Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2000-06-08
2003-04-22
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S072000
Reexamination Certificate
active
06553054
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to laser resonators and associated methods and, more particularly, to chemical laser resonators and associated methods.
BACKGROUND OF THE INVENTION
Chemical lasers, such as chemical oxygen iodine laser (COIL) devices, generate output signals having relatively high intensity. As such, chemical lasers are used in a variety of applications including high energy laser (HEL) weapons and the like.
For purposes of explanation, a conventional chemical laser
10
is depicted in
FIG. 1. A
chemical laser includes a laser cavity
12
having opposed upstream and downstream ends
14
,
16
. A lasing medium flows through the laser cavity in a predetermined flow direction from the upstream end of the laser cavity to the downstream end of the laser cavity. While chemical lasers can include different lasing mediums, a COIL device generally includes a lasing medium containing the species O
2
(
1
&Dgr;) (electronically-excited oxygen) and I
x
(electronically-excited iodine atoms). As shown in
FIG. 1
, the laser cavity typically includes an upstream end defined by a nozzle
18
through which the lasing medium enters the laser cavity and a downstream end through which the lasing medium exits the laser cavity after extraction of some of the medium energy in the form of a laser beam.
FIG. 1
also depicts the resonator structure that cooperates with the laser cavity
12
for producing the output signals having a relatively high intensity level. The resonator of
FIG. 1
is a negative branch standing wave resonator in which the laser beam makes a single pass through a single laser cavity. In this regard, only the collimated passes of the laser beam through the laser cavity are counted with the laser beam also assumed to traverse the laser cavity as an expanding beam, as shown in FIG.
1
. The laser resonator is comprised of several optical components, including a scraper
20
for outcoupling a substantial portion of the laser beam, thereby providing the output signals. The laser resonator also includes a secondary mirror
22
positioned rearwardly of the scraper relative to the laser cavity for reflecting a portion of the laser beam back through the laser cavity. As depicted in
FIG. 1
, the secondary mirror redirects a portion of the laser beam, typically a center portion of the laser beam, in such a manner that the laser beam again expands to its full size prior to being collimated. The laser resonator also includes a primary mirror
24
positioned opposite the scraper and the secondary mirror relative to the laser cavity for collimating the expanding laser beam and for redirecting the collimated laser beam back through the laser cavity.
Since the secondary mirror
22
is a concave mirror positioned rearward of the scraper
20
relative to the laser cavity
12
, the portion of the laser beam redirected by the secondary mirror must pass through focus prior to expanding and being collimated by the primary mirror
24
. As such, the laser resonator of
FIG. 1
is referred to as a negative branch standing wave resonator. Alternatively, the secondary mirror could be a convex mirror positioned proximate to the scraper or rearwardly of the scraper relative to the laser cavity for redirecting a portion of the laser beam back through the laser cavity in such a manner that the redirected portion of the laser beam expands while passing through the laser cavity without passing through focus. As known to those skilled in the art, this alternative type of resonator structure is a positive branch standing wave resonator.
Since the chemicals that produce the gain and/or power are progressively consumed as the laser beam is generated, the gain or amplification provided by the chemical laser progressively drops as the distance of the laser beam from the nozzle
18
increases. By way of illustration,
FIG. 2
a
depicts the gain provided by the laser cavity
12
as a function of the distance from the nozzle with signals that traverse the laser cavity at distances further from the nozzle having lower gain. As such, the resulting outcoupled beam has a strongly peaked (“sugar scooped”) intensity profile as depicted in
FIG. 2
b
. As will be noted, the intensity profile has a gap in the center portion thereof due to beam obscuration caused by the redirection of a portion of the laser beam back through the laser cavity by the secondary mirror
22
. As a result of the peaked intensity profile, the chemical laser
10
of
FIG. 1
does not generate the laser beam as efficiently as desired. Additionally, since the intensity of the laser beam varies across the width of the laser beam, the propagation of the outcoupled laser beam is degraded. In this regard, those portions of the laser beam having a greater intensity will heat the air or other atmosphere through which the laser beam is propagating to a greater degree than those portions of laser beam having a lower intensity. Thus, different portions of the laser beam will be diffracted in unequal amounts by the differently heated air. Moreover, in order to ensure that the average intensity of the laser beam is greater than a predetermined intensity level, that portion of the laser beam having the greatest intensity may have an excessively high intensity which may cause coating damage to the laser optics.
In order to avoid at least some of the shortcomings of the chemical laser
10
depicted in
FIG. 1
, other types of chemical lasers have been designed. As depicted in
FIG. 3
, for example, another conventional chemical laser
30
is designed such that the laser beam passes through the laser cavity
32
twice as a collimated laser beam. In this regard, the laser resonator of the embodiment of
FIG. 3
includes several additional mirrors to invert and translate the laser beam. As described above in conjunction with the embodiment of
FIG. 1
, the gain provided by the lasing medium diminishes in the downstream direction since the chemicals that produce the gain and/or power are progressively consumed as the laser beam is generated. By including mirrors to invert the laser beam following each pass through the laser cavity, one edge of the laser beam is not always nearest the upstream portion of the laser cavity and the other edge of the laser beam is not always nearest the downstream portion of the laser cavity. Instead, each edge of the laser beam is located nearest both the upstream and downstream portions of the laser cavity during different passes through the laser cavity. As such, the intensity profile is no longer “sugar scooped”. In this regard,
FIG. 4
provides a graph depicting the intensity of the outcoupled beam as a function of the distance across the beam which illustrates that the opposed edges of the laser beam have the greatest intensity and the center portion of the laser beam has the lowest intensity. While the intensity profile is more uniform and the chemical laser of
FIG. 3
generally has improved extraction efficiency relative to the chemical laser of
FIG. 1
, the chemical laser of
FIG. 3
disadvantageously requires additional powered optics, i.e., those optical elements, such as mirrors, that reflect or otherwise redirect a collimated laser beam. In addition, the length of the laser resonator can become relatively long since considerable spacing is typically required between the laser cavity and the optical elements.
Another conventional chemical laser
40
that is designed to create a more uniform laser beam intensity profile is depicted in FIG.
5
. In this embodiment, the chemical laser includes first and second laser cavities
42
,
44
to provide smoothing of the beam profile. As shown, the chemical laser of this embodiment is similar to the embodiment depicted in
FIG. 1
with the exception that the laser beam passes through not one, but two laser cavities that are positioned side-by-side. By positioning the laser cavities such that the direction of flow of the lasing medium through one laser cavity
18
is opposite the direction of flow of the lasing medium through the other laser ca
Bauer Arthur H.
Copeland Drew A.
Ullman Alan Z.
Alston & Bird LLP
Ip Paul
Nguyen Dung
The Boeing Company
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