U.S. patent application number 09/968090 was filed with the patent office on 2003-04-03 for active optical system for phase-shifting desired portions of an incoming optical wavefront.
Invention is credited to Hunt, Jeffrey H..
Application Number | 20030063366 09/968090 |
Document ID | / |
Family ID | 25513718 |
Filed Date | 2003-04-03 |
United States Patent
Application |
20030063366 |
Kind Code |
A1 |
Hunt, Jeffrey H. |
April 3, 2003 |
ACTIVE OPTICAL SYSTEM FOR PHASE-SHIFTING DESIRED PORTIONS OF AN
INCOMING OPTICAL WAVEFRONT
Abstract
An active optical system and method for phase-shifting desired
portions of an incoming optical wavefront. A first control optics
assembly receives an incoming optical wavefront and adjusts that
incoming optical wavefront in accordance with first desired
wavelength and beam propagation parameters. A driver element
produces a driver optical wavefront. A second control optics
assembly receives the driver optical wavefront and adjusts that
driver optical wavefront in accordance with second desired
wavelength and beam propagation parameters. A combiner receives an
output from the first control optics assembly and an output from
the second control optics assembly. The combiner provides a
combined, co-linear propagation output wavefront having an initial
beam size. Spatial light modulator (SLM) addressing optics receives
the combined, co-linear propagation output wavefront and produces a
desired beam size for the combined, co-linear propagation output
wavefront. The SLM receives the output from the SLM addressing
optics and provides localized phased shifting of the resulting
wavefront. SLM egressing optics receives the output of the SLM and
returns the beam size of the wavefront to the initial beam size.
The output of the SLM egressing element has desired portions of its
phase shifted relative to the incoming optical wavefront.
Inventors: |
Hunt, Jeffrey H.;
(Chatsworth, CA) |
Correspondence
Address: |
JOHN R. RAFTER
THE BOEING COMPANY
2201 SEAL BEACH BOULEVARD
P.O.BOX 2515
SEAL BEACH
CA
90740-1515
US
|
Family ID: |
25513718 |
Appl. No.: |
09/968090 |
Filed: |
October 1, 2001 |
Current U.S.
Class: |
359/279 |
Current CPC
Class: |
G02F 1/015 20130101;
G02B 26/06 20130101; G02F 2203/18 20130101 |
Class at
Publication: |
359/279 |
International
Class: |
G02F 001/01 |
Claims
1. An active optical system for phase-shifting desired portions of
an incoming optical wavefront, comprising: a) a first control
optics assembly for receiving an incoming optical wavefront and
adjusting that incoming optical wavefront in accordance with first
desired wavelength and beam propagation parameters; b) a driver
element for producing a driver optical wavefront; c) a second
control optics assembly for receiving said driver optical wavefront
and adjusting that driver optical wavefront in accordance with
second desired wavelength and beam propagation parameters; d) a
combiner for receiving an output from the first control optics
assembly and an output from the second control optics assembly,
said combiner providing a combined, co-linear propagation output
wavefront having an initial beam size; e) spatial light modulator
(SLM) addressing optics for receiving the combined, co-linear
propagation output wavefront and producing a desired beam size for
the combined, co-linear propagation output wavefront; f) an SLM for
receiving the output from the SLM addressing optics and providing
localized phased shifting of the resulting wavefront; and, g) SLM
egressing optics for receiving the output of the SLM and returning
the beam size of the wavefront to the initial beam size, the output
of the SLM egressing optics having desired portions of its phase
shifted relative to the incoming optical wavefront.
2. The active optical system of claim 1, wherein said first control
optics assembly, comprises: a) a first wavelength control element
for receiving the incoming optical wavefront; and, b) a first
propagation control element for receiving the output of the
wavelength control element and providing an output to said
combiner.
3. The active optical system of claim 2, wherein said first control
optics further includes a first polarization control element.
4. The active optical system of claim 1, wherein said driver
element comprises a laser.
