Abstract:
An apparatus for attenuating a light signal is disclosed. The apparatus causes optical attenuation in a waveguide, where the waveguide has an input port for receiving a light signal and an output port for output of an attenuated light signal. First, an electric field is generated in at least a portion of the waveguide, such that a first refractive index in that portion of the waveguide is changed to a second refractive index. Next, the light signal in the waveguide is directed from the input port to the output port through the electric field. And lastly, the light signal is attenuated as a function of the electric field. The light signal may be attenuated, for example, by changing the deflection angle, changing the beam collimation width or from emitting part of the light signal from the waveguide before the light signal reaches the output port.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority as a division of allowed U.S. patent application Ser. No. 10/330,785, filed Dec. 26, 2002 published Jul. 1, 2004, now U.S. Pat. No. 7,035.524. The disclosure of the above parent application is incorporated by reference. 

   FIELD OF THE INVENTION 
   The invention relates generally to the field of optical communications networks, and in particular to a variable optical attenuator device. 
   BACKGROUND OF THE INVENTION 
   As bandwidth requirements of communication networks continue to rise, wavelength division multiplexing is being used increasingly to aggregate the traffic of many users onto the optical fiber of backbone networks. For example, using a wavelength division multiplexer (WDM), Eighty or more separate wavelengths or channels of data can be multiplexed into a light signal transmitted on a single optical fiber. If each channel carries 2.5 Gbps (billion bits per second), up to 200 billion bits per second can be delivered on the single optical fiber. 
   However, in wavelength division multiplexing systems, the signal power levels transmitted in an optical fiber depend on the wavelengths. These inter-wavelength discrepancies in optical power levels are caused in part by the use of optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs). The use of EDFAs has revolutionized fiber optics, as they enable WDM data transport over thousands of kilometers. Unfortunately, as EDFAs do not inherently have a flat gain spectrum, there is the problem of uneven gain for different wavelengths. Variable Optical Attenuators (VOAs) provide a solution to this problem by attenuating different wavelengths by different amounts, therefore flattening the gain spectrum. 
     FIG. 1  is a schematic of an exemplary prior aft application of a variable optical attenuator (VOA). A multiple number of VOAs, e.g.,  110 - 1 ,  110 - 2 , and  110 - 3 , each receive an input wavelength, e.g., λ 1 , λ 2 , and λn, respectively. The VOAs attenuate the power of each input wavelength by different amounts and then transmit the attenuated wavelengths to the WDM  112  to be multiplexed into a multi wavelength light signal. This multi wavelength light signal is the amplified by an EDFA optical amplifier  114  and output to a fiber optic cable for transmission. The attenuation for each VOA has been chosen to compensate for the uneven gain spectrum of the optical amplifier  114 . 
   VOAs in current use include either Mach-Zender interferometers which use a thermo-optic effect to cause variation in attenuation or an electronically controlled mechanical means to cause variation in attenuation. One of the significant disadvantages of these typical VOAs is the speed (i.e., long settling or slow response times). Hence for the fast optical switching networks, which need high speed power adjustments on the order of about one nanosecond (1 GHz), current VOAs are inadequate. Therefore what is needed is a VOA with high speed attenuation adjustment that can support fast optical switching networks. 
   SUMMARY OF THE INVENTION 
   The present invention provides techniques, including a system and method, for attenuating a light signal using the electro-optic effect to provide fast attenuation adjustment. One embodiment of the present invention comprises a method for causing optical attenuation in a waveguide, where the waveguide has an input port for receiving a light signal and an output port for output of an attenuated light signal. First, an electric field is generated in at least a portion of the waveguide, such that a refractive index in that portion of the waveguide is changed. Next, the light signal in the waveguide is directed from the input port to the output port through the electric field. And lastly, the light signal is attenuated as a function of the electric field. The light signal may be attenuated, for example, by changing the deflection angle, changing the beam collimation width or from emitting part of the light signal from the waveguide before the light signal reaches the output port. 
   Another embodiment of the present invention comprises a VOA for attenuating a light signal. The VOA includes: a waveguide, having an input port for receiving the light signal and an output port for output of an attenuated light signal; a first lens for collimating the received light signal; a prism formed by an electric field, where the prism changes a deflection of said collimated light signal depending on the strength of the electric field; and a second lens for focusing the changed light signal on or near the output port, where the attenuation of the light signal is dependent on a location of a focal point of the focused light signal with respect to the output port. The location of the focal point varies as a function of the electric field. 
