Abstract:
A variable modulator assembly includes a deformable layer adhered to a compliant layer surface, and an n-phase electrode configuration, n&gt;2, adhered to an opposite surface of the compliant layer. A controller is configured to selectively apply a variable signal to the selected electrodes of the electrode configuration. Application of the variable signal causes the deformable layer to reconfigure to an alternated shape having distinct peaks and valleys. The distance between the peaks and valleys being determined by the value of the applied variable signal, wherein the alternated shape travels in a preferred direction. An optical modulating method includes positioning the variable modulator assembly to receive and reflect light from a light source, monitoring the reflected light, and altering the variable signal to maintain a desired output intensity.

Description:
CROSS REFERENCE  
       [0001]     The following application is related to the present disclosure: U.S. Ser. No. 10/423752, Uma Srinivasan, Eric J. Shrader, entitled CONFIGURABLE GRATING BASED ON SURFACE RELIEF PATTERN FOR USE AS A VARIABLE OPTICAL ATTENUATOR, fully incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     The present exemplary embodiment relates to optical grating assemblies. It finds particular application in conjunction with an assembly and method for an elastomer-based grating assembly based on a travelling surface relief pattern for use as a variable optical attenuator, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.  
         [0003]     Fiber networks normally employ point-to-point links, which are static, where most of the intelligence, provisioning and grooming functions are provided by electronics at the ends of each link. As network architectures grow in size and complexity, however, this approach to building and maintaining network infrastructure will not satisfy the requirements of reliability, efficiency and cost-effectiveness required by service providers. Therefore, the industry is moving to optically reconfigurable networks where optical paths, wavelengths and data rates are dynamically changed to satisfy network system requirements, such as provisioning new wavelengths, balancing data loads and restoring after-service malfunctions.  
         [0004]     Variable optical attenuators (VOA) are used to permit dynamic control of optical power levels throughout a communications, telecommunications or other transmission network. As traffic in networks increase, VOAs may be used to dynamically lower the optical power levels depending on the length of the network route so that appropriate power levels are received at the end receivers. As an example of their usefulness, if a network is providing a wavelength route that is approximately 60 km in length, at a predetermined power, and the network attempts to change the wavelength route to one which is 30 km, it would be expected that excessive power would be delivered to the end receivers of the 30 km route, potentially resulting in a malfunction in the network. A VOA will lower the power output of the switched wavelength to permit a signal of acceptable strength at the end receiver. The VOAs may be maintained at a particular power level for long durations, e.g., several months or even several years. Drift of the output power of the VOA can become an issue under such circumstances.  
         [0005]     Existing VOAs commonly implement mechanical systems to attenuate the light. In one design, attenuation is accomplished by moving two separate optical fibers, and in another by inserting a motor-driven blade or filter in the light path. While these devices have acceptable optical performance, tradeoffs include slow speed, undesirable noise and a potential for mechanical failure.  
         [0006]     Work on a traveling wave concept has primarily been focused in the area of surface acoustic waves, piezoelectrics, and electro-optic modulators where refractive index is changed by the application of electric fields.  
         [0007]     In one particular example, U.S. Pat. No. 6,556,727 to Minakata et al. in its Abstract describes the concept of a traveling wave-type optical modulator having a substrate made of an electro-optic material, optical waveguides fabricated on the top surface of the substrate, and electrodes for modulating an optical wave through the optical waveguide. The substrate is partially thinned from the bottom surface of the substrate to form a first thin portion and a second thin portion so the thickness of the first thin portion is set to be larger than the thickness of the second thin portion, and the optical waveguide is positioned in the first thin portion. Among other concepts, this application does not discuss the use of a traveling wave used to create phase gratings that have corrugations with nearly constant pitch and depth.  
         [0008]     The foregoing material does not address the noted shortcomings of existing systems.  
       BRIEF DESCRIPTION  
