ISOLATION BETWEEN PORTS IN A WAVELENGTH SELECTIVE

SWITCH

Background

[0001] In an optical communication network, optical signals having a plurality of optical channels at individual wavelengths (i.e., channels), are transmitted from one location to another, typically through a length of optical fiber. An optical cross- connect module allows switching of optical signals from one optical fiber to another. A wavelength-selective optical cross-connect, or wavelength selective switch (WSS), allows reconfigurable wavelength-dependent switching, that is, it allows certain wavelength channels to be switched from a first optical fiber to a second optical fiber while letting the other wavelength channels propagate in the first optical fiber, or it allows certain wavelength channels to be switched to a third optical fiber. An optical network architecture based on wavelength- selective optical switching has many attractive features due to the ability to automatically create or re-route optical paths of individual wavelength channels. It accelerates service deployment, accelerates rerouting around points of failure of an optical network, and reduces capital and operating expenses for a service provider, as well as creating a future-proof topology of the network.

[0002] Wavelength selective switches may exhibit undesirable optical coupling between their various input and output ports.

Summary

[0003] In accordance with one aspect of the subject matter discussed herein, an optical device is provided that includes an optical port array, an optical arrangement, a dispersion element, a focusing element and a programmable optical phase modulator. The optical port array has at least one optical input port for receiving an optical beam and a plurality of optical output ports. The optical arrangement allows optical coupling between the at least one optical input port and each of the optical output ports and prevents optical coupling between any one of the plurality of optical output ports and any other of the plurality of optical output ports. The dispersion element receives the optical beam from the at least one optical input after traversing the optical arrangement and spatially separates the optical beam into a plurality of wavelength components. The focusing element focuses the plurality of wavelength components. The 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.

[0004] In accordance with another aspect of the subject matter disclosed herein, the optical arrangement is configured to selectively allow and prevent optical coupling between selected ports by discriminating among different polarization states of optical energy traversing therethrough. In particular, in some embodiments the optical arrangement is further configured to receive the optical beam in any polarization state and provide the optical beam to the dispersion element in a predetermined

polarization state.

[0005] In one particular embodiment, the optical arrangement includes at least one walkoff crystal for spatially separating the optical beams received from any of the ports in optical port array into first and second optical components being arranged in first and second polarization states, respectively. The optical arrangement also includes a first composite half-wave plate for arranging the first optical components into the second polarization state and, optionally, a polarizer for transmitting optical energy in the second polarization state but not the first polarization state. This embodiment of the optical arrangement also includes a Faraday rotator and a second composite half-wave plate. The Faraday rotator and the second composite half-wave plate rotate the spatially separated optical beams received from the optical input port via the polarizer into the first polarization state and rotate the spatially separated optical beams received from the plurality of optical output ports into the second orthogonal polarization state. Brief Description of the Drawings

[0006] FIGs. 1-3 schematically illustrate a cross-sectional view of an optical arrangement that may be incorporated, for example, into a wavelength selective switch, as an optical beam is switched between different ones of the ports.

[0007] FIG. 4 shows an alternative embodiment of the optical arrangement shown in FIGs. 1-3.

[0008] FIGS. 5 A and 5B are top and side views respectively of one example of a simplified optical device such as a free-space WSS that may be used in conjunction with embodiments of the present invention.

Detailed Description

[0009] As detailed below, an optical arrangement is provided which directionally couples one set of optical ports to a different set of optical ports such that optical ports within each of the sets are not optically coupled to any other optical port within its own set. The arrangement may be integrated with an optical device such as optical switch to achieve lower levels of optical coupling between ports.

[0010] FIG. 1 schematically illustrates a cross-sectional view of an optical arrangement 100 that may be incorporated, for example, into a wavelength selective switch for addressing the issues of diversity (the optical loss that occurs when a beam travels from one input port to another input port) and isolation (the optical loss that occurs when a beam travels from an output port to an input port). Four optical ports 10, 20, 30 and 40 are shown and thus in this example the wavelength selective switch in which the optical device 100 may be incorporated is a 1x3 switch. The optical arrangement 100 shown in FIG. 1 will allow the spatial light modulator to direct the optical beams between the Com port 10 and any of the optical ports 20, 30 and 40. More generally, any number n of optical ports may be employed to provide a lxn switching functionality. Optical arrangement 100 includes a walkoff crystal 50, a first composite half-wave plate 60, a polarizer 70, a Faraday rotator 80 and a second composite half-wave plate 90, all of which are optically coupled as shown for processing optical beams that are received by optical port 10, which serves as a Com (common) port and selectively directed to any of the optical ports 20, 30 and 40.

