Abstract:
A photonic switch matrix is disclosed. The photonic switch matrix includes a first pair of power splitters, each power splitter including one input and two output ports and a second pair of power splitters, each power splitter including two input ports and one output port. The photonic switch matrix further includes four optical fibers doped with gain controllable substances under light pumping, the four optical fibers connecting the first pair and the second pair of power splitters, wherein each input port of the second pair of power splitters is connected to an output port of the first pair of power splitters. The photonic switch matrix further includes four multiplexers, each multiplexer coupled with one of the four optical fibers, and at least one light pump connected to each multiplexer, wherein light pumped into a multiplexer defines an optical path of the photonic switch matrix.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0001]     This invention was made with U.S. Government support under contract DASG60-03-C-0021 awarded by the U.S. Army Space and Missile Defense Command. The Government has certain rights in the invention. 
     
    
     CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0002]     Not Applicable.  
       INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC  
       [0003]     Not Applicable.  
       FIELD OF THE INVENTION  
       [0004]     The invention disclosed broadly relates to the field of optics, and more particularly relates to the field of photonic switches for optical communication systems.  
       BACKGROUND OF THE INVENTION  
       [0005]     With the advent of the information era, modern optical communication systems are demanding more and more network capacity to process large volumes of information mixed with data, video, and audio signals. Dense wavelength division multiplexing (DWDM) and large-scale photonic switch matrices are two approaches for boosting overall network capacity. DWDM technology relies on the use of narrow signal channel spacing so that more channels can be utilized within a given wavelength span. DWDM technology, however, cannot evolve indefinitely because it will ultimately run into a technical limit in signal channel spacing. Another technical barrier for DWDM evolution is the chromatic dispersion effect causing signal overlap or cross talk between adjacent channels. A photonic switch matrix, by contrast, boosts the network capacity not by increasing the number of channels, but by enhancing the usage efficiency of existing channels through fast channel reconfiguration (i.e., switching). In principle, a photonic switch matrix does not suffer from evolution limitation and therefore can be cascaded to form a large-scale switch matrix, so long as the accumulated optical insertion loss is within the network overall loss budget. Large-scale switching formation and fast switch reconfiguration rates are the required characteristics for fast computing and image data processing in both military and commercial applications.  
         [0006]     Existing photonic switches mainly rely on electro-optic (EO) or mechanical approaches such as the micro electromechanical systems (MEMS). EO switches have a potential of providing high-speed photonic switching operations. However, these switches are only suitable for small-scale cross-connect applications, because the overall optical insertion loss accumulated during device cascade for large-scale switch matrix formation is too large for practical use. By comparison, MEMS switches have the potential of forming large-scale photonic switch matrixes but at slow switching speed. Similarly, accumulated insertion loss and crosstalk issues are the limiting factors.  
         [0007]     In addition, existing photonic switches as mentioned above are all operated with the aid of external voltage signals, and therefore these switches are in the category of electrical controllable photonic switches. One common drawback shared by these photonic switches is that they are sensitive to electro-magnetic interference (EMI). In uncontrolled operational environments where strong EMI sources may exist, electrical controllable photonic switches are not reliable. All-optical controllable photonic switches alleviate the EMI sensitivity by utilizing optical control signals for photonic switching operations. Despite the obvious technical attractions offered by an all-optical controllable photonic switch matrix, realization of photonic switches has been proven difficult. This is primarily impeded by the lack of suitable light-sensitive materials for practical all-optical photonic switching applications. Secondly, incorporation of light-sensitive materials into a large-scale photonic switch matrix with a compact device size is also not trivial, involving packaging and aligning a large number of control light sources. For these reasons, no viable solutions exist so far for the realization of an all-optical controllable large-scale photonic switch matrix with low insertion loss.  
         [0008]     Very limited research and development efforts exist for the development of all-optical controllable photonic switches. Current efforts are based on integrated optic approaches with various planar waveguide structures. Compared to their electrical controllable counterparts, all-optical controllable photonic switches are inferior in terms of insertion loss, cross talk, switch speed, and device reliability. Thus, photonic switches are used solely for research purposes and are far from being used for practical applications.  
         [0009]     Therefore, a need exists to overcome the problems with the prior art as discussed above, and particularly for an all-optical controllable large-scale photonic switch matrix with low insertion loss.  
