Abstract:
The invention provides a fast variable optical attenuator including a reflective membrane that serves as a varifocal mirror. Electrostatic deflection of the mirror defocuses the optical system and attenuates an optical signal launched to the mirror from an input port to an output port. Since the mirror is generally limited to deflections less than 1 micron, fast attenuation within a small (&lt;5 dB) dynamic range is achieved. The dynamic range is improved by combining the fast attenuator with a slower attenuator. In the preferred embodiment, the slow and fast attenuator are not coupled via optical waveguides, but rather are optically coupled via free space or are integrated in the same device. Advantageously, this compact arrangement reduces the excess loss associated with optical fiber coupling and lowers packaging costs.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of optical attenuators and more specifically to a fast optical attenuator. 
     BACKGROUND OF THE INVENTION 
     Optical attenuators are used to adjust the optical power level in optical signals, for example to equalize levels among WDM channels or to prevent saturation of receivers. Such applications generally require the attenuator to be adjustable over more than 20 dB. Low cost examples of this type of optical attenuator include neutral density filters, attenuating prisms, beam blockers, tilting mirrors and/or systems to bend or off-set optical fibers. Typically, the relatively large mass of the movable attenuating optics in these devices limit the response time to milliseconds. 
     Optical attenuators may also be used to impose information upon the optical signal passing through them, for example telemetry information. However, such applications typically require a response time of microseconds with a dynamic range of about 5 dB. Conventionally, optical attenuators that are capable of achieving fast speed in the order of MHz are based on electro-optics, acousto-optics, or magneto-optics. Traditionally, these fast attenuators are wavelength sensitive, have a relatively high insertion loss, and a relatively high cost. 
     Ideally, an optical attenuator should have a fast response time and be able to adjust the optical power level over a wide range of intensity levels. Unfortunately, the design of a low cost attenuator that has simultaneously high speed and wide dynamic range is very difficult. 
     It is an object of this invention to provide to provide a fast optical attenuator that is relatively low cost. 
     It is another object of this invention to provide an optical attenuator having a wide dynamic range and that can also provide attenuation that varies at high speeds. 
     SUMMARY OF THE INVENTION 
     The instant invention provides a variable optical attenuator that attenuates over a wide dynamic range and that operates at high speeds. The optical attenuator includes a slow attenuator having a large dynamic range and a fast attenuator having a smaller dynamic range. In the preferred embodiment, the slow and fast attenuator are not coupled via optical waveguides, but rather are optically coupled via free space or are integrated in the same device. Advantageously, this compact arrangement reduces the excess loss associated with optical fiber coupling and lowers packaging costs. Moreover, each of the slow and fast attenuators work together to modulate an optical signal from the other such that the total attenuation of the device is the product of the two separate attenuations. 
     In a preferred embodiment, the fast attenuation is provided by a varifocal mirror. Electrostatic deflection of the mirror defocuses the optical system and attenuates an optical signal launched to the mirror from an input port to an output port. Since the mirror is generally limited to deflections less than 1 micron, fast attenuation within a small (&lt;5 dB) dynamic range is achieved. 
     In accordance with the invention there is provided an optical attenuator comprising an input port and an output port, a deformable membrane having a reflective surface disposed to reflect light launched from the input port substantially towards the output port, the membrane supported by a frame having an opening, and deforming means for controllably deflecting the membrane into the opening such that the reflective surface forms a concave mirror having a concavity that determines the optical coupling efficiency between the input and output ports, the membrane deformable between a first position in which light launched from the input port is received at the output port with substantially no attenuation, and a plurality of other positions in which light launched from the input port is inefficiently coupled into the output port. 
     In accordance with the invention there is provided an optical attenuator comprising a first port and a second port, a deformable membrane having a reflective surface disposed to reflect light launched from the first port substantially towards the second port, the membrane supported by a frame having a radially symmetric opening, and deforming means for controllably deflecting the membrane into the opening such that the reflective surface forms a concave mirror having a concavity that determines the optical coupling efficiency between the first and second ports. 
     In accordance with the invention there is provided an optical attenuator comprising: an input port and an output port; a deformable membrane having a reflective surface disposed to reflect light launched from the input port substantially towards the output port, the membrane supported by a frame having an opening; deforming means for controllably deflecting the membrane into the opening such that the reflective surface forms a concave mirror having a concavity that determines a first order optical coupling efficiency between the first and second ports; and a movable member for controllably moving the frame and membrane between a plurality of positions that determine a second order optical coupling efficiency between the first and second ports. 
