Abstract:
An optical isolator includes a birefringent material and a Faraday rotator. The birefringent material receives a forward light propagating in a forward direction and a backward light propagating opposite to the forward direction. The birefringent material has an optical axis, wherein the forward light has a first polarization aligned perpendicular to the optical axis and is configured to pass the first birefringent material substantially along the forward direction. At least a portion of the backward light has a second polarization not perpendicular to the optical axis. The first birefringent material can displace the backward light to form a first displaced backward light. A Faraday rotator can rotate the forward light, and the backward light or the first displaced backward light by a same predetermined angle along the rotation direction.

Description:
CROSS-REFERENCES TO RELATED INVENTIONS 
     The present invention claims priority to commonly assigned Chinese Patent Application No. 200720007963.X, titled “a polarization related free space optical isolator”, filed on Aug. 17, 2007, and Chinese Patent Application No. 200720008166.3, titled “a polarization related free space optical isolator”, filed on Sep. 7, 2007. The disclosures of these related applications are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to optical devices for optical communications. 
     Optical isolator is an optical element that is often used to reduce the backward light in optical transmissions. Optical isolator is a nonreciprocal transmitting device. It allows a light from a light source to pass in a forward direction but prevents light to transmit in a backward direction, thus isolating the light source from the backward light. Free space isolator is a type of optical isolator often used in optical transceivers and tunable lasers. 
     A conventional free space isolator  10 , referring to  FIG. 1 , can include a first polarizer  11 , a Faraday rotator  12 , and a second polarizer  13 . The optical axes of the first polarizer  11  and the second polarizer  13  are oriented at a 45° angle. The Faraday rotator  12  is a nonreciprocal optical element. It can rotate the polarization of an incident light or a backward light by 45° along a same direction  15 . After the incident light passes the first polarizer  11 , its polarization is aligned to be parallel to the optical axis of the first polarizer  11 . The polarization of the incident light is then rotated by 45° along the direction  15  by the Faraday rotator  12  such that the polarization becomes parallel to the optical axis of the second polarizer  13 , allowing incident light to pass through the second polarizer  13 . 
     Backward lights always exist in optical systems. Backward light can include unwanted or astray lights reflected or scattered from various optical elements in the optical system. A backward light typically has randomized polarizations. Its polarization is linearized to be parallel to the optical axis of the second polarizer  13  as it enters the free space isolator  10 . The Faraday rotator then rotates the polarization of the backward light along the same direction  15 , making its polarization perpendicular to the optical axis of the first polarizer  11 . The backward light is thus blocked by the first polarizer  11 , which isolates the backward light from the source direction of the incident light. 
     The above described conventional free space isolator has several drawbacks. It is rather expensive because of the costs of the two polarizing crystals (typically implemented by Polacors) and the Faraday rotator. There is thus a need for a simpler, effective, and less expensive optical isolator. 
     SUMMARY 
     In a general aspect, the present invention relates to an optical isolator that includes a first birefringent material having a surface that can receive a forward light propagating in a forward direction and to receive a backward light propagating opposite to the forward direction, wherein the first birefringent material has an optical axis at about 45 degree angle relative to the surface, wherein the forward light has a first polarization aligned perpendicular to the optical axis and can travel through the first birefringent material as an ordinary ray substantially along the forward direction, wherein at least a portion of the backward light has a second polarization not perpendicular to the optical axis, wherein at least a portion of the backward light travels substantially as an extraordinary ray in the birefringent crystal, wherein the first birefringent material can displace the backward light to form a first displaced backward light; and a Faraday rotator that can rotate the forward light by a predetermined angle along a rotation direction, wherein the Faraday rotator can rotate the backward light or the first displaced backward light by substantially the same predetermined angle along the rotation direction. 
     In another general aspect, the present invention relates to an optical isolator that includes a first birefringent material that can receive a forward light propagating in a forward direction and to receive a backward light propagating opposite to the forward direction, wherein the first birefringent material has an optical axis, wherein the forward light has a first polarization aligned perpendicular to the optical axis and can pass the first birefringent material substantially along the forward direction, wherein at least a portion of the backward light has a second polarization not perpendicular to the optical axis, wherein the first birefringent material can displace the backward light to form a first displaced backward light; and a Faraday rotator that can rotate the forward light by a predetermined angle along a rotation direction, wherein the Faraday rotator can rotate the backward light or the first displaced backward light by substantially the same predetermined angle along the rotation direction. 
     In another general aspect, the present invention relates to an optical isolator that includes a birefringent material that can receive a forward light propagating in a forward direction and to receive a backward light propagating opposite to the forward direction, wherein the birefringent material has an optical axis, wherein the forward light has a first polarization aligned perpendicular to the optical axis and can pass the birefringent material substantially along the forward direction, wherein at least a portion of the backward light has a second polarization not perpendicular to the optical axis, wherein the birefringent material can displace the backward light to form a displaced backward light; a Faraday rotator that can rotate the forward light by a rotation angle between about 40 degrees and about 50 degrees along a rotation direction, wherein the Faraday rotator can rotate the backward light or the displaced backward light by substantially the same rotation angle along the rotation direction; and a polarizer having a polarization axis oriented at about 45 degrees relative to the first polarization, wherein the polarizer can pass the forward light and to pass the portion of the backward light having the second polarization. 
     In another general aspect, the present invention relates to an optical isolator that includes a first birefringent material that can receive a forward light propagating in a forward direction and to receive a backward light propagating opposite to the forward direction, wherein the first birefringent material has a optical axis, wherein the forward light has a first polarization aligned perpendicular to the optical axis and can pass the first birefringent material substantially along the forward direction, wherein at least a portion of the backward light has a second polarization not perpendicular to the optical axis, wherein the first birefringent material can displace the backward light to form a first displaced backward light; a Faraday rotator that can rotate the forward light by a rotation angle between about 40 degrees and about 50 degrees along a rotation direction, wherein the Faraday rotator can rotate the backward light or the first displaced backward light by substantially the same rotation angle along the rotation direction; and a second birefringent material having a polarization axis oriented at 45 degrees relative to the optical axis, wherein the second birefringent material can allow the forward light to pass through along the forward direction and to displace the portion of the backward light having the second polarization not perpendicular to the optical axis to produce a second displaced backward light. 
     Implementations of the system may include one or more of the following. The optical isolator can further include a polarizer having a polarization axis oriented at about 45 degrees relative to the first polarization, wherein the polarizer can pass the forward light and to pass the portion of the backward light having the second polarization. The polarizer, the Faraday rotator, and the first birefringent material are sequentially positioned along the forward direction. The first birefringent material, the Faraday rotator, and the polarizer are sequentially positioned along the forward direction. The polarizer, the Faraday rotator, and the first birefringent material are held in contact with each other to form a unitary component. At least two of the polarizer, the Faraday rotator, and the first birefringent material are separated by a medium or free space. The forward light travels substantially as an ordinary ray in the first birefringent crystal, wherein at least a portion of the backward light travels substantially as an extraordinary ray in the first birefringent crystal. The first birefringent crystal comprises a surface that receives the forward light, wherein the optical axis is at about 45 degree angle relative to the surface. The optical isolator can further include an optical blocker that can block the first displaced backward light. The optical isolator can further include a second birefringent material having a polarization axis oriented at 45 degrees relative to the optical axis, wherein the second birefringent material can allow the forward light to pass through along the forward direction and to displace the portion of the backward light having the second polarization not perpendicular to the optical axis to produce a second displaced backward light. The forward light travels substantially as an ordinary ray in the second birefringent crystal, wherein at least a portion of the backward light travels substantially as an extraordinary ray in the second birefringent crystal. The polarization axis is rotated by about 45 degrees from the optical axis along the rotation direction. The first birefringent material, the Faraday rotator, and the second birefringent material are held in contact with each other to form a unitary component. At least two of the first birefringent material, the Faraday rotator, and the second birefringent material are separated by a medium or free space. The optical isolator can further include an optical blocker that can block the first displaced backward light and the second displaced backward light. The predetermined angle is between about 40 degrees and about 50 degrees. The first birefringent material can displace the backward light to form the first displaced backward light separated more than 5 microns from the forward light. 
     Embodiments may include one or more of the following advantages. The disclosed systems and methods provide a compact and lower cost optical isolating device by using less expensive components and materials. The disclosed optical isolating device can be integrated in a unitary optical assembly that can be easily used in a wider range of applications. The disclosed optical isolating device is more effective in preventing backward light from affecting the operations of a light source. 
     Although the invention has been particularly shown and described with reference to multiple embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a diagram that illustrates the principle of the conventional free space isolator. 
         FIG. 2A  is schematic diagram illustrating an optical assembly with a forward light passing through the optical assembly. 
         FIG. 2B  illustrates the optical assembly of  FIG. 2A  with a backward light blocked by the optical assembly. 
         FIG. 2C  shows an arrangement for an improved optical isolator employing the optical assembly in  FIGS. 2A-2B  illustrating a forward light passing through the optical isolator. 
         FIG. 2D  shows an arrangement for an improved optical isolator of  FIG. 2C  illustrating a backward light blocked by the optical isolator. 
         FIG. 3A  is schematic diagram illustrating another optical assembly with a forward light passing through the optical assembly. 
         FIG. 3B  illustrates the shows the optical assembly of  FIG. 3A  with a backward light isolated by the optical assembly. 
         FIG. 3C  shows an arrangement for an improved optical isolator employing the optical assembly in  FIGS. 3A-3B  illustrating a forward light passing through the optical isolator. 
         FIG. 3D  shows an arrangement for an improved optical isolator of  FIG. 3C  illustrating a backward light blocked by the optical isolator. 
         FIG. 4A  is schematic diagram illustrating another optical assembly with a forward light passing through the optical assembly. 
         FIG. 4B  illustrates the shows the optical assembly of  FIG. 4A  with a backward light isolated by the optical assembly. 
         FIG. 4C  shows the backward light passing the second birefringent crystal in the optical assembly of  FIG. 4A . 
         FIG. 4D  shows the backward light passing the first birefringent crystal in the optical assembly of  FIG. 4A . 
         FIG. 4E  shows an arrangement for an improved optical isolator employing the optical assembly in  FIGS. 4A-4B  illustrating a forward light passing through the optical isolator. 
         FIG. 4F  shows an arrangement for an improved optical isolator  FIG. 4E  illustrating a backward light blocked by the optical isolator. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 2A , an optical assembly  100  includes a birefringent crystal  111 , a Faraday rotator  112 , and a polarizer  113 , which are sequentially positioned along the direction of a forward (i.e. incident) light  131  emitted by a light source (not shown) such as a laser or light emitting diode. The polarizer  113  is a linear polarizer and can be implemented by a polarizing crystal such as a Polacor, available from Polaroid, Corp., a plastic polarizer, or many other forms of polarizers. Materials suitable for the birefringent crystal  111  include Yttrium Vanadate Crystal (YVO 4 ) and calcite, which is typically less expensive than a polarizing crystal. The birefringent crystal  111 , the Faraday rotator  112 , and the polarizer  113  can be separated by free space (or air) or other medium. The birefringent crystal  111 , the Faraday rotator  112 , and the polarizer  113  can also be glued together using an organic or inorganic adhesive material or bonded together by direct optical bonding to form a unitary optical assembly  100 . 
     The forward light  131  first enters the birefringent crystal  111  in the optical assembly  100 . The birefringent crystal  111  has an optical axis  121  that defines an axis of anisotropy in its refractive indices, which in turn defines propagating directions of an ordinary ray or an extraordinary ray propagating in the birefringent crystal  111 . The forward light  131  has a polarization perpendicular to the optical axis  121  and thus propagates through the birefringent crystal  111  as an ordinary ray without changing its direction. (The optical axis  121  is parallel to the viewing plane of  FIG. 2A  and is at an approximately 45° angle to the light-entering face of the birefringent crystal  111 .) 
     After the forward light  131  passes the birefringent crystal  111 , it enters the Faraday rotator  112 . The Faraday rotator  112  can rotate the polarization of the forward light  131  by a rotation angle in the direction  122 . The rotation angle is between about 40 degrees and about 50 degrees, preferably 45°. While the forward light  131  passes the Faraday rotator  112 , its polarizing direction is rotated by the rotation angle (e.g. approximately 45°) along the rotation direction  122 . 
     The polarizer  113  has a polarization axis  123  oriented at an approximately 45° along the direction  122  from the polarizing direction of the forward light  131  in the birefringent crystal  111 . After passing the Faraday rotator  112 , the polarizing direction of the forward light  131  becomes parallel to the polarization axis  123  and can pass through the polarizer  113  to form an output light  132 . 
     Backward lights are often produced in an optical system by unwanted reflections or scatterings of the forward light. Backward lights are typically randomly polarized or partially randomly polarized. Unless properly isolated, backward light can travel back to the light source, which can affect the proper function of the light source. Referring to  FIG. 2B , the randomly polarized backward lights  141  first reaches the polarizer  113  in the optical assembly  100 . The portion of the backward lights whose polarization is parallel to the polarization axis  123  passes through the polarizer  113  to reach the Faraday rotator  112  while the other portions are absorbed by the polarizer  113 . 
     As is known in this art, a Faraday rotator is a nonreciprocal optical element. That is, the Faraday rotator  112  rotates the polarization of the backward light  141  along the same direction  122  as it does to the forward light  131 . After passing the Faraday rotator  112 , the backward light  141  has its polarization rotated by 45° along the direction  122 . The polarization of the backward light  141  is perpendicular to the polarization of the forward light  131 . The polarization of the backward light  141  is aligned in the plane defined by its propagation direction and the optical axis  121 , and thus the backward light  141  travels as an extraordinary ray along deflected direction in the birefringent crystal  111 . The backward light  141  exits the birefringent crystal  111  to form a displaced backward light  142 , which is displaced by a distance from the original propagation direction  143  of the back ward light  141 . As described below, the displaced backward light  142  can be properly blocked or absorbed to prevent it from reaching the light source that produces the forward light  131 . 
     It should be understood that the optical assembly  100  can be made in some other configurations without deviating from the spirit of the invention. The optical axes of the birefringent crystal  111 , the polarizer  113  and the rotation direction of the Faraday rotator  112  can be different. Different materials can be used for the birefringent crystal and the polarizer. The birefringent crystal can have different birefringence or thicknesses for displacing the backward light. For example, the birefringent crystal can have a thickness in the range of 0.1 mm to 0.4 mm in the light transmission direction. The amount of displacement of the backward light, or the distance between the displaced backward light  142  and the forward light  131 , can be more than 5 microns, or in a range from about 10 microns to 40 microns. For example, a YVO4 birefringent crystal can have a thickness of about 0.3 mm. The backward light can be displaced by about 30 microns. 
     The optical assembly  100  can be used to construct an optical isolator  150  as shown in  FIGS. 2C and 2D . The optical isolator  150  can include an opaque optical blocker  160 , the birefringent crystal  111 , the Faraday rotator  112 , and the polarizer  113 . As described above, the birefringent crystal  111 , the Faraday rotator  112 , and the polarizer  113  can be separated by free space (or air) or some other mediums, and bonded together. The opaque optical blocker  160  is positioned adjacent to or in contact with the front outer surface of the birefringent crystal  111 . The optical blocker  160  can be separated from the birefringent crystal  111 . The optical blocker  160  can also be glued to the birefringent crystal  111 , which allows the optical isolator  150  to form a unitary component. 
     In some embodiments, the opaque optical blocker  160  is provided by a housing wall or an outer surface of another optical component in the optical system that the optical assembly  100  is installed. An example of an optical system is a laser system. In other words, a dedicated optical blocker is not provided with the optical assembly  100 . The optical assembly  100  can be used as a stand-alone optical isolator, which simplifies the optical isolator and can further reduce cost. It should be noted that the displaced backward light  142  has a polarization perpendicular to the polarization of the forward light  131 . The light source is not affected by the displaced backward light  142  even if a small portion of it is scattered (e.g. by other optical components in the optical system) and coupled back into the light source. 
     The optical blocker  160  is made of an opaque and preferably, light absorbing, material such as metallic, polymeric, or an inorganic material. In some embodiment, a layer of light absorbing material such as amorphous carbon can be coated on a portion of the outer surface of the birefringent crystal  111 . The optical blocker  160  exposes a portion of the outer surface of the birefringent crystal  111  to allow the forward light  131  to enter the birefringent crystal  111 . The optical blocker  160  can also block and/or absorb the displaced backward light  142 , preventing it from exiting the birefringent crystal  111 , thus isolating the backward light from the light source. The optical blocker  160  can exist in many different forms as long as it provides the above described functions. For example, the optical blocker  160  can be an aperture structure that includes an opening  165  that allows the forward light  131  to enter and pass through the birefringent crystal  111 , the Faraday rotator  112 , and the polarizer  113 . The aperture structure includes an opaque portion  167  that can block and absorb the displaced backward light  142 . 
     In some embodiments, referring to  FIGS. 3A and 3B , an optical assembly  200  includes a polarizer  211 , a Faraday rotator  212 , and a birefringent crystal  213 , which are sequentially positioned along the direction of a forward light (or an incident light)  231  emitted by a light source. The polarizer  211  can be a polarizing crystal such as a Polacor, available from Polaroid, Corp., a plastic polarizer, or other types of polarizers. Materials suitable for the birefringent crystal  213  include Yttrium Vanadate Crystal (YVO 4 ) and calcite which is typically less expensive than a polarizing crystal. The polarizer  211 , the Faraday rotator  212 , and the birefringent crystal  213  can be separated by free space (or air) or other medium. The polarizer  211 , the Faraday rotator  212 , and the birefringent crystal  213  can also be glued together using an adhesive material or bonded together by direct optical bonding to form a unitary optical assembly  200 . 
     The polarizer  211  has a polarization axis  221  that is tilted at a 45° angle relative to the horizontal direction (as viewed in  FIG. 3A ). The forward light  231  first enters the polarizer  211  and maintains its polarization along the polarization axis  221 . As the forward light  231 , the Faraday rotator  212  rotates the polarization of the forward light  231  by about 45° along the direction  222 . 
     An optical axis  232  in the birefringent crystal  213  defines an axis of anisotropy in its refractive indices, which in turn defines propagating directions of an ordinary ray or an extraordinary ray propagating in the birefringent crystal  213 . (The optical axis  232  is parallel to the viewing plane of  FIG. 3A  and is at an approximately 45° angle to the light-exiting face of the birefringent crystal  213 .) The forward light  231  has a polarization perpendicular to the optical axis  223  and thus propagates through the birefringent crystal  213  as an ordinary ray without changing its direction, thereby forming an output light  232  exiting the birefringent crystal  213 . 
     Referring to  FIG. 3B , when the randomly polarized backward lights  241  enters the birefringent crystal  213  in a backward direction, the backward lights  241  is decomposed to travel in two different directions  242  and  245 . A first backward light  243  propagates as an ordinary ray in the birefringent crystal  213  along the direction  242 . A second backward light  245  travels as an extraordinary ray in a different direction  345  in the birefringent crystal  213 , forming a displaced backward light  244  as it exits the birefringent crystal  213  and enters the Faraday rotator  212 . The first backward light  243  and the displaced backward light  244  pass through the Faraday rotator  212  in two parallel paths. The polarization directions of the first backward light  243  and the displaced backward light  244  are both are rotated by 45° along the  222  direction. The polarization direction of the first backward light  243  becomes perpendicular to the polarization axis  221  and is absorbed by the polarizer  211 . The first backward light  243  thus cannot reach the light source. The displaced backward light  244  has its polarization parallel to the polarizing axis  221  and can pass through the polarizer  211 . The displaced backward light  244  can be subsequently blocked or absorbed by an optical blocker ( 260  in  FIGS. 3C and 3D ) and is isolated from the light source. 
     It should be understood that the optical assembly  200  can be made in some other configurations without deviating from the spirit of the invention. The optical axes of the polarizer  211  and the birefringent crystal  213  and the rotation direction of the Faraday rotator  212  can be different. Different materials can be used for the birefringent crystal and the polarizer. The birefringent crystal can have different birefringence or thicknesses for displacing the backward light. For example, the birefringent crystal can have a thickness in the range of 0.1 mm to 0.4 mm in the light transmission direction. The amount of displacement of the backward light, or the distance between the displaced backward light  244  and the forward light  231 , can be more than 5 microns, or in a range from about 10 microns to 40 microns. For example, the birefringent crystal can have a thickness of about 0.3 mm. The backward light can be displaced by about 30 microns. 
     The optical assembly  200  can be used to construct an optical isolator  250  as shown in  FIGS. 3C and 3D . The optical isolator  250  can include an opaque optical blocker  260 , the polarizer  211 , the Faraday rotator  212 , and the birefringent crystal  213 . As described above, the polarizer  211 , the Faraday rotator  212 , and the birefringent crystal  213  can be separated by free space (or air) or some other mediums, and bonded together. The opaque optical blocker  260  is positioned adjacent to or in contact with the front outer surface of the polarizer  211 . The optical is blocker  260  can be separated from the polarizer  211 . The optical blocker  260  can also be glued to the polarizer  211 , which allows the optical isolator  250  to form a unitary component. 
     The optical blocker  260  is made of an opaque and preferably, light absorbing, material such as metallic, polymeric, or an inorganic material. In some embodiment, a layer of light absorbing material such as amorphous carbon can be coated on a portion of the outer surface of the polarizer  211 . The optical blocker  260  exposes a portion of the outer surface of the polarizer  211  to allow the forward light  231  to enter the polarizer  211 . The optical blocker  260  can also block and/or absorb the displaced backward light  244 , preventing it from exiting the polarizer  211 , thus isolating the backward light from the light source. 
     The optical blocker  260  can exist in many different forms as long as it provides the above described functions. For example, the optical blocker  260  can be an aperture structure that includes an opening  265  that allows the forward light  231  to pass through to the polarizer  211 , the Faraday rotator  212 , and the birefringent crystal  213 . The aperture structure includes opaque portion  267  that can block and absorb the displaced backward light  244 . 
     In some embodiments, the opaque optical blocker  260  is provided by a housing wall or an outer surface of another optical component in the optical system that the optical assembly  200  is installed. An example of an optical system is a laser system. In other words, a dedicated optical blocker is not provided with the optical assembly  200 . The optical assembly  200  can be used as a stand-alone optical isolator, which simplifies the optical isolator and can further reduce cost. It should be noted that the displaced backward light  244  has a polarization perpendicular to the polarization of the forward light  231 . The light source is not affected by the displaced backward light  244  even if a small portion of it is scattered (e.g. by other optical components in the optical system) and coupled back into the light source. 
     In some embodiments, referring to  FIGS. 4A and 4B , an optical assembly  300  includes a first birefringent crystal  311 , a Faraday rotator  312 , and a second birefringent crystal  313 , which are sequentially positioned along the direction of a forward light (or an incident light)  331  emitted by a light source. Materials suitable for the first and the second birefringent crystals include Yttrium Vanadate Crystal (YVO 4 ) and calcite, which is typically much less expensive than a polarizing crystal. The first birefringent crystal  311 , the Faraday rotator  312 , and the birefringent crystal  313  can be separated by free space (or air) or other medium. The first birefringent crystal  311 , the Faraday rotator  312 , and the birefringent crystal  313  can also be glued together using an adhesive material or bonded together by direct optical bonding to form a unitary optical assembly  300 . 
     The first birefringent crystal  311  has an optical axis  321  that is parallel to the viewing plane of  FIG. 2A  and is at an approximately 45° angle to the light-entering face of the first birefringent crystal  311 . The second birefringent crystal  313  has an optical axis  323  that is at an approximately 45° angle relative to the optical axis  321 . Specifically, the direction of the optical axis  323  is rotated by 45° along a direction  322  from the optical axis  321 . The Faraday rotator  312  can rotate a polarization of the light through it by a rotation angle along the direction  322 . The rotation angle is also approximately 45°. 
     A forward light  331  having its polarization perpendicular to the optical axis  321  travels without changing direction as an ordinary ray through the first birefringent crystal  311 . The polarization of the forward light  331  is rotated by 45° as it travels through the Faraday rotator  312 . As it exits the Faraday rotator  312 , the forward light  331  has a polarization perpendicular to the optical axis  323  of the second birefringent crystal  313 . The forward light  331  passes through the second birefringent crystal  313  as an ordinary ray without changing its direction, forming an output light  332 . 
     When the randomly polarized backward lights  341  enters the second birefringent crystal  313  in a backward direction, referring to  FIGS. 4B and 4C , the backward lights  341  is decomposed to travel in two different directions  342  and  345 . A first backward light  343  propagates in the second birefringent crystal  313  as an ordinary ray along the direction  342  without changing direction. A second backward light  345  travels as an extraordinary ray in a different direction in the second birefringent crystal  313 , forming a displaced backward light  344  as it exits the second birefringent crystal  313  and enters the Faraday rotator  312 . The first backward light  343  and the displaced backward light  344  pass through the Faraday rotator  312  in two parallel paths. The polarization directions of the first backward light  343  and the displaced backward light  344  are both rotated by 45° along the  322  direction. 
     Referring to  FIGS. 4B and 4D , as it exits the Faraday rotator  312 , the first backward light  343  has its polarization rotated in the plane defined by its propagation direction and the optical axis  321 . Thus the first backward light  343  travels as an extraordinary ray in the first birefringent crystal  311  in a deflected direction to form a displaced backward light  346 . As it exits the Faraday rotator  312 , the displaced backward light  344  has a polarization perpendicular to the optical axis  321  and thus travels as an ordinary ray without changing direction through the first birefringent crystal  311 . The displaced backward light  344  and  346  can subsequently be blocked or absorbed by an optical blocker ( 360  in  FIGS. 4E and 4F ), preventing the them from reaching the light source. 
     It should be understood that the optical assembly  300  can be made in some other configurations without deviating from the spirit of the invention. The optical axes of the birefringent crystals and the angle and the direction of rotation of the Faraday rotator can be different. Different materials can be used for the birefringent crystal. The birefringent crystals can have different birefringence or thicknesses for displacing the backward light. For example, the birefringent crystal can have a thickness in the range of 0.1 mm to 0.4 mm in the light transmission direction. The amount of displacement of the backward light, or the distance between the displaced backward lights  344 ,  346  and the forward light  331 , can be more than 5 microns, or in a range from about 10 microns to 40 microns. For example, the birefringent crystal can have a thickness of about 0.3 mm. The backward lights can be displaced by about 30 microns. 
     The optical assembly  300  can be used to construct an optical isolator  350  as shown in  FIGS. 4E and 4F . The optical isolator  350  can include an opaque optical blocker  360 , the first birefringent crystal  311 , the Faraday rotator  312 , and the second birefringent crystal  313 . As described above, the first birefringent crystal  311 , the Faraday rotator  312 , and the second birefringent crystal  313  can be separated by free space (or air) or some other mediums, and bonded together. The opaque optical blocker  360  is positioned adjacent to or in contact with the front outer surface of the first birefringent crystal  311 . The optical blocker  360  can be separated from the first birefringent crystal  311 . The optical blocker  360  can also be glued to the first birefringent crystal  311 , which allows the optical isolator  350  to form a unitary component. 
     The optical blocker  360  is made of an opaque and preferably, light absorbing, material such as metallic, polymeric, or an inorganic material. In some embodiment, a layer of light absorbing material such as amorphous carbon can be coated on a portion of the outer surface of the first birefringent crystal  311 . The optical blocker  360  exposes a portion of the outer surface of the first birefringent crystal  311  to allow the forward light  331  to enter the first birefringent crystal  311 . The optical blocker  360  can also block and/or absorb the displaced backward lights  344  and  346 , preventing them from exiting the first birefringent crystal  311 , thus isolating the backward light from the light source. The optical blocker  360  can exist in many different forms as long as it provides the above described functions. For example, the optical blocker  360  can be an aperture structure that includes an opening  365  that allows the forward light  331  to enter and pass through the first birefringent crystal  311 , the Faraday rotator  312 , and the second birefringent crystal  313 . The aperture structure includes opaque portion  367  that can block and absorb the displaced backward lights  344  and  346 . 
     In some embodiments, the opaque optical blocker  360  is provided by a housing wall or an outer surface of another optical component in the optical system that the optical assembly  300  is installed. An example of an optical system is a laser system. In other words, a dedicated optical blocker is not provided with the optical assembly  300 . The optical assembly  300  can be used as a stand-alone optical isolator, which simplifies the optical isolator and can further reduce cost. 
     It should be understood that the disclosed optical assemblies and optical isolators can be used in a wide range of optical applications such as laser devices. The disclosed optical assemblies and optical isolators can be made in compact sizes and with inexpensive materials. The disclosed optical assemblies and optical isolators can be produced as unitary components in factories, thus saving the assembly time and cost when they are incorporated into optical devices. 
     The optical blocker for the above disclosed optical isolators can be implemented in many different forms. For example, an optical blocker can include a continuous opaque portion and an edge. The continuous opaque portion can block the displaced backward light. The forward light enters the optical assembly beyond the edge and not covered by the opaque portion. The continuous opaque portion can for example cover substantial portion of the front outer surface of the optical assembly. Alternatively, the continuous opaque portion can be in the form of an island or a pad that only covers the area of the front outer surface where the displaced backward light reaches the optical blocker.