Water impurity in low-loss silica fibre

Water contamination in silica based optical fibre waveguides has been studied. The height of the 950nm water peak was found to vary with fibre pulling speed and reduced sharply from 64 dB/km to 20.5 dB/km as the speed was increased from 0.12 m/sec to 1.47 m/sec. The results have been fitted to a diffusion curve indicating that the water diffuses into the core region from the support tubes used in preform manufacture. Deposition inside a water free Spectrosil tube gave a water peak of only 4 dB/km. A B₂O₃ doped cladding layer was used to provide a barrier to OH migration and the 950nm water peak was reduced from 24 dB/km to 3 dB/km by increasing the B₂O₃ content in the layer from 2 to 4 mole per cent.     

Broadband dispersion-compensating fiber for high-bit-rate transmission network use

The optimum refractive-index profile and drawing temperature were investigated so as to maximize the figure of merit for multicladding broadband dispersion-compensating fibers. Based on the results of the investigation, the authors have fabricated a highly bend-resistant fiber with a 92.6-ps/(nm dB) figure of merit using the modified chemical-vapor deposition method for dispersion compensation in the 1.5-1.6-μm wavelength region. The manufactured dispersion compensator does not suffer bend loss at 1.55 μm for curvatures of radia of 6.3 and 3.3 cm, and it has a 1.1-dB/km bend loss at a curvature of radius of 1.6 cm. Codoping the germanium silicate core with fluorine diminishes the optical loss down to 0.70 dB/km at a 1.55-μm wavelength.     

Comparative study of mid-infrared fibers for modal filtering

We compare the filtering capabilities of two infrared fibers developed to achieve a high rejection ratio of the higher order modes in order to obtain compact modal filters devoted to stellar interferometry. Two types of double-clad fibers are studied: a fiber with a second thin absorbing cladding and a fiber with a second thick absorbing cladding closer to the fiber core; both are single mode around the CO(2) band (10.6 μm). We present the single-mode spectral domain and the nulling capabilities of both fibers for different fiber lengths, comparing simulations with experimental results. We show that the filtering capabilities are improved when the absorbing clad is closer to the fiber core, as the propagation distance needed to filter out these modes is shorter. Thus, to obtain high rejection ratios in compact devices, an absorbing cladding close to the core of the fiber is compulsory in order to suppress cladding modes that could eventually recouple into the waveguide. We present an empirical model that allows determining the minimum filter length, considering only one effective leaky mode with low attenuation, which considerably simplifies the theoretical studies.     

Tapered chalcogenide–tellurite hybrid microstructured fiber for mid-infrared supercontinuum generation

Fibers exhibiting flattened and decreasing dispersion are important in nonlinear applications. Such fibers are difficult to design, particularly in soft glass. In this work, we develop a preliminary design of a highly nonlinear tapered hybrid microstructured optical fiber (TH-MOF) with chalcogenide glass core and tellurite glass microstructure cladding. We then numerically studied its dispersion, loss, and nonlinearity-related optical properties under fundamental mode systematically using the infinitesimal method. The designed TH-MOF exhibits low chromatic dispersion that is similar to a convex function with two zero-dispersion wavelengths and decreases with fiber length from 2 to 5 μm band. The potential use of the TH-MOF in nonlinear applications is demonstrated numerically by a supercontinuum spectrum of 20 dB bandwidth covering 1.96–4.76 μm generated in 2-cm-long TH-MOF using near 3.25-μm fs-laser pump.     

