Reverse path optical combining using an optical commutator

Details Reverse path optical combining using an optical commutator Reverse path optical combining using an optical commutator

2001-04-23

2003-05-27

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C370S537000, C375S260000, C375S295000

Reexamination Certificate

active

06570693

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to broadband communications systems, such as hybrid/fiber coaxial (HFC) systems, and more specifically to an apparatus for combining optical signals from a plurality of optical transmitters.
BACKGROUND OF THE INVENTION
FIG. 1
is a block diagram illustrating an example of one branch of a conventional broadband communications system, such as a two-way hybrid/fiber coaxial (HFC) system, that carries optical and electrical signals. 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 typically transmitted as optical signals in the forward, or downstream, direction along a first communication medium, such as a fiber optic cable
110
. Coupled to the headend equipment
105
are optical nodes
115
that convert the optical signals to radio frequency (RF), or electrical, signals. The electrical 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.
Passive splitter/combiner devices
130
may also be added to the network
100
to split the electrical signals in the forward path, thereby delivering signals to separate portions of the network
100
. Taps
135
then further split the forward signals for provision to subscriber equipment
140
, such as set-top terminals, computers, telephone handsets, modems, and televisions. It will be appreciated that only one branch connecting the headend equipment
105
with the plurality of subscriber equipment
140
is shown for simplicity; however, there are typically several different fiber links connecting the headend equipment
105
with several additional nodes
115
, amplifiers
125
, and subscriber equipment
140
.
In a two-way system, the subscriber equipment
140
can also generate reverse signals that are transmitted upstream through the reverse path to the headend equipment
105
. Such reverse signals may be combined via the splitter/combiner devices
130
along with other reverse signals and then amplified by any one or more of the distribution amplifiers
125
. The signals are then converted to optical signals by the optical node
115
before being provided to the headend equipment
105
. It will be appreciated that in the electrical, or coaxial cable, portion of the network
100
, the forward and reverse path signals are carried on the same coaxial cable
120
.
A detailed example of forward and reverse optical paths that are suitable for use in a broadband communications system is shown in FIG.
2
. Headend equipment
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 portion of the system depending upon the system design. Within the nodes
220
a-n
, an optical receiver
230
a-n
converts the optical signals to electrical signals for delivery through the coaxial portion of the network. Before transmission, a diplex filter
235
a-n
isolates the forward electrical signals from the reverse electrical signals and provides the electrical signals to coaxial cable
240
a-n
for delivery to a plurality of subscriber equipment
245
-
n.
In the reverse path, electrical signals emanating from the plurality of subscriber equipment
245
a-n
are transmitted upstream via the coaxial cable
240
a-n
to the respective node
220
a-n
. The diplex filter
235
a-n
isolates the reverse electrical signals from the forward electrical signals and provides the reverse signals to an optical transmitter
250
a-n
for converting the electrical signals to optical signals for delivery through the fiber portion of the network. The optical signals are then transmitted further upstream via a reverse optical fiber
255
a-n
to an optical receiver
260
a-n
that may also be located within the headend. The optical receiver
260
a-n
converts the optical signals to electrical signals. Each optical receiver
260
a-n
then transmits the electrical signals to a passive splitter/combiner
265
for combining the electrical signals in the conventional electrical manner. Those skilled in the art will appreciate that, at this point, the electrical signals is the same as if the electrical signals from subscriber equipment was combined and carried back to the headend via analog means. Additional equipment within the headend then receives the combined electrical signal and, based on the bandwidth allocation scheme, routes portions of the signal to the correct equipment for further processing.
If additional subscribers are added to the network, it may be necessary to add an additional node
220
to service those subscribers. The new node would require separate fiber links for the forward and reverse paths to the headend and a single coaxial path to connect to the additional subscriber equipment. Additionally, if the operator chooses to optimize the network to accommodate an increase in the amount of reverse signals being transmitted by one optical transmitter due to an increase in interactive services with the subscriber equipment, an operator can accomplish this by decreasing the number of subscriber homes that a node
220
, or path, services. For example, an operator can reduce an existing path that includes
2000
subscriber homes per node to
500
subscriber homes per node, and add three additional paths each including a node to service that portion of the network. It can easily be understood that increasing the size or optimizing the network requires a significant amount of equipment, fiber, and labor.
At certain times, optical signals may be combined via a passive optical combiner, similar to a passive electrical combiner, as long as the optical transmitters, optical combiner, and optical receiver are restricted to a controlled environment. Those skilled in the art will appreciate that when the optical signal from multiple optical transmitters is combined and 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 from 5 Mega Hertz (MHz) to 42 MHz, at the output of the optical receiver. The optical transmitters, therefore, need to transmit the optical signals at different wavelengths in order for the optical receiver to distinguish between them. In a controlled environment, i.e., controlling the temperature of the optical transmitters, the required different wavelengths of the optical signals can then be strictly maintained to avoid drifting due to temperature.
In most real world applications, however, a controlled environment is difficult to achieve. For example, in a broadband communications system, such as the system shown in
FIG. 1
, many components are exposed to the environment, such as varying regions and temperatures. Consequently, optical transmitters may begin transmitting optical signals at a particular wavelength, but due to heat that is imposed upon the transmitter, for example, the afternoon sun, the wavelength drifts. When signals from several transmitters are combined and at least one of the wavelengths drift, the intermodulation distortion that is produced in the optical receiver will then obscure the desired signals. Moreover, it is not desirable for an operator to use different lasers (i.e., at different wavelengths) within the plurality of optical transmitters in order to transmit optical signals at substantially different wavelengths due to the cost of installing a

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