Abstract:
An optical node device includes a light receiving/emitting portion having an input port into which a signal beam is incident and an output port that emits a signal beam of a selected wavelength, a chromatic dispersion device that scatters spatially the signal beam depending on the wavelength of the signal beam, an optical coupler that focuses, onto a two-dimensional plane, beams dispersed by the chromatic dispersion device, a spatial light modulating element arranged so as to receive incident light deployed on an xy plane made up of an x-axis direction deployed according to wavelength and a y-axis direction orthogonal to the x-axis direction, and having numerous pixels arranged in a lattice on the xy plane, and a spatial light modulating element driving portion that drives electrodes of the individual pixels arranged in the xy axial directions in the spatial light modulating element.

Description:
FIELD OF TECHNOLOGY 
     The present invention relates to an optical node device that includes, for example, a wavelength selective optical switching device, a wavelength blocker device, and the like, used in a liquid crystal spatial light modulating element used in the field of optical communications. 
     BACKGROUND ART 
     Wavelength multiplexed optical indication technology is currently used in high-speed high-capacity optical networks to support high-level data communication firms. Advances are being made in deployment of ROADM (Reconfigurable Optical Add Drop Multiplexer) devices that have reconfigurable add and drop functions. The ROADM device is structured from, primarily, a wavelength selective switch (sometimes termed a “WSS”), a wavelength blocker (sometimes termed a “WB”), and the like, to be an optical node device that includes optical filtering functions, optical attenuation functions, and optical switching functions. In order to achieve the ROADM device, the wavelength selective switch must switch light of an arbitrary wavelength in an arbitrary direction. In the wavelength selective switch, an optical beam deflecting element that selects a wavelength and deflects the optical beam to a desired output port is used. In the wavelength blocker device, an optical beam deflecting element is used in order to select a wavelength and either output or not output the optical beam to a desired output port. Patent Citations 1 and 2 propose the use of mechanical dislocation of a MEMS (Micro-Electro-Mechanical System) mirror array as the optical beam deflecting element. Spatial light modulators (sometimes termed “SLMs”) are also known as optical beam deflecting elements. Of these, there are liquid crystal spatial light modulating elements known as LCOS (Liquid Crystal On Silicon) elements that use CMOS technology. Patent Citations 3, 4, and 5 propose the use of LCOS (Liquid Crystal On Silicon) elements. 
     A control method wherein a phase diffraction grating (Optical Phased Array, sometimes termed “OPA”) is formed through a phase modulating function and the diffraction phenomenon is used, and a method wherein the amplitude is controlled through rotation of polarization are well-known As methods for controlling liquid crystal spatial light modulating elements. 
     PATENT CITATIONS 
     
         
         [Patent Citation 1] U.S. Pat. No. 7,725,027 
         [Patent Citation 2] U.S. Pat. No. 7,787,720 
         [Patent Citation 3] U.S. Pat. No. 7,014,326 
         [Patent Citation 4] U.S. Pat. No. 7,822,303 
         [Patent Citation 5] Japanese Unexamined Patent Application Publication No. 2012-108346 
       
    
     In an optical node device that uses a liquid crystal spatial light modulating element, such as an LCOS element, the liquid crystal spatial light modulating element is controlled through diffraction control through phase modulation or through polarization control through amplitude modulation. However, the LCOS element uses driving elements conventionally developed for displaying video signals, where the switching speeds of the individual pixels are driven at the 120 Hz alternating current frequency (frame rate) of a video signal. Because of this, in an optical load device that uses a liquid crystal spatial light modulating element, a flashing phenomenon known as flickering, which varies depending on the period, occurs due to the output signal being powered by a video signal. Moreover, the magnitude of the power variation increases or decreases depending on the amount of optical attenuation. 
     Furthermore, the degree of flickering in the liquid crystal spatial light modulating element depends on the temperature of the use environment. When compared to the room temperature, the temperature of use of optical communication equipment is high, and flickering increases at, for example, 65° C. Given this, because the LCOS element is used in an optical communication application, temperatures are maintained at about room temperature using thermal electric cooling elements (TEC) such as Peltier elements. However, when thermal electric cooling elements are used, this unavoidably increases the size of the equipment and power consumption for the cooling. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides an optical node device that uses a spatial light modulating element that is able to reduce flicker and achieves a light node device that is small and consumes little power, because it does not use thermal electric cooling elements. 
