Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-01-17
2002-10-08
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S281000, C359S284000, C359S324000, C359S312000
Reexamination Certificate
active
06462857
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator and an optical modulation method, and more particularly, the present invention relates to an optical modulator and an optical modulation method for obtaining an optical intensity as a result of modulation by an alternating-current signal of several hundreds or lower megahertz.
2. Description of the Related Art
Conventionally, optical modulators have been configured such that light passes through a magnetic ferrite single crystal, such as an yttrium iron garnet (YIG), a thin-line transducer, a microstrip line, or other suitable element, microwaves are applied thereto, and the optical intensity is modulated by the microwaves. (For example, refer to an article by Makoto Tsutsumi, Tetsuya Ueda, and Others in “Shingaku Giho”, Vol. 98, No. 123 MW
98-41,
OPE
98-33
(1988), pp. 45). A conventional optical modulator is shown in FIG.
1
.
FIG. 1
is a view showing an example of a conventional optical modulator that relates to the present invention described hereinbelow. An optical modulator
10
shown in
FIG. 1
includes a plate-like magnetic garnet single crystal
12
for use as a magnetic ferrite single crystal. A microstrip line
14
defining a transducer is provided in a central portion of one of the primary surfaces of the magnetic garnet single crystal
12
.
An output terminal of a microwave-signal generator
16
is connected to one end of the microstrip line
14
via(a microwave amplifier
18
. In addition, an output terminal of an optional-signal generator
19
is connected to the microwave-signal generator
16
. The optional-signal generator
19
modulates a microwave that has been output from the output terminal of the microwave-signal generator
16
, and it uses an alternating-current signal having a frequency that is lower than that of the microwave for the modulation. The other end of the microstrip line
14
is connected to a terminal resistor
20
.
In addition, a permanent magnet (not shown) is provided near the magnetic garnet single crystal
12
. The permanent magnet is used to apply a dielectric-current magnetic field in a direction that is parallel to the magnetic garnet single crystal
12
and perpendicular to the microstrip line
14
.
Outside of one peripheral surface of the magnetic garnet single crystal
12
, a light source such as a laser-beam source
22
, a polarizer
24
, and a first lens
26
are arranged to be close to the magnetic garnet single crystal
12
in that order. The laser-beam source
22
generates laser beams. The polarizer
24
linearly polarizes laser beams, which have been generated by the laser-beam source
22
, in a predetermined direction. The first lens
26
concentrates the laser beams, which have been generated by the laser-beam source
22
, into the magnetic garnet single crystal
12
. In this way, the laser-beam source
22
is arranged such that laser beam emitted from the laserbeam source
22
is introduced to the magnetic garnet single crystal
12
and modulated by the microwave applied to the microstrip line
14
.
In addition, in the outside of another peripheral surface of the magnetic garnet single crystal
12
, a second lens
28
, an analyzer
30
, a third lens
32
, and a photodiode
34
are arranged to be spaced apart from the magnetic garnet single crystal
12
in that order and arranged to receive the light beam emitted from the magnetic garnet single crystal
12
. The second lens
28
rectifies laser beams transmitted through the magnetic garnet single crystal
12
to parallel beams. The analyzer
30
allows the linearly polarized laser beams to be transmitted in a predetermined direction and the analyzer is arranged to have the crossed-Nicols relationship with the polarizer
24
. The third lens
32
converges the laser beams transmitted through the analyzer
30
. The photodiode
34
detects laser-beam signals.
Also, an output terminal of the photodiode
34
is connected to an input terminal of a photoelectric-current amplifier
36
.
In the optical modulator
10
shown in
FIG. 1
, optical systems (configurations of the magnetic garnet single crystal
12
and the components on two sides thereof) are substantially the same as a measurement optical system for transmission-type photomagnetic effects, such as a Faraday effect. Specifically, a microwave is applied to the magnetic garnet single crystal
12
via the microstrip line
14
defining the transducer to couple the light and the microwave together in the magnetic garnet single crystal
12
, and a polarized state of the light is modulated according to the microwave. The modulated state is then converted by a Faraday optical system into the variation in the intensity for the implementing detection.
FIG. 2
is a view showing a waveform of an optical-signal that is produced without a microwave being applied in the optical modulator
10
shown in FIG.
1
.
FIG. 3
is a view showing a waveform of an optical-signal that is produced with a microwave being applied in the optical modulator
10
shown in FIG.
1
. When the microwave is not applied, as shown in
FIG. 2
, the light transmitted through the magnetic garnet single crystal
12
remains in the state of direct current, and the optical output is constant. However, when the microwave is applied, as shown in
FIG. 3
, a modulated alternating-current component overlaps with a direct-current component.
The microwave applied to the magnetic garnet single crystal
12
is preliminarily modulated according to the alternating-current signal, which has been generated by the optional-signal generator
19
and which has a frequency lower than that of the microwave. Therefore, the intensity of the light led to be incident on the photodiode
34
is also modulated further by the microwave modulated according to the low-frequency signal. When the frequency band of each of the photoreceptor systems (such as the photodiode
34
and the photoelectric-current amplifier
36
) includes the frequency of the signal that modulates the microwave and is lower than the frequency of the microwave, the individual photoreceptor system functions as a low-pass filter in which only the signal for modulating the microwave has sensitivity to the optical signal. Accordingly, the alternating-current component of the optical signal that is output from the photoelectric-current amplifier
36
matches the low-frequency signal that modulates the microwave. As a result, the optical-signal output is modulated according to the alternating-current signal having the frequency lower than the microwave.
The reason that the microwave is first modulated using the low-frequency alternating-current signal and is then applied to the magnetic garnet single crystal
12
is that any frequency band other than the microwave frequency band cannot be propagated through the magnetic garnet single crystal
12
.
In the optical modulator
10
, either when an external magnetic field is not applied to the magnetic garnet single crystal
12
or when an applied external magnetic field applied thereto is extremely weak, the microwave is just propagated therethrough, and optical modulation is implemented by the microwave. When a sufficiently intensive external magnetic field is applied to the magnetic garnet single crystal
12
, a static magnetowave is excited, and optical modulation is implemented by the static magnetowave.
However, conventionally, while attempts have been made to generate an optical modulation phenomenon according to either the microwave or the static magnetowave, no attempt has been successfully made to achieve greatly increased modulation.
For this reason, with the conventional optical modulator having the configuration shown in
FIG. 1
, problems are caused such that the optical-modulation amplitude cannot be easily increased to a desired very large value, and the signal to noise ratio is very small.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide an optical modulator optical and a m
Chiku Shinichiro
Fujii Takashi
Dang Hung Xuan
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Tra Tuyen
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