Abstract:
A method of monitoring a WDM optical signal is provided. The method includes: receiving a WDM optical signal having a plurality of channels; detecting the optical signal after filtering the WDM optical signal with a tunable filter; and reconfiguring at least the center wavelength and bandwidth of the tunable filter optical transfer function to determine a signal performance parameter of the WDM optical signal.

Description:
STATEMENT OF RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/090,722, filed Aug. 21, 2008, entitled “Optical Channel Monitor,” the entire disclosure of which is incorporated by reference in its entirety herein. 
    
    
     BACKGROUND OF THE INVENTION 
     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. 
     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. 
     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. 
     Ideally, monitoring of an optical communications system should be based on the analysis of the actual optical signal waveform across the entire range of wavelengths of interest. Spectral analysis of this type can be performed using a variety of known signal and spectrum analysis equipment. For example, optical signal analyzers are known for determining characteristics of an optical signal such as, for example, power level, enter wavelength, extinction ratio, eye opening, signal-to-noise ratio, polarization dependent loss (PDL), dispersion etc. 
     A number of different devices have traditionally been used as optical channel monitors. For instance, in order to monitor respective channels of a WDM optical communications system a Parallel Detection Monitor (PDM) can be used to determine average and peak power levels, as a function of wavelength, across any desired range of wavelengths. The PDM uses a grating or other device to split or demultiplex a WDM signal into its individual channels or wavelengths and a series of photodetectors to measure the power in each channel or wavelength. Alternatively, a Tunable Filter Monitor (TFM) may be implemented using a fixed filter bandwidth, center wavelength tunable optical filter together with an optical detector. Such a filter transmits or reflects only a narrow portion of the transmission band, which can then be detected by the detector. By scanning the filter center wavelength across the entire transmission band, a full spectral picture of the transmission band can be obtained, and the channel information (e.g., wavelength, power, OSNR) can be extracted using suitable signal processing methods. Tunable filters can be implemented using for example thin film technology to create a Fabry-Perot type filter, with the tuning accomplished either by temperature effects, using liquid crystals, or by tuning the angle at which the signal is incident on the filter. Other technologies for implementing tunable filters include, but are not limited to, tunable Bragg gratings and filters based on the acousto-optic effect. 
     While the above-described devices enable some degree of optical channel monitoring, they tend to suffer from a number of disadvantages. In particular, these devices need to be calibrated for a signal having a particular fixed channel plan. The channel plan may specify a number of parameters such as the center frequency and bandwidth of each channel, the spacing between channels, a power level of each channel and the modulation format used by each channel. In the case of an PDM, the channel spacing and the filtering characteristics such as the bandwidth of each channel must both be selected in advance. While a channel monitor using a tunable filter has the ability to scan across the wavelengths, thus in principle allowing a channel plan with irregular channel spacings to be monitored, such a monitor still requires advance knowledge of the channel bandwidth and modulation format. Therefore these devices cannot be designed to be independent of the network&#39;s channel plan. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method of monitoring a WDM optical signal is provided. The method includes: receiving a WDM optical signal having a plurality of channels; detecting the optical signal after filtering the WDM optical signal with a tunable filter; and reconfiguring at least the center wavelength and bandwidth of the tunable filter optical transfer function to determine a signal performance parameter of the WDM optical signal. 
     In accordance with another aspect of the invention, an optical device is provided that includes an input port for receiving a WDM optical signal and a tunable optical filter with programmable wavelength dependent transmission to change at least the center wavelength and bandwidth of the optical transfer function of the optical signal. The optical device also includes a detector for receiving the optical signal after being modulated by the tunable optical filter. A controller is configured to cause the tunable optical filter to be arranged into optical transfer functions between the input port and the detector which samples different components of the spectrum of the WDM optical signal using at least a first and second optical transfer function between the input port and the detector. The optical transfer functions passband characteristics are optimized to improve the measurement accuracy of different channels or components of the WDM optical signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of one example of an optical device that may be used as an optical channel monitor. 
         FIG. 2  is an enlarged partial view of an SLM that may be employed in the optical device of  FIG. 1 . 
         FIG. 3  shows a plan view of an SLM that may be employed in the optical device of  FIG. 1 . 
         FIG. 4  shows a side view of a wavelength selective switch. 
         FIG. 5  shows a plan view of the surface of the SLM on which the wavelengths are incident, taken along line  2 - 2  in  FIG. 4 . 
         FIG. 6  is another side view of the wavelength selective switch taken along line  4 - 4  in  FIG. 4 . 
