MIT Researchers Strike the Sweet Spot Between Fiber Optic and Copper Cabling

March 03, 2021 by Jake Hertz

At just the width of a human hair, a new type of cable from MIT and Raytheon is said to be ten times faster than USB.

As electronic applications demand faster speeds and more power, interconnects and cabling have become one of the most limiting features of electronic devices.

Applications like data centers require extremely long and bulky cables to connect devices at a given bandwidth, which introduces huge parasitic loads that slow down transmission and consume power. What's worse is that data rates cannot increase without increasing power consumption, thus posing a serious challenge to EEs. 


An image of the new cable bonded to a TX chip.

The new cable bonded to a TX chip. Image used courtesy of Holloway et al

Now, researchers at MIT and Raytheon Technologies have worked to solve this problem, recently publishing a new paper that describes a new kind of cable—the width of a strand of hair—that can achieve rates up to ten times that of a USB.


The Benefits of Fiber Optic Cables in Data Centers

Generally speaking, the two main kinds of cables used in data center applications are copper coaxial cables or fiber optic cables. While many systems will use the two in tandem, each comes with its pros and cons. 


A diagram of a fiber optic cable.

Diagram of a fiber optic cable. Image used courtesy of Fast Metrics


Unlike coaxial cables, which send purely electrical signals, fiber optic cables operate by transmitting photons, resulting in a vast range of benefits. By utilizing light instead of electricity, fiber optics avoid many parasitics that slow down electrical signals. The result is that fiber optics are significantly faster (up to 10 Gbps) and longer than coaxial cables. 

Also, since the transmitted signals are not electric, fiber optics are less susceptible to electromagnetic interference (EMI). This resistance to EMI means that fiber optics don't require significant shielding, allowing the cables to be significantly lighter and thinner than copper with the same data rates. 


Where Fiber Optics Falls Short 

Fiber optics have many advantages to coaxial cables, but they often come up short in durability and upfront costs.

To start, fiber optic cables are thinner and lighter than coaxial cables, making them more susceptible to breaking and causing systems as a whole to be more delicate. Additionally, the upfront cost of installing fiber optic cables can be extremely high.

Beyond just the cabling, fiber optics require specialized test equipment and knowledge to integrate them correctly into a system. Finally, fiber optics often don't mesh well with silicon, making the interface with fiber optic cabling and classical electronics a challenge. 

According to MIT researchers, at this moment, there is no way to detect, amplify, or generate photons in silicon efficiently. From an economic perspective, the expensive and complex integration schemes are not reasonable solutions since this setback. 


Blending the Strengths of Fiber Optics and Copper 

Looking to embrace fiber optics' benefits while avoiding the disadvantages, researchers from MIT and Raytheon have announced a new form of cable that rivals fiber optics while still working electrically. 


Lab setup for the new, hair-thin cable.

Lab setup for the new, hair-thin cable. Image used courtesy of Holloway et al


In their newly-released paper, the researchers explain that the new cable can achieve data rates up to 105 Gb/s. The cable achieves these rates by dividing the 220–335 GHz band into three 35 GHz-wide channels and including two 5 GHz-wide guard bands.

The cable is made of a polymer, resulting in a conduit that is lighter and cheaper to manufacture than traditional copper cables. Their present cable comes in at a cross-sectional area of about 0.4 mm by 0.25 mm, comparable to a strand of hair. 


The diagram of the 3-channel dielectric link transmitter

The diagram of the 3-channel dielectric link transmitter. Image used courtesy of Holloway et al


This experiment resulted in a cable that MIT claims will rival fiber optics in terms of speeds and efficiency while also being compatible with silicon. For this reason, the researchers believe this cable is the perfect blend between fiber optics and copper. 


A New Future For Cables?

Based on the impressive results in efficiency and speed, the researchers believe that applications like server farms could be amongst the first to adopt these new electronic links. They even go as far as to suggest that one day, the new cable could replace consumer electronic cables in homes and offices.