Abstract:
A passive optical network for transmitting digital signals incorporates sub-octave filters for the removal of distortions introduced into the signals as they are transmitted over the fiber optic cable of the network. Stated differently, second order distortions that result when the light beam carrying the digital signals is passed through a fiber optic cable are removed by the sub-octave filter. Further, the employment of another passive optical network on the same fiber optic cable with the present network is provided for. And, considerations for ensuring the compatibility of upstream and downstream transmission frequencies with the sub-octave filters are disclosed.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention pertains generally to systems and methods that enable transmissions of data over optical fibers. More particularly, the present invention pertains to systems and methods for transmitting digital signals over fiber optic networks with subsequent sub-octave filtering to remove second order distortions from the signals. The present invention is particularly, but not exclusively, useful as a system and method for using a Passive Optical Network (PON) to transmit digital signals with subsequent sub-octave filtering. 
       BACKGROUND OF THE INVENTION 
       [0002]    A Passive Optical Network (PON) is essentially an optical network that uses a single fiber optic cable for the transmission of signals from one point (e.g. a service provider) to a plurality of different points (e.g. customer premises). Most likely, the signals to be transmitted will be digital signals. Therefore, in addition to the fiber optic cable, the PON will necessarily include a component (i.e. modem) at the transmit end of the fiber optic cable that modulates digital signals onto a radio frequency (RF) carrier wave. The resulting RF signal is then converted into an optical signal for transmission over the fiber optic cable. At the receive end of the fiber optic cable, the process is reversed. Specifically, a component (modem) reconverts the optical signal to an RF signal, and then demodulates the RF signal for subsequent use. 
         [0003]    An important aspect of a PON is that it can take advantage of the well known transmission of optical signals by Wavelength-Division Multiplexing (WDM). This essentially allows the PON to use one wavelength (λ 1 ) for downstream traffic on the fiber optic cable, while simultaneously using another wavelength (λ 2 ) for upstream traffic. Further, it is possible to have two or more upstream traffic wavelengths (e.g. λ 1  and λ 3 ), and two or more downstream traffic wavelengths (e.g. λ 2  and λ 4 ). This WDM capability, coupled with the point-to-multipoint characteristics of the PON, gives it a distinct advantage over other types of network architectures. Specifically, a PON configuration will reduce the amount of fiber optic cable that is required vis-à-vis a point to point architecture. A potential downside, however, is that fiber optic cables are known to introduce distortions into an optical signal that diminish its clarity. 
         [0004]    Of all the distortions that may be introduced into an optical signal as it transits through a fiber optic cable, the most predominant distortion is the second order distortion. These second order distortions, however, are relatively easily identified. For example, consider an optical signal carrying RF frequencies f a  and f b . It can happen that the fiber optic cable will induce two RF distortion signals at frequencies f a +f b  and f a −f b  into the optical signal as it transits through the fiber optic cable. In the case where f a ≅f b , the second order distortions are f a +f b ≅2f a  and f a −f b ≅0. In this case, f a −f b ≅0 is trivial and 2f a  defines the octave for f a . 
         [0005]    In light of the above, an object of the present invention is to provide a passive optical network with a sub-octave filter that will transmit clear signals over the PON with minimal, if any, distortions at the receive end of the transmission. Another object of the present invention is to provide a passive optical network that effectively removes distortions from a transmitted signal that are induced into the signal by the fiber optic cable of the PON. Still another object of the present invention is to provide a passive optical network with a sub-octave filter for removing second order distortions from transmitted optical signals that is easy to use, is simple to employ and is comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with the present invention, a Passive Optical Network (PON) incorporates a band pass filter for removing second order distortions from an optical signal that are induced when a light beam is transmitted through a fiber optic cable in the PON. In accordance with the present invention, the optical signal from the fiber optic cable is converted to an RF signal, and the RF signal is filtered in the sub-octave bandwidth that includes the RF carrier frequency of the digital signal. The RF signal can then be demodulated for subsequent reception of the digital signal. 
         [0007]    Structurally, the Passive Optical Network (PON) of the present invention includes a transmit modem for modulating a plurality of digital signals onto respective RF carrier frequencies (f). This can be done by either amplitude modulation, frequency modulation, or phase modulation. An optical transmitter with the modem is also used to convert each of these modulated carrier frequencies into an optical signal. A Wavelength-Division Multiplexer (WDM) is then used to combine the optical signal with other, similarly formed optical signals to create a light beam. Importantly, in the light beam each optical signal will have its own separate wavelength (λ). 
         [0008]    For the present invention, an optical fiber cable is provided for transmitting the light beam over the PON between an Optical Line Terminal (OLT) [e.g. a service provider] and a plurality of Optical Network Units (ONU) [e.g. customers]. In detail, the optical fiber will have a first end that is connected to the OLT for receiving the light beam from the transmitter and the WDM. The light beam is then transferred through the optical fiber to its second end. A splitter, which is connected to the second end of the optical fiber, is used for splitting the light beam into subsets. As envisioned for the present invention, each subset will be sent to a respective ONU, and it will include all of the optical signals in the transmitted light beam, albeit at reduced power. 
