Patent Publication Number: US-2017371110-A1

Title: Optical Transceiver With a Mirrored Submount and a Laser Diode for Laser-to-Fiber Coupling

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims benefit of U.S. Provisional Patent Application No. 62/353,777 filed Jun. 23, 2016 by Ning Cheng, et. al. and entitled “Optical Transceiver With a Mirrored Submount and a Distributed Feedback (DFB) Laser for Laser-to-Fiber Coupling,” which is incorporated herein by reference as if reproduced in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not applicable. 
     BACKGROUND 
     Passive optical networks (PONs) have been widely deployed by operators to provide broadband services. PONs include fiber-to-the-homes (FTTHs). There are currently more than one hundred million FTTH users worldwide. Each FTTH user requires an optical network unit (ONU) at the customer&#39;s premise. Thus, cost reduction is important for large scale PON deployment. 
     SUMMARY 
     In some embodiments, an optical device includes a laser diode configured to emit an optical signal, wherein the optical signal diffracts into a plurality of emitted optical signals, and a submount comprising a mirror, wherein the mirror is configured to at least partially reflect and redirect the plurality of emitted optical signals to produce a plurality of reflected optical signals, and wherein the mirror is further configured to substantially reshape a vertical far field angle of the optical signal. 
     In some embodiments, the plurality of reflected optical signals are substantially collimated and focused before being received at an optical fiber. In some embodiments, the mirror is a flat mirror, wherein a plane of the flat mirror is substantially parallel with an active layer of the laser diode. In some embodiments, the laser diode is flip-chip bonded to the submount. In some embodiments, the optical device further includes a lens positioned between the laser diode and the optical fiber, wherein the lens is configured to focus the plurality of emitted optical signals and the plurality of reflected optical signals together. In some embodiments, the mirror is a substantially doubly concave mirror with a substantially toroidal shape. In some embodiments, the mirror is a substantially doubly concave mirror with a substantially toroidal shape, wherein the mirror is configured to reflect light by about 90 degrees. In some embodiments, the mirror is configured to focus the plurality of reflected optical signals toward an acceptance region of an optical fiber, and wherein the acceptance point on the optical fiber is located substantially at an image plane of the mirror. 
     In some embodiments, an optical device includes a laser diode configured to emit an optical signal, wherein the optical signal diffracts into a plurality of emitted optical signals, and a submount comprising a mirror, wherein the mirror is configured to at least partially reflect and redirect the plurality of emitted optical signals to produce a plurality of reflected optical signals, and wherein the mirror is further configured to substantially reshape a vertical far field angle and a horizontal far field angle of the optical signal. 
     In some embodiments, the plurality of emitted optical signals and the plurality of reflected optical signals are substantially collimated and focused before being received by the optical fiber. In some embodiments, the laser is flip-chip bonded to the submount. In some embodiments, the optical device further includes a lens positioned between the laser diode and the optical fiber, wherein the lens is configured to focus the plurality of emitted optical signals and the plurality of reflected optical signals together. In some embodiments, the mirror is a substantially doubly concave mirror with a substantially toroidal shape, and wherein the mirror is configured to focus the plurality of reflected optical signals toward an acceptance point of an optical fiber. In some embodiments, the mirror is a substantially doubly concave mirror with a substantially toroidal shape, and wherein the mirror is configured to reflect light by about 90 degrees. 
     In some embodiments, a method includes generating, via a laser diode disposed on a submount, an optical signal, wherein a mirror is disposed on the submount, wherein the optical signal is emitted from the laser diode as a plurality of emitted optical signals, reshaping a far field angle of the optical signal by reflecting, via the mirror, a portion of the plurality of emitted optical signals to produce a plurality of reflected optical signals, and directing the plurality of emitted optical signals and the plurality of reflected optical signals towards a core of an optical fiber. 
     In some embodiments, reshaping the far field angle comprises reducing a vertical far field angle relative to the laser diode. In some embodiments, the mirror is a flat mirror, wherein a plane of the flat mirror is substantially parallel with an active layer of the laser diode. In some embodiments, the mirror is a substantially doubly concave mirror with a substantially toroidal shape, and wherein an image plane of the mirror is substantially located on the core of the optical fiber. In some embodiments, the mirror is a substantially doubly concave mirror with a substantially toroidal shape and wherein the mirror is configured to reflect light by about 90 degrees. In some embodiments, the method further includes transmitting, via an output aperture of the laser diode, the optical signal to an optical fiber, wherein a portion of the plurality of emitted optical signals are reflected from the mirror to the optical fiber. 
     These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG. 1  is a diagram of a PON. 
         FIG. 2  illustrates a conventional optical coupling scheme employing a lens. 
         FIG. 3  illustrates a portion of the optical coupling scheme in  FIG. 2  where the far field angle of the light emitted from the laser diode. 
         FIG. 4  is a diagram of a transmitter optical assembly (TOSA) according to an embodiment of the disclosure. 
         FIG. 5  is a diagram of a portion of the TOSA shown in  FIG. 4  according to an embodiment of the disclosure. 
         FIG. 6  is a diagram of a portion of the TOSA in  FIG. 4  according to another embodiment of the disclosure. 
         FIG. 7  is a graph illustrating the optical field from the output of the laser diode after being reflected by a mirror on the submount according to an embodiment of the disclosure. 
         FIG. 8  is a diagram of a TOSA according to another embodiment of the disclosure. 
         FIG. 9  is a diagram of a portion of the TOSA according to an embodiment of the disclosure. 
         FIG. 10  is a diagram of a mirror in the TOSA of  FIG. 8  according to various embodiments of the disclosure. 
         FIG. 11  is a flowchart of a method for reducing a far field angle of a laser diode according to an embodiment of the disclosure. 
         FIG. 12  is a diagram of an optical device according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that, although illustrative implementations of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     An optical transceiver in an optical line terminal (OLT) or an ONU of a PON typically includes a bi-directional optical assembly (BOSA). The BOSA includes a TOSA and a receiver optical assembly (ROSA). The TOSA comprises components for optical transmissions, such as a laser diode configured to generate an optical signal, such as light or a light beam, and a receptacle configured to receive an optical fiber. For example, the laser diode generates and emits light toward the optical fiber. However, the light emitted from the laser diode diffracts radially or elliptically as the light travels from the laser diode to the optical fiber. The far field angle of the optical signal emitted from the laser diode is the angle at which the light diverges or diffracts from an output of the laser diode after the light has emitted from the laser diode. The acceptance angle of the optical fiber is the angle at which the optical signal can be received by the optical fiber. Typically, the far field angle on a distributed feedback (DFB) laser is about 20° to 30°. In contrast, the typical acceptance angle of a single mode optical fiber is about 4° to about 5°. This large difference between the far field angle of the optical signal emitted from a laser diode, and the acceptance angle of the optical signal accepted by an optical fiber, results in a coupling loss of light in a TOSA. Thus, the BOSAs used in ONUs and OLTs in a PON experience a low coupling efficiency between the laser diodes and optical fibers, even though lenses are usually used to facilitate the coupling between the laser diodes and optical fibers. 
