P. K. A. Wai

Polarization mode dispersion, decorrelation, and diffusion in optical fibers with randomly varying birefringence

Polarization mode dispersion and the polarization decorrelation and diffusion lengths are calculated in fibers with randomly varying birefringence. Two different physical models in which the birefringence orientation varies arbitrarily are studied and are shown to yield nearly identical results. These models are appropriate for communication fibers. We show that both the length scales for polarization mode dispersion and polarization decorrelation measured with respect to the local axes of birefringence are equal to the fiber autocorrelation length. We also show that the coupled nonlinear Schrodinger equation which describes wave evolution over long length along a communication fiber can be reduced to the Manakov equation. The appropriate averaging length for the linear polarization mode dispersion is just the fiber autocorrelation length but the appropriate averaging length for the nonlinear terms is the diffusion length in the azimuthal direction along the Poincare sphere which can be different, The implications for the nonlinear evolution are discussed.

Nonlinear pulse propagation in the neighborhood of the zero-dispersion wavelength of monomode optical fibers.

Nonlinear pulse propagation is investigated in the neighborhood of the zero-dispersion wavelength in monomode fibers. When the amplitude is sufficiently large to generate breathers (N > 1 solitons), it is found that the pulses break apart if lambda - lambda(0) is sufficiently small, owing to the third-order dispersion. Here lambda(0) denotes the zero-dispersion wavelength. By contrast, the solitary-wave (N = 1) solution appears well behaved for arbitrary lambda - lambda(0). Implications for communication systems and pulse compression are discussed.

Application of the Manakov-PMD equation to studies of signal propagation in optical fibers with randomly varying birefringence

We report on our investigations of the Manakov-polarization mode dispersion (PMD) equation which can be used to model both nonreturn-to-zero (NRZ) and soliton signal propagation in optical fibers with randomly varying birefringence. We review the derivation of the Manakov-PMD equation from the coupled nonlinear Schrodinger equation, and we discuss the physical meaning of its terms. We discuss our numerical approach for solving this equation, and we apply this approach to both NRZ and soliton propagation, We show by comparison with the coupled nonlinear Schrodinger equation, integrated with steps that are short enough to follow the detailed polarization evolution, that our approach is orders of magnitude faster with no loss of accuracy. Finally, we compare our approach to the widely used coarse-step method and demonstrate that the coarse-step method is both efficient and valid.

Stability of solitons in randomly varying birefringent fibers.

The effects of randomly varying birefringence on solitons are studied. It is shown analytically that the evolution equation can be reduced to the nonlinear Schrödinger equation if the variation length is much shorter than the soliton period. The soliton does not split at high values of the average birefringence, but it does undergo spreading and loss of polarization. A soliton with a temporally constant initial state of polarization is still largely polarized after 40z(0) if the normalized birefringence is delta </= 1.3.

Pressure sensor realized with polarization-maintaining photonic crystal fiber-based Sagnac interferometer

A novel intrinsic fiber optic pressure sensor realized with a polarization-maintaining photonic crystal fiber (PM-PCF) based Sagnac interferometer is proposed and demonstrated experimentally. A large wavelength-pressure coefficient of 3.42 nm/MPa was measured using a 58.4 cm long PM-PCF as the sensing element. Owing to the inherently low bending loss and thermal dependence of the PM-PCF, the proposed pressure sensor is very compact and exhibits low temperature sensitivity.

Stable and uniform multiwavelength erbium-doped fiber laser using nonlinear polarization rotation

A novel and simple technique, based on nonlinear polarization rotation (NPR) effect, to generate stable multiwavelength oscillations in an erbium-doped fiber laser is proposed and successfully demonstrated. The NPR effect effectively induced intensity- and wavelength-dependent loss to alleviate mode competition caused by homogeneous gain broadening in erbium-doped fibers. Up to 28-wavelength lasing operation with wavelength spacing of 0.8 nm has been achieved. The outputs had uniform power distributions, and the power fluctuation in each wavelength is smaller than 0.2 dB within a period of an hour.

Soliton at the zero-group-dispersion wavelength of a single-model fiber

It is shown that solitons emerge from initial pulses of arbitrary shape and amplitude whose central frequencies are at the zero-dispersion point. The initial threshold power is substantially reduced from that required for experiments to date. The use of these solitons is thus an attractive alternative to both the linear and the nonlinear communication schemes that have been proposed to date.

Soliton fiber ring laser

A fiber ring laser that can produce nearly transform-limited soliton pulses is simulated. This laser has an erbium-doped optical-fiber amplifier and allows wavelength tuning through the interplay of fiber chromatic dispersion and the round-trip delay time of the laser. We show that a saturable absorber and a frequency limiter are required for the ring laser to self-start, i.e., to mode lock from initial noise and to operate stably. We also show that nonlinear polarization rotation with polarization selectivity inside the ring can act as a saturable absorber.

Crosstalk in a lossy directional coupler switch

Crosstalk due to material absorption in a two-waveguide, symmetric directional coupler switch is investigated. In a material with absorption, it is not possible to completely eliminate the crosstalk by adjusting the coupling length. The coupling length for minimum crosstalk differs from that of lossless systems. Theoretical limits of the lowest achievable crosstalk and the corresponding coupling lengths are calculated. The results show that the effect of absorption on crosstalk is more severe when the devices are designed for low crosstalk. The increase in crosstalk due to absorption can be as high as 20 dB. The material absorption is thus a critical parameter in designing low crosstalk devices. >

Stability of passively mode-locked fiber lasers with fast saturable absorption

The stability of passively mode-locked fiber lasers with fast saturable absorption is examined by a direct numerical approach. The laser operation is described by use of a standard model, closely related to the Ginzburg–Landau equation, which is valid when the change of the laser pulse during one round trip through the laser is small. This equation is then linearized around an equilibrium solution, and the eigenmodes and eigenvalues of the linearized equation are determined numerically. This approach retains the advantage of previous analytical research of permitting one to explore a wide range of parameter space while also permitting one to avoid approximations in the stability calculation. The stability of a figure-eight loop laser with the amplifier in the external loop is then studied by use of this approach.