         FIG. 7   a  illustrates a first light path being reflected off a first single mirror element and a second light path being reflected off a second single mirror element in a DMD, with the two mirror elements being tilted in opposite directions. 
         FIG. 7   b  illustrates a first light path being reflected off a first single mirror element and a second light path being reflected off a second single mirror element in a DMD, with the two mirror elements being tilted in the same direction. 
         FIG. 8  shows seven fibers or ports of a wavelength selective switch (taken along lines  4 - 4  in  FIG. 4 ), any three of which can serve as a 1×2 switch in which the center port serves as the input ports and the outer ports serve as the output ports. 
         FIG. 9  shows the switch of  FIG. 6  in which three ports serve as a 1×2 switch and two sets of other ports  110   1  and  110   2  serve as optical channel monitors. 
     
    
    
     DETAILED DESCRIPTION 
     For a variety of reasons WDM optical communication systems are increasingly being used to carry traffic that have different or varying channel plans. For instance, traffic having mixed transmission rates of 2.5, 10, 40 and even 100 Gb/s may be carried on the same system. In some cases these channel plans may change over time. For instance, if a channel is used to support a service requiring a 100 GHz signal, but at a later time only requires a 10 GHz signal, it would be most efficient to reduce the amount of bandwidth allocated to that channel when it is only being used to carry the 10 GHz signal. In other cases the channel plan employed by the traffic may be unknown to the service operator of the network carrying the traffic. This may be the case, for instance, when the network is carrying traffic (i.e., alien wavelengths) from a third party service operator. As another example, as system needs change over time, it may be desirable to reconfigure the system through the use of a programmable channel plan that can be re-programmed or changed on an as-needed basis. 
     Because of the increased flexibility in the types of traffic that optical communication systems are being required to carry, an optical channel monitor should ideally be designed so that it is independent of the network&#39;s channel plan. That is, an ideal channel monitor should be able to accurately monitor traffic having any arbitrary or programmable channel plan, and preferably without having any a priori knowledge of the channel plan or any other characteristics of the optical signal. In order to accomplish this, the optical channel monitor needs to detect the signal, determine its characteristics (e.g., channel bandwidth), and, based on the characterization, apply an appropriate filter to quickly and accurately acquire information concerning the signal such as its power level. 
     More particularly, a channel plan independent monitor can be formed from a suitable tunable filter in combination with a photodetector. The tunable filter can be used first as a narrowband filter to characterize the signal by measuring the bandwidth and location of the individual channels as the passband of the filter is scanned across the signal spectrum. In this way the signal profile or channel plan can be determined. Given the channel plan, the tunable filter can be reconfigured so that it has an appropriate shape to determine in a single measurement signal performance parameters such as the aggregate signal power level, for instance. That is, the tunable filter is adjusted or tuned so that no components of the signal are removed which would impact the accuracy of the measurement. 
     One example of a tunable filter that may be employed in the present invention incorporates a spatial light modulator (SLM). 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 channel, or a group of 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. 
       FIG. 1  is a block diagram of one example of an optical device  100  that may be used as an optical channel monitor. The optical device  100  employs a SLM which acts as a tunable filter for filtering an incoming optical beam or signal. Optical device  100  includes an input optical fiber  102  capable of introducing a multiple wavelength signal  112  to device  100  and an output optical fiber  104  capable of removing at least a portion of signal  112  from device  100 . A photodetector  120  receives the portion of the optical signal from the output fiber  104 . 
     Optical device  100  also includes a first dispersive grating arrangement  118   a  and a second dispersive grating arrangement. In this example, a first dispersive grating arrangement  118   a  operates to separate the multiple wavelength signal  112  into discrete wavelengths or channels (λ 1 -λ n ) and to focus the wavelengths onto the SLM  108 . Similarly, a second dispersive grating arrangement  118   b  operates to combine the reflected wavelengths from SLM  108  into an output multiple wavelength optical signal  114  and to focus signal  114  into output optical fiber  104 . First and second dispersive grating arrangements may include a combination of gratings, lens (e.g., collimating lens) as well as other optical elements. 
     The SLM  108  is operable to spatially modulate the optical wavelengths. The SLM  108  selectively communicates by reflection one or more of the optical wavelengths to the optical output fiber  104  while modulating the incoming wavelengths such that the SLM  108  acts as a tunable filter. 
     One type of micro-mirror based SLM is a digital micromirror device (DMD). A DMD is comprised of a two-dimensional array of micromirrors or pixels, each actuatable in an ON state or an OFF state. As used herein in the context of a DMD, the terms “micromirror” and “pixel” are used inter-changeably. 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 is accurate and reproducible, and can generally be achieved in about 25 microseconds or less. 
     In the example of  FIG. 1  the SLM  108  is a DMD in which the pixels are formed from a series of micromirrors (not visible in  FIG. 1 ). Typically, each individual wavelength received by SLM  108  is spread over several micromirrors to enable micromirrors  110  to control the amount of light that couples into output fiber  104 . This is most clearly seen in  FIG. 2 , which is an enlarged partial view of SLM  108 . For clarity  FIG. 2  only shows the micromirrors  110  upon which wavelength λ 1  is incident. As with any optical beam, the intensity of wavelength λ 1  is spatially distributed, as indicated by intensity distribution curve  150 . The intensity distribution curve is often a Gaussian distribution, as shown, but is not limited thereto. In operation, on-state micromirrors  110   a  operate to selectively reflect at least some of the energy of wavelength λ 1  to output fiber  104 . In addition, OFF-state micromirrors  110   b  operate to selectively reflect some of the energy in wavelength λ 1  away from output fiber  104 . In this example the OFF-state micromirrors  110   b  reflect the energy to drop port  106 . In this way the SLM  108  can vary the optical power of wavelength λ 1  coupled into output fiber  104  by selectively varying the operating state of some of the micromirrors  110 . The wavelength λ 1  may even be completely attenuated or blocked by placing all the micromirrors  110  on which λ 1  is incident in their OFF-state. The optical power of wavelengths λ 2 -λ n  that is directed to output port  104  by the SLM  108  can be attenuated or otherwise varied in a similar manner. 
     The structure of the SLM-based channel monitor shown in  FIG. 1  is just one example of an optical system in which the embodiments discussed in this application can be applied. For example, in some applications, it may be desirable to direct the reflected beam back along the same path and use a circulator or other means to separate the incoming and outgoing beam. In other embodiments the SLM  108  may selectively communicate one or more wavelengths to the output fiber  104  by transmission instead of reflection. 
       FIG. 3  shows a plan view of the SLM  208  (i.e., SLM  108  in  FIG. 1 ) employed in the optical device of  FIG. 1  as well as spatially separated optical wavelengths λ 1 , λ 2  and λ 3  (represented by beams  310 ,  320  and  330 , respectively) that are focused onto the SLM  108  by the first dispersive grating arrangement  118   a .  FIG. 3  also shows the individual pixels  220  (only one of which is labeled) of the SLM  108  onto which the optical wavelengths λ 1 , λ 2  and λ 3  are incident. Although 3 wavelengths are shown for purposes of illustration, more generally any number of wavelengths may be spatially separated across the SLM  208 . In this example the spectral dispersion axis of the SLM  208  is assumed to be in the horizontal direction and the orthogonal axis along which individual wavelengths are contained is assumed to be in the vertical direction. This orthogonal axis will be referred to herein as the beam axis. However, other orientations are also possible. In the case of a DMD, the pivoting axes of the micromirrors will generally extend parallel to the spectral dispersion axis. As  FIG. 3  shows, the beam diameter for each wavelength in this example extends over about ten pixels. 
     To perform channel monitoring, the SLM  208  in optical device  100  can be used as a tunable filter with a programmable filter function that can adjust the filter passband and center wavelength independently. When terminated with a photodiode, this device can be used to quickly and accurately measure the power of channels with different bandwidths, different modulation formats, and irregular channel spacings. Thus the addition flexibility of this filter to fully program the optical transfer function enables the device to improve speed and accuracy by adapting to future WDM system formats that mix modulation formats and channel spacings. This is valuable because it avoids the need to enforcing rigid channel guidelines or assumptions for optical monitoring purposes. 
     A preferable embodiment of the invention is to use the SLM device as a narrowband filter to first characterize the signal being monitored by actuating into their ON-state a limited number (e.g., one) of columns of the pixels  220 . That is, the passband of the SLM  108  when it is used as a filter can be as narrow as a single pixel. The passband of the SLM  108  can be scanned across the signal spectrum by sequentially actuating into their ON-state successive columns of pixels so that the signal profile or channel plan of the signal can be determined. Once the signal has been characterized in this manner, the SLM  108  can be reconfigured by suitable actuation of the pixels to provide a filter shape that allows the aggregate signal power level to be accurately determined in a single measurement. 
