Integrated optical micro structures for signal processing in the position metrology

Abstract

The miniaturisation of functional components and a high integration depth of sensor systems are the fundamental demands of the micro system technology. Within the, ‘Sonderforschungsbereich 516,’ the development of a position sensor for a micro linear motor is also directed towards these goals; and so a sensor with a size smaller than 10 mm and a measurement accuracy of better than 100 nm is desirable. Additionally, the electromagnetic sensitivity should be minimised. An optical interferometric measurement principle meets these requirements. Light from both of the first diffraction orders from a grating on the motor armature is coupled into a glass-chip waveguide coupler. The interference of the signals results in a beat frequency signal when the grating is shifted. It can be detected as an intensity modulation at the three outputs. The velocity as well as the movement direction of the armature can be derived from the signal phases. The investigations were focused on the coupler design and its numerical field simulations as well as manufacturing and qualification of the coupler itself. Its geometry essentially affects the total sensor length. Refractive index modification by a femtosecond pulse laser was chosen for the creation of the waveguide coupler. This process does not restrict the waveguide geometry to the surface of the glass-chip, and enables arbitrarily positioning of waveguides in the substrate volume. Thus, it also enables non-planar cross section formations of three waveguides e.g. in the form of an equilateral triangle.