Abstract:
A dynamic optical wavelength balancer is described. The apparatus includes a plurality of wavelength selective reflectors. Each wavelength selective reflector reflects optical signals at a predetermined one optical wavelength selected from a plurality of predetermined optical wavelengths. A bi-directional optical variable coupler array has a first port and a plurality, N, of second ports. Each wavelength selective reflector is coupled to a corresponding one of the second ports. The bi-directional optical variable coupler array is responsive to control signals for establishing different degrees of optical couplings between the first port and selected ones of the plurality of second ports.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to optical apparatus, in general, and to an optical wavelength balancer, in particular.  
           [0002]    It is desirable to provide optical signals having multiple wavelength components. One problem with such multiple wavelength component signals is that the different components may be at different power levels for a variety of reasons. It is desirable to be able to adjust the distribution of power among the wavelength components.  
         SUMMARY OF THE INVENTION  
         [0003]    In accordance with the principles of the invention apparatus operable for dynamically adjusting or balancing selected optical wavelengths is provided. The apparatus includes a bi-directional optical variable coupler array coupling a first port to a plurality, N, of second ports. The bi-directional optical variable coupler array is responsive to control signals for establishing selectable bi-directional optical couplings between the first port and the plurality of second ports. The bi-directional optical variable coupler array is operable to selectively adjust the coupling between the first port and each of the second ports. A plurality of wavelength selective reflectors, numbering N, are each coupled to a corresponding one of the second ports, each of the wavelength selective reflectors reflect optical signals at a predetermined one optical wavelength selected from a plurality of predetermined optical wavelengths.  
           [0004]    In accordance with an aspect of the invention the bi-directional optical variable coupler array is formed on a substrate. The substrate comprises LiNbO 3 .  
           [0005]    In accordance with a second embodiment of the invention a second substrate carries the plurality of wavelength selective reflectors. In the second embodiment the second substrate comprises silicon. The second substrate is bonded to the first substrate.  
           [0006]    In accordance with another aspect of the invention each wavelength selective reflector comprises a reflective filter. In the illustrative embodiments each reflective filter comprises a Bragg grating that is a fiber Bragg grating.  
           [0007]    The bi-directional optical variable coupler array comprises a plurality of stages. Each stage comprises a waveguide structure comprising first, second and third legs formed as a “y”, with said second and third legs forming a “v”, and a first electrode proximate said first leg, a second electrode proximate said second leg and a third common electrode. Selection signals applied to the first, second and third electrodes of said stages selectively adjust coupling between said first port and each of said second ports.  
           [0008]    The bi-directional optical variable coupler array is formed on a substrate of electro optic material comprising LiNbO 3 .  
           [0009]    A dynamic optical wavelength balancer in accordance with the principles of the invention includes a plurality of wavelength selective reflectors. Each wavelength selective reflector reflects optical signals at a predetermined one optical wavelength selected from a plurality of predetermined optical wavelengths. A bi-directional optical variable coupler array has a first port and a plurality, N, of second ports. Each wavelength selective reflector is coupled to a corresponding one of the second ports. The bi-directional optical variable coupler array is responsive to control signals for establishing different degrees of optical couplings between the first port and selected ones of the plurality of second ports.  
           [0010]    A micro controller controls the bi-directional optical variable coupler array to establish the different degrees of optical couplings. A bias driver, responsive to the micro controller provides the control signals.  
           [0011]    A detector coupled to the output provides signals representative of the wavelength components. The detector is coupled to the micro controller. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0012]    The invention will be better understood from a reading of the following detailed description taken in conjunction with the several drawing figures in which like reference designations are used to identify like elements in the figures, and in which:  
         [0013]    [0013]FIG. 1 shows a structure in accordance with the principles of the invention;  
         [0014]    [0014]FIG. 2 is a second embodiment in accordance with the principles of the invention;  
         [0015]    [0015]FIG. 3 illustrates a specific structure in accordance with the embodiment of FIG. 2;  
         [0016]    [0016]FIG. 4 illustrates a portion of the structure of FIG. 3 in greater detail;  
         [0017]    [0017]FIG. 5 is a top view of a fiber Bragg grating array in accordance with one aspect of the present invention;  
         [0018]    [0018]FIG. 6 is an end view of the array of FIG. 5; and  
         [0019]    [0019]FIG. 7 illustrates an alternate embodiment of the structure of FIG. 3. 