     Disclosed herein are embodiments of a TOSA that reduce the coupling loss between a laser diode and an optical fiber, and thus improve coupling efficiency and alignment tolerance of an optical signal in between the laser diode and the optical fiber. For example, a TOSA includes a submount comprising a mirror. The mirror is positioned on the submount such that an optical signal emitted from the laser diode is at least partially reflected such that a far field angle of the optical signal emitted from the laser diode is reduced. The disclosed embodiments involve depositing a submount underneath the laser diode and interposing a mirror between the laser diode and the submount. In one embodiment, a laser diode is flip-chip bonded onto the submount comprising a flat mirror. The term flip-chip bonding refers to flipping a component of the TOSA upside down so that a contact point that is typically on the top surface of the component is now flipped to become the bottom surface of the component. The contact point of the component abuts, or is substantially adjacent to, a contact point disposed on the submount. In another embodiment, the laser diode is disposed onto a mirrored submount with a concave mirror. In an embodiment, the concave mirror is a toroidal or spherical mirror. The flat mirror and the concave mirror are positioned on a location of a surface of the submount to reshape the far field angle of the laser diode to match the acceptance angle of the optical fiber. As such, the disclosed embodiments improve coupling efficiency and alignment tolerance, and thus reduce cost. Further, as the coupling loss is reduced, the requirements on the laser diode output power and slope efficiency are reduced. Therefore, the disclosed embodiments may improve laser diode yield and reduce laser diode cost. 
       FIG. 1  is a diagram of a PON  100 . The PON  100  comprises an OLT  110 , a plurality of ONUs  120 , and an optical distribution network (ODN)  130  that couples the OLT  110  to the ONUs  120 . The PON  100  is suitable for implementing the disclosed embodiments. The PON  100  is a communications network that may not require active components to distribute data between the OLT  110  and the ONUs  120 . Instead, the PON  100  may use passive optical components in the ODN  130  to distribute data between the OLT  110  and the ONUs  120 . 
     The OLT  110  communicates with the ONUs  120  and another network. Specifically, the OLT  110  is an intermediary between the other network and the ONUs  120 . For instance, the OLT  110  forwards data received from the other network to the ONUs  120  and forwards data received from the ONUs  120  to the other network. The OLT  110  comprises a transmitter and a receiver. When the other network uses a network protocol that is different from the protocol used in the PON  100 , the OLT  110  comprises a converter that converts the network protocol to the PON  100  protocol and vice versa. The OLT  110  is typically located at a central location such as a central office (CO), but it may also be located at other suitable locations. 
     The ODN  130  is a data distribution system that comprises optical fiber cables, couplers, splitters, distributors, and other suitable components. The components may include passive optical components that do not require power to distribute signals between the OLT  110  and the ONUs  120 . The components may also include active components such as optical amplifiers that do require power. The ODN  130  extends from the OLT  110  to the ONUs  120  in a branching configuration as shown, but the ODN  130  may be configured in any other suitable point-to-multipoint (P2MP) configuration. 
     The ONUs  120  communicate with the OLT  110  and a customer, and act as an intermediary between the OLT  110  and the customer. For instance, the ONUs  120  forward data from the OLT  110  to the customer and forward data from the customer to the OLT  110 . The ONUs  120  comprise an optical transmitter that converts electrical signals into optical signals and transmits the optical signals to the OLT  110 , and an optical receiver that receives optical signals from the OLT  110  and converts the optical signals into electrical signals. The ONUs  120  further comprise a second transmitter that transmits the electrical signals to the customer and a second receiver that receives electrical signals from the customer. ONUs  120  and optical network terminals (ONTs) are similar, and the terms may be used interchangeably. The ONUs  120  are typically located at distributed locations such as customer premises, but they may also be located at other suitable locations. 
     In an embodiment, each of the ONUs  120  and/or the OLT  110  comprises a TOSA and a ROSA. In various embodiments, the TOSA and the ROSA may be both housed within one transistor outline (TO)-can or the TOSA and the ROSA may be housed within separate TO-cans. The TOSA and the ROSA share components of the BOSA, such as a TO-cap surrounding the TOSA and BOSA, a lens positioned on or formed as part of the TO-cap, and a TO-header upon which the components of the TOSA and ROSA are positioned. The TOSA comprises components for optical transmission, and the ROSA comprises components for optical reception. For example, the TOSA comprises a laser diode, and the ROSA comprises a photodiode. The TOSA may also comprise a receptacle configured to receive and secure an optical fiber. In a TOSA, there may be a gap, or a distance, between the laser diode and the optical fiber. The lens is disposed between the laser diode and the optical fiber. 
     When an optical signal is emitted from the laser diode, the optical signal typically diverges, or diffracts, in the gap between the laser diode and the optical fiber. While the lens focuses and directs the optical signal emitted from the laser to the optical fiber during operation, the lens does not reduce the far field angle of the laser diode enough to match the acceptance angle of the optical fiber to reduce coupling loss. Typical schemes of reducing coupling loss to increase coupling efficiency and alignment tolerance involve making changes to the structure of the laser diode and/or the optical fiber. However, these changes to the structure of the laser diode and/or the optical fiber are often times intricate and too costly to implement across the millions of ONUs and OLTs in PONs. 
     Disclosed herein are embodiments of a TOSA that reduce coupling loss between a laser diode and an optical fiber. The embodiments disclosed herein do not involve making complex changes to the structure of the laser diode or the optical fiber. Instead, the embodiments disclosed herein involve interposing a submount comprising a mirror below the laser diode. In an embodiment, the light that is emitted from the laser diode and impinged onto the mirror is at least partially reflected and redirected. In an embodiment, the reflected light reshapes the far field angle of the laser diode. 
     In an embodiment, the laser diode is flip-chip bonded to the submount such that an active layer of the laser diode is adjacent to the submount. The output of the laser diode is relatively close to the submount when the laser diode is flip-chip bonded to the submount. In this embodiment, a mirror may be formed on a surface of the submount proximate to the laser diode such that the mirror is extended past the output of the laser diode. In this embodiment, when an optical signal is emitted from the output of the laser diode, the diverged beams of the optical signal in the vertical direction will be partially reflected by the mirror on the submount. 
     In another embodiment, the laser diode may not need to be flip-chip bonded to the submount. In this embodiment, the submount comprises a plateau that extends from an edge of the laser diode comprising the output. The plateau of the submount comprises the mirror. In this embodiment, the mirror is relatively proximate to the output of the laser diode without having to flip-chip bond the laser diode to the submount. 
     In another embodiment, the submount comprises a concave surface that is bent or curved by about 90° at a predefined distance from the output of the laser diode and the optical fiber. In an embodiment, a concave mirror is formed on the concave surface of the submount. In an embodiment, the concave mirror is a toroidal mirror or a spherical mirror that is radially or elliptically concave. The optical signals impinge on the concave mirror, and the concave mirror reflects the optical signals at substantially 90° and focuses the optical signals toward a point or an area. In an embodiment, the concave surface supports the concave mirror. 
       FIG. 2  illustrates a conventional optical coupling scheme  200  employing a lens  209 . In the optical coupling scheme  200 , the lens  209  is positioned between a laser diode  203  and an optical fiber  206 . In an embodiment, the laser diode  203  is a DFB laser diode or any laser diode suitable for emitting light with the desired characteristics. The laser diode  203  may be employed by an ONU such as the ONUs  120  or an OLT such as the OLT  110  to provide an optical source for gigabit PON (GPON) and Ethernet PON (EPON) applications. In an embodiment, the lens  209  is an aspherical lens or any other lens suitable for focusing light emitted from the laser diode  203 . In an embodiment, the optical fiber  206  is a standard single-mode fiber (SSMF). 
