1.  Field of the Invention
The present invention relates to a variable optical attenuator.  The variable optical attenuator according to the present invention is mainly for use as the DWDM system assembled in an optical fiber communication system and an optical intensity adjuster of the EDFA light amplifier and so forth.  The variable optical attenuator according to the present invention is based on a new type of optical attenuation principle.  By adoption of this type, extreme miniaturization for high-density mounting and improvement in resolving power of an optical attenuation and in weatherproof stability are achieved.  In the variable optical attenuator according to the present invention, by adoption of this type, the structure thereof can also be simplified resulting in reduced cost in constituent parts and reduced number of assembling steps. 
2.  Description of the Related Art
A typical structure of widely used conventional variable optical attenuator will be described with reference to FIG.  5.  A pair of assemblies xe2x80x9cF1xe2x80x9d and xe2x80x9cF2xe2x80x9d of collimator lenses with optical fibers 4, each collimator lens 4 being unitarily formed of a ferrule 2 having an optical fiber 1 attached thereto by coaxially arranging it about the same optical axis, are used by opposing these collimator lenses 4 to each other.  Between the pair of collimator lenses 4 coaxially arranged with each other is inserted an attenuation filter plate 5 which is a thin glass plate having an optical attenuation film 6 formed on the surface thereof so that the thickness thereof is continuously changed. 
The optical fiber 1 is inserted into the central hole 3 of the ferrule 2 to be fixed thereto.  The optical fiber 1 is bonded to a rod lens 4 in coaxial alignment about the same optical axis as that of the rod lens 4 so as to produce the pair of collimator lenses with optical fibers xe2x80x9cF1xe2x80x9d and xe2x80x9cF2xe2x80x9d.  The collimator lenses with optical fibers xe2x80x9cF1xe2x80x9d and xe2x80x9cF2xe2x80x9d are coaxially aligned in series about the optical axes of both the collimator lenses having a gap between the faces of the rod lenses 4.  The continuously-changing-type attenuation filter plate 5 is provided with the optical attenuation film 6 formed thereon.  The continuously-changing-type attenuation filter plate 5 is formed so as to be movable in the direction perpendicular to the optical axis (the arrow direction) by a step motor and a movement mechanism (not shown).  Corresponding to the movement, the thickness of the optical attenuation film at the optical axial position of the collimator lenses with optical fibers xe2x80x9cF1xe2x80x9d and xe2x80x9cF2xe2x80x9d is changed.  Therefore, emitted light from the collimator lens with the optical fiber xe2x80x9cF1xe2x80x9d is continuously attenuated, as the thickness of the continuously-changing-type attenuation filter plate 5 changes, to impinge onto the collimator lens with the optical fiber xe2x80x9cF2xe2x80x9d. 
The above-mentioned conventional attenuation-filtertype motor-driven variable optical attenuator involves the following problems. 
(Reproducibility of Attenuation Characteristics of the Attenuation Filter) It is extremely difficult to produce the continuously-changing-type attenuation filter having constantly equal attenuation characteristics with high stability.  The manufacturing-technically inevitable dispersed errors such as unevenness in the thickness from manufacturing lot to lot of the continuously-changing-type attenuation filters are produced.  Therefore, in the motor-driven filter using a step motor, the relationship between the displacement of the attenuation filter and the optical attenuation cannot be reproduced, so that smooth optical attenuating control is difficult to be performed.  In addition, when a user adjusts the attenuation manually, the dispersion is not a serious problem. 
(Driving Mechanism and High Accuracy in Assembling) The conventional driving mechanism shown in the drawing has an extremely simple structure formed of several parts made by precision working.  However, in the attenuation filter, the driving mechanism has to be arranged to have a plane precisely orthogonal to the optical axis of the collimator lenses with optical fibers.  When errors of the angle due to the accuracy in the driving mechanism are produced, errors due to the angular errors are produced.  In order to prevent these errors from being produced in the structure shown in FIG.  5, the manufacturing accuracy of parts and the assembling accuracy have to be improved.  That is, the cost of constituent parts is increased and numerous number of steps of assembling and adjusting are simultaneously required, resulting in expensive products. 
