Optimization of reversible LPFG for sensing application

We report here for the first time to our knowledge the characterization of mechanically induced long period fiber gratings in novel MSM fiber structure. Reversible grating of same period and length was induced in single mode fiber, multimode fiber and novel multimode-singlemode-multimode (MSM) fiber structure. The spectral response of reversible LPFG in SMF is verified experimentally as well as from simulation results and then compared with the experimental spectral response of reversible LPFG in multimode fiber and MSM fiber structure. Reversible LPFG in novel MSM fiber structure is the most optimized and suitable grating for sensing application. For this grating we have obtained single resonant wavelength over a wide wavelength range and maximum transmission loss peak of around 20 dB.     

Wavelength Tuning of Long-Period Fiber Gratings by Fabrication Using a Tilted Amplitude Mask

We propose a method of tuning resonant wavelengths in the fabrication process of a long-period fiber grating using an amplitude mask tilted relative to the fiber. We irradiated a hydrogenated single-mode fiber with a KrF excimer laser through a tilted amplitude mask with a grating period of 460 Μm. Growth of resonant loss peaks in transmission spectra was measured for various tilt angles. A tuning range up to 329 nm was obtained by tilting up to 45°. The measured results agreed with numerical simulation. It was shown that using only an amplitude mask it is possible to fabricate a long-period fiber grating having resonance at any wavelength throughout the range of 1.2-1.6 Μm. A method to determine the tilt angle for a designed wavelength is discussed.     

Higher-order core mode resonances in a mechanically induced long-period holey fiber grating

We present the spectral analysis of higher-order core mode resonances in a long period holey fiber grating induced mechanically in an asymmetric holey fiber. Calculations based on a fast-Fourier transform mode solver shows that the mode resonances obtained experimentally correspond to the odd- and even-LP_1,1 core modes. Additionally, we analyze the twist and polarization response of these mode resonances in the long period holey fiber grating. The results obtained in this work are of great importance in the design of new all-fiber optical devices that involve couplings of higher order core modes in asymmetric holey fibers.     

Wavelength tunability of L-band fiber ring lasers using mechanically induced long-period fiber gratings

We report on oscillation wavelength control in erbium-doped fiber ring lasers by adjusting the period of a mechanically induced long-period fiber grating (LPFG) inserted into the fiber ring resonator. Pump light is provided by a 974 nm laser diode (LD), the emission of which is coupled into the fiber ring resonator through a wavelength-division multiplexing coupler. Laser oscillation occurs with a threshold pump LD current of 40 mA, corresponding to a threshold pump power of 5 mW. When a periodic pressure of 0.81 N/mm is applied to form the LPFG, the fiber ring laser exhibits the tunable range of 40.9 nm, i.e., from 1563.1 to 1604 nm, by changing the grating period.     

Single- to three-wavelength switchable ytterbium-doped fiber laser based on intracavity induced loss by a long-period holey fiber grating

In this paper, we propose and demonstrate a versatile and simple single-, dual- and three-wavelength switchable ytterbium-doped fiber laser using the intracavity induced-loss generated by a mechanically induced long-period grating (MLPFG) in a holey fiber. The laser net gain in the cavity is reshaped through the MLPFG by adjusting the twist rate and the pressure of the holey fiber in the long-period grating. In this way, the twist response of the MLPFG enables the laser to switch between the single-, dual-, and three-wavelength operations and the tuning of the simultaneous two- and three-wavelengths. These results are of great interest in the design of flexible multiwavelength sources with a tunable capability on their wavelengths.     

A Novel Fabrication Technique of Long Period Fiber Gratings Using a Holographic Optical Element

We develop a new flexible and precise ultraviolet exposure technique to fabricate a long period fiber grating using a computer-generated holographic optical element (HOE). The HOE can generate a striped beam pattern with about a 300 #x03BC;m period from a KrF excimer laser. This method offers (1) ease in varying the grating period and (2) grating period precision. We achieve a grating period within an accuracy of #x00B1;0.04 #x03BC;m. This means that the peak wavelength can be controlled within an accuracy of #x00B1;0.2nm with an operating wavelength of 1.55 #x03BC;m.This paper was originally presented at the 2nd International Conference on Optical Design and Fabrication, ODF2000 which was held on November 15-17, 2000 at the International Conference Center, Tokyo, Waseda University, Japan.     

Generation of ultraviolet broadband light in a single-mode fiber

Ultraviolet broadband light spanning 337–405 nm was produced in a single-mode optical fiber primarily by stimulated Raman scattering. Pulses of 4 ns duration at 337 nm were coupled into a 50 m long ultraviolet-grade fiber featuring single-mode operation in the 320–450 nm range. Significant spectral broadening was achieved with pulses of only ∼10 W peak power. Our experiments demonstrate the potential for a source with ∼10⁴ times the spectral radiance of a quartz tungsten halogen lamp, which is currently used for many applications in this wavelength range. PACS 42.65.Ky; 42.81.Wg     

锁模脉冲波长连续可调谐光纤激光器

利用在腔内加入可调谐光纤光栅滤波器使“8”字形腔掺Yb3+光纤激光器在锁模状态下实现波长连续可调谐.实验中,在保证锁模状态稳定的情况下,通过调节可调谐光纤光栅滤波器,使激光器输出锁模脉冲的中心波长在1 047 nm～1 055 nm范围内连续调谐,重复频率稳定维持在4.9 MHz.在中心波长1 053 nm处,测得锁模脉冲输出平均功率为8.02 mW,光谱带宽1 nm,脉冲宽度为259.3 ps.这种“8”字形腔被动锁模光纤激光器在锁模状态下对波长连续可调谐,并可长时间稳定工作.     

