JP2012530930A - LCD wavelength selection router - Google Patents

Abstract

A polarization-dependent switch that uses polarization diversity to transform the input beam into a defined single polarization direction, followed by chromatic dispersion that broadens the individual wavelength channels on the pixelated switching device. This may be a polarization rotation element whose setting can be controlled by an applied electronic signal. This may leave the polarization direction unchanged, or it may rotate the polarization direction so that the polarization direction is essentially orthogonal to the polarization of the input beam exiting the polarization diversity component. The beam then proceeds to a birefringent wedge element, which refracts light having two orthogonal polarizations to different degrees and is therefore applied to the polarization rotation element through which each wavelength component of the beam has passed. The beam is separated according to the control signal. The beam may then be directed to different ports according to control signal settings. A 2 × 1 switch configuration is shown.
[Selection] Figure 8C

Description

  The present invention relates to high-speed optical switches for use in particularly configurable add-drop multiplexer (ROADM) applications whose operation is wavelength dependent.

  The use of optical wavelengths as optical carriers that carry digital or analog information is known in the field of optical communications. Similarly, different wavelengths may be used to distinguish one set or channel of information from another set or channel. When multiple wavelengths are combined or multiplexed on a single fiber, this is referred to as wavelength division multiplexing (WDM). Such WDM increases the overall bandwidth of the system.

  There is a need in such a system to switch packets of optical information passing along one fiber to any of several other fibers according to the wavelength of the optical signal. Such a switch is known as an optical router. Several wavelength dependent switches and routers exist in the prior art. Nos. 10 / 492,484, 10 / 580,832, 11 / 911,047, and 12 / 066,249, all of which are co-engaged, each incorporated by reference in its entirety. There are various wavelength selective switches and routers, and the incoming optical signal is polarization split into two preferably vertical planes, one of the polarization components so that both polarization components are oriented in one direction Followed by spatial chromatic dispersion. Polarization diversity may be implemented with a polarizing beam splitter or a birefringent walk-off crystal, and chromatic dispersion may be implemented with a diffraction grating. A polarization rotation device, such as a liquid crystal polarization modulator, pixelated along the chromatic dispersion direction so that each pixel operates on a separate wavelength channel, light that passes through each pixel according to a control voltage applied to the pixel. Operates to rotate the polarization of the signal. The polarization modulated signal is then wavelength recombined and polarization recombined by similar dispersion and polarization combining components to the components used to disperse and split the input signal, respectively. In the output polarization recombiner, the direction in which the resulting output signal is derived is determined by whether the polarization of a particular wavelength channel has been rotated by the polarization modulator pixel.

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

  The present invention seeks to provide a new fiber optic wavelength selective switch structure that may be used for channel routing or blocking applications in optical communication and information transmission systems.

  The device is designed as a 2 × 1 switch and uses liquid crystal elements for switching. The addition of multiplexers and demultiplexers adds a configurable optical add-drop multiplexer (ROADM) where these basic 2 × 1 structures have add and drop capabilities to / from several ports. ) To be used advantageously as a core. The switch uses minimal components and can therefore be economically constructed for large scale use in such systems. The switch structure may also incorporate a phase mode or polarization mode LC attenuator cell as a variable optical attenuator for any of the transmission paths through the device. Although the switch structure described in this application relates to a 2 × 1 structure, it is understood that according to the optical reciprocity principle, the described switch can operate as a 1 × 2 switch as well.

  The switch uses polarization diversity that transforms the input beam into a defined single polarization direction, followed by chromatic dispersion that broadens the individual wavelength channels on the pixelated switching device. This may be a polarization rotation element whose settings can be controlled by an electronic signal applied. This may leave the polarization direction unchanged, or it may rotate the polarization direction so that the polarization direction is essentially orthogonal to the polarization of the input beam exiting the polarization diversity component. The beam then proceeds to a birefringent wedge element, which refracts light having two orthogonal polarizations to different degrees and is therefore applied to the polarization rotation element through which each wavelength component of the beam has passed. The beam is separated according to the control signal. The beam may then be directed to different ports according to control signal settings.

  The beam can be guided through a pixelated phase changing element that controls the transmission of each component of the beam by affecting the mode structure of the beam before and after the pixel; Thereby, the ability of the beam to easily couple into the output fiber is controlled.