       SUMMARY OF THE INVENTION  
       [0010]     Briefly, according to an embodiment of the present invention, a 2 by 2 photonic switch matrix is disclosed. The 2 by 2 photonic switch matrix includes a first pair of power splitters, each power splitter including one input and two output ports and a second pair of power splitters, each power splitter including two input ports and one output port. The photonic switch matrix further includes four optical fibers doped with gain controllable substances under light pumping, the four optical fibers connecting the first pair and the second pair of power splitters, wherein each input port of the second pair of power splitters is connected to an output port of the first pair of power splitters. The photonic switch matrix further includes four multiplexers, each multiplexer coupled with one of the four optical fibers, and at least one light pump connected to each multiplexer, wherein light pumped into a multiplexer defines an optical connection path of the photonic switch matrix.  
         [0011]     According to the second embodiment of the present invention, an N by N photonic switch matrix is disclosed. The N by N photonic switch matrix includes a first set of N power splitters, each power splitter including one input and N output ports and a second set of N power splitters, each power splitter including N input ports and one output port. The photonic switch matrix further includes N 2  optical fibers doped with gain controllable substances under light pumping, the N optical fibers connecting the first set and the second set of power splitters, wherein each input port of the second set of power splitters is connected to an output port of the first set of power splitters. The photonic switch matrix further includes N 2  multiplexers, each multiplexer coupled with one of the N 2  optical fibers, and N 2  light pumps, each light pump connected to each multiplexer, wherein light pumped into a multiplexer defines an optical connection path of the photonic switch matrix.  
         [0012]     According to the third embodiment of the present invention, an M by N photonic switch matrix is disclosed. The M by N photonic switch matrix includes a first set of M power splitters, each power splitter including one input and N output ports and a second set of N power splitters, each power splitter including M input ports and one output port. The photonic switch matrix further includes M×N optical fibers doped with gain controllable substances under light pumping, the M×N optical fibers connecting the first set and the second set of power splitters, wherein each input port of the second set of power splitters is connected to an output port of the first set of power splitters. The photonic switch matrix further includes M×N multiplexers, each multiplexer coupled with one of the M×N optical fibers, and M×N light pumps, each light pump connected to each multiplexer, wherein light pumped into a multiplexer defines an optical connection path of the photonic switch matrix.  
         [0013]     The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and also the advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings. Additionally, the left-most digit of a reference number identifies the drawing in which the reference number first appears.  
         [0015]      FIG. 1  is a schematic drawing of a fundamental 1 by 2 all-optical controllable photonic switch with single-ended pumping, according to one embodiment of the present invention.  
         [0016]      FIG. 2  is a revised schematic drawing of the switch of  FIG. 1 , but with double-ended pumping, according to one embodiment of the present invention.  
         [0017]      FIG. 3  is a schematic drawing of a fundamental 2 by 2 all-optical controllable photonic switch of the present invention with single-ended pumping, according to one embodiment of the present invention.  
         [0018]      FIG. 4  is a schematic drawing of a fundamental 2 by 2 all-optical controllable photonic switch with double-ended pumping, according to one embodiment of the present invention.  
         [0019]      FIG. 5  is a schematic drawing of a 4 by 4 all-optical controllable photonic switch, according to one embodiment of the present invention.  
         [0020]      FIG. 6  is a revised schematic drawing of a double-pumping scheme and two single-pumping schemes of one switching path out of total 16 switching paths, for the 4 by 4 photonic switch shown in  FIG. 5 , according to one embodiment of the present invention.  
         [0021]      FIG. 7  is a schematic representation of an N by N all-optical controllable photonic switch, according to one embodiment of the present invention.  
         [0022]      FIG. 8  is a schematic representation of an M by N all-optical controllable photonic switch, according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0023]     The present invention provides an all-optical controllable photonic switch with substantial EMI immunity when the control lasers are not local. The present invention further provides a photonic switch with a potential for forming a large-scale photonic switch matrix with low overall insertion loss. The present invention further provides a cost-effective all-fiber-based photonic switch that contains no moving parts without wearing or bearing issues leading to high device reliability.  
         [0024]     In one embodiment of the present invention, the switching operation is controlled by optical signals rather than electrical voltage signals for EMI immunity. Unlike MEMS photonic switches, the photonic switch of the present invention contains no moving parts for the removal of potential wearing or bearing issues to achieve long-term device reliability. Further, the photonic switch of the present invention can be fabricated at low cost with simple device packaging that does not involve tedious optical alignments or fiber pig-tailing. Also, the photonic switch of the present invention possesses minimum millisecond switching speed comparable to that of mechanical or MEMS photonic switches.  
