Tunable Tm-doped fiber ring laser operating at 1.9 μm band using force-induced fiber grating as wavelength tuner

We report wavelength-tunable operation of a Tm-doped silica fiber laser by using a force-induced long-period fiber grating (LPFG) formed in a fiber ring resonator. The laser output wavelength is tuned by moving the transmission passband that is generated between adjacent resonance wavelengths due to the force-induced LPFG. By changing the grating period around 900 μm, we control the laser output wavelength between 1845 and 1930 nm.     

Prototype CO2 laser-induced long-period fiber grating variable optical attenuators and optical tunable filters

Prototype devices capable of variable attenuation at a fixed wavelength, wavelength tuning at a constant attenuation, and combinations of these spectral characteristics are demonstrated in CO2 laser-induced long-period fiber gratings (LPFGs). These devices are based on controlled flexure by means of a piezoceramic platform. CO2 laser-induced LPFG characteristics along with the fabrication and testing processes of these gratings are discussed. Devices with a optical attenuation of 13 dB and a wavelength tuning of 7 nm are reported.     

Two-Layer Nanocoatings in Long-Period Fiber Gratings for Improved Sensitivity of Humidity Sensors

A relative humidity sensor based on the deposition of electrostatic self-assembled alumina ( $hbox{Al}_{2}hbox{O}_{3}$) and poly(sodium 4-styrenesulfonate) on the cladding of a long-period fiber grating (LPFG) has been designed. The sensitive material has a lower refractive index than that of the fiber cladding, which limits the sensitivity of the LPFG response. In order to enhance its sensitivity, a previous high refractive index coating has been deposited. The overlay thickness is of the order of magnitude of the light wavelength used to interrogate the sensor. A theoretical model of multilayer cylindrical waveguides based on coupled-mode theory has been used to predict the phenomenon. Experimentally, an increased wavelength shift of the attenuation bands (75%) was obtained during the fabrication of the sensor, and, what is more important, the sensitivity was improved by a ratio of almost four. The proposed method improves the performance of LPFG-based sensors characterized by overlays of low refractive index.      

Asymmetric transverse-load characteristics and polarization dependence of long-period fiber gratings written by a focused CO(2) laser

Asymmetric transverse-load characteristics and the polarization dependence of long-period fiber gratings (LPFGs) written by high-frequency CO(2) laser pulses are investigated in detail. It is demonstrated that the resonant wavelength is dependent on the direction of the applied force and on the polarization state of the input light; however, the coupling strength is independent of these parameters. When a transverse load is applied along different orientations of the LPFG, the resonant wavelength may be shifted toward the longer wavelength, the shorter wavelength, or hardly shifted, whereas the absolute value of peak transmission attenuation is linearly decreased with an increase of the applied transverse load, with almost no sensitivity to the load direction. These unique transverse-load characteristics and the polarization dependence are due to the load-induced birefringence that leads to the rotation of optical principal axes in the LPFG.     

基于LPFG微弯特性的高精度位移检测系统

在研究长周期光纤光栅(LPFG)温度及微弯特性的基础上,通过引入聚合物温度增敏封装后的光纤布拉格光栅(FBG)作为解调滤波器,搭建了温度自补偿微位移检测系统.将LPFG粘贴于试件上进行微弯测试,在固定波长处.其插入损耗的变化与弯曲度变化呈线性关系.为解决LPFG带宽宽、谐振波长难以精确测量的问题,选择特定波长的FBG作为滤波器,实现了位移检测系统的功率化解调.同时,对FBG利用聚合物进行了温度增敏封装,使其温度灵敏度与LPFG尽量相同,消除了温度对系统的影响.试验结果表明,传感系统输出的光功率与微位移呈良好的线性关系,位移灵敏度为2μW/mm,分辨力为0.5×10-2 mm.所设计的系统结构简单、灵敏度高、线性度好,不受外界温度干扰.     

Wavelength tuning in optically mode-locked semiconductor fiber ring lasers with linearly chirped fiber Bragg gratings

We demonstrate wavelength tuning in single-wavelength and multiwavelength semiconductor fiber ring lasers that are mode locked with an optically injected control signal. A semiconductor optical amplifier is used to provide gain as well as to function as an optically controlled mode-locking element. Linearly chirped fiber Bragg gratings--single or superimposed--are used to define the lasing wavelengths as well as to provide wavelength tunability and allow for multiwavelength operation. We obtain pulses of tens of picoseconds in duration when we inject a sinusoidal optical control signal into the laser cavity, and we can tune the lasing wavelength(s) over the reflection bandwidth(s) of the grating(s) by simply changing the frequency of the injected control signal.     

