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Masters Course: Experimental Techniques Wavelength – Frequency Measurements Frequency: - unit to be measured most accurately in physics - frequency counters.

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Presentation on theme: "Masters Course: Experimental Techniques Wavelength – Frequency Measurements Frequency: - unit to be measured most accurately in physics - frequency counters."— Presentation transcript:

1 Masters Course: Experimental Techniques 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 Masters Course: Experimental Techniques 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 Masters Course: Experimental Techniques Until 1960 (Wave)Length standard Now, since 1983 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. 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 Masters Course: Experimental Techniques accuracy Time standard (and realization) Cs fountain clock 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

5 Masters Course: Experimental Techniques 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 Masters Course: Experimental Techniques Spectroscopy and Calibration Absolute measurements (wavelength or frequency) Relative measurements (line separations) Spectral referencing (use of atlases) I 2, Te 2, Hollow-cathode lamps, Th-Ar Scanning vs Multiplex spectroscopy

7 Masters Course: Experimental Techniques Example: The Ultraviolet-Visibile Echelle grating spectrometer at VLT

8 Masters Course: Experimental Techniques 8 m mirror at VLT

9 Masters Course: Experimental Techniques

10

11 Detection of all orders on the CCD

12 Masters Course: Experimental Techniques Instant calibration of all orders by illuminating with ThAr lamp

13 Masters Course: Experimental Techniques Science CCDsCalibration CCDs

14 Masters Course: Experimental Techniques Calibration with an Echelle-grating spectrometer Neon lamp + Laser input

15 Masters Course: Experimental Techniques Spectral referencing in laser spectroscopy H 2 O absorption

16 Masters Course: Experimental Techniques Frequency conversion and calibration

17 Masters Course: Experimental Techniques Referencing against tellurium 130Te2

18 Masters Course: Experimental Techniques Precision Doppler Free Spectroscopy 1.Saturation spectroscopy – Lamb dips 2.Polarization spectroscopy 3.Two-photon spectroscopy 4.Molecular beam spectroscopy Bennet hole Bennet peak Burn a fraction out of the velocity distribution

19 Masters Course: Experimental Techniques Lamb Dips Saturation holes Lamb dip in scanning Willis E Lamb Nobel Prize in Physics 1955

20 Masters Course: Experimental Techniques Lamb Dips Saturation in Homogeneous broadening Saturation in Heterogeneous broadening Standing wave field Saturation in Heterogeneous case Weak field probing

21 Masters Course: Experimental Techniques Lamb dip spectroscopy unraveling overlapping lines

22 Masters Course: Experimental Techniques Saturated absorption for referencing + Phase sensitive detection lock-in principle

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

24 Masters Course: Experimental Techniques Sub-Doppler spectroscopy in a beam Reduction of the Doppler width:

25 Masters Course: Experimental Techniques Molecular beam spectroscopy; Two-fold advantage: resolution + cooling

26 Masters Course: Experimental Techniques Laser-based calibration techniques Harmonics + saturation

27 Masters Course: Experimental Techniques P(3) C-X (1,0) R(0) B-X (9,0) line Calibration of H 2 spectral lines (in XUV)

28 Masters Course: Experimental Techniques Frequency measurements in the optical domain with a frequency comb laser Principle: Pulses in time generated by mode-locked laser Frequency spectrum of discrete, regularly spaced sharp lines Is it a pulsed or a CW laser ? f I f 0 =f rep /2 f rep =1/T f n =f 0 + n f rep

29 Masters Course: Experimental Techniques 10 fs, 2 nJ in a 1.7 m fused silica core holey fiber - self phase modulation - shockwave formation - Raman scattering, FWM m f2f Broadening the spectrum to octave spanning

30 Masters Course: Experimental Techniques Frequency measurements in the optical domain Measure rep Measure ce Feedback f:2f interferometer f n =f 0 + n f rep Extend the spectrum to octave spanning

31 Masters Course: Experimental Techniques AOM piezo pump laser Rb atomic clock 10 GHz 15 MHz holey fiber PID temp. control f - 2f CE-detection GPS correction 1:10 12 Kerr-lens 11 fs Ti:Sapphire laser 11 fs, 75 MHz Feedback to stabilize the laser on RF signals

32 Masters Course: Experimental Techniques frequency Int. 0 f0f0 frfr atomic clock CW ultra- stable laser precision spectroscopy f VIS-NIR comb RF beat-note: result Using the FC-laser as an optical reference standard


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