Patent Publication Number: US-2023144555-A1

Title: Apparatus for broadband wavelength conversion of dual-polarization phase-encoded signal

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present U.S. non-provisional patent application is a continuation application and claims the benefit of copending U.S. patent application Ser. No. 17/554,113, filed on Dec. 17, 2021, which is a continuation application and claims the benefit of U.S. patent application Ser. No. 17/145,924, filed on Jan. 11, 2021, which claims the benefit of and priority to U.S. Provisional Patent Application No. 63/113,349, filed Nov. 13, 2020, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates to the conversion of electromagnetic fields from one wavelength (frequency) to another wavelength (frequency). In particular, the present invention relates to an apparatus and method for wavelength (frequency) conversion using a closed-loop design for back-and-forth conversion (i.e., both up-conversion and down-conversion) of an electromagnetic wave for broadband applications without the need for broadband components. 
     BACKGROUND OF THE INVENTION 
     Wavelength conversion of an electromagnetic wave that is used to carry information is often desired to take advantage of available bandwidth, spectrum and in some cases minimize wavelength collision or contention in optical networks. In this particular application, “Up-conversion” refers to a process by which the frequency of a signal is shifted up (i.e., increased). In view of the inverse relationship between the frequency of an electromagnetic wave signal and its wavelength, the wavelength of an up-converted signal is decreased. “Down-conversion” refers to a process by which the frequency of a signal is shifted down (i.e., decreased). In view of the inverse relationship between the frequency of an electromagnetic wave signal and its wavelength, the wavelength of a down-converted signal is increased. Throughout this disclosure, wavelength conversion and frequency conversion will be used interchangeably. 
     The wavelength conversion of an electromagnetic field carrying information and with different attributes (polarization, spatial modes, phase and/or amplitude modulation, etc.) is a cumbersome and tedious process. As waves are converted from one frequency to another, signal integrity is of paramount interest. Of particular importance are attributes such as polarization crosstalk, polarization group delay, timing delay, phase delay, frequency jitter, etc. 
     For example, in phase-encoded dual-polarization systems, a small amount of polarization crosstalk can lead to severe signal degradation leading to an increased bit-error-rate. As a result, wavelength conversion of a dual-polarization signal requires a polarization-independent wavelength conversion device with minimal to zero polarization crosstalk. In the absence of such devices, a wavelength conversion scheme that separates a certain attribute (such as polarization), performs the conversion, and accurately recombines the separated attributes is needed. 
     In the case of a dual-polarization signal, for example, independent wavelength conversion is required for each polarization component of the signal. One solution is to separately convert the wavelength of each polarization component of the signal and then to combine the separately converted polarization components. In this approach, which is referred to as the “polarization diversity” scheme, typically each wavelength conversion is done using a separate yet similar non-linear medium, e.g., a periodically poled lithium niobate (“PPLN”) crystal broadly referred to as one example of a wavelength conversion component. Also, other methods could be employed to perform wavelength conversion. As shown in  FIG.  1   , in a polarization diversity scheme  100 , the polarization components  106  and  104  of a dual-polarization signal  102  having a wavelength λ 1  are separated by a polarization demultiplexer (e.g. polarizing beam splitter)  108  and each polarization component  104 ,  106  is converted to a different wavelength λ 2  by a respective wavelength conversion component such as a PPLN  110 ,  112 . The wavelength-converted polarization components  114 ,  116  are then combined by beam polarization combiner  118  into a dual-polarization optical signal  120  which has a wavelength λ 2 . 
     A similar process is used to convert the dual-polarization optical signal  120  from its wavelength λ 2  back to a dual-polarization optical signal having a wavelength λ 1 . The polarization components  122  and  124  of the dual-polarization optical signal  120  are separated by a polarization beam splitter  126  and each polarization component  122 ,  124  is converted back to wavelength λ 1  by a respective wavelength conversion component(s)  128 ,  130 . The wavelength-converted polarization components  132 ,  134  are then combined by beam polarization combiner  136  into a dual-polarization optical signal  138  which has a wavelength λ 1 . 
     One main drawback of the polarization diversity scheme is that each polarization component experiences a different optical path with different optical components (i.e., different wavelength conversion component(s)). Hence, the optical path lengths for the two polarization components are unequal, which makes it necessary to compensate for a relative delay between the two polarization components. The different optical path lengths also cause the polarization diversity approach to be prone to distortion and instability caused by environmental factors. The polarization diversity approach further requires four wavelength conversion component(s). 
     A well-known solution for such problems is to use a counter-propagating (also referred to as bi-correctional) scheme, which could be free-space or in a waveguide. In the counter-propagating scheme, both polarization components propagate within the same optical path and in different directions and are converted inside the same wavelength converting component(s). Accordingly, the benefit of the counter-propagating scheme is that it provides a similar path length with shared components (i.e., the same waveguides, fibers, nonlinear crystals) for both polarization components of the electromagnetic field, which results in a minimal delay between the two polarization components. 
