Patent Publication Number: US-2017351030-A1

Title: Filter assemblies

Description:
BACKGROUND 
     Wavelength division multiplexing (WDM) is useful for increasing communication bandwidth by sending multiple data channels down a single fiber. For example, a 100 gigabit per second (Gbps) link can be constructed by using four channels operating at 25 Gbps per channel, with each channel operating at a different wavelength. A multiplexer is used to join the signals together before transmitting them down the waveguide, and a demultiplexer is subsequently used to separate the signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate various examples of the principles described below. The examples and drawings are illustrative rather than limiting. 
         FIG. 1A  depicts an example multiplexer system that may use a filter assembly as described herein. 
         FIG. 1B  depicts a side view of an example filter assembly and example optical connector with reference surfaces for passive alignment. 
         FIG. 1C  depicts a bottom view of an example filter assembly and example optical connector with reference surfaces for passive alignment. 
         FIG. 2  depicts an example filter assembly having a mounting substrate and attached filter chips. 
         FIG. 3  depicts an example filter assembly having laterally stacked filter chips. 
         FIG. 4  depicts an example monolithic filter assembly. 
         FIG. 5A  depicts another example monolithic filter assembly. 
         FIGS. 5B-5D  depict example layouts including two monolithic filter assemblies. 
     
    
    
     DETAILED DESCRIPTION 
     Described below are WDM filter assemblies that may be used in a multiplexer/demultiplexer system, where the filter assemblies and an optical connector have complementary references surfaces or features to enable passive alignment of a filter assembly in the optical connector during assembly. 
     In some implementations, the filter assembly may include a mounting substrate and a plurality of filters chips, where each filter chip includes a thin film filter coating on a surface of a different substrate. The filter chips are positioned adjacent to each other in a row. A first edge of a first filter chip is flush with a first edge of the mounting substrate, a second edge of the first filter chip is flush with a second edge of the mounting substrate, and the first edge and second edge share a common corner. The flush edges of the first filter chip and the mounting substrate are reference surfaces, the plurality of filter chips are coupled to the mounting substrate via an epoxy, and the reference surfaces are to mate to connector reference surfaces on a connector. 
       FIG. 1A  depicts an example multiplexer system  100 . In the example of  FIG. 1A , four optical sources with integrated lenses  140  are shown, but any number of optical sources, greater than one, can be used. The optical sources  140  can be any type of light source that emits a light beam  141  in a band of wavelengths, such as a vertical-cavity surface-emitting laser (VCSEL), a distributed feedback laser, and a fiber laser. Light beams  141  are emitted by the optical sources  140  at different wavelengths and impinge on the optical body  120  at different input regions. 
     At each of the input regions of the optical body  120  where a light beam  141  impinges, there is a filter chip that may include a substrate  130  and a wavelength-selective filter  132 . Each wavelength-selective filter  132  reflects light, e.g., at greater than 50% reflectivity, at a first set or group of wavelengths and transmits light, e.g., at greater than 50% transmissivity, at a second set or group of wavelengths. The first set of wavelengths is different from the second set of wavelengths, and each wavelength-selective filter  132  transmits a different second set of wavelengths. For example, the set of wavelengths emitted by optical source  140 - 1  that is transmitted by wavelength-selective filter  132 - 1  is different from the set of wavelengths emitted by optical source  140 - 2  that is transmitted by wavelength-selective filter  132 - 2 , which is different from the set of wavelengths emitted by optical source  140 - 3  that is transmitted by wavelength-selective filter  132 - 3 , and is also different from the set of wavelengths emitted by optical source  140 - 4  that is transmitted by wavelength-selective filter  132 - 4 . In general, the peak wavelength of the optical source  140  is matched to the peak transmission wavelength of the wavelength-selective filter  132  to minimize optical power loss in the system  100 . Wavelength-selective filters  132  can be made of multiple layers of dielectric material having different refractive indices. 
