Analytical design of fiber-optic FM/PM demodulator

Abstract

Abstract In this paper, the concept of a novel optical FM/PM demodulator which can be designed from low-cost passive optical components and can have peak-to-peak bandwidth in the GHz region has been described. The bandwidth can be increased by reducing the single mode fiber length incorporated in the design, which is followed by a corresponding fall in frequency sensitivity of the device. The bandwidth and frequency sensitivity can be traded-off keeping their product a constant. The crossover wavelength can be tuned by changing the length of the optical delay line and the length of the single mode fiber (SMF). The Figure of Merit (FOM) of the demodulator, defined as the bandwidth sensitivity product, has a value of π which is greater than that of a passive hybrid Tee microwave discriminator and those of Mach-Zehnder interferometer-based designs. Calculated third harmonic distortion (3HD) of the demodulated optical FM signal increases with the increase in FM index and with the increase in SMF length for a given modulation frequency. Typical 3HD for the device with SMF length of 1 cm is calculated to be −39.9 dB for FM index = 2 and a modulation frequency of 400 MHz.