Purely axial compression of fiber Bragg gratings embedded in a highly deformable polymer

We demonstrate a tuning device for fiber Bragg gratings with a wavelength tuning range in excess of 65 nm. A purely axial tuning technique using a highly deformable polymer molded in a cylinder shape is used to embed a fiber Bragg grating and to achieve a wavelength tuning range from 1551.7 to 1485.5 nm. The tuning curve is highly linear with a tuning rate of 9.6 nm for every percent of applied strain. The insertion losses of the device, the variations of the full width at half maximum, and the stability of the Bragg wavelength over a working day have been studied and shown to be less than 0.02 dB, 0.14, and 0.2 nm, respectively.     

Passive Temperature-Compensating Technique for Microstructured Fiber Bragg Gratings

The thermal drift of the characteristic wavelength of fiber Bragg gratings (FBGs) photowritten in the core of microstructured fibers (MOFs) is significantly reduced by inserting a liquid of suitable refractive index into their holes. For instance, the spectral range of variations is divided by a factor of 4 over a temperature range larger than 20degC in a six-hole MOF, and the maximum sensitivity is reduced. Such passive FBG temperature compensation technique is of great interest for applications involving accurate sensing free of thermal effects.     

Multiplexed Fiber Bragg Grating Interrogation System Using a Microelectromechanical Fabry–Perot Tunable Filter

This paper describes a fiber Bragg grating strain sensor interrogation system based on a microelectromechanical systems tunable Fabry–Perot filter. The shift in the Bragg wavelength due to strain applied to a sensor fiber is detected by means of a correlation algorithm which was implemented on an embedded digital signal processor. The instrument has a 70 nm tuning range, allowing multiple strain sensors to be multiplexed on the same fiber. The performance of the interrogator was characterized using an optical fiber containing six grating strain sensors embedded in a fiberglass test specimen. The measured root mean square (RMS) strain error was 1.5 microstrain, corresponding to a 1.2 pm RMS error in the estimated wavelength shift. Strain measurements are produced with an update rate of 39 samples/s.      

Tunable graphene-based Q-switched erbium-doped fiber laser using fiber Bragg grating

A graphene-based Q-switched erbium-doped fiber laser (EDFL) with a tunable fiber Bragg grating (TFBG) acting as a wavelength tuning mechanism is proposed and demonstrated. The proposed setup utilizes a newly-developed ‘ferrule-to-ferrule transfer’ technique to obtain a single graphene layer that allows for Q-switch operation in the EDFL using a highly doped-gain medium. A TFBG is used as a wavelength tuning mechanism with a tuning range of 10 nm, covering the wavelength range from 1547.66 nm to 1557.66 nm. The system has a wide repetition rate range of over 206.613 kHz from 1.387 kHz to 208.000 kHz with pulse durations of between 94.80 μs to 0.412 μs. The laser output is dependent on the pump power, with energy per pulse of 4.56 nJ to 16.26 nJ. The system is stable, with power and wavelength variations of less than 0.47 dBm and 0.067 nm. The output pulse train is free from self-mode locking and pulse jitters.     

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.     

Tunable dual-wavelength optical short-pulse generation by use of a fiber Bragg grating and a tunable optical filter in a self-seeding scheme

A simple self-seeding scheme is developed to generate tunable dual-wavelength optical short pulses in a flexible manner and with an increased wavelength-tuning range. The wavelength selection and tuning are achieved by simultaneous use of a fiber Bragg grating and a tunable optical filter. The side-mode suppression ratio of the output pulses is better than 30 dB over a wavelength-tuning range of 33.8 nm. The system is compact and convenient for dual-wavelength tuning.     

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.     

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.     

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.     

Intensity demodulation-based acoustic sensor using dual fiber Bragg gratings and a titanium film

We report on recent experimental results obtained with fiber Bragg grating (FBG) microphone for acoustic measurement. The sensing element is formed by longitudinally sticking ends of two FBGs onto a titanium film, and then being packaged in an aluminum cylinder. Due to wavelength shift of the FBGs induced by the external acoustic disturbance, the corresponding periodic fluctuation in power can be observed on the optical oscilloscope. Theoretical analysis verifies that the optical power variation, result of the titanium film vibration caused by the acoustic disturbance, possesses a linear relationship with the sound pressure in a specific range. A relatively flat frequency response in the range from 100?Hz to 1?kHz with the average signal-to-noise ratio (SNR) above 22?dB is experimentally demonstrated. The maximum sound pressure sensitivity of the proposed sensor is found to be 90?μW/Pa within a sound pressure range 100.3–118.5?dB. The sensing system presents good stability and reliability, and has the advantage of direct self-demodulation.     

Ultra-low-power all-optical switching device by use of ytterbium-doped fiber Bragg gratings

A new, to our knowledge, scheme of all-optical switching is put forward by use of an all-fiber Mach-Zehnder interferometer incorporating ytterbium-doped fiber Bragg gratings. The device utilizes the characteristics of the sharp change of group velocity in transmission with the detuning parameter, delta = 2pi n(1/lambda - 1/lambdaB). The switching is achieved by changing the Bragg wavelength of the ytterbium-doped arm of the interferometer. A very small shift of the Bragg wavelength can lead to a pi phase shift between the two arms, so the power needed to realize complete switching is much lower than that of other schemes. In addition, the device can compensate the dispersion of an optical pulse through the positive group velocity dispersion in transmission provided by the gratings.     

