advanced technologies for optical frequency control and

Advanced Technologies forOptical Frequency Control and Optical Clocks

MURI - Technology for Optical Frequency Control and Optical Clocks

Yoel FinkThomas Greytak

Erich IppenFranz KaertnerDaniel Kleppner

Leslie KolodziejskiJeffrey ShapiroFranco Wong

Massachusetts Institute of TechnologyResearch Laboratory of Electronics

Cambridge, MA 02139

Department of Electrical Engineering and Computer ScienceDepartment of Materials Science and EngineeringDepartment of Physics<[email protected]>

Outline


• Efficient SHG and DFG with chirped-grating PPLN• 3-to-1 self phase-locked optical frequency divider• DFG for comb locking to methane-stabilized HeNe laser

• Novel wide-area GaAlAs oxidized mirrors• Saturable Bragg reflectors for few-cycle lasers

• Novel microstructured fiber for nonlinear optics• Broadband dispersion characteristics of bandgap fibers

Few-cycle laser development• Ti:sapphire, Cr:forsterite, Cr:YAG• Phase sensitive nonlinear optics• Attosecond-precision laser synchronization

Optical frequency metrology with ultracold hydrogen• Hydrogen spectroscopy and frequency referencing • Locking of femtosecond laser comb to single frequency cw laser

Nonlinear optical techniques for comb technology

Ultra-broadband mirrors and saturable absorbers for few-cycle lasers

Cylindrical photonic bandgap fibers

Ultra-low-jitter modelocked diode laser • Locking to visible wavelength reference

•enhanced SPM

•dispersion-managed modelocking•octave-spanning spectrum

•double-chirped mirrors

5-fs Ti:sapphire Laser


Chirped and Double-Chirped Mirrors


Double-Chirped Mirror: Bragg-Wavelength and Coupling Chirped

Air

“Impedance” - Matching

d =h λB/4SiO -Substrate

2 AR-Coating

Bragg-Mirror: TiO / SiO2 2 - LayersλB4

Air

Chirped Mirror: Bragg-Wavelength Chirpedλ1λ2 λ1λ2 >

Negative Dispersion:

λB

--> R. Szipöcs & F. Krausz (Opt. Lett. 19, 201-203, 1994) Computer Optimization

SiO -Substrate

2

SiO -Substrate

2

Designed Measured

100

80

60

40

20

0

REF

LEC

TIVI

TY (%

)

140012001000800600WAVELENGTH, NM

-600

-400

-200

0

200

400

600

GD

D, FS

2

100

80

60

40

20

0

REF

LEC

TIVI

TY (%

)

140012001000800600WAVELENGTH, NM

-600

-400

-200

0

200

400

600

GD

D, FS

2

DCM-Pairs Covering One Octave


Fabricated by Tschudi Group, Darmstadt

8

6

4

2

0

IAC

3020100-10-20TIME DELAY, FSa)

1.0

0.8

0.6

0.4

0.2

0.0SPEC TR UM, a.u.

12001000800600W AVELENGTH, NM

10-5

10-4

10-3

10-2

10-1

100

b)

-12

- 8

- 4

0

PH

AS

E,

RA

DIA

N

140012001000800600W AVELENGTH, NMd)

PO

WE

R,

a.u.

-40 -20 0 20 40TIME, FSc)

FWHM 5 fs

• Nonlinear autocorrelation• Spectrum• Recovered amplitude and phase

5-fs Pulse Characterization


PHASEGROUP VV ≠

Modelockedlaser

gain

∆φ

2nGL/c

Pulse-to-Pulse Phase Slip in a Modelocked Laser


-100

-80

-60

-40

-20

0

RF

spec

tral p

ower

den

sity

[dB]

706050403020100RF Frequency [MHz]

• rf tones reveal dφ/dt

Phase-Dependent 2nd Harmonic


Trf

mTopt = Trf

Stopping the Optical Phase Slip

The optical frequency is locked to an exact multiple of the rf rep rate !


