Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
2001-01-08
2004-04-27
Pascal, Leslie (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S076000, C398S082000, C398S083000, C398S180000, C398S201000, C359S308000, C359S312000
Reexamination Certificate
active
06728487
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acousto-optic filter drive method realizing reduction of a drive apparatus in size for suppressing the time-fluctuations of output characteristics of acousto-optic filters. The invention further relates to an acousto-optic filter applied to the acousto-optic filter drive method, an optical add/drop multiplexer, an optical communication system, and a selective wavelength extension method.
Recently, there has been demanded an optical communication system with a super-long distance and a large capacity so as to construct future multi-media networks. For realizing the large capacity, there has been investigated and developed the wavelength-division multiplexing (WDM) system because it has advantages of utilizing the wide band and the large capacity of optical fibers with efficiency.
Especially in recent years, it has been demanded to realize not only the optical communication system for transmitting/receiving the WDM optical signal between two stations but also an optical communication system having the ADM function, in which an optical signal having a specific wavelength of the WDM optical signal is selectively passed through a repeater station called the “node” disposed midway of the optical transmission line and in which the optical signals at other wavelengths are dropped at that node or another optical signal is added from the node and transmitted to another node. In order to realize the ADM function, there have been extensively investigated the acousto-optic filters (AOTF).
2. Description of the Related Art
The AOTF is an optical part for rotating the polarization state of light to propagate through an optical waveguide by inducing a refractive index change due to the acousto-optic effect in the optical waveguide thereby to separate/select the light of a specific wavelength. One example of the AOTF will be described in the following.
In the AOTF, as shown in
FIG. 12
, optical waveguides
502
and
503
are formed in a substrate
501
made of a piezoelectric crystal. For example, the optical waveguides are formed in the substrate of lithium niobate (LiNbO
3
) by the titanium (Ti) diffusion method. As shown in
FIG. 12
, the optical waveguides
502
and
503
are individually equipped at their input terminals and output terminals with a port Pin and a port Pad, and a port Pth and a port Pdr. The optical waveguides
502
and
503
intersect each other at two portions, which are equipped with polarization beam splitters (PBS)
504
and
509
.
Between the intersecting portions, an inter digital transducer (IDT)
506
is formed over the optical waveguides
502
and
503
. A surface acoustic wave is generated by applying RF signals generated by a signal source
507
, to the IDT
506
to change the refractive indices of the optical waveguides
502
and
503
.
An input light
1
to be inputted to the port Pin is a mixture of a TE mode and a TM mode. This input light
1
is separated by the PBS
504
into the TE mode light and the TM mode light, of which the TM mode light propagates through the optical waveguide
502
and the TE mode propagates through the optical waveguide
503
.
Now, when the surface acoustic wave is generated by applying the RF signal of a predetermined frequency, the refractive indices of the optical waveguides
502
and
503
change. Of the input light
1
, therefore, only the light having a wavelength to interact on the change in the refractive index rotates the polarized light state. The rotation is proportional to the working length for the light in each mode to interact on the change in the refractive index and to the power of the RF signal. The working length is adjusted by the interval between absorbers
505
and
508
for absorbing the surface acoustic wave to be generated over the optical waveguides
502
and
503
across the IDT
506
.
By optimizing the working length and the power of the RF signal, therefore, the TM mode light is transformed into the TE mode light in the optical waveguide
502
, and the TE mode light is transformed into the TM mode light in the optical waveguide
503
.
As a result, the mode-changed light is outputted as a selected light to the port Pdr by the PBS
509
, whereas the light left unchanged in the mode is outputted as the transmitted light to the port Pth.
Here, the transmitted light outputted from the port Pth is prepared by eliminating only the light of the wavelength corresponding to the frequency of the RF signal from the input light
1
inputted to the port Pin. It is, therefore, possible to assume that the AOTF has a rejection function (i.e., band eliminating function).
On the other hand, an input light
2
inputted to the port Pad is likewise separated by the PBS
504
into the TE mode light and the TM mode light. Of these, the TM mode light propagates through the optical waveguide
503
, and the TE mode light propagates through the optical waveguide
502
. Now, when the surface acoustic wave is generated by applying the RF signal of a predetermined frequency, only the light of the predetermined wavelength rotates its polarized light state so that the TE mode light is transformed into the TM mode light in the optical waveguide
502
whereas the TM mode light is transformed into the TE mode light in the optical waveguide
503
. As a result, the light changed in the mode is outputted to the port Pth on the transmitted light side by the PBS
509
, and the light left unchanged in the mode is outputted to the port Pdr on the selected light side.
Here, the selected light outputted from the port Pdr is made by selecting only light at a wavelength corresponding to the frequency of the RF signal from the input light
1
inputted to the port Pin. The transmitted light outputted from the port Pth is made by eliminating only light at a wavelength corresponding to the frequency of the RF signal from the input light
1
inputted to the port Pin and by adding only light at a wavelength corresponding to the frequency of the RF signals, from the input light
2
inputted to the port Pad, to the eliminated wavelength. Therefore, the AOTF can be though to have the optical adding/dropping functions.
Moreover, the AOTF is enabled to change the wavelength of the selected/added/transmitted light by changing the frequency of the RF signal so that it functions as a tunable filter.
When lights at a plurality of wavelengths are to be selected/dropped by the AOTF, on the other hand, a plurality of RF signals having different frequencies are applied to the IDT
506
of the AOTF. Therefore, beats are necessarily generated in the surface acoustic waves by the plurality of RF signals so that the center wavelength of the lights to be selected/dropped fluctuates with time in accordance with the beats. As a result, the optical power at the target wavelength to be selected/dropped will fluctuate with time although the power of the input lights and the power of the RF signals are constant.
Simulations have been performed on the case in which lights at four wavelengths are to be selected by two AOTFs, for example.
These two AOTFs are connected in tandem by connecting the portion Pdr of the AOTF at the front step with the port Pin of the AOTF at the back step. The RF signal for selecting a channel
1
and the RF signal for selecting a channel
3
are applied to the AOTF at the front step. The RF signal for selecting a channel
2
and the RF signal for selecting a channel
4
are applied to the AOTF at the back step. With this construction, the simulations have been made by setting the wavelengths of the four waves to be selected to 1545.6 nm, 1547.2 nm, 1548.8 nm and 1550.4 nm and by setting the working length of the AOTFs to 43.1 mm.
The results are illustrated in FIG.
13
. In
FIG. 13
, the ordinate indicates a transmittance at the unit of dB, and the abscissa indicates a wavelength at the unit of nm.
As seen from
FIG. 13
, the first side lobe on the shorter wavelength side than 1545.6 nm and the first side lobe on the longer wavelength side than 1550.4 nm are at
Fujitsu Limited
Pascal Leslie
Singh Dalzid
Staas & Halsey , LLP
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