Patent Publication Number: US-9419718-B2

Title: Aligning optical components in a multichannel receiver or transmitter platform

Description:
TECHNICAL FIELD 
     Embodiments presented in this disclosure generally relate to passively aligning optical components used for muxing/demuxing a multi-wavelength optical signal. More specifically, embodiments disclosed herein dispose the optical components onto a substrate with pre-fabricated cavities. 
     BACKGROUND 
     The cost of Receiver Optical Sub-Assemblies (ROSA) and Transmitter Optical Sub-Assemblies (TOSA) to a large extent is affected by the cost of packaging. The packaging cost in turn is often driven by the need to actively align the optical components within the ROSA/TOSA with high precision and within tight tolerances. Actively aligning these components also affects the cost of the manufacturing equipment, overall quality, yield, and manufacturability. 
     Multi-wavelength optical sub-assemblies are typically based upon demultiplexing (in the case of a ROSA) and multiplexing (in the case of a TOSA) using thin film filters (TFF) and mirrors to achieve wavelength separation or combination. However, the filters and mirrors require high precision optical alignments through active tuning. Such high precision active alignment increases assembly time and cost. The growth of data centers has increased the demand for cheaper and more compact optical sub-assemblies. 
    
    
     
       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 only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
         FIG. 1  illustrates demultiplexing a multi-wavelength optical signal, according to one embodiment described herein. 
         FIG. 2  illustrates multiplexing optical signals into a multi-wavelength optical signal, according to one embodiment described herein. 
         FIG. 3A  is a sub-mount with cavities for optical components, according to one embodiment described herein. 
         FIGS. 3B-3C  are sub-mounts with aligned optical components, according to one embodiment described herein. 
         FIGS. 4A-4C  illustrate assembling an optical receiver, according to one embodiment described herein. 
         FIG. 5  is a sub-mount with an integrated groove, according to one embodiment described herein. 
         FIGS. 6A-6B  illustrate assembling an optical transmitter, according to one embodiment described herein. 
         FIG. 7  is a method for passively aligning optical components in a multiplexer/demultiplexer, according to one embodiment described herein. 
         FIG. 8  is a wafer including a plurality of optical sub-assemblies, according to one embodiment described herein. 
     
    
    
     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 utilized on other embodiments without specific recitation. 
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     One embodiment presented in this disclosure is a method that includes disposing a mirror into a first cavity such that respective surfaces of the mirror and first cavity are brought into contact and urge the mirror and first cavity into a first predetermined relationship with one another. The method also includes disposing an optical filter into a second cavity such that respective surfaces of the optical filter and second cavity are brought into contact and urge the optical filter and second cavity into a second predetermined relationship with one another. Furthermore, the first and second cavities are arranged relative to each other so that disposing the mirror and optical filter cause the mirror and the optical filter to be passively aligned to perform one of a demultiplexing and multiplexing function using an optical signal incident on the optical filter. 
     Another embodiment presented in this disclosure is an optical device that includes a substrate with a first cavity and a second cavity, each extending from a same surface into the substrate. The device includes a mirror disposed in the first cavity where respective surfaces of the mirror and first cavity are in contact and arrange the mirror and first cavity in a first predetermined relationship with one another. The device also includes at least one optical filter disposed in the second cavity where respective surfaces of the optical filter and second cavity are in contact and arrange the optical filter and second cavity in a second predetermined relationship with one another. Furthermore, the first and second predetermined relationships passively align the optical filter and mirror to perform one of a demultiplexing and multiplexing function using an optical signal incident on the optical filter. 
     Another embodiment described in this disclosure is an optical device with a sub-mount that includes a first substrate comprising a first cavity and a second cavity, each extending from a same surface into the first substrate. The sub-mount also includes a mirror disposed in the first cavity, where respective surfaces of the mirror and first cavity are in contact and arrange the mirror and first cavity in a first predetermined relationship with one another. The sub-mount includes at least one optical filter disposed in the second cavity, wherein respective surfaces of the optical filter and second cavity are in contact and arrange the optical filter and second cavity in a second predetermined relationship with one another. Furthermore, the first and second predetermined relationships passively align the optical filter and mirror to perform one of a demultiplexing and multiplexing function using an optical signal incident on the optical filter. The optical system includes a second substrate with a third cavity where respective surfaces of the first substrate and the third cavity are in contact and arrange the first substrate and third cavity in a third predetermined relationship with one another. The second substrate also includes an optical component configured to one of receive a single-wavelength optical signal from the optical filter and transmit the single-wavelength optical signal to the optical filter. 
