Optical communications – Multiplex – Broadcast and distribution system
Reexamination Certificate
1999-12-23
2004-04-13
Pascal, Leslie (Department: 2633)
Optical communications
Multiplex
Broadcast and distribution system
C398S066000, C398S067000, C398S068000, C398S071000, C398S072000, C398S079000, C398S084000, C398S085000, C398S098000, C398S100000, C398S182000, C398S183000, C398S200000, C385S024000, C385S037000
Reexamination Certificate
active
06721506
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to data transmission over wavelength division multiplexed passive optical networks. More specifically, the invention relates to providing a bandwidth efficient way of delivering multiple broadcast services on such a network using broadband spontaneous emission optical sources.
2. Description of Related Art
Wavelength division multiplexed passive optical networks (WDM PON) can be used to deliver both switched and broadcast services on the same fiber plant. P. P. Iannone, K. C. Reichmann and N. J. Frigo, “High-Speed Point-to-Point and Multiple Broadcast Services Delivered over a WDM Passive Optical Network”, IEEE Photonics Technology Letters, Vol. 10, No. 9, pp. 1328-1330, September 1998. As shown in
FIG. 1
, to deliver broadcast services, a conventional WDM PON system
100
includes at least one broadband optical source
110
, at least one WDM remote node
120
and a plurality of user nodes
130
-
150
. The broadband optical source
110
is coupled to the input port of the WDM remote node
120
. The broadband optical source
110
is modulated to incorporate the service data to be broadcast to the user nodes
130
-
150
. The plurality of user nodes
130
-
150
are coupled to the various outputs of the WDM remote node
120
.
The WDM remote node
120
consists of a wavelength router for distributing data to different user nodes. A wavelength router is also called an arrayed waveguide router (AWG) or waveguide grating router (WGR). The wavelength router has a cyclical property. For example, given a four-output port wavelength router, if the sum of equally separated input wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
, &lgr;
4
are coupled to its input port, then &lgr;
1
will appear at output port
1
, &lgr;
2
at output port
2
etc., in similar ways as any other wavelength demultiplexers. However, if wavelengths &lgr;
5
, &lgr;
6
, &lgr;
7
, &lgr;
8
are coupled to the input port, then &lgr;
5
will again appear at output port
1
, &lgr;
6
at output port
2
etc. In other words, the wavelength demultiplexing property repeats over ranges of wavelengths or frequencies. The smallest range of wavelength over which the cyclical property repeats is called the free spectral range (FSR) of the wavelength router. In broadcast operation, the output spectrum of the broadband optical source
110
, modulated with the broadcast data, contains wavelength components covering at least one FSR of the wavelength router. The wavelength router slices the frequency spectrum of the optical signal produced by the optical source
110
to deliver the broadcast service to the user nodes
130
-
150
. Each user node
130
-
150
gets part of the broadcast signal, albeit at different wavelengths. For example, as shown in
FIG. 1
, one FSR of the wavelength router is shown. However, each node normally sees multiple FSR's and the spectrum of the optical source
110
usually covers more than one FSR. User node
130
receives a bottom slice of the FSR, user node
140
receives a middle slice of the FSR and user node
150
receives a top slice of the FSR. In actuality, the cyclical property of the wavelength router comes from the fact that it is implemented as an interferrometric device.
Multiple broadcast services have also been realized using different FSRs of the wavelength router.
FIG. 2
illustrates a different conventional system architecture for providing multi-band broadcast services in a WDM PON. Specifically, as shown in
FIG. 2
, the system
200
includes a first optical source
205
and a second optical source
210
, a combiner
215
, a WDM remote node
220
and a plurality of user nodes
230
-
250
. The output from the first and second optical sources
205
,
210
are coupled to the combiner
215
, which is also coupled to the input of the WDM remote node
220
. The plurality of user nodes
230
-
250
are also coupled to the various outputs of the WDM remote node
220
. The WDM remote node
220
also incorporates a wavelength router, which has the wavelength or frequency cyclic property, for distributing different broadcast data streams within specific portions of the frequency spectrum to various user nodes. As shown in
FIG. 2
, the illustrations of user nodes
230
-
250
each include a graph depicting the slices of the transmitted spectrum received by each user node.
The first optical source
205
generates the optical signal associated with a first frequency band, B
1
, which encompasses a specific FSR of the router within the WDM node
220
. This optical signal is modulated by the data of the first broadcast service. The second optical source
210
generates the optical signal associated with a second frequency band, B
2
, which encompasses the next FSR of the router within the WDM node
220
. This optical signal is modulated by the data of the second broadcast service. Therefore, as shown in
FIG. 2
, user node
230
receives low end slices of both the first and second frequency bands B
1
and B
2
. Similarly, user node
240
receives intermediate slices of both the first and second frequency bands B
1
and B
2
. User node
250
receives high end slices of the first and second frequency bands. Although
FIG. 2
depicts only two FSR's B
1
and B
2
, it should be appreciated that any number of free spectral ranges may be used to transmit different data streams and to support either broadcast or switched services. Switched services will not be described in this document. Although
FIG. 2
illustrates only three user nodes
230
,
240
,
250
, it should also be appreciated that the number of user nodes is only limited by the number of output ports of the wavelength router in the remote node
220
.
In such broadcast applications, the optical sources
110
,
205
and
210
are broadband sources. Examples of broadband sources include multi-wavelength laser diodes and spontaneous emission sources such as light emitting diodes (LED's) or Erbium doped fiber amplifiers (EDFA's) operated in spontaneous emission mode. The optical sources
110
,
205
and
210
are located in a hub station or a central office of the service provider, which is also called a Host Digital Terminal (HDT) sometimes, and the transmitted signals are connected to the input port of the remote nodes
120
and
220
through a section of feeder optical fiber.
SUMMARY OF THE INVENTION
However, there is a need for producing a system for delivering multiple broadcast services in a spectral efficient way and a scalable way so that the network services can grow easily. Therefore, the invention provides a method and system for delivering multiple broadcast services in a bandwidth efficient and easily growable way in a WDM PON using broadband spontaneous emission optical sources.
The invention relates to using a chirped fiber Bragg grating (FBG) coupled to an optical circulator in a transmitter and/or a receiver used in a WDM PON to select one or more FSR's to be delivered to user nodes. Such an arrangement can be cascaded to combine or separate different sections of the frequency spectrum. In accordance with the exemplary embodiments of the invention, a WDM PON may include multiple transmitters at the hub or central office and multiple receivers located at different user nodes within the PON.
In accordance with the exemplary embodiments of the invention, a filter comprised of an optical circulator and a chirped FBG confines the output from a spontaneous emission source to a desired spectral region. The hub station or central office utilizes such cascaded filters to combine multiple non-overlapping optical spectra from transmitters of different broadband services to be delivered through the feeder optical fiber. At a user node, a band splitter utilizing such cascaded filters separates the combined broadcast spectrum into individual service bands received by separate receivers.
The invention may be implemented to separate the spectral output of a spontaneous emission source such as an EDFA or a praseodymium-doped fiber amplifier (PDFA) into different b
AT&T Corp.
Pascal Leslie
Phan Hanh
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