Patent Publication Number: US-6981804-B2

Title: Vertically integrated optical devices coupled to optical fibers

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Priority is claimed to U.S. Provisional Application No. 60/291,169, filed May 15, 2001 entitled INTEGRATED FIBEROPTIC COMPONENTS, which is incorporated by reference herein. 
   This is a continuation-in-part of U.S. patent application Ser. No. 09/995,214 filed Nov. 26, 2001, now U.S. Pat. No. 6,527,455, entitled MULTILAYER OPTICAL FIBER COUPLER, incorporated by reference herein, which is a continuation of U.S. patent application Ser. No. 09/327,826, filed Jun. 8, 1999, now U.S. Pat. No. 6,328,482 B1, issued Dec. 11, 2001, entitled MULTILAYER OPTICAL FIBER COUPLER, which claims the benefit of U.S. Provisional Application No. 60/088,374, filed Jun. 8, 1998, entitled LOW COST OPTICAL FIBER TRANSMITTER AND RECEIVER and U.S. Provisional Application No. 60/098,932, filed Sep. 3, 1998 entitled LOW COST OPTICAL FIBER COMPONENTS, all of which are incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to optical devices coupled to optical fibers, and particularly to optical fiber-coupled devices that can be formed in large numbers using wafer-level techniques. 
   2. Description of Related Art 
   Optical fibers have by far the greatest transmission bandwidth of any conventional transmission medium, and therefore optical fibers provide an excellent transmission medium. An optical fiber is a thin filament of drawn or extruded glass or plastic having a central core and a surrounding cladding of lower index material to promote internal reflection. Optical radiation (i.e. light) is coupled (i.e. launched) into the end face of an optical fiber by focusing the light onto the core. For effective coupling, light must be directed within a cone of acceptance angle and inside the core of an optical fiber. Because any optical radiation outside the core or acceptance angle will not be effectively coupled into the optical fiber, it is important to precisely align the core with an external source of optical radiation. 
   A fiber optic coupler for coupling optical radiation between an optical device and an optical fiber is disclosed in U.S. Pat. No. 6,328,482 B1, issued Dec. 11, 2001, entitled MULTILAYER OPTICAL FIBER COUPLER, which is incorporated by reference herein. The &#39;482 patent discloses, inter alia, a multiplayer optical fiber coupler that includes a first layer that defines a fiber socket in which an optical fiber is situated, and a second layer coupled to the first layer. 
   It would be an advantage to provide optical fiber-coupled devices that provide functions such as filters, switches, and multiplexers/demultiplexers, and in which the optical fiber is integrated into the optical device. 
   Conventional optical devices generally require costly and time-consuming alignment steps to ensure efficient coupling to optical fibers. For example, one conventional practice for making a fiber-pigtailed transmitter is to assemble an edge-emitting laser diode, an electronics circuit, a focusing lens, and a length of optical fiber and then manually align each individual transmitter. To align the transmitter, the diode is turned on and the optical fiber is manually adjusted until the coupled light inside the fiber reaches a predetermined level. Then, the optical fiber is permanently affixed by procedures such as UV-setting epoxy or laser welding. This manual assembly procedure is time consuming, labor intensive, and expensive. Up to 80% of the manufacturing cost of a fiber-pigtailed module can be due to the fiber alignment step. The high cost of aligning optical fiber presents a large technological barrier to cost reduction and widespread deployment of optical fiber modules. 
   SUMMARY OF THE INVENTION 
   Integrated optical devices are disclosed herein in which one or more optical fibers are vertically integrated with other optical components in a multilayer arrangement. Particularly, the integrated devices include one or more optical fibers inserted into a fiber socket in fiber socket layer, and other optical components vertically integrated into one or more layers aligned with, and attached to the optical fiber socket layer. 
   In one embodiment, a vertically integrated optical device comprises a fiber socket layer comprising a plurality of sockets including a first socket and second socket arranged proximate to each other. A first optical fiber may be situated in the first socket and a second optical fiber may be situated in the second socket. A plurality of component layers are coupled to the fiber socket layer including a first component layer that includes a first optical component and a second component layer that includes a second optical component. The first and second optical components are arranged for optically coupling the first optical fiber with the second optical fiber via the first and second optical components. The first optical component may comprise a lens that defines a central axis, and the first and second optical fibers are aligned offset from the central axis. 
   Optical components that may be included in the structure include an actuable mirror that provides a variable optical attenuator device. The mirror may be partially transparent, and the device may further comprise a photodetector situated opposite the mirror from the optical fibers. Other optical components include an etalon, either passive or actuable. 
   A component layer may comprise a spacer layer that provides a predetermined opening that is hermetically sealed to protect sensitive components, such as MEMS devices. 
   The device may comprise a second fiber socket layer on the structure opposite the first fiber socket layer. One or more optical fibers may be situated in sockets in the second fiber socket layer. The optical fibers in the second socket layer may be optically coupled to the optical fibers in the first socket layer. In one embodiment, a first optical component comprises a first lens that defines a central axis, and first and second optical fibers in the first layer are aligned offset from the central axis, and a second optical component comprises a second lens that defines a second central axis, and a third optical fiber in the second layer is aligned offset from the central axis. A dielectric (e.g. WDM) filter may situated between the first and second lenses, the WDM filter arranged so that an input beam from the first optical fiber interacts with the WDM filter, thereby separating the input beam into a reflected beam that is coupled into the second optical fiber and a transmitted beam that is coupled into the third optical fiber. 
   Also, a method of forming a socket layer for holding a plurality of optical fiber is disclosed, comprising forming a first mask on a first surface of a wafer, the first mask defining a pattern including a first plurality of socket openings, forming a second mask on a second, opposing surface of the wafer, the second mask including a second plurality of socket openings aligned with the first plurality of socket holes. The exposed first surface is etched to between about one-half the thickness of the wafer and the full thickness of the wafer, and then the second surface is etched through the other side to provide a socket between the socket openings in the first and second masks. 
   Additionally, an integrated laser device is disclosed comprising a fiber socket layer including a fiber socket, an optical fiber situated in the fiber socket, a first component layer connected to the socket layer, the first component layer comprising a microlens. A laser layer that comprises a semiconductor material is connected to the first component layer, including a laser facet formed on a surface of the laser layer, a turning mirror formed on the surface, and an in-plane waveguide defined between the laser facet and turning mirror. A partial reflector is situated proximate to the optical fiber, the partial reflector and the laser facet defining a laser cavity. The turning mirror may comprise an etched mirror that is approximately 45° to the surface, thereby providing a 90° turning mirror. An etalon, passive or actuable, may be situated within the laser cavity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of this invention, reference is now made to the following detailed description of the embodiments as illustrated in the accompanying drawing, wherein: 
       FIG. 1  is perspective view of a plurality of wafers, illustrating fabrication of a wafer stack and individual devices. 
       FIG. 2  is a cross-sectional view of a three port, integrated optical fiber filter structure; 
       FIG. 3  is cross-section of an alternative embodiment to  FIG. 2  that comprises a four port fiber filter structure that can be used as a 2×2 fiber coupler; 
       FIG. 4  is a cross-sectional view of a two-port (in-line) fiber 1×1 filter, which is a alternative embodiment to the 1×2 filter shown in  FIG. 2 ; 
       FIGS. 5A ,  5 B,  5 C,  5 D,  5 E,  5 F,  5 G,  5 H,  5 I,  5 J,  5 K, and  5 M are cross-section of wafers that illustrate one fabrication process for making the three-port integrated fiber filter of  FIG. 2 ; 
       FIG. 6  is a cross-sectional view of a multi-channel WDM demultiplexer; 
       FIG. 7  is a cross-section of a variable optical attenuator device; 
       FIG. 8  is a cross-section of the variable optical attenuator device that further includes a photodetector; 
       FIG. 9  is a cross-sectional view of an integrated 2×2 switch that shows a switch mirror in the closed position; 
       FIG. 10  is a cross-sectional view of an integrated 2×2 switch that shows the switch mirror in the open position; 
       FIG. 11  is a cross-sectional view of a dual-pass tunable filter that utilizes a MEMS Fabry-Perot etalon and an angled mirror to select the wavelength; 
       FIG. 12  is a cross section of a laser transmitter; 
       FIG. 13  is a cross-section of an integrated external cavity tunable laser device that emits a single wavelength and is actively tunable across a wavelength range; 
       FIG. 14  is a combination of an in-plane pump laser integrated with a fiber-coupled filter structure; 
       FIG. 15  is a flow chart that illustrates general operations to form a device using the VFI technology; 
       FIGS. 16A ,  16 B,  16 C, and  16 D disclose a two-sided etching method suitable for fabricating socket layers; 
       FIG. 17  is perspective view of a plurality of wafers of alternating diameter aligned and bonded together using the metal soldering technique; 
       FIG. 18  is an exploded view of a smaller diameter wafer and a larger diameter wafer, showing structures used in the metal soldering technique; 
       FIG. 19  is a cross-sectional view of the smaller diameter wafer and the larger diameter wafer of  FIG. 18 ; and 
       FIG. 20  is a cross-section of an integrated fiber receiver. 
   

   DETAILED DESCRIPTION 
   This invention is described in the following description with reference to the figures, in which like numbers represent the same or similar elements. 
