U.S. patent number 3,668,404 [Application Number 05/076,430] was granted by the patent office on 1972-06-06 for electro-optical microtransducer comprising diffractive element monolithically integrated with photoelectric device.
Invention is credited to Kurt Lehovec.
United States Patent |
3,668,404 |
Lehovec |
June 6, 1972 |
ELECTRO-OPTICAL MICROTRANSDUCER COMPRISING DIFFRACTIVE ELEMENT
MONOLITHICALLY INTEGRATED WITH PHOTOELECTRIC DEVICE
Abstract
The position of a photocell is varied along the optical axis of
the light distribution generated by a Fresnel optical system. The
variation of the position of the photocell is caused by the
displacement of a surface area element, thereby translating said
displacement into an electrical signal. Two or more of the three
components of the above-mentioned arrangement, i.e., light source,
Fresnel optics and photocell, can be combined into a compact
integrated structure.
Inventors: |
Lehovec; Kurt (Williamstown,
MA) |
Family
ID: |
22131955 |
Appl.
No.: |
05/076,430 |
Filed: |
September 29, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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692051 |
Dec 20, 1967 |
3546469 |
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Current U.S.
Class: |
250/552;
G9B/7.097; G9B/7.042; 250/237G; 359/565; 438/24; 438/32; 438/69;
250/214.1; 257/82 |
Current CPC
Class: |
H01L
31/00 (20130101); G11B 7/12 (20130101); G02B
27/42 (20130101); G11B 7/085 (20130101); G02B
26/0875 (20130101) |
Current International
Class: |
G11B
7/12 (20060101); H01L 31/00 (20060101); G02B
27/42 (20060101); G11B 7/085 (20060101); G02B
26/08 (20060101); G02b 005/18 (); G02f 001/28 ();
H01l 015/02 () |
Field of
Search: |
;29/572
;250/217SS,237,217S,211J ;313/18D ;317/235 ;350/162ZP |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lawrence; James W.
Assistant Examiner: Grigsby; T. N.
Parent Case Text
CROSS REFERENCE TO OTHER APPLICATION
This a continuation-in-part of U.S. application Ser. No. 692,051
now U.S. Pat. No. 3,546,469.
Claims
1. An integrated monolithic structure of a diffractive optical
element and a solid state photoelectric device, said structure
comprising a thick substrate transparent to radiation, said
photoelectric device substantially thinner than said substrate and
located on a surface of said substrate, said diffractive optical
element located on the same surface of said substrate, said
photoelectric device and said diffractive optical
2. A substrate substantially transparent to a radiation, a solid
electroluminescent emitter of said radiation on one surface of said
substrate, a solid sensor for said radiation on the same surface of
said substrate laterally displaced from said emitter of radiation,
a diffractive optical element on another surface of said substrate,
whereby radiation from said emitter is condensed onto said sensor
by diffraction on said diffractive optical element, said substrate,
said emitter and said
3. The process of preparing an integratedmonolithic structure of an
electro-optical device optically aligned with a diffractive optical
element, said process including the steps of
i. preparing a transparent slab of material,
ii. depositing on a surface of said slab a semiconducting
layer,
iii. preparing said photoelectric device in or on said
semiconducting layer, and
iv. preparing said diffractive optical element on a surface of
said
4. The process of claim 3 whereby said semiconducting film is
grown
6. The process of claim 3 whereby said transparent slab contains
Al.sub.2
7. The process of claim 3 whereby said photoelectric device is a
sensor of
8. The process of claim 7 whereby said sensor contains at least one
p-n
9. The process of claim 3 whereby said diffractive optical element
is a
10. The process of claim 3 whereby said zone plate and said
photoelectric device are positioned on opposite surfaces of said
slab.
Description
BACKGROUND OF THE INVENTION
Transmittance of information is a basic element of modern
civilization and technology. In many cases, the primary information
is available in form of a sound pattern (voice) or else is recorded
in form of a pattern of hills and valleys (phonograph disc). In
order to communicate such information to the human ear,
intermediate steps of translating the information into electrical
signals are frequently required, viz, the telephone conversation
and the record player.
