  We have experimentally created a robust, ultrabright and phase-stable
polarization-entangled state close to maximally entangled Bell-state with $ \%
98 $-fidelity using the type-II spontaneous parametric down-conversion (SPDC)
process in periodically-poled KTiOPO$ _4 $ (PPKTP) collinear crystal inside a
Sagnac interferometer (SI). Bell inequality measurement, Freedman's test, as
the different versions of CHSH inequality, and also visibility test which all
can be seen as the nonlocal realism tests, imply that our created entangled
state shows a strong violation from the classical physics or any
hidden-variable theory. We have obtained very reliable and very strong Bell
violation as $ S=2.78 \pm 0.01 $ with high brightness $ \mathcal{V}_{\rm HV}=
\% (99.969 \pm 0.003) $ and $\mathcal{V}_{\rm DA}= \% (96.751 \pm 0.002) $ and
very strong violation due to Freedman test as $ \delta_{\rm F} = 0.01715 \pm
0.00001 $. Furthermore, using the tomographic reconstruction of quantum states
together a maximum-likelihood-technique (MLT) as the numerical optimization, we
obtain the physical non-negative definite density operator which shows the
nonseparability and entanglement of our prepared state. By having the maximum
likelihood density operator, we calculate some important entanglement-measures
and entanglement entropies. The Sagnac configuration provides bidirectional
crystal pumping yields to high-rate entanglement source which is very
applicable in quantum communication, sensing and metrology as well as quantum
information protocols, and has potential to be used in quantum
illumination-based LIDAR and free-space quantum key distribution (QKD).
