Rapid, high-resolution scanning of flat and curved regions for gated optical imaging

A scanning system for scanning in first and second dimensions a desired surface topology of a sample, the scanning device comprising: a light source for producing a collimated light beam; a first scanning device responsive to the collimated light beam from the light source for producing a first scanned beam in a first dimension with a constant optical path length; and a second scanning device coupled between the first scanning device and the sample for focusing and scanning the first scanned beam in a second dimension onto the surface region of the sample to cause the collimated light beam to scan the surface topology of the sample with a constant optical path length in each of the first and second dimensions of the desired topology of the sample. In a second embodiment of the invention, a beam of light is focused by a first lens before a scanner and the scanner is rotated. Second and third lenses arranged in a 4-f combination are used to image rotated focal spots along a spherical convex surface of a sample while the optical path length stays constant. Slow scanning in other dimensions can be performed by mechanical means.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to optical scanning and particularly to the 
use of special scanning techniques wherein high resolution near-surface 
images can be acquired rapidly in either a flat or a curved surface 
geometry. 
2. Description of the Related Art 
It has been previously demonstrated that ultrafast optical gating 
techniques can be used for defect detection in advanced ceramic materials. 
One of the most promising techniques, due to its low cost and ease of 
implementation, is optical coherence tomography (OCT). This technique is 
based on low coherence fiber interferometry and can produce high 
resolution subsurface images. However, to make devices based on OCT 
practical, the image acquisition time should be fast (hopefully 
approaching video rates). This was recognized and an OCT technique was 
modified so that the image acquisition time was reduced to .about.300 
msec. This was accomplished at a price of reduced spatial resolution since 
the scattered light was not always collected at the focus of the lens. 
Another disadvantage of this technique is that the image is always 
collected in the X-Z plane, where Z represents the depth into the sample 
and X represents one transverse dimension. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide an improved 
scanning device. 
Another object of the invention is to apply special optical scanning 
methods to improve optical techniques used for detecting scattered laser 
radiation from near-surface structures, defects, or imperfections in 
ceramic and other translucent, but highly scattering materials. 
Another object of the invention is to provide an improved scanning device 
wherein the improvements are achieved by reducing image acquisition time 
while keeping spatial resolution high for various surface topologies. 
Another object of the invention is to provide a scanning device which 
produces a two-dimensional line scan of a sample and maintains a constant 
optical path length during the entire scan. 
A further object of the invention is to provide an improved scanning device 
which uses special scanning techniques so that high resolution 
near-surface images can be acquired rapidly in either a flat or a curved 
surface geometry. 
These and other objects of the invention are achieved by providing an 
optical scanning system for developing high-resolution, near-surface 
images of a sample by reducing image acquisition time while keeping 
spatial resolution high for various surface topologies. By using special 
scanning techniques, high resolution near-surface images can be acquired 
rapidly in either one or two dimensions in either a flat or a curved 
surface geometry with a constant optical path length.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The purpose of this invention is to improve on the previously reported OCT 
techniques by providing a special optical scanning arrangement, in 
combination with fast PZT modulation in the reference or the signal arm of 
the interferometer, such that high resolution images may be acquired 
rapidly in a flat or curved surface topology either parallel or 
perpendicular to the surface. The improvement results in a practical 
device capable of obtaining images in a flat or curved topology, parallel 
or perpendicular to the surface, rapidly and with high resolution. 
Additionally, three-dimensional surface profiling can also be accomplished 
on curved surfaces without gating, in a scanning confocal microscope 
configuration. 
In typical OCT the pathlength between the reference and signal beams in the 
reference and signal arms of the interferometer is rapidly varied to 
produce a Doppler shift between the beams. If the sample is moved to 
accomplish this, then the return signal always originates in the focus of 
the collecting lens. However, the sample in general can only be moved with 
a mechanical translation stage, which limits the image collection time to 
&gt;10 seconds. Furthermore, the images can only be acquired perpendicular to 
the surface. A technique used in the prior art reduced the image 
collection time to .about.300 msec. This was accomplished by stretching an 
optical fiber in the reference arm by approximately 3 mm with a PZT . 
