Long Gauge Block Interferometer Using Two Frequency-Stabilised Lasers

The design and construction of a laser interferometer using the method of exact fractions for high-accuracy calibration of gauge blocks up to about one metre in length, are described. The interferometer is based on the classical Michelson arrangement. A two-mode stabilised He-Ne laser emitting light at a wavelength of 633 nm is the primary wavelength reference for the interferometric length measurement, since its frequency is calibrated by heterodyne measurement with an iodine-stabilised He-Ne laser at 633 nm which is used to maintain the national metre standard. A second two-mode stabilised He-Ne laser, at a wavelength of 543 nm, is used in order to define the absolute length. The estimated standard uncertainty (lo) for interferometric length measurements is about 50 nm for a 1 metre gauge block. Results from trial measurements made on various gauge blocks, including a bilateral comparison, are reported.