Beam diverting shutter for a laser beam

A shutter mechanism having a slide element coupled to a support base by a plurality of roller bearings journaled between opposing pairs of guide rods is disclosed. The slide element and support base have corresponding beam apertures in alignment when the shutter is in its open position. Angularly mounted on the slide, adjacent to the slide's beam aperture, is a UV reflective mirror. When the shutter is commended to close, an actuator forces the slide and support base apertures out of alignment, while simultaneously positioning the UV reflective mirror into the position previously occupied by the slide's aperture. A laser beam, previously being transmitted through the aligned apertures, will be reflected by the mirror to a beam stop as a diagnostic instrument mounted on the support base.

FIELD OF THE INVENTION 
The present invention relates to shutter mechanisms for laser systems, 
particularly to an improved design for mechanical shutters. 
BACKGROUND OF THE INVENTION 
The fundamental requirement of a shutter system is to provide a complete 
mechanical blockage of an energy source, such as light. What distinguishes 
shutter systems from other electromagnetic modulation devices is the use 
of a straight-through clearance aperture having no optical surfaces to 
modify the electromagnetic wave front. Typically, shutter systems are 
designed to function as either safety interlocks, as modulators, or in 
both capacities depending on the intended application. 
When used as a safety interlock in an application such as a laser system, 
the shutter is used to block and absorb the laser beam whenever the 
laser's control system receives a request to establish a safe condition. A 
typical control system is comprised of a DC power supply, a relay or solid 
state device that switches the actuator which drives the shutter 
mechanism, and associated microprocessor based logic components. An 
example of such a control system is described in U.S. Pat. No. 4,513,345, 
incorporated herein by reference. 
Generally, shutter systems are mounted directly to the output port of the 
laser head, or in some cases a separate housing is constructed to enclose 
the laser output and shutter mechanism, thereby serving to prevent access 
to the laser beam. The properties of a shutter system, particularly those 
used in conjunction with a laser system, require that they absorb all of 
the incident beam energy, with very little back scattering. Most prior art 
designs have sought to contain the resulting back scatter by utilizing a 
simple enclosure to allow absorption of the back scatter energy into the 
laser head and associated laser components contained in the enclosure. Two 
of the problems associated with this type of design are feedback of 
scattered energy into the optical system and the increased risk of leakage 
from the enclosure, thereby creating a safety hazard. As optical power 
levels escalate, these considerations become increasingly important from a 
design perspective. 
An example of a well-known magnetic flexure type shutter is depicted in 
FIG. 1, which includes a very thin flexible foil 2 which is deflected into 
a beam path 4 by an electromagnet 6. The foil 2 is seen as being attached 
at one end to a collar 8 by a retaining screw 10, and lies parallel to and 
below the beam path 4 when the electromagnet is inactive. The 
electromagnet is of conventional design, having a ferrite core 12 
surrounded by a magnetic winding 14 on a pancake bobbin. In operation, the 
winding 14 is energized, thereby activating the magnet 6 and causing the 
free end 2' of foil 2 to be attracted to magnet 6, moving across beam path 
4. As foil 2 intercepts the beam path, the beam is reflected away at an 
increasing angle of reflection as foil end 2' moves into a biased position 
adjacent the magnet 6. In the full, closed position, as shown in FIG. 1, 
the foil conforms flat as it biases against the magnet, bending into an 
"S" shape as it approaches the retaining screw 10, providing an incident 
angle of about 50.degree. to 60.degree. for beam interception. 
