magnets (e.g. NdFeB in neodymium magnets), where you (a) need much more and (b) cannot pay excessive prices.
In the longer term, we will also probably see new mines in various countries; only this takes some time, and efforts have started only recently, after a long time where the demand was simply not sufficiently high to amortize big investments.
How about recycling? This is certainly a wise thing to do, where large amounts of REE remain packed into compact devices. For example, it would be more than silly not to recycle the neodymium content of the generators and motors of cars, where you can hold with two hands a machine containing 1 kg or more. On the other hand, it is hard to see how recycling of rare earth elements from laser crystals or even active fibers could have a significant impact, except perhaps for the yttrium in the bigger laser crystals.
My conclusion is that there may well be trouble ahead with the supply of rare earth elements, but not for laser technology. The sectors which might be hit are those requiring large amounts – for example, catalysts and magnets. Some sectors of photonics may also be affected to a smaller extent – perhaps the display and lighting sector requiring phosphors. For polishing of optics, we may have to use alternatives to cerium, which however should not be a major problem.
Environmental Concerns about Rare Earth Mining
As a side remark, there are substantial environmental concerns about the mining of rare earth materials. Again, however, it makes a big difference whether you need some milligrams for a device or kilogram amounts. Therefore, good ways to counteract serious pollution issues are not developing rare-earth-free lasers, but building better mines and possibly more efficiently using (and recycling) REE at those places where a lot is used.
10-Year Anniversary of RP Photonics
Posted on 2014-05-16 as a part of the Photonics Spotlight.
Permanent link: http://www.rp-photonics.com/spotlight_2014_05_16.html
Ref.: anniversary article
An an exception, the Photonics Spotlight today covers a non-technical topic which is related to our company history: RP Photonics is celebrating its 10-year anniversary. End of May 2004, the company was founded and officially started in Zürich, Switzerland. Hardly anybody in the laser industry knew about some Dr. Paschotta, who had a reputation only as a scientific researcher. But that changed rapidly – nowadays, RP Photonics is a worldwide very well known company. Our anniversary article tells you how this worked based on a somewhat unusual business model, and what is expected to happen next. Therefore, I can keep this newsletter article very short, hoping you will visit our anniversary article with a single click and learn more there!
- 2014-04-02: Lower Emission Cross-section Leads to Higher Pulse Energy?!?
- 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
- 2013-09-24: 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
- 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
- 2007-10-07: Effective Refractive Index: Correcting a Common Belief
- 2007-09-27: Light Plus Light = Darkness: No Energy Problem, but Quantum Weirdness
- 2007-09-21: Optimum Crystal Length for Frequency Doubling
- 2007-09-13: Using Figures of the Encyclopedia in Your Publications, and Citing the Encyclopedia
- 2007-09-07: Power Scaling in Downward Direction
- 2007-09-01: Stimulated Brillouin Scattering: Lower Peak Power, Stronger Effect?
- 2007-08-27: Distant Healing of Lasers
- 2007-08-23: An OPO Without Resonator Mirrors
- 2007-08-22: Saturation of Pump Absorption - An Important Issue?
- 2007-08-15: Light = Electromagnetic Waves?
- 2007-08-06: Fiber Amplifiers: More ASE for Larger Core with Higher NA?
- 2007-07-30: Fiber Amplifiers: Stronger ASE in Backward Direction
- 2007-07-25: Higher Heat Generation Density, Stronger Thermal Effects?
- 2007-07-16: Mode Competition - Increased or Decreased by Spatial Hole Burning?
- 2007-07-11: What is a Beam Width, Beam Size, and a Beam Waist?
- 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!
- 2007-05-26: Optical Isotropy: Nonlinear Interactions are Different!
- 2007-05-19: Why Strong Birefringence in Fibers Helps
- 2007-05-10: Fundamental Limitation for sigma-tau Product, Gain Efficiency, and Laser Threshold
- 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
- 2007-03-23: Explaining the Nature of Photons to Lay Persons
- 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
- 2007-03-05: More Efficient Frequency Doubling with Shorter Pulses?
- 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
- 2007-01-05: Why the Encyclopedia of Laser Physics and Technology is Successful
- 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
- 2006-11-04: Nd:YVO4 Laser with Polarization-Independent Pump Absorption
- 2006-11-02: Reflection Spectrum of Tilted Dielectric Mirrors
- 2006-10-26: Residual Transmission Through Highly Reflecting Mirrors
- 2006-10-22: Lasers Attract Dust to Cavity Mirrors
- 2006-10-17: A Cute New Imaging Technique Named Compressive Imaging
- 2006-10-16: Using a Current Amplifier for Optical Power Measurements and Recording with a Photodiode
- 2006-10-15: Fivehundred Articles in the Encyclopedia of Laser Physics and Technology
- 2006-10-09: Correct Specifications for Laser Noise - Valuable but Hard to Obtain
- 2006-10-04: Higher-Order Modes of Fibers: a Solution for Single-Mode Guidance with Large Mode Area?
- 2006-10-01: Stability Zones of Laser Resonators
- 2006-09-29: Frequency Dependence of the Conversion Efficiency for Frequency Doubling
- 2006-09-22: Coherence Length of Ultrashort Pulses
- 2006-09-21: Power Scaling Limits of Optical Parametric Amplifiers
- 2006-09-16: Q-switched Lasers: YAG versus Vanadate
- 2006-09-06: Quenching Degrades the Efficiency of Some Ytterbium-Doped Gain Media
- 2006-09-03: Single-Frequency Operation Stabilized by Spatial Hole Burning
- 2006-09-03: Resolution of Conundrum: Threshold Power for Parametric Nonlinear Interactions
- 2006-09-01: Test Yourself with the Photonics Quiz
- 2006-08-23: Lasers with Nonlinear Input-Output Characteristics
- 2006-08-20: Lower Noise from Longer Lasers
- 2006-08-18: Resolution of Conundrum: No Magnetic Field on the Axis of a Coil
- 2006-08-15: The Effect of a Double Pass in a Frequency Doubler
- 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?
- 2006-07-30: Threshold Power for Parametric Nonlinear Interactions
- 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?
- 2006-07-21: Photonics and Laser Technology Blogs – Where Are They?
- 2006-07-16: Spontaneous Emission and Amplifier Noise
- 2006-07-14: Lasers Like it Cool
- 2006-07-10: Strength of Thermal Lensing Effects
- 2006-07-05: 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