利用光纤光栅的高功率掺镱光纤激光器

报道了利用一对光纤光栅作为双包层Yb^3 掺杂光纤激光器的谐振腔，激光二极管光纤模块(LD)进行了抽运，并采用锥形光纤实现了全光纤化结构，获得了高功率双包层光纤激光器。光纤光栅通常是用融接技术实现与双包层光纤的一体化连接的，采用的双包层光纤为内包层为梅花瓣形结构的掺Yb^3 离子的石英光纤，采用的抽运源为中心波长为970nm的半导体激光光纤输出模块，在抽运源电流达到2．4A时，获得了10．8W的光纤激光器单横模输出，输出波长1100．5nm，峰值半峰全宽(FWHM)为0．54nm，激光器斜效率为59％。     

一种测定光纤光栅布拉格反射波长位移的简单方法

报道一种简单的光纤光栅对参考法测定光纤光栅布拉格波长的位移的方法，其原理是利用一个布拉格波长可调谐的光纤光栅去跟踪另一个光纤光栅拉格波长的变化，测得的光栅布拉格波长的温度灵敏度为０．００８８ｎｍ／℃，这和用可调谐半导体激光器测得的结果十分接近。     

基于色散调谐宽带扫频光纤激光器及其在光纤光栅解调中的应用

报道了一种基于色散调谐技术的宽带扫频掺铒光纤激光器。通过在主动锁模光纤激光器腔内引入较大色散,利用锯齿波电信号调制电光调制器的调制频率,可以实现激光器扫频输出。应用扫频激光器可将光纤光栅反射中心波长的变化转变为时域上信号间隔变化,适用于光纤光栅传感解调。搭建了扫频光纤激光器,利用掺铒光纤作为增益介质,研究了扫频范围的影响因素。通过优化实验参数,实验得到的扫频带宽达到43 nm,接近增益带宽,扫频速度为50 Hz。利用搭建的扫频激光器进行了光纤光栅传感解调,灵敏度约为0.68 ms/nm,验证了解调原理。     

基于高双折射光纤布拉格光栅的自动增益控制掺铒光纤放大器

掺铒光纤非均匀展宽引起的空间烧孔现象导致单波长激光并不能完全控制放大器增益,提出了一种新颖的自动增益控制掺铒光纤放大器的结构:即采用高双折射光纤布拉格光栅产生抽运光,其写制光栅的波峰对应的波长分别为1549.3 nm和1549.83 nm,波长间隔为0.53 nm。通过调整偏振控制器,就实现了单激光或双激光的增益控制。这种设计增益控制范围为40 nm(1530~1570 nm),当输入功率在-40~-15 dBm的动态范围内,双激光增益控制的掺铒光纤放大器的平均增益和噪声系数分别约为22.22 dB和8.69 dB,而它们的漂移分别被钳制在0.69 dB和1.51 dB。系统性能测试表明:双激光控制掺饵光纤放大器在稳定性方面比单激光有着明显的优势。     

基于飞秒激光直写光纤光栅的掺镱光纤激光器

 利用飞秒激光微加工技术,可以在光纤纤芯内直写出布拉格光栅,它与传统的光纤光栅制作方法相比,具有耗时短、无需光敏光纤、周期可任意设定、光栅稳定性高等优点。采用800 nm钛宝石飞秒激光器,在Hi1060光纤内写入一支8 mm长的布拉格光栅,光纤光栅的周期为2.9 μm,这是中心波长为1 042 nm的八阶光纤布拉格光栅。将所得光栅与一段有源的双包层光纤熔接,作为激光输出镜,利用975 nm的LD光纤模块作为泵浦源,采用端泵浦技术构成双包层光纤激光器。双包层光纤采用Nufern公司镱(Yb³⁺)离子掺杂双包层光纤,光纤长度3 m。所得激光器的输出功率为71.1 W,中心波长1 042 nm,带宽约为0.8 nm。     

A stable dual-wavelength fiber laser with tunable wavelength spacing using a polarization-maintaining linear cavity

We propose and experimentally demonstrate a stable dual-wavelength erbium-doped polarization-maintaining (PM) fiber laser with tunable wavelength spacing using an all-PM linear cavity that makes use of two reflection peaks from the PM fiber Bragg grating (PM-FBG). Experimental results show stable dual lasing lines with a wavelength separation of ∼0.22 nm and a large optical signal-to-noise ratio (OSNR) of over 40 dB under room temperature. By applying axial strain to the PM-FBG, the center wavelengths of the two lasing lines can be tuned over several nanometers and the wavelength separation between the lasing lines can also be tuned to as small as 0.05 nm, which, to our knowledge, is the smallest wavelength spacing ever obtained from a stable room-temperature dual-wavelength fiber laser. The proposed laser configuration has the advantages of simple structure, low loss, stable dual-wavelength operation and a very small lasing linewidth of ∼5 kHz . PACS 42.55.Wd; 42.81.-i; 42.81.Gs     

