Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-07-30
2001-05-22
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06236483
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical communication systems and, more particularly, to optical code-division multiple access communications systems that transmit data over optical fibers.
2. Description of the Related Art
Recent years have seen rapidly expanding demands for communications bandwidth, resulting in the rise of technologies such as satellite communications, video programming distribution networks such as cable television, and spread-spectrum telephony including, for example, code-division multiple access telephony. Such technologies have become common and well integrated into everyday communications. Growing demand for communications bandwidth has brought significant investments in new communications technologies and in new communications infrastructure. For example, the cable television industry, telephone companies, Internet providers and various government entities have invested in long distance optical fiber networks and in equipment for fiber networks. The addition of this infrastructure has, in turn, spurred demand for bandwidth use, resulting in demand for yet additional investment in new technologies and infrastructure.
Installing optical fibers over long distances is expensive. Additionally, conventional optical fiber or other optical communication networks utilize only a small fraction of the available bandwidth of the communication system. There is consequently considerable interest in obtaining higher utilization of fiber networks or otherwise increasing the bandwidth of optical fiber systems. Techniques have been developed to increase the bandwidth of optical fiber communication systems and to convey information from plural sources over a fiber system. Generally, these techniques seek to use more of the readily available optical bandwidth of optical fibers by supplementing the comparatively simple coding schemes conventionally used by such systems. In some improved bandwidth fiber systems, the optical fiber carries an optical channel on an optical carrier signal consisting of a single, narrow wavelength band and multiple users access the fiber using time-division multiplexing (TDM) or time-division multiple access (TDMA). Time division techniques transmit frames of data by assigning successive time slots in the frame to particular communication channels. Optical TDMA requires short-pulsed diode lasers and provides only moderate improvements in bandwidth utilization. In addition, improving the transmission rates on a TDM network requires that all of the transceivers attached to the network be upgraded to the higher transmission rates. No partial network upgrades are possible, which makes TDM systems less flexible than is desirable. On the other hand, TDM systems provide a predictable and even data flow, which is very desirable in multi-user systems that experience “bursty” usage. Thus, TDM techniques will have continued importance in optical communications systems, but other techniques must be used to obtain the desired communications bandwidth for the overall system. Consequently, it is desirable to provide increased bandwidth in an optical system that is compatible with TDM communication techniques.
One strategy for improving the utilization of optical communication networks employs wavelength-division multiplexing (WDM) or wavelength-division multiple access (WDMA) to increase system bandwidth and to support a more independent form of multiple user access than is permitted by TDM. WDM systems provide plural optical channels each using one of a set of non-overlapping wavelength bands to provide expanded bandwidth. Information is transmitted independently in each of the optical channels using a light beam within an assigned wavelength band, typically generated by narrow wavelength band optical sources such as lasers or light emitting diodes. Each of the light sources is modulated with data and the resulting modulated optical outputs for all of the different wavelength bands are multiplexed, coupled into the optical fiber and transmitted over the fiber. The modulation of the narrow wavelength band light corresponding to each channel may encode a simple digital data stream or a further plurality of communication channels defined by TDM. Little interference will occur between the channels defined within different wavelength bands. At the receiving end, each of the WDM channels terminates in a receiver assigned to the wavelength band used for transmitting data on that WDM channel. This might be accomplished in a system by separating the total received light signal into different wavelengths using a demultiplexer, such as a tunable filter, and directing the separated narrow wavelength band light signals to receivers assigned to the wavelength of that particular channel. At least theoretically, the availability of appropriately tuned optical sources limit the number of users that can be supported by a WDM system. Wavelength stability, for example as a function of operating temperature, may also affect the operational characteristics of the WDM system.
As a more practical matter, the expense of WDM systems limits the application of this technology. One embodiment of a WDM fiber optic communication system is described in U.S. Pat. No. 5,579,143 as a video distribution network with 128 different channels. The 128 different channels are defined using 128 different lasers operating on 128 closely spaced but distinct wavelengths. These lasers have precisely selected wavelengths and also have the well-defined mode structure and gain characteristics demanded for communications systems. Lasers appropriate to the WDM video distribution system are individually expensive so that the requirements for 128 lasers having the desired operational characteristics make the overall system extremely expensive. The expense of the system makes it undesirable for use in applications such as local area computer networks and otherwise limits the application of the technology. As is described below, embodiments of the present invention can provide a video distribution network like that described in U.S. Pat. No. 5,579,143, and embodiments of the invention can provide other types of medium and wide area network applications, making such systems both more flexible and more economical. Unlike the multiple laser WDM system of U.S. Pat. No. 5,579,143, embodiments of the present invention may be sufficiently flexible and cost effective to be used in at least some types of local area networks.
Embodiments of the present invention, as described below, use spread spectrum communication techniques to obtain improved loading of the bandwidth of an optical fiber communication system in a more cost-effective manner than known WDM systems. Spread spectrum communication techniques are known to have significant advantages and considerable practical utility, most notably in secure military applications and mobile telephony. There have consequently been suggestions that spread spectrum techniques, most notably code-division multiple access (CDMA), could be applied to optical communications technologies. Spread spectrum techniques are desirable in optical communications systems because the bandwidth of optical communications systems, such as those based on optical fibers, is sufficiently large that multi-dimensional coding techniques can be used without affecting the data rate of any electrically generated signal that can presently be input to the optical communications system. Different channels of data can be defined in the frequency domain and independent data streams can be supplied over the different channels without limiting the data rate within any one of the channels. From a simplistic point of view, the WDM system described above might be considered a limiting case of a spread spectrum system in that plural data channels are defined for different wavelengths. The different wavelength channels are defined in the optical frequency domain and time domain signals can be transmitted over each of the wavelength channels. From a CDMA perspective, the
Chan James K.
Dutt Birendra
Naraghi Manouher
Chan Jason
CodeStream Technologies Corporation
Haynes and Boone LLP
Leung Christina Y
LandOfFree
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