Lasers optical metrology

Schmitt R, Pavim A (2009) Flexible optical metrology strategies for the control and quality assurance of small series production. Proc SPIE 7389(Optical measurement systems for industrial inspection VI) 738902. doi 10.1117/12.827589 Schmitt R, Mallmann G, Peterka P (2011) Development of a FD-OCT for the inline process metrology in laser structuring systems. Proc SPIE 8082(Optical measurement systems for industrial inspection Vn) 808228. doi 10.1117/12.889365... 

R. Loser, S. Kyle, Alignment and field check procedures for the Leica Laser Tracker LTD 500, Boeing Large Scale Optical Metrology Seminar (1999). 

The absolute frequency of the fundamental IS — 2S transition in atomic hydrogen has now been measured to 1.8 parts in 1014, an improvement by a factor of 104 in the past twelve years. This improvement was made possible by a revolutionary new approach to optical frequency metrology with the regularly spaced frequency comb of a mode locked femto-second multiple pulsed laser broadened in a non-linear optical fiber. Optical frequency measurement and coherent mixing experiments have now superseded microwave determination of the 2S Lamb shift and have led to improved values of the fundamental constants, tests of the time variation of the fine structure constant, tests of cosmological variability of the electron-to-proton mass ratio and tests of QED by measurement of g — 2 for the electron and muon. 

The cornerstone of the recent optical frequency measurements in Paris is the LD/Rb standard laser [51,52,53]. Three identical systems have been built, two at the LPTF and a third in Laboratoire Kastler Brossel. As the two laboratories are linked by two 3 km long optical fibers, it is possible to compare the frequencies of the three systems. The frequency shift due to the fiber has been checked with the highly stabilized titanium-sapphire laser. After a round trip of 6 km through the fibers, a maximum frequency shift of 3 Hz is observed [54]. This shift is completely negligible for the optical frequency measurements. The main metrological features of the LD/Rb laser are a frequency stability (Allan variance) of about 4 X 10-13t-1/2 per laser over 1000 s and a day-to-day repeatability of 400 Hz. 

In this case the electric field would be repetitive with the round trip time. Therefore C(t) is a constant and its Fourier transform is a delta function centered as uc = 0. If it becomes possible to build a laser able to produce a stable pulse train of that kind, all the comb frequencies would become exact harmonics of the pulse repetition rate. Obviously, this would be an ideal situation for optical frequency metrology. 

We summarize hoe some considerations affecting the potential accuracy of a spectroscopic measurement of the IS ->2S transition. We shall not dwell on the challenging problems of laser stabilization and optical frequency metrology, but only on the atomic considerations. In particular, we shall consider the major sources of line broadening and possible systematic shifts. We discuss below some of the factors which govern the accuracy of IS —>2S spectroscopy in the hydrogen trap. 

Despite all of the above-mentioned limitations in accuracy of optical interferometry, it is still widely used in the determination of the wave-numbers of atomic transitions, since optical frequency metrology (synthesis chains, optical frequency combs, etc, 4) does not yet have the wide spectral coverage provided by the broad-band interferometers. As an example, a recent absolute wave-number determination of the Cs D2 resonance line at 852 nm is with a Fabry-Perot interferometer, saturated absorption and a grating-eavity semiconductor laser [76]. These results are of interest to various Cs atomic fountain measurements and lead to better determinations of fundamental constants, such as h/mp and a, [77] as well as of the acceleration due to gravity, g [78,79]. 

Riehle F, Schnatz H, Lipphart B, Zinner G, Kersten P and Helmcke J 1996 "Optical frequency standard based on laser-cooled Ca atoms , Proc. Symposium on Frequency Standards and Metrology, Woods Hole, MA (USA), (World Scientific Publishing, ed. J. C. Bergquist), pp.277 - 82... 

Beeck, M.-A., and Hentschel, W. "Laser Metrology - A Diagnostic Tool in Automotive Development Processes." Optics and Lasers in Engineering 34 (2000) 101-120. 

In the field of metrology a big step forward was the use of frequency combs from cw mode-locked femtosecond lasers. It is now possible to directly compare the microwave frequency of the cesium clock with optical frequencies, and it turns out that the stability and the absolute accuracy of frequency measurements in the optical range using frequency-stabilized lasers greatly surpasses that of the cesium clock. Such frequency combs also allow the synchronization of two independent femtosecond lasers. 

This paper reports a sensitive optical interferometric technique dual frequency modulation (DFM) for measuring and stabilizing a laser frequency by comparison, in a single step, to a radio frequency (rf) standard. Conversely, a low-noise rf source can be stabilized by a laser frequency reference. A prototype has demonstrated a resolution of 2 parts in 10 , but devices currently under development should have a resolution of 10 and an absolute accuracy of 10" . The method may be competitive with the optical frequency synthesis chain in accuracy and its simplicity suggests its convenient use in metrology, high-precision optical spectroscopy, and gravity wave detection. 

Cuypers W, Van Gestel N, Voet A, Kruth J-P, Mingneau J, Bleys P (2009) Optical measurement techniques for mobile and large-scale dimensional metrology. Opt Lasers Eng 47 (3) 292-300... 

The vastly increasing number of applications of laser spectroscopy in basic sciences such as metrology. Chemistry, Biology, Medicine, and for the solution of technical problems will be illustrated by several examples. Even in astronomy lasers have bronght about new techniques such as adaptive optics which increase the angular resolution of optical telescopes by several orders of magnitude or the... 

To relate optical frequencies to the cesium time standard remains a very important issue in precision metrology. In this context the development of the opticai-frequency-comb technique using mode-locked lasers has been very important [9.400]. 

For a number of purposes, the accuracy obtainable by the interferometric measurement of wavelength is not adequate. The most obvious of these purposes is molecular spectroscopy of the lasing molecule itself, which was discussed in Sect. 2. When used as a local oscillator in an astronomical receiver one would also like to know the laser frequency to within a few megahertz so as to know the radial velocity of the observed objects to within a few km/s. In metrology too, where the laser might be used in a chain to link microwave measurements with those made in the optical, high precision is necessary. For such purposes heterodyne measurements, which yield the frequency directly, are to be preferred, and these are now discussed. 

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