Device and method for monitoring alignment utilizing phase conjugation

Monitoring alignment of an element(s) is accomplished with a source of radiation (11), an alignment reflecting assembly (12) embedded within or attached to an element (13) whose alignment is to be monitored and a detection assembly (14). Alignment reflecting assembly (12) includes a conventional mirror (16) that reflects a portion of the incident radiation (E) at a reflection angle that is substantially equal in magnitude and opposite in sign to that of the angle at which the incident radiation is incident thereupon. Alignment reflecting assembly (12) also includes a phase conjugate mirror (18) that reflects incident radiation (E) at a reflection angle that is substantially equal in magnitude and sign to that of the angle at which the incident radiation is incident thereupon, i.e., along the path of the incident radiation. The alignment orientation of alignment reflecting assembly (12) and element (13) is determined by monitoring the so-called retro-reflected radiation from the phase conjugate mirror (18) and the reflected radiation from the conventional mirror (16).

TECHNICAL FIELD 
The present invention relates generally to sensing alignment of an element. 
More particularly, the present invention relates to monitoring alignment 
of elements using both linear and non-linear reflectors. Still more 
specifically, the present invention relates to use of electromagnetic 
radiation, such as light, to monitor the alignment of the desired 
elements, such as optical components. 
BACKGROUND ART 
High accuracy and precision alignment is highly desirable or essential in 
innumerable fields of endeavor. Many techniques for attempting such 
alignment are known. For example, a number of techniques for various 
applications monitor the position of one or more reflections off the 
object(s) to be alignment to determine alignment. However, heretofore 
these reflections obeyed the principal that the angle of reflection 
(arbitrarily defined as being in a positive direction from the normal to 
the object's surface) was equal to the angle of incidence but in the 
negative direction to the normal. As a result, these techniques generally 
had to utilize complicated and costly schemes that yielded alignment 
information whose accuracy and precision was limited. 
I have found a device and method for alignment employing a relatively 
recently discovered class of materials known as phase conjugators. Phase 
conjugate materials provide reflection along the incident path. (A good 
discussion of phase conjugation and materials exhibiting such properties 
in the optical region of the electromagnetic spectrum is furnished in the 
article by Shkunov and Zel'dovich, Optical Phase Conjugation, Scientific 
American, pp. 54-59 Dec. 1985)) 
Phase conjugation has been shown to be useful for several applications, 
particularly in the optical region of the electromagnetic spectrum, as 
explained in the article by Peper, Applications of Optical Phase 
Conjugation, Scientific American, pp. 74-83, (Jan. 1986). Perhaps the most 
common application has been to correct distortions arising in a coherent 
light beam as the beam lases in an optical cavity. U.S. Pat. No. 4,529,273 
and the article by Lindsay and Dainty, Partial Cancellation of Specular 
Refraction in the Presence of a Phase-Conjugate Mirror, Optics 
Communications, Vol. 59, No. 5,6, pp. 405-410 (Oct. 1, 1986), both 
disclose so-called phase conjugate reflector or phase conjugate mirror 
("PCM") configurations for correction of these optical distortions. 
Optical phase conjugators may also be used to construct optical inertial 
navigation sensors such as the gyroscope described in U.S. Pat. No. 
4,681,446 and the accelerometer described in U.S. Pat. No. 4,640,618, and 
to construct an optical interferometer for determination of surface 
deformations as described in U.S. Pat. No. 4,280,764. None of these 
patents or the previously mentioned articles illustrate or suggest using 
PCMs in configurations and with methods that yield highly accurate and 
precise alignment information. 
SUMMARY OF INVENTION 
It is, therefore, an object of the invention to provide a device and method 
for monitoring alignment accurately and precisely using radiation 
reflected from phase conjugate material. 
It is another object of the invention to provide a device and method for 
monitoring alignment, as above, in which alignment is monitored for all 
radiation patterns. 
It is still another object of the invention to provide a device and method 
for monitoring alignment, as above, in which the sensitivity to alignment 
variations is readily selectable and is great for small misalignments. 
It is yet another object of the invention to provide a device and method 
for monitoring alignment, as above, in which alignment perturbations 
within the device itself does not effect monitored alignment accuracy or 
precision. 
It is a further object of the invention to provide a device and method for 
monitoring alignment, as above, in which variations in alignment may be 
dynamically monitored, permitting dynamic correction to misalignment 
conditions. 
It is still a further object of the invention to provive a device and 
method for monitoring alignment, as above, in which the characteristics of 
the alignment monitoring signal are readily tailored to the application. 
These and other objects and advantages of the present invention over 
existing prior art forms will become more apparent and fully understood 
from the following description in conjunction with the accompanying 
drawings. 
In general, device utilizing radiation for monitoring alignment of elements 
includes two elements for reflecting at least a portion of the radiation 
incident thereupon at a first incident angle and a second incident angle, 
respectively. The first element reflects incident radiation at a first 
reflection angle different from the first incident angle. The second 
element reflects incident radiation at a second reflection angle 
substantially equal to the second incident angle. A detector receives the 
radiation reflected from the first element and the radiation reflected 
from the second element and generates at least one signal indicative of 
the alignment of the first element and the second element. 
