Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2000-07-31
2001-09-04
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S305000
Reexamination Certificate
active
06285493
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to apparatus using electro-acoustic-optical devices and to a method of calibrating same and, more particularly, to a signal combiner using such a device, wherein acoustic waves are launched in the device in opposite directions toward each other. Another aspect of the invention relates to an electro-acoustic-optical device having transducers for launching acoustic waves toward each other in the device.
BACKGROUND ART
Electro-acoustic-optical devices include an optically defractive medium capable of having a moving optical grating induced in it in response to acoustic waves propagating therein. An electro-acoustic transducer, usually mounted at one end of the medium, launches acoustic waves in the defractive medium in response to electric excitation of the transducer. Such devices are used in time integrating correlators and apparatus for determining time difference of arrival of two signals.
FIG. 1
is a schematic diagram of a prior art device for determining the time difference of arrival of signals that RF sources
10
and
12
derive. RF sources
10
and
12
are typically continuous wave sources having phases representing positions of objects being tracked. The signals that sources
10
and
12
derive are linearly combined in electronic difference circuit
14
that derives an output signal having an amplitude directly proportional to the difference between the instantaneous amplitudes of the signals sources
10
and
12
derive. The difference signal that circuit
14
derives drives piezoelectric crystal
16
, bonded to one end of optically diffractive medium
17
that forms Bragg cell
18
. Piezoelectric crystal
16
responds to the signal from circuit
14
to launch an acoustic wave in medium
17
. The acoustic wave induces a moving optical grating in medium
17
. Medium
17
is formed as an elongated cell, and crystal
16
is arranged such that acoustic waves propagate in the elongated direction of the cell. Typically medium
17
is made of gallium phosphide (GaP), which is favorably employed because it has high bandwidths of, for example, 2 GHz.
Laser source
20
derives an unmodulated coherent optical beam
22
that illuminates a center portion of cell
18
. Beam
22
is incident on a first front face of cell
18
and is displaced from a line perpendicular to the propagation direction of the acoustic waves in cell
18
by the Bragg angle of the refractive material in medium
17
.
Cell
18
responds to the moving optical grating crystal
16
induces in it to diffract and amplitude modulate the coherent energy in beam
22
. The modulated coherent energy in beam
22
emerges from cell
18
as a series of beamlets propagating from the second, back face of the cell. The deflection angles of beamlets
24
are determined by the diffractive index of the medium
17
where the beam
22
is incident on the medium; the refractive index is determined by the amplitude of the acoustic waves propagating in the cell.
Beamlets
24
are incident on collimating lens
26
which converts the beamlets into parallel beamlets
28
which are incident on photoelectric detector array
30
. Photoelectric detector array
30
includes many detector elements
32
, each of which derives a separate variable amplitude output signal commensurate with the amplitude of the optical energy in the beamlet
28
incident thereon. Detector elements
32
are arranged in linear array
30
that extends in the same direction as the propagation direction of the acoustic energy in cell
18
. Electric leads in bus
33
supply the signals that detector elements
32
derive to processor
34
, which compares the amplitudes of the outputs of detector elements
32
to derive signals indicative of the amplitudes of the optical energy incident on each of detector elements
32
and an indication of which detector
32
has the highest amplitude optical energy incident thereon. Processor
34
responds to the amplitudes of the signals in bus
33
to derive an indication of the difference in time of arrival (i.e., the phase difference) of the signals that sources
10
and
12
derive.
We realize that a problem with the apparatus illustrated in
FIG. 1
is a tendency for difference circuit
14
to combine the output signals of sources
10
and
12
in such a manner that the signal which actually drives piezoelectric crystal
16
is not exactly equal to the difference between the signals of RF sources
10
and
12
. Consequently, when RF sources
10
and
12
are identical to each other and are supplied at exactly the same time, i.e., with the same phase, to difference circuit
14
, the difference circuit frequently does not produce a zero output signal. Consequently, processor
34
does not derive an accurate indication of the time difference of arrival of the signals that sources
10
and
12
derive.
Bragg cell
18
has also been used in time integrating correlators which determine the time difference of arrival of RF signal sources
10
and
12
. The correlator illustrated in
FIG. 2
includes Bragg cell
18
, responsive to a coherent optical wave that RF source
10
amplitude modulates. A moving optical grating is induced in cell
18
in response to RF source
12
.
To these ends, source
10
directly amplitude modulates coherent wave beam
41
laser
40
derives. Coherent wave beam
41
is incident on diverging lens
42
which produces a diverging beam
43
incident on collimating lens
44
. Lens
44
supplies collimated, coherent optical wave beam
45
to a first, input face of Bragg cell
18
, which defracts the optical beam incident on it as a function of the moving optical grating induced in the cell as a result of the acoustic waves that piezoelectric crystal
16
launches in the cell. Crystal
16
responds to a signal including the variations of RF source
12
, as modified by DC bias source
46
and by RF carrier source
50
, typically having a frequency of about 2 GHz. Electronic adder
48
combines the RF output signal of source
12
and the DC bias of source
46
to produce an electronic sum signal that is heterodyned in mixer
52
with the RF carrier wave which source
50
derives. Mixer
52
produces an amplitude modulated electric wave having approximately a 2 GHz carrier. The output signal of mixer
52
drives crystal
16
.
Bragg cell
18
responds to the optical energy in beam
45
and the acoustic wave launched by crystal
16
to produce an amplitude modulated optical beam that drives a spatial filter including focussing lens
54
and collimating lens
56
, such that focussing lens
54
responds to the output beam of Bragg cell
18
and collimating lens
56
produces a collimated beam that is incident on detector elements
32
of detector array
30
. Each of detector elements
32
produces an electric signal having an amplitude indicative of the optical energy incident thereon. Bus
33
supplies these signals to processor
34
which responds to them to indicate the relative time of arrival of the signals of RF sources
10
and
12
at the inputs of laser
14
and crystal
16
, respectively.
A problem with the apparatus illustrated in
FIG. 2
, which is described in an article by Houghton et al., entitled “Spread Spectrum Signal Detection Using a Cross-Correlation Receiver,” HMSO London 1995, is that it ignores transform errors between the different wave domains formed as a result of the signal from RF source
10
being transduced into an optical wave and the RF signal of source
12
being transduced into an acoustic wave.
It is, accordingly, an object of the present invention to provide a new and improved time difference of arrival detecting apparatus.
Another object of the invention is to provide a new and improved electro-acoustic-optical device.
A further object of the invention is to provide a new and improved electro-acoustic-optical device and to an apparatus for using same, wherein waves that are combined in an optical defracting medium are launched in the same wave domain.
An additional object of the invention is to provide an electro-acoust
Carrott David T.
Nayfeh Mahir A.
Dang Hung Xuan
Lowe Hauptman & Gilman & Berner LLP
TASC, Inc.
LandOfFree
Electro-acoustic-optical apparatus and method of calibrating... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electro-acoustic-optical apparatus and method of calibrating..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electro-acoustic-optical apparatus and method of calibrating... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2548856