     Example Embodiments 
     In a ROSA, a received multi-wavelength optical signal may be demultiplexed into a plurality of optical signals with respective wavelengths. Stated differently, the various wavelengths in the received optical signal are separated into different optical signals with different wavelengths. In a TOSA, multiple optical signals with respective wavelength may be multiplexed into a single multi-wavelength optical signal. In one embodiment, these optical sub-assemblies include a plurality of optical filters that is aligned with a mirror to perform the multiplexing/demultiplexing function. The embodiments herein disclose optical sub-assemblies where the optical components performing the multiplexing/demultiplexing function are aligned passively rather than actively. 
     In one embodiment, a sub-mount is etched to include respective cavities that each include at least two adjacent sides for aligning the optical filters and mirror. Moreover, the cavities are arranged on the sub-mount such that when the filters and mirror are disposed in the cavities and contact the two adjacent sides, they align in a manner that enables the multiplexing/demultiplexing function. That is, the filters and mirrors are aligned passively rather than actively where a technician must tune the sub-assemblies. The sub-mount may then be placed on a substrate that includes other components of the ROSA or TOSA. In one embodiment, the substrate is also etched to include a cavity with at least two adjacent sides for passively aligning the sub-mount with optical components disposed on the substrate. 
       FIG. 1  illustrates an optical system  100  for demultiplexing a multi-wavelength optical signal, according to one embodiment described herein. Generally, the system  100  uses a zigzag demultiplexer to separate a multi-wavelength optical signal  110 —i.e., a signal comprising of plurality of wavelengths—into a plurality of output optical signals with different wavelengths. In the embodiment shown, the optical signal  110  comprises four different wavelengths (also referred to as channels) which are separated into the four different output optical signals  121 ,  122 ,  123 , and  124 . 
     The demultiplexing system  100  includes an optical source  105 , a mirror  120 , a plurality of optical filters  115 , lenses  125 , and receivers  130 . The optical source  105  may be an optical fiber, collimator, lens, etc. that transmits the multi-wavelength optical signal  110  in a direction towards the optical filter  115 A. Because there are four different wavelengths in the optical signal  110 , the system  100  includes four different optical filters  115  with respective pass bands for different wavelengths. Specifically, the optical filters  115  may each permit a different range of wavelengths from passing there through but reflects all other wavelengths. As such, the optical filters  115  may be made from a different material or composition, and thus, have different pass bands. For example, optical filter  115 A permits a different range of wavelengths to pass than optical filters  115 B,  115 C, and  115 D. However, the pass band of the optical filters  115  may overlap. In one embodiment, the optical filters may include thin-film-filters. 
     As shown, the optical signal  110  strikes optical filter  115 A which permits one of the wavelengths of the signal  110  to pass through the filter  115 A to generate the first output signal  121 . That is, optical filter  115 A has a pass band range that includes only one of the four wavelengths in the optical signal  110 . Thus, the optical energy with this wavelength passes through the material of the optical filter  115 A, while the other three wavelengths are reflected towards the mirror  120 . The mirror  120  and the optical filters  115  are aligned such that the mirror  120  reflects the remaining three wavelengths of optical signal  110  to the optical filter  115 B which permits only one of the remaining three wavelengths to pass. The optical energy at this wavelength passes through the filter  115 B to generate the second output signal  122 . Notably, the pass band of optical filter  115 B does not need to exclude the wavelength that was removed to generate the first output signal  121  since this wavelength was already separated from the optical signal using filter  115 A. 
     The two remaining wavelengths in optical signal  110  are reflected by filter  115 B to mirror  120  which then reflects the signal  110  to optical filter  115 C. Filter  115 C has a pass band that permits only one of the two remaining wavelengths to pass through the filter  115 C to generate the third output signal  123 . The remaining wavelength is reflected off filter  115 C onto the mirror  120  and onto the optical filter  115 D which has a pass band that permits the last remaining wavelength to pass to generate the fourth output optical signal  124 . Although it is not necessary to have the final optical filter  115 D, it may be preferred to ensure that undesired wavelengths are not then transmitted to other stages in the system  100  and to maintain the same offset as the other three output signals  121 ,  122 , and  123 . In this manner, the system  110  demultiplexes the multi-wavelength optical signal  110  into four different output signals  121 ,  122 ,  123 , and  124  with four different wavelengths. 
