Numerical and experimental analysis of long period gratings in wavelength scale elliptical microfibers

We report the fabrication of long-period gratings （LPGs） in elliptical microfibers with femtosecond laser. Based on the numerical analysis of the modes and the mode coupling condition of elliptical microfibers, an LPG is fabricated with a very short pitch of 10 μm by periodically modifying the fiber surface, which demonstrates very strong polarization-dependent resonances, a very low temperature sensitivity of a few picometers in air, and high temperature sensitivity of -1.62 nm/℃ in refractive index oil.

High Sensitive Long Period Fiber Grating Sensor for Detection of Nitrite

A high sensitive long period fiber grating（LPFG） sensor for the detection of nitrite is proposed, which is realized by coating multiple poly（sodium 4-styrensulfonate）（PSS） and poly（diallyldimethylammonium） chloride （PDDA） layers on the fiber grating surface. The sensitivity of this LPFG sensor is maximum when the number of assembled layers is 70. Under this condition, a nitrite concentration of 3×10^-3 mol/L, which is lower than the National Food Additive Standard, 4.2×10^-3 mol/L, can be distinguished. The sensitivity is further increased by 30% when nitrite was determined in the sucrose solution with a concentration of 65%, which provides a new solution for the best refraction index approaching matched index of the fiber cladding. Compared with chemical methods, this nitrite detection technology offers some advantages, such as high accuracy, non toxicity, high speed, low cost, without chemical reagent, and is suitable for foodstuff security detection.

Numerical Analysis of Fiber Bragg Grating and Long Period Fiber Grating Undergoing Linear and Quadratic Temperature Change

The coupled-mode equations for fiber Bragg grating （FBG） and long period fiber grating （LPFG） undergoing linear and quadratic temperature change were given. The effects of temperature gradient and quadratic temperature change on the reflectivity spectrum of fiber Braggs grating and the transmission spectrum of long period fiber grating were investigated using the numerical simulation, and the dependence relationships of the central wavelength shift, the full-width-athalf-maximum, and the peak intensity upon temperature gradient were also obtained. These relationships may be used to design a novel fiber optical sensor which can simultaneously measure the temperature and temperature gradient.

Periodic structural defects in Bragg gratings and their application in multiwavelength devices

In this paper, we present the finding that periodic structural defects(PSDs) along a Bragg grating can shift the Bragg wavelength. This effect is theoretically analyzed and confirmed by numerical calculation. We find that the Bragg wavelength shift is determined by the defect size and the period of the defects. The Bragg wavelength can be well tuned by properly designing the PSDs, and this may provide an alternative method to fabricate grating-based multiwavelength devices, including optical filter arrays and laser arrays. In regards to wavelength precision, the proposed method has an advantage over the traditional methods, where the Bragg wavelengths are changed directly by changing the grating period. In addition, the proposed method can maintain grating strength when tuning the wavelength since only the period of defects is changed. This will be a benefit for devices such as arrays.

A Novel Electro-Optic Polymer Modulator Based on Long Period Fiber Gratings

A novel high-speed electro-optic (EO) polymer modulator based on long period gratings (LPG) is designed. Using the theory of multi-cladding optical waveguide, we achieved relationship between modulate voltage and LPG's resonant wavelength.

Post-Treatment Techniques for Enhancing Mode-Coupling in Long Period Fiber Gratings Induced by CO2 Laser

Two promising post-treatment techniques, i.e. applying tensile strain and rising temperature, are demonstrated to enhance the mode-coupling efficiency of the CO2-1aser-induced long period fiber gratings （LPFGs） with periodic grooves. Such two post-treatment techniques can be used to enhance the resonant attenuation of the grating to achieve a LPFG-based filter with an extremely large attenuation and to tailor the transmission spectrum of the CO2-1aser-induced LPFG after grating fabrication.

