CCD cameras are commonly used for many imaging applications, as well as in optical instrumentation applications. These cameras have many excellent characteristics for both scene imaging and laser beam analysis. However, CCD cameras have two characteristics that limit their potential performance. The first limiting factor is the baseline drift of the camera. If the baseline drifts below the digitizer zero, data in the background is lost, and is uncorrectable. If the baseline drifts above the digitizer zero, then a false background is introduced into the scene. This false background is partially correctable by taking a background frame with no input image, and then subtracting that from each imaged frame. ("Partially correctable" will be explained in detail later.)

The second characteristic that inhibits CCD cameras is their high level of random noise. A typical CCD camera used with an 8-bit digitizer yielding 256 counts, has 2 to 6 counts of random noise in the baseline. The noise is typically Gaussian, and goes both positive and negative about a mean or average baseline level. When normal baseline subtraction occurs, the negative noise components are truncated, leaving only the positive components. These lost negative noise components can distort measurements that rely on low intensity background.

Situations exist in which the baseline offset and lost negative noise components are very significant. For example, in image processing, when attempting to distinguish data with a very low contrast between objects, the contrast is compromised by the loss of the negative noise. Secondly the measurement of laser beam widths requires analysis of very low intensity signals far out into the wings of the beam. The intensity is low, but the area is large, and so even small distortion can create significant errors in measuring beam width.

The effect of baseline error is particularly significant on the measurement of a laser beam width. This measurement is very important because it gives the size of the beam at the measurement point, it is used in laser divergence measurement, and it is critical for realistic measurement of M², the ultimate criterion for the quality of a laser beam. One measurement of laser beam width, called second moment, or D4, which is the ISO definition of a true laser beam width, is especially sensitive to noise in the baseline. The D4 measurement method integrates all signals far out into the wings of the beam, and gives particular weight to the noise and signal in the wings. It is impossible to make this measurement without the negative noise components, and without other special algorithms to limit the effect of noise in the wings.

Reason #1 To Save Money!

Reason #2 For More Accurate and Reliable Laser Research

Reason #3 For Better Laser Design

So why Spiricon's LBA-100A instead of one of those other guys?

Industrial Applications Of Seeing The Laser Beam 

One of Spiricon's sales representatives recently gave a demonstration of the LBA-100A Advanced Laser Beam Analyzer on an industrial YAG laser. The customer has 10 YAG lasers for cutting and welding. They were getting unacceptable variations in the quality of the trim from two of the machines, and wanted to see if the LBA-100A would help them quantify their beam quality. Following is his report on the demonstration:

"We measured the beam after the point of focus, as it diverges, to an approximate diameter of 1/4". This gave us excellent results. On one laser giving problems we could see a near Gaussian distribution with a clip etch on one side. Even though the beam appeared uniform to them under viewing of an IR viewer, and burn paper showed nearly round patterns, it was obvious with the LBA-100A that there were problems. On a second laser system where they were seeing good cuts, we saw a perfectly uniform, near Gaussian beam".

Most laser engineers and scientists are familiar with beam width, position, divergence angle, Gaussian fit, and such parameters for characterizing a laser beam. M² enables a user to quantitatively evaluate the focusability of the laser beam. It is a measure of how close an actual beam is to a perfect Gaussian single mode beam and is very easy to use in predicting the focused spot properties.

With increasingly sophisticated applications, the demands on the quality of the laser beam have become much greater. Traditional methods of measuring laser beam intensity profile; i.e., burn spots, mode burns, and viewing the reflected beam, are woefully inadequate for assuring the laser quality needed for today's applications. Indeed, lasers are becoming of increasingly high quality. To a large extent this is due to the availability of electronic beam profile instruments. These instruments provide a real time view of the laser beam profile that provides infinitely greater intuition to enable laser optimization. Also, electronic laser beam profilers produce much more accurate quantification of laser beam properties. The accuracy of these measurements enables scientists to fine tune the laser properties to a greater extent than previously possible. New algorithms for laser beam property quantification are discussed, along with the performance improvement of these calculations. In addition, examples are presented of actual situations in which viewing the laser beam has significantly improved its performance.

Laser processing puts increasing demands on beam quality for the process to be cost-competitive. Merely profiling the beam and comparing the profile to a Gaussian fit is no longer adequate, because it does not guarantee a diffraction-limited beam. A 'Gaussian fit' calculation can deceive the user into assuming propagation properties that will not exist in practice. Thus, the Gaussian fit method can lull the user into a false sense of security of laser performance.