WO2010146589A1

WO2010146589A1 - Multiple port wavelength selectable router

Info

Publication number: WO2010146589A1
Application number: PCT/IL2010/000479
Authority: WO
Inventors: Seong Woo Suh; Gil Cohen; Yossi Corem
Original assignee: Oclaro (New Jersey) Inc
Priority date: 2009-06-18
Filing date: 2010-06-17
Publication date: 2010-12-23
Also published as: CN102498427A

Abstract

A multi-port Wavelength Selective Switch (WSS), having a beam steering array to switch input light from one port into any one of a stack of output ports, but with the switch functionality doubled by using a polarization switching architecture which can deflect any of the switched beams in a plane perpendicular to the beam steering plane used by the WSS array. By this means, the port count is doubled, by having two stacks of ports disposed side by side in the switch. The beams are deflected to the desired port level in either of the stacks by means of a beam steering assembly, such as a one-dimensional MEMS array, and at the same time, any beam deflected to any of the port levels can be directed to either of the stacks by means of polarization switching components operative to divert the beams laterally between the two stacks.

Description

MULTIPLE PORT WAVELENGTH SELECTABLE ROUTER

FIELD OF THE INVENTION

The present invention relates to the field of fast optical switches, whose operation is wavelength dependent, especially those having a large number of switchable ports.

BACKGROUND OF THE INVENTION

In the field of optical communications, optical wavelengths are used as optical carriers for carrying digital or analog information. Also, the different wavelengths may be used to discriminate one set or channel of information from another. When a plurality of wavelengths are coupled or multiplexed onto a single fiber, this is called wavelength division multiplexing (WDM). Use of such WDM increases the overall bandwidth of the system.

There is a need in such systems to switch packets of optical information passing along one fiber to any of a number of other fibers, according to the wavelength of the optical signal. Such a switch is known as an optical router. A number of wavelength dependent switches and routers exist in the prior art. In International Patent Applications, published as WO2003/032071 , WO2005/052507, WO2007/029260 and WO2006/123344, all hereby incorporated by reference, each in its entirety, there are disclosed various wavelength selective switches and routers, wherein an input optical signal undergoes spatial wavelength dispersion and polarization splitting in two different planes, which can conveniently be perpendicular planes. The wavelength dispersion may preferably be performed by a diffraction grating, and the polarization-splitting by a polarized beam splitter. A polarization rotation device, such as a liquid crystal polarization modulator, pixelated along the wavelength dispersive direction such that each pixel operates on a separate wavelength channel, is operative to rotate the polarization of the light signal passing through each pixel, according to the control voltage applied to the pixel. The polarization modulated signals are then wavelength-recombined and polarization-recombined by means of similar or the same dispersion and polarization combining components as were used to respectively disperse and split the input signals. At the output polarization recombiner, the direction in which the resulting output signal is directed is determined by whether the polarization of the particular wavelength channel was rotated by the polarization modulator pixel, or not.

In WO2007/029260, there is described a multi-port switchable router, using beam steering elements which can be either an array of Micro-Electro- Mechanical System (MEMS) components, such as micro-mirrors, or a set of serially disposed liquid crystal arrays and wedge shaped birefringent crystals, which generate different angles of propagation to the beam passing therethrough according to the different polarizations of the beams produced by the setting of the liquid crystal array pixels, or a liquid crystal-on-silicon (LCQS) spatial light modulator acting as a phased array. However, all of these devices are limited in the number of ports that can be readily switched without the device becoming impractically large, a router having up to 8 switchable ports being described therein.

The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.

SUMMARY OF THE INVENTION

The present invention seeks to provide a new fiber-optical, wavelength selective switch structure (WSS), such as is used for channel routing applications in optical communication and information transmission systems. In prior art WSS structures, using LC polarization or MEMS beam steering, the number of switchable ports is generally limited. One limitation is that an increase in the number of ports would result in an increase in the height of the component, which would not be practical for integration into an optical communication system. Another limitation is the difficulty in providing suitable optical components, especially the focusing optics, for such a large multi-beam cross section. Exemplary devices described in the present disclosure to overcome this limitation, may comprise a multi-port WSS, typically of up to 1 x 12 way (this being a typical practical limit to the height of currently designed WSS structures for use in deployed commercial systems), with this switch functionality doubled by using a polarization switching architecture which can deflect any of the switched beams in a plane perpendicular to the beam steering plane used by the WSS array. By this means, a total of 24 ports is made available, in two stacks of 12 ports, the stacks being positioned side by side. The beams are deflected to the desired port level in either of the stacks by means of the beam steering assembly, such as any of those described in the above mentioned WO2007/029260. A particularly convenient method of beam steering may use a MEMS array. At the same time, any beam deflected to any of the port levels can be directed to either of the stacks by means of polarization switching components operative to divert the beams laterally between the two stacks. In the switches described in this disclosure, the direction of beam steering deflection for channel selection is performed in what is called the height of the switch, with the ports arranged vertically, while the beam deflection to select to which stack the steered beam is directed is performed in the direction of what is called the width the switch, with the two stacks arranged side-by-side.

