U.S. patent application number 12/426515 was filed with the patent office on 2010-03-18 for ultra-miniaturized thz communication device and system.
This patent application is currently assigned to NEW JERSEY INSTITUTE OF TECHNOLOGY. Invention is credited to John Francis Federici, Haim Grebel, Andrei Sirenko.
Application Number | 20100067918 12/426515 |
Document ID | / |
Family ID | 41265290 |
Filed Date | 2010-03-18 |
United States Patent
Application |
20100067918 |
Kind Code |
A1 |
Federici; John Francis ; et
al. |
March 18, 2010 |
ULTRA-MINIATURIZED THZ COMMUNICATION DEVICE AND SYSTEM
Abstract
Ultra-miniaturized THz spectrometer/multi-channel receiver
devices are provided. THz communication devices employ a THz
transmitter, a THz receiver and a modulator, wherein the THz
transmitter is configured to introduce a THz signal to the
modulator and the THz receiver is configured to receive the THz
signal from the modulator and demodulate the signal. Communication
systems and methods employing the THz spectrometer/multi-channel
receiver devices enable secure communications. Portable THz emitter
devices are provided employing semiconductor lasers and a nonlinear
birefringent waveguide monolithically integrated on the same
substrate.
Inventors: |
Federici; John Francis;
(Westfield, NJ) ; Grebel; Haim; (Livingston,
NJ) ; Sirenko; Andrei; (Basking Ridge, NJ) |
Correspondence
Address: |
GIBSON & DERNIER LLP
900 ROUTE 9 NORTH, SUITE 504
WOODBRIDGE
NJ
07095
US
|
Assignee: |
NEW JERSEY INSTITUTE OF
TECHNOLOGY
Newark
NJ
|
Family ID: |
41265290 |
Appl. No.: |
12/426515 |
Filed: |
April 20, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61046126 |
Apr 18, 2008 |
|
|
|
61046132 |
Apr 18, 2008 |
|
|
|
61051887 |
May 9, 2008 |
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Current U.S.
Class: |
398/158 ;
398/140; 455/130 |
Current CPC
Class: |
G01J 3/12 20130101; G02B
6/12004 20130101; H04B 10/00 20130101; G01J 3/0205 20130101 |
Class at
Publication: |
398/158 ;
398/140; 455/130 |
International
Class: |
H04B 10/00 20060101
H04B010/00; G06F 3/033 20060101 G06F003/033 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] The research leading to the present invention was supported,
in part by the Department of Defense's Technical Service Work Group
(TSWG) through a contract (N41756-04-C-4163) and by the U.S. Army
through a contract (DAAE3003D1015-18). Accordingly, the United
States Government may have certain rights in the invention.
Claims
1. A THz communication device comprising a THz transmitter, a THz
receiver and a modulator, wherein the THz transmitter is configured
to introduce a THz signal to the modulator and the THz receiver is
configured to receive the THz signal from the modulator and
demodulate the signal.
2. The device of claim 1 wherein the THz transmitter is selected
from a GaAs photomixer or an integrated waveguide photomixer.
3. The device of claim 1 wherein the THz transmitter is an
integrated waveguide photomixer.
4. The device of claim 1 wherein the THz receiver is multi-channel
receiver comprising a photonic bandgap THz spectrometer.
5. The device of claim 1 wherein the receiver comprises a
self-focusing element to direct and focus a dispersed THz signal, a
detector array, and a filtering element
6. The device of claim 1 wherein the receiver comprises a photonic
crystal prism and a pin-hole screen.
7. The device of claim 1 comprising a GaAs photomixer comprising
fiber-optic pigtail cables modulated by varying the voltage across
the GaAs device.
8. The device of claim 1 comprising a THz receiver comprising a
bandpass filter.
9. The device of claim 1 integrated on a InP substrate.
10. An apparatus comprising a plurality of THz transmitter and
receivers in accordance with claim 1.
11. A THz communication system comprising at least one THz
transmitter, at least one THz receiver and at least one modulator,
wherein the at least one THz transmitter is configured to introduce
a THz signal to the at least one modulator and the at least one THz
receiver is configured to receive THz signals from the at least one
modulator and demodulate the signals.
12. The system of claim 11 wherein the at least one THz
transmitter, at least one THz receiver and at least one modulator
are mounted on at least one vehicle within a range suitable for the
at least one THz transmitter, at least one THz receiver and at
least one modulator to transmit and receive signals.
12. The system of claim 10 wherein the at least one THz
transmitter, at least one THz receiver and at least one modulator
are mounted on a plurality of vehicles.
