  We report on the realization of a synchronous source of heralded single
photons at telecom wavelengths with MHz heralding rates and high heralding
efficiency. This source is based on the generation of photon pairs at 810 and
1550 nm via Spontaneous Parametric Down Conversion (SPDC) in a 1 cm
periodically poled lithium niobate (PPLN) crystal pumped by a 532 nm pulsed
laser. As high rates are fundamental for multi-photon experiments, we show that
single telecom photons can be announced at 4.4MHz rate with 45% heralding
efficiency. When we focus only on the optimization of the coupling of the
heralded photon, the heralding efficiency can be increased up to 80%.
Furthermore, we experimentally observe that group velocity mismatch inside long
crystals pumped in a pulsed mode affects the spectrum of the emitted photons
and their fibre coupling efficiency. The length of the crystal in this source
has been chosen as a trade off between high brightness and high coupling
efficiency.
