New CMOS Detector Transmits at 520 Mb/s

Daniel C. McCarthy

A spatially modulated light detector fabricated using complementary metal oxide semiconductor (CMOS) technology and developed at Vrije Universiteit Brussel has achieved bit-rate performance of up to 520 Mb/s. The technique provides options for using low-cost CMOS technology that can keep pace with local area networks or improve performance in plastic optical fiber systems, digital video disc reading heads, fast optocouplers and gigabit Ethernet systems.
Professor Maarten Kuijk, whose team developed the detector, explained that semiconductor junctions absorb most of the light entering a CMOS detector. However, incident light absorbed by the substrate reaches the detector hundreds of nanoseconds later, creating a sort of echo effect. "This, combined with the slow speed of carriers in silicon, creates the problem called low-frequency gain. In short, the carriers already present from the previous pulses corrupt new information being received," he said.
This limits transmission speed. The researchers overcame this problem by spatially modulating the incident light through a metal-fingered grid using CMOS metal-2. By registering the difference of photogenerated signals arriving at the CMOS surface in the illuminated and shadowed areas, the device obtained a much faster signal, free of interference from previous pulses or neighboring detectors. Thus its responsivity curve was flat up to 520 Mb/s.
"We hope that our next test chip will be able to demonstrate gigabit performance," said Kuijk. This could lower costs.
The detector is ready for commercial implementation. For example, Kuijk said, it monolithically combines preamplifiers, amplifiers, autogain and decision circuits, multiplexers, printed circuit board line drivers and digital signal processing circuitry on one chip. He and his team are working on coupling their detector with receivers in CMOS, and they hope to see results within months.
"The spatially modified light detectors have a slightly reduced responsivity, but this is largely compensated for by the much lower capacitance of the detector," Kuijk said, adding that a smaller capacitance provides a larger voltage readout.