Abstract:
A programmable encoder is provided that includes at least one optical input for receiving a modulated broadband optical signal and at least one optical output. A dispersion element receives the optical signal from the optical input and spatially separates the optical signal into a plurality of wavelength components. A collimating element is provided for collimating the plurality of wavelength components. An actuatable optical arrangement receives the collimated plurality of wavelength components from the collimating element. The actuatable optical arrangement includes a digital micromirror device (DMD) from which a programmably selected subset of wavelength components are reflected at least twice before being directed to a selected one of optical outputs to thereby encode the CDMA signal.

Description:
STATEMENT OF RELATED APPLICATION 
     This application is related to U.S. patent application Ser. No. 11/781,931, entitled “Multiple Function Digital Optical Switch”, filed on even date herewith, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     As the quantity and speed of data communications over fiber optic systems rapidly increases due to the growing demand from Internet usage and other communications, improved all-optical switching systems are of increased interest to overcome the high cost and slow switching speeds of conventional switches. These conventional switches include, for example, various mechanical switches, electro-optic switches, and thermo-optic switches. In particular, the increased complexity and cost of switching systems that involve switching from an optical signal to an electrical signal and then back to an optical signal have increased the level of interest in improved all-optical switches. 
     An all-optical switch provides switching of an optical signal from one input path to a selected one of a plurality of different output paths without any intermediate conversion of the optical signal to an electrical signal. This is typically accomplished by applying an electrical signal to a switchable element to cause the optical signal to be selectively switched. These electro-optic switches are responsive to the electrical signal to selectively switch the light of the optical signal from the input path to the selected one of the output paths. 
     A variety of approaches are known for making all-optical optical switches, such as, for example, described in U.S. Pat. Nos. 6,687,431, 6,661,948, 6,625,346, and 6,549,699. The switchable element in many of these devices is a Micro-Electromechanical System (MEMS) optical element such as a MEMS mirror. The MEMS mirrors may be individually controllable and movable, e.g., pivotable (or rotatable) under analog (or continuous) control, such that, upon reflection, optical wavelengths are directed from one or more input ports into selected ones of the output ports. In this way, the optical switch may be used to selectively couple the wavelengths between the input and output ports of the system. However, a number of problems arise when such MEMS mirrors operating in an analog manner are used in an optical switch. For example, to avoid instability from mirror structure resonance a means for sensing the precise position of each MEMS mirror may need to be integrated with the MEMS device, which can be difficult and complex to achieve. In addition, switching individual MEMS elements into a MEMS device can cause disturbances in neighboring MEMS elements. In particular, switching MEMS mirrors in a MEMS array has been found to cause an aerodynamic coupling with other mirrors in the array that can disturb mirrors that are intended to remain static. The position of the MEMS mirrors can also drift because the mirrors are generally actuated by a relatively weak electrostatic force that requires a large voltage. The large voltage can cause charging that also disturbs the position of the mirrors. All of these effects become a more significant problem when these devices perform dynamic gain equalization by off-axis alignment of the free space beam as it is coupled into an output port. This technique requires in a much tighter tolerance in the mirror positioning because the attenuation sensitivity as a function of angular positioning level increases nonlinearly as off-axis alignment is extended to achieve higher attenuation levels. The higher sensitivity is the reason complex servo-loops and expensive mirror position sensing systems are disclosed as desirable for this technology. 
     Digital mirror arrays solve many of the problems with their analog counterparts, and a number of such arrays have become available which include an integrated monolithic switching structure. These devices use relatively low voltages to provide individual mirror elements that can be selectively switched between two or more positions with high precision and stability. One class of such digital mirror arrays are spatial light modulators, often referred to as digital micromirror displays or devices (DMDs). Typically, a DMD consists of an array of tiny mirrors (typically, several million per square inch), wherein the angular position of each mirror element is individually controllable between at least two positions that are angularly offset from one another by approximately 10 to 20 degrees, for instance. A mirror base is located behind the mirror elements. The individually addressable mirror elements are tiltably mounted on mechanical hinges, and typically the array of mirror elements overlays a layer of controlling circuitry in the mirror base, all of which is mounted on a semiconductor chip. The mirror face of the DMD is composed of a generally rectangular grid array of rectangular or square mirror elements. A typical mirror element is about 16 micrometers square, and the individual elements are separated from one another by a distance of about 1 micron. Individually controlled tilting of the mirror elements in the array around at least one axis allows energy that is reflected from the mirror face to be formed into a predetermined pattern. Further, the mirror face can be substantially instantaneously reconfigured in response to digital signals to form a different pattern. Such reconfiguration can generally be achieved in about 25 microseconds or less. 
