Abstract:
An improved device, which may act as a variable attenuator, changes the optical intensity of an optical signal by moving a platform onto which a light transmissive structure such as a waveguide is disposed. The light transmissive structure is positioned and aligned to receive an optical signal and positioned and aligned to transmit the optical signal. By moving the light transmissive structure into a position of reduced alignment with an input source, the light transmissive structure may receive less or none of the optical signal, thereby attenuating it. Alternatively, by moving the light transmissive structure into a position of reduced alignment with an output structure, the light transmissive structure may transmit less or none of the optical signal, thereby attenuating its transmission.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This patent application is a continuation-in-part and claims priority of the following related patent applications: (1) provisional U.S. Patent Application Ser. No. 60/233,672 by Ying Wen Hsu, filed on Sep. 19, 2000 and titled “Method For Switching Optical Signals Using Microstructures;” (2) provisional U.S. Patent Application Ser. No. 60/241,762 by Ying Wen Hsu, filed on Oct. 20, 2000, titled “Method for switching optical signals using microstructures;” (3) U.S. Patent Application Ser. No. 09/837,829, now U.S. Pat. No. 6,690,847 by Ying Wen Hsu filed on Apr. 17, 2001 and titled “Optical Switching Element Having Movable Optically Transmissive Microstructure;” (4) U.S. patent application Ser. No. 09/837,817 now U.S. Pat. No. 6,647,170, by Ying Wen Hsu, filed on Apr. 17, 2001 and titled “Optical Switching System That Uses Movable Microstructures To Switch Optical Signals In Three Dimensions,” all patent applications of which are expressly incorporated herein by reference for all purposes. 

   FIELD OF THE INVENTION 
   The field of the invention is devices that change the optical intensity of an optical signal and in particular, devices that use a movable light transmissive structure to change the optical intensity of an optical signal. 
   BACKGROUND 
   There is a class of devices generally referred to as Variable Optical Attenuators (VOAs). A VOA is used to reduce the power of an optical signal so that the resulting power level is within the acceptable range of those devices or instruments working downstream from the VOA. For example, a VOA may be used to equalize the power levels of multiple optical signals before the signals are combined in a DWDM system (Dense Wavelength Division Multiplexing) for high-speed transport. This equalization is required because the multiplexed optical signals will be amplified before being transported an d y excessively high power signals could be lost due to saturation. VOAs may also be required after the signals are multiplexed in a DWDM system to reduce the output power, The reason is that the actual power is dependent on the number of active channels, which can vary over time. 
   A VOA is one of the key components used in fiber optic communication systems. During the past decade, the demand for higher bandwidth driven by the Internet has resulted in a need for mass-producible and low cost optical components. A successful strategy used to reduce cost is to design optical components by leveraging the well-established manufacturing processes taken from the semiconductor industry. A strong interest exists, therefore, to produce VOAs and other optical components from typical semiconductor materials such as silicon, silica, nitrite and others. New developments are also seeking to produce these components using active materials such as gallium arsenide because these materials can be used to produce light generating components. An ultimate goal is to integrate a maximum number of functions on a single substrate to minimize the manufacturing cost. 
   There are prior art methods for adjusting the output power of an optical signal. The most common way to adjust the power of an optical signal is by simply limiting the amount of light transmitted from one fiber to another fiber. This can be accomplished by inserting an object (optically opaque in the wavelength of interest) between the light-carrying fiber and the outgoing fiber. The optically opaque object, usually referred to as a shutter, can be moved in small distances such that the amount of light captured by the receiving fiber can be controlled precisely. Conventional VOAs move the shutter by using precise mechanical stages and motors that have resulted in large and expensive systems. Other techniques rely on optical properties of selective materials such as liquid crystals to affect the amount of light passing through the material. Electro-optics and thermo-optical effects have also been used to affect the amount of light transmitted. 