   A further embodiment of the present invention comprises a VOA for attenuating a light signal. The VOA includes: a waveguide, having an input port for receiving the light signal and an output port for output of an attenuated light signal; a first lens for collimating the received light signal; a second lens formed by an electric field, where the second lens causes a change in a collimation width of the collimated light signal depending on a strength of the electric field; and a third lens for focusing the changed collimated light signal, where attenuation of the light signal is dependent on the changed collimation width. 
   An alternative embodiment of the present invention comprises a VOA for attenuating a light signal. The VOA includes: a waveguide, having an input port for receiving said light signal and an output port for output of an attenuated light signal; a first electric field in said waveguide for collimating said received light signal; a second electric field in said waveguide for changing said collimated light signal depending on a strength of said second electric field; and a third electric field in said waveguide for focusing said changed light signal at or near said output port, wherein attenuation of said light signal is dependent on a location of a focal point of said focused light signal with respect to said output port. 
   Yet another embodiment of the present invention comprises a VOA for attenuating a light signal. The VOA includes: an input port for receiving the light signal; an output port; a waveguide for propagating the light signal from the input port to the output port, and a top electrode on the top clad layer for creating an electric field, where the electric field changes a refractive index of a portion of the top clad layer, such that a part of the light signal is emitted out of the waveguide before the output port. The waveguide includes a core, a top clad layer, and a bottom clad layer, where a part of the top clad layer has an electro-optic material. 
   One aspect of the present invention comprises a system for attenuating a light signal. The system includes: a waveguide comprising input means for receiving the light signal and an output port; means for generating an electric field in at least a portion of the waveguide such that a first refractive index in the portion of the waveguide changes the refractive index; means for directing the light signal in the waveguide from the input means to the output port through the electric field; and means for attenuating the light signal in the waveguide as a function of the electric field. 
   These and other embodiments, features, aspects and advantages of the invention will become better understood with regard to the following description, appended claims and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a know use of a variable optical attenuator (VOA); 
       FIG. 2  is a cross-section of a portion of a waveguide of an embodiment of the present invention; 
       FIG. 3  is a top view of a VOA device of an embodiment of the present invention, which varies optical attenuation by changing the optical beam deflection angle; 
       FIG. 4  is a top view of a VOA device of another embodiment of the present invention, which varies optical attenuation by changing the optical beam collimation; 
       FIG. 5  is a cross-sectional view of a VOA device of yet another embodiment of the present invention, which attenuates the light signal by introducing mode mismatch inside the waveguide; 
       FIG. 6  is an isometric view of a waveguide having a S-shaped core channel of another embodiment of the present invention; 
       FIG. 7  is an isometric view of a waveguide having a straight core channel of an alternative embodiment of the present invention; 
       FIG. 8  is a cross-sectional view of the waveguide of  FIG. 6 ; 
       FIG. 9  is a cross-sectional view of the waveguide of  FIG. 7 ; and 
       FIG. 10  is a graph showing simulation results for the amount of attenuation for a given applied voltage for the waveguides of  FIGS. 6 and 7 . 
   

   DETAILED DESCRIPTION 
   In the following description, numerous specific details are set forth to provide a more thorough description of the specific embodiments of the invention. It will be apparent, however, to one skilled in the art, that the invention may be practiced without all the specific details given below. In other instances, well known features have not been described in detail so as not to obscure the invention. 
     FIG. 2  is a cross-section of a portion of a slab waveguide of an embodiment of the present invention. The slab waveguide includes a core  214  surrounded by cladding material. In  FIG. 2  the surrounding cladding material is depicted as top clad layer  212  and bottom clad layer  216 . On top of and affixed to the top clad layer  212  is a top electrode  218 . Below and affixed to the bottom clad layer  216  is a bottom electrode  220 . A first voltage applied to top electrode  218  and a second voltage applied to bottom electrode  220  causes an electric field between the electrodes. A light signal  222  propagates through the core  214 . When there is no electric field, the top clad layer  212  has refractive index n 1 ; the core has refractive index n 2 ; and the bottom clad layer  216  has refractive index n 3 . The refractive index of the core  214  is greater than the surrounding cladding material, i.e., n 2 &gt;n 1  and n 2 &gt;n 3 . The core is made of an electro-optic (EO) material such as ferroelectric oxide, e.g., LiNbO 3 , PZT, or PLZT, that changes its refractive index in the presence of an electric field. Hence by making one or both of the electrodes a certain shape, an electrically formed lens (or prism) is created when voltages are applied to the top and bottom electrodes, e.g.,  218  and  220 . In addition, any refractive index change of the top or bottom clad layers ( 212  and  216 ) in the presence of the electric field generated by voltages to electrodes  218  and  220  is less than the refractive index change in portion  214 , allowing the slab to function as a waveguide in the presence of an electric field. 