The number of accessible ports is thus doubled, yet without increasing the height of the switch or the complexity of the beam steering mechanism, though the lateral polarization switching mechanism must be added. This polarization switching assembly may advantageously be constructed of a liquid crystal (LC) cell with a birefringent crystal wedge, as described in co-pending US Provisional Patent Application No. 61/213,544 to some inventors of the present application, entitled "Liquid Crystal Wavelength Selectable Router". In this manner, the port count is doubled up without the need for a wide angular swing of the beam steering arrangement, and without excessive height of the switch assembly.

According to further examples, a sequence of more than one polarization switching mechanism can be used in series, as shown in WO2007/029260, to enable switching of the steered output beams in more than two alternative lateral directions, such that the beam steering stack can be tripled-up or even more, thus generating an even larger WSS array. There is thus provided in accordance with an exemplary implementation of the devices described in this disclosure, a multi-port wavelength selective switch comprising:

(i) a two dimensional array of ports adapted to input and to output optical signals,

(ii) a wavelength dispersive element receiving an input optical beam from one of the ports, and dispersing wavelength components thereof in a dispersion plane, (iii) a pixelated beam steering array disposed such that a wavelength components is steered in a plane generally perpendicular to the dispersion plane according to the settings of the pixel of the beam steering array associated with the wavelength component, and (iv) a polarization dependent deflection array comprising:

(a) a pixilated polarization rotation element, adapted to rotate the polarization of light passing through pixels thereof according to control signals applied to the pixels, and

(b) a birefringent element disposed adjacent thereto, the deflection array being oriented such that wavelength components of the input optical beam passing through the deflection array are deflected in a direction generally perpendicular to the beam steering direction, the elements of the switch being further arranged such that the wavelength component of the input optical beam from one of the ports is directed to any other one of the two dimensional array of ports in accordance with:

(c) the setting of the pixel of the polarization rotation component through which the wavelength component passes, and

(d) the setting of the pixel of the beam steering array associated with the wavelength component.

In such a switch, the pixelated beam steering array may advantageously be a reflective MEMS array, and the pixelated polarization rotation component may be a pixelated liquid crystal cell. In an embodiment using a liquid crystal cell, the polarization of a wavelength component passing through a pixel of the liquid crystal cell may be rotated in accordance with the voltage applied across the liquid crystal pixel. In other example implementations of such a switch, the birefringent element may be a birefringent wedge with its taper aligned such that it deflects light of different polarizations in different angular directions generally perpendicular to the beam steering direction. If so, then the angular direction in which the wavelength component is deflected by the birefringent wedge should be dependent on the voltage applied across the pixel of the liquid crystal cell through which the wavelength component passed before impingement on the wedge.

Additionally, in alternative implementations of any of the above- described switches, the two dimensional array of ports may advantageously be arranged in two adjacent stacks, such that the switch provides twice the port capacity to that of a switch of similar height in which the ports are arranged in a one-dimensional array.

In any of these switch implementations, the ports may be arranged such that light can be switched between one port and any of the other ports.

Finally, such switches may further comprise a pixelated liquid crystal attenuator array, such that any wavelength component can be attenuated in passage through the switch.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

Fig.1 illustrates schematically a prior art wavelength selective switch structure (WSS);

Fig. 2 illustrates schematically a wavelength selective switch structure (WSS), constructed and operative according to a preferred embodiment of the present invention; and

Figs. 3 and 4 illustrate schematically alternative methods of generating input beam polarization diversity using a polarization beam splitter or a polarization beam combiner respectively. DETAILED DESCRIPTION

