High resolution cavity-enhanced absorption spectroscopy

with a mode comb.

T. Gherman, S. Kassi, J. C. Vial, N. Sadeghi, D. Romanini

Laboratoire de Spectrométrie Physique, CNRS UMR5588, Univ. J.

Fourier de Grenoble, St Martin d’Hères, France

CRDS User meeting – Cork University, sept-2006

Research at LSP/Grenoble

Spectroscopy

• molecules of amospheric interest (O3, CH4, CO2…) : ICLAS 0.6-1.1µm, CW-

CRDS in the telecom range

• Isotope effects (in NO2) : LIF, CRDS… on a supersonic jet

Trace detection

• Near-mid IR : DFB diode lasers & OF-CEAS

• Blue spectral range ECDL diode + OF-CEAS

New developments…

Mode-Comb CEAS : Detecting radicals in the UV ?

VECSEL lasers (~2.5µm)

Alain Campargue, Samir Kassi (poster)

Remy Jost

Marc Chenevier

Irene Courtillot (poster)

...

Daniele Romanini

Introduction

• Several CRDS/CEAS schemes exist based on different light sources.

• Increasing interest for Broad Band coverage for several applications. Pulsed

dye lasers, lamps, LEDs, and mode-locked femtosecond sources being tested.

• BB CEAS/CRDS: Need to do a compromise dispersion/luminosity: The more

you disperse, the less light you have per spectral element, the longer you have

to accumulate.

• Other problem is spectral quality and stability of the BB source: Dye lasers

and lamps are not very good, contrary to LEDs and ML lasers.

• ML lasers have high beam quality (mode matching…), and adjustable

spectrum (width & spectral range) by using nonlinear optics (fibers, etc)

laser frequency

Tran

smis

sion

LASER detector

Standard CEAS scheme with laser tuning

Sample gas

Broad-bandLASER

modelocked

CCD

arra

y

Wavelength Multiplexed Spectroscopy

Sample gas

laser frequency

Tran

smis

sion

What is a mode-locked laser ?

( )Gain +

Non-linear (Kerr)Medium (Ti:Sa)

Outputcoupler

( )

Laser cavity ( very simplified! )

Laser at work…

Intracavity giant pulseself-sustained by Kerr lensing

Output : periodic series of pulse replicas !

http://www.mpq.mpg.de/~haensch/comb/research/combs.html

Mode spacing1/Δt

Spectrum of a mode-locked laser ... coherent ensemble of single-frequency lasers

in a real case (laser Ti:Sa) the spectrum has ~10 5 modes

Time domain : Periodic train of pulses

Fourier T. => Spectrum of mode-locked laser

Δtδt

1/δt

Mode spacing1/Δt

Spectrum of a mode-locked laser ... coherent ensemble of single-frequency lasers

in a real case (laser Ti:Sa) the spectrum has ~10 5 modes

Time domain : Periodic train of pulses

Fourier T. => Spectrum of mode-locked laser

Δtδt

1/δt

Spectrum of a mode-locked laser - A closer look -

http://www.mpq.mpg.de/~haensch/comb/research/combs.html

The spectrum transmitted by the cavity depends on the overlap of the two combs of modes :

by changing the cavity length, we observe comb beatings, with different periods…

When combs are in tune, the laser spectrum is efficiently transmitted without beatings…

=> “ Magic point ”

“ML-CEAS” : Basic principle

Cavity transmission spectrum

Modelocked laser spectrum

Output Spectrum = Tcav x Input Spectrum

E. R. Crosson et al., REV. SCI. INSTRUM. 70 (1999) 4

Pulses overlap in phase with circulating intra-cavity pulse :

buildup occurs and transmission is efficient !

Pulse stacking : The time-domain point of view

Laser pulses fall in phase with the pulse circulating in the cavity, which gives a coherent buildup of the intracavity field. This is similar to the single-frequency case (CW-CRDS) but the buildup is not a stationary wave…

An alternative point of view in the time domain…

( )

No coherent buildup, instead a decreasing sequence of output pulses is produced (ring-down...), with initial intensity < T2

Cavity injection is much less effective...

Comparison with the case of a lonely pulse

( )

( )( )

1/T 1/T2

CCD

Millennia pump laser, 5W

L1

L2

Spectrograph

M1

M2

PZT Translation stage

Ti:Sa 100fs, 80Mhz

PinHole

M1,2 : high reflectors around 800 nm

Photodiode

opticalisolator

CEAS with a Mode-Locked laser

Beating between mode combs

848 849 850 851 852 853 854 855

0

1000

2000

0

10

20

0

2

4

0.0

1.5

3.0

Wavelength [nm]

00 μm

150 μm

500μm

1000 μm

Transmitted spectra for different displacements of cavity length from the « magic point »

( smaller peaks are from transverse cavity modes )

0 1 2 3 4

0

5

10

0.0

0.5

1.0

0.00

0.25

0.50

0.10

0.15

Time [ms]

00 μm

20 μm

40 μm

150 μm

… distance from

« magic point »

NOTE: When looking at these signals on a fast timescale, one can recover the pulse train again...