     The main drawback of the counter-propagating scheme as shown in  FIG.  2    is the need for broadband components if the two wavelengths of λ 1  and λ 2  are largely separated. In order to perform a conversion to a new wavelength and back to the original wavelength, it is possible to use a cascade of two counter-propagating design. However, as mentioned earlier this scheme will not work unless the MUX and Demuxer components, optical waveguides, etc. within the loop operate at both wavelengths.  FIG.  2    shows an implementation of a counter-propagating scheme  200  in which two consecutive conversions provide back-and-forth conversion from λ 1  to λ 2  and back to λ 1 . As shown in  FIG.  2   , in each conversion both polarization components of a dual-polarization electromagnetic wave counter-propagate within the same loop and are wavelength converted inside the same wavelength conversion component. A first loop  202  and a first wavelength conversion component(s)  204  are responsible for wavelength conversion from λ 1  to λ 2 . The polarization components  206  and  208  of a dual-polarization optical signal  210  having a wavelength λ 1  are separated by a polarization de-multiplexer (e.g. polarizing beam splitter)  212  and each polarization component  206 ,  208  is converted to a different wavelength λ 2  by wavelength conversion component(s)  204 . Polarization component  206  travels in a clockwise direction around first loop  202  and through wavelength conversion component  204 , which converts polarization component  206  from wavelength λ 1  to wavelength λ 2 . Similarly, polarization component  208  travels in a counter-clockwise direction around first loop  202  and through wavelength conversion component  204 , which wavelength-converts polarization component  208  from wavelength λ 1  to wavelength λ 2 . The wavelength-converted polarization components  214 ,  216  are then combined by polarization multiplexer (e.g. polarization beam splitter/combiner)  212  into a dual-polarization optical signal  218  which has a wavelength λ 2 . 
     Second loop  220  and second wavelength conversion component  222  are responsible for wavelength conversion from λ 2  to λ 1 . The polarization components  224  and  226  of a dual-polarization optical signal  218  having a wavelength λ 2  are separated by a polarization de-multiplexer (e.g. polarizing beam splitter/combiner)  228  and each polarization component  224 ,  226  is converted back to wavelength λ 1  by wavelength conversion component  222 . Polarization component  224  travels in a clockwise direction around second loop  220  and through wavelength conversion component  222 , which wavelength-converts polarization component  224  from wavelength λ 2  to wavelength λ 1 . Similarly, polarization component  226  travels in a counter-clockwise direction around second loop  220  and through wavelength conversion component  222 , which wavelength-converts polarization component  226  from wavelength λ 2  to wavelength λ 1 . The wavelength-converted polarization components  230 ,  232  are then combined by polarization multiplexer (e.g. polarization beam splitter/combiner)  228  into a dual-polarization optical signal  234  which has a wavelength λ 1 . 
     In the consecutive counter-propagating scheme shown in  FIG.  2   , each one of wavelength conversion component  204 ,  222  provides only conversion from a wavelength λ 1  to a wavelength λ 2  (wavelength conversion component  204 ) or conversion from a wavelength λ 2  to a wavelength λ 1  (wavelength conversion component  222 ) of the polarization components that pass through it, regardless of the direction in which the polarization component passes through the wavelength conversion component. As described in connection with  FIG.  2   , and as shown in  FIG.  3   , when polarization components  206 ,  208  pass through wavelength conversion component  204  in either a clockwise or a counter-clockwise direction after leaving polarization multiplexer/de-multiplexer (e.g. polarization beam splitter/combiner)  212 , wavelength conversion component  204  converts both polarization components  206 ,  208  from a wavelength λ 1  to a wavelength λ 2 . Likewise, when polarization components  224 ,  226  pass through wavelength conversion component  222  in either a clockwise or a counter-clockwise direction after leaving polarization multiplexer/de-multiplexer (e.g. polarization beam splitter/combiner)  228 , wavelength conversion component  222  converts both polarization components  224 ,  226  from a wavelength λ 2  (e.g., 633 nm) to a wavelength λ 1  (e.g., 1560 nm). It should be noted that both the clockwise and the counterclockwise signals  206  and  208  travel inside the wavelength conversion component  204  with the same polarization and only in the opposite direction. The wavelength converting component  204  works efficiently only for one polarization component. As mentioned earlier, if a dual-polarization wavelength converting component exists with minimum or no polarization crosstalk, then signal could simply be sent through it for up- or down-conversion. 
     The counter-propagating scheme  200  shown in  FIG.  2    provides the benefit of minimum delay since each polarization component travels within the same loop (i.e., experiences similar optical path) and is converted by the same wavelength conversion component within that loop. The counter-propagating scheme also uses only two wavelength conversion components, as compared to the four wavelength conversion components required in the polarization diversity scheme shown in  FIG.  1   . Thus, the counter-propagating scheme can reduce the complexity and cost of providing wavelength conversion as compared to the polarization diversity scheme. 
