Random fiber laser using a cascaded fiber loop mirror

Random fiber lasers(RFLs)have attracted extensive attention due to their rich physical properties and wide applications.Here,a RFL using a cascaded fiber loop mirror(CFLM)is proposed and presented.A CFLM with 10 fiber loop mirrors(FLMs)is simulated by the transfer matrix method and used to provide random feedback.Multiple spikes are observed in both the simulated and measured reflection spectra.The RFL operates in a single longitudinal mode near the threshold and a time-varying multilongitudinal mode at higher pump powers.The RFL exhibits a time-varying radio-frequency spectrum.The Lévy–Gaussian distribution transition is observed,as in many RFLs.The operation mechanism of the lasing longitudinal modes and the impact of complex mode competition and mode hopping on the output characteristics are discussed through experimental and theoretical results.In this study,we unveil an artificial random feedback structure and pave another way for the realization of RFLs,which should be a platform for multidisciplinary studies in complex systems.

A tunable comb filter using single-mode/multimode/polarization-maintaining-fiber-based Sagnac fiber loop

A novel tunable comb filter composed of a single-mode/multimode/polarization-maintaining-fiber-based Sagnac fiber loop is proposed and experimentally demonstrated. The filter tunability is achieved by rotating the polarization controller. The spectral shift is dependent on rotation direction and the position of the polarization controller. In addition, the adjustable range achieved by rotating the half-wave-plate polarization controller is twice higher than that of the quarter-wave-plate one.

Amplification and Compression of Ultrashort Fundamental Solitons in An Erbium-Doped Nonlinear Amplifying Fiber Loop Mirror

A nonlinear amplifying loop mirror constructed from erbium-doped fiber is proposed for simultaneous amplification and compression of ultrashort fundamental solitons. Numerical simulations show that, the proposed device performs efficient high-quality amplification and compression of solitons.

Fiber loop ring-down cavity integrated U-bent single-mode-fiber for magnetic field sensing

A novel magnetic field sensing system based on the fiber loop ring-down technique is proposed in this paper. In the fiber loop, a U-bent single-mode-fiber structure coated with magnetic fluid(MF) serves as the sensing head, and an erbium-doped fiber amplifier(EDFA) is introduced to compensate for the intrinsic loss of the cavity. The ring-down time of the system varies with the change of applied magnetic field due to the tunable absorption coefficient and refractive index of the MF. Therefore, measurement of the magnetic field can be realized by monitoring the ringdown time. The experimental results show that the performance of the system is extremely dependent on the interrogation wavelength, because both the gain of the EDFA and the loss of the sensing head are wavelength dependent.We found that at the optimal wavelength, the ratio of the gain to loss attained its maximum. The sensing system was experimentally demonstrated and a sensitivity of-0.5951 μs∕Oe was achieved.

Recent Progress of Fiber Loop Mirror-Based Sensors in China Jiliang University

The authors overview recent progress of fiber loop mirror based sensors in China Jiliang University. With the unique characteristics of fiber loop mirror made by various inserted fibers, fiber loop mirror based sensors have the potential to be exploited in a variety of wide applications.

Temperature-Independent Torsion Sensor Based on ＂Figure-of-Eight＂ Fiber Loop Mirror

An interrogation sensor system combining the ＂figure-of-eight＂ fiber loop mirror using a single directional 3×3 fiber optic coupler was proposed. One fiber loop mirror was formed by inserting a length of high birefringent optical fiber at the input ports of the 3×3 coupler. Splicing the output ports of the 3×3 coupler between them created the other fiber loop mirror. The introduction of this second loop gave rise to two polarization states of light with the same frequency but different optical phase. The mechanical torsion sensing head was located at the second loop and was exhibited an average modulus torsion sensitivity of 7.9×10^-4 degree/dB. The performance of the sensor was not affected by environmental temperature variations. Keywords： Torsion sensor, fiber loop mirror, interferometer

Wavelength-Tunable Single Frequency Ytterbium-Doped Fiber Laser with Loop Mirror Filter

By using a loop mirror filter, a novel wavelength-tunable single-frequency ytterbium-doped fiber laser is developed to select single longitudinal modes in a linear cavity. The output wavelength could be tuned 2.4 nm intervals range from 1063.3 to 1065.Tnrn with the temperature change of the fiber Bragg grating. The maximum output power could reach 32 m W while the pump power increases to 120 m W. The corresponding optical-to-optical conversion efficiency is 26.7% and the slope efficiency is 33.9%, respectively. The output power fluctuation is below 2%, and its highest signal-to-noise ratio is 60 dB.

Fiber-Loop Ringdown Multi-Function Sensors

We demonstrate a fiber-loop ring down multi-function sensors system, which can be used to measure refractive index and curvature simultaneously. Good agreement has been found between theoretical analyses and experimental results. It has great potential for sensor applications.

