Dynamic Range of Frequency Modulated Direct-Detection Analog Fiber Optic Links

A frequency modulated (FM) analog optical link using an FM discriminator is analyzed. Figures-of-merit are derived, including expressions for the second-order (OIP2) and third-order intercept points (OIP3), spurious-free dynamic range (SFDR) and noise figure (NF). It is found that a discriminator with linear field transmission and constant group delay will produce no third-order or higher-order distortions, and balanced detection will suppress the remaining second-order distortion. Low biasing the discriminator is found not to improve the NF of the link. Residual intensity modulation (IM) of the laser is shown to significantly increase the link's distortion. A discriminator using tenth-order finite-impulse response (FIR) filters implemented in planar lightwave circuits (PLC) is proposed and shown to improve OIP3 by 31 dB over a Mach-Zehnder interferometer (MZI) discriminator.