Patent Publication Number: US-11662532-B2

Title: Coupling multi-channel laser to multicore fiber

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 16/940,528 filed Jul. 28, 2020. The aforementioned related patent application is herein incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     Embodiments presented in this disclosure generally relate to optical devices, and more specifically, to techniques for coupling a multi-channel laser to a multicore optical fiber. 
     BACKGROUND 
     To support increased bandwidth requirements, optical devices may include increasing numbers of optical channels. However, using single-channel optical fibers such as single-mode fiber (SMF) or polarization-maintaining fiber (PMF) for the multiple optical channels occupies a large volume for fiber management, as well as reduces the channel density at the fiber termination, which may require increased packaging size and/or may affect the spacing of components within packaging of a given size. 
     Multicore fibers can significantly reduce a fiber count within the packaging, and in some cases may have a same outer diameter as single-mode optical fibers. However, solutions for optical coupling with the optical cores of the multicore fiber, such as photonic light-wave circuits that fan-in to the relatively small pitch between the optical cores, may impose significant material and/or process costs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above-recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate typical embodiments and are therefore not to be considered limiting; other equally effective embodiments are contemplated. 
         FIGS.  1 A,  1 B, and  1 C  illustrate exemplary implementations of a multicore optical fiber, according to one or more embodiments. 
         FIG.  2    is a diagram illustrating coupling of a multicore optical fiber with a multi-channel laser die, according to one or more embodiments. 
         FIG.  3    is a diagram illustrating exemplary alignment of a multicore optical fiber, a lens, and a multi-channel laser die, according to one or more embodiments. 
         FIG.  4    is an exemplary method of forming an optical device, according to one or more embodiments. 
         FIG.  5    is an exemplary method of rotationally aligning a multicore optical fiber to align with a multi-channel laser die, according to one or more embodiments. 
         FIGS.  6 A,  6 B, and  6 C  illustrate an exemplary sequence of forming an optical device within a Transistor Outline (TO) CAN package, according to one or more embodiments. 
         FIGS.  7 A,  7 B, and  7 C  illustrate an exemplary sequence of forming an optical device within a box-type Transmitter Optical Sub-Assembly package, according to one or more embodiments. 
         FIG.  8    is a diagram illustrating an exemplary optical alignment of a multicore optical fiber, according to one or more embodiments. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially used in other embodiments without specific recitation. 
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     One embodiment presented in this disclosure is a method comprising arranging a laser die on a substrate. The laser die has multiple channels that are arranged with a first planar arrangement proximate to a facet of the laser die. The method further comprises aligning a single lens to the facet, and aligning a multicore optical fiber to the laser die through the single lens. The multicore optical fiber has a plurality of optical cores that are arranged with a second planar arrangement. Aligning the multicore optical fiber to the laser die comprises rotationally aligning the multicore optical fiber to align the second planar arrangement with the first planar arrangement. 
     Another embodiment presented in this disclosure is an optical device comprising a substrate and a laser die arranged on the substrate. The laser die has multiple channels that are arranged with a first planar arrangement proximate a facet of the laser die. The optical further comprises a single lens aligned to the facet, and a multicore optical fiber aligned to the laser die through the single lens. The multicore optical fiber has a plurality of optical cores that are arranged with a second planar arrangement. The multicore optical fiber is rotationally arranged such that the second planar arrangement is aligned with the first planar arrangement. 
     Another embodiment presented in this disclosure is a computer program product comprising a computer-readable storage medium having computer-readable program code embodied therewith. The computer-readable program code is executable by one or more computer processors to perform an operation comprising arranging a laser die on a substrate. The laser die has multiple channels that are arranged with a first planar arrangement proximate to a facet of the laser die. The operation further comprises aligning a single lens to the facet, and aligning a multicore optical fiber to the laser die through the single lens. The multicore optical fiber has a plurality of optical cores that are arranged with a second planar arrangement. Aligning the multicore optical fiber to the laser die comprises rotationally aligning the multicore optical fiber to align the second planar arrangement with the first planar arrangement. 
     EXAMPLE EMBODIMENTS 
     Solutions for optical coupling with a multicore optical fiber include photonic light-wave circuits, which fan-in from separate optical channels to the relatively small pitch between the cores of the multicore fiber. However, implementations of optical devices using photonic light-wave circuits may tend to occupy a relatively large volume, and/or may impose significant material and/or process costs. 