Thus, according to an exemplary embodiment of the device described in this disclosure, a wavelength selective switch is provided,
(I) a first optical port;
(Ii) a polarization conversion element that gives a predetermined polarization direction to light input through the first optical port;
(Iii) a wavelength dispersion element in optical communication with the polarization conversion element, whereby the wavelength component of the light received from the first optical port is dispersed in the dispersion plane;
(Iv) having pixels that are optically coupled to receive the dispersed light and that are generally oriented in the dispersion plane and that rotate the polarization of light passing through each pixel in accordance with a control signal applied to the pixels; A pixelated polarization rotation element configured to rotate the polarization of the wavelength component of the dispersed light according to a control signal applied to the pixel through which the wavelength component passes; and
(V) a light that is in optical communication with the light from the polarization rotation element and is arranged such that light that traverses the pixels of the polarization rotation element is reflected in the first or second direction according to the polarization of the received light. A reflective birefringent element attached,
The element is further directed so that light from the reflective birefringent element reenters the polarization converting element, thereby reconstructing the light back to its original polarization.

  In such a switch, the light is essentially 90 in the first direction and about the polarization of the optical beam when the control signal applied to the pixel is such that it does not produce any change in the polarization of the optical beam. When it is such as to produce a rotation of °, it may be reflected in the second direction. In this arrangement, the first direction may lead to the second optical port and the second direction may lead to the third optical port so that the first optical port is in accordance with the control signal. Optically connected to either the second or third port.

  Other embodiments may further include a switch as described above, wherein the reflective birefringent element is such that light reflected in one of the directions is collinear with respect to the optical path from the first optical port. The switch further includes a circulator that separates light reflected in one direction from light incident from the direction of the first optical port. In such an embodiment, the switch should further comprise an isolator disposed in the optical path in the second direction, wherein the isolator is disposed in the second direction with light directed in the second direction. Oriented so that it cannot enter the optical port.

  In any of the embodiments described above, the pixelated polarizing element may be a pixelated liquid crystal cell, in which case the control signal may be a voltage applied to the electrodes across the pixels of the liquid crystal cell.

  Further embodiments may include a wavelength selective switch according to any of the designs described above, in which the reflective birefringent element is in the form of a wedge, whereby the direction is angularly identified. Direction, or in the form of a block, whereby the direction is the direction identified in the lateral direction.

  In addition, any such switch may direct light input to the first port to either the second and third ports, or light input to either the second and third ports. May be directed to the first port.

  In addition, an alternative embodiment of any of the switches described above may further comprise a pixelated phase changing element that spatially varies phase across the light across the pixel. Controls the transmission of light through the pixels of the polarization rotation element, thereby controlling the ability of light to couple into the output port. In such a switch, the mode structure of the beam of light is degraded by the voltage applied to the pixels of the phase changing element. The pixelated phase change element may comprise a comb-like electrode structure that applies a spatially wavy electric field across the pixels of the pixelated phase element, thereby passing through the comb-like electrode structure. The phase of the light undergoes a corresponding spatially alternating change. In any case, the degradation of the mode structure of the light beam controls the ability of light to couple into the output port fiber.

Any of the described wavelength selective switches may further comprise a polarization mode attenuator that controls the transmission of light through the polarization mode attenuator, the attenuator comprising:
(I) a pixelated birefringent polarization rotating element, wherein the polarization direction of light passing through the pixel of the pixelated birefringent polarization rotating element is rotated according to an electric field applied across the pixel; ,
(Ii) a serial linear polarizer;
Thereby, the attenuation of light passing through the pixels of the polarization mode attenuator depends on the degree to which the polarization of the light traversing the birefringent polarization rotation element is parallel to the polarization direction of the linear polarizer.

  Finally, a further exemplary embodiment comprises (i) a first optical port and (ii) a chromatic dispersion element in optical communication with the first optical port, whereby the first A wavelength dispersion element that disperses the wavelength component of the light received from the optical port, and (iii) a pixel that is generally oriented to receive the dispersed wavelength component and is responsive to a control signal applied to the pixel A pixelated polarization rotation element adapted to rotate the polarization of the light passing through each pixel, whereby the polarization of the wavelength component of the dispersed light is applied to the pixel through which the wavelength component passes A pixelated polarization rotation element that rotates according to the signal, and (iv) light across the pixel of the polarization rotation element is guided in a first or second direction according to the polarization of the light determined by a control signal applied to the pixel. Arranged so that A, and a birefringent element attached countercurrent.

  The present invention will be understood and appreciated more fully from the following detailed description considered in conjunction with the drawings.

