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.
Refractive index fluctuations due to changing environmental conditions or due to air turbulence can significantly influence the measurement uncertainty of interferometric length measurements in air ambiance. A two wavelength interferometer is to some extend capable of measuring synchronously a path difference and the integral refractive index along this path difference. The best performance is achieved using two harmonically correlated optical fields like in a second harmonic interferometer. We developed a new type of a second harmonic two wavelength interferometer based on a double heterodyne interferometer with electronic frequency multiplication of one heterodyne frequency. The optical setup of the interferometer uses a measurement and a reference interferometer, both with spatially separated measurement and reference beam to avoid optical nonlinearities and reduce the influence of mechanical vibrations. The phase difference between measurement and reference interferometer is kept constant which reduces possible nonlinearities of the phase analysis. The system is capable of measuring the integral index of refraction without any effect modulation like e.g. a variation of the path difference which makes the design also applicable for absolute measuring interferometry. First experimental results will be presented
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.