     In some embodiments, a method comprises arranging a laser die on a substrate. The laser die has multiple channels that are arranged with a first planar arrangement proximate to a facet of the laser die. The method further comprises aligning a single lens to the facet, and aligning a multicore optical fiber to the laser die through the single lens. The multicore optical fiber has a plurality of optical cores that are arranged with a second planar arrangement, e.g., arranged linearly within the planar arrangement. Aligning the multicore optical fiber to the laser die comprises rotationally aligning the multicore optical fiber to align the second planar arrangement with the first planar arrangement. 
     Beneficially, the relatively small pitch between the optical cores of the multicore fiber permits a single lens to optically couple all of the optical cores with the multiple channels of the laser die, providing suitable optical performance without imposing significant material and process costs. The relatively small pitch allows other components to be shared, e.g., a single optical isolator shared by the multiple channels. Further, using the single lens to optically couple the multiple channels negates a requirement for a minimum free space channel and/or channel pitch. 
       FIGS.  1 A,  1 B, and  1 C  illustrate exemplary implementations of a multicore optical fiber, according to one or more embodiments. More specifically,  FIG.  1 A  represents a cross-section view (or an end view) of a multicore optical fiber  100  comprising a plurality of optical cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  (also referred to herein as “cores”) and a cladding  110 . The cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  and the cladding  110  may be formed of any materials providing suitable refractive indices, as will be understood by the person of ordinary skill in the art. The cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  are arranged along a line within the plane depicted in  FIG.  1 A  (also referred to herein as the planar arrangement of the cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4 ). 
     Although the multicore optical fiber  100  includes four (4) cores in a linear arrangement, alternative numbers and/or alternative planar arrangements of the cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  are also contemplated. For example,  FIG.  1 B  represents a cross-section view (or an end view) of a multicore optical fiber  120  comprising seven (7) cores  105 - 1 ,  105 - 2 , . . . ,  105 - 7  in a star-shaped planar arrangement.  FIG.  1 C  represents a cross-section view (or an end view) of a multicore optical fiber  130  comprising eight cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  in a circular planar arrangement (e.g., where the cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  are evenly spaced with different radial angles). Other regular and/or irregular planar arrangements are also contemplated (e.g., rectangular). Further, in some embodiments the multicore optical fibers  100 ,  120 ,  130  may include one or more elements (e.g., stress rods) arranged relative to the cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  such that the multicore optical fibers  100 ,  120 ,  130  are polarization-maintaining multicore optical fibers. 
     The multicore optical fibers  100 ,  120 ,  130  may have any suitable dimensioning. In one embodiment, the multicore optical fibers  100 ,  120 ,  130  have an outer diameter d of about 125 microns, although other values are also contemplated. Generally, adjacent cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  may have any suitable spacing within the multicore optical fibers  100 ,  120 ,  130 . In some embodiments, a pitch p between adjacent cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  may be as large as permitted by the outer diameter d of the multicore optical fibers  100 ,  120 ,  130 , as larger spacing may be effective to mitigate interference between optical signals carried on the adjacent cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8 . In one embodiment, corresponding to the outer diameter d of about 125 microns, the pitch p between adjacent cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  may be in the tens of microns (e.g., between about 20 microns and about 30 microns). 
     When optically aligning any of the multicore optical fibers  100 ,  120 ,  130  with a multi-channel laser die, an angular alignment of the cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  (e.g., relative alignment of the cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  within the plane of the planar arrangement) may be performed using two or more of the cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  that are arranged in a line. Any suitable number of the cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  are contemplated. For example, four (4) cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  (shown as angular alignment group  115 ) may be used for angular alignment of the multicore optical fiber  100 , three (3) cores  105 - 2 ,  105 - 5 ,  105 - 7  (shown as angular alignment group  125 ) may be used for the multicore optical fiber  120 , and two (2) cores  105 - 3 ,  105 - 7  (shown as angular alignment group  140 ) may be used for the multicore optical fiber  130 . Notably, not all of the cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  arranged in a particular line need be used for angular alignment (e.g., selecting two or three of the cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  for the multicore optical fiber  100 ), and the selected ones of the cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  need not be adjacent to each other within the line. Further, the line in which the cores  105 - 1 ,  105 - 2 , . . . ,  105 - 8  are arranged need not pass through a center of the multicore optical fibers  100 ,  120 ,  130 , as with the angular alignment group  135  comprising the cores  105 - 1 ,  105 - 8 . 
