  The interferometers being planned for second generation LIGO promise and
order of magnitude increase in broadband strain sensitivity--with the
corresponding cubic increase in detection volume--and an extension of the
observation band to lower frequencies. In addition, one of the interferometers
may be designed for narrowband performance, giving further improved sensitivity
over roughly an octave band above a few hundred Hertz. This article discusses
the physics and technology of these new interferometer designs, and presents
their projected sensitivity spectra.