   Glossary of Terms and Acronyms 
   The following terms and acronyms are used throughout the detailed description: 
                                  InP   Indium Phosphide       MEMS   micro-electro-mechanical system       the &#39;482 patent   U.S. Pat. No. 6,328,482 B1, issued Dec. 11, 2001,           entitled MULTILAYER OPTICAL FIBER COUPLER       VFI technique   Vertical fiber integration technique       VOA   Variable optical attenuator       WDM   Wavelength division multiplexing       WDM filter   A filter, such as a multilayer dielectric coating that           separates an optical signal by wavelength into a reflected           beam and a transmitted beam                    
Overview
 
     FIG. 1  is a diagram that generally illustrates steps for making a vertically integrated device as described herein. First and second socket wafers  101  and  102  are formed with a plurality of fiber sockets shown generally at  105  that are created to hold optical fibers. First, second, and third component wafers  111 ,  112 , and  113 , which can include a variety of optical devices, are situated between the first and second socket wafers. The socket wafers and the component wafers are bonded to provide a wafer stack shown generally at  120  for device integration in the wafer surface-normal (vertical) direction, in contrast to conventional planar waveguide technology. 
   Once the wafer stack has been created, the individual devices on the wafer structure are then broken out by appropriate processes such as “slice and dice” along a grid pattern  121 . One device is shown at  130  after being been broken off from the wafer stack. The optical fibers  140  are then inserted into the sockets in the device  130 . This technology is generally referred to herein as “vertical fiber integration” (“VFI”) technology. Advantageously, the VFI devices are manufacturable in large batches. 
   U.S. patent application Ser. No. 09/327,826, now U.S. Pat. No. 6,328,482 B1, entitled “Multilayer Optical Fiber Coupler”, incorporated by reference herein, discloses a multiplayer structure that includes fiber socket technology to align an optical fiber with other optical components situated on other layers. The fiber socket technology disclosed in the &#39;482 patent is utilized herein in a variety of configurations, with multiple component layers to make ultra-low cost optical fiber components. 
   A variety of devices are disclosed herein as examples that can be implemented using vertical fiber integration technology, including passive optical devices and active optical devices. The passive devices include add/drop filters, and wavelength division multiplexers/demultiplexers, variable optical attenuators, fiber optic switches, and tunable filters. Active devices include fiber optic receivers, laser transmitters and wavelength tunable lasers. Using this technology and these examples, a wide variety of devices can be implemented. In addition to those techniques, additional techniques may be useful such as a wafer level hermetic sealing process disclosed herein, which is useful for the VOA device and any other application that requires space between one layer and another. To illustrate one fabrication process, steps for making the add/drop filter device will be discussed with reference to  FIGS. 5A to 5M ; it should be apparent that the other devices described herein could be implemented using similar techniques. 
   Add/Drop Filter 
   An add/drop filter is a fiber optic device that separates a multi-wavelength input beam into two separate output beams with different wavelengths. Conventionally, add/drop filters may be constructed by using a WDM thin film dielectric filter situated between two collimators. One collimator has two fiber pigtails, one of the pigtails providing the input beam and the other pigtail receiving the beam reflected from the dielectric (e.g. WDM) filter. The other collimator has one fiber pigtail that receives the beam transmitted through the WDM filter. 
     FIG. 2  is a cross-sectional view of a three port, integrated optical fiber filter structure that includes five layers aligned and bonded together, including a first fiber socket layer  201  and a second fiber socket layer  202  that have vertical sockets extending therethrough, dimensioned for receiving the optical fibers. The socket layer comprises any suitable material, such as silicon. As will be described, the sockets are arranged in a predetermined alignment with respect to other optical components in the structure. 
   Component layers  211 ,  212 , and  213  are situated between the first and second fiber socket layers. The first component layer  211  includes a first microlens  221  that has its focal plane proximate to the interface between the first fiber socket layer  201  and the first component layer  211 . The third component layer  213  includes a second microlens  222  that has its focal plane proximate to the interface between the second fiber socket layer  202  and the third component layer  213 . In this embodiment, the microlenses comprise refractive elements. The second component layer has a dielectric thin film coating  225  on one surface to provide a WDM filter. The component layers comprise any suitable material such as glass. 
   The first fiber socket layer  201  comprises a first fiber socket  231  that receives a first optical fiber  241  and a second fiber socket  232  proximate thereto that receives a second optical fiber  242 . The second fiber socket layer  202  comprises a third fiber socket  233  that receives a third optical fiber  243 . The optical fibers  241 ,  242 , and  243  are permanently affixed inside their fiber sockets by optical epoxy  244  and  245 . The optical fibers  241 ,  242 , and  243  typically comprise single mode fibers such as used for telecommunications purposes; however, other optical fibers, such as multimode fibers, may be used. 
   The optical fibers are arranged within their respective sockets so that their ends are proximate to the interface between the socket layer and the component layer. The first and second microlenses are positioned within the structure so that their focal planes are proximate to respective interfaces between the component layer and the socket layer, and therefore the focal planes approximately coincide with the ends of the respective optical fibers. 
   It is a well known property of geometrical optics that a beam of light originating from a point on the focal plane is collimated by the lens into a parallel beam of light. If the point is on-axis, the output beam is parallel to the optical axis. If the point is off-axis, the output beam is at an angle to the lens&#39; optical axis. This property is used in the design of the integrated optical fiber filter herein. 
   The sockets are formed with respect to the microlenses so that the optical fibers are off-axis. Particularly, the first microlens  221  defines a first central optical axis  251  that is offset from the core of the first and the second fibers  241  and  242 . In the embodiment shown in  FIG. 2 , the cores of the first and second fibers are positioned on opposite sides of the first central axis  251  and approximately equidistant therefrom so that light exiting from the first fiber  241  and reflecting from the WDM filter  225  is coupled into the second fiber  242 . The second microlens  222  defines a second central optical axis  252  that is offset from the core of the third fiber  243 . 
   In one example, the first fiber  241  is the input fiber, the second fiber  242  is a reflected output fiber, and the third fiber  243  is a transmitted output fiber. The fiber sockets, microlenses, and WDM filter are all arranged so that input light entering the input fiber is collimated by the first microlens  221  to form an approximately parallel beam with a finite beam angle with respect to the first central axis  251 , due to the off-axis arrangement of the optical fiber. The light beam impinges on the multi-layer WDM filter  225  and the light beam then is split into reflected light and transmitted light depending on the spectral property of the thin film filter  225 . The reflected light beam is tilted back to surface normal direction by the first microlens  221  and coupled into the core of the reflected output fiber  242 . The transmitted light beam from the WDM filter  225  is focused by the second microlens  222  into the core of the transmitted fiber. Again the off-axis arrangement of the second microlens  222  with respect to the fiber  243  tilts the angled beam back to surface normal direction before coupling it into the transmitted output fiber  243 . 
   The dielectric filter  225  can take many forms. The variety of dielectric thin film filters makes the add/drop filter disclosed herein a very useful structure that can be used in a number of applications by choosing a different filter. Possible devices that can be made using this structure include an add/drop WDM filter, a 1480 nm/1550 nm pump coupler or a 980 nm/1550 nm pump coupler, a fiber tap coupler, and/or a 1×2 beam splitter. A wide variety of filters are possible, such as a broadband filter, a narrow band filter, a high pass or a low pass filter, and an amplified spontaneous emission noise rejection filter. This filter can be a simple beam splitter coating. 
     FIG. 3  is cross-section of alternative embodiment to  FIG. 2  that comprises a four port fiber filter structure that can be used as a 2×2 fiber coupler.  FIG. 3  includes, in addition to the elements described with reference to  FIG. 2 , an additional layer  334 , that positions the WDM filter  225  approximately midway between the first and second microlenses  221  and  222 . However, in the embodiment of  FIG. 3  the first and second axes are approximately aligned, rather than being offset. A fourth fiber socket  314  is provided in the second socket layer  202 , and a fourth fiber  344  is situated therein. The fourth fiber socket  314  situates the fourth fiber  344  offset from the second axis  252 . Particularly, the fourth fiber socket  314  positions the fourth fiber  344  on the opposite side of the second axis  252  from the third optical fiber  243 . The third and fourth fibers are approximately equidistant from the second axis  252 ; i.e. the second axis  252  is approximately midway between the third and fourth fibers. In operation, the 2×2 coupler of  FIG. 3  utilizes the fourth fiber  344  to receive a second input, the third fiber  243  receives a second reflected output in addition to the first transmitted output, and the second fiber  242  receives a second transmitted output in addition to the first reflected output. 
     FIG. 4  is a cross-sectional view of a two-port (in-line) fiber 1×1 filter, which is a modification of the 1×2 filter shown in  FIG. 2 . The 1×1 design shown in  FIG. 4  eliminates the reflected output fiber  242 , and provides a single transmitted output on the output fiber  243 , which may be cost effective in applications where only a single output is required. 
     FIG. 5A  is a cross-section of a portion of a double-side polished silicon substrate  501 . A SiO 2  etch mask  502  is deposited on the silicon substrate  501 . In one embodiment the thickness of the silicon wafer is about 500 μm, and the SiO 2  mask  502  is deposited to a thickness of around 10 μm. The SiO 2  mask  502  has a fiber socket pattern  503  that defines a plurality of fiber sockets. In one embodiment the diameter of the fiber sockets is about 126 μm in diameter, which will accommodate standard single mode optical fibers. One method of making the fiber sockets using a two-sided etch hole is described with reference to  FIGS. 16A to 16D . 
     FIG. 5B  shows the silicon substrate  501  with two fiber sockets  504  formed therein, and the SiO 2  mask stripped. The fiber sockets are precision vertical holes etched all the way through the silicon substrate. This process creates the first and second socket layers  201  and  202  shown in  FIG. 2 . The fiber sockets  504  are formed by a process such as dry etching using a deep silicon etch process, such as the Bosch process using a deep RIE etcher, for example. The &#39;482 patent, incorporated by reference, also discloses methods for forming the sockets. 