     In one aspect, an optical node device comprises a light receiving/emitting portion having an input port into which a signal beam is incident and an output port that emits a signal beam of a selected wavelength; a chromatic dispersion device that scatters spatially the signal beam depending on the wavelength of the signal beam; an optical coupler that focuses, onto a two-dimensional plane, beams dispersed by the chromatic dispersion device; a spatial light modulating element arranged so as to receive incident light deployed on an xy plane made up of an x-axis direction deployed according to wavelength and a y-axis direction orthogonal to the x-axis direction, and having numerous pixels arranged in a lattice on the xy plane; and a spatial light modulating element driving portion that drives electrodes of the individual pixels arranged in the xy axial directions in the spatial light modulating element so as to either reflect or transmit each of the beams having an individual wavelength in a respective direction. 
     In one or more embodiments the spatial light modulating element may comprise a plurality of pixels provided at intersecting portions at which a plurality of sets of data lines and a plurality of gate lines respectively intersect, wherein each set of data lines has two data lines; a plurality of switches provided respectively relative to the plurality of sets of data lines, wherein the switches supply a positive polarity signal to one of the two data lines in a set and a negative polarity signal to the other data line, sequentially, by set units, to the plurality of sets of data lines; a horizontal direction driving circuit that drives the plurality of switches, by set units, during a horizontal scanning period; and a vertical direction driving circuit that performs vertical direction driving that selects a plurality of the gate lines with each horizontal scanning period. The plurality of pixels may comprise a liquid crystal element in which a liquid crystal layer is interposed between a mutually facing pixel electrode and common electrode; a first sampling circuit that samples a positive polarity pixel signal and holds it for a specific time period; a second sampling circuit that samples a negative polarity pixel signal and holds it for the specific time period; and a switching circuit that switches alternatingly a positive polarity signal voltage and a negative polarity signal voltage, stored respectively by the first and second sampling circuits, to the pixel electrode by switching, with a specific period that is shorter than a vertical scanning period. The plurality of pixels may further comprise a first buffer amp for performing impedance conversion of a positive polarity signal voltage that is stored by the first sampling circuit; and a second buffer amp for performing impedance conversion of a negative polarity signal voltage that is stored by the second sampling circuit; wherein: the switching circuit may switch alternatingly, with the specific period, the positive polarity signal voltage that is outputted from the first buffer amp and the negative polarity signal voltage that is outputted from the second buffer amp. 
     In one or more embodiments, the first and second buffer amps each comprises an impedance conversion transistor; and a constant current load transistor that can control channel current characteristics with a bias voltage applied to a gate. The constant current load transistor becomes discontinuously active in synchronization with a timing of switching at the specific period of the switch. 
     In another aspect, the optical node device according to one or more embodiments may comprise a light receiving/emitting portion having an input port into which a signal beam is incident and an output port that emits a signal beam of a selected wavelength; a chromatic dispersion device that scatters spatially the signal beam depending on the wavelength of the signal beam; an optical coupler that focuses, onto a two-dimensional plane, beams dispersed by the chromatic dispersion device; a spatial light modulating element arranged so as to receive incident light deployed on an xy plane made up of an x-axis direction deployed according to wavelength and a y-axis direction orthogonal to the x-axis direction, and having numerous pixels arranged in a lattice on the xy plane; and a spatial light modulating element driving portion that drives electrodes of the individual pixels arranged in the xy axial directions in the spatial light modulating element so as to either reflect or transmit each of the beams having an individual wavelength in a respective direction, wherein the spatial light modulating element driving portion divides the surface of use of the spatial light modulating element into at least two parts, wherein a surface area to be used is no more than one half of the total surface area. 
     Here the spatial light modulating element may be an LCOS element having pixels arranged two-dimensionally. 
