Four-wave mixing in non-step-index optical fibers: a sensitive technique for measurement of refractive-index profiles

Investigation of four-wave mixing 6.4-mum core-diameter fiber with 520-610-nm pump light is used to develop a new technique for characterizing the refractive-index profiles of non-step-index fibers. Deviation from the step-index profile is measured. Small variations in the refractive index between different segments of the same commercial fiber is measured with a sensitivity of 0.0001.     

Coupling methods in prospective single-mode fiber integrated optics systems: A progress report

A review of the research progress on the coupling of fibers to fibers, fibers to thin-film waveguides, and thin-film waveguides to thin-film waveguides is presented here. In general, the direct excitation method is preferred for the coupling of two fibers, with efficiency larger than 90%. The direct excitation method is also preferred for the coupling of two thin-film waveguides and thin-film waveguides to fibers where the two component waveguides have vastly different refractive indices. The coupling efficiency depends on matching the field pattern of component waveguides. The laser source to fiber launching is done mostly by direct excitation. The evanescent field coupling method is preferred for the coupling of two thin-film waveguides that have close refractive index profiles, with efficiency larger than 70%.     

Photoinscription of Bragg gratings within a germanosilicate fiber subjected to a high static electric field

The kinetics of Bragg grating growth in germanosilicate fibers subjected to a high static electric field are compared with those obtained without any electric field. The gratings were written by exposure of the fiber core to laser light at 244 or 193 nm. These experiments gave some clues about the mechanisms responsible for both the photosensitivity in germanosilicate fibers and the nonlinear second-order UV-induced susceptibility in silica glasses. The refractive-index modulation proved to be significantly higher in the fibers subjected to an electric field. Furthermore, the change in the fiber's mean effective refractive index as a function of exposure time was not monotonic. This evolution can be explained by the assumption that some electric-field-induced diffusion of electron trapped centers [Ge(1) and Ge(2)] from the fiber core is involved.     

单模及多模光纤折射率分布测量方法研究

通过对几种单模及多模光纤折射率分布测量方法的分析研究，得到单模光纤与多模光纤折射率分布测量方法的根本区别。由于单模光纤芯径比较小，因而只能用波动理论分析其传输机理，其中的远场法和近场法测量都是基于标量亥姆霍兹波动方程，即以单模光纤的基本传输理论进行测量；而多模光纤由于其芯径比较大，故而用射线理论分析其传输原理较为合理。多模光纤的折射近场法和近场扫描法均是以纤芯半径处数值孔径不同，对应的折射模和传导模不同为依据来进行测量的。     

Transmission characteristics of VAD fibers using a multiple burner with single source gas nozzle

A vapor phase axial deposition (VAD) method using a multiple burner with single source gas nozzle is presented. Fibers fabricated by this technique feature very low loss and wide bandwidth characteristics, although the refractive index profile slightly deviates from the ideal, on the outer side of the core part. This technique is expected to provide a simple method of fabricating low loss and wide bandwidth fibers.     

Graded-index characteristics in single-crystal fibers

Single-crystal fibers of Ti(3+):Al(2)O (3)have been grown by the laser-heated pedestal growth technique and shown to exhibit radial refractive-index gradients. A refractive-index increase of approximately 12% in the fiber core with respect to the fiber sidewalls has been measured. The index profile can be fitted with a parabolic model. Postgrowth treatment of Cr(3+):Gd(3)Sc (2)Al (3)O(12)fibers has been shown to produce a uniform cladding region with a graded-index core. The core index is some 12.5% larger than the cladding region, with an index profile shape that is approximately parabolic in nature.     

Generation of soliton-like wave packets and wave packets with linear phase modulation in gaining optical wave-guides with saturable nonlinearity

We have considered the dynamics of soliton-like pulses and stable frequency-modulated self-similar pulses in an active medium with saturable nonlinearity and a parabolic refractive-index profile. We show that, based on gaining gradient fibers with a parabolic distribution of the refractive index and saturable nonlinearity, it is possible to create complexes that ensure generation of laser pulses with a high (above 1 TW) peak power.     

