Abstract:
Optical isolators are important for optical communication systems and serve to reduce the unwanted reflection from the connectors and components in the output side. Conventional optical isolators have two polarizers and a Faraday rotator. The present invention provides simplified isolators which can achieve the optical isolation function.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to optical isolators for optical fiber communication systems and optical instrument. 
       BACKGROUND OF THE INVENTION 
       [0002]    Laser diode is the key transmitter device in optical communication systems, which translate electronic signals into optical signals. With the rapid increase of information demand, it is desirable to increase the transmittance speed. In another word, more information is expected in a single fiber than before, and thus the laser diode should work at a higher transmittance rate. However, it is clear that the higher of the laser speed, the more the backward reflection light will affect the stability of laser diode. Generally speaking, there must be an optical isolator to eliminate the disturbance of the return light when the speed of optical signal speed is lager than 2.5 G/s. Nowadays, the speed of a single wavelength is 10 G/s and even 40 G/s in telecommunication and internet backbone networks. Moreover, with the rapid development of FTTH (Fiber To The Home), there will be a huge demand for high speed laser devices, and thus the optical isolators. 
         [0003]    As shown in  FIG. 1 , traditional polarization dependent optical isolators  10  are composed of three parts. A first polarizer  11  and a second polarizer  13  in which a Faraday rotator  12  is sandwiched between, are arranged to make the angle between polarization axis of first polarizer  11  and polarization axis of said second polarizer  13  to be 45°. The incident polarization beam  14   a  is aligned to the first polarizer  11  almost without any loss of light energy. Then the beam  14   b  goes through Faraday rotator  12  which gives the beam a 45° rotation. Afterwards the light  14   c  goes through the second polarizer  13  transparently since it has an arrangement of 45° axis angle difference with that of first polarizer  11 . For the backward direction, the reflected beam  15   a  goes through the second polarizer  13  and become beam  15   b.  Then beam  15   b  is rotated 45° again by the Faraday rotator  12  in the same direction with the first 45° rotation, because the Faraday rotator will rotate the beam in the same direction no matter which direction the beam is traveling. Therefore, the polarization direction of the reflected light  15   c  is perpendicular to the optical axis of the first polarizer  11 , and thus the backward light  15   c  was totally cut off. 
         [0004]    There is another type of optical isolators without the need of control of the polarization. Such isolators are commonly called polarization independent isolators  20 . Refer to  FIG. 2A ,  2 B, where there is shown a non polarized light beam  21  incident on a first wedge birefringent crystal  22  . The light beam  21  is split into an ordinary beam  210  and an extraordinary beam  21   e,  and allowed to pass through a Faraday rotor  23  and a second wedge birefringent crystal  24 . Polarization of the ordinary beam  210  and the extraordinary beam  21   e  are both rotated 45° by the Faraday rotator  23 . When ordinary beam  210  and the extraordinary beam  21   e  go through the second wedge birefringent crystal  24 , they keep to be still ordinary and extraordinary beam respectively, since the second wedge birefringent crystal  24  is selected to have his axis an 45° angle with that of the first wedge birefringent crystal  22 . The reflected beams  21   r  (see  FIG. 2B ) of these two beams will enter the Faraday rotator  23  and each get additional 45° rotation in the same direction. Therefore, the total angle of rotation is 90° for both beams. When these beams pass through the first wedge crystal  22 , the original ordinary beam  21   ro  will become an extraordinary beam whereas the original extraordinary beam  21   re  will become an ordinary beam. Therefore, both  21   e  and  21   r ″ beams will be directed in directions different from the input light beam  21 . 
         [0005]    Although the traditional isolators as described above will block all polarization mode of the return light from the optical system, it is noted that the main part of the return light is due to the near end reflection, which has the same polarization direction with the output beam  14   d  as shown in  FIG. 1 . According to this invention, an optical isolator without the second polarizer or wedge birefringent crystal is provided in order to reduce cost of manufacturing. 