         [0009]    A plurality of optical receivers are positioned at respective customers (i.e. ONUs) in the network to receive a subset from the light beam. Each optical receiver then functions with a modem to reconvert optical signals in the subset back to their respective modulated carrier frequencies. A sub-octave band pass filter then filters out the second order distortions that are outside the sub-octave of the modulated carrier frequency. Thus, second order distortions are removed from the received signals. 
         [0010]    Once the received signals have been reconverted and filtered, a tuner is used to tune in a selected carrier frequency and to direct the selected carrier frequency to an addressed premise in the ONU. The receive modem then demodulates the tuned carrier frequency to reconstruct its respective digital signal. The digital signal can then be used for its intended purpose. 
         [0011]    Operationally, a method of the present invention for enabling a sub-octave transmission of a digital signal over a passive optical network (PON) relies on establishing a sub-octave bandwidth for each of a plurality of discrete carrier frequencies (f). Initially, the method envisions modulating a digital signal onto a selected carrier frequency (f) and then converting the modulated carrier frequency into an optical signal. With this conversion, the optical signal and the digital signal will both have a same wavelength (λ). Several such optical signals can be correspondingly formed and combined together into the light beam. In the event, the light beam is introduced into the first end of a fiber optic cable and is transmitted through the fiber optic cable from the first end to a second end. 
         [0012]    At the second end of the fiber optic cable, the light beam is split into subsets that each include all of the optical signals of the originally transmitted beam. Each subset of the light beam is then directed to a designated optical receiver at a respective ONU where it is reconverted to the modulated carrier frequency. At this point, the second order distortions that are outside the established sub-octave are filtered from the modulated carrier frequency. A tuner can then be used to tune in a selected modulated carrier frequency, and a receive modem can be used to demodulate the tuned carrier frequency for receipt of its respective digital signal. 
         [0013]    As envisioned for the present invention, establishing the sub-octave involves identifying a first octave bounded by a low carrier frequency MO and a high carrier frequency (f H1 ). This first octave will be used by a forward (downstream) transmit light beam. Importantly, 2f L1 ≧f H1 &gt;f L1 . Also, a second octave is identified which is bounded by a low carrier frequency (f L2 ) and a high carrier frequency (f H2 ). This second octave will be used by a return (upstream) receive light beam, wherein 2f L2 ≧f H2 &gt;f L2 . For the present invention, the forward (downstream) transmit light beam and the return (upstream) receive light beam will include carrier frequencies in a range between 750 MHz and 40 GHz. Further, it is contemplated that embodiments of the present invention may employ two PONs on the same optical fiber cable. For these embodiments, the present invention envisions adding bandwidth below f u  for use by a forward (downstream) transmit light beam (e.g. λ 3 ) in the second PON, and bandwidth below f L2  for use by a return (upstream) receive light beam (e.g. λ 4 ) in the second PON. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
           [0015]      FIG. 1  is a schematic layout of the component elements of a Passive Optical Network (PON) in accordance with the present invention; and 
           [0016]      FIG. 2  is an operational flow chart of the methodology of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Referring initially to  FIG. 1 , component elements of a Passive Optical Network (PON) in accordance with the present invention are shown collectively and generally designated  10 . As shown, the PON  10  includes a fiber optic cable (optical fiber)  12  that interconnects an Optical Line Terminal (OLT)  14  (e.g. a service provider) with a plurality of Optical Network Units (ONU)  16  (e.g. customers). In  FIG. 1 , the ONU  16  is only exemplary, and is shown to be servicing Customer A. 
         [0018]    As indicated in  FIG. 1 , a digital signal  18  that is to be transmitted over the PON  10  is modulated by the modem  20 . For purposes of the PON  10 , this modulation may be either an amplitude modulation, a frequency modulation, phase modulation, or any combination of the three. In any event, the digital signal  18  is modulated onto an RF carrier frequency (f 1 ) in a manner well known in the pertinent art. In  FIG. 1 , it is shown that the modulated carrier frequency  22  (i.e. f 1 ) is established in a sub-octave that is bounded by a low carrier frequency f L1  and a high carrier frequency f H1 . Once the sub-octave is established, the now-modulated carrier frequency  22  is passed to a transmitter  24  where it is converted into an optical signal  26  (i.e. an optical signal with wavelength λ 1 ). In turn, the optical signal  26  (λ 1 ) is sent to a Wavelength-Division Multiplexer  28  (WDM) where it is combined with other optical signals (e.g. λ 3 ) into a light beam  30  for downstream transmission over the fiber optic cable  12 . As shown in  FIG. 1 , the fiber optic cable  12  is connected between the WDM  28  and a splitter  32 . 
