  The detection of gravitational waves from compact binary mergers by LIGO has
opened the era of gravitational wave astronomy, revealing a previously hidden
side of the cosmos. To maximize the reach of the existing LIGO observatory
facilities, we have designed a new instrument that will have 5 times the range
of Advanced LIGO, or greater than 100 times the event rate. Observations with
this new instrument will make possible dramatic steps toward understanding the
physics of the nearby universe, as well as observing the universe out to
cosmological distances by the detection of binary black hole coalescences. This
article presents the instrument design and a quantitative analysis of the
anticipated noise floor.
