Fiber comb lasers for spectroscopy

M. E. Fermann, K. F. Lee, A. Mills, C. Mohr, J. Jiang

IMRA America Inc.

1044 Woodridge Av., MI

Acknowledgements

DESY Ingmar Hartl

Stanford/CREOL Nick Leindecker, Konstantin Vodopyanov

BAE Systems Peter G. Schunemann

Univ. Colorado Thomas Schibli, Jun Ye

Univ. Milano Marco Marangoni, Davide Gatti

Univ. Naples Livio Gianfrani

Overview

• Introduction

• Mid IR comb lasers

Tm fiber combs

OPOs

• Spectroscopy applications

comb referencing of QCLs

QCL spectroscopy

• Summary

Frequency combs & standard molecular spectroscopy techniques

Frequency combs for molecular spectroscopy:

cw: QCLs

Photo-acoustic spectroscopy FTIR

Mass specSingle Line Multi-Line

Spectroscopic Techniques Mass Techniques

Requires ionization

Fragmentation

Measures M/Z ratio

only competes with MS in terms of cost; to compete with MS:

• need broadband mid IR measurements

• need high spectral resolution (≈ 10 – 100 MHz for unambiguous gas analysis)

• need high sensitivity (=sufficient spectral brightness)

For scientific applications:

• narrow linewidth allows proper evaluation of lineshapes,

allowing collision effects to be accounted for.

.

Broadband: gas mixtures

Low power per resolution

bandwidth

Narrow linewidth,

High power,

Slow signal acquisition

1 Line at a time…

Very Sensitive

Wide dynamic range

� � �

How much IR power do we need?

Glowbar spectral power density = 0.01 mW/nm(adapted for 1 GHz resolution FTIR, inversely proportional to FTIR resolution)

Ref. S.P Davis et al., Fourier Transform Spectrometry, Academic Press(2001)

Coherent IR laser source should have:

spectral power density > 1 mW/nm

Incoherent fiber SC spectral power density = 1 mW/nm(at 3000 nm with a fluoride fiber)

Comb technology can give us around 100 times better

sensitivity than incoherent sources with same spectral power

Comb technology should produce 100 times better

sensitivity to make the additional expense of combs tolerable

OPGaAs

Spectral power density of current

coherent mid IR sources

PPLN

GaSechalcogenide

fiber

PPLN

black line = glow bar with 1 GHz res. FTIR

Wavelength (µm)

Red line = incoherent fiber supercontinuum sources

Why mid-IR?

Comparison of molecular absorption linestrength:

Source: HITRAN database

Mid-IR combs promise higher sensitivity for molecular sensing.

courtesy:

S. Diddams

F. Adler

Tm fiber based comb sources

Tm-doped Fiber Properties

efficient cladding pumping at 790 nm

�excellent power scaling capability

(1 kW cw-power demonstrated)

Moulton et al., JSTQE 15, 85 (2009)

Emission cross-section of silica Tm fiber

300 nm

bandwidth

Jackson and King, JLT 17, 948 (1999)

large bandwidth & gain

� ideal for fs-pulse amplification

400 MHz Tm soliton comb oscillator

WDM

TSF DCF

PBS

SA

Graphene

Modulator

SMF

QWP

Low RIN 1564 nm 2 W pump

1.0

0.8

0.6

0.4

0.2

0.0

Inte

nsity

[a.u

.]

-400 0 400Delay [fs]

-1.0

-0.5

0.0

0.5

1.0

Phase [rad]

FWHM 58fs

FROG Intensity Retrieved Phase

1.0

0.8

0.6

0.4

0.2

0.0

In

ten

sity (

a.u

.)

22002100200019001800

Wavelength (nm)

-1.0

-0.5

0.0

0.5

1.0

Ph

ase

(rad

)

retrieved spectrum retrieved phase measured spectrum

Time bandwidth product

0.31, soliton pulse

Intracavity

dispersion

-17,000 fs2

400 MHz Tm soliton amplifier

WDM

TSF DCF

PBS

SA

Graphene

Modulator

SMF

QWP

Low RIN 1564 nm 2 W pump DCF

1.0

0.8

0.6

0.4

0.2

0.0

Inte

nsi

ty (

a.

u.)

205020001950190018501800Wavelength (nm)

Tm Amplifier 8

6

4

2

0

Inte

nsity

(a. u

.)

