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.     

73-nm tuning of a double-clad Yb<Superscript>3+</Superscript>-doped fiber laser based on a hybrid array

We report on a wide wavelength tuning in a double-clad ytterbium-doped fiber laser. The laser cavity consists of an array of broadband high-reflection fiber Bragg gratings and a bulk grating as the output coupler and wavelength selection element. The proposed fiber laser configuration combines a low intracavity loss of the fiber Bragg grating mirrors with a wide wavelength tuning of the bulk gratings. We demonstrate a >70-nm wavelength tuning range, limited only by the available fiber Bragg gratings.     

Switchable dual-wavelength Erbium-doped fiber ring laser with cascaded fiber Bragg gratings

We have experimentally shown wavelength mode switching in a dual-wavelength Erbium-doped single cavity fiber laser where the initial two wavelengths of 1 nm spacing are determined by the cascaded reflection type short-period fiber Bragg gratings having two different centre wavelengths of 1550.5 and 1551.5 nm. The lasing mode depends on the polarization in the ring cavity to migrate from one wavelength to another or operates in both modes in a polarization beam splitter output. To effectively control the polarization in the ring cavity, the polarization controllers were positioned before and after the polarization beam splitter. This method of wavelength switching provides a simple way of mode tuning in dual-wavelength fiber lasers.     

Dynamic strain monitoring by wavelength locking between two fiber Bragg gratings fiber sensing system

We demonstrated a fiber sensing system using the method of wavelength locking of two identical fibers Bragg gratings (FBG) to interrogate the wavelength shift by directly measuring the intensity of the reflection from the sensing Bragg grating. The light source of the fiber sensing system is an EDFA fiber ring laser pumped by a 980 nm laser diode and a narrow bandwidth fiber Bragg grating for the filter of the ring laser resonator. The wavelength shift is converted to the intensity deviation of the reflection from the sensing FBG under strain variation, and is able to achieve real-time sensing of the dynamic strain sensing in civil engineering. The characteristics and key factors to maintain stability of the dynamic strain sensing system are discussed.     

波长可调谐取样光纤光栅激光器的输出特性研究

建立了具有两个取样周期略有不同的取样光纤光栅反射镜的Vernier波长调谐机制的光纤分布布拉格反射式激光器的数值仿真模型,对激光器的输出特性进行了分析. 研究了激光器输出特性随着激光器前后光栅反射镜波长以及前后光栅反射镜反射率的变化关系. 提出了对两取样光栅反射率的优化配置方案,可以使输出激光的边模抑制比有所提高;同时对波长调节方式也进行了探讨;进行了实验研究. 关键词： 取样光纤光栅 光纤激光器 波长可调谐激光器     

互注入锁定产生双波长可调谐光脉冲的实验研究

报道了两个增益开关调制的法布里-珀罗半导体激光器互注入锁定实验方案,可产生双波长可调谐光脉冲。该装置对两个结构基本相同的法布里珀罗半导体激光器做增益开关调制,产生多模光脉冲输出;然后利用两个光纤光栅作为滤波元件,通过调节可调谐光延迟线(VODL2),可使得双波长光脉冲在两个法布里-珀罗激光器之间相互注入锁定．从而在输出端得到高边模抑制比的双波长光脉冲输出。然后再通过应力作用在两个光纤光栅上以改变它们的反射波长和适当调整另一个可调谐光延迟线(VODLI)长度．可得到不同的双波长的光脉冲输出,而重复频率保持524．6MHz不变。在13．2nm调谐范围内边模抑制比高于25dB．系统稳定且波长调节方便。     

Tunable multi-wavelength combined linear-cavity fiber laser source with equally changed wavelength spacing

A tunable multi-wavelength combined linear-cavity fiber laser source with equally changed wavelength spacing is proposed for the first time. The wavelength tuning is realized by stretching or compressing the fiber Bragg gratings incorporated in the laser cavity. To ensure a wavelength tuning with equally increased or decreased wavelength spacing, the gratings are mounted onto a plastic plate with angles between adjacent gratings such that the forces applied to the gratings has an equal force increment. A four-wavelength lasing with equal increased or decreased wavelength spacing is achieved experimentally.     

