Abstract:
An optical assembly includes a photodetector for detecting light signals. An optical fiber receives an input signal and has a light-emitting portion extending in front of the photodetector. A MEMS actuator is located between the light-emitting portion of the optical fiber and the photodetector. The MEMS actuator is controllably deflectable to partially obscure the photodetector and thereby vary the amount of light received.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit under 35 USC 119(e) of prior U.S. provisional application Ser. No. 60/514,014 filed Oct. 27, 2003, the contents of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to the field of photonics, and in particular to an optical assembly including a variable optical attenuator for selectively attenuating an optical signal introduced into an optoelectronic package via an optical fiber and converted to an electrical signal by a photodetector.  
       BACKGROUND OF THE INVENTION  
       [0003]     Variable optical attenuators are used in optical fiber technology for various purposes. For example, one application is to adjust the intensity of a received or transmitted signal so that it best matches the operational range of the optical signal receiver. In this invention, the attenuator also serves to protect the photodetector from damage due to high optical inputs. One such attenuator is described in U.S. Pat. No. 6,066,844, the contents of which are herein incorporated by reference. This solid state device employs membrane technology, which among other things does not permit complete attenuation of the signal. The solid state device can be expensive to make.  
         [0004]     Another type of variable optical attenuator with a profiled blade is described in U.S. Pat. No. 6,246,826 the contents of which are herein incorporated by reference. It includes a mounting base with an actuator formed on the base, the actuator carrying the blade which is moveable across a light beam. The blade is profiled so as to provide a predetermined attenuation of the beam as a function of the displacement of the blade. The blade includes a pattern consisting of a three dimensional notch or protrusion selected to achieve a predetermined attenuation function. This device is of complex construction and also difficult to make.  
       SUMMARY OF THE INVENTION  
       [0005]     The invention employs MEMS (Micro-Electromechanical Systems) technology to provide an effective, easily manufacturable module with a wide dynamic range.  
         [0006]     According to the present invention there is provided an optical MEMS assembly for controlling the amount of light received by a photodetector, said optical assembly being locatable over said photodetector and comprising an optical transmission medium for receiving an input signal and having a light-emitting portion for directing light toward said photodetector; a controllably deflectable actuator; and a light-obscuring member mounted on said actuator for at least partially obscuring said photodetector from said light-emitting portion depending on the deflection state of said actuator arm.  
         [0007]     A novel aspect of this invention is that all components are co-packaged into a single optoelectronic package.  
         [0008]     The optical transmission medium is typically an optical fiber, although the invention is similarly applicable when the optical input is presented to the photodetector from the system fiber by a lens-train design, for example.  
         [0009]     The optical fiber, which preferably extends transversely in front of the photodetector, can be cleaved at an angle at one end to deflect light onto the photodetector. Typically, this angle will be close to 45° so that light passing along the optical fiber will be reflected off the internal end surface directly onto the photodetector. The optical signal can also be presented to the photodetector in the current configuration via a beam splitter rather than the angled fiber, or can be packaged such that a straight cleave fiber or other lens arrangement could be used, e.g. mounting the variable optical attenuator and photodetector vertically.  
         [0010]     The photodetector can be integrated into a common substrate with the MEMS actuator.  
         [0011]     The invention also provides a method of controlling the amount of light received by a photodetector, comprising directing a received input signal toward a photodetector; and displacing a light-obscuring member mounted on a MEMS actuator to at least partially obscure said photodetector. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:— 
         [0013]      FIG. 1  is a perspective view of one embodiment of an optical assembly in accordance with the invention; and  
         [0014]      FIG. 2  is a more detailed view of the region around the photodetector. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     The optical assembly, forming a VOA (Variable Optical Attenuator) shown in  FIGS. 1 and 2  forms a sub-assembly that is designed to sit over a photodetector  16  forming part of a detector assembly  14  for incoming signals transmitted over an optical fiber or other optical transmission medium. The VOA comprises a rectangular substrate  10 , which can be silicon-on-insulator material, or single crystal silicon. The substrate includes on its top surface a landing pad  33  and a capacitor pad  32 .  
         [0016]     A rectangular recess  12  is formed in one of the long sides of the rectangular substrate  10 . This allows for the VOA to sit atop the detector assembly  14  in a saddle configuration.  