2 is a schematic block diagram of the functionality of a fixed add / drop ROADM using 2 × 1 WSS according to the first preferred embodiment of the present invention; FIG. 1B is a schematic diagram of the reflective wavelength selective router used in FIG. 1A, showing the components in more detail. FIG. 6 schematically illustrates a method for generating beam deflection using an LC cell connected in series with a birefringent crystal wedge. FIG. 6 schematically illustrates a method for generating beam deflection using an LC cell connected in series with a birefringent crystal wedge. FIG. 6 schematically illustrates a method for generating beam deflection using an LC cell connected in series with a birefringent crystal wedge. FIG. 6 schematically illustrates a method for generating beam shifts using an LC cell connected in series with a birefringent crystal block. FIG. 6 schematically illustrates a method for generating beam shifts using an LC cell connected in series with a birefringent crystal block. FIG. 6 schematically illustrates a method for generating beam shifts using an LC cell connected in series with a birefringent crystal block. FIG. 4 schematically illustrates a method for constructing an LC cell to perform the polarization switching required for the embodiment of FIGS. FIG. 4 schematically illustrates a method of constructing an LC cell to implement the beam attenuation required for the embodiment of FIGS. FIG. 6 shows the operation of switching the LC cell on or off for single segment transmission of the embodiment of FIG. FIG. 6 shows the operation of switching the LC cell on or off for single segment transmission of the embodiment of FIG. FIG. 6 shows the operation of switching the LC cell on or off for single segment transmission of the embodiment of FIG. FIG. 6 schematically illustrates various reflective polarization mode switching embodiments. FIG. 6 schematically illustrates various reflective polarization mode switching embodiments. FIG. 6 schematically illustrates various reflective polarization mode switching embodiments. FIG. 6 schematically illustrates various reflective polarization mode switching embodiments. FIG. 6 schematically illustrates various reflective polarization mode switching embodiments. FIG. 7B is a more detailed view of the circular configuration of FIG. 7E showing the polarization change of the beam as it traverses the switch assembly for four alternative transmission options. FIG. 7B is a more detailed view of the circular configuration of FIG. 7E showing the polarization change of the beam as it traverses the switch assembly for four alternative transmission options. FIG. 7B is a more detailed view of the circular configuration of FIG. 7E showing the polarization change of the beam as it traverses the switch assembly for four alternative transmission options. FIG. 7B is a more detailed view of the circular configuration of FIG. 7E showing the polarization change of the beam as it traverses the switch assembly for four alternative transmission options. 9 is a truth table showing transmission paths for the four alternative switch positions shown in FIGS. FIG. 6 schematically illustrates a reflected polarization mode switching embodiment incorporating phase attenuation. FIG. 6 schematically illustrates a reflected polarization mode switching embodiment incorporating phase attenuation. FIG. 6 schematically illustrates a reflected polarization mode switching embodiment incorporating phase attenuation. FIG. 6 schematically illustrates a reflected polarization mode switching embodiment incorporating polarization attenuation. FIG. 6 schematically illustrates a reflected polarization mode switching embodiment incorporating polarization attenuation. FIG. 6 schematically illustrates a reflected polarization mode switching embodiment incorporating polarization attenuation. FIG. 6 schematically illustrates a reflected polarization mode switching embodiment incorporating polarization attenuation. FIG. 6 schematically illustrates a reflected polarization mode switching embodiment incorporating polarization attenuation.

Reference is now made to FIG. 1A, which schematically illustrates a block diagram of the functionality of a fixed add / drop ROADM using 2 × 1 WSS according to a first exemplary embodiment of the disclosed device. Show. The ROADM inputs multi-wavelength light having wavelengths λ ₁ , λ ₂ , λ ₃ ,... At its input port 10 and drops a predetermined wavelength at the local drop port 11. The local add port 12 is designed to insert a predetermined wavelength (add) and output the obtained optical signal to the output port 13. The core of the device is an optically switched wavelength selective switch 15 that operates as a 2 × 1 switching router. In the embodiment shown in FIG. 1, the WSS has a through path for wavelength λ ₃ while blocking other wavelengths λ ₁ and λ ₂ with a high extinction rate. In addition, all of the path paths should have variable attenuation capabilities to compensate for different signal strengths originating from different channels, whether through-paths or add-paths.

Reference is now made to FIG. 1B, which is a schematic plan view of a reflective wavelength selective switch that can be used in FIG. 1A, showing the components in more detail. FIG. 1B shows a planar layout of the single channel path of the switch. The input (or output) beam for each port is input (or output) at the fiber interface block, which preferably is a fiber collimator 29 for one port, followed preferably by its output face. A birefringent walk-off crystal 21 such as a YVO ₄ crystal having a half-wave plate 19 on a part thereof is provided. Therefore, the output of each channel has the same polarization direction as indicated by a vertical line for each of the beam outputs, and in the example shown in FIG. 1B, a beam pair arranged in a predetermined plane, which is the plane of the drawing. Prepare. After this polarization decomposition and conversion, these beams can be advantageously subjected to lateral expansion in its same predetermined plane by the anamorphic prism pair 23 in the preferred example shown in FIG. 1B. These laterally expanded beams are also passed to the grating 24 for chromatic dispersion in the same predetermined plane. The dispersed wavelength component is then sent to the lens 25 for convergence on the beam switching and steering module 26, which is shown in FIG. 1B as a pixelated polarization rotating element 27 and a reflecting element. A beam steering device 28 is provided to reflect each switched and steered beam and follow the switch back to the output position of the birefringent crystal and from there back to each output collimator port after recombination To work. This steering is performed in a direction perpendicular to the plane of the drawing and according to one preferred embodiment. The beam steering device may be a MEMS array of mirrors.