Means of translating the voice signal into an electrical signal are
called microphones. My invention concerns a new and improved
microphone by means of a new electro-optical readout of the motion
of minute surface elements of the microphone membrane. The surface
elements can be chosen as the positions of maximum amplitude in the
standing wave pattern of the membrane. Simultaneous pickup of
several selected frequencies from a single membrane is thus
possible by means of several electro-optical microtransducers,
placed at different positions of the membrane. The extremely small
size and weight of my electro-optical microtransducer, of the order
of 100 microns linear dimension and of a few micrograms,
respectively, enable the design of a system containing a large
number of mechanically sharply tuned pickup heads in a small space,
which simulates the function of the human ear and which might
eventually be useful in such important fields as providing hearing
for the deaf and the voice-activated typewriter.
Electrical readout of phonograph discs presently utilizes a
mechanical stylus for translating the hill-and-valley pattern along
the grooves of the disc into electrical signals. The present
invention concerns an optical stylus for an improved phonograph
disc readout, i.e., the intensity of a light beam is modulated by
the pattern of hills and valleys on the disc, and is translated by
a photocell into a corresponding electrical signal. An advantage of
the optical readout as compared to mechanical readout is the
absence of mechanical wear and tear and of damage by shock, which
are incidental to the use of a mechanical stylus. Furthermore,
optical readout permits coating of the grooves of the phonograph
record by a transparent layer of planar outer surface, thereby
facilitating cleaning and avoiding the accumulation of dust in the
grooves. Separate grooves for guiding the optical pickup head along
the grooves carrying the sound pattern can be provided.
Mechanical displacements are translated into corresponding
electrical signals also in test equipment for vibration or for
surface roughness. Means to achieve this are generally known as
transducers. My invention concerns an improved type of transducer
by means of modulation of the intensity of a light beam in a
suitable electro-optical arrangement. My electro-optical
microtransducer modulates comparatively large optical energies by a
mechanical displacement. Thus, there is no need for large
subsequent electrical amplification of the signal. Consequently, my
transducer is comparatively free of electrical noise and electrical
interference.
While optical means of readout of small displacements have been
known for a long time, e.g., the light beam galvanometer which
translates the motion of a mirror into a moving light spot, or else
the interferometric readout of a distance between a movable plate
and a fixed plate, these prior art techniques either require a
large amount of space, or they are not suitable for readout of
minute areas and elevations of the order of a few microns linear
dimensions, as is desirable for readout of sound recordings.
Moreover, the various parts of these known means, i.e., light
source, optical system and photocell, have so far been manufactured
individually, then subsequently assembled into a unit with obvious
loss of compactness, requiring precision workmanship, and thus
involving the possibility of error during assembly, and perhaps of
misalignment subsequent to assembly.
It is an object of this invention to provide an efficient means for
translation of mechanical displacements of a small region into
electrical signals by a compact electro-optical system.
It is another object of this invention to provide an integrated,
compact, electro-optical structure for translating mechanical
displacements of a small surface region into electrical
signals.
It is still another object of this invention to provide a means to
translate a sound pattern into a visible light pattern, indicative
of the frequencies and corresponding intensities of the sound
pattern.
SUMMARY OF THE INVENTION
Briefly, the invention consists of the combination into a compact
electro-optical structure of (i) a coherent light source, (ii) an
optical system which includes a Fresnel lens of short focal length
and which may also include a mirror or mirrors, and (iii) a
photocell of small area, whereby the surface region whose
displacement is to be translated into an electrical signal is
rigidly attached to part of that electro-optical structure. The
photocell is placed at a position in the light beam generated by
the Fresnel lens from the light of the light source, where the
light intensity varies strongly with longitudinal displacement,
i.e., with displacement in the direction of the axis of the light
beam. A longitudinal displacement of photocell versus light beam is
achieved by the aforementioned attachment of part of the
electro-optical system to the displaced surface region, while the
remaining part of the electro-optical system is attached to a rigid
frame against which the displacement of the surface region takes
place. For instance, the surface region can be used as a mirror
reflecting the light beam onto the photocell, the motion of the
surface region in direction to or from the photocell changing the
optical path length between the photocell and the Fresnel lens
through which the light beam passes before being reflected from the
surface. Or else, at least one of the elements of the assembly,
light source, Fresnel lens and photocell, can be attached rigidly
to the surface, whose displacement is to be translated into an
electrical signal.
