This paper presents a novel sensor design and application of long period fiber grating (LPFG) for detection of methane. A styrene-acrylonitrile nano-film incorporating cryptophane A, which is sensitive to methane in close vicinity to the surface, is constructed onto the cladding of long-period grating. For optimal design of the LPFG sensor, the relationship between the resonant wavelength shift and the complex refractive index of sensing film is analyzed based on the coupled-mode theory. The change in refractive index of the sensing film, induced by methane, can easily be obtained as a shift in resonance wavelength. The prepared LPFG sensor with time response of 50 s and good sensitivity (~0.375 nm %(-1)) suitable for the detection of methane below 3.5 vol. % is demonstrated. The response of the sensor (wavelength shift) is linear with methane concentration within our tested range and a detection limit of about 0.2% is estimated for the new sensor.

Long-period fiber grating sensor with a styrene-acrylonitrile nano-film incorporating cryptophane A for methane detection

We proposed and fabricated a long period fiber grating (PS-LPG) for simultaneous measurement of strain and temperature. The is fabricated by inscribing micro-tapers on a pre-tapered fiber using the filament heating method. When the temperature or strain varies, the wavelengths of both transmission notches shift in the same direction, whereas the depths of the notches change in different directions. Therefore, the sensitivity of the PS-LPG can be improved by detecting the depth difference between the two notches, and simultaneous temperature and strain measurements can be achieved from the variation of the wavelength and the depth difference. Experimental results show that the wavelength sensitivities can reach a maximum of respectively −1.5 nm m −1 and for strain and temperature, and the depth sensitivities are as high as −4.6 dB m −1 and −0.06 dB °C−1, respectively.

https://iopscience.iop.org/article/10.1088/2040-8978/17/7/075801/pdf

A phase-shifted long period fiber grating based on filament heating method for simultaneous measurement of strain and temperature

We reported a long period grating (LPG) written in specially designed photonic crystal fiber (PCF) for refractive index (RI) sensing by interrogating the transmitted light power. The outermost ring of clad holes of the PCF is enlarged where the analyte is filled. We showed that the leakage loss of the clad mode increases with the RI in the larger holes. By numerically analyzing the complex couple mode equations for the core mode and the first clad mode, we found the depth of attenuation band in the transmitted spectra and the total transmitted power is sensitive to the leakage loss of the clad mode or the RI in the larger holes. We also demonstrated that the transmitted power is sensitive to the RI even less than that of the silica, which just avoids the limitation that the transmitted light power of LPG in conventional fiber is only sensitive to the RI of the external media higher than that of fiber clad.

/pdf/power-interrogated-refractive-index-sensor-using-long-period-emktzcv6tx.pdf

Power-interrogated refractive index sensor using long period grating in photonic crystal fiber

Characteristics of cladding modes in a photonic crystal fiber (PCF) with triangular air-hole lattice in the cladding are numerically analyzed using a finite element method. The transition for LP 11 cladding mode to core mode with variation of the normalized wavelength has been shown. The transition of the LP 01 cladding mode to the outer silica mode and reorganization of the LP 0 m cladding modes caused by varying the fiber radius has been investigated. By choosing the optimized fiber radius, which is located in the cladding modes’ reorganization region, the sensitivity of the coupled wavelength between the core mode LP 01 and cladding mode LP 03 to surrounding refractive index is increased by a factor of five and reaches to 2660  nm/refractive index unit over the range of 1.40 to 1.42. The sensitivity is competitive with that of long-period grating in PCF in response to changes in refractive indices of the medium contained in the cladding air channels.

Cladding modes in photonic crystal fiber: characteristics and sensitivity to surrounding refractive index

Long period fiber gratings (LPFGs) with different spectral characteristics were fabricated with 1 kHz, 50 fs laser pulses. The contrast of resonant rejection band can be significantly increased by a proper amount of axial stress along a fiber during laser writing or post-processing with lower energy density laser irradiation. Variations of focal condition, pulse energy of laser irradiation and the number of grating periods lead to the generation of resonance rejection band of LPFGs from single-peak to multi-peak plus larger out-of-band loss. The out-of-band loss is primarily caused by Mie scattering from the laser processed cites, and it can be reduced by decreasing the duty cycle of grating pitch instead of lowing down the actual power of laser irradiation.

Characteristic control of long period fiber grating (LPFG) fabricated by infrared femtosecond laser

A sensitive film coated long-period fiber grating with π phase shift introduced at the center is proposed for gas sensing. The sensitivity of the sensor is defined as the rate of change of the transmission at a specific wavelength with the film refractive index. Our analysis shows that it is most favorable to let the coupling strength KL of both parts of the long-period fiber grating be . The sensor has higher sensitivity than a sensor with coupling strength less than and has wider monotonic range than a sensor with coupling strength larger than . The sensitivity of the sensor with coupling strength is qualitatively and numerically analyzed. The sensitivity can be improved by two orders of magnitude by selecting an optimum film thickness. The smaller the index modulation in the fiber core, the higher the sensitivity of the sensor. The sensitivity can be increased to 1.5 times when the index modulation is decreased to 2 times with an optimum film thickness being chosen.

http://iopscience.iop.org/article/10.1088/2040-8978/12/7/075401/pdf

Design of a gas sensor based on a sensitive film coated phase-shifted long-period fiber grating

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Design of a gas sensor based on a sensitive film coated phase-shifted long-period fiber grating