Seldon D. Benjamin

Ultrashort pulse reflection from fiber gratings: a numerical investigation

We consider the linear reflection of ultrashort broadband pulses by uniform and nonuniform narrowband fiber gratings. We examine the effects of grating characteristics, including peak reflectivity, bandwidth, phase response (dispersion and chirp), and apodization on the reflection of such pulses from various gratings. A symmetric transform-limited 1-ps Gaussian pulse is assumed as the ultrashort broadband input to the gratings; the reflected pulses take on significantly different shapes and vary in duration. The prominent features observed are qualitatively explained in order to gain physical insight into the nature of the ultrashort pulse response and corresponding interaction. The results of this study indicate that there is the potential for a new class of applications, including temporal pulse shaping and novel devices for optical communications systems, by combining ultrashort broadband pulses with narrowband fiber gratings.

Ultrafast dynamics of nonlinear absorption in low‐temperature‐grown GaAs

We present the results of a study of the subpicosecond dynamic behavior of optically induced absorption changes in low‐temperature‐grown GaAs. We show that the observed behavior is dominated by mid‐gap trap states, and can be accurately modeled by the rate equations previously developed to describe quasi‐cw results. Our data give the first approximate values for trap emptying times in this material.

Optical characterization of low-temperature-grown GaAs for ultrafast all-optical switching devices

Low-temperature-grown GaAs (LT-GaAs) is a promising material for all-optical switching devices due to its outstanding optical characteristics. In this paper, we outline a simplified model we have developed to describe the dynamics of the carriers in this material. We also report the results of a series of measurements that we have performed to characterize the optical properties of the material. Specifically, we present the first measurements of the two-photon absorption coefficient and the refractive index changes as a function of the growth and annealing temperatures in LT-GaAs. Finally, we show how our model can be used to optimize the material for applications in all-optical switching.

Ultrashort pulse propagation in multiple-grating fiber structures.

We propose a multiple-grating fiber structure that decomposes an ultrashort broadband optical pulse simultaneously in both wavelength and time. As an initial demonstration, we used a transform-limited 1-ps Gaussian pulse centered at 1.55 mu;m as the ultrashort broadband input into a three-grating fiber structure and generated three output pulses separated in wavelength and time with good correlation between experimental results and simulations. This device structure can be used to generate a multiwavelength train of pulses for use in wavelength-division-multiplexed systems or to implement frequency-domain encoding of coherent pulses for optical code-division multiple access.

Applications of ultrashort pulse propagation in Bragg gratings for wavelength-division multiplexing and code-division multiple access

We discuss theoretical and experimental studies on the propagation of ultrashort pulses through fiber Bragg gratings. We also consider several applications in optical communications to be found by combining ultrashort pulses and fiber Bragg gratings: a multiwavelength source for wavelength-division-multiplexed systems and a means for implementing optical code-division multiple access.

Picosecond carrier lifetime and large optical nonlinearities in InGaAsP grown by He-plasma-assisted molecular beam epitaxy

We report the measurement of a fast carrier lifetime and large band-gap-resonant optical nonlinearities in an InGaAsP sample grown by He-plasma-assisted molecular beam epitaxy. Using a 2-microm-thick sample grown on an InP substrate, we observed a carrier lifetime of 15 ps and an index change as large as 0.077 induced by an intense 1-ps pulse at a wavelength of 1.57 microm. Good crystalline structure is maintained in the material during growth, and the absorption spectrum shows a sharp band edge. These properties indicate that materials produced by He-plasma-assisted growth have potential applications in compact ultrafast photonic devices.

Tailoring of trap-related carrier dynamics in low-temperature-grown GaAs

In this letter, we present the results of an experimental study of low-temperature-grown GaAs, which clearly resolves separately both carrier trapping and recombination processes. We extend our previous model to account for the observed carrier dynamics, and show how the material growth and annealing conditions can be adjusted to optimize the material properties for all-optical device applications.

Influence of material growth and annealing conditions on recombination processes in low-temperature-grown GaAs

In this paper we report direct measurements of the recombination times in low-temperature-grown GaAs (LT-GaAs). We investigate the effect of growth and annealing conditions on the recombination dynamics involving mid-gap and localized states.

Materials for all-optical devices

We briefly review the present status of work on all-optical photonic devices and discuss the properties of nonlinear optical materials that control and limit device performance. We then describe some recent work that indicates that As-rich GaAs is a promising material for ultrafast all-optical devices, and conclude with some projections for future progress in this field.

Refractive index and absorption changes in low-temperature-grown GaAs

For all-optical switching device design, the excitation pulse width is one of the essential features to be considered to achieve optimum operation. In this paper we investigate the effect of growth and annealing conditions and the excitation pulse width on refractive index and absorption changes in low-temperature-grown GaAs (LT-GaAs). We use our previously developed rate equation model to validate our interpretations for the different measurements.