A rotating mirror Q-switched CO2 laser was designed and constructed for use as an excitation source in infrared fluorescence studies. Output power of 33 W was reached in the continuous wave (CW) mode. In the pulsed mode peak powers of 3 kW and pulse widths of 200 ns FWHM have been measured. The repetition rate may be varied between 275 and 1000 Hz and single line operation is selectable with a diffraction grating.  The  design is compact and economical. The laser has shown freedom from electrical noise and average power stability of the order of 1 per cent; it has operated reliably for more than 1000 hours with a minimum of maintenance

Hamilton, N

Kelly, JW

Struve, H

ANSTO Publications Online

AAEC/E534
AUSTRALIAN ATOMIC ENERGY COMMISSION
RESEARCH ESTABLISHMENT
LUCAS HEIGHTS RESEARCH LABORATORIES
A HIGH REPETITION RATE Q-SWITCHED CO
 2 LASER
by
N. H A M I L T O N
J.W. K E L L Y
H. STRUVE
March 1982
ISBN 0 642 59733 2
AUSTRALIAN ATOMIC ENERGY COMMISSION
RESEARCH ESTABLISHMENT
LUCAS HEIGHTS RESEARCH LABORATORIES
A HIGH REPETITION PATE Q-SWITCHED C02 LASER
by
N. HAMILTON
J.W. KELLY
H. STRUVE
ABSTRACT
A rotating mirror Q-switched C02 laser was designed and constructed for
use as an excitation source in infrared fluorescence studies. Output power of
33 W was reached in the continuous wave (CW) mode. In the pulsed mode, peak
powers of 3 kW and pulse widths of 200 ns FWHM have been measured. The
repetition rate may be varied between 275 and 1000 Hz and single line
operation is selectable with a diffraction grating. The design is compact and
economical. The laser has shown freedom from electrical noise, and average
power stability of the order of 1 per cent; it has operated reliably for more
than 1000 hours with a minimum of maintenance.
National Library of Australia card number and ISBN 0 642 59733 2
The following descriptors have been selected from the INIS Thesaurus to
describe the subject content of this report for information retrieval
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International Atomic Energy Agency.
CARBON DIOXIDE LASERS; Q-SWITCHING; LASER MIRRORS; PULSE
TECHNIQUES; PERFORMANCE
CONTENTS
1. INTRODUCTION 1
2. DESIGN 1
3. CONSTRUCTION 2
3.1 General 2
3.2 Electrical 3
3.3 Optical 4
3.4 Gas Handling 4
4. PERFORMANCE 5
4.1 CW Mode 5
4.2 Q-switched Mode 5
5. CONCLUSION 6
6. REFERENCES 7
Figure 1 Construction of laser assembly 9
Figure 2 Photograph of laser 10
Figure 3 Window mount assembly 10
Figure 4 Q-switched laser pulse 11
Figure 5 Laser beam profile 11
1. INTRODUCTION
Since the early 1970s, pulsed C02 lasers have been used increasingly as
convenient sources of intense optical radiation and for research into fast
transient effects. As part of the AAEC research program on laser enrichment,
a suitably pulsed C02 laser excitation source was required for time-resolved
infrared emission studies of organic uranyl chelates [Hamilton et al .
forthcoming]. Infrared emiss ion from molecules as large as these had not been
reported elsewhere, and was expected to give v a l u a b l e in format ion on the
excited state of the molecule and its time evolu t ion .
Idea l ly , the excitation source should provide short, monochromatic pulses
at h igh repetition rates and at v ib ra t iona l t r ans i t i on frequencies of the
target molecule. Because the fluorescence s igna l was expected to be extremely
weak, the abi l i ty to operate wi thout the generat ion of electrical noise was
also crucial .
This report describes the des ign, const ruct ion and operat ing
characteristics of the Q-switched C02 laser developed for these infrared
fluorescence studies.
2. DESIGN
When a lasing medium has upper level l ifetimes sufficiently long to al low
a large population inversion to be achieved by the inject ion of energy, Q-
switching, caused by a sudden increase in the qual i ty factor (Q) of the
resonant laser cavity, releases the stored energy in a s i n g l e large pulse.
The cont inuous wave (CW) COp laser is sui table for Q-swi tching , and techniques
which have been appl ied inc lude the use of rotating mirrors or prisms [Kovacs
et al. 1966], mechanica l shutters [Meyerhofer 1968], reactive swi tching
[Bridges 1966], saturable absorbers [Wood and Schwarz 1967] and electro-optic
crystals [Day et al. 1970]. A useful review of CW CO^ lasers was provided by
