The refractive index of air can be directly measured by a refractometer or can be calculated by using updated Edlén's formulas by measuring the parameters of the surrounding air or by using the modified two-colour method. In this paper an overview on the three methods is given by taking into account for their features, uncertainties and application fields.
An overview is given on investigation activities of author’s researcher groups at the PTB, the National Metrology Institute of Germany, for testing, developing and applying of diode lasers to be used as a light source in interferometry. After a short background about laser emission’s principle and types of diode lasers their spectral properties are described in detail with taking their using in interferometry into account. Then different methods of frequency stabilization of diode lasers are briefly discussed. Lastly several examples for application of diode laser interferometers for length measurements are reported.
Demands on dimensional stability of 'high tech' materials relevant for semiconductor industry are growing
considerably. Information about long term stability of materials could be extracted from high resolution length
measurements performed within a relatively short time, e.g. using high finesse Fabry-Perot-resonators. However, the
length changes observed during the short-term measurements can be overlapped by additional length relaxations induced
by even small temperature changes before such measurement is started. This effect is reduced when long-term stability is
studied from length measurements repeated in a larger period of time. This paper describes absolute length
measurements with PTB's Precision Interferometer performed at four gauge block shaped material samples in order to
extract reliable information about their long term stability. The long-term stability was found to be dependent not only on
the material and its age itself but also on the material's history. The latter effect regards a one hour heating to 220°C
applied to one of two identical sample bodies made of glass-ceramics which is still visible in the measurement results of
long-term stability even after a period of almost seven years.
An approach to a homodyne absolute distance interferometer (ADI) was previously presented which makes use of two
extended cavity diode lasers (ECDL). The length measurement is performed by combining variable synthetic wavelength
interferometry and two wavelength interferometry in one setup. In this contribution the ADI was compared to a counting
HeNe laser interferometer up to a length of 10 m.
Form measurements of cylindrical objects are commonly done by mechanical sensing of the rotated specimen by a
stylus. The needed probing force could cause a deformation or an abrasion of the specimen. A new interferometric
measurement technique for form measurements of cylindrical objects with diameters between 0.1 and 2.5 mm is
presented. In this technique the specimen is measured contactless and no rotary table is needed. The specimen is placed
in the centre of an inverse conic mirror and is illuminated by an iodine-stabilized diode laser. The reflected light is
superposed under a slight angle with a reference beam and imaged on a CCD camera. The surface topography of the
specimen can be derived from the reconstructed spatial phase distribution, which is calculated by a spatial phase shifting
algorithm. In order to enhance the measurement range a second laser can be used to generate a synthetic wavelength.
This will allow the quantification of surface variations in the micrometer range with an aimed uncertainty of less than 0.1
&mgr;m. First results on phase measurements of different samples are presented and discussed.
Conventional laser interferometers offers a nanometer resolution but their result are ambiguous if distance variations of more than half a wavelength occur between two measured points. This a rather strong limitation for surface profilometry on surfaces with steps larger than this value. By using multiple wavelengths the accessible range of unambiguousness can be extended to half the result in synthetic wavelength. With three laser diodes emitting in the near IR synthetic wavelengths of approximately 15 micrometers and 290 micrometers could be achieved. This allows calculating the phase of the optical wavelength unequivocal within 145 micrometers . A nanometer resolution was reached with a phase interpolation of 1/100 of the optical wavelength. The laser beams are coupled into an interferometer through a single monomode fiber, and all interference signals are measured by one photo diode simultaneously. This leads to an easy alignment of the optical set-up and avoids the use of polarization optics and retardation plates. The injection currents of the laser diodes are modulated with different frequencies around 1MHz. Using lock-in amplifiers the three interference signals are separated electronically. The high modulation frequencies allow a fast measuring rate of up to 10 kHz. The sample surface as one mirror of the interferometer is scanned by moving the sample with mechanical translation stages in x- and y-direction. These mechanical stages exhibit unwanted vertical movement of up to 250 nm on a travel of several millimeter. By combining the mechanical stages with a piezo driven stage this vertical movement can be corrected resulting in a nanometer resolution in z-direction over a lateral range of several centimeters.
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