     The scheme described in  FIG.  2    shows a method to perform the wavelength conversion from wavelength λ 1  to a wavelength λ 2  and conversion back to wavelength λ 1 . However to the best of the knowledge of the inventors, all of the reported research and literature using the counter-propagating scheme has been done with two wavelengths λ 1  and λ 2  very closely located to each other within a narrow spectral bandwidth usually within the C-band or the L-band. Furthermore, the commercially available optical components, such as polarization beam splitters, circulators, wavelength-division multiplexers, and polarization-maintaining fibers, to name a few, usually have a limited operating band where they can easily handle both wavelengths. For a broadband application, optical components cannot handle both wavelengths, and optical fibers to operate single-mode over a wide spectrum (such as broadband photonic crystal fibers (“PCF”)) are expensive and difficult to work with. Vendors currently have difficulty connecting such fibers to PPLN crystals. A brute-force solution would be to use free-space components and dichroic mirrors to resolve the issue, but this solution would add to the complexity and cost. 
     There is, therefore, a need for a wavelength conversion apparatus and method that overcomes one or more of the above and other deficiencies, simplifies the design, and reduces total cost and complexity. 
     SUMMARY OF THE INVENTION 
     It has now been found that the above and related objects of the present invention are obtained in the form of several related aspects, including an apparatus for wavelength conversion which extends bandwidth to other wavelength bands by wavelength conversion but does not require broadband components. The apparatus in accordance with embodiments of the present invention provides back-and-forth wavelength-conversion (i.e., both up-conversion and down-conversion) of an electromagnetic wave signal between wavelength λ 1  and wavelength λ 2  within the same wavelength-conversion loop. It is herein disclosed that some embodiments provide a single, one-way conversion in either an up or down direction without the need to return to the original wavelength. 
     More particularly, the present invention relates to a wavelength converter. The wavelength converter includes an input for receiving a signal having a first wavelength and a wavelength-conversion loop capable of converting the first wavelength of the signal to a second wavelength and converting the second wavelength to the first wavelength. The wavelength-conversion loop includes a first wavelength-conversion medium and a second wavelength-conversion medium. 
     In at least one embodiment, the wavelength-conversion loop is capable of up-converting the first wavelength to the second wavelength and down-converting the second wavelength to the first wavelength. 
     In at least one embodiment, the wavelength-conversion loop is capable of down-converting the first wavelength to the second wavelength and up-converting the second wavelength to the first wavelength. 
     In at least one embodiment, the first wavelength-conversion medium can convert the first wavelength to the second wavelength and can convert the second wavelength to the first wavelength. 
     In at least one embodiment, a single conversion is performed from the first wavelength to the second wavelength. 
     In at least one embodiment, a single conversion is performed from the second wavelength to the first wavelength. 
     In at least one embodiment, the first wavelength-conversion medium comprises a non-linear medium. 
     In at least one embodiment, the first wavelength-conversion medium comprises periodically poled lithium niobate. 
     In at least one embodiment, the second wavelength-conversion medium can convert the first wavelength to the second wavelength and can convert the second wavelength to the first wavelength. 
     In at least one embodiment, wherein the second wavelength-conversion medium comprises a non-linear medium. 
     In at least one embodiment, the second wavelength-conversion medium comprises periodically poled lithium niobate. 
     In at least one embodiment, the signal comprises a first polarization component and a second polarization component, wherein both the first polarization component and the second polarization component have the first wavelength. The first wavelength-conversion medium converts the first wavelength of the first polarization component to the second wavelength. The second wavelength-conversion medium converts the first wavelength of the second polarization component to the second wavelength. 
     In at least one embodiment, the waveform-conversion loop up-converts the first wavelength to the second wavelength which is spectrally apart from the first one without using broadband components. 
     The present invention also relates to an all-electromagnetic wavelength-division multiplexing transponder. The wavelength-division multiplexing transponder includes a first port; a second port; and a wavelength-conversion medium coupled to the first port and the second port. A first signal received at the first port and having a first wavelength is converted to a second wavelength by the wavelength-conversion medium and is provided to the second port; and a second signal received at the second port and having the second wavelength is converted to the first wavelength by the wavelength-conversion medium and is provided to the first port. 
     In at least one embodiment, the wavelength-conversion medium converts the first wavelength to the second wavelength based on a direction that the first signal travels through the wavelength-conversion medium, and the wavelength-conversion medium converts the second wavelength to the first wavelength based on a direction that the second signal travels through the wavelength-conversion medium. 
     In at least one embodiment, the wavelength-conversion medium comprises a non-linear medium. 
