Abstract:
A controllable attenuator includes a pair of collimators respectively connected to input and output fibers. A pair of reflection devices are respectively positioned behind the pair of collimators opposite to the corresponding input and output fibers. A U-like light path is defined among the pair of collimators and the pair of reflection devices. A neutral density filter is moveably positioned between the pair of reflection devices wherein a moving direction of the filter is preferably parallel to a longitudinal direction of the pair of collimators. An ND filter position indicator such as a potentiometer, is used to dynamically monitor attenuation setting.

Description:
This application claims the benefit of and refers to a provisional application filed Jan. 3, 2001 with a Ser. No. 60/259,393. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the optical power regulators for fiber optical network, and particularly to a variable optical attenuator (VOA) for controllably varying the strength of the optical signals. 
     2. The Related Arts 
     In the fiber optic networking system, the light signals are transited along optical fibers to transfer information from one location to another. It is often desirable to tailor the power of the optical signals within optical fiber networks. For example, the individual components of an optical fiber network may be tested by using a low power optical signal to simulate fiber optic telecommunications or data communications over a long distance. Tailoring of optical signal strengths is expected in automatic optical testing systems, optical signal routing systems, and optical sensor arrays etc. 
     U.S. Pat. No. 5,3111,613 issued on Oct. 5, 1999, describes an attenuator based on polarization modulator (a liquid crystal material). This type attenuator has an issue of controlling PDL (polarization dependent loss), and the associated optical performance is subject to the environmental temperature. 
     U.S. Pat. No. 5,745,634 issued Apr. 28, 1998, describes a voltage-controlled attenuator where the optical path consists of two collimators, one mirror and one filter. The driving means of the filter is constructed by a DC motor, a gearbox, and a rotation disk. There is a lever labeled 30 in FIG. 2B thereof which connects the filter and the rotation disk. This mechanical structure has a concern of difficulties of making the VOA device compact since the optical path of the optical components is essentially parallel to the mechanical structures. Also, the cost of this device is relatively high since the precision gearbox is utilized thereof for implementation of the whole design. 
     U.S. Pat. No. 6,130,984 issued Oct. 10, 2000, discloses a voltage controlled VOA device. A dual fiber collimator is used. In terms of the topical performance, it has a concern of the PDL. Also, the filter driving means, i.e., the motor, is arranged perpendicular to the direction of the fiber or the optical path. This type design results in irregular package size. As illustrated in column 6, lines 5-10 thereof, the device length along the fiber direction is 37 mm, but the device width perpendicular to that fiber direction, is 70 mm. It means that the width is much longer than the length. In respect with the optical system design, there is a desire to have a smaller device width for the compact system equipment. U.S. Pat. No. 6,292,616 demonstrates a U-like frame to form an optical path where the ND filter is designed to be wedge-shaped to reduce the wavelength sensitivity. However this device is subject to the relatively large PDL thus being unsuitable for the high-speed optical network. In general, the PDL requires &lt;0.1 dB for the data transfer rate of 10 Gb/s and 0.05 dB for the data transfer rate of 40 Gb/s. Also, such a device lacks the filter position indicator to precisely monitor the desired position of the ND filter. Thus, the user can not dynamically control the optical attenuation setting. It is hardly integrated into the practical optical network management. 
     In light of the foregoing, it would be desirable to provide improved structures and methods for attenuating optical signals, and particularly with low cost and high reliabilities. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention, a controllable attenuator comprises a first collimator for receiving the incoming light beam and directing the light beam along a first beam path. A first 45° mirror changes the light beam by 90°, and a second 45° mirror successively changes the light beam by another 90°. Then, the light beam enters the second collimator and leaves therefrom via a fiber. A controllable attenuating device, i.e., the associated a ND filter, is located between the first mirror and the second mirror, and is generally laterally moveable in the optical path for varying the attenuation of the outing light beam. 
     Yet, a miniature driving means of the ND filter is provided with a stepping motor connected to a lead screw for driving the ND filter, and a potentiometer place under the ND filter to read the precise position of the ND filter. 
     Advantageously according to the present invention the attenuation can be continuously adjusted over a broad range. The electrical signal from the potentiometer gives a feedback to dynamically tune the attenuation. Also, the whole device is well compacted via unique combination of the optical components and the electrical and mechanical driving device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of a presently preferred embodiment of an electrically controlled variable optical attenuator with potentiometer, according to the invention. 
