The optical transceiver (transmitter and receiver) used for optical-to-electrical conversion is a key component in optical communication systems. In a PON system, an optical transmitter and receiver at the optical line terminal (OLT) or optical network units (ONUs) are typically packaged together to form a bidirectional optical subassembly (BOSA).
The figure below shows the architecture of optical transceiver for an OLT and ONUs. The transmitter optical subassembly (TOSA) consists of a semiconductor laser (Fabry-Perot laser or DFB laser) and a laser driver. The receiver optical subassembly (ROSA) includes a photodiode (PIN or APD), a transimpedance amplifier, a limiting amplifier, and a clock and data recovery circuit. In addition to TOSA and ROSA, a duplex or tripex (WDM filter) is used to separate upstream and downstream wavelengths. The duplex or triplex is usually a thin-film filter, but optical filters (e.g., Bragg grating or a Mach-Zehnder interferometer) based on planar light-wave circuits are becoming a preferred option, as plannar light-wave circuits are more compact, more reliable, and easier to assemble with TOSA and ROSA. As TDM PONs are being deployed on a large scale, significant effort has been put into designing an optical transceiver with improved performance, lower cost, and better reliability. The key challenges in developing optical transceiver for FTTx applications are a higher level of integration, cost-effective packaging, and burst-mode optical transmission technologies in the upstream link. For integration and packaging, optical transceivers are evolving toward plannar light-wave cirsuits and monolithic photonic integrated circuits.