Optical nonlinearities in high power fiber lasers and phase-sensitive amplifiers: novel modeling tools and experiments

My work consists of two main areas. Both focus on experimental and theoretical investigation of phenomena originating from optical nonlinearities in state-of-the-art devices: high power fiber lasers and amplifiers in the first part and phase-sensitive amplifiers for telecom applications in the second part. For rare earth doped fiber lasers and amplifiers with output powers of up to several kilowatts, it is becoming increasingly important to predict inherent nonlinear optical mechanisms for deeper understanding and to facilitate manufacturing. During simulation of such high power devices, computing time can be an issue. First, a detailed, fast converging iterative numerical method is presented to model continuous-wave, bidirectionally pumped ytterbium-doped (Yb-doped) fiber lasers with output powers of several hundreds of watts. The analysis allows of demonstrating the nonlinearityinduced broadening of the spectrum, which modifies power efficiency as well. Using the proposed method, spectral properties are calculated within few minutes using commercial desktop computers and very good agreement with measurements is obtained for up to 708 W output power. The achieved computing time is over 1 order of magnitude below the previously published ones with other numerical methods. For high power fiber amplifiers with novel fiber structures, development of up-to-date tools for mathematical modeling have similarly high importance. Here, an iterative numerical approach is introduced to model continuouswave high power cladding-pumped Yb-doped fiber amplifiers with bidirectional pumping of the star-shaped cladding. Accurate prediction of over 1 kW output power is demonstrated, and the calculation results are in good agreement with experiments. Using the model, furthermore it is shown that the Giles effective overlap integral model parameter in the star-shaped cladding is not a constant but depends on the numerical aperture of the seed laser. While nonlinear optical effects can be parasitic as well, they are successfully exploited e. g. in nonlinear optical crystals for telecom applications. In this work, the first all-optical black-box phase regenerator for binary phase shift keyed signals is demonstrated, constructed from periodically poled lithium niobate (PPLN) nonlinear waveguides. Phase regeneration is performed with different levels of phase noise and constant amplitude noise added to the phase modulated input. The relationship between the signal-to-pump power ratio at the regenerator input and the unwanted phase-to-amplitude noise conversion is determined by experiments. Both PPLN waveguides and fiber-optic parametric amplifiers (FOPAs) are potential candidates as in-line phase-sensitive amplifiers (PSAs) as well. Their performance comparison for multichannel application is convenient, however to date it is not realized. Here a numerical model is introduced to calculate and compare crosstalk between multiple channels of in-line PSA chains built of both kinds of devices. The results show that PPLNs have generally better crosstalk performance for higher channel number as a consequence of the reduced inter-channel crosstalk due to their finite quasi-phase matching band, while FOPAs are better for lower channel number. Channel interaction is investigated in PPLN PSAs experimentally as well. Simultaneous phase squeezing of signal channels with 200 GHz, 100 GHz and 50 GHz frequency spacing in a PPLN PSA is demonstrated. It is observed that additional channels have little impact on the phase squeezing performance, which shows again that PPLNs can be advantageous for multichannel PSA applications.