     Optical switches incorporating DMDs are known and disclosed, for example, in U.S. Pat. Nos. 6,295,154, and 7,203,398 and U.S. Appl. Serial Nos. 2003/0164959 and 2002/0034356. These switches suffer from a number of limitations in extending the devices to higher levels of functionality to support multiple wavelengths or colors or light on each port, larger numbers of switching ports, or additional functionality such as independent control of the optical power level during switching 
     Accordingly, it would be advantageous to provide a multiple function DMD switching device that is capable of performing multiple optical processing functions In particular, it would be advantageous to provide a device that integrates a 1×N switch (where N&gt;1) that can control the power levels directed to multiple output ports in a cross-talk free manner. 
     SUMMARY 
     In accordance with one embodiment of the invention, a programmable encoder is provided that includes at least one optical input for receiving a modulated broadband optical signal and at least one optical output. A dispersion element receives the optical signal from the optical input and spatially separates the optical signal into a plurality of wavelength components. A collimating element is provided for collimating the plurality of wavelength components. An actuatable optical arrangement receives the collimated plurality of wavelength components from the collimating element. The actuatable optical arrangement includes a digital micromirror device (DMD) from which a programmably selected subset of wavelength components are reflected at least twice before being directed to a selected one of optical outputs to thereby encode the CDMA signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a first side view of one embodiment of a wavelength selective switch constructed in accordance with the present invention. 
         FIG. 2  shows a plan view of the surface of the micromirror device on which the wavelengths are incident, taken along line  2 - 2  in  FIG. 1 . 
         FIG. 3  is a perspective view of two mirror elements, which are tiltably mounted so that they can tilt about axis. 
         FIG. 4  is a second side view of the wavelength selective switch taken along line  4 - 4  in  FIG. 1 . 
         FIGS. 5   a  and  5   b  show light paths being reflected off individual mirror elements. 
         FIG. 6  is a side view of another embodiment of the switch taken along lines  4 - 4  in  FIG. 4 . 
         FIG. 7  shows the switch of  FIG. 4  configured as a 1×2 switch with an optical channel monitor (OCM). 
     
    
    
     DETAILED DESCRIPTION 
     As detailed below, a wavelength selective switch that employs a DMD is provided. In contrast to the switches described above, the wavelength selective switch described herein includes a dispersive element to separate the different wavelengths of a wavelength division multiplexed (WDM) or dense wavelength division multiplexed (DWDM) signal, thereby enabling the switching of multiple wavelengths from each port. In addition, the switch can perform other functions in addition to or as an alternative to optical switching. For example, unlike the aforementioned switches, when the switch described herein is configured as a 1×N switch with N&gt;1, it can simultaneously perform dynamic gain equalization or power control on a per wavelength basis without undesirable inter-port crosstalk. 
     It should be noted at the outset that the wavelength selective switch described below operates in a symmetric manner so that any wavelength components directed to any of the output ports can be alternatively directed to any of the input ports. Accordingly, one of ordinary skill in the art will recognize that the switching paths are reciprocal, and thus the terms input and output as used herein are not limited to elements that transmit an optical signal or wavelength component in a single direction relative to the switch. In other words, when light enters the device from any so-called output port, this output port serves as an input port, and similarly, any so-called input port can equally serve as an output port. 
