U.S. patent application number 09/796026 was filed with the patent office on 2002-10-17 for fast optical modulator.
Invention is credited to Cottrell, William J., Ference, Thomas G., Puzey, Kenneth A..
Application Number | 20020149830 09/796026 |
Document ID | / |
Family ID | 25167070 |
Filed Date | 2002-10-17 |
United States Patent
Application |
20020149830 |
Kind Code |
A1 |
Cottrell, William J. ; et
al. |
October 17, 2002 |
FAST OPTICAL MODULATOR
Abstract
A short pulse of radiation is generated by shining radiation
through a magneto-optical material. The material is excited twice
to rapidly change a property of the wave, such as the direction of
the polarization. The first excitation rotates the polarization in
a first direction and the second excitation brings the polarization
back to its initial direction before the first excitation. Although
the time for relaxation from the excitations may be lengthy, a
pulse of light can be produced that is shorter in time than the
time for excitation plus the time for relaxation. Light
experiencing the pair of lengthy relaxations has each cancelling
the effect of the other. The pulse of light has a length that
depends on the time difference between the two excitations and the
spacing between them. The rapid excitations are provided by pulses
of current in a superconductor located near the magneto-optical
material.
Inventors: |
Cottrell, William J.; (Essex
Junction, VT) ; Ference, Thomas G.; (Essex Junction,
VT) ; Puzey, Kenneth A.; (Essex Junction,
VT) |
Correspondence
Address: |
James Marc Leas
37 Butler Drive
South Burlington
VT
05403
US
|
Family ID: |
25167070 |
Appl. No.: |
09/796026 |
Filed: |
February 28, 2001 |
Current U.S.
Class: |
359/280 |
Current CPC
Class: |
G02F 1/212 20210101;
G02F 1/09 20130101; G02F 2201/17 20130101 |
Class at
Publication: |
359/280 |
International
Class: |
G02F 001/295; G02F
001/09 |
Claims
What is claimed is:
1. A method of generating a pulse of radiation, comprising the
steps of: a) directing incident radiation through a material, said
radiation having a first parameter; b) providing a first
stimulation to a first region of said material to excite a first
population of said material into a first excited state, wherein
said material has a time for excitation and a time for relaxation
after said excitation; and c) generating a pulse of radiation in
said material from said incident radiation, wherein said pulse is
shorter in time than said time for excitation plus said time for
relaxation.
2. The method as recited in claim 1, wherein said time for
relaxation is longer than said time for excitation.
3. The method as recited in claim 2, wherein in said providing step
(b) said radiation traveling through said first region stimulated
in said first excited state has said first parameter changed by a
first amount due to said first stimulation, and wherein said
generating step (c) comprises the step of providing a second
stimulation to a second region of said material to excite a second
population of said material into a second excited state wherein
radiation traveling through said second region stimulated in said
second excited state has said first parameter changed by a second
amount due to said second stimulation.
4. The method as recited in claim 3, wherein said radiation
traveling through said first region stimulated in said first
excited state and radiation traveling through said second region
stimulated in said second excited state are combined such that said
first parameter of said combined radiation comprises a linear
combination of said first amount and said second amount.
5. The method as recited in claim 4, wherein said linear
combination of said first amount and said second amount provides
said first parameter with a value approximately equivalent to that
of said incident radiation.
6. The method as recited in claim 5, wherein said second amount is
opposite said first amount.
7. The method as recited in claim 3, wherein said first region of
said material is in a first leg of a Mach-Zehnder interferometer
and said second region of said material is in a second leg of said
Mach-Zehnder interferometer.
8. The method as recited in claim 3, wherein said first region of
said material is in line with said second region of said material,
wherein radiation traveling through said first region stimulated in
said first excited state also travels through said second region
stimulated in said second excited state.
9. The method as recited in claim 3, wherein said pulse of
radiation comprises a first part and a second part, said first part
comprising radiation traveling through said first region stimulated
in said first excited state but not through said second region
stimulated in said second excited state, and wherein said second
part comprises radiation traveling through said first region
stimulated in said first excited state and radiation traveling
through said second region stimulated in said second excited
state.
10. The method as recited in claim 3, wherein said first parameter
comprises polarization, wherein said incident radiation has an
incident polarization in an incident polarization direction, and
wherein said incident radiation traveling through said first region
stimulated in said first excited state has said incident
polarization rotated a first amount in a first direction and
wherein said radiation traveling through said second region
stimulated in said second excited state has its polarization
rotated a second amount in a second direction opposite said first
direction, wherein said second part comprises radiation having a
polarization rotated back to said incident polarization
direction.
11. A method as recited in claim 10, wherein said second amount is
about equal to said first amount.
12. A method as recited in claim 10, further comprising the step of
stopping transmission of a wave having a polarization oriented in
said incident polarization direction.
13. A method as recited in claim 12, wherein said transmission is
stopped using a polarizer or an interferometer.
14. A method as recited in claim 1, wherein said material comprises
a magneto-optic material and wherein said first stimulation
comprises a first magnetic pulse.
15. A method as recited in claim 14, wherein said first magnetic
pulse is provided by a current pulse in a first electrical
conductor.
16. A method as recited in claim 15, wherein said first electrical
conductor comprises a superconductor.
17. A method as recited in claim 15, wherein said second
stimulation comprises a second magnetic pulse, wherein said current
pulse in said first electrical conductor further provides said
second magnetic pulse.
18. The method as recited in claim 17, wherein said second magnetic
pulse has a smaller magnitude in said material than said first
magnetic pulse.
19. The method as recited in claim 18, wherein said first
electrical conductor is spaced more distantly from said second
region than it is spaced from said first region.
20. The method as recited in claim 18, further comprising a current
divider to adjust magnitude of said second current pulse.
21. The method as recited in claim 20, wherein said current divider
comprises a pair of superconductors arranged in parallel, wherein
said current pulse has a frequency sufficient to provide resistance
for each path in said superconductors for dividing said
current.
22. A method as recited in claim 17, wherein said first electrical
conductor crosses said magneto-optic material at a first point and
at a second point.
23. A method as recited in claim 22, wherein said pulse of
radiation has a pulse width, and wherein said pulse width is
determined by the difference in time for light to travel between
said first point and said second point and time for said current
pulse to travel between said first point and said second point.
24. A method as recited in claim 22, wherein said material is
shaped with a U shape and said first electrical conductor crosses
said U shape.
25. A method as recited in claim 22, wherein said first electrical
conductor is shaped with a U shape.
26. A method as recited in claim 15, wherein said first region of
said material is in a first leg of a Mach-Zehnder interferometer
and said second region of said material is in a second leg of said
Mach-Zehnder interferometer .
27. A method as recited in claim 26, wherein said first current
pulse is directed over said first leg of said Mach-Zehnder
interferometer and under said second leg.