     FIG. 5C  is a cross-section of a glass wafer  510  (e.g. fused silica) that will be formed into a component layer with a microlens component. A high selectivity hard mask  511  is deposited on the glass wafer  510  and photolithographically patterned into a pattern that includes an exposed section  512  that has a shape to allow creation of a recessed microlens, as will be described. 
   Referring to  FIG. 5D , the photoresist is deposited onto the wafer assembly and photolithographically patterned into a pattern for microlens fabrication.  FIG. 5D  shows the wafer  510  after photoresist  513  has been spun thereon in a pattern that creates a cylinder  514  of photoresist. 
   Referring to  FIG. 5E , the photoresist is reflowed to form a spherical surface  515  on the photoresist.  FIG. 5E  shows the photoresist cylinder  514  after being reshaped by melting the photoresist in an oven. The surface tension of the melted photoresist creates a spherical surface, which will to act as an etch mask for creating microlenses. 
   Referring to  FIG. 5F , the spherical surface is transferred to glass using a dry etcher. Particularly, the glass wafer  510  is etched using the reflow photoresist/hard mask combination as mask layers to form a microlens  516 .  FIG. 5F  shows the resulting microlens  516  after the photoresist is completely etched away and the spherical surface is transferred onto the glass surface. 
   Referring to  FIG. 5G , the hard mask  511  is stripped in a suitable environment. An anti-reflection (AR) coating  517  may be deposited on the surface of the microlens  516 . 
   The process described above with reference to  FIGS. 5C to 5G  is used to create the microlenses on the component layers such as the first and third component layers  211  and  213  ( FIG. 2 ). 
   Referring now to  FIG. 5H , a glass wafer  520  with a suitable wafer thickness and surface smoothness is provided for forming a dielectric (e.g. WDM) filter thereon. The qualities and dimensions of the glass wafer  520  are determined by the requirements of the final structure. For example, depending on the application, the material in the glass wafer  520  can be a low thermal expansion coefficient glass or fused silica glass. 
   Referring to  FIG. 5I , a suitable dielectric thin film filter coating  521  is deposited on one side of the glass wafer  521  to provide a WDM filter. 
   Referring to  FIG. 5J , the glass wafer  521  is aligned and bonded to the bottom microlens layer  510   a.    
   Referring to  FIG. 5K , the two-wafer stack from the previous step ( FIG. 5J ) is aligned and bonded to the top microlens layer  510   b  to create a three wafer stack. 
   Referring to  FIG. 5L , the three-wafer stack from the previous step ( FIG. 5K ) is aligned and bonded to a top socket layer  501   a  to create four-wafer stack. 
   Referring to  FIG. 5M , the four-wafer stack from the previous step ( FIG. 5L ) is aligned and bonded to a bottom socket layer  501   b  to create the final five-wafer stack. 
   This creates the finished filter structure shown in  FIG. 2 : particularly the top and bottom socket layers  501   a  and  501   b  correspond to the first and second socket layers  201  and  202 , the top and bottom microlens layers  510   a  and  510   b  correspond to the first and third component layers  211  and  213 , and the glass wafer  520  (with the dielectric coating) corresponds to the second component layer  212 . 
   The finished wafer stack is further diced up into chips. Optical fibers are inserted into the fiber sockets with a small amount of epoxy to permanently fix the fiber inside the fiber socket. 
   Wavelength Division Multiplexer and Demultiplexer 
   Currently wavelength division multiplexing (WDM) is causing a revolution in optical fiber communications, since it is the most practical means for increasing the transmission capacity of installed optical fiber cables (e.g. up to 160 fold) without laying new fibers, simply by transmitting multiple wavelengths through the same optical fiber. In a WDM system, multiplexer devices multiplex any number of optical wavelengths into a single fiber at the transmitting end. At the receiving end of the fiber, demultiplexers separate the single beam into its constituent wavelengths. 
     FIG. 6  is a cross-sectional view of a multi-channel WDM demultiplexer, which can also be used as a multiplexer by reversing the inputs and outputs. The embodiment in  FIG. 6  includes a first socket layer  601  and a second socket layer  602  that have a plurality of sockets extending therethrough, and first, second, and third component layers  611 ,  616 , and  613  situated between the first and second socket layers  601  and  602 . The socket layers  601  and  602  comprise any suitable material such as silicon, and the component layers comprise any suitable material such as glass. 
   The first and second socket layers include a plurality of sockets formed in a predetermined alignment with respect to the other optical components in the structure. The first socket layer  601  comprises a first socket  631  that receives a first optical fiber  641 , a third socket  633  that receives third optical fiber  643 , and a fifth socket  635  that receives a fifth optical fiber  645 , all arranged in a proximate relationship to each other. The second fiber socket layer  602  comprises a second fiber socket  632  that receives a second optical fiber  642 , a fourth fiber socket  644  that receives a fourth optical fiber  644 , and a sixth socket  636  that receives a sixth optical fiber  646 , all arranged in a proximate relationship to each other. The optical fibers are arranged within their respective sockets so that their ends are proximate to the interface between the socket layer and the adjacent component layer. The optical fibers typically comprise single mode fibers such as used for telecommunications purposes; however, other optical fibers, such as multimode fibers, may be used. 
   The first and third component layers include a plurality of microlenses. Particularly, the first component layer  611  includes a first microlens  621 , a third microlens  623 , and a fifth microlens  625  whose focal planes are proximate to the interface between the first fiber socket layer  601  and the first component layer  611 . The third component layer  613  includes a second microlens  622 , a fourth microlens  624 , and a sixth microlens  626  whose focal planes are proximate to the interface between the second fiber socket layer  602  and the third component layer  613 . Because each of the optical fibers is arranged within its respective socket so that its end is proximate to the interface between the socket layer and the component layer, the focal planes of the microlenses approximately coincide with the ends of the respective optical fibers. 
   Each of the sockets is aligned with respect to its respective microlens so that its optical fiber is off-axis from the central axes defined by the microlens. Particularly, the first microlens  621  defines a first central optical axis  651  that is offset from the core of the first fibers  641 . The second microlens  622  defines a second central optical axis  652  that is offset from the core of the second fiber  642 . The third microlens  623  defines a third central optical axis  653  that is offset from the core of the third fiber  643 . In the embodiment shown in  FIG. 6 , the cores of the first and third fibers are positioned on opposite sides of the first and third central axes  651  and  653 , and approximately equidistant therefrom so that light from the first fiber  641  reflecting from the dielectric (e.g. WDM) filter  671  is coupled into the third fiber  643 . The fourth microlens  624  defines a fourth central optical axis  654  that is offset from the core of the fourth optical fiber  644 , the fifth microlens  625  defines a fifth central optical axis  655  that is offset from the core of the fifth optical fiber  635 , and the sixth microlens  625  defines a sixth central optical axis  656  that is offset from the core of the sixth optical fiber  636 . 
   The second component layer  612  has a plurality of WDM filters formed on both an upper surface  661  and a lower surface  662 , each having a different center wavelength to select (transmit) a particular predetermined wavelength signal. A first WDM filter  671  is formed on the lower surface proximate to the second microlens  622 , a second WDM filter  672  is formed on the upper surface proximate to the third microlens  623 , a third WDM filter  673  is formed on the lower surface proximate to the fourth microlens  624 , and a fourth WDM filter  674  is formed on the upper surface proximate to the fifth microlens  625 . In this embodiment, four WDM filters are shown for purpose of illustration thereby providing four WDM output wavelengths (and a fifth output that includes all other wavelength(s) not transmitted by the four WDM filters). it should be apparent that the WDM filter/microlens/optical fiber operate as a unit, and that, in other embodiments, additional units can be added as desired. 
   The WDM filters can take many forms including dielectric thin film coatings. The variety of dielectric thin film filters makes the WDM filter disclosed herein a very useful structure that can be used in a number of applications by choosing a different wavelength filter. For example, the WDM filters may comprise beamsplitter coatings, and in such an embodiment an array of 1×N beamsplitters can be provided. 
   In operation, the demultiplexer shown in  FIG. 6  resembles the add/drop filter described with reference to  FIG. 2  in optical principle except that there are several WDM filters with different center wavelengths on the same wafer, and light bounces up and down between the WDM filters until it is transmitted through one of the WDM filters 
   The first fiber  641  is the input fiber. After entering through the input fiber port, light near the center wavelength of the first WDM filter  671  is transmitted therethrough and coupled into the second optical fiber  642  to provides a single wavelength output. Any light not transmitted is reflected toward the second WDM filter  672 , where it is either transmitted and coupled into the third optical fiber  643 , or reflected to the third WDM filter  673 . In this manner, light bounces up and down between the WDM filters until, finally, all the remaining light exits from the structure coupled into the sixth optical fiber  646 . In summary, each time light hits a WDM filter, one wavelength is transmitted, as determined by the WDM filter, while the other wavelengths are reflected. This way, as the input beam reflects from WDM filter to WDM filter, a different wavelength is separated at each interaction with the WDM filter and coupled into a respective fiber. Furthermore, although the structure in  FIG. 6  is described as a demultiplexer with a single input and several single wavelength outputs, it could also be used as a multiplexer by reversing the inputs and outputs; i.e. providing single wavelength inputs to the second, third, fourth, fifth, and/or sixth fibers, and receiving a multiplexed output on the first fiber. 
