Optic element testing method and apparatus

A system is described for testing aspheric optic elements by the interference of light beam components that are respectively directed to the element to be tested and to a reference element, which facilitates the testing. A reference element is deformable in a controlled manner to more closely match the element to be tested, to produce straighter and more even fringes.

BACKGROUND OF THE INVENTION 
One method which has been used to measure the contour of aspheric optic 
devices involves the production of fringes by the interference of two 
components of a coherent light beam, one directed at the optic device to 
be tested and the other at a reference optic device. One system of this 
type includes a Twyman-Green interferometer which includes a laser light 
source, and a reference optic reflector device which matches the reflector 
optic device to be tested. A beamsplitter directs components of the laser 
light onto the two devices and then combines the returned components to 
form a fringe pattern. If the contours of the two reflective devices are 
close, then a pattern of fringes is produced wherein the fringes are 
widely spaced and substantially straight, or circular (so-called null 
fringes) and if there is a moderately small mismatch, then the fringes 
will be closer together and moderately distorted. If the workpiece device, 
or device to be tested, has a significant departure from a spherical 
surface, while the reference device is spherical, then the fringes may be 
too close and too distorted to be interpreted with any accuracy. If the 
maximum density of fringe lines is reduced, or the density of a more 
uniform radius of curvature (i.e., the lines do not vary from almost 
straight to closely curved), the fringe pattern will be easier to analyze. 
An approach often used to overcome the limitation is to insert an aspheric 
null lens (or a series of null lenses) over the test surface to reduce 
fringe density. This is costly and introduces the errors of the null 
lenses into the measurement. 
SUMMARY OF THE INVENTION 
In accordance with one embodiment of the present invention, a method and 
apparatus are provided for measuring an optical property of an optical 
device by interferomically comparing it to a reference element, which 
facilitates such comparison. A coherent light beam is split into two 
components, that travel along two different paths before they are 
recombined to produce a fringe pattern. The system includes a deformable 
optical element (which may be the reference element or another element) 
located along one of the paths, and means for deforming it so the fringe 
pattern can be more accurately analyzed. 
The novel features of the invention are set forth with particularity in the 
appended claims. The invention will be best understood from the following 
description when read in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a system 10 for measuring the configuration of a 
reflective surface 12 of an optical device to be tested, or optic 
workpiece 14. This is accomplished by comparing the surface 12 to the 
reflective surface 16 of a reference optical device 18. A coherent light 
beam 20 from a laser 22 is directed at a beamsplitter 24 which divides the 
beam into two components 26, 28 which initially travel along different 
paths. One component 26 is directed at the workpiece 14, is reflected from 
the workpiece surface 12 back towards the beamsplitter, and is deflected 
by the beamsplitter against a fringe receiving surface 30 on a device 32. 
The other component 28 is reflected off the surface 16 of the reference 
device, and returns to the beamsplitter through which it passes to the 
fringe surface 30. It should be noted that a quarter wave plate and a 
polarizer/analyser pair, are commonly used in such interference system, 
but are well known in the art and are not shown here in order to simplify 
the diagrams to aid in their understanding. 
In one case, it is assumed that the two optical devices 14, 18 have 
identical reflecting surfaces, or geometrically similar surfaces if one of 
the beams is spread or contracted by a telescope 31. In that case, the 
fringes on the fringe surface 30 will be all black or all white, or if 
tilt is introduced for convenience in testing, the fringes will be 
straight, parallel and separated. However, if the two surfaces are of 
different contours, then the fringes will have uneven spacing and will 
generally be curved, with different fringes being curved differently when 
one of the reflective surfaces is considerably aspheric while the other is 
spherical. A slight to moderate deviation of the fringe pattern from a 
series of uniformally spaced parallel fringe lines can be interpreted by 
an expert to indicate the difference between the reflective surfaces 12, 
16 of the two devices. However, if the deviation is very great, then it is 
very difficult for even an expert to determine the configuration of the 
workpiece reflective surface. In some prior art systems, the device 30 
which contains the fringe receiving surface 32, produces electrical 
signals indicating the frame pattern, and the signals are processed by a 
programmed computer to provide a measurement of the curvature of the 
workpiece device surface relative to the reference device. However, 
available fringe sensors and computers coupled thereto cannot accurately 
determine the surface configuration of the workpiece device when the 
fringes are very close and highly curved. 
