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, although we do offer certain advertising opportunities on our website.
If you like this resource, share it with your friends and colleagues, e.g. via social media:
And here are the articles:
Thermal lensing is relevant in many solid-state bulk lasers and amplifiers. Here, a number of important questions on that topic are answered.
It is explained why the term intensity noise is somewhat problematic in laser technology; what is actually meant, is noise of the optical power. Further, the importance of the considered range of noise frequencies is discussed. Depending on the used detector and the measurement time, you may get quite different results for the intensity noise.
It is explained how to apply the saturation intensity or saturation fluence for determining when gain saturation sets in. The example of spiking of a laser illustrates that well.
The host medium of a laser crystal or glass does much more than just suspending the laser-active ions. In particular, phonons of the host medium often play crucial roles for laser operation.
It has been found a couple of years ago that Raman conversion in a fiber gets substantially weaker if the bandwidth of the laser light is increased from below 1~~nm to several nanometers. Here, the physical reason for that is explained and illustrated with some numerical simulations.
It is important to obtain reliable self-starting of passively mode-locked lasers, and thus interesting to consider phenomena which influence that. Here, we discuss the surprisingly strong influences of parasitic reflections and the benefit of using a slow saturable absorber.
Energy levels of tibial solid-stayed laser gain media are grouped in level manifolds; the article discusses some important aspects of those. In practice, that aspect does usually not force us to make laser models substantially more complicated.
It is shown that gain guiding effects can be taken into account in resonator design software when it supports Gaussian apertures. In some situations, the effects on the resonator modes are substantial.
It is well known that an optical bandpass filter can increase the duration of an ultrashort pulse. The article shows that that effect can be quite pronounced, even if the filter only attenuate the extreme spectral wings.
Some relatively simple reasoning would suggest that passively mode-locked lasers with a slow absorbers can hardly be stable -- although they usually are. This article tells the history of how various explanations -- partly wrong ones -- have been found.
A new algorithm has been developed for simulating the amplification of femtosecond pulses in fiber amplifiers, for example. It allows one to properly describe both the time dependence due to gain saturation and the frequency dependencies of optical fields and gain. The details have been published in the open-access journal Optics Express.
It is interesting to consider what conditions are required for obtaining passive Q switching or mode locking of a laser. It turns out that the requirements for those two operation regimes are extremely different.
It is shown with a numerical example that choosing an overly long active fiber for a fiber laser can have quite interesting consequences: not only a reduction of the power conversion efficiency, but also strong ASE, which may even suppress laser operation.
Photonics, being a particularly international enterprise both concerning science and industry, deadly needs that we can preserve a reasonable level of international cooperation. Unfortunately, the recent populist upheaval with attacks on cooperation and fundamental principles raises serious concerns. This article presents various thoughts on economical and political aspects to be carefully observed in order to stabilize the situation.
It is discussed how and when a second opinion can be a useful tool in the technology area, e.g. in the development of laser devices.
It is explained in detail, using a numerical laser model, while the input--output curve of quasi-three-level lasers can be pronouncedly nonlinear. Effectively, one a the slope efficiency which rises for increasing pump power.
Nonlinear frequency conversion is often challenging when light with extremely long or short wavelengths needs to be generated. This article discusses the many reasons for that.
When a light beam is injected into a fiber with some angle against the fiber axis, that tilt will increase the resulting phase delay. On the other hand, higher-order modes exhibit a reduced phase delay. The article discusses that apparent paradox and shows that some relatively common views are in fact wrong.
This article discusses how one can effectively couple light from the fundamental mode of a multimode fiber into a certain higher-order mode. This is surprisingly simple: one only requires a fiber Bragg grating with a properly chosen grating period. The physics behind that mode coupling effect are discussed, and the situation is illustrated with some numerical simulations.
The physics of phase shifts in resonators is relevant in many situations, for example in laser physics and ultra-precise laser measurements. This article discusses a number of interesting aspects in that context and can thus help to improve the understanding of the physics of laser beams and resonators.
The Encyclopedia of Laser Physics and Technology contains a large number of citations to scientific papers, books and other works. Here, it is discussed what criteria are applied for such citations and how new citations are coming in. Also, most articles are now equipped with direct links to the corresponding web pages of the publishers. Finally, it is discussed when it makes sense to decide encyclopedia articles elsewhere.
It appears to be quite simple to measure the absorption spectrum of an active fiber with a white light source and an optical spectrum analyzer. It turns out, however, that fluorescence light captured by guided fiber modes can seriously spoil the results.
We encourage researchers and company people to provide photographs, drawings and videos for use in our Encyclopedia of Laser Physics and Technology. Readers like that, and the contributors obtain a fair recognition.
Recently, a number of new articles have been added to the Encyclopedia of Laser Physics and Technology. This is also useful for both users of the RP Photonics Buyers Guide and for suppliers.
There are different ways for finding suppliers for photonics products. In order to obtain a list of potential suppliers, it is normally useful to use some buyers guide. Several of these exist, and the article discusses some criteria for selecting one.
This article discusses how light could be stored, so that you can take it with you and use it at some later time. These thoughts very nicely show you how a laser works.
This article discusses what determines the linewidth of a single frequency laser - possibly technical noise or quantum noise -- and how the linewidth is affected by further amplification, e.g. in a fiber amplifier.
Many believe that a short upper-state lifetime of a laser crystal is beneficial for Q switching at high repetition rates. This article shows, however, that this is not true. What is essential is a high emission cross section, and a short upper-state lifetime may just result from that.
This article discusses how the gain of the laser gain medium is saturated by short or ultrashort pulses. This is relevant for pulsed amplifiers and Q-switched lasers, for example.
