|Title:||All-optical shutter and polarizer using Fabry Perot laser diode|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Electronic and Information Engineering|
|Pages:||xix, 173 leaves : ill. ; 30 cm|
|Abstract:||Optical networks utilizing fast packet switching are expected to provide the required capacities and flexibility in the next generation high-speed optical networks. A variety of all-optical switching devices using nonlinear interferometers and wavelength conversion have been reported. Here an all-optical shutter using the mutual injection-locking principle of a Fabry Perot laser diode (FP-LD) is demonstrated. When a input signal with a particular wavelength detune and relevant power is injection-locked close to one of the modes of the FP-LD it experiences a mode-shift. When two signals are injection-locked by a FP-LD, the control signal can switch data ON and OFF because of the mode-shift effect. In the absence of the control signal, data is switched ON and in the presence of the control signal, data is switched OFF. An extinction ratio of 16.9 dBm at the switched 10 Gb/s data-rate was obtained for a 3 dB difference in the power of the control signal. The switch-on and switch-off time was reported to be 60 ps and 50 ps respectively and hence the data is switched within one bit period. Real-time automatic polarization stabilization is crucial to the deployment of all-optical switches, add-drop multiplexers, polarization-multiplexed systems, and coherent detection systems in optical networks. Most of the polarization control schemes proposed to date comprises of a polarization rotation unit and a complex feedback control unit. Polarization control is carried out either mechanically (e.g., fiber squeezers/paddles, MEMS) or electro-optically (e.g., liquid crystal or electro-optics crystal and PM-segment fibers). We demonstrate that a mutual injection-locked laser diode functions as an all-optical polarization controller and can be used to re-polarize 10 Gb/s polarization-scrambled NRZ signals. The mutual injection-locked laser diode controls the state-of-polarization (SOP) of the input signal, not by rotating its SOP, but by functioning as an intensity- compensating polarizer. Thus neither complicated SOP characterization nor a speed-limiting feedback control process is required. We further demonstrate how a single mutual injection-locking laser diode can simultaneously re-align the polarizations of two polarization scrambled 10 Gb/s signals. We have also identified a region on the Poincare sphere where the all-optical polarization controller will not be able to compensate for the fluctuating polarization states. This occurs when the TE component of the injected power is less than the threshold power required for injection-locking. The parameter region where the all-optical polarizer fails to work can be quantified as an error region on the Poincare sphere. This error region is directly proportional to the square root of the wavelength detune, and indirectly proportional to the input power and the FP-LD operating current. Hence the error region can be minimized by decreasing the wavelength detune, and increasing the injected power and the operating current of the FP-LD. We have successfully demonstrated the simultaneous re-polarization of two 10 Gb/s polarization scrambled signals using the mutual injection-locking principin. The wavelength detunes and the injected power were chosen in such a way that the two channels injection-locked individually as well as together. Two modulators were used to produce independent data channels. A CW stabilizer signal assisted in the locking and functions as stabilizer by suppressing the free running laser when both the data signals are '0'. The effect of cross talk on the signals was noticed and operating parameters were chosen in order to avoid cross talk.|
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