Abstract:
The invention provides an optical circulator that comprises three ports with the property that light entered through the nth port is output through the (n+1)th port. It can be applied to optical fiber transmission of optical signals. It uses a reflector to make a two-core fiber collimator to be a first port and a second port of the optical circulator so as to minimize the optical circulator volume and to simplify the assembly procedure. A reciprocal crystal and a non-reciprocal crystal are combined to form an optical polarization controller to conquer such technical problems as the conjugate angle of the two-core collimator and the minimal polarization mode dispersion. In particular, the corresponding relation between the Faraday rotator and the birefringent crystal optical axis can be utilized to remove half-wave plates used in ordinary optical circulators, thus lowering manufacturing costs and complexities.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to an optical circulator and, in particular, to an optical circulator that couples optical fibers with optical devices and can be applied to optical fiber transmission of optical signals. 
     2. Related Art 
     An optical circulator is a passive device that has at least three ports for accepting optical fibers. It is featured in that light that enters the circulator through the first port exits through the second port, and light that enters through the second port exits through the third. When the number of ports increases, this principle stays the same. That is, the optical path is not retraceable in the optical circulator, light that enters the nth port exits through the (n+1)th port. 
     Circulators are used for fiber transmission of optical signals. For example, the first ports of two optical circulators may be connected to a data transmitter, the second ports to an optical fiber, and the third ports to a data receiver. The same fiber is then used for transmitting and receiving signals. 
     For anisotropic birefringent crystals, incident light can be classified into extraordinary ray (E-ray) and ordinary ray (O-ray) according to its polarization direction and those two polarization directions are orthogonal. For a linearly polarized beam, the two polarization directions differ by 90 degrees. The O-ray will obey the Snell&#39;s law and the wave propagating direction will be parallel to the energy propagating direction. However, the wave propagating direction of the E-ray is normally not parallel to the O-ray and the energy propagating direction usually differs due to the crystal optical axis. This is called the walk-off phenomenon. 
     When light passes through a reciprocal crystal in the forward optical path, the polarization direction will be rotated by a certain angle; whereas when the light passes through the reciprocal crystal again in the returning path, the polarization direction will be rotated back by the same angle. So the polarization of the light is not changed after the round trip. On the other hand, when light passes through a non-reciprocal crystal in the forward optical path, the polarization direction is rotated by a certain angle; whereas when the light passes through the non-reciprocal crystal in the returning path again, the polarization direction is rotated further by the same angle. Therefore, the change in the polarization of the light is additive in the round trip of the beam. A proper combination of reciprocal crystals and non-reciprocal crystals can generate a particular linearly polarized direction and allow the choice of producing the walk-off phenomenon in order to achieve the above goal of an irretraceable optical path inside the optical circulator. Normal optical circulators use half-wave plates as the reciprocal crystals but not the Faraday rotators, which are non-reciprocal crystals. 
     The design of optical circulators is in whether each port can be distinguished from one another by its axial direction. When different ports of the optical circulator are not in the same axial direction, a polarizing beam splitter (PBS) has to be employed. The product occupies a large volume and costs more, e.g. the technology disclosed in the U.S Pat. No. 5,878,176. To reduce the volume and cost of the product, having different ports in the same axial direction has become the trend of modern designs; see for example the U.S. Pat. No. 5,930,422. Based upon the consideration of lower costs and convenient assembly, the U.S. Pat. No. 5,973,832 discloses a technology to remove half-wave plates by using the relative angle between a multi-layer Faraday rotator and a birefringent crystal optical axis. The U.S. Pat. No. 6,002,512 discloses a technology to reduce the number of half-wave plates used by employing latchable Faraday rotators. The U.S. Pat. No. 5,930,039 discloses a two-core fiber collimator that makes the three ports only need two optical fiber collimators, greatly minimizing the product volume and lowering the manufacturing cost. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide an optical circulator that reduces the volume, lower the cost and solve such problems as the conjugate angle of the two-core collimator and the minimal polarization mode dispersion. 