     These output signals then propagate through respective lenses  125  and receivers  130 . In one embodiment, the receivers  130  are detectors (e.g., photo diodes) that convert the optical energy in the output optical signals  121 ,  122 ,  123 , and  124  into electrical signals. In another embodiment, the receivers  130  are waveguides such as four optical fibers. 
     Moreover, although this disclosure refers to the output signals  121 ,  122 ,  123 , and  124  having respective wavelengths, in some embodiments these output signals may include a range of respective wavelengths. However, in one embodiment, the range of the wavelengths of the output signals  121 ,  122 ,  123 , and  124  may be non-overlapping—i.e., unique. 
       FIG. 2  illustrates an optical system  200  for multiplexing optical signals into a multi-wavelength optical signal, according to one embodiment described herein. Generally, the system  200  uses a zigzag multiplexer to combine (i.e., multiplex) four optical signals  210  with different wavelengths into a multi-wavelength optical signal  225 . The four optical signals  210  may considered as four data channels that are then combined to generate optical signal  225 . Of course, although  FIGS. 1 and 2  illustrate four channel optical systems, the embodiments herein may be used when multiplexing or demultiplexing any number of channels. 
     As shown, optical system  200  includes light sources  205 , optical filters  115 , optical mirror  120 , lens  230  and optical cable  235 . The light sources  205  may be modulated lasers, collimators, optical cables, and the like which respectively output the signals  210 . As discussed above, each of the optical signals  210  include a wavelength (or range of wavelengths) different from the other optical signals  210 . Moreover, by reversing the demultiplexing process described above using the optical filters  115  and the mirror  120 , the optical system  200  can multiplex the four optical filters into the multi-wavelength signal  225 . 
     Starting from the bottom, optical signal  210 D outputted from source  205 D propagates through optical filter  115 D. That is, the material of filter  115 D has a pass band that permits the wavelength of signal  210 D to pass through while reflecting wavelengths outside of this band. Thus, if optical signal  210 D includes an optical signal with a wavelength outside of the pass band, this signal is reflected while only the optical energy with wavelengths in the pass band continues through the filter  115 D and strikes the mirror  120 . 
     Optical signal  210 C outputted from light source  205 C is filtered by optical filter  115 C such that only light within its pass band is permitted to pass through to mirror  120 . In addition, the optical signal outputted from the optical filter  115 D (which is reflected by mirror  120 ) strikes the right side of optical filter  115 C, but because the optical signal is outside the pass band of filter  115 C, it is reflected and combined with the optical signal  210 C. Thus, the optical signal propagating from optical filter  115 C to mirror  120  includes both optical signal  210 D and optical signal  210 C. 
     Optical signal  210 B outputted from light source  205 B strikes optical filter  115 B, and assuming the signal  210 B is within its pass band, passes through filter  115 B and is outputted on its right side. Again, because the optical signal  210 C and  210 D are outside the pass band of optical filter  115 B, these optical signals are reflected back towards the mirror  120  along with the optical signal  210 B. Thus, on the right side of optical filter  115 B, the optical signals  210 B,  210 C, and  210 D have been combined—i.e., multiplexed. 
     Lastly, optical signal  210 A outputted from light source  205 A strikes filter  115 A, and assuming signal  210 A is within its pass band, passes through the filter  115 A and is outputted on its right side where signal  210 A is combined with optical signals  210 B,  210 C, and  210 D. That is, the respective wavelengths of the signals  210 B,  210 C, and  210 D are outside of the pass band of optical filter  205 A, and thus, are reflected upon striking the right side of the filter  205 A and combined with optical signal  210 A to generate the multi-wavelength optical signal  225 . Signal  225  then passes through lens  230  and is introduced into an optical cable  235  or any other type of waveguide or detector. 