However, this has the side effect of moving the gating depth through the 
focus of the image collecting lens. The waist size of a Gaussian beam is 
given by the following equation: 
##EQU1## 
where W.sub.z is the beam waist size as a function of depth Z, W.sub.o is 
the minimum beam waist size and .lambda. is the wavelength. Using this 
equation at a wavelength of 1 .mu.m the beam size has to be at least 30 
.mu.m to have no significant increase in size over a depth of 3 mm. This 
method therefore greatly limits the possible spatial resolution and 
restricts the scans to depth cross-sections. 
The purpose of this invention is to improve on these typical OCT techniques 
in a number of ways. In order to enable optical scanning of the signal 
beam, the modulation of the signal must be separated from the motion of 
either the sample or the light beams. This is accomplished by winding a 
length of fiber on a special high speed-low voltage PZT and changing the 
fiber length by only .about.3 wavelengths. Besides achieving very high 
modulation frequencies (&gt;300 kHz), this also separates the modulation from 
any translation of the sample and enables optical scanning in any 
direction. The focus of the scan can now be scanned with fast, 
commercially available galvanometer mirrors in such a way that the focal 
size does not change and the total optical path length (OPL) from the 
signal fiber output to the focus stays constant. If this condition is 
satisfied, then the gated image will contain the scanned area at the best 
possible resolution. Three scanning techniques of interest have been 
identified, which will now be discussed by referring to the drawings. 
1. A Plane At Any Angle to the Surface Using One Optical Scanner. 
FIG. 1 shows a schematic diagram of a scanning system or arrangement 
wherein an optical fast scan is performed in one dimension parallel to the 
surface, while a slower motion is performed in the other two dimensions by 
mechanical translation stages. 
The scanning system of FIG. 1 is a focused scanning system which allows a 
collimated light beam to be focused and scanned in one dimension. As will 
be explained, the scanning system or arrangement in FIG. 1 is used to 
focus a collimated light beam, such as a laser beam, to a spot on a sample 
by rotating the collimated part of the beam before it reaches the lens. 
That scanning can be accomplished in a straight line and the focus will 
move on a planar surface in that straight line. 
The scanning system of FIG. 1 includes a scanner 11 comprising a rotating 
scanner (not shown) and a scanner mirror (not shown) attached to and 
rotated by the rotating scanner, a flat field lens 13 and mechanical 
translation stage 15. The scanner 11 can be a galvanometer which includes 
the scanner mirror. The flat field lens 13 is a focusing lens. 
An input collimated signal light beam 17 from some collimated light source 
(not shown) incident on the center of the mirror of the scanner 11 is 
reflected by the mirror through the focusing lens 13. The lens 13 focuses 
the collimated light beam to a spot one focal length (1-f) away on a 
sample 19. The scanner 11 is rotated by any suitable means (such as a 
shaft in an exemplary galvanometer--not shown) in the directions shown by 
double arrows 12 to produce a linear scan across the sample 19. The 
mechanical translation stages 15 move the sample in the other two 
dimensions by means well known in the art. 
In operation, when the distance from the center of a scanning galvanometer 
mirror on the scanner 11 to the flat field lens 13 is equal to the focal 
length of the lens 13, the input light beam 17 will scan a flat line 
parallel to the surface of the sample 19 in the focal plane of the lens 13 
without changing the OPL. A flat field lens 13 is designed to keep the 
focus at a minimum in this arrangement. However, even though the beam 17 
is collimated, it has to be at the center of the scanning mirror of the 
scanner 11 for constant OPL. This is different than the requirement for 
confocal scanning microscopy, where only the focus position is important 
and not the OPL. 
