Optica Applicata, Vol. X V , Xo. 1, 19SÔ
Nonlinear laser spectroscopy applied to SF6
Edw ard F . Plik sk i, Romuald Nowicki
I n s titu te of T elecom m unication a n d A coustics, T eclniical U nivei>ity of W rocław , W ybrzeże W y sp iań sk ieg o 27, 50-370 W rocław , P o lan d .
T he in v e stig a tio n s on L am b dip effect in su lfu r h e x a flu o rid e as a reso n a n ce -a b so rb in g m ed iu m u sin g a low po w er cw C 0 2 laser are p re se n te d in th e pap er. T he locations of S F 6 a b s o rp tio n p ea k s are given in re la tio n to th e centres of th e resp e ctiv e C 0 2 em ission lines, such as P 12, P14, PIG, P 18 an d P 20 of 10.6 [ini.
1. Introduction
Xonlincar laser spectroscopy enables the effective elim ination of the Doppler broadening of spectral lines, i.e., the penetration within the Doppler profile, and permits spectroscopic m easurem ents of unusual precision [1 -3]. In experi m ent with nonlinear laser spectroscopy, where the investigated absorber is placed in a cell outside the laser resonator, the absorber molecules are excited sim ultaneously wdtli two laser waves travelling oppositely, i.e., with a saturate wave and a probe w ave. If the laser frequency Q is detuned from the centre co0
of the absorption line, then the two oppositely directed waves, the saturate and the probe ones, will interact with two distinct groups of absorbing spectral profile, i.e., w ith m olecules, the Doppler shift for which is equal to cov = Q
(1 ± » /c ) , where v ■ velocity of absorbing molecules, c - velocity of light. In this
case, the probe wave is attenuated by another group of absorbing molecules and Lamb dip effect cannot be observed [4]. If the laser frequency Î2, i.e., the
frequency of the two opposite waves, coincides with the frequency of the absorp- iion line centre m0, the oppositely travelling probe w ave will interact with the same group of absorbing molecules having no m otion component along the direc tion of th e laser beam. Thus, the probe wave will interact with the group of m olecules saturated by the intense saturate wave, and therefore the absorption of the probe wave will decrease near the frequency of the absorption line centre co0. As a result sharp peak in probe wave transm ission (so-called inverted Lamb dip) can be observed on a laser gain profile as the laser frequency is tuned.
In this paper the experim ent with Lamb dip spectroscopy has been performed for sulfur liaxafluoride as an examplary absorber and a tunable low power cw COa laser as a spectrom eter. The experim ent has been carried out for P12, P l i, PIC, 1*18 and P20 laser em ission lines of 10.C ¡Ain band, i.e., for those ones coinciding w ith S F 6 absorption lines.
2. Experiment and its results
A homemade cw C 0 2 laser described previously in paper [5], has been used in the experim ent. In th e laser used for th e spectroscopic m easurem ents a diffrac tion grating was placed instead of one of th e mirrors. The laser produced an output power of approxim ately 2 W per one preselected emission line. The cavity on the opposite side of the grating was term inated by the concave (7i = 10 m) gold-coated mirror w ith transm ission hole 2.5 mm in diameter [6]. To adjust the position of the coupling-out hole mirror a piezoelectric transducer PZT was used [7]. The laser was tuned by means of saw-tooth signal from an oscilloscope (Fig. 1).
P ig . 1. E x p e rim e n ta l se tu p : D 6 - d iffra c tio n g ra tin g , D - d ia p h ra g m , CH - ch o p p e r (283 Hz), B S - b e a m sp litte r, A - a tte n u a to r , M, - o u tp u t laser m irro r, M2- to ta lly reflec tin g m irro r
The setup of the laser spectrom eter consisted of a 10 cm-long glass absorption cell of diameter of 2.5 cm filled w ith S F 6 and term inated with NaCl Brewster-an- gle windows. Inten se output laser beam , as the saturate w ave, fell onto the absorption cell. According to the theoretical investigations [8 ],in order to obtain m axim al slop of th e first derivative of absorption peak the in ten sity of th e saturate wave should be approxim ately tw ice higher than th at of absorption saturation. This is a necessary condition both for the achievem ent of higher resolution of the first-derivative m ethod of spectroscopy (like in E P R spectro m eters [9]) or for obtaining m axim al frequency stability of output laser radiation during the laser stabilization to the zero of th e first-derivative signal [10].
