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Submitted on 1 Jan 1980
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NEW LASER SYSTEMS
W. Silvfast, O. Wood
To cite this version:
NEW LASER SYSTEMS
W.T. Silvfast and O.R. Wood I1
Be22 TeZephone Laboratories, HoZmdeZ, New Jersey 07733, U.S.A.
Abstract.- Two new types of lasers, using electron-ion recombination as a pumping mechanism, are described. The first, a segmented-plasma-excitation-recombination (SPER) laser, is simple to construct and ~ r o v i d e s ultraviolet, visible and infrared laser outuut from many metal vapors. The second, an annular recombination iaiirpr&&es efficiencies of 5% in the near infrared in several species.
I. Introduction
Two new kinds of lasers that use electron-ion recombination as a pumping mechanism are under development. The first is a simple metal vapor laser device called a segmented-plasma-excitation-recombination
(SPER) laser.
'
The electrode structure for this device consists of a number of thin narrow metal strips (of the lasing species) positioned end to end on an insulating substrate in such a way as to leave a small gap between each of the adjacent strips. When a high-voltage, high-current pulse is applied to the ends of this series of strips a metal vapor plasma is produced in each gap. These high density plasmas then expand into a surrounding background gas and recombine resulting in laser action in the recombining species. To date, a number of lasers in the ultraviolet, visi- ble and infrared have been produced by this technique. SPER lasers are simple to con- struct, scalable in length, volume, and wavelength and are capable of a long oper- ating life.The second type of laser under develop- ment is an annular-gain-region recombina- tion laser2 that is designed specifically to operate at high overall efficiency. This device consists of a large diameter
glass tube with mirrors mounted at each end. In the center of each mirror is a pin elec- trode that extends into the cylinder along its axis. When a high-voltage, high- current pulse is applied to these pins a plasma confined to a cylindrical region along the axis of the tube is produced between the pins. After initiation, this plasma expands radially, cools, and re- combines producing gain in an annular region surrounding the initial plasma. The laser output consists of a "doughnut"- shaped beam emerging from the output coupling mirror. This second laser device should ultimately be capable of producing
5
-
10% efficient lasers primarily in thenear infrared as a result of recombination of singly ionized atomic species.
Since both of these types of lasers use electron-ion recombination as part of their pumping mechanism this process will be briefly described before each type is discussed in more detail.
11. Recombination Lasers
An ideal plasma-recombination laser
would operate in the following manner:
The atoms of some element E are excited by
an electrical current pulse within a gas- eous medium and some fraction are ionized
into state E (Z+l)+ (as shown in Figure 1)
29
C9-440 JOURNAL DE PHYSIQUE
where Z could be any charge state of ele-
ment E.
(1) FORMATION (2) EXPANSION (3) DECAY
(n,-
lod5-
1 0 ~ ~ ~ 6 ~ ) (DENSITY REDUCED) (newlod4
~rn-~)Ez+l+
----
LASER
Fig. 1. Sequence of events necessary to produce recombination lasers in expanding plasmas..
Under suitable conditions the resulting plasma of electrons and ions of element E will expand and thereby reduce in density. Interaction of the expanding hot electrons with a surrounding cool gas will cool the electrons causing the electron-ion re- combination rate to increase rapidly. The recombining electrons will move downward in energy, via collisions with the free elec- tron continuum, through the highly excited
states of +E' until a significant gap in
the energy levels of that charge state is reached. Due to the reduced collisional decay rate across the gap, the bottleneck
caused by the gap causes the population density to build up in levels immediately
above the gap. A population inversion will
then develop with respect to one or more lower levels and laser action can be achieved across the gap if a high decay rate for the lower laser level is present. Two possible decay schemes for the lower- laser-level, based upon either radiative or collisional processes, are possible
depending upon the choice of atomic spe- cies, the charge state of that species and the exact plasma conditions. In either scheme a minimum number of atoms having electrons in the lower laser level is desirable.