         [0025]     A splitter is a transmission coupling device for separately sampling either the forward (incident) or the backward (reflected) wave in a transmission line. A multiplexer is a device that combines multiple inputs at separate wavelengths into an aggregate signal to be transported via a single transmission channel. An optical fiber is a filament of transparent dielectric material, usually plastic or glass, and usually in circular cross section, that guides light through total internal reflection, or by photonic crystal or photonic band-gap structures. Erbium is one of the so-called rare-earth elements on the lanthanide series with an atomic number of 68. Erbium can be placed on an optical fiber for controlling gain, resulting in an erbium doped fiber (EDF). A light pump is an optical signal that excites the erbium atoms in an EDF to increase the intensity of light beams passing through. A laser pump is a device that produces coherent pumping light for erbium doped optical fibers. Input/output ports are the two ends of a photonic device that route optical signals from one end (input) to the other end (output).  
         [0026]     In one embodiment of the present invention, an EDF-based all-optical controllable photonic switch matrix for large-scale (N by N) optical cross-connect applications is disclosed. The switch comprises: (A) N 1 by N splitters; (B) N N by 1 combiners; and (C) N 2  pieces of EDF of a certain length connecting the N 2  output ports of the 1 by N splitters and the N 2  input ports of the N by 1 combiners. In addition, N 2  980 nanometer (hereinafter “nm”) laser pumps are multiplexed to one end of the N2 pieces of EDF cables with one-to-one correspondence by the aid of N 2  980/1550 nm multiplexers. Optionally, another set of N 2  980/1550 nm multiplexers can be multiplexed to the other end of the EDF cables for dual pumping purpose. In this optional configuration, N 2  1 by 2 980 nm power splitters are used to share the same 980 nm laser pump at each EDF path for dual pumping purpose without the need to double the number of laser pumps for cost reduction. The addition of dual 980/1550 nm multiplexers at both ends of each EDF path also helps remove 980 nm pump leakage at both signal input and output ends. Due to the symmetric device configuration, the switching operation is bi-directional.  
         [0027]      FIG. 1  is a schematic drawing of a 1 by 2 EDF all-optical controllable photonic switch with single-ended pumping, according to one embodiment of the present invention. In this configuration, an optical signal from input end  10  is first split into two beam paths of equal power by a 1 by 2 power splitter  13 . Two pieces of EDF cables  18  and  19  are fusion spliced into the two beam paths, respectively, in connection with two output fiber ports  11  and  12 . At each beam path, a 980/1550 nm multiplexer  14  or  15  multiplexes the 980 nm light pump from the laser pump  16  or  17  to the EDF cable  18  or  19 . For switching operations from input end  10  to output end  11 , laser pump  16  is turned on while laser pump  17  is turned off leading to optical amplification (“optical connection”) by EDF cable  18  and optical absorption (“optical disconnection”) by EDF cable  19 .  
         [0028]     When the EDF cable has a sufficient length, the accumulated optical absorption along the EDF becomes significant (typical attenuation of common EDF cables is around 5 dB/m) leading to effective optical disconnection along the fiber path  19 . At the EDF path  18  with optical amplification, the pumping power from laser pump  16  is adjusted to a certain level so that the gain provided by EDF  18  compensates the splitting loss by the 1 by 2 power splitter  13 . Thus, zero-loss optical switching from input end  10  to output end  11  can be achieved. Switching from input end  10  to output end  12  can be achieved in a similar manner. Since the switching operation in this configuration is bi-directional, the switch described above can also be operated reversibly.  
         [0029]     The 1 by 2 EDF photonic switch shown in  FIG. 1  uses a single-pumping scheme at each EDF path. One possible drawback of the single-pumping scheme is the remnant pump leakage at the other end of the EDF that could interfere with either the source or the detector in the networking systems. This potential problem can be alleviated by utilizing the double-pumping scheme shown in  FIG. 2  below.  
         [0030]      FIG. 2  is a revised schematic drawing of the switch of  FIG. 1 , but with double-ended pumping, according to one embodiment of the present invention. With the double-pumping scheme, remnant pump leakage at each end of the EDF path  28  or  29  is de-multiplexed from the signal path  20  and  21 , or,  20  and  22  by the two pairs of 980/1550 nm multiplexers  24  and  25 , or,  26  and  27  located at each end of the EDF path. Naturally, due to the double-pumping scheme, two 980 nm power splitters  210  and  211  are needed to split the power of the laser pumps  212  and  213  for the double-pumping purpose.  