  Similar transmissive embodiments can be implemented as well, in which case the reflective element 28 is replaced by an embodiment of the transmissive steering element and the above described device is used to address the output of the transmitted beam. The input element is repeated to the right side of the beam steering device.

  WSS operates by polarization-dependent beam deflection to switch incoming signals. Reference is now made to FIGS. 2A-2C, which schematically illustrate a first advantageous method of accomplishing this. The drawing shows the switching function of a single pixel element of pixels arranged along the chromatic dispersion direction of the device. Prior to this switching element, each input beam exiting the fiber collimator has the same definition to pass through the switch, beam expander, if used, and chromatic dispersion element, all already shown in FIG. 1B. It is understood that the beam is converted into a closely arranged beam pair having a polarization direction of.

  The switching assembly of FIGS. 2A-2C includes a liquid crystal (LC) cell 20 connected in series to a birefringent crystal wedge 22. The LC cell changes the polarization direction of the passing light according to the voltage applied to the LC cell electrode, and the birefringent crystal wedge deflects the beam according to the polarization of the incident light. Thus, the beam can be guided according to the voltage applied to the LC pixel through which the beam passes. In the exemplary device shown in FIGS. 2A-2C, the incident light has a polarization perpendicular to the plane of the drawing. In FIG. 2A, with the LC voltage applied to fully activate the cell, there is no change in the polarization direction of the light across and the beam passes through the wedge without deviation. In FIG. 2B, the LC is turned off and the polarization rotates essentially through 90 ° to be in the plane of the drawing and is thus detected by the birefringence of the wedge, as shown. In FIG. 2C, an intermediate voltage that rotates the polarization by 45 ° is applied to the LC pixel, thus generating circularly polarized light, whereby each vertical component of this circularly polarized light is in a direction that depends on the polarization of that component. Induced by wedge. Because the two components diverge in free space, the next optical component in the switch signal processing chain is such that the angular deviation provides sufficient spatial separation of the components so that each component can be processed separately. Can be placed at a distance from a large wedge. This embodiment has the advantage that the wedge is thin, thus saving material costs.

  Reference is now made to FIGS. 3A-3C, which schematically illustrate another exemplary method of deflecting the beam according to its polarization. This embodiment is similar to the embodiment of FIGS. 2A-2C with the input LC cell 30 for switching, except that a block of birefringent crystal 32 is used instead of the wedge 22. This has the advantage that the beam is displaced laterally rather than angularly deflected so that it remains parallel. However, this has the disadvantage that it requires a longer block of material than the wedge embodiment of FIGS. 2A-2C to provide sufficient spatial separation of the two polarization components. The configuration of the three states illustrated in FIGS. 3A to 3C is equivalent to the configuration of FIGS.

  Reference is now made to FIG. 4 in its three parts, both of which are LC switching cells 20 or 30 used to implement the polarization switching shown in FIGS. 2A-2C or 3A-3C. The method of constructing is schematically shown. The cells are such that different wavelengths (generated by dispersive elements not shown in the drawing but understood to be part of the device) fall on different pixels of the LC cell as shown on the right hand side of FIG. Furthermore, it should be pixelated along the dispersion direction λ of WSS. The exemplary cell shown in FIG. 4 has a common back electrode 40, shown as COM, while the front electrode, shown as SEG, is shown on the left hand side of FIG. 4 so that each wavelength channel can be switched separately. And divided into pixels or segments 41 along the wavelength dispersion direction. LC material is present between these two electrodes. The rubbing axis of the LC is shown to be oriented at 45 ° with respect to the input polarization direction, so that if no activation voltage is applied between the electrodes of a particular segment, the input polarization is rotated by 90 ° On the other hand, application of an activation voltage will leave the polarization unaffected. According to any of the embodiments of the switching assembly of FIGS. 2A-2C and 3A-3C, the beam is thus output from the device according to whether a voltage is applied to the pixel or segment of that particular wavelength. Will be guided in either direction.

