High-Speed Optical Transceivers for Data Centers

Today's data centres, which underpin software such as cloud computing, artificial intelligence and machine learning, rely on optical transceivers to move information between servers. Hundreds of thousands of such transceivers are installed yearly. Despite this being a mass market, assembly of electronics and optics into a transceiver is still a step-by-step process, which makes it difficult to increase throughput and drive down cost. The EU-funded Caladan project is harnessing micro-transfer printing technology to move from sequential assembly to a manufacturing process in which thousands of transceivers are assembled in one parallel step.
At the transmit side, a Mach-Zehnder Modulator (MZM) driver IC is designed to be compatible with the micro transfer printing process by limiting its footprint to a mere 200µm by 300µm. Such a small footprint allows for dense integration with the long (typically a couple of mm) MZMs implemented in imec’s silicon photonics platform (iSiPP).
However, innovation at the device level only is not sufficient to support the increasing demands for more bandwidth. System level developments towards coherent communication links are hampered by the need for power hungry DSP to correct the phase error between the received signal and the LO at the receiver. One way to completely mitigate this problem is by detecting the phase difference between subsequent symbols (by way of a delay line interferometer), instead of detecting the phase with respect to a fixed reference laser (LO). Analysis shows that optical systems using Differential Phase Shift Keying (DPSK) offer a viable alternative to LO-based coherent systems.