Cascaded‐Gain‐Amplified Fiber Microcavity Laser With Attomolar Detection Limit

ABSTRACT Optical fiber microcavities have attracted considerable interest for their potential in developing multi‐color microlasers and high‐performance sensors. Owing to their high Q‐factor‐enhanced light‐matter interaction, fiber microcavity lasers have been extensively investigated for sensitive biodetection. Nevertheless, it is challenging to detect minute variations in the analyte‐correlated emission against the dominant collective lasing output, hampering the exploration of the ultimate detection limit. Here, we report an ultrasensitive fiber microlaser biosensor with cascaded submonolayer gain on a telecom optical fiber. The cascaded gain from specific bioconjugation improves the laser slope and signal‐to‐noise ratio, thereby lowering the detection limit. The cascaded‐gain‐amplified fiber microlasers achieve a detection limit down to 271 aM for avidin detection, which shows 6000‐fold and 50‐fold improvements compared to the monolayer and submonolayer microlasers, respectively. This microlaser biosensor is then applied to detect the tumor biomarker programmed cell death ligand 1 (PD‐L1), reaching a detection limit of 7.14 pg/mL and a dynamic range of more than three orders of magnitude. This work highlights an ultrasensitive and single‐use sensing technology, showing great potential for applications in disease diagnostics and therapy evaluation.