Reference is now made to Fig. 1 , which illustrates schematically a top view of a prior art wavelength selective switch structure (WSS), such as is described in International Patent Application, Publication No. WO2007/029260, and as used for channel routing applications in optical communication and information transmission systems. The WSS includes a stack 10 of fiber collimators for inputting and outputting the optical signals. Only one collimator is visible in the drawing since the stack is being viewed from the top. In this application, and as is conventionally used in the art, the dispersion plane of the switch is generally called the lateral plane, such that the view in the dispersion plane is called the side view, while the view perpendicular to the dispersion plane is called the plan view, or the view from the top. The drawing insert shows a side view of the stack 10 of fiber collimators. The beam issuing from each fiber collimator is then converted into a pair of closely disposed beams having the same predefined polarization direction for transmission through the WSS. This may be achieved by the use of a birefringent walk-off crystal 19, such as a YVO₄ crystal, having a half wave plate over part of its output face. The output of each input collimator channel is thus converted into a pair of beams having the same polarization direction, disposed in a predetermined plane. After this polarization decomposition and conversion, these beams can then advantageously be laterally expanded in that same predetermined plane, such as by an anamorphic prism pair. These optional beam expansion components, being well known in the art, are not shown in Fig. 1.

These input beams are wavelength dispersed in the plane of the drawing, conveniently by means of a diffraction grating 11. The wavelength dispersed beams are focused by a lens 12 onto a one-dimensional beam steering and switching array 13. In the exemplary switch shown in Fig. 1 , a MEMS array 14 is used for the beam steering, and a pixilated liquid crystal cell 15 for attenuation of the switched beams. For the sake of simplicity, only 3 separate wavelength channels and three pixels are shown in Fig. 1 , though it is to be understood that using a channel spacing of 100GHz or even 50 GHz, the number of channels that will fit into the bandwidth of the device will be much larger than that. The MEMS array steers the signals destined for different output ports in a direction out of the plane of the drawing, i.e. in the direction of the height of the switch, such that output signals are differently directed to enter different fiber optical collimators 10 shown in the side view of the collimator stack.

Reference is now made to Fig. 2, which illustrates schematically a wavelength selective switch structure (WSS), according to one exemplary implementation of the switches of the present disclosure. The WSS of Fig. 2 differs from that of Fig. 1 in that the beam steering and switching array 20 now includes two switching arrays. The first is the reflective MEMS array 14 used for the beam steering, in the same way as in the embodiment of Fig. 1. This steers the beams in the direction of the height of the switch, i.e. in the direction out of the plane of the drawing, to output the WSS at any of the different output port levels of the output port collimator array 10.

The second switching array is positioned in front of the MEMS array 14, and it is operative to switch the beams in a direction generally perpendicular to the beam steering direction. It is shown in Fig. 2 operating as an LC polarization mode switching array, comprising a birefringent crystal wedge 21 and associated pixilated LC cell 22. The operation of such an LC polarization mode switching array is described more fully in co-pending US Provisional Patent Application No. 61/213,544 for "Liquid Crystal Wavelength Selectable Router", herewith incorporated by reference in its entirety. The LC cell 22 changes the polarization direction of light passing through, in accordance with the voltage applied to the LC cell electrodes, and the birefringent crystal wedge 22 laterally deflects the beam in accordance with the polarization of the incident light. The taper of the wedge has to be aligned laterally so that its deflection plane is also lateral, i.e. in the dispersion plane, and perpendicular to the beam steering plane. The beam can thus be deflected according to the voltage applied to the LC pixel 22 through which it is passing. For instance, . for a particular polarization direction, if the LC voltage is applied to fully activate a cell ON, no change in the polarization direction of the traversing light occurs, and the beam passes through the wedge undeviated. If the LC is turned OFF, the polarization of the traversing light is rotated 90 degrees, and is then deflected by the wedge. The wedge orientation is such as to deflect the beam when switched, in the plane of the drawing, i.e. in the dispersion plane. The deflected beam angle, as determined by the birefractivity of the crystal material, is such that the deviation diverts the beam such that it now enters a second collimator array 25, disposed next to the first collimator array 10. Thus, by switching a pixel in the LC polarization mode switching array without changing the beam steering setting, the output beam can be switched between corresponding output port levels in collimator array 10 or 25. By switching both the pixels of the LC array and the beam steering array, the output beam can be switched to any of the output ports in the two side-by-side collimator arrays, 10, 25. As is seen in the side perspective view of the two collimator stacks, one is situated behind the other, both occupying the same height of the WSS.

Finally, the beam steering and switching array 20 may also include an LC attenuation array 15, generally comprising a linear polarizer and a pixilated LC cell, as is known in the art.

