Abstract:
A method of monitoring at least one optical wavelength component of a WDM optical signal being routed through a wavelength selective switch (WSS) includes directing an optical wavelength component from a given input port of the WSS to a selected output port with a selected amount of attenuation. A rejected portion of the optical wavelength component giving rise to the selected amount of attenuation is directed to an optical monitor associated with another output port of the WSS. A power level of the optical wavelength component is determined by pre-calibrating a proportionality between the power level of the wavelength component and the power level of the rejected portion that is directed to the optical monitor.

Description:
BACKGROUND 
       [0001]    Fiber optic communication systems typically employ wavelength division multiplexing (WDM), which is a technique for using an optical fiber to carry many spectrally separated independent optical channels. In a wavelength domain, the optical channels are centered on separate channel wavelengths which in dense WDM (WDM) systems are typically spaced apart by 25, 50, 100 or 200 GHz. Information content carried by an optical channel is spread over a finite wavelength band, which is typically narrower than the spacing between channels. 
         [0002]    Optical channel monitoring is increasingly being used by telecommunications carriers and multi-service operators of fiber optic systems. As the traffic on optical networks increases, monitoring and management of the networks become increasingly important issues. To monitor the network, the spectral characteristics of the composite signal at particular points in the network must be determined and analyzed. This information may then be used to optimize the performance of the network. Optical channel monitoring is particularly important for modern optical networks that use reconfigurable and self-managed fiber-optic networks. 
         [0003]    For example, reconfigurable optical add/drop multiplexers (ROADMs) and optical cross connects, which are used to manipulate individual wavelength channels as they are transmitted along the network, require an optical channel monitor. A ROADM allows dynamic and reconfigurable selection of wavelength channels that are to be added or dropped at intermediate nodes along the network. In a ROADM, for instance, an optical channel monitor can provide an inventory of incoming channels as well as an inventory of outgoing channels and to provide channel-power information to variable optical attenuator (VOA) control electronics so that the power of added channels can be equalized with the pass-through channels. 
         [0004]    One type of optical channel monitor employs a wavelength selective switch (WSS), which is a type of switch configured to perform optical switching on a per wavelength channel basis, and is typically capable of switching any wavelength channel at an input fiber to any desired output fiber. Thus, a 1XN WSS can switch any wavelength channel of the WDM input signal propagating along the input fiber to any of the N output fibers coupled to the WSS. 
         [0005]    U.S. Appl. Serial No. [Docket No. 2062/16] shows an OCM that is incorporated in a WSS. A series of photodiodes are provided, each of which receive optical wavelengths from one of the output ports of the OCM. 
       SUMMARY 
       [0006]    In accordance with one aspect of the invention, a method of monitoring at least one optical wavelength component of a WDM optical signal being routed through a wavelength selective switch (WSS) is provided. In accordance with the method, a first phase pattern to be produced by an SLM is selected from among a first plurality of phase patterns. Each of the phase patterns in the first plurality of phase patterns (i) directs a first optical wavelength component from a first input port of the WSS to a selected output port and (ii) causes the first wavelength component to undergo a first selected amount of attenuation. The first phase pattern is further selected to cause a first rejected portion of the first optical wavelength component to give rise to the first selected amount of attenuation to be directed to an optical monitor associated with another output port of the WSS. A first portion of the SLM is periodically programmed with the first phase pattern. The first optical wavelength component is directed to the first portion of the SLM while the first portion of the SLM is periodically configured with the first phase pattern. 