13. A portable THz emitter comprising at least two semiconductor
lasers and a nonlinear birefringent waveguide monolithically
integrated on the same substrate.
14. The emitter of claim 13 comprising an InP substrate.
15. The emitter of claim 13 wherein the birefringent waveguide
provides phase matching for orthogonally polarized lasers.
16. The emitter of claim 13 wherein the lasers have different
emission energies.
17. A method of communication using THz signals comprising:
generating a THz signal; modulating the phase of the THz signal;
transmitting the modulated THz signal to a THz receiver; and
demodulating the THz signal at the THz receiver.
18. The method of claim 17 further including the step of generating
THz radiation at the beating frequency of at least two lasers to
provide individual laser beams, splitting each of the individual
laser beams, mixing the split laser beams, recombining the laser
beams and coupling the laser beams into fibers.
19. The method of claim 18 comprising applying a voltage to the
modulator.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Nos. 61/046,126, filed Apr. 18, 2008,
61/046,132, filed Apr. 18, 2008 and 61/051,887, filed May 9, 2008,
the entireties of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0003] This invention relates to terahertz (THz) communication
devices and systems and methods of using same.
BACKGROUND OF THE INVENTION
[0004] There has been a rapid expansion in the area of terahertz
technology, apparatus and components using THz technology.
Commensurate with this expansion is the need for smaller, faster,
less expensive and more reliable THz components. THz applications
include secure wireless communications, stand-off detection of
explosives, biological weapons, chemical weapons, hand-held
scanners, and non-destructive testing.
[0005] In the development of future combat systems, vehicles must
be lighter, faster, and consequently not as heavily armored.
Communications between vehicles must be stealthy so that hostile
forces can not identify the presence nor mission objectives via
eavesdropping on communication channels. Unmanned vehicles require
short-distance secure communications links so that they can receive
instructions and transmit data before dispersing to conduct their
remote controlled or autonomous mission.
SUMMARY OF THE INVENTION
[0006] In accordance with at least one embodiment the present
invention provides ultra-miniaturized THz
spectrometer/multi-channel receiver devices.
[0007] In one embodiment an ultra-miniaturized THz
spectrometer/receiver system is provided for hand-held, portable
sensing, monitoring, and detection applications. In another
embodiment an ultra-miniaturized THz spectrometer/receiver is
employed as a multi-channel THz receiver integrated in a THz
secure, wireless communications link. In another embodiment devices
in accordance with the present invention may be employed as
hand-held or hands-free wireless communication devices. The present
inventions can be employed as next-generation wireless
communication devices.
[0008] The ultra-miniaturized THz receiver can receive THz signals
at several different THz frequencies (channels). The
ultra-miniature size of the receiver (.ltoreq.6 mm) enables the
communications link to be used either with an integrated component
in a communications system or with stand-alone or hand-held
communications elements. Other key attributes include minimal
fabrication, operational and maintenance costs of the
ultra-miniaturized receiver. Since the receiver can be mass
fabricated from silicon wafers using standard lithographic
processing (using, for example, 10 i.tm features), the cost per
receiver is relatively low. Furthermore, the lack of moving parts
in the receiver minimizes the cost of fabrication and
maintenance.
[0009] In one embodiment the THz receiver is based on photonic
crystal structures. These structures permit the fabrication of
compact, efficient, and low-cost photonic devices. The typical
dimensions of the present THz receivers are comparable to only a
few THz wavelengths. The present devices differ from those in the
prior art at least in that they (a) are three dimensional in
nature; (b) involve both passive and active elements; (c) employ
imaging as well as filtering aspects; and (d) ultra-miniature
receivers on the order of a few THz wavelengths are realized as the
result of THz photonic crystals being integrated with THz detector
elements.
[0010] In at least one embodiment a THz communication system is
provided for data communication between mobile units. The use of
THz radiation as a method for data communication through air is
intrinsically eye-safe and presents no radiation hazards. There are
no reported adverse health effects due to THz radiation.