The influence of the input beam width on the bandwidth of multimode step index fibers

A detailed theoretical investigation of the influence of the input on the bandwidth of multimode step index fibers is given. The calculations take into account mode coupling, absorption and leaky modes. For 100 m of a typical large numerical aperture fiber the bandwidth can vary with excitation from 1.8 GHz to 15 GHz. The numerical calculations agree with published experimental results.     

Internal Features of Fiber Fuse in a Yb-Doped Double-Clad Fiber at 3 kW

We study internal features of fiber fuse in a Yb-doped double-clad fiber. The samples of fiber fuse are acquired at the power level of 3 kW in an all-fiber forward-pumped master oscillator power amplifier configuration fiber laser that is built specially for fiber fuse analyses. At this high power level, drastic refractive-index redistribution arises in an expended high refractive index area around the bullet-shaped voids of fiber fuse. Electron spin resonance analyses on post-fiber-fuse samples of the Yb-doped double-clad fiber indicate rising Frenkel defect concentration, meanwhile showing a new resonance center that is different from the ones of the Ge-doped fibers studied previously. This new resonance center probably suggests the generation of Al-oxygen hole center, a kind of defect formed during the catastrophic fuse process.     

Lognormal Modes of Dielectric Optical Waveguides

It is shown that, at a critical point where the effective refractive index of a mode coincides with the bulk index of a cladding region, optical waveguide modes exhibiting a lognormal feature are supportable for a hyperlongrange refractive-index profile. As a typical example of waveguides that support lognormal modes a parabolic-index core surrounded by a depressed-index cladding is presented.     

弱渐变抛物型光纤传导模式的截止频率与介电比

在弱渐变假定下,研究了抛物型渐变光纤传导模式的截止频率对折射率参量的依赖关系。采用矢量场法,得到了在弱导条件下传导模式的特征方程和截止频率的近似计算公式。引入了介电比γ,它是由光纤三个折射率参量所确定的常数。在弱导近似的条件下,归一化截止频率仅与γ这一个变量有关,任一渐变抛物型光纤的实际截止频率可通过γ而由归一化截止频率简单求得,从而找到了渐变光纤的实际截止频率与归一化截止频率间的联系。给出了归一化截止频率随γ的变化曲线,归一化截止频率随γ的增加而单调上升,但HElm模截止频率的上升规律与其他模式不同。     

Selective excitation and coupling of high-order optical modes of a microstructured optical fiber by using a fiber-end microtip

We present the selective excitation and coupling of high-order optical modes in microstructured optical fibers (MOFs) by using a fiber-end microtip. After the self-growing of a microtip on the end face of a standard single-mode fiber, it is demonstrated that a fine tuning of the relative displacement between the microtip and MOF can greatly improve the excitation and coupling efficiency of high-order optical modes of the MOF. A 6.85?dB improvement in coupling efficiency of the second-order mode of a homemade small-hole MOF is experimentally achieved, and a sensitivity-enhanced measurement of the refractive index in MOF microholes is demonstrated.     

Numerical calculation of cutoff frequencies of optical fibers by a variational technique

A numerical method for calculating cutoff frequencies of modes of weakly guiding optical fibers is described. Starting from a properly constructed field representation, the method follows a variational approach to obtain an eigenvalue problem for the cutoff frequencies which can be easily solved by a standard numerical routine. The field representation uses an expansion in terms of the Laguerre–Gauss functions in the fiber core and limiting form for small arguments of modified Bessel functions in the fiber cladding. Fibers with power-law refractive index profiles and fibers with a profile showing a dip on the axis are analyzed. Results obtained for modes of arbitrary order (both azimuthal and radial) show that our method is very efficient and accurate.     

Cross-point analysis for a multimode fiber sensor based on surface plasmon resonance

A novel analysis based on surface plasmon resonance (SPR) with a side-polished multimode fiber and a white-light (halogen light) source is presented. The sensing system is a multimode optical fiber in which half of the core has been polished away and a 40 nm gold layer is deposited on to the polished surface by dc sputter. The SPR dip in the optical spectrum is investigated with an optical spectrum analyzer (OSA). In our SPR fiber sensor, the use of liquids with different refractive indices leads to a shift in the spectral dip in the SPR curve. The cross point (CP) of the two SPR spectra obtained from the refractive-index liquid and the deionized water measurements was observed with the OSA. The CP is shifted sensitively in wavelength from 630 to 1300 nm relative to a change in the refractive index of the liquid from 1.34 to 1.46. High sensitivities of 1.9 x 10(-6) refractive-index units (RIUs) in the range of the refractive index of the liquid from 1.40 to 1.44 of 5.7 x 10(-7) RIUs above the value of 1.44 are proposed and demonstrated in our novel SPR analysis.     

