Four Point Photo-Conductance (FPPC) monitoring array for speckle and fringe motion sensing

摘要

We report on the increase in frequency response of a semi-insulating GaAs Four Point Photo-Conductance (FPPC) remote vibration monitoring array. This simple, rugged device works by monitoring the lateral motion of an illumination pattern and has been demonstrated for remote, non-contact vibration sensing applications in the kilohertz regime at a distance over five meters. The FPPC sensor requires only a line-of-sight path to the object being probed (although later models are expected to work also with multimode fiber optics feed). Due to its construction and application to remote sensing, the FPPC device enables measuring vibrating bodies enclosed in a high temperature or pressure environment without adverse effects. This paper reports techniques for increasing the frequency response in these sensors. Experimental results will be given on sensors that have an approximate -3 db rolloff frequency of 10 kHz to 40 kHz. The performance improvements were achieved by replacing the opaque AuGe electrical contacts with transparent Indium Tin Oxide (ITO) contacts. These 80 nm thick films were deposited by RF sputtering at 3 mTorr Argon back pressure. Further frequency response improvements were obtained by increasing the array conductivity with background illumination from a red LED. These improvements are attributed to the reduction in lateral trapped space charge distribution across the active areas. The FPPC devices exhibited rise times of 1.5 microseconds and effective fall times of 100 microseconds. The pulse response exhibited tails with an underdamped oscillatory behavior; the damping increased with increasing background illumination.