Patent Publication Number: US-10761265-B2

Title: Mode matched Y-junction

Description:
This application is a continuation of U.S. patent application Ser. No. 15/928,594, now allowed, which is a continuation of and claims priority to and the benefit of U.S. patent application Ser. No. 15/423,843, now U.S. Pat. No. 9,946,020, which applications are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a waveguide Y-junction, and in particular to a mode-matched waveguide Y-junction with balanced or unbalanced splitting. 
     BACKGROUND 
     With reference to  FIGS. 1 a , 1 b  and 1 c   , a junction section (JS)  4  of a conventional semiconductor waveguide Y-junction splitter  1  receives an input beam at an input port  2  for transmission to an input waveguide  3 , and splits the input beam into two output beams onto two output waveguides  6  and  7  for output to two output ports  8  and  9 , respectively. Typically, the input port  2  and the output ports  8  and  9  are optically coupled to external waveguides (not shown) for transmitting the optical beams to and from the Y-junction splitter  1 . A good Y-junction splitter is characterized by low insertion loss (IL), i.e. the amount of power lost through the Y-junction splitter that does not go to the output waveguides  8  and  9 ; low return loss (RL), the amount of light reflected by the JS  4 ; and good split ratio, e.g. a balanced Y-junction splits evenly 50:50, not 51:49. 
     One problem that arises, especially with a high-index contrast platform, such as Si/SiO2 or SiN/SiO2, is that there can be an abrupt change in mode profile between the optical mode guided just before the JS  4  and the optical mode just after the JS  4 . The abrupt change results in exciting multiple modes past the JS  4 , such as high order guided modes or radiation modes. These parasitic modes can lead to high IL or RL. 
     Another problem arises with the design of unbalanced Y-junctions splitters. Although balanced Y-junction splitters are intuitively designed by symmetry, designing an unbalanced Y-junction splitter with an arbitrary split ratio is non-trivial; especially when low IL is required. 
     An object of the present invention is to overcome the shortcomings of the prior art by providing a more efficient Y-junction waveguide splitter. 
     SUMMARY OF THE INVENTION 
     Accordingly the present invention relates to a splitter for splitting an input beam of light into first and second portions comprising: an input port for launching the input beam of light; an input waveguide including a longitudinal axis, capable of providing adiabatic expansion of the input beam of light from an input end proximate the input port to an output end, which has a width that supports a fundamental mode and a second order mode; first and second output waveguides extending from the output end of the input waveguide on either side of the longitudinal axis, and separated by a gap; and first and second output ports for outputting the first and second portions, respectively. Each of the first and second output waveguides includes: an initial section, including an initial width smaller than the input end of the input waveguide, whereby the initial sections and the gap form a single waveguide capable of supporting a super mode of the input beam of light, which spans the initial sections of the first and second output waveguides and the gap; a mode splitting section extending from the initial section at an acute angle to the longitudinal axis for splitting the super mode of the input beam of light into first and second portions; and a final expansion section extending from the mode splitting section, at least one of the final expansion sections including an expanding width expanding to a same width as the input end of the input waveguide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
         FIG. 1 a    is a schematic plan view of a junction split section of a conventional Y-junction splitter. 
         FIG. 1 b    is a cross-sectional view of the input waveguide of the Y-junction splitter of  FIG. 1 a    illustrating light intensity. 
         FIG. 1 c    is a cross-sectional view of the input waveguide of the Y-junction splitter of  FIG. 1 a    at the junction split illustrating light intensity. 
         FIG. 2 a    is a schematic plan view of a junction split section of a Y-junction splitter in accordance with the present invention. 
         FIG. 2 b    is a cross-sectional view of the input waveguide of the Y-junction splitter of  FIG. 2 a    illustrating light intensity. 
         FIG. 2 c    is a cross-sectional view of the input waveguide of the Y-junction splitter of  FIG. 2 a    at the junction split section illustrating light intensity. 
         FIG. 3  is a schematic plan view of a Y-junction splitter in accordance with the present invention. 
         FIG. 4  is a schematic plan view of a Y-junction splitter in accordance with another embodiment of the present invention. 
         FIG. 5  is a cross-sectional view of the input waveguide of the Y-junction splitter of  FIG. 4  at the junction split illustrating light intensity. 
     
    
    
     DETAILED DESCRIPTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. 
     With reference to  FIGS. 2 a , 2 b , 2 c    and  3 , a mode-matched semiconductor waveguide Y-junction splitter  11  includes a junction split section  14 , which receives an input beam at an input port  12  for transmission to an input waveguide  13 , and splits the input beam into two output beams onto two output waveguides  16  and  17  for output to two output ports  18  and  19 , respectively. The input port  12  may be along a longitudinal axis  15  of the Y junction splitter  11 , which may divide the input waveguide  13  into two symmetrical halves, and provide an axis of symmetry for the two output waveguides  16  and  17 . 
