Numerical analysis of apodized fiber Bragg gratings formation using phase mask with variable diffraction efficiency

Scalar diffraction theory is applied to analyze the intensity distribution in the fiber core during fiber Bragg grating (FBG) fabrication using an apodized phase mask. The averaged diffraction field distribution in fiber core was calculated as a function of optical fiber position. The results show that in a case of apodized FBGs fabrication, averaged field intensity profile, and thus refractive index changes in fiber core have complex form. Moreover, it was shown that the influence of optical fiber position behind the phase mask on average intensity distribution in fiber core decreases with increasing of its diameter.     

Changing photo-written Bragg wavelengths of fiber gratings via one phase mask and four mirrors

A phase mask interferometer is developed to photo-write long gratings with arbitrary Bragg wavelengths. In this system, the fiber Bragg grating is photo-written by UV interference stripes of 193 nm reflected from two rotatable mirrors and two fixed mirrors, where the phase mask is not only used as a beam splitter, but also initialized the reference quantity of Bragg wavelength; the fixed mirrors are used for reducing the incident angle; the rotatable mirrors are used for adjusting the corresponding photo-written Bragg wavelength. It is worth noting that the photo-written length is fourfold as high as the length in the Talbot interferometer with the same mirror size.     

一种新型的利用重构等效啁啾超结构光纤光栅消啁啾技术研究

提出了一种利用重构等效啁啾超结构光纤光栅对啁啾光脉冲进行频域消啁啾和时域脉宽压缩的方法.由于重构等效啁啾技术可实现任意物理可实现滤波特性的光纤光栅,因此所提出的新型消啁啾方法可以针对任意啁啾模型的脉冲.仿真结果表明,对于脉宽为20 ps,啁啾系数为-5,啁啾模型为线性、高斯型、洛仑兹型的啁啾高斯脉冲,其被消啁啾后时间带宽积分别由初始的225,265,250下降到0458,0708,0731,脉宽压缩效果明显.针对商业软件给出的增益开关分布反馈半导体激光器输出光脉冲的模型,实际制作相应的重构等效 关键词： 重构等效啁啾 光纤光栅 啁啾 增益开关分布反馈半导体激光器     

Investigation on the bistability of linearly chirped nonlinear Bragg gratings

Based on the nonlinear coupled mode theory, by using the reversely recursive transmission matrix method, the switching characteristics of linearly chirped nonlinear Bragg gratings (LC-NLBG) have been investigated numerically. The results show that, introducing spatial chirp in fiber gratings is helpful to widen the bistable wavelength range, and the influence of the chirp on the switching-on threshold and the on-off ratio of the bistable switching are obvious, the chirp-dependent bistability is due to the difference of the axial distribution of forward wave intensity in the gratings.     

A phase-shifted linearly chirped fiber Bragg grating with tunable bandwidth

We demonstrate a new method to broaden and adjust the transmission bandwidth of a phase-shifted linearly chirped fiber Bragg grating (PS-LCFBG) after its fabrication. A movable thermal head is used to heat the PS-LCFBG at a small contact point. The original ultra-narrow transmission bandwidth of the PS-LCFBG can be broadened due to both the presence of the passband peak caused by the thermally-induced temporary phase shift and the passband caused by the thermally-induced local Bragg wavelength shift. The transmission bandwidth of the new passband peak can be adjusted by changing the position of the thermal head. The transmission bandwidth of the PS-LCFBG can be broadened from <10 pm to 0.16 nm, and can be tuned from 0.16 to 1.05 nm.     

Analytical performance evaluation of an adjustable PMD compensator using high-birefringence linearly chirped FBG

System performance of a high-speed WDM transmission system in the presence of PMD can be improved significantly by using an adjustable PMD compensator. We have analytically evaluated the performance of a tunable PMD compensator based on high-birefringence linearly chirped fiber Bragg grating. The device can adjust differential group delay in a linearly continuous way without affecting the wavelength outside the bandwidth. Various properties of the device such as relative group delay, differential group delay, reflectivity, etc. are investigated in terms of wavelength and grating length. Results show that under stretched condition the device generates a time delay between fast and slow polarization axes, which is adjustable from 0 to 55 ps and is tunable within 2.4 nm wavelength range.     

