Transmission of return-to-zero pulses in an optical split-step system based on reflecting fiber gratings

A new variety of the “soliton management” in heterogeneous optical media is proposed. The system is composed as a periodic chain of nonlinear fibers with negligible intrinsic group-velocity dispersion (GVD), alternating with sections of unchirped fiber Bragg gratings (FBGs) operating in the reflection regime. Losses due to incomplete reflection are compensated by linear amplifiers. The model may apply to fiber-optic telecommunication links with periodically installed FBG modules, and it may be used for the design of laser setups. By means of extended simulations, we identify small regions in the underlying parameter space where this model, featuring the periodic separation of the Kerr nonlinearity and FBG-induced GVD (hence the name of the “split-step” system), supports stable transmission of RZ (return-to-zero) pulses, i.e., quasi-solitons. The effect of nonzero fiber’s GVD on the stable transmission regime is considered too. Moderately unstable (partly usable) transmission regimes are found in larger regions of the parameter space; they may be of two different types, with the average nonlinearity either undercompensating or overcompensating the GVD. Interactions between the stable RZ pulses are also studied, leading to the identification of a minimum separation between them necessary for the suppression of interaction effects.     

Millimeter wave beams diffracted from reflecting gratings

According to the angular-spectrum method, a radiation beam of an antenna horn in the Fresnal region is decomposed into the plane waves with the fast Fourier transform algorithm and the waves diffracted from a reflection grating are superposed as a diffracted beam. Compared with the reflected beam from the same size mirror, the radiation half width of the diffracted beam from a grating is narrower and its lateral shape is shift. These performances have been experimentally verified at Ka-band by: (1). The relative diffraction efficiencies in the first order with two triangular gratings which is put along propagating direction of a beam produced by a conical lens-horn. (2). H-plane lateral width with power—3dB lapsed of the focus beam from a grating and a mirror in a grating spectrometer for millimeter waves (25–100 GHz).     

Tunable Bragg gratings for fiber lasers

A tuning method of a fiber Bragg grating by the bending compression-tension is considered. With this method, a relative tuning range of the resonance wavelength of the grating of about 5% has been obtained. The changes in the shape of the spectrum and in the grating reflectivity during the tuning process are studied. The reflectivity of the fiber Bragg grating has been found to considerably increase with increasing degree of compression. It is shown that such an increase is caused by compression-induced changes in the refractive index modulation amplitude in the grating. The fiber Bragg grating developed has been applied for tuning the radiation wavelength of an ytterbium fiber laser. The tuning range of the laser has been found to be 45 nm around the central wavelength 1080 nm.     

Improved coating method for uniform polymer layer in infrared hollow fiber

An improved coating method was proposed in order to form a uniform polymer layer in the fabrication of cyclic olefin polymer-coated silver (COP/Ag) hollow fiber. A COP solution was flowed in a closed loop, in which the silver-coated tube was used as a part of the loop. Owing to the constant flowing speed of the COP solution and the airtight flowing environment, a COP layer was uniformly formed. The hollow fibers attain high performance and deliver multi-wavelength laser light from the infrared to the visible simultaneously. The method was successfully applied to the fabrication of practical hollow fibers with 2 m length for the near and mid-infrared lasers.     

Extracting the structure of highly reflecting fiber Bragg gratings by measuring both the transmission and the reflection spectra

We demonstrate a novel method that enables one to measure the structure of highly reflecting fiber Bragg gratings. The method is based on measuring both the transmission and reflection spectra of the grating and applying an inverse-scattering algorithm. The use of the transmission spectrum significantly reduces the sensitivity of the reconstruction to measurement noise, and therefore it significantly decreases the measurement duration. We experimentally demonstrate our method for reconstructing the structure of an apodized grating with a reflectivity of 99.91%.     