Unusual bound modes in asymmetrically graded planar waveguides

 We show analytically that for certain asymmetrically graded-index planar waveguides at the cutoff point where the effective refractive index of a mode coincides with the index of the substrate, there exist unusual bound modes. We have calculated the refractive-index profile that makes these modes possible. Received: 20 July 1996     

Refractive-index profiling of azimuthally asymmetric optical fibers by microinterferometric optical phase tomography

Accurate nondestructive refractive-index profiling is needed in the modeling, design, and manufacturing of optical fibers and fiber devices. Most profile measurement techniques cannot correctly characterize fibers with small or irregular refractive-index variations over their cross sections. Microinterferometric optical phase tomography (MIOPT) is a technique that allows measurement of fiber refractive-index profiles exhibiting such variations. We present the first demonstration, to our knowledge, of MIOPT. The profile of a polarization-maintaining fiber is measured by MIOPT and shown to be in agreement with (destructive) fiber end-face measurements. MIOPT is also applied to the limiting case of a symmetric single-mode fiber.     

利用超格子模型对布拉格光纤的研究

为了研究布拉格光纤的模式特征和传输特性,提出了超格子模型,利用傅里叶级数表示光纤横向折射率分布,利用平面波展开法分析布拉格光纤的能带结构,基于厄米-主斯函数的局域正交函数展开法,从全矢量耦合波动方程出发,得到关于模式传播常量和电场展开系数的本征方程,从而分析布拉格光纤的模式特征。以高折射率芯布拉格光纤为例,实现了该算法,得到基模与二次模的横向电场分布、基模色散曲线和模式双折射。基模的模式双折射可用于衡量算法的精度,结果表明该算法精度较高。超格子模型不仅可以用于研究高折射率芯布拉格光纤,而且同样可以研究低折射率区域导光的布拉格光纤。     

Automatic determination of refractive index profile of fibers having regular and/or irregular transverse sections considering the refraction of light rays by the fiber

A new model, using non-destructive two- and/or multiple-beam interferometric techniques, is suggested for measuring the refractive index profile of fibers having regular and/or irregular cross-sectional shape taking into consideration the refraction of the light rays by the fiber. The proposed model is applied for three different fibers having different cross-sectional shapes and different refractive index profiles. These fibers are PPT, homogeneous fiber, with circular cross-section, graded index optical fiber of circular cross-sectional shape and Dralon fiber of irregular cross-section. To validate the proposed model it is used, firstly, to calculate the index profile for a standard PPT fiber. Secondly, the calculated results for the irregular Dralon fiber and GR-IN optical fiber are compared with that calculated using other conventional method. From this comparison, we recommend that the refraction must be taken into account to obtain accurate results especially for birefringent fibers and graded index optical fibers.     

Effective Raman gain characteristics in germanium- and fluorine-doped optical fibers

We describe the dopant dependence of Raman gain in germanium- and fluorine-doped optical fibers. We clarify, both theoretically and experimentally, the effective Raman gain characteristic in an optical fiber, which is closely related to the fiber's refractive-index profile and electromagnetic field profile. We also show that this experimentally determined relationship can be used to evaluate the effective Raman gain characteristic in a germaninum- or a fluorine-doped optical fiber with an arbitrary index profile.     

反WKB方法的改进

改进了用于确定渐变平面波导折射率分布的反WKB方法。这种渐变折射率分布是从在WKB近似的条件下得到的本征方程用数值方法求得的。用改进的反WKB方法计算3种(指数、高斯和阶跃函数)折射率分布，结果验证了该方法的有效性。先用最小二乘法拟合测量得到的有效折射率。进而求出有效折射率函数，再利用改进的算法分布计算出各自的折射率分布。计算结果同精确值吻合得很好。计算出的波导表明折射率同精确值的绝对误差约为0．1%。