     The optical device  100  described above can be used as a channel monitor to monitor a signal having any arbitrary channel plan. Significantly, no advance knowledge of the channel plan is needed to perform the monitoring function. The channel plan independence of the optical device  100  arises from the ability to spread a single optical channel over multiple pixels. Simply by the proper adjustment of the SLM  108 , which can be readily achieved, for example, through software operating on the SLM through a programmable controller, a signal with any arbitrary channel plan can be monitored. 
     As described above, the channel plan independence of the channel monitor can be achieved with a tunable filter that has a single adjustable passband. The SLM  108 , however, can be configured to have more complex filter shapes, including multiple passbands that each have a different shape. This capability can be used to provide a channel monitor with many additional features. For instance, the SLM  108  can be configured to have a filter shape that suppresses all the channels and allows the aggregate amplified spontaneous emission (ASE) to be detected in a single measurement. Conversely, the SLM  108  can be adjusted so that it removes all the ASE and only allows the channels to be measured by the photodetector. 
     In some embodiments the functionality of the optical channel monitor may be directly integrated with a device that performs additional functions. One example of such a device is an optical switch such as a wavelength selective switch. Such integration offers a number of advantages, including a reduction in space requirements and even possibly eliminating the need for an extra slot in a network rack that would otherwise be occupied by a dedicated channel monitor. Moreover, integrating the OCM within a switch or other device allows the device itself to directly make use of the information provided by the monitor without mediation of system management software. For example, the optical channel monitor can be used to check whether the channels comprising the output signal from the switch have equal power, and if not provide feed-back in order to achieve the required equalization. 
     Another important advantage that arises from the use of an optical switch or other device that is integrated with an optical channel monitor concerns the usual tradeoffs between isolation or sharpness of a filter and the flatness of its passband. In particular, high isolation is needed to distinguish between adjacent channels or between the channels and noise. In general, the tunable filters used in the channel monitor should be as sharp as the filters that are used by other devices in the network to process the optical signal. If the channel monitor and the optical switch or other optical device shares many of the same optical elements, in particular those used to perform filtering, this criterion can be automatically met. 
     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. 4-9 . Additional details concerning this optical switch may be found in co-pending U.S. application Ser. No. 11/781,931 entitled “Multiple Function Digital Optical Switch.” 
       FIG. 4  shows a first side view of the wavelength selective switch  100 . 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. 4 . 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. 4 , 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 SLM  108 . 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. 4 ) and onto the surface of the SLM  108 . 
       FIG. 5  shows a plan view of the surface of the SLM  108  on which the wavelengths are incident, taken along line  2 - 2  in  FIG. 4 . The SLM  108  in this example is a DMD of the type described above, which includes a two-dimensional array of micromirrors or pixels  145 . As indicated in  FIG. 5  by dashed line  150 , the axis of the micromirrors 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  108  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 SLM device  108  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. 6  is a second side view of the wavelength selective switch taken along line  4 - 4  in  FIG. 4 . 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  108  are sufficiently small relative to the optical wavelengths that are employed in the switch  100  so that the light is diffracted off the DMD  108 . Accordingly, depending on the tilt angle of the mirror elements upon which wavelength λ 1  is incident, the DMD  108  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. 1 , 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  108  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  108  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. 7   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  108 . 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. 6 , after traveling along path  180  or  182  after its second reflection off the DMD  108 , 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  108  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  108  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  1402   2 . This can be illustrated in connection with  FIG. 7   b , which shows incoming light path  180  and the outgoing light path  182 . As previously noted in connection with  FIG. 7   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. 7   a  the mirror elements  143  and  147  are tilted in opposite directions. In  FIG. 7   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  108  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. 7   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  108  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. 8 , 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. 8  shows the switch  100  when configured in this manner. Similar to  FIG. 6 ,  FIG. 8 , as well as  FIG. 9  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  employs mirror strips  140   5  and  140   6 . 
     Optical channel monitoring may be performed using any two adjacent ports in the wavelength selective switch  100 , one of which serves as the input and the other of which serves as the output port at which an optical detector can be placed.  FIG. 9  shows the switch  100  of  FIG. 6  in which ports  110   3 ,  110   4  and  110   5  serve as a 1×2 switch and ports  110   1  and  110   2  serve as one optical channel monitor and ports  110   6  and  110   7  serve as another optical channel monitor. Ports  110   2  and  110   6  serve as the inputs and ports  110   1  and  110   7  serve as the 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  may serve as a 1×1 switch. 
     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, as previously mentioned, the functionality of the optical channel monitor may be directly integrated with other devices besides the optical switch described above. Such devices include, without limitation, optical amplifiers, chromatic dispersion controllers, optical add/drop multiplexers, variable optical attenuators, and the like.