Abstract:
A variable optical attenuator is disclosed, which attenuates a beam of light while preserving its polarization substantially independent of wavelength. The beam of light is attenuated by a filter patterned with a grating of blocking stripes with serrated edges, which partially block and partially transmit the beam of light, respectively. The serrated edges provide for low polarization dependent loss. Along a length of the filter, a mark to space ratio of blocking stripe and aperture widths increases. By a linear translation of the filter along its length attenuation can be varied to a desired value. A stepper motor with lead screw can provide a suitable linear translation to give the filter a latching property.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention claims priority from Provisional Patent Application U.S. No. 60/887,967 filed Feb. 2, 2007, which is incorporated herein by reference for all purposes. 

   TECHNICAL FIELD 
   The present invention relates to variable optical attenuators for attenuating the intensity of a beam of light, in particular to optical attenuators which preserve the polarization of the beam of light over a wide range of attenuation levels and wavelengths. 
   BACKGROUND OF THE INVENTION 
   Variable optical attenuators are widely used in optical telecommunications and other applications to regulate the optical power level of optical signals for equalization purposes or to manage the dynamic range and sensitivity of photodetectors in optical receivers. Means of realizing optical attenuation include the utilization of linear neutral density filters, attenuating prisms, beam blockers, tilting mirrors, and mechanisms for bending or off-setting optical fibers. 
   In a prior art solution, Smiley et al. (U.S. Pat. No. 6,167,185) disclose an adjustable optical attenuator that preserves the composition of polarization of a beam of light. The beam attenuator has a cross-section along a plane perpendicular to the direction of propagation of the collimated beam of light in the shape of a wedge. The attenuation is varied using a controller for moving the beam attenuator in order to vary a size of the portion of the wedge within the collimated beam of light. 
     FIG. 1  shows the adjustable optical attenuator  100  consisting of an opaque cone  101  partially blocking a collimated beam of light  102  shown in cross-section. Adjustment of attenuation is effected by linearly moving the opaque cone  101  in a direction of motion  103  in and out of the collimated beam of light  102 . A disadvantage of this arrangement is that the direction of motion  103  must be maintained in precise alignment with the center of the collimated beam of light  102 , such that the attenuation of one polarization  110  is matched by attenuation of the other polarization  120 . The E and H denote the electric and magnetic field orientations of the respective polarizations. 
   In another example of prior art disclosed by Payne et al. (U.S. Pat. No. 6,801,354) is shown in  FIG. 2 . An adjustable optical attenuator  200  comprises a symmetrical array of circular apertures  203  in a membrane  202  forming a 2-D diffraction grating, which is employed to eliminate polarization dependent losses when attenuating a beam of incident light  201 .  FIG. 3  is a top view of an adjustable optical attenuator  300 . A symmetrical array of circular apertures  302  forms a 2-D diffraction grating in a reflective silicon nitride membrane  301 . Reflective posts  303  are disposed inside each circular aperture  302 . By electrostatically raising or lowering the reflective silicon nitride membrane  301  with respect to the reflective posts  303  an attenuation of the beam of incident light reflected from the adjustable optical attenuator  300  is achieved. The circular shape of the apertures  302  is aimed at reducing PDL of the attenuation of the output reflected beam. 
   The fabrication of the adjustable optical attenuator  300  entails many deposition, photolithographic and etching steps, increasing the cost of manufacture. It also operates in reflection, complicating access to the attenuator output beam. There is no inherent latching property, as the removal of the electric drive voltage releases the electrostatic forces that displace the membrane, resulting in a return of the attenuation to some quiescent value. 
   In general, a linear response of attenuation to a control signal is normally preferred. A desired controllable attenuation range is up to 30 dB. Polarization dependent loss (PDL) is preferably below 0.2 dB within the 0 dB to 20 dB attenuation range. Wavelength dependent loss (WDL) is preferably 0.3 dB or less for the fiberoptic telecommunications C-band. For some applications, a variable attenuator is preferred, which retains its attenuation setting even if its power supply is turned off, i.e. having a latching property. 
   An object of the present invention to provide a variable optical attenuator with a widely controllable attenuation range of 30 dB while at the same time maintaining low PDL and low WDL. 
   It is a further object of this invention to provide a variable optical attenuator with a latching property. 
   Finally, it is another object of this invention to provide a variable optical attenuator with linear attenuation response to the driving signal. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention relates to a variable optical attenuator comprising a filter with a plurality of alternating blocking stripes and linear apertures, wherein the blocking stripes are laid out in a zigzag pattern. The zigzag pattern is dimensioned to minimize polarization dependent loss of the filter. 
   Another feature of the present invention provides for a ratio of the stripe width and the aperture width to vary progressively with a distance from an end of the filter, so that a linear translation of the filter in a travel direction through an input optical beam results in a variation of attenuation level of an attenuated output optical beam. 
   Another aspect of the present invention relates to an actuator for linearly translating the filter through the input optical beam such that a loss of drive power to the actuator does not change the attenuation level, thus providing a latching feature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
       FIG. 1  is a diagram of prior art wedge-type adjustable optical attenuator; 
       FIG. 2  is an isometric view of a prior art membrane with a 2-dimensional grating for low polarization dependence attenuation; 
       FIG. 3  is a top view of a prior art 2-dimensional grating for low polarization dependence attenuation; 
       FIG. 4  is a schematic of variable optical attenuator according to the present invention; 
       FIG. 5  is a detailed schematic of an embodiment of a variable optical attenuator; 
       FIG. 6  is a top view of an embodiment of a linear variable grating filter (not to scale); 
       FIG. 7  is a graph of wavelength dependent loss spectra for three different attenuation settings; and 
       FIG. 8  is a graph of polarization dependent loss as a function of attenuation. 
   

