co_2 laser signature problem

CO, Laser Signature Problem

P. C. Claspy and Yoh-Han Pao

A CO2 laser may oscillate at any one of many possible lines. As the cavity length of such a laser is variedover a distance of X/2, a large number of lines oscillate one at a time in succession. The listing of the iden-tities of the sequences of lines constitutes a signature of the laser and is a matter of practical importance inthe operation of these lasers in remote controlled applications without the use of mode-selecting elementsin the cavity of the laser. We report here a rather remarkable result, namely, that the lines P(20) andP(16) of the 10.4-num band compete so effectively relative to all the other modes that they can berelied on to oscillate over quite a wide gain curve and for a wide range of operating conditions.

1. Introduction

With the use of an appropriate frequency-selectingelement (such as a diffraction grating) in the cavity ofthe laser, any CO2 laser with reasonable gain can bemade to oscillate at any one of a large number of pos-sible rotational transitions in both the 9.4-/im and 10.4-u4m wavelength regions. However, such dispersingelements are known to have quite high insertion lossesand in some applications are to be avoided in the inter-est of increased over-all efficiency. For such circum-stances, it is of interest to know whether some linesalways compete effectively relative to other lines, andwhether the relative dominance of such lines persistsover large ranges of operating conditions.

An investigation of this matter was carried out atCase Western Reserve University in support of parallelinvestigations carried out by J. H. McElroy and G.Schiffner at NASA-Goddard Space Flight Center.

Somewhat unexpected but very fortunate resultswere obtained. These results are consistent with andsupported by the results of NASA. The apparatusand experiments are described in Sec. II. The resultsare contained in Sec. III and are summarized in Sec.IV.

II. Apparatus and Experiments

A reasonably stable CO2 laser was constructed specifi-cally for these experiments. The laser cavity con-sisted of two stainless steel disks connected to each otherby three 58-cm Invar rods 19 mm in diameter. Themirrors were mounted on the disks, and the distancebetween the mirrors was approximately 58 cm. The

The authors are with Case Western Reserve University, Cleve-land, Ohio 44106.

Received 12 June 1970.

output mirror was a 3.2-mm thick, flat germanium disk,antireflection coated on the outside and 80% reflectingon the inside. The other mirror was a 4-m Au-coatedsurface attached to a piezoelectric crystal stack. Thelaser tube itself was a 38-cm long, 5-mm i.d., Pyrextube with a Ni cathode. The tube was supported bytwo clamps attached to one of the Invar spacer rods.Cooling water and vacuum lines were connected byTygon tubing to minimize transfer of vibrations tothe cavity. The space between the Brewster windowson the laser tube and mirrors was enclosed by Mylardrift tubes, one on each end, to minimize the effect ofconvection currents. Output power of the laser wasin the range from about 0.75 W to 1.2 W.

The oscilloscope traces exhibited in Sec. III showthe laser output as a function of laser cavity length.Line identification was accomplished using the arrange-ment shown schematically in Fig. 1. The cavity lengthwas swept over a half wavelength distance for the fol-lowing sequence of parameter changes.

1. Signature or line sequence vs pressure with dis-charge current adjusted to optimize laser output at eachpressure.

2. Signature or line sequence vs current at constantpressure.

3. Effect of gross and arbitrary changes in cavitylength.

4. Effect of insertion of loss in the laser cavity inthe form of a rock salt disk and also in the form of anunevenly heated rock salt disk.

5. Effect of gross changes in pressure.

Although individual line sequences may seem to bequite complicated and confusing, it is nevertheless re-markable that except in one or two extreme cases, thelines P(20) and P(16) of the (001)-(100) C02 bandalways compete effectively relative to other lines and may bedepended to appear if the laser is swept over a distance ofabout X/2.

136 APPLIED OPTICS / Vol. 10, No. 1 / January 1971

Fig. 1. Experimental arrangement.

I , i i i I I I . I i

- Q O N X N D.