It is well known that in many situations the nonlinear effect of self-phase modulation (SPM) leads to a broadening of the optical spectrum of an ultrashort pulse. However, this discussion shows that in other cases SPM does not change the spectral width or even reduced it. The created insight is relevant for pulse propagation in optical fibers and in mode-locked lasers, for example.
It is explained that the alignment sensitivity of a laser resonator can be calculated and should often absolutely be taken into account in design optimizations. Suitable resonator design software is crucial to have, but cannot fully replace human expertise. Instead of acquiring such expertise and software, one may consider letting an experienced expert develop such resonator designs.
This article presents some interesting statistical data on the traffic of the RP Photonics website, which is one of the most popular photonics websites worldwide. For example, it shows that the average number of visitors within each week of the year is comparable to the number of visitors of the largest photonics show worldwide. Such information is particularly relevant for advertisers. The letter also discusses various aspects advertisers should carefully think about.
Numerical simulations on a simple fiber amplifier exhibit various rather surprising phenomena concerning amplified spontaneous emission (ASE). This can all be understood based on the numerical model, which calculates not only observable quantities but also everything which happens inside the fiber. Numerical modeling can be recognized as the best way to acquire a thorough understanding of lasers and amplifiers.
This article discusses various problems which one can encounter when trying to attenuate a laser beam. Depending on the method chosen, one may suffer from thermally introduced beam distortions, interference effects, spatial inhomogeneities, or unwanted beam offsets.
The RP Photonics website contains a new tutorial on the modeling of fiber amplifiers and lasers.
This article discusses the responsibilities of supervisors (senior research assistants and professors) in scientific research. Particularly for planning research projects and for handling a possible crisis of the project, it is invaluable for a student and for the effectiveness of the whole research group to get substantial inputs from a supportive supervisor. Unfortunately, common authorship practices, which are not in line with commonly agreed ethical guidelines, support supervisors who neglect their responsibilities. Even the risk of science fraud strongly depends on how supervisors do their job.
Correctly designing nonlinear frequency conversion stages is not easy, but has a lot of advantages, making development projects faster and more efficient. Unfortunately, many people take a trial-and-error approach, wasting a lot of resources.
The RP Photonics website contains comprehensive physics-based tutorials on passive fiber optics and fiber amplifiers.
This article discusses various issues in the context of defining what is the mode radius of an optical fiber. It turns out that beyond the known issues from the context of laser beams, there are additional issues about which one should know in the context of fiber optics.
There are serious concerns about the long-term availability of rare earth elements. Photonics and particularly laser technology are also heavily dependent on such materials. Nevertheless, it turns out that trouble is not to be expected for laser technology.
RP Photonics is celebrating its 10-year anniversary.
It is explained why a passively Q-switched laser tends to emit more energetic pulses if its laser crystal has a low emission cross-section. This sounds counterintuitive but can be understood.
Disputes on technical issues with laser technology can lead into expensive and troublesome litigation cases. Generally, mediation or arbitration can give a good chance to resolve issues without going to court. RP Photonics offers arbitration in such cases - alone or in collaboration with a professional arbitrator.
It is easy to overlook the importance of clear specifications of laser products, either when offering or buying such devices. The consequences of a lack of diligence can be severe, though. Therefore, it can be very important to get an experienced expert involved before things can go wrong.
This article discusses what beam quality is required for efficiently launching light into a multimode fiber, and what beam quality you get out of the fiber end.
A nice example for numerical simulations of beam propagation in Q-switched Nd:YAG lasers shows that unexpected effects can be found with numerical simulations, but not so easily with analytical models or with experiments. However, you then need analytical reasoning to really understand what is going on.
We consider frequency doubling and the reserve process: degenerate parametric oscillation. It turns out that a degenerate OPO is not exactly doing the time-reserved process of frequency doubling. Therefore, it is not surprising that it has a threshold, while the frequency doubler does not. We also consider back-conversion in frequency doublers, which can occur not only due to imperfect phase matching.
This article explains several details of amplified spontaneous emission in fiber amplifiers.
RP Photonics now offers two new newsletters.
RP Photonics has created the RP Photonics Buyers Guide, where you can find many suppliers for photonics items.
Two interesting papers on very different kinds of Raman lasers haven been presented in Optics Express: a Raman DFB fiber laser and a VECSEL with intracavity Raman conversion in combination with sum frequency generation or second-harmonic generation.
There are conflicting definitions of s and p polarization in the scientific literature. What some people call p polarization, is s polarization for others. Obviously, that can cause confusion.
When doing simulations or modeling, a basic question is whether to use commercial software or home-made solutions. The article helps to find a rational decision on this issue, considering a number of important aspects, such as the time to get started, the issue of software validation, flexibility and user friendliness.
After many reports of thin-disk lasers mode-locked with SESAMs, there is now a Kerr lens mode-locked laser, which produces particularly short pulses, demonstrated by O. Pronin et al. at the Max-Planck Institute for Quantum Optics in Garching, Germany.
This article debunks the widespread myth that a more compact optical resonator setup will always be more stable. In fact, the opposite can be the case: the alignment sensitivity can become very high.
Two recent scientific papers discuss the suspected physical origin of a very nasty mode instability, as has been observed in some very high-power fiber amplifiers. The latest paper by A. V. Smith can be considered as reporting breakthrough results, very much clarifying the situation.
The article follows an interesting gedanken experiment: How would our laser technology be affected if all the laser and pump transitions would be 100 times stronger, leading to very short upper-state lifetimes?
The article discusses a number of issues which make the design and development of fiber lasers substantially more complicated and expensive than that of bulk lasers. The reasons for this are manyfold; they include the typically very high optical intensities (which cause strong saturation effects), the typical quasi-three-level peculiarities, ASE, pulse shaping mechanisms, and polarization issues.
A recent paper reports an enormous peak power of 3.8 GW from a chirped-pulse fiber amplifier system. Although it might seem that the limits are pushed away every year, the article shows that there will not be much room for further increases of pulse energy and peak power.
