Abstract:
A method of multiplexing optical signals in a node of an optical network including as inputs a plurality of electrical signals, a plurality of laser transmitters, and as outputs a plurality of optical fibers, (a) generating clock pulses as a first clock frequency; (b) dividing the clock pulses respectively into a number of parallel trigger outputs; (c) sampling the electrical signals respectively by triggering on the parallel trigger outputs; (d) converting the sampled electrical signals to sampled optical signals by modulating respectively the laser transmitters with the sampled electrical signals and outputting respectively the sampled optical signals on the optical fibers; (e) combining the sampled optical signals onto a single optical fiber.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
   The present invention relates to optical communications and, more particularly, to a novel method of multiplexing optical signals onto a single optical fiber. Specifically, the method includes sampling multiple signals and coupling the sampled signals onto a single optical fiber. The optical multiplexing method, of the present invention, useful for both digital and RF analog signals, is applicable and for upgrading existing fiber optic communications networks and in new installations. 
   Multiplexing in optical fiber communications systems may be achieved by several existing technologies. The choice of multiplexing technology has a tremendous impact on the cost, performance and the upgradeability of the network. One multiplexing technology is wavelength division multiplexing (WDM). In WDM, data signals are each modulated on a separate wavelength of light and then coupled onto a single optical fiber. At the remote end of the optical fiber, the different wavelengths are optically separated and individually detected. WDM systems consequently require a large number of laser sources and detectors for each different wavelength. 
   An alternative to WDM that achieves high data rates on a single wavelength channel is known as optical time division multiplexing (OTDM). A typical OTDM system as shown in the diagram of  FIG. 1  includes a single high speed pulsed laser  101 . An optical splitter  103  splits the pulsed laser output. An external modulator  109  is used to modulate each signal onto an optical carrier. Optical delay lines  105 , such as fibers of varying lengths, are used to inter-leave the pulsed signals of the different channels. The optical signals are then optically multiplexed by a combiner  111  onto a single optical fiber. 
   During the lifetime of fiber optic communications networks, there are often situations in which additional fiber is required but not available and upgradeability options are limited. Therefore, a method to easily upgrade a fiber optic communications network installed in the field would be advantageous. Current multiplexing technologies are not readily upgraded once they are installed. For instance, a fiber optic communications network that does not include WDM lasers installed cannot be upgraded using WDM without replacing all the laser transmitters and installing wavelength demultiplexers and/or filters at the receiver end. Upgrading a prior art OTDM network would require the installation of optical delay lines of precise delay values, a difficult procedure performed in the field. 
   There is thus a need for, and it would be highly advantageous to have a method of optical time division multiplexing by signal sampling which can be used to free optical fibers in an existing network that is straightforward to install and requires replacing a minimum number of existing network components. 
   SUMMARY OF THE INVENTION 
   According to the present invention there is provided a method of multiplexing optical signals in a node of an optical network including as inputs a plurality of electrical signals, a plurality of laser transmitters, and as outputs a plurality of optical fibers, including the steps of: (a) generating clock pulses at a first clock frequency; (b) dividing the clock pulses respectively into a number of trigger outputs carrying respective divided portions of the clock pulses at a second clock frequency equal to the first clock frequency divided by the number; (c) sampling the electrical signals respectively by triggering on the divided portions, thereby generating sampled electrical signals wherein sample durations of the sampled electrical signals are substantially equal to widths of the clock pulses; (d) converting the sampled electrical signals to sampled optical signals by modulating respectively the laser transmitters with the sampled electrical signals and outputting respectively the sampled optical signals on the optical fibers; and (e) combining the sampled optical signals onto a single optical fiber. Preferably, the sampling includes operationally connecting the trigger outputs to the respective laser transmitters and operationally connecting the electrical signals to the respective laser transmitters, and more preferably, sampling includes operationally connecting said trigger outputs to respective anodes of the respective laser transmitters and operationally connecting the electrical signals to respective cathodes of the respective laser transmitters. 
   According to the present invention there is provided a method, of multiplexing optical signals in a node of an optical network including a plurality of optical fibers, respectively transmitting a plurality of optical signals, including the steps of: (a) generating clock pulses at a first clock frequency; (b) dividing the clock pulses respectively into a number of trigger outputs carrying respective divided portions of the clock pulses at a second clock frequency equal to the first clock frequency divided by the number; (c) sampling the optical signals respectively by inputting the optical signals respectively into optical modulators and triggering the optical modulators on the divided portions, thereby generating sampled optical signals; and (d) combining the sampled optical signals onto a single optical fiber. 
   According to the present invention there is provided a system for multiplexing downlink optical signals in a site including multiple base transceiver stations, operationally connected to respective laser transmitters; the laser transmitters connected to multiple optical fibers respectively carrying the optical signals including: (a) a clock generating clock pulses at a first clock frequency; (b) a serial to parallel converter dividing the clock pulses respectively into a number of trigger outputs respectively carrying divided portions of the clock pulses at a second clock frequency equal to the first clock frequency divided by said number; (c) a plurality of laser transmitters operationally connected respectively to said trigger outputs; wherein the optical signals are sampled by driving the lasers on the respective divided portions; and (d) an optical coupler combining the optical fibers onto a single output optical fiber carrying a multiplexed optical signal. The system, further includes (e) an optical receiver connected to the output optical fiber, converting the multiplexed optical signal to a multiplexed electrical signal; (f) an RF amplifier connected to an output of said optical receiver, amplifying the multiplexed electrical signal; and (g) an antenna operationally connected to said RF amplifier broadcasting the multiplexed electrical signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
       FIG. 1  is a prior art drawing of a conventional optical time division multiplex system; 
       FIG. 2  is a drawing, according to an embodiment of the present invention, in a communications network using direct modulation laser sources; 
       FIG. 2   a  is a drawing, according to another embodiment of the present invention, in a communications network using direct modulation laser sources; 
       FIG. 3  is a is a drawing, according to an embodiment of the present invention, using external modulation of multiple optical signals; 
       FIG. 4  is a drawing, according to an embodiment of the present invention, using direct modulation in a mobile telephone application. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is of a system and method of multiplexing optical signals onto a single optical fiber. Specifically, the system and method includes sampling multiple signals and optically coupling the sampled signals onto a single optical fiber. The present invention can be readily used to upgrade existing networks in the field and for new installations of fiber optic communications networks. 
   The principles and operation of a system and method of multiplexing optical signals by sampling individually multiple signals and coupling the sampled signals onto a single fiber, according to the present invention, may be better understood with reference to the drawings and the accompanying description. 
   It should be noted, that although the discussion herein relates to optical multiplexing with RF analog signals, the present invention may, by non-limiting example, be alternatively configured as well using digital data signals. 
   Once a signal is multiplexed, according to an embodiment of the present invention, demultiplexing is achieved using any of the methods known in the art. For moderate digital data rates, multiplexing is achieved, for instance, using well-known clock and data recovery circuits in the time domain. For high speeds or high frequency signals, gated optical external modulators may be used. 
   Before explaining embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
   As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
   Referring now to the drawings,  FIG. 2  illustrates a system and method for multiplexing signals carried in several optical fibers into a single optical fiber, thereby freeing optical fibers for additional use in the optical network. An existing optical communications network includes multiple laser transmitters  201   a  to  201   n . Laser transmitters  201   a  to  201   n  are directly modulated respectively, by varying the current bias on the laser diodes of laser transmitters  201  according to an electrical signal generated respectively by signal generators  205   a  to  205   n . The modulated optical signals are carried by optical fibers  215   a  to  215   n . The multiplexing system, according to a present invention, uses a clock  207  that generates a serial clock signal, e.g., serial pulses at a precise frequency. The clock rate, preferably according to Nyquist, is greater than twice the number of multiplexed channels multiplied by the maximum frequency transmitted per channel. The serial clock signal is input to a serial to parallel converter  209 . Serial to parallel converter  209 , has multiple trigger outputs  221   a  to  221   n , a trigger output  221  to each sampling module  203 . According to one embodiment of the present invention, serial to parallel converter  209  outputs a first pulse to sampling module  203   a , a second pulse to sampling module  203   b , and so on, cycling through all sampling modules  203  to sampling module  203   n  and then returning to first sampling module  203   a . On receiving a clock pulse, sampling modules  203   a  to  203   n , are triggered to sample their respective inputs and output the signal samples to laser transmitters  201   a  to  201   n . Preferably, the time duration of the signal samples are similar to the clock pulse width. Consequently, laser transmitters  201   a  to  201   n  respectively have a “non-zero output” only when sampling modules  203   a  to  203   n  respectively are triggered. The term “zero output” refers to the type of modulation used, in digital modulation that is unipolar, “zero output” preferably means an optical output less than the laser threshold whereas in an analog output which is bipolar a “zero output” preferably refers to a bias point near half the peak maximum instantaneous output level of the laser diode. Sampled optical signals output for laser transmitters  201   a  to  201   n  are carried by optical fibers  215   a  to  215   n  respectively. The sampled signals are combined with an optical coupler  211 . The optical time domain multiplexed signal is carried on a single optical fiber  213  freeing n−1 fibers for additional use in the network. In one embodiment of the present invention, sampling modules  203   a  to  203   n  include RF mixers that convolute the clock output with the signal input as input to laser transmitters  201   a  to  201   n.    
   As an example, multiplexing 10 RF analog channels up maximum frequency 100 Mhz uses preferably a clock frequency of 2 Ghz. Laser transmitters  201  are preferably sampled at a frequency of 200 Mhz. The multiple sampled signals are consequently optical time division multiplexed onto a single optical fiber carrying 2 Ghz modulation from multiple laser transmitters. 