2. A Plane Parallel to the Surface Using Two Optical Scanners. 
To enable even faster scanning in the plane parallel to the surface, as in 
confocal scanning microscopy, a different system or arrangement is 
required. The simple scanning system used in confocal scanning microscopy 
cannot be adapted because the OPL is not constant during the scan. 
Therefore, a second scanning system, shown in FIG. 2, is utilized together 
with the scanning system shown in FIG. 1 to enable fast gated scanning in 
a plane. 
FIG. 2 shows the optical arrangement, including a second scanner 25, 
similar in structure and operation to the scanner 11 in FIG. 1. The 
scanner 25 has to be used to keep a constant path length for a light beam, 
when utilized with the first or focusing scanning system of FIG. 1, to 
make a two-dimensional line scan and keep the optical path length constant 
during that entire two-dimensional line scan. More specifically. FIG. 2 
shows a transverse dimension and the scanning within that transverse 
dimension which, when combined with the system of FIG. 1, allows a 
complete two-dimensional scan across a flat surface. In other words, FIG. 
2 shows the optical arrangement that must be used to keep the optical path 
length constant for that second dimension of scanning. 
In FIG. 2, a 4-f lens system 27 and 29 is used together with a scanning 
galvanometer mirror in the scanner 25, in the dimension perpendicular to 
that used in FIG. 1, to produce fast scanning in two dimensions. In the 
arrangement shown in FIG. 2, the OPL stays constant throughout the scan, 
while the scan is performed always in the focus of the lens 13 in FIG. 1. 
In the operation of the system of FIG. 2, a collimated light beam 31, which 
is incident on and reflected from the scanning mirror in the scanner 25, 
is scanned across the face of a flat field lens 27. A second flat field 
lens 29 is also located in the 4-f lens arrangement. Both the first and 
second flat field lenses 27 and 29 are similar in operation to the flat 
field lens 13 in FIG. 1. 
In the 4-f lens arrangement of FIG. 2, the lens 27 is located one focal 
length (1-f) away from the scanner 25, the distance between lenses 27 and 
29 is two focal lengths (2-f), and the distance between the lens 29 and 
the point of combination with the scanner 11 in FIG. 1 (to be explained in 
FIG. 3) is one focal length (1-f) away. Thus, in FIG. 2 there are four 
focal lengths (4-f) distance between the scanner 25 in FIG. 2 and the 
scanner 11 in FIG. 1, which would be combined with the light output of 
FIG. 2. (To be explained in FIG. 3.) 
The flat field lens 13 in FIG. 1 and the two flat field lenses 27 and 29 in 
FIG. 2 are all focusing lenses, arbitrary in size, and are designed to 
have a minimal aberration when they are used to focus collimated light 
down to a focal spot at one focal length (1-f) away from the lenses. For 
example, the lens 27 focuses the light beam 31 down to a focal spot 33 
which is located one focal length (1-f) from each of the lenses 27 and 29. 
FIG. 3 is a three-dimensional representation of the combination of FIGS. 1 
and 2, combined in such a way that two-dimensional scanning occurs. The 
scanning occurs along a flat plane surface, a planar surface, and the 
focal spot will trace along that planar surface and the optical path 
length will stay constant over that entire scan over that flat surface. 
As shown in FIG. 3, the structural elements of FIG. 2 are placed just ahead 
of the structural elements of FIG. 1 to produce a combined system which 
produces a two-dimensional line scan of the sample 19 while keeping an 
optical path length constant during the entire scan over the sample 19. 
As explained before, the scanner 11, flat field lens 13, mechanical 
translation stage 15 (FIG. 1) and sample 19 are the components from FIG. 1 
and operate as explained in relation to FIG. 1 to focus a light beam and 
allow that focused light beam to scan across a sample in one dimension; 
while the scanner 25, and flat field lenses 27 and 29 are the components 
from FIG. 2 and operate as explained in relation to FIG. 2 to produce a 
scan in a second dimension in such a way to keep the optical path lengths 
constant along the focus on the sample itself over the entire scan. 