Quadratic behavior of fiber Bragg grating temperature coefficients

We describe the characterization of the temperature and strain responses of fiber Bragg grating sensors by use of an interferometric interrogation technique to provide an absolute measurement of the grating wavelength. The fiber Bragg grating temperature response was found to be nonlinear over the temperature range -70 degrees C to 80 degrees C. The nonlinearity was observed to be a quadratic function of temperature, arising from the linear dependence on temperature of the thermo-optic coefficient of silica glass over this range, and is in good agreement with a theoretical model.     

Monitoring of non-homogeneous strains in composites with embedded wavelength multiplexed fiber Bragg gratings: A methodological study

For the detection of problems like fastening failures and delamination in composites, the employment of embedded fiber Bragg grating (FBG) sensors is considered an effective tool to measure non-homogeneous internal strains near damage or interfaces of dissimilar materials which in turn can help identify the severity of fracture. In this work wavelength multiplexed FBGs are used to measure strains during gradual adhesive fastening failure in a single lap joint and delamination in a uniaxial composite, during monotonic and fatigue loads. It is shown that such measurements can be effectively used to determine the stresses near the joint and bridging tractions in delamination. The particular method can provide a significant tool in experimental mechanics and fatigue of composites as well as composite assemblies. Experimental measurements are in very good agreement with the numerical analysis concurrently used for the verification of the experimental procedure’s feedback.     

Multiple-wavelength reference based on interleaved, sampled fiber Bragg gratings and molecular absorption

We present a wavelength calibration reference based on interleaved, sampled fiber Bragg gratings stabilized to a molecular absorption line. Such a hybrid reference can provide multiple stable calibration peaks over a wide range of wavelengths. We demonstrate a wavelength reference that has at least 20 peaks suitable for use as calibration references in each of three wavelength regions: 850, 1300, and 1550 nm. We monitored the stability of a 1300-nm reflection peak and found that the standard deviation of the peak wavelength was 0.7 pm over a 70-day period.     

Fast fiber-optic tunable filter based on axial compression on a fiber Bragg grating

We describe the design, fabrication, and performance of a fiber Bragg grating-based tunable optic filter. The filter, driven by two piezostacks, consists of a flexural hinge structure for displacement magnification and a fiber-ferrule assembly for axial compression of the fiber grating. Finite-element analysis was used to design the mechanical structure to achieve the required displacement magnification and the force for grating compression. A passive thermal compensation design was implemented to reduce thermal-induced wavelength drift. A feedback control system with a linear variable differential transformer was employed to control the displacement for accurate wavelength tuning and fine-tuning resolution. This tunable filter has achieved a closed-loop switching time of 17.3 ms, and a passive thermal compensation that reduced the thermal drift of the Bragg wavelength to 1.5 pm/C. The flexural-hinge structure that offers negligible backlash, noise-free motion, no need of lubricants, and no wear ensures its long-term reliability.     

Adaptive demodulation of dynamic signals from fiber Bragg gratings using two-wave mixing technology

A two-wave mixing (TWM) interferometer using photorefractive (PRC) InP:Fe crystal is configured to demodulate the spectral shift of a fiber Bragg grating (FBG) sensor. The FBG is illuminated with a broadband source, and any strain in the FBG is encoded as a wavelength shift of the light reflected by the FBG. The wavelength shift is converted into phase shift by means of an unbalanced TWM interferometer. TWM wavelength demodulation is attractive for monitoring dynamic strains because it is adaptive and multiplexable. Adaptivity implies that it can selectively monitor dynamic strains without active compensation of large quasi-static strains and large temperature drifts that otherwise would cause system to drift. Multiplexability implies that several FBG sensors can be simultaneously demodulated using a single demodulator. TWM wavelength demodulation is therefore a cost-effective method of demodulating several spectrally encoded FBG sensors.     

Innovative design of a tunable laser using liquid crystal tuning elements

An innovative non-mechanical and low power consumption tunable external cavity laser (ECL) using liquid crystal tuning elements is proposed. This contains a gain chip, a collimating lens, tuning elements and a partial-reflection mirror. The proposed tunable ECL can achieve both coarse tuning and fine tuning, and it is designed to lase at wavelength matching the International Telecommunication Union (ITU) channels, which is one of the important requirements in optical communication. The tuning elements include an ITU etalon, a liquid crystal Fabry–Pérot interferometer (LC-FPI) and a fine tuner. Only two parameters are required to tune the wavelength over the whole C-band, namely the voltage over the LC-FPI and the fine tuner. This high reliability cost-effective design proposes a theoretical tuning range of about 80?nm. The LC tuning elements including LC-FPI and fine tuner has been fabricated and tested.     

Wavelength tuning in optically mode-locked semiconductor fiber ring lasers with linearly chirped fiber Bragg gratings

We demonstrate wavelength tuning in single-wavelength and multiwavelength semiconductor fiber ring lasers that are mode locked with an optically injected control signal. A semiconductor optical amplifier is used to provide gain as well as to function as an optically controlled mode-locking element. Linearly chirped fiber Bragg gratings--single or superimposed--are used to define the lasing wavelengths as well as to provide wavelength tunability and allow for multiwavelength operation. We obtain pulses of tens of picoseconds in duration when we inject a sinusoidal optical control signal into the laser cavity, and we can tune the lasing wavelength(s) over the reflection bandwidth(s) of the grating(s) by simply changing the frequency of the injected control signal.     

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。