Abstract:
An optical multiplexer is provided that includes a reconfigurable switching fabric with a number of inputs and outputs. In addition, the reconfigurable switching fabric employs VLSOAs that facilitate at least partial reconfiguration of the switching fabric by serving as switches to add, drop or pass-through one or more optical signals received at the inputs of the reconfigurable switching fabric. In some cases, one or more of the VLSOAs also serve to amplify one or more optical signals so as to compensate for losses that may be experienced by the signal in transit through the switching fabric, or elsewhere. Finally, this example of the optical multiplexer includes a wavelength division multiplexer coupled to the reconfigurable switching fabric and configured to multiplex at least some of the optical signals received from outputs of the reconfigurable switching fabric.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation, and claims the benefit, of U.S. patent application Ser. No. 10/095,539, entitled RECONFIGURABLE OPTICAL ADD-DROP MULTIPLEXER, filed Mar. 11, 2002, which claims priority to U.S. Provisional Patent Application Ser. No. 60/274,420, entitled RECONFIGURABLE OPTICAL ADD-DROP MULTIPLEXER, filed Mar. 9, 2001. All of the aforementioned applications are incorporate herein in their respective entireties by this reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates generally to semiconductor optical amplifiers. More particularly, it relates to lasing semiconductor optical amplifiers used in combination with other optical elements to create a reconfigurable optical add drop multiplexer (OADM).  
         [0004]     2. Description of Related Technologies  
         [0005]     Fiber optic communications systems transmit information optically at high speeds over optical fibers. A typical communications system includes a transmitter, an optical fiber, and a receiver. The transmitter incorporates information to be communicated into an optical signal and transmits the optical signal via the optical fiber to the receiver. The receiver recovers the original information from the received optical signal. These systems are well adapted to transmit information at high speeds from one location to another.  
         [0006]     However, efficient point-to-point transmission alone is not sufficient to construct a practical network. For example, a fiber running from New York to San Francisco may be efficient for transporting traffic from New York to San Francisco, but does not help much in transporting to/from Chicago, for example. To add Chicago to the New York-San Francisco route, traffic must be able to be added or dropped from the stream between New York and San Francisco.  
         [0007]     An add-drop multiplexer (ADM) is the device which accomplishes this function. Many current ADMs are optical-electrical-optical, meaning that an incoming optical signal is converted to electrical form, the add-drop functionality is implemented electrically, and the resulting signals are then converted back to optical form. The two optical-electrical conversions add extra complexity and, strictly speaking, are unnecessary since the input and output signals are both optical.  
         [0008]     Hence, ADMs which are entirely optical (in the sense that the signal remains in optical form while traveling through the ADM) are desirable. For example, in a wavelength division multiplexed (WDM) system, an optical add drop multiplexer (OADM) might work in the following manner. An optical signal carrying a wavelength division multiplexed (WDM) optical signal is input into a wavelength division multiplexer that demultiplexes the WDM optical signal into N single wavelength signals. The OADM is configured with N inputs, N outputs, N add inputs and N drop outputs. Each of the N single wavelength signals is coupled to an input to the OADM. As the N optical signals propagate through the OADM, each channel (i.e., each wavelength) can be dropped, added or passed through the OADM as needed. Dropping a channel means that channel is redirected by the OADM to the drop outputs. Conversely, adding a channel means that channel is received at one of the N add inputs and the OADM directs it to one of the OADM output ports. A pass through occurs when a channel is directed by the OADM from one input to an output. The N optical signals at the outputs of the OADM are wavelength division multiplexed back together into a single WDM optical signal. This signal is then forwarded out over the optical communications system.  
         [0009]     OADMs may be either fixed or reconfigurable. In a fixed OADM, one or more of the channels are always dropped and/or added in the OADM. Thus, the path traveled by the particular optical signal is fixed based on which channel it occupies. In a reconfigurable OADM, switching elements inside the OADM allow each input signal to be dynamically added, dropped or passed through the OADM. Reconfigurable OADMs are preferred due to their increased functionality and flexibility in changing the topology of a network.  
         [0010]      FIG. 1  is an illustration of a fixed OADM  100 . A fixed OADM is configured such that N wavelength division multiplexed (WDM) channels are input into OADM  100  and M channels are dropped and added while the remaining N-M channels pass through OADM  100 . In this implementation, a single WDM optical signal containing four optical channels enters wavelength division demultiplexer  125  from optical fiber  115 . Wavelength division deumultiplexer  125  demultiplexes the WDM signal into four single wavelength optical signals  110 A-D. Incoming channels  110 C and  110 D are dropped to outputs  120 C and  120 D, respectively. Their outgoing counterparts are added from inputs  130 C and  130 D, respectively. The other two channels  110 A and  110 B simply pass through OADM  100 . As one can see from the figure, the paths traveled by the various channels are fixed.  
