Effects of frequency chirping and finite extinction ratio of optical modulators in microwave photonic IFM receivers

Abstract

Frequency-to-power mapping approach based on chromatic dispersion is frequently used in microwave photonic (MWP) instantaneous frequency measurement (IFM) receivers. In this technique, carrier frequency of an unknown microwave signal is estimated according to a specific relationship between the unknown microwave frequency and the ratio of two optical or microwave power functions which is known as amplitude comparison function (ACF). The ACF can be constructed using chromatic dispersion-induced power fading functions of intensity- or phase-modulated optical signals transmitting through dispersive mediums. Practical non-ideal intensity modulators have finite extinction ratio and suffer from frequency chirping. Despite previously published numerous works, the contribution of these parameter in MWP IFMs have not been already studied. In this paper, effects of chirp parameter and finite extinction ratio of intensity modulators on the performance of chromatic dispersion-based MWP IFM receivers are theoretically investigated. It is shown that in general the ACFs of these MWP IFM receivers depend on the chirp parameter of their intensity modulators which is a function of unknown amplitude of the microwave signal whose frequency is intended to be measured and varies with time. This problem cannot be resolved by calibrations and degrades the measurement accuracy. Besides, it is shown that ACFs are different for different finite extinction ratios which degrades the measurement accuracy. Finally, we propose using Mach–Zehnder interferometer (MZI)- and Sagnac interferometer (SI)-based intensity modulators with zero chirp in these MWF IFMs. Analytical results are verified by computer simulations using OptiSystem software.