In the operation of the system of FIG. 3, the collimated light beam 31 is 
incident on the mirror of the rotating scanner 25 and is deflected off of 
that mirror and passes through flat field lens 27 which focuses the beam 
to the focal spot 33. That beam at the focal spot 33 grows again as it 
approaches the flat field lens 29. After it passes through the lens 29, it 
is in a collimated state and is deflected off of the scanner 11 (FIG. 1) 
to the flat field lens 13 (FIG. 1). The light beam 31 is focused by the 
lens 13 before reaching the flat plane of the sample 19 that is to be 
scanned over. 
In summary, FIGS. 1, 2 and 3 show and describe the scanning of a planar 
surface, with FIG. 1 dealing with a scan in a first dimension, FIG. 2 
dealing with a scan in a second dimension and FIG. 3 dealing with a scan 
in both of the first and second dimensions. 
3. A Spherical Convex Surface Using One Optical Scanner and a Slower 
Rotating Mechanical Device. 
To produce either gated or non-gated confocal scans of spherical objects 
such as ball bearings, a special scanning system is required. As shown in 
FIG. 4, only a single scanner and a lens is sufficient to produce a scan 
of a spherical concave surface. As depicted in FIG. 4, the focus is 
tracing a concave sphere during a scan. This type of scanning is well 
known and is used to study various concave objects such as the interior of 
the eye. 
In the operation of the system of FIG. 4, a collimated light beam 41 is 
focused by a flat field lens 43 onto a scanner 45 similar to the scanner 
11 (FIG. 1) or scanner 25 (FIG. 2) to scan over a concave surface 47 in 
one dimension. To scan in two dimensions, FIG. 4 could be combined with 
the system shown in FIG. 2. Such a combination of scanners would produce a 
linear scan in one dimension (using FIG. 2) and a scan over a spherical 
convex surface in the other dimension (using FIG. 4). 
Referring now to FIG. 5, FIG. 5 shows a technique for performing a linear 
scan along a spherical convex surface, with a constant optical path length 
along that convex surface. To produce a scan of a convex surface, an 
arrangement different from that of FIG. 4 is required. 
In FIG. 5 a flat field lens 52 forms a focus before a scanner 55. Scanner 
55 is similar to scanner 11 (FIG. 1) or scanner 25 (FIG. 2). The expanding 
light beam is rotated by scanner 55 as shown by the double arrows 56. The 
focus 53 of light beam 51 that occurs before the scanner 56 is placed one 
focal length away from lens 58. Because of the geometry used in this 
configuration, it is possible to draw a virtual line that represents how 
the focus spot 53 moves as seen by lens 58. That line is represented by 
the dotted line 57. In other words, dotted line 57 describes the motion of 
the focal spot 53 as seen by lens 58. The combined distance from the 
virtual line shown as the dotted line 57 to the line 61 forms, in 
combination with lenses 58 and 59, a 4-f system. The 4-f system includes 
the distance from focus 53 to lens 58 (which is the same distance as from 
virtual line 57 to lens 58), the distance between lenses 58 and 59, and 
the distance from lens 59 to line 61. The distance from the focus 53 to 
the lens 58 is one focal length, the distance between lenses 58 and 59 is 
two focal lengths, and the distance between lens 59 and line 61 is one 
focal length, where the focal lengths of both lenses are the same and 
equal to f. 