The foil 2 is extremely thin to allow for the required flexibility to 
enable the foil to contort into the "S" configuration. This required 
thinness, however, is inherently weak, particularly at stress points in 
the "S" shaped bends, and is also not thermally suitable for conducting 
away heat resulting from absorption of higher power lasers. Another aspect 
of the foil arrangement is that the foil reflects the light beam at an 
angle near that of the unaltered beam path, resulting in unwanted stray 
reflection lines appearing at the target plane as the foil end 2' begins 
to intercept the beam, continuing until the foil end is flat-biased 
against the magnet. Since the reflection is at a very narrow angle with 
respect to the unaltered beam path 4, the reflection cannot be eliminated 
in the device, and the fully closed "S" shape allows for a back scattering 
of laser light, whose effects as previously discussed are undesirable, 
from a safety standpoint and also from a component standpoint. This is due 
primarily as a result of the absorbed back scattered energy, and the 
contaminating of the exposed optics, including the shutter's own 
reflective mirror, as a result of this out gassing. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved automatic 
shutter for use in a laser system, such as an excimer laser, capable of 
blocking and diverting the output of a laser beam; 
It is a further object of the present invention to provide a shutter 
mechanism having an internal configuration in which any reflected energy 
is multiply scattered, thereby preventing leakage of any back scattered 
energy. 
It is a feature of the present invention to use a reflective surface to 
divert the laser beam, when the shutter is in a closed position, to a 
secondary device, such as a diagnostic device. 
It is another feature of the present invention to have the beam stop 
function as a diagnostic mount, for the attachment of diagnostic 
instrumentation, to allow the coupling of such instrumentation without 
disturbing the output delivery system of the laser. 
The objects and features of the present invention are realized by use of a 
shutter system having a slide mechanism mounted on a rod-roller bearing 
support which restricts the slide to a single degree of freedom. The 
rod-roller beam supports provide a rolling contact interface between the 
slide and the shutter's support base. The roller bearings are constructed 
of a hardened precision ground steel and are subjected to only a light 
force load due to the unique rod-roller configuration, thereby allowing 
the slide to function without lubrication. The shutter's support base 
incorporates a beam aperture positioned in alignment with the output beam 
of the laser system. The slide has a corresponding aperture of identical 
dimensions which aligns with the support aperture when the slide is fully 
retracted in the open position, thereby allowing the output beam of the 
laser to egress through the shutter uninterrupted. Mounted on the slide, 
adjacent to the slide aperture, is a dielectric mirror positioned at an 
angled position relation to the beam path. When the shutter mechanism of 
the present invention is commanded to move from the open to the closed 
position, an actuator coupled to the slide causes the slide to ride the 
roller bearings along its single degree of freedom, positioning the mirror 
in the beam path. The beam is reflected off the mirror's angulated surface 
and diverted to a shutter housing aperture. Because the mirror is fixedly 
mounted to the slide, when the slide is in the closed position to deflect 
the output beam, the slide aperture is likewise orthogonally repositioned 
out of alignment with the housing aperture. The close tolerances between 
the slide and the support base, combined with the slide aperture 
repositioning, create a multiple number of reflective surfaces through 
which any back scatter energy would be required to negotiate in order to 
leak from the housing enclosing the slide and mirror components. As a 
result, the problem of back scatter leakage is effectively eliminated. The 
shutter housing aperture is adapted to allow secondary devices to be 
attached to the shutter support base and positioned directly in line with 
the reflected beam path. An interlock safety switch is mounted on the 
shutter support base adjacent to the shutter housing aperture, to ensure 
that a secondary device, either diagnostic or beam stop, is present before 
the laser system will operate.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring now to FIG. 2, a preferred embodiment of the shutter device of 
the present invention is disclosed, indicated in the figure as 100. 
Shutter 100 comprises a rigid shutter support base 110, having a base beam 
aperture 120 centrally positioned therein, as also shown in FIG. 3a. It is 
preferred that support base be constructed of a material which is 
insensitive to ultraviolet energy, such as uncoated aluminum. Referring to 
FIG. 3a and 3b, depicting shutter 100 along its horizontal-longitudinal 
axis, rod guide supports 130 are shown more clearly as being mounted on 
the peripheral edge of support base 110 on either side of beam aperture 
120. On the aperture side, rod guide supports 130 each have a mounting 
slot 140 containing guide rods 150 and 155 positioned at the lower and 
upper ends respectively of slot 140. The rods 150 and 155 preferably have 
a diameter equal to the depth of slot 140 to create a flush internal 
surface on the aperture face of rod guide supports 130. Rods 150 can be 
secured into position by any of several well-known means; as shown in FIG. 