High-purity As-S-Se and As-Se-Te glasses and optical fibers

We describe a procedure for the preparation of As-S-Se and As-Se-Te glasses with low contents of gas-forming impurities (hydrogen, oxygen, and carbon) via melting of extrapure-grade elements in an evacuated silica ampule and purification of the melt by chemical distillation. The impurity concentrations in the glasses thus prepared have been reduced to the following levels: hydrogen, <0.02; oxygen, 0.2; carbon, <0.02; silicon, <0.4 ppm by weight. Using the double-crucible method, we have fabricated glass fibers with various ratios of the core and cladding diameters (1: 25 to 9: 10), protected with a tetrafluoroethylene/1,1-difluoroethylene copolymer coating, which have an average bending strength of 0.5–1 GPa. The minimal optical losses are 150 dB/km at 6.6 μm in multimode As-Se-Te glass fibers and 60 dB/km at 4.8 μm in As-S-Se glass fibers. The effect of microinhomogeneities in the melt on the optical performance of arsenic sulfoselenide glass fibers fabricated by the double-crucible method is examined.     

Fabrication of silicon based glass fibres for optical communication

Silicon based glass fibres are fabricated by conventional fibre drawing process. First, preform fabrication is carried out by means of conventional MCVD technique by using various dopants such as SiCl₄, GeCl₄, POCl₃, and FeCl₃. The chemicals are used in such a way that step index single mode fibre can be drawn. The fibre drawing process consists of various steps such as heating the preform at elevated temperature, diameter monitor, primary and secondary coating, and ultra violet radiation curing. The fibres are then characterized for their geometrical and optical properties. The drawn fibre has diameter of core and cladding to be 8.3 μm and 124.31 μm, respectively whereas non-circularity is found to be 4.17% for core and 0.26% for cladding as seen from phase plot. Mode field diameter is found to be 8.9 μm and 9.2 μm using Peterman II and Gaussian method, respectively. The fabricated fibres showed the signal attenuation of 0.35 dB/km and 0.20 dB/km for 1310 nm and 1550 nm, respectively as measured by the optical time domain reflectometer (OTDR).     

Influence of humidity during the fibre preparation process on transmission loss for ZrF4-based optical fibres

The influence of humidity on the transmission loss of ZrF₄-based optical fibres is clarified. Humidity increases the OH absorption loss as well as the scattering loss in the fibre. The absorption loss at 2.8 μm, due to fundamental stretching vibration of OH ions, has been reduced to 30 dB/km. It is foreseen that fluoride optical fibre with low loss at the 2–4 μm band can be realized by the extremely dry atmosphere process.     

Fabrication of matched cladding and depressed cladding single mode optical fibers

Abstract

Modified chemical vapor deposition method was used to fabricate single mode optical fibers. Both matched and depressed cladding single mode fibers were designed and fabricated. Loss as low as 0.3 dB/km has been obtained at wavelength 1.55 μm.     

Sulfide glass fibers for infrared transmission

Germanium sulfide glasses were drawn into fibers with optical losses lower than 1 dB/m. The glass fibers were coated with Teflon FEP or silicone resin according to their thermal properties. Loss increase caused by coating was not observed. The main problems in obtaining low-loss fibers were investigated. Sulphur evolution in fiber drawing was avoided by choosing the appropriate glass compositions and drawing conditions, OH and SH impurities were reduced to $\text{1}\text{4}$ of initial values using the S₂Cl₂ chemical reaction with water at 400–600°C. The possibility of ultra-low loss fibers using sulfide glasses is discussed in terms of intrinsic losses and impurity-related absorption losses.     

Low-loss,high-purity (TeO<Subscript>2</Subscript>)<Subscript>0.75</Subscript>(WO<Subscript>3</Subscript>)<Subscript>0.25</Subscript> glass

By melting a mixture of high-purity oxides in a platinum crucible under flowing purified oxygen, we have prepared (TeO₂)_0.75(WO₃)_0.25 glass with a total content of 3d transition metals (Fe, Ni, Co, Cu, Mn, Cr, and V) within 0.4 ppm by weight, a concentration of scattering centers larger than 300 nm in size below 10² cm⁻³, and an absorption coefficient for OH groups (λ ∼ 3 μm) of 0.008 cm⁻¹. The absorption loss in the glass has been determined to be 115 dB/km at λ = 1.06 μm, 86 dB/km at λ = 1.56 μm, and 100 dB/km at λ = 1.97 μm. From reported specific absorptions of impurities in fluorozirconate glasses and the impurity composition of the glass studied here, the absorption loss at λ ∼ 2 μm has been estimated at ≤100 dB/km. The glass has been drawn into a glass-polymer fiber, and the optical loss spectrum of the fiber has been measured.     

