Abstract:
An optical arrangement includes an actuatable optical element and a compensating optical element. The actuatable optical element is provided to receive an optical beam having a plurality of spatially separated wavelength components and diffract the plurality of wavelength components in a wavelength dependent manner. The compensating optical element directs the optical beam to the actuatable optical element. The compensating optical element compensates for the wavelength dependent manner in which the wavelength components are diffracted by the actuatable optical element.

Description:
BACKGROUND 
       [0001]    Conventionally, an optical processing device has been used which includes a dispersion element (for example, a grating) to disperse an optical beam and an actuatable optical element allowing each wavelength in each of the dispersed beams to be incident on any of a plurality of output paths. 
         [0002]    One example of such an actuatable optical element is a DMD (Digital Micromirror Device), which includes an array of micromirror elements, each of which is individually actuatable. The DMD may selectively switch an optical path of reflected wavelength components to any one of multiple output paths by adjusting the position of the mirror elements to control the direction in which the wavelength components are reflected. Such optical processing devices can be used to process the wavelengths in an optical beam in a variety of different ways for a variety of different purposes, including switching, wavelength attenuation and wavelength blocking 
       SUMMARY 
       [0003]    In accordance with one aspect of the invention, an optical arrangement is provided which includes an actuatable optical element and a compensating optical element. The actuatable optical element is provided to receive an optical beam having a plurality of spatially separated wavelength components and diffract the plurality of wavelength components in a wavelength dependent manner. The compensating optical element directs the optical beam to the actuatable optical element. The compensating optical element compensates for the wavelength dependent manner in which the wavelength components are diffracted by the actuatable optical element. 
         [0004]    In accordance with another aspect of the invention, an optical processing device includes at least two optical input/output ports for receiving an optical beam and dispersion element for receiving the optical beam from one of the ports and spatially separating the optical beam into a plurality of wavelength components. The device also includes a collimating lens for collimating the plurality of wavelength components and an actuatable optical element. The actuatable optical element is provided to receive the collimated wavelength components from the collimating element and diffract the plurality of wavelength components in a wavelength dependent manner. A compensating optical element is located in an optical path between the dispersion element and the actuatable optical element, The compensating optical element compensates for the wavelength dependent manner in which the wavelength components are diffracted by the actuatable optical element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  shows a simplified example of a wavelength blocker that is based on a MEMs mirror array such as a DMD. 
           [0006]      FIG. 2  shows another example of an optical processing device. 
           [0007]      FIGS. 3   a  is a side view and  FIG. 3   b  is a top view of another example of an optical processing device. 
           [0008]      FIG. 4  shows the relationship between the compensating prism and the DMD in an optical processing device. 
           [0009]      FIG. 5  shows the diffraction of two beams as they pass through a compensating prism and diffract off of s DMD 
           [0010]      FIG. 6   a  shows a short wavelength beam being diffracted from a DMD and  FIG. 6   b  shows a long wavelength beam being diffracted from a DMD. 
           [0011]      FIG. 7  shows one example of a compensating prism. 
           [0012]      FIG. 8  shows one example of a shape that may be provided to the surface of the compensating prism of  FIG. 7 . 
           [0013]      FIGS. 9   a  and  9   b  show the insertion loss over a portion of the C-band for an illustrative optical processing device without compensation for the wavelength dependence on the angle of diffraction from the DMD and with compensation for the wavelength dependence on the angle of diffraction, respectively. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Many optical processing devices direct an incoming and outgoing optical beam along the same optical path. Such devices may include optical switches, waveblockers and optical attenuators.  FIG. 1  shows a simplified example of a wavelength blocker that is based on a MEMs mirror array such as a DMD. In a 1×1 wavelength blocker the fiber array is a single fiber that serves as an input and output port. Often a circulator (not shown) or other means are used to separate the incoming and outgoing beams. If the fiber array includes N fibers, then each fiber serves as an input and output port. Such a device provides N 1×1 wavelength blockers using a common optical fabric and is referred to as a wavelength blocker array. In such a device the launch optics would generally require the fiber array and a series of circulators or the like to separate each of the N incoming beams and the N outgoing beams. 
         [0015]    Another example of an optical processing device is shown in  FIG. 2 . In this particular example, N (e.g., 15) 1×1 switches are formed using a coupling mirror. In this example a less complex, less costly, launch optics arrangement is employed which avoids the need for circulators or the like. The launch optics arrangement  260  includes a fiber array  250  in combination with a lenslet array  200 . The fiber array  250  typically consists of two V-groove plates that secure the input/output fibers.  FIG. 2  shows two fiber pairs; pair one includes fibers  1  and  2  and pair two includes fibers  3  and  4 . 
