Room-temperature operation of widely tunable loss filter

A widely tunable and room-temperature operational loss filter based on a long-period fibre grating (LPFG) fabricated in a B/Ge codoped fibre is reported. The filter exhibits extremely high temperature sensitivity. A maximum spectral shift of -48.1 nm from 10 to 40°C is achieved, corresponding to a thermal tuning efficiency of 1.6 nm/°C. This value is increased by more than one order of magnitude compared with the LPFGs fabricated in standard telecom fibre and even twice that of a LPFG with sensitivity enhanced by a special polymer     

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。     

Strain Characteristics of CO2-Laser-Carved Long Period Fiber Gratings

Unique strain characteristics of long period fiber gratings (LPFG) fabricated by a focused CO₂ laser beam carving periodic grooves on the fiber are investigated for the first time to our knowledge. Resonant wavelength, transmission attenuation, and polarization dependent loss (PDL) of the CO₂-laser-carved LPFG are found to depend strongly on the tensile strain applied, and their strain sensitivities are dependent on the depth of grooves and/or the initial refractive index modulation. The average strain sensitivity of resonant wavelength for LPFG is increased by 229 times and is up to -102.89 nm/mepsiv by means of carving periodic grooves on the fiber. When a stretching force is applied to the LPFG, the resonant wavelength can "blue" shift by -11.84 nm, the absolute value of peak transmission attenuation and the maximum PDL can be increased by 25.913 and 26.535 dB, respectively. The CO₂-laser-carved LPFG combines the features of the three types of LPFGs reported previously, i.e, the CO ₂-laser-induced LPFGs without physical deformation, the corrugated LPFGs fabricated by hydrofluoric acid etching, and the microbend-induced LPFGs. The mechanisms of refractive index modulation in the CO₂-laser-carved LPFGs under tensile strain are rather complicated and may be regarded as a combination of the stress-relaxation-, the groove-, the strain-, and the microbend-induced refractive index perturbations     

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.     

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.     

Novel fiber-optic sensors based on long-period fiber gratings written by high-frequency CO/sub 2/ laser pulses

In this paper, we report a novel long-period fiber grating (LPFG) fabricated by using a new writing technique that is mainly based on the thermal shock effect of focused high-frequency CO/sub 2/ laser pulses at several kilohertz. A number of unique characteristics of such a LPFG, such as bend, torsion, and transverse load, are observed by experiments, for the first time, to our knowledge. Based on these unique features, a novel bend-insensitive LPFG sensor that could solve the problem of cross-sensitivity between bend and other measurands, a novel torsion sensor that can realize absolute measurement of twist rate, and a load sensor that can achieve simultaneous measurement of transverse load and temperature using a single LPFG element are proposed and demonstrated. These unique features of the LPFGs are mainly due to the asymmetrical distribution of the refractive index on the cross section of the LPFG induced by high-frequency CO/sub 2/ laser pulses.     

Sensitivity characteristics of long-period fiber gratings

We present a detailed investigation into the sensitivity of long-period fiber gratings (LPFGs) as a function of temperature, strain, and surrounding refractive index, with particular attention to the higher order cladding modes and the possibilities for ultrasensitive sensors. From a general theoretical analysis, we identify a general sensitivity factor which offers new physical insight into LPFG behavior and represents a useful design aid in conjunction with a set of measurand-specific sensitivity factors. Our analysis reveals the existence of turning points in the mode dispersion characteristics at which ultrasensitive operation may be obtained. In an extensive set of coordinated experiments, we verify the theoretical predictions with close agreement and provide demonstrations of the device behavior close to the turning points. Alternative sensor schemes for temperature, strain, and refractive index based, respectively, on measurement of the dual resonance characteristic of the modes and on the transmission characteristics close to the turning points, utilizing higher order modes of the LPFG, are presented. For two variables at least, we record the highest LPFG sensitivities yet reported     

弯曲曲率对长周期光纤光栅透射谱特性的影响

报道了一种分析长周期光纤光栅(LPFG)透射谱谐振波长和衰减率随弯曲曲率变化的方法。利用耦合模理论和微扰近似推导了长周期光纤光栅透射谱随弯曲曲率变化的一般表达式,得到当弯曲曲率为2.56 m~(-1)时,长周期光纤光栅透射谱分裂,分裂峰的谐振波长和衰减率与弯曲曲率呈非线性变化,但分裂峰的间距与弯曲曲率呈线性变化。利用B-Ge共掺单模光纤写入的长周期光纤光栅证实了理论的正确性,且具有很好的重复性。     