The stage of ionization (the value of
z
in E'+) is an important parameter indesigning a recombination laser. To make a recombination laser that operates on a
particular transition in +'E it is neces-
sary to have as large a fraction of the
initial population in E('+l)+ as possible
since this will be the source of popula- tion for the upper laser level during the recombination process. For the lower ion- ization stages (2=1,2,3) this requires an initial plasma density in the range of
1015
-
1016 cm-3 as shown in Figure 1 (thespecific initial charge state (Z+1) is determined by the electron temperature during the formation of the plasma). In contrast with this relatively high initial density, the optimum plasma conditions for producing a population inversion have been found to lie at a somewhat lower plasma
density (10 l4 or slightly lower).
This lower density is needed to minimize the unwanted collisional depopulation of
the upper-laser-level. These two appar-
The SPER laser appears to be a sig- nificant new type of laser. At present 42 transitions, ranging in wavelength from
1.84 pm to 2983
5
in neutral, single-ion,and double-ion species, have been pro- duced from 8 different metal vapors (Cd, Sn, Zn, Pb, Mg, C, In and Ag). Many of the transitions have never been observed in laser action before by any excitation scheme.
The SPER laser1 consists typically of a number of narrow metal strips of the lasing species positioned end to end on an insulating substrate in such a way as to leave a small gap between each of the
adjacent strips (see Figure 2).
as the excitation pulse.
Of the 42 laser transitions produced
to date several of the more important ones are listed in Table I.
Species Transition Wavelength
Sn I1 2F05/2-2D5/2 .580
Table I. Some of the more important SPER laser transitions in the visible and ultraviolet.
The infrared transitions occur primarily in the neutral atomic species of the vari-
SPHERICALLY
RECOMBINING ous metals as a result of recombination HIGH VOLTAGE
CURRENT PULSE from the single ion, whereas, the visible
and ultraviolet transitions occur in single and double ion species recombining
METAL VAPOR
ARC FORMED IN from double and triple ions. The higher
OUTPUT ionization stages tend to produce shorter
METAL SEGMENTS INSULATING SUBSTRATE
Fig. 2. Schematic diagram of SPER laser. When a high-voltage, high-current pulse is applied to the ends of this series of strips, a high-density metal-vapor plasma is formed in each gap. Once formed, these plasmas (consisting primarily of vaporized strip material) expand hemispherically, cool in the presence of a background gas
at low pressure (typically He at 2
-
10Torr) and recombine producing inversions in the next lower ionization stage.
Current pulses of the order of 50
-
100amperes, 1
-
10 psec duration and havingwavelengths since their ionization poten- tials and consequently their energy levels are inherently more widely separated.
In addition to the shorter wavelength at higher Z, the recombination rate in-
creases with increasing 2. This is illus-
trated in Figure 3 where the temporal dependence of In excited state emission is shown for double ions, single ions and neutrals recombining from triple, double and single ions respectively. The corres- ponding temporal dependence of the laser output for these ion stages is also shown
C9-442 JOURNAL DE PHYSIQUE CURRENT PULSE
7
7
-
SPONTANEOUS EMISSION--
LASER OUTPUT I I I I I I I 0 5 1 0 1 5 2 0 2 5 30 TIME ( p s e c )Fig. 3. Time dependence of spontaneous
emission and laser output resulting from
recombination of first 3 ionization,stages
of indium.
appearance of the laser output is probably due to both the cavity buildup time and the disappearance of inhomogeneities in the plasma produced d-ring formation.
The gain region, which typically corresponds to the visible plasma emission, occupies an approximately 1 cm diameter
spherical region above each gap as shown
in Figure 2. The small-signal gain
appears to be higher on the infrared transitions where laser action has been
observed in a single gap SPER device
whereas the visible and ultraviolet tran-
sition have in some cases required 30 or
more gaps. This suggests that the double and triple ion stages are not yet being as effectively populated during plasma formation. Experiments to optimize the plasma formation of higher ion stages are now in progress.
IV. Efficient Annular Recombination
Lasers
An annular recombination laser is now being studied as a possible device for
making a 5
-
10% efficient laser.Although this device ha,s not yet evolved to the point of demonstrating high effi- ciency, preliminary experiments indicate that the ideas outlined in detail in
Reference 2 appear to be correct. One
possible experimental configuration for such a device is shown in Figure 4. It
consists of 2 pin electrodes installed in
the center of the laser mirrors that are
LASER MIRRORS
7 ANNULAR GAIN REGION
r
. . . - -. . . .
