Abstract:
A wide range tunable filter is provided. A randomly polarized incoming beam is converted into two orthogonally polarized beams. A ½ wave plate and filter block turns these two beams into four beams. Two of the four beams have a single range of wavelengths and two beams have the remaining wavelengths. Each pair of beams is orthogonally polarized. A ½ wave plate and birefringent crystal positioned after the filter block combine the two beams having the single range of wavelengths and combines the two beams having the remaining wavelengths. The invention includes a thermal compensator to correct the angle of the filter with respect to the incident light.

Description:
[0001]    This application claims priority to Provisional Patent Application Serial No. 60/273,107, titled “A Wide-Range Tunable Filter,” filed Mar. 2, 2001, and incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to optical communication, and more specifically, it relates to tunable add/drop filters used in optical communication.  
           [0004]    2. Description of Related Art  
           [0005]    In optical networks, a variety of methodologies have been provided in the prior art for optical switching and interconnecting of the transport network layers.  
           [0006]    Cheung, (“Acousto-Optic Tunable Filters in Narrowband WDM networks: System Issues and Network Applications,” IEEE J. Sele. Area Comm. 8(6), 1015, 1990.) uses four 1×N demultiplexers and N&#39;s 2×2 optical switches. The structure is complicated and the interconnections are difficult.  
           [0007]    An add/drop filter has been proposed by Glance at AT&amp;T. (Glance, “Tunable add/drop optical filter providing arbitrary channel arrangement”, IEEE Photon. Lett, 7(11), 1303, 1995 and U.S. Pat. No. 5,488,500.) This filter seeks to provide the advantage of arbitrary channel arrangement, but still suffers a costly 6 dB optical coupling loss, because of the two-array waveguide grating demultiplexers used in the structure.  
           [0008]    Another type of wavelength-space switch (Dono et al, “A wavelength division multiple access network for computer communication”, IEEE J. Sol. Area Comm., 8(6), 983, 1990.) has been widely used in various WDM networks, for example the IBM Rainbow Network. This structure uses a passive star-coupler that combines and splits the incoming light signals into N receivers. The receivers are built with a tunable filter and select the desired channels. It has the broadcast capability and the control structure of this implementation is very simple. However, an undesirable feature of the broadcast star is that the splitting loss can be very high when the users number is large.  
           [0009]    U.S. Pat. No. RE037,044, titled “Tunable Add/Drop Optical Filter” describes a tunable optical add/drop filter for all-optical wavelength-division-multiplexing (WDM) network applications. This filter can add or drop part of the high transmission capacity signals of a WDM link. It is intended for use to decentralized access points in the access network or as a small core network node to realize branching points in the network topology. It is intended to work in both the wavelength and space domains.  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the present invention to provide embodiments of a wide band tunable filter.  
           [0011]    It is another object of the invention to provide techniques for maintaining the beam propagation directions over the wavelength tuning range of a tunable filter.  
           [0012]    It is another object of the invention to provide techniques for maintaining the beam separation over the wavelength tuning range of a tunable filter.  
           [0013]    Still another object is to provide a four-port device that can add and drop wavelength channels simultaneously.  
           [0014]    Another object of the invention is to provide a thermal compensator to compensate for heat induced pass-band changes.  
           [0015]    These and other object will be apparent to those skilled in the art based on the disclosure herein.  
           [0016]    According to the present invention, in one embodiment of a tunable filter, a randomly polarized incoming beam normally incident upon a birefringent crystal is separated into separate o- and e-ray beams. The E-field (polarization) orientation of the o-ray, after emerging from the crystal, is orthogonal to that of the e-ray. A ½ wave plate placed in one beam causes the two rays to have the same polarization direction. A filter block comprising optically transparent material, and further comprising a mirror and a bandpass filter is placed in the path of the o-ray and the e-ray. Wavelengths within the passband of the filter are passed for the o- and e-rays. The wavelengths not within the passband are reflected from the filter and reflected by the mirror to produce second o-ray and e-ray beams that include all of the light that was not passed by the passband filter. Thus, the filter block turns the o-ray into a top o-ray and a bottom o-ray and turns the e-ray into a top e-ray and a bottom e-ray. In this embodiment, the bottom rays carry light of wavelengths that are within the filter&#39;s pass band and the top rays carry the rest of light.  