Wavelength Shift of Cladding Mode Resonances in a Mechanically Induced LPFG by Twisting the Fiber

Abstract

The long-period fiber grating is mechanically induced over a twisted fiber and its characteristics are investigated. The amplitude as well as the wavelength shift of the resonance is studied in response to the applied twist and pressures. These resonances decrease in amplitude and shift to shorter wavelength side as the applied twist increases. The spectral responses of a grating assembly formed by two grating sections in series, one section with a twisted fiber and other with an untwisted fiber, are also investigated. Shearing stress and photo-elasticity causes the fiber to be circularly birefringent and the mechanically induced grating formed over the twisted fiber region causes the appearance of two resonances shifted away from the resonances of the untwisted grating section. At higher twist rates, resonant wavelength shift becomes insensitive to applied pressures, showing a reduction in the induced linear birefringence. The wavelength shift is almost symmetric with respect to the applied twist rate in clockwise and counterclockwise directions.     

基于周期性矩形压力槽的可调谐长周期光纤光栅

利用机械线加工技术设计矩形压力槽制作了周期为600 μm,周期数为60的长周期光纤光栅.实验研究了所制作长周期光纤光栅的温度和压力特性,并通过调整光纤和压力槽之间的角度改变长周期光纤光栅的谐振波长.采用此技术制作的长周期光纤光栅最大谐振峰值可达15 dB,通过调整光纤与压力槽之间的角度可使谐振波长变化超过12 nm.     

A novel-configuration multi-wavelength Brillouin erbium fiber laser and its application in switchable high-frequency microwave generation

A novel configuration of compound-cavity multi-wavelength Brillouin erbium fiber laser is proposed and experimentally demonstrated. With an incident optical carrier power of 8 dBm, at least 14 lasing lines are obtained with a wavelength spacing of ∼0.08 nm. Stability and power uniformity of the multi-wave-length lasing are ensured by the flat hybrid gain of Brillouin and erbium, the compound-cavity structure, and the four-wave mixing suppression using a long (10 km) single-mode fiber. A stable and frequency-switchable microwave can be achieved by incorporating a fiber Bragg grating filter to select the desired nth-order Stokes wave and beating it with the optical carrier at a photodetector. In our experiment, the 1st-4th-order Stokes waves are filtered respectively and hence a high-quality microwave with a switchable frequency from ∼10 to ∼40 GHz and a tuning step of ∼10 GHz is achieved. The signal-to-noise ratio is measured to be >25 dB.     

含耐高温涂覆层长周期光纤光栅的温度特性研究

利用逐点写入法在耐高温光纤中用红外飞秒激光直接写入了长周期光纤光栅,研究了光栅的高温温度特性,并做了理论分析.通过对含耐高温涂覆层的长周期光纤光栅进行20 ℃~300 ℃的温度传感实验,结果表明：在高温段光栅的谐振波长漂移量与温度之间仍能保持大的灵敏度（0.060 5 nm/℃）和好的线性度,且光纤耐高温涂覆层不受破坏,光纤耐高温涂覆在高温下不会出现碳化现象,光栅传感性能良好.实验证明该方法制作的光栅适合于长期在高温环境下使用,应用价值巨大.     

啁啾光纤光栅在光纤通信系统中的色散补偿

报道一个简单而新颖的、能将均匀光栅改变为线性啁啾光栅的方法,此方法可对光纤光栅的喃啾度和中心波长进行独立调节,实验表明,一长为１０ｃｍ的均匀光栅其啁啾度可增加至０．５ｎｍ／ｃｍ,而中心波长独立可调量达０．７ｎｍ利用这样的可调啁啾光栅在标准单模光纤（Ｇ６５２）中对一脉宽为２５ｐｓ的输入信号在１５５０ｎｍ窗口传输,成功地实现１００ｋｍ的色散补偿。     

Multi-Wavelength-Switchable Double Clad Yb3+-Doped Fiber Laser Based on Reflectivity Control of Fiber Bragg Gratings by Induced Bend Loss

We propose and demonstrate an all-fiber multi-wavelength switchable double-clad Yb³⁺-doped fiber laser based on the reflectivity cavity control by induced bend loss. The wavelength switching is realized by a variable reflecting mirror that employs a cascaded array of three high reflection (>99%) fiber Bragg gratings at 1064, 1080 and 1096 nm, with a bending controller inserted between each grating. The order of the Bragg gratings is decided according to the gain profile of the Yb³⁺-doped fiber and the induced bending loss allows us to select the Bragg-wavelength laser operation. The laser is capable of switching continuously from one wavelength to another, and slope efficiencies over 50% are obtained at each wavelength.