U.S. patent application number 11/535781 was filed with the patent office on 2008-10-23 for non-electronic radio frequency front-end with immunity to electromagnetic pulse damage.
This patent application is currently assigned to THE REGENTS OF THE UNIVERSITY OF CALIFORNIA. Invention is credited to Bijan Houshmand, Chia-Jen Hsu, Bahram Jalali.
Application Number | 20080260323 11/535781 |
Document ID | / |
Family ID | 39872269 |
Filed Date | 2008-10-23 |
United States Patent
Application |
20080260323 |
Kind Code |
A1 |
Jalali; Bahram ; et
al. |
October 23, 2008 |
NON-ELECTRONIC RADIO FREQUENCY FRONT-END WITH IMMUNITY TO
ELECTROMAGNETIC PULSE DAMAGE
Abstract
A non-electronic all-dielectric (NEAD) or non-electronic RF
(NERF) front-end that exploits isolation features of photonics to
eliminate metal electrodes, interconnects and the antenna. An
electro-optic (EO) modulator is integrated with a dielectric
resonance antenna to exploit unique isolation features of
photonics. A doubly (RF and optical) resonant device design
maximizes the receiver sensitivity. High-Q optical disk resonators
and dielectric resonant antennas are integrated to create an
efficient mixing of light and RF fields. The resulting
non-electronic RF technology produces an all-dielectric RF
front-end which provides complete isolation between the air
interface and the ensuing electronic circuitry, enabling the
creation of an RF receiver that is immune to high-power
electromagnetic pulses (EMP) and High Power Microwave (HPM) pulses.
The device can also be configured as a non-intrusive field probe
that co-exists with a conventional receiver and detects a EMP or
HPM attack.
Inventors: |
Jalali; Bahram; (Los
Angeles, CA) ; Hsu; Chia-Jen; (Los Angeles, CA)
; Houshmand; Bijan; (Pasadena, CA) |
Correspondence
Address: |
JOHN P. O'BANION;O'BANION & RITCHEY LLP
400 CAPITOL MALL SUITE 1550
SACRAMENTO
CA
95814
US
|
Assignee: |
THE REGENTS OF THE UNIVERSITY OF
CALIFORNIA
Oakland
CA
|
Family ID: |
39872269 |
Appl. No.: |
11/535781 |
Filed: |
September 27, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60721269 |
Sep 27, 2005 |
|
|
|
Current U.S.
Class: |
385/12 |
Current CPC
Class: |
H01Q 9/0485 20130101;
G01R 29/0885 20130101 |
Class at
Publication: |
385/12 |
International
Class: |
G02B 6/00 20060101
G02B006/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with Government support under Grant
No. FA8750-05-1-0101 (AFRL) awarded by the Defense Advanced
Research Projects Agency (DARPA). The Government has certain rights
in this invention.
Claims
1. An apparatus, comprising: a dielectric resonance antenna
configured to operate as a concentrator for the received
radio-frequency power; and an electro-optic resonant modulator
integrated with said dielectric resonance antenna; wherein said
electro-optic resonant modulator is configured for receiving
radio-frequency power from said dielectric resonance antenna
through Pockels effect; and wherein both said electro-optic
resonant modulator and said dielectric resonance antenna are
non-electronic structures comprising all-dielectric materials
without either metal electrodes or metal transmission lines.
2. An apparatus as recited in claim 1, wherein said electro-optic
resonant modulator comprises a microdisk resonator.
3. An apparatus as recited in claim 2, wherein said microdisk
resonator comprises a crystal material selected from the group
consisting of lithium niobate (LiNbO.sub.3) and lithium tantalite
(LiTiO.sub.3).
4. An apparatus as recited in claim 2, wherein said microdisk
resonator has a curved sidewall, a radius of approximately 1 mm,
and a thickness of approximately 200 micron.
5. An apparatus as recited in claim 1, wherein said resonator
comprises a Fabry-Perot resonator.
6. An apparatus as recited in claim 5, wherein said resonator
comprises a crystal material selected from the group consisting of
lithium niobate (LiNbO.sub.3) and lithium tantalite
(LiTiO.sub.3).
7. An apparatus as recited in claim 1, further comprising means for
reverse poling to break symmetry of resonance in half of the
electro-optic resonant modulator.
8. An apparatus as recited in claim 1: wherein said dielectric
resonance antenna includes a cavity; wherein said electro-optic
resonant modulator is positioned in said cavity; and wherein said
electro-optic resonant modulator has a sidewall spaced apart from
said dielectric resonance antenna by an air gap.
9. An apparatus as recited in claim 8, wherein said dielectric
resonance antenna comprises: a first segment having said cavity;
and a second segment bonded to said first segment.
10. An apparatus as recited in claim 1, further comprising: a prism
optically coupled to said electro-optic resonant modulator; an
input fiber optically coupled to said prism; and an output fiber
optically coupled to said prism.
11. An apparatus as recited in claim 10, wherein said prism, said
input fiber and said output fiber are integrated with said
dielectric resonance antenna.
12. An apparatus as recited in claim 1, wherein said apparatus is
an unpowered passive device.
13. An apparatus as recited in claim 1, wherein said apparatus
provides a high degree of immunity from damage to interconnected
devices from high power electromagnetic pulses (EMP).
