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fiber optics

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Today most optical fibers are based on silica glass technology. Silica fibers are characterized by their low losses (the power is reduced by a factor of 2 when the light has propagated through 15 km) and in addition low nonlinearity (the refractive index changes approximately 10 10^-3 pr meter pr watt of power). However, the low loss makes it possible to accumulate a nonlinear phase shift. In fact, the nonlinearity has been used through the last decade in optical communication systems, for example to counterbalance the group velocity dispersion and in Raman amplifiers. In addition, it is also important to understand the impact of nonlinearities to optimize the performance of a communication system - to mitigate the effects of nonlinear cross phase modulation Four-wave mixing, etc.
In conventional fibers, the nonlinearity mainly arises from the doping of the core with germanium. However, we are currently interested in exploring other ways of achieving high nonlinearity in fibers. One of the ways being investigated is using fibers made from soft glasses. These soft glass fibers are made from chemical compositions such as Chalcogenide or Tellurite. The soft glass fibers have a greater nonlinearity than conventional silica fibers but currently also suffer from a much greater loss. Work is currently being conducted to reduce the loss of these fibers which would make these fibers very attractive for nonlinear applications. Instead of using soft glasses, another possibility under investigation is focused on filling hollow-core photonic crystal fibers with highly nonlinear gases and liquids, thereby increasing the nonlinearity of the fibers. One of the goals of this investigation is to find the right filling of the fibers so that we can achieve a high nonlinear effect with a short piece of fiber. This in turn will make it more feasible to incorporate in a compact and practical device in contrast to the conventional fiber, which requires several kilometers of fiber to achieve the same nonlinearity.
One interesting application of nonlinear effects in optical fibers is phase sensitive amplifiers for telecommunication. Even though current systems can carry massive amounts of data, the demand is rapidly increasing and the capacity will soon be exhausted. New phase sensitive coding relying on phase sensitive optical amplifiers can be used to further increase the capacity. These amplifiers are based on parametric amplification in nonlinear fibers. We are investigating optical parametric amplifiers to understand the physical processes but also in collaboration with the Systems Group at DTU Fotonik, their use in complete optical communication systems.
By utilizing the nonlinearity to compensate for the anomalous group velocity dispersion, it is possible to propagate a soliton pulse in a fiber. If the pulse is spectrally wide, meaning a small temporal pulse (ps - fs regime), the Raman effect will shift the soliton pulse towards longer wavelengths as the lower wavelength components will amplify the higher wavelength components of the pulse. To have conservation of momentum a dispersive wave is generated at lower wavelengths in the normal group velocity dispersion regime as a response to the shift of the soliton. The effect is used to generate light at other wavelengths where no laser source exists. Currently a lot of work is put into obtaining the correct nonlinearity and group velocity dispersion to obtain the desired light generation at a specific wavelength.