Back in 60&#x27;s AT&amp;T make lots and lots of circular waveguide, intending to bury it across the country.  One of the circular modes is dispersionless; then fiber was invented; lot&#x27;s of money lost on circular waveguide.<p>Interesting that mmWavguide is still rectangular.  Wonder if one could do a mmWave fiber?  Don&#x27;t see why not.