  Entangled photon sources (EPS), the major building block for a variety of
quantum communication protocols, are commonly developed by utilizing the
spontaneous parametric down-conversion (SPDC) in $\chi^{2}$ nonlinear bulk
optical materials. While high nonlinearity and long interaction length have
established the superiority of the periodically poled crystals for EPSs, the
phase-matching condition of such crystals is very sensitive to the fluctuation
of the crystal temperature and the laser wavelength. As a result, deploying
such sources outside the laboratory, for example, satellite-based applications,
demands a stringent mass and power budget, thus enhancing the mission
complexity and cost. We report a bright, stable entangled photon source with a
relaxed requirement of crystal temperature and laser wavelength stabilization.
Using a periodically poled KTP crystal inside a polarization Sagnac
interferometer producing degenerate, type-0 phase-matched entangled photon
pairs at 810 nm in an annular ring, we have transformed the SPDC ring into a
"perfect" ring with the help of two common optical elements, axicon, and lens.
Despite the variation of the SPDC ring size from Gaussian to annular of
different diameters due to the change of crystal temperature over $7^{o}$C, and
laser wavelength over 300 GHz, we observe the size of the "perfect" ring to be
constant. The new EPS, having a spectral brightness as high as 22.58 $\pm$ 0.15
kHz/mW collected using single-mode fiber with a Bell's parameter, S = 2.64
$\pm$ 0.05, and quantum state fidelity of 0.95 $\pm$ 0.02, requires a crystal
temperature stability of $\pm$ $0.8^{o}$C, almost five times relaxation as
compared to the previous EPS. The generic scheme can be used for non-collinear
SPDC photons in all crystals to develop EPS at any wavelength and timescales
for resource-constrained applications.