   More recently, it has been desirable to produce VOAs using materials and processes compatible with semiconductor manufacturing processes.  FIG. 1  illustrates an example of a prior art approach where a miniature actuator  10  is fabricated directly on the silicon substrate  20 . Light is conducted into the switching region by an optical fiber  22 . A shutter  24  is positioned between the end of the input fiber  22  and the entrance of the output fiber  26 . The shutter  24  is supported by an actuator/micro-mechanism  10  produced out of silicon. The actuator/micro-mechanism  10  moves the shutter in the direction indicated by the actuation arrow. Electrical interface pads  28  may be coupled to the actuator/micro-mechanism  10  in order to control the actuator/micro-mechanism  10 . By moving the shutter  24 , more or less of the light from the input optical fiber  22  can be allowed to pass into the output optical fiber  26 . This approach is described in U.S. Pat. No. 6,173,105. A wide range of fabrication technologies referred to as MEMS processes (Micro-Electro Mechanical Systems) have been employed successfully to produce these micro-mechanisms. Different methods of actuation are available including electrostatic, thermal and magnetic. The use of MEMS technology allows precise control of the actuator/mechanism  10  as well as batch manufacturing processes. 
   One problem associated with a VOA based on the shutter approach is the difficulty of integrating it with optical systems that use waveguides. Waveguides, by contrast with shutters, are optically transmissive structures. In the typical semiconductor process, different layers of materials are sequentially deposited and patterned. In the shutter approach, the silicon shutter must be located on the same plane as the waveguides and also must be physically larger than the waveguides to provide effective blocking of light. These two requirements make it difficult to produce both shutter and waveguides in the same processing sequence. Although it is possible create the shutter and waveguides separately by breaking up the process and by selective masking, this approach increases potential misalignments and manufacturing complexity. 
   Ideally, a VOA design for integration with a waveguide-based system uses the same processing steps as that used to make waveguides. One choice is to introduce a mechanism into the waveguide that would modulate light. That can be achieved by introducing electro-optical, thermal, or acousto-optical effects into the waveguides. These methods, however, are limited to waveguides made out of certain active materials, which waveguides are generally difficult to manufacture. Another possibility is to use a waveguide with a movable section which acts as a shutter by doping the movable section of the waveguide so as to become opaque. However, all of these methods require significant deviations from standard waveguide manufacturing processes. Therefore, there is a need for a device that changes the optical intensity of an optical signal which uses standard waveguide manufacturing processes. There is also a need for a cost effective method of fabricating such a device. 
   SUMMARY OF THE INVENTION 
   Generally, the device changes the optical intensity of an optical signal by using a light transmissive structure such as a waveguide disposed on a movable platform. 
   Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. However, like parts do not always have like reference numerals. Moreover, all illustrations are intended to convey concepts, where relative sizes, shapes and other detailed attributes may be illustrated schematically rather than literally or precisely. 
       FIG. 1  is a schematic illustration of a prior art variable attenuator which has a shutter. 
       FIGS. 2A and 2B  are schematic illustrations of an example embodiment of a device that changes the optical intensity of an optical signal by using a light transmissive structure such as a waveguide disposed on a movable platform, where  FIG. 2A  illustrates the movable platform in a first position and  FIG. 2B  illustrates the movable platform in a second position. 
       FIG. 3  is a schematic illustration of another example embodiment of a device that changes the optical intensity of an optical signal by using a light transmissive structure such as a waveguide disposed on a movable platform, where the movable platform rotates. 
       FIG. 4  is a schematic illustration of yet another example embodiment of a device that changes the optical intensity of an optical signal by using a light transmissive structure such as a waveguide disposed on a movable platform, where the movable platform is curved and has a prism coupler. 
       FIG. 5  is a schematic illustration of an example embodiment of a device that changes the optical intensity of an optical signal by using a light transmissive structure such as a waveguide disposed on a movable platform, which illustration includes structures associated with the moving platform. 
       FIG. 6  is a schematic graph of the light output versus the offset in microns. 
       FIG. 7  is a schematic cross-sectional view of a movable waveguide having an air gap. 
       FIG. 8  is a schematic cross-sectional view of a stationary waveguide resting on an oxide layer of a substrate. 
       FIG. 9  is a schematic illustration of an example embodiment of a 4×4 optical switch coupled to VOAs. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The improved device for changing the optical intensity of an optical signal uses a light transmissive structure, preferably a movable waveguide, whose position determines the amount of free space through which the optical signal must travel, thereby variably attenuating light. The phrase “light transmissive structure” includes structures that are optically transmissive such as waveguides and optical fibers, but not air gaps, mirrors and shutters. 
