Investigation and Improvement of Thermal Stability of a Chromatic Confocal Probe with a Mode-Locked Femtosecond Laser Source

Abstract

An intentional investigation on the thermal stability of a mode-locked femtosecond laser chromatic confocal probe, which is a critical issue for the probe to be applied for long-term displacement measurement or surface profile measurement requiring long-time scanning, is carried out. At first, the thermal instability of the first prototype measurement setup is evaluated in experiments where the existence of a considerably large thermal instability is confirmed. Then the possible reasons for the thermal instability of the measurement setup are analyzed quantitatively, such as the thermal instability of the refractive index of the confocal lens and the thermal expansion of mechanical jigs employed in the probe. It is verified that most of the thermal instability of the measurement setup is caused by the thermal expansion of mechanical jigs in the probe. For the improvement of the thermal stability of the probe, it is necessary to employ a low thermal expansion material for the mechanical jigs in the measurement setup and to shorten the optical path length of the laser beam. Based on the analysis result, a second prototype probe is newly designed and constructed. The improved thermal stability of the second prototype probe is verified through theoretical calculations and experiments.

A new signal processing method for a differential chromatic confocal probe with a mode-locked femtosecond laser

“…Major components employed in the setup were the same as those employed in the conventional setup [35]. Meanwhile, efforts were made in this paper to reduce the optical path length in the chromatic confocal probe so that the thermal stability of the system could be improved [35,37]. The overall size of the modified setup was reduced to be 200 mm × 250 mm (in the X-Z plane), which was approximately one-third of that of the conventional one (350 mm × 480 mm) while realizing easier alignment of each of the optical components in the system.…”

“…A chromatic probe using an ultrashort pulse laser with high coherence and good beam quality is expected to have a higher resolution than that employing an incoherent chromatic light source, because a coherent light has better focusing properties. The authors have developed a fibre-based dual-detector differential chromatic confocal probe with a mode-locked femtosecond laser [35][36][37]. Meanwhile, a differential algorithm is often employed in a confocal probe to obtain a normalized output for removing the influences of intensity variations and/or the spectral non-smoothness of the light source, as well as the effect of the wavelength dependence of reflectivity of optical components and measurement object.…”

An insight on optical metrology in manufacturing

“…Pulse trains in the reflected measurement beam and those in the reflected reference beam are detected by a balanced cross-correlator using second-harmonic generation (SHG) by a nonlinear optical crystal to obtain the absolute distance to the target reflector [81]. In the proposed method, absolute distance measurement with a submicrometre precision has been achieved at a working distance of 700 m. Figure 12 shows another example of the measurement of the absolute position of a target using a mode-locked femtosecond laser source in a chromatic confocal probe [83][84][85][86]. A mode-locked femtosecond laser beam from a fiber-based laser source has a non-uniform spectrum; this characteristic of the laser source strongly affects the axial response curve in a confocal probe for absolute position measurement.…”

A High-Precision and Wide Range Method for Inner Diameter Measurement

“…Because of the advantages of assuring high-precision measurements, the use of dispersive confocal methods to tiny displacement and morphological studies has recently become a research hotspot. The measuring point does not change when the item is slanted or twisted, and there is no aberration interference [17]. Wang et al [18] proposed employing confocal methods to estimate the radius of unpolished spheres with less than 20 ppm measurement error.…”