dtdφ

=δlaser

modescavitymodes

Ln2mcf

ngL2mcff f2

g1121 =⇒+==

1f 2fLn2

ncf 0g

1 =⇒=δ

Ln2cg

Locking an octave by SHG:

•Phase slip in the frequency domain

Frequency Locking of Laser Modes


Second Harmonic Generation with Octave-Spanning Spectrum

=> Interference between fundamental and 2nd harmonic

2nd Harmonic

eiφ Fundamental

e2iφ

ωo

2ωo


Continuum Generation with Microstructured Fiber

10-6

10-4

10-2

100

Pow

er (a

rb. u

nits)

1600140012001000800600400

Wavelength (nm)

Input Output

Spectrum resulting when 80fs pulses from a Ti:sapphire oscillator are focused into a holey fiber

Strong guiding shifts the zero-dispersion wavelength to the near visible

J. Ranka et al.


-40 -20 0 20 400

4

8

Inte

rfer.

Auto

corre

latio

n (n

orm

.)

Delay Time (fs)

• All solid state• 1.3 µm wavelengths

Chuboda et.al.

1.2 1.4 1.60

1

Spec

tral I

nten

sity

(nor

m.)

Wavelength (µm)

Recent result: SBR stabilized

14 fs Cr:forsterite Laser


DCM

DCM

DCM

DCM

Autocorrelator

Faraday -rotator

Nd :YAG

OC

L

MX

P

λ /2

20-fs Cr4+:YAG Laser

Nd:YVO4Laser – 1064nm

2-cm Cr4+:YAGOC

f = 10 cmDCM

DCM

DCM

SBR

20 fs FWHM

SBR

Ref

l ect

ivi ty

Wavelength (µm)


Overlapping Femtosecond Laser Spectra

Ti:Sapphire, Cr:Forsterite and Cr:YAG


A Compact Prismless Ti:sapphire Laser

• Even more compact• Scaling to higher repetition rates• >500 MHz, ring laser

Future:

2mm Ti:Al2O3φ = 10ο

PUMP

OC 1

OC 2Base Length = 30cm for 82 MHz Laser

L =

20 c

m

BaF2 - wedges

BaF2

Octave-spanning spectrum


Overlapping Femtosecond Laser SpectraSp

ectra

l Pow

er [l

og]

-60

-50

-40

-30

-20

-10

0

1600140012001000800600Wavelength [nm]

Cr:forsterite

Ti:sapphire

30 dB

• Compact, prismlessoctave spanning Ti:sapphire laser

• Stabilized Cr:forsteritelaser

• Laser synchronization300 asec timing-jitter

• Opportunities:

extended frequency comb: 600–1600 nm

and/orsingle-cycle pulse

generation


Balanced Cross-Correlation for Laser Synchronization

Ti:sa

Cr:fo

3mm Fused Silica

SFG

SFG

Rep.-RateControl

(1/496nm = 1/833nm+1/1225nm).

-

+

GD

-GD/2

∆t

Ti:sa

Cr:fo

3mm Fused Silica

SFG

SFG

Rep.-RateControl

(1/496nm = 1/833nm+1/1225nm). Ti:sa

Cr:fo

3mm Fused Silica

SFG

SFG

Rep.-RateControl

(1/496nm = 1/833nm+1/1225nm).

-

+

GD

-GD/2

GDGD

-GD/2

∆t∆t

1.00.80.60.40.20.0C

ross

-Cor

rela

tion

Am

plitu

de

-100 0 100

Time [fs]

100806040200Time [s]

Timing jitter 0.30 fs (2.3MHz BW)

The residual out-of-loop timing jitter measured from 10mHz to 2.3 MHzis 300 as (a tenth of an optical cycle)

Output spectrum: 650-1450nm


Optical Frequency Metrologywith Ultracold Hydrogen

OVERALL GOAL

• Resolve discrepancies in the theory of quantum electrodynamics• Better values for fundamental constants• Test theories of atomic interactions and atomic structure

