Method of forming a sharp edge on an optical device

A method of forming a sharp edge on an optical device. The method involves the steps of placing the optical device in a holding mechanism; grinding one surface of the optical device so the one surface and a surface of the holding mechanism are co-planer; and polishing the one surface of the optical device and the surface of the holding mechanism with felt until an edge on the one surface of the optical device adjacent the surface of the holding mechanism obtains a desired sharpness.

BRIEF SUMMARY OF THE INVENTION 
This invention relates generally to optical device formation and more 
particularly to a method of forming a sharp edge on optical devices. 
The field of optical technology has grown extremely rapidly since the 
advent of space research. Optical experiments are placed aboard spacecraft 
to learn more about optical energy received from various sources within 
the universe. As more is learned more advanced technical apparatuses are 
needed to detect and measure optical phenomenon. One such phenomenon under 
investigation is the effect that magnetic fields have on beams of light 
energy received from light emitting bodies such as the sun. This, of 
course, is the classical Zeeman Effect and various experiments have been 
developed to measure the Zeeman Effect. 
One of the criteria for measuring the magnetic effects on light beams is to 
split a beam of light into two separate beams which are detected and the 
magnetic effect measured on each. The beam of light is required to be 
physically split rather than split by a conventional beam splitter. The 
beam of light is impinged on an edge of an optical wedge on which the 
angled side is optically polished for reflection of the light beam. The 
optical wedge is placed in the path of the light beam so that the edge 
splits the light beam causing one half to pass uninterrupted along one 
side of the optical wedge to an optical detector. The other half of the 
light beam strikes the angled optically polished side of the optical wedge 
and is reflected to another optical detector. The critical factor to be 
considered is ensuring that the maximum amount of each split portion of 
the light beam is detected by their respective detectors. Consequently, 
the edge of the optical wedge cutting the light beam must be as sharp as 
possible. 
Conventionally, edges have been formed on optical apparatuses, such as 
glass, silicon crystals, quartz, and others by cutting strips of optical 
material from an ingot. A groove is cut at an angle into a chuck or 
holding device. The optical material is placed in the groove and held 
thereto by cement, wax, or by any other conventional method. The chuck is 
connected to a motor for rotating and oscillating the optical material 
simultaneously. A container holds a suitable grinding material such as a 
metal lap with grooves and is filled with a grinding bath. The chuck is 
lowered into the bath and rotated and oscillated while in contact with the 
grinding material. In some instances the container is also rotated in the 
same direction as the chuck but at a different speed. After the optical 
material is ground flat, the chuck polishing material is substituted for 
the grinding material and the process is repeated until the desired edge 
sharpness is obtained. 
Conventionally, the polishing material used to form a sharp edge is pitch 
and the polishing bath is a mixture of water and cerium oxide or the like. 
One disadvantage with this prior art method is that a sharpness of only 
about 2 micron can be obtained. A 2 micron sharpness is entirely 
inadequate for physically splitting a beam of light. Firstly, at a 
sharpness of 2 micron a large part of the light beam strikes the edge and 
is reflected in all directions which prevents the detectors from receiving 
this portion of the light beam. Secondly, most light beams which are to be 
split and detected are themselves only a few micron thick and such an edge 
sharpness would randomly reflect most of the light beam away from the 
detectors. Another disadvantage is that when an edge sharpness of less 
than 2 micron is attempted to be obtained on a relatively small wedge 
angle the edge tends to break, become uneven or the optical material peels 
from the edge. This renders the edge useless as a physical optical beam 
splitter. Another disadvantage of this method is that when using 
conventional polishing material such as pitch, the polishing time is very 
consuming. To obtain an edge sharpness of 2 micron it is not unusual to 
polish a surface for about 48 hours. 
Briefly, these and other disadvantages are overcome by providing a method 
of forming a sharp edge on an optical device having the steps of placing 
the optical device in a holding mechanism; griding one surface of the 
optical device so that the one surface and a surface of the holding 
apparatus are co-planer; and polishing the one surface of the optical 
device and the surface of the holding apparatus with felt until an edge on 
the device adjacent to the surface of the holding apparatus has a desired 
sharpness. 
Accordingly, one object of the invention is to provide a new and improved 
method of forming an edge on an optical device. 
Another object of this invention is to provide a method of forming an edge 
having a sharpness less than 2 micron on an optical device. 
Still another object of the present invention is to provide a method of 
forming an edge on an optical device that has a sharpness of about 0.3 
micron. 
