Abstract:
An optical attenuator for use in optical power regulation uses a shutter to control the insertion loss of a gap in a pair of substantially coaxially aligned waveguides, for example a pair of optical fibers. The shutter is part of a MEMS accuator device and is coupled to the movable one of a pair of spaced plates, the movable plate being movable by a voltage applied between the pair of plates. To regulate the power, a small portion of the optical power is abstracted and used to derive a control voltage that is applied to the optical attenuator for varying the attenuation inserted in the optical wave path.

Description:
FIELD OF THE INVENTION 
     This invention relates to an optical attenuator and more particularly to an optical attenuator that uses a micro-electro-mechanical system (MEMS) device for control of the attenuation. 
     BACKGROUND OF THE INVENTION 
     Optical components in optical communication networks need to operate over a wide range of power levels, varying from those emanating from high power sources and amplifiers that may be tens of watts to signals of fractions of microwatts arriving at receivers for detection. Moreover, there is often the need in such networks for a wide dynamic range of optical power sources and receivers. For example, in wavelength division multiplexed (WDM) systems, the power needs are dependent on the number of channels present at a particular time in a WDM system. Also fixed optical attenuators are frequently used to add attenuation to short optical fiber spans to reduce the power impinging on the receiver&#39;s detector and variable attenuators have been proposed for use in WDM networks to regulate the signal power to a level appropriate to the number of channels then active. 
     It is important for a variable optical attenuator to add little excess insertion loss when the signal power is already as low as desirable. 
     It is also important for a variable optical attenuator to be able to respond rapidly to sudden increases in power levels since excessive power levels that are allowed to persist too long often can do considerable damage or otherwise impair operation. 
     The present invention provides a variable optical attenuator that should be able to meet these various requirements. In particular, the invention provides a variable power attenuator that is especially useful for power regulation. 
     The invention involves the use of a novel micro-electro-mechanical system (MEMS) device to provide control of the attenuation. MEMS technology is now a well-recognized technology. Basically, it involves forming a multilayer structure by sequentially depositing and shaping on a silicon substrate a plurality of polysilicon layers that are separated by layers of silicon oxide and/or silicon nitride. The shaping of the layers is generally done by the techniques used in the manufacture of silicon integrated circuits and generally involves the patterning of the layers by photolithographic techniques. The basic principles of MEMS technology are described in a paper entitled “MEM&#39;S the Word for Optical Beam Manipulation” that was published in  Circuits and Devices , July 1997, pp. 11-18. MEMS devices are available from many sources, such as, for example, the MEMS Technology Application Center at North Carolina (MCNC) that custom makes devices on order. 
     Various applications of MEMs devices are described in copending application Ser. No. 09/088,182, filed Jun. 1, 1998 by five of the present applicants and having a common assignee. 
     SUMMARY OF THE INVENTION 
     A variable optical attenuator in accordance with the invention comprises first and second optical waveguides, for example optical fibers that are positioned end to end, preferably slightly offset from coaxial alignment, and spaced apart to define a small gap between adjacent ends of the two optical fibers. These adjacent ends are preferably angle cleaved and coated to be anti-reflective so that an optical signal introduced at the far end of the first or source fiber will pass through the gap and continue along the second fiber with little loss of power, the offset in alignment of cleaved end surfaces of the fiber sections largely compensating for the bending of the optical signal as it exits and enters the cleaved surfaces. To vary the insertion loss introduced by the gap between fiber sections, a MEMS device that includes a movable shutter element is positioned so that the shutter may be interposed in controlled amounts into the gap quickly and easily to vary the amount of optical signal that traverses the gap. Advantageously, typically the shutter surface on which optical signal power to be controlled is incident is coated to be highly opaque or reflective of such incident signal, preferably scattering such signal so that little of it either traverses the gap to enter into the second fiber or is reflected back into the first fiber. 
     Alternatively, if it is desired to utilize such reflected energy, it may be captured by the source fiber and later diverted to a different wave path in any of the known ways to separate out this backward traveling light. 
     For use in optical power regulation, the variable attenuator described is positioned at the output of the power source or amplifier being regulated and a known fraction of the power level at the output of the attenuator is abstracted and detected. The resulting detected signal level is compared with a known reference level that corresponds to the desired power at the output of the attenuator were at the level desired. The two signals are compared and any difference signal is used to control the attenuation provided by the attenuator. 
     The invention will be better understood from the following more detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a side view of the basic elements of one configuration of art optical attenuator using a MEMS device illustrative of the invention. 
     FIG. 2 is a top view showing details of the MEMS device used to vary the attenuation in the attenuator shown in FIG.  1 . 
     FIG. 3 shows schematically an optical power regulator that uses for regulation a variable attenuator in accordance with the invention. 
