OPTICAL INTERCONNECTS part 1

I argue that optical interconnects are likely to become necessary for certain applications, such as internet-router backplanes and possibly personal computers, at data rates that are expected to be reached by Si complementary metal- oxide-semiconductor (CMOS) in the next 5-10 years (2010 – 2015).  This intermediate conclusion will motivate the rest of this dissertation – a detailed investigation into semiconductor optoelectronic modulators as transmitter devices for optical interconnects

ELECTRICAL INTERCONNECTS

Current technology in CMOS electronics operates chip speeds up in the multi- GHz clock rates and transistor gates down to 90 nm.  Moore’s Law does not seem to be slowing down in the near future, according to the ITRS Roadmap [1].   Yet, electrical interconnects do not seem poised to keep up.   Metal wires are currently being used to connect chips to boards (links ~10-cm long), boards to boards (~50 cm), router linecards to each other (~3 m) and so on.   Unfortunately, at frequencies above about 5 GHz, a copper wire along an internet router backplane fails to be a simple electrical signal channel [2].   The many imperfections of the metallic interconnect, such as frequency- dependent loss, impedance mismatching, and skin depth, complicate the transmitter circuitry.  To some extent, these problems can be overcome, at a cost of electrical power consumption.  Since the heat extraction from CMOS chips has recently become a serious problem [1, 3], reducing electrical power consumption has developed into a high-priority design issue. In the following sections, we take a closer look at each of the major issues with electrical interconnects 

FREQUENCY-DEPENDENT LOSS

Mohammed et al. at Intel [4] simulated a 20-inch electrical interconnect on a standard printed circuit (PC) board using parameters for the material FR4.  Their results (shown in their Fig. 2) indicate that there is an insertion loss of about -25 dB at 5 GHz and about -45 dB at 10 GHz.   At higher signal frequencies, the loss in the electrical interconnect gets even worse.  Similar figures are shown in the Ph.D. dissertation of Dr. Azita Emami-Neyestanak in Fig. 2.16 [2].   In absolute terms, this amount of loss is severe, and the variation in loss with respect to frequency causes signal distortion.         

A 10-Gigabit-per-second (Gbps) signal (non-return-to-zero, intensity modulated) that must pass through this channel contains frequency components up to 5 GHz.  These various frequency components suffer a different amount of loss, resulting in a distorted signal at the receiver.  To some degree, a process called equalization can compensate for this aspect of the channel by amplifying frequency components that are more strongly attenuated during the transmission.   Equalization, however, requires foreknowledge of the data transfer rate and of the channel characteristics (or at least some active method of determining the characteristics) and consumes valuable chip area and electrical power [4].    As  the  bit  rates  increase,  the  loss  of  the  electrical  channel  gets  worse,  and equalization schemes will only become less practical. A second consequence of the frequency-dependent loss of the electrical channel is the so-called “aspect-ratio limit” of electrical lines [5, 6].  The capacity of an electrical interconnect system is essentially limited by the aspect ratio of the wires (the cross- sectional area divided by the length squared) used to extract the information.  Since the distance that must be traveled by the signal and the size of the chips are usually fixed in a system, this aspect-ratio maximum can be calculated and thus, the maximum aggregate data rate is known.   In other words, it does not matter what specific architecture is implemented (e.g. many small wires or a few large wires) – filling a particular volume with information-carrying wires will result in the ability to transmit only a certain amount of data per second.  Advanced techniques such as multilevel coding and repeatering can be used to extend this limit somewhat, but again these techniques consume additional power