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Wavelength – Frequency Measurements

Wavelength – Frequency Measurements. Frequency: - unit to be measured most accurately in physics - frequency counters + frequency combs (gear wheels) - clocks for time-frequency Wavelength: - no longer fashionable - unit [m] no longer directly defined

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Wavelength – Frequency Measurements

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  1. Wavelength – Frequency Measurements Frequency: - unit to be measured most accurately in physics - frequency counters + frequency combs (gear wheels) - clocks for time-frequency Wavelength: - no longer fashionable - unit [m] no longer directly defined - always problem of the medium- index of refraction Units: exercise convert - Ångstrom -> nm - eV, J, cm-1, Hz

  2. Notes Laser Spectroscopy, Vol 1 W. Demtröder Chapter 4 Spectroscopic instrumentation (4.1 – 4.4) Spectrographs, Monochromators, Prisms and Gratings Interferometers Fabry-Perot (etalon), Michelson, Mach-Zehnder Wave meters Michelson, Sigma, Fizeau, Fabry-Perot Chapter 9 Frequency measurement/Frequency comb (9.7)

  3. (Wave)Length standard Until 1960 Now, since 1983 A practical realisation of the metre is usually delineated (not defined) today in labs as 1,579,800.298728(39) wavelengths of helium-neon laser light in a vacuum.

  4. Time standard (and realization) the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom 10-13 accuracy Cs fountain clock

  5. Classical Spectrometers Spectral resolution limited by the diffraction determined by total aperture (size of prism or grating)  study this Prisms: how does the dispersion (resolution) depend on geometry/material of prism Grating: study the avantages of the “Echelle grating”

  6. Spectroscopy and Calibration Absolute measurements (wavelength or frequency) Relative measurements (line separations) Spectral referencing (use of atlases) I2, Te2, Hollow-cathode lamps, Th-Ar Scanning vs Multiplex spectroscopy

  7. Example: The Ultraviolet-Visibile Echelle grating spectrometer at VLT

  8. 8 m mirror at VLT

  9. Detection of all orders on the CCD

  10. Instant calibration of all orders by illuminating with ThAr lamp

  11. Science CCD’s Calibration CCD’s

  12. Calibration with an Echelle-grating spectrometer Neon lamp + Laser input

  13. Spectral referencing in laser spectroscopy H2O absorption

  14. Frequency conversion and calibration

  15. Referencing against tellurium 130Te2

  16. Precision Doppler Free Spectroscopy • Saturation spectroscopy – Lamb dips • Polarization spectroscopy • Two-photon spectroscopy • Molecular beam spectroscopy Bennet peak Burn a fraction out of the velocity distribution Bennet hole

  17. Lamb Dips Saturation holes Lamb dip in scanning Willis E Lamb Nobel Prize in Physics 1955

  18. Lamb Dips Saturation in Heterogeneous broadening Standing wave field Saturation in Heterogeneous case Weak field probing Saturation in Homogeneous broadening

  19. Lamb dip spectroscopy unraveling overlapping lines

  20. Saturated absorption for referencing + Phase sensitive detection lock-in principle

  21. Doppler-free two-photon absorption (excitation) Doppler shift: Resonance condition: All molecules, independent of their velocities, absorb at the sum frequency

  22. Sub-Doppler spectroscopy in a beam Reduction of the Doppler width:

  23. Molecular beam spectroscopy; Two-fold advantage: resolution + cooling

  24. Laser-based calibration techniques “Harmonics” + “saturation”

  25. Calibration of H2 spectral lines (in XUV) R(0) B-X (9,0) line P(3) C-X (1,0)

  26. Frequency measurements in the optical domain with a frequency comb laser • Principle: fn=f0 + n frep I frep=1/T f f0=frep/2 • Pulses in time generated by mode-locked laser • Frequency spectrum of discrete, regularly spaced sharp lines • Is it a pulsed or a CW laser ?

  27. Broadening the spectrum to “octave spanning” 1.7 mm 2f f 10 fs, 2 nJ in a 1.7 mm fused silica core holey fiber - self phase modulation - shockwave formation - Raman scattering, FWM ...

  28. Frequency measurements in the optical domain fn=f0 + n frep Measure wrep Feedback f:2f interferometer Measure wce Extend the spectrum to octave spanning

  29. Feedback to stabilize the laser on RF signals PID 15 MHz Rb atomic clock f - 2f CE-detection GPS correction 1:1012 10 GHz holey fiber temp. control PID piezo pump laser AOM 11 fs, 75 MHz Kerr-lens 11 fs Ti:Sapphire laser

  30. Using the FC-laser as an optical reference standard atomic clock VIS-NIR comb f0 fr Int. 0 frequency CW ultra- stable laser D f RF beat-note: result precision spectroscopy

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