Temperature Sensitivity Control and Mechanical Stress Effect of Boron-Doped Long-Period Fiber Gratings

We will present in this work the quantitative analysis of the relationship between the doping concentrations of GeO 2 and B 2 O 3 in the core and cladding regions and the temperature sensitivity of the resonance wavelength shift in long-period fiber gratings (LPFGs). Based on this analysis, the temperature sensitivity was suppressed and enhanced to 0.002 nm/C and 0.28 nm/C, respectively. We will also discuss the effect of the residual mechanical stress on the optical and mechanical properties of LPFGs. In particular, we will present the measurement results of the dependence of the refractive index change and mechanical strength on the residual mechanical stress in the boron-doped fibers with depressed clad and matched clad.     

Tunable optical add-drop multiplexer based on long-period fiber gratings for coarse wavelength division multiplexing systems

We propose and experimentally demonstrate a voltage-controllable add-drop multiplexer with a tunable coupler based on long-period fiber gratings (LPFGs). The tunable coupler is based on cladding mode coupling between two parallel LPFGs with divided coil heaters, which controls the resonant wavelength positions of the two LPFGs. The wavelength tunability of the tunable coupler is measured to be -0.45 nm/degrees C in the temperature range from 20 to 120 degrees C. The large tuning range of optical signals (approximately 50.54 nm) from 1502.32 to 1552.86 nm, which covers both the short (S) and the conventional (C) bands, is achieved. The channel isolation is as high as approximately 40 dB. It has several advantages, including a broad tuning range of wavelength in both the S- and C-bands, high channel isolation, no backreflection, and so on.     

Defect versus nanocrystal luminescence emitted from room temperature and hot-implanted SiO2 layers

Silicon nanocrystals have been synthesized in SiO₂ matrix using Si ion implantation. Si ions were implanted into 300-nm-thick SiO₂ films grown on crystalline Si at energies of 30–55 keV, and with doses of 5×10¹⁵, 3×10¹⁶, and 1×10¹⁷ cm⁻². Implanted samples were subsequently annealed in an N₂ ambient at 500–1100°C during various periods. Photoluminescence spectra for the sample implanted with 1×10¹⁷ cm⁻² at 55 keV show that red luminescence (750 nm) related to Si-nanocrystals clearly increases with annealing temperature and time in intensity, and that weak orange luminescence (600 nm) is observed after annealing at low temperatures of 500°C and 800°C. The luminescence around 600 nm becomes very intense when a thin SiO₂ sample is implanted at a substrate temperature of 400°C with an energy of 30 keV and a low dose of 5×10¹⁵ cm⁻². It vanishes after annealing at 800°C for 30 min. We conclude that this luminescence observed around 600 nm is caused by some radiative defects formed in Si-implanted SiO₂.     

Oxidation and annealing of thin FeTi layers covered with Pd

The hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such as O₂ and H₂O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for hydrogen. In this study we prepared FeTi layers covered with a (4 or 20 nm) thin Pd layer. We used ion beam and sputter profiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of these bi-layers upon annealing up to 300°C in vacuum, air and 10⁻⁵ mbar O₂. The layered structure remains intact up to 150°C. At 200°C in air and O₂, Fe and (some) Ti move towards the Pd surface where they form oxide regions. At higher temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced for the case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 150°C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 200°C, 20 nm Pd may still provide sufficient protection against oxidation.     

Numerical analysis of GeO2-concentration effects in arc-induced long-period fiber gratings under external refractive-index changes

We present a numerical analysis of the response of arc-induced long-period fiber gratings (LPFGs) under ambient refractive-index changes using standard fibers with SiO₂ as cladding material and different GeO₂ concentration in the SiO₂-GeO₂ doped core. We obtain that the LPFG-sensitivity is increased using lower GeO₂ concentrations and it follows a parabolic behavior. Also, we identify two linear regimes using an external index of 1.37: for GeO₂ concentrations between 2 and 3.5 mol %, the LPFG-sensitivity changes with a rate of 3.614 nm per mole fraction of GeO₂, while for higher concentrations the sensitivity changes with a rate of 1.39 nm/mol %. Additionally, for fibers with 2 mol% of GeO₂, the sensitivity calculated is comparable to this obtained in previous works using corrugated and tapered LPFGs in SMF28 fibers. These results are of interest for the design of improved LPFG-sensors.     

