Investigation of Acoustooptic Mode Coupling on a Long-Period Fiber Grating

It is indicated theoretically that the cladding mode generated by a long-period fiber grating (LPFG) can be coupled further to another unsymmetrical cladding mode via acoustooptic effect. A novel technique to tune the transmission spectrum of an LPFG is proposed and demonstrated experimentally by applying an acoustic wave with proper frequency and intensity.     

LPFG包层模对环境折射率的灵敏度分析

从理论和实验两个方面分析了长周期光纤光栅（LPFG）前3阶包层模的透射谱特性与光栅外包层厚度间的关系。LPFG的谐振波长漂移范围随着其外包层厚度的减小而扩大,同时,高阶耦合模LP04模的谐振波长漂移强于低阶模。研究表明,通过设计光纤光栅结构能改善其各阶包层模谐振峰对环境折射率的灵敏度。     

含涂覆层长周期光纤光栅温度特性研究

提出在含涂覆层的标准通信光纤中用800nm红外飞秒激光直接写入了长周期光纤光栅（LPFGs）。从理论上分析了光纤涂覆层对于纤芯基模和包层模有效折射率的影响,并进一步研究涂覆层对LPFGs温度灵敏度的影响。分别对制作的去除涂覆层和含涂覆层LPFGs的低阶包层模进行了温度特性实验。结果表明：涂覆层的存在不仅能成为LPFGs理想的保护层,更重要的是将LPFGs的温度灵敏度提高到0.1173 ,与理论分析吻合。这种含涂覆层光纤光栅若作为温度传感器,将有着良好的机械强度和温度灵敏度。     

LPFG折射率模式响应及其在传感建模中的应用

基于光纤光栅的模式耦合理论,采用严格的数学模型,给出了三层介质长周期光纤光栅(LPFG)的纤芯模和包层模的本征方程,分析了长周期光纤光栅中的纤芯模式和包层模式的有效折射率随外界环境折射率变化的情形,以及外界环境折射率变化对长周期光纤光栅谐振波长的影响.通过在长周期光纤光栅外涂覆折射率对外界环境变化敏感响应的特定薄膜可以显著提高其传感测量的灵敏度.研究结果可为长周期光纤光栅传感器结构参数的优化设计和实际应用提供理论支持.     

薄包层长周期光纤光栅的研究

报道了在薄包层光纤上用高频CO2激光脉冲写入的长周期光纤光栅(LPFG),并分析了该类薄包层LPFG横截面折射率的分布.结合LPFG的腐蚀实验,通过传输谱图的比较发现:较先写入光栅后腐蚀光纤包层的方法,先腐蚀光纤包层后写入光栅的方法更能有效地调整谐振波长.进一步研究发现薄包层LPFG对环境折射率的敏感度更高.     

Effects of cladding radius on the optical characteristics of long-period fiber gratings

Using three-layer vector field model,the dispersion characteristics and spectral response of long-period fiber gratings（LPFGs）with different cladding radii are analyzed.Both the numerical and experimental results demonstrate that LPFG＇s resonant wavelengths shift towards much longer wavelength and the transmission depth is strengthened significantly by decreasing cladding radius.When the cladding radius is less than 20μm,only a single resonant peak over a wavelength range of 600 nm（from 1200 nm to 1800 nm）is achieved,and the sensitivity of LPFG to external refractive index is enhanced significantly.     

级联长周期光纤光栅的温度特性研究

对级联长周期光纤光栅(C-LPFG)的温度敏感特性进行了理论和实验研究。讨论了C-LPFG的谐振波长温度灵敏度与包层模的阶次、光纤光栅周期的关系,进行了20～80℃温度传感实验,实验结果表明,谐振波长与温度成线性响应,升降温数据的重复性比较好,测得其谐振波长温度灵敏度为0.250 1nm/℃、而相同参数的LPFG所测得的谐振波长温度灵敏度为0.131nm/℃。表明,C-LPFG的温度灵敏度远高于同样参数的LPFG。     

Experimental demonstration of enhancing pump absorption rate in cladding-pumped ytterbium-doped fiber lasers using pump-coupling long-period fiber gratings

A long-period fiber grating (LPFG) was inscribed in a single-mode fiber and was spliced with a ytterbium (Yb)-doped double-clad fiber in order to couple pump radiation of the inner cladding into the core in a cladding-pumped fiber laser. The use of an LPFG permits a partial core-pumping scheme in a cladding-pumping fiber laser because a portion of the pump radiation can be coupled to the core by LPFG. The enhancement of the pump absorption of a Yb-doped cladding-pumped fiber laser as the result of pump coupling by LPFG was 35%, and the maximal output power increased by up to 55% when a 20-W pump source is used.     

