Abstract:
A method and an automated apparatus for three-dimensional optical alignment of optical components for testing or assembly purposes comprise use of a visible light vision system to assist active alignment of optical components based on the measured optical output at a wavelength of light outside of the visible range.

Description:
[0001]    This application claims priority benefits under 35 United States Code, Section  119 (e) of co-pending U.S. Provisional Application No. 60/204,964, filed May 17, 2000, for TwoStage Optical Alignment Device (Attorney&#39;s Docket No. CORE-66 PROV). 
     
    
     
       FIELD OF INVENTION  
         [0002]    This invention relates to alignment of optical components for testing and assembly of optical systems operating outside of the visible light range, and more particularly to the alignment of optical components for infra-red optical systems.  
         BACKGROUND OF THE INVENTION  
         [0003]    In the manufacture of optical systems, and more particularly fiber optic communications systems, it is essential to provide efficient optical coupling of optical components for testing, and optionally for assembly, purposes. By way of example, manufacture of certain optoelectronic devices for fiberoptic communications systems may require alignment along orthogonal X-Y-Z axes of a first optical element that has an aperture in the form of an opening or a window that is transparent with respect to light with a wavelength outside of the visible light range, e.g., a Fabry-Perot optical filter or a vertical cavity semiconductor laser, with a light beam emanating from a second optical element, e.g., a laser light source or an optical fiber serving as a light input source. More specifically, the two optical elements must be aligned in an X-Y plane and also precisely spaced along a Z axis that is normal to that plane. It is essential that the alignment process be precise, reliable, repeatable and fast.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0004]    The primary object of this invention is to provide an automated apparatus and method for precise three-dimensional optical alignment of optical components during assembly and inspection.  
           [0005]    A more specific object is to provide an automated apparatus and method for three-dimensional optical alignment that employs a two-dimensional visible light machine vision system to assist active alignment of optical components based on the measured optical output at a wavelength of light outside of the visible range.  
           [0006]    A further object is to provide an apparatus and method for aligning optical elements, one of which is a source of an optical beam with a wavelength outside of the visible range, with a 5 micron precision along the axis of the beam (Z axis) and a 0.25 micron precision in a plane perpendicular to the axis of the beam (the X-Y plane).  
           [0007]    These objects are achieved by providing a motorized X-Y-Z motion apparatus having a movable support member for supporting a first optical element having an aperture for transmittal of light with a wave-length outside of the visible light spectrum and motion-translating means for selectively moving that support member along mutually orthogonal X, Y and Z axes, means for supporting a second optical element in the form of a source of a light beam having a wavelength outside of the visible range in a fixed position relative to the motorized motion apparatus with that light beam directed in the Z-axis direction at the aperture of the first optical element, a visible light vision system using visible light imaging for determining (a) the position of the first optical element relative to said visible light vision system in the X-Y plane and (b) the sharpness of the image of the first element as detected by the visible light vision system, an optical measurement device responsive to the beam for measuring a power-related value of the beam, and a motion control system for causing the motion-translating means to (a) move the movable support member in the X-axis, Y-axis and Z-axis directions as required to achieve X-axis and Y-axis alignment of said first optical element with said visible light vision system and maximize the sharpness of the image, and (b) subsequently move said movable support member first in the Z-axis direction and then in the X-axis and Y-axis directions as required to maximize a power-related value of said beam as measured by said optical measurement device. Other features and advantages of the invention are set forth in or rendered obvious by the following detailed description of the invention which is to be considered together with the accompanying drawing.  
         THE DRAWING  
         [0008]    The drawing schematically illustrates a system embodying the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]    The system shown in the drawing comprises a motorized X-Y-Z motion apparatus  2  that comprises three stages  4 ,  6  and  8 , which are coupled together and carry a platform  10 . Stage  4  is movable bidirectionally in the X-axis direction; stage  6  is movable bidirectionally in the Y axis direction at a right angle to the X axis, and the third stage  8  is movable bidirectionally vertically along the Z axis orthogonal to the X and Y axes. The platform  10  serves as a support for a first optical element  12  which is to be aligned. The platform  10  has an opening  14  to permit light to be transmitted to the optical element  12  from a second optical element  16  that is mounted on a support  18  that is fixed in relation to the motion apparatus  2  and also a visible light vision system  20 . Although not shown, it is to be understood that means are provided for securing optical elements  12  and  16  against movement relative to platform  10  and support  18  respectively. The fixed relationship between motion apparatus  2 , support  18  and vision system  20  is represented schematically in the drawing by the intersecting lines B and S.  
