Broadband diffractive lens or imaging element

A broadband diffractive lens or imaging element produces a sharp focus and/or a high resolution image with broad bandwidth illuminating radiation. The diffractive lens is sectored or segmented into regions, each of which focuses or images a distinct narrowband of radiation but all of which have a common focal length. Alternatively, a serial stack of minus filters, each with a diffraction pattern which focuses or images a distinct narrowband of radiation but all of which have a common focal length, is used. The two approaches can be combined. Multifocal broadband diffractive elements can also be formed. Thin film embodiments are described.

BACKGROUND OF THE INVENTION 
This invention relates generally to diffractive imaging elements or lenses, 
including Fresnel zone plates, Fresnel phase plates, blazed Fresnel phase 
plates or other patterns which focus radiation primarily by diffraction, 
and more particularly to diffractive imaging elements having significantly 
reduced chromatic aberrations to produce a sharp focus and/or produce high 
quality images using much broader bandwidth radiation than is possible 
with conventional diffractive lenses. 
Fresnel zone plates, Fresnel phase plates, and blazed Fresnel phase plates 
can be used to focus and/or image radiation. Blazed Fresnel phase plates 
are described in N. M. Ceglio and H. I. Smith, in "Proceedings VIII Int'l 
Conf. on X-Ray Optics and Microanalysis" (D. R. Beaman R. E. Ogilvie, and 
D. B. Wittry, Eds.), P. 255, Pendell, Midland, Mi., 1980. In addition, 
other diffractive optical elements (e.g., holograms or holographic optical 
components) can also be used to focus or image radiation. All these 
optical elements use primarily diffraction to achieve focus or image 
formation. Since diffractive power (e.g., focal length) is strongly 
wavelength dependent, all of these diffractive lens structures suffer from 
chromatic aberrations. 
Fresnel diffractive structures (e.g., Fresnel zone plates, Fresnel phase 
plates, and blazed Fresnel phase plates) are divided into Fresnel zones 
where the radius of the nth zone is given by 
EQU r.sub.1.sup.2 =nr.sub.1.sup.2 +n.sup.2 .lambda..sup.2 /4 (1) 
where r.sub.1 is the radius of the central zone and .lambda. is the 
wavelength of the radiation to be focused. Such a Fresnel zone structure 
may be viewed as a diffractive lens having a focal length, 
EQU f=r.sub.1.sup.2 /.lambda. (2) 
The focal length of the lens is wavelength dependent; indeed, the 
geometrical pattern itself (i.e., placement of the r.sub.n 's) is 
wavelength dependent. If a Fresnel structure designed to focus radiation 
at one wavelength, .lambda..sub.1, is used with radiation at a different 
wavelength, .lambda..sub.2, there will be a focal error or chromatic 
aberration (from equation (2)). The conventionally "acceptable" bandwidth 
for such a lens is generally taken as 
EQU .DELTA..lambda.=1/N (3) 
where N=total number of zones. Under the conditions of equation (3), the 
performance of the lens is virtually diffraction limited and the focal 
spot size approaches the width of the outermost zone, 
EQU .DELTA.r=r.sub.N -r.sub.N-1 ( 4) 
In practice, the acceptable bandwidth for illumination of the Fresnel 
structure will depend on the application, and will be determined by a 
trade-off between efficiency (i.e., accepting a broader bandwidth) and 
resolution loss (primarily due to chromatic aberrations). 
Thus, diffractive lenses generally produce a sharp focus or a high 
resolution image only if illuminated with sufficiently narrowband 
(.DELTA..lambda.=.lambda./N) radiation. For these reasons, conventional 
diffractive imaging systems or lenses are generally viewed as narrowband 
imaging systems or imaging systems suffering from severe chromatic 
aberrations. 
In many regions of the electromagnetic spectrum (and for other radiation 
such as neutrons, atoms, ions), refractive lenses are not practical so 
that diffractive lenses are all that are available to focus or image the 
radiation. This results either from severe absorption of the radiation in 
materials and/or because the refractive power of available materials is 
not sufficiently different from vacuum for those types of radiation. Under 
such circumstances, there is a great motivation for a scheme which would 
enable diffractive lenses to focus and/or image broadband radiation. 
Indeed, most sources of electromagnetic radiation in these spectral 
regions are broadband, for example, synchrotrons, plasmas, blackbody 
radiation, etc. Diffractive lenses also have properties which would make 
them very useful for application in parts of the electromagnetic spectrum 
where refractive optics already exist. For example, diffractive lenses 
have been easily implemented as bifocal and/or multifocal imaging and 
focusing elements. (Indeed, a bifocal or multifocal diffractive lens may 
simply be considered a hologram.) In addition, diffractive lenses can have 
high dioptric power and at the same time be very thin and easily 
deformable, making diffractive lenses attractive options for intraocular 
lenses and/or contact lenses. U.S. patent application Ser. No. 495,073 
filed Mar. 19, 1990 (now abandoned) describes microthin diffractive lenses 
for intraocular implants and corneal lenses. In these applications, it 
would be highly beneficial, and perhaps essential, that the diffractive 
lenses be able to focus and image broadband radiation and have 
significantly reduced chromatic aberrations. 
