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This paper discusses a method of reducing the polarization dependence of gain (PDG) of a distributed Raman amplifier. Reducing its PDG is important for a Raman amplifier because it is higher than that of an erbium-doped fiber amplifier and can degrade transmission performance. Raman PDG is determined primarily by two factors, namely 1) polarization-mode dispersion (PMD) of the transmission fiber and 2) degree of polarization (DOP) of the pump source. The authors propose a simple analytical model to show the required pump light DOP for a given transmission fiber's PMD and the allowable PDG. For instance, a low pump DOP of 5% produces a low PDG of 0.2 dB under typical fiber PMD conditions, in which the analytical model agrees well with experiment. Subsequently, to achieve the required DOP, the pump source configuration is investigated in detail. The authors used one length of polarization-maintaining fiber (PMF) as an efficient pump depolarizer and evaluated its performance for various pump light spectra. It has been shown that the DOP following the depolarizer is determined simply by Fourier transformation of the pump light spectrum. The analysis in this paper has led to the important result that a Fabry-Perot laser diode pump with a short piece of PMF is effective in achieving a low pump DOP due to its multimode spectrum when the length of the PMF is properly adjusted for the longitudinal-mode spacing frequency. It has been verified that a Raman amplifier's PDG can be reduced by the proposed efficient depolarizer sufficiently for a PDG-reduced Raman amplifier repeater to be applicable to long-haul transmission systems [ABSTRACT FROM PUBLISHER]

The circular-birefringence fiber (CBF) or the so-called twisted fiber provides novel attractive potential for all-optical signal-processing applications. Unlike a standard nonpolarization-maintaining (non-PM) fiber, where the polarization dependence of chi(3) nonlinearity is averaged out by the residual birefringence, the CBF behaves as a perfectly isotropic fiber in terms of nonlinear wave propagation. The CBF, therefore, allows the utilization of the intrinsic absolute polarization dependence of chi (3) nonlinearity in realizing novel functions that can never be obtained with a non-PM fiber. Several types of CBF are successfully fabricated by twisting commercial non-PM fibers at 15 turns/m, and their unique potential is experimentally demonstrated. First, a novel type of all-optical intensity discriminator is realized using the effect of ellipse rotation in CBF, which is absent in a non-PM fiber. The demonstrated device is directly applicable to all-optical data reshaping, pulse compression, pedestral suppression, and enhanced mode-locking of fiber lasers. Next, a 160-Gb/s polarization-insensitive wavelength conversion is demonstrated by using cross-phase modulation in CBF with a circularly polarized probe wave. In addition to its simplicity and robustness, the scheme is readily upgradable to polarization-insensitive all-optical data regeneration [ABSTRACT FROM PUBLISHER]

A femtosecond laser has been used to asymmetrically modify the cladding of fiber containing long-period gratings. Following modification, devices in single-mode fiber are shown to be capable of sensing the magnitude and direction of bending in one plane by producing blue and red wavelength shifts depending upon the orientation of the bend. The resulting curvature sensitivities were -1.62 and +3.82 nmmiddotm. Devices have also been produced using an elliptical core fiber to study the effects of the cladding modification on the two polarization eigenstates. A cladding modification applied on the fast axis of the fiber is shown to affect the light in the fast axis much more significantly than the light in the orthogonal state; this behavior may ultimately lead to a sensor capable of detecting the direction of bending in two dimensions for applications in shape sensing [ABSTRACT FROM PUBLISHER]

The polarization dependence of the coherent detection in optical frequency-domain reflectometry can be exploited to measure the local beatlength along an optical fiber. Reproducibility of the results is given for high and low polarization-mode coupling fibers, and the influence of several effects on beatlength measurement precision, such as spatial reflectometer resolution, fiber twist, and the presence of circular birefringence, is discussed. The experimental results seem to indicate that the typical assumption that the fiber birefringence can be treated as purely linear might not be fully justified. Beatlength overestimates of 10% and more can result. [ABSTRACT FROM PUBLISHER]

