Deposition of overlays by electrostatic self-assembly in long-period fiber gratings

It was proved that the deposition of an overlay material onto a long-period fiber grating causes important shifts in the wavelengths of the typical attenuation bands that are caused by coupling between cladding and core modes [Opt. Lett. 27, 682 (2002)]. A theoretical model for analyzing a multilayer cylindrical waveguide is presented that permits the phenomenon to be understood and predicted. An overlay of higher refractive index than the cladding starts to guide a mode if a certain thickness value is exceeded. This causes large shifts in the resonance wavelength induced by the grating. One important application of this phenomenon to sensors is enhancement of the sensitivity of a long-period fiber grating to ambient conditions. Theoretical results are corroborated with experimental ones obtained by electrostatic self-assembly.     

Magnetic-force-induced long-period fiber gratings

A novel formation method of a long-period fiber grating (LPFG) based on a magnetic-force-induced microbend is proposed and experimentally demonstrated. The LPFG employs a permanent magnet that exerts transversal force to the fiber by attracting a steel coil spring. The transversal force causes periodic microbending to the fiber, and therefore the transmission wave attenuates at the core-to-cladding mode resonance. This device has advantages of ease of fabrication, reconfigurability, and available for any type of fiber.     

Random period arc-induced long-period fiber gratings

We report the fabrication of arc-induced long-period fiber gratings with strong random variations in the period. Long-period fiber gratings with standard deviations in the period from 8.50 to 36.98 μm were fabricated. The spectral position of the resonant bands is determined by the average period value, being similar to that observed in a long-period fiber grating with a fixed period equal to the average period of the random grating. Moreover the notch bands keep the shape characteristics like wideband and depth compared with a long-period grating with a constant period. In addition, their sensitivity to external parameters such as ambient refractive index is not too different with that of fixed period long-period gratings.     

All-optical switching in long-period fiber gratings

Nonlinear pulse propagation in long-period fiber gratings is studied with a mode-locked Q -switched laser pulse approximately 80ps in duration at a wavelength of 1.05 microm . Optical switching, pulse reshaping, and optical limiting are found at intensities in the range of 1-20 GW/cm(2).     

Tunable mechanically induced long-period fiber gratings

We report the experimental characterization of mechanically induced long-period fiber gratings (LPFG's) made by pressing a plate with periodic grooves against a short length of fiber. This filter, which is simple and inexpensive, exhibits transmission spectra and temperature stability similar to those of photoinduced LPFG's. It also offers the unique advantages of being tunable, erasable, and reconfigurable. Its polarization dependence also makes it useful as a polarizer, although it can also be eliminated with simple mechanical designs.     

Optical low-coherence reflectometry characterization of cladding modes excited by long-period fiber gratings

The group refractive-index difference of cladding modes excited by a long-period fiber grating is characterized by use of the technique of optical low-coherence reflectometry, with a precision of <10(-4) . Very good agreement between theoretical and experimental results is demonstrated.     

Bending sensitivity of in-series long-period fiber gratings

The interference fringes formed in a long-period fiber grating pair are sensitive to the loss in the cladding mode, which can be induced by bending or by coating on the fiber. Assuming that there is no loss and that both gratings are identical, the interference fringes are shown to have 100% contrast when each grating has 50% transmissivity. By contrast, if the cladding mode is absorbed or scattered the fringe contrast is shown to be degraded. The spectral behaviors of a long-period fiber grating pair with loss is analyzed by the coupled-mode theory and is compared with experimental results.     

Dynamics of long-period gratings tuned with internal fiber electrodes

The physics of electrically switched long-period grating in a twin-hole fiber with internal electrodes is studied. Dynamic measurements for the two polarizations show how the grating spectra shift in time due to the mechanical stress and heat transfer in the core and the cladding.     

