International Journal of Infrared and Millimeter Waves, Vol. 12, No. 5, 1991

ACOUSTOOPTIC EXTENSION OF THE FREQUENCY TUNABILITY OF CW CO2 LASERS: NEW FIR

LASERS EMISSIONS FROM CH3OH AND 13CH3OH

G. Carelli, N. Ioli, A. Moretti, G. Moruzzi, D. Pereira, ~ and F. Strumia

Dipar t imento di Fisica dell'Universitgt, Pisa P iazza Torricelli 2, 56126 Pisa, I ta ly

Received February 20, 1991

ABSTRACT

We have increased the frequency tunability of our CW waveguide CO 2

lasers by means of an acoustooptic amplitude modulator, operating at the fixed frequency of 90 MHz. The up-shifted, or down-shifted, laser optical sideband can be generated independently by adjusting the orientation of the modulator. The efficiency is larger than 50%. The frequency tunability of

the CO 2 laser around each laser line is thus increased by 180 MHz. To

demonstrate the possibilities of this method, a source composed of the above modulator and of a CW, 300 MHz tunable waveguide CO 2 laser has been used for the search of new large offset FIR laser lines from optically pumped CH3OH and 13CH3OH molecules. As a result 15 and 10 new large offset laser lines were discovered respectively. New assignments of some laser lines are also proposed. We have also measured the Stark

effect, the offset, and the polarization of other already known lines. In particular a Stark effect frequency tuning of about 1 GHz is demonstrated for a laser line at 208.399 Ixm.

Key words :

waveguide CO 2 lasers, new FIR laser lines, CH3OH, 13CH3OH

1_ Permanent address: Dep. EletrSn. Quant, UNICAMP-Campinas-Brazil

449

o195-9271/91/o5oo-o4495o6.5o/o �9 1991 Plenum Publishing Corporation

450 Carelli et al.

INTRODUCTION

The limited tunability (60---80 MHz) of the conventional CO 2 laser pump

source constitutes the main limitation to the number of observed FIR laser emissions from optically pumped molecules. In the recent past, isotopicaUy substituted or sequential lines CO 2 laser have been used in order to increase the number of coincidences with the active molecules. The development of a relatively powerful ( 5 to 15 W in CW regime and 50 -~ 150 W in long pulse regime, respectively), single mode, waveguide (WG) CO 2 laser with a tunability of about 400 MHz [1,2] has been a further important step. This laser has allowed the excitation of pump lines whose offset from the CO 2

laser frequency is larger than the Doppler linewidth so that hundreds of new FIR laser lines from several molecules [1, 3-13] could be observed. A further significant increase in the frequency tunability of the CW CO 2 waveguide lasers cannot be achieved without affecting the output power because of two main reasons : i) the small signal gain decreases for a large detuning from the line center, and this cannot be compensated by a density increase of the active medium [2], ii) the laser length should be reduced in order to increase the cavity free spectral range according to

c 1] Av.-- 2 L

L being the laser cavity length. We describe here an auxiliary technique which extends the tunability by further 180 MHz around any CO 2 laser line. This is obtained by modulating the laser beam intensity at 90.0 MHz. For this purpose we use an efficient acoustooptic modulator located outside the laser cavity. Thus the modulator does not interfere with the laser operation and the frequency of the laser beam is up- or down-shifted by further 90 MHz in addition to the intrinsic tunability of the laser. Another advantage of an efficient external modulator is that the whole tunability profile of the laser is up- or down-shifted. Therefore the laser power at the ends of the tuning range is larger than what would be obtained at the same frequency from a shorter laser cavity without acoustooptic modulator, as shown in fig. 1. A final advantage of the acoustooptic modulator operating in the Bragg regime is that, with a proper orientation of the crystal, only one sideband is generated

New FIR Emissions 451

at one time, thus avoiding any confusion in the interpretation of the results.

