Patent Publication Number: US-2023146759-A1

Title: Polarization beam splitter

Description:
TECHNICAL FIELD 
     The present disclosure relates to waveguides. 
     BACKGROUND 
     A Polarization Beam Splitter (PBS) is a device that splits light in a waveguide based on the polarization state of the light. That is, a PBS can convert a single waveguide carrying two polarizations into two individual waveguides each carrying one polarization. PBSs are also known as polarization multiplexers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A and  1 B  illustrate respective cross-sectional views of a Polarization Beam Splitter (PBS), according to an example embodiment. 
         FIG.  2    illustrates a matrix of longitudinal cross-sections of a PBS that demonstrate how the PBS splits light, according to an example embodiment. 
         FIG.  3    illustrates a graph of TE and TM mode signal transmission over a longitudinal length of a PBS, according to an example embodiment. 
         FIGS.  4 A and  4 B  illustrate graphs of TM and TE mode signal behavior over a longitudinal length of a PBS, according to an example embodiment. 
         FIG.  5    illustrates a cross-sectional view of a variation of a PBS featuring an extended tip introduction configured to reduce Insertion Loss (IL), according to an example embodiment. 
         FIG.  6    illustrates a cross-sectional view of a variation of a PBS configured to provide two-input/two-output functionality, according to an example embodiment. 
         FIG.  7    illustrates a matrix of longitudinal cross-sections of a PBS that demonstrate how the PBS splits light to provide two-input/two-output functionality, according to an example embodiment. 
         FIGS.  8 - 10    illustrate respective use cases for a PBS and/or one or more variations of the PBS, according to an example embodiment. 
         FIG.  11    illustrates a flowchart of a method for performing functions associated with operations discussed herein, according to an example embodiment. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     In one example embodiment, a polarization beam splitter apparatus is provided that comprises first waveguide and a second waveguide. The second waveguide includes a first layer and a second layer. In a first longitudinal segment, the first layer gradually approaches the first waveguide in a first transverse direction, and the first layer is offset from the first waveguide in the first transverse direction by a first gap and in a second transverse direction by a second gap. In a second longitudinal segment, the first and second waveguides are longitudinally oriented, and the second layer is offset from the first waveguide in the first transverse direction by a third gap and is offset from the first layer in the second transverse direction by a fourth gap. In a third longitudinal segment, the first layer includes a length portion having a width in the first transverse direction that gradually decreases along the third longitudinal segment, and the second layer includes a length portion having a width in the first transverse direction that gradually increases along the third longitudinal segment. 
     Example Embodiments 
     Presented herein is an optimized Polarization Beam Splitter (PBS). The PBS may be capable of handling high optical power applications. The PBS may comprise a photonic component configured to separate vertically-polarized light and horizontally-polarized light in one waveguide into two separate waveguides carrying the respective polarizations. For example, the PBS may be designed to adiabatically convert a single waveguide carrying both Transverse Electric (TE)—polarized light and Transverse Magnetic (TM)—polarized light (e.g., TE0 and TM0 signals), into two individual waveguides, one waveguide carrying the TE mode signal and the other waveguide carrying the TM mode signal. 
       FIG.  1 A  illustrates example cross-sectional view  100 A of PBS  105 . PBS  105  includes waveguide  110  and waveguide  115 . Waveguide  115 , in turn, includes layer  120  and layer  125 .  FIG.  1 A  denotes a longitudinal direction of PBS  105  as ‘z’ and a first transverse direction of PBS  105  as ‘x.’ PBS  105  includes longitudinal segment  130 , longitudinal segment  135 , and longitudinal segment  140 . Longitudinal segment  135  is adjacent to longitudinal segment  130 , and longitudinal segment  140  is adjacent to longitudinal segment  135 . 
     In longitudinal segment  130 , layer  120  gradually approaches waveguide  110  in the first transverse direction, and layer  120  is offset from waveguide  110  in the first transverse direction by gap  145 . Gap  145  decreases as layer  120  gradually approaches waveguide  110  in the first transverse direction. As discussed below in connection with  FIG.  1 B , in longitudinal segment  130 , layer  120  is also offset from waveguide  110  in a second transverse direction by a gap. 
     In longitudinal segment  135 , waveguides  110  and  115  are longitudinally oriented. That is, layer  120  may cease gradually approaching waveguide  110  in the first transverse direction such that waveguides  110  and  115  are oriented substantially in the longitudinal direction. In this example, layer  125  is introduced at the border of longitudinal segments  130  and  135 . Layer  125  is offset from waveguide  110  in the first transverse direction by gap  155 . Layer  125  includes length portion  150 , which has a width in the first transverse direction that gradually increases along longitudinal segment  135 . As discussed below in connection with  FIG.  1 B , in longitudinal segment  135 , layer  125  is also offset from waveguide  110  in a second transverse direction by a gap. 
     In longitudinal segment  140 , waveguide  110  branches away from waveguide  115 , while waveguide  115  remains longitudinally oriented. Also, because length portion  150  extends into longitudinal segment  140 , layer  125  has a width in the first transverse direction that gradually increases along longitudinal segment  140 . Layer  120  includes length portion  160 , which has a width in the first transverse direction that gradually decreases along longitudinal segment  140  (e.g., to zero). 