     FIG. 4 shows the position of the shutter in the gap in its rest position in another configuration of the embodiment. 
     It is to be noted that the drawing is not to scale. 
    
    
     DETAILED DESCRIPTION 
     With reference now to the drawing, FIG. 1 shows schematically one configuration of the basic structure of a variable attenuator that uses an element of a MEMS device for control of the attenuation provided to an input optical signal. It includes a pair of waveguides, advantageously optical fibers  11  and  12 , aligned end-to-end. As shown, the two fibers  11  and  12  are slightly offset from coaxial alignment and the adjacent fiber ends  11 A and  12 A are spaced apart to define a gap  13  therebetween. In a different configuration, presently preferred, shown in FIG. 4, the fibers are oriented to come in and out of the plane of the drawing of FIG.  1 . The two adjacent ends are each angle cleaved, and the offset is used to compensate for the bending in light traveling from the first (source) fiber  11  to the second (output) fiber  12 , so that the light may continue with low insertion loss. The angle cleaving serves to minimize entry into the source fiber of any light reflected by the output fiber at the cleaved surface. It also serves to minimize any reflection back into the source fiber of light exiting from the source fiber. Of course, if desired, there may be omitted either or both the angle cleavage and the alignment offset. 
     To introduce variable attenuation, a MEMS device  10  is provided with a shutter  14  that can be inserted a controlled amount into the gap  13  for scattering or absorbing the radiation passing out of the source fiber that is incident on such shutter. Since the signal power is largely concentrated in the core, for maximum effect the shutter advantageously is inserted in a manner the better to intercept the light passing out of the core of the source fiber. Additionally, since it is typically undesirable that any of the light incident on the shutter be reflected back into the core region of the core fiber for travel backwards therein, in such instance it is advantageous to insert the shutter at such an angle into the gap that little reflected light is redirected back into the core section of the source fiber. However, in instances where it is desired to recover the reflected light for utilization, the measures to eliminate re-entry of the reflected light should be avoided. 
     A typical high performance optical fiber comprises a core of relatively small diameter, typically between six and eight microns, and a surrounding cladding typically of about 125 microns diameter. The shutter  14  aligned with the gap typically may have a height of about fifty microns and, when in its rest position, does not block the critical region of the gap between the two fibers. As previously mentioned, in FIG. 4, there is shown more clearly a different relationship between the shutter  14  and the fiber ends in the rest position. In an optical fiber, most of the energy of an optical signal is concentrated in the fiber core and the closely surrounding portion of the cladding. In particular, in a fiber having a core diameter of about eight microns and a cladding diameter of 125 microns, essentially all of the energy of an optical signal traveling in such a fiber is included in a central region of the fiber having a diameter of less than twenty microns. The position of the shutter  14  is controlled by an actuator that includes a pair of spaced plates  15 ,  16 , of which lower plate  15  is fixed and upper plate  16  is a thin layer movable under control of a voltage applied between the two plates. The lower plate rests on substrate  19 , typically of silicon, with an intervening layer of silicon nitride (not shown). To permit such movement the left end of the upper plate is fixed and the right end connected to a lever arm  18 . With zero voltage applied to the actuator, the shutter does not affect the critical gap region. When the shutter is activated by applying sufficient voltage to the actuator, it is moved to interrupt this critical region. 
     The shutter  14  is positioned at the free end of the lever arm  18 . Lever arm  18  is an extension of the top plate  16  of the voltage-controlled actuator formed by the two spaced polysilicon plates  15 ,  16 . The downward movement of the plate  16  is made to cause upward movement of the shutter  14  that is pivoted about a pair of support posts  20 A,  20 B, of which one is seen in side view in FIG.  2 . To provide flexibility to the lever arm  18  and facilitate its upward motion, there is provided a flexure section  17  between the lever arm  18  and the top plate  16 , as seen in the top view of FIG.  2 . This flexure section  17  comprises a pair of thin flexible arm  22 A,  22 B, that extend between the plate  16  and the support posts  20 A,  20 B, respectively. At the ends where the arms  22 A,  22 B are supported, each includes an enlarged end portion  25 A,  25 B. Additionally, a brace section  23  extends between the two arm sections  22 A,  22 B from which extends the lever arm  18 . Advantageously, the shutter  14  was rotated 90 degrees to its operating vertical position upon release by a self-assembly mechanism to perform the rotation automatically upon release. The self-assembly mechanism uses energy stored in a high tensile residual stress metal layer to produce vertical motion, which is then used to rotate and lock the shutter in position as is discussed in a paper entitled, “Low Insertion Loss Packaged and Fiber Connectorized Si Surface-Micromachined Reflective Optical Switch” that appeared in the Proceedings of Solid State Sensors and Actuators Workshop at Hilton Head Island, S.C. on Jun. 8, 1998 by applicants of this application and its teaching is incorporated herein by reference. This paper describes earlier embodiments of the MEMS device and the present application describes a later generation of the MEMS device that is viewed as an improvement. By appropriate design, the lever arm is made to lift the shutter  14  into the gap between the fiber ends by the application of a suitable voltage between the two plates  15 ,  16  that form the actuator. The displacement of the shutter  14  needs to be sufficient to enter the critical region, as has been discussed. It can be appreciated that the movement of the shutter is essentially frictionless because it involves essentially no rubbing between moving parts. 