EXPERIMENTAL APPARATUS

The acoustoopfic modulator is the model AGM-903 from Intra Action Corp. and consists of a driving lithium niobate transducer bounded to a Ge

= -

z

= a+90MHz

/ Frequency

300 MHz

480 MHz

Fig. 1- Output power from different WG lasers as a function of the frequency tuning: a) laser cavity of 300 MHz free spectral range; b) laser cavity of 480 MHz FSR; b) and b') available laser power when the laser beam of curve a) is frequency shifted by the acoustooptic modulator.

crystal operating in the longitudinal acoustic mode. The modulator is designed to operate in the Bragg regime so that the laser beam is modulated at the same frequency ( 90.00 MHz ) and a diffracted beam upshifted or downshifted by 90 MHz emerges from the Ge crystal deflected by an angle of about 10 ~ The laser beam must enter at the Bragg angle for maximum

diffraction efficiency (50=-60 % ), The diffraction efficiency is independent of the frequency tuning of the CO 2 laser in the free spectral range of the longitudinal TEM00 mode. The diffracted beam will then reproduce the whole longitudinal mode up or downshifted by 90 MHz when the CO 2 laser is tuned (fig. 1).

We have tested the reliability of this technique on a 300 MHz tunable WG

452 Carelli el al.

CO 2 laser used as a pump source for the CH3OH and 13CD3OH

molecules. These molecules were chosen because they are well known for

their rich absorption spectrum around 10 ~tm and for the efficiency of their

FIR laser lines.

Two different FIR laser cavities were used. The first is a I00 cm long open

cavity resonator terminated by two spherical copper mirrors 9.5 cm in

diameter and 57.5 cm in focal length. One of the mirrors has a 0.2 cm axial

hole for the pump input, while the FIR output is obtained from a 45 ~

coupling mirror that can be radially adjusted for maximum output power.

The mirror diameter permits a good operation at laser wavelengths up to

about 3 mm. The ratio between cavity length and focal length of the mirrors

was computed according to paraxial optics, in order to assure the best

overlapping between FIR and CO 2 radiation in the cavity, to avoid a

refocalization of the CO 2 radiation on the mirrors, and to separate the

resonant cavity lengths of the TEMoo n modes from the higher order

modes. One mirror is mounted on a precision screw equipped with an

optical encoder giving 1 ktm resolution in the cavity tuning .This cavity

allows the observation of short as well as of long wavelengths, and the

measurements of the relative polarization of the FIR emission with respect

to the CO 2 radiation. Optoacoustic absorption is monitored by an

associated microphone, this is important for finding coincidences between

CO 2 laser emission and CH3OH absorption lines.

The second FIR cavity is a hybrid metal-dielectric WG cavity 144 cm in

length, which allows Stark effect measurements on laser emission [14].

The combination of a tunable pump source with the Stark effect permits to

obtain a frequency tunability of hundreds of MHz in the case of a few FIR

laser lines, as discussed below. In both FIR lasers the wavelength can be

measured by using the cavity itself as a scanning Fabry-Perot

interferometer, calibrated by using FIR laser lines of known frequency.

RESULTS

i)- CH3OH a) -New Laser Lines

The optoacoustic absorption was investigated around each CO 2 laser line,

N e w F I R E m i s s i o n s

and new strong absorptions were discovered, as shown in fig. 2.

453

4 0 0 M H z

4 0 0 M H z

~+90MHz ~+90MH z 1 ,,I z |

o 3 0 0 M H z - 3 0 0 M H z --4

vo+ 90 MHz %+ 90 MHz l ----q I ~ l

3 0 0 M H z 3 0 0 MHz

Fig 2 - a) CO 2 laser power emission as a function of the frequency tuning

around the 9P18 and 9P30 lines; b) CH3OH optoacoust ic

absorption spectrum ; c) FIR laser power output showing also the

Transferred Lamb Dip for a precise pump offset measurement.

Using these absorptions as pump transitions led to the discovery of 15 new

laser lines with a large pump offset. We could also measure the offset

and/or the relative polarization of other, already known, lines, thus increasing the number of the information available for their assignment.

The results are summarized in Table 1, where the subscripts u and d

indicate that the laser lines were observed by using the upshifted and downshifted modulation sideband, respectively.