   DETAILED DESCRIPTION 
   An optical layout of an exemplary variable optical attenuator  400  is shown schematically in  FIG. 4 . Incoming light enters the variable optical attenuator  400  through an input optical fiber  401  and is collimated by collimating lens  421  to produce a collimated beam  420 . A variable grating filter  430 , which is movable in a travel direction  440  substantially perpendicular to the collimated beam  420 , transmits a portion of the collimated beam  420  onto collimating lens  422  for focusing onto an end of an output optical fiber  402 . The portion of collimated beam  420  that is transmitted can be varied by a linear position of the variable grating filter  430  along the travel direction  440 . The input and output optical fibers  401 ,  402  can be replaced by slab waveguides, free-space optical beams or similar optical connections to an external optical system. 
     FIG. 5  shows an embodiment of a variable optical attenuator  500  in greater detail. Incoming light enters the variable optical attenuator  500  through an input optical fiber  501 , an end of which is held in position by tube  511 . From the end of the optical fiber  501  the incoming light is collimated by collimating lens  521  and redirected by a first turning prism  541  into a collimated beam  551 . The collimated beam  551  is incident at substantially normal incidence on variable grating filter  530 , which transmits an attenuated beam  552 , comprising a portion of the collimated beam  551 , onto a second turning prism  542 , thence to collimating lens  522  for focusing onto an end of output optical fiber  502  supported by a second tube  512 . The output optical fiber  502  receives the focused attenuated beam  552  for transmission to other parts of an external optical system. 
   The properties of the variable grating filter  530  are designed so as to change the portion of the collimated beam  551  that gets transmitted as the attenuated beam  552  depending on an incidence distance of the collimated beam  551  on the variable grating filter  530  along filter travel direction  540 . Thus a linear translation of the variable grating filter  530  along filter travel direction  540  results in a variable amount of attenuation of the collimated beam  551 , which varies according to a distance and direction that the variable grating filter  530  is moved. 
   To achieve linear or approximately linear travel of the variable grating filter  530 , several methods may be used. In the above example, a stepper motor  560  is utilized to provide rotation to a lead screw  561 , to which filter holder  562  is engaged with a corresponding threaded section such that the rotation of the lead screw  561  produces a linear translation of the filter holder  562  along the travel direction  540 . The filter holder  562  is prevented from rotating with the lead screw  561  by one or more guide pins  563  (only one shown). When drive current to the stepper motor  560  is switched off, the filter holder  562  supporting the variable grating filter  530  retains its position on the lead screw  561  thereby achieving a desired latching function. 
   In practical applications, the collimated beam  551  does not have to be perfectly collimated, nor does it have to be incident on the variable grating filter  530  at exactly normal incidence. Some angling may even be desirable for reducing deleterious effects due to back reflections from the variable grating filter  530  back into the input optical fiber  501 , ultimately raising a return loss of the variable optical attenuator  500 . The input optical fiber  501  and output optical fiber  502  may also comprise an optical slab waveguide, a free-space beam or other suitable optical connection to an external optical system. 
   Instead of the stepper motor  560 , other types of actuator may be substituted, such as a Micro-Electro-Mechanical Systems (MEMS) actuator, to reciprocate, slide or move the variable grating filter  530  into various positions depending upon the amount of attenuation desired, either predetermined or controlled by a feedback control mechanism. With proper choice of actuator, when power to the actuator is cut, the variable grating filter  530  remains latched in position. Other types of mechanical latches are also possible. 
   The turning prisms  541  and  542  can be triangular prisms having long faces coated with a reflective coating or having a sufficiently high refractive index to provide total internal reflection. Alternatively, small mirrors may be used. 
   With reference to  FIG. 6 , details of an embodiment are shown of the variable grating filter  530 , which is based on variable grating technology that is fabricated using semiconductor compatible processes. Variable grating filter  630  comprises a transparent substrate  631  having on one of its surfaces a plurality of zigzagging blocking stripes  621  alternating with zigzagging apertures  622  defined by serrated leading and trailing edges. The purpose of the serrated edges is to achieve equal blocking or attenuation of both polarizations of the collimated beam ( 551  in  FIG. 5 ). Serration depth  645  must be optimized for reducing PDL and return loss of the variable grating filter  630 . The serrations are defined by a series of teeth, each having an apex angle  646  between 80° and 100°, however 90° is preferred for optimal PDL. Serration pitch  644  is determined by the geometry of the serration depth  645  and the apex angle  646 . 
   The apertures  622  are transparent to transmit light of a desired range of wavelengths. Anti-reflection coating on the apertures  622  may be used to reduce optical losses due to surface reflectivity of the transparent substrate  631 . The blocking stripes  621  are either opaque or highly reflective so as to substantially block light of the desired range of wavelengths from getting transmitted. Gold, chromium, or aluminum are sample materials suitable for opaque coatings of the blocking stripes  621 . 
   The collimated beam of light ( 551  in  FIG. 5 ) is incident on the variable grating filter  630  at approximately normal incidence to form a beam spot  650  with an intensity profile  651 . As the variable grating filter  630  is translated along filter travel direction  640  relative to the beam spot  650 , blocking stripes  621  and apertures  622  can be brought into the beam spot  650  to varying amount, thereby blocking a portion of it from getting transmitted through the variable grating filter  630 . An variably attenuated beam ( 552  in  FIG. 5 ) is thus produced. 
   A ratio of areas of the blocking stripes  621  to the apertures  622  intersecting varies with distance along the filter travel direction  640  of the variable grating filter  630 . For instance, the ratio of areas can be varied by reducing stripe width  641 , increasing aperture width  642 , or a combination of both in a direction along the filter travel direction  640 . Stripe pitch  643  (the spacing between two adjacent blocking stripes  621 ) is optimized for both a slope (resolution) of attenuation against travel distance of the variable grating filter  630  and a return loss affected by reflecting diffraction. 
   Light passage through the variable optical attenuator  500  consists mainly of two components—blocking loss and the mode mismatch loss. 
   Assuming that the power distribution of light incident on the variable grating filter  630  has a Gaussian function, it can then be expressed as: 
   