     Ideally, the Y-junction splitter  11  may be comprised of a silicon on insulator (SOI) waveguide, including a high index silicon core sandwiched between upper and lower cladding layers, comprised of lower index silica; however, other forms and semiconductor materials, e.g., high index cores with refractive indexes between 3 and 4, such as SiN or other Group III/V materials, sandwiched between lower index upper and lower cladding layers with refractive indexes between 1 and 2, such as nitride or other Group III/V materials, are within the scope of the invention. Typically, the input port  12  and the output ports  18  and  19  are optically coupled to external waveguides (not shown) for transmitting the optical beams to and from the Yjunction  11 . 
     The input waveguide  13  expands the input beam of light adiabatically from an input end to an output end, whereby the width of the output end of the input waveguide  13  supports two guided modes (see  FIG. 2 b   ), e.g., the fundamental mode TE 0  (or TM 0  depending on polarization) and the second order mode TE 1  (or TM 1  depending on polarization). There is no need to expand the input waveguide  13  to a width that would support more than two guided modes unless one seeks splitting in more than two output waveguides  16  and  17 . The expansion is smooth and long enough, such that light remains in the TE 0  mode (a.k.a. adiabatic taper) despite the presence of the second order mode. 
     At the JS  14 , a super or hybrid mode is supported, which is defined by an arrangement of waveguides that are in close proximity, such that they share the same optical mode. Ideally, the geometry of the super mode, to the right of the JS  14 , is optimized to achieve maximal overlap integral with the incoming mode of the input waveguide  13  just left to the JS  14  in order to define a mode-matched junction. (see  FIG. 2 c   ) A gap  21  between the first and second output waveguides  16  and  17  on the right-hand side of the JS  14  is typically chosen to be as small as the fabrication process allows, e.g. less than 500 nm, ideally between 50 nm and 300 nm. The gap  21  may be larger if coupling from another mode, other than the fundamental mode, is desirable. An important aspect is that the geometry of the super mode should be selected such that it maximizes the overlap integral between the incoming mode (left of JS  14 ) and the super mode (right of the JS  14 ). With reference to  FIG. 2 c   , the illustrated super mode extends across the gap  21  and into both the first and second output waveguides  16  and  17  on either side thereof, in contrast to  FIG. 1 c   , which illustrates two distinct modes in the conventional Y-junction splitter. 
     Accordingly, at the JS  14  the incoming mode, e.g. TE 0 , is on the left side and the goal is to engineer the dimensions, e.g. widths of the gap  21  and the initial sections of the output waveguides  16  and  17 , such that the incoming TE 0  mode excites, as much as possible, the new TE 0  super mode supported by the input waveguides  16  and  17 . Accordingly, the overall insertion loss of the splitter  11  is kept low by selecting a geometry at the JS  14 , such that almost all of the incoming light, e.g., TE 0  mode, propagates to the super TE 0  mode. This is achieved by first selecting a width of the gap  21  that is as small as possible, e.g., less than 500 nm, ideally between 50 nm and 300 nm, in order to have the super mode fully supported by the initial sections of the input waveguides  16  and  17 . Then the widths of at least the initial sections, e.g., up to the entire length, of the output waveguides  16  and  17 , are varied while tracking the overlap integral between the incoming TE 0  mode at the JS  14  and the super TE 0  after the JS  14 . Mode matching is achieved when the overlap between these two modes is maximized, resulting in a discontinuity between the input waveguide  13  and the initial sections of the output waveguides  16  and  17  that can look counter-intuitive. 
     Therefore, past the JS  14  in the initial sections of the output waveguides  16  and  17  most of the light is still confined in the super TE 0  mode. When it is in this state it is easy to move the spatial power distribution of the mode by changing the size of the output waveguides  16  and  17 . The gap  21  is not increased at least in the initial section in order to maintain the hybridization of the modes, i.e., the initial sections of the output waveguides  16  and  17  are close enough together such that, from the optical stand point, they form a single waveguide. 
     The mode-matching structure of the JS  14  between the input waveguide  13 , and the first and second output waveguides  16  and  17  is provided in order to achieve mode matching between the expanded single mode beam at the output of the input waveguide  13 , and the super mode at the input of the first and second output waveguides  16  and  17 . 