0. 0.0. 0., .- 0,M . 0.

Fig. 5. Sequence of lines with 15-Torr pressure and 8-mA dis-charge current. * Indicates lines of the 9.4-,um band.

Fig. 2. Sequence of lines with 18-Torr pressure and 11-mA dis-charge current.

Fig. 3. Sequence of lines with 17-Torr pressure and 10-mA dis-charge current.

"I0 4 0ON1>I'l ~! 0 N ~";0 0. 0. 2 S>-, ,00_ 0 . 0 0

Fig. 6. Sequence of lines with 14-Torr pressure and 7.8-mA dis-charge current. * Indicates lines of the 9.4-pum band.

Fig. 7. Sequence of lines with 13-Torr pressure and 6.8-mA dis-charge current. * Indicates lines of the 9.4-um band.

1. I i , I i . .

:O : 0 N X N .t0A 0 'l

0 . t00th00000 0 .000000cb 0.0005 0 o00 . .9 0.00 e 0l.1

Fig. 4. Sequence of lines with 16-Torr pressure and 10-mA dis-charge current.

Fig. 8. Sequence of lines with 12-Torr pressure and 6-mA dis-charge current.

January 1971 / Vol. 10, No. 1 / APPLIED OPTICS 137

! i j I I

I"I �� �: 9 I 11 11 - I� 11 . . ;z M R;

I I . I I , I , I I i II0 .0 N 0 '0N 0 -C

-. 0. 0. 0. 0. S IG 0 .I 0

Fig. 9. Sequence of lines with 11-Torr pressure and 5-mA dis-charge current.

Fig. 13. Sequence of lines with 15-Torr pressure and 9-mA dis-charge current. * Indicates lines of the 9.4-,m band.

