Presentation on theme: "Wavelength – Frequency Measurements"— Presentation transcript:
1Wavelength – Frequency Measurements - unit to be measured most accurately in physics- frequency counters + frequency combs (gear wheels)- clocks for time-frequencyWavelength:- no longer fashionable- unit [m] no longer directly defined- always problem of the medium- index of refractionUnits: exercise convert- Ångstrom -> nm- eV, J, cm-1, Hz
2Notes Laser Spectroscopy, Vol 1 W. Demtröder Chapter 4 Spectroscopic instrumentation (4.1 – 4.4)Spectrographs, Monochromators, Prisms and GratingsInterferometersFabry-Perot (etalon), Michelson, Mach-ZehnderWave metersMichelson, Sigma, Fizeau, Fabry-PerotChapter 9 Frequency measurement/Frequency comb (9.7)
3International Standard of Length (Wave)Length standardKrypton (Kr):International Standard of LengthThe 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 1960Now, since 1983A 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.
4Time standard (and realization) the duration of 9,192,631,770 periodsof the radiation corresponding to thetransition between the two hyperfine levelsof the ground state of the cesium 133 atom10-13 accuracyCs fountain clock
5Classical Spectrometers Spectral resolutionlimited by the diffraction determined by total aperture(size of prism or grating) study thisPrisms:how does the dispersion (resolution) depend ongeometry/material of prismGrating:study the avantages of the “Echelle grating”
6Spectroscopy and Calibration Absolute measurements (wavelength or frequency)Relative measurements (line separations)Spectral referencing (use of atlases)I2, Te2, Hollow-cathode lamps, Th-ArScanning vs Multiplex spectroscopy
7Example: The Ultraviolet-Visibile Echelle grating spectrometer at VLT
27Calibration of H2 spectral lines (in XUV) R(0) B-X(9,0) lineP(3)C-X(1,0)
28Frequency measurements in the optical domain with a frequency comb laserPrinciple:fn=f0 + n frepIfrep=1/Tf0=frep /2fPulses in time generated by mode-locked laserFrequency spectrum of discrete, regularly spaced sharp linesIs it a pulsed or a CW laser ?
29Broadening the spectrum to “octave spanning” 1.7 mm2ff10 fs, 2 nJ in a 1.7 mmfused silica core holey fiber- self phase modulation- shockwave formation- Raman scattering, FWM ...
30Frequency measurements in the optical domain fn=f0 + n frepMeasure wrepFeedbackf:2f interferometerMeasure wceExtend the spectrum to octave spanning
31Feedback to stabilize the laser on RF signals PID15 MHzRb atomicclockf - 2fCE-detectionGPScorrection1:101210 GHzholeyfibertemp.controlPIDpumplaserpiezoAOM11 fs, 75 MHzKerr-lens 11 fsTi:Sapphire laser
32Using the FC-laser as an optical reference standard atomicclockVIS-NIR combf0frInt.frequencyCW ultra-stable laserD fRF beat-note: resultprecisionspectroscopy