  Sources of entangled photon pairs are a key component in both fundamental
tests of quantum theory and practical applications such as quantum key
distribution and quantum computing. In this work, we describe and characterize
a source of polarization entangled photon pairs based on two spontaneous
parametric down-conversion (SPDC) crystals in a Sagnac interferometer. Our
source is compact and produces high-quality entangled states in a very flexible
manner. The wavelengths of the photon pairs, around 810 and 1550 nm, the phase
between orthogonal components of the entangled state, and the tangle of the
state are all independently adjustable. In addition to presenting basic
characterization data, we present experimental violations of CHSH and Leggett
inequalities, as well as an instance of the "beautiful" Bell inequality, which
has not previously been tested experimentally.