The measured absorption saturation in ten sity of S F 6 am ounted approxim ate ly to 3.5-6.5 W
x
cm “ 2 Ton·” 1 for P 1 2 -P 2 0 em ission lines of 10.6 pm band [11]. Spectroscopic m easurem ents have been performed for intensity of the saturate w ave of about 1 W X cm “ 2 under pressure of 80 mTorr for S F 6.The saturate w ave, having passed th e absorption cell was reflected from totally reflecting mirror in the opposite direction and having been attenuated in a silicon attenuator, returned through the absorption cell as th e weak probe w ave. As it is known, the probe w ave should be attenuated repeatedly with respect to the saturate w ave in order to obtain contrastive peaks. In our experi m ent the probe wave was attenuated four tim es. The saturate w ave and the
Nonlinear laser spectroscopy applied to S F 6 41 probe one were monitored with non-cooled C dllgTe detectors and sim ultaneously observed w ith an oscilloscope, whereas the laser frequency was scanned in the
c/2L range by means of piezoceramic transducer. In our case the mode distance
was c/2L = 140 MHz (for the resonator length L = 1.07 m), which allowed the
voltage-frequency scaling of the piezoceramic transducer. This step was neces sary for location of the absorpt ion peaks in relation to the laser em ission line centre.
The exam ples of such absorption peaks obtained at PIG, P18 and P20 laser emission lines of 10.6 pm band can be seen in Pigs. 2a, b, c. The observed peaks have been registered sim ultaneously along the C 0 2 laser emission lines (upper
F ig . 2. E x am p les of th e ab so rp tio n p ea k s o b ta in e d a t th e em ission laser lin e s: a - P16, b - P 18, c - P20
curve) which perm itted us to measure the pump offset in frequency scale (Table 1). The absorption peaks have been observed at five CO, laser lines. If, however, the first-derivative method was used, two absorption peaks at P12 emission line of C 0 2 laser, a peak at I’l l line and some additional peaks at PIG and P20 lines have been observed.
The S F 6 absorption lines coinciding' with the respective CO, laser emission lines are shown in Table 2 [12-15].
T a b l e l.T lio lo c atio n s of th e SF„ ab so rp tio n p ea k s a t th e CO" laser linos C 0 2 em issive laser lin e F re q u e n c y shift, in re la tio n to the of an a b so rp tio n p ea k ! laser line ce n te r, [M Hz] P re s e n t w ork Kef. [18] P 1 2 + 1 5 ( ± 1 ) * — - 1 5 ( ± 1)* — I ’ l l + 2 0 ( ± 1 ) * — F I G + 1 1 ( ± 1 ) * + 10.5 — 8 ( ± 1) — 5.0 F I S + 12 ( dt 1) -1- 9.5 P 2 0 + 2 6 ( ± 1 ) 4-10.0 - 35( ± 1) * - 13.0
* th e p ea k s id e n tifie d b y m eans of th e f irs t-d e riv a tiv e m e th o d
T a b l e 2. T he S F 6 a b s o rp tio n lines co inciding w ith th e resp ectiv e CO, laser em ission lines
0 0 2 em issive l a s e r l i n e S F 6 a b s o rp tio n lin e of b a n d [12] [13] [14] [15] P I 2 — — K70 — I ' l l K 25 ± 1 K 3 0 K 28 K2S 1*16 Q 54 ± 1 0 — — — P I S P 4 0 ± 2 P 3 4 P 3 3 — P 2 0 PG9 ± 3 — P 5 9 P 5 9
3. Conclusions
The results of the above experim ent indicate th a t a single-mode tunable cw C 0 2 laser may be a useful radiation source in a laser spectrom eter which allows the investigation of fine structure of molecular system s with high resolution. U nfortu nately, the band width of such a laser spectrom eter is lim ited to the tuning range of the laser frequency, i.e., to the Doppler broadening of a C 0 2 laser line (about •10-70 MHz in th is experim ent). The band of the considered laser spectrometer
Nonlinear laser spectroscopy applied to S F 6 43
m ay be increased by using either a tunable c\v COa laser with the m ixture of a few isotopes of COa m olecule, or a high pressure waveguide C 0 2 laser. In these cases, the tuning range of the laser frequency m ay be increased to a few hundred MHz
[16] and to about 1 GHz [17], respectively.
Acknowledgem ents - T he a u th o rs w ould like to th a n k D r. K . M. A bram ski an d D r. E. M atras
w ho s p e n t m a n y h o u rs d iscussing th e su b je c t an d g re a tly c o n trib u te d to th e re a liz a tio n of th e ex p e rim e n t.
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Received, A u g u st 2d, 1981
Нелинейная лазерная спектроскопия в применении для S F G
В работе представлены исследования по провалу Ламба в поглощающей среде гексафлуорида серы при использовании СО, лазера малой мощности. Представлены расположения пиков поглощения относительно центра эмиссионных линий Р12, Р14, Р16, Р18 и Р20 полосы 10,6 |тм СО, лазера.
