Single-polarization single-mode optical fibers

Strictly speaking, an ordinary axially symmetrical single-mode fiber is a "two-mode" fiber because two orthogonally polarized HE11modes can be propagated in it. This fact results in the instability of the polarization state of the propagated mode when geometrical perturbation exists in the fiber, and also the so-called polarization mode dispersion. These are harmful in some applications of single-mode fibers to communication and measurement. To prevent these adverse effects, single-polarization single-mode (SPSM) optical fibers have been developed. Three basic types of the SPSM fiber are elliptical-core fiber, stress-induced birefringent fiber, and side-pit fiber. This paper describes the principles of these three types, performance obtained experimentally, theoretical approaches, and measurement techniques related to the SPSM fibers. Finally, relevant technical tasks in the future are mentioned.     

Fusion splices for single-mode optical fibers

A practical low loss splicing method based on the discharge fusion for single-mode fibers was developed. Average splice losses of 0.4, 0.2, and 0.1 dB for fibers with 5.2, 7, and 10 μm core diameters, respectively, are obtained by a simple apparatus utilizing the self-alignment effect due to the surface tension of melted fiber ends. The surface tension effect is analyzed both experimentally and theoretically. Experimental splice losses, both after and during heating, coincide with the theoretical estimated values. It was found that the optimum heating temperature for low loss splices is near 2000°C at 8.5 W electric discharge power. Splicing loss causes are examined. The main cause of the practical splice loss is the residual core axis misalignment caused by an insufficient surface tension effect and core eccentricity with respect to cladding.     

Random coupling theory of single-mode optical fibers

A random coupling theory is developed for analyzing the propagation characteristics of the polarization state of light in single-mode optical fibers (including conventional and polarization maintaining fibers) under random disturbances. The basic idea is that the disturbances that a fiber suffers in practice continuously change with time and space, so time-varying coupling will occur along the fiber between two linearly polarized modes HE₁₁ that may propagate in the fiber. A coupled-mode equation of single-mode fibers under random disturbances is derived and solved rigorously with few assumptions. A random coupled-mode equation is derived considering time and space variation. Analytic solutions are obtained and used for analyzing the effect of random birefringence, polarization dispersion, polarization fluctuation, and evolution of the degree of polarization in single-mode fibers and for characterizing fiber properties     

单模光纤模场半径快速测量仪的设计及制作

用FPGA可编程控制器来控制步进电机带动放置在光纤端面远场区域的一个透光圆孔屏障沿光轴线移动,从而调节PIN光电管接收到的光信号强度.用锁相放大器来提高光电信号的信噪比,信号被快速采样后输入FPGA控制器,经过数据处理后显示所测单模光纤的模场半径.测量仪具备较好的测量精度和重复精度.     

Bending losses of coated single-mode optical fibers

Peaks appear in bending losses of coated single-mode fibers due to interference between the guided mode and rays which are radiated from the guided mode and are reflected at cladding-coating boundary. This paper reports derivation of bending loss formulas for coated slab waveguides and coated fibers. Plane wave concepts are also used to explain the appearance of the loss peaks. Measurements were performed by using two coated single-mode fibers. The agreement between theory and experimental results is found to be excellent. It is possible to obtain the refractive index difference from measured peak wavelengths.     

High-isolation polarization-independent optical circulator coupledwith single-mode fibers

A high-isolation polarization-independent optical circulator is reported. The goal is to provide a simple structure that sharply diminishes the degradation on the isolation caused by imperfection of polarization separation in the polarizing beam splitters. The circulator structure is almost the same as that of a conventional circulator except that a birefringent plate is added to each port to achieve high isolation. The effectiveness of the added birefringent plate has been confirmed by calculation and experiment. Insertion loss and isolation of the fabricated circulator coupled with single-mode fibers have been measured to be <1.0 dB and >29.9 dB, respectively, for 1.299 μm wavelength light. A loss ratio over 28.9 dB has been realized     

Polarization control on output of single-mode optical fibers

Polarization control of the light output from single-mode fiber systems is very important in order to connect it to polarization-dependent integrated optical circuits and applications using a heretodyne detection system. In this paper, automatic control of the polarization of light transmitted through the fiber was demonstrated by an electrooptical and electrical feedback system. In this system, we used a 1.6 μm long wavelength semiconductor CW BH laser, low-loss single-mode optical fiber with a loss of 0.2 dB/km, and two thinz-cut LiNbO3modulators tilted 45° relative to each other. The output light from this polarization controller was maintained to be linear while that from the single-mode fiber was unstable and elliptically polarized.     

