Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
2000-09-29
2002-07-23
Epps, Georgia (Department: 2873)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S004000, C385S001000, C385S008000, C359S245000, C359S254000
Reexamination Certificate
active
06424754
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to apparatus for modulating a coherent optical wave and more particularly to an apparatus for modulating a coherent optical wave in response to plural electric signals capacitively coupled to plural portions of an optical waveguide arrangement responsive to the coherent optical wave.
BACKGROUND ART
FIGS. 1 and 2
are respectively a schematic diagram and a cross-sectional view of an apparatus for imposing amplitude modulation on an amplitude modulated coherent wave optical beam laser
10
derives. The amplitude modulation of the beam laser
10
emits is in response to an AC, usually RF, signal that source
12
derives and applies directly to laser
10
. Optical modulator
16
heterodynes the amplitude modulated beam laser
10
derives with the output signal of RF source
14
. Modulator
16
thus derives a coherent wave optical beam having amplitude variations directly proportional to the product of the signals RF sources
12
and
14
derive. Under idealized circumstances, optical modulator
16
derives a coherent wave optical beam having components that are directly proportional to:
AB
sin[(&ohgr;
1
−&ohgr;
2)
t
+(ø
1
−ø
2
)] (1)
AB
sin[(&ohgr;
1
−&ohgr;
2)
t
+(ø
1
+ø
2
)] (2),
where
A and B are respectively the peak amplitudes of the signals sources
12
and
14
derive,
&ohgr;
1
and &ohgr;
2
are respectively the angular frequencies of signals that sources
12
and
14
derive, and
ø
1
and ø are respectively the phase angles of the signals that sources
12
and
14
derive.
The coherent wave optical beam optical modulator
16
derives also frequently includes components that are directly proportional to:
A
sin(&ohgr;
1
t+ø
1
) (3)
B
sin(&ohgr;
2
t+ø
2
) (4).
Optical modulator
16
includes fiber optic waveguide
18
embedded in solid dielectric plate
20
, so that input face
22
of fiber optic waveguide
18
is positioned to be responsive to the coherent wave optical beam laser
10
derives. Modulator
16
includes metal electrodes
24
and
26
, plates coated on the top surface of dielectric plate
20
on opposite sides of fiber optic waveguide
18
. RF source
14
drives electrodes
24
and
26
by virtue of an ungrounded output terminal of source
14
being connected to electrode
24
and a grounded output terminal of source
14
being connected to grounded electrode
26
. Electrodes
24
and
26
are capacitively coupled to fiber optic waveguide
18
so that electric field
19
(FIG.
2
), established between electrodes
24
and
26
, is coupled to the portion of waveguide
18
between electrodes
24
and
26
. As illustrated in
FIG. 2
, electric field
19
penetrates through solid dielectric plate
20
as well as the portion of fiber optic waveguide
18
between electrodes
24
and
26
.
The electric field variations that RF source
14
establishes in the portion of fiber optic waveguide
18
between electrodes
24
and
26
amplitude modulates the coherent wave optical beam laser
10
derives. The resulting coherent wave optical beam in the portion of fiber optic waveguide
18
downstream of electrodes
24
and
26
thus includes the frequency components of RF sources
12
and
14
, as sum and difference frequencies that are amplitude modulated on the optical carrier frequency of laser
10
. The amplitude of the coherent optical wave downstream of electrodes
24
and
26
can thus be considered as the product of the output signals of RF sources
12
and
14
. The portion of optical fiber waveguide
18
downstream of electrodes
24
and
26
supplies the coherent wave optical beam including the products resulting from multiplication of the signals of sources
12
and
14
to a suitable optical-electric transducer
28
. Transducer
28
, typically a photo-electric detector, such as a diode or transistor, derives an electric signal that is a replica of the amplitude variations of the coherent wave optical beam incident on it, i.e., the optical beam at the output of modulator
16
.
A problem with the structure illustrated in
FIG. 1
is that the transfer function of optical modulator
16
in response to the signal that source
14
applies to electrodes
24
and
26
is quite different from the transfer function of laser
10
in response to RF source
12
. These transfer function differences are such that the response time of laser
10
to RF source
12
is considerably different from the response time of modulator
16
to RF source
14
. In addition, laser
10
and modulator
16
have different non-linearities. Calibrating the apparatus illustrated in
FIG. 1
is difficult because of these factors.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for modifying a coherent optical wave in response to at least two electric signals that act on the wave in substantially the same way.
Another object of the invention is to provide a new and improved apparatus for amplitude modulating a coherent optical wave in response to two or more electric signals that are coupled to the coherent beam with substantially the same transformer function and by the same mechanism.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, an apparatus for modulating a coherent constant amplitude optical wave in response to at least first and second AC electric signal sources comprises an optical waveguide arrangement arranged to be responsive to the optical wave. A first pair of electrodes connected to be responsive to the first AC source and capacitively coupled to a first portion of the optical waveguide arrangement modulates the optical wave propagating in the first portion of the optical waveguide arrangement in accordance with the first AC source. A second pair of electrodes connected to be responsive to the second AC source and capacitively coupled to a second portion of the optical waveguide arrangement modulates the optical wave propagating in the second portion of the optical waveguide arrangement in accordance with the second AC source. The first and second portions of the optical waveguide arrangement are coupled together so that the modulated coherent optical wave derived by the first portion and the modulated coherent optical wave derived by the second portion are combined to derive a third modulated coherent optical wave.
In one embodiment, the second portion of the optical waveguide arrangement is cascaded with first portion of the optical waveguide arrangement so that the second portion of the optical waveguide arrangement derives a coherent optical wave including components containing the sum and difference frequencies of the first and second sources.
In a second embodiment, the optical waveguide arrangement includes a third portion connected to be responsive to the coherent optical waves the first and second portions derive. The first, second and third portions are preferably arranged so the coherent optical waves the first and second portions derive propagate toward each other when entering the third portion. The third portion includes an output optical waveguide segment responsive to the coherent optical waves propagating toward each other from the first and second portions. Preferably, the optical waves the first and second portions derive are supplied to a third portion by aligned optical waveguide segments and via one-way mirrors.
In the preferred arrangement of the second embodiment, the output optical waveguide segment is at an oblique angle to the aligned optical segments, to provide a convenient structure for linearly combining the amplitudes of the optical waves in the aligned optical segments.
Each of the first and second portions of the optical waveguide arrangement preferably includes fiber optic waveguides embedded in a solid dielectric medium and comprises first and second spaced electrodes. The first and second spaced electrodes in both portions are carried by the solid dielectric medium and connected to be responsive to the electric sour
Carrott David T.
Nayfeh Mahir A.
Hasan M.
Lowe Hauptman & Gilman & Berner LLP
Tasc, Inc.
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