All-fiber actively Q-switched fiber laser tuned by a pair of temperature controlled fiber Bragg gratings

We report an all-fiber actively Q-switched erbium-doped fiber laser, where the linear laser cavity mirrors are composed of two fiber Bragg gratings (FBGs). The laser oscillation wavelength could be tuned by this pair of temperature controlled FBGs. The Q-switching is achieved by an all-fiber phase modulation device. Using this system, we could obtain stable Q-switched laser pulses output, which could be optimized by tuning the reflection wavelengths of the two FBGs to be adjacent to each other. Instead of being modulated by the FBG filter in high-speed oscillation, this fiber laser system is operating in the Q-switched regime using an all-fiber phase modulator, producing a more stable laser output spectrum.     

两个光纤激光器的互相注入锁定

光纤激光器的相干合成技术是实现高功率、高亮度光纤激光系统的重要技术途径.利用分束器分光使两个光纤激光器的能量互相注入，从而使两个光纤激光器达到锁相锁模实现相干输出. 实验研究表明：在独立运行的情况下，两个光纤激光器的远场光斑只是两个激光器光强的简单叠加；在互相注入锁定的情况下，两个光纤激光器的远场光斑是具有高对比度的干涉条纹，并得到两个激光器的输出模式总为异相模.这与理论分析的结果相一致. 关键词： 光纤激光器 相干合成 锁相     

Double-ring cavity configuration of actively mode-locked multi-wavelength fiber laser with equally tunable wavelength spacing

A novel actively mode-locked multi-wavelength tunable fiber laser with equally increased or decreased wavelength spacing is presented. It is constructed using a double-ring cavity combined with an amplitude modulator and cascaded fiber Bragg gratings (FBGs). The cavity lengths for all FBGs are intrinsically identical due to the double-ring configuration, and simultaneous mode locking of multiple wavelengths is therefore achieved by applying only one mode-locking signal to the modulator. The FBGs are mounted on a organic plate with angles between adjacent gratings such that the forces applied to the FBGs have an equal force increment; thus, stretching or compressing the cascaded FBGs can achieve a wavelength tuning with equally increased or decreased wavelength spacing.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Wideband wavelength tunable fiber ring laser with flattened output power spectrum

A wavelength tunable erbium-doped fiber ring laser with flattened output power spectrum over a broadband is proposed and demonstrated. The power flattening is achieved using a high-birefringence fiber loop mirror, in which a number of high-birefringence fiber sections and polarization controllers are used to get a reflection spectrum that can compensate for the output power spectrum. The wavelength tuning is realized by compressing or stretching the FBGs in the laser cavity. A 1×3 switchable fiber Bragg grating array is incorporated into the fiber ring to get a wideband tuning range of 38 nm, from 1527 to 1565 nm. Within this range, the output power uniformity is controlled within ±0.8 dB. The total output power is about 4 dBm, the 3-dB linewidth is 0.01 nm, and the side mode suppression ratio is more than 48 dB.     

Mode-locked ytterbium fiber laser tunable in the 980-1070-nm spectral range

Spectral tuning of a mode-locked Yb-doped fiber laser over a 90-nm range is reported. Using semiconductor saturable absorber mirrors in a fiber laser cavity incorporating a grating-pair dispersive delay line, we obtain reliable self-starting mode locking over the whole tuning range. The wide tuning range is achieved by optimization of reflection characteristics and bandgap energy of the multiple-quantum-well semiconductor saturable absorber and by proper engineering of the laser cavity.     

Partial scanning frequency division multiplexing for interrogation of low-reflective fiber Bragg grating-based sensor array

Interrogation using partial spectrum scanning for a low-reflective fiber Bragg grating (FBG)-based sensor array is proposed. The sensor head is a Fabry–Perot interferometer (FPI) consisting of low-reflective fiber Bragg gratings. An FPI consisting of low-reflective FBGs (FBG-FPI) has a spectrum with a sinusoidal structure with a period determined by the length of the FBG-FPI. Multiple point sensing is possible by installing multiple low-reflective FBG-FPIs on one fiber and analyzing the reflection spectrum of that fiber by the frequency division multiplexing method. A coherent wavelength-swept light source is necessary to acquire the reflection spectrum; however, but the sweep speed of a commercially available wavelength-swept light source that satisfies the condition is only approximately several tens of hertz. Therefore, we propose to conduct a wavelength sweep by injection current modulation of a laser diode. Wavelength sweeping with a modulated laser diode allows fast sweeping at 100 Hz or more, and the sweep range is as narrow as several hundred picometers. Reading only a part of the reflection spectrum of a low-reflective FBG-FPI sensor array using a modulated laser diode enables high-speed multipoint measurement through frequency analysis. Theoretical requirements for successful interrogation using the partial reflection spectrum are shown. A demonstration experiment to simultaneously measure strain applied to two low-reflective FBG-FPI sensors with a measurement time of 10 ms is reported.     

