The Shack-Hartmann wavefront sensor is composed of a lenslet array generating the spot images from which local slope
is calculated and overall wavefront is measured. Generally the principle of wavefront reconstruction is that the spot
centroid of each lenslet array is calculated from pixel intensity values in its subaperture and then overall wavefront is
reconstructed by local slope of wavefront obtained by deviations from reference positions. Hence the spot image of each
lenslet array has to remain in its subaperture for exact measurement of wavefront. However the spot of each lenslet array
deviates from its subaperture area when wavefront with large local slopes enters the Shack-Hartmann sensor.
In this research, we propose the spot image searching method that finds area of each measured spot image flexibly and
determine the centroid of each spot in its area. Also the algorithms that match these centroids to their reference points
unequivocally even if some of them are situated off the allocated subaperture are proposed. Finally we verify the
proposed algorithm with the test of a defocus measurement through experimental setup for the Shack-Hartmann
wavefront sensor. It has been shown that the proposed algorithm can expand the dynamic range without additional
One of the ever-increasing demands on the performances of heterodyne interferometers is to improve the measurements resolution, of which current state-of-the-art reaches the region of sub-nanometers. So far, the demand has been met by increasing the clock speed that drives the electronics involved for the phase measurement of the Doppler shift, but its further advance is being hampered by the technological limit of modern electronics. To cope with the problem, in this investigation, we propose a new scheme of phase- measuring electronics that reduces the measurement resolution without further increase in clock speed. Our scheme adopts a super-heterodyne technique that lowers the original beat frequency to a level of 1 MHz by mixing it with a stable reference signal generated from a special phase-locked-loop. The technique enables us to measure the phase of Doppler shift with a resolution of 1.25 nanometer at a sampling rate of 1 MHz. To avoid the undesirable decrease in the maximum measurable speed caused by the lowered beat frequency, a special technique of frequency up/down counting is combined to perform required phase- unwrapping simply by using programmable digital gates without 2π ambiguities up to the maximum velocity guaranteed by the original beat frequency.
The thermal radiation from a weld pool is focused on an aperture and the transmitted thermal radiation is monitored at two wavelengths with high-speed single-element detectors. Due to the chromatic aberration introduced in the focusing optics, the transmittance curve of thermal radiation varies by the wavelength. Likewise, the detector field of view varies by the wavelength. Owing to this difference in the transmittance and in the field of view, the local variation in a weld pool can be monitored by processing the two spectroscopic signals from two detectors. In this paper, the algorithms to monitor the weld pool size and the focus shift are presented and the performances of weld pool size monitoring and auto-focus control are shown for a pulsed Nd:YAG laser welding. The size variation monitoring has been applied to the weld depth and weld defects monitoring. The effects of laser power change and weld defects on the weld pool size variation are also shown.
In the COXI (Combined Optical and X-ray Interferometer) system, optical and x-ray interferometers are combined to provide a means for the calibration of transducers with the traceability to the standards of length in the sub-nanometer region. The COXI mainly comprises a laser interferometer, an x-ray interferometer, and a precision translation stage. The laser interferometer used for the COXI instrument was a Michelson type, differential heterodyne interferometer having common optical path. A monolithic x-ray interferometer was made from a silicon single crystal. We have designed a control procedure to operate the COXI instrument for the calibration of nano-transducers and developed a phase demodulator for use with the laser interferometer. The bandwidth, phase resolution, and the measurement uncertainty of the interferometer were found 1 kHz, 0.01 degree, and 0.1 degree, respectively.
The thermal radiation from the weld-pool is measured at two wavelengths through the laser delivery fiber between Nd:YAG laser pulses. The chromatic aberration of delivery optics has been optimized to detect the variation of weld bead width. The design of optics and the signal processing algorithm for optical monitoring is described. Furthermore, the applications of optical monitoring to the detection of power variation and focus shift are shown.