FLS 2010 WorkshopMarch 4th, 2010

Recent Progress in Pulsed Optical Synchronization Systems

Franz X. Kärtner

Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology

Cambridge, MA, USA

2

Acknowledgement

Students

Hyunil ByunJonathan CoxAnatoly KhiloMichelle Sander

Postdocs:

J. Kim (KAIST, Korea)Amir NejadmalayeriNoah Chang

Colleagues and Visitors:

E. Ippen, J. G. Fujimoto, L. Kolodziejski, F. Wong and M. Perrott

DESY: F. Loehl, F. Ludwig, A. Winter

3

Outline Synchronization System Layout for Seeded FEL

Advantages of a Pulsed Optical Distribution System

Low Noise Optical Master Oscillators

Timing Distribution Over Stabilized Fiber Links

Optical-to-Optical Synchronization

RF-Extraction and Locking to Microwave References

Prospects for sub-fs timing distribution

4

Seeded X-ray Free Electron Laser

Long-term sub-10 fs synchronization over entire facility is required.

5

Seeded X-ray Free Electron Laser

J. Kim et al, FEL 2004.

6

Why Optical Pulses (Mode-locked Lasers)?

Real marker in time and RF domain, every harmonic can be extracted at the end station.

Suppress Brillouin scattering and undesired reflections. Optical cross correlation can be used for link stabilization or for optical-to-

optical synchronization of other lasers. Pulses can be directly used to seed amplifiers, EO-sampling, …. Group delay is directly stabilized, not optical phase delay. After power failure system can auto-calibrate.

frequency

… ...

fR 2.fR N.fR

TR = 1/fR

time

7

200 MHz Soliton Er-fiber Laser

• 200 MHz fundamentally modelocked soliton fiber laser

• 167 fs pulses

• 40mW output power

ISO

PBS

λ/4

λ/2

λ/4

collimatorcollimator

WDM980 nm Pump

10 cmSMF

50 cmEr doped fiber 10 cm

SMF

10 cmSMF

10 cmSMF

ISO

PBS

λ/4

λ/2

λ/4

collimatorcollimator

WDM980 nm Pump

10 cmSMF

50 cmEr doped fiber 10 cm

SMF

10 cmSMF

10 cmSMF

J. Chen et al, Opt. Lett. 32, 1566 (2007).

Similar lasers are now commercially available!

K. Tamura et al. Opt. Lett. 18, 1080 (1993).

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Semiconductor Saturable Absorber Modelocked 100MHz - 1GHz Er-fiber Lasers

EDFSBR

output

WDM

Output coupler(coated ferrule)

pump (977nm)

SMF

laser cavity

packaged in a box

1540 1560 1580 16000

0.2

0.4

0.6

0.8

1

wavelength (nm)

inte

nsity

(a.u

.)

17.5nm

sech2 fitmeasured

• Compact, long-term stable femtosecond laser source at GHz reprate

• SBR burning problem solved by SMF buffer and pump-reflective coating on SBR

• 380mW pump 27.4mW output• Optical spectrum FWHM: 17.5nm• Pulse width: 187fs• Repetition rate: 967.4MHz• Intensity noise: 0.014% [10Hz,10MHz]

967.1 967.7-100

-80

-60

-40

-20

0

frequency (MHz)

Optical spectrum RF spectrum

-500 0 5000

2

4

6

8

time delay (fs)

IAC

(a.u

.)

1.54x187fs

Autocorrelation

5”x4”x1.5”

pumpout

Long term output power with 270mW pump

9

Sensitive Time Delay Measurements by

Balanced Optical Cross Correlation

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Reflect fundamentalTransmit SHGTransmit fundamental

Reflect SHG

Type-II phase-matchedPPKTP crystal

Single-Crystal Balanced Cross-Correlator

J. Kim et al., Opt. Lett. 32, 1044 (2007)

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In comparison:Typical microwave mixerSlope ~1 µV/fs @ 10GHz

Single-Crystal Balanced Cross-Correlator

80 pJ, 200 fs 1550nm input pulsesat 200 MHz rep. rate

ML Fiber Laser Timing Jitter Measurement

ModelockedLaser 1

ModelockedLaser 2

HWP

PBS Single crystal balanced

cross-correlator

Oscilloscope

RF-pectrumanalyzer

-1

0

1

-800 0 800

Time delay (fs)D

etec

tor

outp

ut

(V)

Loop filter

J. Kim, et al. , Opt. Lett. 32, 3519 (2007).

12

13

Ultralow timing jitter (<1 fs) in the high frequency range [100 kHz, 10 MHz]J. Kim, et al., Opt. Lett. 32, 3519 (2007).

103

104

105

106

107

-230

-210

-190

-170

-150

-130

-110

Pha

se N

oise

(dB

c/H

z)

Frequency (Hz)

IntegratedMeasuredTheoryNoise Floor

103

104

105

106

1070

1

2

3

4

5

6

Frequency (Hz)

Jitte

r (fs

rms)

Erbium Fiber Laser Phase Noise

Timing Jitter in 200 MHz Fiber Lasers

Noise Floor

Integrated Jitter

Measured JitterDensity

Theory

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Timing - StabilizedFiber Links

15

PZT-based fiber stretcher

Mode-locked laser

Fiber link ~ several hundreds meters

to a few kilometers

isolator

TimingComparison

Faradayrotatingmirror

Cancel fiber length fluctuations slower than the pulse travel time (2nL/c)