There is, in addition, a great interest in x-ray optics in having 
relatively broadband imaging and focusing optics which can approach 
diffraction limited resolution. At soft x-ray wavelengths, Fresnel 
structures (zone plates and phase plates) have demonstrated diffraction 
limited resolution down to about 300A with narrowband 
(.DELTA..lambda.&lt;.lambda./N) illumination. The best performance for 
broadband imaging has been achieved using grazing incidence reflection 
optics, and image resolutions of order .gtoreq.1 .mu.m have been 
demonstrated. There are applications in x-ray microscopy, materials 
analysis, and x-ray matter interaction studies which could benefit from an 
ability to focus and/or image relatively broadband radiation with 
diffraction limited or near diffraction limited performance. 
With such a strong motivation for broadband diffractive optics, there have 
been various attempts to design diffractive lens doublets or triplets to 
correct for and/or reduce the chromatic aberrations in diffractive optics. 
These approaches to chromatic aberration correction are less than 
satisfactory for at least two reasons: (1) They generally involve two or 
more diffractive elements separated by a finite distance. As such, they 
are really a "lens system" or an "optical system", not a simple, compact 
broadband lens. For many applications (e.g., contact lens or intraocular 
lens implants), the "system" approach is not practical. (2) In addition, 
diffractive optical elements typically operate at limited efficiency. For 
example, an ideal Fresnel zone plate diffracts only 10% of the incident 
(narrowband) radiation into its first order focus, and an ideal Fresnel 
phase plate (in the absence of radiation absorption) diffracts 40% of the 
incident narrowband radiation into its first order focus. [However, if the 
Fresnel phase structure is appropriately blazed a blazed Fresnel phase 
plate) it can. in principle. direct 100% of the incident radiation into 
its focal spot.] Thus, an optical system for chromatic aberration 
correction that puts M such structures (each having efficiency .lambda.) 
in series suffers in overall radiation transport efficiency by a factor of 
(.lambda.).sup.M. For example, a triplet (M=3) of zone plate structures 
(.lambda.=0.1) would have an overall efficiency of 0.001. 
SUMMARY OF THE INVENTION 
The invention is a Broadband Diffractive Lens (BBDL), which is produced by 
at least one of two approaches. 
In one embodiment, the diffractive lens is divided into lisegments or 
sectors", each having its own, individualized, narrowband (.DELTA..lambda. 
where .lambda.=wavelength of the radiation) filter which ideally would 
pass the radiation within bandwidth .DELTA..lambda., and reject (i.e., not 
pass; e.g., absorb or reflect) the radiation outside that bandwidth. Each 
sector (or segment) would have its diffractive geometry configured or 
patterned (i.e., it would have a zone plate or phase plate or blazed phase 
plate pattern or other diffractive focusing pattern formed on the sector 
or segment) such that for its bandpass (.DELTA..lambda.) it would have a 
focal length f, and that focal length would be the same for (or not 
significantly different from) all the segments or sectors in the lens. In 
this way, a single lens made up of two or more sectors or segments could 
be illuminated with broadband radiation and produce a single broadband 
focus or broadband image. The invention includes all geometrical shapes 
for the segments or sectors making up the BBDL. For example, the 
individual segments or sectors could be annular, or they could be radial 
or pie shaped segments, or they could even be chosen to have randomly 
shaped boundaries. 
In another embodiment, a "Serial Stack of Minus Filters" (SSMF) is used to 
make up the BBDL. An ideal "minus" filter has the property that it will 
pass all radiation not included in a narrow bandwidth .DELTA..lambda., 
while radiation within bandwidth A), will be rejected (e.g., absorbed or 
reflected) by the minus filter. The minus filter concept is expanded to 
include an idealized "phase" minus filter which has the property that 
radiation within bandwidth .DELTA..lambda. undergoes a phase shift which 
is not 0 or 2.pi. or some multiple of 2.pi. (which can be controlled to 
improve efficiency), whereas the lo radiation outside the .DELTA..lambda. 
spectral band is passed with a phase shift equal to zero, 2.pi. or some 
multiple of 2.pi.. 
In the second embodiment (i.e., SSMF), each minus filter is geometrically 
configured or patterned to produce a diffractive lens with a focal length 
f for radiation within its bandwidth .DELTA..lambda.. Each geometrically 
configured or patterned minus filter acts as a diffractive lens which 
focuses or images the radiation within its bandwidth (.DELTA..lambda.), 
but allows all out-of-bandwidth radiation to pass virtually unaffected. 
The Broadband Diffractive Lens (BBDL) is thereby made up by putting 
together a stack of such minus filters, in series, such that each filter 
modulates a complementary narrow bandwidth (.DELTA..lambda.), and is 
geometrically configured or patterned to produce the same (or 
insignificantly different) focal length as the other geometrically 
configured or patterned minus filters in the stack. Such a stack of ideal 
minus filters, with appropriate diffractive geometrical configurations, 
would thereby be able to focus or image broadband incident radiation. 
