Comparative analysis of optical IQ modulation in four-channel DWDM system in the presence of fiber nonlinearities

Growing demand for capability, high bandwidth and high bit rate transmission has become an important part of optical communication networks in recent years. The key thing to increase the overall performance is to optimize every network component without increasing the cost per transported information bit. One such way of increasing optical network efficiency is the designing of an optical transmitter. The most of today's optical networks use common intensity modulation techniques. These modulation techniques are easy to implement but they are not suitable for long haul transmissions. Next step further form intensity modulations are advanced modulation techniques. Advanced modulation techniques offer higher spectral efficiency and are more suitable for long haul transmission. This paper describes a coherent optical IQ modulator and IQ demodulator, which allows us to perform DQPSK (Differential Quadrature Phase Shift Keying) modulation. The quality of the transmitter and receiver is the key thing in designig any communication system. The quality of the transmitter output signal has a critical influence on the overall system performance. To transmit more and more data, designers has to come up with the new modulation and detection techniques. DQPSK is considered to be high-order modulation technique, which allows us to modulate data more precisely. In this paper we present simulation model of 4-channel DWDM system with three flavors of DQPSK modulation — NRZ-DQPSK (Non Return to Zero DQPSK), CRZ-DQPSK (Chirped Return to Zero DQPSK) and CSRZ-DQPSK (Carrier Suppressed Return to Zero DQPSK). Performance of each flavor of DQPSK is evaluated using BER (Bit Error Rate) and Q-factor measurement.