Magnetic-force-induced long-period fiber gratings

A novel formation method of a long-period fiber grating (LPFG) based on a magnetic-force-induced microbend is proposed and experimentally demonstrated. The LPFG employs a permanent magnet that exerts transversal force to the fiber by attracting a steel coil spring. The transversal force causes periodic microbending to the fiber, and therefore the transmission wave attenuates at the core-to-cladding mode resonance. This device has advantages of ease of fabrication, reconfigurability, and available for any type of fiber.     

Tunable fiber Bragg gratings for application in tunable fiber lasers

The resonance wavelength of the fiber Bragg gratings (FBGs) is tuned using two methods. Tunable FBGs are used as the selecting elements in the cavities of tunable lasers. An ytterbium-doped fiber laser with a wavelength tuning range of 1063–1108 nm and an output power of 6 W, a Raman fiber laser with a wavelength tuning range of 1252–1303 nm and an output power of 3 W, and an erbium-doped fiber laser with a wavelength tuning range of 1530–1580 nm are realized, and their characteristics are studied.     

双面声阵列波束形成的正则化改进算法

双面声阵列波束形成能够区分识别位于不同扫描平面的相干声源,然而该算法在低信噪比条件下识别精度较低。针对此问题提出一种迭代正则化改进算法,通过迭代方法更新正则化矩阵与波束形成输出,在不断提升正则化稳定性、抑制干扰旁瓣的基础上使声学云图主瓣向实际相干声源点处聚焦。数值仿真与实验算例结果显示,改进算法在中高频代表频率下能够正确区分相干声源前后方位,并具有相对原算法更高的识别精度。从而表明:从反问题正则化角度对原算法进行优化改进是理论可行的;正则化矩阵的具体形式与广义逆波束形成输出的空间分辨率紧密相关,且可通过迭代方法将二者整合以提高声源识别精度。     

Broadband fiber Bragg gratings for dispersion management

This paper provides an overview and technology update of a dispersion management component made from chirped fiber Bragg gratings. The history and technology of fiber Bragg gratings (FBGs) have been extensively covered in several excellent review articles, [1-6] book chapters, [7] and books. [8,9] We give a brief overview of fiber Bragg grating technology in this section and then focus on the details concerned with construction and performance of dispersion management devices in optical communications systems with a single broadband long-length grating used in reflection. Sampled chirped grating are sometimes referred to as broadband devices, but they actually have a narrow bandwidth of operation that is periodically repeated across a given spectral range. Although these periodic devices have shown promise as dispersion management devices in communications systems, they will not be discussed in this paper.     

Strong phase-controlled fiber Bragg gratings for dispersion compensation

Dispersion-compensating fiber Bragg gratings with approximately 99.9% reflectivity that are made by continuous apodization and phase control are demonstrated. These strong dispersion-compensating gratings provide precision second-order, third-order, or even more complex dispersion compensation, as well as sufficient transmission isolation to be used at add-drop stages without additional filtering. A 99.84% grating with a constant approximately 700-ps/nm dispersion and a 99.94% grating with dispersion varying linearly from 1000 to -1000 ps/nm are demonstrated.     

间插取样光纤光栅多通道滤波器

对间插取样光纤光栅进行了理论分析和数值模拟,并依照理论分析及数值模拟的结果进行了实验研究,采用逐点扫描曝光工艺首次制作出具有10个以上通道数的光纤光栅梳状滤波器。这种方法工艺简单,控制精度要求低,是一种具有良好实用化前景的多通道光纤光栅滤波器制作技术。     

一种简易的光纤光栅涂覆装置

阐述了对裸光纤光栅进行涂覆的原理和必要性。设计了一种简易可行的光纤光栅涂覆装置。该装置简单灵活,成本较低,尤其适合于长度较长的光纤光栅的一次性涂覆,涂层厚度灵活可调。以啁啾光纤光栅为例进行了涂覆实验,涂层光滑均匀,涂覆后的光栅强度得到了增强,抵抗机械应力的能力得到了提高,使用起来更方便。对光栅在涂覆前后分别作了特性参数测试。结果表明,光栅特性基本上不发生变化,性能未劣化,光纤光栅的涂覆简单易行。     

Novel fiber design for broadband long period gratings

We report a novel fiber design for obtaining long period gratings (LPGs) having a broad transmission spectrum. These gratings are based on guided to guided mode coupling in a staircase profile fiber. Numerical simulations predict 20 dB bandwidths as high as 100 nm and an estimated insertion loss of less than 1 dB. Such broadband LPGs are expected to find applications in many all fiber components.     

