Optical Frequency Combs

Ronald Holzwarth

Max-Planck-Institute for Quantum OpticsGarching, Germany

And

Menlo Systems GmbHMartinsried, Germany

Engelberg, March 6th. 2007

Team

The Team

Ted HänschTh. UdemCh. GohleM. HermannS. KnünzA. OzawaN. KolachevskyE. PetersJ. AlnisK. PredehlT. WilkenJ. Alnis

MPQ

MenloSystems M. MeiM. FischerP. AdelP. Fendel

Frequency comb generation

fM = N frep + f0

Pulsed Lasers

Tasks:

• Create short pulses

• Sustain short pulses in the cavity

Kerr effect for modelocking

Ti:Sapphire

intensity dependent index of refraction

n(I) = n0 + I(t) n2

Or: saturable absorbers

Group Velocity Dispersion

dielectriccoating

substrate

blue

red

red

blue

1978

1978....

Is the mode spacing equal?

I

907nm

972nm851nm

580,000 modes

44 THz

mode lockedTi:Sapph. laser

single modefiber

750 800 850 900 950 1000 1050

-80

-60

-40

-20

0

Wavelength [nm]

Average: 0.054± 0.111 mHz( 3 x 10 -18 )

log In

tens

ityYes!

MPQ 1998

First frequency comb measurement

4ff 7f

λ

x2

70 fs Ti:sapphiremode locked laser

quarz fiber

Csatomic clock

10 MHz

4f 3.5f

x2

x2

f

3.39 µm

848 nm

485 nm

I

MPQ 19993.5 f : 4 f

44 THz gap between848 nm and 972nm

Octave Spanning Spectra

“Photonic Crystal Fiber”

J.C. Knight, W.J. Wadsworth, P. St.J. RusselUniversity of Bath, UK

„Rainbow Fiber“

(Lucent Technologies, 1999)

Spectral Broadening in PCF

Comb detail

Self referenced frequency comb

ωoptical= N ωrep + ωoffset

JILA, MPQ 2000

HistoricOverview:

Harmonic Frequency chain

ONEOptical reference frequency

Frequncy comb system

In real life

Comb System Schematic

Optical frequency comb generator

(OFCG)

Pump and control

electronics

Optical amplifier(OFCG

dependent)

Pump and control

electronics

Nonlinear spectral

broadening(OFCG

dependent)

Control electronics(optional)

NonlinearInterferometer

CEO det.

Control electronics

Beat detectionunit (BDU)

Detectionelectronics

Control and measurement electronics

Radio-frequencyreference

Continuous-waveLaser

EA B C D

(optional)

Opt

ical

laye

rEl

ectro

nic

laye

r

Optical frequencymeasurement

result

I Laser II Stabilization III Application

Other comb sources

Cr:LiSAFin collaboration with Fraunhofer Institut für Lasertechnik, Aachen

Fiber laser

Add free space part to adjust Repetition frequency

Tamura, Ippen et. al. 1994

Laser Specs

1450 1500 1550 1600 1650

-50

-40

-30

-20

-10

0

inte

nsity

[dB]

Wavelength [nm]

-200 -100 0 100 200

0

2

4

6

8

inte

nsity

time [fs]

interferometric AC intensity AC

3 dB bandwidth: 70nm,

autocorrelation length: approx. 100fs

Average power: approx 200 mW / 100 mW

Repetition frequency: 100 MHz / 250 MHz

Offset beat signal

-80

-60

-40

-20

0

0 50 100 150 200 250 300

Frequency (MHz)

RF

leve

l (db

m)

Beat signal free running

-90

-80

-70

-60

-50

-40

19 20 21

Frequency [MHz]

Rf l

evel

[dB]

110 kHz(10 dB)

Beat signal locked

-40 -20 0 20 40

-100

-80

-60

-40

-20

0bandwidth 1 Hz

sp

ectra

l den

sity

[dBm

]

frequency offset from setpoint [Hz]

Ti:Sa Phase Noise 100 mrad

Commercial fiber based frequency comb

Founded in 2001 as a spin of from MPQ to commercialize Frequency comb technology

Difference Frequency comparision

weighted average: -0.99 µHz ± 1.27 µHz ( 2.0x10 )-21

Comparison of two fiber based frequncy combs

Stabilized 1.5 µm laser 2 x 10 -16

0 10000 20000 30000 400004461,5

4462,0

4462,5

4463,0

<PTB> = 4462194,769 ± 0,403 Hz (sd= 73,97 Hz) <MPQ> = 4462194,772 ± 0,463 Hz (sd= 84,90 Hz) fre

quen

cy [k

Hz]

time [sec]

0 10000 20000 30000 40000-300

-200

-100

0

100

200

300

deviation = (-0,0052 ± 0,0381) Hz

frequ

ency

dev

iatio

n [H

z]

time [sec]PTB comb MPQ comb

In collaboration withHarald SchnatzGesine Grosche

100 MHz reference

Test of neighboring comb lines

NIST, Science, 303, 1843 (2004)

Frequency comb

I

Frequency"Oscillations per Second"

450 THz 750 THz0 Hz

The Nobel Prize in Physics 2005

John L. Hall and Theodor W. Hänsch

for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique

What is it good for?