     As shown in  FIG. 2 , the lens  209  is positioned at a point that is a first distance  215  (Z 1 ) from a laser diode  203  and at a second distance  218  (Z f ) from the optical fiber  206 . The laser diode  203  emits optical signal  220  that diverges radially outward from the center point of optical signal  220 , as shown by the dashed lines  212 A. The lens  209  focuses and directs the diverged optical signal, as shown by the dashed lines  212 B, to the core of the optical fiber  206 . The far field angle  230  of the optical signals emitted from the laser diode  203  is typically about 20° to 30°. The acceptance angle  233  of the optical fiber  206  is typically about 4° to 5°. 
       FIG. 3  illustrates a portion  300  of the optical coupling scheme  200 , showing the far field angle  230  of the optical signal  220  as the optical signal  220  is emitted from the conventional laser diode  203 . As shown in  FIG. 3 , the laser diode  203  comprises a substrate  303 , a lower surface  304  of the laser diode  203  that is substantially opposite an active layer  306 , an upper surface  307  of the of the laser diode  203  that is substantially opposite the substrate  303 , and an output aperture  350 . In a traditional TOSA, a lower surface  304  is positioned on a support block, such as a metal block, of the TOSA. The active layer  306  comprises a waveguide and a grating that is configured to generate the optical signal  220 , with the optical signal  220  being emitted by an output aperture  350 . Typically, the upper surface  307  radiates more heat than the lower surface  304 . 
     As shown in  FIG. 3 , optical signal  220  spreads outward as it leaves the output aperture  350 , due to diffraction. The optical signal  220  diffracts into several emitted optical signals  310 ,  313 ,  316 , and  320 , creating an elliptical mode field  318 . Four dashed lines, showing emission angles, are shown in  FIG. 3  for illustrative purposes. In the elliptical mode field  318 , light emitted from the laser diode  203  spreads more in the vertical plane than in the horizontal plane. The far field angle  230  comprises a horizontal far field angle (θ H )  326  and a vertical far field angle (θ V )  323 . The horizontal far field angle  326  is an angle of divergence of optical signal  220  in a horizontal direction and substantially in the same plane as the active layer  306  of the laser diode  203 . The horizontal far field angle  326  of a DFB laser is typically about 20°. The vertical far field angle  323  is an angle of divergence of optical signal  220  in a vertical direction and substantially perpendicular to the active layer  306  of the laser diode  203 . The vertical far field angle  323  of a DFB laser is typically about 30°. 
     However, the mode field of an SSMF that is to accept the emitted light is circular, due to the cross-sectional shape of the fiber. The acceptance angle of the SSMF (i.e., the maximum angle of impinging light as it strikes the end face of the fiber) is about 5°. The angular mismatch between the mode field of the laser diode  203  and the acceptance angle of the optical fiber  206  may result in significant coupling loss between the two components. 
       FIG. 4  is a diagram of a TOSA  400  according to an embodiment of the disclosure. The TOSA  400  comprises a laser diode (or DFB laser)  203 , a submount  406 , a lens  209 , and an optical fiber  206 . The submount  406  comprises a metal with dielectric properties, such as aluminum nitride (AIN) or other suitable materials. The submount  406  comprises a mirror  409  formed or disposed on the submount  406  and adjacent to or abutting the laser diode  203 , and in one embodiment, the end of the laser diode  203  including the output aperture  350 . The mirror  409  may be any metal with reflective properties, such as titanium (Ti), gold (Au), and/or another suitable material. In an embodiment, the mirror  409  is a flat mirror that is disposed on top of submount  406 . The mirror  409  may be coated on the submount  406  at a low cost. In an embodiment, the mirror  409  is a thin planar element that is substantially parallel to the active layer  306  of the laser diode  203 . 
     In an embodiment, laser diode  203  is flip-chip bonded to the submount  406 , where the upper surface  307  is coupled to the submount  406 . In this way, laser diode  203  is flipped upside down so that contact points that are typically on the upper surface  307  are now adjacent to the submount  406 . In an embodiment, the contact points on the upper surface  307  are solder bumps that are bonded to contact points on the submount  406  and/or the mirror  409 . By positioning the active layer  306  of the laser diode  203  adjacent to the submount  406 , heat dissipation in TOSA  400  is improved because the heat radiated from the active layer  306  is dissipated into the submount  406 . Thus, flip-chip bonding the laser diode  203  to the submount  406  improves the performance of the laser diode  203  when operating in high temperatures. 
     As shown in  FIG. 4 , the lens  209  is positioned at a first distance  430  (d 0 ) from the laser diode  203  and a second distance  433  (d i ) from the optical fiber  206 . The first distance  430  may be the same or different from the first distance  215  of  FIG. 2 . The second distance  433  may be the same or different from the second distance  218 . The optical fiber  206  comprises an acceptance region or point  450  at about a center, or core, of the optical fiber  206 . The acceptance angle  233  may be relative to the acceptance region  450  of the optical fiber  206 . In an embodiment, the acceptance region  450  is disposed substantially at a location of the image plane of the lens  209 . 
     As shown in  FIG. 4 , the laser diode  203  emits an optical signal that diverges into at least four emitted optical signals  310 ,  313 ,  413 , and  421 . The emitted optical signals  310 ,  313 , and  413  are optical signals that diverge elliptically from the output aperture  350  of the laser diode  203  in a vertical direction that is perpendicular to the active layer  306  of the laser diode  203 . The mirror  409  on the submount  406  is configured to reflect a portion of the light emitted in a downwardly vertical direction from the laser diode  203 . In an embodiment, the mirror  409  is configured to reflect or redirect downward-emitted light into an upward direction to reshape and reduce the vertical far field angle  323  of the laser diode  203 . The reflection may reduce the vertical far field angle  323  by about half in some embodiments. 
     As shown in  FIG. 4 , the optical signals that diverge downward in a vertical direction are the emitted optical signals  313  and  413 . The mirror  409  reflects emitted optical signals  313  and  413  upwards, as reflected optical signals  416  and  419 , respectively. There may be some emitted optical signals that do not hit the mirror  409 , such as emitted optical signal  421 . 
     Even though the TOSA  400  may only reduce the vertical far field angle  323  of the laser diode  203  in a vertical direction, the TOSA  400  produces an output beam with a more circular shape, thereby substantially matching (or more closely approximating) the circular mode field  318  of the optical fiber  206 . For example, the vertical far field angle  323  of the output beam or the light emitted from the laser diode  203  in the embodiment of the TOSA  400  shown in  FIG. 4  may be reduced from about 30° to about 16°. The horizontal far field angle  326  may remain at about 20°. The reduction in the vertical far field angle  323  provides a better match between the far field angle of the laser diode  203  and the acceptance angle of the optical fiber  206 . It should be noted that a standard DFB laser typically comprises a significantly greater vertical far field angle than a horizontal far field angle. 
     In the TOSA  400 , the lens  209  focuses and directs the emitted optical signals  310  and  421  emitted from the laser diode  203  and the reflected optical signals  416  and  419  to the acceptance region  450  of an optical fiber  206 . The phases of the emitted optical signal  310  and the reflected optical signals  416  and  419  comprise substantially the same phase at the image plane of the lens  209 , thus no optical interference occurs when the optical signals  310 ,  416 ,  419 , and  421  reach the acceptance region  450  of the optical fiber  206 . 
       FIG. 5  is a diagram of a portion  500  of the TOSA  400  according to an embodiment of the disclosure. The portion  500  includes the submount  406 , the mirror  409 , the laser diode  203 , emitted optical signals  310  and  421 , and reflected optical signals  416 ,  419 , and  521 . In an embodiment, the submount  406  is disposed on a TO-header  403  of TOSA  400 . 
