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.     

Polarization Rotation in Twisted Polarization Maintaining Fibers Using a Fixed Reference Frame

In previous studies addressing polarization evolution in optical fibers, the frame of reference in which polarization rotation in twisted optical fibers is described lacks the compatibility with the modeling of some fiber sensor applications. We therefore introduce a formulation of the polarization evolution in a twisted optical and birefringent fiber, which is based on a laboratory coordinate system. We employ coupled mode theory using the fundamental modes of a single mode fiber as a basis. We treat the birefringence of the polarization maintaining fiber and the twist induced birefringence as perturbations introduced into this mode system.      

Analysis of the degree of polarization of optical waves in a single-mode optical fiber with random coupling

A single-mode optical fiber model is proposed with discrete mode-coupling centers having random coupling coefficients at regular intervals. The degree of polarization in a single-mode fiber with random-mode coupling does not approach zero with increasing fiber length. Numerical results show that the ensemble average of the degree of polarization approaches a nonzero value the magnitude of which depends on the coupling intensity, the light source spectrum, the fiber birefringence, and the spatial frequency of coupling centers, The theoretical and numerical results clarify the limitation of the perturbation iteration method for long optical fibers.     

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.     

光谱精确可控Sagnac环滤波器及其在ASE光源中的应用

研究了一种结构新颖光谱特性可精确控制的Sagnac环滤波器,这种滤波器通过在传统Sagnac环中插入两段高双折射光纤和一个偏振控制器构成。利用等效光路和传输矩阵法理论研究了该滤波器的滤波特性,研究结果表明,固定两段高双折射光纤参数,仅通过调整偏振控制器的状态,便可精确控制Sagnac环滤波器的滤波光谱特性。将Sagnac环滤波器用于掺铒光纤（EDF）放大的自发辐射（ASE）光源的光谱平坦滤波,通过调节偏振控制器三个波片的倾斜角度,实验获得了光谱平坦的宽带ASE激光。     

双折射单模光纤的保圆耦合

提出了一种利用双折射单模光纤扭转时产生的圆双折射性获得保圆偏振的新方法，该法应用于光纤耦合器的基本思想是通过有效地改变扭转光纤在耦合区域的单位扭转度，从而达到光纤之间实现保圆耦合的目的。同时，用耦合模方程分析了利用该法构成的２×２单模光纤耦合器的耦合行为，结果表明光纤耦合器达到了较好的保圆耦合。     

Circularly birefringent single-mode optical fibres

The rotation of polarised light which occurs in a twisted anisotropic medium is exploited to produce very high circular birefringence in single-mode optical fibres. Beat lengths of less than 5 mm have already been achieved in some preliminary experiments.     

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     

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.     

Theory and implementation of a Raman active fiber delay line

In an optical delay line made of a single-mode reentrant fiber loop, Raman amplification is used to compensate for recirculating signal losses. Concurrent Stokes noise amplification limits the system performances. A theoretical model shows that the signal-to-noise ratio decays as the reciprocal of the number of signal recirculations. Experimental results obtained with a 760-m-long fiber loop operated atlambda = 1.12 mum are presented. A new pump modulation technique resulting in improved output signal stability is reported whereupon optical delays up to 3 ms were achieved.     

Measurement of the spatial distribution of birefringence in opticalfibers

We describe a technique for the measurement of the birefringence spatial distribution in a single-mode optical fiber with a resolution of 1 m. This technique is based on a polarization optical time-domain reflectometer using a rotary linear polarizer. We report results performed on different types of fibers: standard step-index and dispersion shifted fibers     

Bending effects in biconically tapered single-mode fibers

Biconically tapered single-mode fibers were fabricated, and their characteristics were studied experimentally. The optical throughput was measured as the fiber was being pulled to produce the required radial profile. The tapered single-mode fiber was bent in a simple fixture, and the optical throughput was measured as a function of the bending angle. It is seen that the bending resulted in very strong oscillations of the optical power as the bend angle was varied. At the point when the light in the core was minimal, the cladding region was bright, indicating that the light has moved to the cladding. The propagation characteristics of the tapered single-mode fiber subjected to bending are analyzed using a stepwise approximation. A simple sensor based on this principle is discussed     

Method of measuring optical switch crosstalk attenuationconsidering polarization variation

This paper describes a simple method for measuring the crosstalk attenuation of optical switches that takes into account the polarization variation of incident light. The method involves the use of a laminated film polarizer (LAMIPOL) and a polarization-maintaining fiber (PANDA). Crosstalk attenuation of 2×2 liquid-crystal optical switches for 1.3 μm single-mode fiber is measured using the proposed method and then conventionally with a polarization controller. The similarity of the results confirms the practicality of the proposed method     

A Coupled-Mode Approach to the Analysis of Fields in Space-Curved and Twisted Waveguides

A coupled-mode approach for solving Maxwell's equations in terms of unitary and reciprocal unitary vectors is deduced in a twisted space-curved coordinate system. Application of this method to the analysis of fields in a space-curved and twisted single-mode optical fiber and a twisted rectangular microwave waveguide is presented and compared with the results from existing literature.     

