Structural long period gratings made by drilling micro-holes in photonic crystal fibers with a femtosecond infrared laser.

We present a new method for fabricating structural long-period gratings (LPGs) in photonic-crystal fibers (PCFs). The method is based on periodically drilling holes into the PCF cladding along the length of the fiber by use of a focused femtosecond infrared laser. A very short LPG with only 9 periods and a grating length of < 4 mm exhibited resonance strength of over 20 dB and a polarization dependent loss of 25 dB. The high resonance strength is attributed to the strong modulated mode-field profile caused by the significant perturbation of the fiber geometry. The mechanism of LPG formation is discussed based on coupled local-mode theory.