A laser transmission with very high degree of spectral purity was described. Amplified Spontaneous Emission (ASE) dependence on the oscillator energy and the detuning away from the gain curve center are demonstrated. The effects of both finite laser linewidth and spectral purity on humidity measurements are experimentally demonstrated

Flamant, P. H.

Singh, U. N.

English

NASA Technical Reports Server

N87-10290
HIGH SPECTRAL PURITY, NARROW LINEWIDTH LASER
TRANSMITTER FOR DIAL MEASUREMENTS
U.N. Singh
University of Maryland, Institute for Physical Science
and Technology, College Park, Maryland 20742, U.S.A.
P.H. Flamant
Laboratoire de Meteorologie Dynamique du CNRS,
Ecole polytechnique, 91128 Palaiseau, France
The use of the pulsed, oscillator-amplifier dye laser
as a source of tunable coherent radiation for DIAL measure-
ments is attractive because of its wide tunability, narrow
spectral linewidth and high peak power. However, the multi-
stage, high gain amplifiers, beside generating radiation
within a narrow spectral bandwidth (a few picometers), also
generate uncontrolled emission of a broad spectral nature
(covering tens of nanometers) called amplified spontaneous
emission (ASE). Thus the output radiation has two spectral
components: (1) radiation within the required spectral
bandwidth called signal radiation, and (2) radiation outside
the required bandwidth called noise. The spectral purity
can be defined as the percentage of signal radiation in the
total output. The presence of ASE along with signal _a_ia-
tion is an unwanted interference in DIAL measurements_ ,_
To minimize systematic errors associated with DIAL
measurements, a narrow linewidth laser transmitter with a
very high degree of spectral purity is needed.
A frequency-doubled Nd:YAG-pumped dye laser (Quantel
180C ÷ Jobin Yvon HPHR) was used as a light source in our
experiments. Rhodamine 6G and Carbazine 122 laser dyes were
used for the generation of 580 nm and 720 nm wavelengths. A
spectrometer (Spex 1800) in combination with an Optical
Multichannel Analyzer (OMA) was used for spectral purity
measurements. A multipass white cell with a path length of
150 meters was used along with a ratio meter (Lasers Preci-
sion, Inc. model Rj 7_00) to study the influence of ASE on
humidity measurements?
With a Rhodamine 6G concentration of 4.5 x 10 -4 M/L for
the oscillator and 1.8 x 10-" M/L for the amplifier, the
peak of the frequency doubled Nd:YAG-pumped dye laser gain
curve lies near 580 nm. Figure I illustrates the dependence
of dye laser spectral purity on the oscillator energy,
firstly, when emitted radiation is centered at the peak of
the gain curve and secondly, when it is detuned.
With a Car_azine 122 concentration of 3.0 x 10 -3 M/L
and 1.25 x 10 -J M/L for oscillator and amplifier, respec-
tively, Figure 2 shows the dye laser spectral purity
dependence on oscillator energy.
76
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Figure 3 shows the water vapor absorption line at
723.4734 nm recorded using a white cell of 150 meter path
length• This linewidth was found to be 11.6 pm (FWHM),
which is slightly higher than the value of 11_2 pm and 10.6
pm measured, respectively, by Wilkerson et al _ and Grossmann
et al_
For small values of optical depth, a simple theoretical
calculation predicts a linear variation of the White cell
transmission with spectral purity: transmission approaches
unity as spectral purity approaches zero, and becomes
minimum when the spectral purity is high, i.e. approaches
unity.
Figure 4 shows the transmission of the White cell as a
function of spectral purity for the laser line centered at
the peak of the water vapor absorption line at 723.4734 nm.
The spectral purity at this laser output was varied in a
known way by changing the orientation of the oscillator dye
cell. Figure 4 clearly indicates the error involved in
humidity measurements, if the emitted radiation from the
laser transmitter does not have a high degree of spectral
purity•
CONCLUSIONS
A laser transmission with very high degree of spectral
purity has been described. ASE dependence on the oscillator
energy and the detuning away from the gain curve center are
demonstrated• The effects of both finite laser linewidth
and spectralpurity on humidity measurements are
experimentally demonstrated•
References
i • J. Lefrere, G. Megie, C. Cahen, and P.H. Flamant,
Proc. Twelfth International Laser Radar Conference,
Aix-en-Provence, France, August 13-17, 1984.
• U.N. Singh, "Etude d'un nouveau lidar bi-impulsion bi-
frequence pour la mesure de la vapeur d'eau dans
l'atmosphere", Ph.D. thesis in Physics, l'Universite'
Pierre et Marie Curie (February 1985).
•
•
T.D. Wilkerson, G. Schwemmer, and B. Gentry,
J_ Quant Spectros Radiat. Transfer 22, 315 (1979).
B. Grossmann, C. Cahen and J.L. Lesne,
Proc Twelfth International Laser Radar Conference,
Aix-en-Provence, France, August 13-17, 1984.
77
10
_08
Q.
06
..J
L_J
,.,OA
02
Figure I
m
, /
I / • -_.1 =O.ONM
/ /l 11r 9.1NM
--I _ / • 8.7NM
i / 0 e_M
I / • 7.4NM
/
I / _ 47NM
T , 0 o.oNM
I /
I I
I I I
0 0.5 10 1.5
BYE LASER OSE OUTPUT ENERGY
o
mJ
Spectral purity as a function of oscillator output
energy and detuning from the center of the gain curve
(580 nm).
I0
08
a_ 06
04
0,2
C. C.-
Figure 2
0 I I I I I I I >
0 0.04 0.08 0.12 0.16 0.20 0.24 0.28 mJ
OSCILLATOR OUTPUT ENERGY
Spectral purity versus oscillator output energy at
724 nm.
78
0.3
T ,
0.2-
n
W
a
_J
c_
F--
0_ 0.1
O
0. ,1 I
-2z_
Figure 3
Xo = 723.4734 nm
I I 1 I I 1 _ I I J _ I I _
-20 -16 -12 -B -I, 0 .I, ÷B +12 +16 *20 ÷2l, pm
{X-Xo}
Water vapor absorption spectrum at 723.4734 nm obtained
with pulsed dye laser.
Figure 4
,_ 0.92!
0.886
W
mm
LL
O.B5
0
,.o[
-  o- o Go% k
I I I I I >_
0.2 0.L, 0.6 0.8 1.0
SPECTRAL PURITY
Differential transmission of the white cell at
723.4734 nm versus spectral purity.
79


High Spectral Purity, Narrow Linewidth Laser Transmitter for Dial Measurements

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