4002000-200-400

Delay (fs)

102.4 fs

Ref.: J. Jiang, “500 MHz, 58fshighly coherent Tm fiber soliton laser”

CLEO 2012 PDL, paper CTh5C.4.

2.5 – 5.0 W

SC: 1.1 – 2.3 µmHNLF

TDF

TDF

C.-C. Lee et al., Optics Express 20, 5264 (2012)C.-C. Lee et al., CLEO (2012), paper JTu1M.3.

Graphene Modulator design

Nobel Prize in Physics for 2010 to

Andre Geim & Konstantin Novoselov

Graphene loss modulator for fast fceo control

Small effect at DC dueto gain response

Large effect , sincegain cannot follow

locking performance (500MHz comb)

-80

-60

-40

-20

Inte

nsity

[dB

]

2.42.22.01.81.61.41.21.0Wavelength [µm]

FT-IR measurement OSA measurement

f2f

-80

-60

-40

-20

0

RF

Pow

er [d

Bm

], tr

aces

offs

et b

y 20

dB

-2 -1 0 1 2Frequency Offset [MHz]

fceo lock

-80

-60

-40

-20

0

RF

Pow

er [d

Bm

], tr

aces

offs

et b

y 20

dB

-2 -1 0 1 2Frequency Offset [MHz]

fceo lock

Tm comb lock to SF diode laser

-80

-60

-40

-20

0

RF

Pow

er [d

Bm

], tr

aces

offs

et b

y 20

dB

-2 -1 0 1 2Frequency Offset [MHz]

fceo lock

Tm comb lock to SF diode laser

out of loop beat with SF Yb fiber laser

Coherent SC generated with Tm fiber sysem

0 2 4 6 8 10 12 14 16 180

20

40

60

80

100

G aA sLiN bO3

W avelength (µm )

Tra

nsm

issi

on (

%)

LiNbO3

Advantages of QPM GaAs

Large nonlinear coefficient (94 pm/V)

Non-critical quasi-phase-matching & long interaction lengths for reduced cw threshold (up to 70 mm)

High transparency out to 15 µm

High thermal conductivity (55 W/mK)

Low absorption losses (<0.005 cm-1)

ZnGeP2

Re.: L. A. Eyres et al., All-epitaxial fabrication of thick, orientation-

patterned GaAs films for nonlinear optical frequency conversion,

Appl. Phys. Lett., 79, 904 (2001)

The Future of Mid-IR Frequency Conversion

Quasi-Phase Matched (QPM) GaAs:

mid-IR generation with OP-GaAs OPO Collaboration with:

Nick Leindecker & Konstantin L. Vodopyanov

(CREOL) &Peter G. Schunemann (BAE Systems)

Leindecker et al., Opt. Expr., 30, 7046 (2012)

2.6 – 6.1 µm output

Threshold pump energy ≈ 100 pJ,

Compatible with multi-GHz rep. rates

OPGaAs

crystal

Tm fiber

pump

output

How to lock OPO?

signalidler

• Use spurious doubled signal output from OPO

for OPO cavity length control

• Rep. rate and fceo of OPO automatically locked

OPO lock performance

• cavity length changes move

the cw laser + OPO beat

• feedback beat to OPO mirror

position (5 kHz bandwidth)

• phase lock to pump

• linewidth below 100 mHz

• >80% power in carrier

• <500 mrad cumulative phase

noise

lock

Frequency combs assisted QCL

molecular spectroscopy

• QCL line narrowing

• Precision frequency scanning with QCLs

• Application to molecular spectroscopy

12

10

8

6

4

2

Inte

nsity

x106 x10

6

Frequency

12

10

8

6

4

2

Inte

nsity

x106 x10

6

Frequency

fCEO independent non-linear mixing schemes

① Comb down-conversion (DFG scheme)

② CW-laser up-conversion comb (SFG scheme)

QCLf

DFG n mf f f= − m CEO REPf f mf= + m CEO REPf f nf= +

SFG m QCLf f f= +

120.6604

x106

Frequency1.95

µm1.6

µm9

µm

Gatti et al. Optics Express, 2011, 19, 17520

Comb assisted spectroscopy: Tm fiber comb & 9 µm QCL

1.95 µm

LO

Servo

QCL

Current

Driver

Quantified Line-Narrowing

• Observed line narrowing from 520 kHz to 25 kHz in 1 ms.

