PFC/JA-83-43
A TUNABLE FAR INFRARED LASER
B.G. Danly, S.G. Evangelides, R.J. Temkin, and B. Lax
Plasma Fusion Center
Massachusetts Institute of Technology
Cambridge, MA
02139
December 1983
By acceptance of this article, the publisher and/or recipient
acknowledges the U.S. Government's right to retain a nonexclusive,
royalty-free license in and to any copyright covering this paper.
Abstract
A continuously
tunable
experimental results
tunable C0
the FIR
are presented.
tunable
Accurate
bandwidth
generation of
feasible.
infrared
(FIR)
laser
A high pressure
has
been demonstrated;
(10-12atm)
continuously-
TE laser is used to pump Raman transitions in CH 3 F; the generation
2
of continuously
reported.
far
radiation
frequency
in
and
superradiant
frequency
The generation
tunable,
in
the
250pm-300pm wavelength
bandwidth measurements
emission
of tunable laser
pulses
is
made
and
Consequently,
the
have
is f4-5GHz.
subnanosecond
range
in
the
been
FIR
appears
radiation from 150-10004m by stim-
ulated Raman scattering should be possible using higher pump intensity and/or
other gases.
1
Introduction
We present experimental results on a continuously tunable,
infrared (FIR)
laser.
Recently,
there
has
been
the goal of achieving tunable high power laser
region.
Stimulated Raman
scattering
in
significant
high power far
progress towards
radiation in the
the HF and
HCl has
FIR spectral
been
generate tunable radiation in the 40-250m spectral range.1,2, 3
first
demonstrated
stimulated Raman
gases.
scattering
to
Biron et al.
4
of generating tunable FIR radiation by the
(SRS)
CO2 pump
of
laser
radiation
in
molecular
With the use of a highly focused pump beam and a dielectric waveguide
for the
FIR laser,
1 2 CH 3 F and
time,
the feasibility
employed
13CH3F
FIR
with
emission
offsets
the theory describing
was
of
observed
up
to
30GHz
on
many
from
Raman
transitions
5
resonance.
Since
these tunable Raman lasers has been derived,
in
that
5
,6,7
and several experiments with continuously tunable CO 2 pump lasers have produced
tunable FIR radiation.8,9,
10
,
11
2
,1
We present experimental results
on a 12CH3F
waveguide laser which is tunable from 250um-300pm.
In this
tunable
CH 3 F
laser,
vibrational ground state (g,J),
number,
to
an excited
state
the
C02
laser
pumps
molecules
-
state.
rotational
the
where J is the total angular momentum quantum
level
(u
3
XJ).
The pump transition
Q, or R branch transition, corresponding to J' - J-1, J, J+1.
occurs between
from
levels
+ J'-1) within
(J
the
can
be a P,
The FIR transition
excited
vibrational
The laser pumped emission process occurs via the coherent, Raman process.
The Raman
transition
is
denoted
by
the pump
initial J value.
2
transition
(P,Q,
or
R)
and
the
Experimental Results
The experimental configuration is shown in Fig.
preionized 10-12atm
main
The
laser.
TE
CO 2
The pump laser is a UV-
1.
is
discharge
five
a
by
driven
stage Marx bank which produces 150kV, 36J pulses with a measured 10-90% risetime
of llns, an internal inductance of 122nH and an output impedance of 11.50.
output coupler,
Laser
coupler.
of a 150X/mm diffractiion
consists
cavity
laser optical
beam expanders
and two telescopic
output
energy
from
ranged
grating,
for the
40-120mJ,
a Germanium
grating and
depending
The
on
output
Marx
bank
For emission near the band centers of the
voltage and the emission frequency.
9Pm R and 10m R branches, the laser produces 80-100mJ reliably.
For the 9pm P
and lOpm P branches, only 40-8OmJ near the line center frequencies is obtained.
Continuously tunable output was obtained on the 9R and 1OR branches, but on the
9P and
gain was adequate
10P branches the
frequency.
center
the line
m1OOns, and the bandwidth
The
only for tuning
duration
pulse
output
laser
within ±5GHz about
is
typically
The high pressure CO 2 laser is described
is -4GHz.
6
in detail elsewhere.
The output of the pump laser is focused into a FIR waveguide laser through
a NaCl window (W1).
A fused quartz dielectric waveguide
was used to lower the
diffraction losses in the FIR and thereby reduce the threshold for stimulated
Raman scattering.
waveguide by
guide13,1
4
.