5. The active optical system of claim 1, wherein said driver
element comprises a light emitting diode (LED).
6. The active optical system of claim 1, wherein said driver
element comprises a broadband optical light source.
7. The active optical system of claim 1, wherein said second
control optics assembly, comprises: a) a second wavelength control
element for receiving the driver optical wavefront; and, b) a
second propagation control element for receiving the output of the
second wavelength control element and providing an output to said
combiner.
8. The active optical system of claim 1, wherein said first control
optics assembly further includes a second polarization control
element.
9. The active optical system of claim 1, wherein said combiner
comprises a beamsplitter.
10. The active optical system of claim 1, wherein said combiner
comprises a dichroic optic.
11. The active optical system of claim 1, wherein said combiner
comprises a diffraction grating.
12. The active optical system of claim 1, wherein said SLM,
comprises: a) an avalanche photodiode; b) an electric field across
the photodiode in excess of the breakdown field to cause
avalanching of electrons in the photodiode when the photons from
the driver optical wavefront strike the photodiode, wherein the
avalanching electrons induce a photorefractive response which
changes the index of the index refraction in the photodiode; and,
c) a circuit for regulating the electric field applied across the
photodiode, wherein a thermo-optic response causes a change in the
index of refraction in the photodiode.
13. A method for phase-shifting desired portions of an incoming
optical wavefront, comprising the steps of: a) adjusting an
incoming optical wavefront in accordance with first desired
wavelength and beam propagation parameters; b) producing a driver
optical wavefront; c) adjusting said driver optical wavefront in
accordance with second desired wavelength and beam propagation
parameters; d) combining the adjusted incoming optical wavefront
and the adjusted driver optical wavefront to provide a combined,
co-linear propagation output wavefront having an initial beam size;
e) producing a desired beam size for the combined, co-linear
propagation output wavefront; h) providing localized phased
shifting of the beam size modified combined, co-linear propagation
output wavefront; and, i) returning the combined, co-linear
propagation output wavefront to said initial beam size, the
resulting optical wavefront having desired portions of its phase
shifted relative to the incoming optical wavefront.
14. The method of claim 13, wherein said step of adjusting said
incoming optical wavefront comprises: utilizing a first wavelength
control element for receiving the incoming optical wavefront; and,
utilizing a first propagation control element for receiving the
output of the first wavelength control element.
15. The method of claim 14, wherein said step of adjusting said
incoming optical wavefront comprises: utilizing a second wavelength
control element for receiving the driver optical wavefront; and,
utilizing a second propagation control element for receiving the
output of the second wavelength control element.
16. An active optical system for phase-shifting desired portions of
an incoming optical wavefront, comprising: a) a first control
optics assembly for receiving an incoming optical wavefront and
adjusting that incoming optical wavefront in accordance with first
desired wavelength and beam propagation parameters, said first
control optics comprising first wavelength control element for
receiving the incoming optical wavefront, a first propagation
control element for receiving the output of the wavelength control
element; and a first polarization control element for receiving the
output of said first propagation control element; b) a driver
element for producing a driver optical wavefront, said driver
element comprising a laser; c) a second control optics assembly for
receiving said driver optical wavefront and adjusting that driver
optical wavefront in accordance with second desired wavelength and
beam propagation parameters; d) a combiner for receiving an output
from the first control optics assembly and an output from the
second control optics assembly, said combiner providing a combined,
co-linear propagation output wavefront having an initial beam size;
e) spatial light modulator (SLM) addressing optics for receiving
the combined, co-linear propagation output wavefront and producing
a desired beam size for the combined, co-linear propagation output
wavefront; f) an SLM for receiving the output from the SLM
addressing optics and providing localized phased shifting of the
resulting wavefront; and, g) SLM egressing optics for receiving the
output of the SLM and returning the beam size of the wavefront to
the initial beam size, the output of the SLM egressing element
having desired portions of its phase shifted relative to the
incoming optical wavefront.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to active optical systems and more
particularly to an active optical system for phase-shifting desired
portions of an incoming optical wavefront.