A method utilizing radiation for monitoring alignment of elements, includes 
the steps of reflecting at least a portion of the radiation incident upon 
a first element at a first reflection angle different from the first 
incident angle, reflecting at least a portion of the radiation incident 
upon a second element at a second reflection angle substantially equal to 
the second incident angle and, determining alignment of the first element 
and the second element from the radiation reflected from the first element 
and the radiation reflected from the second element.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT 
FIG. 1 depicts in schematic diagram form an exemplary device, generally 
indicated by the numeral 10, which embodies an exemplary method, in 
accordance with the concepts of the present invention, including a source 
of radiation 11, an alignment reflecting assembly 12 embedded within or 
attached to an element 13 whose alignment is to be monitored and a 
detection assembly 
Referring source 11 may be source of radiation suitable to achieve the 
reflections described more fully hereinafter. One implementation of device 
10 that has been found convenient and suitable for a variety of 
applications employs radiation in the optical region of the 
electromagnetic spectrum. In this instance radiation source 11 may be any 
conventional laser, although it is preferable in many applications to use 
a laser having a high energy output such as an argon-ion laser operating 
at a wavelength of 5145 Angstroms. For other embodiments of device 10 
other energy or radiation may be desirable, such as without limitation 
acoustical energy, infrared energy or microwave radiation. As would occur 
to the ordinarily skilled artisan, the source energy or radiation may be 
conventionally conditioned and directed to the alignment reflection 
assembly 12. Where radiation source 11 is an laser, the laser beam may, 
for example, be passed through polarizers and filters (not shown) and 
redirected by a fold mirror 15. 
Alignment reflecting assembly 12 includes a CM 16 and a PCM 18 in operative 
association, and is affixed to or embedded within element 13 by any 
suitable means for insuring a fixed alignment with alignment reflecting 
assembly 12. In order to more fully appreciate the nature of the operative 
association between CM 16 and PCM 18 and the operation of alignment 
reflecting assembly 12, reference is made to FIGS. 2 through 5. 
FIG 2 presents a CM 20 where radiation 21 incident upon the CM reflecting 
surface 22 at a positive angle i relative to the normal 23 of the CM 
reflecting surface 22 is reflected at a reflection angle -i substantially 
equal in magnitude and opposite in sign to that of the incident angle i. 
CM 20 may be any reflector that reflects incident radiation in this 
manner. 
In FIG. 3 a PCM 24 can be seen to reflect radiation 21 incident upon the 
PCM reflecting surface 25 at a positive angle relative to the normal 26 of 
the PCM reflecting surface 25 at a reflection angle i that is 
substantially equal in both magnitude and sign to that of the incident 
angle i. In other words, PCM 24 reflects radiation back substantially 
along the path of the radiation incident thereupon, a phenomenon known as 
retro-reflection. PCM 24 may be any reflector that reflects incident 
radiation by retro-reflection (such as a substrate of barium titanate). 
A combination of CM 20 and PCM 24 forming one possible exemplary embodiment 
of an alignment reflecting assembly 12 is illustrated in FIGS 4 and 5. A 
partially transmissive CM 20 and PCM 24 have been positioned proximate 
each other such that radiation 21 incident upon alignment reflecting 
assembly 12 along its normal 28 will be retro-reflected by both CM 20 and 
PCM 24. If, solely by way of example, CM 20 is approximately 70 percent 
transmissive of radiation and PCM 24 has a reflectance of approximately 30 
percent, then the reflected radiation energy from CM 20, identified as 
E(CM), will equal (1-0.7) times the incident radiation energy E and the 
reflected radiation energy from PCM 24 will equal the product of 
0.7.times.0.3.times. E. 
In FIG. 5 alignment reflecting assembly 12 has been rotated 
counterclockwise by a tilt angle of .phi. degrees relative to the path of 
the incident radiation. As explained above, PCM 24 will retro-reflect a 
portion of the incident radiation along the path of the incident radiation 
while CM 20 will reflect a portion of the incident radiation at an angle 
from the normal 28 equal in magnitude and opposite in sign to the angle at 
which the radiation is now incident (i.e., twice the tilt angle where the 
magnitude of the incident angle equals the magnitude of the tilt angle). 
Thus, by simply monitoring the difference between the retro-reflected 
radiation from PCM 24 and the reflected radiation from CM 20 and applying 
elementary trigonometry, the orientation of alignment reflecting assembly 
12 may be highly accurately and precisely monitored. This also provides 
highly accurate and precise monitoring of the orientation of any 
element(s) to which alignment reflecting assembly 12 is affixed. 
Returning to FIG. 1, detection assembly 14 can be seen to include a 
detector array 30 and a conventional beam splitter 31 adequate to direct 
the radiation reflected from alignment reflecting assembly 12. Detector 
array 30 may be any conventional detector array suitable for generating a 
signal at least one characteristic of which (such as voltage or current) 
is indicative of the characteristic of the reflected radiation selected to 
be monitored. Where device 10 is to operate in the optical region of the 
electromagnetic spectrum and radiation source 11 is a laser, a 
conventional CCD array may be chosen as detector array 30 and the 
intensity of the reflected light monitored. Detector array 30 and beam 
splitter 31 also must be of sufficient dimensions that detector array 30 
will receive the radiation reflected from alignment reflecting assembly 12 
over the entire range of possible misalignments of interest. 