Thus, by operating the MEMS switch in conjunction with the LC polarization mode switch, a beam input to one of the ports of the 12-collimatror array can be switched in two orthogonal planes such that it can be accessed to any of the 24 ports of the two 12-collimator arrays. Since one of those ports is being used as the input port, this particular embodiment of the WSS of the present invention is a 1 x 23 way WSS. The height of the WSS need be no more than a 12-port WSS, and the increased footprint engendered by the second collimator stack is minimal.

Reference is now made to Fig. 3, which illustrates an alternative method of generating the polarization diversity required at the input to the switch, using a polarization beam splitter (PBS) assembly 32. The input beams 30 from the input collimators, shown as being 9 beams in the example of Fig. 3, are passed into the PBS, in which the p-component of any beam is transmitted undiverted, while the s-component is reflected such that it emerges parallel to the p- component. A half wave plate 33 disposed over the region of the PBS where the diverted beam emerge, changes the s-polarization to p-, such that both beams now have the same p-polarization for transmission through the switch. Any of the 9 beams are then beam steered and laterally deflected as described in the previous examples of Fig. 2.

Reference is now made to Fig. 4, which illustrates an alternative method of generating the polarization diversity required at the input to the switch, using a polarization beam combiner assembly 42. Only a single input beam is shown in Fig. 4, though it is to be understood that multiple beams can be used as in Fig. 3. The input beams 40 from the input collimators, are passed into the PBC, usually made of a birefringent block, such that the s- and p-components of the input beam are diverted into different angular directions. The diverging beams enter a prism 44, where they are refracted to a parallel configuration again, and a half wave plate 43 disposed over the region where the s-component beam emerges, changes the s-polarization to p-, such that both beams now have the same p-polarization for transmission through the switch. Any of the 9 beams are then beam steered and laterally deflected as described in the previous examples of Fig. 2.

It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and sub combinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description. and which are not in the prior art.

Claims

1. A multi-port wavelength selective switch comprising: a two dimensional array of ports adapted to input and to output optical signals; a wavelength dispersive element receiving an input optical beam from one of said ports, and dispersing wavelength components thereof in a dispersion plane; a pixelated beam steering array disposed such that a wavelength components is steered in a plane generally perpendicular to said dispersion plane according to the settings of the pixel of said beam steering array associated with said wavelength component; and a polarization dependent deflection array comprising: a pixilated polarization rotation element, adapted to rotate the polarization of light passing through pixels thereof according to control signals applied to said pixels, and a birefringent element disposed adjacent thereto, said deflection array being oriented such that wavelength components of said input optical beam passing through said deflection array are deflected in a direction generally perpendicular to said beam steering direction, the elements of the switch being further arranged such that said wavelength component of said input optical beam from one of said ports is directed to any other one of said two dimensional array of ports in accordance with: the setting of the pixel of said polarization rotation component through which said wavelength component passes, and the setting of the pixel of said beam steering array associated with said wavelength component.

2. A multi-port wavelength selective switch according to claim 1 wherein said pixelated beam steering array is a reflective MEMS array.

3. A multi-port wavelength selective switch according to claim 1 wherein said pixelated polarization rotation component is a pixelated liquid crystal cell.

4. A multi-port wavelength selective switch according to claim 3 wherein the polarization of a wavelength component passing through a pixel of said liquid crystal cell is rotated in accordance with the voltage applied across said liquid crystal pixel.

5. A multi-port wavelength selective switch according to any of the previous claims wherein said birefringent element is a birefringent wedge with its taper aligned such that it deflects light of different polarizations in different angular directions generally perpendicular to said beam steering direction.

6. A multi-port wavelength selective switch according to claim 5 wherein the angular direction in which said wavelength component is deflected by said birefringent wedge is dependent on the voltage applied across the pixel of said liquid crystal cell through which said wavelength component passed before impingement on said wedge.

7. A multi-port wavelength selective switch according to any of the previous claims, wherein said two dimensional array of ports is arranged in two adjacent stacks, such that said switch provides twice the port capacity to that of a switch of similar height in which the ports are arranged in a one-dimensional array.

8. A multi-port wavelength selective switch according to any of the previous claims, wherein said ports are arranged such that light can be switched between one port and any of the other ports.

9. A multi-port wavelength selective switch according to any of the previous claims, further comprising a pixelated liquid crystal attenuator array, such that any wavelength component can be attenuated in passage through said switch.