[0003] 2. Description of the Related Art
[0004] Many types of active optical systems require the control of
the optical wavefront or phase of a propagating laser beam. When an
image propagates through turbid media, for example, the atmosphere,
random fluctuations in the local index of refraction cause local
fluctuations in the optical path length that the beam traverses.
These fluctuations in path length lead to a randomization of the
phase front contour, causing the image to be obscured. Using an
adaptive optics or active optical control, the original phase state
is restored, allowing the reconstruction of the original image. In
the case of optical communications, the same kind of randomization
can occur. In this case, the adverse result is that the optical
beam cannot be focussed to a diffraction limited (limited by
wavelength) spot, causing loss of information when the beam is
introduced into a small diameter optical element, for example, an
optical fiber. Active control and adaptive optics in this scenario
allows one to reconstruct the original phase state so that the beam
can be focussed to a small spot without loss of information.
Typically, active optical systems make use of adaptive optical
elements that are based on mechanical implementation. One example
of this is a deformable mirror. The mirror contains a number of
small actuators which push or pull on the mirror surface. In doing
so, they compensate for the distortions in the beam phase by making
some parts of the optical path shorter and some parts of the
optical path longer. However, this implementation takes what is
fundamentally an optical problem and turns it into a mechanical
problem. It is desirable to use a non-mechanical system to
accomplish the phase-shifting needed to recreate the original phase
state of the optical beam.
[0005] There have been previous patents to use electro-optical
means to perform adaptive optical processes. U.S. Pat. No.
5,396,364, entitled CONTINUOUSLY OPERATED SPATIAL LIGHT MODULATOR
APPARATUS AND METHOD FOR ADAPTIVE OPTICS, issued to O'Meara et. al,
discusses the use of a spatial light modulator for
electro-optically addressed adaptive optics. A standard SLM is
described, that incorporates an electronically "pixelized"
modulator. The device incorporates a microlenslet array to
physically separate the wavefront into small active areas that form
the pixels. This device has several disadvantages. The electronic
structure must be built directly into the device, causing greater
difficulty in manufacture and limiting the resolution of the device
to the number of electronic structures created. Also, since the
modulation is caused by electronically driven means, instead of
being optically driven, the speed of the device has inherent
limitations.
[0006] U.S. Pat. No. 6,222,667, entitled ELECTRO-OPTIC LIGHT VALVE
ARRAY, issued to Gobeli et, discloses a two-dimensional light valve
array. It uses a pixelized substrate made of lanthanum modified
zirconate-titanate. Electrodes are cut into recesses made in the
substrate. Voltages which are applied to the individual pixels
induce bi-refringence into the pixelized regions. Electronic
control of the bi-refringence affects the light transmittance. The
inventor does not discuss control of phase or wavefront in this
device. As in O'Meara et.al. the device must be pixelized and
electronic driving limits the speed at which controls can be
performed.
SUMMARY
[0007] The present invention is an active optical system and method
for phase-shifting desired portions of an incoming optical
wavefront. A first control optics assembly receives an incoming
optical wavefront and adjusts that incoming optical wavefront in
accordance with first desired wavelength and beam propagation
parameters. A driver element produces a driver optical wavefront. A
second control optics assembly receives the driver optical
wavefront and adjusts that driver optical wavefront in accordance
with second desired wavelength and beam propagation parameters.
[0008] A combiner receives an output from the first control optics
assembly and an output from the second control optics assembly. The
combiner provides a combined, co-linear propagation output
wavefront having an initial beam size. Spatial light modulator
(SLM) addressing optics receives the combined, co-linear
propagation output wavefront and produces a desired beam size for
the combined, co-linear propagation output wavefront. The SLM
receives the output from the SLM addressing optics and provides
localized phased shifting of the resulting wavefront. SLM egressing
optics receives the output of the SLM and returns the beam size of
the wavefront to the initial beam size. The output of the SLM
egressing element has desired portions of its phase shifted
relative to the incoming optical wavefront.