Cavity length tuning by a PZT

Cavity transmission( while slowly tuning its length )

« magic point »

Laser emission

Why we see more than one comb resonances ?f=0 frequency

1

2

3

fine cavity tuning by piezo

↓1

↓2

↓3

These secondary resonances have different intensity: An effect of dispersion

To obtain spectra in transmission, one may either modulate the cavity around a resonance, or use a locking scheme

857 858 859 860 861 862 nm

Effective absorption length F x L /π ∼ 120 m

Acquisition time ~ 10 ms ( cavity length was modulated… )

ML-CEAS : A first demonstration…

Spectrum transmitted by cavity filled with air

Spectrum transmitted bycavity filled with HCCH

laser spectrum

T. Gherman and D. Romanini, Optics Express, 10, 1033 (2002).

BBO

Doubled YAG pump laser, 5W

PM

L1

L2M1

M2

PZT Translation stage

Ti:Sa 100fs, 80Mhz

HR

BBO : doubling crystal

HR : high reflector for cutting the red

M1,2 : high reflectors for 370 - 420 nm

PM : photomultiplier

+ -

gas ingas out

Going to the UV

CCD

Spectrograph

ML-CEAS : Application in the blue…

2 3 7 6 0 2 3 7 8 0 2 3 8 0 0 2 3 8 2 0 2 3 8 4 0 2 3 8 6 0

0

1

2

3

4

5...ra tio & b a s e lin e c o rre c tio n

Abs

orba

nce

[10-7

/cm

]

. . .w ith a c e ty lè n e

Inte

nsity

(a.

u.)

E m p ty c a v ity ...

• Ti:Sa frequency doubled laser : Overtone transition of HCCH with 8 quanta of CH excitation (420 nm)

• Cavity Finesse ~ 3000

• Noise ~ 10–8 / cm

• Acquisition ~ 30 s, (using a basic frequency-locking scheme)

T. Gherman, S. Kassi, A. Campargue, D. RomaniniChem. Phys. Letters 383 (2004) 353

TRSM

ISSI

ON

ML-CEAS : N2 discharge

1000 800 600 400 200 00

5000

10000

15000

20000

Inte

nsity

( C

CD

cou

nts

)

Pixel number

Discharge on Discharge off

Continuous discharge in 1 Torr N2

Cavity: R=98%, L=920 cm, F~150 => Leq~ 50 m

Recording time on CCD ~ 10 s

Band 0-0 of N2+ (B 2Σu

+ - X 2Σg+)

389.0 389.5 390.0 390.5 391.0 391.5

0.97

0.98

0.99

1.00Tr

ansm

ittan

ce

wavelength(nm)

T. Gherman, E. Eslami, D. Romanini, J.-C. Vial, N. Sadeghi, J Phys D 37 (2004) 2408

spin-doubling…Resolution ~ 2E-3 nm( 0.16 cm-1 )

Band 0-0 of N2+ (B 2Σu

+ - X 2Σg+ )

Discharge CW 1 Torr N2Cavity: Finesse ~ 150Exposure time ~ 10 sSimulation : 400°C, 1.6E15 ions/m3

Both sources are sufficiently smooth spectrally…

ML laser emission is highly coherent :

SPATIALLY (TEM00 mode) => Cavity mode-matching

… high cavity injection efficiency

… reproducible & accurate absorption measurements

TEMPORALLY (mode comb) => Cavity comb-locking

… further gain in injection

Lamps easily provide a larger emission spectrum :

Low resolution due to low spectral density and low cavity injection, and to the limited number of spectral elements on a CCD

Less expensive (except for electric consumption ?)

Multiplex CEAS : ML-lasers versus Lamps (or LEDs)

• Robust, simple, multiplex technique

• Real-time, high sensitivity (2x10-9/cm/Hz1/2)

• Quantitative (calibration of cavity finesse by ringdown)

• => UV (efficient femtosecond pulse frequency upconversion)

• Small sample volume…

• Frequency locking of combs allows short acquisition times

even with high finesse cavities…

• Effects of cavity dispersion (e.g. from mirrors) can be

circumvented on relatively large spectral ranges (~>10nm) by

using a small cavity modulation…

ML-CEAS, conclusions & perspectives