At powers above a few tens of watts, we usually add a fan to help remove heat from the sensor. Although Ophir offers 2 fan-cooled sensors rated for 1.1 kW, water cooling is the usual solution for sensors rated for more than a few hundred watts. Most "regular" thermopile type sensors use the water just to remove the heat; some examples are shown below:

Drescher, L., Kornilov, O., Witting, T. et al. Extreme-ultraviolet refractive optics. Nature 564, 91–94 (2018). https://doi.org/10.1038/s41586-018-0737-3

Mauritsson, J. et al. Measurement and control of the frequency chirp rate of high-order harmonic pulses. Phys. Rev. A 70, 021801 (2004).

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Heimann, P. et al. Compound refractive lenses as prefocusing optics for X-ray FEL radiation. J. Synchrotron Radiat. 23, 425–429 (2016).

Focal spot analysis is done using various types of beam profiling technologies. In this article we'll focus mainly on power measurement, but we will say a few words about beam profiling solutions toward the end.

Baez, A. V. A self-supporting metal Fresnel zone-plate to focus extreme ultra-violet and soft X-rays. Nature 186, 958 (1960).

Extreme UVwavelength

Valentin, C. et al. Spectral selection of high harmonics via spatial filtering. In High-Brightness Sources and Light-driven Interactions HW3A.3 (Optical Society of America, 2018).

Schütte, B., Arbeiter, M., Fennel, T., Vrakking, M. J. J. & Rouzée, A. Rare-gas clusters in intense extreme-ultraviolet pulses from a high-order harmonic source. Phys. Rev. Lett. 112, 073003 (2014).

We thank A. A. Ünal and R. Schumann for their support with the laser systems. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie-Sklodowska-Curie grant agreement no. 641789 MEDEA.

Takahashi, E. J., Lan, P., Mücke, O. D., Nabekawa, Y. & Midorikawa, K. Attosecond nonlinear optics using gigawatt-scale isolated attosecond pulses. Nat. Commun. 4, 2691 (2013).

Ultravioletlightexamples

Snigirev, A., Kohn, V., Snigireva, I. & Lengeler, B. A compound refractive lens for focusing high-energy X-rays. Nature 384, 49–51 (1996).

We mentioned the need to minimize footprint. So, how do we make a small sensor that can still measure high powers without overheating? The trick is to use a sensor designed for lower powers so that it's small, and then expose the sensor to the high power beam only for a short time – short enough that the total absorbed heat is low, but long enough for the sensor to measure it. The truth is, though, that this would mean the exposure has to be really short – in fact, shorter than the response time for power measurement! Enter "Pulsed Power" mode. Here's the basic idea:

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Neidel, C. et al. Probing time-dependent molecular dipoles on the attosecond time scale. Phys. Rev. Lett. 111, 033001 (2013).

Since cavemen first figured out how to throw rocks and shoot arrows, our ability to precisely deliver power has come a long way. High-power laser beams, by delivering a lot of power into a small and precisely controlled space, now help us manufacture components that would have been difficult - if not impossible - using purely mechanical means. Automotive and aircraft manufacture, shipbuilding, and similar heavy-industry applications have been dramatically changed by the ever-advancing capabilities of laser technology.

There are a number of important challenges that must be addressed when it comes to measuring high power beams. Some of the main ones:

Wiese, W. L. Smith, M. W. & Glennon, B. M. Atomic Transition Probabilities: Hydrogen through Neon. Technical report, National Standard Reference Data System. (NBS, 1966).

Extreme uv lightlithography

Pan, H. et al. Low chromatic Fresnel lens for broadband attosecond XUV pulse applications. Opt. Express 24, 16788–16798 (2016).

Earlier we said that, although the focus in this article has been power measurement, "we will say a few words about beam profiling solutions toward the end". Okay then…

Ophir offers "Scatter Shields" as an optional accessory. They absorb some of the backscattered light, and reflect some of it back into the sensor's aperture, reducing backscatter by some 70%.

Marr, G. V. Handbook on Synchrotron Radiation: Vacuum Ultraviolet and Soft X-ray Processes Vol. 2 (Elsevier, Amsterdam, 2013).

Of course the meter will need to "know" that the scatter shield has been added; there is a separate calibration factor ("Laser" or wavelength setting) for the "scatter shield in" condition.

