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