         [0011]      FIG. 2  is an illustration of another fixed OADM  200  wherein the switching fabric is a combination of circulators and a wavelength grating. As illustrated, three port circulator  250  is coupled to input  210  of OADM  200 . Circulator  250  is also coupled to fiber Bragg grating  260  and drop output  220 . Three port circulator  270  is also coupled to grating  260  and is coupled to output  240  and add input  230 .  
         [0012]     OADM  200  works in the following way. A WDM optical signal enters OADM  200  through input  210 . The optical signal passes through circulator  250  and encounters grating  260 . Grating  260  reflects a desired wavelength signal back to circulator  250  and passes the other wavelengths of the optical signal to circulator  270 . The reflected signal is directed to drop output  220  by circulator  250 . The remainder of the optical signal that was passed to circulator  270  is directed to output  240 . However, an optical signal of the wavelength that was dropped can also be added to the remainder of the optical signal at circulator  270 . Such a signal enters OADM  200  through add input  230 . This signal then enters circulator  270 , reflects off grating  260  and is combined with the remainder of the original optical signal. This new optical signal is then sent to output  240 .  
         [0013]     In addition to the two devices described above, an OADM can also be based on a crossbar switch. There are a number of devices and techniques that can implement (or attempt to implement) the basic switching of a crossbar. For example, various groups are attempting to develop optical crossbar switches based on MEMS (e.g., micro-mechnical mirrors), BUBBLES, liquid crystal (mirrors), LiNO 3 , or thermal optic switches. However, each of these devices has a loss associated with it when performing the switching function. This aspect of these devices makes their use less attractive in OADMs, particularly since switching may be cascaded in larger crossbars, thus compounding the overall loss through the crossbar. In addition, many of these approaches also suffer from other disadvantages, such as slow switching speed, large size, requiring complicated electronics, excessive intersymbol crosstalk and/or excessive crosstalk between different channels (e.g., WDM channels).  
       BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION  
       [0014]     In general, exemplary embodiments of the invention are concerned with optical multiplexers that include a reconfigurable switching fabric. More particularly, one embodiment of the invention is an optical multiplexer that includes a reconfigurable switching fabric with a plurality of inputs and a plurality of outputs. In addition, the reconfigurable switching fabric employs a plurality of VLSOAs that facilitate at least partial reconfiguration of the switching fabric by serving as switches to add, drop or pass-through one or more optical signals received at the plurality of inputs of the reconfigurable switching fabric. In some implementations, one or more of the VLSOAs also serve to amplify one or more optical signals so as to compensate for losses that may be experienced by the signal in transit through the switching fabric, or elsewhere. Finally, this exemplary implementation of the optical multiplexer includes a wavelength division multiplexer coupled to the reconfigurable switching fabric and configured to multiplex at least some of the optical signals received from outputs of the reconfigurable switching fabric. Among other things then, embodiments of the invention facilitate selective add, drop or pass-through of one or more optical signals, as well as the multiplexing of selected optical signals.  
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0015]      FIG. 1  (prior art) is an illustration of a fixed OADM  100 .  
         [0016]      FIG. 2  (prior art) is an illustration of another fixed OADM  200  based on circulators and a wavelength grating.  
         [0017]      FIG. 3  is an illustration of a reconfigurable OADM  300  according to the invention.  
         [0018]      FIG. 4  is an illustration of another reconfigurable OADM  400  according to the invention.  
         [0019]      FIG. 5A -C are block diagrams of a portion of the switching fabric that can be used in OADMs  300  and  400 .  
         [0020]      FIG. 6  is block diagram of VLSOAs  615  on the outputs of switching fabric  610  to balance the power of the optical signals output of switching fabric  610 .  
         [0021]      FIG. 7  is a block diagram of OADM  700  with VLSOAs  710  and  720  located on the inputs and outputs, respectively, of OADM  700 .  
         [0022]      FIG. 8  is a diagram of a vertical lasing semiconductor optical amplifier (VLSOA)  500  suitable for the present invention.  
         [0023]      FIG. 9  is a flow diagram illustrating operation of VLSOA  500 .  