     In an embodiment, the mirror  409  may be disposed on the submount  406  such that the mirror  409  extends from an edge of the laser diode  203  at a length sufficient enough to reflect a portion of the optical signals that are downwardly vertically emitted from the laser diode  203 . For example, as shown in  FIG. 4 , the mirror  409  extends from an edge of the laser diode  203  comprising the output aperture  350  to an edge of the submount  406 . In another embodiment, the mirror  409  may extend from any point on the surface of the submount  406  under the laser diode  203  to any point past the edge of the laser diode  203  comprising the output aperture  350  so long as a portion of the optical signals emitted from the laser diode  203  are reflected by the mirror  409 . 
     In an embodiment, the mirror  409  may be formed by polishing the surface of the submount  406 . In this embodiment, the submount  406  has a reflective enough surface for reflecting the optical signals emitted from the laser diode  203 . In another embodiment, a metal layer with reflective properties, such Ti and/or Au, is deposited on the surface of the submount  406  to comprise the mirror  409 . 
     In an embodiment, the laser diode  203  is flip-chip bonded to the submount  406 . As shown in  FIG. 5 , the active layer  306  is coupled to the submount  406  instead of to the substrate  303 . In addition, the output aperture  350  of the laser diode  203  is closer to the submount  406  when the laser diode  203  is flip-chip bonded to the submount  406 . This proximity between the output aperture  350  and the mirror  409  enables a greater amount of the emitted optical signals to be reflected than if the output aperture  350  is farther away from the mirror  409 . 
     In an embodiment, the laser diode  203  is coupled to the submount  406  and/or the mirror  409  via electrical contacts  506  and  503 . Electrical contacts  506  and  503  may be formed by soldering together metal contact points on the laser diode  203  to the metal contact points on the submount  406 . As the active layer  306  comprises a waveguide and grating responsible for generating the light, the active layer  306  emits more heat than the substrate  303 . Since the active layer  306  is substantially adjacent to the submount  406 , the submount  406  is configured to dissipate some of the heat from the active layer  306 . In this way, TOSA  400  provides more heat dissipation than a traditional TOSA. 
     The light emitted from the output aperture  350  of the laser diode  203  includes emitted optical signals  310  and  421  and reflected optical signals  416 ,  419 , and  521 . The reflected optical signals  416 ,  419 , and  521  are emitted from the output aperture  350  and reflected from the mirror  409  upwards and to the right. It should be noted that the emitted optical signals  310  and  421  are not reflected from the mirror  409 , but may still be captured by the lens  209 , which focuses the optical signals  310 ,  419 ,  416 ,  521 , and  421  and directs the focused optical signals  310 ,  419 ,  416 ,  521 , and  421  toward the optical fiber. 
       FIG. 6  is a diagram of a portion  600  of the TOSA  400  according to another embodiment of the disclosure. The portion  600  is similar to portion  500 , except that the submount  606  in the portion  600  includes a plateau  609  so that the output aperture  350  is closer to the mirror  409 , i.e., the laser diode  203  does not need to be flip-chip bonded to the submount  606 . The portion  600  of the TOSA  400  includes the submount  606 , the mirror  409 , the laser diode  203 , emitted optical signals  310  and  421 , and reflected optical signals  416 ,  419 , and  521 . 
     The submount  606  is similar to the submount  406  in that the submount  606  may be disposed on either the TO-header  403  or a substrate on the TO-header  403 . The mirror  409  is disposed on the plateau  609 . The plateau  609  is a block comprising the same material as the submount  606 , and includes a substantially planar surface that is substantially parallel to the active layer  306  of the laser diode  203 . In the embodiment shown in  FIG. 6 , the plateau  609  extends from an edge of the laser diode  203  comprising the output aperture  350  to an edge of the submount  606 . In other embodiments, the plateau  609  may extend from the edge of the laser diode  203  to any point on the surface of the submount  606  so long as at least some of the optical signals are reflected by the mirror  409 . A height of the plateau  609  may be based on the distance of the output aperture  350  above the submount  606 . The height of the plateau  609  may be minimal, wherein the mirror  409  is located only a small distance above the surface of the submount  606 . Alternatively, the plateau  609  may have a height that is a large percentage of the distance to the output aperture  350 . When the mirror  409  is disposed relatively near the surface of the plateau  609 , the mirror  409  is located relatively far below the output aperture  350 . For example, in some embodiments the height of the mirror  409  is about  100  micrometers (μm), or a few μm below the output aperture  350 . 
     As shown in  FIG. 6 , the substrate  303  is coupled to the surface of the submount  606 , and the active layer  306  is farther from the submount  606  than the substrate  303 . In this embodiment, the laser diode  203  does not need to be flip-chip bonded to the submount  606 . This is because the output aperture  350  from which optical signals are emitted is proximate to the mirror  409  because of the plateau  609 . In this embodiment, there is no need to flip the laser diode  203  to be closer to the mirror  409  because the plateau  609  brings the mirror close to the output aperture  350  of the laser diode  203 . In the embodiment shown in  FIG. 6 , the optical signals  310  and  421  are emitted from the output aperture  350  of the laser diode  203 , and the reflected optical signals  419 ,  416 , and  521  are reflected from the mirror  409 . 
       FIG. 7  is a graph  700  illustrating the optical field from the output aperture  350  of the laser diode  203  after being reflected by mirror  409  according to an embodiment of the disclosure. With the mirror  409  placed beneath the output aperture  350  of the laser diode  203 , the optical field comprises the emitted optical signals that are emitted by the output aperture  350  and not reflected by the mirror  409  and the reflected optical signals that are emitted by the output aperture  350  and are reflected by the mirror  409 . The optical field of the reflected optical signals is substantially the same as the optical field of the emitted optical signals. The emitted optical signals and the reflected optical signals are then passed through the lens  209 , which is configured to focus the emitted optical signals and the reflected optical signals onto a focal point at an image plane. The image plane may be substantially located on the acceptance region  450  at the core of the optical fiber  206  in some embodiments. A diameter of a core of a standard single mode optical fiber is about 8-10 μm, and a cladding diameter of the standard single mode optical fiber is 125 μm. A mode field diameter of the core of the optical fiber  206  is about 9 μm. In some embodiments, the emitted optical signals and the reflected optical signals are close together within 9 μm after the lens  209  focuses the emitted optical signals and the reflected optical signals. Therefore, the closer together the emitted optical signals and the reflected optical signals are after being focused by the lens  209 , the less coupling loss experienced between the laser diode  203  and the optical fiber  206 . 
     Graph  700  illustrates the optical field along the vertical direction (y-axis  706 ) relative to the distance from the laser diode  203  (x-axis  703 ). Horizontal line  708  represents the height of the mirror  409 . Portion  709  represents the optical field of the emitted optical signals and the reflected optical signals below the mirror  409 , and portion  710  represents the optical field of the emitted optical signals and the reflected optical signals that are above the mirror  409 . Curve  712  represents the optical field of the emitted optical signals, and curve  715  represents the optical field of the reflected optical signals that are reflected off mirror  409 . Curve  718  represents the total optical power of the combined optical fields represented by curve  712  and curve  715 . Note that the optical field vanishes beneath the mirror, so only the portions of the curves above the mirror represent the optical field. 
     In an embodiment, the optical signal  220  emitted from the laser diode  203  may be approximated by an elliptical Gaussian beam. The propagation and diffraction of the optical signal  220  may be analyzed by considering two-dimensional propagations in vertical direction (denoted as x) and the direction toward the optical fiber  206  (denoted as z) without including the horizontal direction since the mirror  409  does not modify the beam propagation in the horizontal direction. The laser diode  203  is positioned at a distance, denoted as h, above the mirror  409 , which is slightly above an optical axis of the optical fiber  206 . The electrical field of a two-dimensional Gaussian beam from emitted optical signal  220  is as follows: 
     
