Researchers at the University of New Mexico and Sandia National Laboratories (both in Albuquerque, NM) have demonstrated a high-speed, wavelength-division-multiplexed (WDM) optical interconnect with a data modulation rate in excess of 1 Gbit/s and a crosstalk rejection ratio in excess of -10 dB at 4-nm channel spacing. Subsequent improvements in circuitry should make data rates of 2.5 Gbit/s easily achievable, according to Julian Cheng, an electrical engineering professor at the University of New Mexico.
The device operates by multiplexing the output of a multiple-wavelength array of vertical-cavity surface-emitting lasers (VCSELs) into a single data stream that transits a single optical fiber and is then demultiplexed by an array of wavelength-selective, resonance-enhanced photodetectors (REPDs). The VCSELs and REPDs were fabricated using a quasi-planar technology for integrating oxide-confined devices on the same substrate that yielded VCSEL bandwidths on the order of 12 GHz and REPD risetimes on the order of 75 ps. The devices were constructed using indium gallium arsenide/gallium arsenide (InGaAs/GaAs) quantum-well (QW) structures. An etched trench approach was used to monolithically integrate the oxide-confined active area apertures into quasi-planar arrays.
The researchers tested the configuration using two VCSELs from the same array with a 3.9-nm wavelength separation between center wavelengths of 917.9 and 921.8 nm. The signals were detected by two wavelength-matched REPDs with a 28-µm optical window. Each VCSEL was individually modulated by a non-return-to-zero pseudorandom data stream, and outputs were multiplexed and transmitted across a single optical fiber to the two REPDs, each mounted in a high-speed package containing a transimpedance amplifier with a 1.25-GHz bandwidth, followed by a limiting amplifier with a 1-GHz bandwidth. The resulting bit error rate was less than 10-11. Optical crosstalk between channels measured using 500-Mbit/s return-to-zero data patterns was -10.4 dB at 917.9-nm REPD and -8.2 dB at 921.8 nm.
More recently researchers have upgraded the circuit with a 5-GHz transimpedance amplifier, a linear grating to provide even wavelength spacing over a bandwidth in excess of 50 nm, and 7-GHz REPDs (see figure). For more details, contact Julian Cheng at email@example.com.
Bit-error-rate performance of a two-channel, 1-Gbit/s/channel WDM link with 4-nm wavelength spacing shows the effect of optical crosstalk. Inset top right shows spectra of the wavelength-matched VCSELs and REPDs; Inset lower left is 1.25-Gbit/s eye diagram at BER = 10-11.