     
       
         
           
             
               
                 
                   
                     ϕ 
                     1 
                   
                   ⁡ 
                   
                     ( 
                     
                       x 
                       , 
                       y 
                     
                     ) 
                   
                 
                 = 
                 
                   
                     A 
                     0 
                   
                   ⁢ 
                   
                     exp 
                     ⁡ 
                     
                       ( 
                       
                         - 
                         
                           
                             
                               x 
                               2 
                             
                             + 
                             
                               y 
                               2 
                             
                           
                           
                             ω 
                             2 
                           
                         
                       
                       ) 
                     
                   
                   ⁢ 
                   
                     exp 
                     ⁡ 
                     
                       [ 
                       
                         jφ 
                         ⁡ 
                         
                           ( 
                           
                             x 
                             . 
                             y 
                           
                           ) 
                         
                       
                       ] 
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   where A 0  and ω are field amplitude and optical beam waist radius respectively, φ is the angle phase. 
   The pattern formed by the blocking stripes  621  can be described by a door function door (x, y). Thus, the loss, IL 1 , of the light due only to the blocking stripes  621  can be obtained: 
   
     
       
         
           
             
               
                 
                   IL 
                   1 
                 
                 = 
                 
                   
                     ∫ 
                     
                       ∫ 
                       
                         ( 
                         
                           
                             
                               ϕ 
                               1 
                             
                             ⁡ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                           * 
                           
                             door 
                             ⁡ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                           ⁢ 
                           
                             ( 
                             
                               
                                 
                                   ϕ 
                                   1 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                               * 
                               
                                 door 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                               * 
                               
                                 ⅆ 
                                 s 
                               
                             
                           
                         
                       
                     
                   
                   
                     ∫ 
                     
                       ∫ 
                       
                         ( 
                         
                           
                             
                               ϕ 
                               1 
                             
                             ⁡ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                           ⁢ 
                           
                             ( 
                             
                               
                                 
                                   ϕ 
                                   1 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                               * 
                               
                                 ⅆ 
                                 s 
                               
                             
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   After passing through the variable grating filter  630 , light is coupled into the optical output  502 , such as optical fiber, the mode distributions through the variable grating filter  630  and the optical output  502  are different and may cause coupling loss due to mode mismatch. The mismatch coupling efficiency can be written as follows: 
   
     
       
         
           
             
               
                 η 
                 = 
                 
                   
                     
                        
                       
                         
                           ∫ 
                           
                             - 
                             ∞ 
                           
                           ∞ 
                         
                         ⁢ 
                         
                           
                             