A further object of the present invention is to provide a method of forming 
an edge having a sharpness of less than 2 micron on an optical device 
without breaking the edge, peeling the optical material away from the 
edge, or forming an uneven edge. 
A still further object of the present invention is to provide a method of 
forming an edge on an optical device that has a sharpness of less than 2 
micron in a relatively short time period. 
The above and further objects of the invention will appear more fully from 
the following detailed description when read in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the method of forming a sharp edge on an optical 
device is shown generally in blocks 1-3. Block 1 illustrates the first 
step of placing the optical device in a holding mechanism. Block 2 
illustrates the next step of grinding one surface of the optical device so 
that the ground surface and the surface of the holding mechanism are 
co-planer. Block 3 illustrates the third step of polishing the one surface 
of the optical device and the surface of the holding mechanism with felt 
until an edge of the surface of the optical device adjacent to the surface 
of the holding mechanism has a desired sharpness. 
Blocks 4-6 represent, more specifically the steps performed in Block 1 and 
Blocks 7-10 represent in greater detail the steps performed in Block 3. 
The step of placing an optical device in a holding mechanism in Block 1, 
includes the step of optically polishing a surface of the optical device 
as illustrated in Block 4. Block 5 illustrates the next step of cutting an 
angled groove into a surface of a holding mechanism. Block 6 illustrates 
the next step of securing the optical device in the groove with the 
optically polished surface contacting the groove. The step of polishing 
the one surface of the optical device with felt to obtain a sharp edge, as 
illustrated in Block 3, includes the step of rotating a felt pad as 
illustrated in Block 7. Block 8 illustrates the next step of applying a 
polishing compound to the rotating felt pad. Block 9 illustrates the next 
step of holding the surface of the holding mechanism and the one surface 
of the optical device in contact with the rotating felt pad. Block 10 
illustrates the last step of polishing the one surface of the optical 
device until the edge formed at the intersection between the optically 
polished surface and the one surface has a sharpness of less than 2 
micron. 
FIGS. 2, 3A, and 3B show that the step of placing an optical device in a 
holding mechanism is performed by obtaining a large blank 10 of optical 
material such as glass, quartz, or the like. The type of material used for 
blank 10 will depend upon the end purpose of the optical device. If the 
device is to be used as a beam splitter glass is the preferred material. 
When making a beam splitter the side thereof that will reflect one half of 
the energy beam is preferrably optically polished so that the entire 
portion reflected will be reflected to an appropriate detector (not 
shown). Blank 10 has a pair of opposed surfaces 12 and 14 of which surface 
12 is to be optically polished using any conventional polishing procedure 
such as pitch polishing. When pitch polishing surface 12, surface 14 is 
attached in any conventional manner to a holding plate 16. A pivot arm 17 
is in contact with holding plate 16 and allows rotational movement of 
plate 16 and blank 10 about a pivot ball 19 of pivot arm 17. Pivot arm 17 
also oscillates plate 16 and blank 10 back and forth as indicated by 
arrows 21. In axial alignment with blank 10 is a support 22 having a 
polishing material 24 thereon which may be any conventional material 
although pitch is preferred. A polishing compound 26 is applied on top of 
the pitch. Preferably, the polishing compound is cerium oxide, mixed with 
water to form a slurry although other compounds such as ferric oxide may 
also be used. Support 22 is connected to shaft 18 or motor 20. When 
energized, motor 20 will rotate support 22, pitch 24, and compound 26 in 
the direction of arrow 27. Motor 20 is energized and blank 10 is lowered 
by pivot arm 17 until surface 12 contacts pitch 24. Blank 10 is held in 
contact with pitch 24 for a time sufficient to optically polish surface 12 
to a flatness of substantially .lambda./10, where .lambda. is the 
wavelength of the energy beam to be reflected by surface 12. 
Upon completeion of optically polishing surface 12, blank 10 is removed 
from holding plate 16. Blank 10 is cut as by a diamond cutter (not shown) 
into strips of optical devices 28 upon which the sharp edge is to be 
formed. 
FIGS. 4, 5, and 6 illustrate the preferred method of securing an optical 
device 28 in a holding mechanism 30 so that a sharp edge can be formed on 
optical device 28. Holding mechanism 30 is preferably cylindrical in shape 
having a diameter substantially the same as the length of optical device 
28. Holding mechanism 30 may be made from any type material although 
preferably from the same type material as optical device 28. 
