Answers to webinar questions: How to monitor laser system performance with power measurement

Thank you to all our participants to the live webinar we held on October 8, 2020 with Laser Focus World “How to monitor laser system performance with power measurement” presented by Félicien Legrand, US/Canada Sales Manager at Gentec-EO.

It was great to engage with existing and new customers and we look forward to follow-up on each of your needs.

The recorded webinar is accessible here for re-watch: https://www.gentec-eo.com/webinar/laser-power-measurement

Please find our answers to the variety of questions received during the presentation below.

We're here to help

Feel free to contact us and share about your needs, we’re happy to help you with your lasers! 

Knowing about your laser wavelength, min/max expected power, min/max expected energy + repetition rate + pulse width if your laser is pulsed, the min/max expected beam size and profile for your application, we will make sure you use the right tools for safe and calibrated laser power, energy, profile measurements in your operations.

Answers to your questions

1- How long is a power meter's calibration valid?

It ultimately depends on your application, but the industry standard is to recalibrate on a yearly basis and this is also our recommendation.

In some instances, it might be preferable to recalibrate more frequently. For example, one may observe the measurement of a detector to drift over time, which implies the calibration sensitivity has changed and recalibration is due to restore it to a proper value.

On the other hand, light usage of a detector in good conditions may not require the user to send it back on a yearly basis. The environment in which lasers and detectors are used plays an important role in establishing the right detector recalibration period and we can offer guidance depending on your application.

2- My detector's surface has marks on it. Does this mean it's broken or can it still be used?

Not necessarily. It depends on how much it is discolored/damaged and the appearance of these marks. If the detector cannot be recalibrated in its actual condition, Gentec-EO can replace the absorber/sensor and recalibrate the detector afterwards.

Light discoloration does not necessarily mean that the detector cannot provide a workable measurement. Please contact us to verify about your detector damage thresholds in power density and energy density for your application.

When marks are found on the surface of a laser power detector or laser energy detector, we recommend to increase your beam spot size or to switch to detectors with higher damage thresholds.

Some of our products have been designed to handle tightly focused beams, with damage thresholds up to 100 kW/cm2 and 300 J/cm2.

3- I want to create a control point and feedback loop in my production area like you mentioned in your presentation. Do I need to go through all the steps of heating up the detector and zero and all that for every measurement?

It is the safest approach for the best accuracy in laser power/energy measurement, but a feedback control loop is usually installed in controlled production environments where temperature can be monitored around lasers and instruments.

A background subtraction can usually be integrated as a fast process and if you measure the delta in power reading before and after the pre-heating step on the detector, you can always use a standardized correction factor on your reading to get back to a repeatable laser power measurement in case the pre-heating step cannot be done.

4- When we turn our 1.5 kW laser up to about 35% of the power, the beam starts to move around a bit. Is there a way to best stabilize the beam? It's in a fixed collimating and focusing optical unit.

First of all a moving beam may not be a big problem for power measurement. It depends how much deviation you get exactly and how large your laser power detector is to be able to fit the beam spot within the effective aperture. Measurement repeatability is better with a fixed, centered beam but even while moving, a traceable measurement can be done.

Then we have seen such beam wanders problems caused by thermal lensing effects which is potentially what is affecting your collimating and focusing optical unit. Mechanical distortions coming from heat accumulation in the laser source or optical elements within the optical unit along the beam path can also be responsible.

We have seen optical cavities deformed by heat accumulation resulting in drastic laser power loss. Monitoring your laser power and beam profile at different steps along the beam path will help troubleshoot and determine precisely which part gets expanded thermally for example, resulting in pointing issues. Changing or modifying this part is then the next option.

Feel free to contact us directly to share more information on what you see!

5- For high power CW meters, I see that a lot of the meters can handle very high power, but the beam spot tends to need to be large. What is the highest power one can measure with a beam spot size of 1 mm?

It depends on the laser wavelength and the detector absorber type. For VIS/NIR lasers, we offer standard detectors with up to 100 kW/cm2 and 300 J/cm2 damage thresholds in power and energy density for flat-top beams, shot continuously at the detector.

