Fiber-Bragg-grating force sensor based on a wavelength-switching actively mode-locked erbium-doped fiber laser

A fiber-Bragg-grating (FBG) transverse-force sensor based on a wavelength-switching actively mode-locked erbium-doped fiber laser is proposed, in which a FBG is used as both the sensing element and the wavelength-selection element of the laser. When a force is applied to the FBG, the induced birefringence in the FBG causes the laser to emit pulses at two close wavelengths, whose separation is proportional to the applied force. To suppress the interference between the two wavelengths, the laser is made to emit at the two wavelengths alternately by use of a polarization-switching technique. The wavelength separation is converted into a time difference by transmission of the laser pulses through a dispersive single-mode fiber, so the wavelength measurement is replaced by the less-expensive time measurement. The output of the sensor is insensitive to temperature and axial strain changes along the FBG. To interrogate similar FBG sensing elements connected in series it is necessary only to change the modulating frequency of an electro-optic modulator to select the corresponding laser cavity. The practicability of this approach was demonstrated experimentally with two multiplexed sensing elements.     

高双折射光纤光栅扭转特性分析与应用

通过对高双折射光纤光栅的特性进行分析,提出了一种新型的可埋入工程材料的扭转传感器结构。基于光纤光栅的模耦合理论和高双折射光纤的扭转特性,建立了扭转高双折射光纤光栅传输光波的数学模型,并分析其光谱特性,仿真结果表明扭转高双折射光纤光栅的反射谱不具备方向性,只与扭转角度大小有关。当扭转角度从0变化到360°时,高双折射光纤光栅的2个主峰波长都随之产生漂移,x方向的光向短波长方向移动,y方向的光向长波长方向移动,并且反射谱峰值逐渐减小,通过前向耦合模理论与反向耦合模理论结合分析并仿真,得出x和y方向上波峰值变化量,以及相应的中心波长的变化量,最后通过实验数据对高双折射光纤光栅的变化参数与扭转角度的关系作出验证。     

Side-hole two-core microstructured optical fiber for hydrostatic pressure sensing

A novel side-hole two-core microstructured optical fiber (STMOF) is proposed for hydrostatic pressure sensing. The two solid fiber cores are surrounded by a few small air holes and two large air holes, and are separated by one small air hole in the center of the cross section of the STMOF. The two large air holes that we called side holes essentially provide a built-in transducing mechanism to enhance the pressure-induced index change, which ensures the high sensitivity of the hydrostatic pressure sensor based on the STMOF. Mode coupling between the two fiber cores of the STMOF has been investigated, which provides a pressure-dependent transmission spectrum by injecting a broadband light into one fiber core of the STMOF on one side and detecting output spectrum on another fiber core on the other side. Our simulations show that there is a one-to-one correspondence between the hydrostatic pressure applied on the STMOF and the peak wavelength shift of the transmission spectrum. A hydrostatic pressure sensor based on an 8 cm STMOF has a sensitivity of 0.111 nm/Mpa for the measurement range from 0 Mpa to 200 Mpa. The performances of hydrostatic pressure sensors based on STMOFs with different structure parameters are presented.     

Measurement of pressure and temperature sensitivities of a Bragg grating imprinted in a highly birefringent side-hole fiber

The sensing characteristics of a Bragg grating imprinted in a specially developed highly birefringent side-hole fiber were investigated in detail. We showed that such a grating has almost identical sensitivities to temperature for both linearly polarized modes LP01(x) and LP01(y) (approximately 5.9 pm/K) and significantly different sensitivities to hydrostatic pressure (-1.93 pm/MPa for LP01(x) and +5.37 pm/MPa for LP01(y) mode). The sensitivity differences versus mode polarization are so high that this effect can be employed for simultaneous sensing of temperature and pressure by interrogation of the wavelength shifts at LP01(x) and LP01(y) modes. Applying interferometric methods, we also measured the sensitivity of the host side-hole fiber to temperature and hydrostatic pressure for each polarization mode. Our results show that there is good agreement between the normalized sensitivities of the host side-hole fiber and those of the grating for the same polarization modes.     

Curvature sensor using a highly birefringent photonic crystal fiber with two asymmetric hole regions in a Sagnac interferometer

A curvature sensor based on a highly birefringent (Hi-Bi) photonic crystal fiber inserted into a Sagnac interferometer is demonstrated. For this purpose, a novel Hi-Bi photonic crystal fiber was designed and fabricated. Half of the microstructured region of the photonic crystal fiber was composed by large diameter holes, while the other half contained small diameter holes. Because of this geometry, the fiber core was shifted from the center and high birefringence appears in the optical fiber. Curvature was applied for three different fiber directions for a range of 0.6-5 m(-1). Temperature and longitudinal strain was also characterized for constant curvature. The configuration showed insensitivity to these two physical parameters.     

