Polarization mode dispersion of spun fibers: an analytical solution

A simple analytical solution is derived from coupled-mode theory to describe the evolution of polarization mode dispersion (PMD) in spun fibers. For practical fibers with a beat length greater than a few meters, the solution is valid for a whole category of periodic spin profiles. We find that the PMD change factor is independent of the intrinsic birefringence of the fiber and the fiber PMD scales linearly with the fiber length in the short length regime. This solution allows us to determine phase-matching conditions for spun fibers, in which the PMD evolves periodically along the fiber. An example of determining the phase-matching conditions of sinusoidal-type spin profiles is given.     

Polarization mode dispersion properties of constantly spun randomly birefringent fibers

We calculate the asymptotic first-order polarization mode dispersion statistics of constantly spun fibers. We show that in the long length regime the mean differential group delay of constantly spun fibers grows linearly with distance at the same rate as for unspun fibers. Conversely, the short length regime of constantly spun fibers may be much longer, even for limited spin rates.     

Polarization mode dispersion of spun fibers with randomly varying birefringence

We show analytically how periodic spinning affects the polarization mode dispersion of a fiber in three different practical regimes that are determined by the values of three length scales: the beat length, the birefringence correlation length, and the spin period. We determine in which limits the spin is effective in reducing the mean differential group delay.     

Low-PMD spun fibers

In the last ten years there has been an increasing interest in the design and realization of low PMD fibers. This goal can be achieved by properly spinning a fiber when it is still in the hot zone. In this chapter, analytical and numerical results on the design of low-PMD fibers by means of spinning techniques are presented.     

Polarization mode dispersion induced pulse broadening in optical fibers

A simple and exact analytical expression is derived for the amount of broadening that a pulse suffers when it is subjected to the combined actions of polarization mode dispersion, chromatic dispersion, and chirping. The theory is then applied to various manifestations of second-order polarization mode dispersions.     

Polarization mode dispersion-induced fluctuations during Raman amplifications in optical fibers

We present a vector theory of Raman amplification and use it to discuss the effect of polarization-mode dispersion (PMD) on the Raman gain process inside the optical fiber used for stimulated Raman scattering. We show that the PMD induces large fluctuations in the amplified signal and reduces the average value of the amplifier gain. In the case of forward pumping, fluctuations are expected to be more than 15% under typical operating conditions and can exceed 50% for fiber with a relatively low value of the PMD parameter. The PMD effects are much less severe in the case of backward pumping. Signal fluctuations reduce to less than 1% in this case when the PMD parameter exceeds 0.05 ps/ radicalkm.     

Polarization decorrelation in optical fibers with randomly varying elliptical birefringence

Polarization decorrelation in single-mode fibers with randomly varying elliptical birefringence is studied. It is found that the effects of ellipticity on the polarization decorrelation length depend on the relative sizes of the beat length and the autocorrelation length of the birefringence fluctuations in the fiber. However, the evolution of the differential group delay remains unaffected by ellipticity.     

Polarization mode dispersion in single mode optical fibers due to core-ellipticity

The variation of polarization mode dispersion (PMD) with V-parameter in single mode optical fibers due to core-ellipticity is studied by performing numerical simulations taking into account both geometrical and thermal-stress-induced birefringences as well as the variation of fiber refractive indices with wavelength. Simple empirical relations are given for calculating the mean PMD for any value of core-ellipticity and V-parameter of a standard single mode fiber. It is observed that the mean PMD saturates for V ? 1.8 leading to very small second order PMD.     

Properties of polarization evolution in spun fibers

We study the properties of polarization evolution in sinusoidally spun fibers. It is found that, similar to linear birefringent fibers, the evolution of the state of polarization exhibits periodicity, which can be measured by distributed measurement, such as those made with a polarization optical time domain reflectometer. The spatial period is linked with the spin parameters and fiber beat length in a simple equation. In combination with a previous finding, it is shown that the spatial period is uniquely related to spun-fiber polarization mode dispersion. This suggests that distributed fiber polarization mode dispersion can be determined throughthe measurement of the spatial period obtained in a distributed measurement.     

Polarization switching of stimulated Raman scattering in optical fibers by dual-frequency pumping

Experiments are presented showing that under dual-frequency, orthogonal-polarization pumping, stimulated Raman scattering in optical birefringent fibers can be suppressed in one of the two fiber axes. For relatively small-frequency spacing and group-velocity mismatch between the pumps, Stokes light is suppressed along the polarization direction of the highest-frequency pump, owing to the orthogonal component of the Raman gain.     

