Todd G. Ulmer

Two-photon absorption-induced self-phase modulation in GaAs-AlGaAs waveguides for surface-emitted second-harmonic generation.

Performance-limiting asymmetric distortion is observed in the spectra of fundamental pulses transmitted through GaAs-Al(0.9)Ga(0.1)As multilayer waveguides designed for surface-emitted second-harmonic generation. This behavior is attributed to refractive-index changes resulting from the accumulation of free carriers created by two-photon absorption in the GaAs layers. Numerical simulations of the intensity-dependent spectra by use of the separately measured two-photon absorption coefficient are shown to be in good agreement with the observed spectra.

160-Gb/s optically time-division multiplexed link with all-optical demultiplexing

An ultrafast single-wavelength optically time-division multiplexed (OTDM) link is described. The link exploits a unique integrated, all-optical serial-to-parallel (S/P) converter based on second-harmonic generation that demultiplexes multiple high-speed optical channels with a single operation. The link is composed of five major components: (1) a high-repetition-rate picosecond-pulse source; (2) a planar waveguide multiplexer that incorporates electroabsorption modulators with integral spot-size converters (SSCs); (3) a dispersion-managed (DM) short-pulse fiber channel; (4) a quasi-phase-matched, resonant-cavity-enhanced AlGaAs waveguide designed for surface-emitted second-harmonic generation (SESHG); and (5) a 775-nm receiver optimized for return-to-zero (RZ) operation. We describe our recent advances with resonant cavity enhancement of the all-optical demultiplexer and the first bit error rate (BER) measurements for this demultiplexing scheme.

Microcavity-enhanced surface-emitted second-harmonic generation for ultrafast all-optical signal processing

By incorporating an integrated microcavity into an optical waveguide structure with vertical quasi-phase-matching, we have realized surface-emitted second-harmonic generation devices that significantly enhance the conversion efficiency for optical pulses in the picosecond and sub-picosecond regimes. We demonstrate both theoretically and experimentally that nonlinear interactions involving short optical pulses can be enhanced by a microcavity, even when the resonance width is substantially narrower than the spectral content of the pulse. The resulting enhancement enables practical signal processing functions such as ultrafast optical time-division demultiplexing at 1.55 /spl mu/m in multilayer AlGaAs structures.

Microcavity-enhanced surface-emitted second-harmonic generation from 200 fs pulses at 1.5 μm

We present theoretical results for short-pulse surface-emitted second-harmonic generation and show that significant cavity enhancement is possible in the femtosecond regime. Experimentally, we demonstrate enhanced surface-emitted second-harmonic generation using 200 fs fundamental pulses near 1.5 μm by use of a 5λSH/2n vertical microcavity in [211]-oriented AlGaAs waveguides. The microcavity height is minimized by allowing the distributed Bragg reflectors that define the vertical cavity to also serve as cladding layers for the fundamental waveguide, thereby increasing the resonance width and hence the overlap with the second-harmonic spectrum.

Ultrafast optical devices for high-speed optical data links

The development of photonic devices for the next generation of optical networks is dependent on advances in ultrafast materials and in the success of waveguide devices comprised of these materials. This includes new methods of producing integrated-optical devices by innovative growth techniques or novel hybridization schemes. We describe aspects of the ultra-fast optical communications program at Georgia Tech that involve the development of hybridized and integrated- optical devices and devices for use in ultrafast optical data links. Two major components are under development: (1) a tapered rib electro-absorption modulator that includes an integrated spot-size converter for hybridization with a passive silica-waveguide tapped delay line. This unique hybridized semiconductor/glass waveguide provides the basic building block of the transmitter multiplexer. (2) a quasi- phase matched multilayer AlGaAs waveguide designed for surface-emitted second-harmonic generation. This device provides an all-optical serial-to-parallel converter and thereby demultiplexes ultrafast optical data streams. We describe our recent advances in materials growth and waveguide design and the impact on the performance of these devices.

Resonant-cavity-enhanced surface-emitted second-harmonic generation in the sub-ps regime

Summary form only given.Surface-emitted second-harmonic generation (SESHG) offers unique advantages in applications such as ultrafast optical time-division de-multiplexing. In this geometry, efficiency enhancements of one to two orders of magnitude have been demonstrated by resonating the SH in a vertical cavity. In contrast to previous work, we explore, for the first time, the short-pulse regime where the spectral filtering effect of a single cavity mode limits the enhancement. We show experimentally that significant enhancement is possible for pulses shorter than 1 ps, thus allowing efficient, ultrafast, all-optical signal processing and frequency conversion.

Optical time-division demultiplexing with resonant-cavity-enhanced surface emitted second-harmonic generation

Summary form only given. We have incorporated a vertical microcavity into an all-optical serial-to-parallel converter based on surface-emitted second-harmonic generation (SESHG). By resonating at the second-harmonic wavelength, the conversion efficiency is improved sufficiently to allow bit error rate (BER) measurements on an SESHG demultiplexer for the first time. This optical time-division demultiplexer has the important advantage that a single device can recover all of the multiplexed channels in a single operation. The SESHG structure is comprised of a quasi-phase-matched (QPM) waveguide imbedded between two distributed Bragg reflectors (DBRs), which form a microcavity resonator.

Optical time-division demultiplexing with surface-emitted second-harmonic generation

Summary form only given. We have previously proposed an ultrafast all-optical demultiplexer based on surface-emitted second-harmonic generation (SESHG) for optically time-division multiplexed (OTDM) communications. This scheme is advantageous in that it provides a serial-to-parallel functionality, allowing all of the multiplexed channels to be recovered with a single device. Here we report a new structure which virtually eliminates linear and two-photon absorption and thereby allows the efficient use of a vertical cavity resonant at the second-harmonic. The combination of reduced loss and the resonant cavity enhancement is expected to yield efficiencies suitable for 160 Gb/s operation.