Fiber loop ring-down spectroscopy with a long-period grating cavity.

We have successfully demonstrated cavity ring-down spectroscopy in a fiber ring with a long-period grating cavity. The cavity couples >99% of light from core to cladding, interacts with the environment surrounding the fiber, and then couples unabsorbed light back into the fiber core. This design provides a relatively large evanescent wave absorption area while maintaining a very low loss for the fiber ring. The effectiveness of this cavity as a sensitive chemical sensor is also demonstrated.