For realizing compact breath sensing device, we have proposed a silica high-mesa waveguide for the gas-cell (breath detection part) because of its low propagation loss. It is, however, still difficult to make breath-sensing due to its total insertion loss because the required length of the waveguide reaches very long of approximately several 10 cm-1 m for the small portion of gas, and thus gas-sensing has not been achieved so far. To compensate the insertion loss, we utilize "amplified" CRDS (cavity ring down spectroscopy) technique to realize gas-sensing in this paper. As a result, we could successfully confirm the gas-sensing of CO 2 with using the waveguide gas-cell for the first time.

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