Multiplexing of Fiber-Optic Long-Period Grating-Based Interferometric Sensors

Two long-period gratings (LPGs) fabricated in series in an optical fiber form a Mach-Zehnder interferometer, producing a sinusoidal channelled spectrum within the characteristic LPG attenuation bands, which has the appearance of an interference fringe pattern. The phase of the fringes is sensitive to changes in the surrounding environmental parameters, such as refractive index. Fabrication of a number of identical cascaded LPG pairs with different separations produces independent fringe patterns of different frequencies within the attenuation bands. The application of basic Fourier techniques to analyze the transmission spectrum allows the phase of each fringe pattern to be determined independently, facilitating the differentiation of external effects acting on each interferometer.     

基于级联长周期光纤光栅的新颖温度传感器

设计了一种基于级联长周期光纤光栅(Long period fiber grating)的新颖温度传感器.理论上得到光栅间光纤的长度随温度增加变化时,级联LPG的干涉峰将线性地向长波方向漂移,其消光比亦将呈现线性变化.实验上制作出了基于级联LPG的光纤温度传感器.利用其某一干涉峰的中心波长和消光比测量温度变化时,测量灵敏度分别为0.0353 nm/℃和0.0684 dB/℃.基于级联长周期光纤光栅的温度传感器具有广泛的应用前景.     

Realization of true-time-delay using cascaded long-period fiber gratings: theory and applications to the optical pulse multiplication and temporal encoder/decoder

This paper proposes and demonstrates a technique for repetition-rate multiplication of an optical pulse train having an arbitrary period or pattern and the optical temporal encoding/decoding for the optical-code-division multiple-access (O-CDMA) system. The technique exploits the difference in the propagation speeds between the core and copropagating cladding modes of a fiber to obtain true-time-delay between the modes traveling in the core mode and the cladding mode, which can be used to achieve pulse multiplication. For the coupling to the cladding mode, long-period fiber gratings (LPGs) were used. A series of cascaded LPGs imprinted in a fiber with a specific separation has been employed to obtain a specific rate of pulse multiplication with a single input pulse. Second, by controlling the separations among the gratings, the temporal encoder/decoder for O-CDMA could be implemented. The principle and the applications of the proposed device are investigated in detail. The effect of the birefringence of fiber and fiber gratings on the system performance in the time and spectral domains is presented. The sensitivity of the cladding modes in a conventional fiber to the perturbations at the cladding has been overcome by replacing the conventional fiber with inner-cladding fiber. The properties and the benefits of using the inner-cladding mode are investigated.     

Stress-Induced Versatile Tunable Long-Period Gratings in Photonic Crystal Fibers

We report a method for generating versatile tunable long-period gratings (LPGs) in photonic crystal fibers (PCFs). The central wavelength and rejection bandwidth of LPGs can be tuned over a broad spectral range with adjustable transmission loss. Spectral fringes with uneven spacing are observed in chirped LPGs, depending on the interaction length and the amount of chirping between the two ends of the LPG, and can be removed by reducing the chirping and increasing the grating length. By utilizing a stress-induced LPG, gain flattening of an erbium-doped fiber amplifier has also been demonstrated.      

级联LPG的原理与应用

因为特有的滤波特性,级联长周期光纤光栅已被设计为优异的多波长滤波和传感器件,为此介绍了级联长周期光纤光栅的工作原理及其应用.     

Tuning of long-period fibre gratings written by CO/sub 2/ laser with resonant transmission below -45 dB

Long-period fibre gratings (LPGs) that were made to have resonant transmission below -45 dB are tuned via their simultaneous bending and straining. It was found that the position of the LPG resonance can be tuned over more than 6 nm, maintaining the level of the resonant transmission below -45 dB. Fine tuning of the resonance transmission without a change in its spectral position is also demonstrated. The reported LPGs should satisfy the strictest requirements on the control of LPG filtering characteristics, especially in terms of the phase response     

干涉型PCF传感器的设计与研究

文章利用长周期光栅的耦合作用,设计了一种级联长周期光栅的PCF(光子晶体光纤)传感器,并对其应变传感特性进行了研究。分析了应变对级联长周期光栅的干涉谱的影响,计算了应变与干涉峰的关系,结果表明,该结构具有较高的灵敏度及较好的线性关系。     

传输矩阵法求解长短周期级联光纤光栅传输谱

基于光纤光栅的模式耦合理论,用传输矩阵法对长短周期级联光纤光栅的传输谱特性进行分析。先由耦合模方程分别得到长周期光纤光栅(LPFG)、连接光纤段和短周期光纤光栅(FBG)的传输矩阵,然后将它们相乘得到总的传输矩阵,最后再结合边界条件求得长短周期级联光纤光栅(CLBG)的传输谱。进一步对CLBG的传输谱进行了数值模拟,并通过实验进行了验证。实验结果表明,CLBG的传输谱中出现了由纤芯模和包层模耦合形成的两个谐振峰,与模拟计算的结果一致,证明了传输矩阵法求解CLBG传输谱的可行性,为CLBG在多参量传感和测量领域的广泛应用提供了理论支持。     

长周期光纤光栅(LPG)均衡滤波器在光纤放大器中的应用

根据耦合模理论 ,给出了两级关联和非关联长周期光纤光栅的透射率表达式 ,从而指出在进行LPG均衡滤波器设计时 ,应采用非关联的光栅 ,可以构成灵活多变的透射谱 ,非常适合光纤放大器的不同增益谱对均衡滤波器传输特性 (透射谱 )的不同要求。以此为基础 ,设计和模拟计算了 LPG均衡光纤宽带放大器的增益及噪声谱。结果表明 ,采用均衡滤波方式是使光纤放大器增益平坦的有效方法     