The article first explains a number of technical aspects, all showing that LEDs can be more energy efficient than any other light sources. Nevertheless, it then shows that rebound effects may well turn this into effectively more consumption, rather than less. The reason is basically that people generally use (and waste) more of what is available cheaply.
When mode-locked high-power lasers were developed, a serious concerning was damage to the SESAMs used for mode locking. It turned out, however, that this issue was much smaller than expected, while other problems came up in the area of dispersion compensation. The concept of power scaling shows more clearly why this had to go that way.
It is explained why it is so essential for cost-effective development and production to engage a highly qualified laser expert very early on. Mistakes made in the early stage can be very expensive.
It is explained that a simple combination of a polarizer and a quarter waveplate can sometimes be used instead of a Faraday isolator. However, this approach has its limitations.
The Encyclopedia of Laser Physics and Technology has become the victim of plagiarism, but RP Photonics fights against that - successfully so far.
The article explains the difference between resolution and accuracy of a measurement, and describes an example case where the accuracy can be much higher than the resolution of a detector chip.
It is discussed why laser diodes, in contrast to active fibers, cannot be made with very small refractive index contrast in order to obtain single-mode guidance in combination with very large mode areas. The key problem arises from the refractive index variations caused by the spatially varying carrier density.
Curious behavior is observed when a nearly rectangular beam shape is generated. This, however, can be well understood considering Fourier optics.
All-in-one concepts, where several functions are integrated into a single component, often look attractive at a first glance. The article shows, however, that moderate advantages often come at a high price. A modular approach is often much more powerful and practical.
It is well known that the phenomenon of spatial walk-off can degrade the beam quality in nonlinear conversion processes. The article shows, however, that this is not necessarily the case, even in cases with strong walk-off.
It is discussed why in the context of modeling of fiber amplifiers and lasers it is normally reasonable to treat nonlinear effects only to a limited extent.
The article explains why threshold values are often given for the onset of nonlinear effects, how Raman and Brillouin scattering lead to nonlinear power losses, and how nonlinear effects can be reduced in fiber amplifiers.
A new paper of Dr. Paschotta appeared in Optics Express. It presents news results on the timing jitter and phase noise of mode-locked fiber lasers.
It is pointed out that the selection process for papers at scientific conferences and journals often favors papers reporting advances in performance, while not rating highly papers which primarily advance our understanding and give new perspectives. In an attempt to improve this, we should think carefully about the right criteria for scoring and accepting papers.
This opinion piece discusses the thesis that the photonics and laser industry is not mature in many places, and what could be done to make life easier.
A record-high power level for a fiber laser has recently been achieved by IPG with an indirect pump scheme. The article explains the reasoning behind this.
It is explained why the beam from a green laser pointer is much better visible in air than for a red laser, even though the difference in output power is not that large. The main reason turns out to be the wavelength-dependent sensitivity of the human eye.
Etalons are often used as spectral filters and are tuned by tilting. The article discusses how tilting affects the transmission wavelengths and the transmission loss via a reduction in beam overlap.
The article explains how to use the Nyquist equations for calculating thermal noise (Johnson noise) in a resistor, particularly in the context of a photodiode circuit.
One would normally expect that forcing a laser to operate on a wavelength deviating from that of the free-running laser will reduce the output power. The article describes various situations, however, where forced tuning even increases the output power.
The article shows that focusing a laser beam does not mean creating a demagnified image of the original beam profile. This has implications in the context of beam quality and beam shaping.
The readers are reminded of additional photonics resources on the RP Photonics website.
Photodetection involves both optical and electrical powers, which can lead to some confusion. The article explains that difference and discusses some important issues, partly related to the square-law dependence of the electrical signal power on the optical input power.
The article discusses the coherence properties of light from incandescent lamps (light bulbs). It shows that it is important to distinguish between spatial and temporal coherence, and that both aspects of coherence are not totally excluded for bulbs.
A case of scientific fraud has been reported at ETH Zurich. It is necessary to foster a serious discussion on scientific integrity, risk factors for science fraud, the responsibility of scientists and group leaders in particular, and on ways to overcome certain corrupt practices (concerning authorships, for example).
It is curious that extremely hot bodies, such as some stars, emit what is called cold light. That term actually has two different meanings. Another interesting point is that many people always prefer warm light, although in offices they would do better with cold light.
The 2009 Nobel Prize in Physics has been awarded to Charles K. Kao, Willard S. Boyle and George E. Smith. The articles focuses on the pioneering work of Charles K. Kao on optical fibers.
Although it seems trivial to calculate the intensity of a laser beam, there is considerable confusion in the literature. A common mistake leads to results which are 2 times too low.
It is explained why lasers can have a low pump threshold power even if the gain medium has a very short upper-state lifetime.
It is shown that even if a truly power-scalable laser head would be developed, this would not lead to a truly power-scalable laser, as optical resonators are not scalable, at least not in a useful sense. It turns out that the resulting limitations are already hit in certain resonator design tasks.
The Encyclopedia of Laser Physics and Technology has been started 5 years ago. It is time to look back and give an overview on the current situation.
The article considers how Fresnel reflections at two nearby interfaces interfere with each other. This applies, for example, to fiber connections or to reflections in thin-film multilayer devices.
A new paper on the timing jitter and phase noise of wavebreaking-free fiber lasers has appeared (in an open-access journal). RP Photonics has been involved in this research. A lot of interesting results have been obtained.
Nichia and OSRAM Opto Semiconductors have developed novel green laser diodes, which have rather promising properties. They may soon lead to products which are very interesting particularly for low-cast mass applications, such as miniature laser projectors.
A recent paper reported an optical transistor, but it is questionable whether that term is appropriate here. The article tries to clarify the issue.
