We fabricate and demonstrate strain-insensitive and high-temperature long-period gratings in endlessly single-mode photonic crystal fiber by use of focused pulses of a CO2 laser and a periodic stress relaxation technique without geometrical deformation and elongation of the fiber. The thermal dependence of mode coupling at 1299.59 nm is 10.9 pm/degrees C from 24 to 992 degrees C, whereas the coefficient of strain sensitivity is -0.192 pm/muepsilon up to the maximum strain of 2.74%epsilon. It is found for what is believed to be the first time that, in contrast with the traditional fiber case, the coupling resonance shifts toward shorter wavelengths under applied strain, indicating that the refractive index of the core is decreased as a result of the rebuilding of tension attributed to the stress-elastic effect, and the cladding modes is highly dispersive because of airholes arranged in the fiber cladding.

We demonstrate the strain-insensitive and high-temperature long-period gratings in photonic crystal fiber. Thermal dependency of resonance wavelength is I0.!9pdC at range from 24OC to 992*C, whereas coefficient of strain sensitivity is -0.192pdp up to 2.74%~.

We demonstrate what is believed to be the first fabrication of a wide-passband, temperature-insensitive, and compact spectral filter based on a pi-phase-shifted long-period grating in an endless single-mode photonic crystal fiber. By introducing a pi-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 pi-phase-shifted long-period gratings are inscribed by the relaxation of mechanical stress with the focused pulses of a CO2 laser and a point-by-point technique without geometric deformation. The length of the spectral filter is approximately 6.6 mm with a sensitivity of 8 pm/ degrees C at a medium temperature range of 23-190 degrees C.

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We demonstrate the first fabrication of highly-compact and temperature-insensitive phase-shifted and multiple phaseshifted long-period gratings using the focused pulses of CO2 laser with point-by-point technique. B y introducing the 3.ir/2, ,i; or 'r/2 phase shifts in the different location of the gratings with 480 im of a period-length and total number about 16-21 periods, the symmetrical and asymmetrical pandpass filters based on those gratings can be obtained. The temperature sensitivity offormed gratings is about 8 .8 pml°C at a temperature range of 23°C -190°C. Such fiber grating component could be used as an all-fiber oscillator in the laser cavity. Long-period gratings with different phase shifts are also investigated with various notches of resonance attenuations. All gratings fabricated are compactness to be packaged into the fiber-optic based devices.

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