Abstract:
Apparatus and methods for tuning an optical filter system of the sort including a filter element which is tuned according the angle of incidence (AOI) of light incident on the filter element by changing the angle of incidence of the light beam incident on the filter element, rather than by rotating the filter element itself. A scan relay changes the angle of the beam incident upon the filter element. An add/drop filter utilizes two scan relays and several mirrors to add, drop, and pass signals without rotating the filter element.

Description:
[0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/344,096, filed Dec. 20, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to apparatus and methods for tuning an optical filter by changing the angle of incidence of the light beam incident on the filter.  
           [0004]    2. Description of the Prior Art  
           [0005]    Certain optical filters (such as Fabre-Perot filters and thin-film interference filters) are tuned by changing the angle of incidence (AOI) of the input beam at the filter. Prior art AOI tuning methods and apparatus have required the filter itself to move in order to change the AOI and tune the filter to a particular frequency.  
           [0006]    [0006]FIG. 1 (Prior Art) is a side view of a conventional scan relay device  100 . Input beam  102  is incident upon scanning mirror  103 . As scanning mirror  103  rotates about an axis perpendicular to input beam  102 , resulting reflected beam  106  is transmitted at different angles. FIG. 1 shows two examples. Scanning mirror  103  position  104   a  (solid line) produces reflected beam  106   a  (solid line). Scanning mirror  103  position  104   b  (dotted line) produces reflected beam  106   b  (dotted line).  
           [0007]    Relay lenses  108  and  110  focus beam  106   a ,  106   b  to form output beams  112   a ,  112   b , respectively. All output beams pass through relayed scan point  114 , but they pass through it at various angels, determined by the angle  104  of mirror  103 .  
           [0008]    A need remains in the art for apparatus and methods for tuning an optical filter by changing the angle of incidence of the light beam incident on the filter, rather than by rotating the filter itself.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides apparatus and methods for tuning an optical filter by changing the angle of incidence of the light beam incident on the filter, rather than by moving the filter itself. Further, it provides apparatus and methods for utilizing such tuning apparatus and methods to accomplish a tunable add/drop filter for use in wavelength-division multiplexed (WDM) optical networks.  
           [0010]    A simple optical filter apparatus for selectively passing or reflecting an input optical signal includes a scan relay for selectively changing the angle of the input optical signal to form a scanned beam, and a filter element of the kind tuned according the angle of incidence (AOI) of light incident on the filter element, the filter element positioned to intercept the scanned beam. The filter element does not rotate with respect to the axis of the input signal.  
           [0011]    The scan relay might comprise a rotating mirror positioned to intercept the input signal and two lenses positioned to pass the reflected beam. The filter element might be a thin film interference filter (TFF) or a Fabry-Perot filter. A detector for detecting any light transmitted by the filter element, and for generating an output signal based on detected light may be included.  
           [0012]    An optical drop filter system of the type having an input port into which an input signal from a optical multiplexing system is transmitted, a pass port through which signals to be returned to the system, and a drop port, through which signals are removed from the system, includes a scan relay for selectively changing the angle of the input optical signal to form an input scanned beam, a filter element of the kind tuned according the angle of incidence (AOI) of light incident on the filter element, the filter element positioned to intercept the input scanned beam and to transmit the input scanned beam or reflect the input scanned beam according to the wavelength of the input scanned beam, and optics for directing any transmitted input scanned beam to the drop port. The first scan relay further directs the reflected input scanned beam to the pass port, and the filter element does not rotate with respect to the optical axis of the input signal.  
           [0013]    An optical add/drop filter of the type having an input port into which an input signal from a optical multiplexing system is transmitted, a pass port through which signals to be returned to the system are transmitted, a drop port, through which signals are removed from the system, and an add port, via which an add signal is injected into the system, the add/drop filter comprises a first scan relay for selectively changing the angle of the input optical signal to form an input scanned beam, a filter element of the kind tuned according the angle of incidence (AOI) of light incident on the filter element, the filter element positioned to intercept the scanned beam and to transmit the input scanned beam or reflect the input scanned beam according to the wavelength of the input scanned beam, and a second scan relay for directing any transmitted input scanned beam to the drop port. The first scan relay directs any reflected input scanned beam to the pass port. The second scan relay selectively changes the angle of any add optical signal to form an add scanned beam. The filter transmits the add scanned beam according to its wavelength, and the first scan relay directs the transmitted add scanned beam to the pass port. Again, the filter element does not rotate with respect to the optical axis of the input signal.  