DeMaria [1973], comparisons of Q-swi tch ing techniques were made by Meyerhofer
[1968] and McDuff [1972], and var ious theories of the Q-swi tching process were
proposed by Arecchi et al . [1964], Midwin te r [1965] and Kaf r i et al. [1971].
In the present work, the rotating mirror was chosen as the Q-switching method,
since mechanical shutters produce double p u l s i n g , even at high shutter speeds;
reactive switching and saturable absorbers provide less f lex ib le control of
the laser and generally produce longer pulses; and suitable electro-optic
crystals are scarce and expensive.
The following requirements were also considered when designing the laser:
(a) Minimum laser-generated electrical noise.
(b) High and variable repetition rates.
(c) Sub-microsecond pulse widths.
(d) High pulse energies and peak powers.
(e) Single frequency operation over the 9.4 and 10.6 ym CO^ laser bands.
(f) Stable T£MQQ mode operation.
(g) Long- and short-term stability of pulse characteristics and
frequency.
(h) Long-term maintenance-free operation,
(i) Optical and electrical safety.
These requirements led to the choice of configuration described in detail
in Section 3. In addition to rotating mirror Q-switching, the system
incorporates intracavity Brewster window sealing of the active volume, line
selection by a plane diffraction grating, water cooling and slow gas flow.
3. CONSTRUCTION
3.1 General
A schematic representation of the laser is shown in Figure 1 and a
photograph of the laser with the protective cover raised is reproduced in
Figure 2. The active volume is contained in a 1.25 m long x 10 mm i.d. quartz
tube, sealed at each end by a Brewster angle window. The length chosen
permits the laser and associated equipment to be accommodated on a
conveniently available massive granite table which provides thermal and
mechanical stability. The tube diameter was determined from conflicting
requirements. Large diameter tubes have low diffraction losses and are easy
to align, whereas small diameter tubes allow the use of cheaper and more
readily available optical components, smaller backing pumps, favour good TEM00
mode stability and diffraction grating line selectivity, and are easier to
cool. The end windows are incorporated in multi-functional window mount
assemblies (see Figure 3) which serve as gas mixture inlet/outlet ports,
supports for the coaxial platinum cathodes, and water jacket seals. The
cooling-water jackets are fabricated from 25 mm i.d. Perspex tube.
3.2 Electrical
The laser excitation energy is provided from a relatively inexpensive
Hipotronics Model 820-50 20 kV, 50 mA d.c. power supply (Figure 1). The two
halves of the laser discharge are electrically in parallel to match the
voltage and current characteristics of the discharge with those of the supply.
A third cylindrical electrode, earthed through the spring contact made by a
molybdenum-quartz seal, acts as a common anode for both halves of the
discharge.
Since a glow discharge exhibits a negative impedance [Cobine 1941], it is
inherently unstable and preferably should be fed from a constant current
supply. An alternative approach is to include sufficient added series
resistance to make the overall impedance positive. In Figure 1, this is
provided by Rl and R2, each comprising four 100 kn, 75 W resistors in series.
Although power is dissipated in these resistors, this system is cheaper than a
more elaborate power supply, and the current regulation is sufficient for
short- and long-term laser output stability. Since the window assemblies are
at high voltage, gas connections are made through sufficient lengths of rigid
polythene tubing to prevent establishment of a glow discharge in the low
pressure gas lines.
The laser is housed under a hinged Perspex cover fitted with a
microswitch safety interlock to switch off the high voltage and ground the
window mounts if the cover is lifted. The cover also provides dust protection
for optical surfaces and, although permitting visual inspection of laser
operation, it is sufficiently opaque to contain scattered infrared radiation
which may cause eye damage.
3.3 Optical
The laser tube assembly and cavity optics are supported in Oriel
precision optical mounts. The resonator cavity is terminated at one end by a
PTR Optics Corp. plane, aluminium diffraction grating of 70 lines/mm blazed
