Measuring High Power Lasers

Mark Slutzki

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.

"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.

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

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:

The result: a bad part.

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.

Prevention is always easier than cure.

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.

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.

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.

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

1. Cooling: All those many tens of kW coming into the sensor must be removed at least as quickly as they come in; otherwise heat can build up inside the sensor and cause serious (read: expensive) damage.
2. Damage: Even if the sensor's body is able to dissipate the expected total power, the absorber surface must be able to handle the power density - all those kW/cm² - to which it will be exposed. At high powers this is much more difficult than at lower powers; "damage threshold" (the maximum power density an absorber can handle, above which there is risk of localized burn damage to the surface) depends on the power level, and as power goes up, a given type of sensor absorber just gets more vulnerable.
3. Backscatter: A typical thermal sensor absorbs ~ 90% (with slight variations depending on wavelength). The 10% back reflection is usually diffuse, so at moderate power levels we don't give it much thought. At 50 kW, though, that's still 5 kW backscatter!
4. Suitability for industrial environments: Sensors designed for high powers are typically large (to help enable the needed heat dissipation). However, in modern production floors, spare real estate is not a cheap commodity! Footprint of instruments must be kept small, heat handling needs notwithstanding. Also, modern production floors use a high degree of automation, so instruments need to be designed for integration into factory networks.
5. Ever increasing powers: We're seeing lasers of higher and higher powers in today's industrial applications. How scalable is the measurement technology being used?

Now let's look at solutions.

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:

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:

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.

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:

• When we measure, there is (usually) no reason to place the sensor in the focal plane – and some very good reasons not to. In the focal plane, workpieces get drilled and welded – and so can sensors. And it's the same number of watts out of focus as in focus, so usually the best way to avoid damage is to find a location where the beam is defocused enough that damage threshold is no longer a concern. This is not always trivial (differences between Gaussian and Flat Top beams, before focus vs. after focus…), and you might want to check with us if you have any questions.
• Use a sensor with an absorber rated for the power density expected. Ophir has developed absorbers with incredibly high damage thresholds; the "LP2", for example, can handle 10 kW/cm² at 1000 W power, and for 10ms pulse widths is rated for 400 J/cm². It also has a flat spectral response, and very low angle dependence.
• The highest power sensors often have a reflective cone, which reflect the beam radially outward with a divergence angle, so that when it reaches the cylindrical absorber around it, the beam's power density has been significantly lowered. This "trick" enables these sensors to handle much higher power density beams than they would otherwise have been able to.

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%.

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.

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:

• Fire the laser for a short, precisely controlled time
• Measure the energy of the resulting "pulse"
• Divide energy by time to get the power

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.

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.

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.

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…

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.

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.