Characterization of birefringence dispersion in polarization-maintaining fibers by use of white-light interferometry

A new method for measuring the birefringence dispersion in polarization-maintaining fibers (PMFs) with high sensitivity and accuracy is presented. The method employs white-light interferences between two orthogonally polarized modes of PMFs. The group birefringence of the fiber is calibrated first. Then the birefringence dispersion and its variation along different fiber sections are acquired by analyzing the broadening of interferograms at different fiber lengths. The main sources of error are investigated. Birefringence dispersions of two PANDA fibers at their operation wavelength are measured to be 0.011 ps/(km nm) and 0.018 ps/(km nm). A measurement repeatability of 0.001 ps/(km nm) is achieved.     

Digital demodulation system for low-coherence interferometric sensors based on highly birefringent fibers

A new type of demodulation system for low-coherence interferometric sensors based on highly birefringent fibers is presented. The optical path delay introduced by the sensor is compensated in four detection channels by quartz crystalline plates of appropriate thickness. The system can be used to decode a single-point sensor with a resolution of 2.5 x 10(-3) or two serially multiplexed sensors with decreased resolution. In a multiplexed configuration each sensor is served by two detection channels. By tilting the quartz plates, we can tune the initial phase shift between interference signals in successive channels to differ by pi/8 or pi/4, respectively, for a single-point or a multiplexed configuration. We transferred the sinusoidal intensity changes into digital pulses by appropriate electronic processing, which eventually allows for an unambiguous phase-shift measurement with a resolution of 1/8 or 1/4 of an interference fringe. The system performance for the measurement of hydrostatic pressure changes and simultaneous changes of hydrostatic pressure and temperature is demonstrated. The pressure sensors are based on side-hole fiber to ensure high sensitivity and an operation range of 2.4 MPa. A new configuration for temperature compensation of hydrostatic pressure sensors is proposed, which is better suited for dynamic pressure measurements. In this configuration the sensing and compensating fibers are located in the same compartment of the sensor housing.     

Study of birefringence of elliptical core photonic crystal fiber using Mathieu function

The birefringence of the elliptical core photonic crystal fiber with circular pores in the cladding has been studied by using higher order Mathieu functions. It is observed that the birefringence decreases with decreasing wavelength. Calculated results also indicate the sensitivity to the radius of the pores in the cladding. High birefringence up to 0.0079 is obtained. The efficacy of this proposed method is proved by comparing the results.     

Polarimetric distributed feedback fiber laser sensor for simultaneous strain and temperature measurements

We report the application of a dual polarization distributed feedback (DFB) fiber laser as a strain and temperature sensor. By measurement of the absolute wavelength of one polarization as well as the polarization beat frequency, strain and temperature were determined simultaneously. The sensor has an accuracy of +3 microepsilon and +/-0.04 degrees C. Self-heating of the DFB fiber laser as a function of pump power was measured with this sensor.     

Design of ultra-flattened nearly zero dispersion and ultrahigh non-linear slot microfibres with ultrahigh birefringence

We proposed a new simple design of microfibre employing an elliptical silica rod in the centre of fibre core region as a slot core for the purpose of controlling the chromatic dispersion properties of the microfibre and enhancing the performance of non-linearity and birefringence. The simulation results show that the proposed slot microfibre has ultra-flattened near-zero dispersion of 0.94 ps/(nm km) for quasi-TE mode over a 50-nm wavelength range, ultrahigh birefringence up to the order of 10^?1, and ultrahigh non-linear coefficients of 38.35 and 37.92 W^?1 m^?1 for the fundamental quasi-TE mode and quasi-TM mode at the wavelength of 1.55 μm. The outstanding advantage of this new design is that nearly zero ultraflattened dispersion, ultrahigh modal birefringence and ultrahigh non-linearity can be realized simultaneously simply using a slot fibre core. Benefiting from its excellent performance, the proposed slot microfibre will have great potential for all-optical signal processing applications.     

基于双折射效应的光纤光栅称重方法研究

与通常的光纤光栅传感器原理不同,本文基于光纤在外界径向载荷作用下产生的双折射效应为原理,建立了一套简单的称重实验装置,并对其检测原理进行了详细的分析。实验表明,这种光纤光栅称重传感器适合大载荷情况的重量检测问题,特别适合于公路计重收费系统中对车辆载重的监测领域,其称重灵敏度可达5kg。     

Intracore fiber bragg gratings for strain measurement in embedded composite structures

An intracore Bragg grating written on a photosensitive fiber core is used for strain measurement in composite specimens under load. The strain information is directly related to the absolute change in the Bragg-reflected wavelength. Fiber Bragg grating (FBG) sensors (fibers with intracore gratings) are thus sensitive to strain that is caused by changes in temperature as well as to load-induced changes. Thus these sensors can be made to be independent of source intensity variations and losses. FBG sensors used for load-induced strain sensing in composite structures and the effects of temperature on them are discussed. A detailed account of the use of such embedded structures as self-monitoring nondestructive testing devices is given.     