Simultaneous temperature and bend sensing with long-period fiber gratings

Long-period gratings (LPG's) written in commercially available boron-codoped fibers operating at wavelengths of <1.1mum are shown to exhibit high temperature and bending sensitivities. Each resonant attenuation band of such LPG's was observed to split in two when the LPG's were bent. The split attenuation bands' separation increased significantly with increasing bend curvature, and the central wavelengths of the split bands provided a measure of temperature. We exploit this effect to allow for simultaneous measurement of temperature and bending in smart-structure applications. The demonstrated novel sensor system is simple and low cost.     

Optical-fiber-to-waveguide coupling using carbon-dioxide-laser-induced long-period fiber gratings

Optical fibers are expected to play a role in chip-level and board-level optical interconnects because of limitations on the bandwidth and level of integration of electrical interconnects. Therefore, methods are needed to couple optical fibers directly to waveguides on chips and on boards. We demonstrate optical-fiber-to-waveguide coupling using carbon-dioxide laser-induced long-period fiber gratings (LPFGs). Such gratings can be written in standard fiber and offer wavelength multiplexing-demultiplexing performance. The coupler fabrication process and the characterization apparatus are presented. The operation and the wavelength response of a LPFG-based optical-fiber-to-waveguide directional coupler are demonstrated.     

Dual-channel microfluidic sensor based on side-hole fiber with two long-period fiber gratings

We propose and demonstrate a dual-channel microfluidic sensor based on a side-hole fiber(SHF)with two longperiod fiber grating(LPFG)structures.There are two air holes in the SHF,which are natural microfluidic channels.We fabricate two LPFGs(long-period gratings LPG-A and LPG-B)in the SHF with the resonance wavelengths of 1268.7 nm and 1385.8 nm,respectively.Results show that the refractive index sensitivities of LPG-A and LPG-B are?76.0 nm/RIU and?71.1 nm/RIU,respectively.One can measure the refractive index of liquid samples in two channels simultaneously.The proposed dual-channel microfluidic sensor has advantages of good linearity response,fluidic technology compatibility,and easy light input/output coupling and system integration,which helps the sensor to have a potential application in environmental detection and food safety detection.     

Mechanically induced long-period fiber gratings on tapered fibers

The characteristics of a mechanically induced long-period fiber grating (MLPFG) made by pressing a pair of grooved plates over single-mode fiber tapers are analyzed. Fiber tapers with a waist length of 80 mm and diameter ranging from 90 to 125 μm, fabricated using the heating and puling method, were used. We observed that the resonance wavelengths shift toward shorter wavelengths as the fiber taper waist diameter is reduced. A maximum shift of 254 nm in the position of the resonance peaks was observed when the fiber diameter was reduced to 90 μm. This technique is particularly suitable for tuning the resonance wavelengths to shorter wavelengths below the limit imposed by the grooved plate period.     

Twist characteristics of the ultraviolet-written long-period fiber gratings

Twist characteristics of the ultraviolet-written long period fiber gratings (UV-LPFGs) are investigated in this paper. It was found experimentally that the resonance wavelength of the UV-LPFG shifts to short side proportional to square of the torsion rate, when it is twisted in both directions of clockwise and counter-clockwise, which is different from that of LPFGs written by CO2 lasers, but similar with that of the corrugated LPFGs. The phenomena can be explained by mechanisms of static compression in twisting and index change caused by photo-elastic effect.     

Temperature impact on mechanically induced long-period fiber gratings

The spectral response of mechanically induced long-period fiber gratings (MLPFG) to the ambient temperature variation was experimentally study. In the MLPFG setting, a pressure rig with aluminum grooved plates on standard telecommunication fiber was used. We found that a relatively low change in temperature can produce a severe impact on the characteristic parameters of the attenuation bands, such as a critical decrease in the contrast and fast shift saturation. When the temperature increases from −10 to 70 °C, all the bands are shifted toward longer wavelengths with a mean sensitivity of approximately 180 pm/°C from 0 to 40 °C in the near-linear region, then they present a flattened zone beyond this temperature. Meanwhile, the contrast of the attenuation bands rapidly decreases from the maximum value to 0 dB with a quasi-cosinoidal behavior. These results are important and have to be considered when MLPFG are applied in a medium with ambient temperature variation. Furthermore, we show that MLPFG can be used as low cost ambient temperature sensors through intensity based measurements.     

