I see, so the first approach would be accurate to find the actual value of the phase accumulated.
But the second approach should work in terms of the incremental phase added to the WG with the increase of WG length, based on your description of the referencing point? For example, the SWG length swept is 4800um, then the total phase accumulated per um should be 166 radian / 4800 um= 0.03458 radian / um
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