Abstract:
An variable optical attenuator according to the present invention has: a first optical fiber for emitting a light beam; a first lens for passing the light beam from the first optical fiber while diffusing the light beam; a second lens for focusing part of the diffused light beam; a second optical fiber for transmitting the focused light beam; and actuator for adjusting the optical path length between the first and second lenses.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a variable optical attenuator as one kind of optical modules.  
           [0003]    2. Description of the Related Art  
           [0004]    In the field of optical communications, a wavelength division multiplex system (WDM) has been developed and put into practical use. In this system, a plurality of multiplexed light beams having wavelengths different from each other propagate through the same transmission channel. In the transmission channel, there are interposed several amplifiers at predetermined intervals. Each amplifier, such as an optical fiber amplifier, amplifies the plurality of light beams in a batch to maintain the predetermined intensity of each of the light beams.  
           [0005]    It is desired that the amplified light beams be provided with substantially the same power to prevent the deterioration of transmission quality. However, since the power gain of the optical amplifier has wavelength dependency, the amplified light beams will not have the same power. Accordingly, to provide the light beams with the same power, an optical demultiplexer first separates the amplified light beams according to their respective wavelengths and then a variable optical attenuator corresponding to each of the light beams attenuates each light beam to the predetermined power. Thereafter, attenuated light beams are again multiplexed by means of an optical multiplexer and then allowed to propagate through the transmission channel.  
         SUMMARY OF THE INVENTION  
         [0006]    A variable optical attenuator according to the present invention comprises a first optical component for emitting a light beam, a first lens for passing the light beam from the first optical component while diffusing the light beam, a second lens for focusing part of the diffused light beam, a second optical component for receiving the focused light beam and transmitting the received light beam, and control means for controlling an optical path length between the first and second lenses. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein:  
         [0008]    [0008]FIG. 1 is a perspective view illustrating a variable optical attenuator according to an embodiment of the present invention;  
         [0009]    [0009]FIG. 2 is a plan view illustrating the attenuator of FIG. 1;  
         [0010]    [0010]FIG. 3 is an enlarged view of region III of FIG. 2;  
         [0011]    [0011]FIG. 4 is an enlarged view of region IV of FIG. 2;  
         [0012]    [0012]FIG. 5 is an enlarged view of region V of FIG. 2;  
         [0013]    FIGS.  6  to  12  are explanatory views illustrating the operation of the attenuator of FIG. 1;  
         [0014]    [0014]FIG. 13 is an explanatory view illustrating the function provided for the attenuator of FIG. 1;  
         [0015]    [0015]FIG. 14 is a characteristic view illustrating the optical path length dependency of the attenuator of FIG. 1;  
         [0016]    [0016]FIG. 15 is a characteristic view illustrating the wavelength of the attenuator of FIG. 1; and  
         [0017]    [0017]FIG. 16 is a characteristic view illustrating the polarization dependency of the attenuator of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0018]    For example, a variable optical attenuator  1  (hereinafter referred to as the VOA  1 ) of FIG. 1 is interposed in between an optical fiber  5  extending from an optical demultiplexer and an optical fiber  7  connected to an optical multiplexer. The VOA  1  attenuates the power of a light beam received from the optical fiber  5  at a desired attenuation rate and delivers the attenuated light beam to the optical fiber  7 .  
         [0019]    The VOA  1  comprises a substrate  3  made of Si and the substrate  3  is formed in the shape of a stepped plate. More particularly, the substrate  3  is provided, on an end portion thereof, with a step  11  protruding from an upper surface  9 , and the step  11  has end faces  13 ,  15 . Incidentally, a rim is integrally formed on the periphery the upper surface  9  and located below the step  11 .  
         [0020]    The step  11  is provided with two grooves  17 ,  19  along the Z direction shown by an arrow in FIG. 1. The grooves  17 ,  19  are open at the end faces  13 ,  15 , and the bottom surfaces of the grooves  17 ,  19  are flush with the upper surface  9 .  
