Abstract:
A method and apparatus are disclosed for filtering an input wavelength-division multiplexed (WDM) signal comprised of N wavelength channels. The disclosed wavelength blocker includes a demultiplexer for producing a plurality of demultiplexed output signals from the input WDM signal and a multiplexer for producing an output WDM signal. A shutter array selectively passes each of the N wavelength channels using a plurality of shutters. The demultiplexer is coupled to the multiplexer using a plurality of waveguides having approximately equal length, in order to reduce multipath interference. Each of the N wavelength channels are selectively passed or blocked using a thermo-optic or electro-optic control signal to control the state of the corresponding shutter. Crosstalk can be reduced using dilation techniques that position two shutters in series, especially where the shutters are thermo-optic Mach-Zehnder switches. Wavelength-selective cross connects and wavelength add-drop multiplexers are also disclosed that employ the novel wavelength blockers.

Description:
FIELD OF THE INVENTION 
     The present invention relates to optical communication networks and, more particularly, to optical devices for routing multi-wavelength optical signals. 
     BACKGROUND OF THE INVENTION 
     When multiple users share a transmission medium, some form of multiplexing is required to provide separable user sub-channels. There are many multiplexing techniques available that simultaneously transmit information signals within the available bandwidth, while still maintaining the quality and intelligibility that are required for a given application. Optical communication systems, for example, increasingly employ wavelength division multiplexing (WDM) techniques to transmit multiple information signals on the same fiber, and differentiate each user sub-channel by modulating it with a unique wavelength of invisible light. WDM techniques are being used to meet the increasing demands for increasing speed and bandwidth in optical transmission applications. 
     In optical communication networks, such as those employing WDM techniques, individual optical signals are often selectively routed to different destinations. Thus, a high capacity matrix or cross-connect switch is often employed to selectively route signals through interconnected nodes in a communication network. Many cross-connect switches used in optical communication networks are either manual or electronic, requiring multiple optical-to-electrical and electrical-to-optical conversions. The speed and bandwidth advantages associated with transmitting information in optical form, however, makes an all-optical network the preferred solution for WDM-based optical networks. Moreover, all-optical network elements are needed to provide the flexibility for managing bandwidth at the optical layer (e.g., on a wavelength by wavelength basis). In addition, it is often desirable to remove light of a given wavelength from a fiber or add light of a given wavelength to the fiber. A device that provides this feature is often referred to as a wavelength add-drop (WAD) multiplexer. 
     Wavelength blockers are optical devices that accept an incoming signal of multiple wavelength channels and independently pass or block each wavelength channel. Wavelength blockers can be used as components in a larger optical communication system, for example, to route a given optical signal along a desired path between a source and destination. Optical cross-connect switches and wavelength add-drop multiplexers, for example, could be implemented using wavelength blockers. A wavelength blocker provides a number of desirable features. First, a network element using wavelength blockers is modular and thus scalable and repairable. Second, network elements using wavelength blockers have a multicasting capability. Third, wavelength blockers are relatively easy to manufacture with high performance. For example, wavelength blockers have only two fiber connections, and it is possible to use a polarization diversity scheme to make them polarization independent. 
     As the demand for optical bandwidth increases in WDM communication systems, it is desirable to increase the number of channels. Unfortunately, an increase in the number of channels provides a corresponding increase in the size, cost and insertion loss of the optical devices in such WDM communication systems. A need therefore exists for improved wavelength blockers that permit optical cross-connect switches, wavelength add-drop multiplexers and other optical devices to be fabricated with reduced size and cost. A further need exists for two-port wavelength blockers that permit optical cross-connect switches and wavelength add-drop multiplexers to be configured without complex waveguide crossings. Yet another need exists for improved wavelength blockers having a frequency spectrum with a generally flat transmission spectrum in both amplitude and phase. 
     SUMMARY OF THE INVENTION 
     Generally, a method and apparatus are disclosed for filtering an input wavelength-division multiplexed (WDM) signal comprised of N wavelength channels. The disclosed wavelength blocker includes a demultiplexer for producing a plurality of demultiplexed output signals from the input WDM signal and a multiplexer for producing an output WDM signal. In addition, a shutter array selectively passes each of the N wavelength channels using a plurality of shutters. According to one aspect of the invention, the demultiplexer is coupled to the multiplexer using a plurality of waveguides having approximately equal length, in order to reduce multipath interference. 