Dispersion and birefringence of irregularly microstructured fiber with an elliptic core

We have investigated the dispersion and birefringence of an irregularly microstructured fiber with an elliptic silica core and irregular airholes. The polarization-dependent output power through the fiber reveals two well-defined principal-axis modes despite the irregularity of airholes. The dispersion of the fiber is measured in the range of 680 to 1000 nm using the Mach-Zehnder interferometric technique with sub-10 fs laser pulses, which yield two zero dispersion wavelengths at 683 and 740 nm for the two principal modes, respectively. The birefringence measured using the wavelength scanning method is about 0.0055 at 800 nm. It is also demonstrated that this irregularly microstructured fiber with high birefringence and short zero dispersion wavelength is useful for the one-octave-spanning supercontinuum generation suitable for an f-2f interferometric system.     

Simultaneous measurement of temperature and pressure by a single fiber Bragg grating with a broadened reflection spectrum

Simultaneous measurement of temperature and pressure with a single fiber Bragg grating (FBG) based on a broadened reflection spectrum is proposed and experimentally demonstrated. A novel double-hole structure of a cantilever beam is designed, and a FBG is affixed on the nonuniform strain area of the cantilever beam. The Bragg reflection bandwidth is sensitive to the spatially gradient strain but is free from the spatially uniform temperature. The wavelength peak shift and the bandwidth broadening of the FBG with a change of temperature and pressure allow for simultaneous discrimination between the temperature and the pressure effects. Standard deviation errors of 1.4 degrees C and 1.8 kPa were obtained with temperature and pressure ranges of 20 degrees C-100 degrees C and 0-80 kPa, respectively. This novel and low-cost sensor approach has considerable potential applications for temperature-insensitive strain measurement.     

Investigation of a half-wave method for birefringence or thickness measurements of a thick, semitransparent, uniaxial, anisotropic substrate by use of spectroscopic ellipsometry

A half-wave method of measurement of wafer birefringence that is based on interference fringes recorded from a uniaxial wafer by use of a standard phase-modulated spectroscopic ellipsometer is investigated. The birefringence of uniaxial wafers is calculated from the extremal points in the recorded oscillating intensities. A formalism is developed to incorporate the change in birefringence with wavelength as a correction factor. The correction explains the overestimation of the birefringence from previous similar research on thick uniaxial sapphire substrates. The enhanced derivative of the birefringence that is due to polarization-dependent intraconduction band transitions is detected. Furthermore, for well-characterized wafers it is shown that this method can be used in wafer-thickness mapping of 4H-SiC and similar uniaxial high-bandgap semiconductors.     

温度应变同时测量的光纤光栅系统的研究

为了同时传感温度和应变,在同一根光纤的两个不同地方写入相同的Bragg光栅,设计制成一种特殊机构的传感系统。由于测量结构特殊,两处光纤光栅的反射峰具有成比例的应变响应系数和不同的温度响应,测出温度后,应变也同时算出。实际测量表明,该系统结构简单,测量精度高,能进行实时测量。     

Simultaneous measurement for liquid level and refractive index based on all-fiber modal interferometer

A simple fiber sensor capable of simultaneous measurement of liquid level and refractive index (RI) is proposed and experimentally demonstrated. The sensing head is an all-fiber modal interferometer manufactured by splicing an uncoated single-mode fiber with two short sections of multimode fiber. The interference pattern experiences blue shift along with an increase of axial strain and surrounding RI. Owing to the participation of multiple cladding modes with different sensitivities, the height and RI of the liquid could be simultaneously measured by monitoring two dips of the transmission spectrum. Experimental results show that the liquid level and RI sensitivities of the two dips are 245.7 pm/mm, ?38 nm/RI unit (RIU), and 223.7 pm/mm, ?62 nm/RIU, respectively. The approach has distinctive advantages of easy fabrication, low cost, and high sensitivity for liquid level detection with the capability of distinguishing the RI variation simultaneously.     

A new visualization technique for short fibers in a slit flow of fiber suspensions

A classical method for visualization of transparent fibers in fiber suspensions using a planar light source is examined by geometrical optics and its defect is revealed. To overcome the defect, a new method using fibers with birefringence is proposed. In the present method the crossed polarizers rotating at a high speed are introduced to observe the fibers with birefringence nearly oriented in the polarizing directions. As a result, the applicability of this method to the visualization of the fibers in a concentrated fiber suspension flow through a slit channel has been confirmed, and its usefulness for the measurement of the fiber orientation and fiber concentration distribution has been demonstrated. Furthermore, our experimental relationship of total shear strain vs. orientation angle in a dilute fiber suspension flow is compared with a theoretical result, and it shows the similar tendency to the theoretical one.     

Method for angular alignment of birefringent fibers in fiber-opticpressure/strain measurement

A simple method for angular alignment of principal birefringence axes of a highly birefringent (HB) polarization-maintaining (PM) optical fiber is presented. The method is based on longitudinal strain-induced cross-coupling phenomena which occur in HB fibers and it can be straightforwardly applied to construct a fiber-optic pressure or strain sensor independent of disturbing thermal effects. The method is also particularly suitable for splicing birefringent fiber pigtails to laser diodes in all-fiber configuration for a fiber-optic pressure (strain) sensor