     One or more embodiments of the present invention enable a reduction in the flicker of the spatial light modulating element regardless of the amount of optical attenuation. Consequently, this enables a stabilization of the level of the beam that is emitted from the spatial light modulating element. Moreover, it does not require cooling elements, thereby making it possible to achieve an optical node device that is small and that consumes little electric power. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating the structure of a wavelength selective optical switching device according to a first embodiment according to the present invention. 
         FIG. 2  is a block diagram illustrating a LCOS element that is the spatial light modulator used in the embodiment. 
         FIG. 3  is a circuit diagram of one pixel of an LCOS element according to the embodiment. 
         FIG. 4  is a circuit diagram illustrating the detailed structure for one pixel of the LCOS element according to the embodiment. 
         FIGS. 5A-C  show a waveform diagram illustrating the applied voltage and the pixel voltage when driving a conventional LCOS element and the LCOS element according to the present embodiment. 
         FIG. 6A  is a circuit diagram illustrating the structure of an LCOS element of a wavelength selective switching device according to a second embodiment according to the present invention. 
         FIG. 6B  is a diagram illustrating one example of an LCOS element display screen according to this embodiment. 
         FIG. 7  is a detailed block diagram of the LCOS element according to the present embodiment. 
         FIG. 8  is a timing chart showing the waveform when driving the LCOS element according to the present embodiment. 
         FIGS. 9A-D  show a waveform diagram illustrating the applied voltage and the pixel voltage when driving a conventional LCOS element and the LCOS element according to the present embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
       FIG. 1  illustrates a structural diagram of a wavelength selective switching device as a typical example of an optical node device. LCOS voltage driving can be performed similarly in the case of a wavelength blocking device as well. The wavelength optical selective switching device  1  is structured from an electric circuit board  1 A and an optical module  1 B. The optical module  1 B of the wavelength selective optical switching device  1  has one input port  11  and a plurality of output ports  12 - 1  through  12 -N. Here the optical signal that is inputted into the input port  11  is a wavelength-division multiplexed optical signal (WDM signal) beam wherein many wavelengths are multiplexed. This WDM signal beam is inputted from an input port  11 , which is an optical fiber, or the like, into a beam converting device  13  that is structured from a collimation lens. A polarization beam splitter  14  is provided on the output side of the beam converting device  13 . The polarization beam splitter  14  has a polarized light beam splitter for separating the WDM beam of the input port into light beams of an s polarization component and a p polarization component, and a wave plate for converting the direction of polarization of one of the light beams into the direction of polarization of the other, to produce two parallel WDM beams. The two light beams are applied to a chromatic dispersion device  15 . The chromatic dispersion device disperses in different directions depending on the wavelength. This can be achieved through, for example, a diffraction grating. The output from the chromatic dispersion device  15  is guided to the spatial light modulating element  17  through an optical coupler  16  such as a lens. The spatial light modulating element  17  is an element that receives the light that has been dispersed at the various wavelengths and changes the direction thereof and reflects it so as to be inputted into an optical coupler  16  for each individual wavelength. A controller  18  of the electric circuit board  1 A is connected to the spatial light modulating element  17 . Given this, the reflected beams from the spatial light modulating element  17  are combined through application to the chromatic dispersion device  15  through the optical coupler  16 . The output from the chromatic dispersion device  15  is returned to the polarization beam splitter  14 , and the direction of polarization of one of the combined reflected beams is rotated by the polarization beam splitter  14 , to be combined. The output of the polarization beam splitter  14  is outputted from the output ports  12 - 1  through  12 -N through the beam converting device  13 . Note that while the input port  11  and the output ports  12 - 1  through  12 -N were used, the device may also be used with the input and output reversed. The input port and output ports are structured from portions that both can receive and emit light. 
     In the present form of embodiment, the spatial light modulating element  17  is achieved through an LCOS element  20 . In the first embodiment, the beam that is applied to the LCOS element  20  is a beam wherein the WDM beam has been deployed into an XY plane depending on the wavelength band. Here the LCOS element  20  is an element wherein 1920 elements are arranged in the direction of chromatic dispersion (the x direction) and 1080 pixels are arranged in the direction perpendicular thereto (the y direction), arranged in a matrix. In this wavelength selective switching device, the direction to which each wavelength is reflected is controlled to enable selection of the light of an arbitrary wavelength. The controller  18  determines the direction of reflection of the beams in the xy plane in accordance with the selected wavelength. The controller  18  has a spatial light modulating element driving portion that is structured so as to control the characteristics of the pixel in a specific location in the x-axial and y-axial directions so as to drive the electrodes of the individual pixels that are disposed in the xy directions of the LCOS element  20  within the spatial light modulating element  17 . 