14. An apparatus as recited in claim 1, wherein said apparatus is a
component of a device selected from the group consisting of a
non-intrusive field sensor that does not require a metal antenna,
metal interconnects, or metal electrodes, a radio frequency
receiver front-end that does not require a metal antenna, metal
interconnects, or metal electrodes, a high-altitude nuclear
electromagnetic pulse immune field probe, and a remote radio
frequency sensor.
15. An apparatus, comprising: a dielectric resonance antenna; and
an electro-optic resonator; said dielectric resonance antenna
comprising a first segment and a second segment bonded to said
first segment; said first segment of said dielectric resonance
antenna having a cavity; said resonator positioned in said cavity;
said resonator having a sidewall spaced apart from said dielectric
resonance antenna by an air gap.
16. An apparatus as recited in claim 15: wherein said resonator
comprises a microdisk resonator; and wherein said microdisk
resonator comprises a crystal material selected from the group
consisting of lithium niobate (LiNbO.sub.3) and lithium tantalite
(LiTiO.sub.3).
17. An apparatus as recited in claim 16, wherein said microdisk
resonator has a curved sidewall, a radius of approximately 1 mm,
and a thickness of approximately 200 micron.
18. An apparatus as recited in claim 15: wherein said resonator
comprises a Fabry-Perot resonator; and wherein said Fabry-Perot
resonator comprises a crystal material selected from the group
consisting of lithium niobate (LiNbO.sub.3) and lithium tantalite
(LiTiO.sub.3).
19. An apparatus as recited in claim 15, wherein reverse poling is
used to break symmetry of said resonator.
20. An apparatus as recited in claim 15, further comprising: a
prism optically coupled to said resonator; an input fiber optically
coupled to said prism; and an output fiber optically coupled to
said prism.
21. An apparatus as recited in claim 20, wherein said prism, said
input fiber and said output fiber are integrated with the said
dielectric resonance antenna.
22. An apparatus as recited in claim 15, wherein said apparatus is
an unpowered passive device.
23. An apparatus as recited in claim 15, wherein said apparatus
provides a high degree of immunity from damage to interconnected
devices from high power electromagnetic pulses (EMP).
24. An apparatus as recited in claim 15, wherein said apparatus is
a component of a device selected from the group consisting of a
non-intrusive field sensor that does not require a metal antenna,
metal interconnects, or metal electrodes, a radio frequency
receiver front-end that does not require a metal antenna, metal
interconnects, or metal electrodes, a high-altitude nuclear
electromagnetic pulse immune field probe, and a remote radio
frequency sensor.
25. An apparatus, comprising: a dielectric resonance antenna; and
an electro-optic crystal resonator; said dielectric resonance
antenna comprising a first segment and a second segment bonded to
said first segment; said first segment of said dielectric resonance
antenna having a cavity; said resonator positioned in said cavity;
said resonator having a sidewall spaced apart from said dielectric
resonance antenna by an air gap.
26. An apparatus as recited in claim 25, wherein said resonator
comprises a microdisk resonator.
27. An apparatus as recited in claim 25, wherein said microdisk
resonator has a curved sidewall, a radius of approximately 1 mm,
and a thickness of approximately 200 micron.
28. An apparatus as recited in claim 25, wherein said resonator
comprises a Fabry-Perot resonator
29. An apparatus as recited in claim 25, wherein said resonator
comprises a crystal material selected from the group consisting of
lithium niobate (LiNbO.sub.3) and lithium tantalite
(LiTiO.sub.3).
30. An apparatus as recited in claim 25, wherein reverse poling is
used to break symmetry of said resonator.
31. An apparatus as recited in claim 25, further comprising: a
prism optically coupled to said resonator; an input fiber optically
coupled to said prism; and an output fiber optically coupled to
said prism; wherein said resonator, said prism, said input fiber,
and said output fiber are integrated with said dielectric resonance
antenna.
32. An apparatus as recited in claim 25, wherein said apparatus is
an unpowered passive device.
33. An apparatus as recited in claim 25, wherein said apparatus
provides a high degree of immunity from damage to interconnected
devices from high power electromagnetic pulses (EMP).
34. An apparatus as recited in claim 25, wherein said apparatus is
a component of a device selected from the group consisting of a
non-intrusive field sensor that does not require a metal antenna,
metal interconnects, or metal electrodes, a radio frequency
receiver front-end that does not require a metal antenna, metal
interconnects, or metal electrodes, a high-altitude nuclear
electromagnetic pulse immune field probe, and a remote radio
frequency sensor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. patent
application serial number 60/721,269 filed on Sep. 27, 2005,
incorporated herein by reference in its entirety.
INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT
DISC
[0003] Not Applicable
BACKGROUND OF THE INVENTION
[0004] 1. Field of Invention
[0005] This invention pertains generally to protecting electronic
equipment from electromagnetic pulses, and more particularly to a
radio frequency (RF) front end designed for immunity to
electromagnetic pulses.
