  We have carried out inelastic neutron scattering experiments to study
magnetic excitations in ordered double perovskite Ca$_2$FeReO$_6$. We found a
well-defined magnon mode with a bandwidth of $\sim$50meV below the
ferri-magnetic ordering temperature ($T_c\sim$520K), similar to previously
studied Ba$_2$FeReO$_6$. The spin excitation is gapless for most temperatures
within the magnetically ordered phase. However, a spin gap of $\sim$10meV opens
up below $\sim$150K, which is well below the magnetic ordering temperature but
coincides with a previously reported metal-insulator transition and onset of
structural distortion. The observed temperature dependence of spin gap provides
strong evidence for ordering of Re orbitals at $\sim$150~K, in accordance with
earlier proposal put forward by Oikawa $\it{et.\,al}$ based on neutron
diffraction [J. Phys. Soc. Jpn., $\bf{72}$, 1411 (2003)] as well as recent
theoretical work by Lee and Marianetti [Phys. Rev. B, $\bf{97}$, 045102
(2018)]. The presence of separate orbital and magnetic ordering in
Ca$_2$FeReO$_6$ suggests weak coupling between spin and orbital degrees of
freedom and hints towards a sub-dominant role played by spin orbit coupling in
describing its magnetism. In addition, we observed only one well-defined magnon
band near magnetic zone boundary, which is incompatible with simple
ferrimagnetic spin waves arising from Fe and Re local moments, but suggests a
strong damping of Re magnon mode.