[0011] The present designs do not require any external light
sources (lasers), which are required in state-of-the-art THz
emitters. This makes the present approach different from THz pulse
generation with visible femtosecond laser operated at the pulse
repetition rate in THz range. The present designs can use
thermoelectric cooling and do not require cryogenic temperatures
like most conventional Quantum Cascade Lasers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] To assist those of ordinary skill in the relevant art in
making and using the subject matter hereof, reference is made to
the appended drawings, wherein:
[0013] FIG. 1 depicts a diagram of a THz secure communications link
in accordance with at least one embodiment of the present
invention;
[0014] FIG. 2 depicts a schematic of a photonic crystal prism
employed as a multi-channel receiver in accordance with at least
one embodiment of the present invention;
[0015] FIG. 3 depicts a diagram of Talbot planes as realized in a
(transverse) Bragg wave guide in accordance with at least one
embodiment of the present invention;
[0016] FIG. 4 depicts a schematic diagram of an embodiment of a
secure THz communication between manned and unmanned vehicles in
accordance with at least one embodiment of the present
invention;
[0017] FIG. 5 is a graphical depiction of an experimental
measurement of THz attenuation through the atmosphere at a
temperature of 22.degree. C. and 8% relative humidity in accordance
with at least one embodiment of the present invention;
[0018] FIG. 6 depicts a schematic breadboard model of a single
channel THz secure communications link in accordance with at least
one embodiment of the present invention;
[0019] FIG. 7 depicts a schematic of a portable THz emitter in
accordance with at least one embodiment of the present
invention;
[0020] FIG. 8 depicts a schematic diagram of fast phase modulation
configuration. The half waveplates are used to rotate the
polarization of the laser beams from external cavity diode laser
(ECDL) 1 and 2 so that the light is polarized parallel to the
polarization axis of the optical fibers.
[0021] FIGS. 9A-9H are graphical depictions of THz detector voltage
output versus time as a function of applied voltage to the
MgO:LiNbO.sub.3 modulator. Applied voltage in FIG. 9A is 20V. The
sawtooth waveform illustrates the timing of the applied voltage
(not to scale) applied to the modulator relative to the THz
detector output. The applied voltage increases in units of 20V with
the last figure (FIG. 9H) corresponding to 160V; and
[0022] FIG. 10 is a graphical depiction of THz detector output for
a thin business card inserted (solid line) between the THz
transmitter and receiver of FIG. 8 and removed (dashed curve).
[0023] It should be noted that the appended drawings illustrate
only typical embodiments of this invention and are therefore not to
be construed as limiting of its scope, for the invention may admit
to other equally effective embodiments. Where possible, identical
reference numerals have been inserted in the figures to denote
identical elements.
DETAILED DESCRIPTION OF THE INVENTION
[0024] In the following description, for purposes of explanation,
specific numbers, materials and configurations are set forth in
order to provide a thorough understanding of the invention. It will
be apparent, however, to one having ordinary skill in the art that
the invention may be practiced without these specific details. In
some instances, well-known features may be omitted or simplified so
as not to obscure the present invention. Furthermore, reference in
the specification to phrases such as "one embodiment" or "an
embodiment" means that a particular feature, structure or
characteristic described in connection with the embodiment is
included in at least one embodiment of the invention. The
appearances of phrases such as "in one embodiment" in various
places in the specification are not necessarily all referring to
the same embodiment.
[0025] Now referring to FIG. 1 a communication device 10 in
accordance with the present invention includes a THz transmitter
20, a THz receiver 40 and a modulator 50.
[0026] A THz transmitter 20 may be selected from a GaAs photomixer
or an integrated waveguide photomixer. Regarding the THz receiver
40, if a single THz channel is required, a single THz detector
(with a bandpass filter) will suffice as a receiver. For multiple
channels, multiple detectors are employed with either multiple
bandpass filters or other such filtering elements. A modulator 50
modulates the THz signal, modulated directly or externally to the
laser, in its amplitude, frequency or phase or some combination
thereof, to encode the propagating THz signal with the stream of
data. Modulation methods for the transmitters are well known in the
art and are described below. A demodulator "decodes" the modulated
THz signal at the THz receiver 40 to retrieve the data stream.
[0027] In one embodiment a THz communications device 10 is provided
having an integrated waveguide photomixer serving as a THz
transmitter 20 and a photonic bandgap THz spectrometer as a
multi-channel receiver 40. A THz transmitter can be modulated
(encoded with data) for example by varying the voltage across a
birefringent waveguide. After transmitting through the atmosphere
over a distance D on the order of at least several meters, the THz
signal is detected using a THz receiver 40 as further depicted in
FIG. 2. Multi-channel operation is realized by increasing the
number of THz transmitters 20 operating at distinct wavelengths
(frequencies).
[0028] Now referring to FIG. 2, a multi-channel receiver 40 may
employ a photonic crystal prism. The collimated THz beam passes
through a pin-hole screen 42. It is then reflected by the upper
portion of the prism and is focused onto the detector array 44.
Owing to the dispersive nature of the prism, only a specific
wavelength is focused per detector-array pixel; all other
wavelengths will be averaged out. Two different channels and
corresponding detector arrays are shown.