Long Period Grating Imprinted on a Flat-Shaped Plastic Optical Fiber for Refractive Index Sensing

A corrugated surface long period grating(LPG)was fabricated on a flat-shaped plastic optical fiber(POF)as a refractive index(RI)sensor by a simple pressing with the heat pressure and mechanical die press print method.The light propagation characteristics of an LPG imprinted on a multi-mode POF were analyzed by the method of geometrical optics.Theoretical and experimental results showed that the structural parameters of the sensor affected the RI sensing performance,and the sensor with a thinner flat thickness,a deeper groove depth of the corrugated surface LPG,and a longer LPG exhibited better RI sensing performance.When the POF with a diameter of 1 mm was pressed with the heat pressure to a flat shape with a thickness of 600μm,an LPG with a period of 300μm,a groove depth of 200μm,and a length of 6 cm was fabricated on it,and the RI sensitivity of 1447%/RIU was obtained with a resolution of 5.494×10^(−6) RIU.In addition,the influences of the POF cladding,tilting of LPG,and bending of the sensing structure were investigated.The results demonstrated that after removing the cladding and tilting or bending the LPG,the RI sensing performance was improved.When the LPG imprinted on the flat-shaped POF was bent with a curvature radius of 6/πcm,the highest sensitivity of 6563%/RIU was achieved with a resolution of 2.487×10^(−9) RIU in the RI range of 1.3330–1.4230.The proposed sensor is a low-cost solution for RI measurement with the features of easy fabrication,high sensitivity,and intensity modulation at the visible wavelengths.

Temperature Compensated Solution Concentration Measurement Based on a Cascaded SMS/LPFG Fiber Structure

In this paper, a hybrid optical fiber structure for solution concentration measurement with the temperature compensation is proposed. The structure consists of long period fiber grating (LPFG) and single mode-multimode-single mode (SMS) fiber structures. The sensing mechanism of the device is studied and verified by experiments. LPFG is sensitive to solution concentration and is affected by temperature crosstalk. SMS structure is not affected by solution concentration, but sensitive to ambient temperature. It can be used as a temperature compensation system. The sensitivity coefficients of LPFG and SMS on temperature and concentration were measured experimentally, and a dual-wavelength matrix was established to realize simultaneous measurement of solution temperature and concentration.

A Novel Method of Edge Filter Linear Demodulation Using Long Period Grating in Fiber Sensor System

A novel method of linear demodulation based on edge filter is presented. An experimental system is built up in which LPG is used as the edge filter. We achieve linear demodulation with a bandwidth of 5nm.

Dual-periodic-microstructure-induced color tunable white organic Ught-emitting devices

In this paper, we demonstrate a color tunable white organic light-emitting devices （WOLEDs） based on the two complementary color strategies by introducing two-dimensional （2-D） dual periodic gratings. It is possible to tune the color in a range between cold-white and warmwhite by simply operating the polarization of polarizer in front of the microstructured WOLEDs. Experimental and numerical results demonstrate that color tunability of the WOLEDs comes from the effect of the 2-D dual periodic gratings by exciting the surface plasmon-polariton （SPP） resonance associated with the cathode/organic interface. The electroluminescence （EL） performance of the WOLEDs have also been improved due to the effective light extraction by excitation and out-coupling of the SPP modes, and a 39.65% enhancement of current efficiency has been obtained compared to the conventional planar devices.

Curvature Sensor Based on a Long-Period Grating in a Fiber Ring Resonator Interrogated by an OTDR

The proposed technique demonstrates a fiber ring resonator interrogated by an optical time domain reflectometer(OTDR),for intensity sensing.By using this methodology,a cavity round trip time of 2.85ms was obtained.For a proof of concept,a long-period grating was inserted in the resonant cavity operating as a curvature sensing device.A novel signal processing approach was outlined,regarding to the logarithmic behavior of the OTDR.Through analyzing the experimental results,an increase in the measured sensitivities was obtained by increasing applied bending.With curvatures performed from 1.8 m^(-1) to 4.5 m^(-1),the sensitivity values ranged from 2.94 dB·km^(-1) to 5.15 dB·km^(-1).In its turn,the sensitivities obtained presented a linear behavior when studied as a function of the applied curvature,following a slope of 0.86×10-3 dB.The advantages of applying this technique were also discussed,demonstrating two similar fiber rings multiplexed in a series of configurations.