     Light beams transmitted by wavelength-selective filters  132 - 1 ,  132 - 2 ,  132 - 3  each travel from surface  101  of the optical body  120 , through the optical body  120 , to impinge upon a reflective focuser  110  coupled to the second surface  102  of the optical body  120 . Each reflective focuser  110  reflects and focuses an incoming light beam back to a different one of the wavelength-selective filters  132  at the input regions of the optical body  120 . Examples of a reflective focuser can include a multi-layer stack of dielectric thin films; a Fresnel lens; a curved mirror lens, such as made with a metallic surface, e.g., gold; and a high-contrast grating reflector. 
     Upon hitting a wavelength-selective filter  132 , at least some portion of the light beam is reflected back toward the second surface  102  of the optical body  120 . Each wavelength-selective filter  132 , except for wavelength-selective filter  132 - 4  closest to the exit region  114 , reflects light to one of the reflective focusers  110 , as discussed above. Each wavelength-selective filter  132  also transmits a light beam from an optical source  140 . Light within the optical body  120  is redirected alternately between the wavelength-selective filters  132  and the reflective focusers  110  until the light hits the wavelength-selective filter  132 - 4  closest to the exit region  114 . 
     Wavelength-selective filter  132 - 4  reflects the light beam from within the optical body  120  to the exit region  114  on the second surface  102  of the optical body  120 . Wavelength-selective filter  132 - 4  also transmits a light beam from the optical source  140 - 4 . The reflected and transmitted light beams together make up the exit light beam that is directed toward the exit region  114 . The exit beam light beam includes at least some light from each of the optical sources  140 , thus, multiplexing the light beams from the optical sources  140 . 
     Coupled to the exit region  114  is an output lens  112  configured to image the light beam to another location, such as the input to a transmission medium  105 . The transmission medium  105  can be, for example, a multimode or single mode optical fiber or planar waveguide. The output lens  112  can also image the beam to an intermediate location. In some implementations, the output lens  112  is not present. 
     The optical body  120  can also operate as a demultiplexer (not shown), where a multi-wavelength light beam enters the optical body  120  at region  114  on the second surface  102  of the optical body  120 . A portion of the multi-wavelength beam is transmitted by the wavelength-selective filter  132 - 4  to a detector. 
     Detectors can be any type of sensor capable of sensing the system operating wavelengths, such as a photodiode. Each detector is positioned to receive a light beam transmitted from a corresponding wavelength-selective filter  132 . The wavelengths reflected by wavelength-selective filter  132 - 4  travel through the optical body  120  until a reflective focuser  110 - 3  coupled to the second surface  102  is reached. Similar to the multiplexer, light is re-directed within the optical body  120  alternately between the wavelength-selective filters  132  and the reflective focusers  110  until the light beam hits a wavelength-selective filter  132  that allows the light to exit the optical body  120 . The light that exits the optical body is then focused by a detector lens onto the active area of a corresponding detector. 
     In some cases, the same optical body  120  can be used for both multiplexing and de-multiplexing signals. For example, the multiplexing portion can be adjacent to the demultiplexing portion, or the multiplexing portion can be interleaved with the demultiplexing region. 
     The filter chips  130 - 1 ,  132 - 1 ;  130 - 2 ,  132 - 2 ;  130 - 3 ,  132 - 3 ;  130 - 4 ,  132 - 4  may be replaced by a filter assembly  139 , as described below. Filter assemblies may be fabricated monolithically or assembled from discrete filter chips. Filters may be manufactured on glass substrates as alternating layers of transparent dielectric materials, such as TiO 2  and SiO 2 . Flatness of the filter chips may be maintained by using stress compensating anti-reflection coatings on the substrate surface opposite the filter coatings and/or by using thicker substrates. Final dimensions and sidewall geometry may be precisely controlled using modern dicing saws or laser dicing techniques. 
     An optical connector  199  that includes the optical block  120 , reflective focusers  110 , and output lens  112  may be manufactured using injection molding. Injection molding may be used to achieve very precise geometric and dimensional control while producing a high volume of parts. The optical connector  199  may be designed with multiple reference surfaces or reference features to mate with external parts, such as a filter assembly  139 . 