A Highly Sensitive Fiber-Optic Refractive Index Sensor Based on an Edge-Written Long-Period Fiber Grating

A novel long-period fiber grating (LPFG) is edge-written without any destructive damage on the fiber based on the thermal shock and rapid cooling effects of the high-frequency CO₂ laser pulses exposure method in this letter. The refractive index disturbance induced by high-frequency CO₂ laser pulses mainly occurs in the edge region of the fiber cladding rather than in the fiber core. An edge-written LPFG with a resonant peak of ~18 dB is obtained using a standard telecommunication fiber. The experimental results show that such a novel LPFG structure has much higher refractive index sensitivity over conventional LPFGs side-written, which can be used as a refractive index sensor, or a tunable filter and modulator by controlling the refractive index sensitive film which is coated on the cladding of such an LPFG.     

半腐蚀长周期光纤光栅光谱特性研究

基于长周期光纤光栅(LPFG)包层有效折射率与包层半径的良好相关性,提出一种半腐蚀长周期光纤光栅的新颖设计。将一根长周期光纤光栅分成等长的两部分,用HF酸腐蚀其中的一半区域,此时整根LPFG可看成具有不同谐振波长的两个半长度LPFG的级联。利用传输矩阵方法和三层介质光纤模型的色散方程分析了该种LPFG的光谱特性:随着腐蚀段包层半径的减小,两个分裂峰谐振波长之间距离增大,且模式越高间距越大。同时实现了应变和温度的同步测量,得到应变和温度的传感精度分别为±8.9με和1.4℃。因此半腐蚀LPFG传感器可为解决LPFG交叉敏感问题提供有效方法。     

LPFG 镀层参数对其折射率传感特性的仿真分析

长周期光纤光栅在传感测量领域具有防爆、防磁、抗干扰等优点,且对周围介质折射率的变化敏感,适用于矿井、油田等易燃、易爆、强电磁干扰环境及具有毒性、腐蚀性等不宜接触的液体的折射率测量。本文采用在长周期光纤光栅包层外沿径向镀一金属薄层的方式,从长周期光纤光栅传感原理着手,利用三包层模型的本征方程,并借助MATLAB进行方程求解和仿真分析,讨论了包层模序数、光栅周期、膜层厚度等参数与谐振峰中心波长偏移的关系。为合理选择光栅结构参数、制作符合要求的长周期光纤光栅用于折射率传感测量提供了依据。     

一种基于LPFG高灵敏度浓度传感器

利用Sagnac光纤环全反射的特性和单根长周期光纤光栅(LPFG)构成Michelson干涉仪,入射光经LPFG后,部分被耦合到包层中传输,经过包层和纤芯传输的光信号经Sagnac光纤环全反射后重新耦合回LPFG,在光栅区域干涉。利用传输矩阵理论分析了干涉结构的光谱特性。用此干涉结构检测了蔗糖溶液浓度,传感灵敏度可达0.065 nmLg-1。     

长周期光纤光栅温度传感器的研究进展

长周期光纤光栅（LPFG）具有与短周期光纤光栅所不同的光谱特性。它无背向反射光谱,不对光源的稳定性产生影响。基于纤芯基模与包层模的谐振耦合原理和热光效应,LPFG温度传感器利用谐振波长的漂移变化实现对温度的测量,具有很高的灵敏度。介绍了LPFG的制作方法、LPFG温度传感器的工作原理及最新研究进展。由于具有插入损耗小、可用于遥测、精度高和抗电磁干扰能力强等优点,LPFG温度传感器将有广阔的应用前景。     

