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The present invention relates to laser systems and, more particularly, to
an improved method and apparatus for producing high-power collimated laser
beams.
Methods and apparatus for producing high-power collimated laser beams have
found many uses and applications in the art, not the least significant of
which is the use of such methods and apparatus in laser research.
Conventional high-power lasers produce a highly coherent light beam,
either pulsed or continuous-wave, having a relatively narrow bandwidth, as
on the order of five angstroms, for example. It has heretofore been
recognized that high beam coherence, which is a measure of the phase
relationship of various beam portions either spatially, i.e., across the
beam profile, or temporally, i.e., along the beam axis, tends to produce
several undesirable effects. For example, amplification of a highly
coherent laser beam may cause interference between adjacent portions of
the beam, and thereby tend to produce spatial or temporal beam "hot
spots", i.e., areas of higher-than-average energy in the beam profile or
length. Because the refractive index of materials varies with beam
intensity, hot spots in the beam may not only cause damage to the various
laser system elements, but also tend to collapse the beam profile, thereby
intensifying the hot spots. This effect is referred to as beam
self-focusing. Moreover, high coherence in a collimated laser beam
increases the tendency for diffraction ring formation around the beam
profile. These inherent problems associated with highly coherent laser
beams have heretofore prompted efforts to reduce beam coherence. However,
no practical method or apparatus has yet been proposed for reducing beam
coherence without detrimentally affecting or, indeed, destroying beam
collimation.
Accordingly, it is a general object of the present invention to provide a
method and apparatus for reducing the spatial and/or temporal coherence of
a collimated laser beam without detrimentally affecting beam collimation.
More specifically, it is an object of the present invention to provide a
method and apparatus for smoothing the profile of a high-power laser beam,
thereby permitting the use of higher beam power densities and, at the same
time, reducing the likelihood of damage to refractive or reflective
elements in the laser system.
In furtherance of the object stated immediately above, it is another object
of the invention to provide a method and apparatus for increasing the
bandwidth, and thereby effectively reducing the coherence, of a collimated
laser beam.
It is a further object of the present invention to provide a method and
apparatus for effectively isolating the downstream laser system elements
from low energy pulses or radiation emanating from the upstream beam
source.
In accordance with the method provided by the present invention, the above
and other objects are accomplished by focusing a collimated, highly
coherent light beam onto a surface preferably fabricated of material
having a relatively low atomic number. A portion of the material surface
is heated by the initial or leading portion of the focused beam energy to
form a hot reflective plasma around the focus point, which plasma reflects
the remaining or trailing portion of the focused beam back onto the
incident beam path. Such plasma reflection not only directly reduces
spatial and temporal beam coherence, but also increased beam bandwidth,
thereby providing further indirect reduction of beam coherence. The
reflected beam portion is recollected to form a collimated light beam with
substantially reduced beam coherence.
The novel features which are considered to be characteristic of the present
invention are set forth in particular in the appended claims. The
invention itself, however, together with additional objects, features and
advantages thereof, may be best understood from the following description
when read in conjunction with the accompanying drawings in which:
FIG. 1 is a schematic diagram of a presently preferred embodiment of an
optical system for practicing the invention;
FIG. 2 is an enlarged view of a portion of FIG. 1 and is useful in
understanding the operation of the invention; and
FIG. 3 is a schematic view of an alternative embodiment of an optical
system for practicing the invention.
Referring to FIG. 1, a bidirectional beam path 10 is defined at one end by
reflective means 12 and at the other end by a polarizer plate 14. A second
polarizer plate 16 is disposed on path 10 and is oriented with respect to
polarizer 14 such that polarizer 16 passes light linearly polarized at an
angle of 45.degree. with respect to linearly polarized light passed by
polarizer 14. A Faraday rotator 18 is disposed on path 10 between
polarizers 14 and 16 to cumulatively rotate the plane of polarization of
linearly polarized light by an angle of 45.degree. during passage
therethrough in either direction.
In the general operation of the embodiment of FIG. 1 as thus far described,
a coherent, highly collimated input laser beam is incident upon polarizer
14 along a path 20 from an amplifier 21, for example, and is directed or
passed by polarizer 14 onto bidirectional beam path 10. The incident beam
may originate from any suitable, conventional laser system (not shown).
Preferably, the incident beam is linearly polarized in the direction of
polarizer 14 so that maximum beam energy is passed onto beam path 10 to
eventually emerge as an output beam. The incident beam is passed through
and rotated by Faraday rotator 18, and then passed by polarizer 16 onto
reflective means 12. Reflective means 12 is effectively disposed normal to
beam path 10 such that a portion of the beam incident thereon is reflected
back onto path 10, through polarizer 16 and Faraday rotator 18, where the
beam polarization is rotated by an additional angle of 45.degree.. After
being cumulatively rotated by rotator 18 during passage therethrough in
both directions, the reflected beam incident upon polarizer 14 is
polarized at an angle of 90.degree. with respect to the input beam, and is
thus reflected by polarizer 14 onto an output beam path 22 to suitable
downstream laser components, exemplified by the amplifiers 23. Output beam
path 22 is orthogonal to input beam path 20. The general provision and
several advantages of the structure thus far described in connection with
FIG. 1 is disclosed and discussed in greater detail in the copending
application at Brueckner et al, Ser. No. 468,176 now U.S. Pat. No.
4,019,151, filed Aug. 26, 1975 and assigned to the assignee hereof. It
will be recognized that, although the incident and reflected beams are
depicted separately with directional arrows in FIG. 1, the paths of such
beams are actually coincident on the bidirectional beam path generally
indicated at 10.
