Abstract:
One embodiment disclosed relates to a method for wavelength-selective switching and equalization. The method includes dispersing an incoming multiplexed signal into wavelength components, focusing the wavelength components onto different portions of a controllable light diffractor array, and controllably diffracting each wavelength component such that each said component is individually attenuated and arrives at the individually selected output without substantial crosstalk between the outputs. Another embodiment relates to a wavelength-selective switching device with integrated equalizer functionality. Another embodiment relates to an optical apparatus for wavelength-selective switching and equalization. The apparatus includes a means for controllably diffracting each wavelength component of a multiplexed input such that each said component is controllably attenuated and directed to arrive at an individually selected output.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to devices for telecommunications. More particularly, the invention relates to devices for wavelength division multiplexed networks. 
   2. Description of the Background Art 
   To further increase the capacity on existing optical networks a number of methods are known. One means is to use a type of wavelength division multiplexing (WDM) technique in order to improve the degree of utilization of the available bandwidth. 
   One problem to overcome in WDM networks relates to managing the frequently changing network operation. Reconfigurable systems are needed in this regard. 
   A different problem in WDM networks pertains to non-uniform gains that are wavelength dependent. For example, erbium doped fiber amplifiers (EDFA) exhibit a non-uniform gain spectrum that differs depending on the WDM channel. It is desirable to be able to equalize such non-uniform gain between channels. 
   SUMMARY 
   One embodiment of the invention relates to a method for wavelength-selective switching and equalization. The method includes dispersing an incoming multiplexed signal into wavelength components, focusing the wavelength components onto different portions of a controllable light diffractor array, and controllably diffracting each wavelength component such that each component is individually attenuated and directed and arrives at the individually selected output without substantial crosstalk between the outputs. 
   Another embodiment relates to a wavelength-selective switching device with integrated equalizer functionality. 
   Another embodiment relates to an optical apparatus for wavelength-selective switching and equalization. The apparatus includes a means for controllably diffracting each wavelength component of a multiplexed input such that each said component is controllably attenuated and directed to arrive at an individually selected output. 
   These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a functional diagram of a 1×2 wavelength-selective switch and equalizer (WSSE) in accordance with an embodiment of the invention. 
       FIG. 2  depicts a cross-sectional view of a N—N step grating light valve™ (GLV®) type device in accordance with an embodiment of the invention. 
       FIG. 3  is a schematic diagram of a 1×2 WSSE in accordance with an embodiment of the invention. 
       FIGS. 4A and 4B  are schematic diagrams depicting a 1×2 WSSE with polarization diversity in accordance with an embodiment of the invention. 
       FIG. 5  schematically shows a polarization diversity module in accordance with an embodiment of the present invention. 
       FIG. 6  schematically shows module output signals in accordance with an embodiment of the present invention. 
       FIGS. 7A and 7B  depict experimental results of a 1×2 WSSE with polarization diversity in accordance with an embodiment of the invention. 
   

   The use of the same reference label in different drawings indicates the same or like components. Drawings are not to scale unless otherwise noted. 
   DETAILED DESCRIPTION 
   As discussed above, reconfigurable systems are needed to manage the continuously changing network operation in modern WDM networks. The reconfigurable systems may utilize controllable switching devices to re-route the optical traffic. One example of such a device is a 1×2 wavelength-selective switch (WSS). The 1×2 WSS is used to divide an incoming WDM line into two WDM lines with complementary spectral components. 
   Another separate device is a dynamic channel equalizer. The equalizer may be used, for example, to ensure the power balance of all the WDM lines. 
   In this specification, the multiplexed signals are referred to generically as “WDM” signals. We intend “WDM” to be inclusive of WDM, DWDM, and other densities of wavelength division multiplexed signals. 
