Temperature-Independent Strain Sensor Based on a Hi-Bi Photonic Crystal Fiber Loop Mirror

This work presents an optical sensor based on a highly birefringent photonic crystal fiber (Hi-Bi PCF) loop mirror. The length of the sensing head is 380 mm and its corresponding wavelength spacing between two interferometer minima is 8 nm. The optical sensor was characterized in strain and in temperature with an uncoated Hi-Bi PCF and with an acrylate coated Hi-Bi PCF. Different results for strain and temperature sensitivity were obtained. Relatively to the strain measurement, the sensor with the uncoated Hi-Bi PCF presents slightly less sensitivity (1.11 ) when compared with coated Hi-Bi PCF (1.21 ). For the temperature measurement and with the uncoated Hi-Bi PCF, the optical sensor is insensitive to temperature (0.29 pm/K).     

Fiber loop ringdown for physical sensor development: pressure sensor

A new method of developing optical fiber pressure sensors by use of a fiber loop ringdown scheme is described. The fiber loop ringdown system is characterized in terms of the ringdown baseline stability, fiber transmission loss, and fiber refractive index. The overall sensor performance is demonstrated by use of sensing forces applied to the sensor head. The current device can sense pressures in the range of 0 to 9.8 x 10(6) Pa, converted approximately from the applied forces. The sensor's linear response, repeatability, detection sensitivity, measuring dynamic range, and temperature tolerance are explored.     

Design of single-polarization wavelength splitter based on photonic crystal fiber

A new single-polarization wavelength splitter based on the photonic crystal fiber (PCF) has been proposed. The full-vector finite-element method (FEM) is applied to analyze the single-polarization single-mode guiding properties. Splitting of two different wavelengths is realized by adjusting the structural parameters. The semi-vector three-dimensional beam propagation method is employed to confirm the wavelength splitting characteristics of the PCF. Numerical simulations show that the wavelengths of 1.3 μm and 1.55 μm are split for a fiber length of 10.7 mm with single-polarization guiding in each core. The crosstalk between the two cores is low over appreciable optical bandwidths.     

Highly sensitive refractive index sensor based on two cascaded special long-period fiber gratings with rotary refractive index modulation

We present a refractive index (RI) sensor based on a fiber Mach-Zehnder interferometer (MZI) formed by two cascaded special long-period fiber gratings (LPFGs) with rotary refractive index modulation (RLPFGs), in which the coupling occurred between the guided mode and the high-order asymmetric cladding mode. The experimental results show that the RI sensitivity of a refractometer with an interaction length of 40?mm is up to 58.8?nm/RI in the range of 1.3344 to 1.3637, which is 3.5 times higher than that of an MZI formed by two normal LPFGs. The temperature sensitivity for the same parameters of an RLPFG-MZI is about 0.03?nm/°C. Such a kind of high-sensitivity, easy-to-fabricate and simple-structure interferometer may find applications in the chemical or biochemical sensing fields.     

一种带自反馈光纤谐振腔的全光纤MZI型梳状滤波器的理论研究

为了改善全光纤马赫-曾德尔干涉仪(MZI)型梳状滤波器(interleaver)的输出特性,提出了由一个2×2光纤耦合器和一个带自反馈光纤谐振腔的光纤耦合器级联组成的改善型的全光纤MZI型梳状滤波器。根据其结构,利用光纤传输理论和矩阵理论推导出了该器件的输出表达式,并进行了数值模拟分析。结果表明：该器件在合理参数的情况下,利用带自反馈光纤谐振腔引入的相位调节效应,使其25 dB截止带宽为46.7 GHz,约占50 GHz频率间隔的93.4%,输出光谱形态近似于方波。当存在传输损耗时,两束干涉光信号的幅度不存在明显的差异,降低了传输损耗对滤波器消光特性的影响,与传统的光纤谐振环辅助非平衡MZI型梳状滤波器相比,不仅降低了传输损耗对滤波器消光特性的影响,而且还将所需的光纤耦合器数目减少至2个,使其在未来的密集波分复用系统中能发挥重要作用。     

