OPTICAL INTERCONNECTION part 1

The benefits of optical interconnects can be attributed to improvements in the information channel and advantages of the higher frequency (or shorter wavelength) of optical electromagnetic waves [6, 7, 9].  For example, the loss of the optical channel is not significantly dependent on frequency.   The higher frequency of optical waves (a wavelength of 1.5 µm has an optical frequency of 200 THz) can be used as a carrier wave that can be modulated at signal frequencies (e.g. 10’s of GHz).

FREQUENCY-DEPENDENT LOSS

The information channel in the case of optical interconnects is either free space (air or glass) or optical fiber (glass).  At the relevant frequencies, both materials are quite transparent – namely, neither material absorbs or scatters much of the energy over a large range of frequencies.  Optical fiber typically offers a loss figure around 0.2 dB/km at the optimum wavelength of 1550 nm.   Thus, almost all of the energy sent into the signal reaches the receiver, especially for a short distance  link.  A 10-m fiber transmits 99.95% of the light that is coupled into the fiber to its output.

The fact that the loss is relatively constant across the wavelength spectrum (e.g. the telecommunications C-band from 1535 nm to 1565 nm) and so low (less than 1 dB over the C-band) implies that a short-distance optical system would have little or no need for equalization or repeaters, saving power and reducing the thermal load.

IMPEDANCE MISMATCHING

Impedance matching in optical systems is achieved simply by utilizing anti- reflection (AR) coatings [6].  Because the refractive index of the materials stays relatively constant over the optical frequency range of interest, an impedance matching layer is sufficient for all signals.     The simplest anti-reflection coating between media with refractive indices n1 and n2 is a single layer of material with a properly chosen thickness (t = λ/(2n)) and index of refraction (n = √(n1n2)).

INTERCONNECT DENSITY

The short wavelength of optical waves enables the focusing of the information down to small areas.  In the context of optical interconnects, this implies a high density of transmitters and receivers in two-dimensional (2D) arrays.   The high 2D density of optical devices is an enormous advantage over the aspect-ratio limited electrical interconnect scheme [6, 7].   Though fiber-based optical systems might be limited a similar aspect-ratio rule, free-space optical systems would not have the same limitation.

CROSS-TALK

Reduced cross-talk due to electromagnetic interference is another significant advantage of optical systems [6].  The short wavelength of optical waves (~1 µm) allows the energy in the beam to be focused down to a small spot at the detector.  Thousands of individual beams can be imaged simultaneously using a single lens without significant cross-talk.