Frequency chirp and mode partition induced mutual constraint on the side-band phase noise of a mode-locking WRC-FPLD fiber ring self-started with a lengthened feedback loop

Abstract

The specific mutual constraint effect of frequency chirp and mode partition on the side-band phase noise and timing jitter is theoretically and experimentally analyzed in a harmonically mode-locked weak-resonant-cavity Fabry–Perot laser diode (WRC-FPLD) fiber ring self-started by fiber-coupled optoelectronic feedback at 10 GHz with a lengthened loop. By inserting a 100 m long single-mode fiber into the feedback loop to self-start the mode-locking, minimal single-side-band phase noise of −70 dBc Hz−1 and −125 dBc Hz−1 are obtained at 100 Hz and 1 MHz offset, respectively. The optimized pulse-train exhibits a timing jitter of 0.67 ps and a pulsewidth of 18.5 ps. A significant degradation is observed for feedback loop lengths longer than 200 m due to the enhanced mode-partition noise contributed by the fiber-ring cavity incorporated in the WRC-FPLD. The theoretical model shows that the mutually coupled fiber-ring cavity and the optoelectronic feedback loop could provide a mode-locking pulse with a minimum phase noise of −135 dBc Hz−1 under the compromise of phase noise responses between the fiber-ring cavity and the feedback loop. These two factors hinder the improvement expected from the longer feedback loop and higher Q-value because the mode-partition noise is greatly enlarged when the multi-mode pulse experiences serious dispersion during distant transmission in the fiber. Under such a mutual constraint, the optimized return-to-zero (RZ) pulsed carrier is on–off keying (OOK) non-RZ data by 10 Gbit s−1, showing a high signal to noise ratio of 11 dB to achieve a receiving sensitivity of −19.2 dBm at a bit error rate (BER) of 10−9. When using the RZ pulsed carrier for data transmission at 10 Gbit s−1 in a passive optical network with a dense wavelength division multiplexing channelization of 200 GHz, the RZ-OOK data-stream still exhibits an error-free performance at a receiving power sensitivity as low as −17 dBm, even though a power penalty of 2.5 dB is added into the BER response.