Effective way of reducing coupling loss between rectangular microwaveguide and fiber

We introduce an anamorphic photonic crystal fiber (PCF) produced by postprocessing techniques to improve the coupling loss between a conventional single-mode fiber and rectangular microwaveguide. One end of the round core is connected with the conventional fiber, and the other end of the rectangular core is connected with the rectangular microwaveguide, then the PCF is tapered pro rata. In this way, the loss of mode mismatch between the output of the conventional fiber and the input of the waveguide would be reduced, which results in enhanced coupling efficiency. The conclusion was confirmed by numerical simulation: the new method is better than straight coupling between the optical fiber and the rectangular microwaveguide, and more than 2.8 dB improvement of coupling efficiency is achieved.     

Polarization-maintaining photonic crystal fiber based quarter waveplate for temperature stability improvement of fiber optic current sensor

Fiber optic quarter waveplate (QWP) is widely used in all kinds of optical fiber systems such as fiber sensing systems. Specifically, in fiber optic current sensor (FOCS) system, which is now applied in power grids to measure current and monitor operation, fiber optic QWP is a key device and has serious impact on temperature stability and accuracy of the system. We fabricated a QWP using polarization-maintaining photonic crystal fiber (PM PCF), and studied its impact on the temperature characteristic of a home-made FOCS system. We also made a QWP with conventional polarization-maintaining fiber (PMF) for comparison purposes. The measured system error of the FOCS prototype using a QWP made of PM PCF between ?40°C and 70°C was 8.45‰, which is five times smaller than with a QWP made of conventional PMF.     

Selective excitation of LP01 mode in multimode fiber using solid-core photonic crystal fiber

To address the overwhelming bandwidth increase in premise backbones, an attractive alternative for selective mode excitation in multimode fiber (MMF) using solid-core photonic crystal fiber (PCF) is presented. The power coupling efficiency, differential mode delay, and bit-error rate performance of several structural designs of solid-core PCF waveguides are investigated for the selective excitation of mode LP01 in a MMF. The achieved coupling efficiency into mode LP01 is above 90% for PCF profiles with seven rings.     

Design of bent photonic crystal fiber supporting a single polarization

In this work, it is shown that the differential loss between the TE- and TM-polarized fundamental modes in a highly birefringent photonic crystal fiber (PCF) can be enhanced by bending the fiber. As a result, a design approach for single-mode single-polarization operation has been developed and is discussed. A rigorous full-vectorial H-field-based finite element approach, which includes the conformal transformation and the perfectly matched layer, is used to determine the single-polarization properties of such a highly birefringent PCF by exploiting its differential bending losses.     

Effects of fiber gripping methods on the single fiber tensile test: I. Non-parametric statistical analysis

Single fiber tensile tests using two different gripping methods were carried out on various fiber lengths. One method (the glue-tab grip method) consists of mounting a fiber onto a rigid tab as specified in ASTM C1557-03 using an adhesive, while the other (the direct grip method) involves directly clamping a fiber using poly(methyl methacrylate) blocks. The tensile moduli obtained by the glue-tab grip as a function of the fiber gauge length are clearly different from those of the direct grip for fiber length between 2 and 10 mm, based on graphical statistical analyses using kernel density and q–q plots. This difference is caused by the gripping effect. In addition, the tensile strains and strengths obtained by the glue-tab grip were different from those of the direct grip at the short fiber gauge lengths (i.e., 2–10 mm). The differences between the tensile properties (i.e., modulus, strain, and strength) for the two grips measured with the 60-mm fiber length were not statistically significant compared to the results with the short fiber lengths.     

