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Absolute-distance interferometry (ADI) with randomly modulated optical wavelength

We present a coherent-optical measurement procedure for contactless distance sensing. The radiation source is a laser diode whose optical wavelength is randomly modulated about 3...12 pm (rms value) by noise modulation of the injection current at a bandwidth of up to 100 kHz. An interferometer serves as a coherent detector. The average beat frequency of the photodetector output is a measure for the absolute distance to the object. Combining the natural phase noise of the laser diode with an additional technical frequency-modulation process, sensitivity can be drastically improved, particularly in the short distance range. Due to the technical current modulation the coherent detection can be used well beyond the coherence length of the laser diode. For small target ranges the absolute distance resolution increases. This measurement behavior is advantageous for approach/docking applications, for example. The measurement procedure was verified experimentally in a distance range up to 4 m.