     In the present embodiment, the flicker is reduced through switching each pixel within the LCOS element  20  at a frequency that is higher than the video frequency, within the period of the video signal. A liquid crystal spatial light modulating element of this type that can be used in a liquid crystal projector is disclosed in Japanese Examined Patent Application Publication 2009-223289. 
       FIG. 2  shows an example of an LCOS element  20  and the driving circuit thereof in the present embodiment. The LCOS element is structured from a pixel portion  31 , a horizontal direction driving circuit  32 , a vertical direction driving circuit  33 , and the like. The pixel portion  31  is structured in an array of pixels  31 - 1 , 1  through  31 - 1920 , 1080 , which is 1920 pixels in the x-axial direction and 1080 pixels in the y-axial direction. In the LCOS element  20 , two systems each of the horizontal signal lines  34   a  and  34   b , data lines  35 - 1   a ,  35 - 1   b , . . . , and switches  36 - 1   a ,  36 - 1   b , . . . , are provided. The horizontal signal line  34   a  supplies, to the switches  36 - 1   a ,  36 - 2   a  . . . , the serial pixel signals for the positive side in relation to the common electrode voltage, where the horizontal signal line  34   b  provides, to the switches  36 - 1   b ,  36 - 2   b , . . . , the serial pixel signals for the negative side. The gate lines  37 - 1 ,  37 - 2 , . . . , from the vertical direction driving circuit  33  are connected to the pixel portion  31 . Moreover, the common electrode lines  38  connect between the individual pixels and the controller  18 . Note that the suffix numbers after the hyphens in the codes in the figure indicate different positions for identical types of structural elements. Moreover, a lowercase alphabetic letter “a” following the suffix number indicates the first of the two systems, and “b” indicates the second system. Note that  FIG. 2  illustrates a portion of all of the structural elements. This can also be applied to resolution standards other than the HD standard, such as 640×480 pixels (the VGA standard), 1400×1050 pixels (the SXGA+ standard), and the like. 
     Moreover, the controller  18  in  FIG. 1  provides, to the horizontal direction driving circuit  32  and the vertical direction driving circuit  33 , various types of clock signals that are generated in order to synchronize the input pixel signals of the two systems that are applied to the horizontal signal lines  34   a  and  34   b . Given this, pixel selection is performed according to the respective horizontal and vertical scans by driving the data lines  35 - 1   a ,  35 - 1   b , . . . and the gate lines  37 - 1 ,  37 - 2 , . . . , synchronized with the input pixel signals. 
     There is the internal circuitry within a single pixel  31 - 1 , 1  in  FIG. 3 . In  FIG. 3 , the data line  35 - 1   a  is connected to the drain of a pixel select transistor Q 1 , and the data line  35 - 1   b  is connected to the drain of a pixel select transistor Q 2 . The gates of these transistors Q 1  and Q 2  are connected in common to a gate line  37 - 1 , and, as illustrated, are respectively connected to storage capacitances C 1  and C 2  in order to store the voltages between the sources and the common electrode line  38 . The transistor Q 1  and the storage capacitance C 1  structure a first sampling circuit, and the transistor Q 2  and the storage capacitance C 2  structure a second sampling circuit. Buffer amps A 1  and A 2  are connected to the terminal portions of the storage capacitances C 1  and C 2 , and are connected/disconnected alternatingly, through switches S 1  and S 2  to the pixel electrode E of the pixel. The capacitor C is a storage capacitor. The switches S 1  and S 2  structure a switching circuit that applies a positive signal voltage alternatingly to the pixel electrode. The display portion of a single pixel is structured with a liquid crystal layer interposed between the pixel electrode E and a common electrode, not shown. 
