  We theoretically and experimentally investigate the spectral tunability and
purity of photon pairs generated from spontaneous parametric down conversion in
periodically poled $\mathrm{KTiOPO_4}$ crystal with group-velocity matching
condition. The numerical simulation predicts that the purity of joint spectral
intensity ($P_{JSI}$) and the purity of joint spectral amplitude ($P_{JSA}$)
can be kept higher than 0.98 and 0.81, respectively, when the wavelength is
tuned from 1460 nm to 1675 nm, which covers the S-, C-, L-, and U-band in
telecommunication wavelengths. We also directly measured the joint spectral
intensity at 1565 nm, 1584 nm and 1565 nm, yielding $P_{JSI}$ of 0.989, 0.983
and 0.958, respectively. Such a photon source is useful for quantum information
and communication systems.
