J-QE/24/ 1//17462
Narrow Bandwidth Emission from a Mirrorless, Far
Infrared, "CH
3F
Laser
S. G. Evangelides, Jr.
L. Carson
B. G. Danly
R. J. Temkin
--------- - 11
Narrow Bandwidth Emission from a Mirrorless, Far
Infrared, 1 3 CH 3F Laser
S. G. EVANGELIDES, JR., L. CARSON, B. G. DANLY,
Abstract-Frequency tuning and linewidth measurements are reported for a pulsed, mirrorless, kW power level, far infrared (FIR)
'34CH 3F laser operating at 245 GHz. The pump laser is an etalon tanable, single-mode CO 2 TEA laser. The FIR frequency spectrum was
measured with 2.5 MHz resolution on individual 100 ns laser output
pulses using harmonic mixing techniques. The linewidth of the amplified spontaneous emission (ASE) was found to be surprisingly narrow,
about 15 MHz, much narrower than the homogeneous linewidth (on
the order of 200 MHz), and narrower than previously reported measurements made with scanning Fabry-Perot interferometers, which
average over many pulses. Frequency tuning of the FIR laser, as the
pump laser frequency is tuned, is nonlinear, possibly due to inhomogeneous broadening of the gain by the K-level substructure of the rotational states. These results indicate that heterodyne receivers capable
of single-shot frequency measurements can be important tools for investigating the properties of Raman FIR lasers.
INTRODUCTION
have studied the fre uency and bandwidth of a
pulsed, laser pumped 3CH 3F laser using heterodyne harmonic mixing techniques. The pump is a stabilized, tunable, CO 2 laser with single longitudinal mode
output. The heterodyne receiver system is capable of
making frequency and bandwidth measurements on a single-shot basis with a resolution of 2.5 MHz. A number of
previous studies have been carried out on the frequency
and bandwidth of CW far infrared (FIR) lasers, including
the work of Matteson and DeLucia [1] on 13 CH 3F. However, very few studies have been carried out on the frequency and bandwidth of pulsed lasers on a single-shot
basis. Studies have been done on a pulsed D 20 laser system operating at 385 yim [2], [3]. The present study differs
from that study in that the D2 0 laser was a cavity laser,
while the present study investigates an amplified spontaneous emission (ASE) laser. Also, the present heterodyne
receiver system uses a YIG-tuned Gunn diode local oscillator (LO) and is considerably smaller and more versatile than that used in previous studies., Further, the
13 CH 3F laser has a K-level substructure, while a single
rotational state is excited in the D 20 laser. The K-level
substructure can result in inhomogeneous broadening of
the laser gain profile, and thus in significantly different
bandwidth and tuning characteristics.
Manuscript received December 29, 1986; revised August 18, 1987. This
work was supported in part by the National Science Foundation under Contract ECS 8413194 and in part by the M.I.T. U.R.O.P. Program.
The authors are with the Plasma Fusion Center and the Department of
Physics, Massachusetts Institute of Technology, Cambridge, MA 02139.
IEEE Log Number 8717462.
MEMBER,
IEEE, AND
R. J. TEMKIN
The linewidth of ASE lasers has been previously measured with a Fabry-Perot interferometer [4]-[7]. The use
of a multimode pump laser in such studies can induce a
wide linewidth in the ASE laser output [4]-[81 either by
exciting many K levels simultaneously or by the Raman
effect. Brown et al. [8] observe a 700 MHz FIR bandwidth in 12 CH3F for a 2-3 GHz bandwidth multimode
pump. They attribute the large ASE bandwidth to the
simultaneous radiation of eight K-level transitions which
are all driven by different modes of the wide bandwidth
pump pulse. Evans et al. [9], using a multimode pump,
observe line center radiation from three K levels simultaneously, giving a total FIR bandwidth of nearly 300
MHz, wider than the pressure broadened width. Lipton
and Nicholson [10] used a tunable single-mode pump and
were able to observe individually the K = 5 and K = 6
levels of 12CH3F with linewidths of about 150 MHz. Measurements of frequency and bandwidth using a scanning
Fabry-Perot interferometer are average quantities resulting from integration over many shots, and hence many,
possibly varying, instantaneous values of frequency and
bandwidth [4],[5], [8]. A measurement made this way will
not contain information about single-shot bandwidth, frequency, or shot-to-shot stability of these quantities. Indeed, shot-to-shot fluctuations are observed in the present
study, verifying the usefulness of the present approach.