       [0009]     In accordance with one aspect of the present exemplary embodiment, there is provided a variable modulator assembly comprising a compliant layer having a first and a second surface. A deformable layer is adhered to the first surface of the compliant layer, and an electrode configuration consisting of a plurality of electrodes is in contact with the second surface of the compliant layer. The electrode configuration is configured for n-phase operation, with n&gt;2. A controller is configured to selectively apply a variable signal to selected electrodes of the electrode configuration, wherein application of the variable signal causes the deformable layer to reconfigure to an altered shape having distinct peaks and valleys, the distance between the peaks and valleys being determined by the value of the applied variable signal, and wherein the altered shape moves in a preferred direction in the plane of the deformable layer.  
         [0010]     In accordance with another aspect of the present exemplary embodiment, there is provided an optical modulating method. The method comprises positioning a variable modulator assembly to receive light from a light source, wherein a deformable layer is adhered to a first surface of a compliant layer of the light source, and deformation of the deformable layer is controlled by selective activation of an electrode configuration adhered to a second surface of the compliant layer. The activation of the electrode configuration is controlled by a controller which 9  generates a variable signal and transmits the variable signal to selected electrodes of the electrode configuration. Activation of the electrodes creates electrostatic charges which deform the deformable layer into a travelling pattern moving in a direction parallel to the plane of the deformable layer corresponding to the activated electrodes.  
         [0011]     In accordance with yet another aspect of the present exemplary embodiment, there is provided a VOA device fabrication method. The method comprises depositing a bus metal layer on a substrate, patterning the bus metal layer, depositing a dielectric layer on the bus metal layer and the substrate, forming a via in the dielectric layer to the bus metal layer, depositing a bottom electrode metal layer in contact with the bus metal layer, the bottom electrode layer being configured for n-phase operation, wherein n&gt;2, spin coating a compliant layer onto the bottom electrode layer and the dielectric layer, and depositing a blanket top electrode/reflector metal layer onto the compliant layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1A  is cross-sectional view of a variable optical modulator system according to concepts of the present application;  
         [0013]      FIG. 1B  is top schematic view of an electrode configuration of the system of  FIG. 1A ;  
         [0014]      FIG. 2A  is cross-sectional view of an exemplary variable optical modulator system prior to application of a signal voltage;  
         [0015]      FIG. 2B  is cross-sectional view of the variable optical modulator system of  FIG. 2A  showing corrugations after application of a signal voltage;  
         [0016]      FIG. 3  is a plot of a theoretical diffraction intensity of an exemplary reflective sinusoidal phase grating for increasing deformation;  
         [0017]      FIG. 4A  is a cross-sectional representation of an exemplary 3-phase traveling wave over one voltage cycle on a 3-phase VOA;  
         [0018]      FIG. 4B  is a 3-dimensional representation of the 3-phase traveling wave of  FIG. 4A ;  
         [0019]      FIG. 5A  is a plot showing a 12-phase voltage across an electrode configuration according to embodiments of the present application at a particular point in time;  
         [0020]      FIG. 5B  is a plot showing the progression of the 12-phase voltage of  FIG. 5A  at a later point in time;  
         [0021]      FIG. 6A  is a plot showing a 3-phase voltage across an electrode configuration according to embodiments of the present application at a particular point in time;  
         [0022]      FIG. 6B  is a plot showing the progression of the 3-phase voltage of  FIG. 6A  at a later point in time;  
         [0023]      FIG. 7  is a plot showing the maximum peak-to-peak voltage experienced by individual electrodes in exemplary VOA devices according to embodiments of the present application as a function of the phase difference between the electrodes;  
         [0024]      FIG. 8  is a plot showing the rms voltage jitter experienced by individual electrodes in exemplary VOA devices as a function of the phase difference between the electrodes;  
         [0025]      FIG. 9  is a plot showing the peak-to-peak voltage jitter experienced by individual electrodes in exemplary VOA devices as a function of the phase difference between the electrodes;  
         [0026]      FIG. 10  is a plot showing an exemplary maximum jitter in the optical power as a function of the phase difference between the electrodes configured according to concepts of the present application;  
         [0027]      FIG. 11  is an exemplary device fabrication procedure for embodiments of the present application; and  
         [0028]      FIG. 12  shows exemplary device electrode configurations for embodiments of the present application. 
     