Apodised fiber Bragg gratings fabricated with a uniform phase mask using Gaussian beam laser

A new exposure scheme for fabricating apodised fiber Bragg gratings by the use of a uniform phase mask and a Gaussian beam laser is demonstrated. Average refractive index is kept nearly constant along the grating to derive a side lobe suppression of 20 dB in the shorter wavelength. This scheme allows the writing of truly apodized fiber Bragg gratings.     

Chirp elimination using a linearly chirped fiber grating

Gain-switched distributed feedback laser diodes (GS-DFB-LD) with high repetition bit rate have large frequency chirp. Conventionally, the F–P spectrum filter and normal dispersion fiber were used to eliminate chirp in GS-DFB-LD pulse sources. In this paper, a new method for eliminating chirp of high repetition pulses generated from DFB laser diodes is proposed. The use of a linearly chirped fiber grating for chirp elimination is discussed theoretically and numerically. A new apodized coupling profile function for the grating, which we call it the inverse Gaussian, is given. Its characteristics are analyzed and compared with other profiles. The numerical results are in agreement with the theoretical analysis.     

Fractional optical Hilbert transform using phase shifted fiber Bragg gratings

In this paper, we demonstrate that an arbitrary order (including both integer and fractional orders) Hilbert transform (HT) of an input optical waveform can be implemented by a simple and practical phase-shifted fiber Bragg grating (PSFBG) operated in reflection mode. The PSFBG consists of two concatenated identical uniform FBGs with a proper phase shift between them. It is proved that both the phase shift of the FBGs and the apodizing profile of the refractive index modulation determine the order of the transform. The simulation results show that the device can perform arbitrary fractional Hilbert transform (FHT) with excellent accuracy for input signals with up to hundreds of GHz bandwidth using feasible FBG structures.     

Wideband tunable linear-cavity fiber laser source using strain-induced chirped fiber Bragg grating

A novel wideband tunable linear-cavity fiber laser source using strain-induced chirped fiber Bragg grating (FBG) and erbium–ytterbium co-doped fibers is proposed and demonstrated for the first time. The strain-induced chirped FBG, which acts as a partial-reflecting mirror, is achieved by bending a uniform FBG bonded at a slant onto the lateral surface of a simply supported beam. A single wavelength lasing with a maximum wavelength tuning range of 17 nm has been achieved experimentally.     

双棱镜改变写入Bragg波长的相位掩模技术

将一种可旋转的双棱镜引入到相位掩模技术中以改变光栅的写入Bragg波长.在该系统中,光纤光栅是由来自可旋转双棱镜所形成的波长为248 nm的紫外干涉条纹写入的,其中,相位掩模被用作 1级衍射光的分束器,通过双棱镜的旋转可改变两写入光束的交叉角.为了初始化Bragg波长的参考值,双光栅的顶角由相位掩模的 1级衍射角和双棱镜的折射率确定.因为在～100 nm范围内两光束的非对称旋转对光栅周期的改变是5×10~(-4) nm,双棱镜引入的光栅的闪耀可忽略.当Bragg波长的移位为1 nm时,棱镜最大的旋转角为～1 degree,最小的旋转角是～2.4 min.与Talbot干涉仪中平面镜的旋转角～23 s/nm相比,该相位干涉仪中棱镜的旋转精度降低了2～3个数量级.     

Simple design technique for a triangular FBG filter based on a linearly chirped grating

A novel and simple design technique for triangular spectrum response of fibre Bragg grating (FBG) is presented based on a linear chirped grating. It is shown that this method is fast and can give a straightforward solution to meet a design target. The numerical simulation examples verified the effectiveness of the design method. A general approach to design for multichannel triangular spectral responses for FBG filters is proposed, which provides a solution that achieves a minimum change of refractive index for the fibre.     

光纤Bragg光栅应变测量中啁啾特性的研究

用光纤Bragg光栅进行应变（静态、动态）测量时,由于光栅轴向各部分受到的应变不同,因而产生布喇格中心波长漂移量发生变化.实际测量中往往忽略此扰动对光纤光栅反射谱中心波长的影响,则必然带来测量误差.采用耦合模理论,分析并研究了Bragg光栅受轴向线性应变、高次应变和非线性周期应变波等情况下的啁啾特性.根据各自啁啾特性的不同,给出了对光纤光栅反射谱中心的影响,以及测量各种应变时的测量条件和范围.     

