Frequency shift of rotation-vibration band oscillation in carbon dioxide lase

Mocker, H. W.

English

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https://ntrs.nasa.gov/search.jsp?R=19690004733 2020-03-12T07:57:22+00:00Z

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Honeywell Report l2044-PR4 1 Octoher 1967 
Prepared by: 
• 
Reviewed by: 
CARBON DIOXIDE LASER 
For Period 1 July to 15 Sept. 1967 
National Aerotaautics and Space Administration 
Marshall Space Flight Center 
Contract No. NAS8-20645 
Hans W. Mocker 
Approved 
J. E. Killpatrick 
HONEYWELL INC. 
Systems and Research Center 
Research Department 
2345 Walnut Street 
St. Pau~, Minnesota 55113 
bY::}-X.~' 
\/. S. Levadi 
Aerospace Sciences 
Research Manager 
I. Introduction ~ 
1-
This is the fourth monthly progress report for the Contral:t NAS8-20645. 
This report is a summary of the results obtained by heterodyning two 
passively stabilized carbon dioxide lasers on the surface of a mercury-
cadmium-telluride detector. The lasers were built on Contract NAS8-l8624 
and were delivered to NASA on 18 July 1967. Reported are the results 
on "Pressure and current dependent shifts in the frequency of oscillation 
of the CO,a las~rtt. The manuscript will be also sent to "Applied Physics 
Letters" for publication. 
Progress made so far on the one-way communication system utili:~ing two I 
CD.; lasers will be reported on 15 October 1967 • 
.. 
• 
Pressure and Current Dependent Shifts in the 
Frequency of Oscillation of the CQa Laser* 
Hans W. Mocker 
Systems & Research Center 
HONEYWELL INC. 
Minneapolis, Minn. 
* This work was supported by NASA Marshall Space 
Flight Center under Contract NAS8-20645 
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Pressure and Current Dependent Shifts in the 
Frequency of Oscillation of the CQa Laser 
Abstract 
The shift of the frequency of oscillation of the 000 1 - 100 0 rotation-vibration 
band at 10.6 micron of a CQa laser has been investigated as a function of 
total pressure, CO2-partial pressure, dis~harge current, and cavity mode 
position. The measurements were made by heterodyning two passively 
stabilized CO2 lasers, both oscillating in a single transverse mode and 
single frequency at the P-branch of the 000 1 - 100 0 rotation-vibration 
band of CQa. The result yielded a 5-8 MHz/Torr frequency shift towards red 
due to an increase in total pressure and a 500-900 kHz/mA shift toward 
blue due to an increase in excitation. The amount of frequency shift due 
to excitation was found to be independent of the location of the cavity 
resonance with respect to the doppler center. The dependence on CQa partial 
pressure was also determined. 
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Since the advent of the CQa laser (1) increasing interest exists in obtaining 
high power levels from oscillation in a single frequency and in a zero 
order tr~sverse mode. The long term frequency stability and the resettability 
of such a frequency controlled CQa laser is affected by pressure and 
current dependent shifts in the center frequency of the 000 1 - 100 0 rotation-
vibration band of CQa. Extensive data has been obtained by several 
authors for the 1.15 ~ and the 6328 J-transition in a He-Ne laser (Ref. 2-4). 
In this paper we repo~t shifts in the frequency of oscillation in a small, 
Jingle mode and single frequency CQa laser due to changes in tlltal pressure, 
CO,a partial pressure, excitation, and cavity mode position. The measure-
ments were made by heterodyning on a mercury-cadmium-telluride detector (5) 
two passively stabi,.lized CQa lasers both having a relative stability of 
3 parts in 1010 • Both lasers were of flowing gaa configuratiol1, identic 
in length and performance and operated from independent gas supplies (Co.., 
" 
N:a, He), power supplies and vacuum pumps. Each laser could be. tuned in 
frequency over approximately 100 MHz with a piezoelectric transducer. One 
laser was used as a reference while the other was varied in pressure, excitation 
and cavity mode position. Both lasers and the heterodyne detector were 
mounted on a spring supported granite slab (3 ton weight). 
The two lasers used in these experiments were of internal mirror design. 
The discharge tube is sealed in a yoke made of the material Cer-Vit* (6) with 
an expansion coefficient a < 10-7 per degree C. The two mirrors. are 
* Owens-Illinois trade name. 
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clamped to the front faces of the Ccr-Vit yoke which is gro~nd flat and 
parallel. In this way the cavity is mechanically rigid and stable and the 
heat dissipated by the discharge tube is thenna11y irsulated from the 
cavity structure. The discharge tube (8mm diameter) has two anodes, one 
at each end, and a center cathode. The gas flow enters the tube at each 
anode and leaves it at the cathode. Doppler shifts introduced by the gas 
flow can produce a broadening but not net frequency shift. The cavity mode 
spacing is 500 MHz. One mirror made of Irtran 2 has a 85% reflecting coat-
ing, the other mirror has a 3m radius of curvature and a gold coating. 