The compactness of the microreadout structure according to my
invention is achieved: (1) by a choice of a Fresnel optical system,
since it can be made in planar form, has small lateral dimensions,
and has an extremely short focal length, and (2) by the choice of
semiconducting components for the light source and the photocell,
which can easily be fabricated by microcircuit technology in a size
of the order of one mil linear dimension. The integrated compact
structure of my invention is achieved by constructing either light
source of photocell or both from semiconducting materials, which
can be integrated with each other or with the Fresnel lens by
microcircuit technology.
Integration of two components A and B as used here means an
inseparable combination, which is already achieved during the
manufacturing process, so that component A is inseparably connected
with a part of component B, before component B is completed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates in cross section an optical arrangement
according to this invention, showing the conversion of the
displacement of an area element into an electrical signal in a
photocell, the area element serving as a mirror in the optical
arrangement.
FIG. 2 illustrates in cross section another optical arrangement
according to this invention, whereby a Fresnel lens is permanently
attached to the area element, whose displacement is converted into
an electrical signal in a photocell.
FIG. 3 illustrates in cross section another optical arrangement
according to this invention for the readout of the displacement of
extremely small area elements.
FIG. 4 illustrates a system of several electro-optical
microtransducers according to this invention.
FIG. 5 illustrates a view in direction of the light beams on the
Fresnel optical lenses of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention combines a light source, Fresnel optical means to
direct and shape the light emitted from the light source into a
suitable beam, a photoelectric cell placed into this light beam for
converting its light energy into an electrical signal, and an
arrangement to modify the light energy incident on the photocell by
the displacement of the surface region, whose motion is to be
translated into an electrical signal. Part of the invention
consists of the electro-optical system and another part of it lies
in its arrangement into a compact, partially or fully integrated
structure. First, the preferred electro-optical arrangement for a
non-integrated assembly will be described, and subsequently the
preferred means to integrate the electro-optical arrangement will
be disclosed. Since the surface regions whose displacement we wish
to translate into an electrical signal are quite small, typically
between a few microns and a few hundred microns, we shall refer to
them as surface elements.
FIG. 1 shows a cross section along the optical axis of an
electro-optical transducer according to this invention; a parallel
monchromatic light beam indicated by the rays 1 through 3 is
focused by a Fresnel lens in the form of a circular zone plate 7
into the point 14. The zone plate consists of a planar substrate 8,
which is transparent to the light beam and carries on one of its
surfaces a series of concentric opaque rings, whose intersects with
the plane of drawing are designated by 9-9', 10-10' and 11-11'. The
center of the opaque rings is the point 12. In general, there will
be more than three transparent zones between the opaque rings.
However, for purpose of explaining the functioning of this
preferred optical system, a sketch of only three zones
suffices.
Fresnel optics, of which zone plate lenses are a special case, has
been known for many years and does not in itself represent a part
of this invention. Thus, only a few comments on zone plate lenses
will be made. The purpose of the opaque rings is to focus the
parallel light beam containing the rays 1 through 3 into the point
14. The focusing action of the zone plate results from the phase
relation at the point 14 among the wavelets emitted from the points
of the transparent zones on the Fresnel lens and involves an
optical principle known as interference. To achieve interference,
the incident light beam has to be coherent. In order for the
wavelets emitted from the points 15, 16 and 17 of the transparent
zones to reinforce each other by interference, the optical path
lengths 15 to 14, 16 to 14 and 17 to 14 must differ by integer
multiples of a vacuum wavelength .lambda.. The wavelengths arrive
then in phase at 14. Their electrical field intensities can then be
added directly to obtain the total field intensity. The light
intensity is in proportion to the square of the total field
intensity. The light intensity at a point other than the image
point 14 can be determined by adding the electric field intensities
arising from the wavelets emitted from 15, 16 and 17, taking into
account their phase differences, and then squaring the resulting
field intensity. It is found that the light intensity decreases
with increasing distance from the image point 14 (at least for not
too large distances), because the individual wavelets are
increasingly out of phase with each other.
Optical path length is the geometrical length multiplied by the
index of refraction. The width of each transparent zone is such
that the optical path length from a point at the outer boundary to
14 differs from the path length from a point at the center circle
of the zone to 14 by + .lambda./4, and the optical path length from
a point at the inner boundary to 14 differs from the optical path
length from a point at the center circle to 14 by - .lambda./4.
