Compensation of thermal expansion in mirrors for high power radiation beams

A mirror for reflecting the high power radiation of laser beams while maintaining the predetermined shape of the reflecting surface. Radiation is reflected by a front, typically dielectric, surface on a transparent substrate. Since some power is unavoidably absorbed at the reflecting surface, the mirror will experience a thermal expansion. Some incident radiation is also transmitted to the opposite side of the substrate where at least a portion of the transmitted radiation is absorbed to a degree preselected to provide thermal expansion of that opposite side complementary to the expansion of the reflecting surface. The two expansions can be adjusted such that the shape of the reflecting surface is affected far less by the incident radiation than in the case where no opposite side absorption is provided.

FIELD OF THE INVENTION 
The present invention relates to mirrors and in particular mirrors with low 
thermal bending for reflecting high power radiation beams. 
BACKGROUND OF THE INVENTION 
In laser isotope separation, in particular uranium enrichment on a plant 
scale, it is required that laser beams of hundreds of watts of power be 
directed over substantial distances such as thousands of meters. It is 
essential that the beams be long in order to insure that the useful energy 
within the laser beams is efficiently utilized in photoexciting or 
ionizing particles of a selected isotope type. At the same time, the 
dimensions of the channels down which the laser beams must pass are not 
much bigger than the beam cross-section itself. It is additionally desired 
that the beam be centered in the channel and not be deflected to graze or 
strike the channel walls. Apparatus of the sort with which such laser 
beams may be employed are illustrated in U.S. Pat. Nos. 3,772,519, and 
3,939,354, incorporated herein by reference and commonly assigned. 
Because the beams utilized in laser enrichment are typically composite 
beams of several colors, as well as the result of the interleaving of 
pulsed radiation from many pulsed laser sources, as for example 
illustrated in U.S. Pat. No. 3,944,947, and further because the radiation 
is likely to be applied through a succession of channels, it is 
anticipated that a number of reflecting surfaces will be required for 
transporting the beams from the source of generation throughout the 
utilization channels, as well as for aligning and redirecting the beams. 
Because of the power densities employed in the laser beams some radiation 
absorption is inevitable at the reflecting surfaces even with the most 
carefully prepared reflectors. As a result of such radiation absorption, 
the reflecting surfaces of the mirrors will increase in temperature 
producing a thermal expansion at the reflecting surface which, by analogy 
to the bimetallic strip, will result in a bending of the mirror and in 
particular of the reflecting surface. Such a bending produces not only an 
undesired shift in the beam direction over the distances of beam 
propagation required, but in addition produces aberations detrimental of 
the beam wave-front which results in defocusing and diverging effects. 
These may be difficult or impossible to correct. 
While it has been proposed, as in U.S. Pat. No. 3,609,589 to provide a 
mirror of layered metallic composition wherein each layer spaced back from 
the reflecting surface has an increased thermal coefficient of expansion 
to compensate for the lower thermal heating of the reflector with distance 
from the reflecting surface, such layers are unsatisfactory. For a first 
reason, reflection by a metallic reflector is less efficient than by a 
layer or layers of dielectric films. In addition, the relative slowness of 
thermal conductivity throughout the mirror substrate prevents such a 
device from being effective in compensating for thermal distortions of the 
mirror surface on all but the most long-term basis under steady state 
illumination conditions. Also, the required precision to which such a 
mirror must be manufactured such that each layer has a precisely 
dimensioned thermal expansion, makes it economically impractical. Finally, 
absorption by a thick front face metallic reflector tends to be higher 
than absorption by a properly designed dielectric mirror, thus limiting 
total power handling capability. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the teaching of the present invention, a mirror for 
reflecting high power laser beams is provided in which a transparent 
substrate has on a first reflecting surface a multi-dielectric layer, 
preferably adapted to provide optimum reflection of the color or colors of 
radiation in the laser beam. Inherent in any such surface, however, is the 
absorption of a portion of the radiation resulting in the heating of the 
reflecting surface and expansion of the underlying substrate. The 
reflecting surface is also provided with a characteristic that results in 
transmission of a portion of the incident laser radiation, apart from that 
portion which is typically scattered. This radiation is permitted to pass 
through the substrate to a rear or opposite surface which has an absorbing 
layer that, in most applications, may be nearly or completely absorbing, 
this rear layer acts to heat the rear surface of the mirror also producing 
a thermal expansion complementary to that of the expansion of the front 
surface. By proper adjusting of the reflecting and rear absorbing 
surfaces, the compensation can be quite complete. The mirror which results 
from such a structure is then greatly superior in its beam maintaining 
characteristics than the mirror without the expansion compensation. 
