Abstract:
The present invention is directed towards a frequency modulated burst mode optical transmitter and systems and methods related thereto. Reverse electrical signals are used to frequency modulate a carrier signal. A carrier detect circuit checks for the presence of a subcarrier signal in received reverse electrical signals. When a subcarrier signal is detected, a laser is turned on and the frequency modulated carrier signal is used to intensity modulate the laser to provide an optical signal. In the absence of a subcarrier signal, the laser is turned off and no optical signals are transmitted. By operating in a burst mode, resources are conserved as optical signals are transmitted only when content-carrying reverse electrical signals are received. A delay circuit may be included to prevent loss of any signal information

Description:
TECHNICAL FIELD 
       [0001]    The present invention is generally related to a communications system and, more particularly, is related to systems and methods for transmitting reverse optical signals by a frequency modulated burst mode transmitter. 
       BACKGROUND OF THE INVENTION 
       [0002]    Hybrid fiber/coaxial (HFC) communications systems transmit signals in a forward and reverse path between a headend and a plurality of subscribers. In the reverse path, a coaxial cable feeder portion connects the subscriber equipment (e.g., cable modems, digital set-top boxes) with an optical node, which conventionally converts the radio frequency (RF) signals received from the subscriber equipment to optical signals, that sits at the input of an optical link. Subsequently, the optical link connects the reverse path from the optical node to a hub or headend where they are processed accordingly. 
         [0003]    Lasers used for reverse path signaling in the conventional approach to HFC network design are intensity modulated by the RF electrical signals that contain information for transmission in the reverse path. Ideally the light intensity from these lasers is proportional to the electrical signals. The light is launched down a reverse path optical fiber and is attenuated by an amount that is a function of the length of that fiber. RF output power levels from conventional optical receivers are a function of the received optical input power. Variations in the length of optical fibers throughout the HFC network result in variations in the received optical power at the input of each optical receiver. Consequently, RF output power is manually adjusted at each optical receiver to compensate for variations in optical loss from link to link. Therefore, there is a need to address the deficiencies and/or inadequacies of reverse optical transmitters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0005]      FIG. 1  is an abridged block diagram of a communications system that is suitable for use in implementing the present invention. 
           [0006]      FIG. 2  illustrates one link in the reverse direction of the broadband communications system of  FIG. 1 . 
           [0007]      FIG. 3  is a block diagram of a first embodiment of a frequency modulated burst mode optical transmitter in accordance with the present invention. 
           [0008]      FIG. 4  is a block diagram of a frequency modulated optical receiver that is suitable for use with the frequency modulated burst mode transmitter of  FIG. 3 . 
           [0009]      FIG. 5  is a block diagram of a second embodiment of a frequency modulated burst mode optical transmitter in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0010]    The preferred embodiments of the invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these 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. Furthermore, all “examples” given herein are intended to be non-limiting. 
         [0011]    The present invention is directed towards a frequency modulated (FM) burst mode optical transmitter that uses received reverse electrical signals to frequency modulate an RF carrier signal which is in turn used to modulate a laser. It is noted that information-carrying reverse electrical signals are transmitted on predetermined carrier frequencies. For teaching purposes, to distinguish the carrier signals of the reverse electrical signals from the RF carrier signal that is frequency modulated, the reverse electrical signal carrier frequencies are referred to herein as subcarrier frequencies, and are generally in the MHz range. For example, reverse electrical signals are typically received at frequencies within the bands of 5 MHz to 45 MHz, 5 MHz to 108 MHz, 5 MHz to 174 MHz, or 5 MHz to 88 MHz, depending upon the application. The signal that is frequency modulated by the FM burst mode optical transmitter of the invention is referred to as the RF carrier signal and, in an exemplary embodiment, is in the GHz range, for example 1.21 GHz. The frequency modulated RF carrier signal is then used to intensity modulate a laser. In this manner, since the optical signal transports the desired information in the frequency domain as opposed to the signal amplitude, longer fiber distances can be used with significant signal attenuation since FM signals are more robust than amplitude modulated signals. The FM burst mode optical transmitter of the present invention includes a carrier detect circuit, which initially detects the presence of a subcarrier signal present in the reverse electrical signals. Conventional optical transmitters typically bias the laser so that the laser continuously generates a reverse optical signal regardless of the presence of a reverse electrical subcarrier signal. By using the carrier detect circuit of the present invention, the laser can be turned on when a reverse electrical signal on a subcarrier frequency is detected, and turned off otherwise. Accordingly, only when a subcarrier signal is detected does the FM burst mode optical transmitter send an optical signal to an optical receiver located further upstream. 
         [0012]      FIG. 1  is an abridged block diagram of a communications system  110  that is suitable for use in implementing the present invention. Typically, the communications system  110  includes a transport network  115  and a transmission network  120 . The transport network  115 , which is fiber optic cable, connects a headend  125  and hubs  130  for generating, preparing, and routing programs and other optical packets over longer distances; whereas the transmission network  120 , which is coaxial cable, generally routes electrical packets over shorter distances. Programs and other information packets received, generated, and/or processed by headend equipment can be broadcast to all subscribers in the system  110 , or alternatively, can be selectively delivered to one or more subscribers. Fiber optic cable  135  connects the transport network  115  to an optical node(s)  140  which converts the packets from optical packets to electrical packets. Thereafter, coaxial cable  145  routes the electrical packets to one or more subscriber premises  150   a - d.    
