Abstract:
An optical switch comprises a programmable optical polarization unit (POLCON) which receives an input optical signal and selects an output signal polarization in response to a control signal. A polarization beam splitter (PBS) splits the selected polarization signal from the POLCON into a first and second orthogonally polarized signals. A feedback circuit couples a feedback signal indicative of the optical signal strength of at least one of the orthogonally polarized signals back to the POLCON which uses it to adjust the output signal polarization. The optical switch may be used with input optical signals having fixed or varying polarizations by utilizing a single or two feedback signals, respectively. The optical switch may be used as an optical Add/Drop unit or incorporated as part of a wavelength division multiplexed (WDM) signal Add/Drop unit.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to an optical switch apparatus for use with Wavelength Division Multiplex (WDM) signals and, more particularly, to a programmable optical switch apparatus implemented using a controllable optical polarization unit. 
     BACKGROUND OF THE INVENTION 
     The architecture of a programmable add/drop node  100  is shown in FIG.  1 . In this scheme, all the WDM signal channels  101  of the system are demultiplexed in demultiplexer  102 . Following this, some of the wavelength channels continue through  103  the add/drop node while other wavelength channels can be dropped or added by add/drop switches  104 . Finally, all the signal channels are multiplexed together by multiplexer  105  to form WDM signal  106  which is passed on to an optical line system (OLS). Within the node  100 , add/drop switches  104  can be installed one channel at a time. In this manner scaling of the number of add/drop channels can be achieved at node  100  without complete disruption of service. Also, any add/drop switch can be reconfigured in response to a local or remote control signal  107  to change a signal channel from a drop to a through state or vice-versa. Consequently, a modular remotely re-configurable switch is advantageous for such an add/drop architecture. 
     A potential problem of this add/drop architecture is in-band crosstalk. It is well known that in-band crosstalk causes severe performance penalties at the receiver in an optical network. [1,2] (Note in this specification, a reference to another document is designated by a number in brackets to identify its location in a list of references found in the Appendix) In-band crosstalk refers to those optical fields that can interfere at a receiver with the signal field to produce spectral beat frequencies which are within the receiver bandwidth. When optical add/drop architectures such as the illustration in FIG. 1 are used in the network, in-band crosstalk can occur in two ways. First, limitations of out-of-passband extinction of the demultiplexer  102  leads to multi-path interference of the signal with itself at the output of the multiplexer  105  of FIG.  1 . While this contribution to in-band crosstalk penalty increases with the number of wavelength channels, the crosstalk level can be low due to a second rejection at the multiplexer  105 . Pires et al. [3] show that a rejection of more than 35dB is required at the demultiplexer in a full mesh WDM ring network in order to sustain nine nodes. Second, in the event of wavelength reuse the added signal channel suffers in-band crosstalk penalty at its receiver due to incomplete extinction of the drop-channel (leak-through)  108  at the switches as shown by  109  in FIG.  1 . The leak-through field  109  of the dropped channel interferes with the added signal field  110  since the spectra of both the added and dropped signal channels are nominally centered at the same wavelength. We have previously measured the in-band crosstalk penalties due to the second mechanism for different data rates and determined that the drop-channel must be suppressed by 32-35 dB to ensure that the in-band crosstalk penalty is less than 1 dB independent of the granularity of the optical network. 
     Thus, there is a continuing need to reduce the in-band crosstalk penalty in Add/Drop apparatuses. 
     SUMMARY OF THE INVENTION 
     In accordance with the apparatus and operating method of the present invention, we disclose an optical switch that eliminates the second type of in-band crosstalk penalty by ensuring that the polarization of an added signal is cross-polarized with respect to the leak-through optical field of a dropped signal. In the event of cross-polarization, there is no interference between the added signal and the leak-through at the receiver. In this case, the leak-through only contributes to non-signal received power which leads to far smaller power penalties. The task of cross-polarizing the added signal is complicated by the randomness of the polarization of leak-through signal. Since the dropped signal originates at a different part of the optical network, its polarization changes with time due to a number of environmental factors. In our optical switch, we provide a feedback signal which controls a polarization rotator [4] to maintain the drop channel field in a fixed state of linear polarization, thereby minimizing any leak-through signal. Thus, our optical switch provides the advantage of eliminating in-band crosstalk power penalties arising from interference between the optical fields of the added signal channel and the leaked-through drop signal channel. Moreover, our optical switch is a modular, remotely re-configurable switch which may be used in a wavelength add/drop node of FIG. 1 to perform three required functions—namely continue-no drop, drop and continue, and drop/add. 
