The ever-growing demand for precision, speed, and scalability in high-energy physics (HEP) experiments has driven the development of sophisticated data acquisition and processing systems. At the heart of this revolution lies optoelectronics, a technology that has transitioned from niche applications to becoming a cornerstone of modern particle physics research. From the early adoption of fiber optics to cutting-edge modular solutions, the evolution of optoelectronics is enabling researchers to unlock the mysteries of the universe.
Early Optical Links in HEP Experiments
The pioneering deployment of optoelectronic links for data transmission in a particle physics experiment dates back to the SLD detector at the SLAC Linear Collider in the late 1990s [1-2]. This implementation used commercial fibre optic components like vertical cavity surface emitting lasers (VCSELs) and p-i-n photodiodes to transmit readout data at 960 Mbps. The SLD detector operated at fairly low radiation levels, enabling the use of optoelectronic devices not specifically designed for radiation tolerance. This initial demonstration nevertheless built confidence in applying the emerging technology of fibre optic data links to meet the high bandwidth readout needs of HEP experiments.
A major early milestone came with the CDF detector at the Tevatron proton-antiproton collider at Fermilab [3], which operated from 2001-2011. The CDF collaboration implemented close to 600 optical links based on edge emitting laser diodes to transmit tracker readout data across long lengths over 300 meters. This represented the first large scale deployment of optical links in a collider experiment with significant radiation exposures up to 2 kGy. The project required designing special laser diode drivers and receiver components hardened for the radiation tolerance required. It also provided an early lesson on the need for thorough reliability assessment and monitoring. Despite excellent radiation resistance results from initial qualification tests, unanticipated device failures emerged after installation from factors like humidity ingress [4]. Adaptations introduced later in the mission enabled successful operation with acceptable failure rates, demonstrating the feasibility of optoelectronic links but also revealing potential pitfalls on the road to reliability.
Optoelectronics in the Era of the HL-LHC
The High-Luminosity Large Hadron Collider (HL-LHC) exemplifies the leap in optoelectronic technology. The HL-LHC will generate unprecedented amounts of data, requiring optical links capable of transmitting terabits per second. These links must also operate reliably in radiation zones exceeding 1 MGy (megagray) over a decade.
In this regards, modular architectures enable easy upgrades and replacements, reducing downtime during maintenance cycles. For instance, the Versatile Link+ system, developed for the HL-LHC, uses modular components to optimize performance and maintainability. As mentioned by Stefano Meroli, responsible for the design and deployment of the optical fibre cable plant of the project:
Modular systems allow researchers to assemble customized solutions from standardized components, simplifying the integration process. This approach also enhances scalability, enabling experiments to grow in complexity without requiring a complete redesign of the data acquisition system.
For example, in optical data links, modularity allows the separation of radiation-sensitive components from less critical ones. By placing sensitive elements in shielded zones, researchers can improve overall system reliability while maintaining performance. Analogous to building blocks, these systems can be easily adapted for future upgrades, ensuring long-term viability.
Emerging Requirements for Next-Generation Systems
The versatile link paradigm aims to satisfy current LHC upgrade requirements using mostly incremental technology advancements like faster VCSELs and silicon photodiodes. But looking beyond the 2030s horizon to proposed future colliders like the 100 TeV FCC-hh [14], even more daunting demands arise. Projections indicate raw data output may reach exabytes per second from detectors operating in extremely high radiation environments with fluences up to 10 times HL-LHC levels approaching 2×10^17 neq/cm2 behind the collision point [15]. Sustaining optoelectronic links in such conditions likely exceeds the limits of current technologies architected primarily for data communications applications rather than extreme irradiation. However promising technology trends suggest potential solutions if focused R&D efforts commence to adapt emerging capabilities for HEP’s unique constraints.
Silicon photonics ranks among the most exciting developments on the horizon for optical links in HEP experiments [16]. This approach utilizes high volume CMOS manufacturing infrastructure to integrate photonics components like light modulators alongside electronics onto silicon wafers. The ability to produce “photonic integrated circuits” offers unprecedented customization to optimize parameters for HEP applications like radiation tolerance, power efficiency, and cost [17]. Intriguing possibilities like wavelength division multiplexing also facilitate aggregating front-end data without separate ASICs.
Beyond customized component solutions, emerging high speed optical interconnect architectures could also address projected bandwidth demands. Multi-level modulation schemes already prevalent in datacenters, like PAM4 encoding, can potentially double throughput over NRZ signaling without increasing line rates [18].
Optical mode division multiplexing leverages multi-core fibers to gain similar capacity enhancements. Most applicable for longer reach links between detector areas and control rooms, novel hollow core and plastic specialty fibers better maintain signal integrity across kilometers of cables [19]. Used in combination, such advanced modulation formats, multiplexing architectures, and specialty fibers should help sustain 10s to 100 Gb/s per link speeds over extended reaches.
Finally, size, mass, and power reduction also rank among major goals to facilitate detector designs with high channel density circuit partitioning. Fortunately the broader electronics industry is likewise pursuing heterogeneous integration of photonics alongside ASICs. Also called co-packaged optics, these technologies promise to co-locate driver electronics within centimeters rather than meters of optical engine components [20]. Direct coupling promises gains in size, mass and power efficiency – all crucial metrics for dense tracking detectors.
As data rates continue climbing, the lengths over which electrical signals can travel without excessive distortion or power droop will keep decreasing. Hence the ultimate vision for future low mass modular detector elements relies critically on advances in heterogeneous optoelectronic integration.
References
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[8] J. Troska et al., “Lessons learned and developments from two generations of Versatile Link optical modules for data transmission in high energy physics experiments,” PoS, vol. TWEPP2021, p. 058, 2022.
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[12] P. Moreira et al., “The GBT-SCA, a radiation tolerant ASIC for detector control and monitoring applications in SLHC experiments,” in Topical Workshop on Electronics for Particle Physics, 2007, vol. 0, pp. 342–346.
[13] The Versatile Link Project collaboration, The Versatile Link Common Project: twiki.cern.ch/twiki/bin/view/Main/VTRx
[14] Future Circular Collider Study collaboration, “Future Circular Collider Conceptual Design Report Vol. 2”, European Phys. J. Spec. Top., vol. 228, pp. 619-1228, 2019.
[15] J. Ebr for the FCC collaboration, “Central detector technologies for the FCC-hh,” Nucl. Instru. Meth. Phys. Res. A, vol. 985, p. 164607, 2021.
[16] SilPho Consortium collaboration, https://silpho.eu/
[17] M. Zeiler et al., “Radiation hardness assurance testing of custom designed silicon photonics,” J. Instrum., vol. 16, no. 12, Dec. 2021.
[18] O. Ozolins et al., “CMOS Analog Front-End Circuits for PAM Optical Links in HEP Experiments,” IEEE Trans. Nucl. Sci., vol. 67, no. 7, pp. 1473–1480, 2020.
[19] M. G. Pulvirenti et al., “Hollow core fibres for high-energy physics experiments photon detection,” J. Phys. Conf. Ser., vol. 1071, no. 1, 2018.