       FIG.  1 A  also shows longitudinal cross-sections  165 ,  170 ,  175 , and  180 . Longitudinal cross-section  165  is in longitudinal segment  130 ; longitudinal cross-section  170  is in longitudinal segment  135 ; longitudinal cross-section  175  is at the border of longitudinal segments  135  and  140 ; and longitudinal cross-section  180  is in longitudinal segment  140 . Longitudinal cross-sections  165 ,  170 ,  175 , and  180  are discussed in greater detail below in connection with  FIG.  2   . 
     In one example, waveguide  110  may be provided with a TE mode signal polarized in the first transverse direction and a TM mode signal polarized in the second transverse direction. PBS  105  may split the TE mode and TM mode signals such that waveguide  110  carries the TE mode signal and waveguide  115  carries the TM mode signal. The TE mode signal may be obtained from waveguide  110 , and the TM mode signal may be obtained from layer  125 . For instance, as illustrated in  FIG.  1 A , PBS  105  may split a combined TE0/TM0 mode signal carried in waveguide  110  into a TM0 mode signal carried in layer  125  and a TE0 mode signal carried in waveguide  110 . 
     While PBS  105  may be configured to split TE- and TM-polarized light across multiple waveguides, it will be appreciated that PBS  105  may also/alternatively be used combine TE- and TM-polarized light across multiple waveguides. That is, PBS  105  may be used for general-purpose polarization multiplexing onto a single waveguide or de-multiplexing from a single waveguide at high powers. Thus, PBS  105  may function as a Polarization Beam Combiner (PBC). For example, consider TM and TE mode signals that are propagating in the right-to-left direction of  FIG.  1 A . If the TM mode signal is provided in layer  125  and the TE mode signal is provided in waveguide  110 , PBS  105  (which is now functioning as a PBC) may output the TM and TE mode signals via waveguide  110 . Other embodiments may be envisioned. 
     PBS  105  may be constructed from any suitable material(s). For instance, waveguide  110  may be a silicon nitride (SiN) waveguide, layer  125  may be a SiN layer, and layer  120  may be a silicon (Si) layer or SiN layer. If layer  120  is a SiN layer, PBS  105  may be referred to as a bilayer-SiN platform. Implementing layer  120  as a SiN layer may further increase the high power-handling capabilities of PBS  105 , lower insertion loss, and shorten the longitudinal length of PBS  105  due to increased TM mode signal confinement. 
     PBS  105  may be of any suitable length in the longitudinal direction. The length of PBS  105  in the longitudinal direction may be the sum of the longitudinal lengths of longitudinal segments  130 ,  135 , and  140 . In one example, the length of PBS  105  in the longitudinal direction may be approximately 150-250 μm (e.g., approximately 200 μm). 200 μm may be a reasonable longitudinal length for a SiN device configured to handle TM polarized light. 
     Waveguide  110 , layer  120 , and layer  125  may have any suitable width in the first transverse direction. In one example, waveguide  110  may have a constant width in the first transverse direction (e.g., approximately 1 μm). Because layer  125  includes length portion  150 , which has a width in the first transverse direction that gradually increases along longitudinal segments  135  and  140 , layer  125  may have a variable width in the first transverse direction. In one example, layer  125  may have a width in the first transverse direction less than 1 μm when layer  125  is first introduced at the border of longitudinal segments  130  and  135 . 
     Similarly, because layer  120  includes length portion  160 , which has a width in the first transverse direction that gradually decreases along longitudinal segment  140 , layer  120  may have a variable width in the first transverse direction. In one example, layer  120  may have a width in the first transverse direction greater than 250 nm in longitudinal segments  130  and  135 . In longitudinal segment  140 , the width of layer  120  in the first transverse direction may gradually decrease along length portion  160  (e.g., to zero). 
     Waveguide  110 , layer  120 , and layer  125  may have any suitable thickness in the second transverse direction. The thicknesses in the second transverse direction of waveguide  110 , layer  120 , and layer  125  may be constant. For example, waveguide  110  and layer  125  may have thicknesses in the second transverse direction of approximately 200-250 nm. The thicknesses in the second transverse direction of waveguide  110  and layer  125  may be the same. 
     The thickness in the second transverse direction of layer  120  may be selected such that PBS  105  can operate effectively at high optical powers. If layer  120  is a Si layer, layer  120  may have a thickness in the second transverse direction of approximately 110 nm. If layer  120  is a SiN layer, layer  120  may have a thickness in the second transverse direction of approximately 200-250 nm. In one example, if layer  120  is a SiN layer, the thicknesses in the second transverse direction of waveguide  110 , layer  120 , and layer  125  may be the same. 
     With continuing reference to  FIG.  1 A ,  FIG.  1 B  illustrates example cross-sectional view  100 B of PBS  105 . In cross-sectional view  100 B, the TM0 and TE0 mode signals shown in  FIG.  1 A  may propagate out of the page.  FIG.  1 B  denotes the second transverse direction of PBS  105  as ‘y.’ Layer  120  is offset from waveguide  110  in the second transverse direction by gap  185 . Layer  125  is offset from waveguide  110  in the second transverse direction by gap  190 . In one example, gap  185  and gap  190  may have the same value. 