                               FT 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     ϕ 
                                     1 
                                     ′ 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       x 
                                       , 
                                       y 
                                     
                                     ) 
                                   
                                 
                                 ) 
                               
                             
                             · 
                             
                               FT 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     ϕ 
                                     1 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       x 
                                       , 
                                       y 
                                     
                                     ) 
                                   
                                 
                                 ) 
                               
                             
                           
                           * 
                           
                             ⅆ 
                             s 
                           
                         
                       
                        
                     
                     2 
                   
                   
                     
                       ∫ 
                       
                         - 
                         ∞ 
                       
                       ∞ 
                     
                     ⁢ 
                     
                       
                         
                            
                           
                             FT 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   ϕ 
                                   1 
                                   ′ 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                               ) 
                             
                           
                            
                         
                         2 
                       
                       ⁢ 
                       
                         
                           ⅆ 
                           s 
                         
                         · 
                         
                           
                             ∫ 
                             
                               - 
                               ∞ 
                             
                             ∞ 
                           
                           ⁢ 
                           
                             
                               
                                  
                                 
                                   FT 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         ϕ 
                                         1 
                                       
                                       ⁢ 
                                       
                                         ( 
                                         
                                           x 
                                           , 
                                           y 
                                         
                                         ) 
                                       
                                     
                                     ) 
                                   
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               ⅆ 
                               s 
                             
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   where φ 1 ′ is the mode distribution of the light, FT(φ 1 ′) and FT(φ 1 ) are the Fourier transform of the light through lens and fiber, respectively. Based on Parseval&#39;s theory, equation (3) can be rewritten as: 
   
     
       
         
           
             
               
                 η 
                 = 
                 
                   
                     
                        
                       
                         
                           ∫ 
                           
                             - 
                             ∞ 
                           
                           ∞ 
                         
                         ⁢ 
                         
                           
                             
                               
                                 ϕ 
                                 1 
                                 ′ 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                             · 
                             
                               
                                 ϕ 
                                 1 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                           * 
                           
                             ⅆ 
                             s 
                           
                         
                       
                        
                     
                     2 
                   
                   
                     
                       ∫ 
                       
                         - 
                         ∞ 
                       
                       ∞ 
                     
                     ⁢ 
                     
                       
                         
                            
                           
                             
                               ϕ 
                               1 
                               ′ 
                             
                             ⁡ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                            
                         
                         2 
                       
                       ⁢ 
                       
                         
                           ⅆ 
                           s 
                         
                         · 
                         
                           
                             ∫ 
                             
                               - 
                               ∞ 
                             
                             ∞ 
                           
                           ⁢ 
                           
                             
                               
                                  
                                 
                                   
                                     ϕ 
                                     1 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       x 
                                       , 
                                       y 
                                     
                                     ) 
                                   
                                 
                                  
                               
                               2 
                             
                             ⁢ 
                             
                               ⅆ 
                               s 
                             
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   Thus, the mode mismatch coupling loss can be calculated as:
 
 IL   2 =−10 log 10 η   (5)
 
   Ignoring the small loss from the lens and the fiber, we have the total loss as a sum of the blocking loss and the mode mismatch loss:
 
 IL=IL   1   +IL   2   (6)
 
   An advantage of this technology is that the door function can be tailored to realize an attenuation characteristic according to a desired function of travel distance of the variable grating filter  630 . 
   An important factor during the design of the variable grating filter  630  is the appropriate consideration of the transmittance of the blocking stripes  621 , which are required to have a sufficiently low transmittance or sufficiently high rejection of light. Even a small portion of light passing through the blocking stripes  621  may interfere with the light passing through the apertures  622 . This interference can generate periodic ripples in the wavelength spectrum, which will increase the wavelength dependent loss (WDL). The effect is especially noticeable at higher attenuation settings, where the power difference between light leaking through the blocking stripes  621  and light transmitted through the apertures  622  is smaller. The effect is not so pronounced at lower attenuation settings. 
   Thus a loss of more than 45 dB for the blocking stripes  621  is normally desired. This can be achieved by coating the blocking stripes  621  with a thick layer of opaque material. Alternatively a multilayer dielectric coating can be applied to make the blocking stripes  621  highly reflective. 
     FIG. 7  presents wavelength dependent loss (WDL) spectra  700  for three nominal attenuation settings: 10 db ( 701 ), 20 dB ( 702 ) and 30 dB ( 703 ). A WDL of 0.3 dB or less is evident over the fiberoptic telecommunications C-band, which corresponds to a wavelength range from 1520 nm to 1570 nm. In any 2 nm band, the WDL is below 0.1 dB. 
   Measured low values of PDL according to the present invention are plotted in  FIG. 8 . The PDL is less than 0.10 dB over the full attenuation range of 0 dB to 32 dB. The PDL decreases as the attenuation setting is lowered.