Abstract:
An optical package includes one or more MEMS mirrors to provide alignment between internal optical components and the signal port(s) on the package (where one or more ports may include optical fiber attachments). Once the components are placed in the package, an electrical signal is used to adjust the deflection profile of the appropriately positioned MEMS mirror(s) until maximum coupling between the internal components and the fibers/ports is obtained. Advantageously, if later signal degradation occurs due to, for example, subsequent physical misalignment of the internal components, corrective electrical signal can be sent to the MEMS mirror(s) to provide correction and re-alignment without having to open the package and physically move the components.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application claims the benefit of Application Ser. No. 60/436,434, filed Dec. 24, 2002. 

   BACKGROUND OF THE INVENTION 
   One of the major advances in recent years has been the increased use of optical communication systems for carrying very large quantities of information with low distortion and at a relatively low cost over great distances. Optical systems are also promising for such purposes as computing because of the inherently high speeds at which they can be operated. For these reasons, considerable development work ha been done in making various photonics packages for use in such systems. Photonics generally refers to devices that share both electronic and optical attributes, such as lasers, which generate coherent light in response to an electrical signal, and photodetectors, which generate an electrical signal in response to light. 
   A fundamental problem in making a photonics package such as a laser source module is the alignment of a device such as a laser source module with an optical waveguide. Conventional packages for photonics arrangements are typically made out of variety of dissimilar materials, such as metal, glass and ceramic, and involve relatively complicated manipulation of components during assembly. That is, the assembly process involves moving the components in three dimensions in order to place the components in the desired locations; for example, alignment of a device to a substrate, alignment of a fiber to a ferrule, alignment of a ferrule to a package and, finally, alignment of the package to the device. These alignment steps depend upon fairly specialized, expensive equipment. Thus, it would be desirable to find an alternative mechanism for performing one or more of these alignment procedures. 
   SUMMARY OF THE INVENTION 
   The need remaining in the prior art is addressed by the present invention, which relates to optical alignment arrangements and, more particularly, to the use of microelectromechanical system (MEMS) mirror elements to provide optical alignment between components in an optical communication system. 
   In accordance with the present invention, one or more MEMS mirrors is disposed in an optical package and positioned to reflect optical signals between an active optical device (e.g., one or more sources/detectors) located in the package and a passive receiver of light (e.g., transmission fiber, optical waveguide) located at a communication port on a package wall. In operation, the active optical device is turned on and the optical coupling between the active device and the passive device is measured. The deflection profile of the MEMS mirror(s) is then adjusted, using an electrical input signal, until maximum coupling is achieved. The use of a MEMS mirror to “fine tune” the coupling between the active and passive devices thus eliminates the need to perform the rigorous mechanical alignment (such as using an x-y alignment table) so prevalent in the prior art. 
   In one embodiment of the present invention, the electrical activation of the MEMS mirror(s) is performed until maximum coupling is obtained, and the positioning of the MEMS mirror is not further adjusted until misalignment is recognized. In an alternative embodiment, an alignment control feedback loop is contained within the package, where the alignment between the active and passive components is continuously monitored and the deflection profile of the MEMS mirror(s) is changed as needed. 
   Other and further embodiments and advantages of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings, 
       FIG. 1  is a block diagram of an embodiment of the present invention; 
       FIG. 2  is a block diagram of a second embodiment of the present invention; 
       FIG. 3  is a block diagram showing a top view of a third embodiment of the present invention; 
       FIG. 4  is a block diagram showing a top view of a fourth embodiment of the present invention; 
       FIG. 5  is a block diagram of a transceiver embodiment of the present invention; and 
       FIG. 6  is a block diagram of yet another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention will best be understood from the detailed description given below, as well as from the accompanying drawings of the embodiments of the present invention which, however, should not be taken to limit the invention to specific embodiments, but rather are for explanation and understanding only. 
