Study on the quality of interference fringes from a pulsed UV source for application in a biprism based fiber Bragg grating writing

This paper presents a study on the quality of interference fringes formed from a pulsed UV (255 nm, 5.6 kHz, and 40 ns) source for an application in writing fiber Bragg gratings (FBGs). The interference fringes of separation of about 8 microm, formed by a biprism of apex angle 2 degrees , were studied for their contrast, evolution of contrast, and positional and intensity stability over a period of 5 min (over 10(6) pulses). Second harmonic UV (255 nm) sources of different spatial coherence and pointing stability characteristics were employed as the inputs. It is established that the UV fringes contrast and interference pattern stability with time is largely controlled by the optical resonator geometry of the fundamental wavelength (510 nm) copper vapor laser (CVL) oscillator. In particular, the generalized diffraction filtered resonator (GDFR) CVL produced the highest quality second harmonic beam with the highest fringes contrast and stability. The implications of these results were studied by employing these UV sources for the fabrication of the C-band FBGs by a 24 degrees apex angle biprism.     

基于狭缝整形的低损耗飞秒光纤光栅制备

为了解决飞秒激光逐点法制备的光纤布拉格光栅（Fiber Bragg Gratings, FBG）损耗较高的问题,利用逐面法完成飞秒光纤光栅制备的实验研究。运用高斯光束传播的基本理论,通过狭缝整形技术对聚焦的飞秒激光能量分布进行等高宽整形,突破在光纤横截面内诱导产生圆形折射率调制的难点,最终制备得到低损耗的飞秒光纤光栅。开展不同狭缝宽度制备FBG的光谱特性对比实验,结果表明：利用光斑直径为5.0 mm的飞秒激光光束刻写FBG时,采用宽度为1.7 mm的狭缝制备得到的FBG插入损耗降低至0.15 dB,短波损耗降低至0.5 dB,验证了基于狭缝整形的低损耗飞秒光纤光栅制备方法的有效性。针对狭缝法制备的FBG反射率分散问题,提出控制折射率匹配液填充量以及调整飞秒激光能量的方法,并优化光束聚焦流程,成功降低FBG反射率的分散度。本研究对推动飞秒光纤光栅在大容量、高链路损耗等环境中的实际应用具有重要意义。     

Single-pulse fiber Bragg gratings and specific coatings for use at elevated temperatures

The technique of recording fiber Bragg gratings (FBGs) with single exposure pulses during the fiber drawing process allows production of such gratings in complex array structures, with high mechanical strength of the fiber and in a simple and cost-efficient way. This is of special interest for the growing field of fiber sensor applications with FBGs. A general advantage of fiber sensor systems is their ability to be used also at elevated temperatures compared with conventional electric or electronic sensors. For this purpose, the fiber itself as well as the grating structure and the fiber coating should be stable under such elevated temperature conditions. We have investigated different coating materials and possibilities of making temperature-stable FBGs of types I and II in the range of 100 degrees C-1000 degrees C with good reflection efficiency by single-pulse exposure during the fiber drawing process.     

Switchable dual-wavelength fiber laser mode-locked by carbon nanotubes

We have proposed a switchable mode-locked fiber laser by means of carbon nanotube saturable absorber and fiber Bragg gratings (FBGs). The single-wavelength mode-locking operation can be switched between 1549.5 and 1559.5 nm, respectively, which correspond to the central wavelengths of two FBGs. With the appropriate setting of polarization controller, the stable dual-wavelength operation can be achieved due to the high stability of saturable absorber based on carbon nanotubes. Our method provides a simple, stable, low-cost, dual-wavelength ultrafast-pulsed source.     

Demonstration of a rapidly strain tuned Er3+-doped fiber laser for sensitive gas detection

We describe a novel combination of a diode-pumped, wavelength-modulated Er(3+)-doped fiber laser light source with a sensitive noise cancellation circuit for detection of acetylene and ammonia. The laser tuning element, a fiber Bragg grating, was mounted in such a way that it could be strained controllably and rapidly, allowing noise cancellation techniques to be applied to the wavelength-modulated output of the fiber laser. The experimental setup is relatively simple and can be extended to other fiber laser wavelengths for which semiconductor lasers are not readily available by selection of a different fiber Bragg grating and gain medium.     

Efficient linearly polarized ytterbium-doped fiber laser at 1120 nm

We report a 20 W linearly polarized, spectrally clean Yb-doped fiber laser at 1120 nm with an optical conversion efficiency of 54%. An excellent polarization extinction ratio of more than 23 dB is obtained using fiber Bragg gratings (FBGs) polarization selection technique at all power levels. The results reveal that a Yb-doped fiber laser at 1120 nm could be a promising replacement compared to Raman fiber lasers.     