     FIGS. 2A and 2B  are illustrations of an example embodiment of a device  30  that changes the optical intensity of an optical signal by using a light transmissive structure such as a waveguide disposed on a movable platform, where  FIG. 2A  illustrates the movable platform in a first position and  FIG. 2B  illustrates the movable platform in a second position. Two stationary waveguides  32 ,  34  are positioned adjacent to the input and output of a movable waveguide  36 . When the movable waveguide  36  is aligned with the stationary waveguides  32 ,  34  as shown in  FIG. 2A , light from the source  38  is guided across through the movable waveguide  36 . When the movable waveguide  36  is moved completely away from the stationary waveguides  32 ,  34 , as in  FIG. 2B , the input light has to traverse across free space  40 . By setting the distance of the free space between the fixed waveguides  32 ,  34  such that a minimal amount of light is captured in the output fixed waveguide  34 , significant optical attenuation can be achieved (e.g., up to 100% attenuation). To adjust the amount of light transmitted, the movable waveguide  36  is inserted into the light path to allow for the desired amount of light to pass through (e.g., up to 100% transmission). The movable waveguide  36  essentially acts as a variable conduit bridging the two junctions. Thus, the improved device  30  uses free space as a means of attenuating light and a movable waveguide  36  to variable adjust the amount of light passing from the input waveguide  32  to the output waveguide  34 . 
     FIG. 3  illustrates another example embodiment of a device for attenuating light by moving a waveguide  50  relative to stationary waveguides  52 ,  54 .  FIG. 3  attenuates light by rotating the movable waveguide  50  so that less or no light is transmitted from the input waveguide  52  into the movable waveguide  50 . In this example, less or no light is transmitted also from the movable waveguide  50  into output wave guide  54 . A maximum amount of light is transmitted when the movable waveguide  50  is aligned with the stationary waveguides  52 ,  54 . When the movable waveguide  50  is rotated such that the entry surface of the movable waveguide  50  is blocked from receiving light from the input stationary waveguide  52 , the transmission of light is completely terminated. Rotating the movable waveguide  50  to an intermediate position makes it possible for a portion of the light to be transmitted. 
     FIG. 4  illustrates yet another example embodiment of a device for using a movable waveguide  60  to attenuate light. In this example, light is attenuated by the air gap  62  between the stationary input waveguide  64  and the movable waveguide  60 . This approach requires relatively larger movement (several millimeters) to translate the movable waveguide  60  in order to completely attenuate light. To couple light laterally into the stationary output waveguide  66 , a prism coupler  68  will be required. The use of prism coupler  68  is well known to those skilled in the art of waveguide designs. In an alternative embodiment to  FIG. 3  or  4 , other light transmissive structures may be used in place of one or more of the waveguides. 
     FIG. 5  illustrates a VOA device which uses a movable waveguide  70  and is fabricated with a MEMS micromachining manufacturing process. The device includes a waveguide  70  integrated on top of a movable platform  72 . The movable platform  72  is supported on springs  74 , which are connected to anchors  76  tied to the substrate. The movable platform  72 , springs  74  and anchors  76  are all preferably produced from the same layer of material. To enable the platform  72  to move, an air gap (not illustrated) underneath the platform  72  is used so that the platform  72  is supported completely on the springs  74 . There are several methods of producing a structure which is capable of being freely suspended; these methods are well known to those skill in the art of micromachining. Materials such as silicon, silica, nitrite and metals have all been made successfully into freely-suspended micro-structures. Any appropriate material may be used in the VOA device. 
   To move the platform  72 , actuators  80  are connected to the platform  72 . A widely used actuator is the inter-digitated structure referred to as “comb fingers” because of their resemblance to combs. Preferably, the actuators  80  of the VOA uses inter-digitated structures. Such inter-digitated structures can be easily produced on the same layer as the platform  72 . A set of comb fingers  84  is patterned onto the movable platform  72 , while an opposing set  82  is patterned and fixed to the substrate. To actuate the actuators  80 , an electrical voltage differential is applied to the fixed electrode  82  and the movable electrode  84 . The resulting voltage differential generates an electrostatic attraction force and pulls the movable platform  72  toward the fixed electrode  82 . Other actuation techniques could also be used. Examples include actuators whose operation is based on thermal, magnetic and/or piezoelectric drives. The design of actuators is well known to those skilled in the art of designing micromachined structures. 