In accordance with the present invention, referring to FIGS. 1 and 2,
reflective means 12 comprises a lens 24 disposed on beam path 10 to focus
a collimated incident beam, generally indicated at 26 (FIG. 2), onto a
target disc 28. Preferably, the focal point 29 of lens 24 is at
substantially the adjacent material surface of disc 28. Thus, the energy
of collimated beam 26 is focused over a very small area of material
surface on disc 28. The initial or leading portion of such focused energy
reaches the surface of disc 28 and heats a portion of the material surface
to form a hot reflective plasma 30 about the focus point. The trailing or
remaining, major portion of the incident beam, i.e., that portion reaching
the disc after plasma formation, is reflected by plasma 30 back toward
lens 24, and is recollimated thereby and passed to polarizer 16. Thus,
lens 24 serves the dual purpose of focusing the collimated beam onto the
material surface for plasma formation, and recollecting and recollimating
that portion of the beam energy reflected by the plasma. In addition to
effectively "scattering" the phase relationship of adjacent beam portions,
plasma 30 scatters somewhat the polarization of the reflected beam, so
that a small portion of the reflected beam is not passed by polarizer 16
and is lost. The remaining, major portion of the reflected beam eventually
appears on output path 22 as described above.
To prevent the focused beam from ionizing the air around focal point 29,
target disc 28 is preferably disposed in an evacuated chamber (not shown).
Disc 28 is preferably fabricated of material having a low average atomic
number. Such material is referred to in the art, and is referred to
hereinafter, as having a low "Z" (wherein the bar indicates average Z).
Examples of preferred materials include polyethylene (Z = 2.66), carbon (Z
= 6) and lithium hydride (Z = 2). Such low Z materials are preferred
because the plasma formed thereby is hotter for a given amount of focused
energy than would be a plasma formed from heavier materials. The
effectiveness of the invention in reducing beam coherence is considered to
be related to plasma temperature. In a working embodiment of the
invention, a target disc 28 fabricated of polyethylene material was found
to yield highly satisfactory results at an estimated plasma temperature of
about two to three million degrees Kelvin. A collimated beam 26 having a
total power density of 2 .times. 10.sup.9 watts/cm.sup.2 and a total
energy of about one joule was focused onto the surface of the polyethylene
disc. The reflected beam had a power density of 2 .times. 10.sup.8
watts/cm.sup.2 and a total energy of 0.15 joule, meaning that 85 percent
of the input beam energy was used in plasma formation or lost in stray
reflections. The spatial coherence of the reflected collimated beam was
reduced by a factor of six when measured with a 40 percent beam overlap.
The bandwidth of the reflected beam was increased from about five
angstroms to about 20 to 40 angstroms giving a temporal coherence length
of 0.25 mm.
In addition to "smoothing" the collimated output beam as intended, the
invention has the additional, somewhat surprising advantage of effectively
isolating the downstream system components, such as amplifiers 23, from
spurious pulses or other undesirable low energy illumination emanating
from the upstream components, such as amplifier 21. For example, pumped
laser amplifiers tend to emit a low energy radiation termed "amplifier
fluorescence" in the art. Such radiation emanating from amplifier 21 would
be of insufficient energy content, even when focused by lens 24, to heat
material 28 to a temperature at which reflective plasma 30 is formed.
Rather, the low energy radiation will be absorbed or scattered by the
disc.
It will be apparent from the foregoing discussion that the embodiment of
the invention shown in FIG. 1, and the method described in connection
therewith, fully satisfy all of the objects and aims set forth above.
Although the invention has heretofore been described in connection with a
specific embodiment thereof, many alternatives, modifications and
variations will suggest themselves to persons skilled in the art in view
of the foregoing description. One such modification is schematically
depicted in FIG. 3, wherein reference numerals identical to those used in
FIGS. 1 and 2 indicate identical elements. In the embodiment of FIG. 3
rotator 18 and polarizer 16 of FIG. 1 are replaced by a conventional
quarter wave plate 32. Plate 32 converts the linearly polarized collimated
beam passed by polarizer 14 into a circularly polarized beam. The
circularly polarized beam is focused onto disc 28, reflected by plasma 30,
recollimated by lens 24 and then reconverted by plate 32 to a linearly
polarized beam having a plane of polarization at an angle of 90.degree.
with respect to the polarization angle of the incident beam. The linearly
polarized reflected beam is directed by polarizer 14 onto output beam path
22.
The invention is described in connection with FIGS. 1-3 primarily in
schematic form, and various mechanical details which will be self-evident
to persons skilled in the art have been omitted. For example, it will be
apparent that, in actual practice, each of the elements depicted in the
drawings comprises a three-dimensional body which must be suitably
mounted. Each of the described elements, with the exception of disc 28, is
itself well known, and suitable apparatus for mounting and positioning
each element to yield the disclosed combination will be self-evident to
the skilled artisan, given the information herein provided. Each burn
caused by a focused beam on the surface of target 28 leaves a small pit or
depression therein. Hence, to insure that focal point 29 is as close as
possible to or, preferably, at the material surface for the next beam to
be focused, suitable means (not shown) must be provided for repositioning
the target disc prior thereto.
Although the target material has been disclosed as a disc 28, it will be
evident that other structural target geometries may be used where desired.
For example, the target material may comprise a small sphere or hollow
shell, on the order of 50 micrometers in diameter, adapted to be
adjustably positioned at lens focus 29. In an automated illumination
system, target disc 28 may conceivably be replaced by a rod of low Z
material having a plane material end surface disposed normal to the beam
path and at which focal point 29 is positioned. After each burn, the
target rod could be automatically moved axially toward lens 24 by an
incremental distance equal to the calculated or empirically-determined
burn pin depth. Accordingly, the invention is intended to embrace the
above-noted and all other modifications and variations as fall within the
spirit and broad scope of the appended claims.
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