       FIG. 1  shows a first side view of one embodiment of a wavelength selective switch  100  constructed in accordance with the present invention. While the switch may be employed in connection with any optical wavelengths, it is particularly suitable for use in connection with optical wavelengths in the C-band (wavelengths between about 1525 to 1565 nm) or the L-band (wavelengths between about 1565-1625 nm). The switch includes an array  110  of parallel input/output optical ports (e.g., fibers) in the plane coming out of the page, only one of which is visible in  FIG. 1 . A collimating lens  115  is positioned to receive light exiting from the fiber array  110 . The collimating lens  115  is centered about the optical axis extending from the middle fiber in fiber array  110 . In this way light exiting the fibers in the fiber array  110  is collimated by lens  115 . The collimated light is then incident upon a transmission grating  120 , which spatially separates the light into its component wavelengths λ 1 , λ 2 , λ 3 , . . . λ N  and directs them through a second lens  125 . Lens  125  performs different functions in the two axes of the lens plane. In the plane of the page in  FIG. 1 , the lens collimates the dispersed wavelengths λ 1  through λ N . In the plane out of the page the lens focuses the collimated beam onto the surface of the micromirror device  130 . The result is a stripe of spatially separated wavelengths that are collimated along the axis of dispersion, and focused perpendicular to that axis, where the stripe is directed between mirror strips  140  (only one of which is visible in  FIG. 1 ) and onto the surface of a micromirror device  130 . 
       FIG. 2  shows a plan view of the surface of the micromirror device  130  on which the wavelengths are incident, taken along line  2 - 2  in  FIG. 1 . The mirror face of micromirror device  130  is populated with thousands of miniature, tiltable, individually controllable mirror elements, typical ones of which are indicated at  140  and  145 . Each mirror element typically has dimensions on the order of microns. For example, one particular such micromirror device  130  is a 768×1024 array of individually controllable mirror elements. Each mirror element is a square about 16 microns on each side and rotates about an axis parallel to the plane of the device  130 . As indicated in  FIG. 2  by dashed line  150 , the axis of the mirror elements extends diagonally from one corner to the opposite corner of each mirror element. 
     Mirror strips  140   1  and  140   2  (“mirror strips  140 ”) are situated over the surface of the micromirror device  130  and extend in a direction parallel to the axis  150 . The mirror strips  140  can be fabricated directly on the lower surface of a transparent window or substrate using conventional lithographic or other techniques. The substrate can be integrated with the micromirror device  130  to form a single optical assembly. An anti-reflective coating may be applied to the upper surface of the window to reduce back reflections. Likewise, an anti-reflective coating may be applied to the nonreflective portions of the lower surface between the individual mirror strips. 
       FIG. 3  is a perspective view of mirror elements  141  and  145 , which are tiltably mounted so that they can tilt about axis  150 . The unactuated surface  50  of individual mirror element  141 , and the surface  52  of individual mirror element  145  normally lie in the same plane, which can be referred to as the off or float state of the mirror elements. By the application of a controlling force to effect the movement of, for example, support member  48 , surface  52  can be tilted around axis  150  out of the common plane. If support  44  holds element  141  in its original position (i.e. its float state), light incident on surface  50  will be reflected to a different location than light incident on surface  52 . Micromirror devices are well known, commercially available components and thus details concerning their structure and fabrication need not be discussed further. 
     Micromirror devices such as micromirror device  130  are generally operated in one of two modes of operation. The first mode of operation is an analog mode, sometimes called beam steering. In this mode of operation the address electrodes, which control the rotation of the individual mirror elements about their respective axes, is charged to a voltage corresponding to the desired deflection of the mirror. Light striking the micromirror device is reflected by the mirror at an angle determined by the deflection of the mirror. The second mode of operation is a digital mode. When operated digitally, each mirror element is fully deflected in either of the two directions about its axis. That is, each mirror element can be positioned in one of three states: two states in which the mirror element are tilted in opposite directions from one another and the off or flat state. Digital operation uses a relatively large voltage to ensure the mirror is fully deflected. For purposes of illustration the present invention will be described in terms of a micromirror device that operates in a digital mode. Such a micromirror device may be conveniently referred to as a digital micromirror device (DMD). In some embodiments of the invention the DMD is preferably selected to have mirror elements sufficiently small so that each wavelength component can reflect off of a relatively large number mirror elements (e.g., 10 or more). Additionally, in some embodiments the spatial fill factor of the mirror elements relative to total area of the mirror array in the DMD is equal to about 85% or more. 
       FIG. 4  is a second side view of the wavelength selective switch taken along line  4 - 4  in  FIG. 1 . This view illustrates the manner in wavelength λ 1  is selectively switched from an input fiber (i.e., fiber  110   4 ) in fiber array  110  to one of two output fibers  110   3  and  110   5  in fiber array  110 . The remaining wavelengths λ 2 -λ N  are selectively switchable in a similar manner. 