     Typically, the outer edge of a Y-junction is a smooth curve; however, in the illustrated example, the width of the input waveguide  13  is wider than the combined widths of the first and second output waveguides  16  and  17  and the gap  21 , resulting in a discontinuity in the form of stepped shoulders at the JS  14  on opposite sides thereof adjacent the first and second output waveguides  16  and  17 , each shoulder including a surface perpendicular to the direction of light propagation. An added advantage of a junction geometry that accomplishes mode matching is the resilience to fabrication processes that its stable maxima brings (e.g., deviation from the optimal geometry will result in added loss due to mode mismatch, but the added loss will be small around the maxima point.) In an exemplary embodiment, the input waveguide  13  may be 2700 nm wide, while the first and second output waveguides  16  and  17  may be 675 nm wide with a 300 nm gap therebetween, leaving a 525 nm shoulder, e.g., a surface perpendicular to the longitudinal axis  15  adjacent outer sides of the output waveguides  16  and  17 , providing the discontinuity. The shoulder may be wider than the gap  21 , but not as wide as the output waveguides  16  and  17 . The width at the output end of the input waveguide  13  is about twice as wide as the combined width of the first and second output waveguides  16  and  17 . 
     In an alternative embodiment, the mode-matching JS  14  includes a subwavelength gratings in the gap  21  alternative to or in combination with the aforementioned discontinuity. Utilizing a high resolution process, such as electron beam lithography, the subwavelength structures (metamaterial) can be provided to have a very fine grating (˜10× smaller than the effective wavelength guided in the input waveguide  13 ) This effectively increases the effective index of the gap  21 , which helps achieve mode matching to the incoming waveguide  13 . Such a variation would reduce the height of the shoulder width, which could increase the widths of the output waveguides to approach the full width of the input waveguide  13 . The subwavelength gratings could start with a high fill ratio in the gap  21  (more high index material than low in a given period of the grating) and adiabatically taper to a low fill ratio (more low index material than high) at the limit of which a full gap would open between the two output waveguides  16 ,  17 . 
     Another variation includes splitting the input beam into a plurality, e.g., more than two, output waveguides; whereby the same mode matching technique may be employed at the JS  14 . 
     With reference to  FIG. 3 , the Y-junction splitter  11  of the present invention comprises the input port  12  optically coupled to an input waveguide  13 , which gradually expands from an input end proximate the input port  12  to the junction split section  14  at an output end. The input waveguide  13  expands linearly or exponentially with a gradually decreasing slope. However, any other smooth taper shape may be used provided that it is long enough to allow adiabatic mode expansion. The expansion is by at least a factor of 2 or between 1.5 and 2.5, e.g., from a 1.2 μm input end to a 2.7 μm output end. An input beam of light is launched via the input port  12  from an external source, and propagates into the input waveguide  13  to the junction split (JS) section  14  along a longitudinal axis  15  and in the direction of propagation, and expands adiabatically to the output end of the input waveguide  13 . The input port  12  may be along the longitudinal axis  15  of the Y junction splitter  11 , which may divide the input waveguide  13  into two symmetrical halves, and may provide an axis of symmetry for the entire Y-junction splitter  11 . 
     The width of the input waveguide port  12  is preferably narrow enough to provide propagation of only a single mode of the input beam. The width of the input waveguide  13  expands providing adiabatic expansion in the core region to a width that supports two guided modes, e.g., the fundamental mode TE 0  (or TM 0  depending on polarization) and the second order mode TE 1  (or TM 1  depending on polarization). There is no need to expand the input waveguide  13  to a width that would support more than two guided modes unless one seeks splitting in more than two output waveguides  16  and  17 . The expansion is smooth and long enough, such that light remains in the TE 0  mode (a.k.a. adiabatic taper) despite the presence of the second order mode. 
     First and second output waveguides  16  and  17 , respectively, extend from the output end of the input waveguide  13  with a gap  21  therebetween, symmetrical about the longitudinal axis  15 , for receiving first and second portions of the input beam according to a desired splitting ratio, e.g.,  50 / 50 , and for outputting the first and second portions to output ports  18  and  19 , respectively. 
     As above, the structure of the mode-matching JS  14  provides that the width of the input waveguide  13  is wider than the combined widths of the first and second output waveguides  16  and  17  and the gap  21 , resulting in a mode-matching discontinuity, e.g., a stepped shoulders at the JS  14  on opposite sides thereof adjacent the first and second output waveguides  16  and  17 , each shoulder including a surface perpendicular to the direction of light propagation  15 . The shoulder may be wider than the gap  21 , but may not as wide as the output waveguides  16  and  17 . The width at the output end of the input waveguide  13  is about twice as wide as the combined width of the first and second output waveguides  16  and  17 . 