   The manufacturing process for the WDM demultiplexer can be accomplished using the principles as described for example with reference to the add/drop filter ( FIGS. 5A to 5M ). One difference is that multiple WDM thin film filters with different center wavelengths are patterned on the same wafer. This task can be achieved using a patterned thin film filter process, such as disclosed in U.S. Pat. No. 3,914,464, entitled “Striped Dichroic Filter and Method for Making the Same”, which is incorporated by reference herein. In this process, a photolithographic liftoff mask is prepared before each thin film filter deposition. The liftoff mask patterns the thin film filter. For a multiple wavelength WDM demultiplexer, multiple thin film deposition and liftoff steps are performed to create the corresponding filters for each of the wavelengths. In principle this structure can be used to produce any WDM demultiplexer including dense WDM demultiplexers; however, it may be less costly to produce coarse WDM demultiplexers (e.g. demultiplexers with wide channel spacing) rather than DWDM (dense WDM) filters with narrow channel spacing (e.g. 100 GHz and 200 GHz). 
   Due to the small size of the parallel optical beams (80 μm diameter typical), the beam widening at each subsequent reflection due to diffraction could become significant in some embodiments if there are more than eight consecutive dielectric filters. If this is the case, relay microlenses (not shown) may be incorporated into the two WDM filter surfaces to effectively collimate the light beam. The relay microlens structure can be made, for example, by bonding two WDM filter wafers, one of which has a relay microlens made on the back surface, so that the relay microlens is sandwiched in the middle between the two WDM filter wafers. Another function of the relay microlenses is to ensure that the light beams strike the WDM filter surfaces with a flat wave front, since the transmission of the WDM filter is sensitive to the incident angle. If the light beam does not strike the WDM surface with a flat wave front, it could cause crosstalk between different wavelength channels. 
   Variable Optical Attenuator (VOA) Arrays 
   Due to the number of channels in WDM networks, and particularly due to the very large number of channels in DWDM networks, there is an urgent market need for variable optical attenuators (VOAs) that can be used to attenuate the optical power in a fiber. An array of the VOAs described herein can be used, for example, to adjust the input power of each of the input beams at each wavelength before multiplexing the beams together in a multiplexer such as discussed with reference to  FIG. 6 . 
   Reference is made to  FIGS. 7 and 8  to illustrate a VOA that includes an active device (e.g. an actuable mirror) that can be controlled to vary the amount of light coupled out.  FIG. 7  is a cross-sectional view of an embodiment that includes a socket layer  701  having a plurality of sockets extending therethrough, and first, second, and third component layers  711 ,  712 , and  713  attached thereto. The socket layer comprises any suitable material such as silicon, and the component layers comprise any suitable material such as glass or silicon. 
   The socket layer  701  includes a plurality of sockets formed in a predetermined alignment with respect to the other optical components in the structure. Particularly, the socket layer  701  comprises a first socket  731  that receives a first optical fiber  741  and a second socket  732  that receives a second optical fiber  742 . The optical fibers are arranged within their respective sockets so that their ends are proximate to the interface between the socket layer and the adjacent component layer. The optical fibers typically comprise single mode fibers such as used for telecommunications purposes; however, other optical fibers, such as multimode fibers, may be used. 
   The first component layer  711  includes a microlens  721  whose focal plane is proximate to the interface between the socket layer  701  and the first component layer  711 . Because each of the optical fibers is arranged within its respective socket so that its end is proximate to the interface between the socket layer and the component layer, the focal planes of the microlens approximately coincides with the ends of the first and second optical fibers  741  and  742 . 
   Each of the first and second sockets  731  and  732  are aligned with respect to the microlens  721  so that the cores of the optical fibers are off-axis from a central axes  751  defined by the microlens. In  FIG. 7 , the cores of the first and second fibers are positioned on opposite sides of the first central axis  651  and approximately equidistant therefrom. 
   The third component layer  713  comprises a MEMS (micro-electro-mechanical system) mirror  761  formed on the upper surface of the layer  713 . The MEMS mirror  761 , which may be approximately centered on the optical axis  751 , is formed in an opening  762  by any suitable technique, and in one embodiment the MEMS mirror comprises single crystal silicon, and the second and third component layers  712  and  713  comprises silicon. A VOA electrode  763  is provided on the upper surface of the layer  713  in electrical contact with the MEMS mirror. The VOA electrode  763  is electrically coupled to a metal-plated hole  764  in the layer  713 , such as a via hole or a deep-etched large through hole plated with metal. Therefore, an electrical control signal can be applied to the MEMS mirror through the bottom side of the device using the metal-plated hole  764  and the VOA electrode  763 . In operation, as voltage is applied to the VOA electrode  763 , the MEMS mirror  761  is pulled down by the electrostatic force between the VOA electrode and the silicon wafer, which acts as the other electrode. 
   For description purposes the first fiber  741  provides an input beam  771 , and the second fiber receives a reflected beam  772  to provide an output, although the inputs and outputs could be reversed. In operation, the two fiber sockets  731  and  732  set the positions of the two fibers  741  and  742  offset from the optical axis  751  of the microlens, and therefore the input beam  771  and the output optical beam  772  form approximately the same angle with the mirror  761  when the mirror is in a neutral position with no voltage applied. As a result, substantially all the optical power will be coupled into the output fiber  742  as long as the MEMS mirror  761  is in the neutral position with no voltage applied. As voltage is increasingly applied to the VOA electrode, the MEMS mirror is pulled down by the electrostatic force between the VOA electrode and the silicon wafer, which acts as the other electrode, misaligning the reflected beam with the core of the output fiber. As misalignment increases the output light decreases in power as coupling efficiency drops. Eventually, the reflected beam will completely miss the output fiber thereby reducing output light power to about zero. 
   The second component layer  712  provides an opening  781  between the microlens  721  and the MEMS mirror  761 . The layer  712  may comprise silicon, and the opening  781  may be formed by a wafer-level process such as DRIE etching that creates a through hole in the wafer. When the component layers  711 ,  712 , and  713  are bonded together such as described elsewhere herein, the through holes become hermetically sealed and thus, the opening  781  is hermetically sealed. Some embodiment of MEMS fiber optic components require hermetic packaging in order to satisfy environmental requirements. By hermetically sealing the opening  781  where the MEMS mirror resides, the other parts of the VOA structure will not require hermetic packaging, which would be the conventional expensive hermetic packaging practice. As a result, very low cost device packaging can be employed. In one method, the component layers can be hermetically sealed by using ring shaped solder patterns. 
     FIG. 8  is a cross-section of an alternative embodiment to the VOA of  FIG. 7 . Many of the elements are the same; however the embodiment of  FIG. 8  additionally includes a photodetector section  805  provided in a third component layer  813 . The photodetector  805  can be monitored to provide input power-level feedback so that a smart VOA device can be made at ultra-low cost. In the embodiment of  FIG. 8 , a MEMS mirror  861  is constructed similar to the MEMS mirror  761  in  FIG. 7  except that the MEMS mirror  861  is partially transparent so that a small percent of the input light beam  771 , as shown at  874 , is transmitted through. The MEMS mirror  861  may be made of single crystal silicon for improved reliability. 
   The photodetector  805  and the third component layer  813  comprises any suitable material. In the embodiment of  FIG. 8 , third component layer  813  comprises a photodetector, such as InP. In some embodiments, the speed of the photodetector is not critical, which is one consideration in selecting a material. The third component layer  813  includes electrodes  863  formed on its upper surface and a metal-coated hole  864 , which provides an electrical connection to the MEMS mirror  861  and the photodetector. During operation, the photodetector is monitored to determine the power level of the input beam. As in  FIG. 7 , the second component layer  712  includes the opening  781 , and wafer level hermetic packaging can be implemented to protect the MEMS mirror  861  at low cost. 
   Fiber Optic Switch 
   Reference is now made to  FIGS. 9 and 10  to describe an integrated 2×2 fiber optic crossbar switch array that uses a mirror with two states to switch an optical input signal between two output fibers. 
     FIG. 9  is a cross-sectional view of an integrated 2×2 switch that includes a first socket layer  901  and a second socket layer  902  that have a plurality of sockets extending therethrough. First, second, third, and fourth component layers  911 ,  912 ,  913 , and  914  are situated between the first and second socket layers. The socket layers comprise any suitable material such as silicon, and the component layers comprise any suitable material such as glass or silicon as appropriate. 
   The first and second socket layers  901  and  902  include a plurality of sockets formed in a predetermined alignment with respect to the other optical components in the structure. The first socket layer  901  comprises a first socket  931  that receives a first optical fiber  941  and a second socket  932  that receives a second optical fiber  942 , arranged in a proximate relationship to each other. The second socket layer  902  comprises a third fiber socket  933  that receives a third optical fiber  643  and a fourth socket  644  that receives a fourth optical fiber  644 , arranged in a proximate relationship to each other. The optical fibers are arranged within their respective sockets so that their ends are proximate to the interface between the socket layer and the adjacent component layer. The optical fibers typically comprise single mode fibers such as used for telecommunications purposes; however, other optical fibers, such as multimode fibers, may be used in some embodiments. 
   The first and fourth component layers  911  and  914  each include a microlens, and may comprise a glass material. Particularly, the first component layer  911  includes a first microlens  921  whose focal plane is proximate to the interface between the first socket layer  901  and the first component layer  911 . The fourth component layer  914  includes a second microlens  922  whose focal plane is proximate to the interface between the second socket layer  902  and the fourth component layer  914 . Because each of the optical fibers is arranged within its respective socket so that its end is proximate to the interface between the socket layer and the component layer, the focal planes of the microlenses approximately coincide with the ends of the respective optical fibers. 