     The dimensions of the mirror elements in DMD  130  are sufficiently small relative to the optical wavelengths that are employed in the switch  100  so that the light is diffracted off the DMD  130 . Accordingly, depending on the tilt angle of the mirror elements upon which wavelength λ 1  is incident, the DMD  130  serves as a blazed grating and the light will be diffracted up and to the right along path  160  or, alternatively, up and the left along path  165 . The diffracted light will reinforce itself as it reflects back toward mirror strips  140   1  or  140   2 , forming a series of points of light at locations represented by various diffraction orders. Likewise, the diffracted light will cancel itself out between the orders. Thus, in order to maximize the light that reaches mirror strips  140   1  or  140   2 , the mirror strips should each be located at one of the diffraction orders for the wavelengths. In the particular example shown in  FIG. 4 , the mirrors  140   1  and  140   2  are located at the −2 order diffraction and the +2 order diffraction, respectively. That is, the light diffracted along paths  160  and  165  represent second order diffraction. More generally, however, the mirror strips  140   2  and  140   2  may be located at any appropriate diffraction order. 
     If the light is incident upon mirror strip  140   1 , it will be reflected back to the DMD  130  along path  175 . On the other hand, if the light is incident upon mirror strip  140   2 , it will be reflected back to the DMD  130  along path  170 . If the mirror elements upon which light beam  170  or  175  is incident are tilted in the opposite position from the mirror elements upon which the initial incoming wavelength λ 1  is incident along path  180 , then the light will be reflected back toward the second collimating lens  125  along paths  180  or  182 , which are parallel to path  180 . This is illustrated in  FIG. 5   a , where for simplicity the light path  180  is shown being reflected off a single mirror element  143  and the light path  182  is shown being reflected off a single mirror element  147  in the DMD  130 . As shown, mirror elements  143  and  147  are tilted in opposite directions. Note that if the optical beam is incident on more than one mirror of the DMD, it is possible to simultaneously direct different portions of the beam to both outputs  180  and  182 . This capability is referred to as programmable multi-casting, which effectively allows a light beam to be split and coupled into multiple output ports. 
     Referring again to  FIG. 4 , after traveling along path  180  or  182  after its second reflection off the DMD  130 , the light will then traverse the grating  120  and the first collimating lens  115  in a reciprocal manner from that described above. If the light has been reflected off the DMD  130  along path  182 , then the light will be received by output port  110   5 . On the other hand, if the light has been reflected off the DMD  130  along path  184 , then the light will be received by output port  110   3 . In this way the wavelength λ 1  can be switched from input port  110   4  to a selected one of the output ports  110   3  and  110   5 . The remaining wavelengths λ 1 -λ N  can be switched in a similar manner. 
     The individual wavelengths may be switched from the input port  110   4  to one of the output ports  110   3  and  110   5  in either a relatively lossless manner or with a controllable amount of attenuation. The attenuation can be imparted by tilting a selected number of the mirror elements upon which the switching wavelength is received from one of the mirror strips  140   1  and  140   2 . This can be illustrated in connection with  FIG. 5   b , which shows incoming light path  180  and the outgoing light path  182 . As previously noted in connection with  FIG. 5   a , for simplicity, the light path  180  is shown being reflected off the single mirror element  143  and the light path  182 ′ is shown being reflected off the single mirror element  147 . In  FIG. 5   a  the mirror elements  143  and  147  are tilted in opposite directions. In  FIG. 5   b  on the other hand, mirror elements  143  and  147  are both tilted in the same direction and as a result the light is reflected off the mirror element  147  along path  182 ′, where it is directed away from the fiber array  110  and thus is lost. By properly tilting any selected number of the mirror elements that reflect components of the switching wavelength incident upon the DMD  130  a second time, such as in the case of mirror element  147 , the light directed to the selected output port or fiber can be attenuated by a selected amount. More specifically, the light can be attenuated in increments equal to the amount of light reflected off a single mirror element.  FIG. 5   b  thus illustrates a general principle of operation of the wavelength selective switch  100 . The first set of mirror elements (e.g., mirror elements  143 ) that the switching wavelength diffracts off of control the output port to which the switching wavelength is directed. The second set of mirror elements (e.g., mirror elements  147 ) that the switching wavelength diffracts off of control the attenuation of the switching wavelength. Note that if mirror element  147  is positioned in the off or float state, the light path  182  could be reflected onto a subsequent one of the mirror strips  140 , which in turn reflects the light back to the DMD  130  from which it could be redirected to one or more additional output ports. 
