Design Con 2015

Challenge: Internal Optical Links in CE Devices--Are We There Yet?—Part II

Holger Hoeltke, Silicon Line, GmbH -November 30, 2012

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Challenge: Transport data to and from displays and image sensors in mobile devices—Part I


Ultra-Low Power Optical Links - A Valid Alternative

An ideal alternative able to eliminate all the shortcomings of electrical cables is ultra-low power optical links. 
They are very small in size and come in the form of active optical cables. Active optical cable basically means that there is no optical connector involved but the conversion from electrical to optical signals and vice versa happens within the electrical connectors assembled at the ends of the optical cable.

Ultra-low power optical links for portable consumer devices are readily available from several manufacturers in Asia, Europe and North America.  Dependent on the application, they come based on polymer optical fibers or as planar optical waveguides.  They may be embedded within FPC or are available as super thin hybrid cables that combine electrical low-speed wires with optical waveguides. They are available as uni-directional or bi-directional solutions.

 

The Power Benefit

Such short-haul optical links exhibit a number of benefits, since photons are constrained by a different set of physical considerations to those of electrons.  Interestingly the key benefit gained from optical links is the low-power consumption needed to transport high-speed data.

Today there is a new series of ultra-low power VCSEL drivers and photodiode transimpedance amplifiers with maximum data rates ranging from 3 Gbps to 12.5 Gbps.  For example, operating a 3Gbps-link at below 10 mW, equals an energy of 3.3 pJ per bit! At 3 Gbps this energy might be comparable to that of electrical links going a few centimeters. However, here the interesting aspect is, that just the same energy is needed to go several meters. Moreover, it is almost the same energy needed to go with 12.5 Gbps over several meters. In this case an active optical cable based on this chip set requires 4 pJ per bit, only!

 


Such low power chips have negligible heat dissipation making them suitable for small active optical cables. The diagram below depicts an optical cable based on the chip set. The outer dimension of the connector that houses all active components is only 3 mm in length, 1.8mm in width, and 1 mm in height.

These dimensions allow such an active optical cable to be fed through a mechanical hinge with only a 2 mm inner diameter.

Signal Integrity Benefit

An optical transport media does not exhibit any practical frequency dependent loss. Consequently, there is no need for complex driver and equalization technologies, which would otherwise compensate for such frequency dependent loss.

The deterministic jitter components in a data signal of an optical link depend mostly on the active components used to convert electrical energy into optical and vice versa.  If done properly, an active optical cable may yield a very nice, clean and open eye, such as the one shown here.

Mechanical Benefits

While optical transport media practically does not have any frequency dependent loss, they will suffer also from regular propagation loss due to material and wavelength dependent attenuation.

However, with 0.4 dB per meter for polymer optical fibers, this propagation loss may be neglected for applications in mobile and consumer electronics.

Polymer optical fibers or planar optical waveguides do have a few more notable features that are pretty interesting and make them superior over their electrical counterparts.  For example, if such an optical waveguide is mechanically stressed with a bending radius of only 1mm, then, even after 1 million bending cycles, there is no real measurable increase of the insertion loss.

The same holds true for a similar test on the twisting possibilities of a planar optical waveguide. If twisted with a twisting length of 5 mm and this repeated 1 million times, the test passed without any significant change to the insertion loss factor.

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