After the application of bend-insensitive single-mode fiber（G.657 class fiber）

some "new" problem occurs:the test results of cut-off wavelength test are sometimes too large or could not be determined.This paper attempts to explore the principle of the G.657 class fiber and the physical meaning of the cut-off wavelength

and the analysis of the in process data during the test

to excavate and interpretate the root causes of these problems.