A distributed optical fiber gas sensing system (100) based on evanescent field sensing, spatially resolved along the fiber (105, 200, 300) and obtained by employing differential absorption lidar techniques is described. The fiber (105, 200, 300) can be arranged in a plurality of ways, and could have a porous cladding (202) sensing fiber (105, 200, 300) as the sensing unit. The sensing fiber (105, 200, 300) has a solid core (201) to guide light and a porous cladding (202) to protect the fiber (105, 200, 300) from physical damage and to enlarge the interface of the evanescent field and the test gas. A pulsed or modulated laser (103), a 3-dB optical coupler (104) and a photomultiplier tube (PMT) (107) form a typical OTDR sensing system. The pulse generator (102) is to control the pulse width, period, duty cycle of the pulsed laser (103). The data acquisition card (106) is to record the amplitude of the reflected light detected by the PMT (107). The controller (101) which usually is a computer is for signal recor
