Abstract:
A method for achieving three-dimensional alignment of a pair of optical components, and apparatus for supporting such method, is initiated by fixing one of the optical components at a selected location on a semiconductor substrate. Subsequently, the other optical component and its associated submount are attached to a pair of coupled motion stages. A reference signal, to which the first optical component has been aligned, is transmitted to the other component and to a detector. That detector is positioned to measure changes in a selected characteristic of the reference signal, such as changes in optical power, as the position of the second optical component and its submount are manipulated. Through the use of a feedback loop, the second component and submount are moved in a pattern until an optimal alignment is converged upon.

Description:
This non-provisional application claims the benefit of U.S. Provisional Application No. 60/233 848 filed on Sep. 20, 2000 titled “Precision Three-Dimensional Opto-Mechanical Assembly Method and Apparatus” by inventors Yakov Kogan and Daryoosh Vakhshoori. 
    
    
     BACKGROUND OF THE INVENTION 
     As it is known in the optical communications art, light signals can be modulated in accordance with associated data signals such that the information is optically conveyed between transmitter and receiver devices. In order for that optical data to be efficiently transmitted, each of the intervening optical components should be in a precise, optimal alignment. Such alignment is typically achieved by transmitting an exemplary light beam through the optical components. As the light beam exits the arrangement of optical components, a measurement is performed of its parameters such as power or spectrum for instance. The optical components can be subsequently moved in relation to each other until an optimum combination of the output light parameters is attained. The process usually has many steps and is fairly slow. Sometimes the duration of the process becomes prohibitive to large volume manufacturing. It is especially true when three or more dimensional alignment is required. 
     Once an optimal alignment of the optical components is achieved, the components are fixed in place using a variety of methods. One of those methods, that is widely used, includes the use of ultra-violet (UV) curable epoxy. The epoxy is disposed on a portion of each component which will enable it to be rigidly fixed to a substrate or other structure. The UV source is subsequently turned on until the epoxy has cured. It is readily apparent that the optical components must remain precisely fixed until the epoxy has cured. It is also important to keep the epoxy layers as thin as possible to minimize the influence of the epoxy shrinkage during cure on the optical alignment. 
     A system is needed that allows fast and precise three-dimensional alignment of components and also allows the components to be held motionless while the epoxy is cured. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, a method and apparatus are provided for optimally aligning a number of optical components in three dimensions. 
     More specifically, a method is disclosed that is initiated by fixing one of the optical components at a selected location on a semiconductor substrate. Subsequently, the other optical component and its associated submount are attached to a pair of coupled motion stages. A reference light beam, to which the first optical component has been aligned, is transmitted to the other component and to a detector. That detector is positioned to measure changes in a selected characteristic of the reference signal, such as changes in optical power, as the position of the second optical component is manipulated. Through the use of a feedback loop, the second component and submount are moved in a pattern until an optimal alignment is converged upon. 
     In accordance with another aspect of the invention, the detector monitors a characteristic of the reference signal and, based upon that measurement, a determination is made regarding the location of the optical component which maximizes that characteristic. That process is repeated while the distance between the components is changed until optimal alignment is achieved. 
     In accordance with a further aspect of the invention, the optical component and its associated submount are fixed on the device substrate at the point of optimal alignment. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Of the drawings: 
     FIG. 1 is a block diagram of a configuration of aligned optical components; 
     FIG. 2 is a block diagram of a system for performing active three-dimensional alignment of the components of FIG. 1; and 
     FIG. 3 is a flow diagram showing the operation of the system of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a block diagram of a configuration  10  of aligned optical components  12  and  14  is shown in relation to the device substrate  18 , component submount  16  and the epoxy  20   a - c  that fixes each in place. Optical components  12  and  14  are not limited to any particular type of optical component however, for exemplary purposes, optical component  12  can be a lens and optical component  14  can be a vertical cavity surface emitting laser (VCSEL). With such an arrangement, the lens  12  focuses pump laser light  22  into the cavity of VCSEL  14 . Alternatively, optical components  12  and  14  could be a pair of lenses, which collectively is part of a larger configuration of optical components. 
     Regardless of the type of component, in order for the overall configuration to operate efficiently, optical component  14  should be optimally aligned with optical component  12  along the optical axis in three dimensions (X, Y and Z). (While it is known in the art, it should be noted that the symbol associated with the Y axis in FIG. 1 indicates that it is orthogonal to the X-Z plane). 
     Once such optimal alignment is achieved, optical component  14  is attached to submount  16 . Simultaneously, the entire structure is attached to substrate  18 . 
     The alignment process used in the prior art allows optical components  12  and  14  to be precisely aligned. However, such a process does not allow simultaneous active alignment along all three axis. More precisely, the aforementioned alignment process includes several independent stages of X-Y-Z motion. These stages each include motion along the X, Y and Z axis of a single component. For example, the first stage includes the positioning and attaching of the submount  16  to the substrate  18 , using a typical accuracy of less than  10  microns. The second stage includes the positioning of optical component  14  relative to submount  16 . A third stage is also used to position optical component  12  in relation to optical component  14 . Accordingly, an improvement to this type of process is one where several components are moved along the X, Y and Z axis at the same time, allowing the alignment process to more quickly converge on an optimum alignment. 
     Referring now to FIG. 2, a block diagram of a system  30  for performing active three-dimensional opto-mechanical alignment and attachment of optical components for fiberoptic devices is shown in relation to optical components  12  and  14 . The system  30  includes a compound movement stage including a bottom servo-driven X-Y-Z motion stage  32  and a top, servo-driven X-Y motion stage  34 . The servo-drive for top motion stage  34  is generated through a feedback loop that includes a photodetector  36  and a control circuit  38 , the output of which is conveyed to top motion stage  34 . 
