Tsai Wei Wu

All-Glass Large-Core Leakage Channel Fibers

Leakage channel fibers (LCFs) have demonstrated their ability to significantly extend the effective mode area of a fundamental mode while maintaining robust single-mode operation. These fibers are designed to have strong built-in mode filtering that effectively suppresses the propagation of all higher order modes while keeping fundamental-mode loss to a minimum, and, therefore, effectively extending the regime of single-mode operation. Recently, all-glass LCFs have been demonstrated as a significant improvement over designs with air holes. These all-glass LCFs can not only be manufactured with much improved consistence and uniformity, but can also be handled and used as conventional fibers. More importantly, mode distortions from collapse of air holes in photonic crystal fibers during splicing and other endface treatments are largely eliminated. We will review some of the recent progress in this area.

Array size scalability of passively coherently phased fiber laser arrays.

We explore, by means of experiments and simulation, the power combining efficiency and power fluctuation of coherently phased 2, 4, 6, 8, 10, 12, 14, 16-channel fiber-laser arrays using fused 50:50 single-mode couplers. The measured evolution of power combining efficiency with array size agrees with simulations based on a new propagation model. For our particular system the power fluctuations due to small wavelength-scale length variations are seen to scale with array size as N(3). Beat spectra support the notion that a lack of coherently-combined supermodes in arrays of increasing size leads to a decrease in combined-power efficiency.

Bend performance of leakage channel fibers.

Bend loss of the first three modes of leakage channel fibers with various designs are studied using finite element method. It is found that very low bend loss at small bend radius can be achieved with large d/Delta. It is also found that best differential mode loss is achieved at large bend radius. It is further found that 2nd order mode loss becomes 9.1 times of fundamental mode loss at small bend radius for the bend orientation where the bending plane crosses centers of two holes and independent of d/Delta. Bend loss dependence of bend orientations are also studied. Excellent agreement between experiment and simulation is obtained.

Model for passive coherent beam combining in fiber laser arrays

We present a new model for studying the beam combining mechanism, spectral and temporal dynamics, the role of nonlinearity, and the power scaling issue of discretely coupled fiber laser arrays. The model accounts for the multiple longitudinal modes of individual fiber lasers and shows directly the formation of the composite-cavity modes. Detailed output power spectra and their evolution with increasing array size and pump power are also explored for the first time. In addition, it is, to our knowledge, the only model that closely resembles the real experimental conditions in which no deliberate control of the fiber lengths (mismatch) is required while highly efficient coherent beam combining is still attained.

The effect of the thermosphere on quiet time plasmasphere morphology

The Naval Research Laboratory SAMI3 (Sami3 is Also a Model of the Ionosphere) code is used to model observed plasmasphere dynamics for 1–5 February 2001, a period of quiet time refilling. The SAMI3 model is driven at high latitudes by the magnetospheric potential calculated by the Weimer05 empirical model, using the observed solar wind. At middle-to-low latitudes, the self-consistent dynamo potential is included, driven by specified winds. During this quiet period we find that the shape of the plasmasphere, at any given time, varies significantly with the wind model even as a similar degree of model-data agreement is recovered for each of the three wind models used. Diurnal oscillations in the model electron density, which are strong when plotted at fixed magnetic local time, are consistent with the degree of variation seen in the measured densities. In all three cases, SAMI3 compares favorably to the electron density measured in situ by the Imager for Magnetopause-to-Aurora Global Exploration spacecraft. Results with no winds or with specific wind effects excluded show that wind-driven E × B drifts shape the plasmasphere, relative to a round plasmasphere with no winds, and reduce the refilling rate, relative to the higher refilling rate found without winds.

Electrostatic reconnection in the ionosphere

Postsunset equatorial plasma bubble merging is examined using the National Research Laboratory code SAMI3/equatorial spread F. It is found that bubbles merge through an “electrostatic reconnection” process. As multiple bubbles develop, the electrostatic potential associated with one bubble can connect with that of a neighboring bubble: this provides a pathway for the low-density plasma in one bubble to flow into the adjoining bubble and merge with it. Additionally, high-speed plasma channels (approximately greater than hundreds of meters per second) can develop during the merging process. Optical data is presented of equatorial plasma bubble evolution that suggests bubble merging occurs in the nighttime equatorial ionosphere.

Seeding equatorial spread F with turbulent gravity waves: Phasing effects

The Naval Research Laboratory (NRL) SAMI3/equatorial spread F (ESF) three-dimensional ionosphere model is used to study the initiation and development of the large-scale plasma bubbles in the postsunset equatorial F region by turbulent gravity waves. The gravity wave turbulence is obtained from a three-dimensional anelastic, finite-volume model. We show that the phasing of gravity waves at conjugate regions in the ionosphere can enhance (in phase) or reduce (out of phase) the effective seed of the instability. The nonlocalized nature of the effective seed may contribute to the observed day-to-day variability of ESF. Additionally, we find that the zonal and vertical wind perturbations associated with the gravity waves are most effective in seeding ESF bubbles; perturbations of the meridional wind are relatively ineffective.

Dynamical, bidirectional model for coherent beam combining in passive fiber laser arrays

We generalize the recently proposed model for coherent beam combining in passive fiber laser arrays [Opt. Express 17, 19509 (2009)] to include the transient gain dynamics and the complication of counterpropagating waves, two important features characterizing actual experimental conditions. The extended model reveals that beam combining is not affected by the population relaxation process or the presence of backward propagating waves, which only serve to co-saturate the gain. The presence of nonresonant nonlinearity is found to reduce the coherent combining efficiency at high power levels. We show that the array lases at the frequencies with minimum overall losses when multiple loss mechanisms are present.