   Referring to  FIG. 1 , there is shown one embodiment of the invention. In particular,  FIG. 1  shows a basic package embodiment of the invention that couples light from a light source to an optical fiber output (or another passive output device, such as an optical waveguide) that is attached to an outside wall of the package. As shown, package  10  includes a laser diode light source  11 , a MEMS mirror  12  and a fiber  13 . Laser diode  11  generates a conical-shaped beam of light  14  which propagates toward MEMS mirror  12 . As mentioned above and will be discussed in detail below, the deflection profile of MEMS mirror  12  is electrically adjusted (through a well-known process) to steer and control the alignment of the beam through a lens system  16  and into fiber  13 . In particular, after MEMS mirror  12  redirects the light beam as beam  15 , lens system  16  focuses light beam  15  toward fiber  13 . In accordance with the present invention, MEMS mirror  12  is activated with an electrical signal to modify its deflection profile so as to redirect beam  15  into the core region of fiber  13 . It will be understood by those skilled in the art that other components can be substituted for fiber  13 . For example, a light detector (such as a photodiode), a mirror, or an optical receiver could be attached at the exit port of package  10  in place of fiber  13 . Package  10  is assembled with the shown components inside the package, then fiber  13  is attached and the package is sealed. Before package  10  is sealed, the alignment of the components inside the package is tested by adjusting MEMS mirror  12  (using an applied electrical signal to change its deflection profile) so that the optical power reaching fiber  13  is at the desired level. After the package is sealed, the light input to fiber  13  can continue to be measured, and further electrical adjustment signals applied to MEMS mirror  12  to provide alignment, since the process of sealing the package may introduce misalignment into the system. Shown in phantom (and exaggerated for the purposes of illustration) is a misalignment between beam  15  and fiber  13 , illustrating an initial misalignment when the components are first placed in package  10  and fiber  13  is attached to output port  17  of package  10 . Unlike prior art arrangements, it is not necessary to precisely align fiber  13  with port  17 , since the deflection profile of MEMS mirror  12 , as controlled by an electrical input signal, is changed to steer the path of beam  15  until alignment is achieved. 
   This package has several advantages over the prior art, due to its use of a MEMS mirror. First, it allows for reduced placement tolerance and placement accuracy of the components inside the package. “Accuracy” is defined as the mean placement position relative to the design placement position. “Tolerance” is the variation in placement position relative to the mean. Second, it allows for monitoring and post-assembly alignment adjustment between the active (e.g., laser) and passive (e.g., fiber) components. That is, the strength of the transmission of light from laser diode  11  to fiber  13  through MEMS mirror  12  can be monitored by measuring the optical output power at fiber  13 . If the output power increases or decreases beyond a specified range, the output power can be brought back into the specified range by improving the re-alignment of laser diode  11  with fiber  13 . The improvement in re-alignment is achieved by using an electrical signal to modify the deflection profile of MEMS mirror  12 . The ability to adjust MEMS mirror  12  thus reduces the variance of coupled optical output power. In particular, if the elements inside of the package go out of alignment after assembly, adjustment of the MEMS mirror can be used to compensate for the misalignment. 
   Referring to  FIG. 2 , there is shown another embodiment of the present invention, this embodiment including a feedback loop within the package to continuously monitor and realign the active optical device(s) with the associated optical fiber (or other passive device). As shown, package  20  includes a laser diode light source  21 , a MEMS mirror  22 , a fiber  23 , and a monitoring photodiode  24 . The output of photodiode  24  is connected by any means known to those skilled in the art to both MEMS mirror  22  and laser diode  21  so that photodiode  24  will provide feedback signals to both devices. 
   In operation, laser diode  21  emits light beam  25  toward MEMS mirror  22 , which divides the light into two new beams  26 ,  28  of a predetermined power ratio, where the majority of the signal power will be within beam  28 , directed to the system and, and a minimal amount of power remaining in beam  26  directed to the feedback photodiode. Emitted beam  25  can be divided into two beams by incorporating a slit into MEMS mirror  22 , or by using any other means that is well-known to those skilled in the art. Each new beam of light  26 ,  28  is focused by a lens system  27  similar to lens system  15  of  FIG. 1 . It will be understood that similar lens systems are used with all of the embodiments of the present invention. Referring back to  FIG. 2 , beam  28  is directed to fiber  23  (or any other suitable passive optical device). Beam  26  is directed to photodiode  24 , which is located at any convenient position inside package  20 . For example, photodiode  24  can be located on a shelf above laser diode  21 . As an alternative to using a split MEMS mirror, a pair of co-located MEMS mirrors may be used, with one having its reflection directed toward fiber  23  and the other having its reflection directed toward monitoring photodiode  24 . 
