OPTICAL INTERCONNECTION PART 2

WAVELENGTH-DIVISION MULTIPLEXING

Modern long-haul telecommunications systems employ a scheme that allows multiple signals on a single fiber.   The technique, known as wavelength-division multiplexing (WDM), is based on modulating each signal on a slightly different carrier frequency.  In other words, the carrier frequencies or wavelengths are all different by an amount somewhat greater than the modulation frequency.  The low-loss optical fiber can support a huge range of wavelengths – the 30-nm-wide telecommunications C-band is equivalent to about 4 THz of bandwidth.   These different wavelengths can all be transmitted down a fiber simultaneously and separated into different signals again at the receiver.  (This technology was greatly enabled in the long-haul market by the invention of a broadband (erbium-doped fiber) optical amplifier (EDFA) which can amplify all those wavelengths at once without cross-talk.)

This strategy is unavailable to electrical interconnects, which typically use baseband modulation (i.e. no carrier wave) [6, 7], though this strategy is not unlike the use of carrier waves in the radio frequency domain transmitting through the “channel” of the atmosphere.   Radio transmitters just emit their signal energy into the air and the

receiver must have a resonant circuit centered on the carrier wave to choose the desired signal to demodulate.   In electrical interconnects, however, we must consider the high frequency of modulation (~ 10 GHz) and the small size of the components (chips sizes

~ 3 mm).    Thus,  while  technically  possible,  free-space  electrical  interconnects  using multiple carrier waves would be complex and probably subject to cross-talk, significant delay, power dissipation, and circuit area [6].

CHALLENGES FOR PRACTICAL OPTICAL NETWORKS

Due to the properties of the available materials and the inherent benefits of modulating a high frequency carrier, optics has significant advantages over electronics for information transmission.   Perhaps an obvious question remains: Why are optical systems not currently ubiquitous?  As discussed above, the advantages of optics are more valuable in systems that operate at high interconnect densities at high bit rates over long distances.  Thus, optical systems are ubiquitous for long-haul telecommunications.  As the demand for internet services and bandwidth has increased in recent years, the networking companies have been installing optical networks at higher bit rates for shorter and shorter distances.

However, a few hurdles remain for optical interconnects for applications such as internet router backplanes and personal computers, even once the bit rates and length scales  reach  the  level  where  optics  could  help.    First,  there  is  the  well-entrenched electrical interconnect technology that currently serves the purposes.  In order to replace this technology, system designers must be convinced that the new technology (optics) is worth the switch.  Any such platform change is seen as an undesirable risk.  Companies have so much money already invested in the current method, it will take additional incentive for them to overcome that risk aversity.  An optical technology that could be slowly  phased  in  and  tested  extensively  for  reliability  under  a  variety  of  adverse conditions would be more attractive.  Second, the cost of optical technology is not yet comparable  to  electronics.    Reliability  and  manufacturing  yields  are  often  not  high enough yet.   Some have argued that once the technology gets a foothold in the marketplace, an increase in volume will reduce the marginal cost of each product, as is common  in  the  consumer  electronics  market  in  general.    While  this  may  be  true,

companies  still  see  a  barrier  to  committing  themselves  before  the  cost  reduction  is proven.  Looking more carefully at the production expenses for optical components, the majority of the cost (60 – 80 percent) is derived from the high cost of packaging optical components, due to the fine alignments required for high performance [10].  For example, the alignment of an optical fiber to a laser diode must be accurate to 0.1 µm [10].  These fine tolerances typically require each device to be aligned one at a time by hand or by expensive automation equipment.   Any increase in packaging parallelism and any relaxation in the alignment tolerances would certainly go a long way towards a practical implementation of optical interconnects.

Several technical challenges are laid out in detail in [7].   There are four major concerns: practical optical systems, integration techniques, receivers, and transmitters. Practical  optical  systems  will  be  discussed  in  the  next  section.    The  optoelectronic devices can be either monolithically or hybridly integrated with the CMOS.  Currently, monolithic integration of III-V devices with CMOS is quite difficult, so a more realistic approach may utilize flip-chip bonding.  Hybrid integration allows the CMOS and III-V devices to be fabricated separately and combined only as a final step.

Receiver circuits and their optoelectronic detectors must continue to develop towards lower capacitance, lower power dissipation, and lower noise designs.   Most optical-interconnect receivers are not likely to be as photon-starved as in telecommunications.  However, the large number and density of individual receivers in optical interconnects (e.g. a thousand channels per chip) requires a design that minimizes the power dissipation in each circuit.

The challenge of designing optoelectronic devices for the transmission side of the link is the focus of this dissertation and thus will be addressed in the following chapters.