[0006] 2. Description of Related Art
[0007] The Graham Commission report, made to U.S. Congress' House
Armed Services Committee on Jul. 22, 2004, concluded that a
"high-altitude nuclear electromagnetic pulse" is one of the few
threats that can hold at risk the continued existence of civil
society in the United States. Directed Energy (DE) weapons,
including high power Electro-Magnetic Pulse (EMP) weapons and High
Power Microwave (HPM) weapons, generate intense pulses of
electromagnetic waves that could damage or destroy sensitive
electronic circuits. The danger is exacerbated by the fact that the
trend towards reduced geometry and voltage renders modern
electronics more susceptible to damage from sources of high-power
spurious EM radiation, including microwave weapons or nuclear
radiation. As an example, a voltage of mere ten volts can punch
through the gate of a modern MOS transistor, while voltage of tens
of kilovolts or larger can be readily generated by EMP or HPM
weapons.
[0008] The most vulnerable devices are high density CMOS digital
circuits and radio frequency electronics hardware, and, in
particular, low noise amplifiers (LNA) of an RF receiver. To meet
the stringent speed and noise requirements, these circuits
typically use highly scaled transistors with low breakdown voltage.
While most components in a system can be protected using Faraday
cages, the front-end components are particularly vulnerable,
because the antenna provides a direct path for high voltage surge
to enter the system. In addition, parasitic or stray capacitances
couple energy into circuits providing additional concerns.
[0009] Because of the low level of received signal, the receiver
circuit is most sensitive to damage from instantaneous voltage
surges. Furthermore, while conventional electrostatic discharge
(ESD) protection schemes may be able to protect low frequency
circuits, presently there are no means to protect high frequency
circuits, including wireless and radar front end electronics, from
EMP or HPM attacks. For example, the traditional ESD protection
approach of using a shunt diode is not applicable at high RF
frequencies, since the additional capacitance of the diode will
compromise the bandwidth and noise performance of the receiver as
illustrated for the low noise amplifier (LNA) front end circuit in
FIG. 1.
[0010] For directed energy test and evaluation (DE T&E)
purposes, sensors are needed to measure electrical fields at high
sample rates and wide dynamic range within EMP or HPM beams. In
addition, the sensors should be non-interfering, non-intrusive,
survivable, and small enough to mount inside targets with limited
space. Photonic techniques using optical carriers to interact with
electromagnetic fields provide a unique isolation feature between
the air interface and the ensuing electronics.
[0011] It will also be appreciated that electro-optic probing
systems using Pockels effect have been widely demonstrated (see,
for example, C. H. Bulmer, "Sensitive, highly linear lithium
niobate interferometric waveguide modulator for electromagnetic
field sensing," Appl. Phys. Lett., vol. 53, pp. 2368-2370, 1988,
incorporated herein by reference in its entirety, and D. H.
Naghski, J. T. Boyd, H. E. Jackson, S. Sriram, S. A. Kingsley, and
J. Latess, "An Integrated Photonic Mach-Zehnder Interferometer with
No Electrodes for Sensing Electric Fields," IEEE J. Lightwave
Technol., vol. 12, no. 6, June 1994, also incorporated herein by
reference in its entirety) and could be considered as a potential
solution. However, metals are used as electrodes for the
electro-optic modulator or as the transmission line linking a metal
antenna to the modulator, and the presence of metal causes two
problems. First, metal electrodes and transmission lines can be
severely damaged or destroyed in EMP and HPM attacks. Second, in an
application as a field probe, these probes must be non-intrusive.
This means that they should not effect a significant change in the
field pattern when placed in front of, or adjacent to a
conventional receiver. For example, non-intrusive survivable
sensors are needed to measure high amplitude fields inside a target
set (e.g., a missile airframe or a computer system) without
altering the EM field inside the structure as if the probe had
never been there. Ideally, a problem should be able to capture the
entire bandwidth and different polarizations of the field, and be
able to withstand extreme power densities ranging from
approximately 1000 W/cm.sup.2 to 10,000 W/cm.sup.2.
[0012] Another problem with prior approaches, and one that is
particularly onerous when it is used in receivers, is the inherent
low sensitivity.
BRIEF SUMMARY OF THE INVENTION
[0013] The foregoing problems are addressed by the present
invention, which generally comprises a Non-Electronic
All-Dielectric (NEAD) or Non-Electronic RF Front End (NERF)
technology that exploits isolation features of photonics. The
advantages of using photonic techniques for electromagnetic field
sensing measurement have been recognized for many years. The theory
of electro-optic (EO) modulation is discussed in detail in "Optical
Waves in Crystals" by Amnon Yariv and Pochi Yeh. In most
electro-optic (EO) probing applications, one takes advantage of the
"Pockels Effect" as the electric field-induced variation of the
refractive index modulates the amplitude or the phase of the
optical carrier. The modulated optical signal is then converted
back to electronic signal by using a photodetector. The use of
optical carriers provides intrinsic electromagnetic isolation
between the incoming field and the electronic instrumentation
connected after the photodetector, making this technique a
promising candidate for creating a survivable HPM sensor.
[0014] One aspect of the invention is to eliminate metal
electrodes, interconnects and the metal antenna found in
conventional equipment. Another aspect of the invention is the use
of a dielectric resonance antenna that behaves as a "concentrator"
for the received RF power. For example, in one embodiment, an EO
modulator is integrated with a dielectric resonance antenna to
exploit unique isolation features of photonics. According to
another aspect of the invention, doubly (RF and optical) resonant
device design maximizes the receiver sensitivity.
