MIKES fibre-coupled differential dynamic line scale interferometer

Abstract

An instrument developed for high accuracy calibration of line scales up to 1.16 m is described. The instrument is based on an earlier design from the early 1990s. Since then, in order to improve performance and achieve smaller uncertainty, large portions of software, mechanics and optics have been redesigned and several uncertainty components better characterized. The software has been developed to be less sensitive to imperfections of the line mark. In order to decrease noise and deformation coupling in interferometric measurement, the interferometer operating principle has been designed to use a fibre-coupled laser light, interferometer optics have been improved for better phase adjustment and detection and a differential interferometer principle has been taken into use. In order to minimize the effects due to deformations of the stone table rail with moving heavy carriage, a separate bed has been designed under line scale supports. The bed with fixed differential reflector prevents deformations of the table from affecting the line scale versus interferometer position. The main properties of the instrument are described and an uncertainty estimate is presented. The expanded uncertainty (k = 2) for line distance calibration of high-quality low-thermal-expansion line scale is U = [(4.5 nm)2 + (43 × 10−9 L)2]½, where L is the measured length. The results of international comparisons support the uncertainty estimate.

High accuracy step gauge interferometer

“…The sensors and the temperature meter are calibrated onsite in a water bath at 20 °C against two calibrated Pt25 standard resistance platinum thermometers. The self-heating of the sensors with a measuring current of 0.5 mA has been determined to be 5 mK [11]. Typical temperature gradients in air between the air temperature sensors are well within ±30 mK, see figure 9.…”

“…To reach target uncertainty, all typical major error sources of dimensional metrology interferometers should be properly eliminated or minimized. These sources include the Abbe error, cosine error, refractive index of medium compensation, thermal expansion compensation, dead path error and drift related errors [11]. Drift minimization requires careful design of the metrology loops.…”

A review of state-of-the-art 1D length scale calibration instruments

“…This system also features an optical probing system which uses a microscope and CCD camera for measurement of line scales. Similar optical systems are used by the MIKES fibrecoupled differential dynamic line scale interferometer [71], the PTB nanometer comparator [37] and the large majority of participants in the EURAMET.L-K7.2006 line scale compariso n exercise (the exceptions either used a microscope centring by eye rather than by electrical means or did not report their method) [72]. At present no alternative methods for measuring line scales appear to be in use for primary calibration systems at the highest levels of performance.…”

Automatic high resolution measurement set-up for calibrating precise line scales

“…They can be calibrated by using different measurement set-ups, depending on their length and precision. Set-ups for calibrating high-precision line scales normally involve a microscope with an optical sensor for capturing and analysing the image of a line marker and a laser interferometer as a traceable measurement standard [4][5][6].…”