         [0007]    In accordance with another aspect of the invention, an optical device is provided with includes an optical port array, a photodetector, a dispersion element, a focusing element, a programmable optical phase modulator and a processor. The optical port array has at least one optical input port for receiving an optical beam and a plurality of optical output ports. The photodetector is optically coupled to a first of the optical output ports. The dispersion element receives the optical beam from the at least one optical input port and spatially separates the optical beam into a plurality of wavelength components. The focusing element focuses the plurality of wavelength components. The programmable optical phase modulator receives the focused plurality of wavelength components. The modulator is configured to steer the wavelength components to a selected one of the optical output ports. The optical device also include a non-transitory computer-readable storage medium comprising instructions that, when executed, control the processor to be configured for programming a first portion of the programmable optical phase modulator with a first phase pattern. The first phase pattern is selected by the processor from among a first plurality of phase patterns that (i) directs a first of the plurality of optical wavelength components from a first of the optical input ports to a selected output port and (ii) causes the first wavelength component to undergo a first selected amount of attenuation. The first phase pattern is further selected to cause a first rejected portion of the first optical wavelength component giving rise to the first selected amount of attenuation to be directed to the first optical output port. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  shows a functional block diagram of one example of a wavelength selective switch (WSS) that includes an integrated channel monitor. 
           [0009]      FIG. 2  shows an example of a periodic, stepped phase shift profile that may be produced across a region of a LCoS device in the y-axis. 
           [0010]      FIG. 3  illustrates one example of a sequence that may be used to route the wavelength components X 1 , X 2 , X 3  and X 4  of an WDM optical signal through the WSS between its input and output ports. 
           [0011]      FIGS. 4A and 4B  are top and side views respectively of one example of a simplified optical device such as a free-space switch that may be used in conjunction with embodiments of the present invention. 
           [0012]      FIG. 5  is a side view of an alternative example of a simplified optical device such as a free-space switch that may be used in conjunction with embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  shows a functional block diagram of one example of a wavelength selective switch (WSS)  100  that includes an integrated channel monitor. For simplicity of illustration the WSS  100  is depicted as an N×2 switch, with N input ports  1101 ,  1102 ,  1103 , . . .  110   n  (“ 110 ”) and two output ports  120  and  130 . In other implementations the WSS may have more than two output ports. 
         [0014]    A switching fabric  140  operates under control of a controller or processor  150  to optically couple the input ports  110  to the output ports  120  and  130  so that the individual wavelength components or channels of a WDM optical signal received at any of the input ports  110  can be selectively directed to either one of the output ports  120  and  130  under the control of a switch controller  150 . 
         [0015]    As further shown in  FIG. 1 , output port  130  terminates in a photodetector  135  such as a photodiode. As explained below the photodetector  135  can be used as an optical channel monitor to measure the power levels and possibly other signal quality parameters of an optical beam that is directed from any of the input ports  110  to the output port  120 . If more than one output port is provided (in addition to output port  130 , which is used for OCM), the photodetector  135  can be used to measure the power levels and possibly other signal quality parameters of an optical beam that is directed from any of the input ports  110  to any of the output ports. 
         [0016]    Switching fabric  140  may incorporate a spatial light modulator (SLM) that can serve as an optical path conversion system. A spatial light modulator (SLM) consists of an array of optical elements (pixels) in which each pixel acts independently as an optical “valve” to adjust or modulate light intensity. An optical signal is dispersed onto the array along a spectral axis or direction so that the individual channels in the signal are spread over multiple ones of the optical elements. Each wavelength component or channel, or a group of wavelength components or channels, may be selectively attenuated or blocked by actuating a select number of optical elements so that the portion of the channel incident on those selected optical elements are directed away from a return path. 
         [0017]    WSS devices often rely on spatial light modulators such as liquid crystal on silicon (LCoS) devices or micro-electromechanical (MEMS) mirror arrays to perform the routing between the input and output ports. For purposes of illustration only and not as a limitation on the subject matter described herein, a WSS that employs a LCoS device as the spatial light modulator will be described. 
         [0018]    LCoS devices include a liquid crystal material sandwiched between a transparent glass layer having a transparent electrode, and a silicon substrate divided into a two-dimensional array of individually addressable pixels. Each pixel is individually drivable by a voltage signal to provide a local phase change to an optical signal, thereby providing a two-dimensional array of phase manipulating regions. Manipulation of individual spectral components is possible once an optical signal has been spatially separated by a diffractive element such as a diffraction grating. The spatial separation of spectral components is directed onto predetermined regions of the LCoS device, which can be independently manipulated by driving the corresponding pixels in a predetermined manner. 
