  We demonstrate a high-brightness source of pairs of indistinguishable photons
based on a type-II phase-matched doubly-resonant optical parametric oscillator
operated far below threshold. The cavity-enhanced down-conversion output of a
PPKTP crystal is coupled into two single-mode fibers with a mode coupling
efficiency of 58%. The high degree of indistinguishability between the photons
of a pair is demonstrated by a Hong-Ou-Mandel interference visibility of higher
than 90% without any filtering at an instantaneous coincidence rate of 450 000
pairs/s per mW of pump power per nm of down-conversion bandwidth. For the
degenerate spectral mode with a linewidth of 7 MHz at 795 nm a rate of 70
pairs/(s mW MHz) is estimated, increasing the spectral brightness for
indistinguishable photons by two orders of magnitude compared to similar
previous sources.