It is explained that a beat note does not always occur when two modes with different optical frequencies are superimposed. The orthogonality of modes is an important issue.
If you have beautiful and useful photographs which may be useful for inclusion in the Encyclopedia of Laser Physics and Technology, you are encouraged to submit them.
It is explained why it is meaningless to specify some signal-to-noise level without the corresponding measurement bandwidth.
Recently, interesting new results on gain-guiding index-antiguiding fibers have been published. That concept has really interesting properties, although pumping such fibers is difficult.
The articles explains with some examples how saving a little money at some locations can kill business. In other words, doing certain key things properly is vital for success.
Optical coherence of lasers and other light sources is often treated as a black-or-white issue, although it actually is not. The article discusses some important considerations.
Recently, the winners of various EPS prizes have been announced. Dr. Paschotta had the honor to serve as the chair of the selection committee. The article also contains some comments on improper behavior in the context of such prizes.
The 5th anniversary of the company RP Photonics Consulting GmbH calls for a short review of the activities and what has been reached.
It is explained why strong interference fringes can occur even when two beams with very different intensities are superimposed.
It is discussed why the minimum time-bandwidth product of a pulse is said to be reached for an unchirped sech^2-shaped pulse, while quantum mechanics always suggest Gaussian wavepackets to have the smallest uncertainty product.
Dr. Paschotta has published two books in the Field Guide series of SPIE: one on Lasers and one on Laser Pulse Generation.
This is a reminder for getting the print version of the Encyclopedia of Laser Physics and Technology at the reduced subscription price.
Currently, miniature laser projection displays for mobile devices such as mobile phones and media players are developed. Although the cost issue appears to be quite challenging, there seems to be a fair chance that cheap mass production of miniature projectors (containing some kinds of red, green and blue lasers) can start soon. That could also unleash many other laser applications.
Some soccer fans use green lasers to irritate players. The involved safety hazards can be substantial if laser pointers with high powers are used, as are easily available.
The article discusses different meanings of stability in the context of laser resonators.
An interesting book is recommended, which illuminates some characteristics of the scientific progress and corrects some common but poorly justified views.
The output of Q-switched fiber lasers often exhibits complicated temporal features such as multiple short spikes. The article shows how can these be understood and simulated with a dynamic fiber laser model.
The article discusses different types of user interfaces for simulation software, based on input forms, interactive graphical interfaces and on input scripts. There are certain trade-offs concerning the easy of getting started and the flexibility. An intelligent combination of approaches can lead to the best solution.
The article points out that the product of a change of beam angle and the beam diameter is often more important to know than the change of beam angle itself. That product is directly related to the optical phase change across the beam profile.
A recent paper in Nature Photonics demonstrates how to use chaotic lasers to generate random numbers at a very high rate.
The article discusses a recent scientific paper, presenting a mode-locked fiber laser with an extremely long laser resonator.
This article explains why many fiber laser products are actually fiber amplifiers, or master oscillator power amplifiers.
A kind of superluminal propagation of light occurs simply when a light beam is focused in free space. This results from the Gouy phase shift.
The validation of numerical simulation software, e.g. for simulating the performance of fiber amplifiers and fiber lasers, is a great challenge. The article discusses various aspects which need to be validated, and shows that very different means are required. The comparison between calculated and experimental results is only one part of the game.
Rate equations for level populations in rare earth ions are often of a kind which is called a stiff set of differential equations. The article discusses this in some detail and draws a number of conclusions, which are relevant e.g. for the modeling of fiber amplifiers and fiber lasers.
Depending on the situation, a tunable bandpass filter in a laser resonator, used for wavelength tuning, may or may not cause a substantial reduction of output power. The article discusses this in some detail.
Changes of the pulse repetition rate of a Q-switched laser usually also affect the pulse duration. This can be avoided with a master oscillator power amplifier setup. The article compares both concepts.
Unpolarized emission of a laser is a necessarily stochastic phenomenon, which cannot occur for a strictly monochromatic beam.
Readers are encouraged to provide photographs for publication in the Encyclopedia of Laser Physics and Technology.
A print version of the Encyclopedia of Laser Physics and Technology will appear soon. It can already be ordered.
It may be surprising that the linewidth of an optical beat signal can be exactly zero, even if the two optical frequencies both have phase noise, which is not even perfectly correlated.
We are often told a very simplified and actually distorted history of the Michelson-Morley experiment and the following development of science. The article tries to correct such views and puts Einsteins accomplishment in a different perspective.
A curious phenomenon is that a SESAM for passive mode locking of a solid-state laser sometimes lives longer when being operated with a more tightly focused resonator mode. The article explains this on the basis of Q-switching instabilities, which are then more firmly suppressed.
Various kinds of all-in-one ultrafast laser systems have been proposed which could be simpler than a traditional system consisting of a mode-locked laser and a regenerative amplifier. It seems though, that this approach has no real chance to lead to cheaper systems with comparable performance. The article gives some reasons for that opinion.
It is sometimes claimed that the transform limit for pulses is strongly related to Heisenbergs uncertainty limit. The article shows that this is nonsense; there is nothing more than a vague similarity. One should not confuse physics (statements on nature) with mathematics.
Fiber laser products often strictly do not contain a fiber laser, but rather a master oscillator fiber amplifier. This may or may not be relevant for the customer. The high sensitivity of such devices to optical feedback can be a problem of which many people are no aware.
It is interesting to consider the contributions of Albert Einstein to the laser. It turns out that these contributions are in a sense similar to those to the atomic bomb: basic physical principles were revealed, while a lot of work remained to be done to get the thing going.
Experience shows that self-starting of passively mode-locked lasers is more easily achieved for those lasers with short resonators. The article explains why.
A frequently encountered question concerns the length of a photon. The article shows that this is not a sensible question, because there is no comprehensive model of a photon to which this question could be applied.