   The movable platform  72  supports a waveguide  70  that bridges two adjacent and stationary waveguides  86 ,  88 . By applying a varying level of electrical voltage to the actuator  80 , the movable waveguide  70  can be moved by any desired amount. For precise movements, the comb fingers of the actuator  80  can be connected to a position sensing circuit, which preferably is coupled to movable and fixed sensing comb fingers  90 , also referred to as position sensing electrodes. The change in the relative position between movable and fixed sensing comb fingers  90  generates a change in the electrical capacitance between the fingers; this change can be detected and converted into electrical voltages through proper detection circuits. Commercial capacitance-to-voltage conversion chips are available. The position signal could also be used in a closed-loop control circuit to hold the movable waveguide  70  in a fixed position. The use of position circuits and control algorithms are well known to those skilled in the art of micromachine control. Other means of sensing such as those based on piezo-resistive, magnetic and/or optical methods are also viable. 
   Referring to  FIG. 5 , an optical signal is connected to the input waveguide  86 , which preferably is mounted on a stationary platform which aligns the input waveguide  86  with the movable waveguide  70 . On command from the system to attenuate power, an electrical voltage is send to the actuator  80  to move the movable waveguide  70 . The actual power of light transmitted can be monitored from the output waveguide  88 , which preferably is mounted on a stationary platform which aligns the outut waveguide  88  with the movable waveguide  70 . Electrical power is applied to the actuator  80  until the desired attenuation is achieved. To lock onto the desired attenuation, the position of the movable waveguide  70  is “fixed” by monitoring the output voltage of the position sensing electrodes  90  or the power optical signal. Buffering or cladding  92  for the waveguides may be used as well. 
     FIG. 6  is a graph of the monitored output light power on the Y axis and the offset in microns on the X axis for a simulated design of a movable waveguide having the following dimensions: 6 micron width, 6 micron height, and 2 mm long. The transmitted power is slightly less than 100% due to loss across the air gap. This loss can be reduced by using an index matching gel or by coating the face of the waveguides with anti-reflection film. As the movable waveguide  70  is moved, light is attenuated until approximately 10 microns of movement. The resulting attenuation for the given geometry is about −27 dB. Higher attenuation is also achievable with further optimization. 
     FIGS. 7 and 8  illustrate cross sectional views of a movable and a stationary waveguide.  FIG. 7  shows a suspended waveguide  100 , while  FIG. 8  shows a stationary waveguide  102  positioned on top of the substrate  104 . The movable waveguide  100  is suspended over an air gap  106  over the substrate  104 . The movable waveguide  100  preferably includes a core  108  surrounded at least partially by a cladding  10  and a buffer  112 . The buffer  112  rests on a silicon layer  114 . Turning to  FIG. 8 , the stationary waveguide  102  preferably includes a core  108  surrounded at least partially by a cladding  110  and a buffer  112 . The buffer  112  rests on a silicon layer  114 , which in turn rests on an oxide layer  116  on the substrate  104 . 
     FIG. 9  illustrates an example of integrating the improved device with an optical switch. For switches with a smaller number of ports, the range of the output power will be small, but for switches having a large number of ports, the range of output power can vary significantly due to the greater number of different paths which can be taken by each optical signal. A large range in the switch output would be undesirable and will require using VOAs to equalize the output. For such an optical switch, the use of any of the improved devices described in this patent specification will greatly simplify the integration of a VOA and the optical switch using the same manufacturing process. For example, input optical fibers  120  are coupled to a 4×4 optical switch  122 . The 4×4 optical switch  122  is coupled to VOAs  124 , each VOA being one of the improved devices described herein. The 4×4 optical switch  122  and VOAs  124  are mounted to a common substrate  126 . Because there are 4 output ports in this example, there are 4 VOAs  124 . Each of the four VOAs  124  is coupled to an output optical fiber  130 . Each VOA  124  may be separately controlled to attenuate the light as desired. 
   While various embodiments of the application have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of the subject invention. For example, each feature of one embodiment can be mixed and matched with other features shown in other embodiments. Features and processes known to those of ordinary skill in the art of optics and semiconductor processing may similarly be incorporated as desired. Additionally and obviously, features may be added or subtracted as desired. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.