     FIG. 1  is a functional diagram of a 1×2 wavelength-selective switch and equalizer (WSSE) in accordance with an embodiment of the invention. Such a WSSE device advantageously combines the prior separate functionalities of a WSS device and a dynamic channel equalizer device. The functional diagram depicts, as an example, an input  102  with four wavelength components (a, b, c and d). The input is received by the 1×2 WSSE  104 . The “1×2” indicates that there is one input WDM signal and two output WDM signals. The WSSE  104  has two outputs  106  and  108 . 
   In this instance, the first output  106  is shown to include the second and fourth wavelength components (b and d), while the second output  108  is shown to include the first and third wavelength components (a and c). In other words, the WSSE  104  selected the first wavelength component (a) to be switched to the second output  108 , the second wavelength component (b) to be switched to the first output  106 , the third wavelength component (c) to be switched to the second output  108 , and the fourth wavelength component (d) to be switched to the first output  106 . 
   Furthermore, the wavelength components are shown to have been equalized by the WSSE  104 . In this instance, the second and fourth components (b and d) have been equalized to a first magnitude, and the first and third components (a and c) have been equalized to a second magnitude that is different than the first. This equalization is also advantageously performed by the WSSE device  104  in accordance with an embodiment of the invention. 
     FIG. 2  depicts a cross-sectional view of a N—N step grating light valve™ (GLV®) type device or ribbon group  200 . Such a device  200  may be advantageously utilized to create a 1×2 WSSE in accordance with an embodiment of the invention. The N—N step GLV® type device  200  includes a group of 2N reflective elements or ribbons. The group of 2N elements are divided into two subgroups of N elements each. In  FIG. 2 , the first subgroup  202  includes elements labeled 1 through N, and the second subgroup  204  includes elements labeled N+1 through 2N. 
   As shown, the device  200  is in a “+1” configuration. In the +1 configuration, within each subgroup ( 202  or  204 ), the reflective elements are configured in steps going from high on the left side to low on the right side. With such a configuration, light incident onto the device is primarily diffracted towards one of the first order, e.g. +1, and none towards the other first order, e.g. −1. As shown in  FIG. 2 , the arrow labeled “0” represents the incident light at an angle perpendicular to the device, and the arrow labeled “+1” represents the positive first-order diffraction (referred to as the “+1” diffraction) which travels towards the upper right. Alternatively, of course, the device could be set in a “−1” configuration. In the −1 configuration, the steps could go from low on the left side to high on the right side. With that configuration of elements, the incident light would be primarily diffracted into the negative first-order diffraction (referred to as the “−1” diffraction) which travels towards the upper left. 
   In accordance with an embodiment of the invention, the steps in each ribbon subgroup are spaced evenly with a vertical spacing of A λ/4N or thereabout, where λ is the wavelength of the light. That vertical spacing minimizes the crosstalk between the +1 diffraction towards the upper right and the −1 diffraction towards the upper left (which couples to the alternative output port  108 , and thus represents undesirable cross-talk). For example, in the +1 configuration shown in  FIG. 2 , the incident light is diffracted into the +1 diffraction (the +1 arrow illustrated) while diffraction into the −1 diffraction (dashed line without arrow) is kept at a minimum. This minimization of crosstalk advantageously enables the device to be used as an optical switch. 
   In a preferred embodiment of the invention, there is a displacement δ between the first subgroup  202  of N elements and the second subgroup  204  of N elements. The displacement δ may be utilized to modulate the diffraction efficiency of the device while the crosstalk between the two 1st orders remains at a minimum. This advantageously enables the device to be used for equalization in addition to the switching functionality. 
