An apparatus for providing a test signal from a device under test (DUT) to a measurement instrument is disclosed. The apparatus includes a probe head configured to receive an electrical signal from the DUT. The probe head includes an electro-optic modulator. The apparatus also includes a control box, which includes an optical source. The optical source is configured to provide an input optical signal to the electro-optic modulator, which is configured to provide an output optical signal based on the electrical signal from the DUT. The control box also includes an optical bias control circuit. Only a bias control signal is provided to the electro-optic modulator.