Infraredspectroscopy Direct comb

spectroscopyTime domain:

CEO phase

Frequency"Oscillations per Second"

450 THz 750 THz

Fiber laser combsDistance

measurement

Harmonic combsPrecision

spectroscopyDissemination

of time and frequncyOptical Frequency Synthesizer

Differencefrequency combs

Optical Frequencies

532 nm or 560 THz

560 000 000 000 000 oscillations per second

Applications

100 000 ultra-stable lasers at onceRevolutionary optical wave meterClockwork for optical atomic clocksUltra-stable microwave source

Tool for fundamental measurementsOptical waveform synthesizer

Source of phase-stabilized femtosecond pulsesKey to attosecond physics

Femtosecond Laser Frequency Combs

An Enabling Technology

Nobel Poster

Measuring the Frequency of Hydrogen with a Laser Comb

Nobel Prize in Physics 2005, Nobel Poster

Hydrogen 1S 2S Transition

Resolving the line

Hydrogen Spectroscopy

max. drift 200 Hz in 3.68 yrs ⇒ f(1s2s)/f(Cs) < 5 x 10-14 yr-1

Hydrogen at MPQ since 1986

History

Why Hydrogen?

Lamb ShiftQED Test

Rydbergconstant

Are fundamental constantsconstant?

Hydrogenvs.

Antihydrogen

H-D Isotopeshift

Are the fundamental constants really constant?

Comparison of optical transitions

ytterbium+

hydrogen

mercury+

Mercury+, 2000-2002, NISTHydrogen, 1999-2003, MPQ

Ytterbium+, 2000-2003, PTB

All

area

Astro

area

quasar spectra+ GUT

Keck/HIRES

ESO VLT

Optical clocks

A clock consists of a oscillator, and a counter that counts these uniform oscillations. The finer the partition of time, the

more accurate the clock can be

From 3500 BC 1656 1918 20021955

Optical atomic clock: 1 267 402 452 899 920 Hz

Sun dial:One oscillationper day

Pendulum clock:One oscillation per second

Quartzuhr:32 768 oscillations Per second

Cesium atomic clock: 9 192 631770 Hz

Optical clock – some candidates

Laser-cooled trapped ions

Hg+, In+, Yb+, Sr+, Ca+, ... Paul trap

Cold neutral atoms:

H, Ca, Sr, Yb, Ag, ...Optical lattice

Molecules:Atomic fountain

I2, C2H2, ...

Atom chip

Shelving Scheme

Clock transition(narrow, forbidden)

cooling transition(broad, dipole allowed)

„Shelving scheme“ (Hans Dehmelt):- Shine in clock laser- Shine in cool laser to see whether the ion is still

In the ground state

Optical vs. Microwave Clocks

femtosecond combs

Yb+

1.0E-16

1.0E-15

1.0E-14

1.0E-13

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1950 1960 1970 1980 1990 2000 2010Year

Frac

tiona

l unc

erta

inty

Microwave atomic clocksOptical frequency standards

Essen's Cs clock

Cs redefinition of the second

Cs fountain clocks

Sr+

Hg+,Yb+,Ca

iodine-stabilised HeNe

H

H

H

Ca

Hg+

GPS as reference

Two frequency combs with slightly different repetition rates

Ti:S laser

Ti:S laser

GaSe

GaSe

125 MHz

125 MHz + ∆

sample

800nm 30 THz

detector

In collaboration with F. Keilmann, MPI Biochemistry Opt. Lett. 29, 1542 (2004)

IR Spectrum mapped to rf

Single shot performance

NH3 cell

classical FTIR

fr=125.130 MHz∆=29.93 Hzτ= 70 µs2cm-1

Load fs pulses into enhancement cavity

6 nJ 400 nJ

112 MHz

Enhancement factor ~ 66

injected: 22 fs pulse duration, 0.65 W average, 200 kW peak

circulating: 27 fs pulse duration, 45 W average, 15 MW peak

MPQ: Nature 436, 234–237 (2005).JILA: Phys. Rev. Lett. 94, 193201 (2005).

Dispersion causes mismatch

pas

sive

cavi

tyla

ser

freq

uen

cy

com

b

Pulse characterization

laser oscillator pulse in build-up cavity

High Harmonic Generation

Setup

Making use of the harmonics

20 40 60 80 100 120 140 160-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

Wavelength/nm

UV Frequency combs

Christoph Gohle et al., Nature 436, 234–237 (2005).

Ultrafast meets Ultrastable

∆ϕ

f0

I(f)

f

fcfr

Phase sensitivityFrequency Comb

Nonlinear interactions with few cycle pulses

High-harmonic generation A. Baltuska et al., Nature 421, 611 (2003)Above-threshold ionization G.G. Paulus et al., , PRL 91, 253004 (2004)Photocathode emission A. Apolonski et al., PRL 92, 073902 (2004)Photocurrents in semiconductors T. Fortier et al., PRL 92, 147403 (2004).......

Light wave oscilloscope

-20

-10

0

10

20

Delay ⊗t [fs]

Phot

oele

ctro

n ki

netic

ene

rgy

[eV]

Vect

or p

oten

tial,

A (t)

[fs

MV/

cm]

L

2 4 8 10 14 18 200 6 12 16 22

50

60

70

80

90

E. Goulielmakis et al., Science 305, 1267 (2004)

An “oscilloscope” for light waves

Above Threshold Ionization (ATI)

“Stereo” ATI

G.G. Paulus et al., , PRL 91, 253004 (2004)

ART SCHALOW

“Never measure anything but frequency!”

Arthur L. Schawlow

End