SCIENCE

• Explore the possibilities for an atomic hydrogen optical clock• Develop and apply techniques for measuring the frequencies of optical transitions• Stimulate the development and application of optical frequency combs

TECHNOLOGY

IMMEDIATE GOAL

Measure absolute frequencies of transitions such as 2S →10S (2-photon, 730 nm)using atomic hydrogen as an optical frequency standard

To pursue ultraprecise spectroscopy of ultracold atomic hydrogen for:


Spectroscopy with a Hydrogen Optical Clock

x 2

486 nm

Lock

x 2 Cold Hydrogen

Lock to 1S-2S (243 nm)

764 nm

Excite 2S-10S

972 nm

• Excite 1S→2S transition (known to about 1 partin 1014.)

• Lock 486 nm laser to 1S→2S • Excite two-photon transition such as 2S→10S• Measure excitation frequencies with an optical

frequency comb, referenced to the 486 nm laser,i.e. referenced to hydrogen


Nonlinear Optical Techniques for Comb Technology

Ultra-broadband SHG with zero group velocity mismatch• 70 nm bandwidth for 1580 nm → 790 nm in PPKTP

Difference-frequency generation of 3.39 µm• locking Ti:sapphire laser comb to CH-stabilized HeNe laser• collaboration between MIT and JILA

3-to-1 frequency divider• self-phase-locked optical parametric oscillator with cascaded nonlinear

interactions in PPLN • provides phase-locked markers at 532, 798, and 1596 nm


Difference Frequency Generation of 3.39 µm

Ti:S fs pulses

cw 3.39 µm output

667 nm 830 nm

Ti:S spectrum

N pairs of modesyield N2 enhancement

N~15,000

CH-stabilized He-Ne@ 3.39 µm

4

}Phase-locked

Servo to lock comb spacing to He-Ne

PPLNdifference-frequency

generator


Self-Phase-Locked OPO Yields3:2:1 Frequency Markers

PZT

PPLNPump

532 nm

Signal 798 nm

Idler 1596 nm

OPOgrating

SHGgrating

Self-phase locking(∆ω=0)

ω

Signal 2ω

Pump 3ω 2ω

2ω


Ultra-Broadband Saturable Absorber Mirrors

• High index-contrast oxidized mirrorsfor broadband low dispersion

• In-based absorber optimized forwavelength – integrated on mirror

• Large area for low power density• Stable, durable structures

delaminated interfacedelaminated interface

AlxOy

GaAs

GaAs substrate

500 µm

GaAs/AlxOy Mirror

500µm

GaAs/InGaAs

AlxOy

Al0.3Ga0.7As

GaAs substrate

AlGaAs/AlxOy Mirror

GOALS

PROGRESS

• Integrated broadband SBRs for Cr:YAG and Cr:forsterite lasers

• Dramatic improvement in layerstability with AlGaAs/AlxOy structures

• Wide area oxidization(>500µm diameter)


Issues Affecting the Distributed Bragg Reflector

Oxidation of AlGaAs Layers

• Edges of mesa less oxidized than at center • Mirror bandwidth a function of position

13µm from mesa edge

R946, 5 hr, 415 COxidized 11/13/2002GaAs/InGaAs

AlxOy

Al0.3Ga0.7As

GaAs substrate99µm from mesa edge

R946, 5 hr, 415 COxidized 11/13/2002


Issues Affecting Saturable Absorber

Sensitivity toGrowth

Temperature

Sensitivity toOxidation

Temperature

Sensitivity toOxidation

Time

Center = ~3mm from wafer edge Center = ~18.4mm from wafer edgeCenter = ~11.4mm from wafer edge

420C 435C

~230 µm

8hrs5hrs


primary gap centered around 1.5 µm

OmniGuide Mirror Fibers


1µm

100µmRegular OmniGuide structure

Modified (defect) structure

Bandgap Fiber with and without Defect


Mirror-guide Dispersion Characteristics

1.2 1.4 1.6 1.8 2 2.2-20

-15

-10

-5

0

5

10

15

20

Metallic

LH

Dis

pers

ion

D (p

s/nm

-km

)