The term polarization is often used in photonics. It can have totally different meanings.
Coherence is a term about which there is significant confusion in the literature. The article gives some examples and encourages a more precise use.
The term modes belongs to the most often abused terms in photonics. The article discusses some examples and shows that the problem is not just sloppy wording, but the risk of losing the essential meaning.
At a first glance, it seems that the balance between dispersive and nonlinear effects in a soliton pulse cannot be maintained e.g. when the fiber dispersion is reduced to one half the original value. A closer inspection shows that this is not the case.
It is often experienced that those mode-locked lasers generating shorter pulses also offer lower average powers and pulse energies. The article explains some common reasons for that correlation.
This article shows that the development of ultrafast fiber lasers is not just applying known features of mode locking to fiber lasers instead of bulk lasers, but rather involves novel aspects, mainly due to the involved nonlinearities and dispersion. This has important consequences, particularly concerning the technical limitations and the complications of the development process, which requires more physics expertise and is likely to take more time.
It is discussed while most nonlinear processes require phase matching to be efficient, while others do not need that. The reason is not that these processes would be phase-insensitive. Instead, phase matching is automatically established in certain situations.
The article emphasizes that there are different meanings of beam quality, which are not all expressed with an M2 factor. Some beams may cause trouble in applications, even though their M2 value is low.
The article discusses the maximum optical power which can be launched from an incandescent bulb into a single-mode fiber.
Laser development projects can easily fail, in the end requiring far more time and resources than originally anticipated. The reason is usually a non-systematic approach. The article gives a fictional example for a failed process, and suggests a better approach.
It can be surprising that the refractive index change caused by cross-phase modulation is twice as large as that for self-phase modulation. The article explains the reason on the basis of the induced nonlinear polarization.
When considering different ways of quantifying the chirp of an ultrashort pulse, one can arrive at definitions which in typical situations may not even agree on the question whether the magnitude of chirp increases or decreases. The article gives some examples.
It is instructive to consider the beam quality of a second-harmonic beam in different cases. It turns out that the full beam quality is normally preserved for a Gaussian pump beam, while the beam quality deteriorates significantly for a multimode case with rectangular beam profile.
The article discusses a scientific paper, reporting that efficient frequency doubling of long laser pulses in KTP is difficult. The reason is related an induced absorption (photodarkening) in the material.
The article shows that in some situations, e.g. in the context of passively Q-switched lasers, the most relevant property of saturable absorber and gain medium is the saturation fluence, and not the saturation intensity.
Laser safety goggles can play an important role in laser safety. However, they must be part of a more comprehensive system of protection layers. The article discusses some essential aspects.
Generation of an idler wave a non-degenerated parametric amplifier or a parametric oscillator is essential, even if the idler output is not utilized. This is because the idler plays an essential role in the nonlinear amplification process. Strong idler absorption in the nonlinear crystal degrades the gain, but does not necessarily spoil the power efficiency.
A new paper on power scalability as a precise concept for the evaluation of laser architectures has been published in the open-access repository arXiv.org. It clearly defines for the first time what power scaling and scalability should mean in the context of lasers.
Solving technical problems step by step is an often reasonable approach, which however normally does not work with lasers. The article explains why, and what has to be done.
The article discusses how temporal and spatial walk-off phenomena in nonlinear crystals are related to the limited phase-matching bandwidth in terms of optical frequency or propagation angles.
Prof. David C. Hanna of the Optoelectronics Research Centre has retired from his position, which he held for many years. The ORC and the international photonics community are greatly indebted to this person.
An article in the Wired magazine reported a method for selectively killing viruses e.g. in human blood. While some original scientific article is interesting, the Wired article contains a misunderstanding of basic aspects. This Spotlight article is intended to clarify the situation.
The article considers the question in which sense a laser gain medium, such as a laser crystal, is in thermal equilibrium during laser operation.
This article points out that the term gain bandwidth in the context of laser gain media is by far more difficult that it seems at a first glance. There are different definitions, and specifications according to such definitions cannot easily be converted into each other. It requires a decent understanding to deal with that term.
It is often seen that the output beam of a frequency doubler is somewhat smaller than the pump beam. The article explains why that is, and points out some interesting details of the conversion process with focused Gaussian beams.
The article creates an intuitive understanding of Fourier spectra by discussing a number of physical examples in the context of ultrashort pulses.
A common belief is that the effective refractive index of a propagation mode e.g. in an optical fiber is a kind of weighted average of the local refractive index, taking into account the intensity distribution. The article shows that this picture is quite wrong.
The article begins with discussing energy conservation for cases with destructive interference, and also mentions issues of quantum weirdness as can be found particularly on the single photon level.
The article discusses how the conversion efficiency e.g. for frequency doubling in a nonlinear crystal depends on the crystal length. It turns out that the answer depends strongly on phase-matching details and the pulse duration.
The article encourages readers to use materials (text and figures) of the Encyclopedia of Laser Physics and Technology in their own publications. The required permission can normally be obtained very quickly and easily. Another options is to cite encyclopedia articles.
Power scaling is usually considered as a strategy to get higher powers from lasers, but it is instructive also to consider power scaling to lower powers. The article discusses two different strategies.
It is common wisdom that lower peak powers cause weaker nonlinear effects. However, the article discusses a case in the context of stimulated Brillouin scattering where the opposite is true. By investigating this in some detail, one can strengthen the understanding not only of Brillouin Scattering, but also of Fourier spectra.
Problems with lasers can often be addressed over a large distance - without the consultant seeing and touching the device. An analysis of the laser design, and particularly the resonator design, is often the central issue. The article also points out that healing is often less efficient than prophylaxis via a laser design which is properly worked out before anyone enters the lab.
Researchers at KTH in Stockholm have demonstrated the first mirrorless optical parametric oscillator (OPO). This is a device with remarkable physical properties, and its fabrication relied on the feat to periodically pole a KTP crystal with a very fine period.