       FIG.  2    is a diagram  200  illustrating coupling of a multicore optical fiber  205  with a multi-channel laser die  210  (also referred to herein as “laser die”), according to one more embodiments. The features illustrated in the diagram  200  may be used in conjunction with other embodiments. For example, the multicore optical fiber  205  may represent any of the multicore optical fibers  100 ,  120 ,  130  that are depicted in  FIGS.  1 A,  1 B,  1 C . Note that the various components and the relative arrangement of the components are not drawn to scale in the diagram  200 . 
     The laser die  210  may include any semiconductor-based laser. In some embodiments, the laser die  210  is formed using III-V material layers that are epitaxially grown on a substrate (e.g., a silicon substrate). The laser die  210  includes a plurality of channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4 , each of which is configured to generate and deliver optical energy. The optical energy delivered by the plurality of channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4  may have the same or differing wavelengths. The plurality of channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4  are arranged with a planar arrangement proximate to a facet  220  of the laser die  210 . The facet  220  of the laser die  210  may be formed using any suitable techniques, such as etching, mechanical sawing, surface grinding, and so forth. In one example, the channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4  are formed as optical waveguides that extend to the facet  220  (e.g., terminate at the facet  220 ). In another example, the optical waveguides extend close to the facet  220  (e.g., within a few microns), such that optical energy exiting the optical waveguides propagates partly through another semiconductor material (e.g., silicon) to exit the facet  220 . 
     In the diagram  200 , a single lens  225  and a single optical isolator  230  are arranged between the laser die  210  and the multicore optical fiber  205 . The lens  225  may be formed of any suitable materials and may have any suitable dimensioning. The lens  225  is aligned to the facet  220 . Optical energy exits the channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4 , which is shown in the diagram  200  respectively as optical signals  235 - 1 ,  235 - 2 ,  235 - 3 ,  235 - 4  (collectively or generically referred to as optical signal(s)  235 ). The optical energy is directed through the lens  225 , through the optical isolator  230 , and toward an endface  240  of the multicore optical fiber  205 . 
     To ensure that each of the cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  optically align with a respective one of the channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4 , the multicore optical fiber  205  may be rotationally aligned with the laser die  210 , such that the planar arrangement of the cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  is aligned with the planar arrangement of the plurality of channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4 . In some embodiments, a six (6)-axis alignment is performed to optically align the multicore optical fiber  205  with the laser die  210 . Notably, an angular alignment of the cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  would not be performed for aligning a single core optical fiber. In some embodiments, the planar arrangement of the cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  corresponds to a plane of the endface  240 . 
     The multicore optical fiber  205  may be rigidly attached with the laser die  210  when rotationally aligned (e.g., using an adhesive or welding process). Thus, when the multicore optical fiber  205  is rotationally aligned with the laser die  210 , the channel  215 - 1  provides the optical signal  235 - 1  to the core  105 - 1 , the channel  215 - 2  provides the optical signal  235 - 2  to the core  105 - 2 , and so forth. 
     In some embodiments, the pitch between adjacent cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  and/or the pitch between adjacent channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4  is small enough that some or all of the optical components may be shared between the cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  and/or the channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4 . As shown in the diagram  200 , the single lens  225  and the single optical isolator  230  are shared by all of the cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4  and the channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4 . In some embodiments, the pitch between adjacent channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4  is small enough that the offset of the channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4  from the optical axis of the lens  225  contributes only negligible aberration without degrading optical coupling, when compared with an on-axis optical system. 
     Using these techniques, packaging density may be increased as fewer optical fibers, optical isolators, lenses, and/or laser dies are needed when optically coupling the laser die  210  and the multicore optical fiber  205 . Further, material and/or process costs during manufacturing may be reduced as fewer components are used, which also corresponds to fewer optical alignment processes. 
     As discussed above, the various components and their relative arrangement are not drawn to scale in the diagram  200 . In some embodiments, and as shown in the diagram  200 , the pitch between adjacent channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4  is greater than the pitch between adjacent cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4 . However, in other embodiments, the pitch between adjacent channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4  may be less than or equal to the pitch between adjacent cores  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4 . 
