Gentec-EO discusses ways of avoiding damage to laser power meters

Even though some laser power meters on the market can withstand tremendous amounts of power, like Gentec EO’s HP400A-100kW (that can sustain 100kW of continuous power), users can still damage one if they are not careful.

To do so, yes users could simply throw at it 150 kW of power, which would exceed the 100 kW it was built to sustain and it would eventually reach breaking point. Assuming that this obvious scenario was not the initial motivation leading to read this article, Gentec EO will explain the physics behind it and go for less obvious ways to damage the meter.

AVERAGE POWER

The first way to damage a power meter is to throw too much average power on it for a prolonged period (like in the previous example). The key factor linking it to damage threshold here relies mostly on the ability of the device to get rid of all the heat that is building up in the device as a result of users firing a laser at it.

So, when a high-average power detector is seen, that means this meter is pretty efficient at absorbing and dissipating heat. It can be damaged by either throwing too much average power on it or by shooting at it for too long.

Besides, by limiting the duration of each shot, way higher average power measurements could be reached with a well-built detector. That’s exactly what Gentec EO did with its PRONTO-250-PLUS, a handheld laser power meter that can give very accurate measurements up to 250W in 5 seconds single shot mode.

When used in continuous measurement mode though, that damage threshold drops to 8W. Great product, but drastically different damage thresholds depending on how it is intended to be used.

POWER DENSITY

To know more about the depths of power density and how to measure it, go check out the previous article Gentec-EO made that explains how to properly calculate it.

But for a quick answer, power density refers to the average power divided by the beam size, so, in simple terms, how concentrated is that average power, or, the other way around, how big is the surface on which the laser power will be spread. Ultimately, how much power is there in each cm2 where the beam hits the detector surface.

If it’s too high, despite Gentec EO relentless efforts to make indestructible power meters, the laser will start to cut through the detector, just like it would cut a thick piece of metal in some applications. Considering users want to measure the exact power of their laser, turning the power down is not an option. The choice is quite simple then, to avoid damage, just make the beam bigger.

How to do it? Because non-diverging laser beams are not of this world (yet!), the beam can easily be made bigger on the detector surface by taking their power measurements further away from the focal plane. Therefore, diluting the power density of their beam.

If we don’t want to simply guess what the distance from the laser source should be to have an adequate beam size, Gentec EO’s spot size & beam waist calculator will do the trick! For scenarios that don’t involve lenses or for very long distances, use the beam divergence & diameter calculator instead.

Gentec-EO’s high-accuracy laser beam measurement instruments help engineers, scientists and technicians in all sorts of laser applications from the factory to the hospital, laboratory and research center. Learn about its solutions for these measurement types:

• Laser power meters
• Laser energy meters
• Laser beam profilers
• Terahertz power meters

ENERGY DENSITY

This way of damaging a power meter is only related to pulsed lasers. The damage threshold can be busted here because, despite having an acceptable average power, a pulsed laser delivers that power not in a continuous manner but in a series of pulses, so in a limited time.

When considering pulses width ranging in the femtoseconds, for example, during these short pulses, the power seen by the detector is way higher than the laser’s average power. The local heat built-up can literally vaporize matter on the detector surface, blasting off parts of its coating at each pulse.

PULSE WIDTH

When using ultra-short pulses like those mentioned above, users are more likely to reach what an electrical breakdown, so the amplitude of the incoming electric field of their laser pulse is so high that electrons from their detector surface will jump into the conduction band, making it behave like a conductive metal.

It could get even further and users could witness multiphoton ionization. That being said, simply remember that by shortening pulse width the damage threshold will be reached way quicker.

To avoid damage, the pulse could be stretched or its energy could be lowered, but in both cases, the power output of the laser would most likely be affected, so it’s not an attractive option to measure its power. The best solution remains the same as when there is too much power density… simply make the beam bigger.

WAVELENGTH ABSORPTIVITY

Wavelength relates to fundamental interactions between each photon emitted by the laser and the absorber material. Each different material absorption spectrum varies, so some may be more sensitive to visible light and others to infrared, etc. So, if a material is a really good absorbent for the laser main wavelength, the heat built-up will be quicker than if it’s not.

At Gentec-EO, its devices are calibrated to account for that, therefore keeping accurate measurements for different wavelengths. It also do try to make this absorption shift as linear and flat as possible. Still, this does mean that at a specific wavelength, it could be safe to measure your laser continuously, but at a more absorbent one, it might not be.

FINAL NOTE

It might be obvious for some, but remember that, just like any other type of optics, the absorber surface should not be touched directly. This might put some contaminant on it and therefore interact with the incoming laser beam. This could affect either accuracy of measurement or damage threshold. But life being what it is, if it happens to be touched with or something is simply dropped on it, here is to properly clean it!

For more information, please visit https://www.gentec-eo.com/.