TULIP Summer School IV  TULIP Summer School IV  Noordwijk Noordwijk  April   April    Wim Wim Ubachs Ubachs Optical frequency combs and frequency comb spectroscopy J
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TULIP Summer School IV TULIP Summer School IV Noordwijk Noordwijk April April Wim Wim Ubachs Ubachs Optical frequency combs and frequency comb spectroscopy J

Hall TW Hnsch for their contributions to the development of laserbased precision spectroscopy including the optical frequency comb technique Frequency Combs A revolution in measuring Nobel 2005 brPage 2br On Pulsed and Continuous wave lasers Conside

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TULIP Summer School IV TULIP Summer School IV Noordwijk Noordwijk April April Wim Wim Ubachs Ubachs Optical frequency combs and frequency comb spectroscopy J




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Presentation on theme: "TULIP Summer School IV TULIP Summer School IV Noordwijk Noordwijk April April Wim Wim Ubachs Ubachs Optical frequency combs and frequency comb spectroscopy J"— Presentation transcript:


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TULIP Summer School IV 2009 TULIP Summer School IV 2009 Noordwijk Noordwijk , April 15 , April 15 18 18 Wim Wim Ubachs Ubachs Optical frequency combs and frequency comb spectroscopy J. Hall T.W. Hnsch for their contributions to the development of laser-based precision spectroscopy including the optical frequency comb technique Frequency Combs: A revolution in measuring Nobel 2005
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On Pulsed and Continuous wave lasers Consider from time and frequency domain perspectives
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Modelocking Modelocking a laser a laser Basic idea: Basic idea: build a

laser cavity that is low build a laser cavity that is low loss for intense pulses, loss for intense pulses, but high but high loss for low loss for low intensity continuous beam intensity continuous beam Solutions: Solutions: Intracavity Intracavity saturable saturable absorber, or Kerr absorber, or Kerr lensing lensing
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A laser running on multiple modes: a pulsed laser SUM And so forth: add 30 waves:
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Ultrafast lasers Pulsing back and forth inside the cavity
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Ultrafast lasers
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Fourier principle for short pulses
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Frequency comb principle Frequency
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Some math: Propagation of a single pulse (described as a wave packet) ik Insert an inverse Fourier transform E( ) for E( ZW ik Propagator Propagation of the field This can be used with exp[ dk Difference between group and phase velocity causes an extra phase When traveling through dispersive medium The carrier/envelop phase continuously changes
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Some math: Propagation of a multiple pulses in a train single nT T is time delay between pulses iN in single single train sin sin single train single train, In the limit With

dispersion single train, CE Phase shift
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Frequency comb principle 'M Frequency ceo = S ceo = ceo = 2 RF frequencies determine the entire optical spectrum! f = n f rep + f ceo tested to <10 -19 level
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Both f rep and f ceo are in the radio-frequency domain can be detected using RF electronics. Measuring f rep is straightforward: Counting Stabilization of Stabilization of rep rep
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Measuring f ceo is more difficult, requires production of a beat signal between a high-f comb mo de and the SHG of a low-f comb mode. f:2f interferometer Detection of

Detection of ceo ceo
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Supercontinuum Supercontinuum generation generation This f This f to to 2f detection scheme requires an octave 2f detection scheme requires an octave wide spectrum wide spectrum spectral broadening in nonlinear medium spectral broadening in nonlinear medium Photonic crystal fiber: Photonic crystal fiber:
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Detection of Detection of ceo ceo f : 2f Beat-note measurement (frequency counter)
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Stabilization of Stabilization of ceo ceo The f The f to to 2f interferometer output is used in a feedback loop. 2f interferometer

output is used in a feedback loop. An AOM controls the pump power to stabilize An AOM controls the pump power to stabilize ceo ceo
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Scanning of Scanning of rep rep Linear cavity required for long Linear cavity required for long range scanning range scanning Multiple reflections on single mirror to increase scan range Multiple reflections on single mirror to increase scan range Scan range determined by: Cavity stability range Alignment sensitivity
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A frequency comb as a calibration tool for spectroscopy laser The frequency of a laser can directly be

determined The frequency of a laser can directly be determined by beating it with the nearest frequency comb mode: by beating it with the nearest frequency comb mode: laser = n f rep + f ceo + f beat Cf: Hnsch and co-workers: atomic hydrogen
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Direct frequency comb spectroscopy Time-domain Ramsey spectroscopcy Full control over pulse timing required Cf : Ramsey spectroscopy Atomic fountain clocks
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QM analysis of pulse sequences Wave function of two-level atom: From Schrdinger equation, and some approximat ions (dipole, rotating wave) the upper state density

can be calculated for two-pulse sequence: T is time between pulses is difference in f ceo between pulses For N pulses: Excited state population N=4 N=3 N=2 the comb superimposed onto the atom
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Feasibility experiment in deep Feasibility experiment in deep UV (Kr atom) UV (Kr atom) With amplification in With amplification in Titanium:Sapphire Titanium:Sapphire Amplification == Phase control Amplification == Phase control 532 nm 2 x 212 nm ionization limit 4p 5p [1/2] 4p =23 ns 212 nm 532 nm ionization pulse time 13.3 ns 60 ns
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Problem with frequency comb

calibration: mode ambiguity Cavity length
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Combs in the VUV and beyond IR UV frequency DUV VUV XUV harmonic conversion frequency comb = high power puls es = 'easy' harmonic generation combination of high peak power and accuracy Harmonic conversion
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Combs in the VUV and beyond Comb is retained in harmonics due to pulse structure Phase control/measurement is the crucial issue
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Measurements at the 7th harmonic (of Ti:Sa) Probing Xe (5p 5p 5d) at 125 nm (Vacuum ultraviolet frequency comb)
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Phase stability (between pulses) in the

VUV (effect on relative phase) pressure dependence: -0.12 (0.29) mrad/mbar= -1.5(3.4) kHz/mbar UV dependence: -8.7(5.8) mrad/ J = -104(70) kHz/
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Novel development: Miniaturisation of frequency comb lasers -V Needle probes ~1 cm Mode-locked diode lasers InP quantum dot material Result from hybrid modelocking