     For example,  FIG.  3    is a diagram  300  illustrating an exemplary alignment of a multicore optical fiber, a lens, and a multi-channel laser die, where a pitch between adjacent channels is less than a pitch between adjacent cores. The features illustrated in the diagram  300  may be used in conjunction with other embodiments. For example, diagram  300  represents one possible implementation of the optical system shown in  FIG.  2   . 
     In the diagram  300 , an inset portion  305  shows the optical signals  235 - 1 ,  235 - 2 ,  235 - 3 ,  235 - 4  exiting the facet  220  of the laser die. The optical signals  235 - 1 ,  235 - 2 ,  235 - 3 ,  235 - 4  are incident on the lens  225  and directed toward the endface  240  of the multicore optical fiber. An inset portion  310  shows the optical signals  235 - 1 ,  235 - 2 ,  235 - 3 ,  235 - 4  being received at the endface  240 . Although not shown here, an optical isolator may be arranged between the lens  225  and the endface  240 . 
     The optical signals  235 - 1 ,  235 - 2 ,  235 - 3 ,  235 - 4  exit along a length of the facet  220  having a distance d 2 , and the optical signals  235 - 1 ,  235 - 2 ,  235 - 3 ,  235 - 4  are received along a length of the endface  240  having a distance d 1 . In some embodiments, the endface  240  of the multicore optical fiber and the facet  220  of the laser die are parallel, and adjacent ones of the optical signals  235 - 1 ,  235 - 2 ,  235 - 3 ,  235 - 4  are equidistant at the endface  240  and at the facet  220 . 
     The lens  225  provides a given magnification imaging the mode size of the optical signals  235 - 1 ,  235 - 2 ,  235 - 3 ,  235 - 4  exiting the facet  220  of the laser die onto the mode size of the cores at the endface  240 . In some embodiments, the magnification of the lens  225  is positive. 
     The magnification of the lens  225  also affects the pitch between adjacent cores of the optical fibers. For example, in a case where the lens  225  has a magnification M=3 the pitch between the optical waveguides of the laser die should be three (3) times narrower than the pitch of the cores in the multicore optical fiber. In some embodiments, the distance d 1  is between about 60-90 microns, which for an implementation having four (4) linearly arranged cores corresponds to a pitch between about 20-30 microns between adjacent cores. Other values of the distance d 1  are also contemplated. The distance d 2  may have any suitable value, e.g. based on the design of the optical system, including the magnification of the lens  225 . For example, the distance d 2  may be approximately 30 microns, which corresponds to an approximately 10 micron pitch between adjacent channels. 
     The size of the lens  225 , the spacing between the lens  225  and the multicore optical fiber, and the spacing between the lens  225  and the facet  220  may be selected based on the distances d 1 , d 2 . In some embodiments, aligning the multicore optical fiber to the laser die through the lens  225  comprises arranging the multicore optical fiber at a distance d 4  from the lens  225 . The distance d 4  is based on a magnification of the lens  225  and is selected to (i) match a mode size of the multiple channels to a mode size of the plurality of optical cores, and (ii) match a pitch between adjacent channels of the multiple channels to a pitch between adjacent cores of the plurality of optical cores. 
     In some embodiments, the distance d 4  between the lens  225  and the multicore optical fiber (i.e., the endface  240 ) is between about two (2) times and about five (5) times a distance d 3  between the lens  225  and the facet  220 . In one non-limiting example, the pitch between adjacent cores of the plurality of optical cores is between about 20 and 30 microns, the distance d 4  is about 3000 microns, and the second distance is about 1000 microns. For this combination of distances d 3 , d 4 , a relatively large aperture of the lens  225  is capable of supporting the multiple channels. When compared with the distances d 3 , d 4 , the relatively small offset of the channels  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4  from the optical axis of the lens  225  contributes only negligible aberration without degrading optical coupling, when compared with an on-axis optical system. 
       FIG.  4    is an exemplary method  400  of forming an optical device, according to one or more embodiments. The method  400  may be used in conjunction with other embodiments, e.g., to form the optical system shown in  FIG.  3   . 