三包层级联长周期光栅的传输谱特性

运用传输矩阵法对三包层级联长周期光纤光栅(LPFG)的传输谱特性进行分析,讨论了级联处光纤的长度、位置以及薄膜参数等对级联光栅传输谱的影响,并对级联长周期光栅和相移长周期光栅的传输谱进行了比较.结果表明两者传输谱在级联光纤长度较小或级联位置靠近光栅两端时具有较强的一致性;而在级联处光纤较长并且级联位置在中间时,两者表现出截然不同的特性,这一点和两包层级联光栅一致.另外,薄膜参数可以更灵活地调节传输谱衰减峰位置和峰值损耗的大小.     

高频CO2激光脉冲写入长周期光纤光栅的方法

在国际上首次提出了高频CO₂激光脉冲写入长周期光纤光栅的方法.该方法与低频CO₂激光脉冲写入法或紫外光等其它写入法相比具有许多独特的优点——效率高、成本低,重复曝光容易、激光加热区域的周期性变化通过扫描振镜的偏振来实现而光纤固定不动,因此特别适合于研制具有特殊用途的新型LPFG.详细分析了单侧CO₂激光入射导致LPFG横截面折射率分布不均匀的形成机理并提出了降低LPFG双折射的方法.     

级联型长周期光纤光栅传输谱的研究

采用级联多个长周期光纤光栅的方法有效拓宽了长周期光纤光栅反射谱带宽.分析了LPFG透射谱的谐振峰的定位问题,并通过级联多段周期相同但长度和微扰不同的LPFG来形成一个级联型的长周期光纤光栅,然后通过使用传输矩阵的方法,对级联光栅的传输谱进行了数值分析.经过分析发现,如果适当地调整级联型长周期光纤光栅的各个参数如级联的段数以及各段的长度和微扰等值,则所得到的这个级联型长周期光栅的透射谱的有效带宽将显著地增大,其透射谱的形状也会发生显著改变.

Abstract：

An improved method of cascaded long-period fiber gratings (LPFGs) is proposed, which can effectively broaden the resonant spectrum bandwidth of LPFGs. The positioning of the resonance peak of transmission spectra in LPFGs was analyzed. A cascaded LPFG was obtained by cascading various LPFGs with the same period but different length and index perturbation. Transfer matrix method was used to numerically calculate its transmission spectrum, and it is found that the effective bandwidth will become wider and the shapes will correspondingly change if the related parameters in cascaded LPFG, such as the number of segments and length and index perturbations of each segment, are adjusted correctly.     

Reduction of transmission spectrum shift of long-period fibergratings by a UV-preexposure method

We examined the photosensitivity and stability in an ultraviolet (UV)-preexposure method that locks the photosensitivity of refractive index by hydrogen loading in the fabrication of long-period fiber gratings (LPFGs). Photosensitivity and stability comparisons are made between the UV-preexposure method, a typical hydrogen-loading method, and a UV-exposure method without hydrogen loading. The UV-preexposure method can enhance the photosensitivity to induce effective refractive index modulations up to 1.6×10^-4, which is sufficient for fabricating LPFGs in a practical manner under the hydrogen-loading condition at a relatively low pressure of 2.9 MPa. The sensitivity is preserved in terms of the length of time at least over 10 000 h. The spectrum changes after the completion of the LPFG writing are more stable than under the typical hydrogen loading. Therefore, the proposed UV-preexposure method is one of the most useful techniques for fabricating precise and stable fiber gratings, because there are no free hydrogen molecules in the fiber when the fiber gratings are written     

飞秒写制长周期光纤光栅的液体浓度测量实验研究

利用逐点写入法在去涂敷层的普通单模通信光纤中直接写入了长周期光纤光栅(LPFG),实验获得的LPFG在C波段的谐振波长1 538.8nm,衰减强度达10.63dB。其氯化钠和蔗糖溶液浓度传感特性实验表明,随着盐溶液浓度的增大,LPFG的谐振波长向左线性漂移,盐浓度每增加1%,波长向左漂移约0.05nm;随着糖溶液浓度从0%增大为50%,LPFG的谐振波长向左非线性漂移量高达约8nm。实验结果表明:飞秒激光逐点写入法操作简单,容易控制诱导的周期性折射率微扰在纤芯和或包层的位置,从而制备出对低浓度液体敏感的LPFG化学传感器;同时飞秒激光写制的LPFG具有较高的外界环境灵敏度,有望用在化学浓度传感中。     