[0015] Another aspect of the invention is the integration of high-Q
optical EO resonators and dielectric resonant antennas to create an
efficient mixing of light and microwave fields. The resulting
Non-Electronic RF (NERF) technology brings about a Non-Electronic
all-dielectric (NEAD) RF front-end, which provides complete
isolation between the air interface and the ensuing electronic
circuitry, thereby creating a wireless receiver front end that is
immune to directed energy attacks.
[0016] In one embodiment, the invention is configured as an RF
receiver front-end that replaces a conventional receiver front-end
(hereafter called "receiver" application). In another embodiment,
the invention is configured as a non-intrusive field probe
(hereafter called "field probe" application") that co-exists with a
conventional receiver and detects a directed energy attack. There
is a particular need for such a non-intrusive field probe, since in
many instances a directed energy attack does not cause immediate
destruction of the conventional receiver and therefore goes
undetected. However, because the receiver circuitry is nevertheless
damaged, the receiver can fail at a later and unpredictable time
that can be during a critical mission.
[0017] Another aspect of the invention is combining a dielectric
resonance antenna with an electro-optic field sensor. In one
beneficial embodiment, this combination is used to create an RF
receiver front end without the use of a metal antenna or metal
interconnects. In another beneficial embodiment, this combination
is used to create an RF receiver front end that contains no
electronic components or circuitry after the antenna. In another
beneficial embodiment, the technology is used as an EMP and HPM
immune field probe. In another beneficial embodiment, the
technology is used as a remote RF sensor. Another aspect of the
invention is use of RF to optical conversion to effect electrical
isolation between receiver electronics and the air interface.
[0018] Another aspect of the invention is use of a dielectric
resonance antenna to reduce the radar cross section and hence to
improve stealth performance of an RF receiver.
[0019] Another aspect of the invention is use a high permittivity
material for reducing the antenna aperture size for applications
related to EMP and HPM immune receivers.
[0020] A still further aspect of the invention is a method for
forming a network of said receivers by multiplexing multiple
devices using optical wavelength division multiplexing (WDM).
[0021] Another aspect of the invention is the use of reverse poling
of the EO resonator to break its symmetry and to maximize the RF to
optical conversion efficiency.
[0022] Further aspects of the invention will be brought out in the
following portions of the specification, wherein the detailed
description is for the purpose of fully disclosing preferred
embodiments of the invention without placing limitations
thereon.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0023] The invention will be more fully understood by reference to
the following drawings which are for illustrative purposes
only:
[0024] FIG. 1 illustrates the response of a conventional Low Noise
Amplifier (LNA) to electromagnetic pulses: (a) simplified circuit
diagram for a Low Noise Amplifier (LNA) showing the high voltage
protection diode at the input node; (b) equivalent circuit model
showing the capacitive loading of the input node; (c) the impact on
the frequency response.
[0025] FIG. 2 is a functional block diagram of a non-electronic all
dielectric (NEAD) radio frequency (RF) sensor comprising a
dielectric resonance antenna with an integrated electro-optic
resonator according to an embodiment of the present invention.
[0026] FIG. 3 is a schematic perspective view of a microdisk
resonator supporting WGM propagation according to an embodiment of
the present invention.
[0027] FIG. 4 is a schematic plan view of the microdisk resonator
shown in FIG. 3.
[0028] FIG. 5 is a schematic perspective view of a Fabry-Perot type
resonator according to an embodiment of the present invention.
[0029] FIG. 6 is a schematic plan view of the resonator shown in
FIG. 5.
[0030] FIG. 7 illustrates the typical output spectrum of both the
microdisk resonator shown in FIG. 3 and FIG. 4 and the F-P
resonator shown in FIG. 5 and FIG. 6.
[0031] FIG. 8 is a schematic plan view illustrating optical
coupling to the microdisk resonator shown in FIG. 3 and FIG. 4
according to an embodiment of the invention.
[0032] FIG. 9 is a schematic plan view illustrating optical
coupling to the Fabry-Perot resonator shown in FIG. 5 and FIG. 6
according to an embodiment of the present invention.
[0033] FIG. 10 illustrates how the modulation efficiency at high
frequency is improved by breaking the symmetry of the EO resonator
and the modulating E-field for a uniform cavity and a field applied
uniformly across the entire cavity for a microdisk resonator and
Fabry-Perot type resonator according to the present invention.
[0034] FIG. 11 illustrates how the modulation efficiency at high
frequency is improved by breaking the symmetry of the EO resonator
and the modulating E-field for a uniform cavity and a field applied
across half of the cavity for a microdisk resonator and Fabry-Perot
type resonator according to the present invention.
[0035] FIG. 12 illustrates how the modulation efficiency at high
frequency is improved by breaking the symmetry of the EO resonator
and the modulating E-field for a half-domain inverted cavity and a
field applied uniformly across the entire cavity for a microdisk
resonator and Fabry-Perot type resonator according to the present
invention.
[0036] FIG. 13 illustrates a Dielectric Resonance Antenna fed by a
monopole according to an embodiment of the invention.
[0037] FIG. 14 illustrates the resonance frequency of
TM.sub.01.delta. mode in a cylindrical DRA.
[0038] FIG. 15 illustrates the electric field pattern associated
with the TM.sub.01.delta. mode in a cylindrical DRA.
[0039] FIG. 16 is a schematic flow diagram showing an embodiment of
integration steps of a DRA combined with a EO microdisk resonator
according to the invention.
