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

Masters Course: Experimental Techniques

Notes

Laser Spectroscopy, Vol 1W. Demtröder

Chapter 4 Spectroscopic instrumentation (4.1 – 4.4)

Spectrographs, Monochromators, Prisms and Gratings

InterferometersFabry-Perot (etalon), Michelson, Mach-Zehnder

Wave metersMichelson, Sigma, Fizeau, Fabry-Perot

Chapter 9 Frequency measurement/Frequency comb (9.7)

Masters Course: Experimental Techniques

Until 1960

(Wave)Length standard

smc 299792458

tcL 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 1,650,763.73 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.

Masters Course: Experimental Techniques

10-13 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

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 ongeometry/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)I2, Te2, Hollow-cathode lamps, Th-Ar

Scanning vs Multiplex spectroscopy

Masters Course: Experimental Techniques

Example: The Ultraviolet-Visibile Echelle grating spectrometer at VLT

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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 ThAr lamp

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Science CCD’s Calibration CCD’s

Masters Course: Experimental Techniques

Calibration with an Echelle-grating spectrometer

Neon lamp + Laser input sinsin Dm

Masters Course: Experimental Techniques

Spectral referencing in laser spectroscopy

H2Oabsorption

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Frequency conversion and calibration

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Referencing against tellurium 130Te2

Masters Course: Experimental Techniques

Precision Doppler Free Spectroscopy

1. Saturation spectroscopy – Lamb dips2. Polarization spectroscopy3. Two-photon spectroscopy4. Molecular beam spectroscopy

Bennet hole

Bennet peak

Burn a fraction out of the velocity distribution

Masters Course: Experimental Techniques

Lamb Dips

Saturation holesLamb dip in scanning

Willis E LambNobel Prize in Physics 1955

Masters Course: Experimental Techniques

Lamb Dips

Saturation inHomogeneous broadening

Saturation inHeterogeneous broadening

Standing wave field

Saturation inHeterogeneous case

Weak fieldprobing

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Lamb dip spectroscopy unraveling overlapping lines

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Saturated absorption for referencing

+ Phase sensitive detection lock-in principle

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Doppler-free two-photon absorption (excitation)

Doppler shift: vk

'

Resonance condition: 212121 ''/ kkvEE if

All molecules, independent of their velocities, absorb at the sum frequency

221

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Sub-Doppler spectroscopy in a beam

xkvvk 00'

Reduction of the Doppler width:

sin' DD 2ln2 0 c

vpD

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

Masters Course: Experimental Techniques

Laser-based calibration techniques

“Harmonics” + “saturation”

Masters Course: Experimental Techniques

P(3) C-X(1,0)

R(0) B-X(9,0) line

Calibration of H2 spectral lines (in XUV)

Masters Course: Experimental Techniques

Frequency measurements in the optical domainwith 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

f0=frep /2

frep=1/T

fn=f0 + n frep

Masters Course: Experimental Techniques

10 fs, 2 nJ in a 1.7 mfused silica core holey fiber

- self phase modulation- shockwave formation- Raman scattering, FWM ...

1.7 m

f2f

Broadening the spectrum to “octave spanning”

Masters Course: Experimental Techniques

Frequency measurements in the optical domain

Measure rep

Measure ce

Feedback

f:2f interferometer

fn=f0 + n frep

Extend the spectrum to octave spanning

Masters Course: Experimental Techniques

AOM

piezopump laser

Rb atomicclock

10 GHz

15 MHz

holey fiber

PID

PIDtemp.control

f - 2fCE-detection

GPScorrection1:1012

Kerr-lens 11 fsTi:Sapphire laser

11 fs, 75 MHz

Feedback to stabilize the laser on RF signals

Masters Course: Experimental Techniques

frequency

Int.

0

f0 fr

atomic clock

CW ultra-stable laser

precisionspectroscop

y

f

VIS-NIR comb

RF beat-note: result

0fnfff represult

Using the FC-laser as an optical reference standard