Long-period Gratings In Photonic Crystal Fiber Research Articles

Transverse-Stress Compensated Methane Sensor Based on Long-Period Grating in Photonic Crystal Fiber

A high-sensitive and transverse-stress compensated methane sensor based on a photonic crystal fiber long-period grating (PCF-LPG) is proposed. The outermost layer of the PCF consists of six large sideholes, five of which are coated with methane-sensitive compound film to achieve methane measurement. Such side-hole structure is helpful for gas sensitive reaction, but not conducive to avoiding external stress interference. Therefore, the last large hole is plated with silver layer to eliminate the cross-sensitivity effect through adding a Surface Plasmon Resonance (SPR) sensing channel with the consideration of photo-elastic effect and material deformation. Results show that the methane gas sensitivity can reach up to 6.39nm/% with the transverse-stress compensation. The sensor is very simple and effective, which provides a new method of gas measurement combined with different actual conditions.

Design and Simulation of Temperature Sensor based on Long Period Grating in Liquid Filled Photonic Crystal Fiber

In this paper, a high sensitivity temperature sensor based on photonic crystal fiber long period grating (PCF-LPG) filled with ethanol is proposed and simulated by full vector finite element method. The relationship between the resonant wavelength shift, and the temperature was analyzed. The results show that the resonant wavelength of the ethanol filled photonic crystal fiber long period grating is proportional linearly with temperature and the highest sensitivity of was achieved, which is 90 times higher than that of conventional LPG temperature sensors.

High-sensitivity photonic crystal fiber long-period grating methane sensor with cryptophane-A-6Me absorbed on a PAA-CNTs/PAH nanofilm.

A high-sensitivity photonic crystal fiber long-period grating (PCF-LPG) methane sensor with cryptophane-A-6Me absorbed on a poly(acrylic acid)-carbon nanotubes/ polypropylene amine hydrochloride (PAA-CNTs/PAH) nanofilm was investigated. The sensing film was coated onto the internal surface of a photonic crystal fiber cladding air holes by an electrostatic self-assembly technique. Based on a finite element method and the coupled local-mode theory, the effects of the sensing film's refractive index (RI) and thickness on the resonant wavelength were theoretically and numerically analyzed. When the sensing film RI decreases from 1.55 to 1.53, and the thickness increases from 100 nm to 200 nm, the resonant wavelength has a blue shift. A higher RI sensitivity with 1.075 × 103 nm RIU-1 is observed for the film thickness of 200 nm. The PCF-LPG methane sensor was fabricated by a pressurized injection method. The sensing experimental result shows that the resonant wavelength of the transmission spectra has a blue shift when the methane concentration increases from 0.0% to 3.5% by volume. The sensor has a good sensitivity of 1.078 nm%-1 and a low detection limit of 0.18% for a film thickness of 210 nm.

Highly sensitive temperature sensor based on an isopropanol-filled photonic crystal fiber long period grating

Abstract A high sensitivity measurement method for temperature has been proposed and investigated based on an isopropanol-filled photonic crystal fiber long period grating (PCF-LPG). Due to the high thermo-optic coefficient (TOC) of isopropanol, the sensitivity of the proposed temperature sensor could be effectively improved by filling isopropanol in the air waveguides of PCF. It can be found that the resonant dip will be split in two dips after filling isopropanol and the two dips have different sensitivities to surrounding temperature. Because of PCF-LPG is sensitive to the refractive index (RI) of internal filled liquid, the isopropanol-filled PCF-LPG temperature sensor has a high sensitivities of 1.356 nm/°C in the range of 20–50 °C. The simplicity and the excellent performance of our proposed device make it potential for the applications of high-precision temperature measurement is required.

