Abstract:
A method of adjusting the attachment of an optical fiber to a laser diode chip within a butterfly package, or any equivalent package, identifies the location of the solder attachment on the optical fiber, thereby moving one or more high power lasers so that their respective beams are incident on the solder attachment. Then, with the application of high power laser pulses according to a predetermined pulse schedule, the solder attachment is heated so that the optical fiber may shift vertically therein. By using an internal or external monitoring means such as an optical power meter, a determination is made when the optical alignment has reached a desired threshold such as when it exceeds a desired optical power output. The pulse schedule may be performed multiple times until a substantially optimized coupling is achieved for the final package.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to fiber-coupled optical assemblies and, more particularly, to a method of non-mechanically optimizing alignment of an optical fiber to an optical output port in a butterfly package.  
       BACKGROUND OF THE INVENTION  
       [0002]     The importance of achieving accurate mutual alignment of individual components in any optical system is well known. The miniature dimensions of components used in modern optical communication systems render such alignment difficult both to achieve and to maintain. For example, one problem in the construction of laser transmitters is that of efficiently coupling the optical output from a laser diode into an optical fiber. To obtain efficient coupling, the fiber end is desirably precisely aligned with the emitting area of the laser. When such alignment is achieved, the fiber is then fixed in place, ideally by a method that ensures alignment is sustained throughout the device lifetime.  
         [0003]     Typically, fiber-coupled diode lasers are packaged in gold plated metal butterfly packages and the fiber is held in alignment with the laser using either epoxy, laser weld, or solder attachment techniques with or without a ferrule. Epoxy attachment is low cost but may have too much thermal expansion for high precision attachments. Furthermore, it is not reliable over a long period of time due to outgassing and alignment shifts arising from aging and temperature cycling. Laser weld techniques are reliable but require costly ferrulization of the fiber and specially designed mounts or clips to allow weld attachment of the ferrulized fiber. The mounts/clips are expensive, large, and may creep over time. Solder attachment techniques, on the other hand, are reliable and low cost, and have become prevalent in the art. However, existing solder attachment techniques tend to use either an integrated heating mechanism and/or a specially configured platform to isolate the heat for a solder reflow. These tend to be expensive and undesirably large.  
         [0004]     The mounting point at which the fiber is soldered desirably has specific material properties in order to work effectively. An acceptable material for the mounting point desirably has a low thermal conductivity (e.g. less than 50 W/m-K) and a thermal expansion coefficient that maintains fiber alignment while the package is heated. The exact thermal expansion property desired may depend on the material to which the laser is mounted, the respective thickness of the fiber mount and laser submount, and/or the temperature profiles expected during operation. The fiber mount material also may be able to be soldered or be able to be plated with a solderable material. During the soldering process, the fiber mount may experience significant stress resulting from differential expansion due to temperature gradients and materials differences. Therefore, the fiber mount desirably has a high tensile strength (e.g. greater than 25 kpsi) to avoid fracturing.  
         [0005]     It is difficult, however, to maintain alignment between the optical component and the fiber when the fiber soldered due to turbulent flows and capillary forces exhibited by the molten solder. For example, the prior art package shown in  FIG. 1  is a butterfly package  100 , where optical fiber  114  is inserted into fiber feed-through  101  and attached to fiber mount  102  with solder attachment  103  so as to obtain a desirable alignment with laser diode chip  112 . However, in such a package, the reflowing of solder attachment  103  creates the above mentioned turbulent flows and capillary forces, thereby causing a misalignment between optical fiber  114  and laser diode chip  112  resulting in an undesirable alignment quality therein. Currently, post soldering adjustments are made to correct this misalignment by physically contacting the optical fiber and/or bending the attachment beyond its yield point, undesirably resulting in permanent deformation. Such methods of physically adjusting the alignment may introduce further stresses, causing the optical fiber to creep over time. Other methods of post-solder adjustments may include the undesirable addition of costly and complicated tooling (e.g., grippers) and high precision linear or rotary stages.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is embodied in a method of aligning an optical fiber attached to a mount pad by a heat sensitive attachment to an adjacent optical source having an optical output port by activating the optical source to emit a signal from the optical output port, aligning a plurality of high-power lasers such that their respective beams are incident on the heat sensitive attachment, activating the plurality of high-power lasers to apply their respective beams on the heat sensitive attachment according to a predetermined pulse schedule, thereby adjusting the fiber at least vertically toward a desired position, activating an optical power meter to obtain a measure of alignment quality between the optical fiber and the optical output port, and repeating until the measure of alignment quality is greater than a predetermined threshold.  