     
    
     DETAILED DESCRIPTION  
       [0020]    [0020]FIG. 1 illustrates the general configuration of apparatus in accordance with the principles of the invention. Optical signals from a source  1000  are applied to an input port  101  of a three port optical circulator  100 . Optical circulator  100  has a second port  103  coupled to optical switch  110 . A third port  105  serves as an output port. Circulator  100  may be any one of a number of known circulators. An isolator may be inserted into the optical path coupling the source of optical signals to port  101  to make port  101  unidirectional. Similarly, an optical isolator may be inserted into the optical path coupled to port  105  so that optical signals flow unidirectionally out from port  105 . Port  103  is a bi-directional port that receives optical signals from port  101  and couples optical signals received at port  103  to port  105 . The polarity of circulator  100  is indicated by directional arrow  102 . Arrows  104 ,  106  show the flow of input optical signals to bi-directional optical waveguide tree  120 . The flow of wavelength selected optical output signals from optical tree  120  to port  103  and out from port  105  is shown by arrows  108 ,  110 . Optical tree  120  is operable to couple port  121  to any of a plurality, n, of ports  123 . Each port of the plurality of ports  123  has coupled thereto a corresponding one of a plurality of reflective wavelength filters  125 . Each reflective wavelength filter  125  is a narrow filter and in the illustrative embodiment may be either a fiber Bragg grating or a dielectric interference filter. Both fiber Bragg gratings and dielectric interference filters are known in the art. Each wavelength filter  125  is selected to reflect optical signals that are only at a specific centerline wavelength designated as λ1-λn. The number of filters  125  utilized is dependant upon the specific application and the incremental wavelength difference between adjacent selected wavelengths. Stated another way, the number of filters  125  is determined by the number of wavelength components and the incremental wavelength, or wavelength granularity between selections. Bi-directional optical variable coupler array  120  receives wavelength selection signals and couples port  121  to selected ones of ports  123  based upon the selection signals. The selection of ones of ports  123  is made based upon the desired wavelength of optical signals desired. Each of the narrow filters  125  reflects optical signals only at the particular center wavelength of the filter and passes or in effect absorbs all other optical signals. Input optical signals received at circulator  100  port  101  are coupled to port  103  and coupled to port  121  of tree  120 . Tree  102  couples the optical signals to selected ones of filters  125 . The selected ones of filters  125  are determined by wavelength select signals received by tree  120 .  
         [0021]    Each selected filter  125  reflects only optical signals at its predetermined wavelength back to port  121  and thence to circulator  100  port  103 . The selected wavelength optical signals are coupled out of circulator  100  at port  105 . In a first embodiment of the invention, bi-directional optical variable coupler array  120  couples one port to N ports. In a second embodiment of the invention, bi-directional optical variable coupler array  120  is formed on a LiNbO 3  substrate or a substrate of other electro-optic material. This embodiment has the advantages of a high wavelength channel count, fast switch speed and small size.  
         [0022]    In a second embodiment in accordance with the invention shown in FIG. 2, 1×N bi-directional optical variable coupler array  120  is again formed on a LiNbO 3  substrate  220  or a substrate of other electro-optic material. Particular details of the 1×N bi-directional optical variable coupler array are not shown on the structure of FIG. 2, however, in this particularly advantageous embodiment of the invention, the plurality of filters  125  is arranged as a fiber Bragg grating array  225  of filters. A plurality, n, of fiber Bragg gratings  225  are provided on a separate substrate  230  that is affixed to substrate  220 . More specifically, a plurality, n, of fiber Bragg gratings  225  are bonded to grooves or channels formed on the surface of a substrate  230 . In the specific embodiment shown, substrate  230  is selected to be a silicon substrate. The end surface  232  of substrate  230  that is adjacent to substrate  220  is polished. End surface  232  is bonded to surface  222  of substrate  220 . Bonding of substrate  220  to substrate  230  may be by any one of several known arrangements for bonding substrates together.  
         [0023]    [0023]FIGS. 3 and 4 show a fiber Bragg grating array  225  with 8 fiber Bragg grating filters λ1-λ8. Each of the fiber Bragg grating filters λ1-λ8 is a separate fiber segment  301 - 308  having a Bragg grating  321 - 328  formed thereon. Each fiber segment is a photosensitive fiber onto which a Bragg grating is formed by using ultraviolet light in conjunction with a different period phase mask for each different filter center wavelength. The forming of Bragg gratings on fibers utilizing such a technique is known in the art. Silicon substrate  230  has a plurality of grooves  401 - 408  formed on a top surface  412 . Each of the grooves  401 - 408  is shown as a “v” groove, but may be of different cross sectional shape, and rather than being shaped as a “groove” may be a channel. By use of the term “channel”, it will be understood that various cross-sectional grooves is included. In the embodiment shown, the grooves or channels may be formed by use of a saw, or by etching or any other process that will permit controlled depth formation of channels. For example, the v-grooves may be formed by providing an oxide masking layer on the silicon substrate, utilizing a photolithography process to define each of the grooves, and applying an etchant to form the grooves  401 - 408 . After the grooves  401 - 408  are formed, the fiber segments  301 - 308  are placed in the grooves  401 - 408  with fixed spacing and are bonded in position with epoxy. The end surfaces  232 ,  333  of substrate  230  as well as the corresponding end faces of fiber segments  301 - 308  are coplanar and polished to optical quality. The corresponding end surface  222  of substrate  220  is likewise polished to optical quality. The fiber Bragg grating array  225  is aligned with the 1×N bi-directional optical variable coupler array  120  substrate  220  and bonded thereto. The bonding may with epoxy or any other method of bonding that provides good optical coupling.  