         [0016]    It should be noted that while for purposes of illustration the example of the launch optics arrangement  260  shown in  FIG. 2  includes a fiber array, more generally the launch optics arrangement  260  may include any type of waveguide array such as a planar waveguide array, for example. Moreover, the waveguides employed in the array may all be of the same type or a combination of different types (e.g., fiber and planar). 
         [0017]    The lenslet array  200  includes inner and outer opposing surfaces  220  and  230  and is formed from silica or another suitably optically transparent material. A series of collimating lens pairs  210   1 ,  210   2 ,  210   3  . . . are arranged on the inner surface  220  of the lenslet array  200 . Each collimating lens pair  210  includes two collimating lenses  212 . Likewise, a series of coupling lens  214   1 ,  214   2 ,  214   3  . . . are formed on the outer surface  230  of the lenslet array  200 . Each collimating lens pair  210  is in registration with one of the coupling lens  214 . For example, in  FIG. 2 , collimating lens pair  210   1  is in registration with coupling lens  214   1  and collimating lens pair  210   2  is in registration with coupling lens  214   2 . Thus, there are twice as many collimating lenses  212  as coupling lenses  214 . 
         [0018]    The pitch of the collimating lenses  212  is the same as the pitch of the fibers in the fiber array  250 . Accordingly, the fiber array  250  and the lenslet array  200  are arranged so that each of the collimating lenses  212  of the lenslet array  200  is in registration with one of the fiber outputs in the fiber array  250 . In some particular implementations the separation between the collimating lenses  212  and the coupling lenses  214  may be about equal to the sum of their individual focal lengths. 
         [0019]    The operation of the launch optics arrangement  260  in  FIG. 2  is as follows. A light beam from an input fiber in each fiber pair is communicated from the fiber array  250  into the lenslet array  200  through the collimating lens  212  with which it is registration. For instance, in  FIG. 2  light from fiber  1  is shown entering its corresponding collimating lens  212  in collimating lens pair  210   1 . The collimating lens  212  directs a collimated beam to the coupling lens  214  with which it is in registration. In  FIG. 2 , the collimated beam from fiber  1  is collimated by collimating lens  212  in collimating lens pair  210   1 , which directs the collimated beam to coupling lens  214   1 . 
         [0020]    The coupling lens  214 , in turn, focuses the beam in a launch plane, where, in the example shown in  FIG. 2 , a coupling mirror  240  is located. The coupling mirror  240  reflects the beam so that it is directed back through the same coupling lens from which it was received (e.g., coupling lens  214   1  in  FIG. 2 ). The coupling lens  214  collimates the reflected beam and directs it back through the lenslet array  200 . Because of the angle through which the beam was reflected by the coupling mirror  240 , the reflected collimated beam is parallel to and spatially offset from the incoming collimated beam. The reflected collimated beam is directed to the output collimating lens of the collimating lens pair that initially received the beam from fiber array  250 . As shown in  FIG. 2 , for example, the beam directed into the lenslet array  200  through the input collimating lens  212  of collimating lens pair  210   1  is directed to the adjacent output collimating lens  212  in collimating lens pair  210   1 . The output collimating lens  212  focuses the reflected beam onto the input of the fiber in registration with the output collimating lens  212 , which in the example shown in  FIG. 2  is fiber  2 . In this way the optical processing device shown in  FIG. 2  directs an input beam received from one fiber (e.g., fiber  1 ) in a fiber pair to another fiber(e.g., fiber  2 ) in the same fiber pair, thus providing a switching function. 
         [0021]    Another example of an optical processing device is shown in  FIGS. 3   a  (side view) and  3   b  (top view). This example employs the same optical launch arrangement as shown in  FIG. 2 , but replaces the coupling mirror  240  with an optical system that includes a DMD  550 , In this particular example, N 1×1 wavelength blockers are formed. If, for instance, the optical processing device includes 15 1×1 wavelength blockers (only three of which are shown in  FIG. 3 ), fiber array  505  would include 30 input/output fibers. 
         [0022]    As shown, the optical launch arrangement  570  is followed by collimating lens  516 , diffraction gratings  522 , scan lens  530 , compensating prism  540  and DMD  550 . As best seen in the top view of  FIG. 3   b , the DMD  550  is tilted with respect to the optical axis of the scan lens  530  in a plane in which the fibers of the launch optics arrangement  570  extend. 
