the research presented in this thesis is on the development of a trace gas detection system based on intracavity absorption spectroscopy (icas). the developed system was capable of detecting nitrous oxide (n2o) and acetylene (c2h2) gas at sub-ppmv levels. the p (12) rotational line of n2o at ~1522.20 nm, and the r (5) and r (4) rotational lines of c2h2 at ~1522.22 nm were used for detection. a fiber bragg grating was incorporated into the icas cavity to enhance the system sensitivity by allowing the selection of strong absorption lines for detection. by operating the fiber laser based icas system at threshold, multiple circulations of the amplified spontaneous emission inside the cavity enhanced the system detection sensitivity. further, the laser wavelength contained multi-longitudinal modes, which improved the system sensitivity. the system sensitivity was explored for two intra-cavity gas cells: a multi-pass herriott cell and a gas cell based on a hollow-core photonic crystal fiber (hc-pcf). the system operated at room temperature and a polarization-maintaining erbium-doped fiber was used as a saturable absorber to help eliminate mode hopping in the laser cavity.