A pair of reflection peaks/transmission dips, at twice the Bragg wavelength, were observed in spectra of a Type I fiber Bragg grating written with the standard phase mask technique. The occurrence of two peaks/dips, rather than one, is attributed to the interleaved refractive index modulations along the fiber core, with the periodicity of the phase mask that has been observed previously in images of gratings that cause destructive interference in a reflected wave at the Bragg condition owing to the pi phase difference between the grating phases. Thus the standard phase mask technique produced an alternative type of pi-phase-shifted grating at twice the design Bragg wavelength.

A fiber Bragg grating sensor fabricated by a phase mask with 536 nm uniform pitch is presented. Two peaks/dips occur, at 785 and 1552 nm, due to reflection/transmission at the Bragg wavelength and at double the Bragg wavelength, and arising from FBG periodicities associated with half the phase mask periodicity and the phase mask periodicity, respectively. It provides simultaneous measurement of temperature and longitudinal strain, with similar intensities in both wavelengths making it better suited for long-distance operation and multiplexing compared with similar schemes.

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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 ±1 were absent during fabrication the features at 1552 nm were not evident. Index Terms-fiber Bragg grating, double of the Bragg wavelength, phase mask, Talbot interferometer.

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