28. A method as recited in claim 15, wherein said second
stimulation comprises a second magnetic pulse, wherein said second
magnetic pulse is provided by a second current pulse in a second
electrical conductor, wherein said first electrical conductor
intersects said material at a first point and said second
electrical conductor intersects said material at a second
point.
29. The method as recited in claim 14, wherein said magneto-optical
material has a more rapid excitation to said first excitation mode
and to said second excitation mode than relaxation from said first
excitation mode and from said second excitation mode.
30. The method as recited in claim 14, wherein said second
stimulation is provided by a second magnetic pulse provided a
specified time after said first magnetic pulse.
31. The method as recited in claim 14, wherein said second
stimulation is provided by a second magnetic pulse provided at the
same time as said first magnetic pulse but in a different region of
said material.
32. The method as recited in claim 14, wherein said incident
radiation is directed to travel in a first direction in said
magneto optical material, wherein said second region is along said
first direction from said first region.
33. The method as recited in claim 14, wherein said incident
radiation is divided into two paths and said pulse comprises
contributions from said two paths.
34. The method as recited in claim 14, wherein said magneto-optical
material comprises a member from the group consisting of Cd, Mn,
Se, Eu, O, S, Bi, C, K, Mg, Al, Br, Te, Cr, Fe, Ba, Y, Gd, Ga, I,
Cl, Ni, Rb, Zn, Rh, Co, and Li.
35. The method as recited in claim 1, wherein said incident
radiation is a continuous optical wave.
36. The method as recited in claim 1, wherein said first
stimulation comprises an interband effect, excitonic effect,
free-carrier effect, or impurity magneto-absorption effect.
37. The method as recited in claim 1, further comprising the step
of generating a train of said pulses, wherein said pulses are
separated by a time less than said time for excitation plus said
time for relaxation.
38. The method as recited in claim 37, wherein said train of pulses
has a rate of 100 GHz or higher.
39. The method as recited in claim 38, wherein said train of pulses
has a rate of 1000 GHz or higher.
40. The method as recited in claim 1, wherein said incident
radiation comprises said first parameter having a first value and
wherein said radiation pulse comprises radiation having said first
parameter different from said first value.
41. A device, comprising an electrical conductor and a
magneto-optical material, wherein said electrical conductor crosses
said magneto-optical material in a first location and in a second
location, wherein said electrical conductor is positioned to
provide a first current pulse stimulating a first excitation of a
first population of said magneto-optical material at said first
location and said electrical conductor is positioned to provide a
second current pulse stimulating a second excitation of a second
population of said magneto-optical material at said second
location, wherein said electrical conductor and said
magneto-optical material are configured to provide said second
excitation of said second population to be opposite said first
excitation of said first population.
42. The device as recited in claim 41, wherein said electrical
conductor comprises a superconductor.
43. The device as recited in claim 41, further comprising a source
of a current pulse, said electrical conductor connected to said
source of current pulse.
44. The device as recited in claim 41, further comprising a source
of light for directing through said magneto-optical material.
45. The device as recited in claim 44, wherein said source of light
provides a continuous wave.
46. The device as recited in claim 44, wherein said second location
is positioned so a first segment of a light wave traveling through
said magneto-optical material experiences only effect of said first
excitation and a second segment of said wave experiences effect of
both said first excitation and said second excitation.
47. The device as recited in claim 41, further comprising a
polarizer.
48. The device as recited in claim 47, wherein said polarizer is
adjusted to prevent transmission of light when no current flows in
said electrical conductor.
49. The device as recited in claim 41, wherein said magneto-optical
material provides a single path for a light wave and said
electrical conductor intersects said single path in two
locations.
50. The device as recited in claim 41, wherein said magneto-optical
material provides two paths for a light wave and said electrical
conductor intersects each of said two paths.
51. A circuit, comprising a first superconductor, a second
superconductor, and a source of a high frequency electrical signal,
said source having a frequency sufficient to provide resistance in
said first superconductor and in said second superconductor for
dividing said signal according to said resistance in each said
superconductor.
52. The circuit as recited in claim 51, wherein said first
superconductor is arranged in parallel with said second
superconductor, further wherein said electrical signal is a current
signal, wherein said circuit divides said current according to said
resistance in said first superconductor and said second
superconductor.
53. A device comprising an electrical conductor and a means for
generating a change in an optical parameter, wherein said
electrical conductor crosses said means in a first location and in
a second location, wherein said electrical conductor is positioned
to provide a first current pulse providing a first stimulation to
said means at said first location and said electrical conductor is
positioned to provide a second current pulse providing a second
stimulation at said second location, wherein said electrical
conductor and said means are configured to provide said second
stimulation opposite said first stimulation.
Description
FIELD OF THE INVENTION
[0001] This invention generally relates to optical modulation. More
particularly, it relates to a device for high data rate modulation
of an optical signal. Even more particularly, the invention relates
to a device that provides a high data rate of magneto-optical
modulation.
BACKGROUND OF THE INVENTION
[0002] Because of the high data rates available, optical fiber is
preferred for high-speed transmission of data, audio, and video.
Binary optical signals consist of low and high intensity signals
traveling through the fiber. A limiting factor in optical fiber
communication networks has been the speed at which light can be
electrically switched or modulated to provide change from a high
intensity signal to a low intensity signal and back to a high
intensity signal. This conversion from electrical to optical signal
is slower than the capability of the fiber. While the optical fiber
can accommodate much higher data rates, commercial techniques for
creating high-speed modulation are presently achieving
approximately 40 billion bits per second, or 40 GHz.
[0003] One method of modulating an optical signal involves
providing a digital optical signal directly from a light source. In
this method light is directly modulated by turning on and turning
off power to a laser source. It is difficult, however, to make
these transitions quickly without introducing non-linear effects
that can degrade the signal. These effects include changes in index
of refraction of material in the laser cavity which effectively
changes the optical path length of the cavity during the pulse,
leading to an effect called chirp, and provides greater dispersion
of the signal as it travels down optical fiber.
[0004] Alternatively, a continuous wave light source can be
externally modulated to provide a digital optical signal. One such
method is electroabsorption modulation. Continuous wave light is
directed through a semiconductor. When current flows in the
semiconductor, enough electrons are moved from valence to
conduction band to provide a population inversion. Light traveling
through the semiconductor with the population inversion is
amplified by stimulated emission. On the other hand, when no
electric current flows, electrons move back to the valence band.
Now the light is absorbed by the low energy electrons, so the light
intensity is diminished as it travels through the semiconductor.
The substantial difference in light intensity when current is
flowing and when current is not flowing provides the on and off
signals. However, this scheme is limited by the time for generation
and relaxation of excited states in the semiconductor.
[0005] A third method, a Mach-Zehnder modulator, provides another
external modulation technique in which a light beam traveling in a
waveguide is split into two paths and then recombined into a single
path where the two beams interfere. A material is provided along
one path that has a refractive index sensitive to applied voltage.