     In at least one embodiment, the wavelength-conversion medium comprises periodically poled lithium niobate. 
     Although specific features, capabilities, and advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated features, capabilities, and advantages. These and other technical features, capabilities, and advantages of the disclosed subject matter, along with the invention itself, will be more fully understood after a review of the following figures, detailed descriptions, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described with reference to the accompanying figures, wherein: 
         FIG.  1    shows a conventional wavelength conversion scheme. 
         FIG.  2    shows another conventional wavelength conversion scheme. 
         FIG.  3    illustrates an aspect of the operation of the conventional wavelength conversion scheme shown in  FIG.  2   . 
         FIG.  4    shows a wavelength conversion scheme in accordance with the present invention. 
         FIG.  5    illustrates an aspect of the wavelength conversion scheme shown in  FIG.  4   . 
         FIG.  6    shows a schematic diagram of an embodiment of a wavelength conversion scheme in accordance with the present invention. 
         FIG.  6 A  illustrates an aspect of the wavelength conversion scheme shown in  FIG.  6   . 
         FIG.  6 B  shows a modification of the embodiment of the wavelength conversion scheme shown in  FIG.  6   . 
         FIG.  7    illustrates the operation of a conventional wavelength-division multiplexing transponder. 
         FIG.  8    shows a schematic diagram of an embodiment of a wavelength-division multiplexing transponder in accordance with the present invention. 
         FIG.  8 A  illustrates an aspect of the operation of the wavelength-division multiplexing transponder shown in  FIG.  8   . 
         FIG.  9    shows output signals provided by the embodiment of the wavelength conversion scheme shown in  FIG.  6   . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG.  4    shows a single-loop counter-propagating wavelength conversion scheme  400  in accordance with embodiments of the present invention. Wavelength conversion is done using a wavelength converting component(s), e.g. a non-linear medium. As is well known to those skilled in the art, mixing a laser input signal and a pump laser signal in a non-linear medium translates the wavelength of the laser input signal to a different wavelength due to sum- and difference-frequency generation. As shown in  FIG.  4   , wavelength conversion scheme  400  uses two such wavelength converting components  414 ,  420 , which makes it capable of converting dual-polarization signals. In preferred embodiments, one or both of wavelength converting components  414 ,  420  may be a periodically poled lithium niobate (“PPLN”) waveguide. 
     An input signal  402  having a wavelength λ 1  is provided to a wavelength conversion loop  404 . A polarization multiplexer/de-multiplexer (e.g. polarizing beam splitter/combiner)  408  separates the input signal  402  into a first polarization component  410  and a second polarization component  412 . The first polarization component  410  travels clockwise around wavelength conversion loop  404  and is provided to wavelength conversion component (such as a PPLN)  414 . Wavelength conversion component  414  converts the wavelength of the first polarization component  410  from λ 1  to λ 2  and provides a converted first polarization component  416  to a processing unit  418 . Similarly, the second polarization component  412  travels counterclockwise around loop  404  and is provided to wavelength conversion component  420 . Wavelength conversion component  420  converts the wavelength of the second polarization component  412  from λ 1  to λ 2  and provides a converted second polarization component  422  to processing unit  418 . 
     The converted first and second polarization components  416 ,  422  are combined in processing unit  418  into a signal having a wavelength λ 2 . This signal is then available to be extracted from processing unit  418  for use outside of wavelength conversion loop  404 . 
     Wavelength conversion loop  404  can also be used to convert a signal having a wavelength λ 2  from wavelength λ 2  back to wavelength λ 1 , the wavelength of input signal  402 . The processing unit  418  separates the signal back into converted first and second polarization components  416 ,  422 . The processing unit provides the converted first polarization component  416  to wavelength conversion component  420 , which converts the wavelength of converted first polarization component  416  from λ 2  back to λ 1 , thereby restoring the original first polarization component  410  traveling in the clockwise direction from wavelength conversion component  420  to polarization multiplexer/de-multiplexer (e.g., polarizing beam splitter/combiner)  408 . Similarly, converted second polarization component  422  is then provided to wavelength converting component  414 , which converts the wavelength of converted second polarization component  422  from λ 2  back to λ 1 , thereby restoring the original second polarization component  412  traveling in the counter-clockwise direction. Both first and second polarization components  410 ,  412  then complete their respective trips around wavelength conversion loop  404  and are re-combined in polarization multiplexer/de-multiplexer (e.g., polarizing beam splitter/combiner)  408  to restore the original input signal  402 , with its wavelength λ 1 . 