     In accordance with the invention there is provided a variable optical attenuator comprising: a first attenuator having a dynamic range less than about 10 dB and a response time of less than about one microsecond; and a second attenuator having a dynamic range greater than about 10 dB at a response time greater than about one millisecond, wherein the first and second attenuators are optically coupled in series. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention will now be described in conjunction with the drawings in which: 
     FIG. 1 is a schematic diagram of a prior art optical attenuator based on the electro-optic effect; 
     FIG. 2 is a schematic diagram of a prior art optical attenuator based on the acoustic-optic effect; 
     FIG. 3 is a schematic diagram of a prior art optical attenuator based on the magneto-optic effect; 
     FIG. 4 is a schematic diagram of a prior art acoustic transducer; 
     FIG. 5 is a schematic diagram of an embodiment of a fast attenuator in accordance with the instant invention including a MEMS membrane having a reflective surface; 
     FIG. 6 a  is a schematic diagram of another embodiment of a fast attenuator in accordance with the instant invention; 
     FIG. 6 b  is a schematic diagram the embodiment shown in FIG. 6 a  where the membrane is deformed into a concave reflector; 
     FIG. 7 is a schematic diagram of an embodiment of a fast attenuator having a wide dynamic range; 
     FIG. 8 is a schematic diagram of another embodiment of a fast attenuator having a wide dynamic range; 
     FIG. 9 is a schematic diagram showing the shape of a beam spot at the position of the receiving end after defocusing for both circular and square membranes; and 
     FIG. 10 is a schematic diagram showing another embodiment of a fast attenuator having an axial design. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As described above, optical attenuators that have a relatively fast (e.g., less than a microsecond) response time are frequently based on electro-optics, acousto-optics, or magneto-optics. 
     Referring to FIG. 1, there is shown an example of a prior art fast attenuator based on the electro-optic effect, e.g., Pockels or Kerr effects. A non-polarized beam of light is launched through a first polarizer  10 , the electro-optic material  12  (e.g., lithium niobate), and a second polarizer (analyzer)  14 . When no voltage is applied to the electro-optic material  12 , there is substantially no loss in intensity observed. When a voltage is applied to the material  12  via ring electrodes  16   a  and  16   b , the direction of the polarized light leaving the first polarizer  10  is rotated by an angle related to the applied voltage, thus affecting the amount of light transmitted through the analyzer  14 . The disadvantages of this type of optical attenuator include relatively high voltage requirements, limited beam diameter capabilities that lead to high insertion loss, material unreliability, and sensitivity to wavelength. 
     Referring to FIG. 2, there is shown an example of a prior art fast attenuator based on acousto-optics. The attenuator includes a piezoelectric transducer  20 , an acousto-optic material  22 , and an acoustic absorber  24 . The transducer  20  provides a sound wave that travels through the acousto-optic material  22 , e.g. as a series of compressions and rarefractions. In regions where the sound pressure is high, the acousto-optic material  22  is compressed slightly, thus locally increasing the index of refraction. This series of local increases in index of refraction forms a grating that defracts an incident beam of light as it travels through the acousto-optic material  22 . The angle of deflection is related to the radio-frequency power applied to the transducer. When no acoustic wave is provided, no light is deflected. The disadvantages of this optical attenuator include wavelength sensitivity and an excessively high cost. 
     Referring to FIG. 3, there is shown an example of a prior art fast attenuator based on magneto-optics. A non-polarized beam of light is launched through a first polarizer  30 , a magneto-optic material  32 , and a second polarizer (analyzer)  34 . When no magnetic field is applied, there is substantially no loss observed. When a magnetic field is applied to the material  32  via magnets  36   a  and  36   b , the direction of the polarized light leaving the first polarizer  10  is rotated by an angle related to the applied voltage, thus affecting the amount of light transmitted through the analyzer  34 . The disadvantages of this type of optical attenuator include high insertion loss and wavelength sensitivity. 
     The prior art fast attenuators described heretofore are based on systems with non-moving parts. Presumably, this is due to the fact that the speed of moving an attenuating optic is limited by the inertia of the object moved. Accordingly, fast attenuators have traditionally been limited to relatively costly systems with non-moving parts. 
     One solution to this is to provide an attenuating object with a relatively small inertia. For example, it is well known that ultrasonic transducers fabricated by MEMS technologies can be driven electrostatically at speeds in the 10 MHz region. Moreover, these transducers are simple and thus show remarkable potential for reliablity. 
     Referring to FIG. 4, there is shown a MEMS ultrasonic transducer including a thin membrane  100 , typically made of silicon nitride (SiN), stretched over the periphery of a frame  102 , which is also typically made of SiN. The structure is formed by MEMS technology on a silicon substrate  104 , thus forming a cavity  106  in the order of 5-50 microns deep between the substrate  104  and the membrane  100 . An electrically conductive material  108 , such as aluminum, is coated on the membrane  100 . The silicon substrate  104  and conductive material  108 , each serve as an electrode of a capacitor for providing a measure of membrane deflection resulting from pressure changes near the membrane  100 . For maximum efficiency the ultrasonic transducers are usually operated at a mechanical resonance. Typically the structures are tensioned by applying a DC voltage of 10-100 volts, such that the mechanical resonance frequencies are very high, e.g., in the 10 MHz range. Preferably, the SiN layer  100  is uniform and the tensioned membrane has a radially symmetric concave shape. Advantageously, MEMS ultrasonic transducer technology produces the cavity  106  without perforating the membrane. The uniform membranes are deflectable into an approximately parabolic shape by an applied DC voltage. 
