A compact and stable interferometer is easily built only with fusion splices. The air-holes of a microstructured fiber are intentionally collapsed in the vicinity of the splices and this broadens the propagating optical mode, allowing coupling from core to cladding modes. The transmission spectrum is sinusoidal and of single frequency, indicating predominant interference between the fundamental core mode (7) and a cladding mode (6). A regular interference spectrum can be observed from 650 nm to 1600 nm with fringe visibility reaching 80%. The fringe spacing is inversely proportional to the distance between the splices. This behavior has a significant impact in optical sensing and communications and so the interferometer can be applied for strain sensing. The device comprises two splices (5) of a microstructured optical fiber (1), said splices (5) determining two regions in which the air-holes (4) are collapsed, separated a length (L) along which said two modes are excited.