A modified method for determining the focal ratio degradation and length properties of optical fibres in astronomy

Abstract

Focal ratio degradation (FRD) is a major contributor to throughput and light loss in a fibre spectroscopic telescope system. We combine the guided mode theory in geometric optics and a well-known model, power distribution model (PDM), to predict and explain the FRD dependence properties. We present a robust method by modifying the energy distribution method (EDM) with f-intercept to control the input condition. This method provides a way to determine the proper position of the fibre end on the focal plane to improve energy utilization and FRD performance, which lifts the relative throughput up to 95% with variation of output focal ratio less than 2%. And this method can also help to optimize the arrangement of the position of focal-plane plate to enhance the coupling efficiency in a telescope. To investigate length properties, we modified PDM by introducing a new parameter, focal distance f, into the original model to make it available for multi-position measurement system. The results show that the modified model is robust and feasible for measuring the key parameter d 0 to simulate the transmission characteristics. The output focal ratio in the experiment does not follow the prediction trend but shows an interesting phenomenon that the output focal ratio increases at first to the peak, then decreases and remains stable finally with increasing fibre length longer than 15m, which provides a reference for choosing appropriate length of fibre to improve the FRD performance for the design of the fibre system in a telescope.

Characterization of multi-clad fibers: approaches to measure the beam divergence angle and its degradation in the fiber

“…Based on geometrical optics, 2α F should be the minimum possible ring width, labeled "no FRD" in Figure 11. The results show a quite stable ring width for the cores of both fibers according to Carrasco [16], but a generally decreasing ring width with increasing α S for the light-guiding layers 2 and 4 in both fibers according to Yan [4]. Correlation to the very different core diameters is weak in this data.…”

“…In addition, this model [16] showed a dependence of the FRD-Parameter D~1 (core diameter) 2 ⁄ . Contrary to that, Yan [4] showed experimental data with decreasing ring width at incident angles between 49 mrad and 167 mrad. To compare these models with our results, Figure 11 shows the ring width β0 - βi and 2α F = 28 mrad for comparison for all investigated light-guiding layers.…”

“…A common method to investigate FRD of fibers is the far field method with selective angular excitation also known as "laser injection method" by Carrasco [16], "inverse farfield method" by Ferwana [10] "parallel laser beam" by Haynes [17] and "collimated beam" by Yan [4] and "ring test" by Belland [2].…”

“…The light's angular distribution will be changed by the fiber itself leading to a degradation of the focal length, called focal ratio degradation (FRD). This topic had been studied for decades leading to an improved understanding of several causes [1], [2], measurement methods [2] and models for description [3], [4].…”

Inverse spatially‐offset Raman spectroscopy using optical fibers: An axicon lens‐free approach

“…In such case, both meridional (preferably due to the cone shown in yellow color) as well as skew (outside the yellow colored cone) rays contribute and the resulting output at the fiber‐exit is of circular shaped with almost uniformly illuminated everywhere. Furthermore, in case of an angulated focusing (shown in Figure C), the contribution of meridional rays rapidly decreases and most of the light is guided through the skew rays which results in an annular or ring‐shaped beam at the fiber‐exit .…”