Abstract:
A device serving as an optical switch or attenuator is disclosed, based on the insertion of a halfwave plate in a collimated beam space between two lenses. All beams have first been aligned to a same polarization state by standard polarization diversity techniques. Moving the halfwave plate into or out of the beam incident thereon enables switching the beams direction based on polarization. Partial insertion of the waveplate attenuates the beam through rotation of polarization state for part of the beam. Preferably, polarizers are used to improve extinction ratio and wavelength flatness.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a polarization insensitive optical device, which can function as an optical switch or as an optical attenuator for application in fiber optics telecommunications.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical switches which steer beams controllably from a launch location to one of may possible destination ports are well known. Once a connection is established by ensuring that two port are optically aliened via an optical path, most switches are bi-directional allowing an optical signal to be propagated from a port to a designation or visa versa. Some of these switches perform a switching function by moving a reflecting or deflecting element into a signal path or light beam; alternatively other switches, perform switching in a more passive manner by varying the refractive index of a material or by rotating the polarization of a light beam prior to it being launched into a beam steering member such as a birefringent crystal directs a beam in dependence upon its polarization state. The latter type of switch typically uses a Faraday rotator for rotating the polarization of light passing therethough in dependence of an applied voltage. An optical switch of this form is described in U.S. Pat. No. 5,694,233 in the name of Wu. Although the switch described by Wu performs its intended function, the switch in accordance with this invention is believed to be advantageous as it allows multiple optical elements to be simultaneously inserted into the path providing additional functionality.  
           [0003]    It is a well known that fabricating an optical switch which is based on the insertion of a deflecting element or reflecting element is very difficult if the switch is to be polarization insensitive.  
           [0004]    It is known to provide an optical attenuator or optical switch in which the collimated beam gets intercepted with some blocking or redirecting means; for example, a moving mirror. It is also known to provide a switch where the polarization state gets rotated with some controllable retardance element such as a liquid crystal cell or an electromagnetically controlled Faraday rotator.  
           [0005]    However, the first class of prior art switch or attenuator based upon the interception of the beam with a mirror or a vane suffers from extremely tight positioning tolerances for the mirror. The second class of switches and attenuators based on liquid crystals or Faraday rotation of polarization suffers from the inability to provide high suppression ratio, especially over a large wavelength band.  
           [0006]    It is an object of this invention to provide with an insertion mechanism that has very loose positioning tolerances.  
           [0007]    A 0 th  order half waveplate retardance depends only marginally on its angular position, and since its thickness is very small, the overall optical impact of misalignment of this plate with respect to the collimated beam is negligible.  
           [0008]    It is a further advantage of this invention to provide a polarization rotation based optical device with a large extinction ratio over a broad wavelength range. This is achieved by the insertion of polarizers in the collimated beam path: a first one aligned with the polarization states of the sub-beams after the first lens and a second one to be inserted with the half waveplate and whose axis is perpendicular to that of the first polarized.  
           [0009]    In summary, it has been found that by inserting an element which rotates the beam&#39;s polarization state into an optical system having a birefringent polarization dependent beam steering block (BPDBS) offers significant advantages over the insertion of a deflecting or reflecting element into a beam&#39;s path; by inserting a polarization rotating element followed by a BPDBS the switch is substantially or nearly polarization insensitive.  
         SUMMARY OF THE INVENTION  
         [0010]    In accordance with the invention, an optical switching or attenuating mechanism is provided based on the insertion of a half waveplate in a collimated beam space, where all beams have first been aligned to a same polarization state by standard polarization diversity techniques. Moving the half waveplate into our out of the beam incident thereon enables switching the beams direction based on polarization, whereas partial insertion of the plate attenuates the beam through rotation of polarization state for part of the beam. In a preferred embodiment, polarizers are used to improve extinction ratio and wavelength flatness. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Exemplary embodiments of the invention will now be described in conjunction with the drawings in which:  
         [0012]    [0012]FIG. 1 is a top view of a polarization switch wherein the switching element is absent from between the back to back graded index (GRIN) lenses;  
         [0013]    [0013]FIG. 2 is a top view of the polarization switch shown in FIG. 1 having a half waveplate fully inserted between the back to back graded index (GRIN) lenses;  
         [0014]    [0014]FIG. 3 is a side view of the switch shown in FIG. 1 absent the switching element between the GRIN lenses;  
         [0015]    [0015]FIG. 4 is a side view of the switch shown in FIG. 2 having a half waveplate fully inserted between the back to back graded index (GRIN) lenses;  
         [0016]    [0016]FIG. 5 is a top view of the switch shown in FIG. 1 and including a polarizing filter between the two GRIN lenses;  
         [0017]    [0017]FIG. 6 is a top view of the switch shown in FIG. 2 including a polarizing filter between the two GRIN lenses;  
         [0018]    [0018]FIG. 7 is a side view of the optical switch shown in FIG. 5;  
         [0019]    [0019]FIG. 8 is a side view of the switch shown in FIG. 6;  
         [0020]    [0020]FIGS. 9 and 10 are side views of the optical switch wherein the switch can operate in an attenuating mode rather than a full switching mode of operation;  
         [0021]    [0021]FIGS. 11 and 12 are side views of the optical switch shown in FIGS. 9 and 10 wherein the switch is configured as an optical attenuator having a half waveplate and crossed polarizing filter inserted partially into the beam&#39;s path; and,  
         [0022]    [0022]FIG. 13 is an isometric view of a half waveplate inserted within rails between two, rod GRIN lenses. 
     
    
     DETAILED DESCRIPTION  
       [0023]    Referring now to FIGS. 1 and 2 an embodiment of an optical switch in accordance with this invention is shown, wherein a first polarization diversity block (PDB)  10   a  is optically coupled with a second polarization diversity block  10   b  via a pair of spaced back-to-back collimating GRIN lenses  16   a  and  16   b . The first PDB  10   a  comprised of a birefringent crystal  12   a  optically coupled and adjacent to a polarization rotating element  14   a  such as a half waveplate. The second PDB is similarly constructed and is comprised of a birefringent crystal  12   b  optically coupled and adjacent to a polarization rotating element  14   b  such as a half waveplate. In FIG. 2 a half waveplate (HWP)  18  is shown inserted into the path: means in the form of a guide rail and actuator to for moving the HWP  18  into or out of the path of the beam are not shown in this figure. Referring now to FIGS. 1 and 3 a beam of unknown polarization is launched into the PDB  10   a  and is split into-two sub-beams having orthogonal polarization states as it passes through the birefringent crystal  12   a ; subsequently, only one of the two sub-beams is passed through a rotator  14   a  and is rotated by 90° so that its polarization matches that of the other sub-beam. FIG. 3 is illustrative of this and also depicts the two sub-beams being re-combined at the output end after passing through the PDB  10   b ; in this instance the beam essentially passes through all of the components as it would have following a straight-through path from port  1  to port  2 . Referring now to FIGS. 2 and 4 the sub-beams after passing through the HWP  18  have their polarization rotated by 90° and therefore walk-off after passing through the walk-off crystal  12   c  which steers the beams toward port  3 . Prior to being incident upon port  3  the sub-beams are combined into a single beam of mixed polarization by the PDB  10   b . Thus, controllable insertion or removal of the HWP  18  determines whether the beam launched into port  1  will be incident upon port  3  or  2  respectively and a 1×2 switch is provided.  
         [0024]    The embodiment shown in FIGS. 5 and 6 is advantageous in that it provides a polarization rotation based optical device with a large extinction ratio over a broad wavelength range. This is achieved by the provision of polarizers  20  and  22  in the collimated beam path: a permanent polarization filter  20  is aligned with the polarization states of the sub-beams propagating through the first GRIN lens  16   a  after the first lens; a second movable polarizing filter optically coupled with the HWP  18  is inserted with the HWP  18 ; the axis of the filter  22  is perpendicular to that of the first polarizing filter  20 . The provision of the first filter  20  provides additional isolation or filtering of the polarized sub-beams exiting the GRIN lens  16   a ; The provision of the second filter  22  provides additional isolation or filtering of the polarized sub-beams entering the GRIN lens  16   b ; essentially  20  is disposed to correct for PDB preceding it, and  22  is disposed to corrects for the drop-in HWP  18  by providing additional filtering. FIGS. 7 and 8 are side views of the embodiment shown in FIGS. 5 and 6.  
         [0025]    [0025]FIGS. 9 and 10 illustrate an embodiment of the invention wherein an attenuator is provided instead of a switch. Light is launched from port  1  to port  2  as shown in FIG. 9. The input beam launched into the device at port  1  is of mixed random polarization and the output beam at port  2  is of mixed polarization. Absent any unwanted coupling losses all of the light launched into port  1  propagates to port  2 . Referring now to FIG. 10 HWP  18  with the filter  22  is shown partially inserted into the beam such that a portion of the beam propagates through the HWP and filter  22 . This is shown in an exploded view of the beam. Since only a portion of the beam passes through the HWP  18 , filter  22  combination, the remaining other portion is un-attenuated and propagates to port  2 . The insertion of the filter combination has the effect of “spilling-off” light away from its destination port  2 . FIGS. 11 and 12 are side views of the attenuator shown in FIGS. 9 and 10.  
         [0026]    In operation the attenuator functions in the following manner. The input beam launched into port  1  impinges upon the first walk-off crystal  12   a  in order to separate the incoming beam into two sub-beams. A first polarization rotator  14   a  is used to align the polarization states of the two sub-beams. Those two sub-beams are then passed through lens  16   a  to collimate these sub-beams at a location between lens  16   a  and lens  16   b . In the collimated path, some space is accommodated in order to be able to drop-in the half waveplate  18  and/or polarizers. The sub-beams are then refocused by the second lens  16   b . After propagating through the GRIN lens  16   b , they traverse the walk-off crystal  12   c , whose deflection direction is different from that of the first walk-off crystal  12   a . The two sub-beams go through a second polarization rotator  14   b  in order to have two orthogonal polarization states for each of the two sub-beams. The third walk-off crystal  12   b , whose deflection direction is essentially the same as that as the first walk-off crystal, is used to recombined the two sub-beams into an output beam of light coupled to a output optical waveguide. Variable and controllable optical attenuation is obtained through partial insertion of the half waveplate in the collimated beam of light.  
         [0027]    When a second output port is connected to the third walk-off crystal  12   b  at a position corresponding to that of the two sub-beams when they are deflected by the second walk-off crystal  12   c , a 1×2 switch is provided. The two output ports are selectable by inserting or removing the half waveplate in the collimated path. One can have a first polarizer whose polarization is aligned with that of the two sub-beams, and or a second cross polarizer attached to the half waveplate in order to improve extinction ratio and decrease wavelength dependency of the switch.  
         [0028]    [0028]FIG. 13 illustrates one method of controlling the HWP  18  which can conveniently be placed between rails  25   a  and  25   b . An actuator, such as a controllable piston or any other form of actuator that is typically used for moving a shutter in our out of a path in an optical device, controllably can be used. An arrow is shown representing the actuator. Of course control circuitry coupled with the actuator is provided by is not shown. The control circuitry is programmed to either operate in a switching mode by fully inserting or fully removing the HWP  18  from the path of the beam passing between the lenses  16   a  and  16   b . Alternatively the control circuitry can be programmed to partially insert or remove the HWP  18  from the path upon receiving a control signal. Feedback circuitry including a properly disposed detector can be provided to ensure a certain level of attenuation of the beam passing therethrough.  
         [0029]    Although the invention described in detail heretofore relates to the controlled insertion of a polarization rotation means in sub-beams of light to realize an optical attenuator or an optical switch, in which the sub-beams originates from the same input beam and whose polarization states have been aligned, other embodiments can be envisaged.