• Possible to line narrow 2 MHz bandwidth mid-IR sources to kHz

level using GPS-referenced combs!

• Narrower line widths possible with optically referenced combs.

Spectral noise density

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

Inte

nsity

10005000-500-1000Frequency (kHz)

1 msQCL: Free RunningComb: RF LockedComb: Free Running

Reconstructed lineshape

Optics Letters. Mills, Gatti, Fermann, et al. 37 (2012) 4083.

1st Application:

Ammonia spectroscopy

Ammonia Spectroscopy

ν2 band of Ammonia

500x10-21

400

300

200

100

0

Inte

nsity

cm

/mol

ecul

e

120011001000900800

Frequency (cm-1

)

Current Hitran line-center accuracy ~ 3 MHz.

Rapid, comb-assisted spectroscopy enables highly reproducible scans over multiple lines.

QCL tuning range

sR(6)

k=1…6160x10-21

140

120

100

80

60

40

20

0

Inte

nsity

(cm

/mol

ecul

e)

111011081106110411021100

Frequency (cm-1

)

150

100

50

0

x10-2

1

1103.481103.461103.44

Tm oscillator can tune 5.5 GHz or 0.2 cm-1 continuously.

Doppler width can be measured to give the thermodynamic

temperature if molecular mass (M) and Boltzmann constant (kB)

is known.

Precise knowledge νline, lineshape, and stability of T, gives kB.

Precision Spectroscopy and Thermometry

2ln 2lineD B

Tk

c M

νν∆ =

Precision Thermometry History using NH3 :

• Previously done with single lines of CO2 lasers

• with huge datasets to reduce the statistic uncertainty

Multi-line fitting

• Remove uncertainty of fit

• Three closely spaced lines

• Same pressure and temperature

Thermometry

Measured temperature dependent on true line shape.

Quantify accuracy of line shape by looking at Td/Tmeasured.

• be independent of pressure

• “= 1”

Linear scaling law (νD~ν0)

• Reduces the statistical uncertainty on Tgas

• Voigt Line shape: TD/Tmeasure has negative slope across a pressure range.

• Demonstrates comb-assisted spectroscopy’s accuracy in observation of collisional effects.

• Voigt profile is insufficient to measure temperature.

• Speed dependent Voigt profile

has 0 slope across pressure range.

Thermometry: Line shape

How about phase-locking of tunable external

cavity QCLs?

Line narrow QCLs ~120 cm-1 tuning range.

-2

-1

0

1

2

Vo

ltag

e

20x10-6151050

Time (s)

Lock Not Engaged Frequency "Lock"Time trace of error signal. Unlocked one rails.

-50

-40

-30

-20

-10

0

10

dB

120100806040200Frequency (MHz)

Spectrum Analyzer: Red: UnlockedBlue: Locked (Div 16=Div 4)

-50

-40

-30

-20

-10

0

10

dB

120100806040200Frequency (MHz)

Spectrum Analyzer: Unlocked

Frequency-locking of EC QCL

EC QCL linewidth = 25 MHz

Error signal of QCL:

Most noise arises from

High bandwidth temperature

control feedback loop

Blue = QCL locked

Linewidth = 3 MHz

CO2 sub-Doppler spectroscopy

4.46 µm CO2

Sub- Doppler spectroscopy with EC QCL

Frequency accurary:

10 - 100 kHz

1.2

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

Vo

ltag

e (

V)

2239.02238.52238.02237.52237.0

Frequency (cm-1

)

10-25

10-24

10-23

10-22

10-21

10-20

10-19

10-18

Stre

ng

th (cm

/mo

lecu

le)

Scan of CO2 absorption features

• Low noise OPO combs pumped by

high power Tm fiber combs

• OPOs are ready for prime-time in molecular

spectroscopy

• Ultimate aim is to be better than mass spectrometry

• QCLs locked to fiber combs offer useful tools for

precise line-shape analysis

• Collisional effects in multiple gas systems can now be

studied

Summary

IMRA Prototypes

• Tm, Ho, Yb fiber amplifiers

• Mid-IR laser technology

• frequency conversion

dual Tm comb system

• cw fiber pump lasers

• Control & feedback

High energy Tm fs source