The C02 beam is coupled into the EH 11 mode of the dielectric
adjusting
the
pump
beam
The FIR emission also propagates
lowest loss mode for this waveguide.
tubes of diameter
Even with
radius
optimum
waveguide mode,
7mm and
at
the
entrance
in the EH 1 1 mode,
to
the
wave-
as this is the
In the experiments reported here, quartz
5mm and length 1.2m were used as the FIR waveguide.
coupling
between
the
free
space
pump
beam
and
the
EH11
the pump beam transmission in an evacuated waveguide was limited
3
This limitation is due to a slight departure from straightness
to about 70%.
of the waveguide
bore.15,1
6
The
FIR emission
exits
the
FIR laser
through a
Teflon window.
Absorption of the pump beam by the FIR laser gas is monitored by a photoacoustic transducer
the entrance
splitting;
to
(P.A.T.)
the
thus,
waveguide.
when
emission fields are
tuning of
the
frequencies.
mounted
the
acoustic
Because
are
the
frequency shifts measured
to
For low pump powers,
resonant.
emission
perpendicular
All
then
signal
is
shifts
measured
relative
of
relative to line
optical
there
maximized,
frequency
spectroscopy
the
is
both
resulting
to
CH 3 F is known
axis
near
AC
Stark
no
the
pump
and
from the Raman
those
line
to high
center
accuracy,
17
center can be accurately converted
to absolute frequencies with the available spectroscopic data.
The measurement
of the Raman tuning behavior of the CH 3 F waveguide
was carried out in two steps.
laser
First, laser emission at a known frequency, such
as that corresponding to line center emission, was obtained.
Then the shift in
FIR emission frequency was measured for a change in the pump laser frequency.
In this
manner,
a
set
of pump
and FIR frequency
obtained; this data yields tuning curves of
shifts
from line
center
is
vs (FIR emission frequency) versus
Vp (pump frequency).
These turning curves can be compared directly with theory.
The measurement
of frequency shifts for the pump and FIR beams was deter-
mined by
several
methods.
Both
the
absolute
FIR emission
frequency
and
the
frequency shifts in the FIR are determined by a scanning Fabry-Perot interferometer (Ui).
meshes,
one of
This
interferometer
which is mounted
electric detector,
and
signal
system consists
on a motor-driven
averaging
of
two wire
translation
electronics.
The FIR
grid inductive
stage,
a pyro-
interferometer
was calibrated with the 496.1im Q(12) emission line of CH 3 F.
A resonable estimate
of the
CO2 laser
4
frequency
could be made using the
grating angle,
since frequency pulling effects were found to be small.
A more
accurate method for measuring the pump laser frequency shift has been recently
incorporated into the experimental
it
system;
involves the use of a piezoelec(12).
trically driven infrared Fabry-Perot interferometer
consists of
X/100 dielectric-coated
two
The overall
96%.
This interferometer
mirrors
Germanium
with
reflectivity
finesse of this instrument was measured with a CW C02 waveAlignment of the Germanium optics proved difficult
guide laser to be 42.
finesse to this
limited the maximum
value.
and
this interferometer
Nevertheless,
is adequate for measuring frequency shifts of 1GHz or more.
The interferometer
monitors the pump laser frequency by detecting a small portion of the pump beam
which is reflected off the NaCl window (WI).
Using the high pressure C02 pump laser and FIR waveguide laser,
tuning experiments
CH 3F were
in
performed.
of the pump laser
measured as a function
The
FIR
emission
frequency
frequency
was
for P and R branch Raman
frequency
transitions in CH3 F.
The best demonstration
from a
CH 3F Raman
available pump
low.
5 6
,
in Fig.2.
laser
laser
Experimental
of the generation of widely tunable
occurs
power
for the
is high,
R branch.
FIR radiation
For these transitions the
and the threshold
for Raman emission
is
results for R branch frequency tuning in CH 3 F are shown
FIR output power in the 1-10kW range was obtained; this corresponds
In Fig.2,
to an efficiency of approximately 0.5%.
the FIR emission frequency
is plotted as a function of the pump laser frequency for Raman emission on the
R(J-19) to R(J-22) transitions.
left; the pump frequency was determined
The theoretical
sion was
tunable
from the
tuning is shown (dashed lines),
center absorptions
error
The experimental
C02 laser
33cm-
1
to
39cm~
1
5
grating position.
and the locations of the line
for each transition are indicated.
from
is shown in the upper
(256pm-300pm).
The far infrared emisAs
predicted,
5 6
,
the
tuning is
asymmetric
about
This
offsets favored.
the
is
due
excited state Raman processes,
emission frequency.
absorption
to
an
line
center,
interference
with
between
negative
ground
pump
state
and
both of which contribute to the gain at the FIR
The discontinuity in Raman tuning occurs
at a pump laser
frequency just above the absorption line center.
