Incident light with a photon energy ħω induces two-photon absorption (TPA) when Eg / 2ħωEg , where Egis the band gap of the photo-absorption layer of a photodiode (PD). Be‐ cause the absorption coefficient is small, photocurrent generated by TPA is too low to be used in conventional optical signal receivers. However, the nonlinear dependence of the photocurrent on the incident light intensity can be used for optical measurements and opti‐ cal signal processing. It has been used for autocorrelation in pulse shape measurements [1], dispersion measurements [2,3] and optical clock recovery [4]. These applications exploit the dependence of the generated photocurrent on the square of the instantaneous optical inten‐ sity. Measurement systems using TPA in a PD can detect rapidly varying optical phenom‐ ena without using high speed electronics.