     With continuing reference to  FIG.  1 A ,  FIG.  2    illustrates matrix  200  of longitudinal cross-sections  165 ,  170 ,  175 , and  180 . Matrix  200  demonstrates how PBS  105  splits the combined TE0/TM0 mode signal carried in waveguide  110  into a TM0 mode signal carried in layer  125  and a TE0 mode signal carried in waveguide  110 . Longitudinal cross-sections  165 ,  170 ,  175 , and  180  are oriented such that the TM0 and TE0 mode signals shown in  FIG.  1 A  propagate into the page. It will be appreciated that longitudinal cross-sections  165 ,  170 ,  175 , and  180  shown in matrix  200  may not be exactly to scale. 
     From left-to-right, each column corresponds to a respective longitudinal cross-section  165 ,  170 ,  175 , and  180 . The top row illustrates the cross-sectional shapes of waveguide  110 , layer  120 , and layer  125  at longitudinal cross-sections  165 ,  170 ,  175 , and  180 . The second row illustrates a heat map of an unused TE Si mode that exists independently from (and does not interfere with) the polarization splitting operation at longitudinal cross-sections  165 ,  170 ,  175 , and  180 . As shown, the unused TE Si mode signal remains in layer  120  as the signal propagates through PBS  105 . The third row illustrates a heat map of the TE0 mode signal at longitudinal cross-sections  165 ,  170 ,  175 , and  180 . The bottom row illustrates a heat map of the TM0 mode signal at longitudinal cross-sections  165 ,  170 ,  175 , and  180 . 
     With reference to the third and bottom rows, in longitudinal cross-section  165 , the TE0 and TM0 mode signals are provided in waveguide  110 . The TE0 mode signal is polarized in the first transverse direction (e.g., horizontally in  FIG.  2   ), and the TM0 mode signal is polarized in the second transverse direction (e.g., vertically in  FIG.  2   ). Layer  120  has not yet fully approached waveguide  110  in the first transverse direction (e.g., gap  145  is relatively large). Layer  120  may gradually approach waveguide  110 , instead of being introduced closer to waveguide  110 , to prevent the TE0 and TM0 mode signals from scattering off the tip of layer  120 . Layer  125  has not yet been introduced at this portion of PBS  105 . 
     In longitudinal cross-section  170 , layer  125  has fully approached waveguide  110  in the first transverse direction. Also, layer  120  has been introduced. The width in the first transverse direction of layer  125  is relatively small. The width in the first transverse direction of layer  120  may be unchanged from the width in the first transverse direction of layer  120  at longitudinal cross-section  165 . 
     Because the TE0 mode signal is polarized in the first transverse direction, and because the width in the first transverse direction of waveguide  110  is greater than the width in the first transverse direction of layer  125 , the TE0 mode signal may remain in waveguide  110 . However, because the TM0 mode signal is polarized in the second transverse direction, and because the thickness in the second transverse direction of waveguide  110  is less than the thickness in the second transverse direction of waveguide  115 , the TM0 mode signal may begin to shift to waveguide  115 . 
     In longitudinal cross-section  175 , the width in the first transverse direction of layer  125  is greater than the width in the first transverse direction of layer  125  at longitudinal cross-section  170 . The width in the first transverse direction of layer  120  may be unchanged from the width in the first transverse direction of layer  120  at longitudinal cross-section  170 . 
     The TE0 mode signal may continue to remain in waveguide  110  because the TE0 mode signal is polarized in the first transverse direction, and the width in the first transverse direction of waveguide  110  is greater than the width in the first transverse direction of layer  125 . Meanwhile, the TM0 mode signal may fully shift to waveguide  115  because the TM0 mode signal is polarized in the second transverse direction, and the thickness in the second transverse direction of waveguide  110  is less than the thickness in the second transverse direction of waveguide  115 . 
     In longitudinal cross-section  180 , waveguide  110  begins to branch away from waveguide  115  in the longitudinal direction, carrying the TE0 mode signal as a first output. The TM0 mode signal is carried in waveguide  115  as a second output. As waveguide  110  continues to branch away from waveguide  115  in the longitudinal direction, the width in the first transverse direction of layer  125  will continue to increase, and the width in the first transverse direction of layer  120  will decrease to zero. At that point, the TM0 mode signal will be fully captured in layer  125 . In one example, when the width in the first transverse direction of layer  120  becomes zero, the width in the first transverse direction of layer  125  may cease increasing and remain at a constant width that is approximately equal to the width in the first transverse direction of waveguide  110 . 
     Thus, PBS  105  may achieve polarization splitting by introducing waveguide  115  adjacent to waveguide  110 . Waveguide  115 —which includes layers  120  and  125  as stacked layers—may be relatively narrow compared to waveguide  110 , which includes a single layer. PBS  105  may be fully/optimally adiabatic; that is, PBS  105  may adiabatically isolate TE- and TM-polarized light into waveguides  110  and  115 . PBS  105  may have one single mode SiN waveguide input (e.g., waveguide  110 ) and two single mode SiN outputs (e.g., waveguides  110  and  115 ). 