This is the highest on the market for commercially-available solutions and we constantly innovate to allow laser manufacturers/integrators/users to benefit from the highest flexibility with their measurements.

For example with a 1064 nm laser, CW, Gaussian beam profile, 1 mm diameter (1/e2), you can measure up to 200 W continuously, safely, with UP52M-300W-QED-D0. Using a beam attenuator system such as BA16K-500F-H9-D0 could be an option to run traceable laser power measurements at higher power levels than 200 W with such a small beam diameter.

6- What should be used to block the beam?

Blocking the laser beam or turning off your laser temporarily at the background subtraction step (to “make a zero”) is useful to remove radiation that is not coming from your laser in the power reading process.

A Gentec-EO detector retrofitted into a beam dump with the same absorbing material and cooling design than your power detector is an excellent way to safely absorb laser power when it is adapted to your laser wavelength, power level and beam size. 

For lasers at eye-safe levels of power, a black painting on a piece of cardboard or plastic can work great. Laser manufacturers sometimes offer the option of an internal shutter or galvo mirrors system allowing to redirect the beam rapidly to an internal or external beam dump.

If redirecting your beam is not an option in your setup and you would like to integrate a removable beam dump without uncoated edges for safe insertion within your beam path, please contact us for further evaluation of your custom needs.

7- If you can't have chilled water for a kW thermopile, is it possible to use city-water, monitor the temperature and compensate the temperature shift over time?

Yes, but laser power measurement will be relative, not absolute/NIST-traceable. Our high-power detectors of the HP Series monitor the cooling water's temperature and flow rate during operation.

Fluctuations of these two quantities are the main sources of noise in laser power measurement with this technology. If during your laser power measurement the water flow rate variation exceeds typically +/- 1 LPM/min or if the water temperature variation exceeds typically +/- 3 C/min, your measurement will not be reliable.

When no dedicated water circulator/chiller system can be used, having a not-actively cooled detector such as the PRONTO Series is an excellent way to provide single shot laser power measurements.

These detectors require an exposure duration of 5 seconds to the laser beam to display NIST-traceable laser power measurements.

8- In our process we need to measure laser power and the width of the beam. Is there test equipment that would measure both at the same time?

A dual-wedge splitter beam attenuator system such as BA16K-500F-H9-D0 will allow you to measure absolute laser power and measure beam profile/beam size simultaneously.

Using a beam profiler such as BEAMAGE-4M with a Camera Lens to image the refection off of the surface of a laser power detector also is an option with virtually no limitation in laser power if the detector is well selected to handle your full laser power.

9- Why does a thermal power meter read a negative value shortly after a beam is blocked?

It takes some time for the detector to reach thermal equilibrium and it’s possible to see the equivalent of an overshoot, but in reverse in the reading when you turn off/block your laser.

It’s also possible that your detector is still cooling down after being placed somewhere in a cooler area or placed in a surface with higher heat capacity and its internal temperature is not yet stabilized.

It’s important to subtract the background radiation (“make a zero”) when the detector readout has stabilized.

10- In a pulsed laser, is it possible to measure energy of every pulse in order to know the energy fluctuation? Then displaying the average power.

Yes. Pyroelectric detectors such as our QE Series are designed to provide calibrated, NIST-traceable laser pulse-to-pulse energy measurements in Joules.

Average power can be measured using a thermopile detector in parallel but the pyroelectric detector offers temporal resolution and by multiplying your energy measurements by the laser repetition rate, you can also get your laser average power displayed automatically in real time within your monitoring interface: MAESTRO display or PC-Gentec-EO software.

11- We have a PRONTO-Si. Is it a thermopile detector? I have used it for CW measurements only so far. I have been hesitant to use it for a pulsed laser, for example a Q-Switched laser. Can it handle that kind of energy?

Thank you for using PRONTO-Si! All-in-one, this photodiode-based detector with a Noise Equivalent Power lower than a thermopile detector (allowing to measure at the nW range up to 800 mW continuously) provides average power measurements in Watts for both CW and pulsed lasers (above 25 Hz typically and below 1 mJ), but not energy measurements in joules.