High sensitivity of taper-based Mach-Zehnder interferometer embedded in a thinned optical fiber for refractive index sensing

A taper-based Mach-Zehnder interferometer (MZI) embedded in a thinned optical fiber is demonstrated as a highly sensitive refractive index (RI) sensor. A RI sensitivity of 2210.84 nm/RIU (refractive index unit) is obtained at the external RI of 1.40, which is ten times higher than that of normal taper- and long-period fiber grating (LPFG)-based sensors. The sensitivity can be further improved by decreasing the diameter of the thinned fiber and increasing the interferometer length of the MZI. The proposed MZIs have lower temperature sensitivities compared with normal fiber sensors, which is a desirable merit for RI sensors to reduce the cross sensitivity caused by thermal drift.     

一种高双折射低限制性损耗光子晶体光纤设计

本文设计了一种适用于长距离光纤通信的新型光子晶体光纤。该光纤包层内椭圆形和圆形空气孔呈交错排列,纤芯两侧为两个小椭圆空气孔。利用有限元分析方法对所设计光纤的传输特性进行分析并对其结构进行了优化,确定了最佳结构。结果表明,波长为1550 nm时,此新型光子晶体光纤在最佳结构下可提供高达3.51×10^-2的高双折射和低至1.5×10^-9 dB/m的限制性损耗。与现存的引入椭圆形空气孔的光子晶体光纤相比,本文中的光子晶体光纤的双折射系数有较大提高,限制性损耗系数降低了5个数量级。另外,本文还详细研究了光子晶体光纤的色散随光子晶体光纤结构的变化以及其布里渊增益特性,并分析了其可制造性。基于其高双折射和低限制性损耗特性,此种光纤可应用于长距离光纤通信系统。     

Characterization of tunable photonic crystal fiber directional couplers

We present a tunable photonic crystal fiber (PCF) directional coupler fabricated by a side-polishing method. The PCF directional coupler was modeled as a typical single-mode fiber-based directional coupler and analyzed using the improved effective-index method (IEIM). The characteristics of the PCF directional coupler such as the coupling coefficient and the coupling ratio were measured and found to be in good agreement with those predicted by the theoretical model. The PCF directional coupler exhibited an insertion loss of approximately 2 dB for a 3 dB coupler and was able to tune the coupling ratio between 0% and 100% by tilting the angle of the top side-polished quartz block against the fixed-bottom quartz block.     

基于光纤耦合器的全光纤链路锁相控制

提出了一种基于光纤耦合器的全光纤链路锁相方法。通过光纤交叉互联使得出射光束与本振激光之间以及光纤端面后向回光相互间在光纤耦合器内进行干涉以获得性能指标,利用随机并行梯度下降算法(SPGD)对性能指标进行盲优化控制,实现多路出射光束在光纤端面处的相位锁定。首先建立了这种锁相方法的模型,讨论其锁相控制的稳态条件。搭建了两路全光纤链路锁相实验,并对两路出射光纤激光进行准直发射,通过相机获得的远场干涉条纹图像来判定锁相精度。实验结果表明,在引入正弦相位扰动幅值4个波长和频率2 Hz的情况下,该锁相方法能够将10 s长曝光图像条纹对比度从开环时的0.25提升至闭环时的0.82,单帧短曝光图像条纹对比度从开环时的0.65提升至闭环时的0.98。     

Effect of beam waists on performance of the tunable fiber laser based on in-line two-taper Mach-Zehnder interferometer filter

A tunable fiber ring laser based on an in-line two-taper Mach-Zehnder interferometer (MZI) filter was realized, and the effect of beam waists of the tapers on performance of the laser was investigated with different beam waists of 70 μm, 49 μm, and 33 μm. Experimental results show that the tunable laser with MZI length of 1 m and beam waist of 49 μm can cover 16.1 nm with tuning steps of 0.07-0.5 nm, a bandwidth of 10 pm, and a side mode suppression ratio (SMSR) of 40-50 dB. Tuning range is not only determined by the number of the cladding modes but is also affected by the filter loss. Tuning step is determined by the differences of the effective refractive indexes between the cladding modes and the core mode. SMSR is determined by the balance between the extinction ratio of the filter and the cavity loss of the laser due to MZI filter.     