A novel adjustable PMD compensator based on a tapered high-birefringence linearly chirped fiber Bragg grating

A novel solution for polarization mode dispersion (PMD) compensation is proposed by applying tension on a tapered high-birefringence linearly chirped fiber Bragg grating. The compensated differential group delay changes with the applied tension, for its cross-sectional area varies nonlinearly along the grating. Its performance as PMD compensator for short pulses fiber-optical transmission system and 10 Gb/s NRZ fiber-optical transmission system is demonstrated experimentally.     

Non-minimum phase reconstruction from amplitude data in fiber Bragg gratings using an adaptive simulated annealing algorithm

Adaptive simulated annealing algorithm used for phase reconstruction in fiber Bragg grating (FBG) is presented. Theory and numerical simulations show that the non-minimum phase of the FBG can be reconstructed by using this method.     

Synthesis of chirped apodized fiber Bragg grating parameters using Direct Tabu Search algorithm: Application to the determination of thermo-optic and thermal expansion coefficients

In this paper, Direct Tabu Search (DTS) is proposed to synthesize the physical parameters of a fiber Bragg grating (FBG) numerically from its reflection response. A reflected spectrum is being calculated by using the Transfer Matrix Method (TMM). Direct search based strategies are used to direct a tabu search. These strategies are based on a new pattern search procedure called Adaptive Pattern Search (APS). In addition, the well-known Nelder-Mead (NME) algorithm is used as a local search method at the final stage of the optimization process. Direct Tabu Search (DTS) is applied for reconstruction of a raised cosine chirped fiber Bragg grating (CFBG) and a Gaussian multi channel fiber grating. The method is then used to synthesize a CFBG from its reflectivity taken at different temperatures. It gives a good estimate of the thermal expansion coefficient and the thermo-optic coefficient of the fiber.     

Novel flat multichannel filter based on fiber Bragg grating and phase shift

In this letter, a novel optical fiber filter based on fiber Bragg grating and phase shift is proposed for wavelength-division-multiplexing (WDM) system applications. As the channel-space of the comb filter based on chirped sampled fiber Bragg grating and phase shift was not accurate and the multi-channel was unable to channel the standard frequency, we discussed and calculated the error between theory and practice which has certificated by simulation. Then a compensatory method with phase shift between the samples was proposed, and a 80-channel comb filter with accurate 50-GHz channel-spacing was simulated, which is a good foundation for fabricating comb filters with accurate transmission peak in future.     

Code synchronization based on lumped time-delay compensation scheme with a linearly chirped fiber Bragg grating in all-optical analog-to-digital conversion

We propose a novel lumped time-delay compensation scheme for all-optical analog-to-digital conversion based on soliton self-frequency shift and optical interconnection techniques. A linearly chirped fiber Bragg grating is optimally designed and used to compensate for the entire time-delays of the quantized pulses precisely. Simulation results show that the compensated coding pulses are well synchronized with a time difference less than 3.3 ps, which can support a maximum sampling rate of 151.52 GSa/s. The proposed scheme can efficiently reduce the structure complexity and cost of all-optical analog-to-digital conversion compared to the previous schemes with multiple optical time-delay lines.     

Temperature-insensitive transverse load sensing with improved accuracy using stress induced birefringence effects of fiber Bragg grating

Different from the basic sensing theory of the fiber Bragg grating (FBG) sensors based on direct spectral analysis, a new method exploiting the polarization properties of FBG for temperature-insensitive transverse load sensing is proposed. In this paper, the birefringence effects on FBG under transverse load are analyzed. The evolution of the polarization dependent loss (PDL) of FBG with wavelength for the transmitted signals with respect to the transverse load is also studied theoretically and experimentally. We proposed utilizing the PDL evolution of FBG to precisely recover the wavelength spacing between the two peaks of x-mode and y-mode for accurate transverse load measurement. Good agreements between experimental results and numerical simulations had been obtained.     

Improving the performance of chirped fiber grating with cladding being etched as sinusoidal function

A new grating structure of cladding being etched as sinusoidal function along the grating length is introduced to improve their performance. When a tension is applied to it, the fiber grating not only exhibits a flattop spectrum with steep edges and high reflectivity, but also has a smoothly time delay curve. The bandwidth utilization defined as the ratio of −1:−30 could be achieved a larger value (>0.90). The −3 dB bandwidth and the bandwidth utilization could be tunable with the increment of tension applied to the grating and the linearly chirped modulus.