For many years, engineers successfully integrated optics and electronics by using a standard silicon CMOS process for small-bandwidth structures like photodetectors. Advances in performance and integration densities continue to energize this niche, such as with the venerable silicon photodiode in terms of functionality. Photodiode advances may fall under the radar a bit, but more visible progress can be seen with ICs like detectors, sensors, LEDs, lasers, and other devices that operate at relatively low bandwidths. They serve barcode scanners, printers, disk players, remotecontrol devices, and other consumer applications. Other markets include medical, industrial, and construction applications. Advanced Photonics, Cal Sensors, Opto Sensors, Silicon Sensors, and UDT Sensors all use silicon and other materials to produce photodetectors and sensors. Some large firms like Agere, GE, Infineon, Perkin-Elmer, and Siemens also make commercially available solid-state photodetectors and sensors.
Products from Texas Advanced Optoelectronics Solutions (TAOS) exemplify this trend. The company’s high-performance optodetectors convert light to voltage, color, digital, and frequency signals (see “Beyond Simple Photodiodes And Phototransistors” at www.electronicdesign.com, Drill Deeper 18789).
The Position Sensitive Photomultiplier Array (PSMArray) family of tiny solid-state light sensors from SensL represent the first commercially available CMOS large-array, detector-based silicon photomultiplier products. According to the company, they surpass the performance of traditional photomultiplier tubes (PMTs) and avalanche photodiodes (APDs).
The PSMArray is an arrayed version of SensL’s novel silicon photomultiplier (SPM) pixels tiled together using flipchip on glass techniques. It operates from 400 to 850 nm.
A MATERIALS CHALLENGE
For higher-bandwidth devices that serve communications and computing functions, though, the greater challenge involves various compound semiconductor materials. Nonetheless, quite a few devices are spread throughout the market, spearheading record-breaking performance levels. Also, many other products are in development or on the cusp of introduction. Companies cite progress in producing higher-performance detectors, waveguides, modulators, lasers, switches, filters, couplers, multiplexers, amplifiers, and other optoelectronic functions. Device development mostly focuses on the use of silicon (Si), germanium (Ge), and indium phosphide (InP). The holy grail for high-bandwidth optoelectronics is to integrate these materials on a standard CMOS silicon process, monolithically or in hybrid form.
Such integration would reduce manufacturing, packaging, and testing costs. It also would increase reliability and improve performance to levels practical with present and future communications and computational needs. Whether it’s possible is unclear, since large-volume applications for such devices don’t exist as of yet.
ON THE MARKET
A recent report by the Optoelectronics Industry Development Association (OIDA) predicts robust growth for optoelectronic components. The group expects steady expansion of the market, both for optoelectronic components and the technologies they will enable, with the overall industry doubling to $1.2 trillion from 2007 to 2017 (Fig. 1).
One of the first complex silicon optoelectronics products to reach commercialization, a highly accurate, multichannel model 2200 dynamic-gain equalizer module from Silicon Light Machines, arrived in 2002. Based on MEMS and CMOS processing, it consists of the patented Grating Light Valve (GLV) circuit, a light engine, and an optical circulator.
An array of parallel aluminum-covered silicon-nitride (SiN2) microribbons is suspended above an air gap. The dynamic ribbons are then deflected up and down in response to an applied electrostatic voltage, acting as variable optical attenuators in response to a light signal. The module is being used in optical wave-division multiplexing (WDM) communications, high-resolution displays, and high-end high-definition video applications.
Manufacturing more complex and integrated optoelectronic devices on a standard process like CMOS, however, is much more challenging than making photodetectors. Silicon’s bandgap is too large, compared to other materials, to effectively operate at the infrared (IR) wavelengths used for optical communications and needed for optical modulator circuits.
The CMOS integration challenge hasn’t stopped researchers from forging ahead, though. Impressive developments have been achieved in the lab for all-silicon devices with wide bandwidths. Last year, Intel researchers announced the world’s fastest optical silicon modulator, writing 30 Gbits of data into a light beam per second.
Three years ago, Intel demonstrated a 10-Gbit/s all-silicon modulator. At one end of the modulator, light enters silicon diodes that split the light beam into two beams that pass through the diodes. Applying a voltage to the diodes shifts the light beams’ phase or position. This shifting encodes data into a binary form of ones and zeros.
The 30-Gbit/s data rate is close to the 40-Gbit/s rate used in modern communications systems. “By slightly altering the chemical makeup of the diodes, we can expect 40-Gbit/s data rates to be ready for commercialization by 2010,” says Intel research fellow Mario Paniccia, director of Intel’s Silicon Photonics Research Lab. “If you take 25 of these silicon lasers and direct them into an array of 25 modulators, you can have a terabit of information all on a piece of silicon the size of a fingernail,” adds Paniccia.
Researchers at IBM developed a photonic silicon modulator that’s 100 to 1000 times faster than previous comparable modulators, and they switch at 10 Gbits/s. The ultra-low-power RF power modulator uses Mach-Zendor modulation (MZM) interferometry and consists of 200-µm long p+-i-n+ active diode regions on an effective chip area of about 0.12 µm2.
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