         [0024]     FIGS.  10 A-C are a perspective view, transverse cross-sectional view, and a longitudinal cross-sectional view of one embodiment of a vertically lasing semiconductor optical amplifier (VLSOA)  500 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]      FIG. 3  is an illustration of a reconfigurable OADM  300  according to the present invention. OADM  300  comprises a switching fabric  390 , which includes two pass inputs  330 A-B and two pass outputs  360 A-B. OADM  300  also includes two add inputs  350 A-B and two drop outputs  340 A-B for a total of four inputs and four outputs to switching fabric  390 . Switching fabric  390  comprises four switching nodes  315 A-D that perform the add, drop and pass-through functionality for OADM  300 . In this embodiment, the nodes  315  are arranged in a two-dimensional matrix where the rows of the matrix are defined by input/output pairs  330 A/ 360 A and  330 B/ 360 B, and the columns of the matrix are defined by add/drop pairs  340 A/ 350 A and  340 B/ 350 B. One node  315  is located at each row/column junction of the matrix.  
         [0026]     In this example, OADM  300  has one input that receives a WDM optical signal  310  carrying two channels (i.e. two optical signals with different wavelengths). Optical signal  310  passes into wavelength division demultiplexer  320 . Wavelength division demultiplexer  320  demultiplexes the signal into two channels each with a different wavelength and inputs them into switching fabric  390  through pass inputs  330 A and  330 B. In this example, each of the channels input at pass input  330 A-B can be dropped to either of the drop outputs  340 A or  340 B. In addition, either of the add inputs  350 A or  350 B can be used to add channels to either of the pass outputs  360 A or  360 B. On the output side, the two optical channels leaving switching fabric  390  are combined by wavelength division multiplexer  370  into a single WDM optical signal  380 .  
         [0027]     Although  FIG. 3  and the accompanying text describe an OADM  300  with two pass inputs, two pass outputs, two drop outputs and two add inputs, the principles illustrated can be straightforwardly extended to OADM switching fabrics with varying numbers of pass inputs, pass outputs, add inputs and drop outputs. For example,  FIG. 4  is an illustration of another embodiment of the invention, OADM  400 , which has a switching fabric  490  comprising N pass inputs  410 A-N and N pass outputs  420 A-N. This configuration also has N add inputs  460 A-N and N drop outputs  470 A-N. In addition, there is a switching node  315  at each intersection of a pass input with an add/drop pair. Other configurations, including those in which the number of pass inputs, pass outputs, add inputs and/or drop outputs differ from each other, will be apparent to one skilled in the art.  
         [0028]     In addition, the principles illustrated in  FIG. 3  can be straightforwardly extended to OADMs with switching fabrics that provide different switching functionality. For example, in OADM  300  in  FIG. 3 , each incoming channel input to the pass inputs  330  may be dropped to either drop output  340 . Similarly, each optical signal input to add input  350  may be added to either pass output  360 . Other embodiments may utilize a different switching functionality. For example, in an alternate embodiment, each pass input  330  and pass output  360  may be limited to a corresponding drop output  340  and add input  350  (or subset of drop outputs and add inputs). For example, the “A” inputs/outputs may be dedicated to a first wavelength so that, for example, the pass input  330 A may only be dropped to drop output  340 A and not to drop output  340 B. Similarly, the add input  350 A may be limited to pass output  360 A. As another example, the OADM switching fabric may only be partially reconfigurable, meaning that some of the inputs and outputs may be hardwired to each other while the remaining inputs and outputs are reconfigurable (either with full crossbar functionality or a subset of fill crossbar functionality).  
         [0029]     As a final example, the switching fabric  390  can also have architectures other than the two dimensional matrix architecture shown in  FIGS. 3 and 4 . Examples of other architectures include Banyan and Clos architectures.  
         [0030]     Referring again to  FIG. 3 , the switching nodes  315  typically will include some sort of splitting and switching of optical signals which, if implemented passively, typically will result in a loss to the optical signal. The result is that optical signals leaving OADM  300  will be weaker than the optical signals entering OADM  300 . However, vertical lasing semiconductor optical amplifiers (VLSOA), as described herein, which have linear gain characteristics can be used to implement the switching functionality in the various nodes of an OADM, resulting in a lossless OADM switch. Using a VLSOA does not avoid the loss that results from implementation of the switching function (e.g., resulting from redirecting, splitting, coupling, and combining). However, the VLSOA can be used to amplify the optical signal, unlike passive switching components. This will compensate for the losses otherwise introduced.  
         [0031]     Constructing an OADM from VLSOAs has further advantages. VLSOAs can be switched quickly so that the overall OADM can be reconfigured quickly. In addition, the electronics required to switch the VLSOAs are relatively simple. In essence, the electronics need only differentiate between turning on the VLSOA (i.e., pumping the VLSOA sufficiently above its laser threshold) and turning off the VLSOA. VLSOAs, because of their gain-clamping characteristics, also have good crosstalk performance. This is important when the OADM adds/drops a WDM optical signal as opposed to single wavelength signals. It is more difficult to construct an OADM  300  from conventional non-lasing SOAs because conventional SOAs have poor intersymbol interference and also poor crosstalk between WDM channels, thus limiting the usefulness of any OADM based on conventional SOAS.  
