25 Jul 2007
Researchers at Intel's Photonics Technology Lab have developed a laser modulator that encodes optical data at 40 Gbit/s.
Intel's long history of innovation in silicon photonics has produced the very first silicon-based laser modulator operating at 40 Gbit/s. Principal engineer Ansheng Liu announced this latest development via the Research@Intel blog:
"As you may know, a photonic integrated circuit (PIC) could provide a cost-effective solution for optical communication and future optical interconnects in computing industry. PICs on silicon platforms have attracted particular interest because of silicon's low cost and high volume manufacturability. "One of the key components needed for silicon PICs is the high-speed silicon optical modulator, which is used to encode data on optical beam. Today's commercially available optical modulators at 10 Gbit/s are based on more exotic electro-optic materials such as lithium niobate and III-V compound semiconductors. These devices have deployed at speeds up to 40 Gbit/s.
"Our goal to achieve similar performance in silicon has been very challenging, because crystalline silicon does not exhibit the linear electro-optic (Pockels) effect used to modulate light in these materials. Engineers are forced to rely on the free-carrier plasma dispersion effect, in which silicon's refractive index is changed when the density of free carriers (electrons/holes) is varied, to modulate light in silicon.
"In 2004, we published in Nature the first silicon modular to reach gigahertz speeds, 50x times faster than previous attempts in silicon. Since then, we scaled the device to 10 Gbit/s, bringing silicon modulation speed to a level comparable to most commercial devices. In January 2007, we designed and fabricated a new type of silicon optical modulator scalable to >10 Gbit/s and demonstrated data transmission at 30 Gbit/s (see also optics.org's previous coverage). The modulator still relies on the free-carrier effect, but its high speed is the result of a unique device design with traveling-wave drive scheme.
"With a similar device configuration, the modulator performance has been further improved by better device packaging to reduce the parasitic effect, better traveling-wave electrode with lower RF attenuation, and better modulator termination circuitry. This has allowed us to reach the goal of data transmission at 40 Gbit/s speed, matching the fastest devices deployed today using other materials.
"The Intel modulator is based on a Mach-Zehnder interferometer with a reverse-biased pn junction in each of the arms. When a reverse voltage is applied to the junction, free carriers - electrons and holes resulting from the n- and p-dopants - are pulled out of the junction, changing its refractive index via the free-carrier effect. The intensity of the light transmitted through the Mach-Zehnder interferometer is modulated by modulating the phase difference between the interferometer's two arms. This modulation can be very fast, because free carriers can be swept out of the junction with a time of approximately 7 ps. The modulator speed is thus limited by the parasitic effects such as RC time constant limit.
"To minimize the RC constant limitation, Intel researchers adopted a traveling-wave drive scheme allowing electrical and optical signal co-propagation along the waveguide. The traveling-wave electrode which is based on a coplanar waveguide was designed to match the velocity for both optical and electrical signals, while keeping the RF attenuation small.
"To operate the traveling-wave modulator, the RF signal is fed into the transmission line using a commercially available driver from the optical input side and the transmission line is terminated with an external resistor. After packaging the modulator on a printed circuit board, the researchers demonstrated that the modulator has a 3 dB bandwidth of ~30 GHz and data transmission capability up to 40 Gbit/s.
"The high-speed silicon modulator could find use in various future applications. For example, a highly integrated silicon photonic circuit may provide a cost effective solution for the future optical interconnects within computers and other devices. With the demonstration of the 40 Gbit/s silicon modulator and the electrically pumped hybrid silicon laser, it will become possible to integrate multiple devices on a single chip that can transmit terabits of aggregate data per second in the near future - truly enabling tera-scale computing."