Graded neutral density filter

If ion lasers are already available in the laboratory, a different method for controlling their intensity with a compnter is needed. One simple and cost-effective method is to use a graded neutral density filter than can be moved with a computer-controlled motorized stage, or alternatively, a combination of a rotating half-wave plate and polarizers. A more expensive, but versatile option is an electroaconstic modulator, but none of these alternatives will be described here. 

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