# Integrated Silicon Photonics for Enabling Next-Generation Space Systems
## 4 Space Radiation Effects
### Integrated Silicon Waveguides
#### 4.2.3 Waveguides: Single-Event Effects  
SEEs in silicon waveguides were theoretically proposed to be a potential source of concern for space systems [80]. An energetic particle can generate a dense cloud of free carriers within the waveguide such that light traveling through this cloud can be absorbed through free-carrier absorption. Figure 10 shows the refractive index change and the absorption profile for different densities of induced EHPs within the waveguide for 1550-nm light [81]. Heavy ions that generate a substantial amount of EHPs can increase the absorption coefficient of the waveguide by several orders of magnitude, which can disrupt the optical signal. The induced EHP density also impacts the refractive index, but  
Figure 9: Example of a typical, silicon-integrated waveguide (after [76]).  
the change is too small and the lateral charge track is too narrow, to produce any severe phase effects.  
This theory was tested experimentally by inducing a dense cloud of EHPs in a waveguide through the two-photon absorption process using a high-energy laser pulse [76]. It was confirmed that when free carriers are present within the waveguide, a fraction of the optical signal is depleted. Figure 11a shows the transient results from an optical signal passing through an induced cloud of EHPs. The y-axis shows the AC-coupled portion of the PD current induced by this perturbation measured via a bias tee. Note that with increasing pulse energy (i.e., the larger density of induced EHPs) the transient peak increases. A collection of transient peaks with respect to laser energy is shown in Figure 11b. This is consistent with the theoretically predicted results by [PERSON] et al. [80]. Historically, a subset of SEEs termed single-event transients (SET) have been observed in electronic parts. In this case, however, the transient exists purely in the optical domain, where no charge collection is taking place and is distinctly different than any other SET. Thus a new type of SEEs that encompasses the present phenomenon is needed, and is termed the optical single-event transient (OSET). An OSET is defined as the \"perturbation of optical power within an integrated optical component that strictly exists in the optical domain and does not affect the functionality of any electronics that precede it\" [76].  
Due to experimental limitations with the TPA process, a maximum transmission loss of \(\sim\)15% was measured. However, simulation results from [PERSON] et al. [80] and [PERSON] et al. [76] predict that 100% transmission loss is possible. This loss can be detrimental to optically modulated data during an ion strike, leading to bit errors and instantaneous drops in a communication link.  
Figure 11: (**a**) Measured OSETs induced by 2.7, 3.1, and 3.4 nJ laser pulse. (**b**) Extracted OSET peak magnitudes across the laser pulse energy that was used to induce the transients (after [76]).  
Figure 10: Simulated refractive index and absorption coefficient of an integrated waveguide for various electron-hole pair densities (after [81]).