Development of coherent mid-infrared source using chalcogenide photonic crystal fibers

Many molecule bonds have vibration frequencies in the mid-infrared band. Thus, this band is of great interest to molecular spectroscopy, material processing and medical applications. However, many optical materials typically used for laser sources experience high losses due to the presence of such vibration bonds making the design of mid-infrared coherent sources challenging. Such task is even more difficult when considering designing all-fiber sources which offer additional compactness and robustness. The goal of this project is to address such challenge by designing all-fiber mid-infrared sources based on chalcogenide fibers and nonlinear effects. Using degenerated parametric conversion of a 2 um pump and a telecom band signal laser, one can generate an idler in the mid-infrared band.

As a first step, the linear and nonlinear performances of several chalcogenide fibers were compared. The results prove the chalcogenide photonic crystal fiber design to be the ideal platform for this project as it offers high nonlinearity and low loss. Numerical simulations provide further improvement of photonic crystal geometry. Our improved designs allow for an extreme shaping of the dispersion to optimize the four-wave-mixing process within the thulium /holmium fiber laser band.

In the meantime, several cavity configurations were tested to boost the thulium/holmium laser performance, that are required for pumping of our chalcogenide fibers. Polarizer-free cavity designs result more compact laser footprint, which moreover enables a higher slope efficiency. Such lasers enable precise measurements of sample fiber optical parameters.

The combined efforts on fiber and laser optimization leads to efficient continuous-wave parametric conversion. Thanks to the excellent mid-infrared transmittance of chalcogenide glass, this high efficiency can be extended to longer pumps in mid-infrared. However, fiber fuse, a result of heat dissipative solitons, is the main constrain for reaching better conversion efficiency.

For ultrafast applications, a sub-cm length chalcogenide photonic crystal fiber generates flat-top, linearly chirped supercontinuum in the normal dispersion region. This pulse length could be compressed to two optical cycles by linear compression. Once again, the experimental results match perfectly with the simulations.

Overall, we show in this thesis that chalcogenide PCFs are promising for highly efficient and low threshold nonlinear optics in the middle infrared, offering new options for light generation in this critical wavelength band.