[0029] Now referring to FIG. 3, Talbot planes as realized in a
(transverse) Bragg waveguide are depicted. Refractive index
variations along the direction of propagation funnel the
electromagnetic wave along the waveguide. See, Su Choi Tsay and H.
Grebel, "Design of transverse holographic optical interconnect", J.
Appl. Optics, 33, 6747 (1994); H. Grebel, J. L. Graziani, S.
Vijayalakshmi, L. Shacklette, K. Stengel, L. Eldada, R. Norwood and
J. Yardley, "Self-imaging chirp holographic optical waveguides",
Appl. Opt., 36, 9391 (1997); J. M. Tobias and H. Grebel,
"Self-imaging in photonic crystals in a sub-wavelength range", Opt.
Letts., 24, 1660 (1999); J. M. Tobias; M. Ajgaonkar and H. Grebel,
"Morphology-dependent transmission through photonic crystals", J.
Opt. Soc. Am. (JOSA) B, 19, 285-291 (2002). J. Shah, D. Moeller, H.
Grebel, O. Sternberg and J. M. Tobias, "Three-dimensional
metallo-dielectric photonic crystals with cubic symmetry as stacks
of two-dimensional screens", accepted by J. Opt. Soc. Am. (JOSA) A,
2004. The waveguide is controllable by variation of period and
An.
[0030] Referring again to the embodiment of FIG. 2, the receiver 40
which is preferably ultra-miniaturized, includes a photonic crystal
structure which acts as a spatial dispersing element (that is, an
element that spatially disperses the THz radiation into its
component "colors" or frequencies), a self-focusing element to
direct and focus the dispersed THz "colors" onto detector array 44,
and a filtering element. Tunable photonic crystal filters have been
demonstrated in the sub-THz (70-110 GHz) frequency range. See, T.
Drysdale, R. J. Blaikie, D. R. S. Cumming, `Calculated and measured
trasmittance of a tunable metallic photonic crystal filter for THz
frequencies`, Appl. Phys. Lett. 83, 5362 (2003). The operating
frequency of the photonic crystal filters is realized in the
appropriate THz frequency band by scaling the dimension of the
photonic crystal lattice constants.
[0031] Self-focusing is achieved through the diffraction of
higher-order modes from the transverse Bragg planes. See, J. M.
Tobias, M. Ajgaonkar and H. Grebel, "Morphology-dependent
transmission through photonic crystals", J. Opt. Soc. Am. (JOSA) B,
19, 285-291 (2002); H. Grebel and J. Tobias, "Study of hybrid
metal-dielectric photonic crystals", QTuF1, Quantum Electronics and
Laser Science 2002, San Francisco, Calif., May 2002; H. Grebel, Z.
Iqbal and A. Lan, "Detection of C.sub.60 using surface enhanced
Raman scattering from metal coated periodic structures", Appl.
Phys. Letts., 79, 3194-3196 (2001); J. M. Tobias and H. Grebel,
"Self-imaging in photonic crystals in a sub-wavelength range", Opt.
Letts., 24, 1660 (1999); S. Vijayalakshmi, H. Grebel, G. Yaglioglu,
R. Dorsinville and C. W. White, "Nonlinear dispersion properties of
sub-wavelength photonic crystals", Appl. Phys. Letts, 78, 1754-1756
(2001).
[0032] The propagating modes interfere in the direction of the beam
propagation along the diffracting planes. Diffraction is periodic
along the direction of propagation. The distance, which separates
the self-imaging planes (also known as Talbot planes), is defined
as the self-imaging distance. The self-imaging distance for a
periodic transverse structure obeys the relationship,
L.sub.s.about..LAMBDA..sup.2/.lamda..sub.n, including inverse
images. Here, .LAMBDA. is the transverse pitch and .lamda..sub.n is
the propagating wavelength in the material, that is
.lamda./n.sub.eff, where n.sub.eff is an effective index of
refraction dependent on the structure as defined below. Use of this
equation is usually limited to pitch values larger than the
propagating wavelength. Nevertheless, if
.LAMBDA..about..lamda..sub.n, L.sub.s is on the order of only a few
wavelengths.
[0033] In one embodiment the THz receiver 40 is fabricated as a
three-dimensional photonic crystal and shaped into the form of a
prism to overcome the obstacle imposed by integration of active THz
detection elements within the photonic crystal structure. The THz
beam, which includes many spectral components or lines, impinges on
the photonic crystal prism from the left through the screen 42
which contains plural pin-holes formed therein. Wavelength
dispersion dependence is such, that only a particular wavelength
will be focused onto a particular detector in the array lying
underneath the prism. All other wavelengths are averaged out so
that the resulting signal can be removed by imaging processing
techniques or the like. The prism is preferably realized using a
hollow structure with a top inclining grating. A stack of
two-dimensional screens is fabricated wherein resonance of each
successive screen is offset such that a spatial chirp is
achieved.
