FBGs in Specialty Optical Fibres for Astrophotonics

Abstract

Astrophotonics is a beautiful combination of the knowledge of photonics and astronomy. It offers the astronomy community unprecedented access to nanoscale light manipulation. In this thesis, I utilized established astrophotonics techniques such as photonic lanterns, and extended the capability of one specific technique - the AFBG OH suppression filters, into the astronomy J band. The achievements can be summarized as follows.

- I explored the inscription of (A)FBGs into two types of 7-core fibers — trench-assisted and non-trench 7-core fibers. The results showed a slight reduction in wavelength offsets in the trench-assisted fibers, which may be attributed to the suppression of core-to-core coupling by the low-index trench. This will require further investigations. We plan to develop a Sagnac-type inscription rig to enable a core-by-core multicore FBG inscription method.

- I proposed a complete design procedure for OH suppression AFBG filters, including detailed analysis of the OH Lambda doublets, detuning curve of the inscription setup and simplified optimization methods. I applied the procedure to the first J-band OH suppression AFBG filters and presented the fabrication results. These demonstrated the potential for AFBGs to be applied to any wavelength band within the J and H bands.

- I proposed a detailed optimization method for the OH suppression AFBG design using artificial neural networks. The optimization method utilized a neural network based forward solver and a genetic algorithm. This novel approach offers the advantage of efficient implementation, requiring reduced computational time and resources, while yielding optimization outcomes of comparable quality to previous methods.

- I built and tested the first J-band OH suppression unit consisting of 3 J-band AFBGs. This will be the beginning of the construction of a J-band OH suppressed spectrograph for celestial Positronium detection.