       FIG. 4  is a circuit diagram illustrating in greater detail the internal circuitry of a single pixel  31 - 1 , 1 . As illustrated in  FIG. 4 , a single pixel of the LCOS element in the present embodiment is structured from pixel select transistors Q 1  and Q  2  for writing positive and negative pixel signals, to storage capacitances C 1  and C 2  that independently store, in parallel, pixel signal voltages of respective polarities, transistors Q 3  through Q 8 , a reflective electrode E, and so forth. The buffer amp A 1  in  FIG. 2  is achieved through a constant current source load transistor Q 7  wherein the value of the electric current can be controlled by the voltage that is applied to the gate, and a transistor Q 3  for impedance conversion. Similarly, the buffer amp A 2  in  FIG. 2  is also structured through a constant current load transistor Q 8  and an impedance conversion transistor Q 4 . The transistor Q 5 , which has the drain thereof attached to the source of the transistor Q 3 , and the transistor Q 6 , which has the drain thereof connected to the source of the transistor Q 4 , are switching transistors corresponding, respectively, to the switches S 1  and S 2  in  FIG. 3 . The constant current source load transistors Q 7  and Q 8  have the gates thereof connected in common to the row interconnection line B for the same pixel, structured so as to enable bias control of the constant current load. 
     The operation of the internal circuitry will be explained next. The horizontal direction driving circuit  32  supplies the positive-side pixel signal for the common electrode voltage of the liquid crystal through the data line  35 - 1   a , through the horizontal signal line  34   a , by the switch  36 - 1   a . Moreover, at the same time, it supplies the negative-side pixel signal for the common electrode voltage through the data line  35 - 1   b , through the signal line  34 - b . The transistors Q 1  and Q 2  turn ON simultaneously through the voltage that is applied to the gates thereof through the gate line  37 - 1 . As a result, the positive-side pixel signal that is supplied from the data line  35 - 1   a  is written to the storage capacitance C 1  through the transistor Q 1 , and the negative-side pixel signal that is supplied from the data line  35 - 1   b  is written to the storage capacitance C 2  through the transistor Q 2 . Following this, the transistors Q 1  and Q 2  are turned OFF simultaneously through the voltage applied to the gates thereof through the gate line  37 - 1 . This causes the pixel signals for the positive side to be stored in the storage capacitance C 1 , and for the negative side to be stored in the storage capacitance C 2 . 
     Following this, the positive-side and negative-side pixel in signals that are stored, respectively, in the storage capacitances C 1  and C 2  are read in through respective buffer amps A 1  and A 2  that are high-input-resistance impedance converting circuits, and are alternatingly selected by switches S 1  and S 2  to alternatingly drive the individual pixels by changing the voltages of the reflector electrodes. Doing so enables the performance of the alternating current driving of the LCOS element at a high-speed in the present embodiment. That is, the LCOS element of the present embodiment makes it possible to prevent flickering through alternating current driving of the LCOS element at a high frequency that is, for example, several dozen times the frame frequency, independent of the writing period of the pixel signal. 
       FIG. 5  is a diagram illustrating the timing with which the voltages are applied to the individual pixels in the LCOS element, showing a comparison of a conventional example of the embodiment in the present application. In the case of driving the LCOS element using the driving circuit for the image display, normally voltages are applied to the individual pixels at 120 Hz alternating currents in order to display the video signal in  FIG. 5  ( a ). However, because the time period over which the voltage is actually applied to the signal is short, the voltage level that is applied to the pixel gradually decreases with the passage of time, as shown in  FIG. 5  ( b ). The result is a flickering phenomenon, described above. 
     In contrast, in the present embodiment the provision of the polarity inverting function in the pixel circuit itself and switching it at a high-speed enables alternating current driving at a high-frequency, not constrained by the vertical scan frequency. The switching frequency in the present embodiment is, for example, between 1.2 kHz and 3.6 kHz, supplying the pixel voltage at a speed that is sufficiently higher than the video frequency, as shown in  FIG. 5  ( c ). Consequently, there is essentially no drop in the pixel voltage, making it possible to prevent the flickering phenomenon. 