Pump saturation effects in laser gain media are often ignored. In many cases, this can be justified, because such effects are rather weak. However, this is not always the case. The article illustrates this with some examples.
The article recalls that as soon as light is propagating in some medium, we have much more than just electric and magnetic fields interacting with each other. So it is not correct to simply view light as an electromagnetic phenomenon.
It is often believed that a fiber amplifier with large core and/or higher NA exhibits stronger ASE. However, this is not true for single-mode amplifiers. The article explains why.
Amplified spontaneous emission in a fiber amplifier is often stronger in backward direction, compared to the forward direction. The article explains why that is.
One usually expects that the higher the density of dissipated power in a laser crystal is, the stronger are the resulting thermal effects. The article shows that in a thin disk laser this is not quite true: reducing the disk thickness increases the power density, while reducing thermal effects.
In the scientific literature, there is some conflict of statements concerning mode competition in lasers. The article explains that this results from a different understanding of the term competition.
This article explains the origins of frequently encountered confusion concerning quantitative measures of the transverse extension of laser beams. Particular problems arise from the inappropriate use of terms such as beam size, waist size, beam width, and beam waist.
It happens that e.g. senior researchers even publicly promote working practices with lasers which are definitely unsafe. The article gives some examples and comments the issue.
The article discusses the challenges of developing laser safety regulations. A key challenge arises from partially conflicting requirements such as clarity, compactness, suitability in many situations, practicability, and legal soundness.
Emission of light with narrow linewidth is a special features of lasers, which can often enough cause enormous trouble. The article gives examples in the context of digital laser projection displays, interference effects in cameras, and problems with nonlinearities in optical fibers.
This article emphasizes that a number of details have to be observed to obtain correct characterization of a laser beam, e.g. concerning beam quality. This is vital for getting reliable predictions of beam propagation e.g. through some focusing optics.
This article strongly recommends to characterize the intensity profile of the pump beam before building the resonator of an end-pumped laser. Plenty of possible problems can be quickly detected and cured at this early stage, while later on the trouble could be substantial. Having a proper beam profiler in a laser lab is essential, and trying without can be a waste of resources.
It is instructive to look at nonlinear crystals such as BBO which can e.g. have a threefold rotational symmetry in terms of nonlinear interactions, while being isotropic in terms of nonlinear optics. The article discusses how that can be.
While it is well known that polarization-maintaining fibers have a strong built-in birefringence, it is often not understood how exactly the birefringence helps. The article explains this on the basis of coherent mode coupling.
By asking how to obtain the lowest possible threshold pump power for a laser, one gets into interesting issues, related to the famous sigma-tau product, the basics of spontaneous emission, and even a bit of quantum optics.
The article discusses how the ease of launching into an optical fiber depends on its effective mode area. The result is somewhat surprising, and certainly instructive.
This article raises a number of straightforward questions on shot noise, and gives the answers. The questions address the origin of shot noise, issues of its measurement, and the dependence on average power level.
One often comes across the question what shape and size of pump intensity profile is ideal for an end-pumped laser. The article shows that a universal answer can not be appropriate: what is best depends on many different aspects. For solid-state lasers operating at higher power levels, a laser design based on a careful analysis can replace a tedious trial-and-error approach.
This article recommends a fascinating book by the Nobel Prize winner Richard Feynman, in which he explains the quantum nature of light - in fact, quantum electrodynamics - in a fascinating way.
The article shows that while avoiding cost overruns in laser development is important, it is at least as important not to endanger the financial return by engaging in poorly planned projects. An example shows that large delays can be caused by such mistakes, and that the caused financial damage can often be dominated by the aspect of time to market, not even the lost money for salaries and infrastructure. The way to minimize such risks is to start with a decent plan in the form of a detailed design document, analyzing all important issues and identifying possible problems at an early stage.
A new method named divided-pulse amplification has been described which can be used for amplifying ultrashort pulses to high energies while avoiding excessive nonlinear phase shifts. Particularly for longer pulse durations in the picosecond domain, the method appears to have some advantages over the common method of chirped-pulse amplification.
The article explains why it is very hard to obtain comprehensive specifications on optical damage e.g. of laser crystals or optical coatings. It also points out that the best strategy to avoid damage problems is often to optimize the device design, rather than to obtain components with higher damage threshold.
For nonlinear frequency conversion of optical pulses, a shorter pulse duration can be beneficial due to the higher peak power. However, other issues can favor longer pulses, and overall the optimum pulse duration or crystal material can strongly depend on the circumstances. The article discusses some of the key issues.
This article is a reminder that usefulness in some sense, and not just performance of some hardware, must be the ultimate criterion for judging the quality of paper submissions, e.g. in laser conferences. It can be very misleading just to see whether there is a device with convincing performance, as this alone may not teach us anything interesting and useful.
It is demonstrated with the example of a simple fiber laser that even severe optical losses within the cavity of a high-gain laser may not have a strong influence on the laser efficiency.
The article reports some remarkable research of Prof. Fritz-Albert Popp on biophotons, which leads to mind-boggling new insight into many phenomena.
While the flood of low-quality papers reporting new laser crystals can be tiring indeed, it is clear that good papers in this area should be very welcome even at a time where hundreds of materials are used already. This article identifies some common problems and tries to make constructive suggestions for authoring such papers.
Many papers presented at the conferences Photonics West in San Jose, January 20-25, and Advanced Solid-State Photonics (ASSP) in Vancouver, January 28-31, addressed the issue of larger mode areas of fibers for high power fiber lasers and amplifiers. The article highlights various approaches. So far, it is not clear how far we will get with those.
The article discusses what the noise figure of an amplifier really means, and uses the example of a chain of two high-gain amplifiers. It turns out that an important ingredient of the definition of the noise figure is often overlooked.