The change in phase introduced by the changing voltage applied to
the material provides for constructive or destructive interference
where the signals recombine. Currently, however, 10-15V is needed
to provide the phase shift, and it has been difficult to provide
high frequency signals at a high voltage to drive the phase
modulator.
[0006] An alternative approach to increase the amount of data that
can be transmitted through an optical fiber is Dense Wave Division
Multiplexing (DWDM), in which many individual signals, each with a
slightly different wavelength, are transmitted through a single
optical fiber at one time. Each of the dozens of signals in the
fiber runs at the 40 GHz data rate, providing a substantially
higher overall data rate. While DWDM increases the data rate
provided by a fiber, the equipment cost for transmission capacity
is higher for DWDM than for faster modulators. Also, errors may be
introduced into the data as a result of a process known as four
wave mixing, in which photons of different wavelengths in a fiber
combine, so data is lost in two channels in the fiber. Two other
photons are generated at different wavelengths, and these may
contribute to noise and errors in other channels in the fiber.
Thus, faster modulation for each wavelength is desirable.
[0007] Two additional techniques to greatly increase modulation
frequency and increase the data rate for transmission in a fiber
have been proposed in commonly assigned U.S. Pat. No. 5,768,002 to
K. A. Puzey, and in a paper "Magneto-Optical Modulator for
Superconducting Digital Output Interface," by Roman Sobolewski, et
al, given at the Applied Superconducting Conference held Sep.
17-22, 2000 ("the Sobolewski paper"). Superconductors allow low
voltage high speed current switching.
[0008] The Puzey technique rapidly switches a superconducting film
between superconducting and non-superconducting states and takes
advantage of the difference in optical properties of the material
in the two states. In the superconducting state, more far-infrared
light is reflected from the material, while in the
non-superconducting state, more is transmitted. Continuous wave
far-infrared light is modulated by an electrical signal provided to
such a superconducting film. After modulation of this far-infrared
light, the signal is then parametrically converted to a shorter
wavelength in the near-infrared range for transmission in a
standard optical fiber. Well known frequency up-conversion
nonlinear optics are used for the conversion.
[0009] The technique described in the Sobolewski paper stimulates
magneto-optic material 10, such as europium monochalcogenides (EuS,
EuTe, EuO, and EuSe) by providing magnetic field 12 from current
pulse 14 in adjacent superconducting signal electrode 16 driven by
a Josephson junction, as shown in FIGS. 1a, 1b. Continuous light
wave 18 is coupled into magneto-optic material 10 through fiber
optic input 19a and exits through fiber optic output 19b. Portion
of light wave 18 traveling in magneto-optical material 10 in
magnetic field 12 has its polarization rotated, a property known as
the Faraday effect. An interferometer is used to provide pulses of
light based on this rotation of the polarization. Because the
excitation of magneto-optical materials occurs in a time measured
in pico-seconds, as shown in FIG. 2a from a paper, "Femtosecond
Faraday rotation in spin-engineered heterostructures," by J. J.
Baumberg, et al, J. Appl. Phys. 75 (10), May 15, 1994 ("the
Baumberg paper"), early investigators recognized that such
magneto-optical microstriplines might provide a way to modulate
signals in the THz (trillion bits per second) range, about two
orders of magnitude higher than present modulation
[0010] The curves in the Baumberg paper, however, show a problem
with the slow relaxation from the excited state that limits the
overall transition time. The relaxation time of magneto-optical
materials from their excited state back to ground state can be much
longer than the time for excitation, as also shown in FIG. 2a from
the Baumberg paper. Thus, there is a very fast excitation rate, on
the order of one picosecond, for Faraday rotation in an applied
magnetic field for a heterostructure. There is also a slow
exponential relaxation rate extending over 250 ps. The slow
relaxation limits the speed at which a magneto-optical material can
operate as an optical modulator. No way to avoid the slow
relaxation has been demonstrated. This lengthy relaxation time
substantially limits the speed of operation of such devices as
compared to the promise of the much more rapid excitation.
[0011] Similarly, in a paper, "Ultrafast magneto-optic sampling of
picosecond current pulses," by A. Y. Elazzabi and M. R. Freeman,
Appl. Phys. Lett. 68 (25) Jun. 17, 1996, data is presented showing
current pulses having a rise time of 15 ps and an exponential fall
time of 250 ps obtained by triggering a photoconductive switch with
an ultrashort laser pulse. The current pulse is used to change the
refractive index of a Bi-substituted yttrium-iron-garnet
ferromagnetic film, and this causes a rotation in the plane of
polarization of polarized light due to the magneto-optic Kerr or
Faraday effect in the ferromagnetic film. The technique allowed a
bandwidth of 82 GHz to be achieved.
[0012] In a paper, "Frequency-dependent Faraday rotation in
CdMnTe," by M. A. Butler, et al, Appl. Phys. Lett. 49 (17), Oct.
27, 1986, data showing a large optical Faraday rotation as a
function of the frequency of an applied magnetic field is presented
for two compositions of the dilute magnetic semiconductor, CdMnTe.
Faraday rotation was observed at frequencies up to about 5 GHz in
small magnetic fields. This material has a very high response to
magnetic excitation (a high Verdet constant), and it has a
relaxation time constant on the order of 100 ps.
[0013] Although a number of authors have suggested advantages to
modulating light based on magneto-optical materials, none suggest a
scheme to overcome the limitation on data rate provided by the slow
relaxation of the magneto-optical materials. Thus, a better system
for more rapidly converting an electrical signal to an optical
signal is needed, and this solution is provided by the following
invention.
SUMMARY OF THE INVENTION
[0014] It is therefore an object of the present invention to
provide a scheme for rapidly modulating an optical signal;
[0015] It is a further object of the present invention to provide a
scheme for rapidly rotating polarization of an optical signal while
eliminating a slow relaxation of the polarization;
[0016] It is a further object of the present invention to provide
that delay associated with relaxation from an excited state of the
magneto-optical material does not affect data rate;
[0017] It is a further object of the present invention to provide
rapid changes between two states in a magneto-optical material to
provide the rapid modulation of the optical signal traveling
through the magneto-optical material;
[0018] It is a further object of the present invention to provide
stimulations to the magneto-optical material for the transition in
each direction so relaxation time does not limit the data rate;
[0019] It is a feature of the present invention that two rotations
of the plane of polarization of light are provided by stimulating
the magneto-optical material twice;
[0020] It is a feature of the present invention that two rotations
of the plane of polarization of light are provided by stimulating
the magneto-optical material with a single current pulse crossing
the material twice;
[0021] It is a feature of the present invention that a
superconductor is used to couple a current pulse which induces a
magnetic pulse stimulation to the magneto-optical material;
[0022] It is an advantage of the present invention that a linear
combination of two high speed stimulations to the magneto-optical
material rotates the polarization back to its original direction
while the effect of the two slow relaxation times on the light
cancel each other out; and
[0023] It is an advantage of the present invention that the optical
modulating is at a much higher data rate than is otherwise
achievable.