           [0017]    A birefringent crystal is positioned after the filter block. A ½ wave plate is attached to the crystal to intercept the top e-ray and bottom e-ray, but not to intercept the top o-ray and the bottom o-ray, The ½ wave plate rotates the polarization direction of top e-ray and bottom e-ray to be orthogonal to the polarizations of the top o-ray and the bottom o-ray. The top o-ray and top e-ray combine in the second birefringent crystal to produce a combined top beam. The bottom o-ray and bottom e-ray combine in the crystal to form a combined bottom beam.  
           [0018]    An embodiment of the invention is thus a three-port device. From one input beam, the invention produces two output beams. The input port carries all the wavelengths injected into the system and the bottom output port carries the light with wavelengths that pass through the filter. The top output port carries the light reflected by the filter.  
           [0019]    Embodiments of the invention include configurations where light reflected by the mirror impinges on a second drop filter, which passes certain wavelengths and reflects all the others. The mirror can be formed from a coating over the incidence surface of filter block, which surface has a non-coated portion to allow entry of the o and e rays. The exit surface of the filter block can have a plurality of bandpass filters positioned to successively pass selected wavelengths. In an alternate embodiment, a series of the devices of FIGS. 1A and 1B are connected to the top output port to operate as a demultiplexer. These principles apply to the embodiments provided below as well.  
           [0020]    The wavelength in the bottom output port is determined by the transmission band of the filter, which can be adjusted by changing the incident angle to the filter. The disadvantage is that, as the incident angle changes, the two output beams shift laterally. This problem is fixed by adding a plane parallel plate (dummy block) in the optical path. The material and thickness of dummy block are the same as those of the filter block. Adding another plane parallel plate to the bottom beam path compensates for the thickness of filter. When the filter block is rotated, the dummy block is correspondingly rotated in the opposite direction. Such arrangement will guarantee that the bottom beam remains in the same location no matter how the filter block is rotated.  
           [0021]    The invention includes embodiments that substitute a first and second polarizing beamsplitter (PBS) and mirror combination for the birefringent crystals of the above-described embodiment.  
           [0022]    Another embodiment is provided which places a bandpass filter coating on the input side of the filter block. Wavelengths within the band pass of the filter coating are transmitted through the filter and the remaining wavelengths are reflected to a corner cube. The corner cube reflects light incident thereon back to a reflective coating placed on the input side of the filter block. The light reflected from this mirror coating is reflected back towards the input direction. This embodiment is a three-port device.  
           [0023]    A 4-port device is provided, which can add and drop wavelengths simultaneously. In this embodiment, the filter block includes a filter coating, an attached mirror and an attached mirror block. The mirror block has an attached mirror. A dummy block with an adherent dummy glass is operatively positioned next to mirror block. In operation, an input beam passes through the filter block and impinges on filter coating. Light having wavelengths within the passband of filter coating will pass through the filter coating and will pass through the dummy glass. The remaining light will be reflected by the filter coating and be further reflected by the mirror, from which the light will propagate through the dummy block. In one embodiment, the filter coating is designed to reflect a narrow wavelength band, such that a single wavelength is reflected therefrom. The reflected channel becomes the main channel of the device. An added channel is introduced into the system by injecting a beam through filter block and mirror block so that the beam reflects from the mirror attached to the mirror block and is made collinear with the other beam reflected from the filter and propagates therewith out of the system.  
           [0024]    The separation between e- and o-ray are not necessary when the incident angle is close to normal incident Since under that condition, the filter response to P- and S-polarization is about the same. The disadvantage of operating in the small incident angle is that the tuning range is small. The advantage is that one does not need to convert the incident polarization into a pure S- or P-state.  
           [0025]    Assuming that when the incident angle onto the filter is fixed, higher temperatures shift the filter pass-band to the longer side. The invention includes a thermal compensator to correct the angle of the filter with respect to the incident light  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1A is a top view of a tunable filter of the present invention.  