To hold optical device 28 a groove 32 is cut into surface 34 of the holding 
mechanism 30 such as by using a diamond saw. To properly form a sharp edge 
on optical device 28 it is preferred that optical device 28 be wedged 
shaped so that the edge formed by the wedge angle can be as sharp as 
possible. The larger the wedge angle the easier it is to form a sharp 
edge. Conversely, if a wedge angle is made small it becomes very difficult 
to form a sharp edge without breakage because the wedge shaped optical 
device would be very thin. Accordingly, groove 32 is cut so that a surface 
36 of groove 32 is formed at an angle .alpha. to surface 34. Surface 38 of 
groove 32 is then cut into surface 34 to complete groove 32. The length of 
surface 38 is preferably the same or less than the length of surface 40 of 
optical device 28 so that a portion 42 will extend above surface 34. 
Preferably, angle .alpha. is an acute angle and more preferably is less 
than 45 degrees. It has been found that an angle of substantially 26 
degrees can be produced with the desired edge sharpness without any edge 
breakage. 
After groove 32 is cut, surfaces 36 and 38 are waxed with conventional 
optical wax and optical device 28 is placed in groove 32 and held secure 
by the wax. Surface 40 abuts surface 38 and optically polished surface 12 
abuts surface 36 causing portion 42 to extend above surface 34. With 
surface 12 abutting surface 36 a sharp edge can be formed without damaging 
the optically polished surface. 
An alternative holding mechanism 44 is shown in FIG. 7. This mechanism 
includes a surface 46 which has been optically polished in the same manner 
as surface 12 of optical device 28. Surface 12 of optical device 28 is 
placed in contact with surface 46 of holding mechanism 44 and because both 
surfaces are polished flat no air gaps remain between the surfaces. This 
causes the surfaces 12 and 46 to be held tightly together. The holding 
mechanism 44 and optical device 28 are cut at an angle .beta. to form 
surface 14 on optical device 28 and a surface 48 on holding mechanism 44. 
Another alternative holding mechanism 50 is illustrated in FIG. 8. This has 
a surface 52 that does not need to be optically polished to any desired 
flatness. Optical device 28 is secured to holding mechanism 50 with screws 
54 so that surface 12 abuts surface 52. Holding mechanism 50 and optical 
device 28 are cut at an angle .gamma. to form surface 14 on optical device 
28 and a surface 56 on holding mechanism 50. 
Referring again to FIGS. 4, 5, and 6, holding mechanism 30 and portion 42 
of optical device 28 are ground in the conventional manner until surfaces 
14 and 34 are substantially co-planer. Preferably, portion 42 is ground 
using a device similar to that shown in FIG. 2 for optically polishing 
surface 12. For grinding, however, pitch 24 and polishing compound 26 is 
removed and replaced with the desired grinding surface (not shown) and 
grinding compound (not shown). Holder 30 with optical device 28 is secured 
to holding plate 16 in a conventional manner. Motor 20 is activated 
causing support 22 and the grinding surface and compound to rotate. Holder 
30 is lowered until portion 42 contacts the grinding surface and then 
activated to oscillate holder 30 and optical device 28. Portion 42 is 
ground down until surface 14 is co-planer with surface 34, as shown in 
FIG. 6. Following the process of grinding, holder 30 with optical device 
28 is removed from holding plate 16. 
By the process of grinding portion 42 until surface 14 is substantially 
co-planer with surface 34, an edge 58 is formed between surfaces 12 and 14 
and the angle formed at edge 58 between surfaces 12 and 14 is angle 
.alpha.. Edge 58 is the edge to be polished to the desired sharpness. When 
using holders 44 and 50 as shown in FIGS. 7 and 8 respectively, surfaces 
48 and 14 and surfaces 56 and 14 are ground in the manner previously 
described to form edge 58 between surfaces 12 and 14 of optical device 28. 
Referring to FIGS. 9 and 10, following the step of grinding, surface 14 of 
optical device 28 and surface 34 of holding mechanism 30 are polished with 
felt 60 for a time sufficient to sharpen edge 58 to the desired sharpness. 
An apparatus 62 is provided for performing the polishing. Apparatus 62 
includes a support plate 64 which is connected to shaft 66 of motor 68. 
Felt pad 60 is secured to support plate 64 as by cementing or other 
adhesive. A polishing compound 70 consisting of a slurry formed of water 
and a polishing material of cerium oxide or ferric oxide is placed in 
contact with felt pad 60. 
Holding mechanism 30 is secured in any conventional manner to a holding 
plate 72 so that surface 34 of holding mechanism 30 and surface 14 of 
optical device 28 are adjacent to polishing compound 70 and felt pad 60. 
