Abstract:
A system for modulating a beam of light in accordance with an input data stream having a data rate greater than 2 MHz, includes a source of light for directing light along a predetermined path; a self-damped diffractive light modulator disposed in the predetermined path and having a plurality of spaced apart self-damped deformable elements being disposed relative to each other and secured at opposite ends and suspended above and movable into a channel containing a gas, and each spaced apart self-damped deformable element having at least one reflective surface. The spaced apart self-damped deformable elements respond to an input data stream and are sufficiently damped to minimize the introduction of data errors into the modulator light beam.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Reference is made to commonly assigned U.S. patent application Ser. No.  09 / 757 , 341  , filed concurrently herewith, entitled “Optical Data Modulation System With Self-Damped Electromechanical Conformal Grating” by Furlani et al, the disclosure of which is incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the modulation of optical signals, and more particularly, to an optical modulation system which includes a self-damped diffractive light modulator. 
     BACKGROUND OF THE INVENTION 
     High-speed optical data modulation systems are used for various applications including optical data storage and communications. These systems require data throughput in the megahertz frequency range. Substantial progress has been made in the development and implementation of microelectro-mechanical (MEMS) based light modulators that operate efficiently at these frequencies. Specifically, Bloom et al. in U.S. Pat. No. 5,311,360 describe an apparatus and method of fabrication for a device for optical beam modulation, known to one skilled in the art as a grating light valve (GLV). Bloom et al. described a similar device in U. S. Pat. No., 5,459,610, with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate stiction between the ribbon and substrate; 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast; 3) solid supports to fix alternate ribbons; and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces. Bloom et al. in U.S. Pat. No. 5,677,783 also presented a method for fabricating the device. 
     According to the prior art, for operation of the GLV device, an attractive electrostatic force is produced by a single polarity voltage difference between the ground plane and the conducting layer atop the ribbon layer. This attractive force changes the heights of the ribbons relative to the substrate. Modulation of the diffracted optical beam is obtained by appropriate choice of the voltage waveform. The voltage needed to actuate a ribbon a certain distance depends on several factors including the tensile stress in the ribbon material and the ribbon length. 
     It is well known that the ribbon elements of the GLV device possess a resonance frequency which depends primarily on the tensile stress, the density, and length of the ribbons. When a ribbon is actuated or released, it rings at its resonant frequency, which is typically between 1 and 15 MHz. The mechanical response of the ribbon elements is damped by the flow and compression of the layer of gas beneath the ribbons. This phenomenon is referred to as squeeze film damping. It depends on the type of gas present, the pressure, film thickness etc. This damping determines the width of the resonant peak associated with the resonant frequency of the ribbons. As a result of this resonant ringing, the maximum frequency at which the GLV device can be operated is limited, and the diffracted light intensity contains undesirable temporal variations. These temporal variations in a data stream give rise to undesired data errors. Therefore, there is a need for a GLV device having increased operating speed and reduced temporal light intensity variations. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an optical modulation system with a self-damped diffractive light modulator for a beam of light in accordance with an input data stream that is particularly suitable for input data rates greater than 2 MHz. 
     This object is achieved by a system for modulating a beam of light in accordance with an input data stream having a data rate greater than 2 MHz, comprising: 
     (a) a source of light for directing light along a predetermined path; 
     (b) a self-damped diffractive light modulator disposed in the predetermined path and having a plurality of spaced apart self-damped deformable elements being disposed relative to each other and secured at opposite ends and suspended above and movable into a channel containing a gas, and each spaced apart self-damped deformable element having at least one reflective surface; 
     (c) means responsive to the input data stream for applying forces to each of the spaced apart self-damped deformable elements to cause the spaced apart self-damped deformable elements to deform and move into the channel so that the spaced apart self-damped deformable elements are movable between first and second positions in accordance with the input data stream; and 
     (d) the self-damped diffractive light modulator modulating the light beam and directing the modulated light to a light utilization device where the modulated light can be recorded or decoded, the spaced apart self-damped deformable elements being sufficiently damped to minimize the introduction of data errors into the modulated light beam. 