     To achieve the demultiplexing and multiplexing functions shown in  FIGS. 1 and 2 , the optical filters  115  and mirror  120  are arranged according to a defined orientation relative to each other. For example, in optical system  100 , the arrangement between the optical filters  115  and the mirror  120  is such that the optical signal reflected from the left side of the filters  115  is reflected by the mirror  120  onto the next adjacent optical filter  115  so the optical signal  110  can be separated into its different wavelengths. Similarly, in optical system  200 , filters  115  and mirror  120  are arranged so that optical signals passing through a lower filter  115  is reflected by the mirror  120  onto the right side of the next adjacent optical filter  115  and combined to form the multi-wavelength optical signal  225 . 
     In one embodiment, the optical filters  115  and mirror  120  are aligned passively rather than actively. As will be described below, a substrate includes alignment features that cause the filters  115  and mirror  120  to align passively as they are disposed onto the substrate. In contrast, an active alignment technique may require a technician to iteratively adjust and test the optical system to determine when the filters  115  and mirror  120  are aligned. Aligning the components actively can take a technician hours to perform, which adds substantial cost to any component that includes optical system  100  or  200 . 
       FIG. 3A  is a sub-mount  300  with cavities for optical components, according to one embodiment described herein. As shown, sub-mount  300  includes cavity  305  and a plurality of cavities  310 . In one embodiment, cavity  300  is etched into the sub-mount  300  with width (W) and length (L) dimensions that substantially match the dimensions of a mirror. As used herein, dimensions that “substantially match” does not necessarily mean the dimensions are exactly the same but rather that the dimensions are close enough such that when a cavity is mated with a component (e.g., the mirror) the position of the mirror is fixed. That is, in this embodiment, the mirror fits snugly into the cavity  300  so that mirror adopts the orientation of the cavity  300 . Moreover, although sub-mount  300  includes a unitary cavity  305  for the mirror, if the mirror is segmented, sub-mount  300  may a respective cavity for each of the segments. 
     Similarly, the plurality of cavities  310  may have dimensions that substantially match the dimensions of the optical filters so that the filters adopt the orientation of the cavities  310 . Thus, the cavities  305  and  310  may be formed on the sub-mount  300  in such a manner that when the mirror  120  and optical filters  115  are placed within the cavities as shown in  FIG. 3B , the filters  115  and mirror  120  are aligned to perform the demultiplexing or multiplexing functions discussed in  FIGS. 1 and 2 . That is, the mirror  120  and filters  115  are aligned passively. In one embodiment, the position and/or orientation of the mirror  120  and filters  115  are not adjusted by a technician during an active tuning process. 
     The material of the sub-mount  300  may be a semiconductor (e.g., silicon), ceramic, or a circuit embedded in plastic or polymer. In one embodiment, the sub-mount  300  may be an interposer or an optical bench with multiple layers of metallization for routing signals through the sub-mount  300 . However, the material of the sub-mount  300  is not limited to the materials mentioned above. Instead, the sub-mount  300  may include any material for which there are fabrication techniques that permit forming the cavities  310  and  305  with enough precision to substantially match the dimensions of the optical filters  115  and mirror  120  to enable passive alignment. 
     In one embodiment, the length and width of the cavity  305  may range from 100 nm up to tens of microns. More specifically, the length and width may range from 1 micron to 5 microns. In one embodiment, the length and width of each of the cavities  310  may range between 100 nm up to tens of microns. More specifically, the length and width of the cavities  310  may be between 0.5 microns and 5 microns. In one embodiment, the length and width of the sub-mount  300  may range from 1 micron to hundreds of microns. More specifically, the length and width of the sub-mount  300  may range from 1 micron to tens of microns. 
       FIG. 3C  illustrates cavities  305  and  310  with dimensions that do not substantially match the dimensions of the filters  115  or mirror  120 . In this embodiment, the cavities  305  and  310  include at least two adjacent sides that are fabricated to passively align the mirror  120  or filters  115 . For example, a filter  115  is placed in one of the cavities  310  and one corner of the filter  115  is mated with the corner formed by the two adjacent sides of the cavity  310 . By aligning a corner of the filter  115  to the corner of the cavity  310  formed by the two adjacent sides, the filter  115  is passively aligned. Stated differently, the cavities  305  and  310  each include at least two adjacent sides that are selected to form at least one an alignment corner  320 . By contacting to two sides on the filter  115  or mirror  120  with the two adjacent sides, the optical components are passively aligned. As such, the cavities  305  and  310  may have dimensions that do not substantially match the dimensions of the filters  115  and mirror  120  and still achieve passive alignment. For instance, the width and length of the cavities  305  and  310  may exceed the width and length of the surfaces of the filters  115  and mirror  120  disposed in the cavities and achieve passive alignment by aligning a corner of the filters  115  and mirror  120  into a corner defined by two adjacent sides of the cavity that is designed to align the optical components to perform a multiplexing or demultiplexing function. 
