G. Ronald Hadley

Peak-power limits on fiber amplifiers imposed by self-focusing

We have numerically investigated the behavior of the fundamental mode of a step-index, multimode (MM) fiber as the optical power approaches the self-focusing limit (P(crit)). The analysis includes the effects of gain and bending (applicable to coiled fiber amplifiers). We find power-dependent, stationary solutions that propagate essentially without change at beam powers approaching P(crit) in straight and bent fibers. We show that in a MM fiber amplifier seeded with its fundamental eigenmode at powers <<P(crit), the transverse spatial profile adiabatically evolves through a continuum of stationary solutions as the beam is amplified toward P(crit).

Bragg fiber design for linear polarization

A new design is presented for Bragg fibers that allows low-loss propagation for linearly polarized light. Predictions based on a simple ray model show that approximately doubling the thickness of the first wall layer results in low losses at TM-like boundaries while keeping TE-like boundary losses manageable. This contrasts sharply with conventional quarter-wave designs that are extremely low loss for TE01 modes but very high loss for linear polarization. We fabricate Bragg fibers based on this design concept in a Si/SiO2 system and verify experimentally that they propagate linearly polarized light with losses less than 6 dB/cm over a 60-nm spectral range.

The complex Jacobi iterative method for three-dimensional wide-angle beam propagation

A new complex Jacobi iterative technique adapted for the solution of three-dimensional (3D) wide-angle (WA) beam propagation is presented. The beam propagation equation for analysis of optical propagation in waveguide structures is based on a novel modified Padé(1,1) approximant operator, which gives evanescent waves the desired damping. The resulting approach allows more accurate approximations to the true Helmholtz equation than the standard Padé approximant operators. Furthermore, a performance comparison of the traditional direct matrix inversion and this new iterative technique for WA-beam propagation method is reported. It is shown that complex Jacobi iteration is faster and better-suited for large problems or structures than direct matrix inversion.

Theoretical treatment of evaporation front drying

Abstract A general formalism is presented for the combined diffusion and forced flow of a binary gas mixture through a porous medium. The resulting equations are then specialized to handle the vapor region of a porous material which is drying according to a receding evaporation front model. Solutions presented for various permeabilities and temperatures give the conditions under which air is present in the pores and allow one to investigate the usefulness of the ‘boiling’ concept for the drying of low permeability materials. Evaporation front motion together with the water loss rate is examined for different curvilinear geometries both analytically and numerically.

High-peak-power (>1.2 MW) pulsed fiber amplifier.

We report results from Yb-doped fiber amplifiers seeded with two microchip lasers having 0.38-ns and 2.3-ns pulse durations. The shorter duration seed resulted in output pulses with a peak power of >1.2 MW and pulse energy of 0.67 mJ. Peak power was limited by nonlinear processes that caused breakup and broadening of the pulse envelope as the pump power increased. The 2.3-ns duration seed laser resulted in output pulses with a peak power of >300 kW and pulse energy of >1.1 mJ. Pulse energies were limited by the onset of stimulated Brillouin scattering and ultimately by internal optical damage (fluences in excess of 400 J/cm2 were generated). In both experiments, nearly diffraction-limited beam profiles were obtained, with M2 values of <1.2. Preliminary results of a pulse-amplification model are in excellent agreement with the experimental results of the amplifiers operating in the low-to-moderate gain-depletion regime.

Design of refractive-index and rare-earth-dopant distributions for large-mode-area fibers used in coiled high-power amplifiers

We have numerically compared the performance of various designs for the core refractive-index (RI) and rare-earth-dopant distributions of large-mode-area fibers for use in bend-loss-filtered, high-power amplifiers. We first established quantitative targets for the key parameters that determine fiber-amplifier performance, including effective LP01 modal area (Aeff, both straight and coiled), bend sensitivity (for handling and packaging), high-order mode discrimination, mode-field displacement upon coiling, and index contrast (manufacturability). We compared design families based on various power-law and hybrid profiles for the RI and evaluated confined rare-earth doping for hybrid profiles. Step-index fibers with straight-fiber Aeff values > 1000 &mgr;m2 exhibit large decreases in Aeff and transverse mode-field displacements upon coiling, in agreement with recent calculations of Hadley et al. [Proc. of SPIE, Vol. 6102, 61021S (2006)] and Fini [Opt. Exp. 14, 69 (2006)]. Triangular-profile fibers substantially mitigate these effects, but suffer from excessive bend sensitivity at Aeff values of interest. Square-law (parabolic) profile fibers are free of modal distortion but are hampered by high bend sensitivity (although to a lesser degree than triangular profiles) and exhibit the largest mode displacements. We find that hybrid (combined power-law) profiles provide some decoupling of these tradeoffs and allow all design goals to be achieved simultaneously. We present optimized fiber designs based on this analysis.

Cavity supermodes for gain‐saturated diode laser arrays

We present a numerical method for the solution of the cavity modes of a multichannel array of laser diodes. The analysis is based on coupled‐mode theory and includes the appropriate boundary conditions for the electric field at each reflecting facet. Saturable gain is also included, with saturation arising from both the forward‐going and reflected waves. We present results which differ from a recent analytic analysis for the case of two channels with different gains and propagation constants. The present work predicts ‘‘cavity supermodes’’ which are similar to the ‘‘symmetric’’ and ‘‘antisymmetric’’ waveguide supermodes previously described in the literature. These cavity modes are characterized by equal intensities in both channels even when the channel gains are grossly different.

Characterization of Si3N4/SiO2 planar lightwave circuits and ring resonators

The large refractive index contrast between silicon nitride and silicon dioxide allows silicon nitride/dioxide planar waveguides to have a small mode size and low radiation bending loss compared with doped silicon dioxide waveguides. Small waveguide bend with low radiation loss can help make small integrated planar lightwave circuits (PLCs), and also high-Q waveguide ring resonators. This presentation will talk about the design, fabrication and characterization of low loss silicon nitride/dioxide planar waveguide devices including waveguide bend, waveguide cross, and leaky mode waveguide polarizer. The key contribution of this work is the use of the lateral mode interference (LMI) 3dB splitter to accurately measure the loss of the planar lightwave circuit devices. We will also talk about the waveguide ring resonators with silicon nitride/dioxide materials. The application for photonic biochemical sensors will also be discussed.

Numerical Modeling of the Drying of Porous Materials

The drying of porous materials represents a challenging class of problems involving several nonlinear transport mechanisms operative in both the liquid and vapor phases. The difficulties encountered in attempting to solve such problems have led most researchers to simplify their models in one of two ways: either 1) moisture migration is ssumed to proceed only in the liquid phase towards a drying surface where it then evaporates, or 2) evaporation occurs at a stationary liquid-vapor interface and is transported to the drying surface as a vapor (evaporation front model). Although there are some situations in which one of these models may be adequate, most drying problems involve the transport of both phases simultaneously. The disposal of nuclear waste canisters in partially saturated geological formations offers a good example of this class of problems, a class for which presently existing solution methods are inadequate.

High-Q integrated on-chip micro-ring resonator

We report a fully integrated high-Q factor micro-ring resonator using silicon nitride/dioxide on a silicon wafer. The micro-ring resonator is critically coupled to a low loss straight waveguide. An intrinsic quality factor of 2.4/spl times/10/sup 5/ has been measured.