       
         
           
             
               
                 
                   
                     
                       E 
                        
                       
                         ( 
                         
                           x 
                           , 
                           z 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         
                           
                             w 
                             0 
                           
                           
                             w 
                              
                             
                               ( 
                               z 
                               ) 
                             
                           
                         
                          
                         
                           exp 
                            
                           
                             [ 
                             
                               - 
                               
                                 
                                   
                                     ( 
                                     
                                       x 
                                       - 
                                       h 
                                     
                                     ) 
                                   
                                   2 
                                 
                                 
                                   
                                     w 
                                     2 
                                   
                                    
                                   
                                     ( 
                                     z 
                                     ) 
                                   
                                 
                               
                             
                             ] 
                           
                         
                          
                         
                           exp 
                            
                           
                             [ 
                             
                               
                                 - 
                                 j 
                               
                                
                               
                                 
                                   
                                     k 
                                      
                                     
                                       ( 
                                       
                                         x 
                                         - 
                                         h 
                                       
                                       ) 
                                     
                                   
                                   2 
                                 
                                 
                                   2 
                                    
                                   
                                       
                                   
                                    
                                   
                                     R 
                                      
                                     
                                       ( 
                                       z 
                                       ) 
                                     
                                   
                                 
                               
                             
                             ] 
                           
                         
                          
                         exp 
                          
                         
                           { 
                           
                             - 
                             
                               j 
                                
                               
                                 [ 
                                 
                                   kz 
                                   - 
                                   
                                     φ 
                                      
                                     
                                       ( 
                                       z 
                                       ) 
                                     
                                   
                                 
                                 ] 
                               
                             
                           
                           } 
                         
                          
                         
                             
                         
                          
                         for 
                          
                         
                             
                         
                          
                         x 
                       
                       &gt; 
                       0 
                     
                   
                    
                   
                     
 
                   
                    
                   where 
                    
                   
                     
 
                   
                    
                   
                     
                       
                         w 
                          
                         
                           ( 
                           z 
                           ) 
                         
                       
                       = 
                       
                         
                           w 
                           0 
                         
                          
                         
                           
                             1 
                             + 
                             
                               
                                 ( 
                                 
                                   z 
                                   / 
                                   
                                     z 
                                     R 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                          
                         
                             
                         
                          
                         represents 
                          
                         
                             
                         
                          
                         the 
                          
                         
                             
                         
                          
                         spot 
                          
                         
                             
                         
                          
                         size 
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       
                         R 
                          
                         
                           ( 
                           z 
                           ) 
                         
                       
                       = 
                       
                         
                           z 
                            
                           
                             [ 
                             
                               1 
                               + 
                               
                                 
                                   ( 
                                   
                                     
                                       z 
                                       R 
                                     
                                     / 
                                     z 
                                   
                                   ) 
                                 
                                 2 
                               
                             
                             ] 
                           
                         
                          
                         
                             
                         
                          
                         represents 
                          
                         
                             
                         
                          
                         the 
                          
                         
                             
                         
                          
                         radius 
                          
                         
                             
                         
                          
                         of 
                          
                         
                             
                         
                          
                         curvature 
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       
                         φ 
                          
                         
                           ( 
                           z 
                           ) 
                         
                       
                       = 
                       
                         
                           
                             tan 
                             
                               - 
                               1 
                             
                           
                            
                           
                             ( 
                             
                               z 
                               / 
                               
                                 z 
                                 R 
                               
                             
                             ) 
                           
                         
                          
                         
                             
                         
                          
                         represents 
                          
                         
                             
                         
                          
                         the 
                          
                         
                             
                         
                          
                         Guoy 
                          
                         
                             
                         
                          
                         phase 
                          
                         
                             
                         
                          
                         shift 
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       
                         z 
                         R 
                       
                       = 
                       
                         
                           π 
                            
                           
                               
                           
                            
                           
                             w 
                             0 
                             2 
                           
                         
                         λ 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where k is the wave number of the laser diode  203 , or DFB laser, output, and w 0  is the waist size of the beam. 
     The mirror  409  on the submount  406  and  606  essentially folds the electrical field for x&lt;0, and the electrical field of the reflected optical signals  416  and  419  may be expressed as: 
     
       
         
           
             
               
                 
                   
                     
                       E 
                       r 
                     
                      
                     
                       ( 
                       
                         x 
                         , 
                         z 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           w 
                           0 
                         
                         
                           w 
                            
                           
                             ( 
                             z 
                             ) 
                           
                         
                       
                        
                       
                         exp 
                          
                         
                           [ 
                           
                             - 
                             
                               
                                 
                                   ( 
                                   
                                     x 
                                     + 
                                     h 
                                   
                                   ) 
                                 
                                 2 
                               
                               
                                 
                                   w 
                                   2 
                                 
                                  
                                 
                                   ( 
                                   z 
                                   ) 
                                 
                               
                             
                           
                           ] 
                         
                       
                        
                       
                         exp 
                          
                         
                           [ 
                           
                             
                               - 
                               j 
                             
                              
                             
                               
                                 
                                   k 
                                    
                                   
                                     ( 
                                     
                                       x 
                                       + 
                                       h 
                                     
                                     ) 
                                   
                                 
                                 2 
                               
                               
                                 2 
                                  
                                 
                                     
                                 
                                  
                                 
                                   R 
                                    
                                   
                                     ( 
                                     z 
                                     ) 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                        
                       exp 
                        
                       
                         { 
                         
                           - 
                           
                             j 
                              
                             
                               [ 
                               
                                 kz 
                                 - 
                                 
                                   φ 
                                    
                                   
                                     ( 
                                     z 
                                     ) 
                                   
                                 
                               
                               ] 
                             
                           
                         
                         } 
                       
                        
                       
                           
                       
                        
                       for 
                        
                       
                           
                       
                        
                       x 
                     
                     &gt; 
                     0 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     For simplicity, the lens  209  may be assumed as a thin lens, the ABCD matrix of the lens  209  is as follows: 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           A 
                         
                         
                           B 
                         
                       
                       
                         
                           C 
                         
                         
                           D 
                         
                       
                     
                     ) 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           1 
                         
                         
                           0 
                         
                       
                       
                         
                           
                             - 
                             
                               1 
                               f 
                             
                           
                         
                         
                           1 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The transformation of a Gaussian beam by a thin lens  209  is as follows: 
     
       
         
           
             
               
                 
                   
                     
                       q 
                       2 
                     
                     = 
                     
                       
                         
                           Aq 
                           1 
                         
                         + 
                         B 
                       
                       
                         
                           Cq 
                           1 
                         
                         + 
                         D 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   where 
                    
                   
                     
 
                   
                    
                   
                     
                       q 
                       1 
                     
                     = 
                     
                       
                         1 
                         
                           R 
                           1 
                         
                       
                       - 
                       
                         j 
                          
                         
                           λ 
                           
                             π 
                              
                             
                                 
                             
                              
                             
                               w 
                               1 
                               2 
                             
                           
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       q 
                       2 
                     
                     = 
                     
                       
                         1 
                         
                           R 
                           2 
                         
                       
                       - 
                       
                         j 
                          
                         
                           λ 
                           
                             π 
                              
                             
                                 
                             
                              
                             
                               w 
                               2 
                               2 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where j represents the imaginary component, w 1  and w 2  are the spot sizes of the Gaussian beam before and after the thin lens  209 , respectively, and f is the focal length of the thin lens  209 . Therefore, 
     
       
         
           
             
               
                 
                   
                     q 
                     2 
                   
                   = 
                   
                     
                       
                         1 
                         
                           R 
                           2 
                         
                       
                       - 
                       
                         j 
                          
                         
                           λ 
                           
                             π 
                              
                             
                                 
                             
                              
                             
                               w 
                               2 
                               2 
                             
                           
                         
                       
                     
                     = 
                     
                       
                         
                           f 
                           
                             R 
                             1 
                           
                         
                         - 
                         
                           j 
                            
                           
                             
                               λ 
                                
                               
                                   
                               
                                
                               f 
                             
                             
                               π 
                                
                               
                                   
                               
                                
                               
                                 w 
                                 1 
                                 2 
                               
                             
                           
                         
                       
                       
                         f 
                         - 
                         
                           1 
                           
                             R 
                             1 
                           
                         
                         + 
                         
                           j 
                            
                           
                             λ 
                             
                               π 
                                
                               
                                   
                               
                                
                               
                                 w 
                                 1 
                                 2 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     For the thin lens  209 , w 1  is about the same as w 2 . Equating the real and imaginary components of both sides of equation (5) results in the following: 
     
       
         
           
             
               
                 
                   
                     
                       
                         1 
                         
                           R 
                           2 
                         
                       
                        
                       
                         ( 
                         
                           f 
                           - 
                           
                             1 
                             
                               R 
                               1 
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         λ 
                         2 
                       
                       
                         π 
                          
                         
                             
                         
                          
                         
                           w 
                           1 
                           4 
                         
                       
                     
                   
                   = 
                   
                     
                       
                         f 
                         
                           R 
                           1 
                         
                       
                       ⇒ 
                       
                         R 
                         2 
                       
                     
                     = 
                     
                       
                         