     PBS  105  may be capable of high power handling, even if layer  120  is a Si layer. Because layer  120  remains sufficiently thin in the first transverse direction, the TE mode signal does not shift to layer  120 , and the TM mode signal does not localize in layer  120 . Also, gap  190  (between layers  120  and  125 ) may prevent layers  120  and  125  from becoming too closely coupled, and may thereby help layer  125  to capture the TM mode signal from layer  120 . 
     Because little or none of the TE or TM mode signals remain confined within layer  120 , PBS  105  may be configured for high power handling (e.g., as a high power TE0/TM0 splitter). PBS  105  may be capable of handling TM mode signals of over 12.9 dB, which is more than 19.5 times what a traditional PBS could handle. Furthermore, PBS  105  may be capable of handling TE mode signals of over 20 dB, which is more than 100 times what a traditional PBS could handle. As a result, Insertion Loss (IL) of PBS  105  may be minimized and PBS  105  may be capable of adiabatic broadband operation. 
     With continuing reference to  FIG.  1 A ,  FIG.  3    illustrates an example graph  300  of TE and TM mode signal transmission over the longitudinal length of PBS  105 . Graph  300  is based on an EigenMode Expansion (EME) simulation. The x-axis of graph  300  represents the longitudinal length of PBS  105  in microns and the y-axis of graph  300  represents the fraction of the TE or TM mode signal that is successfully transmitted. Graph  300  suggests that a longitudinal length of at least approximately 150 μm may be sufficient, but it will be appreciated that PBS  105  may be any suitable longitudinal length. 
     With continuing reference to  FIG.  1 A ,  FIG.  4 A  illustrates graph  400 A, which demonstrates of the behavior of the TM mode signal over the longitudinal length of PBS  105 .  FIG.  4 B  illustrates graph  400 B, which demonstrates of the behavior of the TE mode signal over the longitudinal length of PBS  105 . Graphs  400 A and  400 B are based on an EME simulation. The x-axis of graphs  400 A and  400 B represents the longitudinal length of PBS  105  in microns and the y-axis of graphs  400 A and  400 B represents the first transverse direction in microns. Graphs  400 A and  400 B have the same orientation as cross-sectional view  100 A. 
     Graph  400 A illustrates a heat map of the TM mode signal, and graph  400 B illustrates a heat map of the TE mode signal. As shown, the TM mode signal shifts from waveguide  110  to waveguide  115  over the longitudinal length of PBS  105 . Meanwhile, the TE mode signal remains in waveguide  110  over the longitudinal length of PBS  105 . 
     With continuing reference to  FIG.  1 A ,  FIGS.  5  and  6    illustrate respective variations of PBS  105 . Both variations will be discussed in greater detail below; however, both variations introduce layer  125  before (e.g., in cross-sectional view  100 A, to the left of) the border of longitudinal segments  130  and  135 . More specifically, in longitudinal segment  130 , layer  125  is offset from waveguide  110  in the first transverse direction by gap  155  and is offset from the layer  120  in the second transverse direction by gap  190 . 
       FIG.  5    illustrates cross-sectional view  500  of a variation of PBS  105  featuring an extended tip introduction. Cross-sectional view  500  has the same orientation as cross-sectional view  100 A. By way of explanation, in  FIG.  1 A , the tip of layer  125  may cause back-reflections at the border of longitudinal segments  130  and  135 ; this variation may further reduce IL caused by those back-reflections by introducing layer  125  before the border of longitudinal segments  130  and  135 . As shown, in longitudinal segment  130 , layer  125  is longitudinally oriented. In another example, layer  125  may gradually approach waveguide  110  in the first transverse direction, e.g., aligned with layer  120  in the second transverse direction. Other embodiments may be envisioned. 
       FIG.  6    illustrates cross-sectional view  600  of a variation of PBS  105  configured to provide two-input/two-output functionality. That is, this variation may have two inputs (e.g., waveguides  110  and  115 ) and two outputs (e.g., waveguides  110  and  115 ). Cross-sectional view  600  has the same orientation as cross-sectional view  100 A. 
     As shown, in longitudinal segment  130 , layer  125  gradually approaches waveguide  110  in the first transverse direction and includes a length portion  605  having a width in the first transverse direction that gradually decreases along longitudinal segment  130 . In this example, layer  125  may gradually approach waveguide  110  in the first transverse direction, e.g., aligned with layer  120  in the second transverse direction. 
       FIG.  6    also shows longitudinal cross-sections  610 ,  615 ,  620 , and  625 . Longitudinal cross-section  610  is in longitudinal segment  130 ; longitudinal cross-section  615  is in longitudinal segment  135 ; longitudinal cross-section  620  is at the border of longitudinal segments  135  and  140 ; and longitudinal cross-section  625  is in longitudinal segment  140 . Longitudinal cross-sections  610 ,  615 ,  620 , and  625  are discussed in greater detail below in connection with  FIG.  7   . 
