Analysis of core-mode to radiation-mode coupling in fiber Bragggratings with finite cladding radius

Fiber Bragg gratings (FBGs) are often surrounded with dielectric material whose refractive index is higher than that of cladding. The purpose is to prevent cladding modes because coupling to cladding modes may cause undesirable dips on the short wavelength side of the main Bragg band in the core-mode transmission spectrum. In such FBGs, core-mode to radiation-mode coupling can cause a loss band continuum. Until now, however, reported analyses of this core-mode to radiation-mode coupling have been limited to hypothetical FBGs with an infinite cladding radius. This paper presents, to the best of our knowledge, the first analysis of core-mode to radiation-mode coupling in real FBGs with a finite cladding radius. We derive a formula to calculate the core-mode transmission loss caused by the radiation-mode coupling. We show calculated core-mode transmission spectra that exhibit such fringes as have already been observed experimentally but not yet theoretically estimated     

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     

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.      

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     

Cladding-mode extraction from long-period fibre grating using self-aligned core-mode blocker

A novel method is presented of fabricating a core-mode blocker for use in long-period fibre gratings by forming a self-aligned absorber on the end of the fibre core. The absorber efficiently suppresses the core-mode power and allows the cladding-mode radiation to pass through, thereby resulting in a bandpass output at the resonant wavelengths     

Simultaneous measurement of strain and temperature using single tilted fibre Bragg grating

Tilted fibre Bragg grating (TFBG) with grating planes tilted at an angle of 3deg corresponding to the fibre axis shows core mode and a large number of cladding-mode resonances in its transmission. Discriminating temperature and strain simultaneously using a single tilted fibre Bragg grating is proposed and demonstrated experimentally. The wavelength shifts of the TFBG in response to temperature and strain are investigated. The results show that the cladding modes and core mode have different strain sensitivities while the temperature sensitivities are equal. So, it is possible to monitor the core-mode resonance and the cladding-mode resonances of the TFBG spectrum, allowing the separation of the temperature and strain induced wavelength shifts.     

Multislit interpretation of cascaded fiber gratings

The analogy between the transmission spectrum of a cascaded long-period grating (LPG) and the diffraction pattern of a multislit is presented. Both of them can be expressed with the exact same equation. Only the phase parameters that describe the interference are different. The spectrum of the cascaded LPG and/or a single LPG has been derived into a single analytic equation by diagonalizing the matrix that describes the system. The diagonalizing method that gives the analytic solution of the cascaded device is introduced in detail. Experimentally and theoretically, it is presented that the device composed of N equally spaced identical LPGs has a series of regularly spaced main-loss peaks enveloped by a slowly varying curve determined by the spectrum of the single LPG. Between the adjacent main peaks, there exists N-2 side peaks. With the multislit analogy, the complicated mode coupling in an LPG and/or cascaded LPGs can be explained with the interference among the beams coupled by the series of gratings from an undepleted core mode into a cladding mode. The recoupling back to the core mode does not need to be considered but embedded in the phases of the coupled beams. The analogy holds for any strength of gratings. The phase parameter is proved to be the eigenvalue of the system unit matrix composed of an LPG and the grating-free region between LPGs. The spectral properties of the cascaded LPG are analyzed in detail. As the diffraction pattern of the multislit is enveloped by the curve determined by the shape of a single slit, the interference of the cascaded LPG is enveloped by the curve determined by the specification of a single LPG. The fringe-enveloping curve is also derived in a closed analytic form. As the finesse of the multislit becomes high as the number of slits increases, the finesse of the cascaded LPG increases with the number of the LPGs. The free spectral range decreases with the separation between elements in both cases.     