Averaged bandwidth and frequency measurements may
contain errors due to pump drift and mode purity if the
pump laser is not stabilized against long-term drift. The
heterodyne mixing technique offers the opportunity to investigate single-shot line shape, weak or intermittant signals, and response of the medium to multimode pumps,
as well as effects like gain narrowing in ASE. It is also
possible to estimate the frequency spectrum of an FIR
laser on a single-shot basis by accurately measuring the
temporal structure of the pulse and performing a Fourier
transform. The present approach using a heterodyne receiver has much better frequency resolution and bandwidth than such an approach, and yields absolute frequency values as well. In this paper, we present results
on pump tuning, single-shot line shape, and bandwidth in
the 13 CH 3F laser.
EXPERIMENTAL SYSTEM
3
The ' CH 3F laser is pumped by a CO 2 laser operating
on the 9.63 Am line (9P(32) transition) (see Fig. 1). We
DETECTOR
HETERODYNE
LASER SYSTEM
COUNTER
ISOR
B 2-PASS
M
AMPLIFIER
M
M
103
B
8 M
M
YiG TUNED -6HARMONIC
-6M
GE
R
LOCAL
o
13-16
sc.
wAVE
MIXER
GuioE
TT, E.
DIODE
GHz
36%M
B
GATE
AMP
ETLNOSCILLATOR
AMP -I PASS IH3F
LASER
AMP
DEVICE
STORAGE
SCOPE.
M
B
I
G
R
MIRROR
BREWSTER ANGLE WINDOW
IRIS
GRATING
INVAR RODS
Fig. 2. A schematic block diagram
of the heterodyne
receiver system.
ing threshold. In this two-pass configuration, output powers reached 200-500 W/cm 2 for a pulse duration of 100
3
Fig. 1. The CO 2 pump laser and the 1 CH 3F waveguide laser.
ns. Without the mesh mirror, the outpt power is only 100200 W/cm 2 for a 100 ns pulse. The 13 CH 3 F gas used in
the laser is 99 percent isotopically pure. The pulse shape
use a stabilized low-power pulsed oscillator followed by
and mode quality of the FIR laser were monitored with a
an amplifier to produce pump radiation. The oscillator is
fast risetime video detector.
a Lumonics 101 TEA laser with a cavity that has been
The 'CH 3F laser radiation is heterodyned with a high
dimensionally stabilized by an invar rod frame. It has a harmonic of the local oscillator (LO) on a harmonic mixer
low-finesse intracavity tuning etalon, which allows selecdiode optimized for an 8-15 GHz LO. The LO is an Omtion of a single longitudinal mode. The oscillator can be niyig Gunn diode oscillator tuneable from 13 to 16 GHz,
tuned ±550 MHz (±5 longitudinal cavity modes) about
stable to a few parts in 105 . A portion of the beat fre9P(32) line center with usable output pulse energy of
quency (IF), centered at 420 MHz, is amplified, put
from 100 to 300 mJ. The gas mix in the laser is adjusted
through a bandpass filter, gated to eliminate electrical
so as to suppress the N2 tail, leaving most of the energy
noise, and put into a surface acoustic wave (SAW) anain a gain switched peak of 100-150 ns. This signal is then lyzer (see Fig. 2). The SAW device is a linear dispersive
amplified in two passes through a Lumonics 103 TEA
delay line, the delay being proportional to the frequency,
laser amplifier. The resulting output, in the 1-3 J range,
provided the signal duration is in the 60-400 ns range.
