Abstract:
An active alignment photonics assembly actively couples optical energy between optical devices. An adjustable fiber or other optical carriers, carries an optical signal which is received at a detector where the power level of the optical signal is measured. Based on the power level measurement, the alignment of the fiber or other optical devices contained in the assembly are provided by an optical feed-back loop which controls the position of the fiber or other devices relative to the detector by use of microactuator mechanisms.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to photonics assemblies, and more specifically to a monolithically integrated alignment photonics assembly for actively coupling optical energy between optical devices. 
     2. Description of the Prior Art 
     Compact and simple photonics systems are essential in optical communication applications. Photonics systems require high light transmission efficiencies in order to obtain low error rates. The transmission efficiency is measured as insertion loss for photonics applications and becomes more important for photonics systems working at high data transmission rates. The high data transmission (high bandwidth) rates require the use of single mode and polarization maintaining fiber optics where optical alignment from fiber to fiber, transmitter to fiber, transmitter to modulator, transmitter to multiplexer and fiber to receiver becomes critical to minimizing insertion loss. Optical alignment requirements for single mode fibers are at micron and sub-micron levels as opposed to supermicron levels for lower bandwidth multimode fibers. Optical alignment methods are near the limit of improvement using conventional alignment techniques. For example, single mode fiber connectors using actively aligned ferrules, like that described in the publication “Packaging Technology for a 10-Gb/a Photoreceiver Module”, by Oikawa et al., Journal of Lightwave Technology Vol. 12 No. 2 pp.343-352, February 1994, are typically limited to 0.2 dB insertion loss. The Okiwawa publication discloses an optical coupling system, illustrated in FIG. 1, containing a slant-ended fiber  46  secured in a fiber ferrule  48  where the fiber ferrule  48  is welded to a side wall  50  of a flat package  52  and a microlens  54  is monolithically fabricated on a photodiode  56  where the photodiode  56  is flip-chip bonded to the flat package  52 . An optical signal  58  enters horizontally and is reflected vertically at the fiber&#39;s  46  slant-edge. The microlens  54  then focuses the optical signal  58  on the photodiode&#39;s  56  photosensitive area. 
     As described in the Oikwawa publication, maintaining alignment between the fiber and the photodiode chip is essential for optimal coupling of the optical signal. Misalignment can occur as a result of mechanical stress to the fiber ferrule or thermal fluctuations of the entire system. In an attempt to overcome these factors, complex assembly and fabrication techniques are used. The fiber attachment is a complex ferrule attachment which seeks to optimize the mechanical strength of the attachment and therefore minimize the effects of fiber displacement. Finally, in order to provide a high optical coupling efficiency wide misalignment tolerances must be built in to the photodiode chip during fabrication to compensate for both displacement by the fiber attachment and deformation by temperature fluctuation. 
     Disclosed in U.S. Pat. No. 5,346,583 is an active alignment system for laser to fiber coupling, as illustrated in FIG.  2 . The &#39;583 patent attempts to minimize optical coupling losses by actively coupling optical energy between a source and a transmission medium. A laser  11  directs a beam  10  in the direction of a first mirror  13  and from the first mirror  13  the beam  10  is reflected to a second mirror  17  where the beam  10  is again reflected. The two mirrors are mounted on flexure elements and the flexure elements each have the capability to adjust the beam  10  in one dimension. The beam direction which is determined by the two mirrors  13  and  17  is focused by a lens  18  onto an input aperture for a waveguide contained within a modulator  20 . The modulator  20  splits the beam  10  into two output beams. The two output beams are coupled at a lens  22  and focused onto a pair of fiberoptic fibers  43  and  44 . Fibers  43  and  44  are each connected to electromechanical transducers  23  and  24  respectively, where the transducers have the ability to adjust the input ends of the fibers  43  and  44  in two dimensions. The active adjustment of both the mirrors ( 13  and  17 ) and the fibers ( 43  and  44 ) is accomplished by a controller  27 . The controller  27  receives as input an indication of the amount of light passing through the fibers  43  and  44  from receivers  38  and  39  and supplies corrective feedback to the mirrors ( 13  and  17 ) and the fibers ( 43  and  44 ). 
