Light sources are typically assumed to be monochromatic when these are used for length measurements by interferometry. From the calibrated frequency of the light source, the (vacuum-) wavelength can be calculated on a relative level of 10−12. To our surprise, while implementing a new I2-stabilized Nd:YAG laser, a relative error in the order of up to 10−7 was observed in length measurements by interferometry. Ultimately, a certain amount of ‘parasitic light’, originating from an unwanted mode of...
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