  Reference is now made to FIG. 5 in all its parts, and FIG. 5 shows how to attenuate the input beam of each wavelength channel by using phase mode operation rather than the polarization mode switching shown in the previous figure. 1 schematically shows the structure of an LC cell implementing The common electrode 50 of this embodiment is constructed with a number of separate strips in the form of a comb. The front electrode indicated by SEG is divided into pixels or segments 51 along the wavelength dispersion direction as shown on the left hand side of FIG. 5 so that each wavelength channel can be switched separately. LC material is present between these two electrodes. Since the COM electrode has a comb structure, the height of each segment 51 is divided into a number of separate parts, some of which are under the influence of the electric field of the common electrode, and part of the influence of the electric field. Not below. As a result, when a voltage is applied between the COM electrode and the SEG electrode, different portions of each pixel are subjected to different applied electric fields across the thickness of the pixel. In this embodiment, the rubbing direction of the cell is parallel to the input polarization direction, so that the polarization of the light is not affected by the applied electric field as it passes through the LC. On the other hand, application of a voltage across the cell changes the phase shift of light passing therethrough. The common comb electrode COM is held at the ground potential. When no voltage is applied to a particular segment electrode, there is no electric field variation between the common electrode and that segment electrode, resulting in uniform transmission of light through that segment. On the other hand, when a voltage is applied to the segment electrodes, the electric field across the height of the segments varies periodically between successive “teeth” of the comb, and thus the corresponding between the two electrodes COM and SEG. The refractive index of the liquid crystal material in the region is periodically changed. This periodic variation in refractive index produces a periodic change in the phase of light transmitted through the segment over the height of the segment. This destroys the mode characteristics of the beam, so that the beam cannot be combined in the same way as a uniform mode beam. Thus, the beam that traverses that segment is attenuated. Thus, a beam that traverses a segment is transmitted or blocked depending on whether a voltage is applied to the electrode of that segment.

  As an alternative to the horizontal comb structure applied to the common electrode 50, according to another embodiment of these switches, each segment 51 has a number of narrow strips oriented perpendicular to the drawing direction of FIG. Attenuation can also be achieved by splitting the common electrode. In such an embodiment, it is the phase change that occurs across the width of each segment that results in the destruction of the mode characteristics of the beam that passes through that segment, and thus attenuates the beam that passes through that segment.

  In addition to the phase perturbations created by using the embodiment of FIG. 5, there may be additional diffraction grating effects that can help attenuate the beam passing through the segment. Since the comb spacing is small, typically 100 microns to 20 microns, and at least the narrower of these spacings approaches the wavelength of light used, the application of voltage to the LC segment activates the lattice. When done, this grating action diffracts the light from its path, thus increasing attenuation in addition to diffraction by the phase scrambling action described above.

Reference is now made to FIGS. 6A and 6B, which illustrate the effect of switching the LC cell on or off with respect to single segment transmission of the phase mode operation switch of FIG. When the cell is OFF, as shown in FIG. 6A, no phase difference is generated in the beam segment and therefore there is no attenuation. When the cell is ON, as shown in FIG. 6B, a phase difference is generated across the height of the beam segment, and thus the beam is attenuated. FIG. 6C shows an enlarged cross-section of FIG. 6B, showing the periodic variation between the _two refractive indices n ₁ and n ₂ over the height of the segment. In either of these situations, there is no polarization change of light across.

Reference is now made to FIGS. 7A and 7B, which are similar to the transmission polarization mode switching shown in FIGS. 2A-2C, but instead of a reflective arrangement, according to a further example of an embodiment of the present invention. 1 schematically shows a switch mechanism using the configuration. The birefringent wedge 70 has a reflective surface 71 on the surface facing the impinging direction of incident light. Light passes through the LC cell 74 both before hitting the wedge and after reflection from the wedge. In FIGS. 7A-7E, the LC cell 74 is shown away from the birefringent reflective wedges 70, 71 for clarity, but in practice these two components are used when the reflected beam is in its incident path. Should be close together to ensure that they pass through the same pixels. This same comment is also valid for FIGS. 9A-9C and FIGS. 10A-10E. In the exemplary embodiment shown in FIGS. 7A-7E, when the cell is OFF as in FIG. 7A, the polarization of the incident light is rotated 90 ° from s-polarized light to p-polarized light. In the case of p-polarized light, the light that traverses the birefringent wedge 70 undergoes a shift such that it returns at an angle θ ₁ with respect to its incident direction after being reflected from the wedge. The light then crosses the LC cell again, so that the polarization of the light returns to the s-polarization so that the light has the same polarization as the input polarization with it, but is output in a different propagation direction. Rotate again 90 °. FIG. 7B shows the effect on incident light when the LC cell is ON. In such cases, there is no polarization rotation and the beam across the birefringent wedge maintains the incident s-polarization. As it passes through the wedge, the beam is deflected by a different angle θ ₂ after being reflected from the rear mirror 71. This difference in deflection angle works when enabling the switching of the beam to different ports.