     Although not shown, the two adjacent sides selected to form the alignment corners  320  may include alignment features such as bumps that extend from the sides to contact and align the optical filters  115  and mirror  120 . In one embodiment, a corner of the optical filter  115  or mirror  120  mates with the alignment corner  320  but this is not a requirement. For example, the corners of the filters  115  or mirror  120  may be chamfered, and thus, would not directly contact the alignment corners  320  formed by the two adjacent sides of the cavities  305  and  310 . 
     Furthermore, the cavities  305 ,  310  illustrated in  FIGS. 3A-3C  can be any shape with any number of sides so long as at least two sides are selected to register with respective surfaces of the filters  115  or mirror  120  in order to align the optical components to perform a demultiplexing or multiplexing function. In addition, the cavities  305 ,  310  do not have to form corners where two sides intersect but can form any feature that permits the sides of the cavities to register with respective surfaces of the filters  115  or mirror  120  such that the desired alignment is achieved. 
       FIGS. 4A-4C  illustrate assembling an optical receiver, according to one embodiment described herein. In one embodiment, the optical receiver is a component in a ROSA. As shown in  FIG. 4A , the receiver includes a substrate  400 , electrical integrated circuit (IC)  405 , a detector array  410  of photodiodes, and a cavity  415 . In previous fabrication steps, the IC  405  and detector array  410  are mounted onto the substrate  400 . In one example, the substrate  400  includes one or more metallization layers that provide signal communication between the detector array  410  and the IC  405 . In addition, the cavity  415  is etched into the substrate  400 . In one embodiment, the cavity  415  has width and length dimensions that substantially match the dimensions of the sub-mount  300  illustrated in  FIGS. 3A-3B . As such, when the sub-mount  300  is disposed in the cavity  415  as shown in  FIG. 4B , the sub-mount  300  and the components disposed thereon are passively aligned with one or more components mounted on the substrate  400 —e.g., the detector array  410 . Alternatively, the cavity  415  may have dimensions that do not substantially match the dimensions of the sub-mount  300  but may include two adjacent sides that are selected to form an alignment corner. The sub-mount  300  may be disposed into the cavity  415  and arranged such that respective sides of the sub-mount  300  contact the two adjacent sides which passively aligns the optical components of the sub-mount  300  to one or more optical components disposed on the substrate  400 . 
     In one embodiment, the demultiplexing function performed by the optical filters and mirror on the sub-mount  300  is used to separate a received optical signal that includes four different wavelengths into four optical signals, each comprising one of the four wavelengths. As shown in  FIG. 4C , a lens array  420  is disposed between the optical filters on the sub-mount  300  and the detector array  410 . The array  420  may include four individual lenses each aligned with a respective one of the optical filters. The lenses focus the four optical signals passing through the optical filters onto the photodiodes in the detector array  410 . The photodiodes convert the optical signals into respective electrical signals that may then be amplified using, for example, a transimpedance amplifier that is either located on IC  405  on elsewhere on the substrate  400  proximate to the detector array  410 . The amplified electrical signals may then be processed by the IC  405  and transmitted to other parts of the optical receiver. Eventually, the electrical signals may be converted into data signals that are sent to an external computing device (e.g., a server). 
