David S. Levy

A new design for ultracompact multimode interference-based 2 x 2 couplers

We introduce here a new multimode interference-based 3-dB coupler that has a substantially smaller device geometry than conventional multimode interference structures. This new structure is based on a special multimode region shape and an appropriate launching technique which aids in circumventing the proximity limitations. Simulations show this structure can provide a decrease of a factor of three or more in the device size of the smallest 3-dB MMI coupler made to date.

Fabrication of ultracompact 3-dB 2 x 2 MMI power splitters

Recently proposed tapered MMI devices have been shown to allow for substantial reductions in device geometries, but as of this time such ultracompact devices have not been realized. The fabrication and testing of a series of parabolically tapered 3-dB 2/spl times/2 MMI devices have been accomplished in the InP-InGaAsP double heterostructure system. The results validate previous predictions and yield device geometries about 40% smaller than the shortest previous 3-dB 2/spl times/2 MMI device. The measured splitting ratios of these devices are compared to the results of numerous beam propagation method simulations, based on the finite-difference method, and good correlation is obtained.

Length reduction of tapered N x N MMI devices

The length of N/spl times/N multimode interference-based (MMI) devices scales as the square of the MMI region width, and as a result, the use of these structures for large-N applications can require large chip areas. We discuss the N/spl times/N applications of a recently proposed MMI structure that has smaller device dimensions than conventional multimode interference structures. Numerical simulations of such structures and design rules are presented. Limitations on device performance are discussed.

42.7-Gb/s cost-effective duobinary optical transmitter using a commercial 10-Gb/s Mach-Zehnder modulator with optical filtering

Optical duobinary transmission at 42.7 Gb/s is demonstrated using a commercially available 10-Gb/s LiNbO/sub 3/ modulator. The intrinsic response of the 10-Gb/s electrooptic Mach-Zehnder modulator (MZM) is used to create a low-pass-filtered (LPF) duobinary signal, which shows a minimum optical signal-to-noise ratio (OSNR) requirement penalty of /spl sim/4 to /spl sim/6 dB as compared to nonreturn-to-zero (NRZ) signals. However, when the MZM-LPF duobinary is filtered with a 50-GHz 3-dB bandwidth first-order Gaussian filter, a significant improvement in required OSNR is seen. In contrast, similar optical filtering of NRZ data creates a significant OSNR penalty such that the NRZ and MZM-LPF duobinary data have nearly identical OSNR requirements. Therefore, the MZM-LPF duobinary technique may be a cost-effective approach to high-spectral-efficiency transmission at 42.7 Gb/s.

A multimode interference-based variable power splitter in GaAs-AlGaAs

Bent multimode interference-based variable power splitters are experimentally demonstrated in GaAs-AlGaAs using the principles of general imaging. Simulations are used to investigate sensitivities to device geometry. Results indicate that the devices splitting ratio performance is largely fabrication tolerant.

Enhanced FEC OSNR gains in dispersion-uncompensated 10.7-Gb/s duobinary transmission over 200-km SSMF

We report the experimental comparison of 10.7-Gb/s duobinary transmissions using generic Reed-Solomon forward error correction (FEC) and an enhanced FEC. The coding gains of the two FECs after transmission over a dispersion-uncompensated 200-km standard single-mode fiber (SSMF) link are found to be much larger than those quoted in ideal case, with the enhanced FEC further outperforming the generic FEC by /spl sim/3.5 dB. Numerical simulations show reasonable agreement with the experimental results.

Improved polarization-mode-dispersion tolerance in duobinary transmission

Polarization-mode-dispersion tolerance of a low-pass filter duobinary modulation format is investigated. With the optimum residual dispersion, the optical signal-to-noise ratio penalty at a differential group delay of 40 ps is measured to be 1 dB (at BER=6/spl times/10/sup -5/): 1.5 dB less than that without residual dispersion in a 10.7-Gb/s duobinary transmission. Numerical simulations show reasonable agreement with the experimental results.

Rapid design and fabrication of new photonic integrated circuits for lightwave systems

In this paper we will review our efforts in developing both design and fabrication capabilities for photonic integrated circuits. Design is based on software for CAD and beam propagation simulation of planar waveguide circuits. Fabrication is based on a laser-based rapid prototyping system for patterning and processing waveguide materials.

Size reduction in multimode interference-based NxN couplers using a tapered waveguide geometry

The length of NxN multimode interference-based devices scales as the square of the MMI region width, and as a result, the use of these structures for large-N applications can require large chip areas. We discuss the NxN applications of a recently proposed MMI structure which has smaller device dimensions than conventional multimode interference structures. Numerical simulations of such structures and design rules are presented.