[0024] These and other objects, features, and advantages of the
invention are accomplished by a method of generating an
electromagnetic pulse comprising the step of directing incident
radiation through a material. The radiation has a first parameter.
A first stimulation is provided to a first region of the material
to excite a first population of the material into a first excited
state. The material has a time for excitation and a time for
relaxation after said excitation. From the incident radiation a
pulse of electromagnetic radiation is generated in the material in
which the pulse is shorter in time than time for the excitation
plus the time for relaxation of the material.
[0025] Electromagnetic radiation traveling through the first region
stimulated in the first excited state and radiation traveling
through the second region stimulated in the second excited state
are combined. The first parameter of the combined radiation
comprises a linear combination of the first amount and the second
amount. The linear combination of the first amount and the second
amount provides the first parameter with a value approximately
equivalent to that of the initial radiation. The first region of
the material can be in a first leg of a Mach-Zehnder interferometer
and the second region of the material in a second leg of the
Mach-Zehnder interferometer. Alternatively, the first region of the
material can be in line with the second region of the material,
wherein radiation traveling through the first region stimulated in
the first excited state also travels through the second region
stimulated in the second excited state.
[0026] The second amount is set to be about equal and opposite the
first amount so that slow relaxations cancel each other out.
[0027] The pulse includes a first part and a second part. The first
part includes radiation traveling through the first region
stimulated in the first excited state but not through the second
region stimul ated in the second excited state. The second part
includes radiation traveling through the first region stimulated in
the first excited state and radiation traveling through the second
region stimulated in the second excited state.
[0028] The first parameter can be polarization, wherein the
incident radiation has an incident polarization. The incident
radiation traveling through the first region stimulated in the
first excited state has the incident polarization rotated a first
amount in a first direction. The radiation traveling through the
second region stimulated in the second excited state has its
polarization rotated a second amount in a second direction. The
second part of the pulse includes radiation having a polarization
rotated back to that of the incident radiation.
[0029] Another aspect of the invention is a device comprising an
electrical conductor and a magneto-optical material. The electrical
conductor crosses the magneto-optical material in a first location
and in a second location. The electrical conductor is positioned to
provide a first current pulse stimulating a first excitation of a
first population of said magneto-optical material at the first
location. The electrical conductor is positioned to provide a
second current pulse stimulating a second excitation of a second
population of the magneto-optical material at the second location.
The electrical conductor and the magneto-optical material are
configured to provide the second excitation of the second
population to be opposite the first excitation of the first
population.
[0030] Another aspect of the invention is a method of making an
optical signal comprising the step of providing a material.
Incident radiation is directed at the material.
[0031] The incident radiation includes a first parameter having an
initial value. The incident radiation also includes a first segment
of the wave. A first stimulation is provided to the material to
provide a first change to the first parameter in the first segment
of the wave. A second stimulation is provided to the material to
provide a second change to the first parameter in the first segment
of the wave.
[0032] Another aspect of the invention is a circuit comprising a
first superconductor, a second superconductor, and a source of a
high frequency signal. The source has a frequency sufficient to
provide resistance in the first superconductors and in the second
superconductor for dividing the signal according to the resistance
in each superconductor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] The foregoing and other objects, features, and advantages of
the invention will be apparent from the following detailed
description of the invention, as illustrated in the accompanying
drawings, in which:
[0034] FIG. 1a is a top view of a magneto-optical waveguide of the
prior art crossed by a signal electrode carrying a pulse of current
for generating a magnetic field;
[0035] FIG. 1b is a cross sectional view of the magneto-optical
waveguide of FIG. 1a showing the magnetic field generated by the
current pulse extending parallel to the direction of light
propagation in the magneto-optical waveguide;
[0036] FIG. 2a illustrates the fast excitation and the slow
relaxation of a magneto-optical material as reported in the prior
art;
[0037] FIG. 2b illustrates the effect of the fast excitation and
the slow relaxation of a magneto-optical material on light
transmitted through a polarizer or interferometer, as reported in
the prior art;
[0038] FIG. 3a is a top view of a magneto-optical waveguide crossed
twice by an electrical conductor carrying a pulse of current to
provide magnetic fields for providing two rapid stimulations of the
magneto-optical waveguide for providing two rapid rotations of the
plane of polarization of light traveling through the waveguide;
[0039] FIG. 3b is a top view of the magneto-optical waveguide of
FIG. 3a including a current divider to tune the current pulse used
for providing the second rapid stimulation so as to provide
cancellation of light experiencing slow relaxation;
[0040] FIG. 4 is a diagram showing a change in angle of the plane
of polarization of a wave traveling through the magneto-optical
waveguide after the pulse of current makes its first traversal
across the waveguide;
[0041] FIG. 5a is a graph showing magnetic field as a function of
time from the two current pulses that travel across the
magneto-optical waveguide, showing the first magnetic field pulse
at time t.sub.A and the second magnetic field pulse at time
t.sub.BI;
[0042] FIG. 5b is a graph showing the two fast rotations from the
two rapid excitations of the magneto-optical material and the two
slow rotations from the two slow relaxations of the magneto-optical
material in which the second rotation is in an opposite direction
to the first rotation;
[0043] FIG. 5c is a graph showing the light pulse resulting from a
linear combination of rotations in FIG. 5b;
[0044] FIG. 6a is a graph showing pairs of fast rotations and slow
relaxations provided by current pulses in a modulator of the
present invention;
[0045] FIG. 6b is a graph showing a group of light pulses resulting
from a linear combination of several pairs of rotations;
[0046] FIG. 7a and 7b are graphs showing the effect of a small
amount of non-overlap of a pair of slow relaxations to produce a
noise signal;
[0047] FIG. 7c and 7d are graphs showing a group of light pulses
and noise signals resulting from a linear combination of several
pairs of rotations that do not completely overlap;
[0048] FIG. 8 shows dispersion of a current pulse as it travels in
a conductor;
[0049] FIGS. 9a and 9b show alternate arrangements of electrical
conductor and waveguide while still providing two crossing
points;
[0050] FIG. 10a and 10b show alternate arrangements of electrical
conductor, waveguide and current pulse generator;
[0051] FIG. 11a shows a split waveguide and an alternate
arrangement for providing the two stimulations of the present
invention;
[0052] FIG. 11b shows another split waveguide that provides
inhomogeneous division of the wave;
[0053] FIG. 12 shows an alternate arrangement using an
interferometer instead of a polarizer;
[0054] FIGS. 13a-13c and 13e-13h show cross sectional views of
steps in fabrication of the present invention; and
[0055] FIGS. 13d and 13i show top views of steps in fabrication of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0056] Magneto-optical materials transition from an initial state
to an excited state very rapidly in response to a rapid magnetic
field stimulation but the time for relaxation back to the ground
state is longer. For example, Baumberg found that the time for
relaxation was two orders of magnitude longer than the time for
excitation, as shown in FIG. 2a. The excitation portion of the
curve is almost vertical and time for this excitation appears to be
on the order of one picosecond or less. Relaxation is gradual,
taking about 250 ps. The present inventors recognized the potential
offered by the very fast transition in one direction. They also
recognized that the speed of modulation light to provide a high
data rate in magneto-optical materials is limited by the relatively
long time for the material to return to ground state from the
excited state. The present inventors found a way to use the very
fast excitation while overcoming the problem of slow relaxation.