for 10;6 pm, and at the other end by a II-VI Inc. ZnSe concave output coupler
having 4 m radius of curvature and 80 per cent reflectivity. Originally, an
Oriel 80 per cent reflective Ge output coupler was used, with Harshaw NaCl
windows at the Brewster angle to seal the discharge volume. However, since an
intracavity power of about 1 kW cm"2 in the CW mode resulted in thermal
runaway, causing distortion of the laser optics and a reduction in output
power, a ZnSe output coupler and ZnSe windows at the appropriate Brewster
angle were installed. Unlike NaCl optics, these windows do not require
heating to prevent fogging and hence eliminate the concomitant problem of high
voltage insulation of the window heaters. Both types of window were sealed to
the mounts with Vacseal silicone resin.
For CW operation, a fixed Spawr plane molybdenum mirror is placed at 45°
to the beam tube axis. For Q-switched operation, this mirror is replaced by a
Lincoln Laser model 6-60 beam scanner comprising a hexagonal polygon mirror
integrally mounted on the shaft of a synchronous motor. The scanner is driven
by a variable frequency power supply, thus allowing the laser pulse frequency
to be conveniently adjusted in the range 275 to 1000 Hz.
3.4 Gas Handling
The gas mixture is supplied from a special gas handling rig. Each
constituent gas is metered before mixing. Condensable impurities are removed
by a cold trap, water vapour by a silica gel column, and oxygen by a heated
MnO column with facilities for in situ regeneration. In addition, the gas
mixture may be seeded with measured amounts of additives. The feedstock for
the laser is a mixture of commercial grade C02, dry N2 and He. The total
pressure is measured by a 0-13 kPa capsule gauge before the gas enters the
discharge tube through a Nupro 7 pm stainless steel filter.
4. PERFORMANCE
4.1 CW Mode
Table 1 lists performance details of the laser operating in a multi-line
mode with a plane gold mirror replacing the diffraction grating. Power was
measured with a Scientech Model 36-0001 disc calorimeter. After taking into
consideration diffraction losses and active volume ultilisation, the maximum
CW output power compares favourably with the expected upper limit of about 75
W per metre length for slow gas flow lasers [Rigden and Moeller 1966].
Significantly greater powers can be obtained with high gas flow rates to
provide convective cooling and removal of gas decomposition products [Deutsch
et al. 1969]. However, these require large throughput pumps, and either a
high rate of gas consumption or the complexity of a recirculation and cooling
system. The present laser is economical in gas consumption, using only about
0.1 L of gas mixture per minute, so only infrequent interruptions are required
for gas bottle changes.
TABLE 1
CW OPERATING CONDITIONS
Gas mixture ratio C02/N2/He = 6/7/9
(by volume)
Pressure 700 Pa
Input power 10 kV, 2 x 25 mA
CW output power 33 W
Diffraction loss* 14 per cent
Active volume used* 36 per cent
Average power stability within about 1 per cent
Calculated values.
4.2 Q-switched Mode
Figure 4 shows a Q-switched pulse captured by a Molectron P1-11H
pyroelectric detector and recorded on a Tektronix 7623A storage oscilloscope
with a 100 MHz bandwidth. The ringing following the pulse is typical of this
type of detector. With the laser operating in a multi-line mode, a peak power
of 3 kW can be attained. With the diffraction grating in place and tuned to
the 10P (20) line, peak power is reduced to about 1 kW. In all cases, the
beam can be focused to a spot size of less than 0.1 mm diameter, giving power
densities in the megawatt per cm range. The FWHM pulse width of about 200 ns
is comparable to that produced by a CO,, TEA laser. Although maximum peak
power is achieved at a pulse frequency of about 500 Hz and maximum pulse
energy at about 300 Hz, the pulse shape is largely insensitive to the
rotational speed of the mirror; this indicates that the dynamics of the laser
medium is more important than the rate of mirror alignment. The pulse peak
height reproducibility is about 50 per cent due mainly to the tolerance spread
of ± 30 seconds of arc (± 1 prad) in the vertical alignment angle of the
hexagonal mirror facets.
To establish the beam profile, the divergent beam was probed by a 0.05 mm
