Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-11-05
2002-06-25
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S578000, C359S589000, C359S634000
Reexamination Certificate
active
06411411
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to an optical wavelength selective control apparatus for selecting arbitrary optical signals from wavelength-division-multiplexed optical signals and performing optical transmission.
(2) Description of the Related Art
As well known, wavelength-division-multiplexing (WDM) transmission system using band characteristics of an optical fiber is expected as a transmission system which can increase a transmission capacity or can configure an optical network whose flexibility has been improved because it is easy to drop/add a signal.
In concrete, the WDM transmission system wavelength-division-multiplexes plural optical signals at different wavelengths and transmits them over one optical fiber. Therefore, if multiplexing signals at the same transmission rate, the WDM transmission system can transmit a larger quantity of information by the number of times of wavelength-division-multiplexing than a transmission system which modulates, at a high speed, optical signals at one kind of wavelength and transmits them over one optical fiber. Even with respect to low-speed optical signals, the WDM transmission system can obtain the same transmission capacity as a transmission system which transmits high-speed optical signals at one wavelength, by wavelength-division-multiplexing the low-speed optical signals.
In the above WDM transmission system, wavelengths of transmitted optical signals are required to be spaced to one another to a degree that a signal is not affected by a signal at a neighboring wavelength. There is an optical amplifier having a band larger than ten-odd nanometers, at present. It is therefore possible to realize a WDM transmission system in which the above wavelengths are spaced approximately one nanometer, and such system is being introduced as a real system.
A lot of researches are conducted on optical networks based on the above WDM transmission system in these years. As an example, there is a network having an ADM (Add-Drop Multiplex) function of not only transmitting WDM signals from one point to another point, but also selectively transmitting only an optical signal at a specific wavelength among multiplexed optical signals at a repeating point called a node provided in the course of a transmission line, or receiving signals at other wavelengths at the node, or adding a light of another signal at the node and transmitting the signal to another node. The ADM function is a technique having a feature that can drop/add a signal as in a state of light at will, which is characteristic of the WDM transmission technique.
As an important device for the above WDM transmission system, there is a wavelength division selective optical filter (hereinafter called merely an optical filter, occasionally). For example, there is used the optical filter on the receiving side of the WDM transmission system in order to split wavelength multiplexed optical signals (WDM signals) by wavelength and receive it. The wavelength division selective optical filter in this case eliminates unnecessary signals (other signals) other than a transmitted optical signal, and eliminates noises generated by an optical amplifier provided along the transmission line, at the same time. For this reason, the optical filter is generally required that a transmission band for each wavelength is narrow as much as possible in order to suppress other signals or noises, or a wavelength to be selected is variable in order to select a signal at an arbitrary wavelength.
Such wavelength-selective optical filter is also used as, for example, an optical ADM node or an optical crossconnect apparatus (not shown) in an optical network. Since the optical ADM node needs a function of transmitting (adding) an optical signal at an arbitrary wavelength and receiving (dropping) an optical signal at an arbitrary wavelength, the optical filter is used on both the adding and dropping sides, where a wavelength to be selected is required to be variable.
In the optical crossconnect apparatus, the optical filter is used in a portion where an optical signal is converted from one wavelength to another wavelength. In order to transmit an electric signal with a light at an arbitrary wavelength, the optical filter selects only a light at a desired wavelength among transmittable N-wavelength-multiplexed CW (Continuous Wave) lights, and applies a transmit signal thereto.
As the wavelength selecting optical filter, there is an optical filter (AOTF: Acousto-Optic Tunable Filter) using the acoustooptic effect, for example.
FIG. 14
is a block diagram showing a structure of the AOTF. An AOTF
50
shown in
FIG. 14
has a light input port
50
a
, an optical waveguide
501
, polarization beam splitters (PBSs)
502
and
507
, SAW absorbers
503
and
506
, a finger electrode (IDT)
504
, an SAW cladding (Ti-deeply-diffused region)
505
and light output ports
50
b
and
50
c
. In the AOTF
50
, an optical signal propagated through the optical waveguide
501
interferes with a surface acoustic wave propagated through the SAW cladding
505
so that only a light at a part of wavelengths undergoes polarization-conversion. The splitter (PBS
507
) splits only the polarization-converted light to take out (select) a part of the wavelengths.
In concrete, an RF signal corresponding to a light at a wavelength to be taken out is applied to the IDT
504
(electrode logarithm N, opening length W) to generate a surface acoustic wave (SAW) which applies polarization-conversion to only a light at a wavelength to be taken out, and the surface acoustic wave is propagated through the SAW cladding
505
. At this time, a microwave is generated from the IDT
504
toward the both sides of the SAW cladding
505
, which might affect a polarization-splitting process in the PBSs
502
and
507
. However, the microwave is absorbed by the SAW absorbers
503
and
506
.
When an optical signal is inputted from the input port
50
a
in this state, the optical signal is polarization-split by the PBS
502
, and propagated as an optical signal in a TE mode and an optical signal in a TM mode through the optical waveguides
501
a
and
501
b.
Each of these signals interferes with the above surface acoustic wave propagated through the SAW cladding
505
, whereby only an optical signal at a wavelength which is desired to be taken out is polarization-converted (TE-TM mode conversion). The optical signal so polarization-converted is polarization-split at the PBS
507
so that only an optical signal (selected optical signal) at a wavelength desired to be taken out is outputted from the light output port
50
c
. Optical signals having not been selected are outputted from the other light output port
50
b.
Here, if a temperature of the AOTF
50
(device) is constant in the wavelength selecting process at the AOTF
50
, a relation between a frequency of the above surface acoustic wave and a frequency of the selected optical signal is 1:1. Accordingly, in the AOTF
50
, when a frequency of the RF signal supplied to the IDT
504
is varied, the selected optical wavelength is varied. As this, a wavelength variable selective optical filter is realized with AOTF
50
.
When a plurality of RF signals at different frequencies are mixed and supplied to the IDT
504
, the AOTF
50
can select a plurality of light wavelengths corresponding to frequencies of the RF signals at a time. Namely, the AOTF
50
is very effective as an ADM (multiple wavelength selective optical) filter which can select not only one wave but also a plurality of optical signals at desired wavelengths simultaneously.
Hereinafter, description will be made of an actual system to which the AOTF
50
is applied.
FIG. 12
is a block diagram showing an example of a structure of a WDM transmission system. In a WDM transmission system
600
A shown in
FIG. 12
, unnecessary components such as sidebands and the like of optical signals at different wavelengths generated by light sources (LDs)
610
-
1
through
610
-n are eliminated by band-pass filters (BPF)
6
Chikama Terumi
Okazaki Kazue
Fujitsu Limited
Pascal Leslie
Phan Hanh
Staas & Halsey , LLP
LandOfFree
Optical wavelength selective control apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical wavelength selective control apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical wavelength selective control apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2924984