         [0010]    The visible light vision system  20  comprises an electronic camera  22 , preferably a digital camera, that employs a CCD, MOS or another suitable semiconductor imaging device (not shown). A light source  24  is coupled so as to supply visible light to a housing  28  which is mechanically and optically connected to camera  22  and a second housing  30 , e.g., by barrel members  32  and  34 . Housing  28  contains an optical system comprising a 50/50 beam splitter schematically represented at  36  which (a) directs the visible light from source  24  so as to illuminate optical element  12  and (b) transmits the visible light reflected by element  12  to camera  22 , whereby a visible image of that optical element may be received by the imaging device of camera  22 . In the drawing the visible light path is represented by the solid line arrows. For convenience of illustration, the portion of the visible light path running from beam splitter  36  to optical element  12  is offset from that portion of the light path that extends from optical element  12  to the camera.  
         [0011]    Housing  30  contains a dichroic mirror  40  that is disposed at a 45° angle to the optical axis of camera  22 . Mounted in an extension  42  of housing  30  is an objective lens represented schematically at  44 . Housing  30  has a side opening and connected to that side opening is a tubular barrel  46  which in turn is coupled to an optical measurement device represented schematically at  48 . The optical measuring device may take various forms; preferably it is an optical power meter or an optical spectrum analyzer. In the case where the optical elements are to be aligned for infra-red light testing and assembly, the optical measurement device essentially comprises an IR detector.  
         [0012]    Although not shown, it is to be understood that optical element  12  has an internal aperture in the form of an opening or a window that is transparent to light having a wavelength outside of the visible light range, with the element  12  being, for example, a Fabry-Perot optical filter or a vertical cavity semiconductor laser. The element  16 , which may be a semiconductor laser or an input optical fiber serving as a source of optical power outside of the visible range, is mounted on the support  18  so as to be substantially centered with respect to the optical axis of the visible light vision system, so as to direct a beam of light outside of the visible range through opening  14  and the aperture of optical element  12 . The dichroic mirror  40  is adapted to pass visible light from the visible light vision system through the objective  44  to effect imaging of optical element  12 , and to reflect the non-visible light beam from source  16  to the optical measuring device  48 . The path followed by the non-visible light beam from source  16  is indicated by the broken line arrow.  
         [0013]    The visible light vision system is adapted to determine the X-Y position of optical element  12  in relation to the vision system by measuring the degree of X-Y plane registration of the image of said optical element with the camera&#39;s imaging device, and also is adapted to produce a first output error signal representative of the extent of X-axis and Y-axis misalignment of said first optical element in relation to that imaging device. The vision system is also adapted to determine the sharpness of the image seen by the camera by measuring the intensity gradient of the image, e.g., the contrast, at the periphery of the image, and to produce a second signal that varies as a function of image sharpness. Those signals are applied sequentially to a programmable motion control system  50 . The first signal causes the motor control system to activate the X axis and Y axis stages  4  and  6  so as to move platform  10  in the X-axis and Y-axis directions to the extent required to place the element  12  in alignment with the vision system in the X-Y plane. The second signal produced by the vision system causes control system  50  to effect operation of the Z-axis stage  8  whereby to shift the position of optical element  12  along the Z axis, i.e., along the optical axis of the visible light vision system, until the vision system has determined that the image received by the camera has achieve maximum sharpness. In effect, the vision system and the motion control system coact to provide automatic focusing, with the Z axis stage moving until the maximum intensity gradient is observed at the edges of the image seen by the camera.  
         [0014]    When a light beam with a wavelength outside of the visible range is transmitted by optical element  16  though optical element  12 , it passes through and is transmitted by objective  44  onto mirror  40 , which reflects it to optical measurement device  48 . The optical measurement device  48  is adapted to measure a selected power-related value of the light beam reflected by dichroic mirror  40 . For example, it may be adapted to measure the overall intensity of the beam, or the intensity of a selected mode or the intensity differential between two modes. The optical measurement device produces a feedback control signal which is applied to the motion control system. The latter responds to that signal by causing stages  4  and  6  to effect X-Y plane movement of platform  10  and thereby optical element  12  in a direction to maximize the measured power-related value of the light beam from optical element  16 . That value is at its maximum only when the aperture of optical element  12  has been aligned with the non-visible light beam from optical element  16 .  
