According to the law of Beer-Lambert,a new dual-loop gas concentration measurement system based on the fiber loop ring-down spectroscopy technology is proposed.The gain was controlled by adding an auto gain feedback loop of Erbium Doped Fiber Amplifier(EDFA),therefore,ring-down time is prolonged and measuring accuracy is improved.Influence of Amplified Spontaneous Emission(ASE)Noise of EDFA on measurement accuracy was studied.A method of dealing with experiment data using Adaptive Filter was bring forward.T...