The surface 18 whose displacement is to be recorded electrically,
is placed between zone plate 7 and the focal point 14 and extends
perpendicularly to the axis of the optical system. The light beam
converging onto 14 is reflected from the surface 18 into the point
19, which is at the image position of 14 with respect to 18. When
the plane 18 is displaced downwards into the position 20 indicated
by a dotted line, the image of the focal point moves downwards by
twice this amount to the position 21. A photocell 22, fixed in
position with respect to the zone plate, thus receives a larger
amount of light when the reflecting surface is at position 20 and
the focal point is at 21, than when the reflecting surface is at
position 18 and the focal point is at 19. Conversely, if the
reflecting surface would move a small distance upwards, i.e.,
farther away from the zone plate 7, the light intensity at the
photocell 22 would decrease. However, in the case of such a large
downward movement, that the focal point has passed through the
photocell still closer to the zone plate, the light intensity
incident on the photocell diminishes again. It is obvious from
these considerations that the maximum amplitude of the displacement
must be taken into account in choosing the offset between focal
point 19 in case of undisplaced surface 18 and the location of the
photocell 22. In general, in the arrangement of FIG. 1, the
distance between 22 and 18 must be larger than twice the maximum
displacement. The modulation of the photocell current for a given
displacement of the reflecting surface is increased with the
aperture angle of the optical system and with decreasing
light-sensitive area of the photocell. While the photocell in FIG.
1 has been placed below the focal point 19, i.e., into the
divergent beam, there is a position above the focal point 19, i.e.,
in the convergent beam, which could also be utilized.
The photoelectric cell 22 shown in FIG. 1 consists of a
semiconducting material 23, having p and n-regions separated by a
p-n junction 24. The semiconducting material 23 has a sufficiently
narrow forbidden energy band gap that the incident radiation 1 to 3
is able to generate electron-hole pairs when absorbed by the
material 23. Contacts 25 and 26 connect the photoelectric cell 22
electrically to a load resistor 27 and a power supply 28. The
structure of FIG. 1 translates the mechanical displacement of the
surface region 18 into an electrical signal in the load resistor
27. The p-n junction structure can be used also as a photovoltaic
cell, i.e., generating electric power without need for an external
power source 28.
In the lower part of FIG. 1, a semiconducting light source 30 and
an optical system 31 to shape light emitted from 30 into the
monochromatic parallel rays 1 to 3 are shown. The light source 30
consists of a wafer of single-crystal semiconducting material 32,
having n and p-regions separated by the p-n junction 33. A voltage
from a power supply 36 is applied through the electrical contacts
35 and 34, causing a current flow in the forward direction through
the junction 33, thereby generating light emission by recombination
of electrons and holes.
The optical system 31 consists of a zone plate quite similar to 7,
the light source 30 being located in its focal point. 7 and 31 must
be designed for the same wavelength. If the light source emits
radiation of a range of wavelengths, the zone plates act as a kind
of interference filter to filter out a monochromatic beam. Separate
Fresnel lenses 7 and 31 have been shown in FIG. 1 for sake of
clarity only. A single Fresnel lens can be designed to directly
produce an image of the light source 30 at the point 14, without
the need of a second Fresnel lens to produce first a parallel light
beam.
In the arrangement of FIG. 1, the displaced surface area element
acts as a mirror for the radiation. In the arrangement of FIG. 2,
the zone plate lens 39 is rigidly attached to and moves thus with
the surface area element 40. The surface 40 is a light reflecting
material, whose displacement is to be translated into an electrical
signal. The surface 40 is coated by a transparent body 41 of such a
thickness d that light reflected from its front surface and light
penetrating to its back surface reflected there, and then leaving
the front surface, differ in phase by .lambda./2, i.e., 2 dn =
.lambda./2 for the case of almost normal incidence. Here n stands
for the index of refraction of the body 41. The front surface of
the body 41 carries the metallized regions 42, 43, 44 and 45, whose
boundaries are concentric circles with 46 as center. Two rays 47
and 48 have been indicated in the drawing. These rays arrive with
equal phase at the image point 49 of the light source 52, even
though one comes from the transparent zone 50 and the other comes
from the metallized zone 43 of the zone plate. The equality of
phase is achieved by the phase shift .lambda./2 of the beam 47,
when twice penetrating the layer 41. Thus, all zones, transparent
and metallized, of the zone plate contribute to the light intensity
at 49, providing an increase by a factor 4 as compared to the case
where only the transparent zones would contribute.