   Each of the sockets is aligned with respect to its respective microlens so that its optical fiber is off-axis from the central axes defined by the microlenses. Particularly, the first microlens  921  defines a first central optical axis  951  that is offset from the core of the first and second fibers  941  and  942 . The second microlens  922  defines a second central optical axis  952  that is offset from the core of the third and fourth fiber  943  and  944 . In the embodiment shown in  FIG. 9 , the first and second optical axes  951  and  952  are approximately aligned with each other, and have approximately the same optical power. Furthermore, the cores of the first and second fibers are positioned on opposite sides of the first central axis  951  and approximately equidistant therefrom, and likewise the cores of the third and fourth fibers are positioned on opposite sides of the second central axis  952 , and approximately equidistant therefrom. 
   A mirror  961  that is reflective on both sides is provided approximately equidistant between the first and second microlenses. The mirror  961 , which may be approximately aligned with the optical axes  951  and  952 , is formed by any suitable technique. For example, the mirror can be made by conventional MEMS techniques and in one embodiment comprises single crystal silicon. The MEMS mirror provides two states (e.g. open and closed), and has any suitable configuration; for example it can be a sliding mirror or a torsion mirror. A sliding mirror has one advantage in that, in the event of a power loss, the sliding switch is latched on to the pre-power loss state. 
   The spacing between the microlenses and the mirror is provided respectively by the second and third component layers  912  and  913 , both of which may comprise silicon. In one embodiment the MEMS (micro-electro-mechanical system) mirror  961  is formed on the upper surface of the third layer  913 . Each of the layers  912  and  913  has an opening to allow light to propagate from the microlens to the mirror; particularly, the second layer has an opening  981  between the first microlens and the mirror, and the third layer has an opening  982  between the second microlens and mirror. 
   An electrode  963  is provided on the upper surface of the layer  913  in electrical contact with the mirror  961 . The electrode  963  is electrically coupled to a terminal  964  in the layer  913  that is exposed along the side. The terminal  964  may be formed by any suitable technique such as first creating a via hole or a deep-etched large through hole plated with metal, and then dicing the wafer to expose the metallized hole. Therefore, an electrical control signal can be applied to the MEMS mirror through the terminal  964  and the electrode  963 . In operation in one embodiment, when voltage is applied to the electrode  963 , the MEMS mirror  961  is pulled into one state down by the electrostatic force between the mirror and the adjacent layer, which acts as the other electrode. 
   Reference is now made to  FIG. 10 , which is a cross-section of integrated 2×2 switch shown in  FIG. 9  in a second state. The MEMS mirror  961  is movable between two states (e.g. closed and open). In a first state, shown in  FIG. 9 , the MEMS mirror  961  reflects the inputs from both optical paths of two crossing beams to the adjacent optical fiber. In a second state, shown in  FIG. 10 , the MEMS mirror has been moved out of both optical input paths, thereby allowing the input beam to propagate to the opposite optical fiber. For example, in the first state shown in  FIG. 9 , if the first fiber  941  provides a first input beam  971 , then the first input beam  971  is reflected to provide a first output beam  972  to the second fiber  942 . Similarly, if the third optical fiber  943  receives a second input beam  973 , then it is reflected by the mirror to provide an output beam  974 . However, in the second state as shown in  FIG. 10 , the mirror  961  has been moved to allow the beams to propagate therethrough: particularly, in the second state the first input beam  971  provides the first output beam  974 , and the second input beam  973  provides the first output beam  972 . 
   It may be noted that the two fiber sockets  931  and  932  set the positions of the two fibers  941  and  942  offset from the optical axis  951  of the microlens, and therefore the first input beam  971  and the first output beam  972  form approximately the same angle with the mirror  961  when the mirror is closed as in  FIG. 9 . As a result, substantially all the optical power will be coupled from the first fiber  941  into the second fiber  942  as long as the MEMS mirror  961  is in the reflecting position. 
   In the 2×2 switch embodiment of  FIG. 9 , the MEMS mirror is reflective on both sides, and therefore, when the MEMS mirror is in the optical path, it reflects the two input signals from both sides simultaneously. In another embodiment, a 1×2 switch can be provided by omitting the third optical fiber  943 ; and in such embodiments the mirror  961  need only be reflective on one side. 
   One advantage of the two-state MEMS switch is that it is completely digital: the mirror may be in one of two distinct, mechanically-stable positions. As a result, the fiber switch is insulated from vibration and electrical disturbance problems. 
   The second and third component layers  912  and  913  provide the openings  981  and  982  between the microlens  921  and the MEMS mirror  961 . If, for example the layers  912  and  913  comprise silicon, then the openings  981  and  982  may be formed by a wafer-level process such as DRIE etching that creates a through hole in the wafer. When the four component layers  911 ,  912 ,  913 , and  914  are bonded together such as described elsewhere herein, the through holes become hermetically sealed and thus, the openings  981  and  982  become hermetically sealed. This can be useful because some embodiments of MEMS fiber optic components require hermetic packaging in order to satisfy environmental requirements. By hermetically sealing the openings  981  and  982  where the MEMS mirror resides, the other parts of the switch structure will not require hermetic packaging, which would be the conventional expensive hermetic packaging practice. As a result, a very low cost device packaging can be implemented. In one method, the component layers can be hermetically sealed by using ring shaped solder patterns. 
   Dual-Pass Tunable Filter 
     FIG. 11  is a cross-sectional view of a dual-pass tunable filter that utilizes a MEMS Fabry-Perot etalon and an angled mirror to select the wavelength. The tunable filter is a versatile device, well-suited for wavelength agile networks. 
     FIG. 11  shows the structure of the device, including a socket layer  1101  having a plurality of sockets formed therein, and first, second, and third component layers  1111 ,  1112 , and  113  attached thereto. The socket layer comprises any suitable material such as silicon, and the component layers comprise any suitable material such as glass or silicon. In one embodiment the socket layer comprises silicon, the first component layer comprises glass, and the second and third component layers comprise silicon. 
   The socket layer  1101  includes a plurality of sockets formed in a predetermined alignment with respect to the other optical components in the structure. Particularly, the socket layer  1101  comprises a first socket  1131  that receives a first optical fiber  1141 , a second socket  1132  that receives a second optical fiber  1142 , and a third socket (not shown) that receives a third optical fiber  1143 . The optical fibers are arranged within their respective sockets so that their ends are proximate to the interface between the socket layer and the adjacent component layer. The optical fibers typically comprise single mode fibers such as used for telecommunications purposes; however, other optical fibers, such as multimode fibers, may be used. 
   The first component layer  1111  includes a microlens  1121  whose focal plane is proximate to the interface between the socket layer  1101  and the first component layer  1111 . Therefore, the focal plane of the microlens approximately coincides with the ends of the first, second, and third optical fibers. Each of the first, second, and third sockets are aligned with respect to the microlens  1121  so that the cores of the optical fibers are off-axis from the central axes  1151  defined by the microlens. In one preferred embodiment of the tunable filter, the cores of the first, second, and third fibers are approximately equidistant from the central axis and from each other, so that their ends approximately define an equilateral triangle. 
   The third component layer  1113  comprises a tunable etalon  1161  formed on its upper surface, which provides the tuning mechanism of the dual pass tunable filter. The tunable etalon comprises two high reflectivity thin film mirrors that are separated by a gap, forming a high finesse resonator that controls the resonant wavelength. In some embodiments only one wavelength transmits through the etalon cavity between the two mirror while all other signals are reflected. The tunable etalon  1161  may be constructed by MEMS (micro-electro-mechanical system) techniques. 
   The gap between the two high reflectivity mirrors is controlled electrostatically by applying a voltage. Particularly, by varying the voltage, the optical gap distance (i.e. the optical distance between the two mirror of the etalon) can be varied, which change the wavelength transmitted. An electrode  1163  is provided on the upper surface of the third layer  1113  in electrical contact with the etalon  1161  to provide a system to supply a voltage to the etalon. The electrode  1163  is electrically coupled to a metal-plated hole  1164  in the layer  1113 , such as a via hole or a deep-etched large through hole plated with metal. In operation, as voltage is applied to the electrode  1163 , the etalon  1161  is pulled down by the electrostatic force between the electrode and the silicon wafer, which acts as the other electrode. 
   An angled mirror  1181  is situated below the tunable etalon  1161 . The angled mirror is arranged in a position to reflect light transmitted through the etalon from the first (input) fiber back through the etalon and then to the third optical fiber. 
   The first fiber  1141  provides an input beam  1171 , the second fiber  1142  receives a reflected beam  1172  from the etalon  1161 , and the third optical fiber  1143  receives an output beam  1173  transmitted twice through the etalon and reflected from the angled mirror  1181 . In one embodiment the placement of the angled mirror  1181  is such that the reflection is at the same incidence angle as that of the beam  1171 , although the output beam  1173  is spatially separated from both the input beam  1171  and the etalon-reflected beam  1172 . 
   In operation, the input beam  1171  is incident upon the etalon  1161 , and divides into two beams: the beam  1172  reflected from the etalon that includes all wavelengths not transmitted by the etalon, and the output beam  1173  that comprises the wavelength selected by the etalon. Because the input beam  1171  is incident upon the etalon at an angle, the etalon-reflected beam  1172  is coupled into the second optical fiber  1142  using the off-axis arrangement of the first microlens  1121 . The beam transmitted through the etalon is reflected by the angled mirror  1171 , passing again through the etalon (thereby providing further wavelength selectivity) and then is coupled into the third optical fiber  1143 . 
