Ericsson and the Photonic System Group of McGill University, led by Prof. David Plant, (https://www.photonics.ece.mcgill.ca/Plant/Plant.html) demonstrated a photonic system-on-chip capable of controlling carrier laser’s polarization changes with time, so enabling the use of cost effective co-packaged optics (CPO) transceivers in radio communication systems.
CPO is an emerging technology to meet the demand for high interconnection speed at low power consumption in AI computing clusters and data centers switches. In a recent Ericsson Technology Review, we explained the potential of CPO technology for RAN applications and discussed the gaps to be fulfilled to make it suitable in this new domain.
CPO technology supports ultra-high bandwidth-density (several Terabit-per-second-per square-millimeter), consuming only a few picojoule per transmitted bit. Moreover, it is based on silicon photonic chips that can operate at the high temperatures (>100°C) that may occur close to the Integrated circuits (ICs) inside a Radio Unit (RU). Unfortunately, this does not hold for the light source, which is indeed put in an external laser source (ELS) unit.
This leads to further challenges since the silicon photonics circuits in the CPO transceiver, as the modulator, are very sensitive to the polarization state of the input light. In data centers, a special kind of optical fiber, called polarization maintaining fiber (PMF), is used to couple an external laser source to the CPO modulator. However, this is not best practice for CPO to be used in radio equipment, where standard optical fiber is highly preferred, due its advantages in system cost and reliability.
Ericsson had already patented and tested an ELS based on two laser sources multiplexed in both frequency and polarization. This makes the emitted light basically unpolarized, so that the average power is equal and almost constant for any modulator input polarization state, so allowing the of use standard optical fiber. This solution has been showcased in a joint demo with Nubis Communication at ECOC 2024 (22-26 September 2024, Frankfurt, Germany), one of the largest conference and exhibition on optical communications worldwide. More details can be found in this previous blog.
However, there are situations where the unpolarized ELS does not apply, as for ring modulators used in certain CPO implementations. Ring modulators exhibit a periodic frequency response alternating peaks and valleys. The unpolarized ELS would work only if the modulator frequency period matched the frequency distance of the two lasers of the ELS, which is seldom the case.
The joint photonic research team of McGill University and Ericsson Research also solved this problem, realizing several prototypes of integrated optical chips that rotate the light polarization state until it aligns with the one accepted by the silicon photonic chip in the CPO. The new chip is capable to measure the light polarization state before rotating it, so minimizing the occurrences of outage conditions that affected similar solutions proposed in the past. Remarkably, the chip has been tested in a 100 Gbps experimental transmission setup, demonstrating the first silicon photonic 1310 nm transmitter that compensates for any injected carrier laser state of polarization over 1 km with 4-level pulse Amplitude Modulation (PAM4), below the Hard Decision Forward Error Correction (HD-FEC) threshold, aligning with IEEE 802.3 Ethernet standards.
Moreover, the showcased technology is not only relevant to ELS but applies to all scenarios where polarization control is mandatory, for example it might simplify the optical frontend of coherent pluggable transceivers.
Ericsson collaboration with the Photonic System Group of McGill University started in 2010 with an NSERC (National Science and Engineering Research Council) and was carried over ever since. Professor Plant is a renowned figure in the realm of photonics with hundreds of commonly cited peer-review articles, operating a top-tier lab with unique capabilities. He was also a recipient of the prestigious Killam Fellowship prize for the 2013-2015 period. Throughout those years, with close collaboration with Ericsson, Prof. Plant and his team have delivered cutting edge designs in silicon photonics, generating many world premiere advanced modulation experiments, thus pushing the limits of technologies and knowledge.
