  We experimentally demonstrate a polarization-entangled photon source at 810
nm using a type-II phase-matched PPKTP crystal pumped by a low-cost, broadband
laser diode with a central wavelength of 405 nm and a typical bandwidth of 0.53
nm. The PPKTP crystal is placed in a Sagnac-loop to realize the compact size
and high stability. The downconverted biphotons, the signal and the idler, have
typical bandwidths of 5.57 nm and 7.32 nm. We prepare two Bell states |Psi+>
and |Psi-> with the fidelities of 0.948+-0.004 and 0.963+-0.002. In
polarization correlation measurement, the visibilities are all higher than
96.2%, and in the Bell inequality test, the S value can achieve 2.78+-0.01. To
our knowledge, this experiment is the first to combine a multi-mode pump laser
with a Sagnac-loop configuration. This high-quality and low-cost entangled
photon source may have many practical applications in quantum information
processing.