[0034] The receiver 40 structure may be made three-dimensional by
designing a succession of metallo-dielectric screens. Each
individual screen is patterned using standard silicon processing
techniques on silicon wafers. A wafer bonding technique is used to
attach successive screens into a composite 3-D photonic crystal
structure. In the wafer bonding technique, silicon wafers are
thinned by etching and bonded using heat treatment.
[0035] For an ultra-miniaturized receiver in a secure
communications system, it is contemplated that an appropriate
choice of channel frequencies could be near the three predominate
attenuation frequencies of water vapor (e.g., 0.558 THz, 0.75 THz,
and 0.98 THz) for the communications link. The overall size of the
receiver for such a system would be about 10 wavelengths of 0.558
THz light or roughly 6 mm.
[0036] Secure communications using THz radiation takes advantage of
the limited atmospheric propagation of submillimeter (THz)
electromagnetic radiation to achieve a secure short distance (1-100
m) wireless communications link. This communications link is secure
since the THz radiation (which could be encrypted) is strongly
absorbed by the atmosphere (-4 dB/m) if the frequency of the link
is tuned to specific frequencies (e.g. 0.557 THz). This frequency
corresponds to the frequency at which strong atmospheric absorption
by water vapor occurs.
[0037] Now referring to FIG. 4, in accordance with one embodiment
of the present invention a secure THz communication is established
between 60 vehicles one or more of which may be manned or unmanned.
A THz transmitter 20 on a vehicle 60 is modulated to transmit data
through the atmosphere to a THz receiver 40 on another vehicle 60.
Conceptually, military vehicles are generally congregated in close
proximity to one another before dispersing to conduct their mission
as illustrated in FIG. 4. At such a time, high speed, secure data
transmission can be conducted between the vehicles 60. Outside of
the .about.100 m operating range, the communications signals are
indecipherable.
[0038] The maximum distance of the communications link is
determined by the absorption of THz radiation by the atmosphere.
FIG. 5 shows the attenuation through the atmosphere as a function
of THz frequency. In the example of FIG. 5 THz attenuation through
the atmosphere at a temperature of 22.degree. C. and 8% relative
humidity is shown. The strong absorption features are predominately
due to water vapor absorption. Using the data of FIG. 5, a THz
communication channel tuned to 0.558 THz is attenuated
approximately 1 dB/m. For short distances, the THz receiver 40 will
have sufficient sensitivity to establish a communication link.
However, for longer distances the attenuation is sufficiently large
that the there is insufficient signal to eavesdrop on the
communications channel. For example, a distance of 100 m reduces
the signal by 100 dB (10 orders of magnitude). The presently
disclosed THz communication system includes a THz receiver capable
of high resolution (-10 GHz) at several distinct THz frequencies
(e.g. 0.558, 0.75, and 0.98 THz). Each THz frequency, which can
function as an independent communications channel, is preferably
tuned close to a water absorption line to limit the propagation
distance.
[0039] Two exemplary implementations of a THz transmitter 20 that
could be employed as a part of a THz communications system, a GaAs
photomixer and an integrated photomixer, are described below.
[0040] Now referring to FIG. 6 a single channel THz secure
communications device in accordance with at least one embodiment of
the present invention includes a GaAs photomixer employing
fiber-optic pigtail cables 22 modulated (encoded with data) by
varying the voltage across the GaAs device. After transmitting
through the atmosphere, the THz signal is detected using a THz
receiver 40 with a bandpass filter 46. Multiple channels can be
added by adding additional THz transmitters 20 and detector/filter
pairs that are tuned to different THz frequencies. Fiber-pigtailed
GaAs photomixers are commercially available and can be externally
modulated to encode the transmitted THz radiation. Recent units
available from Picometrix, LLC of Ann Arbor, Mich. have modulation
bandwidths extending to 1 MHz. The photomixer configuration
described above can be implemented by a monolithically integrated
portable THz emitter based on difference frequency generation (DFG)
in artificially birefringent III-V nanostructures. This
implementation utilizes a combination of the latest achievements in
the monolithic integration of InP-based high-power lasers and
nonlinear birefringent active waveguides for efficient difference
frequency generation in the THz spectral range. The approach is
compatible with existing InP technology platform that makes it
scalable for integration into THz emitting arrays. The devices are
compact and consistent with man-portability in size, weight, power
consumption, and geometry. The geometrical size of the InP chip can
be less than 10 mm, while the size of the packaged device can be
less than 30 mm. At such sizes a THz emitter "chip" is easily
affixed to the outside of a vehicle and integrated into a hand-held
unit.