454 Carelli et al.

Table 1- New FIR laser lines from CH3OH

CO 2 FIR Rel. Offset pump (Bm ) Pol. (MHz)

Rel. Press. Comment Int. (mTorr)

9R(22)u 84.9(2) // +163 m 9R(10) 285.56(12) // +52 9R(06)d 53.9(2) // -140 m

67.2(2 ) _1_ -140 m 90.3(2) // -140 m 70.9(3) _1_ -125 vw

203.0(3) // -105 w 9R(4) 253.(1) // +130 9P(08)d 65.7(2) // -142 w

77.4(2) .L -142 w 96.9(2) // -142 w

9P(10)u 45.5(1) // + 65 m 170.3(5) _1_ + 65

9P(18)u 205.0(1) // +163 vs 304.8(2) .1_ +163 m 628.3(4) // +163 m

9P(20) 51.2(5) // +140 9P(26) 43.0 // + 52 w

82.0 .1_ + 52 m 169.1(3) _1_ + 52 w

9P(30)u 116.8(3) _1_ +196 vw 177.5(3) // +196 vw

9P(32) 274.3(4) // -60 9P(40)u 365.8(4) _1_ +225 w 9P(42)u 255.04(4) // +198 s 10R(42)d 167.881 // -125 m

261.587 /! -125 m 464.918 _k -125 w

10R(34)d 223.8(3) // -70 m 10R(20)u 145.05(10) // +147 s

209.03(10) // +147 s 459.4(2) .1_ +147 m

10R(14)d 127.17(8) // -115 m 10R(10)u 124.6(3) // +100 m

190.93106 // +100 s

25 new offset

45 new 45 new 45 new 13 new 13 new

(a) 12 offset (a) 12 offset (a) 12 offset (a) 15 (a) 15 (a) 10 offset (a) 10 offset (a) 10 new

(a) 15 15 13 new 12 new 12 new

offset (a) 9 new

10 offs.+pol. 10 (b) lO (b) 10 new 20 new 10 offset 10 offset 10 new 12 new 13 (c) 13 (c) freq.

New FIR Emissions 455

10P(12) 42.05(30) _1_ -63 (a) 10P(16) 75.9(2) _1_ -41 (a)

99.86(10) _1_ -41 (a) 10P(30) 118.80(5) .1_ +15 m (c)

(a) ref. 24 ; (b) ref. 6 : (c) ref 25

Most of the new lines were preliminarily reported in [15], 8 new laser lines

were independently discovered by Henningsen et al. [16]. Their results are in good agreement with those of Table 1. The 190.9 Ixm laser line was also measured in frequency ( v = 1 570 160. 75 (40) MHz) by heterodyne comparison with already known FIR laser lines [17].

b) - Assignments

In six cases it was possible to discover or to complete the typical triplet emission of CH3OH, which provides useful hints for the assignments. Recently, high resolution Fourier transform spectra of CH3OH were investigated, and most of the lines in the FIR region from 8 to 200 cm-1 [18,19], and in the IR region from 950 to 1100 cm-1 [20] have been assigned. These new data allow a complete revision of all the CH3OH laser lines assignments, at least for the t-n'st two internal rotation states (n=0 and n=l) , with two new important improvements: i- the pump frequency is

predicted with an accuracy within the Doppler line width of the pump

transition; ii- the frequency of the FIR laser lines is predicted with a typical accuracy of + 60 MHz. Although this precision is inferior to that of the

direct frequency measurements, it is one or two order of magnitude better than that available from wavelength measurements. A review of the C H 3 O H FIR laser lines assignments is in press [21]. Here a few comments are given for the lines of table 1, by using the standard level notation (n,x,K; J):

9R(06) and 9P(08) Tentative assignments were proposed in [ 16] as levels belonging to the n=2 torsional state. Therefore no further predictions can be obtained from FT spectra.

456 Carelli et ai.

9R(04) We can propose the assignment shown in fig.3, which is supported by the

good agreement with the pump transition as predicted from the FTS.

g (131) c~ (122) c~

25+

La

24+

9R04 + 130 MHz

exp 1067.5434 FTS 1067.5440

24+

|

\ X \ \

~ \ Lb

\ \ \ ",

\ \

\ Le

Laser" La = 39.52

FIT" La=39.5810 Lb = 56.849 Lc = 237.767

24+

24-

Fig. 3 - Assignments for the laser lines of the CO 2 9R(04)+130 MHz

pump line

9P(18) The pump corresponds to a very strong absorption in the FTS and is easily

assigned, fig. 4. In addition to the lines of table 1, we could also assign a

laser line observed by pumping with a TEA CO 2 laser [22], as a refill

transition in the ground vibrational state.