[0040] FIG. 17 is a schematic flow diagram showing an embodiment of
integration steps of a DRA combined with a Fabry-Perot type
resonator according to the invention.
[0041] FIG. 18 is a schematic partial cutaway view of a DRA
integrated with an EO microdisk resonator according to the present
invention showing an air gap between the periphery of the EO
resonator and the inside walls of the cavity in the DRA.
[0042] FIG. 19 is a schematic partial cutaway view of a segment of
the DRA shown in FIG. 18.
[0043] FIG. 20 is a schematic diagram of a multi-band NEAD receiver
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
1. Front End Architecture: Electro-Optic Isolation
[0044] Referring first to FIG. 2, the architecture of the
Non-Electronic RF Front End (NERF) technology of the present
invention is illustrated. It will be appreciated from the
discussion that follows that the technology can be employed in a
radio frequency receiver, a field probe, or a remote RF sensor, and
other applications where isolation between the input signal and
downstream electronics is required or desirable.
[0045] By way of example, and not of limitation, the invention
employs a probe head 10 that provides isolation between the air
interface and downstream electronic circuitry. In the embodiments
illustrated herein, probe head 10 comprises an electro-optic (EO)
resonator 12 integrated with a dielectric resonance antenna (DRA)
14. Light from a laser 16 is launched into an optical fiber 18 that
serves as an optical signal carrier. The preferred laser source is
a narrow linewidth linearly polarized monochromatic laser which
operates at a wavelength of approximately 1.55 microns. The DRA 14
detects the incoming EM wave (signal) 20 and builds up an efficient
resonant electric field 22 that modulates the nearby EO resonator
through Pockels effect, creating intensity modulation on the
optical carrier. The modulated light is then coupled to a
high-speed photodetector 24 and is converted back to an electrical
form suitable for signal processing.
[0046] Beneficially, the present invention allows for the entire
front end to be made of only dielectric materials (free of any
conducting metal such of metal antenna or metal
interconnects/electrodes). This all-dielectric nature of the front
end significantly increases the damage threshold when attacked by
Electro-Magnetic Pulse (EMP) or High Power Microwave (HPM) weapons
(HPM) weapons, and using an EO sensing technique to pick up the
electrical signal provides a unique form of charge isolation which
prevents EMP or HPM weapons from threatening any electronic system
placed after the photodetector.
[0047] As can be seen from the foregoing, the invention is based on
the use of an EO resonator modulator integrated with a dielectric
resonance antenna. In addition, those two elements form an
innovative doubly resonant device in which RF and optical signals
are in simultaneous resonance. The RF resonance is created by a
Dielectric Resonance Antenna (DRA) and the optical resonance is
sustained in a high-Q optical resonator made from a nonlinear
optical crystal as will be discussed in detail in the following
sections.
2. Optical Resonance: High-Q Optical Resonator
[0048] Referring to FIG. 3 through FIG. 6, two preferred
embodiments of the inventive EO sensor are illustrated that use
resonators made from a material showing strong Pockels effect, such
as lithium niobate (LiNbO.sub.3) and lithium tantalite
(LiTiO.sub.3). FIG. 3 and FIG. 4 illustrate a microdisk resonator
and FIG. 5 and FIG. 6 illustrate a Fabry-Perot (F-P) type
resonator.
[0049] In the microdisk resonator 28, optical resonance is achieved
by confining a linearly polarized optical field 30 in a high-Q
whisper-gallery mode (WGM) along the periphery 32 of the disk
resonator. The output light 34 is then evanescently coupled to an
output fiber 36 using an optic coupling means 38 as will be
described in more detail below. In a Fabry-Perot resonator 40, the
resonance is simply sustained by the two reflecting end mirrors
42a, 42b, and the output light 44 is extracted at an end of the
resonator. As illustrated in FIG. 7, the output spectrum of these
high-Q resonators in frequency domain is a series of Lorentzian
line-shapes, centered at the resonance frequencies of the disk.
Adjacent resonance notches are separated by one free spectral range
(FSR), which is equal to c/2nL, where n is the refractive index of
the resonator material and L is the length of a single round trip
in microdisk or F-P resonator.
[0050] Note that photons will make a number of round-trips in these
high-Q optical resonators. Therefore when a modulating field is
applied to the resonator, the photons interact with the field on
multiple passes which increase the total phase shift accumulated.
When the laser frequency (f.sub.0) is biased at the slope of the
resonance notch, the modulating field can result in direct
intensity modulation on the optical carrier. This is because the
modulating field effectively changes the refractive index and
shifts the resonance spectrum in the frequency domain. Since the
laser is biased at a specific frequency on the resonance slope,
shifting the resonance spectrum results in intensity modulation. In
these configurations, even a small voltage applied across the area
of confinement is enough to induce a change in the resonance
frequency with a magnitude compared to its bandwidth. This forms
the basis for efficient modulation.
3. Fabrication of the EO Resonator
[0051] Referring to FIG. 3 and FIG. 4, in order to achieve the
high-Q WGM resonance required in the present invention, the
sidewall of the microdisk resonator should be shaped into a curved
contour with an optical grade finish. This requires more
sophisticated optical polishing procedures to ensure the
concentricity and surface quality of the disk. This can be easily
achieved, for example, by polishing a z-cut lithium niobate (LN)
disk-shaped resonator. The resonator illustrated in FIG. 3 is a
curved sidewall disk with radius R=1.8 mm and thickness d=300
micron, and expected to have an optical Q in excess of
10.sup.6.