Wideband All-Polarization-Maintaining Yb-Doped Mode-Locked Fiber Laser Using a Nonlinear Optical Loop Mirror

We report on a wide-band and stable mode-locked all-polarization-maintaining fiber laser configuration using a nonlinear optical loop mirror. The central wavelength of the laser is 1080.14nm and the 3dB bandwidth is 20.29nm. The repetition rate of the pulse is 3.28 MHz and the pulse width is 848ps. By tuning the pump power, which is centered at 980nrn, from 300mW to 380mW, we obtain a linearly changed output power from 6row to 7.12roW. The all-polarization-mMntaining fiber configuration is fundamental to the stability of the output power.

Fabrication and formation mechanism of closed-loop fibers by electrospinning with a tip collector

Electrospun nanofibers with designed or controlled structures have drawn much attention. In this study, we report an interesting new closed-loop structure in individual cerium nitrate/polyvinyl alcohol（Ce（NO3）3/PVA） and Na Cl/PVA fibers,which are fabricated by electrospinning with a nail collector. The electrospinning parameters such as voltage and Ce（NO3）3（or Na Cl） concentration are examined for the formation of the closed-loop structure. The results suggest that the increase of the spinning voltage or addition of Ce（NO3）3（or Na Cl） is favorable for the formation of the closed-loop structure, and the increase of loop numbers and the decrease of loop size. Further analyses indicate that the formation mechanism of the closed-loop fibers can be predominantly attributed to the Coulomb repulsion in the charged jets.

Dynamic Free-Spectral-Range Measurement for Fiber Resonator Based on Digital-Heterodyne Optical Phase-Locked Loop

We propose a novel scheme, based on digital-heterodyne optical phase-locked loop with whole-fiber circuit, to dynamically measure the free-spectral-range of a fiber resonator. The optical phase-locked loop is established with a differential frequency-modulation module consists of a pair of acousto-optic modulators. The resonance-tracking loop is derived with the Pound-Drever-Hall technique for locking the heterodyne frequency of the OPLL on the frequency difference between adjacent resonance modes. A stable locking accuracy of about 7 × 10?9 and a dynamic locking accuracy of about 5 × 10?8 are achieved with the FSR of 8.155 MHz, indicating a bias stability of the resonator fiber optic gyro of about 0.1?/h with 10 Hz bandwidth. In addition, the thermal drift coefficient of the FSR is measured as 0.1 Hz/?C. This shows remarkable potential for realizing advanced optical measurement systems, such as the resonant fiber optic gyro, and so on.

Combined optical fiber interferometric sensors for the detection of acoustic emission

A type of combined optical fiber interferometric acoustic emission sensor is proposed. The sensor can be independent on the laser source and make light interference by matching the lengths of two arms,so it can be used to monitor the health of large structure. Theoretical analyses indicate that the system can be equivalent to the Michelson interferometer with two optical fiber loop reflectors,and its sensitivity has been remarkably increased because of the decrease of the losses of light energy. PZT is powered by DC regulator to control the operating point of the system,so the system can accurately detect feeble vibration which is generated by ultrasonic waves propagating on the surface of solid. The amplitude and the frequency of feeble vibration signal are obtained by detecting the output light intensity of interferometer and using Fourier transform technique. The results indicate that the system can be used to detect the acoustic emission signals by the frequency characteristics.

Fiber Optic Sensors and Sensor Networks Using a Time-domain Sensing Scheme

Fiber loop ringdown (FLRD) has demonstrated to be capable of sensing various quantities, such as chemical species, pressure, refractive index, strain, temperature, etc.;and it has high potential for the development of a sensor network. In the present work, we describe design and development of three different types of FLRD sensors for water, cracks, and temperature sensing in concrete structures. All of the three aforementioned sensors were indigenously developed very recently in our laboratory and their capabilities of detecting the respective quantities were demonstrated. Later, all of the sensors were installed in a test grout cube for real-time monitoring. This work presents the results obtained in the laboratory-based experiments as well as the results from the real-time monitoring process in the test cube.

Optic Fiber-Based Dynamic Pressure Sensor

Weigh-in-Motion（WIM） technique is the process of measuring the dynamic tire forces of a moving vehicle and estimating the corresponding tire loads of the static vehicle. Compared with the static weigh station, WIM station is an efficient and cost effective choice that will minimize unneccessary stops and delay for truckers. The way to turn birefringence of single-mode fiber into a prime quality for a powerful and reliable sensor is shown. Preliminary results for the development of a weigh-in-motion （WIM） technique based on sagnac-loop sensor are presented. After a brief description of the sensor and its principle of operation, the theoretical model characterization made in is developed. Then, a full static conditions is presented.

Pump-induced carrier envelope offset frequency dynamics and stabilization of an Yb-doped fiber frequency comb

In this paper, we demonstrate a carrier envelope phase-stabilized Yb-doped fiber frequency comb seeding by a nonlinear-polarization-evolution（NPE） mode-locked laser at a repetition rate of 60 MHz with a pulse duration of 191 fs.The pump-induced carrier envelope offset frequency（ f0） nonlinear tuning is discussed and further explained by the spectrum shift of the laser pulse. Through the environmental noise suppression, the drift of the free-running f0 is reduced down to less than 3 MHz within an hour. By feedback control on the pump power with a self-made phase-lock loop（PLL）electronics the carrier envelope offset frequency is well phase-locked with a frequency jitter of 85 m Hz within an hour.

