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Wavelength – Frequency Measurements

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1 Wavelength – Frequency Measurements
- 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 International Standard of Length
(Wave)Length standard                                                                Krypton (Kr): International Standard of Length The picture shows a device holding a tube of krypton gas. The isotope Kr-86 contained in the tube can be excited so that it emits light. The international standard of length is one meter, which is 1,650, wavelengths of radiation emitted by Kr-86. Until 1960 Now, since 1983 A practical realisation of the metre is usually delineated (not defined) today in labs as 1,579, (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



11 Detection of all orders on the CCD

12 Instant calibration of all orders by illuminating with ThAr lamp

13 Science CCD’s Calibration CCD’s

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

15 Spectral referencing in laser spectroscopy
H2O absorption

16 Frequency conversion and calibration

17 Referencing against tellurium 130Te2

18 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

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

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

21 Lamb dip spectroscopy unraveling overlapping lines

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

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

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

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

26 Laser-based calibration techniques
“Harmonics” + “saturation”

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

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

29 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 ...

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

31 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 pump laser piezo AOM 11 fs, 75 MHz Kerr-lens 11 fs Ti:Sapphire laser

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

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