Proceedings Article | 1 March 2013
KEYWORDS: Monochromatic aberrations, Diffraction gratings, Fiber Bragg gratings, Diffraction, Laser beam diagnostics, Wavefronts, Beam splitters, Reflectivity, Zernike polynomials, Fringe analysis
The effect of aberrations present in the recording beams of a holographic setup is discussed regarding the period and
spectral response of a reflecting volume Bragg grating. Imperfect recording beams result in spatially varying resonant
wavelengths and the side lobes of the spectrum are washed out. Asymmetrical spectra, spectral broadening, and a
reduction in peak diffraction efficiency may also be present, though these effects are less significant for gratings with
wider spectral widths.
Reflecting Bragg gratings (RBGs) are used as elements in a variety of applications including spectral beam combining1,2,
mode locking3,4, longitudinal and transverse mode selection in lasers5,6, and sensing7,8. For applications requiring narrow
spectral selectivity9, or large apertures10, these gratings must have a uniform period throughout the length of the
recording medium, which may be on the order of millimeters. However, when using typical recording techniques such
as two-beam interference for large aperture gratings and phase-mask recording of fiber gratings, aberrations from the
optical elements in the system result in an imperfect grating structure11-13. In this paper we consider the effects of
aberrations on large aperture gratings recorded in thick media using the two-beam interference technique. Previous
works in analyzing the effects of aberrations have considered the effects of aberrations in a single recording plane where
the beams perfectly overlap. Such an approach is valid for thin media (on the order of tens of microns), but for thick
recording media (on the order of several millimeters) there will be a significant shift in the positions of the beams
relative to each other as they traverse the recording medium. Therefore, the fringe pattern produced will not be constant
throughout the grating if one or both beams have a non-uniform wavefront. Such non-uniform gratings may have a
wider spectral width, a shifted resonant wavelength, or other problems. It is imperative therefore to know what the
effects of aberrations will have on the properties of the RBGs. Thus, in this paper we consider the imperfect fringe
pattern caused by the recording beams and its effect on the diffraction efficiency and spectral profile of the recorded
reflecting volume Bragg gratings.
