Comparative Evaluation of Optical Waveguides as Alternative Interconnections for High Performance Packaging

The well-known advantages of optical interconnections include high carrier frequency, low attenuation, high noise immunity, and, in ideal guides, low cross talk. In this paper, a detailed comparison between optical and electrical interconnections is presented, with the emphasis on advantages and drawbacks of optical link utilization. The impact of attenuation, dispersive degradation, and fan out on signal integrity is discussed. Reflections from discontinuities are taken into account in the section on fan out, but the issue of reflection is omitted in the remainder of the paper, not because it is unimportant, but because these reflections do not give either system an advantage. If anything, reflections are more troublesome in optical waveguide circuits than in electrical circuits. Bends in optical waveguides cannot exceed the limitations imposed by the index difference which precludes the existence of abrupt bends that mimic corners or vias in electrical interconnections. According to our results, there is no obvious advantage of using optical interconnections for the distribution of digital signals containing significant frequency components in excess of 10 GHz, unless substantial distances are involved, for which low dispersion optical waveguides could provide a solution. The actual electronic digital signal speeds for which suitable interconnections can be provided still depend as much on the skill of the designer as on the availability of materials or processes. Therefore, in high-speed digital systems, a detailed study has to be performed on the entire system before a decision is made as to which interconnection scheme is superior for a given application. Moreover, the reduction of the number of lines and ports by multiplexing and demultiplexing is limited by the electronics, for both electrical and optical approaches. Retention of waveform amplitude and integrity and tolerance of cross talk eventually become the key parameters that force the transition to the optical channel at ultra-high digital speeds. Approaching tens of picosecond rise time, dispersion and attenuation of electrical signals is detrimental, while optical waveforms are basically unaffected in low dispersion waveguides, making the transition to optical interconnections essential. The implementation of a polyimide optical waveguide in the MCNC package is discussed.