  We propose a novel method for generating broadband spontaneous parametric
fluorescence by using a set of bulk nonlinear crystals (NLCs). We also
demonstrate this scheme experimentally. Our method employs a superposition of
spontaneous parametric fluorescence spectra generated using multiple bulk NLCs.
A typical bandwidth of 160 nm (73 THz) with a degenerate wavelength of 808 nm
was achieved using two beta-barium-borate (BBO) crystals, whereas a typical
bandwidth of 75 nm (34 THz) was realized using a single BBO crystal. We also
observed coincidence counts of generated photon pairs in a non-collinear
configuration. The bandwidth could be further broadened by increasing the
number of NLCs. Our demonstration suggests that a set of four BBO crystals
could realize a bandwidth of approximately 215 nm (100 THz).We also discuss the
stability of Hong-Ou-Mandel two-photon interference between the parametric
fluorescence generated by this scheme. Our simple scheme is easy to implement
with conventional NLCs and does not require special devices.