The result is substantially higher speed modulation than would
otherwise be achieved.
[0057] In brief, the invention provides two closely spaced rapid
magnetic stimulations to the material to excite two populations of
the material. Generally, the second stimulation is oppositely
directed to the first stimulation so it has a counter effect on the
light as the first. The time between the two stimulations and their
physical spacing defines the width of the pulse of light that
emerges. The inventors recognized that the effect of the oppositely
directed second lengthy relaxation on the light can cancel the
effect of the first lengthy relaxation. The result is a light pulse
having a time based on the time between the two rapid excitations
and their physical spacing. And the inventors recognized that a
large number of such pulses could be packed into the time normally
spent waiting for the slow relaxation. Thus, the inventors provided
light pulses at very high data rate, far higher than could
otherwise be provided with the long relaxation time of the
magneto-optical material.
[0058] The invention involves a conductor twice crossing a
magneto-optical material, as shown in FIGS. 3a. In the invention,
continuous light wave 20 passing through polarizer 21 has
polarization 22 pointing in initial direction .theta..sub.o, is
directed through magneto-optical material 24, as shown in FIG. 3a
and FIG. 4. Magneto-optical material 24 can include Cd, Mn, Se, Eu,
O, S, Bi, C, K, Mg, Al, Br, Te, Cr, Fe, Ba, Y, Gd, Ga, I, Cl, Ni,
Rb, Zn, Rh, Co, or Li. Magneto-optical materials include the
materials described in the Sobolewski paper, and europium
monochalcogenides (EuS, EuTe, EuO, and EuSe). Well known
magneto-optical materials include EuS, CrBr.sub.3, terbium gallium
garnet, EuO, EuSe, Y.sub.3Fe.sub.5O.sub.12, CrCl.sub.3,
Cd.sub.0.55Mn.sub.O145Te, and Tb.sub.2Al.sub.5O.sub.12. Many other
magneto-optical materials, such as those listed in the CRC Handbook
of Chemistry and Physics, 80.sup.th edition, pages 12-161 to 12-167
can also be used.
[0059] Light wave 20 may be generated by a source that provides
polarized light, such as a laser, in which case polarizer 21 may be
omitted. Light wave 20 can be plane polarized, circularly
polarized, or elliptically polarized. Polarizer 26 is oriented
perpendicular to polarizer 21 to prevent light with this initial
polarization 22 from being transmitted through polarizer 26, and
then into optical fiber for further transmission through an optical
network (not shown). This non-transmission provides the off-region
of the digital optical signal that will be developed in
magneto-optical material 24. Thus, optical system 28 is set up so
no light is transmitted absent some stimulation of magneto-optical
waveguide 24 to rotate plane of polarization 22 of light wave 20 so
the light can pass through polarizer 26. Although a polarizer 26 is
shown for simplicity, an interferometer configuration can be used
instead, as shown in FIG. 12, and as further described herein
below. Alternatively, the polarizers can be configured so they are
aligned and transmit all incident radiation. Then the stimulation
of the magneto-optical material provides a pulse of radiation with
reduced intensity.
[0060] The on-region of the digital optical signal is provided by a
first rapid magnetic stimulation H.sub.A of first region 24.sub.A
of magneto-optical material 24, as shown in FIG. 3a and in
idealized form in FIG. 5a. The excitation of a first population of
magneto-optical material in first region 24.sub.A causes light wave
20 traveling through first region 24.sub.A to have the direction of
its polarization 22 rotated through angle .theta..sub.A from
initial direction .theta..sub.o, as shown in FIGS. 4 and 5b, and as
described in a book, Handbook of Optics, edited by Michael Bass, et
al, McGraw-Hill, 1995. The stimulation can be an interband effect,
an excitonic effect, an interband or free-carrier effect, or an
impurity magneto absorption effect. This stimulation is provided by
magnetic field H.sub.A induced by pulse of current 30.sub.A
traveling in conductor 32 across magneto-optical material 24 in
region 24.sub.A. As Baumberg found (FIG. 2 and FIG. 5a), this
excitation and the corresponding rotation of polarization 22 in
light wave 20 is very rapid, as shown by the nearly vertical rise
of portion 34a of rotation angle curve 34. Light wave 20 with
rotated polarization 22' rotated at angle .theta..sub.A with
respect to initial angle .theta..sub.o can now penetrate through
polarizer 26 (or the interferometer shown in FIG. 12) and so a
signal can be transmitted into the optical fiber. Because of the
rotation of polarization 22', a significant intensity of light wave
20 can now pass through polarizing filter 26. This is the on-region
of digital signal 42 shown by region 20b in FIG. 5c.
[0061] If left alone, relaxation of magneto-optical material 24
back to an unexcited state is relatively slow, as shown in FIG. 2a
and by portion 34b of curve 34 in FIG. 5b, so light wave 20 with
plane of polarization 22' rotated at an angle between angle
.theta..sub.A and .theta..sub.o, would continue to pass through
filter 26 for a time about equal to the relaxation time. Material
24 gradually de-excites, plane of polarization gradually returns to
.theta..sub.o, and intensity of light wave 20 penetrating polarizer
26 decreases until intensity of light transmitted through
polarizing filter 26 goes back to zero, as shown in FIG. 2a from
the Baumberg paper and FIG. 5b. Since the intensity of light
transmitted through a polarizer goes as the square of the sine of
the angle between the polarization and the transmitting axis of the
polarizer, the intensity of light falls substantially faster than
relaxation of magneto-optical material 24, as shown in FIG. 2b.
However, there is still a substantial time for this decline in
intensity. The present invention provides a scheme in which data
rate is governed by the brief time for the very rapid stimulation
and is not reduced by the lengthy time for this gradual decline in
intensity.
[0062] To provide that the on-signal has a very short duration
uncoupled from the long relaxation time of the magneto-optical
material, a second rapid magnetic stimulation H.sub.B is provided
to a second population of magneto-optical material 24 in second
region 24.sub.B, as shown as shown in FIGS. 3a and 5a. Second rapid
magnetic stimulation H.sub.B is aimed in opposite direction to
first stimulation provided by magnetic field H.sub.A. This second
stimulation is provided by current pulse 30.sub.B returning across
magneto-optical material 24 in region 24.sub.B in opposite
direction from current pulse 30.sub.A. The oppositely directed
magnetic field H.sub.B that current pulse 30.sub.B produces
stimulates magneto-optical material 24 in region 24.sub.B. This
stimulation causes light wave 20 traveling through second region
24.sub.B to have the direction of its polarization rotated through
angle -.theta..sub.B as shown with curve 36 in FIG. 5b, equal in
magnitude to remaining rotation .theta..sub.B in region 24.sub.A
provided by first current pulse 30.sub.A. Like curve 34, curve 36
has rapid excitation portion 36a and slow relaxation portion 36b.
