Abstract:
An optical device includes a walk-off plate, a lens, a half wave plate, a reflective device, and a non-reciprocal device. The walk-off plate is adapted for coupling to a first port and a second port. The half wave plate is positioned between the walk-off plate and the lens. The half wave plate is also configured to change the polarization of the light received from the first port by a first angle. The non-reciprocal device is positioned between the lens and the reflective device, and the non-reciprocal device is also configured to rotate light passing therethrough by a second angle.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to optical technology.  
           [0002]    Optical isolators, variable optical attenuators, and tap monitors are commonly used in optical communication systems and optical measurement systems. An optical isolator is a device generally designed to allow a beam of light to pass through the device in a chosen direction and to prevent the beam of light from passing through the device in the opposite of that chosen direction. A variable optical attenuator is a device generally designed in such a way that the power ratio between a light beam exiting from the device and a light beam entering the device can be adjusted over a variable range. A tap monitor is a device generally designed to monitor the power of a light beam exiting from the device or to monitor the power of a light beam entering the device.  
         SUMMARY OF THE INVENTION  
         [0003]    In one aspect, the invention provides an optical device. The optical device includes a walk-off plate, a half wave plate, a lens, a non-reciprocal device, and a reflector. The walk-off plate is configured to receive a first polarized light as an o-ray from a first port, and to transmit a second polarized light as an o-ray to enter a second port. The half wave plate is configured to receive the first polarized light from the walk-off plate for changing the polarization of the first polarized light by a first angle. The lens is configured to receive the first polarized light from the half wave plate and to transmit the second polarized light into the walk-off plate. The non-reciprocal device is configured to receive the first polarized light from the lens and to transmit the second polarized light into the lens. The non-reciprocal device is also configured to rotate light passing therethrough by a second angle. The reflector is configured to reflect the first polarized light received from the non-reciprocal device to reenter the non-reciprocal device as the second polarized light.  
           [0004]    In another aspect, the invention provides an optical device. The optical device includes a walk-off plate, a lens, a half wave plate, a reflective device, and a non-reciprocal device. The walk-off plate is adapted for coupling to a first port and a second port. The half wave plate is positioned between the walk-off plate and the lens. The half wave plate is also configured to change the polarization of the light received from the first port by a first angle. The non-reciprocal device is positioned between the lens and the reflective device. The non-reciprocal device is also configured to rotate light passing therethrough by a second angle.  
           [0005]    In another aspect, the invention provides a method of directing a first polarized light received from a first port to enter a second port as a second polarized light. The method includes the the following steps: (1) the step of passing the first polarized light through a walk-off plate to enter a half wave plate; (2) the step of passing the first polarized light through the half wave plate to change the polarization of the first polarized light by a first angle and to enter a lens; (3) the step of collimating the first polarized light through the lens to enter a non-reciprocal device; (4) the step of rotating the polarization of the first polarized light by a second angle including passing the first polarized light through the non-reciprocal device; (5) the step of reflecting the first polarized light incident upon a reflective device back as a second polarized light; (6) the step of rotating the polarization of the second polarized light by the second angle including passing the second polarized light through the non-reciprocal device to rotate and to enter the lens; (7) the step of collimating or directing the second polarized light through the lens to enter the walk-off plate; and (8) the step of passing the second polarized light through the lens to enter the second port.  
           [0006]    Among the advantages of the invention may include one or more of the following. Implementations of the invention provide an optical isolator, a variable optical attenuator, and a tap monitor that can have small insertion loss, compact size, and reduced manufacturing cost. Other advantages will be readily apparent from the attached figures and the description below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 a  illustrates an implementation of an optical isolator in the y-z plane.  
         [0008]    [0008]FIG. 1 b  illustrates an implementation of an optical isolator in the x-z plane.  
         [0009]    [0009]FIGS. 1 c  and  1   d  illustrate that light exiting from a PM fiber with the x-polarization becomes light with the y-polarization and does not enter an associated PM fiber.  
         [0010]    [0010]FIG. 2 illustrates an implementation of an optical isolator that includes a tap monitor.  
         [0011]    [0011]FIG. 3 illustrates an implementation of a variable optical attenuator.  
         [0012]    [0012]FIG. 4 illustrates an implementation of a variable optical attenuator that includes a tap monitor.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    The present invention relates to an improvement in optical technology. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the invention will be readily apparent to those skilled in the art and the generic principals herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principals and features described herein.  
         [0014]    The present invention will be described in terms of an optical isolator, a variable optical attenuator, and a tap monitor each having specific components having specific configurations. Similarly, the present invention will be described in terms of components having specific relationships, such as distances or angles between components. However, one of ordinary skill in the art will readily recognize that the devices and systems described can include other components having similar properties, other configurations, and other relationships between components.  
         [0015]    [0015]FIGS. 1 a  and  1   b  illustrate an implementation of an optical isolator  100 , respectively, in the y-z plane and the x-z plane. Optical isolator  100  includes a walk-off plate  140 , a half-wave plate  150 , a lens  160  such as a GRIN lens, non-reciprocal device  170  such as a Faraday rotator, and a reflector  180 . Optical isolator  100  can be coupled to a Polarization Maintenance (“PM”) fiber  110  and a PM fiber  120 . PM fibers  110  and  120  can be fixed with a capillary  130 .  
         [0016]    Walk-off plate  140  is designed in such a way that light entering walk-off plate  140  as an o-ray is not deflected, while light entering walk-off plate  140  as an e-ray is deflected. In one implementation, walk-off plate  140  is designed in such a way that light with the x-polarization enters walk-off plate  140  as an o-ray and light with the y-polarization enters walk-off plate  140  as an e-ray.  
         [0017]    In one implementation, half-wave plate  150  is a device designed to perform the following functions: (1) light with the x-polarization passing through the device in the positive z-direction becomes light with the x+y polarization; (2) light with the x−y polarization passing through the device in the negative z-direction becomes light with the y-polarization.  
         [0018]    In one implementation, lens  160  is a device designed to perform the following functions: (1) light exiting from PM fiber  110  is collimated, and after being reflected by reflective device  180 , reenters PM fiber  120 ; (2) light exiting from PM fiber  120  is also collimated.  
         [0019]    In one implementation, non-reciprocal device  170  is a device designed in such a way that the polarization of light passing through the device, either in the positive or the negative z-direction, is rotated substantially negative 22.5 degrees with respect to the positive z-axis.  
         [0020]    [0020]FIGS. 1 a  and  1   b  illustrate that light  111  exiting from PM fiber  110  with the x-polarization enters PM fiber  120  as light  119  with the x-polarization. More specifically, light  111  exiting from PM fiber  110  with the x-polarization passes through walk-off plate  140  as an o-ray without being deflected and becomes light  112 . Light  112  enters half-wave plate  150  with the x-polarization and exits from half-wave plate  150  as light  113  with the x+y polarization. Light  113  is collimated by lens  160  and exits from lens  160  as light  114 . Light  114  enters non-reciprocal device  170  with the x+y polarization and exits from non-reciprocal device  170  as light  115  with the cos(22.5)x+sin(22.5)y polarization. The polarization of light  114  is rotated substantially negative 22.5 degrees with respect to the positive z-axis. Light  115  is reflected by reflector  180  (e.g., a mirror) and becomes light  116  traveling a direction such that light  116  can be directed into PM fiber  120  using lens  160 .  
         [0021]    Light  116  enters non-reciprocal device  170  with the cos(22.5)x+sin(22.5)y polarization and exits from non-reciprocal device  170  as light  117  with the x-polarization. The polarization of light  116  is rotated substantially negative 22.5 degrees with respect to the positive z-axis. Light  117  passes through lens  160  and becomes light  118 . Light  118  with the x-polarization passes through walk-off plate  140  as an o-ray without being deflected and becomes light  119 . Light  119  enters PM fiber  120  with the x-polarization.  
         [0022]    While light exiting from PM fiber  110  enters PM fiber  120 , light exiting from PM fiber  120  does not enter PM fiber  110 . Therefore, optical isolator  100  provides optical isolation between PM fibers  110  and  120 . The isolation function is described in greater detail below in association with FIGS. 1 c  and  1   d.    
         [0023]    [0023]FIGS. 1 c  and  1   d  illustrate that light  121  exiting from PM fiber  120  with the x-polarization becomes light  129  with the y-polarization and does not enter PM fiber  110 . More specifically, light  121  exiting from PM fiber  120  with the x-polarization passes through walk-off plate  140  as an o-ray without being deflected and becomes light  122 . Light  122  is collimated by lens  160  and exits from lens  160  as light  123 . Light  123  enters non-reciprocal device  170  with the x-polarization and exits from non-reciprocal device  170  as light  124  with the cos(22.5)x−sin(22.5)y polarization. The polarization of light  123  is rotated substantially negative 22.5 degrees with respect to the positive z-axis. Light  124  is reflected by reflector  180  and becomes light  125 .  
         [0024]    Light  125  enters non-reciprocal device  170  with the cos(22.5)x−sin(22.5)y polarization and exits from non-reciprocal device  170  as light  126  with the x−y polarization. The polarization of light  125  is rotated substantially negative 22.5 degrees with respect to the positive z-axis. Light  126  passes through lens  160  and becomes light  127 . Light  127  enters half-wave plate  150  with the x−y polarization and exits from half-wave plate  150  as light  128  with the y-polarization. Light  128  with the y-polarization enters walk-off plate  140  as an e-ray and gets deflected as light  129 . After being deflected by walk-off plate  140 , light  129  does not enter PM fiber  110 .  
         [0025]    As shown in FIG. 2, optical isolator  100  in FIGS. 1 a - 1   d  can be modified to become an optical isolator  200  that includes a tap monitor. More specifically, reflector  180  in FIGS. 1 a - 1   d  is replaced with partial reflector  280 . A photo detector  210  is positioned behind partial reflector  280 . When light  115  is reflected by partial reflector  280  and becomes light  116 , a portion of light  115  transmits through partial reflector  280  and becomes light  217 . Light  217  is monitored by photo detector  210 . Partial reflector  280  can be designed in such a way that the power of light  217  is proportional to the power of light  111  or light  119 . Consequently, the power of light  111  or light  119  can be monitored using light  217 .  
         [0026]    As shown in FIG. 3, optical isolator  100  in FIGS. 1 a - 1   d  can be modified to become a variable optical attenuator (“VOA”)  300 . More specifically, non-reciprocal device  170  in FIGS. 1 a - 1   d  is replaced with a variable non-reciprocal device  370 : Variable non-reciprocal device  370  is a device designed in such a way that the polarization of light passing through the device, either in the positive or the negative z-direction, is rotated by a variable angle φ that can be controlled by external parameters (e.g., electric current).  
         [0027]    In one implementation, variable non-reciprocal device  370  includes a Faraday rotator  320  and an electromagnetic ring  330 . The variable angle φ can be changed by changing the strength of the magnetic field generated by electromagnetic ring  330 . The strength of the magnetic field generated by electromagnetic ring  330  can be controlled by external parameters, such as, electric current.  
         [0028]    In FIG. 3, light  111  exiting from PM fiber  110  with the x-polarization becomes light  114  with the x+y polarization after passing through walk-off plate  140 , half-wave plate  150 , and lens  160 . Light  114  enters variable non-reciprocal device  370  with the x+y polarization and exits from variable non-reciprocal device  370  as light  315  with the cos(45−φ)x+sin(45−φ)y polarization. Here the polarization of light  114  is rotated negative φ degrees with respect to the positive z-axis. Light  315  is reflected by reflector  180  and becomes light  316  traveling in a direction such that light  316  can be directed into PM fiber  120  using lens  160 .  
         [0029]    Light  316  enters variable non-reciprocal device  370  with the cos(45−φ)x+sin(45−φ)y polarization and exits from variable non-reciprocal device  370  as light  317  with the cos(45−2φ)x+sin(45−2φ)y polarization. Here the polarization of light  316  is rotated negative φ degrees with respect to the positive z-axis. Light  317  passes through lens  160  and becomes light  318 . Light  118  includes a component with the x-polarization and a component with the y-polarization. The component with the x-polarization has an power intensity that is proportional to [cos(45−2φ)] 2 , and the component with the y-polarization and proportional to [sin(45−2φ)] 2 .  
         [0030]    The component with the x-polarization passes through walk-off plate  140  as an o-ray without being deflected and becomes light  319   x.  Light  319   x  enters PM fiber  120  with the x-polarization. The component with the y-polarization passes through walk-off plate  140  as an e-ray and gets deflected as light  319   y.  After being deflected by walk-off plate  140 , light  319   y  does not enter PM fiber  120 . Consequently, a portion of light  111  exiting from PM fiber  110  with the x-polarization enters PM fiber  120  as light  119   x  with the x-polarization. The power intensity ratio between the light entering PM fiber  120  and the light exiting from PM fiber  110  is proportional to [cos(45−2φ)] 2 . In the special case that φ=22.5 degrees, a maximum amount of light is transferred from PM fiber  110  to PM fiber  120 .  
         [0031]    As shown in FIG. 4, VOA  300  in FIG. 3 can be modified to become VOA  400  that includes a tap monitor. More specifically, reflector  180  in FIGS. 1 a - 1   d  is replaced with partial reflector  280 . A polarization filter  420  and a photo detector  210  are positioned behind partial reflector  280 . When light  315  is reflected by partial reflector  280  and becomes light  316 , a portion of light  315  transmits through partial reflector  280  and becomes light  417 . Light  417  passes through polarization filter  420  and is monitored by photo detector  210 . Partial reflector  280  and polarization filter  420  are designed in such a way that the power of light  319   x  is proportional to the power of light  417 . Consequently, the power of light  319   x  can be monitored using light  417 .  
         [0032]    Implementations of walk-off plate  140  include one or more of the following. Walk-off plate  140  can be designed in such a way that light with the x-polarization enters walk-off plate  140  as an o-ray and light with the y-polarization enters walk-off plate  140  as an e-ray. Walk-off plate  140  can also be designed in such a way that light with the cos(θ)x+sin(θ)y polarization enters walk-off plate  140  as an o-ray and light with the sin(θ)x-cos(θ)y polarization enters walk-off plate  140  as an e-ray. θ can be an arbitrary angle.  
         [0033]    Implementations of half-wave plate  150  include one or more of the following. Half-wave plate  150  can be designed in such a way that the optical axis of half-wave plate  150  forms a substantially 22.5 degrees angle with respect to the polarization direction of the o-rays in walk-off plate  140 . Half-wave plate  150  can also be designed in such a way that the optical axis of half-wave plate  150  forms a substantially 67.5 degrees angle with respect to the polarization direction of the o-rays in walk-off plate  140 .  
         [0034]    Implementations of lens  160  include one or more of the following. Lens  160  can be a GRIN lens. Lens  160  can also be other type of lenses.  
         [0035]    Implementations of non-reciprocal device  170  include one or more of the following. Non-reciprocal device  170  can be a device designed in such a way that the polarization of light passing through the device is rotated substantially negative 22.5 degrees with respect to the positive z-axis. Non-reciprocal device  170  can also be a device designed in such a way that the polarization of light passing through the device is rotated substantially positive 22.5 degrees with respect to the positive z-axis. Non-reciprocal device  170  can be a Faraday rotator.  
         [0036]    A method and system has been disclosed for providing optical isolators, variable optical attenuators, and tap monitors. Although the present invention has been described in accordance with the implementations shown, one of ordinary skill in the art will readily recognize that there could be variations to the implementations and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.