     In one example, waveguide  110  may be provided with a first TE mode signal and a first TM mode signal, and waveguide  115  may be provided with a second TE mode signal and a second TM mode signal. The first TE mode and first TM mode signals may be split such that waveguide  110  carries the first TE mode signal and waveguide  115  carries the first TM mode signal. Similarly, the second TE mode and second TM mode signals may be split such that waveguide  110  carries the second TM mode signal and waveguide  115  carries the second TE mode signal. Thus, the first and second TM mode signals may be switched in waveguides  110  and  115 . 
     With continuing reference to  FIG.  6   ,  FIG.  7    illustrates matrix  700  of longitudinal cross-sections  610 ,  615 ,  620 , and  625 . Matrix  700  demonstrates how a combined TE0/TM1 mode signal carried in waveguide  115  may be split into a TE0 mode signal carried in waveguide  115  and a TM1 mode signal carried in waveguide  110 , while a combined TM0/TE1 mode signal carried in waveguide  110  may be split into a TE1 mode signal carried in waveguide  110  and a TM0 mode signal carried in waveguide  115 . Longitudinal cross-sections  610 ,  615 ,  620 , and  625  are oriented such that the TM and TE mode signals shown in  FIG.  6    propagate out of the page. It will be appreciated that longitudinal cross-sections  610 ,  615 ,  620 , and  625  shown in matrix  700  may not be exactly to scale. 
     From left-to-right, each column corresponds to a respective longitudinal cross-section  610 ,  615 ,  620 , and  625 . The top row illustrates the cross-sectional shapes of waveguide  110 , layer  120 , and layer  125  at longitudinal cross-sections  610 ,  615 ,  620 , and  625 . The second row illustrates a heat map of the TE0 mode signal at longitudinal cross-sections  610 ,  615 ,  620 , and  625 . The third row illustrates a heat map of the TE1 mode signal at longitudinal cross-sections  610 ,  615 ,  620 , and  625 . The fourth row illustrates a heat map of the TE0 mode signal at longitudinal cross-sections  610 ,  615 ,  620 , and  625 . The bottom row illustrates a heat map of the TM1 mode signal at longitudinal cross-sections  610 ,  615 ,  620 , and  625 . 
     With reference to the second, third, fourth, and bottom rows, in longitudinal cross-section  610 , the TE1 and TM0 mode signals are provided in waveguide  110 , and the TE0 and TM1 signals are provided in waveguide  115 . Layers  120  and  125  have not yet fully approached waveguide  110  in the first transverse direction (e.g., gap  145  is relatively large). 
     In longitudinal cross-section  615 , layers  120  and  125  have fully approached waveguide  110  in the first transverse direction. The width in the first transverse direction of layer  120  may be unchanged from the width in the first transverse direction of layer  120  at longitudinal cross-section  610 . As shown, the TE0 mode signal may remain in layer  120 , and the TE1 mode signal may remain in waveguide  110 . However, the TM0 mode signal may begin to shift to waveguide  115 , and the TM1 mode signal may begin to shift to waveguide  110 . 
     In longitudinal cross-section  620 , the width in the first transverse direction of layer  125  is greater than the width in the first transverse direction of layer  125  at longitudinal cross-section  615 . The width in the first transverse direction of layer  120  may be unchanged from the width in the first transverse direction of layer  120  at longitudinal cross-section  615 . As shown, the TE0 mode signal may continue to remain in layer  120 , and the TE1 mode signal may continue to remain in waveguide  110 . Meanwhile, the TM0 mode signal may fully shift to waveguide  115 , and the TM1 mode signal may fully shift to waveguide  110 . 
     In longitudinal cross-section  625 , waveguide  110  begins to branch away from waveguide  115  in the longitudinal direction, carrying a combined TE1/TM1 mode signal as a first output. A combined TE0/TM0 mode signal is carried in waveguide  115  as a second output. As waveguide  110  continues to branch away from waveguide  115  in the longitudinal direction, the width in the first transverse direction of layer  125  will continue to increase, and the width in the first transverse direction of layer  120  decreases to zero. At that point, the combined TE0/TM0 mode signal will be fully captured in layer  125 . 
     To support both the TM0 and TM1 mode signals, the total longitudinal length of the two-input/two-output variation may be longer than that of PBS  105 . This is because the TM0 mode signal may be closely matched in index to the TM1 mode signal. PBS  105  and/or other variants described herein may ensure that TM1 mode signal is cut-off to prevent strong coupling between the TM0 and TM1 mode signals. The two-input/two-output variation may be capable of high power handling for the TE0, TE1, TM0, and/or TM1 mode signals. In one specific example, layer  120  in may be implemented as an SiN layer to provide high power handling capabilities for the TE0 mode signal. 