     In accordance with the present invention an optical data modulation system with a self-damped diffractive light modulator suitable for 2 MHz data rates is disclosed. The system represents a significant improvement over existing technology in terms of its data throughput, reliability, and manufacturability. The modulator can readily be optimized at standard ambient conditions, which substantially simplifies fabrication and packaging, and reduces per unit costs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of the optical data modulation system of the present invention used in an optical data storage application; 
     FIGS. 2 a - 2   b  are illustrations of a self-damped diffractive light modulator in the un-actuated and actuated state respectively; 
     FIG. 3 is a view perpendicular to the self-damped diffractive light modulator of FIG. 2 showing a suspended un-actuated deformable ribbon element above a channel with the structure of the supporting layers; 
     FIG. 4 is a partially cut-away perspective of a self-damped diffractive light modulator; 
     FIG. 5 is a damped spring-mass system that serves as a model for the transient behavior of a deformable ribbon element; 
     FIGS. 6 a,    6   b  and  6   c  show an activation voltage pulse, ribbon displacement, and modulated light intensity into the 0&#39;th order for an underdamped diffractive light modulator, respectively; 
     FIGS. 7 a,    7   b  and  7   c  show an activation voltage pulse, ribbon displacement, and modulated light intensity into the 0&#39;th order for a self-damped diffractive light modulator, respectively; 
     FIGS. 8 a  and  8   b  are plots of the 0&#39;th order modulated light intensity from a fabricated self-damped diffractive light modulator operating in a contact and noncontact mode, respectively; and, 
     FIG. 9 is a schematic of an alternate embodiment of an optical data modulation system which is used for optical data transmission. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a schematic of an optical data modulation system  100  of the present invention used in an optical data storage application. The optical data modulation system  100  includes a light source  110 , an optical system  120 , a light directing element  130 , a data encoder and modulator driver  140 , a self-damped diffractive light modulator  10 , an optical system  150 , and a light utilization device  200 . The light source  110  is preferably a laser or LED. The light directing element  130  is preferably a mirrored prism. 
     The operation of the optical data modulation system  100  is as follows: Light  112  from the light source  110  is focused by the optical system  120  onto the light directing element  130  which directs the light  112  unto the self-damped diffractive light modulator  10 . The data encoder and modulator driver  140  activates the self-damped diffractive light modulator  10  to modulate the incident light in accordance with an input data stream  160 . Modulated light  122  leaves the self-damped diffractive light modulator  10  and is incident on the light directing element  130 . The light directing element  130  directs the modulated light  122  onto an optical system  150 . The optical system  150  focuses the modulated light  122  onto a light utilization device  200 , which in this embodiment is a high-speed data storage system. Specifically, in this embodiment the light utilization device  200  is an optical data recorder which uses an optically sensitive storage media that consists of a movable light sensitive surface which records data in response to the modulated light  122 . In this way, the input data stream  160  is stored in a digital format on an optically sensitive storage media for subsequent retrieval and use. The optical data modulation system  100  is particularly suitable for operation at data rates above 2 MHz. 
