Strong self-phase modulation in planar chalcogenide glass waveguides.

Single-mode planar waveguides were fabricated from chalcogenide glass compounds with large Kerr nonlinearities. Strong self-phase modulation of subpicosecond pulses along with low linear and nonlinear absorption losses demonstrates the potential for ultrafast, low-power, all-optical processing applications.

Large Kerr effect in bulk Se-based chalcogenide glasses.

High-speed optical communication requires ultrafast all-optical processing and switching capabilities. The Kerr nonlinearity, an ultrafast optical nonlinearity, is often used as the basic switching mechanism. A practical, small device that can be switched with ~1-pJ energies requires a large Kerr effect with minimal losses (both linear and nonlinear). We have investigated theoretically and experimentally a number of Se-based chalcogenide glasses. We have found a number of compounds with a Kerr nonlinearity hundreds of times larger than silica, making them excellent candidates for ultrafast all-optical devices.

Bragg-grating-enhanced polarization instabilities.

Experiments demonstrate a dramatic decrease in polarization-instability threshold as an optical pulse is tuned near the short-wavelength edge of the photonic bandgap formed by a fiber Bragg grating. These enhanced nonlinear interactions and birefringent effects are modeled with coupled-mode numerical simulations. Nonlinearities are shown to increase much more rapidly than the effective birefringence as the pulse wavelength approaches the bandgap edge.

Two-dimensional photonic crystal couplers for unidirectional light output.

We investigate the use of two-dimensional photonic crystal slabs to improve the directionality of output coupling from planar waveguides and distributed-feedback lasers. We present the theory underlying the operation of such structures and design criteria for emission in desired directions. As an example, we demonstrate a vertical coupler that is integrated with an organic distributed-feedback laser, use computer simulations to find its coupling constant and efficiency, and then discuss its feasibility.

Compression of optical pulses spectrally broadened by self-phase modulation with a fiber bragg grating in transmission.

We demonstrate experimentally the compression of optical pulses, spectrally broadened by self-phase modulation occurring in the rod of a mode-locked Q-switched YLF laser, with an unchirped, apodized fiber Bragg grating in transmission. The compression is due to the strong dispersion of the Bragg grating at frequencies close to the edge of the photonic bandgap, in the passband, where the transmission is high. With the systems investigated, an 80-ps pulse, which is spectrally broadened, owing to self-phase modulation, with a peak nonlinear phase shift of D? = 7, is compressed to approximately 15 ps, in good agreement with theory and numerical simulations. The results demonstrate that photonic bandgap structures are promising devices for efficient pulse compression.

All-optical switching in long-period fiber gratings.

Nonlinear pulse propagation in long-period fiber gratings is studied with a mode-locked Q -switched laser pulse approximately 80ps in duration at a wavelength of 1.05 microm . Optical switching, pulse reshaping, and optical limiting are found at intensities in the range of 1-20 GW/cm(2).

Nonlinear propagation in superstructure Bragg gratings.

The existence of solitary waves in superstructure Bragg gratings is experimentally demonstrated, confirming theoretical predictions. We observe nonlinear compression as a result of a combination of the negative dispersion of the grating and the nonlinear phase shift associated with the pulse intensity. We also demonstrate that, in a superstructure Bragg grating, the dispersion is continuously tunable from normal to anomalous, which allows us to manipulate the shape of the transmitted pulse.

Polymer microdisk and microring lasers.

Dye-doped polymer microlasers have been fabricated by photolithography and self-assembly. Microdisk lasers 5 to 30 microm in diameter were photolithographically patterned on thin planar polymer waveguides. We formed polymer microring lasers on thinned silica fibers by dipping the fibers in polymers and allowing the polymer droplets to cure.

Noninvasive detection of changes in membrane potential in cultured neurons by light scattering.

We report a procedure to detect electrical activity in cultured neurons by changes in their intrinsic optical properties. Using dark-field microscopy to detect scattered light, we observe an optical signal that is linearly proportional to the change in the membrane potential. Action potentials can be recorded without signal averaging. We use the dark-field method to show that there are substantial time delays between activity in the soma and in fine distal processes of identified Aplysia neurons. The biophysical basis for the change in optical properties of the neuron was deduced from measurements of the angular distribution of scattered laser light. An analysis of the data indicates that the radial component of the index of refraction of the membrane increases and the tangential components decrease concomitant with an increase in membrane potential. This is suggestive of a rapid reorientation of dipoles in the membrane during an action potential.

All-optical timing restoration using a hybrid time-domain chirp switch.

We demonstrate a hybrid time-domain chirp switch (TDCS) in which the nonlinear chirper is an AlGaAs waveguide and the soliton dispersive delay line is a polarization-maintaining fiber. The hybrid TDCS can restore timing in a switching or transmission system, and when combined with an optical amplifier, it can act as an ultrafast, all-optical regenerator for soliton pulses. The timing restoration concept is applicable to other non-linear materials with negligible walk-off, and the acceptable time window can be tailored by adjusting the width and intensity of a reference pulse.

Waves and turbulence in a tokamak fusion plasma.

The tokamak is a prototype fusion device in which a toroidal Magnetic field is used to confine a hot plasma. Coherent waves, excited near the plasma edge, can be used to transport energy into the plasma in order to heat it to the temperatures required for thermonuclear fusion. In addition, tokamak plasmas are known to exhibit high levels of turbulent density fluctuations, which can transport particles and energy out of the plasma. Recently, experiments have been conducted to elucidate the nature of both the coherent waves and the turbulence. The experiments provide insight into a broad range of interesting linear and nonlinear plasma phenomena and into many of the processes that determine such practical things as plasma heating and confinement.