     More generally, our invention is directed to an optical switch apparatus comprising (1) a programmable optical polarization unit for receiving an input optical signal and for selecting the polarization of an output signal in response to a control signal; (2) a polarization beam splitter (PBS) for splitting the selected polarization output signal from the OSP unit into a first and second orthogonally polarized signals; (3) a feedback circuit for coupling a feedback signal indicative of the optical signal strength of at least one of the orthogonally polarized signals back to the programmable optical polarization unit; and (4) wherein the programmable optical polarization unit adjusts its rotation, in response to the feedback signal, to maintain a fixed polarization of the selected polarization output signal. 
     In accordance with other aspects of the invention, the optical switch apparatus may be incorporated as part of an optical Add/Drop unit or a wavelength division multiplexed (WDM) signal Add/Drop unit. The optical switch apparatus may be used with input optical signals having fixed or varying polarizations by utilizing a single or two feedback signals, respectively. The optical switch apparatus also be implemented using a variety of polarization beam splitters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
     FIG. 1 shows, in accordance with the present invention, an illustrative block diagram of a programmable Add/Drop node for use with wavelength division multiplex (WDM) signals; 
     FIG. 2 shows, illustratively, a first embodiment of an Add/Drop optical switch which may be used in the Add/Drop node of FIG. 1 for controlling the Add/Drop/Continue capability for one wavelength; 
     FIG. 3 shows, illustratively, a second embodiment of an Add/Drop optical switch of the Add/Drop node of FIG. 1 for controlling the Add/Drop/Continue capability for one wavelength; 
     FIG. 4 shows an illustrative block diagram of the electro-optical polarization controller (POLCON) used in the Add/Drop apparatus of FIGS. 2 and 3; and 
     FIG. 5 shows an embodiment of an Drop/Continue optical switch apparatus. 
    
    
     In the following description, identical element designations in different figures represent identical elements. Additionally in the element designations, the first digit refers to the figure in which that element is first located (e.g.,  102  is first located in FIG.  1 ). 
     DETAILED DESCRIPTION 
     Shown in FIG. 1 is an illustrative block diagram of a programmable Add/Drop node  100  for use with wavelength division multiplex (WDM) signals. The Add/Drop node  100  includes one or more Add/Drop switches  104  each of which is programmable under control of control signal  107  to Drop, Drop and Continue, Add, or Continue (i.e., pass) one or more wavelengths from a received WDM signal  101  to form the output WDM signal  106 . 
     FIG. 2 shows, illustratively, a first embodiment of an Add/Drop switch apparatus  200  which may be used with the Add/Drop node  100  of FIG. 1 for controlling the Add/Drop/Continue capability for one or more wavelengths. The Add/Drop switch apparatus  200  includes an electro-optic polarization controller (POLCON)  202  which receives an input optical wavelength signal  201  (e.g., from demultiplexer  102 ) and rotates the polarization of the optical field of input signal  201  to the desired polarization at output  201   a.  As shown, the polarization  230  of the optical field of input signal  201  can be any type and may vary as a function of time. Since the input signal originates at a different part of the optical network, its polarization changes with time due to a number of environmental factors. The variability of the polarization of input signal  201  may be caused, for example, by a variety of environmental factors that affect an optical fiber facility over which the input signal was carried. These environmental factors may include temperature, stress, interference, etc, to which the optical fiber facility is exposed. Generally, the polarization of input signal  201  changes at a 10 to 100 μsec rate. Such a rate of polarization change, however, can be easily compensated for by our feedback arrangement used with POLCON  202 . 