         [0023]    In operation, an optical beam entering the optical launch arrangement  570  from a fiber  502  exits the corresponding collimating lens  514  and comes to a virtual focus in the launch plane  511 . The optical beam is then collimated by a collimating lens  516 . The diffraction grating  522  next diffracts the collimated beam and the scan lens  530  focuses the spectrally dispersed beams onto the DMD  550  after passing through the compensating prism  540 . When set to the pass state, the individual mirrors of the DMD  550  are tilted to reflect the beam nearly back on itself (near Littrow) so that it travels back through the device and exits through the corresponding waveguide  504  of the launch optics arrangement  570 . Alternatively, when set to the blocking state, the individual mirrors of the DMD  550  are actuated so that are tilted at an angle which causes the beam to exit the device (see beam  560  in  FIG. 3   b ). Although  FIG. 3  shows the operation for only the coupled fiber pair  502  and  504 , the coupling described above occurs for all of the fiber pairs. 
         [0024]    Because the DMD  550  is tilted, the distance from the scan lens  530  to the DMD  550  varies from fiber to fiber. The function of the compensating prism  540  is to correct for this path length difference so that the beams from all the fibers are focused on the DMD  550 . The operation of the compensating prism  540  can be more easily seen in  FIG. 4 , which shows the relationship between the compensating prism  540  and the DMD  550 , which may include a transparent window  420 . 
         [0025]    The device shown in  FIG. 3  has an inherent wavelength dependent loss which limits its performance. To understand the source of this wavelength dependent loss, consider that the DMD  550  is composed of an array of micro mirrors and thus behaves as a diffraction grating rather than a mirror. Strictly speaking, the beams are diffracted off of the DMD, not reflected. The angle of diffraction from the DMD  550  in the plane of the fiber array  505  is given by the grating equation: 
         [0026]    Where n is the diffracted order, d is the pixel spacing of the DMD and λ is the wavelength. The angle of diffraction of the beams from the DMD is therefore wavelength dependent. As a result, coupling between an input and output fiber can only be optimized for a single wavelength, and thus an optical beam experiences a wavelength dependent loss when it is directed through the device. 
         [0027]      FIG. 5  shows two beams, a short wavelength beam  370  and a long wavelength beam  380 , as they pass through the compensating prism  540  and diffract off of the DMD  550 . The long wavelength beam  380  diffracts through a larger angle compared to the short wavelength beam  370 . This relationship between wavelength and diffraction angle is made clear with reference to  FIGS. 6   a  and  6   b .  FIG. 6   a  shows the short wavelength beam  370  (solid line) being diffracted from DMD  550 .  FIG. 6   a  also shows the diffraction angle θ d  though which the beam  370  is diffracted.  FIG. 6   b  similarly shows the long wavelength beam  380  (solid line) being diffracted from DMD  550 .  FIG. 6   b  also shows the diffraction angle θ d  though which the beam  380  is diffracted. Comparison of  FIGS. 6   a  and  6   b  shows that the longer wavelength beam  380  diffracts through the larger angle. 
         [0028]    The wavelength dependent loss can be minimized or eliminated by providing a suitable optical element to compensate for the wavelength dependence of the diffracted angle which is introduced by diffraction from the DMD. In general this optical element should be located downstream from the diffraction grating  522  in  FIG. 3 , typically in the vicinity of the DMD  550 . That is, the optical element may be located between the diffraction grating  522  and the DMD  550 . In one implementation, instead of adding an additional optical element to compensate for the wavelength dependency of the diffracted angle, an existing optical element may be modified to perform this function. For example, the shape of the compensating prism  540  in  FIG. 3  may be modified to correct for this loss. In this case the compensating prism  540  both adjusts the path length difference experienced by the beams from each fiber and compensates for wavelength dependent loss arising from DMD diffraction. 
         [0029]    In one particular implementation, a slight twist may be added to one or both of the surfaces of the compensating prism  540 . One example of such a prism is shown in  FIG. 7  for the incoming surface  394  of compensating prism  385 . As shown, the surface  394  causes the long wavelength beam  392  to be refracted at a greater angle of refraction than the short wavelength beam  390 . In this example the surface  394  has the form z=m×y (5 th  term in the Zernike polynomials), which can be thought of as the sum of two crossed cylinders. The shape of such a surface is shown in  FIG. 8 . As a consequence, the correction for the wavelength dependent loss can also be achieved by adding cylindrical surfaces to each surface of the compensating prism  385 . One advantage of this approach is that a cylindrical surface can be generated using conventional optical polishing methods. 
         [0030]    Referring again to  FIG. 4 , in some implementations the DMD  550 , window  420  and compensating prism  540  (or other suitable compensating optical element) may be packaged in a single unit to provide a DMD arrangement that diffracts an optical beam in a wavelength-independent manner. Such a DMD arrangement which may be used in a wide variety of different optical processing devices. 
         [0031]      FIGS. 9   a  and  9   b  show the insertion loss over a portion of the C-band for an illustrative optical processing device without compensation for the wavelength dependence on the angle of diffraction from the DMD and with compensation for the wavelength dependence on the angle of diffraction, respectively. As shown, the amount of loss and the variation in loss is reduced when the wavelength dependence is reduced.