1 Department of Photonics Engineering, Technical University of Denmark2 Fiber Optics, Devices and Non-linear Effects, Department of Photonics Engineering, Technical University of Denmark
The nonlinear process of four-wave mixing (FWM) enables coupling of energy between wavelengths. This is useful for both optical amplification and wavelength conversion. A crucial prerequisite for the process is phase matching. This PhD project investigates how higher order modes (HOMs) in fibers can be used as an additional degree of freedom to fulfill this phase matching requirement. The design of a specialty few moded fiber is discussed. This fiber allows for FWM between a pump in the Ytterbium gain region with a signal at telecommunication wavelengths, hereby generating a new wavelength around 800 nm. Using pulse propagation simulations this process is investigated in details, which includes examining the impact of the overlap integrals and outer diameter (OD) variations along the fiber. Experimentally, it is demonstrated that using a long period grating (LPG), it is possible to convert 99.8 % of the power from the fundamental mode to a specific HOM in the custom designed fiber. Furthermore, it is demonstrated that stable propagation in the considered fiber is possible, without deterioration from mode coupling. Finally, modulation instability and multiple FWM signal and idler lines are demonstrated in the 1 μm wavelength range in the developed large mode area fiber. This is enabled by operating in the LP07 mode, which has anomalous dispersion despite having a mode area of 618 μm2 at 1064 nm. In the experiments the maximum employed pump pulse energy was 105 μJ. This was restricted by the available laser since the fiber is capable of pulse energies of 540 μJ before the onset of dielectric breakdown. This represents a factor of 12 increase compared to photonic crystal fibers (PCFs) with similar anomalous dispersion characteristics. Also a peripheral result was obtained as a continuum from 680 to 1600 nm entirely in the LP07 mode was demonstrated.