         [0032]      FIG. 5A  is an illustration of a portion of the switching fabric of an OADM according to one embodiment of the invention. The broken line-box  315  illustrates a single node in the switching fabric according to  FIGS. 3 and 4 . The embodiment illustrated in  FIG. 5A  comprises a drop path  910 , an add path  920 , and two pass-through paths  930 A-B (one of which goes through node  315  and one of which is external to node  315 ). The pass-through paths  930  couple the pass inputs of the switching fabric to their corresponding pass outputs. Each add path is coupled to a corresponding add input of the switching fabric and each drop path is coupled to a corresponding drop output of the switching fabric. In one embodiment of the invention, the paths are waveguides. However, one skilled in the art will recognize that other embodiments for the paths are possible. For example, the paths could be optical fibers or free space. In addition, a plurality of VLSOAs  500  are coupled to these paths to perform the switching required to pass optical signals to the appropriate outputs.  
         [0033]     VLSOA  500 A is coupled between pass-through path  930 B and drop path  910 . A fiber coupler  950 A splits the optical signal propagating on pass-through path  930 B. A fiber coupler  950 A is an optical component that splits the optical signal into two (or more) different paths. Optical couplers other than fiber couplers may also be used. Part of the optical signal is input to VLSOA  500 A and part is split to VLSOA  500 B. In this embodiment, the power is split 50-50 between VLSOAs  500 A and  500 B, although different splitting ratios may be used in different designs.  
         [0034]     If the optical signal needs to be dropped to the drop path  910  so that it can be output from one of the drop outputs of the OADM, VLSOA  500 A will be turned on and will pass the optical signal to drop path  910 . Fiber coupler  950 B is used to couple the output of VLSOA  500 A to drop path  910 . VLSOA  500 A preferably also amplifies the optical signal as it propagates through the active region, as described herein, to make up for the loss introduced by fiber coupler  950 A and other losses. In an alternate embodiment illustrated in  FIG. 5B , VLSOA  500 I can be positioned on drop path  910  to make up for the losses introduced by fiber coupler  950 B and other losses. When the optical signal is dropped to drop path  910 , VLSOA  500 B can be turned off to block the optical signal from propagating further on pass-through path  930 B. In other cases, such as broadcasting, VLSOA  500 B can be turned on.  
         [0035]     By contrast, when the optical signal is not dropped to drop path  910 , VLSOA  500 B is turned on so that the optical signal continues to propagate on pass-through path  930 B. VLSOA  500 B also amplifies the optical signal as it propagates through the active region to make up for the loss introduced by fiber coupler  950 A. In addition, VLSOA  500 A is turned off so that the optical signal does not propagate on drop path  910  where it might interfere with another optical signal that has been dropped from another node onto drop path  910 .  
         [0036]     When an optical signal needs to be output to one of the pass outputs from one of the add inputs to the switching fabric, the optical signal is input on one of the add paths. In this example, suppose an optical signal is input to add path  920  that needs to be added to pass-through path  930 B. Fiber coupler  950 C will split part of the optical signal to VLSOA  500 C and the remaining portion of the optical signal will continue to propagate on add path  920 . When the optical signal is added to pass-through path  930 B, VLSOA  500 B can be turned off to block any optical signal currently propagating on pass-through path  930 B. VLSOA  500 C is turned on so that the optical signal split from add path  920  can pass onto pass-through path  930 B. Fiber coupler  950 D couples the output of VLSOA  500 C to pass-through path  930 B. In addition to switching the optical signal, VLSOA  500 C can amplify the optical signal to make up for loss.  
         [0037]     When an optical signal propagating on add path  920  is not to be added to pass-through path  930 B, VLSOA  500 C is turned off to block the portion of the optical signal split off from add path  920  by fiber coupler  950 C. However, VLSOA  500 B is turned on to allow the optical signal currently propagating on pass-through path  930 B to continue on pass-through path  930 B.  
         [0038]     As one can see from the  FIG. 5A , half the power of the optical signal propagating on add path  920  is split off by each fiber coupler encountered at each pass-through path  930 . If there are a large number of pass-through paths  930 , the optical signal propagating on add path  920  may become too weak to use reliably. An alternate embodiment, illustrated in  FIG. 5B , counters this problem by positioning VLSOA  500 H on add path  920  to make up for the loss introduced by fiber coupler  950 C and other losses.  
