High-Precision Frequency Locking System Based on Analog-to-Digital Convertor and Asymmetric Mach-Zehnder Interferometer for the Next Generation Coherent PON

In the next generation coherent passive optical network (PON), the local oscillation (LO) laser and the upstream laser in optical network unit (ONU) need to maintain a stable frequency difference with the downstream laser at the optical line termination (OLT). This work designs and packages distributed feedback (DFB) laser chips into transistor outline to provide low-cost light sources. High performance laser control print circuit board makes the wavelength jitter less than ± 0.3 pm, output power greater than 40 mW and Lorentzian linewidth less than 270 kHz, which is suitable for advanced modulation formats. A novel direct frequency locking system based on high-speed analog-to-digital convertor and an indirect frequency locking system based on asymmetric Mach-Zehnder interferometer (AMZI) are demonstrated. Both systems can realize frequency difference stability less than ± 25 MHz between the 1556 nm downstream laser in OLT and LO laser in ONU, which is much more precise than actual system requirement (< ± 100 MHz). By utilizing the periodic transmission frequency characteristics of the AMZI, the 1526 nm upstream laser in ONU can realize a 30 nm ± 25 MHz frequency difference stability with the downstream laser, which is demonstrated for the first time and realize the highest DFB lasers frequency difference locking accuracy of wide wavelength interval (> = 30 nm). This low-cost and high-precision frequency locking system demonstrates excellent potential for application in the next generation coherent PON.