Design and optimization of broadband and polarization-insensitive dual-core photonic crystal fiber coupler

In this paper, we propose a novel (to our knowledge) broadband and polarization-insensitive dual-core photonic crystal fiber (PCF) coupler through the introduction of an elliptical-shaped central air hole to offset the slight birefringence arising from the dual core. With a full vectorial finite element method and anisotropic perfectly matched layers as the external boundaries, the impact of several fiber parameters on the coupling characteristics of dual-core PCF is investigated in detail. Through optimizing the main fiber parameters, including core diameter, size and ellipticity of the central air hole, and refractive index difference, broadband and polarization-insensitive characteristics are achieved in the wavelength range from 0.8 to 1.7 μm. The variation of the coupling ratio is stabilized at 50±1%, and the coupling ratio difference between x polarization and y polarization is less than 2% over the wavelength range. This dual-core PCF makes it easier to develop a 3 dB coupler over a wide wavelength for passive optical networks and large optical systems.     

Design and modeling of a photonic crystal fiber gas sensor

We report the modeling results of an all-fiber gas detector that uses photonic crystal fiber (PCF). The relative sensitivity of the PCF as a function of the fiber parameters is calculated. Gas-diffusion dynamics that affect the sensor response time is investigated theoretically and experimentally. A practical PCF sensor aiming for high sensitivity gas detection is proposed.     

Lensed fiber probes designed as an alternative to bulk probes in optical coherence tomography

We demonstrate a compact all-fiber sampling probe for an optical coherence tomography (OCT) system. By forming a focusing lens directly on the tip of an optical fiber, a compact sampling probe could be implemented. To simultaneously achieve a sufficiently long working distance and a good lateral resolution, we employed a large-mode area photonic crystal fiber (PCF) and a coreless silica fiber (CSF) of the same diameters. A working distance of up to 1270 microm, a 3 dB distance range of 2210 microm, and a transverse resolution of 14.2 microm were achieved with the implemented PCF lensed fiber; these values are comparable to those obtainable with a conventional objective lens having an NA of 0.25 (10 x). The performance of the OCT system equipped with the proposed PCF lensed fiber is presented by showing the OCT images of a rat finger as a biological sample and a pearl as an in-depth sample.     

Modal cutoff properties in germanium-doped photonic crystal fiber

The germanium-doped photonic crystal fiber (PCF) has some characteristics that differentiate it from pure-silica PCF for a germanium element being doped in the core, such as the intensified nonlinearity, the enhanced photosensitivity, and so on. To pave the way for the application of the Ge-doped PCF successfully, it is necessary to study its properties. We investigated the modal cutoff properties of Ge-doped PCF quantitatively by using the beam propagation method. The numerical results show that the effective refractive indices and the normalized frequency V of Ge-doped PCF not only depend on the normalized pitch delta/lambda but also depend on the normalized hole size d/delta, the modal cutoff boundary for the single mode-multimode of the Ge-doped PCF shift to the low d/delta side in contrast to the pure-silica PCF.     

Mode-locked ytterbium fiber lasers

A compact fiber laser is demonstrated with use of a Gires-Tournois compensator and a short length (2-4 cm-long) of highly doped ytterbium (Yb) fiber providing net anomalous group-velocity dispersion. With use of a novel semiconductor saturable absorber mirror based on GaInNAs structure, self-started 1.5-ps-pulse mode-locked operation was obtained at 1023 nm with a repetition rate of 95 MHz. A mode-locked Yb-doped fiber laser was developed without the use of any dispersion compensation technique. Overall group-velocity dispersion was minimized by using a short length of highly doped Yb fiber in a compact amplifying loop cavity. Self-started mode-locked operation was obtained in 980-1030-nm wavelength range with a fundamental repetition rate of 140 MHz.