Abstract:
A variable optical attenuator ( 200 ) comprises a dual fiber collimator ( 202 ) and a reflection component ( 206 ) separated by a linear moveable ND filter ( 204 ). The collimator ( 202 ) includes a ferrule ( 304 ) with dual fibers ( 310, 320 ) therein, and a GRIN lens ( 302 ) spaced from the ferrule ( 304 ) with a distance in compliance with the distance between the reflection component ( 206 ) and the GRIN lens ( 302 ).

Description:
BACK GROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of optical power regulators for fiber optic network, and particularly to a dual fiber variable optical attenuator. 
     2. The Related Art 
     A fiber optical attenuator is an optical component that is intended to control the optical power propagating in the fiber. Optical attenuators are widely used in the optical transmission system and the optical network, and especially in the optical wavelength division multiplexing (WDM) network. The motor driven optical variable attenuator is a new kind of related device used in the WDM network. 
     U.S. Pat. No. 5,745,634 discloses a costly design using two lens for optical coupling and using a normal DC motor to control the motion. 
     Differently, the copending application Ser. No. 09/174,367 having the same assigned with the instant application, discloses a systematic design where a neutral density (ND) filter is placed between two collimators, and a step motor is employed to electrically control the position of the ND filter for obtaining the specific attenuation value. Under this situation, the input and the out put fibers are respectively positioned on two sides of the whole package box. 
     As noted, in the WDM optical network system arrangement, it is preferred to have the input fiber and the corresponding output fiber extend on the same side for compact size consideration. Thus, the copending application Ser. No. 09/255,915 also having the same assignee with the instant application, demonstrates the architecture of duel fiber collimator optical variable attenuator, where both the input fiber and the output fiber are located on one side of the whole package. 
     Anyhow, it is noted because in the copending 09/255,915 design, the reflection of the light occurs on the ND fiber which is actuated/controlled by the step motor. In practice, this type of design may create attenuation instability due to relatively great tolerance/oscillation of the motor motion which results in the inferior reflection effect. 
     Therefore, an object of the invention is to provide a variable optical attenuator not only with a compact size but also very good reliability performance being not influenced by the motion of the step motor. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention, a variable optical attenuator comprises a dual fiber collimator and a reflecting component separated by a linearly moveable ND filter wherein said ND filter is either manually controlled or electrically controlled by a step motor. 
     Another feature of the invention provides significant spacing between the GRIN lens and the reflecting component (such as a mirror) to place the ND filter in between. 
     Another feature of the invention provides spacing between the ND filter and the reflecting component to have the focal point of the GRIN lens located on the reflecting component. 
     Another feature of the invention provides a lateral offset between the axis of the ferrule, a mechanical part, and that of the GRIN lens so as to efficiently capture the reflected light from the reflecting component and the ND filter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a preferred embodiment of a variable optical attenuator, according to the invention. 
     FIG. 2 is a partial enlarged view of the ND filter, the nut and the lead screw of FIG.  1 . 
     FIG. 3 is a diagram showing roughly the optical signal path in FIG.  1 . 
     FIG. 4 is a perspective view of the collimator to show the offset arrangement between the axis of the GRIN lens and that of the ferrule of FIG.  1 . 
     FIG. 5 is a diagram of another embodiment of the variable optical attenuator according to the invention. 
     FIG. 6 is a schematic diagram of the O-rings, the screw drive and the case. 
     FIG. 7 is a diagram to show the relation between the PDL and the attenuation of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     References will now be in detail to the preferred embodiments of the invention. While the present invention has been described in with reference to the specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those, skilled in the art without departing from the true spirit and scope of the invention as defined by appended claims. 
     It will be noted here that for a better understanding, most of like components are designated by like reference numerals throughout the various figures in the embodiments. Attention is directed to FIGS. 1-4 wherein the miniature VOA (variable optical attenuator)  200  comprises a dual fiber collimator  202 , a high reflection mirror  206 , and an ND filter  204  therebetween, wherein the collimator  202  and the mirror  206  are both immoveably mounted to a substrate  208  in the case  216 , while the ND filter  204  is moveably received within the substrate  208  along a transverse direction relative to the axis direction of the collimator  202  or of the reflection mirror  206 . 
     The ND filter  204  is seated on a nut  210  which is mechanically connected to a lead screw  212  which links to a step motor  214  located in the case  216  beside the substrate  208 . 
     The collimator  202  includes a dual fiber ferrule  304  with the input fiber  310  and the output fiber  320  therein, and a GRIN lens  302  relatively secured with each other, as shown in FIG.  3 . 
     Therefore, the input optical signal  306  passes the input fiber  310  of the dual fiber ferrule  304 , the GRIN lens  302 , the ND filter  204 , and coincides on the high reflection broad band mirror  206 . Correspondingly, the reflected signal  308  backwardly passes through the ND filter  204 , the GRIN lens  302  and enters the outer fiber  320  in the dual fiber ferrule  304 . 
