Moving towards polarisation-independent monolithic PICs
As datacentres race towards 200G per lane, polarisation-dependent loss and differential group delay remain significant hurdles in high-speed optical receivers. GlobalFoundries researchers have made significant progress in addressing these challenges.
BY YUSHENG BIAN, MASSIMO SORBARA, WON SUK LEE, SUJITH CHANDRAN, TAKAKO HIROKAWA, ABDELSALAM ABOKETAF, KEVIN DEZFULIAN, RYAN SPORER, KEN GIEWONT AND TED LETAVIC, GLOBALFOUNDRIES
To cope with ever-growing bandwidth demands, datacentre communication applications are transitioning to 200G PAM-4 transmission per optical wavelength (200G per lane), doubling the symbol rate to 106.25 GBaud compared with 53.125 GBaud found in 100G-per-lane systems [1, 2]. However, in 200G-per-lane systems, managing the significant impact of polarisation-dependent loss (PDL) and differential group delay (DGD) on optical receivers is increasingly challenging, particularly in optical interconnection schemes deploying monolithic silicon photonic technology.
Figure 1. GlobalFoundries FotonixTM platform monolithically integrates advanced photonic devices and CMOS components on the same SOI substrate [3].
To facilitate seamless integration between photonic and CMOS components with an optimised process flow (Figure 1), the waveguide layers incorporated into the monolithic silicon photonic platform are typically thinner than those in standalone photonic technologies [3-10]. However, this thinning can result in non-negligible birefringence and PDL, affecting both waveguides and other functional devices. As most optical transmitters are optimised for transverse electric (TE) operation – as opposed to transverse magnetic (TM) operation – PDL becomes pronounced in optical receivers due to the rotating state-of-polarisation in the fibre optic cable. In high-speed data communication systems, it is critical to minimise the net optical path loss and PDL to optimise the system’s overall signal-to-noise ratio (SNR) performance.
Figure 2. (a) Schematic illustration of silicon nitride edge coupler in a
monolithic optical receiver. (b)-(c) Statistical yield distributions of
the TE and TM ILs at 1310 nm.
Alongside PDL, significant DGDs between TE and TM signals arise from birefringence and polarisation mode dispersion in waveguides and other building blocks within the monolithic optical receiver [11]. These DGDs typically surpass 50 percent of a symbol interval in 200G-per-lane transmission, resulting in self-induced inter-symbol interference (ISI) out of the photodetector (PD) output current.
This leads to further closure on the eye diagram and shifting of the optimal sampling of the received PAM-4 signal. This article presents recent advancements in enhancing the functionality of critical monolithic building blocks deployed in an optical receiver front-end, with a primary focus on reducing insertion losses (ILs), net PDL as well as DGD compensation. The performance enhancements are achieved by optimising both the design and processes to improve the balance between the TE and TM paths in a receiver. Combining all improvements, we effectively demonstrate a nearly polarisation-independent receiver circuit with a PDL of only 0.38 dB and negligible DGD, utilising monolithic silicon photonic components.