Abstract Asymmetric microhole-structured long period fiber grating fabricated in single mode, fiber and photonics crystal fiber by use of femtosecond laser pulse drilling is, presented. The periodically structured microholes positioned a few microns away, from the central fiber axis can effectively couple the fundamental core mode into the, forward-propagating cladding mode, thus forming a long period fiber grating. The, results obtained show that the asymmetrical-drilling method can effectively improve, the reliability of the grating fabrication when compared with the symmetrical-drilling, method, owing to the increased tolerance in geometric error of laser pulse, micromachining.