With the ongoing further development of photonic quantum technologies, better single-photon sources are needed. Many applications require photons from independent sources to be indistinguishable. In this thesis a spontaneous parametric down-conversion (SPDC) source, which can simultaneously produce two photon pairs with a wavelength of 795 nm, is characterised and improved. A time-of-flight spectrometer is designed and assembled, allowing the measurement of the joint spectral intensity of the source with a resolution of 0.5 nm. The spectral purity extracted from this distribution substantially exceeded the measured Hong-Ou-Mandel (HOM) interference visibility of 39(2) %, which suggests the presence of strong spatial-spectral correlations in the pump beam. Spatial filtering is used to mitigate these correlations. A short single-mode fiber inserted into the pump indeed removes these correlations, increasing the HOM visibility to 79(15) %. However, the fiber sustains damage within multiple hours due to the high optical peak intensities of the pump pulses, which lowers its transmissivity, rendering this approach impractical. Alternatively, using a 15 µm diameter pinhole in the Fourier plane of a telescope increases the purity to 48(2) %, while providing a steady power transmission. Tighter filtering using a 10 µm diameter pinhole does not enhance the source further as it increases spectral correlations in the generated photon pairs.