Beam power and focal plane location inevitably drift with time and use – a result of aging of components, contamination of the focusing lens by process debris, misalignment of delivery optics, etc. When that happens, the space in which the power density is high enough to affect the material can then move or change shape:

Extreme UVweather

The two laser parameters that are most typically the critical ones in high power laser processes are power density and focus location (and shape). Additional parameters sometimes become important and need to be measured, including pulse energy, actual beam profile (which determines the "focus-ability" of the beam), beam position and size (less than a full profile), and others.

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Manschwetus, B. et al. Two-photon double ionization of neon using an intense attosecond pulse train. Phys. Rev. A 93, 061402 (2016).

Extremeultraviolet lithography

Nature thanks J. Cryan, M. Gaarde and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Here we see a 100 kW beam from a fiber laser, with its power being measured by a 120K-W sensor. The beam first passes through a "BeamWatch" non-contact high-power beam profiler. This unique instrument is based on a physical property of light known as Rayleigh Scattering, where the highly-concentrated light around the laser's beam waist is scattered off air molecules in its vicinity and captured by the camera. This allows for an analysis of the laser's waist without coming into contact with the beam. The result is a beam analyzer with no water cooling required, no moving parts, and no upper limit in the power of the laser being analyzed. And, since it is a camera-based system, it provides data at video rates; this allows the user to see more time-based characteristics of their laser system.

Wang, Y., Yun, W. & Jacobsen, C. Achromatic Fresnel optics for wideband extreme-ultraviolet and X-ray imaging. Nature 424, 50 (2003).

Liao, C.-T., Sandhu, A., Camp, S., Schafer, K. J. & Gaarde, M. B. Beyond the single-atom response in absorption line shapes: probing a dense, laser-dressed helium gas with attosecond pulse trains. Phys. Rev. Lett. 114, 143002 (2015).

We mentioned the 120K-W sensor earlier; this is the first commercial sensor (read: small size, fast response time) for measuring up to 120 kW. It's designed for fiber lasers used in such applications as industrial material processing, military directed-energy applications, and similar. It's very small, considering what it does - 50 cm deep x 50 cm diameter, with a 200 mm aperture. Because of the way it works, it's in a sense almost like a blackbody - less than 1% backscatter, minimizing safety hazards.

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Extremeultraviolet lithography machine

A given process is designed to bring the beam to the needed power density in a precisely controlled location. Consider the following sketch:

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Wu, M., Chen, S., Camp, S., Schafer, K. J. & Gaarde, M. B. Theory of strong-field attosecond transient absorption. J. Phys. B 49, 062003 (2016).

Ophir's Helios is a compact industrial laser power meter designed especially with factory automation in mind. It is based on the same "Pulsed Power" concept as above, except in this case even the pulse width measurement is automatic, using an integrated fast photodetector. It measures up to 12 kW using a short exposure and therefore no water cooling. There are models for Profinet and EtherNet/IP.

Processes requiring less dramatic power levels also benefit; a single high-power beam can be "shared" among multiple parallel processing stations – and because they all use what started as a single beam, there can be much better uniformity and process control across these multiple stations.

Tzallas, P., Charalambidis, D., Papadogiannis, N. A., Witte, K. & Tsakiris, G. D. Direct observation of attosecond light bunching. Nature 426, 267 (2003).

Schroer, C. G. et al. Coherent X-ray diffraction imaging with nanofocused illumination. Phys. Rev. Lett. 101, 090801 (2008).

Extreme UVmeaning

Power – at the sort of levels were talking about, from maybe a few hundred watts to tens of kilowatts – is normally measured using a thermal sensor. Absorbed light becomes heat, and the resulting heat flow is proportional to the beam's power and is measured. The output can be a numeric readout on the display screen of a handheld meter, or perhaps the sensor interfaces directly with software running on a host system…It all depends on the specific needs of a given application.

Galbraith, M. C. E. et al. Few-femtosecond passage of conical intersections in the benzene cation. Nat. Commun. 8, 1018 (2017).

Notice the Alarm and Interlock module on the 16K-W-BB-55; this protects the sensor from overheating in case there is a failure of the water cooling system. Some sensors use a somewhat different design: the temperature rise of the cooling water, and the water's flow rate, are combined to enable measurement of the power. Some sensors using this method are shown below, including a large-format 6 kW sensor, and a unique sensor for measuring up to 120 kW:

Drescher, L. et al. Communication: XUV transient absorption spectroscopy of iodomethane and iodobenzene photodissociation. J. Chem. Phys. 145, 011101 (2016).