In a combined embodiment, a combination of the first two may be utilized, 
i.e., sectors or segments with minus filters. The number of minus filters 
in the stack will be determined by the number of wavelength bands and the 
number of sectors or segments will be determined by the number of foci. 
The invention also comprises the fabrication processes used to make a 
Broadband Diffractive Lens or Imaging Element. These processes are 
described in the present application. 
Although the description of the invention discusses ideal narrowband 
filters and ideal minus filters and ideal Fresnel zone diffractive 
geometries, it is also intended to cover all embodiments which incorporate 
non-ideal approximations to the proposed configurations and concepts. 
The broadband diffractive lens according to the invention is indeed a 
compact lens, not a separated "optical system" and therefore does not 
suffer from the product of inefficiencies of its components. Even in the 
second approach, which uses the series of stacked minus filters (SSMFs), 
each minus filter in the stack modulates only the radiation within its 
design bandwidth and passes the other radiation virtually unaffected. In 
this way, the inefficiencies of the minus filters in the stack do not 
compound serially. 
The Broadband Diffractive Lens will have application in many regions of the 
electromagnetic spectrum and can also be used in the focusing and imaging 
of non-electromagnetic radiation (e.g., in diffractive lenses for focusing 
low energy neutrons or atoms or ions). The BBDL, because it is a 
diffractive lens, can also provide bifocal and multifocal imaging 
capabilities. 
Additionally, the invention is intended to include (but not be restricted 
to) the application of the Broadband Diffractive Lens or Imaging Element 
concept to: 
a. Intraocular lenses with high (greater than 10 diopters) dioptric power. 
b. The correction of chromatic aberration problems in diffractive contact 
lenses (this is intended to apply to diffractive contact lenses used 
either in a stand alone mode or in conjunction with a refractive lens). 
c. The correction of chromatic aberrations in any diffractive optic, 
holographic optic, or binary optic. 
d. Focussing and imaging optics used with broadband lasers, multifrequency 
lasers, wavelength unstable lasers, semiconductor lasers, or diode lasers. 
e. Focussing, imaging, or transfer optics requiring chromatic aberration 
correction for use in: 
1) Optical communications applications 
2) Optical and infra-red radar applications 
3) Point of sale optical scanners 
4) Laser printers 
5) Flat panel displays 
6) Optical encodina 
7) Optical memory devices 
f. White light hologram applications. 
g. The fabrication of inexpensive aspheric, chromatically corrected lenses. 
h. The fabrication of inexpensive, large diameter chromatically corrected 
lenses. 
i. The use of color corrected, binary lens arrays for solar radiation 
collector applications. 
j. Applications requiring color corrected optics which are lightweight, 
such as airborne or space applications. 
k. Applications requiring color corrected optics which are flexible and 
compact and easily placed in areas of limited space or difficult access. 
l. Binary optics applications requiring chromatic aberration correction. 
In the above applications (items a thru 1) the term chromatic correction or 
chromatic aberration correction is intended to include both total or 
partial correction of the chromatic aberrations normally associated with 
prior diffractive structures. The specific degree of chromatic aberration 
correction is a design parameter of the lens or imaging system. 
The BBDL may be particularly useful at infrared, ultraviolet, deep 
ultraviolet, soft x-ray, and hard x-ray wavelengths where options for 
refractive lenses are limited. In such applications, diffractive lenses 
with broadband focusing and imaging capabilities will be a new and welcome 
optical component. 
This invention is intended to include all, but not be restricted to, the 
above described applications of the BBDL.

DETAILED DESCRIPTION OF THE INVENTION 
This invention provides a broadband diffractive lens which is able to 
produce a sharp focus and/or a high resolution image using a significantly 
broader bandwidth of illuminating radiation than the conventionally 
acceptable narrow bandwidth for diffractive lenses. (The conventionally 
acceptable (narrow) bandwidth is defined as .DELTA..lambda.=.lambda./N, 
where .DELTA..lambda. is the conventional (narrow) bandwidth of 
illumination for diffraction limited performance of a diffractive lens, 
.lambda. is the central wavelength within the band of radiation, and N is 
the number of Fresnel zones of the diffractive lens). 
The invention provides for two approaches or a combination thereof to 
achieving a broadband diffractive lens: the diffractive lens is sectored 
or segmented into narrowband filtered regions, each accommodating (i.e., 
focusing or imaging) a distinct narrowband of radiation, but all segments 
or sectors having a common focal length; and the diffractive lens is 
formed of a serial stack of minus filters (SSMF), each accommodating 
(i.e., focusing or imaging) a distinct narrowband of radiation while 
passing all other wavelengths, all filters having a common focal length. 