级联长周期光纤光栅和Bragg光纤光栅的光学特性

基于耦合模理论及传输矩阵法,给出了级联长周期光纤光栅和Bragg光纤光栅系统(CLBG)的耦合模方程与总传输矩阵,理论模拟了CLBG的反射谱,得到的反射峰位置与已知文献中所给出的两个反射峰的位置关系相吻合。在此基础上,模拟了镀膜CLBG之间的光纤长度、薄膜折射率、薄膜厚度等参数对反射谱的影响。由仿真结果可知,CLBG反射峰中受长周期光纤光栅和Bragg光纤光栅共同影响产生的反射峰对外界环境的变化非常敏感,其对薄膜折射率的分辨率较单个镀膜长周期光纤光栅高2个数量级,表明CLBG在薄膜传感领域具有重要的应用价值。     

长周期光纤光栅制作及其应用进展

概述了长周期光纤光栅(LPG)在光纤通信、光纤传感等领域的广泛应用.探讨了强度掩膜紫外光敏纤芯写入法、包层机械和热损伤写入法;并对LPG在光纤通信、光纤传感中使用实例进行了详细分析和说明.     

Low-coherence interrogation scheme for multiplexed sensors based on long-period-grating Mach-Zehnder interferometers

We present a low-coherent interferometric system of multiplexed sensors based on pairs of long-period gratings (LPGs). The LPGs in each pair have approximately 3-dB transitivity and form an in-fiber Mach-Zehnder interferometer. By using a dual-beam Michelson interferometer with two reflective mirrors, the interferometric signature of the LPG pair is obtained fast and accurately by scanning one of the mirrors. For multiple sensors, their center-to-center grating intervals are preset to be discrete with specific spacing, which ensures the interferometric signatures are separated for simultaneous demodulation. A measurement of bending is demonstrated to verify good performances of the system in terms of low crosstalk and high sensitivity.     

基于光子晶体光纤的长周期光栅

介绍在光子晶体光纤（PCF）中写入长周期光栅（LPG）的几种方法，研究基于PCF的LPG的传输特性及其与光栅结构的关系，分析了基于PCF的LPG的温度、应力特性和对环境材料折射率的不敏感性。     

全光纤聚合物电光调制器

设计了一种新型的基于长周期光纤光栅的全光纤聚合物高速电光调制器。应用多层光波导和耦合模理论,分析了这种调制器中长周期光栅的谐振波长随施加在聚合物上调制电压的变化。得出在电压仅为8V时对长周期光栅谐振波长的调制就可达到20nm。     

Novel all-fiber bandpass filter based on hollow optical fiber

We report an in-line all-fiber bandpass filter based on a hollow optical fiber (HOF) core mode blocker along with a long-period fiber grating (LPG) pair. It is theoretically and experimentally confirmed that selective core mode blocking at the HOF region located in the middle of the LPG pair could provide efficient passband channels corresponding to the resonant wavelengths of the LPGs. The filter characteristics in passbands and the nearfield patterns of guided modes in the proposed device are discussed.     

Simultaneous refractive index and temperature measurement usingcascaded long-period grating in double-cladding fibre

A compact scheme for simultaneous temperature and surrounding refractive index (SRI) measurement using two long-period gratings (LPGs) of different periods inscribed side-by-side in a single piece of a double-cladding fibre is presented. One of the LPGs is sensitive to both SRI and temperature, whilst the second is sensitive to temperature only     

Numerical analysis of core-mode to radiation-mode coupling inlong-period fiber gratings

This letter presents, to the best of our knowledge, the first analysis of core-mode to radiation-mode coupling in long-period fiber gratings (LPGs) that are surrounded with dielectric material whose refractive index is higher than that of the cladding. We calculated core-mode transmission spectra through LPGs by integrating numerically the coupled-mode equations between the core mode and a continuum of radiation modes. We show calculated core-mode transmission spectra that exhibit such loss band continuums as have already been observed experimentally but not yet theoretically estimated     

Cascaded coupling: Realization and application to spectral maneuvering

In this paper, we analyze a scheme of three mode coupling referred to as cascaded coupling in an optical fiber by employing a pair of superimposed long period gratings. In the cascaded coupling process, interaction between two modes takes place via an intermediate mode. We show that cascaded coupling gives us an additional degree of freedom to tune the transmission spectrum to achieve desired spectral features by appropriate choice of grating parameters i.e. grating strength, length and wavelength of operation. Applications of the proposed design to achieve flat wavelength response and gain flattening filters for erbium doped fiber amplifiers are presented.     

Dispersion of cascaded fiber gratings in WDM lightwave systems

Fiber gratings operating in the transmission mode can provide high dispersion at wavelengths close to the Bragg resonance. When multiple gratings are cascaded for wavelength division multiplexing (WDM) applications, the net dispersion between the stop bands of any two consecutive gratings is significantly modified. We discuss the dispersion characteristics of such cascaded fiber gratings and propose a dispersion compensator for simultaneous compensation of group-velocity dispersion (GVD) for multiple channels of a WDM lightwave system. We also discuss the impact of the dispersion possessed by cascaded gratings on grating based add-drop multiplexers