    
     DETAILED DESCRIPTION  
       [0029]     With reference to  FIG. 1A , an elastomer-based VOA device  10 , which may be a variable optical attenuator according to concepts of the present application, is illustrated in simplified schematic form. The VOA device  10  is constructed with a bottom electrode configuration  12 , comprising electrodes  12   a - 12   n , on a glass substrate  14 , a compliant layer  16  and a deformable top electrode  18  as shown in the figure. In one embodiment, the bottom electrode configuration  12  is patterned using known photolithography techniques in order to achieve a desired surface relief pattern on the upper surface  20  of the top electrode  18 , the surface relief pattern corresponding to the system&#39;s grating structure. In this exemplary embodiment, the electrode configuration is an interdigitated pattern as illustrated in  FIG. 1B . The compliant layer  16  may be a dielectric, elastomer or electrostrictive material, such as Poly-di-methyl-siloxane (PDMS) formed by known spin-coating or other manufacturing techniques. Piezo-electric materials like poly vinylidene fluoride may also be considered, provided that the frequency of operation is carefully chosen. Top electrode  18  may be an individual or pixel electrode, the electrode may be constructed using at least one of a TFT or CMOS design.  
         [0030]     The upper surface  20  of the deformable top electrode  18  is made to be a reflective surface, which may be a polished surface of the deformable compliant layer  16 , or may be a separate blanket or patterned layer made of reflective material including but not limited to a variety of metals. In one embodiment, the deformable top electrode  18  is directly attached onto an upper surface of the compliant layer  16 . In other embodiments an interposed protective layer (not shown) is provided. The deformable top electrode  18  is conductive as well as reflective. In the configuration shown, the top deformable electrode  18  is made to be conductive and reflective by depositing a thin film of silver.  
         [0031]     The VOA device  10  shown in  FIGS. 1A-1B  comprises a configurable reflection phase grating that generates a nearly sinusoidal surface relief pattern by the application of voltage by the controller  22  to the bottom electrode configuration  12 . The reflective upper surface  20 , of the deformable layer  18 , is designed to be reflective and act as a mirror as shown in  FIG. 2A  when no voltage is applied to the electrode configuration. In this embodiment, the deformable layer  18  is grounded, and the electrode configuration  12  is supplied with a bias and/or variable voltage from a voltage signal generator/controller  22  in order to produce the desired grating structure of the system. As the applied voltage is increased, the compliant layer  16  experiences an electrostrictive force. The compressive stress p experienced by the compliant layer is given by 
 
 p=ε   0 ε r   E   2  
 
 where, E is the electric field generated by the applied voltage, and ε 0 , ε r  are the permittivity of free space and the relative dielectric constant, respectively. The electric field corrugates the upper surface  20  according to the bottom electrode geometry as shown in  FIG. 2B . 
 
         [0032]     The corrugation of the upper surface  20  causes the VOA device to behave like a diffraction grating and higher orders of diffracted light are observed as described by 
 
 d(sin θ   i +sin θ m )=mλ
 
 where, d is the grating constant, θ i  is the incident angle, θ m  is the diffracted angle, m is the diffraction order and λ is the wavelength of the diffracted light. The intensity of light diffracted into the various orders depends on the corrugation depth. 
 
         [0033]     Interference between light reflected from the top and bottom of the corrugation in the phase grating of the upper layer  20  causes the varying intensity and may be expressed as 
 
 I   n = AJ   n   2 [Δm]
 
 where I n  is the intensity of light in the n th  order beam, A is a proportionality constant that depends on the signal intensity, J n  is a Bessel function of order n, Δm is the phase difference, which depends on the corrugation depth h ( 24 ) and is given by Δm=4πh/λ. 
 
