We recently demonstrated trace detection using Cavity Ring Down Spectroscopy (CRDS) coupled with telecom DFB diode lasers. Our scheme exploits optical feedback from a V-shaped cavity back to the laser. We built trace-gas detectors for CH4 and HF, characterized by a low cost, simplicity, compactness and sensitivity. Operating wavelength are 1.312 micrometers for HF and 1.65 micrometers for methane. The optical setup includes a distributed feed-back (DFB) diode laser, temperature stabilized by a Peltier, a collimating lens, 2 steering mirrors, a V-shaped optical resonator and a photodiode. The V-cavity is made of three low-cost super mirrors R 99.995%) and contains the air sample to be analyzed (20cm3). In standard atmospheric conditions the detection limits for 1 second integration time are of 50 ppbv for HF and 200 ppbv for methane. We present an analysis of the mechanisms of cavity injection and laser feedback, allowing to estimate the influence of various parameters on the performances of this type of apparatus. Calculations and results are given, with particular emphasis on the detection limit and the dynamic range.