Speaker
Description
High-capacity optical links are required for the readout of next-generation CERN detectors. These systems must satisfy stringent constraints on radiation tolerance, material budget, and power consumption. Silicon Photonics has emerged as the new industry platform for short-reach optical interconnects, meeting these requirements and offering high levels of integration. In this work, we demonstrate 100 Gb/s data transmission over a single fiber using Silicon Photonic circuits and report on the ongoing development towards co-packaging pixel sensor readout chips with Silicon Photonics chiplets.
Summary (500 words)
Silicon Photonics (SiPh) is a technology that has recently reached a high level of maturity and is now being deployed at scale for data transmission in data centres, high-performance computing, and artificial intelligence. SiPh leverages standard CMOS fabrication processes to manufacture Photonic Integrated Circuits (PICs) in a chiplet format to which optical fibres can be directly attached. Tight co-packaging of these chiplets with detector readout chips (ROCs) offers the opportunity to reduce both power consumption and material budget of the data link by bringing the optical fibers directly to the periphery of the ROC.
We present a 4-channel SiPh transmitter demonstrator achieving an aggregate data rate of 100 Gb/s over a single optical fibre. The demo is based on a chiplet diced from the SystemPIC, a custom-designed PIC developed at CERN. The optical circuit consists of four cascaded micro-ring modulators (MRMs), each operating at 25 Gb/s. The transmitter is based on a wavelength division multiplexing (WDM) architecture in which four independent data streams are multiplexed onto a single optical fibre. Each stream is carried by an individual laser operating in the O-band wavelength range (1271‒1331 nm). In our test bench, the continuous-wave (CW) optical power was supplied to the PIC using commercial off-the-shelf (COTS) external laser sources (ELSs) in a pluggable form factor.
Micro-ring modulators require precise biasing to align their resonance wavelength with the laser carrier, using the thermo-optic effect in silicon. We implemented the thermal tuning of the MRMs using a microcontroller-based system, providing a flexible solution for laboratory characterization and ensuring stable long-term operation under environmental temperature variations. The power-up sequences and control signals used to stabilise the MRMs are presented, demonstrating the effectiveness of this approach in achieving wavelength alignment and maintaining lock across all four channels.
We also present the recent results from the photonic packaging activity, which enabled the assembly of the demonstrators using edge-coupled fibre attachment. In addition, we demonstrate a fully automated optical probing setup for PICs, enabling systematic characterization across production wafers. This approach provides insight into performance variability and supports the selection of good dice for packaging.
We also present the design of a new photonic integrated circuit, PIC4Links, built upon the experimental results of our work to date. PIC4Links was co-designed with SPRINT (Silicon Photonics Radiation-Tolerant Integrated Transmitter) with a matching pad frame that enables direct chip-to-chip wire bonding. The SPRINT integrates a 4 × 25.6 Gb/s micro-ring modulator driver, the necessary thermal tuning circuitry, and a data aggregator for particle sensor readout chips. The SPRINT macro-cell will be instantiated as a serial output block at the periphery of the LA Picopix readout chip. The co-packaging of LA Picopix and PIC4Links will serve as a demonstrator for the use of silicon photonics links in the LHCb VELO Upgrade II detector, where the data rate generated by readout ASICs located close to the interaction point is expected to approach 100 Gb/s. This makes the co-integration of each ROC with a photonic chiplet a highly effective solution.