Picometer-resolution assessment of the period constancy in a FBG phase mask

An ultra-high resolution measurement technique makes the assessment of the period uniformity in a long grating used as a phase mask for Fibre Bragg Gratings (FBG). It comprizes a pair of two identical displacement sensors placed close to each other at a strictly constant spacing flying over the grating under test at an essentially constant velocity. The phase difference between the two sensors is a simple function of the local spatial frequency of the grating. A proper solution of an inverse problem provides the period variation along the grating. The technique is used here to characterize phase masks fabricated by means of a MEBES 4500 electron beam pattern generator. The period does not deviate by more than 3 pm from the prescribed period over a length by more than 100 mm.