9P(40) and 9P(42) Also these lines are easily assigned from the IR spectrum, as shown in fig.

5. Both are transitions with low J values. As a consequence, the laser lines

with the AK=0 selection rule have a too long wavelength to be observed.

New FIR Emissions 457

CO (033) E1 (024) c~

11

10

1 1 l j / L c

FIR Laser : La = 32.81 Lb = 48.78 Lc = 15.92 Ld =44.4

F I T La = 32.7939 Lb = 48.7471 Lc = 15.9440 Ld = 43.9699 refill Le = 15.046

10

P = 9P18 + 163 MHz

exp 1048.6662 FTS 1048.6660

(015) 10

S 9

Fig. 4 - Assignments for the laser fines of the CO 2 9P(18)+163 MHz pump line

10R(42) The third line of the triplet was discovered with a wavelength in agreement with the predicted value [6].

10R(20) Two alternative assignments can be proposed for this triplet: one belonging to the A symmetry [6,23] and one to the E2 symmetry. The first one was disproved because of the absence of the expected triple resonance signal and the observed power decrease as a function of the Stark field [6]. The alternative assignment shown in fig. 6 is, on the contrary, well supported also by the very good agreement with the pump frequency predicted by the FTS.

458 Carelli et al.

A co

(104) c~ (125)

4

Laser : La = 27.33

FIT: L a = 27.3327

9P40 + 225MHz

exp 1027.3898 FTS 1027.3899

4

co co E1 (024) (003)

4

Laser : La = 39.209

F f f : La=39 .2094

9P42 + 198 MHz

exp 1025.3042 FTS 1025.3053

Fig. 5 - Assignments for the CO 2 9P(40) and 9P(42) pump lines.

(032) c~ E 2 (023) c~

3O

29

29

Laser : La = 68.94 Lb= 21.76 Lc = 47.84

FIT: La = 69.1072 Lb = 22.1236 Lc = 47.9111

10R20 + 147 MHz

exp 975.9353 FTS 975.9351

31

Fig. 6 - Assignments for the CO 2 10R(20)+147 MHz pump line

New FIR Emissions 459

10R(10) The assignment for this pump is supported by the good agreement of both

the pump transition and the FIR laser line measured in frequency with the values predicted from the fit of the FTS (fig. 7 ) , even for such high J values. This combination excludes other possible alternative assignments. The third laser line of the triplet is predicted to have a wavelength of 364.855 gm.

E1

33

32

(015) c~ (006) c~

ka ~

Laser : La -- 52.3749 Lb = 80.25

FIT : La = 52.3777 Lb = 79.8841

34

10R10 + 105 MHz

exp 969.1428 FTS 969.1432

32

Fig. 7 - Assignments for the laser lines of the CO 2 10R(10)+105 MHz pump line

c) -Stark Effect The Stark effect on the FIR laser lines was investigated by using a hybrid waveguide FIR laser cavity. The experimental apparatus was arranged so to excite transitions with the AM----Z-_I selection rule. Therefore a FIR laser line splitting into two components, and thus a practical frequency tuning of the laser, is expected for those transitions with the AJ=AK=+I selection rules [14]. Here we report some interesting results.

460 Carelli et al.

9P(18) A splitting into two components was expected for the very strong 205 p.m

laser line. This was observed as shown in fig. 8, where the laser output

power is recorded as a function of the FIR cavity tuning. The laser line is

split into two components as expected, but with a quite different intensity.

; 1 450 500 500 600 V/cm

Fig. 8 - Observed Stark effect line splitting on the 205.14 lam laser line

The corresponding frequency splitting shows a non linear dependence on

the Stark field intensity (fig.9). This apparently anomalous behavior is

explained by considering that the Stark effect is large also for the pump

transition so that, by increasing the electric field, the pump selection rule

changes from AM=+I to separated AM=+I, or AM=-I. In the first case we

can predict a splitting of about 53 MHz.cm/kV, while in the second case

the splitting is reduced to about 40 MHz.cm/kV, as shown in fig. 10. The

predicted values are in agreement with the observed asymptotic splitting of

36+_2 MHz.cm/kV, and the values are larger than expected at low Stark

New F I R Emiss ions

fields.