gives a final pump intensity of 5-15 MW/cm2 . This beam* The bandwidth of the SAW device is 100 MHz. The result
is sent, unfocused, into the FIR laser. A small portion of
of passing the signal through the SAW device is equivathe pump beam is directed into a photon drag detector to lent to a Fourier transform of the IF input into frequency
monitor the CO 2 laser mode structure. This signal was space. The ultimate resolution of the device, 2.5 MHz, is
monitored constantly so that the pump could be fine tuned
achieved under present operating conditions. The output
to maintain single-mode operation or, when desired, two- of the SAW is then displayed on a storage oscilliscope for
mode operation. Another small portion of the pump beam
immediate analysis. The great advantage of using a SAW
was used to monitor the total energy of the pump beam.
device as a "spectrum analyzer" is that it can handle IF
This CO 2 system is the first stage of a high-power CO 2 signals as short as 60 ns, and so is ideally suited for reallaser system which has been described elsewhere [11],
time analysis of short laser pulses on a single-shot basis.
[12].
The level structure of 'CH 3F investigated in this study
The FIR laser is a 1.23 m long cylindrical quartz waveis shown in Fig. 3. Previous studies [13] have found
guide with a 25 mm i.d. This is enclosed in a 1.6 m long,
strong emission near 245 GHz due to the R(4) transition
36 mm i.d. pyrex tube with an NaCl pump beam entrance
in the excited vibrational state (v 3 = 1) and near 248 GHz
window and a polyethylene FIR exit window at Brews- due to emission on the R(4) transition in the ground viter's angle. The NaCl window is perpendicular to the brational state. Emission has also been observed at 388
waveguide axis, but can be tilted 5* from perpendicular
and 412 Am [14] on a transition in the 2v 3 vibrational state.
if FIR feedback is to be kept to a minimum. A mesh mir- In the present study, the laser emitted most strongly on
ror, with a grid spacing of 3 lines/mm, can be fitted across the 245 GHz line, and only results for that line will be
the input end of the quartz waveguide. This reflects 70 presented.
percent of the backscattered FIR radiation into the forRESULTS AND DISCUSSION
ward direction, allowing it a second pass through the gain
medium. This effectively doubles the length of the laser,
Data were taken for two FIR laser configurations: singreatly enhancing the output power and lowering the las- gle-pass ASE and two-pass ASE. The one-pass ASE data
'3 CH
LEVEL
3F
4?
SCHEME
245.6
4
--------- K-5
I245.5
K-4
C
K-4
J=4
= K
~PUMP
K'
C
(D 245.4
0*
'Z
K no
KmI
K 2
K =3
L)
K-4
a) 245.3
0
-J
K
J=4
_______
K'
K'4K
-220
-10
0
+110
*220
Pump offset from 9P(32) in MHz
Ko
2
K 20
3
Fig. 3. Level diagram for ' CH 3F showing the relevant rotational J levels
and the K-level substructure of each. The pump absorption from the
ground vibrational state to the first excited state is indicated by the arrow
labeled Q,. The FIR emission occurs between adjacent rotational levels
and is indicated by an arrow labeled WOFIR.
Fig. 4. Single-pass ASE data. The solid horizontal lines mark the K-level
line center emission frequencies. The solid diagonal lines are the theroetical Raman tuning curves for the designated K levels. Error bars for
the FIR frequencies are only 10 MHz, too small to be shown on these
plots. The individual K-level CO 2 absorption frequencies, relative to
9P(32) line center, can be read off the above plot. They occur at the
intersection of the FIR line center and Raman tuning lines of the given
K level.
are shown in Fig. 4. All the output pulses observed had
bandwidths of 12-20 MHz FWHM for a single-mode
pump [see Fig. 5(a)]. These pulse widths, A P = A w/21r,
are close to the transform-limited value Aco - 1/At
where the pulse duration At is 100 ns in this case. This is
remarkable: at pressures of 5-8 torr, the 13 CH 3F lines are
expected to be homogeneously broadened to widths of
150-240 MHz FWHM. The FIR output intensity of the
single-pass laser is below saturation. If the observed linewidth is due to gain narrowing, then the expected laser
emission linewidth A P for a homogeneous linewidth PH is
[15]
(a)
An2
aL
for unsaturated gain (see [16] for a more detailed discussion). For this laser, aL - 30. This value is accepted as
being the minimum gain necessary to produce a detectable
signal, and is in good agreement with data taken by Biron
et al. on lasing thresholds in 13 CH 3F and 12 CH 3F [17].