         [0039]     Alternately, the fiber couplers  950 C could implement splitting ratios other than 50-50. For example, if there were ten pass-through paths  930 , the fiber couplers  950 C could be designed so that each VLSOA  500 C receives 10% of the power of the optical signal originally input onto add path  920 . Thus, the first fiber coupler  950 C would tap 10% of the power, leaving 90% to propagate further down add path  920 . The second fiber coupler  950 C would tap 11% of the power (11% of 90%=10%), etc. Such an approach may also result in better noise performance. Similar remarks apply to tapping power from the pass-through paths  930 .  
         [0040]     The embodiment in  FIG. 5A  also illustrates VLSOA  500 G coupled to add path  920  and drop path  910  using fiber couplers  950 E and  950 F, respectively. VLSOA  500 G provides a loopback function that allows an optical signal input on add path  920  to be output directly to drop path  910 . When this is desirable, VLSOA  500 G is turned on to amplify and pass the optical signal from add path  920  to drop path  910 . This aspect of the embodiment illustrated in  FIG. 5A  is optional. In another embodiment, VLSOA  500 G is removed but add path  920  is still coupled to drop path  910 . This embodiment can also be used to provide the loopback function.  
         [0041]     Another embodiment of the switching fabric that can be used in an OADM is illustrated in  FIG. 5C . Once again, the functionality of this embodiment will be described using broken-line box  315  which represents a single node from the OADMs illustrated in  FIGS. 3 and 4 . In this embodiment, there are two pass-through paths,  930 B and  930 B 2  in each node  315 . Pass-through path  930 B couples one of the pass inputs of the OADM switching fabric to one of the pass outputs of the switching fabric. Pass-through path  930 B carries the optical signal input to the pass input and can either pass the optical signal to the corresponding pass output or can drop the optical signal on one of the drop paths  910  to one of the drop outputs of the switching fabric. As illustrated, pass-through path  930 B is coupled to drop path  910  through VLSOA  500 A. This part of the embodiment is similar to that in  FIG. 5A  and works in a similar way.  
         [0042]     The difference in this embodiment is that the add paths  920  are coupled to a second pass-through path  930 B 2 . This pass-through path does not couple to a switching fabric pass input or pass output. Pass-through path  930 B 2  is used to carry an optical signal that needs to eventually be added to pass-through path  930 B. As illustrated in broken-line box  315 , add path  920  is coupled to pass-through path  930 B 2  through VLSOA  500 C. In essence, adding an optical signal to pass-through path  930 B 2  is similar to adding an optical signal to pass-through path  930 B in  FIG. 5A . However, also note that pass-through path  930 B 2  is coupled to pass-through path  930 B using VLSOA  500 E (outside of broken-line box  315 ). Preferably, this is done just before pass-through path  930 B is coupled to its corresponding pass output of the switching fabric. When VLSOA  500 E is turned on, the optical signal that is output to the pass output of the switching fabric is the optical signal that was added to pass-through path  930 B 2  through one of the add paths  920 . When VLSOA  500 E is turned off, the optical signal that is output to the pass output of the switching fabric is the optical signal input on pass-through path  930 B at the switching fabric pass input.  
         [0043]     The advantage of this embodiment is that it allows an optical signal to be added from one of the add paths prior to dropping the optical signal from pass-through path  930 B, which was input from the pass input of the switching fabric, to one of the drop paths. For example, referring to  FIG. 3 , this embodiment of the switching fabric allows an optical signal to be added at add input  350 A, that is eventually output from the switching fabric  390  at pass output  360 A while at the same time allowing the optical signal input at pass input  330 A to be dropped to drop output  340 B. In the previous embodiments, this was more difficult because the optical signal added at add input  350 A would have been combined with the optical signal input from pass input  330 A. As a result, both optical signals would be dropped and passed through the switching fabric of the OADM.  
         [0044]     In another embodiment of the invention, the switching fabric of the OADM can be implemented as a full crossbar switch in which each pass input and add input of the switching fabric can be mapped to any of the pass outputs and/or drop outputs. An example of the crossbar switch can be found in copending patent application Ser. No. 10/020,527, entitled “Optical Crossbar Using Lasing Semiconductor Optical Amplifiers,” by Jeffrey D. Walker and Sol P. DiJaili, filed Dec. 15, 2001, which is herein incorporated by reference.  
