Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
2001-08-31
2004-01-06
Healy, Brian (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S001000, C385S002000, C359S238000, C359S245000, C359S254000
Reexamination Certificate
active
06674927
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator, and in particular to an optical modulator having an electric waveguide and an optical waveguide.
Recent optical communication technology has been remarkably developed, leading a trunk information communication network to a high-speed optical communication, so that optical fibers are going to be brought into households. With this development, it has become more and more important to enhance the speed of an optical modulator which puts information on a lightwave, presenting one of the basic communication technologies for transmitting a large amount of information at a high speed.
2. Description of the Related Art
As a prior art optical modulator, a Mach-Zehnder type optical modulator of LiNbO
3
(Lithium Niobate, hereinafter abbreviated as LN), for example, is an optical intensity modulator having a good transmission characteristic, by the combination of an LN optical phase modulator and a Mach-Zehnder type interferometer. It is used for many transmitters of a high optical transmission speed such as 2.4 GHz, 10 GHz, and 40 GHz.
FIG. 12
shows an arrangement of an optical phase modulator
100
′, which is composed of an electric waveguide
20
(generally referred to as electrode) and an optical waveguide
10
. A modulating signal
71
from a modulating signal generator
40
is inputted to the electric waveguide
20
as a modulating signal
71
a
through a driver
50
.
The electric waveguide
20
converts the inputted modulating signal
71
a
into an acting amount (modulating amount)
80
to be provided to the optical waveguide
10
. The acting amount
80
is for providing a modulation to a lightwave
81
which propagates through the optical waveguide
10
.
In case of an optical phase modulation made by an electro-optical effect for example, the acting amount
80
is proportional to the product of a modulation voltage V (electric field E) and its acting interval L.
Since there is a resistance caused by a skin effect in the metal of the electric waveguide
20
in a modulator such as a traveling wave type optical phase modulator, a frequency characteristic of f
1/2
arises as the frequency of the modulating signal increases, thereby narrowing a bandwidth (see characteristic curve A in FIG.
5
A). Accordingly, the acting amount
80
of the travelling wave type optical phase modulator has an attenuation in a high frequency area caused by the skin effect (see characteristic curve B in FIG.
5
A).
Also, the acting amount
80
of a concentrated constant type optical phase modulator has an attenuation in the high frequency area. This attenuation is determined by an impedance-matching resistance R (not shown) between the electric waveguide (electrode)
20
and the driver
50
, and by the frequency characteristic determined by a capacitance C (not shown) and stray capacitances of the electric waveguide
20
. It is to be noted that this frequency characteristic is not shown.
In either optical modulator, an intersymbol interference increases as the attenuation increases, so that an optical waveform deteriorates.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide an optical modulator having an electric waveguide and an optical waveguide which suppresses an attenuation in a high frequency band of an acting amount inputted to the optical waveguide.
In order to achieve the above-mentioned object, an optical modulator according to the present invention comprises: an optical waveguide for propagating a lightwave, an electric waveguide for providing an acting amount, to the optical waveguide, for modulating the lightwave by a modulating signal, and a filter for converting the modulating signal into a signal approximating a frequency characteristic of the acting amount to be provided to the electric waveguide.
FIG. 1
shows a principle (1) of an optical modulator
100
according to the present invention. A modulating signal
71
outputted from a modulating signal generator
40
serves to modulate a lightwave
81
. An electric waveguide
20
provides, to an optical waveguide
10
, an acting amount
80
obtained by converting a modulating signal
72
based on a control method of an optical modulator, e.g. a control method of an electro-optical effect or the like.
The acting amount
80
has a frequency characteristic caused, for example, by stray capacitances of the electric waveguide
20
itself in a concentrated constant type optical phase modulator, and by a skin effect of the electric waveguide
20
itself in a traveling wave type optical phase modulator as will be described later.
Accordingly, when the modulating signal
72
same as the modulating signal
71
is inputted to the electric waveguide
20
, the lightwave
81
which propagates through the optical waveguide
10
becomes a lightwave
82
modulated by the acting amount
80
having the frequency characteristic.
In the present invention, a filter
30
converts the modulating signal
71
into the modulating signal
72
for equalizing the frequency characteristic (not a frequency characteristic of electric waveguide itself of the acting amount
80
to be inputted to the electric waveguide
20
. Thus, the frequency characteristic of the acting amount
80
is approximated to be almost flat in all of the areas from the low frequency area to the high frequency area, enabling the optical modulator
100
to perform an optical modulation independent of the frequency of the modulating signal
71
.
It is to be noted that since
FIG. 1
is a schematic diagram, the driver
50
shown in
FIG. 12
is omitted.
Also, in the present invention according to the above-mentioned invention, the frequency characteristic of the acting amount may comprise a characteristic caused by a skin effect in the electric waveguide.
Namely, as mentioned above, the resistance of the electric waveguide
20
has the frequency characteristic of f
1/2
by the skin effect. Accordingly, the acting amount
80
has the frequency characteristic caused by the frequency characteristic of the resistance. The filter
30
approximates this frequency characteristic of the acting amount
80
.
FIGS. 2A and 2B
show a principle of approximating the frequency characteristic of the acting amount
80
caused by e.g. the skin effect.
FIG. 2B
shows a model of the electric waveguide
20
, in which a modulating source
40
, its internal resistor
42
, the electric waveguide
20
having a skin effect resistance, and a terminal resistor
44
are connected in cascade. The values of the resistors
42
,
44
, and of an impedance Z of the electric waveguide
20
are R
0
.
FIG. 2A
shows a distribution of a voltage v of the electric waveguide
20
. The position of an acting interval where the electric waveguide
20
provides the acting amount
80
to the optical waveguide
10
is indicated by a length (distance) x normalized by a length L of the acting interval. Accordingly, x=0 at an input end of the electric waveguide
20
, and x=1 at the terminating end. Since the voltage v is a function of the distance x and a frequency f of the modulating signal, it is expressed by the following equation (1):
v=v
(
f, x
) Eq.(1)
Accordingly, in the presence of the skin effect, a transfer function (S
21
parameter) indicating the relationship between the voltage v(f, 0) of the input end and the voltage v(f, 1) of the terminating end is expressed by the following equation (2):
20
log
10
v
(
f
,
1
)
v
(
f
,
0
)
=
-
α
f
Eq. (2)
where &agr; is a constant.
If the voltage v(f, x) is normalized by v(f, 0)=1, Eq.(2) assumes the following equation (3), and accordingly, an output voltage v(f, 1) is expressed by the following equation (4):
20 log
10
v
(
f,
1)=−&agr;
{square root over (f)}
Eq.(3)
v
(
f
,
1
)
=
10
-
α
f
20
Eq. (4)
The frequency characteristic of Eq.(4) corresponds to the attenuated amount curve A in
FIG. 5A
as will be described later.
Since being dis
Fujitsu Limited
Healy Brian
Petkovsek Daniel
Staas & Halsey , LLP
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