Grapefruit photonic crystal fiber sensor for gas sensing application

Use of long period gratings (LPGs) formed in grapefruit photonic crystal fiber (PCF) with thin-film overlay coated on the inner surface of air holes for gas sensing is demonstrated. The finite-element method was used to numerically simulate the grapefruit PCF–LPG modal coupling characteristics and resonance spectral response with respect to the refractive index of thin-film inside the holey region. A gas analyte-induced index variation of the thin-film immobilized on the inner surface of the holey region of the fiber can be observed by a shift of the resonance wavelength. As an example, we demonstrate a 2,4-dinitrotoluene (DNT) sensor using grapefruit PCF–LPGs. The sensor exhibits a wavelength blue-shift of ∼820 pm as a result of exposure to DNT vapor with a vapor pressure of 411 ppb v at 25°C, and a sensitivity of 2 pm ppb v −1 can be achieved.

Photonic crystal fiber long-period grating absorption gas sensor based on a tunable erbium-doped fiber ring laser

It has been a persistent challenge to quantify gas emissions from industrial byproducts and warfare agents for environmental protection purposes, especially to detect multiple gas species using one device with high sensitivity and selectivity. To overcome this issue, a photonic crystal fiber (PCF)-based spectroscopic absorption sensing platform is proposed. We combine the PCF and long-period grating (LPG), two fundamental elements of fiber optics, into one functional unit to investigate the coupling properties of various PCF cladding modes. Further, we synchronize the PCF–LPG with a tunable Er-doped fiber ring laser into a thermo-stable and compact absorption spectroscopic sensor as a proof-of-concept for gas sensing. Because the Er-doped fiber ring laser can be tuned within the C-band wavelength, multiple gases can be recorded with the absorption spectra in one sensing operation. As a demonstration, the sensor was tested in NH3 with multiple gases combined, a sensitivity up to 17.3nW/ppm was achieved. The PCF–LPG-based gas sensors are easy-to-fabricate, cost-effective, and can be used in various applications where conventional fiber-optic sensors are not applicable.

Investigation of impact of photonic crystal fiber structure modified by femtosecond laser micromachining on long period gratings׳ sensing characteristics

The sensing characteristics of long period gratings (LPGs) in photonic crystal fiber (PCF) can be changed by using femtosecond laser to modify the PCF waveguide structure although dispersive characteristic plays a key role in determining the sensitivity. Based on the coupled local-mode theory, the coupling behaviors and spectral characteristics of the LPGs in PCF fabricated by a femtosecond laser and a CO2 laser are analyzed which are supported by experiment results. When the distance between the central of fiber core and the peak of the drilled hole, namely the micro-hole diameter is about 3.5μm, the temperature and strain sensitivities are changed by 27% (from 6.20 to 7.81pm/°C) and −21% (from −2.41 to −1.91pm/με) in comparison with the changes of the sensitivities that is induced by CO2 laser. The investigation demonstrates that the local structural changes of PCF have an impact on the sensitivity of LPGs. The investigation demonstrates the versatility of the technique in potential applications to design the desired sensitivity of fiber grating flexibly by forming proper geometrical modulations.

Characterization of external refractive index sensitivity of a photonic crystal fiber long-period grating

Long-period gratings (LPGs) are fabricated in a photonic crystal fiber (PCF) using the symmetric point-by-point CO2 laser irradiation method to explore the sensitivity characterization of PCF–LPG. Numerical simulation is used to guide the investigation. It is found that the refractive index (RI) sensitivity of PCF–LPG depends on the coupled cladding modes as well as the coupled resonance wavelength (RW) of the LPG. Experimental studies show that the longer the RW, the higher the RI sensitivity for the same cladding mode. At similar RWs, the lower the cladding mode, the higher the RI sensitivity of PCF–LPG.