       OBJECTS OF THE INVENTION 
       [0006]    One object of this invention is to provide a simplified optical isolator structure based on a Faraday rotator. The other object is to provide a simplified optical isolator structure based on a quarter-wave plate. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram showing the structure of the traditional polarization dependent optical isolators. 
           [0008]      FIG. 2  is (a) a schematic diagram of a conventional polarization independent optical isolator showing the inputs, and (b) a schematic diagram showing the reflected beams. 
           [0009]      FIG. 3  shows the structure of a polarization dependent optical isolator with one linear polarizer and one Faraday rotator according to this invention. 
           [0010]      FIG. 4  is a schematic diagram showing another polarization dependent optical isolator with one linear polarizer and one quarter-wave plate according to this invention. 
           [0011]      FIGS. 5A ,  5 B show schematic diagrams of polarization independent optical isolators with a wedge birefringent crystal and one Faraday rotator according to this invention. 
           [0012]      FIG. 6A ,  6 B is the schematic diagram of another polarization independent optical isolator with a wedge birefringent crystal and one quarter-wave plate according to this invention. 
           [0013]      FIG. 7  is the schematic diagram of simplified optical isolators with integrated pigtails and collimators according to this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    One embodiment of the present invention of a polarization dependent optical isolator  30  is shown in  FIG. 3 . The incident linear polarization beam  31   a  goes through the first polarizer  32  with alignment , and then passes through the rotator  33  with an 45° rotation in polarization. Parts of the output beam  31   c  will be reflected by output interface (not shown) and forming a reflected beam  34   a.  In this fashion, this reflected beam  34   a  goes back into Faraday rotator  33  and get a polarization angle change of 45° again. Therefore the light beam  34   b  is eventually perpendicular to the optical axis of polarizer  32  and totally blocked. Hence, the final reflected beam  34   c  is very small compared to  31   a . The optical isolation performance of the optical isolators in  FIG. 3  is almost as good as the conventional optical isolators with two polarizers. It is thus evident from the above description that the invention according to  FIG. 3  has the advantage of eliminating the second polarizer  13  as depicted in  FIG. 1  for conventional optical isolators. Therefore, the manufacturing cost of the simplified optical isolators shown in  FIG. 3  according to this invention can be reduced significantly. 
         [0015]    Another embodiment to be provided is a polarization dependent optical isolator  40  without the Faraday isolator. Here, the Faraday isolator  33  shown in  FIG. 3  is replaced by a quarter-wave plate  41  as shown in  FIG. 4 . Said quarter-wave plate is combined with a first polarizer  42  and arranged in such a way that polarization axis of  42  and optical axis of  41  make an angle of 45°. The incident linear polarization beam  43   a  is aligned to go through the first polarizer  42 , and then though the quarter-wave plate  41  to get a circularized polarization of light beam  43   c  to the output interface (not shown). Parts of the output beam  43   c  will be reflected by output interface (not shown) and forming a reflected beam  44   a.  In this fashion, this reflected beam  44   a  goes back into quarter-wave plate  41  and get a linear polarization beam  44   b  with a polarization angle change of 90° with respect to the incident light beam  43   a.  Therefore the light beam  44   b  is eventually perpendicular to the optical axis of polarizer  42  and totally blocked . Hence, the final reflected beam  44   c  is very small compared to  43   a.  The optical isolation performance of the optical isolators in  FIG. 4  is almost as good as the conventional optical isolators with two polarizers and a Faraday rotator. It is thus evident from the above description that the invention according to  FIG. 4  has the advantage of eliminating not only the second polarizer  13  but the Faraday rotator  12  as depicted in  FIG. 1  for conventional optical isolators. Therefore, the manufacturing cost of the simplified optical isolators shown in  FIG. 4  according to this invention can be reduced significantly. 