     Referring to FIGS. 2 a,    2   b  and  3 , the structure of the self-damped diffractive light modulator  10  is shown in cross-sectional views in an un-actuated, actuated, and un-actuated state, respectively. FIGS. 2 a  and  2   b  depict the ribbon structure of the device in the un-actuated and actuated state, respectively. FIG. 3 is the view of the same device (as shown in FIG. 2 a ) in the un-actuated state, but rotated 90 degrees to provide an insight into the layer structure. Referring to FIG. 3, typically a substrate  20  is provided which may be a single crystal silicon wafer or glass. In the case of a single crystal silicon wafer, a bottom conductive layer  22  is generated by heavily doping the silicon near the surface  23  of the substrate  20 . If glass is used as a substrate, the increased conductivity is achieved by depositing a bottom conductive layer  22  on the surface of the substrate  20 . The bottom conductive layer  22  is covered by a protective layer  24  which, for the example of a silicon substrate, can be thermal oxide. A dielectric spacer layer  26  is formed on the protective layer  24  and contains a channel  28  where the active region of the light modulator  10  is located. The channel  28  defines a depth h which is governed by the deposited thickness of the spacer layer  26 , and a length L which is determined by patterning the spacer layer  26 . The spacer layer  26  defines an upper surface level  27 . A plurality of deformable ribbon elements  31  of width w are patterned from a ribbon layer  30  formed atop the spacer layer  26 . The ribbon layer  30  includes a dielectric material, which can be silicon nitride, covered by a conductive and reflective layer  32 . The conductive and reflective layer  32  of every other deformable ribbon element  31  is connected to the bottom conductive layer  22  through an opening  34  that is filled with a thick layer of conducting material  36 . The thickness and tensile stress of the ribbon layer  30  is chosen to optimize performance by influencing the electrostatic force required for actuation and the returning force, which affects the speed, resonance frequency, and voltage requirements of the deformable ribbon elements  31 . 
     For operation of the device, an attractive electrostatic force is produced by a voltage difference between the bottom conductive layer  22  and the reflective and conductive layer  32  atop the ribbon layer  30 . In the un-actuated state (see FIG. 2 a ), with no voltage difference, all of the deformable ribbon elements  31  of the light modulator  10  are suspended above the substrate  20  at the same height. In this state, an incident light beam  14  is primarily reflected into the mirror direction (0 th  order) as a reflected light beam  15 . To obtain the actuated state (see FIG. 2 b ), a voltage is applied to every other deformable ribbon element  31  producing a periodic grating. In the fully actuated state every other deformable ribbon element  31  is in contact with the protective layer  24 . When the height difference between adjacent ribbons is ¼ of the wavelength of the incident light beam  16 , the light beam is primarily diffracted into a 1 st  order 17 and a−1 st  order  18 . One or both of these diffracted orders can be collected and used by an optical system (not shown). Alternatively, in certain applications, the 0 th  order reflected light can be used by the system. When the applied voltage is removed, the forces due to the tensile stress and the bending moment restore the deformable ribbon elements  31  to their original un-actuated state (see FIG. 2 a ). 
     FIG. 4 is a partially cut-away perspective of the self-damped diffractive light modulator  10  shown in FIGS. 2 and 3. The spacer layer  26  has a longitudinal channel  28  with a first and second side wall  28   a  and  28   b,  and a bottom  28   c.  The channel  28  is open to the top and covered by a first and a second set of deformable ribbon elements  31   a  and  31   b.  Each deformable ribbon element  31   a  and  31   b  spans the channel  28  and is secured to the surface of the spacer layer  26  on either side of the channel  27 . The bottom  28   c  of the channel  28  is covered by the protective layer  24 . As mentioned above, the ribbon layer  30  is covered by the conductive and reflective layer  32 . The conductive and reflective layer  32  is patterned such that there is a first and a second conducting region  32   a  and  32   b  with a comb-like structure arranged in an interdigitated manner. The first and second conductive region  32   a  and  32   b  are mechanically and electrically isolated from one another. According to the pattern of the conductive and reflective layer  32  the ribbon layer  30  is patterned in the same manner. As a result, there are the first and the second set of deformable ribbon elements  31   a  and  31   b  spanning the channel  28  and arranged such that every other deformable ribbon element belongs to one set. At the bottom  28   c  of the channel  28  a plurality of standoffs (not shown) may be formed in order to minimize the contact area between the bottom surface of the deformable ribbon elements  31  and the bottom  28   c  of the channel  28 . This reduction in contact area is known to reduce the risk of failure of the deformable ribbon elements  31   a  and  32   b  as a result of adhesion forces, which is also known as stiction failure. 
