Since 2001, with the release of the Canon 400mm f/4 DO IS, we haven’t seen the use of DO (diffractive optics) in super telephoto lenses.
Even though the 400mm DO IS was a successful lens because of its size and weight, due to a somewhat reduced contrast, it could not quite live up to the quality of its larger and heavier "Fluorite" counterparts.

Canon has been filing more DO super telephoto patents recently with the 400mm f/2.8 DO and 600mm f/4 DO. And now, we are finding patents for the 600mm f/5.6 DO and 800mm f/5.6 DO filed. Maybe this means we can
expect a new surge of DO technology in super telephoto lenses in the near future?

I for one would love to carry around a Canon 800mm f/5.6 weighting 6 pounds!
To see the recent DO patents click on the links below:

Canon 600mm f/5.6 DO and Canon 800mm f/5.6 DO patents

Canon 400mm f/2.8 DO and Canon 600mm f/4 DO patents

Thanks Fred, interesting developments.

Definitely interesting, the 400/2.8 is already pretty heavy.