LASEROUTPUT
Fig. 4. Schematic diagram of annular
recombination laser.
unheated gas where the electrons are cooled causing them to recombine with the ions. The recombination process leads to an annular shaped inversion and gain region as shown in Figure 4. The use of pin-type electrodes at the center of the mirrors minimizes cavity losses and allows the maximum efficiency in extracting the laser energy.
The concept of an efficient annular recombination laser arose from a consider- ation of the overall laser efficiency that would result from the product of three
2
separate efficiency factors : 1) The
efficiency of ion production; 2) The
efficiency of recombination of these ions into the upper-laser-level; and 3) The quantum efficiency of the laser (i.e. the laser transition energy divided by the ionization energy). In computing the effi- ciency, it is assumed that the lower-laser- level is rapidly depleted. Conditions for meeting this requirement are discussed in detail in Reference 2.
The efficiency of ion production is perhaps the key unknown factor in this laser excitation process. During the electrical excitation pulse, the principal losses in the efficiency of ion production would be: a) Direct excitation of levels in the neutral species below the upper- laser-level; b) Excitation of electrons into unwanted excited states of the ion; C) Radiation losses by free-free transitions in the electron continuum (bremsstrahlung) and by free-bound transitions (radiative recombination); and d) Thermal heating of
minimized for electron densities of the
order of 1015
-
1016 cmV3 and electrontemperatures of the order of a few eV (the temperature requirements will vary depend- ing upon the specific species being used).
When considering the overall efficiency, a
figure of 65% was assumed for the ioniza- tion efficiency.
Optimizing the recombination effi- ciency into the upper laser level is also
discussed in Reference 2. Suffice it to
say that plasma densities near 1014 cm-3 are necessary during recombination to mini mize electron collisional depopulation of the upper laser level and also to minimize the effect of radiation trapping,an effect which would inhibit the decay of the lower-
laser-level.
Many elements would make suitable candidates for an atomic recombination
laser. A few have been listed in Table I1
along with their potential efficiencies based upon the considerations mentioned above.
Species Wavelength Efficiency
Xe I 2.02 ~.lm 3.3%
A9 I 1.84 5.8
Cd I 1.43 6.3
Pb I 1.31 8.3
Ca I1 0.37 12.1
Table 11,. Possible efficient annular recombination lasers.
C9-444 JOURNAL DE PHYSIQUE
elements were chosen somewhat arbitrarily (with the exception that most have rela-
tively low ionization potentials). It
should be noted that there are no doubt many other possible candidates that could
satisfy the requirements for an efficient recombination laser.
A plasma model requiring densities of
the order of 1015
-
1016 cm-3 duringformation and densities of 10 14 cm-3
during recdmbination points to the use of an expanding plasma when translating the results of this model into the design of
a practical device. The device of Figure
4 is one attempt to produce the conditions outlined above. Only very preliminary data is availaqle for this device. Figure 5 shows a plot of laser intensity output versus distance from the axis for a He-Xe recombination laser at 2.026 um in
-20 -10 0 +I0 -120
RADIAL DISTANCE FROM A X I S (mm) Fig. 5. Laser output versus radial distance from axis showing annular laser output.
which the pins were spaced approximately
6
-
8 cm apart. The initial plasma was ofin an annular region beyond the initial plasma. No measurements have yet been made in a larger device where attempts to optimize the output efficiency will be made.
V. Conclusions
In conclusion, two new types of laser devices, both based upon an expanding plasma recombination model, have been described. The SPER laser has provided many new metal vapor laser transition in the ultraviolet, visible and infrared and holds out the possibility of extending laser output into the vacuum ultraviolet. ~t is one of the simplest lasers to con- struct and has already demonstrated
scalability in wavelength and a long oper- ating life. The annular recombination laser is at an earlier stage of develop-
ment but promises 5
-
10% efficient lasersin the near infrared and possibly in the near ultraviolet. Because of their sim- plicity and wide range of operating wave- lengths (and potential high efficiency of one) both recombination lasers systems
should find many applications in the laser community.
References
1. W. T. Silfvast, L. H. Szeto, and
0. R. Wood 11, Appl. Phys. Lett.
36,
615 (1980).
2 . W. T. Silfvast and 0. R. Wood I1
Submitted to J. Appl. Physics.