         [0034]     To illustrate further, when no variable voltage is supplied from the controller  22 , the reflective surface  20  acts as a mirror, and displacement of an impinging light beam is at substantially zero displacement. As the voltage to the electrode configuration  12  is increased by the controller  22 , displacement, or diffraction, of the light is increased. When the surface relief pattern is displaced by a quarter wavelength, i.e., h=λ/4, the light reflected from the uppermost part of the reflective surface  20  is 180 degrees out of phase with the light reflected from the lowermost part of the reflective surface, and destructive interference occurs. At this point, the light is totally diffracted, and essentially none is reflected.  
         [0035]     Analog control of the reflected light may be achieved by monitoring at least one of the diffracted order wavelengths such as the 1 st  order diffracted wavelength. For example, initially, the 0 th  order and 1 st  order diffraction intensity as a function of voltage is calibrated and this information used to control the intensity of the 0 th  order by monitoring the 1 st  order diffraction. In one implementation, as shown in  FIG. 2B , a light source  26  emits a light beam  28 . A reflected light portion  30  is transmitted to an element  32 , such as a fiber, receiver, or other mechanism. A diffracted light wave  34 , is sensed by a sensor  36 . The sensor  36  may be substantially transparent to the wavelength of the diffracted light  34  for situations where additional testing or use of the diffracted light wave  34  is to be undertaken. Output from the sensor  36  is provided via a feedback line  38  to the controller  22 . Feedback circuitry included in the controller  22  uses the signal obtained from the 1 st  order diffracted wavelength to control the voltage applied to the electrode configuration  12 .  
         [0036]     This design permits for a non-destructive monitoring and controlling of the 0 th  order. Hence, in this example the deformation of the deformable layer  18 , with the reflective surface  20 , is controlled from the 0 th  order to the 1 st  order by an analog control mechanism. By use of this analog control, the intensity output value for the 0 th  order is closely controllable. For example, when no variable voltage is applied (so the surface is essentially a mirror) the intensity output of a beam of light to the component  32  may be substantially 100 percent of the light beam  28 . If the desired output requirements change wherein, e.g., only 75 percent intensity in the 0 th  order is needed, the amount of voltage supplied to the electrode configuration  12  is undertaken to increase the deformation such that 25 percent of the intensity goes into higher order diffracted wavelengths (e.g.,  34 ). More generally, the present design permits analog control from a first displacement of the deformable layer  18  to a second displacement of the deformable layer.  
         [0037]      FIG. 3  shows the theoretical diffraction intensity of a reflective sinusoidal phase grating for increasing deformation, i.e., increasing corrugation depth. The 0 th  order diffraction intensity I 0  ( 40 ) is maximum when Δm=0 and it is essentially zero when Δm=0.75π. In other words, when the reflective surface  20  relief pattern is displaced such that Δm=0.75π, the light reflected from the surface relief pattern of the corrugation is subject to essentially 100 percent destructive interference for the nominally reflecting beam. At this point, all of the reflected light is totally diffracted and essentially none is reflected. Hence in the range where the corrugation dimension h ( 24 ) is controlled, such that Δm is between 0 and 0.75π, an analog control of the light can be achieved. Also shown in the figure are the 1 st  order ( 42 ), 2 nd  order ( 44 ), and 3 rd  order ( 46 ) diffraction intensities.  
         [0038]     As mentioned above the VOA devices are usually maintained at any particular power level for long durations, which may be, e.g., several months or even several years. Long term drift of the output power of the VOA device can become an issue under such circumstances. For a configurable grating based VOA, generating a constant diffracted intensity means the device should produce a surface relief pattern where the corrugation pitch and depth does not change significantly with time. It has been found that cycling the voltage controls the drift more effectively than when the device works at a fixed DC bias. Therefore, an improvement is realized by providing a traveling wave to gratings that are intended to have corrugations with nearly constant pitch and depth of the peaks and valleys of the surface.  