           [0014]    In a particularly efficient embodiment of the present invention, the first scan relay and the second scan relay utilize the same optical elements. For example, the first scan relay and the second scan relay comprise two cylindrical scan lenses. Five mirrors are required to direct the beams. An array of filter panels may also be used, with an actuator selectively placing a filter array element in the path of the scanned beam. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 (Prior Art) is a side schematic view of a conventional optical scan relay.  
         [0016]    [0016]FIG. 2 is a side schematic view of a tunable optical filter according to the present invention using the scan relay of FIG. 1.  
         [0017]    [0017]FIG. 3 is a side schematic view of a tunable add/drop filter according to the present invention using the scan relay of FIG. 1.  
         [0018]    [0018]FIG. 4 is a side ray trace view of the add/drop filter of FIG. 3 configured to provide a relatively low angle of incidence (AOI) at the filter element.  
         [0019]    [0019]FIG. 5 is a side ray trace view of the add/drop filter of FIG. 3 configured to provide a relatively high AOI at the filter element.  
         [0020]    [0020]FIG. 6 a  is an isometric drawing of a second preferred embodiment of an add/drop filter according to the present invention.  
         [0021]    [0021]FIG. 6 b  is an isometric view of a variation on the embodiment of FIG. 6 a  wherein multiple filter elements are used in place of the single filter element of FIG. 6 a.   
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    [0022]FIG. 2 is a side schematic view of a tunable optical filter  200  using the scan relay  100  of FIG. 1. Input beam  102  is provided to scan relay  100 , which provides output beam  112  at a variety of angles, here angle  112   a  and  112   b . Filter  202  is an optical filter which is tuned to a particular wavelength according to the angle of incidence (AOI) of the light beam at the filter. For example, filter  202  could be a thin film interference filter (TFF) or a Fabre-Perot filter. Detector  204  detects the output beam from filter  202 , and provides an output signal  206  based on the detected beam.  
         [0023]    [0023]FIG. 3 is a side schematic view of a tunable add/drop filter  300  using two scan relays similar to scan relay  100  of FIG. 1. Add/drop filter  300  is suitable for use in wavelength-division multiplexed (WDM) optical networking. Add/drop filter  300  provides an input port A for input light  302  wavelengths.  
         [0024]    Wavelengths  304  which are provided back to the system (passed) are reflected from filter  316  and appear at output port B. Signals which are removed from the system for local use (dropped) pass through filter  316  and appear at port C. Signals which are provided to the system (added) are input into port D, pass through filter  316 , and form part of output signal  304 . Ports A-D are generally collimators. Scan mirrors  310   a ,  310   b ,  322   a  and  322   b  must all be synchronized in order to maintain coupling among all of the ports A-D.  
         [0025]    In use, input signal  302  passes through port A and reflects off of scan mirror  310   a . Its path is shown as a dashed line, following the lower path to the left of filter element  316 . From scan mirror  310   a , the input light reflects off of wedge shaped fold mirror  303 , through lenses  312  and  314 , and is incident upon filter element  316 . Portions (wavelengths) of input signal  302  which are to be passed reflect off of filter element  316 , back through lenses  314  and  312 , off mirrors  303  and  310   b , to appear at output port B as pass signal  304 . This passed signal travels the upper path to the left of filter  316  (dotted line).  
         [0026]    Portions of input signal  302  which are to be diverted for local use (dropped) pass through filter element  316 , lenses  318  and  320 , and reflect off mirrors  324  and  322   a  to appear at drop port C as drop signal  306 . This signal follows the upper path to the right of filter element  316  (dashed line).  