Optical properties of GeO2 glass and optical fibers

The optical properties of GeO(2) glass were investigated to clarify its potentiality as an optical fiber material. Glass samples were prepared by the flame hydrolysis and the melting techniques, and their Rayleigh scattering and infrared absorption properties were examined. Optical fibers composed of a pure GeO(2) core and an F-doped GeO(2) cladding were drawn to clarify the scattering loss characteristics. The Rayleigh-scattering intensity obtained from spectral loss measurements on the fibers agreed with that measured in bulk samples, and the intensity relative to that of SiO(2) glass was found to be approximately 3. These results suggest that a minimum loss of 0.15 dB/km is expected at a wavelength of 2 mum.     

High-Purity As2S1.5Se1.5 Glass Optical Fibers

Core–clad optical fibers were fabricated from high-purity As₂S_1.5Se_1.5 glass, and their properties were studied. The arsenic sulfo-selenide was prepared by melting a mixture of high-purity arsenic monosulfide, arsenic, and selenium. Optical fibers with core/clad diameters of 300/400 and 200/400 m were fabricated by the double-crucible method. The minimum loss was found to be 60 ± 20 dB/km at 4.8 m and 200–300 dB/km between 4 and 6 m. The numerical aperture of the fibers was 0.28. A 1.5-m-long section of the fiber transmitted 6-W CO laser radiation. The average bending strength of the 400-m-diameter fibers was 0.8 GPa.     

Characteristics of a long-period fiber grating with reduced cladding for refractive index sensing

The sensitivity to surrounding refractive index (SRI) of a long-period fiber grating (LPFG) can be effectively improved by decreasing the cladding radius. When the cladding is reduced, a three-layer model is necessary to evaluate the effective refractive index (ERI) of the core mode. A variation of SRI can induce a greater resonant wavelength shift when the core mode is coupled to a higher-order cladding mode. However, as the cladding is reduced further, the highest-order cladding mode would be cut off, i.e. the number of cladding modes that a given fiber structure can support would be less; thus, the higher-order cladding modes that can be used for higher sensitivity are limited. Hence, the implementation of high sensitivity for SRI sensing with cladding-reduced LPFGs is dependent on the proper combination of cladding radius and cladding mode order. Based on the vector coupled-mode theory, the transmission spectrum and sensitivity are numerically analyzed with respect to the cladding radius, which shows that the SRI sensitivity of the HE₁₂ mode with cladding radius a ₂?=?20?µm is 32 times as high as that with a ₂?=?62.5?µm and the SRI resolution is available to the order of 10^?7.     

爆炸复合板与轧制复合板界面结构的研究

用透射电镜、扫描电镜和能谱仪等手段对爆炸和轧制复合板界面组织、相结构和成分变化进行了研究。结果发现爆炸复合是由周期性熔化和非熔化构成的波状复合面，它比轧制扩散复合形成的平面积多１／３左右。两种复合方式都有越过界面的元素扩散，爆炸态扩散范围在２５μｍ左右；轧制态Ｆｅ、Ｎｉ、Ｃｒ元素扩散范围在５０μｍ左右，碳元素越过纯Ｎｉ层向不锈钢侧的扩散范围为１００μｍ左右，在此区域发现沿晶界连续析出有Ｍ２３Ｃ６型碳化物。     

Modal field deformation and transition losses in UV side-written optical fibers

The asymmetry of the UV-induced refractive-index profile in photosensitive optical fibers causes a deformation of the modal fields, resulting in transition losses between UV-exposed and unexposed fiber sections up to 0.1 dB for UV-induced index changes of the order of 相似文献   

