Abstract:
An optical device in which crosstalk due to scattering is reduced includes an optical port array having at least one optical input for receiving an optical beam and at least one optical output. The input and outputs extend along a common axis. A dispersion element receives the optical beam from the optical input and spatially separates the optical beam into a plurality of wavelength components. A focusing element focuses the plurality of wavelength components and a programmable optical phase modulator receives the focused plurality of wavelength components. The modulator is configured to steer the wavelength components to a selected one of the optical outputs. The programmable optical phase modulator is oriented with respect to the optical port array so that an axis along which the optical beam is steered is non-coincident with the common axis along which the input and outputs extend.

Description:
BACKGROUND 
     Optical networks use Wavelength Selective Switches (WSS) to dynamically route optical wavelength signals from a source to a destination. WSS devices often rely on wavelength manipulation elements such as liquid crystal on silicon (LCoS) devices or micro-electromechanical (MEMS) mirror arrays to perform the routing. 
     LCoS devices include a liquid crystal material sandwiched between a transparent glass layer having a transparent electrode, and a silicon substrate divided into a two-dimensional array of individually addressable pixels. Each pixel is individually drivable by a voltage signal to provide a local phase change to an optical signal, thereby providing a two-dimensional array of phase manipulating regions. Manipulation of individual spectral components is possible once an optical signal has been spatially separated by a diffractive element such as a diffraction grating. The spatial separation of spectral components is directed onto predetermined regions of the LCoS device, which can be independently manipulated by driving the corresponding pixels in a predetermined manner. 
     Although wavelength selective switches that employ LCoS devices have some very attractive performance characteristics, they also suffer from undesired optical crosstalk, which refers to un-desired power coupled from an input port to an output port at a particular wavelength. 
     SUMMARY 
     In accordance to one aspect of the invention, an optical device is provided in which crosstalk due to scattering is reduced. The optical device includes an optical port array having at least one optical input for receiving an optical beam and at least one optical output. The input and outputs extend along a common axis. A dispersion element receives the optical beam from the optical input and spatially separates the optical beam into a plurality of wavelength components. A focusing element focuses the plurality of wavelength components and a programmable optical phase modulator receives the focused plurality of wavelength components. The modulator is configured to steer the wavelength components to a selected one of the optical outputs. The programmable optical phase modulator is oriented with respect to the optical port array so that an axis along which the optical beam is steered is non-coincident with the common axis along which the input and outputs extend. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are top and side views respectively of one example of a simplified optical device such as a free-space wavelength selective switch (WSS) that may be used in conjunction with embodiments of the present invention. 
         FIG. 2  is a front view of the LCoS device of  FIG. 1  extending in the x-y plane. 
         FIG. 3  shows an example of a periodic, stepped phase shift profile that may be produced across a region of a LCoS device in the y-axis. 
         FIGS. 4 and 5  are front views taken along line A-A in  FIG. 1B  of the port array. 
         FIG. 6  is a side view of one example of a simplified optical device such as a free-space wavelength selective switch (WSS) in which crosstalk is reduced. 
         FIG. 7  shows an alternative embodiment of the optical device shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A and 1B  are top and side views respectively of one example of a simplified optical device such as a free-space WSS  100  that may be used in conjunction with embodiments of the present invention. Light is input and output to the WSS  100  through optical waveguides such as optical fibers which serve as input and output ports. As best seen in  FIG. 1B , a fiber collimator array  101  may comprise a plurality of individual fibers  120   1 ,  120   2  and  120   3  respectively coupled to collimators  102   1 ,  102   2  and  102   3 . Light from one or more of the fibers  120  is converted to a free-space beam by the collimators  102 . The light exiting from port array  101  is parallel to the z-axis. While the port array  101  only shows three optical fiber/collimator pairs in  FIG. 1B , more generally any suitable number of optical fiber/collimator pairs may be employed. 
     A pair of telescopes or optical beam expanders magnifies the free space light beams from the port array  101 . A first telescope or beam expander is formed from optical elements  106  and  107  and a second telescope or beam expander is formed from optical elements  104  and  105 . 
     In  FIGS. 1A and 1B , optical elements which affect the light in two axes are illustrated with solid lines as bi-convex optics in both views. On the other hand, optical elements which only affect the light in one axis are illustrated with solid lines as plano-convex lenses in the axis that is affected. The optical elements which only affect light in one axis are also illustrated by dashed lines in the axis which they do not affect. For instance, in  FIGS. 1A and 1B  the optical elements  102 ,  108 ,  109  and  110  are depicted with solid lines in both figures. On the other hand, optical elements  106  and  107  are depicted with solid lines in  FIG. 1A  (since they have focusing power along the y-axis) and with dashed lines in  FIG. 1B  (since they leave the beams unaffected along the x-axis). Optical elements  104  and  105  are depicted with solid lines in  FIG. 1B  (since they have focusing power along the x-axis) and with dashed lines in  FIG. 1A  (since they leave the beams unaffected in the y-axis). 
     Each telescope may be created with different magnification factors for the x and y directions. For instance, the magnification of the telescope formed from optical elements  104  and  105 , which magnifies the light in the x-direction, may be less than the magnification of the telescope formed from optical elements  106  and  107 , which magnifies the light in the y-direction. 
     The pair of telescopes magnifies the light beams from the port array  101  and optically couples them to a wavelength dispersion element  108  (e.g., a diffraction grating or prism), which separates the free space light beams into their constituent wavelengths or channels. The wavelength dispersion element  108  acts to disperse light in different directions on an x-y plane according to its wavelength. The light from the dispersion element is directed to beam focusing optics  109 . 