The article discusses various aspects of operating lasers and OPOs far above the pump threshold. It turns out that depending on the situation, quite different effects can come into play, and in most (but not all) cases it is most sensible to operate just a few times above threshold.
While heat dissipation and thermal lensing in laser crystals are in principle well known, there are interesting details which are often encountered but not properly understood and handled. The article discusses how heating effects can depend on various parameters, and explains that proper laser designs can avoid a lot of trouble with such matters.
Again and again one encounters the myth that polar bears use fiber optics to stay warm. The article presents some strong arguments against that, and provides further references.
The baffling success of the Encyclopedia of Laser Physics and Technology in terms of search engine ranking, traffic figures and popularity has raised the question how all this was achieved within just over two years. The author explains that the technical content of the encyclopedia is by far of highest importance, while various elements of white-hat search engine optimization play some minor role. Essentially the success of the encyclopedia results from hard work, knowledge and experience, and no optimization tricks could ever replace that.
This article clarifies an issue which often causes confusion: the maximum of some power spectral density can strongly depend on whether the quantity refers to fixed frequency or wavelength intervals. For this reason, there are differing claims e.g. concerning the wavelength of maximum emission of the sun.
Here is a safety warning, reporting about cheap green laser pointers which emit dangerous levels of optical power in a green beam, sometimes even mixed with infrared light of even higher power. Such a laser product can destroy an eye within a fraction of a second.
It is instructive to think about what really makes high technology and a high-tech industry, and to apply these thoughts to current laser technology. The result is that laser technology certainly share essential aspects of high technology, but what happens on the ground is often not really on that level. Huge potentials in terms of cost and performance are wasted, essentially because the required know-how is not there. Continuing education for the technical staff could solve much of this problem.
A discussion of the role of diffraction in optical fibers, particular in single-mode large mode area fibers, improves the understanding of how such devices work, and illuminates a technological area of intense current interest. An apparently fundamental limit of fibers, related to the too weak effect of diffraction at large mode areas, acts as a road block on the way toward output powers of tens or hundreds of kilowatts from a single fiber.
This article shows that it is very instructive to consider in some detail the role of diffraction in laser resonators. This leads to a better understanding of how resonators work, and also of important insight into resonator design issues. Such insight is essential for optimized resonator designs, used as essential parts of laser designs.
This article discusses how a resonant cavity made of two mirrors can transmit most of the incident light, even though one mirror alone would reflect most of the light. A naive photon picture indeed lets this effect appear as a mystery. It is essential to correctly combine the wave nature of light with the photon picture.
The article encourages some amount of healthy skepticism against many efforts of laser modeling, while also pointing out substantial benefits. To realize those at a reasonable cost, one needs to take a number of systematic steps.
A new paper suggests pumping Nd:YVO4 lasers near 888 nm in order to obtain polarization-independent pump absorption. Indeed this approach has some merits, and the demonstrated laser performance is quite good.
The article considers the spectral shift of reflection features of dielectric multilayer mirrors for tilted incidence of light. A simple argument suggests a shift toward longer wavelengths, but the opposite is true.
The weak residual transmission of a highly reflecting mirror is often useful, e.g. for diagnostic purposes related to lasers and laser beams. However, the article warns of some problems which are related to the nonuniformity of the residual transmission. This issue can lead to poor laser alignment or invalid beam quality measurements.
Light forces can pull dusty particles from the air into lasers beams, and from there to the cavity mirrors. This can cause a steady performance degradation.
This article describes a new imaging technique, which is based on a cute method and appears to be substantial practical advantages. It uses a digital micromirror device (DMD) and a single-pixel detector. The images are directly obtained in a somehow compressed format.
This article discusses the manifold advantages of using a current amplifier (possibly a commercial device) instead of a simple electric circuit for optical power measurements with photodiodes. The gained flexibility and efficiency are very much worth the cost of a good current amplifier.
The fivehundredth article has been published in the Encyclopedia of Laser Physics and Technology. The article makes some remarks about the past and future development, and about possible uses of the encyclopedia.
The article shows that there are plenty of pitfalls in the process of making specifications for laser noise. These are related to mathematical difficulties, subtleties of electronic spectrum analyzers, and problems of optical measurement setups. It is actually not easy to avoid all possible problems at the same time, and therefore hard to obtain reliable data. The involvement of an external consultant with a special expertise on noise issues can be extremely valuable both for laser manufacturers and users of laser equipment.
A recent paper proposes a new solution for achieving robust single-mode guidance in fibers with very large mode area. The basic idea is to convert the fundamental mode to a higher-order mode with a large mode area and then convert the light back to the fundamental mode at the end. The mode conversion can be done with a long-period fiber Bragg grating. The article discusses this idea and its limitations.
A detailed understanding of the stability zones of laser resonators turns out to be essential for laser design. In particular, the alignment sensitivity of a laser can be strongly reduced by moving to the optimum stability zone. The article discusses some issues and quotes a helpful reference.
This article discusses reasons for the fact that nonlinear frequency conversion processes in some crystals, such as frequency doubling, tend to become more efficient if the involved beams have short wavelengths.
The relation between the coherence length (or coherence time) and the optical bandwidth of light is usually supposed to be very simple, but interesting issues are encountered in the context of trains of ultrashort pulses from mode-locked lasers. In particular, mode-locked lasers can exhibit very high temporal coherence despite emitting very broad optical spectra. The results are relevant for technical issues of high current interest, in particular in the field of frequency metrology. A recent paper explains a number of issues in surprisingly simple terms.
At a first glance, one may expect that parametric devices can easily be scaled to very high output powers, as they do not have a problem with dissipated heat. However, it turns out that such devices also have power scaling limitations. Two obvious reasons are parasitic absorption and limited size of available crystals. Another one is more subtle but can also be more important: gain guiding can prevent efficient power conversion in high-gain parametric devices. The article explains how this works and gives some references.