     According to the technology disclosed herein, the first port and the second port of the optical circulator are both become a two-core optical fiber collimator using a reflector. Since the same crystal is used repeatedly in the optical path, the volume of the optical circulator can be decreased and the assembly procedure can be simplified. It can further conquer such problems as the conjugate angle of the two-core collimator and the minimal polarization mode dispersion (PMD). Using the property that the polarization state of a light beam will not change when passing through a reciprocal crystal back and forth once, while will change additively when passing through a non-reciprocal crystal back and forth once, the present invention properly combine reciprocal crystals and non-reciprocal crystals to generate a particular polarization direction, to allow the choice of producing the walk-off phenomenon, and to form an optical circulator with an irretraceable optical path therein. In particular, the corresponding relation between the Faraday rotator and the birefringent crystal optical axis can be utilized to remove half-wave plates used in ordinary optical circulators, thus lowering manufacturing costs and complexities. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1A is a schematic view of the reflector. 
     FIG. 1B is a schematic view of the birefringent crystal. 
     FIG. 2 shows an optical path in the x-z plane according to the invention; 
     FIG. 3 shows an optical path in the y-z plane when light enters the second port from the first port a cord into the invention; 
     FIG. 4 shows an optical path in the y-z plane when light enters the third port from the second port according to the invention; 
     FIG. 5A shows the polorization relation of the first port optical path viewing from the x-y plane toward the positive z-axis; 
     FIG. 5B shows the polarizaon relation of the second port optical path viewing from the x-y plane toward the positive z-axis; 
     FIG. 6 is a schematic view of a reflector with the PMD compensation method; 
     FIG. 7 is another design of the PMD compensation method; 
     FIG. 8A shows the optical path in the x-z plane that uses a PMD compensation reflector and no half-wave plates; 
     FIG. 8B shows the optical path in the y-z plane that uses a PMD compensation reflector and no half-wave plates; 
     FIG. 9A shows the polarization relation of the first port optical path viewing from the x-y plane toward the positive z-axis for the design with no half-wave plate according to the invention; and 
     FIG. 9B shows the polarization relation of the second port optical path viewing from the x-y plane toward the positive z-axis for the design with no half-wave plate according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1A, the optical path  10  for one port of a two-core optical fiber collimator  01  subtends an angle 2Θ with the optical path  20  for another port. The optical path  10  is reflected into a reflected beam  10   r  by a reflector  001  according to the principle that the incident angle is equal to the reflecting angle. The reflected beam is then parallel to the other optical path  20  but in the opposite direction. They have a relative displacement in their perpendicular direction. 
     Referring to FIG. 1B, when the reflected beam  10   r  hits a birefringent crystal  002 , the E-ray with a polarization direction parallel to the optical axis generates a walk-off phenomenon and gets a displacement. Beams  10   re,    10   ro  outside the birefringent crystal  002  are parallel to each other and propagate in the same direction as the beam  10   r.    
     Referring to FIG. 2, for the incident light entering the first birefringent crystal  101  from the first port optical path  10 , the optical axis is parallel to the z-axis on the x-z plane. Therefore, there is no walk-off problem. The optical axis of the second birefringent crystal  104  keeps an angle of 45 degrees with the z-axis on the x-z plane. But through the combination of reciprocal and non-reciprocal effects by the Faraday rotator set  102  and the half-wave plate  103 , the entering optical path  10   ft  to the first port is an O-ray, obeying the Snell&#39;s law. The beam  10   ft  that passes through the Faraday rotator set  105  will travel in the z direction after the reflection by the reflector  106  and pass through the Faraday rotator set  105  again. Since the Faraday rotator set is non-reciprocal, the round trip additively changes the polarization state. The beam that passes through the crystal  104  will become an E-ray that will produce the walk-off phenomenon, with an optical path  10   bt.  By adjusting the length of the birefringent crystal  104 , the displacement of the optical path  10   bt  in the x direction can be controlled so as to enter the optical path  20  of the second port. The second port optical path  20  for the incident beam traveling toward the crystal  104  in the +z direction is the same as the one from the first port  10  to the second port  20 . It still obeys the Snell&#39;s law and keeps moving in the +z direction after passing through the Faraday rotator set  105 . It will pass through a half-wave plate  107  and a Faraday rotator set  105  and enters an optical fiber collimator  02 . 