基于周期压力单模光纤长周期光栅的折射率传感特性研究

利用交替放置的两个相同周期性刻槽板,对放置于套管中的单模裸纤和待测媒质施力,形成了具有复合光波导结构的长周期光纤光栅(LPFG)。实验研究了不同折射率待测媒质对LPFG传输谱的影响。结果表明:复合波导光栅的光谱呈现双谐振峰现象,且中心谐振波长随待测媒质折射率增大向短波长方向移动;包层模阶次越高,其中心谐振波长移动量越大。当折射率从1.33变化到1.43时,LP14包层模的中心谐振波长变化了7.2nm,所对应的传感器折射率灵敏度约为2.78×10-4 RIU。     

Analysis of etched long-period fibre grating and its response to external refractive index

Analytical formulas are presented for the first time to describe the shift in the resonance wavelength of a long-period fibre grating (LPFG) in response to etching of the fibre cladding or a change in the external refractive index. The accuracy of the formulas is confirmed by comparison with numerical simulations and experimental results. It is shown that the resonance wavelengths of an etched LPFG are more sensitive to the external refractive index than those of an unetched grating.     

长周期光纤光栅三层结构折射率梯度响应特性研究

基于模式耦合理论和传输矩阵法,使用三层结构圆光波导模型对长周期光纤光栅(LPFG)包层模(HE14)的响应特性进行仿真和分析。通过对比分析外部环境折射率呈梯度分布时与呈均匀分布时的LPFG透射谱,发现谱线中除了损耗峰发生漂移以外,主峰左侧旁瓣深度增加,主峰右侧旁瓣深度减小,且随外部环境折射率的升高,这种变化趋势愈加明显,分析了透射谱发生相应变化的原因。研究结果对使用LPFG进行折射率梯度传感设计具有一定意义。     

Ag-coated Long-period Fiber Grating

Based on coupled-mode theory, the eigenvalue equation of five-layered long-period fiber grating （LPFG） sensor with Ag film and gas-sensitive film overlays are firstly studied. The problem of resolving complex eigenvalue equation on five-layered LPFG is analyzed, and the method of resolution is also given. Then the eigenvalue equation of three-layered metal cladding LPFG is analyzed, and the complex transcendental equation is also discussed. The computing result shows that the coupling between the low order EH modes and the core mode is much stronger than that between the low-order HE modes and the core mode.     

Strain-induced thermally tuned long-period fiber gratings fabricated on a periodically corrugated substrate

This paper reports the characteristics of a strain-induced thermally controlled loss-tunable long-period fiber grating (LPFG) fabricated on a prepatterned corrugated substrate. Periodic microbending at the fiber-substrate interface induces mode coupling between the core and asymmetrical cladding modes that yields narrow-band attenuation as large as -25 dB. Several parameters that affect the performance of the tunable LPFG, such as the diameter of the etched fiber, number of grating periods, curing condition, and thermal expansion properties of the bonding materials, are investigated. This paper also demonstrates the integration of an on-chip thin-film microheater that also functions as a temperature sensor for the LPFG.     

截面折变非对称型长周期光栅高温应变特性

研究了基于高频CO_2激光脉冲单侧写入的截面折变非对称型长周期光纤光栅(LPFG)高温特性及其在高温环境中的轴向应变特性。理论和实验表明在200～1000℃之间长周期光纤光栅的谐振峰漂移呈线性趋势;同时,在高温环境中其谐振峰随轴向应变也呈线性漂移,且不同谐振峰的温度和应变灵敏度也不同。利用这种新型长周期光纤光栅独特的高温应变特性可用一根光纤光栅实现对高温和应变的同时测量。     

Gain-flattening filter using long-period fiber gratings

We propose a novel structure for a gain-flattening filter, in which a conventional long-period fiber grating (LPFG) and a phase-shifted LPFG are written closely. The transmission characteristics are investigated in the case of a closely arranged configuration. For the fabrication of a phase-shifted LPFG, a UV-trimming method to control the amount of the phase shift is examined theoretically and experimentally. Finally, a 67-mm-long LPFG gain-flattening filter designed for an erbium-doped fiber amplifier is fabricated and the measured spectrum is shown