A permeable optical fiber (100, 202, 304, 403, 701) for distributed gas sensing is described. The fiber (100, 202, 304, 403, 701) has got core (101, 502, 602) and cladding (102, 501, 601) structure. It can be arranged in a plurality of ways, but could have a permeable polymer cladding (102, 501, 601) to let the gas go inside the fiber (100, 202, 304, 403, 701) and a silica core (101, 502, 602) to guide light. The polymer cladding (102, 501, 601) is also designed to protect the fiber (100, 202, 304, 403, 701) from physical damage. In the optical fibers (100, 202, 304, 403, 701) some small part of light is distributed in the region of cladding (102, 501, 601), and the permeable cladding (102, 501, 601) makes the interaction between surrounding gas the guided light possible, and then the sensing of the gas surrounding the fiber (100, 202, 304, 403, 701) is realized by the laser absorption spectroscopy. The proposed sensing fiber (100, 202, 304, 403, 701) is preferentially used for long distance gas sensing syste
