Optical power limits in multi-channel wavelength-division-multiplexed systems due to stimulated Raman scattering

General expressions are derived to estimate transmitter power limitations due to stimulated Raman scattering in wavelength-division-multiplexed optical communication systems. These results are applicable to systems containing an arbitrary number of channels with arbitrary (but equal) channel separation.     

Gain saturation in fiber Raman amplifiers due to stimulatedBrillouin scattering

The effects of stimulated Brillouin scattering (SBS) on Raman gain are explored. Measurements of gain in a fiber Raman amplifier show a saturation at low gain levels. Experimental data and a theoretical model are presented, demonstrating that this saturation is due to pump depletion by SBS. This effect also leads to Raman gain fluctuations arising from mode partitioning in a multimode pump laser. Two ways to avoid the deleterious effects of SBS on the performance of Raman amplifiers are suggested. One is to use a multimode semiconductor laser with a modal linewidth of the order of a few hundred megahertz, which leads to a strongly reduced Brillouin gain coefficient. The second approach is to use very short pump pulses in a backward configuration     

Birefringence in curved single-mode optical fibers due to waveguide geometry effect--Perturbation analysis

The field deformation in curved step-index single-mode circular fibers and the birefringence induced by the curved waveguide geometry are analyzed by a perturbation method. We find that the values of the birefringence are proportional toa^{2}/R_{0}^{2}and are much less than those induced by stress. A square optical waveguide is also investigated. We find birefringence values of roughly the same order of magnitude as those for the circular waveguide.     

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.     

High-power InGaAsP edge-emitting LEDs for single-mode optical communication systems

The letter describes 1.3 ?m InGaAsP edge-emitting LEDs which couple 40 to 60 ?W into a single-mode fibre. The 1.0 ns rise times and 30 to 45 nm spectral widths of these LEDs make suitable for optical communication systems operating at data rates as high as 400 Mbit/s.     

Evaluation of extrinsic scattering loss due to crystallization influoride optical fibers

Extrinsic scattering loss caused by crystallization in current fluoride optical fibers is evaluated in order to investigate the possibility of realizing long-length low-loss fibers. In the current fabrication process, which consists of melting, casting, and drawing, crystallization occurs mainly in the latter two heat processes. Crystal initiation and growth are simulated based on the thermal history during these processes. Scattering loss is calculated from the crystal distribution obtained. When the preform diameter is increased, scattering loss increases rapidly. A low scattering loss is essentially expected only for the fibers drawn from small diameter preforms, although drawing has a relatively large effect on crystallization     

高速传输系统用低损耗单模光纤的研究

介绍了国际上低损耗单模光纤的研究进展,从理论上分析了低损耗单模光纤实现的技术途径。通过对石英玻璃物理特性的研究,探索出石英玻璃光纤的退火工艺,以降低光纤的内应力从而降低光纤的衰减。采用自主知识产权的专利技术制造的光纤在1 310、1 550和1 625nm波长的衰减典型值分别为0.320、0.182和0.195dB/km。该光纤的技术指标优于ITU-T国际标准和GB/T规定的光纤指标值。     

Power penalty due to jitter on optical communication systems

Two techniques to assess the performance degradation of optical communications systems due to jitter are compared: a Gaussian approximation and a precise calculation. It is shown that, depending on the jitter distribution, the approximation may either under- or overestimate the performance degradation caused by jitter by a significant amount.     

Power penalty due to decision-time jitter in optical communication systems

The effect of decision-time jitter on the performance of optical communication systems is evaluated theoretically. The jitter-induced noise and the associated power penalty are calculated for the uniform and Gaussian distributions of the time jitter. The results suggest that the time jitter should be ?(4B)?1 to keep the power penalty below 1 dB for a light-wave system operating at a bit rate B.     

Theory of signal light amplification by stimulated Raman scattering in twisted single-mode optical fibers

Characteristics of signal light amplification by stimulated Raman scattering are studied theoretically for the twisted single-mode fibers. Coupling equations for orthogonal modes in the fiber are derived and solved analytically. The optical gain dependence on fiber twist rate, fiber length, and so on are numerically investigated. The Raman gain for the twisted fiber with a rate of 1 turn/m is found to be reduced by 10 dB comparing with the nontwisted fiber for the fiber length less than 2 km, while it is almost independent of twist rate for the fiber longer than 10 km.     

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.     

Theory of backward Rayleigh scattering in polarization-maintaining single-mode fibers and its application to polarization optical time domain reflectometry

Backward Rayleigh scattering in polarization-maintaining single-mode optical fibers has been theoretically analyzed. A simple formula for the backscattered power, which comes back again at an input portion of the fiber, is derived with a recapture factor Sxy. By applying the analysis to a polarization optical time domain reflectometry (POTDR), details of the POTDR have been made clear. The important results are that the fluctuation component in the POTDR is attributable to a polarization beat length of the fiber, and that the quantity of the polarization component occupied in the total back-scattered power is determined by the optical pulse width.     

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     

Fundamental limits in optical communication

This paper addresses fundamental limits on the ability to communicate over optical fiber and optical free-space channels. The underlying limits on the sensitivity of optical receivers dictated by quantum effects are reviewed. Receiver sensitivity limitations caused by background light, dark current, post detection amplifier noise, and transmitter imperfection are then considered. Finally the limitations of the channels related to input coupling, loss, and delay distortion are reviewed. Throughout, comparisons are made between the optical systems and their microwave counterparts to highlight differences and the origins of those differences.     

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.     

Bandwidth limits of nonlinear (soliton) optical communication systems

We have shown that interactions between pulses can lead to a significant reduction of the bandwidth of nonlinear systems. The inclusion of loss is essential and implies that solitons must be launched with a separation of at least 10 times their pulse width for short systems of ~ 30 km and even more for longer systems.     

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     

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.     

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.