  The Hong-Ou-Mandel (HOM) interference is one of the most intriguing quantum
optical phenomena and crucial in performing quantum optical communication and
computation tasks. Lately, twin beam emitters such as those relying on the
process of parametric down-conversion (PDC) have become confident sources of
heralded single photons. However, if the pump power is high enough, the pairs
produced via PDC -- often called signal and idler -- incorporate multiphoton
contributions that usually distort the investigated quantum features. Here, we
derive the temporal characteristics of the HOM interference between heralded
states from two independent narrowband PDC sources. Apart from the PDC
multiphoton content, our treatment also takes into account effects arriving
from an unbalanced beam splitter ratio and optical losses. We perform a
simulation in the telecommunication wavelength range and provide a useful tool
for finding the optimal choice for PDC process parameters. Our results offer
insight in the properties of narrowband PDC sources and turn useful when
driving quantum optical applications with them.
