Strong resonance and a highly compact long-period grating in a large-mode-area photonic crystal fiber

A strong resonance and extremely short length long-period grating (LPG) has been fabricated in a large-mode-area photonic crystal fiber (PCF) by use of a CO2laser heat source. We believe that such a long-period grating in pure silica PCF is the first example of a point-by-point techniquc. The fabrication method is simple and repeatable. The resulting LPG has been developed with eight periodic collapses within a 2.8-mm-long period of the fiber, which gives the strong resonance of core-cladding mode coupling. The lowest mode of LP01is at a 1529.2-nm wavelength with a full width at half-maximum of ∼0.7 nm and a resonance strength of -31.5 dB. The principal advantages of this LPG are that (1) it is temperature insensitive and stable, (2) the device is compact when it is packaged, and (3) it provides practical, low-cost all-fiber filters and PCF-based devices for optical fiber communications and sensing systems.