     The optical output signal  201   a  of POLCON  202  is divided by first fiber Polarization Beam-Splitter (PBS 1 )  203  into its constituent orthogonal polarizations, shown as ↑ and . The state of polarization at each of the two outputs of PBS 1 , i.e., ↑ and  are preserved through the use of polarization maintaining (PM) fibers,  205  and  204 . The 10% couplers or taps,  206  and  207 , on each output path are terminated at photodetectors  208  and  209 , respectively. The two photodetectors—PD↑  209  and PD  208  monitor the power level of each of the two polarization states in the output signal of PBS 1   203 . The electrical photocurrents signals  210  and  211 , from photodetectors  208  and  209 , respectively, are inputs to a control circuit ( 401  of FIG. 4) of POLCON  202 . The ratio of photocurrents signals I PD↑   211 , and I PD   210 , gives the state of polarization of the input signal  201   a  to PBS 1   203  independent of the total power in the signal channel. Thus, if the ratio of photocurrents signals  211  to  212  is low the input signal to PBS 1   203  has more  than ↑ polarization. Conversely, if the ratio of photocurrents signals  211  to  210  is high the input signal to PBS 1   203  has more ↑ than  polarization. Two feedback loops are formed in FIG. 2, one formed by PBS 1 , coupler  206 , PD  208 , and POLCON  202 ; and the second formed by PBS 1 , coupler  207 , PD  207 , and POLCON  202 . The control signal  220  selects whether POLCON  202  is to be in the Drop (and Add), Continue, and Drop and Continue mode and produce, respectively, a , ↑, or combined  and ↑ polarized signal outputted from POLCON  202 . The feedback signals  210  and  211  enable POLCON  202  to adjust its polarization rotation to compensate for any polarization variation (e.g.,  230 ) in input signal  201  and thus maintain constant the selected polarization of the output signal  201 a of POLCON  202 . 
     If the control signal  220  indicates that the input optical signal  201  is to be dropped then POLCON  202  produces a  polarized signal which PBS 1  directs to the Drop port  221 . If the control signal  220  indicates that the optical signal  220  is to be passed through (or continued) Add/Drop switch  200 , then POLCON  202  produces a ↑ polarized signal which PBS 1  directs to fiber  205 . Thus, PBS 1   203  provides drop functionality to the Add/Drop switch apparatus  200 . 
     The add function is provided by PBS 2   222  which receives a  polarized signal at Add port  223  which is combined with any ↑ polarized signal on fiber  205  to produce the output signal  224 . Note, the ↑ polarized signal exists, for example, when control signal  220  selects that POLCON  202  is to be in the Drop and Continue mode (where both the ↑ and  polarized signals are present in signal  201   a ). The output signal  224  of PBS 2  is then multiplexed with the other signal channels by multiplexer  105  to form the WDM signal  106 . The above-described polarization beam splitters, PBS 1  and PBS 2 , may be of a type selected from a group including a prism, a thin glass plate, a partially reflective mirror, a bulk-optic device or other known type. 
     It should be noted that any two fixed orthogonal linear polarizations—illustratively, represented by ↑ and —can be set as the two output polarization states of the signal from POLCON  202 . While the Continue (or through) mode or state of the switch  200  is assigned the ↑-state of polarization, the Drop/Add mode is assigned the -state of polarization. 
     FIG. 4 shows an illustrative block diagram of POLCON  202  used in the Add/Drop apparatus of FIG.  2 . As shown POLCON  202  includes a control circuit  401  and a polarization rotator  404 . The polarization rotator  404  may be a polarization transformer of the type described in U.S. Pat. No. 5,930,414 issued on Jul. 27, 1999 to D. A. Fishman et al, which is incorporated by reference herein. The control circuit  401  includes a difference amplifier  402  which determines the ratio of the two photocurrents  210  and  211 . The output of amplifier  402  is fed to a mode select circuit  403  along with the control signal  220 . The control signal  220  enables mode select circuit  403  to output the proper voltage needed to select the operating mode of control polarization rotator  404 . Depending on particular operating mode selected, control polarization rotator  404  sets the linear state of polarization of its output signal  201   a.  The control signal  220  thus enables each of the Add/Drop switch  200  functions to be executed as follows: 
     Continue-no drop mode: This is achieved by ensuring that the feedback sets the output state of the POLCON  202  to the ↑-polarization state. In other words, the ratio I PD↑ /I PD  is set to a fixed high value (for example 27 dB). Thus as the polarization of the input signal varies over time, the feedback signals I PD↑ /I PD  cause polarization rotator  404  to change its rotation to maintain the polarization of the output signal  201   a  of POLCON  202  at the ↑-polarization state. The upper limit of this ratio is set by the polarization extinction of the PBS. 
