Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-04-23
2003-01-21
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C372S029011
Reexamination Certificate
active
06509994
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to broadband communications systems, such as cable television systems, and more specifically to an optical transmitter and a method of transmitting reverse analog signals within the optical transmitter by a burst-mode technique.
BACKGROUND OF THE INVENTION
A broadband communications system
100
, such as a two-way hybrid/fiber coaxial (HFC) communications system, is depicted in FIG.
1
. Such a system may be used in, for example, a cable television network; a voice delivery network, such as a telephone system; and a data delivery network to name but a few. The communications system
100
includes headend equipment
105
for generating forward signals (e.g., voice, video, or data 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 distribution amplifiers
125
positioned along the communication medium. Taps
130
included in the communications system split off portions of the forward signals for provision to subscriber equipment
135
, such as set-top terminals, computers, telephone handsets, modems, and televisions. It will be appreciated that only one fiber link connecting the headend
105
with a node
115
is shown for simplicity; however, there are typically several different fiber links connecting the headend
105
with several additional nodes
115
, amplifiers
125
, and subscriber equipment
135
.
In a two-way system, the subscriber equipment
135
can also generate reverse electrical signals that are transmitted upstream to the headend equipment
105
. Such reverse signals may be amplified by any one or more of the distribution amplifiers
125
and 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. An example of detailed optical paths including a headend and optical nodes that are suitable for use in an analog broadband communications system
200
is shown in
FIG. 2. 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 each node
220
services a different pocket of the system depending upon the network design. Within the nodes
220
a-n,
an optical receiver
230
a-n
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
located within the headend
205
for further processing.
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. 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 amount 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 of the network requires a significant amount of equipment and fiber.
It will be appreciated that separate reverse fiber paths, or links, are required 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 outputs of optical transmitters, 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 signals 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 amount of reverse signals within the reverse bandwidth. As a result, network operators are redesigning the networks
100
to effectively increase the total reverse signal carrying capacity, for example, by digitizing the reverse analog signals and allowing more signals to be transmitted within the existing reverse bandwidth. More specifically, a simplified digital reverse path that is suitable for use 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 interactive 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 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.
Digitizing the reverse path, however, is an expensive technique to employ and most network operators may not be ready or able to invest in the required capital costs. Typically, network operators that have been operating for a substantial length of time do not have the digital equipment, such as digital transmitters and receivers, required to digitize the reverse signals. In order to accomplish this, the operators would have to substantially upgrade their system to include the digital equipment and may also have to lay extensive routes of fiber. The majority of operators have historically transmitted and received analog signals over an analog HFC system; therefore, due to the expensive undertaking of sending digital reverse signals, most operators would like an intermediate step to enable the efficient, low-cost delivery of reverse signals over their existing HFC system.
In summary, what is needed are devices and networks that are capable of transmitting and combining reverse optical signals, similar to the combining of reverse electrical signals, in order to ensure the reverse bandwidth is ab
Barnhardt III Hubert J.
Bello Agustin
Chan Jason
Couturier Shelley L.
Massaroni Kenneth M.
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