         [0017]     The photodetector  16  can be any suitable photodetector for optical communications, for example, a PIN photodetector or an avalanche photodetector (APD).  
         [0018]     An optical fiber  18  is mounted in a V-groove  20  formed on the top surface of the substrate  10 . The V-groove  20  serves to accurately align the optical fiber  18  with the photodetector  16 .  
         [0019]     The optical fiber  18  has an end portion  18   a  that protrudes beyond the end wall  22  of the recess  12 . The end portion  18   a  terminates in a cleaved end  18   b  angled at 45° lying over the photodetector  16 . Light traveling along the optical fiber  18  is reflected by total internal reflection off the end face of the cleaved end  18   b  and directed downwards toward the photodetector  16 .  
         [0020]     Optional balls lens  24  mounted at the end of the optical fiber  18  focuses light onto the photodetector  16 .  
         [0021]     The other end of the optical fiber  18  has a coupling (not shown) for connection to an external communications optical fiber.  
         [0022]     A cantilevered MEMS actuator arm  26 , which can be made of silicon, is mounted at one end  28  thereof on the substrate  10 . However, the actuator arm  26  could also be made of other suitable materials. The cantilevered arm  26  is thermally actuated and could be of the type described in our co-pending provisional application Ser. No. 60/320,089, the contents of which are herein incorporated by reference. As described in our co-pending application, the actuator arm  26  is mounted alongside a heat sink  30 . The arm is deflected by passing a current through it. The current produces differential heating of the two segments of the arm, which causes the arm to deflect toward the heat sink  30 .  
         [0023]     As better seen in  FIG. 2 , the tip  26   a  of the actuator arm  26  is connected by a bridging link  29  to an opaque rectangular member  27 , referred to as a paddle, which is normally clear of the photodetector  16 . As the arm  26  deflects, the paddle  27  gradually moves under the end  18   b  of the optical fiber  18  and blocks progressively more light from reaching the photodetector  16  as the amount of deflection of the actuator arm  26  increases. It will be appreciated that the shape of the paddle is not critical so long as it is capable of selectively obscuring the photodetector as the actuator arm is displaced. A paddle in this context is a generally flat, blade-like device. Although the opaque member will generally be flat, it could have any solid shape, and need not necessarily be completely opaque so long as it is capable of reducing the amount of light passing through it. Alternatively, the paddle can normally block the light from reaching the photodetector and progressively expose the photodetector as the actuator arm  26  deflects.  
         [0024]     The paddle  27  is also connected to a concertinaed spring element  31 , which permits current to be supplied to one end of the actuator arm  26  through the paddle  27  while allowing deflection of the actuator arm  26 . As the paddle moves in a direction toward the end of the optical fiber  18 , the concertinaed spring element  31  resiliently expands.  
         [0025]     The actuator arm  26  can also act as a shutter allowing the light to be completely blocked if desired.  
         [0026]     Element  32  is a capacitor pad. If desired, control circuits for the optical assembly can be integrated into the portion  34  of the silicon substrate below the capacitor pad  32  using conventional integrated circuit fabrication technology.  
         [0027]     The described device has several advantages over prior art constructions. The variable optical attenuator is planar with the floor of the package. The device can sit directly over the receiver in a saddle-like configuration. It can also use a large chip to facilitate packaging. The use of a paddle shape facilitates wire bonding to the photodetector or any optoelectronics dice placed below the VOA. The device can also act as a jumper chip between other devices.  
         [0028]     The device has zero insertion loss since in the normal position it is completely open. The actuator arm is not located in the light path between the optical fiber and the photodetector. The device also allows control of the overload limit of any co-packaged electronics. An example is the amplifier following the photodetector in this embodiment.  
         [0029]     A typical device has a minimum of 50 μm travel for the end of the actuator arm, 12 V maximum shutter drive, zero insertion loss when the actuator is not powered, and a minimum of 25 dB attenuation range.  
         [0030]     However, these values can be changed by changes to the starting material properties, without changing the nature of the invention described.  
         [0031]     It will be understood by those skilled in the art that the components can be fabricated using MEMS fabrication techniques known in the art.  
         [0032]     The embodiments presented are exemplary only and persons skilled in the art would appreciate that variations to the above described embodiments may be made without departing from the spirit of the invention. The scope of the invention is solely defined by the appended claims.