     The method  400  begins at block  405 , where a multi-channel laser die is arranged on a substrate. The substrate may be formed of any suitable material, such as silicon or ceramic. In some embodiments, arranging the multi-channel laser die comprises epitaxially growing III-V material layers on the substrate. In other embodiments, the multi-channel laser die is attached to another substrate, and arranging the multi-channel laser die comprises attaching the substrates together (e.g., bonding, soldering). 
     At block  415 , a single lens is aligned to the facets of the multi-channel laser die either actively or passively. At block  425 , a multicore optical fiber is aligned to the multi-channel laser die through the single lens. 
     In some embodiments, aligning the multicore optical fiber to the multi-channel laser die comprises arranging the multicore optical fiber at a first distance from the single lens, wherein the first distance is based on a magnification of the single lens and is selected to (i) match a mode size of the multiple channels to a mode size of the plurality of optical cores, and (ii) match a pitch between adjacent channels of the multiple channels to a pitch between adjacent cores of the plurality of optical cores. 
     In some embodiments, aligning the multicore optical fiber to the multi-channel laser die comprises rotationally aligning the multicore optical fiber, which aligns a planar arrangement of a plurality of optical cores with a planar arrangement of multiple channels of the multi-channel laser die (block  430 ). 
     In some embodiments, the multicore optical fiber is rigidly attached with the multi-channel laser die when rotationally aligned, for example, by applying and curing an adhesive or welding process. The method  400  ends following completion of block  425 . 
       FIG.  5    is an exemplary method  500  of rotationally aligning a multicore optical fiber to align with a multi-channel laser die, according to one or more embodiments. The method  500  may be used in conjunction with other embodiments, e.g., within block  430  of  FIG.  4   . 
     The method  500  begins at block  505 , where first spatial coordinates are determined for the multicore optical fiber, at which a first channel has a maximum optical coupling with a first optical core. At block  515 , second spatial coordinates are determined for the multicore optical fiber, at which a second channel has a maximum optical coupling with a second optical core. The first spatial coordinates and/or the second spatial coordinates may be represented as two dimensions or three dimensions. In some embodiments, the first optical core and the second optical core are furthest from each other along a particular dimension. However, the first optical core and the second optical core may be selected according to any other suitable techniques. Further, in other embodiments, spatial coordinates may be calculated for more than two optical cores of the multicore optical fiber. 
     At block  525 , a rotational angle for the multicore optical fiber is determined using the first spatial coordinates and the second spatial coordinates. In some embodiments, the multicore optical fiber is rotated according to the rotational angle and/or spatially translated. In some embodiments, the multicore optical fiber is spatially translated to averaged spatial coordinates, e.g., at a midpoint between the first spatial coordinates and the second spatial coordinates. 
     At block  535 , it is determined that the rotation angle corresponds to an optical coupling, for at least one of the plurality of optical cores, that is less than a threshold value. In some embodiments, the determination is responsive to measurements of test optical signals transmitted from the multi-channel laser die. Generally, the optical coupling being less than the threshold value indicates that an unsuitable optical coupling exists for the at least one optical core. At block  545 , different spatial coordinates are determined for one or both of the first channel and the second channel. The method  500  may return to block  525  using the different spatial coordinates, and may proceed until the determined rotational angle corresponds to a suitable optical coupling for each of the optical cores. In some embodiments, determining the suitable optical coupling comprises determining a spatial balance between the first spatial coordinates and the second spatial coordinates (e.g., if not aligning exactly with the pitch of the multicore optical fiber). The method  500  ends following block  545 . 
       FIGS.  6 A,  6 B, and  6 C  illustrate an exemplary sequence of forming an optical device within a Transistor Outline (TO) CAN package, according to one or more embodiments. The features illustrated in diagrams  600 ,  615 , and  625  may be used in conjunction with other embodiments. For example, a process of assembling the TO CAN package may operate to optically align the components of the optical system shown in  FIG.  2   . Note that the various components and the relative arrangement of the components are not drawn to scale in the diagrams  600 ,  615 , and  625 . 
     In the diagram  600 , the laser die  210  is mounted to a base  605  (representing a first housing component) and conductively connected with leads  610  providing external connectivity to the TO CAN package. The base  605  may be formed of any suitable material, such as a metal. In some embodiments, the laser die  210  is arranged on a substrate, which may operate as a submount that is attached to the base  605  (i.e., arranged on the first housing component). In some embodiments, the submount provides electrical connections between the laser die  210  and the leads  610 . 
