<p>Here we report simultaneous measurement of strain and temperature, carried out by incorporating a long-period fiber grating (LPFG) written on high birefringence photonic crystal fiber (HBPCF), referred to as an HBPC-LPFG, and a Faraday rotator mirror (FRM) concatenated in series with the LPFG. The HBPC-LPFG used as a sensor head was fabricated by irradiating CO<sub>2</sub> laser pulses to unjacketed HBPCF with line-by-line technique. The HBPC-LPFG connected in series with the FRM exhibits a polarization-independent wavelength-dependent loss spectrum in its reflection spectrum, which has two resonance dips (RDs) with different cladding-mode orders, designated as RDs 1 and 2. As the two RDs in the fabricated HBPC-LPFG ended with the FRM have dissimilar cladding-mode orders, their strain sensitivities and their temperature sensitivities are different from each other. For the two RDs with resonance wavelengths of λ<sub>1</sub> = ∼1470.5 nm and λ<sub>2</sub> = ∼1495.1 nm, strain and temperature responses were investigated in an applied strain range from 0 to 1790 με (step: 179 με) and an ambient temperature range from 30°C to 95°C (step: 5°C), respectively. The strain sensitivities of the RDs 1 and 2 were measured to be approximately −0.77 and −1.07 pm / με at room temperature, and the temperature sensitivities of the RDs 1 and 2 were measured as ∼10.44 and ∼8.56 pm / ° C without applied strain, respectively. Owing to their linear and independent responses to strain and temperature, strain and temperature changes applied to the HBPC-LPFG can be simultaneously estimated from the measured wavelength shifts of the RDs 1 and 2 using their predetermined strain and temperature sensitivities.</p>
By incorporating an inline filter based on a polarization-diversified loop structure composed of two fiber Bragg gratings (FBGs) with different resonant wavelengths and three quarter-wave plates (QWPs), we propose a dual-wavelength FBG laser that can independently control the output polarization of dual-lasing lines oscillating at two Bragg resonances. At a special combination of the orientation angles of the QWPs, the inline filter can be arranged so that its transmittance depends on input polarization, and it can also select the output polarization states of reflection spectra of two FBGs independently according to its input polarization. For example, their reflection spectra chosen by the inline filter can be linear horizontally polarized at one Bragg wavelength and linear vertically polarized at the other Bragg wavelength, and vice versa. Moreover, linear horizontally or vertically polarized reflection spectra at both Bragg resonances can also be realized. By controlling the QWPs and a cavity polarizer used as an input polarizer of the filter, switching operation among four possible output polarization sets (i.e., two kinds of orthogonal output polarization sets and two kinds of parallel ones at two Bragg resonances) could be implemented in the proposed fiber laser with the measured polarization extinction ratio of each lasing line more than 20.0 dB.