Instead of monitoring the electrical response, we monitor the diffuse optical response, either as a reflection or transmittance. The phase and amplitude of the diffuse optical signal fit to an appropriate mathematical model provide enough information to determine the tissue absorption and scattering properties. Increased tissue absorption yields a shorter L, or a reduced average time of travel through the tissue, which yields a smaller amplitude and phase shift. Scattering, on the other hand, generally increases L, so that increased scattering will decrease amplitude but increase the phase shift. This is how optical network analysis can pick out differences in L, based upon the absorption and scattering of the tissue.
Thermoelectric coolers stabilize the diode temperature.
Relevant diode laser wavelengths lie in the range of 660 to 1000 nm. A computer controls both the RF switch network and the optical switch to ensure only one modulated laser signal reaches the tissue at any given moment. A source fiber directs light from the switch to the tissue. The APD module casing has machined attachments that hold the source fiber in place. In this configuration, the mean optical penetration depth is about 10 to 15 mm below the skin.