Because the combination of virtual line 57, lenses 58 and 59 and line 61 
forms a 4-f system, the virtual line 57 will transform into the convex 
line 61 in the focus of the lens 59. This transformation only works in a 
4-f lens system. If a single 2f to 2f lens imaging system (not shown) were 
used instead, the line 61 traced by the scanning beam 41 would not be 
spherical. The explanation for this is based on the observation that a 
single lens transforms Z positions in space asymmetrically from 0 to 2f 
into positions from 2f to .infin. and vice versa. However, a 4-f two lens 
system transforms 0 to f positions into f to 2f positions symmetrically, 
preserving the spherical nature of the line 61. The slow scanning in other 
dimensions can be performed by mechanical means. The radius of curvature 
of the convex scan shown in FIG. 5 can be adjusted by changing the 
relative position of focus 53 and scanner 55. Making the distance between 
focus 53 and scanner 55 large increases the radius of curvature of line 
61, and making that distance small decreases the radius of curvature of 
line 61. As an example of a scan over a spherical surface, a prototype 
device performed a 2.times.2 mm scan on the surface of a ball bearing in 
less than 1 sec. 
Even though these scanning techniques were developed for gated optical 
imaging, they can also be applied for confocal scanning microscopy on 
curved and flat surfaces. 
Advantages and New Features of the Invention 
The above-described implementation of optical scanning techniques allows 
fast image acquisition in various surface topologies while keeping high 
spatial resolution and while keeping a constant optical path length during 
the scan. When applied to a convex surface, these optical techniques can 
also dramatically improve the resolution of images obtained with a 
confocal scanning microscope. 
Alternatives 
Aspherical lenses, or other lens combinations may be designed to improve 
the focus of the signal beam. Two-dimensional gated imaging may also be 
feasible with some other surface geometry. An optical polarizer may be 
used in conjunction with the gating techniques to further reduce noise due 
to the surface reflection or to study birefringences of the sample. Lens 
pairs in the 4-f imaging system described in FIG. 5 do not have to be of 
equal focal length. If the focal lengths are different, but if distances 
between different elements are adjusted properly, there is the potential 
to magnify or reduce the final radius of curvature over the initial radius 
of curvature, while still keeping a convex scan in which the OPL is 
constant. 
Therefore, what has been described in a first preferred embodiment of the 
invention is an optical scanning system for developing high-resolution, 
near-surface images from a desired surface topology of a sample, the 
optical scanning system comprising: a light source for producing a 
collimated light beam; a first optical system for directing the collimated 
beam to a first position on a first optical axis; a first scanner device 
having a first center portion for scanning the collimated light beam from 
the light source through the first optical system to the first position on 
the first optical axis with a constant path length in a first dimension; a 
second scanner device having a second center portion for scanning the 
scanned collimated light beam from the first portion on the first optical 
axis along a second optical axis orthogonal to the first optical axis; and 
a second optical system for focusing the collimated light beam onto the 
desired surface topology in a second dimension; the first and second 
scanner devices cooperatively operating to cause the collimated light beam 
to scan the desired surface topology of the sample with a focused constant 
optical path length in both of the first and second dimensions of the 
sample. 
In a second preferred embodiment of the invention, a scanning system for 
scanning in first and second dimensions a convex surface of a sample is 
disclosed. The scanning system comprises: a light source for producing a 
collimated light beam; a scanning mirror having a scanning surface; a 
first focusing lens for focusing the collimated light beam before the 
scanning mirror; second and third focusing lenses optically aligned with 
each other, the combination of the scanning surface of the scanning 
mirror, the first, second and third focusing lenses and the convex surface 
of the sample forming a 4-f system so that the focus of the first lens 
transforms into the convex surface in the focus of the third focusing 
lens, the second focusing lens positioned such that that its focus 
coincides with the on-axis virtual location of the focused collimated 
light beam from the first focusing lens as seen in the scanning mirror, 
the third focusing lens producing at its output a focus of the beam which 
follows a curved line that lies on the surface of the spherical convex 
surface, the focus maintaining a constant optical path length; and 
translation means coupled to the sample for translating the sample to 
produce a two-dimensional scan of the surface of the sample. 
It should therefore readily be understood that many modification and 
variations of the present invention are possible within the purview of the 
claimed invention. It is therefore to be understood that, within the scope 
of the appended claims, the invention may be practiced otherwise than as 
specifically described.