Sectored or Segmented Diffractive Lens (SSDL) 
A particular example of the SSDL is illustrated in FIGS. 1A and B. Shown is 
a Fresnel zone structure 20 divided into two segments 21, 22. In this 
example, the top segment 21 would have its Fresnel zone boundaries given 
by 
EQU r.sub.n.sup.2 =n.lambda..sub.T f+n.sup.2 .lambda..sub.T.sup.2 /4 
from equations (1) and (2). Similarly, the bottom segment 22 would have its 
Fresnel zone boundaries given by 
EQU r.sub.n.sup.2 =n.lambda..sub.B f+n.sup.2 .lambda..sub.B.sup.2 /4 
.lambda..sub.T and .lambda..sub.B are different wavelengths, but the focal 
lengths, f, of the two segments are the same. In addition, the radiation 
incident on the top segment 21 would be filtered by filter 23 to allow 
only a narrowband of radiation .DELTA..lambda..sub.T centered about 
.lambda..sub.T. Similarly, the radiation incident on the bottom segment 
would be filtered by filter 24 to allow only a narrowband of radiation 
.DELTA..lambda..sub.B centered about .lambda.. Although the filters are 
shown in a spaced relationship to the Fresnel structure, the spacing can 
be very small, or the filters may contact the Fresnel structure. 
Alternatively, in place of separate filters 23, 24 the filters may also be 
incorporated into the Fresnel structure itself, e.g., by using suitable 
dyes with the necessary absorption characteristics. In this way, the 
simple diffractive structure illustrated in FIGS. 1A, B could focus 
incident radiation of composite bandwidth .DELTA..lambda..sub.T 
+.DELTA..lambda..sub.B to a single focal spot or it could produce a single 
broadband (i.e., .DELTA..lambda..sub.T +.DELTA..lambda..sub.B) image. 
The invention also includes possible variations in the number of sectors 
(or segments) and variations in their shapes. For example, FIG. 2 shows a 
structure 25 divided into four quadrants 26, 27, 28, 29. These four 
quadrants can be geometrically configured and filtered to accommodate four 
separate bandwidths (.DELTA..lambda..sub.1, .DELTA..lambda..sub.2, 
.DELTA..lambda..sub.3, .DELTA..lambda..sub.4), all having a common focal 
length. However, for reasons of symmetry, it may be decided to use four 
quadrants to accommodate only two separate bandwidths (e.g., 
.DELTA..lambda..sub.1, .DELTA..lambda..sub.2, .DELTA..lambda..sub.2 in the 
four quadrants respectively, i.e., sectors 26, 28 at one wavelength and 
sectors 27, 29 at the other). In general, the invention is intended to 
include expansion and variation of the sectored diffractive lens concept 
to include as much of the electromagnetic spectrum as is desired (e.g., by 
increasing the number of sectors), and/or to control the "color" balance 
in the broadband images by controlling the size of individual sectors 
and/or controlling the transmission through the filters used on specific 
sectors or segments. FIG. 3 provides an additional example of a sectored 
or segmented lens with differently shaped sectors. Shown in FIG. 3 is a 
diffractive structure 30 divided into three annular segments 31, 32, 33, 
i.e., circularly symmetric segments confined within radial bands. As with 
the pie-shaped sectors, the incident radiation within each annular radial 
band would be filtered to pass only a narrowband, .DELTA..lambda., 
centered about a wavelength, .lambda..sub.o, and the Fresnel zones within 
that radial band would satisfy the equation 
r.sub.n.sup.2 =n.lambda..sub.o f+n.sup.2 .lambda..sub.o.sup.2 /4 for 
r.sub.x .ltoreq.r.sub.n .ltoreq.r.sub.y.ltoreq. 
where r.sub.x, r.sub.y are the radial boundaries of the annular segment. 
The other annular segments would accommodate different bandwidths of 
radiation, but, as before, all would have a common focal length. FIG. 4 
illustrates another example of a sectored lens with differently shaped 
sectors. Diffractive structure 34 is divided into four irregularly shaped 
segments 49, 50, 51, 52. 
Broadband Diffractive Lens Using a Serial Stack of Minus Filters 
This particular embodiment of the broadband diffractive lens utilizes the 
minus filter. The minus filter is described in Alfred Thelen, Design of 
Optical Interference Coatings, Ch. 7, pp. 147-155, McGraw Hill (1989). 
FIG. 5A illustrates the effect of an idealized minus filter. The filter 
absorbs radiation only within a narrow band, 
.DELTA..lambda.=.lambda..sub.2 -.lambda..sub.1, and passes al I other 
radiation unaffected. 
This may be achieved using multilayer thin films which provide a high 
reflectivity at a resonant wavelength .lambda..sub.o within the bandwidth 
.DELTA..lambda., as shown in FIG. 5b. Preferably, these films have a 
thickness .lambda..sub.o /4n, where n is the index of refraction of the 
film material. The greater the number of films, the more the filter 
approximates an idealized minus filter. .lambda..sub.o and .DELTA..lambda. 
may be engineered by specifying the spacing (i.e., thickness) of the 
multilayer films and the number of multilayers, respectively. 