An analytic approach to calculate the radiation pattern (including the wavelength dependence, azimuthal distribution, and polarization dependence) of tilted fiber phase gratings using the volume current method (VCM) is introduced in this paper. Results for both uniform and apodized fiber gratings are derived and discussed, and a brief comparison to experimental measurements from practical devices is also made [ABSTRACT FROM PUBLISHER]

Bragg gratings in optical fibers in multimode propagation are investigated experimentally and theoretically. Bragg gratings formed in optical fibers in multimode propagation show multiple reflection peaks or multiple transmission dips in the reflection or transmission spectra, respectively. For standard graded-index multimode fiber, the number of reflection peaks of a Bragg grating depends on excitation condition of propagating modes. The number of reflection peaks of a Bragg grating at around 1.55 μm is 19 for highly multimode excitation and 3-4 for lower order mode excitation. We analyze the phase-matching conditions of the propagating modes and identify half of the reflection peaks as the reflection to the same mode and the rest as the reflection to the neighboring modes. In dispersion-shifted fiber, a Bragg grating at around 0.8 μm in three-mode propagation shows three reflection peaks in the reflection spectrum. The temperature dependence of each reflection peak is similar to that of a conventional Bragg grating in single-mode fiber. Polarization dependence measured on a Bragg grating in multimode graded-index fiber is negligible. An advantage of Bragg gratings in multimode fiber (MMF) and the applications are discussed [ABSTRACT FROM PUBLISHER]

External perturbation such as transverse force can increase the birefringence of most fiber Bragg gratings (FBGs). As a result, the interrogation accuracy of an FBG-based sensor may be significantly deteriorated due to an uncertain wavelength shift caused by the variation of polarization state of the input light. In the experiments, the reflection spectra of the FBG-based strain sensors made of the spun and conventional single-mode fibers were compared under different transverse forces when the polarized and unpolarized light was injected, respectively. It is shown that the spun FBG sensors have lower polarization dependence on, and consequently stronger resistance to, transverse force than the conventional FBG sensors [ABSTRACT FROM PUBLISHER]

The fiber optical parametric amplifier (FOPA) is characterized using a optical time-domain reflectometer-based technique. The measurement technique visualizes how the parametric gain and bandwidth evolves as a function of length. Both gain and bandwidth are measured and compared to theory and a good agreement is found between them, making this technique useful for characterization of FOPAs. The impact of zero dispersion wavelength variations in the fiber and polarization dependence of the measurement technique are also discussed [ABSTRACT FROM PUBLISHER]

The intensity modulation transfer function of a linear optical system is described using a time-dependent Muller-like matrix. Based on this description a novel measurement method for the combined effect of polarization mode dispersion and polarization-dependent loss is introduced [ABSTRACT FROM PUBLISHER]

This paper describes compact variable optical attenuator arrays made by using fiber splicers and polymer-network liquid crystal. Fibers are inserted into the splicers and the gaps between the ends of the fibers are filled with the polymer-network liquid crystal. The residual loss for each array element is low (about 0.8 dB), the attenuation of 1.55-μm light is about 8 dB and the driving voltage is 20 Vrms . The polarization dependence is <2 dB. The power consumption is also very low (30 nW/ch). They are used for channel power equalizers for WDM systems [ABSTRACT FROM PUBLISHER]

In a backward Er-doped superfluorescent fiber source (SFS), we report the observation of large mean wavelength variations (>100 ppm) induced by external perturbations of the fiber birefringence. These previously unreported variations, which need to be reduced to the ppm level for high-accuracy fiber-optic gyroscope applications, are shown to originate first from polarization-dependent gain induced by the polarized pump source, and second from a slight polarization dependence of the SFS wavelength division multiplexing (WDM) coupler and fiber isolator. We demonstrate that these effects can be substantially reduced by incorporating two Lyot fiber depolarizers in the source. The new depolarized SFS exhibits short-term mean wavelength stability of ±2.5 ppm and a long-term drift of ±3 ppm. The latter is probably due mostly to slow variations in the Er-doped fiber temperature, which can be eliminated with a simple temperature control to ~0.1°C [ABSTRACT FROM PUBLISHER]