Fiber fuse phenomenon in triangular-profile single-mode optical fibers

The unsteady-state thermal conduction processes in triangular-profile (TP) optical fibers, which exhibited zero chromatic dispersion near 1.55 /spl mu/m, were studied theoretically with the explicit finite-difference method (FDM). It was estimated that these fibers would exhibit a high-temperature optical absorption on the basis of the high-temperature loss-increase mechanism proposed for step-index (SI) optical fibers. The core-center temperature of the TP fibers changed suddenly and reached over 7/spl times/10/sup 5/ K when a 1.064-/spl mu/m laser power of 1 W was inputted into the core layer heated at 2608 K. This rapid heating of the core initiated the "fiber fuse" phenomenon. The propagation rates of the fiber fuse, estimated at 1.064 /spl mu/m, were in fairly good agreement with the experimentally determined values. It was found that the threshold powers for initiating the fiber fuse are linearly proportional to the roots of the effective core areas of both the SI and the TP optical fibers. This coincides the experimental result reported by Seo et al.     

Fiber fuse phenomenon in step-index single-mode optical fibers

The unsteady-state thermal conduction process in step-index single-mode (SM) optical fiber was studied theoretically with the explicit finite-difference method. We considered a high-temperature loss-increase mechanism, which includes two factors that bring about an increase in the absorption coefficients: 1) electronic conductivity due to the thermal ionization of a Ge-doped silica core and 2) thermochemical SiO production in silica glass. The core-center temperature changed suddenly and reached over 4/spl times/10/sup 5/ K when a 1.064-/spl mu/m laser power of 2 W was input into the core layer heated at 2723 K. This rapid heating of the core initiated the "fiber fuse" phenomenon. The high-temperature core areas were enlarged and propagated toward the light source. The propagation rates of the fiber fuse, estimated at 1.064 and 1.48 /spl mu/m, were in fair agreement with the experimentally determined values. We found that the threshold power for initiating the fiber fuse increases from 0.98 to 1.26 W when the input laser wavelength is increased from 1.06 to 1.55 /spl mu/m.     

Mode field diameter measurements in single-mode optical fibers

The role of the mode field diameter in the characterization of single-mode fibers is examined. The most relevant definitions of this parameter are reviewed, and a comparative analysis of methods for its measurement is performed. All the discussed measurement methods have reached a repeatability and reproducibility which are quite satisfying. Emphasis is given to the requirements posed by the new fiber designs, such as the polarization-maintaining structures. Most of the discussed techniques have been industrialized, and a number of instruments based on them are commercially available; however, it is predicted that the evolution of fiber design will impose new requirements on some of these instruments     

Polarization dispersion measurement on twisted single-mode optical fibers

A new method for measuring polarization dispersion between two eigen polarization modes is presented. It utilizes the measured phase shift corresponding to birefringence difference between two adjacent wavelengths. This method makes it possible to measure small polarization dispersion by using a short test piece even when eigen polarization modes are elliptically polarized. A polarization dispersion of 0.4 ps/km in twisted single-mode fibers can be measured in an approximately 4-m-long fiber by using 1.152- and 1.161-μm lines in a He-Ne laser. Experimental data that polarization dispersion decreases with an increase in fiber twist ate in very close agreement with theoretical results.     

Asymptotic effective cutoff condition in single-mode optical fibers

Recently some experimental investigations have shown that the effective cutoff wavelength of the LP11mode in a single-mode optical fiber decreases with increased distance from the optical source. In this paper such a phenomenon is theoretically investigated confirming that asymptotically the effective cutoff wavelength reaches a minimum value, dependent on few key parameters. Furthermore, attention is focused onto the excess attenuation which characterizes this behavior, leading to a remarkable penalty in the design of the optical link. The wavelength dependence of this parameter seems to be very useful in order to improve the cable manufacturing process.     

