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.
Highlights
Confocal microscopes [1,2] are valuable instruments for non-contact surface profile measurement [3,4], and are employed in various scientific and industrial fields due to their principles that allow three-dimensional profile measurement in the ambient atmosphere
In a chromatic confocal probe, the axial chromatism is used as a space-coding method, in which a different wavelength is associated with each point of the optical axis to provide a mathematical relationship between the surface height and the wavelength focused on the surface [9]; namely, the height information can be obtained without scanning the target in the axial direction
Efforts have been made to improve theAt thermal stability of the chromatic first prototype laser chromatic confocal measurement setup have been verified confocal through probe a femtosecond mode-locked femtosecond laser source
Summary
Confocal microscopes [1,2] are valuable instruments for non-contact surface profile measurement [3,4], and are employed in various scientific and industrial fields due to their principles that allow three-dimensional profile measurement in the ambient atmosphere. In a chromatic confocal probe, the axial chromatism is used as a space-coding method, in which a different wavelength is associated with each point of the optical axis to provide a mathematical relationship between the surface height and the wavelength focused on the surface [9]; namely, the height information can be obtained without scanning the target in the axial direction Based on the analysis results, a second prototype femtosecond laser chromatic confocal probe is newly designed and constructed to improve the thermal stability for displacement measurement where the optical path length of the laser beam is minimized and a low thermal expansion material is employed for mechanical jigs in the optical setup. This paper is focused on the thermal stability of the chromatic confocal probe since the thermal stabilities of the sample, the sample jigs, and the mounting plate change when the probe is employed for different applications
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