         [0016]    Although  FIGS. 3 and 4  show new simplified structures of optical isolators, these arc for polarized light beams. It is necessary to provide new simplified optical isolator structures for polarization independent operation.  FIGS. 5A and 5B  show one new invention embodiment of the polarization independent optical isolator  50  according to this invention. In  FIG. 5 , a first wedge birefringent crystal  51  is combined with a Faraday rotator  52  to form a simplified polarization independent optical isolator. The incident beam  53   a,  which could be any kinds of polarization mode, is input to wedge birefringent crystal  51  and get an ordinary light beam  53   o  and an extraordinary light beam  53   e.  Then the ordinary light beam  53   o  and the extraordinary light beam  53   e  are rotated 45° by the Faraday rotator  52  respectively and form the output light beams  53   b ′,  53   b ″. Parts of the output beam  53   b ′,  53   b ″ will be reflected by output interface (not shown) and forming a reflected beam  54   a  as depicted in  FIG. 5B . In this fashion, this reflected beam  54   a  goes back into the Faraday rotator  52  and get a polarization rotation of 45° again, making the total angle change of 90° with respect to the incident light beam  53   a.  Therefore the light beam  54   b  will undergo an exchange from ordinary to extraordinary and from extraordinary to ordinary when passing through the wedge birefringent crystal  51 . Thus, the final reflected beams  54   c ′,  54   c ″ will be in directions different from the original input light beam  53   a  and will not be coupled into the optical light source (not shown). It is thus evident from the above description that the invention according to  FIG. 5A ,  5 B has the advantage of eliminating the second wedge birefringent crystal  24  as depicted in  FIG. 2  for conventional polarization independent optical isolators. Therefore, the manufacturing cost of the simplified optical isolators shown in  FIG. 5A and 5B  according to this invention can be reduced significantly. 
         [0017]    Another embodiment according to this invention is a polarization independent isolator  60  as shown in  FIG. 6 . A first wedge birefringent crystal  61  is combined with a quarter-wave plat  62  to form a simplified polarization independent optical isolator. The optical axis of said quarter-wave plate  62  is selected to be a 45° of angle with that of the first wedge birefringent crystal  61 .The incident beam  63   a,  which could be any kinds of polarization mode, is input to wedge birefringent crystal  61  and get an ordinary light beam  63   o  and an extraordinary light beam  63   e.  After passing through the quarter-wave plate  62 , these light beams  63   o,    63   e  will become circularly polarized beams,  63   b ′,  63   b ″. Parts of the output beam  63   b ′,  63   b ″ will be reflected by output interface (not shown) and forming a reflected beam  64   a  as depicted in  FIG. 6B . In this fashion, this reflected beam  64   a  goes back into the quarter-wave plate  62  and become linearly polarized light beam with polarization angle change of 90°. Therefore the light beam  64   b  will undergo an exchange from ordinary to extraordinary and from extraordinary to ordinary when passing through the wedge birefringnent crystal  61 . Thus, the final reflected beams  64   c ′,  64   c ″ will be in directions different from the original input light beam  63   a  and will not be coupled into the optical light source (not shown). It is thus evident from the above description that the invention according to  FIG. 6A ,  6 B has the advantage of eliminating not only the second wedge birefringent crystal  24  but the Faraday rotator  23  as depicted in  FIG. 2  for conventional polarization independent optical isolators. Therefore, the manufacturing cost of the simplified optical isolators shown in  FIG. 6A ,  6 B according to this invention can be reduced significantly. 
         [0018]    According to still another embodiment of this invention, the simplified optical isolators may be combined conveniently with pigtails of optical fibers and collimators, which could be any kinds of focusing lens including ball lens, asperical lens and grin lens, to form inline optical isolators  70  as shown in  FIG. 7 . Here,  71 I is a simplified polarization dependent isolator according to this invention, a first fiber pigtail  73  is connected to a first collimator  74  to form input optical unit. Light beam from the input optical unit will be allowed to pass through the  71 I and reach a second collimator  75 , which is connected to a second pigtail fiber  76 . In this manner, the simplified polarization dependent isolators may be conveniently used to form into an inline optical isolator. The simplified polarization dependent isolators include the ones described in  FIG. 3  and  FIG. 4 . To those skilled in the arts, it is clear that the inline optical isolator may be constructed using the simplified polarization independent optical isolators as provided in this invention. The simplified polarization independent isolators include the ones described in  FIG. 5  and  FIG. 6 .