Sampled phase-shift fiber Bragg gratings

A phase-shift fiber Bragg grating (FBG) with sampling is proposed to generate a multi-channel bandpass filter in the background of multi-channel stopbands. The sampled noire fiber gratings are analyzed by Fourier transform theory first, and then simulation and experiment are performed, the results show that transmission peaks are opened in every reflective channel, the spectrum shape of every channel is identical.It can be used to fabricate multi-wavelength distributed feedback (DFB) fiber laser.     

Sensitivity-variable fiber optic pressure sensors using microbend fiber gratings

We report on sensitivity control of fiber optic sensors using microbend long-period fiber gratings. By tuning the phase difference between a pair of periodically aligned rod arrays that sandwiches a single-mode fiber, we can change the coupling strength of the core mode with radiating cladding modes, resulting in variable pressure sensitivity of the transmission power through the fiber. This technique enables control of the measurable range of pressure. When the phase difference is shifted from π toward 0, the measurable range is expanded from 1.7 to 3.3 N/cm and higher using a light-emitting diode as an input light source.     

Analysis of sampled fiber Bragg gratings in polarization-maintaining fiber

A novel type of sampled fiber Bragg gratings （FBGs） written in polarization-maintaining fiber （PMF） is proposed. The reflection spectrum, time delay, and group velocity dispersion （GVD） of the gratings are analyzed. In addition, the reflection spectrum is optimized by apodization. The scheme of multi-wavelength output based on the gratings is proposed, which could be used as a multi-wavelength polarization filter in the density wavelength division multiplexed （DWDM） system.     

Position weighting of fiber Bragg gratings for bandpass filtering

A fiber Bragg grating bandpass filter with tailored refractive-index modulation is investigated. The writing method includes a position weighting technique, which provides flexible and accurate control of the index modulation along the fiber. This weighting is accomplished by adjusting the positions of a large number of overlapping subgratings to control the local grating strength. A 10-cm grating with a truncated sin(x)/x refractive-index modulation function was written and characterized with this method, showing good agreement with simulations. These types of filter are well suited for wavelength-division-multiplexing applications and can be made to have steep edges, low sidelobes, large bandwidth, and low stopband ripple.     

长周期光纤光栅傅里叶模式耦合理论

建立了长周期光纤光栅傅里叶模式耦合理论.在分析同向模式耦合时,发现了同向耦合模式的振幅系数间存在傅里叶变换关系.推导了长周期光纤光栅的同向耦合谱和透射谱的通用表达式.该理论是用傅里叶变换分析得出长周期光纤光栅折射率微扰的空域谱,再对该空域谱进行模式同向耦合分析,从而得到长周期光纤光栅光谱特性的通用表达式.根据该理论模拟分析了长周期光纤光栅在不同长度和微扰幅值时的光谱特性,与传统耦合模理论进行了对比分析.结果表明,该长周期光纤光栅傅里叶模式耦合理论具有简单、精确和高效的特点,与实际长周期光纤光栅的透射谱特性一致.应用该理论可分析无过耦合的任意轴向折射率微扰分布的长周期光纤光栅光谱特性.     

Ultrasensitive long-period fiber gratings for broadband modulators and sensors

We demonstrate long-period fiber gratings whose attenuation can be changed by 25 dB over a 48-nm spectral band, with ambient-index changes of only 2.7 x 10(-4). To achieve this, the fiber waveguide is engineered to induce coupling between the core and a highly ambient-sensitive cladding mode with identical group velocities. The device schematic allows arbitrarily high index sensitivities to be achieved, which makes it an attractive platform for realizing sensors and modulators that respond to small index changes.