Pyroelectric detectors such as our QE Series are recommended to measure pulse energies with Q-switched lasers. Your energy levels will likely be too high to work with PRONTO-Si but feel free to contact your local representative at https://www.gentec-eo.com/contact-us and share about your expected energy levels, as well as laser wavelength, repetition rate, pulse width, and min/max expected beam size/profile so we can make sure to recommend the best measurement solution for your needs!

12- Is the entire presentation recorded?

It is. Please go to https://www.gentec-eo.com/webinar/laser-power-measurement

13- In our process, the laser beam passes through water. Does water temperature affect laser power?

There are likely several parameters to take into account that can affect the transmitted laser power when passing through any medium. This is not our core expertise, but we can say that measurement of your laser power before and after the water reservoir with a well-qualified detector will go a long way to see how it is affecting your transmitted laser power.

The water’s temperature, composition, density, local displacements, absorption at a given wavelength might all have an effect on transmission. You could vary one factor at a time and see the impact on your transmitted laser power by measuring it before and after the water reservoir.

14- Does the power of the laser beam change according to which laser mode we choose? Transverse or longitudinal modes.

Using different modes such as TEM00 vs TM01 for example will modify your beam profile and local intensities (power densities), and can also affect your laser's total power output. Not all modes are  as efficient, depending on the laser cavity design.

Using a calibrated laser power meter and a beam profiler is the best solution to quantify how switching modes is affecting both total laser power and local intensity distributions.

15- How do you calibrate your detectors to get spectral responsivity or detectivity?

Please go to the last page of our technical note How Calibration Works and let us know if you have any remaining questions for your needs.

16- How do you calibrate the measurement of energy using the camera?

Laser beam profilers integrating CMOS or CCD sensors do not come with traceable calibration. It means they are not able to provide absolute laser power or energy measurements.

They are designed to provide an image of the profile of the beam and Gentec-EO beam profilers provide ISO-compliant laser beam diameter measurements (4 sigma, 1/e2, etc).

When combining such beam profiler to a calibrated laser power detector or laser energy detector (by optical sampling or reflection off of the surface of the calibrated detector for example) it is possible to measure the beam profile and power/energy at the same time.

Then by setting up the camera’s exposure time to a fixed value so that your maximum laser power/energy represent 100% of the dynamic range of the camera, you can use the beam profiler for approximate, relative power/energy measurements.

This can be useful when you need to tell approximately if you are at 25%, 50%, 75%, etc, of your maximum laser power using the beam profiler.

17- Why do we need to measure the power at 40% beamwidth?

It is good practice to use an expanded/defocused/diverged beam diameter of 40% to 60% of the detector absorber surface diameter, centered beam, in order to get the best repeatability of measurement.

Large beam diameters also help to be away from the detector’s damage thresholds in power density and energy density. Again, some of our products have been designed to handle tightly focused beams, with damage thresholds up to 100 kW/cm2 and 300 J/cm2 so you get flexibility for your applications!

Additional resources

 

For the ones curious about the setup I have used during the webcast for monitoring laser performance, where an AC component is added to the DC current driving a laser diode in order to modulate laser power and transfer data by hitting a solar cell with the laser beam, while making sure we don’t saturate the cell, here you can watch a fun example of how this setup can be used to transfer signals produced by an electric guitar, wirelessly: https://www.youtube.com/watch?v=8z7gAiLm9RM

In need of training with your Gentec-EO equipment? Let us know! We are happy to help qualify the exact measurement solution for your needs and make sure its integration is quick and easy on your side.

Félicien Legrand is the US/Canada Sales Manager at Gentec-EO, with 11 years of field experience in North America for sales of optical measurement instruments and information technology. He holds a bachelor’s degree in physics from Université Paris-Sud and a master’s degree in commercialization of scientific instrumentation from Université Bordeaux 1 in the heart of business cluster “Route des Lasers”.


Gentec-EO
Gentec Electro-Optics is specialized in laser beam and terahertz source measurement and analysis. With an outstanding 45-year track record of innovation, developing and providing state-of the-art technologies to the laser market, Gentec-EO has become The Expert of the laser beam measurement field. For all sorts of laser applications from the factory to the hospital, laboratory and research center, Gentec-EO offers the broadest range of off-the-shelf and custom solutions, and stands ready to serve you now and in the future.
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