     The shutters may be embodied, for example, as Mach-Zehnder switches, electro-absorption modulators or Y-branch switches. Each of the N wavelength channels in the incoming signal are selectively passed or blocked using a thermo-optic or electro-optic control signal to control the state of the corresponding shutter. According to another aspect of the invention, crosstalk among the various N channels can be reduced using dilation techniques that position two shutters in series, especially where the shutters are thermo-optic Mach-Zehnder switches. The disclosed wavelength blockers may be utilized in wavelength-selective cross connects and wavelength add-drop multiplexers, as well as other optical devices. 
     A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a conventional wavelength blocker; 
     FIG. 2 illustrates a conventional wavelength blocker in accordance with the present invention; 
     FIG. 3 illustrates a wavelength-selective cross connect (WSC) in accordance with the present invention; and 
     FIG. 4 illustrates a wavelength add-drop (WAD) multiplexer in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a conventional wavelength blocker  100 . As shown in FIG. 1, a wavelength blocker  100  is an optical device having two ports  110 - 1 ,  110 - 2  that accept an incoming signal of multiple wavelength channels at a first port  110 - 1  and independently pass or block each wavelength channel, i, to a second port  110 - 2 . A demultiplexer  115 - 1  separates the incoming signal into each component wavelength channel, i, which is then selectively passed or blocked by the corresponding shutter  120 -i (or variable optical attenuators) to a multiplexer  115 - 2 . 
     FIG. 2 illustrates a wavelength blocker  200  in accordance with the present invention. As shown in FIG. 2, the wavelength blocker  200  is comprised of a demultiplexer  201 , a waveguide lens  202  and a multiplexer  203 . The waveguide lens  202  is comprised of a number of equal-length waveguides, WG 1  through WG N , each associated with a corresponding shutter  210 - 1  through  210 -N (hereinafter, collectively referred to as shutters  210 ). In order to reduce multipath interference, the waveguides, WG 1  through WG N , have approximately the same length, for example, using a constant bend radius and have equal straight and bend lengths independently, resulting in adjacent lens arms having nearly exactly the same phase. Thus, no post-trimming should be required. Typically, adjacent lens arms should have an equal length to within a small integer multiple of the corresponding wavelength, λ i . It is noted that since crosstalk is strongest among adjacent waveguides, it is most important that neighboring waveguides have approximately the same length, but that waveguides far separated from each other by other waveguides could have substantially different path lengths. 
     The shutters  210  may be embodied as one or more Mach-Zehnder switches or Mach-Zehnder interferometer shutters, such as those described in M. Okuno et al., “Silica-Based Thermo-Optic Switches,” NTT Review, Vol. 7, No. 5 (September 1995), each incorporated by reference herein. In addition, the shutters  210 -N may be embodied as, e.g., electro-absorption modulators or Y-branch switches. The demultiplexer  201  and multiplexer  203  can be embodied as planar waveguide gratings. It is noted that the waveguide gratings for the demultiplexer  201  and multiplexer  203  need not be the same. 
     In order to selectively pass or block the incoming signal, the shutters  210  are controlled by a thermo-optic or electro-optic control signal (not shown), as appropriate for the selected shutter  210 . If the shutters  210  are thermo-optic Mach-Zehnder switches, or if crosstalk is otherwise a problem, each lens arm, WG 1  through WG N ,should contain two switches in series, i.e., use switch dilation, resulting in reduced crosstalk, but a doubling of the electrical power consumption. 
     The exemplary wavelength blocker  200  handles  40  channels with 100-GHz channel spacing. According to another feature of the wavelength blocker  200 , each wavelength channel from the demultiplixer  201  is optionally carried by two or more equal-length waveguides. Thus, the wavelength blocker  200  includes two or more lens arms (equal-length waveguides) per channel. The two dilated Mach-Zehnder switches  210  associated with the two equal-length waveguides carrying the same demultiplixer output signal work in concert to pass or block the demultiplixer output signal. For a more detailed discussion of this multiple equal-length waveguides per signal arrangement, see U.S. patent application Ser. No. 09/798,501, filed Mar. 2, 2001, entitled “A Wavelength Filter That Operates On Sets of Wavelength Channels,” incorporated by reference herein. Among other benefits, this multiple equal-length waveguides per signal arrangement provides individual passbands having a flat frequency spectrum for each channel and the entire response is completely flat when no channels are dropped or added. 
     The wavelength blocker  200  can be quite compact. It can be shown that the exemplary wavelength blocker  200  has a resulting length of about 9.5 cm in typical silica waveguides and allows for five such devices per five-inch-diameter wafer. 