     Drop-and-continue mode: Depending upon the required extinction between the drop and the through states of the add/drop, the ratio I PD↑ /I PD  can be set. The ratio of signal dropped to the signal continued is determined by the particular application. In a multicast application, the drop signal level may be determined by the number of nodes that are to receive the same optical signal. 
     Drop-and-Add mode: This is achieved by setting the ratio I PD↑ /I PD  to a fixed low value (for example −27 dB). Since the polarization of the added signal can be fixed and set to the -state of polarization, it is naturally cross-polarized with the through (↑) polarization state. Thus as the polarization of the input signal varies over time, the feedback signals I PD↑ /I PD  cause polarization rotator  404  to change its rotation to maintain the polarization of the output signal  201   a  of POLCON  202  at the -polarization state. The leak through signal in the -state of polarization is rejected a second time at PBS 2 . Thus, the total rejection of the drop channel is twice the ratio I PD↑ /I PD . 
     Our Add/Drop switch  200  has two important advantages. First, the Add/Drop switch  200  may be a separate module that can be installed in a specific signal channel at the add/drop node  100  without disrupting service in the other channels. Second, Add/Drop switch  200  may be remotely re-configurable requiring a control signal  220  which specifies just one number, namely the ratio I PD↑ /I PD . The control signal  220  may be provided by a node computer or optical network control computer. 
     Additionally, our Add/Drop optical switch ensures that the polarization of the added signal is cross-polarized with respect to the leak-through optical field of the dropped signal. Because of this cross-polarization, the leak-through signal does not interfere with the added signal when the added signal is detected at a receiver. In this case, the leak-through signal only contributes to non-signal received power which leads to far smaller power penalties. Since our optical switch  200  provides a feedback signal which controls a polarization rotator  404  to maintain the drop channel signal in a fixed state of linear polarization, it thereby minimizes any leak-through signal through optical switch  200 . Thus, our optical switch  200  provides the advantage of eliminating in-band crosstalk power penalties arising from interference between the optical fields of the added signal channel and the leaked-through drop signal channel. Moreover, our optical switch  200  is a modular, remotely re-configurable switch which may be used in a wavelength add/drop node of FIG. 1 to perform three required functions—namely continue-no drop, drop and continue, and drop/add. 
     Shown in FIG. 3 is an alternate Add/Drop switch  200  architecture in which a single bulk-optic PBS 1   301  performs both the add and drop functions. However, this places a stringent restriction on the polarization rejection ratio of PBS 1   301 . If this ratio is less than 35 dB, then the dropped signal will suffer in-band crosstalk penalty due to the added signal channel. 
     FIG. 5 shows an embodiment of an Drop/Continue switch apparatus which is used with a fixed polarization input signal  501 . Since the polarization of the input signal  501  is fixed, the feedback circuit consisting of PBS  503 , coupler  506 , PD  508 , and POLCON  502  is connected only at predetermined times (e.g., setup time). At setup, the rotation in the polarization rotator of POLCON  502  is adjusted to obtain the desired polarization of output signal  501   a  (i.e., ↑-polarization state for the Continue mode, -polarization state for Drop (and Add) mode, and ↑- and -polarization states for the Drop and Continue mode). 
     What has been described is merely illustrative of the application of the principles of the present invention. Other methods and arrangements can be implemented by those skilled in the art without departing from the spirit and scope of the present invention. 
     APPENDIX 
     References 
     [1] E. L. Goldstein, L. Eskildsen and A. F. Elrefaie, “Performance implications of component crosstalk in transparent lightwave networks,” IEEE Photon. Tech. 
     Left., vol.6, no.5, pp.657-660 (1994) 
     [2] C. X. Yu, W-k. Wang, and S. D. Brorson, “System degradation due to multipath coherent crosstalk in WDM network nodes,” J. Lightwave Tech., vol.16, no.8, pp.1380-1386 (1998) 
     [3] J. J. O. Pires, N. Parnis, E. Jones, and M. O&#39;Mahony, “Crosstalk implications in full-mesh WDM ring networks using arrayed-waveguide grating OADMs,” Proc. ECOC&#39;98, pp. 541-542, Sep. 20-24, 1998, Madrid, Spain 
     [4] F. Heismann, “Analysis of a reset-free polarization controller for fast automatic polarization stabilization in fiber-optic transmission systems,” J. Lightwave Tech., vol.12, no.4, pp.690-699