     As discussed, present optical coupling systems use a variety of coupling schemes to obtain efficient coupling within photonics applications. However, many of these schemes use static components which are typically made of different materials and have different thermal expansion coefficients. These differences can cause optical misalignment during temperature changes, which are common in space applications. Photonics systems which are disclosed in the art and attempt to overcome misalignment problems by using active alignment control feedback techniques, typically use discrete bulk optical components and the complexity of the assembly process is increased. The greater the complexity the more assembly costs are increased and reliability decreased. 
     Based on techniques known in the art for photonics coupling schemes, a monolithic alignment assembly for active alignment of an optical fiber core to an optical device is highly desirable. 
     SUMMARY OF THE INVENTION 
     It is an aspect of the present invention to provide an active alignment photonics assembly. Briefly, the photonics assembly includes a fiber optic means for carrying an optical signal and, a first optical element spaced from the fiber optic means and having an input means for accepting the optical signal. Optionally, the photonics assembly may provide a second optical element having an optical coupling means, the second optical element being disposed in an optical signal path between the fiber optic means and the first optical element. To maintain the alignment of the photonics assembly, a sensing element is provided having a means for determining the power of the optical signal at the first optical element and producing a status signal in response thereto. A controller is provided having a means for receiving the status signal and distributing a correction signal and an adjusting means for receiving the correction signal and adjusting the path of the optical signal in response to the correction signal. The adjusting means comprises at least one servo-mechanism controlled microactuator capable of adjusting the fiber optic means, the second optical element, or both simultaneously. 
     It is also an aspect of the present invention to provide an active alignment photonics assembly. Briefly, the photonics assembly includes a fiber optic means for carrying an optical signal, a first optical element spaced from the fiber optic means and having an input means for accepting the optical signal. Optionally, the photonics assembly may provide a third optical element having an optical coupling means, the third optical element being disposed in an optical signal path between the fiber optic means and the first optical element. To maintain the alignment of the photonics assembly, a second optical element is provided spaced from the fiber optical means and having an input means for accepting the divergent power of the optical signal. Further, a sensing element is provided having a means for determining the divergent power of the optical signal at the second optical element and producing a status signal in response thereto. A controller is provided having a means for receiving the status signal and distributing a correction signal and an adjusting means for receiving the correction signal and adjusting the path of the optical signal in response to the correction signal. The adjusting means comprises at least one servo-mechanism controlled microactuator capable of adjusting the fiber optic means, the third optical element, or both simultaneously. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the following specification and attached drawings, wherein: 
     FIG. 1 is an illustration of a known optical coupling system which includes a mounted fiber assembly and a microlens monolithically integrated into a photodiode; 
     FIG. 2 is an illustration of another known optical coupling system which includes an active alignment system for laser to fiber coupling; 
     FIG. 3 is an illustration of a active alignment photonics assembly in accordance with the present invention; 
     FIG. 4 is an illustration of an alternate embodiment of the active alignment photonics assembly illustrated in FIG. 3; 
     FIG. 5 is an illustration of a further alternate embodiment of a active alignment photonics assembly in accordance with the present invention; 
     FIG. 6 is an illustration of a further alternate embodiment of the active alignment photonics assembly in accordance with the present invention; and 
     FIG. 7 is an illustration of an active alignment photonics assembly formed from a monolithic body in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a photonics assembly and, more particularly, to a photonics assembly which provides for active alignment of an optical fiber core to a photodiode detector, an optical fiber, a modulator, a filter, a waveguide, or similar optical receiving device. The preferred embodiment of the present invention, as illustrated in FIG. 3, includes a jacketed fiber optic cable  50 , a fiber core  54 , a plurality of microactuators ( 74 ,  76 , and  78 ), a lens  58 , a photodetector  60 , a sensing element  62 , and a controller  66 . The jacketed fiber optic cable  50  conducts an optical signal  52  through a fiber core  54 . The fiber core  54  is a single mode fiber and is chosen for its ability to operate when modulated at high microwave frequencies (greater than 2 Ghz) and may include polarization maintaining capabilities. The fiber core  54  is mounted on a plurality of microactuators  74 ,  76 , and  78  which are used to align the fiber core  54  and in order to provide optimal adjustment of the fiber core  54 , the microactuators  74 ,  76 , and  78  are oriented to allow adjustment of the fiber core  54  in three dimensions (x, y, and z). The microactuators  74 ,  76 , and  78  adjust the fiber core  54  by a “flexing” motion and may be bimetallic strips which are thermally driven by resistance heating or piezoelectric materials which are electrical current driven and require low electrical power duties. In addition to the preferred bimetallic strips or piezoelectric materials, the microactuators  74 ,  76 , and  78  may also include heated elements, electrical movable elements, bicompound strips, component strips or other materials providing similar “flexing” properties. Alternatively, the microactuators  74 ,  76 , and  78  may be micro-mechanical devices such as micromachined motors, levers, or stepper motors. It is important to note that the selection of microactuators is not limited to those materials or devices which provide microsecond motion responses, but may also include those materials or devices which provide greater than I second response times. 