         [0039]     To create the traveling wave, the electrode configuration  12  is swept with a variable signal (which in this embodiment is an analog signal) by the controller  22 . To accomplish this, the controller  22  may have n-phase (n&gt;2) drive electrodes, rather than the two illustrated in  FIGS. 1B and 2B , and the electrode configuration  12  will have a corresponding number of sets of electrodes as described in more detail below. This creates a phase grating in which there is minimal undesired change in the corrugation depth, and the grating pattern is moving in space, i.e., moving across the reflective surface  20 . For example,  FIG. 4A  shows a 3-phase traveling wave over one voltage cycle on a VOA device  10  having an electrode configuration  12  configured for a 3-phase input.  
         [0040]     Prior to time t= 0  ( 50 ), the VOA device  10  has a smooth reflecting surface  20  as shown. At time t= 0  ( 52 ), an initial 3-phase voltage is applied to the interdigitated electrode configuration  12 , as represented for visualization purposes, e.g., by the patterns shown on each of the electrodes  12   a - 12   c.  This initial voltage causes the reflecting surface  20  to have a corrugation corresponding approximately to a sine wave as shown. At times t= 0 . 33  ( 54 ), t= 0 . 66  ( 56 ), and t= 1  ( 58 ), it may be observed that the 3-phase voltage pattern is moved across the electrode configuration  12  by the controller  22 . Consequently, the corrugation of the reflecting surface  20  moves in phase with the 3-phase voltage across the reflecting surface.  FIG. 4B  shows a 3-dimensional plot of the applied 3-phase voltage ( 60 ) at the bottom electrode configuration  12  for the example shown in  FIG. 4A , as a function of electrode position  62  and time  64 , graphically illustrating the concept of the travelling wave.  
         [0041]     The phase difference between adjacent electrodes of the electrode configuration  12  depends on the specified tolerance of the output power. While a 12-phase voltage provides a grating waveform closer to an actual sinusoidal grating, it has the disadvantage of providing a lower diffraction angle and requires more complicated electronics. A 3-phase voltage, on the other hand, does not approximate an actual sinusoidal grating as well as the 12-phase voltage, but provides a larger diffraction angle and requires less complicated drive electronics than the 12-phase voltage.  FIGS. 5A-5B  show the progression of a 12-phase voltage across the electrode configuration  12  at two different times. At a first time shown in  FIG. 5A , the applied voltage  66  at each of the electrodes  12   a - 12   n  is seen to closely approximate a sine wave voltage. The voltage potential between adjacent electrodes is approximated by a straight line, but because of the number of phases, the voltage curve  66  follows a general approximation of a sine wave. At a later time shown in  FIG. 5B , the applied voltage waveform  66  has advanced across the electrodes  12   a - 12   n,  but still generally approximates a sine wave voltage.  
         [0042]      FIGS. 6A-6B  similarly show the progression of a 3-phase voltage across the electrode configuration  12  at two different times. At a first time shown in  FIG. 6A , the applied voltage  68  at each of the electrodes  12   a - 12 n only loosely approximates a sine wave voltage. However, for many applications, depending on the necessary output tolerance, this approximation is adequate. The voltage potential between adjacent electrodes is again approximated by a straight line, but because of the fewer number of phases, the voltage curve  68  takes on the appearance of a saw-tooth wave at the time shown in the figure. At a later time shown in  FIG. 6B , the applied voltage waveform  68  has advanced across the electrodes  12   a - 12   n,  and the shape has changed noticeably, but still loosely approximates a sine wave voltage.  
         [0043]     The voltage at each electrode at position x for the examples shown in  FIGS. 5A-6B  may be calculated using the equation  
       V   =       V   offset     +         V   pp     2     ⁢     sin   ⁡     (       ω   ⁢           ⁢   t     +     ϕ   n       )               
 