       FIG. 5  is a sub-mount  500  with an integrated groove  505 , according to one embodiment described herein. Sub-mount  500  differs from sub-mount  300  in  FIG. 3  in that sub-mount  500  includes the integrated groove  505  for mounting a light source onto the sub-mount  500 . The groove  505  may be arranged such that an optical source placed in the groove  505  is aligned with the optical filter placed in the bottommost one of the cavities  310 . Thus, when the optical source transmits a multi-wavelength optical signal, the demultiplexing function shown in  FIG. 1  is performed. Moreover, the integrated groove  505  permits the optical source to be aligned passively. That is, disposing the optical source into the groove automatically aligns the optical source to the bottom most optical filter. In one embodiment, a technician does not need to actively adjust the optical source in order to align the source to the optical filters. Of course, if sub-mount  500  is used rather than sub-mount  300  in the optical receiver shown in  FIG. 4A , then the dimensions of the cavity  415  may be modified to accommodate the width and length dimensions of sub-mount  500  which includes the integrated groove  505 . 
     Although integrated groove  505  includes a V-shaped groove, in another embodiment the groove  505  may be U-shaped or any other shape suitable for holding and aligning an optical source. For example, the optical source may be a collimator or the core of an optical cable that is placed within the groove  505 . The optical source may be fixed into the groove  505  using an epoxy. 
     In one embodiment, the sub-mount  500  may be made of the same materials as the sub-mount  300 . In one example, the sub-mount  500  may be made of a semiconductor where the portion of the sub-mount  500  forming the groove  505  has the same crystalline structure of the portion that defines the cavities  310  and  305 . For example, the cavities  305 ,  310  and integrated groove  505  may be formed from a single crystal semiconductor substrate. 
       FIGS. 6A-6B  illustrate assembling an optical transmitter, according to one embodiment described herein. In one embodiment, the optical transmitter is a component in a TOSA. As shown in  FIG. 6A , the transmitter includes a substrate  600 , an electrical IC  605 , optical source array  610 , cavity  615 , lens  620 , connector  625 , and optical cable  630 . The IC  605 , optical source array  610 , and connector  625  may have been mounted on the substrate  600  during previous fabrication steps. In one example, the substrate  600  includes one or more metallization layers that provide signal communication between the IC  605  and the optical array  610 . As shown, the optical source array  610  includes four individual optical sources (e.g., laser assemblies). Each of these optical sources may transmit an optical signal with a different wavelength. The IC  605  may provide control signals that cause the optical sources to transmit their respective optical signals. 
     As shown in  FIG. 6B , the sub-mount  300  shown in  FIG. 3  is disposed within cavity  615 . In one embodiment, the cavity  615  has width and length dimensions that substantially match the dimensions of the sub-mount  300 . As such, when the sub-mount  300  is disposed in the cavity  615 , the sub-mount  300  and the components disposed thereon are passively aligned with one or more components mounted on the substrate  600 . For example, each of the optical sources in the array  610  aligns with a respective one of the optical filters on sub-mount  300 . This alignment is achieved by etching cavity  615  to have dimensions that substantially match the dimensions of the sub-mount  300  and by arranging the cavity  615  at a location of substrate  600  such that the outputs of the optical sources in array  610  align with a respective one of the optical filters on the sub-mount  300 . 
     Once aligned, the optical filters and mirror on sub-mount  300  combine the four optical signals into a multi-wavelength optical signal using the multiplexing function shown in  FIG. 2 . This multi-wavelength optical signal is transmitted into lens  620  which then focuses the signal into the connector  625  and optical fiber  630 . The optical fiber may then transmit the signal to an optical receiver that is external to the optical transmitter shown in  FIGS. 6A and 6B . 
       FIG. 7  is a method  700  for passively aligning optical components in a multiplexer/demultiplexer, according to one embodiment described herein. At block  705 , a cavity is etched into the sub-mount for placing an optical mirror. At block  710 , a plurality of cavities is etched into the sub-mount for placing optical filters. As discussed above, the cavities may have dimensions that substantially match the dimensions of the mirror or optical filters. As such, when the mirror and filters are disposed into the cavities, these components adopt the orientation of the cavities. Moreover, the cavities formed during blocks  705  and  710  are etched at predetermined locations relative to each other so that the optical filters and mirror disposed in the cavities perform a multiplexing/demultiplexing function. Thus, the fabrication techniques or techniques used to form the cavities during block  705  and  710  may be any technique with enough precision to form the cavities to have dimensions that substantially match the dimensions of the mirror and optical filters as well as arrange the cavities relative to each other so that the multiplexing/demultiplexing function is enabled when the mirror and filters are disposed in the cavities. A non-limiting example of fabrication techniques sufficient to achieve these goals is semiconductor fabrication techniques which, in the case of silicon, can currently provide features with a resolution of 22 nm. 