低损耗离子交换玻璃基光波导制备与分析

考虑到离子交换和离子扩散工艺的特殊要求, 设计并熔制了适合于离子交换工艺的硅酸盐玻璃材料SiO₂-B₂O₃-Al₂O₃-R’O-R₂O（R’=Ca, Mg; R=Na, K）. 采用Ag⁺/Na⁺熔盐离子交换和电场辅助离子扩散工艺在这种玻璃材料基片上获得了掩埋式条形光波导. 光学显微镜和电子探针分析表明高折射率的Ag⁺扩散区位于玻璃基片表面以下约10μm处, 形成光波导的芯部. 光波导芯部尺寸约为8μm×8μm, 与单模光纤芯径尺寸相当, 保证了较低的光纤耦合损耗. 对光波导的测量结果得出：在波长为1.5μm处条形光波导的传输损耗约为0.1dB/cm, 与单模光纤的耦合损耗约为0.2~0.3dB. 条形光波导的传输损耗与材料本身的损耗接近, 表现出掩埋式光波导的低损耗特征. 分析表明, 经过进一步优化, 这种光波导制备技术可用于低损耗光波导器件的制作.     

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.     

Acoustic wave propagation in piezoelectric fibers of hexagonalcrystal symmetry

An exact analysis is presented for acoustic wave propagation in cladded acoustic fibers having a core and an infinite thick cladding both made of piezoelectric hexagonal crystal of 6 mm point group symmetry. The crystalline Z axes of both the core and cladding coincide with the fiber axis. A general dispersion equation is derived for all the acoustic modes propagating along the fiber axis. Two simpler and independent equations which represent the dispersion relations of torsional and radial-axial modes can be separated from the general dispersion equation. It has been found that the radial-axial and general flexural modes are piezoelectrically active while the torsional modes are not. Approximate dispersion relations for pure guided modes in weakly guiding weakly piezoelectric fibers which are much simpler than the exact ones are also given. Numerical results are only presented for pure guided modes. Exact and approximate dispersion curves of several lower order pure guided flexural, radial-axial and torsional modes in a weakly guiding ZnS fiber are compared and they are in good agreement     

Suspended core tellurite glass optical fibers for infrared supercontinuum generation

We report the fabrication and characterization of tellurite TeO₂-ZnO-Na₂O (TZN) microstructured suspended core optical fibers (MOFs). These fibers are designed for infrared supercontinuum generation with zero dispersion wavelength (ZDW) at 1.45 μm. The measured losses at this wavelength are approximately 6 dB/m for a MOF with a 2.2 μm diameter core. The effective area of a particular fiber is 3.5 μm² and the nonlinear coefficient is calculated to be 437 W⁻¹km⁻¹. By pumping a 20 cm long fiber at 1.56 μm with a sub-nJ femtosecond laser source, we generate a supercontinuum (SC) spanning over 800 nm in the 1-2 μm wavelength range.     

Cladding and characteristics of LiNbO3 single crystal fibre

Abstract

A magnesium ion indiffusion process is applied to LiNbO₃ single crystal fibres deposited with MgO thin film on their surface. A core–cladding waveguide structure with different refractive index profile is formed in LiNbO₃ single crystal fibres by selecting suitable diffusion parameters such as diffusion temperature, diffusion time and MgO film thickness. An electron probe microanalysis was used to measure the Mg-ion concentration distribution of the Mg diffused layer. The propagation loss of the clad crystal fibre (Mg-diffused crystal fibre) is measured to be 14 times lower than that of the unclad crystal fibre and the transmission modes were observed. The microstructural characteristics of the Mg diffused crystal fibres were also characterized by using Laue X-ray diffraction and scanning electron microscopy. It was shown that a good starting single crystal of the Mg diffused crystal fibres can be preserved by controlling suitable diffusion parameters and matching suitable crystal fibre diameter.