Iodine molecular beam absorption resonance as long-term frequency reference for argon ion laser stabilizatio

Ezekiel, S.

English

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LASER FREQUENCY STABILIZATION USING A 
MOLECULAR BEAM 
National Aeronautics and Space Administration 
Grant NGR 22-009-359 
FINAL REPORT 
covering the period 
February 1, 1969 - December 31, 1969 
Submitted by: Shaoul Ezekiel 
March 15, 1971 
Massachusetts Institute of Technology 
Research Laboratory of Electronics 
Cambridge, Massachusetts 02 13 9 
https://ntrs.nasa.gov/search.jsp?R=19710011968 2020-03-11T20:33:32+00:00Z
SUMMARY OF RESEARCH 
This report is a summary of the work done in a research program 
concerned with laser  frequency stabilization using a molecular beam, 
that w a s  partially sponsored by N. A. S. A. under Grant NGR 22-009-359 
during the period February 1, 1969 - December 31, 1969. 
The objective of this initial phase of the program was'the investiga- 
tion into the use of an iodine absorption resonance, observed in a 
molecular beam, a s  a long-term frequency reference for the stabiliza- 
tion of the 5145 A Argon ion laser. 
0 
Briefly, the stabilization scheme is the following: A single- 
0 
frequency 5145 A laser  beam excites the I2 molecular beam at right 
angles and the absorption is measured by the induced resonance fluo- 
rescence of the I2 molecules. This absorption resonance would then 
be used as a frequency reference in a feedback stabilization system. 
Shifts in the Iz resonance frequency are  negligible, because of the 
unperturbed and isolated conditions in the molecular beam. Since the 
molecular beam can be oriented at right angles to the laser  beam, the 
width of the resonance line is limited to the observation width or,  
in this case, t o  the natural width, which is approximately 100 kHz, 
inferred from lifetime measurements . 
In order to carry out the investigation, a short-term stable 
single-frequency Argon ion laser  is essential, i. e., an Argon laser  
with a spectral width of less  than a few kHz and a frequency drift of 
the order of 1 kHz/sec. 
the' Argon ion laser  short-term stability has not been adequately 
investigated. For  our purposes, we were concerned with two sources 
of short-term instability: one w a s  the fluctuation in the refractive 
index of the gain medium attributable, for example, to  plasma oscilla- 
tions and plasma noise in the discharge tube, and the other was  the 
fluctuation in the physical length of the single longitudinal mode cavity 
comprising two cavities, a long one and a short one coupled together 
by means of a servo loop. 
Unlike the He-Ne and the C02-N,-He lasers,  
In the early part of the program several attempts were made, using 
1 
existing equipment, to obtain a high-resolution I2 absorption spectrum 
observed in a molecular beam that falls within the 5145 A laser  band- 
width. 
lines was  of the order of 10 MHz which was much broader than the I2 
Doppler width of -1 MHz because of the slit geometry in the molecular- 
beam apparatus. 
beam, the laser  frequency must be scanned very slowly at -1 MHz/sec 
to obtain an adequate signal-to-noise ratio in the induced fluorescence 
data. 
limited by the spectral width of the laser  and the drift of the laser  fre- 
quency during a scan period. 
0 
The data showed that the width of the individual I2 resonance 
Because of the low density of I2 molecules in the 
Clearly, the resolution of the observed I2 spectrum w a s  severely 
In order to study the frequency instability of the laser ,  an estimate 
of the laser  spectral width had to be made independently from that 
obtained by observing the I2 resonance width in the beam. A pressure- 
scanned, stable, Fabry-Perot cavity with a spectral width of -1 MHz 
was designed and tested with a frequency stabilized He-Ne laser. 
Because of the rigidity of the cavity, lack of mir ror  adjustment, and 
the use of low expansion coef€icient invar as the spacer, adequate 
stability was achieved for use in our study of the laser  spectral 
width. 
Also, the laser  cavity itself was  redesigned using four 1-in. 
diameter, invar rods, terminated by granite blocks. Several designs 
for a stable short (or Michelson) cavity were investigated and con- 
structed. 
adjustments. 
One particularly attractive design eliminated mir ror  
At the same time, an investigation into techniques for the suppression 
of plasma oscillations in the Argon ion laser  discharge tube was  showing 
a great deal of progress and the remaining period under the N A  S. A. 
grant was devoted to the study of these oscillations. 
work have been reported under “Investigations of Coherent Oscillations 
in an Argon-Ion Laser  Plasma Tube,” Applied Physics Letters, 1 January, 
