Abstract:
An optical isolator ( 100 ) includes an input port ( 10 ), an output port ( 20 ), an optical isolating means ( 30 ) and a mounting tube ( 40 ). The input port includes an optical fiber ( 13 ) having an exposed end, a ferrule ( 12 ) defining a through hole  121  for holding the optical fiber, a molded lens ( 11 ), a sleeve ( 14 ) and a metal holder ( 15 ). The molded lens collimates optical signals transmitted from the optical fiber. The output port is constructed like the input port. The optical isolating means is disposed in an optical path between the input port and the output port. The optical isolating means transmits optical signals in an input direction and blocks reflected optical signals in the reverse direction. The mounting tube accommodates and fixes the input and output ports and the optical isolating means.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to optical isolators for use in optical fiber communication and optical network technology, and more particularly to optical isolators which employ molded lenses. 
     2. Description of the Prior Art 
     In the field of optical fiber communications, problems with the performance of optical devices often arise. One such problem is caused by light reflecting off an end face or another part of an optical device. Such reflections can return to the light source, adversely affecting the light source and deteriorating the quality of communications. Another problem is caused by echoes of transmitted optical signals, which are caused by multiple reflections off the end face or another part of an optical device. The deterioration in performance of a light source due to the return of reflected light has been previously observed in connection with the stability of self-mode locking. Now, devices designed to eliminate reflected lights such as optical isolators, are used in optical fiber communication systems to prevent such deteriorated performance and eliminate reflected light. 
     FIG. 5 shown a conventional optical isolator as disclosed in U.S. Pat. No. 5,557,692. The optical isolator  80  comprises an input port  81 , an output port  82  and an isolating means  83 . The input port  81  comprises an input optical fiber  811  and a first Graded Index (GRIN) lens  812 . The output port  82  comprises an output optical fiber  821  and a second GRIN lens  822 . The isolating means  83  includes a first polarizer  831 , a second polarizer  832  and a liquid crystal cell  833  disposed in the path of the rays from the first polarizer  831  to the second polarizer  832 . 
     The conventional optical isolator  80  using GRIN lenses  812 ,  822  as collimating elements has some disadvantages. Firstly, the GRIN lenses are made using the ion-exchange method. However, this method requires a long time and further steps of polishing after initial formation, so it is difficult and expensive to manufacture. Secondly, some chemicals used in the ion-exchange method contaminate the environment and endanger the fabrication workers. 
     The present invention overcomes the above-described disadvantages of conventional optical isolators by offering an optical isolator having molded lenses which yield higher performance at a lower cost. A copending application Ser. No. 10/172,232 with the same assignee and the same inventors as the present invention discloses similar technology applied to other types of optical components. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an improved optical isolator which employs molded lenses as collimating elements. 
     Another object of the present invention is to provide an optical isolator having high precision lenses which are relatively environmentally friendly to produce. 
     A further object of the present invention is to provide an optical isolator which is easily and cheaply manufactured. 
     To solve the problems of the prior art and to achieve the objects set forth above, an optical isolator of the present invention comprises an input port, an isolating means, an output port and a mounting tube. The input port comprises a ferrule having an optical fiber, a molded lens, a sleeve and a metal holder. The optical fiber has an exposed end and the ferrule defines a through hole for receiving and fixing the optical fiber therein. The ferrule has a rearward face and a forward face. The forward face of the ferrule is ground at an oblique angle and is flush with the exposed end of the optical fiber. The molded lens is cylindrical in shape and has an oblique surface coinciding with that of the ferrule and the exposed end of the optical fiber. A gap is defined between the molded lens and the ferrule. The output port is similar to the input port. The isolating means includes a first polarizer, a second polarizer and a Faraday rotator disposed in the paths of the rays from the first polarizer to the second polarizer. Furthermore, the optical axis of the second polarizer is oriented 45 degrees with respect to the optical axis of the first polarizer. The isolating means is located in the path of light beams from the input port to the output port. 
     Since the present invention employs molded lenses as the collimating elements, the cost and environmental problems associated with GRIN lenses are mitigated and efficiency is improved. 
     Other objects, advantages and novel features of the present invention will be apparent from the following detailed description of the preferred embodiment thereof with reference to the attached drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional diagram of an optical isolator according to the present invention; 
     FIG. 2 is a cross-sectional view of an input port of the optical isolator of FIG. 1; 
     FIG. 3 is a cross-sectional view of a molded lens of the optical collimator of FIG. 2; 
     FIG. 4 is an essential optical paths diagram of the input port of FIG. 2; and 
     FIG. 5 is a schematic view of a conventional optical isolator. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For facilitating understanding, like components are designated by like reference numerals throughout the preferred embodiment of the invention as shown in the various drawing figures. 
     Reference will now be made to the drawings to describe the present invention in detail. 
     Referring to FIG. 1, an optical isolator  100  in accordance with a preferred embodiment of the present invention comprises an input port  10 , an isolating means  30 , an output port  20  and a mounting tube  40 . 
     The input port  10  and the output port  20  are identical in construction. The input port  10  is described as an example. As shown in FIG. 2, the input port  10  comprises a molded lens  11 , a ferrule  12 , an optical fiber  13 , a sleeve  14  and a metal holder  15 . 
     The ferrule  12  is cylindrical in shape and is made of a ceramic, a metal or a plastic material. The ferrule  12  has a forward face  122 , a rearward face (not labeled) and a through hole  121  defined between the forward face  122  and the rearward face (not labeled). A diameter of the through hole  121  is slightly greater than a diameter of the optical fiber  13 . A conical opening (not labeled) coaxial with the through hole  121  is defined in the rearward face (not labeled). The optical fiber  13  with has an exposed end is preferably fixed in the through hole  121  with UV-cured epoxy or 353-ND epoxy. To improve optical performance, the forward face  122  of the ferrule  12  and the exposed end (not labeled) of the optical fiber  13  are ground and polished at an oblique angle relative to an imaginary plane constructed perpendicular to a longitudinal axes of the ferrule  12 . The angle is preferably between 6 and 8 degrees. 