Chromatic dispersion measurement using a multiwavelength frequency-shifted feedback fiber laser

In this paper, we present the realization of a fiber laser source emitting simultaneously over 17 wavelengths spread over the whole C-band. An acoustooptic frequency shifter is placed in the laser ring cavity to suppress the cross-gain saturation effects of the erbium-doped fiber. The emitted wavelengths are fixed by a set of fiber Bragg gratings (FBGs). A power uniformity reaching 6 dB is achieved by inscribing the FBGs while monitoring the laser output. We demonstrate the reliability of this laser as a source for characterization of optical components and networks by the measurement of optical fiber chromatic dispersion. The measurement is performed over the whole telecommunication C-band (1530-1560 nm) using the time-of-flight method. We perform the measurement on three different fibers with different levels of dispersion, namely a standard fiber, a nonzero dispersion shifted fiber, and a dispersion compensating fiber. The results are compared with measurements obtained using a standard network analyzer. The agreement between the two methods is better than /spl plusmn/1%, thus proving the suitability of the developed laser source for this application.     

Studies on bridging tractions – simultaneous bridging tractions and COD measurements

The main objective of this work is to investigate the bridging tractions in a model composite using optical fiber Bragg grating (FBG) sensors written into selected reinforcing fibers. Simultaneously, the crack opening displacement (COD) is measured using a speckle interferometry technique. The measurements are useful in the verification of the relation between the COD and bridging tractions established with the use of the weight function method. Center crack specimens made of epoxy and reinforced with one layer of optical fibers are prepared and tested under remote tension parallel to the fibers. Bragg gratings of 0.17 to 0.38 mm in length are introduced in selected fibers for direct, non invasive, local measurements of axial strains in these fibers. A controlled central crack, bridged by intact fibers, is introduced by a laser technique such that the FBGs are located between the crack faces. The results on the forces obtained from the FBGs and the COD-weight function method show good agreement. The experimental results also compare very well with 3-dimensional numerical simulations of the actual specimen geometry and loading configuration.     

Surface-relief fiber Bragg gratings for sensing applications

We present a new type of fiber Bragg grating (FBG) in which we etch the grating into the flat surface of a D-shaped optical fiber. Instead of being written in the core of the fiber, as are standard FBGs, these surface-relief FBGs are placed in the cladding above the core. These gratings are a viable alternative to standard FBGs for sensing applications. We describe the fabrication process for etching Bragg gratings into the surface of D-fibers and demonstrate their performance as temperature sensors.     

A Parallel Multiplexed Temperature Sensor System Using Bragg-Grating-Based Fiber Lasers

A parallel multiplexed temperature sensor scheme using a Bragg grating-based fiber laser approach has been developed and evaluated. Multiple laser cavities were formed as the active gain media of the system using a common broadband chirped fiber Bragg grating (CFBG) and several normal FBGs, which were used as optical feedback elements, in conjunction with different lengths of erbium-doped fibers (EDFs). These gain media were externally pumped by light from a 1480-nm laser diode (LD) through a 1480-nm 1$times$4 splitter. Normal FBGs were used as the wavelength-selective and sensing elements of the laser system. Simultaneous laser action at three different wavelengths corresponding to channels 1, 3, and 4, respectively, was obtained using this scheme. The temperature was measured over the range from room temperature (27$^circhboxC$) to a maximum of 540$^circhboxC$, which shows the potential of the scheme for quasi-distributed sensor applications.     

Measuring temperature profiles in high-power optical fiber components

We demonstrate a new method for measuring changes in temperature distribution caused by coupling a high-power laser beam into an optical fiber and by splicing two fibers. The measurement technique is based on interrogating a fiber Bragg grating by using low-coherence spectral interferometry. A large temperature change is found owing to coupling of a high-power laser into a multimode fiber and to splicing of two multimode fibers. Measurement of the temperature profile rather than the average temperature along the grating allows study of the cause of fiber heating. The new measurement technique enables us to monitor in real time the temperature profile in a fiber without the affecting system operation, and it might be important for developing and improving the reliability of high-power fiber components.     

光纤光栅器件的长期稳定性及其寿命预期

阐述了紫外写入光纤Ｂｒａｇｇ光栅的光谱特性退化机制及其对器件应用的影响,并给出了预期器件寿命的理论和实验方法。     

Combined time- and wavelength-division-multiplexing demodulation technique of fiber grating sensor arrays using a tunable pulsed laser

Combined time- and wavelength-division-multiplexing demodulation technique of fiber Bragg grating (FBG) sensor arrays using a tunable pulsed laser is proposed and demonstrated. A tunable fiber laser based on a matched FBG is applied. The wavelengths of the sensing FBGs are obtained by detecting the maximum voltages with a photodiode that avoids the complex demodulation process. The advantages of this scheme are simple structure, high signal-to-noise ratio, and the sensing signals obtained by detecting the maximum voltages.     