The light source 52 of FIG. 2 is a p-n junction laser emitting a
highly coherent beam of radiation in a solid angle of about
10.degree. to 15.degree. along the p-n junction plane. This beam is
reflected by the mirror 53 onto the Fresnel lens 39 and imaged by
it into the point 49 in the vicinity of the photocell 51. Contacts
54 and 55 to supply electric power to the light source 52 are
indicated in FIG. 2. The p-n junction is covered on its upper
surface by an insulating film 56, on which the metal contact 57 of
the p-n junction photocell 51 is placed. The metal contact shields
the p-n junction 58 of the photocell 51 against direct illumination
from the laser 52. The second contact to the photocell 51 is
indicated by 59.
The photocell 51 is located below the image point 49. Downwards
displacement of 40 moves the image point 49 closer to 51, while
upwards displacement moves it further away, with a corresponding
increase or decrease, respectively, of the light intensity at the
position of the photocell 51.
The small lateral size of the zone plate, typically about 100
microns diameter, permits applying several readout systems as shown
in FIG. 2 to selected areas of a single microphone membrane. The
motion of several area elements of the same membrane can thus be
translated into electrical signals, each element having its own
readout arrangement of the type of FIG. 2. The readout systems can
be located at the positions of maximum amplitudes of the standing
wave patterns for different frequencies. Thus, some tone quality
selection can be accomplished already in the microphone, in
contrast to customary techniques, accomplishing such a selection in
the electrical circuitry outside of the microphone.
In the case of the vibrating membrane 40 of FIG. 2, the same
surface element is exposed to the light beam, and a portion of the
optical system consisting of light source, zone plate and
photocell, has been permanently attached to it. On the other hand,
in the case of a phonograph record and in applications such as
roughness testing, different area elements are exposed in sequence
to the light beam, and thus none of the three elements, light
source, zone plate lens and photocell, can be permanently attached
to a given area element. In these cases, the surface area element
is merely used as a mirror in the electro-optical system, similar
to the arrangement of FIG. 1. However, since the resolution of the
hill-and-valley structure of such a surface is limited by the size
of the area element used as a mirror, it is desirable to
concentrate the light beam onto a point lying in the reflecting
surface. In general, this requires two optical systems, one for
focusing the incident light beam on or very near to the surface,
and the other for collecting the reflected and scattered beam onto
the photocell.
FIG. 3 shows such an arrangement in a cross section along the
optical symmetry line. The p-n junction laser 60 consists of a
semiconducting material 61 having p and n-conductivity regions
separated by the p-n junction 62. Contacts 63 and 64 are provided
to the p and n-regions for applying a potential from an electric
power supply to pass current in the forward direction through the
p-n junction. For the laser action to occur, the current must be
sufficiently large and the surfaces 65 and 66 at which the p-n
junction terminates must be of optical quality and parallel to each
other, as can be achieved, for instance, by cleaving along parallel
crystallographic planes. A suitable material for laser action is
gallium arsenide, with radiation in the near infrared spectrum at
about 0.8 microns. The laser beam 67 emitted from the junction
region in a narrow cone passes through a solid material 68,
transparent to the wavelength of the laser beam and having planar
surfaces 69 and 70 of optical quality, ground to include a small
angle, typically of a few degrees only, in order that the laser
beam 67 meets the surface 71 at about the intersect with the axis
81 of the zone plate 80, i.e., at the point 73. The laser beam 76
passes through a circular opening 75 in the opaque metal layer 74
on the surface 70. This opening, whose center is at 76, is of such
a diameter that the point 77 is imaged into 73. To achieve this,
the opening 75 should have a radius
where L is the distance from 76 to 77, B is the distance from 76 to
73, and n is the index of refraction of the material 68. The
circular opening 75 represents a one-zone zone plate.
The horizontal arrow 78 in the upper right hand corner indicates a
lateral motion of the surface 71, as caused for instance by the
rotation of a phonograph record. Such a motion shifts the point 73
at which the laser beam 67 meets the surface 71 in a vertical
direction on account of the slope of 71 at 73.
The upper surface 70 of the transparent solid material 68 carries a
zone plate lens 80, consisting of two opaque rings, whose traces
with the plane of cross section are designated by 82-82' and
83-83', and whose center lies at 84. The optical system is designed
to concentrate the cone of radiation emitted from 73 into a point
85 in the vicinity of the photocell 86, which is located adjacent
to the surface 69. The photocell 86 consists of a material 87,
having a p-n junction 88 and the contacts 89 and 90. If the point
73 moves downwards, the image point 85 moves even more downwards
and the light intensity at the p-n junction 88 of the photocell 86
decreases.