2, a simple screw 160 is used to biasly secure the rods into position. 
Preferably rods 150 and 155 are constructed from a case-hardened material 
which is compatible with, and will not deform while subjected to fit 
tolerance operation with the roller bearings discussed infra. For excimer 
laser applications, it is preferred to use stainless steel due to the 
operational cleanliness requirements of the material. 
Referring again to FIG. 3a and 3b, slide shutter element 170 is shown along 
its longitudinal axis, having a slide mounting slot 180 containing slide 
rods 190 and 195 in an upper and lower position respectively, which 
mirrors and is symmetric to the arrangement in rod guide support 130, 
including a securing means (not shown) operationally equivalent to screw 
160. Like support base 110, slide shutter 170 is manufactured from a UV 
insensitive material, preferably uncoated aluminum, and rods 190 and 195 
are likewise manufactured from stainless steel type of material. A 
plurality of hardened roller bearings 200 are journaled between rods 150, 
155, 190 and 195 to allow slide shutter element 170 to move in rolling 
contact along rods 150, 155, 190, and 195 in a single degree of freedom 
along its longitudinal axis. The number of bearings used will be a design 
choice based upon dimensional and operational requirements, as those 
skilled in the art will appreciate. For the design shown in FIGS. 3a and 
3b, six roller bearings were positioned on each side of slide shutter 170 
as described herein. Roller bearings 200 are of conventional design and 
precision ground from a hardened material such as stainless steel. The 
roller bearings 200 are segregated from each other by utilizing synthetic 
buffer strip 210 which is UV tolerant, from a material such as PTFE, such 
as Teflon.RTM., and having a series of individual internal holes (not 
shown) to accommodate each of said roller bearings. Fit tolerances between 
roller bearings 200 and the symmetric guide rod pairs are minimized so 
that movement of slide-shutter 170 is constrained to its longitudinal axis 
with no lateral play. The use of multiple roller bearings in a side 
mounted, fit tolerance relationship as described allows the weight of 
slide shutter 170 to be distributed among each of the roller bearings 200, 
resulting in each of said roller bearings being lightly loaded. This light 
loading allows the slide element to function without the need for 
lubrication and operate within the required signal-to-close/open 
activation time of about 200 ms to about 300 ms. Slide beam aperture 215 
is centrally positioned and in alignment with base beam aperture 120 when 
the shutter mechanism is in the open position. Preferably, the alignment 
between base beam aperture 120 and slide beam aperture 215 is within 0.005 
inches, and the fit tolerance between slide shutter 170 and support base 
110 within 0.020 inches. 
Referring now to FIG. 2, mounted on top of slide shutter 170 is mirror base 
220 (also shown in FIG. 3b), having an angulated support face 230 oriented 
to face the output path of the laser beam 250. The specific angle of 
support face 230 will be operationally dependent, as those skilled in the 
art will readily recognize, based upon the orientation of the output beam 
of the laser system and the desired beam deflection angle. In the 
embodiment depicted in FIG. 2 the support face is angled to provide a 45 
degree angle of incidence between dielectric mirror 240, mounted on 
support face 230, and output beam 250. Mirror 240 is positioned on support 
face 230 such that the bottom edge of the front face 245 is approximately 
aligned or slightly overhanging with respect to the bottom edge 235 of 
support face 230 to facilitate output beam engagement discussed infra. 
Material choices for mirror 240 will likewise be operationally dependent, 
based in principle upon the type of laser system being used. For example, 
for an excimer laser system mirror 240 would be required to have the front 
face be a UV light-reflecting surface, preferably made from a fused silica 
material. An additional feature incorporated in the mirror design is to 
have the back face be a visible light-reflecting surface to facilitate 
alignment of the laser's optical components as will be described infra. 