 with V pp  =peak-to-peak voltage (e.g. 225V), V offset  =V pp /2, and Φ n  being the phase difference between adjacent electrodes. It may be observed in  FIGS. 5A-6B  that the traveling wave goes through a maximum peak-to-peak voltage and minimum peak-to-peak voltage over time per cycle of the traveling wave. For example, by comparing  FIG. 6A  to  FIG. 6B , it is observed that the peak-to-peak voltage is approximately 200 volts in  FIG. 6A , whereas at a different time shown in  FIG. 6B , the peak-to-peak voltage is approximately 170 volts. This short term variation in peak-to-peak voltage constitutes a jitter which has been simulated and has been shown to be higher for the 3-phase voltage than for the 12-phase voltage as can be seen in the respective figures. 
 
         [0044]     The maximum peak-to-peak voltage experienced by the individual electrodes in a VOA device, as a function of the phase difference between the electrodes, is shown in  FIG. 7 . The maximum peak-to-peak voltage is shown for exemplary embodiments of 12 phase ( 70 ), 7 phase ( 72 ), 6 phase ( 74 ), 5 phase ( 76 ), 4 phase ( 78 ), and 3 phase ( 80 ) circuits. It may also be observed for these particular embodiments that the even phases yield the highest peak-to-peak voltages, while the odd phases yield a peak-to-peak voltage that declines as the number of phases is reduced. The maximum peak-to-peak voltage determines the maximum depth of the corrugation that will be possible.  
         [0045]     The maximum jitter in the rms voltage as a function of the phase difference between the electrodes is shown in  FIG. 8 . The maximum rms jitter is shown, as in  FIG. 7 , for 12, 7, 6, 5, 4, and 3 phase circuits ( 82 - 92  respectively). Likewise,  FIG. 9  shows the maximum jitter in the peak-to-peak voltage as a function of the phase difference between the electrodes for the same 12, 7, 6, 5, 4, and 3 phase circuits ( 94 - 104  respectively). Jitter in the maximum peak to peak voltages of the bottom electrodes  12  corresponds to jitter in the corrugation depth, which in turn corresponds to the diffraction optical power. The maximum jitter in the optical power as a function of the phase difference between the electrodes is shown in  FIG. 10  ( 106 - 116 ). For the embodiments represented in the figure, the odd phases ( 108 ,  112 ,  116 ) generally have a lower optical power jitter compared to the even phases ( 106 ,  110 ,  114 ).  
         [0046]     Attention is now directed toward an exemplary device structure and process flow for producing embodiments incorporating concepts of the present disclosure. With reference to  FIG. 11 , an exemplary device fabrication procedure includes the following steps: 
        A bus metal layer  120  is deposited on a substrate  122 , and the bus metal layer  120  is then patterned.     A dielectric layer (e.g. oxide) ( 124 ) is deposited on the bus metal layer  120  and the substrate  122 .     Next, a via  126  is made in the dielectric layer  124  to the bus metal layer  120 , and a bottom electrode metal layer  128  is deposited in contact with the bus metal layer  120 .     An elastomer layer  130  is then spin coated onto the bottom electrode layer  128  and the dielectric layer  124 . Finally, a blanket top electrode/reflector metal layer  132  is deposited onto the elastomer layer  130 .        
 
         [0051]     A top view is provided in  FIG. 12  of the device of  FIG. 11  to further illustrate the exemplary device layouts. The figure is abstracted in order to more clearly show the bus metal  120 , the via  126 , and the bottom electrode metal layer  128 . Exemplary layouts are shown for a 2 phase device  140 , a 3 phase device  142 , a 4 phase device  144 , and a 5 phase device  146 . It should be obvious from the examples provided that the aforementioned process and device layouts can be extended to devices of any number of phases. It should also be noted that the process described, and the device layouts provided are for purposes of explaining embodiments of the present disclosure only, and the disclosure is not limited to the layouts and process provided herein.  
         [0052]     An advantage of the device structure provided in  FIGS. 11 and 12  is that the number of mask layers does not depend on the number of phases. In other words, only three mask layers are required to produce 2, 3, 4, 5 . . . n-phase devices. However, concepts of the present disclosure are intended to include other n-phase embodiments which incorporate the concept of a travelling wave in the deformable layer as described herein. For example, the electrode configuration may be a multi-layer configuration.  
         [0053]     It is also to be appreciated that the signal applied to the bottom electrodes can be different from sinusoidal signals depending on the response required. Sinusoidal signals were used in describing various embodiments, however, the present disclosure is not limited by the waveshape of the applied signals. For example, square wave and sawtooth signals, among others, are suitable for use with devices embodying concepts of the present disclosure.  
         [0054]     Still further, the techniques described herein may also be used in other Micro-Electro-Mechanical-Systems (MEMS) grating-based optical modulators. For example, the foregoing concepts may be used in MEMS grating-based optical modulators which do not use a compliant layer as previously described, but rather the layer is one of air, used to separate top and bottom layers/electrodes. This technique may also be useful for elastomer and other MEMS-based surface relief patterns, e.g., Fresnel lens, electronic holograms, etc.  
         [0055]     It is to be appreciated that features of the foregoing embodiments may be combined with features of other embodiments described herein, and although components may be numbered differently, they may include characteristics of similar components found in the various embodiments.  
         [0056]     While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications, variations, improvements, and substantial equivalents.