How to remove background noise for accurate laser beam profiling and power measurement

In laser beam technology, ‘noise’ simply describes any background interference, which may cause fluctuations in your beam’s measurements. This noise, which typically comes in the form of stray ambient light, can affect both beam diameter and power measurements. In most cases, these erroneous readings are larger than they are supposed to be. 

In this article, we will outline how to minimize background noise and improve the accuracy of your readings when performing both diameter and power measurements. 

Laser beam diameter measurements

Laser beam diameter measurements are typically done with laser beam profiler instruments. Before background noise even comes into play, the accuracy of your measurements depends on the type of devices you use. 

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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 our solutions for these measurement types:

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

How to choose the right beam profiler?

When selecting a profiler for your laser beam measurements, the following factors need to be considered:

• Spectral Range – determine the spectral range that your sensor can safely cover. If you are outside the spectral range, you may need to use special filters and wavelength converters.  
• Beam Size – The camera’s pixels must be small enough to get good resolution. In addition, the beam must fit within the sensor size. 
• Laser power and power density – Beam profiling instruments are very sensitive. Therefore, ensure that your laser’s power and density are within your equipment’s safe operating limits. 

Now that you have chosen your profiler based on the criteria above, you need to tell it which beam diameter measurement method to use. For example, if you want to measure a first-order Gaussian beam, the recommended technique is the D4σ method. 

D4σ, however, includes contributions from pixels from other areas within the active area of the sensor. In other words, it is sensitive to stray background light, thus resulting in an overestimate of your beam’s diameter. 

Reducing background noise during profiling 

ISO standards recommended that background noise be removed from laser beam measurements for accuracy of measurements. Gentec-EO’s powerful profiling software can detect and remove background noise with the click of a mouse. 

The ‘subtract background’ function allows you to specify ambient background light as the baseline measurement, or zero value. In other words, in a stable environment, any measurement taken after the baseline setting will be purely from your laser beam. 

By heading to the toolbar and selecting ‘subtract background,’ the software will inform you to block your laser beam. Once the beam is blocked, the software will begin calculating the background noise based on measurement obtained from 10 frames. Once the noise is determined and set to zero, the laser can be unblocked for a recalibrated, more accurate diameter reading. 

Laser beam power measurements

Now, we will look at reducing background noise and improving power measurement accuracy in power detector instruments. Again, before we look at noise reduction, it is crucial to select the right measurement tools to ensure that you are getting the best possible power readings. 

How to choose the right laser beam sensor?

There are six main parameters that you should consider when selecting the best tool for measuring your laser beam’s power output. These are:

• Wavelength
• Spot size/beam profiles
• Minimum and maximum average power output 
• Maximum and minimum energy output per pulse
• Repetition rate, and
• Pulse width

Obtaining accurate measurements and reducing background noise

At Gentec-EO, we have set out eight general guidelines for achieving the most accurate power measurement when using thermopile power detectors. These guidelines are outlined step-by-step below:

1. Firstly, ensure that the detector is used within its rated specifications (average power, power density, repetition rate, etc.)
2. Ensure that the laser is stable before commencing with measurements. A rule of thumb is to power the laser for at least 20 to 30 minutes beforehand.  
3. Use 40%-60% of the optical aperture.
4. Turn on the laser and heat the Gentec-EO power detector for at least two minutes to achieve thermal equilibrium.
5. Next, turn off (or block) the laser beam for a minimum of two minutes. 
6. As mentioned previously, use the software’s background subtracting feature to eliminate any stray light by setting the baseline to zero. 
7. Turn the laser back on again and heat the Gentec-EO detector for another two minutes.
8. Last but not least, measure the beam’s power using an averaging method for a minimum of one minute. 

Bear in mind that the procedure outlined above is for thermopile detectors. It is not necessary to perform the thermal equilibrium steps with photodiode detectors. 

For example, a photodiode detector connected to a PC was used to determine the average power output for a green laser. As you can see in the bottom-left figure, the initial reading was given as 24.5mW. After we used the software’s automatic background noise baseline features, the reading was reduced to 22.7mW, approximately 7% of the original value.

Accurate laser beam profiling and power measurements depend on two main factors: choosing the right measurement equipment, and eliminating background noise.

At Gentec-EO, our software tools make it easy to identify and remove stray ambient light from your laser beam measurements. If you need assistance selecting a sensor or profiler, or would like to learn more about background noise, feel free to contact our technical support team.