[0052] Referring to FIG. 5, the F-P EO resonator based on lithium
niobate can be fabricated on a waveguide structure (see, for
example, T. Suzuki, J. M. Marx, V. P. Swenson, and O. Eknoyan,
"Optical waveguide Fabry-Perot modulators in LiNbO.sub.3," Applied
Optics, Volume 33, Issue 6, pp.1044-1046, Feb. 20, 1994). For
example, a cavity waveguide 46 can be produced by Ti diffusion on a
lithium niobate substrate. To form a F-P waveguide cavity, the two
end faces need to be polished and coated with dielectric mirrors
42a, 42b.
4. Efficient Coupling of Laser Light Into the Optical Resonator
[0053] Note that optical WGM cannot be excited directly by simple
propagating beams. Accordingly, in an embodiment of the present
invention coupling is achieved through indirect excitation of WGM
using evanescent fields (evanescent coupling). There are numerous
methods for evanescent coupling of light into disk resonators.
However, prism coupling is particularly convenient when dealing
with resonators that are made of high refractive index material
such as lithium niobate.
[0054] FIG. 8 illustrates an embodiment of a microdisk resonator
using prism coupling. In the embodiment shown, a prism 48 couples
the light from an input lensed collimating fiber 50a into EO
resonator 28 and also couples the light out of the EO resonator
into output lensed collimating fiber 50b. The alignment of the EO
resonator 28 with respect to the prism 48 and lensed collimating
fibers 50a, 50b is extremely important in the present invention.
Therefore, with the prism 48 in contact with the resonator 28,
optical alignment is preferably performed by actively determining
the correct position of the input and output fibers 50a, 50b with
respect to the prism 48. FIG. 9 illustrates an embodiment of a
Fabry-Perot waveguide resonator using a collimated lensed fiber 52
to couple light into the resonator. As illustrated in FIG. 6, the
output light is sampled along the same path. In one embodiment
active alignment is performed on the optical sub-assembly (for
microdisk: EO resonator+prism+input/output fibers, for F-P: EO
resonator+fiber) before they are integrated with the DRA. The
entire optical sub-assembly will then be placed inside or in the
vicinity of DRA as will be described below. Active alignment is a
standard manufacturing technique that is used in packaging of
single mode laser transmitters that are the basic building block of
long haul and metropolitan telecommunication networks.
5. RF Modulation
[0055] Unlike conventional Mach-Zehnder traveling-wave devices
which are bandwidth limited by the phase velocity mismatch between
the electrical and optical waves over the active length of the
device, a resonance type modulator described herein has
sufficiently small dimensions that it can be considered a lumped
device. Most importantly, these resonance type devices are not
limited to operation at low RF frequencies. With proper design of
the RF E-field pattern applied on the disk resonator, it is
possible to obtain a modulation response at high RF frequencies up
to tens of GHz. This high frequency modulation is achieved when the
laser frequency is biased at the slope of the optical resonance and
the modulation sideband at the adjacent optical resonance modes
which is one FSR away in the frequency domain. In other words, the
modulation frequency (f.sub.m) is exactly equal to the optical
FSR.
[0056] However, the modulating E-field (E.sub.m) applied to the
resonator should not be uniform along the resonator cavity;
otherwise no sideband will be generated at f.sub.0+f.sub.m and
f.sub.0-f.sub.m. This can be explained by examining the modulated
optical phase after a single roundtrip (refer to, for example, A.
Yariv, Optical Electronics in Modern Communications, New York,
Oxford University Press, 1997, pages 356 to 366, incorporated
herein by reference in its entirety), which is given by
.PHI.(.omega..sub.m)=.delta. sin(.omega..sub.mt-.theta.), where
.delta. = .pi. n 3 r 33 E m L .lamda. sin c ( .omega. m nL 2 c ) .
##EQU00001##
Here L is the roundtrip length and .omega..sub.m is the RF angular
frequency=2.pi.f.sub.m. It is apparent that when f.sub.m=FSR=c/2nL,
the modulated phase shift becomes zero. In other words, the optical
wave propagation time spent in phase with the positive and negative
portions of the RF waves offset each other, resulting in no net
modulation at all. To prevent this from happening, the modulation
field should not be uniformly applied to the EO resonator
cavity.
[0057] As illustrated in FIG. 10 through FIG. 12, in one innovative
embodiment, we break the symmetry by applying reverse poling (also
known as domain inversion) in half of the ferroelectric EO crystal
microdisk or F-P resonator (e.g., z-cut lithium niobate). The
result is reversal of the relative signs of the EO coefficient
(r.sub.33) in two halves of the resonator cavity. Domain inversion
can be done by using patterned electrodes and by applying a large
electric field along the z-axis of the crystal, exceeding the
so-called coercive field E.sub.c, a defined electrode pattern can
be transferred into a corresponding domain pattern (refer to, for
example, M. Yamada, N. Nada, M. Saito, and K. Watanabe,
"First-order quasi-phase matched LiNbO3 waveguide periodically
poled by applying an external field for efficient blue
second-harmonic generation", Appl. Phys Lett. Vol. 62, no. 2, pp.