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.     

SC-type single-mode optical fiber connectors

SC-type single-mode optical fiber connectors specifically developed for subscriber loop networks are discussed. The properties and design of precision zirconia ceramic ferrules which have been found to be ideal for high-performance, low-cost single-mode optical fiber connectors are described. A design approach featuring a plastic-molded rectangular connector housing using a push-pull coupling mechanism which has also been found to be suitable for durable, compact, and low-cost connectors is presented. SC connectors used with 10/125 single-mode fibers exhibit insertion loss of 0.06 dB and return loss of 38.6 dB, with no degradation during and after mechanical and environmental tests. To realize higher packaging density, duplex-ferrule connectors, quadruple-ferrule connectors and optical attenuators have also been developed on the basis of the SC connector design     

A 100-km-long single-mode optical-fiber fault location

A Raman OTDR (ROTDR) fault location for a 100-km-long single-mode optical fiber has been tested. A Ge p-i-n photodiode cooled by liquid nitrogen has been used as a backscattered light receiver. The signal-to-noise ration (SNR) improvement of the cooled Ge p-i-n photodiode from a Ge avalanche photodiode (Ge APD), which operates at room temperature, is 14 dB. Spectral Stokes light power distributions, which have been emitted from a long single-mode fiber, have been measured to estimate the maximum locatable length for a single-mode fiber. It has been shown, from the measurements and calculations, that a 100- km-long and a 30-dB total attenuation at a 1.55-μm wavelength single-mode fiber fault location can be achieved with the aid of an averaging technique. On the basis of these results, a 102-km-long single-mode fiber fault location experiment has been successfully carried out.     

平面手征结构的旋光度精密测量

基于精密步进电机和光纤光谱仪的光学测试系统,优化设计了一种精确测量手征结构样品旋光度的方案。对手征样品旋光性的理论分析结果表明,由于周期性手征结构样品制作过程中90°旋转对称性被打破,线偏振光经过样品后偏振面旋转角度与入射光的偏振面角度有关。实验测量结果直接证明了理论分析的正确性。利用设计的光学测试系统对采用电子束光刻技术制作的周期性平面手征结构样品进行旋光度测量的结果表明,样品在波长为633nm处具有0.05°的旋光度,很好证明了研制的周期性平面手征结构具有一定的旋光性。     

All-silicon integrated optical modulator

The authors describe the operating principle, design, and performance of an all-silicon light modulator at 1.3 μm wavelength. The modulator is based on the plasma effect in silicon and the mode selectivity of single-mode optical fibers, resulting in low polarization dependence and the capability of handling high light intensities. Standard silicon IC technology is used in the fabrication process and the modulator has a vertical structure that takes up a small surface area (the active area matches the single-mode fiber core of 9 μm diameter), simplifying integration with other circuitry on the same chip. The modulator can be directly coupled to a single-mode optical fiber, without using lenses or other bulk optical components. Typical performance of the fabricated modulators is 6 dB insertion loss, 24% modulation depth, and 60 MHz bandwidth with a current drive of 22 mA rms     

Polarization properties of the flint glass fiber

To realize stable characteristics of a current sensor using the Faraday effect in an optical fiber, it is necessary to prevent the influence of birefringent effect on light passing the fiber, which is caused by photo-elastic effect and/or lack of axial symmetry of its structure. The flint glass fiber manufactured from flint glass with very small photo-elastic constant, which has circular core and satisfies the single ode condition,has been developed to solve this problem. This paper reports results of a study to clarify polarization properties of the fiber and to consider a method for highly stabilizing the polarization. From experiments to investigate the relationship between state of polarization of light inserted to the fiber and that emitted from another end, it was confirmed that the birefringence due to the photo-elastic effect and the lack of symmetry are negligibly small. It also became known that the azimuth angle of polarization of the emitted light depends on the shape of the curve of the fiber. Namely, a rotation of the polarization plane is equal to the amount of line integral of the torsional rate along the curve. A countermeasure is necessary for this phenomenon, because characteristics of the current sensor may be influenced if the flint glass fiber coil is deformed by environmental factors. From a consideration of the mechanism of the phenomenon, it is inferred that the problem can be solved by reciprocating light in the fiber to stabilize the polarization. The effect of this method is confirmed experimentally