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。     

双信道解复用长周期光纤光栅的实验研究

长周期光纤光栅是近几年出现的一种波长选择滤波器件。利用其波长选择损耗特性,提出将之用于1310nm/1550nm波分复用系统的解复用。采用电弧熔融刻槽的方法,制备了长周期光纤光栅。并通过控制电弧放电过程和光栅的周期长短及周期数,得到了较为理想的谱特性。结果显示,其通道插入损耗能够小于0.5dB,20dB损耗阻带带宽可达15nm,信道隔离度估计约25dB。     

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.     

Synthesis of long-period fiber gratings with the inverted erbium gain spectrum using the multiport lattice filter model

This paper proposes an accurate modeling technique of concatenated long-period fiber gratings (LPFGs). The proposed technique is then applied to the synthesis of LPFGs for the erbium gain equalization using both the simulated annealing and the steepest descent minimization technique. A piecewise-uniform LPFG is theoretically synthesized according to the inverted gain spectrum of a commercially available erbium-doped fiber amplifier (EDFA) over the range of 1525-1570 nm. Sensitivity analysis of the designed structure is presented by Monte Carlo simulation with regard to the manufactured amplitude mask. To verify a synthesizing technique using the proposed modeling, the piecewise-uniform LPFGs for gain flattening of EDFA are fabricated and their spectra are also presented experimentally.     

基于双光栅级联结构的温度及浓度传感特性测试

研究了长周期光纤光栅（LPFG）级联布拉格光纤光栅（FBG）结构的温度及浓度传感特性。利用飞秒激光直写制作LPFG并级联FBG,且FBG波谷位于1 551.9 nm,LPFG波谷位置为1 560.5 nm。在30~50℃温度变化范围内对传感器温度特性进行测试,并在25℃超净环境下对浓度为3%~30%的葡萄糖溶液进行敏感性测试。实验结果表明:升温过程FBG中心波长发生红移,灵敏度26.36 pm/℃,线性度0.950 8;LPFG中心波长发生蓝移,灵敏度-24.55 pm/℃,线性度0.914 2。降温过程FBG中心波长发生蓝移,灵敏度25.00 pm/℃,线性度0.945 8;LPFG中心波长发生红移,灵敏度为-21.82 pm/℃,线性度0.921 2。FBG对浓度变化不敏感,当浓度由3%增至30%时,LPFG中心波长发生蓝移,灵敏度196.36 pm,线性度0.956 5。结果表明该光纤传感器灵敏度高,线性度好,可以同时动态实现温度和浓度的测量。     

用于S波段光纤拉曼放大器增益平坦的长周期光纤光栅设计

根据实际所测得的S波段光纤拉曼放大器的信号增益谱,通过对长周期光纤光栅具体参数的选定,由两个长周期光纤光栅级联滤波的组合,可以使增益图谱在50 nm(1485-1535 nm)带宽内,增益平坦度达到±0.6dB以内.由三个长周期光纤光栅级联滤波组合,可以使增益图谱在49 nm(1490-1539 nm)带宽内,增益平坦度达到0.5 dB, 55 nm(1485-1540 nm)带宽内,增益平坦度达到1 dB.这对扩大长周期光纤光栅增益平坦滤波器的运用范围,扩大单泵浦S波段光纤拉曼放大器的有效增益带宽有着积极的意义.     

Design of long-period fiber gratings with fast-varying parameters

A transfer matrix of an arbitrary period in a long-period fiber grating (LPFG) is derived. Cascade of such matrixes can easily be used to calculate the overall performance of LPFG, with fast variation in parameters (which is common practice in LPFGs) such as period length, strength, etc. Results show that the difference in transmittivity between our work and traditional coupled-mode theory becomes obvious, when abrupt change such as π phase-shift occurs. This method can be used in the mask design aiming at overall performance optimization through period-by-period pattern adjustment