435-436, 1993, incorporated herein by reference in its
entirety).
[0058] By way of further example, FIG. 10 shows the modulation
response when the E-field is uniformly applied through out the
resonance cavity. As can be seen, no high frequency modulation is
observable in this case. FIG. 11 and FIG. 12 illustrate how the
resulting symmetry removal generates efficient modulation response
at higher frequencies (f.sub.m=FSR). As illustrated in FIG. 11,
when the EO crystal is uniform, applying the E-field to half of the
cavity gives the optimal response (sensitivity). As illustrated in
FIG. 12, if half of the cavity is domain inverted along the z-axis,
the frequency response is doubled compared to the case in FIG.
11.
6. RF Resonance: Dielectric Resonance Antenna
[0059] Efficient coupling of RF power to the optical resonator is
necessary to achieve high receive sensitivity. In prior
conventional approaches, patterned metal electrodes are used to
directly couple the RF electric fields to the disk. As explained
previously, in the present invention we eliminate the metal antenna
and electrodes by using a dielectric resonant antenna (DRA) 14 to
couple the electric field to the EO resonator 12 (microdisk or
F-P). As shown schematically in FIG. 2, this is accomplished by
integrating the high-Q optical resonator with the DRA and using it
to sample the concentrated electric field created by the DRA.
[0060] DRAs have received reasonable attention for the last few
years due to their high radiation efficiency, compact sizes, and
relatively large bandwidth (see, for example, A. Petosa, A.
Ittipiboon, Y. M. M. Antar, D. Roscoe, and M. Cuhaci, "Recent
advances in dielectric resonator antenna technology," IEEE Trans.
Antennas Propagat., vol. 40, pp. 235-48, June 1998, incorporated
herein by reference in its entirety, and R. K. Mongia, A. Ittipibon
and M. Cuhaci, "Measurement of radiation efficiency of dielectric
resonator antennas," IEEE Microwave Guided Wave Letters, vol. 4,
no. 3, pp. 80-82, March 1994, incorporated herein by reference in
its entirety). DRAs are fabricated from materials with low loss and
high relative permittivity. Historically, dielectric resonators
have been generally used as microwave filters and oscillators
packaged in conducting boxes. When placed in an open space, a
dielectric resonator can also act as a radiator and, therefore, can
be used as an antenna. A displacement current standing-wave pattern
can be generated inside the DRA when the resonance mode is properly
excited. This displacement current will produce electromagnetic
radiation. The same principle applies when the DRA works in the
receiving mode. When the incoming radiation matches the resonance
mode of DRA, a significant amount of energy can be concentrated and
stored in the dielectric, leading to a high field build-up in the
vicinity of the structure. FIG. 13 schematically shows a DRA 60 fed
by a monopole 62.
[0061] Design of DRAs requires the knowledge of the resonant
frequencies, quality factor, mode distribution inside the
dielectric structure, and radiation pattern. Antenna geometry and
the dielectric material are design parameters that must be selected
for each application. Electromagnetic simulation of DRAs allows the
optimization of antenna performance by computing the interaction of
the electromagnetic wave with the dielectric material and antenna
geometry. The outcomes of the simulations are the field
distribution inside the DRA, the qualify factor due to radiation
loss, near field and far field patterns. By way of example, the
well-known High Frequency Structural Simulator (HFSS) can be used
for these simulations. The objective is to design for a resonance
frequency that matches the FSR of the integrated EO resonator, and
to arrive at the electric field pattern and optimum location of the
EO resonator within the dielectric resonance antenna that maximizes
the EO modulation efficiency. Cylindrical shape DRA is one of the
most commonly used geometry because of its ease of numerical
analysis. FIG. 14 shows the resonance frequency of the TM.sub.01
mode in a cylindrical DRA for different heights. By changing the
dimension of the DRA, we can precisely control the resonance
frequency of DRA. Since some of the high dielectric constant
materials tend to be hard and brittle, mechanical polishing is a
good way to precisely fabricate the DRA to the desired
dimension.
7. Integration
[0062] In order to reach optimal sensitivity, the EO resonator
sub-assembly needs to be placed at a location where the E-field
build-up is the greatest. Referring to FIG. 15, a cylindrical DRA
operating in TM.sub.01.delta. mode is shown by way of example.
However, the actual shape of the DRA may be different. FIG. 15
shows that the maximum E-field of TM.sub.01.delta. mode is located
along the center axis. The sensitivity is optimal if the EO
resonator sub-assembly is placed in these positions.
[0063] Referring to FIG. 16 and FIG. 17, in one embodiment, the DRA
14 is fabricated in two segments 14a, 14b with the boundary being
defined by the proper position of the EO resonator 28 in the DRA.
One segment 14a includes a recess or cavity 70 for inserting the EO
resonator sub-assembly. The optical subassembly (including, for
example, EO microdisk resonator+prism+fibers as illustrated in FIG.
16 or the EO F-P resonator+fiber as illustrated in FIG. 17) is
inserted into the matching cavity 70 in segment 14a of the DRA, and
the two segments 14a, 14b are bonded together using epoxy or other
bonding technique to complete the assembly. In this way, the EO
resonator assembly is embedded in the DRA. In another embodiment,
the EO resonator sub-assembly is placed on top of the DRA instead
of being embedded in the DRA. In this way, the EO resonator
assembly is only placed in close vicinity of DRA. Accordingly, the
EO resonator is integrated with the DRA. As can be seen from FIG.