The net effect of both stimulations is that a portion of light wave
20 has its polarization 22 experiencing both a rapid clockwise and
a rapid counter clockwise rotation separated in time; the
polarization of light wave 20 that experiences both rotations will
point back in the initial direction .theta..sub.o, and this light
that experiences both rotations will again be blocked by polarizer
26 providing an off-region of digital signal 42, as shown by region
20c in FIG. 5c.
[0063] Amplitude of rotation of radiation may be appropriately
adjusted by spacing electrical conductor 32 more distantly from
second region 24.sub.B than from first region 24.sub.A. The
increased spacing can be achieved by providing a dielectric layer
that is thicker in region 24.sub.B than in region 24.sub.A. In the
Mach-Zehnder embodiment, phase fine tuning or a variable attenuator
can be provided in one leg to match signal from each leg and cancel
effect of slow relaxation. In a single light path embodiment,
parallel current path 32' can be provided, as shown in FIG. 3b, to
bleed off portion 30.sub.B' of current pulse 30.sub.B so remaining
portion 30.sub.B provides stimulation to more nearly match
amplitude of rotation of light experiencing partially relaxed
magneto-optical material in region 24.sub.A. Parallel path 32' may
be a parallel superconducting path or it may have resistor Ra
trimmed to properly make adjustment in current pulse 30.sub.B' to
provide a matching rotation. At very high frequency superconductors
32', 32 both start to exhibit resistance. And these resistances can
be used to provide Ra, Rb to adjust the division of current. Each
resistance can be set by laser trimming dimensions of
superconductor 32', 32.
[0064] In essence, light wave 20 experiences a linear combination
of the two rapid excitations producing two rapid rotations 34a, 36a
and the two slow relaxations producing two slow rotations back,
34b, 36b. First excitation H.sub.A rapidly rotates polarization 22
one way through angle .theta..sub.A, as shown by portion 34a of
curve 34, and light so rotated penetrates polarizer 26 to provide
an on-signal in an optical fiber. Polarization angle of rotation
immediately begins to relax, as shown by portion 34b of curve 34.
However, soon after first excitation H.sub.A, second excitation
H.sub.B rapidly rotates polarization 22 the other way, as shown by
portion 36a of curve 36. Just as this second excitation of
magneto-optical material 24 provides a rotation of polarization 22
countering the effect of the first excitation, the effect of the
first slow relaxation of material 24 on polarization 22 of light
wave 20 is similarly reversed by the equal and opposite effect of
the second slow relaxation. The two slow relaxations are about
equal and opposite in magnitude, so their effects on light wave 20
passing through material 24 cancel, as shown in FIG. 5b. The result
is that light wave pulse 42 emerges from polarizer 26 that is
unaffected by the slow relaxation of magneto-optical material 24,
as shown in FIG. 5c.
[0065] The length of time pulse 42 penetrates through polarizer 26
depends on time t.sub.A for first current pulse 30.sub.A and first
magnetic stimulation H.sub.A at location A in magneto-optical
material 24 and time t.sub.BI for second current pulse 30.sub.B and
second magnetic stimulation H.sub.B at location B in
magneto-optical material 24. It also depends on the time for light
wave 20 that experiences the effects of current pulse 30.sub.A to
traverse the distance between location A and location B in
magneto-optical material 24, t.sub.BL-t.sub.A. This time difference
is t.sub.BI-t.sub.BL, as shown in FIG. 5a, and this is the time
that current pulse 42 can penetrate polarizer 26, as shown in FIG.
5c. Rise times for the two stimulations also contribute to the time
for pulse 42.
[0066] Pulse 42 is the only part of light wave 20 that has
experienced only one rotation, and so this is the only part of
light wave 20 that penetrates polarizer 26, providing the
on-signal. Only for the brief time measured by the time for current
pulse 30.sub.A to traverse the wire between the two stimulation
regions 24.sub.A, 24.sub.B less the time for light to traverse that
spacing can light penetrate polarizer 26. This pulse of light 42 is
far shorter than the time for stimulation plus the time for
relaxation of the magneto-optical material. Thus, the invention
provides a pulse having a time that is much less than the time for
excitation plus the time for relaxation of the magneto-optical
material.
[0067] A closely spaced sequence of such rapid pulses can provide a
digital signal at very high data rate as shown in FIGS. 6a-6b,
which shows data produced by a modulator of the present invention
in which many bits of data fit into the time for relaxation of the
magneto-optical material. Pairs of excitation stimulations are
provided, as shown in FIG. 6a. Portions of one part of each pair
cancel corresponding portions of the other of that pair to provide
discrete pulses, as shown in FIG. 6b. Thus, in the invention, the
two rapid stimulations provide much shorter pulses, much higher
speed modulation, and higher data rate than can be obtained by
providing the first stimulation and then waiting for the material
to relax at its natural relaxation time. The cancellation of one
slow relaxation by the other works because both relaxations are
well behaved curves, typically exponential, and both follow along
identical tracks.
[0068] To the extent two relaxation curves 34b, 36b have different
amplitudes at time t.sub.BI there is a noise signal .DELTA., and
this noise signal limits the number of pulses n that can be
provided during one decay time as shown in FIGS. 7a-7d. The maximum
noise from a series of n stimulations and relaxations, n.DELTA., is
to be kept substantially below the amplitude of a single pulse to
maintain signal integrity, and this provides a limit on the number
of stimulations that can be provided during the time of single
relaxation. The best signal to noise ratio and the largest number
of pulses during a relaxation time is achieved by arranging
stimulations so that the amount of curve non-overlap is a small
fraction of the amplitude of a single pulse.
[0069] Magneto-optical waveguide 24 is formed of a material that is
stimulated by current pulse 30.sub.A traveling through electrical
conductor 32, as described herein above. Electrical conductor 32
can be a superconductor to provide electrical signals at high data
rate over the short distance between pulse generator 35 and
magneto-optical waveguide 24.
[0070] The shape of light pulse 42 is mostly affected by amplitude
and slope of leading edge 36 of stimulating current pulse 30.sub.A,
30.sub.B and by the amount of dispersion of the current pulse, as
shown in FIG. 8. Obtaining optical pulses with steeply sloping
leading and trailing edges requires that electrical pulses
30.sub.A, 30.sub.B have high amplitude, steeply sloping leading
edge 36, and experience little dispersion. Dispersion is
schematically illustrated in FIG. 8 as current pulse broadening 38a
to 38b as current pulse 30.sub.A travels along electrical conductor
32 from pulse generator 35 toward wave guide 24, and then continues
in U-shaped electrical conductor 32 to re-cross wave guide 24 in
the opposite direction. Along with the slow relaxation, trailing
features of first current pulse 30.sub.A are cancelled out because
second current pulse 30.sub.B has a shape that is identical to that
of first current pulse 30.sub.A and produces a rotation curve 36
having a shape comparable to first rotation curve 34.