   In accordance with the present invention, by receiving a portion of the laser output beam, monitoring photodiode  24  is able to sense changes in the laser diode&#39;s output power and will, for example, sense the decrease in output power from laser diode  21  over time (causing the output beam to shift slightly and lose alignment). In accordance with the present invention, monitoring photodiode  24  will receive maximum power when laser diode  21 , lens system  27  and fiber  23  are properly aligned. As misalignments develop, the received power will drop. Feedback circuit  29 , responding to the output from monitoring photodiode  24 , recognizes the drop in power and provides a control signal input to MEMS mirror  22 , which will change the deflection profile of MEMS mirror  22  and steer both beams  26  and  28  until maximum power is restored. 
   Referring to  FIG. 3 , there is shown another embodiment of the present invention.  FIG. 3  illustrates an optical package that is used to couple light from multiple light sources into a single optical fiber output (or other passive output device). This embodiment is particularly well-suited for high power applications. As is well-known in the art, solid state lasers have limited output power. At the present time, their output power is limited by design characteristics to a value of approximately 300 mW. However, much higher power levels are needed for a number of newer applications. Using the embodiment of  FIG. 3 , the output power of multiple laser diodes can be cascaded to obtain a single, high power, output signal. 
   In particular,  FIG. 3  contains a top view of an exemplary high power transmitter package  30 . In this embodiment, a set of three laser diodes, denoted  31 A,  31 B and  31 C are shown, and for this particular case each emits light at essentially the same wavelength. In general, any desired number N of such lasers may be included in a high power transmitter package. Package  30  includes a number of MEMS mirrors equal to the number of lasers in the package, shown in this example as MEMS mirrors  32 A,  32 B and  32 C. Attached to package  30  is a fiber  35  which, as before, can be replaced by any suitable passive optical device or arrangement. In operation, laser diodes  31 A,  31 B and  31 C generate optical beams  33 A,  33 B and  33 C, respectively, where these beams then propagate toward MEMS mirrors  32 A,  32 B and  32 C, respectively. As shown, MEMS mirrors  32 A,  32 B and  32 C are adjusted to direct light beams  33 A,  33 B and  33 C toward fiber  35  as light beams  34 A,  34 B and  34 C, respectively. Before package  30  is sealed, MEMS mirrors  32 A,  32 B and  32 C are individually adjusted, in accordance with the teachings of the present invention, so that maximum light power is received at fiber  35 . In accordance with the present invention, should any misalignment then occur between an exemplary laser diode  31   i  and fiber  35 , the associated MEMS mirror  32   i  can be further adjusted to bring the pair of devices back into alignment. 
   Using the embodiment of the present invention as shown in  FIG. 3  provides a high power optical transmitter without the need to rely only a single laser source. That is, this embodiment allows for the coupling of multiple light sources into a single output signal path to create a high power optical device which has the potential to create power in excess of 1 W. In addition to the advantages identified with other embodiments of this invention, another advantage of this embodiment is that it allows for the use of lower power, lower cost laser diodes. Another advantage of the embodiment of  FIG. 3  is that a selection process may be used to preferentially “add” or “drop” various laser diode sources from use. For example, laser diode  31 A can be selected to be the only laser sending a light beam toward fiber  35  through its associated MEMS mirror  32 A. Alternatively, laser diode  31 B can be selected to be the only laser diode sending a light beam toward fiber  35  through its MEMS mirror  32 B. In another embodiment, laser diodes  31 B and  31 C can be cascaded (other cascaded arrangements may be used). Moreover, another advantage of the embodiment shown in  FIG. 3  is that different laser diodes operating at different wavelengths can be utilized to provide WDM (wavelength division multiplexed) transmission into fiber  35 . 