基于长周期光纤光栅的折射率传感器

长周期光纤光栅具有制作成本低、无背向反射、结构紧凑等优点,且对外界折射率变化有较高的敏感性,特别适用于折射率传感．简要概括了长周期光纤光栅折射率传感的基本原理,介绍了基于长周期光纤光栅的折射率传感器的研究进展,目前的研究重点集中在克服温度交叉干扰、提高传感精度以及拓宽传感范围的方面,比较了各种方案的优缺点．     

Theoretical Analysis of Phase-shifted Long-period Fiber Gratings

The output characteristics of phase-shifted long-period fiber gratings (LPGs) are analyzed using transfer matrix method.The transmissions and their envelops of several kinds of these gratings are given.The characteristics of phase-shifted LPGs and Bragg gratings are compared.The possible applications of phase-shifted LPGs are discussed..     

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.     

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.     

液相外延HgCdTe薄膜组分均匀性对红外透射光谱的影响

用多层模型和膜系传递矩阵计算了HgCdTe/CdZnTe外延薄膜的红外透射光谱，结果表明组分扩散区主要影响透射光谱的干涉条纹和透射率小于10％的区域，而组分梯度区则影响吸收边斜率．横向组分波动也将影响透射光谱的吸收边斜率，当组分均方差小于0.005时，横向组分波动对透射光谱影响可以忽略．用新的组分分布模型计算了HgCdTe/CdZnTe液相外延薄膜的理论透射光谱，并运用非线性二乘法使理论曲线能够很好地与实验结果吻合，从而获得了更加可信的HgCdTe外延薄膜的纵向组分分布和厚度参数．     

Reflectivity-tunable fiber Bragg grating reflectors

We report a novel device that modulates the Bragg reflectivity of a fiber grating by exciting the transverse vibration of the fiber through an acoustic wave. The excitation of the transverse vibration, leading to fiber microbending, induces the coupling of the fiber core mode into cladding modes. This leads to the reduction of core-mode power and hence that of Bragg reflection. This mechanism provides us a means to control the reflectivity after a fiber Bragg grating is fabricated. The numerical results based on a simple micro-bending model agree well in trend with the experimental data     

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     

Pure Bending Characteristic of Tilted Fiber Bragg Grating

a novel structure of the pure macro- bending sensor based on the tilted fiber Bragg grating （TFBG） is proposed. The TFBG located in the half circle with the different diameters is bent at a constant angle with respect to the tilted grating planes. With the variations of the curvature, the core-mode resonance is unchanged and the transmission power of cladding modes detected by the photodiodes varies linearly with curvature, while the ghost mode changes by the form of two-order polynomial. So we can use the transmission power of ghost mode or other cladding modes to detect bending curvature as shape sensor. From a practical point of view, the sensor proposed here is simple, low cost and easy to implement. Moreover, it is possible to make a temperature-insensitive shape sensor due to the same temperature characteristic between the core mode and the cladding modes.     

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.     

UV-Inscribed Optical Fiber Gratings in Mid-IR Range and Their Laser Applications

We have UV-inscribed fiber Bragg gratings (FBGs), long-period gratings (LPGs), and tilted fiber gratings (TFGs) into mid-IR 2m range using three common optical fiber grating fabrication techniques (two-beam holographic, phase mask, and point-by-point). The fabricated FBGs have been evaluated for thermal and strain response. It has been revealed that the FBG devices with responses in mid-IR range are much more sensitive to temperature than that in near-IR range. To explore the unique cladding mode coupling function, we have investigated the thermal and refractive index sensitivities of LPGs and identified that the coupled cladding modes in mid-IR range are also much more sensitive to temperature and surrounding medium refractive index change. The 45 tilted fiber gratings (45-TFGs) as polarizing devices in mid-IR have been investigated for their polarization extinction characteristics. As efficient reflection filters and in-cavity polarizers, the mid-IR FBGs and 45-TFGs have been employed in fiber laser cavity to realize multi-wavelength 2 m Tm-doped CW and mode locked fiber lasers, respectively.     

谐振腔光学参数的测量

由光学谐振腔的透射谱、反射谱特性定义了透射－反射比值谱函数，通过对其精细度的测量，可精确求出谐振腔的光学参数，并通过实验测定了简单光学谐振腔的光学参数。