Because the compensating mirror of the present invention operates directly 
upon the radiation by absorption of it, it is effective over a wide range 
of beam powers. Moreover, because the radiation is directly absorbed from 
the beam itself, and does not have to heat by thermal conduction, the 
compensation effect is just as instantaneous as is the original thermal 
expansion creating the mirror deflection. Also because the mirror of the 
present invention absorbs more nearly equal amounts of thermal radiation 
at both the front reflecting and rear absorbing surfaces, any long-term 
thermal gradient is largely eliminated.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention contemplates a mirror for redirecting high energy 
laser beams and having a front reflecting surface partially absorptive to 
radiation in the incident beam and a rear absorbing surface separated by a 
transparent substrate which permits a portion of the incident radiation to 
be transmitted to the mirror rear surface for absorption there. 
Where high energy laser beams are propagated over substantial distances it 
is often desired that mirrors be employed to reflect the beam for purposes 
of alignment and repositioning of the beam. Thermal energy of significant 
proportions can be absorbed by the mirrors resulting in their thermal 
deformation. While it is known to provide reflection of laser radiation by 
either metallic conducting surfaces or by tuned dielectric layers, in 
state of the art reflector designs, the reflectivity of dielectric layers 
is generally superior, reflecting a greater proportion of the incident 
radiation and absorbing or otherwise scattering a lesser proportion of 
that radiation. 
Reflective dielectric layers are well known in the art and can be provided 
according to known formulate to reflect a frequency or frequencies nearly 
completely as the result of wave cancellation phenomenon from each of the 
interfaces between layers of different dielectric or refractive index. 
In applications of laser enrichment where isotopically selective 
photoexcitation and ionization is desired, the frequencies of radiation in 
the beams propagated throughout the system for this purpose are typically 
of very high spectral purity in order to accomplish the selectivity of 
excitation. While there thus may be three or four colors in a single beam 
of laser radiation for the different energies of excitation desired, it is 
nonetheless possible to provide highly efficient high reflectivity 
surfaces composed of multiple dielectric layers tuned to reflect at those 
frequencies. 
Whereas the reflectivity is indeed very good, approaching 100%, there is 
nevertheless a portion of the radiation, for example 0.2% in good quality 
reflectors, which is not reflected. Of this radiation, a portion is 
typically absorbed at the reflecting surface resulting in the heating of 
that portion of the reflective element and thermal expansion tending to 
create a distorted convex surface. Other portions of the nonreflected 
radiation are typically scattered and transmitted. 
In accordance with the present invention, that portion of the radiation 
which passes through the reflective layers without reflection is utilized 
to provide thermally induced rear surface expansion, compensating for the 
front surface expansion. For this purpose, the reflective dielectric 
layers are deposited upon a transparent substrate which is typically 
optical grade quartz such as the substrate 12 illustrated in FIG. 1, a 
portion of the radiation in an incident beam 14, which is not reflected or 
absorbed at a multilayer dielectric reflecting surface 16, passes through 
the substrate 12 to a rear absorbing layer 18. 
In the case where the portion of the radiation transmitted through the 
substrate 12 by the layer 16 is at least as great as the portion of the 
radiation which is absorbed at the reflecting layer 16, absorption of all 
or part of that radiation by the rear layer 18 will provide thermal 
heating at the rear of the substrate 12 equivalent to the thermal heating 
at the front. In this manner, the thermal expansion of the homogeneous 
substrate 12 can be made the same at both the front, near the reflecting 
surface 16, and at the rear, near the absorbing surface 18. In addition, 
long-term thermal equilibrium for the substrate 12 will be provided with 
no gradient between the front surface 16 and the rear surface 18 thereby 
avoiding the difficulties of maintaining a proper gradient and conditions 
of expansion appropriate to it. 
The technique for providing appropriate multi-dielectric layers for surface 
16 is well known in the art as, for example, presented in "Military 
Standardization Handbook," No. 141, Optical Design. Using such techniques 
and for a given frequency or frequencies, a selection of appropriate 
layers and their order can be made. Fabrication of such layers is a common 
industry technique. For the absorbing layer 18, where it is desired to 
absorb all or nearly all of the radiation transmitted through the 
substrate 12, a fully absorbing material may be utilized. These may be 
optically thick dielectric layers absorbing in the spectral region of the 
applied radiation, or a combination of such layers with thin dielectric or 
thin dielectric and metallic layers. In typical fabrication, the substrate 
12 will be dimensioned approximately one-fourth to one-sixth its diameter 
in thickness, and for precise mounting is typically set between knife 
edges such as the knife edges 20 illustrated in FIG. 1. 
It is additionally contemplated to adjust the absorption of the layer 18 
positionally where desired to vary the degree of heating at different 
locations in the reflector. For example, the substrate 12 edges may lose 
heat more rapidly than the center due to the greater surface area and it 
may be desired to generate more heat in edge regions by thickening the 
absorbing layer 18 or otherwise providing for higher absorption there. 
As thus described, a mirror is presented useful for redirecting and 
aligning radiation in a beam 14 such as from a laser source or sources 22 
for application to one or more channels 24 where isotopically selective 
photoexcitation and ionization takes place in accordance with the teaching 
of the above-referenced United States patents. 
The specific teaching of the invention is intended as exemplary only and 
not as a limitation on the scope of the invention other than as 
specifically provided in the following claims.