     Referring to FIG. 3, the molded lens  11  is substantially cylindrical and has a uniform refractive index. A rearward face  112  of the molded lens  11  forms an oblique angle with an imaginary plane constructed perpendicular to a longitudinal axis of the molded lens  11 . The angle is preferably between 6 and 8 degrees and should be equal to the angle of the forward face  122  of the ferrule  12 . A forward face  111  of the molded lens  11  has an aspherical surface. The rearward face  112  and the forward face  111  are both coated with an antireflective coating to reduce reflection losses. 
     The molded lens  11  may be made entirely using conventional methods such as injection molding. Therefore the molded lens can be formed with a high quality surface and high surface accuracy, and requires no further preparatory operations, such as grinding or polishing. Time required to make the molded lens is short and the cost is low. Furthermore, the antireflective coatings applied to the two end faces of the molded lens do not influence the optical path of transmitted light beams since the molded lens has a uniform refractive index. Finally, the fabrication process does not contaminate the environment or endanger the fabrication workers. 
     The sleeve  14  receives the molded lens  11  and the ferrule  12  therein. The metal holder  15  covers on outer surface of the sleeve  14  to protect the input port  10 . 
     In assembly, the exposed end of the optical fiber  13  is coated with epoxy and is threaded through the conical opening and into the through hole  121  of the ferrule  12 . The ferrule  12  with the attached optical fiber  13  then have a corresponding end thereof ground to a same oblique angle as that of the molded lens  11 . The molded lens  11  and the ferrule  12  with the attached optical fiber  13  are arranged in the receiving cavity of the sleeve  14  so that the forward face  122  of the ferrule  12  is parallel to and separated from the rearward face  112  of the molded lens  11  by a narrow gap defined between the molded lens  11  and the ferrule  12 . This arrangement is designed to assure precise collimation of light beams coming from the optical fiber  13 . The metal holder  15  is attached to the sleeve  14  with epoxy. 
     As shown in FIG. 4, in the present invention, a focal point of the molded lens  11  is located at the point where the through hole  121  intersects with the forward face  122  of the ferrule  12 . Scattered light beams  16  emitted from the optical fiber  13  are refracted at the rearward face  112  of the molded lens  11 , then the light beams  17  are refracted again at the forward face  111  of the molded lens  11  to emerge as parallel light beams  18  from the molded lens  11 . The collimating process of the light beams in the input port  10  is accomplished. 
     Since optical paths are reversible in lenses, light beams from the isolating means  30  directed at a front end of the output port  20  and parallel to a longitudinal axis of the output molded lens (not labeled) can be focused to the exposed end of the output optical fiber (not labeled) at forward face of the output ferrule (not labeled) by the output molded lens (not labeled). 
     As shown in FIG. 1, the isolating means  30  comprises a first polarizer  31 , a Faraday rotator  32 , a second polarizer  33 , and a housing  34 . The first and second polarizers  31 ,  33  are typically made of birefringent crystals, or may be another type of polarizer. The optical axis of the second polarizer  33  is oriented 45 degrees with respect to the optical axis of the first polarizer  31 . The Faraday rotator  32  is disposed in the paths of the light beams from the first polarizer  31  to the second polarizer  33 . The housing  34  holds the polarizers  31 ,  33  and the Faraday rotator  32  together to achieve the isolating function. 
     In operation, the isolating means  30  is located in the path of light beams from the input port  10  to the output port  20 . In the forward direction, the first polarizer  31  of the isolating means  30  separates the incident light from the input port  10  into a first ray, which is polarized along the crystal&#39;s optical axis and which is called an extraordinary ray, and into a second ray, which is polarized in a direction perpendicular to the crystal&#39;s optical axis and which is called an ordinary ray. The light from the first polarizer  31  is then rotated by the Faraday rotator  32 , which rotates the polarized light by 45 degrees. The rotated light is then recombined by the second polarizer  33  and is then output from the output port  20 . 
     In the reverse direction, light from the output port  20  is separated by the second polarizer  33  into a first ray, which is polarized along the crystal&#39;s optical axis and which is called an extraordinary ray, and into a second ray, which is polarized in a direction perpendicular to the crystal&#39;s optical axis and which is called an ordinary ray. When passing back through the Faraday rotator  32 , the light in both rays is rotated 45 degrees. This rotation is nonreciprocal with the rotation of light in the forward direction, so that the ordinary ray from the second polarizer  33  is polarized along the optical axis of the first polarizer  31  and the extraordinary ray from the second polarizer  33  is polarized in a direction perpendicular to the optical axis of the first polarizer  31 . The ordinary and extraordinary rays from the second polarizer  33  have swapped places incident upon the first polarizer  31 , because of this exchange, the light, having passed through the first polarizer  31 , does not leave the first polarizer  31  in parallel rays. The non-parallel light is focused by the molded lens  11  at a point which is not located at the end of the optical fiber  13 . Thus light in the reverse direction is not passed back into the optical fiber  13  of the input port  10 . 
     A mounting tube  40  has a chamber (not labeled) for accommodating and fixing the input and output ports  10 ,  20  and the optical isolating means  30 . Soldering holes  401  are defined between an outside surface (not labeled) of the mounting tube  40  and the chamber (not labeled) of the mounting tube  40 , for soldering the input port  10 , the output port  20 , and the isolating means  30  to an inside of the mounting tube  40 . 
     It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.