_ 0 NA _

4 0. . 00

Fig. 10. Sequence of lines with 10-Torr pressure and 4.4-mA dis-charge current.

~~~~~~1 _!

Fig. 11. Sequence of lines with 15-Torr pressure and 11-mA dis-charge current.

a. 0. 0 0 . 0. 0 0.0

Fig. 12. Sequence of lines with 15-Torr pressure and 10-mA dis-charge current.

'0 ' N ' 0 '0( -, ONAG2 ,,~0. , .0. 0. 0..0.

Fig. 14. Sequence of lines with 15-Torr pressure and 8-mA dis-charge current. * Indicates lines of the 9 .4-,m band.

I I I I I I I t I I I IGO t V7 N O ' N4 O 30NG1:

0 0. 0i. la . I . .

Fig. 15. Sequence of lines with 15-Torr pressure and 7-mA dis-charge current. * Indicates lines of the 9.4-pm band.

I I I i ,, ,I ., , i

!0 v ' N : N . O: NA I

Fig. 16. Sequence of lines with 15-Torr pressure and 6-mA dis-charge current.

138 APPLIED OPTICS / Vol. 10, No. 1 / January 1971

1 4 �� �: "I I . . 1.z 8! W. . . -: - Z .1 .1

0 't 0 N'0-Z N .

Fig. 17. Sequence of lines with 15-Torr pressure and 5-mA dis-charge current.

i 1=, t V V *i C

.s ;X O. - . 1. 0

Fig. 18. Sequence of lines with 15-Torr pressure and 10-mA dis-charge current, cavity length L.

I I i I . I

sn "sI rx0 O z~; _ ri1 .~ N! 4i - = ;:

Fig. 19. Sequence of lines with 15-Torr pressure and 10-mA dis-charge current, cavity length increased to L + 100 m.

'0 N NO '0 0 '0~ .I 0. .. I ! 0. 1 7

Fig. 20. Sequence of lines with 15-Torr pressure and 10-mA dis-charge current, cavity length increased to L + 200 m.

!: I I I i I I I IN '00 0 GO '0.0 N. 1

0. v AGAGI 0.0 I .. 0.

Fig. 21. Sequence of lines with 15-Torr pressure and 10-mA dis-charge current, cavity length decreased to L.

;. .~ .! . '.0. . &.

Fig. 22. Sequence of lines with 15-Torr pressure and 7-mA dis-charge current, fresh gas fill.

7 I I I I

N o '0 0 V NN AGZ: fX ,1 C 7 t~0... 0. W..

Fig. 23. Sequence of lines with 15-Torr pressure and 6-mA dis-charge current, fresh gas fill. * Indicates lines of the 9.4-pm

band.

Z

N - -

Fig. 24. Sequence of lines with 15-Torr pressure and 6-mA dis-charge current, polished NaCl flat in cavity.

January 1971 / Vol. 10, No. 1 / APPLIED OPTICS 139

Fig. 25. Sequence of lines with 15-Torr pressure and 6-mA dis-charge current, polished NaCl flat in cavity, small temperature

gradient i NaCl.

0 . 0 ..

Fig. 26. Sequence of lines with 15-Torr pressure and 6-mA dis-charge current, polished NaCl flat in cavity, large temperature

gradient in NaCI.

0 0

I Ns

Fig. 27. Sequence of lines with 30-Torrcharge current.

pressure and 10-mA dis-

111. Experimental Results

Figures 2-5 are not very informative because thevoltage of the Lansing sweep drive was not sufficientto bring about a complete line sequence with adequaterepetition. As a result, it would not be entirely clearfrom these four traces that P(20) and P(16) are alwayspresent with quite wide gain profiles. Neverthelessthese lines are present, and reruns for very similarconditions confirm this conclusion. The sequence ofFigs. 2-10 represents the changes in line sequence asthe pressure was changed from IS Torr to 10 Torr.

! I I I I :

o .

X. ,f v s

Fig. 28. Sequence of lines with 25-Torr pressure and 10-mA dis-charge current.

In each case, current was adjusted to optimize output.At 9 Torr, the laser did not oscillate. In all cases thegas fill was 4.65% H2 , 17.5% N2, 16.9% C0 2 , and 60.95%He by volume.

Figures 11-17 illustrate the effect of changes in dis-charge current with the pressure maintained constant.It is seen that the lines P(20) and P(16) are alwayspresent with satisfyingly well-rounded gain curves.(Strictly speaking these are not gain curves but areactually the variation of output intensity vs oscillationfrequency and are related to the gain curve.)

Figures 18-21 illustrate the effect of large and arbi-trary changes in the length of the cavity. It seems thatsuch changes do indeed drastically change the identityof the weaker lines and also the sequence in which allthree lines appear. However, P(20) and P(16) stillsurvive marvelously well.

Figures 22 and 23 illustrate the changes in line se-quence brought upon by a new gas fill. Although theoptimum current is less than before refilling (about 6mA vs about 10 mA), there is no change in the factthat P(20) and P(16) show up clearly.

Figures 24-26 show the effects of the insertion of a 5-mm thick, polished NaCi flat in the cavity to producesome loss. In the cases of Figs. 25 and 26, the rocksalt flat was heated unevenly to produce, respectively,moderate and severe temperature gradients in the rocksalt disk. P(20) and P(16) were essentially alwayspresent as long as there was laser oscillation.

Figures 27 and 28 illustrate the effects of operation ata much higher pressure range. It is seen that P(20)is always present, but P(16) does not appear until thepressure is lowered to about 26 Torr. At that pressureit appears in a well rounded gain curve.

IV. Summary of Results

The results of this investigation show that in appli-cations that depend on remote on and off switching ofCO2 lasers, operation should be designed for oscillationat either the P(20) or the P(16) lines. Such lasers willoperate effectively as long as additional features areprovided for recognizing that the laser is oscillating on,say, the P(20) line, and if a feedback signal is avail-able for locking on either to line center or to a passiveabsorption line in an independent molecular gas.

140 APPLIED OPTICS / Vol. 10, No. 1 / January 1971