   For the following calculation, consider the case where N=2. In that case, each subgroup of elements has two elements, and the vertical spacing becomes λ/8 between the elements in each subgroup. In addition, assume a horizontal inter-ribbon gap of zero for simplicity. Based on scalar diffraction, the 1 st-order normalized diffraction efficiencies is calculated to be 
             η     ±   1       =       8     π   2       ⁢       sin   2     ⁡     (       2   ⁢   π   ⁢           ⁢   δ     λ     )       ⁢       cos   2     ⁡     (       π   4     ∓       π   4     ⁢   sgn       )               
where sgn=+1 for the +1 configuration, and sgn=−1 for the −1 configuration. This means that for the +1 configuration
 
             η     +   1       =       8     π   2       ⁢       sin   2     ⁡     (       2   ⁢   π   ⁢           ⁢   δ     λ     )               
for the +1 diffraction, and η −1 =0 for the −1 diffraction. Similarly, when the device  200  is in the −1 configuration, the 1 st-order normalized diffraction efficiency is η +1 =0 for the +1 diffraction, and
 
             η     -   1       =       8     π   2       ⁢       sin   2     ⁡     (       2   ⁢   π   ⁢           ⁢   δ     λ     )               
for the −1 diffraction. Thus, no crosstalk is shown under this scalar calculation and ideal ribbon deflections. Furthermore, the displacement δ may be used to modulate the active diffraction between a minimum value of zero when δ=0 and a maximum value of about 0.81 when δ=λ/4 with no crosstalk for all values of δ.
 
     FIG. 3  is a schematic diagram of a 1×2 WSSE  300  in accordance with an embodiment of the invention. The 1×2 WSSE  300  has one WDM input  102  and two WDM outputs  106  and  108 . The input and outputs correspond to those illustrated in  FIG. 1 . 
   The input  102  may be adapted to launch the input WDM signal from the input optical fiber through freespace, collimated (not shown), and directed onto a static optical grating  302 . The path of the input WDM signal to the grating  302  is depicted in  FIG. 3 . 
   The grating  302  may comprise, for example, a blazed type grating or other device with similar functionality. The grating  302  disperses the wavelength components or channels of the input  102  such that different wavelengths diffract at different angles from the grating  302 . For purposes of simplicity in the illustration,  FIG. 3  depicts the path of one of the wavelength components being diffracted by the grating  302 . 
   A Fourier transform lens (FTL)  304  used in f—f configuration, or device with similar functionality, may be used to map the different angles onto linear channel positions. In other words, the FTL  304  focuses the dispersed wavelength components onto different positions in a controllable light diffractor array  306 . The f—f configuration is illustrated in  FIG. 3  and relates to focal length f of the FTL  304 . In a preferred embodiment, the array  306  comprises an array of N—N step GLV or GLV-type devices or ribbon groups  200 . Each wavelength component is mapped onto a different ribbon group in the array  306 . The simplified illustration in  FIG. 3  depicts one of the wavelength components being mapped to a ribbon group near the middle of the array  306 . 
   Each ribbon group receiving a wavelength component is set to switch that wavelength component either to Output  1  or Output  2 . For example, an N—N step GLV-type device  200  may be set to switch an incident wavelength component to Output  1  by putting the device in the −1 configuration and may be set to switch the incident wavelength component to Output  2  by putting the device in the +1 configuration. In the +1 configuration, the +1 diffraction would be active (while the −1 diffraction would be suppressed). The +1 diffraction is illustrated in  FIG. 3  as traveling from the controllable array  306  back to the FTL  304 . The FTL  304  maps the +1 diffraction to a specific location on the grating  302 . The +1 diffractions for the various channels are then effectively re-multiplexed by the grating  302  and sent to the Output  2 . Similarly, in the −1 configuration, the −1 diffraction would be active (while the +1 diffraction would be suppressed). The −1 diffraction is illustrated in  FIG. 3  as traveling from the controllable array  306  back to the FTL  304 . The FTL  304  maps the −1 diffraction to a separate location on the grating  302 . The −1 diffractions for the various channels are then effectively re-multiplexed by the grating  302  and sent to the Output  1 . 
   Furthermore, in addition to selectively switching a wavelength component as discussed above, the WSSE  300  can also modulate the amplitude of each component, for example, for purposes of channel equalization. In accordance with the preferred embodiment, the amplitude modulation for a particular wavelength component may be accomplished by controlling the displacement δ in the GLV device  200 . 
