Absolute Distance Metrology Using Diode Pumped Nd:YVO4/KTP Green Laser

The paper exploits absolute distance measurement with traceability to the international system (SI) of units of length. In the present work, the diode pumped Nd:YVO4/KTP polarised green laser is used to measure a distance Delta l between two positions without the need for a reference standard. To comprise a synthetic wavelength lambda(s) suitable to the range of the distance, the output wavelength generated by the frequency doubling is shifted by Delta lambda by controlling the injection current, and a set of shifted wavelengths are produced to form lambda(s). The output signal is sent to the interferometer by an optical fibre after being enhanced with a speckle removal unit. The measurement starts by identifying the zero position where no shift in the interference fringes is observed during sweeping the wavelengths, while at the distance Delta l a deviation in the interference pattern is observed. This deviation is analysed by Fourier transformation to extract the phase change Delta phi and determine the distance value. To prove the reliability of absolute distance measuring interferometer, some measurements are made for various ranges with evaluation of both repeatability and uncertainty. The discussed technique can measure 100 mm with uncertainty +/- 16 mu m.