     The present invention recognizes that a wavelength blocker  200  does not need to give access to the dropped channels. Thus, the wavelength blocker  200  in accordance with the present invention employs a transmissive design with evenly distributed lens arms, as shown in FIG.  2 . It is noted that prior techniques configured the waveguide lens in a reflective fashion in order to access the drop channels. See, C. R. Doerr et al., “40-Wavelength Add-Drop Filter,” IEEE Photon. Technol. Lett., Vol. 11, 1437-1439 (1999). When configured in a reflective fashion, the polishing angle and flatness of the reflective facet is generally inaccurate enough to cause large phase differences between adjacent lens arms. In addition, the lens waveguides of a reflective wavelength blocker are arranged in pairs, in order to give room for the waveguides containing the drop channels to reach the facet between the mirror stripes that reflect back the lens waveguides for the express channels, making the environments for adjacent lens waveguides different, resulting in different birefringences for each lens arm. Also, most likely because the waveguide core sidewalls are typically somewhat slanted, there is polarization conversion in the bends and adjacent lens arms curve in different directions at certain points. Thus, the polarization dependence of the reflective grating-lens-grating is generally more than 1 dB, making it unusable for most long-haul systems. 
     The polarization dependence of the wavelength blocker  200  is small. If the polarization dependence is not low enough, however, one can employ a polarization diversity scheme using a polarization splitter and circulator, such as the polarization diversity scheme described in C. R. Doerr et al., “An Automatic 40-Wavelength Channelized Equalizer,” IEEE Photon. Technol. Lett., Vol. 12, 1195-1197 (2000), incorporated by reference herein, since the wavelength blocker is a two-port reciprocal device. 
     FIG. 3 illustrates a wavelength-selective cross connect (WSC)  300  in accordance with the present invention. The wavelength-selective cross connect  300  may be used, for example, in a communication system having multiple fiber rings. As shown in FIG. 3, the wavelength-selective cross connect  300  is an optical device having two input ports  310 - 1  and  310 - 2  and two output ports  310 - 3  and  310 - 4 . An incoming signal received on a given incoming port  310 - 1  and  310 - 2  is selectively (i) passed to the corresponding output port  310 - 3  or  310 - 4 , respectively, in a bar state; or (ii) crossed to the opposite output port  310 - 4  or  310 - 3 , respectively, in a cross state. The wavelength-selective cross connect  300  consists of four wavelength blockers  200 - 1  through  200 - 4 , which may each be embodied as the wavelength blocker  200  discussed above in conjunction with FIG.  2 . 
     In addition, the wavelength-selective cross connect  300  of FIG. 3 includes two power splitters  320 - 1  and  320 - 2  and two power combiners  320 - 3  and  320 - 4 . The power splitters  320 - 1  and  320 - 2  divide the power of an incoming signal in half and the half-power signals are applied to two corresponding wavelength blockers  200 , as shown in FIG.  3 . Likewise, each power combiner  320 - 3  and  320 - 4  combines the power at the output of two alternating wavelength blockers  330 , as shown in FIG.  3 . In this manner, the wavelength-selective cross connect  300  can selectively pass or cross an incoming signal to an appropriate output port, as desired. 
     FIG. 4 illustrates a wavelength add-drop (WAD) multiplexer  400  in accordance with the present invention. The wavelength add-drop multiplexer  400  is an optical device having two ports  410 - 1  and  410 - 2 . An incoming signal of multiple wavelength channels is accepted at a first port  410 - 1  and applied to a power splitter  420 . The half-power signal is then applied in parallel to a wavelength blocker  200  and a demultiplexer  430 . Individual wavelength channels are then either passed by the wavelength blocker  200  or selectively dropped by the demultiplexer  430  to a local destination. In addition, individual wavelength channels are selectively added by a multiplexer  440  in cooperation with the wavelength blocker  200 . The outputs of the wavelength blocker  200  and the multiplexer  440  are combined by a power combiner  450  before being applied to the second port  410 - 2 . 
     When used as a wavelength add-drop multiplexer  400 , the wavelength blocker  200  must impair the express channels as little as possible (express channels pass through the WAD  400 , including the wavelength blocker  200 , without being blocked). In other words, the transmission spectrum of the wavelength blocker  200  must be as flat as possible in both amplitude and phase. As previously indicated, this can be accomplished in accordance with one aspect of the present invention by having all of the path lengths connecting the multplexer and demultiplexer pair  201 ,  203  be the same length, to within a few wavelengths, and ensuring that the shutter  202  connections to the grating  201 ,  203  do not undersample the optical spectrum. However, also as mentioned previously, it is most important that adjacent waveguides have the same path length, and waveguides far separated from each other by several waveguides could have substantially different path lengths. 
     It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.