         [0021]    The distal end of the optical fiber  5  is secured to the groove  17 . Part of the outer circumference surface of the optical fiber  5  is in contact with the bottom surface of the groove  17 . The optical fiber  5  extends outwardly from the end face  15 . The proximal end of the optical fiber  7  is secured to the groove  19 . Part of the outer circumference surface of the optical fiber  7  is in contact with the bottom surface of the groove  19 . The optical fiber  7  extends outwardly from the end face  15 .  
         [0022]    As shown in FIG. 2, rod lenses  21 ,  23  of graded index fiber are coaxially connected to the distal end of the optical fiber  5  and to the proximal end of the optical fiber  7  by fusion splicing, respectively. The rod lenses  21 ,  23  are disposed inside the grooves  17 ,  19 , respectively. The optical fibers  5 ,  7  are equal to each other in outer diameter, and the centers of the rod lenses  21 ,  23  are approximately at the same height from the upper surface  9 .  
         [0023]    There are provided an actuator  24 R, an actuator  24 L, and joint/lock mechanisms  26 , which are of use for fixing and/or displacing an optical reflector  25  in the Z direction and will be described later, on the upper surface  9 .  
         [0024]    The optical reflector  25  for coupling the rod lens  21  optically to the rod lens  23  is located on the side of the end face  13  of the step  11 . As shown in FIG. 3, the reflector  25  has an incidence surface  27  and an outgoing surface  29 , which are each inclined at an angle of 45 degrees to the Z direction shown in the figure and orthogonal to each other. An optical axis  30  that connects between the rod lens  21  and the rod lens  23  is indicated with a chain double-dashed line. The optical axis  30  extending from the rod lens  21  is bent into an angle of 90 degrees once at the incidence surface  27  and outgoing surface  29 , respectively, and then leads to the rod lens  23 .  
         [0025]    For example, a block made of Si may be etched to form the aforementioned reflector  25 . Further, the incidence surface  27  and the outgoing surface  29  have, for example, a metal film such as Al deposited thereon. Incidentally, for example, a metal plate of Al can also be bent to obtain the reflector  25 .  
         [0026]    The reflector  25  is secured by means of a thermosetting adhesive to one end portion  33  of a movable stage  31 . The movable stage  31  made of Si is formed in the shape of a plate extending in the Z direction and placed on the upper surface  9  movably in the Z direction.  
         [0027]    On both sides of the other end portion  35  of the movable stage  31 , there are formed rack teeth  37 ,  39 , respectively. The pitch of the rack teeth  37 ,  39  is 3 μm as shown in FIG. 4. The rack teeth  37 ,  39  mate each with rack teeth  45 ,  47 , which are formed on joints  41 ,  43 .  
         [0028]    The joints  41 ,  43  are formed of Si and the surfaces thereof are covered with silicone oxide. The joints  41 ,  43  kept lifted off the upper surface  9 . The joints  41 ,  43  have comb portions  49 ,  51  formed in one piece therewith opposite to the rack teeth  45 ,  47 . Combs  53 ,  55  are each disposed to face the comb portions  49 ,  51  and are formed in one piece with the rim of the upper surface  9 .  
         [0029]    The joints  41 ,  43  and the comb portions  49 ,  51  are integrally formed in one piece substantially on the center of longitudinal beams  57 ,  59  that extend in the Z direction. The longitudinal beams  57 ,  59  are formed of Si and kept lifted off the upper surface  9 . As shown in FIGS. 3 and 5, both the ends of the longitudinal beams  57 ,  59  are connected to combs  61 ,  63  extending in the X direction. The longitudinal beams  57 ,  59  have elasticity and the center thereof is allowed for elastic displacement in the X direction with nodes at their both ends. Therefore, the joints  41 ,  43  are movable in the X direction.  
