Beam Profiling in the SWIR Range: What You Need to Know
When applications call for beam diagnostics in the SWIR spectral range, specifically the 1.5μm region, there are two practical options available: a phosphor-coated CCD camera or an InGaAs array camera. Here is where it may seem that the decision is easily reached, but it is the wrong solution.
Phosphor-coated CCD array cameras, although they may be reasonably inexpensive, possess significant measurement and imaging limitations. InGaAs array cameras can be more expensive, however, the measurement data for accuracy and graphic displays for true representation are near absolute. Compromising for an easy solution may sacrifice the measurement accuracy needed.
The Challenges of Phosphor Coatings
One major difference between these options stems from the basic equipment: the phosphor-coated camera is simply a standard CCD array camera primarily used for UV to IR applications. It offers a pixel pitch ranging from 3.69um to 7ums. The standard CCD array is shown in Figure 1.
Figure 1. Standard CCD camera array (with protective window removed).
Nearly 20 years ago, it was discovered that by coating the silicon array with a thin layer of phosphor, this type of camera could be used to image the 1550nm region. When the phosphor is illuminated with a 1550nm source, the material converts the source to a visible, 532nm wavelength, one that the standard CCD camera can process and image. The important issue here is that a standard camera has been modified to be used in a different wavelength application.
Figure 2. Typical silicon CCD with each pixel mounted on the substrate. Pixel size can range from 3.69um to 7um and larger.
To adapt a standard CCD camera for use at the 1.5um wavelength calls for the phosphor to be manually applied over the array in a μm thin layer. This often results in varying thicknesses and produces varying results across the beam image. The phosphor is applied to both the pixels and the spacing between each pixel. There is no practical way to apply the phosphor just to each pixels due to their μm-size dimensions. Figure 3 shows how the phosphor is applied to each pixel but also the areas between the pixels.
Figure 3. Phosphor is manually applied over the CCD array in a μm thin layer.
The CCD array, once coated with a thin layer of phosphor, typically appears as in Figure 4.
Figure 4. Typical coated CCD array.
When such an array is imaging a 1550nm source, the phosphor is converting the wavelength to the array not only from each pixel but from the phosphor between the pixels, as well. This can have a significantly negative influence over the conversion process with respect to measurement accuracy and imaging representation.
Consequently, this can influence beam measurements, intensity graphics, and the overall true representation of the 1550nm source.
Dynamic Range and Non-Linearity
One also needs to consider the dynamic range of the phosphor-coated array camera vs the InGaAs array alternative. With the standard CCD camera, after the phosphor has been applied the dynamic range drops to ~30dB. The InGaAs camera, however, is designed for SWIR and has a minimum dynamic range of ~68dB, representing a significant advantage in versatility over the phosphor CCD alternative.
Another limitation to phosphor-coated cameras is the non-linearity of the response. If the software that is being used to support this type of camera does not include the correction factor E, which corrects for the non-linearity of the signal, then accurate measurements and images are compromised. The following chart plots the difference in software between a 1550nm source measured with and without the correction factor included.
Figure 5. Phosphor-coated cameras require the correction factor E to address non-linearity.
Wavelength sensitivity should also be considered when comparing these two camera options. The phosphor CCD camera chart is quite volatile and can produce different results should 1550nm not be the exact wavelength of the source.
Figure 6. Phosphor-coated CCD camera wavelength sensitivity.
When compared to the sensitivity chart of the InGaAs camera, there is a full range of wavelengths suitable for accurate measurements.
Figure 7. InGaAs camera wavelength sensitivity.
The results of these two different technologies can be seen in the images and measurements each produces.
Figure 8. Beam profiles from InGaAs camera (left) and phosphor-coated CCD camera (right).
These images were taken using the same 1550nm source, with no attenuation on either camera. The beam profile on the left was imaged and measured off the InGaAs camera, whereas the image in the right was taken from the phosphor-coated CCD camera. Several conclusions can be made based on these images:
- In this application, the X & Y measurements of the beam on the right display a noisier signal and display with less intensity as compared to the image from the InGaAs camera (left), where the positive/ negative noise on the array is nearly fully balanced out.
- The intensity of the beam characteristics is significantly more visible with this InGaAs camera than the phosphor array on the right. In this application, the detection of a beam ‘wing’ anomaly, shown by the arrow, was an essential piece of data in the transmission of the beam out of the fiber, which had not been detector prior to this analysis.
More specifically, the InGaAs camera measurements showed a 2.08mm x 1.89mm beam diameter using the 4- Sigma method. Due to the sensitivity of the array, the exactness of the array absorption characteristics, the full beam including the wings of the beam (needed for this application) measured the beam as 2.0mm x 1.8mm using the ISO 4-Sigma formula.
Figure 9. Measuring beam diameter.
The following image is an example of a similar 1.5um beam, same size as shown in Figure 9, but due to the reduced sensitivity of the phosphor in the wings of the beam, the measurements report a smaller beam with fewer optical characteristics.
Phosphor-coated CCD cameras have their place in gross beam alignment and applications where essential beam diagnostics are not required. However, due to the wide wavelength range of InGaAs cameras, the very high sensitivity of the array with very low noise, the real characteristics of the 1.5mμm beam can be detected, imaged, and measured with true accuracy. As mentioned before, a phosphor camera can be an Easy Answer, but is the Wrong Solution for accurate beam diagnostic measurements.
For more information on camera-based laser beam profiling, see: https://www.ophiropt.com/laser--measurement/beam-profilers/products/Beam-Profiling/Camera-Profiling-with-BeamGage