We demonstrate an ultralow sample volume optical carbon monoxide sensor with detection sensitivity of $180\mathrm{parts}$ in ${10}^9$ ( $1\sigma$ at $1\mathrm{Hz}$ ). The utilization of a $2.3\mathrm{\mu m}$ surface-emitting laser directly coupled to a $3\mathrm{m}$ hollow capillary fiber as the gas cell is proven to be a compact, sensitive, and cost-efficient gas sensing concept. By mechanical vibration of the fiber, an absorbance resolution of ${10}^{-5}$ is achieved, which is comparable to single-reflective (double-pass) cells. An improvement of sensitivity over the conventional single-reflective cell is thus approximately linearly scaled with the enhancement of the optical path length, which is usually more than 1 order of magnitude.