  In the coming years, the gravitational wave community will be optimizing
detector performance for a variety of astrophysical sources that make competing
demands on the detector sensitivity in different frequency bands. In this paper
we describe a number of technologies that are being developed as anticipated
upgrades to the Advanced LIGO detector, and quantify the potential sensitivity
improvement they offer. Specifically, we consider squeezed light injection for
reduction of quantum noise, detector design and materials changes which reduce
thermal noise, and mirrors with significantly increased mass. We explore how
each of these technologies impacts the detection of the most promising
gravitational wave sources, and suggest an effective progression of upgrades
which culminate in a factor of two broadband sensitivity improvement.