       FIGS.  8 - 10    illustrate respective use cases for a PBS and/or one or more variations thereof.  FIG.  8    illustrates a first use case in which PBS  105  is used in conjunction with a Polarization Splitter-Rotator (PSR) to form a high-power PSR.  FIG.  9    illustrates a second use case in which two back-to-back PBSs  105   a  and  105   b  are used as a high power TE/TM waveguide crossing.  FIG.  10    illustrates a third use case in which PBS  105  is used to generate a polarization bidirectional Input/Output (I/O). While PBS  105  (and/or PBSs  105   a  and  105   b ) are shown in  FIGS.  8 - 10   , it will be appreciated that any suitable PBS or variation thereof may be used. Other use cases may be envisioned. 
       FIG.  8    illustrates an example system  800  configured to form high-power PSR  810 . System  800  includes prong  820 , PBS  105 , and polarization rotator  830 . PBS  105  and polarization rotator  830  make up high-power PSR  810 . In this example, polarization rotator  830  may experience issues with high power handling for TE0 mode signals. PBS  105  may prevent the TE0 mode signal from passing through polarization rotator  830  to mitigate the TE power handling limitations of polarization rotator  830 . As a result, high-power PSR  810  may be an efficient PSR configured to enable increased fiber datalink range. 
     More specifically, prong  820  provides a combined TE0/TM0 mode signal to PBS  105 . PBS  105 , which in this example functions as a TE0/TM0 splitter, may strip the TE0 mode signal from the combined TE0/TM0 mode signal. PBS  105  outputs the TM0 mode signal to polarization rotator  830  and further outputs the TE0 mode signal elsewhere (e.g., to another component in system  800 ). Polarization rotator  830  may rotate the polarization of the TM0 mode signal to produce another TE0 mode signal. 
       FIG.  9    illustrates an example system  900  configured to form a high power TE/TM waveguide crossing. System  900  includes PBSs  105   a  and  105   b , which may be identical or similar to PBS  105  and/or one or more variations described herein. In one example, PBS  105   a  obtains a TE0 mode signal as a first input and a TM0 mode signal as a second input. PBS  105   a  combines the TE0 and TM0 mode signals into a combined TE0/TM0 mode signal. PBS  105   a  provides the combined TE0/TM0 mode signal to PBS  105   b . PBS  105   b  splits the combined TE0/TM0 mode signal into the TM0 mode signal and the TE0 mode signal. PBS  105   b  provides the TM0 mode signal as a first output and the TE0 mode signal as a second output. The net result is that system  900  has switched the TE0 mode signal and the TM0 mode signal. 
       FIG.  10    illustrates an example system  1000  configured to form a polarization bidirectional I/O. System  1000  includes prong  820 , PBS  105 , polarization rotator  830 , and laser  1010 . A low-power TM0 mode signal propagates from prong  820 , to PBS  105 , to polarization rotator  830 . Polarization rotator  830  may rotate the polarization of the low-power TM0 mode signal to produce a low-power TE0 mode signal. Polarization rotator  830  may output the low-power TE0 mode signal. Also, laser  1010  may produce a high-power TE0 mode signal that propagates from laser  1010 , to PBS  150 , to prong  820 . Prong  820  may output the high-power TE0 mode signal. Thus, as shown, PBS  105  may be used in both transmitter and receiver circuits to increase power handling. 
       FIG.  11    is a flowchart of an example method  1100  for performing functions associated with operations discussed herein. Operation  1110  involves, in a first longitudinal segment of an apparatus that includes a first waveguide and a second waveguide including a first layer and a second layer, gradually approaching, by the first layer, the first waveguide in a first transverse direction. Operation  1120  involves, in the first longitudinal segment, offsetting the first layer from the first waveguide in the first transverse direction by a first gap and in a second transverse direction by a second gap. 
     Operation  1130  involves, in a second longitudinal segment of the apparatus adjacent to the first longitudinal segment, longitudinally orienting the first and second waveguides. Operation  1140  involves, in the second longitudinal segment, offsetting the second layer from the first waveguide in the first transverse direction by a third gap and from the first layer in the second transverse direction by a fourth gap. 
     Operation  1150  involves, in a third longitudinal segment of the apparatus adjacent to the second longitudinal segment, gradually decreasing a width in the first transverse direction of a length portion of the first layer along the third longitudinal segment. Operation  1160  involves, in the third longitudinal segment, gradually increasing a width in the first transverse direction of a length portion of the second layer along the third longitudinal segment. 
     Embodiments described herein may include one or more networks, which can represent a series of points and/or network elements of interconnected communication paths for receiving and/or transmitting messages (e.g., packets of information) that propagate through the one or more networks. These network elements offer communicative interfaces that facilitate communications between the network elements. A network can include any number of hardware and/or software elements coupled to (and in communication with) each other through a communication medium. Such networks can include, but are not limited to, any Local Area Network (LAN), Virtual LAN (VLAN), Wide Area Network (WAN) (e.g., the Internet), Software Defined WAN (SD-WAN), Wireless Local Area (WLA) access network, Wireless Wide Area (WWA) access network, Metropolitan Area Network (MAN), Intranet, Extranet, Virtual Private Network (VPN), Low Power Network (LPN), Low Power Wide Area Network (LPWAN), Machine to Machine (M2M) network, Internet of Things (IoT) network, Ethernet network/switching system, any other appropriate architecture and/or system that facilitates communications in a network environment, and/or any suitable combination thereof. 