Long Period Fiber Grating Based Refractive Index Sensor With Enhanced Sensitivity Using Michelson Interferometric Arrangement

The long period fiber grating （LPFG） is widely used as a sensor due to its high sensitivity and resolution. However, the broad bandwidth of the attenuation bands formed by the mode coupling between the fundamental core mode and the cladding modes constitutes a difficulty when the device is used as a conventional sensor. To overcome this limitation, a Michelson interferometer-type sensor configuration has been developed, using an LPFG grating pair formed by coating a mirror at the distal end of the LPFG. This sensor configuration is more convenient to use and is able to overcome the limitations of the single LPFG based sensor as the shifts in the attenuation bands being more easily detectable due to the formation of the sharp spectral fringe pattern in the LPFG based Michelson interferometer. In this work, I studied the LPFG based Michelson interferometer as the refractive index sensor and discussed the sensitivity enhancement of the LPFG based Michelson interferometer as a refractive index sensor by employing higher order cladding modes and by reducing the cladding radius. The results demonstrated the HE17 mode with a cladding radius of 62.5 μm, in the range of surrounding refractive index （SRI） of 1 - 1.45, and its resonant peak showed a wavelength shift of 26.99nm/RIU. When the cladding region was further reduced to 24μm, the resonant peak showed a wavelength shift of 569.88 nm/RIU, resulting in a sensitivity enhancement of nearly 21 times. However, as the cladding region was etched further, then the HE17 order cladding mode and higher mode would be cut off. Therefore, the implementation of high sensitivity for SRI sensing with the reduced cladding in the LPFG based Michelson interferometer is dependent on the proper combination of the cladding radius and cladding mode order.

Study of Modeling Aspects of Long Period Fiber Grating Using Three-Layer Fiber Geometry

The author studied and demonstrated the various modeling aspects of long period fiber grating （LPFG） such as the core effective index, cladding effective index, coupling coefficient, coupled mode theory, and transmission spectrum of the LPFG using three-layer fiber geometry. Actually, there are two different techniques used for theoretical modeling of the long period fiber grating. The first technique was used by Vengsarkar et al who described the phenomenon of long-period fiber gratings, and the second technique was reported by Erdogan who revealed the inaccuracies and shortcomings of the original method, thereby providing an accurate and updated alternative. The main difference between these two different approaches lies in their fiber geometry. Venserkar et al used two-layer fiber geometry which is simple but employs weakly guided approximation, whereas Erdogan used three-layer fiber geometry which is complex but also the most accurate technique for theoretical study of the LPFG. The author further discussed about the behavior of the transmission spectrum by altering different grating parameters such as the grating length, ultraviolet （UV） induced-index change, and grating period to achieve the desired flexibility. The author simulated the various results with the help of MATLAB.

High temperature-sensitivity sensor based on long period fiber grating inscribed with femtosecond laser transversal-scanning method

We propose a high temperature-sensitive long period fiber grating（LPFG） sensor fabricated by using the femtosecond laser transversal-scanning method. The femtosecond pulses scan over the whole fiber core and some part of the cladding region; the modified regions are more extended. It is found that the LPFG-I fabricated by the transversal-scanning method shows higher temperature sensitivity and better temperature uniformity than that of LPFG-II written by the femtosecond laser point-by-point method. The LPFG-I with a temperature sensitivity of 75.96 pm/°C in the range of 25°C–400°C is measured. Moreover, in the range from 400°C to 800°C, a higher temperature sensitivity of 148.64 pm/°C and good linearity of 0.99 are achieved, while the temperature sensitivity of LPFG-II is only 95.55 pm/°C. LPFG-I exhibits better temperature characteristics, which, to the best of our knowledge, has the highest sensitivity in silica fiber temperature sensors.

A polarization-independent torsion sensor based on the near-helical long period fiber grating

A torsion sensor based on the near-helical（NH） long period fiber grating（LPFG） is fabricated by using a high frequency pulsed CO2 laser. Each groove of the NH-LPFG is spirally written in the four sides of a single-mode fiber. The NH-LPFG has a helical periodic vertical index modulation. This is different from the screw-type index modulation of the common helical LPFGs（H-LPFGs） fabricated in a twisted fiber. The torsion and temperature characteristics of the NH-LPFG are experimentally investigated. The temperature sensitivity is about0.0668 nm/℃. The torsion sensitivity is 0.103 nm/（rad/m） and independent of the polarization state of incident light.

