Occurrence of features of fiber Bragg grating spectra having a wavelength corresponding to the phase mask periodicity

Use of a phase mask with 536 nm uniform pitch allowed the fabrication of a fiber Bragg grating for use at a Bragg wavelength of 785 nm. Reflection and transmission features at 1552 nm, i.e. twice the Bragg wavelength, associated with the phase mask periodicity were observed. However, when phase mask orders other than plusmn1 were absent during fabrication the features at 1552 nm were not evident.