Generation of pulse-position modulated signals having high extinction ratios via an IQ modulator

We propose and experimentally demonstrate a method for generating M-ary pulse-position modulation (M-PPM) signals having high extinction ratios (ERs). The method utilizes a dual-path structure, wherein a data-driven Mach-Zehnder modulator (MZM) is embedded in one path and a phase shifter in the other. When the two paths are recombined with a relative π-phase shift, the resulting ER can exceed that of the embedded MZM. We implement the proposed structure using a commercially available in-phase and quadrature (IQ) modulator and generate a 100-Mb/s 256-PPM signal. The dynamic ER of the generated signal is measured to be greater than 40 dB. We achieve a receiver sensitivity of 4.49 photons per bit (PPB) at a target BER of 10^-3 using the generated 256-PPM signal and an optically pre-amplified receiver. This result is only 2.33 dB above the theoretical limit for optically pre-amplified 256-PPM.