Abstract: An optical fiber coupler is disclosed, which comprises two polished substrates each of which comprises a grooved substrate having a polished surface on the open side end thereof and adapted to hold therein an optical fiber and an optical fiber comprising a core, a cladding and two stress-applying parts arranged in the cladding so as to sandwich the core, a part of the optical fiber being polished off such that the polished surface of the fiber is close to the core so as to allow leakage of light power propagating in the core to the outside, the polished optical fiber being embedded in the grooved substrate and fixed thereto in such a manner that the polished surface of the grooved substrate is in flush with the polished surface of the polished optical fiber, with the two polished substrates being in close contact with each other at the polished surfaces thereof to allow coupling of light power, wherein said polished optical fiber is of a construction such that the polished surface thereof is perpendicular to