Wide-passband, temperature-insensitive, and compact π-phase-shifted long-period gratings in endlessly single-mode photonic crystal fiber

We demonstrate what is believed to be the first fabrication of a wide-passband, temperature-insensitive, and compact spectral filter based on a π-phase-shifted long-period grating in an endless single-mode photonic crystal fiber. By introducing a π-phase shift in the middle of a 12-period long-period grating, two symmetrical rejection bands at wavelengths of 1252.65 and 1418.7 nm are obtained with isolation higher than 18 dB and a passband bandwidth of 84.15 nm. The π-phase-shifted long-period gratings are inscribed by the relaxation of mechanical stress with the focused pulses of a CO 2 laser and a point-by-point technique without geometric deformation. The length of the spectral filter is ∼6.6 mm with a sensitivity of 8 pm/°C at a medium temperature range of 23-190°C.