Testbed for coupling starlight into fibers and astrophotonic instruments

ABSTRACT

We assembled a testbed to study coupling of starlight through atmospheric turbulence via astronomical telescopes into astrophotonic devices. The setup allows for varying the turbulence strength and investigating the effects of different levels of adaptive optics correction on the efficiency of integrated optics. In addition to recording optical powers and wavefront errors, focal plane images are captured from which spots sizes and Strehl ratios are also measured. Novel astrophotonic components proposed as alternatives to conventional optical instruments can therefore be qualified in terms of coupling efficiency and throughput on the testbed before they are tested on the sky.

1. INTRODUCTION

Astrophotonic instruments offer an alternative to conventional free-space optics where setups behind astronomical telescopes can be kept at manageable sizes and weights and are therefore less costly and easier to environmentally control. Astrophotonic devices manipulate, i.e. split, filter, disperse, and interfere, starlight via structures of contrasting refractive indices inside waveguides contrary to bulk optics that reflect or refract light at air/glass interfaces.1, 2 This integrated approach reduces the size manyfold and cuts reflection losses common to optical trains. Some astrophotonic technologies like planar beam combiners,3 discrete beam combiners (DBCs)4 and fiber Bragg grating (FBG) OH suppression filters5 will only function as intended if their waveguides sustain a single, fundamental mode. Others like arrayed waveguide gratings (AWGs)6 will have their performance impaired as the number of modes at their intake waveguides increases. The most pressing challenge to photonic devices is efficient coupling into them from free space. For ground-based telescopes, starlight arrives at the entrance pupil with a distorted wavefront due to Earth’s atmosphere. This translates into a speckle intensity pattern at the focal plane that poorly couples into the commonly Gaussian fundamental mode of single-mode waveguides. Adaptive optics (AO) can help improve coupling by correcting the incident wavefront to produce a point spread function (PSF) that is more favorable to waveguides but the quality of correction, i.e. the residual error in the resulting wavefront, depends on the number of actuators, the bandwidth of the control loop, and the brightness of the guide star among other factors. Therefore this correction, in most practical scenarios, is only partial and the theoretical maximum coupling into the single-mode regime cannot be achieved.

Increasing the number of modes at the input waveguide can boost the coupling efficiency even if only a low-order adaptive optics (LOAO) system of ∼ 100 degrees of freedom is employed. This presents a bargain to certain types of astrophotonic devices that can still operate on an input beam of few modes without compromising performance greatly. Alternatively, the few-mode beam in the input waveguide can be converted using a photonic lantern into a number of single-mode beams, where if the numbers of modes and single-mode fibers (SMFs) are matched, the conversion is lossless.7 A multiplexed astrophotonic instrument that has multiple replicas of the same photonic device can then be fed by the SMFs to recover most, if not all, of the flux collected by the telescope.

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