Patent Application
20070297807
The present invention will be described more fully hereinafter with reference to the accompanying drawings in which like numerals represent like elements throughout the several figures, and in which an exemplary embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, the embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. All examples given herein, therefore, are intended to be non-limiting and are provided in order to help clarify the description of the invention.
The present invention is directed towards an optical communications system including a depolarizer or polarization scrambler. Placement of a depolarizer or a polarization scrambler in the optical system mitigates the negative effect of GAWBS noise peaks on the transmission of optical signals. Analog video and QAM transmission are most susceptible to those noise peaks and as a result the signal quality is degraded. It will be appreciated that noise peaks generated by GAWBS in a digital transmission system are usually not picked up by the transmitted signal, and so they do not have a significant impact on the quality of the transmission. A general overview of a typical communications system is described herein below.
Optical transmitters (not shown), which are generally located in the headend facility 105, convert the electrical broadcast signals into optical broadcast signals. In most networks, the first communication medium 115 is a long haul segment that transports the signals typically having a wavelength in the 1550 nanometer (nm) range. The first communication medium 115 carries the broadcast optical signal to hubs 120. The hubs 120 may include routers or switches to facilitate routing the information signals to the correct destination location (e.g., subscriber locations or network paths) using associated header information. The optical signals are subsequently transmitted over a second communication medium 125. In most networks, the second communication medium 125 is an optical fiber that is typically designed for shorter distances, and which transports the optical signals over a second optical wavelength, for example, in the 1310 nm range.
From the hub 120, the signals are transmitted to an optical node 130 including an optical receiver and a reverse optical transmitter (not shown). The optical receiver converts the optical signals to electrical, or radio frequency (RF), signals for transmission through a distribution network. The RF signals are then transmitted along a third communication medium 135, such as coaxial cable, and are amplified and split, as necessary, by one or more distribution amplifiers 140 positioned along the communication medium 135. Taps (not shown) further split the forward RF signals in order to provide the broadcast RF signals to subscriber equipment 110, such as set-top terminals, computers, telephone handsets, modems, televisions, etc. It will be appreciated that only one subscriber location 110 is shown for simplicity, however, each distribution branch may have as few as 500 or as many as 1000 subscriber locations. Additionally, those skilled in the art will appreciate that most networks include several different branches connecting the headend facility 105 with several additional hubs, optical nodes, amplifiers, and subscriber equipment. Moreover, a fiber-to-the-home (FTTH) network 145 may be included in the system. In this case, optical fiber is pulled to the curb or directly to the subscriber location and the optical signals are not transmitted through a conventional RF distribution network.
In a two-way network, the subscriber equipment 110 generates reverse RF signals, which may be generated for a variety of purposes, including video signals, e-mail, web surfing, pay-per-view, video-on-demand, telephony, and administrative signals. These reverse RF signals are typically in the form of modulated RF carriers that are transmitted upstream in a typical United States range from 5 MHz to 40 MHz through the reverse path to the headend facility 105. The reverse RF signals from various subscriber locations are combined via the taps and passive electrical combiners (not shown) with other reverse signals from other subscriber equipment 110. The combined reverse electrical signals are amplified by one or more of the distribution amplifiers 140 and generally converted to optical signals by the reverse optical transmitter included in the optical node 130 before being transported through the hub ring and provided to the headend facility 105.
The optical fiber 210 and the passive and/or active devices 215 inherently all produce, or generate, polarization dependent loss (PDL) or polarization dependent gain (PDG), more or less. As a result, the optical devices 205, 210, 215 generate a local oscillator that interacts with the depolarized scattered light at the receiver and produces a heterodyne signal, thereby imprinting a GAWBS signature onto the RF signals in the RF domain.
Since PDL takes advantage of organized or polarized light, it is able to transfer the noise into the RF domain that is shown as the GAWBS noise peaks. In accordance with the present invention, the polarization scrambler or depolarizer 405 scrambles the light from the optical transmitter 410. The depolarized light is then transmitted downstream to the receiver 220. Advantageously and in accordance with the present invention, the unpolarized light does not cause any effect at the device with polarization dependent loss; therefore, the GAWBS does not transfer noise into the RF domain.
The Detailed Description of a Preferred Embodiment set forth above is to be regarded as exemplary and not restrictive, and the breadth of the invention disclosed herein is to be determined from the following claims as interpreted with the full breadth permitted by the patent laws.