1971, and a reprint of this paper is attached to this report. 
The results of this 
’ 
N. A. S. A. sponsorship, which was to continue for an additional three 
years, was withdrawn in January 1970, because of lack of funds resulting 
2 
from the closing down of the N. A. S.A. Electronics Research Center 
in Cambridge, Massachusetts. 
Publicat ions and Theses 
S ,  Ezekiel, "Laser Frequency Stabilization Using a Primary Frequency 
Reference," Proc. Twenty-third Annual Frequency Control 
Symposium, Atlantic City, N. J. , May 1969. 
T. J. Jach,"Laser-Saturated Iodine Absorption at 5145 A," S. B. Thesis 
Department of Physics, M. I. T. , May 196.9. 
A. J. Mazzella, "Investigation of Properties of a Spherical Mirror 
Fabry-Perot Interferometer and its Role in Laser Frequency 
Stabilization," S. B. Thesis, Department of Physics, M. I. T. , 
June 1970. 
D. C. Galehouse, U. Ingard, T. J. Ryan and S. Ezekiel, "Investigations 
of Coherent Oscillations in an Argon-Ion Laser Plasma Tube," 
Applied Physics Letters, Vol. 18, No. 1, pp. 13-15, 1 January 
1971. 
0 
3 
VOLUME 18 NUMBER 1 APPLIED PHYSICS LETTERS 1 JANUARY 1971 
INVESTIGATIONS OF COHERENT OSCILLATIONS IN AN ARGON ION LASER PLASMA TUBE 
D. C. Galehouse, * U. Ingard, * T. J. Ryan, and S. Ezekielt 
Massachusetts Institute of Technology, Research Laboratmy of Electronics, Cambridge, Massachusetts 02139 
(Received 2 October 1970) 
Coherent fluctuations in the light of an argon ion laser have been investigated and laser op- 
eration free from these fluctuations has been achieved. Oscillations in the lrHz range have 
been identified as anode oscillations and were suppressed by use of an auxiliary cathode. Os- 
cillations related to the longitudinal magnetic field and to striations were eliminated by choice 
of plasma tube operating conditions. Wideband incoherent noise was less than 0.2%. 
We report a method of suppression of low-fre- 
quency noise in an argon ion laser tube. We have 
found that the noise, which appears in the form of 
coherent fluctuations in the laser light, is a result 
of oscillations in the discharge in the vicinity of the 
anode (anode oscillations). The method of noise 
suppression then involves the use of an auxiliary 
cathode similar to that used by Pupp many years 
ago. 
This study was motivated by our need for the 
elimination of noise in the use of the argon ion la- 
se r  in heterodyne spectroscopy of light scattered 
by ripplons, and +so by our efforts to frequency 
stabilize the 5145-A laser line by using an iodine 
transition observed in a molecular beam as  a fre- 
quency reference. 3 1  
Although several other investigators have been 
plagued by anode and other coherent oscillations in 
argon laser plasma tubes, the origin of these os- 
cillations, usually in the range of several kHz to 
several hundred kHz, has not been established. 
Previous efforts to reduce the noise have involved 
/a careful choice of operating parameters, such as 
gas pressure, discharge current, magnetic field, 
tube geometry, location of electrodes, etc. This 
has sometimes led to a relatively “quiet” opera- 
tion of the laser. 
The discharge tube used in our study was made 
of quartz with the anode and cathode mounted at 
right angles to the tube axis. An auxiliary cathode 
was mounted within a cylindrical molybdenum an- 
ode, as shown in Fig. 1.  The bore diameter of the 
plasma tube was 3 mm and the length of the active 
gain region was 40 cm. In this arrangement, two 
independent discharges can be run simultaneously, 
one between the anode and the main cathode, and 
[ ,/WATER JACKET ,,) 
/ I  \ 
3 m m  i.d. CAPILLARY 
FIG. 1. Schematic diagram of argon ion laser discharge 
tube with auxiliary cathode. 
the other one between the anode and the auxiliary 
cathode. The power supplies were filtered but not 
regulated. 
With the auxiliary discharge turned off, strong 
coherent fluctuations in the laser light were ob- 
served. An oscilloscope picture of the correspond- 
ing output signal from a photodetector is shown in 
Fig. 2, together with the frequency spectrum ob- 
tained from a spectrum analyzer. The main dis- 
charge current was 15 A, the gas pressure was 
350 pm, and the axial magnetic field was 350 G. 
The fundamental frequency was found to be inde- 
pendent of the main discharge current but linearly 
I50 IO0 50 0 
FREQUENCY ( k H z 1  
FIG. 2. Fluctuation in the laser light caused by anode 
oscillations in the argon ion discharge tube. Upper: an 
oscilloscope picture of the fluctuations. Time scale 
0.2 msec/cm. Lower: corresponding frequency spectrum 
obtained from a spectrum analyzer. (Main discharge 
current 15  A ,  argon pressure 350pm, axial magnetic 
field 350 G. ) 
13 
