A rotating machine cooled by a cooling medium directed through the rotating machine in a main flow and a secondary flow includes a rotor. The system may be used, for example, to measure urinary 8 hydroxy 2 deoxyguanoisine. The simplicity of the refractive index profiles of the constituent cores provides ease of manufacture of the multiple core structure. A method for building a digital signature is also described that permits signing different parts of an electronic document.