Systems and methods for optically sensing characteristics of a blood flow provide a light source having a driver circuit that includes a source of constant current and a modulator that modulates the constant current at a selected carrier frequency for transmission to the light source. The systems and methods also include a light sensor for receiving light from the light source and producing a modulated output proportional to received light intensity. A receiver circuit including a bandpass filter is coupled to the light sensor to receive the modulated output and having a center frequency at or near the selected carrier frequency to eliminate frequency components above and below the selected carrier frequency.