Input impedance conditions for minimizing the noise figure of an analog optical link

We use an equivalent-circuit model to derive the minimum noise figure (NF) for an amplifierless optical analog link with a perfect lossless input impe dance-matching circuit. This minimum, which is 3 dB, is called the lossless passive match limit. We contrast the link model with an analogous equivale nt-circuit model for a transistor amplifier, which does not have the same l imiting NF under this impedance-matching condition, It turns out that the l ink's 3-dB limit arises from the ohmic nature of the impedance of the devic e that modulates the light, and not from the electro-optic conversion proce sses in the link. In a prior experimental link with near-perfect impedance matching, dissipative loss in our input circuit precluded achieving a measu red NF of less than 4 dB. Investigation of the effects of input impedance m ismatch indicates that mismatch can actually lower the NF to below 3 dB eve n when dissipative loss is present in the input circuit. We have used this mismatch effect to reduce the measured NF of our link to 2.5 dB at 130 MHz. We believe this is the first demonstration of amplifierless-link NF of les s than 3 dB.