Abstract:
An all-optical system for regenerating a first optical signal carried in a first direction on an optical transmission medium and a second optical signal carried in a second direction on the optical transmission medium includes a bi-directional clock recovery loop and a bi-directional optical gate. The bi-directional clock recovery loop includes a first optical clock recovery circuit for recovering a first clock signal from the first optical signal and a second optical circuit for recovering a second clock signal from the second optical signal. The first and second optical circuits of the clock recovery loop share at least some common optical circuit elements. The bi-directional optical gate includes a first non-linear optical light mirror circuit for producing a first regenerated signal based on the first optical signal and the first clock signal and a second non-linear optical light mirror circuit for producing a second regenerated signal based on the second optical signal and the second clock signal. The first and second non-linear optical light mirror circuits share at least some common optical circuit elements.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to methods and systems for regenerating optical signals, and more particularly to a bi-directional all-optical regenerator. 
     DESCRIPTION OF THE PRIOR ART 
     Optical fiber systems have become the physical transport medium of choice in long distance telephone and data communication networks. However, a problem with optical fiber systems is dispersion, which causes the optical pulses to spread. The original optical fiber systems include, in addition to a light transmitter and a light receiver connected by optical fiber, repeaters at various points along the optical fiber path. Repeaters are optical-electrical devices that include a receiver and a transmitter in series with circuitry for amplifying, reshaping, and retiming the signal. The receiver part of the repeater converts the signal on the optical fiber from the optical domain to the electrical domain and the transmitter converts the signal from the electrical domain back to the optical domain. The retiming and reshaping circuitry processes the signal prior to retransmission. 
     Recently, optical network operators have proposed and have begun to introduce all-optical systems. An all-optical system does not include electro-optical repeaters. Rather, all-optical systems use optical line amplifiers, such as rare earth-doped fiber amplifiers, to amplify the optical signals along the route. 
     Optical amplifiers simply amplify the signal and do not include any means for reshaping or retiming the signal. Accordingly, dispersion can be a severe problem in all-optical systems. One solution to chromatic dispersion in all-optical systems is disclosed in U.S. Pat. No. 5,430,822, which discloses dispersion compensating optical fibers. By inserting an appropriate length of dispersion compensating optical fiber into an optical system, dispersion related signal degradation can be compensated. 
     In addition to dispersion compensating fiber, there has been disclosed in U.S. Pat. No. 5,369,520 an optical regenerator. A regenerator differs from a line amplifier in that it not only strengthens the amplitude of the signal, but also reshapes the pulses and removes timing jitter. The regenerator of the &#39;520 patent includes an electrical-optical clock recovery stage and a Sagnac loop optical gate stage. The clock recovery stage generates a periodic optical pulse that matches the clock signal that underlies the incoming data signal. The clock signal is used in the optical gate to generate a retimed regenerated output signal. 
     The purpose of the Sagnac loop of the &#39;520 patent is to use the on-off state of the data signal to meter out single pulses of the clock signal. The clock signal recovered by the electrical-optical clock recovery stage enters the Sagnac loop through a coupler where it is split evenly and traverses the loop in both directions. The signal halves from each direction reconverge at the same optical coupler, and because of their phasing and the fact that they have passed through identical paths, they recombine to couple all of the energy back into the original input port. As long as the loop is kept symmetrical and there is no data signal, the other port from the coupler does not output any clock pulses. The data signal is propagated over a portion of the loop. As a data “one” pulse propagates through a portion of the loop, it travels along side one of the clock signal halves and imparts a phase shift, due to non-linearity of the shared fiber material known as the Kerr effect. The counter-propagating clock signal half is essentially unaffected by the data signal. When the clock signal halves recombine at the coupler, the imbalance introduced by the data signal causes the clock pulse to emerge from the output port of the coupler. Thus, a data signal is used to gate out high quality clock pulses. 
     Recently, there has been proposed an all-optical regenerator that includes an optical clock recovery stage and an optical gate stage. The clock recovery stage is an optical ring with an amplifier and a variable delay line. The size of the ring is selected so that a light pulse makes a complete cycle through the ring during one bit period of the expected incoming data signal, or an integral multiple thereof. The variable delay line is used to fine tune the ring delay with respect to the incoming signal. The optical ring and amplifier form a ring laser that is modulated into a circulating pulse by copropagating it with the incoming data signal. The incoming data signal is amplified and coupled into the clock recovery ring where it shares paths with a portion of the ring laser through a section of optical fiber. The circulating clock and passing data signals are amplified to sufficient levels to cause the material in the shared fiber path to exhibit a non-linear refractive index the Kerr effect. The non-linearity provides a venue for cross modulation. The clock recovery stage includes two outputs. One is a strong, idealized pulse stream from the ring laser representing the recovered clock signal. The other is a sample of the data signal after going through a portion of the ring. Both of these signals are fed into the optical gate stage, which is a Sagnac loop or non-linear optical loop mirror (NOLM). 
     An all-optical regenerator includes several expensive, specialized optical components and acts only on a single optical carrier channel. Each channel requires a separate set of equipment. Additionally, to regenerate carriers traveling in opposite directions, two complete regenerator sets occupying two spaces in an equipment rack are required for each carrier. Thus, regenerators are expensive in terms of both cost and space. It is an object of the present invention to reduce the number of optical components required to regenerate more than one optical carrier. 
     SUMMARY OF THE INVENTION 
     The present invention provides an all-optical system for regenerating a first optical signal carried in a first direction on an optical transmission medium and a second optical signal carried in a second direction on the optical transmission medium. The system includes a bi-directional clock recovery loop and a bi-directional optical gate. The bi-directional clock recovery loop includes a first optical clock recovery circuit for recovering a first clock signal from the first optical signal and a second optical circuit for recovering a second clock signal from the second optical signal. The first and second optical circuits of the clock recovery loop share at least some common optical circuit elements. The bi-directional optical gate includes a first non-linear optical light mirror circuit for producing a first regenerated signal based on the first optical signal and the first clock signal and a second non-linear optical light mirror circuit for producing a second regenerated signal based on the second optical signal and the second clock signal. The first and second non-linear optical light mirror circuits share at least some common optical circuit elements. 
     The bi-directional clock recovery loop includes a first optical signal input arranged to receive the first signal and a second optical signal input arranged to receive the second signal. The clock recovery loop outputs the first clock signal at a first recovered clock signal output and the second clock signal at a second recovered clock signal output. The clock recovery loop also outputs the first optical signal at a first optical signal output and the second optical signal at a second optical signal output. 
     The bi-directional optical gate includes a first recovered clock signal input coupled to the first recovered clock signal output of the bi-directional clock recovery loop and a second recovered clock signal input coupled to the second recovered clock signal output of the bi-directional clock recovery loop. The optical gate also includes a first optical signal input coupled to the first optical signal output of the bi-directional clock recovery loop and a second optical signal input coupled to the second optical signal output of the bi-directional clock recovery loop. The optical gate outputs the first regenerated optical signal at a first regenerated signal output and the second regenerated optical signal output at a second regenerated optical signal output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a high level block diagram of a bi-directional all-optical regenerator according to the present invention. 
     FIG. 2 is an optical circuit diagram of a preferred embodiment of the bi-directional all-optical regenerator of the present invention. 
     FIG. 3 is an optical circuit diagram of an alternative embodiment of the bi-directional all-optical regenerator of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and first to FIG. 1, bi-directional all-optical regenerator is designated generally by the numeral  11 . Regenerator  11  is coupled to an optical transmission cable  13  through a bi-directional line amplifier  15 . Optical fiber  13  carries a first data signal E D  from east to west and a second data signal W D  from west to east. As will be explained in detail hereinafter, regenerator  11  processes W D  and E D  into regenerated signals {overscore (W′ C +L )} and {overscore (E′ C +L )}, respectively. 
     In the drawings, E D  represents the east data signal and W D  represents the west data signal. E′ C  represents a recovered clock signal based upon E D  and W′ C  represents the recovered clock signal based upon W C . The prime indicates that the recovered clock signal has a wave length that is different from the original data signal from which it is recovered. {overscore (E)}′ C  is the regenerated east data signal and {overscore (W)}′ C  is the regenerated west data signal. 
     Regenerator  11  includes a bi-directional clock recovery loop  17  and a bi-directional optical gate  19 . The details of the circuitry of clock recovery loop  17  and optical gate  19  will be discussed in detail with respect to the embodiments of FIGS. 2 and 3. Generally, clock recovery loop  17  includes a first optical signal input  21 , which receives optical signal E D  through a wavelength division multiplexer (WDM)  23 , and a second optical signal input  25 , which receives optical signal W D  through a WDM  27 . Clock recovery loop  17  includes a first optical signal output  29 , which outputs data signal E D , and a second optical signal output  31 , which outputs data signal W D . Finally, clock recovery loop  17  includes a first: recovered clock signal output  33 , which outputs recovered clock signal E′ C , and a second recovered clock signal output  35 , which outputs recovered clock signal W′ C . 
     Optical gate  19  includes a first clock signal input  37 , which is coupled to receive clock signal E′ C  and a second clock signal input  39 , which is coupled to receive clock signal W′ C . Optical gate  19  also includes a first optical signal input  41 , which receives signal E D , and a second optical signal input  43 , which receives optical signal W D . As will be explained in detail hereinafter with respect to FIGS. 2 and 3, optical gate  19  combines optical signal E D  and clock E′ C  to produce a regenerated signal {overscore (E′ C +L )} at a first regenerated optical signal output  45 . Similarly, optical gate  19  combines data signal W D  and clock W′ C  to produce a regenerated signal {overscore (W′ C +L )} at a second regenerated signal output  47 . Regenerated signal {overscore (E′ C +L )} is coupled back to optical fiber  13  through WDM  27  and regenerated signal {overscore (W′ C +L )} is coupled back into optical fiber  13  through WDM  23 . Optical gate  19  finally includes a first optical signal output  49 , which outputs optical signal E D  and a second optical signal output  51 , which outputs optical signal W D . 
     Referring now to FIG. 2, there is shown an optical circuit diagram of a preferred embodiment of the bi-directional all-optical regenerator of the present invention. Signal E D  is received at clock recovery loop  17  from WDM  23  at an input of a WDM  55 . Similarly, data signal W D  is received from WDM  27  at an input of a WDM  57 . A polarization controller  59  is disposed in the optical path of data signal E D  between WDM  23  and WDM  55 . Similarly, a polarization controller  61  is disposed in the optical path of data signal W D  between WDM  27  and WDM  57 . 
     WDM  55  multiplexes data signal E D  with recovered clock signal E′ C  onto a section of optical fiber  63 . The velocity dispersion of fiber  63  is chosen carefully to be near zero at the nominal wavelengths of data signal E D  and clock signal E′ C  to assure that the copropagating waves will remain congruent while traveling together in fiber  63 . A variable optical delay line  65  is disposed in the optical path of recovered clock signal E′ C  into WDM  55  to insure that E D  and E′ C  travel together through optical fiber  63 . Additionally, a polarization controller  67  is disposed in the path of clock signal E′ C , to ensure that clock signal E′ C  and data signal E D  enter fiber  63  in the same polarization state. Data signal E D  is amplified by bi-directional line amplifier  15  and clock signal E′ C  is amplified by a controllable gain bi-directional line amplifier  79  so that the power is high enough to drive fiber  63  into non-linearity to exhibit the Kerr effect. Fiber  63  is coupled to a WDM  69  which separates data signal E D  from recovered clock signal E′ C . Data signal E D  is coupled to optical gate  19  through a WDM  71 . Clock signal E′ C  from WDM  69  is coupled to an optical coupler  73 , which splits clock signal E′ C  into a first portion which is coupled back into clock recovery loop  17  through a WDM  75  and a second portion, which is coupled into optical gate  19  through a variable optical delay line  77 . WDM  75  is bi-directionally coupled to a controllable gain bi-directional line amplifier  79 , which in turn is coupled to a WDM  81 . An output of WDM  81  couples recovered clock signal E′ C  back to WDM  51  through polarization controller  67  and optical delay line  65 . Thus, WDM  65 , optical fiber  63 , WDM  69 , optical coupler  73 , WDM  75 , bi-directional line amplifier  79 , WDM  81 , polarization controller  67  and optical delay line  65  perform a first clock recovery circuit that recovers a clock signal E′ C  from data signal E D . 
     Similarly, WDM  57  multiplexes data signal W D  with recovered clock signal W′ C , onto a section of optical fiber  83 . Optical fiber  83  is coupled to a WDM  85  that separates data signal W D  from recovered clock signal W′ C . Data signal W D  from WDM  85  is coupled into optical gate  19  through a WDM  87 . Recovered clock signal W′ C  from WDM  85  is coupled to an optical coupler  89  that splits recovered clock signal W′ C  into a first portion, which is coupled back to clock recovery loop  17  through WDM  81 , and a second portion, which is coupled into gate  19  through a variable optical delay line  91 . 
     Recovered clock signal W′ C  is amplified by bi-directional line amplifier  79  and coupled back to WDM  57  through WDM  75 . A polarization controller  93  and a variable optical delay line  95  are disposed in the optical path of recovered clock signal W′ C  between WDM  75  and WDM  57  to ensure maximum interaction of recovered clock signal W′ C , and data signal W D  in optical fiber section  83  between WDM  57  and WDM  85 . 
     Thus, WDM  57 , optical fiber  83 , WDM  85 , optical coupler  89 , WDM  81 , bi-directional line amplifier  79 , WDM  75 , polarization controller  93 , and optical delay line  95 , with their respective connections, form a second optical clock recovery that recovers clock signal W′ C  from data signal W D . It will be noted that the first and second clock recovery circuits share WDM  75 , bi-directional line amplifier  79 , and WDM  81 . 
     In the embodiment of FIG. 2, bi-directional optical gate  19  includes a west non-linear optical loop mirror (NOLM)  97  and an east NOLM  99 . West NOLM  97  includes an optical coupler  101 , a WDM  103 , and a WDM  105 . A length of optical fiber  107  is coupled between WDM  103  and WDM  105 . Similarly, east NOLM includes an optical coupler  109  and WDMs  111  and  113  with a section of optical fiber  115  coupled therebetween. A variable gain bi-directional line amplifier  117  is bi-directionally coupled between WDM  71  and WDM  87 . WDM  71  is coupled to WDM  103  of west NOLM  97 , and WDM  87  is coupled to WDM  113  of east NOLM  99 . 
     Recovered clock signal W′ C  is received by west NOLM  97  at optical coupler  101 . Optical coupler  101  splits recovered clock signal W′ C  into two signal halves that traverse west NOLM  97  in opposite directions. WDM  103  multiplexes data signal W D  with the counterclockwise half of recovered clock signal W′ C  onto optical fiber section  107 . Optical delay line  109  is variable so that signals W D  and W′ C  travel through fiber  115  together. Polarization controller  119  and  121  are disposed in the paths of signals W D  and W′ C , respectively, to ensure maximum cross modulation in fiber  107 . WDM  105  separates regenerated west signal {overscore (W′ C +L )}, from original data signal W D . Regenerated signal {overscore (W′ C +L )} is coupled back to WDM  23  through optical coupler  101 . similarly, east NOLM  99  regenerates the east signal by multiplexing recovered clock signal E′ C  with data signal E D  onto optical fiber  115 . Polarization controllers  123  and  125  are disposed in the paths of signals E D  and E′ C , respectively, to ensure maximum cross modulation within optical fiber  115 . Regenerated signal {overscore (E′ C +L )} is coupled back to WDM  127  through optical coupler  109 . It will be noted in FIG. 2 that WDM  71 , WDM  87 , and variable gain bi-directional line amplifier  117  are common to both west NOLM  97  and east NOLM  99 . The gain of line amplifier  117  is controllable to ensure that the power sufficient to drive NOLMs  97  and  99  into non-linearity. Preferably line amplifier is controlled so as not to drive NOLMs  97  and  99  into saturation, thereby to control the bias of NOLMs  97  and  99 . 
     Referring now to FIG. 3, there is shown an alternative embodiment of the bi-directional all-optical regenerator of the present invention. The regenerator of FIG. 3 includes a bi-directional clock recovery loop  17   a  and a bi-directional optical gate  19   a . Clock recovery loop  17   a  includes a WDM  201  and a WDM  203  with a section of optical fiber  205  bi-directionally coupled therebetween. The first optical clock recovery circuit of loop  17   a  includes an optical coupler  207  coupled between WDM  203  and a WI)M  206 . WDM  206  is bi-directionally coupled to a controllable gain bi-directional line amplifier  207 , which in turn is coupled to a WDM  209 . WDM  209  is coupled back to WDM  201  through a polarization controller  211  and a variable optical delay line  213 . Data signal E D  is received from WDM  23  through a polarization controller  215  at WDM  201 . WDM  201  multiplexes data signal E D  with recovered clock signal E′ C  onto optical fiber  205 . WDM  203  separates data signal E D  from recovered clock signal E′ C . Data signal E D  is coupled from WDM  203  to a WDM  217  of optical gate  19   a  through a polarization controller  219 . Recovered clock signal E′ C  is coupled by optical coupler  207  to a WDM  221  of optical gate  19  through a variable optical delay line  223 . 