     FIG. 3  is a top view of a variable optical attenuator (VOA) device  308  of an embodiment of the present invention, which varies optical attenuation by changing the optical beam deflection angle. In  FIG. 3  the top view shows the cores of the waveguides, i.e., core channels  312  and  316  and the slab waveguide core  314 . The lenses or prisms, e.g.,  320 ,  322 ,  324 , and  326 , are located in the slab waveguide core  314 . In one embodiment these lenses or prisms are formed from electric fields produced by electrodes above and below the slab waveguide core  314 . In an alternative embodiment only prisms  322  and  324  are electrically formed, while lenses  320  and  326  are physical lenses. Channel waveguide  313  with core channel  312  is optically coupled to slab waveguide core  314 . Slab waveguide core  314  is optically coupled to channel waveguide  315  with core channel  316 . The VOA  308  has longitudinal axis  310  illustrated by a horizontal dotted line. Slab waveguide core  314  includes collimating lens  320 , prism  322 , prism  324 , and focusing lens  326 . The number, type, and location of the lenses and prisms may vary in other embodiments of the present invention to achieve optical attenuation. 
   A light signal  330  enters the VOA  308  at input port  331  and propagates through channel waveguide  313 , slab waveguide  314 , and channel waveguide  315  to output port  340 . In channel waveguide  313  the light signal  330  travels through channel  312 , and at channel the exit  332 , the light signal  330  enters slab waveguide core  314 . The diverging light signal is collimated by collimating lens  320  into light beam  333 . Light beam  333  is deflected, i.e., the light beam&#39;s direction is changed, by prism  322  to give light beam  334 . The amount of defection is dependent upon the strength of the electric field produced in the waveguide  314  by the electrodes (not shown) of electrically formed prism  322 . The electrically formed prism  324  changes the direction of light beam  334  to be parallel to, but offset from longitudinal axis  310 . Light beam  335  is then converged by focusing lens  326  to focal point  336 , which located at or near the input  337  of core channel  316  of channel waveguide  315 . The focused light beam  341  then proceeds as an attenuated light signal  338  to output port  340  via core channel  316 . 
   When the voltages are off, prisms  322  and  324  are not formed, and the light beam  333  proceeds along the longitudinal axis  310  to lens  326 , where the light beam  333  is focused to a focal point  336  located on the longitudinal axis  310 . The amount of attenuation should be at a minimum for this case. By changing the amount of deflection of light signal  333  produced by prism  322 , the focal point  336  moves up and down the normal to longitudinal axis  310 , i.e., it is offset. The further the focal point is located away from the longitudinal axis  310 , the greater the attenuation as less light enters channel  316 . Thus the amount of light attenuation may be varied as a function of the electric field of prism  322 , i.e., by the amount of defection of the light beam. 
     FIG. 4  is a top view of a VOA  408  of another embodiment of the present invention, which varies optical attenuation by changing the optical beam collimation. In  FIG. 3  the top view shows the cores of the waveguides, i.e., core channels  412  and  416  and the slab waveguide core  414 . The lenses, e.g.,  420 ,  422 ,  424 , and  426 , are located in the slab waveguide core  414 . In one embodiment these lenses are formed from electric fields produced by electrodes (not shown) above and below the slab waveguide core  414 . In an alternative embodiment only lenses  422  and  424  are electrically formed, while lenses  420  and  426  are physical lenses. Channel waveguide  413  with core channel  412  is optically coupled to slab waveguide core  414 . Channel waveguide  413 , having channel  412 , is optically coupled to slab waveguide core  414 . Slab waveguide  414  is optically coupled to channel waveguide  415 , having core channel  416 . The VOA has longitudinal axis  410  illustrated by a horizontal dotted line. Slab waveguide core  414  includes collimating lens  420 , diverging lens  422 , collimating lens  424 , and focusing lens  426 . The number, type, and location of the lenses and prisms may vary in other embodiments of the present invention to achieve optical attenuation. 