                           fR 
                           1 
                         
                         - 
                         1 
                       
                       
                         f 
                         - 
                         
                           
                             λ 
                             2 
                           
                            
                           
                             
                               R 
                               1 
                             
                             / 
                             π 
                           
                            
                           
                               
                           
                            
                           
                             w 
                             1 
                             4 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     For the incident beam, both E(x, z) and E r (x, z), the distance of the object, for example, the laser diode  203  front facet, to the lens  209  is distance  430  (d o ). Then the radius of curvature at the z=d o  is as follows: 
     
       
         
           
             
               
                 
                   
                     R 
                     1 
                   
                   = 
                   
                     
                       R 
                        
                       
                         ( 
                         
                           d 
                           o 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         
                           d 
                           o 
                         
                          
                         
                           [ 
                           
                             1 
                             + 
                             
                               
                                 ( 
                                 
                                   
                                     z 
                                     R 
                                   
                                   / 
                                   
                                     d 
                                     o 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                           ] 
                         
                       
                       = 
                       
                         
                           
                             d 
                             o 
                           
                           + 
                           
                             
                               z 
                               R 
                               2 
                             
                             
                               d 
                               o 
                             
                           
                         
                         ≈ 
                         
                           
                             d 
                             o 
                           
                            
                           
                               
                           
                            
                           for 
                            
                           
                               
                           
                            
                           
                             z 
                             R 
                           
                            
                           
                             &lt;&lt; 
                             
                               d 
                               o 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     The spot-size is as follows: 
         w   1   =w ( d   o )= w   0 √{square root over (1+( d   o   /z   R ) 2 )}≈ w   0   d   o   /z   R  for  z   R   &lt;&lt;d   o    (8)
 
     Therefore, the curvature of the Gaussian beam after passing the lens  209  is as follows: 
     
       
         
           
             
               
                 
                   
                     R 
                     2 
                   
                   = 
                   
                     
                       
                         
                           fR 
                           1 
                         
                         - 
                         1 
                       
                       
                         f 
                         - 
                         
                           
                             λ 
                             2 
                           
                            
                           
                             
                               R 
                               1 
                             
                             / 
                             π 
                           
                            
                           
                               
                           
                            
                           
                             w 
                             1 
                             4 
                           
                         
                       
                     
                     ≈ 
                     
                       
                         
                           fd 
                           o 
                         
                         - 
                         1 
                       
                       
                         f 
                         - 
                         
                           
                             λ 
                             2 
                           
                            
                           
                             
                               z 
                               R 
                             
                             / 
                             π 
                           
                            
                           
                               
                           
                            
                           
                             w 
                             0 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     For R 2 &lt;0 , the lens  209  can focus the diverged Gaussian beam to a narrower beam waist. The phase of the light at the lens  209  for both E(x, z) and E r (x, z) fields are as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         ϕ 
                         E 
                       
                        
                       
                         ( 
                         
                           x 
                           , 
                           
                             d 
                             o 
                           
                         
                         ) 
                       
                     
                     = 
                     
                       
                         - 
                         
                           
                             
                               k 
                                
                               
                                 ( 
                                 
                                   x 
                                   - 
                                   h 
                                 
                                 ) 
                               
                             
                             2 
                           
                           
                             2 
                              
                             
                                 
                             
                              
                             
                               R 
                                
                               
                                 ( 
                                 z 
                                 ) 
                               
                             
                           
                         
                       
                       - 
                       
                         kd 
                         o 
                       
                       - 
                       
                         φ 
                          
                         
                           ( 
                           
                             d 
                             o 
                           
                           ) 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         ϕ 
                         Er 
                       
                        
                       
                         ( 
                         
                           x 
                           , 
                           
                             d 
                             o 
                           
                         
                         ) 
                       
                     
                     = 
                     
                       
                         - 
                         
                           
                             
                               k 
                                
                               
                                 ( 
                                 
                                   x 
                                   + 
                                   h 
                                 
                                 ) 
                               
                             
                             2 
                           
                           
                             2 
                              
                             
                                 
                             
                              
                             
                               R 
                                
                               
                                 ( 
                                 z 
                                 ) 
                               
                             
                           
                         
                       
                       - 
                       
                         kd 
                         o 
                       
                       - 
                       
                         φ 
                          
                         
                           ( 
                           
                             d 
                             o 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     Therefore, on the z=d o  plane, the phase of φ Er (x, d o ) is different from E(x, z). In some embodiments, there may be an interference pattern at the lens  209 . Even though the radius of curvature for E(x, z) and E r (x, z) fields is the same before or after the lens  209 , the small displacement between these two fields creates the interference pattern. 
     For Gaussian beam propagation in free space, the ABCD matrix is as follows: 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           A 
                         
                         
                           B 
                         
                       
                       
                         
                           C 
                         
                         
                           D 
                         
                       
                     
                     ) 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           1 
                         
                         
                           
                             d 
                             i 
                           
                         
                       
                       
                         
                           0 
                         
                         
                           1 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     where d i , is the propagation distance. Then, after a distance of d i , the Gaussian beam is transformed to a beam with radius of curvature 
     
       
         
           
             
               
                 
                   
                     
                       R 
                       3 
                     
                      
                     
                       ( 
                       
                         d 
                         i 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       1 
                       
                         
                           1 
                           / 
                           
                             R 
                             2 
                           
                         
                         + 
                         
                           d 
                           i 
                         
                       
                     
                     = 
                     
                       
                         R 
                         2 
                       
                       
                         1 
                         + 
                         
                           
                             d 
                             i 
                           
                            
                           
                             R 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     when 1+d i R 2  =0, where distance  433  (d i ) may be expressed as 
     
       
         
           
             
               
                 d 
                 i 
               
               = 
               
                 - 
                 
                   1 
                   
                     R 
                     2 
                   
                 
               
             
             , 
           
         
       
     
     the radius of curvature is infinity. This happens at 
     
       
         
           
             
               
                 d 
                 i 
               
               = 
               
                 - 
                 
                   1 
                   
                     R 
                     2 
                   
                 
               
             
             , 
           
         
       