         [0015]    In the preferred embodiment of this invention, the motion control system is programmed so as to automatically cause the motorized Z-axis stage  8  to move the optical element  12  a predetermined amount along the Z axis after the three-axis visible light alignment has been accomplished and before it causes the X and Y stages  4  and  6  to move to maximize the power-related value of the non-visible light beam measured by optical measurement device. This automatic Z-axis movement, which is identified herein as “offset”, is for the purpose of (a) compensating for mechanical misalignment of motion apparatus  2 , vision system  20  and the fixed position of optical element  16  as determined by support  18 , and (b) correcting for optical aberration resulting from the fact that non-visible and visible light behave differently in passing through the objective. The amount of offset is determined by prior measurements.  
         [0016]    In the preferred embodiment of the invention, the motion control system  50  is programmed to achieve alignment by automatically executing a method comprising the following steps:  
         [0017]    1. Vision system  20  measures the position of optical element  12  in the X-Y plane and if the image is not in registration with predetermined X-Y coordinates of the imaging device of camera  22 , the vision system of the camera delivers a control signal to the motion control system which in turn causes movement of X stage  4  and/or Y stage  6  so as to move element  12  into alignment with the visible light vision system.  
         [0018]    2. Next, the visible light vision system measures the sharpness of the image of optical element  12  and delivers a signal to the motion control system which causes it to operate the Z axis stage of the motion apparatus so as to shift optical element  12  incrementally along the Z axis until the image focused on the image plane of camera  22  has achieved maximum sharpness.  
         [0019]    3. Next, motion control system  50  automatically activates the Z axis stage so as to cause the optical element  12  to shift a predetermined amount in a predetermined direction for the purpose of compensating for mechanical misalignments and correcting for chromatic aberration.  
         [0020]    4. Thereafter the optical element  16  is caused to transmit a non-visible light beam through optical element  12  to mirror  40 , and optical measurement device  48  measures the intensity of that beam as reflected by mirror  40  and generates control signals which are applied to the motion control system  50  so as to cause the X-axis stage  4  and/or the Y-axis stage  6  to move in a direction and by an amount sufficient to maximize the power related value measured by the optical measurement device  48 . As noted above, the optical measurement device  48  may measure various power related values of the non-visible light beam, but preferably it is programmed to measure the maximum optical power of the reflected light beam and/or the maximum side mode suppression of that light.  
         [0021]    Thereafter the optical element  12  may be fixed in an optical assembly or subjected to other optical measurements.  
         [0022]    It is to be noted that the invention need not be practiced exactly as hereinabove described and illustrated. For one thing, the invention may be modified by providing more than two optical elements in the system to be aligned. Additionally or alternatively, there may be more than one three-dimensional moving mechanism  2  to move more than one optical component of the system under alignment. Furthermore, it is contemplated that the three-dimensional mechanism  2  can be arranged differently than as illustrated and described. For example, the three-dimensional motion apparatus or device can be arranged so that one axis of motion can move one optical element of the optical system under alignment, together with the vision system and the optical measurement device, and a dual axis motion system can be used to move the second optical element. Further with respect to the invention, it should be noted that the visible light vision system and the optical measurement device may move relative to one another so long as their mutual position is known with sufficient accuracy. Also the invention may be adapted for use with a non-infra-red light source as the optical element  16 . The present invention can also be used if more precise Z-axis alignment is required. In that case, an another step is added to the sequence of steps described hereinabove. During this step the Z-axis position of optical element  12  will be further optimized to obtain a maximum for the optical power-related value measured by optical measurement device  48 .  
         [0023]    In addition to the advantages rendered obvious by the foregoing description, it should be noted that the two-stage alignment process of the present invention makes it possible to obtain precise alignment even though the optical element represented at  12  may not have a precise peripheral configuration or a peripheral configuration that is exactly concentric with its aperture, since the active non-visible light X-Y alignment steps assure that the non-visible light beam is accurately centered with respect to that aperture. Moreover use of a vision system for three-dimensional alignment as herein described allows for faster alignment. The invention also makes possible a reduction in the cost of alignment equipment by reducing the stroke and velocity requirements needed from the super precision-actuators used for fine active alignment. The invention also reduces the cost and complexity of the alignment equipment by promoting the use of conventional two-dimensional machine vision systems for three-dimensional alignment. It also promotes the use of a visible light machine vision system to perform alignment of optical systems with a wavelength of light outside a visible light. A further advantage is that a visible light vision system as described is well known to persons skilled in the art, as are motion control systems and three-axis motion systems. Another advantage is that motion apparatus of the kind contemplated by the invention is commercially available. Moreover, the individual X, Y and Z axis stages are available for separate mounting. Thus, for example, stages  4 ,  6  and  8  may be like the linear motor driven stages sold by Anorad Corporation of Hauppauge, N.Y. and Aerotech Corporation of Pittsburgh, Pa.