Polarization control in a large-stroke optical fiber delay line

In the context of an optical fiber delay-line implementation by use of silica fiber stretching capability, we experimentally demonstrate that highly birefringent fibers preserve their ability to maintain the polarization coherence in spite of the high magnitude of the stresses applied to them during large stretching. Two experimental demonstrations are presented. In the first, polarization cross coupling versus stretching is characterized. This stretching induces a variation of group delay in the span 0-45 cm. The second is a test of the fiber stretcher in an interferometric application.     

Nonlinear control of the propagation of optical pulses in doped optical fibers

Results of study of nonlinear processes occurring during the propagation of light pulses in optical fibers doped with atoms of rare-earth elements under conditions of atomic coherence and interference are presented. For a three-level Λ scheme of interaction, the linear (χ⁽¹⁾) and nonlinear (χ⁽³⁾) susceptibilities of such a medium are calculated. It is shown that the coefficients of Kerr nonlinearity and nonlinear absorption can reach extremely large values and can be negative. The competition between linear and nonlinear processes in the Λ scheme allows one to obtain compensation regimes when the coefficients of dispersion or absorption of the optical fiber material vanish. The efficient control of the optical properties of such a system over wide limits proves to be possible owing to variations in the parameters of light pulses at the input of the medium. The necessary conditions for realizing regimes with “slow” light, as well as for self-compression of a probing pulse on ultimately small spatial scales in a doped optical fiber, were obtained.     

Theoretical study of polarization attraction in high-birefringence and spun fibers

Lossless polarizers are conservative nonlinear optical devices that transform unpolarized light into highly polarized light without polarization-dependent losses. The device proposed here consists of an up to 100-m-long segment of nonlinear highly birefringent or unidirectionally spun fiber pumped from the output end by an intense backward-propagating beam. An initially unpolarized (scrambled) signal beam acquires a degree of polarization close to 100% toward the fiber output.     

Adaptive control of femtosecond pulse propagation in optical fibers

Nonlinear effects present fundamental obstacles to the propagation of femtosecond pulses of detectable energy in single-mode optical fibers, inducing severe distortion even after a very short (a few meters) propagation distance. We show here that adaptive pulse shaping can overcome these limitations by synthesizing pulses that are self-correcting for higher-order nonlinear effects when they are launched in the fiber. This approach would not only affect optical communications but also yield benefits in various disciplines requiring optimized fiber-based femtosecond pulse delivery, for example, nonlinear imaging techniques such as multiphoton microscopy, material processing, and medical diagnostics.     

Ultraviolet-induced absorption losses in hydrogen-loaded optical fibers and in presensitized optical fibers

Reduced hydroxyl formation in presensitized fibers exposed to cw 244-nm light after hydrogen outdiffusion is reported. The OH band in the presensitized fiber shifts toward 1390 nm. In the fully hydrogen-loaded fiber the OH band is centered at 1397 nm and does not shift with fluence.     

光学系统的偏振像差研究

综合论述了偏振像差的理论,给出了具体的例子,指出偏振是影响光学系统性能的一个重要因素,要想提高光学系统的性能,必须控制光学系统的偏振像差,而且光学系统设计必须与光学薄膜的设计结合起来。     

Polarization singularities in optical lattices

Polarization singularities are shown to be unavoidable features of three-dimensional optical lattices. These singularities take the form of lines of circular polarization, C lines, and lines of linear polarization, L lines. The polarization figures surrounding a C line (L line) rotate about the line with winding number +/-1/2 (+/-1). C and L lines permeate the lattice, meander throughout the unit cell, and form closed loops. Surprisingly, every point in a linearly polarized optical lattice is found to be a singularity about which the surrounding polarization vectors rotate with an integer winding number.     

Optical control of vortices in dense media of gas-filled optical fibers

The problem of the formation of optical vortices in a medium composed of three-level atoms is analyzed with allowance for local-field effects in off-resonance Raman regimes of ??-scheme interaction. The possibility of optical control of the vortex dynamics without disturbing their stability is demonstrated in the optically dense medium of a hollow fiber filled with ⁸⁷Rb vapor.     

Active control of long-period fiber-grating-based filters made in erbium-doped optical fibers

Integrating an active medium with a filtering component can bring new functionalities compared with configurations where the two stages (units) are placed in series. We exploit this phenomenon and demonstrate all-optical tunable filters based on long-period fiber gratings written in an active optical fiber. The tuning is obtained via optical pumping at 980 nm.