     
     where R 2 &lt;0 for both E(x, z) and E r (x, z) fields. When the radius of curvature is infinite, the phase is constant on the z=d i  plane, as both fields go through the same transformation. The field at d i =− 1 /R 2  plane as follows: 
                       E        (     x   ,     d   i       )       =         exp        [     -         (     x   -     h   i       )     2         w   3   2          (     d   i     )           ]            {     -     j        [       k        (       d   o     +     d   i       )       -       tan     -   1            (       (       d   o     +     d   i       )     /     z   R       )         ]         }                   for                 x     &gt;   0            
              E   r          (     x   ,     d   i       )       =         exp        [     -         (     x   +     h   i       )     2         w   3   2          (     d   i     )           ]            {     -     j        [       k        (       d   o     +     d   i       )       -       tan     -   1            (       (       d   o     +     d   i       )     /     z   R       )         ]         }                   for                 x     &gt;   0               (   13   )               where  w   3   2 ( d   i )= w   0   2 (1 −d   i   /f ) 2   +w   0   2 ( d   i   /z   r ) 2    
     However, the centers of the transformed Gaussian fields E r  (x,d 0 ) and E(x, z) have a small displacement, h i =hd i /d o . This displacement is required to be smaller than the diameter of the optical fiber  206 . Thus, the distance between the mirror  409  and the laser diode  203  front facet may be small, which is achieved by flip-chip bonding the laser diode  203  to the submount  406 . In addition, the half Gaussian beam for the field E(x, z) is tilted slightly downward by an angle of θ=−h/d o , while the half Gaussian beam for the field E r (x, z) is tilted slightly upward by an angle of θ=−h/d o . If h is significantly less than d o , where h is about 1 μm to about 2 μm and d o &gt;1 millimeter (mm), this tilt may be neglected. 
       FIG. 8  is a diagram of a TOSA  800  according to another embodiment of the disclosure. The TOSA  800  comprises a laser diode  203 , a submount  806 , and an optical fiber  206 . The TOSA  800  is similar to the TOSA  400 , except that the TOSA  800  comprises a submount  806  with a curved or substantially doubly concave surface, and a mirror  809  disposed on the submount  806 . Mirror  809  is also concave instead of flat, being curved in both a longitudinal direction with regard to the top surface of the submount  806  and in a transverse direction. The output aperture  350  of the laser diode  203  faces the substantially doubly concave surface of the mirror  809 . Unlike mirror  409  in  FIGS. 4-5  that is only configured to reshape the vertical far field angle  323  of the laser diode  203 , the mirror  809  is configured to reshape both the vertical far field angle  323  and the horizontal far field angle  326 . 
     The laser diode  203  may or may not be flip-chip bonded to the submount  806 . The surface proximate to the substrate of the laser diode  203  is coupled to submount  806 . The submount  806  is similar to the submount  406  in that submount  806  also comprises a dielectric, such as AIN or another suitable material. Submount  806  may also be disposed on either the TO-header  403  or a substrate. However, unlike submount  406 , submount  806  is substantially L-shaped. 
     The mirror  809  is a curved or substantially doubly concave mirror that is disposed on at least the interior surface of the submount  806 . The mirror  809  is a semi-spherical or toroidal mirror that has a reflective surface, which curves downward to create a curved trough-like shape, curving substantially ninety degrees away from the laser diode  203  in the embodiment shown. Alternatively, other amounts of curvature can be employed. For example, the mirror  809  is a semi-tubularly concave surface including a curving, which is a substantially 90° bend. In an embodiment, the curving is around a vertex of the mirror  809 . In an embodiment, a vertex of the mirror  809  is disposed at a distance  880  from the acceptance region  450  of the optical fiber  206 . The vertex of the mirror  809  may be the center point of the mirror  809  in some embodiments. The distance  880  is based on the optical field of the light emitted from the laser diode  203  and an angle of incidence of light with respect to the mirror  809 . 
     As shown in  FIG. 8 , the laser diode  203  emits light that diverges into at least four emitted optical signals  812 ,  815 ,  818 , and  822 . The emitted optical signals  812 ,  815 ,  818 , and  822  are optical signals that diverge elliptically from the output aperture  350  of the laser diode  203  in a horizontal direction and/or a vertical direction. The horizontal direction is parallel to the active layer  306  of the laser diode  203 , and the vertical direction is substantially perpendicular to the active layer  306  of the laser diode  203 . The mirror  809  on the submount  806  is configured to reflect and re-direct substantially all of the light emitted from the laser diode  203 . 
     In an embodiment, the substantially doubly concave shape of the mirror  809  receives the emitted optical signals  812 ,  815 ,  818 , and  822 , and redirects the reflected optical signals  825 ,  828 ,  831 , and  834  at a substantially 90° angle toward a focal point located at an image plane. The focal point is a point in space at an image plane at which optical signals incident toward the mirror and reflected off the mirror  809  will meet after reflection. In an embodiment, TOSA  800  can be structured so that the focal point of the mirror  809  is substantially on or around the acceptance region  450  on the optical fiber  206 . This way, the reflected optical signals  825 ,  828 ,  831 , and  834  automatically converge toward the core of the optical fiber, thereby reducing the far field angle  230  of the laser diode  203 . In this embodiment, there may be little to no coupling loss between the laser diode  203  and the optical fiber  206 . 
     In some embodiments, a lens  209  may be positioned in between the submount  806  and the optical fiber  206 . However, since the lens  209  is typically used to focus reflected optical signals, such a lens  209  is not needed in this embodiment. This is because the reflected optical signals  825 ,  828 ,  831 , and  834  are substantially focused to a focal point due to the substantially doubly concave shape of mirror  809 . Therefore, the TOSA  800  eliminates the need for a lens  209  in an ONU  120  or OLT  110 . Instead, a less costly glass window can be disposed in place of the lens  209 . 
       FIG. 9  is a diagram of a portion  900  of the TOSA  800  according to an embodiment of the disclosure. The figure shows the substantially doubly concave shape of the mirror  809 . The portion  900  includes the submount  806 , the mirror  809 , the laser diode  203 , emitted optical signals  812  and  815 , and reflected optical signals  828  and  825 . In an embodiment, the submount  806  is disposed on a TO-header of TOSA  800 . In another embodiment, the submount  806  is disposed on a substrate, such as a metal block, which is disposed on the TO-header. 
     As shown in  FIG. 9 , the laser diode  203  is disposed on the submount  806  at a distance  920  away from the mirror  809 . The distance  920  is based on the optical field of the light emitted from the laser diode  203  and an angle of incidence of the emitted light with respect to the mirror  809 . The submount  806  is formed in a substantially L-shape and comprises a substantially doubly concave interior surface  905  upon which the mirror  809  is disposed. In various embodiments, the interior surface  905  may be any shape so long as the interior surface  905  supports the structure of the mirror  809 . The interior surface  905  of the submount  806  may be concave, or curved, so as to support the mirror  809  since mirror  809  is also concave. For example, the interior surface  905  of the submount  806  is a semi-tubularly concave surface including a bend or curving region, which is a substantially regular and continuous 90° bend around a center point of the interior surface  905  in the embodiment shown. In an embodiment, the interior surface  905  may have the same shape as the mirror  809 . 
     In an embodiment, the mirror  809  can extend along the submount  806  so long as the mirror  809  reflects the light emitted from the laser diode  203 . The mirror  809  comprises a vertex  910 . The vertex  910  is the geometric center of the concave structure of the mirror  809 , and the mirror  809  curves radially around the vertex  910 . The mirror  809  is a semi-spherical or toroidal mirror that has a reflective surface, which bulges inward (away from the light emitted from the laser diode  203 ). For example, the mirror  809  is also a semi-tubularly concave surface including a curving region, which is a substantially 90° bend around the vertex  910  of the mirror  809  in the embodiment shown. 
     The light emitted from the output aperture  350  of the laser diode  203  includes emitted optical signals  812  and  815  and reflected optical signals  825  and  828 . The emitted optical signals  812  and  815  may be optical signals in a horizontal direction or a vertical direction that are emitted from the output aperture  350 . The emitted optical signals  812  and  815  impinge on a surface of the mirror  809 . The emitted optical signals  812  and  815  are reflected from the surface of the mirror  809  based on a reflection angle  930  of the mirror  809 . The reflection angle  930  is the angle by which emitted optical signals  812  and  815  are reflected to produce the reflected optical signals  825  and  828 . In an embodiment, the emitted optical signals  812  and  815  are reflected by the mirror  809  at a reflection angle  930  of substantially 90° to form the reflected optical signals  825  and  828 . The reflected optical signals  825  and  828  may be optical signals in a horizontal direction or a vertical direction that are reflected by the mirror  809 . The concave shape of the mirror  809  focuses the reflected optical signals  825  and  828  to a focal point. In an embodiment, the concave shape of the mirror  809  reshapes and reduces the far field angle  230  of the laser diode  203 . Therefore, the portion  900  can be structured such that the image of reflected optical signals  825  and  828  is set to be the acceptance region  450  at a core of the optical fiber  206 . 