[0011] In FIG. 1 two orthogonal states or components of an optical beam are shown as it propagates through the various optical elements. One polarization component (e.g., a vertical or v-component) is denoted by a vertical arrow, and the other polarization component (e.g., a horizontal or h-component) is denoted by a dot. The walk-off direction of walk-off crystal 50 and directions of rotation caused by the half- wave plates and Faraday rotator will be described with respect to polarization components of light beams propagating in the forward or downstream direction, i.e., positive z-direction.

[0012] An optical beam received by the Com port 10 propagates in the forward direction and enters walk-off crystal 50, which splits the optical beam into two orthogonally polarized beams that are spatially displaced with respect to one another. In the example of FIG. 1 this spatial displacement is in the x-direction. The optical beams exit the walk-off crystal 50 and enter a composite half-wave plate 60 having regions 62, 64, 66 and 68 that have their optical axes arranged to rotate the polarization of an incoming beam by 90°. The remaining regions of the composite half-wave plate 60 do not alter the polarization state of an incoming beam.

[0013] Half-wave plate regions 62, 64, 66 and 68 are positioned along the optical paths defined by the optical ports 40, 30, 20 and 10 respectively. The half-wave plate region 68 is positioned to receive the h-polarized component of the optical beam traveling from the Com port 10. As a consequence of the composite half-wave plate 60 the optical beams originating from the Com port 10 are both in the same polarization state (i.e., the v-polarization state).

[0014] The optical beams exit the composite half-wave plate 60 and enter an optional polarizer 70 though which only vertically polarized light passes. Accordingly, only the two spatially displaced optical beam components originating from the Com port 10 will pass through the polarizer 70. [0015] After exiting the optional polarizer 70 the two vertically polarized optical beams originating from the Com port 10 enter a Faraday rotator 80. The Faraday rotator 80rotates the polarization state of the optical beams by 45° in the clockwise direction when the optical beam is traveling in the downstream direction, as shown. A second composite half-wave plate 90 following the Faraday rotator 80 includes a first half-wave plate region 92 that receives the two spatially displaced optical beams from the Com port 10. The second half-wave plate 90 also includes a second half-wave plate region 94 that is positioned along the optical paths defined by the optical ports 40, 30 and 20. For two beams traveling in the same direction, the second half-wave plate region 94 is arranged to rotate the polarization state of the optical beams in the opposite direction from that of the first half-wave plate region 92.

[0016] The first half-wave plate region 92 receiving the optical beams from the Faraday rotator 80 further rotates the optical beams by another 45° in the clockwise direction so that they both exit the first half-wave plate region 92 in the h-polarization state. As shown, the net effect of the Faraday rotator 80 and the first half-wave region 92 is to output an optical beam in a horizontal polarization state if the input optical beam is in a vertical polarization state and output an optical beam in a vertical polarization state if the input optical beam is in a horizontal polarization state. That is, the Faraday rotator 80 and the first half-wave plate 2 together rotate the polarization of a beam in one polarization state to its orthogonal polarization state. Moreover, the order in which the optical beam passes through the Faraday rotator 80 and the first half-wave plate region 92 may be reversed from that shown in FIG. 1.

[0017] After exiting the first half-wave plate region 92 the optical beams are directed further downstream through various other optical components and undergo optical processing by a spatial light modulator (not shown in FIG. 1). The spatial light modulator selectively directs various wavelength components of the optical beams to selected ones of the optical ports 20, 30 and 40. One example of a wavelength selective switch that incorporates a spatial light modulator and the optical arrangement shown in FIG. 1 will be presented below in connection with FIGs.5A and 5B. [0018] The optical arrangement 100 shown in FIG. 1 will allow the spatial light modulator to direct the optical beams between the Com port 10 and any of the optical ports 20, 30 and 40. However, the optical arrangement 100 will also block any optical beams originating from any of the optical ports 20, 30 and 40 which are directed to any of the ports 20 30 and 40 (e.g. an optical beam from optical port 20 will be prevented from reaching the optical port 30 or optical port 40).