     FIGS. 4A and 4B  are schematic diagrams depicting a 1×2 WSSE with polarization diversity in accordance with an embodiment of the invention. A 3-in-1 polarization diversity (PD) module  410  may be configured to receive the WSSE input signal  102  and split the signal  102  into two polarization components. One of the polarization components is then rotated such that the two components have the same predetermined polarization. The PD module  410  may be configured such that said predetermined polarization is consistent no matter the polarization of the input signal  102 . Regarding each WSSE output signal ( 106  or  108 ), the PD module  410  recombines the two polarization component signals in a manner so that the polarization of the output signal ( 106  or  108 ) is the same as that of the original input signal  102 . 
   The insertion of the PD module  410  into the configuration of the WSSE  400  is depicted in  FIG. 4A .  FIG. 4B  is a three-dimensional drawing of the configuration. The splitting of the input signal  102  into two signals is shown, as well as the recombination of the two signals to create each of the output signals  106  and  108 . The construction and operation of such a PD module  410  is described in further detail below in relation to  FIGS. 5 and 6 . 
   Referring now to  FIG. 5 , there is schematically shown a polarization diversity module  410  in accordance with an embodiment of the present invention. As shown in  FIG. 5 , diversity module  410  may comprise a collimator  510 , a bi-refringent crystal  520 , and a polarization rotator  414 . Optical signals may go in and out of diversity module  410  via a pig-tail  550  of a fiber-optic cable, which in turn may be coupled to a circulator. Note that depending on the application, a circulator may also be integrated in diversity module  410 . 
   An optical input signal may enter collimator  510  via pig-tail  550 . Similarly, an optical output signal from collimator  510  may enter pig-tail  550  and propagate out to the rest of the system. Collimator  510  may be a collimating lens configured to direct an input signal into bi-refringent crystal  520 , and to direct an output signal from bi-refringent crystal  520  into pig-tail  550 . 
   In one embodiment, bi-refringent crystal  520  is configured to decompose an input signal from collimator  510  into two orthogonally polarized and spatially separated signals, namely a P-polarized signal on an optical path P 412  and an S-polarized signal on an optical path P 413 . Bi-refringent crystal  520  may be an yttrium vanadate (YVO 4 ) crystal, for example. Note that in  FIG. 5 , symbols  501  (i.e.,  501 A,  501 B,  501 C) represent a P-polarized signal, while symbol  502  represents an S-polarized signal. Also note that in the present disclosure, a P-polarized signal and an S-polarized signal are identified with respect to the plane of incidence at the grating  302 . In particular, a P-polarized signal is in parallel with the plane of incidence, whereas an S-polarized signal is perpendicular to the plane of incidence. The plane of incidence is formed by light beams impinging on, and light beams diffracted from, a grating  302 . 
   A bi-refringent crystal, in general, is a doubly refracting material. That is, a bi-refringent crystal has two indices of refraction. Light entering a bi-refringent crystal along a direction not parallel to the optical axis of the crystal will be divided into two orthogonal beams propagating in different directions. Embodiments of the present invention take advantage of this property to spatially separate an optical input signal into its two polarization state components. 
   Still referring to  FIG. 5 , polarization rotator  414  may be coupled to bi-refringent crystal  520  to convert the S-polarized signal on path P 413  to a P-polarized signal on path P 452 . This allows the first module output signal on path P 452  to have the same polarization state as the second module output signal on path P 457 . In this example, both the first and second module output signals are P-polarized. Both of the module output signals may also be S-polarized by placing polarization rotator  414  in path P 457  instead of in path P 452 . Polarization rotator  414  may be a half-wave plate, for example. A half-wave plate, in general, is an optical component that can be adjusted to rotate a polarization angle by 90 degrees. Thus, a half-wave plate converts an S-polarized signal to a P-polarized signal or vice versa. 
   From paths P 452  and P 457 , the first and the second module output signals, respectively, propagate as the polarization diversified input signals that are adapted to be directed to the grating  302 . 