461

30

5

0 i i i i , I , I , I ~ | ,

0 100 200 300 400 500 600 700 800 900 Electr ic Field Intensi ty - V /cm

25 1,4

- r -

, 20

r . m

15 { D

Fig. 9 - Observed Stark effect splitting of the 205 gm laser line pumped by

the 9P(18) CO 2 line.

1 6 0 ] ~ - - ~ PumpAM=+I

120 ~ PumpAM=- I

.~ g0

0

Fig. 10- Calculated relative intensity of the Stark multiplet of the 205 ~tm laser line

462 Carelli et al.

9P(20)

The line at 51.2 ~tm was tentatively assigned as a (2,2,4;6) - - (2,0,3;5)

transition pumped by an R line [16]. The assignment cannot be confirmed

from the FTS data. However a large and linear Stark effect can be predicted

with a splitting into two lines separated by about 78.5 + 4 MHz.cm/kV.

The prediction is in good agreement with the observed value of 81.5

MHz.cm/kV, as shown in fig. 11.

50

40

N " I -

' 30 e -

20

10

O ~

o

�9 l ' I ' I ' I ' I "

l ! I I !

100 200 300 400 500 600 Electr ic Field - V/cm

Fig. 11- Stark effect splitting of the 51 ~tm laser line pumped by the 9P(20)

CO 2 line

9P(34)

Two new very strong and efficient laser lines were discovered for this

pump line with an offset of +125 MHz [26, 22]. The Stark effect

frequency tuning was found to be particularly large for both lines [27]. The

correct assignment was proposed by Henningsen [28]: the pump is a strong Q branch line of A symmetry (0,1,6;6) of the CO stretch vibration,

and the lower FIR laser level (0,2,5;5) is doubled by mixing with a nearby

New FIR Emissions 463

level belonging to another vibrational mode. The wavelength of the FIR

laser lines can be accurately calculated to be 205.547(5) gm and

208.399(5) p_m respectively, from the analysis of the FTS spectra [21]. A

more accurate investigation of the Stark effect is possible by using the large

frequency tunability of the pump source described in the present paper. The

205 gm line splits into two Stark components at electric fields below 500

V/cm. At higher fields each component is resolved into the single M

components. In fig. 12 only the most intense are lasing, corresponding to

M = 4, 3, 2 respectively.

300 V/cm 700 V/cm

t b

J Fig. 12- Stark effect observed on the 205 gm (9P34) laser line. The laser

power is shown as a function the laser cavity frequency tuning.

Two next longitudinal modes are shown

We also found somewhat different Stark splittings for the pump selection

rule AM--0, and for the pump selection rule AM----k_l, (85(1) MHz.cm/kV

and 101(1) MHz.cm/kV respectively in the case of the most intense

component), as shown in fig. 13. This result is expected from the theory

since the most intense M components for the AM--0 pump are M=+I and

M=-1 respectively. The Stark effect on the 208 gm laser line is larger and

more interesting. In fact, the electric dipole moment of the lower laser level

is modified by the mixing so that the single M components are not resolved

464 CareilJ el al.

with respect to their line width, even at a much higher electric field as

shown in fig 14..

70

60

40

~ 2o

10

0 _-=

0

�9 I �9 I " I " I " I " I

205 ~tm - 9P34 . ~ " ~

"' AM=+1 .,,. ~ "" .,,,,"

1 O0 200 300 400 500 600 E l e c t r i c F ie ld - V / c m

Fig. 13 - Stark splitting of the 205 ~tm (9P34) laser line. The values refer

to the most intense component, and are obtained by using the

pump selection rules AM----&_I and AM--0 respectively.

In addition, the laser line splitting into two components is larger (136.0

(10) MHz.cm/kV), and depends linearly on the field strength, as shown in

fig. 15 The maximum power for each component is obtained at different

frequency settings of the pump CO 2 laser, as shown in fig. 16, where the

transferred Lamb dip [29], corresponding to the pump frequency of each

Stark component, is recorded as a function of the CO 2 laser frequency.

Fig. 17 shows the pump frequency required for each component as a

function of the electric field strength. Also the pump tuning is linear, and it can be seen that the 208 ~tm laser line can be pumped up to a voltage of

about 7 kV/cm when a WG CO 2 laser is used in connection with an

acoustooptic modulator. The corresponding frequency tuning of the FIR laser radiation should be of about 1 GHz, the largest known frequency

tuning of a FIR laser by Stark Effect.