     Unlike the conventional, consecutive counter-propagating scheme shown in  FIG.  2   , in the single-loop scheme shown in  FIG.  4   , each one of wavelength conversion components  414 ,  420  provides both up-conversion and down-conversion of the polarization components that pass through it, depending upon the direction in which the polarization component passes through the wavelength conversion component. As described in connection with  FIG.  4   , and as shown in FIG.  5 , when converted first polarization component  416  passes through wavelength conversion component  420  in a clockwise direction after leaving the processing unit  418 , wavelength conversion component  420  wavelength-converts first polarization component  416  from a wavelength λ 2  to a wavelength λ 1 . In contrast, when second polarization component  412  passes through wavelength conversion component  420  in a counter-clockwise direction after leaving polarization multiplexer/de-multiplexer (e.g., polarization beam splitter/combiner)  408 , wavelength conversion component  420  wavelength-converts second polarization component  412  from a wavelength λ 1  to a wavelength λ 2 . 
     Similarly, when first polarization component  410  passes through wavelength conversion component  414  in a clockwise direction after leaving polarization multiplexer/de-multiplexer  408 , wavelength conversion component  414  wavelength-converts first polarization component  410  from a wavelength λ 1  to a wavelength λ 2 . In contrast, when converted second polarization component  422  passes through wavelength conversion component  414  in a counter-clockwise direction after leaving the processing unit  418 , wavelength conversion component  414  wavelength-converts second polarization component  422  from a wavelength λ 2  (e.g., 1560 nm) to a wavelength λ 1  (e.g., 633 nm). 
       FIG.  6    shows a single-loop counter-propagating wavelength conversion scheme  600  for conversion of an optical input signal in accordance with embodiments of the present invention. An optical input signal  602  having a wavelength λ 1  is provided to a wavelength conversion loop  604  through a circulator  606 . A polarization multiplexer/demultiplexer  608  separates the input signal  602  into a first polarization component  610  and a second polarization component  612 . The first polarization component  610  travels clockwise around wavelength conversion loop  604  and is provided to wavelength conversion component  614  through an optional polarization controller  616  and a wavelength multiplexer (e.g. WDM, dichroic mirror)  618 . Polarization controller  616  can be used to eliminate possible crosstalk between the first and second polarization components  610 ,  612 . The wavelength conversion in wavelength conversion component  614  is based on the sum- and difference-frequency generation in wavelength conversion component (e.g. a nonlinear crystal)  614  using a pump LASER (if needed)  620 . The pump LASER  620  is provided to wavelength conversion component  614  through a beam splitter  622  and wavelength-multiplexer  618 . Wavelength conversion component  614  converts the wavelength of the first polarization component  610  from λ 1  to λ 2  and provides a converted first polarization component  624  to a polarization multiplexer/de-multiplexer  626  through wavelength multiplexer/de-multiplexer  628 . 
     Similarly, the second polarization component  612  travels counter-clockwise around loop  604  and is provided to wavelength conversion component  630  through a wavelength-multiplexer/demultiplexer  638 . The wavelength conversion in wavelength conversion component  630  is based on the sum- and difference-frequency generation in wavelength conversion component  630  using pump laser  620 , which is provided to wavelength conversion component  630  through a beam splitter  622  and wavelength-multiplexer/de-multiplexer  638 . Wavelength conversion component  630  converts the wavelength of the second polarization component  612  from λ 1  to λ 2  and provides a converted second polarization component  632  to polarization multiplexer/de-multiplexer  626  through multiplexer/de-multiplexer  634 . 
     The converted first and second polarization components  624 ,  632  are combined in polarization multiplexer/de-multiplexer  626  into an output signal  636  having a wavelength λ 2 . 
     Wavelength converted signal  636  is then available to be extracted from polarization multiplexer/de-multiplexer  626  for use outside of wavelength conversion loop  604 . In an exemplary embodiment, the output signal  636  can be propagated in free space before being returned to wavelength conversion loop  604 . In another exemplary embodiment, the output signal can be provided to an angle multiplexing system. Systems and methods of angle multiplexing are described in U.S. Pat. No. 10,789,009 which is assigned to the assignee of the present application and is incorporated by reference herein in its entirety. 
     Wavelength conversion loop  604  can also be used to convert output signal  636  from wavelength λ 2  back to wavelength λ 1 , the wavelength of input signal  602 . When the wavelength-converted signal is returned to polarization multiplexer/de-multiplexer  626 , polarization multiplexer/demultiplexer  626  separates it back into converted first and second polarization components  624 ,  632 . Converted first polarization component  624  is then provided to wavelength conversion component  630  through wavelength multiplexer/de-multiplexer  634 , which converts the wavelength of converted first polarization component  624  from λ 2  back to λ 1 , thereby restoring original first polarization component  610 . This wavelength conversion in wavelength conversion component  630  could be based on the sum- and difference-frequency generation in a nonlinear crystal such as a PPLN crystal  630  using pump laser  620 , which is provided to wavelength conversion component  630  through a beam splitter  622  and wavelength conversion component  634 . 