[0019] Continuing with the example of FIG 1, after exiting the 2 ^nd composite half- wave plate 90 and being redirected by the SLM, the optical beam originating from the Com port 10 that is directed by the SLM to optical port40 will first pass through the second half-wave plate region 94 of the second composite half-wave plate 90 and the Faraday rotator 80 when traveling in the upstream direction. The net effect of the second half-wave plate region 94 and the Faraday rotator 80 is to output an optical beam in a horizontal polarization state if the input optical beam traveling in the upstream direction is in a vertical polarization state and output an optical beam in a vertical polarization state if the input optical beam is in a horizontal polarization state when traveling in the upstream direction. That is, the second half-wave plate region 94 and the Faraday rotator 80 together rotate the polarization of an upstream traveling optical beam in one polarization state to its orthogonal polarization state. Moreover, the order in which the optical beam passes through the second half-wave plate region 94 and the Faraday rotator 80 may be reversed from that shown in FIG. 1.

[0020] Since the optical beams traveling in the upstream direction from the SLM to the optical port 40 are in the h-polarization state upon entering the second half-wave plate region 94, they will exit the second half-wave plate region 94 and the Faraday rotator 80 in the orthogonal polarization state. That is, the optical beams will exit the second Faraday rotator 80 in the v-polarization state and thus will pass through the polarizer 70. The optical beams then pass through the composite half-wave plate 60, where the polarization of one the beams will be rotated into the h-polarization state by the half-wave plate region 62. The two beams, which are now in orthogonal polarization states, enter the walkoff crystal 50, which recombines the beams before they are directed to the optical port 40. [0021] In contrast to an optical beam originating from Com port 10 and being directed to any of the optical ports 20, 30 and 40, an optical beam originating from any of the ports 20, 30 and 40 will be prevented from reaching any of the other ports 20, 30 and 40 by polarizer 70. That is, the optical arrangement 100 exhibits a high degree of directivity. This is illustrated in FIG. 2, where an optical beam originates from optical port 20 of the optical arrangement 100 and is prevented from reaching optical port 40 by polarizer 70. Likewise, FIG. 3 shows the optical arrangement 100 when an optical beam is successfully directed from optical port 40 to COM port 10, thereby illustrating that the optical arrangement 100 exhibits a high degree of isolation.

[0022] As previously mentioned, in some embodiments the polarizer 70 is not provided. Without the polarizer 70, the switching of an optical beam from COM 1 to say, port 40 will be unaffected. Likewise, the switching of an optical beam from port 40 to COM port 10 is unaffected. However, the manner in which an optical beam is prevented from being switched between any two of the output ports 20, 30 or 40, such as shown in 2 will be impacted.

[0023] Similar to FIG. 2, FIG. 4 shows the optical arrangement in which an optical beam is switched from port 20 to port 40, except that in FIG. 4 the polarizer 70 is not present. In FIG. 4 the optical beam travels through the optical arrangement as shown in FIG 2 until it reaches the first composite half-wave plate 60. As shown, after each polarization component of the optical beam passes through the walkoff crystal 50, the beam will be directed to the optical port 40. However, instead of being directed along the optical axis of the port 40, the two polarization components will be displaced from the optical axis of the port and, assuming the spacing between adjacent ports is sufficient, the two polarizations will both miss port 40 and any adjacent ports. In this way coupling between ports 20 and 40 is prevented.

[0024] FIGS. 5 A and 5B 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. 5B, a fiber collimator array 101 may comprise a plurality of individual fibers 120i, 120 ₂ and 120 ₃ respectively coupled to collimators 102i, 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. 5B, more generally any suitable number of optical fiber/collimator pairs may be employed.

[0025] Optical arrangement 120 receives the light exiting from the port array 101 and directs the light toward the pair of telescopes described below. Optical arrangement 120 may be an optical isolator of the type described above in connection with FIG. 1.

[0026] 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.

[0027] In FIGs.5A and 5B, 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.5A and 5B 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. 5 A (since they have focusing power along the y-axis) and with dashed lines in FIG. 5B (since they leave the beams unaffected along the x-axis). Optical elements 104 and 105 are depicted with solid lines in FIG. 5B (since they have focusing power along the x-axis) and with dashed lines in FIG. 5A (since they leave the beams unaffected in the y-axis).

[0028] 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.

[0029] 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.

[0030] 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 λι, λ ₂ and λ ₃ are shown in FIG. 5 A being focused on the LCoS device 110 along the wavelength dispersion axis (x-axis).

[0031] As best seen in FIG. 5B, 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.

[0032] The above examples and disclosure are intended to be illustrative and not exhaustive. These examples and description will suggest many variations and alternatives to one of ordinary skill in this art.