New F I R Emiss ions 465

2 200 V / c m 2100 V/cm

Fig. 14- Output power of the 208 I.tm laser line recorded as a function of the frequency tuning of the laser cavity, and for two values of the

electric field. The Stark splitting is equal to two times the cavity

FSR (300 MHz) for a field of 2200 V/crn.

4 0 0

N - r 300

|

.E 200

�9 ~ 1 0 0

0 0 I 000 2000 3000

Electric Field - V/cm

Fig. 15 - Stark splitting of the 208 I.tm laser line.

466 Carell i et al.

0 0 V / c m

TLD

9 8 M H z

Fig. 16- Output power of the two components of the 208 Ilm laser as a

function of the CO 2 laser frequency tuning, and for a Stark field of

2100 V/cm. The transferred Lamb dip (TLD) in the power output

shows the center of the pump transition for each component

150 I I I | I

~ TLD I N

"1- 50

I

c -50

a. -150 E e~

-250 0 1000 2000 3000 4000 5000 6000

E l e c t r i c F i e l d - V / e r n

fig.17- Frequency detuning for the TLD of the two components of the 208 Ixm laser line as a function of the Stark field strength. The Stark tuning is 77 MHz.cm/kV and 59 MHz.cm/kV respectively.

New FIR Emissions 467

ii) - 13CH3OH

a) - New laser lines

New FIR laser lines from this molecule pumped by a CO 2 waveguide laser

were previously reported [30]. High resolution infrared and far-infrared

Fourier transform absorption spectra were employed to investigate

assignments of FIR laser lines [31]. Successive frequency measurements

confirmed the assignment predictions [32]. Here we report 10 new FIR

laser lines discovered by exploring only few pump lines, as shown in

Table 2.

Table 2- New FIR laser lines from 13CH3OH

CO 2 FIR Rel. Offset Rel. Press. Comm. Calculated

pump ( ~tm ) Pol. (MHz) Int. (Pa) (~tm)

9P(28) 97.7(2) # +30 w 9 new

9P(30)d 142.58(8) // - 160 s 11 new 142.532

200.2(5) _1_ - 160 s 11 new 200.138

494.5(5) // - 160 m 11 new 494.335

9P(30) 113.6(1) // -70 s 6 (a) 113.822

147.9(1) _1_ - 70 s 6 (a) 147.932

495.3(5) // - 70 m 6 new 495.005

9P(34) 147.2(1) _1_ - 114 m 13 new

109,8(1) // - 82 w 8 new

70.5(1) .1_ +25 m 18 CH3OH

205,5(1) .1. +125 s 11 CH3OH

9P(34)u 213.6(2) // +144 s 11 new 211.645

335.9(3) 3_ +144 m 11 new 331.675

585.3(6) // +144 m 11 new 584.925

(a) ref. 30

Many additional new laser lines are expected from a more systematic

search. The sample was only 95% enriched in 13C. As a consequence the

468 Carelli et al.

most efficient CH3OH laser lines could also be observed. It is worth noting

that the 205 ktm laser line is classified as a strong line even for this sample.

b) - Assignments

Table 2 shows also the predicted wavelengths of the laser lines assigned

according to [31]. A new assignment can be proposed for the lines pumped

by the 9P(34) as shown in fig. 18.

(0,3,5) E 1 (0,2,6)

11

10

Laser :

FIT :

10 9P(34) + 144 MHz

La = 46.82 Lb = 29.77 Lc = 17.08

La = 47.2488 Lb = 30.1500 Lc = 17.0962 10

Fig. 18 - 13CH3OH assignments for the CO 2 9P(34)+144 MHz pump line

CONCLUSIONS

We have proved the feasibility of a new efficient method for increasing the

frequency tunability of the CO 2 laser used for pumping far-infrared

molecular lasers. The combined use of a WG laser of large tunability, and

of an efficient acoustooptic modulator will permit the discovery of many

new laser lines in the future. We have also demonstrated that the frequency

New FIR Emissions

tunabflity by Stark effect can be further increased.

469

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470 New FIR Emissions

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