         [0027]    [0027]FIG. 1B is a side view of the tunable filter of FIG. 1A.  
         [0028]    [0028]FIG. 2 shows a plane parallel plate (dummy block) added in the optical path.  
         [0029]    [0029]FIG. 3A shows the estimated wavelength shift rate from the center pass band of a filter as a function of incident angle (in air) based on Equation (1).  
         [0030]    [0030]FIG. 3B shows the normalized beam separation, δ/d, as a function of angle of incidence for φ over the range of 0° to 80°.  
         [0031]    [0031]FIG. 3C shows an enlarged view of the area of FIG. 3B for φ over the range of 45° to 55°.  
         [0032]    [0032]FIG. 4 shows a top view of an embodiment that substitutes a first and second polarizing beamsplitter (PBS) and mirror combination for the wave plates and crystals of the embodiment shown in FIGS. 1A and 1B.  
         [0033]    [0033]FIG. 5 shows another embodiment where the main beam is collected by a corner cube.  
         [0034]    [0034]FIG. 6 shows a 4-port device, which can add and drop channels simultaneously.  
         [0035]    [0035]FIG. 7 shows a thermal compensator that is usable in the present invention.  
         [0036]    [0036]FIG. 8 shows a frame of tunable filter using a plan-parallel-plate formed wedge. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    [0037]FIG. 1A is a top view and FIG. 1B is a side view of the structure of a tunable filter. An incoming beam  10  having a random polarization is normally incident on a birefringent crystal  12  having an optical axis  14  that is oriented in the X-Y plane as shown in FIG. 1A. After passing through the crystal, the o- and e-ray are spatially separated. The optical path of the o-ray  16  is propagating in its original direction and the e-ray  18  deviates with respect to the o-ray. The E-field (polarization) orientation of o-ray  16  right after emerging from the crystal  12  is along the Z-direction and that of e-ray  18  is along Y-direction (i.e., they are orthogonal). A ½ wave plate  20  is positioned to intercept o-ray  16 , but not e-ray  18 . The fast axis of ½ wave plate  20  is oriented at 45° degrees from the Y-axis within the Y-Z plane (between the polarization direction of o-ray  16  and e-ray  18 ). This causes the E-field of the o-ray  16  to rotate 90 degrees to the Y-direction. The two rays thus have the same polarization direction when they are incident on the filter block  22 . Filter block  22  comprises an optically transparent material  24  such as glass, and further comprises a mirror  26  and a filter  28 . The figure shows the mirror  26  and the filter  28  fixedly attached to the optically transparent material  24 , however, it is not required that these elements be attached.  
         [0038]    [0038]FIG. 1B is a side view of FIG. 1A. Thus, o-ray  16  is shown to obstruct the view of e-ray  18 . Filter block is placed at an angle with respect to o-ray  16  and e-ray  18 , and both rays travel similar paths parallel to each other. O-ray  16  and e-ray  18  are incident on filter block  22  at an angle and propagate through filter block  22  to filter  28 . Any wavelengths of light that are within the pass band of filter  28  propagate through filter  28 . The remaining light that is not within the pass band of filter  28  is reflected by filter  28  and propagates back through filter block  22  to mirror  26 , which reflects the light out of the filter block. Thus, filter block  22  turns o-ray  16  into top o-ray  30  and bottom o-ray  32  and turns e-ray  18  into top e-ray  34  and bottom e-ray  36 , the views of which are obscured in FIG. 1B by top o-ray  30  and bottom o-ray  32 . The bottom rays carry light of wavelengths that are within the filter&#39;s pass band and the top rays carry the rest of light.  