We expect v - 0.15VH or linewidths of from 23 to 36
MHz for pressures of 5-8 torr. The observed linewidths
are narrower by a factor of two. We also measured the
FIR linewidth at reduced pump intensities, down to 30
percent of the original intensity, and noted no change in
linewidth. Most probably, the observed linewidth is due
to a combination of gain narrowing and coherent feedback, rather than one or the other of these effects alone.
Similar observations have been made on a xenon laser by
Casperson and Yariv [18]. The feedback in our laser is
most likely due to reflections at the waveguide ends arising from impedance mismatches. Weak feedback in a
high-gain single-pass laser, like the present one, will make
it behave like a resonator. For comparison, we made a
(b)
Fig. 5. (a) SAW output showing the frequency spectrum of a typical single-pass ASE laser shot. Horizontal scale: 12.5 MHz/division. Vertical
scale: 2 mV/division. Center frequency: 245.366 GHz 13 CH 3F pressure:
6.0 torr. (b) SAW output showing the frequency spectrum of a typical
two-pass ASE laser shot. Horizontal scale: 12.5 MHz/division. Vertical
scale: 2 mV/division. Center frequency: 245.368 GHz. '3CH 3F pressure: 3.5 torr.
resonator by introducing a partially reflecting mesh mirror
into the beam path of the two-pass laser. The measured
linewidth did not change. We observed cavity modes and
measured a slight cavity pulling near the K = 3 line center.
The dominant FIR output was at line center for pump
laser detunings of -110 MHz, + 110 MHz, and 0 MHz
from [9P(32)] line center. Additionally, at a + 110 MHz
pump detuning, we observed weak output at 245.470 GHz
that tuned with the pump. This corresponds to a K = 3
---------- 11
transition with 6,, = 6, = 110 MHz where 6, is the pump
frequency offset from absorption line center and 6, is the
FIR frequency offset from emission line center. We see,
less frequently, with the same pump detuning, 245.270
GHz, corresponding to K = 4 Raman transition with
6,
= 6s = 47 MHz.
In the two-pass ASE configuration the FIR, output is
250-500 W/cm 2 at pressures of 3.0-4.0 torr, much
stronger than the one-pass configuration. Again, the
bandwidth of the FIR radiation is 12-20 MHz FWHM
[see Fig. 5(b)]. In this instance, we have a source of coherent feedback (the mesh mirror). If there is any other
source of feedback, it can cause apparent line narrowing
by effectively forming a resonator. We have taken great
care to eliminate all possible sources of extraneous feedback so as to avoid this p.oblem. Still, it is harder to unequivocally attribute the narrow width of the two-pass
ASE radiation to gain narrowing.
We were able to observe FIR output at pump detunings
from -110 MHz to +220 MHz (see Fig. 6). The dominant radiation at these pump detunings is always in the
small region containing K = 0, 1, 2, and 3 line centers,
245.370 GHz on average. We observe weak tunable output on the K = 3 transition for pump detunings of + 110
MHz. We see weaker still tuning on the K = 4 transition.
A pump tuned +220 MHz above 9P(32) is only detuned
50 MHz above the K = 4 line center absorbtion frequency. FIR radiation corresponding to K = 4 line center
plus 50 MHz was observed. This emission frequency also
coincides with K = 2 line center.
The reason for this departure from expected linear FIR
tuning may lie in the K-level substructure of each of the
interacting rotational levels. Previous studies of pump
tuning in laser-pumped FIR lasers indicated that for offresonant pumping, the output is dominated by radiation
satisfying the Raman resonance condition 6, = 6,. This
leads to linear tuning of the FIR output when the pump is
tuned. In the case of Fetterman et al. [2], tuning near the
pump absorbtion line center was found to be linear, in
agreement with small-signal theory [19]. However, the
rotational levels of D2 0 involved had no substructure, as
is the case in this experiment. In the study by Danly et
al. [20], a wide-band pump was used, which masked the
effects of the K-level substructure on the tuning behavior.