     A lens  58  receives the optical signal  52  and focuses any divergent optical energy of the optical signal  52  leaving the fiber core  54 . Alternatively, the lens  58  may be excluded from the photonics assembly depending on the coupling requirements of the particular photonics application. The lens  58  may also be substituted for by a mirror, diffraction element, interference element, waveguide or similar optical device having optical coupling characteristics. 
     The optical signal  52  passes through the lens  58  and is focused onto a photodiode detector  60  which converts the optical signal  52  to an electrical signal  53 . As previously described, the photodiode detector  60  may be a fiber, a modulator, a filter, or a waveguide (active or passive). 
     The photodiode detector  60  is electrically connected to a sensing element  62  which, by means of a current meter or similar device, is able to detect the power level and polarization generated by the optical signal  52 . The power level of the signal  52  is compared against a predetermined level of power expected for optimal optical alignment of the fiber core  54  to the photodiode detector  60 . Based on the results of the comparison an electrical status signal  64  is sent to a controller element  66  which actuates the flexing movement of a microactuator by transmitting separate electrical signals  68 ,  70 , and  72  to microactuators  74 ,  76 , and  78  respectively. The electrical signals  68 ,  70 , and  72 , cause the microactuators to “flex” or move in proportion to the strength and duration of the electrical signal and by the flexing action adjust the position of the fiber core  54 . In the preferred embodiment and, as illustrated in FIG. 7, the microactuators ( 74 ,  76 , and  78 ), lens  58 , photodetector  60 , sensing element  62 , controller  66  and their respective alternate embodiments may be fabricated to form a monolithic body  67 . 
     Alternatively, as illustrated in FIG. 6, optimal optical alignment of the photonics assembly may be maintained by receiving the divergent energy of the optical signal  52  at a photodetector, optical fiber, beam splitter, or similar device. A photodetector  61  detects the divergent energy  59  of the optical signal  52 . The photodetector  61 , is electrically connected to the sensing element  62  which detects the divergent energy  59  of the optical signal  52 . The divergent energy  59  is compared against a predetermined level of divergence expected for the signal  52  during optimal signal alignment of the fiber core  54 . Based on the results of the comparison the electrical status signal  64  is sent to the controller element  66  which actuates the flexing movements of the microactuators  74 ,  76 , and  78  in the manner previously described. 
     It should be understood by those of ordinary skill in the art that the principles of the present invention are applicable to many types of photonics assemblies, such as those illustrated in FIGS. 4 and 5. As shown in FIG. 4, a lens  80  is adjusted in a manner similar to that used to adjust the fiber core  54  of FIG.  3 . The lens  80 , as contrasted to the fiber core  54  illustrated in FIG. 3, is adjusted by the “flexing” motion of microactuators  74 ,  76 , and  78  and an optical signal  52  is optimally focused onto a photodector  60 . The “flexing” motion of microactuators  74 ,  76 , and  78  may reorient the lens  80 , or change the optical properties by lens deformation. Alternatively, the lens  80  may be substituted for by a mirror, diffraction element, interference element, waveguide or similar optical coupling device. 
     Further, as illustrated in FIG. 5, both the fiber core  82  and lens  84  may be adjusted by means of microactuators  86 ,  88 ,  90 ,  92 ,  94 , and  96  respectively. A sensing element  98  generates a status signal  100  based on the power of an optical signal  102  at the photodiode detector  104  and sends the status signal  100  to a controller element  108 . The controller element  108  determines the adjustment required at the fiber core  82  and the lens  84  and electrical correction signals  110 ,  112 , and  114  are generated to flex microactuators  86 ,  88 , and  90  respectively; and electrical signals  116 ,  118 , and  120  are generated to flex microactuators  92 ,  94 , and  96  respectively. The assembly illustrated in FIG. 5 allows for simultaneous adjustment of distinct optical elements and therefore provides additional control of the alignment of the fiber  82  relative to the photodiode detector  104 . 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.