Interferometer-less coherent optical range finder

Summary form only given. 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. By measuring the optical frequency difference, target distance can be determined with precision unattainable by direct detection techniques. indeed, a spatial resolution of approximately 15 /spl mu/m was achieved using a widely tunable (75 nanometers) solid state laser.