Communiqué

Terahertz Radios. Far out.

Gigahertz (GHz) is so yesterday, at least for radios.  While processor clocks are still stuck in the GHz region, communications radios of all types are already well into the upper GHz bands and now heading off into the Terahertz (THz) region.  Whoa, that is high frequency.

A THz is 1012 Hz.  That is 1,000,000,000,000 Hz.  Think of it as 1000 GHz.  In terms of cycle time or period, 1 THz is one picosecond or 1000 femtoseconds.  Very short.  In communications we also think in terms of wavelength.  The basic formula for wavelength is:

λ = 300/fMHz

Since 1 THz = 1,000,000 MHz, the wavelength is 0.0003 meter.  That translates to 0.3 mm or 300 micrometers (mm or microns). A very short wavelength.

To put this into perspective, the THz region of the spectrum is between the millimeter wave radio bands and light.  The millimeter wave bands extend from 30 to 300 GHz.  Most of us consider this pretty high frequency, but for communications applications (radar, satellite, etc.) these bands are already in use. 

As for light, the optical spectrum extends from infrared (IR) on the lower frequency end to ultraviolet (UV) on the upper end with visible light in between.  We normally refer to light in terms of wavelength rather than frequency.  IR runs from about 700 nm up to about 2000 nm.  An IR wavelength of 1000 nm is a frequency of 300 THz.   Visible light is in the 400 nm (violet) to 700 nm (red) range.  That’s high frequency, like petahertz (PHz)!

As we run out of electromagnetic frequency spectrum, as we perpetually do, we always head for the higher frequencies where there is more space.  More spectrum means we can accommodate more carriers and we can run higher data rates.  We are at that point now where there is a spectrum crisis of sorts.  We have had so much wireless success with cell phones and LANs/ PANs and other stuff that we have used up most of the available spectrum.  Wireless cannot grow without more space. 

That impetus always pushes R&D to come up with devices (e.g. transistors) that can amplify, oscillate and switch at those higher frequencies.  And so we get them.  We already have transistors, ICs and other devices that can work in the mm wave bands reliably.  Now we need parts to work in the THz region.  It just doesn’t get any easier, does it?

The latest research shows that we scientists are making progress in developing transistors that will work at THz speeds.  Right now, the upper limit is about 700 GHz.  Using GaAs, InP and GaN with special architectures, researchers have made HBT, HEMT and metamorphic HEMT devices to amplify over the 400 to 700 GHz range.  It seems that 1 THz is within sight.  Also when we get into those high frequency ranges we have to think of lasers that are practical frequency sources.

The hard part of this THz research is lack of measurement equipment.  It is time for the test equipment guys to start building us some THz spectrum analyzers, VSAs and scopes.

For researchers in this field, the IEEE has a new publication called the IEEE Transactions on Terahertz Science and Technology.  www.thz.ieee.org

One of the limiting factors of THz radios is their very short potential range.  Line of sight (LOS) is assumed, of course.  Just remember that LOS range is a function of wavelength.  The shorter the wavelength (the higher the frequency) the shorter the range for a given transmit power.  (Remember the Friis formula?) This is usually offset with higher transmitter and receiver antenna gains.  Even so, we are looking at practical ranges of less than 10 meters and even that will depend on high antenna gains as well as active beam forming and beam steering.  Thankfully, we do know how to do that now.  Another limiting factor is the high free space attenuation because of water vapor.   THz apps will probably be indoors.

In any case, we are headed in the THz direction.  It is years away yet, as I said “far out”.  Just thought you would like to know.

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