     FIG. 2 shows a perspective view of the attenuator of FIG.  1 . 
     FIG. 3 is an enlarged perspective illustrative view of the driving device and the associated ND. 
     FIG. 4 shows an equivalent circuit of the potentiometer. 
     FIG. 5 plots the linear relationship between the wiper voltage of the potentiometer and the number of the pulses into the step motor. 
     FIG. 6 describes the measured data of the attenuation vs. the wiper voltage. 
     FIG. 7A plots the PDL measured at wavelength at wavelength of 1550 nm as a function of the attenuation; FIG. 7B plots the PDL measured at wavelength of 1550 nm as a function of the attenuation when the 45 degrees mirror is used thereof; FIG. 7C plots the PDL measured at wavelength of 1550 nm as a function of the attenuation when the prism is used. 
     FIG. 8 is a block diagram of another embodiment of the VOA device where the prisms are used to reflect the light beam. 
     FIG. 9 reveals another embodiment of the VOA device where two photo detectors tap out incoming the outgoing light powers for real time monitoring the optical power in the optical network. 
    
    
     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. 1-3 wherein the electrically controlled variable optical attenuator  1  includes an input port  102 , i.e., the fiber in, and an output port  104 , i.e., the fiber out, arranged on the same side of a mounting base or the VOA case  204 . 
     A first collimator  106  is connected to the input port  102  and a first reflection device  108 , i.e., the first 45 degrees dielectric mirror in this embodiment, is located behind the first collimator  106  and opposite to the input port  102 . A second collimator  114  is connected to the output port  104  and a second reflection device  112 , i.e., the second 45 degrees dielectric mirror in this embodiment, is located behind the second collimator  114  and opposite to the output port  104 . The first collimator  106  and the second collimator  114  are arranged to be parallel to each other. The first reflection device  108  is tilted at 45° relative to the first collimator  106 , and the second reflection device  112  is tilted at 45° relative to the second collimator  114 , wherein the first reflection device  108  and the second reflection device  112  are directed toward each other with a 90° angle therebetween. All the first collimator  108 , the second collimator  114 , the first reflection device  108  and the second reflection device  112  are fixed to a substrate  202  which is fixed to the mounting base or the VOA case  204 . 
     An ND filter  110  is interposed between the first reflection device  108  and the second reflection device  112  to block the light beam path. A stepping motor  116  is located beside the ND filter to move it along a direction perpendicular to a light beam path transmitted between the first reflection device  108  and the second reflection device  112  for controlling attenuation thereof. The ND filter further connects to a potentiometer  118  whereby the electrical signal from the potentiometer  118  gives the precise position of the ND filter  110 . Moreover, an electrical connection port  206  is disposed inside of the VOA case  204  for electrical connection of the stepping motor  116  and the potentiometer  118 . 
     Thus, the main light beam from the input port  102  enters into the first collimator  106  and passes it to directly hit the first reflection  108 , thereby making a 90° direction change. The light further passes the ND filter  110  with attenuation effect occurring thereof, and successively hits the second reflection device  112  to be reflected, with a 90° direction change, toward the second collimator  114 , and finally leaves the second collimator  114  via the output port  104 . The light beam path between the input port  102  and the output port  104  are essentially of a U-like configuration regulated by the first collimator  106 , the second collimator  114 , the first reflection device  108  and the second reflection device  112  with two arms of such a U-like configuration are respectively defined between the first collimator  106  and the first reflection device  108 , and the second collimator  112  and the second reflection device  112  and with the bight between such two arms being defined between the first reflection device  108  and the second reflection device  112  wherein the movement of the ND filter  110  is along the longitudinal direction of the two arms. 
     The ND filter  110  sits upon a lead screw nut  302  as shown in FIG.  3 . The nut  302  itself mechanically lings to a lead screw  304 . The stepping motor  116  is also connected to the lead screw  304 . A silicon rubble coupler connects the step motor and the lead screw. Understandably, using the rubble coupler has advantage of allowing relatively large mechanical misalignment tolerance between the step motor axis and the center axis of the lead screw. It is good for high yield rate manufacturing. As electrical pulses gets into the stepping motor  116 , the motor  116  makes the lead screw  304  to rotate via the ball bearing  306 . The rotating lead screw  304  moves the ND filter holder, i.e., the nut  302 , forward or backward. Moving the nut  304  changes the position of the ND filter  110  and thus the attenuation value is tuned. 