  FIG. 7C shows a situation where the LC cell is partially switched, resulting in circularly polarized output light. This results in a higher insertion loss than in the ON or OFF situation due to the high PDL (polarization dependent loss).

  The embodiment shown in FIGS. 7A-7C (and also below in 7D-7E) utilizes a birefringent wedge 70 to provide a change in direction imparted to the beam according to its polarization. Such a wedge is shown in FIGS. However, as with the embodiment of FIGS. 3A-3C, it is understood that it is feasible to generate a beam direction change according to its polarization using a birefringent block. In this case, instead of polarization-dependent angular deflection, the beam is given a deflection-dependent lateral displacement.

  The embodiment of the switch shown in FIGS. 7A-7C is shown in FIG. 7D with three collimators at the switch input / output port, ie, one collimator 75 that inputs the beam, and FIGS. 7B and 7A, respectively. Collimators 76 and 77 that output beams at two output ports, each corresponding to a switchable diversion angle, are required.

  According to another example of these reflective deflection mode switches, the reflective birefringent wedge can be oriented at an angle such that the p-polarized beam returns along its incident path, as shown in the configuration of FIG. 7E. The s-polarized beam will return along paths with different reflection angles. In such an arrangement, only two collimators 77, 79 are required, but the circulator 78 is necessary to separate the reflected output light from the input light.

  However, the use of such a circulator in a 2 × 1 switch configuration eliminates the need for a single collimator, but has other consequences of switch construction in the form of a need for an isolator at a port without a circulator. Figures 8A-8E schematically illustrate these features. 8A-8D show a more detailed view of the circulator configuration of FIG. 7E, showing the polarization change of the beam as the beam traverses the switch assembly for four alternative transmission options. However, FIG. 7E is illustrated by a 1 × 2 switch and FIGS. 8A-8D show a 2 × 1 configuration. FIG. 8E is a truth table showing the routing of various switch situations as a function of LC cell state.

  Reference is now made to FIG. 8A, which shows the switching geometry of the circulator embodiment of the switch. There are two input ports labeled IN 1 and IN 2 and a single output port labeled OUT. The input to port IN 2 is shown as a dashed line in FIG. 8A because it is not active. The signal input through IN 1 crosses the input collimator COL 1, followed by polarization separation, optional beam expansion, and chromatic dispersion (none shown), covering all of the input and output beam paths. The light enters the refraction switching element 81. The switching element may advantageously be an LC cell. When the cell voltage is set so that a λ / 2 retardation is applied to the beam, the transmitted beam acquires s-polarized light and is normally refracted by the wedge in a direction that is incident on the reflective surface 83 of the birefringent wedge 82. Return to the circulator along its input path and exit the switch at the OUT port.

  In all of FIGS. 8A-8D, the retardation λ / 2 or 0 generated by the birefringent switching element 81 is noted at the bottom edge of the element in the drawing, and the resulting polarization p or s Marked in contact with the transmitted beam. The tilt angles of the birefringent wedge 82 and the birefringent switching element 81 are shown in an exaggerated manner in the drawings of FIGS. 8A-8D to clearly show how the different polarized beams are deflected within the birefringent wedge. Should also be noted. In practice, the tilt angle should be on the order of 15 ° for wedges with a much smaller apex angle, usually on the order of 8 °.

  In FIG. 8B, the switch state is switched by setting the LC cell voltage so that no retardation is applied to the beam, and the beam entering port IN 1 is transmitted to the wedge with p-polarization. The birefringence of the wedge is such that the p-polarized light is refracted at an angle different from the angle at which the s-polarized light is refracted, and the relative orientation of the wedge is such that the beam is reflected backward and towards the port IN 2. It is like leaving the switch assembly. However, in this 2 × 1 switch configuration, it is desired that the input signal be guided from any input port only to the OUT port, and it is necessary to prevent this signal from being output at port IN 2. This is achieved by the use of an isolator 86 in the beam path to port IN 2.

  In FIG. 8C, for the same switch setting, the signal input at port IN 2 passes through the isolator 86 in its forward direction in the low insertion loss direction and is converted to p-polarized light by the LC cell 81 and then In the opposite direction, the beam path of FIG. 8B is followed and finally guided by the circulator 85 to the desired output port OUT.

  In FIG. 8D, the LC cell settings are switched to provide λ / 2 retardation, so that the beam coming from port IN 2 enters the wedge with s-polarization and enters the circulator and port IN 1 Although it is refracted in a direction that does not enter the beam path to which it goes (shown by the dashed line in FIG. 8D), it is absorbed and lost somewhere on the wall or floor of the switch structure.