         [0019]    After the optical signal  26  on light beam  30  has been transmitted over the fiber optic cable  12 , the light beam  30  is split at the splitter  32  into a plurality of subset light beams  30 ′. Importantly, each subset light beam  30 ′ includes all of the optical signals (e.g. λ 1  and λ 2 ) that were combined together at the WDM  28 . Further each subset light beam  30 ′ is then sent to a respective ONU  16 . Operationally, the WDM  34  at ONU  16  (i.e. Customer A) receives the same subset light beam  30 ′ as does every other ONU  16  in the PON  10  (e.g. Customer X). For the specific example of customer A, the optical signal (λ 1 )  26  that is in the subset light beam  30 ′ received by ONU  16 , is sent to a receiver  36  where it is reconverted into its modulated carrier frequency  22 ′ (i.e. f 1 ). This modulated carrier frequency  22 ′ (f 1 ) is then filtered by a band pass filter  38  and is demodulated by the modem  40 . The consequence of this is that the digital signal  18  that is being carried by a filtered carrier frequency  22 ′ is received at the ONU  16  with all impairments caused by second order distortions effectively removed from the digital signal  18 . 
         [0020]    Although the above disclosure has focused on a downstream transmission from OLT  14  to ONU  16 , an upstream transmission from ONU  16  to the OLT  14  is similar and essentially operates in reverse. Specifically, for an upstream transmission, a digital signal  42  is modulated at the modem  40  onto an RF carrier frequency (f 2 ) in a manner as similarly disclosed above for f 1 . In this instance, a modulated carrier frequency  44  (i.e. f 2 ) is established in a sub-octave that is bounded by a low carrier frequency f L2  and a high carrier frequency f H2 . The modulated carrier frequency  44  is then passed to a transmitter  46  where it is converted into an optical signal  48  (i.e. an optical signal with wavelength λ 2 ). In turn, the optical signal  48  (λ 2 ) is sent to the Wavelength-Division Multiplexer  34  (WDM) where it can be combined with other optical signals (e.g. λ 4 ) into a light beam  50  for an upstream transmission over the fiber optic cable  12 . The light beam  50  is then received by OLT  14 , processed through the Wavelength-Division Multiplexer  28  and sent to a receiver  52  where the optical signal  48  in the light beam  50  is reconverted into its modulated carrier frequency  44 ′ (i.e. f 2 ). This modulated carrier frequency  44  (f 2 ) is then filtered by a band pass filter  54 , and it is subsequently demodulated by the modem  20 . The consequence of this is that the digital signal  42  is received at the OLT  14  with all impairments caused by second order distortions being effectively removed from the digital signal  42 . 
         [0021]      FIG. 2  presents a step-by-step methodology, generally designated  56 , which indicates that an initial consideration for an operation of the PON  10  is the establishment of a sub-octave (see block  58 ). Specifically, a sub-octave is established for each transmission (downstream/upstream). To transmit a digital signal  18 / 42  over the PON  10 , block  60  indicates that the digital signal  18 / 42  is modulated onto a carrier frequency  22  (f 1 )/ 44  (f 2 ). Block  62  then indicates that the modulated carrier frequency  22  (f 1 )/ 44  (f 2 ) is converted to an optical signal  26  (λ 1 )/ 48  (λ 2 ). The optical signal  26  (λ 2 )/ 48  (λ 2 ) can then be combined with other such signals at a WDM  28 / 34  and transmitted (downstream/upstream), as a light beam  30 / 50  over the fiber optic cable  12  (see block  64 ). 
         [0022]    Insofar as the light beam  30  is specifically concerned, block  66  indicates that the light beam  30  is split into subset light beams  30 ′. Each subset light beam  30 ′ is then directed to a particular ONU  16  (see block  68 ) where it is converted back (see block  70 ) from an optical signal  26  (λ 1 )/ 48  (λ 2 ) to an RF modulated carrier frequency  22  (f 1 )/ 44  (f 2 ). The RF modulated carrier frequency  22  (f 1 )/ 44  (f 2 ) is then filtered (see block  72 ). More specifically, as indicated above, a unique sub-octave is established for use by each of the band pass filters  38  and  54  to respectively remove second order distortions from the downstream light beam  30  and from the upstream light beam  50 , after the light beams  30 / 50  have been transmitted through the fiber optic cable  12 . 
         [0023]    After the optical signals  26  (λ 1 )/ 48  (λ 2 ) have been reconverted to respective RF modulated carrier frequencies  22 ′ (f 1 )/ 44 ′ (f 2 ), and the second order distortions have been removed from the RF modulated carrier frequencies  22 ′ (f 1 )/ 44 ′ (f 2 ), block  74  indicates a user can tune for a carrier frequency of interest (e.g. modulated carrier frequency  22  (f 1 )). The modulated carrier frequency  22  (f 1 ) is then demodulated by a modem  20 / 40  (see block  76 ) and the digital signal  18 / 42  is received for use without any appreciable impairments caused by second order distortions in the transmission process (see block  78 ). 
         [0024]    While the particular Passive Optical Network with Sub-Octave Transmission as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.