   A light signal  430  enters the VOA  408  at input port  431  and propagates through channel waveguide core  412 , slab waveguide core  414 , and channel waveguide core  416  to output port  440 . In channel waveguide core  414  the light signal  430  travels through core channel  412 , and at channel exit  432  to channel  412 , the light signal  430  diverges into slab waveguide core  414 . The diverging light signal is collimated by collimating lens  420  into collimated light beam  433  with a width  450 . Diverging lens  422  causes light beam  434  to spread out. The amount of divergence is dependent upon the strength of the electric field produced in the slab waveguide core  413  by the electrodes of diverging lens  422 . The collimator lens  424  re-collimates light beam  434  to a light beam  435  with a width  452  of the re-collimated beam that is greater than the width  450  of the collimated light beam  433 . Light beam  435  is then converged by focusing lens  426  to focal point  436  which is located along longitudinal axis  410  at or near the entrance  437  to channel  416  of channel waveguide  415 . The focused light beam  441  then proceeds as attenuated light signal  438  to output port  440  via core channel  416  of channel waveguide  415 . In an alternative embodiment the re-collimated beam width  452  is less than the width  450  of the collimated light beam  433 . 
   When the voltages are off, lenses  422  and  424  are not formed, and the light beam  433  proceeds along the longitudinal axis  410  to lens  426  where the light beam  433  is focused to a focal point  436  located on the longitudinal axis  410  at or near channel entrance  437 . The amount of attenuation is at a minimum for this case. By increasing the amount of divergence of light signal  433  produced by diverging lens  422 , the collimation width  452  is increased, and the amount of light from focused light beam  441  that goes through entrance  437  is decreased. In other words, the amount of light attenuation is a function of the width  452  of the collimation of the light beam. In an alternative embodiment the focal point  436  may also be moved along longitudinal axis  410  by changing the electric field of focusing lens  436 , hence changing the refractive index of lens  426  with respect to the refractive index of the slab waveguide core  414 . 
     FIG. 5  is a cross-sectional view of a VOA  508  of yet another embodiment of the present invention, which attenuates the light signal by introducing mode mismatch inside the waveguide. The waveguide includes a top clad layer  510 , a core  514 , and a bottom clad layer  516 . The top clad layer  510  includes a portion  512  having an electro-optic (EO) material, such as LiNbO 3 , PZT, or PLZT. Positioned on top of and affixed to portion  512  of top clad layer  510  is a top electrode  520 . Bottom electrode  522  is positioned below and affixed to bottom clad layer  516 . The top  520  and bottom  522  electrodes, when there is a voltage applied, produces an electric field  524  (illustrated by the dotted arrows) in the waveguide. The electric field in portion  512  increases the refractive index of portion  512  to a higher refractive index value n 4  (where n 4 &gt;n 1 ). The refractive indexes of the core  514  and bottom clad layer  516  remain the same or change less than that of portion  512 , whether or not there is an electric field. 
   Changing the refractive index of the top clad layer  510  in portion  512 , causes some light to pass out (or “leak out”) of the waveguide. For example, the mode field diameter of a step-indexed fiber is a function of the core diameter, wavelength, and the refractive indexes of the core and clad. As the refractive indexes of the core and clad layers are brought closer together, for example by increasing the refractive index of portion  512 , the mode field diameter gets larger, and the power propagating along core  514  decreases. Specifically, when the refractive index of the top clad layer  510  is increased by the electric field in portion  512 , the beam power confined in the core  514  in the vicinity of portion  512  decreases. Some portion of the light passes from core  514  into portion  510 , as represented by light ray  542 , and the light propagating down core  514  is attenuated. 
   For example, light rays  530  and  532  are normally reflected at the core-clad interface as they propagate along core  514 . With no electric filed, both rays will propagate to the other end of the core  514 . When electric field  524  increases the refractive index of portion  512 , ray  532  at the core-clad interface  540 , is refracted out of the core  514  rather than being reflected (the top electrode  520 , in this case, is transparent). The electric field effectively decreases the critical angle needed for total reflection, so light ray  532  is no longer reflected at interface  540 . Light ray  530  continues to be totally internally reflected. 