     Each of the first and second output waveguides  16  and  17  are divided into plurality of sections  36   a  to  36   d  and  37   a  to  37   d , respectively. Initial sections  36   a  and  37   a  may be straight sections parallel to the longitudinal axis  15  with a constant gap width, e.g., between 50 nm to 500 nm, but ideally less than 300 nm, therebetween, and each preferably having a constant width, e.g., 100 nm to 300 nm for Si/SiO2 or 500 nm to 800 nm for SiN/SiO2, enabling the input mode to stably transform into a super mode. The length of the initial sections  36   a  and  37   a  are typically less than or equal to one half the length of the input waveguide region  13  or in absolute terms preferably greater than 1 μm, preferably less than 20 μm, and preferably between 5 μm and 10 μm. 
     Accordingly, at the JS  14  the incoming mode, e.g., TE 0 , is on the left side and the goal is now to engineer the dimensions, e.g., gap and waveguide widths, of the initial sections of the output waveguides  36   a  and  37   a , such that the incoming TE 0  mode excites, as much as possible, the new TE 0  super mode supported by the initial sections  36   a  and  37   a . Accordingly, the overall insertion loss of the Y-junction splitter  11  is kept low by selecting a geometry at the JS  14 , such that almost all of the incoming light, e.g., TE 0  mode, propagates to the super TE 0  mode. This is achieved by first selecting a width of the gap  21  that is as small as possible, e.g., 50 nm to 300 nm, in order to have the super mode fully supported by the initial sections  36   a  and  37   a . Then the widths of the initial sections  36   a  and  37   a  are varied while tracking the overlap integral between the incoming TE 0  mode and the super TE 0  mode. Mode matching is achieved when the overlap between these two modes is maximized, resulting in a discontinuity between the input waveguide  13  and the initial sections  36   a  and  37   a  of the output waveguides  16  and  17 , respectively, that can look counter-intuitive. 
     Therefore, past the JS  14  in the initial sections  36   a  and  37   a  most of the light is still confined in the super TE 0  mode. When it is in this state it is easy to move the spatial power distribution of the mode by tapering the size of the initial sections  36   a  and  37   a . The gap  21  is not increased in the initial sections  36   a  and  37   a  in order to maintain the hybridization of the modes, i.e., the initial sections  36   a  and  37   a  of the output waveguides  16  and  17  are close enough together such that, from the optical stand point, they form a single waveguide. 
     Once the light is in the super mode, past the JS  14 , a V-splitter is used to split the mode into the two mode splitting sections  36   b  and  37   b . The best way of achieving this is using a V-shaped split, in contrast to a typical s-bend. The V-splitter shape constantly separates the two mode splitting sections  36   b  and  37   b  at an acute angle, e.g., 3° to 15°, from the longitudinal axis  35 , and linearly decouples the output waveguides  36  and  37 , which is found to be simpler and more efficient than an otherwise S-bend. Furthermore, the S-bend may result in high order mode coupling, if the bend radius is not large enough; which is avoided using a V-splitter. 
     Final expansion sections  36   c  and  37   c  gradually, e.g., linearly, expand the first and second output waveguides  16  and  17 , respectively, by a factor of approximately 1.3 to 2, e.g., from 500 nm to 800 nm to 700 μm to 1.6 μm. Ideally, the inner walls of the final expansion sections  36   c  and  37   c  extend coplanar, and are separated by the same angle as the mode splitting sections  36   b  and  37   b ; however, any structure is within the scope of the invention. Final expansion sections  36   c  and  37   c  are used in the V-splitter, once the gap between the input and output waveguide arms  16  and  17  is sufficiently large, such that it avoids having mode overlap. 
     Bend sections  36   d  and  37   d  redirect the ends of the first and second output waveguides  16  and  17 , respectively, to be back parallel with the longitudinal axis  15 , and at the same width as the end of the taper sections  36   c  and  37   c . At the output ports  18  and  19  of the Y-junction  11 , the output waveguides  16  and  17  are back to the initial width, e.g., for single mode propagation, so that the input and output ports  12 ,  18  and  19  have the same widths, e.g., the best waveguide width for low loss propagation. 
     With reference to  FIGS. 4 and 5 , another embodiment of the Y-junction splitter  41  of the present invention comprises an input port  42  optically coupled to an input waveguide  43 , which gradually expands from an input end proximate the input port  42  to a junction split section  44  at an output end. The input waveguide  43  expands linearly or exponentially with a gradually decreasing slope. The expansion is by at least a factor of 2 or between 1.5 and 2.5, e.g., from a 1.2 μm input end to a 2.7 μm output end. An input beam of light is launched via the input port  42  from an external source, and propagates into the input waveguide  43  to the junction split (JS) section  44  along a longitudinal axis  45  and in the direction of propagation, and expands adiabatically to the output end of the input waveguide  43 . First and second output waveguides  46  and  47 , respectively, extend from the output end of the input waveguide  43  for receiving first and second portions of the input beam according to a desired unbalanced splitting ratio, e.g., 25/75, and for outputting the first and second portions to output ports  49   a  and  49   b , respectively. 
