Jonathan Hu

Understanding leaky modes: slab waveguide revisited

Computational methods for determining the complex propagation constants of leaky waveguide modes have become so powerful and so readily available that it is possible to use these methods with little understanding of what they are calculating. We compare different computational methods for calculating the propagation constants of the leaky modes, focusing on the relatively simple context of a W-type slab waveguide. In a lossless medium with infinite transverse extent, a direct determination of the leaky mode by using mode matching is compared with complete mode decomposition. The mode matching method is analogous to the multipole method in two dimensions. We then compare these results with a simple finite-difference scheme in a transverse region with absorbing boundaries that is analogous to finite-difference or finite-element methods in two dimensions. While the physical meaning of the leaky modes in these different solution methods is different, they all predict a nearly identical evolution for an initial, nearly confined mode profile over a limited spatial region and a limited distance. Finally, we demonstrate that a waveguide that uses bandgap confinement with a central defect produces analogous results.

Maximizing the bandwidth of supercontinuum generation in As 2 Se 3 chalcogenide fibers

We describe in detail a procedure for maximizing the bandwidth of supercontinuum generation in As(2)Se(3) chalcogenide fibers and the physics behind this procedure. First, we determine the key parameters that govern the design. Second, we find the conditions for the fiber to be endlessly single-mode; the fiber should be endlessly single-mode to maintain high nonlinearity and low coupling loss. We find that supercontinuum generation in As(2)Se(3) fibers proceeds in two stages--an initial stage that is dominated by four-wave mixing and a later stage that is dominated by the Raman-induced soliton self-frequency shift. Third, we determine the conditions to maximize the Stokes wavelength that is generated by four-wave mixing in the initial stage. Finally, we put all these pieces together to maximize the bandwidth. We show that it is possible to generate an optical bandwidth of more than 4 microm with an input pump wavelength of 2.5 microm using an As(2)Se(3) fiber with an air-hole-diameter-to-pitch ratio of 0.4 and a pitch of 3 microm. Obtaining this bandwidth requires a careful choice of the fibers waveguide parameters and the pulses peak power and duration, which determine respectively the fibers dispersion and nonlinearity.

Computational study of 3–5 μm source created by using supercontinuum generation in As 2 S 3 chalcogenide fibers with a pump at 2 μm

We present simulation results for supercontinuum generation using As(2)S(3) chalcogenide photonic crystal fibers. We found that more than 25% of input power can be shifted into the region between 3 microm and 5 microm using a pump wavelength of 2 microm with a peak power of 1 kW and an FWHM of 500 fs. The broad dispersion profile and high nonlinearity in As(2)S(3) chalcogenide glass are essential for this application.

Pulse Compression using a Tapered Microstructure Optical Fiber

We calculate the pulse compression in a tapered microstructure optical fiber with four layers of holes. We show that the primary limitation on pulse compression is the loss due to mode leakage. As a fibers diameter decreases due to the tapering, so does the air-hole diameter, and at a sufficiently small diameter the guided mode loss becomes unacceptably high. For the four-layer geometry we considered, a compression factor of 10 can be achieved by a pulse with an initial FWHM duration of 3 ps in a tapered fiber that is 28 m long. We find that there is little difference in the pulse compression between a linear taper profile and a Gaussian taper profile. More layers of air-holes allows the pitch to decrease considerably before losses become unacceptable, but only a moderate increase in the degree of pulse compression is obtained.

Flat-gain fiber Raman amplifiers using equally spaced pumps

This paper analyzes the gain flatness of multiwavelength pumped fiber Raman amplifiers using equally spaced pumps with both a fixed and an optimized central pump wavelength. The signal gain ripple using equally spaced pumps is compared with the case in which the pump wavelengths are allowed to vary for two, four, and eight pumps with 20-, 40-, and 80-nm signal bandwidths. The paper shows that using an optimized central pump wavelength with equal pump spacing simplifies system design, while the gain ripple is no more than 0.4 dB larger than the ripple obtained when the pump wavelengths are optimized for the cases considered.

Higher-order mode suppression in chalcogenide negative curvature fibers.

We find conditions for suppression of higher-order core modes in chalcogenide negative curvature fibers with an air core. An avoided crossing between the higher-order core modes and the fundamental modes in the tubes surrounding the core can be used to resonantly couple these modes, so that the higher-order core modes become lossy. In the parameter range of the avoided crossing, the higher-order core modes become hybrid modes that reside partly in the core and partly in the tubes. The loss ratio of the higher-order core modes to the fundamental core mode can be more than 50, while the leakage loss of the fundamental core mode is under 0.4 dB/m. We show that this loss ratio is almost unchanged when the core diameter changes and so will remain large in the presence of fluctuations that are due to the fiber drawing process.

Calculation of the expected bandwidth for a mid-infrared supercontinuum source based on As 2 S 3 chalcogenide photonic crystal fibers

We computationally investigate supercontinuum generation in an As(2)S(3) solid core photonic crystal fiber (PCF) with a hexagonal cladding of air holes. We study the effect of varying the system (fiber and input pulse) parameters on the output bandwidth. We find that there is significant variation of the measured bandwidth with small changes in the system parameters due to the complex structure of the supercontinuum spectral output. This variation implies that one cannot accurately calculate the experimentally-expected bandwidth from a single numerical simulation. We propose the use of a smoothed and ensemble-averaged bandwidth that is expected to be a better predictor of the bandwidth of the supercontinuum spectra that would be produced in experimental systems. We show that the fluctuations are considerably reduced, allowing us to calculate the bandwidth more accurately. Using this smoothed and ensemble averaged bandwidth, we maximize the output bandwidth with a pump wavelength of 2.8 μm and obtain a supercontinuum spectrum that extends from 2.5 μm to 6.2 μm with an uncertainty of ± 0.5 μm. The optimized bandwidth is consistent with prior work, but with a significantly increased accuracy..

Highly efficient cascaded amplification using Pr 3+ -doped mid-infrared chalcogenide fiber amplifiers

We computationally investigate cascaded amplification in a three-level mid-infrared (IR) Pr(3+)-doped chalcogenide fiber amplifier. The overlap of the cross-sections in the transitions (3)H(6)→(3)H(5) and (3)H(5)→(3)H(4) enable both transitions to simultaneously amplify a single wavelength in the range between 4.25 μm and 4.55 μm. High gain and low noise are achieved simultaneously if the signal is at 4.5 μm. We show that 45% of pump power that is injected at 2 μm can be shifted to 4.5 μm. The efficiency of using a mid-IR fiber amplifier is higher than what can be achieved by using mid-IR supercontinuum generation, which has been estimated at 25%. This mid-IR fiber amplifier can be used in conjunction with quantum cascade lasers to obtain a tunable, high-power mid-IR source.

Raman response function and supercontinuum generation in chalcogenide fiber

We show the Raman response function and dispersion curve for a chalcogenide fiber. We then model and reproduce the experimental bandwidth of IR supercontinuum generation using a chalcogenide PCF.

Leakage loss and bandgap analysis in air-core photonic bandgap fiber for nonsilica glasses

We calculate the minimum leakage loss and maximum relative bandgap as a function of the air-filling factor in a photonic bandgap fiber for a refractive index between 1.4 and 2.8. We analyze the mode properties for two maxima of the relative bandgap when we vary the air-filling factor and the refractive index. The maximum relative bandgap accurately predicts the optimal operating air-filling factor corresponding to lowest leakage loss.