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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Some THz System Issues – Part 2 – Safety Date Submitted: November, 2008 Source: Rick Roberts Company: Intel Corporation Address Voice: FAX: E-Mail: [email protected] Re:

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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Submission Title: Some THz System Issues – Part 2 – Safety

Date Submitted: November, 2008

Source: Rick Roberts Company: Intel Corporation


Voice: FAX: E-Mail: [email protected]


Abstract: This contribution presents some desirable system performance issues

Purpose: for discussion

Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.

Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

Rick Roberts, Intel


In this contribution we continue to try to answer the question “in regards to communications, what is the compelling reason to invest in THz”?

For instance, the THz band offers more bandwidth, but we can get more bandwidth using infrared and apparently infrared is easier to generate.

In this contribution, some regard is given to the safety issue. It is found that the maximum allowed power densities for THz are higher than that allowed for near infrared. But it is questionable if the results are compelling for THz since actually emitting signals at these maximum power densities may be impractical.

Rick Roberts, Intel


From 15-08-0410-01 …

  • What will it take to get industry excited about THz?
  • THz will be exciting if and when …
  • THz offers wireless bit rates unmatched by any other wireless technologies
    • Bit rate sells … historically there has been interest in wireless technology that offers more throughput (even at short ranges).
  • Costs are comparable to existing technologies
    • The market wants more for the same cost. Going 10x faster captures the market, but it doesn’t allow charging 10x more.
  • Power/bit is less than existing technology
    • Going 10x faster has to be done with basically the same battery used by the 1x technology.

Rick Roberts, Intel


At 10s of Gbps data rates, will photonic bandgap generation of wireless communications be cheaper than traditional radio?

In traditional radio one can postulate that there is a relationship between cost, data rate and operating frequency … but does bandgap generation of the operating frequency disrupt this model?

Below we assume that the data rate is 1% of the operating frequency (i.e. the higher the operating frequency, the more the bandwidth and hence higher the data rate.



revised chart





10 GHz

100 GHz

1 THz

(10 GHz BW)

10 THz

(100 GHz BW)

100 THz

1 GHz

Rick Roberts, Intel


1 THz

From 15-08-0410-01

But why THz and not near IR1?

1Forin, D.M., et. al., “On field test of a Wavelength Division Multiplexing Free Space Optics Transmission at very high bit rates’, 9th ICT, June, 2007 (where NIR FSO rates of 40 Gbps are reported)


  • Why should industry invest in multi-Gbps THz as opposed to multi-Gbps near-IR?
  • lowest cost?
    • - easier to generate?
  • better performance?
    • lower intrinsic terrestrial noise?
    • higher bit rates, less loss, less power?
  • Safety?
  • We need a compelling argument!

Rick Roberts, Intel


Ionizing Radiation vs. Non-Ionizing Radiation

The energy of an individual photon is given by E=hf (h is Planck’s constant and f is the frequency); and in analogy to Eb, free space optics measures performance by the number of photons per bit (b).

At a frequency of ~2420 Terahertz the photon energy approaches 12.4 eV. At this energy level water molecules can become ionized (body tissue damage). This frequency is well above visible light (450 THz to 750 THz) and is in the ultraviolet region.

So we are only concerned with non-ionizing radiation; that is, we are concerned about absorption of energy (power density) heating the human body.

Rick Roberts, Intel


Not all Wavelengths Enter the Eyeball

Visible light to near infrared (400 nm to 1400 nm) pass through the eye lens and forms an image on the retina.

The problem with passing through the eye lens is

that a very sharply focused image is formed on the

retina which significantly increases the power density.

If the power density is enough to cause localized

“boiling” then a permanent blind spot occurs.

Near Infrared (>700 nm) is particularly dangerous since the body’s protective “blink reflex” is triggered only by visible light and the retina feels no pain as it is damaged.

If the energy does not pass through the lens then the energy (density) is dispersed on the cornea and lens, where the danger is the formation of cataracts.

Rick Roberts, Intel


Absorbed by skin or cornea/lens

Photon Ionization of Water Molecule

Enters the eyeball

Rick Roberts, Intel


MPE1 as power density versus exposure time for various wavelengths.

At 1 THz, λ=300 um

 = 16 dB


1Maximum Permissible Exposure

Rick Roberts, Intel




Safety Link Budget Example

(Safe harbor statement: may not be realistic, may not be correct)

Double Click to Activate Spreadsheet

distance D

Based upon the assumptions, it appears that wavelengths >2.9 um (1 THz is 300 um) could be safely deployed while near infrared wavelengths (<1 um) would exceed the safety limits.

=1 sr



Beam area at distance D is D2.

Rick Roberts, Intel



  • Is the safety argument compelling; that is, is THz always safer than Near Infrared?
  • In our contrived example, Near Infrared (NIR) exceeded safety limits, but not by much. And the parameters could have been easily adjusted to meet the NIR safety standards.
  • In general THz will have more safety margin, but this is not to say that IR couldn’t be safely deployed at 100 Gbps data rates over a short range with a highly focused beam.
  • Call for Contributions: Of interest would be contributions establishing compelling reasons why THz is preferred for communications over IR.

Rick Roberts, Intel


The End – Thanks!

Rick Roberts, Intel