Vacuum wavelength (µm)1.2 1.4 1.6 1.8 2 2.2

-20

-15

-10

-5

0

5

10

15

20

Metallic

LH

Dis

pers

ion

D (p

s/nm

-km

)

Vacuum wavelength (µm)1.2 1.4 1.6 1.8 2 2.2

-20

-15

-10

-5

0

5

10

15

20

Vacuum wavelength (µm)

Dis

pers

ion

D (p

s/nm

-km

) LH

ZD

1.55 µm

Metallic

1.2 1.4 1.6 1.8 2 2.2-20

-15

-10

-5

0

5

10

15

20

Vacuum wavelength (µm)

Dis

pers

ion

D (p

s/nm

-km

) LH

ZD

1.55 µm

Metallic

Bandgap fiber Fiber with cavity mode


Modelocked External External Cavity Diode Laser

NEC external cavity MLLD

OUTPUT

OSC

SELFOCLENS

MIRRORR = 99.9%

5-nm BPF

I gain

VSA

• 500 µm gain section• 50 µm saturable absorber section• λ = 1547nm , f = 9.35 GHz• BPF = 0.7 nm : τ = 6.7 ps• BPF = 5 nm : τ = 3 ps

PoseidonSBO


Residual Phase Noise Measurement

• Vector signal analyzer for low frequency offsets (1-10 MHz)• RF spectrum analyzer for high frequency offsets (10 MHz – 4.5 GHz)

(RF analyzer after mixer has higher sensitivity than for direct detection.)

MLLD

DELAY

OSC AMP AMP3-dBVECTOR SIGNAL ANALYZER

CH1CH2

RF ANALYZER


10 100 1k 10k 100k 1M 10M 100M 1G-160

-150

-140

-130

-120

-110

-100

-90

-80

Spectrum Analyzer Data

Vector Signal Analyzer Data

Spectrum Analyzer Noise Floor

Vector SignalAnalyzer

Noise Floor

L (f)

dBc/

Hz

Offset Frequency (Hz)

Single-Sideband Phase Noise - Results• Hybrid MLLD, 9GHz, 6.7 ps, Poseiden SBO• Vector signal analyzer (0 – 10 MHz)• RF spectum analyzer (10 MHz – 4.5 GHz)

Integrated timing jitter from 10 Hz to 4.5 GHz = 86 fs154 fs including all noise spursSBO jitter = 5.6 fs (10 Hz to 10 MHz)


1kHz – 10MHz : 40 fs

MIT-JILA collaborationD. J. Jones et al. Opt. Lett. 2003

Synchronization of Modelocked Diode Laser to Visible Standard


MIT-JILA collaborationD. J. Jones et al. Opt. Lett. 2003

Timing Jitter Spectral Density of Stabilized MLLD

20 fs jitter (1Hz – 10 MHz)


Outline


• Efficient SHG and DFG with chirped-grating PPLN• 3-to-1 self phase-locked optical frequency divider• DFG for comb locking to methane-stabilized HeNe laser

• Novel wide-area GaAlAs oxidized mirrors• Saturable Bragg reflectors for few-cycle lasers

• Novel microstructured fiber for nonlinear optics• Broadband dispersion characteristics of bandgap fibers

Few-cycle laser development• Ti:sapphire, Cr:forsterite, Cr:YAG• Phase sensitive nonlinear optics• Attosecond-precision laser synchronization

Optical frequency metrology with ultracold hydrogen• Hydrogen spectroscopy and frequency referencing • Locking of femtosecond laser comb to single frequency cw laser

Nonlinear optical techniques for comb technology

Ultra-broadband mirrors and saturable absorbers for few-cycle lasers

Cylindrical photonic bandgap fibers

Ultra-low-jitter modelocked diode laser • Locking to visible wavelength reference

advanced technologies for optical frequency control and

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