         [0031]     Another purpose of pumping scheme is to provide gain for optical amplification as required by a large-scale EDF photonic switch matrix in order to achieve low or no overall insertion loss. Actually, the optical gain provided by the pumping scheme in this configuration may well exceed the splitting loss of the 1 by 2 power splitter  23 . Thus, the present invention possesses a dual-functionality of photonic switching and signal amplifying.  
         [0032]     While  FIGS. 1 and 2  describe an unsymmetrical 1 by 2 EDF photonic switch configuration,  FIG. 3  illustrates a symmetrical 2 by 2 EDF cross-connect photonic switch, in one embodiment of the present invention.  
         [0033]      FIG. 3  is a schematic drawing of a 2 by 2 EDF all-optical controllable photonic switch of the present invention with single-ended pumping, according to one embodiment of the present invention. As seen in  FIG. 3 , four 1 by 2 power splitters  34 ,  35 ,  36 , and  37  are configured to form cross and bar connections between the two input ends  30 / 31  and two output ends  32 / 33 . For cross-switch operations ( 30  to  33  and  31  to  32 ), laser pump  318  is turned on while laser pump  319  is turned off. The pumping power from laser pump  318  is split onto two paths by a 980 nm 1 by 2 power splitter  38 , and then the split pumping beams are multiplexed to the two cross path EDF cables  315  and  316  by two 980/1550 nm multiplexers  311  and  312 , respectively.  
         [0034]     Similarly, for bar-switch operation ( 30  to  32 , and  31  to  33 ), laser pump  319  is turned on while laser pump  318  is turned off. The 980 nm 1 by 2 power splitter  39  splits the pumping power into two paths and then the split pumping beams are multiplexed by two 980/1550 nm multiplexers  310  and  313  to two bar path EDF cables  314  and  317 . The pump sharing scheme of this embodiment of the present invention is beneficial not only because it saves the cost for additional laser pumps, but also because it uses a single laser pump to control each switching state, thus avoiding synchronization issues between multiple laser pumps for switch control. With single-pumping, the 2 by 2 EDF photonic switch can operate bi-directionally.  
         [0035]     The 2 by 2 EDF photonic switch shown in  FIG. 3  uses the single-pumping scheme. The switch of  FIG. 3  can be revised to adopt the double-pumping scheme as shown in  FIG. 4 .  
         [0036]      FIG. 4  is a schematic drawing of a 2 by 2 EDF all-optical controllable photonic switch with double-ended pumping, according to one embodiment of the present invention.  FIG. 4  is similar to  FIG. 3  except that the two 980 nm 1 by 2 power splitters in  FIG. 3  are replaced with two 980 nm 1 by 4 power splitters  48  and  49  with the addition of additional four 980/1550 nm multiplexers  414 ,  415 ,  416 , and  417  for double-pumping purpose. With the double-pumping scheme, the 2 by 2 EDF photonic switch shown in  FIG. 4  is a truly symmetrical 2 by 2 bi-directional cross-connect photonic switch. The EDF photonic switch configuration of the present invention can be readily scaled up as demonstrated by the 4 by 4 EDF photonic switch shown in  FIG. 5 .  
         [0037]      FIG. 5  is a schematic drawing of a 4 by 4 EDF all-optical controllable photonic switch, according to one embodiment of the present invention. In  FIG. 5 , the 4 by 4 EDF photonic switch is composed of eight  1  by 4 power splitters  58  to  515 , and sixteen EDF paths  516  to  531 .  FIG. 5  shows one switching state (input  50  to output  55 ; input  51  to output  57 ; input  52  to output  56 ; input  53  to output  54 ) out of total 4!=4×3×2=24 switching states. Other components in this 4 by 4 EDF photonic switch, not shown in this figure for clarity, are the 980/1550 nm multiplexers, 980 nm 1 by 2 power splitters, and 980 nm laser pumps at each EDF path.  
         [0038]     If the excess losses of the 1 by 4 power splitters can be neglected, each of the four EDF paths at switch-on state should provide 12 dB optical gain to compensate for the splitting and combining losses at both input and output ends. Under these circumstances, a loss-less  4  by 4 EDF photonic switch matrix is hence realized. With an increase in pumping power, a 4 by 4 EDF photonic switch with net optical amplification is achievable. However, excessive pumping power may result in uneven optical amplification among signals at different wavelengths leading to an increase in wavelength dependent loss (WDL). This arises from the gain characteristic of EDF under optical pumping. Two solutions to this problem exist. One is to insert a gain flattening filter at each EDF path to flatten the gain spectral curve of the EDF; the other is to select the EDF cable of a certain length so that both under-pumping and over-pumping of EDF are avoided leading to a flat gain spectral curve of the EDF.  
