Development of a System for High Resolution Spectroscopy with an Optical Frequency Comb

Dept. of Applied Physics, Fukuoka Univ., JST PRESTO, M. MISONO, M. OKUBO, K. DAIRIKI

AIM OF THE STUDY

Precise measure of optical frequency

molecular structure dynamics

High resolutionspectroscopy

Frequency shift, line broadening, line splitting, intensity anomaly, magnetic effect

Optical frequency

EXCITED STATES OF NAPHTHALENE

3B3u(T1)

3B1u(T2)

3B1u

3B3u

1B1u(S1)

1B3u(S2)

1B3u

Singlet Triplet

Intersystemcrossing

1Ag(S0)

4b1u

1B1u

z (a)

y (c)(b)x

PhotonicCrystal Fiber

Ti:Sapphire laser

Nd3+:YVO4laser

Stabilization

EXPERIMENTAL SYSTEM

Single modelaser

High finesse resonator

Stabilization

Frequencycounter

PhotoDiode Beat

measurement

High resolution spectroscopy

1 octave optical frequency comb

HIGH RESOLUTION SPECTROSCOPY

Precise measure of optical frequency

molecular structure dynamics

High resolutionspectroscopy

Frequency shift, line broadening, line splitting, intensity anomaly, magnetic effect

Optical frequency

DOPPLER-FREE TWO-PHOTON ABSORPTION SPECTROSCOPY

ν(1 - u /c) ν(1 + u /c)

Mirror Mirror

ν

u

hν(1 - u /c)

hν(1 + u /c)

2 hν

- Doppler free- Enhancement of light intensity for excitation- Selection rule is different from one photon absorption- Transition in UV region is excited by a visible laser- All molecules contribute to the signal independent of their velocities

PZT

+－

Pump laser

Dye laser

IsolatorPD

PD

PD

λ/4

Color filter

Photomultiplier tube Photon counter

Com

pu

ter

EXPERIMENTAL SETUP

OPTICAL RESONATOR AND SAMPLE CELL

PRECISE MEASUREMENT OF OPTICAL FREQUENCY

Precise measure of optical frequency

molecular structure dynamics

High resolutionspectroscopy

Frequency shift, line broadening, line splitting, intensity anomaly, magnetic effect

Optical frequency

1 OCTAVE OPTICAL FREQUENCY COMB

Frequency

Inte

nsi

ty

optical frequency

fCEO , frep Stabilization

Cs atomic clock in GPS satellites ・ Ultimate precision・ Low cost

Atomic clock　　　　 Precise measure of frequency

fCEO frep （～ 100 MHz ）

GENERATION OF 1 OCTAVE OPTICAL FREQUENCY COMB

PhotonicCrystal Fiber

～ 20 fs

Repetition rate= 100 MHz

Frequencymode separation = 100 MHz

Ultra-short pulse laser

> 1 Octave～ 130 nm

DETECTION OF fCEO

Frequency

Inte

nsi

ty

fCEO

f(2n) = 2nfrep + fCEO

Low frequency end of second harmonics

High frequency end of fundamental

2f(n) = 2nfrep + 2fCEO

Beat frequency fCEO

Fundamental

Second harmonics

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

CONTROL OF frep AND fCEO

Intensitymodulator

End mirror (Cavity length) ： frep, fCEO

Pumping intensity ： fCEO (, frep)

Outputcoupler

RF synthesizer

Cs atomic clock

BEAT NOTE OF REPETITION AND OFFSET

Inte

nsi

ty [

10

dB

/div

]

0 120Frequency [MHz]

Beat by offsetBeat by repetition

fCEO

frep

40

dB

STABILIZATION OF frep

99.999 988

100.000 012f r

ep [

MH

z]

0 600Time [s]

100.000 000

Gate time = 1 s

STABILIZATION OF fCEO

25.00

Fre

que

ncy

[MH

z]

0 700Time [s]

26.12

Gate time = 1 s

SUMMARY

• High resolution spectroscopy with a 1 octave optical frequency comb was studied to elucidate the structure and dynamics of polyatomic molecules.

• An experimental system for Doppler-free two photon absorption spectroscopy was developed.

• A stable mode locked Ti:Sapphire laser was developed and a 1 octave optical frequency comb was realized.

• The repetition rate (mode separation) of the 1 octave optical frequency comb were stabilized.

HIGH PRECISION MEASUREMENT WITH OPTICAL FREQUENCY COMB

flaser = nfrep + fCEO+ fbeat

Frequency

Inte

nsi

ty

frepfCEO fbeat

flaser

OPTICAL SYNTHESIZER

Single modelaser

RF Synthesizer

Opticalfrequency

combH

igh

reso

luti

on

spect

rosc

op

y

flaser

fsyn

flaser = fn + fsyn

FrequencyIn

ten

sity fn

fsyn

flaser fn+

1

frep - fsyn

OPTICAL SYNTHESIZER

0 120Frequency [MHz]

fsyn frep - fsyn

0 120Frequency [MHz]

fsynfrep - fsyn

Frequency shift by AO modulator

STABILIZATION SYSTEM FOR OPTICAL FREQUENCY COMB

PhotonicCrystal Fiber

DM

DMHM

BBO

PD: Photo diodeDM: Dichroic mirrorHM: Half mirror

fCEO

frep

Bandwidth 130 nmPulse width 20 fsRepetition rate 100 MHz

Ti:Sapphirelaser

Intensitymodulator

Cs atomic clock （ GPS satellite ）

RF oscillator RF synthesizer

PD

PD

Prescaler TrackingOscillator

frep

Bandwidth: 300 kHz

FREQUENCY REFERENCE FOR STABILIZATION OF fCEO

25 MHz

Frequency

Sp

ectr

al In

tens

ity [1

0 d

B/d

iv]

[25 kHz/div]

FREQUENCY REFERENCE FOR STABILIZATION OF fCEO

24.93

Fre

que

ncy

[MH

z]

0 2000Time [s]

25.00

DEVELOPMENT OF HIGH RESOLUTION SPECTROSCOPY

precision ～ 10-11Free from limitation of experimental system

Doppler-free spectroscopy

precision ～ 10-6Doppler effect

precision ～ 10-8Measure of optical frequency

Optical frequency comb

This study

～ 4550 cm-1

235 　 　　　 260[nm]

0.3 cm-1

Optical frequency

Laser light

Doppler-limited

Doppler-free

DOPPLER BROADENING