[0071] Magnetic field lines 44.sub.A, 44.sub.B provided by current
pulses 30.sub.A, 30.sub.B along electrical conductor 32 are either
parallel or anti-parallel to the propagation vector of light wave
20 that is being modulated by magneto-optical material 24. Pulse
generator 35 may include Josephson junction technology to provide
high speed current pulses 30.sub.A. CMOS or InP at low temperature,
SiGe, and GaAs technologies can also provide high speed current
pulses.
[0072] Dispersion depends on resistance in a conductor, and
resistance increases with signal frequency. Short current pulses
30.sub.A, 30.sub.B have high frequency. A superconductor operating
at low temperature is particularly desirable to provide electrical
conductor 32 carrying current pulse 30.sub.A, 30.sub.B with low
resistance and low dispersion for providing magnetic fields
H.sub.A, H.sub.B. A superconductor can provide the required pulse
characteristics of high amplitude, steeply sloping leading edge 36,
and low broadening 38a to 38b. Current pulse widths of less than 1
ps have been demonstrated with superconductor, as described in a
paper "Experiments and Simulations of Subpicosecond SFQ Propagation
in Y--Ba--Cu--O Josephson Transmission Lines," R. Adam, et al,
Proceedings of the Applied Superconductivity Conference, Sep.
17-22, 2000, Virginia Beach, Va. Pulses at lower data rate may be
provided along ordinary copper, aluminum or gold wire.
[0073] The angle of Faraday rotation .theta..sub.A of polarization
22 produced in light traveling through a magneto-optical material
is given by:
[0074] ti .theta..sub.A=VBL
[0075] where V is the Verdet coefficient (an intrinsic material
property), B is the magnitude of the magnetic field within the
material and L is the distance the light wave travels within the
magneto-optical material and within the magnetic field. The
material can be thought of as having a circular birefringence
having a magnitude that depends on magnetic field. The sense of
rotation of the plane of polarization experienced by a wave
traveling in the material is dependent on the direction of the
magnetic field within the material in relation to the direction of
propagation of the wave. When the projection of the magnetic field
vector onto the propagation vector is parallel to the propagation
vector one sense of rotation is observed. When anti-parallel, the
opposite sense of rotation is observed.
[0076] The Verdet constant of magneto-optical materials increases
with shorter wavelength and lower temperature. For example, the CRC
handbook on page 12-162 shows a two order of magnitude increase in
Verdet constant as temperature goes from 300K down to 1.45K for
Tb.sub.2Al.sub.5O.sub.12. Thus, to provide largest rotation of the
plane of polarization for a given current pulse it is best to use
short wavelength light and hold the magneto-optical material at low
temperature.
[0077] Magnitude of optical pulse can also be increased by
providing a higher magnetic field by providing a higher current.
Thus, a superconducting wire capable of carrying a high current and
that provides low resistance even at high frequency is desirable.
Materials, such as yttrium barium cuprates, bismuth cuprates,
thallium cuprates, mercury cuprates, and other well known high
temperature superconductors can be used as the superconducting
wire. Low temperature superconductors, such as niobium and tin can
also be used.
[0078] Magnitude of optical pulse can also be increased by methods
as described in the 100-002 application, filed on the same day as
this application and incorporated herein by reference.
[0079] The Sobolewski paper provides data showing a 4.52.degree.
rotation of the polarization of light in a EuSe magneto-optical
wave guide produced by a 2.51 Oe magnetic field provided by a
current pulse of 1 mA flowing in a superconducting Nb-trilayer and
produced by a Josephson junction.
[0080] The fraction of optical power T transmitted through a
polarizer having a polarization rotated through an angle
.theta..sub.A with respect to the angle for blocking light is equal
to
T=sin.sup.2.theta..sub.A
[0081] Thus, for the 4.52.degree. rotation found by Sobolewski, the
power transmitted is 0.6% of the power incident on the polarizer.
The rotation angle and the power transmitted can be increased by
providing a higher current pulse 30.sub.A, longer interaction
length L, or by adjusting temperature, material, or wavelength to
provide a higher Verdet constant V.
[0082] The rotation angle and power transmitted can also be
increased by providing multiple pairs of conductors for carrying
multiple pairs of current pulses across the magneto optical
material timed to provide additional boosts to the rotation angle
of light passing there through, as described in a patent
application filed the same day as this application and having
attorney docket number 100-002, incorporated herein by
reference.
[0083] In any case, light wave 20 is divided into three regions:
first region 20a of light wave 20 experiences none of the rotations
and does not penetrate through polarizer 26, as shown in FIG. 5c.
Second region 20b experiences one of the rotations and some of this
light can penetrate through polarizer 26. Finally, third region 20c
experiences a linear combination of both rotations and this double
rotation moves the plane of polarization back to the initial
direction so no light can penetrate through polarizer 26. Thus,
only a narrow pulse of radiation defined by two rapid excitations
penetrates polarizer 26.
[0084] Oppositely directed magnetic fields 44.sub.A, 44.sub.B
around conductor 32 can be provided by providing electrical
conductor 32 in a U shape to cross magneto-optical material 24 in
two regions 24.sub.A, 24.sub.B. Current pulse 30.sub.A crosses
magneto-optical material in one direction in region 24.sub.A and
current pulse 30.sub.B crosses in a direction opposite the
direction of current pulse 30.sub.A in region 24.sub.B, as shown in
FIG. 3a, providing oppositely directed magnetic fields, 44.sub.A,
44.sub.B and oppositely directed rapid magnetic stimulations
H.sub.A of first region 24.sub.A and H.sub.B of second region
24.sub.B of magneto-optical material 24. In this embodiment a
single current pulse in single electrical conductor 32 is thereby
used twice to provide pulses 30.sub.A, 30.sub.B. Thus, a single
current pulse provides both stimulations to define optical pulse 42
that emerges from polarizer 26. In this embodiment, the time for
current pulse 30.sub.A, 30.sub.B to travel along the U-shaped
region of electrical conductor 32, less the time for light to
travel between points in waveguide 24 crossed by conductor 32, is
the time for optical pulse 42.
[0085] Light wave 20 with both rotations is excluded by polarizing
filter 26. As before, that small portion of light wave 20
experiencing only first rotation 22' penetrates polarization filter
26 and this transmitted light becomes optical pulse 42, as shown
schematically in FIG. 3a and graphically in FIG. 5c.
[0086] In one alternative, electrical conductor 32' can be straight
while magneto-optical material 24' bends in a U shape, as shown in
FIG. 9a. Or both electrical conductor 32" and waveguide 24" can
bend, as shown in FIG. 9b.