         [0007]     In a further embodiment, high-power laser alignment may be achieved by visually aligning the heat sensitive attachment with respect to the plurality of high-power lasers to align the heat sensitive attachment to the plurality of high-power laser beams. In an alternate embodiment, such alignment may be achieved by moving the plurality of high-power lasers to a predetermined position with respect to the heat sensitive attachment. In another alternate embodiment, high-power laser alignment may be achieved by activating a camera system to identify a location of the heat sensitive attachment with respect to the plurality of high-power lasers, and moving the plurality of high-power lasers with respect to the heat sensitive attachment to align the beams of the high-power lasers to the location of the heat sensitive attachment.  
         [0008]     In a further embodiment, the predetermined pulse schedule activates the plurality of high-power lasers to emit 5-8 bursts of 10-14 pulses of optical energy each a pause in between each respective burst of between 170 and 280 milliseconds, each pulse having 10-22 watt pulse power and a pulse width of approximately 22 to 38 milliseconds.  
         [0009]     In another embodiment of the invention, a method of aligning an optical fiber to an optical output port providing an optical signal from an optical source comprises the steps of providing the optical fiber adjacent to the optical output port, the optical fiber being attached to a mount pad with a heat sensitive attachment, activating a localized pulsed heating means over the heat-sensitive attachment according to a predetermined pulse schedule to cause a vertical shift of the fiber, monitoring optical coupling efficiency with a power meter, and repeating as necessary to obtain a predetermined coupling efficiency.  
         [0010]     In an alternate embodiment of the present invention, a method of thermally adjusting an attachment of an optical fiber to a laser diode chip inside a butterfly package comprises the steps of placing the butterfly package on a surface cooled to a predetermined temperature, activating the laser diode chip to emit an optical signal, respectively aligning the attachment and a plurality of high power lasers so that the attachment is substantially within a meeting point of a plurality of beams of the plurality of high power lasers, activating the plurality of high power lasers to emit pulses of optical energy over the attachment, according to a predetermined pulse schedule, activating an optical power meter to obtain a measure of alignment quality between the optical fiber and the laser diode chip and repeating until the measure of alignment quality is greater than a predetermined threshold.  
         [0011]     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0012]     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:  
         [0013]      FIG. 1  is a perspective view of a fiber-coupled optical component package showing the snout feed-through for an optical fiber and the optical fiber soldered to a separate mount component within the package, according to the prior art;  
         [0014]      FIG. 2  is a side plan drawing of an optical fiber with vertical rotational misalignment with respect to an optical component, according to the prior art;  
         [0015]      FIG. 3  is a front plan drawing of the preform in  FIG. 3  with an optical fiber placed within the groove and laser radiation incident on the preform according to the present invention;  
         [0016]      FIG. 4  is a perspective view of a fiber-coupled optical component package with a superimposed cone illustrating possible locations of high power lasers in one embodiment of the present invention;  
         [0017]      FIG. 5  is a flowchart of a method of alignment according to one embodiment of the present invention;  
         [0018]      FIG. 6  is a flowchart showing an exemplary method of aligning a high power laser to a solder attachment, according to the present invention; and 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     Referring now to the drawing, in which like reference numbers refer to like elements throughout the various figures that comprise the drawing,  FIG. 1  shows a butterfly package  100  according to the prior art. The prior art package  100  includes optical fiber  114  inserted through snout feedthrough  101  and attached to fiber mount  102  with solder attachment  103 . Optical fiber  114  is also optically coupled to optical component  112  (e.g. semiconductor laser, laser diode chip, photodetector) mounted on a substrate  111 . Also included in the package shown in  FIG. 1  is a thermistor  116 . If the package includes a thermo-electric cooler (TEC) it is beneath the substrate  111 .  
         [0020]     It can be seen from  FIG. 1  that optical fiber  114  is mounted on a fiber mount  102  that is separate from the substrate  111  on which optical component  112  is mounted. Furthermore, because the fiber is attached when the optical component  112  is in the package  100 , the alignment process may be complicated because there is no available side view of the area where fiber  114  is to be aligned and coupled to optical component  112 . Further, the insertion of fiber  114  through snout feedthrough  101  may require a non-vertical manufacturing process, which may cause complications in the manufacturing process resulting in yield issues. Additionally, optical fiber  114  is attached to fiber mount  102  by application of a drop of molten solder  103 , which may result in a change in the alignment prior to attachment.  