[0009] The present performs phase control on an optical wavefront
without utilizing a deformable mirror to compensate for phase
distortions produced by atmospheric conditions. By altering the
manner in which the imaging device is addressed, the local
refractive index of the two-dimensional medium can be used to
modulate or demodulate the wavefront at a single position within
the wavefront. This results in a phase compensated wavefront.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic view of a preferred embodiment of the
active optical system of the present invention.
[0011] FIG. 2 (Prior Art) is a cross-sectional view of a spatial
light modulator utilized by the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0012] Referring to the drawings and the characters of reference
marked thereon FIG. 1 illustrates a preferred embodiment of the
present invention, designated generally as 10. An incoming optical
wavefront 10 is received by a first control optics assembly,
designated generally as 12. The wavefront of an optical beam is
generally described as the contour of constant phase over the
physical cross-section of the beam. Although any optical source
will have a phase associated with it, in most cases, sources will
be objects which are being imaged or will be from transmitters of
digitally encoded optical beams. The first control optics assembly
12 adjusts the incoming optical wavefront 10 in accordance with
desired wavelength and beam propagation parameters. These
parameters could include, for example, precise wavelength filtering
to the expected signal wavelength, the optical bandwidth of the
incoming signal, or the polarization of the light. The wavelength
may be controlled to fit within the detection range of the
photodiode. It may be more precisely filtered to fit a known input
signal, either from an image or from a digitally encoded
communication beam. The present invention operates with wavelengths
longer than 1 micron. The preferred minimum wavelength is about 1.1
micron. Optical signals from other sources at shorter wavelengths
will effect the operation of the device and should therefore be
eliminated. The assembly 12 preferably includes wavelength control
element 14 such as a color filter, an etalon, a Fabry-Perot
interferometer, a Fizeau interferometer, a diffraction grating, or
a notch filter, etc. A polarization control element 16 polarizes
the wavefront. This may comprise, for example, a polarization
plate, a Brewster's angle polarizer, or a thin film polarizer. The
precise polarizer to be selected depends on the particular
application's engineering requirements such as polarization
rejection ratio, size and weight of the polarizer, and the
wavelength range over which the detector must operate, etc. The
wavefront is then received by a propagation control element 18 such
as a single lens, double lens, refractive elements, reflective
elements or other system up to a fully engineered telescope.
[0013] A driver element 20 for encoding produces a driver optical
wavefront 22. The driver element may comprise, for example, a
laser, a light emitting diode (LED), or broadband optical light
source.
[0014] A second control optics assembly 24 adjusts the driver
optical wavefront 22 in accordance with desired wavelength and beam
propagation parameters. The assembly 24 preferably includes
wavelength control element 26 such as a color filter, an etalon, a
Fabry-Perot interferometer, a Fizeau interferometer, a diffraction
grating, or a notch filter. A polarization control element 28 and a
propagation control element 30 are utilized, as described
above.
[0015] A combiner 32 receives the output 34 from the first control
optics assembly 12 and the output 36 from the second control optics
assembly 24. The combiner 32 provides a combined, co-linear
propagation output wavefront 38 having an initial beam size. In
order for the phase-shifting to occur, the affected and driver
beams must be physically registered in propagation space. The
registration in propagation direction is achieved with appropriate
timing, that is, the pulses enter the phase-shifter overlapped in
time. The registration in the other two dimensions is accomplished
by overlapping the physical cross-sections of the beams. The
combiner 32 allows for this overlapping in cross-section to take
place. The combiner 32 may include, for example, a beamsplitter, a
dichroic optic, or a diffraction grating.
[0016] To provide the correct beam size for matching the SLM, an
SLM addressing optics 40 are provided. The SLM addressing optics 40
may include, for example, a plurality of lenses or curved
reflectors. Typically, the SLM 42 will be on the order of 1
millimeter in diameter, although this may vary somewhat depending
on the application. The active area of the SLM is the only place
where the desired phase-shifting physical effect can take place.