The line 91 indicates the surface of a transparent coating 92
applied to the surface 71, which prevents accumulation of dirt in
the surface 71 and establishes a plane of reference for the spaces
94 and 93, which are attached to 68 and glide over the surface
91.
FIG. 4 shows another preferred arrangement, which locates light
source 100, zone plate 101 and photocell 102 in three parallel
planes along an optical axis normal to these planes. Light from the
light source 100 is imaged by 101 onto the photocell 102, as
indicated by the optical rays bearing arrows.
Either one of three elements, 100, 101, 102, might be attached to a
vibrating system, causing a change in the light flux on the
photocell. However, vibration of the zone plate is preferred, since
the zone plate does not involve electrical contacts. The
arrangement of FIG. 4 is particularly useful, where a large number
of microtransducers operating at different frequencies are
required. Three such transducers, having the elements 100, 101,
102; 103, 104, 105; and 106, 107, 108, are shown, the light sources
100, 103 and 106 being in the plane 109, and the photocells 102,
105 and 108 being in the plane 110. The zone plate 104 is displaced
relative to the plane of 101 and 107, indicated by the dotted line
5'-5", and this displacement causes a diminished light flux on the
photocell 105.
Large arrays of photocells and of light sources can be made
conveniently by microcircuit technology. Large arrays of zone plate
lenses can be made by the same technology on a transparent
substrate. By etching slots of suitable length and width, each zone
plate can be positioned on a stem tuned to a particular vibrational
resonance frequency.
FIG. 5 shows a top view of three such zone plate stems lying in the
plane 5'-5" of FIG. 4. The transparent material 111 has slots 115,
116, 117 and 118, to provide the stems 119, 120, 121, on whose
upper parts are located opaque concentric rings, constituting the
zone plates 101, 104 and 107. The dimensions of the stems are
chosen to have the desired resonance frequency. Considering the
size of a zone plate of only about 100 microns diameter, and a slot
width of about 25 microns, it is seen that about 80 resonance
frequencies can be accommodated side by side on a length of 1
centimeter, corresponding roughly to the number of individual
frequencies available on a piano. The responses of the photocells
can be used to govern the light intensity of a television screen,
each position of the screen corresponding to a particular photocell
and thus a vibrational frequency. In this manner, an acoustical
signal can be translated into a visual one. We may even go a step
further and replace the set of photocells in FIG. 4 by the human
eye, thus translating the sound pattern directly into a visual
pattern. For this purpose, the photocell (eye) is placed exactly in
the image position of the light source for the undisplaced zone
plate, so that displacement in either direction causes a decrease
in the light intensity at the receiver. This is necessary, if the
receiver is capable of perceiving time averages only, as is the
case with the human eye for most of the acoustical frequencies.
However, a receiver which can follow the vibrational frequencies,
will then generate an electrical signal at twice the frequency of
vibrations.
Having explained the principles of the electro-optical
microtransducer on hand of several preferred embodiments, we shall
now provide quantitative design data for an actual zone plate
optics as may be used in this invention.
Consider first a zone plate lens to focus a plane parallel
monchromatic beam of normal incidence into a point of distance z
from the zone plate. This is the situation of the zone plate lens 7
in FIG. 1. The condition for selecting the circles at the centers
of the zones is R.sub. p = R.sub. o + p.lambda., where p is an
integer, R.sub. o is the distance from the center circle of the
innermost zone to the focal point, and R.sub. p is the distance
from the center circle of the zone of index p to the focal point.
Designating the radii of the circles in the centers of the zones by
r.sub. o and ##SPC1##
p b.sub.p (microns) a.sub.p (microns) r.sub.p (microns)
__________________________________________________________________________
0 18 22 20 1 25.4 28.5 27 2 31.3 33.7 32.5 3 35.9 38 37 4 40 42 41
__________________________________________________________________________
In the case that we wish to construct a lens for imaging a point at
the distance z in front of a lens in a medium of index of
refraction n into a point a distance z behind the lens in air, we
have to divide each distance for the lens described in the above
table by (1+ n). For instance, in case of the same wavelength
.lambda. = 0.8 microns as previously, and n = 1.74 (sapphire), we
have to divide by 2.74 and then arrive at z = 200/2.74 .apprxeq. 73
microns, and r.sub. o = 20/2.74 = 7.3 microns. In this manner, the
zone plate lens 80 and distances 85 to 84 and 84 to 73 in FIG. 3
can be chosen. The transparent center disc in the lens 80 in FIG. 3
has then a radius of 5.0 microns. This is also the radius of the
one-zone disc 75.