       FIG. 1B  depicts a side view of an example filter assembly  139  and example optical connector  199  with reference surfaces  190 ,  191  for passive alignment. In some implementations, the corner between reference surfaces  190 ,  191  may be an rounded corner  197 , such as may be obtained by removing a portion of a spherical volume from the corner. An adhesive may be used between the filter assembly  139  and optical connector  199  at reference surface  190  after passive alignment has been attained. 
       FIG. 1C  depicts a bottom view of an example filter assembly  139  and example optical connector  199  with reference surfaces  192 ,  193  for passive alignment. 
       FIG. 2  depicts an example filter assembly  200  having a mounting substrate  210  and attached filter chips  220  positioned adjacent to each other in a row. Four filter chips  220  are shown, but any number of filter chips  220  can be used. As shown in the example of  FIG. 2 , the outer two filter chips in the row,  220 - 1 ,  220 - 4 , are wider than the inner two filter chips,  220 - 2 ,  220 - 3  in the row. If there are more or fewer than four filter chips, the two filter chips at the ends of the row may be wider than the other filter chips in the row. By widening the end filter chips,  220 - 1 ,  220 - 4 , a little more mechanical tolerance may be achieved because more chipping damage is permitted along the edges and more misalignment is allowed with respect to the optical connector. In some implementations, each filter chip  220 - 1 ,  220 - 2 ,  220 - 3 ,  220 - 4  in the row may have the same width. 
     A first edge  220   a  of a first filter chip  220 - 1  is flush with a first edge  211  of the mounting substrate  210  to create a first precision edge  202 . The rest of the filter chips  220 - 2 ,  220 - 3 ,  220 - 4  are pushed against the first filter chip  220 - 1 . A second edge  220   b  of the first filter chip  220 - 1  is flush with a second edge  212  of the mounting substrate, where the first edge  220   a  and second edge  220   b  share a common corner  220   c . An edge of each of the other filter chips  220 - 2 ,  220 - 3 ,  220 - 4  are also pushed flush with the second edge  212  of the mounting substrate  210  to create a second precision edge  204 . The filter chips  220  are coupled to the mounting substrate  210  via an epoxy. In some implementations, the epoxy is transparent at wavelengths reflected by the thin film filters of the filter chips  220 . 
     The first precision edge  202  and the second precision edge  204 , which include the flush edges of the first filter chip  220 - 1  and the mounting substrate  210 , are reference surfaces, and the reference surfaces are to mate to connector reference surfaces on an optical connector, such as connector reference surfaces  192 ,  193  shown in the examples of  FIGS. 1B and 1C . 
     In some implementations, the reference surfaces  202 ,  204  may be flat across the entire surface. In some implementations, the reference surfaces may have any shape and include, for example, serrated teeth. Contact at a single point between each of the references surfaces  202 ,  204  and the corresponding connector reference surfaces is sufficient. For example, the reference surfaces  202 ,  204  or the connector reference surfaces may have a bump that contacts the corresponding surface. Thus, in some implementations, at a minimum, the first edge  220   a  of the first filter chip  220 - 1  and the mounting substrate  210  contact a first connector surface at at least one point (a first point), and the second edge  220   b  of the first filter chip and the mounting substrate  210  contact a second connector surface at at least one point (a second point). However, contact is not limited to one point between corresponding surfaces; contact may occur at two or more points between surfaces. 
       FIG. 3  depicts an example filter assembly  300  having a plurality of filter chips  320  positioned adjacent to each other in a row, such that the filter chips are laterally stacked. Each filter chip  320  includes a thin film filter coating on a surface of a different filter substrate. In contrast to the example of  FIG. 2 , there is no mounting substrate  210 . Thus, in this implementation, the filter substrates are thicker than in the example of  FIG. 2 . The filter chips may have, but are not limited to, the same thickness. Four filter chips  320  are shown, but any number of filter chips  320  can be used. As shown in the example of  FIG. 3 , the outer two filter chips in the row,  320 - 1 ,  320 - 4 , are wider than the inner two filter chips,  320 - 2 ,  320 - 3 . If there are more or fewer than four filter chips, the two filter chips at the ends of the row may be wider than the other filter chips in the row, such that a width of the filter chips  320  is not uniform in the filter assembly. In some implementations, each filter chip  320 - 1 ,  320 - 2 ,  320 - 3 ,  320 - 4  in the row may have the same width. 