     When the uniform MEMS membrane disclosed above includes a reflective surface, the deformable membrane becomes a deformable mirror having a shape dependent on the degree of deflection, the number of electrodes, and the placement of the electrodes. More specifically, the deformable mirror can be constructed to form a concave mirror having a variable focal point. This type, or similar, deformable mirrors have been proposed for various spatial light modulators (e.g., U.S. Pat. Nos. 4,441,791 and 6,147,789, herein incorporated by reference). However, in general, the design of these prior art deformable mirrors is not compatible with the wavelength insensitive, polarization sensitive requirements for optical attenuators used in telecommunication systems. Moreover, these prior art devices are generally limited to on/off states and do not recognize the advantageous of providing variable attenuation. 
     Referring to FIG. 5, there is shown an embodiment of a variable optical attenuator in accordance with the instant invention having a structure similar to the acoustic transducer shown in FIG.  4 . The attenuator includes a thin membrane  200 , preferably made of silicon nitride (SiN), stretched over the periphery of a frame  202 , which is also preferably made of SiN. For example, the structure can be formed by MEMS technology on a silicon substrate  204  to form a cavity  206  about 1-10 microns deep between the substrate  204  and the membrane  200 . An electrically conductive and reflective material  208 , such as gold, is coated on the membrane  200 . Preferably, the surface has a reflectivity greater than about 99%. The silicon substrate  204  and conductive material  208 , each serve as an electrode of a capacitor that provides deforming means for deforming the membrane  200 . When a potential is applied between the two electrodes, electrostatic forces cause the membrane to be attracted to the substrate such that the reflective surface  208  becomes a focussing mirror. Referring to FIG. 5, the deformable membrane becomes an approximately elliptical mirror designed to reflect a beam of light launched from one of its two foci, e.g.,  212 , onto the other focus, e.g.,  214 . Accordingly, an input optical fiber  210  is positioned at a first focus  212  of the elliptical mirror, whereas an output optical fiber  216  is positioned at the other focus  214 . The arrangement shown in FIG. 5, corresponds to a maximum optical coupling arrangement or the low loss state. When the potential applied to the two electrodes is varied, the membrane is deflected to various positions, thus changing the position of the foci and providing variable attenuation for a beam of light launched from the input port  210  and collected at the output port  216 . In other words, when the shape of the mirror  208  changes, the efficiency of the coupling changes, thus providing a controllable variable attenuation. Optionally, the shape of the mirror is further controlled by using multiple electrodes. 
     Preferably, the thickness of the membrane including the reflective layer, the diameter of the deformable portion, and the applied voltage and are selected such that the maximum deflection is less than about 1 μm to achieve a maximum attenuation of about 3-10 dB, or more preferably of about 3-5 dB. Although this relatively small deflection generally limits the dynamic range of the fast attenuator, many applications of fast attenuators only require small dynamic range. For example, when the fast attenuator is used to provide a small amplitude modulation on an optical pulse stream, only small amplitude modulation is desired. 
     The diameter of the deformable membrane is also related to the response time. Preferably, the diameter of the membrane is designed to be about 100 to 300 μm. Membrane diameters varying from about 150 μm to 300 μm have been calculated to be particularly useful. Although it is preferred that the mirror be substantially spherical when deformed, the membrane can also take on other shapes, such as a square. 
     Advantageously, the circular membrane is radially symmetric to provide the wavelength insensitive and polarization insensitive requirements for optical attenuators used in telecommunication systems. 
     Problems arising from the diameter of the beam being larger than the diameter of the membrane, may be circumvented by using a lens designed to reduce the beam diameter (e.g., a SELFOC having more than one-quarter pitch). Alternatively the membrane diameter is designed to be larger than 300 μm, e.g., about 500 μm, but wherein only an area having a diameter of about 300 μm is deformable. Alternatively, a plurality of deformable membranes is disposed in close proximity to one another to receive a single beam of light. 
