Wavelength-tuning Interferometer Research Articles

Non-contact method of thickness measurement for thin-walled rotary shell parts based on chromatic confocal sensor

In this paper, a non-contact method of thickness measurement for thin-walled rotary shell parts based on chromatic confocal sensor is presented. To obtain thickness, a flip method is applied to acquire surface profiles from both sides of the workpiece. The decentration and tilt errors of the workpiece are measured by a centering system consisting of a laser interferometer and a collimated tube. By establishing a unified reference coordinate system, the external and internal surface profiles are reconstructed and the thickness is calculated. A measurement experiment for a sapphire spherical shell workpiece was conducted on a four-axis measurement system and the result is 2.99341 mm. A wavelength-tuning interferometer and a dynamic interferometer were utilized to measure the sapphire spherical shell and an aluminum concave sphere and the results show in good agreement.

Precise Measurement of the Surface Shape of Silicon Wafer by Using a New Phase-Shifting Algorithm and Wavelength-Tuning Interferometer

In wavelength-tuning interferometry, the surface profile of the optical component is a key evaluation index. However, the systematic errors caused by the coupling error between the higher harmonics and phase shift error are considerable. In this research, a new 10N − 9 phase-shifting algorithm comprising a new polynomial window function and a DFT is developed. A new polynomial window function is developed based on characteristic polynomial theory. The characteristic of the new 10N − 9 algorithm is represented in the frequency domain by Fourier description. The phase error of the new algorithm is also discussed and compared with other phase-shifting algorithms. The surface profile of a silicon wafer was measured by using the 10N − 9 algorithm and a wavelength-tuning interferometer. The repeatability measurement error across 20 experiments was 2.045 nm, which indicates that the new 10N − 9 algorithm outperforms the conventional phase-shifting algorithm.

A Real-Time Feedback System to Stabilize Laser Intensity on Wavelength Modulation Interferometer

The phase of an interferogram carries the desired information for various applications such as 3-D profilometry. It can be obtained with a wavelength tuning interferometer by acquiring a series of interferograms while changing the laser wavelength. The phase achieved by the interferometer, however, has severe error due to the laser output power change, which is produced inherently from changing the laser wavelength. To eliminate such phase error, we propose a real-time closed-loop feedback control system to keep the laser output power stable during the wavelength tuning. The control system integrates a photodetector, a data acquisition card, and an electro-optic amplitude modulator with our software to achieve the purpose. The experimental result of the system can steadily output a desired intensity value and the measurement speed is up to 600 KHz. Both its control principles as well as its experimental validation are depicted in this letter.

Simultaneous measurement of surface shape and optical thickness using wavelength tuning and a polynomial window function.

In this study, a 6N - 5 phase shifting algorithm comprising a polynomial window function and discrete Fourier transform is developed for the simultaneous measurement of the surface shape and optical thickness of a transparent plate with suppression of the coupling errors between the higher harmonics and phase shift error. The characteristics of the 6N - 5 algorithm were estimated by connection with the Fourier representation in the frequency domain. The phase error of the measurements performed using the 6N - 5 algorithm is discussed and compared with those of measurements obtained using other algorithms. Finally, the surface shape and optical thickness of a transparent plate were measured simultaneously using the 6N - 5 algorithm and a wavelength tuning interferometer.

Multiple-surface interferometry of highly reflective wafer by wavelength tuning.

The surface shape and optical thickness variation of a lithium niobate (LNB) wafer were measured simultaneously using a wavelength-tuning interferometer with a new phase-shifting algorithm. It is necessary to suppress the harmonic signals for testing a highly reflective sample such as a crystal wafer. The LNB wafer subjected to polishing, which is in optical contact with a fused-silica (FS) supporting plate, generates six different overlapping interference fringes. The reflectivity of the wafer is typically 15%, yielding significant harmonic signals. The new algorithm can flexibly select the phase-shift interval and effectively suppress the harmonic signals and crosstalk. Experimental results indicated that the optical thickness variation of the LNB wafer was measured with an accuracy of 2 nm.

Optical thickness measurement of mask blank glass plate by the excess fraction method using a wavelength-tuning interferometer

The absolute optical thickness of a 140-mm2 mask blank glass plate 3.1mm thickness was measured by three-surface interferometry using a wavelength-tuning Fizeau interferometer. The interference order was determined by the excess fraction method. The wavelength of a tunable laser diode was scanned linearly from 632 to 642nm, and a CCD detector recorded 2000 interference images. Two kinds of optical thicknesses measured by discrete Fourier analysis and phase-shifting were synthesized to obtain the optical thickness with respect to the ordinary refractive index. The optical thickness defined by the group refractive index at the 637nm central wavelength was measured by wavelength scanning. The optical thickness deviation defined by the ordinary refractive index was measured using tunable phase-shifting. The systematic errors caused by nonlinearity in the wavelength tuning were corrected through correlation analysis between the theoretical and observed interference fringes.