  The four alternative switch positions are summarized by the truth table shown in FIG. 8E. As can be seen, in this way, the switch assembly behaves as a 2 × 1 switch, as shown in the situation described in FIGS. 8A and 8C, and either input of ports IN 1 and IN 2 passes the signal. Depending on the switching state of the LC cell pixel, it can be switched alternately to the port OUT.

  When the LC cell is driven so that the retardation is somewhere between 0 and λ / 2, the resulting circular or elliptical polarization is that the input signal input to one port is both the other two ports It should also be understood that it will be guided to. Furthermore, it should be noted that the need for an isolator is limited to a 2 × 1 switching situation. When the same switch structure is used in a 1 × 2 configuration, no further isolators are required as described in FIG. 8B above.

  Reference is now made to FIGS. 9A-9C, which are similar to the switches of FIGS. 7A-7E, according to a further preferred embodiment of the present invention, but with any of the LC cells of FIGS. 1 schematically illustrates a polarization mode LC switch including a similar phase mode attenuator LC cell. In order to equalize the optical throughput at the various ports, it is generally necessary to be able to vary the attenuation through each port. In FIG. 9A, birefringent reflective wedges 90, 91 having switching LC cells 92 are shown. In addition, a phase mode LC attenuator cell 94 is disposed in the beam path so that the switch is provided with attenuation for each port. There is a slight bandwidth penalty for this type of embodiment because the LC switching cell 92 and the LC attenuating cell 94 cannot be perfectly matched spatially. The reason is that the pixel width expected in the attenuator LC cell 94 due to the focused beam on the surface of the mirror 91 is larger than the pixel size of the switching LC 92 because the attenuation LC is farther from the mirror than the switching LC. The switch situation shown in FIGS. 9A-9C already exists in FIGS. 7A-7C by adding a phase mode LC attenuator cell 94 disposed in the beam path so that the attenuation of the signal passing through the switch can be controlled. It is essentially the same as the situation shown.

  It should be noted that the input and output polarization identities for the polarization mode LC switch are important in this embodiment, since the polarization must be the same for the phase mode attenuator to operate correctly. Furthermore, because the input and output polarizations are the same, a high PDL, high efficiency grating can be used in the system.

  Reference is now made to FIGS. 10A-10E, which are similar to the switches of FIGS. 7A-7E according to a further preferred embodiment of the present invention, but in combination with the polarizer 104, the LC cell 102. 1 schematically shows a polarization mode LC switch including a polarization mode attenuator LC cell comprising

  The operation of this embodiment is shown in FIGS. 10D-10E, showing various polarization changes through the device. As it moves from the input port on the left-hand side of the drawing towards the birefringent wedge 108, the beam encounters an attenuator LC cell 102 having a rubbing direction of 45 ° relative to the input polarization direction, thereby showing in FIG. 10E. Thus, if no activation voltage is applied between the electrodes of a particular segment labeled OFF, the input polarization will be rotated by 90 °. Application of an activation voltage labeled ON as shown in FIG. 10D will leave the polarization unaffected. Polarizer 104 is parallel to the input polarization of the beam so that when the LC cell is ON as in FIG. 10D, a polarized beam having an unchanging polarization direction is attenuated by only the insertion loss and passes through. It has its polarization direction. After this stage, the beam proceeds to the switching LC cell 106 and to the reflective birefringent wedge 108, at least for the p-polarized light shown in the drawing, and is reflected from the wedge 108 at its characteristic angle. On the other hand, if the LC cell is OFF as in FIG. 10E, the polarization is rotated by 90 °, so that the beam crosses the polarizer polarization direction 104 and is therefore attenuated by the polarizer extinction rate, “O” is displayed in the drawing. At any voltage between the fully OFF level and the fully ON level, the signal is correspondingly attenuated as it passes through the switch.

  Note that for all of the above example switches, the polarization of the output beam is the same as the polarization of the input beam, regardless of the ON / OFF switching state of the LC, so that the switch is polarization independent. This feature is desirable in optical switch technology because it simplifies the design of the system in which the switch is used.

  Although the embodiment described above shows a 1 × 2 switching configuration, due to the reciprocity principle in optical components, the switch is used in a 2 × 1 configuration, with the exceptions described with respect to the circulator embodiments of FIGS. It is understood that it is possible. Further, such switches can be cascaded, resulting in a multiple switching configuration with compact dimensions and more ports (input or output).

  It will be 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 the various features described above, as well as combinations and subcombinations of variations and modifications to those features that will occur to those skilled in the art upon reading the above description and are not present in the prior art. Including.

