Birefringence and Bragg grating control in femtosecond laser written optical circuits

Abstract:

In this thesis, femtosecond lasers are explored for the fabrication of fiber Bragg gratings (FBGs) in suspended core fibers (SCFs) as well as direct writing of integrated optical devices in bulk fused silica glass.

The FBGs fabricated in suspended core fibers with different core geometries were demonstrated with femtosecond laser exposure through a Talbot interferometer. In this case, the use of a femtosecond laser system was crucial as it eliminates the need for the use of photosensitive fibers, which is the case for SCFs, while the Talbot interferometry setup provided flexibility in the definition of the grating periodicity, while simultaneously protecting the optical components used in the fabrication process from the high intensities reached during exposure in the proximity of the fibers. These Bragg gratings were employed to show simultaneous strain and temperature sensing.

Using a femtosecond laser direct writing system, novel point-by-point fabrication arrangements, with detailed attention to the computer controlled laser beam modulation, were developed in order to correctly introduce modulation of the refractive index profile during the waveguide fabrication process. This technique enabled the development of phase and frequency control required for advanced Bragg grating waveguide (BGW) fabrication and arbitrary spectral shaping. Procedures were demonstrated for the fabrication of chirped and phased shifted BGWs for applications in temporal pulse shaping and spectral shaping that showed significantly improved feature resolutions for sensing applications.

The BGWs were used as a practical sensitive tool to determine and study the waveguide birefringence inherent to the nonlinear absorption processes typical of femtosecond laser-material interaction. The control of form and stress birefringence was developed in order to accomplish the fabrication of integrated optical components for polarization control, like guided wave retarders and polarization beam splitters. Tuning of this waveguide birefringence was achieved by the introduction of stress inducing laser modification tracks that enabled the ability to both enhance or reduce the inherent birefringence. Characterization techniques were developed for the absolute determination of the birefringence based on BGWs spectrum splitting, together with crossed polarizer measurements, while novel data analysis procedures were demonstrated for the study of polarization dependent and polarization independent directional couplers with the introduction of a polarization splitting ratio which is wavelength and coupling length dependent.

All of the improvements made in the understanding of waveguide birefringence control provided increased flexibility to simultaneously fabricate low polarization mode dispersion circuits, as well as more efficient and compact polarization dependent devices. The polarization aspects detailed here, together with the point-by-point fabrication system, may be further developed in the future towards the fabrication of more complex integrated devices that combine spectral, temporal, and polarization control, all achievable with the same femtosecond laser writing system. These flexible processing techniques will open new directions for writing additional functionalities in optical circuits with more compact three-dimensional geometries.


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