Abstract:
A variable optical attenuator in micro-electro-mechanical systems includes a moving shutter for attenuating the energy of a light entering the attenuator, a first optical fiber transversely located on one side of the moving shutter with a first inclined surface facing the shutter, and a second optical fiber transversely located on the other side of the moving shutter with a second inclined surface facing the shutter. The second optical fiber and the first optical fiber are parallel to each other with a space and a shift on the same plane.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a variable optical attenuator (VOA), and more particularly, to a VOA having low return loss in micro-electro-mechanical systems (MEMS).  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    Optical attenuators are devices for attenuating optical power in order to measure sensitivity and balance optical path power transmission in fiber optical systems. Generally, optical attenuators have characteristics as being light in weight, small in volume, easy to use, as well as having high accuracy and stability. In recent years, a variable optical attenuator (VOA) has been developed owing to the advancement of semiconductor manufacturing techniques and MEMS techniques. For example, the U.S. Pat. No. 6,275,320 B1 has disclosed a VOA. The VOA used an actuator in it to make a shutter move or incline at different angles in order to shield the optical path thereof and then to change the amount of the energy of the light outputted.  
           [0003]    [0003]FIG. 1 shows a top view of a VOA  10  in MEMS according to the prior art. Referring to FIG. 1, two optical fibers  11  and  12  on the sides of a moving shutter  13  are disposed and located in a fiber optic locator  14 , respectively. Especially, they are aligned to be in one linear line. With respect to the above, the moving shutter  13  is generally designed as having an inclined surface for decreasing the reflected part of the light waves emitted from the terminal facet  15  of the optical fiber  11 . As a result, the light waves reflected back to the interior of the optical fiber  11  can be reduced. The reason for the above is that the reflected light waves may damage the phase resonant effect in the laser resonant cavity, lower the output power, increase noise, and affect the system function as a whole. Therefore, the return loss within an optical fiber is preferably less than −50 dB in designing specifications of a VOA in MEMS. However, as far as the optical fibers  11  and  12  aligned as a linear line are concerned, a reflection of the light waves may occurred at the terminal facet  15  of the optical fiber when the light waves are emitted from the optical fiber  11  to the air medium. As a result, the return loss can not be effectively lowered to −50 dB by solely the inclined surface of the moving shutter  13 , and the product function would be influenced.  
           [0004]    In addition, according to the fundamental principle of fiber optics, the light waves can be transmitted through long distances by total reflection of light within the optical fiber and a minimal transmission loss of light can be achieved at the other end of the optical fiber. FIG. 2 is a schematic view illustrating the traveling path of a light entering an optical fiber. Referring to FIG. 2, optical fiber  1  is basically consisted of dielectric materials namely core  1   b  and cladding  1   a , wherein the refractive index n 1  of the core  1   b  is slightly higher than the refractive index n 2  of the cladding  1   a , such that a light  3  is total reflected within the core  1   b . Suppose the light  3  is transmitted within the optical fiber and the critical angle of the total reflection is θ c , the principle thereof is indicated as the following equations (1) to (3):  
             n   1 ×sin θ c   =n   2 ×sin 90 °=n   2    (1)  
           sin θ c =n 2 /n 1    (2)  
           θ c =sin −1 ( n   2   /n   1 )   (3)  
           [0005]    In addition, the numerical aperture (NA) represents the largest in-coming incident angle that can cause the total reflection within the core of the optical fiber when a light coupled into the optical fiber. Referring to FIG. 2, suppose θ A  is a largest in-coming incident angle when a light  3  coupled into the optical fiber  1 , θ B  is an included angle between the light  3  in the core  1   b  and a central axis  2  of the optical fiber, and the refractive index of air n 0  equals 1, then  
             n   0 ×sin θ A   =n   1 ×sin θ B   =n   1 ×cos θ C    (4)  
           sin θ A   =n   1 ×cos θ C    (5)  
             NA =sin θ A   ={square root}{square root over (n 1   2 −n 2   2 )}   (6)  
           [0006]    Considering the optical fiber practical for applications, the refractive index n 1  of the core thereof is approximately 1.5 and the refractive index n 2  of the cladding is approximately 1.485; the difference between the two is rather small. When n 2 /n 1 =0.99, the critical angle θ C  is approximately 82°, the largest in-coming incident angle θ A  is approximately 12°, and NA=0.21. In other words, the included angle θ B  between the light  3  and the central axis  2  of the optical fiber is approximately 8°. Therefore, when a light is coupled into the optical fiber, the incident angle thereof has to be less than 12°. Besides, for the purpose that an internal total reflection exists, it is necessary to limit the included angle between the light and the central axis within 8° during the transmission of the light within the optical fiber. Otherwise, the light could not be transmitted within the core of the optical fiber.  
         SUMMARY OF THE INVENTION  
         [0007]    Therefore, an object of the invention is to provide a VOA in MEMS capable of reducing return loss and insertion loss.  
           [0008]    Another object of the invention is to provide a method for making a VOA in MEMS. The method is able to effectively lower return loss and insertion loss, thereby bringing the VOA to conform to specifications of optical fiber system applications.  
           [0009]    The VOA in MEMS according to the invention comprises a moving shutter for attenuating the energy of an optical signal entering the VOA; a first optical fiber transversely located on one side of the moving shutter with a first inclined surface facing the shutter; and a second optical fiber transversely located on the other side of the moving shutter with a second inclined surface facing the shutter. The second optical fiber and the first optical fiber are parallel to each other with a first distance in between, and the central axis of the second optical fiber shifts a second distance relative to the central axis of the first optical fiber.  