     In the case of driving the conventional LCOS element wherein the pixel signal is supplied using a driving circuit for a screen display, and driving with an alternating current frequency of 120 Hz at an ambient temperature of 25° C., the power stability is 0.06 dB when the amount of optical attenuation is 0 dB, and 0.14 dB when the amount of optical attenuation is set to 10 dB. 
     In contrast, in the case of the wavelength selective switching device that uses the LCOS element according to the present embodiment, the power variation at an ambient temperature of 25° C. when the amount of optical attenuation is set to 0 dB is 0.03 dB, and when the amount of optical attenuation is set to 10 dB, there is extremely small variation of 0.04 dB. Next, at an ambient temperature of 65°, the variation in the output power is 0.03 dB when the amount of optical attenuation is set to 10 dB, which is within the acceptable range for optical communication applications. As a result, it is possible to produce a wavelength selective optical switching device with the feature of having low flickering regardless of the amount of attenuation of light. Furthermore, this makes it possible to eliminate the need for temperature control through thermoelectric elements. 
     Second Embodiment 
     A wavelength selective switching device according to a second embodiment according to the present invention will be explained next. In this embodiment as well, a wavelength selectable switching device is used as one example of an optical node device, and the overall structure is identical to that in  FIG. 1 . In the present embodiment, only the spatial light modulating element  17  and the driving device thereof are different. In the present embodiment as well, an LCOS element  50  is used for the spatial light modulating element  17 .  FIG. 6A  is a diagram illustrating the LCOS element, which has a horizontal direction driving circuit  51  that operates in accordance with signals from a controller  60 , a vertical direction driving circuit  52 , and a group of pixels  53 . This LCOS element  50  is also a high definition panel (following the HD standard), made from 1920 pixels in the x direction and 1080 pixels in the y direction, where the size of a single pixel is 8.0 μm, vertically and horizontally. 
       FIG. 7  is a block diagram illustrating the detailed structure of the LCOS element  50 , where a portion of the pixels are omitted. In  FIG. 7 , the horizontal direction driving circuit  51  has a pixels H shift register  54  and a source driver  55 , and the vertical direction driving circuit  52  has a V shift register  56  and a gate driver  57 . Given this, a horizontal clock signal H_Clock and a horizontal start signal H_Start are inputted from the controller  60  into the H shift register  54 . Moreover, the voltages that are to be applied to the individual pixels are inputted as a serial signal from the source driver  55 . Furthermore, a V_Clock signal and a V_Start signal are inputted into the V shift register  56 . The portion shown with diagonal lines in the pixel  53  is a pixel wherein the light beam is illuminated. 
       FIG. 6B  is an explanatory diagram for the method of limiting the use area, using only a portion of the panel, where the portion with the diagonal lines is a region wherein the optical beam is illuminated. The region of illumination by the optical beam is from Yi=400 to Yj=536, so the width thereof is 136 pixels. Consequently, only the pixels of the area that is used need be driven, and it is not necessary to drive the other pixels. Control so as to apply the pixel voltages for only a portion of the pixels in this way is known as partial area of use. In  FIG. 6A , the controller  60 , during partial area use, sends the V_Clock up until Yi, and then drives at the normal speed between Yi and Yj, and then, starting at Yj, outputs a clock signal so as to fast-forward from Yj to the position Yn that is at the end of the y axis. 
       FIG. 8  shows the input signal into the LCOS element  50  for the partial area use method according to the present embodiment. In the case of the partial area use, the V_Start signal that is inputted into the V shift register  56  is sent faster than the normal V_Clock until the use start line Yi. Given this, after arriving at the use location, the driving is performed normally using the H_Start and H_Clock. When the use end line Yj that is the end of the use area is reached, then again there is fast-forwarding to Yn using the V_Clock. Doing so makes it possible to improve the frame rate using a single screen. In the case in the present embodiment, the actual driving frequency for the individual pixels in the use area, with the frequency for the alternating current drive if the entire area were used being 120 Hz, is equivalent to 120 Hz×(1080 pixels) divided by (136 pixels)=925 Hz. In this way, if the area of the screen used is approximately ⅛, then the frequency applied to the pixels is about 8 times, making it possible to reduce the variation in electropotential. Because of this, this is able to reduce the flickering of the screen. The result is greater the smaller the area that is used. 