Further, pump detunings were typically on the order of 5
GHz or greater so the region near pump absorbtion line
center could not be adequately probed. At such pump detunings, K-level structure effects have no effect on tuning
behavior.
For dipole transitions in symmetric top molecules, the
selection rule for K is A K = 0; further, K changing collisions occur on a time scale long compared to the FIR
pulse duration. So the K levels are effectively decoupled.
Each K transition (see Fig. 6) has a different pump absorbtion frequency on the vibrational transition, and a different line center emission frequency on the rotational
transition. The total FIR susceptibility, at a given pump
frequency, is then the sum of the individual K-level sus-
245.5 k
C
0)
.t,
245.4
0)
-K=3
-j
7K4
245.3
-220
-110
0
Pump offset from 9P(32)
0I0
.220
in MHz
Fig. 6. Two-pass ASE data. The solid horizontal lines mark the K-level
line center emission frequencies. The solid diagonal lines are the theoretical Raman tuning curves for the designated K levels. Error bars for
the FIR frequencies are 10 MHz, too small to be shown on these plots.
The individual K-level CO 2 absorption frequencies, relative to 9P(32)
line center, can be read off the above plot. They occur at the intersection
of the FIR line center and the Raman tuning line of the given K level.
ceptibilities, each weighted by population statistical factors. For a single-mode pump x" (W,, K), the susceptibility for the Kth sublevel has two peaks of equal
magnitude, one at Os,K = WOK and the other at Ws,K =
WO, K + Z,, K where WO, K is the K-level line center emission
frequency, 6 p,K is the pump offset for a given K level, and
WsK is the Raman transition frequency. For pump offsets
of less than 300 MHz on either side of 9P(32) absorption
center, the total susceptibility is greatest in the region from
245.350 to 245.393 MHz. In this region, the total susceptibility is dominated by contributions from the emission line center s,K = wo,K portion of each of the individual K-level susceptibilities which overlap in this
region. There is additional enhancement of the susceptibility in this frequency range when 6,,K = 0 for any of the
five K levels. This happens when the pump offset from
9P(32) is between +255 MHz and -169 MHz.
In the single-pass ASE configuration, we occasionally
observed a two-mode FIR output as a result of a two-mode
CO 2 laser pump pulse. FIR output and CO 2 laser output
could be monitored simultaneously to confirm coincidence. Both pump and FIR mode spacings were the same.
The FIR output consisted of two peaks, 15-20 MHz
FWHM, 110 MHz apart.
Radiation from the 248 GHz ground state transition was
also observed, but rarely, and each time accompanied by
radiation from the 245 GHz transition. These data on
simultaneous emissions at 245 and 248 GHz are not comprehensive enough for us to make any verifications of the
existing theories on three-photon transitions [21], [221.
We saw no modification to the tuning due to the ac Stark
effect. With the high pump powers used, the Stark splittings, according to the theory, should be quite large,
hundreds of MHz, quite easy for us to observe. None was
observed. The reason may be that the pump pulse has a
rise time and a fall time of ~50 ns, which is long com-
pared to the time it takes the laser to turn on. The pump
intensity, therefore, is changing over much of the duration of both the pump and the FIR pulses. The magnitude
of the ac Stark splitting, being proportional to the square
root of the pump intensity, will change with the rising or
falling edge of the pump. So the frequency at which there
is gain due to the ac Stark effect is constantly changing;
thus, the gain at a Stark-shifted frequency due to any instantaneous pump intensity does not last long enough for
any appreciable power to build up at that frequency.
