Optical Modulation And Performance Monitoring In Optical Communication Systems

In recent years, there have been increasing demands for Internet bandwidth, which sets new requirements for the transmission capacity and switching functions of the backbone and access networks. In the area of optical communication, a variety of new technologies, equipments, and subsystems are developed to meet the requirements, which leads to a new generation of optical communications. In this paper, we focus on improving the optical performance for the future optical communications, and pin in the optical modulation formats and optical performance monitoring in optical communication systems. The major research achievements are summarized as follows:Modulation format tunable transmitter: Optical transmitter is one of the fundamental modules in optical communication systems. It translates the electrical signal into optical signal and launch it into the fibers. Usually, a transmitter generates a specific data format and is limited to its specific application. To adapt to different applications, it is preferential to generate various formats using one transmitter in a flexible manner. In chapter 2, we design a modulation format tunable transmitter based on dual-parallel Mach-Zehnder modulator. The experiment results show that the duobinary, return to zero-alternate mark inversion (RZ-AMI), and Manchester format can be generated by the proposed transmitter, and it is very flexible to switch between these different formats. On the base of the generation of these formats, it is evident that the signal coding process, such as plus, subtract and XOR operation can be realized in optical domain by the dual-parallel Mach-Zehnder modulator.Frequency offset monitoring for DPSK system: Differential phase shift keying (DPSK) is one of the formats commonly used in new generation optical communications. Using balanced receiver, DPSK signal has fixed threshold, therefore it has better tolerance for power fluctuations in comparison with the OOK signal. To properly operate a DPSK system, it is essential to align the wavelength of the delay interferometer with that of the optical source because a slight misalignment results in large performance degradation. In chapter 3, we proposed a method for frequency offset monitoring using perpendicularly polarized optical carrier. The proposed method has the following advantages: it only requires low speed components for monitoring setup and has high sensitivities especially for small frequency offset. The monitored DC current is nearly linear with the frequency offset, which is a good feedback signal for adjustment of the delay interferometer.In-band OSNR and optical spectrum monitoring: Optical signal to noise ratio (OSNR) is one of the parameters to estimate the signal quality in transmission system. However, in reconfigurable wavelength division multiplexed (WDM) networks, each data channel may go through different physical path. Although the out-of-band noise can be suppressed by the filtering effect of the multiplexing/demultiplexing components, it is the in-band amplified spontaneous emission (ASE) noise that determines the optical OSNR. In chapter 4, we propose and demonstrate a novel in-band optical OSNR and spectrum monitoring technique based on swept coherent detection. The experimental results shows that accurate measurement of the OSNR in a wide range of 16～30 dB can be achieved, and the maximal monitoring error is less than 0.5 dB. With the coherent detection technique, we also realized an in-band ASE spectrum signal spectrum monitoring. The in-band ASE noise spectrum, which reveals the pass-band shape of the filters, can be used to determine the misalignment between the central frequency of the signal and the filters.Gated signal RF spectral analysis technique: In WDM transmission systems, the signal polarization states are random because of the inter-channel cross-phase modulation (XPM) induced polarization scattering effect. As a result, the polarization-assisted methods such as polarization-nulling technique, are not effective any more when they are used for OSNR monitoring. Therefore, an OSNR monitoring method that is not affected by polarization impairments is desirable. In chapter 5, we propose and demonstrate a gated signal RF spectral analysis technique, which takes advantage of the periodic nature of the frame header of synchronous traffic. The experimental results show that our method is robust to fiber nonlinearity induced polarization rotation in WDM systems. In the OSNR range of 10-27 dB, the monitoring error of the proposed method is less than 0.8 dB. In the mean time, we also verify the effectiveness of the proposed scheme for a two-packet interleaved system. The monitoring error is less than 0.6 dB in the OSNR range of 10～26 dB.