Optical coupling method utilizing a lensed fiber integrated with a long-period fiber grating

A novel optical coupling scheme that uses a lensed fiber integrated with a long-period fiber grating (LPFG) is proposed. Two experiments are performed to demonstrate the validity of such a scheme in single-mode-fiber-single-mode-fiber (SMF-to-SMF) and laser-diode-single-mode-fiber (LD-to-SMF) coupling setups. The measured results show that for an appropriate lens radius the addition of a LPFG will lead to a higher coupling efficiency over a longer range of working distance than without the LPFG. Coupling efficiencies of ~78% and 35% are achieved for corresponding working distances of ~250 and 110 mum, 1-dB longitudinal tolerances of ~40 and 26 mum, and 1-dB transverse tolerances of ~7.6 and 2.6 mum for SMF-to-SMF and LD-to-SMF, respectively.     

基于双模LPFG折射率不敏感双参量传感器

环境折射率和环境温度变化是影响光纤应变测量误差的主要因素.本文利用双模光纤纤芯双模式(LP01和LP11)支持特性设计了一款环境折射率不敏感的双模光纤(DMF)长周期光纤光栅LPFG)应变传感器.设计了传感器模型结构,制作了最优化参数的传感器样品.实验测试了DMF-LPFG传感结构对外部环境中应变、温度和折射率的响应....     

聚合物光纤网络器件及其通信链路系统

通过自行研制的聚合物光纤波长转换器和集线器,实现了石英光纤传输介质到聚合物光纤传输介质的工作波长转换,以及多路聚合物光纤传输信息的交换和局域网信息的全聚合物光纤传输。系统测试结果显示,数据交换传输速率达到100Mbps,通信眼图清晰。链路系统传输信号随传输距离成指数衰减,与聚合物光纤的光强衰减规律一致。用商用的发射接收器测得传输距离达到60m以上。聚合物光纤弯曲半径为25mm时,给系统接收端接收信号强度带来0.1~0.4dB的衰减,表明聚合物光纤弯曲对链路系统工作影响较小。     

Simple high-resolution wavelength monitor based on a fiber Bragg grating

A compact and low-cost device for monitoring the peak wavelength of single-peak spectral distributions is presented. The system is based on the transmission properties of a fiber Bragg grating when its period is modulated. Different types of optical signal, such as the emission of distributed-feedback lasers and the reflection of a broadband optical source produced by fiber gratings used in sensor systems, can be measured with this device. We demonstrate that a high wavelength resolution of micro 1 pm can be achieved and that our proposal can be used for real-time monitoring.     

Design of Long-Period Fiber Grating Refractometric Sensors With Linear Response by a Genetic Algorithm

A new method for the design of optical fiber refractometric sensors based on nonuniform long-period fiber gratings (LPFGs) is presented. A specialized genetic algorithm (GA) with properly designed operators is a fundamental element of this method and performs the theoretical synthesis of a necessary nonuniform LPFG period profile. Such a profile yields a linear response of the sensor to the refractive index of the external medium. A distinctive feature of the designed LPFG is a linear gradient of the core and cladding refractive index along the grating length. Also, a new mathematical model of nonuniform LPFGs is presented, the model permits to take into account the effect of some LPFG sections of a lower and others of a higher cladding refractive index than that of the external medium. The application of both the design method and the mathematical model is illustrated with a numerical example.     

Study of the effect of source signal bandwidth on ratiometric wavelength measurement

We derive an analytic equation for a ratiometric wavelength measurement system and analyze the influence of the optical source signal bandwidth. Our investigation shows that in a particular optical sensing system, the higher the bandwidth of the optical signal, the better resolution the system will achieve. Experiments based on two types of optical signals (output signal of a tunable laser and a fiber Bragg grating) were carried out, and experimental results verified both the simulation results and the theoretical analysis.     

Fiber Bragg grating flow sensors powered by in-fiber light

This paper presents an active fiber Bragg grating temperature and flow sensor based on self-heated optical hot wire anemometry. The grating sensors are directly powered by optical energy carried by optical fibers. In-fiber diode laser light at 910 nm was leaked out from the fiber and absorbed by the surrounding metallic coating to raise the temperature and change the background refractive index distribution of the gratings. When the diode laser is turned off, the grating is used as a temperature sensor. When the diode laser is turned on, the resonance wavelength and spectral width change of the self-heated grating sensor is used to measure the gas flow velocity. The grating flow sensors have been experimentally evaluated for different grating length and input laser power. The grating flow sensors have demonstrated a 0.35- m/s sensitivity for nitrogen flow at atmosphere pressure.     

Acousto-optic superlattice modulation in fiber Bragg gratings

The superposition of a long-period grating and a fiber Bragg grating, which we call an optical superlattice, causes high-efficiency narrow-band reflections to be induced on either side of the Bragg wavelength. This effect was recently observed experimentally in a fiber-based acousto-optic superlattice modulator. We develop in detail the theory of optical superlattices in fiber Bragg gratings, treating both the acousto-optic and the fixed-grating cases. Applications include reconfigurable wavelength division multiplexers, fiber lasers and sensors, tunable filters, modulators, and frequency shifters.     

