Wavelength Frequency Measurements Frequency unit to be measured

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 - e. V, J, cm-1, Hz 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) Masters Course: Experimental Techniques

(Wave)Length standard Krypton (Kr): International Standard of Length Until 1960 Now, since 1983 Masters Course: Experimental Techniques 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, 763. 73 wavelengths of radiation emitted by Kr-86. 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.

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 Masters Course: Experimental Techniques Cs fountain clock

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” 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 Masters Course: Experimental Techniques

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

8 m mirror at VLT Masters Course: Experimental Techniques

Masters Course: Experimental Techniques

Masters Course: Experimental Techniques

Detection of all orders on the CCD Masters Course: Experimental Techniques

Instant calibration of all orders by illuminating with Th. Ar lamp Masters Course: Experimental Techniques

Science CCD’s Masters Course: Experimental Techniques Calibration CCD’s

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

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

Frequency conversion and calibration Masters Course: Experimental Techniques

Referencing against tellurium 130 Te 2 Masters Course: Experimental Techniques

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

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

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

Lamb dip spectroscopy unraveling overlapping lines Masters Course: Experimental Techniques

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

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

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

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

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

Frequency measurements in the optical domain with a frequency comb laser • Principle: fn=f 0 + n frep I frep=1/T f 0=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 ? Masters Course: Experimental Techniques f

Broadening the spectrum to “octave spanning” 1. 7 mm 2 f 10 fs, 2 n. J in a 1. 7 mm fused silica core holey fiber - self phase modulation - shockwave formation - Raman scattering, FWM. . . Masters Course: Experimental Techniques f

Frequency measurements in the optical domain fn=f 0 + n frep Measure wrep Feedback f: 2 f interferometer Extend the spectrum to octave spanning Masters Course: Experimental Techniques Measure wce

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

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