   In comparison with conventional tunable filter, this arrangement provides the reflected signal without the use of an external circulator. In conventional tunable filters, the transmitted signal passes through the resonant cavity only once, which limits the dynamic range of the tunable filter. In comparison, by providing a reflecting mirror near the resonant etalon cavity as described herein, the transmitted signal is reflected back through the resonant cavity. Advantageously, this dual-pass arrangement increases the dynamic range of the filter. 
   The spatial orientation of the angled mirror  1181  is defined by any suitable technique. One way is to cut a silicon wafer with a special orientation so that the ( 111 ) plane of the silicon wafer forms the correct orientation. By suitable wet etching of the silicon wafer, the ( 111 ) mirror plane will be exposed. A high reflection coating is then deposited on this surface to form the angled mirror with the desired orientation. 
   The second component layer  1112  provides an opening  1191  between the microlens  1121  and the tunable etalon  1161 . The layer  1112  may comprise silicon, and the opening  1191  may be formed by a wafer-level process such as DRIE etching that creates a through hole in the wafer. When the component layers  1111 ,  1112 , and  1113  are bonded together such as described elsewhere herein, the through holes become hermetically sealed and thus, the opening  1191  is hermetically sealed. 
   Multi-Wavelength Laser Transmitter Device 
   Reference is made to  FIG. 12  to show a waveguide device, and specifically a laser transmitter design (sometimes termed an “external cavity” laser herein) that can be used to create a multi-wavelength laser array. As will be described, the laser output wavelength of this laser device is determined by the alignment between the components in the device, and thus a different wavelength can be predetermined for individual devices by the patterning process. A multi-wavelength array can be created by patterning the devices and dicing them in such a way that multiple lasers at multiple wavelengths are in the same block, each emitting a different wavelength into its respective fiber port. 
     FIG. 12  is a cross section of a laser transmitter that includes a socket layer  1201 , a first component layer  1211  bonded to the socket layer, a second component layer  1212 , a planar Fabry-Perot etalon layer  1213  formed on the second component layer  1212  and situated between it and the first component layer, and a laser layer  1250  bonded to the second component layer. The socket layer  1201  includes a socket  1231  that receives an optical fiber  1241 . 
   The first component layer includes a first microlens  1221  that has its focal plane approximately at the interface between the socket layer and the first component layer. The first microlens  1221  has a central axis  1224  that is arranged slightly off-axis with the core of the optical fiber  1241 . The second component layer comprises a second microlens  1222  having a central axis  1226 . By varying the position of the central axis  1226  laterally with respect to the laser turning mirror  1253  in the manufacturing process as indicated by the arrows  1228  (i.e. from side-to-side), the wavelength can be varied as a result of changing the angle of incidence of the laser emission upon the Fabry-Perot etalon  1213 . 
   The laser layer  1250 , which comprises a suitable semiconductor material such as InP, includes an in-plane waveguide (laser area)  1251 . A laser facet  1252  is made on the bottom surface of the laser layer by etching a vertical wall into the InP semiconductor material. A 90° turning mirror  1253  is defined by etching a 45° slanted surface so that light is reflected upward. Both the vertical facet  1252  and the 90° turning mirror  1253  can be made by ion milling, for example. To protect the etched surfaces, the bottom surfaces of the laser layer are protected by layer  1254  such as a PECVD dielectric layer deposition. 
   A laser cavity is defined between the laser facet  1252  and a partial reflector  1255  that is situated proximate to the end of the fiber  1231 . Particularly, the laser cavity follows a path that for illustration purposes begins at the laser facet  1252  and reflects at about 90° from the turning mirror  1253 . Upon leaving the turning mirror, the light beam begins to expand in the laser substrate due to lack of confinement and broadens to a large area by the time it arrives at the upper surface of the laser. Upon exiting the upper surface, the second microlens  1222  collimates the laser beam before it hits the Fabry-Perot etalon  1213 , which operates to select the laser wavelength. The laser beam is then collimated again by the first microlens  1221  and hits at normal incidence the partial reflector  1255  that forms the other laser facet. Some of the light incident upon the partial reflector  1255  is reflected to provide the output, and some is reflected to provide feedback to the laser. 
   The electrodes of the laser (not shown) may all be provided on the outside of the structure. In one embodiment the laser can be mounted to a heatsink p-side down for heat extraction. 
   Possible advantages of the external cavity laser design described herein include multi-wavelength capability, elimination of wavelength locker, the thermoelectric (TE) cooler is not required, low chirp, high speed direct modulation possible, high power, simple Fabry-Perot dielectric etalon fabrication, no butterfly packaging required for hermetic sealing, integrated photodetector can be included, and no fiber alignment cost since it pre-aligned in the fabrication process. 
   Multi-wavelength by design: The laser wavelength is determined by the incidence angle of the light beam at the Fabry-Perot etalon. The incidence angle, in turn, is defined by the relative position of the upward divergent laser beam with respect to the second microlens  1222 . As a result, by varying the side-to-side position of the etched turning mirror  1253  with respect to the second microlens  1222  in the fabrication process, the lasing wavelength can be varied. Since these devices are fabricated in large quantities on a single wafer, lasers with many different laser wavelengths can be created. By designing the devices to provide multiple wavelengths on the same wafer stack, multi-wavelength laser transmitter arrays can be built. 
   Wavelength locker not necessary: Since Fabry-Perot etalons with very low temperature coefficients can be made, the temperature coefficient of the laser can be made very low. This results in the elimination of the wavelength locker. 
   TE cooler not required: Normal DFB lasers have high temperature coefficient. However, due to the Fabry-Perot etalon which has low temperature coefficient, the external cavity laser may have low temperature coefficient. This may lead to the elimination of a TE cooler. A simple heatsink can be used in place of a TE cooler for lower manufacturing cost. 
   Low chirp, high speed direct modulation: It has been reported that an external cavity laser may have much reduced wavelength chirp in direct modulation, because the wavelength selective element is detached from the laser gain medium; particularly a 15 GHz directly modulated laser has been reported with low chirp in an external cavity laser with fiber Bragg grating as one laser facet, for example in Paoletti et al, “15 Ghz Modulation Bandwidth, Ultralow-Chirp 1.55-μm Directly Modulated Hybrid Distributed Bragg Reflector (HDBR) Laser Source, IEEE Photonics Technology Letters, Vol. 10, No. 12, December 1998, pp. 1691–1693. The direct modulation speed depends on the laser cavity length. Compared to the laser reported therein, a shorter laser cavity length may be achieved using the integrated external cavity laser structure as shown in  FIG. 12 . As a result, even 10 Gb/s direct modulation with low wavelength chirp could be achieved in some embodiments. 
   High power, simple Fabry-Perot laser: Because there is no sophisticated laser regrowth steps involved, external cavity lasers can offer higher power compared to normal DFB lasers. 
   Simple Fabry-Perot dielectric etalon fabrication: The dielectric Fabry-Perot etalon is manufactured with a uniform Fabry-Perot etalon. 
   No butterfly packaging: The external cavity laser does not require butterfly type hermetic package due to the fact that the etched laser surfaces are all protected by dielectric films. An integrated waveguide photodetector may be made on the other side of the vertical laser facet to monitor the laser power output. The waveguide photodetector is reverse biased. 
   No additional fiber alignment cost: The external cavity laser array is naturally integrated with the fiber socket so that fiber alignment costs are eliminated. 
   External Cavity Tunable Laser 
     FIG. 13  is a cross-section of an integrated external cavity tunable laser device that emits a single wavelength and is actively tunable across a wavelength range. The tunable laser in  FIG. 13  uses some of the principles and elements described in the multi-wavelength laser transmitter design of  FIG. 12  described, but instead of the passive etalon in  FIG. 12  it uses a tilting ultra-narrow passband Fabry-Perot MEMS etalon to provide wavelength selection. 
   This device has four layers bonded together including the socket layer  1201  and the first component layer  1211  described above. A laser layer  1350 , which may comprise InP, resembles the laser layer  1250  in  FIG. 12 , including a laser facet  1352  formed on the lower surface, a 90° turning mirror  1353 , an in-plane laser area  1351  between the laser facet and the turning mirror, and the lower surface has a coating  1354  to protect etched surfaces. In addition a second microlens  1322  is formed on the upper surface of the laser layer, which operates to collimate the laser beam from the turning mirror  1353 . The second component layer  1312 , which may comprise silicon, includes an opening  1360  that operates as a spacer between the first and second microlenses  1221  and  1322 . The second component layer  1312  also includes a tilting Fabry-Perot etalon  1361  deposited on a MEMS structure, which is actuable by a using a signal applied to an electrode  1321 . The tilting MEMS Fabry-Perot etalon  1361  provides the wavelength selection mechanism by changing the angle of incidence. A laser cavity is defined between the laser facet  1352  and a partial reflector  1255  that is situated proximate to the end of the fiber  1231 . 
   The external cavity tunable laser of  FIG. 13  shares most of the advantages of the multi-wavelength laser transmitter design of  FIG. 12 . For example, the electrodes of the laser are all on the outside of the structure. The tunable laser can be mounted p-side down to a heatsink for excellent heat extraction. Wafer level hermetic packaging is used for low packaging cost. In some embodiments, additional optical components may be included (e.g. additional component layers) to prevent the tunable laser from mode-hopping. 
   Integrated Pump/Signal Combiner Array 
     FIG. 14  is a combination of an in-plane pump laser integrated with a fiber-coupled filter structure such as disclosed with reference to  FIG. 2 . The resulting device can be used to provide a pump laser beam and combine it with an optical signal to be amplified by an erbium-doped waveguide amplifier for example. Arrays of these devices can be used to pump erbium doped waveguide amplifier arrays. 