   It is to be noted that a P-polarized input signal going into port  481  will come out of port  483  as a P-polarized second module output signal, while an S-polarized input signal going into port  481  will come out of port  482  as a P-polarized first module output signal. An arbitrarily polarized input signal going into port  481  will be split into two spatially separated beams that are both P-polarized as discussed above. P-polarized signals coming back to bi-refringent crystal  520  through port  482 , port  483 , or both are re-combined as an output signal on port  481  and propagate back to the rest of the system. Preferably, to minimize variations in the output signal coming out of port  481 , the first and second module output signals are optically processed in substantially identical fashion in the WSSE  300 . 
   As can be appreciated from the foregoing, the first and second module output signals of diversity module  410  will have the same polarization state regardless of the polarization state of an input signal entering the diversity module. Advantageously, this ensures that light beams propagating in the WSSE  300  will have the same polarization state, thereby mitigating the effects of polarization dependent loss. 
   The optical components of diversity module  410 , such as collimator  510 , bi-refringent crystal  520 , and polarization rotator  414  are preferably, but not necessarily, micro-optical components. Micro-optical components are substantially smaller than regular size optical components, and are thus advantageously more compact. Micro-optical components are commercially available from various manufacturers including Koncent Communications, Inc. of China. Micro-optical implementation is based on very mature technology and has been proven to meet the very stringent requirements of the optical communication industries (such as Telcordia standards). For example, a polarization diversity module  410  with micro-optical components may be fabricated in accordance with an embodiment of the present invention to conform to the following specifications:
         a) operable in telecommunication wavelengths of 1525 nm to 1570 nm (C-band);   b) a first module output signal and a second module output signal with optics axes that are parallel to each other;   c) a first module output signal and a second module output signal having a beam diameter of 1.6 mm (e −2 );   d) a first module output signal and a second module output signal having a beam center-to-center separation of 2.5 mm; and   e) a first module output signal and a second module output signal that are both P-polarized.       

     FIG. 6  schematically shows a first module output signal  610  and a second module output signal  612  in accordance with an embodiment of the present invention. First module output signal  610  may be an output of diversity module  410  coming out of port  482 , while second module output signal  612  may be an output of diversity module  410  coming out of port  483 . In  FIG. 6 , a dimension D 601  represents the separation between the beams of signals  610  and  612 . As mentioned, dimension D 601  may be 2.5 mm. A dimension D 602  represents beam diameter as measured at a point  603 , which may be a point where the amplitude of the beam is 13.5% of peak amplitude. As mentioned, dimension D 602  may be 1.6 mm. 
     FIGS. 7A and 7B  depict experimental results of a 1×2 WSSE with polarization diversity in accordance with an embodiment of the invention. In this instance, the WDM input signal included eight wavelength components (also called channels). The graphs show the signal strength versus wavelength for Output  1 . 
   The left graph  710  in  FIG. 7A  shows the results when the WSSE is set such that each of the eight channels is switched to Output  1 . The right graph  720  in  FIG. 7A  shows the results when the WSSE is set such that each of the eight channels is switched to Output  2 . As seen from the graphs, the WSSE effectively switched each of the eight channels with negligible crosstalk between Outputs  1  and  2 . The difference in amplitudes between a wavelength component switched to Output  1  and one switched away from Output  1  is shown to be greater than 30 dB. 
   The left graph  730  in  FIG. 7B  shows the results when the WSSE is set to switch all but the third channel to Output  1  (the third channel being switched to Output  2 ). This demonstrates the wavelength-selective switching capability of the WSSE device. Conversely, the right graph  740  in  FIG. 7B  illustrates the results when the WSSE is set to switch only the third channel to Output  1  (the remaining channels being switched to Output  2 ). 
   Polarization-dependent loss was measured to be about 0.25 dB. This was advantageously mitigated by the built-in PDM  410  in our prototype device. 
   In the present disclosure, numerous specific details are provided such as examples of apparatus, process parameters, materials, process steps, and structures to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention. 
   While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure. Thus, the present invention is limited only by the following claims.