     In the diagram  615 , a cap  620  (representing a second housing component) is contacted to the base  605 , such that the laser die  210  is arranged in an interior space formed by the base  605  and the cap  620 . The cap  620  may be formed of any suitable material, such as a metal. The lens  225  is arranged at an opening of the cap  620 . As discussed above, the lens  225  may have a positive magnification that images the mode size of the optical signals  235  exiting the laser die  210  onto the mode size of the cores of the multicore optical fiber  205 . By translating the cap  620  relative to the base  605 , the lens  225  may be aligned to a facet of the laser die  210  in two spatial dimensions. Once the lens  225  is aligned to the facet, the cap  620  may be rigidly attached to the base  605  (e.g., through welding). 
     In the diagram  625 , a surface  640  of an optical connector  635  is contacted to a surface  630  of the cap  620 . The optical connector  635  may have any suitable implementation, such as a fiber pigtail or a receptacle. The multicore optical fiber  205  is rigidly attached to the optical connector  635 . In some embodiments, the multicore optical fiber  205  is inserted into an interior space  645  of the optical connector  635 , and retained by the optical connector  635  using any suitable means, such as an adhesive, a friction fit, and so forth. 
     In some embodiments, the optical isolator  230  is arranged in the interior space  645  and is aligned with the multicore optical fiber  205  when inserted. By translating and/or rotating the optical connector  635  relative to the cap  620 , the multicore optical fiber  205  may be aligned, through the optical isolator  230  and the lens  225 , to the facet of the laser die  210  in three spatial dimensions (and rotationally). Once the multicore optical fiber  205  is aligned to the facet, the optical connector  635  may be rigidly attached to the cap  620  (e.g., through welding). In this way, aligning the multicore optical fiber  205  to the laser die  210  comprises attaching the optical connector  635  with a housing component (e.g., attached with the base  605  through the cap  620 ). 
       FIGS.  7 A,  7 B, and  7 C  illustrate an exemplary sequence of forming an optical device within a box-type Transmitter Optical Sub-Assembly (TOSA) package, according to one or more embodiments. The features illustrated in diagrams  700 ,  725 , and  730  may be used in conjunction with other embodiments. For example, a process of assembling the box-type TOSA package may operate to optically align the components of the optical system shown in  FIG.  2   . Note that the various components and the relative arrangement of the components are not drawn to scale in the diagrams  700 ,  725 , and  730 . 
     In the diagram  700 , the box-type TOSA package comprises sidewalls  705 , an interior surface  710  arranged within the sidewalls  705 , and a base  715  attached to the sidewalls  705 . The box-type TOSA package may be formed of any suitable materials. For example, the sidewalls  705 , the interior surface  710 , and/or the base  715  may be formed from metals such as a nickel-cobalt ferrous alloy, cold-rolled steel, or a copper-tungsten composite. In some embodiments, exterior surfaces of the sidewalls  705  may be gold-coated. 
     An opening  720  is defined through one of the sidewalls  705 . Although not depicted, the box-type TOSA package may further comprise conductive leads attached to the base  715  and providing external connectivity to the box-type TOSA package. 
     The laser die  210  is mounted to the interior surface  710  and is conductively connected with the leads providing external connectivity to the box-type TOSA package. In some embodiments, the laser die  210  is arranged on a substrate, which may operate as a submount that is attached to the interior surface  710 . In some embodiments, the submount provides electrical connections between the laser die  210  and the leads. 
     The lens  225  is arranged within the interior space defined by the sidewalls  705 , at the opening  720 . As discussed above, the lens  225  may have a positive magnification that images the mode size of the optical signals  235  exiting the laser die  210  onto the mode size of the cores of the multicore optical fiber  205 . By translating the lens  225  relative to the laser die  210 , the lens  225  may be aligned to a facet of the laser die  210  in three spatial dimensions. Once the lens  225  is aligned to the facet, the lens  225  may be rigidly attached to a structure within the interior space defined by the sidewalls  705 . 
