Operation of a two-photon three level laser

The operation of a two-photon, three level laser system at high temperatures or pressures is made possible by reducing the build up time of the flux of the second lasing transition in the laser discharge region.

Two-photon three level lasers can be defined as lasers that lase at two 
wavelengths where the lower laser level for the first lasing transition is 
the upper laser level for the second lasing transition. More specifically, 
the first lasing transition dumps energy from the top laser level to an 
intermediate laser level. A population inversion is thereby created 
between the intermediate laser level and a lower laser level which results 
in the second lasing transition. 
In some cases, the operation of the two-photon, three level laser is 
restricted to low temperature and/or pressure since the collision 
depletion of the intermediate laser level population reduces the 
population inversion necessary for the second lasing transition. 
SUMMARY OF THE INVENTION 
There is disclosed herein with reference to the accompanying drawings a 
technique for enhancing the operation of two-photon, three level lasers at 
low temperatures and/or pressures and for extending the operation of these 
lasers to elevated pressures and/or temperatures. These improvements are 
achieved by decreasing the build-up time of the laser flux for the second 
lasing transition such that the stimulated transition rates within the 
lasing medium can compete favorably with the undesirable relaxation rates 
which accompany high temperatures and/or pressures. This desired result 
can be achieved by: 
1. Placing the resonator mirrors as close as possible to the lasing medium; 
and/or 
2. Priming the laser with an external optical pump(s) at one or both of the 
lasing wavelengths. 
The first approach reduces the transit time of the lasing radiation through 
any no-gain, or negative gain, medium thereby increasing the total system 
gain which in turn reduces the time needed to build up the lasing 
radiation to significant intensities. 
The second approach allows the lasing radiation to build up from an 
intensity level determined by the external pump lasers which can be many 
orders of magnitude higher than the spontaneous emission intensity levels 
from which the lasing radiation is normally developed. The net effect is 
again to reduce the time needed to build up the lasing radiation to 
significant intensities. When employing the second method, the intensity 
of the external pumps is increased to such an extent that the stimulated 
transition rates within the lasing medium can compete favorably with the 
undesirable relaxation rates which accompany high temperatures and/or 
pressures. 
The successful implementation of the above described techniques for 
extending the operation of two-photon, three level laser systems to 
elevated pressures and/or temperatures is based on the condition that the 
initial energy level populations, i.e. just prior to lasing, do not change 
with temperature and pressure in such a way that the potential improvement 
is negated. Theoretical calculations indicate that the 16 .mu.m CO.sub.2 
bending mode laser, described in U.S. Pat. No. 4,168,474, issued Sept. 18, 
1979, assigned to the assignee of the present invention and incorporated 
herein by reference, satisfies this condition, and is used as an example 
to explain the important features of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The phenomena which provides the basis for the appreciation of the 
disclosed technique for extending the operation of two-photon, three 
wavelength laser systems higher pressures and/or higher temperatures, 
e.g., the ambient conditions of 300.degree. K. and 760 Torr, is typically 
illustrated for the 16 .mu.m CO.sub.2 laser system in FIGS. 1A, 1B and 1C. 
The vibrational energy level diagram for the well-known 16 .mu.m laser 
system is illustrated in FIG. 2 with the three levels corresponding to an 
upper laser level of 00.sup.0 1, an intermediate level 02.sup.0 0, and a 
lower level of 01.sup.1 0. 
The transfer of energy from the upper level to the intermediate level, and 
from the intermediate level to the lower level involves directly only the 
rotational levels from which the lasing originates or terminates. Within 
each vibrational level the non-equilibrium population of these rotational 
levels communicates with the other rotational level populations by 
rotational-rotational energy exchanges, i.e., by rotational relaxation. 
The successful implementation of the disclosed technique requires a 
two-photon, three level laser system having stimulated emission 
transitions which have a common rotational level within the plurality of 
rotational levels contained within the envelope, or manifold, E2 of FIG. 
1B. For the purposes of discussion, a common rotational line J is chosen. 
In the illustration of FIGS. 1A-1C, where J is the rotational quantum 
number, the arrows F indicate the general direction of population flow due 
to rotational relaxation. 