Holding plate 72 is connected; as by rods 74, to a handle 76. Handle 76 is 
pivotably connected at end 78 to a support 80. As illustrated in FIG. 9, 
handle 76, holding plate 72, holding mechanism 30 and optical device 28 
are off-set from the center of felt pad 60. The position of the off-set is 
determined by the direction of rotation of felt pad 60 and the position of 
edge 58. 
To prevent edge 58 from breaking during polishing to a very sharp edge, 
holding mechanism 30 and optical device 28 are held stationary during 
polishing, that is, optical device 28 is not oscillated or allowed to 
rotate. The only item being rotated during the polishing step is felt pad 
60 and polishing compound 70. In addition, to prevent any peeling effect 
on optical device 28 edge 58 is the trailing edge to the rotation. 
Consequently, when edge 58 is positioned as illustrated in FIG. 10, 
holding mechanism 30 and optical device 28 are off-set as illustrated in 
FIG. 9 and felt pad 60 is rotated in the direction indicated by the arrow. 
Although one position of optical device 28 has been described, it should be 
understood that optical device 28 may be off-set at any location from the 
center of felt pad 60. The pad may be rotated in either direction, the 
only criteria being that edge 58 remain a trailing edge to the direction 
of rotation. 
To produce a sharp edge on edge 58, motor 68 is activated to rotate felt 
pad 60 and polishing compound 70 in the direction indicated. Handle 76 
with holding mechanism 30 and optical device 28 attached thereto is 
off-set from the center of felt pad 60 so that edge 28 is the trailing 
edge to the rotation. Optical device 28 is pivoted downward until surface 
34 of holding device 30 and surface 14 of optical device 28 are in contact 
with rotating felt pad 60 and pressure is applied to handle 72. The 
rotating felt pad 60 and polishing compound 70 polish surfaces 34 and 14. 
The required time for polishing is variable and dependent on the amount of 
sharpness desired on edge 58. In the conventional polishing techniques, 
edge 58 can only be sharpened to a sharpness of about 2 micron which may 
take upwards of 48 hours to produce. With the disclosed felt polishing 
technique edge 58 can be sharpened to less than 2 micron down to about 0.3 
micron which is necessary for proper beam splitting. It has been found 
that the time for polishing edge 58 to 0.3 micron using the felt polishing 
technique is only a few hours depending on the pressure applied to handle 
76, the speed of rotation of felt pad 60, and the quality of the ground 
surface. 
When using the holding mechanism 44 and 50 of FIGS. 7 and 8, respectively, 
the polishing step is performed in the same manner as previously described 
for holding mechanism 30 and therefore will not be further described. 
Referring to FIG. 11, if optical device 28 is to be used as a beam 
splitter, upon completion of the polishing step a support device 84 is 
attached to surface 34 and 14 of holding mechanism 30 and optical device 
28, respectively, in any conventional manner such as by using wax. Holding 
mechanism 30 and support device 84 are cut down such as by a diamond 
cutter substantially parallel to the longitudinal axis of optical device 
28 to form surfaces 86 and 88 respectively. Surfaces 86 and 88 are ground 
in the conventional manner as previously set forth and surface 40 of 
optical device 28 is also ground away until a point at substantially 
hatched line 90 is reached. Thus, surface 12 which is the optically 
polished surface becomes the hypotenuse of a triangle so that one half of 
the optical beam will be reflected off surface 12. Upon completion of the 
grinding, holding mechanism 30, support 84, and device 28 are cut into 
desired lengths, and then device 28 is removed from holding mechanism 30 
for use as a beam splitter. When using holding mechanisms 44 and 50 of 
FIGS. 7 and 8, optical device 28 is cut along hatched lines 82 to produce 
a wedge-shaped optical device so that optically polished surface 12 is the 
angled side to properly reflect half the energy Beam. 
Although the product of the method has been described as an energy beam 
splitter, it should be understood that the method may be used to produce a 
variety of products requiring extremely sharp edges on the order of 2 
micron or less. For example, the method may be used to produce sharp edges 
on the entrance and exit slits of a spectrometer to enhance the 
diffraction pattern of the subject energy beam. In addition, the method 
may be used to produce very sharp throw away surgical instruments. 
A method of forming a sharp edge on an optical device has been disclosed. 
Obvious modifications and variations of the method are possible in light 
of the above teachings. For example, the pivotal handle for applying 
pressure to the optical device may be replaced with an automatic spring 
device or hydralically operated plunger system. It is to be understood, 
therefore, that within the scope of the appended claims the method may be 
practiced otherwise than as specifically described and illustrated.