Wavelength switching of picosecond pulses generated from a self-seeded Fabry-Perot laser diode with a tilted fiber Bragg grating formed in a graded-index multimode fiber

We demonstrated the generation of wavelength-switchable picosecond pulses from a self-seeded Fabry-Perot laser diode that used a tilted fiber Bragg grating (FBG) formed in a graded-index multimode fiber as an external optical feedback element, where wavelength switching was achieved by controlling the modal distribution in the FBG. We measured the reflection spectra of multimode FBGs fabricated with different tilt angles and discussed the effects of the tilt angle on wavelength selection. By using a 20?mm long 1.65° tilted FBG and a fiber deformer to control the modal distribution in the FBG, we generated 2?GHz pulses with a wavelength switchable over 14 wavelengths at a spacing of ～0.8?nm.     

Fiber-Optic Sensor Interrogation Based on a Widely Tunable Monolithic Laser Diode

Fiber-optic sensors have experienced great interest in research and development since their invention. Dedicated measurement systems are essential for utilization and development of these sensors. Instruments based on tunable lasers are established devices for the demodulation of such sensors. We introduce and demonstrate a tunable laser measurement system based on a widely tunable monolithic laser diode (TML). It is capable of demodulating different types of fiber-optic sensors, although we focus on the interrogation of fiber Bragg grating (FBG) sensors. The rapid tuning of the laser's wavelength is critical for achieving high measurement rates. However, the high tuning rate is demanding and requires careful characterization. We present a method for this and show, to our knowledge for the first time, data on the rapid tuning through the whole spectrum of this type of laser diode. We propose a modified centroid algorithm that can cope with nonequidistantly sampled spectra caused by the laser's functional principle. Finally, we demonstrate the dynamic performance of the new measurement system in an application: An FBG-based acceleration sensor is demodulated at a 5-kHz sample rate during a vibrational test showing the potential of the approach.      

Analysis of fiber bragg gratings by a side-diffraction interference technique

A new nondestructive, noncontact, and sensitive technique for fiber Bragg grating geometry and index-fault location measurements is presented. Two plane-wave probe laser beams are incident upon the grating from the side at angles that satisfy the Bragg-reflection condition. An interference pattern is formed behind the fiber between the first-order diffracted beam (from one probe beam) and the zero-order transmitted beam (from the second probe beam). The axial grating index modulation and the grating period are functions of the fringe visibility and the fringe period, respectively. The method is sensitive and is applicable even in the case of relatively weak gratings. Unchirped and chirped Bragg gratings have been studied with the proposed technique. We demonstrate accuracies of 1 x 10(-4) for measurement of the index modulation and 0.01 nm for measurement of the period. As well as for the analysis of most already-fabricated gratings, this technique is useful for in situ analysis of a long fiber Bragg grating as such a grating is translated along its axis during the fabrication process.     

Modeling, design, fabrication, and testing of a fiber Bragg grating strain sensor array

The modeling, design, simulation, fabrication, calibration, and testing of a three-element, 15.3 cm fiber Bragg grating strain sensor array with the coherent optical frequency domain reflectometry (C-OFDR) interrogation technique are demonstrated. The fiber Bragg grating array (FBGA) is initially simulated using in-house software that incorporates transfer matrices. Compared to the previous techniques used, the transfer matrix method allows a systemwide approach to modeling the FBGA-C-OFDR system. Once designed and simulated, the FBGA system design is then imprinted into the core of a boron-germanium codoped photosensitive fiber using the phase mask technique. A fiber optic Fabry-Perot interferometric (FPI) strain gauge calibrator is then used to determine the strain gauge factor of a single fiber Bragg grating (FBG), and the results are used on the FBGA. The FPI strain gauge calibrator offers nondestructive testing of the FBG. To test the system, the FBGA is then attached to a 75 cm cantilever beam and interrogated using an incremental tunable laser. Electric strain gauges (ESGs) are then used to independently verify the strain measurements with the FBGA at various displacements of the cantilever beam. The results show that the peak strain error is 18% with respect to ESG results. In addition, good agreement is shown between the simulation and the experimental results.     

High-strength fiber Bragg gratings for a temperature-sensing array

We have successfully demonstrated a one-step laser process of fabricating fiber Bragg grating arrays directly through fiber buffer. A new polysiloxane-based buffer provides high 244-nm transmission and showed no degradation due to UV irradiation or thermal annealing as verified by Weibull analysis of tension tests. The FBG array can withstand over 400-kpsi tension tests and 220/spl deg/C temperature. The spectral quality of the FBG is compatible with the FBGs currently used by the sensing industry and, therefore, can be interrogated with existing instruments. The laser fabrication process is robust with no coating, stripping, or recoating required.     

Distributed measurement of the complex modulation of a photoinduced Bragg grating in an optical fiber

A method of measuring the complex modulation of a Bragg grating is derived from a one-dimensional model of light propagating in an optical fiber. Interference fringes between the Bragg grating and a reference air-gap reflector are measured, and a Fourier transform of the interference fringes generated as a laser is swept through the wavelength is used to compute the complex modulation function of the Bragg grating over a restricted domain. Supporting data, taken by temperature tuning a distributed feedback diode laser, are shown.