An idealized "phase minus" filter, shown in FIGS. 5c and 5e, provides a 
finite, but not 0 or 2.pi. or some multiple of 2.pi. phase shift for 
radiation within a bandwidth .DELTA..lambda.=.lambda..sub.2 
-.lambda..sub.1, but passes all other radiation with a phase shift of 
either zero, 2.pi. or some multiple of 2.pi.. In accordance with the 
invention, a minus filter, "phase-step", or "phase-notch" minus filter (as 
illustrated in FIGS. 5a, 5c, or 5e, respectively) operating on a narrow 
band of radiation .DELTA..lambda..sub.a, centered at wavelength 
.lambda..sub.a (.lambda..sub.1 &lt;.lambda..sub.a &lt;.lambda..sub.2), when 
patterned with a Fresnel structure having Fresnel zones placed at r.sub.n, 
such that 
EQU r.sub.n.sup.2 =n.lambda..sub.a f+n.sup.2 .lambda..sub.a.sup.2 /4 
will focus the incident .DELTA..lambda..sub.a to a virtually diffraction 
limited spot (for .DELTA..lambda..sub.a .gtoreq..lambda..sub.a /N) with a 
focal length f, and will pass the radiation outside bandwidth 
.DELTA..lambda..sub.a virtually unaffected. A second minus or phase minus 
filter placed in series with the first, but operating on a separate narrow 
bandwidth .DELTA..lambda..sub.b centered at .lambda..sub.b (.lambda..sub.1 
&lt;.lambda..sub.b &lt;.lambda..sub.2) and patterned with a Fresnel structure 
having Fresnel zones placed at r.sub.n such that 
EQU r.sub.n.sup.2 =n.lambda..sub.b f+n.sup.2 .lambda..sub.b.sup.2 /4 
will serve to focus the incident .DELTA..lambda..sub.b band to the same 
focal spot and pass the "out of bandwidth" radiation virtually unaffected. 
Thus, a serial stack of such appropriately chosen minus or phase minus 
filters with appropriately patterned Fresnel structures produces a single 
broadband focus of the incident radiation. Such a diffractive lens would 
have a single focal length for the broadband of incident radiation, and, 
as such, could produce high resolution images of broadband sources of 
radiation. 
An embodiment of a phase-step filter with the characteristics shown in FIG. 
5c is shown in FIG. 5d. This is comprised of a "double" stack of 
multilayer thin films such that the first stack has a resonant reflection 
at wavelength .lambda..sub.1 and the second at wavelength .lambda..sub.2. 
Now at each resonance the transmitted radiation undergoes a relative phase 
shift of .pi.. 
So: 
for .lambda.&lt;.lambda..sub.1 the relative phase shift is zero 
for .lambda..sub.1 &lt;.lambda.&lt;.lambda..sub.2 the relative phase shift is 
.pi. 
for .lambda.&gt;.lambda..sub.2 the relative phase shift is 2.pi. 
In this example, the phase shift is .pi.. Preferably, the films have 
thicknesses .lambda..sub.1 /4n.sub.1, .lambda..sub.2 /4n.sub.2, where 
n.sub.1 and n.sub.2 are the indices of refraction of the first and second 
film materials respectively. Again, the greater the number of films the 
more the filter approximates an ideal phase-step filter. 
An embodiment of a phase-notch filter with the characteristics shown in 
FIG. 5e is shown in FIG. 5f. 
Consider a Fresnel zone structure in which the adjacent zones are made up 
of different multilayer thin films, such that the even zone multilayers 
have a resonance at .lambda..sub.2 and the odd zone multilayers have a 
resonance at .lambda..sub.1. 
Then for: 
.lambda.&lt;.lambda..sub.1 the relative phase between adjacent zones is zero 
.lambda..sub.1 &lt;.lambda.&lt;.lambda..sub.2 the relative phase between adjacent 
zones is .pi. 
.lambda.&gt;.lambda..sub.2 the relative phase between adjacent zones is zero 
So that for this Fresnel zone patterned multilayer structure the relative 
phase between adjacent zones is spectrally modulated as a phase notch 
filter. Note that in this example the phase shift is .pi. also, but in 
general it may have any value between 0 and 2.pi.. As above, the film 
thicknesses may be .lambda..sub.1 /4n.sub.1, .lambda..sub.2 /4n.sub.2. 
Although the concept and performance of the broadband diffractive lens or 
imaging element is most easily described and understood in terms of the 
serial stack of minus filters wherein each minus filter element has a 
diffractive structure patterned into it, there are other technological 
methods whereby the same spectral and spatial modulation of the incident 
radiation may be achieved. In such cases the lens or imaging element 
thereby produced will have the equivalent broadband properties as the 
diffractive serial stack of minus filters. An example of such a 
technological equivalent follows: 
Using high resolution, color, photographic transparency films one may be 
presumed to be able to achieve a succession of well defined rings which 
transmit well defined colors. In this case we could produce a pattern of 
color transmitting rings which mimic the colored transmission of a zone 
plate patterned serial stack of minus filters. For example, if we have a 
serial stack of minus filters which are patterned with zone plate 
structures such that 
##EQU1## 
then when illuminated with white light the colored pattern of transmitted 
radiation in the inner zones will be as shown in FIG. 5g. 
______________________________________ 
Radius Transmitted Color 
______________________________________ 
.sup. 0 .fwdarw. r.sub.1B 
black 
r.sub.1B .fwdarw. r.sub.1G 
blue 
r.sub.1G .fwdarw. r.sub.1R 
blue & green 
r.sub.1R .fwdarw. r.sub.2B 
white 
r.sub.2B .fwdarw. r.sub.2G 
green & red 
r.sub.2G .fwdarw. r.sub.2R 
red 
etc. 