         [0019]    A programmable optical phase modulator such as a LCoS device produces a phase shift at a given pixel location in a pixel array which is determined programmatically. Such a modulator can be used in multiple ways, forming virtual lenses, prisms or tilted mirrors among other items. Due to the limited thickness and actuation of a LCoS device, the total phase shift that can be achieved at any given location is limited. This limitation can be circumvented in a LCoS device by application of the segmentation technique similar to that used to form a Fresnel lens by compressing the surface power of a lens into a plano surface. Specifically, the total phase shift desired is usually modulo 2π at the wavelength of interest. The resultant phase is then always less than 2π. Unfortunately, this segmentation technique introduces scattering of light in directions that an un-segmented pattern would not produce. This scattered light is one reason that crosstalk is naturally higher in an LCoS WSS. 
         [0020]    Turning to  FIG. 2 , there is illustrated an example of a periodic, stepped phase shift profile  39  that may be produced across a region of a LCoS device  21  in the y-axis. The periodic, stepped phase shift profile  39  produces a cumulative phase profile  37 . The cumulative steering profile  37  is produced by driving each pixel  19  with a predetermined voltage to provide a desired phase change. As there is a direct relationship between voltage and phase, and a direct relationship between phase and steering angle, a look-up table or the like can be generated which relates the required voltage drive signal with a desired steering angle. The periodic nature of phase is utilized to reduce the required drive voltage. Therefore a periodic, stepped voltage signal will produce the periodic, stepped phase shift profile  39 , which in turn produces the cumulative phase profile  37 , where phase resets  41  occur at multiples of 2π radians. When acting on an incident wavelength component, the phase profile  37  produces a steering angle proportional to or equal to θ. Accordingly, by proper adjustment of the periodic, stepped phase shift profile the wavelength components can be selectively directed to a desired one of the optical fibers. 
         [0021]    When directing an optical beam (e.g., an optical wavelength component) from a particular input port to a particular output port, the beam may be attenuated with a desired degree of attenuation by, for example, directing the optical beam so that it does not completely overlap the output port. The desired attenuation can be achieved by appropriate adjustment of the phase pattern applied to the LCoS so that only part of the optical beam is coupled to the output port. 
         [0022]    The desired amount of attenuation imparted to the optical beam may be achieved with a wide variety of different phase patterns. Application of different phase patterns to the portion of the LCoS on which the beam is incident will cause the rejected light (i.e. the portion of the optical beam not directed to the selected output port) to be directed in different directions. 
         [0023]    Referring again to  FIG. 1 , the solid line  160  represents an optical beam being directed from the selected input port  110   n  to the selected output port  120  with a desired amount of optical attenuation. Likewise, the solid lines  162  represent the rejected light that gives rise to the attenuation. The dashed lines shown in  FIG. 1  represent undesirable crosstalk which arises from light being directed by the LCoS from other input ports  110  to the desired output port  120 . The various phase patterns that direct the optical beam with the same desired amount of attenuation may cause different amounts of crosstalk to arise. Accordingly, it generally will be desirable to a select a phase pattern that reduces this crosstalk to maintain port isolation. 
         [0024]    When selecting a suitable phase pattern to be produced by the LCoS or other SLM for directing an optical beam between a selected pair of ports while producing the desired amount of attenuation, the rejected light can be simply lost by directing it away from any of the ports. This is illustrated in  FIG. 1  by rejected light  162 . Alternatively, a phase pattern to be produced by the LCoS may be selected which directs at least some of the rejected portion of the optical beam to the output  130  equipped with the photodetector  135 . The ratio of the rejected portion of the optical beam to the remaining portion of the optical beam that is directed between the selected pair of input and output ports will be the same for a particular LCoS phase pattern. Thus, the amount of light measured by the photodetector  135  is proportional to the amount of light routed to the selected output port. The actual amount of light routed to the selected output port may be determined by proper calibration, which may be accomplished by measuring the optical power of the input beam and the optical power received by both the selected output port and the photodetector for the selected LCoS phase pattern. 
