Figure vsg.2 : Period Trends. Changes in grating period with time are shown. The dashed lines represent the period derived from the 9611 set of VSG retests (not all facets have LR version 3.+ data prior to this retest.) The VSG retests at 9707 and 9801 (as well as initial LR version 3.+ tests if they exist) are then normalized to this period. The dotted lines represent period variations of +/- 50 ppm from the 9611 value.