In accordance with the invention, applicant can reduce the deviation of the 
fring pattern from a straight line pattern, by deforming the reflective 
surface 16 of the reference optical device to more closely match the 
reflective surface 12 of the workpiece device. The reference device 18 
includes an optical element 34 coupled through several to perhaps one 
thousand piezoelectric actuators 36 to a rigid support or mount 38. It 
should be noted that a variety of actuators are available, including 
magnetostrictive and mechanical lead screws. Each actuator has a 
configuration such as shown in FIG. 4, wherein the actuator 36a is shown 
as having a pair of electrodes 40, 42 on its opposite faces. The actuator 
face with the electrode 40 thereon is bonded to a rear face 44 of the 
reference device 18. The opposite face of the actuator at the electrode 42 
is bonded to the support 38. 
A multiple level voltage source 46 supplies a number of different voltage 
levels, such as sixteen, to a voltage steering switch 47. The switch 47 
has numerous output lines 48 that are each connected to a front electrode 
40 of a different one of the numerous actuators. The rear face of each 
actuator is connected to a grounded line 49. The change in thickness of 
each piezoelectric actuator 36 depends on the voltage applied between its 
opposite faces. By selecting the voltage applied on each output line 48, 
applicant controls the distance D of each small area of the optic device 
18 from the support 38. This permits a change in the average spherical 
curvature or focal length of the reference element, as well as the 
introduction of a controlled deviation from a spherical surface. All of 
this is done to more closely match the contour of the reference device 
surface 16 to the contour of the workpiece device surface 12, so as to 
obtain a fringe pattern which can be more precisely interpreted. 
Telescope systems have been used in the prior art, wherein the portion 
carrying the reflective surface has been deformable. Such deformation has 
been used to compensate for aberrations in the image arising from thermal 
gradients and other causes. Applicant's use of deformable reflective 
surfaces in systems for measuring the configuration of the surface of an 
optical device, and especially an aspherical optical device, aids in the 
manufacture of a wide range of optical devices. 
It is practical, in many instances, to operate the system in FIG. 1 by 
having an expert person manually control each of perhaps one thousand 
switch devices of the voltage steering switch 47 while he views the fringe 
pattern on the surface 30. Such varying can continue until either the 
fringe pattern includes parallel fringe lines, or consists of a pattern 
which can be accurately interpreted to determine the deviation of the 
workpiece device from the reference device. FIG. 2 illustrates a type of 
fringe pattern 57 which is difficult to interpret. By deflecting the 
reference devices at a location corresponding to the location 58 where 
there is a high fringe density, the density and curvature of the fringes 
is reduced. This process may continue until a pattern of the type shown at 
59 in FIG. 3 is obtained which can be accurately interpreted. The 
deviation of contour of the workpiece device from that of the deformed 
reference device can be determined from the fringe pattern. The deviation 
of the reference optical device from its original contour is determined by 
the voltage applied to each actuator. These two deviations can be 
geometrically added to the known original contour of the reference optic 
device, to determine the contour of the workpiece optic device. In 
general, the deflections of piezoelectric actuators at known voltages are 
very repeatable. The contours of the deformed reference surface can then 
be defined to an arbitrary level of precision by structural analysis. 
While an expert person can manually manipulate the switch devices of the 
steering switch, interpret the fringe pattern (if the fringes are curved 
and/or unevely spaced), and geometrically add the deviations, this 
requires considerable time. 