In order to corroborate the bandwidth data described
above, taken with the heterodyne receiver, Fabry-Perot
scans were made of the output of the 13 CH 3F laser at 245
GHz. The Fabry-Perot etalon that achieved the highest
resolution had 5 cm diameter 78 lines/cm Cu mesh mirrors with a free spectral range (FSR) of 1.5 GHz. The
reflectivity of the mirrors was measured to be 0.96. The
total measured finesse was 25-27, yielding a resolution of
60 MHz. Efforts to further increase the resolution of the
etalon were unsuccessful. Changes to larger mirror separation and thus smaller FSR were accompanied by a reduction in the total finesse due to beam diffraction and a
reduction in the overall transmitted intensity. Although
we were unable to achieve a resolution of 15 MHz in order to confirm the heterodyne data, we can still draw a
few important conclusions. First, the linewidths of 60-80
MHz observed with the Fabry-Perot, for single-mode
pumping, fall at the resolution limit of the instrument and
provide only an upper limit on the FIR bandwidth. This
limit is considerably narrower than the expected 120 MHz
due to pressure broadening at 4.0 torr. There was a recognizable difference in line shape and width between the
Fabry-Perot resonances produced by single-mode pumping and those produced by two-mode pumps. The twomode pump widths were much wider, the width being
larger than the pump mode spacing, indicating that there
are two modes in the FIR output. These two modes could
not be cleanly resolved with the Fabry-Perot due to difficulties in making the pump laser operate reliably on two
modes. Finally, scanning through many resonances of the
Fabry-Perot, we looked for evidence of ac Stark-shifted
radiation and none was found. This is in agreement with
the heterodyne receiver data.
CONCLUSIONS
We have observed stable narrow bandwidth radiation
from 13 CH 3F on the 245 GHz line. The single- and double-pass ASE linewidths are the same, 12-20 MHz
FWHM. Previous measurements by Hacker et al. [13] for
gas pressures of from 0.5 to 3.0 torr gave linewidths equal
to the pressure-broadened value of 30 MHz /torr. Our result, for single-pass ASE, which is considerably narrower, is not inconsistent with theoretical predictions for
gain narrowing of ASE in an unsaturated medium. The
two-pass ASE data are not as easy to attribute to gain narrowing alone, and it has to be assumed tht coherent feedback is causing some of the line narrowing. A result of
our investigations into ASE in 13 CH 3F has been to see
how important very weak feedback can be in determining
the output behavior of such lasers. Small amounts of feedback in a mirrorless high gain laser can cause resonator
modes to form the output, and thus significantly narrow
the output pulse in the case that only one mode is excited.
In the case where two or more modes can be excited and
the modes are fairly close and thus difficult to resolve or
the output mode content changes from shot to shot, the
linewidth of the FIR laser will appear much wider when
it is measured by an averaging technique such as a FabryPerot scan.
We have studied pump tuning about absorbtion line
center and have found modifications to linear tuning.
Rather than observing linear FIR tuning with the pump,
we observe, at pump detunings from -110 MHz to + 220
MHz, a dominant signal at line center and weaker signals
that tune with the pump. We attribute this to the K-level
substructure of each J level. We have observed two-mode
FIR response from the medium when excited with a twomode pump. The pump and FIR modes have the same
separation. Observations such as these would be difficult
or impossible using scanning Fabry-Perot interferometric
techniques. These effects could easily be washed out in
the averaging over the many laser shots necessary to make
the measurement.
The present studies indicate that weak feedback effects
at the ends of an ASE laser may significantly affect the
laser threshold and emission bandwidth. These results may
be useful in designing future ASE laser systems in the Xray region where mirror feedback at the laser ends is difficult or impossible to achieve [23].
ACKNOWLEDGMENT
The authors would like to thank B. Lax, P. Woskoboinikow, G. Sollner, and P. Tannenwald for much insightful advice and W. Mulligan for his superb technical
advice and assistance in the lab.
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S. G. Evangelides, Jr., photograph and biography not available at the time
of publication.
L. Carson, photograph and biography not available at the time of publication.
B. G. Danly (M'87) was born in Florida in 1956.
He received the B.A. degree in physics from Haverford College, Haverford, PA, in 1978 and the
Ph.D. degree in physics from the Massachusetts
Institute of Technology, Cambridge, in 1983. His
dissertation research was in the area of quantum
electronics.
He is presently a Research Scientist in the Coherent Electromagnetic Radiation Division at the
Plasma Fusion Center, M.I.T. His areas of research include free-electron lasers, cyclotron resonance masers, and other novel rdiation sources, as well as accelerator
physics and optically-pumped gas lasers.
Dr. Danly is a member of the American Physical Society.
R. J. Temkin, photograph and biography not available at the time of publication.