9 misure ottiche di frequenza

8/13/2019 9 Misure Ottiche Di Frequenza

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Misure ottiche di frequenza


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Confronto di frequenze ottiche Oscillatore locale

Fino a " 100 GHz: oscillatori a microonde (klystron, Gunnoscillators), asserviti in fase ad un oscillatore aquarzo di alta purezza spettrale

Fino a qualcheTHz: laser FIR moleculari calibrati in frequenza

Mixer diodi Schottky, diodi MIM

Laser 1

Laser 2

LO Mixer

f 1

f 2

f mw

Propriet del mixer: mixing bandwith,high-order mixing properties , spectral responsivity

f 1 ! f 2 ! nf mw

f 1

f 2

f mw

Occorre rapportare la frequenza damisurare di modo da portarla nel rangeaccessibile ad un contatore elettronico.


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Dallemicroonde al

visibile

La catena del PTB

9 GHz 456 THz


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How tomeasure the frequency

differencebetween twostabilized

lasers in the

visible region

f 1

Mixe f 2

SFGf 2 + f 1

SHG

f 2 -f 1

phase lock


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Slide dal talk di T. Hnsch a Stoccolma per ilconferimento del premioNobel


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The femtosecond lasers

Frequency comb: mode separation f r=v g /2L , inverse of the round-trip time of the pulse

Fourier transform !

Pulse: T " 20 fs Bandwidth (FWHM)" ! " 25 THz

Large spectral gain medium Ti:sapphire crystal Compensation of group velocity dispersion

# intracavity prism pair or chirped mirrors

Kerr effect self mode-locking! The pulse rate is exactly constant

Fouriertransform

!


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How a mode - locked laser works


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3 possible modes are shown,these are phased such thatthey add coherently at theposition of the dashed line

The output intensity of thelaser is an average over onecycle of the square of themode. For a single mode,the output is constant intime; for multiple modes,

interference forms a pulsetrain.


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! n(t) = n 2I(t)

Temporal Kerr-effectSelf-Phase-Modulation

E E

Spatial Kerr-effect

Self-Focusing

I

x, y

I

x, y ! n(x,y) = n 2I(x,y)

Self-mode-locking indotto da

effetto Kerr ottico Optical Kerr effect: refractive index n(I)=n 0+n 2 I ,

n (t , r ) = n 0 + n 2 I (t , r )


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Spettro del cristallo di Ti:zafro


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Kerr-lens mode-locked Ti:Sapphire laser


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Chirped mirrors


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I n

t e n s

i t y

( a .

u . )

Wavelength (nm)


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Stabilizzazione dellafrequenza diripetizione

Resolution bandwidth dell analizzatore di spettro:10 mHz.

Un circuito di feedback asserve lafrequenza di ripetizione ad unriferimento, agendo su unpiezoelettrico che controlla lalunghezza della cavit.

La stabilit limitata solo dalla qualitdel riferimento.

Il passo del comb rigorosamenteidentico su tutto lo spettro(dimostrato sperimentalmente entro 10 -17 )


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Carrier-envelope phase slips and offsetfrequency

CEO: Carrier-envelope offset


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Confronto di frequenze ottiche

K. R. Vogel, S. A. Diddams, C. W. Oates, E. A. Curtis, R. J. Rafac, W. M.Itano, J. C. Bergquist, R. W. Fox, W. D. Lee, J. S. Wells, and L. Hollberg OPTICS LETTERS / Vol. 26, 102 (2001)

n Hg+ = 2 x 532 360 804 949 559 (124) Hz,

NIST


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Misura dellafrequenza della

transizione 1 S -2S dellidrogeno

2 466 061 413 187 103(46) Hz M. Niering, R. Holzwarth, J. Reichert, P. Pokasov,Th. Udem, M. Weitz, T. W. Hnsch, P. Lemonde,G. Santarelli, M. Abgrall, P. Laurent, C. Salomonand A. Clairon

Phys. Rev. Lett. 84 , 5496 (2000)

1/7 dye " CH4 @3.39 m

4/7 dye il valor medio tra 1/7 dyee dye


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Octave spanning comb

David J. Jones,et al., Science, 288, 635 (2000)

Microstructured optical bers ( photonic crystals ): high power intensity on the core high non-linearity

zero dispersion value in the visible

Jinendra K. Ranka, et al., Opt.Lett. 25, 25 (2000)

Sidebands generated by 4-waves mixing: ! f = f 1 + f 2 " f 3

f 1 f 2 f 3

!! f = f 2 + f 3 " f 1


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Cristalli fotonici

Dal catalogo Thorlabs


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Self-referenced comb

#


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Frequency Combs as 'Gears'

of a Clock All clocks must have a regular, constantor repetitive process or action to markoff equal increments of time. Examplesinclude the movement of the sun acrossthe sky, a pendulum or vibrating crystal,or, in an atomic clock, the natural vibrations of atoms.

But no electronic systemsexist that can directly countthese oscillations. Afrequency comb,functioning like theelectronics in aconventional clock, can beused to divide the

oscillations of opticalclocks into lowerfrequencies that can belinked to microwavestandards and counted.

Today's standard atomicclocks vibrate atmicrowave frequencies,about 9 GHz. Opticalatomic clocks oscillatemuch faster, at about

500,000 GHz, and thusdivide time into smallerunits.


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Misura della transizione 1s-2s con ilcomb autoriferito

M. Fischer et al., PRL 92, 2308 02 (2004)


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Misura con il comb autoriferito

Hg+ = 1 064 721 609 899 143 ! 2.4 Hz,

Absolute Frequency Measurements of the Hg + and Ca Optical Clock Transitions with a Femtosecond Laser Phys.Rev.Lett. 86,4996 2002) Th. Udem, S. A. Diddams, K. R. Vogel, C.W. Oates, E. A. Curtis, W. D. Lee, W. M. Itano, R. E. Drullinger, J.C. Bergquist, and L. Hollberg

Ca = 455 986 240 494 158 ! 26 Hz

Riferimento: maser a idrogeno stabilit 2 ! 10 -13 @ 1s,4 ! 10 -16 su alcuni giorni

Accuratezza calibrata sul NIST-F1 : 1.6 ! 10 -15

! f Ca ! t

# $

& ' f Ca ( 8 ) 10 *14 yr -1

! f Hg +! t

# $

& ' f Hg + ( 30 ) 10 *14 yr -1

9 misure ottiche di frequenza

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