         [0025]    Using the above calibration process, the amount of light directed between any selected pair of ports with a desired amounted of attenuation (corresponding to a particular LCoS phase pattern) can be monitored by the photodetector  135 . Each wavelength being directed from an input port to an output port with a selected degree attenuation can be sequentially monitored in this manner by selecting an LCoS phase pattern that give rises to the selected degree of attenuation while directing the rejected light to the photodetector  135 . 
         [0026]    Individual wavelength components being routed through the WSS may be monitored by the photodetector  135  in a wide variety of different ways.  FIG. 3  illustrates one example of a sequence that may be used in connection with the WSS  100  of  FIG. 1 . In particular,  FIG. 3  shows, as a function of time, the wavelength components X 1 , X 2 , X 3  and X 4  of an WDM optical signal which are routed through the WSS between its input and output ports. 
         [0027]    In  FIG. 3  the horizontal lines terminating in arrows each represent a period of time during which the corresponding wavelength component is routed through the WSS with a selected amount of attenuation. Solid lines represent a period of time during which the rejected light for the corresponding wavelength is simply lost. That is, solid lines represent a period of time during which the corresponding wavelength components are not being monitored because the rejected light is not being directed to the photodetector  135 . On the other hand, dashed lines represent a period of time during which the corresponding wavelength components are being monitored because the rejected light is being directed to the photodetector  135 . 
         [0028]    As  FIG. 3  shows, the rejected light from the different wavelength components is sequentially routed to the photodetector so that various wavelength components can be monitored on a periodic basic. As also shown, in this particular implementation the amount of time during which a given wavelength component is being monitored is much less than the amount of time during which it is not being monitored. In this way the system is better able to monitor all of the wavelength components being routed through the WSS. Of course, the ratio of the time spent monitoring and not monitoring a given wavelength component may vary from application to application. Moreover, in various implementations the amount of time during which a wavelength component is being monitored may differ from wavelength component to wavelength component and may change over time for one or more of the wavelength components. 
         [0029]    For many applications it may be cost prohibitive to dedicate multiple ports which are equipped with a photodetector for simultaneously monitoring wavelength components. Thus, one advantage of the device shown herein is that a single port, equipped with a single photodetector, can be used to monitor multiple wavelength components. 
       Illustrative Wavelength Selective Switch 
       [0030]    One example of a wavelength selective switch in which an optical channel monitor of the type described above may be incorporated will be described with reference to  FIGS. 4-5 . Additional details concerning this optical switch may be found in co-pending U.S. Application Ser. No. [Docket No.  2062 / 17 ] entitled “Wavelength Selective Switch Employing a LCoS Device and Having Reduced Crosstalk,” which is hereby incorporated by reference in its entirety. 
         [0031]      FIGS. 4A and 4B  are top and side views respectively of one example of a simplified optical device such as a free-space WSS  100  that may be used in conjunction with embodiments of the present invention. Light is input and output to the WSS  100  through optical waveguides such as optical fibers which serve as input and output ports. As best seen in  FIG. 1B , a fiber collimator array  101  may comprise a plurality of individual fibers  1201 ,  1202  and  1203  respectively coupled to collimators  1021 ,  1022  and  1023 . Light from one or more of the fibers  120  is converted to a free-space beam by the collimators  102 . The light exiting from port array  101  is parallel to the z-axis. While the port array  101  only shows three optical fiber/collimator pairs in  FIG. 1B , more generally any suitable number of optical fiber/collimator pairs may be employed. 