基于光纤环衰荡技术的锥形光纤磁场测量方法

针对目前光纤磁场传感器解调方法中存在的实用性差且造价昂贵的问题,提出了一种基于光纤环衰荡技术的锥形光纤磁场测量方法,实现了磁场的时间域解调。施加不同强度的磁场时,光在锥形光纤处引起的损耗不同,导致系统衰减时间不同。将系统衰减时间和磁场强度建立联系,实现了磁场测量。为降低传感单元温度和磁场之间的交叉敏感,设计了温度控制电路,将传感单元的温度设定在25.0℃。实验结果表明,该传感器在磁场强度2 mT~9 mT范围内时,磁场强度H和衰减时间τ具有R^(2)=0.98273的线性拟合度,计算得到该传感器的磁场灵敏度为-11.65μs/mT。与光纤磁场传感器的其他解调方式相比,采用光纤环衰荡技术的磁场测量方法,具有响应速度快、成本低、稳定性好的优点。

基于OTDR的智能变电站光纤回路检测改进方法

【目的】在智能变电站建设过程中,为精确检测智能变电站光纤回路中光纤熔接、受压等因素导致的非反射性事件,提出了基于光时域反射仪(OTDR)的智能变电站光纤回路检测改进方法。【方法】通过时钟移相模块进行移相,并进行多次重复测量,对各相位数据进行插值组合,得到更高的采样频率。这样可以在不增加硬件成本的基础上大幅提高OTDR的采样精度。同时通过小波去噪和突变量信号增强技术,抑制噪声,突出增强事件点的小波系数,使有用的信号更加容易被检测出来,提高了智能变电站光纤回路的检测水平。【结果】经过现场试验检测,该方法能够精准地检测出光纤受压、过度弯曲、熔接不良等事件点。【结论】该方法能够有效检测智能变电站光纤回路中存在的异常现象,防止隐蔽性缺陷影响变电站的运行,具有较大的工程应用价值。

10 MHz氢钟信号传递系统

为满足同一科研园区内不同建筑之间10 MHz氢钟信号(HCS)长期稳定度共享的需求,提出了一种低成本、集成化的基于光纤链路的10 MHz HCS传递完整解决方案。该方案采用1 GHz的射频信号对激光光强进行调制,利用光纤实现信号传递。通过将远端反射信号与本地信号和频后直接与待传递的HCS分频鉴相,输出误差信号反馈控制1 GHz信号的频率,实现远端1 GHz信号与本地HCS之间的相位锁定,从而使远端1 GHz信号具有与本地HCS相同的频率稳定度;之后再通过分频器在远端生成10 MHz信号,作为射频参考输出。实验验证了该方案的频率传递保真度,该系统在200 m往返光纤上的附加频率稳定度(艾伦偏差)为1 s平均时间2.4×10^(-13)和10000 s平均时间5.7×10^(-17);在20 km传递距离上,附加频率稳定度(艾伦偏差)为1 s平均时间4.8×10^(-13)和10000 s平均时间2.1×10^(-16)。研究结果表明该系统的长时间频率传递稳定度优于HCS的频率稳定度,可以满足千米范围内氢钟信号共享的需求。

全保偏飞秒光纤激光器异步锁定技术研究

基于全保偏掺铒光纤激光器、锁相环系统和太赫兹测距光路搭建了一套太赫兹双光梳测距系统。所采用的全保偏掺铒光纤激光器重复频率为79.261 MHz。利用压电陶瓷(piezoelectric ceramics,PZT)和步进电机(stepper motor,SM)双级反馈控制的方案,实现了重复频率锁定和重复频率1.54 MHz可调。使用频率计数器对双光梳重复频率锁定效果进行监测,重复频率锁定的峰峰值抖动为±1.5 mHz,抖动标准差为0.4 mHz。将双光梳重频差设置在10 Hz,10 min内重复频率差最大抖动为3 mHz,标准差为0.6 mHz。进一步将异步采样双光梳系统应用于太赫兹测距,测量移动距离的误差为3µm。该系统具有锁定精度高,稳定性强等优势,有望应用于生物无损检测和工业精密加工中。

基于压电陶瓷振镜的单模光纤闭环补偿系统设计

在激光通信过程中,由于卫星平台的振动、机械转台的噪声等会使入射光轴与接收光轴之间存在随机角抖动,影响通信系统的性能。研究了随机角抖动对空间光到单模光纤耦合效率的影响,提出基于CCD相机和快速倾斜镜的闭环补偿系统。在实验室内搭建实验平台,测试了不同抖动幅度对单模光纤耦合效率的影响,验证了补偿系统对提高耦合效率的可行性。实验结果表明,闭环补偿系统最大可提高耦合效率为24%;在同样的抖动频率下,抖动幅度增加时,其补偿效果更明显;补偿系统对于低频抖动的补偿效果比高频更加明显。这些研究内容为解决随机角抖动对单模光纤耦合效率的影响提供了数据参考与技术支持。