Optical techniques for non-contact measurements of distances can be generalized into two main categories: direct detection and coherent detection techniques. In direct detection systems, the time delays of short laser pulses are used to determine target distances. The resultant spatial resolution, however, is quite limited due to the difficulty in measuring small time delays. On the other hand, optical interferometers are used in coherent detection range finders with the laser wavelength being used as the gauge against target distances. Interferometer-based coherent optical range finder can offer nanometer-scale high precision measurements. However, the readout from an interferometer is a sinusoidal function of the path length and hence the measurement range is severely limited. Optical coherent frequency domain reflectometry (CFDR) resembles a hybrid of direct and coherent detection optical range finders. Because the laser frequency is linearly chirped as a function of time in CFDR systems, any imbalance in the lengths of the reference and probe paths results in finite amount of optical frequency difference between the two interfering laser beams.

Enhancement of the photocurrent density generated by internal moving space charge electric fields in semi-insulating GaAs multiple quantum wells is observed when the sample is biased with a high speed, zero-mean square wave electric field which increases the internal space charge electric field strength. Tunability of the space charge field formation time is also demonstrated by adjusting the amplitude of the bias signal. This technique can be used to enhance the sensitivity of optical doppler frequency measurements based on moving space charge field effects in photoconductive semiconductors. Electronic tunability of the space charge field formation time also adds versatility in optical power spectrum analysis applications.

We demonstrate experimentally significant improvement in the sensitivity of photoelectromotive-force (photo-EMF) laser vibrometers using pulsed-light sources. The vibrating surface is discretely sampled by individual laser pulses and recorded by the photo-EMF sensor via the generation of photocurrent pulses whose magnitudes are proportional to the instantaneous surface displacements. With a sufficiently high sampling rate, reconstruction of the vibration wave form can be achieved by conducting envelope (or peak) detection of the resultant series of photocurrent pulses. Significantly higher peak optical power levels of the probe laser pulses, which can be orders of magnitude greater than those of continuous-wave interrogation lasers with the same average power, lead to proportional enhancement in the photo-EMF response and remarkable improvement in detection sensitivity when the photodetection process is initially amplifier noise current limited.