Transcript Lille - GEOCITIES.ws

Numerical and experimental study of the mode
tuning technique effects. Application to the cavity
ring-down spectroscopy.
TU/e
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
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.
The CRDS cavity:
Project Objective
The Helium cooled CW infrared laser diode:
Study dust formation in ArSiH4 plasmas with CRDS
Mode tuning range: 0.5 to 2 cm-1 (6 to 15 GHz)
Mode spacing: 1 to 3 cm-1 (3 to 9 GHz)
Cavity beam waist: 1.72 mm
Define the collective
behaviour of a dust cloud
Power: 0.1 mW; Wavelength: 2 – 6 m
Spot size on the cavity mirrors: 2.4 mm
Effective absorption path length: 350 m
FSR :150 MHz; Fundamental mode FWHM: 100 kHz
Mode linewidth: 0.0003 cm-1 (9 MHz)
N2 cooled InSb photodiode detector
The detuning concept
 Resonance when laser
modes match cavity modes
 changed
 Pulse generator with a high
The laser line width is unknown to
us and is not represented at scale
here.
-5
-1.0x10
-5
0.0
1.0x10
-5
2.0x10
-5
3.0x10
-5
time (s)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-5
-1.0x10
-5
=5 m, L=1 m, v=0.8 mm.s-1
1.0x10
-5
2.0x10
-5
3.0x10
th(s)
-6
2.2x10
-6
2.0x10
-6
1.8x10
-6
1.6x10
-6
1.4x10
-6
1.2x10
-6
m(2,1)
th=exp
3800
(a)
3600
3049
3000
(b)
2800
2400
(d)
(e)
2200
m(4,3)
2.0
2.2
2.0x10
exp(s)
2.0x10
-5
3.0x10
2.4
-5
-1.0x10
-5
0.0
1.0x10
-5
2.0x10
-5
3.0x10
-5
Time (s)
-5
=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.
0
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
10
-7
Experimental ring-down when laser is
detuned
(a)
(b)
-2x10
-5
-1x10
-5
-5x10
-6
0
5x10
-6
-5
-1.0x10
-5
0.0
1.0x10
-5
2.0x10
-5
3.0x10
-5
1x10
-5
2x10
-5
2.6
2.8
3.0
3.2
3.4
R=0.9985, L=1 m, v=0.16
Transmitted signal through CRDS cavity
(a) without and (b) with detuning of the laser.
mm.s-1
I1/I2
m(5,4)
-6
-5
laser switched off
detuned laser signal
injected into the
cavity
detuned laser signal
at 5.25 us
-2.0x10
m(6,5)
1.6x10
1.0x10
Time (s)
m(7,6)
-6
0.0
e
2000
0.01
Time (s)
(c)
2600
m(3,2)
1.2x10
-5
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.
10
4000
Pi*c*Dt/L
2.4x10
-1.0x10
(*) From Poirson et al. J.Opt.Soc.Am.B/ Vol.14, No11
3200
0.1
-2.0x10
-5
FP cavity behavior vs laser detuning
3400
2.6x10
0.00

ct F  I1
   2  e 
L
2  I2

m oscillations minima, =5 m,
L=1 m, v=0.8 mm.s-1
r=0.9995
0.05
=5 m, L=1 m, r=0.9995
Cavity finesse (F)
1/ 2*
(*) From An et al. Optics Letters/ Vol.20, No.9
-6
I2
0.10
Time (s)
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.
 m  m
-6
0.15
v=80 um/s
v=0.16 mm/s
v=0.4 mm/s
v=0.8 mm/s
v=1.6 mm/s
-5
=5 m, L=1 m, r=0.9985
Airy peak secondary oscillations
study
2.8x10
0.20
Time (s)
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.
  2 L   
 
  c  v 
0.0
(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
I1
0.25
-2.0x10
-2.0x10
Normalized signal intensity (a.u.)
0.8
Signal intensity (a.u.)
-2.0x10
0.9
Dt
0.30
Signal intensity (a.u.)
1E-3
Normalized signal intensity (a.u.)
0.01
FP cavity behavior vs piezoelectric
translator speed when laser switched off
1
v=80 um/s
v=0.16 mm/s
v=0.4 mm/s
v=0.8 mm/s
v=1.6 mm/s
1.0
Normalized signal intensity (a.u.)
Normalized signal intensity (a.u.)
r=0.9975
r=0.9980
r=0.9985
r=0.9990
r=0.9995
0.1
Laser is shifted to half the
cavity FSR
FP cavity behavior vs
piezoelectric translator speed
FP cavity behavior vs mirror
reflectivity
1
CRDS schematic
The x axis measures
the cavity length deviation from
its standard 1 meter value, the y
axis measures the relative
transmitted intensity
(Iout_max=1).
  changes when laser
current changes (tens of MHz
or 10-5 mA)
 reaction time in the ns
Plano-concave ZnSe mirrors (R>99.7 %, radius of curvature 1 m). Note
that entrance mirror is coupled to a piezoelectric transducer.
active area)
Simulated Fabry-Pérot
fundamental mode structure,
with r=0.9985, L=1 m and
=5 m.
 Out of resonance zone when
repetition rate (hundreds of
MHz)
(1
mm2
-6
2.4x10
-6
2.8x10
-6
Fth  
R
r2

 3140
1 R
1 r2
Detuning the laser or switching it off generate identical numerical
results