         [0045]     As one skilled in the art will recognize, the configurations described above for the switching fabric can be scaled to function for any number of pass inputs, drop outputs, add inputs and pass outputs. In addition, one skilled in the art will recognize that this switching fabric allows an optical signal from any of the pass inputs to be output to any of the drop or pass outputs. Similarly, this configuration allows any optical signal input on the add input to be output to any of the pass outputs. One skilled in the art will also recognize that the amplification provided by each of the VLSOAs in the switching fabric can be adjusted depending on the strength of the optical signal it is amplifying.  
         [0046]     In an alternative embodiment of the OADM illustrated in  FIGS. 3 and 4 , demultiplexer  320  is configured to demultiplex the incoming WDM optical signal into a plurality of WDM optical signals each having one or more optical channels (i.e. wavelengths). These WDM optical signals can be added, dropped or passed-through by the switching fabric of the OADM in the same manner as the single wavelength optical signals described above. As described herein, VLSOAs  500  can amplify WDM optical signals with substantially less crosstalk than non-lasing SOAs due to the gain clamped characteristics of the VLSOA. Thus, VLSOAs  500  can still be used to perform the switching of the WDM optical signals in this embodiment of the invention.  
         [0047]      FIG. 6  is a block diagram of another embodiment of the invention. In this embodiment, VLSOAs  615 A-N are coupled to the pass outputs of switching fabric  610  in OADM  600 . The outputs of VLSOAs  615 A-N are then coupled to wavelength division multiplexer  370 . VLSOAs  615  are also coupled to microprocessor  620 . It should be noted that the switching fabric  610  of OADM  600  in this embodiment can be any conventional optical switching fabric or one of the switching fabrics of the present invention.  
         [0048]     Due to the reconfigurable nature of OADM  600 , the optical channels output from the pass outputs  640 A-N of switching fabric  610  may have come from pass inputs  650 A-N or from add inputs  670 A-N of switching fabric  610 . Since these signals may have come from different sources and may have traveled unrelated distances and paths to reach OADM  600  (and even traveled different paths through OADM  600 ), it is possible that some of the optical signals output from the pass outputs  640 A-N of switching fabric  610  will have different power levels and signal strengths. By placing VLSOAs  615 A-N on the path of optical channels output from pass outputs  640 A-N, VLSOAs  615  can balance the signal strength of the optical signals by amplify each optical signal to ensure that they all have the same power prior to being passed into wavelength division multiplexer  370 . Microprocessor  620  is coupled to each VLSOA  615 A-N so that it can supply a control signal to the VLSOAs  615 A-N. For example, microprocessor  620  can selectively adjust the amplification provided by each VLSOA  615  in order to achieve the desired output signal strength. In one embodiment, the VLSOAs  615 A-N are implemented as a monolithic array of VLSOAs, as opposed to discrete devices.  
         [0049]     Although the embodiment described above in  FIG. 6  refers to placing VLSOAs  615 A-N on the pass outputs of switching fabric  610 , balancing the optical signals at other locations can be also be achieved using VLSOAs  615 . For example, VLSOAs  615  could also be placed on the drop outputs  660 A-N, the add inputs  670 A-N and/or the pass inputs  650 A-N to amplify and balance the optical signals on these inputs/outputs. Similarly, a microprocessor  620  could also be coupled to these VLSOAs to control the amplification provided by each VLSOA  615 . For OADM&#39;s which use VLSOAs as part of the switching fabric  610 , the amplification provided by these VLSOAs can also be adjusted in order to balance the power in the optical signals.  
         [0050]     Another embodiment of the invention is illustrated in  FIG. 7 . In this embodiment, VLSOAs  710  and  720  are placed on the input and the output of OADM  700  respectively. The demultiplexer  730 , multiplexer  740  and the switching fabric  750  of the OADM introduce loss into the individual channels of the WDM optical signal as they are being switched to the correct output. Unlike the present invention, most OADMs do not have mechanisms in place to make up for these losses. As a result, the optical signals that are output from conventional OADMs are often very weak. By placing VLSOA  720  on the output of OADM  700 , all of the channels of the WDM optical signal output from OADM  700  can be amplified simultaneously. In addition, if the WDM optical signal is weak before entering OADM  700  (due to dispersion, etc.) the losses introduced by OADM  700  may degrade the optical signal to the point where the optical signal is no longer useable. Amplifying the WDM optical signal in VLSOA  710  before it is input into OADM  700  helps to counteract this problem.  
         [0051]     It should be noted that the switching fabric  750  of OADM  700  in this embodiment can be any conventional optical switching fabric or the switching fabric of the present invention. In addition, VLSOAs could also be placed on the add inputs or the drop outputs to amplify the optical signals being added or dropped to counteract the same problems described above.  