In FIG. 1 alignment of the incident radiation with the normal 28 to the 
reflecting plane 32 of the alignment reflecting assembly 12 results in the 
condition depicted in FIG. 4, with both the radiation reflected from CM 16 
and PCM 18 retro-reflected along the incident radiation beam path. The two 
reflected energies E(CM) and E(PCM) combine to form a relatively large 
intensity image 38 shown pictorially above detector array 30. The point on 
detector array 30 receiving the highest image intensity may be arbitrarily 
selected to refer to as the alignment or calibration point 33. 
FIG. 6 depicts device 10 where element 13 has been perturbed and is titled 
at an angle .phi.. As demonstrated in FIG. 5, PCM 18 will continue to 
retro-reflect radiation along the direction of the incident radiation 
because of its operation as a phase conjugator. CM 16 will reflect 
radiation at a total angular deviation of twice the incident angle of 
radiation. Detector array 30 will detect this deviation by a shift in the 
point of maximum reflected energy from CM 16, called the CM centroid 34. 
The distance between the calibration point 33 and the CM centroid 34 
(.delta.) is directly proportional to the magnitude of perturbation of 
element 13 from its alignment orientation. 
The combination of the reflected radiation from both CM 16 and PCM 18 will 
result in interferometric patterns of fringes 35 upon detector array 30. 
The geometry of these patterns are a function of the optical geometry of 
device 10 and can be useful in ascertaining alignment conditions. In FIG. 
7 the fringe image of concentric circles and an elevation through the 
center of the fringe image are indicative of the alignment of the CM 
centroid 34 with calibration point 33, the condition of FIG. 1. In FIG. 9 
there is such a great misalignment that the calibration point 33 and the 
image resulting from the radiation reflected by PCM 18 are totally 
separated from the centroid 34 and the image resulting from the radiation 
reflected by CM 16. Where the misalignment is small one possible fringe 
pattern that may be produced (offset non-circular fringes) is shown in 
FIG. 8. 
In FIG. 10 there exists misalignment by both a counterclockwise rotation of 
the monitored element 13 by an angle of phi degrees (.phi..degree.) and a 
counterclockwise rotation of the detection assembly 14 by an angle psi 
degrees (.psi..degree.). In this instance: the centroid of the energy 
reflected from PCM 18 is now shifted to PCM centroid 36, a distance 
.epsilon. from the calibration point 33; the energy reflected from CM 16 
is still shifted to centroid point 34, a distance .eta. from the 
calibration point 33; and the total distance between centroids must still 
be .delta.. Since .delta. is still proportional to the misalignment of the 
monitored element 13, it should be apparent that even a misalignment in 
the detection assembly 14 does not effect the accuracy or precision by 
which alignment of element 13 is monitored. 
The operation of device 10 is straightforward. Initially element 13 is 
placed in its alignment position and alignment reflection assembly 12 and 
detection assembly 14 are adjusted to produced retro-reflection by both CM 
16 and PCM 18 (which can be most easily determined from the signal 
generated by detection assembly 14 by alignment of both CM centroid 34 and 
PCM centroid 36 at a single calibration point 33 as seen in FIG. 1). 
Thereafter the alignment orientation of element 13 may be monitored 
manually or electronically by monitoring either the separation between CM 
centroid 34 and PCM centroid 36 or fringes 35. 
Several significant aspects of the present invention should now be 
appreciated. First, device 10 will successfully monitor the alignment of 
element 13 for all radiation patterns, including those having plane 
wavefronts, spherical wavefronts or wavefronts of more complex geometries. 
Second, device 10 will function properly with any medium that provides 
phase conjugation of the selected incident radiation used as PCM 18. 
Third, the sensitivity of device 10 to alignment variations is readily 
selectable by careful choice of the initial alignment orientation, the 
geometric and reflectivity of CM 16 and PCM 18 and the use of centroid 
and/or fringe pattern schemes for extraction of alignment information. 
The next important aspect of the present invention emerges because 
alignment is monitored as a function of the difference between 
retro-reflected radiation and conventional reflected radiation: variations 
in incident radiation patterns occurring dynamically during the monitoring 
process will not affect the monitoring of the alignment. It is also to be 
emphasized that the geometry and orientation of CM 16 and PCM 18 may 
assume any configuration by which the radiation reflection characteristics 
described hereinabove are obtained. 
Inasmuch as the present invention is subject to variations, modifications 
and changes in detail, a number of which have been expressly stated 
herein, it is intended that all matter described throughout this entire 
specification or shown in the accompanying drawings be interpreted as 
illustrative and not in a limiting sense. It should thus be evident that a 
device constructed according to the concepts of the present invention, and 
reasonably equivalent thereto, will accomplish the objects of the present 
invention and otherwise substantially improve the art of monitoring the 
alignment of elements with high accuracy and precision.