Radiation Damage in Silicon Photonic Mach–Zehnder Modulators and Photodiodes

Abstract

Radiation-hard optical links are the backbone of read-out systems in high-energy physics (HEP) experiments at CERN. The optical components must withstand large doses of radiation and strong magnetic fields and provide high data rates. Radiation hardness is one of the requirements that become more demanding with every new generation of HEP experiment. Previous studies have shown that vertical cavity surface emitting lasers, on which the current optical links are based, will not be able to withstand the expected radiation levels in the innermost regions of future HEP experiments. Silicon photonics (SiPh) is currently being investigated as a promising alternative technology to address this challenge. We irradiated SiPh Mach-Zehnder modulators (MZMs) with different design parameters to evaluate their resistance against ionizing radiation. We confirm that SiPh MZMs with a conventional design do not show a phase shift degradation when exposed to a 20-MeV neutron fluence of 3 • 10 16 n/cm 2. We further demonstrate that custom-designed MZMs with shallow etch optical waveguides and high doping concentrations in their p-n junctions exhibit a strongly improved radiation hardness over devices with a conventional design when irradiated with X-rays. We also found that MZMs withstood higher radiation levels when they were irradiated at a low temperature. In contrast, larger reverse biases during irradiation led to a faster device degradation. Simulations indicate that a pinch-off of holes is responsible for the device degradation. Photodiodes (PDs) were also tested for their radiation hardness as they are needed in silicon photonic transceivers. X-ray irradiation of building-block germanium-silicon PDs showed that they were not significantly affected.

First 10Gb/s Transmission with radiation-hardened Silicon Photonic Mach-Zehnder Modulators in a Full Transmission System

“…The slab thickness of these standard modulators is 120 nm, providing a good confinement of the optical mode in the waveguide rib. Research at CERN [3], [4] showed that such a phase shifter can be made much more tolerant against radiation, if the slab thickness is increased significantly. The downside is a less confined optical mode and a slightly degraded modulator performance.…”

An Electro-Optical Simulation Methodology for the Analysis of Single-Event Radiation Effects in Photonic Devices

“…Until now, radiation effects studies on PICs have been primarily limited to studies of TID [1,3,4,5], however, recent studies have demonstrated that SEEs may also be a significant area of study for PICs [2]. In particular, there is currently a lack of ability to simulate and model the time domain transient effects that occur as a result of an ionizing particle striking an optical component.…”

“…al. [3] Other research has been done in identifying the mechanisms that cause degradation of the performance of a Mach-Zehnder modulator [4] and to model the effects caused by the TID [1]. A limited set of work has studied SEEs in PICs…”

“…An ion striking a Si PN junction causes the generation of free carriers that are swept away by the electric field in the depletion region (left) and causes a current transient at the circuit nodes connected to the PN junction (right) [ The optical absorption coefficients for single-photon absorption (obtained from [4]) and two-photon absorption (obtained from [5] A graph of Si refractive index versus EHP density for the Soref and Bennet charge-toindex conversion model. As EHP density increases in Si, decreases in refractive index are observed.…”

Radiation tolerance enhancement of silicon photonics for HEP applications

“…Previous work has shown that silicon photonics Mach-Zehnder modulators (MZMs) are relatively insensitive to neutron radiation, but show strong degradation with ionizing X-ray radiation. It was further presented, that with customized MZM designs, increasing the rib height of MZMs (shallow-etch device), the resistance against X-ray radiation can be improved [2,3,4,5,6]. Recently, the influence of forward biasing on the radiation hardness was reported and a strong post-irradiation annealing effect was demonstrated [7].…”