         [0021]     It can be seen that the coupling efficiency achieved by the alignment in the prior art package  100  described above may be undesirably low for high-precision optical fibers and semiconductor lasers. A side plan view is further presented in  FIG. 2  to illustrate one possible type of misalignment that may occur in the prior art. In such a case, laser diode  211  is mounted on laser sub-mount  201 , and is optically coupled to optical fiber  214 , which is attached to fiber mount  202  with heat sensitive attachment  203  (e.g., solder). As is shown in the figure, there is an exaggerated vertical misalignment between the fiber  214  and laser diode  211 , which may cause a considerable degradation of the coupled optical signal strength. Other types of misalignments that may occur may include vertical translational misalignments, for example. Those skilled in the art will recognize, therefore, that such a misalignment, if present in the laser-coupled optical fiber butterfly package  100  of  FIG. 1 , will be complicated to resolve without introducing additional deformations and misalignment.  
         [0022]     A mounting system which uses the present invention addresses this problem by mounting the fiber on a mount pad that is on the same substrate as the optical component and aligning the fiber to the optical component before the substrate is soldered into the package. The heating of the substrate that occurs when it is soldered to the package may disturb the alignment between the optical fiber and the optical component. If the optical device is a single-mode laser, this misalignment may result in a loss of coupled power of as much as 80 percent. For a multi-mode laser, the coupling loss may be as much as 20 percent. The inventors have determined that this misalignment is typically a vertical misalignment and may be corrected by the methods described below.  
         [0023]     In the present invention, with respect to  FIG. 3 , a method is presented for aligning optical fiber  314  attached to mount pad  302  by heat sensitive attachment  301  (e.g., solder) to an adjacent optical source (not shown in  FIG. 3 ) having an optical output port. In this embodiment of the present invention, the optical source (e.g., single mode semiconductor laser, multi-mode semiconductor laser, optical mirror, second optical fiber, semiconductor optical amplifier, optical concentrator, light-emitting diode) is activated to emit an optical signal (not shown in  FIG. 3 ) via an output port (not shown in  FIG. 3 ). The reception of the optical signal by optical fiber  314  may then be monitored by an optical power meter (not shown in  FIG. 3 ) in order to obtain a measurement of alignment quality. The measurement of alignment quality may provide an indication of absolute power reception or, alternately, power reception relative to a predetermined figure. In order to correct any vertical misalignments of optical fiber  314  within heat sensitive attachment  301 , one or more high power heating lasers (not shown in  FIG. 3 ) are aligned over the package such that their respective beams  315  are incident on heat sensitive attachment  301 . The one or more high power lasers are then activated to apply beams  315  on heat sensitive attachment  301  according to a predetermined pulse schedule. Such an application of pulses causes a vertical shift in optical fiber  314 . The pulse schedule may then be repeated until the optical power meter indicates that alignment quality has reached or is greater than a predetermined threshold or otherwise desired value.  
         [0024]     Those skilled in the art will recognize that the optical source having an optical output port in all embodiments of the present invention may alternately be a photo detector with an optical input port. In such an embodiment, the alignment quality of the optical fiber to the photodetector may be obtained by monitoring an output signal of the photodetector, or an output signal of circuitry coupled to the photodetector.  
         [0025]     In a further embodiment of the present invention, the method of aligning the high power lasers to emit their respective beams over the heat sensitive attachment may include visually aligning the heat sensitive attachment with respect to the plurality of high power lasers. Such a visual alignment may be obtained manually or by activating a camera system to identify the location of the heat sensitive attachment with respect to the plurality of high-power lasers using image detection algorithms prevalent in the art, and thereby moving the high-power lasers with respect to the heat sensitive attachment to align the respective beams of the high-power lasers so that they are incident on the heat sensitive attachment.  
         [0026]     In an alternate embodiment, a misaligned laser-coupled optical fiber package may be placed predictively in a predetermined position, so that the one or more high power lasers may be aligned by moving them to corresponding predetermined positions with respect to the heat sensitive attachment, wherein their respective beams will be incident on the heat sensitive attachment.  
         [0027]     In a further embodiment, the predetermined pulse schedule activates the plurality of high-power lasers to emit 5-8 bursts of 10-14 pulses of optical energy each a pause in between each respective burst, each pulse having 10-22 watt pulse power and a pulse width of approximately 22 to 38 milliseconds and preferably about 30 milliseconds. The pause between respective bursts may be from 170 and 280 milliseconds and preferably about 225 milliseconds, for example.  
