Osami Sasaki

Sinusoidal phase modulating interferometry for surface profile measurement

Interferometre a modulation de phase sinusoidale dans lequel une onde de reference est facilement modulee en phase avec un miroir vibrant

Sinusoidal phase modulating laser diode interferometer with feedback control system to eliminate external disturbance

We propose a sinusoidal phase modulating laser diode interferometer that is insensitive to vibrations of the optical components and fluctuations in the optical wavelength of the laser diode. These external disturbances cause fluctuations in the phase of the interference signal. After we analyze the sinusoidal phase modulation in a laser diode interferometer, we describe the method of the feedback control of the injection current of the laser diode to eliminate the phase fluctuations of the interference signal. We construct two sinusoidal phase modulating interferometers for movement measurements and surface profile measurements, respectively. The experimental results make it clear that the interferometers work well in mechanically noisy circumstances.

Analysis of measurement accuracy in sinusoidal phase modulating interferometry.

A theoretical analysis of the measurement accuracy in sinusoidal phase modulating (SPM) interferometry is presented. The measurement accuracy is dependent on multiplicative and additive noise. The characteristics of SPM interferometry in the presence of this noise are made clear. Theoretical results show clearly that SPM interferometry has a high measurement accuracy of the order of 1 nm.

Sinusoidal phase modulating interferometer using the integrating-bucket method

We describe a sinusoidal phase modulating interferometer in which a CCD image sensor detects four values by integrating the time-varying intensity in an interference pattern for intervals of one-quarter period of the phase modulation. The optimum amplitude and phase of the sinusoidal phase modulation are determined. The measurement error caused by the additive noise and the deviation from the optimum phase modulation is analyzed. The experimental results for surface profiles of magnetic sliders show that the sinusoidal phase modulating interferometer proposed here yields a measurement accuracy of the order of 1 nm.

Phase locked laser diode interferometry for surface profile measurement

We propose a new type phase locked interferometer which uses tunability of the wavelength of a laser diode. The phase lock is achieved by controlling the injection current of the laser diode. A CCD image sensor is used as a photodetector to scan electrically a measuring point along the surface of the object. Since this interferometer uses no mechanical elements such as a piezoelectric transducer and galvanomirror, the measurement accuracy is not limited by the mechanical properties. The characteristics of the feedback control system for the phase lock are examined through measurements of surface profiles of the diamond-turned aluminum disks.

Modal wavefront reconstruction based on Zernike polynomials for lateral shearing interferometry: comparisons of existing algorithms

Four modal methods of reconstructing a wavefront from its difference fronts based on Zernike polynomials in lateral shearing interferometry are currently available, namely the Rimmer-Wyant method, elliptical orthogonal transformation, numerical orthogonal transformation, and difference Zernike polynomial fitting. The present study compared these four methods by theoretical analysis and numerical experiments. The results show that the difference Zernike polynomial fitting method is superior to the three other methods due to its high accuracy, easy implementation, easy extension to any high order, and applicability to the reconstruction of a wavefront on an aperture of arbitrary shape. Thus, this method is recommended for use in lateral shearing interferometry for wavefront reconstruction.

Sinusoidal phase modulating interferometer using optical fibers for displacement measurement.

We propose a sinusoidal phase modulating (SPM) interferometer for displacement measurement. The displacement is obtained by processing the detected signal with a microcomputer. The optical configuration of the SPM interferometer is simple because an optical fiber is effectively used to produce sinusoidal phase modulation. The movements of a piezoelectric transducer and the surface profile of a diamond-turned aluminum disk are measured with high accuracy (a few nanometers).

Two-wavelength sinusoidal phase/modulating laser-diode interferometer insensitive to external disturbances

We describe a two-wavelength laser-diode interferometer that is insensitive to external disturbances such as fluctuations in the wavelength of the laser diode and mechanical vibrations of the optical components. In sinusoidal phase-modulating interferometry this insensitivity is easily obtained by controlling the injection current of the laser diode with a feedback control system. Using an equivalent wavelength of 152 microm provided by two single-frequency laser diodes, we can measure the distance, rotation angle, and surface profile measurements with great accuracy.

Double sinusoidal phase-modulating laser diode interferometer for distance measurement

Sinusoidal phase-modulating interferometry is used to detect exactly the sinusoidal phase variation of an interference signal even when the amplitude of the interference signal is varied by modulation of the injection current. We can easily provide a sinusoidal phase-modulating interferometer with a feedback control system that eliminates the phase fluctuations caused by mechanical vibrations. The methods using sinusoidal phase-modulating interferometry improve the resolution of distance measurements. Experimental results show that the thickness of gauge blocks is measured with a resolution of approximately 0.5 microm.

Sinusoidal wavelength-scanning interferometer using an acousto-optic tunable filter for measurement of thickness and surface profile of a thin film

A sinusoidal wavelength-scanning interferometer for measuring thickness and surfaces profiles of a thin film has been proposed in which a superluminescent laser diode and an acousto-optic tunable filter are used. The interference signal contains an amplitude Zb of a time-varying phase and a constant phase alpha. Two values of an optical path difference (OPD) obtained from Zb and alpha, respectively, are combined to measure an OPD longer than a wavelength. The values of Zb and alpha are estimated by minimizing the difference between the detected signals and theoretical ones. From the estimated values, thickness and surface of a silicon dioxide film coated on an IC wafer with different thicknesses of 1 mum and 4 mum are measured with an error less than 5 nm.