         [0030]    Bridges  65 ,  67  extend in the Z direction, opposite to each other, from the center of the teeth side of the combs  61 ,  63 . The bridges  65 ,  67  are also kept lifted off the upper surface  9 . Combs  69 ,  71 , parallel to the combs  61 ,  63 , are secured to ends of the bridges  65 ,  67 , respectively. The aforementioned combs  61 ,  63 ,  69 ,  71  are all formed of Si and kept lifted off the upper surface  9 .  
         [0031]    When viewed from the Z direction, ends of cross beams  73 ,  75 ,  77 ,  79  are connected to the centers of the bridges  65 ,  67 , respectively. The cross beams  73 ,  75 ,  77 ,  79  extend in the X direction and their proximal ends are integrally connected to the rim of the upper surface  9 , respectively. The cross beams  73 ,  75 ,  77 ,  79  have elasticity to make the distal ends thereof displaceable in the Z direction, thereby allowing the bridges  65 ,  67  to be displaced in the Z direction. Therefore, this also makes the aforementioned combs  61 ,  63 ,  69 ,  71 , the longitudinal beams  57 ,  59  and the joints  41 ,  43  displaceable in the Z direction via the bridges  65 ,  67 .  
         [0032]    The bridge  65  is provided with a notch portion  81 , which allows the end portion  33  of the movable stage  31  to move closer to the end face  13  than the comb  61 . That is, the notch portion  81  allows the reflector  25  to be brought closer to the end face  13 .  
         [0033]    Combs  83 ,  85  and  87 ,  89  are located near the combs  61  and  63 , respectively, to mate therewith. The bridge  65  is interposed between the combs  83  and  85 , while the bridge  67  is interposed between the combs  87  and  89 . The combs  83 ,  85 ,  87 ,  89  are formed on the upper surface  9 . In addition, combs  99 ,  101  are formed near the combs  69 ,  71 , respectively, to mate therewith. The combs  99 ,  101  are integrally formed on the rim of the upper surface  9 , respectively.  
         [0034]    As shown in FIG. 4, locks  107 ,  109  are located adjacent to both the sides of the end portion  35  of the movable stage  31 , respectively. The locks  107 ,  109  are kept lifted off the upper surface  9  and provided with rack teeth  111 ,  113  that mate with the rack teeth  37 ,  39  of the movable stage  31 , respectively. There are formed comb portions  115 ,  117  on the locks  107 ,  109  opposite to the rack teeth  111 ,  113 . Comb portions  119 ,  121  are arranged near the comb portions  115 ,  117  to mate therewith and integrally formed on the longitudinal beams  57 ,  59 .  
         [0035]    The locks  107 ,  109  are provided on distal ends of longitudinal beams  123 ,  125  formed of Si. Each of the proximal ends of the longitudinal beams  123 ,  125  is secured to the upper surface  9  so as to be a supporting point. The longitudinal beams  123 ,  125  have elasticity to allow for displacement of the distal ends thereof in the X direction, thereby making the locks  107 ,  109  displaceable in the X direction.  
         [0036]    The following components, having been described above, can be formed on the substrate  3  through the micro-machining technique comprising well-known processes of dummy-layer deposition and etching and constitute a displacement mechanism for displacing the reflector  25  in the Z direction as described later. That is, the components are the movable stage  31 , the joints  41 ,  43 , the longitudinal beams  57 ,  59 ,  123 ,  125 , the locks  107 ,  109 , the combs  53 ,  55 ,  61 ,  63 ,  69 ,  71 ,  83 ,  85 ,  87 ,  89 ,  99 ,  101 , the bridges  65 ,  67 , the cross beams  73 ,  75 ,  77 ,  79 , the comb portions  49 ,  51 ,  115 ,  117 ,  119 ,  121 , and the rack teeth  37 ,  39 ,  45 ,  47 ,  111 ,  113 . Incidentally, the displacement mechanism formed by the micro-machining technique is a type of a micro-electro-mechanical system (hereinafter referred to as the MEMS).  
         [0037]    Now, explained below is the case where the reflector  25  is displaced towards the end face  13  when viewed in the Z direction so that the optical path length between the rod lenses  21  and  23  is made shorter.  