This article explains the differences between the laser crystal materials Nd:YAG and Nd:YVO4 in the context of Q-switched lasers. Nd:YAG is most suitable for high pulse energies at low repetition rates, while Nd:YVO4 allows for the highest repetition rates and shortest pulses. The upper-state lifetime turns out to be of lower importance than is frequently believed.
It is still widely believed that ytterbium-doped gain media (e.g. fibers) cannot exhibit quenching phenomena, degrading the laser or amplifier efficiency, because their simple energy level structure does not permit that. However, quenching phenomena do occur - not via energy transfer upconversion processes, but via color centers. Such a result has been published about nine years ago, but has not been noticed by many. Some devices are particularly sensitive to such effects, in particular ytterbium-doped fiber lasers operating at 975 nm.
This articles describes an experiment where single frequency operation of an ytterbium-doped fiber laser was stabilized rather than prevented by the effect of spatial hole burning. While a superficial look at the experimental setup may have made this attempt to appear as stupid, a close inspection of saturation effects shows how that can work very well - which it did in the experiment.
This article presents the resolution to a physics conundrum in the area of nonlinear optics. Essentially the question was why parametric oscillation involves a threshold pump power, while frequency doubling does not, even though both processes appear to be time-reversed versions of each other. A closer inspection shows that time reversal of frequency doubling in fact leads to a an OPO with an subharmonic input wave.
The Encyclopedia of Laser Physics and Technology now features a new page with the Photonics Quiz. This currently contains 14 questions with a multiple choice of answers. The topics cover laser technology as well as nonlinear optics and fiber optics.
The article discusses a number of possible causes for a nonlinear relation between the pump power and the output power of a laser. These causes include thermal lensing, other thermal effects (e.g. quenching), and a transverse redistribution of excitation density in a quasi-three-level bulk laser.
It may seem surprising that the noise level of a laser can often be reduced simply by making the laser resonator longer. This article shows that the physical reason is quite simple, and discusses the issue in the context of the Schawlow-Townes linewidth of a single-frequency laser. The understanding of such relations can be essential e.g. for developing lasers with minimized intensity and phase noise.
Here is the resolution of a tricky physics conundrum, as recently presented in the Photonics Spotlight: an apparent proof saying that a coil cannot generate a magnetic field on its symmetry axis.
There are interesting physical aspects in situations where pump and second-harmonic waves make a double pass through a frequency doubler crystal. The article discusses what factors influence the total second-harmonic output and how one may obtain virtually zero second-harmonic output power under certain circumstances, where there is destructive interference of the field contributions from both passes. Nevertheless, one does not need to worry that no significant output power may be achieved if one is unlucky with the phase changes upon reflection at the turning mirror.
This article addresses a number of widespread misconceptions on quasi-three-level lasers, related e.g. to the effects of reabsorption effects, the merits of bidirectional pumping, and the cross section data. A profound knowledge of such issues can be very useful for selecting the best suitable laser medium under given conditions.
The article points out that a fair comparison of the performance of large-area single-mode fibers as required for high power fiber devices is not as simple as it might seem. In particular, the effective mode area can be significantly reduced under conditions of bending. Also, there are several other fiber properties to be taken into account.
Quantum theory as well semiclassical theories describe the noise of a laser to be influenced by vacuum fluctuations (zero-point fluctuations of the electromagnetic field) which intrude the laser resonator e.g. via the output coupler. Such theories may appear strange, but allow to accurately and relatively simply calculate the noise properties of various kinds of devices such as lasers, laser amplifiers, optical parametric oscillators and amplifiers, and frequency doublers. The article gives some idea of how this works.
The Photonics Spotlight presents another physics conundrum, this time in the area of nonlinear optics. Essentially the question is why parametric oscillation involves a threshold pump power, while frequency doubling does not, even though both processes appear to be time-reversed versions of each other.
A recent research papers explains a long known curiosity concerning laser beam quality, and furthermore increases the overall understanding of how beam distortions within a laser cavity influence the beam quality. The most important practical implication is that it helps to design laser cavities which make a laser less susceptible to the influence of thermal effects and other aberrations.
Judged in terms of wall-plug efficiency or dollars per watt, the single-atom laser as demonstrated in Pasadena is a total failure. However, the corresponding studies can generate a lot of interesting insight.
Have fun with a tricky physics conundrum: an apparent proof saying that a coil cannot generate a magnetic field on its symmetry axis.
There are currently very few quality blogs on topics like lasers, nonlinear and fiber optics, or photonics. If any are known, readers are welcome to suggest them to be listed in the Photonics Spotlight.
This article discusses some basic issues in the context of amplifier noise and spontaneous emission. In particular, it touches the question whether spontaneous emission noise can be suppressed.
Lasers have quite a few reasons to prefer not too high ambient temperatures: spectral broadening, population of the lower laser level, thermal conductivity, thermal quenching, mode or standing-wave overlap requirements, and material issues. There are some cases, though, where some amount of heat helps.
The effective focal length is not always the most appropriate measure of how strong some lensing effect - e.g. thermal lensing in a gain medium - is. In particular, it is necessary to consider the mode size as well. The peak phase change is sometimes a better criterion.
Laser development projects can fail or at least be strongly delayed if they are carried through without carefully working out a laser design in the beginning, which makes it possible to check various issues while it is comparatively cheap to fix them. This article discusses the possible pitfalls and tells how to organize a development project.
Unclear specifications of doping concentrations in laser crystals and glass fibers cause a lot of confusion. RP Photonics clarifies some issues.
Chromatic dispersion in a cavity as well as the cavity finesse can be precisely measured over a large spectral range with a frequency comb source.
An interesting new technique makes it possible to measure displacements with a resolution of picometers.