     Referring to FIGS. 3,  4 , and  5  the deformable ribbon elements  31  of the self-damped diffractive light modulator  10  can be modeled as a damped-spring- mass system (see E. P. Furlani, “Theory and Simulation of Viscous Damped Reflection Phase Gratings,” J. Phys. D: Appl. Phys, 32 (4), 1999). Referring to FIG. 5, the motion of the center of the deformable ribbon elements  31  is described by the following differential equation,                 2        y            t   2         =         F   e          (   y   )       -     γ                        y          t         -       (       K   s     +     k   gs       )        y                              
     where y(t) is the vertical displacement of the center of the deformable ribbon elements  31  from their un-actuated (up) position, F e (y) is the electrostatic force of attraction, K s , is the spring constant of the deformable ribbon elements  31 , and γ, and k gs  are damping and spring constants due to squeeze film effects as described below. The electrostatic force is given by              F   e          (   y   )       =       K   e                       V   2         [         ɛ   0        s     +     ɛ        (     h   -   y     )         ]     2           ,                          
     where            K   e     =         ɛ   2          ɛ   0        A     2       ,                          
     and A=wL, V is the voltage applied between the bottom conductive layer  22  and the reflective and conductive layer  32  atop the ribbon layer  30 , ∈ 0  and ∈ are the permittivities of free space and the ribbon material  30 , respectively, L is the length of the ribbon, h is the height of the channel  28  that is between 100 nanometers and 300 nanometer; and y is the displacement of the center of the deformable ribbon element  31  from its un-activated position. The ribbon spring constant K s  is given by            K   s     =       4      T     L       ,                          
     where T=T s  ws, and T s , w and s are the tensile stress, width and thickness of the ribbon layer  30 , respectively. The squeeze-film damping and spring coefficients are given by          γ   =         64      σ                   P   a        A         π   6        d              ∑     n   =   odd              ∑     m   =   odd                m   2     +       c   2          n   2               (     m                 n     )     2          [       (       m   2     +       c   2          n   2         )     +       σ   2     /     π   4         ]                 ,              and             k   gs     =         64        σ   2                     P   a        A         π   6        d              ∑     n   =   odd              ∑     m   =   odd                  m   2     +       c   2          n   2               (     m                 n     )     2          [       (       m   2     +       c   2          n   2         )     +       σ   2     /     π   4         ]         .                                  
     where P a  is the ambient pressure, A=Lw, c=w/L, and m and n are summation indices. The parameter σ is given by          σ   =         12                   μ   eff          w   2           P   a          d   2                       ω       ,                          
     where μ eff  is the effective viscosity of the gas, ω=2πf, and f is the frequency of oscillation of the deformable ribbon elements  31  (see T. Veijola, H. Kuisma, T. Ryhanen, “Equivalent-circuit model of squeezed gas film in a silicon accelerometer,” Sensors and Actuators A 48, 1995). 
     After the ribbon has been pulled down. The voltage V is set to zero and the response of the deformable ribbon elements  31  is governed by the                     2        y            t   2         +     γ                        y          t         +       (       K   s     +     k   gs       )        y       =   0     ,                          
     equation The solution of this equation for a damped response is of the form 
     
       
           y ( t )= R exp(−γ/2 m )cos(β t −δ), 
       
     
     where R is the amplitude of oscillation, γ is a damping coefficient, δ is a phase factor and        β   =           [       4        (       K   s     +     k   gs       )        m     -     γ   2       ]       1   /   2         2      m       .                            
     It is instructive to note that because of the functional form of γ, k gs  and K s , the response of the deformable ribbon elements  31  depends in a complex way on numerous device parameters including the dimensions and material properties of the deformable ribbon elements  31 , the gas in the channel  28 , the channel height h, and the ambient temperature and pressure. Therefore, in general, it is difficult to determine specific values for the device parameters that render a desired frequency response of the self-damped diffractive light modulator  10 . For low frequency applications, with data rates in the 100 kHz range, there is a relatively wide range of viable parameter values that render the diffractive light modulator  10  self-damped. Therefore, it is relatively easy to design and fabricate a self-damped diffractive light modulator  10  for low frequency applications. However, for high-frequency applications, with data rates greater than 2 MHz, the range of viable parameters is limited and difficult to determine. 