Insertion of an aperture yielded single transverse mode operation. Po1ar-
ization of the output Wolls obtained by direct insertion of 3 wires 0.0001" 
inches in diameter perpendicular to the beam direction. Different losses 
are obtained for the electric vector oscillating either parallel or per-
pendicular to the wires, similar to the infrared wire-grid polarizer (7). 
Both lasers were oscillating at the same single frequency of the 000 1 -
100 0 rotation vibration band of the CQa transition as was evidenced with a 
Perkin-Elmer 99G spectrometer modified for use at 10 micron. The lasers 
oscillated at a power ~:;vel between 3 and 7 watts. 
The laser beams were combined with a germanium beam splitter and their beat 
was detected with a liquid nitrogen cooled HgCdTe detector. The ti~e constant 
of the detector and the connecting cables yielded a flat respOD.se up to 1 MHz 
and one order of m4gnitude signal reduction at 10 MHz. Beats u.p to 30 MHz 
were detected without preamplification of the s~gnal. The relative stability 
of the 1as~rs was measured with a panoramic spectrum analyz~r and was found 
to be 10 kHz/sec as was determined with a sweep rate of 60 (l/sec) and an 
exposure time of 1 sec. Stability measurements hav, been made in time intervals 
, _'- s __ '4. au;a;k up eli 
3 
ranging from 10-1 to 104 seconds and the obt3ined stability 6v(t) as a 
function of time t (in sec) can be represented by the equation: 
These measured stabilities are comparable to the measurements made by 
Freed (8) on sealed-off Co.; lasers oscillating on a lower power level. 
Each of the measurements to be described below was made in a t:lme span of 
less than 20 min. so that the maximum error due to drift was smaller 
then 350 kHz. 
At the b~ginning of each measurement both lasers were operated with the same 
mixing ratio of the gases CQa:~:He and at the same absolute pressure. 
The laser which was to be operated at variable pressures and excitation was 
initially tuned with thp. piezoelectric transducer so it was oscillating at 
the doppler center. Shifts in the frequency of oscillation could be deter-
mined in two different ways (see Fig 1). Frequency shift could be read 
directly on the panoramic spectrum analyzer which has a calibrated range up 
to 26 MHz. The second method employs a low frequency a.c. modulation (200 Hz) 
in series to a dc voltage on the PZT element of the laser under investigation. 
A lock-in amplifier (PAR) which obtains the phase reference from the ac 
terminal, is used for phase sensitive amplification. A dc voltage tuning 
scan with applied ac voltage yieldsa typical discriminator curve with the 
iRtensity cross-over at the doppler center. The frequency widt~ of oscillation 
is approximately 25-30 MHz. This frequency range is bounded on either aide 
by a spike arising from the inter doppler competition region wh~re the transi-
tion under oscillation is expelled by a new transition that haa a higher gain 
at this frequency. 
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For the pressure and excitation measurements this method has been used in 
the following way: a pressure or excitation change creates a s:tgnal at the 
output of the lock-in amplifier. The dc voltage on the PZT element is now 
set to a new potential to make the signal disappear. From the calibration 
curve of the PZT element the corresponding frequency ahift was determined. 
The Signal to noise ratio at the detector determines the smallest required 
frequency excursion at which an intensity cross-over still can be observed 
'p::s 50kHz). The direction of the frequency shifts was determinE!d by an inde-
pendent measurement of the absolute ~ength change of the piezoE!lectric element 
as a function of the applied voltage. The calibration was found to be 
28kHz/V. 
The influence of pressure on the frequency of oscillation ia shown in Fig 2. 
Both cavity mode-s are located at the beginning of the experiment at the 
center of the doppler curve. All pressure shifts are normalized to the re-
ference laser operating at 11.5 Torr and 10mA of excitation. For the laser 
under investigation the excitation current was adjusted to its initial value 
after every change in pressure, while the partial pressure ratio was kept 
constant. For both described methods of determination of the frequency shift 
we obtain at a partial pressure ra-tio of C~:N; :He • 1: 1:3 a frequency shift 
of 5.3 MHz/Torr with a shift direction towards lower frequencies ("red shift"). 
The influence of the partial pressure of CQa to the center frequency shift is 
shown in Fig 3. With increasing partial pressure of CQa the frequency shift 
increases. The largest shifts observed at a CQa partial pressure of 3.6 Torr 
are 7.4 MHz/Torr. 
The current dependent shifts are plotted in Fig 4. The Irequency shifts are 
.. 
5 
normalized to the reference laser oscillating at lOrnA with both lasers at 
a total press,:,re of 12.0 'I.'orr and the cavity modes tuned to the corresponding 
doppler centers at the beginning of the experiment. Over the entire 
range of excitation the frequency of oscillation increases with increasing 
excitation ("blue shift") and amounts t~ 900 kHz/rnA for low excitation 
