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Application of a tunable near-infrared Erbium doped fiber laser to continuous-wave (cw) cavity ring down spectroscopy (CRDS) is reported. The laser is widely tunable in the spectral region of 1542-1600 nm by temperature tuning with a laser linewidth of $\sim$ kHz. This ultranarrow linewidth is highly desirable for efficient cw laser coupling to a high-finesse ring down cavity} \underline{\textbf{41}}, 6980 (2002).}. Cavity ring down spectra of N$2$O and C$2$H$2$ have been recorded near 1.544 $\mu$m. Measurements of N$2$O concentration have been performed as well with high dynamic range varying from 100 % purity to some tens ppb with a minimum measurable absorption coefficient of about 10$\mathrm{<mrow\; data-mjx-texclass="ORD">-10</mrow>}$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. Preliminary results will be presented.