Wavelength-encoded fiber-optic temperature sensor with ultra-high sensitivity

We present in this paper a wavelength-encoded fiber-optic temperature sensor with ultra-high sensitivity. The sensor consists of a segment of multimode fiber (MMF) with a polymer cladding spliced between two single mode fibers, forming a multimode fiber interferometer. For a temperature sensor with a 55 mm long MMF and a 45 mm long polymer cladding, a temperature sensitivity of −3.195 nm/°C has been achieved over a temperature range of 10 °C which is mainly limited by the spectral range of the light source used in the experiments. It has been found that the high temperature sensitivity is mainly attributed to the high thermo-optic coefficient of the polymer cladding. Other advantages of the temperature sensor reported here include its extremely simple structure and fabrication process, and hence a very low cost.     

Synthesis and physical behaviour of In2S3 films

In₂S₃ layers have been grown by close-spaced evaporation of pre-synthesized In₂S₃ powder from its constituent elements. The layers were deposited on glass substrates at temperatures in the range, 200–350 °C. The effect of substrate temperature on composition, structure, morphology, electrical and optical properties of the as-grown indium sulfide films has been studied. The synthesized powder exhibited cubic structure with a grain size of 63.92 nm and S/In ratio of 1.01. The films grown at 200 °C were amorphous in nature while its crystallinity increased with the increase of substrate temperature to 300 °C. The films exhibited pure tetragonal β-In₂S₃ phase at the substrate temperature of 350 °C. The surface morphological analysis revealed that the films grown at 300 °C had an average roughness of 1.43 nm. These films showed a S/In ratio of 0.98 and a lower electrical resistivity of 1.28 × 10³ Ω cm. The optical band gap was found to be direct and the layers grown at 300 °C showed a higher optical transmittance of 78% and an energy band gap of 2.49 eV.     

一种光子晶体谐振腔的TM模特性

构造了一种二维砷化镓石墨点阵柱状光子晶体谐振腔.利用时域有限差分方法计算了这种光子晶体谐振腔TM_y模的共振峰波长随应力和温度变化的情况.计算结果表明,这种谐振腔TM_y模的共振峰波长随x、y方向的应力变化以及随应力环境的变化均具有较好的线性特性,其应力响应灵敏度为0.013 69 nm/Mpa,且这种谐振腔沿x方向和y方向的应力响应灵敏度相同;同时,这种谐振腔的共振峰主峰随温度呈分段线性变化趋势,且具有最大1.4 nm/℃的温度灵敏度.另外,计算结果显示,这种结构的光子晶体谐振腔具有很好的频率开关特性.     

The Ti/c-Si solid state reaction III. The low-temperature reaction kinetics

Thin Ti layers (≈10nm) are grown on top of a clean Si(111) substrate. Heating these layers initiates a solid state reaction, yielding a monosilicide phase at ≈350°C and a C49 disilicide at ≈450°C. The present study concerns the growth kinetics of both phases by means of ellipsometry. A diffusion-limited growth kinetics is found for the monosilicide formation. However, two growth rates are observed, a fast initial one and a slow terminal growth rate. An enhanced Si diffusion in atomically disordered regions as compared to well ordered regions (grains or clusters) could be an explanation. From the measurements we have found a value of 2×10^-15 cm²/s for the diffusion coefficient at ≈370°C and an activation energy of 0.62 ± 0.1 eV. Both values correspond to the fast process. Subsequently increasing the temperature to ≈450°C permits the growth of the homogeneous C49 TiSi₂ phase. For this process, both planar layer growth and intermixing are observed, however, quantitative results could not be derived from the present study.     

Experimental results of the performance of a laser fiber as a remote sensor of temperature

We present experimental results demonstrating the performance of an erbium-doped silica fiber as a remote temperature sensor. The sensor is based on the fluorescence intensity-ratio change of two spectral bands as a function of temperature in the wavelength interval from 515 to 570 nm. We apply a radiometric analysis to the fluorescence spectrum that we have measured to determine the optimal spectral bands to use in the power ratio of the sensor. The spectral bands used in the power ratio, with best performance, are 525–535 nm/555–565 nm, with a signal–noise ratio of 57 and 56 dB, respectively. The sensor sensitivity is about , and the resolution is approximately .     