diameter wire at a distance of 750 mm from the output coupler. As the cross
section was traversed by the wire, energy scattered normal to the beam axis
was measured by an InfraRed Associates HgCdTe detector. The resulting quasi-
Gaussian profile (Figure 5) confirms that the laser operates in the desired
mode.
Calibration of the diffraction grating micrometer settings for frequency
selection was carried out using an Optical Engineering COp laser spectrum
analyser. Laser line separation was found to be sufficient to allow accurate
and reproducible selection, by using the micrometer setting alone, of the 00°1
- 10°0 transitions within the ranges 10P(8) to 10P(30) and 10R(12) to 10R(20).
To provide a source of trigger signals for associated instrumentation, an
Optron OPB707 reflective sensor was mounted facing the rotating mirror. The
resulting pulses, sharpened by a Schmitt trigger circuit, provide timing
information that is synchronous with the laser pulses. The pulse jitter
(about 30 ps) allows adequate triggering for the observation of processes on a
millisecond time scale. Where more precise triggering is required, pulses
derived from a pyroelectric detector which is directly monitoring the laser
beam are used.
5. CONCLUSION
A laser which has given reliable, trouble-free service for more than a
thousand hours of operation has been described. The only off-line maintenance
period was due to a gradual build-up in the beam tube of metal sputtered from
the cathodes, and to a smaller amount of contamination on the downstream ZnSe
window which caused a slight reduction in output power. The disassembly,
cleaning and reassembly of the laser can be effected simply and conveniently.
The use of this laser has been a key factor in the characterisation of
previously unmeasured infrared emission from gaseous uranyl compounds. Lasers
of this type could be used in related work, such as multiphoton dissociation,
where sub-microsecond pulses are required. They offer an effective
alternative to the chopped beam lasers, and the cumbersome and electrically
noisy (although more powerful) C02 TEA lasers.
6. REFERENCES
Arecchi, F.T., Potenza, G. and Sona, A. [1964] - II Nuovo Cimento, 34(6)1458.
Bridges, T.J. [1966] - Appl. Phys. Lett., 9:174.
Cobine, J.D. [1941] - Gaseous Conductors. Dover, New York.
Day, G.W., Gaddy, O.L. and Jungling, K.C. [1970] - IEEE (Inst. Electr.
Electron. Eng.) J. Quant. Electron., QE6(9)553.
DeMaria, J.A. [1973] - Proc. Inst. Eng. Electr. Eng. (London), 61(6)731.
Deutsch, T.F., Horrigan, F.A. and Rudko, R.I. [1969] - Appl. Phys. Lett.,
15(3)88.
Hamilton, N., Kelly, J .W. , Price, G. and Struve, H. - AAEC/E report in
preparation.
Kafri, 0., Speiser, S. and Kimel, L. [1971] - IEEE (Inst. Electr. Electron.
Eng.) J. Quant. Electron., QE7(3)122.
Kovacs, M.A., Flynn, G .W. .and Javan, A. [1966] - Appl. Phys. Lett., 8(3)62.
McDuff, O.P. [1972] - Techniques of gas lasers. In Laser Handbook (Arecchi,
F.T. and Schulz-Dubois, D.B., Eds.), North-Holland, Amsterdam.
Meyerhofer, D. [1968] - IEEE (Inst. Electr. Electron. Eng.) J. Quant.
Electron., QE4(11)762.
Midwinter, J.E. [1965] - Brit. J. Appl. Phys., 16:1125.
Rigden, J.D. and Moeller, G. [1966] - IEEE (Inst. Electr. Electron. Eng.)
Quant. Electron., QE2(9)365.
Wood, O.R. and Schwarc, S.E. [1967] - Appl. Phys. Lett., 11:88.
to pump
trigger
pulses
1. Quartz tube. 2. Water jacket. 3. Brewster window mount. A-. ZnSe window. 5. Cathode. 6. Anode.
7. Diffraction grating. 8. Rotating mirror. 9. Output coupler. 10. Beam stop. 11. Al backing plate.
12. Hinged cover. 13. Interlock switch. H. Ref lect ive sensor. 15.d.c.power
FIGURE 1. CONSTRUCTION OF LASER ASSEMBLY
10
F I G U R E 2. PHOTOGRAPH OF THE L A S E R
water j acke t
Brewster angle
quartz tube
water seal
gas connection
compression ring
HV cable termination
F I G U R E 3 . W I N D O W M O U N T A S S E M B L Y
DISTANCE FROM BEAM AXIS ( m m )
F I G U R E 5. LASER BEAM PROFILE
Horizontal scale 200 ns cm'1; pulse frequency 300 Hz;
pulse energy 0.7 mJ; average power 210 mW; peak
power 3.1 kW; lasing on the 10P (20) line.
F I G U R E 4. Q-SWITCHED LASER PULSE


High repetition rate Q-switched CO2 laser.

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High repetition rate Q-switched CO2 laser.

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