   The fiber-coupled filter structure is described with reference to  FIG. 2 , including the socket layer  201  that includes first and second sockets  231  and  232  for receiving and positioning first and second optical fibers  241  and  242 , the first component layer  211  bonded to the socket layer  201 , and the second component layer  212  that includes a WDM filter  225  formed on its lower surface. The WDM filter is designed to have a center frequency that transmits the pump laser beam and reflects the signal beam. The first component layer includes the first microlens  221  that defines the first optical axis  251  that is offset from, and approximately equidistant between, the cores of the first and second optical fibers. 
   The filter structure, and specifically the second component layer  212 , is connected to a laser layer  1450 , which may comprise GaAs, for example, which would provide an emitting wavelength of about 980 nm. The laser layer  1450  includes a laser facet  1452  formed on the lower surface, a 90° turning mirror  1453 , an in-plane laser area  1451  between the laser facet and the turning mirror, and the lower surface has a coating  1454  to protect etched surfaces. A Bragg reflector mirror  1455  is formed in the laser layer, which operates together with the laser facet  1452  to form a laser cavity. 
   A second microlens  1456  is formed on the upper surface of the laser layer, which receives the laser beam output from the turning mirror  1453 . A central axis  1457  defined by the second microlens is offset from the propagation direction of the laser beam from the turning mirror  1453 . As a result, when the output of the pump laser strikes the bottom microlens on the other side of the laser substrate, the collimated beam tilts to the right due to the off-axis arrangement of the laser with the second microlens. The pump laser beam, which has a wavelength about the center wavelength than the WDM filter  225 , then transmits through the WDM filter coating. The first microlens  221  is arranged so that the pump laser beam then is coupled into the second optical fiber  242  on the top right. 
   In operation a relatively weak optical signal enters the device through the first optical fiber  241 . The optical signal, which has a wavelength different than the center wavelength of the WDM filter, is reflected by the WDM filter  225 , thereby combining the optical signal with the strong pump laser output generated by the pump laser diode. The combined light beam is then coupled into the second (output) optical fiber  242  using the first microlens  221 . The second (output) fiber may then be connected to an EDWA input port for amplification of the weak signal, using the pump beam to optically pump the erbium-doped fiber. 
   Vertical Fiber Integration Process 
   U.S. patent application Ser. No. 09/327,826, now U.S. Pat. No. 6,328,482 B1, entitled “Multilayer Optical Fiber Coupler”, incorporated by reference herein, discloses fiber socket technology for aligning a single mode fiber with optical components on other lasers. Herein, the fiber socket technology disclosed in the &#39;482 patent may be utilized as part of the process to make ultra-low cost optical fiber components. In this process, referred to as “vertical fiber integration” (VFI) technology, multiple wafers are bonded together into a wafer stack for device integration in the wafer surface-normal (vertical) direction, in contrast to current planar waveguide technology. 
   The VFI technology is a fiber optic component manufacturing technology in which dense two-dimensional array of identical, functional fiber optic devices are created in the surface normal direction of the wafer stack. Each device includes a passively-aligned optical fiber with all necessary fiber passive alignment structure via the fiber socket technology. For example, in a six-inch diameter wafer stack, some 18,000 pre-aligned and vertically integrated devices can be created with 1 mm 2  die sizes. These devices are separated into chips with a suitable number of devices in arrayed form on each chip. As a result of this technology, time consuming active alignment operations are eliminated, and very substantial cost savings (e.g. two orders of magnitudes) may be realized. 
   One advantage of VFI technology is the possibility to achieve ultra-low cost manufacturing of fiber optic components. Therefore it is useful to consider cost in each and every step of the manufacturing process. For example, in addition to photolithographic processing for batch manufacturing, the fiber insertion and device packaging could also be low cost. 
   The possibility of consistent low cost manufacturing is one advantage of vertical fiber integration technology over other fiber optic component manufacturing technologies, for example, that of Digital Optics Corporation (DOC) in North Carolina. In DOC technology, wafers are bonded together into wafer stacks with vertical optical circuits. The wafer stacks are diced into chips and fiber v-groove arrays are then actively aligned and attached to the chips. Apparently the cost of fiber alignment and packaging dominates in this process and the final cost is believed to be significantly higher than that of vertical fiber integration technology. 
     FIG. 15  is a flow chart that illustrates general operations to form a device using the VFI technology. This process can be illustrated with four basic steps: 1) individual wafer processing as shown at  1501 , 2) wafer bonding as shown at  1502 , 3) wafer stack dicing as shown at  1503 , and 4) fiber insertion as shown at  1504 . Reference may also be made to  FIG. 1  to describe these steps. 
   Step  1 . Individual Wafer Processing 
   At  1501 , in a first step a plurality of wafers, which may be silicon, glass or some other suitable material, are obtained and processed in a series of sub-steps using photolithographic means to create two-dimensional arrays of components as required for the particular device to be constructed. Each wafer has a specific pattern with a certain function and/or optical functioning element. The 2-D array of patterns on different wafers are designed with a one-to-one correspondence, so that when the wafers are precisely aligned and permanently bonded together, the patterns on all the wafers form an integrated optical circuit in the surface-normal direction. These elements may include precise vertical holes, microlenses, dielectric thin film filters, mirrors, lasers, and detectors, for example. 
   Sockets are created to receive the optical fibers. One method for creating the sockets is described with reference to  FIGS. 16A to 16D ; however other methods could be used. For single mode fiber applications, the fibers may be spatially positioned with about 1 micron alignment accuracy or less. Since the two-dimensional array of patterns on a wafer can be created using photolithography with location errors of less than 0.1 micron, their locations have negligible error with this process. 
   In one embodiment the fiber sockets comprise photolithographically-defined, vertical through holes (about 500 μm deep) with a diameter of about 126 μm sized to closely match that of the optical fiber. Proper orientation is important, because when the fiber is inserted into the fiber socket, its position and angular orientation are defined by the fiber socket. Positional alignment precision of less than 1 micron can be achieved using the fiber socket. 
   After two or more wafers with precisely defined two-dimensional patterns are being aligned to each other, if two vertically integrated circuits on two opposite sides of the wafer are aligned, all other vertically integrated devices on the same wafer stack are automatically aligned. This feature can be used to eliminate individual active alignment such as used in conventional fiber optic component manufacturing processes. 
   Step  2 . Wafer Bonding 
   At  1502 , in a second step after individual wafers are patterned, they are precisely aligned using alignment fiducials, such as shown in the &#39;482 patent, to each other and the wafers are permanently bonded to provide a wafer stack. Each and every die needs to be permanently bonded. Due to the photolithographic creation of the two-dimensional patterns, when two vertical optical circuits are precisely aligned, all the vertical optical circuits on the wafer stack are aligned. 
   The VFI technology allows many different kinds of materials to be integrated together. Since the thermal expansion properties of the materials can be different, it may be useful to conduct the wafer bonding at lower temperatures to avoid the buildup of thermal stress. A solder bonding method is disclosed with reference to  FIGS. 17 ,  18 , and  19 ; however other method can be used. Examples of bonding methods include anodic bonding, epoxy bonding, metal bonding, glass-frit bonding, wafer direct bonding, and polyimide bonding. If epoxy bonding is utilized, then it may be useful to deposit a thin layer of epoxy, let it begin curing, and then bond the two layers, which would reduce unwanted upwelling of epoxy into the fiber sockets. In embodiments that include glass and silicon layers, anodic bonding is a particularly useful technology for bonding the silicon layer to the glass layer. 
   Step  3 . Wafer Stack Dicing 
   At  1503 , after wafer bonding, the wafer stack is diced into chips as illustrated in  FIG. 1  with a small number of vertical optical circuits on each chip using any suitable technique such as cutting with a diamond saw. This way, devices in individual form or arrayed form can both be made with the same level of manufacturing ease. 
   Step  4 . Fiber Insertion 
   At  1504 , optical fibers are then inserted into the sockets in the chips, such as shown at  140  in  FIG. 1 , and permanently affixed using epoxy for example. Possible epoxy materials include UV-cured epoxy and thermally cured epoxy. 
   Making a Fiber Socket 
   One method for making the vertical fiber alignment hole is a-dry-etched silicon round hole made by using a silicon deep RIE etcher. The etching process may be the Bosch process, although other processes to create a dry etched hole in silicon may be possible. 
   However, the fiber socket may be formed by other methods. In the numerous optical fiber devices disclosed herein, the fiber socket may be created in a number of ways, which should be construed to include all possible ways to create a vertical hole. 
   Silicon holes patterned from both sides: Reference is now made to  FIGS. 16A ,  16 B,  16 C, and  16 D. When creating two fiber sockets with very close proximity as disclosed herein in dual fiber type devices, it may be useful to etch the silicon hole from both sides of the wafer rather than from one side. It has been found experimentally that as the wafer is etched deeper, the etched hole loses fidelity in shape compared to the original photomask. This problem is especially severe when two patterns are closely placed on the original photomask, which creates the so called “microloading” effect. As a result of microloading, etching a mask pattern that begins with two closely-positioned holes and a small gap in between will result in the two holes merging together at the other side. This phenomenon is more severe when photoresist is the etch mask and less severe when an oxide etch mask is used. 
     FIGS. 16A–16D  disclose a method in which an etch mask is patterned on both sides of the wafer, and then etched from each side. 
