Variable pitch grating for diffraction range finding system

Details Variable pitch grating for diffraction range finding system Variable pitch grating for diffraction range finding system

1999-06-17

2002-12-03

Font, Frank G. (Department: 2877)

Optics: measuring and testing

Range or remote distance finding

With photodetection

49

Reexamination Certificate

active

06490028

ABSTRACT:

TECHNICAL FIELD OF INVENTION
The present invention relates to a species of diffractive optical element called chirp gratings as used in diffraction range finders.
BACKGROUND
Range finding by diffraction is comprised of the methods, devices and systems used to measure distance through exploitation of a phenomenon observed with diffraction gratings wherein the displacement between diffraction images of the various diffraction orders can be correlated to the distance from the grating to an observed source of energy illuminating the grating.
As postulated by Huygens over 3 centuries ago and proven experimentally by Thomas Young in 1801, if energy in the form of a periodic wave strikes an aperture, a new wave front is originated at said aperture. Diffraction gratings are surfaces with many adjacent apertures, typically in the form of ruled straight lines. When a wave front strikes a diffraction grating, new wave fronts originate from all apertures in the grating, and these new wave fronts then interfere with each other both constructively and destructively. Along lines of constructive interference, images can be formed in a receiver which is observing the grating. The images are called the diffraction orders.
The zero-order diffraction image appears along a direct line from the receiver to a source of energy (hereinafter called a “target”). Higher-order diffraction images of a target are reconstructed at the receiver, and they appear displaced to the side of the zero-order. As a target is moved toward or away from a grating surface, the relative displacement of a higher-order image from both the zero-order image and other higher-orders images can be used to measure target range.
The behavior of the higher-order diffraction images as a function of target distance has been previously reported in terms of plane gratings, that is, gratings with a fixed spacing between rules. Diffraction range finders that use plane gratings exhibit a characteristic relationship in the displacement of higher-order diffraction images. There is a maximum asymptotic limit to the displacement of the higher-order images which occurs when a target is at a very great distance from the grating. As a target approaches the grating, the displacement between higher-order images collapses. At point-of-contact with the grating, the higher-order images merge with each other and with the central zero-order image. The displacement of the higher-order images as a function of target distance is a parabolic dependency over the excursion of a target from infinite distance to point-of-contact with the grating. Therefore, the accuracy of a range finder made with plane diffraction gratings varies inversely with the square of the distance.
The behavior of diffraction range finders based on plane gratings is shown by example in the schematic diagram, FIG.
1
(
a
), with correlated graph, FIG.
1
(
b
).
FIG.
1
(
a
) illustrates a diffraction range finder for use in the optical regime of light, that is, visible electromagnetic radiation. The device consists of a plane grating
110
, a receiver in the form of a camera
200
, and a structured illumination source in the form of a laser
300
. The bundle of rays traced from the grating to the camera
150
show instances of the field-of-view of the lens
210
received in the camera. On the other side of the grating, the rays
160
are redirected by the action of the grating. Where these rays cross the line
320
representing the structured illumination, there will be a corresponding ray in the bundle
150
. These points of intersection
330
, marked with rectangles, are examples of range points that can be acquired by the range finder. They are mapped in the associated graph, FIG.
1
(
b
), generated with equations (3) and (4). The graph trace
410
shows that along focal plane
220
there can be found a point x of the first-order image which correspond to target range D along the line of structured illumination
320
. The illustrated case is a diffraction range finder using 635 nm illumination with a 1 micron pitch plane grating of 60 mm length as acquired with a camera with a 50 mm focal length lens.
Hyperbolic and parabolic relationships between target distance and image displacements are characteristic of conventional triangulation range finders as well as diffraction range finders made with plane gratings. The phenomenon is easily observed. For example, the stereo separation of human eyes resolves distance more accurately in the region at arm's length than at distances near the limit of the eyes'ability to perceive two dimensional detail. Similarly, in triangulation range finders which use an active method of illumination such as a laser beam, resolution is inversely proportional to the square of the distance. This is a dependency similar to that found with diffraction range finders made with plane gratings.
An explanation for the inverse square relationship of accuracy to distance to be found in the triangulation and stereopsis range finding methods is that these range finders form the image of a target using a lens, and lenses generate images with perspective foreshortening. The mechanism by which a lens works dictates that as objects recede in distance from a lens, the images of objects of the same size will decrease in size on the focal plane of the lens. This is an observation which has informed perspective rendering since the Renaissance.
Perspective foreshortening can be seen in lenses of any focal length. FIGS.
2
(
a
) & (
b
) show a schematic representation a camera
200
of focal plane dimension
220
with two lenses. The perspective foreshortening for the shorter focal length
230
shown in FIG.
2
(
a
) is greater than that for the longer focal length
240
shown in FIG.
2
(
b
), but both lenses show appreciable angles-of-view &thgr;,
235
and
245
.
FIG.
2
(
c
) shows a characteristic graph trace
420
for the change in
255
, view angle &thgr;, as a function of
250
, focal length F, in this instance for a 6.5 mm focal plane, typical for a half inch focal plane array. The angle can be known by
θ
=
2
⁢
 
⁢
tan
⁡
(
X
2
⁢
F
)
(
1
)
where X is the length of the camera focal plane
The graph, FIG.
2
(
c
), shows that as the focal length F increases, the angle of the field-of-view &thgr; narrows, but the asymptotically limited angle &thgr; does not reach zero degrees. The far-field accuracy of lens-based range finders can be increased by increasing the focal length of the lens used in the receiver. This will reduce foreshortening but not eliminate it. Moreover, long focal length lenses, such as telescopes, carry a significant weight and cost penalty in design utility. Depth-of-focus is also sacrificed with increased focal length. Additionally, in the case of range finders that use structured illumination such as a stripe or sheet of light for profilometry, the use of a long focal length will diminish the length of the stripe visible at the receiver and hence reduce the acquired profile length.
Diffraction range finders also may incorporate a lens to form an image of the higher-order diffraction images, but perspective effects of the lens are modified by the diffraction grating. Nonetheless, for diffraction range finders with plane gratings of fixed pitch, as have been used in all reported embodiments of diffraction range finders, a perspective-like effect is observed due to the use of a receiver with a lens to form the diffraction image. Again, the accuracy of the range instrument is inversely proportional to the square of the target distance. Indeed, the relationship between higher-order image deflection displacement and target distance follows the same parabolic shape as is characteristic of ranging systems based on the use of lenses such as triangulation and stereoscopy.
Prior Art
A prior art search was conducted. Patents that presage the present invention, wherein a range finder can be made with diffraction gratings are
U.S. Pat. No. 4,678,324 awarded to Tom DeWitt (now known as Tom Ditto, the co-inventor of the present invention

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