Abstract:
A method and apparatus for providing optical 3R regeneration involving: 1) generating an encoded optical clock signal from at least an optical signal; 2) introducing the encoded clock signal into a delay interference section of a regenerator such that an amplitude modulated clock signal is produced; andoutputting the amplitude modulated clock signal wherein the output amplitude modulated clock signal preserves information present within the input optical signal.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to the field of telecommunications and in particular to a method and apparatus for providing wavelength conversion and 3R (re-amplification, re-timing, and re-shaping) optical signal regeneration of optical communications signals. 
   BACKGROUND OF THE INVENTION 
   To meet the ever-increasing capacity demand of future optical communications networks, a constant and acceptable optical signal quality must be maintained throughout such networks. In particular, methods and apparatus for providing 3R (re-amplification, re-timing, and re-shaping) optical signal regeneration are required due to dispersion, loss, crosstalk and other non-linearities associated with optical fiber, optical components and optical communications through same. 
   SUMMARY OF THE INVENTION 
   I have developed a method and apparatus (3R optical signal regenerator) that provides 3R optical signal regeneration. My method is particularly well suited to provide re-timing and re-shaping and if used in combination with an optical amplifier, it also provides re-amplification. My inventive method and apparatus involves: 1) generating an encoded optical clock signal from at least an optical signal; 2) introducing the encoded clock signal into a delay interference section of the regenerator such that an amplitude modulated clock signal is produced; andoutputting the amplitude modulated clock signal wherein the output amplitude modulated clock signal preserves information present within the input optical signal. 
   Unlike the prior-art, my inventive method and apparatus permits the development of exceedingly simple, low cost devices. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a schematic drawing illustrating the broad concept of 3R signal regeneration with simultaneous wavelength conversion according to my inventive teachings; 
       FIG. 2  is a schematic drawing illustrating a generalized 3R signal regenerator according to my inventive teachings; 
       FIG. 3  is a schematic drawing illustrating the operation of a 3R signal regenerator according to the present invention; 
     FIG.  4 ( a )-( f ) shows in schematic form, alternative forms of a 3R regenerator according to the present invention; 
     FIG.  5 ( a )-( e ) shows in schematic form, alternative forms of a delay interference section according to the present invention; and 
     FIG.  6 ( a )-( b ) depicts a simplified embodiment of my inventive 3R regenerator. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , there is shown in schematic form a diagram depicting the broad concept of 3R optical signal regeneration with simultaneous wavelength conversion. Specifically, an input signal (P in )  110  is mapped onto a clock signal (P in )  120  through the action of 3R regenerator  130  such that regenerated signal (P reg )  140  is produced. As can be appreciated, such a concept comprises: reshaping—wherein the regenerated signal  140  exhibits the general shape of the clock signal  120 ; retiming—wherein the regenerated signal  140  exhibits the general timing of the clock signal  120 ; and alternatively, reamplification—through the action of amplifiers and by selectively choosing clock signal power. Wherein simultaneous wavelength conversion is accomplished, the clock signal  120  exhibits a new, desired wavelength. 
   Turning our attention now to  FIG. 2 , there is shown a generalized 3R signal regenerator  200  according to the present invention. In particular, the 3R signal regenerator comprises a modulation section  210 , a coupling section  220 , and a delay-interference section section  230 . Accordingly, an input signal (P in )  240  in applied to the coupling section  220  and a clock signal (P clk )  250  is applied to the modulation section  210  and then provided to the delay interference section  230 , the output of which is a regenerated signal (P reg )  260  which exhibits the desired characteristics, and in particular, retime, and reshape. Additionally, and as mentioned previously, the regenerated signal  260  may also optionally exhibit a wavelength conversion. 
   In one embodiment, the modulation section  210  comprises a medium or several different media, that permit transmission of a clock signal (P clk )  250  and whose refractive index can be modulated with the input signal (P in )  240 . Of course, the refractive index may be modulated by means of a nolinear effect (i.e., the refractive index changes in the presence of a strong light signal) by means of a voltage change, a temperature change, or current injection. 
   As can be appreciated, materials which may be exploited within the modulation section  210  are, e.g., semiconductor optical amplifiers (SOA) comprising, e.g., InGaAsP, InGaAlAs, Ga,As, etc, absorbers, optical fibers, glasses, chalcogenite glasses, semiconductor, plastic based waveguiding materials, liquids or gases. 
