Numerical and experimental study of Numerical and experimental study of the mode tuning technique effects. the mode tuning technique effects. Application to the cavity ring-down Application to the cavity ring-down

spectroscopy.spectroscopy.J. Remy, G.M.W. Kroesen, W.W. Stoffels

Eindhoven University of Technology, Applied Physics Department, P.O. Box 513, 5600 MB EINDHOVEN, The Netherlands - EU

TU e/TU e/

The Helium cooled CW infrared laser diode:

Mode tuning range: 0.5 to 2 cm-1 (6 to 15 GHz)

Mode spacing: 1 to 3 cm-1 (3 to 9 GHz)

Power: 0.1 mW; Wavelength: 2 – 6 m

Mode linewidth: 0.0003 cm-1 (9 MHz)

N2 cooled InSb photodiode detector (1 mm2 active area)

The CRDS cavity:

Effective absorption path length: 350 m

Cavity beam waist: 1.72 mm

Spot size on the cavity mirrors: 2.4 mm

FSR :150 MHz; Fundamental mode FWHM: 100 kHz

Plano-concave ZnSe mirrors (R>99.7 %, radius of curvature 1 m). Note that entrance mirror is coupled to a piezoelectric transducer.

Project Objective

Study dust formation in Ar-SiH4 plasmas with CRDS

Define the collective behaviour of a dust cloud

We have developed a new simple ring-down technique that does not require the laser to be turned on and off at the right moment. That new method, called “mode tuning”, uses the ring-down cavity mode structure as well as the optical properties of the laser diode itself in order to control the ring-down effect. We

numerically analyzed the Fabry-Pérot (FP) cavity behavior in terms of changes in scanning rates, mirror reflectivity and laser detuning.

CRDS schematic

The detuning concept

Resonance when laser modes match cavity modes

Out of resonance zone when changed

changes when laser current changes (tens of MHz or 10-5

mA)

Pulse generator with a high repetition rate (hundreds of

MHz)

reaction time in the ns

Simulated Fabry-Pérot fundamental mode structure, with r=0.9985, L=1 m and =5 m. The x axis measuresthe cavity length deviation from its standard 1 meter value, the y axis measures the relative transmitted intensity (Iout_max=1).

The laser line width is unknown to us and is not represented at scale here.

-2.0x10-5 -1.0x10-5 0.0 1.0x10-5 2.0x10-5 3.0x10-51E-3

0.01

0.1

1

No

rma

lize

d s

ign

al i

nte

nsi

ty (

a.u

.)

time (s)

r=0.9975 r=0.9980 r=0.9985 r=0.9990 r=0.9995

FP cavity behavior vs mirror reflectivity

=5 m, L=1 m, v=0.8 mm.s-1

Photons keep being injected into the cavity. For high reflectivity, some secondary oscillations appear in one of the feet of the Airy peaks. The ring-up time decreases when the mirrors are more reflective.

Airy peak secondary oscillations study

*2/12

vc

Lmm

m oscillations minima, =5 m, L=1 m, v=0.8 mm.s-1

(*) From An et al. Optics Letters/ Vol.20, No.9

-2.0x10-5 -1.0x10-5 0.0 1.0x10-5 2.0x10-5 3.0x10-5-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Nor

mal

ized

sig

nal i

nten

sity

(a.

u.)

Time (s)

v=80 um/s v=0.16 mm/s v=0.4 mm/s v=0.8 mm/s v=1.6 mm/s

FP cavity behavior vs piezoelectric translator speed

=5 m, L=1 m, r=0.9985

Photons keep being injected into the cavity. For the high speeds (v > 0.5 mm/s), the ring-up time is faster and oscillations can be noticed in one of the feet of the Airy peak.

-2.0x10-5 -1.0x10-5 0.0 1.0x10-5 2.0x10-5 3.0x10-5

0.00

0.05

0.10

0.15

0.20

0.25

0.30Dt

I2

I1

Nor

mal

ized

sig

nal i

nten

sity

(a.

u.)

Time (s)

(a) v=0.8 mm/s (b) v=1.2 mm/s (c) v=1.44 mm/s (d) v=1.6 mm/s (e) v=2.4 mm/s

=5 m, L=1 m, r=0.9995

The oscillations become more visible as the transducer speed gets higher from curves (a) to (f). I1 and I2 are the

first two maxima of each Airy peak and Dt the time delay between them.

-2.0x10-5 -1.0x10-5 0.0 1.0x10-5 2.0x10-5 3.0x10-5

0.01

0.1

1

Nor

mal

ized

sig

nal i

nten

sity

(a.

u.)

Time (s)

v=80 um/s v=0.16 mm/s v=0.4 mm/s v=0.8 mm/s v=1.6 mm/s

FP cavity behavior vs piezoelectric translator speed when laser switched off

=5 m, L=1 m, r=0.9985

The laser is switched off 1.5 s after the light intensity in the cavity reaches its maximum. The logarithmic scale on the vertical axis shows that the cavity ring-down times are strictly identical.

e

I

IF

L

tc2

2 2

1

Cavity finesse (F)

(*) From Poirson et al. J.Opt.Soc.Am.B/ Vol.14, No11

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

2000

2200

2400

2600

2800

3000

3200

3400

3600

3800

4000

3049

e

(b)

(c)(d)

(e)

(a)

Pi*

c*D

t/L

I1/I

2

314011 2

2

r

r

R

RFth

-2x10-5 -1x10-5 -5x10-6 0 5x10-6 1x10-5 2x10-5

(b)

(a)

Sig

nal i

nten

sity

(a.

u.)

Time (s)

-2.0x10-5 -1.0x10-5 0.0 1.0x10-5 2.0x10-5 3.0x10-510-7

10-6

10-5

10-4

10-3

10-2

10-1

100

Sig

nal i

nten

sity

(a.

u.)

Time (s)

laser switched off detuned laser signal

injected into the cavity

detuned laser signal at 5.25 us

FP cavity behavior vs laser detuning

R=0.9985, L=1 m, v=0.16 mm.s-1

Experimental ring-down when laser is detuned

Transmitted signal through CRDS cavity (a) without and (b) with detuning of the laser.

Detuning the laser or switching it off generate identical numerical results

1.2x10-6

1.4x10-6

1.6x10-6

1.8x10-6

2.0x10-6

2.2x10-6

2.4x10-6

2.6x10-6

2.8x10-6

1.2x10-6 1.6x10-6 2.0x10-6 2.4x10-6 2.8x10-6

th=

exp

m(7,6)

m(4,3)

m(5,4)

m(6,5)

m(3,2)

m(2,1)r=0.9995

exp

(s)

th(s

)

Laser is shifted to half the cavity FSR