The application of new, advanced technology in measurement devices, can help both designers and users of high-power laser systems to optimize and control their processes, so they can confidently accomplish their goals and achieve consistently good results.

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Several standard Ophir meters offer "Pulsed Power" mode, meaning they "do the math" automatically; the user is prompted to enter the "pulse width", and the readout is in units of power. "Pulsed Power" mode enables the use of standard, small and inexpensive thermal sensors to measure powers as high as 10 kW - since total amount of heat to be dissipated by the sensor is actually low.

Bengtsson, S. et al. Space–time control of free induction decay in the extreme ultraviolet. Nat. Photon. 11, 252–258 (2017).

Refraction is a well-known optical phenomenon that alters the direction of light waves propagating through matter. Microscopes, lenses and prisms based on refraction are indispensable tools for controlling light beams at visible, infrared, ultraviolet and X-ray wavelengths1. In the past few decades, a range of extreme-ultraviolet and soft-X-ray sources has been developed in laboratory environments2,3,4 and at large-scale facilities5,6. But the strong absorption of extreme-ultraviolet radiation in matter hinders the development of refractive lenses and prisms in this spectral region, for which reflective mirrors and diffractive Fresnel zone plates7 are instead used for focusing. Here we demonstrate control over the refraction of extreme-ultraviolet radiation by using a gas jet with a density gradient across the profile of the extreme-ultraviolet beam. We produce a gas-phase prism that leads to a frequency-dependent deflection of the beam. The strong deflection near to atomic resonances is further used to develop a deformable refractive lens for extreme-ultraviolet radiation, with low absorption and a focal length that can be tuned by varying the gas pressure. Our results open up a route towards the transfer of refraction-based techniques, which are well established in other spectral regions, to the extreme-ultraviolet domain.

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Rupp, D. et al. Coherent diffractive imaging of single helium nanodroplets with a high harmonic generation source. Nat. Commun. 8, 493 (2017).

Parameters that are not controlled can unexpectedly change what the process is doing and where it's doing it. That can make your process unpredictable; in the case of an industrial, commercial process - it can eat into the profits the process is supposed to be generating.

The way to prevent this is to monitor the relevant parameters of the beam with an appropriate level of accuracy. That way, you can catch any drift before it becomes a problem, and deal with it proactively.

Santoro, G. et al. Use of intermediate focus for grazing incidence small and wide angle X-ray scattering experiments at the beamline P03 of PETRA III, DESY. Rev. Sci. Instrum. 85, 043901 (2014).

Semushin, S. & Malka, V. High density gas jet nozzle design for laser target production. Rev. Sci. Instrum. 72, 2961–2965 (2001).

It's also worth mentioning that when using Fiber Adapters at these high powers, the adapters themselves also need to be cooled! Note that highest power sensor for which we have standard FO adapters is the 400 W FL400A-BB-50. The "regular" adapters are not rated for more than that. With high power lasers, the delivery fiber itself is water cooled, as must be the fiber connectors. Ophir offers several models of QBH water-cooled fiber optic adapters.

Extreme uv lightuses

Meijer, J.-M. et al. Observation of solid–solid transitions in 3D crystals of colloidal superballs. Nat. Commun. 8, 14352 (2017).

He, X. et al. Spatial and spectral properties of the high-order harmonic emission in argon for seeding applications. Phys. Rev. A 79, 063829 (2009).

Chollet, M. et al. The X-ray pump–probe instrument at the Linac Coherent Light Source. J. Synchrotron Radiat. 22, 503–507 (2015).

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As we mentioned, a sensor needs to be able to withstand not only the total power it will face but also the power density. Some important ways to prevent damage:

L.D. and B.S. performed the prism experiments. B.S. performed the lens experiments. O.K. carried out the simulations. All authors discussed the results and contributed to writing the manuscript.

"Exotic" applications such as military directed-energy weapons, once of real interest only to sci-fi authors, are now reaching maturity. Experts often mentioned the "magic number" of 100 kW, the power level needed to make such things practical. Thanks largely to advances in fiber lasers and their scalability, industrial materials-processing systems operating at 50 kW and even 75 kW are almost standard items now.