         [0027]    A signal  308  that is to be added into the system is input at add port D. After reflecting off of mirrors  322   b  and  324 , it passes through lenses  320  and  318 , passes through filter  316 , passes through lenses  314  and  312 , reflects off mirrors  303  and  310   b , and appears as part of output signal  304 . Added signal  308  follows the lower path to the right of filter  316  and the upper path to the left of filter  316  (dotted lines).  
         [0028]    The wavelengths destined to drop or pass are selected by rotating scan mirrors  310   a ,  310   b ,  322   a  and  322   b . This is done not to change the path of the light, but rather to change the angle at which the light is incident upon filter  316 . Since filter  316  is a narrow band filter tuned by the AOI of the incident light, this selects the wavelengths reflected and transmitted by the filter.  
         [0029]    Those skilled in the art will appreciate that a drop filter could be constructed in a very similar manner, by simply removing the add port D and associated optics.  
         [0030]    [0030]FIG. 4 is a side ray trace view of add/drop filter  300  of FIG. 3 configured to provide a relatively low angle of incidence (AOI) at filter element  316 . In this configuration, scan mirrors  310   a ,  310   b ,  322   a  and  322   b  are rotated toward a plane perpendicular to the axis of the light. FIG. 5 is a side ray trace view of add/drop filter  300  of FIG. 3 configured to provide a relatively high AOI at the filter element. In this configuration, scan mirrors  310   a ,  310   b ,  322   a  and  322   b  are rotated toward a plane parallel to the axis of the light.  
         [0031]    [0031]FIG. 6 a  is an isometric drawing of a preferred embodiment of add/drop filter similar in function to add/drop filter  300  of FIG. 3. In this embodiment, the scanning system has been folded by adding a pair of system fold mirrors,  622   a  and  622   b , and by using cylindrical scan lenses  612  and  614  in place of scan lenses  312 ,  314 ,  318  and  320 . Two spherical lenses stacked together could be used in place of a cylindrical lens. Hence, two scan relays are still used, but they utilize the same optical elements. The embodiment of FIG. 6 a  has several advantages. Only one set of scan mirrors  610   a ,  610   b  must be synchronously rotated, and hence a single rotating mechanism  624  may be used. In addition, all of the ports A-D are located in one area.  
         [0032]    In use, input signal  302  passes through port A (a collimator) and reflects off of scan mirror  610   b . Its path follows the upper left-hand path before filter element  316 . From scan mirror  610   b , the input light reflects off of wedge shaped fold mirror  603 , passes through lenses  612  and  614 , reflects off mirror  622   b  and is incident upon filter element  616 . Portions (wavelengths) of input signal  302  which are to be passed reflect off of filter element  616  and mirror  622   b , back through lenses  614  and  612 , off mirrors  603  and  610   a , to appear at output port B as pass signal  304 . This passed signal travels the upper right-hand path after reflection off filter  616 .  
         [0033]    Portions of input signal  302  which are to be diverted for local use (dropped) pass through filter element  616 , reflect off mirror  622   a , pass through lenses  614  and  612 , and reflect off mirrors  603  and  610   a  to appear at drop port C as drop signal  306 . This signal follows the lower right-hand path. A signal  308  that is to be added into the system is input at add port D. It reflects off mirrors  610   b  and  603 , passes through lenses  612  and  614 , reflects off mirror  622   b , passes through filter  616 , reflects off mirror  622   a , passes through lenses  614  and  612 , reflects off mirrors  603  and  610   a , and appears as part of output signal  304 . Added signal  308  follows the central path, shown as a dashed line.  
         [0034]    [0034]FIG. 6 b  is an isometric view of a variation on the embodiment of FIG. 6 a  wherein multiple filter panels  628  form a filter array  626  used in place of single filter element  616  of FIG. 6 a . Linear actuator  630  moves filter array back and forth in order to locate the selected filter element  628   a, b, c , etc., such that signal beams are incident upon it. Those skilled in the art will appreciate that a filter wheel with a rotating actuator, or a variety of other configurations and mechanisms, could be used to select which filter panel is used.