Characteristic research on mechanically induced long-period fiber gratings

A full experimental characterization of mechanically induced long-period fiber gratings （MLPFGs） fabricated by pressing a plate with periodic grooves against a short length of fiber is presented. This technique enables a good control over the gratings＇ isolation loss peaks and has high repeatability. The spectra characteristics of MLPFGs are studied with the change of the parameters including pressure, period, and temperature. The produced MLPFGs have low insertion loss （〈 0.2 dB） and the loss peaks can be higher than 20 dB. A large tunability of the resonant wavelength （〉 14 nm） is achieved through adjusting pres- sure grooves＇ period. The center wavelength temperature sensitivities of 0.057, 0.086 nm/℃, and resonant peak temperature sensitivities of 0.230, 0.312 dB/℃, are achieved for jacketed and unjacketed fibers, respectively. These MLPFGs, which are simple and inexpensive, also offer the unique advantages of being tunable, erasable, and reconfigurable.     

基于长周期光纤光栅的FBG解调方法北大核心CSCD

研究了基于长周期光纤光栅(long-period fiber gratings,LPFG)的光纤布拉格光栅(fiber Bragg gratings,FBG)解调方法,该方法利用LPFG的透射谱作为边沿滤波器解调FBG,其解调性能主要取决于LPFG的透射光谱深度。详细分析了CO_(2)激光写制法对透射光谱的影响,制作了透射光谱深度为25 dB的LPFG,并采用该光栅搭建了FBG解调装置,将FBG粘贴于金属应变片表面,测试了其静态与动态响应特征。实验结果表明:该方法制作简单,成本低,具有较好的线性动态范围,可以实现较高灵敏度的FBG解调。     

Spectral polarization-dependent loss of cascaded long-period fiber gratings

We have measured the spectral polarization-dependent losses of cascaded long-period fiber gratings composed of single, double, and triple identical gratings and fabricated by exposure of a UV laser beam on one side of a fiber. The measurement was made by use of the Mueller matrix method for scanning the wavelength of the incident beam. The polarization-dependent loss was dramatically increased as the number of cascaded gratings was increased. The formation of the UV-induced geometric birefringence in the fiber grating and its relationship to the fine interference fringes formed by cascading gratings are discussed.     

Synthesis of long-period fiber gratings with a Lagrange multiplier optimization method

Based on the Lagrange multiplier optimization (LMO) method, a new synthesis approach for designing complex long-period fiber grating (LPG) filters is developed and demonstrated. The proposed synthesis method is a simple and direct approach. It was used to efficiently search for optimal solutions and constrain various parameters of the designed LPG filters according to practical requirements. The inverse scattering algorithm is a good tool for designing FBG filters; as well as for designing transmission-type fiber grating filters like LPGs. Compared to the results of the discrete layer-peeling (DLP) inverse scattering algorithm for LPGs, the synthesized LPGs, using the LMO approach, are more flexible and workable for the constraint conditions can be easily set in the user-defined cost functional, such as the limitation on the maximum value of the refractive index modulation and the parameters of the initial guess in LMO algorithm. Linear transmission LPG filters and EDFA gain flattening filters are synthesized and analyzed systematically by using the proposed method with different parameters. Moreover, as a variation-based method, we find that the convergence and the synthesized results of the LMO method are strongly dependent on the initial guess parameters.     

Fabrication of long-period fiber gratings by twisting a standard single-mode fiber

A new method of fabrication of long-period fiber gratings by twisting of a standard single-mode fiber at high temperature is presented. The method relies on the fact that there always exists some core-cladding eccentricity in the optical fiber. Therefore, when the fiber is twisted, its core follows a helicoidal path inside the cladding. The transmission spectrum of the helicoidal long-period fiber grating that is produced contains several dips that correspond to resonances with the fiber cladding modes.