For the purposes of discussion the disclosed technique will be described in 
its application to a 16 .mu.m CO.sub.2 bending mode laser such as that 
schematically illustrated in FIG. 3. The vibrational energy level diagram 
for this laser is illustrated in FIG. 2. The major interactions of the 16 
.mu.m CO.sub.2 bending mode laser are also illustrated in FIG. 2. A 
detailed description of the operation of this 16 .mu.m laser is provided 
in U.S. Pat. No. 4,168,474 referenced earlier. The lasing medium, LM, of 
the laser system of FIG. 3 consists of a gas composition including a 
lasing gas CO.sub.2 an energizing gas N.sub.2, and, optionally, a buffer 
gas He which is present within a laser discharge region LD. The laser 
medium is electrically excited by a discharge pulse from the electrical 
excitation source ES. 
As is a conventional CO.sub.2 laser, the 00.sup.0 1 level of CO.sub.2 and 
the vibrational states of N.sub.2 are preferentially pumped by the 
electrical discharge. After the discharge pulse, the excited N.sub.2 
transfers much of its excitation energy to the upper CO.sub.2 level, 
00.sup.0 1, via vibrational-vibrational collisions, while the lower level, 
01.sup.0 0, loses much of its excitation energy by 
vibrational-translational collisions with the He atoms of the laser gas 
mixture LM. 
The population inversion on the 9.4 .mu.m laser transition created by 
transfer of vibrational energy from N.sub.2 to the upper CO.sub.2 level, 
00.sup.0 1, is transferred to the intermediate CO.sub.2 level, 02.sup.0 0, 
by stimulated emission caused by an externally delivered 9.4 .mu.m laser 
pulse from pulse source PS1, at an optimum time after the discharge pulse 
from the excitation source ES. The optimum time corresponds to the time 
when the potential population inversion for 16 .mu.m lasing is maximum. 
This stimulated transfer generates a population inversion between the 
intermediate level, 02.sup.0 0, and the lower level, 01.sup.1 0, which 
results in laser emission at 16 .mu.m. 
Referring to FIG. 1A, the 9.4 .mu.m lasing effectively "burns a hole" RL1 
in the rotational manifold E1 for example at the rotational level J.sub.o 
-1 and the rotational relaxation F feeds the population from the remaining 
rotational lines of the manifold E1 into the "hole" RL1. This transfer of 
energy from the entire manifold E1 to the hole RL1 of FIG. 1A of the upper 
level is a desirable result. 
On the other hand, the 9.4 .mu.m lasing transition produces an 
overpopulated rotational level RL2 in the intermediate laser level of FIG. 
1B, for example at the rotational level J.sub.o of the manifold E2 and the 
corresponding rotational relaxation F reduces the overpopulation. The 9.4 
.mu.m lasing transition thereby generates the population inversion 
necessary for 16 .mu.m lasing transition between the intermediate level, 
02.sup.0 0, and the lower level, 01.sup.1 0, but some of this inversion is 
lost by rotational relaxation within the rotational manifold E2. While 
this loss can be considered to be a positive factor with respect to the 
9.4 .mu.m lasing transition, it reduces the population inversion for the 
16 .mu.m transition and thus for this lasing transition the loss is 
detrimental. Once again, as stated above, this conclusion is valid only 
when the two lasing transitions have a common rotational level such that 
the lower rotational laser level for the first photon transition is the 
upper rotational laser level for the second photon transition. 
The second photon lasing, i.e., 16 .mu.m, produces a rotational level 
population situation in the lower laser level 01.sup.1 0 which is 
analogous to that produced in the intermediate laser level 02.sup.0 0 by 
the first photon lasing, i.e., 9.4 .mu.m. However, in the second photon 
lasing transition the rotational relaxation depletion of the overpopulated 
RL3 of the manifold E3 of FIG. 1C is desirable inasmuch as it increases 
the population inversion for the second photon lasing transition by 
reducing the bottle-necking in the lower laser level, for example at the 
rotational level J.sub.o -1 in the manifold E3. 
Thus, it is apparent from the above discussion that rotational relaxation 
has a positive influence in two-photon, three level laser system in the 
upper and lower laser levels, but has a negative effect at the 
intermediate laser level. This experimentally verified effect of 
rotational relaxation on the performance of two-photon, three laser level 
systems accounts in part for the generally accepted requirement to operate 
such laser systems at low pressure, i.e., between 5 and 50 Torr, and low 
temperatures, i.e. less than 220.degree. K. These operational restrictions 
impose significant engineering requirements on the system for cooling and 
sealing. 
