Optical strain gauge

An optical strain gauge for measuring the strain in a structural member comprising:

• (a) a mechanical grating device fixedly attached to the structural member for modulating an incident beam of light by diffraction;
• (b) at least one source of light;
• (c) an optical system for directing light from the at least one source of light onto the mechanical grating device and a sensor for receiving light reflected from the mechanical grating device for producing an output signal;
• (d) the mechanical grating device including:
• (i) an elongated ribbon element including a light reflective surface, such elongated ribbon element having a predetermined resonant frequency;
• (ii) a substrate and a pair of end supports for supporting the elongated ribbon element at both ends over the substrate;
• (iii) at least one intermediate support between the end supports so that there are suspended portions of the elongated ribbon element;
• and
• (iv) a drive circuit for applying a force to the elongated ribbon element to cause the suspended portions of the elongated ribbon element to deform at the resonant frequency between first and second operating states;
• (e) output circuitry responsive to the output signal produced by the sensor for extracting a frequency dependent signal which represents the strain in the structural member that caused a variation in the resonant frequency;
• and
• (f) an output device responsive to the extracted frequency dependent signal for producing a representation of the strain in the structural member.