Sapphire window laser edge annealing

A sapphire window is laser edge annealed using a CO.sub.2 laser spot illuminating along a path following the edge of the window so as to heat the edges to remove or reduce sub surface defects which can cause stress fractures.

The invention described herein may be manufactured and used by and for the 
government of the United States for governmental purpose without payment 
of royalty therefor. 
STATEMENT OF RELATED APPLICATION 
The present patent application is related to applicant's copending 
application Ser. No. 08/608,805, filed: Feb. 27, 1996, entitled "Method 
for Thermally Testing with a Laser the Edge of a Sapphire Window". 
FIELD OF THE INVENTION 
The present invention relates to the field of laser annealing. More 
particularly, the present invention relates to laser edge annealing of 
sapphire windows which may be used in high speed missiles, missile 
interceptors, rockets, seekers, and or atmospheric re-entry vehicles. 
BACKGROUND OF THE INVENTION 
Various high velocity missiles, missile interceptors, rockets, seekers and 
atmospheric re-entry vehicles may require the use of a window through 
which optical signals, such as laser beams or infrared radiation, may be 
used to acquire land based facilities or airborne targets. Specifically, 
high performance interceptor missiles employ infrared seekers to track 
incoming targets and guide the interceptor to these targets. An essential 
component of the interceptor is the seeker window which must be 
transparent to the infrared radiation and be able to structurally 
withstand aerodynamic pressure loading and intense aerodynamic heating. 
The preferred seeker window material is single crystal sapphire because 
sapphire optical properties are well suited for infrared transmission and 
sapphire thermal mechanical properties make the sapphire resistant to 
thermal stress fracture relative to other available window materials. 
These sapphire windows are subjected to excessive stress during flight. 
These flight stresses may cause cracking, breakage or failure of the 
window. The sapphire window is potentially a performance-limiting 
component of a high velocity interceptor missile. Typically, the edge of 
the window is secured to a window frame through edge clamping using 
molding. In some specific applications, the window may be made of sapphire 
and the window edge may be beveled and adapted in shape to be mounted 
within a vehicle window frame and securely clamped by the window molding. 
Computer analyses and thermal stress fracture testing of sapphire windows 
indicate that failure under simulated flight loading occurs at the window 
edge because the highest thermal stresses that cause fracture occur at the 
window edge and because the window is weakest at the edges from machining 
damage during fabrication which produces microscopic flaws in a small 
layer at the window edge surface. Machining damages the top layer which 
has much lower strength than the undisturbed bulk single crystal sapphire. 
The optical surfaces of the window surface area are also subject to 
machining and polishing damage but the nature of the flight induced 
thermal stresses and polishing of these flat surfaces are such that 
thermal fracture is much less likely to occur at these surfaces than at 
the window edges. There exists a need to manufacture windows with 
increased resistance to stresses at the edges of the windows. 
For application as a high velocity flight window, the sapphire window is 
polished for improved optical transmission through the window. Chemical 
etch polishing, abrasive polishing, and flame polishing may be used to 
polish ceramic materials. Abrasive polishing is preferred because of the 
precision and simplicity of the abrasive polishes without the use of 
dangerous chemicals used in chemical etch polishing nor the use of 
imprecise flame polishing or flame annealing. Flame polishing has been 
shown to increase the strength of small sapphire specimens by up to an 
order of magnitude, but flame polishing degrades the optical quality of 
abrasively polished optical surfaces. 
Sapphire is known to be a relatively hard ceramic. Hard ceramic materials 
generally fail due to tension stresses. Sapphire high velocity flight 
windows may be subject to tension stresses at the edge. There exists a 
need to improve the stress resistance of edges of high velocity ceramic 
windows without the use of a high temperature annealing process applied to 
the entire window. High temperature annealing of the ceramics can improve 
the tensile strength, but disadvantageously may destroy the optical 
precision of the polished surface. Bulk high temperature annealing of 
single crystal sapphire near the melt temperature increases structural 
strength, but deforms optical surfaces. Low temperature annealing of 
unpolished windows may increase tensil strength but may not completely 
reduce imperfections leading to fracture failures under stress, and will 
not reduce stress introduced in the edge during post annealing edge 
machining. These and other disadvantages are solved or reduced using the 
present invention. 
SUMMARY OF THE INVENTION 
An object of this invention is to improve edge stress resistance of ceramic 
materials. 
Another object of this invention is to improve the edge stress resistance 
of sapphire windows without degrading the optical surface of the windows. 
Yet another object of the present invention is to edge anneal ceramic 
materials for improving edge stress resistance without introducing large 
temperature gradients and thermal stresses which may weaken or fracture 
the ceramic materials. 
Still another object of the present invention is to edge anneal sapphire 
windows for improved thermal stress resistance without degrading the 
optical properties of the optical surface. 
Still another object of the present invention is to provide a method of 
edge annealing sapphire windows for improved thermal stress resistance 
without degrading the optical properties of the optical surface. 
The present invention covers edge annealing of optical ceramic materials. 
