An optical detector for detecting radio frequency (RF) signals, the optical detector comprising a light source and a photodetector, and an electrical circuit comprising a position dependent capacitor and a bias voltage source adapted for providing a bias voltage for biasing the position dependent capacitor, the position dependent capacitor comprising an electrode and a membrane being displaceable in reaction to RF signals incident on the membrane, the membrane being metallized, has a thickness of less than 1 μm and a quality factor, Qm, of at least 20,000, and the distance between the membrane and the electrode being less than 10 μm.