All-fiber linear polarization and orbital angular momentum modes amplifier based on few-mode erbium-doped fiber and long period fiber grating

A few-mode erbium-doped fiber（FM-EDF） is fabricated using modified chemical vapor deposition in combination with liquid solution. The core and cladding diameters of the fiber are approximately 19.44 and 124.12 μm,respectively. The refractive index difference is 0.98%, numerical aperture（NA） is 0.17, and normalized cut-off frequency at 1550 nm is 6.81. Therefore, it is a five-mode fiber, and can be used as a higher-order mode gain medium. Furthermore, a long period fiber grating（LPFG） is fabricated, which can convert LP〈01 mode to LP11 mode, and its conversion efficiency is up to 99%. The first-order orbital angular momentum（OAM） is also generated by combining the LPFG and polarization controller（PC）. Then, an all-fiber amplification system based on the FM-EDF and LPFG, for LP11 mode and first-order OAM beams, is built up. Its on-off gain of the LP11 mode beam is 37.2 d B at 1521.2 nm. The variation, whose transverse mode field intensity of first-order OAM is increased with the increase of pumping power, is obvious. These show that both the LP11 mode and first-order OAM beams are amplified in the all-fiber amplification system. This is a novel all-fiber amplification scheme,which can be used in the optical communication fields.

A variational principle analysis of surface acoustic waves propagating under periodic metal grating with finite thickness

A theoretical method is presented,which analyzes properties of surface acoustic waves propagating on metallic gratings with finite thickness by combining finite element method with variational principle on surface acoustic waves propagating on periodic metal gratings. Based on D.P.Chen and Haus theory,a finite element method is used to investigate the effects of metallic gratings upon the propagation of surface acoustic waves.The coupling-of-modes parameters contributed by mechanical loading are expressed by the matrix derived from the finite element method.Consequently D.P.Chen and Haus theory can also be applied to analyze the properties of surface acoustic waves propagating on metallic gratings with finite thickness and arbitrary shape.Finally,the characteristics of surface acoustic waves propagating under gold and aluminum or silver gratings on a few piezoelectric crystals are studied.Numerical results of the coupling-of-modes parameters of the surface acoustic waves are obtained.

Compact vector twist sensor using a small period long period fiber grating inscribed with femtosecond laser

We propose and demonstrate a sensitive vector twist sensor based on a small period long period fiber grating(SP-LPFG)fabricated with a femtosecond(fs)laser.The fabricated SP-LPFG is compact in size(2.8 mm)and shows strong polarization dependent peaks in its transmission spectrum due to the vectorial behavior of high-order cladding modes.Twist sensing is realized by monitoring the polarization dependent peaks,since the polarization of input light changes with fiber twist.The proposed sensor can be interrogated by the peak intensity and wavelength,with high twist sensitivity that reaches 0.257 dB/deg and 0.115 nm/deg,respectively.

Ultrasensitive liquid level sensor based on slice-shaped composite long period fiber grating

In this paper,a novel liquid level sensor with ultra-high sensitivity is proposed.The proposed sensor is configured by a sliceshaped composite long period fiber grating(SSC-LPFG).The SSC-LPFG is prepared by polishing two opposite sides of a composite multimode-single-mode-multimode fiber structure using a CO;laser.The method improves the sensitivity of the sensor to external environment.Based on the simulation calculation,a liquid level sensor with a length of 3 mm is designed.The experimental transmission spectrum agrees well with the simulation result.The experimental results show that the sensitivity reaches 7080 pm/mm in the liquid level range of 0-1400 μm in water.The temperature sensitivity is24.52 pm/℃ in the range of 20℃-90℃.Due to the ultra-high sensitivity,good linearity,and compact structure,the SSC-LPFG has potential application in the field of high-Drecision liquid level measurement.

Multiple directional enhanced light source through a periodic metal grating structure

In recent years, semiconductor quantum dots （QDs） have been widely used as photon sources in quantum optics due to their special properties, such as high quantum effi- ciency, narrow and tunable emission spectrum, easy manipulation, and so on. The spontaneous emission of QDs also depends on the surrounding environment.