     A first edge  320   a  from a corner  320   c  of a first filter chip  320 - 1  serves as a first reference surface  302 . The rest of the filter chips  320 - 2 ,  320 - 3 ,  320 - 4  are pushed against the first filter chip  320 - 1 . A second edge  320   b  from the corner  320   c  of the first filter chip  320 - 1  serves a second reference surface  304 . An edge of each of the other filter chips  320 - 2 ,  320 - 3 ,  320 - 4  is also flush with the second edge  320   b  of the first filter chip  320 - 1  and form part of the second reference surface  304 . The filter chips  320  are coupled to each other via an epoxy on adjacent surfaces. In some implementations, the epoxy is transparent at wavelengths transmitted and reflected by the thin film filters of the filter chips  320 . 
     The reference surfaces  302 ,  304  are to mate to connector reference surfaces on an optical connector, such as connector reference surfaces  192 ,  193  shown in the examples of  FIGS. 1B and 1C . As discussed above with respect to reference surfaces  202 ,  204 , reference surfaces  302 ,  304  may be flat across the entire reference surface, or the reference surfaces may be any shape and make contact at at least one point between each of the references surfaces  302 ,  304  and the corresponding connector reference surfaces is sufficient 
     In some implementations, the filter chips  320  may be positioned in a staggered manner relative to the second edge  320   b  of the first filter chip  320 - 1 , such that each of the filter chips  320 - 2 ,  320 - 3 ,  320 - 4  are not necessarily flush with the second edge  320   b.    
       FIG. 4  depicts an example monolithic filter assembly  400 . With a monolithic filter assembly, there are no discrete filter chips to be assembled. Instead, each filter  420  of a first plurality of filters is patterned in a strip on a first substrate  401 , and the strips are positioned parallel to each other in a row. Four strips of filters  420  are shown, but any number of filters  420  can be used. A first filter  420 - 1  of the first plurality of filters  420  is flush with a first edge  420   a  extending from a corner  420   c  of the first substrate  401 , and each of the first plurality of filters  420  are flush with a second edge  420   b  extending from the corner  420   c  of the first substrate  401 . As shown in the example of  FIG. 4 , the outer two filters in the row,  420 - 1 ,  420 - 4 , are wider than the inner two filters,  420 - 2 ,  420 - 3 . If there are more or fewer than four filter chips, the two filter chips at the ends of the row may be wider than the other filter chips in the row, such that a width of the filter chips  420  is not uniform in the filter assembly. In some implementations, each filter chip  420 - 1 ,  420 - 2 ,  420 - 3 ,  420 - 4  in the row may have the same width. 
     To form the filters  420  on the first substrate  401 , a liftoff technique may be used that places a photoresist layer on the substrate prior to depositing the thin film filter layers. Then the photoresist layer is removed. In the regions where the photoresist layer was not applied, the filter layers in those regions will remain. In the regions where the photoresist layer was present, the deposited thin film filter layers are removed. The process can be repeated to create multiple filters on the single substrate. In some implementations, the filters  420  may be immediately adjacent, while in other implementations, there may be a gap between two neighboring filters. 
     The first edge  420   a  and the second edge  420   b  of the first substrate  401  are a first reference surface  402  and a second reference surface  404 , respectively. The first and second reference surfaces  402 ,  404  are to mate to a first and second connector reference surface, respectively, on a connector, such as connector reference surfaces  192 ,  193  shown in the examples of  FIGS. 1B and 1C . As discussed above with respect to reference surfaces  202 ,  204 , reference surfaces  402 ,  404  may be flat across the entire reference surface, or the reference surfaces may be any shape and make contact at at least one point between each of the references surfaces  402 ,  404  and the corresponding connector reference surfaces is sufficient. 
       FIG. 5A  depicts another example monolithic filter assembly  500 . Again, in this implementation, there are no discrete filter chips to be assembled. Rather, two monolithic filter assemblies  520   z ,  521   z , such as described in the example of  FIG. 4  above, may be used together. 
