Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-04-23
2002-10-08
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C725S129000, C375S259000, C375S316000, C375S355000
Reexamination Certificate
active
06462851
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to broadband communications systems, such as a hybrid/fiber coaxial (HFC) cable television system, and more specifically to a communications network allowing a method for transmitting reverse optical signals in the broadband communications system by sub-sampling a limited bandwidth within the reverse bandwidth.
BACKGROUND OF THE INVENTION
A conventional broadband communications system
100
, such as a two-way hybrid/fiber coaxial (HFC) communications system, that carries analog and optical signals is depicted in FIG.
1
. The communications system
100
includes headend equipment
105
for generating forward signals that are transmitted in the forward, or downstream, direction along a first communication medium, such as a fiber optic cable
110
. Coupled to the headend
105
are optical nodes
115
that convert the optical signals to radio frequency (RF) signals. The RF signals are further transmitted along a second communication medium, such as coaxial cable
120
, and are amplified, as necessary, by one or more express amplifiers
130
positioned along the communication medium. Tap amplifiers
135
are typically positioned along the end of the express lines to tap off the RF signals, for example, in three different directions. Taps
140
then further split off portions of the forward signals for provision to subscriber equipment
145
, such as set-top terminals, computers, modems, and televisions. It will be appreciated that there are typically several different fiber links connecting the headend
105
with several additional nodes
115
, amplifiers
130
,
135
, and subscriber equipment
145
.
In a two-way system, the subscriber equipment
145
can also generate reverse electrical signals that are transmitted in the reverse, or upstream, direction to the headend equipment
105
. Any one or more of the distribution amplifiers
130
,
135
may amplify such reverse signals. The signals are then converted to optical signals by the optical node
115
before being provided to the headend equipment
105
.
Conventionally, an analog communications system transmits and receives the forward and reverse signals in the analog domain.
FIG. 2
is a block diagram of an example of an optical link network that includes a headend and optical nodes in further detail. This example is suitable for use in the analog broadband communications system
200
. A headend
205
generates and transmits optical signals via optical transmitters
210
a-n
downstream through their respective fiber links
215
a-n
. It will be appreciated that there are a plurality of optical transmitters
210
a-n
transmitting optical signals to a plurality of nodes
220
a-n
, where, depending upon the network design, each node
220
typically services a different portion of the system. Within the nodes
220
a-n
, an optical receiver
230
a-n
, among other operations, converts the optical signals to electrical signals. A diplex filter
235
a-n
then isolates the forward electrical signals from the reverse path and provides the electrical signals to coaxial cable
240
a-n
for delivery to the subscriber equipment
225
a-n.
In the reverse path, electrical signals emanating from subscriber equipment
225
a-n
are transmitted upstream via the coaxial cable
240
a-n
to the node
220
a-n
. The diplex filter
235
a-n
isolates the reverse signals from the forward path and provides the signals to an optical transmitter
245
a-n
for conversion of the electrical signals to optical signals. The optical signals are then transmitted upstream, via an optical fiber
248
a-n
, to an optical receiver
250
a-n
that may be located within the headend
205
, where the information is processed.
If additional subscriber homes are added to the network
200
, it may be necessary to add an additional node
220
that includes separate links for the forward and reverse path to address the additional subscriber equipment within the homes. Additionally, if the operator chooses to optimize the network
200
to accommodate an increase in the amount of reverse signals being transmitted by one optical transmitter, an operator can accomplish this by decreasing the number of subscriber homes that a node
220
services. For example, an operator can reduce an existing network that includes 2000 subscriber homes per node to 500 subscriber homes per node, and add three additional nodes to the network. It can easily be understood that increasing the size or optimizing the network requires a significant amount of equipment and fiber.
It will be appreciated that separate reverse fiber paths, or links
248
a-n
, are required for each node because reverse optical signals cannot be combined like reverse electrical signals. More specifically, those skilled in the art will appreciate that when the light from multiple optical transmitter outputs, where each output has a specific wavelength, is applied simultaneously to an optical receiver, intermodulation distortion results. If the differences between these received wavelengths are sufficiently small, the intermodulation distortion produced in the optical receiver will obscure the desired electrical signals, which are, for example, signals within the range from 5 MHz to 42 MHz, at the output of the optical receiver. The drift in wavelength encountered in conventional optical transmitters makes this condition likely to happen.
Recently, new broadband applications, such as interactive multimedia, Internet access, and telephony, are increasing the number of reverse signals within the reverse bandwidth. As a result, network operators are redesigning networks to effectively increase the total reverse signal carrying capacity, for example, by digitizing the reverse analog signals and, therefore, allowing more digital signals to be transmitted within the existing reverse bandwidth. More specifically, a simplified digital reverse communications path that can be used in a broadband communications system to digitize analog signals is depicted in FIG.
3
. Digitizing the optical signals as shown in
FIG. 3
allows the operator to increase the reverse signal carrying capacity that is demanded by the growing number of customers and broadband applications.
Briefly, a plurality of digital transmitters
305
a-n
, each including an analog-to-digital (A/D) converter
308
a-n
, receives analog electrical signals from a number of pieces of connected subscriber equipment and converts the analog electrical signals to digital optical signals. Linked, via fiber optic cable
309
a-n
, to each digital transmitter
305
a-n
is a digital receiver
310
a-n
that includes a digital-to-analog (D/A) converter
315
a-n
and which is located further upstream in the network
300
. The D/A converter
315
a-n
converts the received digitized optical signals back to analog electrical signals for delivery to the headend and further processing. An example of a similar digital reverse path is discussed further in commonly assigned, copending patent application Ser. No. 09/102,344, filed Jun. 22, 1998, in the name of “Digital Optical Transmitter”, the disclosure of which is incorporated herein by reference.
To address the new broadband applications and interactive services, system operators are focusing on efforts to drive fiber deeper into neighborhoods and directly into subscribers' homes. The operators need a cost-effective way to add more signals within the existing bandwidth and make two-way capable networks truly two-way active.
FIG. 4
is a block diagram of one example illustrating a combination network including an HFC analog network and an overlayed digital network for increasing the signal carrying capacity for broadband applications. In this example, the network
400
has a portion of the system
402
that carries the traditional forward and reverse signals, which can be analog and/or digital signals, using, for example, existing communications equipment, such as analog and digital headend equipment
405
to generate and process analog signals. These analog signals, such as cable televisi
Barnhardt III Hubert J.
Couturier Shelley L.
Massaroni Kenneth M.
Pascal Leslie
Phan Hanh
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