     Similarly, converted second polarization component  632  is then provided to wavelength conversion component  614  through wavelength-division multiplexer  628 , which converts the wavelength of converted second polarization component  632  from λ 2  back to λ 1 , thereby restoring original second polarization component  612 . This wavelength conversion in wavelength conversion component  614  could be based on the sum- and difference-frequency generation in nonlinear crystal such as a PPLN crystal  614  using pump laser  620 , which is provided to wavelength conversion component  614  through a beam splitter  622  and wavelength-division multiplexer  628 . 
     Both first and second polarization components  610 ,  612  then complete their respective trips around wavelength conversion loop  604  and are re-combined in polarization multiplexer/de-multiplexer  608  to restore the original input signal  602 , with its wavelength λ 1 . The input signal  602  is then returned to circulator  606 , from which it is available to be extracted. 
     To aid in understanding the operation of wavelength conversion loop  604 ,  FIG.  6 A  shows the circulation of first polarization component  610  in a clockwise direction around wavelength conversion loop  604  using numbered arrows  1 - 14 . Numbered arrows  1 - 4  and  11 - 14  indicate the portions of wavelength conversion loop  604  where first polarization component  610  has a wavelength λ 1 . Numbered arrows  5 - 10  indicate the portions of wavelength conversion loop  604  where first polarization component  610  has a wavelength λ 2 . As shown in  FIG.  6 A , these portions are the “inside” portions of wavelength conversion loop  604  (i.e., between wavelength conversion components  614 ,  630 ). 
     It should be understood that reversing the directions of numbered arrows  3 - 12  in  FIG.  6 A  would show the circulation of second polarization component  612  in a counter-clockwise direction around wavelength conversion loop  604 . Numbered arrows  1 - 4  and  11 - 14  would indicate the portions of wavelength conversion loop  604  where second polarization component  612  has a wavelength λ 1 . Numbered arrows  5 - 10  would indicate the portions of wavelength conversion loop  604  where second polarization component  612  has a wavelength λ 2 . 
     Wavelength conversion scheme  600  can be implemented in free space or using optical waveguides, and optical fibers, including as appropriate single-mode fibers, multimode fibers, and polarization-maintaining fibers. The wavelength conversion scheme  600  can be implemented based on commercially available optical components. 
     In accordance with an embodiment of the present invention, the optical input signal  602  can have a wavelength λ 1  that is in the C band. In accordance with another embodiment of the present invention, the output signal  636  at polarization beam splitter  626  can have a different wavelength λ 2  that is also in the C band. In accordance with yet another embodiment of the present invention, the optical input signal  602  can have a wavelength λ 1  of 1560 nm. In accordance with still another embodiment of the present invention, the output signal  636  at polarization beam splitter  626  can have a wavelength λ 2  of ˜631 nm (satisfying the energy conservations). In accordance with another embodiment of the present invention, the pump laser  620  can have a wavelength of 1060 nm. Given the right selection of pump laser (if needed) and wavelength conversion component it is possible to generate a broad spectral range at the second wavelength λ 2  independent of the first wavelength λ 1 . None of the components are required to work at both λ 1  and λ 2 . In accordance with yet another embodiment of the present invention, the output signal  636  provided at polarization beam splitter  626  can be at a very broad wavelength range even though wavelength conversion scheme  600  is implemented without using broadband components. 
     In accordance with yet another embodiment of the present invention, the wavelength conversion scheme  600  shown in  FIG.  6    can be used to provide a single, one-way conversion in either an up or down direction without the need to return to the original wavelength. For example, in a communication system, in order to avoid wavelength collision or contention, a wavelength can be converted to another wavelength and continue propagating in the communication system without the need to reconvert back to the original wavelength. In accordance with still another embodiment of the present invention, the first and second wavelength conversion components  614 ,  630  of wavelength conversion scheme  600  are capable of converting the first wavelength to the second wavelength and are capable of converting the second wavelength to a third wavelength (which could be the first wavelength). 
     A well-known method to perform wavelength conversion is to use a nonlinear process. In this case, usually, a pump laser is needed, and the wavelength conversion medium could be a nonlinear crystal and any form of a nonlinear medium. In some of the non-linear process, the pump laser  620  is undepleted. In view of this fact, in accordance with another embodiment in accordance with the present invention, the wavelength conversion scheme  600  can employ a pump recycling which recirculates residual pump power from one nonlinear conversion medium to be used in another one (or in the same wavelength conversion component). As shown in  FIG.  6 B , in the wavelength conversion scheme  600 , pump recycling can be implemented by connecting wavelength-division multiplexers  628 ,  634  to one another via connection  640 . In alternative embodiments, pump recycling can be implemented using a highly reflective coating inside each wavelength conversion component  628 ,  634 . 
