Analysis and suppression of intermodulation distortion effects in mach - zehnder modulator for analog optical transmission systems

Abstract

The Mach-Zehnder (MZ) interferometer has been extensively used as an electro-optic modulator to overcome the limitations of frequency chirping, phase noise and nonlinearity of intensity modulation in analog radio frequency (RF) transmission over optical fiber communication system. The transfer function of MZ modulator is a sine wave function of the input voltage. For this reason, when multiple carrier frequencies pass through it, intermodulation signal products other than the original frequencies are produced and signal distortion is found in the output. In analog transmission, many channels are allocated close to one another and as a result intermodulation distortion (IMD) in a channel causes interference to other channels. The MZ modulators generate two main IMD terms - composite second order (CSO) and composite triple beat (CTB) which severely limit the performance of the analog optical transmission system. In this thesis an attempt is made to suppress IMD effects in conventional MZ modulator (MZM) and dual parallel MZ modulator (DPMZM). In conventional MZM, an analysis is carried out to derive the expression of CSO and CTB incorporating the parameters like- RF dc bias voltage, input optical phase shift and input signal amplitude etc. It is found that, when the value of optical phase shift is small ( 0.05) and dc biasing voltage (Vb ) is equal to nV (where n  1 2,3 2), the effect of IMD in MZM is minimum. But the performance of the conventional MZM deteriorates in high bandwidth and long haul optical communication system. In such a system DPMZM is used which can handle large amount of power as well as high bandwidth. The performance of DPMZM is also investigated in terms of IMD. Results show that the effect of CSO is minimum, when the DPMZM is operated in quadrature. To reduce CTB effect, the optimum value of power splitting ratio is determined. It is found that when the value of optical power splitting ratio is   0.76 and electrode length ratio is  1.5 , the effect of CTB is minimum. The findings of this thesis may be used to design a high bandwidth and long haul analog optical transmission system.