15, the E-field is not exclusively confined to only inside the DRA
structure. It is also possible for a strong E-field to penetrate
out of the DRA structure. Since the E-field strength eventually
decays outside the DRA structure, the EO subassembly should be
placed as closely to the DRA boundary as possible while still
leaving an air gap between the DRA and the EO resonator (e.g., see
FIG. 18 and 19, as well as the discussion below).
[0064] Referring to FIG. 3 and the previous discussion, the optical
field in a WGM is primarily located along the periphery 32 of the
EO disk resonator 28. In order to achieve high-Q WGM oscillation,
the refractive index inside the EO resonator should be
significantly larger than that of the medium surrounds it.
Therefore, in the preferred embodiment, the sidewalls of the EO
resonator are not placed in direct contact with the DRA material.
Instead, an air gap is left between the edge of the disk 28 and the
surrounding walls of the cavity 70 in the DRA, as shown in FIG. 18
and FIG. 19. Only a thin air gap is needed because most of the
optical field is confined within the disk resonator when sufficient
index difference is present (n=1 in air and n=2.2 in lithium
niobate). Note that the air gap will not impact the performance of
DRA since its dimension is almost negligible when compared to the
RF wavelength. Only the rim of the crystal needs to be suspended in
the air because the WGM mode field is primarily located at the rim.
The EO resonator can still be supported by the DRA on the bottom
center as illustrated.
8. Broadband Channelization
[0065] From the foregoing discussion, it will be appreciated that
RF modulation generates sidebands around the optical frequency,
f.sub.0. These frequency sidebands also need to be located inside
the optical resonance in order to achieve efficient modulation
(e.g., RF to optical conversion). Therefore, only RF modulation
frequencies at integral multiples of the optical FSR of the
resonator are able to efficiently modulate the optical carrier.
While this requirement may limit the system to narrow band RF
operation, a broadband system is contemplated through RF
channelization. As illustrated in FIG. 20, such a receiver can be
created by incorporating an array of resonators of different sizes
each with a different FSR corresponding to a specific RF frequency
band of interest. Also, as illustrated in FIG. 20, an array of
lasers at different wavelengths can be used to address different
channels.
[0066] As can be seen from the foregoing discussion, the present
invention provides a technology that allows for immunity to high
power electromagnetic pulse attack. It will be appreciated that the
invention provides a number of novel features which include, but
are not limited to, the following:
[0067] 1. Combining a dielectric resonance antenna with an
electro-optic sensor.
[0068] 2. Use of an electro-optic sensor combined with a dielectric
resonance antenna to create an RF receiver front end without the
use of a metal antenna or metal interconnects.
[0069] 3. Use of an electro-optic sensor integrated with a
dielectric resonance antenna to create an RF receiver front end
that contains no electronic components or circuitry after the
antenna.
[0070] 4. An electro-optic field sensor that uses a spatially
asymmetric EO resonator.
[0071] 5. Use of RF to optical conversion to effect electrical
isolation between receiver electronics and air interface.
[0072] 6. Use of the technology to mitigate damage from high power
electromagnetic pulses.
[0073] 7. Use of the dielectric resonance antenna to reduce the
radar cross section and hence to improve stealth performance of an
RF receiver.
[0074] 8. Use of a high permittivity material for reducing the
antenna aperture size for applications related to EMP and HPM
immune receivers.
[0075] 9. Use of the technology as a EMP and HPM immune field
probe.
[0076] 10. Use of the technology as a remote RF sensor.
[0077] 11. A method for forming a network of said receivers by
multiplexing multiple devices using optical wavelength division
multiplexing (WDM).
[0078] 12. Method for creating a field probe by an electro-optic
resonator that lacks azimuthal symmetry.
[0079] 13. Use of reverse poling of the EO resonator to break its
azimuthal symmetry and to maximize the RF to optical conversion
efficiency.
[0080] 14. An electro-optic field sensor front-end that is a
passive device with no electrical power required.
[0081] Although the description above contains many details, these
should not be construed as limiting the scope of the invention but
as merely providing illustrations of some of the presently
preferred embodiments of this invention. Therefore, it will be
appreciated that the scope of the present invention fully
encompasses other embodiments which may become obvious to those
skilled in the art, and that the scope of the present invention is
accordingly to be limited by nothing other than the appended
claims, in which reference to an element in the singular is not
intended to mean "one and only one" unless explicitly so stated,
but rather "one or more." All structural, chemical, and functional
equivalents to the elements of the above-described preferred
embodiment that are known to those of ordinary skill in the art are
expressly incorporated herein by reference and are intended to be
encompassed by the present claims. Moreover, it is not necessary
for a device or method to address each and every problem sought to
be solved by the present invention, for it to be encompassed by the
present claims. Furthermore, no element, component, or method step
in the present disclosure is intended to be dedicated to the public
regardless of whether the element, component, or method step is
explicitly recited in the claims. No claim element herein is to be
construed under the provisions of 35 U.S.C. 112, sixth paragraph,
unless the element is expressly recited using the phrase "means
for."
* * * * *