     With reference to FIG. 3, the potentiometer  118  is placed under the lead screw nut  302 . The wiper  308  sticks on the nut  302 . This arrangement makes the potentiometer  118  as a position indictor of the ND filter  110 . As shown in FIG. 4, the potentiometer  118  is equivalent to a resistor  402  and the wiper  308  acts as a linear variable resistor  404 . It is required to have a power supply such as 5 Volts to electrically connect to the potentiometer  118 . It also requires a voltage meter to read out the wiper electrical signal. All these can be done the electrical connection port  206 . 
     In order to move the ND filter  110 , the user needs to send electrical pulses to the stepping motor  116 . The ND filter will move when the lead screw  304  rotates. Understandably, the moving distance is proportional to the number of the electrical pulses into the stepping motor  116 . The wiper signal, i.e., the electrical voltage, is linearly dependent upon the moving distance of the ND filter  110 . It means that the wiper voltage reading and the number of the electrical pulses into the stepping motor  116 , have the linear relationship with each other. FIG. 5 shows the measured data of the wiper reading and the number of the electrical pulses where the curve is substantially straight, thus confirming the linear relationship therebetween. 
     One of the unique design feature is that the sending electrical pulses to the stepping motor  116  results in changes of the wiper voltage reading and the attenuation value. It means that there is a relationship between the attenuation and the wiper voltage signals. The measured data of the attenuation with its corresponding wiper voltage are shown in FIG. 6 whereby the corresponding curve provides the optical system designers with the feedback loop of dynamically tuning the attenuation. Also, the wiper voltage reading provides the crash stop signals for the ND filter of hitting the substrate walls. For example, the minimum and maximum voltages in FIG. 6 are 0.72 V and 4.21 V, respectively. These minimum and maximum voltages are crash stop signals. For the normal operation, the wiper signal should be in a range of the minimum and the maximum voltages such as 0.72 V&lt; the wiper reading &lt;4.21 V in FIG.  6 . Basically, the curve in FIG. 6 is a desired feature of a VOA device for the optical network management. 
     The PDL will be a problem for the high speed of the optical network of 10 Gb/s or above. When a regular mirror such as bare gold mirror used in the aforementioned prior art U.S. Pat. No. 6,292,616, the measured PDL at wavelength of 1550 nm for such a VOA has relative large value, close to 0.15 dB for attenuation up to 35 dB (FIG.  7 A). It can not meet requirement of PDL&lt;0.1 dB for the optical network with the transfer rate of 10 Gb/2. In this invention, the specified 45 degrees dielectric mirror is utilized for the PDL control. Then the PDL can reach &lt;0.1 dB for attenuation up to 30 dB, as shown in FIG.  7 B. 
     Another embodiment is shown in FIG. 8 where the first reflection device  108  and the second reflection device  112  of FIG. 1 are replaced by two prisms  508  and  512 . The feature of the total light reflection, i.e., 100% reflection, of the prisms  508 ,  512 , can further reduce the PDL and minimum insertion loss (IL) of the device. FIG. 7C shows the PDL is less than 0.04 dB for attenuation even over 30 dB, which definitely meets requirement of the PDL &lt;0.05 dB for the high speed (&gt;10 Gb/s) optical network such as 40 Gb/s transfer rate. 
     FIG. 9 demonstrates another embodiment where the first reflection device  108  and the second reflection device  112  of FIG. 1, are replaced by first and second beam splitters  608  and  612 . The first beam splitter  608  reflects most of light, but taps out a small percentage, i.e., 1%-5%, of the light intensity. Such a tap-out light beam is detected by a photo diode/detector  602  which just sits behind the splitter  608 . Similarly, the second beam splitter  612  also reflects mot of light and also taps out a similar small percentage, i.e., 1%-5%, of the incoming light. A second photo diode/detector  604  is placed behind the second beam splitter  612  and detects the tap-out signal from the second beam splitter  612 . The tap-out signals from the first photo detector  602  and the second photo detector  604  gives the actuarial values of the incoming and outgoing light power, respectively. It means that this VOA device provides option of real time monitoring light power in the optical network, thus resulting in great advantage for the optical network management. 
     In conclusion, the invention uses two 45 degrees dielectric mirrors or prisms incorporating the potentiometer in the attenuator so as to be able to precisely achieve the relative small value of the PDL meeting the industry standard which can not be reached by the prior arts. 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.