Next, we shall describe quantitatively the change in light energy
at the optical axis, as we move away from the focal point z by a
distance .delta., considering again a plane parallel beam of normal
incidence on the lens. The decrease in light intensity arises
primarily (small second-order effects will be ignored here) from
the fact that the wavelets emitted from the various zones are
increasingly out of phase. For instance, the path difference
##SPC2##
2.pi. p.delta./(z + .delta.) = 2.pi., i.e., .delta. = z/(p - 1) for
the zone of largest index p, the preferred phase angle relationship
of zones between the innermost and the p.sup. th zone is completely
lost and their contributions to light intensity vanish by
interference. Thus, the useful range of .delta. for modulations of
the light intensity is 0 .ltoreq. .delta. .ltoreq. z/(N - 2), where
N = p + 1 is the number of zones, assuming that zones of all
indices p up to a maximum value have been used. In the case of N =
5 and z = 200 microns, 0 < .delta. < 66 microns. This
suggests a displacement of about 30 microns between the position of
the photocell and the focal point in case of membrane at its
average, i.e., rest position. The usable range of membrane
displacement is then about .+-. 10 microns, considering that the
mirror action of the membrane 18 in FIG. 1 shifts the displacement
of the focal point by .+-. 20 microns. The analysis of the
dependence of light energy for lateral displacements y from the
optical axis has to take into account the interference effects
among rays emitted from different points of the same circle r. It
is well known that this leads to a Bessel function of zero order of
the argument y r 2.pi./z.lambda., which passes through zero at a
value 2.4 of the argument, i.e., at y = 2.4 z.lambda. /r 2.pi..
Next, we shall describe the preparation of the various components
with particular emphasis on integration of the various components.
Integration of light source, photocell and zone plate is possible
since they can be made from compatible materials, i.e., solid
materials which can be bonded to each other on account of
compatible physical and chemical properties. Examples of such
compatible materials are: gallium arsenide as material for light
source and germanium as material for the photocell; or else,
sapphire as material for the transparent body 8 in FIG. 1, which is
the substrate for an epitaxial silicon film as the semiconducting
material 23 of the photocell 22.
Integration of the electro-optical micro-readout system is
accomplished by combining at least two of the three components,
light source, Fresnel lens and photoelectric cell, into a compact,
inseparable, solid structure. Such an integration is possible by
the use of compatible technologies in the production of light
source, zone plate and photocell. The compatible technologies are
those used for semiconductor microcircuits. Semiconductor light
sources operating on the principle of p-n junction injection, and
semiconducting photocells such as p-n junction photoelements, can
be prepared by microcircuit technology, which includes the
so-called photoresist technique. Preparation of zone plates
involves deposition of well-defined metallized areas on a
transparent substrate. Well-defined metallized areas, e.g., for the
gate electrode configurations of silicon MOS transistors, are made
on the transparent silicon oxide by the photoresist technique.
Definition of boundaries of microcircuit electrode configurations
to a precision better than 1 micron can be achieved. Thus the
photoresist technique is eminently suitable for preparing zone
plate lenses.
As a specific example, we shall discuss the integration of the zone
plate 7 in FIG. 1 with the photocell 22. We start with a plane
parallel optically polished slab of sapphire 8. On the surface of
this slab an epitaxial single-crystal layer of silicon 23 is
deposited by a well-known high temperature vapor deposition
process. The epitaxial silicon layer is processed into the p-n
junction structure of the photocell 22 by standard microcircuit
technology. Using the photoresist technique, the silicon layer is
selectively removed from the outer regions of the sapphire and the
Fresnel lens 8 is then deposited by metallization and selective
etching, using again the photoresist technique. In this manner, an
integrated, inseparable combination of zone plate lens and
photocell is produced by using the compatible technologies of
vaporization, photoresist, selective etching and epitaxial
deposition.
