Study on several processing schemes for optical polarization shift keying modulation format

Research and development based on optical networking technologies are growing very rapidly. It is expected that the evolution of broadband services will further accelerate the demand for capacity in optical communication networks with higher channel bit rate. The demand of high transmission capacity and better system reliability has lead to the recent research in spectrally efficient advanced optical modulation formats. As modulation format is one of the most important factors in the system, the choice of the suitable modulation format in transmission system is required. With significant advantages, one of the advanced modulation format, polarization shift keying (PolSK) signal offers promising alternatives to the existing modulation formats for future long haul transmission system. However, there are very few research works have been done so far regarding PolSK signal. Hence, it is worth investigating the several processing about this modulation format. This thesis presents our study about several processing about PolSK signal. Several processing like polarization-insensitive widely tunable wavelength conversion, simultaneous multichannel wavelength conversion and transmission performance investigation using optical phase conjugation for PolSK signal are experimentally investigated. For future high capacity all-optical network based on PolSK, wavelength conversion, a key technique, will be very much required. In this work, a fiber-based polarization-insensitive and widely tunable all-optical wavelength conversion for PolSK signal is experimentally demonstrated by means of four-wave-mixing (FWM) in highly nonlinear fiber (HNLF). In our scheme, preservation of state of polarization (SOP) of input PolSK signal is possible on both of the converted spectral components in FWM spectrum which is not possible otherwise in conventional scheme. Polarization-insensitivity is one of the features of the proposed wavelength converter. This scheme is compared with the conventional wavelength converter regarding polarization sensitivity issues. Wavelength tunable operation with high conversion performance is also achieved. Processing on simultaneous multichannel wavelength conversion using a single device is another very important function for future photonic network. In realistic situation, multichannel signal to the converter have different channel-group delay and state of polarization (SOP), since these signals come from different nodes and are transmitted to different links in the photonic network. In this situation, performing simultaneous multichannel wavelength conversion using a single device leads to performance degradation of the converted channels. Hence, in multiwavelength converter, it is very useful to mitigate such degradation and convert simultaneously multichannel signals with equal performances. Using our proposed polarization-insensitive wavelength converter, the wavelengths of four 10-Gb/s PolSK channels are simultaneously converted without any performance degradation for the converted channels. We successfully achieved high conversion performances for all the converted channels even though the input PolSK channels have different delay time and SOP. Investigation on transmission performance against the dispersion is another important issue for the photonic network. We experimentally investigate the transmission performance against dispersion for PolSK signal. Optical phase conjugation (OPC) is one of the attractive methods for improving the transmission performance. However, employing OPC is very difficult with the conventional wavelength converter using this modulation format. In the current research, the optical phase conjugator is based on our proposed polarization-insensitive wavelength converter. This wavelength converter can preserve the input signal SOP at the wavelength converted spectral component which is phase conjugated. Hence it can be used as an optical phase conjugator for PolSK signal which is not possible otherwise in conventional wavelength converter. Moreover, the proposed optical phase conjugator is polarization-insensitive. Therefore, employing the proposed optical phase conjugation scheme will increase the signal robustness for PolSK signal in long haul transmission system for future photonic network. The results of the current research provide some new finding for the PolSK modulation format. For PolSK transmission system in future photonic network, the current experimental investigations and results will be very useful.