Electromagnetic fields that remain totally polarized under propagation

It is remarked that, in general, a totally polarized field becomes partially polarized at the output of an optical system, even though the field propagates through non-polarizing devices. Two kinds of fields are shown in the present work, which maintain the totally-polarized character at the output of any (deterministic) non-polarizing first-order optical system. This property is satisfied by uniformly totally polarized beams and also by those fields whose electromagnetic degree of coherence [T. Setälä, J. Tervo, A.T. Friberg, Opt. Lett. 29 (2004) 328.] equals 1.