Asymmetric laser gating for robust low power half-duplex optical link in camera-on-tip surgical endoscopes

Camera-on-tip surgical endoscopes for cardiovascular imaging capable of real-time hyperspectral imaging require high speed optical communication links that operate within the strict power and footprint constraints at the distal end of a surgical guidewire. To address these constraints a half-duplex reflective link was previously demonstrated in which a Surface Normal Electroabsorption Modulator (SNEAM) served as a dual-functional optical transceiver at the distal tip requiring 5 V bias voltage, which is nevertheless incompatible with low power CMOS driver technology. Additionally, practical half-duplex operation requires time coordinated gating of the up- and down-stream channels. Adopting direct laser gating introduces thermally induced wavelength chirp, producing a time-varying power penalty that erodes the available link margin. This increases the possibility of corrupted camera image frames in the upstream channel and risks misdiagnosis. In this work, a SNEAM with an optimized Fabry-Perot (FP) cavity has been designed and fabricated. The device achieves ∼10 dB extinction ratio with a 3 V pp swing and the operating bias is reduced to below 3 V, with ∼1 mW device power consumption at the distal end. A novel asymmetric gating strategy is proposed and experimentally validated, in which only the downstream laser is gated while the upstream laser operates continuously. Error free downstream operation (BER < 10 -9 ) was experimentally validated at 0 dB downstream to upstream optical isolation. This asymmetric gating strategy improves link robustness against variable bending loss with only ∼0.2 mW additional static power overhead during the downstream time slot.