     As described above in connection with input port  110   4  and output ports  110   3  and  110   5 , the wavelength selective switch serves as a 1×2 switch. Many other switch configurations are possible using the 1×2 switch as a fundamental building block. For instance, any number of such 1×2 switches may be formed using a single DMD, provided that the appropriate number of mirror strips  140  are employed. In  FIG. 4 , for example, seven fibers or ports are shown, any three of which can serve as 1×2 switch in which the center port serves as the input ports and the outer ports serve as the output ports. For instance, one switch may be formed by ports  110   1 ,  110   2  and  110   3 , with port  110   2  serving as the input port, while another switch may be formed by ports  110   5 ,  110   6  and  110   7 , with port  110   6  serving as the input port.  FIG. 6  shows the switch  100  when configured in this manner. Similar to  FIG. 4 ,  FIG. 6 , as well as  FIG. 7  described below, is a side view of the switch  100  taken along lines  4 - 4  in  FIG. 4 . In this example the 1×2 switch associated with ports  110   1 ,  110   2  and  110   3  employs mirror strips  140   3  and  140   4  and the 1×2 switch associated with ports  110   5 ,  110   6  and  110   7  employ mirror strips  140   4  and  140   5 . 
     The various embodiments of the wavelength selective switch  100  described above may be provided with optical channel monitoring (OCM) capabilities. This can be accomplished using any two adjacent ports, one of which serves as the OCM input and the other of which serves as the output port at which an optical detector can be placed.  FIG. 7  shows the switch  100  of  FIG. 4  in which ports  110   3 ,  110   4  and  110   5  serves as a 1×2 switch and ports  110   1  and  110   2  serve as one OCM and ports  110   6  and  110   7  serve as another OCM. Ports  110   2  and  110   6  serve as the OCM inputs and ports  110   1  and  110   7  serve as the OCM outputs at which detectors  112  and  114  are located. It should be noted that more generally any two adjacent ports in the wavelength selective switch  100  serve as a 1×1 switch. This switch can be programmably operated as an OCM by using its wavelength selective attenuation capability to raster a narrow band of time varying wavelengths onto the output port detector. This rastering can be calibrated to convert the detector output as a function of time to the optical input power as a function of wavelength. 
     Rather than being used in DWDM systems, the optical device described herein can be used as a programmable decoding processor in code division multiple access (CDMA) systems. This is an attractive device for such an application because the configuration of the mirror elements can direct a programmable set of wavelength components to a given output port, and it can do so without disturbing the coherence of these wavelength components relative to each other. This feature is ideally suited for a CDMA system in which a unique “code” is defined for each channel. Because the DMD is a low cost device with many mirrors, it can readily realize a potentially large number of unique codes which makes a CDMA system with larger number of channels. The larger number of channels could be used to send additional unique flows of information, or they could be used as a high resolution decoder that is capable of emulating the code of any number of lower resolution devices. Such a device also could be used together with dynamic code or spectrum hopping to enhance the security of transmission of the CDMA system. The current device should perform particularly well in this application because it has a very high spectral selectivity to improve signal to noise ratio, and multiple output ports, which can be used to provide two independently programmable channels simultaneously, or the ability to provide a code-difference signal whereby the received power of a particular code is contrasted with its spectral complement in the other port to offset the noise produced by other channels. Note that this complementary signal (difference) arrangement with the high spectral resolution and high DMD could also effectively be used as a code-identifier to search for CDMA codes in an optical signal. 
     Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, while the wavelength selective switch as described above employ a fiber array in which the various ports are arranged in parallel with one another, in other embodiments of the invention two or more of the various ports may be arranged to be non-parallel to one another. In this way back reflections into the ports can be avoided when the mirror elements of the DMD are in their off or float state. In addition, while the optical switch has been described as a device that performs switching functionality, the device disclosed herein more generally may be employed for a variety of other purposes instead of, or in addition to, switching. For instance, it has already been mentioned that the device can be used as an optical channel monitor. The device may also be used in applications such as programmable multicasting to direct wavelength components to more than one output port. Another application includes chromatic dispersion control, which can be accomplished by suitably adjusting the path lengths traveled by each of the wavelength components passing through the device.