Abstract:
A method and apparatus for substantially reducing OBI on optical networks in which a number of source lasers operate simultaneously and the output from the lasers is subsequently combined in the network. The method involves operating the laser in burst mode such that the laser is operational only during transmission periods.

Description:
TECHNICAL FIELD 
     This invention relates generally to the field of optical communications systems and in particular to a method and apparatus for reducing optical beat interference associated with broadband, fiber-to-the-home systems. 
     BACKGROUND OF THE INVENTION 
     The communications industry has been looking eagerly toward providing broadband, two-way communications to business and homes alike. Potential applications include computer networking, telephony, interactive multi-media and entertainment such as video-on-demand. Accordingly, the industry has been viewing fiber-to-the-home (FTTH) technologies as an attractive way to provide pervasive broadband communications. Advantageously, once optical fiber is installed to a user&#39;s home or business, the bandwidth can be upgraded as demand warrants without requiring the installation of new cabling and the attendant cost and disruption. 
     While many FTTH approaches are possible, one particularly attractive approach is described in United States Provisional Patent Application Serial No. 60/075,362 filed on Feb. 20, 1998 by the present inventors for the present assignee and incorporated herein by reference. With this FTTH approach, communications signals directed to users (downstream) are transmitted through a passive optical network (PON) and are split for distribution to a number of users as appropriate. Conversely, signals that emanate from the users are combined as they proceed upstream through the PON. 
     Unfortunately, however, passive optical networks that utilize combining signals may suffer from a degradation known in the art as optical-beat interference (OBI) whenever two or more output sources (lasers) operate simultaneously. The RF spectrum of OBI produced by multiple lasers at a common photodetector is substantially proportional to the convolution of the electric-field spectral densities of each operating laser. Importantly, if portions of the spectra are closely spaced in optical frequency, OBI may fall in a desired signal band. 
     Consequently, a need exists for methods and apparatus that reduces OBI in optical communications systems. 
     SUMMARY OF THE INVENTION 
     An advance is made over the prior art in accordance with the principles of the present invention directed to a method and apparatus for reducing OBI in optical communications systems. Accordingly, our inventive method and apparatus operates source lasers whose output is subsequently combined in a burst mode, thereby substantially reducing the occurrence of OBI. 
     Viewed from one aspect, the present invention is directed to a method for reducing OBI on optical networks in which a number of source lasers operate simultaneously and the output from the lasers is subsequently combined in the network. The method involves operating the laser in burst mode such that the laser is operational only during transmission periods. Further, the method provides for a sensor or optional control signal to initiate laser operation. 
     Viewed from another aspect, the present invention is directed to an apparatus for operating a laser in burst mode such that OBI is substantially reduced. 
     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a simplified block diagram of an optical communications network suitable for the present invention; and 
     FIG. 2 is a simplified block diagram of an apparatus according to the teachings of the present invention for incorporation into the network of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     A preferred embodiment of the invention will now be described while referring to the figures, several of which may be simultaneously referred to during the course of the following description. 
     With reference now to FIG. 1, there it shows an optical network that is subject to the OBI problems overcome by the present invention. Specifically optical network 100, which is generally a passive optical network is characterized by a headend 101 which is typically connected to another, larger network or networks 110. Generally, signals originating within the other networks 110, propagate through the headend 101 and out through optical plant 102 where it is split by splitter 103 for distribution to a number of individual subscriber premises 104-107 by way of individual subscriber access 104(a)-107(a). Signals originating at individual subscriber premises 104-107, are transmitted via respective individual subscriber access 104(a)-107(a) to combiner 103, along optical plant 102 to headend 101, for conveyance out to other networks 110. As noted previously, when the individual subscribers attempt to transmit optical signals simultaneously, OBI results with such network configurations. 
     While not explicitly shown in FIG. 1, our inventive method and apparatus will work with any network topology in which multiple optical signals are combined in the network medium. For example, well known bus or star topologies that use optical signaling would benefit from our invention. 
     FIG. 2 shows in block diagram form, our inventive apparatus that overcomes OBI when using network configurations such as that of FIG. 1. Specifically, transmission apparatus 200, which generally resides in subscriber premises (104-107 of FIG. 1) is optically connected by access 210 which corresponds to one of the subscriber access lines 104(a)-107(a) shown previously. Accordingly, a data source 203 generates subscriber data which may modulate an rf carrier in modem 202, or alternatively be sent directly to a laser module 201 as baseband data. As is shown in the figure, the output signal of modem 202 is presented to a laser module 201 which generates laser light in response and onto access 210. 
     Our inventive method and apparatus substantially eliminates OBI by operating the laser in a burst mode. As can be readily appreciated by those skilled in the art, the laser should be turned on rapidly just prior to the transmission of a first bit of data and shut off rapidly shortly after the transmission of a final bit. Accordingly, bias control 204, detects the output of modem and turns on the laser 201 as the first bit of data is transmitted. 
     Additionally, the bias control 204 must determine in advance what an appropriate bias current should be prior to turning on the laser 201. Consequently, an appropriate bias current may be stored in local memory 211. In typical operation, the bias current will not change from burst to burst. Nevertheless, during the burst operation of the laser 201, a photocurrent out of the laser may be optionally monitored by a photocurrent monitor 212 in the bias control 204. Advantageously, if the laser power determined by such monitoring is not optimal, a new bias may be selected by the bias control 204 for biasing a subsequent burst. 
     At the beginning of a session, the laser 201 may be set to a default bias value. After several bursts, the bias may be advantageously varied until an optimal bias value is obtained. As should be apparent, the bias control 204 need not actively sense a signal output from the modem 202. Alternatively, a control signal output from the modem 202 or directly from data source 203 may activate the bias control 204 as well. 
     While the invention has been shown and described in detail in the context of a preferred embodiment, it will be apparent to those skilled in the art that variations and modifications are possible without departing from the broad principles and spirit of the invention which should be limited solely by the scope of the claims appended hereto.