     Networks through which communications propagate can use any suitable technologies for communications including wireless communications (e.g., 4G/5G/nG, IEEE 802.11 (e.g., Wi-Fi®/Wi-Fib®), IEEE 802.16 (e.g., Worldwide Interoperability for Microwave Access (WiMAX)), Radio-Frequency Identification (RFID), Near Field Communication (NFC), Bluetooth™, mm.wave, Ultra-Wideband (UWB), etc.), and/or wired communications (e.g., T1 lines, T3 lines, digital subscriber lines (DSL), Ethernet, Fibre Channel, etc.). Generally, any suitable means of communications may be used such as electric, sound, light, infrared, and/or radio to facilitate communications through one or more networks in accordance with embodiments herein. Communications, interactions, operations, etc. as discussed for various embodiments described herein may be performed among entities that may be directly or indirectly connected utilizing any algorithms, communication protocols, interfaces, etc. (proprietary and/or non-proprietary) that allow for the exchange of data and/or information. 
     In various example implementations, entities for various embodiments described herein can encompass network elements (which can include virtualized network elements, functions, etc.) such as, for example, network appliances, forwarders, routers, servers, switches, gateways, bridges, load-balancers, firewalls, processors, modules, radio receivers/transmitters, or any other suitable device, component, element, or object operable to exchange information that facilitates or otherwise helps to facilitate various operations in a network environment as described for various embodiments herein. Note that with the examples provided herein, interaction may be described in terms of one, two, three, or four entities. However, this has been done for purposes of clarity, simplicity and example only. The examples provided should not limit the scope or inhibit the broad teachings of systems, networks, etc. described herein as potentially applied to a myriad of other architectures. 
     Communications in a network environment can be referred to herein as ‘messages’, ‘messaging’, ‘signaling’, ‘data’, ‘content’, ‘objects’, ‘requests’, ‘queries’, ‘responses’, ‘replies’, etc. which may be inclusive of packets. As referred to herein and in the claims, the term ‘packet’ may be used in a generic sense to include packets, frames, segments, datagrams, and/or any other generic units that may be used to transmit communications in a network environment. Generally, a packet is a formatted unit of data that can contain control or routing information (e.g., source and destination address, source and destination port, etc.) and data, which is also sometimes referred to as a ‘payload’, ‘data payload’, and variations thereof. In some embodiments, control or routing information, management information, or the like can be included in packet fields, such as within header(s) and/or trailer(s) of packets. Internet Protocol (IP) addresses discussed herein and in the claims can include any IP version 4 (IPv4) and/or IP version 6 (IPv6) addresses. 
     To the extent that embodiments presented herein relate to the storage of data, the embodiments may employ any number of any conventional or other databases, data stores or storage structures (e.g., files, databases, data structures, data or other repositories, etc.) to store information. 
     Note that in this Specification, references to various features (e.g., elements, structures, nodes, modules, components, engines, logic, steps, operations, functions, characteristics, etc.) included in ‘one embodiment’, ‘example embodiment’, ‘an embodiment’, ‘another embodiment’, ‘certain embodiments’, ‘some embodiments’, ‘various embodiments’, ‘other embodiments’, ‘alternative embodiment’, and the like are intended to mean that any such features are included in one or more embodiments of the present disclosure, but may or may not necessarily be combined in the same embodiments. Note also that a module, engine, client, controller, function, logic or the like as used herein in this Specification, can be inclusive of an executable file comprising instructions that can be understood and processed on a server, computer, processor, machine, compute node, combinations thereof, or the like and may further include library modules loaded during execution, object files, system files, hardware logic, software logic, or any other executable modules. 
     It is also noted that the operations and steps described with reference to the preceding figures illustrate only some of the possible scenarios that may be executed by one or more entities discussed herein. Some of these operations may be deleted or removed where appropriate, or these steps may be modified or changed considerably without departing from the scope of the presented concepts. In addition, the timing and sequence of these operations may be altered considerably and still achieve the results taught in this disclosure. The preceding operational flows have been offered for purposes of example and discussion. Substantial flexibility is provided by the embodiments in that any suitable arrangements, chronologies, configurations, and timing mechanisms may be provided without departing from the teachings of the discussed concepts. 
     As used herein, unless expressly stated to the contrary, use of the phrase ‘at least one of’, ‘one or more of’, ‘and/or’, variations thereof, or the like are open-ended expressions that are both conjunctive and disjunctive in operation for any and all possible combination of the associated listed items. For example, each of the expressions ‘at least one of X, Y and Z’, ‘at least one of X, Y or Z’, ‘one or more of X, Y and Z’, ‘one or more of X, Y or Z’ and ‘X, Y and/or Z’ can mean any of the following: 1) X, but not Y and not Z; 2) Y, but not X and not Z; 3) Z, but not X and not Y; 4) X and Y, but not Z; 5) X and Z, but not Y; 6) Y and Z, but not X; or 7) X, Y, and Z. 