         [0039]    Referring again to FIG. 1A, top o-ray  30  obscures the view of bottom o-ray  32  and top e-ray  34  obscures the view of bottom e-ray  36 . A birefringent crystal  40 , with its optical axis  41  oriented in the same direction as optical axis  14 , is positioned after the filter block  22 . A ½ wave plate  42 , with its fast axis oriented 45 degrees from the Y axis in the Y-Z plane is attached to the crystal  40 , to intercept top e-ray  34  and bottom e-ray  36 , but not to intercept top o-ray  30  and bottom o-ray  32 . The ½ wave plate  42  rotates the polarization direction of top e-ray  34  and bottom e-ray  36  to be in the Z-direction right before they enter crystal  40 . Since top o-ray  30  and bottom o-ray  32  do not pass through the ½ wave plate, their direction of propagation is in the Y-direction right before they enter the crystal  40 . As shown in FIGS. 1A and 1B, top o-ray  30  and top e-ray  34  combine to produce a combined top beam  50 . As shown only in FIG. 1B, bottom o-ray  32  and bottom e-ray  36  combine to form a combined bottom beam  52 . The view of combined bottom beam  52  is obscured by combined top beam  50  in FIG. 1A.  
         [0040]    An embodiment of the invention is thus a three-port device. From one input beam  10 , the invention produces two output beams are  50  and  52 . The input port carries all the wavelengths injected into the system and the bottom output port carries the light with wavelengths that pass through the filter. The top output port carries the light reflected by the filter (referring to FIG. 1B).  
         [0041]    As required, detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely exemplary of the present invention that may be embodied in various systems. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to variously practice the present invention.  
         [0042]    It should be recognized by those skilled in the art that embodiments of the invention include configurations where light reflected by mirror  26  impinges on a second drop filter, which passes certain wavelengths and reflects all the others. The mirror  26  can include a coating over the incidence surface of filter block  22 , which surface has a non-coated portion to allow entry of the o and e rays. The exit surface of the filter block can have a plurality of bandpass filters positioned to successively pass selected wavelengths. In an alternate embodiment, a series of the devices of FIGS. 1A and 1B are connected to the top output port to operate as a demultiplexer. These principles apply to the embodiments provided below as well.  
         [0043]    Filter Tunability  
         [0044]    The wavelength in the bottom output port is determined by the transmission band of the filter, which can be adjusted by changing the incident angle to the filter. In FIG. 1B, if the filter block  22  is slightly rotated along its Y-axis with the pivot at any point in the XZ plane, the angle of incidence of o-ray  16  and e-ray  18  to the filter  28  is changed. The disadvantage of the setup shown in FIGS. 1A and 1B is that, as the incident angle changes, the two output beams shift laterally. The setup shown in FIG. 2 fixes this problem. In FIG. 2, a second dummy block  60  is added in the optical path. The material and thickness of dummy block  60  are the same as these of filter block  22 . Dummy glass  62  is added to the bottom beam path to compensate for the thickness of filter  28 . When the filter block is rotated, the dummy block  60  is correspondingly rotated in the opposite direction. Such arrangement will guarantee that the bottom beam remains in the same location no matter how the filter block is rotated.  
         [0045]    Equation 1 estimates the dependence of wavelength shift as a function of incident angle.  
         2 n*d  cosθ= mλ   
         sin φ= n  sinθ         ⇒       Δ                 λ       Δ                 φ         =     -     λ        [       sin                 φcos                 φ         n     *   2       -       sin   2        φ         ]                                 
         [0046]    θ: incident angle in the spacer layer of FP filter  
         [0047]    φ: incidnt angle in the air  
         [0048]    n*: effective refraction index  
         [0049]    [0049]FIG. 3A shows the estimated wavelength shift rate from the center pass band of filter  28  as a function of incident angle (in air) based on Equation (1). At zero degrees incidence, the wavelength shift rate is zero. It is seen that around 50° of incident angle, the wavelength shift rate is maximized, which is about 5 nm/degree. To cover the whole C-band of telecommunication system (1525 nm to 1565 nm),) the filter block has to rotate ±30°.  
         [0050]    The separation between the top and the bottom beam is expressed as follows.  