FIG. 5 shows a system 60 wherein the fringe receiving surface 62 is 
photosensitive to generate electrical signals on an output 64 representing 
the pattern of fringes on the surface 62. The output 64 is delivered to a 
control processor 63 which controls the voltage steering switch 47 to 
apply a voltage to each of the actuator elements 36 of the reference optic 
device 18. The voltages (above zero) are initially applied to each region 
of the reference device 18 which corresponds to a region on the fringe 
receiving surface 62 where there is a high density of fringes (e.g., 
corresponding to location 58 in FIG. 2). The process is iterative to 
reduce the fringe density and to straighten the fringes. The controller is 
programmed to generate signals on its output 67 representing the deviation 
in curvature of the two optic devices 18, 14A, from the fringe pattern and 
from the deviation of the referece device 18 from its original 
configuration due to the voltages on each of the numerous actuators. The 
signals on line 67 representing the deviations are received by a data 
reduction circuit 68, which geometrically adds the deviations to the 
original curvature of the reference device to indicate the curvature of 
the workpiece optic device. A fringe reading system of the type shown in 
FIG. 5, except it has no controller for varying the contour of the 
reference device, is available from Zygo Corporation of Middlefield, 
Connecticut. It is named the Mark III Phase Measuring Interferometer and 
is used with a computer program from Zygo Corporation named ZAPP (for Zygo 
Automatic Pattern Processor). 
The operation of the control processor 63 is represented by the flow 
diagram of FIG. 7. In keeping with normal practice, for testing a 
nominally spherical surface workpiece device, the reference surface is 
considered initially spherical, and tilt is introduced to produce a series 
of fringe lines, although the reference could be aspheric and/or null 
fringes could be employed. A first step in the operation of the control 
processor is indicated at 100, where it analyzes the fringe pattern to 
determine whether it is "O.K." or "not O.K." If the spacing of the fringes 
is at least a certain minimum, and none of the fringes have more than a 
certain radius of curvature therealong, then the fringe pattern will be 
"O.K." If the fringe pattern is not "O.K.," then the next step is as 
indicated at 102, which is to vary the actuator voltages at the locations 
corresponding to location on the fringe sensor of greatest fringe density 
and smallest radius of curvature. This continues until the fringe pattern 
is acceptable. 
When the fringe pattern is acceptable, the next step, indicated at 104, is 
for the control processor to examine the fringe pattern and from that 
determine the workpiece contour with respect to the reference device. A 
next step indicated at 106 is for the control processor to examine the 
pattern of voltages applied to the actuators that deform the reference 
device to determine the contour of the deformed reference device with 
respect to the undeformed reference device. A next step indicated at 108 
is to determine the undeformed reference contour, with signals 
representing the undeformed contour having been previously fed into the 
central processor. A final step indicated at 110 is to determine the 
workpiece contour from the combination of the three measurements which are 
the deviation of the devices as determined from the fringe pattern, the 
deformation of the reference device as determined from the voltages 
applied, and the original undeformed reference contour. Signals 
representing the workpiece contour can be delivered to the data reduction 
circuit 67, where the contour of the workpiece device is displayed, such 
as on a screen with numerous lines thereon that follow locations of equal 
elevation above a spherical surface, in addition to a number representing 
the average focal length or average radius of curvature of the surface of 
the workpiece device. 
Although the measuring system is useful for measuring the contour of 
reflective surfaces, it can also be used to measure the refractive 
characteristics of lenses such as lens 80 in FIG. 6. Of course, each 
surface of an aspheric lens can be measured by determining the contour 
from reflections from that surface, but it can also be determined by 
passing light through the lens. In FIG. 6, the system 82 is similar to 
that of FIG. 1, except that a flat mirror 86 is positioned behind a lens 
workpiece device 80 so that light passes twice through the lens 80. A 
reference reflector 88 should have about half the focal length of the lens 
80, in order that the fringe pattern on a surface 90 will have fringe 
lines that are not too curved or too close. Either the reference reflector 
88 or the flat reflector 86 (or both) is deformable. 
FIG. 8 illustrates another system 114 wherein the reference optical device 
116 includes two reflectors 118, 120, with one of them 120 being 
deformable and the other 118 being nondeformable. In this system, the 
light 122 from a laser source 124 is split, with one component 126 passing 
along a test arm or path 128 which includes the workpiece optical device 
130 that is to be tested, while the other component 132 passes along the 
reference path or arm 133 along which the optical element surfaces are 
known. The electrically controlled actuators 134 which serve as 
controllable means for deforming the first reflector 120, deform a 
reflector optical element 135 which is substantially flat. It is generally 
easier to construct a flat precision optical surface than a curved one, 
which can reduce the cost of the system. 
A common type of deviation of an optical surface from a desired 
configuration, is the presence of circular or ring-shaped zones that 
deviate from the desired configuration. Such zones are often introduced 
into a mirror surface during polishing. FIG. 9 illustrates three common 
types of deviation of an optical surface from a desired optical surface 
indicated in solid lines at 140. One common deviation is a hump 142 in the 
center of the optical surface. Another type is a low band 144 near the 
half-radius of the surface. Another common type is a roll-off 146 at the 
edge of the surface. FIG. 10 illustrates one arrangement of actuators 150 
for correcting deformations that occur in a circle of a ring-shaped band 
in an optical element 152. The actuators 150 are arranged substantially 
along an imaginary ring 154. Each actuator such as 156 applies a moment or 
torque indicated at 158, to tilt the optic device. FIG. 11 illustrates the 
actuator 156, which includes a post 158 having an inner end 160 anchored 
in the optical element 152. The actuator also includes a piezoelectric 
actuator element 162 mounted on a base 164 and positioned to engage the 
outer end 166 of the post. The actuator element 162 applies force to the 
outer end of the post which causes its inner end 160 to turn in the 
direction 158 to tilt up the radially outer portion 168 of the optical 
element 152. A plurality of such actuators is spaced about the periphery 
of the optical element, to tilt up a ring-shaped area at the edge of the 
optical element that had a roll-off near its edge. FIG. 11 illustrates 
another actuator 170 wherein the piezoelectric actuator element 172 is 
positioned to apply torque in a direction 174 opposite to the 
piezoelectric element 162. 
FIG. 12 illustrates another system, wherein the deformable optical element 
182 is located along a test arm or path 184 of the system, along which a 
light component 186 passes, which reflects off the workpiece element 188. 
A reference optical element 190 is a nondeformable mirror. The deformable 
device 192 which includes the deformable element 182, is similar to the 
deformable device 120 of FIG. 8, in that it includes a substantially flat 
element which can be manufactured at a relatively low cost (although a 
curved deformable element could be used here). 
The terms "light" herein includes electromagnetic radiation of any 
frequency, and the term "optical element" includes an element which acts 
in accordance with such light. The invention can be used for wavelengths 
far from the visible, as in testing a radar dish antenna. The present 
invention can be used to compare the reflective and refractive properties 
of acoustic devices, where the interaction of an acoustic test element 
with acoustic radiation is to be compared with the interaction of a test 
acoustic element with acoustic radiation. The same setup, using a 
deformable acoustic test element instead of an optical test element, can 
be used. 
Thus, the invention provides systems for measuring the surface of a 
workpiece optical device or element by comparing that surface to a 
reference surface, which facilitates such measurements especially where 
the workpiece surface is highly aspheric. A deformable element (which may 
be the reference optical device or an auxiliary device) is position along 
the reference path or the test path. The deformable element can be 
distorted in a controlled manner, while a fringe pattern is produced, to 
produce a fringe pattern that can be more easily analyzed than if the 
deformable element were not deformed. 
Although particular embodiments of the invention have been described and 
illustrated herein, it is recognized that modifications and variations may 
readily occur to those skilled in the art, and consequently, it is 
intended that the claims be interpreted to cover such modifications and 
equivalents.