Abstract:
An optical device includes an optical port array having first and second optical inputs for receiving optical beams and a first plurality of optical outputs associated with switching functionality and a second plurality of optical outputs associated with channel monitoring functionality. A dispersion element receives the optical beam from an input and spatially separates the beam into a plurality of wavelength components. The focusing element focuses the wavelength components. The optical path conversion system receives the plurality of wavelength components and selectively directs each one to a prescribed one of the optical ports. The photodetectors are each associated with one of the optical outputs in the second plurality of optical outputs and receive a wavelength component therefrom. The controller causes the optical path conversion system to simultaneously direct each of the wavelength components to a different one of the optical outputs of the second plurality of optical outputs.

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 1×N 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. Pat. Appl. Publ. No. 2010/0046944 shows an optical channel monitor that is incorporated in a WSS. This is accomplished by using the functionality of a 1×1 switch that is available in a 1×N WSS. In particular, the output of the 1×1 switch terminates with a photodiode. In this way, the power of any individual channel can be measured. 
         [0006]    While the use of a 1×1 WSS to form an OCM is useful when the optical switching technology is sufficiently fast, this technique is not suitable when used with switches that do not have relatively fast response times. In particular, the optical switching time, the photodiode settling time and the number of channels being monitored determine the OCM loop speed, i.e., the time needed to monitor each channel one time. For many applications OCM loop speeds of less than 1 second, and ideally less than 0.1 second, are desired. Accordingly, the switch and photodiode settling times need to be sufficiently fast to interrogate many channels, which may approach or even exceed 100 in number. To accomplish a 0.2 second loop speed with a photodiode settling time of 1 ms and 100 channels, the optical switching time must also be 1 ms. While this is feasible with some technologies such as digital micro-mirror devices (DMDs) it is not practical with other technologies such as liquid crystal and Liquid Crystal on Silicon (LCoS) technologies. 
       SUMMARY 
       [0007]    In accordance with one aspect of the invention, an optical device is provided. The optical device includes an optical port array, a dispersion element, a focusing element, an optical path conversion system, a plurality of photodetectors and a controller. The optical port array has at least first and second optical inputs for receiving optical beams and at least a first plurality of optical outputs associated with switching functionality and a second plurality of optical outputs associated with channel monitoring functionality. The dispersion element receives the optical beam from an optical input and spatially separates the optical beam into a plurality of wavelength components. The focusing element focuses the plurality of wavelength components. The optical path conversion system receives the plurality of wavelength components and selectively directs each of the wavelength components to a prescribed one of the optical ports. The plurality of photodetectors are each associated with one of the optical outputs in the second plurality of optical outputs and receive a wavelength component therefrom. The controller causes the optical path conversion system to simultaneously direct each of a plurality of wavelength components to a different one of the optical outputs of the second plurality of optical outputs. 
     
    
     
