Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-11-30
2002-11-19
Chan, Jason (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C385S037000, C385S024000
Reexamination Certificate
active
06483618
ABSTRACT:
CROSS-REFERENCED TO A RELATED APPLICATION
Reference is made commonly assigned copending patent application serial number, filed simultaneously herewith in the name of Weller-Brophy, Laura and entitled “Narrow Band Wavelength Division Multiplexer and Method of Multiplexing Optical Signals.”
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to optical demultiplexers and more specifically relates to wavelength division demultiplexers.
2. Technical Background
Wavelength divisions multiplexers are used in optical communication networks to combine various optical signals (channels) carried by two or more optical wavelengths into a single, common carrier (for example, an optical waveguide such as a single fiber). Wavelength division demultiplexers are used in optical communication circuits to separate a plurality of signals transmitted on a common carrier based upon the wavelength of the light onto which the signal is modulated. Wavelength division multiplexers and demultiplexers typically include various combinations of optical elements for performing the combination and separation function respectively. The most common of such components are band edge dichroic filters, which reflect light having wavelengths above or below a certain characteristic wavelength into a first transmission path while allowing the remaining light (i.e. the light having wavelengths below or above the characteristic wavelength) to be transmitted through the band edge filter into a second transmission path. Such edge filters are not ideal in that they have a transition zone surrounding the characteristic wavelength. See, for example,
FIG. 1
, which shows the characteristics of a hypothetical ideal band edge filter F
1
that does not exist or is otherwise extremely expensive to create, and
FIG. 2
, showing the characteristics of an actual band edge filter F
2
as commonly used in these types of devices. Incident light having a wavelength in the transition zone (e.g. &lgr;
4
) is partially reflected and partially transmitted. When a band edge filter only partially transmits or reflects incident light that is supposed to be entirely transmitted or reflected, the band edge filter reduces the intensity of the light signal that is transmitted through the intended transmission path while introducing noise into the other path (i.e. transmission path). To avoid such signal loss and noise, either the separation between the channels must be large enough so that no channels fall within the transition zone of the filter, or the filter must be nearly ideal so as to have a transition zone smaller than the channel separation. To accommodate more signals on a single optical fiber trunk line, designers must decrease channel (i.e. wavelength) separation, which makes the non-ideal band edge filters less practical for use in a wavelength division multiplexers and demultiplexers.
The following description of prior art is directed to both multiplexers and demultiplexers, because these devices are similar to one another and generally a multiplexer will function as a demultiplexer when the input and output are reversed so as to separate (with a demultiplexer) instead of combining (with a multiplexer) different wavelength signals.
U.S. Pat. No. 5,652,814 issued to Pan et al. discloses a wavelength division demultiplexer made up entirely of such band edge filters. This demultiplexer is shown in FIG.
3
. As illustrated, a first filter
271
reflects signals having wavelengths &lgr;
1
through &lgr;
4
while transmitting signals having wavelengths &lgr;
5
through &lgr;
8
. A second filter
272
receives signals having wavelengths &lgr;
1
through &lgr;
4
and reflects signals having wavelengths &lgr;
1
and &lgr;
2
while transmitting signals having wavelengths &lgr;
3
and &lgr;
4
. Similarly, a third filter
273
receives signals having wavelengths &lgr;
5
through &lgr;
8
and reflects signals having wavelengths &lgr;
5
and &lgr;
6
while transmitting signals having wavelengths &lgr;
7
and &lgr;
8
. Additional band edge filters
274
through
277
are provided as a final separation stage. Because the band edge filters are not ideal, the demultiplexer disclosed in Pan et al. would exhibit large levels of signal loss and crosstalk, particularly when the channel separation is small.
To overcome these difficulties, wavelength division demultiplexers have been constructed with wavelength channel dropping components that include a combination of an optical circulator and various fiber Bragg gratings (FBGs). An example of such a demultiplexer is disclosed in U.S. Pat. No. 5,754,718 issued to Duck et al. This demultiplexer is illustrated in FIG.
4
. As shown at the left side of
FIG. 4
, eight channels having wavelengths &lgr;
1
through &lgr;
8
are transmitted into port
1
of an optical circulator
610
. All these signals are transmitted out of circulator
610
at port
2
. These signals are then passed through the four FBGs that are configured to reflect the signals of non-adjacent wavelengths &lgr;
2
, &lgr;
4
, &lgr;
6
,and &lgr;
8
back into port
2
of optical circulator
610
. The remaining non-adjacent wavelengths are transmitted into port
1
of a second optical circulator
612
. First circulator
610
transmits the signals of wavelengths &lgr;
2
, &lgr;
4
, &lgr;
6
, and &lgr;
8
, which are reflected into port
2
, out of port
3
. Two FBGs reflect wavelengths &lgr;
2
and &lgr;
6
and provided at port
3
of optical circulator
610
, reflect signals having wavelengths &lgr;
2
and &lgr;
6
back into port
3
of optical circulator
610
. Optical circulator
610
transmits these signals from port
4
. Signals of wavelengths &lgr;
4
and &lgr;
8
, however, which exit port
3
of circulator
610
, are transmitted through the FBGs to a band edge filter
626
. Band edge filter
626
transmits signals of wavelength &lgr;
4
and reflects signals of wavelength &lgr;
8
(signals &lgr;
5
, &lgr;
6
, and &lgr;
7
are already removed from the optical path). Similarly, a band edge filter
628
separates signals of wavelengths &lgr;
2
and &lgr;
6
, which exit port
4
of circulator
610
. Optical circulator
612
and band edge filters
622
and
624
similarly separate the signals of wavelengths &lgr;
1
, &lgr;
3
, &lgr;
5
, and &lgr;
7
. As will be apparent, the wavelengths of the signals transmitted to each of band edge filters
622
through
628
are not in adjacent channels. Therefore, any transition zone present in band edge filters
622
through
628
does not necessarily degrade the strength of the signals or the ability to separate the signals based on their wavelengths.
While the wavelength division demultiplexer shown in
FIG. 4
overcomes the above noted problems relating to channel separation using band edge filters, the construction of such a demultiplexer is quite expensive due to the very large number of FBGs and other necessary optical components. In addition, the demultiplexer shown in
FIG. 4
is designed to separate eight optical channels. If the number of channels to be separated were increased, the demultiplexer would need to be substantially redesigned.
U.S. Pat. No. 5,748,350 is directed to both wavelength division multiplexers and demultiplexers. The multiplexers are illustrated, for example, in
FIGS. 1A
, and
6
A of this patent.
FIGS. 7A and 7B
illustrate 4n×1 wavelength division multiplexers.
FIGS. 8A and 8B
of this patent illustrate 4n×1 wavelength division demultiplexers, while
FIGS. 9
,
10
A and
10
B show devices that function as wavelength division multiplexers and demultiplexers. These multiplexers and demultiplexers utilize multi port optical circulators, fiber Bragg gratings and band pass wavelength division couplers. The multiplexers disclosed in this reference require fiber Bragg gratings for more than 50% of the optical channels, and typically, optical bandpass filters for each of the optical channels to be multiplexed. That is, a separated bandpass coupler is required for each channel.
As stated above, an exemplary multiplexer is shown
Amin Jaymin
Weidman David L.
Weller-Brophy Laura A.
Bello Agustin
Chan Jason
Corning Incorporated
Short Svetlana
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