         [0039]     Similarly, the pumping scheme for the 4 by 4 EDF photonic switch in  FIG. 5  can be either single-pumping or double-pumping.  
         [0040]      FIG. 6  is a revised schematic drawing of a double-pumping scheme and two single-pumping schemes of one EDF path out of total 16 EDF paths, for the 4 by 4 EDF photonic switch shown in  FIG. 5 , according to one embodiment of the present invention.  FIG. 6  illustrates total three pumping possibilities (one double-pumping scheme and two single-pumping schemes) that can be used for one EDF path out of total 16 EDF paths. The double-pumping scheme pumps the two ends of the EDF path  62  from one laser pump  64  split by a 980 nm 1 by 2 power splitter  63 . Naturally, two 980/1550 nm multiplexers  60  and  61  are included to multiplex the pumping light onto the EDF cable from both ends. As for single-pumping schemes, only one 980/1550 nm multiplexer either  60  or  61  is used to multiplex the pumping light from the pump  64  to either end of the EDF cable  62 . Obviously, the pumping scheme shown in  FIG. 6  should be applied to each EDF path of total 16 EDF paths to complete the 4 by 4 EDF photonic switch configuration.  
         [0041]     An N by N EDF photonic switch configuration as shown in  FIG. 7  can be formed similarly based on an embodiment of the present invention.  FIG. 7  is a schematic representation of an N by N EDF all-optical controllable photonic switch, according to one embodiment of the present invention. In this configuration, 2N 1×N power splitters  731 ,  732 , . . . ,  733 , and  741 ,  742 ,  743  are connected by N 2  EDF cables  751 ,  752 , . . . , and  759 . Similarly, double-pumping or single-pumping schemes can be chosen for each EDF path out of total N 2  EDF paths, as illustrated in  FIG. 6 . Optical cross-connections between input ports  711 ,  712 , . . . ,  713  and output ports  721 ,  722  . . . ,  723  can be realized by turning on the laser pumps of the corresponding EDF paths while all other laser pumps of the remaining EDF paths are kept off.  
         [0042]     More generally, an M by N EDF photonic switch configuration as shown in  FIG. 8  can be formed similarly based an embodiment of the present invention.  FIG. 8  is a schematic representation of an M by N EDF all-optical controllable photonic switch, according to one embodiment of the present invention. In this configuration, M  1  by N power splitters  831 ,  832 , . . . and  833 , and N M by 1 power couplers  841 ,  842 , . . . , and  843  are connected by M×N EDF cables  851 ,  852 , . . . , and  859 . Similarly, double-pumping or single-pumping schemes can be chosen for each EDF path out of total M×N EDF paths, as illustrated in  FIG. 6 . Optical cross-connections between input ports  811 ,  812 , . . . ,  813  and output ports  821 ,  822 , . . . ,  823  can be realized by turning on the laser pumps of the corresponding EDF paths while all other laser pumps of the remaining EDF paths are kept off.  
         [0043]     The embodiment of the present invention mentioned above uses single 1 by N power splitters for optical power splitting and combining between the input and output ports. The illustrated single 1 by N power splitters can be further extended to cover multiple power splitters cascaded for the same functionality of 1 by N power splitting. For instance, a single 1 by 4 power splitter can be replaced by three 1 by 2 power splitters cascaded in serial to achieve the required 1 by 4 power splitting functionality. Thus, use of such multi-stage cascaded power splitters is contemplated as within the scope of the appended claims.  
         [0044]     While the invention has been described in the context of EDF that represents erbium doped optical fiber, it will be readily understood by the skilled artisan that the term “EDF” can be interpreted to include any of erbium doped light guides such as an erbium doped waveguide (EDW) with either two-dimensional (planar) or three-dimensional (3D) light-wave circuitry configuration. Also, other components described such as the power splitters and multiplexers can be either all-fiber based or planar- or 3D-waveguide circuitry based. Thus, any light guided media that serve to provide a dual function of optical amplification and optical absorption under the control of pumping light are contemplated as within the meaning of the term “EDF” and such a configuration structure either all-fiber based or planar- or 3D-waveguide circuitry based is intended to be included within the scope of the invention.  
         [0045]     Although specific embodiments of the invention have been disclosed, those having ordinary skill in the art will understand that changes can be made to the specific embodiments without departing from the spirit and scope of the invention. The scope of the invention is not to be restricted, therefore, to the specific embodiments. Furthermore, it is intended that the appended claims cover any and all such applications, modifications, and embodiments within the scope of the present invention.