     The bottom X-Y-Z motion stage  32  is coupled to a gripper arm referred to as the submount gripper arm  40 . The top X-Y motion stage  34  is effectively mounted on the bottom X-Y-Z motion stage  32  such that the X-Y plane in which motion stage  34  can move is perpendicular to the Z-axis movement of bottom stage  32 . Additionally, a second gripper arm referred to as the component gripper arm  42  is directly attached to top motion stage  34 . The system  30  allows the submount  16  and optical component  14  to move as a unified assembly in the X, Y and Z directions while optical component  14  can move independently in the X-Y plane along the surface of the submount  16 . 
     Three dimensional optimization of the position of optical component  14  is achieved by passing light beam  22  through components  12  and  14  and onto photodetector  36 . It should be noted that optical component  12  has previously been aligned to light beam  22 . A search operation is performed wherein the power detected by photodetector  36  is monitored while the top motion stage  34  is moved in a predetermined pattern in the X-Y plane. Once the optimization in X-Y plane is complete, bottom motion stage  32  starts moving along the Z axis in predetermined increments. Each time bottom motion stage  32  is moved, the top motion stage adjusts the position of component  14  until the power detected by photodetector  36  is maximized. That process is repeated for a number of predetermined increments along the Z-axis. Once completed, the top  34  and bottom  32  motion stages are returned to the position where the highest optical power was detected. 
     More specifically, referring to FIG. 3, the operation of system  30  procedes as follows. The system  30  is initialized by attaching optical component  12  to substrate  18 , mounting optical component  14  in gripper arm  42  and mounting submount  16  in submount gripper arm  40  (Step  50 ). Subsequently, epoxy is disposed on substrate  18  in the general vicinity where submount  16  will be positioned (Step  52 ). Epoxy is also disposed on submount  16  in the vicinity where optical component  14  will be positioned (Step  54 ). Once the components are coupled to the grippers and the epoxy is disposed, control circuit  38  transmits a signal to X-Y motion stage  34  that causes it to move optical component  14  into contact with submount  16  (Step  56 ). Additionally, the bottom X-Y-Z motion stage  32  receives a signal that causes it to move the assembly of optical component  14  and submount  16  to the vicnity on substrate  18  where it will be positioned (Step  58 ). It should be noted that optical component  14  and submount  16  move together at this point in the process since they are both essentially mounted on bottom X-Y-Z motion stage  32 . At this point in the process, the simultaneous X-Y-Z alignment operation can begin. 
     Next, active alignment is initiated in the plane perpendicular to the optical axis (i.e. the X-Y plane). During the active alignment process, control circuit  38  transmits signals to the top motion stage  34  that responsively moves the components in relation to the by photodetector  36 . Using the top X-Y motion stage  34 , driven by the feedback loop, optical component  14  is moved relative to the substrate  18  until photodetector  36  detects maximum optical power(Step  60 ). Once maximum power is detected, the position of optical component  14  and submount  16  is moved along the Z axis by manipulating the bottom motion stage  32 . Simultaneously, top motion stage  34  moves component  14  in the X-Y plane until a power maxima is recorded (Step  62 ). That motion results in the coordinated movement of optical component  14  and the submount  16  as one assembly. However, as that motion occurs, the top X-Y motion stage  34  will continue to move optical component  14  relative to submount  16  in response to the output of photodetector  36  and, hence, the feedback loop. After the bottom motion stage  32  has moved through a desired distance range along the Z-axis, it is returned to the Z-axis position of maximum recorded optical power. The top motion stage  34  is also returned to the position of maximum recorded optical power in the X-Y plane (Step  64 ). In this manner, alignment is achieved along the X, Y and Z axis simultaneously. 
     Once optimal alignment is achieved, UV light sources are turned on to begin the epoxy cure process, fixing the position of submount  16 , optical component  14  and optical component  12  (Step  66 ). Next, gripper arms  40  and  42  are signaled to release the associated components, and the entire assembly is ready for post cure operations (Step  68 ). 
     The present invention provides numerous advantages such as allowing true three dimensional active alignment and ensuring coordinated motion of the components during that alignment process without the need for complex and expensive control circuits. It also allows the epoxy layers to remain relatively thin such that shrinkage during the curing process is avoided. In addition, the system is compact and easy to implement, i.e. it has a small footprint which reduces ABBE errors. Finally, the present invention reduces the cost and complexity of the alignment equipment. 
     It should be noted that the system described hereinabove could be modified without departing from the scope of the present invention. For example, system  30  can be used to align devices other than optical components. Additionally, the submount can be manufactured of more than one piece. Further, a different mechanical arrangement can be used to achieve a similar motion of the components, i.e. simultaneous motion of the individual components along the X, Y and Z axis. Further still, it is anticipated that the top motion stage  34  can be an X-Y-Z motion stage as opposed to simply an X-Y motion stage. 
     Additionally, the feedback system can incorporate a device other than a photodetector to measure optical power or another selected characteristic of reference light signal  22 . In fact, other optical parameters such as wavelength, side mode suppression, full width at half maximum of a pass band, in isolation or combination, can be used to optimize alignment. Further, the present invention can be used for performing other positioning, aligning and attachment operations . Components attachment methods other than UV cured epoxy can be utilized. The axis of motion of the top and bottom stages can be not parallel to each other depending on the optical alignment and components mounting requirements. 
     It will be recognized that many configurations similar to those described above can be designed using different values, combinations and architectures which will yield the same results as the claimed invention. Thus, while this invention has been particularly shown and described with references to preferred embodiments herein, it is understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the appended claims.