Consequently, this step is essential to assure that both affected
and driver beams enter that area of the detector.
[0017] The SLM 42 receives the output from the SLM addressing
optics 40 and provides localized phased shifting of the resulting
wavefront, as will be described in greater detail below. SLM
egressing optics 44 receives the output of the SLM 42 and returns
the beam size of the wavefront to the initial beam size, i.e. the
beam size of output 38. The resulting waveform 46 has desired
portions of its phase shifted relative to the incoming optical
wavefront 10. Further optical processes may require a beam diameter
that differs from that required for the SLM. These optics allow for
the modification of the beam diameter or spot size.
[0018] Referring now to FIG. 2 a preferred embodiment of the SLM 42
is illustrated. This is fully described and claimed in U.S. Pat.
No. 5,521,743, issued to Holmes et al, incorporated herein by
reference. This Figure shows a cross section of a three-layer
photon counting photorefractive spatial light modulator with
avalanche photodiode structure. A photon 48 is shown striking a
positive doped semiconductor layer 50, causing an avalanche 52 of
electrons to be released, the second layer is either a negative
layer or an insulator 54, and the third layer is a negative layer
56. A charge is placed across the device by electrodes 58 and 60
connected to voltage source 62 and circuit resistance 64. In this
manner electric field 65 is created across the device and if the
photodetector is properly designed, it can be operated in the
Geiger mode.
[0019] The overall performance of the device is enhanced by
hot-carrier assisted absorption, the Franz Keldysh effect and by
Gunn domain formation. These effects enhance the photoionization
and avalanche gain. A spiked or alternating voltage waveform can
also increase the sensitivity of the device. The device can be
stacked in parallel or in series for improved primary electron
quantum efficiency or multi-wavelength operation.
[0020] Avalanche photoelectron gain and ohmic heating are combined
to drive the thermo-optic effect, as may be observed in existing
silicon avalanche photodiodes. Using the avalanche process, and
operating in the Geiger mode, one photon can cause the excitation
of hundreds of millions of carrier electrons in a semiconductor. By
utilizing the electrical energy supplied by external fields, the
optical energy of a single absorbed quantum is multiplied
sufficiently to induce a change of the optical properties of the
spatial light modulator material. The localized current causes
localized ohmic heating; the heating modifies the local carrier
density and electronic structure. This results in a refractive
index change that is proportional to the average supplied
electrical current. Since the refractive index change is so
localized no pixellation is required allowing for simplicity of
fabrication and low cost manufacturing. The localized change in the
index of refraction causes a localized change in the optical path
length at that position in the wavefront. Consequently, only those
localized positions, as addressed by the driver beam, will
experience the change in optical path length. The wavefront will
then be phase-shifted at the position in question as a result of
the optical path length change.
[0021] This present invention can be used in a number of optical
applications. For example, suppose a user is imaging an object
through a long distance in the atmosphere. Without a phase
correction, the image will be smeared out and its features may be
unresolvable. With the present active phase corrector in place, the
scrambled optical phase can be reconstructed to its original
condition before the atmosphere introduced aberrations. Another use
is for long-distance optical communications. When an optical signal
is received, it is introduced into an optical fiber for signal
handling and processing. Unfortunately, phase distortions will not
allow the beam to be focused to a small spot, so that not all the
signal will go into the fiber, causing a loss of encoded
information. Phase correction enables the entire optical signal to
be focused into the fiber. In optical microlithography used in
semiconductor processing, it is essential to hold the laser to a
small controlled focus spot. The environment in which the
processing occurs causes huge distortions to occur and phase
control is essential for good yields. This device can be used in
that environment to compensate for phase distortions that are
caused there.
[0022] Thus, while the preferred embodiments of the devices and
methods have been described in reference to the environment in
which they were developed, they are merely illustrative of the
principles of the inventions. Other embodiments and configurations
may be devised without departing from the spirit of the inventions
and the scope of the appended claims.
* * * * *