(5mA/cur) and reduces to 500 kHz/rnA for strong excitation (l5mA/c~). Over 
the corresponding range of excitation the output power increases from 3 
to 7 Watts. 
In Fi'g 5 we have plotted the frequency shifts due to current changes as a 
function of the relative cavity mode position. The reference laser was 
oscillating at the doppler center while the cavity mode of the second laser 
was placed a measureable amount off the doppler center to highE!r and lower 
frequencies. Excitation increases and decreases were made by LI • + lmA at 
each mode position and the frequency shifts were measured. Although a total 
frequency range of over 25 MHz was investigated no dependence of the frequency 
shift with' cavity mode position was found. The two horizontal lines in 
Fig.S for 61· ± lmA represent the arithmetic average of all measurements over 
the 25 MHz scanning range • 
To summarize the experimental results we have found a red shift with in-
creasing pressure and a blue shift with increasing excitation, the latter 
independent of the cavity mode position with respect to the doppler center. 
These results can be interpreted on the basis of a change in plasma parameters 
as a function of pressure and excitation. The index of refraction of a 
plasma is given by the equation (9): 
-.. -------____ --___ 0 __ ----_.,." .... ------__ .... P-. "'_""",4--
Index of refraction 
n -= 1 - 1 ND p~ .1 --~ . 2 IDE" oW-
~ 
contribution from el ( . "!:'ons 
6 
2rr O'm Nm + 
( , 
OJ 
atoms or 
molecules 
in ground 
state 
2fT E 0'1" N. 
" 1 
< 1 ) 
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ions and excited 
atoms 
An increase in pressure enhances the magnitude of the second term and 
therefore reduces the frequency of oscillation. a can be evaluated from 
the Lorentz-Lorenz equation. For the gas mixtures and pressures as used 
in our experiments ty~ical contributions to the frequency shift are 10-12 
(1) 
MHz/Torr which compares with the measured value of 5-8 MHz/Torr. The tenD 
due to ionized components cannot be evaluated due to little kncwledge of 
the dissociation rates in the gas mixture. The contribution from the 
electron term is determined by the electron density. Measurements of the 
electron density by Carswell and Wood (10) for a C~:N2:He mixture at a 
pressure of 5 Torr yielded an electron d~nsity of He = 2 x 1010 (l/~) 
and measurements made by Witteman (11) for a ~:He = 1:7 mixture yielded a value 
for N equal 2 x lefl (1/cm3 ). If all ~f the observed frequency shift with 
increasing excitation is attributed· to a change in electron density the 
corresponding electron density changes between 5 and l5mA/cm'2 amounted to 
approximately 2 x 1012{1/c~) which is a large but not unreasonable number 
for the large current densities and pressures used in the reported experi-
ments. A significant contribution to tPe reduction in index may arise from 
dissociation of C~ molecules and creation of components such as CO and NO 
that have a smaller polarizability than CQa and therefore contribute to an 
increase in the frequency of oscillation. 
Acknowledgment is made to P. Gust3fson for assistance in the design 
of the lasers and to B. 'Stinson for laboratory assistance. 
,. ~ . -
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.. 
.-
REFERENCES 
1. C.K.N. Patel, Phys. Rev. Letters, 13, 619(1964). 
Appl. Phys. Letters, 7, 15(1965). 
2. A. Szoke and A. Javan, Phys. Rev., Vol. 145, l370Kay 1966). 
3. A.L. Bloom and D.L. Wright. Appl. Optics, 5, 1528(1966). 
4. A.D. White, Appl. Phys. Letters, Vol. 10, No.1, (January 1967). 
5. P.W. Kruse, Appl. Optics, Vol. 4, No.6, (June 1965). 
6. A.D. White, Appl. Optics, Vol. 6, No.6, (June 1967). 
1. Y.R. Bird and M •. Parrish, Jr., Joum. Opt. Soc. of America, 
Vol. 50, No.3, (Sept. 1960). 
8. Ch. Freed, IEEE Joum. Quantum Electronics, Vol. QE-3, 6(1967). 
9. J.B. Gerardo and J.T. Verdel'en, Proc. IEEE, 52, 690(1964). 
10. A.I. Carswell and J.I. Wood, Journ. Appl. Ph,s., Vol. 38, 
No.7, 3028(1961). 
11. W.J. Witteman, Appl. Phys. Letters, Vol. 10, 12(15 June 1967) • 
1 
-~ 
List of Figures 
Figure 1: Experimental set-up of the frequency shift measurement with 
spectrum analyzer and doppler center indicator. 
Figure 2: Shift in frequency of oscillation of the 000 1 - lOon rotation 
vibration band of CG,a as a function of total gas prE~ssure. Gas 
mixing ratio CQa:N,a:He • 1:1:3. Excitation current: 10mA 
(Reference laser operating at 11.5 Torr with 10mA e>:citation). 
Figure 3: Absolute p"ressure gradient of frequency shift as a function of 
Figure 4: Shift in fr~quency of oscillatiOn of the 000 1 - 100 0 rotation 
vibration band of CQa"and output power as a function of discharge 
current. Gas mixing ratio: CQa:~:He. 1:1:3. (Reference laser 
operating at 12.0 Torr and lamA excitation). 
Figure 5: Shift in frequency of oscillation for a change in excitation of 
, 6I = ±lmA for different cavity mode positions. .(Reference laser 
operating at 12.0 Torr and l3.0mA excitation). 
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Carbon dioxide laser Progress report, 1 Jul. - 15 Sep. 1967

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