         [0038]    Referring to FIG. 6, the rack teeth  39  ( 37 ) engage with the rack teeth  113  ( 111 ) of the lock  109  ( 107 ), thereby preventing the movable stage  31  from being displaced in the Z direction. The rack teeth  39  ( 37 ) of the movable stage  31  also engage with the rack teeth  47  ( 45 ) of the joint  43  ( 41 ). As shown in FIG. 7, application of a voltage between the lock  109  ( 107 ) and the longitudinal beam  59  ( 57 ) causes an electrostatic force to act upon the comb portion  117  ( 115 ) and the comb portion  121  ( 119 ). This in turn causes the lock  109  ( 107 ) to be displaced in the X direction towards the longitudinal beam  59  ( 57 ) to disengage the rack teeth  113  ( 111 ) from the rack teeth  39  ( 37 ). As shown in FIG. 8, application of a voltage between the comb  61  and the comb  85  ( 83 ) in this condition cause the comb  61  to be displaced towards the comb  85  ( 83 ) due to an electrostatic force while the cross beams  73  and  75  are elastically deformed. Therefore, this also causes the movable stage  31  to be displaced towards the comb  85  ( 83 ) along with the comb  61 , the longitudinal beam  59  ( 57 ) and the joint  43  ( 41 ). That is, the reflector  25  is displaced towards the end face  13 .  
         [0039]    As shown in FIG. 9, turning off the voltage between the lock  109  ( 107 ) and the longitudinal beam  59  ( 57 ) causes the rack teeth  113  ( 111 ) of the lock  109  ( 107 ) to mate again with the rack teeth  39  ( 37 ) of the movable stage  31 , so that the lock  109  ( 107 ) secures the movable stage  31 . Then, as shown in FIG. 10, application of a voltage between the comb portion  51  ( 49 ) of the joint  43  ( 41 ) and the comb  55  ( 53 ) opposite thereto causes the rack teeth  47  ( 45 ) of the joint  43  ( 41 ) to be disengaged from the rack teeth  39  ( 37 ) of the movable stage  31 . Then, as shown in FIG. 11, turning off the voltage between the comb  61  and the comb  85  ( 83 ) in this condition causes the joint  43  ( 41 ) to return to the same position as that shown in FIG. 6 in the Z direction due to the restoring force of the cross beams  73  and  75 . Then turning off the voltage between the comb portion  51  ( 49 ) and the comb  55  ( 53 ) causes the rack teeth  47  ( 45 ) of the joint  43  ( 41 ) to engage again with the rack teeth  39  ( 37 ) of the movable stage  31  as shown in FIG. 12.  
         [0040]    As can be seen from FIGS. 6 and 12, the one cycle of the aforementioned operations causes the movable stage  31  or the reflector  25  to be displaced by a pitch of the rack teeth  39  ( 37 ), or 3 μm, towards the end face  13  in the Z direction. Incidentally, for simplicity, the operations of the comb portion  69  and the comb  99  have not been explained in the foregoing, however, the comb portion  69  and the comb  99  act in the same manner as the comb  61  and the comb  85  ( 83 ), respectively.  
         [0041]    Thus it can be said that a pair of the comb  61  and combs  83 ,  85 , a pair of the comb  69  and comb  99 , and the bridge  65  constitute the actuator  24 R utilizing an electrostatic force for making the optical path length shorter. And it can be also said that a pair of the comb  63  and combs  87 ,  89 , a pair of the comb  71  and comb  101 , and the bridge  67  constitute the actuator  24 L utilizing an electrostatic force for making the optical path length not shorter but longer. Further, it can be said that the joint/lock mechanisms  26  are constituted by the joints  41 ,  43 , locks  107 ,  109 , and comb  53 ,  55  utilizing electrostatic forces.  