       FIG. 10  is a diagram  1000  of a mirror  809  according to various embodiments of the disclosure. Mirror  809  is a concave shape and comprises a concave surface  1010 . For example, the mirror  809  shown in  FIG. 10  is a toroidal mirror. A toroidal mirror is an aspherical mirror or a form of a parabolic reflector which has a different focal distance  920  depending on a deflection angle  930  of the mirror. In various embodiments, the mirror  809  may be spherical, as well as elliptic, parabolic, or hyperbolic, depending on the location of the acceptance region  450  of the optical fiber  206  and the location of the output aperture  350  of the laser diode  203 . The base of the mirror  809  has a rectangular shape. However, it should be appreciated that the base of the mirror  809  may be any shape so long as the mirror  809  may be formed on the interior surface  905  of the submount  806 . 
     The concave surface  1010  comprises a vertex  910 . The vertex  910  is a point or an area at the center of the concave surface  1010 . In an embodiment, the mirror  809  or the concave surface  1010  curves around the vertex  910 . The deflection angle  930  of the mirror  809  is about 90°. However, as should be appreciated, the deflection angle  930  of the mirror  809  can be any angle such that laser diode  203  emits optical signals that are reflected off of the concave surface  1010  onto a focus point, which is at an acceptance point of the optical fiber  206 . In an embodiment, the deflection angle  930  can be based on the focal distance  920 , the distance  880  from the mirror  809  to the acceptance region  450  of the optical, and/or the optical field of the optical signals emitted from the laser diode  203 . 
     As shown in  FIG. 10 , output aperture  350  of the laser diode  203  emits the emitted optical signals  812 ,  815 ,  818 , and  822 , which impinge upon the concave surface  1010  of mirror  809 . As shown in  FIG. 10 , the emitted optical signals  812 ,  815 ,  818 , and  822  impinge at different points on the concave surface  1010  of the mirror  809 . The concave surface  1010  of the mirror  809  reflects the emitted optical signals  812 ,  815 ,  818 , and  822  from the different points on the concave surface  1010  as reflected optical signals  825 ,  828 ,  831 , and  834 , respectively. The reflected optical signals  825 ,  828 ,  831 , and  834  are reflected from the different points on the concave surface  1010  to a focus point  1020  on an image plane  1030 . The concave shape of the mirror  809  enables the reflected optical signals  825 ,  828 ,  831 , and  834  to be focused and substantially collimated at the distance  880  from the mirror  809 . The image plane  1030  is a plane in which the reflected optical signals  825 ,  828 ,  831 , and  834  are substantially collimated. The focal point  1020  is a point or area on the image plane  1030  at which reflected optical signals  825 ,  828 ,  831 , and  834  are substantially collimated. In an embodiment, an acceptance region  450  of an optical fiber  203  can be positioned substantially at the focal point  1020  of the reflected optical signals  825 ,  828 ,  831 , and  834 . In this way, the optical signals emitted from the laser diode  203  are accepted by the optical fiber  206  without experiencing coupling loss due to diffraction of the optical signals emitted by the laser diode  203 . 
       FIG. 11  is a flowchart of a method  1100  for reducing a far field angle of a laser diode according to an embodiment of the disclosure. The method  1100  is implemented by an optical transceiver comprising TOSAs  400  or  800  or portions  500 ,  600 , or  900  when performing laser-to-fiber coupling. At step  1100 , an optical signal is generated. For example, laser diode  203  generates the optical signal  220 . The laser diode  203  emits the optical signal  220  as one or more emitted optical signals  310 ,  313 ,  316 ,  320 ,  413 ,  421 ,  812 ,  815 ,  818 , and  822 . In an embodiment, the laser diode  203  may be coupled to submount  406 ,  606 , or  806 . In an embodiment, a mirror may be disposed on the submount. The mirror may be mirror  409  or  809 . At step  1120 , reshaping a far field angle of the optical signal by reflecting, via the mirrored submount, a portion of the optical signal to produce a reflected optical signal. For example, a far field angle  230  of the emitted optical signal is reshaped by reflecting, via either mirror  409  or mirror  809 , a portion of the emitted optical signal to produce a reflected optical signal. The reflected optical signal is similar to reflected optical signal  416 ,  419 ,  521 ,  825 ,  828 ,  831 , or  834 . 
     In an embodiment where the mirror is the flat mirror  409 , a lens  209  may be used to further direct the emitted optical signals and the reflected optical signals to the optical fiber  206 , and the vertical far field angle  323  is reduced. In an embodiment where the mirror is a substantially doubly concave mirror  809 , a lens is not required, and both the vertical far field angle  323  and the horizontal far field angle  326  are reduced. At step  1130 , the reflected optical signals are directed towards a core of an optical fiber. For example, the reflected optical signals are directed to an acceptance region  450  of the optical fiber  206 . For example, the acceptance region  450  may be located at a core of the optical fiber  206 . In an embodiment, the optical transceiver is structured such that at the focus point, the reflected optical signals are substantially collimated together at the acceptance region  450  of the optical fiber  206 . 
       FIG. 12  is a diagram of an optical device  1200  according to an embodiment of the disclosure. The optical device  1200  is suitable for implementing the disclosed embodiments described above, such as an ONU  120  or an OLT  110  that includes TOSAs  400  or  800  or portions  500 ,  600 , or  900 . The optical device  1200  comprises ingress ports  1210 ; a receiver unit (Rx)  1220  coupled to the ingress ports  1210  and configured for receiving data; a processor, logic unit, or central processing unit (CPU)  1230  coupled to the Rx  1220  and configured for processing the data; a transmitter unit (Tx)  1240  coupled to the processor  1230 ; and egress ports  1250  coupled to the Tx  1240  and configured for transmitting the data; and a memory  1260  coupled to the processor  1230  and configured for storing the data. In an embodiment, the optical device  1200  is an ONU  120  or an OLT  110 . In such an embodiment, Tx  1240  and/or the Rx  1220  are optical transceivers included in an ONU  120  or an OLT  110 . The Tx  1240  and/or the Rx  1220  may include the one of the TOSAs  400  or  800  or portions  500 ,  600 , or  900 . The optical device  1200  may also comprise optical-to-electrical (OE) components and/or electrical-to-optical (EO) components coupled to the ingress ports  1210 , the Rx  1220 , the Tx  1240 , and the egress ports  1250  for egress or ingress of optical or electrical signals. 
     The processor  1230  is implemented by any suitable combination of hardware, middleware, firmware, and software. The processor  1230  may be implemented as one or more CPU chips, cores (e.g., as a multi-core processor), field-programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), or digital signal processors (DSPs). The processor  1230  is in communication with the ingress ports  1210 , Rx  1220 , Tx  1240 , egress ports  1250 , and memory  1260 . In an embodiment, the processor  1230  comprises an optical module  1270 . In an embodiment, the optical module  1270  may be configured to control the laser diode  203 . 
     The memory  1260  comprises one or more disks, tape drives, or solid-state drives and may be used as an over-flow data storage device, to store programs when such programs are selected for execution, and/or to store instructions and data that are read during program execution. The memory  1260  may be volatile and/or non-volatile and may be read-only memory (ROM), random-access memory (RAM), ternary content-addressable memory (TCAM), or static random-access memory (SRAM). 
     In an embodiment, the disclosure includes a means for emitting an optical signal, wherein the optical signal diffracts into a plurality of emitted optical signals, and a means for at least partially reflecting and redirecting the emitted optical signals to produce a plurality of reflected optical signals, and wherein the mirror is further configured to reshape a far field angle of the optical signal. 
     In an embodiment, the disclosure includes a means for emitting an optical signal, wherein the optical signal diffracts into a plurality of emitted optical signals, at least partially reflecting and redirecting the plurality of emitted optical signals to produce a plurality of reflected optical signals, and wherein the mirror is further configured to reshape a far field angle of the optical signal, and receiving the plurality of emitted optical signals and the plurality of reflected optical signals. 
     In an embodiment, the disclosure includes a means for generating an optical signal, wherein a mirror is disposed on the submount, wherein the optical signal is emitted from the laser diode as a plurality of emitted optical signals, a means for reshaping a far field angle of the optical signal by reflecting a portion of the plurality of emitted optical signals to produce a plurality of reflected optical signals, and directing the plurality of emitted optical signals and the plurality of reflected optical signals towards a core of an optical fiber. 
     While several embodiments have been provided in the present disclosure, it may be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and may be made without departing from the spirit and scope disclosed herein.