     In the diagram  730 , a surface  740  of the optical connector  635  is contacted to a surface  735  of one of the sidewalls  705 . By translating and/or rotating the optical connector  635  relative to the surface  735 , the multicore optical fiber  205  may be aligned, through the optical isolator  230  and the lens  225 , to the facet of the laser die  210  in three spatial dimensions (and rotationally). Once the multicore optical fiber  205  is aligned to the facet, the optical connector  635  may be rigidly attached to the sidewalls  705  (e.g., through welding). In this way, aligning the multicore optical fiber  205  to the laser die  210  comprises attaching the optical connector  635  with a housing component (e.g., attached with the sidewalls  705 ). 
       FIG.  8    is a diagram  800  illustrating an exemplary optical alignment of a multicore optical fiber  205 , according to one or more embodiments. The features illustrated in  FIG.  8    may be used in conjunction with other embodiments, e.g., an exemplary connection of the multicore optical fiber  205  with the optical connector  635  as shown in  FIGS.  6 C and  7 C . 
     In the diagram  800 , a ferrule  805  surrounds the multicore optical fiber  205  and ensures alignment of the multicore optical fiber  205  during connector mating. The ferrule  805  may be formed of any material having suitable rigidity, such as ceramic, stainless steel, plastic, or tungsten carbide. The ferrule  805  and the multicore optical fiber  205  may be rigidly attached to each other using any suitable techniques, such as adhesive or crimping. In some cases, an end of the ferrule  805  may be polished after rigidly attaching the multicore optical fiber  205 , e.g., to provide an improved optical interface. 
     Once rigidly attached with the multicore optical fiber  205 , the ferrule  805  is inserted into a split sleeve  810 . A connector body  815  surrounds the assembly of the multicore optical fiber  205 , the ferrule  805 , and the split sleeve  810 . In some embodiments, the ferrule  805  may be permitted to rotate within the connector body  815  to support the angular alignment of the cores of the multicore optical fiber  205 . In some embodiments, one or more additional optical components may be housed within the connector body  815 . 
     Various techniques have been described for optical coupling a multi-channel laser die with a multicore optical fiber, which may include a polarization-maintaining multicore optical fiber. In some embodiments, a single lens is used to couple with multiple channels of the laser die, which eliminates a requirement for a minimum free space channel or channel pitch. Having a narrower channel pitch permits a single optical isolator to be shared by all channels. 
     Certain types of packaging provide relatively large working distances (e.g., about 1 mm object distance from the laser die to the lens, and about 3-5 mm image distance from the lens to the multicore optical fiber, in a TO CAN package), which corresponds to a large lens aperture that supports multiple channels. Although some or all of the multiple channels have an offset from an optical axis of the optical system, the relatively large working distances contribute only negligible aberration without degrading optical coupling, when compared with an on-axis optical system. 
     Further, optical coupling a multi-channel laser die with a multicore optical fiber may leverage existing assembly processes for single-channel optical coupling. For example, the laser die may be arranged on a substrate, a lens is actively or passively aligned in two dimensions (e.g., placing a cap including the lens), and an active fiber pigtail or receptacle is aligned in three dimensions and actively rotationally aligned to align the planar arrangements of the channels of the laser die and the cores of the multicore fiber. In some cases, the same materials and assembly equipment may be used as for single-channel optical coupling. 
     One exemplary application of the techniques described herein is assembling a multi-channel directly-modulated or continuous-wave laser package. Current packaging techniques may include the entire transmitter inside an expensive hermetic package, although only the laser source requires temperature regulation. In contrast, the techniques described herein enable a single small and inexpensive laser package that supports all of the laser channels. The package can be efficiently wavelength-stabilized with a single thermoelectric cooler that cools only the laser source. With the increase in packaging density, all of the laser channels may fit easily into existing transceiver module form factors. 
     In the preceding, reference is made to embodiments presented in this disclosure. However, the scope of the present disclosure is not limited to specific described embodiments. Instead, any combination of the described features and elements, whether related to different embodiments or not, is contemplated to implement and practice contemplated embodiments. Furthermore, although embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the scope of the present disclosure. Thus, the preceding aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). 
     Aspects of the present disclosure are described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments presented in this disclosure. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The flowchart and block diagrams in the Figures illustrate the architecture, functionality and operation of possible implementations of systems, methods and computer program products according to various embodiments. In this regard, each block in the flowchart or block diagrams may represent a module, segment or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     In view of the foregoing, the scope of the present disclosure is determined by the claims that follow.