   Referring to  FIG. 4 , there is shown another embodiment of the present invention. In particular,  FIG. 4  illustrates an arrangement that couples light from multiple light sources to multiple optical fibers (or other optical outputs). As shown, package  40  includes multiple laser diodes  41 A,  41 B,  41 C (where, as before, any desired number of laser diodes can be used). Package  40  also includes multiple MEMS mirrors  42 A,  42 B,  42 C, where the number of MEMS mirrors is equal to the number of laser diodes. Attached to package  40  are multiple fibers  45 A,  45 B and  45 C, which could also comprise any sort of passive light receiver. Laser diodes  41 A,  41 B and  41 C generate optical beams  43 A,  43 B and  43 C, respectively, which then propagate toward MEMS mirrors  42 A,  42 B and  42 C. In accordance with the present invention, each MEMS mirror is separately adjustable (using an electrical input signal to control the deflection profile of the device) to direct the output beams  44 A,  44 B,  44 C toward their respective fibers  45 A,  45 B and  45 C. 
   As an advantage of this embodiment is that it uses MEMS mirrors to couple multiple light sources to multiple or single fibers in an optoelectronics package. It allows for isolation and independent control of multiple devices requiring different wavelengths and/or different output power levels. It allows multiple signals (perhaps all the same wavelength) to be sent over multiple fibers. Another advantage of this embodiment is that it allows multiple wavelengths to be transmitted over multiple fibers. For example, laser diode  41 A can be set to emit light at a first wavelength λ 1 , laser diode  41 B can be set to emit light at a second wavelength λ 2 , and laser diode  41 C can be set to emit light at a third wavelength λ 3 . 
   Referring to  FIG. 5 , there is shown an embodiment of a single fiber transceiver, that is, a system that can both transmit and receive optical signals simultaneously over a single fiber. The transmit function and the receive function can be performed simultaneously when a first wavelength is used for transmission and a second, different wavelength is used for reception.  FIG. 5  is a package embodiment  50  of the invention that couples light from a single laser diode  51  to a single optical fiber output  55 . Simultaneously, an input light signal propagating along fiber  55  is applied as an input to photodiode  52  (or any other suitable optical receiving device). In this embodiment, the single fiber  55  serves as both an output receiver and an input transmitter. Package  50 , as shown, also includes MEMS mirrors  53  and  54 , where MEMS mirror  54  is used to adjust the alignment between optical fiber  55  and photodiode  52 , and MEMS mirror  53  is used to adjust the alignment between laser diode  51  and optical fiber  55 . By “decoupling” the alignment between the source (laser) and detector (photodiode) with respect to the communication fiber  55  shared by both devices, in accordance with the present invention, the alignment between each device and fiber  55  may be individually controlled and corrected without having to adjust the position of fiber  55  (which would impact all other alignments). 
   Referring to  FIG. 6 , there is shown another embodiment of the invention.  FIG. 6  illustrates a package embodiment that uses MEMS mirrors  63 ,  64  and  65  in conjunction with other passive optical elements to modify light beams in various ways. Package  60  includes fibers  61  and  62 , as well as MEMS mirrors  63 ,  64  and  65 . This embodiment also includes a passive optical element  66  that is placed between MEMS mirrors  64  and  65 . Passive optical element  66  may comprise a semiconductor optical amplifier, a fiber amplifier, or other suitable passive components. 
   As shown, fiber  62  provides a means to input a light beam into package  60 . As will be understood by those skilled in the art, any active source of light may be used at the input to package  60  in place of fiber  62 . For example, a laser diode may be directly coupled to the input port. Light beam  67  enters package  60  from fiber  62  (or any other appropriate source) and is directed toward MEMS mirror  64 , which then reflects the light beam toward MEMS mirror  65  as beam  68 . One or more passive devices  66  can be placed in the path of light beam  68  in order to modify the characteristics of light beam  68 . For example, device  66  can be an amplifying fiber that increases the power of the propagating signal. After the beam passes through device  66 , it will impinge and then reflect off of MEMS mirror  65 . The reflected beam from MEMS mirror  65  then propagates toward MEMS mirror  63 , where it is again reflected, this time toward output fiber  61  (or any other suitable output element). An advantage of this arrangement is that a highly integrated package incorporating wavelength control, optical amplification and/or modulation can easily be formed. The use of a set of three MEMS mirrors, in accordance with the present invention, allows for independent control of: (1) the alignment between input fiber  62  and the input of device  66  and (2) the alignment between the output of device  66  and output fiber  61 . 
   While the present invention has been described with specificity, additional advantages and modifications will readily occur to those skilled in the art. Therefore, the present invention in its broader aspects is not limited to the specific details shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concepts as defined by the claims appended hereto.