High sensitivity of taper-based Mach-Zehnder interferometer embedded in a thinned optical fiber for refractive index sensing

A taper-based Mach-Zehnder interferometer (MZI) embedded in a thinned optical fiber is demonstrated as a highly sensitive refractive index (RI) sensor. A RI sensitivity of 2210.84 nm/RIU (refractive index unit) is obtained at the external RI of 1.40, which is ten times higher than that of normal taper- and long-period fiber grating (LPFG)-based sensors. The sensitivity can be further improved by decreasing the diameter of the thinned fiber and increasing the interferometer length of the MZI. The proposed MZIs have lower temperature sensitivities compared with normal fiber sensors, which is a desirable merit for RI sensors to reduce the cross sensitivity caused by thermal drift.     

Fiber Bragg grating location by a side-scatter technique based on cladding-mode coupling

At wavelengths slightly shorter than the normal resonance for a fiber Bragg grating, radiation can be coupled into cladding and radiation modes that become visible external to the fiber. We describe experimental measurements on a fiber Bragg grating under broadband illumination, showing the axial variation and angular distribution of side-scattered radiation. The scattering signal can locate the grating position subject to an offset in the backscatter direction. The spectrum of scattered radiation is consistent with a published theoretical model of mode coupling by a grating in a finite cladding fiber.     

Switchable dual-wavelength single-longitudinal-mode erbium-doped fiber laser using an inverse-Gaussian apodized fiber Bragg grating filter and a low-gain semiconductor optical amplifier

We present a stable and switchable dual-wavelength erbium-doped fiber laser. In the ring cavity, an inverse-Gaussian apodized fiber Bragg grating serves as an ultranarrow dual-wavelength passband filter, a semiconductor optical amplifier biased in the low-gain regime reduces the gain competition of the two wavelengths, and a feedback fiber loop acts as a mode filter to guarantee a stable single-longitudinal-mode operation. Two lasing lines with a wavelength separation of approximately 0.1 nm are obtained experimentally. A microwave signal at 12.51 GHz is demonstrated by beating the dual wavelengths at a photodetector.     

A miniaturized sensor consisting of concentric metallic nanorings on the end facet of an optical fiber

A polarization-independent optical sensor is created by fabricating a concentric gold ring grating with a period of 900 nm on the end facet of an optical fiber. The sensing function of this miniaturized device is realized by sending white light as a probe to the gold rings and collecting the response signal in the back-reflection through the optical fiber. A pronounced peak due to the Rayleigh anomaly of the gold ring grating is observed in the reflection spectrum, the center wavelength of which is sensitive to the change in the environmental refractive index of the fiber end facet. Theoretical analysis not only shows excellent agreement with the experimental results, but also gives insights into the mechanisms of this kind of sensor. Using the center position of the Rayleigh peak as the response signal, a high sensitivity dλ/dn of 900 nm per unity refractive index is realized for this sensor and a resolution of Δn/n ≈ 1% is demonstrated in preliminary experiments. The sensitivity is solely determined by the period of the grating.     

All-fiber dynamic gain equalizer based on a twisted long-period grating written by high-frequency CO2 laser pulses

A novel dynamic gain equalizer for flattening Er-doped fiber amplifiers based on a twisted long-period fiber grating (LPFG) induced by high-frequency CO(2) laser pulses is reported for the first time to our knowledge. Experimental results show that its transverse-load sensitivity is up to 0.34 dB/(g.mm(-1)), while the twist ratio of the twisted LPFG is approximately 20 rad/m, which is 7 times higher than that of a torsion-free LPFG. In addition, it is found that the strong orientation dependence of the transverse-load sensitivity of the torsion-free LPFG reported previously has been weakened considerably. Therefore such a dynamic gain equalizer based on the unique transverse-load characteristics of the twisted LPFG provides a much larger adjustable range and makes packaging of the gain equalizer much easier. A demonstration has been carried out to flatten an Er-doped fiber amplifier to +/-0.5 dB over a 32 nm bandwidth.     

Design of binary long-period fiber grating filters by the inverse-scattering method with genetic algorithm optimization

An approach is presented to the design of binary long-period fiber grating (LPFG) filters based on the Gel'fand-Levitan-Marchenko (GLM) inverse-scattering method and genetic algorithm optimization. The nonuniform coupling strength of the binary grating can be realized by varying the local duty ratio. A coupled-mode theory combined with the Poisson sum formula for treating the binary index perturbation is developed for the application of the GLM synthesis method. Since the coupled-mode theory, which smears out the discrete coupling nature, can be regarded only as an approximation to the modeling of a binary LPFG, we use instead the transfer-matrix model to analyze the coupling behavior of a nonuniform binary LPFG. Based on the synthesized grating patterns from the GLM method, a real-coded genetic algorithm with the transfer-matrix model is used to compensate for the discrepancies resulting from use of the coupled-mode theory and to optimize the design. We exemplify the above procedure by designing a flatband LPFG filter and a high-visibility all-fiber Mach-Zehnder filter.