   One implementation based upon an SOA (not explicitly shown in  FIG. 2 ) may operate as follows. An input signal (P in )  240  modulates the refractive index of the medium within the SOA by exploiting a refractive-index-related carrier depletion effect. Alternatively, the input signal (P in )  240  could be guided into a photodiode resulting in the production of a photocurrent. This photocurrent is then injected into the SOA or is used to vary a voltage over the SOA which in turn changes a carrier density in the SOA resulting in a modulation of the refractive index. In yet another implementation, a Kerr-effect could be exploited to modulate the refractive index in a fiber (not shown) upon the application of an input signal (P in )  240 . As should be apparent to the reader, many other implementations are easily envisioned. 
   With continued reference now to  FIG. 2 , the coupling section  220  directly introduces the input signal (P in )  240  into the modulation section  210  or, alternatively, introduces and modifies the input signal (P in )  240  such that it may utilized to effect a change in the refractive index of the modulation section  210 . In one simple embodiment, the coupling section  220  may comprise an optical coupler, that couples the input signal (P in )  240  into a signal path of the clock signal (P clk )  250 , which subsequently modulates the refractive index of a generally, non-linear material. Such a simple coupler could be placed, for example, before the modulation section  210  or between the modulation section  210  and the delay interference section  230 . In another embodiment, the coupling section  220  may comprise an optical circulator positioned between the modulation section  210  and the delay interference section  230 . In yet another embodiment, the coupling section  220  may comprise an optical photodiode, which translates the optical input signal (P in )  240  into a signal current. This resulting signal current is then introduced into the modulation section  210  where it effects a change in the refractive index. 
   Continuing with this discussion of  FIG. 2 , the delay interference section  230  comprises an optical splitter (not shown) an optical combiner (not shown) and multiple optical paths therebetween (not shown). Advantageously, the splitter and the combiner could be the same device. In operation, the delay interference section  230  is used to split the clock signal (P clk )  250  into two signals which then propagate for different times along an optical interference path until they are subsequently recombined through the action of the combiner. The combiner, then introduces interference (either constructive or destructive) into the output, (P conv )  260  depending upon the relative phase relations between the two, split signals. It may be advantageous, if there is positioned in the optical interference path, a phase-shifter and/or a gain/absorbing section. The time delay Δt of the two split signals which results from one of the split signals traversing the optical interference path, is approximately represented by Δt=N•Δt clk , where N=1,2,3 . . . , and where the introduced •Δt clk  is substantially the time delay between subsequent clock signal pulses. 
   Turning our attention now to  FIG. 3 , there is shown in schematic form a device illustrating the operation of the present invention. In particular,  FIG. 3  shows a 3R signal regenerator  300  which accepts as input, input signal P in    310  at λ 1  and clock signal P clk    320  at λ 2  and subsequently outputs regenerated signal P conv    330  at λ 2 . In operation, the input signal P in    310  modulates directly or indirectly by one of the aforementioned processes the refractive index of a modulation section (not explicitly shown in  FIG. 3 ) and thereby a phase Φ cw  (not shown) of the clock signal P clk    320 . The intensity of the refractive index change is adapted such that it modulates the phase of the clock signal P clk    320  by an amount approximately larger than 0 but not much larger than +πor −π. The rise time of the induced phase shift depends upon the underlying process that is used. Normally, it is almost an instantaneous process, which is limited by the pulse width of the P in    310  signal, whereas the decay of the induced phase is typically slower. 
   Subsequent to the modulation section, the P clk    320  signal is guided into a delay interference section where it is split and subsequently recombined through the action of splitter  350  and combiner  370 , respectively. The split signals traverse at least two separate paths or “arms”, one including a delay loop  360  and another including a phase-shifter  365 . The phase shifter could be on the other arm as well. A gain or absorbing section could be placed on one or both of the arms. 
   The clock signal traversing the shorter interferometer path exhibiting the phase shift first reaches the coupler and “opens a switching window” in the non-bit inverting operation mode respectively and closes the switching window in the bit inverted operation mode on the output port  380   b . At a time Δt later when the clock signal traversing the longer interferometer path, the phase difference is reset and the switching window in the output port  380   b  closes opens, respectively. If a second input data pulse follows, the switching window is reopened by setting the phase of the clock in the shorter interferometer path again or remains closed respectively. In order to obtain substantial constructive and destructive interference of subsequent clock pulses into the output, it is necessary to choose the delay time Δt such that it is substantially the same as the time delay of subsequent clock pulses Δt clk . 
