Coupling light into optical fibres near the diffraction limit

1. Coupling light into optical fibres near the diffraction limit Anthony Horton & Joss Bland-Hawthorn Anglo-Australian Observatory Ground-based and Airborne Instrumentation for Astronomy SPIE Orlando May 2006

2. Overview • Why is the near diffraction limited case different from seeing limited? • Single-mode fibres • Few-mode fibres • Fibre modes • Initial results: • Coupling efficiency, optimal F/ratio, F/ratio dependence, De-centre sensitivity, wavelength dependence • Next steps • Summary

3. Why any different? • Short answer: Astrophotonics • Longer answer: • Astrophonic devices such as OH suppression fibres and Integrated Photonic Spectrographs have the potential to revolutionise astronomy. • However many potentially useful photonic devices have been conceived as single-mode, meaning they cannot be fed with the multi-mode devices commonly used in astronomy. • Astrophotonics therefore provides a motivation to move away from multi-mode fibres where possible, and this is easiest done near the diffraction limit.

4. Single-mode fibres • Can we simply use single-mode fibres instead? • Single-mode fibres are already in use in astronomy, for optical interferometry. • However it is hard to efficiently couple light into SMFs: • ~80% maximum coupling efficiency with a clear circular aperture telescope. • ~70% maximum coupling efficiency with a central obstruction of 20% of the primary diameter. • Efficiency is further reduced in proportion to Strehl ratio when there are aberrations present. • On large telescopes SMFs aren’t practical without AO, and even then they place strong demands on AO performance.

5. Few-mode fibres • While some important astrophotonic devices were conceived as single mode, they can in general be made to accept at least a few modes: • OH suppression fibres must suppress the lines separately for each mode, this can be done either with a single, more complex AFBG or by using multiple AFBGs and few-to-single-mode convertors. • Integrated Photonic Spectrographs can be made to work with a few modes, at least for moderate resolutions. • We expect coupling to get easier with more modes. • FMFs may allow the integration of astrophotonic devices and efficient coupling without stretching AO capabilities.

6. Fibre modes

7. Max. coupling efficiency NA = 0.1, wavelength = 1.5 microns

8. Optimal F/ratio NA = 0.1, wavelength = 1.5 microns

9. F/ratio dependence NA = 0.1, wavelength = 1.5 microns

10. De-centre sensitivity NA = 0.1, wavelength = 1.5 microns

11. Wavelength dependence NA = 0.1, focal ratio optimised for 1.6 microns

12. Next steps • Use simulated atmospheric phase screens with partial correction to investigate the dependence of FMF fibre coupling on the level of AO correction and seeing as a function of the number of modes. • Determine the number of modes needed for acceptable throughput levels for a range of realistic usage conditions. • Consider both image plane and pupil plane coupling (pupil plane preliminary results already obtained) and lenslet arrays of various geometries to determine the best approach for FMF integral field spectroscopy.

13. Summary • New astrophotonic devices provide a strong motivation to move away from MMFs. • SMFs, while ideal for the astrophotonic devices, are difficult to couple light into and make strong demands on AO performance. • Few-mode fibres are a potential compromise, offering easier coupling at the expense of device complexity. • Initial diffraction limit results show high coupling efficiencies and MMF-like behaviour with only a few (~10) modes. • Work is continuing to study more realistic situations with imperfect AO correction.