     Referring to FIG. 3, the items denoted  102   a  and  105   a  are Faraday rotators that rotate the polarization direction of the light passing through them counterclockwise by 45 degrees. The first port optical path  10  perpendicularly enters the first birefringent crystal  101 . The optical axis has an angle of 45 degrees with the z-axis. There will be walk-off phenomena occurring on two perpendicular optical paths of two polarization directions on the y-z plane. The two optical paths are  10   fo  and  10   fe.  The beam  10   fe,  an E-ray type beam, does not change its polarization direction after passing through the Faraday rotator  102   a  and the half-wave plate  103 . The polarization direction of the beam  10   fe,  an O-ray type beam, will rotate by 90 degrees after passing through the Faraday rotator  102   b  and the half-wave plate  103 . This change in the polarization direction is shown in details in FIG. 5 a.  At the moment, the beams  10   fo  and  10   fe  will have the same polarization direction and are O-rays for the second birefringent crystal  104 . The beam  10   fe  is reflected by the reflector  106  after passing through the Faraday rotator  105   a  and re-enters the Faraday rotator  105   a.  The beam  10   fo  travels an extra distance after passing through the Faraday rotator set  105 , gets reflected by the lower portion of the reflector  106  and then re-enters the Faraday rotator  105   b.  The disclosed invention utilizes this extra optical path covered by the beam  10   fo  to compensate the optical path difference between the beams  10   fo  and  10   fe  in the first birefringent crystal  101 . This method can effectively solve the technical problem of polarization mode dispersion (PMD). 
     The polarization directions of the beams  10   fo  and  10   fe  traveling along the z direction are rotated by 90 degrees due to non-reciprocal effects. They are both E-rays for the second birefringent crystal  104  and thus will generate walk-off phenomena. The polarization of the beam  10   fe  in the z direction will rotate another 90 degrees after passing through the half-wave plate  103  and the Faraday rotator  102   a,  whereas that of the beam  10   fo  will stay the same after passing through the half-wave plate  103  and the Faraday rotator  102   a.  Therefore, the beam  10   fe  traveling in the z direction will generate walk-off phenomena after passing through the first birefringent crystal  101 , which can couple with the beam  10   fo  and enter the second port. 
     Referring to FIG. 4, the light emanating from the second port optical port  20  has the same optical path as that in FIG. 3 before passing through the Faraday rotator  105   a,    105   b.  The polarization relation is shown in FIG. 5, wherein the beams  20   fe  and  20   fo  are not reflected. The beam  20   fe  goes through the Faraday rotator  104   a  and the half-wave plate  107  and the polarization direction rotates by 90 degrees. It is an O-ray when entering the third birefringent crystal  108 . The polarization of the beam  20   fo  does not change after passing through the Faraday rotator  105   b  and the half-wave plate  107 . When entering the third birefringent crystal  108  it is an E-ray that produces walk-off phenomena and combine with the beam  20   fe  to form the second port optical path  30 . 
     Referring to FIG. 5A, it shows the polarization relation of the first port optical path viewing from the x-y plane toward the positive z-axis. 
     The first port optical path  10  perpendicularly enters the first birefringent crystal  101 . The entering beam is divided into two perpendicular beams  10   fo  and  10   fe  due to the walk-off phenomenon. The beam  10   fe  does not change its polarization direction after passing through the Faraday rotator  102   b  and the half-wave plate  103 . The polarization direction of the beam  10   fo  rotates by 90 degrees after passing through the Faraday rotator  102   b  and the half-wave plate  103 . The beams  10   fo  and  10   fe  are the same polarization direction O-rays for the second birefringent crystal  104 . The beam  10   fe  is reflected by the reflector  106  after passing through the Faraday rotator  105   a  and re-enters the Faraday rotator  105   a.  The beam  10   fo  travels an extra distance after passing through the Faraday rotator  105   b,  gets reflected by the lower portion of the reflector  106  and then re-enters the Faraday rotator  105   b.  The invention utilizes this extra optical path covered by the beam  10   fo  to compensate the optical path difference between the beams  10   fo  and  10   fe  in the first birefringent crystal  101 . 
     Referring to FIG. 5B, it shows the polarization relation of the second port optical path viewing from the x-y plane toward the positive z-axis. 
     The second port optical path  20  perpendicularly enters the first birefringent crystal  101 . The entering beam is divided into two perpendicular beams  20   fo  and  20   fe  due to the walk-off phenomenon. The beam  20   fe  goes through the Faraday rotator  105   a  and the half-wave plate  107  and the polarization direction rotates by 90 degrees. It is an O-ray when entering the third birefringent crystal  108 . The polarization of the beam  20   fo  does not change after passing through the Faraday rotator  105   b  and the half-wave plate  107 . When entering the third birefringent crystal  108  it is an E-ray that produces walk-off phenomena and combine with the beam  20   fe  to form the second port optical path  30 . 