______________________________________ 
This transmitted color pattern could be simulated using high resolution 
photographic transparency film or by a number of other filter 
technologies. This patent is intended to relate to all techniques of 
diffractive optics combined with techniques of filtration to produce color 
corrected diffractive lenses. 
An illustration of a serial stack 35 of two minus filters 36, 37 patterned 
to provide a common focus for two separate bands of radiation is shown in 
FIG. 6. The first minus filter 36 focuses or images a first band 
.DELTA..lambda..sub.1 centered at .lambda..sub.1 while passing a second 
band .DELTA..lambda..sub.2 centered at .lambda..sub.2 unaffected. The 
second band is focused or imaged by second minus filter 37 which passes 
the first band unaffected. Thus, stack 35 focusses or images the combined 
band .DELTA..lambda..sub.1 +.DELTA..lambda..sub.2 at a common focus or 
image plane. Although minus filters 36, 37 are shown in a spaced 
relationship, the spacing can be very small or the filters can contact 
each other. 
The invention also includes broadband diffractive lenses made up of serial 
stacks which include non-ideal approximations to the minus filters and 
phase minus filters discussed herein. 
The invention also includes the options of patterning the minus filter 
stack with Fresnel zone plate patterns (i.e., alternately transparent and 
opaque Fresnel zones), with Fresnel phase plate patterns (i.e., providing 
a phase shift difference of .pi. between adjacent Fresnel zones), with 
blazed Fresnel phase plate patterns (i.e., Fresnel structures wherein the 
phase of the incident wave is adjusted continuously from 0 to 2.pi. over 
each adjacent pair of Fresnel zones), with generalized holographic 
diffractive patterns, or with any approximation to the above-cited 
diffractive patterns. 
Broadband Multifocal Diffractive lens 
It is also intended as part of this invention to include the application of 
the above-described Broadband Diffractive Lens to bifocal and multifocal 
diffractive lens schemes, e.g., intraocular or corneal lenses as described 
in U.S. patent application Ser. No. 495,073 filed Mar. 19, 1990. For 
example, FIG. 7 shows a bifocal, sectored diffractive lens 38. In this 
case, the lens is divided into six sectors, 39, 40, 41, 42, 43, 44. The 
top three sectors 39, 40, 41 are filtered and configured for a single 
focal length, f.sub.1, for three bands of radiation (e.g., a narrowband in 
the red .DELTA..lambda..sub.R, in the green .DELTA..lambda..sub.G, and in 
the blue .DELTA..lambda..sub.B). The bottom three sectors 42, 43, 44 are 
filtered and configured for a different focal length, f.sub.2, for the 
same three bands of radiation (.DELTA..lambda..sub.R, 
.DELTA..lambda..sub.G .DELTA..lambda..sub.B). In this way, a bifocal 
Broadband Diffractive Lens can be made. In similar fashion, the invention 
can be extended to produce multifocal (i.e., three or more foci) broadband 
diffractive lenses. It is also intended, as part of this invention, that 
by simple extension of these concepts, bifocal and multifocal diffractive 
lenses with "designer" (i.e., specifically designed or custom designed) 
bandwidths can be constructed. For example, it is intended to include the 
possibility of applications which have a specific bandwidth (e.g. 
.DELTA..lambda..sub.R, .DELTA..lambda..sub.G, .DELTA..lambda..sub.B) for 
focal length f.sub.1, but a very different or slightly different bandwidth 
for focal length f.sub.2, etc. Such a capability could be quite useful for 
applications in which the radiation coming from a distance (e.g., need 
longer focal length) has a spectrum quite different from the radiation 
coming from nearby sources (e.g., need shorter focal length). In this 
application, a bifocal broadband diffractive lens with different 
bandwidths (suited to the spectra of the different sources) for the 
different foci could be quite useful. 
It is also intended, as part of this invention, to include broadband 
diffractive lenses which are a combination or composite of both 
approaches, for example, a serial stack of minus filters which are 
sectored. In such a lens the need for filtering the sectors is obviated by 
the presence of the serial stack of minus filters. Such a composite 
broadband diffractive lens could serve ideally as a bifocal or multifocal 
broadband diffractive lens. For example, FIG. 8 shows a sectored minus 
filter 45 having three sectors 46, 47, 48. A plurality of sectored minus 
filters 45 can be stacked, similar to the stack arrangement shown in FIG. 
6. For example, four minus filters 45 each having three sectors 46, 47, 
48, can be stacked. The four minus filters provide for four different 
wavelength bands (.DELTA..lambda..sub.1, .DELTA..lambda..sub.2, 
.DELTA..lambda..sub.3, .DELTA..lambda..sub.4); thus the number of bands 
determines the number of minus filters in the stack. The number of 
segments in each minus filter determines the number of foci. For example, 
the four minus filter sectors in position 46 all have one focus, while the 
four minus filter sectors in position 47 have a second focus and the four 
in position 48 have a third focus. The minus filter stacks in each of the 
sectors can be the same (though they need not be) so that each of the 
three sectors will focus the same broad bandwidth of radiation. However, 
the diffractive patterning in the three sectors is different so that the 
lens will be tri-focal. As shown, the diameters of the three focal 
segments 46, 47, 48 are different, but, in general, they can be the same. 