(Restriction in Package Miniaturization) It is manufacturing-technically difficult to miniaturize the product by reducing the entire length of the continuously changing type attenuation filter.  The arrangement of the collimator lenses with optical fibers xe2x80x9cF1xe2x80x9d and xe2x80x9cF2xe2x80x9d is a structure in which the optical fibers are extended to the right and left as shown in FIGS.  1A and 1B. In this type of driving mechanism, since the attenuation filter is moved in the direction orthogonal to the optical axis of the collimator lenses with optical fibers, it is required to arrange a step motor and a driving mechanism.  Therefore, the longitudinal length of the package exceeds the sum of lengths of optical fibers xe2x80x9cF1xe2x80x9d and xe2x80x9cF2xe2x80x9d of the collimator lenses with optical fibers while the width of the package is dependent on spaces for the movement of the attenuation filter and the arrangement of the related mechanisms, so that the size is increased.  Accordingly, this is not suitable for a small-sized package for highdensity mounting on an optical communication system.  By the way, the package dimensions of the smallest attenuation filter of this type of conventional attenuator available in the market are 50 mm in length, 25 to 45 mm in width, and 10 to 12 mm in height.  In a half-inch printed circuit board package used heavily in the communication system, the height of devices to be assembled is limited to substantially 8.5 mm, so that the above-mentioned attenuator cannot be attached thereto to be used. 
It is an object of the present invention to provide a reflecting-mirror-type variable optical attenuator based on a new principle so as to solve the problems of the above-mentioned conventional attenuation-filter-type motor-driven variable optical attenuator. 
In order to achieve the above-mentioned object, a reflecting-mirror-type variable optical attenuator according to the present invention is formed based on a new optical attenuation principle, wherein attention is paid to the phenomenon that the optical axis of a light beam emitted from one optical fiber deviates from the optical axis of a collimator lens with optical fibers, in which two optical fibers are attached to a rod lens in symmetrical positions off-centered from the optical axis of the rod lens, by the angle according to the off-centered distance on the surface of the rod lens.  That is, by changing the distance between the surface of a reflecting mirror arranged to be linearly movable in the optical axial direction back and forth and the surface of the rod lens, input light from one optical fiber is attenuated to be output to the other optical fiber. 
To this end, according to one aspect of the present invention, there is provided a reflecting-mirror-type variable optical attenuator comprising a rod lens, a pair of optical fibers connected to one end face of the rod lens at end faces of the optical fibers, optical axes of the optical fibers being symmetrically arranged about the optical axis of the rod lens and in parallel to each other, and a reflecting mirror disposed in the other end face side of the rod lens linearly movable in the direction of the optical axis of the rod lens having a reflecting surface in the direction perpendicular to the optical axis thereof, wherein the quantity of light from one of the pair of optical fibers, which is reflected by the reflecting mirror via the rod lens to impinge on the other of the pair of optical fibers via the rod lens, is adjustable by positioning the reflecting mirror. 
According to another aspect of the present invention, there is provided a reflecting-mirror-type variable optical attenuator comprising a mount, a rod lens supported by the mount, a pair of optical fibers, connecting means for the optical fiber pair connecting end faces of the pair of optical fibers to one end face of the rod lens so that the optical axis of each optical fiber is arranged symmetrically about the optical axis of the rod lens and in parallel to each other, and a reflecting mirror, supporting and guiding means for supporting the reflecting mirror to have a reflecting surface thereof in the direction perpendicular to the optical axis of the rod lens and to be linearly movable in the direction of the optical axis thereof, and driving means for driving the supporting and guiding means so as to adjust the quantity of light from one of the pair of optical fibers, which is reflected by the reflecting mirror via the rod lens to impinge on the other of the pair of optical fibers via the rod lens again. 
The connecting means for the optical fiber pair is a ferrule in which a plurality of optical fibers are arranged in the central hole of the ferrule symmetrically about the central axis of the ferrule and in parallel thereto so as to be joined to the rod lens. 
The ferrule has a central square hole in which two optical fibers are arranged symmetrically about the central axis of the ferrule and in parallel thereto. 
The ferrule has two holes symmetrically about the central axis of the ferrule, in which two optical fibers are respectively arranged symmetrically about the central axis of the ferrule and in parallel thereto. 
The mount is a housing member having a stepped cylindrical hole having a smaller cylindrical hole section and a larger cylindrical hole section in the axial direction, the rod lens being supported by the smaller cylindrical hole section, the supporting and guiding means for supporting the reflecting mirror being supported by the larger cylindrical hole section to be linearly movable by pin-grove connection, and a motor for driving the supporting and guiding means for supporting the reflecting mirror being fixed to the end face of the larger cylindrical hole section. 
The supporting and guiding means for supporting the reflecting mirror is provided with a female thread portion, a male thread portion to be screwed with the female thread potion being driven by a motor. 
The motor is a step motor and the male thread portion is disposed in the output shaft of the motor. 
The reflecting mirror has a gold coating formed on a glass plate or a metallic plate by sputtering or plating. 
The reflecting mirror has a gold coating formed on a glass plate or a metallic plate by sputtering or plating to yield a required attenuation.  In addition to the attenuation absorbed by the reflecting mirror itself, the attenuation can be thus adjusted in accordance with position of the mirror. 
The reflecting mirror has a multi-layer filter coating formed on a glass plate or a metallic plate for only reflecting light within a specific wave length range. 
The rod lens has a shaft having a longitudinal pitch of 0.25.