Silicon-photonics-based ultimate-simplified coherent optics with reduced DC leakage for next-generation optical access networks

Coherent passive optical network (PON) is a promising architecture that offers high sensitivity and support for advanced modulation formats. However, fully coherent systems have high complexity and cost, making them unsuitable for cost-sensitive user-end applications. Additionally, in coherent access architectures, the lasers of optical network units (ONUs) in the upstream are typically kept on rather than being switched off to avoid wavelength drift, with the Mach–Zehnder modulators (MZMs) biased at the null point for modulation. However, the limited extinction ratio (ER) of the MZM introduces direct current (DC) leakage, which degrades system performance and limits the number of users the network can support. To address this issue, we design and fabricate the first silicon-photonics-based ultimate-simplified coherent-optics chip for ONU. The simplified transmitter employs a single MZM combined with a high-speed optical switch (OS), while the simplified receiver adopts a hybrid and a standalone photodetector (PD) for coherent detection with Alamouti coding. By integrating the OS, the upstream achieves a maximum ER of 63.93 dB, while the OS switching speed meets the guard interval requirements of PON systems. Compared to a traditional single MZM with a 26 dB ER, our design achieves an additional 37.93 dB improvement, which significantly increases the user capacity in PON systems. Comprehensive experiments were conducted to validate the proposed design. Ultimately, leveraging this ultimate-simplified coherent-optics chip, a 100 Gbps bidirectional transmission was successfully demonstrated with a power budget exceeding 29 dBm, verifying its feasibility and cost-effectiveness for next-generation coherent optical access networks.