     The wavelength conversion schemes described herein have broad applicability, such as, for example, wavelength-converting transmitter/receivers, broadband constellation transparent detectors, ROADM (reconfigurable optical add/drop multiplexers), and optical wavelength-division multiplexing transponders. An optical wavelength-division multiplexing transponder is a wavelength converter that converts a received signal after receiving it and transmits the same signal in another wavelength. Such devices work for both wavelengths. The operation of a conventional optical-electronic-optical (OEO) WDM transponder  700  is shown in  FIG.  7   . The WDM transponder  700  receives an optical signal  702  having a wavelength λ 1  and electrically converts it to an optical signal  704  having a wavelength λ 2  The WDM transponder  700  then transmits the optical signal  704 . Similarly, the WDM transponder  700  can receive the optical signal  704  having a wavelength λ 2  and electrically convert it to the optical signal  702  having a wavelength λ 1  before transmitting the optical signal  702 . The main disadvantage of OEO WDM transponder  700  is the optical-to-electrical and electrical-to-optical conversions that are required. An all-optical WDM transponder would achieve the same results without the disadvantageous optical-to-electrical and electrical-to-optical conversions required by OEO WDM transponder  700 . 
     In another embodiment in accordance with the present invention, an all-optical WDM transponder is provided.  FIG.  8    shows an all-optical WDM transponder  800  which can operate at any given wavelength. An input signal  802  having a wavelength λ 1  is provided at the first port of the WDM transponder  800  and will be converted to the new wavelength λ 2  at a second port of the WDM transponder  800 . 
     An input signal  802  having a wavelength λ 1  is provided to a polarization beam splitter  806 , which separates the input signal  802  into a first polarization component  808  and a second polarization component  810 . The first polarization component  808  is provided to wavelength converting component  812  through wavelength-division multiplexer  814 . Pump laser  816  is also provided to wavelength converting component  812  through wavelength-division multiplexer  814 . Wavelength converting component  812  converts the wavelength of the first polarization component  808  from λ 1  to λ 2  and provides a converted first polarization component  818  to a polarization beam splitter  820  through wavelength-division multiplexer  822 . 
     Similarly, the second polarization component  810  is provided to wavelength converting component  812  through wavelength-division multiplexer  822 . Pump laser  824  is also provided to wavelength converting component  812  through wavelength-division multiplexer  822 . Wavelength converting component  812  converts the wavelength of the second polarization component  810  from λ 1  to λ 2  and provides a converted second polarization component  826  to polarization beam splitter  820  through wavelength-division multiplexer  814 . The converted first and second polarization components  818 ,  826  are combined in polarization beam splitter  820  into an output signal  804  having a wavelength λ 2 . Output signal  804  is then available for transmission. 
     The operation of WDM transponder  800  when an input signal having a wavelength λ 2  is provided at the second port of the WDM transponder  800  and is converted to the new wavelength λ 1  at the first port of the WDM transponder  800  will now be described. Referring to  FIG.  8 A , an input signal  830  having a wavelength λ 2  is provided to the polarization beam splitter  820 , which separates the input signal  830  into a first polarization component  834  and a second polarization component  836 . The first polarization component  834  is provided to wavelength converting component  812  through wavelength-division multiplexer  822 . Pump laser  824  is also provided (if required) to wavelength converting component  812  through wavelength-division multiplexer  822 . Wavelength converting component  812  converts the wavelength of the first polarization component  834  from λ 2  to λ 1  and provides a converted first polarization component  838  to polarization beam splitter  806  through wavelength-division multiplexer  814 . 
     Similarly, the second polarization component  836  is provided to wavelength converting component  812  through wavelength-division multiplexer  814 . Pump laser  816  is also provided (if required) to wavelength converting component  812  through wavelength-division multiplexer  814 . Wavelength converting component  812  converts the wavelength of the second polarization component  836  from λ 2  to λ 1  and provides a converted second polarization component  840  to polarization beam splitter  806  through wavelength-division multiplexer  822 . The converted first and second polarization components  834 ,  836  are combined in polarization beam splitter  806  into an output signal  844  having a wavelength Output signal  844  is then available for transmission. 
     As described above, a signal having a wavelength λ 2  coming from the second port of WDM transponder  800  will be converted to wavelength λ 1  in the first port of WDM transponder  800 . Thus, WDM transponder  800  works for both wavelengths as the input wavelength. If the input wavelength is λ 1 , then the output wavelength will be λ 2  and vice versa. WDM transponder  800  is format transparent (i.e., it does not matter what signal constellation (phase encoding) the input signals use), can work for both single and double-polarization signals, and can be implemented easily for a broadband application. One main advantage of WDM transponder  800  is its almost instantaneous response time, which makes it suitable for very high bit rate wavelength conversion applications. 
       FIG.  9    shows the experimental demonstration of the constellation pattern provided by wavelength conversion scheme  600  of  FIG.  6    for the consecutive up- and down-conversion of dual-polarization QPSK data. The 1560 nm dual-polarization QPSK data was up-converted to a wavelength of 633 nm and consecutively down-converted back to the wavelength of 1560 nm. No delay compensation or any amplitude balancing between the two polarizations of the QPSK data was needed. 