VOLUME 18 NUMBER 1 APPLIED PHYSICS LETTERS 1 JANUARY 
0 
150 200 250 300 350 400 
ARGON PRESSURE (am) 
FIG, 3. Fundamental frequency of anode oscillation a s  
a function of argon pressure. (Argon pressure was mea- 
sured on a thermocouple gauge, calibrated for air.) 
dependent on gas pressure, at least over a limited 
range, as shown in Fig. 3. 
With the auxiliary discharge turned on, and the 
current adjusted to a value above 2.5 A, the coher- 
ent noise spectrum in Fig. 2 was completely elimi- 
nated. This occurred in the entire range of avail- 
able discharge currents up to 20 A and magnetic 
fields up to 650 G in the important pressure range 
200-400 pm. In this region the laser light was 
free from any coherent oscillations in the frequency 
range of the photodetector response, which was 
approximately 10 MHz. 
Although this noise suppression represents our 
main result, we wish to mention that our noise 
studies have also resulted in identification of other 
oscillations. For example, at axial magnetic fields 
exceeding a threshold value that depends on the gas 
pressure, we have observed coherent narrow-band 
oscillations in the 100-kHz range which appear to 
be related to the Kadomtsev instability. The fre- 
quency increased with the discharge current, as 
shown in Fig. 4, but was found to be independent of 
the magnetic field (above its threshold value), at 
least in the range of magnetic fields (< 750 G) 
available in our experiment. The threshold value 
of the magnetic field was found to decrease with 
pressure. For example, at an argon pressure of 
200 pm the critical magnetic field was 650 G, 
whereas at 150 pm it was 250 G. Because of limi- 
tations in our magnetic field power supply we were 
not able to determine the critical magnetic field at 
pressures above 200 pm. 
At gas pressures below 100 pm, other oscilla- 
tions were observed, probably related to striations. 
Striations a re  known to occur only below a certain 
pressure-dependent critical value of the discharge 
current. At the discharge currents used in our ex- 
periment the conditions for occurrence of striations 
were satisfied only for pressures below 100 pm. 
14 
I P - 
N 
1971 
d 
100 
T I I I I I I I I I I  
0 2 4 6 8 IO 12 14 16 18 20 
DISCHARGE CURRENT (A I  
FIG. 4. Oscillation frequency of Kadomtsev-related 
instability as  a function of discharge current and argon 
pressure. 
These oscillations were unaffected by the auxiliary 
discharge and the magnetic field. 
In addition to the coherent oscillations described 
above, we have detected wideband incoherent back- 
ground noise in the laser light. The amplitude of 
the noise depended on the argon pressure and al- 
ways decreased with increasing discharge cur- 
rent. At a pressure of 300 pm, a discharge cur- 
rent of 15 A, and a magnetic field of 350 G, the 
peak-to-peak noise in a bandwidth of 10 Hz-10 MHz 
was less than 0.2% of the laser light. 
A more extensive discussion of the various types 
of oscillations and their dependence on various 
parameters will be published elsewhere. This will 
also include questions not mentioned in this letter, 
such a s  the influence of the external circuit com- 
ponents on the various types of oscillations and the 
possible use of these oscillations in plasma diag- 
nostics. 
In conclusion, then, we can say that for the dis- 
charge tube configuration utilized, the use of an 
auxiliary or  secondary discharge between an aux- 
iliary cathode and the anode makes it possible to 
obtain a laser operation free from coherent oscil- 
lations over a range of useful operating conditions. 
This range, in our case, for low values of the 
axial magnetic field, say, less than 650 G, cor- 
responds to argon pressures between 200 and 400 p 
independent of discharge current. 
We wish to thank R. Benford for valuable techni- 
cal assistance. This research was sponsored in 
part by the U. S. Navy Office of Naval Research 
(Grant N00014- 67-A-0204-0019), NASA Grant 
(NGR-009-359), and the Joint Services Electronics 
Program [Contract DA28-043-AMC-02536 (E)]. 
*Dept. of Physics. 
tDept. of Aeronautics and Astronautics. 
‘W. Pupp, Physik. Z. 34, 756 (1933). 
VOLUME 18 NUMBER 1 APPLIED PHYSICS LETTERS 1 JANUARY 1971 
'R.H. Katyl andU. Ingard, Phys. Rev. Letters 2, 
%. Ezekiel and R. Weiss, Phys. Rev. Letters 2, 91 
4S. Ezekiel and R. Weiss, International Quantum Elec- 
tronics Conference, Miami, Florida, 1968, Paper 16P-2 
248 (1968). (unpublished). 
(1968). Energy L, 230 (1960). 
5B. B. Kadomtsev and A. V. Nedospasov, J. Nucl. 
6W. Pupp, Physik. 2. g9 844 (1933). 
15 


Laser frequency stabilization using a molecular beam Final report, 1 Feb. - 31 Dec. 1969

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