Processes 
The invention is intended to include, but not be restricted to, the 
following processes for fabrication of the Broadband Diffractive Lens or 
Imaging Element. 
The deposition of multilayer thin films of, but not limited to materials 
such as magnesium fluoride, calcium fluoride, lithium fluoride, 
zinc-sulfide, and quartz onto an optically transparent or optically 
reflecting substrate made of a material such as glass or plastic 
(transparent), or metallized glass or plastic (reflective). Typical 
materials are mentioned in "Optical Waves in Layered Media", by Pochi Yeh, 
(1988). Said deposition to be accomplished by any of a variety of 
processes including, but not restricted to, vacuum evaporation, sputtering 
(discharge sputtering or ion beam sputtering), chemical vapor deposition, 
atomic layer epitaxy or molecular beam epitaxy techniques. 
The patterning of a diffractive structure into or onto the multilayer thin 
film using lithographic techniques and processes including, but not 
restricted to, optical, ultraviolet, x-ray or e-beam lithography, and/or 
ion beam implantation. Subsequent etching of the pattern into the 
multilayer using any of a variety of processes including, but not 
restricted to, liquid etching, plasma or discharge etching, and/or 
sputtering via ions or neutral particles. 
FIGS. 11-14 show serial stacks of diffractive structures etched into 
multilayer thin films, each diffractive structure having its own specific 
diffractive geometry--wherein, the serial stack may be produced as 
follows: 
a) The initial multilayer film is deposited, lithographically patterned and 
then etched. The etched grooves are subsequently filled with a polymer or 
other transparent material of suitably chosen refractive index to produce 
a relatively flat surface. Then, the process is repeated on top of this 
first etched multilayer (or on top of a thin layer of planarizing 
material), in order to produce a second element of the stack of 
multilayers. The process is repeated to produce as many elements of the 
serial stack of multilayers as is desired. This is illustrated in FIG. 11. 
b) FIGS. 12 and 13 show an etched diffractive pattern in a multilayer film 
is produced on one or both sides of a thin transparent substrate material. 
A serial stack of such etched multilayers may thereby be accomplished by 
appropriately registering and stacking the substrate supported patterns on 
top of one another--as illustrated in FIGS. 12 and 13. 
The production of a phase minus or phase notch diffractive structure 
comprised of a Fresnel zone or diffractive structure in which adjacent 
zones are made up of different multilayer thin films, accomplished by 
producing the "odd-zone" etched multilayer diffractive structure on one 
side of a thin, transparent substrate and the "even-zone" etched 
multilayer diffractive structure on the opposite side of the thin, 
transparent substrate, is illustrated in FIG. 14. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention is a lens or imaging element which achieves focusing and/or 
imaging primarily by diffraction and is capable of focusing and/or imaging 
radiation of a bandwidth much broader than conventional diffractive lenses 
or imaging elements using the above described principles. The invention 
includes, but is not limited to, the following specific embodiments. 
Embodiment 1: A sectored or segmented diffractive lens is made of a number 
of sectors, as few as 2 or as many as 20 or more, in which the individual 
sectors have narrowband filters and the diffractive patterns in the 
individual sectors are Fresnel zone plate patterns (i.e., amplitude 
modulated zones). The shapes of the sectors or segments can be chosen to 
meet the needs of applications. The sectors or segments can be annular 
rings, pie shaped sectors, or even randomly shaped segments. 
Embodiment 2: The same as Embodiment 1 except that the diffractive patterns 
in the individual sections are Fresnel phase plate patterns (i.e., phase 
modulated zones). 
Embodiment 3: The same as Embodiment 1 except that the diffractive patterns 
in the individual sectors are blazed Fresnel phase plates of modulo k 
(where k is an. integer), i.e., where the phase shift through the pattern 
is varied continuously from 0 to 2.pi.k over 2k adjacent Fresnel zones as 
shown in FIG. 9. 
Embodiment 4: The same as Embodiment 3 except that the diffractive patterns 
in the individual sectors are blazed Fresnel phase plates for which the 
integer modulo (k) may be varied throughout the radial distribution. For 
example, FIG. 10 shows a BFPP such that over zones 1 through 4 the phase 
shift provided by the pattern varies continuously from 0 to 4.pi. (i.e., 
modulo but over zones 5 and 6 the phase shift is varied continuously from 
0 to 2.pi. (i.e., modulo 1), whereas over zones 7 through 14 the phase 
shift is varied continuously from 0 to 8.pi. (i.e., modulo 4), etc. 
Embodiment 5: The same as Embodiment 1 except that the diffractive pattern 
in the individual sectors can be any generalized holographic focusing 
pattern of the amplitude modulation or phase modulation type. 
Embodiment 6: All the options of Embodiments 1 thru 5 are extended to 
produce bifocal or multifocal broadband diffractive lenses or imaging 
elements as described above. 