     The novel and inventive wavelength conversion scheme in accordance with embodiments of the present invention provide several benefits and advantages. The wavelength conversion scheme has a broadband application although it does not require broadband components. It works from continuous-wave to high bit-rate signals since it has an almost instantaneous response time. The wavelength conversion scheme is phase-insensitive, which makes it suitable for phase-encoded signals. In other words, any input signal (regardless of its properties such as polarization, phase, temporal shape, and bitrate) will be converted to another wavelength at the output while preserving the properties of the input signal enabling using more of the spectrum where commercial hardware is not available. 
     Since both wavelength conversions are performed in the same loop, the wavelength conversion scheme provides high environmental stability (e.g., reduces the effect of temperature fluctuations) and it reduces the effects of amplitude noise (i.e., distortion) of the converted signal. Since both polarization components travel only in different directions within the same loop, polarization mode dispersion (PMD) is minimized and there is no need to add a delay-line to compensate for PMD. Also, the counter-propagating scheme in the same loop automatically results in a balanced wavelength conversion for both polarizations and reduces the bit error rate penalty due to an imbalance between the two orthogonal polarizations. This resolves the need to balance the two polarizations&#39; amplitudes using a polarization-dependent optical attenuator. 
     The wavelength conversion scheme in accordance with the present invention reduces complexity and cost. The single-loop counter-propagating wavelength conversion scheme in accordance with embodiments of the present invention is less complex than a comparable conventional polarization diversity wavelength conversion scheme and is less costly as it only uses two wavelength conversion devices (e.g., PPLNs, nonlinear crystal, nonlinear fibers, etc.) for wavelength conversion. A WDM transponder in accordance with embodiments of the present invention uses only one wavelength conversion device (e.g., a PPLN). 
     The single-loop counter-propagating wavelength conversion scheme in accordance with embodiments of the present invention also enables the implementation of all-optical wavelength conversion and an all-optical WDM transponder. 
     Finally, due to residual pump recycling in optical embodiments in accordance with the present invention, residual pump laser power from one wavelength conversion device can be used in another wavelength conversion device (or in the same wavelength conversion device), thereby reducing laser pump power consumption considerably and thus increasing total power efficiency. 
     While this invention has been described in conjunction with exemplary embodiments outlined above and illustrated in the drawings, it is evident that the principles of the present invention may be implemented using any number of techniques, whether currently known or not, and many alternatives, modifications, and variations in form and detail will be apparent to those skilled in the art. Modifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the present invention. For example, the components of the systems and apparatuses may be integrated or separated. Also, the system may be implemented in free space, in a waveguide, in optical fiber(s), or a combination of these. Furthermore, the operations of the systems and apparatuses disclosed herein may be performed by more, fewer, or other components, and the methods described may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. While the dual-polarization has been used as an example and shown experimentally for this design, however other attributes may be considered such as spatial modes, etc. It should be mentioned that it has been shown experimentally that more than one input signal could be sent to the system for wavelength conversion from λ 1  to λ 2  and back to λ 1 . 
     In addition, although embodiments in accordance with the present invention have been shown and described with regard to optical signals, it should be understood that the principles of the present invention are not limited to wavelength conversion involving optical signals but encompass as well embodiments that implement wavelength conversion of electromagnetic wave signals generally. Information or any kind of data can be stored as electromagnetic waves (e.g., generated by LASER, optical beam, radio frequency (RF) signals, other types of electromagnetic wave signals, to name a few), which can be transmitted and/or reflected between structures or within structures in various transmission media (e.g., free space, vacuum, crystals, nonlinear media, optical waveguides, optical fibers, to name a few). The terms “electromagnetic wave signal” and “electromagnetic wave beam” are used herein interchangeably. Electromagnetic radiation or electromagnetic beam as used herein may include any kind of electromagnetic signal, including a LASER beam or signal, a MASER beam or signal, an optical beam or signal, or any type of wired or wireless signal, including acoustic waves, radio waves, IR radiation, UV radiation, microwave-band transmission, or any combination of more than one of the foregoing. While referred to herein sometimes simply as a LASER beam or signal, other types of optical signals and other types of electromagnetic radiation transmissions, including radio waves, microwaves, IR, UV, and combinations of bandwidths of wavelengths of electromagnetic radiation, whether guided, shaped, phased, or none of the foregoing, are also intended to be included. 
     Accordingly, the exemplary embodiments of the invention, as set forth above, are intended to be illustrative, not limiting, and the spirit and scope of the present invention are to be construed broadly and limited only by the appended claims and not by the foregoing specification. 
     In addition, unless otherwise specifically noted, the articles depicted in the drawings are not necessarily drawn to scale.