Two techniques for beam profiling are described in this section: the knife-edge technique and the CCD camera technique.
The knife-edge slit technique is a method of quantifying the width of a laser beam. The technique involves chopping the beam with a knife and measuring the transmitted power as the blade cuts through the beam. The measured intensity versus knife position yields a curve that is the integrated beam intensity in one direction. By measuring the intensity curve for several directions, the original beam profile can be reconstructed – but not in real time – using algorithms developed for x-ray tomography. Instead of a knife, sometimes a scanning slit is used. In this case, the intensity is integrated over the slit width. The resulting measurement is equivalent to the original cross section convolved with the profile of the slit. The advantages of the knife-edge technique are that it can measure very small spot sizes down to 1 μm, and can be used for high power lasers. The disadvantages are that the knife-edge technique does not offer real-time readout, measures the integrated intensities in the x and y directions and not the actual 2D spatial profile (integrating intensities can be hard to interpret for complicated beam profiles), and does not work for pulsed laser sources. Knife-edge beam profiling instruments often only work for continuous wave lasers because of the extra complexity of synchronizing the rotating knife-edge and the laser pulses.
The CCD camera technique is simple: attenuate and shine a laser onto a CCD and measure the beam profile directly. It is for this reason that the camera technique is the most popular method for laser beam profiling. The most popular cameras used are silicon CCDs that have sensor diameters that range up to 25 mm (1 inch) and pixel sizes down to a few micrometres. These cameras are also sensitive to a broad range of wavelengths, from deep UV, 200 nm, to near infrared, 1100 nm; this range of wavelengths encompass a broad range of laser gain media. The advantages of the CCD camera technique are:
It captures the 2D beam profile in real-time
Software typically displays critical beam metrics, such as D4σ width, in real-time
Sensitive CCD detectors can capture the beam profiles of weak lasers
Resolution down to about 5 μm
CCD cameras with trigger inputs can be used to capture beam profiles of low-duty-cycle pulsed lasers
CCD’s have broad wavelength sensitivities from 200 to 1100 nm
The disadvantages of the CCD camera technique are that attenuation is required for high power lasers, and CCD sensor size limited to about 1 inch.
The D4σ width is sensitive to the beam energy or noise in the tail of the pulse because the pixels that are far from the beam centroid contribute to the D4σ width as the distance squared. To reduce the error in the D4σ width estimate, the baseline pixel values are subtracted from the measured signal. The baseline values for the pixels are measured by recording the values of the CCD pixels with no incident light. The finite value is due to dark current, readout noise, and other noise sources. For shot-noise-limited noise sources, baseline subtraction improves the D4σ width estimate as , where N is the number of pixels in the wings. Without baseline subtraction, the D4σ width is overestimated.
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