         [0032]    A pair of telescopes or optical beam expanders magnifies the free space light beams from the port array  101 . A first telescope or beam expander is formed from optical elements  106  and  107  and a second telescope or beam expander is formed from optical elements  104  and  105 . 
         [0033]    In  FIGS. 4A and 4B , optical elements which affect the light in two axes are illustrated with solid lines as bi-convex optics in both views. On the other hand, optical elements which only affect the light in one axis are illustrated with solid lines as plano-convex lenses in the axis that is affected. The optical elements which only affect light in one axis are also illustrated by dashed lines in the axis which they do not affect. For instance, in  FIGS. 4A and 4B  the optical elements  102 ,  108 ,  109  and  110  are depicted with solid lines in both figures. On the other hand, optical elements  106  and  107  are depicted with solid lines in  FIG. 4A  (since they have focusing power along the y-axis) and with dashed lines in  FIG. 4B  (since they leave the beams unaffected along the x-axis). Optical elements  104  and  105  are depicted with solid lines in  FIG. 4B  (since they have focusing power along the x-axis) and with dashed lines in  FIG. 4A  (since they leave the beams unaffected in the y-axis). 
         [0034]    Each telescope may be created with different magnification factors for the x and y directions. For instance, the magnification of the telescope formed from optical elements  104  and  105 , which magnifies the light in the x-direction, may be less than the magnification of the telescope formed from optical elements  106  and  107 , which magnifies the light in the y-direction. 
         [0035]    The pair of telescopes magnifies the light beams from the port array  101  and optically couples them to a wavelength dispersion element  108  (e.g., a diffraction grating or prism), which separates the free space light beams into their constituent wavelengths or channels. The wavelength dispersion element  108  acts to disperse light in different directions on an x-y plane according to its wavelength. The light from the dispersion element is directed to beam focusing optics  109 . 
         [0036]    Beam focusing optics  109  couple the wavelength components from the wavelength dispersion element  108  to a optical path conversion system. In this example the optical path conversion system is a programmable optical phase modulator, which may be, for example, a liquid crystal-based phase modulator such as a LCoS device  110 . The wavelength components are dispersed along the x-axis, which is referred to as the wavelength dispersion direction or axis. Accordingly, each wavelength component of a given wavelength is focused on an array of pixels extending in the y-direction. 
         [0037]    As best seen in  FIG. 4B , after reflection from the LCoS device  110 , each wavelength component can be coupled back through the beam focusing optics  109 , wavelength dispersion element  108  and optical elements  106  and  107  to a selected fiber in the port array  101 . As discussed in more detail in the aforementioned co-pending U.S. Application, appropriate manipulation of the pixels in the y-axis allows selective independent steering of each wavelength component to a selected output fiber. 
         [0038]    In one particular embodiment, the LCoS  110  is tilted about the x-axis so that it is no longer in the x-y plane and thus is no longer orthogonal to the z-axis along which the light propagates from the port array  101 . Stated differently, a skewed angle is formed between the z-axis and a direction in the plane of the modulator perpendicular to the wavelength dispersion axis. Such an embodiment is shown in  FIG. 5 , which is a side-view similar to the side-view shown in  FIG. 4B . In  FIG. 5  and  FIGS. 4A and 4B , like elements are denoted by like reference numerals. By tilting the LOCS  110  in this manner crosstalk arises from scattered light can be reduced. 
         [0039]    While the optical path conversion system employed in the particular wavelength selective switch shown in  FIGS. 4-5  is based on a programmable optical phase modulator (e.g., a LCoS device), more generally other technologies may be employed instead, including, for instance, MEMs-based devices such as DMDs. 
         [0040]    The controller  150  shown in  FIG. 1  may be implemented in hardware, software, firmware or any combination thereof. For example, the controller may employ one or more processors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic, or any combinations thereof. When the controller is implemented partially in software, a device may store computer-executable instructions for the software in a suitable, non-transitory computer-readable storage medium and may execute the instructions in hardware using one or more processors to perform the techniques of this disclosure.