Sinusoidal Wavelength-scanning Research Articles

Three-dimensional step–height measurement using sinusoidal wavelength scanning interferometer with four-step phase-shift method

A sinusoidal wavelength scanning interferometer with the four-step phase-shift method is demonstrated for a three-dimensional profile measurement. The interference phase-shift signal generated by the sinusoidal wavelength scanning contains information about optical path difference covering a nm-μm scale structure. The interference phase-shift signal was obtained by the four-step phase-shifting method. The sinusoidal wavelength shifting with a frequency of approximately 180 Hz and tunable range of 5.7 nm was performed by the Littman–Metcalf external resonator-type tunable laser with a center of 772.1 nm. The full-field surface profile measurement was conducted by a charge coupled device image sensor with an accuracy on a nm scale. The surface profiles of gauge blocks with a step height up to about 10 μm were successfully measured with a surface profile irregularity of 3.8×10−2 μm. The deviations of two measured surfaces of upper and lower steps were estimated to be 1.6×10−2 μm and 3.1×10−2 μm, respectively.

Extension of distance measurement range in a sinusoidal wavelength-scanning interferometer using a liquid-crystal wavelength filter with double feedback control

The optical path difference (OPD) and amplitude of a sinusoidal wavelength scanning (SWS) are controlled with a double feedback control system in an interferometer, so that a ruler marking every wavelength and a ruler with scales smaller than a wavelength are generated. These two rulers enable us to measure an OPD longer than a wavelength. A liquid-crystal Fabry-Perot interferometer (LC-FPI) is adopted as a wavelength-scanning device, and double sinusoidal phase modulation is incorporated in the SWS interferometer. Because of a high resolution of the LC-FPI, the upper limit of the measurement range can be extended to 280 microm by the use of the phase lock where the amplitude of the SWS is doubled in the feedback control. The ruler marking every wavelength is generated between 80 microm and 280 microm, and distances are measured with a high accuracy of the order of a nanometer in real time.

Thickness and Surface Profile Measurement by a Sinusoidal Wavelength-Scanning Interferometer

We propose a sinusoidal wavelength-scanning interferometer for measuring thickness and surface profile of a thin film. The interference signal contains phase modulation amplitude Z and phase $aL which are related to the positions and profiles of the reflecting surfaces, respectively. By reducing the difference between the detected signal and the estimated signal using the multidimensional nonlinear least-squares algorithm, we estimate values of Z and $aL. Experimental results show that the front and rear surfaces of a silica glass plate of 20 $mUm-thickness could be measured with an error less than 10 nm.

Real-time measurement of one-dimensional step profile with a sinusoidal wavelength-scanning interferometer using double feedback control

A linear CCD image sensor is used to electrically scan a measuring point and measure 1-D step-profiles in real time with a sinusoidal wavelength-scanning (SWS) interferometer using double feedback control. In this interferometer, the optical path difference (OPD) and the amplitude of the SWS are controlled so that a ruler marking every wavelength and a ruler with scales smaller than a wavelength are generated. These two rulers enable us to measure an OPD longer than a wavelength in real time. Two different step profiles with step heights of 1 and 20 μm, respectively, are measured with a measurement error of less than 8 nm. Measuring time for one measuring point is 0.04 s.

Sinusoidal wavelength-scanning interferometer with a superluminescent diode for step-profile measurement

In sinusoidal phase-modulating interferometry an optical path length (OPD) larger than a wavelength is measured by detection of sinusoidal phase-modulation amplitude Z(b) of the interference signal that is produced by sinusoidal scanning of the wavelength of a light source. A light source with a large scanning width of wavelength is created by use of a superluminescent laser diode for the error in the measured value obtained by Z(b) to be smaller than half of the central wavelength. In this situation the measured value can be combined with a fractional value of the OPD obtained from the conventional phase of the interference signal. A sinusoidal wavelength-scanning interferometer with the light source measures an OPD over a few tens of micrometers with a high accuracy of a few nanometers.