Claims (18)

A wavelength selective switch,
A first optical port;
A polarization conversion element that provides a predetermined polarization direction to light input through the first optical port;
A wavelength dispersion element in optical communication with the polarization conversion element, whereby a wavelength component of light received from the first optical port is dispersed in a dispersion plane;
Rotating the polarization of light passing through each pixel in accordance with a control signal applied to the pixel, having pixels that are optically coupled to receive the dispersed light and that are generally oriented in the dispersion plane A pixelated polarization rotation element adapted to rotate the polarization of the wavelength component of the dispersed light according to the control signal applied to the pixel through which the wavelength component passes When,
A direction that is optically in communication with the light from the polarization rotation element and that crosses the pixels of the polarization rotation element is reflected in a first or second direction according to the polarization of the received light. A reflective birefringent element attached,
The element is further directed such that light from the reflective birefringent element reenters the polarization converting element, thereby reconfiguring the light back to its original polarization.   The light is in a first direction when the control signal applied to the pixel does not produce any change in polarization of the optical beam, and the control signal applied to the pixel is the optical signal. 2. A wavelength selective switch according to claim 1, wherein the wavelength selective switch is reflected in the second direction when it is such as to produce an essentially 90 [deg.] Rotation about the polarization of the beam.   The first direction communicates with a second optical port, the second direction communicates with a third optical port, whereby the first optical port communicates with the second and third optical ports according to the control signal. The wavelength selective switch according to claim 1, which is optically connected to one of the ports.   The reflective birefringent element is oriented such that the light reflected in one of the directions returns collinearly with respect to an optical path from the first optical port, and a switch is The wavelength selective switch according to claim 1, further comprising a circulator for separating light reflected in the one direction from light incident from the direction of the optical port.   The device further includes an isolator disposed in the optical path in the second direction, and the isolator is configured such that light guided in the second direction enters an optical port disposed in the second direction. The wavelength selective switch according to claim 4, wherein the wavelength selective switch is oriented so as to be impossible.   6. The wavelength selective switch according to claim 1, wherein the pixelated polarization element is a pixelated liquid crystal cell.   The wavelength selective switch according to claim 6, wherein the control signal is a voltage applied to electrodes across the pixels of the liquid crystal cell.   The wavelength selective switch according to any one of claims 1 to 7, wherein the reflective birefringent element is in the form of a wedge, whereby the direction is an angularly identified direction.   9. A wavelength selective switch according to any one of claims 1 to 8, wherein the reflective birefringent element is in the form of a block, whereby the direction is a direction identified in a lateral direction.   The wavelength selective switch according to claim 3, wherein the light input to the first port is guided to one of the second and third ports.   4. The wavelength selective switch according to claim 3, wherein light input to one of the second and third ports is guided to the first port. 5.   Further comprising a pixelated phase modifying element, wherein the pixelated phase modifying element controls transmission of light passing through the pixels of the polarization rotation element by spatially changing the phase across the light traversing the pixel, thereby The wavelength selective switch according to any one of claims 1 to 11, which controls the ability of light coupled into an output port.   The wavelength selective switch according to claim 12, wherein a mode structure of the light beam is lowered by a voltage applied to a pixel of the phase changing element.   The pixelated phase changing element comprises a comb-like electrode structure, the comb-like electrode structure applying a spatially wavy electric field across the pixels of the pixelated phase element, thereby forming the comb-like electrode structure 13. A wavelength selective switch according to claim 12, wherein the phase of the light passing therethrough undergoes a corresponding spatially alternating change.   The wavelength selective switch of claim 13, wherein the degradation of the mode structure of the beam of light controls the ability to couple into an output port fiber. A polarization mode attenuator, further comprising a polarization mode attenuator that controls transmission of light passing through the polarization mode attenuator, the polarization mode attenuator comprising:
A pixelated birefringence polarization rotation element, wherein the polarization direction of light passing through the pixel of the pixelation birefringence polarization rotation element is rotated according to an electric field applied across the pixel; and
With a serial linear polarizer,
16. The attenuation of light passing through the pixels of the polarization mode attenuator depends on the degree to which the polarization of light traversing the birefringent polarization rotation element is parallel to the polarization direction of the linear polarizer. The wavelength selective switch according to item.   17. A wavelength selective switch according to any one of claims 1 to 16, wherein the arrangement of elements renders the switch polarization independent. A wavelength selective switch,
A first optical port;
A wavelength dispersion element in optical communication with the first optical port, whereby a wavelength component of light received from the first optical port is dispersed;
Pixelation having pixels that are generally oriented to receive the dispersed wavelength components, and rotating the polarization of light passing through each pixel in response to a control signal applied to the pixels A polarization rotation element, whereby the polarization of the wavelength component of the dispersed light rotates according to the control signal applied to the pixel through which the wavelength component passes; and
Birefringence arranged and directed so that light traversing the pixel of the polarization rotation element is guided in a first or second direction according to the polarization of the light determined by the control signal applied to the pixel And a chromatic dispersion element.

Images