The graphical illustration of FIG. 4 illustrates the expected dependence of 
laser output efficiency and energy on gas pressure, showing the effects of 
rotational relaxation. FIG. 5 depicts the expected dependence of laser 
pulse width on pressure, showing the effects of rotational relaxation. 
Having identified rotational relaxation, as illustrated in FIGS. 1A-1C, as 
a factor restricting the operation of two-photon, three level laser 
systems at higher temperatures and pressures, the negative effect of 
rotational relaxation at the intermediate laser level can be minimized by 
decreasing the build-up time of the second photon laser flux. This desired 
condition can be achieved by either increasing the rate at which the 
intensity of the second photon laser flux is increased, or by establishing 
an initial bias intensity level through the use of an external source of 
radiation, such as a pump laser source, thereby reducing the time needed 
to reach the desired magnitude of laser flux for the second lasing 
transition. 
The former method of achieving the desired decrease in build up time of the 
second photon laser flux can be implemented as shown with reference to 
FIG. 3 by positioning the mirrors M1 and M2 as close as possible to the 
laser discharge region LD defined within the laser tube LT by the laser 
windows W1 and W2. The distance L between the mirrors defines the optical 
cavity of the laser system 10. The length l of the laser medium LM is 
defined to be the distance between the laser windows W1 and W2. 
The intensity I of the second photon laser flux within the laser cavity 
defined by the mirrors M1 and M2 is given approximately as follows: 
EQU I=I.sub.o e.sup.N.alpha.l.spsp.2.sup./L 
where .alpha. is the gain of the laser medium; l is the length of the laser 
medium, L is the distance between the mirrors, N corresponds to the number 
of passes within the laser cavity, and, I.sub.o is the initial intensity. 
It is apparent from the above relationship that the build-up rate of the 
intensity I of the laser flux can be increased by minimizing the distance 
L between the mirrors M1 and M2. While the embodiment of FIG. 3 
illustrates the mirrors being external to the windows, the mirrors can be 
located within the laser tube adjacent to the laser discharge region. 
A second approach for achieving the desired objective of decreasing the 
build-up time of the second photon laser flux intensity involves the use 
of another external source of radiation, such as the pulsed laser source 
PS2, to establish a preset bias, or level, of the second photon laser flux 
intensity within the laser cavity to thereby reduce the necessary 
additional laser flux intensity build-up time required to support the 
lasing transition between the intermediate laser level and the lower laser 
level. In other words, I.sub.o is increased. The reduction in required 
additional laser flux intensity build-up results in a corresponding 
decrease in the required build-up time. 
Several techniques exist for generating the second photon laser flux in the 
external laser source, PS2, shown in FIG. 3. In the example of the 16 
.mu.m CO.sub.2 bending mode laser used to illustrate the disclosed 
concept, the external laser source, PS2, could be a small 16 .mu.m 
CO.sub.2 laser constructed using the teaching of referenced U.S. Pat. No. 
4,168,474. The 16 .mu.m laser output from PS2 can be injected into the 
optical path using a turning mirror M3 and a dichroic mirror, DM, as shown 
in FIG. 3. Dichroic mirrors, or gratings, which might also be used to 
combine the pump beams from PS1 and PS2 are available in the present 
technology. 
The teaching of the disclosed invention for improving the operation of a 
two-photon laser does not specifically require that the external laser PS2 
operate in a pulsed mode. Continuous wave, CW, operation of this laser is 
also acceptable and is considered to be a part of the teaching of this 
application. 
Thus, a two-photon, three level laser system which heretofore has operated 
successfully only at low temperatures and/or pressures where an energy 
loss to adjoining rotational lines is minimal, can now be operated at high 
temperatures, e.g., above 200.degree. K., and/or high pressures, e.g., 
above 50 Torr, and the potential energy loss resulting from high 
temperature and/or high pressure operation to adjoining rotational lines 
can be minimized by reducing the build-up time of the laser flux intensity 
at the intermediate laser level as described above. 
Furthermore it is clear that the same procedures which extend the operation 
of these lasers to high temperatures and/or pressures, can also be used to 
improve their performance at low temperatures and/or pressures.