Experimental results
for Raman tuning
been obtained; they are shown in Fig.
P transitions
at large
our laser in
this
transition.
The
3.
on the P branch in CH 3 F have also
Because of the higher thresholds for
J and the lower C02 laser
region,
location
Raman
of
tuning data
the P(35)
line
output power available from
was
only obtained
center
absorption
R20 C02 laser transition are also indicated in Fig.
3.
on the
P(35)
and the
10.21pm
Data shown as circles
are referenced to the C02 laser line frequency; the pump laser frequency shifts
were measured by the interferometer for this data.
The data shown as squares
are referenced
the pump frequency shifts
to the P(35)
were calculated
from the
for the
frequencies
Stokes
absorption line center;
grating position
is ±1.5GHz in
frequencies are t1.75GHz and ±3GHz for
respectively.
this range
Fig. 3
Raman
being
serve
transitions in
to
tuning
limited
both
the
by
These
the
the
transitions
pump
of
allow
Experimental
Errors
and
obtained
available
generation
cases.
circled
by ±4.5GHz was
demonstrate
CH3 F.
for this data.
for
boxed
about
the
intensity.
tunable
additional
FIR
error
the
pump
data
points,
line
center,
The
data
radiation
in
on
frequencies
to
P
be
obtained beyond those available with R-branch tuning.
The presence
of AC
Stark shifts in a tunable
FIR laser
can
result in a
nonlinear tuning of the FIR emission frequency with the pump frequency.
6
Our
experimental conditions precluded any determination of the relevance of the AC
Stark shift to frequency tuning in this FIR laser.
of
6MW/cm 2 ,
the AC Stark shifts should be
6
For the pump laser intensity
1.4GHz.
For the pump intensities,
pump bandwidth (4-5
GHz)
tion, the
AC
shift,
measure.
However,
Stark
may be important
and experimental error encountered in this investigaif present,
the effects
and measurable
of AC
is relatively
Stark shifts
small
and difficult
on frequency
tuning
to
curves
for high pump intensities or narrow-bandwidth,
high resolution studies of tunable FIR Raman lasers.
The FIR laser bandwidth was measured as a function of the CO 2 laser pressure
and is
shown in Fig.
4.
The bandwidth
of the pump laser radiation increases
with increasing laser gas pressure.
For atmospheric pressure CO2 lasers, both
the pump
are
typically
pump
laser,
and
FIR laser
bandwidths
pressure (-10atm) operation
of
frequency tunable
both the
were measured
emission,
to be '4-5GHz.
the
This
bandwidth of a tunable FIR laser.
of frequency
tunable
pump and
represents
subnanosecond
pulses
for high
is necessary
to obtain
FIR laser
the
first
emission
bandwidths
determination
in
the
FIR may
Such a subnanosecond
both compact and relatively inexpensive,
The existence
which
However,
of the
These results indicate that the production
similar laser pumped molecular lasers.
electron lasers.1 8
1GHz.
be
possible
FIR laser, being
would compare favorably with FIR free
of a simple,
tunable high power,
short pulse
FIR source could benefit solid state physics research significantly.18
7
with
I
Conclusions
The experimental
transitions,
on R and P branch
tunable far infrared
molecules.
The
results presented here not only demonstrate Raman tuning
generation
straightforward
but they also demonstrate the
feasibility
of
by stimulated
Raman
scattering
in polyatomic
of
these
principles
to
extension
other
FIR
laser gases should result in the production of tunable laser radiation at other
FIR frequencies.
Alternatively,
the availability of a higher power pump laser
source would also allow the range of FIR laser tuning to be extended.
the emission bandwidth results reported here,
able,
subnanosecond
the generation of frequency tun-
FIR pulses in these laser pumped molecular
feasible.
8
Based on
lasers appears
I
Figure Captions
Fig.
1.
Tunable
FIR Laser
Mirrors; II,
12:
System.
D=Detector;
Interferometers;
P1,
Wi,
P2:
W2:
Windows;
Plotters;
S.
F.M.,
A.:
Averaging Electronics; R: Ramp Generator; P.A.T.: Photoacoustic
Transducer.
Fig. 2.
R Branch Tuning Data for CH3 F
Fig. 3.
P Branch Tuning Data for CH 3 F
Fig. 4.
FIR Laser Emission Bandwidth
9
M.:
Signal
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Addison-Wesley,
0oatm CO 2 LASER
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