     FIG. 16A  is a cross-section of a silicon wafer  1601  that has a first oxide etch mask  1611  formed on its upper surface and a oxide etch second mask  1612  formed on its lower surface. Both masks include openings where the optical fibers are to be formed, and the openings are aligned. Particularly, a first set of openings is aligned about a first centerline  1621 , and a second set of openings is aligned about a second centerline  1622 . In one method, the steps of patterning the oxide masks on both sides of the wafer include growing a thermal oxide on both sides of a double-side polished wafer, then the first mask  1611  is formed on the upper surface by photolithography and etching of the oxide film, and then the lower surface is aligned and the second mask  1612  side is formed by photolithography and etching the oxide film on the lower surface. 
   Referring to  FIG. 16B , the upper surface exposed through the mask  1611  is etched about one-half to substantially more than one-half of the thickness of the wafer  1601 , but without going through the lower surface. Under suitable conditions, deep DRIE etching creates trenches with a reentrant profile.  FIG. 16B  is a cross-section that shows first and second etched holes  1631  and  1632 , etched respectively about the first and second centerlines, which is the result of etching from the upper surface. 
   Referring to  FIG. 16C , the lower surface exposed through the second mask  1612  is etched by a process such as deep RIE etching until first and second sockets  1641  and  1642  are formed respectively about the centerlines  1621  and  1622 . 
   Referring to  FIG. 16D , the first and second oxide masks are stripped using a suitable process, such as hydrofluoric acid etching to form the final socket wafer. 
   It has been found that etching from both sides of the wafer as described herein results in well-defined rims on both sides of the fiber hole. In some embodiments, the hole diameters on the photomask on the insertion side of the hole may be made larger than that of the other side to facilitate the fiber insertion process. 
   Other methods for forming the fiber socket: Although the dry etched silicon hole, etched from both sides is a preferred method for creating the socket wafer for fiber passive alignment, other methods of making the fiber socket are possible with varying degrees of convenience and performance. 
   For example other methods include wet etching of a diamond shaped vertical hole in a (110) silicon wafer, and plating a round hole using a LIGA process on the back of the microlens wafer. In the LIGA process, tall cylinders of polymer are created on a wafer surface, and thick metal is plated using the cylinders as molds. After the polymer is removed, round through holes in metal are created. 
   Still another possibility in making the fiber socket is by dry etching through a material other than silicon. 
   Shape of the fiber socket: The shape of the hole may be varied as may be useful or necessary. For example, round holes with vertical grooves on the vertical sidewalls can be used to facilitate the epoxy in escaping from the bottom of the round hole during the fiber insertion process. 
   Surface orientation of the fiber socket wafer: Since the fiber socket defines the position of the optical fiber, the side of the fiber socket wafer with the highest precision should be the side of the fiber socket wafer directly bonded to the microlens wafer. 
   If the fiber socket is created using an etch mask on only one surface of the wafer, that wafer surface should be the surface that is bonded to the microlens wafer; otherwise there may not be sufficient precision to ensure efficient coupling. 
   If the fiber socket wafer is created by etching from both wafer surfaces, the wafer surface with the smaller fiber hole diameter should be bonded to the microlens wafer so that the fibers are more precisely positioned by the fiber sockets. 
   Wafer Bonding Process Using Solder Bonding 
   Reference is now made to  FIGS. 17 ,  18 , and  19 . One embodiment of wafer bonding process for the device structures is metal solder bonding. One advantage of this process is the low temperature bonding, which could lead to room temperature bonding capability. In this process multiple embedded electrical thin film heaters are individually activated by running electrical current through them, which melts and reflows the solder wires nearby. The solder wires bond the wafers together without heating the whole wafer. 
     FIG. 17  is perspective view of first, second, third, fourth, and fifth wafers  1701 ,  1702 ,  1703 ,  1704 , and  1705  aligned and bonded together. The wafers are arranged with alternating wafer diameters; particularly, the first, third, and fifth wafers have a larger diameter than the second and fourth wafers. 
     FIG. 18  is an exploded view of the second and third wafers  1702  and  1703 . As illustrated in  FIG. 18 , the larger diameter wafers such as the third wafer  1703  have electrical contact areas including first terminal pads  1801  and second terminal pads  1802  that are connected to heat a solder layer  1803 . The second wafer  1702 , which has a smaller diameter, has a solder layer  1805  in a pattern that matches the solder layers  1803  on the opposing surface of the third wafer. 
   In the wafer stack of  FIG. 17 , the terminal pads  1801  and  1802  extend beyond the edges of the smaller diameter second and fourth layers, so that small electrical contact probes can reach in and provide electrical current to each of the terminal pads. 
     FIG. 19  is a cross-sectional view of the second and third wafers, showing the solder layer  1805  on second (smaller) wafer, the opposing solder layer  1803  on the third (larger) wafer, and a heater structure, which is connected to the terminal pads  1801  and  1802 , that includes a metal conductive layer  1901  such as tungsten, covered by an electrical insulator such as an oxide film. 
   On the smaller diameter wafers, metal solder patterns may be formed by photolithographic liftoff processes on both surfaces of the wafers. On the larger diameter wafers, the metal (e.g. tungsten) heater patterns are formed first, followed by an oxide layer which covers the metal heater patterns, and followed by another layer of solder pattern (e.g. gold-tin) which is directly above the metal heater pattern but insulated from the metal heater pattern by the oxide layer. Both sides of the larger diameter wafers are provided with this structure, except on the outer facing surfaces. 
   After precise wafer alignment between the two wafers, the two wafers to be bonded are held down by pressure. Pulsed electrical current is sent through the terminal pads to the metal heater wires individually. The generated heat reflows each individual solder pattern in a controlled way without causing significant thermal expansion of the wafer. The reflowed solder pattern balls up due to surface tension and makes contact to the solder pattern on the adjacent wafer. The two solder patterns melt together. This way, any small gap between the two solder patterns is bridged and a constant spacing between the two wafers is maintained by the other solder patterns which are not activated (heated). In some embodiments it may be necessary to place the wafer bonding setup inside an inert environment to facilitate the solder reflow process. 
   Although  FIGS. 18 and 19  show a linear pattern for purposes of illustration, some embodiments can utilize other configurations, such as a circular configuration near the circumference of the wafers. Such a configuration would effectively seal the volume within the circular pattern. 
   Anti-Reflection Coating 
   An anti-reflection coating may be desirable for every optical surface in the vertical stack. The AR coating step is done after optical patterns such as microlenses have been formed. In the devices disclosed herein, the steps of AR coating may not be discussed specifically for each device, but may be implemented as desired. 
   Wafer Level Hermetic Packaging 
   In fiber optic devices such as lasers, detectors and MEMS switches, it is frequently necessary to enclose environmentally sensitive devices inside a hermetically sealed metal package. Conventionally, these metal packages are expensive, and they exacerbate the difficulty of manufacturing fiber optic components. 
   Using the vertical fiber integration technology, it is possible to achieve hermetic packaging on a wafer level, for all of the devices contained in the wafer. Particularly, the sensitive spaces such as MEMS cavities are sealed off from the outside environment by the two adjacent wafers bonded together. In the case of solder bonding, the spaces are sealed off by suitably designed solder rings around the cavities. These metal solder rings reflow during the wafer bonding process and hermetically seal the sensitive areas from the outside environment, without any special wafer bonding arrangement. For added reliability, two rings may be used to encircle the same cavity. 
   With the sensitive areas hermetically sealed by the solder rings, the reliability of a fiber optic component depends on the reliability of the fiber socket, the fiber, and the epoxy. With a suitably chosen uv- or thermal-cured epoxy, it should not be necessary to hermetically seal the fiber sockets in order for the fiber optic component to pass Bellcore environmental tests. Therefore low cost manufacturing of previously hermetically sealed fiber optic components is possible. 
   Fiber Optic Device Packaging 
   Because the vertical fiber integration technology provides automatic fiber passive alignment with ultra-low cost, the goal of the device packaging is to provide a rugged fiber device package, rather than to maintain fiber alignment as is currently done in conventional fiber optic device packaging. 
   The packaging processes may employ low cost injection molding or epoxy potting to encapsulate the vertically integrated optical circuit, which has fibers already inserted into the fiber sockets and fixed permanently with epoxy. Suitable strain relief rubber boots may be provided to ensure the fully packaged devices withstand fiber side pull tests. 
   Fiber Optic Receiver 
     FIG. 20  is a cross-section of an integrated fiber receiver, which illustrates an example of a device that can be constructed with the metal solder technique. The receiver in  FIG. 20  has a socket layer  2001 , a component layer  2002 , and a photodetector layer  2003  that is bonded to the component layer by a metal solder technique, such as described with reference to  FIGS. 17 ,  18 , and  19 . As a result of this bonding technique, a metal solder layer  2004  is situated between the component and photodetector layers, which provides wafer level hermetic packaging. Particularly, the metal solder technique creates a hermetically-sealed opening  2010  between the microlens  2021  and the photodetector section  2002 . 
   The socket layer  2001 , which may comprise silicon, includes a socket  2041  for receiving an optical fiber  2041 . The component layer  2002  includes a microlens  2021  having a central axis that is aligned with the core of the optical fiber. The photodetector layer includes a photodetector  2005 , comprising for example a InGaAs detector, that is arranged to receive input light from the optical fiber  2041  focused by the microlens  2021 . The photodetector  2005  and the photoconductor layer  2003  comprise any suitable material, such as InP. Electrical connection can be provided by any suitable connection, such as using wire bonding in the open area or using a via hole  2051  on the photodetector layer. The solder layer  2004 , or another electrode may be used to connect the photodetector with a monitoring device. 
   In operation, the input signal from the optical fiber  2031  is focused by the microlens  2021  and hits the photodetector area  2005 . The optical energy is converted to electrical signal by the photodetector, which then provides an appropriate output.