Photonic lantern behaviour and implications for instrument design

Abstract

Photonic lanterns are an important enabling technology for astrophotonics with a wide range of potential applications including fibre Bragg grating OH suppression, integrated photonic spectrographs and fibre scramblers for high resolution spectroscopy. The behaviour of photonic lanterns differs in several important respects from the conventional fibre systems more frequently used in astronomical instruments and a detailed understanding of this behaviour is required in order to make the most effective use of this promising technology. To this end we have undertaken a laboratory study of photonic lanterns with the aim of developing an empirical model for the mapping from input to output illumination distributions. We have measured overall transmission and near field output light distributions as a function of input angle of incidence for photonic lanterns with between 19 and 61 cores. We present the results of this work, highlight the key differences between photonic lanterns and conventional fibres, and illustrate the implications for instrument design via a case study, the design of the PRAXIS spectrograph. The empirical photonic lantern model was incorporated into an end-to-end PRAXIS performance model which was used to optimise the design parameters of the instrument. We describe the methods used and the resulting conclusions. The details of photonic lantern behaviour proved particularly important in selecting the optimum on sky field of view per fibre and in modelling of the instrument thermal background.Comment: 18 pages, 14 figures, presented at SPIE Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation 2014, to be published in Proc. SPIE 915

First demonstration of OH suppression in a high-efficiency near-infrared spectrograph

“…For efficient performance the multimode input of each photonic lantern must be fed at the correct focal ratio to excite approximately 19 modes. In fact the throughput of a photonic lantern is a non-trivial function of the input focal ratio, but has been well characterised empirically (Horton et al 2014). Consideration of this throughput dependence is of central importance in the design of the fore-optics, which will be discussed next.…”

The photonic lantern

“…Thus photonic lanterns have applications wherever there is a need to convert light between single-mode and multimode systems, especially if the distribution of light between output states (cores or modes) is unimportant. One example is in astronomical instrumentation, where the multimode light delivered by a telescope needs to be spectrally-filtered by single-mode fibre Bragg gratings [1,3,[10][11][12]18,21,23,25,26,28,31,37,39,41,49,54,64,66]. Another is in optical telecommunications, where individual channels of information delivered by single-mode fibres need to be multiplexed via orthogonal combinations of modes in a common few-mode fibre [20,33,35,36,40,42,43,[46][47][48]52,53,57,58,67,[69][70][71][72][73][74][75][76][77][78][79].…”

“…This means that the path lengths can be dramatically different within a single device, making mode scrambling very efficient. The effect of mode scrambling has been observed in MM-SM-MM lantern pairs made by both techniques [32,34,63,64].…”

Seeking celestial positronium with an OH-suppressed diffraction-limited spectrograph

“…The optimum field of view therefore depends on the site conditions (i.e. seeing), and the level of adaptive optics correction, as well as the throughput of the photonic lanterns, which is a function of injected focal ratio [25]. The best S/N is achieved by injecting the maximum target light into the smallest possible field of view, so minimising the night-sky contamination of the signal.…”