Optical waveguides – Directional optical modulation within an optical waveguide – Acousto-optic
Reexamination Certificate
2001-07-27
2004-03-02
Healy, Brian (Department: 2874)
Optical waveguides
Directional optical modulation within an optical waveguide
Acousto-optic
C385S011000, C359S199200
Reexamination Certificate
active
06701031
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acousto-optical tunable filter which can appropriately filter/select/drop/add lights with a plurality of frequencies (wavelengths) and a method of driving the acousto-optical tunable filter. Further, the present invention relates to an optical add/drop multiplexer using the acousto-optical tunable filter.
In recent years multimedia communication, such as the Internet has been spreading quickly. In the field of communication technology, research and development of optical communication technology, which allows ultra-long distance communication and large-capacity communication, has been diligently undertaken in order to cope with the substantial increase in traffic volume from this high proliferation of multimedia communications. To cope with further increase in traffic volume, measures have been taken to increase the speed of transmission of wavelength-division multiplexing and to increase high-density multiplexing of the wavelength-division multiplexing (hereinafter abbreviated “WDM”) transmission.
In particular, in recent years there has been a need for an optical communication system having an ADM (add-drop multiplexer) function, as well as an optical communication system for sending/receiving a WDM optical signal between two stations. In such an optical communication system having the ADM function, only an optical signal with a predetermined frequency from within the WDM optical signal, would be selectively transmitted through repeater stations, which are called node, and which are provided in an optical transmission line. An optical signal with a frequency other than the predetermined frequency would be dropped therefrom in the node, and another optical signal would be added thereto in the node to be transmitted to another node. Hence, considerable research has been made on the acousto-optical tunable filter (hereinafter abbreviated “AOTF”) for realizing the ADM function.
2. Description of the Related Art
The AOTF is an optical component which induces a refractive index change in an optical waveguide by an acousto-optical effect, and rotates the polarization state of light propagating through the optical waveguide to separate/select an optical signal.
FIG. 18
is a schematic view showing the structure of an example of a conventional AOTF.
As shown in
FIG. 18
, a port Pin, a port Pad, a port Pth and a port Pdr are provided at input ends and output ends of optical waveguides
51
,
52
, respectively. The optical waveguides
51
,
52
intersect with each other at two points, and polarization beam splitters (hereinafter abbreviated “PBS”)
53
,
55
are respectively provided at the intersections. The optical waveguides
51
,
52
are formed on a substrate as a piezoelectric crystal in the AOTF and, for example, may be formed on a substrate of lithium niobate (LiNbO
3
) by titanium (Ti) diffusion.
An interdigital transducer (hereinafter abbreviated “IDT”)
54
is formed on the waveguides
51
,
52
between the intersections where the polarization beam splitters are provided. The IDT
54
generates a surface acoustic wave by an RF signal supplied from oscillators
301
-
1
to
301
-
4
and changes the refractive indexes of the optical waveguides
51
,
52
. Two absorbers
56
,
57
are provided for absorbing the surface acoustic wave, which is unnecessary for changing the refractive indexes of the optical waveguides
51
,
52
, and are formed to interpose the IDT
54
. An interval between the absorbers
56
,
57
is an action length.
A substrate part
16
of the AOTF is structured to include the optical waveguides
51
,
52
, the PBSs
53
,
55
, the IDT
54
and the absorbers
56
,
57
on the substrate.
Input light
1
, which is inputted into the port Pin, is light with a TE mode and a TM mode coexisting therein. The input light
1
is demultiplexed by the PBS
53
to TE mode light and TM mode light and the TM mode light propagates through the optical waveguide
51
and the TE mode light propagates through the optical waveguide
52
. It should be mentioned that, when the surface acoustic wave is generated by applying an RF signal with a predetermined frequency, the refractive indexes of the optical waveguides
51
,
52
change. For this reason, only light whose frequency interacts with this refractive index change, from among the input light
1
, rotates its polarization state. The amount of the rotation is proportional to the action length in which the lights in the respective modes interact with the refractive index change and power of the RF signal.
Therefore, by optimizing the action length and the power of the RF signal, the TM mode light is converted to the TE mode light in the optical waveguide
51
, and the TE mode light is converted to the TM mode light in the optical waveguide
52
. As a result, the light, whose mode is converted, is outputted by the PBS
55
to the port Pdr as selected light, and the light, whose mode is not converted, is outputted to the port Pth as transmitted light.
Thus, the transmitted light, which is outputted from the port Pth, is the input light
1
inputted into the port Pin from which only the light with the frequency corresponding to the frequency of the RF signal is removed. Hence, it is possible to assume that the AOTF has a rejection function (band-eliminating function).
Meanwhile, input light
2
, which is inputted into the port Pad, is also demultiplexed by the PBS
53
to the TE mode light and the TM mode light, and the TM mode light propagates through the optical waveguide
52
and the TE mode light propagates through the optical waveguide
51
. When the surface acoustic wave is generated by applying the RF signal with the predetermined frequency, only light with a predetermined frequency rotates its polarization state, and the TE mode light is converted to the TM mode light in the optical waveguide
51
and the TM mode light is converted to the TE mode light in the optical waveguide
52
. As a result, the light, whose mode is converted, is outputted by the PBS
55
to the port Pth on the transmitted light side of the ATOF, and the light, whose mode is not converted, is outputted to the port Pdr on the selected light side of the AOTF.
The selected light, which is outputted from the port Pdr, is the input light
1
inputted into the port Pin from which only the light with the frequency corresponding to the frequency of the RF signal is selected. Further, the transmitted lights which is outputted from the port Pth, is the input light
1
inputted into the port Pin from which only the light with the frequency corresponding to the frequency of the RF signal is removed, and only the light with the frequency corresponding to the frequency of the RF signal, out of the input light
2
, which is inputted into the port Pad, is added to the removed frequency. Hence, it is possible to assume that the AOTF has an optical add/drop function.
Furthermore, by changing the frequency of the RF signal, the AOTF can change the frequency of light to be selected/added/transmitted, and hence it also functions as a variable wavelength selective filter.
FIG. 19
is a view showing a relationship between a frequency of selected light and an RF frequency for the case of selecting one frequency in the AOTF. The horizontal axis of
FIG. 19
indicates the frequency of the selected light displayed in THz, and the vertical axis thereof indicates the RF frequency of an RF signal displayed in MHz. As shown in
FIG. 19
, the frequency of the selected light is proportional to the frequency of the RF signal.
FIG. 19
clearly shows, for example, to select light with a frequency of 193.4 THz, it is suitable to supply an RF signal with a frequency of 174.4 MHz to the IDT
54
. To select light with a frequency of 194.2 THz, it is suitable to supply the RF signal with a frequency of 175.2 MHz to the IDT
54
.
Meanwhile, to select lights with a plurality of frequencies (wavelengths) in the AOTF, a plurality of RF signals, whose frequencies are different from each other, are supplied t
Fujitsu Limited
Healy Brian
Lin Tina
Staas & Halsey , LLP
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