     FIG. 6  shows a waveguide  610  having an S-shaped core channel  618  of another embodiment of the present invention. Waveguide  610  has top clad layer  612  having an EO material and a bottom clad layer  616  with a non-EO material. The core channel  618  has a non-EO material and is formed within the bottom clad layer  616  (see  FIG. 9  for cross-sectional view). The core  618  has a curved shape, e.g., S-shape. In other embodiments the core  618  maybe straight or otherwise curved. The core  618  dimensions include the width  630  of the bottom clad layer  616  and the lateral shift  634  from the entrance  636  into core  618  to the exit  638  from core  618 . A light signal  620  enters the core channel  618  at entrance  636  and propagates in the core channel  618  as light signal  622  until the channel exit  638 . Electrodes (not shown) are positioned above top clad layer  612  and below bottom clad layer  616  to create an electric field in and around core channel  618 . The electric field increases the index of refraction in the top clad layer and causes&#39; a portion of the light signal  622  to leak out of the waveguide  610 . In addition there is significant light leakage in the curved areas  640  and  642  of core  618 , because the critical angles needed for total reflection of the beam can no longer be met due to the bends in the core. Thus light signal  624  is attenuated by controlling the electric field, i.e., the voltage on the electrodes. 
     FIG. 7  shows a waveguide  650  having a straight core channel  656  of another embodiment of the present invention. Waveguide  650  has top clad layer  652  having an EO material and a bottom clad layer  654  with a non-EO material. The core channel  656  has a non-EO material and is formed within the top clad layer  652  (see  FIG. 8  for cross-sectional view). The core  656  has a non-curved shape, e.g., straight. In other embodiments, the core  656  has a curved shape, e.g., S-shape. A light signal  660  enters the core channel  656  at entrance  664  and propagates in the core channel  656  until the channel exit  666 . Electrodes (not shown) are positioned above top clad layer  652  and below bottom clad layer  654  to create an electric field in and around core channel  656 . The electric field increases the index of refraction in the top clad layer  652  and causes a portion of the light signal in core  656  to leak out of the waveguide  650 . Thus light signal  662  is an attenuated version of light signal  660 , where the attenuation is controlled by controlling the electric field, i.e., the voltage on the electrodes. 
     FIG. 8  is a cross-sectional view of the waveguide  650  of  FIG. 7  along view line YY. The waveguide  710  includes top clad layer  712 , core  714 , and bottom clad layer  716 . Top clad layer  712  comprises EO material. Core  714  is formed within top clad layer  712 . The core  714  and bottom clad layer  716  have non-EO material.  FIG. 8  shows a ridge type placement of the core  714  above the bottom clad layer  716 . 
     FIG. 9  is a cross-sectional view of the waveguide  610  of  FIG. 6  along view line XX. The waveguide  810  includes top clad layer  812 , core  814 , and bottom clad layer  816 . Top clad layer  812  has EO material. The core  814  is formed within bottom clad layer  816 . The core  814  and bottom clad layer  816  have non-EO material.  FIG. 8  shows a buried type placement of the core  814  in the bottom clad layer  816 . 
   A simulation was conducted using the waveguide  650  having the straight channel core with both ridge type ( FIG. 8 ) and buried type ( FIG. 9 ) cores. The simulation also used using the waveguide  610  having the S-channel core with both ridge type ( FIG. 8 ) and buried type ( FIG. 9 ) cores. The refractive index of the bottom clad was about 1.563. The refractive index of the core was about 1.567. The refractive index of the top clad (with the EO material having electro-optic coefficient of 100 picometers per volt) was about 1.563. For  FIG. 6  the width  630  (and  632 ) was about 4 mm and the lateral shift  634  about 0.125 mm. The core had a 7.times.7 μm cross-section. 
     FIG. 10  is a graph showing the results of a simulation for the amount of attenuation for a given applied voltage for the waveguides of  FIGS. 6 and 7 . The y-axis  912  gives the attenuation in dB (power) per cm and the x-axis  910  gives voltage applied across the electrodes in volts per 10 μm. The EO coefficient is 100 ρm/volt. Curves  920  and  922  show the attenuation for a curved core such as in  FIG. 6 , and curves  924  and  926  show attenuation for a straight core such as in  FIG. 7 . Curves  920  and  924  are for the ridge type core of  FIG. 8 . Curves  922  and  926  are for the buried type core of  FIG. 9 . From curves  920  and  922 , the S-shaped core channel of  FIG. 6  gives a wider dynamic range, e.g., &gt;15 dB, when compared to the straight channel core of  FIG. 7 . However, the straight core channel (curves  924  and  926 ) does allow finer control of the attenuation. 
   The specification and drawings are to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that additions, subtractions, deletions, and other modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims.