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NON LINEAR MIR GENERATION IN AMMONIA PUMPED BY A CONTINUOUSLY TUNABLE CO2
LASER
M. Bernardini, F. d’Amato, M. Giorgi, S. Marchetti
To cite this version:
M. Bernardini, F. d’Amato, M. Giorgi, S. Marchetti. NON LINEAR MIR GENERATION IN AMMO- NIA PUMPED BY A CONTINUOUSLY TUNABLE CO2 LASER. Journal de Physique Colloques, 1987, 48 (C7), pp.C7-439-C7-442. �10.1051/jphyscol:19877104�. �jpa-00226920�
JOURNAL DE PHYSIQUE
Colloque C7, supplement au n012, Tome 48, decembre 1987
NON LINEAR MIR GENERATION IN AMMONIA PUMPED B Y A CONTINUOUSLY TUNABLE CO, LASER
M. BERNARDINI, F. D'AMATO, M. GIORGI and S. MARCHETTI" )
E N E A , DIP-TIE Frascati, PB 6 5 , 1-00144 Frascati ( ~ o m a ) , Italy
ABSTRACT
By using high power continuously tunable C02 laser we have frequency analyzed the superfluorescant (S.F.) MIR emission of ammonia around the aR(6,K=OI1 ,2) multiplet near the 9R( 16) C02 llne. I n particular the short pulse without tail produced b y high gain hlgh pressure laser allows the analysis of the SF process without perturbation. We have compared the tlme delay and time width of the SF pulses to some experimental conditions ,taking into account the self-focusing/defocusing of the pump pulse, b y means of some simple relations of the SF process.
INTRODUCTION
Raman scattering I s the most suitable process to produce Intense peak power i n reglons where It i s not possible to generate it with direct sources. Our group has produced many works around these processes i n the infrared ( 1-3). I n particular we have produced tunable FIR emisslon b y using an high pressure continuously tunable C02 laser i n a superflorescent configuration(4-5).
Generally also a superradiant MtR emlssion,related to the FIR scattering I s produced i n these experiments. In fact i n a simple three levels two flelds Raman scattering In
NH3
the FIR emission has a spontaneous gain larger than the MIR one,
so without a suitable expwimental system the MIR Raman scattering cannot be observed. On the contrary i t i s possible to observe the superradiant MIR emission (SF) which takes plece when the FIR Raman scattering i s saturated so that a MIR transition can be inverted , end also It i s possible to observe a resonant four wave mixing MIR emission ( R)which i s present when both the FIR and pump powers are intense. On the contrary of the Raman emlsslon ,both these ones are not tunable.The SF emission i s induced b y a spontaneous polarization of the pump inverted gas ,which takes origin by a spontaneous fluorescence. A general frequency scheme of a l l the superradiant transitlons is reported i n fig 1.
The analysis of the SF process has been reported i n ref( 6-71, To perform a correct exporimentat analysis i t i s v e r y Important that the pump pulse and the SF pulse are time separeted and the time pump pulse i s very shorter than this delay. This condition can be easily obtained when high power pulses are produced i n hlgh pressure gas lasers. A complete analysis needs to take into account the diffraction effects and also the effective focuslng/defocuslng of the pump beam. This fact i s very complicated. So we can restr ict the analysis to some condi tlons:
1 ) The effective pumped volume has a mean section equal the mean pump beam I n the medium 2 ) diffraclion effects are not considered
So we can relate the process conditions to two times: td the tlme delay and tw the time width of the SF pulse. Now we have (7):
("permanent address : IFAM-CNR. Via del Giardino 7, 1-56100 Pis.,. Italy
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877104
J O U R N A L DE PHYSIQUE
J- 1 RM=RFWM
r=Ramen
SF= S u p e r f l u o r e s c e n t
-
=MIR- = FIR
-
= p u m p/ ( A u ) l n ( ~ ) - ' / ~ ( 1 ) t w 2 o c T ~ / g (2)
where A i s the mean section, and N i s the pressure. Now taking Into account that i n our experiments T 2 0 c 1 /N, we can obtain:
t , 2 / t d = ~ i n ( ~ ) - 1 ' 2 ( 3 ) , g & f W -2 ( 4 ) at flxed pressure, and
tw DC.
N- '
( 5 ) , A(N) I / ( N ~ td ) (6) i n a f l r s t approxlmatlon at fixed frequency.The effective radius of the pumped volume Is proportional to A ~ / ~ . To foresee the focusing defocuslng effect we can use the approximate linear expansion( valid only at low pressures)( 8)
A ' / ~ G C I-nnl .and nnl (u)
=Zk(
Bq
(v-vK))/(C+ (v-v,?) (7) where In Eq. 7 we have considered the effect of a l l the ammonia lines of the multiplet(5); ak i s the relative intensity and C can be fixed to - 1 5 ~ m ' ~ whlch i s the extimated power broadening In our experimentSo by uslng EQ. ( 1 - 7 ) we can lastly analyze our SF data vs the frequency and the pressure.
EXPERIMENTAL AND RESULTS
About 6 0 0 - 8 0 0 m J I n 4 0 ns pulses can be produced around the 9R 16 C02 line, b y our modified 1 umonlcs 8 2 0 hlgh pressure laser.
1_
P.D.1
raman cell'I
tunable T E A
12
F i a 2 P . D > = ~ h o t o n drag ,G=grating,p=pump,r=RFWM emission,SF=
superflorescent emission
The power i s focused into a 2 m long cell. Superradiant FIR or MIR emissions can be analyzed separately( 4-5). I n particular the MIR emission ( m j ) i s sep~rated by the resldual pump pulse by meens of grating and time end frequency detected with a lens and a photon drag posed et 5 0 cm from the grating (Fig.2).Two different f r e q u e n k a r e easily separated: the SF aP(8,K=0,1,2) line and the R aP(6,k=O11 ,2) line( 3,5). These emissions are essy to label also when the photon drag i s posed i n a position where both the lines are detected. I n f m t i n fi g.3 the MIR emission i n thls detection conflguratlon i s reported for two different pump frequencies respect to the aR( 6,O) line.
a
b
Fig3: R! left pulse) and SF (rigth) at 7 torr: a)=-0.1 cm-I ,b)=0.5 c m - I pump detuning While the R pulse is very short and synchronous w i t h the pump pulse , the SF i s delved and enlarged. So i n thls experimental configuration we have directly the delay of the SF pulse. I n fig 4 i t I s reported tw vs the pressure at two different pump detunings w i t h the dominant aR( 6 ,k=O) line which we have used to f i x the frequency origin.
n 14 -
* 1 2 -
5 B
l o :c 8
1 :;
1
2.-5
-
- I -+.5 cm
- 1
- . l cm 2
t 0
A,
0 t/N 1 ( 10 t o r r units1 2 3 0::L
1 0 A 10 I t o r r 20 I 30 IF1g4 t vs the 1 /N at different frequencles Flg 5: r-radlus of resldual pump beam (b3- -.2 cm'i)
C7-442 J O U R N A L DE PHYSIQUE
I n f ig.5 i t Is reported the A( N) (the radius) data ( Eq.6) vs the pressure at fixed frequency.
In the same figure the relative measured pump focusing i s also reportedsfhe g(v) curve can be obtained from Eq.4. We expect that this curve i s similar to the absorption coefficient taking Into account the power brodenlng. Besldes the abs rptlon coefficient can be directly measured. A l l these data are reported i n f i g 6. ,with 0.8 cm- power broadening
? .
Finally we can compared the A data from Eq. 3 w i t h the experimental self focused/defocused residual pump radius ( r ) and also the extimated diffrection index (Eq. 7). A l l these data ,with a suitable vertical shift , are reported i n fig 7.
We note that large agreement I n the frequency behaviour i s obt i n
d
i n ths red side o f the rnultlplet , while i n the blue side the agreement between theA l l 3
data and the measured residual pump beam radlus i s poor .Maybe this effect can be produced b y some forbidden llnes Induced by the strong electric fleld.- 1 detunlng 0 ( cm
-
) 1 -1 Fig:7 residual radius(r), diffraction frequency,
Fig 6: g(\) )and the pump absorption 1 /2
vs the frequency Index( n), and A vs the frequency
f = f o r b i d d e n t i n e CONCLUSION
We have analyzed the SF emission i n ammonia vs the frequency b y using a frequency tunable pump source. The hypothesis of a mean inverted section related to a pump mean focused/defocused section i s well confirmed .Many data polnt out some anomalous effects I n the blue side which can be related to some forbidden lines.
REFERENCES
1 ) M.Bernardini ,M.Oiorgl ,A.Palucci ,S.Ribezzo and S.Marchettt:Europhys. Lett. 2 ,695( 1986) 2)M.Oiorgi,S.Marchetti,A.Palucci,S.Rlbezzo:N.C1MENTO D 5
,
3 9 3 ( 1985)3) M. Bernardini ,M.Oiorgi,A.Pelucci ,S.Ribezzo:Opt. Comm.57 4 3 5 ( 1986)
4 ) M. Bernardlni,M. Oiorgi, S.Marchettl:' Single line high efficiency, tunable FIR generation b y resonant four wave mixing i n CH3F pumped b y a multiatmosphere tunable C02 laser ' to be published i n Opt. Comm. 1 9 8 7
5) M. Bernardini ,M. Glorgi, S.Marchettl: 'A continuosly NH3 Raman laser i n the 60- 150 prn wavelength region b y pumping w i t h a continuously tunable multiatmopheric C02 laser' to be published i n Int. J. of Inf. & MM waves 1 9 8 7
6) F.P. Mattar : Appl. Phys. 17 53 ( 1978)
7) C. M. Rnwden, F.M.Mattar: SPlE Vol. 3 6 9 , pag15 1 - 163, Max Born Centenary Conference 1983 , ed. by the Soc. of Photo-optical Instr. , Bellingham WA USA
8)I.A. Al-saidi,D.J. Biswas, C.A.Emshary,R.O.Hariison: Opt. Comm. 52 ,336 ( 1985)