     Understandably, similar to the aforementioned two copending applications with the same assignee, in the instant application the ND filter  204  is moveably linearly intercept between the collimator  202  and the mirror  206  by cooperation of the step motor  214 , the lead screw  212  and the nut  210 , for attenuation. 
     FIG. 4 shows the sleeve  309  encloses the GRIN lens  302  and the ferrule  304  wherein the center axis  303  of the GRIN lens  302  is laterally offset from the center axis  305  of the ferrule  304 . This is a feature of the invention, and will be illustrated with reasons later. 
     FIGS. 5 and 6 show another embodiment of the invention wherein the ND filter  504  is also mounted to a nut  511  actuated by the screw lead  510  which is further linked to an adjustable screw driver  506 . The screw driver  506  can be installed to the case knob  508  via a pair of O-rings  608  as shown in FIG.  6 . 
     Several features are provided in the invention. 
     (1) In comparison with the two aforementioned copending applications in which the ND filter is directly coated with a high reflection layer; on the surface facing to the collimator, thus resulting in instability due to oscillation of motor motion, oppositely in the invention the reflection mirror  206  is immoveably mounted in the attenuator  200 , and spaced from the ND filter  204  and at the opposite side of the ND filter  204  relative to the collimator  202 . The securement of the mirror in the invention assures stability of the attenuation. 
     (2) In the invention, because the reflecting mirror  206  is disposed distantly from the ND filter  204 , it is arranged to have the mirror  206  is located around the focal point of the GRIN lens  302 , where the focal length is D 1  for precisely and fully reflecting the signals thereof with the GRIN lens  302  functioning as a converging lens. 
     (3) Different from the prior art which generally has the ferrule and the GRIN lens closely attached to each other for easily controlling the transmitted signal path, oppositely in the invention, to comply/cooperate with distance D 1  between the GRIN lens  302  and the mirror  206 , the spacing/distance between the GRIN lens  302  and the ferrule  304  of the collimator  202  is correspondingly dimensioned as D 2  wherein D 1 ≈D 2 . The reason is that in the invention, the reflecting mirror  206  and the ferrule  304  are arranged to be placed about both focus points (front focus and back focus) of the GRIN lens  302  wherein the GRIN lens  302  may function as a converging lens to have the incoming/forward light converging on the mirror  206  at a point where a center axis of the GRIN lens  302  passes, while simultaneously the reflecting/backward light from the mirror  206  may be directed in a parallel manner to be guided to the fiber end in the ferrule  304 . 
     (4) Different from the prior art which generally has the center axis of the ferrule aligned with the center axis of the GRIN lens, in the invention because the ferrule  304  and the GRIN lens  302  is spaced from each other with a significant distance D 2 , the center axis of the dual fiber ferrule and the center axis of the GRIN lens are intentionally laterally offset from each other to optimize the optical path due to the existing 8 degrees of the GRIN lens  302  and the ferrule  304 . Understandably, this lateral offset is linearly proportion to the distance D 2 . It is noted that FIG. 3 is only a rough sketch without precisely/clearly showing such an offset and shift of the signal paths thereof, while FIG. 4 does a little bit exaggeratedly. 
     (5) In the invention, the ND filter is tilted for lowering the PDL (Polarization Dependent Loss). 
     Therefore, in comparison with the aforementioned prior art designs, the invention has the following advantages: 
     (1) In U.S. Pat. No. 5,745,634, the variable optical attenuator requires two collimators. In opposite, the invention only need one, thus reducing the manufacturing cost. 
     (2) In the copending application Ser, No. 09/174,367, the in-and-out fibers are placed on both sides of the whole package case, It may bring some inconvenience of mounting VOA on the system board since the customer needs to create more space for taking care of both side fibers. Understandably, this makes it difficult to design the compact system board. The invention solves this problem by using dual fiber collimator whereby the in-and-out fibers are placed on the: same side of the VOA package. 
     (3) The invention is of a low PDL (Polarization Dependent Loss) design. As noted, PDL is caused by the difference of the P light and the S light after transmitting the ND filter. Because the attenuation changes 20 dB along the 4 mm length of the filter, the metal material coating causes/requires a tiny wedge on the glass plate. Thus, this wedge plate is essentially a polarization dependent device. Generally, PDL is more than 0.3 dB. 
      The copending application Ser. No. 09/255,915 uses an extra glass slab between the ND filter and the dual fiber collimator to compensate PDL due to the ND filter, thereby increasing the insertion loss of the device and complicating the structures thereof. In opposite, in our invention the lower PDL is obtained by only tilting the ND filter within preferably a 5-20 degrees range, and particularly at a 11 degrees angle. Therefore, there is no need to use an extra glass slab for PDL compensation. FIG. 7 shows in the invention the PDL is less than 0.1 dB for attenuation up to 30 dB. 
     (4) The two O-rings  608  tightly seal the gap between the screw driver  506  and the VOA box for protecting the VOA from moisture attach and enhancing the reliability. 
     While the present invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims. 
     Therefore, person of ordinary skill in this field are to understand that all such equivalent structures are to be included in the scope of the following claims.