[0041] Now referring to FIG. 7 in one embodiment a portable THz
emitter 100 includes semiconductor lasers 110 and 120 and nonlinear
birefringent waveguide 150, all of which may be monolithically
integrated on the same substrate 170. Substrate 170 is preferably
an InP substrate. Birefringent waveguide 150 provides phase
matching for orthogonally polarized lasers 110 and 120.
Birefringent waveguide 150 may have for example .DELTA.n=0.02.
Lasers 110 and 120 may have different emission energies, for
example, h.omega..sub.1=0.785 eV (.lamda..sub.1=1.58 .mu.m) and
h.omega..sub.2=0.81 eV (.lamda..sub.1=1.53 .mu.m), respectively.
Fine-tuning of the waveguide birefringence is preferably controlled
by the external electric field applied perpendicular to the plane
of the waveguide 150. Multiple contacts (V.sub.1, V.sub.2, etc.)
provide additional degrees of freedom in achieving a perfect
phase-matching condition. Radiation of lasers 110 and 120 may be
coupled with the help of passive waveguides 112 and 122 into the
active waveguide 150.
[0042] In one embodiment the THz emitter 100 is operable for the
frequency range 0.5-7 THz. Semiconductor lasers with different
emission energies (wavelengths) may be employed. For example, in
one embodiment laser 110 is a distributed feedback (DFB) laser with
TM polarization. The polarization of the laser 110 is controlled by
tensile strain in the active region. Power of laser 110 is 80 mW
and the laser line width is 1 MHz. Laser 120 is a conventional
distributed feedback (DFB) laser with TE polarization. Power of
this laser is 100 mW and the laser line width is 1 MHz.
Orthogonally polarized radiation of these lasers is coupled to the
passive integrated waveguides and is mixed in the active
birefringent waveguide section.
[0043] Phase matching of the laser radiation is determined by the
design of the active birefringent waveguide 150:
n(.omega..sub.1)=n(.omega..sub.2)=n(.omega..sub.2-.omega..sub.1).
Birefringence in the waveguide 150 is produced both by the
nanostructures-based active layer and by geometry. In this
particular example, the difference between the laser frequencies
.omega..sub.2-.omega..sub.1 falls in the THz frequency range
(.about.6 THz). Using a typical power of the semiconductor laser of
80 -100 mW and the Manley-Rowe relation for the conversion limit:
P.sub.THz=P.sub.laser(.omega..sub.2-.omega..sub.1)/.omega..sub.1,
the estimated upper limit for the THz emission power is 3 mW.
[0044] To make the size of the THz emitter 100 comparable to that
of conventional optoelectronic devices, such as electro-absorption
modulated lasers (EMLs), modulators, and amplifiers, the components
of the THz emitter 100 can be monolithically integrated on the same
semiconductor chip. InP-based technology of Metal-Organic Vapor
Phase Epitaxy (MOVPE) in the regime of selective area growth may be
employed to utilize existing technology for the lasers and passive
waveguides, commonly used for telecommunications. The exemplary
design in FIG. 7 can be easily modified or scaled to produce arrays
of THz emitters with a fixed or variable frequency.
[0045] The phase-matching condition is controlled by the design of
the birefringent waveguide 150, which in a contemplated embodiment
is comprised of InP-based nanostructures such as quantum well
layers, quantum dots, vertical nanowires and the like.
[0046] In contrast to current THz emitters, which employ THz pulse
generation with visible femtosecond laser operated at the pulse
repetition rate in THz range, external light sources (lasers) are
not required in the present designs. The present designs can employ
thermoelectric cooling and do not require cryogenic temperatures
like most of conventional Quantum Cascade Lasers.
[0047] The advantages of using waveguides composed of
nanostructures are twofold. First, efficient nonlinear interaction
is achieved due to the high value of nonlinear coefficient
.chi..sup.(2) of III-V materials (e.g., .chi..sup.(2) of GaAs is
equal to 240 pmV.sup.-1). Second, the tremendous increase of the
surface area is expected to modify the energy exchange between the
three interacting electromagnetic waves controlled in general by
the Manley-Rowe law. At the extended surface of the nanowires, the
energy flow that goes beyond the Poynting vector should modify the
limit for conversion of the optical power into THz radiation
Phase Modulation
[0048] The rate of the scanning can be increased by recognizing
that the initial phase of the THz wave in the photomixing process
is determined by the phase difference of the two lasers. In other
words, shifting the phase of one of the infrared lasers by 2.pi.
