Relative contributions of scattering, diffraction and modal diffusion to focal ratio degradation in optical fibres

Abstract

Focal ratio degradation (FRD) is a major contributor to light loss in astronomical instruments employing multimode optical fibres. We present a powerful diagnostic model that uniquely quantifies the various sources of FRD in multimode fibres. There are three main phenomena that can contribute to FRD: scattering, diffraction and modal diffusion. We propose a Voigt FRD model where the diffraction and modal diffusion are modelled by the Gaussian component and the end‐face scattering is modelled by the Lorentzian component. The Voigt FRD model can be deconvolved into its Gaussian and Lorentzian components and used to analyse the contribution of each of the three major components. We used the Voigt FRD model to analyse the FRD of modern astronomical grade fibre for variations in (i) end‐face surface roughness, (ii) wavelength, (iii) fibre length and (iv) external fibre stress. The elevated FRD we observed was mostly due to external factors, i.e. fibre end effects such as surface roughness, subsurface damage and environmentally induced microbending caused by the epoxy, ferrules and fibre cable design. The Voigt FRD model has numerous applications such as a diagnostic tool for current fibre instrumentation that show elevated FRD, as a quality control method for fibre manufacture and fibre cable assembly and as a research and development tool for the characterization of new fibre technologies.

Integrated photonic building blocks for next-generation astronomical instrumentation I: the multimode waveguide

“…Secondly, beyond ~F/10 it can be seen that the output F/# is lower than the injected F/# which means that the light diverges into a larger cone angle at the output than that corresponding to the injected light at the input. This phenomenon is called focal ratio degradation (FRD) and results from the various attributes of the waveguides (such as index profile inhomogeneities, stress at the facets due to polishing, etc) causing light to couple to higher order modes [22]. This unwanted property is present in MM optical fibers as well [22], however unlike MM fibers these waveguides offer a narrow window (F/7.5 to F/10) within which no degradation is observed.…”

“…This phenomenon is called focal ratio degradation (FRD) and results from the various attributes of the waveguides (such as index profile inhomogeneities, stress at the facets due to polishing, etc) causing light to couple to higher order modes [22]. This unwanted property is present in MM optical fibers as well [22], however unlike MM fibers these waveguides offer a narrow window (F/7.5 to F/10) within which no degradation is observed. This is the first time, to the best of our knowledge, that FRD has been explored in optical waveguides.…”

The influence of motion and stress on optical fibers

“…Two, there is not one single source of FRD, but rather several affects of fiber polishing, mounting, and on-telescope application that drive the various causes of a divergent output light cone that we generally refer to under the generic term of FRD. (Haynes et al 2011) …”

“…However, this advantage comes at a cost as fibers constitute an added element in the instrument's optical path. Moreover, optical fibers are not entirely stable light guides, and while their characteristics have been studied in a comprehensive and systematic way by several groups (Ramsey 1988;Schmoll et al 2003;Crause et al 2008; Poppett & Allington-Smith 2010b), little work has been focused on their behavior during periods of motion and accumulated stress, with notable and valuable exceptions (Craig et al 1988;Clayton 1989;Avila 1998;Bryant et al 2010;Haynes et al 2011;Bryant et al 2011). As both current (Bershady et al 2004;Kelz et al 2004;Smith et al 2004;Roth et al 2005;Kelz et al 2006b;Tuttle et al 2008;Wilson et al 2010) and proposed instrumentation (Navarro et al 2010;Saunders et al 2010) is making heavy use of optical fibers, with many instruments requiring repeated motion of the fiber optics, a clear understanding of their properties under these conditions is needed.…”

“…Two, there is not one single source of FRD, but rather several affects of fiber polishing, mounting, and on-telescope application that drive the various causes of a divergent output light cone that we generally refer to under the generic term of FRD. (Haynes et al 2011) However, despite these challenges, several groups have quantified FRD via a number of different techniques. (Ramsey 1988;Craig et al 1988;Schmoll et al 1998;Avila 1998;Carrasco & Parry 1994;Schmoll et al 2003;Crause et al 2008;Murphy et al 2008;Haynes et al 2008;Brunner et al 2010;Poppett & Allington-Smith 2010a) FRD is typically defined as any increase in the output angle (i.e output f-ratio, hereafter f/out) of light when compared to the input angle.…”