         [0052]      FIG. 8  is a diagram of a vertical lasing semiconductor optical amplifier (VLSOA)  500  suitable for the present invention. The VLSOA  500  has an input  812  and an output  814 . The VLSOA  500  further includes a semiconductor gain medium  820 , with an amplifying path  830  coupled between the input  812  and the output  814  of the VLSOA  500  and traveling through the semiconductor gain medium  820 . The VLSOA  500  further includes a laser cavity  840  including the semiconductor gain medium  820 , and a pump input  850  coupled to the semiconductor gain medium  820 . The laser cavity  840  is oriented vertically with respect to the amplifying path  830 . The pump input  850  is for receiving a pump to pump the semiconductor gain medium  820  above a lasing threshold for the laser cavity  840 .  
         [0053]      FIG. 9  is a flow diagram illustrating operation of VLSOA  500  when it is used as an amplifier. The VLSOA  500  receives  990  an optical signal at its input  812 . The optical signal propagates  991  along the amplifying path  830 . The pump received at pump input  850  pumps  992  the semiconductor gain medium above a lasing threshold for the laser cavity  840 . When lasing occurs, the round-trip gain offsets the round-trip losses for the laser cavity  840 . In other words, the gain of the semiconductor gain medium  820  is clamped to the gain value necessary to offset the round-trip losses. The optical signal is amplified  993  according to this gain value as it propagates along the amplifying path  830  (i.e., through the semiconductor gain medium  820 ). The amplified signal exits the VLSOA  500  via the output  814 .  
         [0054]     Note that the gain experienced by the optical signal as it propagates through VLSOA  500  is determined in part by the gain value of the semiconductor gain medium  820  (it is also determined, for example, by the length of the amplifying path  830 ) and this gain value, in turn, is determined primarily by the lasing threshold for the laser cavity  840 . In particular, the gain experienced by the optical signal as it propagates through each VLSOA  500  is substantially independent of the amplitude of the optical signal. This is in direct contrast to the situation with non-lasing SOAs and overcomes the distortion and crosstalk disadvantages typical of non-lasing SOAs.  
         [0055]      FIGS. 10A-10C  are a perspective view, transverse cross-section, and longitudinal cross-section, respectively, of one embodiment of VLSOA  500  according to the present invention, with  FIG. 10B  showing the most detail.  
         [0056]     Referring to  FIG. 10B  and working from bottom to top in the vertical direction (i.e., working away from the substrate  502 ), VLSOA  500  includes a bottom mirror  508 , bottom cladding layer  505 , active region  504 , top cladding layer  507 , confinement layer  519 , and a top mirror  506 . The bottom cladding layer  505 , active region  504 , top cladding layer  507 , and confinement layer  519  are in electrical contact with each other and may be in direct physical contact as well. An optional delta doping layer  518  is located between the top cladding layer  507  and confinement layer  519 . The confinement layer  519  includes a confinement structure  509 , which forms aperture  515 . The VLSOA  500  also includes an electrical contact  510  located above the confinement structure  509 , and a second electrical contact  511  formed on the bottom side of substrate  502 .  
         [0057]     VLSOA  500  is a vertical lasing semiconductor optical amplifier since the laser cavity  540  is a vertical laser cavity. That is, it is oriented vertically with respect to the amplifying path  530  and substrate  502 . The VLSOA  500  preferably is long in the longitudinal direction, allowing for a long amplifying path  530  and, therefore, more amplification. The entire VLSOA  500  is an integral structure formed on a single substrate  502  and may be integrated with other optical elements. In most cases, optical elements which are coupled directly to VLSOA  500  will be coupled to the amplifying path  530  within the VLSOA. Depending on the manner of integration, the optical input  512  and output  514  may not exist as a distinct structure or facet but may simply be the boundary between the VLSOA  500  and other optical elements. Furthermore, although this disclosure discusses the VLSOA  500  primarily as a single device, the teachings herein apply equally to arrays of devices.  
         [0058]     VLSOA  500  is a layered structure, allowing the VLSOA  500  to be fabricated using standard semiconductor fabrication techniques, preferably including organo-metallic vapor phase epitaxy (OMVPE) or organometallic chemical vapor deposition (OMCVD). Other common fabrication techniques include molecular beam epitaxy (MBE), liquid phase epitaxy (LPS), photolithography, e-beam evaporation, sputter deposition, wet and dry etching, wafer bonding, ion implantation, wet oxidation, and rapid thermal annealing, among others.  