     Additionally, unless expressly stated to the contrary, the terms ‘first’, ‘second’, ‘third’, etc., are intended to distinguish the particular nouns they modify (e.g., element, condition, node, module, activity, operation, etc.). Unless expressly stated to the contrary, the use of these terms is not intended to indicate any type of order, rank, importance, temporal sequence, or hierarchy of the modified noun. For example, ‘first X’ and ‘second X’ are intended to designate two ‘X’ elements that are not necessarily limited by any order, rank, importance, temporal sequence, or hierarchy of the two elements. Further as referred to herein, ‘at least one of’ and ‘one or more of’ can be represented using the ‘(s)’ nomenclature (e.g., one or more element(s)). 
     In one form, an apparatus is provided. The apparatus comprises: a first waveguide; a second waveguide including a first layer and a second layer; and a first longitudinal segment, a second longitudinal segment adjacent to the first longitudinal segment, and a third longitudinal segment adjacent to the second longitudinal segment, wherein: in the first longitudinal segment, the first layer gradually approaches the first waveguide in a first transverse direction, and the first layer is offset from the first waveguide in the first transverse direction by a first gap and in a second transverse direction by a second gap, in the second longitudinal segment, the first and second waveguides are longitudinally oriented, and wherein the second layer is offset from the first waveguide in the first transverse direction by a third gap and is offset from the first layer in the second transverse direction by a fourth gap, and in the third longitudinal segment, the first layer includes a length portion having a width in the first transverse direction that gradually decreases along the third longitudinal segment, and the second layer includes a length portion having a width in the first transverse direction that gradually increases along the third longitudinal segment. 
     In one example, the first waveguide is a silicon nitride waveguide and the second layer is a silicon nitride layer. In a further example, the first layer is a silicon layer. In another further example, the first layer is another silicon nitride layer. 
     In one example, in the first longitudinal segment, the second layer is offset from the first waveguide in the first transverse direction by the third gap and is offset from the first layer in the second transverse direction by the fourth gap. In a further example, in the first longitudinal segment, the second layer is longitudinally oriented. In another further example, in the first longitudinal segment, the second layer gradually approaches the first waveguide in the first transverse direction and includes a length portion having a width in the first transverse direction that gradually decreases along the first longitudinal segment. 
     In one example, the first waveguide is configured to receive a transverse electric mode signal polarized in the first transverse direction and a transverse magnetic mode signal polarized in the second transverse direction, the first waveguide is configured to output the transverse electric mode signal, and the second layer of the second waveguide is configured to output the transverse magnetic mode signal. 
     In another form, a method is provided. The method comprises: in a first longitudinal segment of an apparatus that includes a first waveguide and a second waveguide including a first layer and a second layer, arranging the first layer so as to gradually approach the first waveguide in a first transverse direction; in the first longitudinal segment, offsetting the first layer from the first waveguide in the first transverse direction by a first gap and in a second transverse direction by a second gap; in a second longitudinal segment of the apparatus adjacent to the first longitudinal segment, longitudinally orienting the first and second waveguides; in the second longitudinal segment, offsetting the second layer from the first waveguide in the first transverse direction by a third gap and from the first layer in the second transverse direction by a fourth gap; in a third longitudinal segment of the apparatus adjacent to the second longitudinal segment, gradually decreasing a width in the first transverse direction of a length portion of the first layer along the third longitudinal segment; and in the third longitudinal segment, gradually increasing a width in the first transverse direction of a length portion of the second layer along the third longitudinal segment. 
     In another form, another apparatus is provided. The other apparatus comprises: a first waveguide configured to receive a transverse electric mode signal polarized in a first transverse direction and a transverse magnetic mode signal polarized in a second transverse direction and to output the transverse electric mode signal; a second waveguide including a first layer and a second layer configured to output the transverse magnetic mode signal; and a first longitudinal segment, a second longitudinal segment adjacent to the first longitudinal segment, and a third longitudinal segment adjacent to the second longitudinal segment, wherein: in the first longitudinal segment, the first layer gradually approaches the first waveguide in the first transverse direction, the first layer is offset from the first waveguide in the first transverse direction by a first gap and in the second transverse direction by a second gap, and the second layer is offset from the first waveguide in the first transverse direction by a third gap and is offset from the first layer in the second transverse direction by a fourth gap, in the second longitudinal segment, the first and second waveguides are longitudinally oriented, and wherein the second layer is offset from the first waveguide in the first transverse direction by the third gap and is offset from the first layer in the second transverse direction by the fourth gap, and in the third longitudinal segment, the first layer includes a length portion having a width in the first transverse direction that gradually decreases along the third longitudinal segment, and the second layer includes a length portion having a width in the first transverse direction that gradually increases along the third longitudinal segment. 
     One or more advantages described herein are not meant to suggest that any one of the embodiments described herein necessarily provides all of the described advantages or that all the embodiments of the present disclosure necessarily provide any one of the described advantages. Numerous other changes, substitutions, variations, alterations, and/or modifications may be ascertained to one skilled in the art and it is intended that the present disclosure encompass all such changes, substitutions, variations, alterations, and/or modifications as falling within the scope of the appended claims.