Study of the 110°C TL peak sensitivity in optical dating of quartz

As the 110°C TL emission in quartz uses the same luminescence centers as the OSL emission, the 110°C TL signal from a test dose may be used to monitor the OSL sensitivity change. It is thus important to study the relationship between the 110°C TL peak and the OSL sensitivity in studies related to optical dating from quartz. We have conducted a series of experiments using sedimentary quartz, where the annealing temperatures were varied between 260 and 1000°C before the measurement of OSL and 110°C TL sensitivities. Another series of experiments on two sedimentary quartz samples investigated the 110°C TL peak and OSL dose-dependent sensitivity change after different annealing temperatures. In these experiments, the 110°C TL and OSL signals from the test dose are shown to have similar sensitization characteristics: the 110°C TL sensitivity change is proportional to the OSL sensitivity change if the annealing temperature is lower than 500°C. It is concluded that the 110°C TL signal can be used to correct the OSL sensitivity change in the single-aliquot additive-dose protocol.     

All-Fiber Dual-Parameter Sensor Based on Cascaded Long Period Fiber Grating Pair Fabricated by Femtosecond Laser and CO2 Laser

An all-fiber dual-parameter sensor based on cascaded long period grating pair fabricated by femtosecond laser and CO₂ laser has been proposed and realized both theoretically and experimentally. The resonant wavelengths of LPFGs are 1557.80 nm and 1590.88 nm. In the strain range of 0–400 με, strain sensitivities are ?7.2 pm/με for C-LPFG and ?1.6 pm/με for F-LPFG. In the temperature range of 30–70°C, temperature sensitivities are ?41.1 pm/°C for C-LPFG and ?21.2 pm/°C for F-LPFG. By analyzing the resonant wavelength characterization, the proposed sensor can be efficiently used for dual-parameters measurement with promising application prospect and great research reference value.     

Magnetic properties in BaFe12O19 nanoparticles prepared under a magnetic field

It was observed that the nanocrystallites of BaFe₁₂O₁₉ formed at 140°C under a 0.25 T magnetic field exhibited a higher saturation magnetization (6.1 emu/g at room temperature) than that of the sample (1.1 emu/g) obtained under zero magnetic field. Both of the two approaches yielded plain-like particles with an average particle size of 12 nm. However, the Curie temperature (T_c), a direct measuring of the strength of superexchange interaction of Fe³⁺–O²⁻–Fe³⁺, increased from 410°C for the nanoparticles prepared without an external field applied to 452°C for the particles formed under a 0.25 T magnetic field, which indicates that external magnetic fields can improve the occupancy of magnetic ions and then increase the superexchange interaction. This was confirmed by electron paramagnetic resonance and Mössbauer spectrum analysis. The results present in this paper suggest that in addition to oxygen defects, surface non-magnetic layer and a fraction of finer particles in the superparamagnetic range, cation vacancies should be responsible for the decreasing of saturation magnetization in magnetic nanoparticles.     

Trajectory-property relationships in MOD-derived YBCO films

A study of the physical and chemical changes during processing in MOD-derived YBCO films was performed. Fully processed films were 70–85% of theoretical density. The sintering rate increased substantially in the compositional range F/Ba = 1.8–1.5. The activation energy for sintering decreased above a P(H₂O) dependent threshold temperature. XRD indicated this temperature/composition threshold also corresponded to YBCO nucleation, suggesting ex situ YBCO forms in contact with a melt. The ramp rate and P(H₂O) were used to control F/Ba trajectories, which were correlated to performance. The nucleation of YBCO was strongly dependent on processing conditions. Nucleation temperature was varied by at least 60 °C in the study. The optimal YBCO nucleation temperature in the 300–800 nm films was around 725 °C. a-axis grains dominated the microstructures of films where YBCO nucleated at <700 °C. Large second phases, but no a-axis grains, were found when the nucleation temperature was >750 °C.     