[0087] In yet another alternative, two current pulses from the same
generator can be provided in the same direction across
magneto-optical material 24 to provide the same effect if one
electrical conductor is located above and the other below
magneto-optical material 24, as shown in FIG. 10a. Time delay to
set the length of optical pulse 42 can be provided by adjusting the
relative length of electrical conductors 50a, 50b between generator
35 and magneto-optical material 24. Separate current pulse
generators can also be used, and in this case optical pulse width
is also determined by the separation in time between the current
pulses at magneto-optical material 24. A similar result is obtained
by providing current pulses in electrical conductors 51a, 51b
extending in opposite directions from pulse generator 35, as shown
in FIG. 10b.
[0088] Just as an optical pulse can be obtained from a linear
combination of rotations of light wave 20 in sequence, as shown in
FIGS. 3, 9a-9b, 10a-10b, an optical pulse can also be obtained from
a linear combination of rotations of polarizations of two waves
traveling in different paths which combine, as shown in FIG.
11a.
[0089] Continuous light wave 52 traveling along Mach-Zehnder
interferometer 54 is divided into light waves 52a, 52b in two
magneto-optical material paths, 54a, 54b. Current pulse 60
traveling in electrical conductor 62 crosses over path 54a some
time before crossing under path 54b. Current pulse 60 stimulates a
rapid change in magnetic field in magneto-optical waveguide 54a
slightly before stimulating a second rapid but opposite change in
magnetic field in magneto-optical waveguide 54b. These two equal
but opposite changes in magnetic field in waveguides 54a, 54b cause
rotations of light waves 52a, 52b through equal angles but in
opposite directions. Light wave 52a traveling in one of the two
paths 54a of waveguide 54 has its plane of polarization rotated
through angle .theta..sub.B in one direction while a short time
later light wave 52b traveling in other path 54b has its plane of
polarization rotated through angle -.theta..sub.B in the opposite
direction.
[0090] Thus, the arrangement shown in FIG. 11a provides a time
delay because of the time for current pulse 60 to travel across
space 64 between two paths 54a, 54b of waveguide 54. Light with
rotation from path 54a reaches intersection 66 before any light
with counter rotation from path 54b reaches intersection 66.
Polarizer 68 is adjusted to exclude initial continuous light wave
52. Polarizer 68 will also exclude light that includes a linear
combination of contributions of oppositely rotated light from both
paths 54a, 54b when light from path 54b reaches intersection 66.
Only during the brief interval of time for current pulse 60 to
travel distance 64 between paths 54a, 54b when light having
rotation .theta..sub.B from path 54a reaches intersection 66 and
before light having counter rotation .theta..sub.B from path 54b
reaches intersection 66 does light pulse 70 penetrate polarizer 68,
and this interval of time defines light pulse 70. Thus, two fast
excitations in two parallel paths mask the slow time-varying
relaxation of magneto-optical material in waveguides 54a, 54b. In
addition, pulse of light 70 is again defined by amplitude, leading
edge, and dispersion of fast excitation current pulse 60.
[0091] For more precise cancellation of polarizations along the two
paths, intensity matching of delayed pulse with earlier pulse can
be provided, for example, by providing unequal division of initial
continuous light 52 by providing path 54a' wider than path 54b', as
shown in FIG. 11b. Since some relaxation occurs during the time
before the second stimulation, a decrease in intensity is provided
so the initial second signal in path 54b' matches the intensity of
the partially relaxed first signal in path 54a'.
[0092] Alternatively, optical path can be longer in one arm 54a, to
provide interference where light waves 52a, 52b recombine (see FIG.
12). In this case polarizer 68 is not needed.
[0093] An interferometer can be used in all embodiments described
herein above instead of polarizer 26 to exclude light having
polarization pointing in initial direction .theta..sub.o, as shown
in FIG. 12. Interferometer 70 includes beam splitter 72 that
directs half of polarized light wave 20 from polarizer 21 into each
of two arms. First arm 74 has magneto-optical material 24 and
electrical conductor 32, as illustrated in FIG. 3a and described
herein above. Second arm 76 includes a longer or shorter optical
path, such as provided by phase delay plate 78, to provide a 180
degree phase shift to the portion of light wave 20 traveling this
path. Where they recombine at beam splitter 80, light traversing
the two arms are out of phase and interfere destructively. Thus,
the output of interferometer 70 is normally dark, just as unrotated
light is unable to pass through polarizer 26 in FIG. 3a. However,
when the portion of light wave 20 traveling through magneto-optical
material 24 has its polarization rotated, there will no longer be
destructive interference and signal will emerge at beam splitter
80. Interferometer 70 provides advantage in that absorption by
polarizer 26 is avoided so more light is transmitted.
[0094] The layered structure of the present invention is fabricated
using techniques common to integrated circuit manufacture. In one
embodiment of the process, about a 500-Angstrom thick layer of
Yttrium Stabilized Zirconium (YSZ) 80 is deposited on about a 500
micrometer thick silicon substrate 82, as shown in FIG. 13a.
Approximately 1000 Angstroms of yttrium barium copper oxide (YBCO)
superconductor 84 is then deposited on YSZ layer 80, as shown in
FIG. 13b. Using standard photolithography and etch techniques,
about a 20 micrometer wide microstripline or electrical conductor
32 is formed from YBCO superconductor 84, as shown in FIGS. 13c and
13d to carry current pulses 30.sub.A, 30.sub.B. Other
superconductors can be used, Alternatively, other superconducting
materials, as described herein above, and other substrates such as
lanthanum aluminate, strontium titanate, magnesium oxide, can be
used.
[0095] In the next step about 500 Angstroms of YSZ 86 and then
about 500 Angstroms of silicon nitride 88 is deposited, as shown in
FIGS. 13e, 13f. YSZ is a good diffusion barrier to silicon. Nitride
is a good barrier to water and other ionics. Next, about 5 microns
of magneto-optical material 90, such as EuSe is deposited and
etched to form wave guide 24, as shown in cross sectional view in
FIG. 13g and in top view in FIG. 13i, so that waveguide 24 is
crossing over superconducting electrical conductor 32. Finally,
about a 5000 Angstrom thick layer of gold 92 is deposited over the
entire surface to act as a ground plane, as shown in FIGS. 13h.
Although not shown in FIG. 13d, superconducting wire or
magneto-optical waveguide can be formed in a U shape or other shape
described herein above to provide the double stimulation to
magneto-optical material 90 as provided in the present
invention.
[0096] While several embodiments of the invention, together with
modifications thereof, have been described in detail herein and
illustrated in the accompanying drawings, it will be evident that
various further modifications are possible without departing from
the scope of the invention. Nothing in the above specification is
intended to limit the invention more narrowly than the appended
claims. The examples given are intended only to be illustrative
rather than exclusive.
* * * * *