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Beam Profiling for Small Beam Widths or Spot Size

Posted by Melinda Dragone on 9/14/2015 to App Notes
Beam Profiling for Small Beam Widths or Spot Size

Beam width or spot size is an important parameter to identify in selecting the optimal beam profiler. Scanning slit beam profilers are ideal for measuring small Gaussian beams. DataRay Inc.’s Beam’R2 and BeamMap2 are capable of measuring beams as small as 2 µm using knife-edge slits. Camera-based beam profilers, such as the WinCamD and BladeCam series, are ideal if a true two-dimensional image is desired. This is essential when viewing non-Gaussian beams. The smallest beam size that a camera can accurately measure is dependent on the camera’s pixel size, but an external microscope objective can be used to magnify a small beam.

Cameras

The minimum beam size that a Camera-based profiler can accurately measure is dependent upon the pixel dimensions of the camera sensor. To achieve accurate measurements, we recommend limiting the spot size to about 10 times the camera’s pixel size. Software settings can also optimize the accuracy when measuring small beams. The WinCamD-XHR and BladeCam-XHR have the smallest pixel size at 3.2 x 3.2 µm and therefore are able to measure beam sizes down to 32 µm.

For a more in depth explanation as to why the pixel size limits the beam size, please see our application note WinCamD Series Diameter Accuracy, Precision & Resolution.

Software Recommendations

When measuring a small beam size there are a few settings in the software that will optimize your results. We recommend changing the Capture Resolution to FULL and choosing a smaller Capture Block, as shown in the image below. We also recommend choosing No filtering in the Filter pull-down menu.

Microscope Objectives

WinCamD series cameras may be used with microscope objectives to perform near-field re-imaging of beams that would normally be too small for the camera to measure accurately. The microscope objective essentially magnifies the beam (10X, 20X, etc.) and reimages the ‘larger’ beam on the camera sensor. This method requires more setup than using a scanning slit beam profiler, but provides two-dimensional data that cannot be obtained with a scanning slit profiler and may be more cost effective.

The software allows you to enter the magnification of the microscope objective so that the calculated values displayed represent the actual beam dimensions.

Scanning Slit Beam Profilers

DataRay Inc. offers three series of scanning slit beam profilers: Beam’R2, BeamMap2, and BeamScope-P8. Scanning slit beam profilers are available with Si, Ge, InAs, and InGaAs detectors that are capable of seeing wavelengths from 190 nm to 3.9 µm. The Beam’R2 and BeamMap2 series can measure beam widths from 5 µm to 4 mm, or as small as 2 µm in Knife Edge mode. When looking to measure very small beams or beams with short working distances with the Beam’R2 or BeamMap2, DataRay offers external optics in the form of the LensPlate2 lens assembly. The LensPlate2 reimages an external beam waist onto the scanning plane of a BeamMap2 or Beam’R2. It employs a pair of diffraction‐limited aspheric lenses coated for the wavelengths of interest. These lenses are designed for use with the small beams typical of fiber optics and integrated optics.

Knife Edge Mode

On a beam narrower than the slit, the slit acts as two opposed knife-edges. As the slit scans the profile, this gives first the integral of the actual beam profile, then a flat top while the beam energy is totally within the slit width, and then the inverse integral of the profile. The beam width of very small beams can be calculated from this integral of the beam profile. A screenshot showing this in the DataRay software is visible at the top of this post.

Conclusion

Beam width or spot size is an important parameter to identify in order to select the optimal beam profiler. Scanning slit beam profilers are ideal for measuring small Gaussian beams. DataRay’s Beam’R2 and BeamMap2 are capable of measuring beams as small as 2 µm using Knife Edge slits. Camera-based beam profilers are ideal if a true two-dimensional image is desired. This is essential when viewing non-Gaussian beams. The smallest beam size that a camera can accurately measure is dependent on the camera’s pixel size, but an external microscope objective can be used to magnify a small beam.

Application Note

An Application Note related to the topic described above is available for review here.

DataRay is available to discuss these options for you to assist in the selection of the best system for your needs. Please contact us at sales@dataray.com.