     Beam focusing optics  109  couple the wavelength components from the wavelength dispersion element  108  to a programmable optical phase modulator, which may be, for example, a liquid crystal-based phase modulator such as a LCoS device  110 . The wavelength components are dispersed along the x-axis, which is referred to as the wavelength dispersion direction or axis. Accordingly, each wavelength component of a given wavelength is focused on an array of pixels extending in the y-direction. By way of example, and not by way of limitation, three such wavelength components having center wavelengths denoted λ 1 , λ 2  and λ 3  are shown in  FIG. 1A  being focused on the LCoS device  110  along the wavelength dispersion axis (x-axis). 
     As best seen in  FIG. 1B , after reflection from the LCoS device  110 , each wavelength component can be coupled back through the beam focusing optics  109 , wavelength dispersion element  108  and optical elements  106  and  107  to a selected fiber in the port array  101 . 
       FIG. 2  is a front view of the LCoS device  110  extending in the x-y plane. 
     The three wavelength components λ 1 , λ 2  and λ 3  are spatially separated along the wavelength dispersion axis (x-axis). As will be discussed below, appropriate manipulation of the pixels  19  in the y-axis allows selective independent steering of each wavelength component to a selected output fiber. 
     A programmable optical phase modulator such as a LCoS device produces a phase shift at a given pixel location in a pixel array which is determined programmatically. Such a modulator can be used in multiple ways, forming virtual lenses, prisms or tilted mirrors among other items. Due to the limited thickness and actuation of a LCoS device, the total phase shift that can be achieved at any given location is limited. This limitation can be circumvented in a LCoS device by application of the segmentation technique similar to that used to form a Fresnel lens by compressing the surface power of a lens into a plano surface. Specifically, the total phase shift desired is usually modulo 2π at the wavelength of interest. The resultant phase is then always less than 2π. Unfortunately, this segmentation technique introduces scattering of light in directions that an un-segmented pattern would not produce. This scattered light is a major reason the crosstalk is naturally higher in an LCoS WSS. 
     Turning to  FIG. 3 , there is illustrated an example of a periodic, stepped phase shift profile  39  that may be produced across a region of a LCoS device  21  in the y-axis. The periodic, stepped phase shift profile  39  produces a cumulative phase profile  37 . The cumulative steering profile  37  is produced by driving each pixel  19  with a predetermined voltage to provide a desired phase change. As there is a direct relationship between voltage and phase, and a direct relationship between phase and steering angle, a look-up table or the like can be generated which relates the required voltage drive signal with a desired steering angle. The periodic nature of phase is utilized to reduce the required drive voltage. Therefore a periodic, stepped voltage signal will produce the periodic, stepped phase shift profile  39 , which in turn produces the cumulative phase profile  37 , where phase resets  41  occur at multiples of 2π radians. When acting on an incident wavelength component, the phase profile  37  produces a steering angle proportional to or equal to θ. Accordingly, by proper adjustment of the periodic, stepped phase shift profile the wavelength components can be selectively directed to a desired one of the optical fibers. 
       FIG. 4  is a front view taken along line A-A in  FIG. 1B  of port array  101 . As shown the optical fibers  1201 ,  1202 ,  1203 , . . . extend in a 1×N configuration along the y-axis. When the LCoS device  110  steers a light beam from one optical fiber (serving as an input port to the WSS  100 ) to another optical fiber (serving as an output port to the WSS  100 ) such as from the optical fiber  1203  to the optical fiber  1202 , for example, the beam is steered along a single direction (the y-axis), either in a positive or negative direction, as indicated by the arrows  122  in  FIG. 4 . In other words, the steering axis is coincident with the optical port axis along which the optical fibers  1201 ,  1202 ,  1203  . . . are aligned. That is, the steering direction is not a function of the optical fiber or port to which the beam is being steered. 
     As previously mentioned, the scattered light that gives rise to crosstalk is preferentially aligned with the direction in which the beam is steered. That is, in  FIG. 4  light is scattered along that axis indicated by arrows  122 . As a result the scattered light is largely aligned with the ports, thereby causing the undesired crosstalk. This crosstalk can be reduced by rotating the beam steering axis (which is also the scattering axis along which light is scattered) so that it is no longer coincident with the optical port axis. Such an arrangement is shown in  FIG. 5 , in which the beam steering axis along which the light beam is steered to ports  1201 ,  1202 ,  1203  . . . is represented by arrows  1301 ,  1302 ,  1303  . . . , respectively. As a consequence, as the beam is steered to any particular port, scattered light will not also be scattered to that port. 
     One way to accomplish this misalignment between the beam steering axis and the optical port axis is shown in  FIG. 6 , which is a side view of one example of a free-space WSS similar to the WSS shown in  FIG. 1B . In  FIGS. 1B and 6 , like elements are denoted by like reference numerals. As shown, the plane in which the LCoS device  110  extends is no longer orthogonal to the axis along which the light propagates from the port array  101 . That is, in the particular example of  FIG. 6 , the LOCS  110  is tilted about the x-axis so that it is no longer in the x-y plane and thus is no longer orthogonal to the z-axis along which the light propagates from the port array  101 . Stated differently, a skewed angle is formed between the z-axis and a direction in the plane of the modulator perpendicular to the wavelength dispersion axis (i.e., the x-axis in  FIG. 6 ). 
     Because of the skewed angle that is employed, the distance between the beam focusing optics  109  and the LCoS device  110  will be different along different portions of the LCoS device  110 . In order to properly focus the light beam on any portion of the LCoS device  110  an optional focus correction element can be placed in the optical path of the system. For instance, as shown in  FIG. 7 , a focus correction element such as a low angle wedge prism  111  can be placed between the beam focusing optics  109  and the LCoS device  110 . 
     Although the invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as example forms of implementing the claimed invention.