     As above, with reference to  FIGS. 2 and 3 , the structure of the mode-matching JS  41  provides that the width of the input waveguide  43  is wider than the combined widths of the first and second output waveguides  46  and  47  and the gap  48 , resulting in a mode-matching discontinuity, e.g., stepped shoulders at the JS  44  on opposite sides thereof adjacent the first and second output waveguides  46  and  47 , each shoulder including a surface perpendicular to the direction of light propagation  45 . The shoulder may be wider than the gap, but not as wide as the output waveguides  46  and  47 . The width at the output end of the input waveguide  43  is about twice as wide as the combined width of the first and second output waveguides  46  and  47 . 
     In an alternative embodiment, the mode-matching JS  44  may include a subwavelength gratings in the gap  48  alternative to or in combination with the aforementioned discontinuity, as hereinbefore disclosed with reference to the JS  14 . 
     Each of the first and second output waveguides  46  and  47  are divided into a plurality of sections  46   a  to  46   d  and  47   a  to  47   d , respectively. Initial sections  46   a  and  47   a  form an unbalanced region, as section  46   a  comprises a straight sections parallel to the longitudinal axis  45  with a constant width, e.g., 100 nm to 300 nm for Si/SiO2 or 500 nm to 800 nm for SiN/SiO2, while section  47   a  comprises an initial expansion section, linearly expanding from the JS  44  to section  47   b . Ideally, the inner walls of the initial unbalanced sections  46   a  and  47   a  are separated by a constant gap width, e.g., between 50 nm to 500 nm, but ideally less than 300 nm, while the outer wall of the initial section  47   b  tapers away from the longitudinal axis  45 ; however, other arrangements are within the scope of the invention. The length of the initial sections  46   a  and  47   a  are typically between one half to two times the length of the Y-junction core region  43  or in absolute terms preferably at least 5 μm, preferably less than 20 μm, and preferably between 5 μm and 10 μm. 
     In order to create the unbalanced Y-junction  41 , an unbalanced region is used where light is confined in a hybrid mode or super mode. The unbalanced region is defined as a region where the width of one of the junction arms, i.e.,  47   a , is tapered, i.e., expands. This breaks the symmetry of the super mode adiabatically, which results in shifting the power distribution towards one arm or another (See  FIG. 5 ). Arbitrary splitting ratios can be achieved by properly selecting the un-balanced region&#39;s length and by tapering the symmetrical super mode to an asymmetrical with sought power distribution. 
     Once the light is in the super mode, past the JS  44 , a V-splitter is used to split the mode into the two mode splitting sections  46   b  and  47   b . The V-splitter shape constantly separates the two mode splitting sections  46   b  and  47   b  at an acute angle, e.g., 3° to 15°, from the longitudinal axis  45 , and linearly decouples the output waveguides  46  and  47 , which is found to be simpler and more efficient than an otherwise S-bend. Furthermore, the S-bend may result in high order mode coupling, if the bend radius is not large enough; which is avoided using a V-splitter. The V-splitter sections  46   b  and  47   b  have constant, but different widths, e.g.,  46   b  is 500 nm to 800 nm wide, while  47   b  is 700 μm to 1.6 μm wide. 
     Final expansion section  46   c  gradually, e.g., linearly, expands the first output waveguides  36  by a factor of approximately 1.3 to 2, e.g., from 500 nm to 800 nm to 700 μm to 1.6 μm, to equal the expansion provided by the initial expansion section  47   a . Ideally, the inner walls of the final expansion sections  46   c  and  47   c  extend coplanar, and are separated by the same angle as the mode splitting sections  46   b  and  47   b ; however, any structure is within the scope of the invention. Final expansion sections  46   c  and  47   c  are used in the V-splitter, once the gap between the input and output waveguide arms  46  and  47  is sufficiently large, such that it avoids having mode overlap. 
     Bend sections  46   d  and  47   d  redirect the ends of the first and second output waveguides  46  and  47 , respectively, to be back parallel with the longitudinal axis  45 , and at the same width as the end of the final expansion sections  46   c  and  47   c.    
     Lithographic rounding at the interfaces may be included to provide a smooth transition between sections. 
     The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.