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.