         [0024]    Turning now to FIG. 5, the apparatus of FIG. 2 is shown with 1×N bi-directional optical variable coupler array  120  shown in greater functional detail. 1×N bi-directional optical variable coupler array  120  is formed from an array of 1×2 optical switches  501 - 507  and waveguides  521 - 535 . Switches  501 - 507  are selectively operated by a microprocessor or micro-controller  550  that responds to wavelength signals indicating a desired optical wavelength and determines which optical switches  501 - 507  to operate to couple optical signals to corresponding fiber Bragg gratings  125  of array  225 . In operation, a wavelength selective detector  1005  is utilized to monitor output optical signals from circulator  100  and to provide signals representative of the power of each wavelength component at the output port  105  of circulator  100 . Micro controller  550  utilizes the signals receive from detector  1005  to control bias voltage driver  553  to adjust the level of optical signals at desired wavelength components reflected to circulator port  103 . Micro controller  550  determines the levels of the various wavelength components and may vary the levels. The variation in levels may be in accordance with predetermined levels that are provided to micro controller  550 , or in accordance with algorithms provided to micro controller  550 . The level of each wavelength component may be varied from zero to a maximum level and is determined by the operation of switches  501 - 507 . Operation of switches  501 - 507  is determined by the selective application of bias voltages to switches  501 - 507  by bias voltage driver  553 .  
         [0025]    [0025]FIG. 6 illustrates a 1×2 switch  501  that is appropriate for use in the 1×N bi-directional optical variable coupler array  120  of the invention. Switch  501  is a bi-directional, polarization independent 1×2 switch design. It includes a waveguide that forms a “y” having first, second and third waveguide legs  521 ,  522 ,  529 . The waveguides  521 ,  522 ,  529  are formed on a substrate utilizing known fabrication methods for forming optical waveguides on electro optic substrates such as LiNbO 3 . Switch  501  further includes three electrodes  601 ,  602 ,  603  that are used to determine the optical path through switch  501 . The application of bias voltage V to electrodes  601 ,  602 ,  603  determines the degree of coupling between waveguide portion  521  and waveguide portions  522  and  529 . The high voltage switch  501  can couple both TE and TM mode signals. Switch  501  has an on-off ratio of greater than  20  dB. In a reflective design, a double pass produces 40 dB of isolation. With this building block switch structure other sized switches may be provided. In operation, each switch  501  acts as a variable bi-directional coupler that is operated by appropriate selection of bias voltage to determine the amount of coupling between one port and two other ports.  
         [0026]    Although switch  501  is shown in detail in FIG. 6, each of the switches  501 - 507  is of the same construction and all are fabricated on a single substrate  220  in the illustrative embodiment. The waveguides  521 - 535  are formed utilizing any of the known techniques for formation of waveguides in electro-optic substrates.  
         [0027]    [0027]FIG. 7 illustrates another embodiment of the invention in which the reflective filters  525 - 535  are formed on the same substrate  720  as the 1×N Switch. The substrate is LiNbO 3  or another electro optic material. Each filter  725  is formed on a waveguide  525 - 528 ,  532 - 535  formed on substrate  720 . Each waveguide has a photosensitive region onto which a Bragg grating is formed. Operation of the structure of FIG. 7 is the same as that of FIG. 5.  
         [0028]    It should be apparent to those skilled in the art that although certain structures shown in the drawing figures illustrate only a 1×8 bi-directional optical variable coupler array and  8  wavelengths, the number of wavelengths and the size of the 1×N bi-directional optical variable coupler array is a matter of design selection to provide the desired number of selectable wavelengths. For example, 1×16 and 1×32 bi-directional optical variable coupler arrays can be built. If it is desired to accommodate a larger number of wavelengths, cascading several stages can accommodate more wavelengths. For example, to accommodate  128  wavelengths, a 1×4 bi-directional optical variable coupler array can be cascaded with four 1×32 bi-directional optical variable coupler arrays.  
         [0029]    Various other changes and modifications may be made to the illustrative embodiments of the invention without departing from the spirit or scope of the invention. It is intended that the invention not be limited to the embodiments shown, but that the invention be limited in scope only by the claims appended hereto.