     Referring to FIG. 6, it shows a schematic view of a reflector with the PMD compensation method. 
     In the PMD compensation method, one portion of one surface of the reflector is coated with an HR film to reflect E-rays while the other portion is coated with an AR film for O-rays to pass through. The other surface as a whole is coated with an HR film for reflecting O-rays. 
     Referring to FIG. 7, it shows another design of the PMD compensation method. 
     In this PMD compensation method, both surfaces of the reflector  106  are coated with AR films placed between the half-wave plate  103  and the second birefringent crystal  104 . A reflector  109  with coated with an HR film is placed after the Faraday rotators  105   a,    105   b.    
     Referring to FIG. 8A, it shows optical paths in the x-z plane that use a PMD compensation reflector and no half-wave plates. 
     The optical axes of the first birefringent crystal  101  and the third birefringent crystal  108  on the x-y plane and the x-z plane are adjusted to have an angle of 45 degrees with the y axis and the z axis, respectively. Due to the reflection principle, reciprocal crystals can be omitted. Two sets of counterclockwise and clockwise rotating non-reciprocal crystals can be employed to achieve the goal of polarization control. Through such designs, the present invention has the advantages of fewer crystals and simpler assembly. 
     Referring to FIG. 8B, it shows optical paths in the y-z plane that use a PMD compensation reflector and no half-wave plates. 
     The optical axes of the first birefringent crystal  101  and the third birefringent crystal  108  on the x-y plane and the x-z plane are adjusted to have an angle of 45 degrees with the y axis and the z axis, respectively. Due to the reflection principle, reciprocal crystals can be omitted. Two sets of counterclockwise and clockwise rotating non-reciprocal crystals can be employed to achieve the goal of polarization control. Through such designs, the present invention has the advantages of fewer crystals and simpler assembly. 
     Referring to FIG. 9A, it shows the polarization relation of the first port optical path viewing from the x-y plane toward the positive z-axis for the design with no half-wave plate according to the invention. 
     The entering beam is divided into two perpendicular beams  10   fo  and  10   fe  due to the walk-off phenomenon. The polarization direction of the beam  10   fe  rotates by −90 degrees after passing through the set of Faraday rotators  102   a  and  105   a.  The polarization direction of the beam  10   fo  rotates by 90 degrees after passing through the set of Faraday rotators  102   b  and  105   b.  The beams  10   fo  and  10   fe  are the same polarization direction O-rays for the second birefringent crystal  104 . The beam  10   fe  is reflected by the reflector  106  after passing through the Faraday rotator  105   a  and re-enters the Faraday rotator  105   a.  The beam  10   fo  travels an extra distance after passing through the Faraday rotator set  105   b,  gets reflected by the lower portion of the reflector  106  and then re-enters the Faraday rotator  105   b.  The invention utilizes this extra optical path covered by the beam  10   fo  to compensate the optical path difference between the beams  10   fo  and  10   fe  in the first birefringent crystal  101 . 
     Referring to FIG. 9B, it shows the polarization relation of the second port optical path viewing from the x-y plane toward the positive z-axis for the design with no half-wave plate according to the invention. 
     The second port optical path  20  perpendicularly enters the first birefringent crystal  101 . The entering beam is divided into two perpendicular beams  20   fo  and  20   fe  due to the walk-off phenomenon. The polarization direction of the beam  20   fe  rotates by −90 degrees after passing through the set of Faraday rotators  102   a  and  105   a.  The polarization direction of the beam  20   fo  rotates by 90 degrees after passing through the set of Faraday rotators  102   b  and  105   b.  The beam  20   fo  is an E-ray while entering the third birefringent crystal  108 . It combines with the beam  20   fe  to form the second port optical path  30  due to the walk-off phenomena. 
     Two sets of counterclockwise and clockwise rotating non-reciprocal crystals can be employed to achieve the goal of polarization control. The drawing shows the polarization relation. 
     Effects of the Invention 
     The present invention is an optical circulator. Since crystals are used repeatedly, the optical circulator volume can be minimized and the assembly procedure can be simplified. The disclosed can further conquer such technical problems as the conjugate angle of the two-core collimator and the minimal polarization mode dispersion. In particular, the corresponding relation between the Faraday rotator and the birefringent crystal optical axis can be utilized to remove half-wave plates used in ordinary optical circulators, thus lowering manufacturing costs and complexities. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.