         [0042]    Suppose the reflector  25  is displaced opposite to the end face  13  in the Z direction, the optical path length between the rod lenses  21  and  23  is elongated. This also is performed by the aforementioned cycle of operations except for using the combination of the electrostatic forces between the comb  63  and the combs  87 ,  89  and between the comb  71  and the comb  101 .  
         [0043]    As can be seen from FIG. 2, the aforementioned VOA  1  allows a light beam demultiplexed by the optical demultiplexer to be emitted from the rod lens  21  that is secured to the end portion of the optical fiber  5 , and then the light beam impinges on the incidence surface  27  of the reflector  25 . Subsequently, the light beam is reflected on the incidence surface  27  and the outgoing surface  29  of the reflector  25  to impinge on the rod lens  23  and propagate through the optical fiber  7  to the optical multiplexer.  
         [0044]    As shown in FIG. 13, the light beam emitted from the rod lens  21  diffused along the optical axis  30  so that the diameter of the light beam gradually increases toward the reflector  25  due to the effect of diffusion of the rod lens  21 . Therefore, only a part of the diffused light beam can be incident on the end surface of the rod lens  23  and then is focused with the rod lens  23  to propagate to the optical fiber  7 . Accordingly, the power of the light beam propagating through the optical fiber  7  is attenuated compared to the power of the light beam propagating through the optical fiber  5 .  
         [0045]    Suppose that the reflector  25  is displaced by a length L in the Z direction opposite to the rod lenses  21 ,  23  or the end face  13  as shown in FIG. 13. This displacement of the reflector makes the distances between the incidence plane  27  and the rod lens  21  and between the outgoing plane  29  and the rod lens  23  longer by the length L, respectively, while the distance between the incidence plane  27  and the outgoing plane  29  is the same. Accordingly, the displacement of the reflector  25  makes the optical length between the rod lenses  21  and  23  longer and the increase of the optical path length being twice times longer than the length L. The increase of the optical path length provides an increased area of incidence for the light beam on a virtual plane including the end surface of the rod lens  23 . Consequently, the intensity of the light beam incident on the end surface of the rod lens  23  is reduced, thus causing the optical attenuation to be increased. On the contrary, when the optical path length is made shorter, the area of incidence of the light beam on the virtual plane is decreased and the optical attenuation is thereby reduced. In other words, according to the VOA  1 , displacement of the reflector  25  leads to the variation of the optical path length extending between the rod lenses  21  and  23 , and thereby results in the adjustment of the optical attenuation.  
         [0046]    The aforementioned VOA  1  has no members that absorb the light beam. Light-absorbing members would generate a greater amount of heat as the power of the light beam received increases, leading to damage thereto in some cases. The prior-art variable optical attenuator has a light-absorbing film, which is provided on an optical axis, for absorbing a light beam and thereby attenuating the power of the light beam. In contrast to the prior-art variable optical attenuator, the VOA  1  is applicable to an optical transmission of a larger power.  
         [0047]    Furthermore, for example, the VOA  1  is only required to displace the reflector  25  by 1750 μm to provide an optical attenuation of 30 dB. This allows the displacement mechanism for displacing the reflector  25  to be constituted by a type of the aforementioned so-called MEMS. In addition to this, the VOA  1  is provided with a simple optical system comprising lenses and an optical reflector, so that the VOA  1  is small in size.  
         [0048]    In contrast to the VOA  1 , according to the prior-art VOA provided with the aforementioned light-absorbing film that varies in thickness in one direction, it is possible to adjust the optical attenuation by displacing the light-absorbing film in the aforementioned direction. However, the prior-art VOA is required to displace the light-absorbing film on the order of 1 cm to provide an optical attenuation of 10 dB, for example. Accordingly, in addition to an increase in the maximum thickness of the light-absorbing film, it is necessary to employ a stepping motor or the like to constitute the displacement mechanism for displacing the light-absorbing film. This restricts the miniaturization of the variable optical attenuator.  
         [0049]    A prior-art variable optical attenuator that employs a magneto-optical effect requires a complicated optical system comprising a Faraday rotator, a birefringent crystal, a permanent magnet, a polarizer, and an analyzer. This also makes it difficult to reduce the size of the prior-art VOA.  