The edge annealing increases the tensile strength of the window at the 
edge. The edge annealing can be accomplished without serious degradation 
of the polish of any optical surfaces of the optical ceramic which is in 
the form of a polished window. Fractures along the edges is the most 
common failure mechanism of the sapphire window. The invention broadly 
covers edge annealing of optical ceramics. In the preferred form of the 
invention, a sapphire window is first polished to optical specification 
and then placed in an oven for a general low temperature anneal in 
combination with the laser edge annealing. This edge anneal processing 
improves the edge tensile strength without degrading the optical 
properties of the polished surface. These and other advantages will become 
more apparent from the following detailed description of the preferred 
embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention is described with reference to the 
figures using reference designations as shown in FIGS. 1 and 2. Referring 
to FIGS. 1 and 2, an optical means 10, which is preferably a 
carbon-dioxide (CO.sub.2) laser, illuminates an optical ceramic 12. The 
optical ceramic 12 is preferably a high performance seeker sapphire 
window. The optical ceramic 12 has an optical surface 13 defined by an 
edge 15, which may comprise a vertical edge 13 and a beveled surface 14. 
The ceramic 12 may be edge strength limited, in that, during use under 
stress, failure, if any, is likely to occur at the edge 15. The beveled 
surface 14 is machined into a shape adapted for purposes of mounting the 
optical ceramic 12. The optical ceramic may be mounted into a window 
frame, not shown. Aerodynamic heating subjects the optical ceramic 12 to 
stresses during use. 
The laser 10 illuminates the edge 15 to heat treat the sapphire window edge 
to anneal out any surface or subsurface damage and reduce the window 
susceptibility to thermal stress fracture. The edge annealing increases 
the thermal stress fracture resistance of the ceramic optical window. The 
CO.sub.2 laser is suitable for performing the window edge treatment 
because the absorption of CO.sub.2 wavelength laser radiation in sapphire 
is relatively high causing a very thin layer of the surface exposed to the 
laser to heat up. Other similar lasers and other corresponding optical 
ceramics can be used and treated in a similar fashion. Depending on the 
laser power and the exposure time, the thin surface layer may be heated to 
the sapphire melt temperature to remove or reduce machining defects in the 
sapphire. 
The laser 10 projects a laser beam 16 having a defined spot size 18. Lasers 
10 are well known to be controlled to predetermined power levels, 
direction, and duration. Those skilled in the art of optics can readily 
design control means to control the laser 10 to illuminate the edge 15 of 
the optical ceramic 12 along the path 20. The spot illumination 18 
substantially illuminates the edge 15 and insubstantially illuminates the 
optical surface 13 thereby causing the edge 15 to heat up while the 
remaining bulk portion of the ceramic 12, and particularly the optical 
surface 13, does not substantially heat up to the melt or anneal 
temperature. 
Laser annealing is more controllable than flame polishing and can be 
performed at temperatures close to the melt temperature to minimize the 
effect of thermal gradients, yet low enough to maintain shape stability of 
the fabricated window. After a slow cool down, the optical surfaces could 
be finished to meet optical specification without creating imperfection at 
the edges 15. No finishing of the edges 15 is necessary because the edges 
15 typically have no optical requirements and are not use as the primary 
optical surface 13. The specification of the edges 15 are typically 
limited only by the mounting specification of the window frame. The laser 
10 may illuminate the ceramic 12 through an oven window, not shown, while 
the ceramic 12 is in a low temperature annealing oven. 
The optical means 10 in the form of a CO.sub.2 laser requires power in the 
laser spot of typically 100 W/cm2 to 300 W/cm2 applied for one to several 
seconds to anneal an optical ceramic 12 in the form of a sapphire seeker 
window of approximate dimensions 10 cm wide.times.20 cm long.times.0.5 cm 
thick. The ceramic would normally be pre-heated in a conventional oven to 
a temperature slightly below its annealing temperature. The laser would 
then raise the temperature of a local spot above the annealing 
temperature. By scanning the beam, all areas of the ceramic requiring 
annealing are successively illuminated and thereby annealed. The bulk 
ceramic is then slowly cooled to room temperature. For a typical spot size 
of 1 cm.times.1 cm, this technique requires a total power in the laser 
beam of 100 W to 300 W. Assuming 50% losses in laser power from optics and 
other sources, this requires a total laser power in the range of 200 W to 
600 W. The laser spot is located typically on the window edge by means of 
an aperture. For a laser as the optical means 10, the laser can be located 
at any practical distance from the optical ceramic, e.g., 1 m to 100 m or 
more, with suitable optics to steer the beam and control beam divergence. 
The aperture that controls the spot size and location is typically located 
close to the optical ceramic, e.g., mm to cm, depending on the optics used 
to steer and focus the beam. 
The present invention uses an optical illumination means to spot anneal 
optical ceramics along the edges of the ceramic to improve the stress 
resistance of the optical ceramic. In the preferred form of the invention, 
the laser is a CO.sub.2 laser and the optical ceramic is a beveled 
sapphire window. Other improved, enhanced, alternative or modified 
illumination sources and optical ceramic materials may used as well. 
However, those enhancements, improvements and modifications may 
nonetheless fall within the spirit and scope of the following claims.