Embodiment 7: A broadband diffractive lens or imaging element is made up of 
a serial stack of minus filters, and the diffractive pattern is a Fresnel 
zone plate (i.e., amplitude modulated) or any amplitude modulated 
holographic focusing or imaging element. 
Embodiment 8: The same as Embodiment 7 except that the broadband 
diffractive lens or imaging element is made up of serial stack of 
"phase-minus" filters, as described above, and the diffractive pattern is 
a Fresnel phase plate or blazed Fresnel phase plate (of modulo k, as in 
Embodiment 3, or of variable modulo as in Embodiment 4), or any phase 
modulated holographic focusing or imaging element. 
Embodiment 9: A composite broadband diffractive lens or imaging element 
made up of a sectored stack of minus filters or phase-minus filters, and 
which can be used for monofocal or multi-focal imaging. 
Embodiment 10: Includes Embodiments 1 through 9 in which approximations to 
idealized minus filters, or phase-minus filters, or idealized diffractive 
patterns are used. 
Embodiment 11: Includes Embodiments 1 through 10 for application in any 
region(s) of the electromagnetic spectrum and for focusing or imaging 
other particles or radiation, such as neutrons, electrons, ions, atoms, 
etc. 
EXAMPLES 
Example 1 is an example of Embodiment 3. A lens that can image radiation 
over a bandwidth from 500 to 560 nm with about a 1 cm focal length and a 
spatial resolution less than 20 microns (which is not diffraction limited) 
is formed of a three sector blazed Fresnel phase plate. Each of three 
sectors is designed to focus a 20 nm bandwidth of radiation. The first 
sector should have the following characteristics: 
r.sub.1 =71.41 microns 
N=50 
Diameter .gtoreq.1 mm 
.DELTA.r.sub.min 5.apprxeq.microns 
filter: transmits from 500 to 520 nm 
##EQU2## 
EQU r.sub.n.sup.2 nr.sub.1.sup.2 +n.sup.2 .lambda..sub.1.sup.2 /4 n=1, N 
where .lambda..sub.1 510 nm 
The second sector should have the following characteristics: 
r.sub.1 =72.8 microns 
N=49 
Diameter.gtoreq.1 mm 
.DELTA.r.sub.min .apprxeq.5.2 microns 
filter: transmits from 520 to 540 nm 
##EQU3## 
where .lambda..sub.2 530 nm 
The third sector should have the following characteristics: 
r.sub.1 =74.16 microns 
N=47 
Diameter.gtoreq.1 mm 
.DELTA.r.sub.min .apprxeq.5.4 microns 
filter: transmits from 540 to 560 nm 
##EQU4## 
where .lambda..sub.3 =550 nm 
The first sector is designed to operate at 510 nm, and focuses the 
radiation from 500 to 520 nm. The second sector is designed to operate at 
530 nm and focuses the radiation from 520 to 540 nm. The third sector is 
designed to operate at 550 nm and focuses the radiation from 540 to 560 
nm. 
Example 2 is an example of Embodiment 7. A lens with similar performance to 
Example 1 can be fabricated using minus filters. Three minus filters are 
patterned with Fresnel zone plate patterns. The first patterned minus 
filter has the following characteristics: 
r.sub.1 =71.41 microns 
N=50 
Diameter.gtoreq.1 mm 
.DELTA.r.sub.min .apprxeq.5 microns 
EQU r.sub.n.sup.2 =nr.sub.1.sup.2 +n.sup.2 .lambda..sub.1.sup.2 /4 n=1, N 
##EQU5## 
where .lambda..sub.1 =510 nm 
The first minus filter rejects radiation in the band from to 520 nm and 
passes all out-of-bandwidth radiation. The second patterned minus filter 
for this lens design has the following characteristics: 
r.sub.1 =72.8 microns 
N=49 
Diameter.gtoreq.1 mm 
.DELTA.r.sub.min .apprxeq.5.2 microns 
##EQU6## 
where .lambda..sub.2 =530 nm 
This minus filter rejects radiation in the band from 520 to 540 nm. 
The third patterned minus filter for this lens design has the following 
characteristics: 
r.sub.1 =74.16 microns 
N=47 
Diameter.gtoreq.1 mm 
.DELTA.r.sub.min .apprxeq.5.4 microns 
##EQU7## 
where .lambda..sub.3 =550 nm 
The minus filter rejects radiation in the band from 540 to 560 nm. 
The first minus filter is designed to operate at 510 nm, and focuses the 
radiation from 500 to 520 nm. The second minus filter is designed to 
operate at 530 nm and focuses the radiation from 520 to 540 nm. The third 
minus filter is designed to operate at 550 nm and focuses the radiation 
from 540 to 560 nm. Thus the bands that are "rejected" by each minus 
filter are focused or imaged by the diffractive pattern on the 
corresponding minus filter and pass through all the other minus filters 
without other effect, so that all the bands are focused or imaged to the 
same point or plane. Changes and modifications in the specifically 
described embodiments can be carried out without departing from the scope 
of the invention which is intended to be limited only by the scope of the 
appended claims.