         [0050]    The VOA  1  is provided with an optical system comprising lenses and a reflector whose optical characteristics have low wavelength dependency. Accordingly, the VOA  1  can control the optical attenuation independently with respect to the wavelength of the light beam, thereby making it possible to readily provide the same power for a plurality of light beams having different wavelengths.  
         [0051]    In contrast to this, the prior-art variable optical attenuator that employs the magneto-optical effect comprises optical components having a high wavelength dependency and therefore causes the optical attenuation to depend largely on the wavelength. More specifically, suppose that a light beam of wavelength 1565 nm is attenuated by means of the prior-art variable optical attenuator of this type under the condition that an optical attenuation of 30 dB is provided for a light beam of wavelength 1535 nm. In this case, an optical attenuation of approximately 33 dB is provided for the light beam of wavelength 1565 nm. That is, there is a difference by 3 dB in the optical attenuation between the light beams of wavelengths 1535 nm and 1565 nm.  
         [0052]    On the other hand, a prior-art shutter-type variable optical attenuator that employs a shutter to control the interruption of the optical path and thereby vary the attenuation of a light beam is described in IEEE Journal of Selected Topics in Quantum Electronics, vol. 5, No. 1, January/February 1999, pp18-25. According to the variable optical attenuator of the shutter type, since the mode field diameter has wavelength dependency, the optical attenuation has a high wavelength dependency. More specifically, suppose that a light beam of wavelength 1600 nm is attenuated by means of the prior-art variable optical attenuator of this type under the condition that an optical attenuation of 12.8 dB is provided for a light beam of wavelength 1530 nm. In this case, an optical attenuation of approximately 12.0 dB is provided for the light beam of wavelength 1600 nm. Likewise, suppose that a light beam of wavelength 1600 nm is attenuated under the condition that an optical attenuation of 13.5 dB is provided for a light beam of wavelength 1530 nm. In this case, an optical attenuation of approximately 12.5 dB is provided for the light beam of wavelength 1600 nm. That is, the wavelength dependency is increased as the optical attenuation is increased.  
         [0053]    Therefore, to attenuate a plurality of light beams having different wavelengths by means of these prior-art variable optical attenuators, it is necessary to set the optical attenuation of each variable optical attenuator in connection with the wavelength of the light beam. This makes the control of the attenuator complicated.  
         [0054]    On the other hand, the VOA  1  is provided with the optical system comprising lenses and a reflector whose optical characteristics have no dependency on the polarization of the light beam. Therefore, the VOA  1  provides an optical attenuation that has no dependency on polarization. This makes it possible for the VOA  1  to attenuate the light beam independently from the polarization thereof.  
         [0055]    In contrast to this, the aforementioned shutter-type variable optical attenuator has a high polarization dependency of optical attenuation. This is because the light beam interrupted by the shutter is diffracted to spread and the spread light beam is incident upon an optical fiber, which has polarization dependency of reflectivity near the circumferential rim of the core thereof. Accordingly, for the shutter-type optical attenuator, it is necessary to set the optical attenuation in connection with the polarization of the light beam. This makes the control complicated and in some cases makes it difficult to provide the light beam with stable power.  
         [0056]    [0056]FIG. 14 illustrates the optical path length dependency of the optical attenuation of the VOA  1 . That is, FIG. 14 shows the relationship between the length of the optical axis  30  extending between the rod lenses  21  and  23  and the attenuation of the light beam having a wavelength of 1550 μm. For convenience, the abscissa shown in FIG. 14 denotes the increment of the optical path length compared to the shortest optical path length, where the reflector has been brought the closest to the end face  13 . More specifically, the abscissa represents variations in optical path length provided when the reflector  25  is displaced opposite to the end face  13  in the Z direction. As can be seen from FIG. 14, the insertion loss of the VOA  1  or the optical attenuation for the shortest optical path length is 0.3 dB while displacement of the reflector  25  opposite to the end face  13  by 1750μm or increasing the optical path length by 3500 μm provides an optical attenuation of 30 dB. The VOA  1  thus can provide the range of optical attenuation from 0.3 dB to 30 dB by varying the optical path length.  
