A fiber in-line, multimode coupling interferometer with a trench-embedding, fiber taper probe is proposed and fabricated by femtosecond-laser-induced water breakdown. The reflection-type taper probe is used for gas refractive index (RI) detection from 1.0001143 to 1.0002187 and temperature sensing from 50 degrees C to 500 degrees C. The largest RI sensitivity of the taper probe embedded with a trench at a width of 18.4 mu m is 669.502 nm/RIU for hybrid nitrogen and helium. Temperature sensitivity is 9.97 pm/degrees C and it shows good linearity through the whole testing range. The new-type multimode interferometer is appropriate for high-accuracy gas RI detection of micrometer-scale spaces and wide-range temperature compensation can be realized. (C) 2014 Optical Society of America