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Abstract
A mode-locked femtosecond laser, which is often referred to as the optical frequency comb, has increasing applications in various industrial fields, including production engineering, in the last two decades. Many efforts have been made so far to apply the mode-locked femtosecond laser to the absolute distance measurement. In recent years, a mode-locked femtosecond laser has increasing application in angle measurement, where the unique characteristics of the mode-locked femtosecond laser such as the stable optical frequencies, equally-spaced modes in frequency domain, and the ultra-short pulse trains with a high peak power are utilized to achieve precision and stable angle measurement. In this review article, some of the optical angle sensor techniques based on the mode-locked femtosecond laser are introduced. First, the angle scale comb, which can be generated by combining the dispersive characteristic of a scale grating and the discretized modes in a mode-locked femtosecond laser, is introduced. Some of the mode-locked femtosecond laser autocollimators, which have been realized by combining the concept of the angle scale comb with the laser autocollimation, are also explained. Angle measurement techniques based on the absolute distance measurements, lateral chromatic aberration, and second harmonic generation (SHG) are also introduced.
Highlights
The angle and length are among the most fundamental parameters that determine the form of an object [1,2]
A limited result over an angle range of approximately 400 arc-seconds is indicated in the figure, each of the comb modes is verified to be identified over an angular range of approximately 6°, which is limited by the spectral width of the mode-locked femtosecond laser, as well as the resolving power of the diffraction grating employed in the setup
[73]. aberration, When a collimated displacement anincident object with thevia enhancement of the laser autocollimation femtosecond is the made incident a simple lens in such way that the displacement laser axis is aligned to be Figure 11laser shows optical setuptofor measurement of theasmall angular of an object coaxialonwith respect to aberrations the optical axis of the lens, the optical modes inofthe laser are based the chromatic of a simple lens, where the principle thefemtosecond laser autocollimation focused at different points on the optical axis of the lens
Summary
The angle and length are among the most fundamental parameters that determine the form of an object [1,2]. For measurement of the angular displacement or angular position of an object with a fixed axis of rotation, rotary encoders are often employed. Rotary encoders are capable of carrying out high-precision angle measurement over an angular range of 360◦ and can be employed as a feedback sensor for the control of the axial position of an object. Optical fiber gyros are not appropriate for applications in production engineering, such as the evaluation of the angular error motion of a precision linear slide or the measurement of the freeform or the aspheric form of an optical component, due to their drift characteristics associated with their principle based on the detection of angular velocity. Angle Measurement Methods Based on the Discrete Modes of a Mode-Locked Femtosecond
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