         [0028]     Furthermore, in another embodiment of the invention, two high power heating lasers may be positioned symmetrically on the outer surface of cone  409 , having height H, radius R, and corresponding plume angle A. High power lasers  455  and  457 , for example, may be placed symmetrically on the outer edge of the cone, so that their beams are incident on the heat sensitive attachment. Alternately, high power lasers  475  and  477  may be symmetrically placed further down the outer surface. In one embodiment of the invention the laser may have a working distance of 5 cm (2 inches), plume angle A may be 60 degrees, height H may be 4.4 cm (1.73 inches), and radius A may be 2.5 cm (1 inch). Alternately, they may be positioned arbitrarily as long as their respective beams are incident on the heat sensitive attachment and provide heating of the heat sensitive attachment to 70-110 degrees Celsius, in one embodiment.  
         [0029]      FIG. 5  shows a method of aligning an optical fiber to an optical output port of an optical component in a package, according to one embodiment of the present invention. In optional step  502 , the surface on which the optical package is to be placed may be cooled to a predetermined desired operating temperature, which may be, for example, 20-25 degrees Celsius, so that any misalignment may be measured at that temperature. In a further optional step  504 , a thermoelectric cooler (TEC) (not shown) of the optical package may be deactivated so that it does not interfere with heating in the alignment process. In this exemplary embodiment, the TEC is activated after the alignment process to determine if the optical fiber and optical component are aligned at the desired operating temperature. If it is expected that the device will operate at several different temperatures, alignment measurements may be taken at each of these temperatures to optimize the overall alignment of the optical fiber to the optical component.  
         [0030]     Alternately, if the optical package does not include a TEC, such a step is omitted. Step  506  provides the optical package and places it on a surface—the cooled surface of step  502 , if step  502  was performed. In step  508 , one or more high power lasers are aligned over the optical package so that their respective optical beams are incident on a heat sensitive attachment of the optical package. Step  510  activates a power meter that receives its input from the optical fiber, and obtains measures of the alignment quality between the optical fiber and optical output port. The optical power meter may obtain alignment quality measurements continuously throughout the rest of the method, or, alternately, it may provide a measure of alignment quality upon the completion of each pulse schedule. Step  512  activates the optical component to emit an optical signal from the optical output port. In general, steps  508 - 512  may be performed in any order. Step  514  activates the one or more high power lasers to emit pulses of optical energy on the heat sensitive attachment according to a predetermined pulse schedule. In one embodiment, the predetermined pulse schedule activates the plurality of high-power lasers to emit 5-8 bursts of 10-14 pulses of optical energy each with a 170 to 280 millisecond pause, and preferably a 225 millisecond pause, between each respective burst and each pulse having 10-22 watt pulse power and a pulse width of between 22-38 milliseconds and preferably about 30 milliseconds. The temperature of the package may be monitored to control the application of laser energy so that the package reaches a desired temperature range but does not exceed it. In the exemplary embodiment of the invention, the temperature of the package may be measured using a temperature measuring device such as a thermometer, thermocouple, thermistor or other such device. For devices that include thermo-electric coolers (TECs), this temperature range may be, for example, 90-110 degrees Celsius. For devices without TECs the range may be, for example, 70-90 degrees Celsius. In step  516 , a determination is made whether the alignment quality observed by the optical power meter is greater than or equal to a desired threshold value. If it is not, the method proceeds to step  514 , where the pulse schedule is repeated. Once the alignment quality threshold is reached, the method ends in step  518 .  
         [0031]     In an alternate embodiment of the present invention, the predetermined pulse schedule that is repeatedly applied to achieve desirable alignment of the optical fiber and output port may comprise a plurality of predetermined pulse schedules, wherein the process applies a predetermined pulse schedule corresponding to a current measure of alignment quality. The process may, therefore, apply a different predetermined pulse schedule for each measure of alignment quality, or each range of values of alignment quality.  
         [0032]      FIG. 6  is a flow chart showing a method of aligning the high power lasers to the heat sensitive attachment, described above, and using visual alignment with a camera system. Step  601  activates and initializes the camera system, which may be a Powervision System, available from Acuity Imaging, Inc., for example. It is contemplated, however, that any similar vision system may be used. In step  603 , by using data obtained by the camera system, a processor or manual operator identifies the location of the heat sensitive attachment, which may be a metal solder, for example. A motion vector may then be generated to move the high power lasers in alignment with the heat sensitive attachment, in step  605 . Further data from the camera system may be obtained in step  607  to determine if the high power lasers are within substantial alignment with the heat sensitive attachment. If they are not, the method iterates from step  603 . If alignment is confirmed, however, the method ends in step  609 . Those skilled in the art will recognize that moving the high power lasers to achieve alignment with the heat sensitive attachment may be alternately performed by moving the package containing the heat sensitive attachment to achieve alignment with the high power lasers.  
         [0033]     Although illustrated and described above with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.