The Ti/c-Si solid state reaction I. An ellipsometrical study

This paper is the first of a series of three articles in which we present the results and analyses of an extended study of the c-Si/Ti solid state reaction. In this paper we will discuss the spectroscopic ellipsometric investigation. Thin (≈10nm) Ti films are grown on clean Si(111) surfaces and are subsequently heated. The Si indiffusion and the Si-Ti intermixing are continuously registered by three-wavelengths ellipsometry. Two metastable intermediate phases are observed to form before the final state is obtained Spectroscopic ellipsometry (E = 2−4.5 eV) is used to characterize the as-deposited layer, the metastable intermediate phase and the final state. Analysis of these spectra shows that: (1) Si and Ti intermix during the initial Ti deposition, (2) a fast reordering of the Ti atoms occurs when the system is slightly heated (≈175°C), (3) a metastable, probably monosilicide phase with a large Si concentration gradient is obtained at ≈350°C, (4) a homogeneous metastable TiSi₂ forms at ≈450°C, at ≈700°C a roughened TiSi₂ layer with a surplus of c-Si is formed.     

A photonic crystal biosensor with temperature dependency investigation of micro-cavity resonator

A two dimensional photonic crystal biosensor implemented by waveguides and microcavity is theoretically investigated. The designed structure has high quality factor about 15,000 and sensitivity approximately 141.67 nm/RIU, which are important parameters in biosensing applications. Also there is a linear dependency between resonant wavelength shift and refractive index changes. Since water is the main component of human organism, the temperature and wavelength dependence of proposed microcavity is investigated. The results show that the structure has good temperature stability. The temperature sensitivity is about −0.0142 nm/°C.     

Quantitative measurements of Ge surface segregation during SiGe alloy growth

Ge segregation during the growth of Si_{1 − x}Ge_x alloys (x = 5, 10, 20, and 40%) was studied using X-ray photoelectron spectroscopy. The alloys were grown in thicknesses up to 20.0 nm at 500°C to measure quantitatively the amount of segregated surface Ge. The length of alloy needed to reach steady-state growth edge was found to decrease with increasing alloy concentration (4.8, 2.8, 2.4, and 2.0 nm, respectively). It was found that each alloy had a complete monolayer of Ge on the surface and an increasing amount of segregated Ge in the second layer (20, 55, 80, and 95%, respectively) during steady-state growth. An increase in the temperature of alloy growth (400–750°C) resulted in an increase in the leading edge of alloy growth but did not change the amount of segregated Ge during steady-state growth. We propose that film stress is responsible for the amount of Ge segregation.     

Structure and optical properties of sol–gel derived Gd2O3 waveguide films

Pure and rare earth doped gadolinium oxide (Gd₂O₃) waveguide films were prepared by a simple sol–gel process and dip-coating method. Gd₂O₃ was successfully synthesized by hydrolysis of gadolinium acetate. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were used to study the thermal chemistry properties of dried gel. Structure of Gd₂O₃ films annealed at different temperature ranging from 400 to 750 °C were investigated by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that Gd₂O₃ starts crystallizing at about 400 °C and the crystallite size increases with annealing temperature. Oriented growth of (4 0 0) face of Gd₂O₃ has been observed when the films were deposited on (1 0 0) Si substrate and annealed at 750 °C. The laser beam (λ=632.8 nm) was coupled into the film by a prism coupler and propagation loss of the film measured by scattering-detection method is about 2 dB/cm. Luminescence properties of europium ions doped films were measured and are discussed.     

Multiple quantum well structures and high-power lasers of gaas- algaas grown by metalorganic vapor phase epitaxy (MOVPE)1

GaAs/AlGaAs GRIN-SCH type multiple quantum well lasers with four wells of 11 nm GaAs, grown in an MOVPE chimney reactor, exhibit an output power as high as 110 mW/facet (CW, 30°C; 5 μm stripe) and 1.3 W/facet (pulsed, 30°C; 53 μm stripe) until catastrophic optical damage occurs. 2000 hours life tests conducted at 60°C and 15 mW CW show no noticeable degradation for the 5 μm stripe laser with a reflective coating on both facets. Raman spectroscopy on similar multiple quantum well structures with 65 GaAs wells is used to ascertain that the wells have minimum residual aluminum- content.