Attached to rear longitudinal face 260 (as shown in FIG. 4) of shutter 
slide 170 is actuator means 270. In the described embodiment, actuator 
means 270 is a pneumatic actuator such as those manufactured by BIMBA, and 
is operably coupled to longitudinal face 260 by actuating rod 280. While 
the depicted embodiment describes a pneumatic actuator, other actuator 
means meeting the signal-to-close/open time limitations operationally 
required may be substituted, such as electromagnetic or the like. For use 
in an excimer laser system, the pneumatic system manufactured by BIMBA was 
chosen due to its signal-to-close/open actuation time of about 300 ms. 
Referring again to FIGS. 2 and 3a, a housing 290, manufactured from a 
material such as uncoated aluminum, encloses the shutter slide and mirror 
base combination and includes an incoming beam port 300 designed to 
transmit only the incoming light of beam 250. With the shutter mechanism 
in the open position, as shown, slide shutter 170 is retracted, thereby 
positioning mirror base 220 and attached mirror 240 away from beam 250, 
while simultaneously aligning slide aperture 215 with base aperture 120 to 
allow beam 250 unimpeded egress for operational use. Referring now to FIG. 
4, when the shutter is commanded to move to the closed position, through a 
laser control means not shown but well known by those skilled in the art, 
actuator 270 forces slide shutter 170 to move forward along the roller 
bearing/guide rod arrangement previously described. As slide shutter 170 
moves forward, mirror 240 is positioned to reflectively engage beam 250, 
while slide aperture 215 is repositioned out of alignment with base 
aperture 120, with the slide's bottom face 310 closing off and effectively 
light-sealing base aperture 120. This light sealing aspect of the present 
invention is achieved as a result of the close tolerances, of about 0.020 
inches, between slide shutter 170 and support base 110; as well as the 
repositioning of slide aperture 215 out of alignment with base aperture 
120. In order to leak from housing 290, back scattered energy would have 
to be reflected through slide aperture 215, re-reflected through 
shutter/support tolerance gap 217, and again reflected through support 
aperture 120, as shown in FIG. 4. As a result, the problems associated 
with leakage of back scattered energy is, for the most part, eliminated. 
It is to be noted that as a result of the alignment of the bottom edges 235 
and 245 of the support face 230 and mirror 240, respectively, the mirror 
intercepts beam 250 ahead of edge component 235 encroaching the path of 
beam 250, to immediately divert beam 250 without any inadvertent 
reflections. It is preferred that the operation time from open to close 
occur in about 0.200 ms. As shown in the preferred embodiment in FIGS. 3b 
and 4, beam 250 is reflectively diverted through 90.degree., wherein 
diverted beam 320 is directed to diverter port 330. Coupled to diverter 
port 330 and rigidly mounted on the peripheral edge of shutter support 
base 110 is diagnostic instrument means 340. Preferably, the means for 
coupling diagnostic instrument is of universal design to accommodate a 
variety of instruments for optical analysis or diagnosis. When diagnostic 
instrument means 340 is removed, and interlocks activated by the open 
switch a device such as a collimating telescope can be attached to 
diverter port 330 and used to facilitate alignment of the laser optics by 
utilizing the visible light reflection off the back face of mirror 240, as 
previously described. To ensure the laser system being used cannot operate 
unless an instrument means is present to block diverted beam 320, a safety 
interlock switch 350 is mounted at the aperture. This arrangement allows 
the alignment of the optics by remote access, thereby eliminating the need 
for disassembly of the laser delivery systems. 
While the invention has been described in connection with what is presently 
considered to be the preferred embodiments, it is to be understood that 
the invention is not limited to the disclosed embodiments, but on the 
contrary, covers various modifications and equivalents included within the 
spirit and scope of the following claims. Therefore, persons of ordinary 
skill in this field are to understand that all such equivalents are 
included within the scope of the claims.