       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  illustrates one example of a sequence that may be used in connection with a device having a series of N (where N is equal to or greater than 2) WSSs each having 5 output ports and an OCM having N photodiodes receiving light from N output ports. 
           [0010]      FIGS. 3A and 3B  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. 
           [0011]      FIG. 4  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 
       [0012]      FIG. 1  shows a functional block diagram of one example of a wavelength selective switch (WSS)  100  that includes an integrated channel monitor. As shown, three distinct functions are depicted: two 1×n WSSs, represented by WSSs  110  and  120 , and an optical channel monitor  130  (OCM). It should be noted, however, that as will be described below, the different functions may be incorporated into a single physical switching device. 
         [0013]    WSS  110  includes an input port  112  and output ports  114   1 ,  114   2 ,  114   3 ,  114   4  and  114   5  (“ 114 ”). A switching fabric  116  optically couples the input port  112  to the output ports  114  so that an optical signal received at the input port  112  can be selectively directed to one of the output ports  114  under the control of a switch controller  140 . Similarly, WSS  120  includes an input port  122  and output ports  124   1 ,  124   2 ,  124   3 ,  124   4  and  124   5  (“ 124 ”). A switching fabric  126  optically couples the input port  122  to the output ports  124  so that an optical signal received at the input port  122  can be selectively directed to one of the output ports  124  under the control of the switch controller  140 . 
         [0014]    OCM  130  is similar to WSSs  120  and  130  except that each of its output ports terminates in a photodetector such as a photodiode. In particular, OCM  130  includes an input port  132  and output ports  134   1 ,  134   2 ,  134   3 ,  134   4  and  134   5  (“ 134 ”). A switching fabric  136  optically couples the input port  132  to the output ports  134  so that an optical signal received at the input port  132  can be selectively directed to one of the output ports  134  under the control of the switch controller  140 . Photodiodes  150   1 ,  150   2 ,  150   3 ,  150   4  and  150   5  receive light from optical outputs  134   1 ,  134   2 ,  134   3 ,  134   4  and  134   5 , respectively. 
         [0015]    It should be noted that while the WSSs  110  and  120  and the OCM  130  are depicted as having five output ports, more generally any number of output ports may be employed, and this number may be the same or different among the three functional elements. That is, WSS  110 , WSS  120  and OCM  130  may have the same or a different number of output ports. 
         [0016]    Because the OCM has multiple output ports that are each equipped with a photodiode, multiple channels can be monitored simultaneously, thereby increasing the OCM loop speed. For instance, with only 1 photodiode, a 100-channel measurement would take 100 sequential samples with switch and settle times between each sample. If, for instance, a 1×20 WSS with 20 photodiodes were used, then each photodiode could be sampled nearly simultaneously, with 20 channels being detected in parallel. This would reduce the loop time by a factor of 20 from in comparison to time needed with a conventional arrangement. In this way a target loop time of 0.2 seconds with a settling time of 1 ms could support a switching time of 39 ms. Such a switching time is practical for use with liquid crystal-based switching technologies. 
         [0017]    Individual channels may be simultaneously routed to the OCM  130  for monitoring in a wide variety of different ways.  FIG. 2  illustrates one example of a sequence that may be used in connection with a device having a series of N (where N is equal to or greater than 2) WSSs each having 5 output ports and an OCM having N photodiodes receiving light from N output ports. As shown, channels wavelengths  1 ,  2 ,  3 ,  4  and  5  are routed in sequence to the five outputs of the first WSS. Wavelengths  6 ,  7 ,  8 ,  9  and  10  are routed in sequence to the five outputs of the second WSS. This process continues for each WSS, with the final wavelengths N, N+1, N+2, N+3, N+4 and N+5 being routed in sequence to the five outputs of the N th  WSS. 
         [0018]    Since the OCM has N outputs, one channel from each of the N WSSs can be monitored simultaneously. For instance, with such an arrangement channels or wavelengths  1 ,  6 ,  11 ,  16  . . . N can be simultaneously monitored. Then, after these channels have been monitored, channels  2 ,  7 ,  12 ,  17  . . . N+1 can be simultaneously monitored, followed by channels  3 ,  8 ,  13 ,  18  . . . N+2, and so on. Finally, the monitoring sequence may be completed by simultaneously monitoring channels  5 ,  10 ,  15 ,  20  . . . N+4, after which the entire sequence may be repeated. 
         [0019]    For many applications it may be cost prohibitive to build a multi-port WSS that is solely dedicated for use as an OCM with multiple photodiodes. However, the cost diminishes substantially if the functionality of an OCM could be incorporated as an adjunct to a device that includes the functionality of one or more WSS if most of the optical elements used in the WSS(s) are also used to implement the functionality of the OCM. In this case, the incremental cost of the additional WSS can be small, making an OCM having multiple photodiodes a viable alternative. 
         [0020]    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. 3-4 . 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.” 
         [0021]      FIGS. 3A and 3B  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. 3B , a fiber collimator array  101  may comprise a plurality of individual fibers  120   1 ,  120   2  and  120   3  respectively coupled to collimators  102   1 ,  102   2  and  102   3 . 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. 
         [0022]    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 . 
         [0023]    In  FIGS. 3A and 3B , 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. 3A and 3B  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. 3B  (since they have focusing power along the y-axis) and with dashed lines in  FIG. 3B  (since they leave the beams unaffected along the x-axis). Optical elements  104  and  105  are depicted with solid lines in  FIG. 3B  (since they have focusing power along the x-axis) and with dashed lines in  FIG. 3B  (since they leave the beams unaffected in the y-axis). 
         [0024]    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. 
         [0025]    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 . 
         [0026]    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. By way of example, and not by way of limitation, three such wavelength components having center wavelengths denoted λ 1 , λ 2  and λ 3  are shown in  FIG. 3A  being focused on the LCoS device  110  along the wavelength dispersion axis (x-axis). 
         [0027]    As best seen in  FIG. 3B , 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. 
         [0028]    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. 4 , which is a side-view similar to the side-view shown in  FIG. 3B . In  FIG. 4  and  FIGS. 3A and 3B , like elements are denoted by like reference numerals. By tilting the LOCS  110  in this manner crosstalk arises from scattered light can be reduced. 
         [0029]    While the optical path conversion system employed in the particular wavelength selective switch shown in  FIGS. 3-4  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.