Light modulator of waveguide type

Details Light modulator of waveguide type Light modulator of waveguide type

2000-04-10

2003-02-18

Bovernick, Rodney (Department: 2874)

Optical waveguides

Temporal optical modulation within an optical waveguide

Electro-optic

C385S004000, C385S008000, C385S009000, C385S129000

Reexamination Certificate

active

06522792

ABSTRACT:

TECHNICAL FIELD
This invention relates to an optical waveguide modulator configuration, particularly, an optical waveguide modulator configuration preferably applied to waveguide type optical intensity-modulators, phase-modulators, and polarization scramblers employed in high speed and large capacity optical fiber-communication systems and wavelength division multiplexing systems.
BACKGROUND ART
With the recent advances in high speed and large capacity optical fiber-communication systems, from the viewpoint of broad bandwidth, low chirp and low propagation loss characteristics, waveguide type external modulators using substrates made of lithium niobate (LiNbO
3
: hereinafter often abbreviated to “LN”) are being realized, rather than conventional diodes which are direct-modulation type.
FIG. 1
is a cross sectional view showing an example of a conventional optical waveguide modulator.
An optical waveguide modulator
10
, as shown in
FIG. 1
, has a substrate
1
made of “LN” etc., a Mach-Zehnder type interferometer
2
, formed by thermal diffusion of Ti into the substrat
1
, a travelling wave-type signal electrode
3
and ground electrodes
4
made of Au that is applied directly on the optical waveguide
2
, or on a nearby surface.
Moreover, for lowering the absorption loss of the lightwave travelling in the optical waveguide
2
by the travelling wave-type signal electrode
3
and the ground electrodes
4
and matching of velocity between the lightwave and microwave travelling on the signal electrode
3
, a buffer layer
5
made of silicon dioxide (SiO
2
) is formed between the substrate
1
and the signal electrode
3
and the ground electrode
4
.
Furthermore, with the developments in recent optical communication systems, multi-functions as well as high speeds and large capacity are required. In particular, the wavelength-multiplexing in the same optical waveguide, the switching and the exchanging of optical transmission guides are sought. Such communication systems are being realized with a wavelength division multiplexing method (hereinafter often abbreviated to “WDM system”) using an optical fiber amplifier (hereinafter often abbreviated to “EDFA”).
The WDM system transmit, by a single optical fiber, multiple lightwaves having different wavelengths from the corresponding optical sources, to each lightwave being modulated by one of the different signals. That is, the system requires to prepare multiple optical modulators each connected with the corresponding optical source, and any one of the signals modulated by the multiple optical modulators is transmitted by a single optical fiber. The EDFA is provided in its transmission guide to amplify the gain of transmitted lightwave.
The WDM system enables the transmission capacity of the whole communication system to be increased without augmenting the number of optical fibers and the bit rate of each signal.
The WDM system requires the transmission condition of each lightwave to be constant. However, there is a problem that received intensity of an optical signal at the detector sometimes fluctuate in each transmitted lightwaves, on account of the wavelength dependency of the EDFA's gain and the change of the output power with time from each optical source, etc.
To overcome this problem, the integration of an attenuator with each of the optical modulator is being attempted.
FIG. 2
is a top plan view showing an example of a conventional optical waveguide modulator to which an attenuator is integrated. FIGS.
3
(
a
) and
3
(
b
) are cross sectional views of the optical modulator shown in FIG.
2
.
FIG. 3
a
is a cross sectional view of an optical modulation part, taken on line A-A' of
FIG. 2
, and
FIG. 3
b
is a cross sectional view of an attenuator part, taken on line B-B' of FIG.
2
.
A conventional optical waveguide modulator
30
shown in
FIG. 2 and 3
has a substrate
11
made of a material having an electrooptic effect, a first interferometer
12
and a second interferometer
13
formed by thermal diffusion of Ti into the substrate. Then, it has a buffer layer
14
made of silicon dioxide, etc. formed on the substrate
11
. On the buffer layer
14
are formed a first signal electrode
15
, first ground electrodes
16
, a second signal electrode
17
and second ground electrodes
18
.
Electrical inputs of the first and the second signal electrodes
15
and
16
, are connected with external electric power supplies
21
and
22
, respectively, the output of the first signal electrode
15
being terminated via a resistor “R” and a capacitor “C”. Metal-cladding type waveguide polarizers
23
and
24
are provided in the input and output sides of the optical modulator
30
.
The first interferometer
12
, the first signal electrode
15
and the first ground electrodes
16
constitute an optical modulation part
28
. The second optical waveguide
13
, the second signal electrode
17
and the second ground electrodes
18
constitute an attenuator part
29
. The first signal electrode
15
and the first ground electrodes
16
constitute an electrode for modulation. The second signal electrode
17
and the second ground electrodes
18
constitute an electrode for attenuation. And, the first interferometer
12
is in series connected with the second interferometer
13
in the boundary “H” between the optical modulation part
28
and the attenuator part
29
. The arrow in
FIG. 2
depicts a travelling direction of a lightwave.
The buffer layer
14
is formed to prevent the absorption of the lightwave guiding in the optical waveguide by the modulation electrode and the attenuator electrode.
When a lightwave having a wavelength of &lgr;
1
is incident into the optical waveguide modulator
30
, it is on-off switched and thereafter its intensity is controlled in attenuator part
29
. That is, by compulsive attenuation of the intensities of specific optical signals having large output powers, the intensity of each optical signal having different wavelengths, is equalized in the whole communication system.
Such an optical waveguide modulator, as shown in
FIG. 1
, is desired to be enhanced in modulation efficiency in view of reducing the load for a high frequency driver. Thus, the distance between the optical waveguide and the travelling type signal electrode and electrode gap are required to be shorter and narrower, respectively, to lower the driving voltage of the optical modulator.
However, as shown in the optical waveguide modulator
10
in
FIG. 1
, when the buffer layer
5
is formed between the substrate
1
and the travelling type signal electrode
3
or the like, the distance between the optical waveguide
2
and the signal electrode
3
is inevitably increased and thereby the driving voltage can not be efficiently lowered.
Moreover, such an optical waveguide modulator as in
FIGS. 2 and 3
, is required to have relatively longer interaction length in optical modulation part
28
to realize low driving voltage. However, in the optical waveguide modulator having above-mentioned configuration, the attenuator part
29
can not have sufficient length because of limitation in wafer size. As a result, attenuator part
29
requires a very high driving voltage.
If the driving voltage is being higher, an electric discharge sometimes occur in the electrodes of the attenuator part
29
, resulting in the destruction of the optical waveguide modulator
30
itself. Thus, the above optical modulator does not have a sufficient reliability.
In addition, if the driving voltage is being higher, there is practical problem that a DC drift due to the buffer layer
14
tends to be larger.
It is an object of the present invention to provide a new optical waveguide modulator configuration capable of reducing driving voltage in an optical modulation part or an attenuator part.
DESCRIPTION OF THE INVENTION
The first optical waveguide modulator applying the present invention has a substrate made of a material having an electrooptic effect, an optical waveguide to guide a lightwave, travelling wave-type signal electrode, ground electrodes

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