     Similarly, in the case of partial use in the X direction, it is possible to advance in the X direction using a faster H_clock. Moreover, after applying the voltage to the Yj line, it is not necessary to send the V lines up until Yn after displaying Yj, given the provision of the reset function in the V shift register, making it possible to achieve an even faster frame rate. Similarly, the provision of the reset function in the H shift register as well makes it possible to achieve a higher frame rate. If resetting is through the V_Start and H_Start, instead of adding dedicated reset lines, the reset method can be achieved with a reduced number of signal lines. 
       FIG. 9  is a diagram illustrating a comparison of the timing with which voltages are applied to the individual pixels in the LCOS element, comparing the conventional example and the case of the embodiment according to the present invention. When driving the LCOS element with a driving circuit for an image display, normally the pixel voltages are applied to the individual pixels at the alternating current of 120 Hz for displaying the video signal in  FIG. 9  ( a ). However, in this case, as illustrated in  FIG. 9  ( b ), the voltage levels applied to the pixels gradually decline with the passage of time, as shown in  FIG. 9  ( b ). 
     In contrast, in the present embodiment in the areas in the LCOS element other than those that are used are fast-forwarded, making it possible to essentially increase the drive frequency as shown in  FIG. 9  ( c ), without being constrained by the normal verticals scan frequency. Because of this, there is essentially no decrease in the pixel voltages, as shown in  FIG. 9  ( b ), making it possible to prevent the flickering phenomenon. 
     In the case of driving the LCOS element as shown above, when the amount of optical attenuation is set to 0 dB, in an ambient temperature of 25° C., the variation in the output power is less than 0.001 dB, and when the amount of optical attenuation is set to 10 dB, the variation in output power is an extremely small variation, at 0.014 dB. Next, at an ambient temperature of 65° C., when the amount of optical attenuation is set to 10 dB, the variation in the output power is 0.05 dB, which is in the acceptable range for an optical communication application. Because of this, it is possible to eliminate the need for temperature control using thermoelectric cooling elements. 
     Note that while in this embodiment a portion of the LCOS element screen was defined as the use area, and as long as the use area is no more than ½, the driving frequency can be increased by the inverse thereof, that is, can be increased to more than twice the original driving frequency. Consequently, although it is possible to set arbitrarily the numbers and sizes of areas used, in order to obtain the effect of reducing flickering it may be necessary to set the use area to no more than ½ or ⅓. 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the present disclosure. Accordingly, the scope of the present disclosure should be limited only by the attached claims. While in the embodiments the explanations were for frequency selective switching devices, the present invention can be applied to a variety of optical node devices that use spatial light modulating elements, for example, the present invention can be applied to a variety of devices such as wavelength blockers and wavelength equalizers. 
     One or more embodiments of the present invention can reduce flickering through supplying voltages to the individual pixels at a speed that is independent of and faster than the video frequency for a spatial light modulating element such as an LCOS element. Consequently, it is well suited for use in optical node devices having, for example, wavelength selective switching devices, wavelength blocker devices, and the like, that have liquid crystal spatial elements. 
     EXPLANATION OF CODES 
     
         
         
           
               1 : Wavelength-Selective Optical Switching Device 
               1 A: Electric Circuit Board 
               1 B: Optical Module 
               11 : Input Port 
               12 - 1  through  12 -N: Output Ports 
               13 : Beam Converting Device 
               14 : Polarization Beam Splitter 
               15 : Chromatic Dispersion Device 
               16 : Optical Coupler 
               17 : Spatial Light Modulating Element 
               18 ,  60 : Controllers 
               20 ,  50 : LCOS Elements 
               31 : Pixel Group 
               32 : Horizontal Direction Driving Circuit 
               33 : Vertical Direction Driving Circuit 
               34   a ,  34   b : Horizontal Signal Lines 
               35 - 1   a ,  35 - 1   b , . . . : Data Lines 
               36 - 1   a ,  36 - 1   b , . . . : Switches 
               37 - 1  through  37 -N: Gate Lines 
             A 1 , A 2 : Buffer Amps 
             S 1 , S 2 : Switches