Optimizing transverse photo-electromotive force detectors for laser ultrasonic applications

The sensitivity of the adaptive photoelectromotive-force (photo-EMF) detectors arranged in the conventional transverse configuration and optimized for laser ultrasonic applications is analyzed. It is shown that for the devices based on bipolar photoconductors operating under typical conditions, i.e. with approximately pixels in the detected speckle-like signal wave, the amplifier input noise current and the probe beam wavelength of the optimal matching with the preamplifier can be achieved with a detected signal beam power P S greater than ≈0.1 mW. The limit on the sensitivity of these optimized photo-EMF detectors (with the inter-electrode spacing equal to 5 optimal fringe periods) found to be determined by the noise of the detector itself and is approximately a factor of 14 below the fundamental limit set by the shot noise of the detected signal power. In particular, for and the detection frequency range of the minimal detectable phase modulation amplitude is rad, corresponding to the detection of ≈0.13 nm ultrasound induced surface displacement amplitude. For lower signal laser power the sensitivity of the detection configuration proves to be limited by the noise of the preamplifier and deteriorates linearly with the detected signal power. Significant growth of the cut-off frequency makes difficult to deploy the typically more effective monopolar photoconductors in such optimal small-size devices.