         [0013]    In the reverse, or upstream, direction, subscriber premises equipment, such as set-top boxes or cable modems, generate reverse electrical signals. The optical node  140 , which includes an optical transmitter, converts the reverse electrical signals into optical signals for further routing to the hubs  130 . The hubs  130  then route the optical signals to the headend  125  for further processing. 
         [0014]      FIG. 2  illustrates one link in the reverse direction of the broadband communications system  110  of  FIG. 1 . A tap  210  receives the reverse electrical signals from a subscriber  150  and combines the signals with other reverse electrical signals being transmitted on that path. An amplifier  215  amplifies the combined electrical signals as necessary. At the demarcation point between the transmission network and the optical links is the optical node  140  which includes an optical transmitter  220 . Reverse electronics  225 , such as amplifiers and other configuration modules, prepare the signals for conversion into optical signals by laser  228 . The optical signals are then transported across optical fiber  135  to an optical receiver  230 , which is included in either the headend  125  or the hub  130 . The optical receiver  230  converts the optical signals back into electrical signals via a photodiode  235  and reverse electronics  238  further condition the signal as required. The reverse electrical signals, which have been combined from various subscribers  150   a - d , are then provided to headend equipment. As mentioned, however, the optical signals are susceptible to signal attenuation in the case of analog optical signal transport, or require expensive digital electronics in order to convert the optical signals into a digital optical signal. The present invention, in contrast, transports frequency modulated optical signals where the information of the reverse signals is carried in the frequency domain, rather than the signal amplitude, so that long fiber can be used without incurring significant signal losses. 
         [0015]      FIG. 3  is a block diagram of a first embodiment of an optical node  300  that includes an exemplary embodiment of an FM burst mode optical transmitter  340  of the present invention. Feeder legs  305   a - d  receive reverse electrical signals from subscribers located on four different paths. Any number of feeder legs can be input to the optical node  300 . Diplex filters  310   a - d  isolate the reverse electrical signals from the forward, or downstream, signals. Reverse electronics  315  then amplify, combine, and configure the signals in a known manner. The output of reverse electronics  315  is then input to the FM burst mode optical transmitter  340 . 
         [0016]    In the exemplary embodiment shown in  FIG. 3 , the FM burst mode optical transmitter  340  includes an FM modulator  320 , a carrier detect circuit  325 , and a laser  330 . As shown in  FIG. 3 , the output of the reverse electronics  315  is input to the FM modulator  320  and the carrier detect circuit  325  of the FM burst mode optical transmitter  340 . The FM modulator  320  uses the reverse electrical signals to modulate an RF carrier signal. In an exemplary embodiment, a 1.21 GHz carrier signal is frequency modulated with the reverse electrical signals. The carrier detect circuit  325  detects the presence of a subcarrier signal in the reverse electrical signals. Typically, electrical noise, interference and other undesired ingress signals are received at the feeder legs  305   a - d ; thus some sort of reverse electrical signals are present at the optical node  300  regardless of the presence of a subcarrier signal with desired information. The carrier detect circuit  325  can detect the presence of a subcarrier signal in the reverse electrical signals and control the laser  330  accordingly. More specifically, when a subcarrier signal is detected, the carrier detect circuit  325  turns the laser  330  on so that it can be intensity modulated with the frequency modulated 1.21 GHz RF carrier signal. When a carrier signal is not detected, the laser  330  is turned off so that no optical signals are transmitted upstream. In a further exemplary embodiment, the carrier detect circuit  325  can control the FM modulator  320  so that the carrier signal (for example, a 1.21 GHz signal) is frequency modulated with the reverse electrical signals only when a subcarrier signal is detected. In yet a further embodiment, the carrier detect circuit  325  can control the means used to perform the modulation of the laser, for example the carrier detect circuit  325  may control an optical modulation modulator. Additional information regarding the transmission of frequency modulated optical signals can be found in copending U.S. patent application Ser. No. 11/683,640 entitled “Reverse Path Optical Link using Frequency Modulation”, filed on Mar. 8, 2007, and U.S. Pat. No. 6,509,994 entitled “Burst Mode Analog Transmitter”, filed on Apr. 23, 2001, the disclosures and teachings of which are hereby incorporated by reference. 
         [0017]      FIG. 4  is a block diagram of a hub  400  that includes an exemplary embodiment of a reverse FM optical receiver  420  that is suitable for use with the FM burst mode transmitter  340  of  FIG. 3 . The reverse FM optical receiver  420 , which is coupled to the FM burst mode transmitter  340 , receives the optical signal that is present only when reverse signals in the subcarrier frequency band(s) are present at the input to the carrier detect circuit  325  in the burst mode optical transmitter  340 . A photodiode  405  converts the optical signals back into electrical signals. Subsequently, an FM demodulator  410  demodulates the electrical signals. Reverse electronics  415  further condition the signal prior to further transmission to headend equipment. 
         [0018]      FIG. 5  is a block diagram of an optical node  500  that includes an FM burst mode optical transmitter  540  in accordance with a further embodiment of the present invention. The FM burst mode optical transmitter  540  includes a delay circuit  510 . The delay circuit  510  delays the frequency modulated electrical signals by an appropriate time in order to allow the carrier detect circuit  325  to detect the presence of a subcarrier signal and then turn on the laser  330 . In this manner, reverse signals are not lost due to any time delays by the carrier detect circuit  325 . 
         [0019]    Accordingly, systems and methods have been described that enable a frequency modulated burst mode optical transmitter. It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementation set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.