Optical: systems and elements – Optical modulator
Reexamination Certificate
2002-12-17
2004-06-01
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
C359S238000, C385S002000, C398S186000
Reexamination Certificate
active
06744546
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multilevel light-intensity modulating circuit (or apparatus) for producing a multilevel modulated optical signal.
2. Description of the Related Art
In accordance with the requirements for increasing the transmission capacity in optical communication systems, improvement of the efficiency of using optical bandwidth is required in wavelength-division multiplexed transmission systems, thereby producing an important goal of performing band suppression of the optical spectrum so as to arrange a plurality of wavelength channels at narrow spacing.
In order to achieve this goal, multilevel (or multivalued) signals are used so as to decrease the bit rate, thereby suppressing the spectrum width. For example, in comparison with conventional methods, such as two-level light-intensity modulation methods, when 2
n
amplitude levels are defined in signal transmission, the same amount of data (as that transmitted in the conventional method) can be transmitted at a bit rate of 2/2
n
and the spectrum width can also be suppressed to approximately 2/2
n
as much as the band necessary for the conventional methods.
FIG. 7
shows an example of the structure of the conventional multilevel (here, four-level) light-intensity modulating circuit, where the light source
66
is not included in the modulating circuit (refer to S. Walkin et al., “A 10 Gb/s 4-ary ASK Lightwave System”, ECOC 97, Conference Publication No. 448, pp. 255-258, 1997).
In the figure, two two-level (or binary amplitude-shifted) electric signals having the same power are respectively input into two input terminals
61
and
62
. The power of one of the two electric signals is attenuated to half by using the attenuator
63
, and two signals are then combined by the power combiner
64
, thereby producing a four-level (or quaternary amplitude-shifted) electric signal. This four-level electric signal is applied to the light-intensity modulator
65
, in which the intensity of an optical carrier output from the light source
66
is modulated, thereby producing a four-level modulated optical signal.
FIGS. 8A
to
8
D show eye patterns which can be observed in a numerical calculation for producing a four-level electric signal by using two two-level electric signals, and further producing a four-level modulated optical signal.
That is, two two-level electric signals, whose eye patterns are respectively shown in
FIGS. 8A and 8B
, are electrically combined using a system as shown in
FIG. 7
, so that a four-level electric signal as shown in
FIG. 8C
is produced. This signal is used for intensity modulation of the optical carrier, thereby producing a four-level modulated optical signal as shown in FIG.
8
D.
Here, a Mach-Zehnder light-intensity modulator is commonly used as the light-intensity modulator
65
.
FIG. 9A
is a diagram showing the response characteristics obtained when the Mach-Zehnder light-intensity modulator is used for two-level intensity modulation. As is clearly shown by the figure, amplitude distortion in each level of mark “1” and mark “0” in the two-level electric signal is suppressed, that is, a two-level modulated optical signal having preferable characteristics is obtained.
However, when intensity modulation of the optical carrier is performed using a four-level electric signal, amplitude distortion at level “0” and level “3” is suppressed, but amplitude distortion at levels “1” and “2” is increased, as shown in FIG.
9
B.
In addition, the response characteristics of the Mach-Zehnder light-intensity modulator is non-linear; thus, in order to equalize each interval between adjacent levels in the four-level modulated optical signal, a four-level electric signal, in which the interval between levels “1” and “2” is narrowed in advance, must be produced. This condition is also required when intensity modulation is performed using a multilevel (more than four-level) electric signal, and it is inevitable to suitably define the interval between the intermediate levels, and amplitude distortion should be suppressed.
It is desirable, therefore, to provide a multilevel light-intensity modulating circuit for suppressing the amplitude distortion regarding intermediate levels, caused by the conversion from a multilevel electric signal to a multilevel modulated optical signal.
SUMMARY OF THE INVENTION
The present invention provides a multilevel light-intensity modulating circuit comprising:
an optical distribution section for distributing an input optical carrier into n-channel optical carriers, where n is an integer of 2 or greater;
n light-intensity modulators into which the n-channel optical carriers are respectively input, wherein each light-intensity modulator modulates intensity of the input optical carrier by using an input two-level electric signal and outputs a two-level modulated optical signal;
an optical phase control section for producing a phase difference between the n-channel two-level modulated optical signals which are respectively output from the n light-intensity modulators;
a light-intensity control section for assigning a different light intensity to each of the n-channel two-level modulated optical signals which are respectively output from the n light-intensity modulators; and
an optical coupling section for combining the n-channel two-level modulated optical signals obtained via the optical phase control section and the light-intensity control section, and outputting a 2
n
-level modulated optical signal, wherein:
the phase difference produced by the optical phase control section and the different light intensity assigned by the light-intensity control section are defined in advance so as to produce the 2
n
-level modulated optical signal.
The optical phase control section may be positioned at the input or output side of at least one of the n light-intensity modulators.
The light-intensity control section may be positioned at the input or side of at least one of the n light-intensity modulators.
As a typical example, the light-intensity control section has a structure for respectively attenuating the light intensities of (n−1) channel input signals to 1/2, 1/4, . . . , 1/2
n−1
as high as the original light intensities.
In a specific example,
n=2;
the optical distribution section has a distribution ratio of 1:1;
the light-intensity control section defines the light-intensity ratio between the 2-channel modulated optical signals as 2:1±8%;
the optical phase control section provides a phase difference of 90°±3% between the 2-channel modulated optical signals; and
the optical coupling section has a structure for coupling the 2-channel modulated optical signals and producing the four-level modulated optical signal.
The present invention also provides a multilevel light-intensity modulating circuit comprising:
an optical distribution section for distributing an input optical carrier into n-channel optical carriers, where n is an integer of 2 or greater;
n light-intensity modulators into which the n-channel optical carriers are respectively input, wherein each light-intensity modulator modulates intensity of the input optical carrier by using an input two-level electric signal and outputs a two-level modulated optical signal;
an optical phase control section for producing a phase difference between the n-channel two-level modulated optical signals which are respectively output from the n light-intensity modulators; and
an optical coupling section for combining the n-channel two-level modulated optical signals obtained via the optical phase control section, and outputting a 2
n
-level modulated optical signal, wherein:
a distribution ratio of the optical distribution section, a coupling ratio of the optical coupling section, and the phase difference produced by the optical phase control section are defined in advance so as to produce the 2
n
-level modulated optical signal.
The optical phase control section may be positioned at the input or output side of at least one of the n light-intensity m
Kani Jun-ichi
Nakamura Takuya
Takachio Noboru
Teshima Mitsuhiro
Epps Georgia
Harness & Dickey & Pierce P.L.C.
Hasan M.
Nippon Telegraph and Telephone Corporation
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