   In another embodiment of the present invention, shown in  FIG. 2   a , sampling may be achieved by driving laser transmitters  201   a  to  201   n  directly with both triggering pulses and outputs from generators  205   a  to  205   n  without requiring intervening sampling modules  203   a  to  203   n . In  FIG. 2   a , laser drivers  217   a  to  217   n  receive clock pulses from trigger outputs  221   a  to  221   n  of serial to parallel converter  209  and drive the respective anodes of laser transmitters  201   a  to  201   n . Outputs of RF signal generators  205   a  to  205   n  are connected to respective attenuators  219   a  to  219   n  to achieve appropriate signal levels, so as not to cause excess distortion in laser transmitters  201   a  to  201   n , and operationally connected to the respective cathodes of laser transmitters  201   a  to  201   n . The embodiment shown in  FIG. 2   a  has an advantage over the embodiment of  FIG. 2  since the embodiment of  FIG. 2   a  requires fewer parts than the embodiment of  FIG. 2  and does not require filters to provide isolation. 
   The present invention according to the embodiments shown in  FIG. 2  and  FIG. 2   a  have an advantage that with the exception of a simple, low cost and easy to install optical coupler  211 , all the additional equipment required for multiplexing is in the electrical domain and may added into an existed network at relatively low cost. 
   Another possible configuration of the present invention is shown in  FIG. 3 . In the embodiment of  FIG. 3 , signal generators  205   a  to  205   n  are input to laser transmitters  201   a  to  201   n . Modulated optical outputs from laser transmitters  201   a  to  201   n  are input to external modulators  301   a  to  301   n . The serial clock signal output from clock  207  is input to serial to parallel converter  209 . Serial to parallel converter  209 , has multiple trigger outputs  221   a  to  221   n , one trigger output  221  to each external modulator  301 . Serial to parallel converter  209  outputs a first pulse to external modulator  301   a , a second pulse to external modulator  301   b , and so on, cycling through all the external modulators  301   a  to external modulator  301   n  and then returning to first external modulator  301   a . On receiving a clock pulse, external modulators  301   a  to  301   n , are triggered to sample their respective optical inputs and output the signal samples to optical fibers  215   a  to  215   n  respectively. Optical fibers  215   a  to  215   n  are optically combined by optical coupler  211  into a single optical fiber  213 . 
   The embodiment of  FIG. 3  is appropriate for fast clock rates. For instance, multiplexing 5 data channels operating at 2.5 Gbs (gigabit/sec) according to the present invention, requires clock  207  to output pulses at a frequency at least 25 Gbs. Alternatively, an embodiment similar to that of  FIG. 3  using externally modulation may be configured using laser transmitters with fast rise and fall times of for instance 40 ps, characteristic of, for instance, a mode locked pulsed laser. 
   Another embodiment of the present invention, shown in  FIG. 4  is an application of optical time domain multiplexing in the field of mobile phone cellular communications. Referring to  FIG. 4 , base transceiver stations  405   a  to  405   n  are located at the same site. Base transceiver stations  405   a  to  405   n  belong to different cellular services. Alternatively, base transceiver stations  405   a  to  405   n  belong to the same cellular service but operate at frequency channels different from each other. Downlink (mobile receive) signals are output through uplink/downlink duplexers  409   a  to  409   n  and operationally connected, (after suitable attenuation, attenuators not shown) to laser transmitters  201   a  to  201   n . As in the embodiments shown in  FIG. 2  and  FIG. 2   a , clock  207  is operationally connected to respective laser transmitters  201   a  to  201   n  through serial to parallel converter  209  and through respective laser drivers  217   a  to  217   n . Optical outputs  215   a  to  215   n  from laser transmitters  201   a  to  201   n  are optically combined in optical coupler  211 ; the multiplexed optical output is carried by single optical fiber  213 . The optical signal carried by optical fiber  213  is received by an optical receiver  408  and converted into a multiplexed electrical signal. The multiplexed electrical signal is connected to an input of RF amplifier  410  and transmitted through an antenna duplexer  411  to antenna  413  for broadcasting. Mobile units  407   a  to  407   n  receive respectively signals from base transceiver stations  405   a  to  405   n . The uplink (mobile transmit) signal path from antenna  413  to base station transceivers  405   a  to  405   n  is not shown in  FIG. 4 . 
   As an example, of the embodiment shown in  FIG. 4 , three base transceiver stations each have distinct downlink frequency bands within 800-900 Mhz. Clock  207  operates for instance at 6 Ghz. and laser transmitters  201   a  to  201   n  are sampled at 2 Ghz. However, RF amplifier  410  has a frequency response of for instance 1 Ghz. Therefore, RF amplifier  410  is not fast enough to amplify the sampling transitions of the signals and therefore sufficiently distortion free signals from base transceiver stations  405   a  to  405   n  are transmit from antenna  413 . 
   Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. 
   While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.