     In one embodiment, the cavity formed at block  705  is etched into the same surface of the sub-mount as the plurality of cavities etched during block  710 . Moreover, although shown as two separate blocks, etching the cavities for the mirror and the optical filters may occur during the same etching step, and thus, the cavities may have the same depth. In another example, the cavity formed at block  705  is etched at a different time than the cavity formed at block  710 . Thus, the cavities may have different depths. Furthermore, in one embodiment, the number of cavities formed during block  710  may be the same as the number of optical filters—i.e., a one to one relationship. Alternatively, the optical filters may be combined into a single structure rather than being, for example, four individual structures. For instance, the four optical filters may be formed from the same material that is then processed to form the four filters, or the four filters may have been attached using an adhesive to form a unitary structure. In either case, at block  710  only one cavity may be formed which has dimensions that substantially match the unitary structure that includes the different optical filters rather that forming a plurality of cavities for each of the optical filters. 
     At block  715 , the mirror and optical filters are disposed into the respective cavities in the sub-mount. Because the dimensions of the cavities substantially match the dimensions of the mirror and optical filters, once placed, the mirror and optical filters are aligned passively and able to perform multiplexing or demultiplexing as described above. 
     At block  720 , the sub-mount is disposed into a cavity of a substrate in an optical receiver or transmitter. In one embodiment, the dimensions of the cavity in the substrate substantially match the dimensions of the sub-mount. Thus, placing the sub-mount into the cavity passively aligns the sub-mount to one or more components on the substrate. For example, if the substrate is part of an optical receiver, the sub-mount is aligned with an optical source on the substrate such that a multi-wavelength optical signal transmitted by the optical source is incident upon one of the optical filters on the sub-mount. The optical filters and mirror then perform a demultiplexing function on the multi-wavelength signal. Alternatively, if the substrate is part of an optical transmitter, the sub-mount may be aligned with a plurality of optical sources that each transmits an optical signal onto a respective optical filter. The optical filters and mirror then perform a multiplexing function to combine the optical signals. 
     In one embodiment, the sub-mount or the substrate may include one or more visual alignment features (e.g., fiducial markers) such as a cross or circular target that improve the ability for a technician or automated machine to place the optical filters and mirror onto the sub-mount or to place the sub-mount onto the substrate. For example, the alignment features may be etched into, or formed on, the sub-mount or substrate. 
       FIG. 8  is a wafer  800  including a plurality of optical assemblies  805 , according to one embodiment described herein. As shown, the assemblies  805  are repeated throughout the wafer  800 . In one embodiment, each optical assembly  805  is a sub-mount  300  like the one shown in  FIG. 3A . For example, the semiconductor wafer  800  may be etched to form the cavities  310  and  305  in each of the assemblies  805 . Once the cavities are formed, a technician or an automated machine may place the optical filters and mirror into the cavities of each assembly  805 . The wafer  800  may then be diced along the vertical lines  815  and horizontal lines  810  to separate the optical assemblies  805 . The assemblies  805  may then be mounted onto a substrate of an optical receiver or transmitter as shown in  FIGS. 4B and 6B . In this manner, the sub-mounts  300  may be fabricated at a wafer level rather than individually. 
     In another embodiment, each optical assembly  805  includes a substrate for an optical receiver or transmitter like the ones shown in  FIGS. 4B and 6B . The wafer  800  may be processed to include, for example, the metallization layers that route electrical signals between the IC  405  shown in  FIG. 4A  and the detector array  410 . In addition, the substrates in the assemblies  805  may be etched to include the cavity  415  for holding the sub-mount  300 . A technician or an automated machine may then dispose a respective sub-mount  300  into the cavities  415  in each of the optical assemblies  805 . As discussed above, the sub-mount  300  may have been formed on a different wafer and then diced into individual components which are then disposed onto the assemblies  805  of wafer  800 . Once the transmitters or receivers are assembled, the wafer  800  is diced along the lines  810  and  815  to form individual transmitters or receivers. In this manner, the optical receivers or transmitters shown in  FIGS. 4B and 6B  may be fabricated at a wafer level rather than individually. 
     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.