   As can be appreciated, there are a variety of ways in which a coupling unit such as that shown in  FIG. 2  as reference  220 , could be implemented. With reference now to FIG.  4 ( a )-( f ) there is shown such variety of implementation. FIG.  4 ( a ) shows a simple combiner (coupler)  410  positioned before a modulation section  420  and delay interference section  430 . It permits coupling of both an input clock signal and input signal into a modulating section in a co-propagating manner. 
   A filter could be placed between the modulation section and the delay interference section to extract the input-signal, since the input-signal is not needed in the delay interference section. This filter could be a tunable wavelength filter or a special coupler, etc. FIG.  4 ( b ) depicts a coupler  410  positioned between the modulation section  420  and delay interference section  430 . An input signal is introduced in a counterpropagating manner. Advantageously, no filter is required to separate the clock from the control signal. Additionally, optical isolators may be utilized at an input side if desired. 
   FIG.  4 ( c ) includes a circulator  440 , positioned between the modulation section  420  and the delay interference section  430 . The circulator  440  introduces an input signal into the modulation section  420 . The circulator  440  maps a clock signal into the delay interference section  430  without distortions. 
   FIG.  4 ( d ) includes both an optical coupler  410  and a circulator  440 . A clock signal and an input signal are introduced via the optical coupler  410 . This configuration assumes that a reflection from the delay interference section  430  is extracted via the circulator  440  which includes a regenerated pulse. 
   FIG.  4 ( e ) shows my inventive 3R regenerator having a grating  420 ( a ) which introduces an input signal into the modulation section  420 . The grating  420 ( a ) may be placed in a fiber or waveguide in front of, or after the modulation section  420 . 
   Finally, FIG.  4 ( f ) includes a photo diode  450  which may be used to detect an input signal and effect a modulation of current (or voltage) of a nonlinear medium. Of course, many other types and variations of these are possible and readily apparent to one skilled in the art. 
   FIG.  5 ( a )-( e ) shows variations to a delay interference section of my inventive 3R regenerator. As can be readily appreciated, interferometric delay schemes utilize light paths of different length or, alternatively, have different propagation speed for different parts of a light signal. They contain, for example, light splitters and combiners in the form of optical couplers, gratings, mirrors, polarization splitters, higher order mode couplers. The coupler may have, for example, symmetric or asymmetric splitting ratios or even be tunable. 
   With reference now to FIG.  5 ( a ), an interference section is formed by two couplers and two interference arms of different lengths. FIGS.  5 ( b ) and  5 ( c ) are similar,  5 ( b ) shows an apparatus having 2 different length fibers. In FIG.  5 ( c ), a coupler is followed by two light guiding means. The guiding means are terminated at a reflecting surface such that light is reflected back to the coupler. The lengths of the two light paths is different. In FIG.  5 ( d ), an interferomenter is shown, where one part of the light is reflected back at probability R 1  and the remaining part of the light is reflected back with a probability R 2 , a little later. Subsequently, the two backward reflected parts interfere into the output. Finally, in FIG.  5 ( e ), the clock signal is introduced with a certain polarization state into a birefringement crystal or other material. The two signals having different polarization exit the material with a certain delay relative to one another. They are subsequently rotated into the same polarization state and combined, so that they can constructively or destructively interfere. 
   Finally, FIG.  6 ( a ) shows my inventive 3R regenerator implemented with fiber. Specifically, the regenerator  600  includes a modulation section  610 , an input signal coupling unit and a delay interference section  630 , wherein this delay interference section  630  has a birefringent fiber, a phase shifter  634  and a polarizer,  636 . The birefringent fiber  632  could be a polarization mode dispersion fiber that exhibits different group velocity for light of different polarization. The length of the fiber determines the delay between different modes. As can be seen in  FIG. 6 , at an end of the fiber distal to the modulation section  610 , is located a phase shifting element  634 . This phase shifter could be a half-wave plate, a polarization controller or another suitable element. Further distal, there is located a polarizer, that is positioned such that approximately one-half of the light which traverses the fiber is coupled into an output. 
   In alternative implementations, additional phase shifters, wave-plates or polarizers may be inserted between the modulation section and the delay section thereby assuring more stable operation. Generally, it is recommended to temperature control the whole unit or part of it to ensure stable operation. An input signal may be coupled into a SOA in counter-propagating operation. An example of such a configuration is shown in FIG.  6 ( b ). 
   Various additional modifications of this invention will occur to those skilled in the art. Nevertheless, all deviations from the specific teachings of this specification that basically rely upon the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.