Multiwavelength digital holography with meter scale synthetic wavelengths at micrometer precision
Digital holography enables high-precision quality control in machining production and has already been introduced to several multi-axis systems1, 2. To meet the demanding measurement tasks in the quality control of complex components, accuracies in the sub-micrometer range with measurement ranges larger than several centimeters are required. Previous measurements have shown the potential of multiwavelength digital holography to allow unambiguous ranges of few millimeters3. We present multiwavelength digital holographic measurements using synthetic wavelengths with two meters down to a few micrometers, potentially enabling measurements with meter-scale unambiguity at sub-micrometer accuracy. Measurements on a 10 cm step-height sample have been conducted using the compact digital-holographic sensor HoloTop NX for various multi-axis systems, supplied by an Ondax LMFC single frequency diode laser at 632.852 nm and the tunable laser Hübner C-Wave used in the wavelength range of 480.786 nm – 632.852 nm. The latter offers a frequency stability of 150 MHz on a time scale of several hours. The maximum laser drift during data acquisition was observed to be 0.02 pm. Thus, at the 2 m synthetic wavelength, this results in a maximum synthetic wavelength error of 200 mm. Random noise of 20 mm at the largest used synthetic wavelength of 2 m requires multiple synthetic wavelengths to get down to micrometer precision: Eight nested synthetic wavelengths from 2 μm to 2 m and numerical refocusing of the hologram were used to evaluate a milled sample with multiple step heights, machined on a Hermle C32U machine tool. Ten repetitive measurements confirm a machining uncertainty of 9 μm for this sample at its maximum step height of 10 cm.