         [0057]    [0057]FIG. 15 shows the wavelength dependency of the VOA  1 . Solid lines A, B, C indicate the results of measurement of optical attenuations obtained by varying the wavelength of the light beam on the condition that the optical path length between the rod lenses  21  and  23  of VOA  1  is adjusted to provide an optical attenuation of 10 dB, 20 dB, and 30 dB in FIG. 14, respectively. As can be seen from the solid line C in FIG. 15, when the optical path length of the VOA  1  is adjusted to provide an optical attenuation of about 30 dB, the difference in optical attenuation between wavelengths 1530 nm and 1580 nm is as extremely low as 0.36 dB or less. Also as can be seen from the solid lines A, B, the differences in optical attenuation between wavelengths 1530 nm and 1580 nm are 0.18 dB and 0.25 dB at the optical path lengths having the optical attenuation of about 10 dB and 20 dbB, respectively. Thus it can be said from this fact that the optical attenuation of the VOA  1  is independent of wavelength.  
         [0058]    [0058]FIG. 16 shows the polarization dependency of optical attenuation of the VOA  1 . A Dashed line D, a solid line E, and a chain double-dashed line F indicate the results of measurement of polarization dependency losses obtained by varying the wavelength of the light beam at constant intervals on the condition that the optical path length between the rod lenses  21  and  23  being each adapted to provide an optical attenuation of 10 dB, 20 dB, and 30 dB in FIG. 14, respectively. The polarization dependency loss means the difference in optical attenuation between two light beams having polarization planes orthogonal to each other at each wavelength. As can be seen from FIG. 16, polarization dependency losses are 0.05, 0.1, and 0.21 dB or less within the wavelength range from 1530 to 1580 nm at the optical path lengths having the optical attenuation of about 10 dB, 20 dB and 30 dB, respectively. It can be thus said from this fact that the optical attenuation of the VOA  1  is independent of polarization of the light beam.  
         [0059]    Incidentally, the present invention is not limited to the aforementioned embodiment but may be modified in a variety of ways.  
         [0060]    For example, in the embodiment, a rod lens of a grated index fiber was employed to diffuse or focus a light beam. However, the present invention is not limited thereto but may employ any lenses so long as the lenses can diffuse or focus a light beam. The lenses may include a spherical lens, an aspherical lens, or a combination of a plurality of lenses.  
         [0061]    In the embodiment, the rod lenses and optical fibers were connected to each other by fusion splicing, however, may be disposed spaced apart from each other. Nevertheless, the method for securing the rod lens and the optical fiber connected to each other by fusion splicing to the groove is still preferable since the method facilitates the alignment of the optical axis of the lens with that of the optical fiber.  
         [0062]    Furthermore, in the embodiment, the reflector was formed in the shape of a bent plate, however, may be formed in the shape of a block such as a prism which is formed of an optically transparent material and has reflective planes orthogonal to each other.  
         [0063]    Furthermore, the actuators employed an electrostatic force, however, may be provided with an electromagnet to use the electromagnetic force thereof. Nevertheless, it is still preferable that the actuator utilizes the electrostatic force since such an actuator can be readily reduced in size.  
         [0064]    Still furthermore, in the embodiment, an optical fiber was employed as an optical component for allowing light into the VOA and delivering light from the VOA. However, the optical component is not limited thereto but may be any one such as an optical waveguide so long as the optical component can allow the light beam to propagate therethrough while maintaining the phase and amplitude of the light beam at an appropriate level.  
         [0065]    Furthermore, in the embodiment, the variable optical attenuator was interposed in between the optical demultiplexer and optical multiplexer. However, the variable optical attenuator according to the present invention can be incorporated into an optical demultiplexer and/or an optical multiplexer.