Long-period gratings inscribed in photonic crystal fiber by symmetric CO_2 laser irradiation

Long-period gratings (LPGs) inscribed in endlessly single mode (ESM) photonic crystal fibers (PCFs) with symmetric and asymmetric CO2 laser irradiation are investigated both numerically and experimentally. Parallel results from conventional single mode fibers (SMFs) are presented for comparison. Theoretical predictions, transmission measurements, and near-field imaging indicate that, regardless of the fiber type, symmetric index perturbation induced by laser irradiation with the aid of a 120° gold-coated reflecting mirror results in LP(0n) symmetric mode coupling, while asymmetric irradiation without using the mirror leads to LP(1n) asymmetric mode coupling. Our results show that, because of the azimuthally anisotropic hexagonal cladding structure, symmetric irradiation yields far more reproducible LPGs in PCFs than asymmetric irradiation. On the other hand, the irradiation symmetry has little effect on the reproducibility of LPGs inscribed in SMFs due to the isotropy of its all-solid cladding structure.

Fiber humidity sensors with high sensitivity and selectivity based on interior nanofilm-coated photonic crystal fiber long-period gratings

Abstract A photonic crystal fiber (PCF) long-period grating (LPG) humidity sensor has been developed with high sensitivity and selectivity for nondestructive detection of moisture ingression into structures that can potentially lead to corrosion. We have proposed two types of nanofilms to be coated on the surface of air channels in the grating region by electrostatic self-assembly deposition processing. The primary nanofilm does not affect LPG properties such as resonance wavelength or transmission intensity which can impact sensing characteristics; however it increases the sensitivity by changing the refractive index of the surrounding material. The secondary nanofilm is used for selectively adsorbing analyte molecules of interest. The experimental results reveal that, compared to the conventional fiber LPGs and exterior nanofilm-coated PCF-LPG, the interior nanofilm-coated PCF-LPG humidity sensors have both the most sensitive resonance intensity change of 0.00022%/10 −3 dBm from relative humidity (RH) of 38% to 39% and average wavelength shift of 0.0007%/pm for a relative humidity variation from 22% to 29%. The proposed sensor shows excellent thermal stability as well.

A highly‐birefringent fiber loop mirror temperature sensor demodulation based on a long‐period grating in photonic crystal fiber with differential processing

AbstractA highly‐birefringent fiber loop mirror temperature sensor demodulation based on a long‐period grating (LPG) in photonic crystal fiber (PCF) with differential processing was proposed.Utilizing the property temperature insensitive and the filtering function of the LPG in PCF, the intensities of two signals located respectively within the positive and negative linear region of the LPG's transmission spectrum can be monitored simultaneously and accurately. The noise of fiber sensors is eliminated effectively by the algorithm of two signals, since the two signals (also including the noise of fiber sensors) transmit through the same way to the monitor. Experimental results show that the value of the algorithm remains constant when the power of light source varies ±10% and the relative error between the measured temperature change and the true change is only 0.34%. Compared with the sensing without the algorithm, the accuracy increases from∼97.3 to ∼99.7%. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:176–179, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26460

Simultaneous strain and temperature measurement using a highly birefringence fiber loop mirror and a long-period grating written in a photonic crystal fiber

We present a new design for simultaneous strain and temperature measurement using a high-birefringence fiber loop mirror (HiBi-FLM) concatenated with a temperature-insensitive long-period grating (LPG) written in a photonic crystal fiber (PCF). The FLM acts as a sensor head, while the LPG in PCF serves as a filter to convert wavelength variation to optical power change. By measuring the wavelength variation and the power difference of two near peaks in the spectral response of this configuration, simultaneous strain and temperature measurement is obtained.

Photonic crystal fiber long-period gratings for biochemical sensing

We present experimental results showing that long-period gratings in photonic crystal fibers can be used as sensitive biochemical sensors. A layer of biomolecules was immobilized on the sides of the holes of the photonic crystal fiber and by observing the shift in the resonant wavelength of a long-period grating it was possible to measure the thickness of the layer. The long-period gratings were inscribed in a large-mode area silica photonic crystal fiber with a CO2 laser. The thicknesses of a monolayer of poly-L-lysine and double-stranded DNA was measured using the device. We find that the grating has a sensitivity of approximately 1.4nm/1nm in terms of the shift in resonance wavelength in nm per nm thickness of biomolecule layer.