           [0010]    In one aspect, the oblique angle of the second inclined surface is the same as the oblique angle of the first inclined surface as long as the reflection of the optical signal within the first optical fiber and the insertion loss thereof are reduced. In one embodiment, the first oblique angle is preferably 8°, which effectively decrease the reflected light.  
           [0011]    In another aspect, an angle difference exists between the oblique angles of the first inclined surface and the second inclined surface. Especially, the angle difference has a particular range that allows it to decrease the reflection of the optical signal within the first optical fiber as well as lower the insertion loss thereof.  
           [0012]    It shall be noted that, in the aforesaid aspects, the design of the oblique angle of the first inclined surface prohibits the reflected part of the optical signal at the first inclined surface of the first optical fiber from causing total reflection within the first optical fiber, and the second distance determines the first distance and the oblique angle of the first inclined surface.  
           [0013]    Therefore, the VOA in MEMS according to the invention is capable of effectively reducing the return loss to under −50 dB as a main advantage, thereby elevating the product performance.  
         DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
         [0014]    Descriptions shall be given below with the accompanying drawing for illustrating the VOA in MEMS according to the invention.  
           [0015]    [0015]FIG. 3 shows a top view of a VOA in MEMS according to the present invention. Referring to FIG. 3, the VOA  100  in MEMS according to the invention comprises two optical fibers  111  and  112 , and a moving shutter  113 . Wherein, the two optical fibers  111  and  112  are disposed and located within a fiber optic locator  114 , respectively. The fiber optic locator  114  employed in the invention may be a V-shaped groove, a planar locating groove, a planar locating bump or other devices formed by fiber optic locating methods.  
           [0016]    It shall be noted that, first of all, the first and second fiber optic locators  114  are formed to be parallel to each other on a same plane, and are transversely located on the two sides of the moving shutter  113 , respectively, such that a space L exists between terminal facets  115   a  and  115   b  of the two optical fibers  111  and  112 , respectively. Secondly, a shift S exists between the two fiber optic locators  14 . Thirdly, the two terminal facets  115   a  and  115   b  of the optical fibers  111  and  112  facing the moving shutter  113  are both pared to have an inclined surface with an oblique angle θ, respectively, and the terminal facets  115   a  and  115   b  are parallel to each other. Fourthly, referring to FIG. 4, because of the circumstances that the terminal facets of the optical fibers are pared as inclined surfaces, a reflected light wave  116 a produced at the inclined terminal facet  115   a  is unable to meet the transmission mode of fiber optics, and therefore the reflected light wave  116   a  fails to cause total reflection within the optical fiber  111 . In other words, the light wave  116   a  is unable to be transmitted within the core  111   b  of the optical fiber, and the return loss is effectively lowered. Considering the above, the aforesaid shift S exists as a result of the oblique angle θ, and detailed description shall be given below for illustrating the relationship between the shift S, space L, and oblique angle θ of a VOA in MEMS.  
           [0017]    Referring to FIG. 5, based upon the refraction principle, a deviated angle α is produced when a light wave  117  within the optical fiber  111  travels to the terminal facet  115   a  with an oblique angle θ. As a result, it is necessary for the terminal facet  115   b  of the optical fiber  112  paralleling to the optical fiber  111  to have a same oblique angle θ and a shift S relative to the optical fiber  111 , so that the optical fiber  112  is able to accept the light wave  117  passing through the terminal facet  115   a . That is, the shift S may be varied according to the degree of the oblique angle θ of the optical fibers  111  and  112 . The equations (7) and (8) listed below explain the relationship between the refractive index n 1  of the core of the optical fiber  111 , the oblique angle θ of the terminal facet  115   a , the refraction angle α of the light wave  117 , the refractive index no of air, the space L and the shift S:  
             n   1 ×sin θ= n   0 ×sin(θ+α)   (7)  
             S=L ×tan α  (8)  
           [0018]    Therefore, it is concluded from the equations (7) and (8) that, the shift S is practically determined by the space L and the oblique angle θ while the refractive index n 1  of the core of the optical fiber  111  and the refractive index n 0  of air are known, meaning that the shift S may be adjusted according to the space L and the oblique angle θ.  
           [0019]    In an embodiment of the invention, the terminal facets  115   a  and  115   b  of the optical fibers  111  and  112  are pared as inclined surfaces having an oblique angle of  80 . In addition, the refractive index n 1  of the core (glass material) of the optical fibers  111  and  112  is 1.5 and the refractive index n 0  is 1. According to these conditions and the equations (7) and (8), the refraction angle α is calculated as approaching 4°. It is worth noticing that the reflected light wave is soon diverged with the terminal facets of the optical fibers being pared to have oblique angles between 6° and 12°, and thus the return loss is lowered. Such design of inclined surfaces not only enables the VOA in MEMS to conform to specifications of optical fiber communication applications, but also decreases the return loss within the optical fiber and lowers the insertion loss.  
           [0020]    Summing up the above, descriptions of the preferred embodiments according to the present invention have been illustrated in detail. However, it is to be understood that the embodiments described herein are merely illustrative of the principles of the invention, namely, a wide variety of modifications thereto may be effected by persons skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 shows a top view of a VOA in MEMS according to the prior art.  
         [0022]    [0022]FIG. 2 is a schematic view for illustrating the traveling path of a light entering an optical fiber.  
         [0023]    [0023]FIG. 3 shows a top view of a VOA in MEMS according to one aspect of the invention.  
         [0024]    [0024]FIG. 4 is a schematic view for illustrating the traveling path of a light in FIG. 3 after being reflected at the terminal facet of the optical fiber.  
         [0025]    [0025]FIG. 5 is a schematic view for illustrating the traveling path of a light in FIG. 3 after passing through the terminal facet of the optical fiber.