(which corresponds to a path length change of one infrared
wavelength .about.0.8 .mu.m) is equivalent to shifting the phase of
the THz beam by 2.pi. (which corresponds to a path length change of
.about.0.3 mm). Furthermore, rather than mechanically scanning a
distance of .about.1 .mu.m, since the rate of such scanning would
be limited to the 10 s of kHz range with piezo-actuators, one can
directly modulate the phase of one of the lasers using an
opto-electronic Lithium Niobate phase modulator. Since the speed of
Lithium Niobate modulators can be as high as the gigahertz range,
one can eliminate the speed limitations due to mechanical scanning
in acquiring a THz waveform.
[0049] Now referring to FIG. 8, an experimental apparatus for rapid
continuous wave (CW) detection of the THz phase and amplitude is
shown. THz radiation is generated at the beating frequency of two
Littman external cavity diode lasers (Sacher Lion TEC520) operating
near 0.78 .mu.m. For these experiments, they are detuned by 0.6 nm
which corresponds to 0.3 THz. The output of each laser is evenly
split using the first pair of beam splitters. The phase modulator
is inserted into one of the beams from ECDL #1. After splitting and
passing one beam through the modulator, the light from the two
lasers are combined with another pair of beam splitters. The
combined laser light is coupled into polarization maintaining
optical fibers and delivered to both the THz transmitter and
receiver. The transmitter and receiver are low-Temperature-Grown
GaAs bowtie-type photo-conductive dipole antennae (PDA). The total
optical power on both channels is .about.12 mW. A bias of 20 V DC
is applied to power the THz transmitter.
[0050] THz radiation is generated by photomixing of the two laser
beams in the THz transmitter. The generated THz wave can be
presented as a product of electric fields,
E.sub.THz.about.E.sub.1E.sub.2.about.E.sub.1
sin(.omega..sub.1t+.phi..sub.1)E.sub.2
sin(.omega..sub.2t+.phi..sub.2).about.E.sub.1E.sub.2[cos
(.DELTA..omega.t+.DELTA..phi..sub.0)] (1)
where .DELTA..omega.=.omega..sub.1-.omega..sub.2,
.DELTA..phi..sub.0=.phi..sub.1-.phi..sub.2, E.sub.1 and E.sub.2 are
the amplitudes of infrared EDCL electric fields at the frequencies
.omega..sub.1 and .omega..sub.2, and phases .phi..sub.1 and
.phi..sub.2, respectively. In Eq. (1) it is assumed that the
polarization of the two beams from ECDL #1 and #2 are parallel so
that the vector dot product of the infrared E fields simply becomes
a scalar multiplication.
[0051] The electro-optic phase modulator, which is inserted into
the optical path of the ECDL #1 beam that will drive the THz
transmitter, is oriented so that the applied voltage induces a
change in refractive index along the polarization axis of the
infrared laser beam. By varying the applied voltage to the phase
modulator, the optical path length experienced by the propagating
laser beam varies proportionally. Adding the addition phase shift
.phi..sub.m(t) induced by the modulator into Eq. (1) gives
E.sub.THz(t).about.E.sub.1E.sub.2[cos(.DELTA..omega.t+.DELTA..phi..sub.0-
+.phi..sub.m(t))] (2)
where the time dependant phase shift can be expressed as
.phi. m ( t ) = 2 .pi. .lamda. [ n e 3 r 33 2 ] LV ( t ) d . ( 3 )
##EQU00001##
[0052] In Eq. (3), L is the length of the MgO:LiNbO.sub.3 crystal,
.lamda. is the vacuum wavelength of the ECDL laser, n.sub.e is the
refractive index of the crystal at zero voltage, r.sub.33 is the
electro-optic coefficient, and d is the crystal thickness (and the
electrode spacing). Since the phase shift is proportional to the
applied voltage, a linear phase shift requires a linear increase in
voltage. After passing through free space to the THz detector, the
THz beam acquires a phase shift .phi..sub.p.
[0053] The detected THz signal is determined by mixing
(multiplying) the incoming THz radiation of Eq. (2) with the two
infrared ECDL signals present at the THz detector:
E.sub.det.about.E.sub.THz(E.sub.1E.sub.2).about.E.sub.THz(t,
.phi..sub.p)E.sub.1 sin(.omega..sub.1t+.phi..sub.3)E.sub.2
sin(.omega..sub.2t+.phi..sub.4). (4)
where .phi..sub.3 and .phi..sub.4 represent the phase of the beams
from ECDL #1 and #2, respectfully, at the THz receiver. Generally,
.phi..sub.1.noteq..phi..sub.3 and .phi..sub.2.noteq..phi..sub.4
since the optical path length from the lasers to the transmitter is
not equal to the path length from the lasers to the receiver.