     FIGS. 6 a,    6   b  and  6   c  illustrate the activation and response of an underdamped diffractive light modulator. Specifically FIGS. 6 a,    6   b  and  6   c  show plots of an activation voltage pulse  42 , ribbon displacement  44 , and modulated light intensity  46  into the 0&#39;th order for an underdamped diffractive light modulator. The underdamped diffractive light modulator has substantially the same structure and operation as the self-damped diffractive light modulator  10  except that its deformable ribbon elements  31  tend to ring (oscillate) upon activation as described above. FIG. 6 a  shows an input voltage pulse  42  that is applied between the bottom conductive layer  22  and the reflective and conductive layer  32  atop the ribbon layer  30 . FIG. 6 b  shows the response of the deformable elements  31  to the input voltage pulse  42  of FIG. 6 a.  Specifically, it shows the displacement  44  of the center point of the deformable ribbon elements  31 . FIG. 6 c  shows a profile of the modulated light intensity  46  into the 0&#39;th order. The 0&#39;th order corresponds to the modulated reflected light. It is instructive to note that the modulated light intensity  46  of an underdamped light modulator is characterized by an oscillatory temporal variation due to the ringing of the underdamped deformable elements  31 . This oscillatory temporal variation is undesired for high-frequency optical data modulation because it causes data errors. 
     FIGS. 7 a,    7   b  and  7   c  illustrate the activation and response of a self-damped diffractive light modulator  10 . Specifically, FIGS. 7 a,    7   b  and  7   c  show plots of an activation voltage pulse  52 , ribbon displacement  54 , and modulated light intensity  56  into the 0&#39;th order for a self-damped diffractive light modulator  10 , respectively. FIG. 7 a  shows an input voltage pulse  52  that is applied between the bottom conductive layer  22  and the reflective and conductive layer  32  atop the ribbon layer  30 . FIG. 7 b  shows the response of the deformable elements  31  to the input voltage pulse  52  of FIG. 6 a.  Specifically, it shows the displacement of the center of the deformable elements  31 . FIG. 7 c  shows a profile of the modulated light intensity  56  into the 0&#39;th order that is generated by a self-damped diffractive light modulator  10  in response to the input voltage pulse  52 . It is instructive to note that the modulated light intensity  56  of the self-damped diffractive light modulator  10  exhibits a minimal temporal oscillation of the modulated light. This is desired for high-frequency optical data modulation because it provides an error free representation of the input data stream  160 . 
     FIGS. 8 a  and  8   b  show experimental data for the light intensity from a self-damped and underdamped modulator, respectively. A self-damped light modulator  10  for use at a 2 MHz data rate was fabricated. The deformable ribbon elements  31  in this modulator have the follows parameters: ribbon thickness=170 nm, ribbon tensile stress=1100 MPa, L=60 μm, w=4 μm and h=150 nm. The gas in the channel  28  is air at standard temperature and pressure, which simplifies device packaging. Modification of the gas type, temperature and pressure can be used to increase damping, but requires more complex and expensive packaging. The experimental response of the fabricated self-damped light modulator  10  to a 2 MHz input data stream is shown in FIG. 8 a.  This figure depicts the 0&#39;th order light intensity. Because the light modulator is self-damped, the modulated light intensity has minimal temporal variation, which is less than 20% of the peak modulated intensity. Therefore, this light modulator is well suited for use with 2 MHz data rates. The fabricated self-damped diffractive light modulator  10  functions in contact mode, whereby the deformable ribbon elements  31  are displaced vertically by 150 nm when actuated, and make mechanical contact with the bottom of the channel  28 . The self-damped diffractive light modulator  10  is preferably of this contact-mode type. For optimum diffraction efficiency, the vertical displacement upon actuation needs to be approximately ¼ of the wavelength of the incident light  112 . 