Practical microbending loss formula for single-mode optical fibers

Microbending loss in a step-index single-mode fiber is formulated in an expression which provides an explicit dependence on wavelength λ and relative index difference Δ. For the permissible mean bending radiusRdaggercorresponding to a given loss,Delta^{3/2}Rdagger/lambdais the functionfof only a normalized frequency υ. This property resembles that for the uniform-bending loss. However,fin the microbending loss depends less critically on υ than in the uniform-bending loss.     

大模场宽频单模光子晶体光纤的设计与制备

文章对无尽单模光子晶体光纤进行了理论设计,并提出了微结构光纤的制造工艺技术和相应的光纤拉丝工艺参数,制造出了较大模场的宽频单模光子晶体光纤,该光纤的芯直径为13.1 μm,微孔直径为3.2 μm,孔间距为8.2 μm,1 550 nm波长的模场直径为14.6 μm,1 550 nm的衰减为0.6 dB/km,1 383 nm水峰为14.9 dB/km.截止波长测试和近场光强分布检验表明,该光子晶体光纤确实具备较宽的单模工作范围.     

Performance of optical cable composed of dispersion-shifted single-mode fibers

A low-loss dispersion-shifted single-mode fiber cable has been fabricated. The index-profile of the fiber is nearly parabolic and the relative index difference is 0.8 percent. An average cable loss is 0.229 dB/km at 1.55 μm, and excellent loss stability has been achieved in the cabling process and in the temperature and mechanical test.     

FAST EVALUATION OF THE CHARACTERISTICS OF SINGLE-MODE OPTICAL FIBERS

This paper describes a method for fast engineering evaluation of the transmission characteristicsof single-mode optical fibers.A versatile microcomputer program is presented which can be utilized to ana-lyze mode field characteristics of single-mode fibers with arbitrary refractive index profiles.Our computationshows that the mathematical algorithm and the corresponding programs developeod in this paper are rela-tively simple and accurate enough for the engineenring design of single-mode fibers for optical communica-tion systems.     

Deformation of single-mode optical fibers under static longitudinal stress

The deformation of single-mode fibers resulting from a longitudinally applied static force has been measured experimentally by means of high resolution heterodyne interferometry and analyzed theoretically using the second-order theory of elasticity and the photoelastic effect. Both the elongation of the fiber and the phase change of light propagating through the fiber have been measured as a function of tensile force. The values of the elastic constants measured for fibers with pure silica core and B2O3doped cladding areE = 6.41 times 10^{10}N/m²for the Young's modulus,delta = -4.0for the nonlinearity constant of the longitudinal strain, andbeta = -2.3for the nonlinearity constant of the transverse strain. For unit elongations up to 0.3 percent, no creep, hysterisis, or relaxation effects have been observed within a resolution of one part in 10⁴.     

Refractive-index-profile determination of single-mode optical fibers by a propagation-mode near-field scanning technique

A new technique is presented for determining the refractive-index profile of a single-mode optical fiber. The profile is calculated from the measurement of the near-field mode pattern by a new equation derived from the scalar wave equation, A result of computer simulation shows that the method is useful for practical use in measurement systems.     

Simulation of fiber fuse phenomenon in single-mode optical fibers

The unsteady-state thermal conduction process in single-mode (SM) optical fiber was studied theoretically with the explicit finite-difference method (FDM). In the numerical calculation it was assumed that the electrical conductivity of the core layer increased rapidly above 1323 K. The core-center temperature changed suddenly and reached over 3/spl times/10/sup 5/ K when an optical power of 1 W was input into the core layer heated at 1373 K. This rapid heating of the core initiated the "fiber fuse" phenomenon. The high-temperature core areas, whose radiation spectrum extended over a wide range from the infrared to the ultraviolet regions, were enlarged and propagated toward the light source at a rate of about 0.7 m s/sup -1/. This rate was in fair agreement with the experimentally determined value.     

On the theory of backscattering in single-mode optical fibers

A new theory of backscattering in single-mode fibers is presented. It allows backscatter waveforms to be predicted for fibers of any refractive-index profile or scattering-loss distribution. The results agree with experimental data and provide confirmation of an earlier, more restricted theory.