     Referring to FIG. 6, there is shown another embodiment of a fast attenuator in accordance with the instant invention including lenses  330  and  336  optically coupled to the input  310  and output  316  fibers. FIG. 6 a  illustrates the low loss state, wherein the reflective surface  308  is not deformed, i.e., is substantially flat, and wherein light launched from the input fiber  310  is substantially coupled into the output fiber  316 . In FIG. 6 b , the reflective surface is electrostatically deformed to form a concave reflective surface. As a result, the beam of light launched from the input fiber  310  is not effectively coupled into the output fiber  316 , thus providing attenuation of the optical signal. Variable attenuation is provided by varying the applied potential (e.g., typically by applying a voltage from about 10-100 V). In one embodiment, the focussing mirror is designed to provide the extra optical power required for efficient coupling, and the lenses are somewhat shorter or longer than those required for efficient coupling with reflection off a flat mirror. Alternatively, the optical fibers are thermally expanded core fibers, and/or include other means for increasing the optical coupling efficiency. 
     The fast optical attenuators depicted in FIGS. 5 and 6 have been calculated to provide insertion losses varying from about 0.02 dB to 0.6 dB, maximum attenuation less than about 10 dB, and microsecond response times. More specifically, it has been calculated that a deflection of 0.5 μm can correspond to an attenuation to about 2 dB. 
     Advantageously, the varifocal optical attenuators described above provide modulation with a rate in the 1 MHz range, and small dynamic range (up to 30%), for imposing dither, telemetry and other in-band modulation on optical data. 
     In accordance with another aspect of the instant invention, there is provided an optical attenuator including the MEMS attenuator described above and a slow, high dynamic range (&gt;20 dB) attenuator for providing both fast modulation with the slow, precise and high range attenuation properties characteristic of the conventional slow attenuators. 
     Referring to FIG. 7, there is shown an embodiment of an optical attenuator including a MEMS varifocal mirror  440  for providing fast attenuation with a dynamic range less than about 5 dB within about one microsecond, and a graded neutral density filter  442  for providing slow attenuation with a dynamic range greater than about 20 dB within about a millisecond. A mirror  446  directs a beam of light launched along an optical path  448  to the neutral density filter  442 , which is controllably moved in and out of the optical path  448  with a stepping motor  444  to provide the slow attenuation. The varifocal mirror  440  is disposed in the optical path in series with the neutral density filter  442  to provide the fast modulation. The two attenuators are controlled independently. 
     Referring to FIG. 8, there is shown another embodiment of an optical attenuator in accordance with the instant invention, wherein a varifocal mirror  540  for providing fast modulation is supported on a tilting substrate  542  for providing the slow attenuation. Separate input control is provided for each attenuator (i.e., the fast  540  and the slow  542 ). Preferably, the substrate  542  is tilted using a piezoelectric crystal, thermal deformation, or electronic charge as shown in FIG.  8 . The optical attenuator further includes a single lens  560  disposed between the combination attenuator  540 , 542  and the input  510  and output  516  optical fibers, which are supported by a double bore tube  550 . 
     In operation, a beam of light launched from input optical fiber  510  is transmitted through the lens  560  and is incident on the reflective surface of the varifocal mirror  540 . The reflected light is retransmitted through the lens  560  and is directed to the output optical fiber  516  with a coupling efficiency related to the degree of deflection of the varifocal mirror  540  and an angle of the tilting substrate  542 . Low speed attenuation input is provided via electrodes  580   a  and  580   b , which electrostatically attract one side of the substrate  542  closer to the electrodes. High speed attenuation input is provided via the electrodes within the varifocal mirror as discussed above. Advantageously, this embodiment allows the fast and slow attenuation to be provided simultaneously and controlled independently. 
     Notably, a square membrane is particularly attractive for the embodiment illustrated in FIG.  8 . Referring to FIG. 9, the beam spot produced at the output port by the square membrane is elliptical in shape. Deformation of the square membrane expands the output spot along the major axis only. This means that if the tilting platform moves the spot along the minor axis, and the membrane expands the spot along the major axis, the fast and slow attenuation mechanisms will function independently, without interfering with the other&#39;s operation. Only small deflections of the substrate  542  in a direction that moves the beam spot in a direction parallel to the minor axis of the ellipse, are necessary to provide very sensitive slow attenuation. Notably, when the membrane is square only two opposing sides of the membrane need to be supported. 
     Referring to FIG. 10, there is shown another embodiment of a fast attenuator having axial design. More specifically, the axial design includes a wafer bonded membrane chip  601  and top chip  602 . The membrane chip  601  supports membrane  603 . The top chip  602  includes a slot for supporting a lens  604 , which is secured within the slot with an adhesive. A double bore tube  605  supporting input  606  and output  607  optical fibers is secured to the top chip  602  with an adhesive. A cavity  608 , or air gap, separates the lens  604  and the double bore tube  605 . This embodiment has the advantage of being relatively compact (e.g., less than 7 mm in length) and relatively stable. 
     Of course, numerous other embodiments can be envisaged without departing from the spirit and scope of the invention. For example, the fast and slow attenuators are not limited to those disclosed herein, other attenuators are also possible.