谱线和耦合系数对光纤激光器相互注入锁定的影响

相互注入锁定是相干合成技术中最具发展前景的方案之一. 在两个光纤激光器相互注入锁定的基础上,利用可调节光纤光栅改变其中一个激光器的中心谱线和利用不同的分束器改变两个激光器间的耦合系数,同时监测两个激光器的远场光斑和谱线,判断激光器谱线和耦合系数对相互注入锁定的影响. 通过理论和实验得到：提高耦合系数可以提高锁定的频率差;两个激光器的谱线越接近锁定的效果越好,当激光器的谱线超过一定范围时,两个激光器将不能被锁定. 关键词： 光纤激光器 相干合成 相位锁定     

High frequency microwave signal generation using dual-wavelength emission of cascaded DFB fiber lasers with wavelength spacing tunability

A dual-wavelength fiber laser source based on two cascaded phase-shifted fiber Bragg gratings is presented. The gratings are written in an erbium-doped fiber, each configuring the cavity of a distributed feedback fiber laser. The spacing between lasing modes is controlled dynamically by the use of piezoelectric actuators. A continuous tuning range of 5-724 pm of the wavelength difference, which is equivalent to a photodetected 0.72-92 GHz range, is obtained. Efficient generation from the L to the W microwave and millimeter bands has been achieved by heterodyne photodetection of the dual-wavelength optical signal.     

Tunable filter based on a pair of special long-period fiber gratings and its application in fiber ring laser

A filter based on a Mach-Zehnder interferometer (MZI) formed by cascading a pair of longperiod fiber gratings (LPFGs) written by CO₂ laser in a twisted single-mode fiber is proposed. The transmission spectrum of the filter is tuned by modifying the bending curvature of the MZI, and thus wavelength tuning is achieved. And a tunable erbium-doped fiber (EDF) ring laser employing the filter as a wavelengthselection component is experimentally demonstrated. The laser can be tuned continuously from 1573.05 to 1595.75 nm with a side-mode suppression ration of ∼50 dB over the total tuning range.     

Single-frequency thulium-doped distributed-feedback fiber laser

We have successfully demonstrated a single-frequency distributed-feedback (DFB) thulium-doped silica fiber laser emitting at a wavelength of 1735 nm. The laser cavity is less than 5 cm long and is formed by intracore UV-written Bragg gratings with a phase shift. The laser is pumped at 790 nm from a Ti:sapphire laser and has a threshold pump power of 59 mW. The laser has a maximum output power of 1 mW in a single-frequency, single-polarization radiation mode and is tunable over a few nanometers. To the best of the authors' knowledge, this is the first report of a single-frequency DFB fiber laser that uses thulium as the amplifying medium. The lasing wavelength is the longest demonstrated with DFB fiber lasers and yet is among the shortest obtained for thulium-doped silica fiber lasers.     

32路光纤激光相干阵列的相位锁定

报道了32路光纤激光相干阵列的相位锁定实验研究。搭建了32路光纤激光相干阵列实验系统,基于现场可编程逻辑阵列(FPGA)设计制作了高速高精度相位控制器。当相位控制器执行随机并行梯度下降(SPGD)算法对各路激光的相位进行锁定时,相干阵列输出的激光功率与不进行相位锁定时相比提高了约26倍。     

分布反馈式光纤激光器的光热调谐方法

针对现有光纤激光器调谐方法通常不能兼顾调谐幅度和调谐速率的问题,提出了一种基于光热效应的调谐方法,有效兼顾了调谐幅度和调谐速率的要求。分析了光纤激光器的输出波长随泵浦功率变化而改变的实验现象,并以980 nm泵浦源自身调谐为例,进行了静态和动态实验,结果表明:基于光热效应的分布反馈式光纤激光器静态调谐幅度达392 MHz/100 mA,动态调谐速率超过2 kHz,验证了光纤激光器光热调谐方案的可行性。     

光纤激光器自组织相干阵列的理论分析

 分析了光纤激光器自相干阵列最简单的情况——两个光纤激光器之间的相位锁定过程。建立了该过程的数学模型，并对它的时间特性和输出光束的模式进行了理论和数值分析，讨论了运行频率差和耦合系数对锁相过程的影响。对于运行频率相等的两个光纤激光器，当两个光纤激光器初始相位差不为0时，相位锁定后的激光器阵列输出模式为异相模；对于运行频率不等的情形，给出了锁相运行对两个光纤激光器运行频率差的要求，并分析了降低这种要求的几种方法。