         δ=2 d  tanψ cosφ 
           n  sinψ=sinφ         ⇒   δ     =       2      d                 sin                 φ                 cos                 φ           n   2     -       sin   2        φ                                   
         [0051]    ψ: incident angle inside the substrate of filter block  
         [0052]    n: refraction index of the substrate of filter block  
         [0053]    d: filter block thickness  
         [0054]    [0054]FIG. 3B shows the normalized beam separation, δ/d, as a function of angle of incidence for φ over the range of 0° to 80°. FIG. 3C shows an enlarged view of the area of FIG. 3B for φ over the range of 45° to 55°. It is seen that the maximum beam separation occurs at φ˜49.5°, with δ/d=0.778. When the incident angle is off 3° from 49.5°, δ/d becomes 0.774, which drops about 0.5% from its peak value. By comparing FIG. 3A to FIGS. 3B and 3C, it can be seen that the peak wavelength shift and peak beam separation occurs almost at the same incident angle. With a 3 mm thick filter block, the maximum beam separation in the output port is about 2.334 mm. When the system is operated at 50°±3° of incident angle, the beam separation is in the range of 2.322 to 2.334 mm. The insertion loss due to the 12 μm range beam walk is less than 0.1 dB.  
         [0055]    One embodiment of the present invention is a tunable filter that has a filter operation angle of 50°±3°. The filter block substrate could be fused silica having a thickness of 3 mm. The wavelength tuning range of such a device can be designed at 30 nm and the separation of the two beams would be 2.334 mm.  
         [0056]    [0056]FIG. 4 shows a top view of an embodiment that substitutes a first and second polarizing beamsplitter (PBS) and mirror combination for the crystals  12  and  40  of the embodiment shown in FIGS. 1A and 1B. An incoming beam  70  having a random polarization is normally incident on a PBS  72 . The horizontally (P-) polarized beam  74  passes through PBS  72  and the vertically (S-) polarized beam  76  is reflected. The beam  76  reflected from PBS  72  is again reflected from reflector  78  and then passes through ½ wave plate  80 , which is oriented to rotate the vertically polarized beam  76  to the horizontal polarization. The beams  74  and  76  pass through filter block  82  which operates on beams  74  and  76  in the same way that filter block  22  operated on beams  16  and  18  in the embodiment of FIGS. 1A and 1B. Thus, beam  74  is acted upon by filter block  82  to produce a top beam  84  and a bottom beam  86  (obscured in this view by top beam  84 ). Thus also, beam  76  is acted upon by filter block  82  to produce a top beam  88  and a bottom beam  90  (obscured in this view by top beam  88 ). Horizontally polarized beams  88  and  90  pass through PBS  92 . The ½ wave plate  94  rotates the horizontal polarization of beams  84  and  86  to the vertical position so that after reflection from reflector  96 , beams  84  and  86  are reflected from PBS  92  such that top beam  84  combines with top beam  88  to produce top output beam  98  and bottom beam  86  combines with bottom beam  90  to produce bottom output beam  100  (obscured in this view by beam  98 ). Thus, the embodiment of FIG. 4 is a three-port device.  
         [0057]    [0057]FIG. 5 shows another embodiment where the main beam is collected by a corner cube. The incident beam  100  impinges on the filter coating  102 , which is located on filter block  104 . Only the light ( 100 ′) having wavelengths within the passband of the filter coating  102  can pass through the filter coating. All the rest of light ( 100 ″) is reflected by the filter coating and then hits the corner cube  106 . The corner cube  106  sends the light  100 ″ back to the filter block  104  where it is reflected by the mirror coating  108  on the filter block  104 . The reflected light  100 ″ is directed to the output channel (mainstream). Wavelength tuning is achieved by rotating the filter block  104  with pivot at the intersection between the filter coating  102  and the incident beam  100 . At the same time, the corner cube  106  is rotated 2*theta with the same pivot point (i.e., twice the angular rotation as the filter block  104 ). Under such conditions, the incident angle to the corner cube s stays the same while the filter block is rotating. (The corner cube does not need to be laterally displaced if it is rotated with the pivot at the intersection between the filter coating and the incident beam.) Therefore, the separation between the incident beam  100  and the second output beam (main stream)  100 ″ remains unchanged when the filter block  104  is rotated. As in the embodiment of FIG. 2, the dummy block  110  rotates at an angle theta but in the opposite direction to keep the dropped beam  100 ′ at the same position. As in the embodiments of FIG. 1A, 1B and FIG. 4, this embodiment can be used with birefringent filters and/or polarizing beamsplitters to separate and recombine the o ray and e ray polarization components.  
         [0058]    [0058]FIG. 6 shows a 4-port device, which add and drop wavelength channels simultaneously. Filter block  120  has an adherent filter coating or separated filter piece  122  and a mirror  124  and mirror block  126 , which has an attached mirror  128 . A dummy block  130  with an adherent dummy glass  132  is operatively positioned next to filter block  120 . In operation, an input beam  134  passes through filter block  120  and impinges on filter coating  122 . Light ( 136 ) having wavelengths within the passband of filter coating  122  will pass through the filter coating  122  and will pass through dummy glass  132 . The remaining light  138  will be reflected by filter coating  122  and be further reflected by mirror  124 , from which the light  138  will propagate through the dummy block  130 . An added channel is introduced into the system by injecting a beam  140  through filter block  120  and mirror block  126  so that beam  140  reflects from mirror  128  and is made collinear with beam  138  and propagates therewith out of the system. As in the embodiments of FIG. 1A, 1B, FIG. 4 and FIG. 5, this embodiment can be used with birefringent filters and/or polarizing beamsplitters to separate and recombine the o ray and e ray polarization components.  
         [0059]    In all of the tunable devices described herein, one can remove all the polarization control elements when the device is operating at a small incident angle. When the device is designed for small angle operation, the tuning range is smaller.  
         [0060]    [0060]FIG. 7 shows a thermal compensator that is usable in the present invention. Normally, the angle of incidence of the filter is set by the screw  150  position, which is controlled by the screw controller  152 . As the temperature increases, the length of the thermal compensator  154  increases due to thermal expansion. This makes the angle of incidence of the beam  156  onto the filter  158  to decrease. Assuming that when the incident angle is fixed, higher temperatures shift the filter pass-band to the longer side. Since the incidence angle accordingly decreases, the pass-band wavelength of the device will stay the same. The figure also shows the beam  156  as it propagates through filter block  160  and impinges on drop filter  158 . Light  162  that has wavelengths that are within the passband of the filter  158  passes therethrough. Light  164  that reflects from drop filter  158  is then reflected from mirror  166 . In the illustrated embodiment, the center of rotation  166  is at the bottom of filter block  160 . As in the embodiments of FIG. 1A, 1B, and FIGS.  4 - 6 , this embodiment can be used with birefringent filters and/or polarizing beamsplitters to separate and recombine the o ray and e ray polarization components.  
         [0061]    In conclusion, the tunable optical filter of the present invention utilizes the wavelength tunability of bandpass filters and controls the filter incident angle to tune the central wavelength of the filter. Polarization elements are used to make the incident polarization either pure S- or P-polarized. The polarization element includes walk-off crystals, PBSs and wave plates. Plane-parallel plates are used to make the propagation direction of the two output beams independent with the incident angle. Another alternative is to use a plane-parallel-plate formed wedge as a tunable filter frame, shown in FIG. 8, to maintain the propagation direction of the two output beams independent with the incident angle. This scheme is useful to enlarge the separation between the two output beams. By using dual plane-parallel plate  170 ,  172 , the position of the dropped beam  174  is made to be independent of the incident angle of incident beam  173 . Plate  170  has an attached mirror  178 . Plate  172  has an attached bandpass filter  180 .  
         [0062]    Using the filter block shown in FIG. 2 or the plane-parallel-plate formed wedge as a tunable filter frame, the directions of the two output beams are independent with the angle of incidence. The location variation of the main beam can be minimized by properly choosing the operation range of the incident angle. In FIG. 2, properly choosing the pivot of rotation makes the beam location on the filter to be independent of the incident angle. When the incident angle is around 45 degrees, the pivot is about d/3 from the front surface of the filter block.  
         [0063]    The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims.