High reflectivity fibre gratings produced by incubated damage using a 193 nm ArF laser

Effect of phase mask alignment on fiber Bragg grating spectra at harmonics of the Bragg wavelength

“…The presence of two peaks at double the Bragg wavelength was, as noted above, attributed to the alignment of the FBG writing system [11], and this might be why the peak/dip spacing at twice the Bragg wavelength changed from 0.3 nm in a stationary writing system [12] to 0.45 nm when a scanning method was used, despite all gratings being written with the same phase mask and fiber type [22]. Indeed the alignment of optical components in FBG fabrication systems has been shown by other authors to be responsible for small changes in spectra near the Bragg wavelength.…”

“…This removes the potential complication of higher-order mode effects on the spectral details and thus, for a standard FBG, a single narrowband peak is expected. However, in previous work by the authors and others there are a number of examples where different harmonics of the Bragg wavelength have two closely spaced peaks despite the fiber being singlemoded, while a single peak is observed at the Bragg wavelength [11,15,16]. It has been proposed that this double-peak structure arises from the interleaved π out-of-phase grating planes of periodicity Λ pm (as discussed above), which produces a type of π-phase-shifted FBG in which the normal reflection peak has a dip at its center due to reflections from neighboring planes being cancelled [12].…”

“…Indeed FBGs resonances in phase-mask-produced gratings have been observed at double the Bragg wavelength [11,12] and at several of the harmonics of either the Bragg wavelength or twice the Bragg wavelength, namely 2λ B =3, λ B =2 and 2λ B =5 [10,13]. On the other hand, when just the 1 diffraction orders were present in an FBG writing arrangement involving a Talbot interferometer, features at twice the Bragg wavelength (expected near 1550 nm for Λ pm 536 nm) were not observed [14].…”

“…Interestingly, Dyer et al [11] in summarizing their experiments (in which they reported on damage gratings using 193 nm radiation in fiber of unspecified diameter), noted the occurrence of either a single peak or a double peak at 2λ B in different gratings. They attributed the double peak to fiber misalignment, noting that a 1 mrad angle between the phase mask surface and the fiber produced a separation between the two peaks of ∼1 nm, meaning that the Talbot planes were not co-linear with the fiber axis.…”

“…They attributed the double peak to fiber misalignment, noting that a 1 mrad angle between the phase mask surface and the fiber produced a separation between the two peaks of ∼1 nm, meaning that the Talbot planes were not co-linear with the fiber axis. They conjectured that this produced a Fabry-Perot cavity in which a transmission maximum occurred at the grating reflectance peak since certain grating planes of periodicity Λ pm were π out-of-phase with similar planes elsewhere along the fiber core [11]; such a condition will not occur for perfect alignment. This and the previous descriptions are virtually equivalent, as standard π-phase-shifted gratings are a Fabry-Perot effect arising from two distinct grating sections along the fiber core that are shifted in phase with respect to each other, and which usually, through a large value of the DC refractive index change, have a very narrow passband in the middle of the grating's transmission dip [17].…”