     The second optical clock recovery circuit of clock recovery loop  17   a  includes WDM  203 , optical fiber  205 , and WDM  201 , as well as WDM  209 , bi-directional line amplifier  208 , and WDM  206 . Additionally, the second optical clock recovery circuit includes an optical coupler  225  which couples recovered clock signal W′ C  from WDM  201  to WDM  209 . Recovered clock signal W′ C  is coupled from WDM  206  to WDM  203  through a polarization controller  227  and a variable optical delay line  229 . Data signal W D  is coupled to the second clock recovery circuit by WDM  203 . A polarization controller  231  is disposed in the optical path of data signal W D  between WDM  27  and WDM  203 . Data signal W D  is coupled from WDM  201  to a WDM  233  of optical gate  19   a  through a polarization controller  235 . Recovered clock signal W′ C  is coupled from WDM  201  to WDM  221  of optical gate  19   a  through optical coupler  225  and a variable optical delay line  237 . Thus, in the embodiment of FIG. 3, the first and second optical clock recovery circuits share a common optical fiber  205  and a common bi-directional line amplifier  208 . 
     In the optical gate of the embodiment of FIG. 3, a controllable gain bi-directional line amplifier  239  is coupled between WDM  217  and WDM  233 . Data signal W D  is received at WDM  233  and amplified by bi-directional line amplifier  239 . Similarly, data signal E D  is received at WDM  217  and amplified by bi-directional line amplifier  239 . Data signal E D  is coupled from WDM  233  to a WDM  241  of an NOLM  243 . Data signal W D  is coupled from WDM  217  to a WDM  245  of NOLM  243 . An optical fiber  247  is coupled between WDM  241  and  245 . Recovered clock signals W′ C  and E′ C  are multiplexed into NOLM  243  by WDM  221 . The multiplexed signals W′ C  and E′ C  are split by an optical coupler  249 , and each of signals W′ C  and E′ C  travel in both directions around NOLM  243 . The line carrying signals W′ C  and E′ C  in the clockwise direction is split by an optical coupler  250 , and the line carrying signals W′ C  and E′ C  in the counterclockwise direction is split by an optical coupler  252 . Data signal E D  and recovered clock signal E′ C  are multiplexed by WDM  241  onto optical fiber  247  to produce regenerated signal {overscore (E′ C +L )}. Similarly, data signal W D  and recovered clock signal W′ C  are multiplexed onto optical fiber  247  to produce regenerated signal {overscore (W′ C +L )}. Regenerated signal {overscore (W′ C +L )} is separated from data signal W D  at, WDM  241  and regenerated signal {overscore (E′ C +L )} is separated from data signal E D  by WDM  245 . Regenerated signals {overscore (E′ C +L )} and {overscore (W′ C +L )} are separated from each other at WDM  221 . Regenerated signal {overscore (W′ C +L )} is coupled back to optical fiber  13  by WDM  23 . Similarly, regenerated signal {overscore (E′ C +L )} is coupled back to fiber  13  by WDM  27 . 
     From the foregoing, it may be seen that the present invention provides a bi-directional all-optical regenerator that is economical in terms of both cost and facilities rack space. The clock recovery circuits and the and the NOLM or NOLMs share common optical circuit elements. By causing signals to travel in opposite directions through the common optical circuit elements, the total number of elements is reduced without affecting performance of the regenerator. 
     The present invention has been described and illustrated with reference to preferred embodiments. Those skilled in the art will recognize that features and subcombinations of elements described and illustrated may be used independently of or in combination with other elements and subcombinations. For example, optical gate  19   a  may be used in combination with clock recovery loop  17 , or optical gate  19  may be used in combination with clock recovery loop  17   a , all as would be apparent to one skilled in the art, given the benefit of the foregoing disclosure. Accordingly, the spirit and scope of the present invention is set forth in the appended claims.