Simplifying Eq. (4) using Eq. (2) gives
E.sub.det(t).about.E.sub.1.sup.2E.sub.2.sup.2
cos(.phi..sub.m(t)+.phi..sub.p+(.phi..sub.1-.phi..sub.3)+(.phi..sub.2-.ph-
i..sub.4)) (5)
since the harmonic at 2.DELTA..omega. is removed by the detection
electronics. Consequently, the voltage produced by the THz detector
varies sinusoidally with a linear increasing applied voltage. Of
course, one also has the option of generating other functional
variations by electronically controlling the functional form of
.phi..sub.m(t).
[0054] If a linear voltage ramp waveform with a maximum voltage
swing corresponding to a 2.pi. phase shift were applied to the
modulator, then output voltage of the THz receiver would vary over
a complete cycle of the cosine of Eq. (5). The output of the THz
receiver can then be recorded with a digital lock-in amplifier that
locks to the ramp modulation frequency. However, if the voltage
swing corresponds to a phase shift that were either smaller than or
larger than 2.pi., the output voltage from the THz receiver would
not be perfectly sinusoidal. The preference for a complete 2.pi.
phase shift in the modulator is illustrated in FIGS. 9A-9H. For
voltages below the equivalent of 2.pi. phase shift, (20-100V plots)
the output waveforms are not complete sinusoids. For voltages that
are too large (140 and 160V), a voltage swing larger than one cycle
is observed. When an object is inserted between the THz TX and RX
which modifies the phase shift of the propagating THz beam
.phi..sub.p, the measured phase of the RX waveform shifts as well.
To illustrate this effect, a thin business card was inserted
between the THz transmitter and receiver of FIG. 8. When the phase
modulator voltage is set correctly, the phase of the THz receiver
waveform shifts by 1.6 .mu.s corresponding to a 0.32.pi. phase
shift of the THz wave as shown in FIG. 10.
[0055] Using THz time-domain systems, the maximum measured data
rate for THz wireless communication has been reported to be 1
Mbit/s. Moller, L.; Federici, J.; Sinyukov, A; Xie, C.; Lim, H.;
Giles, R., "Data encoding on terahertz signals for communication
and sensing", Optics Letters, 33:4, 393-395 (2008). Data is encoded
on the THz pulse train by modulating the bias voltage applied to
the THz transmitter. There are two limitations to this data rate:
the first limitation is the electronic bandwidth (420 kHz) of the
THz receivers, the second is the repetition rate (.about.80 MHz) of
the Ti:Sapphire laser that is used to generate and detect the THz.
Using the present method, increasing the bandwidth of the THz
receivers beyond 80 MHz, the data rate of the fast phase modulation
system exceeds that of a time-domain system.
[0056] This opto-electronic method is roughly 3 orders of magnitude
faster than mechanical scanning methods. Utilizing the rapid phase
modulation method enables MHz data rates for THz communication and
can be applied for phase modulation in accordance with the present
invention. In one embodiment phase modulation can be achieved using
a Lithium Niobate phase modulator which can operate in the GHz
range. The phase of the THz radiation can be directly modulated
through a 2.pi. phase shift. By varying the applied voltage to the
modulator 50, the optical path length experienced by the
propagating laser beam varies proportionally. The speed of a
Lithium Niobate phase modulator can be optimized in a
communications system with a function generator in the hundreds of
MHz range and a THz receiver having a large bandwidth response,
preferably greater than 420 kHz and more preferably 80 MHz or
greater.
[0057] The present inventions can be employed as wireless
communication devices, and applied in any environment where
deployment of same would be necessary or desirable, including but
not limited to airports, military installations, mobile military
units, vehicles and the like.
[0058] Applicants have attempted to disclose all embodiments and
applications of the described subject matter that could be
reasonably foreseen. However, there may be unforeseeable,
insubstantial modifications that remain as equivalents. While the
present invention has been described in conjunction with specific,
exemplary embodiments thereof, it is evident that many alterations,
modifications, and variations will be apparent to those skilled in
the art in light of the foregoing description without departing
from the spirit or scope of the present disclosure. Accordingly,
the present disclosure is intended to embrace all such alterations,
modifications, and variations of the above detailed
description.
[0059] All references cited herein are incorporated fully by
reference.
* * * * *