1 km fiber: travel time = 10 μs ~100 kHz BW

Timing-Stabilized Fiber Links

2 Link Test System

J. Cox et al. CLEO 2008.

Experimental Apparatus~300 m optical fiber

Piezo stretcher

200 MHz Laser EDFA

Invar Board

Motor

In-LoopPPKTP Out-of-Loop

PPKTP

Balanced Cross-Correlation Signals

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-2000

0

2000

Link 1 In Loop S-Curve, slope = 21 mV/fs

Sig

nal (

mV

)

t ime (ms)

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-2000

0

2000

Link 2 In Loop S-Curve, slope = 18 mV/fs

Sig

nal (

mV

)

t ime (ms)

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-2000

0

2000

Out of Loop S-Curve, slope = 20 mV/fs

Sig

nal (

mV

)

t ime (ms)

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2

2000

4000

6000

8000100 Hz PZT Modulation

Pha

se S

hift

(fs)

t ime (ms)

Link 1

Link 2

Out ofLoop

Results – Timing Jitter

360 as (rms) timing jitter from 1 Hz to 100 kHz3.3 fs (rms) timing jitter from 35 μHz to 100 kHz

10-4

10-3

10-2

10-1

100

101

102

103

104

105

10-6

10-4

10-2

100

102

104

Jitte

r Spe

ctra

l Den

sity

(fs2 /H

z)

Frequency (Hz)10

-410

-310

-210

-110

010

110

210

310

410

50

0.5

1

1.5

2

2.5

3

Inte

grat

ed J

itter

(fs

rms)

Out-off loop jitter limited by quantum noise

20

Optical-to-Optical Synchronization

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Optical-to-Optical Synchronization

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Ti:sapphire Laser + Cr:Forsterite Laser

Ti:sapphire Cr:forsterite

Spanning over 1.5 octaves

5fs 30 fs

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Sub-femtosecond Residual Timing Jitter

J. Kim et al, EPAC 2006.

Long-term drift-free sub-fs timing synchronization over 12 hours

Balanced optical cross-correlator based on GDD (T. Schibli et al, OL 28, 947 (2003))

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Optical-to-RF Conversion or

Optical-to-RF Lockingnecessary for

Locking OMO to RMO

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Direct Extraction of RF from Pulse Train

AM-to-PM conversion,Temperature driftof photodetectors and mixers

…

RF frequency

… ...

fR 2fR nfR

TR = 1/fR

time Photodetector

t

TR/n

E. Ivanov et al, IEEE UFFC 52, 1068 (2005). IEEE UFFC 54, 736 (2007).

26

Direct Extraction of RF from Pulse Train

RF frequency

… ...

fR 2fR nfR

TR = 1/fR

time Photodetector

t

TR/n

55 fs driftin 100 sec

A. Bartels et al, OL 30, 667 (2005).

More in: B. Lorbeer et al, PAC 2007.

MicrowaveSignal

27

Balanced Optical-Microwave Phase Detector(BOM-PD)

J. Kim et al., Opt. Lett. 31, 3659 (2006).

Electro-optic sampling of microwave signal with optical pulse train

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Optoelectronic Phase-Locked Loop (PLL)Regeneration of a high-power, low-jitter and drift-free microwave signal whose phase is locked to the optical pulse train.

Balanced Optical-Microwave Phase Detector (BOM-PD) Regenerated

Microwave Signal Output

Tight locking of modelocked laser to microwave reference

Balanced Optical-Microwave Phase Detector (BOM-PD) Stable Pulse

Train Output

Modelocked Laser

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Testing Stability of BOM-PDs

BOM-PD 1: timing synchronizationBOM-PD 2: out-of-loop timing characterization

RMS timing jitter integrated in 27 μHz – 1 MHz: 6.8 fs

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Long-Term Stability: 6.8 fs drift over 10 hours

J. Kim et al, Nature Photonics 2, 733 (2008).

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Delay Locked Looop: 2.9 fs drift over 8 hours

J. Kim et al, submitted to CLEO 2010.

RMS timing jitter integrated in 0.1 Hz – 1MHz, 2.4 fs

32

1 GHz diode pumped CrLiSAF Laser: Modelocked

Jointly with Jim Fujimoto

Prospects for Attosecond Timing Distribution(100 MHz Cr:LiSAF Laser, SSB scaled to 1GHz)

U. Demirbas, submitted to CLEO 2010

Cr:LiSAF Laser Phase Noise

-280

-250

-220

-190

-160

-130

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07Frequency (Hz)

Phas

e N

oise

(dB

c/H

z)

0

50

100

150

200

250

Jitte

r (as

rms)

Noise Floor

Theory

Measured Jitter Density

IntegratedJitter

Cr:LiSAF Laser Phase Noise

-280

-250

-220

-190

-160

-130

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07Frequency (Hz)

Phas

e N

oise

(dB

c/H

z)

0

50

100

150

200

250

Jitte

r (as

rms)

Noise Floor

Theory

Measured Jitter Density

IntegratedJitter

33

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Conclusions Long term stable (10h) sub-10 fs timing distribution system

is completed.

True long term stability (forever): Implement Polarization Control

Master Oscillators commercially available + amplifier >400mW of output power > 10 links)

300 m Fiber Links, over 10h < 5 fs ( < 1fs possible)

Optical-to-Optical Synchronization, over 12h < 1fs

Optical-to-Microwave Synch., over 10h < 7fs ( < 1fs possible)

Solid-State Lasers show timing jitter [1kHz – 10 MHz] < 200as (<50as)

Continued development of this technology seems to enable < 100as long term stable timing distribution.