         [0059]     The optical signal amplified by the VLSOA  500  is confined in the vertical direction by index differences between bottom cladding  505 , active region  504 , and top cladding  507 , and to a lesser extent by index differences between the substrate  502 , bottom mirror  508 , confinement layer  519 , and top mirror  506 . Specifically, active region  504  has the higher index and therefore acts as a waveguide core with respect to cladding layers  505 ,  507 . The optical signal is confined in the transverse direction by index differences between the confinement structure  509  and the resulting aperture  515 . Specifically, aperture  515  has a higher index of refraction than confinement structure  509 . As a result, the mode of the optical signal to be amplified is generally concentrated in dashed region  521 . The amplifying path  530  is through the active region  504  in the direction in/out of the plane of the paper with respect to  FIG. 10B .  
         [0060]     The choice of materials system will depend in part on the wavelength of the optical signal to be amplified, which in turn will depend on the application. Wavelengths in the approximately 1.3-1.7 micron region are currently preferred for telecommunications applications, due to the spectral properties of optical fibers. The approximately 1.28-1.35 micron region is currently also preferred for data communications over single mode fiber, with the approximately 0.8-1.1 micron region being an alternate wavelength region. The term “optical” is meant to include all of these wavelength regions. In one embodiment, the VLSOA  500  is optimized for the 1.55 micron window.  
         [0061]     In one embodiment, the active region  504  includes a multiple quantum well (MQW) active region. MQW structures include several quantum wells and quantum wells have the advantage of enabling the formation of lasers with relatively low threshold currents. In alternate embodiments, the active region  504  may instead be based on a single quantum well or a double-heterostructure active region. The active region  504  may be based on various materials systems, including for example IAlGaAs on InP substrates, InAlGaAs on GaAs, InGaAsP on InP, GaInNAs on GaAs, InGaAs on ternary substrates, and GaAsSb on GaAs. Nitride material systems are also suitable. The materials for bottom and top cladding layers  505  and  507  will depend in part on the composition of active region  504 .  
         [0062]     Examples of top and bottom mirrors  506  and  508  include Bragg reflectors and non-Bragg reflectors such as metallic mirrors. Bottom mirror  508  in  FIG. 10  is shown as a Bragg reflector. Top mirror  506  is depicted as a hybrid mirror, consisting of a Bragg reflector  517  followed by a metallic mirror  513 . Bragg reflectors may be fabricated using various materials systems, including for example, alternating layers of GaAs and AlAs, SiO 2  and TiO 2 , InAlGaAs and InAlAs, InGaAsP and InP, AlGaAsSb and AlAsSb or GaAs and AlGaAs. Gold is one material suitable for metallic mirrors. The electrical contacts  510 ,  511  are metals that form an ohmic contact with the semiconductor material. Commonly used metals include titanium, platinum, nickel, germanium, gold, palladium, and aluminum.  
         [0063]     In this embodiment, the laser cavity is electrically pumped by injecting a pump current via the electrical contacts  510 ,  511  into the active region  504 . In particular, contact  510  is a p-type contact to inject holes into active region  504 , and contact  511  is an n-type contact to inject electrons into active region  504 . Contact  510  is located above the semiconductor structure (i.e., above confinement layer  519  and the semiconductor part of Bragg reflector  517 , if any) and below the dielectric part of Bragg reflector  517 , if any. For simplicity, in  FIG. 10 , contact  510  is shown located between the confinement layer  519  and Bragg reflector  517 , which would be the case if Bragg reflector  517  were entirely dielectric. VLSOA  500  may have a number of isolated electrical contacts  510  to allow for independent pumping within the amplifier. This is advantageous because VLSOA  500  is long in the longitudinal direction and independent pumping allows, for example, different voltages to be maintained at different points along the VLSOA. Alternately, the contacts  510  may be doped to have a finite resistance or may be separated by finite resistances, rather than electrically isolated.  
         [0064]     Confinement structure  509  is formed by wet oxidizing the confinement layer  519 . The confinement structure  509  has a lower index of refraction than aperture  515 . Hence, the effective cross-sectional size of laser cavity  540  is determined in part by aperture  515 . In other words, the confinement structure  509  provides lateral confinement of the optical mode of laser cavity  540 . In this embodiment, the confinement structure  509  also has a lower conductivity than aperture  515 . Thus, pump current injected through electrical contact  510  will be channeled through aperture  515 , increasing the spatial overlap with optical signal  521 . In other words, the confinement structure  509  also provides electrical confinement of the pump current. Other confinement techniques may also be used, including those based on ion implantation, impurity induced disordering, ridge waveguides, buried tunnel junctions, and buried heterostructures.  
         [0065]     The above description is included to illustrate various embodiments of the present invention and is not meant to limit the scope of the invention. From the above description, many variations will be apparent to one skilled in the art that would be encompassed by the spirit and scope of the invention. The scope of the invention is to be limited only by the following claims.