     Non-contact mode operation is often used to eliminate failure of deformable ribbon elements  31  due to sticking to the substrate (also known as stiction failure). Such devices also operate with a vertical displacement that is ¼ of the wavelength of the incident light  112 , but have a height h approximately equal to the wavelength. The experimental response of a diffractive light modulator designed for non-contact mode operation is shown in FIG. 8 b.  Even though the input data stream is now only 100 KHz, the 0&#39;th order intensity shows several cycles of underdamped oscillations. For this device, the parameters of the deformable ribbon elements  31  are as follows: ribbon thickness=170 nm, ribbon tensile stress=1100 MPa, L=120 μm, w=4 μm and h=600mn. This example illustrates the difficulty associated with designing diffractive light modulators that are self-damped and are capable of responding to a 2 MHz input data stream. 
     In high-speed optical data modulation system applications the oscillation of the deformable ribbon elements  31  must be kept to a minimum to avoid data errors. Specifically, any oscillation of the deformable ribbon elements  31  about their equilibrium position gives rise to an output signal. Moreover, if an oscillation is of sufficient amplitude, it will register as a data bit error. The criteria for a an optical data modulation system that is viable for data rates above 2 MHz are as follows: The self-damped diffractive light modulator  10  must be capable of producing a pulse of modulated light of intensity of constant amplitude I m  that has a temporal duration τ≦250 ns. Moreover any undesired oscillations of the deformable ribbon elements  31  must be limited so that the intensity of the modulated light resulting from such oscillations is less than 20 % of I m . 
     FIG. 9 is a schematic of an alternate embodiment of optical data modulation system in which it is used for optical data transmission. The optical data modulation system  300  includes of a light source  110 , a light transmission system  320 , a light directing element  130 , a data encoder and modulator driver  140 , and input data stream  160 , a self-damped diffractive light modulator  10 , an light transmission system  330 , a light sensor  340  and a data decoder  350 . The light source  110  is preferably a laser or LED. The light directing element  130  is preferably a mirrored prism, the light transmission systems  320  and  330  are preferably optical fiber systems, and the light sensor  340  is preferably a photodiode. 
     The operation of the optical data modulation system  300  is as follows: Light  112  from the light source  110  is transmitted by the light transmission system  320  onto the light directing element  130  which directs the light  112  unto the self-damped diffractive light modulator  10 . The data encoder and modulator diver  140  activates the self-damped diffractive light modulator  10  to modulate the incident light in accordance with an input data stream  160 . The modulated light  122  leaves the self-damped diffractive light modulator  10  and is incident on the light directing element  130 . The light directing element  130  directs the modulated light  122  onto the light transmission system  330 . The light transmission system  330  directs the modulated light  122  onto a light sensor  340 . The light sensor  340  outputs data into a data decoder  350  which outputs the decoded data in the form of and output data stream  360  for use in a variety of optical transmission and communications equipment. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
       10  self-damped diffractive light modulator 
       14  incident light beam 
       15  reflected light beam 
       16  incident light beam 
       17  diffracted light beam 1 st  order 
       18  diffracted light beam −1 st  order 
       20  substrate 
       22  bottom conductive layer 
       23  surface of the substrate 
       24  protective layer 
       26  spacerlayer 
       27  upper surface level 
       28  channel 
       28   a  side wall of channel 
       28   b  side wall of channel 
       28   c  bottom of channel 
       30  ribbon layer 
       31  deformable ribbon elements 
       31   a  first set of deformable ribbon elements 
       31   b  second set of deformable ribbon elements 
       32  conductive and reflective layer 
       32   a  first conducting region 
       32   b  second conducting region 
       34  opening 
       36  conducting material 
       42  voltage pulse 
       44  underdamped ribbon displacement 
       46  underdamped modulated light intensity 
       52  voltage pulse 
     Parts List Cont&#39;d 
       54  self-damped ribbon displacement 
       56  self-damped modulated light intensity 
       100  optical data modulation system 
       110  light source 
       112  incident light 
       120  optical system 
       122  modulated light 
       130  light directing element 
       140  data encoder and modulator driver 
       150  optical system 
       160  input data stream 
       200  light utilization device 
       300  optical data modulation system 
       320  light transmission system 
       330  light transmission system 
       340  light sensor 
       350  data decoder 
       360  output data stream