  Spin-orbit coupling plays an important role in various properties of very
different materials. Moreover efforts are underway to control the degree and
quality of spin-orbit coupling in materials with a concomitant control of
transport properties. We calculate the frequency dependent optical conductivity
in systems with both Rashba and Dresselhaus spin-orbit coupling. We find that
when the linear Dresselhaus spin-orbit coupling is tuned to be equal to the
Rashba spin-orbit coupling, the interband optical conductivity disappears. This
is taken to be the signature of the recovery of SU(2) symmetry. The presence of
the cubic Dresselhaus spin-orbit coupling modifies the dispersion relation of
the charge carriers and the velocity operator. Thus the conductivity is
modified, but the interband contribution remains suppressed at most but not all
photon energies for a cubic coupling of reasonable magnitude. Hence, such a
measurement can serve as a diagnostic probe of engineered spin-orbit coupling.
