Introduction
Laser power detectors (photodiode-based detectors) can be affected by ambient light sources such as laboratory lighting, sunlight, or reflections from surrounding optics. Since photodiodes detect all incident photons within their spectral response, these unwanted sources contribute to the measured signal leading to erroneous results.
The OPM150 Optical Power Meter uses calibrated photodiode detector heads covering wavelength ranges from 250 nm to 2490 nm depending on the detector type, allowing accurate measurements of visible and infrared lasers.
To ensure accurate results in the presence of ambient light, calibrated optical filters (e.g., spectral filters) can be used to attenuate unwanted light while maintaining measurement accuracy. The OPM150 software supports filter calibration curves, allowing the instrument to automatically compensate for filter transmission and report the correct optical power of the laser.
Table of Contents
Measurement Principle
The detector head in the OPM150 contains a photodiode that produces a current proportional to incident optical power.
Measured signal: Pmeasured = Plaser + Pambient
Where:
- Plaser = power from the laser source
- Pambient = power from room light and stray radiation
Three methods are available for the suppression of ambient light:
- Subtracting the ambient light signal via software
- Attaching a tube to the front of the detector or use of other mechanical light shielding
- Optical filtering
Software based background suppression
The simplest method is to use the “Deviation” function in the OPM150 software. This function stores the measured signal at the moment of activation of the function and subtracts this value from all subsequent measurements.
For the application of background suppression:
- Block or turn off the laser source to be measured.
- Start the measurement and click the “Deviation” button:
- Unblock or turn on the laser source to be measured.
Note that this solution is only effective, if the background light level is constant. Room lighting typically fluctuates at 100Hz or 120Hz which will therefore be imperfectly suppressed.
Mechanical shielding
Mechanical shielding is also a simple method but is only feasible where sufficient room is available and where the ambient disturbance is not colinear with the lase beam to be measured. The OPM150 series detector heads come with a threaded “nose” on the body. Tubes may be fitted onto the front of the detector which effectively reduces the field of view of the detector. Since laser beams are typically collimated, they can traverse the length of the tube unhindered. The longer the tube, the more effective the suppression of ambient light will be.
Optical filtering
Using a filter reduces unwanted components, even if they are colinear with the laser beam to be measured:
Pmeasured = Tfilter (λ) ·Plaser + Tfilter, ambient ·Pambient
If the filter strongly attenuates ambient wavelengths while passing the laser wavelength (i.e., Tfilter,ambient << Tfilter(λ), the ambient contribution becomes negligible.
Available Filter Options
Typical filters used with the OPM150 include:
Spectral Filters
Used to block unwanted wavelengths (for example room light) while transmitting the laser wavelength.
Examples:
- Bandpass filters
- Short-pass filters
- Long-pass filters
Neutral Density Filters
Used mainly to reduce the optical power level impinging on the photodiode, thus increasing the power which the detector is capable of measuring.
Examples supplied as accessories:
- OD1 filter → ~10% transmission
- OD2 filter → ~1% transmission
These filters are supplied with calibration data files so the system can compensate for their attenuation.
Hardware Setup
Step 1 — Assemble the Measurement System
Required components:
- OPM150 Optical Power Meter base module
- Compatible detector head (e.g., Si, Ge, or InGaAs)
- Calibrated filter in its filter holder
Procedure:
- Connect the detector head to the OPM150 base module.
- Screw the filter holder onto the detector front thread.
- Verify alignment so the beam fully passes through the filter aperture.
- Load the filter calibration file corresponding to the serial number of the mounted filter.
Multiple holders may be cascaded if additional filter functionality is required.
Loading the Filter Calibration File
To maintain measurement accuracy, the filter’s transmission spectrum must be “known” by the OPM150 software.
Each filter is supplied with a calibration file containing wavelength-dependent transmission values.
File Structure
The file contains:
Filter Name
Start wavelength
End wavelength
Transmission values every 10 nm
The transmission data must match the wavelength range and spacing to be accepted by the software. Also, the wavelength range of the filter must be within the wavelength range of the detector head onto which the filter has been installed.
Procedure
- Launch the OPM150 control software.
- Select the pull-down menu Channel 1 or Channel 2, depending on which detector has the filter installed.
- Select “Load Filter” to load the calibration file supplied with the filter.
- Select the appropriate filter file from the list of files available. The filter files have the name format *.flt.
- Set the laser wavelength in the software to match the measurement wavelength. This ensures the detector calibration and filter correction are applied correctly.
Once loaded, the software automatically corrects measured power values based on the filter transmission curve. If the filter is physically removed, the loaded filter file must be deselected, in order to accurately measure without the filter.
Best Practices
- Use Spectral Filters When Possible
A narrow bandpass filter centered on the laser wavelength significantly reduces ambient light.
- Shield the Detector
Even with filters, stray reflections can enter the detector. If space allows, optical tubes are recommended.
Troubleshooting
Problem | Cause | Solution |
Measured power too low | Filter attenuation not corrected | Load filter calibration file |
Fluctuating signal | Ambient light modulation | Improve shielding |
Detector saturation | Laser too strong | Use OD filters |
Unexpected wavelength response | Wrong detector type | Select proper detector head |
Summary
Using calibrated filters with the OPM150 Optical Power Meter enables accurate laser power measurements even in environments with significant ambient light.
Key points:
- Filters suppress unwanted light reaching the detector.
- Calibration files ensure transmission characteristics are compensated.
- The OPM150 software supports several filter curves for automated correction.
- Combining spectral filtering, proper shielding and attenuation (if necessary), yields the best measurement accuracy.
About the Author:
Donovan Ellis
Product Specialist & Sales
Donovan Ellis studied computer engineering in Hamilton, Ontario, and today works as Artifex Engineering’s sales and product specialist for measuring instruments. Through daily contact with customers, he knows their questions and challenges firsthand — from designing and planning to calibrating and building devices, he understands the everyday problems and tricks of the trade.