     To prevent the top plate  16  from touching the bottom plate  15  and shorting the capacitor formed by the two plates, spacers (not shown) can be provided protruding from the bottom surface of the top plate  16  and holes (not seen) can also be provided in the bottom plate  15  aligned with the posts such that the posts can pass through the holes to rest on the nitride-covered silicon substrate  119  underlying the bottom plate  15  without touching the plate if the applied voltage is sufficient. Typically the mass of the shutter assembly comprising the top plate  16 , the flexure section  17 , the lever  18  is sufficiently light and the material is sufficiently rigid that the posts  20 A and  20 B can keep it suspended appropriately in the absence of any applied voltage. 
     The extension arm  18  typically may be about  500  microns long and capable of rotation around the pivot point to provide a desired mechanical advantage. As discussed, the pivot point is provided by the support posts  20 A and  20 B and the lever arm is coupled to the top plate of the actuator by the two flexible arms  22 A,  22 B. Their size is chosen to permit an adequate lever rotation angle for a specified separation change between the two plates, without requiring an excessively high voltage to move the top plate sufficiently to provide the necessary lever arm rotation angle. Typical dimensions are for each plate to be about 200 μm on a side with plate  15  about 0.5 μm thick and plate  16  about between 1.5 and 3.0 μm thick with a spacing between plates of about 2 μm. The length of the lever is between 18 to 30 μm long, its width between 1.5 and 3.0 μm, and its thickness between about 1.5 and 2.0 μm, the shutter is, about 50 μm square and the width of the gap  13  between the two waveguides  11 ,  12  is about 20 μm. Of course, these dimensions can be varied, although they should all be in the micrometer range. 
     Instead of DC, the control voltage preferably should be an alternating voltage of a frequency sufficiently high, for example 100 kilocycles per second, to be higher than the mechanical resonant frequency of the MEMS device. This is to avoid the known charging effect on the actuation that might occur with d-c actuation. Because of the electrostatic nature of the control operation, very little amount of electric power is needed for control. 
     As seen in FIG. 3, for power regulation, there is inserted in the waveguide  32  transmitting the optical signal to be regulated a variable attenuator  10  of the kind previously described that includes a MEMS device for control of signal power that passes through the attenuator. A known, typically small, fraction of the optical signal at the output of the attenuator is abstracted by way of a coupler  33  of any suitable form and this optical signal is detected and amplified to a suitable voltage in a conventional optical detector-amplifier  34 . This detected voltage forms an input to the differential amplifier  36 . The other input voltage to the amplifier  36  is provided by the reference voltage source  37 . This reference source provides a voltage that would match the voltage provided by the detector-amplifier  34  if the signal power abstracted were at the desired level. Accordingly, any difference in the amplified voltage level detected and the reference voltage acts as an error signal that will be amplified by the amplifier  37  and available for control of the attenuator  10 . If desired, smaller amounts of the signal power may be abstracted, and after detection amplified for use in deriving the control, with a resultant saving in the signal power being abstracted. As was discussed, the voltage applied for control is best provided as an alternating voltage. 
     It should be apparent that the arrangement described can be modified by abstract control power upstream of the variable attenuator rather than downstream of it. Also one can readily devise circuit alternatives, other than use of a differential amplifier, to control the attenuation with a sample of its input power. 
     It should also be apparent that various other arrangements available for control can be used. In particular, when warranted, a more sophisticated circuit to provide the desired feedback transfer function can be employed, such as one that also controls the rate of change, for example PID control, in place of the differential amplifier arrangement shown. 
     It should be apparent that because of the small size of the devices involved and the fabrication techniques employed, which are very similar to those employed in the manufacture of monolithic integrated circuits, the invention can be used with fiber ribbons and silica or silicon waveguides to form monolithic arrangements that include a large number of such devices. 
     It should be understood that the specific embodiment described is merely illustrative of the general principles of the invention. Various modifications may be devised by a worker in the art without departing from the spirit and scope of the invention. For example, the optical waveguides involved can be of the known planar type using a planar surface on which are formed waveguides that depend on index of refraction differences. In particular, the variable optical attenuator should be useful in a variety of other ways in optical networks. The attenuator can be used to control the power level of individual channels either into or out of add/drop multiplexers or cross-connect optical networks.