An integrated structure of the p-n junction laser light source 60
and photocell 86 shown in FIG. 3 can be made as follows:
We start with a gallium arsenide injection laser 60. We then
deposit on one of its surfaces, parallel to the junction, an
epitaxial germanium film 87. A single crystal germanium film can be
deposited on a single crystal gallium arsenide because of a
similarity in crystal lattice structure. The p-n junction 88 is
produced in the germanium film by suitable impurity diffusion into
selected areas, the remaining part of the surface of the film being
protected against diffusion by a silicon oxide film.
A fully integrated structure, encompassing light source, zone plate
optics and photocell, can be made as follows: The light emitting
surface of the integrated combination of gallium arsenide light
source and germanium photocell just described and shown in FIG. 3,
is coated with a vapor deposited silicon oxide film of a few
microns thickness, and is then fused to a low melting point glass
body. The outer surface of this glass body is then provided with
the zone plate optical systems 75 and 80 by metal deposition and
selective removal using the photoresist technique.
The numerical example for a zone plate lens given previously
concerned rings of a diameter less than 100 microns. The thickness
of semiconducting microcircuit components usually arises from
mechanical considerations rather than electrical considerations.
For mechanical support, a substrate thickness of about 100 microns
is sufficient. Thus, electro-optical microtransducers can be made
in a size of about (100 microns).sup.3 = 10.sup. .sup.-6 cm.sup.3.
Considering an average density of about 5 g/cm.sup.3, we arrive at
a weight of only 5 micrograms. Taking into account the great
difficulties in assembling and aligning separate components of an
optical system of such a small size, the advantage of using an
integrated electro-optical system becomes obvious.
For purposes of illustration, we have selected photocells and light
sources of the semiconductor p-n junction type. It should be
understood, however, that this invention is not limited to these
particular components. For instance, microplasms, generated in
semiconductor vs. metal junctions or in MOS transistors by the high
field avalanche process, can be used also as the light source of my
invention. Photocells of the polycrystalline film type, e.g., the
well-known cadmium sulphide photoconductive cells, can be used
instead of p-n junction type photocells. It should also be stressed
that p-n junction light sources in the sub-laser regime, are quite
useful for my invention, and the high intensity, high
monochromaticity and high coherence of laser beams is not
necessarily required in most cases.
While zone plate lenses of radial symmetry have been shown in FIGS.
1 through 5 for purposes of illustration, linear zone plate
gratings consisting of a set of parallel opaque lines of
appropriate spacings and widths can be used also. Furthermore,
Fresnel optical systems need not be limited to structures having
gratings of materials of different optical properties. For
instance, it is well known that the sum of distances between the
focal points of a rotational ellipsoid and any point on its surface
is a constant. Consider a coherent monochromatic light source
placed at one focal point and a photocell at the other. All rays
arriving at the photocell from points of the surface of the
ellipsoid are then in phase, i.e., the ellipsoid acts as Fresnel
optical lens for our purposes. Displacement of light source,
ellipsoid or photocell causes phase differences between various
beams, resulting in a decreased light intensity at the photocell.
Thus, the zone plate lens on the membrane in FIG. 2 might be
replaced by a section of a rotational ellipsoid concave toward
light source and photocell, which are positioned side by side as in
FIG. 3 in a plane parallel to the membrane at or near the positions
of the focal points of said ellipsoid, the vibration of the
membrane changing the distances between the focal points and light
source and photocell. The section of the rotational ellipsoid can
be metallized to provide reflection. While the arrangement in
question will operate also on the principle of ordinary reflection
optics, the dependence of photocell signal on displacement is much
stronger for the case of Fresnel optics, i.e., interference
optics.
While we have dwelt in detail on the translation of vibrational
energy into electric energy by means of a light beam and a
photocell, it is obvious that my invention can be utilized also to
translate vibrational energy into a pattern of varying optical
density on a photographic film by merely replacing the stationary
photocell by a photographic film moving in a direction
perpendicular to the optical axis. Thus, my invention is eminently
suited for a movie-sound recording system.
As many apparently widely differing embodiments of my invention may
be made without departing from the spirit and scope thereof, it is
to be understood that my invention is not limited to the specific
embodiments hereof, except as defined in the appended claims, in
which photoelectric device includes any structure for transferring
radiant energy into electric circuit energy or vice versa;
diffractive optical element means any structure for shaping
optically a coherent beam of radiation incident on said structure
by diffraction on said structure; integrated monolithic structure
means a rigid solid structure which cannot be disassembled into its
components such as light source, optical element or photocell,
without destruction of at least one of said components.
* * * * *