     In the example of  FIG. 5A , for a first monolithic filter assembly  520   z , each filter of a first plurality of filters  520  is patterned in a strip on a first substrate  501 , and the strips are positioned parallel to each other in a row. Two strips of filters  520  are shown in the example of  FIG. 5A , but any number of filters  520  can be used. A first filter  520 - 1  of the first plurality of filters  520  is flush with a first edge  520   a  extending from a corner  520   c  of the first substrate  501 , and each of the first plurality of filters  520  are flush with a second edge  520   b  extending from the corner  520   c  of the first substrate  501 . The second edge  520   b  serves as a reference surface  504 . 
     For a second monolithic filter assembly  521   z , each filter of a second plurality of filters  521  is patterned in a strip on a second substrate  511 , and the strips are positioned parallel to each other in a row. Two strips of filters  521  are shown in the example of  FIG. 5A , but any number of filters  521  can be used. A first filter  521 - 1  of the second plurality of filters  521  is flush with a third edge  521   a  extending from a corner  521   c  of the second substrate  511 , and each of the second plurality of filters  521  are flush with a fourth edge  521   b  extending from the corner  521   c  of the second substrate  511 . The fourth edge  521   b  may serve as a reference surface  505 . Additionally, at least one of the third edge  512   a  and the fourth edge  521   b  of the second substrate  511  is used as a reference surface. 
     In the example implementation of  FIG. 5A , the first edge  520   a  of the first monolithic filter assembly  520   z  serves as a first reference surface  502 . The third edge  521   a  of the second monolithic filter assembly  521   z  is pushed against the surface opposite the first edge  520   a  of the first monolithic filter assembly  520   z . The second edge  520   b  of the first monolithic filter assembly  520   z  and the fourth edge  521   b  of the second monolithic filter assembly  521   z  are flush, creating a second reference surface  504 - 505 . The first and second reference surfaces  502 ,  504 - 505  are to mate to a first and second connector reference surface, respectively, on a connector, such as connector reference surfaces  192 ,  193  shown in the examples of  FIGS. 1B and 1C . In this implementation, the use of adhesive between the first and second monolithic filter assemblies  520   z ,  521   z , may be foregone, to eliminate a potential point of failure. 
     In some implementations, the third edge  521   a  of the second substrate  511  is coupled via epoxy or other adhesive to an edge of the first substrate  501  that is opposite the first edge  520   a , as shown in the example of  FIG. 5B . In this case, the first edge  520   a  of filter  520 - 1  mates to a first connector reference surface  590 , and reference surface  504  of the first monolithic filter assembly  520   z  mates with a second connector reference surface  592 , while reference surface  505  of the second monolithic filter assembly  521   z  does not make contact with the second connector reference surface  592 . 
     Alternatively, in some implementations, the third edge  521   a  of the second substrate  511  is to be coupled via epoxy or other adhesive to a mechanical feature  595  on the connector, as shown in the example of  FIG. 5C . In this case, the first edge  520   a  of filter  520 - 1  mates to the first connector reference surface  590 , and reference surface  504  of the first monolithic filter assembly  520   z  mates with the second connector reference surface  592 . The reference surface  505  of the second monolithic filter assembly  521   z  also mates with the second connector reference surface  592 . 
     In some implementations, the fourth edge  521   b  of the second substrate  511  is an additional reference surface  505 , and the additional reference surface  505  is to mate to a third connector reference surface  594  on the connector, as shown in the example of  FIG. 5D . In this case, the first edge  520   a  of filter  520 - 1  mates to the first connector reference surface  590 , and reference surface  504  of the first monolithic filter assembly  520   z  mates with a second connector reference surface  593 . The reference surface  505  of the second monolithic filter assembly  521   z  also mates with a third connector reference surface  594 , where the second connector reference surface  593  and the third connector reference surface  594  are distinct. 
     In some implementations, the third connector reference surface  594  may be the same as the second connector reference surface  593 . 
     As used in the specification and claims herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.